Design, Synthesis and Properties of D-A-D' Asymmetric Structured

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Design, Synthesis and Properties of D-A-D' Asymmetric Structured Electrochromic Polymers Based on Fluorenone as Acceptor Units Junlei Liu, Lin Li, Ruoteng Xu, kaili Zhang, Mi Ouyang, Weijun Li, Xiaojing Lv, and Cheng Zhang ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00103 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Polymer Materials

Design,

Synthesis

and

Properties

of

D-A-D'

Asymmetric Structured Electrochromic Polymers Based on Fluorenone as Acceptor Units Junlei Liu, Lin Li, Ruoteng Xu, Kaili Zhang, Mi Ouyang*, Weijun Li, Xiaojing Lv, Cheng Zhang*

State Key Laboratory Breeding Base for Green Chemistry Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014, P. R. China. KEYWORDS D-A-D' asymmetric structure; electrochromic polymer; multicolor; fluorenone; conjugated polymers；electrochemical polymerization ABSTRACT: Two novel conjugated polymers based on a donor-acceptor-donor’ (D-A-D’) asymmetric structure, using fluorenone as the acceptor unit linked with different donor units on both sides, were designed and synthesized, namely poly2-(2,3-dihydrothieno[3,4b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)-9H-fluoren-9-one(PSWE)

and

poly2-(4-

(diphenylamino)phenyl)-7-(thiophen-2-yl)-9H-fluoren-9-one(PSWT). Compared with the symmetrical

structure

polymer

poly(2,7-Di-thiophen-2-yl)-fluoren-9-one(PSWS),

the

asymmetric-structure polymers exhibit lower redox potentials and bandgap values, and more redox peaks, and thus showed a richer variety of colors. Moreover, the introduction of 3,4Ethylenedioxythiophene (EDOT) improved PSWE’s response speed under the near infraredvisible band, and enhanced its optical contrast in the near ultraviolet spectrum. The introduction of triphenylamine improved PSWT’s optical contrast in the near infrared and

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visible spectra. It could be inferred that the polymers with asymmetric structure(D-A-D’) exhibit more redox sites and meta-stable states with respect to the symmetrical structure(DA-D), which was attributed to the change in the distribution of the electronic cloud by replacing one donor (D) with another (D’) in the polymer monomer, and the electrochromic properties of the polymers were improved. 1. INTRODUCTION Polymer electrochromic (PEC) materials have received extensive attention due to their wide variety of structures, rich color range, high optical contrast, good process-ability, adjustable band, and fast response, and are considered as one of the development directions for the nextgeneration of EC materials.1-3 At present, the multicolour display properties of PEC materials are generally achieved through molecular structure design and modification, such as donor-acceptor (D-A) structure design and the copolymerization of different monomers.4-7 Copolymerization is an effective structural adjustment method that can combine the advantages of two or more monomers to improve the electrochromic properties of polymers.8,9 We have used copolymerization to synthesize a series of multicolor display polymer materials.10-12 However, the randomness of the copolymerization is not conducive to the in-depth study of the relationship between the molecular structure and the discoloration properties of the PEC material. D-A structures can effectively regulate the energy band structure, and then tailor the photoelectric properties of polymers.13,14 Thompson15, Amb16,17 and coworkers successfully applied D-A structure to prepare multicolor display PEC materials.18 Among them, the electronwithdrawing groups used as electron acceptors mainly include benzothiadiazole, quinoxaline, benzoxadiazole and thiophenepyrazine.19-22 The molecular structure of fluorenone23,24 is easily chemically modified, and it can improve the electrochromic performance of materials as electron


