Noncovalent Approach to One-Dimensional Ion Conductors: Enhancement of Ionic Conductivities in Nanostructured Columnar Liquid Crystals

Coarse-Grained Simulations of Columnar Ionic Liquid Crystals: Comparison with Experiments

We simulate a homologous series of guanidinium-based columnar ionic liquid crystals (ILCs) using coarse-grained molecular dynamics (MD) simulations with the Martini force field. We systematically vary the length of alkyl side chains, ILC-n (n = 8, 12, 16), and compare our results with previous experimental findings. Experimentally, ILC-8 exhibits a narrow mesophase window and weak columnar order, while ILC-12 and ILC-16 display a broad mesophase window and high columnar order. The MD simulations show that ILC-8 forms a percolated structure, whereas the longer chain analogues self-assemble into columns, with columnar assembly becoming more prominent as the side chain length increases, in qualitative agreement with the experiments. Furthermore, the intercolumnar distance increases monotonically with increasing side chain length and decreases with increasing temperature. Finally, we find that the diffusion coefficient and ionic conductivity decrease substantially with increasing chain length, consistent with experimental observations. We attribute this decrease in mobility to the formation of hexagonally ordered columns, which restrict transport more than percolated networks.

Homeotropic orientation of an ion-channel forming mesophase induced by nanotemplate wetting

Anodic aluminum oxide (AAO) membranes were used as templates to control orientation of an ion-channel forming columnar mesophase obtained by self assembly of a wedge-shaped sulfonate molecule. Inside the AAO structure, the director vector of the mesophase is oriented parallel to the pore axis due to the confinement effect. The molecular arrangement induced by the spatial confinement within the pores is extended over several microns into the remnant film on the AAO surface. The homeotropic alignment of the channels promotes unidimensional ion conduction through the film plane, which is manifested by a considerable increase in conductivity relative to isotropic samples.

Aquatic Functional Liquid Crystals: Design, Functionalization, and Molecular Simulation

Aquatic functional liquid crystals, which are ordered molecular assemblies that work in water environment, are described in this review. Aquatic functional liquid crystals are liquid-crystalline (LC) materials interacting water molecules or aquatic environment. They include aquatic lyotropic liquid crystals and LC based materials that have aquatic interfaces, for example, nanoporous water treatment membranes that are solids preserving LC order. They can remove ions and viruses with nano- and subnano-porous structures. Columnar, smectic, bicontinuous LC structures are used for fabrication of these 1D, 2D, 3D materials. Design and functionalization of aquatic LC sensors based on aqueous/LC interfaces are also described. The ordering transitions of liquid crystals induced by molecular recognition at the aqueous interfaces provide distinct optical responses. Molecular orientation and dynamic behavior of these aquatic functional LC materials are studied by molecular dynamics simulations. The molecular interactions of LC materials and water are key of these investigations. New insights into aquatic functional LC materials contribute to the fields of environment, healthcare, and biotechnology.

Towards Nematic Phases in Ionic Liquid Crystals - A Simulation Study

Ionic liquid crystals (ILCs) are soft matter materials with broad liquid crystalline phases and intrinsic electric conductivity. They typically consist of a rod-shaped mesogenic ion and a smaller spherical counter-ion. Their mesomorphic properties can be easily tuned by exchanging the counter ion. ILCs show a strong tendency to form smectic A phases due to the segregation of ionic and the non-ionic molecular segments. Nematic phases are therefore extremely rare in ILCs and the question of why nematic phases are so exceptional in existing ILCs, and how nematic ILCs might be obtained in the future is of vital interest for both the fundamental understanding and the potential applications of ILCs. Here, we present the result of a simulation study, which highlights the crucial role of the location of the ionic charge on the rod-like mesogenic ions in the phase behaviour of ILCs. We find that shifting the charge from the ends towards the centre of the mesogenic ion destabilizes the liquid crystalline state and induces a change from smectic A to nematic phases.

