| Smart materials that respond to chemical stimuli with color or luminescence changes are highly desirable for daily-life and high-tech applications.Among many candidates,metal-organic frameworks(MOFs),a relatively new class of porous crystalline hybrid materials,have attracted much attention because of their high versatility in functionalization through modification about the metal nodes,the organic linkers,and the pores.In particular,the variable porous frameworks can be designed with specific electronic properties to afford responsivity to specific chemicals through host-guest interactions.One of the most effective ways to make MOFs multifunctional is introducing building units with special physical/chemical properties into MOFs framework.The electron-deficient 4,4'-bipyridinium unit is easy to accept electrons under different external stimuli and can be converted into the radical state,and its structure does not change significantly before and after the transition.Therefore,the ligands containing bipyridinium unit have been widely used in the construction of reversible stimulation-responsive MOFs.In order to expand the structure and properties of the stimulation-responsive MOFs of pyridinium family and obtain new functions and structural-functional relationships,tripyridinium tricarboxylate amphoteric ion ligand 2,4,6-tri(1-(4-carboxyphenyl)pyridinium)-4-pyridinium([H3tcbpp]3+),2,4,6-tri(1-(4-carboxyphenyl)pyridinium)-4-benzene([H3tcbpph]3+)were used to obtain a series of functional MOFs,and their stimulus responses were investigated.The main research contents are as follows:1.Synthesis and functionality of MOFs based on tris(pyridinium)tricarboxylate ligand(1)Compound 1-Eu([Eu4(OH)4(tcbpp)2(H2O)9]Cl8·2H2O·6Et OH)was synthesized by[H3tcbpp]Cl3 ligand and Eu3+ion under solvothermal conditions.Single-crystal X-ray diffraction(SXRD)analysis demonstrates that complex 1-Eu crystallizes in the triclinic system with space group P1 and shows 3D frameworks.The framework is based on an asymmetric tetranuclear secondary building unit(SBU)of[Eu4(?3-OH)4(COO)6(H2O)9]2+,all Eu(III)ions are eight-coordinated in the dodecahedron geometry with severe distortion toward tetragonal antiprism.And the connection of the[Eu4(?3-OH)4]8+clusters by the ligands leads to a cationic 3D framework.From the topological point of view,the 3D framework can be simplified to a 3,6-connected net with point symbol{4.62}2{42.610.83}.Two identical frameworks related by translation along the a-axis interpenetrate each other to occupy the space inindividual frameworks.It shows reversible and discriminative chromic response to aliphatic amines and aniline through different host-guest interactions between electron-deficient pyridinium and electron-rich amines.The size-and shape-selective response to aliphatic amines is ascribed to the radical formation through host-guest electron transfer,whereas the response to aniline is ascribed to the formation of sandwich-type acceptor-donor-acceptor complexes.The MOF is capable of reversible anion exchange with various anions and shows selective and discriminative ionochromic response to iodide,bromide,and thiocyanate,which is attributed to the formation of charge-transfer complexes.The above chromic behaviors are accompanied by efficient quenching of Eu(III)photoluminescence.The MOF represents a multi-stimuli dual-output responsive system.It can be used for discrimination and identification of anions and amines.The potential use in invisible printing,reusable sensory films,and optical switches was demonstrated by the ink and the membrane made of the MOF and organic polymers.(2)Nine isostructural MOFs were synthesized from different transition metal ions and[H3tcbpp]3+or[H3tcbpph]3+.Four of them,1-Cd,1-Mn,1-Ni and 2-Co,were structurally determined by X-ray single crystal diffraction.The MOFs show two-dimensional(2D)coordination networks with unique four-fold interlocking.It is interesting that 1-Cd,1-Ni,1-Zn,2-Cd,and 2-Zn show reversible photochromic properties under the irradiation of xenon light,and 1-Cd also shows strong fluorescence.The photochromic properties and the concomitant fluorescent change were investigated in details for 1-Cd.Electron paramagnetic resonance spectroscopy(ESR)and UV-Vis spectroscopy showed that the photochromic process involves radical formation through photoinduced electron transfer.According to the structure,there are short contacts between the electron-rich pyridinium groups and the electron-rich carboxylic groups or the chloride ion.These short contacts can be assumed to the efficient pathways for photoinduced electron transfer.Concomitant with the photochroimic process,gradual fluorescence quenching was observed for1-Cd,which permits modulation of the fluorescence intensity with photoirradiation time.In addition,the fluorescence sensing properties of 1-Cd for UO22+ions were investigated. |
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