Diffusion, spin and reaction control in geminate reverse electron transfer

Kinetic analyses of geminate radical escape yields in terms of a simple ("e xponential'') reaction scheme with first-order rate constants of separation and geminate recombination have been widely used in the literature, e.g. t o evaluate rate constants of reverse electron transfer (k(-et)). Here we de monstrate the limited value of such rate constants by formally analysing, i n terms of the exponential model, the diffusion coefficient (viz. viscosity ) dependence of the radical escape yield as theoretically calculated in the framework of diffusion-dependent electron transfer theory (unified treatme nt of non-contact photoinduced forward and geminate reverse electron transf er). It is shown that, while the true electron transfer rate constant is ke pt constant, the apparent rate constant k(-et) from the exponential model u ndergoes a wide variation as a function of diffusion coefficient and the ra te of spin conversion. Nevertheless, the function k(-et)(D) represented in a double log plot for various rates of spin conversion provides a useful ma p suitable to assign characteristic regions of diffusional, spin and reacti on control of the geminate process. As an application to real systems the e xperimental example of the [Ru(bpy)(3)](2+)/methylviologen system is recons idered. Here a magnetic field effect on the k(-et)(D) dependence is useful to corroborate the non-contact formation of the radical pair in the photoch emical forward electron transfer reaction.