Effects of a sulfur substituent on the behavior and generation mode of aroyloxyl radicals in the photolysis of diaroyl peroxides

Continuous light irradiation of bis(p-methylthiobenzoyl) peroxide (p-MeSBPO) at 300 nm in acetonitrile gave only p-methylthiobenzoic acid as a product. On pulsed laser photolysis the absorption spectrum due to intermediate p-methylthiobenzoyloxyl radicals was observed in the region 400–800 nm, and the radical yield was significantly decreased in the presence of 1,3-pentadiene or oxygen as a triplet quencher. The results indicate that p-MeSBPO is decomposed from the excited triplet state and the radicals exclusively undergo hydrogen-atom abstraction from the solvent. The origin of the effect is discussed on the basis of experimental and theoretical results.