Chiral Liquid Crystal Trimer Exhibiting an Optically Uniaxial Smectic

Article pubs.acs.org/JPCC

Chiral Liquid Crystal Trimer Exhibiting an Optically Uniaxial Smectic Phase with a Double-Peak Polarization Daiki Tsuji,† Yoichi Takanishi,‡ Jun Yamamoto,‡ and Atsushi Yoshizawa*,† †

Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, 036-8561, Japan ‡ Department of Physics, Graduate School of Science, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8562, Japan S Supporting Information *

ABSTRACT: We prepared a chiral liquid crystal trimer consisting of three different mesogenic units connected via flexible spacers with different parity. We then investigated its liquid crystalline properties using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). A POM study revealed that the trimer has a phase sequence of isotropic liquid−optically uniaxial smectic A (SmA*)−helical smectic C (SmC*) on cooling. However, XRD measurements revealed a discontinuous decrease in layer spacing occurring at T − TIA* = −4 K in the SmA* phase. Double-peak polarization in the current response and tristable switching were observed in the low-temperature SmA* phase below T = TIA* − 4 °C. In a binary phase diagram of the trimer and its enantiomer, the transition temperature from the low-temperature SmA* phase to the SmC* phase increased continuously with decreasing optical purity. The results indicate that the low-temperature SmA* phase forms an optically uniaxial structure in which the molecules tilt to the same direction in each layer, as they do in the SmC phase. We discuss the organization of the supermolecules in the unusual smectic phase.

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decyloxybenzylideneamino)cinnamate (DOBAMBC),34 many rod-like ferro- and antiferroelectric liquid crystals possessing molecular chirality have been reported. Furthermore, achiral bent-shaped molecules exhibit ferro- and antiferroelectricity in their biaxial B phases.15,16,31,35 Tilt of achiral bent-shaped molecules induces macroscopic chirality in the fluid smectic liquid crystal phases.36 Recently, spontaneous ferroelectric ordering was observed in a fluid orthorhombic SmA phase of an achiral bent-core liquid crystal possessing a terminal trisilane unit.37 This is the highest-symmetry layered ferroelectric phase. Although a polar uniaxial SmA phase has not yet been found, electro-optical response is known to occur in some chiral SmA (SmA*) phases. In the SmA* phase, an external electric field induces a molecular tilt because of the electroclinic effect.38−41 The magnitude of the induced tilt depends on the value of the electroclinic coefficient, and induced angles usually lie in the 5− 10° range. Electroclinic switching occurs usually in the SmA* phase close to the SmA*−SmC* transition. Recently, unusual double-peak polarization in the current response was observed in some de Vries type SmA* phases.42−45 “De Vries-like” liquid crystal materials are characterized by their maximum layer contraction of ≤1% upon transition from the SmA phase to the SmC phase.30 De Vries proposed some diffuse cone models

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.1−17 Particularly, molecular design for frustrated liquidcrystalline phases has been an attractive field of materials science. A primary factor in the thermotropic liquid-crystalline phases is the gross molecular shape of compounds. Microsegregation produced by amphiphilicity within a molecule is another important factor for producing a higher-order system.3,4 Coupling or competition between chirality-induced twisting power and such factors can produce chiralitydependent frustrated phenomena such as twist grain boundary phases,18 blue phases,19 and ferroelectric−ferrielectric−antiferroelectric phase transitions.20−23 Among them, the phase transition behavior of a polar system is an interesting phenomenon.24−32 Not only is it of value in application such as fast-response displays but also it is of fundamental interest in relation to synclinic or anticlinic ordering of the molecules.33 Macroscopic polarization density, characteristic of ferroelectric phases, is stabilized by dipolar interactions. These are weakened as materials become more fluid and take higher symmetry, limiting ferroelectricity to crystals and to biaxial smectic liquid crystals. Since the appearance of ferroelectricity in the chiral smectic C (SmC*) phase of (S)-2-methylbutyl 4-(4© 2012 American Chemical Society

Received: December 2, 2011 Revised: March 18, 2012 Published: March 23, 2012 8678

dx.doi.org/10.1021/jp2115934 | J. Phys. Chem. C 2012, 116, 8678−8687

The Journal of Physical Chemistry C

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Figure 1. Molecular structure of the chiral trimer.

