Synthesis And Fluorescence Properties Of Luminescent Metal-Organic Frameworks

Metal-organic frameworks（MOFs）as a new class of materials are composed of metal ions and organic ligands.They have been used in catalysis,gas adsorption and fluorescence recognition due to the diverse structure,tunability of pore channels and high specific surface area.Luminescent metal-organic frameworks（LMOFs）contain two kinds of luminescent units including metal ions and organic ligands with antenna effect or electron transfer,which exhibit fluorescence activity to respond to external stimuli.Therefore,LMOFs can be used as fluorescent probes to detect metal cations,inorganic anions,pesticide molecules and biological small molecules.In this work,the transition metal ion including Zn（II）,Co（II）and Ni（II）were selected to synthesize five novel water-stable MOFs based on nitrogen-containing ligands and carboxylic acid-containing ligands.The structures and properties of the complexes were characterized.The main research contents are as follows:1.Three novel water stability MOFs 1-3 were constructed under solvothermal conditions based on the ligands of 3,5-bis（1-imidazoly）pyridine（Bimpy）and 1,4-naphthalene dicarboxylicacid（1,4-ndc）,namely,[Zn（Bimpy）（1,4-ndc）]·H₂O（1）,[Ni（Bimpy）（1,4-ndc）·2H₂O]（2）,[Co（Bimpy）（1,4-ndc）·2H₂O]（3）.Single crystal X-ray diffraction shows that complex 1 is a 2D layered structure,which is linked with each other by theπ-πstacking to get a 3D supramolecular structure.Complexes 2 and 3 are isomorphic 2D structure.The 2D layer structures are connected by hydrogen bonding to form 3D supramolecules.The fluorescence experiments show that complex 1 exhibits good luminescence performance in both aqueous solution and p H=2-10 solutions.In the presence of other interfering ions,complex 1 shows an excellent selectivity for Fe³⁺in aqueous solution.It is worth noting that complex 1 can recognize ascorbic acid（AA）by fluorescence“turn-on”,and has high sensitivity(K_sv=1.47×10⁴M^-1)and low detection limit(LOD=6.12×10^-7M).It is noteworthy that the fluorescence intensity of Fe³⁺@1 fluorescence-silent system can be recovered（turn-on）with the addition of AA.The luminescent results show that complexes 2and 3 have high selectivity and sensitivity for the detecting of Fe³⁺and Mn O₄^-in aqueous solution.In addition,complexes 2 and 3 can be used for the high-efficiency detection of pesticide 2,6-Dich-4-NA（DCN）,and the LOD are 3.9×10^-7M and 5.5×10^-7M,respectively.The recognition mechanism of complexes 1-3 for different analytes were investigated by X-ray powder diffraction（PXRD）,X-ray photoelectron spectroscopy（XPS）and ultraviolet-visible absorption spectroscopy（UV-vis）.2.Two MOFs were successfully synthesized by solvothermal method using the ligands of 4,4′,4″-nitrilotrisbenzoic acid（H₃TNB）,1,2-di（4-pyrigdl）ethylene)（DPE）and1,4-bis（1-imidazoly）benzene（Bimb）:[Zn₃（TNB）（DPE）₂]·2H₂O·DMF（4）,[Co₃（TNB）₂（Bimb）]·2H₂O（5）.The single-crystal X-ray diffraction analysis shows that complex 4 is a 3D+3D→3D double intersperse network structure.Complex 5 is a 3D structure.Luminescence tests suggest that complex 5 can detect Fe³⁺and Pb²⁺as a‘‘turn-off’’fluorescence probe in PBS solution with high sensitivity and selectivity.Additionally,with the change of concentration of Fe³⁺and Pb²⁺,an obvious colour changes can be observed under ultraviolet light.Notably,the Fe³⁺@4 system exhibits a fluorescence‘‘turn-on’’signal after adding reduced glutathione（GSH）due to the reduction reaction,which indicates that complex4 can be designed as an“on-off-on”fluorescence probe to detect Fe³⁺ion and reduced GSH.In addition,the“on-off-on”fluorescent sensor can be successfully applied to detect of reduced GSH in serum and garden stuff with high recovery range.Fluorescence performance results show that complex 5 exhibits high sensitively and selectively for Fe³⁺with low detection limit.In addition,complex 5 can specifically recognize DCN and the LOD is8.2×10^-8M^-1.