A novel multielectrochromic copolymer based on 1-(4-nitrophenyl)-2,5-di(2-thienyl)-1H-pyrrole and EDOT

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A novel multielectrochromic copolymer based

on 1-(4-nitrophenyl)-2,5-di(2-thienyl)-1H-pyrrole and EDOT

Serhat Varis, Metin Ak, Idris M. Akhmedov, Cihangir Tanyeli, Levent Toppare

*

Department of Chemistry, Middle East Technical University, Ankara 06531, Turkey Received 1 June 2006; received in revised form 23 January 2007; accepted 5 February 2007

Available online 12 February 2007

Abstract

Copolymer of 1-(4-nitrophenyl)-2,5-di(2-thienyl)-1H-pyrrole (NTP) with 3,4-ethylene dioxythiophene (EDOT) was electrochemically synthesized and characterized. Resulting copolymer film has distinct electrochromic properties. It has five different colors (light red, red, light grey, green, and blue). At the neutral state kmaxvalue due to the p–p*transition was found to be 550 nm and Egwas calculated as

1.6 eV. Double potential step chronoamperometry experiment shows that copolymer ﬁlm has good stability, fast switching time (1.2 s) and high optical contrast (42%).

Electrochromic device based on P(NTP-co-EDOT) and poly(3,4-ethylenedioxythiophene) was constructed and characterized. Oxi-dized state of the device shows light red color whereas reduced state blue color. At interval potentials device has good transparency and colors of the device are yellow and grey. Maximum contrast (T%) and switching time of the device were measured as 23% and 1.1 s at 650 nm. Electrochromic devices have good environmental and redox stability.

Keywords: Electrochemical polymerization; Conducting copolymers; Electrochromic devices

1. Introduction

Among the electroactive organic polymers, polythioph-ene derivatives have occupied prime position due to its high conductivity, good redox reversibility, swift change of color with potential, and stability in environment [1]. Polythi-ophenes retain extensive interest due to the technological applications such as non-linear optical devices[2], polymer light emitting diodes[3], gas sensors[4], organic transistors

[5]and electrochromic devices[6].

Electrochromics (ECs) are materials which exhibit diﬀer-ent color as a function of applied potdiﬀer-ential. Both inorganic and organic materials have been used as EC materials. A wide variety of electrochomic materials are presently known, ranging from metal oxides such as WO3 [7] and mixed-valence metal complexes like prussian blue [8] to

organic molecules. In the realm of organic electrochromic systems, viologens[6]and conjugated polymers[9]have been shown to display electrochromism. There is still a lot of scope for further improvement in terms of switching speeds, stability, contrast, and ease of synthesis and processing.

For conducting polymers, electrochromism is related to doping–undoping process. The doping process modiﬁes the polymer electronic structure, producing new electronic states in the band gap, causing color changes. Electronic absorption shifts bathochromically upon doping, and the color contrast between the undoped and doped states is related to the polymer band gap. A major focus in the study of electrochromic polymeric materials has been that of con-trolling their colors by main-chain and pendant group structural modiﬁcation and copolymerization. Copolymer-ization can leads to an interesting combination of the properties.

For that matter, we synthesized a novel copolymer of (NTP) wih 3,4-ethylenedioxythiophene (EDOT). EDOT is a popular choice as a substituted monomer since it

*

Corresponding author. Tel.: +90 312 210 32 51; fax: +90 312 210 32 00.

E-mail address:toppare@metu.edu.tr(L. Toppare).

www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 603 (2007) 8–14

Electroanalytical Chemistry

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produces a low band gap polymer with high stability and good conductivity[10]. EDOT can give rise to noncovalent intramolecular interactions with adjacent thiophenic units and thus induce self-rigidiﬁcation of the p-conjugated sys-tem in which it is incorporated[11–13]. We report here the electrochemical copolymerization of a new monomer with EDOT. The resultant copolymers were characterized via cyclic voltammetry, SEM and conductivity measurements. The optoelectrochemical and electrochromic properties, such as the relative luminance, change of color upon redox switching, and long-term switching stability of the copoly-mer were determined.

