New bent core mesogens with exceptionally high clearing points

A homologous series of novel two-ring alkoxythiophene-containing liquid crystals has been synthesized using a recently developed cyclization methodology to construct the heterocyclic core. The new heterocyclic unit is expected to confer high negative dielectric anisotropy and high dielectric biaxiality when incorporated into suitable mesogenic structures. The materials reported here possess nematic phases, with the longer chain homologues displaying smectic mesomorphism. In all cases the clearing points are significantly enhanced when compared with other two-ring thiophene-based mesogens. The synthesis, mesomorphic behaviour and stability are discussed, and the mesomorphic behaviour is compared with the analogous 4-alkoxy-4'-cyanobiphenyl derivatives synthesized by Gray.     

Synthesis and mesomorphic properties of mesogens containing 1,1,2,2-tetrahydroperfluorodecaoxy terminal chains

One series of two-ring and two series of three-ring mesogens with 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-heptadecafluorodecaoxy terminal chains containing tolane or biphenyl units have been synthesized. Their mesomorphic properties were observed and measured by optical polarizing microscopy and differential scanning calorimetry. The three-ring mesogens only exhibit a wide smectic C phase when the length of their hydrocarbon chains is intermediate.     

Solid-phase combinatorial syntheses of mesomorphic 4-alkoxyphenyl 4-alkoxybenzoylaminobenzoates

Eighteen two-ring and 100 three-ring benzenoid amides were synthesised using a solid-phase combinatorial method involving acylation or benzoylation and palladium(0)-catalysed carbonylation of a secondary amine, obtained by the reductive amination of 4-iodoaniline and a backbone amide linker. The purity of the products obtained was high enough for investigation of their thermal properties. All the three-ring derivatives were shown to be mesomorphic, but the two-ring derivatives were not. The mesomorphic behaviour and the transition temperatures of the three-ring derivatives were virtually identical to those of samples obtained by liquid-phase synthesis and purified by column chromatography and recrystallisation.     

Nematogenic and smectogenic liquid crystals from new heterocyclic isoflavone derivatives:Synthesis,characterization and X-ray diffraction studies

A new series of liquid crystals comprising eight heterocyclic isoflavone esters,7-alkanoyloxy-3-[4’-(3-methylbutyloxyphenyl)]- 4H-1-benzopyran-4-ones exhibiting enantiotropic nematic(N) and smectic C(SmC) phases were synthesized and investigated.The mesomorphic properties of all derivatives were investigated by means of differential calorimetry and polarized optical microscopy. Wide angle X-ray diffraction technique was employed to investigate the molecular packing associated with the intermolecular interaction as well as the correlation between the thermal behaviour of these compounds with their anisotropy properties within a mesophase.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

New 1,3-dioxan-type ionic liquid crystal compounds having a large dielectric constant

New pyridinium-type thermotropic ionic liquid crystal materials having a 1,3-dioxan ring in its central core: N-substituted-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides (6) were synthesized. The mesomorphic behaviour of these compounds and dielectric constant perpendicular to the molecular axis were measured. The principal features of these compounds are that they exhibit a smectic A phase over a wide temperature range including room temperature - for example 6f: g -9 SmA 181 I (°C)-and they have a large dielectric constant perpendicular to the molecular axis.     

Synthesis and mesomorphic properties of some bisegmented compounds derived from 4,4'-disubstituted biphenyl

New unsymmetrical disubstituted biphenyls have been synthesized. Two different chains (one hydrocarbon and the other fluorinated) are linked through an acetamide bond to 4-methoxy-4'-hydroxybiphenyl. Their mesomorphic properties have been characterized by light microscopy and by differential thermal analysis showing the peculiar contribution of the hydrocarbon chain and the fluorinated tail. The enhancement of the hydrocarbon moiety from one to twelve methylene groups leads to a decrease of the clearing temperatures; furthermore the enantiotropic behaviour is changed to monotropic. The variation of the fluorinated moiety generates strong effects on the transition temperatures. These mesomorphic properties are compared to those of their monocatenar perfluorinated analogues incorporating a connector of the ester or monosubstituted amide type.     

