Organic Electronics 14 (2013) 1094–1102

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Organic Electronics

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A new electrochromic copolymer based on dithienylpyrroleand EDOT

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.01.036

⇑ Corresponding authors. Tel.: +90 3125868390x8528; fax: +903125868091 (S. Tirkes), tel.: +90 2862180018x2415; fax: +902862180536 (F. Algi).

E-mail addresses: stirkes@atilim.edu.tr (S. Tirkes), falgi@comu.edu.tr(F. Algi).

Melek Pamuk Algi a, Zahide Öztas� a, Seha Tirkes b,⇑, Atilla Cihaner b, Fatih Algi a,⇑a Laboratory of Organic Materials (LOM), Çanakkale Onsekiz Mart University, TR-17100 Çanakkale, Turkeyb Chemical Engineering and Applied Chemistry, Atilim Optoelectronic Materials and Solar Energy Laboratory (ATOMSEL), Atilim University, TR-06836Ankara, Turkey

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

Article history:Received 9 December 2012Received in revised form 29 January 2013Accepted 30 January 2013Available online 18 February 2013

Keywords:Multicolor electrochromismCamouflageDithienylpyrroleEDOT

A new compound, namely diethyl 2,5-di(thiophen-2-yl)-1H-pyrrole-3,4-dicarboxylate(1), was copolymerized with 3,4-ethylenedioxythiophene (EDOT) via electrochemicalmethod. The copolymer exhibits multicolor electrochromic property: It is found that thecopolymer, poly(1-co-EDOT), has a specific optical band gap (1.71 eV) to reflect and/ortransmit reddish brown color in the neutral state, and it can be switched to reddish orange,orange, yellowish green and blue colors upon oxidation in a low switching time (1.0 s).Importantly, these colors are essential for camouflage and/or full color electrochromicdevice/display applications. In addition to these, the obtained copolymer has a colorationefficiency of 173 cm2/C at 500 nm.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Functional conjugated polymers have attractedconsiderable attention since they are promising candidatesto be amenable for use in a variety of advanced technolog-ical applications such as sensors [1–3], light emittingdiodes [4,5], photovoltaic cells [6–8], transistors [9,10],electrochromic devices [11–13], and optical displays [14].Especially, conjugated polymers have been envisaged asone of the most useful electrochromic materials due totheir multicolors with the same material under externallyapplied potentials. Also, they have high optical contrast ra-tio between various redox states with a short responsetime, high redox stability and long cycle life under ambientconditions. Furthermore, they can be coated onto largearea surfaces by spin and spray coating [15] or roll-to-rolltechnique [16]. More importantly, all of these properties

can be adjusted by structural design of the startingmaterials [17–22].

Recently, significant effort has been devoted to designand syntheses of novel, simple and effective functionaland solution-processable polymeric materials [17,23–25].Among these materials, dithienylpyrrole (SNS) is one ofthe most useful core units in order to obtain materials withdistinct properties [19,26–37]. For instance, the flavinfunctionalized polydithienylpyrrole (PSNS) fabricated byelectrochemical methods could be used for the detectionof 2,6-diamidopyridine, which is a biologically importantredox-active molecule [33]. Furthermore, it was recentlyshown that a chemiluminescent PSNS functionalized withluminol appendages could be used for the detection ofreactive oxygen species [34]. In addition, by the help offluorescent substituents like naphthalene and fluorene,PSNS based polymers could exhibit both fluorescent andelectrochromic properties [38–40]. It was also shown thatthe intrinsic properties could easily be controlled throughrational design of the backbone structures. Nonetheless,examples of processable fluorescent and electrochromicpolymeric materials based on SNS unit have still been rare

M.P. Algi et al. / Organic Electronics 14 (2013) 1094–1102 1095

and newer ones are welcome. Moreover, SNS based mate-rials can be amplified to create viable materials.

In this context, SNS based polymeric systems withelectron-withdrawing units such as ester groups have notbeen investigated so far probably due to the difficultiesin the synthesis. Herein we wish to report the synthesisand properties of a novel polymeric material, which isbased on a novel SNS bearing strong electron-withdrawingsubstituents (1). In addition, electrochemical copolymeri-zation of 1 and 3,4-ethylenedioxythiophene (EDOT) wereinvestigated for electrochromic applications. It was notedthat the copolymer, P(1-co-EDOT), had a specific opticalband gap (1.71 eV) to reflect and/or transmit reddishbrown color in the neutral state, and it could be switchedto reddish orange, orange, yellowish green and blue colorsupon oxidation. Importantly, these colors are essential forcamouflage and/or full color electrochromic device/displayapplications.

