Optical Materials 34 (2012) 1095–1101

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Optical Materials

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Electrochemical synthesis, characterization and electrochromic propertiesof a copolymer based on 1,4-bis(2-thienyl)naphthalene and pyrene

Bin Wang a, Jinsheng Zhao a,⇑, Chuansheng Cui a, Min Wang b, Zhong Wang b, Qingpeng He a

a Shandong Key Laboratory of Chemical Energy-Storage and Novel Cell Technology, Liaocheng University, 252059 Liaocheng, PR Chinab The Central Laboratory of Liaocheng Hospital, 252000 Liaocheng, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 October 2011Received in revised form 1 January 2012Accepted 5 January 2012Available online 23 January 2012

Keywords:Electrochemical polymerizationConjugated copolymerSpectroelectrochemistryElectrochromic device1,4-Bis(2-thienyl)naphthalenePyrene

0925-3467/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.optmat.2012.01.009

⇑ Corresponding author. Tel./fax: +86 635 8539607E-mail address: j.s.zhao@163.com (J. Zhao).

Electrochemical copolymerization of 1,4-bis(2-thienyl)naphthalene (BTN) with pyrene is carried out inacetonitrile (ACN) solution containing sodium perchlorate (NaClO4) as a supporting electrolyte. Charac-terizations of the resulting copolymer P(BTN-co-pyrene) are performed by cyclic voltammetry (CV),UV–vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy(SEM). The P(BTN-co-pyrene) film has distinct electrochromic properties and exhibits three different col-ors (yellowish green, green and blue) under various potentials. Maximum contrast (DT%) and responsetime of the copolymer film are measured as 37.8% and 1.71 s at 687 nm. An electrochromic device(ECD) based on P(BTN-co-pyrene) and poly(3,4-ethylenedioxythiophene) (PEDOT) is constructed andcharacterized. Neutral state of device shows green color while oxidized state reveals blue color. ThisECD shows a maximum optical contrast (DT%) of 24.4% with a response time of 0.43 s at 635 nm. The col-oration efficiency (CE) of the device is calculated to be 349 cm2 C�1 at 635 nm. In addition, the ECD alsohas satisfactory optical memories and redox stability.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Electrochromic materials show a reversible optical change abil-ity in absorption or transmittance upon electrochemically oxidizedor reduced, which has stimulated the interest of scientists over thepast decades [1]. Electrochromic materials exhibit at least two dis-tinct color states and may give multiple colors, depending on thestructure of the material. A wide variety of electrochromic materi-als are presently known, ranging from metal oxides and mixed-valence metal complexes to organic molecules and conjugatedpolymers [2]. In recent years, electrochromic conjugated polymershave gained a lot of attention due to their several advantages overinorganic compounds, such as low cost, processability, high opticalcontrast ratio, multi-colors with the same material, high stabilityand long cycle life with fast response time [3]. To achieve colorchange in electrochromic polymers, absorption in the visible re-gion should be monitored by means of an externally applied poten-tial. Upon doping, electronic states change due to the formation ofpolaronic and bipolaronic bands causing a change in the absorptioncharacteristics of the polymer [4,5].

There are numerous applications for electrochromic materialssuch as optical displays [4], smart windows [6] and electrochromicdevices [7–9]. Electrochromic behavior in polymers such as

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.

polypyrrole, polyaniline or polythiophene and their derivativeshas been studied deeply. Polypyrrole has been extensively utilizedas an electrochromic material and can be easily synthesized chem-ically or electrochemically with a varying range of optoelectronicproperties available through alkyl and alkoxy substitution [4]. Thinfilms of neat polypyrrole are yellow in the dedoped insulating stateand black in the doped conductive state [10]. Polyaniline films arepolyelectrochromic materials which exhibit switching among yel-low–green–blue and black colors [10]. As a class of excellent elec-trochromic materials, polythiophenes have occupied primeposition because of their high electrical conductivity and goodredox property. They exhibit fast response time, outstanding sta-bility and high contrast ratios in the visible and near-infrared re-gions [11]. Synthesis of new polythiophene derivatives with theability to tailor the electrochromic properties is an important partof conducting polymer research [12,13]. Among the polythio-phenes, the polymers having the structure like thiophene-aryl-ene-thiophene have been synthesized and characterized [8,14].Recently, our group synthesized the 1,4-bis(2-thienyl)naphthalene(BTN) monomer and studied the electrochromic properties ofpoly(1,4-bis(2-thienyl)naphthalene) (PBTN), the PBTN film pre-sents reasonable electrochromic properties [15]. On the otherhand, pyrene is also an important aromatic monomer, it is interest-ing to note that fine tuning in the band gap and neutral state colorof the polymer can be achieved by copolymerization with pyrene[16,17]. Furthermore, copolymerization offers an effective way of

