pubs.acs.org/Langmuir © 2009 American Chemical Society
Modulation of Morphology and Conductivity of Mixed-Valence Tetrathiafulvalene Nanofibers by Coexisting Organic Acid Anions Kazuo Tanaka,† Tomoyuki Kunita,† Fumiyasu Ishiguro,† Kensuke Naka,‡ and Yoshiki Chujo*,† †
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, and ‡Department of Chemistry and Materials Technology, Graduate School of Science and Technology, Kyoto Institute of Technology, Sakyo-ku, Kyoto 606-8585, Japan Received January 19, 2009. Revised Manuscript Received February 22, 2009 We describe here the facile preparation of conductive casting films consisting of mixed-valence tetrathiafulvalene (TTF) nanofibers. Self-assembled nanofibers of mixed-valence TTF with organic acid anions were grown during film deposition, and the conductivities of the resulting films were significantly altered by the type of the anion. In particular, the casting films containing heptafluorobutyrate, propanesulfonate, and undecanesulfonate work as semiconductors (10-2 S/cm). It was revealed that anion species played a crucial role in the formation of the nanofibers and the morphology of the film was varied.
Introduction The fabrication of well-ordered molecular arrays is of considerable interest in modern science and technology for developing molecular devices.1 One-dimensional (1D) conductive materials have been studied with the expectation for potential applications not only as a molecular wire but also as an interconnect module in the integration of molecular circuits.2 In particular, organic conducting 1D nanostructures have attracted much attention due to their low density and flexibility for molecular design compared with the inorganic nanostructures.3 Various chemical synthetic methods have been established to accomplish 1D growth of nanostructures,4 among which template-directed synthesis is the most widely used method for generating metallic or metal oxide nanowires.5 On the other hand, creation of template-free approaches is still required for future practical applications of the organic 1D nanostructures.6,7 Tetrathiafulvalene (TTF)-based charge-transfer (CT) complexes have been known to form conducting crystals consisting of segregated stacks of TTF and acceptors.8 The TTF radical *Corresponding author. E-mail:
[email protected]. Fax: +81-75-383-2605. Telephone: +81-75-383-2604. (1) (a) Li, J.; Kamata, K.; Watanabe, S.; Iyoda, T. Adv. Mater. 2007, 19, 1267. (b) Horie, M.; Sakano, T.; Osakada, K. J. Organomet. Chem. 2006, 691, 5935. (2) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362. (3) (a) Haung, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (b) Chiu, J.-J.; Kei, C.-C.; Perng, T.-P.; Wang, W.-S. Adv. Mater. 2003, 15, 1361. (c) Liu, H.; Zhao, Q.; Li, Y.; Liu, Y.; Lu, F.; Zhuang, J.; Wang, S.; Jiang, L.; Zhu, D.; Yu, D.; Chi, L. J. Am. Chem. Soc. 2005, 127, 1120. (d) Gan, H.; Liu, H.; Li, Y.; Zhao, Q.; Li, Y.; Wang, S.; Jiu, T.; Wang, N.; He, X.; Yu, D.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 12452. (4) Ahmadi, T. S.; Wang, Z. L.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (5) Xia, Y.; Yang, P.; Sun, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (6) Akutagawa, T.; Ohta, T.; Hasegawa, T.; Nakamura, T.; Christensen, C. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5028. (7) Yasui, K.; Kimizuka, N. Chem. Lett. 2005, 34, 248. (8) (a) Iyoda, M.; Hasegawa, M.; Miyake, Y. Chem. Rev. 2004, 104, 5085. (b) Segura, J. L.; Nartın, N. Angew. Chem., Int. Ed. 2001, 40, 1372. (c) Bryce, M. R. J. Mater. Chem. 1995, 2, 1481–1496. (d) Kitamura, T.; Nakaso, S.; Mizoshita, N.; Tochigi, Y.; Shimomura, T.; Moriyama, M.; Ito, K.; Kato, T. J. Am. Chem. Soc. 2005, 127, 14769. (e) de Caro, D.; Malfant, I.; Savy, J.-P.; Valade, L. J. Phys.: Condens. Matter 2008, 20, 184012. (f) Iyoda, M.; Hasegawa, M.; Enozawa, H. Chem. Lett. 2007, Vidal-Gancedo, J.; 36, 1402. (g) Puigmartı-Luis, J.; Laukhin, V.; Perez del Pino, A.; Rovira, C.; Laukhina, E.; Amabilino, D. B. Angew. Chem., Int. Ed. 2007, 46, 238. Ujaque, G.; Rovira, C.; Lledos, (h) Puigmartı-Luis, J.; Minoia, A.; Perez del Pino, A.; A.; Lazzaroni, R.; Amabilino, D. B. Chem.;Eur. J. 2006, 12, 9161.
