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Interlayer Interactions Induced by Amphiphilicities of a Rod-Like Molecule Produce Frustrated Structures in Conventional Calamitic Phases Atsushi Yoshizawa,*,† Ayumi Nishizawa,† Kazuhito Takeuchi,† Yoichi Takanishi,‡ and Jun Yamamoto‡ Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki UniVersity, 3 Bunkyo-cho, Hirosaki, 036-8561, Japan, and Department of Physics, Graduate School of Science, Kyoto UniVersity, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8562, Japan ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: August 31, 2010

We prepared a rod-like molecule, 4-[4-(7-hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3-difluorobenzene, and investigated its physical properties using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The compound was found to exhibit nematic, smectic A, and smectic C phases. A smectic-like layer ordering was detected using XRD at low temperatures of the nematic phase. The nematic phase changed to a smectic A phase when cooled, with no accompanying enthalpy change. Analyses using XRD and POM revealed that the smectic C phase consists of three states: conventional SmC with a monolayered structure, monolayered SmC′ possessing an additional weak bilayered character, and SmC′′ possessing both monolayered and bilayered structures. Furthermore, a discontinuous increase in birefringence of a homeotropically aligned sample of the compound was observed in the SmC′′ phase. Interlayer interactions organized by hydrophobic-hydrophilic amphiphilicity and orthogonal-tilt amphiphilicity are discussed to explain the appearance of the unusual liquid-crystalline phases with a hierarchical structure. Introduction Supermolecular assemblies with well-defined morphologies are fundamental components for structural formation in biological systems and for application to production of novel functional materials. For those reasons, investigation of the driving forces underlying this self-assembly process is an important research topic.1-14 Particularly, molecular design for frustrated liquidcrystalline phases with a hierarchical structure has been an attractive field of materials science. Supermolecules comprising preorganized mesogenic units have been designed because they can form secondary and tertiary structures with unique physical properties. Microsegregation produced by amphiphilicity within a molecule is another important concept for producing higher ordered systems.2 Novel layered structures have also been realized through the microsegregation effect produced by newly designed mesogenic block molecules carrying a semiperfluorinated chain. Lemieux et al. proposed a molecular design combining a structural element that produces the formation of a SmC phase with one that promotes the formation of a SmA phase within a mesogen.15,16 Many mesogenic molecules have been prepared. However, their structures are becoming complex: they less and less resemble a rod-like molecule. We have designed an amphiphilic liquid crystal (I) possessing a terminal hydroxy group that can produce a bilayered smectic structure and a 2,3-difluoro-1,4-diphenylbenzene unit that can produce a tilted smectic structure (Figure 1). In fact, 2,3difluoro-1,4-diphenylbenzene has been used as a useful building block for nematic and smectic liquid crystals.17 Many fluorinated liquid crystals have been designed and investigated, not only in pursuit of fundamental interest but also to explore their * To whom correspondence should be addressed. Fax: +81 172 393558. Phone: +81 172 393558. E-mail: [email protected]. † Hirosaki University. ‡ Kyoto University.

Figure 1. Molecular structure of compound I.

potential functions for use in various applications.18,19 The terminal alcohol is expected to form noncovalent interactions, which can realize a supermolecule system.20 Herein, we report a simple rod-like compound exhibiting optically conventional calamitic phases with an enthalpy-driven hierarchical structure. Experimental Section Preparation of Materials. 2,3-Difluoro-4-(4-hydroxyphenyl)1-(4-hexylphenyl)benzene was purchased from Midori Kagaku Co., Ltd. Purification of the final product was conducted using column chromatography over silica gel (63-210 µm; Kanto Chemical Co. Inc.) with subsequent recrystallization. The purity of the final compound was confirmed using elemental analysis (EA 1110; CE Instruments Ltd.). The structure was elucidated using infrared (IR) spectroscopy (FTS-30; Bio-Rad Laboratories Inc.) and proton nuclear magnetic resonance (1H NMR) spectroscopy (JNM-ECA500; JEOL). 4-[4-(7-Hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3difluorobenzene (I). Potassium carbonate (140 mg, 1.0 mmol) was added to a solution of 2,3-difluoro-4-(4-hydroxyphenyl)1-(4-hexylphenyl)benzene (294 mg, 0.8 mmol) and 7-bromo1-heptanol (206 mg, 1.0 mmol) in cyclohexanone (15 mL). The reaction mixture was stirred at 85 °C for 6 h and then at 105 °C for 1 h. After filtration of the precipitate, the solvent was removed by evaporation. The residue was purified on silica gel using column chromatography with a dichloromethane and ethyl acetate (11:1) mixture as the eluent. It was then recrystallized with a hexane and ethanol (5:2) mixture, giving the desired

