Synthesis and electrochromic properties of a novel conducting polymer film based on dithiafulvenyl-triphenylamine-di(N-carbazole)

Accepted Manuscript Title: Synthesis and electrochromic properties of a novel conducting polymer film based on dithiafulvenyl-triphenylamine-di(N-carbazole) Author: Ping Zhou Zhongquan Wan Yanning Liu Chunyang Jia Xiaolong Weng Jianliang Xie Longjiang Deng PII: DOI: Reference:

S0013-4686(15)31020-3 http://dx.doi.org/doi:10.1016/j.electacta.2015.12.094 EA 26243

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

6-8-2015 13-12-2015 14-12-2015

Please cite this article as: Ping Zhou, Zhongquan Wan, Yanning Liu, Chunyang Jia, Xiaolong Weng, Jianliang Xie, Longjiang Deng, Synthesis and electrochromic properties of a novel conducting polymer film based on dithiafulvenyl-triphenylamine-di(N-carbazole), Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.12.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and electrochromic properties of a novel conducting polymer film based on dithiafulvenyl-triphenylamine-di(N-carbazole)

Ping Zhou, Zhongquan Wan, Yanning Liu, Chunyang Jia , Xiaolong Weng, Jianliang Xie, Longjiang Deng

State Key Laboratory of Electronic Thin Films and Integrated Devices, National Engineering Research Center of Electromagnetic Radiation Control Materials, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China.

 Corresponding author. Tel.: +86 28 83201991; Fax: +86 28 83202569. Email: [email protected] (C. Y. Jia)  Corresponding author. Tel.: +86 28 61830196; Fax: +86 28 83202569. Email: [email protected] (L. J. Deng) 1

Graphical abstract

2

Highlights 1. A novel monomer dithiafulvenyl-triphenylamine-di(N-carbazole) was synthesized. 2. The polymer film based on the monomer was electrochemically polymerized. 3. The polymer film exhibits interesting electrochromic properties. 4. The polymer film has color changing from light yellow to green to blue grey. 5. The polymer film has a maximum optical contrast of 40%.

3

Abstract A strong dithiafulvenyl (DTF)-substitued triphenylamine (TPA) hybrid electron donor was introduced

to

N-position

of

carbazole

(CBZ)

successfully

to

synthesize

the

dithiafulvenyl-triphenylamine-di(N-carbazole) (DTF-TPA-DCBZ) monomer, then its polymer film (poly(DTF-TPA-DCBZ)) was deposited on ITO coated glass substrate by electrochemical polymerization through cyclic voltammetry. Spectroelectrochemical and electrochromic properties of the polymer film were measured through some commonly used methods: cyclic voltammetry, colorimetry, chronoamperometry, ultraviolet-visible and electrochemical impedance spectroscopy analysis. The deposited poly(DTF-TPA-DCBZ) is a multi-color electrochromic film, which has color changing from light yellow to green to blue grey reversibly when different voltage was applied. The polymer film has a broad absorption band at around 650 nm in the visible region, its coloring response time is 4 s and bleaching response time is 8 s. Coloration efficiency of the polymer film is 23.77 cm2 C-1, meanwhile maximum optical contrast of the polymer film reaches to 40% at the wavelength of 658 nm.

Keywords: Electrochromism, Dithiafulvenyl, Carbazole, Triphenylamine, Polymer film

4

1. Introduction Electrochromism is defined as the reversible and visible changes (color changes in naked eyes) of electrochromic materials in transmittance and/or reflectance induced by external voltages [1]. Electrochromic devices (ECDs) have potential applications in many fields, such as smart windows [2-4], antiglare rear-view mirrors [5], photovoltaic devices [6] and displays [7]. A good electrochromic device must possess high coloration efficiency (CE), high optical contrast (ΔT%), short response time, good stability, and optical memory [8]. The most important component of an ECD is the film made by electrochromic materials. There are three main kinds of electrochromic materials: inorganic oxides, organic small molecules and conducting polymers. Transition metal oxides WO3 [9], NiO [10], V2O5 [11] are typical and widely used as inorganic electrochromic materials. Organic small molecules, such as viologen [12] and oligothiophene [13] are usually used in electrochromism. Additionally, some organic-metallic hybrid polymers also have good electrochromic properties [14]. In recent years, conducting polymers (CPs) [15, 16]: polypyrrole (PPy), polyaniline (PANI), polythiophene, polyethylene-dioxythiophene (PEDOT), polycarbazole and their derivatives have received extensive attentions because of their many advantages (low cost, fast switching speed, high optical contrast, and ease of processability) over inorganic materials in electrochromism [17]. To tailor electrochromic properties of conducting polymers, two methods are often used to create multicolor chromophores [18]. One is modification of the polymer structure, that is tuning the band gap of CPs through backbone or side chain structural modification, and the other is copolymerization of distinct monomers. Carbazole (CBZ) is a well-known hole-transporting and light-emitting unit [19], and its polymers have various applications in electroluminescent and electrochromism for their