2. Experimental 2.1. Chemicals

AlCl3(Aldrich), succinyl chloride (Aldrich), 4-nitroani-line (Sigma), dichloromethane (Sigma–Aldrich), toluene (Merck), acetonitrile (ACN) (Merck), LiClO4 (Aldrich), poly(methyl metacrylate) (PMMA) (Aldrich), propylene carbonate (PC) (Aldrich) were used without further puriﬁ-cation. Thiophene (Aldrich) was distilled before use. Tetra-butylammonium tetraﬂuoroborate (TBAFB) (Aldrich), p-toluene sulfonic acid (PTSA) (Sigma), 3,4-ethylenedioxy-thiophene (EDOT) (Aldrich) were used as received. The monomer 1-(4-nitrophenyl)-2,5-di(2-thienyl)-1H-pyrrole was synthesized according to the literature procedure[15]. 2.2. Instrumentation

A Voltalab PST50 model potentiostat was used for the CV and chronocoulometry studies. FTIR spectra were recorded on a Nicolet 510 FTIR spectrometer. Surface mor-phologies of the homopolymer ﬁlms were investigated by JEOL JSM-6400 scanning electron microscope. Varian Cary 5000 UV–Vis spectrophotometer was used in order to con-duct the spectroelectrochemical experiments of copolymer and characterize the devices. Colorimetry measurements were done via Minolta CS-100 spectrophotometer.

2.3. Electrochemistry

The oxidation reduction behavior of monomer in the presence of EDOT was investigated by CV. The voltammo-grams were recorded in LiClO4(0.1 M)/ACN solvent-elec-trolyte couple using a system consisting of a potentiostat and a CV cell with Pt foil working and counter electrodes

and a Ag/Ag+ reference electrode. The electrochemistry experiments were carried out at room temperature under nitrogen atmosphere. For conductivity, spectroelectro-chemical measurements and SEM imaging, P(NMT) was synthesized via cyclic voltammetry by sweeping potential between 0.4 and 1.0 V, whereas P(NMT-co-EDOT) and PEDOT ﬁlms were prepared by constant potential electrol-ysis (1.3 V) in LiClO4 (0.1 M)/ACN solvent-electrolyte couple. As stated before [15] potentiodynamic conditions work better for P(NMT) ﬁlm formation on electrode. 2.4. Synthesis of copolymers of NTP with EDOT

EDOT was used as the comonomer for the synthesis of conducting copolymer of 1-(4-nitrophenyl)-2,5-di(2-thie-nyl)-1H-pyrrole (NTP). NTP (50 mg) was dissolved in 5 ml of ACN and 5 lL of EDOT were introduced into the single compartment electrolysis cell. LiClO4was used as supporting electrolyte. Constant potential of 1.3 V was applied for 10 min under the inert atmosphere. After elec-trolysis, the ﬁlm was washed with ACN to remove the sup-porting electrolyte and the monomers. Scheme 1 shows copolymerization reaction.

2.5. Spectroelectrochemistry

In order to carry out the spectroelectrochemical experi-ments, copolymer ﬁlms were deposited on ITO-coated glass. They were used both for the spectroelectrochemistry and electrochromic measurements in ACN/LiClO4 using Ag/Ag+ as the reference and a Pt wire as the auxiliary electrodes.

2.6. Preparation of the gel electrolyte

The gel electrolyte for electrochromic device was pre-pared by using LiClO4:ACN:PMMA:PC in the ratio of 3:70:7:20 by weight. After the dissolution of LiClO4 in ACN, poly(methyl methacrylate) (PMMA) was plasticized by 1,2-propylenecarbonate in order to form a highly trans-parent and conductive gel.

3. Results and discussion 3.1. Cyclic voltammetry

The oxidation/reduction behavior of copolymer was investigated by CV using LiClO4 (0.1 M)/ACN

S S N NO₂ + S O O S O O S S N NO₂ S _O O 1.3 V ACN/LiClO4 NaClO₄

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solvent-electrolyte couple. Experiments were carried out in an electrolysis cell equipped with indium doped tin oxide (ITO) coated glass plate as the working, Pt wire counter and Ag/Ag+reference electrodes.

