Photoinduced ω-bond dissociation in the higher excited singlet (S2) and lowest triplet (T1) states of a benzophenone derivative in solution

Authors:

• Minoru Yamaji,
• Susumu Inomata,
• Satoru Nakajima,
• Kimio Akiyama,
• Seiji Tobita,
• Bronisław Marciniak

Abstract

Photochemical properties of photoinduced ω-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown to undergo photoinduced ω-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical (PTR) at room temperature. The quantum yield (Φ_rad) for the radical formation was found to depend on the excitation wavelength, i.e., on the excitation to the excited singlet states, S₂ and S₁ of BBPS; Φ_rad(S₂) = 0.65 and Φ_rad(S₁) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet state independent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (α_rad) of the C-S bond fission in the lowest triplet state (T₁) of BBPS was determined to be unity. The agreement between Φ_rad(S₁) and α_rad values indicates that the C-S bond dissociation occurs in the T₁ state via the S ₁ state due to a fast intersystem crossing from the S₁ to the T₁ state. In contrast, the wavelength dependence of the radical yields was interpreted in terms of the C-S bond cleavage in the S₂ state competing with internal conversion from the S₂ to the S ₁ state. The smaller value of Φ_rad(S₂) than that of Φ_rad(S₁) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S₂ state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematic energy diagram for the ω-bond dissociation of BBPS. © 2005 American Chemical Society.