International audienceA strong drawback of the photoinitiators of cationic polymerization or photoacids is the photosensitivity for short and energetic wavelengths preventing their general use (specialized photochemical equipment with safety concerns must be used). In the present paper, a novel iodonium salt bearing a naphthalimide moiety (naphthalimide-Ph-I+-Ph) is proposed as a one-component photoinitiator/photoacid operating at longer and safer wavelengths (i.e. violet light emitting diodes at 365, 385 nm and 395 nm). It allows the polymerization of various formulations (methacrylates, epoxides, vinyl ethers). A high reactive function conversion for multifunctional monomers can be achieved: e.g. 50% for a diepoxide under air, >90% for a divinylether (with a very high rate of polymerization Rp), almost 100% for an epoxide/vinyl ether blend (very high Rp) under air, and 85% for methacrylates (high Rp) in laminate (43% under air). These results are above the ones obtained with a thianthrenium salt chosen as a reference e.g. a lower epoxy conversion ∼25% and a clearly lower Rp for the diepoxide polymerization. ESR-spin trapping, laser flash photolysis, steady state photolysis and molecular orbitals calculations support the formation of Ph˙ and naphthalimide-Ph-I˙+ as well as the generation of H+, thereby explaining the photoinitiation step mechanism

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.