Silyloxy-substituted anthraquinones as Type I photoinitiators for visible light-induced radical polymerization

Among the photoinitiators that can be activated using ultraviolet-visible light-emitting diodes, Type I photoinitiators often contain sulfur, nitrogen, and phosphorus and may affect human health and the environment, whereas Type II photoinitiators typically contain only carbon, hydrogen, and oxygen, as exemplified by anthraquinone derivatives, but require coinitiators. Hence, sulfur-, nitrogen-, and phosphorus-free Type I photoinitiators are highly desirable. In our pursuit of such photoinitiators, we examined the ability of different silyloxyanthraquinones to initiate radical photopolymerization upon irradiation at 405 nm and found that some achieved high conversion in the absence of a coinitiator. The initiation mechanism was probed by analyzing the photolysis products, electron spin resonance spectroscopy, and isotope labeling experiments. The 1-substituted silyloxy compounds acted as Type I photoinitiators, generating isopropyl radicals as the initiating species. These compounds are among the very few known Type I photoinitiators with an anthraquinone skeleton that are sensitive to 405 nm visible light. The findings of this study facilitate the design of clean initiators free of the sulfur, nitrogen, and phosphorus commonly present in other Type I initiators. Silyloxy-substituted anthraquinones were evaluated as Type I photoinitiators for the visible light-induced radical polymerization of an acrylate monomer. Structure–activity correlations were established, and a mechanistic explanation of the observed activity trend was proposed. Isopropyl radicals were generated from compounds with a silyloxy group at position 1 and acted as the initiating species. The 1-substituted silyloxy anthraquinones synthesized herein are among the few Type I photoinitiators that have an anthraquinone skeleton and are sensitive to visible light (405 nm).