Scheme 1. Diethyl 2,5-dibromo-1H-pyrrole-3,4-dicarboxylate.

Scheme 2. Diethyl 2,5-di(thiophen-2-yl)-1H-pyrrole-3,4-dicarboxylate.

2. Experimental

All chemicals were purchased from Sigma AldrichChemicals or Merck Company and used as received unlessotherwise noted. FTIR spectra were recorded on Perkin El-mer Spectrum 100 model FTIR with an attenuated totalreflectance (ATR). 1H (400 or 300 MHz) and 13C (100 or75 MHz) NMR spectra were recorded on a Bruker DPX-400 or Ultrashield 300 NMR Spectrometers. Combustionanalysis was carried out by using a LECO CHNS-932 ana-lyzer. High resolution mass spectra (HRMS) were recordedon Waters SYNAPT MS system. UV–Vis and fluorescencemeasurements were recorded on Varian Cary 50 andVarian Cary Eclipse spectrophotometers, respectively.Melting points were determined on a Schorrp MPM-H2model apparatus and are uncorrected. Column chromatog-raphy was performed on silica gel (60–200 mesh) fromMerck Company. TLC was carried out on Merck 0.2 mm sil-ica gel 60 F254 analytical aluminum plates. The synthesesof 3 [41] and 4 [42] were carried out according to pub-lished procedures.

0.1 M TBAClO4 dissolved in acetonitrile (ACN) was usedas electrolyte solution. A platinum disk (0.02 cm2) and aplatinum wire were used as working and counter elec-trodes, respectively, as well as a Ag/AgCl reference and aAg wire pseudo-reference electrodes (calibrated externallyusing 10 mM solution of ferrocene/ferrocenium couplewhich is an internal standard calibrated to be 0.44 V inACN solution both vs. Ag/AgCl and vs. Ag wire). Repetitivecycling was used to obtain the polymer films. Electro-optical properties were investigated by using an indiumtin oxide (ITO, Delta Tech. 8–12 ohm, 0.7 � 5 cm) elec-trode as well as a platinum wire as counter electrodeand a Ag wire as a pseudoreference electrode. For thespectroelectrochemical measurements, the polymer filmwas coated on ITO electrode via cyclic voltammetry orconstant potential electrolysis in 0.1 M ACN/TBAClO4. Inorder to break in the polymer film, it was switched be-tween 0.0 and 1.25 V in a monomer-free electrolytic solu-tion. Electroanalytical measurements were performedusing a Gamry PCI4/300 potentiostat–galvanostat. The

electro-optical spectra were monitored on a SpecordS600 spectrometer.

2.1. Synthesis of diethyl 2,5-dibromo-1H-pyrrole-3,4-dicarboxylate (3)

Br2 (0.11 mL, 2.1 mmol) was dropwise added to amagnetically stirred solution/suspension of diethyl-1H-pyrrole-3,4-dicarboxylate (2) (211.2 mg, 1 mmol) andNaHCO3 (252 mg, 3 mmol) in CH2Cl2 (20 mL) at 0 �C. Afterthe addition was completed, the mixture was stirred atroom temperature until 2 was totally consumed (TLC,overnight). The mixture was filtered to remove the solidpart, washed with water (100 mL) and dried over MgSO4.The solvent was evaporated and the residue was subjectedto column chromatography on silica gel eluting with 5%CH3OH–CHCl3 (v/v) to give 2: Rf = 0.56; 347 mg; 94% yield;white solid; m.p. 102 �C; 1H NMR (300 MHz, CDCl3) d/ppm:9.50 (bs, 1H, –NH), 4.33 (q, J = 7.1 Hz, 4H, –CH2), 1.35(t, J = 7.1 Hz, 6H, –CH3); 13C NMR (75 MHz, CDCl3) d/ppm:162.7, 117.5, 104.2, 61.2, 14.1; FTIR (ATR, cm�1): 3106,3059, 3006, 2973, 2934, 2870, 2639, 1714, 1677, 1492,1460, 1446, 1375, 1291, 1214, 1191, 1068, 1024, 862,800, 783, 765, 678; Anal Calcd. for C10H11Br2NO4: C,32.55; H, 3.00; N, 3.80; Found: C, 32.53; H, 3.05; N, 3.82.HRMS Calcd. for NaC10H11Br2NO4: 389.8952; Found:389.8951.

