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 dyes eosin 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 (kq) in the region of -1.9 eV < ΔGet < -0.2 eV do not decrease as predicted by Marcus theory, but they show a small increase with decreasing ΔGet. 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 ΔGet are not entirely explained, suggesting that new, refined theoretical models may be required. © The Royal Society of Chemistry and Owner Societies.