New thioxanthone and xanthone photoinitiators based on silyl radical chemistry

The ability of a new efficient photoinitiator (TX–Si), based on the well-known thioxanthonechromophore 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 xanthonechromophore (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.