1096 B. Wang et al. / Optical Materials 34 (2012) 1095–1101

controlling the electrochromic properties of conducting polymers.Copolymers can lead to an interesting combination of the proper-ties observed in the corresponding homopolymers [18]. Besides,it is well known that the pursuit of new high-quality electrochro-mic materials is still the main goal of scientists in the field of elec-trochromic devices. The neutral green dual type electrochromicdevices are desirable [8,19].

According to above considerations, in this work, the electro-chemical copolymerization of BTN with pyrene is carried out in0.2 M NaClO4/ACN solution (Scheme 1). The spectroelectrochemi-cal and electrochromic properties of the P(BTN-co-pyrene) areinvestigated in details. The copolymer P(BTN-co-pyrene) film hasfast response time and high optical contrast when compared withPBTN homopolymer. In addition, we constructed and characterizeddual type electrochromic devices based on P(BTN-co-pyrene) andPEDOT in details. The electrochromic device shows green color atneutral state.

2. Experimental

2.1. Materials

1,4-Bis(2-thienyl)naphthalene (BTN) monomer was synthesizedas reported previously by our group [15]. Pyrene (Acros Organics,98%), commercial high-performance liquid chromatography gradeacetonitrile (ACN, Tedia Company, Inc., USA), poly(methyl methac-rylate) (PMMA, Shanghai Chemical Reagent Company), propylenecarbonate (PC, Shanghai Chemical Reagent Company), lithium per-chlorate (LiClO4, Shanghai Chemical Reagent Company, 99.9%) and3,4-ethylenedioxythiophene (EDOT, Aldrich, 98%) are used directlywithout further purification. Sodium perchlorate (NaClO4, ShanghaiChemical Reagent Company, 98%) is dried in vacuum at 60 �C for24 h before use. Other reagents are all used as received without fur-ther treatment. Indium-tin-oxide-coated (ITO) glass (sheet resis-tance: <10 X h�1, purchased from Shenzhen CSG DisplayTechnologies, China) is washed with ethanol, acetone and deion-ized water successively under ultrasonic, and then dried by N2 flow.

2.2. Electrochemistry

Electrochemical synthesis and experiments are performed in aone-compartment cell with a CHI 760 C Electrochemical Analyzerunder computer control, employing a platinum wire with a diame-ter of 0.5 mm as working electrode, a platinum ring as a counterelectrode, and a silver wire (Ag wire) as a pseudo reference elec-trode. The working and counter electrodes for cyclic voltammetric(CV) experiments are placed 0.5 cm apart during the experiments.All electrochemical polymerization and CV tests are taken in ACNsolution containing 0.2 M NaClO4 as a supporting electrolyte. Thepseudo reference electrode is calibrated externally using a 5 mMsolution of ferrocene (Fc/Fc+) in the electrolyte (E1/2(Fc/Fc+)= 0.20 V vs. Ag wire in 0.2 M NaClO4/ACN) [11] and all the potentialsmentioned follow are vs. the Ag wire electrode. The half-wave po-tential (E1/2) of Fc/Fc+ measured in 0.2 M NaClO4/ACN solution is0.28 V vs. SCE. Thus, the potential of Ag wire was assumed to be0.08 V vs. SCE [8]. All of the electrochemical experiments are carriedout at room temperature under nitrogen atmosphere.

S

S +

electrochemicapolymerization

Scheme 1. Electrochemical copolyme

2.3. Characterizations

FT-IR spectra are recorded on a Nicolet 5700 FT-IR spectrome-ter, where the samples are dispersed in KBr pellets. Scanning elec-tron microscopy (SEM) measurements are taken by using a HitachiSU-70 thermionic field emission SEM. UV–vis spectra are carriedout on a Perkin-Elmer Lambda 900 UV–vis–near-infrared spectro-photometer. Digital photographs of the polymer films are takenby a Canon Power Shot A3000 IS digital camera. Colorimetry mea-surements are obtained by a Coloreye XTH Spectrophotometer(GretagMacbeth).

