**Part 5**

**Applications of Photochemistry** 

192 Molecular Photochemistry – Various Aspects

Schweitzer, C. and R. Schmidt (2003). Physical mechanisms of generation and deactivation

Scurlock, R. D. and P. R. Ogilby (1988). Spectroscopic Evidence for the Formation of Singlet

Shao, Y. H., M. Head-Gordon, et al. (2003). The spin-flip approach within time-dependent

Stockert, J. C., M. Canete, et al. (2007). Porphycenes: Facts and prospects in photodynamic

Szabo, A. and N. S. Ostlund (1996). *Modern Quantum Chemistr - Introduction to Advanced* 

Tsubomura, T. M., S. (1960). Molecular Complexes and their Spectra. XII. Ultraviolet

Worth, G. A. and L. S. Cederbaum (2004). BEYOND BORN-OPPENHEIMER: Molecular

Yarkony, D. R. (1998). Conical intersections: Diabolical and often misunderstood. *Accounts of* 

Yarkony, D. R. (2005). Escape from the double cone: Optimized descriptions of the seam

Zewail, A. H. (2000). Femtochemistry: Atomic-scale dynamics of the chemical bond. *Journal* 

Zewail, A. H. (2000). Femtochemistry. Past, present, and future. *Pure and Applied Chemistry*,

therapy of cancer. *Current Medicinal Chemistry*,14, 9: 997-1026.

*Electronic Structure Theory*. Mineola, Dover Publications, INC.

space using gateway modes. *Journal of Chemical Physics*,123, 13.

*Journal of the American Chemical Society*,82: 5966-5974.

Molecular-Oxygen (1-Delta-6-O-2) Upon Irradiation of a Solvent Oxygen (3-Sigma-G-O(-)-2) Cooperative Absorption-Band. *Journal of the American Chemical Society*,110,

density functional theory: Theory and applications to diradicals. *Journal of Chemical* 

Absorption Spectra Caused by the Interaction of Oxygen with Organic Molecules.

Dynamics Through a Conical Intersection. *Annual Review of Physical Chemistry*,55:

of singlet oxygen. *Chemical Reviews*,103, 5: 1685-1757.

2: 640-641.

127-158.

72, 12: 2219-2231.

*Physics*,118, 11: 4807-4818.

*Chemical Research*,31, 8: 511-518.

*of Physical Chemistry A*,104, 24: 5660-5694.

**9** 

*1,2Portugal 3Brazil* 

**Solar Photochemistry for Environmental** 

**for Industrial Wastewater Treatment** 

Josino Costa Moreira3 and Luis Filipe Vieira Ferreira1

*2Centro Interdisciplinar de Investigação e Inovação,* 

*Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa,* 

*3Centro de Estudos da Saúde do Trabalhador e Ecologia Humana,* 

*Escola Nacional de Saúde Pública, Fundação Oswaldo Cruz, Rio de Janeiro,* 

**Remediation – Advanced Oxidation Processes** 

Anabela Sousa Oliveira1,2, Enrico Mendes Saggioro3, Thelma Pavesi3,

*1Centro de Química-Física Molecular and Institute of Nanosciences and Nanotechnology,* 

*Escola Superior de Tecnologia e Gestão, Instituto Politécnico de Portalegre, Portalegre,* 

Photochemistry is the chemistry induced by light. Being the sun the most abundant and widespread light (and consequently energy) source on earth, it is obvious that solar light can also induce chemical reactions. There are several classes of organic pollutants (organic dyes, pharmaceuticals, polycyclic aromatic hydrocarbons, polychlorinated pesticides, polychlorinated dibenzodioxins, dibenzofurans and biphenyls) that by the seriousness of the risks they pose to environment and human health are considered priorities for environmental monitoring by the most important environmental agencies. In this chapter we will show how solar light can be advantageously used for environmental remediation, leading to the destruction of environmentally relevant molecules, especially when they are present in industrial wastewaters. In fact, solar light can greatly contribute to the remediation (going from the partial decomposition to the complete destruction) of those environmental pollutants. This solar remediation action can be effective either through direct photolysis and photodegradation (light induced chemical bond cleavage leading to the formation of smaller compounds) or as being the photon source that triggers the processes of their photocatalytic degradation (solar photocatalysis through advanced

Advanced Oxidation Processes (AOPs) are an emergent and promising methodology for the degradation of persistent environmental pollutants, refractory to other environmental decontamination / remediation treatments. AOPs are methods of advanced photocatalysis that use the highly oxidant and non selective hydroxyl radicals, which are able to react with almost all classes of organic compounds leading to their total (complete) mineralization or to the formation of more biodegradable intermediates. The method has the advantage that it

**1. Introduction** 

oxidation processes).

## **Solar Photochemistry for Environmental Remediation – Advanced Oxidation Processes for Industrial Wastewater Treatment**

Anabela Sousa Oliveira1,2, Enrico Mendes Saggioro3, Thelma Pavesi3, Josino Costa Moreira3 and Luis Filipe Vieira Ferreira1 *1Centro de Química-Física Molecular and Institute of Nanosciences and Nanotechnology, Instituto Superior Técnico, Universidade Técnica de Lisboa, Lisboa, 2Centro Interdisciplinar de Investigação e Inovação, Escola Superior de Tecnologia e Gestão, Instituto Politécnico de Portalegre, Portalegre, 3Centro de Estudos da Saúde do Trabalhador e Ecologia Humana, Escola Nacional de Saúde Pública, Fundação Oswaldo Cruz, Rio de Janeiro, 1,2Portugal 3Brazil* 

### **1. Introduction**

Photochemistry is the chemistry induced by light. Being the sun the most abundant and widespread light (and consequently energy) source on earth, it is obvious that solar light can also induce chemical reactions. There are several classes of organic pollutants (organic dyes, pharmaceuticals, polycyclic aromatic hydrocarbons, polychlorinated pesticides, polychlorinated dibenzodioxins, dibenzofurans and biphenyls) that by the seriousness of the risks they pose to environment and human health are considered priorities for environmental monitoring by the most important environmental agencies. In this chapter we will show how solar light can be advantageously used for environmental remediation, leading to the destruction of environmentally relevant molecules, especially when they are present in industrial wastewaters. In fact, solar light can greatly contribute to the remediation (going from the partial decomposition to the complete destruction) of those environmental pollutants. This solar remediation action can be effective either through direct photolysis and photodegradation (light induced chemical bond cleavage leading to the formation of smaller compounds) or as being the photon source that triggers the processes of their photocatalytic degradation (solar photocatalysis through advanced oxidation processes).

Advanced Oxidation Processes (AOPs) are an emergent and promising methodology for the degradation of persistent environmental pollutants, refractory to other environmental decontamination / remediation treatments. AOPs are methods of advanced photocatalysis that use the highly oxidant and non selective hydroxyl radicals, which are able to react with almost all classes of organic compounds leading to their total (complete) mineralization or to the formation of more biodegradable intermediates. The method has the advantage that it

Solar Photochemistry for Environmental Remediation

treatment.

revised in the literature.

by traditional treatment.

– Advanced Oxidation Processes for Industrial Wastewater Treatment 197

these effluents are generally released into water bodies (rivers or sea), being usually designated as wastewaters (industrial or domestic). Deficient or incomplete wastewater treatment can lead to surface and groundwater contamination. The extensive use of chemicals such as pesticide, fertilizers, pharmaceuticals, detergents, etc. and soil deposition of urban solid residues are the most important causes of water contamination (Tchobanoglous, et al., 1993). Because of the risk posed to public health by consumption of contaminated water, special care must be taken in water source preservation and water

Water and wastewater treatment sequences consists of several different mechanical, physical, chemical and biological treatments that frequently include harrowing, filtration, flocculation, sedimentation, sterilization and chemistry oxidation of organic pollutants, among others. After physical treatment (filtration and sedimentation) the water still has considerable amounts of organic matter (including organic contaminants), which, in general, can be efficiently degraded under biological treatment (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003). The gaseous effluents are often released to the air through tall chimneys and their main treatments include masking, adsorption on active carbon, contact liquid method, combustion and biological treatment (Davis & Cornwell, 1998; Kiely, 1998; Nevers, 2000). The methods for the treatment of water and wastewater (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003) and gases (Davis & Cornwell, 1998; Eckenfelder, 2000; Nevers, 2000) are deeply

However, the treatment of wastewater containing some organic substances cannot be achieved by traditional processes, because they resist to biological degradation (biorecalcitrant or persistent organic pollutants - POPs) or they are not completely removed

Nowadays, the persistent organic pollutants (POPs) are a matter of great importance, because they cannot be eliminated by the ordinary water or wastewater treatments (Davis &

POPs are xenobiotic chemicals of natural or anthropogenic origin witch accumulated in the environment and biota, due to theirs highly refractory chemical structures and physicalchemical properties. Structurally they are polycyclic conjugated compounds (polycyclic aromatic hydrocarbons) or they have a high number of halogen atoms, especially chlorine or bromine (pesticides, polychlorinated dibenzodioxins – PCDDs -, polychlorinated dibenzofurans – PCDFs -, polychlorinated biphenyls – PCBs -, brominated flame retardants, etc). Because most POPs are semi-volatile they suffer long range transport and can be found anywhere, even in distant regions where they have never been produced or released. POPs have a lipophylic and hydrophobic characters and so they consequently bioaccumulate in fatty tissues of organisms and are capable of bioaccumulating or biomagnificating into food chains, reaching extremely high concentrations (in comparison with their environmental concentrations) on the top species (Baird, 1999). Many of these compounds are biologically actives possessing mutagenic and/or carcinogenic or even endocrine disruption properties. Although several of them have natural sources, the fast industrial development since the

Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Nevers, 2000).

**1.2 Treatment of wastewaters containing persistent organic pollutants** 

can be applied to a large set of different matrixes and that decontamination occurs through pollutants degradation instead of their simple phase transfer. These methodologies become even more attractive when they use the sunlight as energy source, and they are generally identified as Solar Photocatalysis. The most representative solar photocatalysis treatments are semiconductor photocatalysis (Titanium dioxide, TiO2, is the most used semiconductor) and photo-Fenton. Using TiO2 or photo-Fenton, highly oxidant hydroxyl radicals are produced to promote the degradation of environmental contaminants.

Photochemistry is not only the aim of solar photocatalysis in what regards source energy and basic mechanism of action but it also furnishes powerful analytical tools for the study, monitoring and understanding of the main photodegradation processes and theirs mechanisms of action. So, many environmental pollutants and effluents which contain them had already their photolytic and photocatalytic photodegradation mechanisms investigated and have been treated through direct photolysis and/or solar activated advanced oxidation technologies in different environmental segments. Although most of the studies reported in literature are performed in laboratorial conditions, most often with artificial irradiation simulating solar light, there are also studies performed in pilot plants at industrial scale. In fact, there are several technological solutions using solar light and photocatalysis being applied in remediation / detoxification pilot plants. Those technologies (that use batch or continuous reactors) as well as the chemical substances or type of effluents that have already been treated with or without solar concentration capabilities will be revised in this chapter.

In our opinion solar photochemistry through advanced oxidation process is an elegant application of fundamental photochemistry that is close to reach a wide industrial use and that in this way well deserves to be included in a reference work on photochemistry and their most relevant applications in modern world. Despite the impressive volume of data published in the last 30 years, the questions related to the actual trends and future involvement of advanced oxidation processes in environmental remediation applications is a hot topic as reflected by the increasing number of publications on the filed in the most recent years.

### **1.1 Treatment of industrial wastewaters**

Generally the term wastewater refers to any residual fluid released into the environment and that contains polluting potential. The equivalent term effluent, which means to spill, derives from the latin *effluente.*

In the last decades the growing environmental awareness led to the implementation of national, international and communitary legislation (Simonsen, 2007) which prohibits or severely restricts the discharge into the environment of untreated industrial effluents containing various classes of substances (a list of controlled or restricted organic pollutants is found in Metcalf & Eddy, 2003, chapter 2, pages 99 – 104). Therefore, particular attention has been devoted to the development of methodologies for industrial wastewater treatment able to destroy or reduce the concentration of restricted chemicals within the allowed legal limits (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Tchobanoglous at al., 1986; Nevers, 2000).

Effluents discharges into the environment may be liquid and gaseous. Domestic sewage and several different industrial effluents are the main sources of liquid effluents. After treatment,

can be applied to a large set of different matrixes and that decontamination occurs through pollutants degradation instead of their simple phase transfer. These methodologies become even more attractive when they use the sunlight as energy source, and they are generally identified as Solar Photocatalysis. The most representative solar photocatalysis treatments are semiconductor photocatalysis (Titanium dioxide, TiO2, is the most used semiconductor) and photo-Fenton. Using TiO2 or photo-Fenton, highly oxidant hydroxyl radicals are

Photochemistry is not only the aim of solar photocatalysis in what regards source energy and basic mechanism of action but it also furnishes powerful analytical tools for the study, monitoring and understanding of the main photodegradation processes and theirs mechanisms of action. So, many environmental pollutants and effluents which contain them had already their photolytic and photocatalytic photodegradation mechanisms investigated and have been treated through direct photolysis and/or solar activated advanced oxidation technologies in different environmental segments. Although most of the studies reported in literature are performed in laboratorial conditions, most often with artificial irradiation simulating solar light, there are also studies performed in pilot plants at industrial scale. In fact, there are several technological solutions using solar light and photocatalysis being applied in remediation / detoxification pilot plants. Those technologies (that use batch or continuous reactors) as well as the chemical substances or type of effluents that have already been treated with or without solar concentration capabilities will be revised in this chapter. In our opinion solar photochemistry through advanced oxidation process is an elegant application of fundamental photochemistry that is close to reach a wide industrial use and that in this way well deserves to be included in a reference work on photochemistry and their most relevant applications in modern world. Despite the impressive volume of data published in the last 30 years, the questions related to the actual trends and future involvement of advanced oxidation processes in environmental remediation applications is a hot topic as reflected by the increasing number of publications on the filed in the most

Generally the term wastewater refers to any residual fluid released into the environment and that contains polluting potential. The equivalent term effluent, which means to spill,

In the last decades the growing environmental awareness led to the implementation of national, international and communitary legislation (Simonsen, 2007) which prohibits or severely restricts the discharge into the environment of untreated industrial effluents containing various classes of substances (a list of controlled or restricted organic pollutants is found in Metcalf & Eddy, 2003, chapter 2, pages 99 – 104). Therefore, particular attention has been devoted to the development of methodologies for industrial wastewater treatment able to destroy or reduce the concentration of restricted chemicals within the allowed legal limits (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003;

Effluents discharges into the environment may be liquid and gaseous. Domestic sewage and several different industrial effluents are the main sources of liquid effluents. After treatment,

produced to promote the degradation of environmental contaminants.

recent years.

**1.1 Treatment of industrial wastewaters** 

Tchobanoglous at al., 1986; Nevers, 2000).

derives from the latin *effluente.*

these effluents are generally released into water bodies (rivers or sea), being usually designated as wastewaters (industrial or domestic). Deficient or incomplete wastewater treatment can lead to surface and groundwater contamination. The extensive use of chemicals such as pesticide, fertilizers, pharmaceuticals, detergents, etc. and soil deposition of urban solid residues are the most important causes of water contamination (Tchobanoglous, et al., 1993). Because of the risk posed to public health by consumption of contaminated water, special care must be taken in water source preservation and water treatment.

Water and wastewater treatment sequences consists of several different mechanical, physical, chemical and biological treatments that frequently include harrowing, filtration, flocculation, sedimentation, sterilization and chemistry oxidation of organic pollutants, among others. After physical treatment (filtration and sedimentation) the water still has considerable amounts of organic matter (including organic contaminants), which, in general, can be efficiently degraded under biological treatment (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003). The gaseous effluents are often released to the air through tall chimneys and their main treatments include masking, adsorption on active carbon, contact liquid method, combustion and biological treatment (Davis & Cornwell, 1998; Kiely, 1998; Nevers, 2000). The methods for the treatment of water and wastewater (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003) and gases (Davis & Cornwell, 1998; Eckenfelder, 2000; Nevers, 2000) are deeply revised in the literature.

However, the treatment of wastewater containing some organic substances cannot be achieved by traditional processes, because they resist to biological degradation (biorecalcitrant or persistent organic pollutants - POPs) or they are not completely removed by traditional treatment.

### **1.2 Treatment of wastewaters containing persistent organic pollutants**

Nowadays, the persistent organic pollutants (POPs) are a matter of great importance, because they cannot be eliminated by the ordinary water or wastewater treatments (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Nevers, 2000).

POPs are xenobiotic chemicals of natural or anthropogenic origin witch accumulated in the environment and biota, due to theirs highly refractory chemical structures and physicalchemical properties. Structurally they are polycyclic conjugated compounds (polycyclic aromatic hydrocarbons) or they have a high number of halogen atoms, especially chlorine or bromine (pesticides, polychlorinated dibenzodioxins – PCDDs -, polychlorinated dibenzofurans – PCDFs -, polychlorinated biphenyls – PCBs -, brominated flame retardants, etc). Because most POPs are semi-volatile they suffer long range transport and can be found anywhere, even in distant regions where they have never been produced or released. POPs have a lipophylic and hydrophobic characters and so they consequently bioaccumulate in fatty tissues of organisms and are capable of bioaccumulating or biomagnificating into food chains, reaching extremely high concentrations (in comparison with their environmental concentrations) on the top species (Baird, 1999). Many of these compounds are biologically actives possessing mutagenic and/or carcinogenic or even endocrine disruption properties. Although several of them have natural sources, the fast industrial development since the

Solar Photochemistry for Environmental Remediation

**2.1 Theory of advanced oxidation** 

– Advanced Oxidation Processes for Industrial Wastewater Treatment 199

and inorganic ions. The partial oxidation can be enough to decrease toxicity enabling biological degradation, but is essential to verify if the intermediary products formed are not more toxic than the parent compound under treatment. In the last 30 years several books (Bahnemann, 1999; Halmann, 1995; Pelizzetti & Serpone, 1989; Schiavello, 1988) and reviews (Byrne, et al., 2011; Dusek, 2010; Gogate & Pandit; 2004a, 2004b; Legrini, et al., 1993; Linsebigler, et al, 1995) were published on the subject. Blake, 2001 contains more than 1200 references on the subject. AOPs can remediate all different types of organic pollutants in liquid, gaseous or solid media, reason why they are used on the remediation of contaminated waters, liquid or gaseous effluents and also on the treatment of different hazardous wastes namely on contaminated soils. Some of the above mentioned reviews present comprehensive compilations of the substances and residues already mineralized

Although different advanced oxidation processes use several different reaction systems, all of them have the same chemical characteristic: i.e., the production and use of hydroxyl radicals (OH•) (Eckenfelder, 2000; Metcalf & Eddy, 2003). Hydroxyl radicals are highly reactive species that are able to attack and destroy even the most persistent organic molecules that are not oxidized by the oxidants as oxygen, ozone or chlorine (Eckenfelder, 2000). Table 1 shows oxidation potential of the hydroxyl radical and compares it with others

Hydroxyl radical is the most powerful oxidant after fluorine; it is able to initiate several oxidation reactions leading to complete mineralization of the original organic substances and their subsequent degradation products. Hydroxyl radical reacts will all classes of

Hydrogen abstraction produces organic radicals able to react with molecular oxygen and

OH• + RH → R• + H2O (1)

R• + O2 → R O•2 → (2)

using different advanced oxidation processes (Blake, 2001, Legrini, et al., 1993).

commons oxidants used in chemical oxidation (Fox & Dulay, 1993).

Fluorine 3.06 Hydroxyl radical 2.80 Atomic oxygen 2.42 Ozone 2.08 Hydrogen peroxide 1.78 Hypochlorite 1.49 Chlorine 1.36 Chlorine dioxide 1.27 Molecular oxygen 1.23

Table 1. Oxidation potential of most common oxidizing agents.

organics mainly by hydrogen abstraction:

originating peroxyl radicals.

 **Oxidizing agent Oxidation potential, Volt** 

late nineteenth century lead to an enormous increase either on the quantity and on the diversity of the persistent organic pollutants from anthropogenic origin present in the environment. Conjugation of their above mentioned characteristics determines that these compounds represent a high risk to public and environmental health.

Several of those substances have already been classified as prioritary substances for environmental monitoring (see Baird, 1999, chapter 7, pages 293 to 379). Dibenzodioxins, dibenzofurans, polychlorinated biphenyls and organochlorinated pesticides join the list of priority organic pollutants of World Health Organization (WHO), United Nations Environmental Program (UNEP) and other Environmental Protection Agencies (Kiely, 1998; Metcalf & Eddy, 2003). The Stockholm Convention regulates this matter worldwide. This Convention presents a list of POPs (originally 12 substances: aldrin, dieldrin, endrin, chlordane, PCDDs, PCDFs, BHC, DDT, heptachlor, mirex, PCBs, toxaphene). Nowadays there are other under consideration: HCH, chlordecone, hexabromobiphenyl, hexa and heptabromobiphenyl ether, pentachlorobenzene tetra and pentabromodiphenyl ether e perfluorooctanosulfonic acid and its salts) which production, use and trading are banned or severely restricted (United Nations Environment Programme, 2005; Stockholm Convention on Persistent Organic Pollutants, 2005; Oliveira, et al., 2004, 2008, 2011). There are many other synthetic substances that have been identified as priority pollutants for environmental monitoring by the United States Environmental Protection Agency (USEPA) based on theirs probable or confirmed carcinogenic, mutagenic, teratogenic or acute toxicity characteristics. Among them we can mention volatile organic compounds, agricultural fertilizers and chlorinated residues resultant from disinfection processes at water public supply systems. Many of those substances can either be found in the air (as is the case of the volatile organic compounds) or in surface and groundwater and they reach the reception media through domestic or industrial wastewater systems or due to drain-off from agriculture (as appends with pesticides and fertilizers). There are also several substances (i.e. dyes, pharmaceuticals, etc) some of them specially synthesized to be resistant to degradation and conventional wastewater treatment processes are not able to remove them efficiently (Eckenfelder, 2000). Although these substances are not classified as prioritary pollutants, their negative impact in aquatic life and the changes of physical-chemical characteristics of the water bodies even when present in low concentrations make the control of their concentration very important.

Once the use and discharge of bioactive organic substances in the different environmental segments is not easy to eliminate and appears extremely difficult to control its essential to develop new powerful, clean and safe environmental remediation technologies for their treatment especially for the biorecalcitrant organic pollutants. One of the new most promising technologies available uses hydroxyl radical, a highly reactive chemical species that can attack and destroy organic molecules and is denominated advanced oxidation processes (Eckenfelder, 2000; Metcalf & Eddy, 2003).

### **2. Advanced oxidation processes**

Advanced oxidation processes (AOPs) is the common name of several chemical oxidation methods used to remediate substances that are highly resistant the biological degradation. Although oxidation can be total, frequently a partial oxidation is sufficient to decrease the toxicity of the biorecalcitrant compound enabling their final treatment by conventional biological treatment. The complete oxidation leads to mineralization and yields CO2, H2O

and inorganic ions. The partial oxidation can be enough to decrease toxicity enabling biological degradation, but is essential to verify if the intermediary products formed are not more toxic than the parent compound under treatment. In the last 30 years several books (Bahnemann, 1999; Halmann, 1995; Pelizzetti & Serpone, 1989; Schiavello, 1988) and reviews (Byrne, et al., 2011; Dusek, 2010; Gogate & Pandit; 2004a, 2004b; Legrini, et al., 1993; Linsebigler, et al, 1995) were published on the subject. Blake, 2001 contains more than 1200 references on the subject. AOPs can remediate all different types of organic pollutants in liquid, gaseous or solid media, reason why they are used on the remediation of contaminated waters, liquid or gaseous effluents and also on the treatment of different hazardous wastes namely on contaminated soils. Some of the above mentioned reviews present comprehensive compilations of the substances and residues already mineralized using different advanced oxidation processes (Blake, 2001, Legrini, et al., 1993).

### **2.1 Theory of advanced oxidation**

198 Molecular Photochemistry – Various Aspects

late nineteenth century lead to an enormous increase either on the quantity and on the diversity of the persistent organic pollutants from anthropogenic origin present in the environment. Conjugation of their above mentioned characteristics determines that these

Several of those substances have already been classified as prioritary substances for environmental monitoring (see Baird, 1999, chapter 7, pages 293 to 379). Dibenzodioxins, dibenzofurans, polychlorinated biphenyls and organochlorinated pesticides join the list of priority organic pollutants of World Health Organization (WHO), United Nations Environmental Program (UNEP) and other Environmental Protection Agencies (Kiely, 1998; Metcalf & Eddy, 2003). The Stockholm Convention regulates this matter worldwide. This Convention presents a list of POPs (originally 12 substances: aldrin, dieldrin, endrin, chlordane, PCDDs, PCDFs, BHC, DDT, heptachlor, mirex, PCBs, toxaphene). Nowadays there are other under consideration: HCH, chlordecone, hexabromobiphenyl, hexa and heptabromobiphenyl ether, pentachlorobenzene tetra and pentabromodiphenyl ether e perfluorooctanosulfonic acid and its salts) which production, use and trading are banned or severely restricted (United Nations Environment Programme, 2005; Stockholm Convention on Persistent Organic Pollutants, 2005; Oliveira, et al., 2004, 2008, 2011). There are many other synthetic substances that have been identified as priority pollutants for environmental monitoring by the United States Environmental Protection Agency (USEPA) based on theirs probable or confirmed carcinogenic, mutagenic, teratogenic or acute toxicity characteristics. Among them we can mention volatile organic compounds, agricultural fertilizers and chlorinated residues resultant from disinfection processes at water public supply systems. Many of those substances can either be found in the air (as is the case of the volatile organic compounds) or in surface and groundwater and they reach the reception media through domestic or industrial wastewater systems or due to drain-off from agriculture (as appends with pesticides and fertilizers). There are also several substances (i.e. dyes, pharmaceuticals, etc) some of them specially synthesized to be resistant to degradation and conventional wastewater treatment processes are not able to remove them efficiently (Eckenfelder, 2000). Although these substances are not classified as prioritary pollutants, their negative impact in aquatic life and the changes of physical-chemical characteristics of the water bodies even when present in low concentrations make the control of their concentration very important. Once the use and discharge of bioactive organic substances in the different environmental segments is not easy to eliminate and appears extremely difficult to control its essential to develop new powerful, clean and safe environmental remediation technologies for their treatment especially for the biorecalcitrant organic pollutants. One of the new most promising technologies available uses hydroxyl radical, a highly reactive chemical species that can attack and destroy organic molecules and is denominated advanced oxidation

Advanced oxidation processes (AOPs) is the common name of several chemical oxidation methods used to remediate substances that are highly resistant the biological degradation. Although oxidation can be total, frequently a partial oxidation is sufficient to decrease the toxicity of the biorecalcitrant compound enabling their final treatment by conventional biological treatment. The complete oxidation leads to mineralization and yields CO2, H2O

compounds represent a high risk to public and environmental health.

processes (Eckenfelder, 2000; Metcalf & Eddy, 2003).

**2. Advanced oxidation processes** 

Although different advanced oxidation processes use several different reaction systems, all of them have the same chemical characteristic: i.e., the production and use of hydroxyl radicals (OH•) (Eckenfelder, 2000; Metcalf & Eddy, 2003). Hydroxyl radicals are highly reactive species that are able to attack and destroy even the most persistent organic molecules that are not oxidized by the oxidants as oxygen, ozone or chlorine (Eckenfelder, 2000). Table 1 shows oxidation potential of the hydroxyl radical and compares it with others commons oxidants used in chemical oxidation (Fox & Dulay, 1993).


Table 1. Oxidation potential of most common oxidizing agents.

Hydroxyl radical is the most powerful oxidant after fluorine; it is able to initiate several oxidation reactions leading to complete mineralization of the original organic substances and their subsequent degradation products. Hydroxyl radical reacts will all classes of organics mainly by hydrogen abstraction:

$$\bullet \text{OH} \bullet + \text{RH} \rightarrow \text{R} \bullet + \text{H}\_2\text{O} \tag{1}$$

Hydrogen abstraction produces organic radicals able to react with molecular oxygen and originating peroxyl radicals.

$$\mathsf{R}\bullet + \mathsf{O}\_{2} \rightarrow \mathsf{R}\bullet \mathsf{O}\_{2} \rightarrow \tag{2}$$

Solar Photochemistry for Environmental Remediation

(Ikehata & El-Din, 2004; Mills & Hunte, 1997).

(Galindo et al., 2000, Ikehata & El-Din, 2006).

hydrogen peroxide and UV radiation.

**2.2.2 Ozone + hydrogen peroxide** 

formation.

peroxide.

(Metcalf & Edie, 2003).

radiation (UV) according to:

**2.2.1 Ozone + UV** 

– Advanced Oxidation Processes for Industrial Wastewater Treatment 201

peroxide, ozone + UV + hydrogen peroxide and hydrogen peroxide + UV are the most used commercial processes (highlighted in italic in Table 2) and its use will be analyzed below

The hydroxyl radical production is achieved by ozone irradiation (O3) with ultraviolet

O3 + hUV → O2 + singlet oxygen (6)

Ozone photolysis in air with moisture produces two hydroxyl radicals by each ozone molecule while when the same reaction occurs in water the hydroxyl radicals produced suffer rapid recombination and hydrogen peroxide is readily formed. Due to this later reaction, the process in water is not economically viable, since a lot of energy as to be imparted to the system to keep the adequate hydroxyl radical concentration. However, ozone + UV process is efficient on degradation in gaseous phases. The efficiency is even higher if the compounds also undergo direct photolysis by ultraviolet radiation. Figure 1 presents a scheme of an advanced oxidation treatment unity using ozone and UV radiation

For compounds that do not efficiently absorb ultraviolet radiation the yield of the degradation processes can be increased adding hydrogen peroxide once the latter when in contact with ozone undergoes an additional reaction, further promoting hydroxyl radical

Figure 2 presents a scheme of an advanced oxidation unity using ozone and hydrogen

UV irradiation of water with hydrogen peroxide also leads to hydroxyl radical formation

However, frequently the process is not economically viable due to the low absorption extinction coefficient of hydrogen peroxide; this fact determines the use of high concentrations of hydrogen peroxide so that hydroxyl radicals are produced in the adequate amount. The combination of the later process with ozone promotes a better efficiency on the use of UV radiation. Figure 3, presents a scheme of an advanced oxidation unit using ozone,

Although the technologies presented above have reached commercial application, especially in industrial wastewater treatment and water disinfection all of them use high amounts of

**2.2.3 Hydrogen peroxide + UV and ozone + UV + hydrogen peroxide** 

Singlet oxygen + H2O → OH•+ OH• (in moistured air) (7)

Singlet oxygen + H2O → OH• + OH• → H2O2 (in water) (8)

H2O2 + 2 O3 → OH• + OH• + 3 O2 (9)

Electrophilic additions may also occur (Legrini et al., 1993).

$$\text{OH}\bullet + \text{PhX} \rightarrow \text{HOPhX}\bullet \tag{3}$$

Electron transfer reactions,

$$\rm OH^{\bullet} + \rm RX \to \rm RX^{\bullet} + \rm OH^{\bullet} \tag{4}$$

and reactions between hydroxyl radicals,

$$2\,\mathrm{OH}^{\bullet} \to \mathrm{H}\_{2}\mathrm{O}\_{2} \tag{5}$$

Hydroxyl radical is characterized by a non-selective attack; this is an extremely useful characteristic for an oxidant to be used on environmental remediation. Other relevant and important characteristics are the existence of several possible pathways for hydroxyl radical production and the fact that all reactions occur at normal temperature and pressure. AOPs advantageously promote complete degradation of pollutants being remediated while classical treatments usually only transfer target pollutants to another phase, leading to the production of secondary residues (slugs) that require further treatment or deposition. Therefore, the advanced oxidation process is a good method for environmental decontamination (Linsebigler et al., 1995).

AOPs versatility is favoured also by the existence of various pathways to produce hydroxyl radicals, which enables a high adaptability to any specific environmental remediation problem. The advanced oxidation process can degrade all types of organic compounds in water therefore they are widely used in industrial wastewater remediation.

### **2.2 Technologies used in the production of hydroxyl radicals**

Advanced oxidation processes enclose several different treatments options: as ozone, hydrogen peroxide, ultraviolet radiation, ultrasound, homogeneous and heterogeneous photocatalysis, photocatalytic disinfection and also their combination (Hoffmann et al., 1995). The use of hydroxyl radicals to promote chemical oxidation it is the common feature of all AOPs. Table 2 shows several of the chemical oxidation technologies available.


Table 2. Technologies used in the production of hydroxyl radicals.

The AOPs classification is frequently based on the use or not of ozone on the production of hydroxyl radicals. The classification can also be based on the use or not of irradiation and on the number of phases (homogeneous or heterogeneous). Ozone + UV, ozone + hydrogen peroxide, ozone + UV + hydrogen peroxide and hydrogen peroxide + UV are the most used commercial processes (highlighted in italic in Table 2) and its use will be analyzed below (Metcalf & Edie, 2003).

### **2.2.1 Ozone + UV**

200 Molecular Photochemistry – Various Aspects

Hydroxyl radical is characterized by a non-selective attack; this is an extremely useful characteristic for an oxidant to be used on environmental remediation. Other relevant and important characteristics are the existence of several possible pathways for hydroxyl radical production and the fact that all reactions occur at normal temperature and pressure. AOPs advantageously promote complete degradation of pollutants being remediated while classical treatments usually only transfer target pollutants to another phase, leading to the production of secondary residues (slugs) that require further treatment or deposition. Therefore, the advanced oxidation process is a good method for environmental

AOPs versatility is favoured also by the existence of various pathways to produce hydroxyl radicals, which enables a high adaptability to any specific environmental remediation problem. The advanced oxidation process can degrade all types of organic compounds in

Advanced oxidation processes enclose several different treatments options: as ozone, hydrogen peroxide, ultraviolet radiation, ultrasound, homogeneous and heterogeneous photocatalysis, photocatalytic disinfection and also their combination (Hoffmann et al., 1995). The use of hydroxyl radicals to promote chemical oxidation it is the common feature

The AOPs classification is frequently based on the use or not of ozone on the production of hydroxyl radicals. The classification can also be based on the use or not of irradiation and on the number of phases (homogeneous or heterogeneous). Ozone + UV, ozone + hydrogen

of all AOPs. Table 2 shows several of the chemical oxidation technologies available.

water therefore they are widely used in industrial wastewater remediation.

**2.2 Technologies used in the production of hydroxyl radicals** 

**Processes with ozone Processes without ozone** 

*Ozone + H2O2 + UV* Oxidation supercritical

Table 2. Technologies used in the production of hydroxyl radicals.

*Ozone + UV* Photocatalysis (UV+ photocatalyst)

Ozone + TiO2 H2O2 + UV + iron salts (Foto-Fenton) Ozone + TiO2 + H2O2 H2O2 + iron salts (Fenton reagent)

Ozone at high pH (8-10) *H2O2 + UV* 

*Ozone + H2O2* Ultrasound

Ozone + Ultrasound

OH• + PhX → HOPhX• (3)

OH• + RX → RX•**+** + OH **-** (4)

2 OH• → H2O2 (5)

Electrophilic additions may also occur (Legrini et al., 1993).

Electron transfer reactions,

and reactions between hydroxyl radicals,

decontamination (Linsebigler et al., 1995).

The hydroxyl radical production is achieved by ozone irradiation (O3) with ultraviolet radiation (UV) according to:

$$\text{O}\_3 + \text{hu}\_{\text{UV}} \rightarrow \text{O}\_2 + \text{singlet oxygen} \tag{6}$$

$$\text{Single oxygen} + \text{H}\_2\text{O} \rightarrow \text{OH} \bullet + \text{OH} \bullet \text{ (in moistured air)}\tag{7}$$

$$\text{Single oxygen} + \text{H}\_2\text{O} \rightarrow \text{OH} \bullet + \text{OH} \bullet \rightarrow \text{H}\_2\text{O}\_2 \text{ (in water)}\tag{8}$$

Ozone photolysis in air with moisture produces two hydroxyl radicals by each ozone molecule while when the same reaction occurs in water the hydroxyl radicals produced suffer rapid recombination and hydrogen peroxide is readily formed. Due to this later reaction, the process in water is not economically viable, since a lot of energy as to be imparted to the system to keep the adequate hydroxyl radical concentration. However, ozone + UV process is efficient on degradation in gaseous phases. The efficiency is even higher if the compounds also undergo direct photolysis by ultraviolet radiation. Figure 1 presents a scheme of an advanced oxidation treatment unity using ozone and UV radiation (Ikehata & El-Din, 2004; Mills & Hunte, 1997).

