Structural and optical studies of fluoride ion binding using N-heteroaromatic ligands

8-Hydroxyquinoline and imidazole, two important N-heteroaromatic systems, have a strong affinity towards various anions via their acidic OH or NH protons. Three receptor ligands, 5-(1H-benzo[d]imidazol-2-yl)quinolin-8-ol (1), 5-(benzo[d]thiazol-2-yl)quinolin-8-ol (2), and 4-(1H-benzo[d]imidazol-2-yl)benzene-1,3-diol (3), were synthesized, and their fluoride (F^-) ion binding properties were investigated. These ligands could selectively bind F^- ions, and their respective F^- complexes, namely, 1-TBAF, 2-TBAF, and 3-TBAF (TBAF = tetrabutylammonium fluoride), were characterized using single crystal X-ray analysis, NMR, UV-vis, Hirshfeld surface (HS) analysis and computational studies. Their solid-state structural analysis revealed that in each case, the F^- ion is strongly bound within the receptor molecule scaffold through NH, OH, and aryl-CH hydrogen bonding interactions. In the case of 1-TBAF and 3-TBAF, F^- binding is further strengthened by the inclusion of water molecules, forming fluoride-water cluster complexes. ¹H NMR study exhibited the disappearance of NH and OH proton signals upon the addition of TBAF, indicating that the NH and OH protons are the primary sites for F^- ion coordination. UV -vis absorbance spectra was used to study their lowest sensing limits towards F^- ions. Ligands 1 and 2 showed good responsiveness towards F^- ions at concentrations ≥0.5 μM, with obvious changes occurring at F^- concentrations of 180 and 12 μM, respectively. HS analysis quantified non-covalent interatomic contacts between the atoms and fluoride ions, revealing that 1-TBAF contributed a maximum of 19.2% of all atoms⋯F contacts to the HS. Density functional theory studies were carried out on the 1-TBAF, 2-TBAF, and 3-TBAF crystal systems, and the band structure and total density of states of the systems were evaluated. The optical response of 2-TBAF was also investigated.