Journal of Electroanalytical Chemistry 645 (2010) 50–57

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Journal of Electroanalytical Chemistry

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A novel multichromic copolymer of 1,4-bis(3-hexylthiophen-2-yl)benzeneand 3,4-ethylenedioxythiophene prepared via electrocopolymerization

Cheng Zhang *, Cheng Hua, Genghao Wang, Mi Ouyang, Chunan MaState Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering and Materials Science, Zhejiang University of Technology,Hangzhou, PR China

a r t i c l e i n f o

Article history:Received 9 February 2010Received in revised form 12 April 2010Accepted 14 April 2010Available online 18 April 2010

Keywords:Conjugated copolymerEDOTThiophene derivativeMultichromismSpectroelectrochemistry

1572-6657/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jelechem.2010.04.009

* Corresponding author. Address: Zhejiang UniversLaboratory Breeding Base for Green Chemistry SyHangzhou, PR China. Tel./fax: +86 579 88320253.

E-mail address: czhang@zjut.edu.cn (C. Zhang).

a b s t r a c t

Copolymer based on 1,4-bis(3-hexylthiophen-2-yl)benzene (BHThBe) and 3,4-ethylenedioxythiophene(EDOT) is electrochemically synthesized and characterized. Electrochemical methods, FTIR and 1H NMRanalyses confirm that the resulting polymer is a copolymer rather than a blend or a composite of therespective homopolymers. Moreover, the spectroelectrochemical and electrochromic properties of thecopolymer films are investigated. According to the spectroelectrochemical analyses, the copolymer filmreveals distinct electrochromic properties from that of the BHThBe homopolymer film and shows five dif-ferent colors under various potentials. At the neutral state of the copolymer, the p ? p� transition absorp-tion peak is located at 401 nm with a shoulder around 510 nm and the energy gap is calculated as 1.86 eV.The copolymer film shows a maximum optical contrast (DT%) of 36% in visible region with a switchingtime of 1.8 s and of 76% in the near-IR region with a switching time of 1.4 s. The new multichromiccopolymer is thermally stable up to 315 �C and is soluble in chloroform. SEM images illustrate that thecopolymer film presents a much smoother surface than that of the respective homopolymers.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of high levels of electronic conductivity inthe doped forms of p-conjugated structures, tremendous interestshave been paid to design and fabricate various functional conju-gated polymers applied in electronic devices [1–4]. As one of therepresentatives, electrochromic (EC) conjugated polymers havedrawn a lot of attentions due to their tunable energy gap (Eg), higherswitching rate, excellent processability and low costs [5,6], whichhave been widely used in the fields of displays [7], energy-saving‘‘smart” windows [8] and memory devices [9]. For EC polymers,the electrochromism is related to the changing of band gaps duringthe doping-dedoping process. Thus, it is an effective way to achievedifferent kinds of EC properties via adjusting the electronic charac-ter of the p-orbit along the neutral polymer backbone, includingmain chain and pendant group structural modification and copoly-merization [10–12].

Polythiophenes are interesting EC materials due to their facileEg tunability through structural modification and have been re-ported extensively in past few years [13,14]. But the pure electro-chemically prepared polythiophenes have the ‘‘polythiophene

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ity of Technology, State Keynthesis Technology, 310014

paradox” and insoluble in organic solvents, which restrict themfor further applications. In order to overcome these drawbacks,various thiophene-based oligomers have been invented [15,16].Recently, Chan and co-workers have reported a serial of oligomerscontaining alternating phenylene and alkyl-substituted thienyleneunits whose polymers are regioregular and soluble in organic sol-vents [17]. Reynolds and co-workers have designed three discreteoligomeric systems and have successfully constructed the first oli-gomer-based photopatterned EC device [18].

On the other hand, copolymerization is a promising strategy toobtain materials with better properties than those of the respectivehomopolymers. Poly(3,4-ethylenedioxythiophene) (PEDOT) hasdominated much of the recent EC copolymers literatures due toits rapid switching and excellent stability in its doped form. For in-stance, several groups have reported copolymer films based onEDOT exhibiting multichromism or a tunable electrochromism[19–21].