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acceptors. The D-A-D symmetric structure conjugated polymer has a narrow band gap of in the polymer due to the meso-nesical action of the molecule, and has the dual properties of the donor group and the acceptor group.25 Work on PEC materials with symmetric D-A-D structures at the ends of fluorenone attached to the same donor group has also been reported recently.26 Due to the internal racemization of conjugated macromolecules with a D-A-D structure, they not only possess a narrow bandgap, but also have the dual characteristics of donor group and acceptor group.25 Therefore, polymers based on the symmetrical D-A-D structure tend to improve the photoelectric properties27,28 relative to D-A structures with fluorenone. For example, Önal28 and his group reported on a soluble electrochromic polymers

by introducing the same donor

units(ProDOT) at both ends of fluorenone. The materials showed higher coloring efficiency and two different colors. Loganathan29 and Argun30 and coworkers introduced thiophene or EDOT and its derivatives on both ends of fluorenone to design symmetrical D-A-D structured PEC polymers, and achieved a variety of colour changes from dark-orange to live-green or sun exposure to gray-purple. At the same time, high contrast ratio and excellent electrochemical performance were also obtained. However, considering the simple characteristics of the D-A structure and the uncertain polymer structures caused by the electrochemical copolymerization of different D-A monomers, further studies should be carried out on the relationships between molecular structure and electron cloud distribution, the metastable state of polymers, and the multicolor display performance, to understand the electrochromic mechanism thoroughly. In this paper, we designed two asymmetric D-A-D' structure monomers with fluorenone as electron acceptor, thiophene as donor (D) and EDOT or triphenylamine as donor (D'), respectively. Compared to the D-A-D symmetric structure of the polymer with fluorenone as acceptor (thiophene-


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fluorenone-thiophene),31 asymmetric structures have more redox sites and obvious multipoint absorption characteristics, which produce more color changes. Moreover, the other EC performances of the D-A-D' polymers were studied, such as optical contrast, response time, and coloration efficiency. Our asymmetric structure molecular design strategy provides a new method of the performance regulation of polymeric electrochromic materials. 2. EXPERIMENTAL Three monomers, (2,7-Di-thiophen-2-yl)-fluoren-9-one (SWS), 2-(2,3-dihydrothieno[3,4b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)-9H-fluoren-9-one

(SWE)

and

2-(4-

(diphenylamino)phenyl)-7-(thiophen-2-yl)-9H-fluoren-9-one (SWT), were synthesized according to Scheme 1 and their corresponding polymers, PSWS, PSWE and PSWT respectively, were prepared according to Scheme 2. The detailed synthetic routes are provided in the Supporting Information. Scheme 1. Chemical Structures and Synthetic Route of SWS, SWE and SWT.


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Scheme 2. Electrochemical polymerization of PSWS, PSWE and PSWT.

3.RESULTS AND DISCUSSION 3.1. Electrochemistry Figure 1 shows the cyclic voltammograms (CVs) of the polymerization of 1mM SWS, SWE and SWT in CH2Cl2/ACN (3/2, v/v) containing 0.1 M TBAP as a supporting electrolyte on ITO cycling from 0 V to 1.4～1.6 V at a scan rate of 100 mV/s for each electrochemistry cycles. With increasing of scanning cycles, the irreversible oxidation of the monomers SWS and SWE appears clearly on the first cycle followed by typical polymer film growth loops, which indicates the deposition of electroactive polymeric films on to the electrodes.25 The SWT monomer has better reversibility than others on the first cycle, and it shows better growth loop characteristics with the increasing scanning cycles. But the difference in their irreversible oxidation peaks is probably due to the differential effects of the D-A-D′ asymmetric structure on the electronic cloud density of the main chain.29


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Figure 1 (a), (b), (c) show SWS, SWE and SWT monomer cyclic voltammograms curves of the electropolymerization in the solution with 0.1 M TBAP as the electrolyte (DCM:ACN, 3:2 (v/v)) at a scan rate of 100 mV·s−1.

