Ferrielectric Smectic C Phases Stabilized Using a Chiral Liquid Crystal

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Ferrielectric Smectic C Phases Stabilized Using a Chiral Liquid Crystal Oligomer Anna Noji,† Naoki Uehara,† 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, and Department of Physics, Graduate School of Science, Kyoto UniVersity, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8562, Japan ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: October 2, 2009

Achiralliquidcrystaloligomer,(R)-1-methylheptyl4′-{8-[4-(5-octylpyrimidin-2-yl)phenyloxy]octanoyloxy}biphenyl4-carboxylate, was prepared. Its physical properties were investigated using optical microscopy, differential scanning calorimetry, and X-ray diffraction. The oligomer was found to exhibit two ferrielectric smectic C phases with a wide temperature range between ferroelectric and antiferroelectric smectic C phases. Furthermore, ferrielectric-like ordering was observed in a racemic mixture of the enantiomers. The oligomeric effect can produce highly chirality-dependent ordering in the racemic system. Introduction Investigation of chirality is an exciting area of liquid crystal science. Appearance of ferroelectricity and antiferroelectricity in chiral tilted smectic phases is an interesting phenomenon. It is not only attractive for applications to fast-response displays; it also attracts fundamental interest related to synclinic or anticlinic ordering of the molecules.1-9 The frustration between synclinic-ferroelectricity and anticlinic-antiferroelectricity in chiral smectic C phases causes temperature-induced successive phase transitions, as characterized by a devil’s staircase10 and thresholdless V-shaped switching induced by an applied electric field.11,12 At the outset of disclosing an antiferroelectric SmC*A phase in MHPOBC, three other SmC*-like phases were observed.13 Chandani et al. designated these phases as SmC*, R SmC*, β and SmC*γ in order of decreasing temperature,14 identifying SmC*β as the ordinary ferroelectric SmC* phase. Gorecka et al. soon proved that SmC*γ is a ferrielectric phase.15 Isozaki et al. confirmed that an antiferroelectric subphase might emerge 10,16,17 Mach et al. reported the first between SmC*γ and SmC*. β direct structural observation of distinct multilayer periodicities of the subphases using resonant X-ray scattering.18,19 They confirmed the three- and four-layer periodicities in what they respectively called ferri 1 and ferri 2 phases. Nguyen et al. 20 The ferri 2 phase was found to identified the ferri 1 as SmC*. γ have antiferroelectric characteristic.21 Other ferrielectric subphases induced by successive phase transitions have been observed. Fukuda et al. proposed that subphases are represented as SmC*A (qT) where qT ) F/(A + F), where F is the number of synclinic layers in one periodicity. Furthermore, A is the number of anticlinic layers in one periodicity.10 The SmC*γ and the ferri 2 phases are, respectively, SmC*A(1/3) and SmC*A(1/2). Figure 1 presents schematic illustrations of antiferroelectric phase SmC*A(0), ferrielectric ferri 1 subphase SmC*A(1/3), antiferroelectric-like ferri 2 subphase SmC*A(1/2), and ferroelectric phase SmC*A(1). Some theoretical and experimental works have been undertaken to explain the appearance of the ferrielectric phases.12,22-27 Recently, Emely* To whom correspondence should be addressed. E-mail: ayoshiza@ cc.hirosaki-u.ac.jp. † Hirosaki University. ‡ Kyoto University.

