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Electrochromic and spectroelectrochemical properties of novel 4,4′-bipyridilium–TCNQ anion radical complexes
Chemical Physics Letters 579 (2013) 105–110
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Electrochromic and spectroelectrochemical properties of novel 4,40 -bipyridilium–TCNQ anion radical complexes Guoming Wang, Xiangkai Fu ⇑, Jun Deng, Xuemei Huang, Qiang Miao College of Chemistry and Chemical Engineering, Research Institute of Applied Chemistry Southwest University, The Key Laboratory of Applied Chemistry of Chongqing Municipality, Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), Chongqing 400715, PR China
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Article history: Received 17 April 2013 In final form 20 June 2013 Available online 27 June 2013
a b s t r a c t Three novel electrochromic materials 7,7,8,8-tetracyanoquinodimethane (TCNQ) anion radical salts with substituted 4,40 -bipyridilium derivatives (monosubstituent-4,40 -bipyridilium) were prepared. The structure of the complexes was characterized by Elemental analyses, Solid IR spectra and UV–vis spectroscopy. The electrochromic behaviors and electrooptical properties of the complexes were investigated by cyclic voltammetry and UV–vis absorption spectra. Electrochromic devices based on monosubstituent 4,40 bipyridilium–TCNQ anion radical salts (abbreviated as MBTS) were fabricated which exhibited green– magenta color change. Their color reversibility was excellent with high color-change efficiency after 1000 cycles of the transmittance and transmittance change. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ‘‘Viologens’’ (1,10 -disubstituted-4,40 -bipyridilium salts) [1–3], have focused attention of many research groups due to both their theoretical interest and practical applications. The ready availability of 4,40 -bipyridine and the ease of varying the nature of the quaternizing agent have allowed intensive research into the electrochromic properties of such ‘viologens’ [4]. Of the three common viologen redox states (bipm2+ ? bipm + ? bipm0), dication is the most stable and colorless when pure unless optical charge transfer with counter anion occurs. The viologen radical cations in particular are highly colored with high molar absorption coefficients owing to optical charge transfer between (formally) +1 and zero valent nitrogens. Radical cations with short alkyl chains are blue (blue-purple in concentrated solution), and becoming crimson as the alkyl chain length increased due to the increased dimerization [5]. The main applications of bipyridilium systems are smart windows [6,7], electrochromic displays [8,9], car rear-view mirrors [10,11] and electronic paper [12]. However, monosubstituted 4,40 bipyridilium derivatives used for electrochromic materials have seldom been reported due to their monocations’ instability [13]. In this Letter, star-shaped substituent 4,40 -bipyridinlium–TCNQ salts could overcome this defect and can be used as promising candidates for electrochromic materials. On the other hand, 7,7,8,8-tetracyanoquinodimethane (TCNQ) is a well-known electron acceptor which readily forms charge–transfer (CT) complexes with an electron donor such as amine compounds [14–16]. The charge–transfer salts were almost ⇑ Corresponding author. Fax: +86 023 68254000. E-mail address: [email protected] (X. Fu). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.06.037
established immediately after TCNQ was first synthesized and applications are very common now [17–37]. These TCNQ-based materials are considerably interesting due to their extensive and novel electrical, electrochemical, and magnetic properties arising from the characteristic p-stacking of the TCNQ moieties into columns. TCNQ as well as its derivatives have been extensively used as electron acceptor molecules [38], light emitting diodes [39], data storage devices [40], organic field-effect transistors [41], potentiometric sensors [24] and incorporated into electrochromic devices [25]. With this feature, the combination of two moieties, i.e. viologen and TCNQ, might result in a new type of novel cathodic–anodic composite electrochromic materials which might possess unique properties. What is more, the complexes between viologen salts and TCNQ anion radical have not been previously reported. Therefore, a series of novel cathodic-anodic composite ECMs (MBTS) were synthesized by incorporating cathodic material viologen with anodic material TCNQ anion radical and their corresponding ECDs were assembled for the first time. The structures of complexes MBTS were shown in Scheme 1, and the films of these ECMs were investigated by measuring the changes in ECDs charge, optical and electrochemical methods. Furthermore, the electrochromic coloration and response times of the ECDs were also investigated in detail. 2. Experimental details 2.1. General All manipulations in this Letter involving air-sensitive reagents were performed in an atmosphere of dry argon. The chemicals and reagents, unless otherwise specified, were purchased from Aldrich,
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(w), 1506 (mC@C),1400 (s), 809 (dCH). The bands at 2198 and 2169 are attributed to m (CN) of TCNQ1.
3. Results and discussion 3.1. Electrochemical properties and characterization
Scheme 1. Synthetic route of MBTS 3a–c.
Acros, and TCI Chemical Co. and used as received. All the solvents were further purified before use. All the new synthesized compounds were characterized by Elemental analyses, Solid IR spectra and UV spectra. Elemental analyses were performed on a Costech ECS 4010 instrument; values agreed with the calculation. Solid IR spectra (400–4000 cm1) were obtained using potassium bromide (KBr) pellets with a Polari’s FT-IR spectrometer. UV spectra were performed on a Unico UV-4802H UV–visible spectrophotometer connected to a computer. Cyclic voltammetry was carried out on CHI 650B electrochemical workstation using three-electrode system. All ECDs were fabricated, sealed and tested at room temperature. 2.2. General procedure for the synthesis of LiTCNQ Purple solid: Yield: 92%. To a boiling solution of 10.2 g (0.05 mol) of TCNQ in 1000 ml of acetonitrile was added a boiling solution 20.0 g (0.15 mol) of lithium iodide in 50 ml of acetonitrile. Purple crystals separated from the dark brown solution. The mixture stood for 4 h at room temperature and the purple solid was collected, then washed on the filter with acetonitrile until the washings were bright green. The solid was then washed with a large volume of ether. 2.3. Preparation of viologen derivatives 2a–c The viologen derivatives were prepared with three alkyl halides. Their structure was characterized according to literature methods [3,42]. 2.4. General procedure for the synthesis of MBTS Twenty milliters of MeOH solution of 2a–c (1.0 mmol) was added to 20 mL boiling MeOH of LiTCNQ (213 mg, 1.0 mmol). The mixture was refluxed, a colored solid appeared, which was filtered, washed with MeOH and Et2O in turn, dried in vacuum (Scheme 1). 2.4.1. Preparation of 3a Blue-black solid: Yield: 54.5%. M.p.: 248–250 °C. Anal. Calc. for C23H15N6: C, 73.59; H, 4.03; N, 22.39. Found: C, 73.64; H, 4.12; N, 22.20%. FT-IR (KBr, cm1): 2187, 2170 (mCN, TCNQ1), 1638, 1618 (s), 1579, 1506 (s), 1505 (mC@C), 1400 (s), 819 (dCH), 620. The bands at 2187 and 2170 are attributed to m (CN) of TCNQ1. 2.4.2. Preparation of 3b Purple-black solid: Yield: 90%. M.p.: 202–203 °C. Anal. Calc. for C27H21N6: C, 75.50; H, 4.93; N, 19.57. Found: C, 75.46; H, 4.78; N, 19.70%. FT-IR (KBr, cm1): 2186, 2171 (mCN, TCNQ1), 1637 (s), 1580, 1522 (w), 1507 (mC@C), 1401 (s), 820 (dCH). The bands at 2186 and 2171 are attributed to m (CN) of TCNQ1. 2.4.3. Preparation of 3c Purple solid: Yield: 54.5%. M.p.: 189–190 °C. Anal. Calc. for C29H19N6: C, 77.14; H, 4.24; N, 18.62. Found: C, 77.18; H, 4.43; N, 18.35%. FT-IR (KBr, cm1): 2198, 2169 (mCN, TCNQ1), 1637, 1581
