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

Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

https://iopscience.iop.org/article/10.1143/JJAP.45.597/pdf

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

In this critical review we present examples of coordination complexes with efficient chiral transfer determining stereochemistry at the metal centre and throughout the overall molecular assembly. The general features controlling the transmission of chirality are presented. The transfer of chirality are considered here with the special purpose of obtaining a molecular material displaying a particular property or function. Coordination complexes in fields as diverse as chiral luminescent materials, homochiral MOFs, chiral liquid crystals, enantioselective sensors, chiroptical switches, and magnetochiral compounds are presented (162 references).

Chiral transfer in coordination complexes: towards molecular materials

Recent advances in the field of organic molecular electronics include an increasingly significant role of discotic structures based on all-benzenoid polycyclic aromatic hydrocarbons (PBAHs), reflected by the growing research activities in this field. Progress has profited largely from an iterative approach based on cooperation between synthetic organic chemists and physicists, both in academia and industry. The current paper deals with this class of compounds, describing the preparation of large, yet well-defined, nanoscale PBAH moieties, which then self-assemble to highly ordered, supramolecular arrays with advantageous electronic properties. Progressive miniaturisation leads from thin films as one-dimensional charge-transport layers, to single columns as potential nanowires or data storage elements, to single molecules and nanoscale (opto)electronically active components. The use of ‘large’
				(diameter ≥1 nm) disks ensures a columnar arrangement, which can be further modified by peripheral substitution to encode bulk 3-dimensional packing, orientation at interfaces, thermal properties, and processability. Adaptation of processing techniques from solution, melt or the vapour phase plays a crucial role for the (supra)molecular arrangement, which can be controlled from the macroscopic down to the nanometre scale. Finally, the characteristics of each new material can be evaluated in terms of supramolecular order, electronic device performance, single molecule properties, etc. This paper gives a brief overview of synthetic methods and molecular design, followed by evaluation of their columnar structures over various length scales down to nanoscale, processing techniques, and device performance/electrical characterisation.

From graphite molecules to columnar superstructures - an exercise in nanoscience

The spontaneous self-organization of small molecular entities to form soft materials with well-defi ned supramolecular assemblies is currently a topic of great interest in areas that range from chemistry and biology to materials science. [ 1 ] Over the past decade, non-covalent forces such as hydrogen-bonds, metal-ion-to-ligand coordination, electrostatic, π – π stacking, dipole-dipole, or hydrophilic-hydrophobic interactions have been identifi ed as enabling the construction of a broad range of complex superstructures from specifi cally engineered small molecular building blocks. [ 2 ] Incorporation of molecules into larger entities by non-covalent forces has enormous potential for materials science due to the possibility of bridging the gap between the molecular and the macroscopic scale in terms of structural order, when precise control of such a self-assembly process is achieved. [ 3 ]

Tuning the Columnar Organization of Discotic Polycyclic Aromatic Hydrocarbons

Chiral oxovanadium(IV), copper(II), and palladium(II) ‚-diketonates show a room-temperature columnar mesophase which undergoes ferroelectric switching. All the compounds were obtained as liquid crystals at room temperature, and crystallization or melting processes were not detected by differential scanning calorimetry carried out to -20 °C. The mesophase was investigated by optical microscopy, DSC and X-ray diffraction, and identified as a rectangular columnar ( P21). The flowerlike texture observed for all the compounds led us to deduce a high tilt angle (ca. 40°) of the molecules with respect to the column axis. Circular dichroism has confirmed the existence of a helical arrangement within the column. This result is in accordance with the so-called columnar mode found at low frequencies (ca. 10 -3 Hz) in dielectric spectroscopy studies. The electrooptical response of these materials has been examined by means of a photomultiplier. The results obtained can be explained by considering a strong influence of the high tilt angle found in the mesophase.

/pdf/switchable-columnar-metallomesogens-new-helical-self-4n934z3yfl.pdf

R. Iglesias

Papers

Switchable Columnar Metallomesogens. New Helical Self-Assembling Systems