ARTICLE pubs.acs.org/Langmuir
Electrochromic Properties of Polythiophene Polyrotaxane Film Taichi Ikeda*,†,§ and Masayoshi Higuchi†,‡ †
Functional Materials Chemistry Group, Organic Nanomaterials Center (ONC), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, 305-0044, Japan § Experimental Physics, Max Planck Institute for Polymer Research (MPI-P), Ackermannweg 10, D-55128, Mainz, Germany ‡ International Center for Materials Nanoarchitectonics (MANA), NIMS, Namiki 1-1, Tsukuba, 305-0044, Japan
bS Supporting Information ABSTRACT: The electrochromic properties of a polythiophene polyrotaxane film consisting of a polythiophene backbone wrapped by the tetra-cationic cyclophane, cyclobis(paraquat-p-phenylene), were characterized. A naked reference polythiophene film, i.e., polythiophene without tetra-cationic cyclophane, was also characterized. The surface morphology and thickness of the film (L) were observed by atomic force microscopy. The surface of the naked reference polythiophene film has micrometer-scale polythiophene aggregates, which causes the darker color of the film and smaller color contrast in the electrochromic process. The polythiophene polyrotaxane gives a more homogeneous and brighter colored film owing to the suppression of molecular interactions between the polythiophene chains by the tetra-cationic cyclophanes. Potential-step chronoamperometric measurement provided the area density of the oxidizable sites (Γ) and the apparent diffusion coefficient of the charge transport in the film. From linear relationship between L and Γ, the concentrations of the oxidizable sites in the polythiophene polyrotaxane and naked reference polythiophene films were calculated to be 1.3 and 2.4 mmol cm-3, respectively. Interestingly, the polythiophene polyrotaxane film afforded a significantly larger apparent diffusion coefficient than the naked reference polythiophene film. This result suggests that the ratedetermining step of the charge transport is not the electron hopping between the polythiophene chains but the transport of chargecompensating counterions from the solvent into the polythiophene. We believe that the counteranions of the tetra-cationic cyclophane provide a pathway allowing the charge-compensating counteranions to migrate from the solvent to polythiophene. The polythiophene polyrotaxane film showed faster color change than the naked reference polythiophene film in the electrochromic reaction. These results indicate that our polythiophene polyrotaxane is a better electrochromic material than the naked reference polythiophene.
’ INTRODUCTION The development of organic electronics devices is one of the central issues in materials science. Semiconductive polymers have many applications in devices such as solar cells,1,2 fieldeffect transistors,3,4 sensors,5,6 electroluminescent devices,7,8 and electrochromic materials.9-11 In order to improve device performance, many research groups have proposed various kinds of chemical modifications to the semiconductive polymers.1-11 Recently, some groups have reported new functionalization protocols,12,13 in which the conductive polymer is wrapped with macrocycles. These semiconductive polymers are called “insulated molecular wires.”14 The insulated molecular wires have some advantages because the molecular and electronic interactions between the semiconductive polymer chains are suppressed by the insulating layer (macrocycles).12-14 As a result, some researchers reported improved solubility15,16 and optical properties17,18 of the insulated molecular wires as compared to naked molecular wires. However, insulated molecular wires inherently have a drawback. Since the electron hopping between the semiconductive polymers is interfered by the insulating layer (macrocycle), the conductivity of the bulk material should be lower than that of the noninsulated semiconductive polymer.12 r 2011 American Chemical Society
Recently, we have reported new insulated molecular wire based on polythiophene polyrotaxane,19 which consists of polythiophene backbone wrapped with tetra-cationic cyclophane [cyclobis(paraquat-p-phenylene)]20 (Figure 1a). We can prepare polythiophene polyrotaxane thin film through the electrochemical polymerization of the monomer [2]rotaxane. The polythiophene polyrotaxane film is electrochemically active, and its color is controllable by the redox reaction of the polythiophene backbone. Therefore, the polythiophene polyrotaxane film is available as an electrochromic material. To the best of our knowledge, the electrochromic property of the insulated molecular wire has not been reported so far. The objective of this study is to clarify whether our polythiophene polyrotaxane is superior to naked reference polythiophene (Figure 1b) as an electrochromic material. We characterized the electrochromic properties of the polythiophene polyrotaxane and naked reference polythiophene films. Then, we compared these properties. We observed the surface morphology and thickness of the films by atomic force microscopy (AFM) measurements. The oxidation (doping) process Received: November 3, 2010 Revised: February 2, 2011 Published: February 23, 2011 4184
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Figure 1. Chemical structures of (a) polythiophene polyrotaxane (P6TRx) and (b) naked reference polythiophene (P6T). The monomer units of polymers a and b are thiophene [2]rotaxane (6TRx) and dumbbell-shaped sexithiophene (6T), respectively.