Figure 1. Schematic illustrations of the periodic structures of antiferroelectric SmC*A(0) phase, ferrielectric SmC*A(1/3) phase, ferrielectric SmC*A(1/2) phase, and ferroelectric SmC*A(1) phase.

anenko and Osipov proposed that the effective coupling determined using a combination of spontaneous polarization, discrete flexoelectric effect, and an initial direct polarization coupling between adjacent layers stabilizes the ferrielectric phases.28 Chirality-dependent dipole-dipole interaction and electrostatic dipole-quadrupole interaction between positionally correlated molecules located in adjacent layers are thought to play an important role in stabilizing these phases. Widely various antiferroelectric and ferroelectric liquid crystals have been prepared. Ferrielectric phases have been observed at narrow temperatures of some chiral compounds possessing antiferroelectric or ferroelectric phases (or both). Decreasing the optical purity, a ferrielectric phase vanishes. Recently, Nishiyama et al. reported a chiral twin molecule with wide temperatures of a ferrielectric phase (Figure 2).29 In this case, the ferrielectric phase also disappears concomitantly with decreasing optical purity. Some mixtures of antiferroelectric chiral liquid crystals with highly chiral dopants of the same handedness were reported to exhibit ferrielectric phases with a range of 30 K.30 Synclinic or anticlinic ordering can exist for achiral forms. However, to our knowledge, no ferrielectric ordering has been reported for achiral systems. On the other hand, asymmetric switching in a ferrielectric liquid crystal has been of interest in prospective devices.31,32 The molecular design for ferrielectric liquid crystals is now an important issue not only because of their unusual phase structures but also because of their applications to optical devices. A liquid crystal oligomer is defined as a preorganized system and has several mesogenic units within a single molecule.33,34 Linear liquid-crystal dimers, in which two mesogenic units are

10.1021/jp907291b  2009 American Chemical Society Published on Web 11/23/2009

Ferrielectric Phases Stabilized Using LC Oligomer

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Figure 2. Chiral twin molecule with wide temperatures of a ferrielectric phase. The molecular structure was reported earlier by Nishiyama et al.29

Figure 3. Molecular structure of compound (R)-1.

connected via a flexible spacer, show pronounced odd-even effects of the spacer on the phase transition. The two mesogenic units are coparallel or inclined because of the parity of the all extended spacer. Therefore, preorganized synclinic or anticlinic ordering can be designed in a single dimeric molecule. For this study, we designed a novel asymmetric chiral dimer (R)-1, as shown in Figure 3. It was found to exhibit ferrielectric phases with a wide temperature range between antiferroelectric and ferroelectric SmC* phases. Furthermore, ferrielectric-like ordering was observed in the racemic mixture between (R)-1 and its enantiomer (S)-1. Experimental Section Preparation of Materials. Purification of the final product was conducted using column chromatography over silica gel (63-210 µm; Kanto Chemical Co. Inc.) using a dichloromethane/ ethyl acetate (20:1) mixture as the eluent, followed by recrystallization from ethanol. 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). (R)-1-Methylheptyl 4-Methoxycarbonyloxybiphenyl-4′carboxylate. Triphenylphosphine (1.23 g, 4.69 mmol) in THF (26 mL) was added to a solution of 4-methoxycarbonyloxybiphenyl-4′-carboxylic acid (1.16 g, 4.26 mmol), (S)-2-octanol (0.55 g, 4.26 mmol) and diethylazodicarboxylate (1.86 g, 4.28 mmol) in tetrahydrofuran (THF, 110 mL). The reaction mixture was stirred at room temperature for 3 h. After filtration of the precipitate, the solvent was removed by evaporation. The residue was purified using column chromatography on silica gel with dichloromethane. The desired product was obtained. Yield: 0.78 g (48%). (R)-1-Methylheptyl 4-Hydroxybiphenyl-4′-carboxylate. An aqueous ammonia solution (20%, 7 mL) was added to 35 mL of a solution comprising (R)-1-methylheptyl 4-methoxycarbonyloxybiphenyl-4′-carboxylate (0.69 g, 1.81 mmol) in ethanol. The reaction mixture was stirred at room temperature for 2 h. The solvent was removed by evaporation. Water was added to the residue, and the solution was extracted with diethylether. The combined organic layers were dried over anhydrous sodium sulfate. After the drying agent and the solvent were removed, the desired product was obtained without further purification. Yield: 0.48 g (82%). 8-[4-(5-Octylpyrimidin-2-yl)phenyloxy]octanoic Acid. Potassium carbonate (0.27 g, 1.95 mmol) was added to a solution of 5-octyl-2-(4-hydroxyphenyl)pyrimidine (0.37 g, 1.30 mmol)

