Ethylenedioxythiophene derivatized polynapthalenes as active materials for electrochromic devices

Electrochimica Acta 96 (2013) 82–89

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Ethylenedioxythiophene derivatized polynapthalenes as active materials for electrochromic devices Caixia Xu a , Jinsheng Zhao a,∗ , Junsheng Yu b,∗ , Chuansheng Cui a a

Shandong Key Laboratory of Chemical Energy-storage and Novel Cell Technology, Liaocheng University, 252059 Liaocheng, PR China School of Optoelectronic Information, State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China (UESTC), Chengdu, Sichuan 610054, PR China b

a r t i c l e

i n f o

Article history: Received 18 August 2012 Received in revised form 27 December 2012 Accepted 9 February 2013 Available online 20 February 2013 Keywords: Electrochemical polymerization Spectroelectrochemistry Electrochromic devices Naphthalene

a b s t r a c t Two bis(ethylenedioxythiophene)naphthalene monomers 1,4-bis(2-(3,4-ethylenedioxythiophene))naphthalene (M1) and 2,6-bis(2-(3,4-ethylenedioxythiophene))-naphthalene (M2) were synthesized, and corresponding polymer poly(1,4-bis(2-(3,4-ethylenedioxythiophene))-naphthalene) (P1) and poly(2,6-bis(2-(3,4-ethylenedioxythiophene))-naphthalene) (P2) were electrochemically synthesized and characterized. Characterizations of the resulting polymers were performed by cyclic voltammetry, UV–vis spectroscopy and scanning electron microscopy. The oxidation potential of monomer M2 was lower than that of the M1, and the conjugation lengths of both the M2 and P2 were longer than the corresponding M1 and P1, respectively. Spectroelectrochemical analysis revealed that both the polymer films had good electrochromic properties and exhibited multi-electrochromic behaviors. Besides, the corresponding devices P1/Poly(3,4-ethylenedioxythiophene) (PEDOT) device and P2/PEDOT device showed satisfactory optical contrast (T%), fast response time and excellent cyclic voltammetry stability. The electrochromic properties of two EDOT-naphthalene-EDOT style polymers are dependent on the position of substitution. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Conjugated polymers obtained by polymerization of suitable redox active monomers come to the attention at present, in particular as electrochromic materials. Recently, electrochromic (EC) polymers have drawn a lot of attentions due to their different colors obtained from the same material at different redox states (multichromism), the ease of band gap control with structure modification toward generating different colors, fast response times and superior coloration efficiencies [1]. These polymers are usually based on thiophene [2], pyrrole [3], phenylene, fluorene, or carbazole moieties [4]. Poly(3,4-ethylenedioxythiophene) (PEDOT) is also relatively important member in the electrochromics polymer family. PEDOT displays many interesting properties including low band gap and fast switching with a high contrast ratio over a long period of time in single or dual electrochromic devices [5,6]. In recent years, the kind of poly(heterocycles-arylene-heterocycle)s have been drawn attentions due to their excellent electrochromic properties [7]. As a kind of rationalized resonance contributor, naphthalene not only can constitute a more localized electronic structure but also provides more sites for structural modification.

∗ Corresponding authors. Tel.: +86 635 8539607; fax: +86 635 8539607. E-mail addresses: [email protected] (J. Zhao), [email protected] (J. Yu). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.02.038

Besides, polynaphthalene and their derivatives have unique optical and non-linear optical properties and have potential applications in the fabrication of sensors, second battery, electrochromic and electroluminescence devices [8–10]. Recently, the electrochromic properties of naphthalene containing polymers have been attracted increasing interesting [11–13]. A number of symmetrical bithienylnaphthalene and bis(ethylenedioxythiophene)naphthalene monomers have been synthesized and electrochemical polymerization [14,15]. Our group have reported the electrochromic and fluorescence properties of poly(1,4-bis(2-thienyl)-naphthalene) (PBTN) [16]. The electrochemical and spectroelectrochemical properties of poly(1,4-bis(2-(3,4-ethylenedioxythiophene))-naphthalene) have been studied by Fraind et al. [7]. However, there are still no reports on the electrochromic properties and the electrochromic devices of the bis(ethylenedioxythiophene)naphthalene based polymers, and the effects on the electrochromic properties of the EDOT-naphthalene-EDOT style polymers have also remained to be study. In this paper, two bis[2-(3,4-ethylenedioxythiophene)]naphthalene monomers including 1,4-bis(2-(3,4-ethylenedioxythiophene))-naphthalene (M1) and 2,6-bis(2-(3,4-ethylenedioxythiophene))-naphthalene (M2) were synthesized, and their corresponding polymers P1 and P2 were electrochemically synthesized and characterized. Based on the detailed studies of the

