Silica Nanohybrids: Synthesis and Macroscopic Alignment

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Langmuir 2007, 23, 12350-12355

Chromonic/Silica Nanohybrids: Synthesis and Macroscopic Alignment Mitsuo Hara, Shusaku Nagano, Norihiro Mizoshita,† and Takahiro Seki* Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Chikusa, Nagoya 464-8603, Japan ReceiVed May 28, 2007. In Final Form: August 20, 2007 Some dye molecules self-aggregate to exhibit a lyotropic columnar liquid crystal state (chromonic liquid crystal) via π stacking in relatively highly concentrated aqueous solutions. In this work, the chromonic liquid crystal structure was immobilized, for the first time, with silica networks by way of the sol-gel condensation process. The immobilization of the columnar structure was successfully attained in the presence of 2-(2-aminoethoxy)ethanol, which favorably mediates the interface between the anionic charge of the dye aggregates and the silica network. Without this molecule, the sol-gel process gave rise to a transformation from columnar to lamellar structure. Both spin-coating and dipcoating methods gave essentially the same results. In the dip-coated films, the dye molecules were aligned over a large area with orientation orthogonal to the lifting direction.

1. Introduction Liquid crystals provide varieties of ordered structures and dynamic functions.1 The dynamic molecular cooperativity in liquid crystals is of particular use for creating many types of nanostructures. One of the practical and fascinating approaches to produce nanostructures is to immobilize the mobile liquid crystalline state by radical polymerizations or sol-gel condensation processes. For lyotropic liquid crystalline states, the latter process is of special significance and widely applied to fabricate defined mesoporous materials. To date, a vast number of studies have been undertaken to fabricate mesoporous silica materials2-6 using various surfactant-templated nanostructures. To attain new functions in optics and molecular electronics, etc., macroscopic uniaxial alignment of the hexagonal columnar mesostructured silica is of importance. In this context, many efforts have been made to align macroscopically the nanohybrids and resulting mesoporous materials by applying external electric7 and magnetic fields,8 depositions on a silicon (110) surface,9 rubbed polyimide films,10 Langmuir-Blodgett films,11 and photo-oriented polymer films,12-15 and incorporation into a porous alumina membrane.16 * To whom correspondence should be addressed. Fax: +81-52-7894669. E-mail: [email protected]. † Present address: Toyota Central Research Laboratory. (1) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38. (2) Lu, G. Q., Zhao, X. S., Eds. Nanoporous Materials: Science and Engineering; Imperial College Press: London, 2004; Vol. 4. (3) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (4) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (5) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (6) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (7) Kuraoka, K.; Tanaka, Y.; Yamashita, M.; Yazawa, T. Chem. Commun. 2004, 1198. (8) Yamauchi, Y.; Sawada, M.; Sugiyama, A.; Osaka, T.; Sakka, Y.; Kuroda, K. J. Mater. Chem. 2006, 16, 3693. (9) Miyata, H.; Kuroda, K. J. Am. Chem. Soc. 1999, 121, 7618. (10) Miyata, H.; Kawashima, Y.; Itoh, M.; Watanabe, M. Chem. Mater. 2005, 17, 5323. (11) Miyata, H.; Kuroda, K. AdV. Mater. 1999, 11, 1448. (12) Kawashima, Y.; Nakagawa, M.; Seki, T.; Ichimura, K. Chem. Mater. 2002, 14, 2842. (13) Kawashima, Y.; Nakagawa, M.; Ichimura, K.; Seki, T. J. Mater. Chem. 2004, 14, 328. (14) Fukumoto, H.; Nagano, S.; Kawatsuki, N.; Seki, T. AdV. Mater. 2005, 17, 1035. (15) Fukumoto, H.; Nagano, S.; Kawatsuki, N.; Seki, T. Chem. Mater. 2006, 18, 1226.