2.4. Spectroelectrochemistry

Spectroelectrochemical data are recorded on Perkin-ElmerLambda 900 UV–vis–near-infrared spectrophotometer connectedto a computer. A three-electrode cell assembly is used where theworking electrode is an ITO glass, the counter electrode is a stain-less steel wire, and an Ag wire is used as pseudo reference electrode.The polymer films for spectroelectrochemistry are prepared bypotentiostatically deposition on ITO electrode (the active area:0.8 cm � 2.0 cm). The measurements are carried out in ACN solu-tion containing 0.2 M NaClO4.

2.5. Preparation of the gel electrolyte

A gel electrolyte based on PMMA and LiClO4 is plasticized withPC to form a highly transparent and conductive gel. ACN is also in-cluded as a high vapor pressure solvent to allow an easy mixing ofthe gel components. The composition of the casting solution byweight ratio of ACN:PC:PMMA:LiClO4 is 70:20:7:3. The gel electro-lyte is used for construction of the polymer electrochromic devicecell [20].

2.6. Construction of ECDs

ECDs are constructed using two complementary polymers,namely P(BTN-co-pyrene) as the anodically coloring material andPEDOT as the cathodically coloring material. Both P(BTN-co-pyr-ene) and PEDOT films are electrodeposited on two ITO glasses(the active area: 1.8 cm � 2.5 cm) at 1.35 and 1.4 V, respectively.Electrochromic device is built by arranging the two polymer films(one oxidized, the other reduced) facing each other separated by agel electrolyte.

3. Results and discussion

3.1. Electrochemical polymerization and characterization of polymers

3.1.1. Electrochemical polymerizationThe anodic polarization curves of 0.005 M BTN, 0.005 M pyrene

and the BTN/pyrene mixture at 1:1 in 0.2 M NaClO4/ACN solutionare shown in Fig. S1 (see Supporting Information). The onset oxida-tion potential (Epa onset) of BTN in the solution is 1.11 V (Fig. S1,curve a), while that of pyrene is 1.10 V (Fig. S1, curve b). The differ-ence of the onset oxidation potential between BTN and pyrene is0.01 V, implying that the electrochemical copolymerization is

l

S

S m n

rization route of BTN and pyrene.

B. Wang et al. / Optical Materials 34 (2012) 1095–1101 1097

readily to be achieved [21]. As can been seen from curve c inFig. S1, the Epa onset of the BTN/pyrene mixture is 1.08 V, which islower than those of BTN and pyrene, indicating the existence ofthe interaction between two monomers in 0.2 M NaClO4/ACN [22].

The successive CV curves of 0.005 M BTN, 0.005 M pyrene andthe BTN/pyrene mixture (0.005 M BTN and 0.005 M pyrene) in0.2 M NaClO4/ACN are illustrated in Fig. 1. As shown in Fig. 1a,the CV curves of BTN show the unsymmetrical redox peaks andthe reduction peak potential at 0.86 V, while the correspondingoxidation waves are overlapped with the oxidation waves of theBTN monomer and cannot be observed clearly [23]. The polymeri-zation of pyrene presents the cathodic peak potential at 1.08 V(Fig. 1c). However, the CV curve of the BTN/pyrene mixture exhib-its a reduction peak at 0.80 V (Fig. 1b), which is different fromthose of BTN and pyrene, indicating the formation of a new copoly-mer consisting of both BTN and pyrene units [24,25]. In addition, ascan be seen from Fig. 1, there is an obvious increase of current den-sity of the BTN/pyrene mixture compared with those of BTN andpyrene, which can also imply the formation of a copolymer [25].

3.1.2. Electrochemistry of the P(BTN-co-pyrene) filmFig. 2a shows the electrochemical behavior of the P(BTN-co-pyr-

ene) film (prepared on platinum wire by sweeping the potentialsfrom 0 to 1.35 V for three cycles) at different scan rates between25 and 250 mV s�1 in 0.2 M NaClO4/ACN. The copolymer film showsa single and well-defined redox process (Fig. 2a). The current re-sponse is directly proportional to the scan rate indicating that thecopolymer film is electroactive and adheres well to the electrode[23]. The scan rate for the anodic and cathodic peak current densitiesshows a linear dependence as a function of the scan rate as illustratedin Fig. 2b. This demonstrates that the electrochemical processes ofthe copolymer are reversible and not diffusion limited [24].