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O n-BuLi O

O

SnBu3

S

TH F

l2

l2 ) 2C 3 Ph (P HF Pd T

Ph (P Pd

) 2C 3

S O

O M1

2.3. Instrumentation

S

S O

(s, 2H, ArH), 7.815 (s, 4H, ArH), 6.248 (s, 2H), 4.363–4.295 (dd, 8H). FT-IR (KBr, cm−1 ): 1600, 1490, 1326 (C C, C C), 1182, 1070 (C O C), 977, 914 (C S C), 880, 815 (1,2,4-trisubstituted benzene ring) (see supporting information Fig. S1–3).

O

SnBu3Cl

S

O

O

O

O

O M2

83

S

Scheme 1. Synthetic routes of the monomers M1 and M2.

electrochromic properties of two polymers, the positional substitution effects on the polymers were also discussed. Besides, the corresponding dual type electrochromic devices employing P1 or P2 as the anodically coloring materials and PEDOT as the cathodically coloring material were constructed and characterized. The results of this study showed that the position of substitution can help manipulate the effective conjugation length of the polymers, which provided an effective way to tailor the spectral characteristics and improve the electrochromic properties of the polymers. 2. Experimental 2.1. Materials 3,4-Ethylenedioxythiophene (EDOT, 98%), bis(triphenylphosphine) dichloropalladium (Pd(PPh3 )2 Cl2 ), 1,4- and 2,6dibromonaphthalene and were all purchased from Aldrich Chemical and used as received. Commercial high-performance liquid chromatography grade acetonitrile (ACN, Tedia Company, INC. USA), dichloromethane (DCM, Sinopharm Chemical Reagent CO., Ltd., China), poly(methyl methacrylate) (PMMA, Shanghai Chemical Reagent Company), propylene carbonate (PC, Shanghai Chemical Reagent Company), n-butyllithium (n-BuLi) and lithium perchlorate (LiClO4 , Shanghai Chemical Reagent Company, 99.9%) were all used directly without further purification. Tetrahydrofuran (THF, J&K Chemical Co. Beijing China) was distilled over Na/benzophenone prior to been used, sodium perchlorate (NaClO4 , Shanghai Chemical Reagent Company, 98%) was dried in vacuum at 60 ◦ C for 24 h before use. Other reagents were all used as received without further treatment. Indium-tin-oxide-coated (ITO) glass (sheet resistance: <10  −1 , purchased from Shenzhen CSG Display Technologies, China) was washed with ethanol, acetone and deionized water successively under ultrasonic, and then dried by N2 flow.

1 H NMR spectroscopy studies were carried out on a Varian AMX 400 spectrometer and tetramethylsilane (TMS) was used as the internal standard for 1 H NMR. FT-IR spectra were recorded on a Nicolet 5700 FT-IR spectrometer, where the samples were dispersed in KBr pellets. Scanning electron microscopy (SEM) measurements were taken by using a Hitachi SU-70 thermionic field emission SEM. UV–vis spectra were carried out on a Shimadzu UV–2550 spectrophotometer. Digital photographs of the polymer films and device cell were taken by a Canon Power Shot A3000 IS digital camera.

