Synthesis and characterization of non‐symmetric chiral dimers

A new class of cholesterol‐based non‐symmetric dimers have been synthesized and characterized. They comprise O‐alkylated cinnamic acid and pro‐mesogenic cholesterol segments interlinked covalently through a methylene spacer varying in its length and parity. All the dimers and some of the key precursors have been studied for their phase behaviour. All the intermediates show mesomorphism. Importantly, the thermal properties of dimers are found to be critically dependent on the parity of the flexible spacer. The dimers with odd‐parity spacer display chiral nematic and/or twist grain boundary phases. In contrast, the dimers with even‐parity spacer are either crystalline or exhibit metastable chiral nematic and/or twist grain boundary phases with the exception of one compound for which two unknown mesophases have been observed. The odd–even effect was found to be indistinct for selective reflection wavelengths of the chiral nematic phase. For some dimers, a variation in the pitch of the chiral nematic phase as a function of temperature was observed. Cyclic voltammetry experiments revealed the electrochemical properties of a representative liquid crystal dimer.     

Novel liquid crystals and dopants (for induced ferroelectricity) possessing a chiral 2-oxetanone unit. A comparison with corresponding dioxolanones

The mesomorphic properties, induced spontaneous polarizations, and response times of a new type of liquid crystal and ferroelectric liquid crystal mixtures containing di- or trisubstituted 2-oxetanones as a chiral unit are described. By heating above 140°C oxetanones, bearing a three core mesogenic part, suffer cycloelimination of carbon dioxide yielding liquid-crystalline olefines with nematic, smectic C and higher ordered smectic phases, which are best suited as host materials for induced S_c* phases using the corresponding oxetanones as dopants. Compared with 1,3-dioxolan-4-ones recently synthesized and investigated, the oxetanones show minor mesomorphism.     

Ferroelectric liquid crystals and dopants containing the chiral thiirane unit. A comparison with analogous oxiranes

The synthesis and mesomorphic properties of a new type of ferroelectric liquid crystals and dopants (for induced ferroelectric phases) containing cis or trans disub-stituted thiiranes as the chiral unit are described. A comparison of the thiiranes with the corresponding oxiranes is given. In the thiirane series the tendency to exhibit mesophases is less pronounced. S*_C phases occur for three aromatic cores in the mesogenic part of the molecule. The temperature ranges of the liquid crystal phases are broader and the transition temperatures are higher in the trans compared with cis configured thiiranes and oxiranes. Cis configured thiiranes exhibit a higher spontaneous polarization than the corresponding trans compounds.     

Influence of hydrogen bonding on phase abundance in ferroelectric liquid crystals

The synthesis and characterization of five hydrogen-bonded ferroelectric liquid crystal complexes (HBFLCs) prepared from mesogenic p-n-alkoxy benzoic acids and non-mesogenic propionic/butyric acids with different chiral centres are reported. Complementary intermolecular hydrogen bonding is confirmed through IR study. HBFLCs are found to exhibit chiral nematic (N*), smectic C* (SmC*) and smectic G* (monotropic) phases in their cooling profiles during polarizing thermal microscopy and differential scanning calorimetry. Phase coexistence regions are observed above the IN* transition. The chiral nematic to smectic C* transition is found to be of first order. The temperature variation of spontaneous polarization exhibited by these HBFLC complexes in their SmC* phase is presented. The effect of non-covalent interaction imparted by the soft hydrogen bonding in these LC complexes on enhanced or induced thermal stability of tilted LC phases is discussed.     

Synthesis and mesomorphic behaviour of novel chiral nematic side chain liquid crystalline polysiloxanes

A series of novel thermotropic side chain liquid crystalline polymers was synthesized by grafting copolymerization of a mesogenic monomer, 4-allyloxybenzoyl-4′?-(4-n-alkylbenzoyl)–p-benzenediol bisate and a chiral monomer, menthyl undecylenate. The mesogenic monomers exhibited nematic threadlike textures during heating and cooling. The polymers showed thermotropic liquid crystalline properties with a broad mesomorphic region over a range of 100°C. The polymers exhibited a cholesteric mesophase with a colourful Grand-Jean texture when the content of chiral units was greater than 15?mol?%; the others exhibited nematic threadlike textures. All of the polymers were thermally stable over 300°C, and most were laevorotatory as the chiral monomer.     

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.     

具有氟烷基边链和手性中心液晶化合物的合成及液晶性研究

本文设计合成了十个含氟烷基边链和手性中心的液晶化合物，并通过DSC和偏光显微镜对它们的液晶性进行了研究。其中二环系液晶化合物不显示液晶相或仅显示单边近晶A相。三环系液晶化合物中较长的氟烷基边链有利于近晶相的形成，且当液晶核另一端的烷氧基链的长度适中时，在氟烷基边链和液晶核之间具有手性中心的液晶分子显示了手性近晶C相和其它液晶相。     

A novel class of materials for ferroelectric liquid crystals containing siloxy chain end groups

Novel liquid crystals containing a siloxy chain as an end tail group instead of an alkyl chain were synthesized. The substitution effects were studied for ferroelectric liquid crystal materials. It was found that the temperature range for the chiral smectic C phase was reduced and shifted to lower temperature in comparison with the analogous alkyl chain derivatives. The crystallinity of the siloxy chain derivatives decreased and cholesteric phases were not observed. The influence of siloxy chains on ferroelectric liquid crystal properties, especially spontaneous polarizations and tilt angles, also greatly depended upon the mesogenic group structure. The X-ray diffraction results showed that the end tail group occupied a larger thickness in the chiral smectic C layer for the siloxy chain derivative than that for the alkyl chain derivative.     