stirred at room temperature for 14 h. After filtration of the precipitate, the solvent was removed by evaporation. The residue was purified using column chromatography on silica gel with a toluene and ethyl acetate (2:1) mixture as the eluent. (R)-1-Methylheptyl 4-(5-hydroxypentyloxy)biphenyl-4′-carboxylate was obtained as white solid. Yield: 1.77 g (47%). (R)-1-Methylheptyl 4-(5-hydroxypentyloxy)biphenyl-4′-carboxylate (1.73 g, 4.2 mmol), N,N′-dicyclohexylcarbodiimide (1.3 g, 6.3 mmol), and 4-(N,N-dimethylamino)pyridine (0.077 g, 0.63 mmol) were added to a solution of 4-(4methyloxycarbonyloxyphenyl)benzoic acid (1.14 g, 4.2 mmol) in dichloromethane (130 mL). The resulting solution was stirred at room temperature for 13 h. The precipitated materials were removed by filtration. After removal of the solvent by evaporation, the residue was purified using column chromatography on silica gel with a dichloromethane:ethyl acetate (20:1) mixture as the eluent. Subsequently, the obtained solid was washed from hexane, giving (R)-1-methylheptyl 4′-{5-[4-(4methoxycarbonyloxyphenyl)benzoyloxy]pentyloxy}biphenyl-4carboxylate as a white solid. Yield: 1.63 g (58%). An aqueous ammonia solution (20%, 40 mL) was added to 35 mL of a solution comprising (R)-1-methylheptyl 4′-{5-[4-(4methoxycarbonyloxyphenyl)benzoyloxy]pentyloxy}biphenyl-4carboxylate (1.6 g, 2.4 mmol) in 100 mL of an ethanol:chloroform (1:1) mixture. The reaction mixture was stirred at room temperature for 12 h. The solvent was removed by evaporation. The residue was washed from hexane, giving the desired product as a white solid. Yield: 1.42 g (97%). 7-[4-(5-Octylpyrimidin-2-yl)phenyloxy]heptanoic acid. Potassium carbonate (0.59 g, 4.2 mmol) was added to a solution of 5-octyl-2-(4-hydroxyphenyl)pyrimidine (0.79 g, 2.8 mmol) and ethyl 7-bromoheptanoate (0.66 g, 2.8 mmol) in cyclohexanone (16 mL). The reaction mixture was stirred at 120 °C for 5 h. After filtration of the precipitate, the solvent was removed by evaporation. The residue was purified using column chromatography on silica gel with a dichloromethane:ethyl acetate (20:1) mixture as the eluent. The intermediate product, ethyl 7-[4-(5-octylpyrimidin-2-yl)phenyloxy]heptanoate, was obtained. Yield: 0.98 g (66%). Ethyl 7-[4-(5-octylpyrimidin-2-yl)phenyloxy]heptanoate (0.85 g, 1.9 mmol) was added to a solution of KOH (0.21 g, 3.7 mmol) in 18 mL of an ethanol:water (19:1) mixture. The resulting solution was stirred under reflux for 3 h. The solution was acidified using aq. HCl. The solution was extracted using dichloromethane. The organic layers were combined, dried over magnesium sulfate, filtered, and evaporated. Recrystallization from ethanol gave the desired compound. Yield: 0.61 g (76%). (R)-5-{4-[4-(1-Methylheptyloxycarbonyl)phenyl]phenyloxy}pentyl 4′-{7-[4-(5-octylpyrimidin-2-yl)phenyloxy]heptanoyloxy}biphenyl-4-carboxylate, (R)-1. (R)-1-Methylheptyl 4′-{5-[4-(4-hydroxyphenyl)benzoyloxy]pentyloxy}biphenyl-4-carboxylate (202 mg, 0.33 mmol), N,N′-

that describe the SmA phase as a lamellar structure in which mesogens have a tilted molecular orientation and random azimuthal distribution.46−48 In de Vries I, the in-layer director tilts in the same direction, although the azimuthal angle is disordered from layer to layer.47 In de Vries II, the molecules are tilted, but the azimuthal angle varies even in a single layer.48 Furthermore, a cybotactic SmC structure is thought to be another model for the de Vries SmA. It would be a smectic phase with cybotactic SmC domains with some characteristic size both in layers and across layers.49 A de Vries SmA−SmC phase transition is described as an ordering of the azimuthal distribution that results in zero layer contraction. The chiral version of this phase, termed de Vries SmA*, has attracted particular attention because of the absence of the zigzag defects as a consequence of associated minimal layer shrinkage across the SmA*−SmC* transition. Supermolecules comprising mesogenic units have been designed because they can form secondary and tertiary structures with unique physical properties. We present herein a preprogrammed molecular design for frustrated polar liquidcrystalline phases. We designed a chiral liquid crystal trimer possessing a dipole moment along the long axis (Figure 1). The three mesogenic units are connected via flexible spacers with different parity. The compound exhibited optically uniaxial SmA* and biaxial SmC* phases. The layer spacing decreased discontinuously at T − TIA* = −4 K (TIA*: isotropic liquid to SmA* transition temperature) in the SmA* phase. It decreased continuously with decreasing temperature. Furthermore, unusual double-peak polarization in the current response was observed in the low-temperature SmA* phase below T = TIA* − 4 °C. We discuss the organization of the supermolecules in the unusual smectic phase.