of the films was characterized in terms of the apparent diffusion coefficient of the charge transport in the films by potential step chronoamperometry. We confirmed that the charge transport in the polythiophene polyrotaxane film is significantly faster than that in the naked reference polythiophene film. This was an unexpected result because the charge transport by electron hopping was considered to be interfered by the tetra-cationic cyclophane. Faster charge transport is attractive, since it causes a quick response of the electrochromic reaction. In this paper, we discuss the reasons why the charge transport in the polythiophene polyrotaxane film is faster than that in the naked reference polythiophene film.
’ EXPERIMENTAL SECTION Materials. Tetrabutylammonium perchlorate (TBA 3 ClO4, electro-
chemical grade) and acetonitrile (MeCN, spectrophotometric grade) were purchased from Sigma-Aldrich Co. and Kanto Chemical Co. Inc., respectively. Thiophene [2]rotaxane (6TRx) was synthesized according to our previously reported method. The indium-tin-oxide (ITO)-coated glass substrate (surface resistivity 8-12 Ω) was purchased from Aldrich Co.
Preparation of Polythiophene Polyrotaxane (P6TRx) Film. The electrochemical polymerization of thiophene [2]rotaxane (6TRx) was carried out by the potentiostatic and potentiodynamic methods. In the case of the potentiostatic method, the applied voltage (E) was kept at þ1.0 V. In the case of the potentiodynamic method, the applied voltage was scanned from þ0.2 V to þ1.2 V. The MeCN solution (concentrations of 6TRx and TBA 3 ClO4 were 0.5 mM and 0.1 M, respectively) was put in the voltammetry cell (VC-4, BAS Inc.); then, the ITO-coated glass substrate (working electrode), Pt wire (counter electrode), and saturated calomel electrode (SCE, reference electrode) were immersed in the solution. The applied voltage on the working electrode was controlled using the ALS Electrochemical Analyzer, Model 612B (ALS Co.). After electrochemical polymerization, the P6TRx film on the ITO-coated glass substrate was rinsed carefully with pure MeCN to remove the 6TRx solution. The electrochemical polymerization of the dumbbell-shaped sexithiophene (6T) was also carried out under the same conditions to obtain the naked reference polythiophene film (P6T). Atomic Force Microscopy (AFM) Measurement. AFM images were obtained in air under ambient conditions with the S-image controlled by SPI 4000 probe station (SII Nano Technology Inc.). We scratched the film with Ag wire to make a clear boundary of the P6TRx film and bare ITO surface. We approached the AFM tip (SI-DF20, SII
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Figure 2. AFM images of (a) polythiophene polyrotaxane film, (b) naked reference polythiophene film, and (c) section profile of polythiophene polyrotaxane film on ITO-coated glass substrate. Nano Technology Inc.) on this boundary using an optical microscope. The topographic image was obtained using the DFM mode.
Electrochemistry and Electrochromic Characterization. Electrochemical characterizations of the P6TRx film on the ITO-coated glass substrate were carried out in MeCN solution containing 1.0 M TBA 3 ClO4 with Pt counter and SCE reference electrodes. For the electrochromic characterization, MeCN solution containing 1.0 M TBA 3 ClO4 was put in the quartz cell for spectroscopy (T-5 UV10, Toso Co.). The P6TRx film deposited on the ITO-coated glass substrate, Pt counter electrode, and Ag wire pseudo reference electrode were immersed in the quartz cell; then, this cell was set to the spectrometer (UV-2550, Shimadzu Co.) and each electrode was connected to the electrochemical analyzer (Model 612B, ALS Co.).
