SOLVENT EFFECTS ON THE PHOTOINDUCED ELECTRON-TRANSFER REACTION BETWEEN EXCITED SINGLET PYRENE AND INDOLE

The quenching of the excited singlet of pyrene was studied in acetonit rile (MeCN), propionitrile (PrCN), methanol (MeOH) and ethanol (EtOH). Rate constants for the quenching processes and their activation param eters 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 kc al 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 que nching was confirmed by laser flash photolysis. The radical anion and triplet state of pyrene are both produced in the quenching process. Th e quantum yields of these species were determined in the four solvents . The quantum yield of radical ions follows a good correlation with th e cage escape rate constants calculated with the Eigen equation. The r ate constants for back electron transfer to the triplet and ground sta te were estimated from the quantum yields and the cage escape rate con stants. They are independent of the solvent.