Inspired by the aforementioned contributions to recent ad-vances in oligomers designs, we have recently taken interest inpreparing a phenylene/alkyl-substituted thiophene/EDOT basedcopolymer material which is soluble in chloroform and possessmultichromic property. In this work, 1,4-bis(3-hexylthiophen-2-yl)benzene (BHThBe) is synthesized via Grignard coupling reaction[22]. The electrochemical polymerization of BHThBe and copoly-merization reaction based on BHThBe and EDOT are achieved bypotentiostatic electrolysis in a solution of LiClO4/acetonitrile

C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57 51

electrolyte-solvent couple. The resultant copolymer presents goodthermal stability and rapid switching responses either in the visi-ble or near-IR regions, and exhibits multichromism (brick red, kha-ki, yellow green, green and midnight blue colors) with thevariation of the applied potentials.

2. Experimental

2.1. Materials

2-Bromo-3-hexylthiophene (Huicheng chemical, 98%), 1,4-dibromobenzene (ACROS, 99%), 1,10-bis(diphenylphosphino)ferro-cene palladium dichloride (Pd(dppf)Cl2) (J&K Chemical, 99%), mag-nesium powder (Alfa-Aesar, 99.8%), 3,4-ethylenedioxythiophene(EDOT) (Aldrich, 98%), and lithium perchlorate anhydrous (LiClO4)(ACROS, 99 + %) were used without further purification. Diethylether (Et2O) (Sigma–Aldrich, HPLC) and acetonitrile (ACN) (Sig-ma–Aldrich, HPLC) were distilled from CaH2 and then stored over4A molecular sieves. 1,4-Bis(3-hexylthiophen-2-yl)benzene(BHThBe) was synthesized according to the literature procedure[22] with some modification.

2.2. Equipments

Neutral state polymer samples peeled off from the ITO electrodewere pressed in a diamond cell and then examined between 4000and 650 cm�1 by a Nicolet 6700 Fourier-transform infrared spec-trometer (FTIR) (Thermo Fisher Nicolet, USA), equipped with anitrogen cooled MCT/A detector and a contiuum infrared-micro-scope. The micro-FTIR spectra of polymer samples were recordedin transmission mode as sum of 128 scans at 8 cm�1 resolution.UV–vis spectra were recorded on a Shimadzu UV-1800 UV–visspectrophotometer (Shimadzu, Japan). 1H and 13C NMR spectra ofthe synthesized products were recorded on a Bruker AVANCE III500 MHz instrument (Bruker, Switzerland) using chloroform-d(CDCl3) as the solvent. A GCT Premier spectrometer (Waters,USA) using the electron impact (EI+) mass spectra technique wasused to perform the mass spectrometry (MS) analysis. A CHI660C electrochemical analyzer (CH Instruments, China) was ap-plied to conduct the electrochemical measurements. Thermogravi-metric analysis of the neutral state polymer sample which wasloaded into the crucible was performed on a Diamond TG/DTA6300 (Perkin Elmer, USA) under a nitrogen stream in the tempera-ture range of 90–750 �C with a heating rate of 10 �C min�1. SEMmeasurements were taken by using a Hitachi S-4800 scanningelectron microscopy (Hitachi, Japan). Digital photographs of thepolymer films were obtained through the Canon E0S 500D (Canon,Japan) digital camera.

2.3. Synthesis of monomer BHThBe

2-Bromo-3-hexylthiophene (4.94 g, 20 mmol) in a dry Et2O(15 mL) was added dropwise to a stirred suspension of magnesiumpowder in anhydrous Et2O (15 mL) at room temperature. After stir-red for 2 h, the obtained Grignard reagent was added into a dry Et2Osolution of 1,4-dibromobenzene (2.12 g, 9 mmol) and Pd(dppf)Cl2