5

outstanding photoactive and electroactive properties [20]. The 3,6-, 2,7- and N-, positions of CBZ can be easily functionalized and covalently linked to polymer systems. Polymers based on 3,6-linked CBZ are interesting multi-color system, while they have higher band gap compared to 2,7-linked polycarbazoles, so many functional groups are introduced to 3,6-linked polycarbazole to change its band gap in electrochromism [21, 22]. Polymers containing triphenylamine (TPA) unit are another high performance functional material for their strong electron-donating and hole-transporting/injecting properties [23]. Electron-rich triarylamines are easily oxidized to form stable polarons and the oxidation process is always associated with noticeable changes of color, which is the reason why many TPA-based polymers show electrochromic behaviors [24, 25]. Therefore, TPA unit is usually incorporated into electrochromic conducting polymers to form new chromophores. Polymer films based on CBZ and TPA units have been widely used as electrochromic materials for their superior properties [26-28]. Dithiafulvenyl (DTF) can be regarded as a small version of the fulvene family characterized by a terminal electron-donating group, which has been proved to be an excellent donor segment [29]. When DTF unit is connected to TPA unit as an additional electron donor, it forms a strong DTF-substituted TPA hybrid donor [30, 31]. In this paper, we synthesized a monomer dithiafulvenyl-triphenylamine-di(N-carbazole) (DTF-TPA-DCBZ) successfully by introducing the DTF-substitued TPA hybrid donor to N-position of CBZ, which has one DTF unit and two CBZ units separated by a central TPA unit. Then polymer film (poly(DTF-TPA-DCBZ)) based on the synthesized monomer was deposited on ITO coated glass substrate by electrochemical polymerization through cyclic voltammetry. Spectroelectrochemical and electrochromic behaviors of the polymer film were studied in detail, which indicate that the introduced DTF-substitued TPA

6

hybrid electron donor can enhance electrochemical and electrochromic properties of the resulting polymer film.

2. Experimental

2.1. Materials TPA and CBZ were purchased from Astatech. Toluene was pre-dried over 4 Å molecular sieves and distilled under nitrogen atmosphere before use. 4,5-Bis(methylthio)-1,3-dithiol-2-one was synthesized according to literature [32]. Other chemicals and solvents were purchased from commercial sources and used without any further purification. ITO coated glass substrate which has the conductance in 15 Ω/cm2, was also purchased from commercial source. The ITO coated glass substrate was cut into pieces with the size of 0.8 cm × 4 cm, then ultrasonic washed with acetone, ethanol, deionized water for 15 minutes respectively, and immerged in ethanol for use. 0.1 M tetrabutylammonium perchlorate (TBAP) in dichlormethane (DCM) was employed as electrolyte in the tests.

2.2. Characterization Electrochemical behaviors of the synthesized DTF-TPA-DCBZ monomer were examined by cyclic voltammetry (CV) of the CHI660C electrochemical workstation in DCM solution containing 1.0 × 10-3 M monomer and 0.1 M TBAP. Additionally, Pt disk was used as the working electrode, Cu sheet as the counter electrode, and Ag/AgCl in saturated KCl solution as the

7

reference electrode. Electrochemical processes of the poly(DTF-TPA-DCBZ) film were characterized by CV and electrochemical impedance spectroscopy (EIS) analysis using three-electrode system on the CHI660C electrochemical workstation, in which the polymer film deposited on ITO coated glass substrate was served as the working electrode, Cu sheet as the counter electrode, Ag/AgCl in saturated KCl solution as the reference electrode, and 0.1 M TBAP/DCM as the supporting electrolyte. Surface morphology of the polymer film was observed by scanning electron microscopy (SEM, JEOL, JSM-7600F). A spectrophotometer (SP60, X-Rite) was used to test chromaticity of the polymer film, which was switched to different states by amperometric i-t method on the electrochemical workstation before tests. Optical property and kinetic feature of the poly(DTF-TPA-DCBZ) film were carried out on spectrophotometer (UV-2550, SHIMADZU) and CHI660C electrochemical workstation. A home-made three-electrode system was empolyed in the tests, in which the film deposited on ITO coated glass substrate was used as the working electrode, Pt wire as the counter electrode, Ag wire as the reference electrode, and 0.1 M TBAP/DCM in cuvette as the electrolyte. We tested absorption spectra of the polymer film on the spectrophotometer under voltages of -0.4 V, 0 V, 0.4 V, 0.6 V, 0.8 V, 1.0 V, 1.2 V, 1.6V, 1.8 V driven by amperometric i-t method of the electrochemical workstation, respectively. Then through combination of the spectrophotometer and CHI660C electrochemical workstation, we tested its kinetic feature of transmittance at 658 nm. To totally measure CE, ΔT% values of the polymer film, the kinetic tests were conducted under voltage interval between the bleached state (-0.4 V) and colored state (1.8 V).

8

2.3. Synthesis Synthetic

routes

of

poly(DTF-TPA-DCBZ)

are

shown

in

Scheme

1.

4-Diphenylamino-benzaldehyde (Compound 1), 4-[Bis-(4-iodo-phenyl)-amino]-benzaldehyde (Compound 2) and 4-[Bis-(4-carbazol-9-yl-phenyl)-amino]-benzaldehyde (Compound 3) were prepared according to literatures [33, 34].

Scheme 1 is here

2.3.1.

Synthesis of dithiafulvenyl-triphenylamine-di(N-carbazole) monomer(DTF-TPA-DCBZ)

4,5-Bis(methylthio)-1,3-dithiol-2-one (0.828 mmol, 0.17 g) and compound 3 (0.830 mmol, 0.50 g) were dissolved in boiling toluene (10 mL) under nitrogen atmosphere, then adding 5 mL P(OEt)3. The mixture was refluxed for 4 h, then cooled to room temperature and added 50 mL DCM. The resulting mixture was washed with brine and dried with magnesium sulfate. After removing solvent of the solution by rotary evaporation, the residue was purified by column chromatography (petroleum ether-DCM, 1/1.3, v/v), yielding a yellow powder (0.26 g, 40%). HRMS-EI (m/z): [M + H]+ calcd for C48H35N3S4, 782.0290; found, 782.1675. 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.20 (d, 4H), 7.53 (m, 8H), 7.48 (m, 4H), 7.44 (m, 4H), 7.35 (m, 6H), 7.28 (d, 2H), 6.52 (s, 1H), 2.48 (m, 6H).

