Liquid crystal dimers and oligomers

A liquid crystal dimer is composed of molecules containing two mesogenic groups linked via a flexible spacer. Initial interest in these materials stemmed from their ability to act as model compounds for semi-flexible main chain liquid crystal polymers but are now of fundamental interest in their own right because their behaviour is significantly different to that of conventional low molar mass liquid crystals. Recently research has begun to focus also on higher monodisperse oligomers such as trimers and tetramers consisting of molecules containing either three or four mesogenic units, respectively, linked via flexible spacers. In this review the most recent developments in our understanding of structure–property relationships in liquid crystal dimers and higher oligomers is discussed.     

Liquid crystal dimers and higher oligomers: between monomers and polymers

The underlying theme of this Critical Review is the relationship between molecular structure and liquid crystalline behaviour in a class of materials referred to as liquid crystal oligomers. For the purposes of this review, a liquid crystal oligomer will be defined as consisting of molecules composed of semi-rigid mesogenic units connected via flexible spacers. Much of the review will be devoted to structure-property relationships in the simplest oligomers, namely dimers, in which just two mesogenic units are connected by a single spacer. Along the way we will see how this molecular architecture has been exploited to address issues in a range of quite different areas and has given rise to potential applications for these materials. On the whole, only compounds in which the mesogenic units are linked essentially in a linear fashion will be considered while structures such as liquid crystal dendrimers and tetrapodes fall outside the scope of this review. The review will be of interest not only to scientists working directly in this area but in particular to those interested in understanding the relationships between structure and properties in polymers, and those designing materials for new applications.     

Nanostructured assemblies of liquid-crystalline supermolecules: from display to medicine

ABSTRACT

Liquid crystal oligomers have a supermolecular structure in which two or more mesogenic units are interconnected through flexible spacers. They can form various molecular packing structures in their liquid-crystalline phases. We present flexible LC oligomers stabilising optically isotropic phases, i.e. blue phases (BPs) and chiral conglomerate phases. Hysteresis free switching, high contrast and wide viewing angle are obtainable in the amorphous BPIII without surface treatment. Spontaneous mirror symmetry breaking in layered phases of achiral flexible linear trimers produces periodic nanostructured surfaces. Then we demonstrate the effects of liquid-crystallinity on anticancer activities. Biological systems have links with liquid crystallinity. We found a molecular assembly of mesogenic molecules possessing an active site as a novel therapeutic approach against solid cancer cells.     

Molecular design of flexible liquid crystal oligomers stabilising the chiral frustrated phases

ABSTRACT

Chirality induces structural frustration in liquid crystal systems, producing various kinds of chiral frustrated phases, for example, twist grain boundary (TGB) phases, blue phases (BPs) and dark conglomerate (DC) phases. Almost all molecules exhibiting these frustrated phases have a rigid shape. Especially, a bent–core unit is regarded as a key structure for BPs and DC phases. This paper describes that some flexible liquid crystal oligomers being far from a rigid bent–core molecule stabilise these phases. The LC oligomers have a supermolecular structure in which mesogenic units are connected via flexible spacers. By designing intermolecular interactions, they can exhibit various molecular packing structures in the liquid-crystalline phases as follows: chiral dimers inducing TGB phases, U-shaped and T-shaped oligomers stabilising BPs and achiral liquid crystal trimers exhibiting DC phases. I discuss how the designed liquid crystal oligomers produce the chiral frustrated phases.     

Theory of main chain nematic polymers with spacers of varying degree of flexibility

Main chain liquid crystal polymers are modelled as either worms or jointed rods. In reality they are composed of mesogenic units (rods) linked by spacers with varying degrees of flexibility. We present a molecular model to describe non-homogeneous nematic polymers. The model takes account of molecular parameters, such as the lengths of the mesogenic group and the spacer units, and the interactions between them. The spacers are found to have an order differing from the mesogenic units. If the spacer is not very long and thus in effect is inflexible, one end of the spacer can retain to some extent the orientation of the other end, allowing orientational correlation between spacers mediated by the intermediate mesogenic unit. This is important in giving the chain a global rod-like behaviour as the nematic field becomes strong or the temperature low. The nematic order of the two components (mesogens and spacers), the nematic-isotropic transition as well as the latent entropy are examined. Furthermore, the anisotropic conformations of the polymers are investigated, which show either rod-like or random walk behaviour. Comparison of our results with experiment is found to be satisfactory.     

