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Liquid Crystal (LC) Materials

 Liquid crystals have properties between those of conventional liquid and those of solid crystal. For instance, a liquid crystal shows fluidity like a liquid, but it also demonstrates optical anisotropy like a crystal. Liquid crystal molecules are directionally oriented, but positionally not oriented. Small molecule-based and polymer-based liquid crystals are known. The structure of a liquid crystal involves rigid π-electron systems bearing flexible long alkyl chains. Many liquid crystal molecules are calamitic shaped with a group for polarization, but planar molecules are also known. We can control the temperature which shows a liquid crystal phase by modifying the length of alkyl chain. A practical liquid crystal has a mesophase around room temperature. In addition to an application for a liquid crystal display, liquid crystal materials are expected to be organic semiconductors. A semiconductor having a liquid crystal phase, the so-called liquid crystal semiconductor, spontaneously undergoes a molecular orientation and self-assembly.1,2)
 The various liquid-crystal phases can be characterized by the type of ordering. Among them, there are mainly nematic, smectic, cholesteric, and discotic phases. We can introduce chirality into a liquid crystal molecule giving chiral nematic and chiral smectic phases.
(1) Nematic phase
 Calamitic shaped molecules are oriented one-dimensionally. The individual molecule can be relatively movable along the long axis direction. This phase belongs to the most flexible liquid crystal with large fluidity and small viscosity. Calamitic shaped cyanobiphenyls with large dielectric anisotropy (Δε) enable control of the molecular orientation by applying an electrical field. A liquid crystal display of a twisted nematic (TN) system3) is fabricated from a nematic liquid crystal.

(2) Smectic phase
 There is a two-dimensional layered structure caused by more positional limitations compared with that of a nematic phase. A smectic phase is harder than a nematic phase, because the movable range of the unit molecules is relatively narrow. A nematic phase sometimes changes to a smectic phase by decreasing the temperature. Diversity of the layered structures demonstrates many kinds of smectic phases.

(3) Cholesteric phase (Chiral nematic phase)
 This phase is usually observed from cholesterol derivatives. The unit molecules are oriented one-dimensionally similar to a normal nematic phase, but the molecular orientation shows a twisted helical arrangement between layers. This is due to an asymmetric carbon (chiral center) in the cholesterol molecule. Accordingly, a cholesteric phase is called a chiral nematic phase. This chiral phase exhibits optical rotation, selective optical scattering, circular polarization, and dichroism. Recently, a research development on a ‘blue phase’ received much attention.4) This phase is observed between temperatures of the cholesteric phase and an isotropic liquid. One difficulty is that we can find the blue phase in a narrow temperature range of 1-2 degrees. However, one can widen the temperature range more than several dozens of degrees, when a polymer slightly forms in the blue phase (polymer-stabilized blue phase).5)

(4) Discotic phase
 Formation of a discotic phase requires a discotic aromatic molecule such as phthalocyanine,6) triphenylene,7) hexabenzocoronene8) and so on, although nematic and smectic phases require calamitic molecules. A discotic molecule usually forms a one-dimensional columnar structure (columnar phase) by stacking the molecules. A research area on organic electronics focuses on the discotic phase, because electrical conduction may occur along the molecular stacking direction. On the other hand, a rare example was reported that a chemical modification of a discotic molecule provided a three-dimensionally stacked cubic phase,9) whereas discotic molecules normally stack one-dimensionally.

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