**3. Photocyclic initiating systems**

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

378 Holographic Materials and Optical Systems

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 One of the reasons that could be responsible for the lower sensitivity of type II photoinitiating systems (PIS) compared to type I is their inherent chemical mechanisms. In type I PIS, the molecules are cleaved after light absorption: this is an intramolecular fast reaction. On the other side, for type II PIS, the reactions are bimolecular and can be limited by diffusion process of the photosensitizer (PS) and coinitiator (Co) (see **Figure 4**). The actual reaction rate constants of electron transfer (or H‐abstraction) in such conditions can be evaluated from a simple encounter complex kinetic model (see [45] and ref. herein for more details) and is generally lowered when the viscosity of the resin increases. As a result, the efficiency of reaction is greatly impacted and the PIS becomes inefficient with low radical quantum yields.

One way to overcome the lack of reactivity of conventional type II photoinitiating systems (PIS) is to develop one molecule type II PIS where the PS and the coinitiator (Co) are chemically linked together (see **Figure 4b**). In this case, reaction rates are independent of the diffusion process and highly sensitive systems can be obtained [71–74]. However, such kind of single molecule type II photoinitiating systems (PIS) suffer of proprietary structure and synthesis costs which makes them tricky to use.

In order to enhance type II sensitivity, many groups have developed more complex photoini‐ tiating systems by adding a third component into the PIS formulation leading to the so‐called three‐component photoinitiating system. Indeed, the photopolymerization efficiency of type II PIS can be greatly improved by introducing an additive, which yields to an additional radical formation through reaction with one photoproduct arising from the photochemical reaction [75–78]. The use of photoinitiating systems based on three components keeps the tremendous flexibility of the light sources and the determination of the actinic wavelength, because the wide variety of dyes that may be used in type II PIS is still large. Three different kinds of additives can be used: (1) latent species that create reactive centers after reaction, (2) molecules that are oxidized and (3) molecules that are reduced [47, 48, 79, 80]. The first class of usable additives are species that leads to the formation of reactive centers after reaction, containing chain transfer agents such as S‐H, P‐H, Si‐H or Ge‐H‐based molecules. They have found only limited applications and will not be discussed further. Among oxidable molecules that can be selected, one can find amine derivatives such *N*‐methyldiethanolamine [23, 24] or triethyla‐ mine [79] as common electron donor reported to date for the photoinitiating systems in the literature. To circumvent the toxicity of alkyl amines, aromatic amines such as *N*‐phenylglycine are available [32]. Triarylalkyl borates, sulfur‐ or tin‐ containing compounds, sulfinates [81] have also been reported as well as, amides, ethers, ferrocene, metallocenes, ureas, salts of xanthanates, salts of tetraphenylboronic acid, etc. It should be mentioned that the oxidation potential of the donor plays a key role in the mechanism of electron transfer.

**Figure 4.** Reaction scheme of conventional type II photoinitiating systems (PIS) (a) and unimolecular type II PIS (b); PS: photosensitizer, Co: coinitiator, R•: initiating radical, kdiff: diffusion rate constant, k‐diff: separation rate constant, kdiss: rate constant of dissociation, ket: electron transfer rate constant.

Electron acceptor additives (i.e., reducible additives) most commonly used are iodonium salts which exhibit a low reduction potential [14, 17, 82–85]. It is possible to introduce bromo compounds instead of the iodonium salts [24, 86]. Triazine derivatives are one of the most common electron acceptors used as third component for PIS based on dye/borate salts [87], while alcoxypyridinium derivatives were also used as a third component to increase the sensitivity of borate salts based two component PIS [88]. Other oxidative additives can be selected such as peroxides, sulfonium and pyridinium salts, iron arene complexes, and hexaarylbisimidazole as alternatives to iodoniums or triazines. The reduction potential of the third component is an important criterion to select the compound [46, 47].