2.3.2.

Preparation of the poly(DTF-TPA-DCBZ) film

The polymer film was deposited on ITO coated glass substrate by CV on the electrochemical workstation in a three-electrode system, in which ITO coated glass substrate was used as the

9

working electrode, Cu sheet as the counter electrode, and Ag/AgCl in saturated KCl solution as the reference electrode. 0.0060 g DTF-TPA-DCBZ monomer and 0.34 g TBAP were dissolved in 10 mL DCM to form an uniform reaction system by ultrasonic. The polymerization process was conducted between -0.6 V and 2.0 V at a scanning rate of 50 mV/s for 3 cycles (Fig. 1). After scanning, the prepared film was washed with DCM to remove inorganic salt and oligomers on the surface, then dried in air at room temperature. The poly(DTF-TPA-DCBZ) film deposited on ITO coated glass substrate has an active surface area of 0.8 cm × 2 cm.

Fig. 1 is here

3. Results and discussion

3.1. Cyclic voltammetry Electrochemical polymerization process of DTF-TPA-DCBZ monomer on ITO coated glass substrate was recorded in Fig. 1. As the CV scan continued, poly(DTF-TPA-DCBZ) film was formed on the ITO coated glass substrate successfully. The rises in the redox wave current imply that thickness of the deposited polymer film on electrode is increasing [35], meanwhile, color of the deposited polymer film can change reversibly during the scanning process. Furthermore,

electrochemical

behaviors

of

DTF-TPA-DCBZ

monomer

and

poly(DTF-TPA-DCBZ) film were studied in detail, as depicted in Fig. 2, in which each line represents three full scans. Cyclic voltammograms were tested between different voltage ranges

10

for three cycles to study electrochemical reversibility of the monomer and the polymer film when potentials were limited to different values [36]. Corresponding redox peaks of the monomer are located at Eox 1 = 0.68 V, Eox 2 = 0.91 V, Eox 3 = 1.25 V, and Ered 1 = -1.03 V, Ered 2 = 0.16 V, Ered 3 = 0.93 V, which were readed from the CV curve tested between -2.0 V and 2.0 V in Fig. 2a. Moreover, onset oxidation potential of the monomer was Eonset = 0.51 V. From Fig. 2a we can see when applied potentials are below 1.8 V, electrochemical behaviors of the monomer are reversible, while further oxidation of the monomer over 1.8 V was somewhat irreversible. As DTF unit is an excellent electron donor segment, it is easily oxidized to lose electron when voltage was applied on. It is deduced that peripheric DTF unit of DTF-TPA-DCBZ monomer was first oxidized, then followed by oxidation of central electron-rich TPA unit, and further oxidation process occurred in the two peripheral CBZ units. Oligomerization and formation of electroactive polymer film on the working electrode would occur in reactive radicals of CBZ units during continuous scanning. The first oxidation peak in Fig. 2a was assigned to formation of radical cation state in DTF unit, the second oxidation peak assigned to stable radical cation or dication state in TPA unit, and the third attributed to reactive radicals in the two CBZ units, seen in scheme 2 [36-38]. There are three reduction peaks of poly(DTF-TPA-DCBZ) film in the CV curves (Fig. 2b) located at -0.39 V, 0.30 V, 0.65 V, however, only two oxidation peaks can be found at 0.64 V and 1.57 V. What’s more, the oxidation peaks are not clear. When scanning between -0.5 V to 2.0 V, electrochemical reversibility of the polymer film is not well. Differences in the redox potentials between the synthesized monomer and its corresponding polymer film can be attributed to the extended conjugation after polymerization [39].

11

Fig. 2 is here

Scheme 2 is here

Fig. 3 shows electrochemical behaviors of the poly(DTF-TPA-DCBZ) film tested by sweeping voltages between -0.4 V and 1.8 V at different scanning rates from 50 to 300 mV/s. From Fig. 3a we can see that the polymer film exhibited a redox process during the scanning, and peak current density (j) rises with scanning rate increasing. What’s more, redox potential difference increases slightly when the scanning rate was higher than 100 mV/s, beause ohmic drop of the solution occurred or rapid scanning rate is bad for keeping electrochemical reversibility of the polymer film. It is a linear dependence between square root of scanning rate and redox peak current densities, as illustrated in Fig. 3b, indicating that electroactive poly(DTF-TPA-DCBZ) film is reversible and nondiffusional limited [40].

Fig. 3 is here

3.2. Morphology Morphology of poly(DTF-TPA-DCBZ) film was investigated by SEM. SEM images of the polymer film on ITO coated glass substrate were given in Fig. 4. An accumulation of small particles with the diameter of about 90 nm is found on surface of the polymer film, and there are dense holes among the accumulation (Fig. 4a). Besides, from Fig. 4b we know that the polymer film has a thickness of about 670 nm. The special porous morphology of the polymer film has a

12

great influence on improving electrochromic properties of the poly(DTF-TPA-DCBZ) film, which made the redox process easily achieved when voltages were applied.

Fig. 4 is here

3.3. Electrochromic properties of the polymer film

3.3.1.

Colorimetry

Colors of the polymer film are measured by colorimetric analysis. To quantitatively define the color of electrochromic films, three parameters L* (luminance), a* (hue), b* (saturation) are often used. Table 1 presents the tested L*, a*, b* parameters of the poly(DTF-TPA-DCBZ) film at different states and their corresponding optical images under voltages of -0.4 V, 1.2 V, 1.8V, respectively. It demonstrates that the deposited poly(DTF-TPA-DCBZ) is a multi-color electrochromic film, which has color changing from light yellow to green to blue grey.

