An overview on photoinduced bimolecular electron transfer (ET) in constrained reaction media

Understanding kinetic and mechanistic details of photoinduced electron transfer (PET) reactions in constrained media is an area of intense research in photochemical sciences. Well celebrated Marcus outer-sphere electron transfer (ET) theory predicted the unique bell-shaped correlation of the ET rates with changing reaction exergonicity (-ΔG⁰), universally known as Marcus Inversion (MI) behavior, which is now unambiguously demonstrated from experimental finding involving intramolecular ET reactions, but remains quite deceptive even today involving bimolecular ET reactions. For last about one and half decades, our group has made an extensive effort to understand the intricate details of the kinetics and energetic of the bimolecular PET reactions under various solution conditions that made us to realize why MI behavior mostly remains elusive for bimolecular PET reactions in solution phase and also to understand what are the requisite solutions conditions that can lead us to observe MI behavior conclusively involving bimolecular PET reactions. Our study revealed that constrained reaction media (like micelles, reverse micelles, ionic liquids, etc.), that cause the diffusion of the reactants as well as the solvent relaxation dynamics to become exceedingly slow, can influence the bimolecular PET reactions to take place essentially under non-diffusive reaction condition and with non-equilibrium solvent reorganization. In essence thus, in constrained media, the bimolecular ET reactions effectively occur following two-dimensional ET mechanism than the conventional one-dimensional ET mechanism, which in turn cause the MI behavior to be observed easily even for bimolecular PET reactions. In the proposed talk these aspects of the bimolecular PET reactions will be presented and discussed based on the results obtained from our studies on such reactions in constrained reaction media. (author)