An Approach to a Model Free Analysis of Excited-State Proton Transfer Kinetics in a Reverse Micelle

Time resolved area normalized emission spectra (TRANES) of a photoacid dye HPTS, confined within the water pools of reverse micelles (RMs), are analyzed for a direct and model free observation of excited state proton transfer (ESPT) kinetics. When area normalized emission spectra of HPTS at different times are overlapped in a single window, we find population of RO-* (PTS-*) form of HPTS increases at the cost of a concomitant decreasing of the population of ROH* (HPTS*) form as time progresses. Migration of excited state population from ROH* form to RO-* form causes the emergence of an isoemissive point in TRANES, which retains throughout the entire course of ESPT process. An estimation of ESPT time scale is obtained either from the population depleting rate of ROH* form or from the population increasing rate of RO-* form; both are practically the same here. Emergence of an isoemissive point in TRANES implies that there are only two kinetically and reversibly coupled emitting species (ROH* and RO-*) of HPTS are present within the water pools of RMs. Continuous spectral relaxations of HPTS due to excited state solvation dynamics apparently have no effect on the ESPT kinetics of HPTS within the RMs; otherwise spectral shifting, caused by the excited state solvation, would have destroyed the isoemissive point of TRANES.