The Construction Of Luminescent Coordination Polymers Based On Nitrogen Heterocyclic Carboxylic Acid Ligands And Their Fluorescence Recognition For Small Organic Molecules

Coordination polymers are formed by self-assembly of metal ions or metal clusters as inorganic metal centers and bridged organic ligands.Due to their diverse structure,adjustable pore size,good stability,high specific surface area and porosity,and post-treatment modification,they have broad application prospects in sensing,gas separation and adsorption,drug delivery,catalysis,electrochemical storage,magnetic properties,etc.Luminescent coordination polymers,also known as fluorescent metal organic frameworks（LMOFs）,as an important branch of metal organic skeleton materials,have the advantages of long fluorescence life,high quantum yield,sensitivity to the surrounding environment,low cost and simple detection process.They are important members of the sensing material family,and can be used as fluorescent sensors to detect nitroaromatic compounds,organic amines and other small organic molecules.The selection of ligands plays a very important role in the construction of LMOFs,especially the nitrogen-containing heterocyclic carboxylic acid ligands are widely used because of their diverse coordination atoms and rich coordination modes.In this paper,using 6-mercaptonicotinic acid and 5-（1,2,4-triazol-1-yl）isophthalic acid as organic ligands,transition metal ions such as Zn（Ⅱ）,Mn（Ⅱ）,Cd（Ⅱ）,Cu（Ⅰ）and rare earth metal Eu（Ⅲ）as metal centers,four novel luminescent coordination polymers materials were synthesized by solvothermal method,and were comprehensively characterized by various test methods.The specific experimental contents are as follows:1.A novel heterometallic compound{[Cu₆Zn₄（mna）₆（en）₂（H₂O）₄}·2H₂O}_n（1）containing Cu₆S₆ cluster and Zn₄en₂（COO）₄ was synthesized by using 6-mercaptonicotinic acid（6-mna）as an organic ligand and Cu Cl,Zn（OAc）₂·2H₂O metal salt under solvothermal conditions.Using 6-mercaptonicotinic acid（6-mna）as an organic ligand,a three-dimensional luminescent compound{[Cd₄（mna）₄（en）₂]}·4H₂O}_n（2）was synthesized under solvothermal conditions with Cu SCN and Cd（OAc）₂·2H₂O metal salts.A three-dimensional compound{[Mn₃（mna）₃]}·4H₂O}_n（3）was synthesized by using 6-mercaptonicotinic acid（6-mna）as an organic ligand and Cu SCN and Mn（OAc）₂·4H₂O metal salts under solvothermal conditions.The three compounds were fully characterized by single crystal X-ray diffraction,elemental analysis,powder X-ray diffraction,infrared spectrum and thermogravimetric analysis.Interestingly,it shows obvious red fluorescence under the ultraviolet lamp.The fluorescence experiment shows that compound 1 can effectively detect nitroaromatic compounds（NACs）,including 2-nitrophenol,4-nitrophenol,2,4-dinitrochlorobenzene,nitrobenzene,2-nitrotoluene,4-nitrotoluene,1,3-dinitrobenzene and pentachloronitrobenzene.The corresponding Ksv values are 7.20×10³ M^-1,5.41×10³ M^-1,5.31×10³ M^-1,4.40×10³ M^-1,4.01×10³ M^-1,3.96×10³ M^-1,3.62×10³ M^-1and 2.33×10³ M^-1,corresponding detection limits are 9.51×10^-7 M,1.125×10^-6 M,1.149×10^-6 M,1.242×10^-6 M,1.30×10^-6 M,1.31×10^-6 M,1.403×10^-6 M and 1.933×10^-6 M.At the same time,the fluorescence sensing experiment shows that compound 2 has good yellow luminescence performance and can effectively detect nitroaromatic compounds（NACs）in water through fluorescence quenching.The 5-cycle cycle experiment proves that compound 2 has good reproducibility and can be used as an excellent fluorescence probe for detecting NACs.2.A new Ln-MOF:{[Eu₄（taip）₄（ox）（OH）₂（H₂O）₄]·3H₂O}_n（4）was synthesized by using5-（1,2,4-triazol-1-yl）isophthalic acid as organic ligand and rare earth metal Eu³⁺under solvothermal conditions.Its structure was characterized by single crystal X-ray diffraction,infrared spectroscopy,thermogravimetric analysis and powder X-ray diffraction,showing a fascinating 3D network structure containing butterfly Eu₄（OH）₂clusters.Due to the antenna effect,it shows strong red light emission.Its fluorescence characteristics in various organic solvents have been studied,and the results showed that it can be used as a fluorescent sensing material for detecting organic amines,and the corresponding quenching mechanism has been studied.It is found that electron transfer is the main reason for its fluorescence quenching.