Experimental and Computational Studies of Selective Recognition of Hg<sup>2+</sup> by Amide Linked Lower Rim 1,3-Dibenzimidazole Derivative of Calix[4]arene: Species Characterization in Solution and that in the Isolated Complex, Including the Delineation of the Nanostructures

Amide linked lower rim 1,3-dibenzimidazole derivative of calix[4]arene, L has been shown to be sensitive and selective to Hg<sup>2+</sup> in aqueous acetonitrile solution based on fluorescence spectroscopy, and the stoichiometry of the complexed species has been found to be 1:1. The selectivity of L toward Hg<sup>2+</sup> has been shown among 11 M<sup>2+</sup> ions, viz., 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>, Ca<sup>2+</sup>, and Mg<sup>2+</sup> studied, including those of the mercury group and none of these ions impede the recognition of Hg<sup>2+</sup> by L. Role of the solvent on the recognition of Hg<sup>2+</sup> has been demonstrated. The role of calix[4]arene platform and the benzimidazole moieties in the recognition of Hg<sup>2+</sup> by L has been delineated upon performing such studies with five different molecules of relevance as reference molecular systems. The binding cores formed by the receptor L and the reference compounds have been established based on the single crystal XRD structures, and the preferential metal ion binding cores have been discussed. The binding of Hg<sup>2+</sup> with L has been further established based on <sup>1</sup>H and <sup>13</sup>C NMR, ESI MS, absorption, and fluorescence lifetime measurements. Some of these techniques have been used to establish the stoichiometry of the species formed. The complex species formed between L and Hg<sup>2+</sup> have been isolated and characterized and found to be 1:1 species even in the isolated complex. Whereas transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) provided the nanostructural behavior of L, the TEM and SEM demonstrated that the mercury complex has different characteristics when compared to L. The TEM, SEM, and powder XRD studies revealed that whereas L is crystalline, that of the mercury complex is not, perhaps a reason for not being able to obtain single crystals of the complex. Binding characteristics of Hg<sup>2+</sup> toward L have been established based on the DFT computational calculations.