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Electrochimica Acta 54 (2009) 5694–5702

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

lectrochemical and optical properties of biphenyl bridged-dicarbazoleligomer films: Electropolymerization and electrochromism

ermet Koyuncub,c,∗, Burak Gultekina, Ceylan Zafera,∗∗, Hakan Bilgili a, Mustafa Cana,erafettin Demica, Ismet Kayab, Siddik Icli a

Solar Energy Institute, Ege University, 35100 Bornova, Izmir, TurkeyDepartment of Chemistry, Faculty of Sciences and Arts, Canakkale Onsekiz Mart University, 17020 Canakkale, TurkeyCan Vocational School, Canakkale Onsekiz Mart University, 17400 Canakkale, Turkey

r t i c l e i n f o

rticle history:eceived 10 February 2009

a b s t r a c t

4,4′-Di(N-carbazoyl)biphenyl monomer (CBP) was synthesized and coated onto ITO–glass surface byelectrochemical oxidative polymerization. Its CV shows two distinct one-electron and stepwise oxida-

eceived in revised form 3 May 2009ccepted 4 May 2009vailable online 13 May 2009

eywords:arbazole

tion processes occurred at 1.29 and 1.61 V. By using this property, the monomer was electrochemicallypolymerized separately at these oxidation states and thus, two different oligomer films were obtainedafterwards. Their spectro-electrochemical and electrochromic properties were also investigated. Switch-ing ability of the oligomers was evaluated by kinetic studies upon measuring the percent transmittance(%T) at their maximum contrast point, indicating that these oligomers were found to be suitable material

s.

lectrochemical polymerizationlectrochromic devices

for electrochromic device

. Introduction

Carbazole containing polymers are of interest due to theirpplications in electrochromic devices, hole transport layers,lectro-xerography, microcavity photoconduction, and as photo-oltaic components that provide a very efficient matrix as aurrent carrier transport [1–5]. These polymers also constituten important part of the photoconductive polymers and organichotoreceptors [6]. These polymers obtained have also variousdvantageous properties; such as, high charge carrier mobilities,igh thermal and photochemical stabilities [7,8]. Carbazole couldlso be easily functionalized at its 3,6- [9,10], 2,7- [11], or N-ositions [12], and then covalently linked into polymeric systems,oth in the main chain [13] as building blocks and in a side chain asendent groups [14].

Due to the ease of formation of relatively stable radical cationspolarons), carbazole readily polymerize at electrochemical poly-

erization process. The fist report of electropolymerization ofarbazole was carried out by Ambrose and Nelson [15]. They havexhibited that carbazole could polymerize from 3, 6 and 9, posi-ions and also noted that coupling could proceed through the 1 and

∗ Corresponding author at: Can Vocational School, Canakkale Onsekiz Mart Uni-ersity, 17400 Canakkale, Turkey. Tel: +90 286 4167705: fax: +90 286 4167706.∗∗ Corresponding author. Tel.: +90 232 3886023; fax: +90 232 3886027.

E-mail addresses: sermetkoyuncu@hotmail.com (S. Koyuncu),eylan.zafer@ege.edu.tr (C. Zafer).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.05.014

© 2009 Elsevier Ltd. All rights reserved.

8 positions; however, these positions are sterically hindered due torigid structure of carbazole.

Electrochromism refers to the reversible color change of elec-troactive materials, during the electrochemical redox reaction.Electrochromic polymers have received increasing attention dueto their potential for structurally controllable HOMO–LUMO bandgap, fast switching speeds, high contrast ability, and easily process-ing [16–19]. With these properties they become more advantageousthan the ones which are in inorganic nature. The optical changein these electrochromic polymers is influenced by a low electriccurrent at low potentials in the order of a fraction of a volt toa few volts. The color is determined by the band gap, definedas the onset of the �–�* transition [20]. Then, band gap controlbecomes an important issue in the construction of dual-polymer-based electrochromic devices where a low band gap (cathodiccoloring) polymer is matched with a high band gap (anodic col-oring) polymer to obtain a high degree of contrast during theswitching process [21]. A material which has more than two redoxstates may exhibit electrochemically several colors and it can betermed as poly-electrochromic [22].

