Naphthoquinone-Based Oxime Esters for Free Radical Photopolymerization under Sunlight or a Blue Light-Emitting Diode

Free radical polymerization initiated under visible light or sunlight remains one of the major challenges in the photopolymer chemistry area. Because of the low light intensity of modern light sources (light-emitting diodes, LEDs) and the mismatch of their absorption spectra with that of irradiation sources, commercial or usual photoinitiators often exhibit poor polymerization efficiency under blue light or sunlight. In this work, eight naphtho[2,3-d]thiazole-4,9-dione benzaldehyde-based oxime esters were developed as type I photoinitiators and in multicomponent photoinitiating systems with good light absorption properties in the visible region, especially at 405 and at 450 nm. Compared to the benchmark photoinitiator (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, TPO), some of our initiators furnished similar or better photoinitiation performance than the reference photoinitiator upon exposure to LED@405 nm. Compared to TPO, most of the newly proposed photoinitiators used in multicomponent photoinitiating systems could furnish performance on par with that of the reference system upon exposure to LED@405 nm and LED@450 nm. Among the most interesting results of this study, good final acrylate function conversions could be obtained during the free radical polymerization of trimethylolpropane triacrylate within 1 h of sunlight exposure and by using our type I photoinitiators in solar multicomponent photoinitiating systems when combined with iodonium salt and aromatic amine.