Isabel M. Saez

Transmission and amplification of information and properties in nanostructured liquid crystals.

In recent years the design of chemical structures of liquid-crystalline materials has developed rapidly, and in many cases changed radically. Since Reinitzers days, liquid crystals have either been classed as rodlike or disclike, with combinations of the two leading to phasmidic liquid crystals. The discovery that materials with bent molecular structures exhibited whole new families of mesophases inspired investigations into the liquid-crystal properties of materials with widely varying molecular topologies-from pyramids to crosses to dendritic molecules. As a result of conformational change, supermolecular materials can have deformable molecular structures, which can stabilize mesophase formation, and some materials that are non-mesogenic, on complexation form supramolecular liquid crystals. The formation of mesophases by individual molecular systems is a process of self-organization, whereas the mesophases of supramolecular systems are formed by self-assembly and self-organization. Herein we show 1) deformable molecular shapes and topologies of supermolecular and self-assembled supramolecular systems; 2) surface recognition processes of superstructures; and 3) that the transmission of those structures and their amplification can lead to unusual mesomorphic behavior where conventional continuum theory is not suitable for their description.

Supermolecular liquid crystals

In this article we review recent work on the development and study of the properties of self-organising supermolecular and supramolecular materials, that are in size comparable to small proteins. We take the concept of the creation of dendritic liquid crystals, and apply it to the creation of new materials with single identifiable entities, so that they are monodisperse or are single compounds. We show how functionality can be in-built into such materials so that self-organising functional systems can be created.

Molecular complexity and the control of self-organising processes

In this article we investigate the complexity of the molecular architectures of liquid crystals based on rod-like mesogens. Starting from simple monomeric systems founded on fluoroterphenyls, we first examine the effects of aromatic core structure on mesophase formation from the viewpoint of allowable polar interactions, and then we model these interactions as a function of terminal aliphatic chain length. By incorporating a functional group at the end of one, or both, of the aliphatic chains we study the effects caused by intermolecular interfacial interactions in lamellar phases, and in particular the formation of synclinic or anticlinic modifications. We then develop these ideas with respect to dimers, trimers, tetramers, etc. We show, for dendritic systems, that at a certain level of molecular complexity the local mesogenic interactions become irrelevant, and it is gross molecular shape that determines mesophase stability. The outcome of these studies is to link the complexity of the molecular interactions at the nanoscale level, which lead to the creation of the various liquid-crystalline polymorphs, with the formation of mesophases that are dependent on complex shape dependencies for mesoscopic supermolecular architectures.

A liquid-crystalline silsesquioxane dendrimer exhibiting chiral nematic and columnar mesophases

A hexadecamer, first-generation, octasilsesquioxane liquid-crystalline dendrimer was synthesized by a platinum-catalyzed hydrosilylation reaction of the parent first-generation vinyl octasilsesquioxane dendrimer with a modified, laterally substituted mesogen. The structure and purity of the octasilsesquioxane substrate was confirmed by 1H, 13C, and 29 Si NMR spectroscopy, microanalysis, and size exclusion chromatography (SEC). The mesogenic substrate was found to exhibit only a chiral nematic phase, whereas the resulting hexadecamer dendrimer displays enantiotropic chiral nematic, disordered hexagonal columnar, and disordered rectangular columnar phases, with a glass transition below room temperature. The lateral or side-on attachment of the mesogen to the dendritic core was found to be a key design feature in the formation of the mesophases.

Supermolecular liquid crystal dendrimers based on the octasilsesquioxane core

Liquid crystalline dendrimers are proving to be a fascinating class of materials that possess unusual physical properties. The self-assembling process involved in the formation of the mesomorphic state apparently deforms the spherical disposition of the supermolecular dendrimers to give rod-like entities that pack together to yield disordered Smectic A and smectic C mesophases.

An optically-active liquid-crystalline hexa-adduct of [60]fullerene which displays supramolecular helical organization

Polyaddition of mesogenic moieties to C60 were found to yield chiral supermolecular nanoparticles which exhibit iridescent helical chiral nematic phases.

