4.1. Langmuir-Blodgett films

The most efficient way to obtain the arrangement of the large dye molecules is to take advantage from their intrinsic self-assembling ability. Hydrophobically modified chemicals that exhibit NLO properties can be efficiently organized as 2D structure using the Langmuir-Blodgett technique. Formation of the coherent and stable Langmuir film at liquid-air interface is mandatory for the further step, the transfer on solid substrate. The self-assembling properties of the NLO compounds could be tuned by chemical modification of the chromophore with various groups with suitable hydrophobicity, but the chemical modification will also influence the optical properties.

Tang et al. [70] report a study on the influence of the terminal group of the modified amphiphilic azobenzene chromophore on the Langmuir-Blodgett monolayer and multilayer formation. The azobenzene NLO derivative was modified with various electron acceptor groups (acetyl, nitro, and cyano), and the self-assembling behavior of the resulted compounds was investigated, to produce monolayers and multilayers deposited onto hydrophobically treated quartz substrate.

The experimental data confirm the molecular modeling conclusion that the packing of modified azobenzene molecules at the water-air interface and the transfer of the film are both due to the equilibrium of non-covalent interactions in the aggregates, dipol-dipol and π-π stacking interactions, respectively. The packing density and the electronic coupling vary strongly with the chemical structure of the functional group (see Figure 11).

The possibility to control the internal morphology and stability of the film through the balance of non-covalent interactions leads to the preparation of highly ordered multilayered film with large second-order susceptibility.

Most of the NLO chromophores could not be chemically modified to achieve suitable properties for spreading as Langmuir monolayers at the air-liquid or liquid-liquid interfaces; thus, the formation of Langmuir-Blodgett film is restricted to compound with a required degree of amphiphilicity. At the same time, many NLO materials processed by using Langmuir-Blodgett technique exhibit poor mechanical stability and limited surface area for deposition. A more sophisticated method to ensure the orientation of the chromophore molecules in the polymeric host network is to produce self-assembled thin film with the aid of film-forming matrix.

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required for photonic applications, such as thermal and chemical stability, high loading of the active molecule, and stability of the chromophore orientation [68]. Highly cross-linking of sidechain polymers enhances the stability of the thin film together with the chromophore moiety

Another covalent bonded variant is main-chain NLO polymer, with chromophores linked in the backbone at either end of the chromophore or linked at both ends of the chromophores, to form the polymer backbone. The dye fragment can be processed in various configurations: a head-to-tail (isoregic), head-to-head (syndioregic), or in random head-to-tail and head-to-head

The most efficient way to obtain the arrangement of the large dye molecules is to take advantage from their intrinsic self-assembling ability. Hydrophobically modified chemicals that exhibit NLO properties can be efficiently organized as 2D structure using the Langmuir-Blodgett technique. Formation of the coherent and stable Langmuir film at liquid-air interface is mandatory for the further step, the transfer on solid substrate. The self-assembling properties of the NLO compounds could be tuned by chemical modification of the chromophore with various groups with suitable hydrophobicity, but the chemical modification will also influence

Tang et al. [70] report a study on the influence of the terminal group of the modified amphiphilic azobenzene chromophore on the Langmuir-Blodgett monolayer and multilayer formation. The azobenzene NLO derivative was modified with various electron acceptor groups (acetyl, nitro, and cyano), and the self-assembling behavior of the resulted compounds was investigated, to produce monolayers and multilayers deposited onto hydrophobically treated

The experimental data confirm the molecular modeling conclusion that the packing of modified azobenzene molecules at the water-air interface and the transfer of the film are both due to the equilibrium of non-covalent interactions in the aggregates, dipol-dipol and π-π stacking interactions, respectively. The packing density and the electronic coupling vary strongly with

The possibility to control the internal morphology and stability of the film through the balance of non-covalent interactions leads to the preparation of highly ordered multilayered film with

Most of the NLO chromophores could not be chemically modified to achieve suitable properties for spreading as Langmuir monolayers at the air-liquid or liquid-liquid interfaces; thus, the formation of Langmuir-Blodgett film is restricted to compound with a required degree of amphiphilicity. At the same time, many NLO materials processed by using Langmuir-Blodgett technique exhibit poor mechanical stability and limited surface area for deposition. A more sophisticated method to ensure the orientation of the chromophore molecules in the polymeric host network is to produce self-assembled thin film with the aid of film-forming matrix.

the chemical structure of the functional group (see Figure 11).

arrangement.

configurations (aregic) (2) [69].

