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

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India E-mail : hpal@barc.gov.in Manuscript received 27 October 2017, accepted 01 November 2017 Understanding the kinetic and mechanistic details of the electron transfer (ET) reactions in constrained media is an area of intense research in chemical sciences. The well known Marcus outer-sphere ET theory predicted an interesting inversion behavior for the ET rates with reaction exergonicity (–ΔG0 ), which remains debatable for last more than fifty years for bimolecular ET reactions, though clear Marcus Inversion (MI) behaviors have been well documented for many intramolecular ET systems. For bimolecular ET reactions in conventional solvents, the observed reaction rate at the higher exergonicities usually reach a saturation limit equal to the bimolecular diffusional rate (kd), displaying the typical Rehm-Weller behavior than the theoretically predict MI behavior. Though many explanations have been put forward to justify the observed Rehm-Weller behavior for the bimolecular ET rates with reaction exergonicity, research had been continuing to find out the means that can overcome the limitations associated with the bimolecular ET reactions and can lead to appearance of the theoretically predicted MI behavior even for the bimolecular ET reactions. With this perspective and considering that the ET reactions are the most fundamental reactions occurring ubiquitously in chemistry and biology, extensive studies have been pursed on bimolecular ET reactions for last many decades to understand all the intricate details that can modulate the kinetics of such fundamental reactions. For last about one and half decades, our extensive efforts to understand bimolecular photoinduced ET (PET) reactions under various experimental conditions have made us to explore the requisite situations for the reaction environments that can favorably assist in realizing the MI behavior for bimolecular PET reactions. It has been comprehended from our studies that using constrained reaction media like microherogeneous surfactant assemblies like micelles, reverse micelles, etc., or high viscosity and slow relaxing solvents like ionic liquids, the MI behavior is observed very easily for bimolecular PET reactions, assisted jointly by ultraslow reactant diffusion and exceedingly slow solvent relaxation dynamics compared to the ET rates. In this article we provide an overview of various aspects of bimolecular ET reactions bringing out how the constrained reaction media help in realizing MI behavior even for such bimolecular ET reaction