Unusual Hypochlorous Acid (HClO) Recognition Mechanism Based on Chlorine–Oxygen Bond (Cl−O) Formation

One of the most important endogenous reactive oxygen species, hypochlorous acid (HClO), is involved in numerous pathological and physiological processes. Herein, a near‐infrared fluorescence probe (CyHR) was designed and synthesized for ultrafast (within 0.2 s), sensitive (limit of detection=39.44 n m ), and selective response to HClO. The reaction mechanism was systematically analyzed by MS, 1 H NMR spectroscopy, HPLC‐MS techniques, and theoretical calculations. The results indicated that HClO can be recognized by CyHR, which is based on chlorine–oxygen (Cl−O) bond formation. To the best of our knowledge, this study is the first to find Cl−O bonds among organic aromatic compounds, given that Cl−O bonds are common among inorganics. Through biological experiments, CyHR was successfully applied to image exogenous and endogenous HClO in macrophage cells (RAW 264.7). Thus, CyHR is a promising tool for HClO‐related physiological and pathological studies and may provide a means for designing HClO‐specific fluorescence probes.