Construction,Fluorescence And Catalytic Properties Of MOFs Based On Bipyridyl Tetracarboxylic Acid Ligands

Metal-organic frameworks（MOFs）have attracted much attention due to their excellent luminescence properties,good magnetic properties,better adsorption properties,and efficient catalytic activities.Among them catalysis is one of the earliest proven applications and becomes one of the most promising applications of MOFs.A large amount of CO₂ emission will lead to the occurrence of various abnormal climates and frequent natural disasters.Much effort has been devoted to reducing the CO₂ content,and studies have found that the cycloaddition of CO₂ and epoxides（CCE）is atomically economical,but CO₂ has a certain chemical inertness,and the reaction is difficult to carry out.Therefore,it is necessary to develop catalysts for CCE.MOFs can be used as high-efficiency catalysts for CO₂ fixation due to their advantages such as post modifiable active sites,excellent selectivity and easy separation.Luminescent MOFs（LMOFs）have attracted much attention because of their tunable,stable structures and large specific surface areas for good selectivity,sensitivity,and recyclability.It is widely used in white light,detection of pollutants,p H sensing and anticounterfeiting fields.We chose bipyridyl tetracarboxylic acid ligands to synthesize five novel MOFs,and mainly studied their luminescent properties and catalytic activities.The main work contents are as follows:1.Fluorescence investigation of Ln-MOFs with 2,6-di（2,4-dicarboxyphenyl）-4-（pyridin-4-yl）pyridineHerein,four water stable novel microporous Ln-MOFs,namely[Ln₃（L~1）₂（HCOO）（H₂O）₅]·14H₂O（Ln=Eu,Tb,Dy and Gd）,have been synthesized by a solvothermal method using a new 2,6-di（2,4-dicarboxyphenyl）-4-（pyridin-4-yl）pyridine ligand（H₄L~1）and Ln³⁺.Structural analysis shows that the four MOFs are isostructural,and the 1-Eu will be described as an example.Crystallographic studies revealed that 1-Eu is a 3D framework in which 1D chains with alternating triple and single carboxylate bridges are interlinked by the L~1 ligands,and contains uncoordinated carboxyl O atoms,exposed N atoms,coordinated water molecules.As expected,1-Eu exhibits a strong red light emission at room temperature and good chemical stability in common solvents,even in aqueous solution of p H=2-13.Therefore,the strong luminescence,good stability,uncoordinated carboxylic acid O atom,bare N and coordinated water molecules in the framework make 1-Eu an excellent luminescence sensor for detecting Cu²⁺,Cr₂O₇^2-and Cr O₄^2-in various water bodies,and at the same time,the quenched 1-Eu sample recovered the luminescence intensity after washing with water,indicating that 1-Eu can serve as a multi responsive luminescent sensor for Cu²⁺,Cr₂O₇^2-and Cr O₄^2-.Moreover,1-Eu exhibits a reversible luminescence switch in response to acid-base vapor stimuli.Finally,we explored the luminescent sensing mechanism of 1-Eu toward the above different analytes.2.N-methylated 1-Eu MOF efficiently catalyzes the cycloaddition of CO₂ and epoxides.Bifunctional ionic metal-organic frameworks（MOFs）containing Lewis acid sites（unsaturated metal sites）and halide ions（Cl^-,Br^-and I^-）have attracted increasing attention due to their extra high activity for the CCE.1-Eu microporous channels with uncoordinated pyridyl N atoms,which are easy to be modified by N-methylation.Therefore,three bifunctional N-methylation 1-Eu MOFs,1-Eu-Me X（X=Cl^-,Br^-and I^-）,were successfully prepared from the 1-Eu MOF by a post-synthetic modification（PSM）method.1-Eu-Me X can efficiently catalyze the cycloaddition reaction without any cocatalyst and solvent.Among them,the 1-Eu-Me I catalyst displays the highest catalytic performance.Therefore,our work represents a rare demonstration of ionic MOFs as heterogeneous catalysts for efficient CO₂ fixation with epoxides.3.Synthesis,crystal structure and luminescence properties based on extended bipyridinium tetracarboxylic acid Pb（Ⅱ）complexA novel lead coordination compound based on extend bipyridinium ligand,[Pb₄L~2Cl₆（H₂O）₂]was successfully assembled under solvothermal condition and characterized.Single crystal structure studies reveal that compound 5 is a 2D layer.Adjacent Pb²⁺ions are doubly bridged by isophthalates andμ₂-Cl ions to form an infinite one dimensional（1D）ladder-like chain.The chains are further interconnected through the thiazolothiazolyl bipyridinium moiety to generate a 2D layer.And hydrogen bond interaction based on C-H···Cl and C-H···O forms the supramolecular 3D structure.Additionally,Solid state fluorescence shows 5 emit pale green fluorescence,which can be assigned to the inter-ligandπ*-πand/orπ*-n transitions.