Dynamics and mechanism of radical formation in a highly sensitive oxime photoinitiator as revealed by time-resolved absorption and EPR measurements

Reaction dynamics and mechanism of radical formation of a photoinitiator, Irgacure OXE01, were investigated by transient absorption spectroscopy and time-resolved EPR measurements. Femtosecond transient spectroscopy revealed that the S1 state underwent ultrafast intersystem crossing and N-O bond cleavage with a time constant of 1.6 ps, resulting in the production of benzoyloxyl and iminyl radicals. The T1 state also yielded benzoyloxyl and iminyl radicals with a time constant of 300 ps. These radicals decreased in several ns time range by the geminate recombination in the radical pair in competition with the diffusional dissociation into free radicals. Time-resolved EPR measurement indicated that the radical pair produced from the T1 state effectively led to the production of the free radicals. These results strongly suggested the attractive potential via the exchange interaction between the two radicals in the pair suppressed the dissociation process of the radical pair.