**5. Influence of photoinitiating system on holographic recording**

In summary, this survey of the literature shows that most of the photoinitiating systems used for holographic recording are simple classical type I or type II PIS. Three components combi‐ nation in holographic resins are less common. Moreover, the exact photochemistry underlying the initiating radical generation is not very well‐known, and the effect of the PIS photochem‐ istry on hologram formation was rarely questioned [126–128]. Furthermore, it is difficult to compare and gather holographic recording resin results, many different PIS systems are available and can be associated to various polymerizable resin, with or without binders, for use in different optical setup with diverse photonic parameters.

However, in a recent work, the influence of photochemistry on the holographic recording efficiency was performed by our group in collaboration with Bayer material science team [129]. It was shown that the prediction and interpretation of the holographic performance of a photosensitive resin containing a type II photo initiating systems is directly related to the reactivity of the dyes excited states involved and to the intrinsic properties of the photopoly‐ merizable medium. Indeed the radical quantum yield of the dyes (RB and SFH+ ) coupled to borates salt electron donor coinitiator was fixed by the viscosity of the holographic resin matrix and the redox properties of the dyes and borates. A method was proposed to calculate the initial yield of initiating radicals. It was found that this radical formation quantum yield directly governs not only the maximum rate constant of photopolymerization, but also the final diffraction efficiency.

With this in mind, we have recently tailored different three‐components photoinitiating systems for holographic recording with the advantage that all measurements were performed on the same holographic resin formulation, under fixed experimental conditions in a given holographic recording setup [130–132]. Furthermore, the holographic results were compared to the visible curing of the holographic resin formulation followed by real‐time FTIR spectro‐ scopy [130–134]. RT‐FTIR allowed the study of free radical polymerization (FRP) by following the disappearance of C‐C double bonds in monomer (see Ref. [131–133] for more detail): it permitted to measure the final monomer conversion into polymer and the rate of double bond consumption during the polymerization reaction *R*c (s‐1) (see **Figure 8a**). The IR spectra were recorded during sample irradiation using a green laser diode emitting at 532 nm (Roithner Lasertechnik, 50 mW) which was adapted to the FTIR spectrometer by a light guide (see **Figure 8a**). The irradiation intensity was adjusted at 25 mW/cm2 on the sample. To prevent the diffusion of oxygen into the sample during the irradiation, experiments were carried out by laminating the resin between two polypropylene films and two CaF2 windows. The thickness of the sample was adjusted using a 25‐µm Teflon spacer. The spectra were recorded between 600 and 3900 cm‐1. The kinetics of the polymerization were measured by following the disappearance of the C‐C bond stretching signal at 1637 cm‐1. The degree of conversion was directly related to the decrease in peak area at 1637 cm‐1 according to:

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

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.

use in different optical setup with diverse photonic parameters.

final diffraction efficiency.

**5. Influence of photoinitiating system on holographic recording**

In summary, this survey of the literature shows that most of the photoinitiating systems used for holographic recording are simple classical type I or type II PIS. Three components combi‐ nation in holographic resins are less common. Moreover, the exact photochemistry underlying the initiating radical generation is not very well‐known, and the effect of the PIS photochem‐ istry on hologram formation was rarely questioned [126–128]. Furthermore, it is difficult to compare and gather holographic recording resin results, many different PIS systems are available and can be associated to various polymerizable resin, with or without binders, for

However, in a recent work, the influence of photochemistry on the holographic recording efficiency was performed by our group in collaboration with Bayer material science team [129]. It was shown that the prediction and interpretation of the holographic performance of a photosensitive resin containing a type II photo initiating systems is directly related to the reactivity of the dyes excited states involved and to the intrinsic properties of the photopoly‐

borates salt electron donor coinitiator was fixed by the viscosity of the holographic resin matrix and the redox properties of the dyes and borates. A method was proposed to calculate the initial yield of initiating radicals. It was found that this radical formation quantum yield directly governs not only the maximum rate constant of photopolymerization, but also the

With this in mind, we have recently tailored different three‐components photoinitiating systems for holographic recording with the advantage that all measurements were performed on the same holographic resin formulation, under fixed experimental conditions in a given holographic recording setup [130–132]. Furthermore, the holographic results were compared to the visible curing of the holographic resin formulation followed by real‐time FTIR spectro‐ scopy [130–134]. RT‐FTIR allowed the study of free radical polymerization (FRP) by following the disappearance of C‐C double bonds in monomer (see Ref. [131–133] for more detail): it permitted to measure the final monomer conversion into polymer and the rate of double bond

merizable medium. Indeed the radical quantum yield of the dyes (RB and SFH+

laser). A diffraction efficiency around 51% corre‐

) coupled to

used to record holograms at 488 nm (Ar+

386 Holographic Materials and Optical Systems

$$C = \frac{\left(A\_{1637}\right)\_0 - \left(A\_{1637}\right)\_{t0}}{\left(A\_{1637}\right)\_0} \tag{1}$$