Thus, the immobilization of fluid ordered lyotropic liquid crystalline states by the sol-gel condensation processes constitutes a central strategy in constructing desired nanostructures. To date, almost all of the templates employed for these strategies are limited to “typical surfactant aggregates” of low-molecularweight surfactants3-16 or amphiphilic block copolymers17,18 that exhibit the lyotropic liquid crystal nature. We propose herein a new templating system of chromonic dye aggregates (chromonic lyotropic liquid crystals; chromonic LLCs). The chromonic LLCs are composed of disclike or planklike aromatic dye molecules with hydrophilic units at the peripheries, and they can spontaneously self-assemble via π-π stacking interaction and stack face to face to form columnar structures in aqueous solutions.19-33 In this Article, we report the first demonstration of the immobilization of columnar structure of the chromonic LLC by formation of silica networks. As a typical example for the chromonic LLC material, C.I. direct blue 67 (B67) is employed here. The resulting hybrid films are found to possess macroscopic (16) Yamaguchi, A.; Uejo, F.; Yoda, T.; Uchida, T.; Tanamura, Y.; Yamashita, T.; Teramae, N. Nat. Mater. 2004, 3, 337. (17) Zhu, H. G.; Jones, D. J.; Zajac, J.; Roziere, J.; Dutartre, R. Chem. Commun. 2001, 2568. (18) Goltner, C. G.; Berton, B.; Kramer, E.; Antonietti, M. AdV. Mater. 1999, 11, 395. (19) Vasilevskaya, A. S.; Generalova, E. V.; Sonin, A. S. Russ. Chem. ReV. 1989, 58, 904. (20) Lydon, J. E. Curr. Opin. Colloid Interface Sci. 2004, 8, 480. (21) Tiddy, G. J. T.; Mateer, D. L.; Ormerod, A. P.; Harrison, W. J.; Edwards, D. J. Langmuir 1995, 11, 390. (22) Ichimura, K.; Momose, M.; Fujiwara, T. Chem. Lett. 2000, 1022. Ruslim, C.; Hashimoto, M.; Matsunaga, D.; Tamaki, T.; Ichimura, K. Langmuir 2004, 20, 95. (23) Ichimura, K.; Fujiwara, T.; Momose, M.; Matsunaga, D. J. Mater. Chem. 2002, 12, 3380. (24) Fujiwara, T.; Ichimura, K. J. Mater. Chem. 2002, 12, 3387. (25) Matsunaga, D.; Tamaki, T.; Akiyama, H.; Ichimura, K. AdV. Mater. 2002, 14, 1477. (26) Matsunaga, D.; Tamaki, T.; Ichimura, K. J. Mater. Chem. 2003, 13, 1558. (27) Ruslim, C.; Matsunaga, D.; Hashimoto, M.; Tamaki, T.; Ichimura, K. Langmuir 2003, 19, 3686. (28) Ruslim, C.; Hashimoto, M.; Matsunaga, D.; Tamaki, T.; Ichimura, K. Langmuir 2004, 20, 95. (29) Iverson, I. K.; Casey, S. M.; Seo, W.; Tam-Chang, S.-W. Langmuir 2002, 18, 3510. (30) Tam-Chang, S.-W.; Seo, W.; Iverson, I. K.; Casey, S. M. Angew. Chem., Int. Ed. 2003, 42, 897. (31) Tam-Chang, S.-W.; Seo, W.; Rove, K.; Casey, S. M. Chem. Mater. 2004, 16, 1832. (32) Schneider, T.; Lavrentovich, O. D. Langmuir 2000, 16, 5227. (33) Schneider, T.; Artyushkova, K.; Fulghum, J. E.; Broadwater, L.; Smith, A.; Lavrentovich, O. D. Langmuir 2005, 21, 2300.

10.1021/la701557q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007

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Figure 2. UV-vis spectra of 7.5 wt % B67 solution containing 1 wt % Emal20C (-) and the spin-coated B67/silica hybrid films without (- - -) and with (‚ ‚ ‚) AEE.

