Ion-pairing control of excited-state electron-transfer reactions - Effect of cations on cationic reactants

The rate constants for the oxidative quenching of *Ru(bpy)(3)(2+) by MV2+ (k(q)) and the cage escape yields (eta(ce)) Of the redox products (Ru(bpy)(3)(3+) and MV.+) were determined as a function of added electrolytes (Cl- salts of Li+, Na+, Cs+, Ca2+, La3+) and temperature (10-60 degrees C) in aqueous solution. At 25 degrees C and constant [Cl-], k(q) is independent of the cation. There is, however, a specific cation effect on eta(ce) (La3+ > Ca2+ similar to Li+ > Na+ > Cs+), which is attributed to differences in the rate constants of cage escape (k(ce)) due to variations in the bulk properties of the solution (viscosity, dielectric constant); the rate constants of back electron transfer within the cage are essentially independent of the nature of the electrolyte cation. The reactant cations are extensively ion-paired by Cl- in bulk solution and within the quenching solvent cage. However, the electrolyte cations do not have any effect on the rates of electron transfer between the cationic species. (C) 1997 Elsevier Science S.A.