Influence of halogen substituent on the mesomorphic properties of five-ring banana-shaped molecules with azobenzene wings

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B₆ phases, a B₄-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B₆-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-induced trans–cis-isomerisation on the prepared compounds.     

Liquid crystalline properties of 4-isopropyl- and 4-isopropoxy- N -(4- n -alkoxysalicylidene)anilines

New homologous series of 4-isopropyl- and 4-isopropoxy- N -(4- n -alkoxysalicylidene)anilines were synthesized. The phase transitions of these homologues were determined by differential scanning calorimetry and polarizing optical microscopy. No enantiotropic liquid crystalline phases were observed for homologues having a terminal isopropyl group; as the terminal alkoxyl chain is lengthened, smectic C and nematic phases occur monotropically. The members with an isopropoxy substituent exhibit an enantiotropic nematic phase as the terminal alkoxyl chain is lengthened; the octyloxy member exhibits a monotropic smectic C phase as well as an enantiotropic nematic state.     

Bifurcational analysis of the isotropic-discotic nematic phase transition in the presence of extensional flow

A bifurcational analysis is performed on a version of Doi's equation of nematodynamics that describes the non-equilibrium isotropic-discotic nematic phase transition in the presence of steady uniaxial extensional flow. The disc-like molecular geometry and the degenerate extensional flow-induced orientation are shown to be the source of a complex bifurcation and multistability behaviour involving two physically equivalent biaxial nematic phases, one uniaxial nematic phase and one uniaxial paranematic phase. Depending on the temperature and the extension rate, the isotropic-discotic nematic transition, involving the two biaxial nematic phases and the uniaxial paranematic phase, may be continuous (2nd order), discontinuous (1st order), or it may exhibit a tricritical non-equilibrium phase transition point. A validation procedure on the validity of the predictions is implemented. The predictions presented here find practical applications in the industrial spinning of mesophase carbon fibres, and also provide new results that increase the present fundamental understanding of the rheology of discotic nematic liquid crystals.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Phase diagrams of cholesteric liquid crystals obtained with a generalized Landau-de Gennes theory

Phase diagrams of chiral nematic liquid crystals are studied within the framework of a generalized Landau-Ginzburg-de Gennes theory. Using the parametrization of Grebel, Hornreich, and Shtrikman for the tensor order parameter Q, all relevant elastic terms are included for the helicoidal phase and the blue phases of chiral nematic liquid crystals up to fourth order in Q and its gradient ∂Q. The influence of the additional elastic terms on the phase diagrams of the chiral nematic phases is then investigated. The theory correctly describes the variation of the pitch with temperature and the induced biaxiality of the cholesteric phase. The results resolve the discrepancies encountered by Hornreich and Shtrikman in the comparison of experiment and theory. New features in the topology of the phase diagrams of blue phases, like re-entrant phase transitions, are predicted.     

Phase diagrams of cholesteric liquid crystals obtained with a generalized Landau-de Gennes theory

Abstract

Phase diagrams of chiral nematic liquid crystals are studied within the framework of a generalized Landau-Ginzburg-de Gennes theory. Using the parametrization of Grebel, Hornreich, and Shtrikman for the tensor order parameter Q, all relevant elastic terms are included for the helicoidal phase and the blue phases of chiral nematic liquid crystals up to fourth order in Q and its gradient ?Q. The influence of the additional elastic terms on the phase diagrams of the chiral nematic phases is then investigated. The theory correctly describes the variation of the pitch with temperature and the induced biaxiality of the cholesteric phase. The results resolve the discrepancies encountered by Hornreich and Shtrikman in the comparison of experiment and theory. New features in the topology of the phase diagrams of blue phases, like re-entrant phase transitions, are predicted.     

Synthesis,characterisation and mesomorphic behaviours of non-symmetrically substituted 1,2,4- and 1,3,4-oxadiazole derivatives

Two series of non-symmetrically substituted bent-core mesogens derived from the central 3,5 and 2,5 cores of the 1,2,4- and 1,3,4-oxadiazole derivatives, respectively, containing ether and ester linkage have been synthesised by several straightforward synthetic procedures, and their mesomorphic behaviour was studied by optical polarising microscopy (OPM) and differential scanning calorimetry (DSC). The last five homologues of the 1,2,4-oxadiazole series exhibit a monotropic nematic phase on cooling, while the 1,3,4-oxadiazole derivatives do not show any liquid crystalline properties but rather a crystal to isotropic transition is observed by OPM. We show that the bent angles of the 1,2,4- and 1,3,4-oxadiazole rings are crucial in observing the liquid crystal (LC) phases formed, and also that the length of the alkyl substituent is observed to have a significant effect on the nature of the LC phases.     

