Elsevier

Electrochimica Acta

Volume 354, 10 September 2020, 136672
Electrochimica Acta

Synthesis, electrochemistry and electrochromic properties of donor-acceptor conjugated polymers based on swivel-cruciform monomers with different central cores

https://doi.org/10.1016/j.electacta.2020.136672Get rights and content

Highlights

  • Two D-A conjugated polymers based on swivel-cruciform monomers with different A cores were synthesized by electrochemical polymerization.

  • PBTDQL exhibited the obviously rapider electrochromic switching rate than pBTBBT, which was attributed to the lower charge-transfer resistance and faster ion-diffusion behavior.

  • The charge transport processes play key roles on the switching rate of electrochromic conjugated polymers.

Abstract

Two swivel-cruciform D-A conjugated monomers (BTBBT and BTDQL) combining 5,5′-bibenzo[c] (Lv et al., 2017; Higginbotham et al., 2018; Dai et al., 2017) [1,2,5]thiadiazole (BBT) and 2,2′,3,3′-tetramethyl-6,6′-biquinoxaline (DQL) as the A units and 2,2′-bithiophene (BT) as the D unit are synthesized. The corresponding polymer films (pBTBBT and pBTDQL) are smoothly obtained on ITO-glass via electrochemical polymerization. Owing to the structural similarity, BTBBT and BTDQL enable to be electropolymerized under the same conditions, resulting in the similar thickness and surface morphology of their corresponding polymer films. Interestingly, pBTDQL displays the coloration time of 0.9 s at λmax and 0.5 s at 900 nm, which is much shorter than that of pBTBBT with the coloration time of 6.3 s at λmax and 3.0 s at 900 nm under the same conditions. The obviously rapider electrochromic switching rate of pBTDQL is attributed to its lower oxidation potentials and higher redox charge as evidenced by cyclic voltammetry results, which indicate an easier and more charge injection during electrochemical process. The electrochemical impedance spectroscopy results further demonstrate that pBTDQL exhibits the lower charge-transfer resistance and ion-diffusion resistance. These data indicate that charge transport property of conjugated polymer plays a key role on the electrochromic switching rate of pBTBBT and pBTDQL, and additionally DQL could serve as an electron-accepting building block for the constructions of faster switching conjugated polymer-based electrochromic materials.

Introduction

Conjugated polymers are garnering more and more interest in electrochromic (EC) applications such as smart window, reflectance mirrors, military camouflage and displays, owing to excellent processability, diverse colors, high optical contrast, fast switching rate and remarkable memory effect [[1], [2], [3], [4], [5]]. For display applications, the rapid switching rate in the sub-second magnitude order is particularly desired with the exclusion of required color switch involving three primary colors red, green and blue-to-transmissive [6,7]. However, the switching time of many conjugated polymer-based electrochromic (PEC) materials is more than 1s, presently, which is incapable of meeting the commercial application in displays. So, the design and synthesis of PEC materials with rapider switching rate still remains significant. Meanwhile, it is very important to deeply understand the mechanism for EC switching kinetics.

Based on the current research results, firstly, the ion-diffusion rate in PEC films is strongly related to the switching rate [8,9]. Lee et al. achieved an ultrafast switching rate (switching time is less than 10ms) by constructing the nanotubes array structure of PEDOT [10]. Steiner et al. fabricated the free-standing 3D nanostructured conjugated polymer film by using styrenic templates, which exhibited greatly enhanced EC switching speed [11]. These reports showed that reducing the ion-diffusion resistance by introducing nanostructures, resulting in the shortened ion-diffusion distance and the increased ion-diffusion channels, contributed to the rapider EC switching rate. On the other hand, the charge transport process may be an important influence factor on the switching rate of PEC materials. Xiong et al. reported that a hybrid EC material deriving from polyaniline (PANI)-grafted single-walled carbon nanotubes (SWCNTs) revealed the enhanced EC switching rate in comparison to the pure PANI [12]. They also synthesized C60-PANI:PSS hybrid materials which equally exhibited the increased switching rate [13]. The work makes us know that the introduction of high conductivity of components such as SWCNTs and C60 into PEC materials facilitates charge transport, resulting in the reduced charge-transfer resistance, and then leading to the faster switching rate. Despite these improved EC switching rate in these researches, the modified PEC materials oftentimes possess the multiple changes in structures, which makes the understanding for the intrinsic mechanism of electrochromic kinetics complicated and uncertain.

