MATERIALS CHEMISTRY COMMUNICATIONS Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules T. Niori,"T. Sekine,'J. Watanabe,*.a T. Furukawab and H. Takezoeb 'Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan bDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan The synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer. Because of this characteristic packing, spontaneous polarization appears parallel to the layer and switches on reversal of an applied electric field. This is the first obvious example of ferroelectricity in an achiral smectic phase and is ascribed to the CZVsymmetry of the molecular packing.Ferroelectric liquid crystals are of considerable theoretical and technological interest' and it has been recognized that a tilted smectic phase made up of chiral molecules can be ferroelectric. Chirality of the molecules and their tilted association into a smectic layer, reducing the overall symmetry of the liquid crystals, are essential for conventional ferroelectric liquid Since the essential requirement is the introduction of polar symmetry in the system, ferroelectric liquid crystals need not necessarily be chiral. For this reason, a great deal of attention has been directed, theoretically and experimentally, towards non-chiral ferroelectric system^.^-'^ Tournilhac et u1.' reported the first experimental example to our knowledge.They have synthesized polyphilic molecules comprising three or more chemically different subunits, for example, a fluoroalkyl group on one edge and a fluoromethyl group on another edge, and found for the first time that these achiral molecules can form ferroelectric smectic liquid crystals as a result of the segregation of their units into homogeneous microdomains. Watanabe et uL9 reported that a ferroelectric smectic phase with CZV symmetry7 may be formed from some main-chain types of liquid crystal polymers, if two different aliphatic spacers with odd numbers of carbons are incorporated into the backbone in a regularly alternating fashion and they segregate into different microdomains.Further, we have also found that a novel nematic liquid crystal with polar ordering is formed from a rod-like aromatic polyester with polarity along its chain axis". The polarity in this nematic phase appears along the nematic director as determined from the measurement of the second harmonic generation;"," the strong dipole-dipole interaction between the polar molecules is considered to be the origin of the polarity.6 In this report, we describe a distinct ferroelectric smectic liquid crystal which is formed from a banana shaped achiral molecule. The origin of the ferroelectricity is ascribed to the polar CZv symmetry which is obtained from the efficient packing of the banana-shaped molecules into a smectic layer.Matsunaga and co-~orkers~~~~~have synthesized some banana-shaped materials and found that they can form smectic liquid crystals. However, no detailed analyses of their structure and properties have been reported. These materials, which form a smectic phase, are particularly interesting since, because of their characteristic shape, they form a peculiar smectic phase in which the banana-shaped molecules are closely packed and are all aligned in the direction of bending [see Fig 1 (a)].This type of structure is not unknown in the smectic field since it has already been encountered in main-chain polymers and twin dimers.l4.'' For a quantitative description of this structure, we need to consider the space group of the layered structure.As illustrated in Fig. l(b), each layer has biaxiality, that is in- layer anisotropy exists and the refractive indices are different in the bent direction (y axis) and in the direction normal to the y axis direction (x axis). The space group is analogous to the crystallographic CZV1gro~p.~,~,'~There is a two-fold axis along the y axis and there are mirror planes perpendicular to the x and z axes. Since there is no mirror plane perpendicular to the two-fold axis, spontaneous polarization can be expected to arise along the y axis (bent direction). btYEl*si Fig. 1 (a) Possible smectic structure which may be formed by the banana shaped molecules and (b)its space-group symmetry J. Muter.Chem., 1996, 6(7), 1231-1233 1231 In order to obtain this type of ferroelectric smectic liquid crystals, we prepared compound 1. 1 The banana shape of the molecule comes from linking two benzylideneaniline groups to 1,3-dihydroxybenzene by an ester linkage. The phases observed on heating this material are crystal, S2, S1 and isotropic, as shown in eqn. (1). 