| Abstract
| - Photochemical properties of photoinduced ω-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) insolution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown toundergo photoinduced ω-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical(PTR) at room temperature. The quantum yield (Φrad) for the radical formation was found to depend on theexcitation wavelength, i.e., on the excitation to the excited singlet states, S2 and S1 of BBPS; Φrad(S2) = 0.65and Φrad(S1) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet stateindependent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (αrad) of theC−S bond fission in the lowest triplet state (T1) of BBPS was determined to be unity. The agreement betweenΦrad(S1) and αrad values indicates that the C−S bond dissociation occurs in the T1 state via the S1 state dueto a fast intersystem crossing from the S1 to the T1 state. In contrast, the wavelength dependence of theradical yields was interpreted in terms of the C−S bond cleavage in the S2 state competing with internalconversion from the S2 to the S1 state. The smaller value of Φrad(S2) than that of Φrad(S1) was proposed tooriginate from the geminate recombination of singlet radical pairs produced by the bond dissociation via theS2 state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematicenergy diagram for the ω-bond dissociation of BBPS.
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