Figure 1. Chemical structures of the materials used in this work.

in-plane alignment of the chromonic structure when the dipping deposition is adopted. In the course of this investigation, we also found that the columnar B67 aggregates transform to a lamella phase in a simple hybridization with silica. The immobilization of the columnar structure is successfully attained when an appropriate mediating molecule, 2-(2-aminoethoxy)ethanol, is present. The significant role of mediator molecules in the structural stabilization of LLC materials33-35 is emphasized. 2. Experimental Section 2.1. Materials. C.I. direct blue 67 (B67) and an anionic surfactant poly(oxyethylene) lauryl ether sodium sulfonate (Emal20C) were kindly supplied by Nippon Kayaku Co., Ltd. and Kao Co., respectively. 2-(2-Aminoethoxy)ethanol (AEE) was purchased from Acros Organics. The chemical structure of the materials is shown in Figure 1. Pure water (Milli-Q grade, 18 MΩ cm) was used to prepare the dye solutions. Tetraethoxysilane (TEOS), ethanol, and concentrated hydrochloric acid were purchased from Kanto Chemical Co. 2.2. Methods. B67/silica hybrid films were prepared from a mixture of a dye solution and a sol solution. The precursor sol solution composed of TEOS, water, ethanol, and concentrated hydrochloric acid was stirred at 70 °C for 2 h before use. The nanostructured B67/silica hybrid films were prepared on glass substrates by two preparation methods as follows. 2.2.1. Spin-Coating Method. In this method, the concentration of B67 was 9.7 wt %. These dye solutions containing AEE or without AEE were mixed with the above-mentioned sol solution, and the mixtures were stirred for 1 min at room temperature. The final molar ratios of components in these mixtures were TEOS (1):B67 (0.2): AEE (0 or 0.5):water (103):ethanol (4):concentrated hydrochloric acid (0.01). Emal20C was added at a concentration of 1 wt % to B67 in these mixtures as described in the literature.26-28 The B67/silica hybrid films were prepared on glass substrates by spin-coating of these mixtures (first spin, 1000 rpm for 5 s; second spin, 2000 rpm for 30 s). The film thickness was ca. 650 nm. In X-ray diffraction measurements, a thicker film was obtained (ca. 1.5 µm thickness) by multiple (typically five times) spin-coating. 2.2.2. Dip-Coating Method. The mixture composed of the dye solution (B67 concentration: 5.8 wt %) and the sol solution was prepared in the same way as described above. The final molar ratios of components in the mixture for the dip-coating films were TEOS (1):B67 (0.2):AEE (0.5):water (184):ethanol (4):concentrated hydrochloric acid (0.01). Emal20C was also added at a concentration of 1 wt % to B67 in the mixture. For the dip-coating, glass substrates (34) Yokoi, T.; Yoshitake, H.; Tatsumi, T. Chem. Mater. 2003, 15, 4536. (35) Che, S.; Garcia-Bennett, A. E.; Yokoi, T.; Sakamoto, K.; Kunieda, H.; Terasaki, O.; Tatsumi, T. Nat. Mater. 2003, 2, 801.

were immersed into the mixture and then lifted up at 10 cm min-1. The film thickness was ca. 100 nm. 2.3. Measurements. The optical anisotropy of B67 molecules was evaluated by polarized optical microscopic observations using a BX51 (Olympus Co.). X-ray diffraction (XRD) and in-plane XRD measurements were performed on an ATX-G diffractometer (Rigaku Co.) using Cu KR radiation. UV-vis absorption spectra in both non-polarized and polarized modes were taken on an Agilent 8453A spectrophotometer (Agilent Technologies). For the polarized measurements, a Glan-Thomson-type polarizer was placed in front of the samples.