12 Advanced Surface Engineering Research

4.1. Langmuir-Blodgett films

the optical properties.

quartz substrate.

large second-order susceptibility.

Figure 11. The representation of monolayer structure with various packing densities for azobenzene NLO compounds bearing acetyl, nitro, and cyano groups (adapted with the permission from reference [70]).

Usually, Langmuir-Blodgett films are obtained from Langmuir film deposited at the air-water interfaces with the well-known orientation of the molecule with the hydrophobic groups in air and hydrophilic ones immersed in water.

The driving forces of coherent monolayer formation are the hydrophobic interactions between the nonpolar parts and the interaction between the polar groups and the water subphase. Wang et al. [71] propose an unconventional method to prepare NLO LB films based on the molecular electrostatic interaction of hydrogen bonding in a chromophore modified with urea. The new synthesized optic active compound is 1-(10'-[(10-nitro)-6,7-azobenzenl]-ether-decyl)- 3-(tetracosa-12,14-diynyl)urea (NAEDTDU), which exhibits an unusual packing behavior at air-water interface as it is proven by the П-A isotherm (see Figure 12).

The recorded specific molecular area of NAEDTDU is 35 Å<sup>2</sup> /molecule, smaller than the molecular area of the usual urea-containing derivatives in Langmuir film (50 Å<sup>2</sup> /molecule).

Figure 12. П-A isotherm of Langmuir film of NAEDTDU (adapted with the permission from reference [71]).

When assuming a traditional orientation of NAEDTDU molecule in the monolayer, with the urea polar group in the water subphase and both hydrophobic alkyl chain and nitro-azobenzene group in air, the molecular modeling suggests specific surface area larger than 50 Å2 /molecule. To obtain the observed 35 Å2 /molecule value, it is presumable that the nitro group of chromophore molecule lies on the air-water interface. Langmuir NAEDTDU monolayer can be transferred onto a hydrophobically modified quartz and silicon substrates using conventional vertical upward dipping technique and produces Langmuir-Blodgett multilayers. In this nontraditional LB monolayer, a peculiar aggregation of the chromophore molecules is suggested, i.e., the formation of a stable and linear network based on the hydrogen bonding between the urea groups.

The noncentrosymmetric arrangement of the NAEDTDU molecule in LB films generates relatively large intensities of SHG signal recorded. The thermal stability of SHG activity of LB multilayers was improved after photopolymerization of diacetylene moieties in film, due to the restriction of the movement of the chromophores and stabilization of their ordered orientation.

Using electric field in assembling process, the oriented adsorption of chromophores is facilitated, and the density of oriented optic active molecules increases. The preparation of the hybrid film using LbL deposition of aromatic diazonium salt-linked silicon sol (ADS) and DRCB chromophore on ITO glass previously functionalized with (3-mercaptopropyl)-trimethoxysilane (MTPS)

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Figure 13. The structure of the ADS-DRBC hybrid thin film (adapted with the permission from reference [74]).

The UV irradiation of deposited multilayered films results in transformation of the electrostatic interaction between layers in covalent bonds, leading to a significant increase of the thermal and chemical stability of the final material. The architecture assembles with the aid of the electric field ensuring a high degree of molecular orientation of chromophores; thus, large

In order to rational design the host-guest material, molecular dynamic simulation is used for quantum chemical calculation. The structures of considered molecules are simulated by using molecular dynamics, and relevant electrical properties, such as polarizability α and β, could be computed with density functional theory (DFT). For example, Makowska-Janusik (3) reports the investigation of the influence of polymeric matrix on the NLO properties of the pyrazoloquinoline derivatives in poly(methyl methacrylate) matrix. According to the chemical structure of the chromophore molecules, the best description of the pyrazoloquinolines-PMMA systems could be obtained by using various approaches, such as point-dipole or distributed molecular response models, with good concordance with the experimental data. Molecular simulation is also performed in order to elucidate the origin of the NLO behavior in composite materials, since in experimental approach it is not possible to separate the contributions of each component. The molecular modeling methods can provide explanations about the nature of the guest-host interaction based on the separate different contribution of the components to the

second-harmonic generation signal of the LbL film was observed.

is presented in Figure 13.