formation through reaction with one photoproduct arising from the photochemical reaction [75–78]. The use of photoinitiating systems based on three components keeps the tremendous flexibility of the light sources and the determination of the actinic wavelength, because the wide variety of dyes that may be used in type II PIS is still large. Three different kinds of additives can be used: (1) latent species that create reactive centers after reaction, (2) molecules that are oxidized and (3) molecules that are reduced [47, 48, 79, 80]. The first class of usable additives are species that leads to the formation of reactive centers after reaction, containing chain transfer agents such as S‐H, P‐H, Si‐H or Ge‐H‐based molecules. They have found only limited applications and will not be discussed further. Among oxidable molecules that can be selected, one can find amine derivatives such *N*‐methyldiethanolamine [23, 24] or triethyla‐ mine [79] as common electron donor reported to date for the photoinitiating systems in the literature. To circumvent the toxicity of alkyl amines, aromatic amines such as *N*‐phenylglycine are available [32]. Triarylalkyl borates, sulfur‐ or tin‐ containing compounds, sulfinates [81] have also been reported as well as, amides, ethers, ferrocene, metallocenes, ureas, salts of xanthanates, salts of tetraphenylboronic acid, etc. It should be mentioned that the oxidation

potential of the donor plays a key role in the mechanism of electron transfer.

**Figure 4.** Reaction scheme of conventional type II photoinitiating systems (PIS) (a) and unimolecular type II PIS (b); PS: photosensitizer, Co: coinitiator, R•: initiating radical, kdiff: diffusion rate constant, k‐diff: separation rate constant, kdiss:

Electron acceptor additives (i.e., reducible additives) most commonly used are iodonium salts which exhibit a low reduction potential [14, 17, 82–85]. It is possible to introduce bromo compounds instead of the iodonium salts [24, 86]. Triazine derivatives are one of the most common electron acceptors used as third component for PIS based on dye/borate salts [87], while alcoxypyridinium derivatives were also used as a third component to increase the sensitivity of borate salts based two component PIS [88]. Other oxidative additives can be selected such as peroxides, sulfonium and pyridinium salts, iron arene complexes, and

rate constant of dissociation, ket: electron transfer rate constant.

380 Holographic Materials and Optical Systems

The dyes reported in the literature as photosensitizers for type II photoinitiating systems can be used for three‐component photoinitiating systems (PIS): coumarin dyes, xanthene dyes, acridine dyes, thiazoles dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, aminothiaryl methanes, merocyanines dyes, squarylium dyes, pyridinium dyes, etc. Many studies reported that the photopolymerization efficiency, kinetics and mechanistic reactivity of this type of systems [41, 46–49, 64, 75–79, 86, 88–93]. However, for type II and three‐components PIS, the selection of dye must respect some key criteria:


A full description of the electron transfer reactions occurring in three‐components (and type II) PIS is summarized in **Figure 5**. Four electron transfer reactions are identified: the first one (**Figure 5** [1]) is the thermal electron transfer (ET) which should be avoided; the second one is the photoinduced electron transfer (PET) (**Figure 5** [2]) and is discussed in point iv. (vide supra), the third one is the so‐called back electron transfer (BET) (**Figure 5** [3]), an intra‐encounter complex recombination of reduced photosensitizer (PS) an oxidized coinitiator (Co) which gives the reactants back and lower the quantum yield of radical generation. Up to this point, this thermodynamic approach covers also the type II photoinitiating systems requirements. Finally, if ever the fourth electron transfer reaction occurs (**Figure 5** [4]), the reduced PS is oxidized by the third component and the PS is regenerated while a second initiating radical is produced. This leads to the so‐called photocyclic (or photocatalytic) initiating systems (PCIS) (vide infra). Detailed explanation of PCIS kinetics and thermodynamics can be found in Ref. [95]. However, obtaining a photocyclic behavior in three‐component systems is not straigh‐ forward and great care must be taken when combining dyes and coinitiators.

**Figure 5.** Thermodynamics of an oxidative three‐components PCIS [1], ground state reaction (ΔET GS) [2], excited state reaction (ΔET\* ) [3], back electron transfer (BET, BET) [4], PS regeneration (ET PS • + ) in photocyclic initiating system (PCIS vide infra).