Under the same conditions, the onset oxidative potential of SWS is 1.3 V while the onset oxidation potential of SWE and SWT are 1.1 V and 0.9 V, respectively, for the first oxidative process.This is due to the introduction of EDOT and triphenylamine increasing the electronic cloud density of the system, thereby lowering oxidation potential of SWE and SWT.31 Cyclic voltammogram curves show the oxidation potentials of SWS, SWE and SWT are 1.48 V, 1.28 V and 1.08 V/1.21 V, respectively. Among them, SWS has a higher oxidation potential at 1.48 V which is due to the formation of radical cations.32 SWT contains two pairs of reversible redox peaks at 1.08 V/0.82 V and 1.21 V/0.94 V that come from the influence of two different donors. SWE only has an oxidation potential, which is attributed to the similar spatial structure and small steric hindrance of the thiophene and EDOT as donor units, and thus it facilitates the redistribution of conjugated electrons, resulting in the overlapping of the oxidation peaks of the


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two

groups.

These

results

indicate

that

SWE

and

SWT

are

prone

to

electrochemical polymerization and the asymmetric structure brings new changes to the redox peaks. To further investigate the influence of the molecular structure on the electronic energy state, the monomer electronic cloud distribution and energy levels of the electronic transitions were calculated using the DFT B3LYP method. As is well known, the HOMO and LUMO levels of molecules are related to the electron-withdrawing ability of the acceptor and the electrondonating ability of the donor, in which the stronger the donating ability of the donor is, the higher the HOMO level will be.9 Due to the similar LUMO levels for the three monomers, the difference in HOMO levels (triphenylamine>EDOT>thiophene) makes SWT have the lowest energy band gap. As shown in Figure 2, the orbitals pictures of the LUMOs for the monomers SWS, SWE and SWT have their electron densities mainly distributed at the fluorenone group. In other words, the LUMO positions of monomers SWS, SWE and SWT depend mainly on the acceptor unit.33,34

Figure 2 HOMO–LUMO orbital pictures of monomers SWS, SWE and SWT. The monomer electronic cloud distribution and energy levels of the electronic transitions were calculated using the DFT B3LYP method.


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As a result, the LUMO level of the three monomers changes little (SWS (3.3 eV), SWE (3.0 eV) and SWT (2.9 eV)). As a comparison, the electronic orbitals of HOMOs for the three monomers change significantly. Among them, SWS’s electronic cloud distribution is relatively uniform due to its symmetrical structure, with an energy level of −5.8 eV. SWE’s HOMO is unevenly distributed because of the introduction of the EDOT group. Furthermore, SWT′s HOMO’s electronic orbital picture is mainly distributed at the triphenylamine group. The bandgaps of SWS, SWE and SWT were calculated as 2.5 eV, 2.3 eV and 1.9 eV, respectively. This result is consistent with the spectroelectrochemistry and electrochemical bandgaps of monomers as shown in Table 1. Thus, it is demonstrated that the asymmetric D-A-D' structure directly affects the electronic cloud distribution and bandgap.

Figure 3 FTIR spectra of monomers and polymers, the polymer films were prepared from a reaction medium containing 3 mM monomer and 0.1M TBAP in CH2Cl2/ACN(3/2,v/v).


8

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Figure 3 shows the FTIR spectra of monomers and polymers. In the spectra of the pure PSWS, PSWE and PSWT films, the peak at 1715 cm-1 is the C=O stretching vibration. This indicates the presence of fluorenone groups in the polymer. The medium-intensity infrared absorption peak at 786 cm-1 is attributed to the C-C stretching vibration on the thiophene, and it proves the existence of thiophene or thiophene derivated groups in the polymer. The strong absorption peak at 1086 cm-1 can be ascribed to the C-O-C asymmetric stretching vibration, which shows that the polymer contains the EDOT units. The weak absorption peak at 1318 cm-1 is the C-N stretching vibration in the aromatic tertiary amine, which implies the existence of the triphenylamine groups. In addition, the ortho C-H absorption of thiophene is strong in the fingerprint region 750690 cm-1. The single replacement of benzene and its derivatives are tip (strong) absorption peak in 710-690 cm-1. The absorption peaks of thethree polymers at 750-690 cm-1 have disappeared in the FTIR spectra after the electrochemical deposition. This demonstrates that the polymer based on the corresponding monomer is successfully synthesized via electropolymerization.


