Photoinduced electron transfer in hydrogen bonded donor-acceptor systems. Free energy and distance dependence studies and an analysis of the role of diffusion

The free energy dependence of electron transfer in a few small-molecule don or-acceptor systems having hydrogen-bonding appendages was studied to evalu ate the role of diffusion in masking the inverted region in bimolecular PET reactions. A small fraction of the probe molecules associate and this led to the simultaneous observation of unimolecular and diffusion-mediated quen ching of the probe fluorescence. Free energy dependence studies showed that the unimolecular electron transfer obeys Marcus behavior and the diffusion -mediated electron transfer obeys Rehm-Weller behavior. The absence of an i nverted region in bimolecular PET reactions is thus attributed to diffusion . The results of the free energy dependence studies suggest that distance d ependence of electron transfer plays a role in masking the inverted region. To ascertain this aspect we have carried out a study of the distance depen dence of electron transfer in the hydrogen-bonded donor-acceptor systems. F or a system in the normal region an exponential rate decrease was observed. For a system in the inverted region it was observed that the rate depends very feebly on distance. Thus distance dependence studies did not confirm t he prediction of enhanced rates at larger distances in the inverted region.