The photochemical properties of α-cleavage of C–O bond in highly excited triplet states (Tn 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 (Φrad) 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 (Φdec) of the decomposition in the dissociative highly excited triplet state (TR) 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 TR state was discussed.
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Photochemical & Photobiological Sciences
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