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Anisotropic Self-Aggregation of an Anthracene Derivative: Formation of Liquid-Crystalline Physical Gels in Oriented States Takashi Kato,* Takaaki Kutsuna, Kazuhiro Yabuuchi, and Norihiro Mizoshita Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received April 5, 2002
Introduction Self-assembly is a useful process to prepare functional molecular materials.1-4 For example, one-dimensional molecular assembly in solution has attracted attention because it leads to the formation of organic physical gels, which have potential applications as soft functional materials.4-9 A variety of gelators have been reported.4-9 Recently, it has been shown that thermotropic liquid crystals can be physically gelated by hydrogen-bonded molecules such as amino acid and sugar derivatives.2,10-16 Significant electrooptical properties have been induced for nematic liquid-crystalline (LC) physical gels in twisted nematic cells.13 Moreover, a positive effect on hole transport has been observed for discotic LC physical gels.14 Thus, the LC physical gels can be regarded as new dynamically functional composites. * To whom correspondence should be addressed. E-mail: kato@ chiral.t.u-tokyo.ac.jp. Fax: +81-3-5841-8661. Tel: +81-3-58417440. (1) Molecular Self-Assembly, Organic Versus Inorganic Approaches; Fujita, M., Ed.; Structure & Bonding Vol. 96; Springer: Berlin, 2000. (2) Kato, T. Science 2002, 295, 2414. (3) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071. (4) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (5) (a) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (b) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485. (c) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (d) Hanabusa, K.; Shirai, H. Kobunshi Ronbunshu 1998, 55, 585. (e) Melendez, R. E.; Carr, A. J.; Linton, B. R.; Hamilton, A. D. Struct. Bonding 2000, 96, 31. (f) Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; McLeish, T. C. B.; Pitkeathly, M.; Radford, S. E. Nature 1997, 386, 259. (g) Tamaoki, N.; Shimada, S.; Okada, Y.; Belaissaoui, A.; Kruz, G.; Yase, K.; Matsuda, H. Langmuir 2000, 16, 7545. (h) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 412. (6) Brotin, T.; Utermo¨hlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J.-P. J. Chem. Soc., Chem. Commun. 1991, 416. (7) Pozzo, J.-L.; Clavier, G. M.; Colomes, M.; Bouas-Laurent, H. Tetrahedron 1997, 53, 6377. (8) (a) Terech, P.; Bouas-Laurent, H.; Desvergne, J.-P. J. Colloid Interface Sci. 1995, 174, 258. (b) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (c) Lu, L.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Langmuir 2000, 16, 20. (d) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084. (e) Placin, F.; Desvergne, J.-P.; Lasse`gues, J. C. Chem. Mater. 2001, 13, 117. (9) (a) Tian, H. J.; Inoue, K.; Yoza, K.; Ishi-i, T.; Shinkai, S. Chem. Lett. 1998, 871. (b) Hishikawa, Y.; Sada, K.; Watanabe, R.; Miyata, M.; Hanabusa, K. Chem. Lett. 1998, 795. (10) Kato, T.; Kutsuna, T.; Hanabusa, K.; Ukon, M. Adv. Mater. 1998, 10, 606. (11) Kato, T.; Kondo, G.; Hanabusa, K. Chem. Lett. 1998, 193. (12) Yabuuchi, K.; Rowan, A. E.; Nolte, R. J. M.; Kato, T. Chem. Mater. 2000, 12, 440. (13) Mizoshita, N.; Hanabusa, K.; Kato, T. Adv. Mater. 1999, 11, 392. (14) Mizoshita, N.; Monobe, H.; Inoue, M.; Ukon, M.; Watanabe, T.; Shimizu, Y.; Hanabusa, K.; Kato, T. Chem. Commun. 2002, 428. (15) Mizoshita, N.; Kutsuna, T.; Hanabusa, K.; Kato, T. Chem. Commun. 1999, 781. (16) (a) Kato, T. Struct. Bonding 2000, 96, 95. (b) Kato, T.; Mizoshita, N.; Kanie, K. Macromol. Rapid Commun. 2001, 22, 797.
Figure 1. Comparison of the phase transition behavior of liquid-crystalline gels: (a) type I, Tsol-gel > Tiso-lc; (b) type II, Tsol-gel < Tiso-lc.