2.4. Electrochemistry Electrochemical synthesis and experiments were performed in a one-compartment cell with a CHI 760 C Electrochemical Analyzer under computer control, employing a platinum wire with a diameter of 0.5 mm as working electrode, a platinum ring as counter electrode, and a silver wire (Ag wire) as pseudo reference electrode. All the potentials mentioned follow were vs. the Ag wire electrode. The working and counter electrodes for cyclic voltammetric (CV) experiments were placed 0.5 cm apart during the experiments. The pseudo reference electrode was calibrated externally using a 5 mM solution of ferrocene (Fc/Fc+ ), the potential of Ag wire was assumed to be 0.03 V vs. SCE in ACN/DCM (1:1, by volume) solution containing 0.2 M NaClO4 [17] and 0.08 V vs. SCE in ACN solution containing 0.2 M NaClO4 [18]. All of the electrochemical experiments were carried out at room temperature under nitrogen atmosphere. 2.5. Spectroelectrochemistry Spectroelectrochemical data were recorded on Shimadzu UV-2550 spectrophotometer connected to a computer. A threeelectrode cell assembly was used where the working electrode was an ITO glass, the counter electrode was a stainless steel wire, and an Ag wire was used as pseudo reference electrode. The polymer films for spectroelectrochemistry were prepared by potentiostatically deposition on ITO electrode (the active area: 0.9 cm × 2.0 cm). The thickness of the polymer films grown potentiostatically on ITO is controlled by the total charge passed through the cell. The measurements were carried out in ACN solution containing 0.2 M NaClO4 for M1 and in a mixture of DCM and ACN solution (1:1 by volume) containing 0.2 M NaClO4 for M2.

2.2. Synthesis of M1 and M2 2.6. Preparation of the gel electrolyte As shown in Scheme 1, M1 and M2 were synthesized via Stille cross coupling reaction. 3,4-Ethylenedioxythiophene was converted to its stannyl derivative by treating it with n-BuLi and SnBu3 Cl. A Stille coupling reaction between 1,4dibromonaphthalene and 2-tributylstannyl-3,4-ethylenedioxythiophene gave the compound M1. The same reaction between 2,6dibromonaphthalene and 2-tributylstannyl-3,4-ethylenedioxythiophene gave the compound M2. The title compound M1 is yellow solid. 1 H nuclear magnetic resonance (1 H NMR) (CDCl3 , 400 M Hz, ppm): ı = 8.096 (m, 2H, ArH), 7.576 (s, 2H, ArH), 7.542 (m, 2H, ArH), 6.501(s, 2H), 4.284–4.229(dd, 8H). Fourier transform infrared spectroscopy (FT-IR) (KBr, cm−1 ): 1569, 1502 (C C, C C), 1182, 1070 (C O C) 977, 914 (C S C), 775 (1,2,3,4tetrasubstituted benzene ring). The purified product M2 is glistening yellow solid. 1 H NMR (CDCl3 , 400 MHz, ppm): ı = 8.141

A gel electrolyte based on PMMA and LiClO4 was plasticized with PC to form a highly transparent and conductive gel. ACN was also included as a high vapor pressure solvent to allow an easy mixing of the gel components. The composition of the casting solution by weight ratio of ACN:PC:PMMA:LiClO4 was 70:20:7:3. The gel electrolyte was used for construction of the polymer electrochromic device cell [19]. 2.7. Construction of the electrochromic devices (ECDs) Polymer films (P1 and P2) as the anodically coloring materials and PEDOT as the cathodically coloring material. Both anodically and cathodically coloring polymers were electrochemically deposited onto the ITO-coated glass (the active

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Fig. 2. CV curves of the polymer films P1, P2 and PEDOT (prepared on platinum wires by sweeping the potentials three cycles) at scan rates 100 mV s−1 in monomer free electrolyte.

Eon,m (0.76 V), which might indicate that the conjugation length of the M2 is longer than that of the M1. The CV curves of M2 shows one pair of redox peaks at 0.5–0.8 V, and the increase in the current density of oxidation/reduction also indicates the generation of the electroactive conducting polymer on the surface.