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EXPERIMENTAL SECTION Preparation of Materials. 5-Octyl-2-(4-hydroxyphenyl)pyrimidine was purchased from Midori Kagaku Co. Ltd. (R)-1Methylheptyl 4-hydroxybiphenyl-4-carboxylate was prepared using our reported method.50 Purification of the final product was conducted using column chromatography over silica gel (63−210 μm; Kanto Chemical Co. Inc.) with subsequent recrystallization. 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). (R)-1-Methylheptyl 4′-{5-[4-(4-hydroxyphenyl)benzoyloxy]pentyloxy}biphenyl-4-carboxylate. Triphenylphosphine (2.41 g, 9.19 mmol) in tetrahydrofuran (THF, 30 mL) was added to a solution of (R)-1-methylheptyl 4hydroxybiphenyl-4-carboxylate (3.0 g, 4.26 mmol), pentane1,5-diol (1.92 g, 18.4 mmol), and diethyl azodicarboxylate (4.0 g, 4.28 mmol) in THF (230 mL). The reaction mixture was 8679



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was 0.1 Vpp. The sample was inserted into a 13-mm-thick planar cell, the temperature of which was controlled using a temperature control unit (DB1230; Chino Corp.).

dicyclohexylcarbodiimide (100 mg, 0.5 mmol), and 4-(N,Ndimethylamino)pyridine (6 mg, 0.05 mmol) were added to a solution of 7-[4-(5-octylpyrimidin-2-yl)phenyloxy]heptanoic acid (145 mg, 0.35 mmol) in dichloromethane (6 mL). The resulting solution was stirred at room temperature for 5 h. The precipitated materials were removed by filtration. After removal of the solvent by evaporation, the residue was purified using column chromatography on silica gel with dichloromethane as the eluent. Recrystallization from ethanol gave the desired product as a white solid. Yield: 200 mg (64%). 1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.57 (s, 2H, Ar−H), 8.35 (d, 2H, Ar−H, J = 9.1 Hz), 8.09 (d, 2H, Ar−H, J = 8.4 Hz), 8.07 (d, 2H, Ar−H, J = 8.3 Hz), 7.63−7.59 (m, 6H, Ar−H), 7.55 (d, 2H, Ar−H, J = 8.6 Hz), 7.17 (d, 2H, Ar−H, J = 8.6 Hz), 6.99 (d, 4H, Ar−H, J = 8.7 Hz), 5.17 (sext, 1H, −OC*H(CH3), J = 6.9 Hz), 4.39 (t, 2H, −COOCH2−, J = 6.6 Hz), 4.05 (t, 4H, −OCH2−, J = 6.4), 2.63−2.58 (m, 4H, aliphatic−H), 1.93−1.82 (m, 8H, aliphatic−H), 1.70−1.53 (m, 10H, aliphatic−H), 1.35−1.27 (m, 21H, aliphatic−H), 0.88 (t, 6H, −CH3, J = 6.6 Hz). IR (KBr) νmax/cm−1: 2927, 2854, 1753, 1709, 1605. Elemental anal. calcd. for C64H78N2O8: C, 76.61; H, 7.84; N, 2.79. Found: C, 76.68; H, 7.88; N, 2.86. The enantiomer (S)-1 was obtained using a similar method to that used for (R)-1 from (R)-2-octanol. 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.). The XRD patterns of the homeotropically aligned sample during a cooling process were obtained using a real-time X-ray diffractometer (MicroMax−007HF; Rigaku Corp.) 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 Xray apparatus was equipped with a platform arrangement and a two-dimensional detector (Image intensifier and CCD C929901; Hamamatsu Photonics KK). Then X-rays were generated at 40 kV and 20 mA; the sample was irradiated with a Cu Kα Xray beam, with confocal mirrors to correlate the incident X-ray beam and to increase its intensity. Each diffraction pattern was obtained using the detector at a camera distance of ca. 730 mm. Electro-optical studies were conducted using commercially available evaluation cells (EHC Co. Ltd., Japan). Spontaneous polarization and optical tilt across the temperature range of the smectic phases were measured using standard electro-optical techniques. The ITO-coated glass sandwich cells were constructed with 5 μm spacers; the unidirectionally buffed inner surfaces had been coated with a polyimide aligning agent. The optical tilt angle was obtained by determining the extinction direction when an electric field was applied to the specimen. A regulated DC power supply (Kikusui Electronics Corp.) was used for the DC field. The spontaneous polarization was measured using a current-pulse technique. Dielectric measurements were carried out using an LCR meter (LCR Hitester 3532−50; Hioki E.E. Corp.) at frequencies between 42 Hz and 5 MHz. The measuring field

