Development of structural complexity by liquid-crystal self-assembly

Nonsymmetrical cholesterol dimers constituting regioisomeric oxadiazole and thiadiazole cores: an investigation of the structure–property correlation

The effect of a bent angle on the stabilization of liquid crystalline and organogel self-assemblies.

Synthesis and self-assembly of bent core polycatenar mesogens with binding selectivity to Hg2+

Target compounds can self-assemble into Cub_I/Pm3̄nLC phases and organogels, simultaneously; these compounds have binding selectivity to Hg²⁺.

Self-assembled helical columnar superstructures with selective homochirality

Helical columnar polymeric complexes with selective homochirality have been achieved from side-chain discotic liquid crystalline polymers doped with chiral acceptors.

Body-centred cubic packing of spheres - the ultimate thermotropic assembly mode for highly divergent dendrons

We have synthesized sodium tris(alkoxy)benzoates in which one of the three alkyl chains branches further into three C₁₈H₃₇ chains. These AB5 hyperbranched minidendrons melt directly into a body-centred cubic (BCC) mesophase formed by spherical "micelles". In contrast, their non-branched counterparts display various mesophases before they turn into BCC upon heating. This agrees with the predictions from a numerical geometric model that relates the shape of the molecular wedge to the type of mesophase they adopt. The spheres were found to shrink in volume upon heating and expand upon cooling, as molecules, in some cases nearly half of them, are ejected and reintegrated in the spheres. The ejection of dendrons is caused by their lateral thermal expansion. The BCC appears to be the ultimate mesophase for the extremely divergent wedges such as the hyperbranched minidendrons. In dendrons with chains of unequal length, the sphere size is fixed by the shorter chains, the longer ones back-folding or interdigitating to effectively widen the wedge. This new understanding of their assembly will help in designing new dendrons, e.g. for better encapsulation of guest molecules.

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications

Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photochromism, liquid crystallinity, and fabrication techniques has enabled some fascinating functional materials which can be driven by ultraviolet, visible, and infrared light irradiation. Nanoscale particles have been incorporated to widen and diversify the scope of the light-driven liquid crystalline materials. The developed materials possess huge potential for applications in optics, photonics, adaptive materials, nanotechnology, etc. The challenges and opportunities in this area are discussed at the end of the Review.

Induction of bicontinuous cubic liquid-crystalline assemblies for polymerizable amphiphiles via tailor-made design of ionic liquids

The lyotropic liquid-crystalline behavior of polymerizable amphiphiles has been tuned by using tailor-made ionic liquids as solvents so as to induce the formation of bicontinuous cubic assemblies having 3D interconnected nanochannels. The fixation of the amphiphilic assemblies has been successfully achieved by in situ photopolymerization.

Efficient Light-Induced Phase Transitions in Halogen-Bonded Liquid Crystals

Here, we present a new family of light-responsive, fluorinated supramolecular liquid crystals (LCs) showing efficient and reversible light-induced LC-to-isotropic phase transitions. Our materials design is based on fluorinated azobenzenes, where the fluorination serves to strengthen the noncovalent interaction with bond-accepting stilbazole molecules, and increase the lifetime of the cis-form of the azobenzene units. The halogen-bonded LCs were characterized by means of X-ray diffraction, hot-stage polarized optical microscopy, and differential scanning calorimetry. Simultaneous analysis of light-induced changes in birefringence, absorption, and optical scattering allowed us to estimate that <4% of the mesogenic units in the cis-form suffices to trigger the full LC-to-isotropic phase transition. We also report a light-induced and reversible crystal-to-isotropic phase transition, which has not been previously observed in supramolecular complexes. In addition to fundamental understanding of light-responsive supramolecular complexes, we foresee this study to be important in the development of bistable photonic devices and supramolecular actuators.

Passing Current through Electrically Conducting Lyotropic Liquid Crystals and Micelles Assembled from Hybrid Surfactants with π-Conjugated Tail and Polyoxometalate Head

The solvent-mediated ability for molecularly encoded self-assembly into states of higher order (micelles, lyotropic liquid crystals) embodies the basis for many applications of surfactants in science and society. Surfactants are used frequently in recipes for nanoparticle synthesis. Because ordinary surfactants comprise insulating constituents (alkyl groups as side-chains and charged organic heads), such nanostructures are wrapped in an electrically inactive barrier, and this is a large disadvantage for future developments in nanotechnology. Implications of micelles with electrically conducting walls made from either "metallic" or "semiconducting" surfactants are huge, also in other areas such as nanoelectrocatalysis or micellar energy storage. We cross this frontier by replacing not only the hydrophilic chain but also the hydrophilic head by electronically conducting entities. We report the synthesis of surfactants with oligo para-phenylene-ethynylene as a π-conjugated side-chain attached to a redox-active, inorganic polyoxometalate cluster as charged head. It is proven that electronic communication between head and tail takes place. Hybridization on the molecular level leads to the emergence of advanced surfactant features such as semiconductor properties (Egap = 2.6 eV) in soft lyotropic systems (micelles, liquid crystals).