3.1.1. Electrochemical properties In order to study the mechanism of the electrochromism and electrochemistry for MBTS, electrochemical measurements were performed by using cyclic voltammetry at room temperature in a conventional three-electrode system using platinum disk (0.02 cm2) as working electrodes, platinum wire as counter electrodes and Ag/AgCl in 3 M NaCl (aq) solution as a reference, with 0.1 M TBAP (Tetrabutylammonium perchlorate) in CH3CN as the electrolyte. All of the electrochemistry experiments were carried out at room temperature under nitrogen atmosphere. The full scan-range cyclic voltammetry curves were shown in Figure 1, from which two couples of redox waves were observed. The cyclic voltammetry data was recorded simultaneously during 100 cyclic voltammetry experiments. After 100 successive wide-range potential cycling between 0.75 and +0.75 V, three MBTS similar welldefined reversible redox couple (Ep, 1/2) that was centered at about 0.32 V, 0.23 V for 3a; 0.35 V, 0.23 V for 3b; 0.36 V, 0.22 V for 3c. From cyclic voltammograms analysis, it was observed that all of the three complexes unchanged much after 100 cycling, which indicates the three TCNQ complexes showed fine electrochemical reversibility and stabilities. Usually, compared with traditional 1,10 -disubstituent-4,40 -bipyridilium derivatives, monosubstituted 4,40 -bipyridilium derivatives used for electrochromic materials have seldom been reported due to their monocations’ instability [13]. However, in this Letter, when it combine with TCNQ to form the monosubstituted 4,40 -bipyridilium–TCNQ anion radical complexes 3a–c, they also performed superior cycling stabilities and well-defined reversibility. The mechanism of cathode–anode composite electrochromic compound has been reported [43]. In the curve of complex 3a, the first cathodic peak at 0.26 V, corresponds to the reaction between +1 and zero valent nitrogens of viologen part of 3a[44], and this can be expressed as in Scheme 2a. While the first anodic peak at +0.17 V was due to the oxidation of TCNQ anion radical [45]. As can be illustrated in Scheme 2b. In addition, the complexes MBTS of propylene carbonate (PC) solution were intensely green color in the neutral state owing to the coloration of TCNQ radical anion. While when the voltage applied, the color changes from green to magenta, which actually due to the reduction of viologen at the cathode part i.e., the coloration of monosubstituent 4,40 -bipyridilium radical. The mechanism and color change of complexes MBTS can be expressed in Scheme 2c.
3.1.2. UV study of solution Investigation of the electronic properties of MBTS measured by UV–vis in CH3CN, revealed that the individual chromophores were enhanced in the electronic spectrum ground state. As seen in Figure 2, the neutral TCNQ absorbs only at 395 nm. However, when it combine with monosubstituent-4,40 -bipyridilium, new absorption bands appear at 260, 420, 680 and 743 nm. The absorption band around 260 nm might be assigned to the p–p⁄ transitions which due to intramolecular transition of the viologen unit [46– 48], while the last two bands located at 680 and 750 nm were related to the absorption of TCNQ anion radical [49–52]. It was also noted that the absorbance at 420 nm attributed to the charge– transfer band was still observed. Furthermore, it was clear that the absorption maximum (kmax) in the spectra of three complexes
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Figure 1. Cyclic voltammetry of complexes 3a–c in 0.1 M TBAP/CH3CN vs. Ag/AgCl. The first, 50th and 100th cycles are shown and the scan rate is 50 mV/s.
(a)
(b)
(c) Scheme 2. The mechanism and color change of complexes MBTS.
and ion conducting layer [44]. However, in this Letter, a newly simple liquid EC device was applied without additioning conductive polymers and supporting electrolyte. In this device, an indium tin oxide (ITO)-coated glass surface (conductive side inwards) and a refective metallic surface, spaced a fraction of a millimetre apart (35 40 0.7 mm, 50 X/sq), form the two electrodes of the cell, with a solvent containing electroactive chemical species. The electrolyte solution was prepared by dissolving MBTS and 0.1 M of Ferrocene (abbreviated as Fc) in propylene carbonate (PC). Special attention must be paid to avoid any contamination from moisture and oxygen. The solution electrolyte needs to be bubbled with argon gas before use, and all the containers should be dried in oven. The prepared electrolyte was injected into the electrochromic device later using syringe. The ECD was assembled as the structure including, conductive ITO glass ||MBTS + Fc|| PC|| conductive ITO glass. 3.3. Electrochromic properties
Figure 2. UV–vis absorption spectra of neutral TCNQ and TCNQ anion radical salts 3a–c (0.02 M) in CH3CN at room temperature.
were similar. Normally change was observed for the different substituted groups.
3.2. Electrochromic devices construction The most common electrochromic device configuration is a solid EC device. In this configuration, a thin film cathode electrochromic material layer and an anode electrochromic material counterelectrode layer are separated by a polymer electrolyte
3.3.1. Spectroelectrochemistry properties study The best way of examining the changes in optical properties of electrochromic material upon voltage applied is spectroelectrochemistry. The spectroelectrochemical and electrochromic properties of the ECDs based on resultant three TCNQ complexes were studied by applying stepped potentials sequentially from 2.0 to 3.5 V. As shown in Figure 3, three distinguishable absorbance peaks at around 420, 745 and 850 nm were observed in the neutral state. When the voltage applied to the ECDs, the absorbances of the bands with kmax at 745 and 752 nm decreased, while a new band appears at 400 nm, which correspond to the formation of neutral TCNQ band [53]. Figure 4 shows photograph of ECD with MBTS as ECMs in the neutral state and different colored state when the potential of 3.0 V was applied. The three substituent 4,40 -bipyridilium–TCNQ anion radical complexes were intensely green color at 0 V (Figure 4 a), with high molar absorption coefficients owing to optical complete charge transfer. When ±3.0 V potential was applied between the ECM layers, the color of the ECDs changed from green to magenta (Figure 4 b). The color change is due to the reduction of viologens at the cathode part whereas the other TCNQ anion radical at the anodic part is oxidized. Once the potential is switched again to 0 V, the device recovers its original green color, proving that the electrochromic device works satisfactorily. Table 1 summarizes the maximum absorption wavelength (kmax), optical band gaps (Eg) values, redox couple (Eox), Ep,1/2 of three TCNQ complexes quite clearly. The optical band gap of the
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Figure 3. UV/vis/NIR spectroelectrochemistry for MBTS on ITO glass, at different potential stepping from 2.0 to 3.5 V with 0.5 V decrement, respectively. Applied potentials are the following: (a) 0 V; (b) 2.0 V; (c) 2.5 V; (d) 3.0 V; (e) 3.5 V.