### **2.2.2 Ozone + hydrogen peroxide**

For compounds that do not efficiently absorb ultraviolet radiation the yield of the degradation processes can be increased adding hydrogen peroxide once the latter when in contact with ozone undergoes an additional reaction, further promoting hydroxyl radical formation.

$$\text{H}\_2\text{O}\_2 + 2\text{ O}\_3 \rightarrow \text{OH}\bullet + \text{OH}\bullet + 3\text{ O}\_2\tag{9}$$

Figure 2 presents a scheme of an advanced oxidation unity using ozone and hydrogen peroxide.

### **2.2.3 Hydrogen peroxide + UV and ozone + UV + hydrogen peroxide**

UV irradiation of water with hydrogen peroxide also leads to hydroxyl radical formation (Galindo et al., 2000, Ikehata & El-Din, 2006).

However, frequently the process is not economically viable due to the low absorption extinction coefficient of hydrogen peroxide; this fact determines the use of high concentrations of hydrogen peroxide so that hydroxyl radicals are produced in the adequate amount. The combination of the later process with ozone promotes a better efficiency on the use of UV radiation. Figure 3, presents a scheme of an advanced oxidation unit using ozone, hydrogen peroxide and UV radiation.

Although the technologies presented above have reached commercial application, especially in industrial wastewater treatment and water disinfection all of them use high amounts of

Solar Photochemistry for Environmental Remediation

extremely attractive (Byrne et al., 2011; Malato et al., 2002, 2009).

UV lamps

Effluent before treatment

H2O2

**3. Advanced oxidation processes with sunlight** 

Pandit, 2004a, 2004b).

activate catalytic processes.

processes (Fujishima et al., 2000; Pirkanniemi & Sillanpaa, 2002).

– Advanced Oxidation Processes for Industrial Wastewater Treatment 203

However, because AOPs are essential for treatment of resistant substances in wastewater the most recent research efforts were on the development of more efficient energy processes. AOPs that do not need UV irradiation to activate hydroxyl radical production and that alternatively can use sunlight (wavelengths greater than 300 nm) for the same propose are

> Effluent after treatment

Fig. 3. Scheme of advanced oxidation unit using ozone, hydrogen peroxide and UV light.

O3

The degradation of persistent organic pollutant using advanced oxidation processes with sunlight as energy source have as great advantage their lower costs. There are two advanced oxidation processes that enable the use of sunlight as energy source: heterogeneous photocatalysis using semiconductors and homogeneous photocatalysis using photo-Fenton

We can compare the solar emission spectra (starting at 300 nm) with the absorption spectra of titanium dioxide and of Fenton reactant (Malato et al., 2002). Heterogeneous photocatalysis activated by sunlight uses near ultraviolet solar spectrum (wavelength under 380 nm) and homogeneous photocatalysis by photo-Fenton uses a larger portion of solar spectrum (wavelength up to 580 nm). Both processes are efficient in the photodegradation of persistent organic pollutants, they are a innovative way of using a renewable energy and they are very promising technologies in what regards environmental remediation (Gogate &

Photocatalysis is the combination of photochemistry and catalysis, a process where light and catalysis are simultaneously used to promote or accelerate a chemical reaction. So, photocatalysis can be defined as "catalysis driven acceleration of a light induced reaction". Direct light absorption is one of photocatalysis bigger advantages compared to thermally

expensive reagents (hydrogen peroxide and ozone) and consume a lot of energy on UV radiation generation. So, their application is restricted to processes where more economic alternatives are not viable.

Fig. 1. Scheme of advanced oxidation treatment unit using ozone and UV radiation.

Fig. 2. Scheme of advanced oxidation unit using ozone and hydrogen peroxide.

$$\mathrm{H\_2O\_2 + hv\_{UV} \to OH^\cdot + OH^\cdot} \tag{10}$$

Taking in account their cost, advanced oxidation processes can be used in integrated treatment systems for water, and domestic or industrial wastewaters where prior to biological treatment an advanced oxidative degradation of toxic and refractory substances is performed (Mantzavinos & Psillakis, 2004; Oller et al., 2007; Zapata et al., 2010). Frequently, the primary attack promoted by hydroxyl radical is sufficient to produce less toxic compounds that can already undergo biological treatment. This integrated treatment scheme can lead effectively to global treatment cost reduction. At different times of the advanced oxidative treatment it is advisable to perform toxicity tests with different microorganisms commonly used in biological treatment. These toxicity tests will help to determine the moment that the advanced oxidation process can already be substitute by the biological treatment (Fujishima et al., 2000).

expensive reagents (hydrogen peroxide and ozone) and consume a lot of energy on UV radiation generation. So, their application is restricted to processes where more economic

Fig. 1. Scheme of advanced oxidation treatment unit using ozone and UV radiation.

O3 and effluent

O3 residue UV lamps

O3 residue

Fig. 2. Scheme of advanced oxidation unit using ozone and hydrogen peroxide.

biological treatment (Fujishima et al., 2000).

Effluent before treatment

H2O2

 H2O2 + hUV → OH• + OH•(10) Taking in account their cost, advanced oxidation processes can be used in integrated treatment systems for water, and domestic or industrial wastewaters where prior to biological treatment an advanced oxidative degradation of toxic and refractory substances is performed (Mantzavinos & Psillakis, 2004; Oller et al., 2007; Zapata et al., 2010). Frequently, the primary attack promoted by hydroxyl radical is sufficient to produce less toxic compounds that can already undergo biological treatment. This integrated treatment scheme can lead effectively to global treatment cost reduction. At different times of the advanced oxidative treatment it is advisable to perform toxicity tests with different microorganisms commonly used in biological treatment. These toxicity tests will help to determine the moment that the advanced oxidation process can already be substitute by the

O3

Effluent after treatment

Effluent after treatment

alternatives are not viable.

O3

Effluent before treatment However, because AOPs are essential for treatment of resistant substances in wastewater the most recent research efforts were on the development of more efficient energy processes. AOPs that do not need UV irradiation to activate hydroxyl radical production and that alternatively can use sunlight (wavelengths greater than 300 nm) for the same propose are extremely attractive (Byrne et al., 2011; Malato et al., 2002, 2009).

Fig. 3. Scheme of advanced oxidation unit using ozone, hydrogen peroxide and UV light.

### **3. Advanced oxidation processes with sunlight**

The degradation of persistent organic pollutant using advanced oxidation processes with sunlight as energy source have as great advantage their lower costs. There are two advanced oxidation processes that enable the use of sunlight as energy source: heterogeneous photocatalysis using semiconductors and homogeneous photocatalysis using photo-Fenton processes (Fujishima et al., 2000; Pirkanniemi & Sillanpaa, 2002).

We can compare the solar emission spectra (starting at 300 nm) with the absorption spectra of titanium dioxide and of Fenton reactant (Malato et al., 2002). Heterogeneous photocatalysis activated by sunlight uses near ultraviolet solar spectrum (wavelength under 380 nm) and homogeneous photocatalysis by photo-Fenton uses a larger portion of solar spectrum (wavelength up to 580 nm). Both processes are efficient in the photodegradation of persistent organic pollutants, they are a innovative way of using a renewable energy and they are very promising technologies in what regards environmental remediation (Gogate & Pandit, 2004a, 2004b).

Photocatalysis is the combination of photochemistry and catalysis, a process where light and catalysis are simultaneously used to promote or accelerate a chemical reaction. So, photocatalysis can be defined as "catalysis driven acceleration of a light induced reaction". Direct light absorption is one of photocatalysis bigger advantages compared to thermally activate catalytic processes.

Solar Photochemistry for Environmental Remediation

TiO2 (h+ VB) + OH-

TiO2 (e-

TiO2 (e-

– Advanced Oxidation Processes for Industrial Wastewater Treatment 205

Fig. 4. Heterogeneous photocatalysis on a semiconductor (TiO2) particle surface.

Presence of oxygen is essential in all oxidative degradation processes

because its reaction with TiO2 provides another hydroxyl radical source.

TiO2 (h+VB) + H2O adsorv → TiO2 + OH• adsorv + H+ (12)

Once formed hydroxyl radicals promote the already mentioned oxidation reactions that degrade the persistent organic pollutants. The electrons promoted to the conduction band are also able to reduce the oxygen available in the surroundings to superoxide radicals.

When hydrogen peroxide is added the speed of the photodegradation significantly increase

Although the nature of all oxidizing species formed on semiconductor surface after light absorption is controversial, all authors agree that hydroxyl radical is the main oxidizing species formed on semiconductor surface. Effectively all detected intermediary species during photodegradation of polycyclic aromatic hydrocarbons and halogenated organic compounds are typically hydroxylated structures (Bahnemann, 1999; Oliveira et al., 2004b; Xavier et al., 2005). More difficult to clarify is if oxidation proceeds by direct or indirect route, directly by holes or by hydroxyl radical formed from them, bonded to surface or in solution. On the other hand, the strong correlation between speed of degradation and the concentration of pollutants absorbed on catalyst surface also suggests and reinforces the hypothesis that the species responsible for the photodegradation are hydroxyl radicals formed on the surface of the photocatalyst. Laser flash photolysis and electronic

CB) + O2 → TiO2 + O2•**-**

adsorv → TiO2 + OH• adsorv (13)

(15)

TiO2 (h+ VB) + RXadsorv → TiO2 + RX•**+** adsorv (14)

CB) + H2O2 → TiO2 + OH**- +** OH• (16)

Nowadays, photocatalysis appears as an excellent tool for final treatments of samples containing persistent organic pollutants (POPs) when compared to classical treatments (Doll & Frimmel, 2005; Hincapié, 2005). In a near future they can turn in one of the most used technologies for POPs remediation.

### **3.1 Heterogeneous photocatalysis using semiconductors – TiO2/UV**

Heterogeneous photocatalysis is a (sun)light activated process that produces reducing and oxidizing species able to promote mineralization of organic pollutants using a semiconductor (TiO2, ZnO, etc) as catalyst. The interaction of a photon with the catalyst produces an electron/hole pair on it. Excited electrons can be transferred to chemicals (reduction) into the semiconductor particle environment and at the same time the catalyst accepts electrons of oxidized specie. The neat flux of electrons in both directions is null and the catalyst stays unaltered. The mechanism and the electron/hole generation processes of heterogeneous photocatalysis is addressed in several reviews (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Nevers, 2000) that also present exhaustive list of organic residues remediated already by the method.

The ability of heterogeneous photocatalysis to eliminate organic pollutants from gaseous or aqueous (Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003) streams was largely demonstrated. Polycyclic aromatic hydrocarbons, pentachlorophenol, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 4,4'-DDT, dichlorobiphenyls and dichlorodibenzodioxins were already mineralized by heterogeneous photocatalysis (Blake, 2001; Pelizzetti & Serpone, 1989). Once TiO2 only uses about 10% of the available solar radiation, several groups are working on improving TiO2 visible light absorption (Anandan & Yoon, 2003; Chan et al., 2011; Fox & Dulay, 1993; Gupta & Tripathi, 2011; Janus & Morawski, 2010; Linsebigler et al., 1995; Mills & Hunte, 1997; Mourao et al., 2009; Reynaud et al., 2011; Roy et al., 2011; Zhang et al., 2004) to improve photocatalysis efficiency.

### **3.1.1 Heterogenous photocatalysis mechanism**

Semiconductors (as TiO2, ZnO, CdS, ZnS, etc.) have a typical electronic structure composed by a fully occupied valence band (VB) and an empty conduction band (CB). This typical semiconductor's electronic structure enable they can act as sensitizers of light induced oxidation processes. Photocatalysis action mechanism can be visualized on Figure 4. In this scheme the valence and conduction band of a semiconductor are represented over a spherical semiconductor particle.

The semiconductor (TiO2) absorbs photons with enough energy to promote an electron from the valence band to the conduction band and on the process an electron/hole pair is formed. The energy of the absorbed photon has to be equal or higher than that of the semiconductor "band-gap". The process can be described in a simple way by the following set of equations:

$$\text{TiO}\_2 + \text{hv} \rightarrow \text{h^\*}\_{\text{VB}} + \text{e}\_{\text{CB}} \tag{11}$$

While the electron is promoted to the conduction band a hole is produced in the valence band; this hole has a high oxidative power (+1 to +3.5 V, depending on semiconductor and pH) able not only to oxidize the water absorb on semiconductor surface producing hydroxyl radicals but also able to oxidize hydroxide ions, OH- , or the substrate itself, RX (Figure 4).

Nowadays, photocatalysis appears as an excellent tool for final treatments of samples containing persistent organic pollutants (POPs) when compared to classical treatments (Doll & Frimmel, 2005; Hincapié, 2005). In a near future they can turn in one of the most used

Heterogeneous photocatalysis is a (sun)light activated process that produces reducing and oxidizing species able to promote mineralization of organic pollutants using a semiconductor (TiO2, ZnO, etc) as catalyst. The interaction of a photon with the catalyst produces an electron/hole pair on it. Excited electrons can be transferred to chemicals (reduction) into the semiconductor particle environment and at the same time the catalyst accepts electrons of oxidized specie. The neat flux of electrons in both directions is null and the catalyst stays unaltered. The mechanism and the electron/hole generation processes of heterogeneous photocatalysis is addressed in several reviews (Davis & Cornwell, 1998; Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003; Nevers, 2000) that also present

The ability of heterogeneous photocatalysis to eliminate organic pollutants from gaseous or aqueous (Eckenfelder, 2000; Kiely, 1998; Metcalf & Eddy, 2003) streams was largely demonstrated. Polycyclic aromatic hydrocarbons, pentachlorophenol, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 4,4'-DDT, dichlorobiphenyls and dichlorodibenzodioxins were already mineralized by heterogeneous photocatalysis (Blake, 2001; Pelizzetti & Serpone, 1989). Once TiO2 only uses about 10% of the available solar radiation, several groups are working on improving TiO2 visible light absorption (Anandan & Yoon, 2003; Chan et al., 2011; Fox & Dulay, 1993; Gupta & Tripathi, 2011; Janus & Morawski, 2010; Linsebigler et al., 1995; Mills & Hunte, 1997; Mourao et al., 2009; Reynaud et al., 2011; Roy et al., 2011; Zhang

Semiconductors (as TiO2, ZnO, CdS, ZnS, etc.) have a typical electronic structure composed by a fully occupied valence band (VB) and an empty conduction band (CB). This typical semiconductor's electronic structure enable they can act as sensitizers of light induced oxidation processes. Photocatalysis action mechanism can be visualized on Figure 4. In this scheme the valence and conduction band of a semiconductor are represented over a

The semiconductor (TiO2) absorbs photons with enough energy to promote an electron from the valence band to the conduction band and on the process an electron/hole pair is formed. The energy of the absorbed photon has to be equal or higher than that of the semiconductor "band-gap". The process can be described in a simple way by the following set of equations:

TiO2 + h → h+VB + e-

While the electron is promoted to the conduction band a hole is produced in the valence band; this hole has a high oxidative power (+1 to +3.5 V, depending on semiconductor and pH) able not only to oxidize the water absorb on semiconductor surface producing hydroxyl

CB (11)

, or the substrate itself, RX (Figure 4).

**3.1 Heterogeneous photocatalysis using semiconductors – TiO2/UV** 

exhaustive list of organic residues remediated already by the method.

et al., 2004) to improve photocatalysis efficiency.

**3.1.1 Heterogenous photocatalysis mechanism** 

radicals but also able to oxidize hydroxide ions, OH-

spherical semiconductor particle.

technologies for POPs remediation.

Fig. 4. Heterogeneous photocatalysis on a semiconductor (TiO2) particle surface.

$$\rm TiO\_2\ (h^\*\nu\_B) + H\_2O\_{ads\text{-}v} \rightarrow TiO\_2 + OH^\cdot \+ \_{ads\text{-}v} + H^+ \tag{12}$$

$$\mathrm{TiO\_2}\left(\mathrm{h^{\*}}\mathrm{vB}\right) + \mathrm{OH^{\*}}\_{\mathrm{adsvar}} \longrightarrow \mathrm{TiO\_2} + \mathrm{OH^{\*}}\mathrm{s}\_{\mathrm{adsvar}}\tag{13}$$

$$\text{TiO}\_2\text{ (h}^\*\text{}\_\text{VB}\text{)} + \text{RX}\_\text{ads}\text{cov}\rightarrow\text{TiO}\_2 + \text{RX}\bullet^\*\_\text{ads}\text{cov}\tag{14}$$

Once formed hydroxyl radicals promote the already mentioned oxidation reactions that degrade the persistent organic pollutants. The electrons promoted to the conduction band are also able to reduce the oxygen available in the surroundings to superoxide radicals. Presence of oxygen is essential in all oxidative degradation processes

$$\text{TiO}\_2\text{(e}\_\text{C3}\text{)} + \text{O}\_2 \rightarrow \text{TiO}\_2 + \text{O}\_2\text{\textbullet} \tag{15}$$

When hydrogen peroxide is added the speed of the photodegradation significantly increase because its reaction with TiO2 provides another hydroxyl radical source.

$$\text{TiO}\_2\text{(e}\_{\text{CH}}\text{)} + \text{H}\_2\text{O}\_2 \rightarrow \text{TiO}\_2 + \text{OH} \cdot \text{H} \cdot \text{OH} \bullet \tag{16}$$

Although the nature of all oxidizing species formed on semiconductor surface after light absorption is controversial, all authors agree that hydroxyl radical is the main oxidizing species formed on semiconductor surface. Effectively all detected intermediary species during photodegradation of polycyclic aromatic hydrocarbons and halogenated organic compounds are typically hydroxylated structures (Bahnemann, 1999; Oliveira et al., 2004b; Xavier et al., 2005). More difficult to clarify is if oxidation proceeds by direct or indirect route, directly by holes or by hydroxyl radical formed from them, bonded to surface or in solution. On the other hand, the strong correlation between speed of degradation and the concentration of pollutants absorbed on catalyst surface also suggests and reinforces the hypothesis that the species responsible for the photodegradation are hydroxyl radicals formed on the surface of the photocatalyst. Laser flash photolysis and electronic

Solar Photochemistry for Environmental Remediation

**3.2 Homogeneous photocatalysis - photo-Fenton process** 

2006):

removing iron at the end of the treatment.

– Advanced Oxidation Processes for Industrial Wastewater Treatment 207

new insights in this subject with several classes of nanoscale materials (some of them including already titanium or other catalysts) that are being already evaluated as functional materials for water purification (Biswas & Wu, 2005; Wang et al., 2008; Xu et al., 2011): metal-containing nanoparticles, carbonaceous nanomaterials, zeolites and dendrimers (Savage & Diallo, 2005). The use of light to activate such nanoparticles opens up new ways to design green oxidation technologies for environmental remediation (Kamat & Meisel, 2002, 2003; Savage & Diallo, 2005). Due to their high-specific surface area, nanoparticles exhibit enhanced reactivity when compared with their bulk counterparts by several reasons such as the proportion of surface sites at edges or corners, the presence of distorted highenergy sites, contributions of interfacial free energies to chemical thermodynamics, the effects of altered surface regions, and quantum effects. Nanoparticles can be easily deposited or anchored onto various surfaces or used as a tailored film. These facilities can improve the adsorption of desirable chemicals, such as organics and heavy metals onto film surfaces. (Biswas & Wu, 2005; Kamat & Meisel, 2002, 2003). TiO2 nanoparticules have been extensively studied for oxidative and reductive transformation of organic and inorganic species present as contaminants water (Wang et al., 2008; Xu et al., 2011). Ashasi et al. (2001) synthesized N-doped TiO2 nanoparticles that were capable of photodegraded methylene blue under visible light and Bae & Choi (2003) have synthesized visible light-activated TiO2 nanoparticles based on TiO2 modified by ruthenium-complex sensitizers and Pt deposits.

Fenton's reagent is another extremely useful source of hydroxyl radicals and a potent oxidant of organic compounds. It was first described at the end of the XIX century and consists in a process in homogeneous phase, in which an aqueous hydrogen peroxide solution and Fe2+ (ferrous) ions, in acidic conditions (pH = 2-4), generate hydroxyl radicals, in a process that is not activated by light (Nogueira et al., 2007; Pignatello et al., 2006):

 Fe2+ + H2O2 → Fe3+ + OH- + OH• (17) When Fenton process occurs under solar radiation, degradation rate increases significantly. Although the oxidizing power of Fenton reaction was known for more than one hundred years, only recently it was discovered that Fenton reaction can be accelerated by irradiation with ultraviolet or visible light ( <580 nm), making it a photocatalytic process (Fe2+ is regenerated). The so-called photo-Fenton reaction produces additional hydroxyl radicals and leads to the photocatalyst reduction by light (Nogueira et al., 2007; Pignatello et al.,

Fe3+ + H2O + hUV-Vis → Fe2+ + H+ + OH• (18)

The great advantage of this process when compared with heterogeneous photocatalysis with TiO2 it is its sensibility to light up to 580 nm. When compared with TiO2 photocatalysis, this process allows a more efficient use of sunlight. The contact between the pollutant and the oxidizing agent is more effective once the process occur in homogeneous phase. The disadvantages of the photo-Fenton process are the treatment aggressivity due to low pH required (usually below 4), the high consumption of hydrogen peroxide and the need of

paramagnetic resonance proved to be helpful on the elucidation on the nature of intermediary species formed during photocatalytic degradation processes (Bahnemann, 1999; Botelho do Rego and Vieira Ferreira, 2001, Fox & Dulay, 1993, Oliveira et al., 2004b).

### **3.1.2 Photocatalysts**

The ideal semiconductor to be used as photocatalyst must be photoactive, able to absorb ultraviolet and visible radiation, photostable, chip and biologically and chemically inert. TiO2, ZnO e CdS are the most studied photocatalysts. Titanium dioxide has all above mentioned characteristics of a good photocatalyst and is in fact the photocatalyst with higher fotocatalytic activity on organic matter decomposition. This quality of titanium dioxide made him the reference semiconductor to establish and compare the photocatalytic activity of other semiconductor materials. TiO2 photocatalysis also obeys to green chemistry key principles (Anastas & Warner, 1998; Hermann et al., 2007).

TiO2 occurs in three crystals forms: anatase, rutile and brokite. Anatase is the photocatalytic active form. However, different semiconductor batches have present different photocatalytic activities from batch to batch and between different producers. It is also difficult to reproduce the photocatalytic activity between laboratories. Because of that, TiO2 P25 from Degussa is currently accepted as the standard titanium dioxide. TiO2 Degussa P25 without any treatment is used on phenol degradation for comparative proposes of the photocatalytic reactors performance. Degradation of 4-chlorophenol is also a standard reaction for certification of titanium dioxide photocatalytic activity.

TiO2 Degussa P25 is the standard form of TiO2; it is a powder available commercially with a purity of 99.5% (70:30 anatase : rutile), with a superficial area of 50 ± 15 m2/g, its not porous and have cubic particles of rounded edges and a average particle diameter of 21 nm. However TiO2 particles does not exist isolated but as complex irreducible primary aggregates of about 1 m.

To perform a heterogeneous photocatalytic reaction activated by light it is necessary to use semiconductors with the adequate "band-gap" to be activated by solar energy. TiO2 have a high bang-gap, of 3.2 eV, being consequently activated only by radiation below 380 nm, i.e., using only 10% of the sunlight spectrum. However, metal oxides with high band-gap, as TiO2, go on being strongly used on photocatalysis since they are usually resistant to photocorrosion. Although photocatalysts with lower band-gaps present bigger sensitivity to solar spectrum, they are not frequently used because they experience strong photocorrosion, being globally less effective.

Maximizing the efficiency of photocatalysis is an object of great challenge for scientists. Many efforts have been devoted to extend the photocatalytic properties into the visible region (Anandan & Yoon, 2003; Asahi et al., 2001; Augugliaro et al., 2006; Chan et al., 2011; Emeline et al., 2008; Fox & Dulay, 1993; Gupta & Tripathi, 2011; Janus & Morawski, 2010; Linsebigler et al., 1995; Mills & Hunte, 1997; Mourao et al., 2009; Reynaud et al., 2011; Roy et al., 2011; Zhang et al., 2004). Approaches such as doping the TiO2 with transition metal ions or the deposition of a noble metal on semiconductor particles have been successfully used. Inclusion of iron into TiO2 particles, for example, has been effectively used in the degradation of chlorinated organic compounds. Additionally nanotechnology is providing

paramagnetic resonance proved to be helpful on the elucidation on the nature of intermediary species formed during photocatalytic degradation processes (Bahnemann, 1999; Botelho do Rego and Vieira Ferreira, 2001, Fox & Dulay, 1993, Oliveira et al., 2004b).

The ideal semiconductor to be used as photocatalyst must be photoactive, able to absorb ultraviolet and visible radiation, photostable, chip and biologically and chemically inert. TiO2, ZnO e CdS are the most studied photocatalysts. Titanium dioxide has all above mentioned characteristics of a good photocatalyst and is in fact the photocatalyst with higher fotocatalytic activity on organic matter decomposition. This quality of titanium dioxide made him the reference semiconductor to establish and compare the photocatalytic activity of other semiconductor materials. TiO2 photocatalysis also obeys to green chemistry

TiO2 occurs in three crystals forms: anatase, rutile and brokite. Anatase is the photocatalytic active form. However, different semiconductor batches have present different photocatalytic activities from batch to batch and between different producers. It is also difficult to reproduce the photocatalytic activity between laboratories. Because of that, TiO2 P25 from Degussa is currently accepted as the standard titanium dioxide. TiO2 Degussa P25 without any treatment is used on phenol degradation for comparative proposes of the photocatalytic reactors performance. Degradation of 4-chlorophenol is also a standard reaction for

TiO2 Degussa P25 is the standard form of TiO2; it is a powder available commercially with a purity of 99.5% (70:30 anatase : rutile), with a superficial area of 50 ± 15 m2/g, its not porous and have cubic particles of rounded edges and a average particle diameter of 21 nm. However TiO2 particles does not exist isolated but as complex irreducible primary

To perform a heterogeneous photocatalytic reaction activated by light it is necessary to use semiconductors with the adequate "band-gap" to be activated by solar energy. TiO2 have a high bang-gap, of 3.2 eV, being consequently activated only by radiation below 380 nm, i.e., using only 10% of the sunlight spectrum. However, metal oxides with high band-gap, as TiO2, go on being strongly used on photocatalysis since they are usually resistant to photocorrosion. Although photocatalysts with lower band-gaps present bigger sensitivity to solar spectrum, they are not frequently used because they experience strong photocorrosion,

Maximizing the efficiency of photocatalysis is an object of great challenge for scientists. Many efforts have been devoted to extend the photocatalytic properties into the visible region (Anandan & Yoon, 2003; Asahi et al., 2001; Augugliaro et al., 2006; Chan et al., 2011; Emeline et al., 2008; Fox & Dulay, 1993; Gupta & Tripathi, 2011; Janus & Morawski, 2010; Linsebigler et al., 1995; Mills & Hunte, 1997; Mourao et al., 2009; Reynaud et al., 2011; Roy et al., 2011; Zhang et al., 2004). Approaches such as doping the TiO2 with transition metal ions or the deposition of a noble metal on semiconductor particles have been successfully used. Inclusion of iron into TiO2 particles, for example, has been effectively used in the degradation of chlorinated organic compounds. Additionally nanotechnology is providing

key principles (Anastas & Warner, 1998; Hermann et al., 2007).

certification of titanium dioxide photocatalytic activity.

**3.1.2 Photocatalysts** 

aggregates of about 1 m.

being globally less effective.

new insights in this subject with several classes of nanoscale materials (some of them including already titanium or other catalysts) that are being already evaluated as functional materials for water purification (Biswas & Wu, 2005; Wang et al., 2008; Xu et al., 2011): metal-containing nanoparticles, carbonaceous nanomaterials, zeolites and dendrimers (Savage & Diallo, 2005). The use of light to activate such nanoparticles opens up new ways to design green oxidation technologies for environmental remediation (Kamat & Meisel, 2002, 2003; Savage & Diallo, 2005). Due to their high-specific surface area, nanoparticles exhibit enhanced reactivity when compared with their bulk counterparts by several reasons such as the proportion of surface sites at edges or corners, the presence of distorted highenergy sites, contributions of interfacial free energies to chemical thermodynamics, the effects of altered surface regions, and quantum effects. Nanoparticles can be easily deposited or anchored onto various surfaces or used as a tailored film. These facilities can improve the adsorption of desirable chemicals, such as organics and heavy metals onto film surfaces. (Biswas & Wu, 2005; Kamat & Meisel, 2002, 2003). TiO2 nanoparticules have been extensively studied for oxidative and reductive transformation of organic and inorganic species present as contaminants water (Wang et al., 2008; Xu et al., 2011). Ashasi et al. (2001) synthesized N-doped TiO2 nanoparticles that were capable of photodegraded methylene blue under visible light and Bae & Choi (2003) have synthesized visible light-activated TiO2 nanoparticles based on TiO2 modified by ruthenium-complex sensitizers and Pt deposits.

### **3.2 Homogeneous photocatalysis - photo-Fenton process**

Fenton's reagent is another extremely useful source of hydroxyl radicals and a potent oxidant of organic compounds. It was first described at the end of the XIX century and consists in a process in homogeneous phase, in which an aqueous hydrogen peroxide solution and Fe2+ (ferrous) ions, in acidic conditions (pH = 2-4), generate hydroxyl radicals, in a process that is not activated by light (Nogueira et al., 2007; Pignatello et al., 2006):

$$\rm Fe^{2+} + H\_2O\_2 \rightarrow Fe^{3+} + OH + OH^\bullet \tag{17}$$

When Fenton process occurs under solar radiation, degradation rate increases significantly. Although the oxidizing power of Fenton reaction was known for more than one hundred years, only recently it was discovered that Fenton reaction can be accelerated by irradiation with ultraviolet or visible light ( <580 nm), making it a photocatalytic process (Fe2+ is regenerated). The so-called photo-Fenton reaction produces additional hydroxyl radicals and leads to the photocatalyst reduction by light (Nogueira et al., 2007; Pignatello et al., 2006):

$$\rm Fe^{3+} + H\_2O + h\nu\_{UV\,V\,lb} \rightarrow Fe^{2+} + H^+ + OH^+ \tag{18}$$

The great advantage of this process when compared with heterogeneous photocatalysis with TiO2 it is its sensibility to light up to 580 nm. When compared with TiO2 photocatalysis, this process allows a more efficient use of sunlight. The contact between the pollutant and the oxidizing agent is more effective once the process occur in homogeneous phase. The disadvantages of the photo-Fenton process are the treatment aggressivity due to low pH required (usually below 4), the high consumption of hydrogen peroxide and the need of removing iron at the end of the treatment.

Solar Photochemistry for Environmental Remediation

**4.2 Reactors with solar collectors** 

below the four most commonly used.

**4.2.1 Thin film fixed bed reactor** 

the maximum radiation available.

**4.2.2 Parabolic trough reactor** 

(Bahnemamm, 1999; Malato et al., 2002).

used, the rest being lost by various causes.

2009).

– Advanced Oxidation Processes for Industrial Wastewater Treatment 209

As the temperature plays no role in photochemical reactions, their technological applications typically only use low and medium temperature collectors, which have a much more economical construction. An important difference between these two reactors is that the first type of concentrators uses both direct and diffuse radiation while the concentrators collectors only use direct radiation. In terms of collector and reactor design itself, the systems have much in common with that of conventional thermal collectors; however, as the effluent to be cleaned must be directly exposed to sunlight, the absorber must be transparent to the photons. As temperature is not important, the systems are not thermally isolated. Most photocatalytic remediation systems involve wastewaters (Duran et al., 2010, 2011; Malato et al., 2007a ,2007b; Marugan et al., 2007; Miranda-Garcia et al., 2011; Navarro et al., 2011; Oyama et al., 2011; Vilar et al., 2009; Zayani et al., 2009), but the appropriated technology for gas phase photocatalytic processes is also possible (Lim et al.,

Several types of solar reactors for effluents decontamination have been tested. We describe

This reactor, whose simplified diagram is presented in Figure 5, is one of the first nonconcentrators solar reactors, so it can use the total solar radiation (direct and diffuse) for the photocatalytic process. Quantities of direct and diffuse radiation reaching earth are nearly identical, so concentrator reactors, by not using the diffuse radiation, profit from only half of

The most important part of the reactor is a tilted fixed dish (0.6 m x 1.2 m) coated with a thin film of photocatalyst, typically Degussa P25 TiO2, which is continuously washed with a film of about 100 m from the wastewater to be treated at a rate of 1 to 6.5 liters per hour

This reactor directly concentrates sunlight by a factor from 5 to 50. Tracking of solar radiation is done by a single or dual motors system that allow the continued alignment of the solar concentrator with the sun and various reactors can be connected in series or in parallel. In the parabolic trough reactor, the reflector has a parabolic profile and the tube where the photocatalytic reaction takes place is in its focus, in this way, only the light that enters parallel in the reflector can be focused on the reaction tube (Figure 6). This type of reactor is being used in solar decontamination circuits installed in the United States (Albuquerque, Sandia National Laboratories, and California, Lawrence Livermoore Laboratories) and Spain (Plataforma Solar de Almeria), Figure 7 (Navntoft et al., 2009).

The concentrated radiation is focused into a tube containing an aqueous suspension of TiO2 and the effluent to be treated. In fact, only about 60% of the radiation collected is effectively

The use of sunlight instead of artificial light for photo-Fenton activation, besides increasing the efficiency, also significantly decreases the cost of treatment. Therefore photo-Fenton is a great advance towards industrial implementation of photocatalysis processes (Brillas et al., 2009; Nogueira et al., 2007; Pignatello et al., 2006). Foto-phenton's process ability to treat water containing various pollutants (Kavitha & Palanivelu, 2004; Soon & Hameed; 2011; Umar et al., 2010) was already proved.

### **4. Industrial units of wastewater treatment by photocatalysis**

### **4.1 Solar collectors for photochemical processes**

The use of light activated advanced oxidation processes requires the development of dedicated photochemical solar technology that include the design of efficient solar photons collection technologies and the direction of those photons to the appropriate reactor in order to promote the photodegradation of the persistent organic pollutants to be remediated. For solar photochemical processes it is more interesting the collection of photons with high-energy and low wavelength, since typically the majority of the photocatalysis processes use solar radiation in the ultraviolet (300-400 nm). The exception is photo-Fenton process, which uses all sunlight below 580 nm. Usually radiation with wavelengths higher than 600 nm does not have any utility for photocatalysis processes. Solar flux at ground level is about 20 to 30 W per square meter, so sun approximately provides 0.2 to 0.3 moles of photons per square meter per hour (Bahnemamm, 1999; Malato et al., 2002).

The equipment that makes the efficient collection of photons is the solar collector. This equipment represents the largest source of operating costs of a photocatalysis unit for treatment of effluents. Solar collectors can be classified into three types according to the level of solar concentration achieved, which is usually directly related to the temperature reached by the system. So we have solar collectors which are non concentrators, moderately concentrators or highly concentrators. They can also be called concentrators of low (<150 °C), medium (150 - 400 ° C) and high (> 400 ° C) temperature.

The non concentrating collectors or low temperature collectors are static and usually consist of flat plates directed towards the sun with a certain inclination, depending on the geographic location. Its main advantage is their great simplicity and low cost.

The moderately concentrating collectors or medium temperature collectors concentrate the sun 5 to 50 times; to achieve this concentration factor the equipment must be able to continuously follow the sun. The parabolic and holographic are such type of collectors. Parabolic collectors have a parabolic reflecting surface which concentrates the radiation in a tubular collector located at the focus of the parabola and can have uni or biaxial movement. Holographic collectors consist of reflective surfaces like convex lenses that deflect radiation and concentrate it in a focus.

The highly concentrating collectors or high temperature collectors have a punctual focus rather than a linear one and they are based on a paraboloid with solar tracking. These reactors ensure solar concentrations from 100 to 10,000 times, reason why they require high precision optical elements.

