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Electrochemical synthesis, characterization and electrochromic properties of a copolymer based on 1,4-bis(2-thienyl)naphthalene and pyrene
Optical Materials 34 (2012) 1095–1101
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Electrochemical synthesis, characterization and electrochromic properties of a copolymer based on 1,4-bis(2-thienyl)naphthalene and pyrene Bin Wang a, Jinsheng Zhao a,⇑, Chuansheng Cui a, Min Wang b, Zhong Wang b, Qingpeng He a a b
Shandong Key Laboratory of Chemical Energy-Storage and Novel Cell Technology, Liaocheng University, 252059 Liaocheng, PR China The Central Laboratory of Liaocheng Hospital, 252000 Liaocheng, PR China
a r t i c l e
i n f o
Article history: Received 22 October 2011 Received in revised form 1 January 2012 Accepted 5 January 2012 Available online 23 January 2012 Keywords: Electrochemical polymerization Conjugated copolymer Spectroelectrochemistry Electrochromic device 1,4-Bis(2-thienyl)naphthalene Pyrene
a b s t r a c t Electrochemical copolymerization of 1,4-bis(2-thienyl)naphthalene (BTN) with pyrene is carried out in acetonitrile (ACN) solution containing sodium perchlorate (NaClO4) as a supporting electrolyte. Characterizations of the resulting copolymer P(BTN-co-pyrene) are performed by cyclic voltammetry (CV), UV–vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The P(BTN-co-pyrene) film has distinct electrochromic properties and exhibits three different colors (yellowish green, green and blue) under various potentials. Maximum contrast (DT%) and response time of the copolymer film are measured as 37.8% and 1.71 s at 687 nm. An electrochromic device (ECD) based on P(BTN-co-pyrene) and poly(3,4-ethylenedioxythiophene) (PEDOT) is constructed and characterized. Neutral state of device shows green color while oxidized state reveals blue color. This ECD shows a maximum optical contrast (DT%) of 24.4% with a response time of 0.43 s at 635 nm. The coloration efficiency (CE) of the device is calculated to be 349 cm2 C1 at 635 nm. In addition, the ECD also has satisfactory optical memories and redox stability. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Electrochromic materials show a reversible optical change ability in absorption or transmittance upon electrochemically oxidized or reduced, which has stimulated the interest of scientists over the past decades [1]. Electrochromic materials exhibit at least two distinct color states and may give multiple colors, depending on the structure of the material. A wide variety of electrochromic materials are presently known, ranging from metal oxides and mixed -valence metal complexes to organic molecules and conjugated polymers [2]. In recent years, electrochromic conjugated polymers have gained a lot of attention due to their several advantages over inorganic compounds, such as low cost, processability, high optical contrast ratio, multi-colors with the same material, high stability and long cycle life with fast response time [3]. To achieve color change in electrochromic polymers, absorption in the visible region should be monitored by means of an externally applied potential. Upon doping, electronic states change due to the formation of polaronic and bipolaronic bands causing a change in the absorption characteristics of the polymer [4,5]. There are numerous applications for electrochromic materials such as optical displays [4], smart windows [6] and electrochromic devices [7–9]. Electrochromic behavior in polymers such as
⇑ Corresponding author. Tel./fax: +86 635 8539607. E-mail address: [email protected] (J. Zhao). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2012.01.009
polypyrrole, polyaniline or polythiophene and their derivatives has been studied deeply. Polypyrrole has been extensively utilized as an electrochromic material and can be easily synthesized chemically or electrochemically with a varying range of optoelectronic properties available through alkyl and alkoxy substitution [4]. Thin films of neat polypyrrole are yellow in the dedoped insulating state and black in the doped conductive state [10]. Polyaniline films are polyelectrochromic materials which exhibit switching among yellow–green–blue and black colors [10]. As a class of excellent electrochromic materials, polythiophenes have occupied prime position because of their high electrical conductivity and good redox property. They exhibit fast response time, outstanding stability and high contrast ratios in the visible and near-infrared regions [11]. Synthesis of new polythiophene derivatives with the ability to tailor the electrochromic properties is an important part of conducting polymer research [12,13]. Among the polythiophenes, the polymers having the structure like thiophene-arylene-thiophene have been synthesized and characterized [8,14]. Recently, our group synthesized the 1,4-bis(2-thienyl)naphthalene (BTN) monomer and studied the electrochromic properties of poly(1,4-bis(2-thienyl)naphthalene) (PBTN), the PBTN film presents reasonable electrochromic properties [15]. On the other hand, pyrene is also an important aromatic monomer, it is interesting to note that fine tuning in the band gap and neutral state color of the polymer can be achieved by copolymerization with pyrene [16,17]. Furthermore, copolymerization offers an effective way of
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controlling the electrochromic properties of conducting polymers. Copolymers can lead to an interesting combination of the properties observed in the corresponding homopolymers [18]. Besides, it is well known that the pursuit of new high-quality electrochromic materials is still the main goal of scientists in the field of electrochromic devices. The neutral green dual type electrochromic devices are desirable [8,19]. According to above considerations, in this work, the electrochemical copolymerization of BTN with pyrene is carried out in 0.2 M NaClO4/ACN solution (Scheme 1). The spectroelectrochemical and electrochromic properties of the P(BTN-co-pyrene) are investigated in details. The copolymer P(BTN-co-pyrene) film has fast response time and high optical contrast when compared with PBTN homopolymer. In addition, we constructed and characterized dual type electrochromic devices based on P(BTN-co-pyrene) and PEDOT in details. The electrochromic device shows green color at neutral state. 2. Experimental 2.1. Materials 1,4-Bis(2-thienyl)naphthalene (BTN) monomer was synthesized as reported previously by our group [15]. Pyrene (Acros Organics, 98%), commercial high-performance liquid chromatography grade acetonitrile (ACN, Tedia Company, Inc., USA), poly(methyl methacrylate) (PMMA, Shanghai Chemical Reagent Company), propylene carbonate (PC, Shanghai Chemical Reagent Company), lithium perchlorate (LiClO4, Shanghai Chemical Reagent Company, 99.9%) and 3,4-ethylenedioxythiophene (EDOT, Aldrich, 98%) are used directly without further purification. Sodium perchlorate (NaClO4, Shanghai Chemical Reagent Company, 98%) is dried in vacuum at 60 °C for 24 h before use. Other reagents are all used as received without further treatment. Indium-tin-oxide-coated (ITO) glass (sheet resistance: <10 X h1, purchased from Shenzhen CSG Display Technologies, China) is washed with ethanol, acetone and deionized water successively under ultrasonic, and then dried by N2 flow. 2.2. Electrochemistry Electrochemical synthesis and experiments are performed in a one-compartment cell with a CHI 760 C Electrochemical Analyzer under computer control, employing a platinum wire with a diameter of 0.5 mm as working electrode, a platinum ring as a counter electrode, and a silver wire (Ag wire) as a pseudo reference electrode. The working and counter electrodes for cyclic voltammetric (CV) experiments are placed 0.5 cm apart during the experiments. All electrochemical polymerization and CV tests are taken in ACN solution containing 0.2 M NaClO4 as a supporting electrolyte. The pseudo reference electrode is calibrated externally using a 5 mM solution of ferrocene (Fc/Fc+) in the electrolyte (E1/2(Fc/Fc+) = 0.20 V vs. Ag wire in 0.2 M NaClO4/ACN) [11] and all the potentials mentioned follow are vs. the Ag wire electrode. The half-wave potential (E1/2) of Fc/Fc+ measured in 0.2 M NaClO4/ACN solution is 0.28 V vs. SCE. Thus, the potential of Ag wire was assumed to be 0.08 V vs. SCE [8]. All of the electrochemical experiments are carried out at room temperature under nitrogen atmosphere.
