Design,Synthesis,Electropolymerization And Electrochromic Properties Of Conjugated Thiophene Polymers Based On Swivel-Cruciform Configuration

Polymer-based electrochromic(PEC)materials are considered to be the most promising a new generation of electrochromic materials due to its wide color modulation range,high contrast and shading efficiency,fast response speed and good workability.In this paper,a series of D-A conjugated polymers with cruciform-swivel configuration and based on their multilayer composite polymer films was constructed from the cruciform-swivel D-A conjugated thiophene monomers using electrochemical polymerization method with the purpose of exploring the relationships between the monomer configuration,polymer structure and electrochromic response speed,and trying to develop a new method of adjusting the PEC material performance.In the first part,we designed and synthesized two swivel-cruciform D-A conjugated monomers(ST-BBT and DT-BBT)based on 5,5'-bibenzo[c][1,2,5]thiadiazole as the acceptor center and different conjugation length of thiophene for donor arms,and meanwhile two similar linear structure of D-A conjugated monomers(ST-BT and DT-BT)were also synthesized.The corresponding polymers(pST-BT,pDT-BT,pST-BBT and pDT-BBT)were obtained on the surface of ITO glass via electrochemical polymerization.The cyclic voltammetry polymerization curves and spectral analysis showed that the swivel-cruciform monomers tended to form the product with a relatively higher degree of polymerization during the electrochemical polymerization process.The possible reason is that the introduction of the swivel-cruciform configuration into the monomer backbone increased the solubility of the monomers or corresponding oligomers,allowing it to obtain a higher efficiency of electrode reaction.The relatively larger volume and distortion of space configuration may make the polymer possess a more loose accumulation and even microporous structure,resulting from the enhancement of the velocity of ion-migration which is beneficial to speed up the switching rate of materials.Electrochemical impedance spectroscopy results showed that pST-BBT had a relatively lower ion diffusion resistance than those of pST-BT.Calculation results showed that the response time pST-BBT and pDT-BBT possessed an obviously faster switching rate than those of pST-BT and pDT-BT,especially in the coloring processes.Summarizing these results,we thought the introduction of the cross structure could enhance the response speed of the PEC materials effectively,5,5'-bibenzo[c][1,2,5]thiadiazole might be a potential molecular construction unit used for the preparation of high performance of PEC material.In the second part,we have developed a new swivel-cruciform D-A conjugated monomer(DT-DQL)based on 2,2',3,3'-tetramethyl-6,6'-biquinoxaline as a acceptor center and double thiophenes as donor arms.Combining the monomer DT-BBT in the first chapter,through the cyclic voltammtery polymerization,the two monomers DT-BBT and DT-DQL were electropolymerized under the same conditions.The one aim is to investigate their electrochemical controllable polymerization behavior.Within the scope of 10 cycles,the thickness of its corresponding polymer films(pDT-BBT and pDT-DQL)increased with the increase of cycle number presented a good linear growth trend,which made us enable to precisely study the relationship between film thickness and electrochromic properties of polymer based materials.The test results showed that the relationship between the manageable film thickness of pDT-BBT and pDT-DQL film thickness and optical contrast and response speed displayed a linear growth trend.Notably,pDT-DQL exhibited the relatively faster electrochromic response speed compared with pDT-BBT under the same thickness,and the coloring time of pDT-DQL was only 0.6s,but the coloring time of pDT-BBT was close to 3.0s.Ac impedance results showed pDT-DQL than pDT-BBT had a lower ion diffusion impedance.Cyclic voltammograms showed that pDT-DQL than pDT-BBT had the higher redox activity and lower redox potential.DQL may serve as a more excellent molecular construction unit to design the synthesis of quick response PEC material,and D-A conjugated polymers in the nature of A unit may have an important influence on the response speed of PEC material.In the third part,a series of double composite films based on different properties of pDT-BBT and pDT-DQL were obtained by electrochemical multilayer polymerization.The test results showed that changing the cascading order between pDT-BBT and pDT-DQL had no significant effect on optical contrast and response speed of double composite films,and were just the neutralization of properties of each layers when the layers of thickness remained the same.The response speed of double composite films would be faster with the increasing of the proportion of pDT-DQL in the double composite films.On the contrary,the response speed of double composite films would be slower with the increasing of the proportion of pDT-BBT in the double composite films.Accordingly,we thought the structure of electrochemical multilayer composite membrane may contribute to realize the coordination effect of different properties of polymer film,thus improving the performances of the polymer films.In the range of controllable film thickness,the electrochromic properties of the polymer films could be effectively controlled by adjusting the proportion of each layer polymer films in the double composite films.In addition,depending on the model of the double composite films structure,the double composite membrane being composed of pPHT lying inside layer and pTPAT lying at the external layer was prepared on the surface of the ITO electrode through electrochemical multilayer polymerization.According to the results of previous studies,pPHT had no electrochromic behavior because of its low electroactivity and irreversible redox process,while pTPAT possessed a distinct electrochromic phenomenon.The test results showed that the response speed of pPHT/pTPAT double composite film is relatively slower than the simple pTPAT film.The results of electrochemical impedance spectroscopy displayed the pPHT/pTPAT films formed a smaller double layer capacitance in the process of the electrode reaction in comparison to the pTPAT films.Summarizing the two results,we concluded that the introduction of the pPHT film into the interface between the electrode and pTPAT may inhibit the charge transfer between the electrode and electrochromic layer,causing electrochromic layer difficult to be oxidized,which made the change color need a longer time to complete.The results showed that the electrode charge injection process may have important effects on the response speed of the PEC materials.