Benzophenones in the higher triplet excited states

Abstract

Transient phenomena of benzophenone (BP) in the higher triplet excited state (T_n) have been investigated by the two-colour two-laser excitation method. Triplet energy transfer from BP(T_n) to quenchers (Q) occurred within the duration of a laser pulse (5 ns) to give Q(T₁) with higher triplet energy than that of BP(T₁). The quantum yield of the triplet energy-transfer quenching of BP(T_n) by CCl₄ was found to be 0.0023 ± 0.0002 from the bleaching of the transient absorption of BP(T₁) and the absorbed photon number. It appears that internal conversion from BP(T_n) to BP(T₁) is the predominant process. The lifetimes (τ_Tn) of BP(T_n) and several substituted benzophenones (BPs) in the higher triplet excited state [BPs(T_n)] were estimated from the dependence of the Q concentration on the efficiency of the triplet energy-transfer quenching of BP(T_n) by Q, and found to be 110–450 ps, depending on the nature of the substituents on the BPs. The effect of the substituents on τ_Tn may be explained by the energy gap between the T_n and T₁ states, because the main deactivation pathway for BPs(T_n) is the internal conversion process. In contrast, the substituent effect on the lifetimes of BPs(T₁) cannot be explained by the energy gap law. The transient behaviour of Q(T₁) depends on the properties of the quencher. Sequential triplet energy transfer from Q(T₁) to BP occurred for p-dichlorobenzene and tert-butylbenzene as quenchers, while Q(T₁) reacted partly with Q to form triplet excimers (³Q₂*) for benzene, chlorobenzene, and o-dichlorobenzene as quenchers. When CCl₄ was used as the quencher, the homolytic cleavage of a C–Cl bond of CCl₄(T₁) occurred to give Cl˙ and Cl₃C˙ radicals.