Thermotropic Ionic Liquid Crystals

Design of Viologen-Based Liquid Crystals Exhibiting Bicontinuous Cubic Phases and Their Redox-Active Behavior

We have succeeded in developing viologen-based liquid-crystalline materials forming bicontinuous cubic phases. They are composed of amphiphilic zwitterions with a viologen ionic-head-group and sulfonyl-imide-type acids. In the bicontinuous cubic liquid-crystalline assemblies, the ionic-head groups of the amphiphiles align along a gyroid minimal surface, forming a 3D continuous viologen layer. The ionic state of the viologen-moieties can be tuned from a dication state (V²⁺) to a radical mono cation state (V^1+•) by UV irradiation and/or electric field. This redox reaction proceeds in bulk, accompanying the change of their color from colorless to purple. Interestingly, they preserve the 3D molecular assembled structures beyond the redox reaction, which has been confirmed by polarizing optical microscopy and X-ray diffraction measurements.

Tuning Coulombic interactions to stabilize nematic and smectic ionic liquid crystal phases in mixtures of charged soft ellipsoids and spheres

We have investigated the effect of electrostatic interactions in mixtures of soft ellipsoids and spheres based on the well-known Gay-Berne (GB) and Lennard-Jones (LJ) potential, respectively. These model systems, in their original version, that is without any electrostatic charge, have been thoroughly investigated in the literature both as pure components and mixtures. In particular, mixtures of particles of different shapes, such as spheres and ellipsoids, tend to phase separate because of the excluded volume effects. Common ionic liquid crystals, based on imidazolium or other quaternary ammonium salts, are usually composed of roughly elongated (although flexible) cations and roughly spherical anions, that is, particles with a similar shape such as the GB and LJ models. Therefore, in this work, we present the results of molecular dynamics simulations of mixtures of positively charged GB and negatively charged LJ particles as models of ionic liquid crystals. Interestingly, by modulating the charge of the particles it is possible to stabilize isotropic, nematic, smectic and crystalline ionic phases. The relative weight of Coulomb (a radial, therefore isotropic interaction) and van der Waals (an anisotropic interaction) contributions is a key parameter to tune the stability of various mesophases.

Phase Behavior and Physical Properties of New Biobased Ionic Liquid Crystals

Protic ionic liquids (PILs) have emerged as promising compounds and attracted the interest of the industry and the academy community, due to their easy preparation and unique properties. In the context of green chemistry, the use of biocompounds, such as fatty acids, for their synthesis could disclose a possible alternative way to produce ILs with a low or nontoxic effect and, consequently, expanding their applicability in biobased processes or in the development of bioproducts. This work addressed efforts to a better comprehension of the complex solid-[liquid crystal]-liquid thermodynamic equilibrium of 20 new PILs synthesized by using fatty acids commonly found in vegetable oils, as well as their rheological profile and self-assembling ability. The work revealed that their phase equilibrium and physical properties are significantly impacted by the structure of the ions used for their synthesis. The use of unsaturated fatty acids and bis(2-hydroxyethyl)ammonium for the synthesis of these biobased ILs led to a drastic decreasing of their melting temperatures. Also, the longest alkyl chain fatty acids promoted higher self-assembling and more stable mesophases. Besides their sustainable appeal, the marked high viscosity, non-Newtonian profile, and very low critical micellar concentration values of the PIL crystals here disclosed make them interesting renewable compounds with potential applications as emulsifiers, stabilizers, thickeners, or biolubricants.