Electrochromic properties of poly-N-vinylcarbazole were real-ized around 1980 by Desbene-Monvernay et al. [23] and alsostudied Chevrot and coworkers [24–26]. They have presented poly-

N-vinylcarbazole to be clear colorless when neutral state, greenat oxidized state. It is very difficult to prepare green polymerfilms which can absorb in two different regions (blue and red) inUV–vis region at neutral state. Up to date, absorption of blue andred regions was succeeded only by donor–acceptor type polymers

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S. Koyuncu et al. / Electroch

hich have green color [27–29]. Although having high oxidationotential and the low transmittance change (contrast) comparingo the donor–acceptor polythiophene derivatives synthesized ear-ier, poly-CBP-2 is the first green polymer having fully donor moietyn its neutral state.

We report here the synthesis of 4,4′-di(N-carbazolyl)biphenylonomer (CBP) through the use of carbazole and 4,4′-

ibromobiphenyl [3]. UV–vis, FT-IR, 1H NMR and CV were usedor the structural characterization. Its CV measurements showedhat it has two separate redox states observed at 1.29 and 1.61 V.n order to get naturally two different oligomers (poly-CBP-1 andoly-CBP-2), CBP monomer was polymerized and coated onto

TO–glass surface by two individual scanning electrochemicalxidation states [30]. Two different electrochromic devices fromhese corresponding oligomers were prepared and characterizedy spectro-electrochemical processes.

. Experimental

.1. Synthesis of biphenyl bridging carbazole (CBP) monomer

CuI (0.057 g, 0.3 mmol), 18-crown-6 (0.026 g, 0.1 mmol),2CO3 (1.66 g, 12 mmol), carbazole (2 g, 12 mmol) and N,N′-imethylacetamide (DMA) (5 ml) were added to a round-bottomask and vigorously stirred at 165 ◦C under argon. After 2 h,,4′-dibromobiphenyl (1.9 g, 6 mmol) as a solution in hot DMA10 ml) was added into the mixture slowly. The final mixture waseated to reflux for 16 h. The reaction solution was poured into00 ml of water and the precipitated crude product was collected,ried and re-crystallized from CHCl3/n-hexane (1:1) to afford pureompound as colorless crystals. Yield: 1.2 g, 82%.

UV–vis (�max, nm) (MeOH): 224, 256 and 296. FT-IR (cm−1):037 (C–H aromatic); 1598, 1496 (C C phenyl); 1228 (C–N); 1HMR (CHCl3-d): ı8.18, (d, 2H, Ar-Hdd′ ); 7.91, (d, 2H, Ar-Hff

′); 7.70d, 2H, Ar-Hee

′); 7.49, (d, 2H, Ar-Haa′); 7.44, (t, 2H, Hbb

′); 7.31 (t, 2H,cc

′) (Scheme 1).

.2. Electrochemical polymerization and characterization

Electrochemical measurements were performed using aHI660B electrochemical workstation from CH Instruments

Scheme

Acta 54 (2009) 5694–5702 5695

(Austin, TX, USA). The oligomers were synthesized from a reac-tion medium containing 2.0 × 10−3 M CBP monomer and 0.1 MLiClO4–NaClO4 in acetonitrile (ACN) solution via repetitive cyclingat a scan rate of 250 mV/s. The oligomers were coated on platinum(0.02 cm2) or indium–tin oxide (ITO, 8–12 �, 0.8 cm × 5 cm). Theactive area of coated oligomers on ITO was adjusted to 1 cm2.

