The synthesis and properties of fluoro-substituted analogues of 4-butyl-4′-[(4-butylphenyl)ethynyl]biphenyls

The synthesis of 4-butyl-4′-[(4-butyl-2,6-difluorophenyl)ethynyl]biphenyl and its higher fluorinated analogues is presented and discussed. Correlations between molecular structure and mesomorphic properties for presented compounds as well as other known from the literature analogues have been drawn. The dielectric study of four synthesised compounds and their mixtures are presented and discussed. Trifluoro-substituted analogues are trade off between low dielectric anisotropy of difluorinated compounds and lower clearing points of tetrafluorinated ones.     

λ-Shaped azomesogens with polar chloro- and nitro-substituents

A new homologous series of λ-shaped mesogenic azo esters with polar chloro- and nitro- substituents has been synthesised by attaching a rigid 2-chloro-4-nitrophenylazo group to a resorcinol moiety by a central azo linkage. Both phenolic hydroxyl groups are esterified by 4-n-alkoxybenzoyl groups. All the compounds synthesised have been characterised using a combination of elemental analysis and standard spectroscopic methods. The mesomorphic properties of the compounds have been determined by polarising optical microscopy and differential scanning calorimetry. The lower members of the series are non-mesogenic, the middle members exhibit a nematic mesophase, and higher members exhibit a smectic A mesophase. The effect of polar chloro-, nitro-, and long lateral aromatic substituents, on mesomorphism is discussed.     

Effect of the linking unit on the twist-bend nematic phase in liquid crystal dimers: a comparative study of two homologous series of methylene- and ether-linked dimers

ABSTRACT

Two series of bimesogens with phenyl benzoate mesogenic units were prepared: one series having a heptamethylene spacer and the other a pentamethylene spacer with two ether-linking groups. These materials were prepared to provide experimental backing to the widely held hypothesis that methylene-linked bimesogens are more likely to exhibit the twist-bend nematic mesophase than their ether-linked counterparts. Several of the methylene-linked materials exhibited nematic and N_TB mesophases, whereas the analogous ether-linked materials gave only nematic phases albeit with significantly higher clearing points. Virtual N–N_TB transition temperatures for both methylene- and ether-linked bimesogens were extrapolated by constructing binary phase diagrams with the well-studied twist-bend material CB9CB. Contrary to our expectations these virtual transition temperatures were in most cases higher for the ether-linked bimesogens than in the analogous methylene compounds, this runs counter to reported theories and hypotheses that the incorporation of ether-linking groups should serve to destabilise the N_TB phase.     

Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to ?60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.     

Synthesis and mesomorphism of tetrafluoro substituted 4-cyano oligophenyls

ABSTRACT

The synthesis and characterisation of new liquid crystal (LC) compounds based on terphenyl core are described. New family of dielectrically positive compounds presents alternative molecular approach to the conventional LC design. Correlations between molecular structure and mesomorphic properties for compounds being cyano terminated analogues as well as other known from the literature have been drawn. Compounds are characterised by ¹H NMR spectroscopy and mass spectrometry (electron ionisation) analysis. They show monotropic nematic behaviour in broad temperature range, confirmed by a polarising thermomicroscopy, differential scanning calorimetry. Detailed synthetic procedures are attached. Synthesised compounds stand as promising components of medium to highly birefringent liquid crystalline mixtures.     

Synthesis,characterisation and liquid crystalline properties of some Schiff base-ester central linkage involving 2, 6- disubstituted naphthalene ring system

