VOLUME 16, NUMBER 20
OCTOBER 5, 2004
© Copyright 2004 by the American Chemical Society
Communications Formation of Helical Hybrid Silica Bundles Yonggang Yang, Miho Nakazawa, Masahiro Suzuki, Mutsumi Kimura, Hirofusa Shirai, and Kenji Hanabusa* Department of Functional Polymer Science, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan Received June 24, 2004 The chemistry of organic-inorganic hybrid materials is developing very fast.1 Bridged polysilsesquioxanes are a family of hybrid organic-inorganic materials prepared by sol-gel processing of monomers that contain a variable organic bridging group and two or more trifunctional silyl groups. By varying organic groups, the properties of materials, such as porosity, thermal stability, and refractive index, could be modulated. These materials are suitable for ceramics precursors2 and catalyst supports.3 Furthermore, by partially removing organic groups in the bridged polysilsesquioxanes, microporous silica was obtained, which can act as a shapeselective base catalyst.4 Generally, polysilsesquioxanes are amorphous solids.5 Recently it was found that the hybrids can be structured; for example, periodic mesoporous hybrid silicas were obtained using surfactants (1) Boury, B.; Corriu, R. J. P. Chem. Commun. 2002, 795. Laine, R. M.; Sanchez, C.; Giannelis, E.; Brinker, C. J. Organic/Inorganic Hybrid Materials - 2000: Symposium Held April 24-28, 2000, San Francisco, CA; Materials Research Society: Warrendale, PA, 2001; vol. 628. (2) Corriu, R. J. P. Angew. Chem., Int. Ed. 2000, 39, 1376. (3) Lindner, E.; Schneller, T.; Auer, F.; Mayer, H. A. Angew. Chem., Int. Ed. 1999, 38, 2154. (4) Katz, A.; Davis, M. E. Nature 2000, 403, 286. (5) (a) Shea, K. J.; Loy, D. A.; Webster, O. W. J. Am. Chem. Soc. 1992, 114, 6700. (b) Corriu, R. J. P.; Moreau, J. J. E.; The´pot, P.; Wong Chi Man, M. Chem. Mater. 1992, 4, 1217.
or polymers as templates.6,7 Especially by being combined with nanotechnology,8 the hybrid materials can be shaped into nanofibers,9 hollow tubes, and spheres.10 And lamellar-structured hybrids have also been obtained by the self-organization of the bridges.11 Helical nanofibers are the most interesting of the nanomaterials.12 With respect to the applications of helical inorganic materials, the hybrid helical bundles can be used as asymmetric reaction catalysts,13 helical sensors,14 and optical materials.15 Based on trans-1,2diaminocyclohexane derivates,16 Moreau’s group developed a procedure for the preparation of helical bundles by self-organization of the gelator molecules through H-bonds.9 For future applications, new bridged polysilsesquioxanes with helical structures should be designed (6) (a) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (b) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2001, 123, 8520. (7) (a) Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. (b) Yang, Q.; Kapoor, M. P.; Inagaki, S. J. Am. Chem. Soc. 2002, 124, 9694. (8) Van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (9) Moreau, J. J. E.; Vellutini, L.; Wong Chi Man, M.; Bied, C. J. Am. Chem. Soc. 2001, 123, 1509. (10) Moreau, J. J. E.; Vellutini, L.; Wong Chi Man, M.; Bied, C. Chem. Eur. J. 2003, 9, 1594. (11) (a) Moreau, J. J. E.; Vellutini, L.; Wong Chi Man, M.; Bied, C.; Bantignies, J.-L.; Dieudonne´, P. J. Am. Chem. Soc. 2001, 123, 7957. (b) Moreau, J. J. E.; Pichon, B. P.; Wong Chi Man, M.; Bied, C.; Pritzkow, H.; Bantignies, J.-L.; Dieudonne´, P.; Sauvajol, J.-L. Angew. Chem., Int. Ed. 2004, 43, 203. (c) Dautel, O. J.; Le`re-Porte, J.-P.; Moreau, J. J. E.; Wong Chi Man, M. Chem. Commun. 2003, 2662. (12) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 5008. (13) Soai, K.; Osanai, S.; Kadowaki, K.; Yonekubo, S.; Shibata, T.; Sato, I. J. Am. Chem. Soc. 1999, 121, 11235. (14) Bodenho¨fer, K.; Hierlemann, A.; Seemann, J.; Gauglitz, G.; Koppenhoefer, B.; Go¨pel, W. Nature 1997, 387, 577. (15) Hodgkinson, I.; Wu, Q. H. Adv. Mater. 2001, 13, 889. (16) (a) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (b) Van Esch, J.; Schoonbeek, F.; De Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J. 1999, 5, 937.