10.1021/jp106269m  2010 American Chemical Society Published on Web 10/04/2010

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product: yield 245 mg (64%). 1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 7.52 (d, 2H, Ar-H, J ) 8.6 Hz), 7.50 (d, 2H, Ar-H, J ) 8.0 Hz), 7.28 (d, 2H, Ar-H, J ) 8.0 Hz), 7.25-7.20 (m, 2H, Ar-H), 6.99 (d, 2H, Ar-H, J ) 9.2 Hz), 4.02 (t, 2H, -OCH2, J ) 6.3 Hz), 3.68-3.65 (m, 2H, HO-CH2), 2.67 (t, 2H, Ar-CH2, J ) 7.7 Hz), 1.66 (q, 2H, -CH2, J ) 7.7 Hz), 1.60 (q, 2H, -CH2, J ) 6.9 Hz), 1.51-1.28 (m, 12H, -CH2), 1.23 (t, 1H, -OH, J ) 5.4 Hz), 0.90 (t, 3H, -CH3, J ) 7.2 Hz). IR (neat) νmax/cm-1: 3292, 1609, 1516, 1456, 1109, 813. Elemental Anal. Calcd for C31H38O2F2: C 77.47, H 7.97. Found: C 77.68, H 8.22. Liquid-Crystalline and Physical Properties. The initial phase assignments and corresponding transition temperatures for the products were determined using thermal optical microscopy with a polarizing microscope (POL, Optiphoto; Nikon Corp.) equipped with a microfurnace (FP82; Mettler Inst. Corp.) and a control unit (FP80). Temperatures and enthalpies of transitions were investigated using differential scanning calorimetry (DSC, DSC 6200 calorimeter; Seiko Corp.). Electrooptical studies were conducted using commercially available evaluation cells (E. H. C. Co. Ltd., Japan). The homogeneously aligned cells were made with 5 µm spacing. The inner surfaces had been coated with a polyimide aligning agent and had been buffed unidirectionally. The homeotropically aligned cells were made with 5 µm spacing; the inner surfaces had been coated with cetyltrimethylammonium bromide. The optical tilt angle was determined by finding the extinction direction when a sample was rotated. The XRD patterns of the sample during cooling processes were obtained using a real-time X-ray diffractometer (D8 Discover; Bruker AXS GmbH) equipped with a hot stage and a temperature-control processor. A sample was put on a convex lens, which was then placed in a custom-made temperature stabilized holder (stability within (0.1 °C). The phase transition of the sample under the X-ray beam was monitored by observing the texture simultaneously using polarized light microscopy with a CCD camera. The X-ray apparatus was equipped with a crosscoupled Go¨bel mirror on a platform system with a twodimensional position-sensitive proportional counter (PSPC) detector (HI-Star; Bruker AXS GmbH). Then X-rays were generated at 45 kV and 20 mA; a parallel Cu KR X-ray beam was used to irradiate the sample. Each diffraction pattern was obtained using the 2D PSPC detector at a camera distance of 300 mm. The correlation length along the layer normal (ξ) was determined using the Ornstein-Zernike expression as follows. First, the X-ray profile as a function of 2θ is converted to a scattering function of q according to

q ) (4π/λ) sin θ

(1)

By fitting the X-ray profiles using the following Lorentzian equation,

I(q) )

I0 1 + (q - q0)2ξ2

+ background

(2)

the correlation length ξ is determined. Here, I0 and q0, respectively, signify the peak height and the peak position of q. Results and Discussion Liquid-Crystalline Properties. The phase transition behavior of compound I was investigated using polarized optical mi-