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Figure 4 Scan rate dependence of PSWS, PSWE and PSWT film for p-doping in 0.1 MTBAP solvent-electrolyte couple (CH2Cl2:ACN, 3:2(v/v)) at a scan rate 50, 100, 150, 200,250, 300, 350 and 400 mV/s. As shown in Figure 4, the cyclic voltammogram curves of PSWS with anodic scans are irreversible, and there is only a pair of redox potentials (1.43 V/1.21 V), while the CV curves of the PSWE and PSWT have multiple oxidation potentials. Among them, PSWE’s redox potentials are 1.23 V/0.98 V and 0.98 V/0.53 V, and the PSWT’s redox potentials are 1.26 V/0.97 V, 0.97 V/1.06 V. Therefore, it can be inferred that asymmetric structured polymers may have more redox sites than symmetrical ones. It was obvious that all the three polymer displayed the relative lower oxidation potentials than those of their corresponding monomer (Table 1), indicating the higher HOMO levels from monomer to polymer, which could be ascribed to the increased


10

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conjugation effect of polymer structure. And their LUMO levels are close to those of monomers, which might be attributed to the same acceptor of fluorenone group in them. Moreover, the anodic and cathodic peak currents of the PSWE and PSWT’s increase as the scan rate increases linearly in the process of P-doping, indicating that the polymer has excellent adhesion to the ITO substrate, and the redox process is non-diffusion controlled. Table 1 The electrochemical spectra of the materials. Absorption Materials

λ(nm)

Eox,onset

Hexp/ Lexp

HDFT/LDFT

Eg,DFT

Eg,opt

Eg,elec

λabs1

λabs2

(V)

(eV)

(eV)

(eV)

(eV)

(eV)

SWS

347

468

1.3

-6.1/-3.9

-5.8/-3.3

2.5

2.2

1.8

PSWS

379

/

1.1

-5.9/-3.8

/

/

2.1

/

SWE

354

471

1.1

-5.9/-3.8

-5.3/-3.0

2.3

2.1

1.7

PSWE

365

/

0.7

-5.5/-3.8

/

/

1.8

/

SWT

363

478

0.9

-5.7/-3.7

-4.8/-2.9

1.9

2.0

1.6

PSWT

366

/

0.8

-5.6/-3.5

/

/

2.2

/

The HOMO energy levels ( ) were determined by = −(Eox, onset+ 4.80) (eV). exp Eg,opt= 1240/λonset (eV), and the LUMO energy levels (L ) = Hexp + Eg, opt. Eox, onset, λonset and Eg,opt are the onset oxidation potential, onset wavelength of maximum absorption peak and optical bandgap of the material respectively. Hexp

Hexp

3.2. Spectroelectrochemistry The electrochemical spectra of the three polymers are shown in Table 1. The intrinsic absorption peaks of various monomers and their polymers are almost identical. The absorption wavelengths of the π-π* transition are redshifted for the three polymers (PSWS, PSWE and PSWT) compared to the absorption wavelengths for their monomers, because of the increased conjugation length. In other words, as the molecular conjugation length increases, the electron cloud density of molecule decreases and the energy of the π-π* electron transition increases,


