Ratiometric Fluorescence Sensing of Fluoride Ions by an Asymmetric Bidentate Receptor Containing a Boronic Acid and Imidazolium Group

The synthesis of the first examples of anion receptors that utilize boron-fluoride interactions and (C-H)(+)center dot center dot center dot F--type ionic hydrogen-bond interactions in the binding of F ions is reported herein. o-, m-, and p-Phenylboronic acids were linked to naphthoimidazolium through a methylene group. On the basis of fluorescence and F-19 NMR studies, we have confirmed that the addition of fluoride to a boron center occurs prior to the formation of (C-H)(+)center dot center dot center dot F--type ionic hydrogen bond with the imidazolium moiety. More importantly, these investigations have demonstrated that only the receptor bearing the ortho-directed boron and imidazolium. exhibits enhanced fluoride binding. The increased binding ability of the asymmetric bidentate receptor of ortho-boronic acid and imidazolium, towards F- enables it to sense fluoride ions in a 95:5 CH3CN/HEPES aqueous solution. The fluorescence responses to different anions were also explored; the ortho-boron-imidazolium. receptor displayed ratiometric fluorescence changes and a high selectivity towards fluoride ions over other anions (Cl-, Br-, CH3CO2-, HSO4-, and H2PO4-). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)