Figure 2. Optical textures of compound I on a glass slide with a cover glass in (a) the SmC phase at 116 °C, (b) the SmC′ phase at 109 °C, and (c) the SmC′′ phase at 97 °C.

croscopy and differential scanning microscopy (DSC). The transition temperatures determined using polarized optical microscopy at a cooling rate of 1 °C · min-1 were those of Iso Liq 146.5 °C N 123.5 °C SmA 117.0 °C SmC 109.8 °C SmC′ 98.2 °C SmC′′. The melting point was 103 °C. The schlieren texture was observed for the planarly aligned domains of a sample on the glass in the N phase. Concomitantly with decreasing temperature, the molecules aligned homeotropically and the texture became completely dark in the N phase. The

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Figure 3. Photomicrograph of the SmC′′ phase of a homeotropically aligned sample of compound I at 97 °C.

N-SmA transition was confirmed by observing their mutual changes in viscosity. The SmA-SmC transition increased the birefringence, which results from biaxiality of the SmC phase. Three different textures were observed for the sample in the SmC phase. Although it remains unclear whether they are liquidcrystalline phases with an inherent symmetry or different structures of a SmC phase, the three states are tentatively classified as SmC, SmC′, and SmC′′ phases on cooling. Photomicrographs of the sample on the glass in the SmC, SmC′, and SmC′′ phases are presented in Figure 2. Characteristic fluctuation for a conventional SmC phase was observed in the homeotropically aligned region for the SmC and SmC′′ phases, although it was not detected for the SmC′ phase. We observed textures of a sample in a homeotropically aligned cell. Birefringence increased slightly with decreasing temperature in the SmC and SmC′ phases. However, it showed a discontinuous increase in the SmC′′ phase near the SmC′-SmC′′ transition, as depicted in Figure 3. The birefringence changed in a region within the sample with clear borders. The bright region was spread. Fluctuation was apparent in both of the dark and bright regions (Figure 3). Therefore, the change in birefringence is thought to occur in the SmC′′ phase. Figure 4 shows photomicrographs of the SmC, SmC′, and SmC′′ phases of a sample in the homogeneously aligned cell. Dark and bright domains corresponding to two azimuthal angles were observed. However, no significant difference was found in texture among the three subphases. Temperature-dependent tilt angles determined by POM are presented in Figure 5. The SmC′-SmC′′ transition temperature of a sample in the homogeneously aligned cell was slightly higher than that on the glass. The tilt angle increases concomitantly with decreasing temperature in the SmC and SmC′′ phases, although it is almost constant in the SmC′ phase. Therefore, the marked increase in birefringence of the homeotropically aligned sample in the SmC′′ phase cannot be explained solely by the change in the tilt angle. Figure 6 shows a cooling DSC thermogram for compound I. The Iso-N transition accompanied an enthalpy change of 1.2 kJ · mol-1. Surprisingly, no transition enthalpy was detected from the N phase to the SmC′′ phase. An N-SmA phase transition usually accompanies an enthalpy change, except for some dimesogenic compounds.21 Phase Structures. We investigated structures of the nematic and smectic phases of compound I using X-ray diffraction. Figure 7 portrays XRD patterns in the small angle region for compound I in the N phase. No diffraction in the small angle region was found in the XRD pattern in the N phase at 140 °C.

Figure 4. Photomicrographs of (a) the SmC phase at 110 °C, (b) the SmC′ phase at 102 °C, and (c) the SmC′′ phase at 95 °C of a planar unidirectionally aligned sample of compound I.

However, a peak at 2θ ) 3.14° appeared at 133 °C; the peak became sharp and its intensity increased concomitantly with decreasing temperature in the N phase. Appearance of the sharp peak in the N phase is unusual behavior because only a broad halo is usually observed in a conventional N phase of a rodlike molecule. The sharp peak at 2θ ) 3.10° at 125 °C corresponds to a spacing of 28.5 Å. The molecular length of compound I using MOPAC is estimated as 31 Å. To confirm the transition behavior, the texture of the sample under the X-ray

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Figure 5. Temperature dependence of an apparent tilt angle as determined by POM in the SmC, SmC′, and SmC′′ phases of compound I.