’ RESULTS AND DISCUSSION AFM Measurement of Electrochemically Deposited Films. In order to prepare the polythiophene polyrotaxane film on the ITO-coated glass substrate, we tested two methods: potentiostatic and potentiodynamic methods. The potentiodynamic method has been claimed to give films with superior adhesion, smoothness, and optical properties.21 In fact, the potentiodynamic method was better than the potentiostatic method in the case of the polythiophene polyrotaxane film. However, in the case of the naked reference polythiophene, the potentiostatic method was better. The reason for this will be discussed later with the help of AFM images. In this study, we prepared five kinds of polythiophene polyrotaxane and naked reference polythiophene films with different thicknesses by the potentiodynamic and potentiostatic methods, respectively. The morphology and thickness of the polythiophene polyrotaxane film and naked reference polythiophene film were investigated by AFM. Figure 2a shows the surface topography of the polythiophene polyrotaxane film. One can see that the surface is covered with nanometer-scale globular objects. It is considered that polymerized thiophene [2]rotaxane near the electrode surface aggregated to form the globular objects, which were then deposited on the surface. This inhomogeneity is too little to be detected by optical microscope observation. Therefore, we can conclude that the polythiophene polyrotaxane film is inhomogeneous at the nanoscopic scale but homogeneous at the macroscopic scale. On the other hand, the AFM image of the naked reference polythiophene film shows the presence of micrometer-scale objects on 4185
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Table 1. Summary of Data for Polythiophene Polyrotaxane Film Γ,b sample
a
L, nm
mol cm
Dapp-d,c -2
2 -1
cm s
Dapp-dd,e d
Td, sec
cm2 s-1
Tdd,f s
4.1 10-9 1.6 10-9 7.0 10-9 5.9 10-9
0.16 0.19
4.8 10-9 0.06 1.1 10-8 0.07
P6TRx-3 70 ( 10 10.0 10-9 3.7 10-9
0.26
9.1 10-9 0.11
0.30
1.1 10-8 0.13
0.37
1.7 10-8 0.16
P6TRx-1 29 ( 5 P6TRx-2 54 ( 8 P6TRx-4 89 ( 8 P6TRx-5 125 ( 9
-9
12.2 10
-9
16.6 10
-9
4.9 10
-9
5.1 10
a
Film thickness obtained by AFM height profile. b Area density of oxidizable sites obtained by potential step chronoamperometry (E = þ0.2 V f þ1.2 V). c Apparent diffusion coefficient of charge transport in doping process. d Response time for completing 90% of doping process. e Apparent diffusion coefficient of charge transport in dedoping process. f Response time for completing 90% of dedoping process. Dapp-d and Dapp-dd were obtained by the method proposed by Aurbach et al.24
Table 2. Summary of Data for Naked Reference Polythiophene Film Γ,b a
Dapp-d,c -2
2 -1
sample L, nm
mol cm
P6T-1 20 ( 3
4.2 10-9 1.6 10-10 -9
cm s
Td, s
cm2 s-1
Tdd,f s
0.11
3.0 10-10
0.09
P6T-2 31 ( 3 P6T-3 38 ( 3
2.5 10 5.6 10 7.4 10-9 3.6 10-10
0.20 0.22
4.0 10-10 7.2 10-10
0.10 0.13
P6T-4 41 ( 3
9.3 10-9 1.7 10-10
0.35
3.4 10-10
0.18
0.37
6.3 10-10
0.24
-9
P6T-5 50 ( 4 11.7 10
-10
Dapp-dd,e d
-10
2.8 10
Figure 3. Potential step chronoamperometry of the film deposited on ITO-coated glass substrate in 1.0 M TBA 3 ClO4/MeCN solution. (a) i-t profile of P6TRx-2 film, (b) Cottrell plot of P6TRx-2 film, (c) i-t profile of P6T-5 film, and (d) Cottrell plot of P6T-5 film.
