A new quinoline-based tripodal thiourea has been synthesized, which exclusively binds fluoride anion in DMSO, showing no affinity for other anions including, chloride, bromide, iodide, perchlorate, nitrate and hydrogen sulfate. As investigated by ¹H NMR, the receptor forms both 1:1 and 1:2 complex yielding the binding constants of 2.32(3) (in log β₁ ) and 4.39(4) (in log β₂ ), respectively; where quinoline groups are protonated by the fluoride-induced proton transfer from the solution to the host molecule. The 1:2 binding is due to the interactions of one fluoride with NH binding sites of urea sites and another fluoride with secondary ⁺NH binding sites within the tripodal pocket. The formation of both 1:1 and 1:2 complexes has been confirmed by the theoretical calculations based on density functional theory (DFT).

A thiourea-based tripodal receptor L substituted with 3-nitrophenyl groups has been synthesized, and the binding affinity for a variety of anions has been studied by ¹H NMR titrations and nuclear Overhauser enhancement spectroscopy experiments in dimethyl sulfoxide-d₆. As investigated by ¹H NMR titrations, the receptor binds an anion in a 1:1 binding mode, showing the highest binding and strong selectivity for sulfate anion. A competitive colorimetric assay in the presence of fluoride suggests that the sulfate is capable of displacing the bound fluoride, showing a sharp visible color change. The strong affinity of L for sulfate was further supported by UV-vis titrations and density functional theory (DFT) calculations. Time-dependent DFT calculations indicate that the fluoride complex possesses a different optical absorption spectrum (due to charge transfer between the fluoride and the surrounding ligand) than the sulfate complex, reflecting the observed colorimetric change in these two complexes. The receptor was further tested for its biocompatibility on primary human foreskin fibroblasts and HeLa cells, exhibiting an excellent cell viability up to 100 μM concentration.