As the temperature plays no role in photochemical reactions, their technological applications typically only use low and medium temperature collectors, which have a much more economical construction. An important difference between these two reactors is that the first type of concentrators uses both direct and diffuse radiation while the concentrators collectors only use direct radiation. In terms of collector and reactor design itself, the systems have much in common with that of conventional thermal collectors; however, as the effluent to be cleaned must be directly exposed to sunlight, the absorber must be transparent to the photons. As temperature is not important, the systems are not thermally isolated. Most photocatalytic remediation systems involve wastewaters (Duran et al., 2010, 2011; Malato et al., 2007a ,2007b; Marugan et al., 2007; Miranda-Garcia et al., 2011; Navarro et al., 2011; Oyama et al., 2011; Vilar et al., 2009; Zayani et al., 2009), but the appropriated technology for gas phase photocatalytic processes is also possible (Lim et al., 2009).

### **4.2 Reactors with solar collectors**

208 Molecular Photochemistry – Various Aspects

The use of sunlight instead of artificial light for photo-Fenton activation, besides increasing the efficiency, also significantly decreases the cost of treatment. Therefore photo-Fenton is a great advance towards industrial implementation of photocatalysis processes (Brillas et al., 2009; Nogueira et al., 2007; Pignatello et al., 2006). Foto-phenton's process ability to treat water containing various pollutants (Kavitha & Palanivelu, 2004; Soon & Hameed; 2011;

The use of light activated advanced oxidation processes requires the development of dedicated photochemical solar technology that include the design of efficient solar photons collection technologies and the direction of those photons to the appropriate reactor in order to promote the photodegradation of the persistent organic pollutants to be remediated. For solar photochemical processes it is more interesting the collection of photons with high-energy and low wavelength, since typically the majority of the photocatalysis processes use solar radiation in the ultraviolet (300-400 nm). The exception is photo-Fenton process, which uses all sunlight below 580 nm. Usually radiation with wavelengths higher than 600 nm does not have any utility for photocatalysis processes. Solar flux at ground level is about 20 to 30 W per square meter, so sun approximately provides 0.2 to 0.3 moles of photons per square meter per hour (Bahnemamm, 1999;

The equipment that makes the efficient collection of photons is the solar collector. This equipment represents the largest source of operating costs of a photocatalysis unit for treatment of effluents. Solar collectors can be classified into three types according to the level of solar concentration achieved, which is usually directly related to the temperature reached by the system. So we have solar collectors which are non concentrators, moderately concentrators or highly concentrators. They can also be called concentrators of low (<150 °C),

The non concentrating collectors or low temperature collectors are static and usually consist of flat plates directed towards the sun with a certain inclination, depending on the

The moderately concentrating collectors or medium temperature collectors concentrate the sun 5 to 50 times; to achieve this concentration factor the equipment must be able to continuously follow the sun. The parabolic and holographic are such type of collectors. Parabolic collectors have a parabolic reflecting surface which concentrates the radiation in a tubular collector located at the focus of the parabola and can have uni or biaxial movement. Holographic collectors consist of reflective surfaces like convex lenses that deflect radiation

The highly concentrating collectors or high temperature collectors have a punctual focus rather than a linear one and they are based on a paraboloid with solar tracking. These reactors ensure solar concentrations from 100 to 10,000 times, reason why they require high

geographic location. Its main advantage is their great simplicity and low cost.

**4. Industrial units of wastewater treatment by photocatalysis** 

Umar et al., 2010) was already proved.

Malato et al., 2002).

and concentrate it in a focus.

precision optical elements.

**4.1 Solar collectors for photochemical processes** 

medium (150 - 400 ° C) and high (> 400 ° C) temperature.

Several types of solar reactors for effluents decontamination have been tested. We describe below the four most commonly used.

### **4.2.1 Thin film fixed bed reactor**

This reactor, whose simplified diagram is presented in Figure 5, is one of the first nonconcentrators solar reactors, so it can use the total solar radiation (direct and diffuse) for the photocatalytic process. Quantities of direct and diffuse radiation reaching earth are nearly identical, so concentrator reactors, by not using the diffuse radiation, profit from only half of the maximum radiation available.

The most important part of the reactor is a tilted fixed dish (0.6 m x 1.2 m) coated with a thin film of photocatalyst, typically Degussa P25 TiO2, which is continuously washed with a film of about 100 m from the wastewater to be treated at a rate of 1 to 6.5 liters per hour (Bahnemamm, 1999; Malato et al., 2002).

### **4.2.2 Parabolic trough reactor**

This reactor directly concentrates sunlight by a factor from 5 to 50. Tracking of solar radiation is done by a single or dual motors system that allow the continued alignment of the solar concentrator with the sun and various reactors can be connected in series or in parallel. In the parabolic trough reactor, the reflector has a parabolic profile and the tube where the photocatalytic reaction takes place is in its focus, in this way, only the light that enters parallel in the reflector can be focused on the reaction tube (Figure 6). This type of reactor is being used in solar decontamination circuits installed in the United States (Albuquerque, Sandia National Laboratories, and California, Lawrence Livermoore Laboratories) and Spain (Plataforma Solar de Almeria), Figure 7 (Navntoft et al., 2009).

The concentrated radiation is focused into a tube containing an aqueous suspension of TiO2 and the effluent to be treated. In fact, only about 60% of the radiation collected is effectively used, the rest being lost by various causes.

Solar Photochemistry for Environmental Remediation

et al., 2002).

– Advanced Oxidation Processes for Industrial Wastewater Treatment 211

surface that surrounds a circular reactor, as shown in Figure 7. They had shown to provide better efficiency in the treatment of low pollutant concentration effluents. This reactor is an effective combination of the two reactors types described above (Bahnemamm, 1999; Malato

This type of reactor without concentration consists of a transparent box with an internal structure similar to that shown in Figure 8, through which is pumped the suspension containing the pollutant and the photocatalyst. It has the advantage of using the total

Figure 9 shows a diagram of a photocatalytic installation that can be alternatively used for heterogeneous TiO2 photocatalysis or for homogeneous photo-Fenton photocatalysis (or any other of the treatments previously described). In both cases the catalyst (TiO2 or iron) must

radiation and be very simple to operate (Bahnemamm, 1999; Malato et al., 2002).

Fig. 7. Simplified diagram of a ompound parabolic collecting reactor.

**4.2.4 Double skin sheet reactor** 

Fig. 8. Simplified diagram of a double sheet reactor.

be separated at end of treatment to be recycled and reused.

**4.3 Industrial units** 

At Almeria reactors, the total volume of effluent are about 400 liters, but the illuminated tube of about 180 meters contains only about half of that volume of effluent, which moves at speeds between 250 to 3500 liters per hour (Bahnemamm, 1999; Malato et al., 2002).

Fig. 5. Simplified diagram of a thin film fixed bed reactor.

Fig. 6. Simplified diagram of a plant using parabolic trough reactors.

### **4.2.3 Compound parabolic collecting reactor**

This reactor, whose simplified diagram is presented in Figure 7, is an open reactor without solar concentration. Basically this reactor differs from the conventional open parabolic reactor in the form of the reflectors. These reactors are static collectors with a reflective surface that surrounds a circular reactor, as shown in Figure 7. They had shown to provide better efficiency in the treatment of low pollutant concentration effluents. This reactor is an effective combination of the two reactors types described above (Bahnemamm, 1999; Malato et al., 2002).

### **4.2.4 Double skin sheet reactor**

210 Molecular Photochemistry – Various Aspects

At Almeria reactors, the total volume of effluent are about 400 liters, but the illuminated tube of about 180 meters contains only about half of that volume of effluent, which moves at

speeds between 250 to 3500 liters per hour (Bahnemamm, 1999; Malato et al., 2002).

Fig. 5. Simplified diagram of a thin film fixed bed reactor.

Fig. 6. Simplified diagram of a plant using parabolic trough reactors.

This reactor, whose simplified diagram is presented in Figure 7, is an open reactor without solar concentration. Basically this reactor differs from the conventional open parabolic reactor in the form of the reflectors. These reactors are static collectors with a reflective

**4.2.3 Compound parabolic collecting reactor** 

This type of reactor without concentration consists of a transparent box with an internal structure similar to that shown in Figure 8, through which is pumped the suspension containing the pollutant and the photocatalyst. It has the advantage of using the total radiation and be very simple to operate (Bahnemamm, 1999; Malato et al., 2002).

Fig. 8. Simplified diagram of a double sheet reactor.

### **4.3 Industrial units**

Figure 9 shows a diagram of a photocatalytic installation that can be alternatively used for heterogeneous TiO2 photocatalysis or for homogeneous photo-Fenton photocatalysis (or any other of the treatments previously described). In both cases the catalyst (TiO2 or iron) must be separated at end of treatment to be recycled and reused.

Solar Photochemistry for Environmental Remediation

– Advanced Oxidation Processes for Industrial Wastewater Treatment 213

Solar driven AOPs proved to be an excellent environmental remediation method to destroy persistent organic compounds not treatable by biological processes. In many cases, they allow the degradation of several persistent organic toxic pollutants decreasing the toxicity of the effluents released into the environment. These methods are particularly suitable for treating recalcitrant substances including those requiring special attention (hazardous or controlled ones). Several organochlorinated substances (dioxins, PCBs, etc) are persistent and sufficient toxic to disturb the environmental health and must be degraded prior to their environmental release. Without being exhaustive on the list of applications and systems to

**5. Applications of photocatalysis on the treatment of industrial effluents** 

Fig. 10. Simplified diagram of a unit for dyes degradation with thin film fixed reactors.

discharged into the environment in aqueous media.

Water is essential for life and therefore a key resource for humanity. Although it may seem that the water is very handy on our planet that is not true. Of the entire planet's water, 97.5% of the water is salty, among the remaining 2.5%, 70% is frozen and the rest is largely inaccessible in underground aquifers or as soil moisture. In fact, less than 1% of world potable water is available for immediate human consumption and even that is not uniformly distributed around the globe. For this reason, methodologies such as advanced oxidation processes that allow the maintenance of water quality are essential (Andreozzi et al., 1999; Chong et al., 2010; Comninellis et al., 2008; Matilainen & Sillanpaa, 2010). The problematic of water treatment and industrial wastewater treatment are inseparable issues since these industrial effluents constitute a major source of water contamination and are usually

The classical treatment processes of drinking water include treatment with ozone and filtration through granular activated carbon beds. Photocatalysis emerged as a promising tool for the treatment of water (and for the degradation of persistent substances even when they are present in low concentrations or complex matrices). So the advanced oxidation processes have been widely reported as an appropriate remediation methodology of all kinds of biorecalcitrants pollutants in water and industrial wastewater and their application to large-scale treatment facility is already being implemented, as discussed in section 4.

be treated, we will refer some examples that we consider most significant.

The surface area of the solar collector depends essentially on the effluent to be treated, mandatorily of the type and concentration of the contaminant, and on solar irradiation conditions and the location where treatment plant will be installed. The lifetime of the catalysts depends on the type of effluent to be treated and of the desired treatment final quality. In the end, the toxicity of the treated effluent must always be evaluated.

The project of an industrial effluent decontamination plant by photocatalysis requires a careful selection of the type of reactor to use, the arrangement of the reactor at the installation (series or parallel), the operation mode of the photocatalyst (fixed or suspended), the system for recycling catalysts and flow velocity, among others. The concentration of photocatalyst is also a key parameter and must be adjusted according to the following basic principles: for suspensions of TiO2, the speed of reaction is maximum for concentrations among 1 to 2 grams of TiO2 per liter of effluent to be treated, when the optical path is small (1-2 cm maximum). When the optical path is substantially higher, the appropriate concentration of photocatalyst is several hundred milligrams per liter. Anyway, when TiO2 concentrations is too high there is an internal filter effect and the rate of photodegradation decreases due to excessive opacity of the solution, which itself inhibits the illumination of the photocatalyst.

Fig. 9. Simplified diagram of a photocatalytic effluent treatment plant.

If the treatment plant is intended for treatment of a specific effluent, it does not need to be versatile and will be very similar to that shown in Figure 10. On the other hand, if an installation needs to treat various types of wastes, it must have the versatility to adapt to the optimal photodegradation conditions of the various types of effluents, e.g., having different types of solar collectors and reactors (as is the case of Almeria solar platform) and the project will be much more complicated.

The surface area of the solar collector depends essentially on the effluent to be treated, mandatorily of the type and concentration of the contaminant, and on solar irradiation conditions and the location where treatment plant will be installed. The lifetime of the catalysts depends on the type of effluent to be treated and of the desired treatment final

The project of an industrial effluent decontamination plant by photocatalysis requires a careful selection of the type of reactor to use, the arrangement of the reactor at the installation (series or parallel), the operation mode of the photocatalyst (fixed or suspended), the system for recycling catalysts and flow velocity, among others. The concentration of photocatalyst is also a key parameter and must be adjusted according to the following basic principles: for suspensions of TiO2, the speed of reaction is maximum for concentrations among 1 to 2 grams of TiO2 per liter of effluent to be treated, when the optical path is small (1-2 cm maximum). When the optical path is substantially higher, the appropriate concentration of photocatalyst is several hundred milligrams per liter. Anyway, when TiO2 concentrations is too high there is an internal filter effect and the rate of photodegradation decreases due to excessive opacity of the solution, which itself inhibits the

quality. In the end, the toxicity of the treated effluent must always be evaluated.

Fig. 9. Simplified diagram of a photocatalytic effluent treatment plant.

project will be much more complicated.

If the treatment plant is intended for treatment of a specific effluent, it does not need to be versatile and will be very similar to that shown in Figure 10. On the other hand, if an installation needs to treat various types of wastes, it must have the versatility to adapt to the optimal photodegradation conditions of the various types of effluents, e.g., having different types of solar collectors and reactors (as is the case of Almeria solar platform) and the

illumination of the photocatalyst.

### **5. Applications of photocatalysis on the treatment of industrial effluents**

Solar driven AOPs proved to be an excellent environmental remediation method to destroy persistent organic compounds not treatable by biological processes. In many cases, they allow the degradation of several persistent organic toxic pollutants decreasing the toxicity of the effluents released into the environment. These methods are particularly suitable for treating recalcitrant substances including those requiring special attention (hazardous or controlled ones). Several organochlorinated substances (dioxins, PCBs, etc) are persistent and sufficient toxic to disturb the environmental health and must be degraded prior to their environmental release. Without being exhaustive on the list of applications and systems to be treated, we will refer some examples that we consider most significant.

Fig. 10. Simplified diagram of a unit for dyes degradation with thin film fixed reactors.

Water is essential for life and therefore a key resource for humanity. Although it may seem that the water is very handy on our planet that is not true. Of the entire planet's water, 97.5% of the water is salty, among the remaining 2.5%, 70% is frozen and the rest is largely inaccessible in underground aquifers or as soil moisture. In fact, less than 1% of world potable water is available for immediate human consumption and even that is not uniformly distributed around the globe. For this reason, methodologies such as advanced oxidation processes that allow the maintenance of water quality are essential (Andreozzi et al., 1999; Chong et al., 2010; Comninellis et al., 2008; Matilainen & Sillanpaa, 2010). The problematic of water treatment and industrial wastewater treatment are inseparable issues since these industrial effluents constitute a major source of water contamination and are usually discharged into the environment in aqueous media.

The classical treatment processes of drinking water include treatment with ozone and filtration through granular activated carbon beds. Photocatalysis emerged as a promising tool for the treatment of water (and for the degradation of persistent substances even when they are present in low concentrations or complex matrices). So the advanced oxidation processes have been widely reported as an appropriate remediation methodology of all kinds of biorecalcitrants pollutants in water and industrial wastewater and their application to large-scale treatment facility is already being implemented, as discussed in section 4.

Solar Photochemistry for Environmental Remediation

al., 2007; Le-Minh et al., 2010).

**6. Conclusion** 

technology.

chapter.

**8. References** 

**7. Acknowledgements** 

No. 1, pp. 5-18.

Press, London.

*Technology*, Vol. 60, No. 8, pp. 2187-2193.

*Photochemistry Reviews,* Vol. 7, No. 4, pp. 127-144.

– Advanced Oxidation Processes for Industrial Wastewater Treatment 215

compounds (Auriol et al., 2006; Dalrymple et al., 2007; Gueltekin & Ince, 2007; Homem & Santos, 2011; Khetan & Collins, 2007; Santos et al., 2009) . Research has show that advanced oxidative processes, which generate very active oxidative species such as the hydroxyl radicals, are promising tools for the destruction of pharmaceuticals compounds (Gültekin, et

Advanced oxidation processes offer a consistent path to the treatment of recalcitrant substances that can not be treated by conventional effluents treatments. Either TiO2 mediated photocatalysis or Fenton related methodologies offer feasible alternatives for the treatment of dyes, organochlorinated substances (pesticides, dioxines, furanes, PCBs, etc.) and pharmaceutical products, enabling the decomposition of such substances. Those methods, which are very attractive from the point of view of sustainable and green chemistry because they can use solar light as energy source, are being increasingly tested in several treatment plants (some of them pilot plans) with the help of solar collecting

The authors thank to Fundação para a Ciência e Tecnologia (FCT, Portugal) for financial support through projects PTDC/QUI/65510/2006 and PTDC/QUI/70153/2006. J.C. Moreira and E.M. Saggioro thanks Faperj and ENSP/FioCruz for Master grants. A.S. Oliveira thanks M.E.A.M.R. Vieira Ferreira for critical reading of initial version of this

Anandan, S.; Yoon, M. (2003). Photocatalytic activities of the nano-sized TiO2-supported Y-

Anastas, P.T; Warner, J.C. (1998). *Green Chemistry: Theory and Practice*, Oxford University

Andreozzi R; Caprio V; Insola A.; Marotta, R. (1999). Advanced oxidation processes (AOP) for water purification and recovery. *Catalysis Today*, Vol. 53, No. 1, pp. 51-59. Atheba, P.; Robert, D.; Trokourey, A.; Bamba, D.; Weber, J.V. (2009). Design and study of a

Asahi, R; Morikawa, T; Ohwaki, K; Aoki, K; Taga, Y. (2001). Visible-light photocatalysis in nitrogen-doped titanium dioxides. *Science*, Vol. 293, No. 5528, pp. 269-271. Augugliaro, V.; Litter, M.; Palmisano, L.; Soria, J. (2006). The combination of heterogeneous

zeolites. *Journal of Photochemistry and Photobiology C:Photochemistry Reviews*, Vol. 4,

cost-effective solar photoreactor for pesticide removal from water. *Water Science and* 

photocatalysis with chemical and physical operations: A tool for improving the photoprocess performance. *Journal of Photochemistry and Photobiology C:* 

Typical examples of water pollutants that were efficiently mineralized by photocatalysis are effluents from industries containing dyes (Guillard et al., 2003), pesticides (Burrows et al., 2002; Marinas et al., 2001) and the effluents from the paper industry (Peiro et al., 2001). Various applications are also known for the decontamination of waste gases (Fu et al., 1996; Hay & Obee, 1999) including those involving self-cleaning surfaces (Hashimoto & Watanabe, 1999).

### **5.1 Industrial effluents containing dyes**

The dyes are common industrial residues present in wastewaters of different industries, ordinarily in textile dyeing process, inks, and photographic industries, among others. The environmental aspects of the use of dyes, including their degradation mechanisms in various environment compartments, have been a target of increasingly interest. It is estimated that nearly 15% of world production of dyes is lost during synthesis and dyeing process. Concomitantly, the major problem related with dyes is the removal of their colour from effluents. The non treated effluents frequently are highly colored and then particularity susceptible to public objection when disposed in water bodies. The dye concentration in residual waters can be smaller than others contaminants, but because of its high molar absorption coefficients they are visible even in very low concentrations. So, methodologies of effluents discoloration became very relevant. The oxidation processes are very much used in treatment of dye containing effluents (Khataee & Kasiri, 2010; Oliveira et al., 2008, 2011; Rauf & Ashraf, 2009; Saggioro et al., 2011; Soon & Hameed, 2011). Figure 10 presents a pilot unit commonly used on the degradation of dyes with thin film fixed bed **(**Guillard et al., 2003).

### **5.2 Effluents containing pesticides and pharmaceuticals**

The photodegradation and mineralization of pesticides and pharmaceuticals has been widely studied because of the danger they represent to the environment and also due to the highly recalcitrant nature of some of these compounds. For a comprehensive review of pesticide degradation see Blake, 1999 or some of the reviews listed here (Atheba et al., 2009; Bae & Choi, 2003; Felsot et al., 2003). Either titanium photocatalysis or Fenton parent methodologies usually promote rapid destruction of persistent pesticides.

Municipal water recycling for industrial, agricultural, and non-potable municipal may contain several different pharmaceuticals including antibiotics, hormones and other endocrine disruptors, sulphonamides, antipyretics, etc. Those are present in municipal sewage, largely as a result of human use and/or excretion. Much of the concern regarding the presence of these substances in wastewater and their persistence through wastewater treatment processes is because they may contribute to directly or indirectly affect the environmental and human health (Exall, 2004; Vigneswaran & Sundaravadivel, 2004).

In spite of the variable removal of antibiotics during conventional waste water treatment processes, many of these chemicals are often observed in secondary treated effluents. Conventional water and wastewater treatment are inefficient for substantially removing many of these compounds. While there appears to be no standard treatment for removal of all residual pharmaceuticals under conventional treatment processes, there is a strong opinion that advanced oxidation processes can be used for the effective removal of these compounds (Auriol et al., 2006; Dalrymple et al., 2007; Gueltekin & Ince, 2007; Homem & Santos, 2011; Khetan & Collins, 2007; Santos et al., 2009) . Research has show that advanced oxidative processes, which generate very active oxidative species such as the hydroxyl radicals, are promising tools for the destruction of pharmaceuticals compounds (Gültekin, et al., 2007; Le-Minh et al., 2010).

### **6. Conclusion**

214 Molecular Photochemistry – Various Aspects

Typical examples of water pollutants that were efficiently mineralized by photocatalysis are effluents from industries containing dyes (Guillard et al., 2003), pesticides (Burrows et al., 2002; Marinas et al., 2001) and the effluents from the paper industry (Peiro et al., 2001). Various applications are also known for the decontamination of waste gases (Fu et al., 1996; Hay & Obee, 1999) including those involving self-cleaning surfaces (Hashimoto &

The dyes are common industrial residues present in wastewaters of different industries, ordinarily in textile dyeing process, inks, and photographic industries, among others. The environmental aspects of the use of dyes, including their degradation mechanisms in various environment compartments, have been a target of increasingly interest. It is estimated that nearly 15% of world production of dyes is lost during synthesis and dyeing process. Concomitantly, the major problem related with dyes is the removal of their colour from effluents. The non treated effluents frequently are highly colored and then particularity susceptible to public objection when disposed in water bodies. The dye concentration in residual waters can be smaller than others contaminants, but because of its high molar absorption coefficients they are visible even in very low concentrations. So, methodologies of effluents discoloration became very relevant. The oxidation processes are very much used in treatment of dye containing effluents (Khataee & Kasiri, 2010; Oliveira et al., 2008, 2011; Rauf & Ashraf, 2009; Saggioro et al., 2011; Soon & Hameed, 2011). Figure 10 presents a pilot unit commonly used on the degradation of dyes with thin film fixed bed **(**Guillard et al.,

The photodegradation and mineralization of pesticides and pharmaceuticals has been widely studied because of the danger they represent to the environment and also due to the highly recalcitrant nature of some of these compounds. For a comprehensive review of pesticide degradation see Blake, 1999 or some of the reviews listed here (Atheba et al., 2009; Bae & Choi, 2003; Felsot et al., 2003). Either titanium photocatalysis or Fenton parent

Municipal water recycling for industrial, agricultural, and non-potable municipal may contain several different pharmaceuticals including antibiotics, hormones and other endocrine disruptors, sulphonamides, antipyretics, etc. Those are present in municipal sewage, largely as a result of human use and/or excretion. Much of the concern regarding the presence of these substances in wastewater and their persistence through wastewater treatment processes is because they may contribute to directly or indirectly affect the environmental and human health (Exall, 2004; Vigneswaran & Sundaravadivel, 2004).

In spite of the variable removal of antibiotics during conventional waste water treatment processes, many of these chemicals are often observed in secondary treated effluents. Conventional water and wastewater treatment are inefficient for substantially removing many of these compounds. While there appears to be no standard treatment for removal of all residual pharmaceuticals under conventional treatment processes, there is a strong opinion that advanced oxidation processes can be used for the effective removal of these

Watanabe, 1999).

2003).

**5.1 Industrial effluents containing dyes** 

**5.2 Effluents containing pesticides and pharmaceuticals** 

methodologies usually promote rapid destruction of persistent pesticides.

Advanced oxidation processes offer a consistent path to the treatment of recalcitrant substances that can not be treated by conventional effluents treatments. Either TiO2 mediated photocatalysis or Fenton related methodologies offer feasible alternatives for the treatment of dyes, organochlorinated substances (pesticides, dioxines, furanes, PCBs, etc.) and pharmaceutical products, enabling the decomposition of such substances. Those methods, which are very attractive from the point of view of sustainable and green chemistry because they can use solar light as energy source, are being increasingly tested in several treatment plants (some of them pilot plans) with the help of solar collecting technology.

### **7. Acknowledgements**

The authors thank to Fundação para a Ciência e Tecnologia (FCT, Portugal) for financial support through projects PTDC/QUI/65510/2006 and PTDC/QUI/70153/2006. J.C. Moreira and E.M. Saggioro thanks Faperj and ENSP/FioCruz for Master grants. A.S. Oliveira thanks M.E.A.M.R. Vieira Ferreira for critical reading of initial version of this chapter.

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**10** 

*1,2Tunis 3France* 

**High Power Discharge Lamps and** 

**Their Photochemical Applications:** 

**An Evaluation of Pulsed Radiation** 

Georges Zissis3 and Jean Pascal Cambronne3 *1Ecole Supérieure des Sciences et Techniques de Tunis, 2Institut Supérieur de Pêche et d'Aquaculture de Bizerte* 

*LAPLACE (Laboratoire Plasma et Conversion d'Energie)* 

*3Université de Toulouse; UPS, INPT;* 

Lotfi Bouslimi1,2, Mongi Stambouli1, Ezzedine Ben Braiek1,

The photochemical applications of the ultraviolet (UV) radiation develop with rate accelerated so much in the field of general public technologies as in lighting, descriptive, and imagery, and too of the advanced technologies (treatment and engraving of surfaces, air, water and agro-alimentary treatment). The radiation sources used are generally high,

In the past decades, gas discharge lamps have gained widespread use in industrial applications. Due to their unique design properties concerning spectral, electrical and geometrical features, all types of gas discharge lamps can been found in technical applications. Mercury based lamps are the workhorses in many applications upgraded by their relatives, the metal halide versions. The low and medium pressure mercury lamps are usually used as sources of UV radiation. Low pressure mercury lamps are used extensively for disinfection of drinking water, packing material and air. Medium pressure lamps are applied in printing industry to dry inks and cure adhesives, in waste water treatment plants to reduce the total organic compounds (TOC) and as a competing technology to low pressure versions in germicidal applications. Metal halide doped versions of medium and high pressure mercury

The control of the spectral distribution of energy is considered as the main parameter affecting the system flexibility and the product quality. However, even though the lamp characteristics have an important impact on the spectral distribution of radiation, the power supply characteristics cannot be neglected. Indeed, the temporal characteristics of the

Indeed, in the case of the high pressure lamps, the significant interactions between particles, it is difficult, with traditional power supply (electromagnetic ballasts) to move the energy

lamps open the possibility to adjust spectral output to specific requirements.

system are controlled mainly by the used power supply.

**1. Introduction** 

medium and low pressure gas discharge lamps.


## **High Power Discharge Lamps and Their Photochemical Applications: An Evaluation of Pulsed Radiation**

Lotfi Bouslimi1,2, Mongi Stambouli1, Ezzedine Ben Braiek1, Georges Zissis3 and Jean Pascal Cambronne3 *1Ecole Supérieure des Sciences et Techniques de Tunis, 2Institut Supérieur de Pêche et d'Aquaculture de Bizerte 3Université de Toulouse; UPS, INPT; LAPLACE (Laboratoire Plasma et Conversion d'Energie) 1,2Tunis 3France* 

### **1. Introduction**

222 Molecular Photochemistry – Various Aspects

Zapata, A.; Malato, S.; Sanchez-Perez, J.A.; Oller, I.; Maldonado, M.I. (2010). Scale-up

pesticide-contaminated water. *Catalysis Today*, Vol. 151, No. 1-2, pp. 100-106. Zayani, G.; Bousselmi, L.; Mhenni, F.; Ahmed, G. (2009). Solar photocatalytic degradation of

Zhang, X.H.; Li, W.Z.; Xu, H.Y. (2004). Application of zeolites in photocatalysis. *Progress in* 

reactor. *Desalination*, Vol. 246, No. 1-3, pp. 344-352.

*Chemistry*, Vol. 16, No. 5, pp. 728-737.

strategy for a combined solar photo-Fenton/biological system for remediation of

commercial textile azo dyes: Performance of pilot plant scale thin film fixed-bed

The photochemical applications of the ultraviolet (UV) radiation develop with rate accelerated so much in the field of general public technologies as in lighting, descriptive, and imagery, and too of the advanced technologies (treatment and engraving of surfaces, air, water and agro-alimentary treatment). The radiation sources used are generally high, medium and low pressure gas discharge lamps.

In the past decades, gas discharge lamps have gained widespread use in industrial applications. Due to their unique design properties concerning spectral, electrical and geometrical features, all types of gas discharge lamps can been found in technical applications. Mercury based lamps are the workhorses in many applications upgraded by their relatives, the metal halide versions. The low and medium pressure mercury lamps are usually used as sources of UV radiation. Low pressure mercury lamps are used extensively for disinfection of drinking water, packing material and air. Medium pressure lamps are applied in printing industry to dry inks and cure adhesives, in waste water treatment plants to reduce the total organic compounds (TOC) and as a competing technology to low pressure versions in germicidal applications. Metal halide doped versions of medium and high pressure mercury lamps open the possibility to adjust spectral output to specific requirements.

The control of the spectral distribution of energy is considered as the main parameter affecting the system flexibility and the product quality. However, even though the lamp characteristics have an important impact on the spectral distribution of radiation, the power supply characteristics cannot be neglected. Indeed, the temporal characteristics of the system are controlled mainly by the used power supply.

Indeed, in the case of the high pressure lamps, the significant interactions between particles, it is difficult, with traditional power supply (electromagnetic ballasts) to move the energy

High Power Discharge Lamps

**2.1 Ultraviolet radiation** 

Fig. 1. Ultraviolet bandwidth

Fig. 2. Ultraviolet lamps for water disinfection

sterilization of surfaces, air and water

fluorescent inspection purposes.

importance.

**2. Overview of UV-lamp applications** 

section 5.

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 225

electrical and spectral measurements carried out on high and medium pressure mercury lamps operated in pulsed current, are compared with the square wave operation for the same consumption in section 4. The paper is finally summarized with some conclusions in

Like visible light, Ultraviolet light (UV) is a classification of electromagnetic radiation having a wavelength bandwidth between 100 and 400nm, between the X-ray portion of the spectrum and the visible portion (Fig. 1). UV radiation is subdivided into four wavebands, which we use for a wide range of applications. These four subgroups within the UV

spectrum are located in the 100nm - 380nm waveband (Meulemans, 1986):

UV lamp TL 55W UV lamp TL-30 Watts 12 tubes UV lamp

conjunction with UVA light for artificial accelerated aging of materials.





distribution of the electronic cloud compared to Local Thermodynamic Equilibrium (LTE). However, by using short pulses of current one can hope to obtain such a result and to modify of this fact the distribution of the atomic excitation and the spectral distribution of the radiation (mainly visible and ultraviolet). Former works showed that the form of the current wave imposed on the lamp could be selected so as to improve the production of the radiation (Chalek, 1981; Brates, 1987; Chammam et al., 2005; Mrabet et al., 2006; Bouslimi et al., 2009a, 2009c). It remains, for these sources, to optimize the parameters of excitation (form, amplitude, frequency, duration of pulses) according to those of the discharge (natural of the mixture gas, energy spectral distribution).

Current UV radiation technology is dominated by two techniques, the continuous radiation and pulsed-radiation. The first technique provides a lower-level constant-flux UV radiation. The second technique provides radiation doses through flashing a source lamp. The effect of this pulsing technique is to provide short pulses of higher energy into the system.

The technology of the electronic pulsed supply is a field of studies relatively new related to the development of the generators, switches and electric applications of high energy, with weak durations and face of fast rise. These pulsed operations created by current or voltage pulses produce a pulsed light rich in UV.

The pulsed light system rich in UV radiation from 100 to 400 nm seems to be a promising alternative for the decontamination of the foodstuffs, and the sterilization of packing. Its effectiveness is now fully proven in experiments for decontamination on the surface of the products. Recent studies show the effectiveness of this treatment on products in powder form in fine layer. Bacteria in vegetative form, the ascosporous of moulds, the viruses and the parasites are destroyed by this instantaneous contribution of energy. Many works was completed on the biological effects of the UV carried out an excellent bibliographical analysis on this subject (Mimouni, 2004; Dunn et al., 1997a, 1997b; Dunn et al., 1990; Jagger, 1967; Fine, 2004).

The UV radiation as a disinfection technique has been also proven in multiple industrial applications, especially in the water treatment. Applications for water UV treatment are numerous: Potabilization of water, waste water treatment, treatment of seawater for aquaculture and shellfish culture. An historic perspective on UV disinfection has been published in several review articles (Groocock, 1984; Schenck, 1981; USEPA, 1996; Wolfe, 1990; Zoeteman et al., 1982).

The general objective of this work consists in studying the effect of the current pulses, provided by a feeding system (prototype) designed in our laboratory, on the spectral radiant flux emitted by two types of lamps: high and medium pressure mercury vapour lamps. The first is used mainly for screen printing, copying, and light curing adhesives and varnishes, and the second is germicidal gas-discharge lamps, intended particularly for water treatment. The spectral results obtained by two mode of current, highlight and evaluate the effectiveness of the pulsed current on the radiation production in the ultraviolet and the visible part of the spectrum.

In the remainder of this chapter, we present in the second section an overview of the ultraviolet applications. In the third section, we explore some special lamps for technical applications and their power suppliers. The experimental results of time-dependent electrical and spectral measurements carried out on high and medium pressure mercury lamps operated in pulsed current, are compared with the square wave operation for the same consumption in section 4. The paper is finally summarized with some conclusions in section 5.

### **2. Overview of UV-lamp applications**

### **2.1 Ultraviolet radiation**

224 Molecular Photochemistry – Various Aspects

distribution of the electronic cloud compared to Local Thermodynamic Equilibrium (LTE). However, by using short pulses of current one can hope to obtain such a result and to modify of this fact the distribution of the atomic excitation and the spectral distribution of the radiation (mainly visible and ultraviolet). Former works showed that the form of the current wave imposed on the lamp could be selected so as to improve the production of the radiation (Chalek, 1981; Brates, 1987; Chammam et al., 2005; Mrabet et al., 2006; Bouslimi et al., 2009a, 2009c). It remains, for these sources, to optimize the parameters of excitation (form, amplitude, frequency, duration of pulses) according to those of the discharge (natural

Current UV radiation technology is dominated by two techniques, the continuous radiation and pulsed-radiation. The first technique provides a lower-level constant-flux UV radiation. The second technique provides radiation doses through flashing a source lamp. The effect of

The technology of the electronic pulsed supply is a field of studies relatively new related to the development of the generators, switches and electric applications of high energy, with weak durations and face of fast rise. These pulsed operations created by current or voltage

The pulsed light system rich in UV radiation from 100 to 400 nm seems to be a promising alternative for the decontamination of the foodstuffs, and the sterilization of packing. Its effectiveness is now fully proven in experiments for decontamination on the surface of the products. Recent studies show the effectiveness of this treatment on products in powder form in fine layer. Bacteria in vegetative form, the ascosporous of moulds, the viruses and the parasites are destroyed by this instantaneous contribution of energy. Many works was completed on the biological effects of the UV carried out an excellent bibliographical analysis on this subject (Mimouni, 2004; Dunn et al., 1997a, 1997b; Dunn et al., 1990; Jagger,

The UV radiation as a disinfection technique has been also proven in multiple industrial applications, especially in the water treatment. Applications for water UV treatment are numerous: Potabilization of water, waste water treatment, treatment of seawater for aquaculture and shellfish culture. An historic perspective on UV disinfection has been published in several review articles (Groocock, 1984; Schenck, 1981; USEPA, 1996; Wolfe,

The general objective of this work consists in studying the effect of the current pulses, provided by a feeding system (prototype) designed in our laboratory, on the spectral radiant flux emitted by two types of lamps: high and medium pressure mercury vapour lamps. The first is used mainly for screen printing, copying, and light curing adhesives and varnishes, and the second is germicidal gas-discharge lamps, intended particularly for water treatment. The spectral results obtained by two mode of current, highlight and evaluate the effectiveness of the pulsed current on the radiation production in the ultraviolet and the

In the remainder of this chapter, we present in the second section an overview of the ultraviolet applications. In the third section, we explore some special lamps for technical applications and their power suppliers. The experimental results of time-dependent

this pulsing technique is to provide short pulses of higher energy into the system.

of the mixture gas, energy spectral distribution).

pulses produce a pulsed light rich in UV.

