Jan H. Porada

Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers

Significance The appearance of new nematic liquid crystal (LC) equilibrium symmetry (ground state) is a rare and typically important event. The first and second nematics were the helical phase and blue phase of chiral molecules, both found in 1886 in cholesteryl benzoate by Reinitzer, discoveries that marked the birth of LC science. The third nematic, the achiral uniaxial phase, also found in the 19th century, ultimately formed the basis of LC display technology and the portable computing revolution of the 20th century. Despite this achievement, the 20th can claim only the fourth nematic, the lyotropic biaxial phases found by Saupe. Now, early in the 21st, the heliconical structure of the fifth nematic is observed, an exotic chiral helix from achiral molecules. Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called “twist–bend” nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d ∼ 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q ∼ 2π/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d ∼ in CB(CH2)7CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be θTB ∼ 25° and the full pitch of the director helix to be pTB ∼ 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend.

A Twist-Bend Chiral Helix of 8nm Pitch in a Nematic Liquid Crystal of Achiral Molecular Dimers

Significance The appearance of new nematic liquid crystal (LC) equilibrium symmetry (ground state) is a rare and typically important event. The first and second nematics were the helical phase and blue phase of chiral molecules, both found in 1886 in cholesteryl benzoate by Reinitzer, discoveries that marked the birth of LC science. The third nematic, the achiral uniaxial phase, also found in the 19th century, ultimately formed the basis of LC display technology and the portable computing revolution of the 20th century. Despite this achievement, the 20th can claim only the fourth nematic, the lyotropic biaxial phases found by Saupe. Now, early in the 21st, the heliconical structure of the fifth nematic is observed, an exotic chiral helix from achiral molecules. Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called “twist–bend” nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d ∼ 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q ∼ 2π/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d ∼ in CB(CH2)7CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be θTB ∼ 25° and the full pitch of the director helix to be pTB ∼ 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend.

Microemulsions with novel hydrophobic ionic liquids

The phase behaviour of different ternary mixtures containing water, a hydrophobic ionic liquid (IL) and Triton X-100 have been examined as a function of surfactant concentration and temperature, maintaining equal volumes of water and IL. In all previously published studies on water–IL microemulsions the hydrophobic IL contained hexafluorophosphate (PF6−) as anion. As this anion is not stable towards hydrolysis, our aim was to replace it by the hydrolysis-stable anion bis-triflimide (NTf2−). The challenge was to find a suitable cation, which, in combination with the chosen anion, forms microemulsions with an efficiency equal or greater than reported values. The cation leading to the most efficient microemulsion was [ali336]+, which is based on the phase transfer catalyst aliquat 336. Thus the IL [ali336]NTf2 turned out to be a suitable choice for a hydrophobic room temperature IL which forms microemulsions of efficiencies comparable to those of water–[bmim]PF6 systems, whilst being stable towards hydrolysis.

Design of liquid crystals with ‘de Vries-like’ properties: carbosilane-terminated 5-phenylpyrimidine mesogens suitable for chevron-free FLC formulations

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the orthogonal SmA phase to the tilted SmC phase. In an effort to expand the library of ‘de Vries-like’ liquid crystals required for the formulation of chevron-free ferroelectric liquid crystal mixtures, we report the synthesis of a homologous series of tricarbosilane 5-phenylpyrimidine liquid crystals QL16-n using an improved synthetic route, and the characterization of their liquid crystalline and ‘de Vries-like’ properties. Measurements of orientational order parameters S2 and effective molecular lengths Leff by monodomain 2D X-ray scattering suggest that ‘de Vries-like’ behavior in series QL16-n is due to the combined effect of an increase in S2 and a decrease in bilayer interdigitation, thus causing a smectic layer expansion that compensates for the molecular tilt in the SmC phase. We also show how the optical tilt angle in the SmC phase may be optimized for SSFLC displays—without compromising ‘de Vries-like’ properties—by shortening the tricarbosilane end-group to a dicarbosilane. Two of the new materials reported herein, 5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]-2-hexyloxypyrimidine (QL16-6) and 2-hexyloxy-5-[4-(12,12,14,14-tetramethyl-12,14-disilapentadecyloxy)phenyl]pyrimidine (QL24-6) rank among the best ‘de Vries-like’ materials reported heretofore, with broad SmC phases and reduction factors R of 0.17 and 0.18, respectively, at a reduced temperature T − TAC = −10 K. We also show that inverting the orientation of the 5-phenylpyrimidine core in the homologous series QL17-n causes a suppression of ‘de Vries-like’ properties. These results suggest that non-covalent core–core interactions in the intercalated smectic bilayers formed by these mesogens may influence ‘de Vries-like’ behavior.

