Transient effect in fluorescence quenching by electron transfer. 4. Long-range electron transfer in a nonpolar solvent

The transient effect in fluorescence quenching observed in a viscous nonpolar solvent, liquid paraffin, was measured and analyzed in order to study the mechanism of electron transfer fluorescence quenching in such solvents. The method of analysis is similar to that adopted in previous papers (J. Phys. Chern. 1995, 99, 5354; 1996, 100, 4064) to study electron transfer in polar solvents. The difference from the previous method is that a function of the form A exp[-b(r - r(0))] was used as the electron transfer rate constant k(r) instead of the Marcus equation and that the dependence of the steady-state fluorescence intensity on the quencher concentration was also analyzed in addition to the fluorescence decay at high quencher concentrations (greater than or equal to 50 mM). The parameters A and b and the diffusion coefficient D were determined from these analyses. The distribution of quenching distance was calculated using these parameter values. The distribution shows that the quenching occurs at distances longer than the contact distances of the fluorescer and quencher molecules. It was concluded that, even in nonpolar liquid paraffin, quenching occurs by long-range electron transfer rather than by exciplex formation as long as the Delta G of electron transfer is negative and far from 0 eV. The relation between electron transfer and exciplex formation in nonpolar solvents was discussed.