TEMPERATURE AND SPECIFIC ANION EFFECTS IN PHOTOINDUCED INTERMOLECULARELECTRON-TRANSFER REACTIONS

The rate constants for the bimolecular oxidative quenching of Ru(bpy) (3)(2+) by methylviologen (k(q)) and the electron-transfer recombinati on (k(rec)) reaction of the redox products were determined as a functi on of the ionic strength and temperature in aqueous solution. At 25 de grees C for Na+ salts at constant mu, k(q) and k(rec) are in the order H-2 PO4- < CH3CO2- similar to HPO42- similar to SO42- much less than ClO4-; activation energies are independent of mu and the anion (simila r to 16 kJ/mol), except for ClO4- where E(a) is similar to 30% lower. The trends in k(q) and k(rec) are attributed to the existence of ion-p aired aggregates of the cationic reactants and anions; correlations wi th the sizes and hydration enthalpies of the anions can be made. It is suggested that ClO4-, with the smallest hydration radius and the leas t negative hydration enthalpy, is more closely ion-paired within the R u(bpy)(3)(2+) (or Ru(bpy)(3)(3+)) interligand pockets, ''tethering'' t he MV(2+) (or MV(.+)) into closer proximity, thereby lowering the acti vation barrier and increasing the rate of electron transfer.