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 aoion receptors that utilize boron-fluoride interactions and (C-H)⁺-F∼-t:ype 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 ¹⁹F NMR studies, we have confirmed that the addition of fluoride to a boron center occurs prior to the formation of (C-H)⁺-F∼-type ionic hydrogen bond with the imidazolium moiety. More importantly, these investigations have demonstrated that only the receptor bearing the oriho-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 orfho-boron-imidazolium receptor displayed ratiometric fluorescence changes and a high selectivity towards fluoride ions over other anions (Cl^-, Br^-, CH₃CO2^-, HSO4 ^- and H₂PO4^-).