Conductivity of a series of (4,8)-connected lanthanide metal–organic frameworks with [Ln4O5] clusters and high selectivity for visual sensing of nitrophenol compounds

Abstract

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-(2H-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 Pmmn 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 Hdtztp³⁻ 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 p-nitrophenol (4-NP) is 38.5 nM.