3.1.3. FT-IR spectra of PBTN, P(BTN-co-pyrene) and polypyrene filmsThe FT-IR spectra of PBTN, P(BTN-co-pyrene) and polypyrene

are shown in Fig. S2 (see Supporting Information). These polymers

Fig. 1. Successive CV curves of (a) 0.005 M BTN, (b) 0.005 M BTN and 0.005 M pyrene, (c)density.

are synthesized potentiostatically in the solution of 0.2 M NaClO4/ACN containing 0.005 M BTN and 0.005 M pyrene monomers ortheir mixture. According to the spectrum of PBTN (Fig. S2, spec-trum a), the band around 1574 cm�1 is ascribed to the stretchingvibrations of phenylene rings, and the bands at 1506 and1456 cm�1 are due to the stretching vibrations of thiophene rings[15], the strong absorption peak at 797 cm�1 is attributed to theout-of-plane bending vibrations of CAH bond in b-position of the2,5-disubstitued thiophene rings [26] and the 762 cm�1 band is as-signed to the out-of-plane vibration of the 4 adjacent CAH bonds inthe substituted phenylene rings [27]. In the spectrum of polypy-rene (Fig. S2, spectrum c), the bands at 1633, 1599 and1487 cm�1 are related to the C@C stretching vibration of pyrenerings, the peaks at 844 and 815 cm�1 are assigned to the out-of-plane bending vibration of the CAH bonds of substituted benzenerings, the band at 681 cm�1 could be ascribed to the newly formedCAC bond between pyrene units [17,28]. The above mentionedbands of PBTN and polypyrene could also be found in the FT-IRspectrum of P(BTN-co-pyrene) (Fig. S2, spectrum b). Comparedwith corresponding homopolymers, the band at 800 cm�1 and762 cm�1 in the spectrum of P(BTN-co-pyrene) originate fromthe CAH bond in b-position of the 2,5-disubstitued thiophene ringsand the 4 adjacent CAH bonds in the substituted phenylene rings,respectively, indicating the presence of BTN units in the copoly-mer. While the bands at 844 and 815 cm�1 in the copolymer as-cribed to the substituted benzene ring in pyrene units can alsobe found. All the above features indicate that P(BTN-co-pyrene)contains both BTN and pyrene units.

3.1.4. MorphologyThe morphologies of polymer films are investigated by scanning

electron microscopy (SEM). The PBTN, P(BTN-co-pyrene) andpolypyrene films are prepared by constant potential electrolysisfrom the solution of 0.2 M NaClO4/ACN containing relevant mono-mers on ITO electrodes and dedoped before characterization. TheSEM images of these polymer films are shown in Fig. 3. The PBTN

0.005 M pyrene in 0.2 M NaClO4/ACN. Scan rates: 100 mV s�1. j denotes the current

Fig. 2. (a) CV curves of the P(BTN-co-pyrene) film at different scan rates between25 and 250 mV s�1 in the monomer-free 0.2 M NaClO4/ACN. (b) Scan ratedependence of the anodic and cathodic peak current densities graph. jp.a and jp.c

denote the anodic and cathodic peak current densities, respectively.

1098 B. Wang et al. / Optical Materials 34 (2012) 1095–1101

film exhibits a porous structure like coral grown with small gran-ules (Fig. 3a) and polypyrene shows an accumulation state of clus-ters of globules (Fig. 3c). While P(BTN-co-pyrene) shows porousstructure with smaller granules grown than that of PBTN(Fig. 3b) and the approximate diameters of these granules are inthe range of 150–500 nm, which are moderately different fromtwo corresponding homopolymers. The difference of morphologybetween P(BTN-co-pyrene) and homopolymers also confirms theoccurrence of copolymerization between BTN and pyrene.

3.1.5. Optical properties of polymersThe UV–vis spectra of homopolymers and their copolymer

deposited on ITO electrode are shown in Fig. 4. In the neutral state,polypyrene film exhibits an absorption band at 357 nm due to thep–p⁄ transition (Fig. 4, spectrum a). As shown by the spectrum b inFig. 4, the neutral state PBTN exhibits the p–p⁄ electron transitionpeak at about 397 nm. However, it should be noted that a well-de-fined maximum absorption band centered at 399 nm is observed inspectrum c of Fig. 4, which is attributed to the p–p⁄ transition ofthe neutral state P(BTN-co-pyrene) copolymer backbone. Com-pared to the homopolymers, the P(BTN-co-pyrene) copolymerhas a broad absorption band. Besides, the optical band gap (Eg) ofpolymers are calculated from their low energy absorption edges(konset) (Eg = 1240/konset) [29]. The Eg of the P(BTN-co-pyrene) filmis calculated as 2.35 eV, which is lower than those of PBTN(2.48 eV) and polypyrene (2.66 eV).

