Diffusion, spin and reaction control in geminate reverse electron transfer
2001
Kinetic
analyses of geminate radical escape yields in terms of a simple (“exponential”) reaction scheme with first-order
rate constants of separation and geminate recombination have been widely used in the literature, e.g. to evaluate
rate constants of reverse electron transfer (k−et). Here we demonstrate the limited value of such rate constants
by formally analysing, in terms of the exponential model, the diffusion coefficient (iz. viscosity) dependence
of the radical escape yield as theoretically calculated in the framework of diffusion-dependent electron
transfer theory (unified treatment of non-contact photoinduced forward and geminate reverse electron transfer).
It is shown that, while the true electron transfer rate constant is kept constant, the apparent rate constant
k−et from the exponential model undergoes a wide variation as a function of diffusion coefficient and the rate of spin
conversion. Nevertheless, the function k−et(D) represented in a double log plot for various rates of spin
conversion provides a useful map suitable to assign characteristic regions of diffusional, spin and reaction
control of the geminate process. As an application to real systems the experimental example of the [Ru(bpy)3]2+/methylviologen
system is reconsidered. 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 photochemical
forward electron transfer reaction.
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