Benzo‐Crown‐Ether Functionalized O‐BODIPY Probes for Cations – A Selective Fluorescent Probe for Ba²⁺

Abstract

Herein, we report the synthesis and sensing characteristics of 4,4’‐methoxy‐substituted BODIPY fluorescent probes (OBODIPYs) 3, 4 and 5 equipped with differently sized benzo‐crown ethers (cf. Scheme 1, 3 (benzo‐15‐crown‐5), 4 (benzo‐18‐crown‐6) and 5 (benzo‐21‐crown7)). O‐BODIPYs 3, 4 and 5 exhibited in comparison to their known F‐BODIPY analogues 3a, 4a and 5a (cf. Scheme 1) an improved solubility in aqueous medium and higher fluorescence quantum yields. Fluorometric study in aqueous solutions of 3, 4 and 5 in the presence of different cations show cation induced fluorescence enhancements (FE). Compared to the benzo‐crown ether substituted F‐BODIPY analogues 3a, 4a and 5a, we found for the free O‐BODIPYs 3, 4 and 5 higher fluorescence quantum yields (ϕf ) but lower cation induced FEs. We show that in aqueous medium the fluorescence quenching process (OFF switching), a photoinduced electron transfer, in O‐BODIPYs 3, 4 and 5 is less effective and consequently sensitive and selective ON switching of the fluorescence by cations, too. Albeit these observations the novel benzo‐21‐crown‐7 equipped fluorescent probe 5 exhibits a good fluorometric Ba2+ selectivity and Ba2+ sensitivity in conjunction to their superior aqueous solubility.