Figure 6. DSC thermogram of compound I on cooling. The cooling rate was 1 °C · min-1.

beam was observed simultaneously using a CCD camera (see the Supporting Information). The texture change of the sample attributable to the N-SmA transition occurred at 123 °C. Therefore, the N phase was found to exhibit smectic-like ordering with a monolayered structure, which can explain the N-SmA transition without enthalpy change. Investigation of dynamic behavior of the molecules in the N phase is in progress. Figure 8 shows XRD patterns in the small angle region for compound I in the SmA, SmC, SmC′, and SmC′′ phases. A sharp peak at 2θ ) 3.07°, corresponding to the layer spacing of 28.7 Å, is apparent in the SmA phase. Therefore, the SmA phase is inferred to have a monolayered structure in which the interlayer permeation of tails occurs to some degree. Another peak at 2θ ) 1.68°, corresponding to a spacing of 52.4 Å, appeared in the SmC′ phase. The two reflection peaks are positioned in one direction. The SmC′ phase is thought to have a weak bilayered periodicity. Alternatively, another substructure with a periodicity of 52.4 Å can exist in the monolayered structure. From SmC′ to SmC′′, the second peak corresponding to the bilayered periodicity became sharp and the intensity increased. Monolayered and bilayered structures are thought to coexist in the SmC′′ phase. Figure 9 presents XRD patterns in a wide angle region for compound I in the SmC, SmC′, and SmC′′ phases. Broad peaks around 19° reveal that neither SmC′ nor SmC′′ has positional order within the layer. The SmC′ and SmC′′ phases have no hexatic structure within the layer. Figure 10 shows temperature dependences of (a) the monolayered periodicity length and (b) the bilayered periodicity length in the liquid-crystalline phases. Figure 11 shows temperature dependences of relative peak intensities of those peaks corresponding to the monolayered and bilayered periodicities. Phase transitions detected through textural observations using a CCD

Figure 7. XRD patterns in the small angle region for compound I in the N phase at (a) 140 °C, (b) 133 °C, and (c) 125 °C.

camera are indicated in each figure. The monolayered periodicity length in the N phase increases concomitantly with decreasing temperature. In the SmA phase, it shows a plateau. The peak intensity of the monolayered periodicity increases continuously in the N and SmA phases, which is consistent with the fact that the N-SmA transition does not accompany the enthalpy change. The monolayered periodicity length shows a continuous decrease concomitantly with decreasing temperature in the three SmC subphases. The bilayered periodicity length also decreases in the SmC′ and SmC′′ phases with decreasing temperature. The intensity of a peak corresponding to the monolayered periodicity increases concomitantly with decreasing temperature in the SmC and SmC′ phases. However, it shows a discontinuous and marked decrease at the SmC′-SmC′′ transition, suggesting that reorganization of the constituent molecules occurs at the SmC′-SmC′′ transition. The intensity of a peak corresponding

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Figure 8. XRD patterns in the small angle region for compound I in (a) the SmA phase at 120 °C, (b) the SmC phase at 112 °C, (c) the SmC′ phase at 104 °C, and (d) the SmC′′ phase at 95 °C.