(0.1 g, 0.14 mmol) in a ice bath. The solution was heated to 30 �Cand stirred for 36 h under nitrogen atmosphere. The reaction mix-ture was poured into 2 M HCl, and the aqueous layer was extractedwith chloroform. The evaporation residue was purified by elutionchromatography on a silica column with n-hexane. 2.63 g (71%) ofBHThBe was obtained as a yellow liquid. 1H NMR (500 MHz, CDCl3):d (ppm) 7.46 (s, 4H, benzene); 7.24 (d, 2H, thiophene); 6.99 (d, 2H,thiophene); 2.69 (t, 4H, 2 � ThACH2AC5H11); 1.63 (m, 4H,2 � ThACH2ACH2AC4H9); 1.31 (m, 12H, 2 � ThAC2H4AC3H6ACH3);

0.86 (t, 6H, 2 � ThAC5H10ACH3). 13C NMR (125 MHz, CDCl3): d(ppm) 138.8; 137.3; 133.7; 129.6; 129.3; 123.8; 31.6; 31.0; 29.2;28.7; 22.6; 14.1. EM: m/z (relative intensity) 410.2 (M+, 100%).HRMS: calculated for C26H34S2 m/z 410.2136, found m/z 410.2148.

2.4. Electrochemistry

The electrochemical tests were recorded in a conventionalthree-electrode cell equipping an ITO/glass electrode (CSG holdingCo., LTD, Rs 6 10 X h�1) as the working electrode which wassequencely washed with ethanol, acetone and deionized water un-der ultrasonic before use. The counter electrode was made from aplatinum sheet, and a double-junction Ag/AgCl electrode (silverwire coated with AgCl in saturated KCl solution, 0.1 M LiClO4 inACN solution as the second junction) was applied as the referenceelectrode. All of the electrochemical experiments were carried outat 25 �C under nitrogen atmosphere unless specified. The homopol-ymer films of 5 mM BHThBe (PBHThBe) and 5 mM EDOT (PEDOT)were prepared by constant potential electrolysis (1.20 V and1.30 V) in 0.1 M LiClO4/ACN electrolyte–solvent couple,respectively.

2.5. Synthesis of copolymer P(BHThBe–EDOT) based on BHThBe andEDOT

The concentrations of BHThBe and EDOT for electrocopolymer-ization were controlled as 5 mM and 5 mM, respectively. LiClO4

(0.1 M) was used as the supporting electrolyte. Constant potentialof 1.25 V was applied for 5 min under the nitrogen atmosphere.After the electrolysis, the obtained film was washed with cleanACN for several times to remove the supporting electrolyte andthe oligomers/monomers. The hypothetic synthetic approach ofcopolymer based on BHThBe and EDOT was outlined in Scheme 1.

3. Results and discussion

3.1. Electrochemical polymerization

The polarization curves of BHThBe, EDOT and the mixture ofBHThBe/EDOT in 0.1 M LiClO4/ACN are shown in Fig. 1. The onsetpotentials for anodic current of BHThBe and EDOT are 1.14 V and1.26 V, respectively. The difference of the onset oxidation poten-tials between monomer BHThBe and monomer EDOT is 0.12 V,implying that the electrochemical copolymerization is readily tobe achieved [23]. The onset potential of the BHThBe/EDOT mixtureis 1.12 V, which is lower than those of BHThBe and EDOT, indicat-ing the existence of the interaction between two monomers in Li-ClO4/ACN solution [24].

Fig. 2 displays the successful cyclic voltammograms (CV) ofBHThBe, EDOT and the mixture of the two monomers in ACN con-taining 0.1 M LiClO4 with ITO as working electrodes at a potentialscan rate of 100 mV s�1. As can be seen from the figure, an irrevers-ible oxidation of the monomer appears clearly on the first cycle fol-lowed by typical polymer film growth loops for all three systems,which is indicative of the deposition of electroactive polymericfilms on electrodes.

The redox waves and increments between successive cycles ofthe BHThBe/EDOT mixture (broad oxidation and reduction wavesbetween �0.40 V and +1.10 V, Fig. 2b) are completely differentfrom those of the BHThBe and the EDOT. These might be indica-tions of copolymer formation which need to be supported by othermeans of characterization. CV curve of the BHThBe (Fig. 2a) showsan oxidation peak at +1.00 V and a reduction peak at +0.87 V. Andthe pure EDOT (Fig. 2c) in LiClO4/ACN reveals an oxidation peak at+0.20 V and a reduction peak at �0.68 V.

