Substituent Effects on Photoinitiation Ability of Coumarin-Based Oxime-Ester Photoinitiators for Free Radical Photopolymerization

International audienceIn this paper, a series of coumarin chromophore-based oxime-esters was designed and synthesized as visible light photoinitiators (PIs). Interestingly, upon exposure to irradiation of a LED at 405 nm, the investigated oxime-esters (OXEs) could undergo a direct photocleavage, followed by decarboxylation, generating active free radicals capable to effectively initiate acrylate polymerization. Seven of the 10 investigated OXEs have never been synthesized in the literature. Markedly, the new proposed structures exhibit also thermal initiation ability and can be used as dual photo and thermal initiators. In addition, the oxime-ester system was also paired with an iodonium salt and exhibited a higher efficiency in free radical polymerization (FRP). The chemical mechanisms and the structure/reactivity/efficiency relationships were investigated through different techniques including real-time Fourier Transform Infrared Spectroscopy (RT-FTIR), UV-visible absorption spectroscopy, fluorescence (time-resolved or steady state), cyclic voltammetry as well as molecular modelling calculations. As a proof of their remarkable polymerization performance, the new OXEs were also used at 405 nm for direct laser write applications