Most associate liquid crystals with their everyday use in laptop computers, mobile phones, digital cameras, and other electronic devices. However, in contrast to their rodlike (calamitic) counterparts, first described in 1907 by Vorländer, disklike (discotic, columnar) liquid crystals, which were discovered in 1977 by Chandrasekhar et al., offer further applications as a result of their orientation in the columnar mesophase, making them ideal candidates for molecular wires in various optical and electronic devices such as photocopiers, laser printers, photovoltaic cells, light-emitting diodes, field-effect transistors, and holographic data storage. Beginning with an overview of the various mesophases and characterization methods, this Review will focus on the major classes of columnar mesogens rather than presenting a library of columnar liquid crystals. Emphasis will be given to efficient synthetic procedures, and relevant mesomorphic and physical properties. Finally, some applications and perspectives in materials science and molecular electronics will be discussed.

Die meisten Menschen verbinden mit dem Begriff Flüssigkristalle sofort die Displays von Laptop‐Computern, Mobiltelefonen, Digitalkameras und anderen elektronischen Geräten. Im Unterschied zu ihren stäbchenförmigen (calamitischen) Verwandten, die erstmals 1907 von Vorländer beschrieben wurden, bieten aber die 1977 von Chandrasekhar entdeckten scheibenförmigen (diskotischen) Flüssigkristalle andere vielversprechende Verwendungsmöglichkeiten. Ihre einzigartige Anordnung in columnaren Mesophasen macht sie zu idealen Kandidaten für molekulare Drähte in vielen optischen und elektronischen Geräten, beispielsweise in Photokopierern, Laserdruckern, Solarzellen, organischen Leuchtdioden und Feldeffekttransistoren oder zur holographischen Datenspeicherung. Einer Übersicht über die verschiedenen Mesophasentypen und Charakterisierungsmethoden columnarer Flüssigkristalle folgen ausgewählte Beispiele aus den Hauptklassen columnarer Mesogene unter besonderer Berücksichtigung effizienter Syntheseverfahren, mesomorpher Eigenschaften und der für Anwendungen relevanten physikalischen Eigenschaften der Flüssigkristalle. Abschließend werden Anwendungsbeispiele und Perspektiven für einen Einsatz in den Materialwissenschaften und der molekularen Elektronik vorgestellt.

Unsymmetrical benzo[15]crown-5 ethers 5 with one lateral ortho-terphenyl unit bearing alkoxy side chains of varying chain lengths (C5-C14) were prepared from 3,4-dialkoxyphenylbromides 2. Complexation with metal salts MX (M = Na, Cs) afforded the corresponding derivatives MX5. The uncomplexed crown ethers 5 h and 5 i, with dodecyloxy and tetradecyloxy side chains, respectively, exhibit liquid crystalline properties. In the series of complexed crown ethers, liquid crystal properties appeared as early as NaI5 f with C9H19 side chains. Whereas the uncomplexed 5 h,i form smectic mesophases, the complexed NaI5 g and NaI5 h exhibit textures typical of columnar mesophases. These results were supported by X-ray diffraction measurements (WAXS, SAXS), which revealed smectic (5 h,i), rectangular columnar (NaI5 g), and hexagonal columnar (NaI5 h) mesophases. As the liquid crystalline phase might retain packing features of the solid-state structure, single-crystal X-ray analyses were also performed for some of the uncomplexed and complexed crown ether derivatives. The complex NaI(3)5 a displays a sandwich-type structure, with the crown ether cores mutually antiperiplanar and maintaining an almost perfect crown conformation. In contrast, non-mesogenic uncomplexed crown ether 5 b displays a layer-type ordering in the solid phase.

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Novel guanidinium chlorides 6a-f which are tethered to a pentaalkyloxytriphenylene unit have been prepared by a five-step sequence from hexaalkyloxytriphenylenes 1a-f. The mesomorphic properties of 6a-f were studied by DSC, POM, and X-ray diffraction. Columnar rectangular mesophases were found for derivatives 6b-f with long alkyl chains. Detailed SAXS experiments revealed a change of the common Col r (C2/m) phase to Col r (P2m) phase with increasing chain length. Derivative 6a with pentyl side chain displayed only unidentified crystalline or plastic crystalline properties.