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.     

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.     

Molecular surface and order parameters in liquid crystals

The surface model for solute ordering in nematics, which is based on the decomposition of the orientational potential according to the contributions of surface elements, leads to a simple procedure for the calculation of both orientational properties and the cholesteric order induced by a chiral compound. However, a realistic representation of the molecular surface accessible to the solvent is required. The rolling sphere algorithm, applied to the ensemble of atomic van der Waals spheres of a molecule, provides a natural determination of such a surface, sinceit smoothes away the smallscaledetails. Thesurfaceordering model implemented with the rolling sphere smoothing of the surface is described and applied to several molecular systems. It is shown that the orientational order parameters are substantially independent of the rolling sphere radius identified with the average curvature of the solvent molecular surface. On the contrary, a sensible dependence on such a parameter emerges for the chirality order parameter, this behaviour pointing out the role of the shape of solvent molecules in the chirality recognition of solutes. A fair agreement is obtained in the comparison with experimental data.     

Towards in Silico Liquid Crystals. Realistic Transition Temperatures and Physical Properties for n‐Cyanobiphenyls via Molecular Dynamics Simulations

We study the important n‐cyanobiphenyl (with n= 4–8) series of mesogens, using modelling and molecular dynamics simulations. We are able to obtain spontaneously ordered nematics upon cooling isotropic samples of 250 molecules. By using the united‐atom force field developed herein, we show that the experimental isotropic–nematic transition temperatures are reproduced within 4 K, allowing a molecular‐level interpretation of the odd–even effect along the series. Other properties, like densities, orientational order parameters and NMR residual dipolar couplings are also reproduced well, demonstrating the feasibility of predictive in silico modelling of nematics from the molecular structure.     

Molecular mechanics modelling of structure/property relationships in liquid crystals

Molecular mechanics calculations are reported of structure/property correlations in some liquid crystals. An atom-atom intermolecular potential is used to calculate minimum energy configurations of pairs of mesogenic molecules. Observed effects of dipole-dipole association, twist sense in chiral nematics and properties of flexible nematogens are confirmed by the calculations. It is concluded that molecular mechanics can be a useful technique for predicting liquid-crystalline properties of potentially mesogenic molecules.     

A correlation between the absolute configuration of alkyl aryl sulfoxides and their helical twisting powers in nematic liquid crystals

In this paper, for the first time, a systematic experimental and theoretical analysis of the cholesteric induction due to solutes whose chirality is originated only by a single stereogenic center has been carried out. The twisting power beta of a series of alkyl aryl sulfoxides has been determined in several nematic solvents. The sign of beta, which reflects the handedness of the induced helical arrangement of the solvent molecules, correlates with the configuration of the stereogenic sulfur in the nematic solvents E7, Phase 1083, and ZLI 2359: (S)-configurated dopants induce (M)-chiral nematics. (S)-Configurated cyclic sulfoxides, which are forced to adopt a different conformation with respect to the parent acyclic compounds, induce, instead, right-handed chiral nematics. The experimental data have been interpreted in the light of the surface chirality model, which allows the calculation of beta in terms of the molecular properties of the dopant, namely, the anisotropy and helicity of its molecular surface. The calculations reliably reproduce the behavior experimentally observed. The more flexible, open-chain compounds investigated induce chiral nematics of opposite handedness in MBBA and Phase 1053: temperature-dependent experiments point out the importance of the conformation in determining the effective sign of beta. The results have been discussed in terms of different conformation populations in these latter solvents with respect to E7, Phase 1083, and ZLI 2359.     

Opto-thermal reorientation of nematics with two-fold degenerate alignment

The preferred direction of alignment of the liquid crystal molecules in nematics with two-fold degenerate alignment can be affected substantially by changing the temperature or by applying an electric field. As a result, an almost in-plane switching of the molecules occurs. Here, we report an opto-thermal reorientation effect in a nematic with two-fold degenerate alignment due to a local heating of the liquid crystal by a high power laser beam. The mechanism of this phenomenon is discussed. The opto-thermal reorientation of the molecules makes it possible to visualize the temperature distribution in the illuminated cell and some applications can be foreseen.     

On the molecular interpretation of the static dielectric properties of nematic liquid crystals

This paper shows that on the basis of the temperature dependence of the principal static permittivities of nematics epsilon (T) and epsilon (T), using the Maier-Meier equations, the following quantities can be obtained: (i) the angle between the dipole moment vector and the long axis of the mesogenic molecules, (ii) the square of the apparent molecular dipole moment mu2app(T) and (iii) the nematic order parameter S(T).     