For electrochromic device (ECD) fabrication, PEDOT wasdeposited electrochemically onto 18.75 cm2 ITO–glass from an ACNsolution containing 2 × 10−3 M EDOT and 0.1 M LiClO4–NaClO4 byapplying static voltage at 1.2 V (vs. a non-aqueous Ag/Ag+). A plat-inum wire was used as a counter electrode and non-aqueousAg/Ag+ as a reference. After coating of oligomers were on ITO/glasssurface via electroxidative polymerization, the films were rinsedwith ACN. HOMO and LUMO energy levels of oligomers werecalculated according to the inner reference ferrocene redox cou-ple E◦(Fc/Fc+) = +0.41 V vs. Ag/AgCl in ACN by using the formulaEHOMO = −e(Eox − EFc) + 4.8 [31]. Onset values of oxidation poten-tials were taken account while calculating HOMO energy levels.LUMO energy levels were calculated by subtracting the optical bandgap from HOMO level. Conductivity measurements of the oligomerfilms were carried out by a Keithley 2400 electrometer using four-point probe technique.

2.3. Spectrochemical and spectro-electrochemicalcharacterization

UV–vis spectra were recorded by Analytic Jena Speedcord S-600diod-array spectrophotometer. The absorption spectra of monomerand oligomer were recorded in CHCl3 (liquid phase) or ITO/glasstransparent film (solid phase). The optical band gaps (Eg) ofmonomer and oligomers were calculated from their absorptionedges [32].

The data obtained from UV–vis spectra and cyclic voltamme-try were used for spectro-electrochemical measurements of CBPoligomers on ITO/glass transparent film [33]. These measurementswere carried out to consider absorption spectra of this oligomer film

under applied voltage. The spectro-electrochemical cell consists ofa quartz cuvette, an Ag wire (RE), Pt wire counter electrode (CE) andITO/glass as transparent working electrode (WE). Measurementswere carried out in the 0.1 M TBAPF6 as supporting electrolyte inACN.

1.

5 imica Acta 54 (2009) 5694–5702

2

ODn

2

tbetap

2

anitme

3

3

lticcCadslot

N7tc

tLs0rd

3

i(iet(

(Fig. 2) exhibits two oxidation peaks attributed to its polaronic andbipolaronic states, respectively [35]. Because of these two separateoxidation states, two different products can be obtained from theelectrochemical polymerization of CBP monomer. After carrying

696 S. Koyuncu et al. / Electroch

.4. Structural characterization

FT-IR spectra were recorded by a PerkinElmer FT-IR Spectrumne by using ATR system (4000–650 cm−1). 1H NMR (Bruker AvancePX-400) spectra were recorded at 25 ◦C in CDCl3 and TMS as inter-al standard.

.5. Preperation of the gel electrolyte

A gel electrolyte prepared by from LiClO4:ACN:PMMA:PC inhe ratio of 3:70:7:20 by weight was used as conducting layeretween anodically and cathodically conducting polymer in thelectrochromic devices. Just after the dissolution of LiClO4 in ace-onitrile, poly(methyl metacrylate) (PMMA) was plasticized bydding 1,2-propylenecarbonate (PC) in order to form a highly trans-arent and conductive gel [34].

.6. Atomic force microscopy (AFM) studies

AFM measurements were carried out at room temperature andmbient conditions by using Ambios Q-Scope 250 instrument. Theon-contact mode (wave-mode) was used to take topographic

mages. 20 �m scanner equipped with silicon tips with 10 nmip-curvature and ITO coated glass substrate was used for measure-

ents. The system is covered with acoustic chamber to preventlectromagnetic noises which may effect the measurements.

. Results and discussion

.1. Synthesis and characterization

CBP monomer were synthesized and purified by a standarditerature procedure [3]. Determination of chemical structures ofhe materials (i.e. CBP monomer and the polymers obtained fromt) was performed from their FT-IR and 1H NMR spectra. Signifi-ant changes in the spectral properties were observed for initialompounds and the products. For example, in FT-IR spectrum ofBP, aromatic C–H vibration was observed at 3037 cm−1 and char-cteristic N–H vibration at 3412 cm−1 for secondary amines wasisappeared by substitution on the biphenyl bridge. Due to thisubstitution, aromatic C–H vibration signals shifted to low energyevel, owing to conjugation. Also C C signals and C–N signals werebserved at each spectrum (1595 and 1228 cm−1, respectively). Inhe oligomers whole peaks broadened and slightly shifted.