Cyclometalated Platinum(II) Metallomesogens Based on Half-Disc-Shaped β-Diketonate Ligands with Hexacatenar: Crystal Structures, Mesophase Properties, and Semiconductor Devices

A series of new half-disc-shaped platinum(II) complexes [Pt(ppy)(ALⁿ-6OC_nH_2n+1)] (Pt-An), [Pt(ppyF)(ALⁿ-6OC_nH_2n+1)] (Pt-Bn), and [Pt(ppyCF₃)(ALⁿ-6OC_nH_2n+1)] (Pt-Cn) (ALⁿ-6OC_nH_2n+1 = 1,3-bis(3,4,5-trialkoxyphenyl)propane-1,3-dionato; n = 1, 6, 12) with concise structures have been designed and synthesized, in which 2-phenylpyridine (ppy) derivatives were used as cyclometalated ligands and hexacatenar β-diketonate derivatives ALⁿ-6OC_nH_2n+1 as auxiliary ligands. The single-crystal data of the methoxy diketonate analogues Pt-A1, Pt-B1, and Pt-C1 indicate that they all display excellent square planarity. These platinum(II) complexes show a certain emission tunability (ranging from λ = 506-535 nm) by the introduction of fluorine or trifluoromethyl into ppy. Thermal studies reveal that the fluorine-substituted complexes are liquid crystals but the trifluoromethyl-substituted complexes are not. The platinum(II) complexes Pt-A12, Pt-B6, and Pt-B12 can form a hexagonal columnar mesophase via intermolecular π-π interactions. In addition, compared to the reported platinum(II) metallomesogens, Pt-A12 and Pt-B12 exhibit improved ambipolar carrier mobility behaviors in semiconductor devices at the liquid crystal states.

Current Topics in Ionic Liquid Crystals

Ionic liquid crystals (ILCs), that is, ionic liquids exhibiting mesomorphism, liquid crystalline phases, and anisotropic properties, have received intense attention in the past years. Among others, this is due to their special properties arising from the combination of properties stemming from ionic liquids and from liquid crystalline arrangements. Besides interesting fundamental aspects, ILCs have been claimed to have tremendous application potential that again arises from the combination of properties and architectures that are not accessible otherwise, or at least not accessible easily by other strategies. The current review highlights recent developments in ILC research, starting with some key fundamental aspects. Further subjects covered include the synthesis and variations of modern ILCs, including the specific tuning of their mesomorphic behavior. The review concludes with reflections on some applications that may be within reach for ILCs and finally highlights a few key challenges that must be overcome prior and during true commercialization of ILCs.

Bicontinuous Cubic and Hexagonal Columnar Liquid Crystalline Ion-Conductors at Room Temperature in Ion-Doped Dendritic Amphiphiles

A bicontinuous cubic (Cubbi) liquid crystalline (LC) phase consisting of three dimensional (3D) conducting networks is a promising structural platform for ion-conductors. For practical applications using this fascinating LC structure, it is necessary to suppress crystallization at room temperature (RT). Herein, we report the Cubbi structure at RT and the morphology–dependent conduction behavior in ionic samples of a non-crystallizable dendritic amphiphile. In the molecular design, branched alkyl chains were used as an ionophobic part instead of crystallizable linear alkyl chains. Two ionic samples with Cubbi and hexagonal columnar (Colhex) LC phases at RT were prepared by adding different amounts of lithium salt to the amphiphile. Impedance analysis demonstrated that the Cubbi phase contributed to the faster ion-conduction to a larger extent than the Colhex phase due to the 3D ionic networks of the Cubbi phase. In addition, the temperature–dependent impedance and electric modulus data provided information regarding the phase transition from microphase-separated phase to molecularly mixed liquid phase.

Ionic N-phenylpyridinium tetracatenar mesogens: competing driving forces in mesophase formation and unprecedented difference in phase stabilisation within a homologous series

Ionic, tetracatenar liquid crystals containing an N-phenylpyridinum core are described; many of these compounds display a SmA phase, something extremely rare in tetracatenar materials. The competing forces driving mesophase formation lead to an unprecedented difference in phase stabilities for SmA and Colh phases.

Nanostructured Virus Filtration Membranes Based on Two-Component Columnar Liquid Crystals

Here we report columnar liquid-crystalline (LC) nanostructured membranes that highly remove viruses and show sufficient water permeation. These membranes were prepared by employing two-component liquid crystals that exhibit tetragonal columnar phases. The membranes exhibited virus rejection values of >99.99% (log₁₀ reduction value (LRV) > 4) and water flux ranging from 19 to 61 L m^-2 h^-1 (operation pressure: 0.3 MPa). These membranes were fabricated by photopolymerization of a fan-shaped diol molecule and imidazolium ionic liquid mixture, followed by subsequent removal of the ionic liquid. The rejection values and water flux depend on the fraction of ionic liquid. These results show new design strategies of materials for the water treatment nanostructured membranes that remove pathogens and contaminants.

