Dynamic Fluorescence Sensing of Bromide Ions by Photochromic Bi(III)‐Coordination Polymers Based on a Ligand Integrated by Naphthalene Diimides and Pyridinium in Solution and Films

Abstract Bismuth(III)‐based complexes have garnered increasing attention in fluorescence sensing due to their environmentally friendly and sustainable characteristics. A Bismuth(III) coordination polymer (CP),1‐Cl based on a naphthalene diimides(NDI)‐pyridinium is synthesized by an in situ reaction method. Notable for its sensitivity to visible light, 1‐Cl shows excellent photochromic properties, and the integration of NDI and pyridinium in one ligand makes photogenerated radicals more stable. Structural analysis and theoretical calculations are employed to investigate the potential pathway of photoinduced electron transfer (ET) during the photochromic process. Notably, in aqueous solutions, 1‐Cl displays an extraordinary fluorescence enhancement response to bromide ion (Br^‐), resulting in a distinct transition from yellow to orange in color. The potential mechanism of fluorescence sensing has been revealed through single‐crystal X‐ray diffraction analysis. This insight highlights a continuous substitution process where the Cl⁻ ions are successively replaced by Br⁻ ions. Consequently, a single‐crystal‐to‐single‐crystal transformation (SCSC) occurs, yielding the intermediate species, 1‐Cl‐Br, which ultimately transforms into the final product, 1‐Br. Finally, the photochromic film is successfully prepared and applied to practical applications such as ink‐free printing, information anti‐counterfeiting, and the visual detection of Br⁻ ions. This work combines photochromism with fluorescence sensing, broadening the research field and practical application of photochromic materials.