Langmuir 2007, 23, 10913-10915
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Solubilization of Single-Walled Carbon Nanotubes by Supramolecular Complexes of Barbituric Acid and Triaminopyrimidines Atsushi Ikeda,* Yasunori Tanaka, Kazuyuki Nobusawa, and Jun-ichi Kikuchi Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma 630-0192, Japan ReceiVed September 5, 2007 In this letter, we report that single-walled carbon nanotubes (SWNTs) can be dissolved in organic solvents through the formation of admixtures with barbituric acid‚triaminopyrimidine (BA‚TP) complexes using mechanochemical high-speed vibration milling (HSVM) and sonication methods. In contrast, neither BA nor TP alone were capable of solubilizing SWNTs. Moreover, the glutarimide (GI)‚TP complex was also found to be incapable of solubilizing SWNTs because the two carbonyl groups and one imino group of GI are located on only one side of the molecule such that the GI‚TP complex cannot form the desired hydrogen-bonding network. These results strongly suggest that the formation of a hydrogen-bonding network makes possible the formation of multipoint interactions with the surfaces of the SWNTs.
The easy availability of single-walled carbon nanotubes (SWNTs) has spurred and sustained the exploration of their outstanding unique physical and chemical properties.1,2 However, in spite of their vast potential, the number of applications has been extremely limited because of the difficulty of dissolving SWNTs in most solvents.3 Much research has been devoted to overcoming this problem by adopting a supramolecular approach using solubilizing agents4-7 because it involves noncovalent modification of the SWNTs while incurring little damage. This approach is expected to be simple and easy to perform and more capable of functionalizing small molecules as solubilizing agents compared with oligomers or polymers,5 which generally have a high solubilizing ability with respect to SWNTs as a result of multipoint interactions with the SWNT surface. Noncovalent modifications, however, require the use of molecules with a large π system, such as porphyrin6 and pyrene7 derivatives. Although the larger π systems promise a high solubilizing ability * Corresponding author. Fax: +81-743-72-6099. E-mail: aikeda@ ms.naist.jp. (1) Iijima, S. Nature 1991, 354, 56-58. (2) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787-792. (3) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483-487. (4) (a) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J. P.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593-596. (b) Li, H. P.; Zhou, B.; Lin, Y.; Gu, L. R.; Wang, W.; Fernando, K. A. S.; Kumar, S.; Allard, L. F.; Sun, Y. P. J. Am. Chem. Soc. 2004, 126, 1014-1015. (c) Chichak, K. S.; Star, A.; Altoe´, M. V. R.; Stoddart, J. F. Small 2005, 1, 452-461. (d) Maeda, Y.; Kanda, M.; Hashimoto, M.; Hasegawa, T.; Kimura, S.; Lian, Y. F.; Wakahara, T.; Akasaka, T.; Kazaoui, S.; Minami, N.; Okazaki, T.; Hayamizu, Y.; Hata, K.; Lu, J.; Nagase, S. J. Am. Chem. Soc. 2006, 128, 12239-12242. (5) (a) Star, A.; Steuerman, D. W.; Heath, J. R.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 2508-2512. (b) Cheng, F. Y.; Zhang, S.; Adronov, A.; Echegoyen, L.; Diederich, F. Chem.sEur. J. 2006, 12, 6062-6070. (c) Chen, J.; Liu, H. Y.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034-9035. (d) Dieckmann, G. R.; Dalton, A. B.; Johnson, P. A.; Razal, J.; Chen, J.; Giordano, G. M.; Mun˜oz, E.; Musselman, I. H.; Baughman, R. H.; Draper, R. K. J. Am. Chem. Soc. 2003, 125, 1770-1777. (e) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Tassi, R. E. R. Nat. Mater. 2003, 2, 338-342. (f) Numata, M.; Asai, M.; Kaneko, K.; Bae, A.-H.; Hasegawa, T.; Sakurai, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 5875-5884. (6) Murakami, H.; Nomura, T.; Nakashima N. Chem. Phys. Lett. 2003, 378, 481-485. (7) Tomonari, Y.; Murakami H.; Nakashima, N. Chem.sEur. J. 2006, 12, 4027-4034.
of SWNTs, it is difficult to extend the π system of these molecules through covalent bonds. We have now examined supramolecular complexes such as barbituric acid‚triaminopyrimidine (BA‚TP) complexes8 as the solubilizing agents and have found that they can dissolve SWNTs in DMF or NMP. We have already reported the higher solubilization and debundling of purified and pristine SWNTs by the formation of SWNT‚solubilizing agent complexes using a mechanochemical high-speed vibration milling (HSVM) technique.9 Mixtures of pristine SWNTs (1.00 mg)10 and 1, 2, 3, 4, 5, 1 + 4, 2 + 4, 3 + 4, and 3 + 5 ([1] ) [2] ) [3] ) [4] ) [5] ) 17.0 µmol) were placed in an agate capsule together with two agate mixing balls and mixed vigorously at 1800 rpm for 20 min using a high-speed vibration mill (MM200, Retsch Co. Ltd.). The solid mixtures obtained were dissolved in 10.0 mL of organic solvent to produce black emulsions. On the other hand, a dispersion was formed via sonication by adding the pristine SWNTs (1.00 mg), the 3 + 4 mixture ([3] ) [4] ) 17.0 µmol), and 10.0 mL of a DMF solution to a 10 mL glass vial. The solution was sonicated using an ultrasonic bath (70 W, 42 kHz, 1510 J-DTH Branson Ultrasonic Corp.) on cooling to 5 °C for 3 h. After centrifugation (14 000 rpm, 20 min, 20 °C), all of the nondispersed SWNTs were removed from each solution.