Thus, in the case of three‐components PIS, two mechanisms have been observed leading to two general classes (see **Figure 6**):


Application of High Performance Photoinitiating Systems for Holographic Grating Recording http://dx.doi.org/10.5772/66073 383

this thermodynamic approach covers also the type II photoinitiating systems requirements. Finally, if ever the fourth electron transfer reaction occurs (**Figure 5** [4]), the reduced PS is oxidized by the third component and the PS is regenerated while a second initiating radical is produced. This leads to the so‐called photocyclic (or photocatalytic) initiating systems (PCIS) (vide infra). Detailed explanation of PCIS kinetics and thermodynamics can be found in Ref. [95]. However, obtaining a photocyclic behavior in three‐component systems is not straigh‐

forward and great care must be taken when combining dyes and coinitiators.

**Figure 5.** Thermodynamics of an oxidative three‐components PCIS [1], ground state reaction (ΔET

\* ) [3], back electron transfer (BET, BET) [4], PS regeneration (ET

Thus, in the case of three‐components PIS, two mechanisms have been observed leading to

**a.** Parallel reactions in which the coinitiators Co and the additive A react with the excited

**b.** (b) Sequential reactions in which, for example, the Co reacts first through PET with the dye excited state (**Figure 6b**) leading to reduced PS and a first initiating radical. Then, the reduced PS can react with the additive A to regenerate the PS and give a second initiating

reaction (ΔET

system (PCIS vide infra).

radical.

two general classes (see **Figure 6**):

382 Holographic Materials and Optical Systems

state of the dye independently (**Figure 6a**);

GS) [2], excited state

) in photocyclic initiating

PS • +

**Figure 6.** General reaction mechanism occurring in three‐component photoinitiating systems, (a) parallel mechanisms, (b) photocyclic initiating system (PCIS).

In parallel (i.e., independent and competitive) reactions, the total yield of radicals depends on the reactivity of each coinitiator with the excited state and is not very interesting. The sequential reactions present more attractive features. This mechanism yields two advantages: the additive (A) leads to the formation of supplementary initiating radical, and the ground state dye is recovered and can be involved in further photoreactions. Therefore, a real cyclic photoreaction occurs until complete depletion of reactants. Moreover, the maximum theoretical quantum yields of PCIS are two, meaning that one absorbed photon can give two initiating radicals.

#### **4. Photoinitiating systems for holographic recording**

Holographic polymerization recording is a very particular application of photopolymeriza‐ tion: a three‐dimensional image is built through inhomogeneous polymerization when the interference pattern illuminates the photosentive medium [5–7]. Indeed, photopolymerization is ideally suited for such application as the reaction can be spatially and temporally controlled. Holographic elements, reversible holograms and switchable holographic gratings have been created by this method [5, 96–99]. Since the 1970s, photopolymers were developed in holog‐ raphy as media able to record an interference pattern or small series of pattern, through building‐up of refractive index variations or relief profiles [100–102]. During volume hologram recording in a photopolymer, a complex fringe pattern with small features at the micrometer scale is recorded. The local incident variations of irradiance induce inhomogeneous photopo‐ lymerization of the photosensitive recording medium at the sub‐micrometer scale leading to refractive index modulations in the hologram. Many physical and chemical processes are involved in the holographic recording: photochemical conversion of the sensitizer, of the monomer, mass transport (being a consequence of the formation of spatial concentration gradients of monomer and sensitizer), hardening of the polymer matrix, etc. Therefore, formulating the material requires a fair knowledge of all these processes, which is not a straightforward way as many parameters must be taken into account [4, 103, 104].

Many PIS can be used for holographic recording applications. Titanocene derivative Bis(η5‐ 2,4‐cylcopentadien‐1‐yl)‐bis (2,6‐difluoro‐3‐(1H‐pyrrol‐1‐yl)‐phenyl) titanium, Irgacure®784, was used for the optimization of several high index organic monomers into high optical quality acrylate oligomer‐based formulation and compared to the type II PIS [105]. However, even if it is a one molecule process by‐products that can alter recoding are formed [106]. Moreover, by introducing with these type I titanocene compounds, nanoparticles in the holographic polymer, the effect of nanoparticles concentration and size, as well as the benefic effect of chain transfer agent were studied [107, 108]. Triphenylphosphine (TPO) type I PIS was used for initiating thiol‐ene photopolymerization in a composite holographic resin containing nano‐ particles in a way to reduce shrinkage and enhance Δ*n*sat [109]. Near UV type I photoinitiator Irgacure 1700 was used to study the effect of surface modified ZrO2 and TiO2 nanoparticles, allowing high refractive index modulation and better stability against UV light [110].