**Figure 8.** RT‐FTIR (a) and Holographic grating recording (b) setup.

where (*A*1637)0 and (*A*1637)*<sup>t</sup>* were the area of the IR absorption band at 1637 cm‐1 of the sample before exposure and at time *t*, respectively.

Holographic gratings were recorded in transmission at 514 nm for a spatial frequency of 1000 lines mm‐1 transmission grating in the resin with a 514 nm actinic laser. For that purpose, the samples are prepared by embedding the photopolymerizable formulation between two glass‐substrates. Calibrated glass beads were used as spacers to guarantee the thickness of the system around 20 µm. The photopolymerizable system was irradiated by the sinusoidal interference pattern of two incident s‐polarized beams of equal intensity, corresponding to a total power density of 25 mW/cm2 on the photosensitive sample with a beam diameter of 2.5 cm (**Figure 8b**). Inhomogeneous polymerization reaction and dye bleaching took place leading to a modulation of the refractive index, giving rise to thick phase volume diffraction gratings. The fact that no chemical posttreatment was needed for this recording medium used, allowed the continuous follow up of the process during exposure with an inactinic reading light beam (HeNe laser at 633 nm) which was more or less diffracted (see **Figure 8b**). The diffraction efficiency at 633 nm (*η*) was defined by the ratio of the intensity of the first diffraction order to the diffracted plus transmitted light intensities.

The holographic resin was a mixture of different monomers and additives: the choice of the formulation was governed by earlier experiments performed in the field of visible curable systems and use of fluorinated acrylate monomers for the recording of holographic polymer‐ dispersed liquid crystals (LC) transmission gratings. This self‐developing formulation contained:


An example of RT‐FTIR monomer conversion of the holographic resin as a function of irradiation time is given on **Figure 9b**. The photoinitiaitng system was based on Safranine O (SFH+ ) as photosensitizer (PS), EDB as an electron donor and Triazine A electron acceptor (see **Figure 3** for molecular structures) as the third component. It can be seen that going from two components type II to three‐components PIS increases both the rate of conversion *R*c and the final conversion *C*<sup>f</sup> . The same observation is valid for measured diffraction yield as a function of irradiation time (**Figure 9a**): both the formation building up time of the diffraction grating and the final diffraction efficiency increase.

Application of High Performance Photoinitiating Systems for Holographic Grating Recording http://dx.doi.org/10.5772/66073 389

where (*A*1637)0 and (*A*1637)*<sup>t</sup>*

388 Holographic Materials and Optical Systems

contained:

(SFH+

final conversion *C*<sup>f</sup>

and the final diffraction efficiency increase.

as primary oligomer;

easily with acrylic monomers;

before exposure and at time *t*, respectively.

the diffracted plus transmitted light intensities.

were the area of the IR absorption band at 1637 cm‐1 of the sample

Holographic gratings were recorded in transmission at 514 nm for a spatial frequency of 1000 lines mm‐1 transmission grating in the resin with a 514 nm actinic laser. For that purpose, the samples are prepared by embedding the photopolymerizable formulation between two glass‐substrates. Calibrated glass beads were used as spacers to guarantee the thickness of the system around 20 µm. The photopolymerizable system was irradiated by the sinusoidal interference pattern of two incident s‐polarized beams of equal intensity, corresponding to a total power density of 25 mW/cm2 on the photosensitive sample with a beam diameter of 2.5 cm (**Figure 8b**). Inhomogeneous polymerization reaction and dye bleaching took place leading to a modulation of the refractive index, giving rise to thick phase volume diffraction gratings. The fact that no chemical posttreatment was needed for this recording medium used, allowed the continuous follow up of the process during exposure with an inactinic reading light beam (HeNe laser at 633 nm) which was more or less diffracted (see **Figure 8b**). The diffraction efficiency at 633 nm (*η*) was defined by the ratio of the intensity of the first diffraction order to

The holographic resin was a mixture of different monomers and additives: the choice of the formulation was governed by earlier experiments performed in the field of visible curable systems and use of fluorinated acrylate monomers for the recording of holographic polymer‐ dispersed liquid crystals (LC) transmission gratings. This self‐developing formulation

