Conformational degrees of freedom and stability of splay-bend ordering in the limit of a very strong planar anchoring

We study the self-organization in a monolayer (a two-dimensional system) of flexible planar trimer particles. The molecules are made up of two mesogenic units linked by a spacer, all of which are modeled as hard needles of the same length. Each molecule can dynamically adopt two conformational states: an achiral bent-shaped (cis-) and a chiral zigzag (trans-) one. Using constant pressure Monte Carlo simulations and Onsager-type density functional theory (DFT), we show that the system consisting of these molecules exhibits a rich spectrum of liquid crystalline phases. The most interesting observation is the identification of stable smectic splay-bend (S_{SB}) and chiral smectic-A (S_{A}^{*}) phases. The S_{SB} phase is also stable in the limit, where only cis- conformers are allowed. The second phase that occupies a considerable portion of the phase diagram is S_{A}^{*} with chiral layers, where the chirality of the neighboring layers is of opposite sign. The study of the average fractions of the trans- and cis- conformers in various phases shows that while in the isotropic phase all fractions are equally populated, the S_{A}^{*} phase is dominated by chiral conformers (zigzag), but the achiral conformers win in the smectic splay-bend phase. To clarify the possibility of stabilization of the nematic splay-bend (N_{SB}) phase for trimers, the free energy of the N_{SB} and S_{SB} phases is calculated within DFT for the cis- conformers, for densities where simulations show stable S_{SB}. It turns out that the N_{SB} phase is unstable away from the phase transition to the nematic phase, and its free energy is always higher than that of S_{SB}, down to the transition to the nematic phase, although the difference in free energies becomes extremely small when approaching the transition.

Controlling spontaneous chirality in achiral materials: liquid crystal oligomers and the heliconical twist-bend nematic phase

Liquid crystal oligomers, namely dimers, trimers and tetramers, consisting of cyanobiphenyl and benzylideneaniline-based mesogenic units connected by either linear or bent alkoxy or alkyl spacers are reported. These materials, although built from achiral molecules, show the spontaneously chiral heliconical twist-bend nematic (N_TB) phase. We report the relationships between the shape of the oligomer, and the N_TB phase stability, the temperature dependence of the helical pitch length and tilt angle, birefringence, and elastic constants.

Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen

Nematic twist-bend phases (NTB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature NTB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to NTB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable NTB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding

A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (nOS) of varying terminal chain length (n) to obtain the CB6OBA:nOS complexes. Complexes with n ≤ 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for n = 4–7, and finally, a hexatic-type smectic X phase for n = 3–7. Mixtures with n = 8–10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic Ab phase and the same hexatic-type SmX phase. Supramolecular trimers, CB6OBA:CB6OS and CB6OBA:1OB6OS, formed between CB6OBA and dimeric stilbazoles [(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4’-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1’-biphenyl]-4- carbonitrile (CB6OS), exhibit nematic and twist-bend nematic phases, and are the first hydrogen-bonded trimers consisting of unlike donor and acceptor fragments to do so.

Pentaerythritol Derived Tetrapode Exhibiting a Nematic-Like Mesophase at Ambient Temperatures

The nematic liquid-crystalline phase exhibits average orientational order, with no positional organisation. So-called modulated nematic phases exhibit this same orientational order with an additional spatially periodic modulation of the nematic director, the most common of which is the twist-bend nematic phase. We report a pentaerythritol derived tetrapode which exhibits a nematic-like mesophase at ambient temperature, and we denote this new mesophase 'N_X ' to indicate a nematic phase of unknown structure. X-ray scattering experiments refute the possibility of positional order, yet optical textures are consistent with a periodic structure. We suggest that the mesophase exhibited by this material is a new type of nematic-like mesophase with some form of modulated structure. We find the N_X phase to exhibit an electrooptic response consistent with a nematic-like phase.

Distinct differences in the nanoscale behaviors of the twist-bend liquid crystal phase of a flexible linear trimer and homologous dimer

We synthesized the liquid crystal dimer and trimer members of a series of flexible linear oligomers and characterized their microscopic and nanoscopic properties using resonant soft X-ray scattering and a number of other experimental techniques. On the microscopic scale, the twist-bend phases of the dimer and trimer appear essentially identical. However, while the liquid crystal dimer exhibits a temperature-dependent variation of its twist-bend helical pitch varying from 100 to 170 Å on heating, the trimer exhibits an essentially temperature-independent pitch of 66 Å, significantly shorter than those reported for other twist-bend forming materials in the literature. We attribute this to a specific combination of intrinsic conformational bend of the trimer molecules and a sterically favorable intercalation of the trimers over a commensurate fraction (two-thirds) of the molecular length. We develop a geometric model of the twist-bend phase for these materials with the molecules arranging into helical chain structures, and we fully determine their respective geometric parameters.