When redox behavior of NTP was investigated via cyclic voltammetry, a color change between yellow and blue col-ors was observed, while a greenish cloud was formed around the electrode due to the partial dissolution of olig-omers. Positive potentials higher than 1.0 V promotes less adhesion of deposits due to the oxidation and degradative crosslinking of polymer at 1.13 V (second anodic peak). Hence, potentials lower than 1.13 V seem to be most useful in obtaining insoluble, adherent and electroactive ﬁlms on the electrode. Under these conditions, the monomer gives a

polymer, which is subsequently oxidized at the same poten-tial to produce polarons balanced with ClO4 counterions. Reduction of this polymer below 0.4 V causes the neutral-ization of polarons accompanied with the loss of ClO₄. Hence, the resulting short chains dissolve and no ﬁlm depo-sition on the electrode surface takes place[15](Fig. 1a).

In order to investigate the CV behavior of the copoly-mer, we performed CV studies in the presence of EDOT under same experimental conditions. There was a drastic change in the voltammogram, both the current increase between consecutive cycles and the oxidation potential of the material were diﬀerent than those of NTP and EDOT, which, in fact, could be interpreted as the formation of copolymer (Fig. 1b and c).

3.2. Scan rate dependence of the peak currents

Polymer films prepared by constant potential electroly-sis (1.3 V) were washed with AN, and their redox switching in monomer-free electrolyte showed a single, well-defined redox process. The current response was directly propor-tional to the scan rate as can be seen inFig. 2a, indicating that the polymer film was electroactive and adhered well to the electrode[14]. The scan rate dependence of the anodic and cathodic peak currents shows a linear dependence as a function of the scan rate as illustrated inFig. 2b. This dem-onstrates that the electrochemical processes are reversible and not diffusion limited, even at very high scan rates. 3.3. FTIR spectra

FTIR spectrum of the copolymer shows the following absorption peaks: 3103 cm 1 (aromatic C–H stretching), 1518 cm 1 (asymmetric Ar–NO2 stretching), 1342 cm

1 (symmetric Ar–NO2stretching), 843 cm 1(C–N stretching for Ar–NO2), and 3020 cm 1 (C–Ha stretching of thio-phene). The broad band observed at around 1639 cm 1

Fig. 1. Cyclic voltammograms (a) CV of NTP; (b) CV of EDOT; (c) CV

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proves the presence of polyconjugation and peaks at 2925 and 2870 cm 1are related to –CH2CH2–. C–O–C stretch-ing arisstretch-ing from ethylene-dioxy group and C–S stretchstretch-ing appear at 693 cm 1and 1242 cm 1, respectively. The inten-sity of the absorption band of the monomer at 774 cm 1 arising from C–Ha stretching of thiophene moiety disap-peared completely. This is an evidence of the polymeriza-tion from 2,5 posipolymeriza-tions of thiophene moiety of the monomers. The strong absorption peaks at 1113, 1087 and 630 cm 1 were attributed to the incorporation ClO₄ ions into the polymer ﬁlm during doping process. Results of the FTIR studies clearly indicated the copolymerization was successfully achieved.

3.4. Conductivities of the ﬁlms

The conductivities of the ﬁlms were measured via four probe technique. The conductivity of electrochemically prepared homopolymer was 6.0· 10 4

S cm 1; whereas P(NTP-co-EDOT) ﬁlm was measured as 2.5· 10 3

S cm 1. Introducing EDOT into the polymer chain increased the conductivity.

3.5. Scanning electron microscopy (SEM)

Surface morphologies of copolymer were investigated by Scanning Electron Microscope. SEM micrograph of P(NTP-co-EDOT) was diﬀerent than those of from both P(NTP) and PEDOT (Fig. 3). This diﬀerence could be attributed to copolymerization.