Table 1 is here

3.3.2.

UV-vis spectra

Optical properties of the electrochromic film at different redox states were tested by ultraviolet-visible (UV-vis) spectroscopy. Redox reaction takes place when different voltages are applied on electrochromic film, resulting in changes of the inner structure, so absorbance spectra of the electrochromic film at different states can be different. We selected some voltages around

13

the potentials which drove the polymer’s color to change to observe absorbance spectra of the polymer film at different states, recorded in Fig. 5b. Absorbance spectra of DTF-TPA-DCBZ monomer and poly(DTF-TPA-DCBZ) film are also tested. As shown in Fig. 5a, the monomer displayed strong absorption below 430 nm, having two absorbance peaks located at 383 nm and 341 nm, but it was almost transparent above 430 nm. While poly(DTF-TPA-DCBZ) film exhibited absorption from 300 nm to 800 nm, which is assigned to π-π* transitions in conjugated carbazole moieties and introduction of the strong electron-donating DTF-substitued TPA unit increased degree of π-conjugation in the polymer backbone [41]. Once the polymer film was driven to oxidized states by applying higher voltages, absorbance spectra changed a lot. From Fig. 5b we can find a broad absorption peak located at around 650 nm in the visible region when the polymer film was oxidized, because the oxidization process leaded to formation of bipolaron state in DTF and TPA units of the polymer, while it diminished when applied voltage was higher than 1.6 V [42]. The intrinsic absorption characters of poly(DTF-TPA-DCBZ) film located in the visible region indicate that it is a potential electrochromic material.

Fig. 5 is here

3.3.3.

Response time

Response time is an important parameter of electrochromic material, defining as the time required in coloring or bleaching process. Fast response time partly ensures practical applications of electrochromic devices [43]. In this article, response time was calculated for reaching 90% of the maximum transmittance change between bleached (-0.4 V) and colored (1.8 V) states of the

14

polymer film at the wavelength of 658 nm. As shown in Fig. 6c, coloring response time of the poly(DTF-TPA-DCBZ) film is 4 s, and bleaching response time is 8 s. The time of the electrochromic film switching to its bleached state is a bit long , beacause the reduction process becomes slow once the polarization of CBZ unit is formed [44].

Fig. 6 is here

3.3.4.

Coloration efficiency

Coloration efficiency (CE) is another crucial parameter to study electrochromic phenomenon. CE values are usually used to compare performances of various electrochromic materials and devices. High CE value means that a small amount of charge can make a large transmittance change of the material, which is directly related to injected charge and contrast ratio of the corresponding electrochromic material [45]. The change in optical density (ΔOD) at a specific wavelength over the charge consumed per unit electrode area (Qd) is used to define CE. Equations are given below [46]:



OD( ) Qd

and OD( )  log

T T0

where η denotes the CE, T0 represents the transmittance of an electrochromic film in bleached

15

state, and T represents the transmittance in colored state at a given wavelength. A worthy electrochromic material should be equipped with high CE. As described in Fig. 6, a maximum ΔT% of the polymer film reaches to 40% at the wavelength of 658 nm in the visible region. Higher ΔT% of the polymer film ensures its application in electrochromism. Fig. 6a and Fig. 6b exhibits chronoamperometry curves and in-situ transmittance of the polymer film at 658 nm under voltage interval between -0.4 V and 1.8 V, respectively. According to these two sets of data, plots of optical density versus charge density were drawn, which are illustrated in Fig. 7a. It is calculated that CE value of the polymer film is 23.77 cm2 C-1 at 658 nm. However, from Fig. 6b we can find that reversibility of the poly(DTF-TPA-DCBZ) film is poor, because there exits attenuation in value of ΔT%. Therefore, high potentials applied on poly(DTF-TPA-DCBZ) film would lead to inactivation of the polymer film’s electroactivity. When the polymer film was oxidized to its colored state, electrochemical coupling of electroactive CBZ units would take place [47], so the oxidized polymer film was hard to return to its entirely bleached state, leading to a longer bleaching time. To further measure reversibility of the polymer film voltage interval was changed between to -0.4 V and 1.4 V, as illustrated in Fig. 6d and Fig. 6e. It is apparently that reversibility of the polymer film is much better when higher voltage was reduced from 1.8 V to 1.4 V. Under this condition, ΔT% of the polymer film is only 22%, and its CE value was calculated to be 13.2 cm2 C-1 (Fig. 7b). ΔT% of the polymer film decreased a lot, because the polymer film didn’t reach its entirely colored state under 1.4 V.

Fig. 7 is here

16

3.4. Electrochemical impedance spectra Electrochemical impedance spectroscopy (EIS) is a very useful tool to investigate bulk and interfacial electrical properties of many materials, including conducting polymers [48, 49]. Electrochemical behaviors of poly(DTF-TPA-DCBZ) film were further studied by EIS analysis, in which an AC voltage of small amplitude (5 mV) was imposed when the frequency changed from 100 kHz to 0.01 Hz. We tested electrochemical impedance spectra of the polymer film under voltages of -0.4 V, 0.4 V, 0.6 V, 1.0 V, 1.2 V, 1.4 V, 1.6 V, 1.8 V, respectively, and the resulted Nyquist plots are depicted in Fig. 8. As applied voltages changed from negative to positive, the polymer film transformed from its bleached state to colored state. In many cases, Nyquist plots are composed of two parts: a semicircle in high frequency region which is principally related to charge-transfer-controlled process, and a line with slope related to diffusion-limited process in low frequency region [50]. Electrochemical behavior of poly(DTF-TPA-DCBZ) film is primarily a charge-transfer-controlled process because of the higher slope and larger arc occurred in Nyquist plots of the polymer film [51, 52].