Scheme 1. Electrochemical copolymerization reaction of BHThBe and EDOT.

Fig. 1. Anodic polarization curves of (A) 5 mM BHThBe, (B) the mixture of 5 mMBHThBe and 5 mM EDOT and (C) 5 mM EDOT in ACN containing 0.1 M LiClO4 at ascan rate of 5 mV s�1.

Fig. 2. Cyclic voltammogram curves of (a) 5 mM BHThBe, (b) the mixture of 5 mMBHThBe and 5 mM EDOT and (c) 5 mM EDOT in 0.1 M LiClO4/ACN solutions at a scanrate of 100 mV s�1.

Fig. 3. Cyclic voltammogram curves of (A) PBHThBe, (B) P(BHThBe–EDOT) and (C)PEDOT films in monomer free solution of 0.1 M LiClO4/ACN at a scan rate of150 mV s�1.

52 C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57

3.2. Electrochemistry of polymer films

The films of PBHThBe, P(BHThBe–EDOT) and PEDOT were pre-pared via potentiostatic polymerization under the same consumedcharge and their CV curves in ACN containing 0.1 M LiClO4 are gi-ven in Fig. 3. As seen in Fig. 3A, the PBHThBe film exhibits one mainoxidation wave at 1.36 V and the main reduction peak around0.85 V. While the PEDOT film displays a broad redox processbetween �0.80 and 0.60 V as depicted in Fig. 3C. The P(BHThBe–EDOT) film exhibits an intermediate CV response with two revers-

ible redox processes over a wide potential range (Fig. 3B). Thoughthe CV response of homopolymer composites or blends can alsogive the similar behavior, the reversible redox processes ofP(BHThBe–EDOT) between 0.00 and 1.20 V can be attributed tothe existence of BHThBe and EDOT units to the polymeric chain[25].

3.3. Scan rate dependence of the peak current densities

Copolymer films of BHThBe/EDOT prepared by constant poten-tial electrolysis (1.25 V) were washed with clean ACN, and their CVcurves in monomer free electrolyte present two broad redox pro-cesses at about +0.40 V and +0.90 V. The current density responseincreases with the increasing of the scan rate as shown in Fig. 4,indicating that the obtained copolymer film has good electrochem-ical activity and adhesion [26,27]. The scan rate dependence of theanodic and cathodic peak current densities presents a lineardependence on the scan rate as illustrated in the insert of theFig. 4, which demonstrates that the electrochemical processes ofP(BHThBe–EDOT) are not diffusion limited and reversible even athigh scan rates.

3.4. FTIR spectra of polymers

Fig. 5 presents the FTIR spectra of PBHThBe, P(BHThBe–EDOT)and PEDOT. All the samples were electrodeposited on ITO elec-trodes potentiostatically. As seen from the spectrum of the purePBHThBe (Fig. 5a), the bands at 2954, 2924 and 2858 cm�1 areattributed to the stretching modes of CAH in aliphatic alkyl chains.While bands around 1600 and 1546 cm�1 are ascribed to thestretching of phenylene rings, and the bands at 1500 and1458 cm�1 are due to the stretching of thiophene rings [28]. The

Fig. 4. Cyclic voltammogram of the P(BHThBe–EDOT) in monomer free solution of0.1 M LiClO4/ACN at different scan rates. Insert: peak current density vs. scan rategraph.

Fig. 5. FTIR spectra of (a) PBHThBe prepared at 1.20 V, (b) P(BHThBe–EDOT)prepared at 1.25 V with mole concentration fraction of BHThBe/EDOT at 5/5 mMand (c) PEDOT prepared at 1.30 V.

Fig. 6. (A) 1H NMR spectrum of BHThBe monomer in CDCl3. (B) 1H NMR spectrum ofhomopolymer (PBHThBe) in CDCl3. (C) 1H NMR spectrum of copolymer [P(BHThBe–EDOT)] in CDCl3.