1967; Fine, 2004).

1990; Zoeteman et al., 1982).

visible part of the spectrum.

Like visible light, Ultraviolet light (UV) is a classification of electromagnetic radiation having a wavelength bandwidth between 100 and 400nm, between the X-ray portion of the spectrum and the visible portion (Fig. 1). UV radiation is subdivided into four wavebands, which we use for a wide range of applications. These four subgroups within the UV spectrum are located in the 100nm - 380nm waveband (Meulemans, 1986):

Fig. 1. Ultraviolet bandwidth

UV lamp TL 55W UV lamp TL-30 Watts 12 tubes UV lamp

Fig. 2. Ultraviolet lamps for water disinfection


High Power Discharge Lamps

facilitate superior bonding.

harden.

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 227

optimal wavelengths are known. We find that the sources most frequently used are medium

Photochemistry is also used in the curing (polymerization) of specially formulated printing inks and coatings. Since it was originally introduced in the 1960's, UV curing has been widely adopted in many industries including automotive, telecommunications, electronics,

Ultraviolet curing (commonly known as UV curing) is a photochemical process in which high-intensity ultraviolet light is used to instantly cure or "dry" inks, coatings or adhesives. Offering many advantages over traditional drying methods; UV curing has been shown to increase production speed, reduce reject rates, improve scratch and solvent resistance, and

Using light instead of heat, the UV curing process is based on a photochemical reaction. Liquid monomers and oligomers are mixed with a small percent of photoinitiators, and then exposed to UV energy. In a few seconds, the products - inks, coatings or adhesives instantly

UV curable inks and coatings were first used as a better alternative to solvent-based products. Conventional heat- and air-drying works by solvent evaporation. This process shrinks the initial application of coatings by more than 50% and creates environmental pollutants. In UV curing, there is no solvent to evaporate, no environmental pollutants, no loss of coating thickness, and no loss of volume. This results in higher productivity in less

UV-VIS spectroscopy is one of the main applications of photochemistry. It allows us to determine the concentration of a molecule in a sample, and sometimes, it can aid in identifying an unknown molecule. The molecule being tested must absorb light in the ultraviolet (about 200 to 400nm) or the visible (about 400 to 700nm) range in order to be detected by this equipment. A light beam containing multiple wavelengths gets passed through a small container holding your sample, and the computer records which

Another emerging UV application is the photocatalysis. This is the photoactivation of a surface covered with TiO2 (sometimes doped) which is causing a hyper-hydrophilicity.This process

Usually, this type of process uses UV-C (<180 nm). Xenon lamps with low-pressure radiation at 172nm are well positioned for this application. However, today by doping the layer of TiO2, we get a photoactivatable produce materials with wavelengths larger located in the UV-A (380 nm). Lamps or dielectric barrier to Xe2 or Xe-halide combinations seem to

The UV disinfection process corresponds to the inactivation of microorganisms, following a modification of their genetic information: the UV affect the DNA double helix, as well as RNA, cells, blocking all biochemical processes used for their reproduction. The maximum efficiency of UV disinfection depends on the energy emitted (with peaks near 200 and

leads us to make self-cleaning surfaces using the following procedure in figure 4.

pressure mercury lamps (possibly doped with iron iodide) and xenon lamps.

graphic arts, converting and metal, glass and plastic decorating.

time, with a reduction in waste, energy use and pollutant emissions.

wavelength(s) were absorbed, and at which intensity.

be the most promising sources.

**2.3 Mechanism of UV disinfection** 

Practical application of UV disinfection relies on the germicidal ability of UVC and UVB and depends on artificial sources of UV. The most common sources of UV are commercially available low and medium pressure mercury arc lamps (Fig. 2).

### **2.2 UV applications in photochemistry**

Photochemistry is the study of the action of light on chemical reactions. In a more precise, it includes works whose purpose is to determine the nature of the reactive excited states of molecules obtained by absorption of light, to study the deactivation process of these states, especially those that lead to products different reagents and irradiated to establish the mechanisms by which rearrangements occurring intra-and intermolecular initiated by radiation (Hecht, 1920).

The chemical reactions induced by light indirectly as a result of electronic energy transfer are an area of study and implementation has long been known (F. Weigert, 1907) and highly developed now. In general, photo-chemical processes are part of different modes of deactivation of molecules previously made in their metastable excited states by absorption of a photon.

Ideally, a photochemical process is performed by irradiating the sample with monochromatic light, since the reaction may depend on the excitation wavelength. Most of polychromatic light sources; the wavelength is selected or required by filters or by a monochromator (Hecht, 1920). Light sources are now almost always discharge tubes containing either xenon or mercury vapor alone or in carefully selected impurities. Some of these lamps are very powerful and they can consume tens of kilowatts of electricity.

The first application of photochemistry was the isomerization of benzene in the liquid state: Under the influence of radiation from a mercury vapor lamp (253.7 nm), the isomerization of benzene in liquid product benzvalene and fulvene, while in the field of wavelength range 166-200 nm, irradiation produces more benzene told Dewar (Fig.3).

Fig. 3. Isomerization of benzene in liquid

Today industrial, photochemistry has made its biggest breakthrough in the field of setting polymers on different surfaces, such as the "drying" of printing inks and the manufacture of electronic circuits. The notion of quantum efficiency is very important in photochemistry. For that performance is great each application needs a specific wavelength. Although its

Practical application of UV disinfection relies on the germicidal ability of UVC and UVB and depends on artificial sources of UV. The most common sources of UV are commercially

Photochemistry is the study of the action of light on chemical reactions. In a more precise, it includes works whose purpose is to determine the nature of the reactive excited states of molecules obtained by absorption of light, to study the deactivation process of these states, especially those that lead to products different reagents and irradiated to establish the mechanisms by which rearrangements occurring intra-and intermolecular initiated by

The chemical reactions induced by light indirectly as a result of electronic energy transfer are an area of study and implementation has long been known (F. Weigert, 1907) and highly developed now. In general, photo-chemical processes are part of different modes of deactivation of molecules previously made in their metastable excited states by absorption

Ideally, a photochemical process is performed by irradiating the sample with monochromatic light, since the reaction may depend on the excitation wavelength. Most of polychromatic light sources; the wavelength is selected or required by filters or by a monochromator (Hecht, 1920). Light sources are now almost always discharge tubes containing either xenon or mercury vapor alone or in carefully selected impurities. Some of

The first application of photochemistry was the isomerization of benzene in the liquid state: Under the influence of radiation from a mercury vapor lamp (253.7 nm), the isomerization of benzene in liquid product benzvalene and fulvene, while in the field of wavelength range

Today industrial, photochemistry has made its biggest breakthrough in the field of setting polymers on different surfaces, such as the "drying" of printing inks and the manufacture of electronic circuits. The notion of quantum efficiency is very important in photochemistry. For that performance is great each application needs a specific wavelength. Although its

these lamps are very powerful and they can consume tens of kilowatts of electricity.

166-200 nm, irradiation produces more benzene told Dewar (Fig.3).

Fig. 3. Isomerization of benzene in liquid

available low and medium pressure mercury arc lamps (Fig. 2).

**2.2 UV applications in photochemistry** 

radiation (Hecht, 1920).

of a photon.

optimal wavelengths are known. We find that the sources most frequently used are medium pressure mercury lamps (possibly doped with iron iodide) and xenon lamps.

Photochemistry is also used in the curing (polymerization) of specially formulated printing inks and coatings. Since it was originally introduced in the 1960's, UV curing has been widely adopted in many industries including automotive, telecommunications, electronics, graphic arts, converting and metal, glass and plastic decorating.

Ultraviolet curing (commonly known as UV curing) is a photochemical process in which high-intensity ultraviolet light is used to instantly cure or "dry" inks, coatings or adhesives. Offering many advantages over traditional drying methods; UV curing has been shown to increase production speed, reduce reject rates, improve scratch and solvent resistance, and facilitate superior bonding.

Using light instead of heat, the UV curing process is based on a photochemical reaction. Liquid monomers and oligomers are mixed with a small percent of photoinitiators, and then exposed to UV energy. In a few seconds, the products - inks, coatings or adhesives instantly harden.

UV curable inks and coatings were first used as a better alternative to solvent-based products. Conventional heat- and air-drying works by solvent evaporation. This process shrinks the initial application of coatings by more than 50% and creates environmental pollutants. In UV curing, there is no solvent to evaporate, no environmental pollutants, no loss of coating thickness, and no loss of volume. This results in higher productivity in less time, with a reduction in waste, energy use and pollutant emissions.

UV-VIS spectroscopy is one of the main applications of photochemistry. It allows us to determine the concentration of a molecule in a sample, and sometimes, it can aid in identifying an unknown molecule. The molecule being tested must absorb light in the ultraviolet (about 200 to 400nm) or the visible (about 400 to 700nm) range in order to be detected by this equipment. A light beam containing multiple wavelengths gets passed through a small container holding your sample, and the computer records which wavelength(s) were absorbed, and at which intensity.

Another emerging UV application is the photocatalysis. This is the photoactivation of a surface covered with TiO2 (sometimes doped) which is causing a hyper-hydrophilicity.This process leads us to make self-cleaning surfaces using the following procedure in figure 4.

Usually, this type of process uses UV-C (<180 nm). Xenon lamps with low-pressure radiation at 172nm are well positioned for this application. However, today by doping the layer of TiO2, we get a photoactivatable produce materials with wavelengths larger located in the UV-A (380 nm). Lamps or dielectric barrier to Xe2 or Xe-halide combinations seem to be the most promising sources.

### **2.3 Mechanism of UV disinfection**

The UV disinfection process corresponds to the inactivation of microorganisms, following a modification of their genetic information: the UV affect the DNA double helix, as well as RNA, cells, blocking all biochemical processes used for their reproduction. The maximum efficiency of UV disinfection depends on the energy emitted (with peaks near 200 and

High Power Discharge Lamps

**2.4 Biological effects of UV radiation** 

radiation of shorter lengths waveform.

Fig. 6. UV-region spectrum of Sun

Ozone formation

these elements, or for the direct synthesis of vitamin D.

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 229

inactivation of E. coli. The action spectra of E. coli peaks at wavelengths near 265nm and near 220nm. It is convenient that the 254nm output of a low pressure lamp coincides well

The effects of ultraviolet radiation on living organisms are due to its photochemical action. The best known are the erythema or "sunburn", for which the area of activity is between 320 and 280 nm (with a maximum at 297 nm), and "tan", which involves training, migration and oxidation of melanin, and whose field of activity is wider towards longer wavelengths, which allows you to tan without the risk of rash using products such as filters stopping the

> Germicidal effect

In terms of medical treatment, in addition to its use in some diseases of the skin, ultraviolet was mainly used for the treatment of rachitis; its action has the effect of the conversion of vitamin D sterols: direct radiation (sterols present in the skin), irradiation of food containing

As for dermatological applications in the fight against diseases such as vitiligo and psoriasis, there are two types of treatment. The first consists in irradiating the skin with a UV-A radiation at 308 nm, which inhibits locally the patient's immune system by calming for period more or less limited effects of the disease. For this application, dermatologists now use lasers. However, dielectric barrier lamps using a mixture of Xe-Cl2 begin to appear. The advantages of systems using these lamps are numerous: they are easy to handle, they require less maintenance, they are lighter and can be portable, compared to a laser. They produce a lower UV power and thus limit the risk of burns. The second method of treatment is called "PUVA". PUVA therapy is a method that combines a photosensitizing drug (in the series of psoralen) administered orally and irradiation of the skin lesions to be treated by long ultraviolet (UVA). The comparison of the effectiveness of each of psoralens used does not show clear-cut superiority of one or the other of them. Their general tolerance appears to be satisfactory, except for minor digestive problems. The tolerances are checked blood and liver in each case respectively by the blood counts, blood count and assay of transaminases. Elevated levels of these enzymes involves discontinuation of treatment is followed by a rapid normalization. It has been shown that the presence of radiation, psoralens are all capable of combining with the pyrimidine bases of DNA chains. This can lead to very different metabolic changes, which would explain why the PUVA appears to have

Antirachitic effect

Effect of pigmentation

with the inactivation peak near 265nm (Wright & Cairns, 1998).

260nm), more precisely; it corresponds to output energy of 253.7 nm (absorption peak of UV radiation by micro-organisms) (Wright & Cairns, 1998; Sonntag et al., 1992).

Fig. 4. The process of self-cleaning surfaces

Absorbed UV promotes the formation of bonds between adjacent nucleotides, creating double molecules or dimmers (Jagger, 1967). While the formation of thymine-thymine dimers are the most common, cytosine-cytosine, cytosine-thymine, and uracil dimerization also occur. Formation of a sufficient number of dimmers within a microbe prevents it from replicating its DNA and RNA, thereby preventing it from reproducing. Due to the wavelength dependence of DNA UV absorption, UV inactivation of microbes is also a function of wavelength. Figure 5 presents the germicidal action spectra for the UV

Fig. 5. Comparison of the action spectrum for E. coli inactivation to the absorption spectrum of nucleic acids (Wright & Cairns, 1998)

inactivation of E. coli. The action spectra of E. coli peaks at wavelengths near 265nm and near 220nm. It is convenient that the 254nm output of a low pressure lamp coincides well with the inactivation peak near 265nm (Wright & Cairns, 1998).

### **2.4 Biological effects of UV radiation**

228 Molecular Photochemistry – Various Aspects

260nm), more precisely; it corresponds to output energy of 253.7 nm (absorption peak of UV

Absorbed UV promotes the formation of bonds between adjacent nucleotides, creating double molecules or dimmers (Jagger, 1967). While the formation of thymine-thymine dimers are the most common, cytosine-cytosine, cytosine-thymine, and uracil dimerization also occur. Formation of a sufficient number of dimmers within a microbe prevents it from replicating its DNA and RNA, thereby preventing it from reproducing. Due to the wavelength dependence of DNA UV absorption, UV inactivation of microbes is also a function of wavelength. Figure 5 presents the germicidal action spectra for the UV

> **190 210 230 250 270 290 310 Wavelength (nm)**

Fig. 5. Comparison of the action spectrum for E. coli inactivation to the absorption spectrum

**E. coli killing**

radiation by micro-organisms) (Wright & Cairns, 1998; Sonntag et al., 1992).

Fig. 4. The process of self-cleaning surfaces

**0**

of nucleic acids (Wright & Cairns, 1998)

**0.2**

**0.4**

**DNA absorption**

**0.6**

**0.8**

**Relative units** 

**1**

**1.2**

**1.4**

The effects of ultraviolet radiation on living organisms are due to its photochemical action. The best known are the erythema or "sunburn", for which the area of activity is between 320 and 280 nm (with a maximum at 297 nm), and "tan", which involves training, migration and oxidation of melanin, and whose field of activity is wider towards longer wavelengths, which allows you to tan without the risk of rash using products such as filters stopping the radiation of shorter lengths waveform.

Fig. 6. UV-region spectrum of Sun

In terms of medical treatment, in addition to its use in some diseases of the skin, ultraviolet was mainly used for the treatment of rachitis; its action has the effect of the conversion of vitamin D sterols: direct radiation (sterols present in the skin), irradiation of food containing these elements, or for the direct synthesis of vitamin D.

As for dermatological applications in the fight against diseases such as vitiligo and psoriasis, there are two types of treatment. The first consists in irradiating the skin with a UV-A radiation at 308 nm, which inhibits locally the patient's immune system by calming for period more or less limited effects of the disease. For this application, dermatologists now use lasers. However, dielectric barrier lamps using a mixture of Xe-Cl2 begin to appear. The advantages of systems using these lamps are numerous: they are easy to handle, they require less maintenance, they are lighter and can be portable, compared to a laser. They produce a lower UV power and thus limit the risk of burns. The second method of treatment is called "PUVA". PUVA therapy is a method that combines a photosensitizing drug (in the series of psoralen) administered orally and irradiation of the skin lesions to be treated by long ultraviolet (UVA). The comparison of the effectiveness of each of psoralens used does not show clear-cut superiority of one or the other of them. Their general tolerance appears to be satisfactory, except for minor digestive problems. The tolerances are checked blood and liver in each case respectively by the blood counts, blood count and assay of transaminases. Elevated levels of these enzymes involves discontinuation of treatment is followed by a rapid normalization. It has been shown that the presence of radiation, psoralens are all capable of combining with the pyrimidine bases of DNA chains. This can lead to very different metabolic changes, which would explain why the PUVA appears to have

High Power Discharge Lamps

electronic ballasts (O'Brian et al., 1995; Phillips, 1983).

for broadband UV-radiation between 150 and 300nm.

of surface modification, cleaning, curing and disinfection.

directly consumer related applications.

starting and operating voltages.

et al., 2009b).

**3. Lamps for technical applications and their power suppliers** 

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 231

In view of that, practical application of UV disinfection depends on artificial sources of UV and their mode of electrical power supplier. The most common sources of UV are commercially available low and medium pressure mercury arc lamps. The power suppliers (named ballasts) for mostly lamps may be characterized as either electromagnetic or

In addition to low, medium and high pressure mercury discharge lamps, mercury short arc lamps with high operating pressures are found wherever high brightness and good imaging is required, for example in steppers for micro-lithography or as ultra-high pressure types in projectors. Besides the huge field of specialty lighting (stage-studio-TV, floodlights, effectlighting and car headlights) with special focus on the response function of human eyes, these lamps are also used in reprographic machines, photo-chemistry, medical applications and by the tanning industry. Thus covering the whole field from pure industrial use to

Pure rare gas fillings are used in flash-lamps for pumping the active medium of solid state lasers, whereas long arc xenon lamps satisfy the request for simulating solar radiation in chambers to test the radiation resistance of textiles and colours. Highly stable deuteriumlamps are operating in UV spectrometer and analytical instruments (HPLC, LC) as a source

In addition to the above mentioned lamp types, excimer lamps have gained increased interest during the last decade due to their quasi monochromatic spectrum. Intense and efficient UV generations of these lamps have revealed their potentials in the application field

Discharge lamps are a source of light in which light is produced by the radiant energy generated from a gas discharge. A typical mercury arc lamp consists of a hermetically sealed tube of UV -transmitting vitreous silica or quartz with electrodes at both ends (Phillips, 1983). The tube is filled with a small amount of mercury and an inert gas, usually argon. Argon is present to aid lamp starting, extend electrode life, and reduce thermal losses. Argon does not contribute to the spectral output of the lamp. Most gas discharge lamps are operated in series with a current-limiting device. This auxiliary, commonly called ballast, limits the current to the value for which each lamp is designed. It also provides the required

Ballasts are classified into two major types: electromagnetic ballasts and high-frequency electronic ballasts. The conventional ballast, made of a simple electromagnetic coil, has many significant disadvantages, such as large size, heavy weight, including low-frequency humming, low efficiency, poor power regulation, and high sensibility to voltage changes, etc. Since the electronic ballast can overcome these drawbacks. The high operating frequency allows to the ballast to be smaller and lighter-weight than the electromagnetic ballast. Unfortunately, there is a serious problem of acoustic resonance when the lamps operate in certain frequency range; this phenomenon is even severe for low-wattage lamps. These types of ballasts is more widely developed and used in many applications (Bouslimi

contradictory effects. However, it might be a good alternative to chemotherapy against some types of skin cancer because UV radiations associated with psoralens have the power to destroy the offending cells.

The effects of UV on microorganisms depend on the doses, ranging from the reduction of vital processes (cell division, cell motility, synthesis of nucleic acid) to the destruction of organisms. The germicidal action, observed during exposure to UVC radiation type, is most effective when the wavelength is between 250 and 260 nm (253.7 nm). At this level, the UVC damage the nucleic acids of microorganisms, causing the amount of energy following implementation (afigfoessel.fr):


The germicidal action has received applications where mercury vapour lamps are used (253.7 nm): surface sterilization of food products or pharmaceuticals in their packaging, disinfection of objects, air and water (difficult because of the absorption if the water is not pure).

Today we do not really know the answer of microorganisms to UV radiation, but, empirically, we know what is the ultraviolet dose required to kill different microorganisms to a certain percentage (usually for the treatment of water, this is between 90 and 99%).

For water treatment (potable and tertiary) where the rate of destruction of microorganisms required no more than 2log (99%), now the most commonly lamps used are UV lamps, low pressure (using amalgam thereby obtaining high power of about 100 W / lamp) and HID lamps "medium pressure" (pure mercury lamps with power ratings up to 5 kW, see more in some cases). Some systems based on the phenomenon of photo-catalysis are emerging in the market. Regardless of the application cited above photo-biological lamps used do not produce the optimal wavelength.

### **2.5 Other UV applications**

The following table summarizes some other UV wavelength applications "optimal"


Table 1. Various UV wavelength applications

contradictory effects. However, it might be a good alternative to chemotherapy against some types of skin cancer because UV radiations associated with psoralens have the power to

The effects of UV on microorganisms depend on the doses, ranging from the reduction of vital processes (cell division, cell motility, synthesis of nucleic acid) to the destruction of organisms. The germicidal action, observed during exposure to UVC radiation type, is most effective when the wavelength is between 250 and 260 nm (253.7 nm). At this level, the UVC damage the nucleic acids of microorganisms, causing the amount of energy following

A bacteriostatic effect in the case of low radiation level of the cell. In this case it

A bactericidal effect in the case of a significant radiation at the cellular level. In this case

The germicidal action has received applications where mercury vapour lamps are used (253.7 nm): surface sterilization of food products or pharmaceuticals in their packaging, disinfection

Today we do not really know the answer of microorganisms to UV radiation, but, empirically, we know what is the ultraviolet dose required to kill different microorganisms to a certain percentage (usually for the treatment of water, this is between 90 and 99%).

For water treatment (potable and tertiary) where the rate of destruction of microorganisms required no more than 2log (99%), now the most commonly lamps used are UV lamps, low pressure (using amalgam thereby obtaining high power of about 100 W / lamp) and HID lamps "medium pressure" (pure mercury lamps with power ratings up to 5 kW, see more in some cases). Some systems based on the phenomenon of photo-catalysis are emerging in the market. Regardless of the application cited above photo-biological lamps used do not

of objects, air and water (difficult because of the absorption if the water is not pure).

The following table summarizes some other UV wavelength applications "optimal"

**KrCl (222 nm)** 

Photolysis of to

 Inactivation of microorganism UV curing for

Photochemical

hydrogen peroxide

**XeBr (282 nm)** 

 Inactivation of microorganisms UV curing for printing processes

printing processes

vapour deposition

destroy the offending cells.

implementation (afigfoessel.fr):

produce the optimal wavelength.

**2.5 Other UV applications** 

 Cleaning of surfaces Photochemical

vapour deposition Modification of structure and composition of surfaces Activation of surfaces UV matting Ozone generation

Table 1. Various UV wavelength applications

**Xe2 (172 nm)** 

it is destroyed.

continues to live while unable to reproduce.

In view of that, practical application of UV disinfection depends on artificial sources of UV and their mode of electrical power supplier. The most common sources of UV are commercially available low and medium pressure mercury arc lamps. The power suppliers (named ballasts) for mostly lamps may be characterized as either electromagnetic or electronic ballasts (O'Brian et al., 1995; Phillips, 1983).

### **3. Lamps for technical applications and their power suppliers**

In addition to low, medium and high pressure mercury discharge lamps, mercury short arc lamps with high operating pressures are found wherever high brightness and good imaging is required, for example in steppers for micro-lithography or as ultra-high pressure types in projectors. Besides the huge field of specialty lighting (stage-studio-TV, floodlights, effectlighting and car headlights) with special focus on the response function of human eyes, these lamps are also used in reprographic machines, photo-chemistry, medical applications and by the tanning industry. Thus covering the whole field from pure industrial use to directly consumer related applications.

Pure rare gas fillings are used in flash-lamps for pumping the active medium of solid state lasers, whereas long arc xenon lamps satisfy the request for simulating solar radiation in chambers to test the radiation resistance of textiles and colours. Highly stable deuteriumlamps are operating in UV spectrometer and analytical instruments (HPLC, LC) as a source for broadband UV-radiation between 150 and 300nm.

In addition to the above mentioned lamp types, excimer lamps have gained increased interest during the last decade due to their quasi monochromatic spectrum. Intense and efficient UV generations of these lamps have revealed their potentials in the application field of surface modification, cleaning, curing and disinfection.

Discharge lamps are a source of light in which light is produced by the radiant energy generated from a gas discharge. A typical mercury arc lamp consists of a hermetically sealed tube of UV -transmitting vitreous silica or quartz with electrodes at both ends (Phillips, 1983). The tube is filled with a small amount of mercury and an inert gas, usually argon. Argon is present to aid lamp starting, extend electrode life, and reduce thermal losses. Argon does not contribute to the spectral output of the lamp. Most gas discharge lamps are operated in series with a current-limiting device. This auxiliary, commonly called ballast, limits the current to the value for which each lamp is designed. It also provides the required starting and operating voltages.

Ballasts are classified into two major types: electromagnetic ballasts and high-frequency electronic ballasts. The conventional ballast, made of a simple electromagnetic coil, has many significant disadvantages, such as large size, heavy weight, including low-frequency humming, low efficiency, poor power regulation, and high sensibility to voltage changes, etc. Since the electronic ballast can overcome these drawbacks. The high operating frequency allows to the ballast to be smaller and lighter-weight than the electromagnetic ballast. Unfortunately, there is a serious problem of acoustic resonance when the lamps operate in certain frequency range; this phenomenon is even severe for low-wattage lamps. These types of ballasts is more widely developed and used in many applications (Bouslimi et al., 2009b).

High Power Discharge Lamps

**4.2.1 Lamp characteristics** 

investigation are consigned in the table below.

Table 2. Characteristics of the studied lamp

our electrical and spectral measurements.

figures 8 and 9.

**4.2.2 Spectral results (Bouslimi et al., 2009b)** 

from figure 8 and 9 above are summarised and given in Table 3.

2009b).

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 233

provides flexibility in the integration of the two current source AC/DC converters with the full-bridge inverter. It allows obtaining a low-frequency rectangular wave with one or more pulses superimposed on each half cycle. The current in each source is controlled by a regulating circuit. D1 and D2 are anti-return fast diodes (Bouslimi et al., 2008, 2009a,

This power supply allows as more studying the energy effectiveness and the photometric behaviour of various gas-discharge lamps (low, medium and high pressure), and this with an aim of evaluating the visible and ultraviolet radiation and of comparing it with the

We also note that the proposed current pulsed power supply can be more exploited in photochemical applications exactly for the treatment of water whose needs a variation of the amplitude and the duration of the pulse (UV dose) according to the virus and bacteria

The main characteristics of filling, geometrical and electric of the discharge lamp used in this

Characteristics Rating values Diameter (mm) 18.2 Inter-electrode length (mm) 72 Total mercury mass (mg) 70 Argon pressure at the ambient temperature (torr) 10 Power (W) 400 I arc (A) 3.2 V arc (V) 140

The lamp operates vertically through a current inverter and all the measurements have been done in a steady state after the flux and circuit stabilization. Below, we present the results of

Relative average spectral flux was recorded for a rectangular current and a pulsed current. In these two modes, the power provided to the lamp was the same one. Thus, it is possible to evaluate the influence of the current pulses on the radiation production effectiveness in the ultraviolet part and the visible part of the spectrum. Theses results are illustrated in

If you look at the figures above we see that the difference between the spectral results for both modes of operation is small. Better to see the difference, we calculate the total flux through each band. The results for the average values of the total spectral flux determined

continuous radiation for the same consumption by the discharge lamps.

lifespan (Severin et al, 1984). It cans also feeding power lamps going until 3kW.

**4.2 The radiation produced by high pressure lamp in pulsed operation** 

The structure of the electronic pulsed power supply developed in our laboratory presents several advantages in this domain. The main advantage of the proposed topology is to provide to the lamp a various shapes of current (square wave, rectangular and pulses) with optimization of the excitation parameter (form, amplitude, frequency, number and duration of pulses).

### **4. Experimental results**

We present in this section, the effect of the current pulses, provided by the feeding system designed in our laboratory, on the ultraviolet and visible spectral flux emitted by two types of lamps: high and medium pressure mercury vapour lamps. In order to highlight and evaluate the effectiveness of the pulsed current on the radiation production, we give a comparison of spectral results obtained by two mode of excitation, rectangular and pulsed current.

### **4.1 Structure of the pulsed power supply**

The bloc diagram of the lamp circuitry is shown in figure 7. The lamp is supplied mainly through an inverter connected with two electrical separate sources: the first source provides a rectangular wave current and the second provides a pulsed current.

The rectangular wave operation is achieved using a (DC) constant current source (S1) in conjunction with an electronic full bridge IGBTs inverter and an active protection system that allows protecting the IGBTs and the drivers against the over-voltage at the time of starting and the hot restarting of the lamp or by an unexpected opening of the circuit.

Fig. 7. Bloc diagram of the pulsed power supply

The pulsed operation is achieved by the second (DC) current source (S2) switched by a pulse switching circuit (transistor T3). The control signals for the pulse switching circuit and full wave bridge are ensured by a microcontroller (PIC16F628). The microcontroller provides flexibility in the integration of the two current source AC/DC converters with the full-bridge inverter. It allows obtaining a low-frequency rectangular wave with one or more pulses superimposed on each half cycle. The current in each source is controlled by a regulating circuit. D1 and D2 are anti-return fast diodes (Bouslimi et al., 2008, 2009a, 2009b).

This power supply allows as more studying the energy effectiveness and the photometric behaviour of various gas-discharge lamps (low, medium and high pressure), and this with an aim of evaluating the visible and ultraviolet radiation and of comparing it with the continuous radiation for the same consumption by the discharge lamps.

We also note that the proposed current pulsed power supply can be more exploited in photochemical applications exactly for the treatment of water whose needs a variation of the amplitude and the duration of the pulse (UV dose) according to the virus and bacteria lifespan (Severin et al, 1984). It cans also feeding power lamps going until 3kW.

### **4.2 The radiation produced by high pressure lamp in pulsed operation**

### **4.2.1 Lamp characteristics**

232 Molecular Photochemistry – Various Aspects

The structure of the electronic pulsed power supply developed in our laboratory presents several advantages in this domain. The main advantage of the proposed topology is to provide to the lamp a various shapes of current (square wave, rectangular and pulses) with optimization of the excitation parameter (form, amplitude, frequency, number and duration

We present in this section, the effect of the current pulses, provided by the feeding system designed in our laboratory, on the ultraviolet and visible spectral flux emitted by two types of lamps: high and medium pressure mercury vapour lamps. In order to highlight and evaluate the effectiveness of the pulsed current on the radiation production, we give a comparison of spectral results obtained by two mode of excitation, rectangular and pulsed

The bloc diagram of the lamp circuitry is shown in figure 7. The lamp is supplied mainly through an inverter connected with two electrical separate sources: the first source provides

The rectangular wave operation is achieved using a (DC) constant current source (S1) in conjunction with an electronic full bridge IGBTs inverter and an active protection system that allows protecting the IGBTs and the drivers against the over-voltage at the time of starting and the hot restarting of the lamp or by an unexpected opening of the circuit.

D1

D2

T3

Inverter

+



Switching circuit PIC16F628

The pulsed operation is achieved by the second (DC) current source (S2) switched by a pulse switching circuit (transistor T3). The control signals for the pulse switching circuit and full wave bridge are ensured by a microcontroller (PIC16F628). The microcontroller

a rectangular wave current and the second provides a pulsed current.

+

+


of pulses).

current.

**4. Experimental results** 

(S2)

Current sources

Fig. 7. Bloc diagram of the pulsed power supply

(S1)

**4.1 Structure of the pulsed power supply** 

The main characteristics of filling, geometrical and electric of the discharge lamp used in this investigation are consigned in the table below.


Table 2. Characteristics of the studied lamp

The lamp operates vertically through a current inverter and all the measurements have been done in a steady state after the flux and circuit stabilization. Below, we present the results of our electrical and spectral measurements.

### **4.2.2 Spectral results (Bouslimi et al., 2009b)**

Relative average spectral flux was recorded for a rectangular current and a pulsed current. In these two modes, the power provided to the lamp was the same one. Thus, it is possible to evaluate the influence of the current pulses on the radiation production effectiveness in the ultraviolet part and the visible part of the spectrum. Theses results are illustrated in figures 8 and 9.

If you look at the figures above we see that the difference between the spectral results for both modes of operation is small. Better to see the difference, we calculate the total flux through each band. The results for the average values of the total spectral flux determined from figure 8 and 9 above are summarised and given in Table 3.

High Power Discharge Lamps

Total flux (u.a)

**4.2.3 Discussion** 

the pulsed mode.

Spectral Bandwidth (nm)

Pulsed mode

Relative progress

two feeding modes of current: rectangular and pulsed

confirmed by the results found by (Chammam et al., 2005).

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 235

Table 3. Comparison between the relative total flux of UV and visible radiation bands for

We note a clear increase in all the lines measured in the pulsed mode for the same power as in rectangular mode. However, the increase is particularly marked in the ultraviolet band spectrum and limited to the visible (Table 3). We can say that the pulsed mode favors the short wavelengths emission (UV band). This increase is mainly due to rising temperatures in

The increase in the UV and visible radiation in pulsed mode compared to the rectangular is

In this part, we present experimental results (electric and spectral) for a medium pressure lamp. This special lamp is provided by the Canadian company Trojan-UV. It is intended for water treatment because it has a broad emission band in the UV and visible spectral range.

**4.3 The radiation produced by medium pressure lamp in pulsed operation** 

The geometrical and electrical provided with this lamp are shown in Table 4 below:

Characteristics values Inter-electrode length (cm) 25 Diameter (mm) 22 Nominal Arc current (Arms) 6,8 Maximum arc current (Arms) 7,9 Nominal arc voltage (Vrms) 440±5% Maximum arc voltage (Vrms) 550 Power (W) 3000 Table 4. Electrical and geometrical characteristics of the medium pressure lamp

Rectangular mode 4100 57512

(7 pulses per half period) 5510 60296

(%) 34,4 4,84

total UV Visible 200-400 400-700

Fig. 8. Spectral flux UV with two supplying modes: (a) rectangular current; (b) with pulsed current

Fig. 9. Spectral flux visible with two supplying modes: (a) rectangular current; (b) with pulsed current


Table 3. Comparison between the relative total flux of UV and visible radiation bands for two feeding modes of current: rectangular and pulsed

### **4.2.3 Discussion**

234 Molecular Photochemistry – Various Aspects

200 240 280 320 360 400

200 240 280 320 360 400

Wavelength (nm)

400 450 500 550 600 650 700

400 450 500 550 600 650 700

Wavelength (nm)

Fig. 9. Spectral flux visible with two supplying modes: (a) rectangular current;

Fig. 8. Spectral flux UV with two supplying modes: (a) rectangular current;

0

(b) with pulsed current

0

(b) with pulsed current

4000

8000

12000

16000

Spectral flux visible (ua)

0

4000 8000

12000

100

200

300

Spectral flux UV (ua)

100

200

300

0

(b) Pulsed mode

(b) Pulsed mode

16000 (a) Rectangular mode

(a) Rectangular mode

We note a clear increase in all the lines measured in the pulsed mode for the same power as in rectangular mode. However, the increase is particularly marked in the ultraviolet band spectrum and limited to the visible (Table 3). We can say that the pulsed mode favors the short wavelengths emission (UV band). This increase is mainly due to rising temperatures in the pulsed mode.

