Synthesis, Structures And Optical, Electric Properties Of Viologen-Functionalized Metal-Organic Frameworks

Metal-organic frameworks（MOFs）have become one of the hotspot among worldwide researchers for their fascinating network topologies and the potentially wide application in many fields.In recent years,the design and synthesis of organic linkers with different functional groups are very critical to target MOFs with desired properties.Viologen compounds can express different redox states due to the electron transfer process caused by the external stimuli of light or electricity,and the reversible transitions between different states often induce regular modulation of optical,electrical and magnetic properties.The study of introducing viologen groups into MOFs to construct multi-functionalized MOFs is still in the embryonic stage.We designed and synthesized two viologen-functionalized organic linkers and assembled several MOFs with fascinating structures and properties.Viologen modification endows the material with excellent photochromic properties and other modulated optical and electrical properties.In addition,photocatalytic hydrogen production property of MOFs has also been studied.This paper includes four parts as follows:1.A novel MOF（compound 1）with 3-connected（10,3）-a network is assembled by Tb³⁺ ion and viologen functionalized m-benzenedicarboxylate ligand,1-（3,5-dicarboxybenzyl）-4,4’-bipyridinium chloride.Compound 1 featuring left and right helix alternately arranged into a 3D structure,in which two nets with different chirality result in the 2-fold interpenetrated architecture.The reversible photochromism is realized due to the formation of radicals caused by photoinduced electron transfer from carboxyl oxygen atoms to pyridyl nitrogen atoms.The photochromic component of the organic linkers can modulate the luminescence of tetranuclear terbium cluster under irradiation of UV light,which is defined as photoluminescence switch behavior.The different protonation levels of uncoordinated N atoms with varying p H enables the MOF as a potential fluorescent p H sensor.At the same time,complex 1 has high photocatalytic activity for the degradation of methyl orange（MO）under UV light irradiation.Thus,multi-photofunctionality,viz.photoluminescence,photochromism as well as the derivate fluorescent response to irradiation and p H have been combined in the compound 1 for the first time.2.The H₂L₁（10）NO₃^-ligand was obtained by replacing the chloride ions of previous ligand with nitrate ions,and two chiral MOFs（compound 2 and compound 3）with interesting interdigital architecture were assembled with Eu³⁺ ion and H₂L₁（10）NO₃^-ligand.They have the similar structure,while the difference is the complex 2 balances the charge with free nitrate ions,and the complex 3 is the coordination of formate ions.Introduction of the viologen unit endows MOFs with excellent photochromism due to the electron transfer process.Intramolecular energy transfer from the metal center to the colored state of the ligands accounts for fluorescent quenching.Furthermore,Eu-MOF displays interesting second-order nonlinear optical and piezoelectric switch properties resulted from the combination of inherent chirality and photo-induced charge separation upon irradiation.Our work successfully integrates photochromism,luminescence,NLO,and piezoelectric properties into a single MOF,and points the way to an effective strategy for constructing multifunctional MOF-based switchable materials.3.A series of compounds（compound 4-7）with the spatial structure ranging from discrete 0D,infinite 1D,2D layers to 3D networks were constructed with ligand H₂L₁（10）Cl-and zinc salts of different anions.Chloride and bromide ions in the compound 4 and 5 occupy the coordination sites,resulting in 0D and 1D structures.Sulfate and nitrate ions in the compound 6 and 7 serve as counter anions in the pores,so compound 6 and 7 exhibit 2D and 3D structures.Compound 4,6 and 7 display excellent photochromism properties.The further research about the effects of anions on their structure and properties were studied.4.We have designed and synthesized a viologen modified porphyrin ligand with a large conjugated system: 5,10,15,20,-tetrakis [4-[N-（4-pyridyl）pyridyl]phenyl] porphine tetrachloride.A novel 3D network（compound 8）was obtained by Cu ion and porphyrin ligand.Compound 8 possesses all the requirements of visible light photocatalysis: a large conjugated organic ligand endowing it with wonderful absorption and utilization of visible light,the copper center with good catalytic activity,and the viologen as an excellent electron transfer carrier.The study of photocatalytic property shows that compounds 8 can simultaneously act as photosensitizers and catalysts for photocatalytic hydrogen production,but less efficient.With the addition of appropriate photosensitizer,the catalytic efficiency has been greatly improved.We further screened and optimized the conditions of photocatalytic hydrogen production.In summary,we have designed and synthesized a series of metal organic framework materials with viologen groups,and their photochromic properties and the mechanism of photochromism were studied.At the same time,the mechanism of photochromism-induced optical and electrical properties was studied,which provided the direction for the study of multi-functional MOFs.In addition,the performance of functionalized MOFs in the use of visible light photocatalytic hydrogen production was also studied.