C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57 53

FTIR spectrum of pure PEDOT (see Fig. 5c) shows bands at 1508,1484 and 1303 cm�1, which originate from the stretching of C@Cand CAC in thiophene rings [29]. The bands at 1142 and1061 cm�1 are assigned to the stretching modes of the ethylenedi-oxy groups [30]. And the vibration modes from the CAS bonds inthe thiophene rings can also be found at 941 and 841 cm�1. Com-pared with the corresponding homopolymers, the spectrum ofP(BHThBe–EDOT) (see Fig. 5b) possesses all the characteristicbands of the above mentioned homopolymers. For instance, thebands at 2954, 2924 and 2858 cm�1 indicate the presence ofBHThBe units. And the bands at 1504, 1076 and 837 cm�1, origi-nated from EDOT units, can also be found. All the above featuresindicate that P(BHThBe–EDOT) contains both BHThBe and EDOTunits.

3.5. 1H NMR spectra of polymers

In order to further verify the copolymerization reaction takenplace between BHThBe and EDOT, comparison 1H NMR spectra ofthe monomer BHThBe, the neutral state PBHThBe and the neutralstate P(BHThBe–EDOT) recorded in CDCl3 are given in Fig. 6. Asshown in Fig. 6B, the homopolymer of the BHThBe contains eightgroups of protons which are located in the lower field compared

with that of the monomer BHThBe (Fig. 6A) due to the high conju-gation length of the polymer chain [27]. Besides, the disappearanceof the proton signal in thiophene rings proves the fact that thepolymerization reaction takes place in the a-positions of thiophenerings. It should be noted that the proton signals located at 7.3 ppmin the Fig. 6B and C are assigned to the undeuterated CDCl3.

The spectrum of P(BHThBe–EDOT) (Fig. 6C) shows that it notonly possesses the protons belonging to the BHThBe chain sections,but also contains the characteristic proton signal of the EDOT chainsections located at about 4.4 ppm (j, aliphatic CAH) [20]. As wellknown, pure PEDOT is insoluble in chloroform. Coupled with theanalysis of 1H NMR and FTIR spectra, we suggest that theP(BHThBe–EDOT) is a copolymer of BHThBe and EDOT, rather thana composite of the respective homopolymers.

3.6. Thermogravimetric analysis

The thermal stability of EC polymers is very important for theirpotential applications. Fig. 7 displays the TG curves of thePBHThBe, P(BHThBe–EDOT) and PEDOT films at a heating rate of10 �C min�1 from 90 �C to 750 �C, respectively. An initial slightweight loss is observed for all polymer films between 90 �C and200 �C, which should be assigned to the evaporation of the residualsolvent or the trapped water in polymer films [31]. The percentagerelated to the solvent/water loss is in the range of 3–5%. The TGcurve of PBHThBe (Fig. 7A) shows the onset temperature of weightloss at around 357 �C. Besides, two obvious weight losses ofPBHThBe can be observed. One is located between 370 �C and450 �C with a weight loss of 23%. And the other is located in therange of 556–678 �C and the loss is measured to be 21%. The PEDOTfilm begins to loose weight at around 312 �C and the maximumdecomposition temperature is between 316 �C and 368 �C(Fig. 7C). For comparison, the P(BHThBe–EDOT) film is stable upto 315 �C where the beginning of the significant weight loss is ob-served (Fig. 7B). The residue of the three polymers at 750 �C is 22%for PBHThBe, 15% for PEDOT and 19% for P(BHThBe–EDOT). Allthese results described above imply that the incorporation of

Fig. 7. TG curves of (A) PBHThBe, (B) P(BHThBe–EDOT) and (C) PEDOT polymerfilms under nitrogen atmosphere at a heating rate of 10 �C min�1 from 90 �C to750 �C.

54 C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57

BHThBe units (containing benzene rings) into PEDOT improves thethermal property of the formed P(BHThBe–EDOT) film.

3.7. Morphology

The polymer films of BHThBe, BHThBe–EDOT and EDOT wereprepared by constant potential electrolysis from the solution of0.1 M LiClO4/ACN containing relevant monomers on ITO elec-trodes. And the morphologies of the neutral PBHThBe,P(BHThBe–EDOT) and PEDOT films are investigated comparativelyby SEM (see Fig. 8) after depoding at �0.70 V for 30 min in blanksolutions.

