Bridge-Caging Strategy in Phosphorus-Substituted Rhodamine for Modular Development of Near-Infrared Fluorescent Probes

Replacement of the bridging oxygen atom in rhodamine with phosphorus is one of the most efficient ways for bright near-infrared (NIR) fluorophores with wavelengths over 700 nm. However, the organophosphorus bridge is more versatile than just being a spectrum tuner, it is also a profound solubility booster and photostability enhancer, as proved by a series of phosphorus-substituted rhodamines (PRBs). A unique bridge-caging strategy for efficiently manipulating fluorescence has further been innovated in example PRB2. Consistent with theoretical calculations, the formation of organophosphinate by a caging group as a fluorescence-controller locks the spirolactone into a colorless and nonfluorescent form, whereas decaging, a process induced by a specific stimulus, results in a ring-opened form, which yields strong fluorescence. The bridge-caging strategy is feasible for the modular development of NIR probes. Efficient in vivo imaging of photoillumination, hydrogen peroxide, and enzyme have been achieved on the PRB2 scaffold as a photoactivatable fluorophore, PRB2-hν; fluorescent indicator, PRB2-H₂O₂; and fluorogenic enzyme substrate, PRB2-NTR, respectively.