Synthesis and electropolymerization of 1,2-bis(thiophen-3-ylmethoxy)benzene and its electrochromic properties and electrochromic device application

Solid State Sciences 12 (2010) 1199e1204

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Synthesis and electropolymerization of 1,2-bis(thiophen-3-ylmethoxy)benzene and its electrochromic properties and electrochromic device application Metin Ak a, *, Mine Sulak Ak b, Gülbin Kurtay b, Mustafa Güllü b, Levent Toppare c a

Pamukkale University, Department of Chemistry, 20017 Denizli, Turkey Ankara University, Department of Chemistry, 06100 Ankara, Turkey c Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 October 2009 Received in revised form 9 March 2010 Accepted 21 March 2010 Available online 2 April 2010

A new thiophene-based monomer; 1,2-bis(thiophen-3-ylmethoxy)benzene (BTMB) has been synthesized and chemical structure of the monomer was characterized. Polymerization of BTMB and characterization of the resulting polymer P(BTMB) were performed. Spectroelectrochemical analysis of the P (BTMB) reflected electronic transitions at 400 nm, 520 nm and w720 nm, corresponding to pep* transition, polaron and bipolaron band formation respectively. Switching ability was evaluated by a kinetic study via measuring the transmittance (%T) at the maximum contrast. Dual type all polymer electrochromic device (ECD) based on P(BTMB) and poly(ethylene dioxythiophene) (PEDOT) was constructed. Spectroelectrochemistry and switching ability of the devices were investigated by UVevis spectroscopy. Ó 2010 Elsevier Masson SAS. All rights reserved.

Keywords: Conducting polymers Electrochromic device Spectroelectrochemistry

1. Introduction The last few decades have been marked by the growing importance taken by organic conducting polymers. Recent advances in the field of the conducting polymers have led to a variety of materials with great potentials for practical applications such as batteries [1,2], electronic devices [3], sensors and capacitors [4], electromagnetic radiation shielding, antistatic coating, gas separation membranes, nonlinear optics and electrochromic devices (ECDs) [5,6]. Electrochromism is related to doping-undoping process in conducting polymers. The polymer electronic structure is modified by the doping process, producing new electronic states in the band gap, causing color changes. Electronic absorption wavelength of polymer shifts to higher wavelengths with doping process (bathochromic effect). Color change upon doping is due to this bathochromic effect. A major focus in the study of electrochromic polymeric materials was on controlling their colors by main-chain and pendant group structural modification and copolymerization [7e9]. An ECD is an electrochemical cell, composed of optically transparent electrodes coated with complementary electrochromic materials and separated by an electrolyte, which may be liquid or solid. ECDs have been developed for mirrors, optical displays, camouflage materials, spacecraft thermal control, and solar control

* Corresponding author: Tel.: þ902582963595; fax: þ902582963535. E-mail address: [email protected] (M. Ak). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.03.013

glazings for “smart windows” [10e12]. Electrochemical techniques such as cyclic voltammetry, coulometry, chronoamperometry and spectroscopic measurements were employed for characterization of both polymer electrodes and electrochromic devices, which enables one to decide whether they are suitable for commercial purposes. The requirements for high performance electrochromic device are high electrochromic efficiency, short response time, good stability, optical memory, optical contrast and color uniformity [13]. Synthesis of new polythiophene derivatives with the ability to tailor the EC properties is an important part of conducting polymer research [14,15]. One of the problem for the full technological utilization of polymer semiconductors is that they display generally a much lower charge carrier mobility than inorganic materials. In conducting polymers charge carrier mobility is usually limited by disorder effects, which prevent efficient inter-chain hopping and lead to materials with one-dimensional electronic properties. Crosslinked conducting polymers with electronically connected nodes are excellent candidates among super structured CPs; with such polymers there should be no need for inter-chain coupling or inter-chain electronic transfer [16-19]. For investigate electrochromic properties of a crosslinked CP sutructure we synthesis 1,2-bis(thiophen-3-ylmethoxy)benzene (BTMB) with more than one thiophene unit. Electrochemical polymerization of BTMB was achieved in acetonitrile (AN) using tetrabutylammonium tetrafluoroborate (TBAFB) as the supporting electrolyte. The resultant products were characterized via cyclic voltammetry (CV), Fourier

