Syntheses,Structures And Optical-electro Properties Of Coordination Polymers Based On Viologen Ligands

Early research involving MOFs mainly focused on the synthesis of the structures,while their performance has been paid more and more attention in recent years.As for the synthesis of MOFs materials,it has shifted from structure-oriented to application-oriented.Being one of the vital molecules in many redox processes,viologens are well-known for their photoelectrochromism,electron-accepting ability and redox activity.Because of the interesting properties of bipyridinium derivatives,if bipyridinium units are directly incorporated into the backbone of polymeric frameworks,we could envision designing new solid-state materials with predetermined or improved physical and chemical properties.So far,a lot of photochromic materials have been reported in the literature,but the study of metal-organic hybrid materials is very limited.In this thesis,two viologen functionalized ligands and a series of coordination polymers based on them are prepared and well characterized.And the stimuli-induced chromic and luminescent behaviors are also thoroughly studied.Our work mainly includes the following two parts:1.We designed a viologen-functionalized m-benzenedicarboxylate organic ligand,i.e.,1,1?-bis?3,5-dicarboxybenzyl?-4,4?-bipyridinium dichloride?H₄L₁Cl₂?.Self-assembly of this linker with different metal salts by solvothermal method afforded three coordination compounds [Zn₂?L₁??Cl?₂]n?1?,[Cd₃?L₁??Cl?₆?MeOH?₂]n?2?and [Eu₂?L₁??NO₃?₄?HOCH₂CH₂OH?₂]n?3?.The results show that 2 and 3 exhibit remarkable photochromic behavior.Upon light stimuli,viologen radical generated with the electron transfer from the electron donating group to the accepting viologen moiety.Meanwhile,the crystal color turns blue from pale yellow.Notably,the colored photoproduct can be decolored under the heat treatment.Complex 3 shows metal-centered luminescence attributed to the intense emissive character of rare-earth metals.The emission intensity decreases gradually along with the exposure time in light and recovers after heating treatment.Therefore,complex 3 is a dual functional material with photochromic and light controlled fluorescent switch properties.2.A viologen-functionalized amino-acid ligand,i.e.,1-?4-L-5-phenylalaninyl?-4,4?-bipyridinium chloride was prepared.The reactions of this ligand with cadmium salts by solvothermal method at different temperatures yield four novel complexes {[Cd₃?L₂?₆?H₂O?₆]×11NO₃×14H₂O}n?4?,{[Cd?L₂??NO₃??H₂O?]×NO₃×2H₂O}n?5?,{[Cd₈?L₂?₈?ClO₄?₈?H₂O?₁₂]×ClO₄×2Cl×27H₂O}n?6?and {[Cd₁₅?L₂?₄?Cl?₃₂?H₂O?₇]×4H₂O}n?7?.Although the favorable geometry for electron transfer from the carboxylate group to the bipyridinium moiety is created in transformed structures,the introduction of anions leads to structural changes and the formation of different charge transfer interactions with electron-accepting bipyridinium units in the ground state,which causes they exhibit the multicolor photochromic properties.To have insight in the photochromic mechanism,PXRD patterns,IR spectra,UV-Vis absorption spectra and election spin resonance measurements were performed for the detailed characterization and analysis.Interestingly,complex 7 also exhibits rapid reversible electrochromic behavior.With the voltage varied in the range of-0.8 V to 0 V,the color of the material switches between colorless and violet.To our best knowledge,the MOFs materials possessing both photochromic and electrochromic features remain highly limited.