Selective anion recognition by inhibition of excited state intramolecular proton transfer process via hydrogen bonding interaction and efficient deprotonation: Spectroscopic and theoretical investigation

Anion (X) recognition fluorescent chemosensor 5-(4-Fluoro-phenyl)-2-hydroxy-nicotinonitrile (FP2HN) having both –NH and –OH groups in its two tautomeric forms has been synthesized and its excited state intramolecular proton transfer (ESIPT) reaction has been investigated by UV–vis, fluorescence, 1H NMR spectroscopy in combination with computational calculations. The experimental findings show that FP2HN selectively recognizes F−, AcO− and H2PO4− ions through the formation of –N(O)H⋯X hydrogen bonding (HB) complex. The binding constant obtained by non-linear fit predicts the binding order of F− > AcO− > H2PO4− > Cl−. The ESIPT process of FP2HN is inhibited either by the fluoride induced deprotonation of acidic proton of FP2HN or by the formation of a strong –N(O)H⋯X intermolecular hydrogen bonding complex. Further insights into the recognition mechanism of the receptor–anion complexes using ab initio calculations have been performed which support the experimental binding affinity order of F− > AcO− > H2PO4− > Cl−.