Fundamentally Different Distance Dependences of Electron-Transfer Rates for Low and High Driving Forces

The distance dependences of electron-transfer rates ( k_ET) in three homologous series of donor-bridge-acceptor compounds with reaction free energies (Δ G_ET⁰) of ca. -1.2, -1.6, and -2.0 eV for thermal charge recombination after initial photoinduced charge separation were studied by transient absorption spectroscopy. In the series with low driving force, the distance dependence is normal and k_ET decreases upon donor-acceptor distance ( r_DA) elongation. In the two series with higher driving forces, k_ET increases with increasing distance over a certain range. This counterintuitive behavior can be explained by a weakly distance-dependent electronic donor-acceptor coupling ( H_DA) in combination with an increasing reorganization energy (λ). Our study shows that highly exergonic electron transfers can have distance dependences that differ drastically from those of the more commonly investigated weakly exergonic reactions.