Novel calix[4]arene-based receptors with bis-squaramide moieties for colorimetric sensing of anions via two different interaction modes

Novel calix[4]arene-based anion receptors 1–3 with bis-squaramide moieties were designed and synthesized. Their sensing properties with various anions were investigated by UV–vis, ESI-MS, and 1H NMR spectra. The experimental results revealed that there were two types of interaction modes where receptor 1 bound biologically important H2PO4−, F−, and AcO− ions selectively over other anions in the hydrogen bonding interaction mode, while receptors 2 and 3 displayed deprotonation behavior with basic anions (F−, AcO−, and H2PO4−) via acid–base interaction mode. Receptors 2 and 3 were proved to be efficient ‘naked-eye’ sensors for F− and AcO− with marked color changes.