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 Vorlander, 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.

Discotic Liquid Crystals: From Tailor-Made Synthesis to Plastic Electronics

Chiral Salen Complexes: An Overview to Recoverable and Reusable Homogeneous and Heterogeneous Catalysts

Ligand environment and the structure of schiff base adducts and tetracoordinated metal-chelates

Transition metal liquid crystals: advanced materials within the reach of the coordination chemist

The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent–solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.

Amplification of chirality in liquid crystals

Liquid crystal properties of metal–salicylaldimine complexes.: Chemical modifications towards lower symmetry

A thermochromic derivative of salicylideneaniline, N,N′-bis(salicylidene)-p-phenylenediamine (BSP), has been prepared and subjected to structural and optical studies in the crystalline state. An X-ray crystallographic analysis has shown that the molecules are planar and are stacked in a parallel fashion to form one-dimensional columns. The interplanar spacing has been found to be quite short, suggesting the existence of an intermolecular charge-transfer interaction. The molecule contains fairly short O–H··· N hydrogen bonds, the strength of which is manifested in an OH stretching absorption band in the infrared region, showing considerable broadening and a low-frequency shift. The BSP crystals are remarkably thermochromic, and visible absorption and emission spectral changes with temperature have been interpreted in terms of an intramolecular proton transfer from the hydroxyl oxygen to imine nitrogen through the O–H··· N hydrogen bond. The emission spectra have also been examined under high pressures.

Structure and Optical Properties of a Thermochromic Schiff Base. Thermally Induced Intramolecular Proton Transfer in the N,N′-Bis(salicylidene)-p-phenylenediamine Crystals

The crystal structure of N,N′-disalicylidene-p-phenylenediamine (BSP) has been determined at 108 K. This compound shows a thermochromic-type behavior in the crystalline state; this chromism has been interpreted in terms of an intramolecular proton transfer from the hydroxyl oxygen to the imine nitrogen through the O–H···N hydrogen bond. In comparison with the room-temperature structure, no significant differences in the molecular structure and the molecular packing in the lattice have been observed. The room-temperature and low-termperature structures of another N,N′-salicylideneaniline derivative, N,N′-disalicylidene-1,6-pyrenediamine (DSPY), have also been determined. In contrast to BSP, the absence of a thermal proton-transfer process in DSPY is suggested by optical measurements. Comparative studies of these two compounds indicate that a conjugation system which is not included in a π-electron configurational change by the proton transfer must be coplanar with the hydrogen-bonded chelate ring for the t...

Structure and optical properties of a thermochromic schiff base. Low-temperature structural studies of the N,N'-disalicylidene-p-phenylenediamine and N,N'-disalicylidene-1,6-pyrenediamine crystals

Liquid crystalline copper(II) complexes of N-salicylideneaniline derivatives. Mesomorphic properties and a crystal structure

Two kinds of N-salicylideneaniline derivatives, N-tetrachlorosalicylideneaniline (CL4SA) and N-tetrachlorosalicylidene-1-pyrenylamine (CL4SPY), have been prepared and subjected to structural and optical studies in the crystalline state. A thermochromic-type behavior of the former crystal has been observed as a shift of the absorption edge, while that of the latter crystal is much less pronounced. Fairly short O–H···N hydrogen bonds have been found in both compounds through crystallographic studies, which may be mainly due to a steric effect of chlorine substituents. The difference in the proton transfer behavior of CL4SA and CL4SPY should presumably be caused by an intermolecular charge-transfer interaction in the latter crystal.

Structure and optical properties of thermochromic schiff bases. Charge transfer interaction and proton transfer in the N-tetrachlorosalicylideneaniline and N-tetrachlorosalicylidene-1-pyrenylamine crystals.

Naomi Hoshino

Papers

Liquid crystal properties of metal–salicylaldimine complexes.: Chemical modifications towards lower symmetry

Structure and Optical Properties of a Thermochromic Schiff Base. Thermally Induced Intramolecular Proton Transfer in the N,N′-Bis(salicylidene)-p-phenylenediamine Crystals

Structure and optical properties of a thermochromic schiff base. Low-temperature structural studies of the N,N'-disalicylidene-p-phenylenediamine and N,N'-disalicylidene-1,6-pyrenediamine crystals

Liquid crystalline copper(II) complexes of N-salicylideneaniline derivatives. Mesomorphic properties and a crystal structure

Structure and optical properties of thermochromic schiff bases. Charge transfer interaction and proton transfer in the N-tetrachlorosalicylideneaniline and N-tetrachlorosalicylidene-1-pyrenylamine crystals.