Complex liquid-crystal nanostructures in semiflexible ABC linear triblock copolymers: A self-consistent field theory

We show that two series of ABC linear triblock copolymers possess sequences of order-to-order phase transitions between microphase-separated states, as the degree of flexibility of the semiflexible middle B-blocks varies. The spatial and orientational symmetries of these phases, some of them containing liquid-crystal ordering, are analysed in comparison with related structures previously determined experimentally and theoretically. A theoretical framework based on the self-consistent field treatment of the wormlike-chain model, which incorporates the Flory-Huggins and Maier-Saupe interactions in the free energy, is used here as a basic foundation for numerical calculations. We suggest that tuning the flexibility parameter, which reduces to the concept of degree of polymerization in the coil-like limit and characterizes the chain-persistency in the rod-like limit, provides a promising approach that can be used to design the resulting microphase-separated structures in semiflexible copolymer melts.

Maximizing Headgroup Repulsion: Hybrid Surfactants with Ultrahighly Charged Inorganic Heads and Their Unusual Self-Assembly

Nonequilibrium states of matter are arousing huge interest because of the outstanding possibilities to generate unprecedented structures with novel properties. Self-organizing soft matter is the ideal object of study as it unifies periodic order and high dynamics. Compared to settled systems, it becomes vital to realize more complex interaction patterns. A promising and intricate approach is implementing controlled balance between attractive and repulsive forces. We try to answer a fundamental question in surfactant science: How are processes like lyotropic liquid crystals and micellization affected, when headgroup charge becomes so large that repulsive interactions are inevitable? A particular challenge is that size and shape of the surfactant must not change. We could realize the latter by means of new hybrid surfactants with a heteropolyanion head [EW₁₁O₃₉]^n- (E = P^V, Si^IV, B^III; n = 3, 4, 5). Among the unusual self-assembled structures, we report a new type of micelle with dumbbell morphology.

Reversible Polymorphism, Liquid Crystallinity, and Stimuli-Responsive Luminescence in a Bola-amphiphilic π-System: Structure-Property Correlations Through Nanoindentation and DFT Calculations

We report the design, synthesis, detailed characterization, and analysis of a new multifunctional π-conjugated bola-amphiphilic chromophore: oligo-(p-phenyleneethynylene)dicarboxylic acid with dialkoxyoctadecyl side chains (OPE-C₁₈-1). OPE-C₁₈-1 shows two polymorphs at 123 K (OPE-C₁₈-1') and 373 K (OPE-C₁₈-1″), whose crystal structures were characterized via single crystal X-ray diffraction. OPE-C₁₈-1 also exhibits thermotropic liquid crystalline property revealing a columnar phase. The inherent π-conjugation of OPE-C₁₈-1 imparts luminescence to the system. Photoluminescence measurements on the mesophase also reveal similar luminescence as in the crystalline state. Additionally, OPE-C₁₈-1 shows mechano-hypsochromic luminescence behavior. Density functional theory (DFT)-based calculations unravel the origins behind the simultaneous existence of all these properties. Nanoindentation experiments on the single crystal reveal its mechanical strength and accurately correlate the molecular arrangement with the liquid crystalline and mechanochromic luminescence behavior.

Induction of an Infinite Periodic Minimal Surface by Endowing An Amphiphilic Zwitterion with Halogen-Bond Ability

We have designed an amphiphilic zwitterion with an iodine‐substituted imidazolium cation. Although it forms a layered assembly with flat interfaces, the addition of an equimolar amount of bis(trifluoromethane)sulfonimide results in the formation of a bicontinuous cubic liquid‐crystalline assembly with a primitive‐type infinite periodic minimal surface, where its zwitterionic headgroup sits regularly. IR measurements revealed that halogen bond between the iodine atoms on the imidazolium cation and the anions is involved in its molecular‐assembly behavior. The present results clearly indicate the potential utility of halogen bonding to control the dimensionality and continuity of the ionic/nonionic interface of amphiphiles in bulk and consequent mesophase patterns, which may be a significant new molecular technology for precisely arranging functional molecules on a 3D continuous interfaces.

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.

