Analyte-Induced Restriction of Intramolecular Rotation: Turn-On aza-BODIPY Sensor for Biothiol Detection in Aqueous Media

Two alternate aza-BODIPY fluorophores bearing a formyl-functional group were synthesized and evaluated for biothiol sensing. Both provide high binding constants (up to 9800 M^–1) and low limits of detection (as low as 1 μM). More importantly, this study introduces an unconventional “turn-on” sensing mechanism, in which fluorescence enhancement arises from analyte-induced restriction of intramolecular rotation rather than analyte-triggered cleavage of quencher. This alternative strategy provides a promising platform for the development of next-generation fluorescent probes.