Structural Design of Zinc-Directed Heterometallic Uranyl Organic Frameworks: Multifunction Detection of Cr2O72– and Metamitron Contaminants from Water

Based on the hard and soft acid and base (HSAB) theory, three heterometallic uranyl organic frameworks (UOFs), namely, [Zn(UO₂)(npa)₂(2,2′-bpy)(H₂O)] (1), [Zn(UO₂)(nip)₂(2,9-dm-1,10-phen)] (2), and [Zn(UO₂)(ntp)₂(4,7-dm-1,10-phen)(H₂O)]·(H₂O) (3), have been solvothermally synthesized by using a variety of different substituted nitro-benzenedicarboxylic acid (H₂npa = 3-nitrophthalic acid, H₂nip = 5-nitroisophthalic acid, H₂ntp = 2-nitroterephthalic acid) and uranyl zinc acetate, in the presence of N-bearing coligands, including 2,2′-bipyridine (2,2′-bpy), 2,9-dimethyl-1,10-phenanthroline (2,9-dm- 1,10-phen), and 4,7-dimethyl-1,10-phenanthroline (4,7-dm-1,10-phen). The results of single-crystal X-ray diffraction analysis show that each of them presents a heterometallic interaction between the uranyl and Zn²⁺ centers. UOFs 2 and 3 adopt three-dimensional (3D) frameworks with different architectures, while UOF 1 exhibits a one-dimensional (1D) chain assembly. Furthermore, 1 can be used as a bifunctional luminescent sensor for the detection of metamitron (MMT) and Cr₂O₇^2– in aqueous solution with the limit of detection (LOD) for 4.06 × 10^–6 and 2.52 × 10^–6 M, respectively. The sensing mechanism was also investigated in detail through ultraviolet–visible (UV–vis) absorption spectroscopy, density functional theory calculations (DFT), and fluorescence lifetime analysis. This work provides valuable guidance for the facile and effective design and construction to employ UOFs as multiresponsive fluorescence sensors for the detection of environmental pollutants in aqueous solution.