2-(Methylselanyl)benzoate: A Quantitative and HOCl-Selective Fluorophore Uncaging Trigger.

Chemical uncaging-mediated controlled fluorophore release is a foundational strategy in the modular design of fluorescent probes, enabling incorporation of structurally diverse fluorogenic reporters. However, simultaneous attainment of high selectivity for hypochlorous acid (HOCl), rapid reaction kinetics, and quantitative fluorophore uncaging remains a key challenge owing to HOCl's intrinsic high reactivity and poor selectivity. These properties frequently cause off-target oxidation, incomplete uncaging, or fluorescence signal attenuation, thereby compromising probe fidelity. To address this limitation, we report 2-(organoselanyl)benzoates─including 2-(methylselanyl)benzoate (MSeB), 2-(benzylselanyl)benzoate (BzSeB), and 2-(phenylselanyl)benzoate (PSeB)─as a new class of HOCl-selective uncaging motifs. Conjugation of these triggers to HBP, a Nile Red-derived fluorogenic scaffold featuring an optically tunable phenolic group, yielded the corresponding HOCl-responsive probes BPMSeB, BPBzSeB, and BPPSeB. These probes are proposed to operate through a cascade mechanism: HOCl selectively oxidizes the selenide to the selenoxide, triggering spontaneous nucleophilic addition of water, followed by a five-membered intramolecular cyclization that releases HBP and elicits a robust turn-on red fluorescence response. Notably, MSeB delivers optimal performance owing to its enhanced selenium nucleophilicity, which is attributable to greater electron density at the selenium atom, as well as minimal steric hindrance. These two factors cooperatively accelerate the initial oxidation step and promote efficient nucleophilic addition-cyclization, resulting in near-quantitative, rapid fluorophore release under physiological conditions. Consequently, BPMSeB supports high-fidelity imaging of HOCl─detecting both exogenous and endogenous pools in live cells and capturing transient HOCl surges in the brains of living mice during systemic inflammation.