A nonadiabatic description of electron transfer reactions involving large free energy changes

The electron transfer reaction within a donor-acceptor, solvent separated ion pair (²A·⁺...²D·^-) leading to excited singlet, excited triplet, and ground state products is studied within the framework of the theory of nonradiative transitions. Inner sphere as well as outer sphere rearrangement processes are included. The nonadiabatic model presented here predicts an almost linear dependence of lnk upon the free energy change ΔF⁰ in the abnormal free energy region in contrast to the quadratic dependence predicted by conventional Marcus electron transfer theory. Experimental evidence concerning the quantum yield and kinetics of the chemiluminescent electron transfer reaction between the anion radical and the cation radical of 9,10-diphenylanthracene is presented to demonstrate that the nonadiabatic electron transfer description results in a closer agreement between theory and experiment for the abnormal free energy region than does an adiabatic theory or its generalized version containing a preexponential factor, κ, which might be calculated by a Landau-Zener type formula. However, the present theory still falls short in quantitatively explaining the relatively large observed rates leading to the ground state products, which involve large changes in the free energy ΔF⁰.It is argued that second order transfer processes involving strongly coupling intermediate states can resolve the remaining discrepancy.