Fig. 8 is here Fig. 9 is here

Three equivalent circuits used for matching impedance spectra of the polymer film under different voltages are presented in Fig. 9, which were fitted by ZSimpWin software, and the fitting errors for each component are less than 10%. Rs refers to electrolyte resistance between working and reference electrodes, that is a significant factor in the impedance of an electrochemical cell.

17

Double-layer

capacitance

(Cdl),

charge-transfer

resistance

(Rct)

on

the

working

electrode/electrolyte interface reflect charge transfer influence on impedance of the electrochemical cell. Q represents constant-phase element attributed to rough surface of poly(DTF-TPA-DCBZ) film, Cc denotes coating capacitance and Rp indicates polarization resistance of the polymer film. Zw is Warburg impedance created by diffusion depending on the frequency of potential perturbation [53, 54]. Equivalent circuit is exactly combination of these components and reflects dynamic characteristics of the electrode system, which agrees with the electrochromic property of the polymer film mainly induced by the charge-transfer process. Different impedance spectra and equivalent circuits for the polymer film under different voltages further suggested that the film’s inner structure have changed when voltages are applied on. As shown in Fig. 8a, Nyquist plots of the polymer film under different voltages (which are corresponding to the redox process took place in the polymer film) have same arcs. While there is another smaller arc occurred in Nyquist plots of the polymer film under voltages varied from 1.0 V to 1.6 V followed by the first one, then disappeared when the polymer film was driven to the state under 1.8 V, and these changes are reversible during the tests, presented in Fig. 8b explicitly. When applied voltages varied from 1.0 V to 1.6 V, the polymer film was oxidized to stable radical cation or dication states in TPA unit, or reactive radical cations in CBZ unit. As a result, polarization of electroactive CBZ or TPA units in the polymer film becoming significant. Rp of the polymer film increased and the polarization process was fast, so the second arc occurred, becoming the product of higher polarization impedance [55].

18

4. Conclusion In summary, a novel electrochromic film of poly(DTF-TPA-DCBZ) has been prepared by electrochemical polymerization through cyclic voltammetry. An excellent electron-donating group DTF unit was introduced to the polymer side chain as a pendent, forming a strong hybrid electron donor through connecting to electron-rich TPA unit. A porous film with an accumulation of small particles was deposited on ITO coated glass substrate, and the polymer film can change its color reversibly from light yellow to green to blue grey. The polymer film has a broad absorption band in the visible region at around 650 nm, its coloring response time is 4 s and bleaching response time is 8 s. Coloration efficiency and maximum optical contrast of the polymer film are 23.77 cm2 C-1 and 40% at the wavelength of 658 nm, respectively. Poly(DTF-TPA-DCBZ) is proved to be a promising candidate for electrochromic material because of its specific spectroelectrochemical and electrochromic properties.

Acknowledgements We thank the National Natural Science Foundation of China (Grant No. 21272033, 21402023) and Fundamental Research Funds for the Central Universities (Grant No. ZYGX2014J026) for financial support.

19

References [1] P.R. Somani, S. Radhakrishnan, Electrochromic materials and devices: present and future, Mater. Chem. Phys. 77 (2003) 117-133. [2] R.D. Clear, V. Inkarojrit, E.S. Lee, Subject responses to electrochromic windows, Energy Buildings 38 (2006) 758-779. [3] R. Baetens, B.P. Jelle, A. Gustavsen, Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: A state-of-the-art review, Sol. Energy Mater. Sol. Cells 94 (2010) 87-105. [4] J.R. Jennings, W.Y. Lim, S.M. Zakeeruddin, M. Gratzel, Q. Wang, A redox-flow electrochromic window, ACS Appl. Mater. Interfaces 7 (2015) 2827-2832. [5] D.R. Rosseinsky, R.J. Mortimer, Electrochromic Systems and the Prospects for Devices, Adv. Mater. 13 (2001) 783-793. [6] L.M. Huang, C.W. Hu, H.C. Liu, C.Y. Hsu, C.H. Chen, K.C. Ho, Photovoltaic electrochromic device for solar cell module and self-powered smart glass applications, Sol. Energy Mater. Sol. Cells 99 (2012) 154-159. [7] F.C. Krebs, Electrochromic displays: The new black, Nat. Mater. 7 (2008) 766-767. [8] V.K. Thakur, G. Ding, J. Ma, P.S. Lee, X. Lu, Hybrid materials and polymer electrolytes for electrochromic device applications, Adv. Mater. 24 (2012) 4071-4096. [9] C. Liao, F. Chen, J. Kai, Electrochromic properties of nanocomposite WO3 films, Sol. Energy Mater. Sol. Cells 91 (2007) 1282-1288. [10] G.A. Niklasson, C.G. Granqvist, Electrochromics for smart windows: thin films of tungsten oxide and nickel oxide, and devices based on these, J. Mater. Chem. 17 (2007) 127-156. [11] C. Lamsal, N.M. Ravindra, Optical properties of vanadium oxides-an analysis, J. Mater. Sci. 48 (2013) 6341-6351. [12] M. Li, Y. Wei, J. Zheng, D. Zhu, C. Xu, Highly contrasted and stable electrochromic device based on well-matched viologen and triphenylamine, Org. Electron. 15 (2014) 428-434. [13] C.B. Nielsen, A. Angerhofer, K.A. Abboud, J.R. Reynolds, Discrete Photopatternable π-Conjugated Oligomers for Electrochromic Devices, J. Am. Chem. Soc. 130 (2008) 9734-9746.

