Quantitative Probing of Photoisomerization‐Driven pH Changes in Metastable‐State Merocyanine–Spiropyran Photoacids

Photoacids are light-responsive molecular systems that convert optical energy into chemical potential through proton release, enabling precise spatiotemporal control of local acidity. Among these, merocyanine-spiropyran (MC-SP) metastable-state photoacids are particularly attractive because photoisomerization is directly coupled to reversible light-driven acid-base switching under mild conditions relevant to chemical and biological systems. Despite significant advances in molecular design, the quantitative relationship between photoisomerization and proton output remains largely unexplored. Here, we systematically investigate four structurally engineered MC-SP photoacids exhibiting distinct switching regimes, ranging from highly reversible to partially irreversible behavior. UV-vis spectroscopic analysis resolves photoisomer populations and switching kinetics, revealing pronounced substituent-dependent effects on electronic structure, photoisomerization dynamics, and photoacidity. To directly quantify proton generation, we developed a spectrally orthogonal near-infrared (NIR)-emissive pH probe that enables real-time, interference-free monitoring of pH changes during continuous photoirradiation. Quantitative analysis of both light-driven MC-to-SP conversion and dark-driven thermal SP-to-MC recovery, combined with fluorescence-based pH measurements, establishes a direct quantitative relationship between molecular photoisomerization, proton release, and proton uptake. This integrated approach establishes a mechanistic link between photoacid structure, photoisomerization dynamics, and proton output, providing design principles for metastable-state photoacids with tunable, light-controlled acidity for chemical and biological applications.