Figure 4. Colors of a sandwich-type MBTS in ECDs. (a) Neutral state, (b) MBTS at 3 V.
Table 1 Electrical and optical data of the three TCNQ complexes. Complexes Eox (V) 3a 3b 3c
EP,1/2 (V)
konset (nm) Eg (eV) 0 V
0.38, 0.17 0.32, 0.23 456 0.44, 0.14 0.35, 0.23 468 0.43, 0.15 0.36, 0.22 458
2.72 2.65 2.71
±3 V
Green Magenta Green Magenta Green Magenta
electrochromic materials were calculated from their low energy absorption edges (konset) according to Planck equation (Eg = 1241/ konset) [54]. As seen in Figure 3, the kmax and konset of the absorption spectra of 3a on ITO glass were at 398 nm and 456 nm, corresponded to a band gap of 2.72 eV; for 3b, the kmax and konset of the absorption spectra were observed at 400 and 458 nm, corresponded to a band gap of 2.65 eV; While for 3c, the kmax and konset of the absorption spectra were observed at 396 and 458 nm, corresponded to a band gap of 2.71 eV. 3.3.2. Switching and stability study Switching and stability are two important properties of an electrochromic material and its corresponding ECD [55]. The three TCNQ anion radical salts of materials on the ITO glass were switched by repeating the potential steps using 3.0 V and +3.0 V with a residence time of 5 s in the PC solution. The optical contrast (DT%), which can be defined as the transmittance difference between the redox states, was monitored as a function of time at the wavelength in the visible region. In order to investigate DT% of the three TCNQ salts of materials between 3.0 V and +3.0 V, three absorption wavelength 400 nm, 745 nm,
850 nm were chosen to monitor. As shown in Figure 5, the maximum DT% at 745 nm of the complex was found to be 36% for 3a, 41% for 3b, 46% for 3c, respectively. To compare with this result, compounds MBTS exhibited superior stability and optical properties. Response time, one of the most important characteristics of electrochromic materials, is the time needed to perform a switching between the two colored states of the materials [56]. Quantification of the switching time was performed by defining a change in 95% of the total absorbance span, as the naked eye could not distinguish the color change after this point [57]. As shown in Figure 6, the time required attain 95% of total transmittance difference for reduction process and oxidation process is found to be 1.77 s, 1.7 s for 3a; 1.90 s, 0.99 s for 3b; 0.97 s, 0.99 s for 3c, which indicate the three MBTS have faster response time than traditional viologens alone. The coloration efficiency (CE) is also an important characteristic for the electrochromic materials when making a comparison between electrochromic materials and devices [40]. The CE value can be calculated by using the equations and given below [58]:
MOD ¼ log
Tb MOD and CEðgÞ ¼ Qd Tc
ð1Þ
where Tb and Tc are the light transmittances in the neutral state and reduced state of the ECD at certain wavelength (k), 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). Qd is the amount of injected/ejected charge per unit sample area. On the basis of these equations, the DOD and CE were
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3.3.3. Stability of ECD Redox stability is another important parameter for ECDs. The stability of devices toward multiple redox switching usually limits the application of electrochromic materials in ECD utility [59]. A thousand cycles of the transmittance and transmittance change for viologen–TCNQ complex 3c were measured and the result was illustrated in Figure 7. The complex 3c presented a stable and reproducible electrochromic process at 745 nm between 3.0 V and +3.0 V even after multiple scans, e.g., the retained DT% is 42% for 3c after the first 1000 cycles. The results indicate that there is no obvious decrease of activity between first cycle and 1000 cycles. 4. Conclusion
Figure 5. Optical transmittance changes for 3a–c at 745 nm in 0.01 M Fc/PC under subsequent double potential steps between 3.0 and +3.0 V.
In summary, three novel electrochromic materials MBTS were synthesized in this Letter. Cyclic voltammetry (CV), UV–vis and FT-IR analyzes confirm the resultant structure. MBTS can be described as the charge–transfer (C–T) complexes and cathode–anode composite electrochromic materials. The ECDs based on C–T complexes were assembled and three distinguishable absorbance peaks at around 400, 745 and 850 nm at applied potential were observed. According to the spectroelectrochemical analyze, these ECMs reveal distinct electrochromic properties from that of corresponding ECDs and show green–magenta color change for MBTS under applied potentials. Their color and reversibility were excellent with high coloration efficiency after 1000 cycles of the transmittance and transmittance change. Moreover, the number of EC circulation for most of our ECDs was over 104, and the response times for the electrochromic processes were less than common viologen alone. These results will make these ECMs promising candidate for applications in electrochromic devices and displays. Acknowledgements
Figure 6. Optical switching studies for ECDs based on MBTS 3a–c were monitored at 745 nm.
All authors herein are grateful to the support from National Ministry of Science and Technology Innovation Fund for High-tech Small and Medium Enterprise Technology (No. 09C26215112399) and National Ministry of Human Resources and Social Security Start-up Support Projects for Returned oversea Students to Business, Office of Human Resources and Social Security Issued 2009 (143). References
Figure 7. Transmittance at a monochromatic wavelength of 745 nm as a function of time for complex 3c. (First cycle, solid line; after 1000 cycles, dotted line.)
calculated at 750 nm to be 0.60 and 645 cm2/C for 3a, 1.06 and 825 cm2/C for 3b, 0.74 and 796 cm2/C for 3c, which had high coloration efficiency.
[1] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [2] C.L. Bird, A.T. Kuhn, Chem. Soc. Rev. 10 (1981) 49. [3] P.M.S. Monk, The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,40 -Bipyridine, J. Wiley and Sons, Chichester, 1998. [4] R.J. Mortimer, Electrochim. Acta 44 (1999) 2971. [5] G. Bara, N. Larinab, L. Grinisc, V. Lokshinb, R. Gvishi, Sol. Energy Mater. Sol. Cells 99 (2012) 123. [6] S. Panero, B. Scrosati, M. Baret, B. Cecchini, E. Masetti, Sol. Energy Mater. Sol. Cells 39 (1995) 239. [7] Y. Kim, J. Do, E. Kim, G. Clavier, L. Galmiche, P. Audebert, J. Electroanal. Chem. 632 (2009) 201. [8] P.M.S. Monk, J. Electroanal. Chem. 432 (1–2) (1997) 175. [9] K. Bange, Sol. Energy Mater. Sol. Cells 58 (1) (1999) 1. [10] D.R. Rosseinsky, R.J. Mortimer, Adv. Mater. 13 (2001) 783. [11] N.R. Lynam, A. Agrawal, Automotive applications of chromogenic materials, in: C.M. Lampert, C.G. Granqvist (Eds.), Large Area Chromogenics: Materials and Devices for Transmittance Control, SPIE Optical Engineering Press, Bellingham, Washington, USA, 1990, p. 46. [12] P.M.S. Monk, F. Delage, S.M. Costa Vieira, Electrochim. Acta 46 (2001) 2195. [13] G. Wang, X.K. Fu, J. Huang, C.l. Wu, L. Wu, Q.l. Du, Org. Electron. 12 (2011) 1216. [14] D.S. Acker, R.J. Harder, W.R. Hertler, W. Mahler, L.R. Melby, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 82 (1960) 6408. [15] L.R. Melby, W. Mahler, W.E. Mochel, R.J. Harder, W.R. Hertler, R.E. Benson, J. Am. Chem. Soc. 84 (1962) 3374. [16] J.B. Torrance, Mol. Cryst. Liq. Cryst. 126 (1985) 55.