Crystalline structure and thermotropic behavior of alkyltrimethylphosphonium amphiphiles

Quaternary organophosphonium salts bearing long alkyl chains are cationic surfactants of interest owing to their physical and biological properties. In the present work, the crystal structure and thermotropic behavior of the homologous series of alkyltrimethylphosphonium bromides (nATMP·Br), with the alkyl chain containing an even number (n) of carbon atoms from 12 to 22, have been examined within the 0-300 °C range of temperatures. These compounds were shown to be resistant to heat up to ∼390 °C. The phases adopted at different temperatures were detected by DSC, and the structural changes involved in the phase transitions have been characterized by simultaneous WAXS and SAXS carried out in real-time, and by polarizing optical microscopy as well. Three or four phases were identified for n = 12 and 14 or n ≥ 16, respectively, in agreement with the heat exchange peaks observed by DSC. The phase existing at room temperature (Ph-I) was found to be fully crystalline and its crystal lattice was determined by single-crystal X-ray diffraction methods. Ph-II consisted of a semicrystalline structure that can be categorized as Smectic-B with the crystallized ionic pairs hexagonally arranged in layers and the molten alkyl chain confined in the interlayer space. Ph-II of 12ATMP·Br and 14ATMP·Br directly isotropicized upon heating at ∼220 °C, whereas for n ≥ 16, it converted into a Smectic-A phase (Ph-III) that needed to be heated above ∼240 °C to become isotropic (Ph-Is). The correlation existing between the thermal behavior, phase structure and length of the alkyl side chain has been demonstrated.

Pressure-induced ionic liquid crystal in 1-dodecyl-3-methylimidazolium tetrafluoroborate

The phase behaviors of 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C₁₂MIM][BF₄]) had been investigated by means of Raman spectroscopy and polarized optical microscopy under pressure values up to 2.0 GPa at the temperature of 80.0 °C.

Development of Glassy Bicontinuous Cubic Liquid Crystals for Solid Proton-Conductive Materials

Glassy bicontinuous cubic liquid crystals are developed to be a matrix having a hydrophilic infinite periodic minimal surface (IPMS). They function as a scaffold for water, leading to the formation of a 3D continuous hydrogen-bonding network of water molecules along the IPMS. This material design is advantageous for developing novel electrolytes with rigidity and high proton conductivity.

Bolaamphiphilic liquid crystals based on bis-imidazolium cations

Rare examples of bolaform ionic materials containing cyanometallates and their lamellar mesomorphic supramolecular architectures are characterized.

Dimension-controlled ion-pairing assemblies based on π-electronic charged species

This feature article summarizes the recent progress in the study of ion-pairing assemblies based on π-electronic ion pairs, including anion complexes of π-electronic molecules.

Photoresponsive ionic liquid crystals assembled via halogen bond: en route towards light-controllable ion transporters

We demonstrate that halogen bonding (XB) can offer a novel approach for the construction of photoresponsive ionic liquid crystals. In particular, we assembled two new supramolecular complexes based on 1-ethyl-3-methylimidazolium iodides and azobenzene derivatives containing an iodotetrafluoro-benzene ring as XB donor, where the iodide anion acted as an XB acceptor. DSC and X-ray diffraction analyses revealed that the preferred stoichiometry between the XB donors and acceptors is 2 : 1, and that the iodide anions act as bidentate XB-acceptors, binding two azobenzene derivatives. Due to the high directionality of the XB, calamitic superanions are obtained, while the segregation occurring between the charged and uncharged parts of the molecules gives rise to a layered structure in the crystal lattice. Despite the fact that the starting materials are non-mesomorphic, the halogen-bonded supramolecular complexes exhibited monotropic lamellar liquid-crystalline phases over broad temperature ranges, as confirmed with polarized optical microscopy. Due to the presence of the azobenzene moieties, the LCs were photoresponsive, and a LC-to-isotropic phase transition could be obtained by irradiation with UV light. We envisage that the light-induced phase transition, in combination with the ionic nature of the LC, provides a route towards light-induced control over ion transport and conductance in these supramolecular complexes.

Bis-imidazolium salts with alkyl sulfates as counterions: synthesis and liquid crystalline properties

A series of flexibly linked bis-imidazolium salts with alkyl sulfate anions having different chain lengths (10, 12, 14) was investigated for their thermotropic behavior.

