Photoinduced Electron-Transfer Reactions: A Study of the Diffusion-Controlled and Activation-Diffusion-Controlled Processes

The diffusion-controlled electron transfer rate constants (k(d)) of several quenching reactions of ruthenium complexes [Ru(L)(3)](2+)* = bpy, phen, and 4,7-(CH3)(2)phen) with [Fe(CN)(6)](3-) were experimentally determined at different concentrations of NaNO3. From these rate constants, the effective values of viscosity coefficients for NaNO3 solutions were calculated using EMSA (exponential mean spherical approximation) and EF (Eigen-Fuoss) approaches in order to take into account the mean force potential between reactants. The reliability of the effective parameters were checked through calculations of the rate constants of the reaction [IrCl6](2-)+ [Ru(bpy)(3)](2+)* in these NaNO3 solutions. The rate constants of this reaction were also obtained by fluorescence quenching measurements. The agreement between the two sets of data (experimental and predicted) is excellent. The trends of association (k(d)) and dissociation (k-(d)) rate constants for 2+/3, 2+/2, and 2+/2+ reactions in NaNO3 solutions 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(NH3)(5)Cl](2+) complexes from the observed (quenching) rate constant. The trend of electron-transfer rate constant in NaNO3 for this reaction was rationalized by using the Marcus electron-transfer treatment.