A WEAK DELTA-G-DEGREES DEPENDENCE OF BACK ELECTRON-TRANSFER WITHIN THE GEMINATE REDOX PAIRS FORMED IN THE QUENCHING OF EXCITED RUTHENIUM(II) COMPLEXES BY METHYLVIOLOGEN

The oxidative quenching by MV2+ of the lowest luminescent excited states of nine Ru(II)-diimine complexes in H2O/CH3CN was studied by pulsed laser flash photolysis. From a knowledge of the spectral characteristics of the excited states, the fraction of the excited states quenched, and the yield of MV.+ generated in the flash, the efficiency of escape of the redox products into bulk solution (F1) was calculated; values of F1 were also obtained for a variation of the composition of the mixed solvent, for changes in the ionic strength, and for quenching by Rh(phen)3(3+). From the values of F1 were calculated kappa-bet/kappa-dis ratios, where kappa-bet is the rate constant of back electron transfer between the geminate redox pair (Ru(III)...MV.+) within the solvent cage and kappa-dis is the rate constant of escape of the redox pair out of the cage. By assuming that kappa-dis is a constant value for these identically charges species, the variation of kappa-bet was found to be weakly dependent on DELTA-G-degrees of the back-electron-transfer reaction (-1.4 to -2.0 eV) within the "inverted Marcus" region. For those complexes that deviated from the general behavior with unusually low values of F1, the variation was attributed to the large size of the ligands that would increase the intrapair distance and decrease the value of kappa-dis. Possible origins of the weak dependence of kappa-bet on DELTA-G-degrees are discussed.