Transient photocurrents induced by laser flash photolysis of aromatic carbonyl compounds in solution

Abstract

Photocurrents induced by 25 ns laser flashes (λ = 347.1 nm) were measured in order to elucidate the mechanism of the reaction of triplet excited benzophenone (BP) and BP derivatives with unsaturated compounds whose triplet energy E T is lower than that of BP compounds. It was found that the irradiation of BP compounds alone in various solvents gave rise to the formation of transient currents indicating the existence of exciplexes formed by triplet excited BP compounds and ground state solvent molecules. The quantum yields φ(i) of free ions increased with increasing dielectric constant ϵ of the solvent. The highest values of φ(i) (about 0.1) were obtained with acetonitrile solutions. Upon addition of unsaturated compounds φ(i) was reduced in all cases. If E T of the added compound was lower than E T of the BP compound, φ(i) decreased steadily with increasing additive concentration ( e.g. in the case of styrene). If, however, E T of the unsaturated compound was higher than E T of the ketones (such as in the cases of methyl methacrylate and acrylonitrile), φ(i) was only slightly decreased and levelled off with increasing additive concentration. Thus, it was concluded that free ions were formed stemming from the interaction of unsaturated compounds with triplet excited BP compounds. From this finding it was inferred that exciplexes with CT character are generated which precede the formation of free ions. The following ketones were used: benzophenone (BP), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 3,3′,4,4′-tetramethoxycarbonylbenzophenone (TMCB). As a standard reaction pertaining to the formation of photocurrents the reaction 3TMCB * + diethylaniline (DEA) was used. Its rate constant was determined as (7.5 ± 1) × 10 9 M −1 s −1.