Transient absorption spectra and lifetimes of benzophenone ketyl radicals in the excited state

The absorption spectrum of benzophenone ketyl radicals in the D-1 excited state (BPH.(D-1)), 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.(D-0)), was directly observed using nanosecond-picosecond two-color two-laser flash photolysis. The whole spectral shape of BPH.(D-1) 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 D-1 excited state (BPDH.(D-1)) 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.(D-1) 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 D-1-state lifetime. The decay processes of the D, state including the radiative and nonradiative relaxation processes and the chemical reaction are discussed quantitatively.