To obtain ambiguous evidence for the formation of complexes between BA and TP in DMF, we measured the 1H NMR spectrum (8) (a) Bent, H. A. Chem. ReV. 1968, 68, 587-648. (b) Yagai, S. J. Photochem. Photobiol. C 2006, 7, 164-182. (9) (a) Ikeda, A.; Hayashi, K.; Konishi, T.; Kikuchi, J. Chem. Commun. 2004, 1334-1335. (b) Ikeda, A.; Hamano, T.; Hayashi, K.; Kikuchi, J. Org. Lett. 2006, 8, 1153-1156. (c) Ikeda, A.; Nobusawa, K.; Hamano, T.; Kikuchi, J. Org. Lett. 2006, 8, 5489-5492. (10) We purchased the SWNTs used in this study from Carbon Nanotechnologies Inc. and 1, 4, and 5 from Aldrich. Compounds 2 and 3 were prepared as described in a previous paper.12
10.1021/la702747r CCC: $37.00 © 2007 American Chemical Society Published on Web 09/26/2007
10914 Langmuir, Vol. 23, No. 22, 2007
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Figure 2. TEM images of SWNTs: (A) 1 + 4‚SWNTs, (B) 2 + 4‚SWNTs, and (C) 3 + 4‚SWNT complexes.
Figure 1. Vis-NIR absorption spectrum of SWNTs in a DMF solution after HSVM in the presence of 1 (green dotted line), 2 (blue dotted line), 3 (red dotted line), 4 (black dotted line), 5 (orange dotted line), 1 + 4 (green solid line), 2 + 4 (blue solid line), 3 + 4 (red solid line), and 3 + 5 (orange solid line) and after sonication in the presence of 3 + 4 (black solid line) at 25 °C (1 mm cell). Scheme 1. Schematic Presentation of (A) 3 + 4 and (B) 3 + 5 Mixtures
of the 3 + 4 mixture ([3] ) [4] ) 4.84 mM) in DMF-d7. In Figure S1B,C, peaks for the two kinds of amino groups of 3 (5.09 and 5.42 ppm) were found to have shifted by 6.11 and 6.19 ppm, respectively, to lower magnetic fields (1.02 and 0.77 ppm). However, the peak for the imino group of 4 (11.17 ppm) was noticeably absent in the 3 + 4 mixture. Because it is known that the peaks of the imino groups shift to a lower magnetic field by the formation of hydrogen bonds with carbonyl groups, these results clearly indicate the successful formation of the 3 + 4 complex. Figures 1 and S2 show the vis-NIR absorption spectra of these DMF and NMP solutions, respectively. The sole solubilizing agents of 1, 2, 3, 4, and 5 hardly solubilized any SWNTs (Figure 1, green, blue, red, black, and orange dotted lines). However, the mixed solubilizing agents 1 + 4, 2 + 4, and 3 + 4 showed a moderately higher solubility of SWNTs. In particular, the visNIR spectrum for a solution of SWNTs obtained using 3 + 4 exhibited sharp, well-resolved peaks (Figure 1, red solid line). In contrast, broadened peaks were obtained with 1 + 4 and 2 + 4 (Figure 1, green and blue solid lines). Because these sharp van Hove peaks are characteristic of debundled, individually dispersed SWNTs, these results indicate that 3 + 4 has a higher ability to debundle SWNTs in DMF than do 1 + 4 and 2 + 4. For comparison, we employed glutarimide (5) instead of BA to confirm that the occurrence of hydrogen bonding on the SWNT surface is important for the solubilization of SWNTs. Although 5 has a similar structure to 4, 5 cannot form the hydrogen-bonding network as shown in Scheme 1B because 5 has two carbonyl groups and one imino group on only one side. As expected,
complex 3 + 5 showed a far lower solubility of SWNTs (Figure 1, orange solid lines). These results indicate that the formation of a hydrogen-bonding network makes possible the formation of multipoint interactions with the SWNT surface, thereby stabilizing the interactions between the BA‚TP complexes and SWNTs (Scheme 1A). Moreover, the Raman spectra of these samples showed nearly identical sharp peaks with a shoulder near the high-frequency range of 1578-1588 cm-1. These peaks are assigned to the tangential modes of graphite (G modes) (Figure S3).11 The low-frequency range of 160-280 cm-1 corresponds to the radial breathing modes11 whose frequencies are dependent on the tube diameter. No recognizable difference existed between them, indicating that the selectivity for metallic and semiconducting SWNTs was scarcely influenced by the alkyl chains of the triaminopyrimidine derivatives. The disorder-induced D mode at 1250-1350 cm-1 was observed for all of our SWNT samples. In particular, the 3 + 4‚SWNTs complex obtained using the HSVM method showed a higher intensity of the D mode relative to the G mode than that obtained using the sonication method. Because the D peaks typically obscured the spectral signal from sp3-bonded carbon, these results suggested that the SWNTs obtained