Transition temperatures ( C) and Δ
Appearance of ferroelectricity and antiferroelectricity in chiral tilted smectic phases is an interesting phenomenon. It is not only attractive for use in applications to fast-response displays (Goodby et al., 1991; Walba, 1995); it also attracts fundamental interest related to synclinic or anticlinic ordering of the molecules (Lagerwall & Giesselmann, 2006; Lemieux, 2007; Nishiyama, 2010). The frustration between synclinic-ferroelectricity and anticlinic-antiferroelectricity in chiral smectic C phases causes temperature-induced successive phase transitions (Fukuda et al., 1994; Inui et al. 1996; Isozaki et al., 1993; Matsumoto et al., 1999; Osipov & Fukuda, 2000; Sandhya et al., 2009; Takezoe et al., 2010). When ferroelectric and antiferroelectric phases have equal free energy, intermediate ferrielectric sub-phases with a degenerated energy level can appear between ferroelectric and antiferroelectric phases.
At the outset of disclosing antiferroelectric SmC*
Some theoretical and experimental studies have been undertaken to explain the appearance of ferrielectric phases (Cepic & Zeks, 2001; Cepic et al., 2002; Fukuda et al., 1994; Johnson et al., 2000; Matsumoto et al., 1999; Osipov & Fukuda, 2000; Yamashita & Miyajima, 1993). Chirality is probably prerequisite for the appearance of the ferrielectric phases. Emelyanenko and Osipov proposed that effective coupling that is determined using a combination of spontaneous polarization, discrete flexoelectric effect, and an initial direct polarization coupling between adjacent layers stabilizes the ferrielectric phases (Emelyanenko & Osipov, 2003). The Emelyanenko-Osipov model predicts only mesophases with periodicity of 8, 5, 7, and 9 layers between SmC* and SmC*
With respect to liquid-crystalline materials, ferrielectric phases have been observed for narrow temperature ranges of some highly chiral compounds. By decreasing the optical purity, a ferrielectric phase vanishes (Fukui et al., 1989; Gorecka et al., 2002). Nishiyama et al. reported a chiral twin molecule with wide temperatures of a ferrielectric phase (Nishiyama et al., 2001). 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 (Jaradat at al., 2006). Asymmetric switching in a ferrielectric phase has been of interest in prospective devices (Jaradat et al., 2008, 2009). The molecular design for ferrielectric liquid crystals now constitutes an important issue not only because of their unusual phase structures but also because of their applications to optical devices.
Recently, we designed an asymmetric chiral dimer (
We prepared a homologous series of liquid crystal oligomers and observed their physical properties. This report describes structure-property relations of liquid crystals and presents an origin for stabilizing the ferrielectric phases of the present chiral oligomeric system.
For use in this study, 5-alkyl-2-(4-hydroxyphenyl)pyrimidine, 5-octyloxy-2-(4-hydroxyphenyl)pyridine, and 4-(4-hexyloxyphenyl)-1-(4-hydroxyphenyl)-2,3-difluorobenzene were purchased from Midori Kagaku Co. Ltd. The final compounds were prepared using a similar method to that used for (
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.57 (s, 2H, Ar-H), 8.35 (d, 2H, Ar-H,
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.57 (s, 2H, Ar-H), 8.35 (d, 2H, Ar-H,
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.57 (s, 2H, Ar-H), 8.34 (d, 2H, Ar-H,
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.57 (s, 2H, Ar-H), 8.36 (d, 2H, Ar-H,
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.33 (d, 1H, Ar-H,
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.09 (d, 2H, Ar-H,
1H NMR (500 MHz, solvent CDCl3, standard TMS) δH/ppm: 8.57 (s, 2H, Ar-H), 8.35 (d, 2H, Ar-H,
2.2. Physical properties
The initial phase assignments and corresponding transition temperatures for the final products were determined using polarized optical microscopy (POM) with a polarizing microscope (Optiphot-pol; Nikon Corp.) equipped with a hot stage (FP82; Mettler Inst. Corp.) and a control processor (FP80; Mettler Inst. Corp.). The heating and cooling rates were 5 C min-1. Temperatures and enthalpies of transition were investigated using differential scanning calorimetry (DSC, DSC6200; Seiko Instruments Inc.). The materials were studied at a scanning rate of 5 C min-1 after encapsulation in aluminium pans. The X-ray diffraction (XRD) patterns of the powder samples on cooling processes were obtained using a real-time X-ray diffractometer (D8 Discover; Bruker AXS GmbH). A sample was put on a convex lens, which was placed in a custom-made temperature stabilized holder (stability within ±0.1 C). The textural observations were conducted using polarized light microscopy with a CCD camera. The X-ray apparatus was equipped with a cross-coupled Göbel mirror on a platform system with a two-dimensional position-sensitive proportional counter (PSPC) detector (HI-Star; Bruker AXS GmbH). X-rays were generated at 40 kV and 40 mA; a parallel Cu Kα X-ray beam was used to irradiate the sample.