5. Theoretical modeling

optical properties.

## 4.2. Layer-by-layer deposited thin films

The layer-by-layer (LbL) method to produce thin films is a versatile and convenient technique for the fabrication of thin films, based on the electrostatic attraction between the materials in different layers successively deposited. The most relevant advantage of LbL technique is represented by the precise control of the composition and thickness of the final film, which can be easily tuned from the chemical structure of the polyelectrolytes and deposition parameters. Acentric supramolecular architecture, as essential requirement for preservation of NLO properties, is a challenge to be achieved from chromophores and polyelectrolytes in LbL film technique. The conventional LbL technique should be adjusted in order to ensure the density and stability of the optic active molecules in the obtained film, with particular attention to be paid to the orientation of chromophore [72].

Facile fabrication of a second-order nonlinear optical films with superior properties and stability was reported [73] using a surface sol-gel synthesis of ZrO2 layers and subsequent layer-bylayer (LbL) deposition of the nonlinear optical (NLO)-active azobenzene-containing polyanion and poly(diallyldimethylammonium chloride). The resulted material is an organic/inorganic hybrid multilayer film with noncentrosymmetrically orientated azobenzene chromophores. The specific orientation of the NLO-active azobenzene chromophores is produced by the strong repulsive interactions between the negatively charged ZrO2 and the sulfonate groups of the sulfonate-modified azobenzene chromophore.

The SHG signal could be increased by increasing the number of deposition cycles and also with the increase of the azobenzene graft ratio in the polyion. Wang et al. [74] proposed a method to fabricate an organic/inorganic hybrid NLO film by electric field-induced layer-by-layer deposition technique. The compound used to produce alternative layers is a new synthesized polycation from aromatic diazo group linked silicon and a chromophore molecule 2-({4-[4-(2 carboxy-2-cyano-vinyl)-phenylazo]-phenyl}-methyl-amino)-ethyl acid (DRCB) as anion.

Synthesis and Nonlinear Studies on Selected Organic Compounds in Nanostructured Thin Films http://dx.doi.org/10.5772/intechopen.79522 15

Figure 13. The structure of the ADS-DRBC hybrid thin film (adapted with the permission from reference [74]).

Using electric field in assembling process, the oriented adsorption of chromophores is facilitated, and the density of oriented optic active molecules increases. The preparation of the hybrid film using LbL deposition of aromatic diazonium salt-linked silicon sol (ADS) and DRCB chromophore on ITO glass previously functionalized with (3-mercaptopropyl)-trimethoxysilane (MTPS) is presented in Figure 13.

The UV irradiation of deposited multilayered films results in transformation of the electrostatic interaction between layers in covalent bonds, leading to a significant increase of the thermal and chemical stability of the final material. The architecture assembles with the aid of the electric field ensuring a high degree of molecular orientation of chromophores; thus, large second-harmonic generation signal of the LbL film was observed.

### 5. Theoretical modeling

When assuming a traditional orientation of NAEDTDU molecule in the monolayer, with the urea polar group in the water subphase and both hydrophobic alkyl chain and nitro-azobenzene

phore molecule lies on the air-water interface. Langmuir NAEDTDU monolayer can be transferred onto a hydrophobically modified quartz and silicon substrates using conventional vertical upward dipping technique and produces Langmuir-Blodgett multilayers. In this nontraditional LB monolayer, a peculiar aggregation of the chromophore molecules is suggested, i.e., the formation of a stable and linear network based on the hydrogen bonding between the urea

The noncentrosymmetric arrangement of the NAEDTDU molecule in LB films generates relatively large intensities of SHG signal recorded. The thermal stability of SHG activity of LB multilayers was improved after photopolymerization of diacetylene moieties in film, due to the restriction of the movement of the chromophores and stabilization of their ordered orientation.

