Chiral Construction And Properties Of Viologen-Functionalized Metal-Organic Frameworks

Metal-organic frameworks(MOFs)are inorganic-organic hybrid crystalline materials with infinite ordered space structure,self-assembling by metal ions/ion clusters and organic ligands via coordination bonds.MOFs have been utilized as the efficient platform for the research of functional materials in recent years because of their designable structure and adjustable performance.In addition to the excellent performance of conventional MOFs,chiral MOFs show remarkable potential applications in chiral molecular recognition,circularly polarized luminescence and asymmetric catalysis.Viologen functionalized MOFs exhibit excellent redox property,possess electrochromic,photochromic,thermochromic and second-order nonlinear properties,etc.Chiral viologen-functionalied MOFs integrate chiral and viologen active sites,and have broad application prospects in the fields of intelligent anticounterfeiting,information encryption and storage,chiral stimulation.In this paper,chiral construction of viologen-functionalized MOFs was carried out by three different strategies,and chiral viologen-functionalized MOFs with photoelectric activity and intrinsic polarity were obtained.The specific work contents are as follows:1.In order to obtain chiral viologen-functionalized MOFs,a pair of chiral viologen ligand 1-(3,5-dicarboxybenzyl)-1 '-(R/S-2,3'-dihydroxypropy l)-4,4'-bipyridinium dichloride(H3(R/S-L1)Cl2)were designed and synthesized,and a two-dimensional planar compound {[Eu(R-Li)(NO3)2(H2O)](H2O)}n(Eu-MOF)was obtained by selfassembly of the ligand and Eu3+.Eu-MOF crystallizes in achiral space group due to the disorder of hydroxyl groups in the structure.Under UV irradiation,the electrontransfer of Eu-MOF from the oxygen atom of electron donor group to the viologen moiety of electron-deficient produce viologen radicals,and the color change from yellow to blue.In addition,when the ITO film of Eu-MOF was applied by the voltage from-0.2 V to-1.37 V,the viologen cationic captured electrons to form neutral radicals,accompanied by the color change from yellow to purple.The purple sample was placed in the air,and can return to yellow,indicated that Eu-MOF possess the reversible electrochromic property.In addition,Eu-MOF possess a typical red luminescence,which can be reversibly changed by light/electric stimulation.The fluorescence is regulated by energy transfer from luminescent group to viologen absorber moiety,resulting from partial overlap of absorption and emission spectra.2.Taking advantage of the diversity of synthetic methods,chiral viologenfunctionalized MOFs was constructed by the mixed-ligand strategy.In this chapter,a pair of chiral viologen compounds {[Cd2(L2)2(D-Lm)(H2O)2]·5H2O}n(D-MOF)and{[Cd2(L2)2(L-Lm)(H2O)2]·5H2O}(L-MOF)were prepared by D/L-tartaric acid(D/LLm),1-(3,5-dicarboxybenzyl)-4,4'-bipyridine chloride(H2L2Cl)and Cd2+through solvothermal reaction.D/L-MOF show electron-transfer under UV light,and produce viologen radicals,accompanying by color change from colorless to blue.Then blue sample under heat can return to colorless,showing reversible photochromic properties of D/L-MOF.In addition,when ITO film of D/L-MOF were applied by the voltage from-0.8 V to-1.1 V,the cationic viologen was reduced to produce neutral viologen radicals,meaning a good electrochromic property,accompanying by the color change of D/L-MOF from white to purple black.Furthermore,the D/L-MOF exhibited circularly polarized luminescence properties.3.It is also an effective strategy to construct chiral viologen-functionalized MOFs through spontaneous resolution of achiral viologen ligands.Here,a bidentate viologen ligand 1,3-bis(4-pyridyl-1-pyridinio)benzene dihexafluorophosphate(L3(PF6)2)was designed and synthesized,and a pair of helical enantiomeric compounds{[Cd3(L3)2(Ln)Br10](PF6)})(P-MOF and M-MOF)were obtained by L3(PF6)2 and CdBr2·4H2O through spontaneous resolution,where P and M represent the righthanded and left-handed helix,respectively.The space groups of compounds P-MOF and M-MOF are P6122 and P6522,respectively,which belong to the chiral space group.the conglomerate crystals are racemic(MOF-rac),but each single crystal is single chiral.MOF-rac materials display obvious color changes under light and electric stimulation,and possess photochromic and electrochromic properties.