Photo-induced electron transfer studies in donor-bridge-acceptor molecules

Electron transfer reactions through Donor-Bridge-Acceptor (DBA) molecules are important as they constitute a fundamental chemical process and are of intrinsic importance in biology, chemistry, and the emerging field of nanotechnology. Electron transfer reactions proceed generally in a few limiting regimes; nonadiabatic electron transfer, adiabatic electron transfer and solvent controlled electron transfer. This study is going to address two different regimes (nonadiabatic and solvent controlled) of electron transfer studies. In the nonadiabatic limit, we are going to explore how the electron tunneling kinetics of different donor-bridge-acceptor molecules depends on tunneling barrier. Different parameters like free energy, reorganization energy, and electronic coupling which govern the electron transfer were quantitatively evaluated and compared with theoretical models. In the solvent controlled limit we have shown that a change of electron transfer mechanism happens and the kinetics dominantly depends on solvent polarization response.;This study comprises of two different kinds of Donor-Bridge-acceptor molecules, one having a pendant group present in the cleft between the donor and acceptor hanging from the bridge and the other having no group present in the cleft. The electron transfer kinetics critically depend on the pendant unit present in the cavity between the donor and the acceptor moieties. The electronic character of the pendant unit can tune the electronic coupling between the donor and the acceptor. If the cavity is empty then solvent molecule(s) can occupy the cavity and can influence the electron transfer rate between donor and acceptor. It has been shown that water molecules can change the electron transfer pathways in proteins. This study has experimentally shown that few water molecules can change the electron transfer rate significantly by forming a hydrogen bonded structure between them. This experimental finding supports the theoretical predictions that water molecules can be important in protein electron transfer.;Understanding the issues outlined in this work are important for understanding and controlling electron motion in supramolecular structures and the encounter complex of reactants. For example, the efficiency of electron tunneling through water molecules is essential to a mechanistic understanding of important biological processes, such as bioenergetics. Also, the influence of friction and its role in changing the reaction mechanism should enhance our understanding for how nuclear motions affect long range electron transfer.