and ethyl 8-bromooctanoate (0.33 g, 1.30 mmol) in cyclohexanone (11 mL). The reaction mixture was stirred at 120 °C for 6 h. After filtration of the participate, 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 8-[4-(5-octylpyrimidin-2-yl)phenyloxy]octanoate, was obtained. Yield: 0.51 g (86%). Ethyl 8-[4-(5-octylpyrimidin-2-yl)phenyloxy]octanoate (0.47 g, 1.00 mmol) was added to a solution of KOH (0.27 g, 4.80 mmol) in an ethanol/water (19:1) mixture. The resulting solution was stirred under reflux for 3 h. The solution was acidified using aqueous 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: 0.39 g (88%). (R)-1-Methylheptyl 4′-{8-[4-(5-Octylpyrimidin-2-yl)phenyloxy]octanoyloxy}biphenyl-4-carboxylate, (R)-1. (R)-1-Methylheptyl 4-hydroxybiphenyl-4′-carboxylate (0.14 g, 0.42 mmol), N,N′-dicyclohexylcarbodiimide (0.13 g, 0.63 mmol), and 4-(N,Ndimethylamino)pyridine (0.007 g, 0.06 mmol) were added to a solution of 8-[4-(5-octylpyrimidin-2-yl)phenyloxy]octanoic acid (0.18 g, 0.42 mmol) in dichloromethane (20 mL). The resulting solution was stirred at room temperature for 7 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. Recrystallization from ethanol gave the desired product. Yield: 0.26 g (83%). 1H NMR (500 MHz, CDCl3): δ ) 8.59 (s, 2H, Ar-H), 8.38 (d, 2H, Ar-H, J ) 8.6 Hz), 8.10 (d, 2H, Ar-H, J ) 8.6 Hz), 7.62 (d, 2H, Ar-H, J ) 8.6 Hz), 7.62 (d, 2H, Ar-H, J ) 8.6 Hz), 7.18 (d, 2H, Ar-H, J ) 8.6 Hz), 6.99 (d, 2H, Ar-H, J ) 8.6 Hz), 5.17 (sext, 1H, -OC*H(CH3), J ) 6.3 Hz), 4.05 (t, 2H, -OCH2, J ) 6.3 Hz), 2.60 (t, 2H, Ar-CH2-, J ) 7.5 Hz), 2.60 (t, 2H, -OCOCH2-, J ) 7.5 Hz), 1.87-1.72 (m, 5H, aliphatic-H), 1.67-1.28 (m, 27H, aliphatic-H), 1.35 (d, 3H, -OC*H(CH3), J ) 6.5 Hz), 0.88 (t, 6H, -CH3, J ) 6.9 Hz); IR (KBr): 2925, 2853, 1753, 1715, 1610, 1586 cm-1. Elemental analysis. Calculated for C47H62N2O5: C, 76.80; H, 8.50; N, 3.81. Found: C, 76.60; H, 8.93; N, 3.79. The enantiomer (S)-1 was obtained using a method similar to that used for (R)-1 from (R)-2-octanol. Characterization of LC Properties. The initial phase assignments and corresponding transition temperatures for the final product were determined using polarized optical microscopy (POM) with a polarizing microscope (Optiphot-pol; Nikon Corp.) equipped with a hot stage and FP80 control processor

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Figure 4. Optical textures of compound (R)-1 on a glass slide with a cover glass in (a) the ferro phase at 112.5 °C, (b) the ferri-H phase at 109.9 °C, (c) the ferri-L phase at 93.7 °C, and (d) the antiferro phase at 83.3 °C.

Figure 6. Applied voltage dependence of the apparent tilt angle in (a) the ferri-H phase and (b) the ferri-L phase of (R)-1. The cell gap was 5 µm.

