Tuning the Aggregate Emission Behavior of Phenothiazine-Derived Coumarin for Ratiometric Sensing of Hypochlorous Acid

Despite significant progress in the design of fluorescent probes, the development of ratiometric fluorescent hypochlorous acid (HOCl) probes with high specificity still remains a challenge. Phenothiazine-derived-coumarin (PTZC) fluorophores have been shown to respond to HOCl with high specificity; however, their faint fluorescence in aqueous media presents a barrier to their utility in the design of ratiometric HOCl fluorescent probes. To tackle this issue, we herein demonstrate a molecular design strategy of step-by-step increasing the steric hindrance of the substituent at the 3-position of the PTZC fluorophore to improve the emission behavior of the formed aggregates. Single-crystal analysis reveals that the weaker intermolecular π-π interaction and the higher rigidity of the phenyl group in the aggregate state are the two critical factors for the higher QY of PBF. Further experimental results demonstrated that this improved fluorescence allows PBF to respond ratiometrically to HOCl with high specificity and sensitivity. Finally, we utilize PBF for ratiometric fluorescence imaging of the fluctuations of endogenous HOCl during the progression of an acetaminophen-induced liver injury cell model and mouse model. Overall, the design strategy presented here not only offers a promising avenue for the advancement of ratiometric fluorescent HOCl probes but also broadens the application of AIE dyes in ratiometric fluorescent probe design.