Lewis acidic stiborafluorenes for the fluorescence turn-on sensing of fluoride in drinking water at ppm concentrations

Prompted by the importance of fluoride in fields such as drinking water analysis and 18F-positron emission tomography, we have synthesized and investigated three Lewis acidic organostiboranes containing the (2,2'-biphenylene) phenylantimony(V) moiety as a common subunit and a catecholate (1), a tetrachlorocatecholate (2) or an alizarin (alizarin = 1,2-dihydroxyanthraquinone) ligand (3). DFT calculations show that the Lewis acidity of these stiboranes arises from the presence of a low lying antimony-centered orbital. While 1 shows no measurable fluoride anion affinity in 7/3 vol. THF-water mixtures, spectrophotometric titrations show that the more electron deficient stiboranes 2 and 3 bind fluoride anions in the same medium with binding constants (K) of 13 500 (+/- 1400 M-1) for 2 and 16 100 (+/- 1100) M (1) for 3. Formation of the fluoride complexes has been confirmed by isolation of TAS[2-F] and TAS[3-F] (TAS = tris(dimethylamino) sulfonium), the structures of which have also been determined. While formation of [2-F](-) does not trigger a marked photophysical response, a color change from yellow to dark red is observed upon conversion of 3 into [3-F](-) in CH2Cl2. This color change, which has been investigated using TD-DFT calculations, is also accompanied by an increase in the fluorescence of the alizarin chromophore at 616 nm. Owing to its high fluoride affinity and fluorescence turn-on properties, 3 can be used in biphasic water-CH2Cl2 mixtures to assay sub-ppm fluoridation levels in tap water or bottled infant drinking waters.