Design,Synthesis And Properties Of Cross-linked Thiophene Derivatives Electrochromic Polymers

With the global energy consumption and environment worsening dramatically,energy saving and environmental friendly materials have attracted extensive attention.Electrochromic material is a new type of functional material that maybe can change the way of human life and contribute to make use of energy rationally.Conjugated polymers(CPs)have been identified as one of the most potential electrochromic materials due to easy of structure modification,outstanding coloration efficiency,color adjustment,fast switching time,high contrast and multicolor switching.The mechanism of conjugated electorchromic polymers mainly refers to electronic and ionic transport,which is mainly related to the structure of polymers.A series of cross-linked structure polymers were contructed successfully through rational designation in this paper.Meanwhile,the influence of different molecular configuration on the cross-linked structure of the polymers and the electrochromic properties of the corresponding polymers were investigated by this paper.In the first part,we introduced phenyl,biphenyl,terphenyl moiety into dithienylpyrrole(SNS)structure to form the phenyl,biphenyl,terphenyl brideged-SNS(PhSNS,BPhSNS,TPh SNS)and achieved four thiophene sites for polymerization in the peripheral part in the molecular structure.It is beneficial to generate the cross-linked polymer structure by electrochemical polymerization.The three monomers may possess different molecular configuration due to the variation number of phenyl bridges or the variation steric hindrance of bridge units.The cyclic voltammetry curves exhibited that PhSNS,BPhSNS,TPhSNS possess similar oxidative properties except for one new oxidative peak appearing at the higher potential in the CV curves of Ph SNS.Theoretical calculation results indicated that it should be attributed to the different molecular configuration between PhSNS and BPh SNS,TPhSNS.Two SNS units oof Ph SNS exhibited different twist angle between thiophene and pyrrole rings(20.2°,40.2°)compared with two similar SNS units of BPhSNS and TPhSNS.It enabled that PhSNS possess relatively the large energy gap(~0.4 eV)between HOMO-1 and HOMO compared with the nearly same HOMO-1 and HOMO of BPhSNS and TPhSNS.It causes that PhSNS and BPh SNS,TPhSNS showed different electrochemical and optical properties.However,the three monomers exhibited similar onset oxidation potential.The cyclic voltammetry curves of the corresponding polymers pPhSNS,pBPhSNS and pTPhSNS showed similar oxidative properties.These enabled them to exhibit similar electrochromic properties except for pTPhSNS polymers.Both pPhSNS and p BPhSNS polymers showed similar multicolor behavior of yellow,greyish-green,gray.They displayed optical contrast of 40%(1100 nm)and fast switching time of 1 s,which enabled them with potential application in electrochromic area.For pTPhSNS polymer,it displayed poor electrochromic properties due to the poor solubility of TPhSNS with a large rigid terphenyl unit.Interestingly,the pTPh SNS-EDOT copolymer exhibited reasonable optical contrast of 40%(1100 nm),fast switching time of 0.37 s(1100 nm)and good electrochemical stability compared with pTPh SNS.The favorable eletrochromic properties of pTPhSNS-EDOT copolymers may be attributed to form the expected cross-linked polymer structure through introduction flexible EDOT units into the p TPh SNS structure.In the second part,we introduced phenyl,biphenyl unit into 3,6-dithiophencarbazole(BTC)structure to form phenyl or biphenyl bridged BTC derivatives(BTCPh,BTCBPh)and achieved four thiophene sites for polymerization in the peripheral part in the molecular structure.Similarly,both BTCPh and BTCBPh may form the cross-linked polymer through electrochemical polymerization.The different numbers of bridge phenyl units or the variation steric hindrance of bridge units enabled the two compounds to exhibit different molecular configuration.The cyclic voltammetry curves,theoretical calculations and UV-vis absorption displayed that the two compounds exhibited similar electrochemical and optical properties.The cyclic voltammetry of the corresponding pBTCPh and pBTCBPh polymers showed similar spectroelectrochemistry properties.At the neutral state,both pBTCPh and pBTCBPh exhibited the similar maximum absorption peak at 410 nm.And also,both them displayed the similar maximum absorption peak at 660 nm and 1100 nm at the oxidative state.Besides,the optical contrast of pBTCPh and pBTCBPh was 25%(410 nm),40%(660 nm),50%(1100 nm).However,the two polymers exhibited different switching time.The switching time of pBTCPh was just the half of p BTCBPh.SEM reflected that it was attributed to different surface morphology of them.Compared with pBTCBPh polymer film,p BTCPh film displayed more loose particles morphology,which was beneficial to ion insert or desert process during the process of electrochromism.Furthermore,pBTCPh polymer film exhibited faster charge injection ability compared to pBTCBPh,which may reflect that the former possessed the faster ionic transport.It may also contribute to the electrochromic process.In the third part,we introduced thiophene,bithiophene,and terthiophene units into tetraphenylmethane structure to form new tetraphenylmethane derivatives(TPhTTh,TPhTBTh,TPhBTTh BBr),which also possess four thiophene sites for polymerization in the peripheral part in the molecular structure.It is beneficial to form cross-linked polymer structure due to the unique tetrahedron structure.Meanwhile,in the system of tetraphenylmethane as the core,a carbon atom with the sp3 hybrid character exists,which can break the conjugation between every two peripheral arms.Thus a series of thiophene polymers with different definite conjugation lengths will be obtained via changing the number of thiophene in the peripheral arms.Therefore,we investigated the electrochemical and electrochromic properties of the definite conjugation lengths cross-linked polymers.Also,the structure of the three monomers was characterized by NMR and MS.The theoretical calculations of the three monomers indicated that their conjugated level increased with the increase of the number of thiophene.And correspondingly,the UV-vis absorption of them showed red-shift and the onset oxidation of them exhibited lower potential.From the cyclic voltammetry polymerization curves,it is obvious that pTPh TTh polymers can't be obtained by electrochemical polymerization.It may be attributed to good solubility of p TPhTTh,so as to it can't well deposited on ITO.However,the CV polymerization curves of TPh TBTh and TPhBTThBBr displayed that they can well deposited on the ITO and inform pTPh TBTh,pTPhBTTBBr polymers.The UV-vis absorption showed that the latter exhibited red shift with 40 nm.At the neutral state,pTPhTBTh displayed yellow while p TPhBTTBBr revealed orange-yellow color.Besides,the spectroelectrochemistry of the two polymers indicated that pTPhBTTBBr films exhibited obvious spectral behavior.Compared with pTPhTBTh,pTPhBTTBBr films exhibited high optical contrast of 55%(1100 nm),faster switching time and good stability,which reflect that pTPh BTTBBr films displayed better electrochromic properties.It may be attributed to pTPhBTTBBr films with more loose particles,as seem from SEM of the two polymers.Besides,pTPh BTThBBr films possessed better conjugated level,which may be beneficial to the electron transport and the redox behavior.