10.1021/cm0489993 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/11/2004
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Scheme 1. Molecular Structure of Compounds (S, S)-1 and (R, R)-1
and synthesized. And the mechanism of the helical structure formation should be studied also. Organic bolaamphiphilic monomers have been very well-studied recently.17 Sugar-based,18 peptide-based, and nucleobase-containing19 bolaamphiphilic monomers could form helical nanofibers. Other gelators with C2-symmetry have also been confirmed to form helical bundles.20 In this paper, C2-symmetric silsesquioxanes (S, S)-1 and (R, R)-1 were designed and synthesized. (Scheme 1 and Supporting Information) Both the urea and amide groups can support the self-organization of molecules. The sol-gel polymerization procedure was carried out as follows. Compound (S, S)-1 (10 or 100 mg) was dissolved in 1 mL of 1,4-dioxane under heating. When it formed a gel at room temperature, HCl (aq., 1 mL, 4 M) was injected under strong stirring. Ten min later, the mixture was kept under static conditions at 25 °C for 4 days. The obtained gel was subsequently washed with water, methanol, and acetone and finally dried in air. Compound (S, S)-1 can gel in many polar solvents (Supporting Information); the solubility is higher in
Figure 1. (a) TEM image and (b) SEM image of the xerogels prepared from frozen dioxane solution of compound (S, S)-1 (10 mg of compound (S, S)-1/1 mL dioxane). (c) SEM image of the xerogels prepared from frozen dioxane solution of compound (S, S)-1 (100 mg of compound (S, S)-1/1 mL dioxane). (d) SEM image of the hybrid silica (preparation condition: compound (S, S)-1 (10 mg) in the mixture of dioxane and water (2 mL, vol %, 50:50) under the catalyst HCl).
protic solvents than in aprotic ones. It is insoluble in nonpolar solvents such as n-hexane and cyclohexane. In dioxane, compound (S, S)-1 forms an opaque gel consisting of three-dimensional networks (Figure 1a and b). The diameters of the gel fibers ranged from 20 to 100 nm. Many bundles were identified from the SEM image (Figure 1b). The bundles were very straight and only twisted slightly in some cases. Although the precursor is a chiral compound, and the CD spectrum (Figure 2) showed that chiral aggregates can be formed, the gel fibers formed in both a low concentration (10 mg of compound (S, S)-1 in 1 mL of dioxane) and a high concentration (100 mg of compound (S, S)-1 in 1 mL of dioxane) do not show any helical structures (Figure 1c); the hybrid silica fibers prepared in a low concentration were not helical either (Figure 1d). But when the concentration of compound (S, S)-1 was increased, a very interesting phenomenon happened. The hybrid silica fibers twisted each other to form left-handed helical bundles (Figure 3a and b). We can clearly recognize the left-handed helical bundles are constructed by gathering several thinner bundles in which the fibers are oriented parallel to the axis of the thinner bundles. The diameter of the bundles ranged from hundreds of nanometers to several micrometers. Although the mechanism of concentration effect is not yet clear, the interaction between the surfaces of nanofibers should be the main factor.
Figure 2. CD spectrum of the precursors (S, S)-1 and (R, R)-1 (5 mg of precursor in the mixture of ethanol (0.2 mL) and n-hexane (0.8 mL)).
