Electron Transfer Rate Maxima at Large Donor−Acceptor Distances

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor–acceptor distance. We report here on electron transfer rate maxima at donor–acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor–photosensitizer–acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor–acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.