The increase in the UV and visible radiation in pulsed mode compared to the rectangular is confirmed by the results found by (Chammam et al., 2005).

### **4.3 The radiation produced by medium pressure lamp in pulsed operation**

In this part, we present experimental results (electric and spectral) for a medium pressure lamp. This special lamp is provided by the Canadian company Trojan-UV. It is intended for water treatment because it has a broad emission band in the UV and visible spectral range. The geometrical and electrical provided with this lamp are shown in Table 4 below:


Table 4. Electrical and geometrical characteristics of the medium pressure lamp

High Power Discharge Lamps

0

0

2500

5000

Spectral flux UV-B (ua)

7500

5000

10000

Spectral flux UV-A (ua)

15000

20000

320 340 360 380 400

W avelength (nm )

Rectangular mode

285 290 295 300 305 310 315

Rectangular m ode

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 237

0

Fig. 11. Spectral flux band UV-A with two feeding modes of current: rectangular and pulsed

W avelength (nm) W avelength (nm)

Fig. 12. Spectral flux band UV-B with two feeding modes of current: rectangular and pulsed

5000

10000

15000

20000

320 340 360 380 400

W avelength (nm )

Pulsed mode

285 290 295 300 305 310 315

0

2500

Spectral flux UV-B (ua)

5000

7500

Pulsed m ode

### **4.3.1 Electrical measurements**

In this part we will present some electrical measurements carried out under pulsed current. To power the lamp at rated power of 3 kW, were overlaid seven pulse of amplitude equal to 4 A on a rectangular current level of 5.5 A in each half cycle of the rectangular current. The pulse duration is about 0.5 ms and the base frequency of the rectangular current is 50 Hz. In figure 10 we represent, the current and the instantaneous power consumed by the lamp in pulsed mode (Bouslimi et al., 2008).

Fig. 10. Instantaneous Current and power in the lamp in pulsed mode, A: Current (5 A/div), B: Power (2 kW/div), Time: 5 ms/div

Note that the instantaneous peak of power in the medium pressure lamp reaches almost twice the level. Thus, it is because the impulses that are causing successive short duration peaks of high power. The radiation produced, called pulsed light, is required by some photochemical applications such as disinfection of wastewater or drinking.

### **4.3.2 Spectral flux measurements in ultraviolet and visible band**

For this lamp, in order to evaluate the influence of pulses on the spectral flux of ultraviolet and visible radiation, spectral measurements are performed with a rectangular and pulsed current. The results obtained for the same power consumed by the lamp are shown in figures (11, 12, 13 and 14).

In this part we will present some electrical measurements carried out under pulsed current. To power the lamp at rated power of 3 kW, were overlaid seven pulse of amplitude equal to 4 A on a rectangular current level of 5.5 A in each half cycle of the rectangular current. The pulse duration is about 0.5 ms and the base frequency of the rectangular current is 50 Hz. In figure 10 we represent, the current and the instantaneous power consumed by the lamp

Fig. 10. Instantaneous Current and power in the lamp in pulsed mode, A: Current (5 A/div),

Note that the instantaneous peak of power in the medium pressure lamp reaches almost twice the level. Thus, it is because the impulses that are causing successive short duration peaks of high power. The radiation produced, called pulsed light, is required by some

For this lamp, in order to evaluate the influence of pulses on the spectral flux of ultraviolet and visible radiation, spectral measurements are performed with a rectangular and pulsed current. The results obtained for the same power consumed by the lamp are shown in

photochemical applications such as disinfection of wastewater or drinking.

**4.3.2 Spectral flux measurements in ultraviolet and visible band** 

**4.3.1 Electrical measurements** 

in pulsed mode (Bouslimi et al., 2008).

B: Power (2 kW/div), Time: 5 ms/div

figures (11, 12, 13 and 14).

Fig. 11. Spectral flux band UV-A with two feeding modes of current: rectangular and pulsed

Fig. 12. Spectral flux band UV-B with two feeding modes of current: rectangular and pulsed

High Power Discharge Lamps

Spectral bands (nm)

Relative increase

**4.3.3 Discussion of results** 

Relative tota flux (u.a)

visible.

**5. Conclusion** 

lamps and their power suppliers are showed.

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 239

Rectangular 11330 16495 29415 57108 308235

Pulsed 14760 20195 35945 70826 380810

(%) 30,2 22,4 22,1 24,2 23,5

Table 5. Comparison between the relative total flux of UV and visible radiation bands for

In figures (11, 12, 13 and 14) there is a clear increase in the flux of all the spectral lines measured in pulsed mode for the same power in rectangular mode. However, the increase is particularly important for the band UVC spectrum, dominated by the 254 nm line and in particular the molecular line 265 nm, very used to destroy bacteria. Increases in the UVA, UVB and visible, important, too, are substantially identical (about 23%). The increase of the radiation is mainly due to the increase of the electron temperature in the medium pressure discharge lamp. Note that for this lamp the increase is important both in the UV than in the

In this work, we have exposed the UV radiation and its applications in photochemistry. The mechanism of UV disinfection and the biological effects are also presented. Some discharge

In a great part of this work, we have showing some experimental results carried out on two

An attempt to raise the efficacy and to improve the performance was made by going to pulse operation instead of operating the arc on a rectangular wave power supply. It is

types of mercury lamp, considered as UV sources: high and medium pressure.

possible with this method to increase the efficacy to sufficiently high values.

two feeding modes of current: rectangular and pulsed (medium pressure lamp)

UVC UVB UVA UV total Visible 200-280 280-315 315-400 200-400 400-700

Fig. 13. Spectral flux of UV-C band with two feeding modes of current: rectangular and pulsed

Fig. 14. Spectral flux of visible band with two feeding modes of current: rectangular and pulsed.


Table 5. Comparison between the relative total flux of UV and visible radiation bands for two feeding modes of current: rectangular and pulsed (medium pressure lamp)

### **4.3.3 Discussion of results**

238 Molecular Photochemistry – Various Aspects

**Wavelength (nm) Wavelength (nm)**

Fig. 13. Spectral flux of UV-C band with two feeding modes of current: rectangular and

Fig. 14. Spectral flux of visible band with two feeding modes of current: rectangular and

200 220 240 260 280

Pulsed mode

400 500 600 700

Wavelength (nm)

Pulsed mode

0

0

20000

40000

Spectral flux Visible (ua)

60000

80000

500

1000

Spectral flux UV-C (ua)

1500

2000

2500

200 220 240 260 280

Courant rectangulaire

400 500 600 700

Wavelength (nm)

Rectangular mode

0

0

pulsed.

20000

40000

Spectral flux Visible (ua)

60000

80000

pulsed

500

1000

Spectral flux UV-C (ua)

1500

2000

2500

In figures (11, 12, 13 and 14) there is a clear increase in the flux of all the spectral lines measured in pulsed mode for the same power in rectangular mode. However, the increase is particularly important for the band UVC spectrum, dominated by the 254 nm line and in particular the molecular line 265 nm, very used to destroy bacteria. Increases in the UVA, UVB and visible, important, too, are substantially identical (about 23%). The increase of the radiation is mainly due to the increase of the electron temperature in the medium pressure discharge lamp. Note that for this lamp the increase is important both in the UV than in the visible.

### **5. Conclusion**

In this work, we have exposed the UV radiation and its applications in photochemistry. The mechanism of UV disinfection and the biological effects are also presented. Some discharge lamps and their power suppliers are showed.

In a great part of this work, we have showing some experimental results carried out on two types of mercury lamp, considered as UV sources: high and medium pressure.

An attempt to raise the efficacy and to improve the performance was made by going to pulse operation instead of operating the arc on a rectangular wave power supply. It is possible with this method to increase the efficacy to sufficiently high values.

High Power Discharge Lamps

942, 1990

and Their Photochemical Applications: An Evaluation of Pulsed Radiation 241

Dunn, J., Burgess, D. & Leo, F., Investigation of pulsed light for terminal sterilization of WFI

Dunn, J., Clark, RW. ,Asmus ,JF. , Pearlman , JS. , Boyer, K., Painchaud , F. & Hofmann , GA.

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The spectral flux results obtained highlight and evaluate the effectiveness of the pulsed current on the radiation production in the ultraviolet and the visible part of the spectrum.

We also note that the improvement of the production of radiation considered, interested many photochemical applications and field lighting.

The applications of the pulsed supply with short duration and sharp dismounted front are considered as relatively recent techniques. It allows us to study in the future, the dynamic behavior of the discharge lamps and their instantaneous effects on the microorganisms in various water treatments (drinking water, waste water, seawater for aquaculture and shellfish culture).

### **6. References**


The spectral flux results obtained highlight and evaluate the effectiveness of the pulsed current on the radiation production in the ultraviolet and the visible part of the

We also note that the improvement of the production of radiation considered, interested

The applications of the pulsed supply with short duration and sharp dismounted front are considered as relatively recent techniques. It allows us to study in the future, the dynamic behavior of the discharge lamps and their instantaneous effects on the microorganisms in various water treatments (drinking water, waste water, seawater for aquaculture and

Bouslimi, L., Chammam, A., Ben Mustapha, M., Stambouli, M. & Cambronne, J.P. (2009a).

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'

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**11** 

*Poland* 

**The Comparison of the Photoinitiating** 

Light-induced polymerization reaction is largely encountered in many industrial applications. For example, laser direct imaging, graphics arts, holography, and dental materials require irradiation in the visible light region to benefit from laser technologies or simply to avoid UV damaging effects on skin [1]. The basic idea is to readily transform a liquid resin or a soft film into a solid film upon light exposure to form either a coating as developed in the UV curing area or an image as used in the (laser) imaging area. The starting resin is in fact a formulation that consist in an oligomer, a monomer, a photoinitiating system, and various additives depending on the applications (formulation

The imaging technology industries where lasers are very often used currently, appear in high-tech sectors combining photochemistry, organic and polymer chemistry, physics, optics, electronics such as (i) microelectronics – photoresists for the printed circuits, integrated circuits, very large and ultralarge scale integration circuits and laser direct imaging (LDI) technology that allows to write complex relief structures for the manufacture of microcircuits or to pattern selective areas in microelectronic packaging, and so on, (ii) graphic arts – manufacture of conventional printing plates, computer-to-plate technology that directly helps to reproduce a document on a printing plate, and so on (iii) 3D machining (or three-dimensional photopolymerization or stereolithography) which is giving the possibility to make objects for prototyping applications, (iv) optics – holographic recording and information storage, computer generated and embossed holograms, manufacture of optical elements (diffraction grating, mirrors, lenses, waveguide, array illuminators, and

Great effort is taken at present in the design the new photosensitive systems being able to work in well-defined conditions. As far as the polymerization reactions are concerned in UV

display applications), design of structured materials on the nanoscale size.

curing and imaging areas, they are mostly based on a radical process.

**1. Introduction** 

 \*

Corresponding Author

agents, stabilizers, pigments, fillers, etc.).

**Ability of the Dyeing Photoinitiating** 

**Systems Acting via Photoreducible** 

**or Parallel Series Mechanism** 

Janina Kabatc\* and Katarzyna Jurek *University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering,* 

Zoeteman, B.C.J., Hrubec, J., de Greef, E. & Kool, H.J. (1982). Mutagenic activity associated with by-products of drinking water disinfection by chlorine, chlorine dioxide, ozone, and UV-irradiation. *Environmental Health Perspectives*, 1982, vol.46, pp. 197- 205.

## **The Comparison of the Photoinitiating Ability of the Dyeing Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism**

Janina Kabatc\* and Katarzyna Jurek *University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Poland* 

### **1. Introduction**

242 Molecular Photochemistry – Various Aspects

Zoeteman, B.C.J., Hrubec, J., de Greef, E. & Kool, H.J. (1982). Mutagenic activity associated

205.

with by-products of drinking water disinfection by chlorine, chlorine dioxide, ozone, and UV-irradiation. *Environmental Health Perspectives*, 1982, vol.46, pp. 197-

> Light-induced polymerization reaction is largely encountered in many industrial applications. For example, laser direct imaging, graphics arts, holography, and dental materials require irradiation in the visible light region to benefit from laser technologies or simply to avoid UV damaging effects on skin [1]. The basic idea is to readily transform a liquid resin or a soft film into a solid film upon light exposure to form either a coating as developed in the UV curing area or an image as used in the (laser) imaging area. The starting resin is in fact a formulation that consist in an oligomer, a monomer, a photoinitiating system, and various additives depending on the applications (formulation agents, stabilizers, pigments, fillers, etc.).

> The imaging technology industries where lasers are very often used currently, appear in high-tech sectors combining photochemistry, organic and polymer chemistry, physics, optics, electronics such as (i) microelectronics – photoresists for the printed circuits, integrated circuits, very large and ultralarge scale integration circuits and laser direct imaging (LDI) technology that allows to write complex relief structures for the manufacture of microcircuits or to pattern selective areas in microelectronic packaging, and so on, (ii) graphic arts – manufacture of conventional printing plates, computer-to-plate technology that directly helps to reproduce a document on a printing plate, and so on (iii) 3D machining (or three-dimensional photopolymerization or stereolithography) which is giving the possibility to make objects for prototyping applications, (iv) optics – holographic recording and information storage, computer generated and embossed holograms, manufacture of optical elements (diffraction grating, mirrors, lenses, waveguide, array illuminators, and display applications), design of structured materials on the nanoscale size.

> Great effort is taken at present in the design the new photosensitive systems being able to work in well-defined conditions. As far as the polymerization reactions are concerned in UV curing and imaging areas, they are mostly based on a radical process.

<sup>\*</sup> Corresponding Author

The Comparison of the Photoinitiating Ability of the Dyeing

consideration [3].

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 245

from an electron donor to the photo-excited dye and the generation of radicals followed by either proton transfer from radical cation of electron donor or bond cleavage in electron donor is more common [3]. The intrinsic characteristics of two-component initiator systems leads to numerous kinetic limitations. For example, since the back electron transfer step is invariably thermodynamically feasible, back electron transfer and radical recombination decrease the potential concentration of free radical active centers. Furthermore, an inefficient radical is often produced simultaneously in this electron transfer/proton transfer reaction step because the dye-based radical is not active for initiation but is able to terminate a growing polymer chain [3]. These cumulative effects significantly limit polymerization kinetics of two-component initiator systems and tend to make visible light polymerization less attractive, than UV photocuring in applications where reaction rate is a primarily

Some dyes absorbing in the visible region have been reported to be photoreduced in the presence of amines [1]. These compounds belong to the families of xanthenes, fluorones, acridines, phenazines, thiazenes, and so on. For example, methylene blue is well known to react from its triplet state with amine to initiate the photopolymerization of acrylates. The photoreduction is accompanied with an important photobleaching of the dye, rendering the photopolymerization of thick samples under visible light. The photobleaching is not so important in the case of xanthenes or fluorones, although the polymerization can be very efficient. Very good efficiencies were reported using thionine, rose bengal, eosin Y, erythrosin, riboflavin, polymethine dyes as photosensitizers, and co-initiators, such as amines, sulfinates, carboxylates, organoborate salts [1]. In the case of amine as co-initiator, the reaction involves a hydrogen abstraction from a amine to semireduced form of a dye. But in the case of organoborate salts acting as a co-initiator, the reaction involves an electron transfer from borate anion to polymethine dye in its excited singlet state. These systems are able to shift the spectral sensitivity of photopolymers up to the red region of the visible spectrum. However, dye/co-initiator systems were not developed significantly in the industry. Very often, dark reactions take place that lead to poor shelf life of the formulation, an effect that was detrimental to their industrial use for a long time. In addition, the conversion of the monomer to polymer was generally limited. Indeed, for most of the industrial applications, conversion of more than 60% have to be reached, a goal that is difficult to achieve with conventional dye/co-initiator photoinitiating systems (PIS) [1].

In the last decade, three-component photoinitiating systems have emerged as an attractive alternative for visible light polymerization based on numerous demonstrations that the kinetic effectiveness of a two-component electron/proton transfer initiator system can be

Like the two-component system, the three-component (PIS) include a light absorbing moiety, an electron donor (ED) and an electron acceptor (EA). In such systems, the third component is supposed to scavenge the chain-terminating radicals that are generated by the photoreaction between other two components or produce the additional initiating radicals. This process leads to an increase of the free radical polymerization rate. Therefore, certain additives improve the polymerization efficiency, leading to the development of the so-called three-component photoinitiating systems [3-12]. Three-component initiator systems have consistently been found to be faster, more efficient, and more sensitive than their twocomponent counterparts [3]. The mechanism involved is rather complex and is based on

improved by the addition of a third component.

In this chapter, we will focus on photosensitive systems that are used in free radical photopolymerization reactions. We will give the most exhaustive presentation of potentially interesting systems developed on a laboratory scale together with the characteristic of their excited-state properties. We will also show how modern time resolved laser spectroscopy techniques allows to probe the photophysical/photochemical properties as well as the chemical reactivity of a given photoinitiating system [2].

### **2. Properties of photoinitiating system**

A photoinitiating system (PIS) consist at least in a photoinitiator (I). Very often, a co-initiator (coI), a radical scavenger (RS) or a photosensitizer S can be added. Basically, a photoinitiating system leads to radicals that can initiate the polymerization (1).

$$\begin{array}{ccccc} \text{I} & \xrightarrow{\text{h}\mathfrak{O}} & \text{R} \bullet & \xrightarrow{\text{M}} & \text{RM} \bullet & \xrightarrow{\text{I}} & \text{Polymer} \end{array} \tag{1}$$

The photoinitiator (I) is usually an organic molecule. Upon excitation by light, (I) is promoted from its ground singlet state S0 to its first excited singlet state S1 and then converted into its triplet state T1 via a fast intersystem crossing. In many cases, this transient T1 state yields the reactive radicals R that can attack a monomer molecule and initiate the polymerization [1, 2].

Radicals of photoinitiators are produced through several following typical processes:


The spectral absorption range of photoinitiator is a decisive factor: the wavelength range of the (I) absorption has to match the spectral emission range of the light source. Therefore, when pigmented or colored media are used, a spectral window has to be found to excite. It may happen that the direct excitation of photoinitiator is impossible. In that case, a photosensitizer (S) must be added. The role of sensitizer is to absorb the light and to transfer the excess of energy to the photoinitiator through the well-known energy transfer process. The process is efficient only if the energy level of a donor is higher than that of an acceptor.

The panchromatic sensitization of free radical polymerization under visible light can occur in a presence of the dye alone (one-component) or in a presence of two-, three- or multicomponent photoinitiating systems composed of dye molecule (sensitizer) and second compound acting as a co-initiator (either as electron or hydrogen atom donor).

Commonly, visible-light activated initiators are typically two-component initiator systems: a light-absorbing photosensitizer and co-initiator. In this type of photoinitiating system, the photo-excited dye may act as either an electron acceptor (for example, if an amine is used as the second component), or an electron donor (for example, when an iodonium salt is used as the second component) [3]. Athouugh both reaction pathways are known, electron transfer

In this chapter, we will focus on photosensitive systems that are used in free radical photopolymerization reactions. We will give the most exhaustive presentation of potentially interesting systems developed on a laboratory scale together with the characteristic of their excited-state properties. We will also show how modern time resolved laser spectroscopy techniques allows to probe the photophysical/photochemical properties as well as the

A photoinitiating system (PIS) consist at least in a photoinitiator (I). Very often, a co-initiator (coI), a radical scavenger (RS) or a photosensitizer S can be added. Basically, a

The photoinitiator (I) is usually an organic molecule. Upon excitation by light, (I) is promoted from its ground singlet state S0 to its first excited singlet state S1 and then converted into its triplet state T1 via a fast intersystem crossing. In many cases, this transient

A photoscission of a C-C, C-S, C-B and C-P bonds (most cleavable compounds are

 An hydrogen abstraction reaction between (I) and (coI), which plays the role of a hydrogen donor (such as an alcohol, a thiol, etc.); two radicals are formed: one on an

The spectral absorption range of photoinitiator is a decisive factor: the wavelength range of the (I) absorption has to match the spectral emission range of the light source. Therefore, when pigmented or colored media are used, a spectral window has to be found to excite. It may happen that the direct excitation of photoinitiator is impossible. In that case, a photosensitizer (S) must be added. The role of sensitizer is to absorb the light and to transfer the excess of energy to the photoinitiator through the well-known energy transfer process. The process is efficient only if the energy level of a donor is higher than that of an acceptor. The panchromatic sensitization of free radical polymerization under visible light can occur in a presence of the dye alone (one-component) or in a presence of two-, three- or multicomponent photoinitiating systems composed of dye molecule (sensitizer) and second

Commonly, visible-light activated initiators are typically two-component initiator systems: a light-absorbing photosensitizer and co-initiator. In this type of photoinitiating system, the photo-excited dye may act as either an electron acceptor (for example, if an amine is used as the second component), or an electron donor (for example, when an iodonium salt is used as the second component) [3]. Athouugh both reaction pathways are known, electron transfer

compound acting as a co-initiator (either as electron or hydrogen atom donor).

**RM Polymer** (1)

that can attack a monomer molecule and initiate the

photoinitiating system leads to radicals that can initiate the polymerization (1).

**M**

Radicals of photoinitiators are produced through several following typical processes:

**R**

chemical reactivity of a given photoinitiating system [2].

**2. Properties of photoinitiating system** 

**I**

T1 state yields the reactive radicals R

donor and another on (I),

based on the benzoyl chromophore),

An electron transfer process between (I) and (coI).

polymerization [1, 2].

**h**

from an electron donor to the photo-excited dye and the generation of radicals followed by either proton transfer from radical cation of electron donor or bond cleavage in electron donor is more common [3]. The intrinsic characteristics of two-component initiator systems leads to numerous kinetic limitations. For example, since the back electron transfer step is invariably thermodynamically feasible, back electron transfer and radical recombination decrease the potential concentration of free radical active centers. Furthermore, an inefficient radical is often produced simultaneously in this electron transfer/proton transfer reaction step because the dye-based radical is not active for initiation but is able to terminate a growing polymer chain [3]. These cumulative effects significantly limit polymerization kinetics of two-component initiator systems and tend to make visible light polymerization less attractive, than UV photocuring in applications where reaction rate is a primarily consideration [3].

Some dyes absorbing in the visible region have been reported to be photoreduced in the presence of amines [1]. These compounds belong to the families of xanthenes, fluorones, acridines, phenazines, thiazenes, and so on. For example, methylene blue is well known to react from its triplet state with amine to initiate the photopolymerization of acrylates. The photoreduction is accompanied with an important photobleaching of the dye, rendering the photopolymerization of thick samples under visible light. The photobleaching is not so important in the case of xanthenes or fluorones, although the polymerization can be very efficient. Very good efficiencies were reported using thionine, rose bengal, eosin Y, erythrosin, riboflavin, polymethine dyes as photosensitizers, and co-initiators, such as amines, sulfinates, carboxylates, organoborate salts [1]. In the case of amine as co-initiator, the reaction involves a hydrogen abstraction from a amine to semireduced form of a dye. But in the case of organoborate salts acting as a co-initiator, the reaction involves an electron transfer from borate anion to polymethine dye in its excited singlet state. These systems are able to shift the spectral sensitivity of photopolymers up to the red region of the visible spectrum. However, dye/co-initiator systems were not developed significantly in the industry. Very often, dark reactions take place that lead to poor shelf life of the formulation, an effect that was detrimental to their industrial use for a long time. In addition, the conversion of the monomer to polymer was generally limited. Indeed, for most of the industrial applications, conversion of more than 60% have to be reached, a goal that is difficult to achieve with conventional dye/co-initiator photoinitiating systems (PIS) [1].

In the last decade, three-component photoinitiating systems have emerged as an attractive alternative for visible light polymerization based on numerous demonstrations that the kinetic effectiveness of a two-component electron/proton transfer initiator system can be improved by the addition of a third component.

Like the two-component system, the three-component (PIS) include a light absorbing moiety, an electron donor (ED) and an electron acceptor (EA). In such systems, the third component is supposed to scavenge the chain-terminating radicals that are generated by the photoreaction between other two components or produce the additional initiating radicals. This process leads to an increase of the free radical polymerization rate. Therefore, certain additives improve the polymerization efficiency, leading to the development of the so-called three-component photoinitiating systems [3-12]. Three-component initiator systems have consistently been found to be faster, more efficient, and more sensitive than their twocomponent counterparts [3]. The mechanism involved is rather complex and is based on

The Comparison of the Photoinitiating Ability of the Dyeing

first described by G. B. Schuster et al. [19, 20].

(see Scheme 1).

dye.

boron bond cleavage and the formation of free radicals [22].

**2.1 Polymethine dyes as sensitizer in photoinitiating system** 

by a polymethine bridge as shown by the general structure 1.

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 247

Polymethine dyes were first synthesized in 1856 by Greville Williams. Classical polymethine dyes are cationic molecules in which two terminal nitrogen heterocyclic subunits are linked

> **<sup>N</sup> <sup>N</sup> R1 R1 n**

(1) In the ensuing 150+ years, thousands more cyanines have been synthesized due to demand based on diverse applications of these versatile dyes [18]. As it is known, these dyes present intense absorption and fluorescence bands in the green-red visible region of the electromagnetic spectrum and exhibit high fluorescence quantum yields. The best known application of these dyes is in the laser field, where they showed higher laser efficiency than rhodamine dyes. Besides their use as laser dyes, polymethines have also shown very good performance as sensitizing dyes in free radical photopolymerization, with the idea of using the photopolymers in industrial applications, such as photoimaging, holography, computer-to-plate, and so on. They have been used as sensitizer dye with organoborate or 1,3,5-triazine derivatives as a radical generating reagent. The ion pair composed of cyanine dye cation and an alkyltriarylborate anion was

The work of Schuster and co-workers [19, 20] on the photochemistry of cyanine borates led to the preparation of the color-tunable, operating in the visible region commercial photoinitiators [21]. This research group discovered that, photolysis of 1,4 dicyanonaphthalene containing an alkyltriphenylborate leads to one electron oxidation of alkyltriphenylborate salts yielding an alkyltriphenylboranyl radical that undergoes carbon-

The laser flash photolysis data allows one to describe the mechanism of the polymerization initiation process. The initiation step of the reaction involves alkyl radical formation as a result of photoinduced electron transfer from borate anion to the excited singlet state of cyanine dye, followed by the rapid cleavage of the carbon-boron bond of the boranyl radical

Scheme 1 summarizes possible primary and secondary processes, which may occur during the free radical photoinitiated polymerization with the use of cyanine borate initiators; where kBC denotes the rate of the carbon-boron bond cleavage, the reverse step is designated as k-BC, and kbl is the rate constant of the free radicals cross-coupling step yielding bleached

As it was mentioned above this chapter reports the use of polymethine dye as a part of a three-component photoinitiating system for radical polymerization in the visible region of the spectrum, together with an alkyltriphenylborate salt and different additives as coinitiators. In the study, we examined the ability of the systems formed by Cy/borate salt,

**X**

chemical secondary reactions. It was reported, that different radical intermediates generated during the irradiation and in the subsequent polymerization reaction react with the additive to give new reactive radicals.

The development of new photoinitiating systems remains an interesting challenge. In specific areas, for example in graphic arts or in conventional clear coat and overprint varnish applications, the photoinitiators must exhibit particular properties, among them a high photochemical reactivity leading to high curing speeds.

Kim et al. [4], used the thermodynamic feasibility and kinetic considerations to study photopolymerization initiated with rose bengal or fluorescein as photosensitizer to investigate the key factors involved with visible-light activated free radical polymerization involving three-component photoinitiating systems. Many of the same photosensitizers used for two-component electron-transfer initiating systems may also be used in threecomponent ones [3]. Examples include coumarin dyes, xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, *p*-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes, and pyridinium dyes [3, 13-17].

A number of kinetic mechanisms have been suggested to explain the enhanced kinetics and sensitivity for three-component initiatior systems.

There are few mechanisms of free radicals generation in dyeing three-component photoinitiating systems:


The photoreducible series mechanism is the well-known representative kinetic mechanism for three-component photoinitiating systems. Until now, photoreducible series mechanism for (PIS) containing camphoquinone or methylene blue dye have been reported as a representative kinetic mechanism. However, alternative kinetic mechanisms must be considered since a variety of dyes used in three-component initiator systems impose different thermodynamic and kinetic constraints. For this study, we used three-component photoinitiator systems containing thiacarbocyanine dye (Cy). This dye has excellent attributes that make it attractive for these mechanistic studies. Because this photosensitizer has both reduction potential as well as oxidation potential, the photo-excited dye allows thermodynamically feasible direct interactions with an electron donor as well as with an electron acceptor simultaneously.

In this chapter, the efficiency of the three-component photoinitiating system based on thiacarbocyanine dye to induce visible light polymerization of triacrylate monomer will be described. The ability of both photoinitiating systems formed by Cy/*n*butyltriphenylborate/second co-initiator and Cy/1,3,5-triazine derivative/heteroaromatic thiol to initiate polymerization under visible light will be reported.

To understand their efficiency in terms of monomer conversion, the photochemistry of these systems was investigated by means of steady state and time resolved spectroscopy.

### **2.1 Polymethine dyes as sensitizer in photoinitiating system**

246 Molecular Photochemistry – Various Aspects

chemical secondary reactions. It was reported, that different radical intermediates generated during the irradiation and in the subsequent polymerization reaction react with the additive

The development of new photoinitiating systems remains an interesting challenge. In specific areas, for example in graphic arts or in conventional clear coat and overprint varnish applications, the photoinitiators must exhibit particular properties, among them a high

Kim et al. [4], used the thermodynamic feasibility and kinetic considerations to study photopolymerization initiated with rose bengal or fluorescein as photosensitizer to investigate the key factors involved with visible-light activated free radical polymerization involving three-component photoinitiating systems. Many of the same photosensitizers used for two-component electron-transfer initiating systems may also be used in threecomponent ones [3]. Examples include coumarin dyes, xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, *p*-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes, and pyridinium dyes [3, 13-17].

A number of kinetic mechanisms have been suggested to explain the enhanced kinetics and

There are few mechanisms of free radicals generation in dyeing three-component

The photoreducible series mechanism is the well-known representative kinetic mechanism for three-component photoinitiating systems. Until now, photoreducible series mechanism for (PIS) containing camphoquinone or methylene blue dye have been reported as a representative kinetic mechanism. However, alternative kinetic mechanisms must be considered since a variety of dyes used in three-component initiator systems impose different thermodynamic and kinetic constraints. For this study, we used three-component photoinitiator systems containing thiacarbocyanine dye (Cy). This dye has excellent attributes that make it attractive for these mechanistic studies. Because this photosensitizer has both reduction potential as well as oxidation potential, the photo-excited dye allows thermodynamically feasible direct interactions with an electron donor as well as with an

In this chapter, the efficiency of the three-component photoinitiating system based on thiacarbocyanine dye to induce visible light polymerization of triacrylate monomer will be described. The ability of both photoinitiating systems formed by Cy/*n*butyltriphenylborate/second co-initiator and Cy/1,3,5-triazine derivative/heteroaromatic

To understand their efficiency in terms of monomer conversion, the photochemistry of these

systems was investigated by means of steady state and time resolved spectroscopy.

to give new reactive radicals.

photoinitiating systems:

Parallel series mechanism.

electron acceptor simultaneously.

photochemical reactivity leading to high curing speeds.

sensitivity for three-component initiatior systems.

Photoreducible series mechanism (dye/amine/iodonium salt),

thiol to initiate polymerization under visible light will be reported.

Photooxidizable series mechanism,

Polymethine dyes were first synthesized in 1856 by Greville Williams. Classical polymethine dyes are cationic molecules in which two terminal nitrogen heterocyclic subunits are linked by a polymethine bridge as shown by the general structure 1.

### (1)

In the ensuing 150+ years, thousands more cyanines have been synthesized due to demand based on diverse applications of these versatile dyes [18]. As it is known, these dyes present intense absorption and fluorescence bands in the green-red visible region of the electromagnetic spectrum and exhibit high fluorescence quantum yields. The best known application of these dyes is in the laser field, where they showed higher laser efficiency than rhodamine dyes. Besides their use as laser dyes, polymethines have also shown very good performance as sensitizing dyes in free radical photopolymerization, with the idea of using the photopolymers in industrial applications, such as photoimaging, holography, computer-to-plate, and so on. They have been used as sensitizer dye with organoborate or 1,3,5-triazine derivatives as a radical generating reagent. The ion pair composed of cyanine dye cation and an alkyltriarylborate anion was first described by G. B. Schuster et al. [19, 20].

The work of Schuster and co-workers [19, 20] on the photochemistry of cyanine borates led to the preparation of the color-tunable, operating in the visible region commercial photoinitiators [21]. This research group discovered that, photolysis of 1,4 dicyanonaphthalene containing an alkyltriphenylborate leads to one electron oxidation of alkyltriphenylborate salts yielding an alkyltriphenylboranyl radical that undergoes carbonboron bond cleavage and the formation of free radicals [22].

The laser flash photolysis data allows one to describe the mechanism of the polymerization initiation process. The initiation step of the reaction involves alkyl radical formation as a result of photoinduced electron transfer from borate anion to the excited singlet state of cyanine dye, followed by the rapid cleavage of the carbon-boron bond of the boranyl radical (see Scheme 1).

Scheme 1 summarizes possible primary and secondary processes, which may occur during the free radical photoinitiated polymerization with the use of cyanine borate initiators; where kBC denotes the rate of the carbon-boron bond cleavage, the reverse step is designated as k-BC, and kbl is the rate constant of the free radicals cross-coupling step yielding bleached dye.

As it was mentioned above this chapter reports the use of polymethine dye as a part of a three-component photoinitiating system for radical polymerization in the visible region of the spectrum, together with an alkyltriphenylborate salt and different additives as coinitiators. In the study, we examined the ability of the systems formed by Cy/borate salt,

The Comparison of the Photoinitiating Ability of the Dyeing

**N CCl3**

**N N**

**Cl3C**

**ClO4**

**S N**

Calorimeter.

**SH**

all runs after 5 min of irradiation.

Scheme 2. Compounds used in this study

**O**

**<sup>N</sup> H3C**

**OCH3**

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 249

**1,3,5- Triazine derivatives** 

**T1 T2** 

**Other co-initiators** 

**<sup>O</sup> <sup>N</sup> H3C**

**E1B E1BB2** 

**SH**

**2.1.1 Kinetic key factors for visible-light activated free radical polymerizations** 

**MS MI K1 EPM** 

The efficiency of different combinations of polymethine dye and additives as (PIS) for the polymerization of triacrylate, was evaluated using the differential scanning calorimetry (DSC), under isothermal conditions at room temperature, using a photo-DSC apparatus constructed on the basis of the TA Instruments DSC 2010 Differential Scanning

The different formulations, in molecular ratio of each component, for dye studied are detailed in Table 1. No significant photopolymerization was detected in the absence of the dye. Figures 1-4 show the corresponding kinetic observed for N,N'-diethylthiacarbocyanine dye, and Table 1 shows the final conversions, polymerization rates and inhibition times for

**N N**

**N N N** **OCH3**

**Cl3C**

**Cl3C**

**O**

**B**

**O O** **O**

**CH3 N**

**O**

**O**

Scheme 1. Primary and secondary processes occuring during the free radical photoinitiated polymerization with the use of cyanine borate photoinitiators.

Cy/borate salt/different derivatives, and Cy/1,3,5-triazine/heteroaromatic tiol to initiate polymerization under visible light (Scheme 2).

### **Photosensitizer**

**Cy** 

**Co-initiators:** 

**polymer**

Scheme 1. Primary and secondary processes occuring during the free radical photoinitiated

Cy/borate salt/different derivatives, and Cy/1,3,5-triazine/heteroaromatic tiol to initiate

**Photosensitizer** 

**CH**

**CH CH**

**Cy** 

**Co-initiators:** 

**Onium salts** 

**B2 NO NOB2 I** 

**Bp** 

**OCH3**

**N** N **BF4**

**OCH3 N**

**OCH3 BF4**

**<sup>N</sup> <sup>X</sup>**

**S N**

**OCH3 N BF4**

**isomerization fluorescence**

polymerization with the use of cyanine borate photoinitiators.

polymerization under visible light (Scheme 2).