The UV–vis spectra of the obtained copolymers with 2:1, 1:1 and1:2 BTN/pyrene feed ratios are also studied (see Supporting Informa-tion Fig. S3), the absorption spectra of the obtained copolymers showcontinuous red-shift and reduction of the band gap as the feed ratioof BTN/pyrene increases. Fine tuning in the band gap is achieved bytailoring the co-monomer feed ratio of copolymerization.

Table 1 summarizes the onset oxidation potential (Eonset), max-imum absorption wavelength (kmax), low energy absorption edges(konset), HOMO and LUMO energy levels and the optical band gaps(Eg) values of PBTN, polypyrene and the copolymers (prepared withthe feed ratio of BTN/pyrene at 2:1 (COP 1), 1:1 (COP 2) and 1:2(COP 3), respectively) quite clearly. HOMO energy levels of themare calculated by using the formula EHOMO = �e(Eonset + 4.4) (vs.SCE) and LUMO energy levels (ELUMO) of them are calculated bythe subtraction of the optical band gap from the HOMO levels[30,31]. The Eonset values of the polymers are obtained from theirCV curves in monomer-free 0.2 M NaClO4/ACN solution (see Sup-porting Information Fig. S4).

3.2. Electrochromic properties of P(BTN-co-pyrene) film

3.2.1. Spectroelectrochemical properties of P(BTN-co-pyrene) filmSpectroelectrochemistry is the best way of examining the

changes in optical properties of a polymer on ITO upon appliedpotentials [32]. The P(BTN-co-pyrene) film coated ITO (preparedpotentiostatically at 1.35 V in 0.2 M NaClO4/ACN solution mixingwith 0.005/0.005 M BTN/pyrene) is switched between 0 and1.30 V in monomer-free 0.2 M NaClO4/ACN solution in order to ob-tain the in situ UV–vis spectra (Fig. 5). At the neutral state, thecopolymer film exhibits an absorption band at 403 nm due to thep–p⁄ transition. As shown in Fig. 5, the intensity of the P(BTN-co-pyrene) p–p⁄ electron transition absorption decreases whiletwo charge carrier absorption bands located at around 687 nmand more than 1050 nm increase dramatically upon oxidation.The appearance of charge carrier bands could be attributed to theevolution of polaron and bipolaron bands.

While there are many methods to quantify and represent color,one of the most widely applicable method to measure the color ofmaterials illuminated by a standard light source is the 1976 CIELAB (or L⁄a⁄b⁄) with the value of L⁄ representing how the measuredmaterial is light vs. dark, the value a⁄ representing how red vs.green, and b⁄ representing how yellow vs. blue the material is[33]. It is interesting to find that the P(BTN-co-pyrene) film showsa multicolor electrochromism. During the oxidation process, yel-lowish green color of the film at neutral state (0 V) turns into greencolor at intermediate doped state (1.10 V), and then into blue colorat full doped state (1.30 V). These colors and corresponding L⁄, a⁄,b⁄ values are given in Fig. 6.

3.2.2. Electrochromic switching of P(BTN-co-pyrene) film in solutionIt is important that polymers can switch rapidly and exhibit a

noteworthy color change for electrochromic applications [34]. Forthis purpose, double potential step chronoamperometry techniqueis used to investigate the switching ability of P(BTN-co-pyrene)film between its neutral and full doped state (Fig. 7). The dynamicelectrochromic experiment for P(BTN-co-pyrene) film is carriedout at 687 nm. The potential is interchanged between 0 V (the neu-tral state) and 1.30 V (the oxidized state) at regular intervals of 6 s.One important characteristic of electrochromic materials is theoptical contrast (DT%), which can be defined as the transmittancedifference between the redox states. The DT% of the P(BTN-co-pyr-ene) is found to be 37.8% at 687 nm, as shown in Fig. 7.

