Durable Light-Driven pH Switch Enabled by Solvation Environment Tuning of Metastable Photoacids

Metastable photoacids (mPAH), such as protonated merocyanine, are organic molecules that release protons under illumination, providing spatiotemporal control of pH. While merocyanine-type photoacids enable fully reversible pH cycling in water, their solubility is limited and chemical stability <24 hours. In recent studies, structure modifications have been the main direction in improving photoacid properties. In this work we introduce a new pathway to increase stability and solubility of photoacids by tuning their solvation in binary solvent mixtures. We show that a modified solvation environment supports highly reversible, large and stable pH modulation. We demonstrate >350 hours of light-induced pH cycling with 10 times more protons released per cycle in water-DMSO mixtures compared to pure water. This solva-tion engineering approach can be applied to other metastable photoacids, serving as a steppingstone towards practical use in applications where long-term stability is critical.