**Cy ....**

**Cy**

**<sup>1</sup> kel \* BuB(Ph)3**

**Cy BuB(Ph)3 ....**

**Cy**

**B**

**<sup>h</sup> Cy .... BuB(Ph)3**

**electron transfer Cy <sup>+</sup> Bu**

**S**

B

**M kp**

**kBl**

**bleaching product Bu +**

**+**

**Cy + B(Ph)3**

**I Cl**

**kBC**

**k-BC**

**Cy BuB(Ph)3 .... k-el**

### **1,3,5- Triazine derivatives**

Scheme 2. Compounds used in this study

### **2.1.1 Kinetic key factors for visible-light activated free radical polymerizations**

The efficiency of different combinations of polymethine dye and additives as (PIS) for the polymerization of triacrylate, was evaluated using the differential scanning calorimetry (DSC), under isothermal conditions at room temperature, using a photo-DSC apparatus constructed on the basis of the TA Instruments DSC 2010 Differential Scanning Calorimeter.

The different formulations, in molecular ratio of each component, for dye studied are detailed in Table 1. No significant photopolymerization was detected in the absence of the dye. Figures 1-4 show the corresponding kinetic observed for N,N'-diethylthiacarbocyanine dye, and Table 1 shows the final conversions, polymerization rates and inhibition times for all runs after 5 min of irradiation.

The Comparison of the Photoinitiating Ability of the Dyeing

 2-Mercaptobenzimidazole 1,3,5-Triazine

2-Mercaptobenzimidazole/1,3,5-Triazine

246

**Dye [M]** 

Time [min]

**B2 [M]** 

0

triazine derivative.

**Co-initiator** 

inhibition time.

15

Rate of polymerization [mW]

30

45

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 251

0

**Molar ratio B2/other additives**

4

8

Double bond conversion [%]

Fig. 4. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) using photoinitiating systems based on the polymethine dye, heteroaromatic thiol and 1,3,5-

**B2** 5 10-3 5 10-3 0 0 6.5 0.502 10 **NO** 5 10-3 5 10-3 5 10-4 10 1 1.54 22 **NOB2** 5 10-3 1 10-2 1 10-2 1 0 9.38 46 **Bp** 5 10-3 5 10-3 5 10-4 10 0 3.20 39 **I** 5 10-3 5 10-3 3 10-3 1.67 0.8 1.11 10 **T1** 5 10-3 5 10-3 5 10-2 0.1 0 4.92 31 **T2** 5 10-3 5 10-3 1 10-2 0.5 1.4 2.24 30 **E1B** 5 10-3 5 10-3 1 10-2 0.5 1 5.86 28 **E1BB2** 5 10-3 1 10-2 1 10-2 1 1.5 2.11 25 **MS** 5 10-3 5 10-3 5 10-2 0.1 9 2.02 24 **K1** 5 10-3 5 10-3 1 10-1 5 9 0.84 15 **EPM** 5 10-3 5 10-3 5 10-3 1 0 0.261 4 **T1** 5 10-3 0 5 10-2 0 3 0.388 7 **MS** 5 10-3 0 5 10-2 0 8 0.29 10.7 **T1 + MS** 5 10-3 0 5 10-2 0 9 0.63 13 Table 1. Molar composition of the samples, corresponding B2/other additive molar ratio, final conversion obtained after 5 min of irradiation, maximum polymerization rate RP and

It has been reported that to enhance the kinetics of a visible-light activated initiation process, it is important to: (1) retard the back electron transfer and recombination reactions and (2) use the secondary reaction step to consume the nonproductive dye-based radical and thereby regenerate the original photosensitizer (dye) [3]. Figures 1-4 provide a comparison of the visible-light activated free radical polymerizations initiated by two-component

**Other Additives [M]** 

12

2345

 Cy/MI Cy/T1 Cy/MI/T1

**Final conversion (%)** 

Time [min]

**RP [mol/s]** 

**Inhibition time [s]** 

Fig. 1. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) (2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate/1-methyl-2-pyrrolidinone) using different photoinitiating systems based on the polymethine dye and onium salts.

Fig. 2. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) using different photoinitiating systems based on the polymethine dye and 1,3,5-triazine derivatives.

Fig. 3. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) using different photoinitiating systems based on the polymethine dye and other additives.

0

0

0

15

Double bond conversion [%]

Fig. 3. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) using different photoinitiating systems based on the polymethine dye and other additives.

30

15

Double bond conversion [%]

Fig. 2. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) using

different photoinitiating systems based on the polymethine dye and 1,3,5-triazine

 CyB2 CyB2/E1B CyB2/MS CyB2/EMP CyB2/K1 Cy/E1BB2

30

15

Double bond conversion [%]

Fig. 1. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) (2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate/1-methyl-2-pyrrolidinone) using different

30

45

2345

 CyB2 CyB2/NO CyB2/Bp CyB2/I Cy/NOB2

> CyB2 CyB2/T1 CyB2/T2

 CyB2 CyB2/E1B CyB2/MS CyB2/EMP CyB2/K1 Cy/E1BB2

Time [min]

2345

Time [min]

2345

Time [min]

 CyB2 CyB2/NO CyB2/Bp CyB2/I Cy/NOB2

photoinitiating systems based on the polymethine dye and onium salts.

 CyB2 CyB2/T1 CyB2/T2

2345

Time [min]

2345

Time [min]

2345

Time [min]

0

0

derivatives.

0

100

200

Rate of polymerization [mW]

300

400

150

Heat flow [mW]

300

150

300

Rate of polymerization [mW]

450

600

750

Fig. 4. Comparative study of the photopolymerization of TMPTA/MP mixture (9:1) using photoinitiating systems based on the polymethine dye, heteroaromatic thiol and 1,3,5 triazine derivative.


Table 1. Molar composition of the samples, corresponding B2/other additive molar ratio, final conversion obtained after 5 min of irradiation, maximum polymerization rate RP and inhibition time.

It has been reported that to enhance the kinetics of a visible-light activated initiation process, it is important to: (1) retard the back electron transfer and recombination reactions and (2) use the secondary reaction step to consume the nonproductive dye-based radical and thereby regenerate the original photosensitizer (dye) [3]. Figures 1-4 provide a comparison of the visible-light activated free radical polymerizations initiated by two-component

The Comparison of the Photoinitiating Ability of the Dyeing

rate of initiation from the onset of polymerization.

triacrylate compared with the two-component systems.

fluorescence quenching by co-initiators was first studied.

max [nm] 556 max [mol-1dm3cm-1] 113 000 Es [kJ/mol] 203 <sup>f</sup> 0.05 0 [ps] 139, 392 Eox [V/SCE] 1.0 Ered [V/SCE] -1.34

Maxiumum absorption wavelength max, molar extinction coefficient max, singlet state energy Es, fluorescence quantum yield f, singlet state lifetime 0, half-wave oxidation and reduction potentials Eox and Ered, respectively.

values close to the diffusion rate constant (kq = 2 1010 M-1s-1).

Table 2. Photophysical and Electrochemical Properties of Polymethine Dye

The quenching rate constants kq of the singlet excited state by co-initiators tested were determined in ethyl acetate:1-methyl-2-pyrrolidinone mixture (4:1) (Table 3), and showing

the secondary reaction step.

**2.1.2 Excited state reactivity** 

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 253

of the original photosensitizer (dye) in the secondary reaction step. The enhanced conversion relative to the two-component initiator system also arises from production of two radicals: an active initiating butyl radical and an active alkoxy, triazinyl, picolinium ester, thyil or phenyl radical. These results are supported by Table 1, which illustrates the reaction rate as well as the final conversion of monomer with photoinitiating systems under study. The data clearly indicate that the three-component initiator system (Cy/B2/second co-initiator) is the most effective in overall radical active centre production as well as the

On the contrary, the system Cy/T1 exhibits a good reactivity with both higher rate of polymerization and final conversion. However, the best results were obtained for the threecomponent system CyB2/T1 and CyB2/T2. The addition of 1,3,5-triazine derivative to the CyB2 system increased the polymerization rate as well as the final conversion of the

Finally, all these kinetic results provide very useful information in terms of the selection criteria for each component of photoinitiating system. Because once photosensitizer with both reduction and oxidation potentials is selected, the kinetic pathway is controlled by selection of an electron donor or an electron acceptor based on the thermodynamic feasibility, thereby influencing the conversion and rate of polymerization kinetic data.

As before, these kinetic differences of two kinetic pathways are ascribed to differences in the efficiency of retarding back electron transfer as well as regenerating photosensitizer through

Because polymethine dye tested exhibits medium fluorescence quantum yield (Table 2), the

**Cy** 

initiator systems (CyB2, Cy/MS and Cy/T) with the corresponding three-component PIS. These examples clearly show that the three-component initiator systems produce the highest rates and final conversion as predicted.

Figures 1-3 demonstrate that the photoreducible series mechanism (Cy/B2/second coinitiator) produces the highest conversion and the fastest rates of polymerization. In such photoinitiating system, since Cy\* reacts directly with borate salt (ED), this kinetic pathway can prevent photon energy wasting steps such as back electron transfer [23]. But it is well known, that in a case when stable alkyl radical (initiating radical) is formed as a result of carbon-boron bond cleavage in boranyl radical (product of primary photochemical reaction) the back electron transfer process does not occur. Therefore, in the EA-based secondary reaction step, the dye-based radical can be consumed and photosensitizer (Cy) can be regenerated.

The parallel-series mechanism (Figure 4 (Cy/thiol/triazine)) showed intermediate reaction kinetics because this kinetic pathway simultaneously involves both the photoreducible and photooxidizable mechanisms in the primary photochemical reaction. These results are also supported by Table 1 which illustrates that the photoreducible series mechanisms (Cy/B2/second co-initiator) produced the highest reaction kinetics and photo-sensitivity then the alternative kinetic pathway.

The comparison of Cy/MS and Cy/T systems also illustrates the importance of preventing of back electron transfer reaction. Grotzinger and coworkers reported that when 1,3,5 triazine derivative accepts an electron, it produce 1,3,5-triazine radical anion which fragments to produce an active, initiating 1,3,5-triazynyl radical and a chloride anion [12]. Thus, triazine (T) accepts an electron from Cy\*, and the product obtained undergoes a rapid unimolecular fragmentation reaction that limits back electron transfer. Because of the reduced back electron transfer between the Cy\* and T, Cy/T system leads to the generation of higher concentrations of active centers than Cy/MS system (however, complete bleaching of the dye in the photochemical reactions results in the low conversion in the twocomponent systems and the conversion reaches < 10 %.

On the other hand, the excited dye wastes photon energy in an electron transfer process between dye and co-initiator because of the back electron transfer competes with separation of gemine radical pair. It has generally been reported than only 10% of the absorbed light energy may be used for photo-induced electron transfer in the bimolecular organic electron transfer reaction [23]. Hence the Cy/MS initiator system only reached ~ 10 % of final conversion. The Cy/MS/T three-component initiator system produced enhancened conversion about 13 %.

As expected, this behavior is strongly dependent on the composition of the photoinitiating system. The photoinitiating ability of the (PIS) under study depends mostly on the nature of the co-initiator. The use of diphenyliodide or N-phenylethylmaleimide in the CyB2 photoinitiating system leads to poor and slow conversion of the monomer.

On the other hand, the Cy/B2/second co-initiator photoinitiating systems produced dramatically enhanced conversion ranging from 15 to 46 % because of effective retardation of the recombination reaction step and consumption of the dye-based radical to regenerate of the original photosensitizer (dye) in the secondary reaction step. The enhanced conversion relative to the two-component initiator system also arises from production of two radicals: an active initiating butyl radical and an active alkoxy, triazinyl, picolinium ester, thyil or phenyl radical. These results are supported by Table 1, which illustrates the reaction rate as well as the final conversion of monomer with photoinitiating systems under study. The data clearly indicate that the three-component initiator system (Cy/B2/second co-initiator) is the most effective in overall radical active centre production as well as the rate of initiation from the onset of polymerization.

On the contrary, the system Cy/T1 exhibits a good reactivity with both higher rate of polymerization and final conversion. However, the best results were obtained for the threecomponent system CyB2/T1 and CyB2/T2. The addition of 1,3,5-triazine derivative to the CyB2 system increased the polymerization rate as well as the final conversion of the triacrylate compared with the two-component systems.

Finally, all these kinetic results provide very useful information in terms of the selection criteria for each component of photoinitiating system. Because once photosensitizer with both reduction and oxidation potentials is selected, the kinetic pathway is controlled by selection of an electron donor or an electron acceptor based on the thermodynamic feasibility, thereby influencing the conversion and rate of polymerization kinetic data.

As before, these kinetic differences of two kinetic pathways are ascribed to differences in the efficiency of retarding back electron transfer as well as regenerating photosensitizer through the secondary reaction step.

### **2.1.2 Excited state reactivity**

252 Molecular Photochemistry – Various Aspects

initiator systems (CyB2, Cy/MS and Cy/T) with the corresponding three-component PIS. These examples clearly show that the three-component initiator systems produce the highest

Figures 1-3 demonstrate that the photoreducible series mechanism (Cy/B2/second coinitiator) produces the highest conversion and the fastest rates of polymerization. In such photoinitiating system, since Cy\* reacts directly with borate salt (ED), this kinetic pathway can prevent photon energy wasting steps such as back electron transfer [23]. But it is well known, that in a case when stable alkyl radical (initiating radical) is formed as a result of carbon-boron bond cleavage in boranyl radical (product of primary photochemical reaction) the back electron transfer process does not occur. Therefore, in the EA-based secondary reaction step, the dye-based radical can be consumed and photosensitizer (Cy) can be

The parallel-series mechanism (Figure 4 (Cy/thiol/triazine)) showed intermediate reaction kinetics because this kinetic pathway simultaneously involves both the photoreducible and photooxidizable mechanisms in the primary photochemical reaction. These results are also supported by Table 1 which illustrates that the photoreducible series mechanisms (Cy/B2/second co-initiator) produced the highest reaction kinetics and photo-sensitivity

The comparison of Cy/MS and Cy/T systems also illustrates the importance of preventing of back electron transfer reaction. Grotzinger and coworkers reported that when 1,3,5 triazine derivative accepts an electron, it produce 1,3,5-triazine radical anion which fragments to produce an active, initiating 1,3,5-triazynyl radical and a chloride anion [12]. Thus, triazine (T) accepts an electron from Cy\*, and the product obtained undergoes a rapid unimolecular fragmentation reaction that limits back electron transfer. Because of the reduced back electron transfer between the Cy\* and T, Cy/T system leads to the generation of higher concentrations of active centers than Cy/MS system (however, complete bleaching of the dye in the photochemical reactions results in the low conversion in the two-

On the other hand, the excited dye wastes photon energy in an electron transfer process between dye and co-initiator because of the back electron transfer competes with separation of gemine radical pair. It has generally been reported than only 10% of the absorbed light energy may be used for photo-induced electron transfer in the bimolecular organic electron transfer reaction [23]. Hence the Cy/MS initiator system only reached ~ 10 % of final conversion. The Cy/MS/T three-component initiator system produced enhancened

As expected, this behavior is strongly dependent on the composition of the photoinitiating system. The photoinitiating ability of the (PIS) under study depends mostly on the nature of the co-initiator. The use of diphenyliodide or N-phenylethylmaleimide in the CyB2

On the other hand, the Cy/B2/second co-initiator photoinitiating systems produced dramatically enhanced conversion ranging from 15 to 46 % because of effective retardation of the recombination reaction step and consumption of the dye-based radical to regenerate

photoinitiating system leads to poor and slow conversion of the monomer.

rates and final conversion as predicted.

then the alternative kinetic pathway.

conversion about 13 %.

component systems and the conversion reaches < 10 %.

regenerated.

Because polymethine dye tested exhibits medium fluorescence quantum yield (Table 2), the fluorescence quenching by co-initiators was first studied.


Maxiumum absorption wavelength max, molar extinction coefficient max,

singlet state energy Es, fluorescence quantum yield f, singlet state lifetime 0,

half-wave oxidation and reduction potentials Eox and Ered, respectively.

Table 2. Photophysical and Electrochemical Properties of Polymethine Dye

The quenching rate constants kq of the singlet excited state by co-initiators tested were determined in ethyl acetate:1-methyl-2-pyrrolidinone mixture (4:1) (Table 3), and showing values close to the diffusion rate constant (kq = 2 1010 M-1s-1).

The Comparison of the Photoinitiating Ability of the Dyeing

of the formation of the dye-based radical and boranyl radical.

parallel-series mechanism: Cy/thiol/triazine.

**2.2 Mechanism of free radicals formation** 

**2.2.1 Photoreducible series mechanisms** 

(Dye

based radical (Dye

recombination of the both radicals.

420 nm shows that the signal of (Dye

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 255

salt, Cy/B2/N-methylpicolinium ester, Cy/B2/acetal, Cy/B2/thiol, Cy/B2/triazine, (ii)

From the transient absorption spectra obtained during laser flash photolysis, the ground state photobleaching of sensitizer under addition of borate salt, heteroaromatic thiol or 1,3,5 triazine derivative can be observed at 420 nm. Laser flash photolysis experiments were carried out in acetonitrile solution, exciting at 355 nm. Accordingly, it can be seen in Scheme 3 that the depletion increases with increasing concentration of borate salt, as a consequence

As illustrated in Scheme 3, the kinetic pathway involves electron transfer and carbon-boron bond cleavege from borate salt to the photo-excited dye (Dye\*) and produces an active initiating radical (such as butyl radical) as the primary photochemical reaction. The second onium salt (N-alkoxypyridinium or diphenyliodonium salt), as an electron acceptor, consumes an inactive radical and produces another active radical (alkoxy or phenyl), thereby regenerating the original dye in the secondary reaction step. The regenerated (PS) may re-enter the primary photochemical reaction. This kinetic pathway is designed as a photoreducible series mechanism. It is the well-known representative kinetic mechanism for three-component initiators. In this mechanism, the second co-initiator increases the photopolymerization kinetics in two ways: (1) it consumes an inactive dye-based radical

) and produces an active initiating radical, thereby regenerating the original (PS) (dye),

) to boranyl radical within the gemine radical pair; as well with the

) formed from the interaction Dye/B2 (with excess of

) and (B2

) radicals

and (2) it reduces the recombination reaction of dye-based radical and boranyl radical.

Unfortunately, the latter species can not be observed under our experimental conditions. The initiating radicals in this case could come mainly from the boranyl radical, which undergoes rapid and irreversible fragmentation as a result of carbon-boron bond cleavage. It should be noted that this reaction will compete with the back electron transfer from dye-

As stated above, when carbocyanine dye is used with borate salt as co-initiator the excited singlet state is quenched with the rate close to the diffusion rate constant, observing an increase in the signal of dye-based radical: as borate salt acts as an electron donor, the

(Schemes 1 and 3). Monitoring the dye radical formation at 420 nm leads to the observation of a increasing absorbance of the dye-based radical, in line with the results obtained for heteroaromatic thiol. This demonstrates that the reaction between carbocyanine dye-excited state and borate salt behaves similary to that of thiol. From all these results, the low conversion observed in the photopolymerization for Cy/MS photoinitiating system could be

Turning now to the study of the three-component system, transient absorption spectroscopy at

borate salt with respect to other additives) decreases under addition of N-alkoxypyridinium

electron transfer reaction of sensitizer excited state and B2 leads to (Dye

explained by a low quantum yield of radical formation from MS+.


Table 3. Fluorescence Quenching Data kq and Gibbs Free Energy Gel Changes for Thiacarbocyanine Dye with Co-initiators Tested.

### **2.1.3 Thermodynamics of photo-induced electron transfer reaction**

Before investigating the kinetic mechanisms for efficient design of photoinitiator systems, thermodynamic feasibility for electron transfer reactions must be verified. The Rehm-Weller equation was used to predict the thermodynamic feasibility for electron transfer reaction as shown below [24]. In this study, N,N'-diethylthiacarbocyanine dye was selected as photosensitizer because allows thermodynamic feasibility for direct simultaneous interaction with an electron donor as well as with an electron acceptor previously described. B2 or MS are used as electron donor (ED) and NO, Bp, I, T1, T2, E1B, K or EMP is used as (EA).

Because of the redox properties of the dye (Table 2) and the co-initiators, the mechanism for the quenching of sensitizer's excited state likely involves an electron transfer process. The values of the Gibbs free energy change for the photoinduced electron transfer Get on excited state is given by the Rehm Weller equation (2) [24].

$$
\Delta G\_{el} = E\_{ox} - E\_{red} - E \, ^\circ + \text{C} \tag{2}
$$

where:

Eox and Ered are the half-wave oxidation and reduction potentials for the acceptor (Cy; Ered = -1.34 V/SCE) and the donor (B2; Eox = 1.16 V/SCE), respectively, and E\* is the energy of the excited state. The coulombic term C is usually neglected in polar solvents.

The Get values are very useful for determining the potential kinetic pathway. As can be seen in Table 3, the calculated values for the intermolecular singlet electron transfer reactions are favorable, indicating that the dye can be reduced in the presence of the electron donors, such as: B2 or heteroaromatic thiol or oxidized with 1,3,5-triazine derivative.

From these results, one can conclude that the carbocyanine dye reacts with the co-initiators mainly through the quenching of the first excited singlet state. The reaction proceeds through the formation of a geminate radical pair, which can recombine through a back electron transfer process or separate into free radicals. The latter process explains the formation of the dye-based radical when alkyltriphenylborate salt is used as a quencher, or the radical cation of the dye when 1,3,5-triazine is used instead.

These results can lead to two initiator systems with two corresponding thermodynamically feasible kinetic pathways, which are (i) photoreducible series mechanism: Cy/B2/onium salt, Cy/B2/N-methylpicolinium ester, Cy/B2/acetal, Cy/B2/thiol, Cy/B2/triazine, (ii) parallel-series mechanism: Cy/thiol/triazine.

### **2.2 Mechanism of free radicals formation**

254 Molecular Photochemistry – Various Aspects

**B2** 9.51011 -1.93 **NO** 6.151010 -64.64 **Bp** 1.961010 -67,.06 **T1** 3.61010 -27.02 **E1B** 7.42109 -26.44 **Thiol** 2.781010 -12.54

Table 3. Fluorescence Quenching Data kq and Gibbs Free Energy Gel Changes for

Before investigating the kinetic mechanisms for efficient design of photoinitiator systems, thermodynamic feasibility for electron transfer reactions must be verified. The Rehm-Weller equation was used to predict the thermodynamic feasibility for electron transfer reaction as shown below [24]. In this study, N,N'-diethylthiacarbocyanine dye was selected as photosensitizer because allows thermodynamic feasibility for direct simultaneous interaction with an electron donor as well as with an electron acceptor previously described. B2 or MS are used as electron donor (ED) and NO, Bp, I, T1, T2,

Because of the redox properties of the dye (Table 2) and the co-initiators, the mechanism for the quenching of sensitizer's excited state likely involves an electron transfer process. The values of the Gibbs free energy change for the photoinduced electron transfer Get on

Eox and Ered are the half-wave oxidation and reduction potentials for the acceptor (Cy; Ered = -1.34 V/SCE) and the donor (B2; Eox = 1.16 V/SCE), respectively, and E\* is the energy of the

The Get values are very useful for determining the potential kinetic pathway. As can be seen in Table 3, the calculated values for the intermolecular singlet electron transfer reactions are favorable, indicating that the dye can be reduced in the presence of the electron

From these results, one can conclude that the carbocyanine dye reacts with the co-initiators mainly through the quenching of the first excited singlet state. The reaction proceeds through the formation of a geminate radical pair, which can recombine through a back electron transfer process or separate into free radicals. The latter process explains the formation of the dye-based radical when alkyltriphenylborate salt is used as a quencher, or

These results can lead to two initiator systems with two corresponding thermodynamically feasible kinetic pathways, which are (i) photoreducible series mechanism: Cy/B2/onium

donors, such as: B2 or heteroaromatic thiol or oxidized with 1,3,5-triazine derivative.

\*

*G E E EC el ox red* (2)

**2.1.3 Thermodynamics of photo-induced electron transfer reaction** 

Thiacarbocyanine Dye with Co-initiators Tested.

excited state is given by the Rehm Weller equation (2) [24].

excited state. The coulombic term C is usually neglected in polar solvents.

the radical cation of the dye when 1,3,5-triazine is used instead.

E1B, K or EMP is used as (EA).

where:

**kq [M-1s-1] Gel [kJ/mol]** 

From the transient absorption spectra obtained during laser flash photolysis, the ground state photobleaching of sensitizer under addition of borate salt, heteroaromatic thiol or 1,3,5 triazine derivative can be observed at 420 nm. Laser flash photolysis experiments were carried out in acetonitrile solution, exciting at 355 nm. Accordingly, it can be seen in Scheme 3 that the depletion increases with increasing concentration of borate salt, as a consequence of the formation of the dye-based radical and boranyl radical.

### **2.2.1 Photoreducible series mechanisms**

As illustrated in Scheme 3, the kinetic pathway involves electron transfer and carbon-boron bond cleavege from borate salt to the photo-excited dye (Dye\*) and produces an active initiating radical (such as butyl radical) as the primary photochemical reaction. The second onium salt (N-alkoxypyridinium or diphenyliodonium salt), as an electron acceptor, consumes an inactive radical and produces another active radical (alkoxy or phenyl), thereby regenerating the original dye in the secondary reaction step. The regenerated (PS) may re-enter the primary photochemical reaction. This kinetic pathway is designed as a photoreducible series mechanism. It is the well-known representative kinetic mechanism for three-component initiators. In this mechanism, the second co-initiator increases the photopolymerization kinetics in two ways: (1) it consumes an inactive dye-based radical (Dye ) and produces an active initiating radical, thereby regenerating the original (PS) (dye), and (2) it reduces the recombination reaction of dye-based radical and boranyl radical.

Unfortunately, the latter species can not be observed under our experimental conditions. The initiating radicals in this case could come mainly from the boranyl radical, which undergoes rapid and irreversible fragmentation as a result of carbon-boron bond cleavage. It should be noted that this reaction will compete with the back electron transfer from dyebased radical (Dye ) to boranyl radical within the gemine radical pair; as well with the recombination of the both radicals.

As stated above, when carbocyanine dye is used with borate salt as co-initiator the excited singlet state is quenched with the rate close to the diffusion rate constant, observing an increase in the signal of dye-based radical: as borate salt acts as an electron donor, the electron transfer reaction of sensitizer excited state and B2 leads to (Dye ) and (B2 ) radicals (Schemes 1 and 3). Monitoring the dye radical formation at 420 nm leads to the observation of a increasing absorbance of the dye-based radical, in line with the results obtained for heteroaromatic thiol. This demonstrates that the reaction between carbocyanine dye-excited state and borate salt behaves similary to that of thiol. From all these results, the low conversion observed in the photopolymerization for Cy/MS photoinitiating system could be explained by a low quantum yield of radical formation from MS+.

Turning now to the study of the three-component system, transient absorption spectroscopy at 420 nm shows that the signal of (Dye ) formed from the interaction Dye/B2 (with excess of borate salt with respect to other additives) decreases under addition of N-alkoxypyridinium

The Comparison of the Photoinitiating Ability of the Dyeing

oxidation potential of (Dye

of interactions (Schemes 3-6).

**Photoisomerization Fluorescence**

radial formation.

**Dye**

reaction between (Dye

**B**

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 257

salt, iodonium salt, 1,3,5-triazine derivative, N-methylpicolinium ester or other co-initiators (Schemes 3-6). This indicates that onium salt, triazine or other co-initiators (with the

interaction of dye excited state with borate salt (Schemes 1-6). At the same time, the photobleaching of sensitizer ground state is lowered when second co-initiator is added to

recorvery of the dye ground state. The reaction is expected to proceed through an electron transfer process from dye-based radical to second co-initiator. From the value of the

to 0.08 eV (e.g. –12.54 kJmol-1 to 7.72 kJmol-1), this value would lead to a fast rate constant

**diff**

**H3C N**

**ClO4**

Scheme 4. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, borate salt and N-methylpicolinium ester. Inset: Transient absorption spectra of: (A) cyanine dye in a presence of borate salt recorded 50 ns after laser puls (squares) presented dye-based radical formation and (B) for cyanine dye in presence of equimolar ratio of tetramethylammonium *n*-butyltriphenylborate and N-methylpicolinium perchlorate recorded 100 ns after laser puls (circles) presented N-methylpicolinium ester

**O**

**R O**


0,00

Absorbance

0,01

0,02

0,03

**+ Dye diff ET ET**

 **C-B bond cleavage**

**+**

**B**

**B**

) (Eox = 1.0 V/SCE), the free energy of the electron transfer

) and second co-initiator is estimated to be in a range from –0.13 eV

the Dye/B2 system. This means that the reaction of second co-initiator with (Dye

) formed from the

 **C-O bond cleavage**

**O**

400 500 600 700

Wavelength [nm]

B Dye + B2 + Ester (E1B)

Dye + B2

**H3C**

**Dye +**

A

**H3C**

**N**

**N +**

**R O**

> **O R O**

) leads to

exception of heteroaromatic thiol) react with the Dye-based radical (Dye

Scheme 3. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, borate salt and other onium salt. Inset: Left: Transient absorption spectra recorded 100 ns after laser flash (355 nm) for dye in MeCN (squares) and 500 ns after lash for dye in presence *n*-butyltriphenylborate salt presented dye-based radical formation. In the midst of: Kinetic traces for dye-based radical decay at 610 nm in the presence of various amount of N-methoxy-*p*-phenylpyridinium salt. The concentration of quencher is marked in Figure. Right: The Stern-Volmer plot of the fluorescence quenching of cyanine dye-based radical by onium salt.

0,0 2,0x10-7 4,0x10-7 6,0x10-7 8,0x10-7

Time [s]

**\***

 0 M 1\*10-3 M 1\*10-2 M 5\*10-2 M

**ET**

Dye

**Onium salt**

**diff**

7,0x10<sup>4</sup>

**N OCH3**

1,4x10<sup>5</sup>

kobs

2,1x10<sup>5</sup>

2,8x10<sup>5</sup>

0,00 0,02 0,04 0,06 0,08

) [M]

**polymer**

**monomer**

**Ph3B <sup>+</sup>**

**B**

or

**I**

Quencher concentration (NO <sup>+</sup>


Dye **<sup>+</sup>**

**+**

Dye

Scheme 3. Mechanism of the reactions occurring in three-component photoinitiating systems

absorption spectra recorded 100 ns after laser flash (355 nm) for dye in MeCN (squares) and 500 ns after lash for dye in presence *n*-butyltriphenylborate salt presented dye-based radical formation. In the midst of: Kinetic traces for dye-based radical decay at 610 nm in the presence of various amount of N-methoxy-*p*-phenylpyridinium salt. The concentration of quencher is marked in Figure. Right: The Stern-Volmer plot of the fluorescence quenching of

based on carbocyanine dye, borate salt and other onium salt. Inset: Left: Transient

**\* ET**

Dye

**N OCH3**


0,00

Absorbance

0,05

400 500 600 700

Wavelenght [nm]

**B**

Dye **<sup>B</sup>**

**<sup>I</sup>** or **<sup>+</sup>**

cyanine dye-based radical by onium salt.

**N**

**I**

**monomer**

or

**+** 

**polymer**

**diff**

**h**

Dye radical

(Dye)TT

0,000 0,009 0,018 0,027 0,036

**CH3O**

Absorbance

salt, iodonium salt, 1,3,5-triazine derivative, N-methylpicolinium ester or other co-initiators (Schemes 3-6). This indicates that onium salt, triazine or other co-initiators (with the exception of heteroaromatic thiol) react with the Dye-based radical (Dye ) formed from the interaction of dye excited state with borate salt (Schemes 1-6). At the same time, the photobleaching of sensitizer ground state is lowered when second co-initiator is added to the Dye/B2 system. This means that the reaction of second co-initiator with (Dye ) leads to recorvery of the dye ground state. The reaction is expected to proceed through an electron transfer process from dye-based radical to second co-initiator. From the value of the oxidation potential of (Dye ) (Eox = 1.0 V/SCE), the free energy of the electron transfer reaction between (Dye ) and second co-initiator is estimated to be in a range from –0.13 eV to 0.08 eV (e.g. –12.54 kJmol-1 to 7.72 kJmol-1), this value would lead to a fast rate constant of interactions (Schemes 3-6).

Scheme 4. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, borate salt and N-methylpicolinium ester. Inset: Transient absorption spectra of: (A) cyanine dye in a presence of borate salt recorded 50 ns after laser puls (squares) presented dye-based radical formation and (B) for cyanine dye in presence of equimolar ratio of tetramethylammonium *n*-butyltriphenylborate and N-methylpicolinium perchlorate recorded 100 ns after laser puls (circles) presented N-methylpicolinium ester radial formation.

The Comparison of the Photoinitiating Ability of the Dyeing

reaction is exergonic and to be feasible.

secondary reaction step.

mechanism.

**2.2.2 Parallel-series mechanism** 

detected at 510 nm (Scheme 7).

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 259

Similarly, if the deactivation of the excited state of the dye proceed through a photoinduced electron transfer with heteroaromatic thiol or triazine (Schemes 6 and 7), the dye-based radical and (Dye+) are formed, respectively. In this case, one can assume that the oxidation and reduction potentials of dye are 1.0 V/SCE and –1.34 V/SCE, respectively. But the oxidation potential of thiol and reduction potential of triazine are in the range from 0.69 V/SCE to 0.90 V/SCE and –0.84 V/SCE, respectively. This leads to the calculation of Gel values of –12.54 and –27.02 kJ/mol for thiol and triazine, respectively, showing that this

In summary, when N,N'-diethylthiacarbocyanine dye is used as photosensitizer, the kinetic pathway is seen for the three-component initiator system composed of onium salt, picolinium ester, cyclic acetal, 1,3,5-triazine derivative or maleimide as second co-initiator enhances photopolymerization kinetics as previously described. As an example, the Cy/B2/NO photoinitiator system may be used to explain photoreducible series mechanism (Scheme 3). Because Cy/NO is not a thermodynamically feasible system, the primary photoinduced electron transfer reaction only proceeds between photo-excited dye and borate salt. Then, subsequent electron transfer involves the electron acceptor (NO) in a

On the other hand, under conditions where photosensitizer (dye) has both reduction and oxidation potentials, the photoexcited dye may act as both an electron donor and an electron acceptor, resulting in a parallel-series mechanism [3]. In this kinetic pathway, the electron transfer between the excited dye molecule and an electron donor competes with an electron transfer between the excited dye and an electron acceptor, as the primary photochemical reaction. The Cy/thiol/1,3,5-triazine photoinitiating system provides example of the combined parallel-series mechanism. Because Cy/MS initiator system is thermodynamically feasible (which did produce free radical active centers as a two-component initiator system; Figure 4) and Cy/T system is also thermodynamically feasible (free radical active centers were also produced), the Cy/MS/T initiator system may engage in the parallel-series

For such system composed of polymethine dye/1,3,5-triazine derivative, the photobleaching of the ground state increases with increasing concentration of triazine. This indicates that the photochemical reaction between Dye-excited state and T yields to the formation of transient species. According to the electron transfer reaction, the radical anion (T-) is easily

It can be seen from Scheme 7 that (T-) is formed within the laser pulse, as a consequence of its formation mainly in the electron transfer process from the excited singlet state of dye to triazine. This leads to the formation of radical cation of dye and the radical anion of triazine. The latter species afterwards loses chloride anion to give the initiating radical (T-), as was demonstrated for other triazine derivatives in presence of rose bengal [12]. Interesingly, the recorded cyclic voltammogram for T1 in acetonitrile (Figure 5 right) exhibits an irreversible reduction wave at –0.84 V/SCE and a noticeable oxidation wave at 1.270 V/SCE indicating a cleavage process within the radical anion (T-). It is expected that the chloride anion is

Scheme 5. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, borate salt and cyclic acetal.

Scheme 6. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, borate salt and heteroaromatic thiol.