Electro-optical studies were conducted using commercially available evaluation cells (E. H. C. Co., Ltd., Japan). The inner surfaces had been coated with a polyimide aligning agent and had been buffed unidirectionally. The cells were made with 5 µm spacings. Switching current and optical tilt angle across the temperatures of tilted smectic phases were measured using standard electro-optic techniques (Goodby et al., 1991). 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.
3. Results and discussion
We prepared a homologous series of the chiral dimesogenic compound and investigated the effects of terminal chain, central spacer, core structure, and chiral moiety of the chiral dimesogenic compound on appearance of the ferrielectric phase.
3.1. Effects of the central spacer
We investigated effects of parity of the central spacer of the compounds on the phase transition behaviour. Transition temperatures and associated entropy changes, Δ
|6||• 71.6||[• 50.8
|8||• 78.5||[• 69.3
Figure 4 depicts optical textures of SmC*α, Ferro, Ferri-L, Anti, SmI*, and SmX phases of (
Figure 5 portrays a cooling thermogram of (
Odd–even effects were observed not only for the phase sequence but also for the Iso-Ferro (or SmC*α) phase transition temperature. The transition temperatures of the even-numbered series are higher than those of the odd-numbered series. However, it is noteworthy that such an odd–even effect was not observed for the associated entropy changes. Typical liquid crystal dimers show marked odd–even effects not only on the transition temperature but also on the associated entropy changes. The effects on the entropy changes are interpreted as follows (Imrie & Luckhurst, 1998). In the isotropic phase, approximately half the conformers of an even-membered dimer are essentially linear; for an odd–membered dimer, only 10% are linear. A synergy exists between conformation and orientational order. Therefore, many of the bent conformers are converted to a linear form at the transition to the nematic phase for even-membered dimers, which enhances the orientational order of the nematic phase, engendering a larger nematic–isotropic entropy than would be expected for a monomer. For odd-membered dimers, however, the difference in free energy between the bent and linear conformers is such that the orientational order of the nematic phase is insufficient to convert bent into linear conformers. Consequently, the orientational order is not enhanced and a smaller nematic-isotropic entropy is expected. In the present system, compound (
Ferrielectric properties in the Ferri-L phase of compound (
Layer spacings in the Ferro and Ferri-L phases of compound (
Figure 7 shows the binary phase diagram between compounds (
To summarize the effects of the central spacer on the appearance of the ferrielectric phases of the chiral dimesogenic compound, the compounds possessing an even-numbered spacer show both Ferri-H and Ferri-L phases with a wide temperature range, although the compounds possessing an odd-numbered spacer show only a Ferri-L phase. Furthermore, no significant difference was found in the electro-optical properties in the Ferri-L phase between even– and odd-membered series. Both even– and odd-membered compounds have a monolayer structure in the smectic phases. Unusual entropy change observed at the Iso-Ferro or Iso-SmC*α of the odd-membered compounds indicates that they exist as a linear conformer in the Ferro and Ferri-L phases.
3.2. Effects of the terminal chain
We prepared compound (
Recent reports describe that the octyloxy derivative (
The wide temperature ranges of the ferrielectric phases enable observation of the switching current in the Ferri-H phase [Fig. 10(a)] and that in the Ferri-L phase [Fig. 10(b)].
Two asymmetric peaks in the Ferri-H phase suggest that switching between two ferroelectric states occurs via one intermediate state. Figure 11 shows a schematic model for the switching behaviour assuming that the intermediate state has four-layer periodicity.
With respect to the Ferri-L phase, its three asymmetric peaks suggest that switching between two ferroelectric states occurs via two intermediate states. Figure 12 shows a model for the switching behaviour assuming that the intermediate state has three-layer periodicity.
Asymmetric switching was observed in both the Ferri-H and Ferri-L phases. We have no explanation for the asymmetric switching. It is possible that the Ferri-H and Ferri-L phases have a more complex periodicity.
A ferrielectric phase normally arises at the lower temperature end of the ferroelectric smectic C phase. A direct transition is unusual. The introduction of the alkoxy tail to the chiral oligomeric system produces marked stability of the ferrielectric phases.
