A NONADIABATIC DESCRlPTlON OF ELECTRON TRANSFER INVOLVlNG LARGE FREE ENERGY CHANGES

The electron transfer reaction within a donor-acceptor, solvent separated ion pair (‘AZ . . . . *D:) leading to IXcited singlet, excited triplet, and ground stale products is studied within the iramcwork of tic theorv of nonradiative Cransitions. Inner sphere as well as Ouier sphere rearrangement processes are included. The nonadiabruc model presented here predicts an almost linear dcpendcncc of Ink upon the free energy change & in tie abnormal free cncr6y region in contrast to the quadratic depcndcncc predicted by conventional Marcus electron transfer theory. Experimental evidence concerning the quantum yield and kinetics of rhe chemilumincscent electron transfer rcaction between the anion radical and the cation radical of 9.lO-diphenylanlhraccnc is presented to demonstrate that the nonadhbatic electron transfer description results in a closer agreement between theory and experiment for the abnormid free energy region rhan does an adiabatic theory or its gcncmiized version contai%g a precxponcntial factor, K. which might be calcularcd by a Landau-Zencr type formula. However, the present theory still falls short in quantitatively explaining rhe rclalively large observed rates leading to the ground state products, which involve Iargc changes in the free energy AF’. It is argued that second order transfer processes involving strongly coupling intermediate states can resolve the remaining discrepancy.