Solvent effects on the photoinduced electron transfer reaction between excited singlet pyrene and indole

Abstract

The quenching of the excited singlet of pyrene was studied in acetonitrile (MeCN), propionitrile (PrCN), methanol (MeOH) and ethanol (EtOH). Rate constants for the quenching processes and their activation parameters were determined. The activation enthalpies are −2.9 kcal mol −1 (MeCN), −1.4 kcal mol −1 (PrCN), −0.2 kcal mol −1 (MeOH) and 0.0 kcal mol −1 (EtOH). The activation entropies are −30.0 cal K −1 mol −1 (MeCN), −26.4 cal K −1 mol −1 (PrCN), −21.8 cal K −1 mol −1 (MeOH) and −22.8 cal K −1 mol −1 (EtOH). The electron transfer nature of the quenching was confirmed by laser flash photolysis. The radical anion and triplet state of pyrene are both produced in the quenching process. The quantum yields of these species were determined in the four solvents. The quantum yield of radical ions follows a good correlation with the cage escape rate constants calculated with the Eigen equation. The rate constants for back electron transfer to the triplet and ground state were estimated from the quantum yields and the cage escape rate constants. They are independent of the solvent.