Fig. 1. Cyclic voltammogram curves of M1 (a) and M2 (b) in 0.2 M NaClO4 /ACN and NaClO4 /ACN/DCM solutions at a scan rate of 100 mV s−1 , respectively.

area: 1.8 cm × 2.0 cm) with the same polymerization charge (2.7 × 10−2 C). Before sealing the ECD, the anodically coloring polymer films, P1 or P2 were fully reduced and the cathodically coloring polymer PEDOT was fully oxidized, in order to achieve complementary operating conditions. To provide the ion exchange between the electrochromic layers we coated ITOs with gel electrolyte and the ITOs were positioned so that the electrochromic layers faced each other. 3. Results and discussion 3.1. Electrochemical polymerization and characterization of P1 and P2 3.1.1. Electrochemical polymerization Electrochemical polymerization of the monomers were carried out in ACN solution containing 0.2 M NaClO4 for M1 and in a mixture of DCM and ACN solution (1:1 by volume) containing 0.2 M NaClO4 for M2 since M2 has poor solubility in ACN. Fig. 1a shows the cyclic voltammograms (CV) of M1 which has a onset oxidation potential (Eon,m ) at 1.02 V and a peak potential (Ep,m ) at 1.14 V. As the CV scan continued, P1 film is formed on the working electrode surface. The increase in the redox wave current densities imply that the amount of conducting polymers deposited on the electrode are increasing [17]. The CV curves of M1 shows one pair of redox peaks at 0.67–0.90 V. Compared with M1, the M2 (see Fig. 1b) has lower

3.1.2. Electrochemistry behavior of the polymer films All the polymer films including P1, P2 and PEDOT (prepared on platinum wires by the cyclic voltammetry for three cycles) rinsed with ACN to remove the traces of unreacted monomers and examined in monomer-free electrolyte by CV. Fig. 2 shows the electrochemical behavior of these polymer films at scan rates 100 mV s−1 . The CV curves of the P1 shows a onset oxidation potential at 0.65 V with the oxidation and reduction peak at 0.80 and 0.68 V, respectively. And the CV curves of the P2 shows the onset oxidation potential at–0.01 V with the main oxidation and reduction peak at 0.64 and 0.56 V. The onset oxidation potential of P2 is lower than P1 and higher than PEDOT (−0.60 V), the reason for which might be due to the different effective conjugation length of the polymers. In this respect, the substituent effect of the EDOT-naphthalene-EDOT style polymers can be used to tune the properties of the polymers. Besides, the electrochemical behaviors of the polymer films at different scan rates between 25 and 300 mV s−1 in monomer free electrolyte were also carried out, as shown in Fig. S4 (see supporting information). And, the linear relationships between the peak currents and the scan rates were observed for both the P1 and the P2 polymers (Fig. S4), which indicates that the electroactive polymer films were well adhered and the redox processes were not diffusion controlled [20]. 3.1.3. Optical properties The UV–vis absorption spectra of M1 and M2 monomers in DCM and their respective polymer films deposited on ITO electrode are shown in Fig. 3. In general, the absorption behavior of monomer M2 and polymer P2 are distinctly different from that of M1 and P1. The absorption maximum (max ) of the monomers M1 and M2 (upper right inset, Fig. 3) are centered at 335 and 352 nm, respectively. The max of M2 monomer exhibits a 17 nm red shift compared to that of M1. From the molecular structures of both monomers, one would expect that the 2,6-diEDOT substituted monomer M2 is more coplanar and with more stronger interaction between the EDOT and naphthalene moieties, and, in turn, more extended conjugation in the 2,6-diEDOT substituted monomer. According to this result, the optical band gap values of M1 and M2 were found to

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Table 1 The onset oxidation potential (Eonset ), maximum absorption wavelength (max ), HOMO and LUMO energy levels and optical band gap (Eg ) of M1, M2, P1, and P2. Compounds

Eonset , vs.(Ag-wire) (V)

max (nm)/onset (nm)

Eg a (eV)

HOMOb (eV)

LUMOc (eV)

M1 M2 P1 P2

1.02 0.76 0.65 0.01

335/389 352/399 427/510 495/603

3.18 3.11 2.43 2.06

−5.50 −5.19 −5.13 −4.44

−2.32 −2.08 −2.70 −2.38

a b c

Calculated from the low energy absorption edges (onset ), Eg = 1240/onset . HOMO = −e(Eonset + 4.4) (Eonset vs. SCE). Calculated by the subtraction of the optical band gap from the HOMO level.