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RESULTS AND DISCUSSION Liquid-Crystalline Properties. The phase transition behavior of compound (R)-1 was investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The transition temperatures and enthalpies of the transitions at a cooling rate of 1 °C·min−1 were those of the isotropic liquid (Iso), 136.7 °C (21.4 kJ·mol−1), chiral smectic A (SmA*), 122.4 °C, and chiral smectic C (SmC*), 120.0 (73.1 kJ·mol−1), crystal. Figure 2 shows a DSC cooling thermogram

Figure 2. DSC thermogram of (R)-1 on cooling. The cooling rate was 1 °C·min−1.

for (R)-1. The SmA*−SmC* phase transition did not accompany enthalpy change. The melting point was 125.6 °C. Figure 3 depicts optical textures of those phases by a sample of (R)-1 on a glass plate with a cover glass. A typical fan texture in the planar alignment region and a dark texture in the homeotropic region were observed in the SmA* phase. On cooling to the SmC* phase, birefringence in the homeotropic region increased, and helical pitch bands appeared in the fanshaped texture. These observations reveal that the uniaxial− biaxial phase transition occurs, and the helical structure in which the helical axis is parallel with respect to the layer normal forms. Although the helical structure was not formed completely, the helical pitch is estimated as about 2 μm. Figure 4 portrays the temperature dependence of the layer spacing for (R)-1 in the SmA* phase on cooling. Because of crystallization, the layer spacing in the SmC* was not obtainable. The molecular length of (R)-1 using MOPAC is estimated as 64 Å. Therefore, the SmA* phase has a monolayered structure. The layer spacing shows a discontinuous decrease at T − TIA* = −4 K. Then, it decreases continuously with decreasing temperature. The discontinuous decrease indicates that the molecules start to tilt with respect to the layer normal at 132 °C in the optical uniaxial SmA* phase. Electro-Optical Properties. Clear tristable electro-optical switching was observed in the SmA* phase below 132 °C. The high-temperature SmA* above 132 °C and the low-temperature SmA* below 132 °C are denoted, respectively, as SmA*-H and SmA*-L. Figure 5 shows optical textures of (R)-1 in a homogeneously aligned cell in the SmA*-L phase at 124.6 °C with ±8 V·μm−1 or without an electric field. The extinction 8680



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direction was rotated counterclockwise with an AC field of +8 V·μm−1, but it was rotated clockwise with an AC field of −8 V·μm−1. Applying an electric field causes a distinct change in interference color from yellow to red, which is related to the increasing birefringence. In the SmA*-H phase, electro-optical response was also observed. The extinction direction was rotated slightly with an AC field; however, application of the electric field did not cause a marked change in the interference color (see Supporting Information Figure S1). Figure 6

Figure 6. Temperature dependence of a saturated tilt angle in the SmA* phase of (R)-1 with an electric field of 8 V·μm−1.

portrays the temperature dependence of a saturated tilt angle corresponding to the electrically induced ferroelectric state with an electric field of 8 V·μm−1. The tilt angle increases continuously up to 16° with decreasing temperature. (R)-1 exhibits a large electroclinic effect in the SmA*-L phase. The maximum tilt angle obtained using XRD is 10°. The change in birefringence with tilt and the large electroclinic effect indicate that the SmA*-L phase has a de Vries character. On the other hand, the main characteristic associated with de Vries smectic phases is that the smectic layer thickness is nearly constant in the smectic area. However, the layer spacing in the SmA*-L phase decreases with decreasing temperature. We consider two hypotheses as follows. The layer spacings in SmA*-H and SmA*-L phases are shorter than the molecular length. The layer shrinkage is attributed to the increase in interlayer permeation. The other hypothesis is that the molecular tilt in each layer increases upon cooling, as occurs also in conventional SmC phases, but the azimuthal angle is disordered from layer to layer. We observed the temperature dependence of electrical response in switching current under triangular waves in the SmA* phase of (R)-1. The results are shown in Figure 7. No switching current was detected at T − TIA* = −3 K [Figure 7a] in the SmA*-H. However, the double-peak polarization current per half-period of the switching voltage appeared at T − TIA* = −4.5 K. The peaks became larger with decreasing temperature [Figures 7b,c] in the SmA*-L. The two peaks were well separated and appeared on opposite sides of the 0 V level of the applied field. In addition to the double peaks, a single sharp peak characteristic to ferroelectric switching appeared around the 0 V field in the low-temperature region of the SmA*-L phase close to the SmA*-L−SmC* transition [Figure 7d]. Double-peak polarization is usually observed in tilted smectic phases of an antiferroelectric liquid crystal and an extremely

Figure 3. Optical texture of (R)-1 on a glass slide with a cover glass in (a) the SmA* phase at 133.5 °C, (b) the SmA* phase at 131.5 °C, and (c) the SmC* phase at 120.0 °C. Scale bar = 100 μm.