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resulting in a redshift in the absorption wavelength. Because of the different size of redshift (PSWS>PSWE>PSWT), the degrees of conjugation within the polymers are different. As shown in Figure 5, the absorption peak of PSWS at 379 nm is assigned to the π –π* transition, and the film is amber under neutral conditions. The absorption intensity of PSWS at 379 nm decreases as the voltage gradually increase, and the absorption intensity increases to 718 nm. Therefore, the doping level of the polymer film gradually increases with voltage. Relatively, the bipolarons of the PSWS grow slowly, indicating that it is difficult to form high doping conditions for the symmetric structure with the increase in doping level, and the polymer film shows medium aquamarine color. The intrinsic absorption peak of the PSWE is located at 365 nm. The maximum absorption peaks at 439 nm and 588 nm are the π–π* transition, leading to a khaki color in the polymer film under neutral conditions. The doping level of the polymeric backbone increases as the voltage increases. The absorption peak at 439 nm in the near-UV region disappears gradually, the absorption intensity at approximately 600 nm in the visible region gradually increases, and the dipole state gradually develops. At this point, the color of the polymer gradually changes from khaki to dark blue and has obvious multicolor display characteristics. The intrinsic absorption peak of PSWT is 363 nm under neutral conditions, and the π-π* transfer (π-π* transition) peaks are at 465 nm and 640 nm, respectively. The polymer film shows a wheat color. As the voltage increases, the absorption intensity at 639 nm is greater than the absorption intensity at 465 nm. The film exhibits a brown and dark sea-green color.


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Figure 5 The polymer film of PSWS, PSWE and PSWT UV-vis-NIR absorbance test pattern.

UV-vis-NIR spectroscopy was used to monitor the change in spectral absorption from neutral to oxidized state in a 0.1M TBAP/CH2Cl2/ACN (3:2, v/v) solution. As shown in Figure 5, new absorption peaks appear at 439 nm and 640 nm for PSWE and PSWT, respectively, implying the existence of new meta-stable structures. Compared to the symmetric structure, the introduction of the EDOT and triphenylamine electron-donor groups increases the absorption range and absorption of the molecule significantly. At the same film thickness, the PSWS and PSWT have a higher absorbance except for PSWE at 439 nm, with the increase in the voltage. Therefore, there are two kinds of color changes for PSWS and PSWE, and the PSWT has the characteristics


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of a multicolor display. Accordingly, polymers with synchronous absorption changes have less electrochromic colors, while the nonsynchronous absorption change of the polymers have more color-changing characteristics. 3.3 Thickness and Optical contrast

Figure 6 The relationships between the polymer film thickness and the cycle numbers. The control of the thickness of the film is beneficial to the systematic study of the relationship between the structure and properties of the polymer. Figure 6 shows the relationship between the film thickness of three polymers and the number of CV cycles. It is easy to see that there is a linear relationship between film thickness and the cycle numbers for polymers based on asymmetrical structure within 30 cycles. However, for PSWS, its film thickness stops increasing significantly after 26 cycles. That is, the deposition rate and the dissolution rate of the PSWS film are almost balanced. For the comparison of the three films, we have controlled the thickness of the three polymer films to approximately 600 nm.


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Figure 7 Contrast test of PSWS(a), PSWE(b) and PSWT(c). At the same film thickness, the optical contrasts (∆T%), response times and coloration efficiencies (CE) of the three polymers (located at 400, 650 and 1100 nm, respectively) were shown in Figure 7 and Table 2. The optical contrasts were taken under a 5 s voltage pulse at 95% of the maximum transmittance. Compared with PSWS, the PSWT film shows better optical


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contrasts (3.9%, 36.1% and 75.9% compare to 2.5%, 20.3 and 40.7%, at 400, 650 and 1100 nm, respectively). In the visible wavelength spectrum, the response time of the PSWE is 0.8 s at the maximum transmittance, which is faster than PSWS(1.4 s). Therefore, the asymmetric structure improves the photoelectric properties of the polymer in some respects. The coloration efficiency (CE) is also an important characteristic for electrochromic materials. The CE is not only related to the contrast and response speed of materials, but also can indirectly reflect the doping degree of the PEC materials. This means that the doping degree of the material changes synchronously with the change of CE. As shown in Table 2, a lower CE value corresponds to a lower doping level. The CE values of PSWE and PSWT are higher than PSWS for the fully doped state. This is in agreement with the theory that the asymmetric structure enhances the coloration efficiency in this system. Table 2 Spectra and kinetic properties of various materials. Materials