Liquid-crystalline mesogens based on cyclo[6]aramides: distinctive phase transitions in response to macrocyclic host-guest interactions

Producing macrocyclic mesogens that are responsive to guest encapsulation presents a significant challenge. Cyclo[6]aramides, a type of macrocycle with a hydrogen-bond-constrained backbone, exhibit thermotropic lamellar, discotic nematic, hexagonal, and rectangular columnar mesophases over a considerably wide temperature range, including at room temperature. Additionally, cyclo[6]aramides show unusual mesophase transitions from lamellar to hexagonal columnar phase mediated by macrocyclic host-guest (H-G) interactions between the macrocycles and alkylammonium salts. The phase transition, triggered by an organic guest engaging in H-G interactions with a macrocyclic cavity, provides a novel strategy for manipulating the properties of liquid-crystalline materials. The crystal structure of a homologous cyclo[6]aramide reveals a disk-shaped, near-planar molecular backbone that facilitates intermolecular π-π stacking and leads to columnar assembly.

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.

Stabilization of the bicontinuous cubic phase in siloxane-terminated mesogens, 1,2-bis[4'-(n-(oligodimethylsiloxyl)alkoxy)benzoyl]hydrazine

The introduction of oligodimethyl siloxane segments at the termini of the alkyl tails has been employed to stabilize the bicontinuous cubic (Cub(bi)) phase of a chain-core-chain type molecule having a 1,2-bis(benzoyl)hydrazine central core with two chains attached at the 4' position of each benzoyl moiety. In this study, three silylated molecules, bis-C10Si2, bis-C10Si3, and C10Si2-C8C=C, were synthesized, where "CnSim" represents the number of carbon and silicon atoms in the chain and "bis" indicates the two chains being the same, whereas the last one is asymmetric with respect to the core. The phase behaviors were examined by using polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction techniques. All three compounds form Cub(bi) phases and their syntheses were compared including their parent compound bis-C18. It was clearly revealed that the introduction of oligodimethyl siloxane segments effectively suppresses the crystallization at low temperatures, and as a result stabilizes the Cub(bi) phases, in an extreme case down to room temperature. The semi-quantitative analyses in terms of lattice constant and three-dimensional electron density map help us to better understand the self-assembly process in the Cub(bi) phases. The study also revealed that the introduction of oligodimethyl siloxane segments is not only an alternative for the hydrocarbon segment but also is able to provide a versatile design strategy for obtaining stable Cub(bi) phases.

Supramolecular Columnar Liquid Crystals with Tapered-Shape Simple Pyrazoles Obtained by Efficient Henry/Michael Reactions

A straightforward synthesis of mesogenic pyrazoles starting from benzaldehydes by a combination of efficient Henry and Michael reactions led to novel supramolecular liquid crystals. The mesogens are fluorescent 3,5-dimethyl-4-(di or trialkoxyphenyl)pyrazoles and, in spite of the tapered shape of these molecules and their structural simplicity (only one phenyl ring), columnar liquid-crystal phases were formed that are stable at room temperature. The self-assembled structure was studied by XRD and the columnar cross section contains two molecules on average with an antiparallel arrangement of pyrazoles interacting through hydrogen bonds. In contrast, the single-crystal structure of a trimethoxy analog did not show hydrogen-bonded pyrazoles but chains of head-to-tail arranged molecules.

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.

Induced stabilization of columnar phases in binary mixtures of discotic liquid crystals

Three discotic liquid-crystalline binary mixtures, characterized by their extent of bidispersity in molecular thickness, were investigated with molecular dynamics simulations. Each equimolar mixture contained A-type (thin) and B-type (thick) discogens. The temperature-dependence of the orientational order parameter reveals that A-type liquid samples produce ordered phases more readily, with the (hexagonal) columnar phase being the most structured variant. Moderately and strongly bidisperse mixtures produce globally-segregated samples for temperatures corresponding to ordered phases; the weakly bidisperse mixture displays microheterogeneities. Ordered phases in the B-type liquid are induced partially by the presence of the A-type fluid. In the moderately bidisperse mixture, order is induced through orientational frustration: a mixed prenematic-like phase precedes global segregation to yield nematic and columnar mesophases upon further cooling. In the strongly bidisperse mixture, order is induced less efficiently through a paranematic-like mechanism: a highly-ordered A-type fluid imparts order to B-type discogens found at the interface of a fully-segregated sample. This ordering effect permeates into the disordered B-type domain until nematic and columnar phases emerge upon further cooling. At sufficiently low temperatures, all samples investigated exhibit the (hexagonal) columnar mesophase.

A modular approach towards functional supramolecular aggregates – subtle structural differences inducing liquid crystallinity

A modular approach towards photo-responsive supramolecular liquid crystals is described, allowing efficient screening of complementary binding blocks for the design of functional materials.

Mirror symmetry breaking by mixing of equimolar amounts of two gyroid phase-forming achiral molecules

Mirror symmetry breaking was realised by mixing of two achiral molecules both of which form themselves achiral bicontinuous cubic phases.