Response time, one of the most important characteristics ofelectrochromic materials, is the necessary time for 95% of the fulloptical switch (after which the naked eye could not sense the colorchange) [35]. The optical response time of P(BTN-co-pyrene) is

Fig. 3. SEM images of (a) PBTN, (b) P(BTN-co-pyrene) and (c) polypyrene deposited potentiostatically on ITO electrode.

Fig. 4. UV–vis spectra of (a) polypyrene, (b) PBTN and (c) P(BTN-co-pyrene)deposited on ITO at the neutral state.

B. Wang et al. / Optical Materials 34 (2012) 1095–1101 1099

found to be 1.71 s from the reduced to the oxidized state and 0.59 sfrom the oxidized to the reduced state at 687 nm. Compared toPBTN homopolymer film, the copolymer P(BTN-co-pyrene) filmhas high optical contrast and fast response time [15].

Table 1The onset oxidation potential (Eonset), maximum absorption wavelength (kmax), low energyvalues of PBTN, polypyrene and the copolymers (prepared with the feed ratio of BTN/pyre

Compounds Eonset, vs. (Ag-wire) (V) kmax (nm)/konset (nm

PBTN 0.86 397/501COP 1 0.80 405/541COP 2 0.87 399/528COP 3 0.92 390/518Polypyrene 0.93 357/466

a Calculated from the low energy absorption edges (konset), Eg = 1240/konset.b Calculated by the subtraction of the optical band gap from the HOMO level.

3.3. Spectroelectrochemistry of electrochromic devices (ECDs)

3.3.1. Spectroelectrochemical properties of ECDA dual type ECD consisting of P(BTN-co-pyrene) and PEDOT con-

structed and its spectroelectrochemical behaviors are also studied.Before composing the ECD, the anodically coloring polymer filmP(BTN-co-pyrene) is fully reduced and the cathodically coloringpolymer PEDOT is fully oxidized. The spectroelectrochemical spec-tra of the P(BTN-co-pyrene)/PEDOT device as a function of appliedpotential (between�0.8 V and 1.5 V) are given in Fig. 8. The copoly-mer is in its neutral state at�0.8 V, where the absorption at 418 nmis due to p–p⁄ transition of the copolymer. At that state, PEDOT doesnot reveal an obvious absorption at the UV–vis region of the spec-trum and the device reveals green color (Fig. 8, Inset). As the appliedpotential increases, the copolymer layer starts to get oxidized andthe intensity of the peak due to the p–p⁄ transition decreased.Meanwhile, PEDOT layer is in its reduced state, which leads to anew absorption at 635 nm due to the reduction of PEDOT, and thedominated color of the device is blue at 1.5 V (Fig. 8, Inset).

3.3.2. Switching of ECDKinetic studies are also done to test the response time of P(BTN-

co-pyrene)/PEDOT ECD. Under a potential input of �0.8 and 1.5 Vat regular intervals of 2 s, the optical response at 635 nm, as illus-trated in Fig. 9. The response time is found to be 0.43 s at 95% of the

absorption edges (konset), HOMO and LUMO energy levels and optical band gap (Eg)ne at 2:1, 1:1 and 1:2, respectively).

) Ega (eV) HOMO (eV) LUMOb (eV)

2.48 �5.34 �2.862.29 �5.28 �2.992.35 �5.35 �3.002.39 �5.40 �3.012.66 �5.41 �2.75

Fig. 5. Spectroelectrochemical spectra of P(BTN-co-pyrene) films on ITO electrodeas applied potentials between 0 V and 1.30 V in monomer-free 0.2 M NaClO4/ACNsolution.

Fig. 6. Colors and corresponding L⁄, a⁄, b⁄ values of the P(BTN-co-pyrene) film atdifferent applied potentials.

Fig. 7. Electrochromic switching response for P(BTN-co-pyrene) film monitored at687 nm in 0.2 M NaClO4/ACN solution between 0 V and 1.30 V with a residencetime of 6 s.

Fig. 8. Spectroelectrochemical spectra of the P(BTN-co-pyrene)/PEDOT device atvarious applied potentials between �0.8 and 1.5 V. Inset: Colors of the P(BTN-co-pyrene)/PEDOT device at �0.8 V (the neutral state) and 1.5 V (the oxidized state).

Fig. 9. Electrochromic switching, optical transmittance change monitored at 635 nmfor P(BTN-co-pyrene)/PEDOT device between�0.8 V and 1.5 V with a residence timeof 2 s.