Figure 5. A cooling DSC thermogram of (R)-1 at a scanning rate of 5 °C min-1.

(FP82; Mettler Instruments Corp.). The heating and cooling rates were 5 °C min-1. Photomicrographs were taken using a camera (Olympus Digital Camera C-5050 ZOOM; Olympus Optical Co. Ltd.) with an attached polarizing microscope (Optiphot-pol; Nikon Corp.). Temperatures and enthalpies of transition were investigated using differential scanning calorimetry (DSC) with a calorimeter (DSC6200; Seiko Instruments Inc.). The materials were studied at a scanning rate of 5 °C min-1 after being encapsulated in aluminum pans. The X-ray diffraction (XRD) patterns of the homeotropically aligned samples on 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 coated with surfactant for homeotropic alignment, which was placed in a custom-made temperature-stabilized holder (stability within (0.1 °C). The textural observations were performed by polarized light microscopy using a CCD camera. The X-ray apparatus was equipped with a cross-coupled Go¨bel mirror on a platform system with a two-dimensional positionsensitive proportional counter (PSPC) detector (HI-Star; Bruker AXS GmbH). 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 PSPC detector at a camera distance of 300 mm for a short counting time of 30 s. Electro-optical studies were conducted using commercially available evaluation cells (E. H. C. Co. Ltd., Japan). Spontaneous polarization and optical tilt across the temperature range of tilted smectic phases, as well as optical transmission as a function of applied field, were measured using standard electro-

Figure 7. Saturated tilt angle corresponding to the electrically induced ferroelectric state of (R)-1 with an electric field of 8 V µm-1 as a function of reduced temperature (the temperature below the isotropic transition). The cell gap was 5 µm.

optic techniques.35 The cells were made with 5 µm spacings. The inner surfaces had been coated with a polyimide aligning agent and had been unidirectionally buffed. The optical tilt angle was determined by finding the extinction direction when an electric field was applied to the specimen in increasing or decreasing steps. A Kikusui Electric Regulated DC Power Supply was used to supply the d.c. field. The spontaneous polarization was measured using a current-pulse technique. Results and Discussion Liquid-Crystalline Properties. The phase transition behavior of (R)-1 was investigated using POM and DSC. In fact, (R)-1 was found to exhibit a ferroelectric phase, two ferrielectric phases, and an antiferroelectric phase. The ferrielectric phases were clearly distinguished from the antiferroelectric or ferroelectric phase under a polarized microscope because the characteristic texture with constant motion as domains form, coalesce, and disappear was observed, as had been reported for monomeric36 and dimeric materials.29 Figure 4 depicts optical



Figure 8. Spontaneous polarization (Ps) for (R)-1 as a function of an applied voltage at a frequency of 100 Hz. The cell gap was 5 µm.

Figure 10. (a) Peak intensity of the sharp peak in the small-angle region and (b) layer spacing corresponding to the peak as a function of reduced temperature (the temperature below the isotropic transition).

Figure 11. The tilt angle as determined by POM or X-ray for (R)-1 in the ferro, ferri-H, ferri-L, and antiferro phases as a function of reduced temperature (the temperature below the isotropic transition).

Figure 9. X-ray diffraction intensity profiles in the small angle region of (R)-1 in (a) the ferro phase at 114 °C, (b) the ferri-H phase at 104 °C, (c) the ferri-L phase at 94 °C, and (d) the antiferro phase at 84 °C. The figures in the right-hand side column are photomicrographs of the homeotropically aligned sample at given temperatures taken by a CCD camera.