to the bilayered periodicity is weak in the SmC′ phase. It shows a slight increase concomitantly with decreasing temperature in the SmC′ and SmC′′ phases; then, it exhibits a marked increase around 97 °C in the SmC′′ phase. The discontinuous increase in the peak intensity corresponding to the bilayered periodicity is thought to be related to the marked increase in birefringence of the homeotropically aligned sample, as depicted in Figure 3. Some chiral smectic C liquid crystals exhibit an intermediate ferrielectric phase with a 3- or 4-layer periodicity and an antiferroelectric phase below the SmC* phase.22 Anticlinic ordering can exist for achiral forms. However, no ferrielectric ordering has been observed for achiral systems. Very recently, we reported that a ferrielectric-like ordering can exist in a racemic mixture of a chiral liquid crystal oligomer and the enantiomer.23 A ferrielectric phase is clearly distinguished from an antiferroelectric or ferroelectric phase under a polarized microscope because its texture fluctuated with constant motion as domains form, coalesce, and disappear. Such a texture was not observed in the SmC′ phase of compound I. The SmC′ phase is not an intermediate ferrielectric-like SmC phase. However, the SmC′′ phase exhibits a significant increase in birefringence of the homeotropically aligned sample and has a bilayered arrangement. Each is often observed in an anticlinic SmC phase. An extinction direction of a planarly aligned sample in the SmC′′ phase is along the layer normal if the SmC′′ phase has an anticlinic structure. However, the extinction direction in the SmC′′ phase differed from that in the SmA phase. Therefore, we can exclude the possibility that the SmC′′ phase is an anticlinic SmC phase. Molecular Organization Models. In the low-temperature region of the N phase, a sharp peak in the small angle region signifies the existence of a cybotactic structure with layer ordering in the nematic phase. The correlation length along the

layer normal in the N phase increases as it approaches the N-SmA phase transition. It is estimated as 590 Å at 133 °C and 1000 Å at 125 °C. About 30 layers might exist in each cybotactic smectic-like ordering at 125 °C near the N-SmA transition. Hydrophilic interactions between terminal hydroxyl groups in adjacent layers can produce cybotactic layer orderings, as presented in Figure 12. After cooling, cybotactic layer structures can mutually adhere to form a monolayered smectic A phase. In the SmC phase, the layer spacing decreases concomitantly with decreasing temperature. The tilt angle is estimated using XRD measurements according to θ ) cos-1(LC/LA), where θ is the tilt angle and where LA and LC, respectively, denote layer spacings in the SmA and SmC phases. The maximum tilt angle in the SmC phase is 16.4°, which is larger than the optically determined tilt angle of 13°. Therefore, the SmC phase has a monolayered structure in which the long axis tilts slightly more than the core part. In the SmC′ phase, the layer spacing decreases concomitantly with decreasing temperature, although the optical tilt angle shows an almost constant value, as presented in Figure 5. Furthermore, the SmC′′ phase has a weak bilayered periodicity. A possible explanation for the results described above is interlayer permeation caused by interactions between terminal hydroxy groups or aliphatic tails in adjacent layers, as presented in Figure 13a, which can induce the bilayered character. In the SmC′′ phase, both monolayered and bilayered spacings decrease on cooling, and the optical tilt angle increases. Peak intensity corresponding to the monolayered structure shows a discontinuous and marked decrease at the SmC′-SmC′′ transition. Then, it increases concomitantly with decreasing temperature in the SmC′′ phase, indicating that a significant change in the monolayered molecular ordering occurs at the SmC′-SmC′′ transition. In contrast, a discontinuous increase in peak intensity of the bilayered

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Figure 10. Temperature dependences of (a) the monolayered periodicity lengths in the N, SmA, SmC, SmC′, and SmC′′ phases and (b) the bilayered periodicity lengths in the SmC′ and SmC′′ phases.

Figure 9. XRD patterns in the wide angle region for compound I in (a) the SmC phase at 112 °C, (b) the SmC′ phase at 103 °C, and (c) the SmC′′ phase at 99 °C.

periodicity is apparent in the SmC′′ phase. When cooling to the SmC′′ phase, lateral tilt interactions occur again. The molecules are tilted with respect to the layer normal, which accompanies the interlayer correlation.24 Intralayer tilt correlation appearing in the SmC′′ phase might produce hydrogen bonding between the terminal alcohol in adjacent layers, which increases the longitudinal order. Consequently, closer packing with respect to the lateral direction of molecules between adjacent layers is thought to be induced by the hydrogen bonding in the SmC′′ phase. Two adjacent molecules are combined into a quasi-single dimer. The molecules exhibit frustration whether they exist as a quasidimeric structure or as a monomeric structure. Hydrogenbonded liquid crystals have led to discovery of new mesomorphic behaviors.25 Recently, trimeric liquid crystals assembled using both hydrogen and halogen bonding have been reported.26 A discontinuous increase in birefringence of the homeotropically aligned sample in the SmC′′ phase is explained in terms of the formation of a dimeric structure as follows. Generally speaking, birefringence of a homeotropically aligned SmC texture depends

Figure 11. Temperature dependences of relative peak intensities for the monolayered periodicity (closed rhombi) in the N, SmA, SmC, SmC′, and SmC′′ phases and for the bilayered periodicity (closed triangles) in the SmC′ and SmC′′ phases.

