Imidazopyridine–fluoride interaction: solvent-switched AIE effects via S⋯O conformational locking

Abstract

Convenient and straightforward methods have been developed for the detection and quantitative estimation of fluoride anions by two novel imidazopyridine-based carboxamides (C1 and C2) in organic and aqueous media. The synthesized molecules exhibit high selectivity and enhanced sensitivity towards recognizing fluoride ions through the ‘turn on’ fluorogenic mode. The fluoride ion-driven deprotonation of the N–H proton and the subsequent spectral changes have been characterized by ¹H and ¹⁹F NMR spectroscopic techniques. Single-crystal XRD elucidates the structural evidence for deprotonation upon the addition of fluoride anions, wherein C1.S (C1⁻.2H₂O.TBA⁺) and C2.S (C2⁻.2H₂O.TBA⁺) salts were generated. Accordingly, C1 unveiled broad absorption bands at λ_max 340 nm, whereas C2 displayed similar bands at λ_max 308 nm in UV-vis spectra. The peaks shift to λ_max 350 nm and λ_max 341 nm after deprotonation. C1 itself exhibits the aggregation induced emission (AIE) phenomenon in water–DMSO (1 : 1). This is the first report of an imidazopyridine moiety being involved in a solvent-switched AIE effect via S⋯O conformational locking. UV-vis and fluorescence spectra of the compounds were obtained to predict the stoichiometry, binding constant, and the limit of detection (LOD). The actual structures of carboxamides and the resulting deprotonated carboxamide salts have been confirmed by single-crystal X-ray diffraction data. Different intra- and intermolecular H-bonds, namely O–H⋯N, O–H⋯O, N–H⋯O, C–H⋯N and C–H⋯O, play a significant role in the crystal packing of these compounds.