3. Results 3.1. Optical Measurements. 3.1.1. Spin-Coated Films. Figure 2 shows UV-vis absorption spectra of the B67 solution (-) and the B67/silica hybrid films in the absence (- - -) and presence (‚ ‚ ‚) of AEE measured at room temperature. In the aqueous solution, the spectrum of B67 dye exhibited a sharp peak with the absorption maximum (λmax) at 530 nm. The position of λmax and the spectral feature indicate that B67 adopts an H-type aggregated state, implying the formation of B67-columnar structure.23,27 On the other hand, the hybrid film without AEE exhibited a broadened absorption band with the λmax at 566 nm accompanied by an appearance of a shoulder at longer wavelengths around 700 nm, suggesting a disruption of H-aggregated columnar structure leading to multifarious stacking states of B67. In contrast, the UV-vis absorption spectrum of the silica hybrid film containing AEE gave λmax at 532 nm, and the spectral feature was almost the same as that of the B67 aqueous solution. These facts suggest that H-like aggregate and thus the columnar structure in the B67 solution could be retained in the hybrid film when AEE is present. 3.1.2. Dip-Coated Films. A hybrid film in the same composition was also prepared by the dipping method. Essentially the same spectral features were obtained for the dip-coated films, but the resulting film had a clear in-plane optical anisotropy at a macroscopic scale as indicated in Figure 3a. The dichroic ratio (D) defined as A⊥/A|, where A⊥ and A| are the absorbance at λmax taken in perpendicular and parallel to the polarized probing beam, respectively, was 1.5. The optical microscopic image taken through a polarizer also indicated the anisotropic nature of the hybrid film (Figure 3b). The B67 molecules preferentially aligned orthogonal to the lifting direction. Here, the dipping and lifting directions coincide with each other. The term “lifting direction” is adopted in this article because a film is formed during the lifting process. Figure 4a shows the polarized absorption spectra of the dipcoated hybrid film containing AEE. B67 molecules also aligned preferentially orthogonal to the lifting direction with D ) 2.3, a larger value than that observed for the film without AEE. Polarized optical microscopic observation also indicated the in-

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Figure 3. Polarized UV-vis spectra of the dip-coated B67/silica hybrid film without AEE deposited onto a glass substrate (a). A⊥ and A| indicate the spectrum monitored with polarized incident light perpendicular and parallel to the dipping direction, respectively. Optical microscopic images using polarized incident light of the dip-coated B67/silica hybrid film without AEE (b). The doubleheaded arrows show the direction of polarized incident light.

Figure 4. Polarized UV-vis spectra of the dip-coated B67/silica hybrid film containing AEE deposited onto a glass substrate (a). A⊥ and A| indicate the spectrum monitored with polarized incident light perpendicular and parallel to the dipping direction, respectively. Optical microscopic images using polarized incident light of the dip-coated B67/silica hybrid film containing AEE (b). The doubleheaded arrows show the direction of polarized incident light.

plane anisotropic nature of the film (Figure 4b). The obvious contrast of the polarized microscopic images taken in the parallel and orthogonal direction further indicates the achievement of macroscopic alignment. In the above manners, B67 molecules are preferentially aligned orthogonal to the lifting direction, regardless of the presence of AEE. However, some significant differences observed in the spectral feature and dichroic ratio should be of significance. These spectroscopic data in connection with the nanostructures formed will be discussed below. 3.2. X-ray Diffraction Measurements. 3.2.1. Spin-Coated Films. To obtain structural features of the hybrid film, the XRD measurement was performed (Figure 5). Contrasting results were obtained in the cases in the presence (-) and absence (- - -) of AEE. For the film without AEE, clear diffraction peaks (shown as 1) were observed at 7.3° and 14.7° for the hybrid film. These peaks were attributed to the (100) and (200) diffractions of an ordered lamellar structure with a spacing period (d) of 1.32 nm. To our knowledge, this is the first observation that aggregated B67 adopts a lamellar structure. Considering the length of short axis of B67 molecule of 1.05 nm27 and the lamella spacing of 1.32 nm in the hybrid film, we assume that the hybrid film has

Figure 5. XRD patterns of the spin-coated B67/silica hybrid films with (-) and without (- - -) AEE.

the lamellar layers in which the dye and silica sheets are alternatively piled up parallel to the substrate plane. The reason for the formation of lamellar structure along the short axis of B67 is still not clear, but it seems that this phenomenon may be related to slippage direction of the molecular plane between the dyes during the disruption of the columnar structure. If this is the case, the molecular slippage preferentially occurs in the direction of long axis and a planar structure is formed with a thickness of short axis.

