Four new luminescent-organic frameworks exhibiting highly sensing of nitroaromatics: An experimental and computational insight

Abstract

Four new metal–organic frameworks (MOFs) namely {[Cd(L)(bpp)]·H2O}n (1), {[Zn(L)(bpp)]·H2O}n (2), {[Cd(L)(bpz)]·H2O}n (3), [Zn(L)(bib)(H2O)]n (4), (H2L = 1,4-bis(4-carboxylbenzyl)piperazine acid; bpp = 1,3-bis(4-pyridyl)propane; bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole, bib = 1,4-bis(imidazol-1-yl)butane)) have been designed, synthesized and characterized. The MOFs 1 and 2 are isostructural and possess 4-connected dia topology with rare 8-fold interpenetrating network. In 3, the dianionic ligand L and bpz ligands link binuclear Cd(II) centres to generate a 2D layer framework with a double stranded chain. While in 4, the Zn(II) centres are bridged by L and bib ligands to furnish a 2D framework, which further generates a 3D supramolecular structure through the hydrogen bonding interactions formed between coordinated water molecules and carboxylate groups. The MOF 1 have been utilized as a possible luminescent sensor to detect nitroaromatic compounds (NACs). The results indicates that 1 showed good sensitivity towards p-nitrophenol (PNP) in comparison to other NACs by the decline in its luminescent intensity. The detection limit for PNP was found to be extremely low (1.27 ppm in solution) which represents one of the most efficient MOF-based sensors for PNP. The most plausible mechanism associated with sensing have been explained with the aid of theoretical calculations which indicated that a simultaneous electron/energy transfer phenomenon operating between 1 and NACs contributes toward the decline in the luminescence intensity.