3.6. Electrochromic properties of conducting polymer The best way of examining the changes in optical prop-erties of conducting polymers upon voltage change is spec-troelectrochemistry. It also gives information about the electronic structure of the polymer such as band gap (Eg)

and the intergap states that appear upon doping. P(NTP-co-EDOT) ﬁlm was potentiostatically synthesized at 1.3 V on ITO electrode. Electrolyte solution was composed of 0.01 M NTP, 0.01 M EDOT and ACN/LiClO4 (0.1 M). The spectroelectrochemical and electrochromic properties of the resultant copolymer were studied by applying poten-tials ranging between 0.7 V and +1.1 V in monomer free ACN/LiClO4 (0.1 M) medium. At the neutral state kmax

Fig. 2b. Peak current vs scan rate graph.

Fig. 3. SEM micrograph of (a) P(NTP), (b) PEDOT, (c) P((NTP)-co-EDOT).

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value due to the p–p* _{transition of the copolymer was} found to be 550 nm and Egwas calculated as 1.6 eV. Upon applied voltage, reduction in the intensity of the p–p* tran-sitions and formation of charge carrier bands were observed. Thus, appearance of peaks around 815 nm and >1040 nm could be attributed to the evolution of polaron and bipolaron bands, respectively (Fig. 4).

The colors of the electrochromic materials were defined accurately by performing colorimetry measurements. CIE system was used as a quantitative scale to define and com-pare colors. Three attributes of color; hue (a), saturation (b) and luminance (L) were measured and recorded. The P(NTP-co-EDOT) film has distinct electrochromic proper-ties. It shows five different colors in neutral and oxidized states. These colors and corresponding L, a, b values were given inFig. 4a.

The following table is a good summary of comparison of the homopolymer, copolymer and PEDOT. kmax value of

the copolymer is between those of the PEDOT and the homopolymer. As seen from the table, introduction of EDOT to the polymer chain leaded to a tremendous decrease in the band gap. These numerical values also sup-port the copolymerization phenomena (seeTable 1). 3.7. Electrochromic switching

The ability of a polymer to switch immediately and exhi-bit a noteworthy color change is significant for electrochro-mic applications. Electrochroelectrochro-mic switching studies can investigate these types of properties. The experiments car-ried out by spectroelectrochemistry proves the ability of P(NTP)-co-EDOT switch between its neutral and doped states with a change in transmittance at a fixed wavelength. During the experiment, the % transmittance (%T) at 550 nm of the polymer is measured using a UV–Vis spec-trophotometer. The polymer film was synthesized on ITO-coated glass slides. The %T was then monitored at kmax while the polymer was switched between 0.6 V and 1.0 V. The contrast is measured as the difference between %T in the reduced and oxidized forms and noted as 42 %T. As seen inFig. 5, copolymer has a reasonable stability and switching time of 1.2 s.

The optical contrast of the homopolymer at its kmaxwas 13%[15]. The copolymerization with EDOT increased the optical contrast from 13% to 42%, via copolymerization switching time was also found to be shorter (1.2 s) com-pared to the one for pristine P(NMT) (1.7 s)[15].

Fig. 4. Spectroelectrochemical spectrum of P(NTP-co-EDOT) as applied potentials between 0.6 and +1.0 V in ACN/ LiClO4 (0.1 M). (a) 2D,

(b) 3D.

Table 1

kmaxand Egvalues of homopolymer, copolymer and PEDOT

Homopolymer Copolymer PEDOT

kmax(nm) 400 544 600

Eg(eV) 2.15 1.7 1.6

Fig. 5. Electrochromic switching, optical absorbance change monitored at 550 nm for P(NTP)-co-EDOT between 0.6 V and 1.0 V.

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3.8. Spectroelectrochemistry of electrochromic devices (ECDs)

A dual-type ECD consists of two electrochromic materi-als (one anodically coloring, the other cathodically color-ing) deposited on transparent ITO, placed in a position to face each other and a gel electrolyte was applied in between. The anodically coloring polymer ﬁlm P(NTP-co-EDOT) was fully reduced and the cathodically coloring polymer (PEDOT) was fully oxidized prior to construction of electrochromic devices (ECD).