Four new mesogenic homologous series, each containing a 6-alkoxy 2-naphthoic acid and Schiff base-ester as central linkage, have been synthesised by esterification of 4-{[(4-hydroxyphenyl) imino] methyl} phenyl 4-propoxy benzoate, 4-{[(4-hydroxyphenyl) imino] methyl} phenyl 4- (pentyloxy) benzoate, 4-{[(4-hydroxyphenyl)imino]methyl}-2-methoxyphenyl 4- nitrobenzoate and 4-{[(4-hydroxyphenyl)imino]methyl}-2-methoxyphenyl 4- chlorobenzoate with different 6-alkoxy 2-naphthoic acid to give Series-A, -B, -C and -D, respectively. These compounds were characterised by elemental analysis, Fourier transform infrared, ¹H nuclear magnetic resonance, ultraviolet-visible and mass spectral studies. Their mesomorphic behaviour was studied by polarising optical microscope (POM) with a heating stage. POM data were compared with differential scanning calorimetry thermograms. In Series-A and -B all compounds exhibit mesomorphism. Series-A compounds exhibit a enantiotropic nematic mesophase, while a smectic A mesophase is observed from the butoxy derivative and persists up to the last member of the homologou series. Series-B compounds also exhibit the enantiotropic nematic mesophase, while the smectic A mesophase is observed from the ethoxy derivative and persists up to the last member of the homologou series. The mesomorphic properties of both series are compared with each other and the other structurally related Series-C and –D compounds. In Series-C and -D all compounds exhibit the only nematic mesophase; no smectic mesophase is observed even for higher members of the homologous.

The aim of the research was to synthesise and characterise novel liquid crystalline compounds containing 2,6-disubstituted naphthalene and to study their mesomorphic properties.     

Biphenylyl-1,2,4-oxadiazole based liquid crystals – synthesis,mesomorphism, effect of lateral monofluorination

ABSTRACT

Aiming at developing a new type of materials for functional applications, three series of new compounds were prepared, 3-(biphenylyl)-1,2,4-oxadiazoles, Series I; laterally mono-fluorinated 3-(biphenylyl)-1,2,4-oxadiazoles, Series II; and 5-(biphenylyl)-1,2,4-oxadiazoles, Series III. Liquid crystalline properties of these compounds were studied using differential scanning calorimetry and polarizing optical microscopy. Ten of the thirteen compounds in Series I are mesogenic forming either N or SmA phases depending on the length-to-width molecular ratio. Mesogens of the positional isomers, Series III are both N and SmA. Monofluorination of the biphenylyl group at either the central or the terminal phenylene rings, Series III, resulted in the formation of nematic mesogens with low phase transition temperatures and broad nematic phase range.     

Ethylenedioxythiophene as a novel central unit for bent-core liquid crystals

We have discovered that ethylenedioxythiophene (EDOT) can be used as a central unit for the synthesis of bent-core liquid crystals (BC LCs). Two series of EDOT-based BC LCs are prepared via Sonogashira coupling reaction. The mesophase behaviour of all the compounds was characterised using a combination of polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. EDOT-based three-ring compounds were found to be nonliquid crystalline, while all the four derivatives of five-ring series, including a branched alkoxy chain derivative, display enantiotropic nematic phase over wide temperature range. The bent angle of these compounds is about 153°, which falls in between typical rod-like and banana liquid crystals. The transition temperature of branched alkoxy chain compound is lower than straight alkoxy chain compounds. The detailed XRD investigations of all the mesogens corroborate the presence of nematic phase.     

Synthesis and properties of hydroxy tail-terminated cyanobiphenyl liquid crystals

Two series of new hydroxy tail-terminated cyanobiphenyl compounds are described. The 4′-ω-hydroxyalkynyl-4-cyanobiphenyl compounds (1a–1g) were synthesised as the key intermediates to the 4′-?ω-hydroxyalkyl-4-cyanobiphenyl compounds (2a–2g) obtained upon reduction of the acetylenes. Many of these ω-hydroxyalkynyl and ω-hydroxyalkyl cyanobiphenyl compounds exhibit nematic mesophases and they also serve as precursors for the synthesis of other interesting materials. Using density functional theory, we calculate the dipole moment of all relevant ω-hydroxyalkynyl and ω-hydroxyalkyl cyanobiphenyl compounds and find a correlation between the calculated dipole moments and measured crystalline to nematic or isotropic liquid transition temperatures.     