The layer-by-layer (LbL) method to produce thin films is a versatile and convenient technique for the fabrication of thin films, based on the electrostatic attraction between the materials in different layers successively deposited. The most relevant advantage of LbL technique is represented by the precise control of the composition and thickness of the final film, which can be easily tuned from the chemical structure of the polyelectrolytes and deposition parameters. Acentric supramolecular architecture, as essential requirement for preservation of NLO properties, is a challenge to be achieved from chromophores and polyelectrolytes in LbL film technique. The conventional LbL technique should be adjusted in order to ensure the density and stability of the optic active molecules in the obtained film, with particular attention to be

Facile fabrication of a second-order nonlinear optical films with superior properties and stability was reported [73] using a surface sol-gel synthesis of ZrO2 layers and subsequent layer-bylayer (LbL) deposition of the nonlinear optical (NLO)-active azobenzene-containing polyanion and poly(diallyldimethylammonium chloride). The resulted material is an organic/inorganic hybrid multilayer film with noncentrosymmetrically orientated azobenzene chromophores. The specific orientation of the NLO-active azobenzene chromophores is produced by the strong repulsive interactions between the negatively charged ZrO2 and the sulfonate groups

The SHG signal could be increased by increasing the number of deposition cycles and also with the increase of the azobenzene graft ratio in the polyion. Wang et al. [74] proposed a method to fabricate an organic/inorganic hybrid NLO film by electric field-induced layer-by-layer deposition technique. The compound used to produce alternative layers is a new synthesized polycation from aromatic diazo group linked silicon and a chromophore molecule 2-({4-[4-(2-

carboxy-2-cyano-vinyl)-phenylazo]-phenyl}-methyl-amino)-ethyl acid (DRCB) as anion.

/molecule value, it is presumable that the nitro group of chromo-

/molecule.

group in air, the molecular modeling suggests specific surface area larger than 50 Å2

To obtain the observed 35 Å2

14 Advanced Surface Engineering Research

4.2. Layer-by-layer deposited thin films

paid to the orientation of chromophore [72].

of the sulfonate-modified azobenzene chromophore.

groups.

In order to rational design the host-guest material, molecular dynamic simulation is used for quantum chemical calculation. The structures of considered molecules are simulated by using molecular dynamics, and relevant electrical properties, such as polarizability α and β, could be computed with density functional theory (DFT). For example, Makowska-Janusik (3) reports the investigation of the influence of polymeric matrix on the NLO properties of the pyrazoloquinoline derivatives in poly(methyl methacrylate) matrix. According to the chemical structure of the chromophore molecules, the best description of the pyrazoloquinolines-PMMA systems could be obtained by using various approaches, such as point-dipole or distributed molecular response models, with good concordance with the experimental data. Molecular simulation is also performed in order to elucidate the origin of the NLO behavior in composite materials, since in experimental approach it is not possible to separate the contributions of each component. The molecular modeling methods can provide explanations about the nature of the guest-host interaction based on the separate different contribution of the components to the optical properties.

A particular kind of host-guest material exhibiting NLO properties is obtained by using semiconducting nanoparticles (quantum dots) as optical active materials. The interface phenomena play an important role in tuning the NLO properties of these materials.

obtained when heating the system to the glass transition temperature (T)g of the polymer. Langmuir-Blodgett films, which exhibit better NLO properties, are obtained from Langmuir film deposited at the air-water interfaces with the well-known orientation of the molecule with the hydrophobic groups in air and hydrophilic ones immersed in water. A convenient technique for the fabrication of thin films is the layer-by-layer (LbL) method which presents the advantage of the precise control of the composition and thickness of the final film. Both theoretical modeling and experimental approach should be addressed, in order to find the best matching between the host and guest material. In addition, the influence of the surface and film formation method should be considered, with a particular attention to the thermo- or

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17

The difference between the energy of the highest occupied molecular orbital

and the lowest unoccupied molecular orbital (eV)

\*

1 Faculty of Chemistry, Department of Physical Chemistry, University of Bucharest, Bucharest,

2 Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis,

photosensitive chromophores.