Synthesis and characterization of liquid crystalline polymethacrylates,polyacrylates, and polysiloxanes containing 4-methoxy-4′-hydroxy-α-methylstilbene-based mesogenic groups

The synthesis and characterization of polymethacrylates and polyacrylates containing 4-methoxy-4′-hydroxy-α-methylstilbene side groups attached either directly or through flexible spacers containing eleven, eight, six, three, and respectively two methylenic units, and of the polysiloxanes containing the same mesogenic group connected through flexible spacers containing eleven, eight, six, and respectively three methylenic units are described. All polymers exhibit thermotropic liquid crystallinity. The nature of the mesophase is determined by the spacer length. However, the nature of the polymer backbone determines the thermal stability of the mesophase. That is, for the same spacer length and similar polymer molecular weight, the most flexible polymer backbone leads to the highest isotropization temperature.     

Preliminary communication Bi-mesogenic organosiloxane liquid crystal materials exhibiting antiferroelectric phases

The first antiferroelectric low molar mass organosiloxane liquid crystal materials are presented. The molecules are bi-mesogens consisting of two chiral mesogenic units attached, via methylene spacers, symmetrically to a siloxane group. Three series of compounds with different laterally substituted halogens on the phenyl ring nearest to the chiral centre have been studied. For siloxane groups containing three silicon atoms all of the materials exhibit antiferroelectric phases over a 60 C temperature range. The tilt angle in the SmC* A is, within experimental uncertainties, independent of temperature and close to 45 degrees for the three series with a spontaneous polarization greater than 110nC cm -2.     

Liquid crystal tetramers: influence of molecular shape on liquid crystal behaviour

The synthesis and characterization of two homologous series of tetramers in which four mesogenic units are linked via three alkyl spacers are reported. Both series contain a hexamethylene central spacer while the length of the two outer spacers, n, is varied from three to 12 methylene units. The two series differ only in the substitution pattern around the inner two mesogenic units. The series in which one mesogenic unit is attached to the central spacer in the 4-position while the other is connected at the 3-position is referred to as the n-p6m-n series, while in the n-m6m-n series both inner units are attached in the 3-position. All the members of the n-p6m-n series exhibited a nematic phase while no liquid crystallinity was observed for the n-m6m-n series. The thermal behaviour of this series is compared with that of the n-p6p-n series and also with that of the corresponding series containing a pentamethylene central spacer. The trends observed are interpreted in terms of the average molecular shapes of these tetramers.     

Odd liquid crystalline dimers can be linear

The crystal and molecular structures of two liquid crystalline symmetric dimers containing the mesogenic group α,α′‐dimethylbenzalazine and flexible polymethylenic spacers of different parity (even and odd) are reported. They show nematic phases with strong odd–even effects. The unusual result we have found is that the odd dimer crystallizes adopting a molecular conformation in which the two mesogenic groups are aligned with respect to each other, as for the even dimer. The crystal packing of the two dimers is also analogous.     

Transimination: An efficacious reaction for the synthesis of macrocycles

A new family of main-chain liquid crystal polymers containing azomethine units have been synthesized and characterized. Our attention has been focused on their strong tendency to generate macrocyclic dimers (AB)² constituted with two mesogenic rods and siloxane spacers. In favourable conditions (diluted solution, acid catalysis) macrocycles may be rapidly formed and purely obtained by fractionation. Synthetic aspects and main thermotropic properties are discussed. In spite of their large size some macrocycles have been obtained as single crystals and then characterized by X-ray diffraction.     

Synthesis and characterization of main-chain thermotropic aromatic polyazomethines with siloxane spacers

New main-chain liquid crystal copolymers have been synthesized in the polyazomethine series from two polycondensation routes (polyhydrosilylation and polyimination). The mesogenic units are separated with siloxane spacers. The obtained copolymers have been characterized by size-exclusion chromatography and NMR and FT-IR spectroscopies; their mesomorphic behavior has been investigated by polarizing microscopy, differential scanning calorimetry and RX diffraction. Moreover, when the polycondensation reactions are carried out in solution or when the copolymers are kept in a solvent, we observed the formation of macrocyclic dimers, which have been isolated and characterized.     

Trimeric and tetrameric liquid crystalline thiadiazole derivatives

Novel liquid crystalline 2-phenyl-1,3,4-thiadiazole based oligomers with three and four rigid aromatic units linked by a flexible central unit have been investigated by polarizing microscopy. The synthesis of these compounds and the influence of structural variations on the mesomorphic properties are described. The combination of suitable mesogenic moieties with appropriate central units leads to oligomers which exhibit S_c phases.     

Liquid crystalline compounds having a tribranched structure: substituted malonic esters consisting of two Schiff's base units and a cholesteryloxyalkyl substituent

New tribranched thermotropic liquid crystal compounds were synthesized and their liquid crystalline properties studied by differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. The compounds are the bis-{10-[4-(4-alkylphenyliminomethynyl)phenoxy]decyl} 2-[6-(cholesteryloxy)hexyl]malonates and the corresponding alkoxy derivatives. These compounds contain three mesogenic units, two identical Schiff's base type mesogens and one cholesteryl either moiety, interconnected in a tribranched structure via spacers. The cholesterly moiety is attached to the malonic acid core through an oxyhexamethylene spacer while the two Schiff's-base moieties are attached through oxydecamethylene spacers. The terminal alkyl group of the Schiff's base unit is either a butyl or decyl group, and the alkoxy terminal group is either a butoxy or decyloxy chain. All the compounds form only an enantiotropic smectic phase, most probably of the smectic C type. The larger spacings determined by small angle X-ray diffraction range from 3.3 to 4.1 nm, which are much shorter than the end-to-end distance (5.9-7.4 nm) of the molecules estimated using molecular models assuming an all trans extended conformation for all the alkyl spacers.     