Figure 12. Possible organization model for molecules in the N phase of compound I.

on a projected component of the long axis to the layer plane, as presented in Figure 13a. The birefringence increases concomitantly with increasing tilt angle. In this system, the

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Figure 13. Possible organization model for the molecules in (a) the SmC′ phase and (b) the SmC′′ phase of compound I. Red molecules represent a quasidimeric structure, whereas blue molecules represent a monomeric structure.

temperature-dependent tilt angle in the SmC′′ phase cannot explain the discontinuous increase in birefringence. However, monomeric molecules and quasidimeric molecules coexist in the SmC′′ phase. The projected component of the quasidimeric molecule is longer than that of the monomeric molecule, as presented in Figure 13b. Conclusion This report describes the phase transition behavior of a simple rod-like molecule in which a 2,3-difluoro-1, 4-diphenylbenzene unit and a hydroxy group are connected via a flexible methylene spacer. Compound I was found to exhibit the following phase sequence: high-temperature nematic phase with a conventional structure, low-temperature nematic phase with a layer ordering, monolayered smectic A phase, monolayered smectic C phase, monolayered smectic C′ phase possessing a weak bilayered periodicity, and smectic C′′ phase possessing both monolayered and bilayered structures. Interlayer interactions attributable to equal hydrophilic properties of both terminals in each layer can induce cybotactic structures with layer ordering in the lowtemperature N phase. Interlayer permeation produces a bilayered

periodicity in the SmC′ phase. In the SmC′ phase, tilt ordering in each layer organizes strong interactions between the terminal alcohol in adjacent layers, which forms a quasidimeric molecule. The interlayer reorganization of the constituent molecules produces a discontinuous change in birefringence of the homeotropically aligned SmC′′ texture. Coupling between two amphiphilicities, i.e., hydrophobic-hydrophilic and orthogonaltilt, can regulate interlayer interactions in a simple rod-like molecule, producing unusual frustrated structures in the calamitic

Figure 14. Amphiphilicities of compound I.

Frustrated Structures in Conventional Calamitic Phases phases (Figure 14). The present approach yields a dynamic design for supramolecular assemblies with a hierarchical structure. Acknowledgment. This work was partially supported by a Grant for Hirosaki University Institutional Research and a Grantin-Aid for Challenging Exploratory Research from JSPS (21655045). Supporting Information Available: Photomicrographs of (a) the N phase at 140 °C, (b) the N phase at 133 °C, (c) the N phase at 125 °C, and (d) the SmA phase at 120 °C of a sample of compound I under the X-ray beam. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Goodby, J. W.; Mehl, G. H.; Saez, I. M.; Tuffin, R. P.; Mackenzie, G.; Auzely-Velty, R.; Benvegnu, T.; Plusquellec, D. Chem. Commun. 1998, 2057. (b) Goodby, J. W.; Saez, I. M.; Cowling, S. J.; V. Go¨rtz, V.; Draper, M.; Hall, A. W.; Sia, S.; Cosquer, G.; Lee, S.-E.; Raynes, E. P. Angew. Chem., Int. Ed. 2008, 47, 2754. (2) (a) Tschierske, C. J. Mater. Chem. 2001, 11, 2647. (b) Tschierske, C. Chem. Soc. ReV. 2007, 36, 1930. (3) (a) Kato, T. Science 2002, 295, 2414. (b) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38. (4) Imrie, C. T.; Henderson, P. A. Chem. Soc. ReV. 2007, 36, 2096. (5) Lim, Y.-B.; Moon, K.-S.; Lee, M. J. Mater. Chem. 2008, 18, 2909. (6) Yoshizawa, A. J. Mater. Chem. 2008, 18, 2877. (7) Nguyen, H.-T.; Destrade, C.; Malthete, J. AdV. Mater. 1997, 9, 375. (8) Donnio, B.; Buathong, S.; Bury, I.; Guillon, D. Chem. Soc. ReV. 2007, 36, 1495. (9) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Richardson, R. M.; Whitehouse, I. J.; Rebrov, E. A.; Muzafarov, A. M. Macromolecules 2000, 33, 5549. (10) Paleos, C. M.; Tsiourvas, D. Liq. Cryst. 2001, 28, 1127.