2.2. Synthesis of diethyl 2,5-di(thiophen-2-yl)-1H-pyrrole-3,4-dicarboxylate (1)

To an argon degassed solution of compound 3 (185 mg,0.5 mmol) and 4 (400 mg, 1.1 mmol) in dry toluene(25 mL) was added Pd(PPh3)4 (20 mg) as catalyst and thesolution was heated under reflux until all the startingmaterials were consumed (TLC, 5 h). The flask was cooledand the solvent was removed under reduced pressure.The residue was filtered through a short pad of silica gelby eluting with CH2Cl2 to give 4: Rf = 0.70; 168 mg; 90%yield; m.p. 129–130 �C; 1H NMR (400 MHz, CDCl3)d/ppm: 8.70 (s, 1H, –NH), 7.40 (dd, J = 4–2 Hz, 2H, H3),

Scheme 3. Synthesis of compound 1.

Fig. 1. (a) Cyclic voltammogram and (b) electropolymerization of 1 in 0.1 M TBAClO4/ACN at a scan rate of 100 mV/s on a Pt disk working electrode vs. Ag/AgCl.

Fig. 2. (a) Electropolymerization of 1 by potential scanning between 0.0 and 1.50 V, and (b) cyclic voltammogram of obtained product in 0.1 M TBAClO4/ACN at a scan rate of 100 mV/s on Pt working electrode vs. Ag/AgCl.

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Fig. 3. (a) Cyclic voltammograms of P1 film at a scan rate between 20 and 200 mV/s with a 20 mV/s increment on Pt disk working electrode, and(b) relationship of anodic (ia) and cathodic peak currents (ic) as a function of scan rate for P1 in 0.1 M TBAClO4/ACN vs. Ag/AgCl.

Fig. 4. Cyclic voltammograms of 1, EDOT and a mixture of 1 and EDOTwith a feed-ratio of 1:1 in 0.1 M TBAClO4/ACN at 100 mV/s, ITO workingelectrode. Inset: 6.5 mM EDOT cycled between �0.75 and 1.25 V in 0.1 MTBAClO4/ACN at 100 mV/s.

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7.35 (dd, J = 5–2 Hz, 2H, H5), 7.07 (dd, J = 5–4 Hz, 2H, H4),4.28 (q, J = 8 Hz, 4H, –CH2), 1.30 (t, J = 8 Hz, 6H, –CH3);13C NMR (100 MHz, CDCl3) d/ppm: 164.6, 131.3, 128.1,127.5, 127.4, 126.6, 114.8, 60.9, 14.1; FTIR (ATR, cm�1):3106, 3100, 2975, 2956, 1689, 1550, 1486, 1437, 1369,1341, 1298, 1248, 1215, 1112, 1015, 857, 785, 701; UV–Vis (CH3CN, kmax, nm): 230, 312; Anal Calcd. for C18H17NO4-

S2: C, 57.58; H, 4.56; N, 3.73; S, 17.08; Found: C, 57.55; H,4.52; N, 3.75; S, 17.02. HRMS Calcd. for NaC18H17NO4S2:398.0497; Found: 398.0482 (see Schemes 1 and 2).

3. Results and discussion

The synthesis of 1 was started with the bromination ofdiethyl-1H-pyrrole-3,4-dicarboxylate (2) with Br2 in thepresence of NaHCO3 to give 3 followed by Stille reactionof 3 with 4 [41], which provided the product 1 in 90% yield(Scheme 3). 1 was initially characterized on the basis of 1H,13C NMR, elemental and HRMS analysis, which firmlyestablished the structure [42].

The electrochemical behavior of the monomer 1 wasstudied in an electrolyte solution of 0.1 M TBAClO4 dis-solved in ACN. As shown in Fig. 1a, during anodic scan,the monomer 1 exhibits two oxidation peaks at 1.15 Vand 1.45 V vs. Ag/AgCl. The formation of a new peak at0.85 V during reverse scan was followed by an increasein intensity after each successive scan between 0.0 V and1.25 V (Fig. 1b), which indicates the deposition of a con-ducting polymer film on the surface of the working elec-trode. On the other hand, the oxidation of 1 at 1.15 Vwas followed by a second irreversible oxidation peak(1.45 V) which can be attributed to the overoxidation ofoligomeric species on working electrode. In order to provethis assumption, as seen from Fig. 2, potentiodynamicpolymerization was tried to carry out between 0.0 V and1.50 V and unfortunately, at the end a non-uniform andpoorly electroactive polymer film was obtained due tothe overoxidation.

