Ion-Pairing Control of Excited-State Electron-Transfer Reactions. Quenching, Charge Recombination, and Back Electron Transfer

The rate constants for the oxidative quenching of *Ru(bpy)32+ by MV2+ (kq) and the charge recombination reaction between Ru(bpy)33+ and MV•+ in bulk solution (krec) and the cage escape yields of the redox products (ηce) were determined as a function of added electrolytes (Na+ salts of oxyanions and halides) and temperature (10−60 °C) in aqueous solution. At 25 °C and constant [anion], kq and krec are a function of the specific anion, decreasing in the order ClO4- ≫ SO42- ∼ HPO42- > H2PO4- ∼ CH3CO2- and I- > Br- > Cl- > F-. Activation energies for kq and krec for ClO4- are ∼30% lower than the average values for the other salts. Values of ηce show anion-specific trends opposite to those for kq and krec. The reactant cations are extensively ion-paired by the dominant anions in bulk solution, and a similar situation is proposed to exist within the quenching solvent cage. The electron-transfer component of quenching for ion-paired species (kip) was extracted from kq by use of the Olson−Simonson treatment; ΔH⧧ (activation enthalpy) and λ (solvent reorganization energy) were evaluated for kip and back electron transfer within the solvent cage (kbt) and were found to be smallest for ClO4- and I-. The correlation that exists between kip or kbt and the standard free energy of hydration (ΔG°hyd) of the anions indicates that the rates of electron transfer between the cationic reactants are greatest in the presence of anions that have the most weakly-held hydration sphere and the greatest ability to break the water structure. The rate constants of quenching and charge recombination and the yields of redox products can be fine-tuned through the variation of the supporting electrolyte as well as the variation of temperature and ionic strength.