be 3.18 and 3.11 eV, respectively. In the meanwhile, the max of P1 was observed at about 427 nm, and, the max of P2 was observed at about 495 nm. This bathochromic shift in the –* absorption of P2 might be attributed to the more coplanar molecular structure of the monomer of M2, and then the resulted polymer P2 had less kinks and longer effective conjugation length. In addtion, the optical band gap (Eg ) of the polymers were calculated from their low energy absorption edges (oneset ) (Eg = 1240/oneset ). The Eg of the P2 polymer film was calculated as 2.06 eV, which was somewhat lower than that of the P1 polymer (2.43 eV), clearly due to the extended conjugation length. Besides, the M2 and P2 have higher HOMO levels than M1 and P1, respectively, since polymers with longer effective conjugation length are prone to oxidative doping. Table 1 clearly summarizes the maximum absorption wavelength (max ), the absorption onsets wavelength (onset ) and the optical band gap (Eg ) of the M1, M2, P1, and P2 quite clearly. From the above data, it can be concluded that the substitution positions of the diEDOT units on the naphthalene ring substantially affect the optical and structural properties of the monomers and their respective polymers.

3.2. Electrochromic properties of the polymer films 3.2.1. Spectroelectrochemical properties of the polymer films Spectroelectrochemistry is a useful method for studying the changes in the absorption spectra and the information about the electronic structures of conjugated polymers as a function of the applied potential difference [21]. The P1 and P2 films were electrodeposited onto ITO (the active area was 0.9 cm × 2.0 cm) with the same polymerization charge of 3.2 × 10−2 C at 1.2 and 1.0 V, respectively. The P1 film was switched between 0 and 1.1 V. At the neutral state, the polymer P1 film exhibits an absorption band at 427 nm due to the –* transition. As shown in Fig. 5a, the intensity of the P1 –* electron transition absorption decreased while two charge carrier absorption bands located at around 670 nm and longer than 800 nm increased dramatically upon oxidation. The appearance of charge carrier bands could be attributed to the evolution of polaron and bipolaron bands. The neutral form of P1 is yellow in color. Stepwise oxidation of the polymer shows that the color changes from yellow to blue, while yellowish green, green and light blue colors exist at intermediate potentials.

3.1.4. Morphology The morphologies of polymer films were investigated by SEM. The P1 and P2 films were prepared by constant potential electrolysis from the solution of 0.2 M NaClO4 /ACN and NaClO4 /ACN/DCM containing relevant monomers on ITO electrodes, respectively, and dedoped before characterization. The SEM images of polymer films are shown in Fig. 4. The P1 film exhibits an accumulation state of small globules, and dense holes were also found among the clusters (Fig. 4a). However, P2 film presents a loose spongy network structure (Fig. 4b), which is different from P1. These morphologies facilitated the movement of doping anions into and out of the polymer film during doping and dedoping, in good agreement with the good redox activity of P1 and P2 films.

Fig. 3. UV–vis absorbtion spectra of P1 and P2 inset: M1 and M2 in dichloromethane.

Fig. 4. SEM images of P1 (a) and P2 (b) deposited potentiostatically onto ITO electrode.

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Fig. 6. Electrochromic switching, optical absorbance change monitored at 427 nm for P1 between 0.0 and 1.1 V, and at 495 nm for P2 between −0.4 and 1.0 V with a same residence time of 4 s.

reduced to the oxidized state at 427 nm, and the optical response time of P2 was found to be 1.19 s from the reduced to the oxidized state at 495 nm. The faster switching response of P2 film than that of P1 film could be ascribed to the faster dopant ion diffusion during the redox process. The satisfactory optical contrast and response time make P1 and P2 promising electrochromic materials for smart windows. The coloration efficiency (another important parameter of electrochromic materials) has been used extensively for performance comparison of devices containing different types of elctrochromic materials. It is defined as the change in the optical density (OD) for the charge consumed per unit electrode area (Q) [23]. The corresponding equations are given below [24]: OD = lg Fig. 5. (a) Spectroelectrochemical spectra of P1 films on ITO electrode as applied potentials between 0 V and 1.1 V in monomer-free 0.2 M NaClO4 /ACN solution (b) Spectroelectrochemical spectra of P2 films on ITO electrode as applied potentials between −0.4 V and 1.0 V in monomer-free 0.2 M NaClO4 /ACN/DCM solution.