Model of a disclination core in nematics

The core structure of a disclination line of strength 1 in uniaxial nematics is determined by using a space-dependent mean-field approach somehow based on the rod-like molecular model. It is shown that, due to distortion, biaxiality arises at all points of the defect core, except at the centre line where symmetry dictates uniaxiality; the orientational distribution of the rod-like molecules is there 'oblate', however. Although, as is well known, the configuration of a 1 defect is stable only under limiting conditions (as compared with either escaping in the axial direction or splitting into two 1/2 lines), the simple example developed here is indicative of a method which can readily be extended to more realistic, if mathematically more complex, situations such as 1/2 lines, layers close to solid boundaries, etc.     

The calculation of the elastic constants of nematics

A molecular model for nematics is proposed. Its elastic constants are calculated and the attendant approximations are discussed. The results are compared with those of existing models.     

A surface-alignable micellar nematic

The lyotropic mesophases of alkali metal salts of 4-trans-pentyl cyclohexanoic acid have been studied. The lithium salt is of particular interest in that it forms a lyotropic nematic phase, a hexagonal phase, and another columnar phase, possibly with a rectangular lattice. Unlike most nematics, this phase can assume a parallel alignment on a treated surface in much the same way as thermotropic (molecular) nematics. When the nematic dries, a columnar phase grows into it, showing a strong preferential direction of the column axes. This system should be useful in studying the nematic-columnar transition and in other investigations which require an aligned micellar nematic phase.     

Flexible Molecules with Defined Shape—Conformational Design

Chemists have for a long time considered molecules simply in terms of their constitution. The importance of molecular shape was recognized perhaps for the first time by the semio-chemists, who were interested in the interactions of fragrant substances with a receptor. Apart from the case of rigid molecules, our ideas and concepts of molecular shape simply reflect an instantaneous situation, because flexible molecules take advantage of the whole range of conformation that is available to them. Nature frequently utilizes flexible molecules whose conformational space is restricted, in other words, molecules that can adopt only a few preferred conformations. This review discusses some of the principles that nature employs in order to impart a defined shape to flexible molecules. The purposeful application of these principles then allows the conformational design of molecular skeletons. For corrigendum see DOI: 10.1002/anie.199215401 For corrigendum see DOI: 10.1002/anie.199216571     

Vitrifying star-shaped liquid crystals: synthesis and application in cholesteric polymer networks

In this paper, the formation of glass-forming reactive mesogens, that do not crystallize upon cooling, but vitrify and form supercooled LC phases, is described. These molecules exhibit broad range LC phases and enable us to carry out photopolymerization in a broad range of temperatures. From such reactive mesogens densely crosslinked networks in which the liquid crystalline order is permanently fixed are formed by photopolymerization. For this purpose eight novel low molecular mass LC materials with photopolymerizable acrylate groups have been synthesized and the detailed experimental procedures are given. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. Comparing the twin molecules which we have described before with the novel three- and four-armed stars, we found that the supercooled LC phase in the three-armed stars has a stability superior to that in both twin molecules and four-armed stars. In the three-armed star triple-4 with a suitable substituent pattern, the supercooled nematic phase is stable at room temperature for at least nine months. Photo-DSC experiments show that the final conversion after 10 min of UV-irradiation for the threearmed star molecule triple-4 is as high as for the smaller molecules twin-4 and mono-4 over the whole temperature range. Doped with a suitable chiral molecule the novel nematics formed cholesteric phases which were used to make cholesteric polymer networks by photopolymerization.     

Vitrifying star-shaped liquid crystals: synthesis and application in cholesteric polymer networks

In this paper, the formation of glass-forming reactive mesogens, that do not crystallize upon cooling, but vitrify and form supercooled LC phases, is described. These molecules exhibit broad range LC phases and enable us to carry out photopolymerization in a broad range of temperatures. From such reactive mesogens densely crosslinked networks in which the liquid crystalline order is permanently fixed are formed by photopolymerization. For this purpose eight novel low molecular mass LC materials with photopolymerizable acrylate groups have been synthesized and the detailed experimental procedures are given. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. Comparing the twin molecules which we have described before with the novel three- and four-armed stars, we found that the supercooled LC phase in the three-armed stars has a stability superior to that in both twin molecules and four-armed stars. In the three-armed star triple-4 with a suitable substituent pattern, the supercooled nematic phase is stable at room temperature for at least nine months. Photo-DSC experiments show that the final conversion after 10 min of UV-irradiation for the threearmed star molecule triple-4 is as high as for the smaller molecules twin-4 and mono-4 over the whole temperature range. Doped with a suitable chiral molecule the novel nematics formed cholesteric phases which were used to make cholesteric polymer networks by photopolymerization.     