Optoelectrochemical spectra of the dual type ECD as a function of applied potential (from 2.0 V to 2.2 V) are given inFig. 6. Maximum absorption at 510 nm revealing light red color was observed due to p–p* _{transition upon} application of positive voltages. At that state, PEDOT did not reveal an obvious absorption at the UV–Vis region of the spectrum and device revealed light red color.

At moderate potentials device have good transparency and colors of the device are yellow and grey at 0.7 V and 0.4 V, respectively. When the applied potential decreased, due to reduction of PEDOT layer, blue color became dominant and a new absorption observed at 610 nm. The observed colors and corresponding parame-ters L, a, b values are shown inTable 2. This device reveals four colors as was the case for the one where the homopol-ymer was utilized as the anodically coloring polhomopol-ymer. 3.9. Switching of ECDs

Response time, one of the most important characteristics of electrochromic devices, is the time needed to perform a switching between two states. Chronoabsorptometry was performed to estimate the response time of the device and its stability during consecutive scans. Switching between square wave potentials 2.0 V and 2.2 V with a residence time of 5 s, the optical contrast (%DT) at 650 nm was found 23% with 1.2 s. switching time by UV–Vis spectrophotome-ter (Fig. 7).

3.10. Stability of ECDs

Cyclic voltammetry is employed by monitoring current change to ﬁgure out the long-term stability for devices.

Fig. 6. Spectroelectrochemical spectrum of the device as applied poten-tials between 2.0 and +2.2 V in ACN/NaClO4/LiClO4(0.1 M). (a) 2D,

(b) 3D.

Table 2

Colorimetry properties of copolymer and device

Material Potential (V) L a b Copolymer ﬁlms 0.6 47 19 12 0.1 62 2 29 0.3 72 14 36 1.1 63 1 26 Copolymer/PEDOT device 1.8 52 11 2 0.0 66 1 2 0.7 61 23 1 1.8 51 21 28

Fig. 7. Electrochromic switching, optical contrast change monitored at 650 nm for device between 2.0 V and 2.2 V.

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Cyclic voltammetry studies showed that the electrochromic devices operated stably with applied voltage of 2.4 V and 2.0 V with 500 mV/s scan rate under atmospheric condi-tions. P(NTP-co-EDOT)/PEDOT device could be repeat-edly switched up to 500 cycles. As seen from Fig. 8, almost all initial electroactivity was maintained proving that ECD has reasonable environmental and redox stability.

4. Conclusion

The synthesis of a new copolymer from 1-(4-nitro-phenyl)-2,5-di(2-thienyl)-1H-pyrrole NTP and EDOT was successfully achieved in ACN/LiClO4(0.1 M) solvent-elec-trolyte couple. Copolymer was characterized by CV, SEM and FTIR studies. Spectroelectrochemical studies and elec-trochromic characterization methods showed that copoly-merization with EDOT not only decreases the band gap Eg but also enhanced the electrochromic properties such as optical contrast and switching time.

Spectroelectrochemical analyses revealed that the copolymer has an electronic band gap of 1.6 eV. The

con-trast is measured as the diﬀerence between %T in the reduced and oxidized forms and noted as 42 %T at 550 nm. In the second part of the study, dual-type complementary colored polymer ECD were assembled with a conﬁguration of ITO/P(NTP-co-EDOT) i gel electrolyte i PEDOT/ITO and its characteristics were examined. Electrochromic switching study results showed that optical contrast (%DT) and switching time were 23% and 1.2 s at 650 nm. In addition, the device has good environmental and redox stability.

Acknowledgements

The authors gratefully thank the DPT-2005K120580, BAP2005-01-03-06 and TUBA Grants.

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Fig. 8. Cyclic Voltammogram of the device as a function of repeated scans 500 mV/s: after 1st cycle (plain), after 500 cycles (dash).