**•** 45 wt% of a hexafunctional aliphatic urethane acrylate oligomer (Ebecryl 1290, Cytec) acting

**•** 22.5 wt% of 1,1,1,3,3,3‐hexafluoroisopropyl acrylate and 22.5 wt% of vinyl neononanoate, both from Sigma‐Aldrich (France). The vinyl ester monomer is known to copolymerize very

**•** 5 wt% of *N*‐vinyl‐2‐pyrrolidinone (Sigma‐Aldrich, France) which is a standard additive

**•** 5 wt% of Trimethylpropane tris (3‐mercaptopropionate) (Sigma‐Aldrich, France) which is a trifunctional thiol able to increase the photopolymerization rate in air and leading to higher monomer conversion. These compounds were used as received. The chemical structures of the reagents used for the preparation of holographic resin can be found in Ref [132].

An example of RT‐FTIR monomer conversion of the holographic resin as a function of irradiation time is given on **Figure 9b**. The photoinitiaitng system was based on Safranine O

of irradiation time (**Figure 9a**): both the formation building up time of the diffraction grating

) as photosensitizer (PS), EDB as an electron donor and Triazine A electron acceptor (see **Figure 3** for molecular structures) as the third component. It can be seen that going from two components type II to three‐components PIS increases both the rate of conversion *R*c and the

. The same observation is valid for measured diffraction yield as a function

introduced in photopolymerizable systems here to favor compounds solubility;

**Figure 9.** Evolution of the diffraction efficiency η during grating writing (a) of SFH+ PIS and corresponding monomer conversion curves as a function of irradiation time (b).

Moreover, besides RT‐FTIR and holographic measurements, a complete and detailed study of the photochemical reaction of the PIS was performed. For this purpose, steady state (UV‐Vis, fluorescence) and time resolved (laser flash photolysis, time correlated single photon counting, etc.) spectroscopies were used. The first system was based on the singlet excited state reactivity of Bipyrromethene‐BF2 complexes (EMP) combined with an amine (EDB) and two electron acceptor (TA, I250) [130]. The two other systems were based on the reactivity of the triplet excited state of two dyes, RB and Safranine O (SFH+ ) [131, 132]. In the first one, the reactivity of RB was compared to the reactivity of SFH+ when combined to NPG electron donor coinitiator and HABI as additive third component [131]. In the last system, SFH+ was combined with one electron donor (EDB) and one electron acceptor. However, it was demonstrated that no photocycle occurred in these PIS (see **Figure 9** [2, 3]). [133] Indeed, besides the nature of the excited state involved in the radical photogeneration process, it was shown that EMP PIS exhibited a photocyclic behavior (i.e., forms a PCIS) while RB and SFH+ PIS presented a parallel behavior (see **Figure 10**).

In order to understand correctly Scheme 2 in **Figure 10**, Hexaarylbiimidazole (HABI) deriva‐ tives deserve a little more explanations. HABI (Scheme 1 in **Figure 10**) was first synthesized by Hayashi and Maeda (see [134] and ref. Herein). It was proved that the two imidazolyl rings are twisted almost 90° relative to each other [134]. The bond energy of C‐N in ClHABI is very low, which easily leads to the homolytic cleavage of ClHABI when exposed to the UV light or heated [135] to a pair of triarylimidazolyl radicals (lophyl radicals = L•). The lophyl radical (L•) is known to be a poor initiator of free radical polymerization, because of both high stability [135–137]. However, the lophyl radical is an excellent hydrogen atom abstractor, and this can be exploited in initiation using a hydrogen donor coinitiator (Scheme 2 in **Figure 10**).

**Figure 10.** Photochemical mechanism underlying the radical photogeneration of [1] pyrromethene‐based photocyclic initiating system (PCIS) [2], RB and SHF+ where L• stands for lophyl radical, i.e., the moiety of a HABI molecule (see text) [3], SFH+ with one reductant and one oxidant coinitiators.

Application of High Performance Photoinitiating Systems for Holographic Grating Recording http://dx.doi.org/10.5772/66073 391


exhibited a photocyclic behavior (i.e., forms a PCIS) while RB and SFH+

In order to understand correctly Scheme 2 in **Figure 10**, Hexaarylbiimidazole (HABI) deriva‐ tives deserve a little more explanations. HABI (Scheme 1 in **Figure 10**) was first synthesized by Hayashi and Maeda (see [134] and ref. Herein). It was proved that the two imidazolyl rings are twisted almost 90° relative to each other [134]. The bond energy of C‐N in ClHABI is very low, which easily leads to the homolytic cleavage of ClHABI when exposed to the UV light or heated [135] to a pair of triarylimidazolyl radicals (lophyl radicals = L•). The lophyl radical (L•) is known to be a poor initiator of free radical polymerization, because of both high stability [135–137]. However, the lophyl radical is an excellent hydrogen atom abstractor, and this can be exploited in initiation using a hydrogen donor coinitiator (Scheme 2 in **Figure 10**).

