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)(3)(2+) by MV(2+) (k(q)) and the charge recombination reaction between Ru(bpy)(3)(3+) and MV(.+) in bulk solution (k(rec)) and the cage escape yields of the redox products (eta(ce)) were determined as a function of added electrolytes (Na+ salts of oxyanions and halides) and temperature (10-60 degrees C) in aqueous solution. At 25 degrees C and constant [anion], k(q) and k(rec) are a function of the specific anion, decreasing in the order ClO4- >> SO42- similar to HPO42- > H2PO4- similar to CH3CO2- and I- > Br- > Cl- > F-. Activation energies for k(q) and k(rec) for ClO4- are similar to 30% lower than the average values for the other salts. Values of eta(ce) show anion-specific trends opposite to those for k(q) and k(rec). 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 (ki,) was extracted from k(q) by use of the Olson-Simonson treatment; Delta H-double dagger (activation enthalpy) and lambda (solvent reorganization energy) were evaluated for k(ip) and back electron transfer within the solvent cage (k(bt)) and were found to be smallest for ClO4- and I-. The correlation that exists between k(ip) or k(bt) and the standard free energy of hydration (Delta G degrees(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.