Design,Synthesis And Chromic Properties Of Viologen-dicarboxyl Compounds

Chromic materials that change color in response to external physical or chemical stimuli have been evergreen topics of research for a variety of actual and potential applications in our daily life and various high-tech areas.Viologen compounds?1,1'-disubstituted 4,4'bipyridiniums?possess good reversible redox capability with remarkable color change.Since the seventies of the last century,the research and applications of viologen-based compounds in electro-/photochromic materials and redox catalysts of biological system have attracted more and more attentions.In this work,two kinds of viologen-based dicarboxylic compounds were designed and synthesized and the first piezochromism induced by generation of radicals via electron transfer was reported.By adjusting the content of lattice water and counter anions in their crystalline structure,the effects on electron transfer were elucidated.At the same time,two metal-organic coordination polymers were also obtained by using them as organic ligands,which show good properties in piezo-/photo-/hydrochromism and detection of different amines.This dissertation covers the following two parts:1.Synthesis and properties of [p-/m-H₂bpybdc]Cl₂ compoundsDesign and development of stable multi-functional chromic materials have always been the hotspot and challenge.Since viologen-based compounds with good redox capability have potential discoloration ability,introducing viologen moiety into the structure may give materials new controllable multifunctional chromic behaviors.Therefore,a series of viologen-based organic compounds was synthesized and studied systematically.?i?The compound bpybdc×6H₂O?1?6H₂O?was synthesized firstly.The yellow crystal undergoes reversible color change to red under high pressure,to green after decompression,and finally back to yellow upon standing at ambient pressure.Ultraviolet-visible spectroscopy,X-ray photoelectron spectroscopy,electron paramagnetic resonance X-ray diffraction and DFT calculations suggested that the piezochromism is owing to the formation of radicals via pressure-induced electron transfer from carboxylate to pyridinium,without crystallographic phase transition.It was proposed that electron transfer was induced by pressure-forced reduction of intermolecular donor-acceptor contacts.Additionally,the electron transfer can also be induced by dehydration without any other stimuli,such as light or electricity,which gives a stable green anhydrous radical phase.The compound not only demonstrates new chromic phenomena for viologen compunds but also represents the first example of organic mechanochromism and hydrochromism associated with radical formation via electron transfer.?ii?A systematic study on photo-/piezochromic properties of six pseudopolymorphs,including the inner salt?p-bpybdc,1?x H₂ O,x=0,2,6?and the chloride salt?[p-H₂bpvbdc]Cl₂,2?y H₂ O,y=0,2,4?was reported,which provide unique model systems for studying the relationship between the water-perturbed structures and electron transfer behaviors.We show that the higher the lattice water content,the less sensitively the compounds?either chloride or inner salts?respond to light and pressure.The impedimental impact of lattice water can be associated with its high ionization potential,high polarity,and strong hydrogen bonding capability.It is the first time that the impact of lattice water on solid-state electron transfer of viologen-compounds is recognized and analyzed.The piezochromic study suggests that pressure is a more powerful tool to induce electron transfer than light.?iii?After a systematic study of [p-H₂bpybdc]Cl₂ compounds,we change the counter anions?2-BF4,2-PF6,2-Cl O4?and the substituents?dicarboxylic acid from the para-to the meta-position,[m-H₂bpybdc]Cl₂,3?to see the potential effects on the properties of these compounds.The experimental results demonstrate that the change of the substituent position does not affect photo-/piezochromism obviously,but the species of counter anions do to some degree.2.Coordination polymers using [p-/m-H₂bpybdc]Cl₂ as ligandsCoordination polymers have become a very important platform for the design of multifunctional materials.After the study of [p-/m-H₂bpybdc]Cl₂ organic compounds,we synthesize two metal-organic coordination polymers using them as ligands,and investigate structures and properties systematically.?i?A flexcible zigzag-chain Cd^II coordination polymer using [p-H₂bpybdc]Cl₂ as ligand was synthesized.The compound?[Cd?p-bpybdc??NO3?2]?H₂O,4?shows reversible piezochromism,with modulations in threshold pressure and color compared to the free ligand.However,the piezochromism can be suppressed upon coordination of the same ligand in a rigid three dimensional framework?5?with the same metal ion.Herein,two basic requirements were proposed for viologen-based piezochromism: appropriate donor-acceptor contacts providing electron transfer pathways,and the structural flexibility allowing pressure to further reduce the contacts.In addition,the very fast photochromic response of the compound at ambient pressure was also studied.?ii?A one-dimensional porous Cd^II coordination polymer?[Cd2Cl?m-bpybdc?2?H₂O?4]?NO3?3?7H₂O,6?was synthesized,using [m-H₂bpybdc]Cl₂ as ligand,which shows reversible discriminative responses toward to different low-carbon amines.The differentiable color changes are attributable to electron transfer from the analyte molecules to viologen,which is affected by interacting sites and dimensions of the amine-accessible channels in the material.The material also shows reversible photo-and hydrochromism owing to stimuli-induced reversible electron transfer.The compound can be deposited in paper through spraying the mixture solution of the metal salt and the ligand simply.The paper can be used as portable test strips for visual and differentiable detection of amines and erasable inkless printing medium.