Polyhedral liquid crystal silsesquioxanes

Synthetic routes towards monodisperse polyhedral liquid-crystalline silsesquioxanes are described. The liquid-crystalline phase behaviour of these structurally uniform materials, with various silsesquioxane core and mesogenic side-chain structures, were analysed and compared with those of related oligomeric and polymeric materials. Common features of the solid-state behaviour of these organic/inorganic hybrids, which form self-organized structures in their soft-matter states, were identified. Copyright

Liquid crystals with restricted molecular topologies: supermolecules and supramolecular assemblies

The term liquid crystal is often associated with fascinating compounds that exhibit unusual melting or solubilisation properties, however, it also represents a unique collection of mesophases that exist between the solid state and the amorphous liquid. As such, this unique state of matter can be accessed by a wide variety of materials from low molar mass to polymeric systems. In this Feature Article we describe some recent studies concerning the liquid-crystalline behaviour of ‘in-between’ materials that have discrete molecular structures, and which are oligomeric but not low molar mass or polymeric systems. Thus, these materials could be described as having supermolecular architectures. We examine some of their mesophase properties and their abilities to form supramolecular assemblies. The development of liquid crystals that have large molecular structures or consist of large scale assemblies is one step towards creating novel self-organising systems which are of a similar dimension to certain biological materials, such as proteins.

Diastereoisomerically Pure Fulleropyrrolidines as Chiral Platforms for the Design of Optically Active Liquid Crystals

Incorporation of [60]fullerene (C(60)) within self-organizing systems is conceptually challenging but allows us to obtain materials which combine the characteristics (anisotropy, organization) of condensed mesophases with the properties of C(60) (photo- and electrochemical activity). Here, we report on the synthesis, characterization, and liquid-crystalline properties of four optically active fullerodendrimers, which are chiral at the point of conjunction between the fullerene scaffold and the mesogenic moieties. Thus, the novelty of this study is to take advantage of the asymmetric carbon atom created during the 1,3-dipolar cycloaddition reaction on C(60) in order to induce mesoscopic chirality in the materials. Four diastereoisomeric fulleropyrrolidines ((R,S)-1, (R,R)-1, (S,R)-1, and (S,S)-1) were synthesized and associated with a second-generation nematic (N) dendron to give fullerodendrimers ((R,S)-2, (R,R)-2, (S,R)-2, and (S,S)-2) which display chiral nematic (N*) phases. The absolute configurations of the stereogenic centers were determined by X-ray crystallography, 1D and 2D NMR experiments, and circular dichroism (CD) spectroscopy. The liquid-crystalline properties of the fullerodendrimers were studied by polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The fulleropyrrolidine derivatives 2 exhibit supramolecular helicoidal organizations with a right-handed helix for the (R,S)-2 and (R,R)-2 diastereoisomers and a left-handed helix for the (S,R)-2 and (S,S)-2 diastereoisomers. This result suggests that the self-organization of such supermolecular materials can be controlled at the molecular level by the introduction of only one chiral center.

Reversible long range network formation in gold nanoparticle - nematic liquid crystal composites

Nanoparticles (NPs) are dispersed into liquid crystals (LCs) to create ordered NP assemblies and thereby modify the LC and NP properties. Although low NP concentrations are normally used to avoid aggregation, high concentrations can lead to new organization through coupling of the interparticle attractive forces with the LC elastic properties. Gold nanoparticles (AuNPs) with mesogenic coatings, tailored to be highly miscible in the liquid phase of n-alkyl-cyanobiphenyl LCs, form reversible micron-scale networks on cooling at the clearing point by enrichment of the NPs at the nematic-isotropic liquid interfaces. The network topology and LC director field orientation are controlled by the cooling rate, surface alignment, film thickness, AuNP concentration and ligand shell composition. Thin film networks consisted of branches and circular areas of LC enriched in AuNPs. Nucleating nematic droplets evolve into homeotropic alignment of the host nematic matrix, accompanied by birefringent disclination lines and loops. Thick film AuNP networks in LCs form complex structures with stable radial director configurations in small domains and Schlieren domains elsewhere. Controlled formation of networks via the use of LC phase transitions offers an additional approach to produce quasi-periodic NP assemblies that are both long range and reversible in nature.