New lyotropic nematics of double chain surfactants

The phase behaviour of N,N-dialkyl-N,N-dimethyl ammonium bromide double chain surfactants was investigated by polarization microscopy, ²H-N.M.R. spectroscopy, and surface and magnetic field alignment in order to find new lyotropic nematic systems. Decanol was utilized to cause a rod to disc transition in the nematic phase. The sign of the anisotropy of the diamagnetic susceptibility Δx was inverted by substituting the bromide counter-ion with the benzenesulphonate ion. The phase behaviour was modified within a wide range by varying the length of the two alkyl chains. Using these tools we were able to prepare any of the four kinds of uniaxial nematic phases: rod-like and disc-like aggregates with both negative and positive anisotropy of the diamagnetic susceptibility. Several new binary, ternary and quaternary systems of these double chain surfactants and some experiments are described to classify the different nematic phases.     

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.     

Preliminary communication - The effect of a substituent in the central ring on the liquid crystalline properties of di(4-alkoxycarbonylphenyl) terephthalates

Four series of di(4-alkoxycarbonylphenyl) terephthalates with different lateral substituents in the central ring and lengths of the terminal alkyl groups were synthesized. The influence of the lateral substituent on thermodynamical parameters of the phase transitions and elastic properties of the nematic phases (ratio of bend elastic constant to diamagnetic anisotropy K3/Delta chi) were studied by polarizing optical microscopy, differential scanning calorimetry and observations of the Freedericksz transition. It was found that increasing the alkyl chain length and the substituent size in the sequence H, Cl, Br, NO2 results in reduction in stability of the crystal and (as frequently observed in other cases) the liquid crystal phases. The changes in the bend elastic constant and the melting enthalpy caused by the substituent variation are correlated with each other, a fact that is explained by the dependence of short-range lamellar packing on size and polarity of substituent.     

Cybotactic nematic phases with wide ranges in photoresponsive polycatenars

ABSTRACT

We report on the synthesis and phase behaviour of new light-responsive polycatenars. The new materials represent tetracatenar molecules with four terminal chains which are substituted at 2,3,4-position in one benzene ring at one terminus and at the other terminus only one chain is present. The liquid-crystalline behaviour of the prepared materials was characterised by differential scanning calorimetry (DSC), polarised optical microscope (POM) and X-ray diffraction (XRD). We investigated the effect of changing the type of one terminal chain, while keeping the remaining three chains fixed on the mesomorphism of these materials. All of the tetracatenars exhibit nematic and smectic C phases. By investigating the nematic phase with XRD it was found that it represents nematic phase with cybotactic clusters of the SmC type (N_CybC). Moreover, these nematic phases exist over wide temperature ranges for all compounds. Additionally, the photosensitivity of these polycatenars was studied upon light irradiation.     

Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.     

Synthesis and properties of some novel high birefringence phenylacetylene liquid crystal materials with lateral substituents

We have synthesized and investigated new 3-ring phenylacetylene liquid crystals with high birefringence values (Δn) to improve the nematic temperature range while retaining a high optical anisotropy. In the case of modifying the terminal ring, the introduction of branched alkoxy chains, fluorine or methyl groups shifted the nematic phase to lower temperatures. In order to minimize the influence of the substituent on Δn, the incorporation of lateral methyl groups was chosen as the most profitable method for obtaining wide and low temperature nematic phases. With these results in mind, we studied the effect of the position of the methyl group on the physical properties. The most effective methyl position for improving the nematic temperature range was on the central ring. From the standpoint of obtaining high Δn values and low viscosity, the central ring was also the best position.     

Does Topology Dictate the Incidence of the Twist‐Bend Phase? Insights Gained from Novel Unsymmetrical Bimesogens

We prepared a significant number of unsymmetrical liquid‐crystalline dimers that exhibit the twist‐bend nematic phase; a state of matter that exhibits spontaneous breaking of mirror symmetry and, for some materials, a microsecond electrooptic response. A number of novel unsymmetrical bimesogens were synthesized and in comparing their thermal behaviour to previous literature examples, we have uncovered an unexpected relationship between the thermal stability of the nematic and N_TB phases. This relationship demonstrates that molecular shape dictates the incidence of this fascinating phase of matter and leads us to speculate as to the existence of “twist‐bend nematic phases” on length scales beyond those of the molecule.     