3.3. Effects of the mesogenic core structure
We introduced a phenylpyridine core or a 2,3-difluro-1,4-diphenylbenzene core into the chiral oligomeric system instead of a phenylpyrimidine core and investigated the liquid-crystalline properties. Figure 13 shows the molecular structure and phase transition properties of compound (
Figure 14 depicts optical textures of Ferro, Ferri-H, Ferri-L, and Anti phases of (
Figure 15 shows molecular structure and phase transition properties of compound (
A phenylpyridine core having larger tilting ability than a phenylpyrimidine core enhances the stability of the Ferro and Anti phases; however, it does not affect that of the ferrielectric phases. Intermolecular tilting interactions between adjacent mesogenic units make no large contribution to the stability of the ferrielectric phases. A 2,3-difluro-1,4-diphenylbenzene core making the mesogenic units to align along the director induces a SmA phase but eliminates both of the Ferri-H and Ferri-L phases. A phase sequence of SmA-Ferro-Ferri is often observed for some monomeric chiral compounds. However, the ferrielectric phases of the present chiral oligomeric system are thought unlikely to coexist with a SmA phase.
3.4. Effects of chirality
The appearance of ferrielectric phases is known to be highly dependent on the optical purity of the system. Actually, ferrielectric phases are seen only in high enantiomer excess areas. We reported that ferrielectric-like ordering was observed in a racemic mixture of (
We prepared compound (
3.5. Origin for stabilizing the ferrielectric phases
Electrical response studies of compound (
3.5.1. Preorganized effects
According to the XRD measurements, both the Ferri-H and Ferri-L phases have a monolayer structure. Comparison of tilt angles determined by POM with those determined by XRD of (
3.5.2. Interlayer interactions
It is generally accepted that the appearance of a ferrielectric phase is explained in terms of macroscopic chirality, i.e. helicity or spontaneous polarization. In this system, the Ferri-H phase is destabilized by decreasing optical purity. However, the macroscopic chirality does not affect the stability of the Ferri-L phase. Another model is necessary to explain the stabilization of the ferrielectric phase. We infer interlayer interactions among the preorganized molecules in adjacent layers via chiral recognition to discuss the stability of the Ferri-L phase. Interlayer interactions are known to govern tilt– and helical-correlation between molecules in adjacent layers (Yoshizawa et al., 1995; Yoshizawa & Nishiyama, 1995). Two molecular arrangements for the chiral interaction exist as presented in Fig. 20. One is a parallel twin ordering in which two oligomer units are coparallel, thereby inducing synclinic ordering.
The other is a bent twin ordering in which they are inclined with respect to each other, inducing anticlinic ordering. The alkoxy tail substituted to the pyrimidine ring stabilizes the Ferri-L phase much more than the Ferri-H phase. Introduction of the alkoxy tail is thought to enhance the electrostatic dipole-quadrupole interaction and to stabilize the Ferri-L phase. The chiral dimeric system can induce favourable positional correlation among molecules in adjacent layers.
3.5.3. Molecular organization model
According to the theoretical study (Emelyanenko & Osipov 2003), ferrielectric phases are stabilized by two factors:
chirality-dependent direct polarization coupling between adjacent layers; and
electrostatic dipole-quadrupole interaction between positionally correlated molecules in adjacent layers.
Furthermore, our experimentally obtained results indicate that interlayer interaction via chiral recognition between preorganized molecules in adjacent layers is important for stabilization of the ferrielectric phases of the present dimesogenic system.
Figure 21 shows models for molecular organization in the Ferro, Ferri-H, Ferri-L, and Anti phases of the chiral system. A circle or cross is inserted at each interlayer region.
Circles represent synclinic ordering in adjacent layers and crosses represent anticlinic ordering. In the Ferro phase, the tilt direction in each layer is the same between adjacent layers and polarization has the same direction. The Ferro phase consists of parallel twin orderings. In the Ferri-H phase, the two successive layers of the four periodic layers have the same tilt and polarization directions, whereas the other two layers have alternative ones. Such is the case for SmC*
With respect to the dimesogenic compounds with an odd-numbered spacer, many of bent conformers are converted to a linear form at the transition to the liquid-crystalline phase. This conversion accompanies a
We prepared a homologous series of chiral dimesogenic compounds and investigated their structure-property relations. The dimesogenic compounds possessing an even-numbered spacer show the Ferri-H and Ferri-L phases, whereas those possessing an odd-numbered spacer show the Ferri-L phase. The Ferri-H and Ferri-L phases were found to have a four-layer periodicity as SmC*
We thank Professors J. Yamamoto and Y. Takanishi for X-ray measurements. We also thank Dr. I. Nishiyama for fruitful discussion. This work was partially supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (No. 22350078) and a Grant for Hirosaki University Institutional Research.
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