New dimeric liquid crystals with chiral flexible spacers

In order to obtain chiral compounds of low molecular weight expected to exhibit mesogenic and ferroelectric properties, the asymmetric carbon was introduced into the central part of the molecule. The series consists of nine chiral dimers in which the chiral flexible spacer derives from S-2-chlorosuccinic acid. The mesomorphic properties of the compounds were studied and all of them were found to be mesogenic. From n = 6 onwards the mesophases were chiral smectic C. Some analogous compounds derived from S-2-chloroglutaric acid were also prepared; these were not found to be mesogenic. The thermal instability of the 2-chlorosuccinic acid derivatives prevented the study of their ferroelectric properties. We attempted to perform this study for several mixtures involving 3-methyladipic acid derivatives and both 2-chlorosuccinic and 2-chloroglutaric acids derivatives. All the mixtures studied show chiral smectic C phases. The mixtures containing 2-chlorosuccinoyl derivatives show thermal instability; however those containing 2-chloroglutaroyl derivatives are stable, but do not show significant values of spontaneous polarization.     

New isothiocyanato liquid crystals containing thieno[3,2-b]thiophene central core

New isothiocyanato liquid crystal (LC) materials based on thieno[3,2-b]thiophene core have been synthesised and characterised. Their mesomorphic and physical properties were evaluated. The results indicate that most of the new compounds exhibit only enantiotropic smectic A phases with high clearing points, and the difluorinated compounds (A3 and A6), in particular, also display a stable nematic phase with low melting points. Compared with the corresponding biphenyl and thiophene-phenyl analogues, these thieno[3,2-b]thiophene-based compounds possess increased birefringence (the highest value found to be 0.40) and mesogenic phase intervals. These new thieno[3,2-b]thiophene-based LCs exhibit a high birefringence and a high clearing point and are of potential use as components of high birefringence mixtures.     

Synthesis and mesomorphic properties of chiral liquid crystal dimers derived from azobenzene and substituted naphthol

Symmetrical liquid crystal dimers bis{2-alkyl-(S)-(+)-2-(6-[4-4′-decyloxyphenylazo)-benzoyloxy]-2-naphthyl)propinate} have conveniently been designed, synthesised and their mesomorphic properties were investigated. These dimers possess two identical mesogenic units with each of them attached to the terminal end of a flexible spacer (–C _n H_2n ; where n = 6–10). The respective mesogenic unit was made up from a naphthyl ring attached to the alkyloxylated azobenzene via the ester (COO–) bond. The chiral moiety attached to the 6-position of the naphthyl ring possesses a terminal carboxylic group COO– which linked up the mesogenic unit and the spacer. The thermal and optical properties for the ultimate compounds at different temperature were studied explicitly with the aids of differential scanning calorimetry and polarising optical microscopy. All dimers exhibit unambiguously the monotropic nematic and smectic A phases. The homeotropic alignment of the molecules within the anisotropic region was further confirmed through the application of homogeneously treated cell.     

Some novel liquid crystalline derivatives of 4-carboxyphthalimide

A series of novel esters of 4-carboxyphthalimides has been prepared. Differential scanning calorimetry and polarizing optical microscopy studies reveal that N-substituted 4-cholesteryloxycarboxyphthalimides exhibit a chiral nematic (cholesteric) phase in the range 120-205°C. Unexpectedly, esters containing well-known mesogenic groups such as 4-cyano- or 4-methoxy-phenyl do not form liquid crystal phases.     

Mesogenic properties and the effect of 1,2,4-trisubstitution on the central benzene nucleus of a three-ring mesogen

Four mesogenic homologous series have been synthesized by fixing a rigid 4-substituted phenylazo group to a resorcinol moiety. In series I and II, one phenolic -OH group is esterified by 4-n-alkoxybenzoyl groups; in series III and IV both the phenolic -OH groups are esterified by 4-n-alkoxybenzoyl groups. All the homologues of series III and IV exhibit low melting smectic C phases. Monoesters of series I and II, having free lateral hydroxy group with strong hydrogen bonding, exhibit high temperature nematic phases. The effect of different substituents on mesomorphic properties is discussed. One homologue of series III was doped with a chiral dopant and its spontaneous polarization evaluated.     