One interesting feature of the LC physical gels is thermoreversible phase transitions.10-15 They show two independent transitions: the isotropic-anisotropic transitions of the liquid crystals and the sol-gel transitions induced by the association and dissociation of the gelators. Two types of gels exhibiting a different order of sol-gel transition temperatures (Tsol-gel) and isotropic-anisotropic transition temperatures (Tiso-lc) have been obtained: the gels of Tsol-gel > Tiso-lc (type I)10,12,13 and Tiso-lc > Tsol-gel (type II)15 (Figure 1). The Tsol-gel’s of the gels are tunable by varying the gelators of different chemical structure. The type I gels form finely and randomly dispersed fibrous networks.10,12,13 Recently, we have found that anisotropic fibrous aggregates are oriented perpendicular to the long axes of smectic LC molecules for type II gels.15 In this case, liquid crystals function as a template for the anisotropically oriented aggregation. If we could develop such a methodology to orient functional molecules, oriented molecular wires could be obtained. Here, we report that anisotropic aggregation of a nonhydrogen-bonded aromatic molecule, an anthracene derivative, can occur through π-π interactions in nematic and smectic liquid crystals. Anthracene derivatives were reported to gelate common organic solvents.6-8 A dialkoxy anthracene has been found to gelate biphenyl liquid crystals by one-dimensional self-assembly in their LC state. Experimental Section Preparation of Anisotropic Gels. The chemical structures of compounds used in this study are shown in Chart 1. The LC materials 4′-pentyl-4-biphenylcarbonitrile (5CB, Aldrich) and 4′-octyl-4-biphenylcarbonitrile (8CB, Aldrich) are commercially available. Anthracene derivative 1 was synthesized according to the procedure reported by Pozzo et al.7 For the preparation of anisotropic gels, 1 and the liquid crystals were mixed in a sealed test tube. The mixtures were heated to isotropic states and then cooled to appropriate temperatures. Characterization of Anisotropic Gels. The phase behavior of the samples was determined by differential scanning calorimetry (DSC) measurements and microscopic observation. DSC measurements were performed with a Mettler DSC 30. Heating and cooling rates were 5 °C/min in all cases. Transition temperatures were taken at the maximum of transition peaks. A polarizing optical microscope, an Olympus BH2 equipped with
10.1021/la025809z CCC: $22.00 © 2002 American Chemical Society Published on Web 08/09/2002
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Chart 1. Chemical Structures of the Compounds Used in This Study
a Mettler FP82HT hot stage, was used for visual observation. The morphologies of self-assembled fibers of 1 formed in LC solvents were studied using a polarizing optical microscope and a scanning electron microscope (SEM). The mixtures of 1 and liquid crystals in isotropic states were placed between glass slides. This glass cell was cooled to an appropriate temperature. SEM samples were prepared by immersing the glass cell in hexane to extract the LC solvent followed by drying at room temperature. The cover glass was removed, and the samples were shaded with platinum. SEM observation was performed with a Hitachi S-900S. Anisotropic Self-Aggregation of Compound 1. A parallel rubbed cell was made of glass slides covered with rubbed polyimide layers on its surface. The rubbing directions of two glass slides were parallel to homogeneously align LC molecules. Microscopic observation of self-assembled fibers was performed on the samples as described above. Polarizing Infrared Spectroscopy. Polarized infrared spectra were taken with a Jasco MFT-2000 spectrometer equipped with polarizers and a Mettler FP82 hot stage. Samples were prepared in a parallel rubbed cell using KBr plates instead of glass slides.
Results and Discussion Cyanobiphenyl compound 8CB is a room-temperature smectic liquid crystal. It exhibits a nematic phase between 38 and 32 °C and a smectic A phase below 32 °C on cooling. Figure 2a shows the phase diagram of the mixtures of 1 and 8CB on cooling. Compound 1 gelates 8CB. The mixture of 8CB with 9.1 mol % of 1 exhibits a smectic A phase at 42 °C and then forms a smectic gel at 17 °C. Fibrous network aggregates of 1 are observed under a polarizing optical microscope in the gel state. The fibers form in the smectic A phase of 8CB on cooling of the mixtures, leading to the formation of smectic LC gels. The aggregation of 1 should be driven by π-π interactions between the anthracene and dipolar forces between the other moieties.6 5CB is a room-temperature nematic liquid crystal which exhibits only a nematic phase below 34 °C. For mixtures of 1 and 5CB, nematic LC gels are also formed by self-assembly of 1 as shown in Figure 2b. The Tsol-gel’s are lower than the Tiso-lc’s for all gelated samples. Several hydrogen-bonded molecules were reported to gelate LC cyanobiphenyls.10-15 It is of interest that even such a non-hydrogen-bonded aromatic molecule can gelate aromatic anisotropic liquids. The Tsol-gel’s of the mixtures based on 1 are much lower than those of the gels10-15 based on H-bonded gelators. The enthalpy changes on the sol-gel transitions suggest that the interactions among compound 1 are weaker than those among H-bonded gelators. For example, the enthalpy change on the sol-gel transition is 14.0 kJ mol-1 for the mixture of 8CB with 9.1 mol % of 1, while the change is 23.9 kJ mol-1 for the mixture of 8CB with the same content of a H-bonded sugar derivative.12 We have found that the fibers of 1 develop anisotropically in the aligned smectic A phase of 8CB as shown in Figure 3a. The mixtures of 1 and 8CB in isotropic states were placed in a parallel rubbed cell at 80 °C. When they
Figure 2. Phase diagrams of the mixtures of 1 with (a) 8CB and (b) 5CB.