S S
+
2.3. Characterizations FT-IR spectra are recorded on a Nicolet 5700 FT-IR spectrometer, where the samples are dispersed in KBr pellets. Scanning electron microscopy (SEM) measurements are taken by using a Hitachi SU-70 thermionic field emission SEM. UV–vis spectra are carried out on a Perkin-Elmer Lambda 900 UV–vis–near-infrared spectrophotometer. Digital photographs of the polymer films are taken by a Canon Power Shot A3000 IS digital camera. Colorimetry measurements are obtained by a Coloreye XTH Spectrophotometer (GretagMacbeth). 2.4. Spectroelectrochemistry Spectroelectrochemical data are recorded on Perkin-Elmer Lambda 900 UV–vis–near-infrared spectrophotometer connected to a computer. A three-electrode cell assembly is used where the working electrode is an ITO glass, the counter electrode is a stainless steel wire, and an Ag wire is used as pseudo reference electrode. The polymer films for spectroelectrochemistry are prepared by potentiostatically deposition on ITO electrode (the active area: 0.8 cm 2.0 cm). The measurements are carried out in ACN solution containing 0.2 M NaClO4. 2.5. Preparation of the gel electrolyte A gel electrolyte based on PMMA and LiClO4 is plasticized with PC to form a highly transparent and conductive gel. ACN is also included as a high vapor pressure solvent to allow an easy mixing of the gel components. The composition of the casting solution by weight ratio of ACN:PC:PMMA:LiClO4 is 70:20:7:3. The gel electrolyte is used for construction of the polymer electrochromic device cell [20]. 2.6. Construction of ECDs ECDs are constructed using two complementary polymers, namely P(BTN-co-pyrene) as the anodically coloring material and PEDOT as the cathodically coloring material. Both P(BTN-co-pyrene) and PEDOT films are electrodeposited on two ITO glasses (the active area: 1.8 cm 2.5 cm) at 1.35 and 1.4 V, respectively. Electrochromic device is built by arranging the two polymer films (one oxidized, the other reduced) facing each other separated by a gel electrolyte. 3. Results and discussion 3.1. Electrochemical polymerization and characterization of polymers 3.1.1. Electrochemical polymerization The anodic polarization curves of 0.005 M BTN, 0.005 M pyrene and the BTN/pyrene mixture at 1:1 in 0.2 M NaClO4/ACN solution are shown in Fig. S1 (see Supporting Information). The onset oxidation potential (Epa onset) of BTN in the solution is 1.11 V (Fig. S1, curve a), while that of pyrene is 1.10 V (Fig. S1, curve b). The difference of the onset oxidation potential between BTN and pyrene is 0.01 V, implying that the electrochemical copolymerization is
electrochemical polymerization
S S
m
Scheme 1. Electrochemical copolymerization route of BTN and pyrene.
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readily to be achieved [21]. As can been seen from curve c in Fig. S1, the Epa onset of the BTN/pyrene mixture is 1.08 V, which is lower than those of BTN and pyrene, indicating the existence of the interaction between two monomers in 0.2 M NaClO4/ACN [22]. The successive CV curves of 0.005 M BTN, 0.005 M pyrene and the BTN/pyrene mixture (0.005 M BTN and 0.005 M pyrene) in 0.2 M NaClO4/ACN are illustrated in Fig. 1. As shown in Fig. 1a, the CV curves of BTN show the unsymmetrical redox peaks and the reduction peak potential at 0.86 V, while the corresponding oxidation waves are overlapped with the oxidation waves of the BTN monomer and cannot be observed clearly [23]. The polymerization of pyrene presents the cathodic peak potential at 1.08 V (Fig. 1c). However, the CV curve of the BTN/pyrene mixture exhibits a reduction peak at 0.80 V (Fig. 1b), which is different from those of BTN and pyrene, indicating the formation of a new copolymer consisting of both BTN and pyrene units [24,25]. In addition, as can be seen from Fig. 1, there is an obvious increase of current density of the BTN/pyrene mixture compared with those of BTN and pyrene, which can also imply the formation of a copolymer [25]. 3.1.2. Electrochemistry of the P(BTN-co-pyrene) film Fig. 2a shows the electrochemical behavior of the P(BTN-co-pyrene) film (prepared on platinum wire by sweeping the potentials from 0 to 1.35 V for three cycles) at different scan rates between 25 and 250 mV s1 in 0.2 M NaClO4/ACN. The copolymer film shows a single and well-defined redox process (Fig. 2a). The current response is directly proportional to the scan rate indicating that the copolymer film is electroactive and adheres well to the electrode [23]. The scan rate for the anodic and cathodic peak current densities shows a linear dependence as a function of the scan rate as illustrated in Fig. 2b. This demonstrates that the electrochemical processes of the copolymer are reversible and not diffusion limited [24]. 3.1.3. FT-IR spectra of PBTN, P(BTN-co-pyrene) and polypyrene films The FT-IR spectra of PBTN, P(BTN-co-pyrene) and polypyrene are shown in Fig. S2 (see Supporting Information). These polymers
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are synthesized potentiostatically in the solution of 0.2 M NaClO4/ ACN containing 0.005 M BTN and 0.005 M pyrene monomers or their mixture. According to the spectrum of PBTN (Fig. S2, spectrum a), the band around 1574 cm1 is ascribed to the stretching vibrations of phenylene rings, and the bands at 1506 and 1456 cm1 are due to the stretching vibrations of thiophene rings [15], the strong absorption peak at 797 cm1 is attributed to the out-of-plane bending vibrations of CAH bond in b-position of the 2,5-disubstitued thiophene rings [26] and the 762 cm1 band is assigned to the out-of-plane vibration of the 4 adjacent CAH bonds in the substituted phenylene rings [27]. In the spectrum of polypyrene (Fig. S2, spectrum c), the bands at 1633, 1599 and 1487 cm1 are related to the C@C stretching vibration of pyrene rings, the peaks at 844 and 815 cm1 are assigned to the out-ofplane bending vibration of the CAH bonds of substituted benzene rings, the band at 681 cm1 could be ascribed to the newly formed CAC bond between pyrene units [17,28]. The above mentioned bands of PBTN and polypyrene could also be found in the FT-IR spectrum of P(BTN-co-pyrene) (Fig. S2, spectrum b). Compared with corresponding homopolymers, the band at 800 cm1 and 762 cm1 in the spectrum of P(BTN-co-pyrene) originate from the CAH bond in b-position of the 2,5-disubstitued thiophene rings and the 4 adjacent CAH bonds in the substituted phenylene rings, respectively, indicating the presence of BTN units in the copolymer. While the bands at 844 and 815 cm1 in the copolymer ascribed to the substituted benzene ring in pyrene units can also be found. All the above features indicate that P(BTN-co-pyrene) contains both BTN and pyrene units. 3.1.4. Morphology The morphologies of polymer films are investigated by scanning electron microscopy (SEM). The PBTN, P(BTN-co-pyrene) and polypyrene films are prepared by constant potential electrolysis from the solution of 0.2 M NaClO4/ACN containing relevant monomers on ITO electrodes and dedoped before characterization. The SEM images of these polymer films are shown in Fig. 3. The PBTN
Fig. 1. Successive CV curves of (a) 0.005 M BTN, (b) 0.005 M BTN and 0.005 M pyrene, (c) 0.005 M pyrene in 0.2 M NaClO4/ACN. Scan rates: 100 mV s1. j denotes the current density.
