Upper Rim Allyl- and Arylazo-Coupled Calix[4]arenes as Highly Sensitive Chromogenic Sensors for Hg²⁺ Ion

The syntheses and chromogenic properties of calix[4]arenes, carrying 5,17-bisallyl-11,23-bis(p-X-phenyl)azo 3a−c, 5,11,17-triallyl-23-(p-X-phenyl)azo 4a−c, and 5,17-bis(hydroxypropyl)-11,23-bis(p-X-phenyl)azo groups on the upper rims 5a,b, are described. Unexpectedly, UV/vis spectra of the very popular 4-(4-nitrophenyl)azophenol-coupled calix[4]arenes 3c and 4c did not show any shift in λ_max when 10 different metal perchlorates were added separately to the host in a methanol−chloroform (v/v = 1/399) cosolvent. In contrast, the absorption spectra of calix[4]arenes with either 4-methoxyphenylazo (3b−5b) or 4-phenylazo (3a−5a) on the upper rim showed substantial bathochromic shifts (Δλ = 128−162 nm) upon the addition of soft metal ions (such as Hg²⁺, Cr³⁺, and Cu²⁺). The 4-(4-methoxyphenyl)azophenol-coupled calix[4]arenes (the 3b−5b series) are found to be highly sensitive for mercury ion (Hg²⁺) among the 10 different metal ions screened. Strong interactions between Hg²⁺ ion and the 4-(4-methoxyphenyl)azophenol(s) as well as the p-allyl groups are corroborated by the ¹H NMR studies of 3a,b·Hg²⁺ complexes. Furthermore, Job's plots revealed 1:1 binding stoichiometry for all these p-allyl- and arylazo-coupled calix[4]arenes with transition metal ions, and Benesi−Hilderbrand plots were used for the determination of their association constants.