textures of those phases. The typical fan texture in planar alignment regions and dark texture attributable to the shortpitch helical structure in homeotropic regions were observed in the ferroelectric phase (Figure 4a). In the higher temperature ferrielectric phase (ferri-H), a Schlieren texture with two- and four-brush singularities appeared in the homeotropic regions. The two-brush singularities are not followed in the synclinic

phase but can be generated with anticlinic ordering. Therefore, the ferri-H phase is thought to be a ferri-2 phase possessing an anticlinic structure denoted as SmC*A(1/2). With decreasing temperature, a characteristic texture with vigorous constant movement was observed. Figure 4c presents a lower-temperature ferrielectric phase (ferri-L). The Schlieren texture disappeared, and a platelet-like texture with blue and green colors appeared in the homeotropic region. The constant motions in the ferri-L phase were also observed, but they were weaker than those in the ferri-H phase. Cooling to the antiferroelectric phase, the homeotropic alignment exhibited a dark texture, and the constant movement did not exist (Figure 4d). There is no evidence that (R)-1 exhibits the SmC*R phase. The SmC*R phase usually appears between SmA and ferreoelectric SmC* phases of a compound having intermediate phases. However, (R)-1 does not possess an SmA phase. Therefore, it is not unusual that (R)-1 does not exhibit the SmC*R phase. Figure 5 shows a cooling DSC thermogram of (R)-1. An unusually large enthalpy change was observed at the Iso-to-SmC* transition. Neither the ferro-to-ferri-H nor the ferri-H-to-ferri-L transition accompanied enthalpy change. The transition temperatures detected by DSC were slightly lower than those by

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Figure 14. Optical tilt angle as a function of the applied voltage of the mixture containing 45 mol % of (R)-1 at 96.2 °C in the ferri-L phase. The cell gap was 5 µm.

Figure 12. Optical textures in the contact region between (R)-1 and (S)-1 on a glass slide with a cover glass with decreasing temperature.

Figure 15. Schematic sketches for (a) parallel twin ordering and (b) bent twin ordering. The elliptical shapes represent mesogenic units within the molecules.

Figure 13. A binary phase diagram for mixtures of (S)-1 and (R)-1.

POM. The phase transition temperatures (°C) detected by POM and transition enthalpies (kJ mol-1) (in parentheses) for (R)-1 are as follows: Cry 83.8 (49.4), antiferro 85.2 (2.2), ferri-L 98.5 (-), ferri-H 111.8 (-), ferro114.2 (19.8) Iso. Electro-optical Studies. Ferrielectric properties were studied using measurements of the apparent switching angle as a function of the applied voltage. Figure 6 presents the applied voltage dependence of the apparent tilt angle in the ferri-H and ferri-L phases of (R)-1. In both ferri-H and ferri-L phases, when the applied voltage is increased, the apparent tilt angle increases and reaches a saturated value corresponding to the electrically induced ferroelectric ordering via characteristic multistep change.29,37 Figure 7 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 accuracy of the measurements was (0.5°. The tilt angle increases with decreasing temperature in the ferroelectric and ferri-H phases, although it decreases slightly with decreasing temperature in the ferri-L and antiferroelectric phases. Spontaneous polarization (Ps) for (R)-1 in the ferro, ferri-H, ferri-L, and antiferro phases was observed as a function of an applied voltage at a frequency of 100 Hz. The results are presented in Figure 8. The Ps values in the ferro and antiferro phases with a voltage of 10 V were, respectively, +23 and +5 nC cm-2. They increase almost monotonically with increasing

