The Preparation And Properties Of Long Chain Alkyl-Containing Thienoviologen Derivatives

Viologen is a class of 4,4′-substituted bipyridine salts.It has unique electrochemical properties and strong electron acceptability with three stable redox states.It has been widely used in electrochromic,redox battery,photocatalytic reaction and many other fields.However,the large energy gap,weak absorption of visible light and poor luminescence of traditional viologens greatly limit their further development in electrochromism.Methods such as introducing conjugated groups or‘heavy’main group elements between the bipyridine of viologen skeleton can overcome the above problems through improving the conjugation degree of viologen,reducing the optical energy gap,and significantly enhancing the visible light absorption.The planar rigid structure not only achieves the conformational locking of traditional viologens,but also makes their derivatives show different colors in the reduction state,which is conducive to improving their electrochromic properties.It is worth noting that the color change of various redox states of viologens are also influenced by N-position substituents,which provides a new method for designing and synthesizing novel high-performance photoelectric functional materials.Thiophene has attracted much attention because of its high conductivity,good redox reversibility and strong visible light absorption ability.Therefore,it is not difficult to imagine that the introduction of thiophene between the two pyridine units would further reduce the optical band gap of the violet derivatives and improve their absorption capacity for visible light.At present,thiophene viologen derivatives have shown some applications in electrochromism.However,the electrochromic devices prepared with thienoviologen derivatives still have the problems such as low color contrast,low coloring efficiency,poor stability and so on.Therefore,thiophene and multi-chain liquid crystals with dendritic gallic acid alkylation derivatives are introduced into the thiophene viologen system to design and prepare thienoviologen derivatives containing long chain alkyl groups.The thiophene group can improve the conjugation degree of viologen,and the long flexible alkyl chain can adjust the self-assembly performance of the system.By combining the advantages,the electrochromic devices with better contrast,coloring efficiency,and stability can be prepared,thus developing more unique and stable electrochromic devices.On the other hand,dendritic gallic acid alkylated derivatives are mostly used in the preparation of liquid crystals,while there are few reports on thienoviologen derivatives,and few functional materials for liquid crystals have been developed on this basis.Therefore,this work will also explore the structure of such long-chain alkyl thienoviologen derivatives in liquid crystal,laying the foundation for the extension of viologen material application.The main work and innovation of this paper are as follows:（1）Six thienoviologen derivatives containing long chain alkyl groups with excellent electrochromic properties(TVC₆,DTVC₆,TVC₁₂,DTVC₁₂,TVC₁₆,DTVC₁₆)were designed and prepared by simultaneously introducing thiophene and alkylated derivatives of gallic acid into the viologen system.The compounds were characterized by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry.The thermal stability of the six compounds was investigated by thermogravimetric analysis,which provided information about their stability at different temperatures.To investigate the effects of introducing thiophene and long chain dendritic gallic acid into the viologen system,the UV-Vis absorption spectra of the thiophene-based viologen skeleton and the six derivatives were compared.The results showed that introducing these groups resulted in a red shift of the UV-Vis absorption spectrum,an increase in the molar extinction coefficient,a decrease in the gap between the HOMO and LUMO orbitals,and a greater effect when more thiophene groups were introduced.In addition,the fluorescent properties of the six derivatives were studied through excitation and emission spectra,fluorescence lifetime,and quantum yield measurements.The cyclic voltammetry curves obtained using cyclic voltammetry were used to study the oxidation-reduction properties of the compounds,and the possible mechanisms and processes of the redox reaction were proposed.Furthermore,theoretical UV-Vis absorption spectra and orbital energy distributions were calculated for the precursor,a single-thiophene-based viologen derivative,and a linked-thiophene-based viologen derivative using density functional theory.The theoretical UV-Vis absorption spectra results were consistent with the experimental UV-Vis spectra measurements,and the calculated orbital energy distributions explained the trends in the UV-Vis absorption properties of the precursor and thiophene-based viologen derivatives.Moreover,the calculations supported the idea that introducing thiophene and long chain dendritic gallic acid into the viologen system can lower the energy of the compounds,resulting in a red shift of their absorption spectra.These research results have enriched our understanding of this type of compound and provided theoretical support for its future applications.（2）TVC₆ and DTVC₆ electrochromic concept verification devices were prepared,and the color-changing phenomena were explained by electrochemical spectroscopy,chemical reduction,and electron paramagnetic resonance.It was found experimentally that TVC₆ had higher coloring time and efficiency than DTVC₆,which was attributed to the constraint effect of linked thiophene on electron transfer in the structure of DTVC₆.In order to expand the potential applications of these compounds,electro-fluorescence color-changing experiments were performed on devices made of TVC₆ and DTVC₆.It was found that their fluorescence could be quenched under applied voltage,with TVC₆ exhibiting an enhancement process,but gradually decreasing as the voltage increased.Furthermore,bromide ions of TVC₆ and DTVC₆were replaced by tetrafluoroborate ions,and an ion gel was synthesized through in-situ thermal polymerization under the synergistic effect of methyl methacrylate monomer and 1-butyl-1-methylpyrrolidinium bis（trifluoromethylsulfonyl）imide ionic liquid.Experimental results showed that the synthesized ion gels could achieve electrochromism.Finally,the liquid crystal structures of these long-chain alkyl-substituted thiophene-based viologen derivatives were explored.The liquid crystal structures of TVC₆ and DTVC₆were analyzed by DSC curves and X-ray diffraction,and they were found to have crystal structures,while TVC₁₂,TVC₁₆,DTVC₁₂,and DTVC₁₆ tended to form hexagonal columnar phases with p6mm symmetry at high temperatures.These research results have expanded the application fields and enriched the performance studies of this type of compound.