Molecular structure of CBP monomer was also identified by 1HMR spectrum, recorded in CDCl3. Characteristic doublet signals at.70 and 7.91 ppm were attributed to biphenyl which pointed thathe reaction was completed. Remaining signals were attributed toarbazole moiety.

Electrochemical polymerization was carried out in a reac-ion medium containing 2.0 × 10−3 M CBP monomer and 0.1 MiClO4–0.1 M NaClO4 mixture in ACN via repetitive cycling at acan rate of 250 mV/s. Oligomer films deposited onto ITO (8–12 �,.8 cm × 5 cm) surface were rinsed with plenty of ACN for theemoval of inorganic salts and other organic impurities formeduring the process.

.2. Optical and electrochemical properties

UV–vis spectra of CBP monomer and two oligomers were exam-ned both in liquid (CHCl3) and solid phase (ITO/glass surface)

Fig. 1). Electronic absorption and optical band gap data are shownn Table 1. UV–Vis spectrum of CBP monomer in liquid phasexhibits an absorption band with a maximum at 296 nm attributedo �–�* transition of carbazole moiety. The absorption maximum�max) of first oligomer (poly-CBP-1) exhibits a 134 nm red shift

Fig. 1. UV–vis spectra of CBP monomer (a) in CHCl3 solution, poly-CBP-1 (b) andpoly-CBP-2 (c) on the ITO/glass surface.

as compared to CBP monomer. Due to having larger conjugationaccording to CBP-1, a new broad band between 600 and 950 nmis observed in the absorption spectrum of the second oligomer(poly-CBP-2). While the low energy edge of the absorption spec-trum of CBP is at 322 nm which corresponds to band gap (Eg)of 3.85 eV for poly-CBP-1, it is observed at 492 and 980 nm forpoly-CBP-2 s are, corresponding to a band gap of 2.52 and 1.30 eV,respectively.

In the conjugated polymers, band gap is decreased by theincrease of the conjugation on the polymer backbone. Although ini-tial monomer is same, two different oligomeric films were obtainedby repeated electrochemical cycles at different scan region. Becauseof difference in their backbones, two films have different color andabsorb in different range in the UV–vis region. Poly-CBP-2 has moreconjugation than poly-CBP-1 which could be polymerized fromdouble side, then poly-CBP-2 absorbs in low energy region and alsoband gap of this oligomer is lower than that of poly-CBP-1 whichwas polymerized from only one side.

The electrochemical properties of CBP monomer and thepolymers were investigated by cyclic voltammetry. CBP monomer

Fig. 2. CV of CBP monomer in TBAPF6–ACN, scan rate 100 mV/s, Ag/AgCl.

S. Koyuncu et al. / Electrochimica Acta 54 (2009) 5694–5702 5697

Table 1HOMO and LUMO energy levels and optical band gap (Eg), conductivity values of CBP monomers and oligomers.

Compounds Oxidized groups and oxidation potentials (V) HOMO (eV) LUMOa (eV) Eg, optical band gap (eV) Conductivity (S/cm)

Carbazole, polaronic state Carbazole, bipolaronic state

CBP+1.39b +1.61b

+1.22c −5.61 −1.76 3.85 –

Poly-CBP-1+1.14b, 1.09b

−5.29 −2.80 2.49 1.8 × 10−5+0.9c +1.46b, +1.31b

Reversible Semi-reversible

Poly-CBP-2+1.12b, +1.07b +1.38b, +1.29b

−5.20 −3.90 1.30 1.2 × 10−3+0.81c ReversibleReversible

op0ttisp(

Fb

a Calculated by the subtraction of the optical band gap from the HOMO level.b Peak potentials.c Onset potentials.

ut this so-called polymerization, while the first oligomericroduct (poly-CBP-1) obtained from a electrochemical cycle at–1.3 V exhibited a new reversible redox couple at 1.02–1.08 V,he second one (poly-CBP-2) obtained from 0 to 1.7 V showedwo reversible redox couples at 1.06–1.12 and 1.30–1.36 V. The

ncrease in the current intensity of these reversible peaks after eachuccessive cycle clearly indicates the formation of an electroactiveolymer which deposited onto the surface of the working electrodeFig. 3).

