Conformational Locking With Built‐in Recognition: A Generalizable Strategy for Reversible SO 2 Detection

ABSTRACT The development of fluorescent sensors capable of simultaneous ultrafast response and high reversibility poses a significant challenge, as these two performance metrics are typically constrained by a trade‐off. Here, we present a conformational locking strategy with built‐in recognition, realized through a tetracoordinate boron‐bridged fluorophore (Py‐BF 2 ). In this design, the B←N Lewis pair rigidifies the molecular framework while extending π‐conjugation, yielding an electronic structure that promotes a pronounced intramolecular charge transfer (ICT) state with a substantially enlarged Stokes shift relative to the unlocked analogue. This integrated architecture not only enhances fluorescence efficiency but also exploits the ICT state for sensitive SO 2 detection. The resulting film‐based sensor achieves a response time of under 2 s, a detection limit of 1 ppm, and remarkable cycling stability exceeding 100 cycles without performance degradation, effectively overcoming the conventional speed‐reversibility trade‐off. Practical applicability is validated through rapid screening of SO 2 residues in grapes and raisins, with results corroborated against the Chinese national standard method (GB 5009.34–2022) and enabling compliance determinations within minutes. This work establishes a generalizable design paradigm that unifies signal transduction and analyte recognition within a single molecular scaffold for reversible gas sensing applications.