Free energy dependence of the diffusion-limited quenching rate constants in photoinduced electron transfer processes

Abstract

Electron-transfer rate constants were determined by means of lifetime measurements for the fluorescence quenching of 9,10-dicyanoanthracene by aromatic amines and methoxybenzenes as electron donors, and for the quenching of the synthetic dyeseosin Y and phenosafranine by a series of p-benzoquinones as electron acceptors. All determinations were carried out in acetonitrile at 298 K. The quenching rate constants (k_q) in the region of –1.9 eV < ΔG_et < –0.2 eV do not decrease as predicted by Marcus theory, but they show a small increase with decreasing ΔG_et. Although this behaviour is in qualitative agreement with the current theories for reactive systems in the diffusion limit region, a closer analysis of the experimental data showed that several aspects of the dependence of the k_q on ΔG_et are not entirely explained, suggesting that new, refined theoretical models may be required.