Designing Squaraine Dyes with Bright Deep‐Red Aggregation‐Induced Emission for Specific and Ratiometric Fluorescent Detection of Hypochlorite

Abstract Development of ratiometric fluorescent hypochlorite probes with strong long wavelength fluorescence in aqueous medium, high resistance to photobleaching, high sensitivity and selectivity, and low biological toxicity remains a challenge. In this work, a molecular design strategy is proposed that can transform the traditional squaraine dyes (SQs) with aggregation‐caused quenching character into aggregation‐induced emission (AIE)‐active luminogens by functionalizing the end‐groups with tetraphenylethylene units and further introducing hydrophilic sulfonate group as the side chains. The resulting TPE‐SQ5 not only emits strong deep‐red fluorescence with a high quantum yield of 11.0% and high photostability, but more encouragingly can serve as a ratiometric fluorescent hypochlorite probe with high selectivity and sensitivity (detection limit: 5.6 n m ), which indeed is the first report for SQs. The detailed sensing mechanism study demonstrates that the oxindole product with sulfonate substitution is responsible for the ratiometric fluorescent response. Furthermore, TPE‐SQ5 nanoparticles with high biocompatibility and low cytotoxicity are successfully used for ratiometrically imaging exogenous and endogenous hypochlorite in living cells.