Mechanistic Investigation of a Dual Bicyclic Photoinitiating System for Synthesis of Organic–Inorganic Hybrid Materials

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Introduction
Following the pioneering work of G. Ciamician 1 , light is nowadays a significant sustainable energy source for the triggering of chemical processes, especially material synthesis. Light offers several major advantages over classic thermal sources, such as spatial and temporal control, ease of use, cost reduction, room temperature process… Polymeric materials can be easily synthesized under light using various processes such as free radical photopolymerization (FRP) 2-8 , cationic ring opening photopolymerization [2][3][4][5][6][7][9][10][11][12] , anionic photopolymerization [4][5]7,13 , thiol-ene step growth reaction [14][15] , photoclick polymerization [16][17] or hybrid organic-inorganic curing [18][19][20][21][22] . Several types of photoinitiating systems (PISs) have been developed in order to optimize the conversion of light into chemical energy and to improve the initial quantum yield in reactive species.  Among the materials formed under light, organic/inorganic hybrid materials present interesting properties due to the elegant combination of the properties of organic polymers and inorganic materials through the in-situ formation of interpenetrated network or generation of chemical bonds between organic and inorganic components. [18][19][20][21][22] They are conventionally synthesized through a two-step process: a first sol-gel thermal reaction produces a liquid organic based polysilicate network which is further photopolymerized, leading to a solid cross-linked hybrid system. Beside this approach, an innovative process has been proposed in which a concomitant organic-inorganic photopolymerization occurs in a one-step water-and solvent free approach. UV light triggers the generation of acids or bases capable of catalyzing simultaneously both sol-gel reaction and organic photopolymerization. [34][35][36][37] The fabrication of such hybrid materials generally involves both the photopolymerization of multifunctional (meth)acrylates or epoxides, and the photocatalyzed hydrolysis-condensation of metal alkoxide inorganic precursors (i.e. alkoxysilanes). In most cases, the widely available (meth)acrylate monomers (or oligomers) and free radical photoinitiators make poly(meth)acrylates-based hybrid materials much more attractive in the photocuring field. As a matter of course, the photoinitiating system responsible for simultaneous generation of radical and proton initiating species becomes definitely essential in the design and synthesis of poly(meth)acrylates-based hybrid materials.
Due to the lack of light absorption in the visible light region, the photoacid generator which is often an iodonium or sulfonium salt, seems to be the limits for developing poly(meth)acrylatesbased hybrid materials under more environmental and sustainable light (i.e. visible light LEDs).
Recently, a visible light photoacid generator with fast protonic acid release behavior has been reported by combining isopropylthioxanthone (ITX) with an iodonium salt (IOD + ). 37 The addition of a thiol (RSH) to a blend of ITX and IOD + is able to initiate simultaneously the free radical photopolymerization of dimethacrylates and the cationic polymerization of epoxides under visible light LED with high monomer conversion rates. 27 This innovative three-component photoinitiating system is very promising to design and prepare poly(meth)acrylate-based organic-inorganic hybrid materials. Nevertheless, the photochemical mechanism for efficient production of radical and protonic acid initiating species by photolysis of this three-component photoinitiating system is still unclear up to now. The aim of this paper is to understand the photochemical mechanism of this innovative dual-mechanistic photoinitiating system by Laser Flash Photolysis. Primary photoreactions of ITX with either IOD + or RSH are first studied. The resulting ITX photoproducts are found to be involved in secondary dark reactions with these co-initiators, leading to additional formation of reactive species such as radicals that are evidenced by LFP. The photogenerated protons are detected at the microsecond scale for the first time with a proton-sensitive molecular probe and the proton production quantum yield is also calculated. These mechanistic considerations are finally correlated to the synthesis results of poly(meth)acrylates, polysiloxane and the corresponding hybrid materials.

