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

Abstract

Abstract The synthesis of the first examples of anion receptors that utilize boron–fluoride interactions and (C–H) + ··· 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 19 F 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 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 CH 3 CN/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 – , CH 3 CO 2 – , HSO 4 – , and H 2 PO 4 – ).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)