The authors have no conflict of interest to declare.

SHG Second-harmonic generation

α Molecular polarizability (esuD)

Tg Glass transition temperature (C)

Ludmila-Otilia Cinteza<sup>1</sup> and Maria Marinescu<sup>2</sup>

University of Bucharest, Bucharest, Romania

LbL Layer-by-layer deposition technique

\*Address all correspondence to: maria7marinescu@yahoo.com

μtot Dipole moment (D)

Q Quadrupole moment (D)

βtot First-order hyperpolarizability (esuD)

Conflict of interest

List of abbreviations

HOMO-LUMO gap

Author details

Romania

An innovative approach is reported by Britton et al. [75], to significantly increase the nonlinear properties when combining in the polymeric matrix a low symmetry phthalocyanine derivative with CdSe/CdS quantum dots. The new synthesized metal-free chromophore 2,3-bis[2<sup>0</sup> -(200-hydroxyethoxy)ethoxy]-9,10,16,17,23,24-hexa-n-butoxyphthalocyanine exhibits by itself an unusual electric properties due to the high dissymmetry of electronic density in the molecule. The addition of CdSe/CdS quantum dots to the phthalocyanine in chloroform solution produces a negligible effect on the values of the imaginary component of the third-order optical susceptibility and nonlinear absorption coefficient βeff. However, when the dye is embedded into a polycarbonate matrix, a twofold increase of the abovementioned optical properties is recorded, while the incorporation of both phthalocyanine and CdSe quantum dots into the polymeric film produces a three-times increase of the third-order optical susceptibility compared to the chloroform solution of the dye. The chromophore molecules and CdSe/CdS nanoparticles are in close proximity when they are locked together in the polymeric film; thus, the heavy atom effect could manifest, resulting in a significant improvement of NLO properties.

## 6. Conclusion

The synthesis of the most important classes of the organic compounds, alkyne, dye, heterocycle, thienyl, and ferrocene compounds, was presented. A lot of important reactions, like Sonogashira, Suzuki, Heck, Huisgen, Wadsworth-Emmons, Vilsmeier-Haack-Arnold, Wittig, etc., were employed for the synthesis of the new compounds with nonlinear optical applications. The synthesis of the new molecules possessing privileged structures, with a couple donor-acceptor or "push-pull" molecules, was the purpose of the last decades. Besides, new unorthodox structures, which do not possess this system, but with very good NLO properties and very good SHG signal, were reported, and the number of these increase every year. The most important parameters in the characterization of the new NLO organic compounds are the HOMO-LUMO energy gap and the first-order hyperpolarizability βtot. Small energy gaps and high hyperpolarizabilities define the best compounds with nonlinear optical applications.

Thin films were fabricated by incorporating substances with NLO properties in a large variety of polymers to improve the performance of the resulted hybrid system. Polymethyl methacrylate is the most used polymer, but silica matrix is preferred. Also, Disperse Red 1 in a silica glass allows uniform dispersion and arrangement of the chromophore molecules in the crosslinked polymeric matrix. The new hybrid films are fabricated by sol-gel synthesis. The morphology of the silica matrix is very important for a good NLO behavior. Each class of organic compounds possesses a specific method for the fabrication of the thin films in order to increase their NLO properties. Thus, azo-photosensitives dyes are encapsulated. Noncentrosymmetric arrangement for the dipolar push-pull chromophores embedded in a polymeric matrix is obtained when heating the system to the glass transition temperature (T)g of the polymer. Langmuir-Blodgett films, which exhibit better NLO properties, are obtained from Langmuir film deposited at the air-water interfaces with the well-known orientation of the molecule with the hydrophobic groups in air and hydrophilic ones immersed in water. A convenient technique for the fabrication of thin films is the layer-by-layer (LbL) method which presents the advantage of the precise control of the composition and thickness of the final film. Both theoretical modeling and experimental approach should be addressed, in order to find the best matching between the host and guest material. In addition, the influence of the surface and film formation method should be considered, with a particular attention to the thermo- or photosensitive chromophores.
