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.

A new single‐molecule switching concept relying on the E‐field‐dependent orientation of a push‐pull system (via its dipole moment) is presented. As first steps towards realizing the E‐field‐triggered single‐molecule motion, the synthesis of turnstile 1, which is designed to bridge a gap within a carbon nanotube junction, is reported. Turnstile 1 consists of a hexakis(m‐phenyleneethynylene) macrocycle decorated with phenanthrene‐functionalized oligo(phenyleneethynylene) (OPE) wires at opposite ends, as well as a para‐connected push‐pull rod as rotator unit. The similarity of the rotor dimension with the surrounding macrocycle guarantees efficient π‐stacking between both subunits when an E‐field is applied. To introduce a dipole moment into the rotator, the structure is terminally functionalized with a nitrile group on one side and with a dimethylamino group on the opposite end. Synthetic protocols based on Sonogashira–Hagihara couplings were developed to build up the macrocycle scaffold. The highly functionalized open precursor A is the key building block of the sequence as it allows a twofold intramolecular palladium‐catalyzed cyclization reaction to be performed to obtain the target turnstile in a very good yield of 68 %. The target structure was fully characterized by NMR spectroscopy and mass spectrometry. Furthermore, 1H‐1H NOESY NMR experiments pointed to a pseudo‐rotation of the push‐pull rod within the turnstile structure on the time scale of the NMR experiment.

SummaryBased on 5-(4-hydroxyphenyl)-2-octylpyrimidine 8, 5-phenylpyrimidine derivatives 3–7, 9 with different spacer chain lengths (C2 up to C6) and different terminal polar groups (Br, Cl, N3, OH, CN) were synthesized by etherification and nucleophilic substitution. The mesomorphic behaviour of these compounds was investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS and SAXS) and revealed smectic A mesophases for bromides, chlorides and azides 3, 4 and 6. For these compounds a maximum phase width was observed for the C5 spacer regardless of the terminal group, whereas the hydroxy- and cyano-substituted derivatives 5 and 7, respectively, were non mesomorphic and showed only melting transitions.

A convergent synthesis of a series of 4,4¢-functionalized 1,1¢-biisoquinolines via 1-chloro-4-hydroxyisoquinoline and substituted biphenyl-and phenylpyrimidine ethers as building blocks is described. The latter were prepared by Williamson etherification of the respective 4-hydroxybiphenyl and -phenylpyrimidine precursors with dibromoalkanes, allowing variation of the spacer lengths. 1-Chloro-4-hydroxyisoquinoline was obtained from N-phthalimidoglycine ethyl ester through a Gabriel-Colman reaction as a key step. Linkage of the building blocks by etherification in the presence of potassium carbonate gave the isoquinolines, which were submitted to a nickel(II) chloride mediated homocoupling to yield the ligand systems.