Syntheses,structures And Fluorescence Sensing Activities Of MOFs Based On Flexible Polycarboxylic Acid Ligands

Metal-organic frameworks(MOFs)are a class of coordination polymers formed by self-assembly of metal ions and organic building units.They have high specific surface area,permanent porosity,controllable structures and properties.Among them,LMOFs(Luminescent Metal-Organic Frameworks)can be used as luminescent probes or luminescent sensing materials for the detection of ions,organic compounds,biological macromolecules and other substances.It has the characteristics of simple,fast,high sensitivity and high selectivity,and has broad application prospects in the field of luminescent chemistry and biosensor.In this disseration,eight novel complexes were synthesized by the reaction of the flexible polycarboxylic acid ligands with metal ions with d¹⁰ electronic configuration Zn²⁺,Cd²⁺in the presence of different nitrogen co-ligands.The investigation of luminescence sensing properties showed that they had multiple luminescence responses toward metal ions,antibiotics and other substances.The main work of this disseration includes the following three aspects:1.ThreeZn-MOFs,namely{[Zn(L1)_0.5(bpea)]?0.5DMF}_n(1),{[Zn(L1)_0.5(ipt)]?H₂O·DMF}_n(2),[Zn₂(L1)(5,5'-dmbpy)₂(H₂O)(DMF)]_n(3),were synthesized based on flexible tetracarboxylic acid 1,3-bis(3,5-dicarboxyphenoxy)benzene(H₄L1).The luminescence sensing properties of organic solvents,ions and antibiotics were studied.Complexes 1 and 2 have three-dimensional two-fold interpenetrating pillared-layer structures,in which two-dimensional layer was formed by carboxylic acid and Zn²⁺ions and pillared by nitrogen-containingco-ligands,1,2-dipyridylethane(bpea)and3-(4'-imidazolobenzene)-5-(pyridine-4'-yl)-1,2,4-triazole(ipt).Complex 3 exhibits a one-dimensional chain structure.The carboxylate acid ligands display three different coordination modes.Due to the existence of ether oxygen atoms,the aromatic rings in the ligands could form different dihedral angles from 34.65°to 78.14°.The results showed that:(1)the luminescence quenching of complexes 1 and 2 occurred in the aqueous solutions of Fe³⁺,Cr O₄^2-,Cr₂O₇^2-,and nitrofurazone(NFZ),with the detection limits of 0.35-3.78?M.These complexes have excellent anti-interference and recyclability;(2)complex 3 has luminescence quenching toward Co²⁺,Ni²⁺,and C₂O₄^2-in H₂O,and the detection limit is0.10-4.36?M.The luminescence quenching mechanism analysis shows that the luminescence quenching of these complexes in Fe³⁺,Cr O₄^2-,Cr₂O₇^2-aqueous solutions mainly comes from internal filtering effect of stimulated luminescence and the weak interaction between the framework structure and the target analytes.The luminescence quenching in Co²⁺,Ni²⁺solutions mainly comes from the change of the structure of the complex.2.Three Cd-MOFs were synthesized based on 1,3-bis(3,5-dicarboxybenzoxy)benzene(H₄L1)of flexible tetracarboxylic acid:{[Cd₂(L1)(bpp)(H₂O)₂]?H₂O?DMF}_n(4)?[Cd₂(L1)(5,5'-dmbpy)₂(H₂O)(DMF)]_n(5)and{[Cd_1.5(L1)]?H₂O·2DMF}_n(6).The L1^4-ligand in complex 4 connected Cd²⁺ions to form a two-dimensional layered structure,which was futher connected by 1,3-dipyridyl propane(bpp)ligands to form a three-dimensional network structure.Complex 5 is isomorphic to complex 3,which also exhibits one-dimensional chain structure linked by carboxylic acid.The six-coordinated Cd1²⁺and Cd2²⁺centers in complex6 are bridged by L1^4-ligands to form a three-dimensional network structure.The dihedral angles of benzene rings at either end of L1^4-ligands are 65.68°,80.23°and 79.62°respectively.The results of luminescence analysis show that:(1)complex 4 displays significant luminescence enhancement in THF,and luminescence quenching in NFZ aqueous solution(the detection limits was 0.85?M);(2)the luminescence of complex 5 was quenched in Co²⁺and Ni²⁺aqueous solution,while enhanced in Zn²⁺aqueous solution(the detection limit was 0.21-0.27?M)and quenched in the aqueous solution of nitroimidazole antibiotics:metronidazole(MDZ),ornidazole(ODZ)and ronidazole(RDZ),the detection limit was1.18-1.46?M;(3)the luminescence of complex 6 was quenched in Cr³⁺and Pb²⁺aqueous solution(the detection limit were 1.36?M and 0.71?M).The luminescence quenching mechanism analysis shows that the quenching of complex 5 in Co²⁺and Ni²⁺solutions may be due to the change of the structure of the complex,and the quenching in antibiotic solutions may be due to the internal filtering effect of stimulated luminescence.The quenching of complex 6 results from the weak interaction between O and Cr³⁺,Pb²⁺in the framework.3.Two Cd-MOFs,{[Cd₃(L2)₂(bpea)₃(H₂O)₄]?2H₂O·4DMF}_n(7)and[Cd₃(L2)₂(4,4'-bpy)]_n(8),were synthesized from flexible tricarboxylic acid3,5-bis[(dicarboxyphenyl)oxy]benzoic acid(H₃L2).These two complexes can detect Fe³⁺,Cr O₄^2-,Cr₂O₇^2-in aqueous solution by luminescence quenching,the detection limits were0.21-0.44?M for complex 7 and 0.90-1.31?M for complex 8.In particular,the luminescence of complex 8 was also quenched in Pb²⁺aqueous solution(the detection limits was 1.43?M).When vitamin C was added into the system,Fe³⁺and Cr₂O₇^2-could be effectively distinguished.The results of anti-interference,recycling,and cross test,UV-Vis absorption,reduction and time response test show that these two complexes have good selectivity,cyclicity and sensitivity.We also explored the practical application performance of two complexes,and their detection ability was verified in tap water by standard addition method.High recoveries in the range of 96.3-103.4%were obtained,indicating that complexes 7 and 8are expected to be employed for the detection of Fe³⁺,Cr O₄^2-,Cr₂O₇^2-and Pb²⁺in actual samples.The quenching of Fe³⁺,Cr O₄^2-,and Cr₂O₇^2-may be due to inner filter effect and the synergistic effect of weak interaction,and the quenching of Pb²⁺may be due to the weak interaction between Pb²⁺and O atoms.Through the structural analysis and luminescence properties test of the above eight novel LMOFs,It was found that the construction units,structures and sensing medium all have influence on the sensing behavior of the complexes.The ether oxygen atoms introduced as the active sites of Lewis acids and the inner filter effect of luminescence together affect the sensing process.Therefore,the functional groups and flexible structures of ligands play an important role in regulating the structure,luminescent properties and detection ability of complexes.