Photoinduced electron transfer in hydrogen bonded donor-acceptor systems. Study of the dependence of rate on free energy and simultaneous observation of the Marcus and Rehm-Weller behaviors

Hydrogen bonding networks play a very important role in biological electron-transfer processes. The free energy dependence of electron transfer in a few small-molecule donor-acceptor systems, having hydrogen bonding appendages, were studied by fluorescence lifetime quenching measurements. Two types of electron transfers take place in these systems. A fraction of the molecules associates and exists as hydrogen bonded species and electron transfer in this segment is unimolecular. A major fraction of the donors and accepters freely diffuse in the medium and electron transfer is bimolecular in this segment. Free energy dependence studies showed that the former obeys the Marcus equation and the latter follows the Rehm-Weller behavior. The absence of the inverted region in bimolecular charge separation reactions is thus attributed to diffusion in the moderately large driving force regime.