Lower Rim 1,3-Di{4-antipyrine}amide Conjugate of Calix[4]arene: Synthesis, Characterization, and Selective Recognition of Hg²⁺ and Its Sensitivity toward Pyrimidine Bases

The structurally characterized lower rim 1,3-di­{4-antipyrine}­amide conjugate of calix[4]­arene (L) exhibits high selectivity toward Hg²⁺ among other biologically important metal ions, viz., Na⁺, K⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, and Ag⁺ as studied by fluorescence, absorption, and ESI MS. L acts as a sensor for Hg²⁺ by switch-off fluorescence and exhibits a lowest detectable concentration of 1.87 ± 0.1 ppm. The complex formed between L and Hg²⁺ is found to be 1:1 on the basis of absorption and fluorescence titrations and was confirmed by ESI MS. The coordination features of the mercury complex of L were derived on the basis of DFT computations and found that the Hg²⁺ is bound through an N₂O₂ extending from both the arms to result in a distorted octahedral geometry with two vacant sites. The nanostructural features such as shape and size obtained using AFM and TEM distinguishes L from its Hg²⁺ complex and were different from those of the simple mercuric perchlorate. L is also suited to sense pyrimidine bases by fluorescence quenching with a minimum detection limit of 1.15 ± 0.1 ppm in the case of cytosine. The nature of interaction of pyrimidine bases with L has been further studied by DFT computational calculations and found to have interactions through a hydrogen bonding and NH-π interaction between the host and the guest.