A widely used type II PIS is the combination of phenanthrenequinone (PQ) with methacrylates (MA) monomers and oligomers (PMMA) [111–116]: in this case, the PQ is supposed to directly react with PMMA in its triplet state by hydrogen abstraction [112, 113]. Among other Type II PIS, methylene blue (MB) and rose Bengal (RB) have been tested in crylamide and polyviny‐ lalcohol films [42] under 633 or 514 nm irradiation, diffraction efficiencies of 65 and 35%, and sensitivities of 30 and 100 mJ cm‐2 have been, respectively, reached, with a spatial resolution of *ca.* 1000 lines mm‐1.

The influence of photonic and chemical parameters on the holographic recording capabili‐ ties and photochemical bleaching process of a series of xanthene dyes such as RB, eosin Y (EY), erythrosin B (ErB), fluorescein (F) and rhodamine B (RoB) has been investigated with triethanolamine (TEA) as electron donor Co [117, 118]. The photobleaching efficiency, i.e., the ability of the PS to lose their tint, followed the order ErB > EY > RB > RoB > F. The highest photobleaching rate constant of ErB PIS was invoqued to explain the higher diffraction effi‐ ciency obtained compared to EY, RB, RoB and F under the same experimental conditions. More recently, the same family of xanthenic dyes was theoretically and experimentally in‐ vestigated [119].

A system using EY, F, MB and thionine (TH) as photosensitizers with morpholine, dimethy‐ laminoethanol and piperidine as electron donors, and an oligourethane‐acrylates resin has been used for holographic applications such as interferometry and pattern recognition systems [120]. A sufficiently high speed of recording in the 460–540 nm range has been noticed. Despite higher absorbance in the longer wavelength, holographic experiment in the red light with the MB systems outlined low reactivity and slow recording rates. More recently, combi‐ nation of MB and trimethylamine (TEA) as coinitiator was still used as type II PIS in holo‐ graphic recording systems bearing metallic ions as dopers to enhance holographic sensitivity efficiency [121, 122].

is ideally suited for such application as the reaction can be spatially and temporally controlled. Holographic elements, reversible holograms and switchable holographic gratings have been created by this method [5, 96–99]. Since the 1970s, photopolymers were developed in holog‐ raphy as media able to record an interference pattern or small series of pattern, through building‐up of refractive index variations or relief profiles [100–102]. During volume hologram recording in a photopolymer, a complex fringe pattern with small features at the micrometer scale is recorded. The local incident variations of irradiance induce inhomogeneous photopo‐ lymerization of the photosensitive recording medium at the sub‐micrometer scale leading to refractive index modulations in the hologram. Many physical and chemical processes are involved in the holographic recording: photochemical conversion of the sensitizer, of the monomer, mass transport (being a consequence of the formation of spatial concentration gradients of monomer and sensitizer), hardening of the polymer matrix, etc. Therefore, formulating the material requires a fair knowledge of all these processes, which is not a

straightforward way as many parameters must be taken into account [4, 103, 104].

allowing high refractive index modulation and better stability against UV light [110].

*ca.* 1000 lines mm‐1.

384 Holographic Materials and Optical Systems

vestigated [119].

