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 M V2+ (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 elect rolytes (Cl- salts of Li+, Na+, Cs+, Ca2+, La3+) and temperature (10-6 0 degrees C) in aqueous solution. At 25 degrees C and constant [Cl-], k(q) is independent of the cation. There is, however, a specific catio n 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 wi thin the cage are essentially independent of the nature of the electro lyte cation. The reactant cations are extensively ion-paired by Cl- in bulk solution and within the quenching solvent cage. However, the ele ctrolyte cations do not have any effect on the rates of electron trans fer between the cationic species. (C) 1997 Elsevier Science S.A.