110
G. Wang et al. / Chemical Physics Letters 579 (2013) 105–110
[17] M. Azcondo, L. Ballester, S. Golhen, A. Gutierrez, L. Ouahab, P. Delhanes, J. Mater. Chem. 9 (1999) 1237. [18] L. Ballester, C. Alonso, M. Fonari, A. Gutierrez, M. Mitewa, M. Perpinan, K. Suwinska, J. Alloys Compd. 323–324 (2001) 138. [19] R. Bozio, I. Zanon, A. Girlando, C. Pecile, J. Chem. Soc., Faraday Trans. 2 (1977) 235. [20] C. Campana, K.R. Dunbar, X. Ouyang, Chem. Commun. (1996) 2427. [21] J. Fox, B. Foxman, D. Guarrera, J. Miller, J. Calabrese, A. Reis, J. Mater. Chem. 6 (1996) 1627. [22] A. Graja, J. Farges, A. Brau, P. Dupuis, Synth. Met. 70 (1995) 1233. [23] Hasanudin, N. Kuroda, T. Sugimoto, Synth. Met. 120 (2001) 90. [24] M. Higo, X. Lu, U. Mazur, K. Hipps, Thin Solid Films 384 (2001) 90. [25] P. Hooper, M. Newton, G. McHale, M. Willis, Semicond. Sci. Technol. 12 (1997) 455. [26] M. Khatkale, P. Devlin, J. Chem. Phys. 70 (1979) 1851. [27] M. Kim, Y. Kim, S. Moon, S. Choi, Bull. Korean Chem. Soc. 17 (1996) 424. [28] P. Krysinski, Adv. Mater. Opt. Electron. 8 (1998) 121. [29] P. Kunkeler, P. Koningsbruggen, J. Cornelissen, A. vander Horst, A. vander Kraan, A. Spek, J. Haasnoot, J. Reedijk, J. Am. Chem. Soc. 118 (1996) 2190. [30] M. Le Cointe, M. Lemee-Cailleau, H. Cailleau, B. Toudic, J. Mol. Struct. 374 (1996) 147. [31] J. Miller, J. Zhang, W. Reiff, D. Dixon, L. Preston, A. Reis, E. Gebert, M. Extine, J. Troup, A. Epstein, M. Ward, J. Phys. Chem. 91 (1987) 4344. [32] H. Staab, J. Weikard, A. Ruckemann, A. Schwogler, Eur. J. Org. Chem. (1998) 2703. [33] M. Suarez, A.M. Bond, R. Compton, J. Solid State Electro-chem. 4 (1999) 24. [34] Symons, P. G. Ph.D. Thesis, Monash University, Melbourne, 1998; p177. [35] S. Yamaguchi, R. Potember, Synth. Met. 78 (1996) 117. [36] H. Zhao, R. Heintz, K.R. Dunbar, J. Am. Chem. Soc. 118 (1996) 12844. [37] H. Zhao, R. Heintz, X. Ouyang, G. Grandinetti, J. Cowen, K.R. Dunbar, Conductors and Magnets, Kluwer, Dordrecht, 1999, p. 353. [38] Hasanudin, N. Kuroda, T. Sugimoto, Synth. Met. 120 (2001) 1045.
[39] M.A. Khan et al., Appl. Phys. 40 (2007) 6535. [40] C. Ran, H. Peng, L. Ren, W. Zhou, Y. Ling, Z. Liu, J. Phys. Chem. 111 (2007) 631. [41] M. Cano, J. Luis Avila, M. Luis Avila, M.L. Mena, J. Ping arron, R. Rodr iguezAmaro, J. Electroanal. Chem. 615 (2008) 69. [42] M. Lamberto, Elizabeth.E. Rastede, J. Decker, Françisco M. Raymo, Tetrahedron Lett. 51 (2010) 5618. [43] J. Mortimer Roger, Electrochim. Acta 44 (18) (1999) 2971. [44] X. Tu, X.k. Fu, Q.l.. Jiang, Z.J. Liu, G.D. Chen, Dyes Pigm. 88 (2011) 39. [45] A.R. Harris, A.K. Neufeld, A.P. O’Mullane, A.M. Bond, R.J.S. Morrison, J. Electrochem. Soc. 152 (2005) C577. [46] P. Cea, C. Lafuente, J.S. Urieta, M.C. Lopez, F.M. Royo, Langmuir 14 (1998) 7306. [47] R.M. Silverstein, G. Clayton Bassler, T.C. Morril, Spec-trometric Identification of Organic Compounds, 5th ed., John Wiley and Sons Inc., 1991. [48] J. Deng, X.K. Fu, G. Wang, L. Wu, J. Huang, Electrochim. Acta 85 (2012) 195. [49] A. Yamagishi, T. Masui, F. Watenabe, J. Coll. Interf. Sci. 72 (1979) 154. [50] L.R. Melby, R.J. Harder, W.R. Hertler, W. Mahler, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 84 (1962) 3374. [51] A. Remleaum, A.M. Hermann, F.E. Stewart, F. Gut-mann, J. Phys. Chem. 73 (1969) 513. [52] A. Yamagishi, F. Watenabe, J. Coll. Interf. Sci. 59 (1977) 181. [53] Anthony.P. O’Mullane, N. Fay, A. Nafady, Alan M. Bond, J. Am. Chem. Soc. 129 (2007) 2066. [54] K. Colladet, M. Nicolas, L. Goris, L. Lutsen, D. Vanderzande, Thin Solid Films 7 (2004) 451. [55] G. Wang, X.k. Fu, J. Huang, L. Wu, Q.l. Du, Electrochim. Acta 55 (2010) 6933. [56] B. Yigitsoy, S. Varis, C. Tanyeli, I.M. Akhmedov, L. Toppare, Electrochim. Acta 52 (2007) 6561. [57] G. Wang, X.k. Fu, J. Huang, L. Wu, J. Deng, J. Electroanal. Chem. 661 (2011) 351. [58] C. Bechinger, M.S. Burdis, J.-G. Zhang, Solid State Commun. 101 (1997) 753. [59] W.A. Gazotti, J.R.G. Casalbore-Micelli, A. Geri, M.A. De-Paoli, Adv. Mater. 10 (1998) 60.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Electrochromic and spectroelectrochemical properties of novel 4,40 -bipyridilium–TCNQ anion radical complexes Guoming Wang, Xiangkai Fu ⇑, Jun Deng, Xuemei Huang, Qiang Miao College of Chemistry and Chemical Engineering, Research Institute of Applied Chemistry Southwest University, The Key Laboratory of Applied Chemistry of Chongqing Municipality, Key Laboratory of Eco-environments in Three Gorges Reservoir Region (Ministry of Education), Chongqing 400715, PR China
a r t i c l e
i n f o
Article history: Received 17 April 2013 In final form 20 June 2013 Available online 27 June 2013
a b s t r a c t Three novel electrochromic materials 7,7,8,8-tetracyanoquinodimethane (TCNQ) anion radical salts with substituted 4,40 -bipyridilium derivatives (monosubstituent-4,40 -bipyridilium) were prepared. The structure of the complexes was characterized by Elemental analyses, Solid IR spectra and UV–vis spectroscopy. The electrochromic behaviors and electrooptical properties of the complexes were investigated by cyclic voltammetry and UV–vis absorption spectra. Electrochromic devices based on monosubstituent 4,40 bipyridilium–TCNQ anion radical salts (abbreviated as MBTS) were fabricated which exhibited green– magenta color change. Their color reversibility was excellent with high color-change efficiency after 1000 cycles of the transmittance and transmittance change. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ‘‘Viologens’’ (1,10 -disubstituted-4,40 -bipyridilium salts) [1–3], have focused attention of many research groups due to both their theoretical interest and practical applications. The ready availability of 4,40 -bipyridine and the ease of varying the nature of the quaternizing agent have allowed intensive research into the electrochromic properties of such ‘viologens’ [4]. Of the three common viologen redox states (bipm2+ ? bipm + ? bipm0), dication is the most stable and colorless when pure unless optical charge transfer with counter anion occurs. The viologen radical cations in particular are highly colored with high molar absorption coefficients owing to optical charge transfer between (formally) +1 and zero valent nitrogens. Radical cations with short alkyl chains are blue (blue-purple in concentrated solution), and becoming crimson as the alkyl chain length increased due to the increased dimerization [5]. The main applications of bipyridilium systems are smart windows [6,7], electrochromic displays [8,9], car rear-view mirrors [10,11] and electronic paper [12]. However, monosubstituted 4,40 bipyridilium derivatives used for electrochromic materials have seldom been reported due to their monocations’ instability [13]. In this Letter, star-shaped substituent 4,40 -bipyridinlium–TCNQ salts could overcome this defect and can be used as promising candidates for electrochromic materials. On the other hand, 7,7,8,8-tetracyanoquinodimethane (TCNQ) is a well-known electron acceptor which readily forms charge–transfer (CT) complexes with an electron donor such as amine compounds [14–16]. The charge–transfer salts were almost ⇑ Corresponding author. Fax: +86 023 68254000. E-mail address: [email protected] (X. Fu). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.06.037
established immediately after TCNQ was first synthesized and applications are very common now [17–37]. These TCNQ-based materials are considerably interesting due to their extensive and novel electrical, electrochemical, and magnetic properties arising from the characteristic p-stacking of the TCNQ moieties into columns. TCNQ as well as its derivatives have been extensively used as electron acceptor molecules [38], light emitting diodes [39], data storage devices [40], organic field-effect transistors [41], potentiometric sensors [24] and incorporated into electrochromic devices [25]. With this feature, the combination of two moieties, i.e. viologen and TCNQ, might result in a new type of novel cathodic–anodic composite electrochromic materials which might possess unique properties. What is more, the complexes between viologen salts and TCNQ anion radical have not been previously reported. Therefore, a series of novel cathodic-anodic composite ECMs (MBTS) were synthesized by incorporating cathodic material viologen with anodic material TCNQ anion radical and their corresponding ECDs were assembled for the first time. The structures of complexes MBTS were shown in Scheme 1, and the films of these ECMs were investigated by measuring the changes in ECDs charge, optical and electrochemical methods. Furthermore, the electrochromic coloration and response times of the ECDs were also investigated in detail. 2. Experimental details 2.1. General All manipulations in this Letter involving air-sensitive reagents were performed in an atmosphere of dry argon. The chemicals and reagents, unless otherwise specified, were purchased from Aldrich,
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(w), 1506 (mC@C),1400 (s), 809 (dCH). The bands at 2198 and 2169 are attributed to m (CN) of TCNQ1.
3. Results and discussion 3.1. Electrochemical properties and characterization
Scheme 1. Synthetic route of MBTS 3a–c.
Acros, and TCI Chemical Co. and used as received. All the solvents were further purified before use. All the new synthesized compounds were characterized by Elemental analyses, Solid IR spectra and UV spectra. Elemental analyses were performed on a Costech ECS 4010 instrument; values agreed with the calculation. Solid IR spectra (400–4000 cm1) were obtained using potassium bromide (KBr) pellets with a Polari’s FT-IR spectrometer. UV spectra were performed on a Unico UV-4802H UV–visible spectrophotometer connected to a computer. Cyclic voltammetry was carried out on CHI 650B electrochemical workstation using three-electrode system. All ECDs were fabricated, sealed and tested at room temperature. 2.2. General procedure for the synthesis of LiTCNQ Purple solid: Yield: 92%. To a boiling solution of 10.2 g (0.05 mol) of TCNQ in 1000 ml of acetonitrile was added a boiling solution 20.0 g (0.15 mol) of lithium iodide in 50 ml of acetonitrile. Purple crystals separated from the dark brown solution. The mixture stood for 4 h at room temperature and the purple solid was collected, then washed on the filter with acetonitrile until the washings were bright green. The solid was then washed with a large volume of ether. 2.3. Preparation of viologen derivatives 2a–c The viologen derivatives were prepared with three alkyl halides. Their structure was characterized according to literature methods [3,42]. 2.4. General procedure for the synthesis of MBTS Twenty milliters of MeOH solution of 2a–c (1.0 mmol) was added to 20 mL boiling MeOH of LiTCNQ (213 mg, 1.0 mmol). The mixture was refluxed, a colored solid appeared, which was filtered, washed with MeOH and Et2O in turn, dried in vacuum (Scheme 1). 2.4.1. Preparation of 3a Blue-black solid: Yield: 54.5%. M.p.: 248–250 °C. Anal. Calc. for C23H15N6: C, 73.59; H, 4.03; N, 22.39. Found: C, 73.64; H, 4.12; N, 22.20%. FT-IR (KBr, cm1): 2187, 2170 (mCN, TCNQ1), 1638, 1618 (s), 1579, 1506 (s), 1505 (mC@C), 1400 (s), 819 (dCH), 620. The bands at 2187 and 2170 are attributed to m (CN) of TCNQ1. 2.4.2. Preparation of 3b Purple-black solid: Yield: 90%. M.p.: 202–203 °C. Anal. Calc. for C27H21N6: C, 75.50; H, 4.93; N, 19.57. Found: C, 75.46; H, 4.78; N, 19.70%. FT-IR (KBr, cm1): 2186, 2171 (mCN, TCNQ1), 1637 (s), 1580, 1522 (w), 1507 (mC@C), 1401 (s), 820 (dCH). The bands at 2186 and 2171 are attributed to m (CN) of TCNQ1. 2.4.3. Preparation of 3c Purple solid: Yield: 54.5%. M.p.: 189–190 °C. Anal. Calc. for C29H19N6: C, 77.14; H, 4.24; N, 18.62. Found: C, 77.18; H, 4.43; N, 18.35%. FT-IR (KBr, cm1): 2198, 2169 (mCN, TCNQ1), 1637, 1581