PSWS

PSWE

PSWT

Wavelength/ nm

Switching Time Optical contrast /% CE (cm2C-1) /s

1100

1.1

42.7

14.1

650

1.4

20.3

37.1

400

0.4

2.5

6.7

1100

0.7

37.6

108.0

650

0.8

19.6

92.2

400

0.5

13.3

96.1

1100

1.5

75.9

157.8

650

1.2

36.1

85.5

400 2.6 3.9 82.8 Coloration efficiency(CE) can be calculated by means of the equation: CE(η)=ΔOD/Qd, where ΔOD is the change in optical density, ΔOD=log(Tox/Tred), Toxand Tred are the transmittance of the colored and bleached state, respectively, Qd=Q/S, Q is the charge passed during the redox process, and S is the electrode effective area.


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4. CONCLUSION In this work, two kinds of polymers based on a D-A-D’ asymmetric structure were designed and synthesized using fluorenone as the electron acceptor, thiophene as the donor (D) and EDOT or triphenylamine as an additional donor (D'). A series of electrochemical and spectroscopic tests were performed on the polymers. The results show that the polymers with D-A-D′ asymmetric structure have more visible color changes in the polarized state. This result is due to the D-A-D′ asymmetric structure, which causes a change in the electron cloud distribution. The number of redox-active points, meta-stable states and colored species increases, compared with those of the symmetrically structured polymer PSWS. Furthermore, the D-A-D′ asymmetric structure significantly improves the coloration efficiency of polymer in almost all regions (UV-vis-NIR). At the same time, the films response speed and optical contrast (∆T%) also improves. These results demonstrated that the introduction of different donor groups on both sides of the same acceptor unit can improve the properties of the electrochemical and optical chemistry of the polymer. This work therefore has broader implication for the general design of functioned polymer materials. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The experiment section includes the synthesis of the monomers (SWS, SWE, SWT), the electrochemical

polymerization

of

monomers,

and

the

electrochemical

and

spectroelectrochemical tests. (PDF) AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Tel: +86-571-88320508 (M.O.). *E-mail: [email protected]. Tel: +86-571-88320508 (C.Z.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully thanks for the supports from National Natural Science Foundation of China (51573165, 51673174, 51603185 ， 51703199), Natural Science Foundation of Zhejiang Province (LQ18E030015, LZ17E030001, LY15E030006) and Education Department of Zhejiang Province (Y201534880). REFERENCES (1) Wang, B.; Zhao, J. S.; Cui, C. S.; Liu, J. F.; He, Q. P. Electrosynthesis and characterization of an electrochromic material from poly(1,4-bis(3-methylthiophen-2-yl)benzene) and its application in electrochromic device. Sol. Energ. Mat. Sol. C. 2012, 98, 161-167. (2) Ding, G. Q.; Cho, C. M.; Chen, C. X.; Zhou, D.; Wang, X. B.; Yan, A.; Tan, X.; Xu, J. W.; Lu, X. H. Black-to-transmissive electrochromism of azulene-based donor–acceptor copolymers complemented by poly(4-styrene sulfonic acid)-doped poly(3,4-ethylenedioxythiophene). Org. Electron. 2013, 14, 2748–2755. (3) Neo, W. T.; Loo, L. M.; Song, J.; Wang, X. B.; Cho, C. M.; Chan, H. S.; Zong, Y.; Xu. J. W. Solution-processable blue-to-transmissive electrochromic benzotriazole-containing conjugated polymers. Polym. Chem-UK. 2013, 4, 4663–4675. (4) Lv, X. J.; Li, W. J.; Ouyang, M.; Zhang, Y. J.; Wright, D. S.; Zhang, C. Polymeric electrochromic materials with donor-acceptor structures. J. Mater. Chem. C 2017, 5, 12-28.


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Design, Synthesis and Properties of D-A-D' Asymmetric Structured

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