Dynamic Fluorescence Modulation via Light‐Driven Competitive Protonation

ABSTRACT

This work illustrates a design strategy for strongly fluorochromic systems via competitive protonation. To do this, we demonstrate a material system consisting of a spiropyran–merocyanine photoacid and a strongly proton‐responsive fluorophore. The idea is based upon a reversible proton‐exchange mechanism in which light and temperature control proton partitioning between two spectrally distinct emissive states with high chromatic contrast. Unlike conventional spiropyran systems that rely on colorimetric changes, both sides of this switch are strongly fluorescent, enabling large, reversible color changes with high optical contrast. In polar protic solvents, optical excitation transiently increases the acidity of the merocyanine, driving proton transfer to the fluorophore and causing a large change in fluorescence wavelength. Thermal equilibration reverses this process by favoring proton capture on the merocyanine site. The switching is reversible over many write–erase cycles and shows minimal evidence of fading or photobleaching, as it arises from equilibrium proton redistribution rather than chemical transformation. The proton‐switch concept enables re‐writable fluorescent information storage and visible‐light thermal mapping in thin liquid films and extends to polymeric solid matrices.