3.1.1. Electrochemical properties In order to study the mechanism of the electrochromism and electrochemistry for MBTS, electrochemical measurements were performed by using cyclic voltammetry at room temperature in a conventional three-electrode system using platinum disk (0.02 cm2) as working electrodes, platinum wire as counter electrodes and Ag/AgCl in 3 M NaCl (aq) solution as a reference, with 0.1 M TBAP (Tetrabutylammonium perchlorate) in CH3CN as the electrolyte. All of the electrochemistry experiments were carried out at room temperature under nitrogen atmosphere. The full scan-range cyclic voltammetry curves were shown in Figure 1, from which two couples of redox waves were observed. The cyclic voltammetry data was recorded simultaneously during 100 cyclic voltammetry experiments. After 100 successive wide-range potential cycling between 0.75 and +0.75 V, three MBTS similar welldefined reversible redox couple (Ep, 1/2) that was centered at about 0.32 V, 0.23 V for 3a; 0.35 V, 0.23 V for 3b; 0.36 V, 0.22 V for 3c. From cyclic voltammograms analysis, it was observed that all of the three complexes unchanged much after 100 cycling, which indicates the three TCNQ complexes showed fine electrochemical reversibility and stabilities. Usually, compared with traditional 1,10 -disubstituent-4,40 -bipyridilium derivatives, monosubstituted 4,40 -bipyridilium derivatives used for electrochromic materials have seldom been reported due to their monocations’ instability [13]. However, in this Letter, when it combine with TCNQ to form the monosubstituted 4,40 -bipyridilium–TCNQ anion radical complexes 3a–c, they also performed superior cycling stabilities and well-defined reversibility. The mechanism of cathode–anode composite electrochromic compound has been reported [43]. In the curve of complex 3a, the first cathodic peak at 0.26 V, corresponds to the reaction between +1 and zero valent nitrogens of viologen part of 3a[44], and this can be expressed as in Scheme 2a. While the first anodic peak at +0.17 V was due to the oxidation of TCNQ anion radical [45]. As can be illustrated in Scheme 2b. In addition, the complexes MBTS of propylene carbonate (PC) solution were intensely green color in the neutral state owing to the coloration of TCNQ radical anion. While when the voltage applied, the color changes from green to magenta, which actually due to the reduction of viologen at the cathode part i.e., the coloration of monosubstituent 4,40 -bipyridilium radical. The mechanism and color change of complexes MBTS can be expressed in Scheme 2c.
3.1.2. UV study of solution Investigation of the electronic properties of MBTS measured by UV–vis in CH3CN, revealed that the individual chromophores were enhanced in the electronic spectrum ground state. As seen in Figure 2, the neutral TCNQ absorbs only at 395 nm. However, when it combine with monosubstituent-4,40 -bipyridilium, new absorption bands appear at 260, 420, 680 and 743 nm. The absorption band around 260 nm might be assigned to the p–p⁄ transitions which due to intramolecular transition of the viologen unit [46– 48], while the last two bands located at 680 and 750 nm were related to the absorption of TCNQ anion radical [49–52]. It was also noted that the absorbance at 420 nm attributed to the charge– transfer band was still observed. Furthermore, it was clear that the absorption maximum (kmax) in the spectra of three complexes
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Figure 1. Cyclic voltammetry of complexes 3a–c in 0.1 M TBAP/CH3CN vs. Ag/AgCl. The first, 50th and 100th cycles are shown and the scan rate is 50 mV/s.
(a)
(b)
(c) Scheme 2. The mechanism and color change of complexes MBTS.
and ion conducting layer [44]. However, in this Letter, a newly simple liquid EC device was applied without additioning conductive polymers and supporting electrolyte. In this device, an indium tin oxide (ITO)-coated glass surface (conductive side inwards) and a refective metallic surface, spaced a fraction of a millimetre apart (35 40 0.7 mm, 50 X/sq), form the two electrodes of the cell, with a solvent containing electroactive chemical species. The electrolyte solution was prepared by dissolving MBTS and 0.1 M of Ferrocene (abbreviated as Fc) in propylene carbonate (PC). Special attention must be paid to avoid any contamination from moisture and oxygen. The solution electrolyte needs to be bubbled with argon gas before use, and all the containers should be dried in oven. The prepared electrolyte was injected into the electrochromic device later using syringe. The ECD was assembled as the structure including, conductive ITO glass ||MBTS + Fc|| PC|| conductive ITO glass. 3.3. Electrochromic properties
Figure 2. UV–vis absorption spectra of neutral TCNQ and TCNQ anion radical salts 3a–c (0.02 M) in CH3CN at room temperature.
were similar. Normally change was observed for the different substituted groups.
3.2. Electrochromic devices construction The most common electrochromic device configuration is a solid EC device. In this configuration, a thin film cathode electrochromic material layer and an anode electrochromic material counterelectrode layer are separated by a polymer electrolyte
3.3.1. Spectroelectrochemistry properties study The best way of examining the changes in optical properties of electrochromic material upon voltage applied is spectroelectrochemistry. The spectroelectrochemical and electrochromic properties of the ECDs based on resultant three TCNQ complexes were studied by applying stepped potentials sequentially from 2.0 to 3.5 V. As shown in Figure 3, three distinguishable absorbance peaks at around 420, 745 and 850 nm were observed in the neutral state. When the voltage applied to the ECDs, the absorbances of the bands with kmax at 745 and 752 nm decreased, while a new band appears at 400 nm, which correspond to the formation of neutral TCNQ band [53]. Figure 4 shows photograph of ECD with MBTS as ECMs in the neutral state and different colored state when the potential of 3.0 V was applied. The three substituent 4,40 -bipyridilium–TCNQ anion radical complexes were intensely green color at 0 V (Figure 4 a), with high molar absorption coefficients owing to optical complete charge transfer. When ±3.0 V potential was applied between the ECM layers, the color of the ECDs changed from green to magenta (Figure 4 b). The color change is due to the reduction of viologens at the cathode part whereas the other TCNQ anion radical at the anodic part is oxidized. Once the potential is switched again to 0 V, the device recovers its original green color, proving that the electrochromic device works satisfactorily. Table 1 summarizes the maximum absorption wavelength (kmax), optical band gaps (Eg) values, redox couple (Eox), Ep,1/2 of three TCNQ complexes quite clearly. The optical band gap of the
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Figure 3. UV/vis/NIR spectroelectrochemistry for MBTS on ITO glass, at different potential stepping from 2.0 to 3.5 V with 0.5 V decrement, respectively. Applied potentials are the following: (a) 0 V; (b) 2.0 V; (c) 2.5 V; (d) 3.0 V; (e) 3.5 V.