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S

NBS Benzoyl Peroxide

S Br S

2

S Br

+

OH

K2CO3 / DMF

OH

O O

S

BTMB

All electrochemistry experiments were carried out using a Voltalab PST 50 model potentiostat/galvanostat. 1H NMR spectrum of the monomer was taken by using a 400 Mhz NMR spectrometer (Bruker DPX-400). FTIR spectra were recorded on a Nicolet 510 FTIR spectrometer. Agilent 8453 UVevis spectrophotometer was used in order to perform the spectroelectrochemical studies of the polymer and the characterization of the devices. Colorimetry measurements were done via Minolta CS-100 spectrophotometer. 2.3. Synthesis of 3-bromomethylthiophene

Scheme 1. Synthesis route for monomer.

transform infrared (FTIR) spectroscopy and conductivity measurements. Spectroelectrochemical properties of the polymer were investigated via UVevis spectrophotometer. Second part of the study was devoted to construct the electrochromic device with a configuration of ITO/P(BTMB)jjGel ElectrolytejjPEDOT/ITO. Spectroelectrochemistry and switching ability of the devices were investigated by UVevis spectroscopy and cyclic voltammetry.

A solution of 3-bromomethylthiophene was prepared by adding a mixture of (8.85 g, 0.05 mol) N-bromosuccinimide and (0.05 g, 20.6 mmol) of benzoyl peroxide batch-wise to a refluxing solution of 3-methylthiophene (5 g, 0.05 mol) in carbon tetrachloride (30 ml). After 3e5 h refluxing the mixture was cooled and succinimide was filtered. Carbon tetrachloride was evaporated and the resulting product was highly lacrimatory oil. The yield was 65% (Scheme 1). 1H NMR (CDCl3): d ¼ 7.40 (d), 7.20 (s), 4.50 (s).

2. Experimental section 2.4. Synthesis of 1,2-bis(thiophen-3-ylmethoxy) benzene 2.1. Materials 3-Methylthiophene, carbontetrachloride, N-bromosuccinimide, benzoyl peroxide, 1,2-dihydroxy benzene, potassium carbonate, dichloromethane, acetonitrile, propylene carbonate (PC), tetrabutylammonium tetrafluoroborate (TBAFB), polymethylmetacrylate (PMMA) and acetic acid were purchased from Aldrich and used as received without further purification. 2.2. Instrumentation A three-electrode cell containing an ITO coated glass slide as the working electrode, a platinum foil as the counter electrode, and a silver wire as the pseudo-reference electrode were used for electrodeposition of polymer films via potentiostatic or potentiodynamic methods. The pseudo-reference was calibrated externally using a 5 mM solution of ferrocene (Fc/Fcþ) in the electrolyte (E1/2(Fc/Fcþ) ¼ þ0.30 V vs. Ag wire and þ0.1 V vs. Ag/Agþ in 0.1 M AN/TBAFB). The potentials are reported vs.Ag/Agþ. To remove dissolved oxygen from the electrochemical cell the solution was purged with nitrogen for 5e10 min prior to the experiment, and a “blanket” of inert gas was maintained above the solution during the experiment.