Figure 4. Temperature dependence of layer spacing in the SmA* phase of (R)-1.

Figure 5. Optical textures of (R)-1 between homogeneously aligned glass plates in the SmA* phase at 124.6 °C with ±8 V·μm−1 or without an electric field. 8681



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for systems in which a molecule contains a structurally incompatible terminal such as siloxane and hydrocarbon chains.44 Ghosh et al. supported the antiferroelectric-like model based on their observation of the double-peak current in the de Vries SmA* phase of a fluorinated antiferroelectric liquid crystal.45 Neither enthalpy change nor clear discontinuity in the X-ray diffractograms was observed at the SmA*-L−SmC* transition. However, we cannot exclude the possibility that it is a weak first-order transition. We then consider the possibility of SmA*L being short-pitch SmC* and the low-temperature SmC* being antiferroelectric SmC*A. To clear a phase sequence of (R)-1, a miscibility study between (R)-1 and its homologue, (R)-5-{4-[4-(1-methylheptyloxycarbonyl)phenyl]phenyloxy}pentyl 4′-{7-[4-(5-octylpyrimid in-2-yl)phenyloxy]hexanoyloxy}biphenyl-4-carboxylate [(R)-2], was carried out. (R)-2 exhibits a phase sequence of Iso−SmC*−Ferri− SmC*A−SmI*A−Cry. The SmC* phase of (R)-2 was found not to mix continuously with the SmA*-L phase of (R)-1, and the SmC* phase of both compounds proved to be miscible across the full composition range [see Supporting Information Figure S2]. These results support that the SmA*-L phase is not a SmC*α phase and that the SmC* phase is not a SmC*A phase. Although further investigation of the SmA*-L−SmC* transition is necessary, we assume the diffuse cone model of de Vries I for the SmA*-L phase at present (Figure 8). Without an

Figure 7. Electrical response in the switching current in the SmA* phase of (R)-1 at (a) T − TIA* = −3 K, (b) T − TIA* = −7 K, (c) T − TIA* = −9 K, and (d) T − TIA* = −14 K on application with a triangle wave switching field of ±8 V·μm−1 at a frequency of 15 Hz. The cell gap was 5 μm.

short-pitch SmC* phase with very small director tilt.30,31 The unusual short-pitch SmC* phase is designated as SmC*α. With respect to antiferroelectric liquid crystals, it is attributed to polarization switching between the antiferroelectric state and the electric-field-induced ferroelectric state. There are two ferroelectric states with different sign of spontaneous polarization. The ground state of antiferroelectric structure transforms to a ferroelectric one of both signs of the field if the field applied is above a threshold field. A SmC*α phase is always found directly below a SmA* phase, and its temperature range is narrow (ca. 1−2 K). It can be quite difficult to distinguish the phase from SmA*. The electro-optical response in a SmC*α phase is explained by Lagerwall as follows.51 If the pitch is short, the incommensurate antiferroelectric aspect of the helix can produce hysteresis in the electro-optic response and two peaks in the current response, i.e., the characteristics of antiferroelectric switching, even in SmC*. The electro-optic and dielectric behavior of the SmC*α phase is explainable as a result of helix distortion at low field strength and unwinding/ rewinding at high field strength. If the SmA*-L phase is a SmC*α phase, the complex switching behavior near the SmA*L−SmC* transition [Figure 7d] can be interpreted as follows. A short-pitch SmC* helix in parts of the sample that no longer rewinds during the process could give ferroelectric switching overlaid with the helical antiferroelectric. Recently, double-peak polarization was observed in some de Vries SmA* phases. Two mechanisms were proposed for the double-peak polarization. Wang et al. reported the double-peak polarization in a de Vries material for the first time.42,43 They argued that the alternating electric field lifts the degeneracy of the azimuthal angle which gives rise to the double-peak polarization. Clark et al. reported that a first-order SmA*−SmC* transition is necessary to observe double peaks.43 Karpernaum et al. reported the origin of the large electroclinic effect in a “de Vries”-type ferroelectric liquid crystal.52 Using high-resolution synchrotron X-ray experiments, they demonstrated that the compound exhibits a weak first-order SmA*−SmC* transition. Prasad et al. proposed an antiferroelectric-like ordering to explain the double-peak polarization current in the de Vries SmA* phase

