Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed.

In this context, naphthalimide was selected as a possible candidate to act as a covalently linked photosensitizer for diaryliodonium salts [7]. Therefore, in the present paper, a series of five new photosensitive iodonium salts (noted 2-6 in Figure 1; synthesis described in Scheme 1) differing by the counter-anions and resulting from a coupling between both a naphthalimide chromophore and an iodonium moiety is proposed to improve the light absorption properties in the 350-380 nm range. Interestingly, Speedcure 938 i.e., bis-(4-tert-butylphenyl)-iodonium hexafluorophosphate is totally ineffective to initiate a polymerization at such wavelength without taking recourse to a photosensitizer. The approach presented in this work is interesting to generate initiating species through an intramolecular process (here a monocomponent system) instead of an intermolecular reaction in a two-component photosensitizer/diaryliodonium system. Especially in this work, the higher efficiency of the monocomponent system over the bicomponent system making using of the benchmark Speedcure 938 is clearly evidenced. The polymerization of a benchmark epoxy monomer i.e., (3,4-epoxycyclohexane)methyl 3,4-epoxycyclo-hexylcarboxylate (EPOX) is monitored by time resolved infrared spectroscopy. The performance of the proposed iodonium salts is compared to a commercial reference, namely Speedcure 938 (See Scheme 2), exhibiting a significant absorption only in the 230-300 nm range. A comparison of the performances obtained while using different counter anions is also proposed. The photochemical cleavage processes are studied by electron spin resonance experiments and molecular orbital (MO) calculations.
In this context, naphthalimide was selected as a possible candidate to act as a covalently linked photosensitizer for diaryliodonium salts [7]. Therefore, in the present paper, a series of five new photosensitive iodonium salts (noted 2-6 in Figure 1; synthesis described in Scheme 1) differing by the counter-anions and resulting from a coupling between both a naphthalimide chromophore and an iodonium moiety is proposed to improve the light absorption properties in the 350-380 nm range. Interestingly, Speedcure 938 i.e., bis-(4-tert-butylphenyl)-iodonium hexafluorophosphate is totally ineffective to initiate a polymerization at such wavelength without taking recourse to a photosensitizer. The approach presented in this work is interesting to generate initiating species through an intramolecular process (here a monocomponent system) instead of an intermolecular reaction in a two-component photosensitizer/diaryliodonium system. Especially in this work, the higher efficiency of the monocomponent system over the bicomponent system making using of the benchmark Speedcure 938 is clearly evidenced. The polymerization of a benchmark epoxy monomer i.e., (3,4-epoxycyclohexane)methyl 3,4-epoxycyclo-hexylcarboxylate (EPOX) is monitored by time resolved infrared spectroscopy. The performance of the proposed iodonium salts is compared to a commercial reference, namely Speedcure 938 (See Scheme 2), exhibiting a significant absorption only in the 230-300 nm range. A comparison of the performances obtained while using different counter anions is also proposed. The photochemical cleavage processes are studied by electron spin resonance experiments and molecular orbital (MO) calculations.

Results
The different iodonium salts 2-6 were first investigated for their photoinitiating ability for the cationic polymerization of epoxides in thin (See Figure 2) and thick (See Figure 3) samples upon irradiation with a LED at 365 nm. Remarkably, it can be observed that 3, 5, and 6 lead to good polymerization performances that are clearly better than that of the reference Speedcure 938 for which no polymerization occurred. These results are in agreement with their significant light absorption properties in the 350-400 nm region (See Figure 4). On the opposite, as the reference Speedcure 938 does not absorb for λ > 300 nm [5] so that no polymerization is possible upon irradiation at 365 nm with this compound. Conversely, the maximum absorption wavelength of 3, 5, and 6 is close to 345 nm with a molar extinction coefficient higher than 10,000 M −1 cm −1 so that the absorption is sufficient to efficiently initiate a polymerization process. From the theoretical calculations (See Figure 5), it can be observed that the naphthalimide chromophore strongly participates to the highest occupied molecular orbital (HOMO), destabilizing the HOMO level and