1,2-Dihydroxy benzene (8.98 g, 0.065 mol) and anhydrous potassium carbonate (35.95 g, 0.26 mol) were added into 20 ml of dichloromethane in a two necked flask under nitrogen atmosphere. The mixture was stirred about an hour. 3-Bromomethylthiophene (23.01 g, 0.13 mol) was added dropwise. The reaction mixture was refluxed for 8 h at 60  C, and after cooling to room temperature the mixture was poured into 10 mL ice water. After 1 h, the pH of the mixture was adjusted to 5 by adding acetic acid. The crude solid powder was filtered and recrystallized from acetone. The light brown crystals were obtained with % 75 yields (Scheme 1). 1H NMR spectrum of the BTMB is given in Fig. 1 IR (KBr) n cm1: 3448, 1569, 1502, 1373, 1246, 1203, 1120, 997, 785, 741, 692, 635, 594; C16H14O2S2 (CDCl3, 400 MHz, ppm): d ¼ 7.2e7.4 (t; 2H; d;2H), 7.15 (s; 2H, J ¼ 3.6 Hz), 6.9-7.0 (d; 2H, J ¼ 5.5 Hz; d; 2H, J ¼ 5.5 Hz), 5.15 (s, 4H). 2.5. Synthesis of conducting polymer of BTMB Preparative electrochemical polymerization was performed under potentiostatic conditions in a one compartment cell. 40 mg BTMB (0.013 M) were dissolved in 10 ml AN where 0.2 M TBAFB (0.66 g) was used as the supporting electrolyte. Electrolyses were

Fig. 1. 1H NMR spectrum of monomer.

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2.6. Investigation of spectroelectrochemistry and switching time Spectroelectrochemical analyses of the polymer were carried out to understand the band structure of the product. For spectroelectrochemical studies, the polymer film was deposited potentiostatically at 1.8 V from a solution of 40 mg BTMB (0.013 M) in AN/TBAFB (0.2 M) solventeelectrolyte couple on indium tin oxide-coated glass slide (ITO). UVevis spectra of the film were recorded at various potentials in monomer free AN/TBAFB (0.2 M) solution. A square wave potential step method coupled with optical spectroscopy was used to investigate the switching times and contrast for these polymers. In this double potential step experiment, the potential was set at an initial potential (where the conducting polymer was in one of its extreme states) for 5 s and stepped to a second potential for another 5 s, before being switched back to the initial potential. During the experiment the percent transmittance (%T) and switching times at lmax of the polymer were measured using a UVevis spectrophotometer. Fig. 2. Cyclic voltammogram of BTMB in AN/TBAFB at 100 mV scan rate.

carried out at 1.8 V in a cell equipped with Pt working and counter electrodes and silver wire pseudo reference electrode at room temperature for 1 h. The free standing films were washed with AN several times to remove unreacted monomer and TBAFB.

Fig. 3. (a) In situ electropolymerization of BTMB, (b) absorbance recorded at different wavelengths during the electropolymerization.

Fig. 4. (a) CV of polymer at different scan rates. (b) Peak current vs. scan rate graph (Ipa: anodic peak current value, Ipc: cathodic peak value, r2:lineer regression value).

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Table 1 Electronic and electrochromic properties of the P(BTMB) and its device. Polymer/device

lmax

Redox state

La

Further oxidized Oxidized Neutral Further oxidizedb Oxidizedb Neutralb

43

aa

ba

Color

Eg (eV)

12

40

Blue

2.41

70 38 49

38 28 32

38 18 33

Green Red Red

e

55 34

21 0

30 26

Green Blue

(nm) P(BTMB)

P(BTMB) /PEDOT Device

400

400

605 a b

CIE L  a  b system: luminance (L), hue (a) and saturation (b). Redox states for PEDOT layer.

2.7. Preparation of the gel electrolyte Gel electrolyte was prepared using TBAFB: AN: PMMA: PC in the ratio of 3:70:7:20 by weight. After TBAFB was dissolved in AN, PMMA was added into the solution. In order to dissolve PMMA, vigorous stirring and heating was required. PC, as a plasticizer, was introduced to the reaction medium when all of the PMMA was completely dissolved. The mixture was stirred and heated until the highly conducting transparent gel was produced. 2.8. Construction of electrochromic device P(BTMB) was utilized as the anodically, and PEDOT as the cathodically coloring electrochromic materials. P(BTMB) was potentiostatically deposited on ITO in 0.2 M TBAFB/AN at þ1.8 V. 0.01 M solution of EDOT in 0.2 M TBAFB/AN was used to deposit the PEDOT film onto ITO electrode at þ1.4 V. It is important to balance the charge capacities of the devices prior to assembling the devices. Otherwise, there would be incomplete electrochromic reaction and residual charges would remain during the redox process. Therefore, redox charges of the anodically and cathodically coloring polymers were matched by chronocoulometry. In order to obtain the complementary operating conditions, anodically coloring polymers were in neutral states and the cathodically coloring polymer was fully oxidized (bipolaronic state). By sandwiching the gel electrolyte between the anodically and the cathodically coloring polymers the device was constructed. 3. Results and discussion

