Description:
<jats:title>Abstract</jats:title><jats:p>Photoswitchable acid–base pairs, whose p<jats:italic>K</jats:italic><jats:sub>a</jats:sub> values can be reversibly altered, are attractive molecular tools to control chemical and biological processes with light. A significant, light‐induced p<jats:italic>K</jats:italic><jats:sub>a</jats:sub> change of three units in aqueous medium has been realized for two thermally stable states, which can be interconverted using UV and green light. The light‐induced p<jats:italic>K</jats:italic><jats:sub>a</jats:sub> modulation is based on incorporating a 3‐H‐thiazol‐2‐one moiety into the framework of a diarylethene photoswitch, which loses the heteroaromatic stabilization of the negatively charged conjugate base upon photochemical ring closure, and hence becomes significantly less acidic. In addition, the efficiency of the photoreactions is drastically increased in the deprotonated state, giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of the ground‐ and excited‐state potential energy surfaces, as indicated by quantum‐chemical calculations.</jats:p>