Chromonic LC/Silica Nanohybrids

Figure 6. Top view of the apparatus of in-plane XRD measurement (a). The φ scan profile of in-plane XRD measurement for the dipcoated B67/silica hybrid film containing AEE (b). The sample was set in the direction that the dipping direction coincided with the parallel direction to the grazing-incidence X-ray at φ ) 0°.

On the other hand, a clear peak was observed at 1.3° (d ) 6.78 nm) in the hybrid film containing AEE. In this way, the addition of a relatively small amount of AEE (a molar ratio of 0.5 of the total) induced the obvious structural change of the hybrid film. Judging from the knowledge of H-aggregate formation from the spectroscopic data and a reasonable coincidence with X-ray data by Ruslim et al.,27 this spacing should indicate the average distance between the columns of B67 aggregates. Aqueous solutions of B67 adopt a nematic columnar phase or a middle phase. The nematic columnar phase possesses only an orientational order, whereas the middle phase has an additional positional order of a hexagonal array. Both phases similarly provide a single diffraction peak;27 it is thus somewhat difficult to assign the phase. Because the d value of the film well agreed with that obtained for the aqueous solution in the nematic state (Supporting Information), we assume that the nematic columnar phase is fixed by the silica network. The possibility that the existence of AEE largely changes the lamella spacing can be ruled out because the higher order diffraction peaks that are typical in the lamella phase are not observed. A small peak at 7.3° was also observed, which can be attributed to an existence of a minor amount of lamella structure in the film. With respect to the wide-angle diffraction region, a diffraction corresponding to 0.34 nm was consistently observed for the systems both with and without AEE. This period can be assignable to a stacking distance between B67 molecules. 3.2.2. Dip-Coated Film. For the film containing AEE, we further evaluated the alignment of the B67 molecules by inplane XRD measurements for the dip-coated films. Figure 6a shows a scheme of the top view of the apparatus of in-plane XRD measurements. The sample was set in a direction so that the lifting direction coincided with that of the grazing-incidence of X-ray at (φ ) 0°). The detector was set at 26° where the diffraction derived from the π-stacking repeating distance (0.34 nm) of the B67 aggregates could be detected. Figure 6b shows the in-plane diffraction intensity profile for the hybrid film in the rotation (φ scan). Two diffraction peaks were observed at φ ) -75° and +100°. This result indicates that B67-columnar aggregates were aligned in the direction parallel to the lifting direction, which is consistent with the result of polarized UV-

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Figure 7. Structural models of the B67/silica hybrid films without (a) and with (b) AEE. The arrow indicates the lifting direction.

vis spectra (Figure 4a). The degree of orientation, D.O. (%), of the B67-columnar aggregates was calculated by the following equation,36

D.O. )

∑

360° Wi × 100 360°

where Wi (deg) is the full-width degree at half-maximum of the diffraction peak in the in-plane XRD profile, with the subscript i being the number of peaks. The value of D.O. of the B67 aggregate reached 87%, which means that the B67-columnar structure was highly aligned along the lifting direction. This value nearly equals that obtained for a dip-coated silica mesochannel templated with LC rodlike micelles of cetyltrimethylammnonium chloride prepared on a photoaligned polymer film (D.O. ) ca. 90%).15

