Stepwise two-color laser photolysis studies of α-cleavage in highly excited triplet states of α-acyl-4-phenylphenols

Abstract

The photochemical properties of α-cleavage of C–O bond in highly excited triplet states (T_n with n > 2) of p-biphenyl acetate and p-biphenyl benzoate (Me-OBP and Ph-OBP) in solution were investigated in comparison with those in the lowest excited singlet and triplet states by using single laser and sequential two-color two-laser photolysis techniques. Upon 266 nm laser photolysis of Me-OBP and Ph-OBP, occurrence of C–O bond cleavage in the excited singlet state was recognized from the observation of the formation of p-phenylphenoxy radical (PPR) in the transient absorption. The quantum yields (Φ_rad) of the PPR formation were determined to be 0.29 and 0.24 for Me-OBP and Ph-OBP, respectively. Triplet sensitization using acetone (Ac) provided efficient formation of the lowest triplet states (T₁) of Me-OBP and Ph-OBP, and the molar absorption coefficients of the triplet–triplet absorption were determined. No photochemical reactions were found in the T₁ state. Upon 355 nm laser flash photolysis of the T₁ states of Me-OBP and Ph-OBP, formation of PPR accompanied with decomposition of the triplet state was confirmed in the transient absorption. This observation indicated that α-cleavage proceeds in the highly excited triplet state. The quantum yields (Φ_dec) of the decomposition in the dissociative highly excited triplet state (T_R) were determined to be 0.25 and 0.15 for Me-OBP and Ph-OBP, respectively. The reaction mechanism for α-bond cleavage in the T_R state was discussed.