A widely used type II PIS is the combination of phenanthrenequinone (PQ) with methacrylates (MA) monomers and oligomers (PMMA) [111–116]: in this case, the PQ is supposed to directly react with PMMA in its triplet state by hydrogen abstraction [112, 113]. Among other Type II PIS, methylene blue (MB) and rose Bengal (RB) have been tested in crylamide and polyviny‐ lalcohol films [42] under 633 or 514 nm irradiation, diffraction efficiencies of 65 and 35%, and sensitivities of 30 and 100 mJ cm‐2 have been, respectively, reached, with a spatial resolution of

The influence of photonic and chemical parameters on the holographic recording capabili‐ ties and photochemical bleaching process of a series of xanthene dyes such as RB, eosin Y (EY), erythrosin B (ErB), fluorescein (F) and rhodamine B (RoB) has been investigated with triethanolamine (TEA) as electron donor Co [117, 118]. The photobleaching efficiency, i.e., the ability of the PS to lose their tint, followed the order ErB > EY > RB > RoB > F. The highest photobleaching rate constant of ErB PIS was invoqued to explain the higher diffraction effi‐ ciency obtained compared to EY, RB, RoB and F under the same experimental conditions. More recently, the same family of xanthenic dyes was theoretically and experimentally in‐

Many PIS can be used for holographic recording applications. Titanocene derivative Bis(η5‐ 2,4‐cylcopentadien‐1‐yl)‐bis (2,6‐difluoro‐3‐(1H‐pyrrol‐1‐yl)‐phenyl) titanium, Irgacure®784, was used for the optimization of several high index organic monomers into high optical quality acrylate oligomer‐based formulation and compared to the type II PIS [105]. However, even if it is a one molecule process by‐products that can alter recoding are formed [106]. Moreover, by introducing with these type I titanocene compounds, nanoparticles in the holographic polymer, the effect of nanoparticles concentration and size, as well as the benefic effect of chain transfer agent were studied [107, 108]. Triphenylphosphine (TPO) type I PIS was used for initiating thiol‐ene photopolymerization in a composite holographic resin containing nano‐ particles in a way to reduce shrinkage and enhance Δ*n*sat [109]. Near UV type I photoinitiator Irgacure 1700 was used to study the effect of surface modified ZrO2 and TiO2 nanoparticles, If type II photoinitiating systems (PIS) are widely used in holographic recording, the use of three‐components PIS is less common. The efficiency of a three‐component PIS based on hexaarylbisimidazole derivative (HABI), associated to a chain transfer agent 2‐mercaptoben‐ zooxazole (MBO) and 2,5‐bis[[(4‐diethylamino)phenyl] methylene] cyclopentanone (DEAW) as the PS incorporated into high optical quality acrylate oligomer‐based formulations was reported [105] (see **Figure 7**).

**Figure 7.** Structures of some PIS compounds; Bis(η5‐2,4‐cylcopentadien‐1‐yl)‐bis (2,6‐difluoro‐3‐(1H‐pyrrol‐1‐yl)‐phe‐ nyl) titanium: Irgacur®784, 2,4,6‐Trimethylbenzoyl‐diphenylphosphine oxide: Darocur® TPO, phenanthrenequinone: PQ, methylene blue: MB, triethanolamine: TEA; 2‐mercaptobenzooxazole: MBO, 2,5‐bis[[(4‐diethylamino)phenyl] methylene] cyclopentanone: DEAW, hexaarylbisimidazole: HABI, 3‐3'‐carbonyl‐bis‐7‐diethylaminocoumarin: KC.

Another three‐component photosensitive resin, based on a 3‐3'‐carbonyl‐bis‐7‐diethylamino‐ coumarin (KC) PS, *N*‐phenylglycine (NPG) and diphenyl iodonium chloride (DPI) has been used to record holograms at 488 nm (Ar+ laser). A diffraction efficiency around 51% corre‐ sponding to a refractive index modulation of 0.013 was obtained [123]. The efficiency of PIS based on new original synthesized dyes that could be sensitive to He‐Ne laser line (632.8 nm), a HABI derivative and 3‐mercapto‐4‐methyl‐4H‐1,2,4‐triazole (MTA), was studied [124]. A 80% diffraction efficiency was obtained, with good physical and chemical stability under ambient conditions. The singlet excited state reactivity of bipyrromethene‐BF2 complexes (also known as BODIPY) was used in amine‐free photosensitive hydrophobic binder resin as an alternative to xanthene dyes‐based redox PIS [125]. The unbleached final gratings showed diffraction efficiency of 85% with good sensitivity in the 457–520 nm range.