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cation can work as an acceptor, and the mixture of neutral TTF and TTF radical cation can form self-assembled mixed-valence CT complexes.9 The casting films obtained from the solutions of the mixed-valence TTF can show electrical conductivity as a semiconductor (ca. 10-2 S/cm), which makes these organic CT salts interesting candidates for structurally well-defined microscopic conducting wires to meet the need of ever increasing complexity and miniaturization of electronic devices.10 However, when these crystalline organic materials are prepared, severe conditional controls in the crystal growth and the fiber formation are required for obtaining sufficient conductivity. In addition, the difficulties for regulating the anisotropy and the morphology of the fibers still remain. Herein, we describe the facile preparation of conductive nanofibers consisting of the mixed-valence TTF. Self-assembled nanofibers of the mixed-valence TTF with organic anions can be constructed with a simple casting method, and the casting film can work as a semiconductor. In particular, the conductivity of the casting films was significantly influenced by the type of the coexisting anions in the nanofibers. The present Article indicates that the anion species played a crucial role in the formation of the nanofibers due to the differences of their crystallinity. This study presents a simple and valid methodology for regulating the film conductivity of the TTF nanofibers.
Results and Discussion In this study, we used iodobenzene diacetate as an oxidizer for TTF and strong acids which are promised to be the anion species in the nanofibers.11 We synthesized the TTF radical cation salts (9) (a) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden, P. E. Phys. Rev. 1979, 19, 7301. (b) Naka, K.; Ando, D.; Wang, X.; Chujo, Y. Langmuir 2007, 23, 3450. (c) Iyoda, M.; Hasegawa, M.; Kuwatani, Y.; Nishikawa, H.; Fukami, K.; Nagase, S.; Yamamoto, G. Chem. Lett. 2001, 1146. (10) (a) Yoshizawa, M.; Kumazawa, K.; Fujita, M. J. Am. Chem. Soc. 2005, 127, 13456. (b) Kitahara, T.; Shirakawa, M.; Kawano, S.; Beginn, U.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 14980. (c) Lyskawa, J.; Salle, M.; Balandier, J.-Y.; Le Derf, F.; Levillain, E.; Allain, M.; Viel, P.; Palacin, S. Chem. Commun. 2006, 2233. (d) Chiang, P.-T.; Chen, N.-C.; Lai, C.-C.; Chiu, S.-H. Chem.;Eur. J. 2008, 14, 6546. (e) Azov, V. A.; Gomez, R.; Stelten, J. Tetrahedron 2008, 64, 1909. (f) Aprahamian, I.; Olsen, J.-C.; Trabolsi, A.; Stoddart, J. F. Chem.;Eur. J. 2008, 14, 3889. (11) Giffard, M.; Mabon, G.; Leclair, E.; Mercier, N.; Allain, M.; Gorgues, A.; Molinie, P.; Neilands, O.; Krief, P.; Khodorkovsky, V. J. Am. Chem. Soc. 2001, 123, 3852.
Published on Web 03/23/2009
DOI: 10.1021/la900219b
6929
Article
Tanaka et al. Scheme 1. Schematic Illustration of the Preparation of the Casting Films Containing Mixed-Valence TTF Nanofibers
with a variety of organic strong acids such as heptafluorobutyrate ([C3F7CO2 ]), trifluoroacetate ([CF3CO2 ]), henicosafluoroundecanoate ([C10F21SO3 ]), methanesulfonate ([MeSO3 ]), propaneundecanesulfonate ([C11H23SOsulfonate (C3H7SO3 ), 3 ]), benzenesulfonate ([PhSO3 ]), and chloride ([Cl ]). The general procedure for the preparation of the casting films consisting of TTF nanofibers is outlined in Scheme 1. The TTF radical cation was synthesized via oxidation of TTF using iodobenzene diacetate in the presence of various kinds of strong acids.11 To an acetonitrile solution of iodobenzene diacetate and the acid was added TTF under vigorous stirring at room temperature. The color of the solution changed from yellow to deep brown within several minutes. To confirm the electron-transfer reaction, a UV-vis absorption spectrum for the reaction mixture was measured after stirring for 5 min (Figure 1a). Two evident absorption bands derived from the π-π* transition of the TTF cation radical appeared at 437 and 583 nm after oxidation in all the cases.12 This indicates that the electron-transfer reaction proceeded in the solution and resulted in the formation of the TTF radical cation. The TTF radical cations with the desired organic anion salts were isolated by reprecipitation into diethyl ether after 1 h of stirring. The purity of the cations was confirmed from the absorption spectra of the acetonitrile solutions (Figure 1b). After washing with toluene and diethyl ether, the TTF radical cation salts and neutral TTF were mixed, and then the mixture solution was cast on a substrate. Purple-colored casting films were obtained after drying at ambient temperature. Conductivity was evaluated with a four-probe method as the average of five distinct points on the casting films. The casting films used in this study were several microns thick on the substrates. We initially investigated the structure and the properties of the casting films prepared with HCl and iodobenzene diacetate. The feed molar ratio (TTF/Cl-) between TTF and chloride anion was 0.71. The existence of the mixed-valence state of TTF was detected as a broad absorption around 2000 nm in the UV-vis/ NIR absorption spectra (Figure 2).8a,9 We executed powder X-ray diffraction (XRD) measurements (Figure 3). The films provided the characteristic peaks at 2θ = 12°, 26°, and 35°, which might originate from the [TTF+/Cl-] crystal structure.8b,9b We observed at least 2 μm rods from the scanning electron microscopy (SEM) image (Figure 4). The conductivity of the samples was measured with the four-probe method; however, the casting films showed slight conductivity under a detectable range (