applied voltage. Although a marked difference is shown in Ps values between in the ferro and antiferro phases, electro-optical switching occurs via one process in each phase in the applied voltage region up to 50 V. The Ps values in the ferri-H and ferri-L phases increase with increasing applied voltage in the lower voltage region in the same trend as that in the antiferro phase. Above a certain voltage, they increase discontinuously with a large slope. Threshold voltages in the ferri-H and the ferri-L were 25 and 35 V, respectively. The threshold voltage in the ferri-L phase is higher than that in the ferri-H phase. The applied voltage dependencies of Ps in the ferrielectric phases suggest that electro-optical switching comprises two different processes in the applied voltage up to 50 V. X-ray Diffraction Studies. Figure 9 portrays X-ray diffraction intensity profiles in the small angle region of (R)-1 in the ferro, ferri-H, ferri-L, and antiferro phases. The phase transition of the sample was monitored by observing the texture using a CCD camera. A sharp peak at around 2θ ) 1.9° and the weak second-order peak at around 2θ ) 3.9° are apparent in each phase. Figure 10 presents temperature dependencies of (a) the peak intensity of the sharp peak and (b) layer spacing corresponding to the peak. The peak intensity is almost identical in the ferro, ferri-H, and ferri-L phases, although it increases at the ferri-L-to-antiferroelectric phase transition, indicating that the antiferroelectric phase has a stronger layer structure than the other phases. The correlation between layer spacing and the stability of the antiferroelectric phase has also been reported.38,39 The layer spacing decreases continuously in the ferro, ferri-H, and ferri-L phases, although it increases at the ferri-L-toantiferroelectric phase transition. The extended molecular length was obtained from MOPAC (MOPAC-6/PM3) as about 50 Å.



Figure 16. Possible models for molecular organization in the ferroelectric, ferri-H, ferri-L, and antiferroelectric phases of (R)-1. The Ps direction is indicated by arrows into or out of the page, and the tile direction is indicated by solid lines. Open circles represent synclinic ordering in adjacent layers; crosses represent anticlinic ordering. The elliptical shapes represent mesogenic units within the molecules.

Therefore, the smectic phases of (R)-1 have a monolayer structure. Figure 11 portrays the temperature dependence of the tilt angle determined through POM and X-ray measurements. The difference between them reveals the difference in a tilt angle between the molecular long axis and the mesogenic parts. The optical tilt angle is attributed to the tilt of the mesogenic parts; therefore, the tilt of the mesogenic parts is larger than that of the long axis. Temperature dependence of the layer spacing can result from coupling between change in the molecular length caused by the tilt of the mesogenic parts and interlayer permeation of the terminal chain. Chiral Effects. It is generally accepted that the appearance of ferrielectric phases is highly dependent on the optical purity of the system.26,27 Actually, ferrielectric phases are seen only in high enantiomer excess areas. We investigated the effect of optical purity on the stability of the ferrielectric phases of (R)1. A contact study between compound (R)-1 and its S-isomer was conducted. Figure 12 shows optical textures in the contact region with decreasing temperature. The ferroelectric SmC* phase of both enantiomers proved to be miscible, and the smectic C phase was induced in the central region at 112.4 °C. With decreasing temperature, the ferri-H phase with vigorous movement appeared and spread to the central region (Figure 12a). Then, the ferri-H phase disappeared and the SmC*-like texture with mild movement appeared, indicating that the ferri-H phase changes to the ferri-L phase over a wide range of optical purity (Figure 12b). The constant movement was also observed in the central region, suggesting that ferrielectric-like ordering can exist in a nonchiral system. Figure 13 depicts a binary phase diagram for mixtures of (S)-1 and (R)-1. Stabilities of both the ferroelectric and antiferroelectric phases were found to be independent of the optical purity. The ferro-to-ferri-H transition