a
Film thickness obtained by AFM height profile. b Area density of oxidizable sites obtained by potential step chronoamperometry (E = þ0.2 V f þ1.2 V). c Apparent diffusion coefficient of charge transport in doping process. d Response time for completing 90% of doping process. e Apparent diffusion coefficient of charge transport in dedoping process. f Response time for completing 90% of dedoping process. Dd-Ox and Ddd-Red were obtained by the method proposed by Aurbach et al.24
the polythiophene film (Figure 2b). These large-scale objects of polythiophene were detectable as black dots in an optical microscope image. Therefore, the film appears dark in color, as discussed later. Since the molecular interaction between the semiconductive polymers is quite strong, we can easily obtain large insoluble aggregates after polymerization. In the case of the polythiophene polyrotaxane, tetra-cationic cyclophane can suppress the molecular interactions between the polythiophene chains. As a result, we can suppress the formation of the micrometer-scale aggregates and obtain a fine homogeneous film. In the case of the naked reference polythiophene, we could not obtain a uniform film by the potentiodynamic method. Even when we applied the potentiostatic method, it was difficult to obtain the thick film owing to the partial abruption of the film from the polymer surface. The micrometer-scale aggregates are considered to weaken the adhesion of new film on the surface. The height profile of the AFM image affords the thickness of the films (Figure 2c). The results for the polythiophene polyrotaxane and naked reference polythiophene films are summarized in Table 1 and Table 2, respectively. Chronoamperometric Characterization of the Film. In order to characterize the area density of the oxidizable sites and the charge transport kinetics of the film, we conducted potential step chronoamperometric measurements.22 Figure 3a shows the current (i) versus time (t) plot when the applied voltage (E) on the P6TRx-2
Figure 4. Relationship between area density of oxidizable sites and thickness of polythiophene polyrotaxane film.
film (film thickness: 54 ( 8 nm) was stepped from þ0.2 to þ1.2 V (vs SCE). In this process, we could observe the current flow based on the oxidation of the polythiophene backbone (doping). From the integral of the current flow, we quantified the oxidizable sites in the polythiophene polyrotaxane film. The area density of the oxidizable sites (Γ) is summarized in Table 1. As shown in Figure 4, it is clear that the area density depends on the film thickness (L). From the slope of this linear relationship between Γ and L, we calculated the concentration of the oxidizable site in the polythiophene polyrotaxane film to be 1.3 mmol cm-3. Figure 3c shows i-t plot when the applied voltage on the P6T5 film (film thickness: 50 ( 4 nm) was stepped from þ0.2 to þ1.2 V (vs SCE). Using the same procedure for the polythiophene polyrotaxane film, we calculated the concentration of the oxidizable sites in the naked reference polythiophene film to be 2.4 mmol cm-3. This value is almost twice of that for the polythiophene polyrotaxane film. This is a reasonable result, 4186
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Langmuir because the polythiophene backbone provides the oxidizable sites, and only half of the molecular mass in the polythiophene polyrotaxane film is due to the polythiophene backbone. The molecular weights of the dumbbell-shaped sexithiophene repeating unit and tetra-cationic cyclophane with four perchlorate ions are 903.2 and 918.5, respectively. The time course change of the current flow gives us the charge transport kinetics in the film. As shown in parts b and d of Figure 3, the i-t-1/2 profile did not show a Cottrell-type behavior.22 In the initial stage, the external resistance, which is composed of ohmic drops (in the solution and in the electrode bulk) and slow interfacial charge-transfer kinetics, causes the experimental current to be significantly lower than the theoretical current.23,24 In the later stage, the finite thickness of the film leads to lower experimental currents.23,25 Aurbach et al. reported a practical method for obtaining the diffusion coefficient from the potential step chronoamperometric i-t curve, which does not exhibit a Cottrell-type behavior.24,26 On the basis of their method (see the Supporting Information), we calculated the apparent diffusion coefficient of the charge transport in the doping and dedoping processes (Dapp-d and Dapp-dd). Table 1 and Table 2 summarize the calculated Dapp-d and Dapp-dd for the polythiophene polyrotaxane and naked reference polythiophene films, respectively. We confirmed the Dapp-d and Dapp-dd