Compartmentalization of reactants in different regions of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate/heptane/water reverse micelles and its influence on bimolecular electron-transfer kinetics.

Sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micellar medium has been used to study the photoinduced electron-transfer (ET) reactions between some coumarin derivatives and amines, namely, aniline (AN) and N,N-dimethylaniline (DMAN) at different w(0) (w(0) = [water]/[AOT]) values, to explore the appearance of Marcus inversion and also the possible role of w(0), if any, on the Marcus correlation curves. The coumarin derivatives are found to partition between the heptane-like and the water-like phases of the reverse micelles, and their locations have been confirmed by time-resolved anisotropy measurements. Fluorescence quenching is found to depend both on the location of the coumarin molecules and on the hydrophobicity of the amine donors. Various aspects such as the effect of differential partitioning of the quenchers, the location of the probes in the two phases, the diffusion of the reactants in the micellar phase, etc. have been considered to rationalize the fluorescence quenching rates in reverse micelles. Rotational relaxation times and the diffusion parameters estimated from the anisotropy results do not show good correlation with the observed quenching rates indicating that the diffusion of reactants has no role in the quenching kinetics in reverse micelles. Marcus inversion behavior has been observed for the coumarin-amine systems in the water-like phase at a relatively high exergonicity of approximately 1.2 eV suggesting that the solvent reorganization energy contributes fully to the free energy of activation for the ET reactions in the present systems. This is in accordance with the fast solvent relaxation dynamics reported in reverse micelles. Quenching rates in the water-like phase are found to decrease or increase marginally with increasing w(0) for the coumarin-DMAN and coumarin-AN systems, respectively. This is explained on the basis of the changing solubility of these amines in the water-like phase with changing w(0) values of the reverse micelles. In the heptane-like phase, no clear inversion in the quenching rate versus free energy plot could be observed because the study could not be extended to higher exergonicity due to nonsolubility of the dye C151 in this phase. Present results, especially in the water-like phase, suggest that the confinement of reactants in micellar media can effectively remove the influence of reactant diffusion on bimolecular ET rates and thus make the systems more conducive for the observation of the Marcus inverted region.