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The UV–vis spectra of the obtained copolymers with 2:1, 1:1 and 1:2 BTN/pyrene feed ratios are also studied (see Supporting Information Fig. S3), the absorption spectra of the obtained copolymers show continuous red-shift and reduction of the band gap as the feed ratio of BTN/pyrene increases. Fine tuning in the band gap is achieved by tailoring the co-monomer feed ratio of copolymerization. Table 1 summarizes the onset oxidation potential (Eonset), maximum absorption wavelength (kmax), low energy absorption edges (konset), HOMO and LUMO energy levels and the optical band gaps (Eg) values of PBTN, polypyrene and the copolymers (prepared with the feed ratio of BTN/pyrene at 2:1 (COP 1), 1:1 (COP 2) and 1:2 (COP 3), respectively) quite clearly. HOMO energy levels of them are calculated by using the formula EHOMO = e(Eonset + 4.4) (vs. SCE) and LUMO energy levels (ELUMO) of them are calculated by the subtraction of the optical band gap from the HOMO levels [30,31]. The Eonset values of the polymers are obtained from their CV curves in monomer-free 0.2 M NaClO4/ACN solution (see Supporting Information Fig. S4). 3.2. Electrochromic properties of P(BTN-co-pyrene) film
film exhibits a porous structure like coral grown with small granules (Fig. 3a) and polypyrene shows an accumulation state of clusters of globules (Fig. 3c). While P(BTN-co-pyrene) shows porous structure with smaller granules grown than that of PBTN (Fig. 3b) and the approximate diameters of these granules are in the range of 150–500 nm, which are moderately different from two corresponding homopolymers. The difference of morphology between P(BTN-co-pyrene) and homopolymers also confirms the occurrence of copolymerization between BTN and pyrene.
3.2.1. Spectroelectrochemical properties of P(BTN-co-pyrene) film Spectroelectrochemistry is the best way of examining the changes in optical properties of a polymer on ITO upon applied potentials [32]. The P(BTN-co-pyrene) film coated ITO (prepared potentiostatically at 1.35 V in 0.2 M NaClO4/ACN solution mixing with 0.005/0.005 M BTN/pyrene) is switched between 0 and 1.30 V in monomer-free 0.2 M NaClO4/ACN solution in order to obtain the in situ UV–vis spectra (Fig. 5). At the neutral state, the copolymer film exhibits an absorption band at 403 nm due to the p–p⁄ transition. As shown in Fig. 5, the intensity of the P(BTNco-pyrene) p–p⁄ electron transition absorption decreases while two charge carrier absorption bands located at around 687 nm and more than 1050 nm increase dramatically upon oxidation. The appearance of charge carrier bands could be attributed to the evolution of polaron and bipolaron bands. While there are many methods to quantify and represent color, one of the most widely applicable method to measure the color of materials illuminated by a standard light source is the 1976 CIE LAB (or L⁄a⁄b⁄) with the value of L⁄ representing how the measured material is light vs. dark, the value a⁄ representing how red vs. green, and b⁄ representing how yellow vs. blue the material is [33]. It is interesting to find that the P(BTN-co-pyrene) film shows a multicolor electrochromism. During the oxidation process, yellowish green color of the film at neutral state (0 V) turns into green color at intermediate doped state (1.10 V), and then into blue color at full doped state (1.30 V). These colors and corresponding L⁄, a⁄, b⁄ values are given in Fig. 6.
3.1.5. Optical properties of polymers The UV–vis spectra of homopolymers and their copolymer deposited on ITO electrode are shown in Fig. 4. In the neutral state, polypyrene film exhibits an absorption band at 357 nm due to the p–p⁄ transition (Fig. 4, spectrum a). As shown by the spectrum b in Fig. 4, the neutral state PBTN exhibits the p–p⁄ electron transition peak at about 397 nm. However, it should be noted that a well-defined maximum absorption band centered at 399 nm is observed in spectrum c of Fig. 4, which is attributed to the p–p⁄ transition of the neutral state P(BTN-co-pyrene) copolymer backbone. Compared to the homopolymers, the P(BTN-co-pyrene) copolymer has a broad absorption band. Besides, the optical band gap (Eg) of polymers are calculated from their low energy absorption edges (konset) (Eg = 1240/konset) [29]. The Eg of the P(BTN-co-pyrene) film is calculated as 2.35 eV, which is lower than those of PBTN (2.48 eV) and polypyrene (2.66 eV).
3.2.2. Electrochromic switching of P(BTN-co-pyrene) film in solution It is important that polymers can switch rapidly and exhibit a noteworthy color change for electrochromic applications [34]. For this purpose, double potential step chronoamperometry technique is used to investigate the switching ability of P(BTN-co-pyrene) film between its neutral and full doped state (Fig. 7). The dynamic electrochromic experiment for P(BTN-co-pyrene) film is carried out at 687 nm. The potential is interchanged between 0 V (the neutral state) and 1.30 V (the oxidized state) at regular intervals of 6 s. One important characteristic of electrochromic materials is the optical contrast (DT%), which can be defined as the transmittance difference between the redox states. The DT% of the P(BTN-co-pyrene) is found to be 37.8% at 687 nm, as shown in Fig. 7. Response time, one of the most important characteristics of electrochromic materials, is the necessary time for 95% of the full optical switch (after which the naked eye could not sense the color change) [35]. The optical response time of P(BTN-co-pyrene) is
Fig. 2. (a) CV curves of the P(BTN-co-pyrene) film at different scan rates between 25 and 250 mV s1 in the monomer-free 0.2 M NaClO4/ACN. (b) Scan rate dependence of the anodic and cathodic peak current densities graph. jp.a and jp.c denote the anodic and cathodic peak current densities, respectively.
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Fig. 3. SEM images of (a) PBTN, (b) P(BTN-co-pyrene) and (c) polypyrene deposited potentiostatically on ITO electrode.
3.3. Spectroelectrochemistry of electrochromic devices (ECDs)
Fig. 4. UV–vis spectra of (a) polypyrene, (b) PBTN and (c) P(BTN-co-pyrene) deposited on ITO at the neutral state.
found to be 1.71 s from the reduced to the oxidized state and 0.59 s from the oxidized to the reduced state at 687 nm. Compared to PBTN homopolymer film, the copolymer P(BTN-co-pyrene) film has high optical contrast and fast response time [15].
