Strategic Construction of Highly Stable Metal-Organic Frameworks Combining Both Semi-Rigid Tetrapodal and Rigid Ditopic Linkers: Selective and Ultrafast Sensing of 4-Nitroaniline in Water.

In this work, we have designed two new 3D metal-organic frameworks (MOFs), {Zn4(TPOM)(1,4-NDC)4} n (1) and {Ni2(TPOM)(1,4-NDC)2(H2O)2} n (2), utilizing both semi-rigid tetrapodal neutral linker, tetrakis(4-pyridyloxymethylene)methane (TPOM) and rigid ditopic anionic linker, 1,4-naphthalene dicarboxylic acid (H2(1,4-NDC)). On the basis of the single-crystal X-ray diffraction, 1 has a 3D structure with star-shaped pores arising from four-fold symmetry due to the presence of a paddle-wheel core [Zn2(O2CC12H6)4(C6H4N)2] as a subunit, whereas 2 consists of a zig-zag 3D framework with strong hydrogen bonding between the coordinated water molecules and coordinated carboxylate groups. Their thermogravimetric analysis indicates an extraordinary thermal stability: 1 up to 400 °C and 2 up to 350 °C. In addition to elemental microanalysis and spectroscopic characterization (UV-vis and infra-red spectroscopy), the bulk phase purity of 1 and 2 as well as hydrolytic stability of 1 are established by powder X-ray diffraction. Exploiting the luminescence nature of 1, both solvent-dependent fluorescence properties and sensing of various amines in aqueous medium are demonstrated. It exhibits good sensing ability toward 4-nitroaniline (4-NA) and 2,6-dichloro-4-nitroaniline (2,6-DCNA; a broad spectrum pesticide belonging to toxicity class III) with the lowest detection limit of 88 ppb and 0.28 ppm, respectively. The mechanism of action has been established through Stern-Volmer plots, time-resolved fluorescence studies, spectral overlap, and density functional theory calculations. The recyclability and stability of 1 after sensing experiments also reveal no change in its crystallinity. Furthermore, selectivity test and time-dependent detection for 4-NA have been successfully demonstrated. For practical applications, naked eye detection of 4-NA using test paper strips is also displayed.