Chart 1. Molecular structures of the compounds used in this study.
Absorption and fluorescence spectra were recorded at room temperature with an Analytik Jena Specord 210 UV-visible spectrometer and a Horiba Jobin Yvon Fluoromax 4 spectrofluorimeter respectively. Compounds were dissolved in acetonitrile and placed into 1 cm  1 cm quartz cuvette.
The fluorescence quantum yield (Φfluo) of ITX was determined with quinine sulfate dihydrate in 1N H2SO4 as reference (Φfluo = 0.546 38 ), using a conventional procedure. 39 Maximal optical density above 300 nm was maintained below 0.1 in order to avoid inner filter effects. Two excitation wavelengths (360 and 375 nm) and two slit sizes (1 and 2 nm) were used and the results were averaged on these four experiments.
The relaxed triplet state energy of ITX was calculated in the same manner as reported elsewhere [40][41] . Redox potentials were determined by cyclic voltammetry, as described in Ref  from the ratio between the incident energy and that of one photon at 355 nm. The number of generated protons Nprotons was monitored through the decrease of the 520 nm band of QR, assuming a quantitative relation between QR disappearance and H + production in the considered range.
Finally, quantum yield was calculated as the ratio of these two quantities (Eq. 4). Results were averaged on three repetitions and the associated standard deviation was estimated.
ITX based photoinitiating systems were used to trigger the free radical photopolymerization and the sol-gel process. The concentration of ITX was 2.5 mol% with respect to the monomer and/or organic precursor. For two-component systems, 1.5 mol% of co-initiator (IOD + or RSH) was contained. For three-component systems, 1.5 mol% of co-initiator (IOD + or RSH) and 0.15 mol% of another component (RSH or IOD + , respectively) were both incorporated. To prepare the hybrid materials, a mixture of PDMOS (80 mol%) and SR348C (20 mol%) was used and all other mol% were calculated on the basis of the total amount of PDMOS and SR348C.
Photopolymerization kinetics and monomer conversions were followed by real-time FTIR spectroscopy using a Vertex 70 from Bruker Optics operating in a rapid scan mode and being equipped with a nitrogen liquid cooled MCT detector. [45][46] The sampling interval was 0.1 s and the resolution was 4 cm -1 . The irradiation was provided by a 395 nm LED device (Roithner LaserTechnik) at 20 mW.cm -2 under air. The intensity of the LED was measured using a calibrated fiber optic spectrometer (Ocean Optics, USB4000). The photopolymerization reaction was carried out under air at room temperature, using polypropylene film and BaF2 pellet. Room humidity was kept constant at almost 43 -45% by a hygrometer and consistent with previous work. 37 Each experiment was repeated at least three times to ensure a good reproducibility. Conversion of SR348C was followed by the decrease of the area of its twisting vibration band at 1310 cm -1 and calculated from: A0 and At represent respectively the absorption band area before exposure and at exposure time t. Areas were corrected with a reference band (at 1510 cm -1 ) in order to avoid any error from displacement of the 1310 cm -1 band. The absorbance decrease of the CH3 symmetric stretching vibration band centered at 2848 cm -1 (distinctive from CH3 methanol vibration modes) was monitored to follow the methoxysilyl hydrolysis ratio in PDMOS. The maximum rate of conversion Rc max was determined as the maximum of the first derivative of the conversion with time.

Results and Discussion
Isopropylthioxanthone (ITX) has been considered as one of the most important photoinitiator in the field of photopolymerization. 33 It has been reported to act as both efficient photosensitizer for iodonium salts to trigger the cationic polymerization or sol-gel process, and effective type II photoinitiator for free radical polymerization, owing to its attracting photochemical and redox properties. Table 1 shows the photochemical and redox properties of ITX which were found very close to those reported in the literature. [47][48][49][50][51][52][53][54][55][56][57] Due to the known short lifetime of the first excited singlet state (τS) and the high intersystem quantum yield (ΦISC), ITX is expected to react from its lower triplet state which can be detected using laser flash photolysis. The transient absorption spectrum of ITX in argon saturated acetonitrile ( Figure S1) exhibits a photobleaching at 385 nm and two peaks at 310 and 640 nm corresponding to the triplet -triplet absorption.

ITX / IOD + as primary photoreaction
The reaction of ITX triplet state ( 3 ITX) with strong electron acceptors such as iodonium salts should result in ITX radical cation (ITX •+ ). Therefore, the photoinduced electron transfer reaction between 3 ITX, cumenyl(tolyl)iodonium tetrakis(pentafluorophenyl)borate (IOD + , Chart 1) was firstly evaluated to clearly address the roles of each component in generation of proton and radical species. The corresponding Gibbs free energy can be calculated from the Rehm-Weller equation (Eq. 6) 58-59 : where Eox(D) refers to the oxidation potential of the electron donor D and Ered (A) to the reduction potential of the electron acceptor A. C stands for a coulombic term which is often neglected in polar solvents such as acetonitrile. Ered(IOD + ) was measured as -0.70 V / SCE, in agreement with reported values for iodonium salts [60][61][62] . The reaction was found to be exergonic with ΔGET = -0.50 eV.
The kinetic of the reaction between 3 ITX and IOD + was studied by Laser Flash Photolysis (LFP) at 640 nm. By increasing the concentration of IOD + , 3 ITX was found to decay faster. The quenching rate constant kq 3ITX/IOD+ was determined from the Stern-Volmer plot as 6.210 9 L.mol -Interestingly, when the triplet state signal at 310 nm and 640 nm was progressively quenched by increasing amounts of IOD + , a long-lived transient could be detected at 340 nm and around 800 -850 nm (Figure 1). The electron transfer reaction being exergonic, this new transient was confidently attributed to the radical cation of ITX (ITX •+ ). A similar band has also been reported above 800 nm in the literature. 54 It should be noted that weak transient signal between 400 and 450 nm is traditionally attributed to cation radicals originating from TX derivatives. [62][63][64][65][66] Such a band is also visible on Figure 1, albeit at much lower intensity than those at 340 and 800 -850 nm.
Therefore, it appears reasonable to study the behavior of ITX •+ at 340 nm and 800 -850 nm.  These results clearly reveal the photocyclic behavior of the ITX / IOD + / RSH photoinitiating system. Radicals are produced by the reaction between 3 ITX and IOD + followed by a secondary dark reaction between ITX •+ and RSH which produces both radicals and protons (Scheme 1). This [RSH] combination is expected to initiate both free radical polymerization and sol-gel process, enabling the simultaneous one-pot synthesis of the organic-inorganic hybrid material.