Synthesis and characterisation of thermomesogenic polysiloxanes with 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione units in the main chain

The synthesis of the first two liquid crystalline polysiloxanes bearing 3,6-diaryl-2,5-dihydropyrrolo-[3,4-c]pyrrole-1,4-dione units in their main chain are described. Investigations on their thermotropic phase behaviour by polarising microscopy reveal nematic or smectic enantiotropic phases, respectively.     

Unusual thermal behaviour of mesogens containing an N,N-dialkylaminomethylene lateral substituent

An N,N-dialkylaminomethylene lateral substituent can be used to reduce the melting temperatures of nematogens having a long rigid core. However, compared with a single lateral alkoxy chain, this lateral substituent does not enhance the thermodynamic stability of the mesophase. DSC curves and optical microscopy show that after melting, a new solid appears in the liquid crystal phase or in the isotropic phase of these compounds. NMR spectra of the nematic phase and of the solid phase show that a thermal reaction releases dialkylamine into the medium and produces a disubstituted 2 H -indazole in rather good yield. Further heating of each individual mixture leads to a second wide range enantiotropic nematic phase. The purified disubstituted 2 H-indazole has a larger nematic range and the nematic phase is stable at high temperature, indicating that the 2 H -indazole motif can be used to build new mesogenic structures. The temperature dependence of the 13C chemical shifts of one of the initial compounds was obtained. The results indicate that the two chains on the nitrogen atom are equivalent and more or less folded back in the same way along the mesogenic core.     

Phase transfer Pd(0)/Cu(I) catalysed polymerization reactions 7. Synthesis and thermotropic behaviour of 1,4-bis[2-(3',3'-difluoro-4',4'-di-n-alkyloxyphenyl)-ethynyl]benzene dimers

The symmetrically difluorinated aryl-acetylene dimers, 1,4-bis[2-(3',3'-difluoro-4',4'-di-n-alkyloxyphenyl)ethynyl]benzenes (n = 7-12), were prepared by a one pot phase transfer. Pd(0)/Cu(I) catalysed, three step coupling of 1,4-diiodobenzene with 2-methyl-3-butyn-2-ol and the appropriate 1-bromo-3-fluoro-(4-n-alkyloxy)benzene. All members of the series display enantiotropic nematic and smectic C phases as well as an additional smectic mesophase in the sequence C-S-S_C-N-I. The textures observed by polarized optical microscopy strongly support identification of the second smectic phase as a S_E mesophase. The S_C temperature window increases as the length of the n-alkyloxy substituent increases, whereas the stability of the nematic and S_E temperature windows decrease. In these fluorinated dimers, destabilization of mesomorphic behaviour by lateral fluorine substitution is compensated by stabilization due to the large length to breadth ratio of the mesogen, such that melting and crystallization are depressed more than isotropization when compared to the non-fluorinated dimers.     

Phase transitions in novel disulphide-bridged alkoxycyanobiphenyl dimers

The synthesis, characterization and mesomorphic properties of the first examples of novel disulphide-bridged alkoxycyanobiphenyl dimers are reported. The thermal behaviour of these mesogens was investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffractometry. The dimers with a shorter spacer exhibit only the nematic phase while dimers with a longer spacer display nematic as well as smectic phases. X-ray diffraction experiments reveal the intercalated structure of the SmA phase of these dimers and the presence of short range SmA-like order in the N phase (cybotactic nematic) of all the compounds, except the one with the shortest spacer.     

Complex formation in mixtures of 4,4'-disubstituted diphenyldiacetylenes with cyanobiphenyls

We investigated 1:1 mixtures of a variety of 4,4'-disubstituted diphenyldiacetylenes (PTTP series) and cyano-containing mesogens for possible complex formation. Complexes were formed when alkyl/alkoxy cyanobiphenyls (CB series) were mixed with alkyl/alkoxy PTTP homologues. We found complexes having a melting temperature higher than that for either component, and complexes with lower melting temperatures. A fluoro substituent on the PTTP gave an enhanced nematic phase and a cyclohexane ring in place of a benzene ring yielded an enhanced smectic A phase, but a cyano on the PTTP gave poor liquid crystalline properties. Phase diagrams were constructed for PTTP-24/5-CB and PTTP-24/80-CB mixtures. Both complexes formed had melting temperatures higher than those for either component. These complexes were recrystallized and shown by X-ray studies to be 1:1 complexes. The PTTP-24/5-CB complex showed only a monotropic nematic phase, but the PTTP-24/80-CB complex showed induced smectic A and hexatic B phases in addition to an enantiotropic nematic phase.