Results
The different iodonium salts 2-6 were first investigated for their photoinitiating ability for the cationic polymerization of epoxides in thin (See Figure 2) and thick (See Figure 3) samples upon irradiation with a LED at 365 nm. Remarkably, it can be observed that 3, 5, and 6 lead to good polymerization performances that are clearly better than that of the reference Speedcure 938 for which no polymerization occurred. These results are in agreement with their significant light absorption properties in the 350-400 nm region (See Figure 4). On the opposite, as the reference Speedcure 938 does not absorb for λ > 300 nm [5] so that no polymerization is possible upon irradiation at 365 nm with this compound. Conversely, the maximum absorption wavelength of 3, 5, and 6 is close to 345 nm with a molar extinction coefficient higher than 10,000 M −1 cm −1 so that the absorption is sufficient to efficiently initiate a polymerization process. From the theoretical calculations (See Figure 5), it can be observed that the naphthalimide chromophore strongly participates to the highest occupied molecular orbital (HOMO), destabilizing the HOMO level and decreasing the HOMO-LUMO (lowest unoccupied molecular orbital) gap in the iodonium salts 2-6. As a result of this, a bathochromic shift of the absorption spectra for 2-6 is observed compared to that of the reference Speedcure 938.
decreasing the HOMO-LUMO (lowest unoccupied molecular orbital) gap in the iodonium salts 2-6. As a result of this, a bathochromic shift of the absorption spectra for 2-6 is observed compared to that of the reference Speedcure 938.
The light absorption properties for the different iodonium salts originating from the iodonium salt and not from the counter anions, the difference of reactivity between 2-6 can be ascribed to the counter anion effect. Indeed, it is well established that low nucleophilicity counter anions are more favorable (SbF6 − ; PF6 − ; bis(trifluoromethane)sulfonamide) by preserving the cationic propagating centers compared to higher nucleophilic counter anions (Cl − ; TfO − ) [1,3,5].   decreasing the HOMO-LUMO (lowest unoccupied molecular orbital) gap in the iodonium salts 2-6. As a result of this, a bathochromic shift of the absorption spectra for 2-6 is observed compared to that of the reference Speedcure 938.
The light absorption properties for the different iodonium salts originating from the iodonium salt and not from the counter anions, the difference of reactivity between 2-6 can be ascribed to the counter anion effect. Indeed, it is well established that low nucleophilicity counter anions are more favorable (SbF6 − ; PF6 − ; bis(trifluoromethane)sulfonamide) by preserving the cationic propagating centers compared to higher nucleophilic counter anions (Cl − ; TfO − ) [1,3,5].

Discussion
It is well established that the iodonium salts can be cleaved (C-I bond) upon light irradiation leading to the formation of strong acids (r1) [1,3,5 and references herein] in agreement with a photoacid behavior: This behavior is observed here with 3, 5, and 6. Indeed, during the irradiation of the solutions of iodonium salts in presence of methyl red, appearance of the protonated form of this pH-indicator was clearly observed by UV-visible spectroscopy. This photoacid ability is also in full agreement with their initiating properties for the polymerization of epoxides [61].
To further investigate the cleavage process, molecular orbital calculations were carried out. From the two possible cleavages (noted A and B in Figure 5), the cleavage B is more favorable than the cleavage A due to a lower C-I bond dissociation energy (BDE): 49.7 vs. 57.4 kcal/mol for cleavage B and A, respectively. This latter result is also in agreement with the electron spin resonance experiments. Indeed, the ESR-ST spectrum obtained upon irradiation of 3 in the presence of PBN as a spin-trap agent in tert-butylbenzene showed the formation of an aryl (Ar • )/PBN radical adduct (hyperfine coupling constants hfcs: aN = 14.3 G and aH = 2.2 G; reference values in [62]).  . Figure 4. UV-vis absorption spectra for the most reactive iodonium salts (3, 5, and 6) in acetonitrile for a concentration of 1 × 10 −4 M.