New antiferroelectric liquid crystalline materials containing a keto group and two lactate groups

New chiral liquid crystalline materials containing a keto group and two lactate units attached to the mesogenic core have been synthesized and studied. All the new compounds show the paraelectric SmA, ferroelectric SmC* and antiferroelectric SmC*_A phases each over rather broad temperature ranges. The properties of the phases were characterized by DSC, electro-optical and dielectric studies. The spontaneous polarization, tilt angle, helical pitch, and switching times were determined for the polar phases.     

Small angle X-ray scattering study of chiral side chain liquid crystalline polymers in 5CB and 8CB solvents

Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P_4M and P_11M) and their mixtures (2?wt?%) in the low molar mass nematogenics 4′-n-pentyl-4-cyanobiphenyl (5CB) and 4′-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A₁ phases were observed for the polyacrylates with four (P_4M) and eleven (P_11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P_4M exhibits an injected smectic phase, whereas P_11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected.     

Ferroelectric liquid crystalline polymers based on mesogens with halogen-containing terminal groups

A new series of halogen-containing side chain ferroelectric liquid crystal polymers was synthesized. Mesophases were characterized by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and molecular simulation. The behaviour of the liquid crystalline phase was investigated with variation of chiral centres, spacer units and grafted ratios. It was found that the thermal stability and temperature range of the chiral smectic C phase decreased with increasing length of the oligo-oxyethylene spacer, and decreasing mesogenic group content. The bulky substituent attached to the chiral centre reduces molecular packing in smectic liquid crystal phases, which disturbs the orientation of the side chain liquid crystal polymer. Furthermore, the influence of molecular structure on electrooptical properties of FLCPs has been studied by broad band dielectric spectroscopy (from 0.1 to 1 ×10 6 Hz).     

Optically-active comb-branch liquid crystalline polymers having the chiral center in the flexible spacer unit. III. Copolymers of (S)-2-(4′-cyano-4-biphenylyloxy)-1-methylethoxy ethyl methacrylate and di[6-(4-methoxy-4′-oxybiphenyl) hexyl]-2-methylene butane-1,4-dioate

A new series of copolymers with optically-active liquid crystalline side chain units has been synthesized from the comonomers (S)-2-[2-(4′-cyano-4-biphenylyloxy-1-methylethoxy]ethyl methacrylate ( 1 ) and di[6-(4-methoxy-4′-oxybiphenyl)hexyl]-2-methylene butane-1,4-dioate ( 4 ). Chiral nematic phases were exhibited by two members of the series, rich in monomer 1 , while a smectic phase was exhibited in copolymers rich in 4 . While it was thought possible that ordered chiral liquid crystalline phases may be induced by copolymerizing chiral mesogenic monomers with mesogenic derivatives of itaconic acid where the high side chain density encourages greater ordering in the system, no evidence of smectic C^* phases could be found in the present systems. © 1993 John Wiley & Sons, Inc.     

Functional Janus dendrimers containing carbazole with liquid crystalline,optical and electrochemical properties

ABSTRACT

Novel liquid crystalline Janus dendrimers that combine a mesogenic block and an electroactive block have been synthesised. The mesogenic block is based on two third-generation Percec-type dendrons bearing six or eight terminal dodecyloxy alkyl chains, whereas the electroactive blocks are formed by one or two carbazole units. The liquid crystal behaviour was investigated by polarised-light optical microscopy, differential scanning microscopy and X-ray diffraction. The Janus dendrimers with one electroactive unit exhibited cubic or columnar liquid crystal phases, whereas the Janus dendrimers with two electroactive units did not show liquid crystalline behaviour. The UV-vis absorption and emission properties of the Janus dendrimers were investigated. The spectra suggested the existence of π-π stacking and the formation of aggregates in the solid state. Electrodeposition of the carbazole-containing dendrimers afforded semi-globular particles in which the number of electropolymerizable units and the flexible or rigid character of the linker have a decisive influence in the particle size.     

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.     

Synthesis and characterization of achiral banana‐shaped liquid crystalline molecules containing bisnaphthyl moieties

Three analogous series of symmetric banana‐shaped liquid crystalline molecules containing bisnaphthyl units were synthesized and characterized. The effects of linking groups (on the side wings) and lateral meta‐fluoro substitutions (on the middle outer rings) on the mesogenic properties were examined. The type of mesophase depends on the lengths of terminal alkoxy chains. Thus, achiral molecules with shorter flexible chains (n = 8) exhibit a rectangular columnar (B₁) phase, while analogous derivatives with longer flexible chains (n = 12) display the B₂ phase. All lateral meta‐fluoro substituted analogues (series II) possess the lowest isotropization temperatures and the narrowest mesophasic ranges of the B₁ and B₂ phases. The B₁ and B₂ phases were confirmed by X‐ray diffraction, polarizing optical microscopy (POM) and electro‐optical (EO) switching experiments. An electric field‐induced transition from an antiferroelectric (tristable) state to a ferroelectric (bistable) state was observed in the EO measurements. Spontaneous polarization (by switching current response), tilt angle of chiral domains (by POM), and transmittance–voltage measurements of the B₂ phase in related compounds have been surveyed in this study.