Similarly, if the deactivation of the excited state of the dye proceed through a photoinduced electron transfer with heteroaromatic thiol or triazine (Schemes 6 and 7), the dye-based radical and (Dye+) are formed, respectively. In this case, one can assume that the oxidation and reduction potentials of dye are 1.0 V/SCE and –1.34 V/SCE, respectively. But the oxidation potential of thiol and reduction potential of triazine are in the range from 0.69 V/SCE to 0.90 V/SCE and –0.84 V/SCE, respectively. This leads to the calculation of Gel values of –12.54 and –27.02 kJ/mol for thiol and triazine, respectively, showing that this reaction is exergonic and to be feasible.

In summary, when N,N'-diethylthiacarbocyanine dye is used as photosensitizer, the kinetic pathway is seen for the three-component initiator system composed of onium salt, picolinium ester, cyclic acetal, 1,3,5-triazine derivative or maleimide as second co-initiator enhances photopolymerization kinetics as previously described. As an example, the Cy/B2/NO photoinitiator system may be used to explain photoreducible series mechanism (Scheme 3). Because Cy/NO is not a thermodynamically feasible system, the primary photoinduced electron transfer reaction only proceeds between photo-excited dye and borate salt. Then, subsequent electron transfer involves the electron acceptor (NO) in a secondary reaction step.

### **2.2.2 Parallel-series mechanism**

258 Molecular Photochemistry – Various Aspects

**CH**

**S N CH3**

**initiating radical**

**R C O OCH2CH2**

**B +**

**S + H**

**N**

**O O R**

**primary radicals formation electron transfer**

**Dye +**

based on carbocyanine dye, borate salt and cyclic acetal.

**el**

**fast process**

> **slow process**

based on carbocyanine dye, borate salt and heteroaromatic thiol.

**el**

**X N SH**

**diff**

**B**

**Dye [ ] \***

**Photoisomerization Fluorescence**

**diff**

**X N SH** **N S CH CH3**

**B**

**B +**

**B +**

Scheme 5. Mechanism of the reactions occurring in three-component photoinitiating systems

**B + Dye**

**+ Dye**

Scheme 6. Mechanism of the reactions occurring in three-component photoinitiating systems

 **C-B bond cleavage**

**O O R**

**hydrogen abstraction**

**initiating radical**

**bond cleavage C B**

**Hydrogen abstraction**

 **<sup>X</sup>**

**X**

**CH**

**N S CH CH3**

**CH**

**B**

**X**

**CH**

**S N CH3**

**h**

On the other hand, under conditions where photosensitizer (dye) has both reduction and oxidation potentials, the photoexcited dye may act as both an electron donor and an electron acceptor, resulting in a parallel-series mechanism [3]. In this kinetic pathway, the electron transfer between the excited dye molecule and an electron donor competes with an electron transfer between the excited dye and an electron acceptor, as the primary photochemical reaction. The Cy/thiol/1,3,5-triazine photoinitiating system provides example of the combined parallel-series mechanism. Because Cy/MS initiator system is thermodynamically feasible (which did produce free radical active centers as a two-component initiator system; Figure 4) and Cy/T system is also thermodynamically feasible (free radical active centers were also produced), the Cy/MS/T initiator system may engage in the parallel-series mechanism.

For such system composed of polymethine dye/1,3,5-triazine derivative, the photobleaching of the ground state increases with increasing concentration of triazine. This indicates that the photochemical reaction between Dye-excited state and T yields to the formation of transient species. According to the electron transfer reaction, the radical anion (T-) is easily detected at 510 nm (Scheme 7).

It can be seen from Scheme 7 that (T-) is formed within the laser pulse, as a consequence of its formation mainly in the electron transfer process from the excited singlet state of dye to triazine. This leads to the formation of radical cation of dye and the radical anion of triazine. The latter species afterwards loses chloride anion to give the initiating radical (T- ), as was demonstrated for other triazine derivatives in presence of rose bengal [12]. Interesingly, the recorded cyclic voltammogram for T1 in acetonitrile (Figure 5 right) exhibits an irreversible reduction wave at –0.84 V/SCE and a noticeable oxidation wave at 1.270 V/SCE indicating a cleavage process within the radical anion (T-). It is expected that the chloride anion is

The Comparison of the Photoinitiating Ability of the Dyeing


Potential [mV]

**2.3 Photoinitiation efficiency** 

concentration of the co-initiators.

evidences the electron transfer process from sensitizer to triazine.



Current [A]

0

10

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 261




Current [A]

Fig. 5. Cyclic voltammograms of Dye (Cy) and 1,3,5-triazine derivative (T) in 0.1 M

tetrabutylammonium perchlorate solution in dry acetonitrile as the supporting electrolyte.

At 510 nm (Scheme 7) it is possible to observe an increase of (T-) signal, which clearly

From all these experiments, it turns out that the photoreactions from the excited state of the carbocyanine dye are very efficient with both borate salt and second co-initiator. These photoreactions lead to the formation of initiating species, and therefore, to the conversion of monomer. A rough estimate of the diffusion rate constant can be given by equation 3:

> 8 3 *<sup>d</sup> RT <sup>k</sup>*

This leads to the value of kd = 1.84 106 M-1s-1 for the monomer used. Consequently, the quantum efficiency of dye excited state deactivation by a given quencher Q will depend on kd [Q] for most of the photoreactions reported in Table 3. Therefore, the relative efficiency of the corresponding photochemical processes will be mainly dependent on the

In the case of the three-component photoinitiating systems, the highest concentration of additive makes the excited state quenched by second co-initiator, leading to initiating

react with second co-initiator leading to the recorvery of the dye ground state and additional initiating species (Schemes 3-7). The fact that three-component photoinitiating systems have higher efficiences than two-component ones is in good agreement with the expected reaction of second co-initiator with the dye-based radical. The combination of coinitiators B2 and others have clearly a beneficial effect on the photopolymeryzation process. By contrast, in a case of three-component photoinitiating system composed of dye/thiol/triazine, the deactivation of the (Cy) excited state will be mainly governed by the photoreaction with both thiol and 1,3,5-triazine derivative (Scheme 7). This leads to the

radical (after electron transfer process) and the dye-based radical (Dye

formation of initiating radicals and radical cation (Dye+).

0

300

600


Potential [mV]

(3)

). This latter is able to

expelled in a fast time scale, preventing the (Dye+)/(T-) system to undergo a back electron transfer process.

Scheme 7. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, heteroaromatic thiol and 1,3,5-triazine derivative. Inset: Left: Transient absorption spectra of cyanine dye in a presence of 2-mercaptobenzothiazole (MS) recorded: 1 s (squares), 4 s (circles) and 10 s (triangles) after laser pulse presented the thiyl and dye-based radicals formation. In: Transient absorption spectra of cyanine dye in a presence of 2-mercaptobenzothiazole (MS) recorded 100 ns after laser pulse (circles) in acetonitrile solution. Right: Transient absorption spectra of cyanine dye in a presence of 2,4 bis-(trichloromethyl)-6-(4-methoxy)phenyl-1,3,5-triazine (T) recorded 50 ns after laser pulse presented the 1,3,5-triazinyl radical formation.

Fig. 5. Cyclic voltammograms of Dye (Cy) and 1,3,5-triazine derivative (T) in 0.1 M tetrabutylammonium perchlorate solution in dry acetonitrile as the supporting electrolyte.

At 510 nm (Scheme 7) it is possible to observe an increase of (T-) signal, which clearly evidences the electron transfer process from sensitizer to triazine.

### **2.3 Photoinitiation efficiency**

260 Molecular Photochemistry – Various Aspects

expelled in a fast time scale, preventing the (Dye+)/(T-) system to undergo a back electron


**\***

**Electron transfer**

**I**

**S N CH**

**CH CH**

**S N**

Absorbance

**S N**

> **S N**

400 500 600 700

**+**

**+ H**

**X N S**

**X N SH**

Wavelength [nm]

**X N SH**

0,18 1,3,5-Triazine radical anion

transfer process.

0,08

0,00

0,04

Absorbance

**N N N Cl3C CCl2**

**N N N Cl3C CCl3**

**OCH3**

**OCH3**

**+**

**+ Cl**

presented the 1,3,5-triazinyl radical formation.

570 600 630 660 690

**OCH3**

**N N N Cl3C CCl3**

Wavelength [nm]

**Electron transfer**

**I**

**S N CH**

**CH CH**

**S N**

0,00 0,01 0,02

Absorbance

Thiyl radical

350 400 450

Wavelength [nm]

Dye radical

**Photoisomerization Fluorescence**

**I**

**I**

**S N CH**

> **S N CH**

**diff diff**

**CH CH**

**h**

**CH CH**

Scheme 7. Mechanism of the reactions occurring in three-component photoinitiating systems based on carbocyanine dye, heteroaromatic thiol and 1,3,5-triazine derivative. Inset: Left: Transient absorption spectra of cyanine dye in a presence of 2-mercaptobenzothiazole (MS) recorded: 1 s (squares), 4 s (circles) and 10 s (triangles) after laser pulse presented the thiyl and dye-based radicals formation. In: Transient absorption spectra of cyanine dye in a presence of 2-mercaptobenzothiazole (MS) recorded 100 ns after laser pulse (circles) in acetonitrile solution. Right: Transient absorption spectra of cyanine dye in a presence of 2,4 bis-(trichloromethyl)-6-(4-methoxy)phenyl-1,3,5-triazine (T) recorded 50 ns after laser pulse

From all these experiments, it turns out that the photoreactions from the excited state of the carbocyanine dye are very efficient with both borate salt and second co-initiator. These photoreactions lead to the formation of initiating species, and therefore, to the conversion of monomer. A rough estimate of the diffusion rate constant can be given by equation 3:

$$k\_d = \frac{8RT}{3\eta} \tag{3}$$

This leads to the value of kd = 1.84 106 M-1s-1 for the monomer used. Consequently, the quantum efficiency of dye excited state deactivation by a given quencher Q will depend on kd [Q] for most of the photoreactions reported in Table 3. Therefore, the relative efficiency of the corresponding photochemical processes will be mainly dependent on the concentration of the co-initiators.

In the case of the three-component photoinitiating systems, the highest concentration of additive makes the excited state quenched by second co-initiator, leading to initiating radical (after electron transfer process) and the dye-based radical (Dye ). This latter is able to react with second co-initiator leading to the recorvery of the dye ground state and additional initiating species (Schemes 3-7). The fact that three-component photoinitiating systems have higher efficiences than two-component ones is in good agreement with the expected reaction of second co-initiator with the dye-based radical. The combination of coinitiators B2 and others have clearly a beneficial effect on the photopolymeryzation process.

By contrast, in a case of three-component photoinitiating system composed of dye/thiol/triazine, the deactivation of the (Cy) excited state will be mainly governed by the photoreaction with both thiol and 1,3,5-triazine derivative (Scheme 7). This leads to the formation of initiating radicals and radical cation (Dye+).

The Comparison of the Photoinitiating Ability of the Dyeing

*Polymer Chemistry*, Vol. 48, pp. 2594-2603.

**4. Acknowledgment** 

**5. References** 

(MNiSW) (grant No N N204 219734).

47, pp. 887-898.

*47*, pp. 3131-3141.

*47*, pp. 576-588.

4251.

3677.

1119-1123.

100, p. 1973

p.2326.

Vol. *38*, pp. 2057-2066.

*Chemistry*. Vol. *45*, pp. 3626-3636.

[10] Kabatc, J. (2010). *Polymer*. Vol. *51*, pp. 5028-5036.

Photoinitiating Systems Acting via Photoreducible or Parallel Series Mechanism 263

This work was supported by The Ministry of Science and Higher Education of Poland

[1] Tarzi, O.I.; Allonas, X.; Ley, C.; Fouassier, J.P. (2010). *Journal of Polymer Sciences: Part A:* 

[2] Fouassier, J.P.; Allonas, X.; Lalevée, J.; Dietlin C. (2010). *Photochemistry and Photophysics of Polymer Materials*, Ed. Allen, N.S., John Wiley & Sons, Inc,pp. 351-420. [3] Kim, D.; Stansbury, J.W. (2009) *Journal of Polymer Science: Part A: Polymer Chemistry*. Vol.

[4] Kim, D.; Stansbury, J. W. (2009). *Journal of Polymer Science: Part A: Polymer Chemistry*. Vol.

[5] Padon, K.S.; Scranton, A. B. (2000). *Journal of Polymer Science: Part A: Polymer Chemistry*.

[7] Kabatc, J.; Zasada, M.; Pączkowski, J. (2007). *Journal of Polymer Science: Part A: Polymer* 

[8] Kabatc, J.; Pączkowski, J. (2009). *Journal of Polymer Science: Part A: Polymer Chemistry*. Vol.

[9] Kabatc, J. (2010). *Journal of Polymer Science: Part A: Polymer Chemistry*. Vol. *48*, pp. 4243–

[11] Cavitt, T. B.; Hoyle, C. E.; Kalyanaraman, V.; Jönsson, S. (2004). *Polymer*. Vol. *45,* pp.

[12] Grotzinger, C; Burget, D.; Jacques, P.; Fouassier, J.P. (2003). *Polymer.* Vol. 44, pp. 3671-

[15] Oxman, J.D.; Jacobs, D.W.; Trom, M.C.; Sipani, V.; Ficek, B.; Scranton, A.B. (2005) *Journal* 

[18] Mishra, A.; Behera, R.K.; Behera, P.K.; Mishra, B.K.; Behera, G.B. (2000). *Chem Rev*. Vol.

[19] Chatterjee, S.; Davis, P.D.; Gottschalk, P.; Kurz, M.E.; Sauerwein, B.; Yang, X.; Schuster,

[20] Chatterjee, S.; Gottschalk, P.; Davis, P.D.; Schuster, G.B. (1988). *J Am Chem Soc.* Vol. 110,

[21] For examples of cyanine dyes see: (a) Gottschalk, P.; Neckers, D.C.; Schuster, G.B.

(1980). *US Patent 4 772 530*; (1987). *Chem Abstr* 107:187434n; (1988). *US Patent* 4 842 980; (1987). *Chem Abstr* 107:187434n; (b) Gottschalk, P. (1989). *US Patent* 4 874 450;

[14] Palazzotto, M.C.; Ubel, F.A.; Oxman, J.D.; Ali, Z.A. (2000) *U.S. Patent* 6,017,660.

*of Polymer Science: Part A: Polymer Chemistry*. Vol. 43, pp. 1747-1756.

[6] Kabatc, J.; Pączkowski, J. (2005). *Macromolecules*. Vol. *38*, pp. 9985-9992.

[13] Oxman, J.D.; Ubel, F.A.; Larson, G.B. (1989) *U.S. Patent* 4,828,583.

[16] Oxman, J.D.; Jacobs, D.W. (1999) *U.S. Patent* 5,998,495. [17] Oxman, J.D.; Jacobs, D.W. (2000) *U.S. Patent* 6,025,406.

G.B. (1990). *J Am Chem Soc.* Vol. 112, p. 6329.

(c) Weed, G.; Monroe, B.M. (1992). *US Patent* 5 143 818.

It was show that three-component photoinitiating systems acting via photo-reducible series mechanism producess the highest rates of polymerization and final conversion of monomer (Figure 6).

Fig. 6. The comparison of the photoinitiating ability of the dyeing photoinitiating systems acting via photoreducible series mechanism and parallel series mechanism.

### **3. Conclusions**

In this chapter, we have characterized two different kinetic mechanisms using thermodynamic feasibility and key kinetic factors with three-component visible light photoinitiating systems containing thiacarbocyanine dye as a photosensitizer. We used the Rehm-Weller equation to verify the thermodynamic feasibility for the photoinduced electron transfer reaction. Based on this, we have suggested two different kinetic mechanisms, which are (i) photoreducible series mechanism (Cy/B2/second co-initiator) and (ii) parallel series mechanism (Cy/thiol/triazine). In addition, based on experimental kinetic data, we have evaluated two kinetic pathways. The photo-DSC kinetic experiments revealed that the photoreducible series mechanism produced the highest rates of polymerization and final conversion of monomer values. It was found, that threecomponent PIS showed the best performance. Laser spectroscopy studies allowed the understanding the processes that may explain the behavior observed in terms of photopolymerization. The sensitizer reacts mainly throught a singlet electron transfer mechanism from the borate salt or heteroaromatic thiol to the dye and from the dye to the triazine derivative. Beneficial side-reactions were shown to limit the photobleaching of the dye, resulting in higher final monomer conversion.

Although, these two kinetic pathways presented here can not govern the detailed interactions in all initiator mechanisms, this approach will provide useful information for selection criteria for each component, as well as provide a straightforward manner for classifying the photopolymerization process.

### **4. Acknowledgment**

This work was supported by The Ministry of Science and Higher Education of Poland (MNiSW) (grant No N N204 219734).

### **5. References**

262 Molecular Photochemistry – Various Aspects

It was show that three-component photoinitiating systems acting via photo-reducible series mechanism producess the highest rates of polymerization and final conversion of monomer

CyB2/E1B

CyB2/T2

Photo-reducible series mechanism

CyB2/T1

CyB2/I

acting via photoreducible series mechanism and parallel series mechanism.

CyB2/Bp

Cy/NOB2

CyB2/NO

CyB2

dye, resulting in higher final monomer conversion.

classifying the photopolymerization process.

Parallel series mechanism

Cy/T1/MS

CyB2/EPM

CyB2/K1

Cy/E1BB2

Photoinitiating system

Fig. 6. The comparison of the photoinitiating ability of the dyeing photoinitiating systems

In this chapter, we have characterized two different kinetic mechanisms using thermodynamic feasibility and key kinetic factors with three-component visible light photoinitiating systems containing thiacarbocyanine dye as a photosensitizer. We used the Rehm-Weller equation to verify the thermodynamic feasibility for the photoinduced electron transfer reaction. Based on this, we have suggested two different kinetic mechanisms, which are (i) photoreducible series mechanism (Cy/B2/second co-initiator) and (ii) parallel series mechanism (Cy/thiol/triazine). In addition, based on experimental kinetic data, we have evaluated two kinetic pathways. The photo-DSC kinetic experiments revealed that the photoreducible series mechanism produced the highest rates of polymerization and final conversion of monomer values. It was found, that threecomponent PIS showed the best performance. Laser spectroscopy studies allowed the understanding the processes that may explain the behavior observed in terms of photopolymerization. The sensitizer reacts mainly throught a singlet electron transfer mechanism from the borate salt or heteroaromatic thiol to the dye and from the dye to the triazine derivative. Beneficial side-reactions were shown to limit the photobleaching of the

Although, these two kinetic pathways presented here can not govern the detailed interactions in all initiator mechanisms, this approach will provide useful information for selection criteria for each component, as well as provide a straightforward manner for

CyB2/MS

Cy/T1

Cy/MS

(Figure 6).

0

**3. Conclusions** 

4

8

12

Relative Rate of Polymerization

16

20


**12** 

*Spain* 

Miguel A. Rodríguez

**Light-Induced Iminyl Radicals:** 

**Generation and Synthetic Applications** 

R<sup>2</sup> N

**Iminyl**

R1

*Departamento de Química, Unidad Asociada al C.S.I.C., Universidad de La Rioja* 

The photochemistry of the carbon-nitrogen double bond was first explored in depth in the seventies (Padwa, 1977; Pratt, 1977). The low photochemical reactivity of this bond is due to the deactivation of the excited state of the imine by *E*–*Z* isomerization processes, which do not have any synthetic utility because of their very low energy barrier for thermal conversion (Padwa & Albrecht, 1974a, 1974b). However, the presence of an electronegative atom on the nitrogen opens up the field of reactivity. Studies on the energies of the N–O bonds in acyloximes show that this bond can easily undergo homolytic cleavage induced by ultraviolet light, a process that leads to iminyl radicals (Okada et al., 1969). The use of radicals has proven to be a useful tool in organic synthesis (Renaud & Sibi, 2001; Togo, 2004; Zard, 2003). In particular, the cyclization of nitrogen-centred radicals, such as aminyl or iminyl radicals (Chart 1), is a valuable procedure for the preparation of nitrogen heterocycles (Fallis & Brinza, 1997; Zard, 2008). It is therefore necessary to have effective

The direct way to create nitrogen radicals involves the homolytic cleavage of N–X bonds, while the addition of a radical to an unsaturated nitrogen functional group, such as a nitrile or an imine derivative, constitutes the most widely used indirect method. The cleavage of the bond may be triggered thermally or photochemically. This chapter focuses on the photochemical generation of iminyl radicals and the study of their reactivity. Firstly, the methods for the production of iminyl radicals will be reviewed and the reactivity of this species will be examined, highlighting its ability to be added to unsaturated systems, with particular emphasis on intramolecular cyclization reactions. At this point, the regioselectivity in the formation of five- and six-membered rings will be analysed. In terms of the course of these reactions, both experimental results and theoretical calculations on the reaction mechanism will be discussed. Finally, the last part of the chapter will be devoted to

**1. Introduction** 

Chart 1.

methods for the production of these radicals.

**Aminyl**

N R2

R1


## **Light-Induced Iminyl Radicals: Generation and Synthetic Applications**

Miguel A. Rodríguez

*Departamento de Química, Unidad Asociada al C.S.I.C., Universidad de La Rioja Spain* 

### **1. Introduction**

264 Molecular Photochemistry – Various Aspects

[22] Lan, L.Y.; Schuster, G.B. (1986). *Tetrahedron Lett*. Vol. 27, p. 4261. [23] Gould, I.R.; Farid, S. (1993). *J Phys Chem.* Vol. 97, pp. 13067-13072.

[24] Rehm, D.; Weller, A. (1970). *Isr J Chem.* Vol. 8, pp. 259.

The photochemistry of the carbon-nitrogen double bond was first explored in depth in the seventies (Padwa, 1977; Pratt, 1977). The low photochemical reactivity of this bond is due to the deactivation of the excited state of the imine by *E*–*Z* isomerization processes, which do not have any synthetic utility because of their very low energy barrier for thermal conversion (Padwa & Albrecht, 1974a, 1974b). However, the presence of an electronegative atom on the nitrogen opens up the field of reactivity. Studies on the energies of the N–O bonds in acyloximes show that this bond can easily undergo homolytic cleavage induced by ultraviolet light, a process that leads to iminyl radicals (Okada et al., 1969). The use of radicals has proven to be a useful tool in organic synthesis (Renaud & Sibi, 2001; Togo, 2004; Zard, 2003). In particular, the cyclization of nitrogen-centred radicals, such as aminyl or iminyl radicals (Chart 1), is a valuable procedure for the preparation of nitrogen heterocycles (Fallis & Brinza, 1997; Zard, 2008). It is therefore necessary to have effective methods for the production of these radicals.

Chart 1.

The direct way to create nitrogen radicals involves the homolytic cleavage of N–X bonds, while the addition of a radical to an unsaturated nitrogen functional group, such as a nitrile or an imine derivative, constitutes the most widely used indirect method. The cleavage of the bond may be triggered thermally or photochemically. This chapter focuses on the photochemical generation of iminyl radicals and the study of their reactivity. Firstly, the methods for the production of iminyl radicals will be reviewed and the reactivity of this species will be examined, highlighting its ability to be added to unsaturated systems, with particular emphasis on intramolecular cyclization reactions. At this point, the regioselectivity in the formation of five- and six-membered rings will be analysed. In terms of the course of these reactions, both experimental results and theoretical calculations on the reaction mechanism will be discussed. Finally, the last part of the chapter will be devoted to

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 267

O

O

A slight modification of this method allowed the homolytic substitution by iminyl radicals at selenium (Fong & Schiesser, 1993). The photolysis of thiohydroxamic esters derived from the *O*-carboxymethyl oxime derivatives of 2-(benzylseleno)benzaldehyde gave 1,2 benzoselenazoles in 70% yield (Scheme 3). Similarly, Barton oxalate esters of oximes were used as starting materials. In this case, the formation of the iminyl radical takes place after a double decarboxylation (Boivin et al., 1994). An analogous thermal decomposition procedure involving decarboxylation and loss of formaldehyde to obtain an iminyl radical

O



N S N

SeCH2Ph

Se N

70%

N

N

78%

N O

Scheme 2.

Scheme 3.

O

SeCH2Ph

species to propagate the chain.

N O

N

S

h

has been also described from peresters (Leardini et al., 2001).

S

O

N

O

benzene

Although not used from a synthetic point of view, ketoxime diurethanes also lead to the formation of radicals (Hwang et al., 1999). Of greater applicability is the use of ketoxime xanthates (Gagosz & Zard, 1999), which allow the preparation of a variety of substituted 1 pyrrolines when the appropriate structure is irradiated (Scheme 4). The photoreaction has great versatility and gives yields between 72 and 88%, which exceeds those of the thermal process (Gagosz & Zard, 1999). The authors postulate that iminyl radicals generated by homolysis of the N–O bond eventually escape from the solvent cage and then undergo cyclization. The resulting cyclised radical finally adds to the sulfur atom of the starting oxime xanthate, thus producing the cyclised dithiocarbonate and regenerating the iminyl

h

(visible) <sup>N</sup>

O

further synthetic applications of these reactions, particularly in the synthesis of different polycyclic heteroaromatic compounds and the preparation of natural products.

### **2. Methods for the generation of iminyl radicals**

Different methods have been developed for the light-induced formation of iminyl radicals. The homolytic cleavage of N–X bonds is a well known and widely used method for the production of this kind of radical, while the use of radical addition to nitriles is much more limited. These two alternatives will be described in detail below.

### **2.1 Homolytic cleavage of N–X bonds**

A range of different unsaturated nitrogen derivatives (C=N–X) have been used as starting materials, where the heteroatom X can be oxygen, nitrogen, sulfur or halogen. Of these, the most widely used approach is the cleavage of the N–O bond and these systems will therefore be the starting point in this section.

### **2.1.1 Cleavage of N–O bonds**

The average bond energies of C=N, N–O and C–O bonds are 147, 53 and 86 kcal/mol, respectively (Petrucci et al., 2011). The application of energy in the form of heat or light to C=N–O–C structures should consistently fragment the N–O bond. To the best of my knowledge, the first reactions to involve the formation of iminyl radicals were the photolysis of oxadiazoles (Newman, 1968a, 1968b; Cantrell & Haller, 1968; Mukai et al., 1969), benzo[*c*]isoxazole (Ogata et al., 1968), oxadiazolinone (Sauer & Mayer, 1968) and aromatic oxime benzoates (Okada et al., 1969). In the latter case, dimerization of the radical occurred to give the major product (Scheme 1). The homolytic cleavage of similar *O*-acyl aromatic ketoximes was also observed and this took place in the triplet excited state when the photolysis was conducted in the presence of triplet sensitizers (Yoshida et al., 1975).

Scheme 1.

Since the nineties, interest in the photochemical generation of iminyl radicals has increased greatly. In a pioneering study by the group of Boivin & Zard (Boivin et al., 1994), modified Barton esters, prepared from *O*-carboxymethyl derivatives of oximes, were irradiated to give an initial N–O bond cleavage, which was followed by decarboxylation and loss of formaldehyde (Scheme 2), a process that provides a mild and very useful source of iminyl radicals. These radicals can evolve by subsequent intramolecular cyclization to give a fivemembered ring and subsequent transfer of a pyridylthiyl group from the starting Barton ester in the absence of an external trap.

### Scheme 2.

266 Molecular Photochemistry – Various Aspects

further synthetic applications of these reactions, particularly in the synthesis of different

Different methods have been developed for the light-induced formation of iminyl radicals. The homolytic cleavage of N–X bonds is a well known and widely used method for the production of this kind of radical, while the use of radical addition to nitriles is much more

A range of different unsaturated nitrogen derivatives (C=N–X) have been used as starting materials, where the heteroatom X can be oxygen, nitrogen, sulfur or halogen. Of these, the most widely used approach is the cleavage of the N–O bond and these systems will

The average bond energies of C=N, N–O and C–O bonds are 147, 53 and 86 kcal/mol, respectively (Petrucci et al., 2011). The application of energy in the form of heat or light to C=N–O–C structures should consistently fragment the N–O bond. To the best of my knowledge, the first reactions to involve the formation of iminyl radicals were the photolysis of oxadiazoles (Newman, 1968a, 1968b; Cantrell & Haller, 1968; Mukai et al., 1969), benzo[*c*]isoxazole (Ogata et al., 1968), oxadiazolinone (Sauer & Mayer, 1968) and aromatic oxime benzoates (Okada et al., 1969). In the latter case, dimerization of the radical occurred to give the major product (Scheme 1). The homolytic cleavage of similar *O*-acyl aromatic ketoximes was also observed and this took place in the triplet excited state when the photolysis was conducted in the presence of triplet sensitizers (Yoshida et

R1, R2 = Ar, Me 43 - 50%

O

Ph <sup>+</sup>

Since the nineties, interest in the photochemical generation of iminyl radicals has increased greatly. In a pioneering study by the group of Boivin & Zard (Boivin et al., 1994), modified Barton esters, prepared from *O*-carboxymethyl derivatives of oximes, were irradiated to give an initial N–O bond cleavage, which was followed by decarboxylation and loss of formaldehyde (Scheme 2), a process that provides a mild and very useful source of iminyl radicals. These radicals can evolve by subsequent intramolecular cyclization to give a fivemembered ring and subsequent transfer of a pyridylthiyl group from the starting Barton

O

R2

N N R1

<sup>R</sup><sup>2</sup> <sup>+</sup> ...

R1

polycyclic heteroaromatic compounds and the preparation of natural products.

**2. Methods for the generation of iminyl radicals** 

**2.1 Homolytic cleavage of N–X bonds** 

therefore be the starting point in this section.

**2.1.1 Cleavage of N–O bonds** 

al., 1975).

N O

Scheme 1.

O

Ph

ester in the absence of an external trap.

h

R2

N

R1

R2

R1

limited. These two alternatives will be described in detail below.

A slight modification of this method allowed the homolytic substitution by iminyl radicals at selenium (Fong & Schiesser, 1993). The photolysis of thiohydroxamic esters derived from the *O*-carboxymethyl oxime derivatives of 2-(benzylseleno)benzaldehyde gave 1,2 benzoselenazoles in 70% yield (Scheme 3). Similarly, Barton oxalate esters of oximes were used as starting materials. In this case, the formation of the iminyl radical takes place after a double decarboxylation (Boivin et al., 1994). An analogous thermal decomposition procedure involving decarboxylation and loss of formaldehyde to obtain an iminyl radical has been also described from peresters (Leardini et al., 2001).

### Scheme 3.

Although not used from a synthetic point of view, ketoxime diurethanes also lead to the formation of radicals (Hwang et al., 1999). Of greater applicability is the use of ketoxime xanthates (Gagosz & Zard, 1999), which allow the preparation of a variety of substituted 1 pyrrolines when the appropriate structure is irradiated (Scheme 4). The photoreaction has great versatility and gives yields between 72 and 88%, which exceeds those of the thermal process (Gagosz & Zard, 1999). The authors postulate that iminyl radicals generated by homolysis of the N–O bond eventually escape from the solvent cage and then undergo cyclization. The resulting cyclised radical finally adds to the sulfur atom of the starting oxime xanthate, thus producing the cyclised dithiocarbonate and regenerating the iminyl species to propagate the chain.

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 269

The formation of six-membered heterocyclic rings by intramolecular cyclization between an iminyl radical and an olefin was first reported by Rodríguez and co-workers (Alonso et al., 2006). Direct irradiation of 2-vinylbenzaldehyde *O*-acetyloximes induced N–O bond cleavage and led to the formation of an iminyl radical, which was able to add to a vinyl group to give isoquinolines after aromatisation through the formal loss of a hydrogen atom (Scheme 7). Analysis of the Stern-Volmer plots (Turro, 1991) for the quenching of the photoreactivity of acyloximes in the presence of common triplet-state quenchers shows that both excited states, singlet and triplet, undergo the same N–O fracture (Alonso et al., 2008). Direct and sensitized laser experiments also led to the same conclusion (Lalevée et al., 2002). According to ab initio molecular orbital calculations on the singlet excited states of acyloximes, the oscillator strength for the n–\* S0 S1 transition should be 0.014, with a max of 233 nm, while the –\* S0 S2 transition should have an oscillator strength of 0.2, with a max of 212 nm, which indicate that S2 is the spectroscopic state while S1 is an excited dark state. Relaxation from S2 leads directly to N–O bond cleavage due to the coupling between

N

26 - 38%

N

R

64%

N

R

The reaction is also effective for carbon-carbon triple bonds (Alonso et al., 2006). In the intramolecular version, the addition of a nitrogen-centred radical should generate an isoquinolyl radical, which may evolve by atom abstraction since the use of 2-propanol-*d*7 as solvent led to 4-deuteroquinoline, while the use of methanol-*d*1 led to nondeuterated

h / Pyrex *t*- Butanol

(CD3)2CDOH

D

N

The light-induced intermolecular attack of the iminyl radical on a carbon-carbon triple bond has also been reported (Alonso et al., 2006). As shown in Scheme 9, irradiation of

N

CH3OD

H

N

the imine \* and the \* N–O orbitals (Alonso et al., 2008).

N *t*-Butanol

<sup>R</sup> R = H, Me, Ph

OAc

Scheme 7.

Scheme 8.

isoquinoline (Scheme 8).

N

OAc

h / Pyrex

Scheme 4.

One alternative is to use oxime ethers as starting compounds. In a first paper, the group of Narasaka published the thermal treatment of γ,δ-unsaturated *O*-(2,4-dinitrophenyl)oximes with sodium hydride and 3,4-methylenedioxyphenol, which gave 3,4-dihydro-2*H*-pyrroles after intramolecular cyclization (Uchiyama et al., 1998). The authors considered that this reaction proceeded with formation of an iminyl radical through an initial one-electron transfer from the sodium phenolate and expected that a similar electron transfer would occur on irradiation of the oxime ether in the presence of a sensitizer. Indeed, irradiation of γ,δ-unsaturated *O*-aryloximes in the presence of 1,5-dimethoxynaphthalene (DMN) as a sensitizer led to 1-pyrrolines through cyclization of an iminyl radical (Mikami & Narasaka, 2000; 2001). The reaction was carried out in the presence of 1,4-cyclohexadiene (CHD) in order to trap the radical resulting from the intramolecular addition to the alkene (Scheme 5).

Scheme 5.

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers have been studied by steady-state and laser flash photolysis methods (de Lijser & Tsai, 2004; de Lijser et al., 2007). On the basis of these experiments, the formation of iminyl radicals is rationalized. On the other hand, Narasaka's group also studied the effect of substituting an ether oxime by an acetate oxime (Kitamura et al., 2005; Kitamura & Narasaka, 2008). The sensitized photoreaction in acetonitrile again led to the formation of five-membered rings in good yields (Scheme 6).

Scheme 6.

The formation of six-membered heterocyclic rings by intramolecular cyclization between an iminyl radical and an olefin was first reported by Rodríguez and co-workers (Alonso et al., 2006). Direct irradiation of 2-vinylbenzaldehyde *O*-acetyloximes induced N–O bond cleavage and led to the formation of an iminyl radical, which was able to add to a vinyl group to give isoquinolines after aromatisation through the formal loss of a hydrogen atom (Scheme 7). Analysis of the Stern-Volmer plots (Turro, 1991) for the quenching of the photoreactivity of acyloximes in the presence of common triplet-state quenchers shows that both excited states, singlet and triplet, undergo the same N–O fracture (Alonso et al., 2008). Direct and sensitized laser experiments also led to the same conclusion (Lalevée et al., 2002). According to ab initio molecular orbital calculations on the singlet excited states of acyloximes, the oscillator strength for the n–\* S0 S1 transition should be 0.014, with a max of 233 nm, while the –\* S0 S2 transition should have an oscillator strength of 0.2, with a max of 212 nm, which indicate that S2 is the spectroscopic state while S1 is an excited dark state. Relaxation from S2 leads directly to N–O bond cleavage due to the coupling between the imine \* and the \* N–O orbitals (Alonso et al., 2008).

Scheme 7.