Fig. 3. Repeated potential scan of CBP monomer between 0 and 1.3 V (poly-CBP-1) (a)

ig. 4. Scan rate dependence of poly-CBP-1 film on a Pt disk electrode in 0.1 M LiClO4–Na= 250 mV/s, c = 150 mV/s, d = 100 mV/s and e = 50 mV/s).

The scan rate dependence experiments for both oligomer filmsshowed a linear relationship between peak current and the scanrate, indicating non-diffusional redox process (Figs. 4 and 5).

As a result of the spectro-electrochemical measurements, bothelectronic structure of the CBP oligomers and their optical behavior

upon redox switching were clarified. During the oxidation pro-cess of poly-CBP-1 film, by increasing the electrochemical dopingthe band at 430 nm intensified and a new broad band devel-oped between 650 and 1000 nm. Besides, during the oxidation

and 0–1.7 V (poly-CBP-2) (b) in 0.1 M LiClO4–NaClO4–ACN, scan rate 250 mV/s.

ClO4/ACN solution at different scan rates between 50 and 500 mV/s (a = 500 mV/s,

5698 S. Koyuncu et al. / Electrochimica Acta 54 (2009) 5694–5702

F O4–Nab

p4c(

CctHro

ig. 5. Scan rate dependence of poly-CBP-2 film on a Pt disk electrode in 0.1 M LiCl= 250 mV/s, c = 100 mV/s and d = 50 mV/s).

rocess of poly-CBP-2 film, the diminishing in the first band at36 nm and the intensification in the second band at 802 nmlearly indicate the formation of bipolaronic species in the processFig. 6).

Furthermore, HOMO and LUMO energy levels and band gap ofBP monomer and oligomers were calculated to investigate the

harge-injection and charge-transport properties and to evaluatehe HOMO and LUMO energy levels of these molecules as well. TheOMO binding energy of these molecules with respect to the fer-

ocene/ferrocenium couple standard was about +0.41 V, which wasbtained from the onset potential of the oxidation at CV (Table 1).

Fig. 6. Spectro-electrochemical measurements o

Fig. 7. Electrochromic switching, optical absorbance monitored (a) poly-CBP-

ClO4/ACN solution at different scan rates between 50 and 500 mV/s (a = 500 mV/s,

For CBP molecule, the HOMO level was calculated as−5.61 eV. At theend of the polymerization, due to enlargement of �-conjugation,the oxidation peaks shifted to a lower potential and the HOMOenergy level increased (−5.29 eV for poly-CBP-1 and −5.20 eV poly-CBP-2). Due to no reductive groups in the structure (i.e. –NO2, COOHor –CN), it is not possible to observe any reduction peaks in the

cathodic scan region. Besides, LUMO energy level of these oligomerswere calculated by subtracting the optical band gap from HOMOenergy level (−2.80 and −3.86 eV, respectively). A proposed mech-anism for oxidation and polymerization reactions is presented inScheme 2.

f poly-CBP-1 (a) and poly-CBP-2 (b) films.

1 film at 430 nm (0–1.3 V) and (b) poly-CBP-2 film at 802 nm (0–1.7 V).

S. Koyuncu et al. / Electrochimica Acta 54 (2009) 5694–5702 5699

eme 2

3

epcCc1tco

Sch

.3. Electrochromic switching

Double potential step chronoamperometry was carried out tostimate the response time of the oligomer films. Electrochromicarameters of the oligomer films were extracted by analysis ofhanges in transmittance of the absorption band (430 nm for poly-BP-1 and 802 nm for poly-CBP-2) with respect to time while a

onstant switching the potential between 0 and 1.3 V for poly-CBP-and 0–1.7 V for poly-CBP-2 with a residence time of 10 s. During

he switching, the % transmittance at the wavelength of maximumontrast was measured by using a UV–vis spectrophotometer. Theptical contrasts were measured as a difference in %T between neu-

Fig. 8. AFM images of poly-CBP-1 (a) and poly-CB

.

tral and oxidized forms (%�T) and found to be 23 and 17% forpoly-CBP-1 (430 nm) and poly-CBP-2 (802 nm), respectively (Fig. 7).Furthermore, the response time were calculated as 2.2 and 2.4 s forthese oligomers, respectively. All of these results suggest that poly-CBP-1 and poly-CBP-2 films exhibit a reasonable electrochromiccharacters.