Figure 4. Colors of a sandwich-type MBTS in ECDs. (a) Neutral state, (b) MBTS at 3 V.
Table 1 Electrical and optical data of the three TCNQ complexes. Complexes Eox (V) 3a 3b 3c
EP,1/2 (V)
konset (nm) Eg (eV) 0 V
0.38, 0.17 0.32, 0.23 456 0.44, 0.14 0.35, 0.23 468 0.43, 0.15 0.36, 0.22 458
2.72 2.65 2.71
±3 V
Green Magenta Green Magenta Green Magenta
electrochromic materials were calculated from their low energy absorption edges (konset) according to Planck equation (Eg = 1241/ konset) [54]. As seen in Figure 3, the kmax and konset of the absorption spectra of 3a on ITO glass were at 398 nm and 456 nm, corresponded to a band gap of 2.72 eV; for 3b, the kmax and konset of the absorption spectra were observed at 400 and 458 nm, corresponded to a band gap of 2.65 eV; While for 3c, the kmax and konset of the absorption spectra were observed at 396 and 458 nm, corresponded to a band gap of 2.71 eV. 3.3.2. Switching and stability study Switching and stability are two important properties of an electrochromic material and its corresponding ECD [55]. The three TCNQ anion radical salts of materials on the ITO glass were switched by repeating the potential steps using 3.0 V and +3.0 V with a residence time of 5 s in the PC solution. The optical contrast (DT%), which can be defined as the transmittance difference between the redox states, was monitored as a function of time at the wavelength in the visible region. In order to investigate DT% of the three TCNQ salts of materials between 3.0 V and +3.0 V, three absorption wavelength 400 nm, 745 nm,
850 nm were chosen to monitor. As shown in Figure 5, the maximum DT% at 745 nm of the complex was found to be 36% for 3a, 41% for 3b, 46% for 3c, respectively. To compare with this result, compounds MBTS exhibited superior stability and optical properties. Response time, one of the most important characteristics of electrochromic materials, is the time needed to perform a switching between the two colored states of the materials [56]. Quantification of the switching time was performed by defining a change in 95% of the total absorbance span, as the naked eye could not distinguish the color change after this point [57]. As shown in Figure 6, the time required attain 95% of total transmittance difference for reduction process and oxidation process is found to be 1.77 s, 1.7 s for 3a; 1.90 s, 0.99 s for 3b; 0.97 s, 0.99 s for 3c, which indicate the three MBTS have faster response time than traditional viologens alone. The coloration efficiency (CE) is also an important characteristic for the electrochromic materials when making a comparison between electrochromic materials and devices [40]. The CE value can be calculated by using the equations and given below [58]:
MOD ¼ log
Tb MOD and CEðgÞ ¼ Qd Tc
ð1Þ
where Tb and Tc are the light transmittances in the neutral state and reduced state of the ECD at certain wavelength (k), 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). Qd is the amount of injected/ejected charge per unit sample area. On the basis of these equations, the DOD and CE were
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3.3.3. Stability of ECD Redox stability is another important parameter for ECDs. The stability of devices toward multiple redox switching usually limits the application of electrochromic materials in ECD utility [59]. A thousand cycles of the transmittance and transmittance change for viologen–TCNQ complex 3c were measured and the result was illustrated in Figure 7. The complex 3c presented a stable and reproducible electrochromic process at 745 nm between 3.0 V and +3.0 V even after multiple scans, e.g., the retained DT% is 42% for 3c after the first 1000 cycles. The results indicate that there is no obvious decrease of activity between first cycle and 1000 cycles. 4. Conclusion
Figure 5. Optical transmittance changes for 3a–c at 745 nm in 0.01 M Fc/PC under subsequent double potential steps between 3.0 and +3.0 V.
In summary, three novel electrochromic materials MBTS were synthesized in this Letter. Cyclic voltammetry (CV), UV–vis and FT-IR analyzes confirm the resultant structure. MBTS can be described as the charge–transfer (C–T) complexes and cathode–anode composite electrochromic materials. The ECDs based on C–T complexes were assembled and three distinguishable absorbance peaks at around 400, 745 and 850 nm at applied potential were observed. According to the spectroelectrochemical analyze, these ECMs reveal distinct electrochromic properties from that of corresponding ECDs and show green–magenta color change for MBTS under applied potentials. Their color and reversibility were excellent with high coloration efficiency after 1000 cycles of the transmittance and transmittance change. Moreover, the number of EC circulation for most of our ECDs was over 104, and the response times for the electrochromic processes were less than common viologen alone. These results will make these ECMs promising candidate for applications in electrochromic devices and displays. Acknowledgements
Figure 6. Optical switching studies for ECDs based on MBTS 3a–c were monitored at 745 nm.
All authors herein are grateful to the support from National Ministry of Science and Technology Innovation Fund for High-tech Small and Medium Enterprise Technology (No. 09C26215112399) and National Ministry of Human Resources and Social Security Start-up Support Projects for Returned oversea Students to Business, Office of Human Resources and Social Security Issued 2009 (143). References
Figure 7. Transmittance at a monochromatic wavelength of 745 nm as a function of time for complex 3c. (First cycle, solid line; after 1000 cycles, dotted line.)
calculated at 750 nm to be 0.60 and 645 cm2/C for 3a, 1.06 and 825 cm2/C for 3b, 0.74 and 796 cm2/C for 3c, which had high coloration efficiency.
[1] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [2] C.L. Bird, A.T. Kuhn, Chem. Soc. Rev. 10 (1981) 49. [3] P.M.S. Monk, The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,40 -Bipyridine, J. Wiley and Sons, Chichester, 1998. [4] R.J. Mortimer, Electrochim. Acta 44 (1999) 2971. [5] G. Bara, N. Larinab, L. Grinisc, V. Lokshinb, R. Gvishi, Sol. Energy Mater. Sol. Cells 99 (2012) 123. [6] S. Panero, B. Scrosati, M. Baret, B. Cecchini, E. Masetti, Sol. Energy Mater. Sol. Cells 39 (1995) 239. [7] Y. Kim, J. Do, E. Kim, G. Clavier, L. Galmiche, P. Audebert, J. Electroanal. Chem. 632 (2009) 201. [8] P.M.S. Monk, J. Electroanal. Chem. 432 (1–2) (1997) 175. [9] K. Bange, Sol. Energy Mater. Sol. Cells 58 (1) (1999) 1. [10] D.R. Rosseinsky, R.J. Mortimer, Adv. Mater. 13 (2001) 783. [11] N.R. Lynam, A. Agrawal, Automotive applications of chromogenic materials, in: C.M. Lampert, C.G. Granqvist (Eds.), Large Area Chromogenics: Materials and Devices for Transmittance Control, SPIE Optical Engineering Press, Bellingham, Washington, USA, 1990, p. 46. [12] P.M.S. Monk, F. Delage, S.M. Costa Vieira, Electrochim. Acta 46 (2001) 2195. [13] G. Wang, X.K. Fu, J. Huang, C.l. Wu, L. Wu, Q.l. Du, Org. Electron. 12 (2011) 1216. [14] D.S. Acker, R.J. Harder, W.R. Hertler, W. Mahler, L.R. Melby, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 82 (1960) 6408. [15] L.R. Melby, W. Mahler, W.E. Mochel, R.J. Harder, W.R. Hertler, R.E. Benson, J. Am. Chem. Soc. 84 (1962) 3374. [16] J.B. Torrance, Mol. Cryst. Liq. Cryst. 126 (1985) 55.
