Effects of localized triplet exciton on reactivity of photoinduced ω-bond dissociation in naphthyl phenyl ketones having π,π* lowest triplet (T1) states studied by laser flash photolysis

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl) benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S-0) to the excited singlet states, S-3, S-2, and S-1 of BNMPS; Phi(rad)(S-3) 0.77 and Phi(rad)(S-2) Phi(rad)(S-1) 1.0. By using triplet sensitization of p- phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T-1( pi,pi*) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S-1) and Phi(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T-1 state via the S-1 state via a fast intersystem crossing from the S-1 to the T-1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S-3 state competing with internal conversion from the S-3 to the S-2 state. The smaller value of Phi(rad)(S-3) than those of Phi(rad)(S-1) and Phi(rad)(S-2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S-3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S-1(n, pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.