Figure 8. Model for double-peak polarization.

electric field, the molecules are tilted in the same direction in each layer, although the azimuthal angle is disordered from layer to layer. It is not clear how the chirality plays on the interlayer correlation. The present double-peak polarization is explainable as a result of the electric field induced into a SmC* phase. Chiral Effects. We investigated the effect of optical purity on the phase transition behavior of (R)-1. Figure 9 depicts a binary phase diagram for mixtures of (R)-1 and its enantiomer (S)-1.The SmA*-H−SmA*-L transition was detected by observing the electro-optical switching. Figure 10 depicts optical textures of SmA*-L and SmC* phases of the mixture containing 10 mol % of (S)-1 on a glass plate with a cover glass. A typical fan texture in the planar alignment region and a dark texture in the homeotropic region were observed in the SmA*L phase. On cooling to the SmC* phase, birefringence in the homeotropic region increased, and helical pitch bands appeared in the fan-shaped texture. Figure 11 portrays the temperature dependence of a saturated tilt angle corresponding to the electrically induced ferroelectric state with an electric field of 8 V·μm−1. The tilt angle increases continuously up to 16° with decreasing temperature, exhibiting no discontinuous change at the SmA*-L−SmC* transition. Figure 12 shows the electrical response in the switching current in the SmA*-L and SmC* phases of the mixture containing 10 mol % of (S)-1. Double8682



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Figure 9. Binary phase diagram for mixtures of (R)-1 and its enantiomer (S)-1. Transition temperatures were observed on cooling. The SmA*-H to SmA*-L transition was detected using electro-optical measurements. Other transitions were observed using POM.

Figure 11. Temperature dependence of a saturated tilt angle corresponding to the electrically induced ferroelectric state with an electric field of 8 V·μm−1 of the mixture containing 10 mol % of (S)-1.

Figure 12. Electrical response in the switching current in the mixture containing 10 mol % of (S)-1 in (a) the SmA*-H phase at T − TIA* = −3 K, (b) the SmA*-L phase at T − TIA* = −5 K, (c) the SmA*-L phase at T − TIA* = −9 K, and (d) the SmC* phase at T − TIA* = −15 K on application of ±40 V at a frequency of 15 Hz.

Figure 10. Optical texture of the mixture containing 10 mol % of (S)-1 on a glass slide with a cover glass in (a) the SmA*-L phase at 128.5 °C and (b) the SmC* phase at 119.0 °C. Scale bar = 50 μm.

peak polarization was observed in the SmA*-L phase [Figure 12b]. Close to the low-temperature border of the phase, another sharp peak characteristic ferroelectric switching appeared [Figure 12c]. A single sharp peak was visible in the SmC* phase. The ferroelectric switching was also observed with an AC field at a frequency of 2 Hz, indicating that the SmC* phase is not a SmC*A phase. This is consistent with the results obtained from the miscibility study between (R)-1 and (R)-2. However, we cannot exclude a possibility that the relaxation into the anticlinic structure for this large trimer is too slow to be detected under the present conditions. In the present system, the complex switching behavior in the lowtemperature region of the SmA*-L phase is thought to result from coexistence of a metastable SmC* phase in the vicinity of the SmA*-L−SmC* transition. The Iso−SmA*-H phase transition is independent of the optical purity. The SmA*-L phase was visible in mixtures containing 0−25 mol % of (S)-1. The SmA*-H−SmA*-L transition temperature is almost

independent of the optical purity, indicating that helicity does not play an important role in the phase transition. The SmA*L−SmC* transition temperature increases with decreasing optical purity, and the SmA*-L disappears in the mixture containing 35 mol % of (S)-1. The chirality-dependent transition behavior of the SmA*-L phase is similar to that of a blue phase. The Iso Liq−BPIII transition temperature is independent of the optical purity, whereas the BPIII to N* transition temperature increases with decreasing optical purity.53 The BPIII−N* transition is described in terms of the double-twist to single-twist transition. The SmA*-L phase appears even in a mixture with 50% ee. A completely dark state in the homeotropic alignment region of the mixture was observed, and no helical pitch band in the planar region could be detected (see Supporting Information Figure S3). There is no significant difference in optical texture of the SmA*-L phase between mixtures with different enantiomer excess. The 8683



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homeotropic texture changed markedly at the SmA*-L−SmC* transition of the mixture containing 10 mol % of (S)-1. However, the electric-field-induced tilt angle increased continuously with decreasing temperature in the SmA*-L and SmC* phases. It showed no discontinuous increase at the transition. Furthermore, the maximum tilt angle in the SmA*-L phase was 15°. We investigated the temperature dependence of the layer spacing for this binary system. Figure 13 presents the results for

Figure 14. Comparison of the temperature dependence of layer spacing among (R)-1, the mixture containing (S)-1, and the racemic mixture.