Ionic-surfactants-based thermotropic liquid crystals

Ionic surfactants can be combined with various functional groups through electrostatic interaction, resulting in a series of thermotropic liquid crystals (TLCs).

Ionic transport in nematic liquid crystals and alignment layer effects on electrode polarization

The physical properties of a liquid crystal-ionic liquid system were investigated. Low-frequency dielectric spectroscopy for 4-cyano-4'-pentylbiphenyl (5CB) doped with 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF ₄) for the nematic and isotropic phase of host substances was performed. We obtained electrical conductivity values in the range from 298.2 K to 313.2 K and the conductivity anisotropy was confirmed. Further study of the relaxation process for bmim ⁺ allowed us to extract the relaxation frequencies and amplitudes from experimental data and confirm the temperature scaling; the thickness of the interfacial layers was estimated for the homogeneous and homeotropic alignments of the prepared composite. An attempt to unfold the ion contribution on the charge transport was made in order to better understand the electrode polarization process. In this work, the influence of the alignment layer and phase state on the interfacial layer formation in liquid crystal media will be explained better.

Functional Liquid Crystals towards the Next Generation of Materials

Since the discovery of the liquid-crystalline state in 1888, liquid crystal science has made great advances through fusion with various technologies and disciplines. Recently, new molecular design strategies and new self-assembled structures have been developed as a result of the progress made in synthetic procedures and characterization techniques. Since these liquid crystals exhibit new functions and properties derived from their nanostructures and alignment, a variety of new functions for liquid crystals, such as transport for energy applications, separation for environmental applications, chromism, sensing, electrooptical effects, actuation, and templating have been proposed. This Review presents recent advances of liquid crystals that should contribute to the next generation of materials.

Noncovalent Approach to Liquid-Crystalline Ion Conductors: High-Rate Performances and Room-Temperature Operation for Li-Ion Batteries

We report advanced liquid-crystalline (LC) electrolytes for use in lithium-ion batteries (LIBs). We evaluated the potential of LC electrolytes with a half cell composed of Li metal and LiFePO4 which is a conventional positive electrode for LIBs. Low-molecular-weight carbonates of ethylene carbonate or propylene carbonate were incorporated into the two-dimensional (2D) nanostructured electrolyte composed of mesogen-containing carbonate and lithium bis(trifluoromethylsulfonyl)imide. The incorporation of low-molecular-weight carbonates increased the ionic conductivity with maintaining 2D nanostructures in the LC state. High-power performances at relatively high current densities induced by higher ionic conductivities have been achieved by LC electrolytes with low-molecular-weight carbonates. Furthermore, room-temperature operation of LIBs using LC electrolytes is reported for the first time. In the research field of electrolytes for LIBs, we demonstrate the progress of a new category of LC electrolytes.

Dynamics and ionic conductivity of ionic liquid crystals forming a hexagonal columnar mesophase

For the first time, the molecular mobility of two linear-shaped tetramethylated guanidinium triflate ionic liquid crystals (ILCs) having different lengths of alkyl chains was investigated using a combination of broadband dielectric (BDS) and specific heat spectroscopy (SHS).

Mesostructure and physical properties of aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl sulfate doped with divalent sulfate salts in the liquid and the mesomorphic states

Mesomorphic liquid crystal character and effect on physical properties.

Crystal structures of ordered and plastic-crystalline phases of iso-butyllithium by X-ray powder diffraction

The crystal structures of solid, donor-free i-BuLi at different temperatures were determined from X-ray powder diffraction data.

Repulsion between oppositely charged rod-shaped macromolecules: Role of overcharging and ionic confinement

The interaction between two oppositely charged rod-shaped macro-ions in a micro-ion solution is investigated via Monte Carlo simulations of the primitive model. The focus is on the asymmetry in rod and/or ion charge, i.e., conditions where oppositely charged objects can repel one another. For equally and oppositely charged rods with asymmetric z:1 micro-ions, repulsion may be induced by overcharging one of the rods with the z valent ions. For asymmetrically charged rods in a symmetric z:z micro-ion solution, a repulsive interaction-at separation of the order of one ion diameter-can arise via an unbalanced osmotic pressure contribution from the ionic atmosphere in the inter-rod space, and an attractive interaction-at a smaller separation-may occur due to a "squeezing out" of the micro-ions from the space between the rods (with a consequent gain in entropy). The thermodynamics of each mechanism is investigated in terms of rod charge and size and micro-ion valence, size, and concentration. Our findings contribute to the understanding of the complex role of charge asymmetry on the interaction of, for example, oppositely charged polyelectrolytes, functionalized nanotubes, and rod-like biomolecules, e.g., viruses.