Electrochemically obtained polymer film (P1) waswashed with ACN to remove the monomer and oligomericspecies. Then, the redox behavior of polymer was investi-gated in a monomer-free TBAClO4/ACN electrolytesolution. The polymer showed a quasi-reversible redoxcouple (Epa = 1.00 V, Epc = 0.89 V vs. Ag/AgCl). The linearrelationship of peak currents as a function of the scan ratesrevealed a non-diffusional redox process and firmly depos-ited polymer film on Pt electrode (Fig. 3).

Fig. 5. Electrochemical polymerization of: (a) 6.5 mM EDOT between �0.75 and 1.50 V, (b) a mixture of 6.5 mM of both 1 and EDOT with a feed ratio of 1:1between �0.75 and 1.25 V and (c) cyclic voltammograms of poly(1-co-EDOT) and PEDOT in 0.1 M TBAClO4/ACN at a scan rate of 100 mV/s by potentialscanning between �1.0 and 1.25 V on Pt working electrode vs. Ag/AgCl.

Scheme 4. Electrochemically obtained poly(1-co-EDOT).

Fig. 6. (a) Cyclic voltammograms of poly(1-co-EDOT) film at scan rates between 20 and 100 mV/s with 20 mV/s increments. (b) Relationship of anodic (ia)and cathodic peak currents (ic) as a function of scan rate for copolymer 0.1 M TBAClO4/ACN vs. Ag/AgCl.

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In order to investigate the optical properties of the poly-mer P1, electrochemical deposition onto ITO surface viarepetitive cycling between 0.0 V and 1.5 V was attempted.Disappointingly, however, the deposition of an electroac-

tive polymer film on working electrode was not successful.This prompted us to consider about the copolymerizationof 1 with EDOT. The proximity of oxidation peaks of 1(1.15 V) and EDOT (1.48 V) (Fig. 4) was in favor.

Fig. 7. FTIR spectra of 1 and poly(1-co-EDOT).

Fig. 8. SEM images of PEDOT: (a) 4000� magnification, (b) 25,000� magnification, and poly(1-co-EDOT), (c) 4000� magnification, and (d) 25,000�magnification.

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Fig. 9. Optical absorption spectra and colors of poly(1-co-EDOT) film on ITO in 0.1 M TBAClO4/ACN scanned between �1.0 and 1.20 V vs. Ag wire.

Fig. 10. T% change of neutral and oxidized forms of poly(1-co-EDOT) at open circuit potential.

1100 M.P. Algi et al. / Organic Electronics 14 (2013) 1094–1102

As seen from the inset of Fig. 5, EDOT did not exhibitany electroactivity up to 1.25 V, indicating that there wasno polymerization of EDOT. In order to clarify this, beforecopolymerization, EDOT was tried to polymerize by cyclicvoltammetry technique between �0.75 and 1.25 V (see in-set Fig. 5). However the deposition of PEDOT was notachieved. On the other hand, if a mixture of 1 and EDOTwas used, the electropolymerization was carried out suc-cessfully between �0.75 and 1.25 V and an increase inthe current peak was observed, which can be attributedto the electron transfer from 1 to EDOT.

Based on the foregoing results, the synthesis of copoly-mer poly(1-co-EDOT) was performed via cyclic voltamme-try in 0.1 M TBAClO4/ACN solution (Fig. 5a) and then

compared to PEDOT obtained by repetitive cycling(Fig. 5b).

An apparent difference in redox behaviors of the PEDOTand poly(1-co-EDOT) can be seen from the cyclicvoltammogram in Fig. 5c. The voltammograms show thatthe oxidation of copolymer film) was shifted to a higherpotential (Epa = 0.30–0.43 V) when compared to PEDOT(Epa = 0.30 V), indicating the formation of the copolymer(Scheme 4).

Furthermore, the copolymer film was stable and pre-served its electroactivity even at high scan rates. The inten-sity of peak currents linearly increased with increasingscan rates, which revealed a non-diffusional redox process(Fig. 6).

Fig. 11. Chronoabsorptometry experiments for poly(1-co-EDOT) film on ITO in 0.1 M TBAClO4/ACN while the polymer was switched between �1.0 and1.20 V with a switch time of 10, 5, 2, and 1 s at 500 nm.