and platinum wire as counter electrodes and a silver wire pseudo reference electrode. Measurements were carried out in TBAFB (0.2 M)/AN solventeelectrolyte couple at room temperature under nitrogen atmosphere at 100 mV. The cyclic voltammogram of BTMB in AN revealed an reduction peak at þ0.65 V and two oxidation peaks at þ1.1 V and þ1.6 V (Fig. 2). Increase in the current density with increasing scan number suggests a polymer film on the electrode surface. At potential 1.27 V (a), the electrode has sufficient oxidizing character to oxidize the monomer to its radical cation. The anodic current increases (aeb). Monomer oxidation is immediately followed by chemical coupling that affords oligomers in the vicinity of the electrode. Once these oligomers reach a certain length, they precipitate onto the electrode surface where the chains can continue to grow. The electroactivity of the polymer deposited onto the WE can be monitored by the appearance of a peak corresponding to the reduction of the oxidized polymer while scanning in the cathodic direction (c) at 0.72 V. A second positive scan reveals another oxidation peak at a lower potential than the monomer oxidation peak (d) (nearly 0.87 V). This is due to the neutral polymer now getting oxidized. Another noticeable fact is the increase in polymer oxidation peak current in the second and subsequent scans. As the peak current is directly proportional with the electrode area, this increase in the peak current could be attributed to an increase of the WE area as predicted by RandleseSevcik equation. 3.2. Spectroelectrochemical investigation of in-situ polymerization In situ electropolymerization of BTMB was carried out in a solution containing 0.013 M BTMP in TBAFB (0.2 M)/AN solventeelectrolyte couple by constant potential of 1.8 V. During the electrolysis, UVevis spectra were taken at different time intervals. Fig. 3a shows the visible region of the absorption spectra for the oligomer and the polymer samples deposited for every 10 s time interval. At the beginning of the polymerization, oligomers show two absorption bands, one narrow at w350 nm and the other broad band at 670 nm. Upon electropolymerization the polymers showed an absorption band shift to longer wavelengths (400 and 740 nm). This is attributed to the extension of the p-conjugation as a consequence of polymerization. The absorption spectra of the polymers at different deposition times showed linear increase with time until a certain point (Fig. 3b). After this time linearity continues with a different slope. Linearity change may be due to slower polymerization after a certain time.

3.1. Cyclic voltammetry 3.3. Scan rate dependence of the peak currents The oxidation/reduction behaviors of the monomer BTMB were investigated by CV. The system consisted of a potentiostat and a CV cell containing ITO glass slide (nearly 1 cm2 area) as working

One of the first characteristics of a CV of a conducting polymer (CP) film that one searches for is the dependence of the peak

Fig. 5. Spectroelectrochemical spectra of P(BTMB) film at applied potentials between 0.6 and þ1.4 V and corresponding colors for different applied potentials.

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Fig. 6. Spectroelectrochemical spectra of P(BTMB) device at applied potentials between 2.3 and þ1.5 V and corresponding colors with different applied potentials.

