Study of diffusion-controlled and activation–diffusion-controlled electron-transfer processes from quenching measurements

Abstract

The diffusion-controlled rate constants, k d , of various quenching reactions, [Ru(L) 3] 2+∗ (L = bpy, phen and 4,7-(CH 3) 2phen) + [Fe(CN) 6] 3−, were measured through fluorescence measurements. From them, the effective values of viscosity coefficients for several methanol + water mixtures were calculated. These coefficients were checked through calculations of the rate constants of the reaction [IrCl 6] 2− + [Ru(bpy) 3] 2+∗, which were also obtained by fluorescence quenching measurements. The agreement between the two sets of data (experimental and predicted) is excellent. Besides, the trends of association, k d , and dissociation, k − d , rate constants for 2+/3−, 2+/2− and 2+/2+ reactions in methanol–water mixtures are discussed. The use of effective diffusion coefficients for estimating k d and k − d allowed us to obtain the intrinsic electron transfer rate constant, k et , for the activation-diffusion-controlled process between [Ru(bpy) 3] 2+∗ and [Co(NH 3) 5Cl] 2+ complexes from the observed (quenching) rate constant. The influence of methanol–water mixtures on k et was rationalized by using the Marcus electron-transfer treatment.