Self-assembly of chemically linked rod-disc mesogenic liquid crystals

A series of new molecular discs (RDn, here n is the number of carbon atoms between the rod and disc mesogens) was synthesized via the chemical attachment of six cyanobiphenyl calamitic (rod) mesogens (R) linked to the triphenyl discotic (disc) mesogen (D) with a series of six alkyl chain linkages (n = 6-12). In this study, phase structures, transitions, and liquid crystalline (LC) behavior of the RD12 compound with 12 carbon atoms in each alkyl chain linkage between the rod and disc mesogens were investigated. Differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction (WAXD), and selected area electron diffraction (SAED) allowed us to identify three ordered phases below the isotropization temperature: nematic (N) LC and K1 and K2 crystalline phases. On the basis of the structural results obtained via 2D WAXD experiments on oriented samples and SAED experiments on single crystals, the K1 crystalline unit cell was determined to be triclinic with the dimensions of a = 1.36 nm, b = 1.45 nm, c = 2.11 nm, alpha = 85 degrees, beta = 100 degrees, and gamma = 50 degrees. The K2 phase was metastable with respect to the K1 phase. It also possessed a triclinic unit cell with a = 1.40 nm, b = 1.51 nm, c = 1.92 nm, alpha = 87 degrees, beta = 117 degrees, and gamma = 62 degrees. Molecular packing models for the crystalline phases were proposed on the basis of the diffraction results. In the whole range of ordered structures, it was found that RD12 molecular discs are intercalated. Both triphenyl discotic mesogens and cyanobiphenyl calamitic mesogens are completely interdigitated.     

Synthesis and mesogenic properties of azo-dye liquid crystals

Azo-dye liquid crystals with strong amino electro-donating and moderately flexible carboxylate electron-withdrawing groups were synthesized, and their mesogenic properties were studied.     

Amphiphilic and mesogenic carbohydrates IX. Perfluoroalkyl substituted monosaccharides: a new group of amphiphilic liquid crystals

The thermal properties of twelve 'single-tailed' carbohydrate-based liquid crystal materials which have a perfluoroalkyl chain (n-C4F9, n-C6F13, n-C8F17) linked to the polar sugar head group are described. All reported amphiphiles, the 6-deoxy-6-C-perfluorohexyl-L-altrose 1, the (5R)-xylopentoses 2, 3 and 4, the 5,6-dideoxy-6-C-perfluoroalkyl-D-glucofuranoses 5 and 6, the 2,3-O-perfluoroalkylidene-D-gulopyranoses 7, 8 and 9 and the 3,4-O-perfluoroalkylidene- D-altropyranoses 10 and 11 form mesophases of the smectic A type. Even a C4F9-chain (compounds 2, 7, and 10) is long enough to cause liquid crystalline behaviour.     

Magnetic anisotropy of liquid crystals based on lanthanide mesogenic complexes

The values of magnetic anisotropy of smectic A-phases for a number of lanthanide complexes (LH)₂LM(NO₃)₂, where M=Nd, Eu, Gd, Tb, Dy, Ho, and Er, and LH is a Schiff's base), were measured. These values are two orders of magnitude larger than those normally found for diamagnetic liquid crystals and are well correlated with magnetic birefringence constants and molecular mangetic anisotropy of nomesogenic lanthanide diketonates. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 694–697, April, 1999.     

A new convenient synthesis of functionalized 2,3-dihydro-4-pyridones

A simple, efficient and diastereoselective route to 2,3,6-trisubstituted 2,3-dihydro-4-pyridones from methyl 3,5-dioxohexanoate, aryl aldehydes and ammonium acetate is described.     

Synthesis and thermotropic behaviour of liquid crystals containing tolane-based mesogenic units

Tolane-based liquid crystal materials with three different terminal groups: trifluoromethyltolanes, cyanotolanes, and nitrotolanes, and with different chain lengths of alkenyloxy groups, have been synthesized. The phase behaviour of these liquid crystal materials has been characterized by differential scanning calorimetry, optical polarized light microscopy and X-ray diffraction. The mesophase behaviour of the materials was shown to be significantly influenced by the polarity of the terminal groups. The trifluoromethyltolanes exhibit an enantiotropic smectic E mesophase and the cyanotolanes exhibit a monotropic nematic mesophase. The most polar among the three types of liquid crystal material; the nitrotolanes do not display any liquid crystalline phases, except for the homologue with an eleven methylene unit chain.     

Synthesis and thermotropic behaviour of liquid crystals containing tolane-based mesogenic units

Abstract

Tolane-based liquid crystal materials with three different terminal groups: trifluoromethyltolanes, cyanotolanes, and nitrotolanes, and with different chain lengths of alkenyloxy groups, have been synthesized. The phase behaviour of these liquid crystal materials has been characterized by differential scanning calorimetry, optical polarized light microscopy and X-ray diffraction. The mesophase behaviour of the materials was shown to be significantly influenced by the polarity of the terminal groups. The trifluoromethyltolanes exhibit an enantiotropic smectic E mesophase and the cyanotolanes exhibit a monotropic nematic mesophase. The most polar among the three types of liquid crystal material; the nitrotolanes do not display any liquid crystalline phases, except for the homologue with an eleven methylene unit chain.     

Synthesis and characterization of new ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (S_A), a chiral smectic C (S_C) and a crystal E (C_r E) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n = 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic S_A and Cr E phases.     

