Steady-State, One-, and Two-Color Laser Flash Photolysis Studies of α-Bond Cleavage ofS-Acyl-4-phenylthiophenols in Solution

Abstract

Photochemical properties of alpha-cleavage of the C-S bond in excited states of p-biphenyl thioacetate and p-biphenyl thiobenzoate (Me-SBP and Ph-SBP) in solution are investigated using steady-state and laser flash photolyses in comparison with those of S-phenyl thiobenzoate, where the photo-Fries rearrangement was reported to be absent. Although Me-SBP and Ph-SBP decompose upon 254 nm photolysis in acetonitrile irrespective of the amount of the dissolved oxygen, no definite photoproducts due to the photo-Fries rearrangement were found. Laser flash photolysis (266 nm) of these molecules reveals the occurrence of the C-S bond cleavage in the excited state based on the observation of the formation of the biphenylylthiyl radical (BTR) in the transient absorption. Quantum yields (Phi(rad)) of the BTR formation were determined to be 0.20 and 0.15 for Me-SBP and Ph-SBP, respectively. Triplet sensitization of Ph-SBP using xanthone (XT) as a sensitizer shows that the lowest triplet (T(1)) state of Ph-SBP is dissociative for the C-S bond with an efficiency of >or.56. In contrast, triplet sensitization of Me-SBP using acetone as a sensitizer demonstrates the efficient formation of triplet Me-SBP, and the molar absorption coefficient of the triplet-triplet absorption was determined. No photochemical reactions are found in the T(1) state of Me-SBP. Upon 355 nm laser flash photolysis of the T(1) state of Me-SBP, the formation of BTR is confirmed in the transient absorption. This observation indicates the C-S bond cleavage in a highly excited triplet (T(n)) state of Me-SBP. The quantum yield (Phi(dec)) of the alpha-cleavage in the T(n) state of Me-SBP was determined to be 0.69. Photochemical features of Me-SBP and Ph-SBP are discussed from the viewpoint of the spin-multiplicity of the bond dissociative states.