Lower Rim 1,3-Di{4-antipyrine}amide Conjugate of Calix[4]arene: Synthesis, Characterization, and Selective Recognition of Hg<sup>2+</sup> and Its Sensitivity toward Pyrimidine Bases DessingouJayaraman TabbasumKhatija MitraAtanu HingeVijaya Kumar RaoChebrolu P. 2012 The structurally characterized lower rim 1,3-di­{4-antipyrine}­amide conjugate of calix[4]­arene (<b>L</b>) exhibits high selectivity toward Hg<sup>2+</sup> among other biologically important metal ions, <i>viz</i>., Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Fe<sup>2+</sup>, Co<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, Cd<sup>2+</sup>, Hg<sup>2+</sup>, Pb<sup>2+</sup>, and Ag<sup>+</sup> as studied by fluorescence, absorption, and ESI MS. <b>L</b> acts as a sensor for Hg<sup>2+</sup> by switch-off fluorescence and exhibits a lowest detectable concentration of 1.87 ± 0.1 ppm. The complex formed between <b>L</b> and Hg<sup>2+</sup> 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 <b>L</b> were derived on the basis of DFT computations and found that the Hg<sup>2+</sup> is bound through an N<sub>2</sub>O<sub>2</sub> 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 <b>L</b> from its Hg<sup>2+</sup> complex and were different from those of the simple mercuric perchlorate. <b>L</b> 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 <b>L</b> 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.