20

[14] M. Higuchi, Electrochromic Organic–Metallic Hybrid Polymers: Fundamentals and Device Applications, Polym. J. 41 (2009) 511-520. [15] K. Hyodo, Electrochromism of conducting polymers, Electrochim. Acta 39 (1994) 265-272. [16] P.M. Beaujuge, J.R. Reynolds, Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices, Chem. Rev. 110 (2010) 268-320. [17] E.K. Unver, S. Tarkuc, Y.A. Udum, C. Tanyeli, L. Toppare, The effect of the donor unit on the optical properties of polymers, Org. Electron. 12 (2011) 1625-1631. [18] A.A. Argun, P.H. Aubert, B.C. Thompson, I. Schwendeman, C.L. Gaupp, J. Hwang, N.J. Pinto, D.B. Tanner, A.G. MacDiarmid, J.R. Reynolds, Multicolored Electrochromism in Polymers: Structures and Devices, Chem. Mater. 16 (2004) 4401-4412. [19] Y. Tao, C. Yang, J. Qin, Organic host materials for phosphorescent organic light-emitting diodes, Chem. Soc. Rev. 40 (2011) 2943-2970. [20] H.M. Wang, S.H. Hsiao, Enhancement of redox stability and electrochromic performance of aromatic polyamides by incorporation of (3,6-dimethoxycarbazol-9-yl)-triphenylamine units, J. Polym. Sci. Part A: Polym. Chem. 52 (2014) 272-286. [21] F. Baycan Koyuncu, S. Koyuncu, E. Ozdemir, A new multi-electrochromic 2,7 linked polycarbazole derivative: Effect of the nitro subunit, Org. Electron. 12 (2011) 1701-1710. [22] D. Witker, J.R. Reynolds, Soluble Variable Color Carbazole-Containing Electrochromic Polymers, Macromolecules 38 (2005) 7636-7644. [23] P. Kochapradist, N. Prachumrak, R. Tarsang, T. Keawin, S. Jungsuttiwong, T. Sudyoadsuk, V. Promarak,

Multi-triphenylamine-substituted

carbazoles:

synthesis,

characterization,

properties, and applications as hole-transporting materials, Tetrahedron Lett. 54 (2013) 3683-3687. [24] C.H. Yang, F.J. Liu, L.R. Huang, T.L. Wang, W.C. Lin, M. Sato, C.H. Chen, C.C. Chang, The application of triphenylamine-based hole-transporting materials in electrochromic devices, J. Electroanal. Chem. 617 (2008) 101-110. [25] S. Beaupré, J. Dumas, M. Leclerc, Toward the Development of New Textile/Plastic Electrochromic Cells Using Triphenylamine-Based Copolymers, Chem. Mater. 18 (2006) 4011-4018. [26] Y. W. Chuang, H.J. Yen, J.H. Wu, G. S. Liou, Colorless triphenylamine-based aliphatic 21

thermoset epoxy for multicolored and near-infrared electrochromic applications, ACS Appl. Mater. Interfaces 6 (2014) 3594-3599. [27] B. Hu, Y. Zhang, X. Lv, M. Ouyang, Z. Fu, C. Zhang, Electrochemical and electrochromic properties of two novel polymers containing carbazole and phenyl-methanone units, J. Electroanal. Chem. 689 (2013) 291-296. [28] G.S. Liou, S.H. Hsiao, H.W. Chen, Novel high-Tg poly(amine-imide)s bearing pendent N-phenylcarbazole units: synthesis and photophysical, electrochemical and electrochromic properties, J. Mater. Chem. 16 (2006) 1831-1842. [29] F. Meyers, J.L. Bredas, J. Zyss, Electronic structure and nonlinear optical properties of push-pull polyenes: theoretical investigation of benzodithia polyenals and dithiolene polyenals, J. Am. Chem. Soc. 114 (1992) 2914-2921. [30] Z. Wan, C. Jia, Y. Duan, X. Chen, Z. Li, Y. Lin, Novel organic sensitizers containing dithiafulvenyl units as additional donors for efficient dye-sensitized solar cells, RSC Adv. 4 (2014) 34896-34903. [31] Z. Wan, C. Jia, Y. Duan, X. Chen, Y. Lin, Y. Shi, Novel organic dye employing dithiafulvenyl-substituted arylamine hybrid donor unit for dye-sensitized solar cells, Org. Electron. 14 (2013) 2132-2138. [32] K.B. Simonsen, N. Svenstrup, J. Lau, O. Simonsen, P. Mørk, G.J. Kristensen, J. Becher, Sequential Functionalisation of Bis-Protected Tetrathiafulvalene-dithiolates, Synthesis 1996 (1996) 407-418. [33] W. Xu, B. Peng, J. Chen, M. Liang, F. Cai, New Triphenylamine-Based Dyes for Dye-Sensitized Solar Cells, J. Phys. Chem. C 112 (2008) 874-880. [34] Z. Wan, C. Jia, Y. Duan, L. Zhou, J. Zhang, Y. Lin, Y. Shi, Influence of the antennas in starburst triphenylamine-based organic dye-sensitized solar cells: phenothiazine versus carbazole, RSC Adv. 2 (2012) 4507-4514. [35] C. Xu, J. Zhao, M. Wang, C. Cui, R. Liu, Electrosynthesis and characterization of a donor–acceptor

type

electrochromic

material

from

poly(4,7-dicarbazol-9-yl-2,1,3-

benzothiadia-zole) and its application in electrochromic devices, Thin Solid Films 527 (2013) 232-238. [36] K. Karon, M. Lapkowski, A. Dabuliene, A. Tomkeviciene, N. Kostiv, J.V. Grazulevicius, 22

Spectroelectrochemical

characterization

of

conducting

polymers

from

star-shaped

carbazole-triphenylamine compounds, Electrochim. Acta 154 (2015) 119-127. [37] D. Gulfidan, E. Sefer, S. Koyuncu, M.H. Acar, Neutral state colorless electrochromic polymer networks: Spacer effect on electrochromic performance, Polymer 55 (2014) 5998-6005. [38]

E.

