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.

The self-assembly of amphiphilic molecules in aqueous solution into lyotropic liquid crystals (LCs), characterised by soft yet long-range ordered nanoscale structures, constitutes a fascinating phenomenon at the heart of soft matter science which can be employed in a manifold creative ways. Particularly interesting structures may arise as a result of functionalization of the LC with appropriate guest molecules, adopting the order of their host. Here we combine cat-and anionic surfactants to form a liquid crystalline colloidal suspension of carbon nanotubes (CNTs), which by virtue of the spontaneously formed hexagonal columnar LC structure are uniaxially aligned over macroscopic areas. The nanotube concentration can be so high, with sufficiently uniform alignment, that the mixture becomes a fluid linear polariser, the anisotropic optical properties of CNTs having been transferred to macroscopic scale by the LC.Moreover, thin and highly aligned filaments can be drawn and deposited in selected directions on arbitrary surfaces, after which the LC template can be rinsed away.Combined with recently developed methods for CNT fractionation according to chirality, the technique would yield an unprecedented degree of control in the practical realization of carbon nanotube-based devices and materials.2

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.

Rather uncommon but pronounced odd-even effects in the clearing temperatures of liquid crystals are found for three new homologous series of discotic tetraphenylenes [figure: see text]. Detailed similarities to the well-known odd-even effects in calmitic mesogens point towards a more general understanding of odd-even effects in liquid crystals.The phase transition temperatures in many homologous series of liquid crystals show odd-even effects similar to the well-known odd-even effects in non-mesogenic organic compounds like carboxylic acids. For discotic liquid crystals only a few series with a sufficient number of consecutive homologues are available and only a small part of these series show odd-even effects at all. We now synthesized and investigated three series of discotic tetraphenylenes each ranging from the n=7 to the n=16 homologues. For these compounds an anomalous odd-even effect is found: The ascending and descending transition temperatures with increasing number of methylene groups in the side chains exhibit an inversion of this alternation which is always found between the n=12 and n=14 homologues. An overview of odd-even effects in liquid crystals is presented and we discuss the nature and the origin of odd-even effect in discotic mesogens.