Probing Proton Delivery in Microheterogeneous Water Structures Using a Proton-Dependent Isomerization Reaction of a Merocyanine Photoacid

Water structure and proton dynamics in complex environments, such as mixed electrolytes, biological environments, and microdroplet surfaces, are often hypothesized to affect reaction thermodynamics, kinetics, and selectivity. Toward better understanding the influence of water microphases in complex mixtures, this study leverages the proton-dependent recovery kinetics of a merocyanine photoacid in acetonitrile (ACN) and dimethyl sulfoxide (DMSO) over a range of water mole fractions χ_w. We report that the rates of recovery, <i>k</i>_obs, do not scale linearly with χ_w. In DMSO, which is a strong hydrogen bond acceptor, <i>k</i>_obs is quite slow until χ_w ∼ 0.7 and increases linearly beyond that value. This observation implies that the reaction requires the establishment of an extended hydrogen bond network that can only be afforded beyond a threshold. In contrast, the recovery rate in the more weakly hydrogen bond acceptor, ACN, shows three distinct regions as a function of increasing χ_w, implying isolated water molecules χ_w < 0.2, water nanopools 0.2 < χ_w < 0.6, and extended hydrogen bond networks χ_w > 0.6. Furthermore, when adding a model surfactant, cetyltrimethylammonium bromide (CTAB), to the ACN-H₂O mixtures, a sharp decline in the recovery rates is observed beyond χ_w ∼ 0.8. This behavior is consistent with the formation of micelles, likely incorporating the photoacid and limiting their access to the otherwise largely hydrogen-bonded network of water. This study informs the design principles of water delivery and proton access for creating tailored protonic environments for tuning reactivity.