Ripaud,

P.

Leriche,

N.

Cocherel,

T.

Cauchy,

P.

Frere,

J.

Roncali,

Tristhienylphenylamine--extended dithiafulvene hybrids as bifunctional electroactive species, Org. Biomol. Chem. 9 (2011) 1034-1040. [39] E. Sefer, F. Baycan Koyuncu, Naphthalenediimide bridged D-A polymers: Design, synthesis and electrochromic properties, Electrochim. Acta 143 (2014) 106-113. [40] B. Wang, J. Zhao, C. Cui, J. Liu, Q. He, Electrosynthesis and characterization of an electrochromic

material

from

poly(1,4-bis(3-methylthiophen-2-yl)benzene)

and

its

application in electrochromic device, Sol. Energy Mater. Sol. Cells 98 (2012) 161-167. [41] V. Promarak, S. Ruchirawat, Synthesis and properties of N-carbazole end-capped conjugated molecules, Tetrahedron 63 (2007) 1602-1609. [42] J. Qu, Y. Suzuki, M. Shiotsuki, F. Sanda, T. Masuda, Synthesis and electro-optical properties of helical polyacetylenes carrying carbazole and triphenylamine moieties, Polymer, 48 (2007) 4628-4636. [43] J. Jensen, M. Hösel, I. Kim, J.S. Yu, J. Jo, F.C. Krebs, Fast Switching ITO Free Electrochromic Devices, Adv. Funct. Mater. 24 (2014) 1228-1233. [44] X. Fu, C. Jia, S. Wu, X. Weng, J. Xie, L. Deng, Electrosynthesis and characterization of a novel electrochromic film based on poly (4,4′-di(N-carbazolyl)triphenylamine), Synth. Met. 188 (2014) 104-110. [45] C.L. Gaupp, D.M. Welsh, R.D. Rauh, J.R. Reynolds, Composite Coloration Efficiency Measurements of Electrochromic Polymers Based on 3,4-Alkylenedioxythiophenes, Chem. Mater. 14 (2002) 3964-3970. [46] C. Xu, J. Zhao, J. Yu, C. Cui, Ethylenedioxythiophene derivatized polynapthalenes as active materials for electrochromic devices, Electrochim. Acta 96 (2013) 82-89. [47] H.M. Wang, S.H. Hsiao, Substituent effects on electrochemical and electrochromic properties of aromatic polyimides with 4-(carbazol-9-yl)triphenylamine moieties, J. Polym. Sci. Part A: Polym. Chem. 52 (2014) 1172-1184. 23

[48] C. Fernández-Sánchez, C.J. McNeil, K. Rawson, Electrochemical impedance spectroscopy studies of polymer degradation: application to biosensor development, Trac-Trend. Anal. Chem. 24 (2005) 37-48. [49] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamental and Applications, Wiley, New York, USA, 1980. [50] Y. Liu, C. Jia, Z. Wan, X. Weng, J. Xie, L. Deng, Electrochemical and electrochromic properties of novel nanoporous NiO/V2O5 hybrid film, Sol. Energy Mater. Sol. Cells 132 (2015) 467-475. [51] B. Gadgil, P. Damlin, T. Ääritalo, C. Kvarnström, Electrosynthesis of viologen cross-linked polythiophene in ionic liquid and its electrochromic properties, Electrochim. Acta 133 (2014) 268-274. [52] X. Xia, J. Tu, Y. Mai, R. Chen, X. Wang, C. Gu, X. Zhao, Graphene sheet/porous NiO hybrid film for supercapacitor applications, Chem. Eur. J. 17 (2011) 10898–10905. [53] Garmy Instruments, Application notes: Basics of Electrochemical Impedance Spectroscopy, Pennsylvania, USA, 2010. [54] C.N. Cao, J.Q. Zhang, An Introduction to Electrochemical Impedance Spectroscopy, Science Press, Beijing, 2002. [55] C. Ma, J. Zheng, S. Yang, D. Zhu, Y. Bin, C. Xu, Electrochromic kinetics of nanostructured poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) film on plastic substrate, Org. Electron. 12 (2011) 980-987.