Figure 13. Temperature dependence of layer spacing for the mixture containing 25 mol % of (S)-1 in the SmA*-H, SmA*-L, and SmC* phases.

the mixture containing 25 mol % of (S)-1. The layer spacing shows a discontinuous decrease at the SmA*-H−SmA*-L transition similar to that of (R)-1. Then, it decreases continuously with decreasing temperature. It shows no discontinuous change at the SmA*-L−SmC* transition. Another possibility is that the SmA*-L phase has a cybotactic SmC structure. We estimated the correlation length of the layer ordering along the layer normal in the SmA*-H, SmA*-L, and SmC* phases of the mixture containing 25 mol % of (S)-1. It increases continuously from 1500 Å in the SmA*-H phase to 1700 Å in the SmC* phase with decreasing temperature. Therefore, we can exclude a cybotactic SmC structure for the SmA*-L phase. Figure 14 shows a comparison of the temperature dependence of layer spacing among (R)-1, the mixture containing 25 mol % of (S)-1, and the racemic mixture. The layer spacings for (R)-1 are longer than those for the others in the SmA*-H phase, about 0.2 Å. We have no explanation for this difference. The layer spacing for the racemic mixture shows a discontinuous decrease at the SmA−SmC transition; then it decreases continuously with decreasing temperature. No significant difference exists for the temperature dependence of the layer spacing between SmA*-L−SmC* and SmA−SmC. Preliminary dielectric spectroscopy measurements on (R)-1 and the mixture containing 25 mol % of (S)-1 were carried out. Figure 15 shows temperature dependence of the dielectric strength for (R)-1 and the mixture. The maximum dielectric strength for (R)-1 is found in the SmA*-H near the SmA*-H− SmA*-L transition. It should be noted that this behavior is observed at a transition between a SmA phase and a very shortpitch SmC* phase. Then, it drops at the transition to the SmA*-L phase and increases continuously with decreasing temperature in the SmA*-L phase. Because of crystallization, no

Figure 15. Temperature dependence of dielectric strength for (a) (R)1 and (b) the mixture containing 25 mol % of (S)-1 in their liquidcrystalline temperature range.

dielectric spectrum was observed in the SmC* phase of (R)-1. The dielectric strength for the mixture also shows the maximum in the SmA*-H near the SmA*-H−SmA*-L transition. After it drops at the transition to the SmA*-L phase, it increases with decreasing temperature in the SmA*-L phase. It shows a discontinuous decrease at the SmA*-L− SmC* transition. The SmC* phase has a too weak dielectric 8684



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Molecular Organization Model. We present a possible explanation for the molecular organization in the SmA*-L phase. At present, we infer that molecules are tilted in the same direction in each layer and that the interlayer correlation of the azimuthal angle is disordered. The trimer (R)-1 forms a monolayer structure in which the molecules are tilted with respect to the layer normal. The trimer has both odd-numbered and even-numbered spacers. It has a hockey-stick-like shape if we assume the trimers with all-trans conformations of the spacers. The central biphenyl unit is tilted with respect to the molecular long axis, whereas the other terminal cores are inclined with respect to the other. The trimer is thought to have an inherent tilt within the molecule. The inherent tilt angle between the central biphenyl axis and the long axis is estimated as 28° using MOPAC (MOPAC-6/PM3). Figure 17 shows a

response to be SmC*. A SmC*−SmC*A transition is known to accompany such a drop in dielectric response. Therefore, the molecular organization in the SmC* is thought to be different from that in a conventional SmC* phase. Figure 16 shows the

Figure 17. Schematic model for molecular organization in the SmA*-L phase of (R)-1.