ITX / RSH as primary photoreaction
Due to positive electron transfer Gibbs free energy (0.54 eV), neat electron transfer between 3 ITX and RSH can be excluded. One should then consider the well-known photoreduction of aromatic ketones by hydrogen donors (such as thiols), which could occur through coupled electron/proton transfer. 52,67 In the present case, this reaction led to the formation of ketyl ITXH • and thiyl RS • radicals. Indeed, 3 ITX signal at 640 nm was efficiently quenched when increasing amounts of RSH and the quenching rate constant kq 3ITX/RSH was determined as 1.110 9 L.mol -1 .s -1 . Interestingly, this value is much higher than for aliphatic thiols 47,68 , and in the same order of magnitude for aromatic thiols. 52  Moreover, it is shown that the photoreaction of 3 ITX with RSH and the secondary dark reaction of ITXH • with IOD + also lead to a photocyclic behavior in which radicals and protons are generated, enabling the synthesis of hybrid materials (Scheme 2).

A dual bicyclic initiating system
The previous results showed that the three place which are both able to initiate the polymerization, as recently reported for different systems. 70 It is then possible to favor one of the two pathways by finely adjusting the relative concentrations of IOD + and RSH, knowing the rate constants in the medium.

Scheme 3.
Proposed mechanism of reactions for the photocyclic initiating system involving ITX, IOD + and RSH.

Photogeneration of proton
The two different mechanisms described above show that whatever the photoreaction which primarily occurred ( 3 ITX / IOD + or 3 ITX / RSH), radicals were undoubtedly formed. Protons should also be generated in both cases, which is a hypothesis that needs to be verified. Detection of photogenerated protons at the microsecond scale by LFP had been already reported in the literature. 71 Therefore, in order to prove the photogeneration of acid, we decided to apply a similar method. It should be noted that the detection of protons generated within the second reaction of a three-component photocyclic system has never been reported up till now.
Quinaldine Red (QR) was used as the molecular probe to detect the release of protons. QR exhibits an absorption maximum at 520 nm, far above from the absorption range of ITX and without overlap with the spectra of the transient species ( Figure S4). UV-vis spectra were recorded before and after laser irradiation (Figure 4b), confirming the decrease of the 520 nm band of QR after laser excitation, and the growing shoulder at 365 nm was attributed to QRH + .

20
The same behavior is also observed at 520 nm for photocycle (b) (Figure 5a). UV-visible spectra show a shoulder similar to that previously observed (Figure 5b). It can be conclude that the photogeneration of acid actually occurs through the two photocyclic pathways. UV-vis spectra shown in Figures 4 and 5 were obtained with the same number of laser pulses.
However, it clearly appears that the photolysis signal of QR at 520 nm was lower for photocycle (a) as compared to (b) (Figures 4 and 5). It suggests that the photoacid generation efficiency is higher when 3 ITX first reacted with IOD + . In order to quantify this difference in reactivity, proton production quantum yield ΦH+ was estimated by steady-state spectroscopic measurements. Values of ΦH+ = 0.12 (standard deviation σ = 0.012) and 0.017 (σ = 0.003) for photocycles (a) and (b), respectively, confirmed the differences observed by LFP. Pathway (a) is then almost 7-times more efficient than pathway (b). This may be explained by a better reactivity of ITX •+ with RSH compared to ITXH • with IOD + in acetonitrile, as attested by the rate constants of these reactions (1.210 9 and 10 6 M -1 .s -1 respectively).