values are independent of the film thickness. The average Dapp-d value of the naked reference polythiophene film is (2.4 ( 0.7) 10-10 cm2 s-1. This value is comparable to those reported for the polythiophene derivatives.24 On the other hand, the average Dapp-d value of the polythiophene polyrotaxane film is (4.2 ( 1.5) 10-9 cm2 s-1. Interestingly, the apparent diffusion coefficient of the polythiophene polyrotaxane film is significantly larger than that of the naked reference polythiophene film. The apparent diffusion coefficient is considered to be affected by two processes.25 One is the electron hopping between the semiconductive polymer chains, and the other is the transport of chargecompensating counterions from the solvent to the semiconductive polymer. Recently, some groups concluded that the latter process is the rate-determining step of the charge transport.23,24 This is important because the charge transport kinetics is independent from the kinetics of the electron hopping between the semiconductive polymers. Therefore, we can overcome the inherent drawback of the insulated molecular wires12 in the case of the electrochromic system. However, if the electron hopping process becomes slower than the diffusion of the charge-compensating counterions, the electron hopping process should be the rate-determining step. Fortunately, our polythiophene polyrotaxane is a partially insulated molecular wire. Namely, the 3,4ethylenedioxy-thiophene (EDOT) unit is not encapsulated by the tetra-cationic cyclophane. The larger Dapp-d value of the polythiophene polyrotaxane film suggests that the electron hopping between the polythiophene chains in this film is not interfered significantly, and the diffusion process of the chargecompensating counterions is the rate-determining step. Why can the charge-compensating counterions diffuse faster in the polythiophene polyrotaxane film? We believe that the counteranions of the tetra-cationic cyclophane can provide a pathway allowing the charge-compensating counterions to migrate from the solvent to polythiophene. Therefore, the counteranions in the polythiophene polyrotaxane film can diffuse faster than those in the naked reference polythiophene film. Recently, Cacialli et al. also reported the importance of the counterion mobility to improve the performance of the electroluminescent devices using
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Figure 5. Colors of films on ITO-coated glass plates. Polythiophene polyrotaxane film at (a) E = 0.0 V and (b) E = þ1.0 V. Naked reference polythiophene film at (c) E = 0.0 V and (d) E = þ1.0 V (vs Ag wire pseudo reference electrode).
the insulated molecular wires.17 The faster doping process in the polythiophene polyrotaxane film is an attractive property for their use as an electrochromic material. In both cases of the polythiophene polyrotaxane and naked reference polythiophene films, the Dapp-dd values are larger than the Dapp-d values (Table 1 and 2). The average Dapp-dd values of the polythiophene polyrotaxane and naked reference polythiophene films are 1.1 ( 0.4 10-8 and 4.8 ( 1.7 10-10 cm2 s-1. After doping, configuration or orientation of the polythiophene changes and the charge-compensating counterion expands the polymer network.27,28 Therefore, the release of the counteranions from the doped polythiophene film takes place more easily than the insertion of those into the ground state polythiophene film. Electrochromic Property of Polythiophene Polyrotaxane Film. The spectroelectrochemistry of the polythiophene polyrotaxane film has already been reported in our previous paper.19 When the applied voltage (E) on the polythiophene polyrotaxane film deposited on the ITO-coated glass substrate was changed from 0.0 to þ1.2 V, the π-π* absorption band (around 540 nm) was bleached owing to the oxidation of the polythiophene backbone. As a result, the color of the polythiophene polyrotaxane film changes from red to pale blue (Figure 5, parts a and b). The naked reference polythiophene film deposited on the ITO-coated glass substrate also shows electrochromism. However, this film is darker in color than the polythiophene polyrotaxane film (Figure 5, parts c and d). This is because micrometer-scale large aggregates of the naked polythiophene are deposited on the film surface as discussed before (Figure 2b). The color contrasts in the electrochromic process were evaluated using the CIE L*a*b* color system, where L* (lightness), a* (green-red) and b* (blue-yellow) are used to parametrize the color space.29 With CIE L*a*b*, the color contrast in the electrochromic process is defined as the following equation.29 ΔE ¼ ðΔL2 þ Δa2 þ Δb2 Þ0:5
ð1Þ
