Calix[4]amido crown functionalized visible sensors for cyanide and iodide anions

This study comprises the design and development of calix[4] arene-amido-based ionophores by varying structural stringency and steric hindrance at the lower rim to probe the anion sensing properties. The ionophores are prepared, purified, and characterized using various analytical techniques. The molecular structure of the most active ionophore I is established by single-crystal X-ray characterisation. Out of various anions investigated, iodide and cyanide show the highest sensitivity towards the ionophores investigated. Both anions are sensitive enough to give a visibly distinct color change. The binding properties of the ionophores are established with ¹H & ¹²⁷I NMR, fluorescence, and UV-vis spectroscopy, revealing that three ionophores strongly interact with CN^- and I^-. The binding constants are calculated via Benesi-Hildebrand plots using absorption data. The time-dependent ¹H NMR revealed strong hydrogen bonding between the OH and NH groups of the ionophore and cyanide anion. The ¹²⁷I NMR shows the highest 27.6 ppm shift after 6 h for ionophore I. The crystal structure revealed hydrogen bonding of N-H protons of the amide pendulum and phenolic oxygen of the calix rim. The Job's plot depicted the possibility of a 1 : 1 complex of ionophores with both anions.