Effect of lateral substituents on the mesomorphic properties of side-chain liquid crystalline polysiloxanes containing 4-[(S)-2-methyl-1-butoxy]phenyl 4-(alkenyloxy)benzoate side groups

The synthesis of side-chain liquid crystalline polysiloxanes containing either 4-[(S)-2-methyl-1-butoxy]phenyl 4-(alkenyloxy)benzoate or laterally fluoro-, chloro-, bromo-, and methoxy-substituted 4-[(S)-2-methyl-1-butoxy]phenyl 4-(alkenyloxy)benzoate mesogenic side groups is presented. The mesomorphic properties of the synthesized polymers have been characterized by optical polarizing microscopy, differential scanning calorimetry, and X-ray diffraction measurements. The effects of spacer length and lateral substituent on the mesomorphic properties of the obtained polymers are examined. The five polymers which contain three methylene units in the spacers show no mesophase, while the five polymers which contain eleven methylene units in the spacer display smectic mesomorphism. Among the other fifteen polymers which contain respectively four, five, or six methylene units in the spacers, those with small fluoro and chloro substituents reveal respectively an S_A phase, while those with bulky bromo and methoxy substituents show no liquid crystalline behavior. The experimental results demonstrate that introducing a bulky lateral substituent into the mesogenic core of a polymer depresses the tendency to form a mesophase. Furthermore, the technique of thermally stimulated current has been used to study the dipolar relaxation mechanisms in a side-chain liquid crystalline polysiloxane. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2793–2800, 1997     

某些含二维液晶基元的热致液晶高分子的向列态纹影织构

发现某些含二维液晶基元的热致液晶高分子容易产生反向壁织构与含有高强度向错点(|S|大于1)的向列态纹影织构。这类液晶高分子的织构与形态学特点与含一维棒状液晶基元的其他液晶高分子不同。     

A Nanohelicoidal Nematic Liquid Crystal Formed by a Non‐Linear Duplexed Hexamer

The twist‐bend modulated nematic liquid‐crystal phase exhibits formation of a nanometre‐scale helical pitch in a fluid and spontaneous breaking of mirror symmetry, leading to a quasi‐fluid state composed of chiral domains despite being composed of achiral materials. This phase was only observed for materials with two or more mesogenic units, the manner of attachment between which is always linear. Non‐linear oligomers with a H‐shaped hexamesogen are now found to exhibit both nematic and twist‐bend modulated nematic phases. This shatters the assumption that a linear sequence of mesogenic units is a prerequisite for this phase, and points to this state of matter being exhibited by a wider range of self‐assembling structures than was previously envisaged. These results support the double helix model of the TB phase as opposed to the simple heliconical model. This new class of materials could act as low‐molecular‐weight surrogates for cross‐linked liquid‐crystalline elastomers.     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

Liquid crystal polymers. IV. Thermotropic polyesters with flexible spacers in the main chain

Two closely related series of polyesters that contain mesogenic units interconnected by flexible spacers along the main chain were prepared and characterized for their liquid crystal properties. All of these polymers showed theotropic behavior, which was examined by DSC, hot-stage microscopy on a polarizing microscope, small-angle light and wide-angle x-ray scattering methods, and visual observation of stir-opalescence of the polymer melts. The effect of the length of the flexible spacer and the nature of the substituent, which is on the central aromatic ring of the mesogenic unit, on the stability and molecular order of the mesophase for each of the polymers was investigated and the results are discussed on the basis of the thermodynamic data obtained.     

SYNTHESIS AND PROPERTIES OF NEW MESOGEN-JACKETED LIQUID CRYSTALLINE POLYMERS*

Some new mesogen-jacketed liquid crystalline polymers (MJLCP) with polymer backbones, spacers, andmesogenic units of different structures were synthesized by radical polymerization. The mesomorphic behavior of thesepolymers was examined using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Theirliquid crystallinity is influenced by the variation of polymer backbone, spacer, mesogenic unit and its terminal groups. Theresults show that 1) a more flexible polymer main-chain is more favorable to the formation of a liquid crystal phase, while 2)a flexible spacer will decrease the "Jacket Effect" and the liquid crystallinity and 3) a subtle modification of the terminalgroups on the mesogenic unit may also have a significant influence on properties of the polymers.