Novel Liquid-Crystal Phases Formed with Introduction of Chirality
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Novel Liquid-Crystal Phases Formed with Introduction of Chirality Smectic liquid-crystalline systems are molecular structures whose constituent molecules are arranged in a series of layers with the molecules’ axes perpendicular to the plane of the layers. The introduction of chiral, or “handed,” compounds into such a system can lead to a wide variety of helical, polar, and frustrated macrostructures. I. Nishiyama of the Yokoyama Nanostructured Liquid-Crystal Project at JST in Japan, J.W. Goodby of the University of Hull, and their colleagues have recently demonstrated how the introduction of chirality into smectic liquid crystals can lead to novel liquid-crystal phases. As reported in the August 24 issue of Chemistry of Materials (p. 3212), Nishiyama, Goodby, and JST colleagues J. Yamamoto and H. Yokoyama have investigated the molecular assembly of mixtures of the R and S chiral enantiomorphs (handedness) of bis{4’-(1-methylheptyloxycarbonyl)biphenyl-4-yl}alkanedioates. Five homologues of this chiral molecule can be distinguished: SSn (n = 2–5) and RRn (n = 4), each of which demonstrates a different phase-transition behavior. All forms of this enantiomorphic compound are present as an isotropic liquid at temperatures above ~105ºC and as a smectic liquid crystal at sufficiently low temperatures. Beyond that, interesting new phase transformations were observed, depending on the chiral composition of the materials. The even-membered chiral molecules (SS2, SS4, and RR4) were seen to transition into a mesophase, denoted as M2, that exhibited a striped “parquet-like” texture. In addition, SS4 and RR4 formed another mesophase (M 1) between the isotropic liquid phase and M2. This M1 phase is characterized by its optical isotropy, regardless of orientation. Peaks observed during differential scanning calorimetry (DSC) experiments verify that M1 and M2 are thermodynamically real phases rather than simply transient or metastable molecular organizations. Furthermore, these phases represent “highly chiral” versions of the conventional antiferroelectric phase observed at intermediate temperatures. The SS4–RR4 phase diagram (Figure 1) summarizes the compositional and temperature features of this enantiomorphic liquid-crystal system. X-ray diffraction (XRD) analysis of the M1 and M2 phases showed only broad scattering features, indicating a liquidlike short-range order in both. The pattern obtained from M1, the optically isotropic phase, is a ring of scattered intensity, indicating the possible formation of a layered structure. In contrast, discrete difMRS BULLETIN/OCTOBER 2004
Figure 1. Phase diagram between chiral molecule SS4 and the racemic mixture (SS4:RR4 = 1:1). Reprinted in part with permission from Chem. Mater. 16 (17) (August 24, 2004) p. 3213. ©2004 American Chemical Society.
fraction spots are observed for the M2 phase, indicating that the assembled molecules with “parquet-like” texture have some three-dimensional (3D) crystalline order as well. These results were further investigated by measuring the frequency depen
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