**1. Introduction**

Photopolymerization is a chemical reaction where organic molecules exposed to UV or visible photons react to form macromolecules corresponding to high molecular weight molecules, i.e.,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the polymer. Photopolymerizable resins usually contain a photoinitiating system (PIS) which converts light into chemical energy, a mixture of monomers, oligomers and additives [1]. The most versatile process is the free radical photopolymerization (FRPP) which offers the most important choice of materials and potential PIS. The efficiency of the PIS is determined by the light absorption properties of the photosenzitizer (PS, the molecules that absorb photons, like organic dyes), given by Beer‐Lambert's law, the quantum yield of initiating radicals and the reactivity of these radicals towards the monomer. It is recognized that the photoinitiating systems is the corner stone of photopolymerization process. One application of photopolymerization is holographic recording [2, 3], for example, for information storage [4–7]. Holography also represents an interesting and growing field due to an increasing demand in security. The development of such applications is directly governed by the characteristics of the material available for holographic recording. Photopolymerizable media are promising candidates. In that case, the performance of the material is related to the photosensitivity of the photopolymer and the diffraction efficiency which could be obtained. Therefore, the design of systems for highest performances is still an interesting challenge. For instance, Acrylic monomers, which are currently used in these recording systems, have opened up in this field interesting possibil‐ ities, due to their attractive features (the complete absence of wet processing, high flexibility of the formulation, high diffraction efficiencies) [8, 9].

#### **2. Basics of photoinitiating systems**

Historically, two classes of photoinitiating systems were defined depending on the mecha‐ nisms of light conversion into chemical potential (i.e., radicals). In the first class, the photoi‐ nitiating system contains one molecule (the photoinitiator PI) which is promoted into a dissociative excited state after light absorption and undergoes a homolytic (or heterolytic) cleavage through a Norrish I photoreaction [10–12]. These type I photoinitiators produce two radicals that could be both reactive toward free radical photopolymerization (FRPP) (**Figure 1a**).

$$\begin{array}{|c|c|}\hline \textbf{(a) type 1:}\\\hline \\\\ \textbf{P} & \xrightarrow{\mbox{\bf hv}} \mathsf{P} & \xrightarrow{\mbox{\bf k}\_{\text{co}}} \mathsf{P}\*\xrightarrow{\mbox{\bf k}\_{\text{co}}} \mathsf{R}\*\_{1}+\mathsf{R}\*\_{2} \xrightarrow{\mbox{\bf M}} \mathsf{P}\mathsf{Q}\mathsf{M}\mathsf{mer} \\\\ \hline \\ \textbf{(b) type 1:} \\\\ \textbf{PS} & \xrightarrow{\mbox{\bf k}\_{\text{v}}} \mathsf{PS}\*+\mathsf{Co} \xrightarrow{\mbox{\bf k}\_{\text{v}}} \mathsf{PS}\*^{\star/\*+}+\underbrace{\mathsf{Co}^{\*\prime/\*}}\_{\mathsf{R}}\*\underbrace{\mathsf{M}}\_{\mathsf{P}} \mathsf{Pol}\mathsf{pr}\mathsf{mer} \\\\ \hline \\ \textbf{PS} & \xrightarrow{\mbox{\bf k}\_{\text{v}}} \mathsf{PS}\*+\mathsf{Co} \xrightarrow{\mbox{\bf k}\_{\text{v}}} \mathsf{PS}\*+\mathsf{Co}^{\*} \\\\ & \xrightarrow{\mbox{\bf k}\_{\text{v}}} \mathsf{R}\*.\end{array}$$

**Figure 1.** Type I and type II photoinitiating systems (PIS), PI: photoinitiator, PS: photosensitizer, Co: coinitiator, R•: ini‐ tiating radical, M: monomer, kdiss: rate constant of dissociation, ket: electron transfer rate constant, k‐H: hydrogen ab‐ straction rate constant.

Among type I photoinitiators, one can find hydroxylalkylphenones, benzylketals, benzoin ether derivatives, α‐aminoketones and acylphosphine oxides (**Figure 2**). In these compounds, the cleavage rate constants are high, leading to very good quantum yields of radical generation ΦR. The quantum yield of radical generation is defined as the number of radicals formed divided by the number of absorbed photons. Some type I photoinitiating systems exhibit quantum yield as high as 0.8–1.0 [13, 14]. However, the vast majority of type I photoinitiators is only reactive under UV light [14, 15]. Only a limited set of available type I photoinitiating systems absorbs in the blue or green region. For instance, a bisbenzoylphosphine oxide derivative (Irgacure 819) exhibits an absorption spectrum extending up to around 410–420 nm. The great advantage of this class of acylphosphine is the efficient photobleaching ability that increases the photoinitiation efficiency. This is especially useful when high thickness of photopolymer is needed. However, oxygen inhibition impacts the efficiency of this class of acylphosphine oxide, which limits their applications [10]. A titanocene derivative (CG‐784) absorbing in the green region was applied to the photopolymerization of some high‐index organic monomers and incorporated into acrylate oligomer‐based formulations, which enables irradiation at 546‐nm light source [16]. Unfortunately, it seems that this compound does not produce enough initiating radicals to achieve the appropriate monomer conversion [17]. Recently, a visible light photoinitiator based on acylgermanium structure was developed, exhibiting high reactivity under 550‐nm irradiation. However, its main drawback relies on the availability of the molecule (proprietary structure and synthesis) [18].

