Ba ²⁺ ‐Selective and pH‐Stable Dibenzo‐18‐Crown‐6 Equipped BODIPYs with Pronounced Fluorescence Enhancement in Aqueous Solution

Sensitive and selective fluorescent sensing of Ba 2+ has attracted significant attention due to its relevance in environmental monitoring, industrial applications, and biological systems, as Ba 2+ is highly toxic. Herein, we report on Ba 2+ selective fluorescence sensing in aqueous solution by a fluorescent probe consisting of a dibenzo‐18‐crown‐6 as a Ba 2+ binding unit (ionophore) and a tetramethylated 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) fluorophore as a fluorescence reporter. This fluorescent probe showed an extraordinary Ba 2+ induced fluorescence enhancement (FE) by a factor of 185 ± 20 independently of the pH value (from 4 to 10), a high Ba 2+ sensitivity with a limit of detection of (0.60 ± 0.03) mM, and a high Ba 2+ selectivity. Moreover, the high Ba 2+ ‐induced FE is caused by an off‐switching of multiple fluorescence quenching processes within the fluoroionophore by Ba 2+ . Here, we observed that a cation switches off a photoinduced electron transfer (PET) and hampers aggregation between BODIPY dyes in aqueous solution.