The main conclusions obtained from the SEM are as follows: (1)PBHThBe film exhibits an accumulation state of clusters of glob-ules. And the approximate diameters of these globules are in therange of 500–1000 nm; (2) PEDOT film presents a loose spongynetwork structure; (3) the surface of the P(BHThBe–EDOT) filmalso shows an accumulation state of clusters of globules, but theapproximate diameter of each globule (less than 250 nm) is muchsmaller than that of the PBHThBe. The smooth surface morphologyof the P(BHThBe–EDOT) film could be attributed to the effect ofcopolymerization, which is important when considering thiscopolymer for device applications.

3.8. Spectroelectrochemical property and electrochromic switching ofPBHThBe film

Spectroelectrochemical analysis is a powerful way to investi-gate the optical switches and contrasts of EC conducting polymersupon potential change [32,33], which provides insights into the

Fig. 8. SEM images of (A) PBHThBe (1.20 V), (B) P(BHThBe–ED

electronic structure of the conducting polymer. As can be seenfrom Fig. 9, the absorption maximum of the monomer BHThBeand the neutral state PBHThBe are centered at 296 and 396 nm,respectively. The difference between the kmax corresponding tothe monomer and the corresponding polymer for BHThBe, whichis about 100 nm, is owing to the increased conjugation length inthe polymer [34].

The spectroelectrochemical and electrochromic properties ofthe PBHThBe are investigated by applying potentials ranging be-tween 0.60 V and 1.20 V in monomer free 0.1 M LiClO4/ACN solu-tion. As shown in Fig. 9, the intensity of the PBHThBe p ? p�

electron transition absorption decreases while two charge carriersabsorption bands located at 640 nm and longer than 1100 nm in-crease dramatically upon oxidation. An isosbestic point ofPBHThBe can be found at 460 nm. In addition, PBHThBe film pre-sents dual-color electrochromism, which exhibits yellow (neutralstate) and blue (doped state) during the redox loop. Structurallysimilar polymers were reported by Reynolds and co-workers [18]and Chan and co-workers [17] to show very similar phenomena.

Table 1 reveals the electrochromic switching behavior ofPBHThBe film monitored at 396 and 1100 nm in 0.1 M LiClO4/ACN solution between 0.60 V and 1.10 V with a residence time of8 s. The optical contrast (DT%) for PBHThBe at 396 nm and1100 nm are 24% and 30%, respectively. Besides, the optical switch-ing time of PBHThBe at 396 and 1100 nm are 4.9 s (95%) and 4.5 s(95%).

3.9. Spectroelectrochemical property and electrochromic switching ofPEDOT film

The spectroelectrochemistry and the electrochromic switchingbehavior of the as-prepared PEDOT film are given for comparison.As depicted in Fig. 10, the neutral state PEDOT exhibits the p ? p�

electron transition peak at about 597 nm, and its absorption inten-sity decreases with the increase of the applied potential, while thedoped PEDOT film presents intense charge carrier bands at around880 nm and longer than 1100 nm. The isosbestic point of PEDOT islocated at around 720 nm. As can be seen in Table 1, the opticalcontrast (DT%) and the optical switching time of the as-preparedPEDOT at 597 and 1100 nm are 44% and 1.1 s (95%), 35% and1.9 s (95%), respectively.

3.10. Spectroelectrochemical property of P(BHThBe–EDOT) film

Fig. 11 depicts the different UV–vis absorbance curves of theP(BHThBe–EDOT) copolymer film under various applied potentialsranging from �0.40 V to 1.20 V. Copolymer films were synthesizedon ITO glass slides by constant electrolysis at voltage of 1.25 V in0.1 M LiClO4/ACN solution mixing with 5/5 mM BHThBe/EDOT. Asshown in Fig. 11, a well-defined maximum absorption band cen-tered at 401 nm with a shoulder located at around 510 nm are ob-served, which are attributed to the p ? p� transition of the neutral

OT) (1.25 V, 5 mM/5 mM) and (C) PEDOT (1.30 V) films.

Fig. 11. UV–vis spectroelectrochemical spectra of P(BHThBe–EDOT) films on ITOglass as applied potentials between �0.40 V and 1.20 V in 0.1 M LiClO4/ACNsolution.

