Acid–base reaction of a cationic hydration probe in vicinity of anionic interface of AOT reverse micelles

Abstract

The excited-state proton transfer (ESPT) of a cationic super-photoacid, N-methyl-7-hydroxyquinolium, has been studied within the water pool of an aerosol-OT reverse micelle (RM). Time-resolved emission spectra were obtained and analyzed to reveal the effect of environmental heterogeneity on the ESPT process. The ESPT was found to involve two main emissive species, namely the excited cationic and keto forms of the photoacid and was facilitated by scenarios that expose them to an abundance of water molecules. The dynamic charge-state transition (from positive to neutral) induced by the ESPT dictated the location and diffusion of the probe with the modulation of the Coulombic interaction between the probe and anionic interface of the RM. The non-exponential behavior of the ESPT was observed using time-resolved intensity profiles and rationalized in terms of the heterogeneity along the reaction coordinates involving the hydrogen-bond network, polarity, and viscosity of the confined water pool within the RM. It was found that the disruption of the hydrogen-bond network in the vicinity of the interface resulted in the decrease of polarity that retarded the overall ESPT by modulating the energetics of the reactant and the product in the ESPT. Solvation dynamics was also considered and found to occur on timescales faster than the ESPT process, profoundly for larger water pools.