New thioxanthone and xanthone photoinitiators based on silyl radical chemistry

The ability of a new efficient photoinitiator (TX-Si), based on the well-known thioxanthone chromophore linked to a disilylacetylene moiety and exhibiting a red-shifted absorption, to initiate both free radical polymerization and cationic polymerization is checked. A comparison with the parent compound (isopropylthioxanthone) is provided. High rates of polymerization and high final conversions are obtained. The migration of TX-Si out of the polymer film is noticeably reduced (3-fold factor) compared to that observed with ITX. A similar modification of the xanthone chromophore (XT-Si) also leads to an efficient cleavable photoinitiator and opens a door for the design of a new class of potentially interesting compounds for UV curing applications. As characterized by ESR spin trapping, TX-Si and XT-Si generate silyl radicals under light irradiation evidencing a Type I photoinitiator character. The analysis of the excited state processes through laser flash photolysis and molecular orbital calculations allows explaining the observed photochemical behavior.