are cooled to 30 °C, a homogeneously aligned smectic A phase is formed in the cell. On further cooling, the fibers of 1 grow anisotropically. For example, the mixture of 8CB with 7.4 mol % of 1 forms the aligned smectic A phase at 42 °C, and the fibers of 1 grow at 10 °C. The growing direction of the fibers is perpendicular to the rubbing direction as shown in Figure 3a. Anisotropically grown fibers of 1 are also obtained from the mixture of 1 and 5CB. Figure 3b shows the SEM image of the oriented fibers grown in the aligned nematic phase of 5CB. The growing direction of fibers is also perpendicular to the rubbing direction. These results show that even nonhydrogen-bonded aromatic molecules can form oriented self-assembled fibers in aligned nematic and smectic media. The formation of oriented fibers is induced not by the rubbing treatment itself but by the anisotropic environment of the LC media, because an isotropic butanol gel introduced in a parallel rubbed cell gives randomly dispersed fibers. No oriented fibers of 1 are observed when the mixtures of 1 and cyanobiphenyl liquid crystals are placed in a normal glass cell without any surface treatment, causing random orientation of liquid crystals. Figure 3c shows the SEM image of the randomly grown fibers of 1 formed in 8CB. The structures of the oriented fibers of 1 with 5CB and 8CB in the rubbed cell were examined by polarizing infrared spectroscopy. Figure 4 shows the polar plot of the absorbance for the CN group stretching of 8CB (2226 cm-1) and the out-of-plane bending of the aromatic C-H of 1 (891 cm-1) for the mixture of 8CB with 7.4 mol % of 1 in the rubbed cell. The CN group vibration is parallel to the long axis of the 8CB molecule, while the C-H vibration is perpendicular to the long axis of 1. Both IR bands exhibit dichroism. Dichroic ratios are 4.6 for the peak at 2226 cm-1 and 4.2 for the peak at 891 cm-1, respectively. The same spectral features are observed for
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Figure 3. SEM images of self-assembled fibers of 1 formed in (a) a homogeneously aligned smectic A phase of 8CB, (b) a homogeneously aligned nematic phase of 5CB, and (c) a smectic A phase of 8CB without any rubbing treatment.
Figure 5. Schematic illustration of anisotropic self-assembly of 1 in a homogeneously aligned nematic state. Figure 4. Polar plot of the IR bands of the mixture of 8CB and 1 containing 7.4 mol % of 1 at room temperature.
the fibers of 1 with aligned 5CB. These results indicate that the aggregated molecules of 1 are oriented with their long axes parallel to the director of the liquid crystals. As the stacking direction of the molecules of 1 is perpendicular to the molecular long axis, it is reasonable that the oriented fibers of 1 grow perpendicularly to the molecular long axes of 1 and the LC molecules. The molecular packing of 1 in organic solvents was reported to stack in an antiparallel fashion.6 The anisotropic aggregation of 1 in the liquid crystals is schematically illustrated in Figure 5. In summary, we have reported here the control of the molecular self-assembly process using the anisotropic
environment of LC templates. This is the first example that oriented aromatic fibers are formed through π-π interactions in anisotropic templates. It is of interest that the direction of π-π stacking can be anisotropically controlled. This material design may enable us to control one-dimensional structures of functional molecules such as electroactive molecules17 on the micrometer or nanometer scale. Acknowledgment. We thank JSR Corporation for support of the measurement of polarized infrared spectra. K.Y. and N.M. are thankful for financial support by the Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists. LA025809Z (17) Scho¨n, J. H.; Kloc, C.; Batlogg, B. Nature 2000, 406, 702.