3.3.1. Spectroelectrochemical properties of ECD A dual type ECD consisting of P(BTN-co-pyrene) and PEDOT constructed and its spectroelectrochemical behaviors are also studied. Before composing the ECD, the anodically coloring polymer film P(BTN-co-pyrene) is fully reduced and the cathodically coloring polymer PEDOT is fully oxidized. The spectroelectrochemical spectra of the P(BTN-co-pyrene)/PEDOT device as a function of applied potential (between 0.8 V and 1.5 V) are given in Fig. 8. The copolymer is in its neutral state at 0.8 V, where the absorption at 418 nm is due to p–p⁄ transition of the copolymer. At that state, PEDOT does not reveal an obvious absorption at the UV–vis region of the spectrum and the device reveals green color (Fig. 8, Inset). As the applied potential increases, the copolymer layer starts to get oxidized and the intensity of the peak due to the p–p⁄ transition decreased. Meanwhile, PEDOT layer is in its reduced state, which leads to a new absorption at 635 nm due to the reduction of PEDOT, and the dominated color of the device is blue at 1.5 V (Fig. 8, Inset). 3.3.2. Switching of ECD Kinetic studies are also done to test the response time of P(BTNco-pyrene)/PEDOT ECD. Under a potential input of 0.8 and 1.5 V at regular intervals of 2 s, the optical response at 635 nm, as illustrated in Fig. 9. The response time is found to be 0.43 s at 95% of the
Table 1 The onset oxidation potential (Eonset), maximum absorption wavelength (kmax), low energy absorption edges (konset), HOMO and LUMO energy levels and optical band gap (Eg) values of PBTN, polypyrene and the copolymers (prepared with the feed ratio of BTN/pyrene at 2:1, 1:1 and 1:2, respectively).
a b
Compounds
Eonset, vs. (Ag-wire) (V)
kmax (nm)/konset (nm)
Ega (eV)
HOMO (eV)
LUMOb (eV)
PBTN COP 1 COP 2 COP 3 Polypyrene
0.86 0.80 0.87 0.92 0.93
397/501 405/541 399/528 390/518 357/466
2.48 2.29 2.35 2.39 2.66
5.34 5.28 5.35 5.40 5.41
2.86 2.99 3.00 3.01 2.75
Calculated from the low energy absorption edges (konset), Eg = 1240/konset. Calculated by the subtraction of the optical band gap from the HOMO level.
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Fig. 5. Spectroelectrochemical spectra of P(BTN-co-pyrene) films on ITO electrode as applied potentials between 0 V and 1.30 V in monomer-free 0.2 M NaClO4/ACN solution.
Fig. 8. Spectroelectrochemical spectra of the P(BTN-co-pyrene)/PEDOT device at various applied potentials between 0.8 and 1.5 V. Inset: Colors of the P(BTN-copyrene)/PEDOT device at 0.8 V (the neutral state) and 1.5 V (the oxidized state).
Fig. 6. Colors and corresponding L⁄, a⁄, b⁄ values of the P(BTN-co-pyrene) film at different applied potentials.
Fig. 9. Electrochromic switching, optical transmittance change monitored at 635 nm for P(BTN-co-pyrene)/PEDOT device between 0.8 V and 1.5 V with a residence time of 2 s.
where Tb and Tc are the transmittances before and after coloration, respectively. DOD is the change of the optical density, which is proportional to the amount of created color centers. g denotes the coloration efficiency (CE). DQ is the amount of injected charge per unit sample area. The CE of the P(BTN-co-pyrene)/PEDOT device (the active of area is 1.8 cm 2.0 cm) is calculated to be 349 cm2 C1 at 635 nm.
Fig. 7. Electrochromic switching response for P(BTN-co-pyrene) film monitored at 687 nm in 0.2 M NaClO4/ACN solution between 0 V and 1.30 V with a residence time of 6 s.
maximum transmittance difference from the neutral state to oxidized state and 0.27 s from the oxidized state to the neutral state, and optical contrast (DT%) is calculated to be 24.4%. The P(BTN-copyrene)/PEDOT device has fast response time (0.43 s) and high optical contrast (24.4%) when compared with the PBTN/PEDOT device which has 0.57 s and 10% response time and optical contrast, respectively [15]. The coloration efficiency (CE) is one of the most important parameter of the electrochromic device. The CE can be calculated by using the equations and given below [36]:
Tb DOD and g ¼ DOD ¼ lg Tc DQ
3.3.3. Open circuit memory of ECD The optical memory in the electrochromic devices is an important parameter because it is directly related to its application and energy consumption during the use of ECDs [24]. The optical spectrum for P(BTN-co-pyrene)/PEDOT device is monitored at 635 nm as a function of time at 0.8 V and 1.5 V by applying the potential for 1 s for each 200 s time interval. As shown in Fig. 10, at green colored state device shows a true permanent memory effect since there is almost no transmittance change under applied potential or open circuit conditions. In blue colored state device is rather less stable in terms of color persistence, however this matter can be overcome by application of current pulses to freshen the fully colored states. 3.3.4. Stability of ECD The stability of the devices toward multiple redox switching usually limits the utility of electrochromic materials in ECD appli-
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and 0.43 s at 635 nm, respectively. The coloration efficiency (CE) of the device is calculated to be 349 cm2 C1 at 635 nm. In addition, the device shows satisfactory optical memories and redox stability. Considering these results, P(BTN-co-pyrene) is a promising candidate for electrochromic layers in ECDs. Acknowledgements The work was financially support by the National Natural Science Foundation of China (No. 20906043), the Promotive research fund for young and middle-aged scientists of Shandong Province (2009BSB01453), the Natural Science Foundation of Shandong province (ZR2010BQ009) and the Taishan Scholarship of Shandong Province.
Fig. 10. Open circuit stability of the P(BTN-co-pyrene)/PEDOT device monitored at 635 nm.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.optmat.2012.01.009. References
Fig. 11. Cyclic voltammogram of P(BTN-co-pyrene)/PEDOT device as a function of repeated with a scan rate of 500 mV/s.
