Azocalix[4]arene with Three Distal Ethyl Ester Residues As a Highly Selective Chromogenic Sensor for Ca2+ Ions

Abstract Three azocalix[4]arenes with distal ethyl ester residues, 5-phenylazo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (2), 5-(o-methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (3), 5-(p-Methylphenyl)azo-25,26,27-tris[(ethoxycarbonyl)methoxy]-28-hydroxycalix[4]arene (4), were synthesized and their binding properties with metal ions were investigated by ultraviolet (UV)/visible spectroscopy. The chromogenic behavior of these compounds upon metal ion complexation indicates a specific selectivity toward Ca2+ ion in the presence of other cations tested. The stoichiometry of 3 to Ca2+ ion in the complex is 1:1 and the stability constant of the complex is 1.28×104 m −1.