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.

Revolutionary developments in the fabrication of nanosized particles have created enormous expectations in the last few years for the use of such materials in areas such as medical diagnostics and drug-delivery, and in high-tech devices. By its very nature, nanotechnology is of immense academic and industrial interest as it involves the creation and exploitation of materials with structural features in between those of atoms and bulk materials, with at least one dimension limited to between 1 and 100 nm. Most importantly, the properties of materials with nanometric dimensions are, in most instances, significantly different from those of atoms or bulk materials. Research efforts geared towards new synthetic procedures for shape and size-uniform nanoscale building blocks as well as efficient self-assembly protocols for manipulation of these building blocks into functional materials has created enormous excitement in the field of liquid crystal research. Liquid crystals (LCs) by their very nature are suitable candidates for matrix-guided synthesis and self-assembly of nanoscale materials, since the liquid crystalline state combines order and mobility at the molecular (nanoscale) level. Based on selected relevant examples, this review attempts to give a short overview of current research efforts in LC-nanoscience. The areas addressed in this review include the synthesis of nanomaterials using LCs as templates, the design of LC nanomaterials, self-assembly of nanomaterials using LC phases, defect formation in LC-nanoparticle suspensions, and potential applications. Despite the seeming diversity of these research topics, this review will make an effort to establish logical links between these different research areas.

Nanoparticles in Liquid Crystals: Synthesis, Self-Assembly, Defect Formation and Potential Applications

In this article, we probe the formation of liquid crystal and soft crystal phases as a consequence of minimising the free volume of the system either through design engineering of molecular shape o...

What makes a liquid crystal? The effect of free volume on soft matter

The diversity of phase morphologies observed recently in star-branched liquid-crystalline and polymeric compounds containing at least three immiscible segments is reviewed. Bolaamphiphiles and facial amphiphiles with rodlike aromatic cores, two end-groups, and one (T-shape) or two (X-shape) chains attached laterally to the core, form numerous honeycomblike liquid-crystal phases, as well as a variety of novel lamellar and 3D-ordered mesophases. Molecular self-organization is described in bulk phases and in thin films on solid and liquid surfaces, as well as in Langmuir–Blodgett films. The remarkably reversible formation of mono- and trilayer films is highlighted. In the bulk, T-shaped “rod–coil” molecules without appended end-groups form predominantly lamellar phases if the core is a straight rod, but the bent-core variety forms hexagonal honeycombs. Furthermore, self-assembly of “Janus”-type molecules, is discussed. Also covered is the diversity of morphologies observed in miktoarm star terpolymers, i.e., polymers with three different and incompatible arms of well defined lengths. Similarities and differences are highlighted between the liquid-crystal morphologies on the 3–15 nm scale and the polymer morphologies on the scale of 10–100 nm. A separate section is dedicated to computer simulations of such systems, particularly those using dissipative particle and molecular dynamics. Of special interest are the recently synthesised X-shaped tetraphilic molecules, where two different and incompatible side-chains are attached at opposite sides of the rodlike core. The tendency for their phase separation produces liquid-crystal honeycombs with cells of different compositions that can be represented as a plane paved with different colored tiles. The independent variation of chain length and “color” creates the potential for developing a considerable range of complex new 2D and 3D soft nanostructures. Analogous X-shaped rod–coil compounds with unequal side groups are also of considerable interest, forming tubular lyotropic structures capable of confining strings of guest molecules.

/pdf/self-assembly-at-different-length-scales-polyphilic-star-7398bbivcx.pdf

Self-Assembly at Different Length Scales: Polyphilic Star-Branched Liquid Crystals and Miktoarm Star Copolymers

According to a new design strategy for “de Vries-like” liquid crystal materials, which are characterized by a maximum layer contraction of ≤1% upon transition from the SmA phase to the SmC phase, we report the synthesis and characterization of two homologous series of organosiloxane mesogens. The design of these new materials is based on a frustration between one structural element that promotes the formation of a SmC phase (a trisiloxane-terminated side-chain) and one that promotes the formation of a SmA phase (either a chloro-terminated side-chain or a 5-phenylpyrimidine core). Measurements of smectic layer spacing d as a function of temperature by small-angle X-ray scattering (SAXS) combined with optical tilt angle measurements revealed that the mesogens 5-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)-undecyloxy)phenyl)-2-(1-alkyloxy)pyrimidine (3(n)) undergo SmA−SmC phase transitions with maximum layer contractions ranging from 0.5% to 1.4%. A comparison of reduction factors R and f suggests that thi...