cations. Therefore, redox stability is another important parameter for ECD [34]. For this reason, the P(BTN-co-pyrene)/PEDOT device is tested by cyclic voltammetry of the applied potential between 0.8 and 1.4 V with 500 mV/s to evaluate the stability of the device (Fig. 11). After 500 cycles, 86.8% of its electroactivity is retained and there is no obvious decrease of activity between 500 cycles and 1000 cycles. These results show that this device has reasonable redox stability. 4. Conclusions A new copolymer based on BTN and pyrene is electrochemically synthesized and characterized. At the neutral state of the copolymer, the p–p⁄ transition absorption peak is located at 399 nm and the optical band gap is calculated as 2.35 eV. P(BTN-co-pyrene) copolymer film has distinct electrochromic properties and shows three different colors (yellowish green, green and blue) under various potentials. The copolymer film shows a maximum optical contrast (DT%) of 37.8% and a response time of 1.71 s at 687 nm which are higher and faster than those of the homopolymer of BTN (PBTN, 24% and 1.78 s). Further, an ECD is also assembled from P(BTN-copyrene) in the sandwich configuration, ITO/P(BTN-co-pyrene)//gel electrolyte//PEDOT/ITO. The color of the constructed device switched between green and blue on the application of potential between 0.8 and 1.5 V. Electrochromic switching study results show that optical contrast (DT%) and response time are 24.4%
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Electrochemical synthesis, characterization and electrochromic properties of a copolymer based on 1,4-bis(2-thienyl)naphthalene and pyrene Bin Wang a, Jinsheng Zhao a,⇑, Chuansheng Cui a, Min Wang b, Zhong Wang b, Qingpeng He a a b
Shandong Key Laboratory of Chemical Energy-Storage and Novel Cell Technology, Liaocheng University, 252059 Liaocheng, PR China The Central Laboratory of Liaocheng Hospital, 252000 Liaocheng, PR China
a r t i c l e
i n f o
Article history: Received 22 October 2011 Received in revised form 1 January 2012 Accepted 5 January 2012 Available online 23 January 2012 Keywords: Electrochemical polymerization Conjugated copolymer Spectroelectrochemistry Electrochromic device 1,4-Bis(2-thienyl)naphthalene Pyrene
a b s t r a c t Electrochemical copolymerization of 1,4-bis(2-thienyl)naphthalene (BTN) with pyrene is carried out in acetonitrile (ACN) solution containing sodium perchlorate (NaClO4) as a supporting electrolyte. Characterizations of the resulting copolymer P(BTN-co-pyrene) are performed by cyclic voltammetry (CV), UV–vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The P(BTN-co-pyrene) film has distinct electrochromic properties and exhibits three different colors (yellowish green, green and blue) under various potentials. Maximum contrast (DT%) and response time of the copolymer film are measured as 37.8% and 1.71 s at 687 nm. An electrochromic device (ECD) based on P(BTN-co-pyrene) and poly(3,4-ethylenedioxythiophene) (PEDOT) is constructed and characterized. Neutral state of device shows green color while oxidized state reveals blue color. This ECD shows a maximum optical contrast (DT%) of 24.4% with a response time of 0.43 s at 635 nm. The coloration efficiency (CE) of the device is calculated to be 349 cm2 C1 at 635 nm. In addition, the ECD also has satisfactory optical memories and redox stability. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Electrochromic materials show a reversible optical change ability in absorption or transmittance upon electrochemically oxidized or reduced, which has stimulated the interest of scientists over the past decades [1]. Electrochromic materials exhibit at least two distinct color states and may give multiple colors, depending on the structure of the material. A wide variety of electrochromic materials are presently known, ranging from metal oxides and mixed -valence metal complexes to organic molecules and conjugated polymers [2]. In recent years, electrochromic conjugated polymers have gained a lot of attention due to their several advantages over inorganic compounds, such as low cost, processability, high optical contrast ratio, multi-colors with the same material, high stability and long cycle life with fast response time [3]. To achieve color change in electrochromic polymers, absorption in the visible region should be monitored by means of an externally applied potential. Upon doping, electronic states change due to the formation of polaronic and bipolaronic bands causing a change in the absorption characteristics of the polymer [4,5]. There are numerous applications for electrochromic materials such as optical displays [4], smart windows [6] and electrochromic devices [7–9]. Electrochromic behavior in polymers such as
⇑ Corresponding author. Tel./fax: +86 635 8539607. E-mail address: [email protected] (J. Zhao). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2012.01.009
polypyrrole, polyaniline or polythiophene and their derivatives has been studied deeply. Polypyrrole has been extensively utilized as an electrochromic material and can be easily synthesized chemically or electrochemically with a varying range of optoelectronic properties available through alkyl and alkoxy substitution [4]. Thin films of neat polypyrrole are yellow in the dedoped insulating state and black in the doped conductive state [10]. Polyaniline films are polyelectrochromic materials which exhibit switching among yellow–green–blue and black colors [10]. As a class of excellent electrochromic materials, polythiophenes have occupied prime position because of their high electrical conductivity and good redox property. They exhibit fast response time, outstanding stability and high contrast ratios in the visible and near-infrared regions [11]. Synthesis of new polythiophene derivatives with the ability to tailor the electrochromic properties is an important part of conducting polymer research [12,13]. Among the polythiophenes, the polymers having the structure like thiophene-arylene-thiophene have been synthesized and characterized [8,14]. Recently, our group synthesized the 1,4-bis(2-thienyl)naphthalene (BTN) monomer and studied the electrochromic properties of poly(1,4-bis(2-thienyl)naphthalene) (PBTN), the PBTN film presents reasonable electrochromic properties [15]. On the other hand, pyrene is also an important aromatic monomer, it is interesting to note that fine tuning in the band gap and neutral state color of the polymer can be achieved by copolymerization with pyrene [16,17]. Furthermore, copolymerization offers an effective way of
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controlling the electrochromic properties of conducting polymers. Copolymers can lead to an interesting combination of the properties observed in the corresponding homopolymers [18]. Besides, it is well known that the pursuit of new high-quality electrochromic materials is still the main goal of scientists in the field of electrochromic devices. The neutral green dual type electrochromic devices are desirable [8,19]. According to above considerations, in this work, the electrochemical copolymerization of BTN with pyrene is carried out in 0.2 M NaClO4/ACN solution (Scheme 1). The spectroelectrochemical and electrochromic properties of the P(BTN-co-pyrene) are investigated in details. The copolymer P(BTN-co-pyrene) film has fast response time and high optical contrast when compared with PBTN homopolymer. In addition, we constructed and characterized dual type electrochromic devices based on P(BTN-co-pyrene) and PEDOT in details. The electrochromic device shows green color at neutral state. 2. Experimental 2.1. Materials 1,4-Bis(2-thienyl)naphthalene (BTN) monomer was synthesized as reported previously by our group [15]. Pyrene (Acros Organics, 98%), commercial high-performance liquid chromatography grade acetonitrile (ACN, Tedia Company, Inc., USA), poly(methyl methacrylate) (PMMA, Shanghai Chemical Reagent Company), propylene carbonate (PC, Shanghai Chemical Reagent Company), lithium perchlorate (LiClO4, Shanghai Chemical Reagent Company, 99.9%) and 3,4-ethylenedioxythiophene (EDOT, Aldrich, 98%) are used directly without further purification. Sodium perchlorate (NaClO4, Shanghai Chemical Reagent Company, 98%) is dried in vacuum at 60 °C for 24 h before use. Other reagents are all used as received without further treatment. Indium-tin-oxide-coated (ITO) glass (sheet resistance: <10 X h1, purchased from Shenzhen CSG Display Technologies, China) is washed with ethanol, acetone and deionized water successively under ultrasonic, and then dried by N2 flow. 2.2. Electrochemistry Electrochemical synthesis and experiments are performed in a one-compartment cell with a CHI 760 C Electrochemical Analyzer under computer control, employing a platinum wire with a diameter of 0.5 mm as working electrode, a platinum ring as a counter electrode, and a silver wire (Ag wire) as a pseudo reference electrode. The working and counter electrodes for cyclic voltammetric (CV) experiments are placed 0.5 cm apart during the experiments. All electrochemical polymerization and CV tests are taken in ACN solution containing 0.2 M NaClO4 as a supporting electrolyte. The pseudo reference electrode is calibrated externally using a 5 mM solution of ferrocene (Fc/Fc+) in the electrolyte (E1/2(Fc/Fc+) = 0.20 V vs. Ag wire in 0.2 M NaClO4/ACN) [11] and all the potentials mentioned follow are vs. the Ag wire electrode. The half-wave potential (E1/2) of Fc/Fc+ measured in 0.2 M NaClO4/ACN solution is 0.28 V vs. SCE. Thus, the potential of Ag wire was assumed to be 0.08 V vs. SCE [8]. All of the electrochemical experiments are carried out at room temperature under nitrogen atmosphere.