The L*, a*, b* values at the ground and doped states of the polythiophene polyrotaxane film are 41, 35, -8 and 60, -9, -5, respectively, calculating the ΔE* value to be 48. In the case of the 4187
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the naked reference polythiophene film. AFM measurements show that the surface of the polythiophene polyrotaxane film is more homogeneous than that of the naked reference polythiophene film. The latter film has micrometer-scale aggregates on the surface owing to strong molecular interactions between the polythiophene chains. In the case of the polythiophene polyrotaxane film, the molecular interactions between the polythiophene chains are suppressed by the tetra-cationic cyclophanes. As a result, polythiophene polyrotaxane gives a brighter colored film than the naked reference polythiophene. In addition, the color contrast of the polythiophene polyrotaxane film in the electrochromic process is better than that of the naked reference polythiophene film. Chronoamperometric measurements confirmed that the apparent diffusion coefficient of the charge transport in the polythiophene polyrotaxane film is significantly larger than that in the naked reference polythiophene film. This result suggests that the rate-determining step of the charge transport is not the electron hopping between the polythiophene chains but the transport of charge-compensating counterions from the solvent to the polythiophene. We believe that the counteranions of the tetra-cationic cyclophane provide a pathway allowing the charge-compensating counterions to migrate from the solvent into the polythiophene in the case of the polythiophene polyrotaxane. As a result, the polythiophene polyrotaxane film shows a faster response time than the naked reference polythiophene film in the electrochromic reaction. These results indicate that our polythiophene polyrotaxane is superior to the naked reference polythiophene as an electrochromic material. We believe that this study provides useful information for overcoming the drawbacks of insulated molecular wires. Figure 6. Potential steps (top), simultaneous current (middle), and absorbance at 540 nm (bottom) of polythiophene polyrotaxane film on ITO-coated glass substrate.
naked reference polythiophene, the L*, a*, b* values at the ground and doped states are 38, 19, 19 and 57, -9, 5, respectively, calculating the ΔE* value to be 37. The smaller L* value of the naked reference polythiophene film than the polythiophene polyrotaxane film reflects the darker color of the film. The larger ΔE* value of the polythiophene polyrotaxane film indicates that the polythiophene polyrotaxane is superior to the naked reference polythiophene as an electrochromic material. Figure 6 shows the changes in the absorbance at 540 nm and the simultaneous current response upon repeatedly stepping the potential between 0.0 and þ1.0 V. It is evident that the absorbance change is driven by the current flow. We obtained the response times (Td and Tdd) for completing 90% of the electrochromic reaction on the basis of potential step chronoamperometric i-t data (Table 1 and 2). The response time depends on the film thickness. The redox reactions accompanying the electrochromism were completely reversible. No deterioration of the polythiophene polyrotaxane film was confirmed over 150 redox cycles (see the Supporting Information). The reversibility of the redox reaction for the naked reference polythiophene film is almost the same as that for the polythiophene polyrotaxane film.
’ CONCLUSION We have characterized a polythiophene polyrotaxane film deposited on an ITO-coated glass substrate and measured its electrochromic property and compared the results with those for
’ ASSOCIATED CONTENT
bS
Supporting Information. Procedure to calculate the apparent diffusion coefficient (Dapp-d) and the reversibility of the redox reaction. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ81-29-860-4721.
’ ACKNOWLEDGMENT This work was financially supported by Grant-in-Aid for Young Scientists B, 22750175. ’ REFERENCES (1) Chen, J.-W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709. (2) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954. (3) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.; C€oelle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.; Salleo, A.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. Adv. Mater. 2009, 21, 1091. (4) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070. (5) Peng, H.; Zhang, L.-J.; Soeller, C.; Travas-Sejdic, J. Biomaterials 2009, 30, 2132. (6) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339. (7) Sun, Q.-J.; Li, Y.-F.; Pei, Q.-B. J. Display Technol. 2007, 3, 211. 4188
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