Pyridine-Spiropyran Derivative as a Persistent, Reversible Photoacid in Water

A highly versatile water-soluble pyridine-spiropyran photoswitch is reported which functions as photoacid in a wide pH range. Under neutral conditions, the open-ring merocyanine (MC^-) exists to 48% and closes quantitatively by irradiation with visible light, while the reverse reaction occurs rapidly in the dark or by irradiation at 340 nm. The different pK_a of the pyridine nitrogen in the closed spiropyran (4.8) and open merocyanine form (6.8) leads to a reversible proton release in a pH range of 3-7. Only negligible hydrolytic decomposition was observed in the pH range from 1 to 12. The application of potentially harmful UV light can be circumvented due to the fast thermal ring-opening except for pH values below 3. Its photoacidic properties make this compound an effective pH-regulating photoswitch in water and enable controlled proton-transfer processes for diverse applications. Additionally, all of the involved protonated states of the compound exhibit discriminative fluorescence features within certain pH ranges, which even expands its utility to a light-controllable, pH-sensitive fluorophore.