Dynamics of the Frederiks transition in nematics consisting of disc-like molecules Thermal dependence of a bend viscosity coefficient

We report measurements of the dynamics of the magnetic Frederiks transition in nematics consisting of disc-like molecules. In this paper the results are presented for three 2, 3, 6, 7, 10, 11-hexakis(p-alkoxybenzoyloxy)triphenylenes, which exhibit a normal nematic phase, and for three 2, 3, 7, 8, 12, 13-hexa(alkanoyloxy)truxenes, which exhibit an inverted nematic phase. We find that the thermal dependence of a bend viscosity coefficient (γ*₁) can be accurately described by the expression, γ*₁ ∼ S² exp (E_a/kT). The absolute value of γ*₁ is found to be higher (by a factor of 10-100) than is commonly encountered in nematics consisting of rod-like molecules.     

Sulphur ligated siamese twin mesogens

The synthesis of 3,3'-sulphinyl-bis-[alkyl 4-(4-substituted benzoyloxy)-benzoates] and 3,3'-sulphonyl-bis-[alkyl 4-(4-substituent-benzoyloxy)-benzoates] is described. These new nematic compounds are of the ligated siamese twin type. By microscopy and DSC calorimetry the thermal behaviour was investigated. Many of the new compounds transform on quenching to the glassy nematic state with glass temperatures above room temperature. Above the glass temperature the substances behave like highly viscous but ordinary nematics. The substances may be used for the construction of new thermo-electrooptic displays. By means of dielectric and X-ray investigations we were able to show that the two halves of the twin molecules are in an antiparallel orientation in the nematic state. In mixtures of the new compounds with rod-like nematics reentrant nematic behaviour was observed.     

Functional polymers and sequential copolymers by phase transfer catalysis. 25. Transformation of a monotropic mesophase into an enantiotropic one by increasing the molecular weight of the polymer and by copolymerization

The influence of molecular weight on thermal transitions and on the thermodynamic parameters was studied for two polymers based on 4,4′-dihydroxy-α-methylstilbene with either 1,9-dibromononane (HMS-C9 polyethers) or 1,11-dibromoundecane (HMS-C11 polyethers). HMS-C9 polyethers present an enantiotropic nematic mesophase over the entire range of molecular weights and a monotropic smectic mesophase for polymers of number average molecular weights higher than 17,000. The low molecular weight HMS-C11 polyethers are only crystalline. On increasing their molecular weight, the polymers become monotropic nematics, and at higher molecular weights, enantiotropic nematics. Up to a composition containing as little as 20 mol % nonane structural units, the random copolyethers based on 1,9-dibromononane, 1,11-dibromoundecane, and 4,4′-dihydroxy-α-methylstilbene (HMS-C9/11 copolyethers) exhibit on cooling a phase diagram resembling that of HMS-C9 polyether. HMS-C9/11 containing about a 1/1 mole ratio between the two spacers presents both smectic and nematic enantiotropic mesophases. These results suggest that the phase diagram of random liquid crystalline copolymers is controlled by the shorter spacer. The thermodynamic parameters of isotropization for both polyethers and copolyethers are compared and suggest that copolymerization does not significantly decrease the degree of order of the mesogenic units in the mesomorphic phase.     

Nematic Liquid Crystals as Media for Real-Time Holography

We report on a novel type of orientational photorefractive devices consisting of dye-doped or pure nematic liquid crystals. These systems can be employed as the recording media for real-time (dynamic) holography. Optical addressing of liquid crystal molecules is facilitated due to the natural tendency of molecular self-assembling in nematics and their easy reorientation upon action of an electric field. This field can be produced by incoming light via photoconductivity either in the bulk or at the surface of the nematic liquid crystal layer. We present an experimental study of dynamic self-diffraction of light on thin phase holograms formed in a nematic liquid crystal panel. Optically addressed spatial light modulators designed by us can be the active elements of image processing systems and in this communication we demonstrate the optical correlator performing image recognition.     

Structural and Optical Anisotropy of Biaxial Nematics

It is shown that biaxial thermotropic nematics have anomalously small parameters of biaxial orientational order of molecules, which are one or two orders of magnitude lower than the limit of their detection by NMR. This accounts for the dramatic discrepancy between the optical and NMR data on biaxiality of these compounds.