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 χw. We report that the rates of recovery, kobs, do not scale linearly with χw. In DMSO, which is a strong hydrogen bond acceptor, kobs is quite slow until χw ∼ 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 χw, implying isolated water molecules χw < 0.2, water nanopools 0.2 < χw < 0.6, and extended hydrogen bond networks χw > 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 χw ∼ 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.