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Herein, we report on highly Ba2+ selective fluorescence sensing in water by a fluorescent probe consisting of a benzo‐21‐crown‐7 as a Ba2+ binding unit (ionophore) and a tetramethylated BODIPY fluorophore as a fluorescence reporter. This fluorescent probe showed a Ba2+ induced fluorescence enhancement (FE) by a factor of 12±1 independently of the pH value and a high Ba2+ sensitivity with a limit of detection (LOD) of (17.2±0.3) μM. Moreover, a second fluorescent probe consisting of the same BODIPY fluorophore, but a benzo‐18‐crown‐6 as a cation‐responsive binding moiety, showed an even higher FE upon Ba2+ complexation by a factor of 85±3 and a lower LOD of (13±3) μM albeit a lower Ba2+ selectivity. The fluorescence sensing mechanism of Ba2+ was further investigated by time‐resolved fluorescence as well as transient absorption spectroscopy (TAS) and it turned out that within these probes a blocking of a photoinduced electron transfer (PET) by Ba2+ is very likely responsible for the FE. [ABSTRACT FROM AUTHOR]

Herein, we report on fluorescent probes which enable the selective quantification of Na+ and K+ in water by fluorescence enhancement (FE) independent of the pH value. These fluorescent probes, so called fluoroionophores, consist of benzo‐crown‐ether derivatives as ionophores, a benzo‐15‐crown‐5 (cf. 5) or a benzo‐18‐crown‐6 (cf. 6), respectively, to selectively bind either Na+ or K+ and a 4,4‐difluoro‐1,3,5,7‐tetramethyl‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) fluorophore. The fluoroionophore 5 shows a FE factor of 7.3 in the presence of 1000 mM Na+ and 6 of 4.7 upon addition of 250 mM K+ in an acidic environment. The dissociation constants (Kd) for 5+Na+ is 276 mM and for 6+K+ is 18 mM enabling the fluorometric determination of biological relevant Na+ or K+ levels. The fluorescence intensities of 5 and 6 were not impacted over a broad range of proton concentrations (pH stable from 3.5 to 9.5) and by the presence of other biologically relevant cations. [ABSTRACT FROM AUTHOR]