3.4. AFM images

The morphologies of the electrodeposited films were studied byAFM. Fig. 8 shows the images of film surfaces poly-CBP-1 and poly-CBP-2 electropolymerized with 250 mV/s scan rate after 30 cycles.

P-2 (b) films (scan rate: 250 mV, 30 cycle).

5700 S. Koyuncu et al. / Electrochimica Acta 54 (2009) 5694–5702

Fig. 9. Spectro-electrochemical measurements of poly-CBP-1/PEDOT (a) and poly-CBP-2/PEDOT (b) devices.

S. Koyuncu et al. / Electrochimica Acta 54 (2009) 5694–5702 5701

F EDOT+

Arnrfufifsfith

3

ooAd

3

I

ig. 10. Electrochromic switching, optical absorbance monitored for poly-CBP-1/P2.2 V) (b).

s a typical films for deposited by electrochemical polymerization,ough films were obtained. The RMS (root mean surface) rough-ess of the poly-CBP-1 and poly-CBP-2 are 34.39 and 56.14 nm,espectively. The morphology of two oligomers is quite differentrom each other. The film properties can be affected by the molec-lar structure of the oligomer system. Poly-CBP-1 forms less roughlms because of lower crosslink formation degree and shows uni-

ormly dispersed nodular structure. CBP-1 polymerizes from oneide and forms linear chains. On the other side CBP-2 polymerizesrom both sides and crosslink formation degree is higher. The AFMmage of the poly-BCP-2 shows agglomerated non-uniform struc-ure. Because of these agglomerates, the roughness of poly-CBP-2 isigher.

.5. Conductivity

The electrical conductivity is a function of the conjugation lengthf the polymer. Since the available number of polymerization sitesf poly-CBP-2 is a highly poly-conjugated and crosslinked oligomer.s a result of this property it has a two fold better electrical con-uctivity than poly-CBP-1 (Table 1).

.6. Spectro-electrochemistry of electrochromic devices (ECDs)

There are two electrochromic layer deposited onto transparentTO/glass surface in a dual type ECDs. The first (cathodic coloring)

Fig. 11. CV of poly-CBP-1/PEDOT (a) and poly-CBP-2/PEDOT (b) device as fu

device 440 nm (−1.0 to +1.7 V) (a) and poly-CBP-2/PEDOT device 550 nm (−1.0 to

and the second (anodic coloring) materials are placed in a positionto face each other and the gel electrolyte is applied in between thesetwo layers. When the anodic coloring polymer film (poly-CBPs) isfully oxidized, the catodic coloring polymer film (PEDOT) is fullyreduced [34]. By the application of voltage to the fist device pre-pared from poly-CBP-1 and PEDOT, the broad band between 400and 700 nm and the band between 650 and 950 nm attributed topoly-CBP-1 and PEDOT layer, respectively, increases. On the otherhand, the second device prepared from poly-CBP-2 and PEDOTshows the weak band, attributed to poly-CBP-2, at 440 nm in neu-tral state. The band intensify and the new broad band are formedbetween 500 and 700 nm by the applied potential (Fig. 7). Besides,the band between 650 and 950 nm attributed to PEDOT layer isalso observed. The neutral and oxidized state photos of the ECDsare presented in Fig. 9.