Formation of hexagonal columnar phases by heterocyclic pyrimidine derivatives

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds derived from pyrimidine as core group are reported. These compounds were prepared by condensation reactions of appropriate acetophenones and benzonitriles in the presence of trifluoromethanesulphonic anhydride. They were characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis, and their phase transitions characterized and studied by thermal analysis and polarizating microscopy. These compounds exhibit hexagonal columnar (Col_h) phases, as expected for disk-like molecules; the formation of columnar phases was found to be dependent on the numbers of alkoxy side chains attached. For those compounds having the same numbers of flexible side chains attached, the one with a preferred unsymmetric structure exhibited better mesomorphic properties. The observed improved mesomorphic behaviour of these compounds over other similar all-carbon heterocyclic compounds is attributed to the greater polarization of nitrogen atoms in the core ring.     

New azobenzene containing bent-core liquid crystals based on disubstituted resorcinol

New bent-core molecules with 4,6-dichlororesorcinol or 4-chloro-2-methylresorcinol as the central unit, and azobenzene with different alkyloxy chain length as side arms were synthesised. The mesophase behaviour of the new compounds was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical measurements. It is found that 4,6-dichlororesorcinol is more conducive towards mesomorphism than 4-chloro-2-methylresorcinol. The liquid crystalline properties of all of the prepared compounds are greatly affected by the lateral substitution on the outer ring. 4,6-Dichlororesorcinol-based compounds without lateral substitution show nematic phases with cybotactic cluster of the SmC-type (N_CybC). Moreover, depending on the chain length, the nematic phase appears as enantiotropic phase for the shortest homologue and as monotropic phase for the higher homologues.     

Banana-shaped mesogens: effect of lateral substituents on seven-ring esters containing a biphenyl moiety

The synthesis and characterization of five homologous series of symmetrical compounds composed of banana-shaped molecules containing a biphenyl moiety are reported. All these compounds are non-Schiff's bases and are esters. The effects of lateral substituents such as fluoro, methyl and ethyl in the side arms of these molecules are examined. These substituents have a strong influence in reducing the clearing temperatures. Banana phases such as B1, B2 and B6 were observed in the above series of compounds. The mesophases were characterized by a combination of polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic studies.     

New isothiocyanatotolane liquid crystals with terminal but-3-enyl substitute

Four kinds of new but-3-enyl-based isothiocyanate liquid crystals composed of tolane core and but-3-enyl terminal group (A1–A4) were synthesised via seven step reactions based on 2-(4-bromophenethyl)-1,3-dioxolane, and four n-butyl analogues B1–B4 as comparison structures were also prepared. The mesomorphic properties and physical properties of the compounds were investigated. Single fluorinated compounds A2 and A3 exhibit monotropic nematic phase, and they have lower melting enthalpy and higher clearing points than those of the comparison compounds B2 and B3. The non-fluoro-substituted compound A1 and difluorinated compound A4 exhibit no nematic phase. Replacement of n-butyl chain by but-3-enyl as terminal group is enabled to increase birefringence (Δn ~ 0.394–0.430) and reduce rotational viscosity. These isothiocyanatotolanes with terminal but-3-enyl substitution have potential application for high birefringence mixtures.     

Self-assembly of azo molecules to mesogenic phasmid-like materials through inter-molecular hydrogen bonding

A series of acid-functionalised new azo compounds was synthesised and characterised. The constituent molecules self-organise to form dimers through inter-molecular hydrogen bonding, resulting in phasmid-like compounds. This was once considered to be a promising molecular architecture to form biaxial nematic phases. The mesomorphic properties of these new azo compounds were studied using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. Investigations revealed that these compounds form nematic and columnar mesophases. A few more compounds were synthesised by replacing the??N=N- linkage in these compounds with??CH=N- and??COO- to study the effect of different linkages on the mesomorphic properties in such molecular systems. All were found to be liquid crystalline. The compounds with a??N=N- linkage are more conducive to mesomorphism and are thermally very stable. The effect of number of alkoxy chains on the mesomorphic properties of this system was also studied. To the best of our knowledge, at present, only a handful of phasmid-like mesogenic compounds, formed by the intermolecular hydrogen bonding, are known.     

Novel high birefringent isothiocyanates based on quaterphenyl and phenylethynyltolane molecular cores

We report the synthesis of new laterally alkyl substituted high birefringent quaterphenyl and phenylethynyltolane liquid crystals with isothiocyanate terminal group. The thermal and optical properties such as birefringence are measured and discussed based on their molecular structures. The compounds exhibit birefringence (Δn) in the range of 0.44–0.63, and are expected to be appropriate components of high birefringent nematic mixtures for various applications.     