**Figure 10.** Photochemical mechanism underlying the radical photogeneration of [1] pyrromethene‐based photocyclic

where L• stands for lophyl radical, i.e., the moiety of a HABI molecule (see

behavior (see **Figure 10**).

390 Holographic Materials and Optical Systems

initiating system (PCIS) [2], RB and SHF+

with one reductant and one oxidant coinitiators.

text) [3], SFH+

PIS presented a parallel

**Table 1.** Holographic and RT‐FTIR characterization of the 16 PIS combination [131–133]. *η*<sup>f</sup> : final diffraction yield, *C*<sup>f</sup> : final conversion in FRP, *R*η: maximum rate of grating formation, *R*c: maximum rate of monomer conversion.

**Figure 11.** (a) Reversible photodissociation of HABI (L2) into two lophyl radicals L•; (b) reaction of lophyl radical with hydrogen donor (RH).

Indeed, we have here a very unique combination of photosensitzers, coinitiators and third components showing very different mechanisms of radical photogeneration. With the three PIS presented here, 16 different combinations of PS‐Co, PS‐additive and PS‐Co‐additive were measured both in real‐time FTIR (RT‐FTIR) and holographic recording. The results in term of both FRP and holographic recording of these combinations are summarized in **Table 1**. The performance of the PIS toward homogeneous‐free radical photopolymerization are given by the final conversion *C*<sup>f</sup> and maximum rate of conversion *R*c (s‐1), while the gratings are charac‐ terized by their final diffraction yield ηf and maximum rate of grating formation *R*η (s‐1).

**Figure 12.** Final diffraction efficiency of grating recording ηf as a function of final monomer conversion *C*<sup>f</sup> .

The existence of a relationship between the evolution of monomer conversion under uniform irradiation (FRP) and that of diffraction efficiency under holographic exposure is not straight‐ forward. In **Figure 11**, the final diffraction yield ηf is plotted as a function of the final monomer conversion *C*<sup>f</sup> .

**Figure 12** shows that no clear correlation exists between the final conversion *C*<sup>f</sup> achieved in homogeneous FRP and grating efficiency ηf .

The picture is completely different when the maximum rate of grating formation *R*η are plotted as a function of the corresponding maximum rate of monomer conversion *R*c. As can be seen in **Figure 13**, a monotonic curve is obtained despite the various photochemical reactions and photopolymerization kinetics of the 16 PS‐Co combinations.

**Figure 13.** Maximum rate of grating formation *R*η as a function of the maximum rate of conversion *R*c.

The faster is the monomer conversion *R*c, the faster is the building up of the diffraction grating *R*η. Therefore, as the holographic resin is always the same for all these experiments, the key role in the grating formation is the number of active radicals locally created and capable of initiating the polymerization. Indeed, the fast polymerization of the monomer mixture in the bright fringes of the interference pattern results in the fast formation of refractive index modulation in the medium. On **Figure 13**, it is clearly seen that two regimes are present, each one showing a quasi linear relation between *R*η and *R*<sup>c</sup> with a saturation effect occurring for *R*<sup>c</sup> higher than around 4 s‐1. At low *R*c, i.e., lower than 4 s‐1, the mass transport of reactants (PIS, monomers, fillers, etc.) needed to build the index modulation is not limited by a too fast jellification of the medium during polymerization and high refractive index modulation can be obtained. While at higher *R*c (i.e., >4 s‐1), the faster polymerization of the resin in the bright areas leads to a sooner freezing of the resin, preventing the mass transport effect needed for high index modulation building up, resulting in lower grating efficiency.

Thus, the coupling between PIS and holographic grating recording is not easy, and a fine tuning of the photonic parameters, holographic material, with a good comprehension of the photochemistry underlying the radical photogeneration is needed to tailor photopolymeriz‐ able systems to holographic recording, i.e., PIS‐resin couple. Moreover, during our work [131–133], it appears that holographic recording reveals differences in photoinitiating sys‐ tem reactivity that are not detected with classical RT‐FTIR measurements.
