Temperature and specific anion effects in photo-induced intermolecular electron-transfer reactions

Abstract

The rate constants for the bimolecular oxidative quenching of *Ru(bpy) ²⁺₃ by methylviologen (k _q ) and the electron-transfer recombination (k _rec ) reaction of the redox products were determined as a function of the ionic strength and temperature in aqueous solution. At 25°C for Na⁺ salts at constantμ, k _q andk _rec are in the order H₂PO ⁻₄ <CH₃CO ⁻₂ ∼HPO ²⁻₄ ∼SO ²⁻₄ ≪ClO ⁻₄ ; activation energies are independent of μ and the anion (∼16kJ/mol), except for ClO ⁻₄ , whereE _a is ∼30% lower. The trends ink _q andk _rec are attributed to the existence of ion-paired aggregates of the cationic reactants and anions; correlations with the sizes and hydration enthalpies of the anions can be made. It is suggested that ClO ⁻₄ , with the smallest hydration radius and the least negative hydration enthalpy, is more closely ion-paired within the Ru(bpy) ²⁺₃ (or Ru(bpy) ³⁺₃ ) interligand pockets, “tethering” the MV²⁺ (or MV^.+) into closer proximity, thereby lowering the activation barrier and increasing the rate of electron transfer.