H. Sun et al., OXIDATIVE QUENCHING OF THE EXCITED-STATE OF TRIS(2,2'-BIPYRIDINE)RUTHENIUM(2- VARIATION OF SOLUTION MEDIUM AND TEMPERATURE() ION BY METHYLVIOLOGEN ), Journal of physical chemistry, 98(19), 1994, pp. 5058-5064
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.