Synthesis,Structures And Properties Study Of Metal-organic Frameworks With Lanthanide And Bipyridine Carboxylic Acids

Metal organic frameworks?MOFs?have been concerned as new porous materials during nearly 20 years by scientists.MOFs use metal ions or ion clusters as the center and the organic ligand as the skeleton to build a special topology structure through the regular arrangement.MOFs have good nature and important applications such as luminescence,electricity,magnetism,adsorption,gas storage,separation,catalysis and so on.MOFs based on lanthanide metals?Ln-MOFs?have excellent optical properties and are widely used in luminescence tuning,light-emitting diodes,ion or molecular detection,optical conversion photonic devices,LEDs,color screens,fluorescence sensing and biosensor applications.These applications is of great significance to the increasingly popular cross-disciplinary researches such as electronic technology and biotechnology.The lanthanide metal has an affinity for oxygen and is easily coordinated with O atom.Therefore,the organic compounds containing O atoms in a carboxylic acids are used as ligands for Ln-MOFs,such as Nitrogen-containing heterocycle with carboxylic acids.The appropriate ligands could improve the luminescence intensity of lanthanide metals and form metal organic frameworks with excellent luminescent properties.In this paper,2,2'-bipyridine-5,5'-dicarboxylic acid?bpydc?as the appropriate ligand and the lanthanide metal salt was prepared to synthesis five compounds by solvothermal method.Five compounds are [Ln₂?bpydc?₃?DMA?₂] ·2DMA?Ln= Sm?1?,Eu?2?,Tb?3??,[Sm₂?bpydc?₃?DMF?₂?H₂O?] ·2DMF?4?,[Sm₂?bpydc?₃?DMA?₂?H₂O?_0.5] ·1.5DMA?5?.Compounds 1-3 have same structures with different lanthanide metals but they have different properties of luminescence.Compound 4 has a 3D diamond-shaped pores using the lanthanide metal nitrate.Compound 5 uses the second ligand to obtain a better crystal and single crystal data.The metal ions in five compounds all coordinate with the O atoms.The differences are the source of O atoms and different coordination modes.The different coordination mode between the metal ions and ligand molecules or solvent molecules could be achieved by changing the reaction temperature,pressure,PH and the auxiliary of other condition,which could build many kinds of MOFs with different pores and structures.The five compounds were studied and analyzed by X-ray powder diffraction,thermogravimetric analysis,infrared spectroscopy and fluorescence spectroscopy.Compound 2?Eu?is chosen because of its qualites of stability and excellent luminescence and used to detect the eight metal ions of Al³⁺,Fe³⁺,Li⁺,Cd²⁺,Mg²⁺,Mn²⁺,Zn²⁺,Ca²⁺ qualitatively?soaked in DMA,the concentration of metal ion is 10-3mol / L?.By visual observation under the UV lamp,it is found that compound 2?Eu?emits the blue light in place of the original red color in the 0.01 mol / L Al³⁺ DMA solution and hardly emits light in the 0.01 mol / L Fe³⁺ DMA solution.The phenomenon of changes are confirmed by fluorescence spectra.Since Ln-MOFs have excellent optical properties,the compounds have been studied chosen from the five stable prepared compounds with good light-emitting properties.According to the different luminescent properties of compounds 2?Eu?and 3?Tb?,the metal centers were modified and doped with different ratios of Eu³⁺ and Tb³⁺ for preparing compound 6-9.With the increasing ratios of Tb³⁺ / Eu³⁺,the colors of the series of compounds gradually change from red to orange until green under UV lamp.Meanwhile,the emission characteristic peaks of Eu shift to the emission characteristic peaks of Tb in the fluorescence spectrum.The changes of fluorescence properties of these four compounds are also investigated by the fluorescence lifetime and quantum yield.