OXIDATIVE QUENCHING OF THE EXCITED-STATE OF TRIS(2,2'-BIPYRIDINE)RUTHENIUM(2- VARIATION OF SOLUTION MEDIUM AND TEMPERATURE() ION BY METHYLVIOLOGEN )

The rate constant (k(q)) for the quenching of Ru(bpy)(3)(2+) by methy lviologen (MV(2+)) has been determined as a function of the mole fract ion of CH3CN (X(AN)) in aqueous mixtures; k(q) goes through a minimum at X(AN) similar to 0.4, which is suggested to occur because of the dy namic solvent effect for the electron-transfer reaction in the mixed s olvents, whereby the relaxation dynamics of the solvent is a strong fu nction of its composition. The cage escape yield (eta(ce)) for the rel ease of the redox products into bulk solution has been determined as a function of ionic strength, temperature, and X(AN); the apparent acti vation energy for the back electron transfer (E(bt)) between the gemin ate redox pair, Ru(bpy)(3)(3+) and MV(.+), within the solvent cage pro duced upon quenching is obtained from the data through the application of the simple cage model. E(bt) is independent of ionic strength in a queous solutions, but is strongly dependent on solvent composition in mixed solvents. However, E(bt) decreases smoothly from 9.6 kJ mol(-1) in aqueous solution to -5.5 kJ mol(-1) in CH3CN-rich solvents as X(AN) is increased, requiring that a modification of the simple model be ma de. The concept of a kinetically important reorientation of the gemina te redox pair is introduced to account for the experimental observatio ns; the simple solvent cage model can be viewed as a limiting case in the modified model.