Dendron-mediated self-assembly, disassembly, and self-organization of complex systems have been investigated. The most ideal building blocks would need to display shape persistence in solution and in the solid state, since only this feature provides access to the use of complementary methods of structural analysis. Most supramolecular dendrimers are chiral even when they are constructed from nonchiral building blocks and are equipped with mechanisms that amplify chirality. This poses additional challenges associated with the understanding of the structural origin of chirality in different supramolecular structures through combinations of structural analysis methods. While many supramolecular structures assembled from dendrimers and dendrons resemble some of the related morphologies generated from block-copolymers, they are much more complex and are not determined by the volume ratio between the dissimilar parts of the molecule.

Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems

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

An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.

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Bent-core liquid crystals: polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Ionic Liquid Crystals: Versatile Materials.

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous electric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominated the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity).

Current topics in smectic liquid crystal research.

Even though the average molecular direction in smectic A phases is perpendicular to the smectic planes, most molecules in these phases have rather large tilt angles because the orientational order parameter S in all smectic A phases is significantly less than unity. This molecular tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains. From the differences between d and l , S values from 0.78 to 0.84 are obtained, which yield average molecular tilt angles from 20° to 17°. It is predicted that all smectic A phases with monomolecular layers will follow the same pattern. The concept of “inherent tilt” should apply to all orthogonal smectic phases, thermotropic as well as lytropic.

Why the Molecules are Tilted in all Smectic A Phases, and How the Layer Thickness Can be Used to Measure Orientational Disorder*

Structure and dynamics of pyramidic liquid crystals by deuterium NMR and x-ray diffraction

Deuterium NMR and X-ray diffractometry studies of the lyomesophases of the disodium/cromoglycate-water system are reported. The deuterium NMR studies were made on three specifically labelled compounds, viz. disodium cromoglycate-6,8,6′,8′-d4, disodium cromoglycate-3,3′-d2 and disodium cromoglycate-10-d1. The deuterium spectra of these species consist of doublets caused by the corresponding average quadrupolar interaction in the N and M phases; however, in phase III only the methine deuteron of disodium cromoglycate-10-d1 could be seen. The splittings of the chromone moiety deuterons, 3,6 and 8, are essentially identical over the whole temperature and concentration range of the N and M phases, ranging between 50 and 65 kHz. The methine deuteron splitting in the M phase is similar to that of the chromone deuterons, but is considerably higher (115 to 117 kHz) in phase III. The X-ray diffraction studies were made on a 20 wt% solution as a function of temperature between −15°C and 60°C. Two diffractio...

Structural and Orientational Characteristics of the Disodium/Cromoglycate-Water Mesophases by Deuterium NMR and X-Ray Diffraction

The preparation and mesomorphic properties of four members of the homologous series of dimers, alkandioic acid bis\[pentakis(n -heptanoyloxy)phenyl] esters \[(C6 H13 COO)5 C6 OC(O)(CH2)(n-2)/2]2 with n= 6, 8, 12 and 14, are reported. They are henceforth referred to as T n. Between -100 C and the isotropic liquid (I) the four compounds exhibit, with increasing temperature, the phase sequence Cr P I, where Cr is a solid and P is an intermediate phase which is crystalline for T6 and T8, and columnar discotic for T12 and T14. The mesophases of T12 and T14 are completely miscible with that of the corresponding monomer, hexaheptanoyloxybenzene (BHA7) and thus are identified as of type Col rd. This assignment is confirmed by X-ray measurements which also provide the lattice parameters for the two-dimensional columnar array. Deuterium and carbon-13 NMR measurements indicate that the side chains in the P(Cr) phases of T6 and T8 are dynamically disordered, while the benzene core is rigid. In the P(Col) mesophases o...

Mesomorphic and dynamic properties of discotic alkanoyloxybenzene dimers as studied by X-ray and NMR: The effect of spacer length

By X-ray diffraction, one can determine the average tilt angle, the average layer thickness, the average intermolecular distance, and the intensities of the inner and outer rings of the skewed cybotactic nematic phase, all independently of each other. We have investigated the behavior of these parameters as a function of temperature in the presence of a magnetic field for the C6, C8, C10 and C12 members of the bis-(4′-n-alkoxybenzal)-2-chloro-1,4-phenylenediamine series.

X-Ray Studies of Liquid Crystals VIII: A Study of the Temperature Dependence of the Directly Observed Parameters of the Skewed Cybotactic Nematic Phase of Some Bis-(4′-n-Alkoxybenzal) -2-Chloro-1,4-Phenylenediamines

N. Spielberg

Papers

Why the Molecules are Tilted in all Smectic A Phases, and How the Layer Thickness Can be Used to Measure Orientational Disorder*

Structure and dynamics of pyramidic liquid crystals by deuterium NMR and x-ray diffraction

Structural and Orientational Characteristics of the Disodium/Cromoglycate-Water Mesophases by Deuterium NMR and X-Ray Diffraction

Mesomorphic and dynamic properties of discotic alkanoyloxybenzene dimers as studied by X-ray and NMR: The effect of spacer length

X-Ray Studies of Liquid Crystals VIII: A Study of the Temperature Dependence of the Directly Observed Parameters of the Skewed Cybotactic Nematic Phase of Some Bis-(4′-n-Alkoxybenzal) -2-Chloro-1,4-Phenylenediamines