4. Discussion 4.1. On the Nanostructures of the Hybrid Films. It is found here that a relatively small amount of AEE brings about a significant change in the nanostructures of the hybrid films. The structural models of the nanohybrids consisting of B67 and silica network based on the spectroscopic and X-ray diffraction data are drawn in Figure 7. Without AEE, multifarious aggregation states are involved in the hybrid film as shown by the spectroscopic measurements. Thus, the aggregation state of B67 in the film greatly differs from that in the aqueous or sol solutions. This fact suggests that the columnar aggregates of B67 formed in the aqueous solution are distorted to a large extent at some stage of the formation of silica network. X-ray measurements reveal that a lamellar structure is formed in the resulting film (Figure 7a). In the presence of AEE, in contrast, both spectroscopic and X-ray data revealed the retention of the columnar structure in the film, which is macroscopically aligned along the lifting direction in the dipcoating process (Figure 7b). In the presence of AEE, a peak at (36) Rigaku Corporation Handbook of X-ray Structure Analysis; Rigaku Corp.: 1999.

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1.3° (d ) 6.78 nm) is observed. We also measured the average columnar distance of the B67/sol mixture solution, and the distance was found to be 7.35 nm (Supporting Information). The columnar distance in the hybrid film became slightly narrower. It seems plausible that the evaporation of solvents and the silica condensation induce some shrinkage of the nanostructure. The average distance obtained in this work is somewhat larger than those obtained by Ruslim et al. (d ) ca. 5 nm).27 The discrepancy may result from the difference in the amount of Emal20C involved, which is not described in their paper. Next, a question arises as to which stage the lamella structure is formed in the case without AEE. The phase behavior of pure B67 in water at various concentrations was investigated by Ruslim et al.27 Pure B67 exhibits nematic columnar and middle (hexagonal columnar) phases; however, the lamella structure has not been observed in the aqueous solution. The LLC phase of B67 in the sol mixture and an aqueous solution at the concentration corresponding to the precursor sol were evaluated by UV-vis absorption spectroscopy, and small and wide-angle X-ray scattering measurements (Supporting Information). Both samples consistently exhibited a relatively sharp peak with λmax at 530 nm in the absorption spectrum and diffraction peaks at 1.2° (d ) 7.35 nm) and 26.0° (d ) 0.34 nm) in the X-ray diffraction profile. These data clearly indicate that the B67 aggregates form the columnar structure with essentially the same LC structure in both systems. In other words, the columnar LLC structure of B67 is stable enough and invariable in the modified conditions of the precursor sol solution in the highly acidic state as far as the B67 concentration is maintained. Despite the above facts, the resulting dry hybrid film exhibits the lamellar structure. Therefore, a transformation from the columnar to lamellar structure should take place at some stage during the formation of silica networks and the successive drying process. We assume that at least three factors can affect the aggregation state. First, the solution is too far concentrated in the final stage of the drying process. Second, the anionic molecules (B67 in this case) exhibit poor miscibility with a silica sol-gel precursor solution due to the electric repulsion because both are negatively charged. In this respect, it is known that anionic surfactants can be applied for templating of silica only when a mediator molecule such as aminopropyl ethoxysilane is present. Third, the solubility of B67 is poor in ethanol, which is a required solvent for obtaining a homogeneous solution of the sol precursor. Precipitation of B67 in the precursor occurs above 7.5 wt % of ethanol content. In any case, it is stressed here that the new nanostructure, the lamellar structure, is first obtained in the process of hybridization with silica. 4.2. The Role of AEE in the Silica Hybridization. The addition of AEE brought about the stabilization of the columnar structure of B67 in the silica network. Most probably, AEE behaves as a mediator between the B67-columnar aggregates and silica network. It has been reported that the organic/inorganic interfacial boundaries are varied by neutralization balance of electrostatic charge at the organic/inorganic interfaces, such as S+I-, S-I+, S+X-I+, etc. (S+ or S-, cationic or anionic surfactants; I+ or I-, cationic or anionic silicates; and X-, counter anions). Therefore, the nanostructures of the hybrid materials depend on the interfacial charge balance.34,35 Thus, the neutralization of charged surfaces at the organic/inorganic interface is important for the retention of nanostructures of the hybrid materials.34,35,37,38 In the present case, both the B67 columns with sulfonic acid unit (37) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (38) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317.