Fig. 9. UV–vis spectroelectrochemical spectra of PBHThBe films on ITO glass asapplied potentials between 0.60 V and 1.20 V in 0.1 M LiClO4/ACN solution (insert:absorption spectrum for BHThBe in ACN).

Table 1Electrochromic switching responses of PBHThBe and PEDOT monitored at differentwavelength in 0.1 M LiClO4/ACN solution.

Optical contrast (DT%) Switching time

PBHThBe 24% (396 nm)a 4.9 s (396 nm, 95%)a

30% (1100 nm)a 4.5 s (1100 nm, 95%)a

PEDOT 44% (597 nm)b 1.1 s (597 nm, 95%)b

35% (1100 nm)b 1.9 s (1100 nm, 95%)b

a Between 0.60 V and 1.10 V with a residence time of 8 s.b Between �0.60 V and 0.70 V with a residence time of 5 s.

Fig. 10. UV–vis spectroelectrochemical spectra of PEDOT films on ITO glass asapplied potentials between �0.60 V and 0.70 V in 0.1 M LiClO4/ACN solution.

Table 2kmax and Eg of PBHThBe, P(BHThBe–EDOT) and PEDOT.

PBHThBe P(BHThBe–EDOT) PEDOT

kmax (nm) 396 401 597Eg (eV) 2.63 1.86 1.64

1 For interpretation of color in Figs. 4 and 9–13, the reader is referred to the webversion of this article.

Fig. 12. Multichromic behavior of P(BHThBe–EDOT) (prepared potentiostatically at1.25 V with BHThBe/EDOT ratio of 5 mM/5 mM) at �0.10 V (brick red), 0.20 V(khaki), 0.40 V (yellow green), 0.80 V (green) and 1.20 V (midnight blue). (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57 55

state copolymer backbone, and both of them decrease with theincreasing of potential. Besides, the appearance of charge carrierbands (at around 630 nm, 780 nm and 1100 nm) could be attrib-uted to the evolution of polaron and bipolaron bands. By compar-ing with the UV–vis spectrum of PBHThBe, we speculate that theshoulder band at 510 nm and the charge carrier band at 780 nmare the consequences of introducing EDOT units into PBHThBebackbone.

Table 2 summarizes the maximum absorption wavelength(kmax) and the energy gap (Eg) of the PBHThBe, P(BHThBe–EDOT)and PEDOT quite clearly. The kmax value of the copolymer is be-tween those of the PBHThBe and the PEDOT. Besides, the effectof copolymerization between BHThBe and EDOT leads to an obvi-ous decrease in the Eg comparing with that of PBHThBe, which

indicates that the introduction of EDOT units into PBHThBe can re-duce the Eg of the copolymer.

Furthermore, it is interesting to find that the P(BHThBe–EDOT)film shows a multicolor electrochromism. In order to study therange of colors, a wide interval of potentials (1.20 to �0.10 V)was applied on the as-prepared P(BHThBe–EDOT) films. At1.20 V, the copolymer film shows a midnight blue color whichpasses to a green color at 0.80 V. With the decrease of potential,the copolymer film turns into yellow green (0.40 V), khaki color(0.20 V), and eventually to a brick red1 at �0.10 V (see Fig. 12),respectively. These different colors corresponding to various dopedand neutral states have also been confirmed by the CV tests of theP(BHThBe–EDOT) film. This multicolor property possesses signifi-cant potential applications in smart windows or displays.

3.11. Electrochromic switching of P(BHThBe–EDOT) film

Electrochromic switching time and optical contrast (DT%) aretwo of the most important characteristics for EC devices. Fig. 13describes the EC switching response of P(BHThBe–EDOT) filmmonitored at visible region (401, 510 and 628 nm) in 0.1 M

Fig. 13. Electrochromic switching response for P(BHThBe–EDOT) film monitored at401 nm, 510 nm and 628 nm (visible region) in 0.1 M LiClO4/ACN solution between�0.4 V and 1.20 V with a residence time of 5 s.