1100 B. Wang et al. / Optical Materials 34 (2012) 1095–1101

maximum transmittance difference from the neutral state to oxi-dized state and 0.27 s from the oxidized state to the neutral state,and optical contrast (DT%) is calculated to be 24.4%. The P(BTN-co-pyrene)/PEDOT device has fast response time (0.43 s) and highoptical contrast (24.4%) when compared with the PBTN/PEDOT de-vice which has 0.57 s and 10% response time and optical contrast,respectively [15].

The coloration efficiency (CE) is one of the most importantparameter of the electrochromic device. The CE can be calculatedby using the equations and given below [36]:

DOD ¼ lgTb

Tc

� �and g ¼ DOD

DQ

where Tb and Tc are the transmittances before and after coloration,respectively. DOD is the change of the optical density, which is pro-portional to the amount of created color centers. g denotes the col-oration efficiency (CE). DQ is the amount of injected charge per unitsample area. The CE of the P(BTN-co-pyrene)/PEDOT device (the ac-tive of area is 1.8 cm � 2.0 cm) is calculated to be 349 cm2 C�1 at635 nm.

3.3.3. Open circuit memory of ECDThe optical memory in the electrochromic devices is an impor-

tant parameter because it is directly related to its application andenergy consumption during the use of ECDs [24]. The optical spec-trum for P(BTN-co-pyrene)/PEDOT device is monitored at 635 nmas a function of time at �0.8 V and 1.5 V by applying the potentialfor 1 s for each 200 s time interval. As shown in Fig. 10, at greencolored state device shows a true permanent memory effect sincethere is almost no transmittance change under applied potential oropen circuit conditions. In blue colored state device is rather lessstable in terms of color persistence, however this matter can beovercome by application of current pulses to freshen the fully col-ored states.

3.3.4. Stability of ECDThe stability of the devices toward multiple redox switching

usually limits the utility of electrochromic materials in ECD appli-

Fig. 10. Open circuit stability of the P(BTN-co-pyrene)/PEDOT device monitored at635 nm.

Fig. 11. Cyclic voltammogram of P(BTN-co-pyrene)/PEDOT device as a function ofrepeated with a scan rate of 500 mV/s.

B. Wang et al. / Optical Materials 34 (2012) 1095–1101 1101

cations. Therefore, redox stability is another important parameterfor ECD [34]. For this reason, the P(BTN-co-pyrene)/PEDOT deviceis tested by cyclic voltammetry of the applied potential between�0.8 and 1.4 V with 500 mV/s to evaluate the stability of the device(Fig. 11). After 500 cycles, 86.8% of its electroactivity is retainedand there is no obvious decrease of activity between 500 cyclesand 1000 cycles. These results show that this device has reasonableredox stability.

4. Conclusions

A new copolymer based on BTN and pyrene is electrochemicallysynthesized and characterized. At the neutral state of the copoly-mer, the p–p⁄ transition absorption peak is located at 399 nmand the optical band gap is calculated as 2.35 eV. P(BTN-co-pyrene)copolymer film has distinct electrochromic properties and showsthree different colors (yellowish green, green and blue) under var-ious potentials. The copolymer film shows a maximum optical con-trast (DT%) of 37.8% and a response time of 1.71 s at 687 nm whichare higher and faster than those of the homopolymer of BTN (PBTN,24% and 1.78 s). Further, an ECD is also assembled from P(BTN-co-pyrene) in the sandwich configuration, ITO/P(BTN-co-pyrene)//gelelectrolyte//PEDOT/ITO. The color of the constructed deviceswitched between green and blue on the application of potentialbetween �0.8 and 1.5 V. Electrochromic switching study resultsshow that optical contrast (DT%) and response time are 24.4%

and 0.43 s at 635 nm, respectively. The coloration efficiency (CE)of the device is calculated to be 349 cm2 C�1 at 635 nm. In addition,the device shows satisfactory optical memories and redox stability.Considering these results, P(BTN-co-pyrene) is a promising candi-date for electrochromic layers in ECDs.

Acknowledgements

The work was financially support by the National Natural Sci-ence Foundation of China (No. 20906043), the Promotive researchfund for young and middle-aged scientists of Shandong Province(2009BSB01453), the Natural Science Foundation of Shandongprovince (ZR2010BQ009) and the Taishan Scholarship of ShandongProvince.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.optmat.2012.01.009.

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