Tuning the appearance of Marcus inversion region for intermolecular electron transfer (ET) reactions in micellar media

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India E-mail : manojk@barc.gov.in, hpal@barc.gov.in Manuscript received 15 December 2009, accepted 16 December 2009 In this mini-review we present a concise account of the results of our research on photoinduced electron transfer (ET) processes In micellar media, which we have been actively pursuing in our group for last one decade. Though Marcus inversion behavior ls easy to observe for Intramolecular ET reactions as well as for charge recombination reactions In contact ion pairs, the difficulty of observing such inversion behavior for Intermolecular ET reactions under diffusive condition Is a long lasting problem. The two most Important factors that act against the observation of Marcus inversion for Intermolecular ET reactions are : (I) diffusion of the reactants, which imposes a maximum limit for the observed reaction rate such that the true ET rates, which are faster than the diffusional. rate of the reactants, are not possible to be estimated for such systems, and (li) the non-availability of suitable donor and acceptor series to reach a very high reaction exergonicity ( -ΔG0) such that It Is difficult to reach the farthest part of the Inverted region where the true ET rates in the donor-acceptor encounter pairs become inherently slower than the diffusional rate of the reactants. In our studies, different micellar systems have been Judicially used as the reaction media to overcome the above limiting factors and accordingly the Marcus Inversion behavior for the intermolecular ET reactions could be observed very easily. In these studies, the advantages of the micellar systems have been taken with respect to the following considerations, i.e. (I) the diffusion of the reactants Inside the micelles will be highly retarded such that the ET reactions can effectively be considered as occurring under a non-diffusive condition of the reactants and (ii) the solvent relaxation dynamics in micellar media will be substantially slow such that the ET reactions In micelles wilI effectively occur under a situation of non-equilibrium solvent reorganization where the effective contribution of the solvent reorganization energy towards the activation barrier for the ET reactions will be reduced significantly and accordingly the onset of the Marcus inversion will be shifted towards the lower exergonicity region In comparison to that under very fast solvent relaxation condition. In fact, our studies in different micellar media using various donor-acceptor systems have unequivocally established for the first time the easy observation of Marcus Inversion region for Intermolecular ET reactions. Besides the observation of Marcus inversion region, we have also Investigated other Important aspects that might change the kinetics of ET reactions In micellar media, namely the effect of Intervening surfactant chain, effect of temperature, distant dependence or the reactants, etc. In this short review we briefly present the results of these studies, with special emphasis to the observation of Marcus Inversion and Its tuning using micellar systems