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 M V(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 e lectrolytes (Na+ salts of oxyanions and halides) and temperature (10-6 0 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 s alts. Values of eta(ce) show anion-specific trends opposite to those f or k(q) and k(rec). The reactant cations are extensively ion-paired by the dominant anions in bulk solution, and a similar situation is prop osed to exist within the quenching solvent cage. The electron-transfer component of quenching for ion-paired species (ki,) was extracted fro m k(q) by use of the Olson-Simonson treatment; Delta H-double dagger ( activation enthalpy) and lambda (solvent reorganization energy) were e valuated for k(ip) and back electron transfer within the solvent cage (k(bt)) and were found to be smallest for ClO4- and I-. The correlatio n that exists between k(ip) or k(bt) and the standard free energy of h ydration (Delta G degrees(hyd)) of the anions indicates that the rates of electron transfer between the cationic reactants are greatest in t he presence of anions that have the most weakly-held hydration sphere and the greatest ability to break the water structure. The rate consta nts of quenching and charge recombination and the yields of redox prod ucts can be fine-tuned through the variation of the supporting electro lyte as well as the variation of temperature and ionic strength.