268 Molecular Photochemistry – Various Aspects

One alternative is to use oxime ethers as starting compounds. In a first paper, the group of Narasaka published the thermal treatment of γ,δ-unsaturated *O*-(2,4-dinitrophenyl)oximes with sodium hydride and 3,4-methylenedioxyphenol, which gave 3,4-dihydro-2*H*-pyrroles after intramolecular cyclization (Uchiyama et al., 1998). The authors considered that this reaction proceeded with formation of an iminyl radical through an initial one-electron transfer from the sodium phenolate and expected that a similar electron transfer would occur on irradiation of the oxime ether in the presence of a sensitizer. Indeed, irradiation of γ,δ-unsaturated *O*-aryloximes in the presence of 1,5-dimethoxynaphthalene (DMN) as a sensitizer led to 1-pyrrolines through cyclization of an iminyl radical (Mikami & Narasaka, 2000; 2001). The reaction was carried out in the presence of 1,4-cyclohexadiene (CHD) in order to trap the radical resulting from the intramolecular addition to the alkene (Scheme 5).

N

Me

Me

N

H

77%

78%

Me

Me

CHD N

Ph

77%

N

Ph

SCOSMe

N

N O

Scheme 4.

Ph

Scheme 5.

Scheme 6.

S

SMe

h

<sup>N</sup> <sup>h</sup> > 320 nm

Me

DMN

Ph

<sup>N</sup> <sup>h</sup> > 300 nm

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers have been studied by steady-state and laser flash photolysis methods (de Lijser & Tsai, 2004; de Lijser et al., 2007). On the basis of these experiments, the formation of iminyl radicals is rationalized. On the other hand, Narasaka's group also studied the effect of substituting an ether oxime by an acetate oxime (Kitamura et al., 2005; Kitamura & Narasaka, 2008). The sensitized photoreaction in acetonitrile again led to the formation of five-membered rings in

> DMN, CHD CH3CN

Me

OAr

Ar = *p*-CN-C6H4

good yields (Scheme 6).

Ph

OAc

The reaction is also effective for carbon-carbon triple bonds (Alonso et al., 2006). In the intramolecular version, the addition of a nitrogen-centred radical should generate an isoquinolyl radical, which may evolve by atom abstraction since the use of 2-propanol-*d*7 as solvent led to 4-deuteroquinoline, while the use of methanol-*d*1 led to nondeuterated isoquinoline (Scheme 8).

Scheme 8.

The light-induced intermolecular attack of the iminyl radical on a carbon-carbon triple bond has also been reported (Alonso et al., 2006). As shown in Scheme 9, irradiation of

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 271

Finally, dioxime oxalates are also precursors of iminyl radicals. Irradiation of these compounds in the presence of 4-methoxyacetophenone as a sensitizer allowed the

Although N–N bonds are generally stronger than N–O bonds, homolysis of N–N bonds also offers an attractive alternative for the generation of iminyl radicals. Irradiation of hydrazones led to the formation of azines (Takeuchi et al., 1972) or, in the presence of a good hydrogen donor, to a mixture of amines and imines (Binkley, 1970) after N–N bond scission. Iminyl radicals are also formed by the reaction of photochemically generated *tert*-butoxyl radicals with primary or secondary alkyl azides (Roberts & Winter, 1979). However, these procedures are of little synthetic utility. In contrast, as shown in Scheme 12, irradiation with a sunlamp of readily accessible thiocarbazone derivatives in the presence of catalytic amounts of hexabutylditin provides adducts that arise from subsequent intramolecular cyclization of an iminyl radical and transfer of an iminodithiocarbonate group (Callier-Dublanchet et al., 1997). The cleavage of the N–N bond seems to be relatively slow and the

60%

N Ph

NH2

<sup>N</sup> <sup>N</sup>

1,2-H shift

<sup>S</sup> SMe

NPh

N

N

N R1

NH

<sup>N</sup> <sup>N</sup>

P O

N R1

N Ph N

MeO OMe

preparation of dihydropyrroles and phenanthridines (Portela-Cubillo et al., 2008).

optimum substituent on the thiocarbazone moiety has yet to be determined.

N

N H

N N Ph

Interest in the photolytic cleavage of the nitrogen-nitrogen single bond in phenylhydrazones has increased in recent years because results from systematic studies indicate that both the aminyl and the iminyl radicals have DNA-cleaving ability (Hwu et al., 2004). Upon UV

NH

<sup>N</sup> <sup>N</sup>

<sup>N</sup> <sup>N</sup>

N R1

N R1

Cyclohexane (Bu3Sn)2 cat.

NH <sup>N</sup> Ph +

S h / sunlamp

**2.1.2 Cleavage of N–N, N–S and N–Br bonds** 

P

MeO OMe

N Ph

R1 = 2-deoxyribose

N Ph

O

MeS

Scheme 12.

N

H

NH

<sup>N</sup> <sup>N</sup>

<sup>N</sup> <sup>N</sup>

HN

N R1

> N R1

Scheme 13.

N N Ph

313 nm

benzophenone *O*-acetyloxime in the presence of tolane or dimethyl acetylenedicarboxylate induces a tandem (cascade) process of intermolecular addition/intramolecular cyclization with formation of isoquinoline in good yields.

Scheme 9.

Similarly, the iminyl radicals generated by the action of UV light are capable of reacting with alkynyl Fischer carbenes (Blanco-Lomas et al., 2011). The reaction led to the formation of the new carbene complex together with another product, identified as a seven-membered compound (Scheme 10). Based on mechanistic studies, it is proposed that the iminyl radical would attack the alkynyl carbene at the alkynyl carbon (1,4-addition) to form the carbene complex, while 1,2-addition at the carbene carbon should lead to the cyclic structure.

Scheme 10.

An aromatic ring can also be used as an unsaturated system. This kind of attack by iminyl radicals was previously observed when the photolysis of aromatic ketone *O*-acyloximes took place in aromatic solvents and this process involves a homolytic aromatic substitution (Sakuragi et al., 1976). Starting from appropriately substituted oximes (Scheme 11), different phenanthridines were obtained in good to excellent yields after intramolecular cyclization onto a phenyl ring followed by rearomatisation (Alonso et al., 2006). In the case of aldehyde *O*-acyloxime (R = H) the iminyl radical partially decomposed into a nitrile (Bird et al., 1976).

Scheme 11.

Finally, dioxime oxalates are also precursors of iminyl radicals. Irradiation of these compounds in the presence of 4-methoxyacetophenone as a sensitizer allowed the preparation of dihydropyrroles and phenanthridines (Portela-Cubillo et al., 2008).

### **2.1.2 Cleavage of N–N, N–S and N–Br bonds**

270 Molecular Photochemistry – Various Aspects

benzophenone *O*-acetyloxime in the presence of tolane or dimethyl acetylenedicarboxylate induces a tandem (cascade) process of intermolecular addition/intramolecular cyclization

R

Ph R = CO2Me, Ph 74 - 76%

Similarly, the iminyl radicals generated by the action of UV light are capable of reacting with alkynyl Fischer carbenes (Blanco-Lomas et al., 2011). The reaction led to the formation of the new carbene complex together with another product, identified as a seven-membered compound (Scheme 10). Based on mechanistic studies, it is proposed that the iminyl radical would attack the alkynyl carbene at the alkynyl carbon (1,4-addition) to form the carbene

An aromatic ring can also be used as an unsaturated system. This kind of attack by iminyl radicals was previously observed when the photolysis of aromatic ketone *O*-acyloximes took place in aromatic solvents and this process involves a homolytic aromatic substitution (Sakuragi et al., 1976). Starting from appropriately substituted oximes (Scheme 11), different phenanthridines were obtained in good to excellent yields after intramolecular cyclization onto a phenyl ring followed by rearomatisation (Alonso et al., 2006). In the case of aldehyde *O*-acyloxime (R = H) the iminyl radical partially decomposed into a nitrile (Bird et al., 1976).

h / Pyrex

*t*-Butanol

R R R = H, Ph, Me 40 - 91%

complex, while 1,2-addition at the carbene carbon should lead to the cyclic structure.

R

N N

25%

H Ph

Ph

N

N

EtO <sup>H</sup> 9%

Ph

Ph

Ph +

(CO)5W N OEt

R

Ph

R

with formation of isoquinoline in good yields.

N

OAc (CO)5W

Ph

OEt

Ph

Scheme 9.

Scheme 10.

Scheme 11.

OAc

h / Pyrex

R R

<sup>h</sup> / CH3CN <sup>+</sup> Ph N

N

OAc

Ph low-pressure Hg

Although N–N bonds are generally stronger than N–O bonds, homolysis of N–N bonds also offers an attractive alternative for the generation of iminyl radicals. Irradiation of hydrazones led to the formation of azines (Takeuchi et al., 1972) or, in the presence of a good hydrogen donor, to a mixture of amines and imines (Binkley, 1970) after N–N bond scission. Iminyl radicals are also formed by the reaction of photochemically generated *tert*-butoxyl radicals with primary or secondary alkyl azides (Roberts & Winter, 1979). However, these procedures are of little synthetic utility. In contrast, as shown in Scheme 12, irradiation with a sunlamp of readily accessible thiocarbazone derivatives in the presence of catalytic amounts of hexabutylditin provides adducts that arise from subsequent intramolecular cyclization of an iminyl radical and transfer of an iminodithiocarbonate group (Callier-Dublanchet et al., 1997). The cleavage of the N–N bond seems to be relatively slow and the optimum substituent on the thiocarbazone moiety has yet to be determined.

Scheme 13.

Interest in the photolytic cleavage of the nitrogen-nitrogen single bond in phenylhydrazones has increased in recent years because results from systematic studies indicate that both the aminyl and the iminyl radicals have DNA-cleaving ability (Hwu et al., 2004). Upon UV

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 273

olefins on the *O*-alkyl side chains, to give 4,5-dihydrooxazoles or 5,6-dihydro-4*H*-1,3 oxazines, respectively, or *exo*-1,5 cyclization onto an olefin on the iminyl side chain, to yield 2-alkoxy-1-pyrrolines (Scheme 15). The experimental results indicate that dihydrooxazole formation is more favourable than cyclization to 1-pyrrolines, a finding that has been

In the previous section, direct methods of generating an iminyl radical by homolytic cleavage of an N–X bond have been examined. An alternative to these methods is the photochemical formation of another radical and subsequent addition to an unsaturated nitrogen derivative, such as an azide or a nitrile. This indirect approach was first reported by Roberts's group (Cooper et al., 1977). A photochemically generated *tert*-butoxyl radical reacted with primary or secondary alkyl azides to produce iminyl radicals (Scheme 16),

h / (*t*-BuO)2

5-Bromovaleronitrile was used to establish the rate constant of cyclization of the 4 cyanobutyl radical to the cyclopentiminyl radical (Griller et al., 1979), as determined by kinetic measurements using electron paramagnetic resonance (EPR) spectroscopy, and this

With respect to intermolecular reactions, a couple of examples have been reported where photochemically generated radicals add to acetonitrile (Engel et al., 1987; de Lijser & Arnold, 1997). Carbon-centred radicals prefer to add to the carbon of the nitrile to give the iminyl radical (de Lijser & Arnold, 1998), a preference that has been rationalized by a timeresolved infrared study on the photochemistry of *O*-fluoroformyl-9-fluorenone oxime in acetonitrile solution and by ab initio molecular orbital calculations (Bucher et al., 2006). In addition to the N–O bond cleavage, upon laser flash photolysis (266 nm) the short-lived transient fluoroformyl radical and the transient iminyl radical were detected (Scheme 18).

Boryl radicals also add to nitriles. Photochemically formed *tert*-butoxyl radicals were capable of abstracting hydrogen from borohydride anions to form a borane radical anion,

h / (Bu3Sn)2

N

<sup>C</sup>≡C = 1.2 × 105 s-1 at 80 ºC). The reaction is initiated by

C N N

<sup>C</sup>≡N = 4.0 × 104 s-1 at 80 ºC, almost an order of magnitude slower than the

rationalized by semiempirical MNDO molecular orbital calculations.

**2.2 Addition of a radical to an unsaturated nitrogen derivative** 

H N3

which ultimately yielded a ketone after hydrolysis.

Scheme 16.

was found to be *k*<sup>c</sup>

Br

Scheme 17.

analogous 5-hexynyl radical (*k*<sup>c</sup>

photolysis of hexabutylditin (Scheme 17).

C N

photolysis, modified 2'-deoxyadenosine containing a photoactive phenylhydrazone moiety undergoes efficient homolytic cleavage to give aminyl and iminyl radicals (Kuttappan-Nair et al., 2010). Both radicals evolve by recombination as the main pathway (Scheme 13). Alternatively, it is reasonable to propose that these radicals react with other DNA base moieties, leading to intra- and interstrand cross-linking.

Although thermal reactions are more commonly used (Zard, 2008; Esker & Newcomb, 1993), sulfanyl imines can also be employed as precursors of iminyl radicals in photochemical reactions (Guindon et al., 2001). Irradiation of benzothiazolylsulfanylimines induces the cleavage of the N–S bond and facilities a tandem process of intramolecular iminyl radical cyclofunctionalization/hydrogen transfer to afford *syn*-*anti* 1-pyrrolines with high levels of 1,2-induction in both steps (Scheme 14).

Scheme 15.

The generation of alkoxyiminyl radicals can be addressed by photolysis of *N*-bromo imidates (Glober et al., 1993). These radicals can undergo *exo*-1,5 and *exo*-1,6 cyclization onto olefins on the *O*-alkyl side chains, to give 4,5-dihydrooxazoles or 5,6-dihydro-4*H*-1,3 oxazines, respectively, or *exo*-1,5 cyclization onto an olefin on the iminyl side chain, to yield 2-alkoxy-1-pyrrolines (Scheme 15). The experimental results indicate that dihydrooxazole formation is more favourable than cyclization to 1-pyrrolines, a finding that has been rationalized by semiempirical MNDO molecular orbital calculations.

### **2.2 Addition of a radical to an unsaturated nitrogen derivative**

In the previous section, direct methods of generating an iminyl radical by homolytic cleavage of an N–X bond have been examined. An alternative to these methods is the photochemical formation of another radical and subsequent addition to an unsaturated nitrogen derivative, such as an azide or a nitrile. This indirect approach was first reported by Roberts's group (Cooper et al., 1977). A photochemically generated *tert*-butoxyl radical reacted with primary or secondary alkyl azides to produce iminyl radicals (Scheme 16), which ultimately yielded a ketone after hydrolysis.

Scheme 16.

272 Molecular Photochemistry – Various Aspects

photolysis, modified 2'-deoxyadenosine containing a photoactive phenylhydrazone moiety undergoes efficient homolytic cleavage to give aminyl and iminyl radicals (Kuttappan-Nair et al., 2010). Both radicals evolve by recombination as the main pathway (Scheme 13). Alternatively, it is reasonable to propose that these radicals react with other DNA base

Although thermal reactions are more commonly used (Zard, 2008; Esker & Newcomb, 1993), sulfanyl imines can also be employed as precursors of iminyl radicals in photochemical reactions (Guindon et al., 2001). Irradiation of benzothiazolylsulfanylimines induces the cleavage of the N–S bond and facilities a tandem process of intramolecular iminyl radical cyclofunctionalization/hydrogen transfer to afford *syn*-*anti* 1-pyrrolines with high levels of

N

<sup>R</sup> Me

R H Me OMe

CO2Et

+

*syn anti*

*syn*:*anti* 1:9 1:16 1:7

R = alkyl; R1 = CH2CH=CH2

R = alkyl; R1 = CH2CH2CH=CH2

R = CH2CH2CH=CH2; R1 = alkyl

N

Yield (%) 83 84 78

<sup>R</sup> Me

CO2Et

moieties, leading to intra- and interstrand cross-linking.

Bu3SnH AIBN / -23ºC

N

R

O

N

N CH2Br

R

O

CH2Br

CH2Br

OR<sup>1</sup>

The generation of alkoxyiminyl radicals can be addressed by photolysis of *N*-bromo imidates (Glober et al., 1993). These radicals can undergo *exo*-1,5 and *exo*-1,6 cyclization onto

1,2-induction in both steps (Scheme 14).

R

Scheme 14.

R O

Scheme 15.

N Br

<sup>S</sup> <sup>S</sup>

N

Me

CO2Et

<sup>h</sup> / THF <sup>N</sup>

h

h

R1

h

AIBN = Azobisisobutylonitrile

5-Bromovaleronitrile was used to establish the rate constant of cyclization of the 4 cyanobutyl radical to the cyclopentiminyl radical (Griller et al., 1979), as determined by kinetic measurements using electron paramagnetic resonance (EPR) spectroscopy, and this was found to be *k*<sup>c</sup> <sup>C</sup>≡N = 4.0 × 104 s-1 at 80 ºC, almost an order of magnitude slower than the analogous 5-hexynyl radical (*k*<sup>c</sup> <sup>C</sup>≡C = 1.2 × 105 s-1 at 80 ºC). The reaction is initiated by photolysis of hexabutylditin (Scheme 17).

Scheme 17.

With respect to intermolecular reactions, a couple of examples have been reported where photochemically generated radicals add to acetonitrile (Engel et al., 1987; de Lijser & Arnold, 1997). Carbon-centred radicals prefer to add to the carbon of the nitrile to give the iminyl radical (de Lijser & Arnold, 1998), a preference that has been rationalized by a timeresolved infrared study on the photochemistry of *O*-fluoroformyl-9-fluorenone oxime in acetonitrile solution and by ab initio molecular orbital calculations (Bucher et al., 2006). In addition to the N–O bond cleavage, upon laser flash photolysis (266 nm) the short-lived transient fluoroformyl radical and the transient iminyl radical were detected (Scheme 18).

Boryl radicals also add to nitriles. Photochemically formed *tert*-butoxyl radicals were capable of abstracting hydrogen from borohydride anions to form a borane radical anion,

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 275

The rate constants of other processes involving radicals have also been measured. For example, in the bimolecular combination reactions to form the corresponding azides, in some cases values close to the diffusion controlled limit have been measured, *k*2 ~ 3 × 108 M-1 s-1 at -28 ºC (Portela-Cubillo et al., 2009), while others range between 102 and 109 M-1 s-1 at -35 °C depending on steric factors (Griller et al., 1974). Iminyl radicals also abstract hydrogen from different species. Specifically, the rate constants for the reaction of the iminyl radical shown in Scheme 20 (R1 = Me; R2 = H; R3 = R4 = Ph) with thiophenols has been estimated at

Iminyl radicals have also been studied by theoretical DFT calculations. Since these species could evolve through cyclization to the five- or six-membered rings, the course of the ring formation has been calculated (Alonso et al., 2008). The choice of B3PW91 was made on the basis of previous results where this functional proved to give satisfactory results in radical chemistry (Pace et al., 2006; Pinter et al., 2007). As can be seen from Table 1 and Figure 1, the calculated relative free energies for the transition states of the cyclization to the five- and six-membered ring when no other factor is present are 12.6 and 16.0 kcal/mol, respectively, at the B3PW91/6-31+G\* level, which indicates a preference for the formation of the 1-pyrroline ring. The presence of an aromatic ring as a spacer makes these values 6.4 and 5.6 kcal/mol, respectively, and indicates a small preference for the construction of the six-membered ring (Table 1). These predictions were corroborated by

<sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup>

Iminyl Radical 0.0 0.0 0.0 0.0 Transition State 12.6 16.0 6.4 5.6 Product -5.2 -7.6 -10.6 -27.7 Table 1. Free energies (*G*, kcal/mol) for the cyclization step to five- or six-membered rings,

DFT calculated structures and energies have also been used to understand the influence of some substituents on the reactivity of iminyl radicals (Alonso et al., 2010). The most significant is the reduction of the energy barrier, from 14.6 to 13.2 kcal/mol at the B3PW91/6-31G\* level, when a methyl group is located on the iminic carbon (Figure 2). This

relative to the corresponding iminyl radical, at the B3PW91/6-31+G\* level

change is probably due to electronic effects.

N

R2 R3

R4

R1

R1

*k*H ~ 107 M-1 s-1 at 25 ºC (Le Tadic-Biadatti et al., 1997).

Scheme 20.

experimental results.

N R2

R4

R3

which added to cyanides to give iminyl radical adducts (Giles & Roberts, 1983). Similarly, these *tert*-butoxyl radicals reacted with primary amine-boranes to give amine-boryl radicals, which can be intercepted by addition to the CN group (Kirwan & Roberts, 1989).

Scheme 18.

### **3. Characterization, kinetic data and calculations**

In order to perform an adequate characterization of iminyl radicals, it is necessary to have valid precursors. In this sense, oxime esters seem to be suitable substrates for these experiments. The photochemistry of 9-fluorenone oxime phenylglyoxilate was investigated by laser flash photolysis at 355 nm, using time-resolved EPR spectroscopy (Kolano et al., 2004). The first step in the reaction would be cleavage of the N–O bond to give, initially, the 9-fluorenoneiminyl radical and the benzoylcarbonyloxy radical (Scheme 19). This process led to the detection, among others, of a set of transient signals, a 1:1:1 triplet centred at 3455.7 G (*J*N = 9.7 G) attributed to the iminyl radical. Previous hyperfine splitting values obtained for iminyl radicals were in the range 9.1 to 10.3 G (Griller et al., 1974).

Scheme 19.

The group of Walton has devoted a great deal of attention to iminyl radicals. They used EPR spectroscopy to detect and characterize several N-centred radicals (McCarroll & Walton, 2000). Of particular interest is the study on the pentenyliminyl radicals (Portela-Cubillo et al., 2009). These radicals selectively closed in the 5-*exo* mode, irrespective of the substitution pattern around the C=C double bond of the pentenyl chain (Scheme 20). DFT computations have been used to model the experimental results obtained. The rate constant for the parent compound (R1 = Ph; R2 = R3 = R4 = H) was *k*<sup>c</sup> 5-*exo* = 8.8 × 103 s-1 at 27 ºC, while the presence of two phenyl groups at the end of the C=C bond (R1 = Me; R2 = H; R3 = R4 = Ph) gave rise to a cyclization rate constant of *k*<sup>c</sup> 5-*exo* = 2.2 × 106 s-1 at 25 ºC (Le Tadic-Biadatti et al., 1997). The rate of cyclization is slower for an iminyl with two H atoms at the terminus of the C=C double bond (R3 = R4 = H) than that with two phenyl groups (R3 = R4 = Ph), probably due to the higher stability of the phenyl conjugated C-centred radical formed for the later during the cyclization reaction.

Scheme 20.

274 Molecular Photochemistry – Various Aspects

which added to cyanides to give iminyl radical adducts (Giles & Roberts, 1983). Similarly, these *tert*-butoxyl radicals reacted with primary amine-boranes to give amine-boryl radicals,

In order to perform an adequate characterization of iminyl radicals, it is necessary to have valid precursors. In this sense, oxime esters seem to be suitable substrates for these experiments. The photochemistry of 9-fluorenone oxime phenylglyoxilate was investigated by laser flash photolysis at 355 nm, using time-resolved EPR spectroscopy (Kolano et al., 2004). The first step in the reaction would be cleavage of the N–O bond to give, initially, the 9-fluorenoneiminyl radical and the benzoylcarbonyloxy radical (Scheme 19). This process led to the detection, among others, of a set of transient signals, a 1:1:1 triplet centred at 3455.7 G (*J*N = 9.7 G) attributed to the iminyl radical. Previous hyperfine splitting values

355 nm Ph <sup>N</sup> <sup>+</sup>

The group of Walton has devoted a great deal of attention to iminyl radicals. They used EPR spectroscopy to detect and characterize several N-centred radicals (McCarroll & Walton, 2000). Of particular interest is the study on the pentenyliminyl radicals (Portela-Cubillo et al., 2009). These radicals selectively closed in the 5-*exo* mode, irrespective of the substitution pattern around the C=C double bond of the pentenyl chain (Scheme 20). DFT computations have been used to model the experimental results obtained. The rate constant for the parent

two phenyl groups at the end of the C=C bond (R1 = Me; R2 = H; R3 = R4 = Ph) gave rise to a

rate of cyclization is slower for an iminyl with two H atoms at the terminus of the C=C double bond (R3 = R4 = H) than that with two phenyl groups (R3 = R4 = Ph), probably due to the higher stability of the phenyl conjugated C-centred radical formed for the later during

obtained for iminyl radicals were in the range 9.1 to 10.3 G (Griller et al., 1974).

O

CH3CN

O

5-*exo* = 8.8 × 103 s-1 at 27 ºC, while the presence of

5-*exo* = 2.2 × 106 s-1 at 25 ºC (Le Tadic-Biadatti et al., 1997). The

O

O

Ph

N

H3C

F

O

<sup>F</sup> <sup>+</sup>

which can be intercepted by addition to the CN group (Kirwan & Roberts, 1989).

N O

N O

Scheme 18.

Scheme 19.

266 nm

**3. Characterization, kinetic data and calculations** 

N O

compound (R1 = Ph; R2 = R3 = R4 = H) was *k*<sup>c</sup>

cyclization rate constant of *k*<sup>c</sup>

the cyclization reaction.

O

O

F

O

The rate constants of other processes involving radicals have also been measured. For example, in the bimolecular combination reactions to form the corresponding azides, in some cases values close to the diffusion controlled limit have been measured, *k*2 ~ 3 × 108 M-1 s-1 at -28 ºC (Portela-Cubillo et al., 2009), while others range between 102 and 109 M-1 s-1 at -35 °C depending on steric factors (Griller et al., 1974). Iminyl radicals also abstract hydrogen from different species. Specifically, the rate constants for the reaction of the iminyl radical shown in Scheme 20 (R1 = Me; R2 = H; R3 = R4 = Ph) with thiophenols has been estimated at *k*H ~ 107 M-1 s-1 at 25 ºC (Le Tadic-Biadatti et al., 1997).

Iminyl radicals have also been studied by theoretical DFT calculations. Since these species could evolve through cyclization to the five- or six-membered rings, the course of the ring formation has been calculated (Alonso et al., 2008). The choice of B3PW91 was made on the basis of previous results where this functional proved to give satisfactory results in radical chemistry (Pace et al., 2006; Pinter et al., 2007). As can be seen from Table 1 and Figure 1, the calculated relative free energies for the transition states of the cyclization to the five- and six-membered ring when no other factor is present are 12.6 and 16.0 kcal/mol, respectively, at the B3PW91/6-31+G\* level, which indicates a preference for the formation of the 1-pyrroline ring. The presence of an aromatic ring as a spacer makes these values 6.4 and 5.6 kcal/mol, respectively, and indicates a small preference for the construction of the six-membered ring (Table 1). These predictions were corroborated by experimental results.


Table 1. Free energies (*G*, kcal/mol) for the cyclization step to five- or six-membered rings, relative to the corresponding iminyl radical, at the B3PW91/6-31+G\* level

DFT calculated structures and energies have also been used to understand the influence of some substituents on the reactivity of iminyl radicals (Alonso et al., 2010). The most significant is the reduction of the energy barrier, from 14.6 to 13.2 kcal/mol at the B3PW91/6-31G\* level, when a methyl group is located on the iminic carbon (Figure 2). This change is probably due to electronic effects.

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 277

yield by irradiation of the corresponding dioxime oxalate in the presence of 4-

An indirect method to form iminyl radicals by addition of a radical to a nitrile has been used to synthesize heteropolycycles. The process involves a cascade reaction from a photochemically generated radical. The formation of a tetracyclic heteroarene initiated by irradiation of an aromatic disulfide, with cleavage of an S–S bond, is shown in Scheme 23. The intermolecular addition of the sulfanyl radical to isonitriles gave an imidoyl radical, which was able to add to a nitrile to provide an iminyl radical that cyclizes to complete the

methoxyacetophenone as a photosensitizer (Portela-Cubillo et al., 2008).

cascade reaction (Camaggi et al., 1998; Nanni et al., 2000).

<sup>N</sup> <sup>N</sup>

Ph

N

N

N

OAc

Scheme 21.

Scheme 22.

R1

R2

OAc

S

OAc

OAc h / Pyrex

CH3CN N

h / Pyrex CH3CN

h / Pyrex CH3CN

h / Pyrex CH3CN

<sup>R</sup><sup>3</sup> <sup>R</sup><sup>1</sup>

N N

R2

N

48%

56%

90%

N

N

N

R3

39 - 53%

N

S

N

Fig. 1. Free energy diagram (*G*, kcal/mol) for the cyclization of the simplest iminyl radical, at the B3PW91/6-31+G\* level

Fig. 2. DFT optimised structures and energy barriers, relative to the corresponding iminyl radical, at the B3PW91/6-31G\* level

### **4. Preparation of polycyclic heteroaromatic compounds and natural products**

Direct irradiation of *O*-acyloximes has also been employed for the preparation of several polycyclic heteroaromatic compounds. As shown in Scheme 21, the use of a combination of reagents with five- and six-membered rings and cyclization of the intermediate iminyl radicals onto phenyl, thiophenyl or pyridinyl rings led to a variety of fused rings with different heteroatoms on the structure in good to excellent yields (48 to 90%) (Alonso et al., 2010). Thus, 2*H*-pyrazolo[4,3-*c*]quinoline, thieno[3,2-*c*]isoquinoline and benzo[*c*][1,7] naphthyridine derivatives have been prepared.

The versatility of this methodology has been exploited in the preparation of some interesting natural products. On using the appropriate structure (Scheme 22), direct irradiation of *O*acyloximes allowed the preparation of several phenanthridine derivatives (Alonso et al., 2010), such as the alkaloid trisphaeridine (R1,R2 = OCH2O; R3 = H) or the vasconine precursor (R1 = R2 = OCH3; R3 = CH2CH2OH), which can also be used to obtain assoanine, oxoassoanine and pratosine (Rosa et al., 1997). Trisphaeridine was also prepared in 59%

**TS-6**

16.0

R = H, 14.6 kcal/mol R = Me, 13.2 kcal/mol

N

Fig. 1. Free energy diagram (*G*, kcal/mol) for the cyclization of the simplest iminyl radical,

Fig. 2. DFT optimised structures and energy barriers, relative to the corresponding iminyl

**4. Preparation of polycyclic heteroaromatic compounds and natural products**  Direct irradiation of *O*-acyloximes has also been employed for the preparation of several polycyclic heteroaromatic compounds. As shown in Scheme 21, the use of a combination of reagents with five- and six-membered rings and cyclization of the intermediate iminyl radicals onto phenyl, thiophenyl or pyridinyl rings led to a variety of fused rings with different heteroatoms on the structure in good to excellent yields (48 to 90%) (Alonso et al., 2010). Thus, 2*H*-pyrazolo[4,3-*c*]quinoline, thieno[3,2-*c*]isoquinoline and benzo[*c*][1,7]

The versatility of this methodology has been exploited in the preparation of some interesting natural products. On using the appropriate structure (Scheme 22), direct irradiation of *O*acyloximes allowed the preparation of several phenanthridine derivatives (Alonso et al., 2010), such as the alkaloid trisphaeridine (R1,R2 = OCH2O; R3 = H) or the vasconine precursor (R1 = R2 = OCH3; R3 = CH2CH2OH), which can also be used to obtain assoanine, oxoassoanine and pratosine (Rosa et al., 1997). Trisphaeridine was also prepared in 59%

0.0

**TS-5**

12.6

N


N

N H

R


at the B3PW91/6-31+G\* level

N

radical, at the B3PW91/6-31G\* level

naphthyridine derivatives have been prepared.

yield by irradiation of the corresponding dioxime oxalate in the presence of 4 methoxyacetophenone as a photosensitizer (Portela-Cubillo et al., 2008).

An indirect method to form iminyl radicals by addition of a radical to a nitrile has been used to synthesize heteropolycycles. The process involves a cascade reaction from a photochemically generated radical. The formation of a tetracyclic heteroarene initiated by irradiation of an aromatic disulfide, with cleavage of an S–S bond, is shown in Scheme 23. The intermolecular addition of the sulfanyl radical to isonitriles gave an imidoyl radical, which was able to add to a nitrile to provide an iminyl radical that cyclizes to complete the cascade reaction (Camaggi et al., 1998; Nanni et al., 2000).

Scheme 22.

Scheme 21.

Light-Induced Iminyl Radicals: Generation and Synthetic Applications 279

N I

sunlamp / (Bu3Sn)2

N

N

CN N

CN N

N

O

N

O

(route B)

O

21%

skeleton is created from *N*-acyl-*N*-(2-iodobenzyl)cyanamide.

N

The biologically active alkaloid luotonin A has been obtained using cascade radical cyclization *via* an iminyl radical from 4-oxo-3,4-dihydroquinazoline-2-carbonitrile (Bowman et al., 2005). In a similar way, the vinyl radical attacked the cyano group and the resulting iminyl radical cyclized to the alkaloid (Scheme 25, route A). More recently, an alternative cyclization cascade process has been described for the preparation of luotonin A (Servais et al., 2007). As shown in Scheme 25 (route B), under radical conditions the pyrroloquinazoline

The photochemical generation of iminyl radicals can be performed in a direct way, which involves the homolytic cleavage of N–X bonds, or by an indirect method, which involves the addition of a radical to an unsaturated nitrogen functional group, such as an azide or a nitrile. Regarding the reactivity of this kind of radical, its ability to be added to unsaturated systems (double and triple bonds, aromatic and heteroaromatic compounds) has been

N

43%

N

O

sunlamp / (Me3Sn)2

N

N

Scheme 25.

**5. Conclusion** 

O

N

O

N

N

O

N

CN I

CN

N

(route A)

### Scheme 23.

The preparation of rings A-D of the alkaloids camptothecin and mappicine has been achieved in good yield using this protocol (Bowman et al., 2001; 2002). After light-induced cleavage of hexamethylditin and abstraction of the iodine atom to give a vinyl radical, subsequent 5-*exo* cyclization onto the nitrile and 6-*exo* cyclization of iminyl radical followed by aromatisation led to the four ring skeleton (Scheme 24).

Scheme 24.

Scheme 25.

278 Molecular Photochemistry – Various Aspects

CN

S

S N

The preparation of rings A-D of the alkaloids camptothecin and mappicine has been achieved in good yield using this protocol (Bowman et al., 2001; 2002). After light-induced cleavage of hexamethylditin and abstraction of the iodine atom to give a vinyl radical, subsequent 5-*exo* cyclization onto the nitrile and 6-*exo* cyclization of iminyl radical followed

sunlamp /

CN

S S

Scheme 23.

Scheme 24.

NC

N

N

h

by aromatisation led to the four ring skeleton (Scheme 24).

(Me3Sn)2 I

O

NC

N

N

O

N

40%

S

73%

N

NC

N

N

O

O

N S

NC

NC

N

The biologically active alkaloid luotonin A has been obtained using cascade radical cyclization *via* an iminyl radical from 4-oxo-3,4-dihydroquinazoline-2-carbonitrile (Bowman et al., 2005). In a similar way, the vinyl radical attacked the cyano group and the resulting iminyl radical cyclized to the alkaloid (Scheme 25, route A). More recently, an alternative cyclization cascade process has been described for the preparation of luotonin A (Servais et al., 2007). As shown in Scheme 25 (route B), under radical conditions the pyrroloquinazoline skeleton is created from *N*-acyl-*N*-(2-iodobenzyl)cyanamide.

### **5. Conclusion**

The photochemical generation of iminyl radicals can be performed in a direct way, which involves the homolytic cleavage of N–X bonds, or by an indirect method, which involves the addition of a radical to an unsaturated nitrogen functional group, such as an azide or a nitrile. Regarding the reactivity of this kind of radical, its ability to be added to unsaturated systems (double and triple bonds, aromatic and heteroaromatic compounds) has been

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### **6. Acknowledgment**

Financial support from the Ministerio de Ciencia e Innovación of Spain (CTQ2011-24800) and Universidad de La Rioja (API11/20) is gratefully acknowledged.

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## *Edited by Satyen Saha*

There have been various comprehensive and stand-alone text books on the introduction to Molecular Photochemistry which provide crystal clear concepts on fundamental issues. This book entitled "Molecular Photochemistry - Various Aspects" presents various advanced topics that inherently utilizes those core concepts/techniques to various advanced fields of photochemistry and are generally not available. The purpose of publication of this book is actually an effort to bring many such important topics clubbed together. The goal of this book is to familiarize both research scholars and post graduate students with recent advancement in various fields related to Photochemistry. The book is broadly divided in five parts: the photochemistry I) in solution, II) of metal oxides, III) in biology, IV) the computational aspects and V) applications. Each part provides unique aspect of photochemistry. These exciting chapters clearly indicate that the future of photochemistry like in any other burgeoning field is more exciting than the past.

Molecular Photochemistry - Various Aspects

Molecular Photochemistry

Various Aspects

*Edited by Satyen Saha*

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