Solvent effects on rates of photochemical reactions of rose bengal triplet state studied by nanosecond laser photolysis

Bimolecular rate constants of energy transfer, electron-transfer, and H-atom abstraction reaction have been determined by following the transient absorption bands of the triplet states of rose bengal [(3)(RB2-)*] at about 1020 nm with various substrates using nanosecond laser flash photolysis. For electron acceptor (1,4-benzoquinone), electron-transfer takes place via an exciplex. Exciplex formation was found to be predominant in high viscosity solvents rather than low viscosity solvents. For electron donor (3,3',5,5'-tetramethylbenzidine), the rate constants are independent of solvent viscosity, indicating that the contribution to an exciplex formation is small. The H-atom abstraction reactions of (3)(RB2-)* with H-donors show that the rate constants in water are higher than those in ethanol and in THF. The negative slopes of the Hammett plots indicate that the reaction center of [(3)(RB2-)*] has a highly electrophilic nature, which increases with solvent polarity, suggesting the polar transition states. (C) 1999 Elsevier Science S.A. All rights reserved.