current (ip) on the scan rate (n). According to electrochemical treatments, for a behavior dominated by diffusion effects, ip is proportional to n1/2, whilst for a material localized on the electrode surface, such as a CP film, ip is proportional to n. However, this is so only for CP films that are not inordinately thick, not inordinately compact and not doped with very large or sluggish dopant ions which have inordinately small diffusion coefficients. If any of the latter conditions prevail ip can be proportional to n1/2. BTMB film prepared via constant potential electrolysis (1.8 V) was washed with AN and its cyclic voltammogram in monomerfree electrolyte showed a single, well-defined redox process (Fig 4a). The current response was directly proportional to the scan rate indicating that the polymer film was electroactive and adhered well to the electrode. The scan rate for the anodic and cathodic peak currents shows a linear dependence as a function of the scan rate as illustrated in Fig. 4b. This demonstrates that the electrochemical processes are not diffusion limited and reversible even at very high scan rates. 3.4. Spectroelectrochemistry of P(BTMB) The electrochemical switching between neutral state and oxidized state of conjugated polymers is accompanied by changes in electronic transitions upon doping and dedoping, which is reason why the conjugated polymers are useful in electrochromic applications such as, smart windows, mirrors, etc. Spectra were recorded while the polymer was oxidized by stepwise increasing the potential. To investigate the spectroelectrochemical behavior of the polymer, films were deposited onto ITO-coated glass slides in TBAFB/AN potentiostatically. Spectroelectrochemical properties were studied in a monomer free solution. The lmax value for the pep* transition in the neutral state of polymer was found to be 400 nm, revealing red color. The electronic band gap defined as the onset energy for the pep* transition was found to be 2.41 eV. Upon increase in the applied voltage, a new absorption band at 520 nm was observed due to evolution of charge carriers (polarons), which was accompanied by gradual decrease in the intensity of the bands at lmax. Polymer has green color at moderate potentials and at high doping level (at 1.4 V) the extreme oxidation was achieved and bipolaron bands was observed (at 720 nm), where color of the polymer turned blue. Fig. 5 shows spectroelectrochemical spectra of P(BTMB) film at applied potentials between 0.6 and þ1.4 V and polymer’s color with different applied potentials. The colors of the electrochromic materials were defined accurately by performing colorimetry measurements. The CIE system was used as a quantitative scale to define and compare colors. Three attributes of color, hue (a), saturation (b) and luminance (L), were measured, and these are recorded in Table 1.

3.5. Spectroelectrochemistry of electrochromic devices ECDs While constructing the electrochromic device, the anodically coloring polymer film P(BTMB) was in neutral state and the cathodically coloring polymer PEDOT was fully oxidized (bipolaronic state). Upon application of a voltage, one of the polymer films was oxidized, whereas the other was neutralized resulting in a color change. Device has blue color at negative potentials, red color at positive potentials and green color in moderate potentials. The observed colors with the colorimetry parameters L, a, b values are shown in Table 1. Spectroelectrochemistry experiments were performed to investigate the changes of the electronic transitions of the ECD, with the increase of applied potential. Fig. 6 represents the absorption spectrum of the ECD, recorded by different applied voltages between 2.3 V and 1.5 V. At 2.3 V the PEDOT layer was in its neutral state (blue), where the absorption at 610 nm was due to pep* transition of the PEDOT. Since the oxidized state of P(BTMB) layer is also blue at this potential, device had blue color. As the applied potential was increased the polymer layer started to get neutralized, where the absorption at 400 nm was due to pep* transition of the polymer. Meanwhile, PEDOT layer was in its oxidized state showing no pronounced absorption at the UVevis region of the spectrum, thus the color of the device was red.

Fig. 7. Electrochromic switching, optical transmittance change monitored for P(BTMB) film at 400 and 800 nm.

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polymer of BTMB was synthesized potentiostatically in acetonitrile (AN)/tetrabutylammonium tetrafluoroborate (TBAFB) solventeelectrolyte couple and characterized via CV and UVevis spectroscopy. Spectroelectrochemistry analysis of P(BTMB) reflected electronic transitions at 400 nm, 520 nm and w720 nm, referring to pep* transition, polaron and bipolaron band formation, respectively. Band gap of the P(BTMB) was found as 2.41. Switching ability of the homopolymer was evaluated by a kinetic study upon measuring the %T at the maximum contrast point. We also successfully established the utilization of dual-type complementary colored polymer electrochromic devices using P(BTMB)/poly(3,4ethylenedioxythiophene) (PEDOT) in sandwich configuration. The switching ability and spectroelectrochemical properties of the electrochromic device were investigated by UVevis spectrophotometry and cyclic voltammetry. This device exhibits moderate switching voltages (1.5 to 2.3 V) and reasonable switching times (1.7 s). Device switches between red, green and blue colors with a maximum optical contrast (% DT) of 35%. Fig. 8. Electrochromic switching, optical transmittance change monitored for P(BTMB) device at 605 nm.