3.7. Switching of ECDs

In order to investigate response time, transmittance at the max-imum contrast wavelength and electrochromic switching behaviorof the prepared ECDs, they were operated under a repeated redox

stepping conditions [36]. For poly-CBP-1/PEDOT device, switchingbetween −1.0 and +1.7 V with a residence time of 10 s, the opticalcontrast (%�T) at 440 nm was found to be 16% and switching timewas measured as 1.8 s. On the other hand for poly-CBP-2/PEDOTdevice, switching between −1.0 and +2.2 V with a residence time of

nction of repeated scans at 500 mV/s (a: 1st cycle, b: 1000th cycle).

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702 S. Koyuncu et al. / Electroch

0 s, the optical contrast (%�T) at 550 nm was found to be 19% andesponse time was measured as 2.4 s (Fig. 10).

Sotzing et al. in 2007 reported to dual type solid state ECDaving 30 cm2 switching area, 30% contrast ratio and 0.6 s responseime based on poly-(3,6-bis2-(3,4-ethylenedioxy)thienyl)N-

ethylcarbazole and poly(3,4-ethylnedioxythiophene) [37]. Whenompared with this device, the efficiencies of poly-CBPs/PEDOTevices have high voltage consumption, low contrast ratio andigh response time.

.8. Stability of ECDs

Redox stability is another important parameter for ECDs. For thiseason, the electrochromic devices were tested by cycling of thepplied potential with 500 mV/s. These results indicate that poly-BP-1/PEDOT device had better stability than poly-CBP-2/PEDOTevice after 1000 cycle (Fig. 11). High operation voltage and highlyonjugated structure can be reason for low stability of poly-CBP-/PEDOT device.

. Conclusion

In this paper, we report the synthesis and characterization of,4′-di(N-carbazolyl)biphenyl monomer via Ullmann coupling andhen its electrochemical polymerization onto the ITO/glass trans-arent surface. It was found that CBP monomer has two separatexidation potentials at +1.29 and +1.61 V and they are a one-electrontepwise oxidation process which occurs on carbazole molecules.ue to these separate oxidation potentials, two different oligomericroducts (poly-CBP-1 and poly-CBP-2) were obtained by repeatedcans at these potential. The presence of two oxidizable groupsi.e. two carbazole rings) in CBP monomer gives rise to an efficientross-linking in the electro-generated oligomeric films at the higherxidation potential (0–1.7 V cycle). As a result, these oligomer filmshow reversible electrochemical oxidation, strong color changes,igh coloration efficiency and reasonable contrast ratios were alsobserved by switching through modulation of applied potential. Byhe use of these properties, two different ECDs were prepared andhese devices showed reasonable contrast ability. The ECD, poly-BP-1/PEDOT, showed a better stability than poly-CBP-2/PEDOTfter a 1000-cycle test.

cknowledgements

We acknowledge the supports from the State Planning Organiza-ion of Turkey (DPT) and Technical and Scientific Research Councilf Turkey (TUBITAK).

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Acta 54 (2009) 5694–5702

References

[1] J.L. Reddinger, G.A. Sotzing, J.R. Reynolds, Chem. Commun. 15 (1996) 1777.[2] A. Kruzinauskiene, A. Matoliukstyte, A. Michaleviciute, J.V. Grazulevicius, J.

Musnickas, V. Gaidelis, V. Jankauskas, Synth. Met. 157 (2007) 401.[3] P.J. Low, M.A.J. Paterson, D.S. Yufit, J.A.K. Howart, J.C. Cherryman, D.R. Tackley,

R. Brook, B. Brown, J. Mater. Chem. 15 (2005) 2304.[4] N.D. MacClenaghan, R. Passalacqua, F. Loiseau, S. Campagna, B. Verheyde, A.

Hameurlaine, W. Dehaen, J. Am. Chem. Soc. 125 (2003) 5356.[5] P. Trannekar, T. Fulghum, A. Baba, D. Patton, R. Advincula, Langmuir 23 (2007)

908.[6] J.V. Grazulevicius, P. Strohriegl, J. Pelichowski, K. Pelichowski, Prog. Polym. Sci.

