Solvent effects on intermolecular electron transfer processes

Abstract: The solvent effects on the rate constants and activation parameters of several intermolecular electron transfer reactions were investigated. The studies were carried out in protic (methanol, ethanol) and aprotic (acetonitrile, propionitrile) solvents. It was found that rate constants are generally higher in the protic solvents and that activation enthalpies are generally small and in some cases negative. The negative values were found in acetonitrile, while for the same reaction positive values are observed in alcoholic solvents. Large negative values of activation entropies were observed. They are more negative in the aprotic solvents. These observations can be explained by considering the entropy changes caused by the solvent reorganization on forming the transition state. The solvent effect on the quantum yield of the charge separation process was also investigated. For the system pyrene-indole the free-ions quantum yield changes in accordance with the trend of the cage escape rate constant. On the other hand, in the triplet quenching of dibenz-anthracene by nitrobenzenes the charge separation process is much less efficient in acetonitrile than in methanol. These results cannot be explained if the solvent is considered as a dielectric continuum