Stability of merocyanine-type photoacids in aqueous solutions

Over the past years, protonated merocyanines (MEHs) have been used as photoacids to control various chemical, material, and biological processes using visible light. For the applications under aqueous conditions, stability of this type of photoacid has been a concern. While hydrolysis of merocyanines is well known, this work showed that deprotonation of MEH to form merocyanine is not necessary for the hydrolysis of MEH. The decomposition products were identified by ultraviolet-visible spectroscopy and liquid chromatography-mass spectrometry. Comparing the behaviors of different MEHs under different conditions indicates that the hydrolysis is catalyzed by OH-and thus MEHs are more stable at a lower pH. Modifying an MEH with an electron-donating group conjugated to the double bond significantly improved its stability. Photostability of an MEH was tested by conducting 100 irradiating/recovering cycles, and the photoacid showed good photostability.