Photoinduced electron transfer in hydrogen bonded donor-acceptor systems. Study of the dependence of rate on free energy and simultaneous observationof 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 f ew small-molecule donor-acceptor systems, having hydrogen bonding appendage s, were studied by fluorescence lifetime quenching measurements. Two types of electron transfers take place in these systems. A fraction of the molecu les associates and exists as hydrogen bonded species and electron transfer in this segment is unimolecular. A major fraction of the donors and accepte rs freely diffuse in the medium and electron transfer is bimolecular in thi s segment. Free energy dependence studies showed that the former obeys the Marcus equation and the latter follows the Rehm-Weller behavior. The absenc e of the inverted region in bimolecular charge separation reactions is thus attributed to diffusion in the moderately large driving force regime.