Acid‐Responsive Two‐photon Absorption Switch via Cocrystal‐to‐Salt‐to‐Cocrystal Conversion

Abstract Stimuli‐responsive organic cocrystals are typically governed by single‐crystal to single‐crystal (SCSC) transformation, remaining a long‐standing challenge due to disruption of long‐range order structure throughout the whole solid‐state process. Herein, we demonstrate for the first time an acid‐responsive two‐photon absorption (TPA) switch based on reversible cocrystal‐to‐salt‐to‐cocrystal SCSC transformation. While both para ‐N‐dipyridylanthracene and meta ‐N‐dipyridylanthracene coassemble with 1,2,4,5‐tetracyanobenzene (TCNB) into cocrystals via donor–acceptor interactions and hydrogen bonds, only the former cocrystal exhibits broad‐window TPA activity alongside enhanced solid‐state photoluminescence quantum yield (PLQY). Theoretical calculations confirm that positional isomerism of electron donors modulates molecular coplanarity and donor–acceptor interactions in cocrystals, directly controlling TPA cross‐sections. Remarkably, para ‐N‐dipyridylanthracene undergoes diprotonation by exposure to trifluoroacetic acid, enabling the conversion of the cocrystal to a salt form, which can be reverted upon triethylamine treatment. This cocrystal‐to‐salt‐to‐cocrystal transformation switches the TPA characteristics with reversible color changes between upconversion orange emission and Stokes yellow luminescence for 20 cycles, to the best of our knowledge, which is an unprecedented performance for stimuli‐responsive organic materials. Based on the unique acid‐responsive emission, we also apply the SCSC transformation in display devices and information storage. This study elucidates structure‐property relationships governing TPA in cocrystals and establishes a new paradigm for stimuli‐responsive optical switches.