Nanostructured discotic Pd(ii) metallomesogens as one-dimensional proton conductors

Bis(isoquinolinylpyrazolate) Pd(ii) metallomesogens may be a promising step forward in the design of highly-stable proton conducting water-free electrolyte materials.

Facilitated Ion Transport in Smectic Ordered Ionic Liquid Crystals

A novel ionic mixture of an imidazolium-based room-temperature ionic liquid containing ethylene-oxide-functionalized phosphite anions is fabricated, which, when doped with lithium salt, self-assembles into a smectic-ordered ionic liquid crystal through Coulombic interactions between the ion species. Interestingly, the smectic order in the ionic-liquid-crystal ionogel facilitates ionic transport.

The plastic crystalline A15 phase of dimethylaminoalane, [N(CH3)2-AlH2]3

A plastic crystalline phase of dimethylaminoalane has been discovered at T > 332 K. The phase transitions solid - plastic phase - liquid are fully reversible. The plastic crystalline phase exhibits a cubic unit cell, space group Pm3[combining macron]n, in which the dimethylaminoalane molecules rotate and adopt a structural arrangement reminiscent of the A15 phase.

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.

Ionogels Based on Poly(methyl methacrylate) and Metal-Containing Ionic Liquids: Correlation between Structure and Mechanical and Electrical Properties

Ionogels (IGs) based on poly(methyl methacrylate) (PMMA) and the metal-containing ionic liquids (ILs) bis-1-butyl-3-methlimidazolium tetrachloridocuprate(II), tetrachloride cobaltate(II), and tetrachlorido manganate(II) have been synthesized and their mechanical and electrical properties have been correlated with their microstructure. Unlike many previous examples, the current IGs show a decreasing stability in stress-strain experiments on increasing IL fractions. The conductivities of the current IGs are lower than those observed in similar examples in the literature. Both effects are caused by a two-phase structure with micrometer-sized IL-rich domains homogeneously dispersed an IL-deficient continuous PMMA phase. This study demonstrates that the IL-polymer miscibility and the morphology of the IGs are key parameters to control the (macroscopic) properties of IGs.

Designing the topology of ion nano-channels in the mesophases of amphiphilic wedge-shaped molecules

The wedge-shaped amphiphiles bearing sulfonate groups at the tip of the wedge are prone to form ion nano-channels upon exposure to a humid atmosphere. During swelling, water molecules preferentially accumulate in polar regions of the system resulting in the formation of a lyotropic phase. In this work, the details of the structure formation processes occurring upon swelling in water vapour, including determination of the size and topology of the ion nano-channels, are explored. The electron density profiles across the channel are obtained from the fits of the X-ray scattering data with two- and three-phase structural models the applicability of which is critically analysed. The results show that the ion channel size correlates not only with water uptake but also with the molecular architecture such as the structure of the rigid molecular fragment bearing a polar group. These findings can help optimising the ion transport for development of ion-selective membranes.

Fused π-conjugated imidazolium liquid crystals: synthesis, self-organization, and fluorescence properties

Fused π-conjugated imidazolium compounds bearing two or three long alkoxy chains and various counter anions were systematically prepared following our reported synthetic route.

π-Electron Systems That Form Planar and Interlocked Anion Complexes and Their Ion-Pairing Assemblies

Interactions between designed charged species are important for the ordered arrangements of π-electron systems in assembled structures. As precursors of π-electron anion units, new arylethynyl-substituted dipyrrolyldiketone boron complexes, which showed anion-responsive behavior, were synthesized. They formed a variety of receptor-anion complexes ([1+1] and [2+1] types) in solution, and the stabilities of these complexes were discussed in terms of their thermodynamic parameters. Solid-state ion-pairing assemblies of [1+1]- and [2+1]-type complexes with countercations were also revealed by single-crystal X-ray analysis. In particular, a totally charge-segregated assembly was constructed based on negatively and positively charged layers fabricated from [2+1]-type receptor-anion complexes and tetrabutylammonium cations, respectively. Furthermore, the [1+1]-type anion complex of the receptor possessing long alkyl chains exhibited mesophases based on columnar assembled structures with contributions from charge-by-charge and charge-segregated arrangements, which exhibited charge-carrier transporting properties.