Supramolecular assembly-mediated lithium ion transport in nanostructured solid electrolytes

Supramolecular solid polymer electrolytes provide mechanical integrity and well-defined ion-conducting paths for rapid ion transport that can be applied in high-performance lithium-ion batteries.

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.

Mirror symmetry breaking in fluorinated bent-core mesogens

New fluorinated bent-core liquid crystals exhibiting helical nano-crystallite phases composed of chiral domains with opposite handedness and polar smcetic phases.

Synthesis and self-assembly of luminescent hexacatenar molecules incorporating a 4,7-diphenyl-2,1,3-benzothiadiazole core

A luminescent hexacatenar molecule forms an oblique columnar liquid crystal, gels in various organic solvents, and has binding selectivity to Li⁺ in DMSO–CH₂Cl₂ solution.

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.

Chemical Programming of the Domain of Existence of Liquid Crystals

This work illustrates how enthalpy and entropy changes responsible for successive phase transitions of cyanobiphenyl-based liquid crystals can be combined to give cohesive free energy densities. These new parameters are able to rationalize and quantify the demixing of the melting and clearing processes that occur in thermotropic liquid crystals. Minor structural variations at the molecular level can be understood as pressure increments that alter either the melting or clearing temperatures in a predictable way. This assessment of microsegregation operating in amphiphilic molecules paves the way for the chemical programming of the domain of existence of liquid-crystalline phases.

Zeolite-like liquid crystals

Zeolites represent inorganic solid-state materials with porous structures of fascinating complexity. Recently, significant progress was made by reticular synthesis of related organic solid-state materials, such as metal-organic or covalent organic frameworks. Herein we go a step further and report the first example of a fluid honeycomb mimicking a zeolitic framework. In this unique self-assembled liquid crystalline structure, transverse-lying π-conjugated rod-like molecules form pentagonal channels, encircling larger octagonal channels, a structural motif also found in some zeolites. Additional bundles of coaxial molecules penetrate the centres of the larger channels, unreachable by chains attached to the honeycomb framework. This creates a unique fluid hybrid structure combining positive and negative anisotropies, providing the potential for tuning the directionality of anisotropic optical, electrical and magnetic properties. This work also demonstrates a new approach to complex soft-matter self-assembly, by using frustration between space filling and the entropic penalty of chain extension.

Zeolites with regular porous structures are widely used as gas adsorbents and scaffolding for catalysts. Poppe et al. report a liquid crystal with zeolite-like structure by self-assembly of polyphilic molecules with π-conjugated rod-like cores into a honeycomb formed by pentagonal/octagonal channels.

Nanoobjects formed by ionic PAMAM dendrimers: hydrophilic/lipophilic modulation and encapsulation properties

In this paper, we present the preparation and properties of some ionic PAMAM derivatives, which combine hydrophilic and lipophilic carboxylic acid chains as counter-ions for all protonable inner and outer amino groups. The amphiphilic nature of the final ionic codendrimers and, hence, their self-assembling features can be modulated by using different ratios between hydrophilic and lipophilic chains. In the bulk, these new materials self-organize into smectic A liquid crystal phases. In water, they self-assemble into different types of nano-objects depending on the molecular composition. The study of the morphology of these nano-structures, their cytotoxicity and their capability to encapsulate a lipophilic anticancer drug are reported herein. Some of these nanoobjects are non-cytotoxic and present good drug trapping ability, which make them interesting nanocarriers for applications in nanotechnology and biomedicine.

Self-assembly concepts for multicompartment nanostructures

Compartmentalization is ubiquitous to many biological and artificial systems, be it for the separate storage of incompatible matter or to isolate transport processes. Advancements in the synthesis of sequential block copolymers offer a variety of tools to replicate natural design principles with tailor-made soft matter for the precise spatial separation of functionalities on multiple length scales. Here, we review recent trends in the self-assembly of amphiphilic block copolymers to multicompartment nanostructures (MCNs) under (semi-)dilute conditions, with special emphasis on ABC triblock terpolymers. The intrinsic immiscibility of connected blocks induces short-range repulsion into discrete nano-domains stabilized by a third, soluble block or molecular additive. Polymer blocks can be synthesized from an arsenal of functional monomers directing self-assembly through packing frustration or response to various fields. The mobility in solution further allows the manipulation of self-assembly processes into specific directions by clever choice of environmental conditions. This review focuses on practical concepts that direct self-assembly into predictable nanostructures, while narrowing particle dispersity with respect to size, shape and internal morphology. The growing understanding of underlying self-assembly mechanisms expands the number of experimental concepts providing the means to target and manipulate progressively complex superstructures.