28 (2003) 1297.[7] A. Kimoto, J.S. Chao, K. Ito, D. Aoki, T. Miyake, K. Yamamoto, Macromol. Rapid

Commun. 26 (2005) 597.[8] J.P. Chen, F.L. Labarthet, A. Natansohn, P. Rochon, Macromolecules 32 (1999)

852.[9] M. Grigoras, M.C. Antonia, Eur. Polym. J. 41 (2005) 1079.10] A. Kimoto, J.S. Cho, M. Higuchi, K. Yammamoto, Macromolecules 37 (2004) 5531.

[11] N. Bloudin, M. Leclerc, Acc. Chem. Res. 41 (2008) 1110.12] M. Watanabe, M. Nishiyama, T. Yamamoto, Y. Koie, Tetrahedron Lett. 41 (2000)

481.13] J.P. Chen, A. Natansohn, Macromolecules 32 (1999) 3171.14] J. Qu, R. Kawasaki, M. Shiotsuki, F. Sonda, T. Masuda, Polymer 47 (2006) 6551.15] J.F. Ambrose, R.F. Nelson, J. Electrochem. Soc. 115 (1968) 1159.16] G. Sonmez, Chem. Commun. 42 (2005) 5251.17] G. Sonmez, H.B. Sonmez, C.K.F. Shen, R.W. Jost, Y. Rubin, F. Wudl, Macro-

molecules 38 (2005) 669.18] A.A. Argun, P.H. Aubert, B.C. Thomson, I. Schwendeman, C.L. Gaupp, J. Hwang,

N.J. Pinto, D.B. Tanner, A.G. MacDiarmit, J.R. Reynolds, Chem. Mater. 16 (2004)4401.

19] A. Durmus, G.E. Gunbas, P. Camurlu, L. Toppare, Chem. Commun. 31 (2007)3246.

20] G. Sonmez, H.B. Sonmez, C.K.F. Shen, F. Wudl, Adv. Mater. 16 (2004) 1905.21] I. Schwendeman, R. Hickman, G. Sonmez, P. Schottland, K. Zong, D.M. Welsh,

J.R. Reynolds, Chem. Mater. 14 (2002) 3118.22] G. Sonmez, H. Meng, F. Wudl, Chem. Mater. 16 (2004) 574.23] A. Desbene-Monvernay, P.C. Lacaze, J.E. Dubois, P. Desbene, J. Electroanal. Chem.

152 (1983) 87.24] K. Faid, D. Ades, A. Siove, C. Chevrot, J. Chim. Phys. 89 (1992) 1019.25] K. Faid, A. Siove, D. Ades, C. Chevrot, Synth. Met. 55–57 (1993) 1656.26] K. Faid, D. Ades, A. Siove, C. Chevrot, Synth. Met. 63 (1994) 89.27] G.E. Gunbas, A. Durmus, L. Toppare, Adv. Mater. 20 (2008) 691.28] P.M. Beaujuge, S. Ellinger, J.R. Reynolds, Adv. Mater. 9999 (2008) 1.29] A. Cihaner, F. Algi, Adv. Funct. Mater. 18 (2008) 1.30] Z. Wie, Q. Wang, J. Xu, Y. Nie, Y. Du, H. Xia, J. Polym. Sci. A: Polym. Chem. 46

(2008) 5232.31] K. Colladet, M. Nicolas, L. Goris, L. Lutsen, D. Vanderzande, Thin Solid Films 7

(2004) 451.32] P. Wagner, P.H. Aubert, L. Lutsen, D. Vanderzande, Electrochem. Commun. 4

(2002) 912.33] C. Bohn, S. Sadki, A.B. Brennan, J.R. Reynolds, J. Electrochem. Soc. 149 (2002)

34] F. Wudl, H. Meng, G. Sonmez, Chem. Matter 16 (2004) 574.35] D. Witker, J.R. Reynolds, Macromolecules 38 (2005) 7636–7644.36] B. Sankaran, J.R. Reynolds, Macromolecules 30 (1997) 2582.37] V. Seshadri, J. Padilla, H. Bircan, B. Radmard, R. Draper, M. Wood, T.F. Otero, G.A.

Sotzing, Org. Electron. 8 (2007) 367.