A new fluoride ion colorimetric sensor based on azo–azomethine receptors

<div><p>Four new colorimetric chemosensors, receptors <b>1a</b>–<b>1d</b>, containing electron-donating moieties appended to the azophenol moiety were synthesised. The dyes were characterised by elemental analysis, infrared, UV–vis and nuclear magnetic resonance (NMR) spectra. Spectral characteristics of the dyes were investigated in four organic solvents of differing polarity and their chromogenic behaviours towards various anions. The sensors showed a colour change upon addition of fluoride ion with a substantial bathochromic shift. No significant colour changes was observed upon addition of any other anions. The significant changes of absorption bands and colour show that chemosensor was selective towards fluoride ion over other anions such that Cl⁻, Br⁻ and could not cause any colour change. The sensing action was further confirmed by UV–vis titration. In addition, ¹H NMR experiments were carried out to explore the nature of interaction between receptors and F⁻.</p>