Bond-cleavage mode of O-acyl anthracene-9-methanol derivatives in the singlet excited state and their abilities to initiate radical and cationic photopolymerizations

Substituent effects on the bond-cleavage mode in the singlet excited state as well as on the initiation ability for photopolymerization were investigated on the title anthracenemethanol derivatives (1). Analysis of substituent effects on the excited-state reactivity and product distribution revealed that intramolecular electron transfer in 1 having the aroyl group is responsible for its excited-state deactivation while the heterolytic and selective CH2O bond cleavage is induced on forming a singlet exciplex with some charge-transfer character. It was suggested that substituted benzoate ion-bridged anthrylmethylcarbonium ion is a key intermediate in the photodecomposition process of aroyl-substituted derivatives. On the other hand, less electron-withdrawing acetyl-substituted derivative undergoes the heterolytic CH2O bond and homolytic CH2OC(O) bond cleavages to form geminate anthrylmethyl carbonium-acetate ion pair and anthrylmethyloxyl-acetyl radical pair, respectively. An examination of the ability of 1 to initiate radical and cationic photopolymerizations allowed us to propose that O-anisoyl anthracene-9-methanol is a likely candidate for novel hybrid-type photoinitiator.