A novel merocyanine photoacid for visible light-controlled pH modulation

Merocyanine-based photoacids generate high proton concentrations under visible light irradiation. In the past decade, it has been established that these photoacids offer significant advantages over photoacid generators (PAGs) and hydroxyaryl photoacids, enabling better spatiotemporal control of proton transfer reactions in bulk media. In this study, we modified the core structure of the first generation of meroyanine photoacids. We developed a novel photoacid with color tuning capabilities and high solubility in polar organic solvents. Specifically, by incorporating a cationic benzoindolium moiety as an acceptor, we have altered the photoacid's light absorption properties. Unlike the first generation of indolium-based merocyanine photoacids, this photoacid can now be activated with green light (λ_max = 525 nm) as well as blue (λ_max = 450 nm) and ultraviolet (λ_max = 365 nm) lights. Furthermore, the novel photoacid exhibits high photo stability, photo-acidity (Π=3.28±0.08) and moderate reverse reaction rate (k = 1.08 × 10^-3±0.00017 s^-1) in solution. We envision that with improved color tuning capabilities, this class of photoacids will be a more versatile tool for controlling proton-induced reactions in different systems, including biological reactions.