24

Figure captions

Fig. 1. Preparation curve of poly(DTF-TPA-DCBZ) film by cyclic voltammetry in DCM solution containing 1.0×10-3 M monomer and 0.1 M TBAP. Fig. 2. Cyclic voltammograms of DTF-TPA-DCBZ monomer (a) and poly(DTF-TPA-DCBZ) film (b) in 0.1 M TBAP/DCM solution with a scanning rate of 100 mV/s between various potentials, each line represents full three scans. Fig. 3. (a) CV curves of poly(DTF-TPA-DCBZ) film at different scanning rates from 50 to 300 mV/s in 0.1 M TBAP/DCM solution. (b) Square root of scanning rate dependence of the redox peak current densities. Fig. 4. (a) Surface and (b) cross-sectional SEM images of poly(DTF-TPA-DCBZ) film. Fig. 5. (a) UV-vis absorption spectra of DTF-TPA-DCBZ monomer and poly(DTF-TPA-DCBZ) film. (b) UV-vis absorption spectra of poly(DTF-TPA-DCBZ) film under different voltages. Fig. 6. Chronoamperometry (a) and in-situ transmittance (b,c) curve of poly(DTF-TPA-DCBZ) film at 658 nm under voltage interval between -0.4 V and 1.8 V with a residence time of 20 s; Chronoamperometry (d) and in-situ transmittance (e) curve of the polymer film at 658 nm under voltage interval between -0.4 V and 1.4 V with a residence time of 15 s. Fig. 7. Variation of the optical density (ΔOD) versus charge density (Qd) of poly(DTF-TPA-DCBZ) film under voltage interval between -0.4 V and 1.8 V (a), -0.4 V and 1.4 V (b). Fig. 8. (a) and (b) Nyquist plots of poly(DTF-TPA-DCBZ) film under different voltages in 0.1 M TBAP/DCM solution. Fig. 9. (a) Equivalent circuit for poly(DTF-TPA-DCBZ) film under -0.4 V, 0.4 V, 0.6V. (b) Equivalent circuit for poly(DTF-TPA-DCBZ) film under 1.0 V, 1.2 V, 1.4 V, 1.6 V. (c) Equivalent circuit for poly(DTF-TPA-DCBZ) film under 1.8 V.

25

2.0

Current / mA

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential / V (vs Ag/AgCl)

Fig. 1.

26

0.10

(a) -0.6 V-2.0 V 0 V-1.8 V 0 V-1.6 V -2.0 V-2.0 V

Current / mA

0.05 0.00 -0.05 -0.10 -2

-1

0

1

2

Potential / V (vs Ag/AgCl)

2.0

(b)

Current / mA

1.5

-0.5 V-1.4 V -0.5 V-2.0 V

1.0 0.5 0.0 -0.5 -1.0 -0.5

0.0

0.5

1.0

1.5

2.0

Potential / V (vs Ag/AgCl)

Fig. 2.

27

2.5

(a)

2.0

j / mA cm-2

1.5 1.0 0.5

-1 50 mV s -1 80 mV s -1 100 mV s -1 150 mV s -1 200 mV s -1 250 mV s -1 300 mV s

0.0 -0.5 -1.0 -1.5 -2.0

-0.5

0.0

0.5

1.0

1.5

2.0

Potential / V (vs Ag/AgCl)

2.0 1.5

(b)

jp.a

j / mA cm-2

1.0 0.5 0.0 -0.5 -1.0 jp.c

-1.5 -2.0

50

100

150

200 250 1/2 1/2 V / (mV s-1)

300

Fig. 3.

28

Fig. 4.

29

1.2

(a)

DTF-TPA-DCBZ poly(DTF-TPA-DCBZ) film

Absorbance / a.u.

1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength / nm

1.4

-0.4 V 0V 0.4 V 0.6 V 0.8 V 1.0 V 1.2 V 1.6 V 1.8 V

(b)

Absorbance / a.u.

1.2 1.0 0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

900

Wavelength / nm

Fig. 5.

30

(a)

10

Current / mA

5 0 -5 -10 0

50

100

150

200

150

200

t/s

90

(b)

Transmittance / %

80 70 60 50 40 0

50

100 t/s

90

(c)

4s

8s

Transmittance/%

80 70 60 50 40 30

35

40

45

50

55

60

65

70

75

t/s

31

(d)

Current / mA

10

5

0

-5 0

100

200

300

400

500

t/s

(e)

Transmittance / %

80 75 70 65 60 55 0

100

200

300

400

500

t/s

Fig. 6.

32

0.35

(a) 0.30 0.25

OD

0.20 0.15 0.10 0.05 0.00 0

2

4

6

8

10

12

14

16

18

Charge density / mC cm-2

0.12

(b) 0.10

OD

0.08 0.06 0.04 0.02 0.00 6

8

10

12

14

16

Charge density / mC cm-2

Fig. 7.

33

5000

(a)

-0.4 V 0.4 V 0.6 V 1.0 V 1.2 V 1.4 V 1.6 V 1.8 V

- Z" / ohm

4000 3000 2000 1000 0

0

1000

2000

3000

4000

5000

Z' / ohm

400 350

-0.4 V 0.4 V 0.6 V 1.0 V 1.2 V 1.4 V 1.6 V 1.8 V

(b)

- Z" / ohm

300 250 200 150 100 50 0 50

100

150

200

250

300

350

400

Z' / ohm

Fig. 8.

34

Fig. 9.

35

CHO

CHO POCl3, DMF 1,2-dichloroethane

KI, KIO3, CH3COOH

reflux, 12h

N

reflux , 4h

N

N I

1

I 2

CHO H N N Cu, K2CO3, 18-crown-6 1,2-dichlorobenzene reflux, 48h

N

N

3 S S S

S

S

S

S S

O N

P(OEt)3, toluene reflux, 4h

N

N

DTF-TPA-DCBZ

S S S

electrochemical polymerization

S

N N

N

n

poly(DTF-TPA-DCBZ)

Scheme 1. Synthetic routes of poly(DTF-TPA-DCBZ).

36

S

S

S

S S

S

S

S

-e N

N N

N

N

N

-e

S

S

S

S

S

S

S

S

N

N N

N

N

N

-e

S

S

S

S

S

S

S

S

N

N HC

CH N

N

N

N

Scheme 2. Mechanism of oxidation process for DTF-TPA-DCBZ.

37

Table 1. Color parameters and optical images of poly(DTF-TPA-DCBZ) film. Voltage (V) -0.4

L* 73.86

a* 0.47

b* 17.57

Color Light yellow

1.2

64.53

-3.34

11.86

Green

1.8

57.72

-0.7

9.56

Blue grey

38