Nematic compounds and mixtures with high negative dielectric anisotropy

Syntheses, mesogenic and dielectric properties of three-ring alkylcyclohexyl, alkoxy and alkylphenyl benzoates substituted with four fluorine atoms or with two fluorine atoms and simultaneously a cyano group or two cyano groups are described. Their properties are compared with analogous three-ring phenyl tolanes. Conformation analysis and the calculation of dipole moments with the quantum chemistry method are performed to explain the experimental results. Several four fluorine-substituted two-ring tolanes and biphenyls are also prepared, which are used as a solvent decreasing viscosity, melting point and clearing point of the formulated mixtures. Examples of mixtures prepared from the esters and tolanes with high dielectric anisotropy are given. Curiosity is observed that 4?-(4-ethoxyphenyl)-4-pentyloxy-2,2?,3,3?-tetrafluorotolane is not the mesogenic compound, while analogous ester 4?-ethoxy-2,3-difluorobiphenyl-4-yl 2,3-difluoro-4-pentyloxybenzoate exhibits a broad temperature range of the nematic phase.     

Synthesis and mesomorphic properties of 4-cyanophenyl and 4-nitrophenyl 4- n -alkoxytetrafluorobiphenyl-4'-carboxylates

Four homologous series of 4-cyanophenyl and 4-nitrophenyl 4- n -alkoxytetrafluorobiphenyl-4'carboxylates have been synthesized. Their mesomorphic properties were measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between the properties and chemical structures of these compounds has been studied; the effects of terminal alkoxy chain length, polarizable end groups and the position of tetrafluorophenylene in the rigid core on mesomorphic behaviour are discussed in detail.     

The effect of hydrogen bonding, molecular shape, dipole moments, and chain length on the mesomorphism of some D-glucose and D-glucosamine derivatives

Diverse 4-substituted-benzyl 1-O-alpha- and beta-D-glucopyranosides, 4-substituted-benzyl 2-deoxy2-trifluoroacetamido-alpha- and beta-D-glucopyranosides and 4-substituted-benzyl 2-acetamido2-deoxy-alpha- and beta-D-glucopyranosides have been synthesized. They were prepared directly by a one-step alkylation of commercially available carbohydrate starting materials. The 4-substituted-benzyl 2-deoxy-2-trifluoroacetamido-alpha- and beta-D-glucopyranosides were cleaved to yield the corresponding free amines. The influence of hydrogen bonding, terminal dipole moments, molecular shape and chain length on the thermotropic mesomorphic behaviour of these benzyl-substituted carbohydrate amphiphiles is discussed. Comparisons between the mesomorphic behaviour of the new benzyl-substituted carbohydrates and that of the related phenyl-substituted carbohydrates, as well as the non-aromatic n-alkyl D-glucosides, are made. Although the new carbohydrates do not exhibit lyotropic phase behaviour, the 4-substitutedbenzyl 2-deoxy-2-trifluoroacetamido-alpha- and beta-D-glucopyranosides have been found to gel water at very low sugar concentrations.     

Synthesis of fluorinated naphthylphenylacetylenic and naphthylphenyldiacetylenic liquid crystals

Four series of fluorinated tolanes, naphthylphenylacetylenes and naphthylphenyldiacetylenes were prepared and characterized. Most of the fluorinated tolanes exhibit no mesomorphic behaviour. However, most of the fluorinated naphthylphenylacetylenes and naphthylphenyldiacetylenes form enantiotropic nematic phases. The dielectric anisotropy and birefringence of some selected compounds were measured by a guest-host method. All of those studied show relatively high dielectric anisotropy and birefringence.     

Liquid crystalline polybutadiene diols with chiral thiol side-chain units

New chiral thiols with three aromatic rings and the azo group in the molecular core have been synthesized and grafted on polybutadiene diols backbone. The resulting diols possess the OH end groups promising for preparation of the liquid crystal ordered networks. The mesomorphic and physical properties of the synthesized thiols and resulting diols have been studied by polarizing optical microscopy, differential scanning calorimetry, X-ray studies and dielectric spectroscopy. The resulting side-chain diols show the liquid crystalline behaviour. The effects of side-chain structure of the thiols and density of grafting on the polybutadiene diols has been studied and discussed.     