3.6. Switching of the P(BTMB) and it’s ECD The switching time is defined as the time elapsed between the highest and lowest transmittance values which was calculated from transmittance changeetime graph. The switching time of the polymer was determined by monitoring the %T change at 400 and 800 nm through switching the applied potential in a square wave form between 0.6 and 1.4 V with a residence time of 5 s. Applied potentials, which corresponds to the extreme states of the polymer, were obtained from the spectroelectrochemistry studies. As seen in Fig. 7, P(BTMB) has 21% and 40% optical contrast values at 400 and 800 nm, respectively, with a 1.4 s switching time. 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. On switching between square wave potentials (þ1.5 V and 2.3 V) with a residence time of 5 s, the optical contrasts (DT %) at 605 and 400 nm were found to be 35% and 18%, respectively, with 1.7 s switching time (Fig. 8). Switching time and optical contrast are comparable with the ones in the literature [20]. 4. Conclusion A new thiophene monomer, 1,2-bis(thiophen-3-ylmethoxy) benzene (BTMB) was synthesized and characterized. Conducting

Acknowledgements One of us (M. Ak) gratefully thanks PAU-BAP(BSP008) project. We also thank TUBA and AU-BAP (09B4240003) support. References [1] S.R. Sivakkumar, P.C. Howlett, B. Winther-Jensen, M. Forsyth, D.R. MacFarlane, Electrochim. Acta 54 (27) (2009) 6844. [2] C. Li, X. Yin, L. Chen, Q. Li, T.J. Wang, Phys. Chem. C 113 (30) (2009) 13438. [3] H.E. Katz, J. Huang, Annu. Rev. Mater. Sci. 39 (2009) 71. [4] J. Janata, M. Josowıcz, Nature 2 (2003) 19. [5] M. Ak, C. Tanyeli, I.M. Akhmedov, L. Toppare, Thin Solid Films 516 (2008) 4334. [6] A. Yildirim, S. Tarkuc, M. Ak, L. Toppare, Electrochim. Acta 53 (2008) 4875. [7] O. Turkarslan, M. Ak, C. Tanyeli, I.M. Akhmedov, L. Toppare, J. Polym. Sci. A: Polym. Chem. 45 (2007) 4496. [8] A. Cihaner, J. Electroanal. Chem. 605 (2007) 8. [9] M. Ak, M.S. Ak, M. Güllü, L. Toppare, Smart Mater. Struct. 16 (6) (2007) 2621. [10] C.G. Granqvist, Sol. Energy Mater. Sol. C 91 (17) (2007) 1529. [11] M. Ak, A. Durmus, L. Toppare, Solid State Sci. 9 (9) (2007) 843. [12] B. Sui, X. Fu, J. Solid State Electrochem. 13 (12) (2009) 1889. [13] S. Varis, M. Ak, I.M. Akhmedov, C. Tanyeli, L. Toppare, Solid State Sci. 8 (12) (2006) 1477. [14] A. Cihaner, F. Algi, Electrochim. Acta 54 (6) (2009) 1702. [15] P. Berdyczko, W. Domagala, A. Czardybon, M. Lapkowski, ). Synthetic Metals 159 (21e22) (2009) 2240. [16] C. Weder, Chem. Commun. (2005) 5378. [17] E. Zhou, Z. Tan, Y. Yang, L. Huo, Y. Zou, C. Yang, Y. Li, Macromolecules 40 (2007) 1831e1837. [18] M. Ak, L. Toppare, Mater. Chem. Phys. 114 (2009) 789. [19] Z.H. Wang, C. Li, E.M. Scherr, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. Lett. 66 (1991) 1745. [20] P.M. Beaujuge, J.R. Reynolds, Chem. Rev. 110 (2010) 268.