Structure And Properties Of Luminous Metal-Organic Frameworks Based On Tetracarboxylic Acid Ligand

Metal-organic frameworks(MOFs)are a class of porous coordination polymers constructed by self-assembly of metal ions or metal cluster unit and organic bridging molecules(organic ligands).They have high specific surface area,high porosity,and structural functions.The advantages of controllability and diversity have important research value in the fields of adsorption,separation,catalysis,luminescence,and sensing,and so on.Among them,especially in recent years,as a light-emitting material,it can be used for various small molecule sensing,which has attracted widespread attention from scientists.To design and control the fluorescence emission of three-dimensional MOFs,the first step is to select the d10 elements,for example,Zn,Cd and other transition metal ions.Another key factor is that choice of ligands.The rigidity or flexibility,conjugation,groups,and size of the ligands can be used to design the cavity structure and chemical properties of MOFs,which are the regulation of their luminescence and selective fluorescence sensing performance of fundamental factor.Therefore,a series of semi-rigid and flexible tetracarboxylic acid ligands were selected in this paper,and a series of three-dimensional MOFs with fluorescent properties were designed and synthesized with transition metal Zn and Cd ions.The structure,physical and chemical properties,fluorescent luminescence,and small molecule sensing of materials had been studied.(1)Three MOFs with similar chemical components(M2:L1:Sol3)and topology structures were synthesized by hydrothermal method in different solvents using semi-rigid tetracarboxylic acid ligands:[Zn4(bptc)2(NMP)3(DM-F)(H2O)2]n(1-a),[Cd4(bptc)2(NMP)3(DMF)2(H2O)1]n(1-b)and {[Zn2(bptc)(DM-A)(H2O)2]·(DMA)2·H2O}n(2).Further we studied the fluorescence sensing properties of these three materials for nitroaromatic explosives(NACs)and metal cations.Founding that they have excellent fluorescence quenching effects on nitroaromatic explosives and iron(?)cations with great selectivity,sensitivity.In addition,studying the luminous effects of these luminescent MOFs on the encapsulation of lanthanide(?)cations.Founding that after encapsulating different lanthanide(?)cations,which the luminous intensity was remarkably enhanced,generating different light colors.(2)3-(3',5 '-dicarboxyphenoxy)phthalic acid(H4L)as a flexible tetracarboxylic acid ligand was selected to coordinated with Zn and Cd to assemble two novel 2-dimensional fluorescent MOFs,[Zn2L(H2O)3]n(1),{[Cd3(HL)2(H2O)2](DMA)}n(2)under hydrothermal conditions.The two compounds formed two-dimensional layers with different structures,and the two-dimensional layers were further expanded into three-dimensional structures through hydrogen bonds.The phase purities of compounds 1-2 were characterized by powder X-ray diffraction.The thermal stabilities and fluorescent properties were further investigated by thermogravimetry and solid-state fluorescence emission.This work provides an important theoretical and practical basis for the further design of the light-emitting flexible MOFs.