Conductivity of a series of (4,8)-connected lanthanide metal–organic frameworks with [Ln ₄ O ₅ ] clusters and high selectivity for visual sensing of nitrophenol compounds

A series of lanthanide (Ln) metal-organic frameworks, namely [Nd₄(Hdtztp)₂O(OH)₄(H₂O)₄]·5(H₂O)·DMF (Nd4), [Sm₄(Hdtztp)₂O(OH)₄(H₂O)₄]·5(H₂O)·DMF (Sm4), [Tb₄(Hdtztp)₂O(OH)₄(H₂O)₂(DMF)₂] (Tb4), and [Er₄(Hdtztp)₂O(OH)₄(H₂O)₂(DMF)₂] (Er4), were synthesized by using 2,5-bis-(2<i>H</i>-tetrazol-5-yl)-terephthalic acid (H₄dtztp) and lanthanide nitrate as raw materials under solvothermal conditions. X-ray single crystal diffraction reveals that the Ln4 compounds crystallize in the orthorhombic <i>Pmmn</i> space group. In these structures, four Ln ions are interconnected through four μ-3 OH and one μ-2 O bridge to form a [Ln₄O₅] cubane, which is surrounded by eight ligands to form 8-connected building units. These 8-connected building units are linked to 4-connected Hdtztp^3- forming the 4,8-connected three-dimensional (3D) frameworks of Ln4. The phase and purity of Ln4 polycrystalline samples were confirmed by elemental analysis, thermogravimetric analysis (TG) and powder X-ray diffraction (PXRD). The conductivity test proves the existence of π-π stacking electron transfer pathways between tetrazolium and aromatic carbon rings in the crystal, where the conductivity of Nd4 with the largest atomic radius is one order of magnitude higher than that of Er4 with the smallest atomic radius. Due to the excellent fluorescence performance and water stability, Tb4 can be used as a fluorescent probe for nitro compounds, and its detection limit of <i>p</i>-nitrophenol (4-NP) is 38.5 nM.