MOFs Based On Pyridine Carboxylic Acid And Viologen Ligands: Structure,Chemical Sensing And Photochromic Properties

Metal-organic frameworks（MOFs）have potential applications in many fields due to its features of variable composition,structure and functional post-modification.Among them,its applications in chemical sensing materials and photochromic materials have attracted extensive attention of scientists.In the chemical sensing materials,the fluorescent MOFs sensing materials can realize the analysis and detection of the analysts through the change of fluorescence intensity;while MOFs based on the viologen ligands have photochromic properties caused by photo-induced electrons transfer owing to its lack of electron nature on one hand,and on the other hand,they may produce visual color responses to some different kinds of electron-rich materials.In this thesis,seven MOFs were constructed by combing lanthanide metal ions and transition metal ions with pyridine carboxylic acid and viologen ligands,and the synthesis,structure,chemical sensing and photochromic properties were studied in detail.This thesis is mainly divided into the following five chapters:The first chapter is the introduction,which introduces the research background and practical application of MOFs in fluorescence sensing and photochromic materials,forecasts the application prospects of fluorescent sensing and photochromic MOFs,and puts forward the basis of this paper.In the second chapter,a series of isomorphic Ln-MOFs（Eu-MOF-1,Tb-MOF-2,Gd-MOF-3）and mixed-metal Eu_xTb_yGd_1-x-y-DPNC were synthesized by the self-assembly of Ln^Ⅲ ion and H₃DPNC（H₃DPNC=5-（3’,5’-dicarboxylphenyl）nicotinic acid）.By adjusting the molar ratios of Ln^Ⅲ and different excitation wavelengths,a novel single-phase mixed-metal lanthanide white light-emitting MOF Eu_0.06Tb_0.04Gd_0.9-DPNC（MOF-4）was successfully prepared under the excitation of365 nm.It shows obvious fluorescence quenching for MnO₄^-and obvious fluorescence enhancement for folic acid（FA）in water system.In addition,by adding FA into the quenched system Eu_0.06Tb_0.04Gd_0.9-DPNC@MnO₄^-,the"turn-on plus"system was successfully obtained.Based on this,we also designed a new Boolean logic gate operation.In the third chapter,a three-dimensional cationic pcu framework Co-MOF-5with 1D channels was synthesized through solvothermal method by using viologen ligand[H₂L¹]Cl₂、N₃^-and Co（II）,which has reversible electrochemical redox and photochromic behaviors.Photochromism of Co-MOF-5 is caused by the production of viologen free radicals in the photoinduced electron transfer process.The carboxyl groups on the organic ligand and the nitrate ions in the pore act as the electron donors,transferring electrons to the viologen parts that lack electrons.More importantly,Co-MOF-5 also exhibits different color responses to volatile amines of different types（primary,secondary,and tertiary）,which is attributed to the different degree of interactions between the amines and Co-MOF-5 due to the different sizes of substituents on amine molecules and the different number of N-H bonds of amine molecules.In addition,a portable detection paper was successfully prepared by loading Co-MOF-5 on the filter paper,which provides a simple method for visually detecting various volatile amines,and the filter paper can also be used as an ink-free and erasable printing medium.In chapter four,two MOFs of Mn-MOF-6 and Mn-MOF-7 with one-dimensional zigzag chain and wavy chain structures were synthesized by slovothermal method and room temperature volatilization methods respectively with viologen ligands[H₂L²]Cl₂ and[H₂L¹]Cl₂ and transition metal Zn²⁺and Mn²⁺.They were characterized by IR,X-ray single crystal diffraction,element analysis,XPS,XRD,EPR,UV-Vis and so on.Both of them show obvious reversible electrochemical redox and photochromic properties in air.The fifth chapter mainly summarizes the research contents of all previous chapters and gives the final further conclusions.