using the HSVM method were shorter and the carbons in the terminal ends of the SWNTs changed from sp2 to sp3. The solubilities of SWNTs solubilized by HSVM for 20 min were compared with those solubilized by sonication for 3 h. The vis-NIR spectra for the solutions of SWNTs obtained using 3 + 4 and solubilized by HSVM and sonication methods showed that the solutions of SWNTs solubilized by HSVM were obtained with higher solubilities and in shorter time periods than those solubilized by sonication (Figure 1, red and black solid lines). These results indicate that the solubilization of SWNTs by HSVM is superior to that by sonication. The morphology of the SWNTs was observed by TEM (Figure 2A-C). The 1 + 4‚SWNT complex appears in bundles with diameters in the range of 4.5-10 nm (Figure 2A). Bundles with diameters below 4.5 nm were not observed. This result is consistent with the broadened peaks observed in the vis-NIR spectrum. However, the 2 + 4‚SWNT and 3 + 4‚SWNT complexes have mean diameters in the ranges of 1.0-2.0 nm and 1.0-1.6 nm, respectively, as seen in the TEM micrographs (Figure 2B,C), indicating that an SWNT with a diameter of around 1 to 2 nm was partially debundled. We also used atomic force microscopy (AFM) to investigate the dispersion state of the SWNTs formed after evaporation of a single drop of DMF solution on mica, followed by washing with methanol for 1 + 4 and DMF for 2 + 4 and 3 + 4, respectively. (Figure 3A-C). Figure 3A-C clearly shows that the SWNT length increased in the order of 3 + 4 complex > 2 + 4 complex > 1 + 4 complex. These results indicate that the (11) (a) Ravindran, T. R.; Jackson, B. R.; Badding J. V. Chem. Mater. 2001, 13, 4187-4191 (b) Paloniemi, H.; A ¨ a¨ 1ritalo, T.; Laiho, T.; Liuke, H.; Kocharova, N.; Haapakka, K.; Terzi, F.; Seeber, R.; Lukkari J. J. Phys. Chem. B 2005, 109, 8634-8642 (12) Bauer, T.; Thomann, R.; Mu¨lhaupt, R. Macromolecules 1998, 31, 76517658.
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Langmuir, Vol. 23, No. 22, 2007 10915
and S4F,G). These values are comparable to the diameters estimated from the TEM images. In contrast, the SWNT length of the sonication method was longer than that of the HSVM method (Figure 3D). These results for the SWNTs lengths are consistent with the conclusion derived from the Raman spectroscopy studies. In summary, we have succeeded in solubilizing SWNTs in organic solvents through the formation of supramolecular complexes between BA and TP prepared using both HSVM and sonication methods. However, BA or TP alone do not have the ability to behave as solubilizing agents for SWNTs. Moreover, because 5 cannot form a hydrogen-bonding network, 3 + 5 shows a far lower solubility of SWNTs than does 3 + 4. These results confirm the importance of the hydrogen-bonding network. We are now examining the feasibility of applying this technique to other supramolecular complexes not only to explore the practicality of this method as a general means for preparing SWNT solutions but also to investigate potential applications of the supramolecular complex‚SWNT mixtures developed within our laboratory. Figure 3. Tapping mode AFM images of SWNTs: (A) 1 + 4‚ SWNTs, (B) 2 + 4‚SWNTs, and (C) 3 + 4‚SWNTs complexes obtained using HSVM, and (D) 3 + 4‚SWNTs complex obtained using sonication. Each scan area is 2.0 µm × 2.0 µm.
3 + 4 complex with long alkyl chains has a high solubilizing ability and can solubilize longer SWNTs. Furthermore, bundled SWNTs were seen in the AFM images in Figures 3A and S4E; that is, the observed heights were 1.3-1.6 nm. This height is smaller than that observed by TEM. This reason implied that the 1 + 4 complex on mica was not considerably removed by washing with methanol. However, the height profiles (each 1.0-1.5 nm) of the 2 + 4‚SWNT and 3 + 4‚SWNT complexes indicate the existence of individual nonaggregated SWNTs (Figures 3B,C
Acknowledgment. We thank Professor Michiya Fujiki and Dr. Masanobu Naito of the Nara Institute of Science and Technology for the use of their Jasco V-570 spectrophotometer. This study was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Supporting Information Available: 1H NMR spectra of 3, 4, and 3 + 4. Vis-NIR absorption spectrum of SWNTs in a 1-methyl2-pyrrolidone solution. Raman spectra of thin films. Tapping mode AFM images of SWNTs. This material is available free of charge via the Internet at http://pubs.acs.org. LA702747R