Liquid-crystalline calcium carbonate: biomimetic synthesis and alignment of nanorod calcite

Liquid-crystalline CaCO₃ crystals were obtained by bio-inspired crystallization through amorphous CaCO₃. These calcite nanorods were macroscopically aligned by applying mechanical force to the liquid-crystalline phase.

Liquid-crystalline CaCO₃ has been prepared for the first time. The nanorods of CaCO₃ calcite are obtained by bio-inspired crystallization through aqueous colloidal precursors of amorphous CaCO₃ stabilized by poly(acrylic acid). The synthesized calcite nanocrystals have well-tuned morphologies that are preferable for formation of liquid-crystalline phases in concentrated aqueous colloidal solution. The one-dimensional alignment of calcite crystals is achieved by mechanical shearing of the aqueous colloidal solution showing liquid-crystalline phases. These CaCO₃-based liquid crystals formed by a self-organization process in mild conditions may have great potential for use as environmentally friendly materials.

Synthesis and Optical Properties of Imidazole- and Benzimidazole-Based Fused π-Conjugated Compounds: Influence of Substituent, Counteranion, and π-Conjugated System

Fused π-conjugated imidazolium chlorides having hydrogen (1-Cl), octyloxy (2-Cl), N,N-dibutylamino (3-Cl), trifluoromethyl (4-Cl), and cyano (5-Cl) groups substituted on the benzene ring at the 2-position of imidazole were prepared. Counteranion exchanges from chloride to bis(trifluoromethanesulfonyl)imidate (2-TFSI) and tetrafluoroborate (2-BF4) were performed. The optical properties of these compounds (absorption and emission wavelengths, fluorescence quantum yield, and solvatochromism) were influenced by both the substituent and anion character, which was investigated by theoretical calculations using the density functional theory (DFT) and symmetry-adapted cluster-configuration interaction (SAC-CI) methods. Fused π-conjugated benzimidazolium chlorides having N,N-dibutylamino (6-Cl) and cyano (7-Cl) groups were also prepared to observe the different solvatochromic shifts.

2D assemblies of ionic liquid crystals based on imidazolium moieties: formation of ion-conductive layers

Organization of imidazolium ionic liquids into 2D ion-conductive pathways enhances the ionic conductivity of the imidazolium-based ionic liquid crystals.

Highly conducting lyotropic liquid crystalline mesophases of pluronics (P65, P85, P103, and P123) and hydrated lithium salts (LiCl and LiNO₃)

Demand for ionically conducting materials, as membranes and electrodes, is one of the driving forces of current research in chemistry, physics, and engineering. The lithium ion is a key element of these materials, and its assembly into nanostructures and mesophases is important for the membrane and electrode technologies. In this investigation, we show that hydrated lithium salts (such as LiCl·xH2O and LiNO3·xH2O, x is as low as 1.5 and 3.0, respectively) and pluronics (triblock copolymer such as PX where X is 65, 85, 103, and 123) form lyotropic liquid crystalline mesophases (LLCM), denoted as LiY·xH2O-PX-n (Y is Cl(-) or NO3(-), and n is the salt/PX mole ratio). The structure of the mesophase is hexagonal over a broad salt concentration and transforms to a cubic mesophase and then to disordered gel phase with an increasing salt content of the mixtures. The mesophases are unstable at low salt contents and undergo a phase separation into pure pluronics and salt-rich LLCMs. The salt content of the ordered mesophase can be as high as 30 mole ratio for each pluronic, which is a record high for any known salted phases. The mesophases also display high ac ionic conductivities, reaching up to 21 mS/cm at room temperature (RT), and are sensitive to the water content. These mesophases can be useful as ion-conducting membranes and can be used as media for the synthesis of lithium-containing nanoporous materials.

Bio-based ionic liquid crystalline quaternary ammonium salts: properties and applications

In the present work, we describe the preparation, properties, and applications of novel ionic liquid crystalline quaternary ammonium salts (QSs) of 3-pentadecylphenol, a bio-based low-cost material derived from cashew nut shell liquid. Amphotropic liquid crystalline phase formation in QSs was characterized using a combination of techniques, such as DSC, PLM, XRD, SEM, and rheology, which revealed the formation of one, two, and three dimensionally ordered mesophases in different length scales. On the basis of these results, a plausible mechanism for the formation of specific modes of packing in various mesophases was proposed. Observation of anisotropic ionic conductivity and electrochemical stability suggests their application as a solid electrolyte.