S S
+
2.3. Characterizations FT-IR spectra are recorded on a Nicolet 5700 FT-IR spectrometer, where the samples are dispersed in KBr pellets. Scanning electron microscopy (SEM) measurements are taken by using a Hitachi SU-70 thermionic field emission SEM. UV–vis spectra are carried out on a Perkin-Elmer Lambda 900 UV–vis–near-infrared spectrophotometer. Digital photographs of the polymer films are taken by a Canon Power Shot A3000 IS digital camera. Colorimetry measurements are obtained by a Coloreye XTH Spectrophotometer (GretagMacbeth). 2.4. Spectroelectrochemistry Spectroelectrochemical data are recorded on Perkin-Elmer Lambda 900 UV–vis–near-infrared spectrophotometer connected to a computer. A three-electrode cell assembly is used where the working electrode is an ITO glass, the counter electrode is a stainless steel wire, and an Ag wire is used as pseudo reference electrode. The polymer films for spectroelectrochemistry are prepared by potentiostatically deposition on ITO electrode (the active area: 0.8 cm 2.0 cm). The measurements are carried out in ACN solution containing 0.2 M NaClO4. 2.5. Preparation of the gel electrolyte A gel electrolyte based on PMMA and LiClO4 is plasticized with PC to form a highly transparent and conductive gel. ACN is also included as a high vapor pressure solvent to allow an easy mixing of the gel components. The composition of the casting solution by weight ratio of ACN:PC:PMMA:LiClO4 is 70:20:7:3. The gel electrolyte is used for construction of the polymer electrochromic device cell [20]. 2.6. Construction of ECDs ECDs are constructed using two complementary polymers, namely P(BTN-co-pyrene) as the anodically coloring material and PEDOT as the cathodically coloring material. Both P(BTN-co-pyrene) and PEDOT films are electrodeposited on two ITO glasses (the active area: 1.8 cm 2.5 cm) at 1.35 and 1.4 V, respectively. Electrochromic device is built by arranging the two polymer films (one oxidized, the other reduced) facing each other separated by a gel electrolyte. 3. Results and discussion 3.1. Electrochemical polymerization and characterization of polymers 3.1.1. Electrochemical polymerization The anodic polarization curves of 0.005 M BTN, 0.005 M pyrene and the BTN/pyrene mixture at 1:1 in 0.2 M NaClO4/ACN solution are shown in Fig. S1 (see Supporting Information). The onset oxidation potential (Epa onset) of BTN in the solution is 1.11 V (Fig. S1, curve a), while that of pyrene is 1.10 V (Fig. S1, curve b). The difference of the onset oxidation potential between BTN and pyrene is 0.01 V, implying that the electrochemical copolymerization is
electrochemical polymerization
S S
m
Scheme 1. Electrochemical copolymerization route of BTN and pyrene.
n
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readily to be achieved [21]. As can been seen from curve c in Fig. S1, the Epa onset of the BTN/pyrene mixture is 1.08 V, which is lower than those of BTN and pyrene, indicating the existence of the interaction between two monomers in 0.2 M NaClO4/ACN [22]. The successive CV curves of 0.005 M BTN, 0.005 M pyrene and the BTN/pyrene mixture (0.005 M BTN and 0.005 M pyrene) in 0.2 M NaClO4/ACN are illustrated in Fig. 1. As shown in Fig. 1a, the CV curves of BTN show the unsymmetrical redox peaks and the reduction peak potential at 0.86 V, while the corresponding oxidation waves are overlapped with the oxidation waves of the BTN monomer and cannot be observed clearly [23]. The polymerization of pyrene presents the cathodic peak potential at 1.08 V (Fig. 1c). However, the CV curve of the BTN/pyrene mixture exhibits a reduction peak at 0.80 V (Fig. 1b), which is different from those of BTN and pyrene, indicating the formation of a new copolymer consisting of both BTN and pyrene units [24,25]. In addition, as can be seen from Fig. 1, there is an obvious increase of current density of the BTN/pyrene mixture compared with those of BTN and pyrene, which can also imply the formation of a copolymer [25]. 3.1.2. Electrochemistry of the P(BTN-co-pyrene) film Fig. 2a shows the electrochemical behavior of the P(BTN-co-pyrene) film (prepared on platinum wire by sweeping the potentials from 0 to 1.35 V for three cycles) at different scan rates between 25 and 250 mV s1 in 0.2 M NaClO4/ACN. The copolymer film shows a single and well-defined redox process (Fig. 2a). The current response is directly proportional to the scan rate indicating that the copolymer film is electroactive and adheres well to the electrode [23]. The scan rate for the anodic and cathodic peak current densities shows a linear dependence as a function of the scan rate as illustrated in Fig. 2b. This demonstrates that the electrochemical processes of the copolymer are reversible and not diffusion limited [24]. 3.1.3. FT-IR spectra of PBTN, P(BTN-co-pyrene) and polypyrene films The FT-IR spectra of PBTN, P(BTN-co-pyrene) and polypyrene are shown in Fig. S2 (see Supporting Information). These polymers
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are synthesized potentiostatically in the solution of 0.2 M NaClO4/ ACN containing 0.005 M BTN and 0.005 M pyrene monomers or their mixture. According to the spectrum of PBTN (Fig. S2, spectrum a), the band around 1574 cm1 is ascribed to the stretching vibrations of phenylene rings, and the bands at 1506 and 1456 cm1 are due to the stretching vibrations of thiophene rings [15], the strong absorption peak at 797 cm1 is attributed to the out-of-plane bending vibrations of CAH bond in b-position of the 2,5-disubstitued thiophene rings [26] and the 762 cm1 band is assigned to the out-of-plane vibration of the 4 adjacent CAH bonds in the substituted phenylene rings [27]. In the spectrum of polypyrene (Fig. S2, spectrum c), the bands at 1633, 1599 and 1487 cm1 are related to the C@C stretching vibration of pyrene rings, the peaks at 844 and 815 cm1 are assigned to the out-ofplane bending vibration of the CAH bonds of substituted benzene rings, the band at 681 cm1 could be ascribed to the newly formed CAC bond between pyrene units [17,28]. The above mentioned bands of PBTN and polypyrene could also be found in the FT-IR spectrum of P(BTN-co-pyrene) (Fig. S2, spectrum b). Compared with corresponding homopolymers, the band at 800 cm1 and 762 cm1 in the spectrum of P(BTN-co-pyrene) originate from the CAH bond in b-position of the 2,5-disubstitued thiophene rings and the 4 adjacent CAH bonds in the substituted phenylene rings, respectively, indicating the presence of BTN units in the copolymer. While the bands at 844 and 815 cm1 in the copolymer ascribed to the substituted benzene ring in pyrene units can also be found. All the above features indicate that P(BTN-co-pyrene) contains both BTN and pyrene units. 3.1.4. Morphology The morphologies of polymer films are investigated by scanning electron microscopy (SEM). The PBTN, P(BTN-co-pyrene) and polypyrene films are prepared by constant potential electrolysis from the solution of 0.2 M NaClO4/ACN containing relevant monomers on ITO electrodes and dedoped before characterization. The SEM images of these polymer films are shown in Fig. 3. The PBTN
Fig. 1. Successive CV curves of (a) 0.005 M BTN, (b) 0.005 M BTN and 0.005 M pyrene, (c) 0.005 M pyrene in 0.2 M NaClO4/ACN. Scan rates: 100 mV s1. j denotes the current density.