**Figure 2.** Examples of commercially available type I photoinitiators.

the polymer. Photopolymerizable resins usually contain a photoinitiating system (PIS) which converts light into chemical energy, a mixture of monomers, oligomers and additives [1]. The most versatile process is the free radical photopolymerization (FRPP) which offers the most important choice of materials and potential PIS. The efficiency of the PIS is determined by the light absorption properties of the photosenzitizer (PS, the molecules that absorb photons, like organic dyes), given by Beer‐Lambert's law, the quantum yield of initiating radicals and the reactivity of these radicals towards the monomer. It is recognized that the photoinitiating systems is the corner stone of photopolymerization process. One application of photopolymerization is holographic recording [2, 3], for example, for information storage [4–7]. Holography also represents an interesting and growing field due to an increasing demand in security. The development of such applications is directly governed by the characteristics of the material available for holographic recording. Photopolymerizable media are promising candidates. In that case, the performance of the material is related to the photosensitivity of the photopolymer and the diffraction efficiency which could be obtained. Therefore, the design of systems for highest performances is still an interesting challenge. For instance, Acrylic monomers, which are currently used in these recording systems, have opened up in this field interesting possibil‐ ities, due to their attractive features (the complete absence of wet processing, high flexibility of

Historically, two classes of photoinitiating systems were defined depending on the mecha‐ nisms of light conversion into chemical potential (i.e., radicals). In the first class, the photoi‐ nitiating system contains one molecule (the photoinitiator PI) which is promoted into a dissociative excited state after light absorption and undergoes a homolytic (or heterolytic) cleavage through a Norrish I photoreaction [10–12]. These type I photoinitiators produce two radicals that could be both reactive toward free radical photopolymerization (FRPP)

**Figure 1.** Type I and type II photoinitiating systems (PIS), PI: photoinitiator, PS: photosensitizer, Co: coinitiator, R•: ini‐ tiating radical, M: monomer, kdiss: rate constant of dissociation, ket: electron transfer rate constant, k‐H: hydrogen ab‐

the formulation, high diffraction efficiencies) [8, 9].

**2. Basics of photoinitiating systems**

376 Holographic Materials and Optical Systems

(**Figure 1a**).

straction rate constant.

Even if type I photoinitiators can exhibit high quantum yields of radicals, their main drawback is their limited spectral sensitivity to the UV—blue region of the electromagnetic spectrum. By contrast, type II PIS are versatile initiators for UV curing systems and visible light photopo‐ lymerization. Indeed, the combination of organic dyes and coinitiators provides tremendous flexibility in the selection of irradiation wavelength from the UV to the near infra‐red region.

**Figure 3.** Examples of commercially available type II photosensitizers (PS) and coinitiators (Co).

Thus, on the contrary to type I PIs which are monomolecular, type II photoinitiating systems rely on the combination of two molecules (see **Figure 1b**). The first molecule absorbs the photon. It is the chromophore and is often called the photoinitiator (PI) or the photosensitizer (PS). The second one could be an electron donor, an electron acceptor or a hydrogen donor (see **Figure 1b**), the so‐called coinitiator (Co), which gives the initiating radicals R• through photoreaction [19, 20]. After light absorption, the photosensitizer (PS) is promoted into one of its electronic excited states (singlet and/or triplet) from which the photochemical reaction occurs with the coinitiator (Co). One important feature of type II PIS is that the photosensitizer (PS) and the coinitiator (Co) must be selected to prevent any dark thermal reaction. Thus, photopolymerization occurs only in irradiated zones allowing a full control of the polymeri‐ zation in time and space. In typical type II PIS, photosensitizers with good absorption features in the UV‐blue region can be selected among benzophenones [21–28], thioxanthones [29–34], camphorquinone [35–37], benzyls [22, 38] and ketocoumarin derivatives [39–41]. For visible light PIS, the PS can be selected in whole panel of organic dyes such as, coumarins [41], xanthenic dyes [42–44], cyanine dyes [45], thiazine dyes [42], phenazine dyes and pyrrome‐ thene dyes [46–48]. The hydrogen donor coinitiators are generally amines [36, 49–54], ethers [55–57], sulfides [57–61] and thiols [61–63](see **Figure 3**). However, the ketyl radical formed on the PS moiety is generally unreactive with respect to the double bonds and could even act as a terminating agent towards the growing chain [17, 49, 64, 65]. Coinitiators reacting through electron transfer are borate salts [66, 67], iodonium salts [68–70] or triazine derivatives [47] which lead to the production of radical after a photodissociative electron transfer reaction with excited PS. However, these two components systems have limited efficiency compared to type I systems.
