Carbazole-fused coumarin based oxime esters (OXEs): efficient photoinitiators for sunlight driven free radical photopolymerization

Seventeen carbazole-fused coumarin-based oxime esters were developed as monocomponent and photocleavable (Type I) initiators of polymerization displaying excellent light absorption properties in the visible range. Compared to the benchmark photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), some of them (namely OXE-1 and OXE-5) could furnish photoinitiation performance on par with that of the reference compound upon irradiation with a LED at 405 nm. Photoinitiation mechanism of these photoinitiators is proposed, supported by theoretical calculations, the detection of CO2 during photopolymerization by mean of Fourier transform infrared spectroscopy (FTIR) and the detection of radical species by electron spin resonance (ESR). Through direct laser write, different objects exhibiting an excellent spatial resolution could be obtained. Parallel to the photoinitiating ability, the different photoinitiators also showed a thermal initiation behavior, meaning that these structures can serve both as thermal-or as photo-initiators on demand. Due to the high sensitivity of these structures to sunlight, OXE-1 and OXE-5 were also investigated as solar photoinitiators (reactive also in Central Europe in winter) and excellent monomer conversions could be obtained within one hour using a multifunctional acrylate monomer (Ebecryl 605). To the best of our knowledge, these two oxime esters constitute the first examples of sunlight activable oxime esters ever reported in the literature