Application of dual bicyclic initiating system for synthesis of organic-inorganic material
The ability of the bicyclic system to generate initiating radicals and protons simultaneously can be employed to perform simultaneously a free radical photopolymerization (FRP) and a photoacid catalyzed sol-gel reaction (PSG) under LED at 395 nm. A difunctional methacrylate monomer (SR348C) was selected for attesting radical initiation of organic polymerization and a poly(dimethoxysiloxane) sol-gel precursor (PDMOS) was used for proton-catalyzed photoinduced sol-gel process (Chart 1).
The diffusion rate constant kdiff in these monomers can be estimated according to the Stokes-Einstein equation. 72 Values of 510 6 and 10 9 M -1 .s -1 were found, respectively, for SR348C and PDMOS. Because kq ITX/IOD+ and kq ITX/RSH measured in acetonitrile are both greater than these values, they can be considered as equal in SR348C and PDMOS. Therefore, the two photochemical pathways were differentiated by introducing ten times more (in mol%, based on the monomer) of one of the co-initiators with respect to the second one. In SR348C, the dark reaction rate constants for photocycles (a) and (b) are greater than (or almost equal to) kdiff and these reactions are also diffusion-limited. The same holds true for pathway (a) in PDMOS. However, the dark reaction in pathway (b) is clearly not limited by the diffusion as k ITXH•/IOD+ is lower than kdiff.
For sake of comparison, the two-component systems ITX/IOD + or ITX/RSH were first analyzed.
The conversion profiles of the acrylate monomer during light exposure under air are displayed in Figure 6. Final conversions and maximum polymerization rates are collected in Table 2. Final conversions were found to be low for these two-component systems, most probably because of a relatively low production of radicals. In addition, aryl radicals produced from the interaction ITX/IOD + are very sensitive to molecular oxygen, leading to inactive peroxy radicals (kO2 ≈ 10 10 M -1 .s -1 73 , also limited to kdiff). On the contrary, thiyl radicals are known to overcome oxygen inhibition in free radical polymerization. [14][15][74][75] This could explain the difference between the two-component systems. Therefore, more ITX is still available for light absorption, leading to an increased reactivity of photocycle (b). The efficiency of the photosol-gel reaction was studied by monitoring the conversion of the SiOCH3 groups of PDMOS. In the absence of IOD + , only 29% of the alkoxy functions were hydrolyzed after 200 s without IOD + (Figure 7). This 29% hydrolysis ratio observed in presence of only RSH may be directly due to residual water present in the reaction medium. By contrast, the presence of IOD + led to fast and efficient hydrolysis. Any of the two photocycles (a) or (b) led to a fast and efficient hydrolysis. In fact, the production of protons after reaction of 3 ITX with RSH relies on an electron transfer from ITXH • to IOD + followed by a rapid deprotonation of ITXH + (photocycle (b)). In the case of photocycle (a), any hydrogen donor (such as water or CH3OH liberated from PDMOS during the process) can react with ITX •+ to produce a proton in photocycle (a). This was the reason why two-and threecomponent systems present similar results for pathway (a).
Comparing the two photocycles, it appears that pathway (a) led to a faster hydrolysis of the PDMOS. This can be related to the value of acid quantum yield of 0.12 determined previously. As These results nicely illustrate the conclusion obtained from the previous mechanistic study, and the successful synthesis of both organic and inorganic networks demonstrates the simultaneous generation of free radicals and protons through the bicyclic photoinitiating system. Moreover, the ability of this system to mediate the one-pot light-induced synthesis of organic-inorganic hybrids has also been clearly established.

Conclusions
The photochemical mechanism of a new bicyclic photoinitiating system for concomitant synthesis of organic-inorganic hybrid materials under LED irradiation was elucidated. Triplet state of ITX can primary react with an iodonium salt IOD + or a tetrafunctional thiol RSH, and the ITX photoproduct (ITX •+ or ITXH • ) further reacts (respectively RSH and IOD + ) to increase the formation of acid and radicals. The proton generation was assessed at the microsecond scale with a proton-sensitive probe, and the acid quantum yield was also calculated. Free radical photopolymerization of dimethacrylate monomer and sol-gel reaction were performed using this photocyclic initiating system. Polymerization results were in good agreement with the conclusions drawn from mechanistic study. Finally, a hybrid material was successfully produced, attesting the dual initiating character of the bicyclic system. UV-vis absorption spectra of ITX / IOD + and ITX / IOD + / RSH systems before and after laser irradiation; photobleaching kinetics at 385 nm of ITX, ITX / RSH and ITX / RSH / IOD + systems; UV-vis absorption spectra of protonated and non-protonated Quinaldine Red. This material is available free of charge via the Internet at http://pubs.acs.org.