Fig. 14. Electrochromic switching response for P(BHThBe–EDOT) film monitored at1100 nm (near-IR region) in 0.1 M LiClO4/ACN solution between �0.4 V and 1.20 Vwith a residence time of 5 s.

Fig. 15. Cyclic voltammogram of the P(BHThBe–EDOT) film on the ITO electrode asa function of repeated scans. Scan rate: 500 mV s�1. Insert: variation of anodic (jpa)and cathodic (jpc) peak current densities as a function of number of cycles (n).

56 C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57

LiClO4/ACN solution between square wave potentials �0.40 V(neutral state) and 1.20 V (oxidized state) with a residence timeof 5 s. The optical contrasts of the copolymer at 401, 510 and628 nm are 21%, 23% and 36%, respectively. And the copolymerswitches very rapidly between its neutral and oxidized states andachieves more than 95% of its optical contrasts in the visible regionin less than 2 s. P(BHThBe–EDOT) shows the switching times of1.3 s at 401 nm (95%), 0.6 s at 510 nm (100%) and 1.8 s at 628 nm(95%). The faster switching response of copolymer film than thatof PBHThBe film could be ascribed to the introduction of EDOTunits into the polymer backbone resulting in the faster dopantion diffusion during the redox process. This fast switching responseand reasonable contrast ratio at all three wavelengths makes theP(BHThBe–EDOT) film a promising material for application in ECdevices.

Fig. 14 shows the EC switching response of the P(BHThBe–EDOT) film at near IR (nIR) wavelength (1100 nm). The as-preparedcopolymer film reveals a 76% optical contrast and a 1.4 s switchingtime (95%) at 1100 nm, which are sufficient for nIR EC devices.

3.12. Stability of the electrochromic film P(BHThBe–EDOT)

The stability of EC materials toward long-term switching be-tween the neutral and oxidized states is one of the most important

factors for the application of EC materials in device utilities. The as-prepared P(BHThBe–EDOT) film on an ITO electrode were cycled500 times between its neutral state (�0.40 V) and the oxidizedstate (+1.10 V) under atmospheric condition (Fig. 15). After 500 cy-cles, the CV curve of P(BHThBe–EDOT) reveals the film retaining84% of its original electroactivity and the changes in anodic (jpa)and cathodic peak current densities (jpc) are 18% and 19%, respec-tively. These results imply that the P(BHThBe–EDOT) film has areasonable environmental and redox stability and could be as apromising candidate material for EC devices.

4. Conclusions

A novel soluble multichromic copolymer based on the BHThBeand EDOT is successfully prepared via electrocopolymerization.The structure and properties of the as-prepared polymer are char-acterized and investigated through electrochemical methods, FTIRspectra, 1H NMR spectra and thermogravimetric analyses. FTIR and1H NMR characterizations indicate that the obtained P(BHThBe–EDOT) polymer contains both BHThBe and EDOT units, confirmingthe formation of copolymer structure. The thermogravimetric anal-yses discover that this copolymer is thermally stable up to 315 �C.Besides, investigation of the electrochromic properties of polymervia spectroelectrochemsitry and evaluation of their switching re-sponses are also achieved. P(BHThBe–EDOT) copolymer film exhib-its excellent multicolor electrochromism (five colors), low bandgap and fast switching response. The onset energy for the p ? p�

transition (Eg) of P(BHThBe–EDOT) copolymer film is found to be1.86 eV and kmax is 401 nm. And the copolymer film shows a max-imum optical contrast (DT%) of 36% in visible region with a switch-ing time of 1.8 s and of 76% in the near-infrared region with aswitching time of 1.4 s. With its smooth surface, soluble in chloro-form and reasonable redox stability, P(BHThBe–EDOT) copolymerfilm is expected to be applied in EC papers or other areas of ECdisplays.

Acknowledgements

The authors gratefully thank the supporting of National BasicResearch Program of China (2010CB635108), Natural Science Foun-dation of Zhejiang Province, China (Y4090260) and Major Scienceand Technology, Special and Priority Themes of Zhejiang Province,China (2009C14004).

C. Zhang et al. / Journal of Electroanalytical Chemistry 645 (2010) 50–57 57

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