A new class of liquid crystals: methylene-1,4-dihydropyridines

A group of liquid crystal materials which contain the novel methylene-1,4-dihydropyridine substructure were synthesized and their mesogenic properties examined. Three main classes of liquid crystal compounds which differ in the structure of the aromatic core group (phenyl, azobenzene and diphenylacetylene) attached to the nitrogen of the 1,4-dihydropyridine group were studied. The synthesis of the methylene-1,4-dihydropyridine group was accomplished in excellent yield by a Knoevenagel condensation of a 4-pyridone intermediate with an active methylene compound. The liquid crystal materials prepared thus far which contain this methylene-1,4-dihydropyridine structure all possess broad enantiotropic smectic A phases and one example also possesses a tilted smectic C phase. These mesogens may possess useful properties such as high birefringence.     

Thermal properties of 4-alkyl-4′-cyanotolanes: a new series of liquid crystals

The thermal properties of a new series of liquid crystals, the 4-alkyl-4′-cyanotolanes, were measured. The DuPont 990 thermal analyser was used to measure the temperatures and heats of the transitions. The entropy changes in both the crystal to mesophase and mesophase to isotropic liquid transitions are found to be a function of the length of the alkyl chain on the molecules.     

Synthesis and characterisation of imidazolium-based ionic liquid crystals bearing a cholesteryl mesogenic group

A series of cholesteryl-containing imidazolium chlorides and imidazolium tetrachloroaluminates were synthesised, and the chemical structure, liquid crystalline behaviour and ionic conductivity were characterised by several technical methods. Whereas the imidazolium chlorides show chiral smectic A (SmA*) phase on heating and cooling cycles, the imidazolium tetrachloroaluminates display chiral nematic (N*) phase, which is uncommon for ionic liquid crystals (ILCs). The imidazolium chlorides display similar phase transition temperature and entropy, indicating the cholesteryl component influence predominately on the phase transition rather than the different alkyl substituent groups. The imidazolium tetrachloroaluminates show lower melting point temperatures and lower clear point temperature than the imidazolium chlorides. The mesophases exist at rather moderate temperatures. Non-mesomorphic imidazolium tetrachloroaluminate(III) salts with short alkyl substituents have been known for a long time, and the synthesised imidazolium tetrachloroaluminates are the first examples of tetrahalogenoaluminate(III)-containing ILCs. For the imidazolium tetrachloroaluminates, imidazolium cations combine loosely with AlCl₄^? ions because AlCl₄^? ions are large and occupy more space in spite of the hydrogen bond and electrostatic attraction interaction, indicating that the layer structure can be destroyed easily to form N* phase on heating.     

A new look at the reduced-gradient-following path

 The mathematical structure of the reduced-gradient-following (RGF) path introduced by Quapp et al. (1988 J. Comput. Chem. 19:1087) is reviewed and analyzed. We report two new algorithms to evaluate the RGF path. The RGF path is also compared mathematically and computationally with the gradient extremals path. An example of the evaluation of the RGF path is also reported. Received: 21 May 2001 / Accepted: 27 September 2001 / Published online: 9 January 2002     

The influence of vicinal threo-difluorination on electro-optic and mesogenic properties of propyleneoxy-linked nematic liquid crystals

A protocol for the preparation of a series of liquid crystals is presented, where the structures carry a central propyleneoxy motif carrying two vicinal threo-C–F bonds. The negative dielectric anisotropy (−Δ?) of the resultant liquid crystals has been explored. The stereoelectronic relationship between the vicinal C–F bonds was anticipated to orientate the fluorine atoms gauche to each other when the propylene chain is extended. This is shown to be the case. This orientation of the C–F bonds generates a net dipole perpendicular to the molecular axis, a prerequisite for the design of dielectrically negative liquid crystals. However the molecules adopt a conformation where one C–F bond reinforces the net molecular dipole, and the other has a counter effect, thus the introduction of the motif has an almost neutral effect on dielectric anisotropy (Δ?_virt) of these candidate liquid crystals. However, introduction of the difluoro motif raises the melting points of the liquid crystals and increases their conformational rigidity.     

New calamitic thermotropic liquid crystals of 2-hydroxypyridine ester mesogenic core: mesophase behaviour and DFT calculations

ABSTRACT

New three groups of 2-hydroxypyridine ester-based liquid crystals named, 5-[2-(4-substitutedphenyl)diazenyl]pyridin-2-yl 4?-alkoxybenzoate were synthesised. Each group differs from each other by the terminal polar substituent X (CH₃O, Cl and H) which contains five compounds with different numbers (n) of carbons in the alkoxy chain. Mesophase behaviour was investigated for the prepared homologues by differential scanning calorimetry and polarised light microscopy. Elemental analyses, FTIR and ¹HNMR spectroscopy were used for structure confirmation of the prepared compounds. Density functional theory (DFT) theoretical calculations are estimated to prove the experimental data. Stability ranges and the type of the mesophases observed for the investigated compounds were found to be mainly dependent on the length of the alkoxy chain, the polarity, as well as dipole moment and charge distribution. Moreover, the high terminal charge distribution in the case of the methoxy derivative could effect the end-to-end interactions resulting in a nematic phase rather than the ordered Smectic A phase observed in the case of Cl derivatives and the unsubstituted homologues which are non-mesomorphic. Results of the DFT discussed are found to be consistent with the present experimental investigations.     

A facile synthesis of novel tricyclic 4-pyridones

A new and efficient synthesis of tricyclic 4-pyridone analogs through the intramolecular Heck coupling cyclization was described. This reaction features mild conditions and good functional group tolerance allowing for the preparation of several novel tricyclic 4-pyridone analogs.