J. Phys. Chem. B, Vol. 114, No. 42, 2010 13311 (11) Donnino, B.; Guillon, D.; Bruce, D. W.; Deschenaux, R. Metallomesogens. ComprehensiVe Coordination Chemistry II: From Biology to Nanotechnology; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2003; Vol. 7, pp 357-627. (12) Peltz, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 11, 707. (13) Reddy, R. A.; Tschierske, C. J. Mater. Chem. 2006, 16, 907. (14) Ryu, J.-H.; Lee, M. Struct. Bonding (Berlin, Ger.) 2008, 128, 63. (15) Li, L.; Jones, C. D.; Magolan, J.; Lemieux, R. P. J. Mater. Chem. 2007, 17, 2313. (16) (a) Roberts, J. C.; Kapernaum, N.; Giesselmann, F.; Lemieux, R. P. J. Am. Chem. Soc. 2008, 130, 13842. (b) Roberts, J. C.; Kapernaum, N.; Song, Q.; Nonnenmacher, D.; Ayub, K.; Giesselmann, F.; Lemieux, R. P. J. Am. Chem. Soc. 2010, 132, 364. (17) (a) Hird, M.; Toyne, K. J.; Slaney, A. J.; Goodby, J. W. J. Mater. Chem. 1995, 5, 423. (b) Hird, M.; Toyne, K. J. Mol. Cryst. Liq. Cryst. 1998, 323, 1. (c) Hird, M. Chem. Soc. ReV. 2007, 36, 2070. (18) Kirsch, P.; Bremer, M. Angew. Chem., Int. Ed. 2000, 39, 4216. (19) Pauluth, D.; Tarumi, K. J. Mater. Chem. 2004, 14, 1219. (20) Bruce, D. W. Struct. Bonding (Berlin, Ger.) 2008, 126, 161. (21) Yoshizawa, A.; Kurauchi, M.; Kohama, Y.; Dewa, H.; Yamamoto, K.; Nishiyama, I.; Yamamoto, T.; Yamamoto, J.; Yokoyama, H. Liq. Cryst. 2006, 33, 611. (22) (a) Isozaki, T.; Fujisawa, T.; Takezoe, H.; Fukuda, A.; Hagiwara, T.; Suzuki, Y.; Kawamura, I. Jpn. J. Appl. Phys. 1992, 31, L1435. (b) Inui, S.; Iimura, N.; Suzuki, T.; Iwane, H.; Miyachi, K.; Takanishi, Y.; Fukuda, A. J. Mater. Chem. 1996, 6, 671. (c) Matsumoto, T.; Fukuda, A.; Johno, M.; Motoyama, Y.; Yui, T.; Seomun, S, -S.; Yamashita, M. J. Mater. Chem 1999, 9, 2051. (23) Noji, A.; Uehara, N.; Takanishi, Y.; Yamamoto, J.; Yoshizawa, A. J. Phys. Chem. B 2009, 113, 16124. (24) Yoshizawa, A.; Kikuzaki, H.; Fukumasa, M. Liq. Cryst. 1995, 18, 351. (25) For reviews, see, e.g.: Kato, T. Hydrogen-Bonded Systems, Handbook of Liquid Crystals; Demus, D., Gray, J. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 2B, pp 969-978. (26) (a) Metrangolo, P.; Pra¨sang, C.; Resnati, G.; Liantonio, R.; Whitwood, A. C.; Bruce, D. W. Chem. Commun. 2006, 3290. (b) Pra¨sang, C.; Nguyen, H. L.; Horton, P. N.; Whitwood, A. C.; Bruce, D. W. Chem. Commun. 2008, 6164.

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