A series of spectra was collected at various potentials ranging from–0.4 V to 1.0 V as shown in Fig. 5b. P2 film switches between a red neutral state, a gray intermediate state, and a blue oxidized state. Neutral form of P2 film gives rise to –* absorption band centered at 495 nm. Electrochemical oxidation of the polymer resulted in a decrease in the –* transitions and in an increase in the transitions at 681 and longer than 900 nm which were characteristic of polarons and bipolarons respectively (Fig. 5b). Although the backbone component is the same, the optical properties of P1 and P2 are different depending on the position of substitution. 3.2.2. Electrochromic switching of P1 and P2 film in solution Electrochromic switching studies were carried out to obtain an insight into changes in the optical contrast with time during repeated potential stepping between reduced and oxidized states. One important characteristic of electrochromic materials is the optical contrast (T%), which can be defined as the transmittance difference between the redox states. The T% was found to be 31.66% at 427 nm for P1 and 27.45% at 495 nm for P2, as shown in Fig. 6. The P1 and P2 have high optical contrast when compared with the poly(1,4-bis(2-thienyl)-naphthalene) (PBTN) [16] (see Table 2). Response time, one of the most important characteristics of electrochromic materials, is the necessary time for 95% of the full optical switch (after which the naked eye could not sense the color change) [22]. The optical response time of P1 was found to be 2.12 s from the

T  b

Tc

and  =

OD Q

where Tb and Tc are the transmittances before and after coloration, respectively, and  denotes the coloration efficiency (CE). The CE of the P1 film was calculated to be 117.06 cm2 C−1 at 427 nm and the P2 film was calculated to be 180.21 cm2 C−1 at 495 nm. The results clearly showed that the coloration efficiency of P2 is much greater than that of P1. 3.3. Spectroelectrochemistry of ECDs 3.3.1. Spectroelectrochemical properties of ECDs The dual type ECD consisting of polymer (P1 or P2) film and PEDOT film constructed and theirs spectroelectrochemical behaviors were studied. The spectroelectrochemical spectra of the P1/PEDOT device as a function of applied potential (between −0.8 V and 1.2 V) are given in Fig. 7a. At its neutral state, there was an absorption at 427 nm which is due to –* transition of the P1. As the applied potential increased, the polymer layer started to get oxidized and the intensity of the peak due to the –* transition decreased, which leaded to a new absorption at around 610 nm due to the reduction of PEDOT, and the dominated color of the device was blue at 1.2 V. As shown in Fig. 7b, the P2/PEDOT device spectroelectrochemical behaviors were also studied. The device color from red shifted to blue with a clear and homogeneous change of color. It is interesting to find that the P1/PEDOT device can switch between green and blue, and the P2/PEDOT device can switch between red and blue, the related colors are belonging to the primary colors (red, green and blue). These results make P1 and P2 paramount choice as anodic electrochromic materials in the completion of RGB color space.

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Table 2 The optical contrast (T%), response time and coloration efficiency (CE) of the P1, P2, PBTN, P1/PEDOT device and P2/PEDOT device. Compounds

Optical contrast (T %)

Response time

Coloration efficiency (CE)

P1 P2 PBTNa P1/PEDOT device P2/PEDOT device

31.66% (427 nm) 27.45% (495 nm) 24% (700 nm) 33.27% (645 nm) 32.59% (610 nm)

2.12 s (427 nm) 1.19 s (495 nm) 1.78 s (700 nm) 0.29 s (645 nm) 0.65 s (610 nm)

117.06 180.21 124.8 342.92 226.08

a

Data were taken from Ref. [16].

