Salt effects on the charge separation yield in photoinduced electron transfer reactions: The effect of excited state multiplicity

The electron transfer quenching of the excited singlet and triplet states o f 9-methylcarbazole (MCBz) by p-dicyanobenzene (DCNB) was investigated in s everal solvents. While the singlet quenching rate constant is diffusion con trolled in all the solvents, the triplet quenching rate constants increase with the solvent polarity. In acetonitrile, free radical ions are produced in high yield from both excited states, while a very low yield is observed in low polarity solvents. The effect of two salts, tetrabutylammonium pechl orate (TBAP) and LiClO4 was investigated in THR In the presence of the salt s, an enhancement of the yield of free radical ions was observed. Two diffe rent concentrations of DCNB were employed such that the effect of the salts on either the singlet or the triplet state processes could be separated. A different efficiency was observed depending if the reaction proceeds by th e singlet or triplet state.