The elusive thermotropic biaxial nematic phase in rigid bent-core molecules

The biaxial nematic liquid crystalline phase was predicted several decades ago. Several vigorous attempts to find it in various systems resulted in mis-identifications. The results of X-ray diffraction and optical texture studies of the phases exhibited by rigid bent-core molecules derived from 2,5-bis-(p-hydroxyphenyl)-l,3,4-oxadiazole reveal that the biaxial nematic phase is formed by three compounds of this type. X-ray diffraction studies reveal that the nematic phase of these compounds has the achiral symmetry D_2h, in which the overall long axes of the molecules are oriented parallel to each other to define the major axis of the biaxial phase. The apex of the bent-cores defines the minor axis of this phase along which the planes containing the bent-cores of neighboring molecules are oriented parallel to each other.     

光致变色液晶高分子研究（Ⅲ）──含胆甾介晶基元和偶氮苯光色基元侧链共聚硅氧烷的液晶行为

用热台偏光显微镜、Ｘ射线衍射和ＤＳＣ研究了含胆甾介晶基元和偶氮苯光色基元侧链共聚硅氧烷（ＰＳＩ）的液晶性．将非介晶基元并入液晶均聚物，所得共聚物的液晶态类型不变，仍显示近晶型织构，在ＰＳＩ共聚物中，保持液晶性的含介晶基元单体的最低极限组成为６０ｍｏｌ％．在液晶性存在的范围内，共聚物的清亮点由１３０℃升至１７０℃，液晶共聚物的热稳定性随非介晶组分含量的增加而增强．     

Coumarin based emissive rod shaped new schiff base mesogens and their zinc(II) complexes: synthesis,photophysical, mesomorphism,gelation and DFT studies

New homologous series of coumarin Schiff base derived from 6-aminocoumarin and their zinc(II) complexes have been synthesised. The spectroscopic characterisations, photophysical properties, phase transition temperature, characterisation of phase and gelation behaviour are reported. The ligand is non-mesogenic at lower 4-n-alkoxy chain length (n = 4) and mesogenic for longer chains (n > 4). For small chains (n = 5, 7, 8), ligand displays monotropic nematic or nematic-smectic A phase sequences, whereas longer homologues (n = 12, 14, 16 and 18) display only enantiotropic smectic A phase. The ligands and their Zn(II) complexes are fluorescent in nature. Interestingly, ligands exhibit gelation property only in polar solvents, whereas Zn(II) complexes discourage gelation. The effect of 4-n-alkoxy chain length on the gelation properties was also discussed. Density functional theory calculations show broad agreement with observed molecular conformation, dipole moment, molecular orbitals and polarisability of the coumarin Schiff base molecules and their Zn(II) complexes.     

Bent-shaped liquid crystals based on 4-substituted 3-hydroxybenzoic acid central core – Part II

The synthesis and mesomorphic properties of new series of bent-shaped liquid crystals are presented. The properties of the materials were tuned by the character of the lateral substituent in position 4 of the central aromatic ring and length of terminal alkyl chains. Further modification of physical properties was achieved by the introduction of an additional ester unit in the terminal alkyl chain. The physical properties of the studied materials were determined by differential scanning calorimetry, polarising optical microscopy and for selected compounds by x-ray diffraction. The influence of structural changes as well as the orientation of ester linkages (in comparison with previously described materials) is discussed. The addition of the fifth ester unit into the structure of the studied compounds leads to significant changes in their mesomorphic behaviour.     

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACT

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.     

Preparation and properties of laterally multifluorinated benzoxazole-based nematic mesogens

Series of laterally multifluorinated heterocyclic compounds, 2-(2?,3-difluoro-4?-alkoxy-1,1?-biphenyl-4-yl)-benzoxazole derivatives (nPF(2)PF(3)Bx), are prepared and characterised. They mainly display enantiotropic nematic mesophases with wider mesophase ranges of 12–107°C (heating process) and 22–134°C (cooling process) than the corresponding analogues. The enhanced nematic mesophase stability is achieved via slightly increasing inter-ring twist angle with inter-ring lateral fluorine substitute in biphenyl unit, as well as through improving the molecular polarity with multifluorine substitutes. Meanwhile, two inter-ring lateral fluorine atoms lead to a decrease in melting/clearing points and a wide nematic mesophase range, which makes it possible for heterocyclic mesogens nPF(2)PF(3)Bx to use in nematic liquid crystal display mixtures.     

Enantiotropic Nematics From Cross‐Like 1,2,4,5‐Tetrakis(4′‐alkyl‐4‐ethynylbiphenyl)benzenes and Their Biaxiality Studies

The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross‐shaped compound 3 , which showed enantiomeric nematic phase behavior. This cross‐like core structure could alternatively be viewed as two fused V‐shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface‐induced biaxiality was evidenced by optical studies. Cluster‐size analysis within the nematic phase of compound 3 revealed the formation of meta‐cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small‐angle 2D X‐ray diffraction patterns of magnetic‐field‐aligned samples indicated that the nematic phase was composed of small smectic C‐like clusters with the tilting of molecules within the clusters. The wide‐temperature‐range enantiomeric nematic phase of cross‐like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent‐rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher‐order liquid‐crystalline phases and crystallization, for future biaxial investigations.     

Viscometry of Nematic Liquid Crystals

Viscometric experiments have played a significant part in establishing continuum theory for nematic liquid crystals. Such theory supports the hypothesis that observed non-Newtonian behaviour stems from competition between the aligning influences of flow and solid surfaces, and as a consequence predicts rather unusual scaling for the apparent viscosity. This paper first describes such scaling and its subsequent experimental confirmation emphasising the full implications of the latter. The relevance of the theory thus established, there follows a simple analysis of alignment in shear flow which leads to conditions on material coefficients necessary to ensure consistency with observations. Our discussion turns next to an account of solutions exhibiting non-Newtonian behaviour, and also mention of a recent stability analysis which attempts to discriminate between the different solutions that are possible. The final section considers oscillatory shear flow and presents an analysis of an instability which can occur in certain nematics.     

Synthesis and Mesomorphic Characterisation of Chiral Homologues

Abstract

Chiral liquid crystals have attracted considerable interest as they exhibit a good variety of modulated phases. We have synthesised a homologous series viz., 4-(4^!-n-alkoxy benzoyloxy) benzylidene-4^!!?1-(s)-methyl propoxy anilines, incorporating a terminal chiral centre, inorder to obtain better understanding of the relationship between molecular structure and appearance of SmC* phase in the molecules. It is observed that, in the present series the lower members upto butyl are pure nematogens, while pentyl to hexadecyl derivatives exhibit classical smectic as well as nematic mesophases. An additional smectic C* phase is observed in the middle octyl to dodecyl homologues. The homologues have been characterised by IR, NMR and DSC. Their mesomorphic properties have been compared with structurally related homologous series.