First Examples of de Vries-like Smectic A to Smectic C Phase Transitions in Ionic Liquid Crystals

In ionic liquid crystals, the orthogonal smectic A phase is the most common phase whereas the tilted smectic C phase is rather rare. We present a new study with five novel ionic liquid crystals exhibiting both a smectic A as well as the rare smectic C phase. Two of them have a phenylpyrimidine core whereas the other three are imidazolium azobenzenes. Their phase sequences and tilt angles were studied by polarizing microscopy and their temperature-dependent layer spacing as well as their translational and orientational order parameters were studied by X-ray diffraction. The X-ray tilt angles derived from X-ray studies of the layer contraction and the optically measured tilt angles of the five ionic liquid crystals were compared to obtain their de Vries character. Four of our five mesogens turned out to show de Vries-like behavior with a layer shrinkage that is far less than that expected for conventional materials. These materials can thus be considered as the first de Vries-type materials among ionic liquid crystals.

A New Volume-Based Approach for Predicting Thermophysical Behavior of Ionic Liquids and Ionic Liquid Crystals

Volume-based prediction of melting points and other properties of ionic liquids (ILs) relies on empirical relations with volumes of ions in these low-melting organic salts. Here we report an accurate way to ionic volumes by Bader's partitioning of electron densities from X-ray diffraction obtained via a simple database approach. For a series of 1-tetradecyl-3-methylimidazolium salts, the volumes of different anions are found to correlate linearly with melting points; larger anions giving lower-melting ILs. The volume-based concept is transferred to ionic liquid crystals (ILs that adopt liquid crystalline mesophases, ILCs) for predicting the domain of their existence from the knowledge of their constituents. For 1-alkyl-3-methylimidazolium ILCs, linear correlations of ionic volumes with the occurrence of LC mesophase and its stability are revealed, thus paving the way to rational design of ILCs by combining suitably sized ions.

Doped ionic liquid crystals as effective weakly alignment media for polar solutes

The ionic liquid crystal 1-dodecyl-3-methylimidazolium tetrafluoroborate slightly doped with water is presented as a promising NMR alignment medium for the measurement of residual dipolar couplings for polar molecules dissolved therein.

Superfluorinated Ionic Liquid Crystals Based on Supramolecular, Halogen-Bonded Anions

Unconventional ionic liquid crystals in which the liquid crystallinity is enabled by halogen-bonded supramolecular anions [Cn F2 n+1 -I⋅⋅⋅I⋅⋅⋅I-Cn F2 n+1 ](-) are reported. The material system is unique in many ways, demonstrating for the first time 1) ionic, halogen-bonded liquid crystals, and 2) imidazolium-based ionic liquid crystals in which the occurrence of liquid crystallinity is not driven by the alkyl chains of the cation.

Key Developments in Ionic Liquid Crystals

Ionic liquid crystals are materials that combine the classes of liquid crystals and ionic liquids. The first one is based on the multi-billion-dollar flat panel display industry, whilst the latter quickly developed in the past decades into a family of highly-tunable non-volatile solvents. The combination yields materials with a unique set of properties, but also with many challenges ahead. In this review, we provide an overview of the key concepts in ionic liquid crystals, particularly from a molecular perspective. What are the important molecular parameters that determine the phase behavior? How should they be introduced into the molecules? Finally, which other tools does one have to realize specific properties in the material?

Diketonylpyridinium Cations as a Support of New Ionic Liquid Crystals and Ion-Conductive Materials: Analysis of Counter-Ion Effects