Novel high birefringence dibenzothiophenylacetylene liquid crystals

We have designed, synthesized and evaluated dibenzothiophenylacetylene homologues as new heterocyclic molecules which were expected to have high δα values and short wavelength UV absorption spectra. The dibenzothiophenyl acetylene homologues exhibited wide nematic phase ranges and had very high extrapolated δn of over 0.6 and directly measured values of 0.5. The compounds with two dibenzothiophenyl rings were colourless, so that we have succeeded in obtaining colourless super-high δn liquid crystals. We have also analysed the contribution of the transition band to the refractive indices and the birefringence. From the analysis, the main origins of δn for compounds 1 and 2 are lower MOs than the HOMO and this results in the compatibility of large δn and lack of colour. The dibenzothiophenylacetylene compounds behaved according to our design expectations.     

Novel polymerizable bent‐shaped monomeric molecules

Novel bent‐shaped mesogens based on substituted naphthalene‐2,7‐diol, possessing a double bond at the end of terminal branches, have been synthesized and characterized. A variety of banana‐mesophases has been found, depending on the type of lateral substitution. Increasing the length of the alkyl chain leads to the appearance of the SmC_AP_A phase for two of the compounds synthesized. Under an electric field a transition into the SmC_SP_F phase takes place and a tilt angle close to 45° has been found. These materials are appropriate for the preparation of polymeric materials.     

Fourth-order nematic elasticity and modulated nematic phases: a poor man’s approach

We propose an extension of Frank-Oseen’s elastic energy for bulk nematic liquid crystals which is based on the hypothesis that the fundamental deformations allowed in nematic liquid crystals are splay, twist and bend. The extended elastic energy is a fourth-order form in the fundamental deformations. The existence of bulk spontaneous modulated or deformed nematic liquid crystal ground states is investigated. The analysis is limited to bulk nematic liquid crystals in the absence of limiting surfaces and/or external fields. The non deformed ground state is stable only when Frank-Oseen’s elastic constants are positive. In case where at least one of them is negative, the ground state becomes deformed. The analysis of the stability of the deformed states in the space of the elastic parameters allows to characterise different types of deformed nematic phases. Some of them are new nematic phases, for instance a twist – splay nematic phase is predicted. Inequalities between second-order elastic constants which govern the stability of the twist–bend, splay–bend, and splay–twist states are obtained. Their stability in respect to triple splay–bend–twist deformations is investigated.     

The influence of lateral substituents on the occurrence of a transition between two polar smectic phases with antiferroelectric properties

The synthesis and mesomorphic properties of two homologous series of bent-core (BC) compounds derived from 2,7-dihydroxynaphthalene are reported. The two series differ from one another by an electron-withdrawing or an electron-donating lateral substituent on the middle phenyl ring of the arms of the BC. Very interestingly, contrasting results are obtained in which polar phases are observed in compounds containing a chloro substituent and apolar smectic C and nematic phases are obtained when this substituent is replaced by a methyl group. The mesophases have been characterised by using a combination of polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction measurements, electro-optical and dielectric studies.     

Effect of terminal aryl rings of liquid crystal trimers on liquid crystalline properties

Liquid crystalline properties of five series of biphenyl derivatives connected with p-nitrophenyl, phenyl, 2-naphthyl, 9-phenathryl and bromo as the terminal groups via flexible spacers were studied. All the compounds exhibited nematic phase. The nematic–isotropic or isotropic–nematic transition properties resulted in an odd–even effect as the length of the spacers was varied, in which the even members exhibited higher values, for the compounds with p-nitrophenyl, phenyl, 2-naphthyl and 9-phenanthryl groups, while the transitional properties of the compounds substituted with bromo atom as the terminal group instead of the aryl groups did not show such odd–even effect. These results suggest that the aryl rings, such as p-nitrophenyl, phenyl, 2-naphthyl and 9-phenanthryl groups, play a role of mesogen while the terminal bromo atom does not. The effect of the terminal aryl rings on the liquid crystalline properties will be reported.