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and the silica networks have the negative charged surface. The electric repulsion in the interface should destabilize the columnar structure in the high concentrated state, which can lead to the transformation to the lamellar structure when AEE is absent. AEE can neutralize the B67/silica interfaces in which the amine unit and ethylene oxide unit of AEE strongly interact with the sulfonic acid units on the B67 columns and the silica network, respectively. The B67-columnar structure should be stabilized in the silica network via this effect. With respect to the phase order of the resulting film, the structure fixed as the nematic columnar phase and the middle (hexagonal columnar) phase, which appears in a more concentrated state in an ordinary aqueous solution, was not observed. 4.3. In-Plane Anisotropic Structure. The dip-coating process in known to cause the orientation of rodlike LC substances aligned parallel to the lifting direction.15,39 It seems that the uniform alignment in the present system also stems from the same effect. As shown in this work, the immobilization of the chromonic LLC structure and macroscopically uniform alignment is successfully performed by adopting the dipping method using AEE. Thus far, the large-scale alignment of chromonic LLCs is known to be performed by shear fields.29-32 The dipping method can be an alternative method for macroscopic alignment of chrominic LLC state, which should be of technical significance. Interestingly, the B67 dye molecules are preferentially aligned macroscopically, although the layers are separated by the silica sheet also for the case without AEE. B67 molecules exhibit interlayer orientational correlations despite the fact that they are separated by the silica layer. This fact is reasonably explained by an assumption that the column to lamella phase transition occurs during the immobilization process with the silica network. Without the lyotropic liquid crystallinity characteristic of the chromonic columnar dye aggregates, such macroscopic orientation would not be rationalized. The dichroic ratio in the lamellar structure (D ) 1.5) became smaller than that in the columnar one (D ) 2.3). The reduction of the orientational order should reflect a ravel of the columnar aggregates.

5. Conclusions We successfully attained the immobilization of the chromonic LLC structure of B67 aggregate with silica network under specific conditions and addition of AEE. When the hybrid film is prepared by the dip-coating, the in-plane alignment control of the nanostructure over a large area can be achieved; thereby the chromonic columns are aligned parallel to the lifting direction (the absorption transition of B67 being orthogonal to the column axis). On the other hand, when the same procedure is achieved without AEE, the transformation from the columnar to lamellar structure occurred during the fixation with the silica network. Interestingly, the molecular orientation imposed by the dipping method is retained also in the lamellar structure. We anticipate that the method presented here will propose new opportunities in the fabrication of optical devices such as a new type of lightpolarizing device. One of the significant advantages to construct organic/inorganic nanohybrids is that the robust inorganic network chemically and mechanically stabilizes the structure and functions of the organic molecules. We expect that this method may be extended to immobilizations of other types of functional molecular assemblies such as optically and electrically active discotic liquid crystals, etc. (39) Lu, Y.; Gangull, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, E.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364.

Chromonic LC/Silica Nanohybrids

Acknowledgment. We thank D. Matsunaga at Nippon Kayaku Co. and Dr. T. Tamaki at AIST for kindly providing B67 and Emal20C. We also thank T. Hikage for technical assistance with the X-ray measurements. This work was supported by JSPS GrantIn-Aid for Scientific Research A (16205019) and B (19350056), MEXT Scientific Research for Priority Areas (No. 446 to S.N.), and CREST program of Japan Science and Technology Agency.

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Supporting Information Available: UV-vis absorption spectrum in the precursor sol solution containing B67 (Figure S1) and X-ray diffraction data (Figure S2) in comparison with those in B67 aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA701557Q