Ionic liquid crystals (ILCs) allow the combination of the high ionic conductivity of ionic liquids (ILs) with the supramolecular organization of liquid crystals (LCs). ILCs salts were obtained by the assembly of long-chained diketonylpyridinium cations of the type [HOO^R(n)pyH]⁺ and BF₄⁻, ReO₄⁻, NO₃⁻, CF₃SO₃⁻, CuCl₄²⁻ counter-ions. We have studied the thermal behavior of five series of compounds by differential scanning calorimetry (DSC) and hot stage polarized light optical microscopy (POM). All materials show thermotropic mesomorphism as well as crystalline polymorphism. X-ray diffraction of the [HOO^R(12)pyH][ReO₄] crystal reveals a layered structure with alternating polar and apolar sublayers. The mesophases also exhibit a lamellar arrangement detected by variable temperature powder X-ray diffraction. The CuCl₄²⁻ salts exhibit the best LC properties followed by the ReO₄⁻ ones due to low melting temperature and wide range of existence. The conductivity was probed for the mesophases in one species each from the ReO₄⁻, and CuCl₄²⁻ families, and for the solid phase in one of the non-mesomorphic Cl⁻ salts. The highest ionic conductivity was found for the smectic mesophase of the ReO₄⁻ containing salt, whereas the solid phases of all salts were dominated by electronic contributions. The ionic conductivity may be favored by the mesophase lamellar structure.

Ionic Liquid Crystals: Versatile Materials

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.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Supramolecular liquid crystalline dendrimers with a porphyrin core and functional carboxylic acid dendrons

Supramolecular dendrimers prepared via H-bonding between porphyrin core and bifunctionalised dendrons, incorporated in the periphery, display photoluminescence properties and self-assembly into smectic mesophase.

Nickel oxide nanoparticles/ionic liquid crystal modified carbon composite electrode for determination of neurotransmitters and paracetamol

Ionic liquid crystals mimic the natural bio-based ionic liquid crystals such as cell membrane structures in their interactions with drugs.

Construction of gyroid-structured matrices through the design of geminized amphiphilic zwitterions and their self-organization

Gemini amphiphilic zwitterions exhibit thermotropic bicontinuous cubic liquid-crystalline phases having a 3D continuous ionic domain.

Ionic Self-Assembly and Red-Phosphorescence Properties of a Charged Platinum(II) 8-Quinolinol Complex Associated with Ammonium-Based Amphiphiles

A series of ionic associates based on the platinum(II) chelate of 5-sulfo-8-quinolinol, [Pt(qS)2](2-), and ammonium-based amphiphiles is described. At variance with the prototypical neutral complex Pt(q)2 (q=8-quinolinol), these dianionic fluorophores, functionalized at the periphery with sulfonate groups, can be associated by the ionic self-assembly approach with various ammonium cations, such as (H2n+1Cn)2Me2N(+) (n=12, 16, 18) or complex ammonium cations carrying three Cn carbon chains (n=12, 14, 16) and an additional amide group. Investigations of their luminescence properties in solution, in the solid state, and, when possible, in thin films revealed that the phosphorescence properties in condensed phases are directly correlated to intermolecular interactions between the luminescent [Pt(qS)2](2-) centers. Of particular interest is also the formation of a columnar liquid-crystalline phase around room temperature (between -25 and +180 °C), as well as the very good film-forming ability of some of these fluorophores from organic solvents.

Insights on the isotropic-to-smectic A transition in ionic liquid crystals from coarse-grained molecular dynamics simulations: the role of microphase segregation

We have investigated the role of microphase segregation as the driving force in the stabilization of thermotropic ionic liquid crystals of smectic type. To this end we have applied the heterogeneity order parameter, initially proposed for ionic liquids, to the coarse-grained molecular dynamics simulation results for a model system of an imidazolium nitrate ionic liquid crystal, [C16mim][NO3], whose phase diagram was recently studied. We have found that the heterogeneity order parameters become larger when the system goes through the transition from the isotropic phase to the smectic A phase as the temperature decreases. This can be understood by considering that, in the smectic A phase, the layered structure allows the tail groups to have a degree of aggregation larger than that in the isotropic phase. Our results highlight the role of microsegregation in the stabilization of ionic liquid crystals, which cannot be revealed by the commonly used translational and orientational order parameters used to describe liquid crystal phases.

Triazolium based ionic liquid crystals: effect of asymmetric substitution

A new series of asymmetrical 1-dodecyl-3-alkyl-triazolium bromides has been synthesized to investigate the effect of asymmetric substitution on their phase behavior.