M.P. Algi et al. / Organic Electronics 14 (2013) 1094–1102 1101

Also, the formation of copolymer was proved by usingIR spectrum. As seen in Fig. 7, the IR spectrum of 1 showsstrong peaks at 693 cm�1 (for a-hydrogens of thiophenerings), and 843 cm�1 (for b/b0-hydrogens of thiophenerings). In addition, the peaks at 1020 cm�1 and1208 cm�1 are for ethoxy groups and etheric groups,respectively. The decrease in intensity of the peak at697 cm�1 reveals the coupling at a-position. The existenceof the peak at 1700 cm�1 attributed to the carbonyl groupof ester unit confirms the presence of 1 in the copolymerbackbone.

As seen in Fig. 8, the morphological difference betweenPEDOT and copolymer is another evidence for the copoly-mer formation. SEM images show that PEDOT has smooth-er surface (Fig. 8a and b) and the copolymer film has a fibriland globule-like morphology (Fig. 8c and d).

Fig. 12. Cyclic voltammetry stability of poly(1-co-EDOT) film at a scan rat

The applications of electrochromic polymers requireidentification of spectroelectrochemical properties for dif-ferent redox states by means of the alterations in the elec-tronic absorption spectra. As it is seen in Fig. 9, thecopolymer film deposited on working electrode (ITO) hasan absorption band centered at 500 nm (2.48 eV) in itsneutral state. The calculation of the band gap of the copoly-mer from its commencement on the low energy end givesthe value of 1.71 eV. During oxidation, the intensity of va-lence-conduction band (p–p� transition) at 500 nm startedto decrease associated with hypsochromic shift, which isnot the case for PEDOT (p–p� transition at 615 nm) [43]and a new absorption band at 765 nm, revealing the for-mation of the polarons, started to intensify. Upon oxida-tion, the copolymer showed a color change from reddishbrown (L = 54.34, a� = 17.58, b� = 14.57) to reddish orange,

es of 100 mV/s between �1.00 V and 1.20 V in 0.1 M TBAClO4/ACN.

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and to orange, and then to yellowish green and finally toblue (L = 67.74, a� = �4.09, b� = �4.25). The course of colorchange can be seen as inset in Fig. 9. Importantly, thesecolors are essential for camouflage and/or full color elec-trochromic device/display applications. Thus, the obtainedcolors make this copolymer highly valuable. Furthermore,the memory study showed that the polymer film wasmostly stable in its neutral state when compared to its oxi-dized state. Oxidized polymer film slowly turned intopseudo-oxidized state with open circuit potential underambient conditions (Fig. 10).

The coloration efficiency (CE) is one of the importantparameters for the electrochromic polymeric materials interms of power efficiency. CE is equal to the ratio of opticaldensity (DOD) to the amount of charge (Qd, injected/ejected) and it can be calculated by using following equa-tions [44]:

CEðkÞ ¼ DODðkÞ=Qd and DODðkÞ ¼ log½ToxðkÞ=TredðkÞ�

The CE of the copolymer film was calculated as173 cm2/C at 500 nm. This value is almost same as CE ofPEDOT at 615 nm [43].

In addition to CE, switching time, which is the requiredtime passage between two different colors under appliedpotentials, is another significant parameter. For this pro-cess, square-wave potential technique was used to calcu-late the percent transmittance between �1.00 and 1.20 Vwith switching time of 10, 5, 2, and 1 s (Fig. 11). Thecopolymer film showed 32.9% of transmittance at 500 nmwith a response time of 1.0 s. When the switching timewas changed from 10 s to 1 s, electro-optical propertiesof the copolymer film was retained without significantchanges.

Also, the electrochemical stability of the copolymer filmstudied under ambient conditions showed that the poly-mer film retained its electroactivity after 100 cycles(Fig. 12).

4. Conclusions

To conclude, demonstrated herein is a new dithienyl-pyrrole with strong electron-withdrawing substituentsthat was copolymerized with EDOT electrochemically. Thiscombination provided a new electrochrome which had areversible redox switching and also exhibited multielectro-chromic behavior; reddish brown at the neutral state,reddish orange, orange, and yellowish green at theintermediate state and blue at the oxidized state. This widecolor range makes the copolymer a good candidate forelectrochromic device, display and/or camouflageapplications.

Acknowledgements

The authors gratefully acknowledge the financialsupport from the Scientific and Technological ResearchCouncil of Turkey (TUBITAK, Grant Nos. 109R009,110T488), Atilim University (ATÜ-ALP-1011-02, ATÜ-BAP-1011-05), Çanakkale Onsekiz Mart University and

The Turkish Academy of Sciences (TUBA). M.P.A. is in-debted to TUBA for graduate fellowship.

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