Effect of donor-acceptor distance on the rate of electron transfer reactions

The transient effect observed in fluorescence quenching in solution was measured and analyzed to study the effect of donor-acceptor distance on the rate of electron transfer reactions. Theoretical and experimental fluorescence decay curves were compared to determine the electron transfer rate parameters involved in either the Marcus equation or an exponentially decaying function. These parameters were used to evaluate the distance dependence of the electron transfer rate k(r) and the distribution of reaction distance. The distance dependence is different in solvents of different polarity: in nonpolar solvent, the slope of the logk(r) vs r plot changes with the free energy change of the reaction, while in polar solvent it remains almost unchanged. The origin of this difference was discussed. Electron transfer reactions at very high quencher (donor) concentrations were also studied. Electron transfer much faster than the prediction of the Marcus theory was found experimentally and the dependence of its rate on the energy gap studied. It was concluded that electron transfer occurs adiabatically between the donor and acceptor interacting strongly at short distances.