A supramolecular Tröger's base derived coordination zinc polymer for fluorescent sensing of phenolic-nitroaromatic explosives in water

Abstract

A V-Shaped 4-amino-1,8-napthalimide derived tetracarboxylic acid linker (<b>L</b>; bis-[<i>N</i>-(1,3-benzenedicarboxylic acid)]-9,18-methano-1,8-naphthalimide-[<i>b</i>,<i>f</i>][1,5]diazocine) comprising the Tröger's base (TB) structural motif was rationally designed and synthesised to access a nitrogen-rich fluorescent supramolecular coordination polymer. By adopting the straight forward precipitation method, a new luminescent nanoscale Zn(ii) coordination polymer (<b>TB-Zn-CP</b>) was synthesized in quantitative yield using Zn(OAc)₂·2H₂O and tetraacid linker <b>L</b> (1 : 0.5) in DMF at room temperature. The phase-purity of as-synthesised <b>TB-Zn-CP</b> was confirmed by X-ray powder diffraction analysis, infra-red spectroscopy, and elemental analysis. Thermogravimetric analysis suggests that <b>TB-Zn-CP</b> is thermally stable up to 330 °C and the morphological features of <b>TB-Zn-CP</b> was analysed by SEM and AFM techniques. The N₂ adsorption isotherm of thermally activated <b>TB-Zn-CP</b> at 77 K revealed a type-II reversible adsorption isotherm and the calculated Brunauer-Emmett-Teller (BET) surface area was found to be 72 m² g^-1. Furthermore, <b>TB-Zn-CP</b> displayed an excellent CO₂ uptake capacity of 76 mg g^-1 at 273 K and good adsorption selectivity for CO₂ over N₂ and H₂. The aqueous suspension of as-synthesized <b>TB-Zn-CP</b> showed strong green fluorescence (<i>λ</i>_max = 520 nm) characteristics due to the internal-charge transfer (ICT) transition and was used as a fluorescent sensor for the discriminative sensing of nitroaromatic explosives. The aqueous suspension of <b>TB-Zn-CP</b> showed the largest quenching responses with high selectivity for phenolic-nitroaromatics (4-NP, 2,4-DNP and PA) even in the concurrent presence of other potentially competing nitroaromatic analytes. The fluorescence titration studies also provide evidence that <b>TB-Zn-CP</b> detects picric acid as low as the parts per billion (26.3 ppb) range. Furthermore, the observed fluorescence quenching responses of <b>TB-Zn-CP</b> towards picric acid were highly reversible. The highly selective fluorescence quenching responses including the reversible detection efficiency make the nanoscale coordination polymer <b>TB-Zn-CP</b> a potential material for the discriminative fluorescent sensing of nitroaromatic explosives.