91 7 "C 156 4 "C crystal S, --12 2 "C 139 9 "C (4.90 kcal mol -') (2.86 kcal mol -') 161 4 "C S1 isotropic (1) 158 1"C (5.30 kcal mol-l) The smectic phase that draws immediate attention is the S1 phase. Its X-ray diffraction p?ttern shows an inner sharp reflection with a spacing of 37.4 A and an outer broad reflection with a spacing of ca. 4.6A. The spacing between layers is approximately the length of the molecule in the conformation illustrated above, which is a strong indication that the mol- ecules in each layer are in the bent form.The outer broad reflection indicates the disordered lateral packing of the mol- ecules within each layer as seen in conventional SA and Sc phases.17 The viscosity of the S1 phase, similar to the SA and Sc phases, is low and a fan-like texture is observed on cooling from the isotropic melt. The most notable observation from microscopy is that the homeotropic texture, which can be seen by shearing a thin specimen between glass plates, shows distinct birefringence (see Fig. 2). This means that a c-director exists in the layers, and hence that the bent molecules are packed with the molecules aligned in the direction of bending and parallel to the layers.Furthermore, it is a requirement that the long-range orientation correlation of the c-director is main- tained from one layer to another. The X-ray and optical microscopic observations, thus, lead to the layer structure proposed in Fig. 1. In ferroelectric materials, the macroscopic polarization should change its sign on reversal of the applied electric field. To confirm this, the switching current was examined in a pseudo-planar cell, by the triangular wave method,'* where Fig. 2 Optical microscopic texture observed in the homeotropically ahgned S1phase which was prepared by sheanng between glasses. The dark areas are due to air bubbles. 1232 J.Muter. Chem., 1996, 6(7), 1231-1233 55s 9 Q)c 0g -00) Q) -0 -5 2 Pn ([I -10 0.0 0.5 1.o 1.5 tls Fig. 3 Switching current curve obtained by applying a triangular voltage wave (k8.9 V pm-', 1 Hz) at 150 "C.The spontaneous polanz- ation was estimated to be about 50 nC cm-'. the layer normal lies parallel to the glass surface but is randomly oriented between domains. Fig. 3 shows the switch- ing current curve obtained by applying a triangular voltage wave (k8.9 V pm-l, 1 Hz). When the polarity of the electric field changes, a switching current peak is clearly observed. The spontaneous polarization was estimated to be ca. 50 nC cm-2. It should be further noted that the two states under positive and negative electric fields cannot be distinguished by a polarizing optical microscope.Thus, we can conclude that the S, phase of this material is ferroelectric with the tip of the bent molecule orienting to the electric field and reversing its orien- tation on reversal of the polarity of the field. Relative permittivity measurements can also support the existence of spontaneous polarization. Fig. 4 shows the tem- perature dependence of the relative permittivity measured at 1 kHz. In the S1 phase, the relative permittivity is much larger than that in the other phases, namely, crystal, S2 and isotropic. The large relative permittivity is attributed to the so-called Goldstone mode, in which the spontaneous polarization har- moniously responds to the field.From the dispersion of the relative permittivity, its relaxation frequency was determined to be below 100 Hz, though it could not be determined exactly. In conclusion, this system consisting of banana-shaped mol- ecules forms a biaxial S1phase as shown by X-ray and optical microscopic observations. The polarization and relative permit- tivity measurements indicate that the s, phase is ferroelectric. A spontaneous polarization exists parallel to the layers and switches on field reversal. This is the first obvious example of ferroelectricity exhibited by an achiral smectic phase, which is ascribed to the CZvsymmetry of the packing of the banana- shaped molecules into a layer. 10 10 8 8 2 2 n.. 0 40 60 80 100 120 140 160 TPC Fig. 4 The temperature dependence of the real and imagmary parts of the relative permittivity measured at 1 kHz on cooling from the isotropic melt References 1 J.W. Goodby, Ferroelectric Liquid Crystals, Gordon and Breach, Philadelphia, 1991 R. B. Mayer, Mol. Cryst. Liq. Cryst., 1977,40, 33. R. G. Petschek and K. M. Wiefling, Phys. Rev. Lett., 1988,59,343. R. H. Tredgold, J. Phys. D: Appl. Phys., 1990,23, 119. F. Biscarini, C. Zannoni, C. Chiccoli and P. Pasini, Mol. Phys., 1991, 73,439. 6 J. Lee and S.-D. Lee, Mol. Cryst. Liq. Cryst., 1994,254, 395. 7 P. E. Cladis and H. R. Brand, Liq. Cryst., 1993, 14, 1327. 8 F.Tournilhac, L. M. Blinov, J. Simon and S. V. Yablonsky, Nature, 1992,359,621. 9 J. Watanabe, Y. Nakata and K. Shimizu, J.Phys. 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