Assessment of three azophenol calix[4]arenes as chromogenic ligands for optical detection of alkali metal ions

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Abstract
Three novel chromogenic cone conformational calix[4]arene tetraesters bearing nitrophenylazophenol residues (mono-, di- and tetra-substituted) in the ester portion of the molecules have been synthesized and shown to display a dramatic change in absorbance spectrum upon complexation with lithium, and to a lesser extent sodium, in the presence of a base. An instantaneous colour change from yellow to red corresponding to a λmax shift from 380 to 520 nm, is noted upon the addition of lithium perchlorate to a solution containing any of the three ligands with the intensity of the colour being concentration dependent. The best selectivity for lithium against sodium and potassium was exhibited by the mono-substituted derivative, ligand 4, (approximately 70-fold selectivity in both cases). Although this is somewhat less than that required for clinical applications, it is anticipated that it can be further improved using a variety of approaches, which are the subject of further investigations. The interference from potassium is too small to be measured.

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