Discussion
It is well established that the iodonium salts can be cleaved (C-I bond) upon light irradiation leading to the formation of strong acids (r1) [1,3,5 and references herein] in agreement with a photoacid behavior: This behavior is observed here with 3, 5, and 6. Indeed, during the irradiation of the solutions of iodonium salts in presence of methyl red, appearance of the protonated form of this pH-indicator was clearly observed by UV-visible spectroscopy. This photoacid ability is also in full agreement with their initiating properties for the polymerization of epoxides [61].
To further investigate the cleavage process, molecular orbital calculations were carried out. From the two possible cleavages (noted A and B in Figure 5), the cleavage B is more favorable than the cleavage A due to a lower C-I bond dissociation energy (BDE): 49.7 vs. 57.4 kcal/mol for cleavage B and A, respectively. This latter result is also in agreement with the electron spin resonance experiments. Indeed, the ESR-ST spectrum obtained upon irradiation of 3 in the presence of PBN as a spin-trap agent in tert-butylbenzene showed the formation of an aryl (Ar • )/PBN radical adduct (hyperfine coupling constants hfcs: aN = 14.3 G and aH = 2.2 G; reference values in [62]).

Discussion
It is well established that the iodonium salts can be cleaved (C-I bond) upon light irradiation leading to the formation of strong acids (r1) ( [1,3,5] and references herein) in agreement with a photoacid behavior: This behavior is observed here with 3, 5, and 6. Indeed, during the irradiation of the solutions of iodonium salts in presence of methyl red, appearance of the protonated form of this pH-indicator was clearly observed by UV-visible spectroscopy. This photoacid ability is also in full agreement with their initiating properties for the polymerization of epoxides [61].
To further investigate the cleavage process, molecular orbital calculations were carried out. From the two possible cleavages (noted A and B in Figure 5), the cleavage B is more favorable than the cleavage A due to a lower C-I bond dissociation energy (BDE): 49.7 vs. 57.4 kcal/mol for cleavage B and A, respectively. This latter result is also in agreement with the electron spin resonance experiments. Indeed, the ESR-ST spectrum obtained upon irradiation of 3 in the presence of PBN as a spin-trap agent in tert-butylbenzene showed the formation of an aryl (Ar • )/PBN radical adduct (hyperfine coupling constants hfcs: a N = 14.3 G and a H = 2.2 G; reference values in [62]).
The wavelength 405 nm is important in 3D printing and laser write experiments [63], however, the light absorption properties of the new proposed iodoniums is not high sufficient to initiate a polymerization process at this wavelength. Recently, the possibility to create charge transfer complexes (CTC) between electron rich amines (e.g., trimethylaniline (TMA) in Scheme 2) and electron poor iodonium salts has been reported in the literature (r2) [63]. These CTC can be photolyzed at 405 nm, leading to an efficient release of aryl radicals Ar (here Tol ) (r3) to initiate radical polymerization processes.
This approach has notably been used here in laser write experiments @405 nm for the 5/TMA system in trimethylolpropane triacrylate (TMPTA) used as a trifunctional acrylic monomer ( Figure 6). No polymerization was observed while using 5 or TMA alone, showing the requirement of this combination through a CTC. The wavelength 405 nm is important in 3D printing and laser write experiments [63], however, the light absorption properties of the new proposed iodoniums is not high sufficient to initiate a polymerization process at this wavelength. Recently, the possibility to create charge transfer complexes (CTC) between electron rich amines (e.g., trimethylaniline (TMA) in Scheme 2) and electron poor iodonium salts has been reported in the literature (r2) [63]. These CTC can be photolyzed at 405 nm, leading to an efficient release of aryl radicals Ar • (here Tol • ) (r3) to initiate radical polymerization processes.
This approach has notably been used here in laser write experiments @405 nm for the 5/TMA system in trimethylolpropane triacrylate (TMPTA) used as a trifunctional acrylic monomer ( Figure  6). No polymerization was observed while using 5 or TMA alone, showing the requirement of this combination through a CTC.

Synthesis of the New Iodonium Salts
The general synthetic pathway for the iodonium salts 2-6 is depicted in Scheme 1. The procedure is described in detail below. The procedure has been inspired from [64].

Synthesis of the New Iodonium Salts
The general synthetic pathway for the iodonium salts 2-6 is depicted in Scheme 1. The procedure is described in detail below. The procedure has been inspired from [64].

