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

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Abstract

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.

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