Liquid crystalline paracyclophane derivatives

Various paracyclophane derivatives incorporating 4,4'-biphenyl, 2,5-diphenyl-1,3,4-thiadiazole, phenyl benzoate and 2,6-disubstituted naphthyl rigid cores were synthesized and their mesomorphic behaviour was studied using polarizing microscopy, DSC and X-ray diffraction. Most of these macrocyclic compounds possess liquid crystalline properties with unexpectedly high clearing temperatures compared to those of conventional calamitic mesogens. In this way, the coupling of two appropriate rigid units using flexible chains to form a macrocycle constitutes a new and powerful approach towards mesophase induction and stabilization. The types of mesophase formed by these macrocycles do not depend only on the nature of the bridging chains, but also strongly on the structure of the rigid aromatic system. The smectic A phase and the E phase are formed by polyetherbiphenylophanes. Poly-ethercyclophanes incorporating the 2,5-diphenylthiadiazole rigid core form nematic and smectic C phases. The nematic phase is the only mesophase when the rigid core is the phenyl benzoate unit. No mesomorphic properties could be detected for macrocycles which featured either the benzyl phenyl ether moiety or the 2,6-disubstituted naphthalene unit in their constitution.     

Non-symmetric calamitic liquid crystal dimers containing troponoid and benzenoid units

Five types of non-symmetric calamitic dimers were synthesized to investigate the effect of the core structure and length of the spacer on mesomorphic properties. Two non-symmetric dimers containing a troponoid and benzenoid unit showed smectic A and C phases whereas the corresponding benzenoid dimers showed no mesophase. Non-symmetric dimers with a three-ring system showed smectic A and C phases with higher transition temperatures than the two-ring system. We propose packing models for these non-symmetric dimers by considering the direction of the dipole moments of the ring structures and microsegregation between the polar units and the non-polar chains.     

Synthesis, characterization and mesomorphic properties of Ag(I) and Pd(II) complexes containing the pyridyl and tetrazoyl rings: crystal structure of [C30H46N10Ag • ClO4]

The synthesis of new monodentate heterocyclic ligands 5-(4-pyridyl)-2-alkyltetrazole (L¹a,b) and 4-[5-(2-alkyltetrazole)]aryl-4'-pyridinecarboxylate (L²a,b,c) containing two or three aromatic or heterocyclic rings (tetrazole, pyridine and benzene) and preparation of their corresponding silver(I) and palladium(II) complexes (Ia,b,c and IIa,b,c) are described. The thermal behaviour of the ligands and complexes was characterized by polarizing optical microscopy. The ligands and the complexes Ia,b,c and IIc showed no liquid crystalline phase. The complexes IIa,b showed mesomorphic behaviour, exhibiting smectic A enantiotropic mesomorphism X-ray diffraction measurements for complex Ia showed monodentate coordination of N-pyridine, and no coordination on the nitrogen atoms of the tetrazole ring.     

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 1 H and 13 C 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.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

Mesomorphism of complexed 2,6-disubstituted pyridine ligands: crystal and molecular structure of two bent-core pyridines

Bent-core, polycatenar pyridines are reported, synthesized via a Siegrist coupling of di- and tri-alkoxybenzylidene fragments with 2,6-dimethylpyridine. None of these pyridines was liquid crystalline. Isomeric materials based on a 3,5-disubstituted pyridine core were mesomorphic in hydrogen-bonded complexes, whereas these new materials were not. However, the new pyridines did form mesomorphic metal complexes with silver salts. Some insight into this behaviour is gleaned from single crystal structure determinations of the model systems, 3,5-bis(3',4'-dimethoxystyryl)pyridine and 2,6-bis(3',4'-dimethoxystyryl)pyridine.     

Synthesis,characterization and thermal properties of 4,4'-bis(4-n-alkoxybenzoyloxy)benzylideneanilines and bis(4-benzylidene-4'-n-alkoxyaniline) terephthalates

Series of novel thermotropic liquid crystalline compounds which serve as models for mesogenic diols have been synthesized. The mesogen molecules are constructed from 1,4-disubstituted benzene rings linked through ester and azomethine units. Based on the number of phenyl rings, a tetrad or pentad classification has been assigned to the core molecule. The structures of these compounds were established by FTIR and NMR spectroscopy, while the mesomorphic behaviour has been confirmed using polarizing optical microscopy and DSC. All the compounds of the homologues series are mesomorphic in nature. The lower members of the series are nematogenic, while the higher members exhibit both nematic and smectic character, with high melting behaviour and wide mesophase ranges.