A Lyotropic Chiral Smectic C Liquid Crystal with Polar Electrooptic Switching

Thermotropic and lyotropic liquid crystals share a common state of matter with many overlapping featur es such as phase structure and symmetry. For example t he thermotropic smectic A phase (SmA) has a well-known analogue in lyotropics, the so called la mellar Lα phase. Therefore it is quite astonishing, that there are only very few reports on the existence of a lyotropic analogue to the thermotropic smectic C (SmC) phase. Furthermore, so far there has been no report on a chiral lyotropic SmC* phase, which is expected to show macroscopic chirality effects such as elicity and ferroelectricity.

Phasmidic indigoid liquid crystals

Two new phasmidic mesogens with an integrated indigo core have been synthesized and their thermotropic properties were investigated by polarization microscopy and differential scanning calorimetry. Their surprising textural features are reminiscent of those known from some bent core mesogens. Common properties and differences are discussed.

The 2D-correlated structures of a lyotropic liquid crystalline diol with a phenylpyrimidine core

Even though the fluid tilted smectic C (SmC) phase is one of the most common phases in thermotropic liquid crystals, the lyotropic analogue to SmC is practically unknown so far. One of the rare examples, a lyotropic liquid crystalline diol, which was reported in 1991 to show this phase, was re-examined by means of detailed X-ray diffraction and polarised optical microscopy. Although the diol possesses a smectic C promoting phenylpyrimidine core, it was not possible to verify the existence of a SmC-analogue lyotropic phase. Instead, the phase diagram shows the existence of five different lyotropic phases identified as nematic, lamellar Lα and hexagonal as well as two novel complex 2D-correlated lyophases. Results obtained by electron density calculations and X-ray diffraction data strongly suggest that the structure of one of these phases possesses a pseudo-c2mm symmetry and is built up of two different kinds of columns, whereas the other one is a ribbon phase of the oblique plane group p2.

Liquid Crystallinity and Supramolecular Organization of Regioisomeric Isatin Derivatives: An Experimental and Theoretical Study

Spatially organized chromophores can be beneficial for advanced applications like for example, organic solar cells, laser technology or non-linear optic devices as well as supramolecular photochemistry. Of particular interest are non-static ordered forms of molecular organization as for example, liquid crystals. With this in mind we synthesised four new regioisomeric isatin derivatives by Suzuki-Miyaura coupling of 4-dodecyloxyphenylboronic acid with all four possible regioisomers of bromoisatin. Liquid crystalline properties are found for 5-(4-dodecyloxyphenyl)isatin, while the other regioisomers do not display a mesomorphic behaviour. The synthesis, physicochemical investigations including polarization microscopy, differential scanning calorimetry and X-ray investigations are discussed and accompanied with density functional theory calculations with respect to the target molecules and their possible H-bonded aggregates. Two distinct setups of supramolecular assemblies for such isatin derivatives are discussed and a model for the mesophase is proposed.

Planar and distorted indigo as the core motif in novel chromophoric liquid crystals

Symmetrically bis-substituted indigo derivatives with long peripheral alkyl chains were synthesised by the reductive condensation of corresponding isatin derivatives. Their thermotropic mesomorphism was investigated with respect to different substitution patterns, which include the position and lateral modifications of the substituents. A systematic investigation of structure–property relationships revealed that only substitution at the 6 and 6′ positions affords the calamitic shape necessary to form smectic or nematic liquid crystalline phases. This finding is rationalised on the basis of a structural analysis of N,N′-diacetyl indigo and the consequences for 5,5′- and 6,6′-bis-substituted derivatives are discussed. Some of the liquid crystalline substances exhibit dichroism, which is especially pronounced in highly ordered phases. In addition, the 6,6′ substitution leads to significantly enhanced activity with respect to the photochemical trans–cis isomerization of N,N′-diacetylated indigo derivatives.

About the nanostructure of the ternary system water – [BMIm]PF6 – TX-100

HYPOTHESIS Many efforts have been made to formulate water-IL microemulsions. One of the most intensely studied systems is H2O - 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6) - 4-octylphenol polyethoxylate (TX-100) and it is not questioned that this system forms microemulsions. The nanostructures observed for traditional microemulsions are postulated with the surfactant being adsorbed at the interface such that the hydrophilic EO sides intrude into the water domains, while the hydrophobic hydrocarbon chains are immersed into [BMIm]PF6. However, the high polarity of [BMIm]PF6 and the observation that [BMIm]PF6 mixes well with oligoethylene oxides but hardly with non-polar solvents like toluene or alkanes are not in line with this picture. EXPERIMENTS We re-studied the ternary system H2O - [BMIm]PF6 - TX-100 by measuring phase diagrams, determining tie-lines, and carrying out ROESY NMR and PFG NMR measurements. FINDINGS We found that the hydrophobic part of the surfactant interacts neither with water nor with [BMIm]PF6, while both solvents interact with the hydrophilic part of the surfactant. We suggest that the surfactant is not adsorbed at the interface between water and the IL, but forms normal spherical or elongated micelles or even continuous aggregates with the hydrocarbon chains forming the interior of the aggregates.