Dipyrenylcalix[4]arene-A fluorescence-based chemosensor for trinitroaromatic explosives

A new chemosensor-based approach to the detection of nitroaromatics is described. It involves the analyte-induced quenching of excimer emission of a dipyrenyl calix[4]arene (L). The chemical and photophysical properties of the complexes formed between L and mono-, di-, and trinitrobenzene, and di- and trinitrotoluene were studied in acetonitrile and chloroform by using ¹H NMR, UV/Vis, and fluorescence spectroscopy. Fluorescence spectroscopy revealed that the trinitroaromatics engendered the largest response among the various substrates tested, with the sensitivity for these analytes being correspondingly high. Quantitative analysis of the fluorescence titration profile generated from the titration of L with TNT provided evidence that this particular functionalized calix[4]arene receptor allows for the detection of TNT down to the low ppb level in CH₃CN. A single-crystal X-ray diffraction analysis revealed that in the solid state the complex L·TNT consists of a supramolecular crystalline polymeric structure, the formation of which appears to be driven by intermolecular π-π interactions between two pyrene units and a TNT molecule held at a distance of 3.2-3.6 Å, as well as by intra- and intermolecular hydrogen-bonds among the amide linkages. Nevertheless, the changes in the ¹H NMR, UV/Vis, and fluorescence spectrum, including sharp color changes, are ascribed to a chargetransfer interaction arising from complementary π-π overlap between the pyrene subunits and the bound trinitroaromatic substrates. A number of ab initio calculations were also carried out and, considered in concert, they provide further support for the proposed charge-transfer interactions, particularly in the case of L·TNT.