According to our previous work [14], the swivel-cruciform configuration has been proven to effectively improve the switching rate of PEC materials, because it can render the conjugated polymers with porous structure to reduce the ion-diffusion resistance. In this work, we further designed and synthesized two new donor-acceptor (D-A) conjugated monomers (BTBBT and BTDQL) based on 5,5′-bibenzo[c] [1,2,5]thiadiazole (BBT) and 2,2′,3,3′-tetramethyl-6,6′-biquinoxaline (DQL) as the different A cores and 2,2′-bithiophene (BT) as the same D arms (see Scheme 1), which both exhibit the same molecular configuration of swivel-cruciform. Subsequently, the target polymer films (pBTBBT and pBTDQL) were formed on the surface of ITO-glass by electrochemical polymerization under the same conditions. Their electrochemical properties and EC switching performance were deeply investigated to reveal their structure-property relationship. Through the establishment of this system, we attempted to fully reveal the intrinsic mechanism of electrochromic kinetics of PEC materials, and in the meantime, obtain the novel PEC film with faster switching rate.

Section snippets

Chemicals

Thionyl chloride (99.5%), pyridine (99.8%), 2,3-butanedione (98%), n-Butyllithium solution (2.5 M in hexanes), [1,1′-biphenyl]-3,3′,4,4′-tetraamine (98%, Scheme 1) and 2,2′-bithiophene (BT, 98%, Scheme 1), tetrabutyl-ammonium hexafluorophosphate (TBAPF6, 98%), commercial HPLC grade acetonitrile (ACN, 99%), dichloromethane (DCM, 99%) were purchased from Aladdin and used as received. The key intermediates 5,5′-bibenzo[c] [1,2,5]thiadiazole and 4,4′,7,7′-tetrabromo-5,5′-bibenzo[c] [1,2,5

Synthesis

The synthetic routes to the two D-A conjugated monomers BTBBT and BTDQL are shown in Scheme 1. The key precursor 4,4′,7,7′-tetrabromo-5,5′-bibenzo[c] [1,2,5]thiadiazole (4Br-BBT) is obtained via two step reaction: cyclization from 1,1′-bi(cyclohexa-1,5-diene)-3,3′,4,4′-tetraamine to gain BBT and then bromination of BBT with bromine to afford BBT tetrabromide. Another key precursor 5,5′,8,8′-tetrabromo-2,2′,3,3′-tetramethyl-6,6′-biquinoxaline (4Br-DQL) is also acquired through two step reaction:

Conclusion

In summary, two D-A conjugated polymers pBTBBT and pBTDQL based on two swivel-cruciform monomers containing 5,5′-bibenzo[c] [1,2,5]thiadiazole (BBT) and 2,2′,3,3′-tetramethyl-6,6′-biquinoxaline (DQL) as the A cores and 2,2′-bithiophene (DT) as the D arms are designed and synthesized via electrochemical polymerization. Under the identical experimental conditions, pBTDQL reveals an obviously faster EC switching rate than pBTBBT. The EIS results demonstrate that the rapider EC switching

Credit author statement

Shuanma Yan: Writing - original draft, Investigation, Formal analysis; Haichang Fu: Methodology, Data curation; Yujie Dong: Resources; Weijun Li: Conceptualization, Project administration, Funding acquisition, Writing - review & editing; Yuyu Dai: Software; Cheng Zhang: Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful for supports from the National Natural Science Foundation of China (51603185, 51673174), and Zhejiang Provincial Natural Science Foundation of China (LY19E030006, LQ19E030016 and LZ17E030001), the Key Research and Development Program of Zhejiang (YS2017YFGH000734).

References (31)

  • P.M. Beaujuge et al.

    Color control in π-conjugated organic polymers for use in electrochromic devices

    Chem. Rev.

    (2010)
  • S.I. Cho et al.

    Fast electrochemistry of conductive polymer nanotubes: synthesis, mechanism, and application

    Acc. Chem. Res.

    (2008)
  • S.I. Cho et al.

    Nanotube-based ultrafast electrochromic display

    Adv. Mater.

    (2005)
  • R. Dehmel et al.

    3D nanostructured conjugated polymers for optical applications

    Adv. Funct. Mater.

    (2015)
  • S.X. Xiong et al.

    Water-processable polyaniline with covalently bonded single-walled carbon nanotubes: enhanced electrochromic properties and impedance analysis

    ACS Appl. Mater. Interfaces

    (2011)
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