3.3.2. Switching of ECDs Kinetic studies were done to test the optical contrast and response time of P1/PEDOT device. Under a potential input of −0.8 and 1.2 V at regular intervals of 4 s, this ECD showed a maximum optical contrast (T%) of 32.57%, a response time of 0.65 s, and a coloration efficiency (CE) of 226.08 cm2 C−1 at 610 nm (see the support information Fig. S5). The optical response of the P2/PEDOT device was also investigated (Fig. 8). The ECD was found to have 33.27% optical contrast with a response time of 0.29 s and CE of 342.92 cm2 C−1 at 645 nm. Compared with the P1/PEDOT device, the P2/PEDOT device

Fig. 8. Electrochromic switching response for P2/PEDOT device film monitored at 645 nm between −0.8 V and 1.2 V with a residence time of 4 s.

revealed fast response time and high coloration efficiency so the P2 is more suitable for electrochromic device. Table 2 summarizes the optical contrast (T%), response time and coloration efficiency (CE) of the P1, P2, PBTN [16], P1/PEDOT device and P2/PEDOT device. 3.3.3. Open circuit memory of ECDs The optical memory in the electrochromic devices is an important parameter because it is directly related to its application and energy consumption during the use of ECDs [25]. As shown in Fig. 9, the optical spectrum for P1/PEDOT device was monitored at 610 nm as a function of time at −0.8 V and 1.2 V by applying the potential for

Fig. 7. Spectroelectrochemical spectra of P1/PEDOT device (a) and P2/PEDOT device (b) as applied potentials between −0.8 V and 1.2 V.

Fig. 9. Open circuit memory of P1/PEDOT device monitored by single-wavelength absorption spectroscopy at 610 nm. –0.8 and 1.2 V pulse are applied for 1 s every 200 s to recover the initial transmittance.

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Acknowledgements The work was financially supported by the National Natural Science Foundation of China (20906043), the Promotive research fund for young and middle-aged scientists of Shandong Province (2009BSB01453), the Natural Science Foundation of Shandong province (ZR2010BQ009), the Open Foundation of the State key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201114) and the Taishan Scholarship of Shandong Province. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.02.038. References Fig. 10. Cyclic voltammogram of P1/PEDOT device as a function of repeated with a scan rate of 500 mV s−1 .

1 s for each 200 s time interval. The same procedure is repeated on P2/PEDOT device at 645 nm (see supporting information Fig. S6). At neutral colored state device shows a true permanent memory effect since there is almost no transmittance change under applied potential or open circuit conditions. In blue colored state device is rather less stable in terms of color persistence; however this matter can be overcome by application of current pulses to freshen the fully colored states.

3.3.4. Stability of ECDs The stability of the ECD for long-term switching between oxidized and neutral states is important for practical applications. The P1/PEDOT device was tested by cyclic voltammetry of the applied potential between −0.8 and 1.2 V with 500 mV s−1 to evaluate the stability of the device (Fig. 10). The above observation of the ability to switch between oxidized and reduced states of the ECDs, 95.13% of its electroactivity is retained after 1000 cycles for P1/PEDOT device. And the P2/PEDOT device was also tested (see supporting information Fig. S7). 89.37% of its electroactivity is retained after 1000 cycles for P2/PEDOT device. Both have good redox stability, theirs long lifetime means they have good effective charge compensation ability.

4. Conclusions In this study, two different substituted bis(ethylenedioxythiophene)naphthalene polymers were synthesized and characterized. It has been determined that electrochemical and electrochromic properties of the polymers were greatly influenced by the different substituted. Compared with P1, P2 has more charge trapping capacity and lower band gap. According to the spectroelectrochemical analyses, the P1 and P2 showed excellent multicolor electrochromism. Besides, the polymers P1, P2 and theirs corresponding electrochromic devices showed reasonable optical contrasts and fast response time. It is interesting to find that the P1/PEDOT device can switch between green and blue, and the P2/PEDOT device can switch between red and blue, the related colors are belonging to the primary colors (red, green and blue). Thus, P1 and P2 are promising candidates for anodic electrochromic materials owing to the spectroelectrochemical properties and short synthetic routes.

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