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The UV–vis spectra of the obtained copolymers with 2:1, 1:1 and 1:2 BTN/pyrene feed ratios are also studied (see Supporting Information Fig. S3), the absorption spectra of the obtained copolymers show continuous red-shift and reduction of the band gap as the feed ratio of BTN/pyrene increases. Fine tuning in the band gap is achieved by tailoring the co-monomer feed ratio of copolymerization. Table 1 summarizes the onset oxidation potential (Eonset), maximum absorption wavelength (kmax), low energy absorption edges (konset), HOMO and LUMO energy levels and the optical band gaps (Eg) values of PBTN, polypyrene and the copolymers (prepared with the feed ratio of BTN/pyrene at 2:1 (COP 1), 1:1 (COP 2) and 1:2 (COP 3), respectively) quite clearly. HOMO energy levels of them are calculated by using the formula EHOMO = e(Eonset + 4.4) (vs. SCE) and LUMO energy levels (ELUMO) of them are calculated by the subtraction of the optical band gap from the HOMO levels [30,31]. The Eonset values of the polymers are obtained from their CV curves in monomer-free 0.2 M NaClO4/ACN solution (see Supporting Information Fig. S4). 3.2. Electrochromic properties of P(BTN-co-pyrene) film
film exhibits a porous structure like coral grown with small granules (Fig. 3a) and polypyrene shows an accumulation state of clusters of globules (Fig. 3c). While P(BTN-co-pyrene) shows porous structure with smaller granules grown than that of PBTN (Fig. 3b) and the approximate diameters of these granules are in the range of 150–500 nm, which are moderately different from two corresponding homopolymers. The difference of morphology between P(BTN-co-pyrene) and homopolymers also confirms the occurrence of copolymerization between BTN and pyrene.
3.2.1. Spectroelectrochemical properties of P(BTN-co-pyrene) film Spectroelectrochemistry is the best way of examining the changes in optical properties of a polymer on ITO upon applied potentials [32]. The P(BTN-co-pyrene) film coated ITO (prepared potentiostatically at 1.35 V in 0.2 M NaClO4/ACN solution mixing with 0.005/0.005 M BTN/pyrene) is switched between 0 and 1.30 V in monomer-free 0.2 M NaClO4/ACN solution in order to obtain the in situ UV–vis spectra (Fig. 5). At the neutral state, the copolymer film exhibits an absorption band at 403 nm due to the p–p⁄ transition. As shown in Fig. 5, the intensity of the P(BTNco-pyrene) p–p⁄ electron transition absorption decreases while two charge carrier absorption bands located at around 687 nm and more than 1050 nm increase dramatically upon oxidation. The appearance of charge carrier bands could be attributed to the evolution of polaron and bipolaron bands. While there are many methods to quantify and represent color, one of the most widely applicable method to measure the color of materials illuminated by a standard light source is the 1976 CIE LAB (or L⁄a⁄b⁄) with the value of L⁄ representing how the measured material is light vs. dark, the value a⁄ representing how red vs. green, and b⁄ representing how yellow vs. blue the material is [33]. It is interesting to find that the P(BTN-co-pyrene) film shows a multicolor electrochromism. During the oxidation process, yellowish green color of the film at neutral state (0 V) turns into green color at intermediate doped state (1.10 V), and then into blue color at full doped state (1.30 V). These colors and corresponding L⁄, a⁄, b⁄ values are given in Fig. 6.
3.1.5. Optical properties of polymers The UV–vis spectra of homopolymers and their copolymer deposited on ITO electrode are shown in Fig. 4. In the neutral state, polypyrene film exhibits an absorption band at 357 nm due to the p–p⁄ transition (Fig. 4, spectrum a). As shown by the spectrum b in Fig. 4, the neutral state PBTN exhibits the p–p⁄ electron transition peak at about 397 nm. However, it should be noted that a well-defined maximum absorption band centered at 399 nm is observed in spectrum c of Fig. 4, which is attributed to the p–p⁄ transition of the neutral state P(BTN-co-pyrene) copolymer backbone. Compared to the homopolymers, the P(BTN-co-pyrene) copolymer has a broad absorption band. Besides, the optical band gap (Eg) of polymers are calculated from their low energy absorption edges (konset) (Eg = 1240/konset) [29]. The Eg of the P(BTN-co-pyrene) film is calculated as 2.35 eV, which is lower than those of PBTN (2.48 eV) and polypyrene (2.66 eV).
3.2.2. Electrochromic switching of P(BTN-co-pyrene) film in solution It is important that polymers can switch rapidly and exhibit a noteworthy color change for electrochromic applications [34]. For this purpose, double potential step chronoamperometry technique is used to investigate the switching ability of P(BTN-co-pyrene) film between its neutral and full doped state (Fig. 7). The dynamic electrochromic experiment for P(BTN-co-pyrene) film is carried out at 687 nm. The potential is interchanged between 0 V (the neutral state) and 1.30 V (the oxidized state) at regular intervals of 6 s. One important characteristic of electrochromic materials is the optical contrast (DT%), which can be defined as the transmittance difference between the redox states. The DT% of the P(BTN-co-pyrene) is found to be 37.8% at 687 nm, as shown in Fig. 7. Response time, one of the most important characteristics of electrochromic materials, is the necessary time for 95% of the full optical switch (after which the naked eye could not sense the color change) [35]. The optical response time of P(BTN-co-pyrene) is
Fig. 2. (a) CV curves of the P(BTN-co-pyrene) film at different scan rates between 25 and 250 mV s1 in the monomer-free 0.2 M NaClO4/ACN. (b) Scan rate dependence of the anodic and cathodic peak current densities graph. jp.a and jp.c denote the anodic and cathodic peak current densities, respectively.
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Fig. 3. SEM images of (a) PBTN, (b) P(BTN-co-pyrene) and (c) polypyrene deposited potentiostatically on ITO electrode.
3.3. Spectroelectrochemistry of electrochromic devices (ECDs)
Fig. 4. UV–vis spectra of (a) polypyrene, (b) PBTN and (c) P(BTN-co-pyrene) deposited on ITO at the neutral state.
found to be 1.71 s from the reduced to the oxidized state and 0.59 s from the oxidized to the reduced state at 687 nm. Compared to PBTN homopolymer film, the copolymer P(BTN-co-pyrene) film has high optical contrast and fast response time [15].
