Design, Synthesis, and Characterization of pH-Resettable Photoswitches Mimicking the GFP Fluorophore Structure

Light-controlled molecular switches based on double bond isomerization can represent classical binary systems in logic gates. Here, starting from the biomimetic photoswitch 1 and combining computational and experimental techniques, we designed the insertion of a third control element (the "reset button"), proposing an appropriate structural modification capable of altering the electronic distribution within the molecule. Thus, the substitution on the pyrrolidinone nitrogen atom of 1 with a methane sulfonic (in 2a) or toluene sulfonic (in 2b) functional groups furnished molecules capable of alternating between two stable equilibrium forms by light irradiation. The addition of KOH deprotonates the phenolic moiety of the molecular photoswitches, providing systems in which the Z isomer becomes thermally unstable and spontaneously evolves into the starting E isomer. Re-establishing the initial phenolic form (e.g., by addition of acetic acid), the two molecules re-establish both isomers relative stability and their photochemical properties. These molecules represent a prototype of a potentially pH-resettable photoswitch with possible applications in pH-sensitive organic materials.