Design of Liquid Crystals with “de Vries-like” Properties: Frustration between SmA- and SmC-Promoting Elements

According to a new design strategy for “de Vries-like” liquid crystal materials, we report the synthesis and characterization of an organosiloxane mesogen with a 5-phenylpyrimidine core that forms SmA and SmC liquid crystal phases. This new material is characterized by a maximum layer contraction of 1.2% upon SmA−SmC phase transition and is comparable to the best “de Vries-like” materials reported heretofore.

Design of liquid crystals with "de Vries-like" properties: organosiloxane mesogen with a 5-phenylpyrimidine core.

The atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4′-heptyl[1,1′-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76), and electroclinic coefficients ec were measured as a function of the dopant mole fraction x1 in the chiral SmA* phase at T
				−
				TC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ec values reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1 in 2-PhP is amplified by achiral 2-phenylpyrimidine additives (5 mol%) that are longer than 2-PhP; in the best case, ec is amplified by a factor of 3.2 with 5-(tetradecyloxy)-2-(4-(tetradecyloxy)phenyl)pyrimidine (3g), which is almost twice as long as 2-PhP. However, no amplification is observed in a 4 mol% mixture of (R)-1 in NCB76 using the same series of additives. A correlation between ec values and the temperature range of the SmA* phase suggests that the amplification of ec with increasing length of the additive 3 in the (R)-1/2-PhP mixture is due primarily to a decrease in the tilt susceptibility coefficient α as the second-order SmA*–SmC* phase transition moves away from the tricritical point. Measurements of smectic layer spacing as a function of T
				−
				TC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with T
				−
				TC for the pure host 2-PhP fits to a square-root law, which indicates that the second-order SmA–C transition is nearly tricritical. On the other hand, the corresponding variation in dA/dC with T
				−
				TC for a 5 mol% mixture of 3g in 2-PhP fits to a linear relation, which indicates that the second-order SmA–C transition approaches typical mean-field behavior.

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants : amplification of the electroclinic coefficient using achiral additives

Two isometric series of chloro-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine mesogens QL8-m/n and QL9-m/n (m + n = 16), with the chloro-terminated alkoxy chain tethered to either the pyrimidine ring or the phenyl ring, were synthesized and their liquid crystalline properties characterized by polarized optical microscopy and differential scanning calorimetry. Based on the analysis of mesogenic properties and correlations to electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level, we present evidence suggesting that the SmA-promoting effect of chloro end-groups is not due to strong polar interactions at the layer interfaces, as previously postulated in the literature. Instead, the evidence suggests that the SmA-promoting effect is due to the electron-withdrawing effect of the chloro end-group, which should reduce electrostatic repulsion between alkoxy chains and, consequently, increase van der Waals interactions between aromatic cores in the orthogonal SmA phase.

Elucidating the smectic A-promoting effect of halogen end-groups in calamitic liquid crystals

We report the synthesis and characterization of organosiloxane 2-phenylpyrimidine mesogens with chiral (R,R)-2,3-difluorooctyloxy side-chains. These compounds form chiral SmA* and SmC* phases, and show remarkable electro-optical and alignment properties as surface-stabilized ferroelectric liquid crystals, including one of the fastest optical rise times (11 µs at T − TAC = −10 K and 6 V µm−1) for an organosiloxane mesogen.

Jeffrey C. Roberts

Papers

Design of Liquid Crystals with “de Vries-like” Properties: Frustration between SmA- and SmC-Promoting Elements

Design of liquid crystals with "de Vries-like" properties: organosiloxane mesogen with a 5-phenylpyrimidine core.

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants : amplification of the electroclinic coefficient using achiral additives

Elucidating the smectic A-promoting effect of halogen end-groups in calamitic liquid crystals

Fast switching organosiloxane ferroelectric liquid crystals