Synthesis,Structure And Properties Of Metal-Organic Framework Materials Based On Dithienylethylenecarboxylic Acid And Pyridine Ligands

As an emerging class of crystalline materials that combine stability and porosity with the tunability of traditional organic compound,Metal-organic Frameworks（MOFs）have attracted considerable attention in various fields.In recent years,thermally responsive,chemically responsive,mechanically responsive,and electrochromic smart MOFs materials have been explored.However,these stimuli-responsive materials either rely on the constant addition of chemicals or require invasive stimuli,which are not only harmful to the environment and humans,but some of these responses are irreversible.Therefore,photoresponsive MOFs materials stand out.To avoid these problems,we directly integrate photochromic units as linker into the backbone of MOFs structures,thereby realizing MOFs materials with remote controllability.Perfluorodiarylene has become one of the most promising target molecules due to its excellent thermal stability and fatigue resistance.In recent years,the construction of MOFs based on perfluorodiarylene has been favored due to its excellent performance in information protection,light-controlled gas adsorption and separation,etc.Based on this,the research carried out in this paper is as follows:Dicarboxylic acid 1,2-bis[2-methyl-3-thienyl-5-carboxy]perfluorocyclopentene（H₂BCM）and amide bispyridine（bpfb）as mixed ligands and zinc nitrate hexahydrate as metal Ions construct a fluorescence-switching metal-organic framework[Zn（BCM）₂（bpfb）]under solvothermal conditions.Its structure was characterized by single crystal X-ray diffraction,powder X-ray diffraction,thermogravimetric analysis,infrared spectroscopy and N₂ adsorption and desorption.Single crystal X-ray diffraction shows that[Zn（BCM）₂（bpfb）]belongs to the monoclinic P2₁/n space group,a=15.1003（6）?;b=8.5099（3）?;c=22.3527（8）?;β/?=102.018（2）;The binuclear zinc cluster[Zn₂（COO）₄N₂]is regarded as a node,and[Zn₂（COO）₄N₂]is interconnected with BCM^2-and bpfb into a three-dimensional（3D）framework.The structure of H₂BCM is filled in an antiparallel conformation with two coordination modes,the distance between the two activated carbon atoms is 3.790<4.2?for the bidentate bridge coordination modes,indicating that[Zn（BCM）₂（bpfb）]can exhibit optical activity.And for the monodentate chelate coordination modes due to the NH···O hydrogen bond is formed between the adjacent ligand N2 and O4,the distance between the two activated carbon atoms is 4.478>4.2?.[Zn（BCM）₂（bpfb）]exhibited reversible photochromic behavior in solid or methanol solution by UV-Vis absorption spectroscopy.In addition,[Zn（BCM）₂（bpfb）]can also be used as a fluorescent switch for precise reading and hiding of information.Two photochromic metal-organic framework materials[Zn₂（BCM）₂（4,4′-bipy）]and[Cd（BCM）_0.5（4,4′-bipy）_0.5]were successfully synthesized constructed the dicarboxylic acid 1,2-bis[2-methyl-3-thienyl-5-carboxy]perfluorocyclopentene（H₂BCM）and 4,4′-bipyridine（4,4′-bipy）with Zn（NO₃）₂·6H₂O and Cd（NO₃）₂·4H₂O under the same conditions,respectively.It can be seen from the coordination environment diagram that the metal center Cd（II）is hexacoordinated,and is coordinated with four H₂BCM ligands and two 4,4’-bipy ligands,respectively,forming an octahedral geometry.The metal center Zn（II）has two coordination modes（hexa-coordination and tetra-coordination）,octahedral and tetrahedral geometry alternately,and the distance between the two activated carbon atoms of both materials is less than 4.2?.Since the pore occupancy of[Zn₂（BCM）₂（4,4′-bipy）]is 10%,the specific surface area and pore size distribution of its open-ring and closed-ring isomer are quite different.We attempted to separate CO₂ using the difference between open-ring and closed-ring isomer,and found that the adsorption capacity of open-ring increased by 13.5%compared with closed-ring isomer.And the relationship between the photoisomerization rates of the three materials was explored,and the effect of photoisomerization on the electronic structures of the three materials was also explored.