Photoinduced intramolecular hydrogen atom transfer in 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)-benzothiazole studied by laser flash photolysis

2-(2-Hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl) benzothiazole (HBT) underwent hydrogen atom transfer to give the tautomer in both the singlet excited state and the triplet excited state. In the singlet excited state, the keto form produced by excited state hydrogen atom transfer underwent isomerization around the quasi-double bond to give the trans-keto form. In the triplet excited state, HBO and HBT were equilibrated between the enol form and the cis-keto form and the equilibrium constant was determined by triplet sensitization and quenching experiment by using laser ash photolysis in benzene at room temperature. From these results, we have revealed the energy diagram of the hydrogen atom transfer reaction of HBO and HBT both in the singlet excited state and the triplet excited state.