110
G. Wang et al. / Chemical Physics Letters 579 (2013) 105–110
[17] M. Azcondo, L. Ballester, S. Golhen, A. Gutierrez, L. Ouahab, P. Delhanes, J. Mater. Chem. 9 (1999) 1237. [18] L. Ballester, C. Alonso, M. Fonari, A. Gutierrez, M. Mitewa, M. Perpinan, K. Suwinska, J. Alloys Compd. 323–324 (2001) 138. [19] R. Bozio, I. Zanon, A. Girlando, C. Pecile, J. Chem. Soc., Faraday Trans. 2 (1977) 235. [20] C. Campana, K.R. Dunbar, X. Ouyang, Chem. Commun. (1996) 2427. [21] J. Fox, B. Foxman, D. Guarrera, J. Miller, J. Calabrese, A. Reis, J. Mater. Chem. 6 (1996) 1627. [22] A. Graja, J. Farges, A. Brau, P. Dupuis, Synth. Met. 70 (1995) 1233. [23] Hasanudin, N. Kuroda, T. Sugimoto, Synth. Met. 120 (2001) 90. [24] M. Higo, X. Lu, U. Mazur, K. Hipps, Thin Solid Films 384 (2001) 90. [25] P. Hooper, M. Newton, G. McHale, M. Willis, Semicond. Sci. Technol. 12 (1997) 455. [26] M. Khatkale, P. Devlin, J. Chem. Phys. 70 (1979) 1851. [27] M. Kim, Y. Kim, S. Moon, S. Choi, Bull. Korean Chem. Soc. 17 (1996) 424. [28] P. Krysinski, Adv. Mater. Opt. Electron. 8 (1998) 121. [29] P. Kunkeler, P. Koningsbruggen, J. Cornelissen, A. vander Horst, A. vander Kraan, A. Spek, J. Haasnoot, J. Reedijk, J. Am. Chem. Soc. 118 (1996) 2190. [30] M. Le Cointe, M. Lemee-Cailleau, H. Cailleau, B. Toudic, J. Mol. Struct. 374 (1996) 147. [31] J. Miller, J. Zhang, W. Reiff, D. Dixon, L. Preston, A. Reis, E. Gebert, M. Extine, J. Troup, A. Epstein, M. Ward, J. Phys. Chem. 91 (1987) 4344. [32] H. Staab, J. Weikard, A. Ruckemann, A. Schwogler, Eur. J. Org. Chem. (1998) 2703. [33] M. Suarez, A.M. Bond, R. Compton, J. Solid State Electro-chem. 4 (1999) 24. [34] Symons, P. G. Ph.D. Thesis, Monash University, Melbourne, 1998; p177. [35] S. Yamaguchi, R. Potember, Synth. Met. 78 (1996) 117. [36] H. Zhao, R. Heintz, K.R. Dunbar, J. Am. Chem. Soc. 118 (1996) 12844. [37] H. Zhao, R. Heintz, X. Ouyang, G. Grandinetti, J. Cowen, K.R. Dunbar, Conductors and Magnets, Kluwer, Dordrecht, 1999, p. 353. [38] Hasanudin, N. Kuroda, T. Sugimoto, Synth. Met. 120 (2001) 1045.
[39] M.A. Khan et al., Appl. Phys. 40 (2007) 6535. [40] C. Ran, H. Peng, L. Ren, W. Zhou, Y. Ling, Z. Liu, J. Phys. Chem. 111 (2007) 631. [41] M. Cano, J. Luis Avila, M. Luis Avila, M.L. Mena, J. Ping arron, R. Rodr iguezAmaro, J. Electroanal. Chem. 615 (2008) 69. [42] M. Lamberto, Elizabeth.E. Rastede, J. Decker, Françisco M. Raymo, Tetrahedron Lett. 51 (2010) 5618. [43] J. Mortimer Roger, Electrochim. Acta 44 (18) (1999) 2971. [44] X. Tu, X.k. Fu, Q.l.. Jiang, Z.J. Liu, G.D. Chen, Dyes Pigm. 88 (2011) 39. [45] A.R. Harris, A.K. Neufeld, A.P. O’Mullane, A.M. Bond, R.J.S. Morrison, J. Electrochem. Soc. 152 (2005) C577. [46] P. Cea, C. Lafuente, J.S. Urieta, M.C. Lopez, F.M. Royo, Langmuir 14 (1998) 7306. [47] R.M. Silverstein, G. Clayton Bassler, T.C. Morril, Spec-trometric Identification of Organic Compounds, 5th ed., John Wiley and Sons Inc., 1991. [48] J. Deng, X.K. Fu, G. Wang, L. Wu, J. Huang, Electrochim. Acta 85 (2012) 195. [49] A. Yamagishi, T. Masui, F. Watenabe, J. Coll. Interf. Sci. 72 (1979) 154. [50] L.R. Melby, R.J. Harder, W.R. Hertler, W. Mahler, R.E. Benson, W.E. Mochel, J. Am. Chem. Soc. 84 (1962) 3374. [51] A. Remleaum, A.M. Hermann, F.E. Stewart, F. Gut-mann, J. Phys. Chem. 73 (1969) 513. [52] A. Yamagishi, F. Watenabe, J. Coll. Interf. Sci. 59 (1977) 181. [53] Anthony.P. O’Mullane, N. Fay, A. Nafady, Alan M. Bond, J. Am. Chem. Soc. 129 (2007) 2066. [54] K. Colladet, M. Nicolas, L. Goris, L. Lutsen, D. Vanderzande, Thin Solid Films 7 (2004) 451. [55] G. Wang, X.k. Fu, J. Huang, L. Wu, Q.l. Du, Electrochim. Acta 55 (2010) 6933. [56] B. Yigitsoy, S. Varis, C. Tanyeli, I.M. Akhmedov, L. Toppare, Electrochim. Acta 52 (2007) 6561. [57] G. Wang, X.k. Fu, J. Huang, L. Wu, J. Deng, J. Electroanal. Chem. 661 (2011) 351. [58] C. Bechinger, M.S. Burdis, J.-G. Zhang, Solid State Commun. 101 (1997) 753. [59] W.A. Gazotti, J.R.G. Casalbore-Micelli, A. Geri, M.A. De-Paoli, Adv. Mater. 10 (1998) 60.