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.

The intensities of X-ray scattering from three compounds has been measured photographically at a number of temperatures. The substances studied were 4,4′di-n-octyloxyazoxybenzene (OOAB) and its di-n-heptyl homologue (HOAB) in their nematic and smectic C phases and ethyl-4-(4′acetoxybenzylidene)aminocinnamate (EABAC) in its nematic and smectic A phases. In all cases except the smectic C phase of HOAB the distribution function f d(β) describing the orientation of the local director for clusters of ≳ 10 molecules relative to the average director has been determined. The form of f d(β) is close to that predicted by simple mean field theory although the magnitude of the order parameters ⟨P 2⟩ in general differ significantly. from the simple mean field values. The results suggest that f d(β) is close to the true singlet distribution so that the dominant angular fluctuations are co-operative and involve several molecules. For the smectic phases properties of the density wave f(z) which is the smectic layer distr...

Distribution functions in three liquid crystals from X-ray diffraction measurements

http://feigensonlab.mbg.cornell.edu/publications/Mills2008orderparams.pdf

Order parameters and areas in fluid-phase oriented lipid membranes using wide angle X-ray scattering.

Liquid crystalline epoxy based thermosetting polymers

A previous paper demonstrated that the orientational disorder present in all smectic A phases causes most molecules to make fairly large angles with the normal to the smectic plane. Thus, the molecular directions form a diffuse cone around the layer normal. It is now argued that even the preferred directions of the molecules in a smectic A phase will make a nonzero angle with the layer normal, and from the data examined, this angle, ϑm, appears to be about 10–15 ° for the compound TCOOB (trans‐1,4‐cyclohexane‐di‐n‐octyloxybenzoate). The smectic A‐to‐C phase transition is found to have two aspects in the diffuse‐cone model: the distribution of the molecular directions around the layer normal loses its rotational symmetry at the phase transition temperature TA–C, and the angle ϑm starts increasing sharply at a temperature Tt. In most cases these two temperatures are probably equal. The increase of ϑm with decreasing temperature in the smectic C phase of TCOOB can be described by a (Tt−T)0.35 dependency superimposed on a much smaller linear temperature dependency already present in the smectic A phase.

The description of the smectic A and C phases and the smectic A–C phase transition of TCOOB with a diffuse‐cone model

A general formula is derived for the x‐ray diffraction from an assembly of elongated molecules, and it is shown that this formula is too complex to be solved. To enable the calculation of the molecular cylindrical distribution function (mcdf) and the order parameter from the x‐ray data, the approach taken in the literature has been to assume that only molecules approximately perpendicular to the scattering vector contribute significantly to the scattered radiation. Calculations on a model indicate, however, that this ``literature approximation'' is totally unjustified in the derivation of the mcdf, and, consequently, that the mcdf's obtained from x‐ray data must be regarded as of no value. For the order parameter, the model calculations show that the errors made in using the ``literature approximation'' are not as large as in the case of the mcdf, and it appears that under favorable conditions the x‐ray data may yield approximate values for the order parameter.

On the Calculation of the Molecular Cylindrical Distribution Function and the Order Parameter from X‐Ray Diffraction Data of Liquid Crystals

On the basis of evidence in the literature, it is concluded that the basic molecular packing underlying all smectic structures is a herringbone arrangement of the molecular planes with an orthogonal lattice of the molecular centers. Starting from this basic packing, two sets of five orthorhombic and three monoclinic smectic layer structures are derived which can all be classified with crystallographic space‐group symbols. One set is for centrosymmetric ’’molecules,’’ the other for noncentrosymmetric ’’molecules.’’ A second classification of the same set of structures is developed on the basis of the symmetry of the molecular positions. This classification is equally specific but more easily visualized, and it leads to simpler classification symbols. With an additional symbol to indicate order or disorder in the stacking of the smectic layers, a set of 14 possible smectic structures is obtained (each structure allowing centrosymmetric as well as noncentrosymmetric ’’molecules’’), and all known layered smec...

Two classification systems for smectic phases, based on symmetry and order

On the basis of theoretical considerations and experimental evidence it is proposed that in the smectic H phase of TBBA all molecular planes are essentially parallel to the direction of tilt. It is shown that this model does not fit the data on the smectic H phase of BBEA, and so for this phase the previously suggested herringbone packing is retained. A theoretical model is developed for the forces governing the packing in these smectic H phases. This model combines the dipole model of McMillan with interactions between neighboring phenylene groups. The model predicts a parallel arrangement for TBBA and a herringbone arrangement for BBEA, in agreement with the conclusions reached in the first part of the paper.

On the structure of the smectic H phase

The formula for the intensity of the radiation scattered by a liquid with complex molecules is discussed. It is shown that the choice of the molecular center is not arbitrary but determined by the condition that the distance between two molecular centers should have no influence upon the orientation of the molecules concerned. Also it is pointed out that Fourier inversion of the intensity formula is not always possible.

Adriaan de Vries

Papers

The description of the smectic A and C phases and the smectic A–C phase transition of TCOOB with a diffuse‐cone model

On the Calculation of the Molecular Cylindrical Distribution Function and the Order Parameter from X‐Ray Diffraction Data of Liquid Crystals

Two classification systems for smectic phases, based on symmetry and order

On the structure of the smectic H phase

Scattering of X Rays by Liquids with Complex Molecules. The Choice of the Molecular Center