Photopolymerization Reactions Monitored by Real Time Fourier Transformed Infrared Spectroscopy (RT-FTIR)
The weight percent of the photoinitiator is calculated from the monomer content (w/w). The photosensitive thin formulations (~25 μm of thickness) were deposited on a BaF2 pellet under air for the CP of EPOX. The 1.4 mm thick samples of EPOX were also polymerized under air into a rounded plastic mold of ~7 mm diameter and 1.4 mm of thickness. For thin and thick samples, the evolution of the epoxy group content of EPOX was continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100, Tokyo Japan) at about 790 and 3700 cm −1 , respectively. The procedure used to monitor the photopolymerization profiles has been described in detail in refs [65,66].

UV-Visible Absorption Spectroscopy
The UV-Visible absorbance properties of the different iodoniums were studied by JASCO V730 UV-visible spectrometer (JASCO, Tokyo Japan). To demonstrate the formation of acid upon irradiation, methyl red was added as pH indicator as reported in [67].

Photopolymerization Reactions Monitored by Real Time Fourier Transformed Infrared Spectroscopy (RT-FTIR)
The weight percent of the photoinitiator is calculated from the monomer content (w/w). The photosensitive thin formulations (~25 µm of thickness) were deposited on a BaF 2 pellet under air for the CP of EPOX. The 1.4 mm thick samples of EPOX were also polymerized under air into a rounded plastic mold of~7 mm diameter and 1.4 mm of thickness. For thin and thick samples, the evolution of the epoxy group content of EPOX was continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100, Tokyo Japan) at about 790 and 3700 cm −1 , respectively. The procedure used to monitor the photopolymerization profiles has been described in detail in refs [65,66].

UV-Visible Absorption Spectroscopy
The UV-Visible absorbance properties of the different iodoniums were studied by JASCO V730 UV-visible spectrometer (JASCO, Tokyo Japan). To demonstrate the formation of acid upon irradiation, methyl red was added as pH indicator as reported in [67].

Electron Spin Resonance (ESR) Spin Trapping (ESR-ST)
Electron spin resonance-spin trapping experiments were carried out using an X-band spectrometer (Magnettech MS400, Magnettech, Berlin, Germany). The radicals were observed under nitrogen saturated media at room temperature. N-tert-Butylnitrone (PBN) was used as a spin trap agent in acetonitrile [66]. ESR spectra simulations were carried out using PEST WINSIM software.

Laser Write Experiments
A computer programmed laser diode with spot size around 50 µm was utilized for spatially controlled irradiation to produce specific 3D patterns using 5/TMA in TMPTA monomer. Finally, the printed patterns were analyzed through a numerical optical microscope (DSX-HRSU, OLYMPUS corporation, Tokyo, Japan) [66].

Molecular Modeling
Frontier Molecular Orbital calculations were performed by the Gaussian 03 suite of programs (Pittsburgh, PA, USA) [68,69]. The simulation of UV absorption spectra and the calculations of the C-I bond dissociation energies (BDE) were done at the density functional theory level (UB3LYP/LANL2DZ) on the relaxed geometries calculated at this level.

Conclusions
In the present paper, the design and the synthesis of a new series of five iodonium salts combining within a unique chemical structure the naphthalimide chromophore and the iodonium scaffold are presented. Compared to the reference structure Speedcure 938, the proposed iodoniums are more reactive at 365 nm to initiate the cationic polymerization of epoxides. Bis(trifluoromethane)sulfonimide was found as the most interesting counter anion, the best photoinitiating abilities in both radical and cationic polymerizations being determined for the iodonium salt possessing this counteranion. The development of new 3D printing resins @405 nm based on the cationic polymerization of epoxides and based on the elaboration of a CTC between an iodonium salt and an amine has allowed to polymerize at longer wavelength than what is theoretically possible on the simple basis of the absorption spectrum of the iodonium salt. The formation of CTC to red-shift the absorption spectrum of a photoinitiator and to allow the polymerization at longer wavelength is an efficient tool that deserves to be further investigated. Especially, by mean of this innovative approach, benchmark UV photoinitiators that do not exhibit an absorption in the visible range could be revisited in the context of this recent approach.