Alpha-bond dissociation of p-phenylbenzoyl derivatives in the higher triplet excited state studied by two-color two-laser flash photolysis.

Photochemical properties in the lowest singlet and triplet excited states (S(1) and T(1)) and in the higher triplet excited states (T(n)) of p-phenylbenzoyl derivatives (PB-X) having C-O and C-S bonds were investigated in solution using a stepwise two-color two-laser flash photolysis technique. PB-Xs (X = OPh and SPh as a leaving group) undergo alpha-bond dissociation in the S(1) state, while the C-O bonds in PB-OH and -OMe were stable upon the 266-nm laser flash photolysis. The T(1)(pi,pi*) states of PB-X were efficiently produced during the 355-nm laser flash photolysis of PB-X in the presence of benzophenone as a triplet sensitizer. The T(1)(pi,pi*) states of PB-Xs deactivate to the ground-state without producing any intermediates. However, when PB-Xs (X = OPh and SPh) in the T(1) states (PB-X(T(1))) were excited upon the 430-nm laser flash photolysis, their disappearance was observed. These observations indicate that PB-X(T(1)) (X = OPh and SPh) is excited to PB-Xs in the T(n) states which decompose through the C-X bond cleavage. From the transient absorption measurements, quantum yields (Phi(dec)) of the disappearance of PB-X(T(1)) were determined while bond dissociation energies (BDE) of the C-X bonds of PB-X were calculated by computations. On the basis of the Phi(dec) and the BDE values, it was shown that the rates of the decomposition process of PB-X in the T(n) states were expressed in an energy gap low form as a function of BDE. Features of the C-X bond cleavage of PB-X in the T(n) states were discussed.