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Figure 3. (a) and (b) SEM images of the left-handed helical hybrid silica (preparation condition: compound (S, S)-1 (100 mg) in the mixture of dioxane and water (2 mL, vol %, 50:50) under the catalyst HCl). (c) and (d) SEM images of the right-handed helical hybrid silica (preparation condition: compound (R, R)-1 (100 mg) in the mixture of dioxane and water (2 mL, vol %, 50:50) under the catalyst HCl).
Figure 4. SEM image of the hybrid silica (preparation condition: compound (S, S)-1 (100 mg) in the mixture of HOAc (1 mL) and H2O (50 µL)).
Before hydrolysis and polymerization, -Si(OEt)3 groups should be on the surface of the nanofibers. Although the chiral groups of compound (S, S)-1 may induce the arrangement of -Si(OEt)3 groups in helix on the surface of nanofibers, the nanofibers are straight due to the weak interaction between -Si(OEt)3 groups. During hydrolysis and polymerization, ethoxy groups were partially replaced with hydroxy groups. The interaction between nanofibers was enhanced by both Si-O-Si and H bonds. In a high concentration, the interactions between fibers became sufficient to induce the bundles into helix. Considering the fact that BET surface area of this fibrous material is 48 m2/g, the obtained hybrid silica could be used as chiral hybrid catalyst. And TG analysis showed that thermostablity of the hybrid silica was higher than that of the precursor (Supporting Information). When compound (R, R)-1 was selected as (17) Shimizu, T. Macromol. Rapid Commun. 2002, 23, 311. (18) Shimizu, T.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 2812. (19) Shimizu, T.; Iwaura, R.; Masuda, M.; Hanada, T.; Yase, K. J. Am. Chem. Soc. 2001, 123, 5947. (20) Sumiyoshi, T.; Nishimura, K.; Nakano, M.; Handa, T.; Miwa, Y.; Tomioka, K. J. Am. Chem. Soc. 2003, 125, 12137.
the precursor, right-handed helical bundles were prepared at high concentration (Figure 3c and d). Stereochemical effect of connecting links on superamolecular assemblies indicates that the morphologies were affected by the lengths of the alkylene chains and their even or odd carbon numbers.18,21 Here, with respect to the hybrids prepared from compound (S, S)1, different morphologies were identified by varying the solvents, i.e., fibrous structure in dioxane and ultrathin membrane structure in acetic acid (Figure 4). When compound (S, S)-1 (100 mg) was dissolved in HOAc (1 mL) with a trace of water (50 µL) at room temperature, a homogeneous solution was formed; then an opaque gel was formed on standing overnight. The SEM image shows a kind of ultrathin membrane morphology. FTIR spectra showed that the packing of the precursors in this membrane was somewhat different from that of the precursors in the helical bundle. In conclusion, two new chiral silsesquioxanes have been designed and synthesized. The morphologies of the polysilsesquioxanes are sensitive to the concentration and solvents. The concentration of monomer is a magic factor on the formation of helical bundles. In the mixture of dioxane and water under the catalyst of HCl, a lower concentration of monomer will enable the fabrication of straight nanofibers and higher concentration of monomer will allow the production of a helical hybrid. Acknowledgment. This work was supported by the Grant-in-Aid for 21st Century COE Program and a grant (15350132) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Supporting Information Available: Synthesis, characterization, and data of the gelation properties of compound (S, S)-1; TG spectra of (S, S)-1 and the hybrid silica synthesized from (S, S)-1; and FT-IR spectra of hybrid silicas synthesized in dioxane and acetic acid. These materials are available free of charge via the Internet at http://pubs.acs.org. CM0489993 (21) Tomioka, K.; Sumiyoshi, T.; Narui, S.; Nagaoka, Y.; Iida, A.; Miwa, Y.; Taga, T.; Nakano, M.; Handa, T. J. Am. Chem. Soc. 2001, 123, 11817.