model for molecular packing of (R)-1 in a single layer and that for the phase structure. The strong dipole moment due to the two ester units exists along the long axis, and the dipole−dipole interaction can cause antiparallel alignment of adjacent molecules within a layer. The antiparallel alignment of the hockey-stick-like molecules might induce in-layer anticlinic orientation of the terminal cores between adjacent molecules. Recent theoretical results suggest that calamitic materials combining low orientational and high lamellar order are able to exhibit de Vries-like behavior.54,55 de Vries-like molecules usually have structurally incompatible termini. Although the trimer (R)-1 has no such incompatible terminus of the molecule, it is thought to combine the low orientational order due to the anticlinic terminal cores and high lamellar order because of the central dipole moment. From a C-13 NMR study of a smectic liquid crystal,56 the interlayer correlation in a SmC phase is explained as follows. The core parts are tilted in the same direction within each layer, producing the positional order of the tail around the layer normal. This positional order formed in each layer is thought to produce the long-range correlation of the direction of the core part over the layers. In the chiral system, the relative orientation of the chiral tail with respect to the core part depends on the absolute configuration of the chiral center and the steric hindrance. The interlayer permeation of the chiral tail might cause a twist interaction between the cores in adjacent layers. As a result of the twist interaction, the core parts form to the helicoidal structure in the SmC* phase. In this system, the central biphenyl cores are tilted in the same direction within each layer. However, the terminal cores have in-layer anticlinic

Figure 16. Temperature dependence of the relaxation frequency for (a) (R)-1 and (b) the mixture containing 25 mol % of (S)-1 in their liquid-crystalline temperature range.

temperature dependence of the relaxation frequency for (R)-1 and the mixture. The relaxation frequency for the mixture shows a discontinuous increase in the SmC* phase near the SmA*-L−SmC* transition of the mixture. These results indicate that the SmA*-L and SmC* phases have differing basic structures. This preliminary dielectric spectroscopy measurement suggests a possibility that (R)-1 has a phase sequence of SmA*−SmC*α−SmC*A. We cannot rule out that the SmA*-L phase is a SmC*α phase. Systematic investigation on dielectric response of this chiral trimer is in progress. Chiral effects on the phase transition behavior can lead to the following hypothesis. The molecules are tilted in the same direction within each layer in the SmA*-L phase as they are in the SmC phase of the racemic mixture. The azimuthal angle is disordered from layer to layer in the SmA*-L phase, whereas it forms a helical distribution on moving from layer to layer in the SmC* phase. Chirality can induce disordered interlayer correlation in the SmA*-L phase. Decreasing the optical purity decreases the disorder; the SmA*-L−SmC* transition temperature increases. Chirality generally does not induce disorder, but it induces twisting. We will discuss this point in the following section. 8685



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orientation which disturbs the effective interlayer twist interaction. Decreasing the optical purity stabilizes the SmC* phase. Analogy to chirality-dependent phase transition behavior between BPIII and SmA*-L suggests that the trimer can induce two helical interactions along different directions. Twist deformation of the director is forbidden in a smectic phase. However, the terminal cores are thought to have a low orientational order with a partially nematic-like organization in a layer. A lateral twist interaction between neighboring terminal cores can exist to some degree. Therefore, it is not unrealistic that the trimer induces two helical interactions: long-range interlayer twist interaction along the layer normal and shortrange in-layer twist interaction between neighboring terminal cores within a layer. Competition between those longitudinal interlayer and lateral inlayer interactions is thought to induce the disordered azimuthal angle from layer to layer in the SmA*L phase. Decreasing the optical purity, the unfavorable in-layer interaction is much more weakened than the interlayer twist interaction. Therefore, the SmA*-L−SmC* transition temperature increases. Coupling between chirality and in-layer anticlinicity might produce the frustrated polar smectic phase.

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ASSOCIATED CONTENT

REFERENCES

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We designed a chiral trimer consisting of three different mesogenic units connected via flexible spacers with different parity. The trimer was found to show an optically uniaxial smectic phase exhibiting double-peak polarization in the current response. Although we cannot rule out that the phase is a very short-pitch SmC* phase at present, the optically uniaxial structure is discussed in terms of a de Vries I type model in which the molecules are tilted in the same direction in each layer and the azimuthal angle is disordered from layer to layer. The double-peak polarization is explainable as a result of electric-field-induced transition of the azimuthal angle. Chirality and the hockey-stick-like oligomeric structure play an important role in randomization of the azimuthal angle, and they induce the optically uniaxial phase.

S Supporting Information *

Optical textures of (R)-1 between homogeneously aligned glass plates in the SmA* phase at 133.9 °C with ±8 V·μm−1 or without an electric field. Binary phase diagram for mixtures of (R)-1 and (R)-2. Optical textures of a mixture of (R)-1 (75 mol %) and (S)-1 (25 mol %) on a glass slide with a cover glass in (a) the SmA*-H phase at 134 °C, (b) the SmA*-L phase at 128 °C, and (c) the SmC* phase at 120 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81 172 393558. Tel.: +81 172 393558. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 23107503) on the Innovative Areas: “Fusion Materials” (Area no 22006) from MEXT. 8686



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Chiral Liquid Crystal Trimer Exhibiting an Optically Uniaxial Smectic

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