3.3.1. Spectroelectrochemical properties of ECD A dual type ECD consisting of P(BTN-co-pyrene) and PEDOT constructed and its spectroelectrochemical behaviors are also studied. Before composing the ECD, the anodically coloring polymer film P(BTN-co-pyrene) is fully reduced and the cathodically coloring polymer PEDOT is fully oxidized. The spectroelectrochemical spectra of the P(BTN-co-pyrene)/PEDOT device as a function of applied potential (between 0.8 V and 1.5 V) are given in Fig. 8. The copolymer is in its neutral state at 0.8 V, where the absorption at 418 nm is due to p–p⁄ transition of the copolymer. At that state, PEDOT does not reveal an obvious absorption at the UV–vis region of the spectrum and the device reveals green color (Fig. 8, Inset). As the applied potential increases, the copolymer layer starts to get oxidized and the intensity of the peak due to the p–p⁄ transition decreased. Meanwhile, PEDOT layer is in its reduced state, which leads to a new absorption at 635 nm due to the reduction of PEDOT, and the dominated color of the device is blue at 1.5 V (Fig. 8, Inset). 3.3.2. Switching of ECD Kinetic studies are also done to test the response time of P(BTNco-pyrene)/PEDOT ECD. Under a potential input of 0.8 and 1.5 V at regular intervals of 2 s, the optical response at 635 nm, as illustrated in Fig. 9. The response time is found to be 0.43 s at 95% of the
Table 1 The onset oxidation potential (Eonset), maximum absorption wavelength (kmax), low energy absorption edges (konset), HOMO and LUMO energy levels and optical band gap (Eg) values of PBTN, polypyrene and the copolymers (prepared with the feed ratio of BTN/pyrene at 2:1, 1:1 and 1:2, respectively).
a b
Compounds
Eonset, vs. (Ag-wire) (V)
kmax (nm)/konset (nm)
Ega (eV)
HOMO (eV)
LUMOb (eV)
PBTN COP 1 COP 2 COP 3 Polypyrene
0.86 0.80 0.87 0.92 0.93
397/501 405/541 399/528 390/518 357/466
2.48 2.29 2.35 2.39 2.66
5.34 5.28 5.35 5.40 5.41
2.86 2.99 3.00 3.01 2.75
Calculated from the low energy absorption edges (konset), Eg = 1240/konset. Calculated by the subtraction of the optical band gap from the HOMO level.
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Fig. 5. Spectroelectrochemical spectra of P(BTN-co-pyrene) films on ITO electrode as applied potentials between 0 V and 1.30 V in monomer-free 0.2 M NaClO4/ACN solution.
Fig. 8. Spectroelectrochemical spectra of the P(BTN-co-pyrene)/PEDOT device at various applied potentials between 0.8 and 1.5 V. Inset: Colors of the P(BTN-copyrene)/PEDOT device at 0.8 V (the neutral state) and 1.5 V (the oxidized state).
Fig. 6. Colors and corresponding L⁄, a⁄, b⁄ values of the P(BTN-co-pyrene) film at different applied potentials.
Fig. 9. Electrochromic switching, optical transmittance change monitored at 635 nm for P(BTN-co-pyrene)/PEDOT device between 0.8 V and 1.5 V with a residence time of 2 s.
where Tb and Tc are the transmittances before and after coloration, respectively. DOD is the change of the optical density, which is proportional to the amount of created color centers. g denotes the coloration efficiency (CE). DQ is the amount of injected charge per unit sample area. The CE of the P(BTN-co-pyrene)/PEDOT device (the active of area is 1.8 cm 2.0 cm) is calculated to be 349 cm2 C1 at 635 nm.
Fig. 7. Electrochromic switching response for P(BTN-co-pyrene) film monitored at 687 nm in 0.2 M NaClO4/ACN solution between 0 V and 1.30 V with a residence time of 6 s.
maximum transmittance difference from the neutral state to oxidized state and 0.27 s from the oxidized state to the neutral state, and optical contrast (DT%) is calculated to be 24.4%. The P(BTN-copyrene)/PEDOT device has fast response time (0.43 s) and high optical contrast (24.4%) when compared with the PBTN/PEDOT device which has 0.57 s and 10% response time and optical contrast, respectively [15]. The coloration efficiency (CE) is one of the most important parameter of the electrochromic device. The CE can be calculated by using the equations and given below [36]:
Tb DOD and g ¼ DOD ¼ lg Tc DQ
3.3.3. Open circuit memory of ECD The optical memory in the electrochromic devices is an important parameter because it is directly related to its application and energy consumption during the use of ECDs [24]. The optical spectrum for P(BTN-co-pyrene)/PEDOT device is monitored at 635 nm as a function of time at 0.8 V and 1.5 V by applying the potential for 1 s for each 200 s time interval. As shown in Fig. 10, at green colored state device shows a true permanent memory effect since there is almost no transmittance change under applied potential or open circuit conditions. In blue colored state device is rather less stable in terms of color persistence, however this matter can be overcome by application of current pulses to freshen the fully colored states. 3.3.4. Stability of ECD The stability of the devices toward multiple redox switching usually limits the utility of electrochromic materials in ECD appli-
B. Wang et al. / Optical Materials 34 (2012) 1095–1101
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and 0.43 s at 635 nm, respectively. The coloration efficiency (CE) of the device is calculated to be 349 cm2 C1 at 635 nm. In addition, the device shows satisfactory optical memories and redox stability. Considering these results, P(BTN-co-pyrene) is a promising candidate for electrochromic layers in ECDs. Acknowledgements The work was financially support by the National Natural Science Foundation of China (No. 20906043), the Promotive research fund for young and middle-aged scientists of Shandong Province (2009BSB01453), the Natural Science Foundation of Shandong province (ZR2010BQ009) and the Taishan Scholarship of Shandong Province.
Fig. 10. Open circuit stability of the P(BTN-co-pyrene)/PEDOT device monitored at 635 nm.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.optmat.2012.01.009. References
Fig. 11. Cyclic voltammogram of P(BTN-co-pyrene)/PEDOT device as a function of repeated with a scan rate of 500 mV/s.
cations. Therefore, redox stability is another important parameter for ECD [34]. For this reason, the P(BTN-co-pyrene)/PEDOT device is tested by cyclic voltammetry of the applied potential between 0.8 and 1.4 V with 500 mV/s to evaluate the stability of the device (Fig. 11). After 500 cycles, 86.8% of its electroactivity is retained and there is no obvious decrease of activity between 500 cycles and 1000 cycles. These results show that this device has reasonable redox stability. 4. Conclusions A new copolymer based on BTN and pyrene is electrochemically synthesized and characterized. At the neutral state of the copolymer, the p–p⁄ transition absorption peak is located at 399 nm and the optical band gap is calculated as 2.35 eV. P(BTN-co-pyrene) copolymer film has distinct electrochromic properties and shows three different colors (yellowish green, green and blue) under various potentials. The copolymer film shows a maximum optical contrast (DT%) of 37.8% and a response time of 1.71 s at 687 nm which are higher and faster than those of the homopolymer of BTN (PBTN, 24% and 1.78 s). Further, an ECD is also assembled from P(BTN-copyrene) in the sandwich configuration, ITO/P(BTN-co-pyrene)//gel electrolyte//PEDOT/ITO. The color of the constructed device switched between green and blue on the application of potential between 0.8 and 1.5 V. Electrochromic switching study results show that optical contrast (DT%) and response time are 24.4%
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