temperature decreases with decreasing optical purity, whereas the ferri-H-to-ferri-L transition temperature increases slightly and then decreases concomitantly with decreasing optical purity. It is noteworthy that the ferri-L phase does not disappear with decreasing optical purity close to 0% ee. Figure 14 presents the optical tilt angle of the mixture containing 45 mol % of (R)-1 as a function of the applied voltage at 96.2 °C. A multistep tilt angle change was observed. Constant movement was also seen in mixtures containing 0-50 mol % of (R)-1 in the ferri-L phase. The results indicate that a ferrielectric phase appears in a system with a low degree of chirality, and a ferrielectric-like ordering exists in the racemic mixture. Molecular Organization Model. We discuss an origin for stabilizing the ferrielectric phases of the chiral oligomeric system. The typical phase sequence of a compound having the intermediate phases is ferro-SmC*A(1/2)-SmC*A(1/3)-antiferro, as shown in Figure 1. We assume that ferri-H and ferri-L are SmC*A(1/2) and SmC*A(1/3), respectively. Nishiyama et al. reported that symmetrically substituted chiral twin compounds exhibit a wide-temperature-range ferrielectric phase.29 Therefore, the chiral interaction plays an important role in stabilizing the ferrielectric phases. We apply twin ordering organized by chiral recognition to our model. We assume that the spacers of the oligomer adopt all-trans conformation and that the two mesogenic units are coparallel. Two molecular arrangements for the twin ordering exist, as shown in Figure 15. One is a parallel structure in which two oligomer units are coparallel (Figure 15a); the other is a bent structure in which they are inclined with respect to each other (Figure 15b). Figure 16 shows models for molecular organization in the ferroelectric, ferri-H, ferri-L, and antiferroelectric phases of (R)-1. A circle or cross is inserted at each interlayer region. Circles represent synclinic ordering in

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adjacent layers, and crosses represent anticlinic ordering. In the ferroelectric phase, the tilt direction in each layer is the same between adjacent layers, and polarization has the same direction. The ferroelectric phase consists of parallel twin orderings. In the ferri-H, the two successive layers of the four periodic layers have the same tilt and polarization directions, whereas the other two layers have alternative ones. This is the case for SmC*A(1/ 2). The sequence of bent-parallel-bent-parallel-bent for the twin ordering can be seen at five nodes in the periodic four layers. In the Ferr-L, two layers of the periodic three layers have the same tilt and polarization directions, whereas the other layer has alternative ones. This is the case for SmC*A(1/3). The sequence of parallel-bent-bent-parallel can be seen at four nodes in the periodic three layers. In the antiferroelectric phase, tilt and polarization directions alternate between adjacent layers. The antiferroelectric phase consists of bent twin orderings. The present models can explain the applied voltage dependence of Ps values in each phase. The ferro phase has all circle nodes, but the antiferro phase has all cross nodes. These patterns are consistent with the fact that a marked difference in Ps values exists between the ferro and antiferro phases, whereas electrooptical switching occurs via one process in each phase. The ferri-H and ferri-L phases have both circle and cross nodes, which might produce electro-optical switching via two different processes. Cross and circle nodes appear as alternatives in the ferri-H phase, although two successive cross nodes exist in the ferri-L phase. This might be a reason for the higher threshold voltage in the ferri-L than in the ferri-H phase. We subsequently discussed chiral effects on ferrielectric phases’ stability. According to Emelyanenko and Osipov, we assume that coupling between spontaneous polarization and discrete flexoelectric effect stabilizes the ferrielectric phases. The ferri-H phase is destabilized by decreasing optical purity, suggesting that chirality-dependent direct polarization coupling between adjacent layers stabilizes the ferri-H phase. On the other hand, the stability of the ferri-L phase might be governed by electrostatic dipole-quadrupole interaction between positionally correlated molecules located in adjacent layers. The flexoelectric interaction depends on the shape and polar unit of the molecule. For that reason, the ferri-L phase can exist in mixtures with low optical purity. Conclusions A novel chiral liquid crystal oligomer, (R)-1, was prepared and found to exhibit two ferrielectric phases with a wide temperature range in addition to ferroelectric and antiferroelectric phases. The lower-temperature ferrielectric phase appears even in the forms with a low optical purity. Moreover, the ferrielectric-like ordering exhibits in a racemic mixture of (R)-1 and the enantiomer. The stability of the ferrielectric phases is explained in terms of the preorganized oligomeric effect. Our findings yield fundamental insights into clinicity in smectic liquid crystals in addition to a new approach to molecular design for the threshold-less V-shaped switching materials. Acknowledgment. We thank Dr. Isa Nishiyama for valuable discussions. This work was supported in part by a Grant for Hirosaki University Institutional Research.

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JP907291B

Ferrielectric Smectic C Phases Stabilized Using a Chiral Liquid Crystal

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