Transient Absorption Spectra and Lifetimes of Benzophenone Ketyl Radicals in the Excited State

The absorption spectrum of benzophenone ketyl radicals in the D1 excited state (BPH•(D1)), generated by hydrogen abstraction of triplet benzophenone from cyclohexane as a solvent and the sequential excitation of the benzophenone ketyl radical in the ground state (BPH•(D0)), was directly observed using nanosecond−picosecond two-color two-laser flash photolysis. The whole spectral shape of BPH•(D1) with peaks at 350 and 480 nm was detected for the first time. The absorption spectra and lifetimes were obtained for a series of ketyl radicals (BPDH•) in the D1 excited state (BPDH•(D1)) of benzophenone derivatives (BPDs) such as 4-methyl-, 4,4‘-dimethyl-, 4-methoxy-, 4,4‘-dimethoxy-, 4-trifluoromethyl-, 4-fluoro-, 4,4‘-difluoro-, 4-chloro-, and 4-bromobenzophenones. The absorption spectra of BPDH•(D1) were found to be significantly affected by the number and electronic character of the substituents on the phenyl ring. BPDH• which has a large conformational change between the ground and emitting states showed a shorter D1-state lifetime. The decay processes of the D1 state including the radiative and nonradiative relaxation processes and the chemical reaction are discussed quantitatively.