Linear symmetric liquid crystal trimers exhibiting supramolecular chiral architectures

We prepared a homologous series of achiral liquid crystal trimers (I-n) in which two phenylpyrimidine units and one biphenyl unit were connected via flexible spacers, and investigated the physical properties. All the trimers possessing odd-numbered methylene spacers exhibited soft crystalline chiral conglomerate phases. X-ray diffraction measurements reveal that they have an intercalated layer structure. On the other hand, the trimers possessing even-numbered spacers showed nematic and smectic C phases. We investigated the surface structures of odd-membered trimers in the soft crystalline phases using scanning electron microscopy. Trimers I-3 and I-5 were found to form cylindrical tubes, whereas trimers I-7, I-9 and I-11 toroidal pits. We discuss the formation of diverse supramolecular architectures in terms of the anisotropy of the chirality transfer.

Order parameters, orientational distribution functions and heliconical tilt angles of oligomeric liquid crystals

Twist-bend (TB) phases possess a local helical structure with a pitch length of a few nanometers. X-ray scattering experiments on aligned samples of dimeric and oligomeric materials allows the orientational order parameters, orientational distribution functions and heliconical tilt angles to be calculated.

Twist-bend nematic liquid crystals based on thioether linkage

“Thioether”-based twist-bend nematogens.

A novel nematic-like mesophase induced in dimers, trimers and tetramers doped with a high helical twisting power additive

From the observation of a previously undiscovered nematic-like mesophase (NX) by Archbold et al., we report on several new binary liquid-crystalline mixtures between the high helical twisting power dopant RM1041 and a selection of dimers with varying average bend angles and conformational landscapes. We also report on mixtures between RM1041 and oligomeric LC materials. We find that dimers and oligomers exhibit not only chiral nematic and twist-bend modulated phases, but also the same NX phase reported by Archbold, indicating that this state of matter (the structure of which is yet to be definitively characterised) is exhibited by a wide range of materials. Mixtures of the dimer CB9CB with a selection of different chiral dopants suggest that it is the helical twisting power of the chiral additive that is responsible incidence of the NX phase.

Supramolecular organization of liquid-crystal dimers - bis-cyanobiphenyl alkanes on HOPG by scanning tunneling microscopy

2D supramolecular organization of a series of six cyanobiphenyls bimesogens deposited on highly oriented pyrolytic graphite (HOPG) is studied by scanning tunneling microscopy (STM). The adsorbates are 1,ω-bis(4-cyanobiphenyl-4'-yl)alkanes (CBnCB) with different lengths of their flexible alkyl spacer (containing from 7 to 12 methylene groups). Microscopic investigations at the molecular resolution allow for detailed analysis of the effect of the alkyl spacer length on the type and the extent of the resulting 2D organization. It was demonstrated that bimesogens with shorter spacers (7 and 8 methylene units) organize in a similar manner characterized by the formation of two types of differently ordered monolayers: dense packed, wherein the molecules are oriented in one direction and ordered into parallel rows (layer structure), or less densely packed where they are organized into a chiral windmill-like structure. For derivatives with longer spacers (ranging from 9 to 12 methylene units) the additional effect of parity of carbon atoms in the spacer (even versus odd) is observed. In this range of the spacer lengths even membered bimesogens are also organized in a typical layer structure. However, odd-membered dimers exhibit a much more complex 2D supramolecular organization with a larger unit cell and a helical arrangement of the molecules. Careful comparison of this structure with the 3D structural data derived from the X-ray diffraction investigations of single crystals indicates that for these bimesogens a clear correlation exists between the observed complex 2D supramolecular organization in the monolayer and the organization in one of the crystallographic planes of the 3D nematic twist-bent phase.

A Nanohelicoidal Nematic Liquid Crystal Formed by a Non-Linear Duplexed Hexamer

The twist-bend modulated nematic liquid-crystal phase exhibits formation of a nanometre-scale helical pitch in a fluid and spontaneous breaking of mirror symmetry, leading to a quasi-fluid state composed of chiral domains despite being composed of achiral materials. This phase was only observed for materials with two or more mesogenic units, the manner of attachment between which is always linear. Non-linear oligomers with a H-shaped hexamesogen are now found to exhibit both nematic and twist-bend modulated nematic phases. This shatters the assumption that a linear sequence of mesogenic units is a prerequisite for this phase, and points to this state of matter being exhibited by a wider range of self-assembling structures than was previously envisaged. These results support the double helix model of the TB phase as opposed to the simple heliconical model. This new class of materials could act as low-molecular-weight surrogates for cross-linked liquid-crystalline elastomers.

Optically active bimesogens incorporating branched central spacers

In the current fascination with liquid crystalline dimers, bimesogens and oligomers the role of the central spacer in these systems has perhaps been somewhat neglected. In compound 1, a phenyl 4-cyanobenzoate bimesogen, the central spacer incorporates a methyl group at the 2-position and is therefore chiral. The helical twisting power of 1, measured in both 5CB and E7, was found to be 0.36 and 0.35 μm-1 wt%-1 respectively. Compound 1 exhibited a monotropic chiral nematic phase, however no twist-bend modulated phase was observed. We prepared a number of analogues of 1 incorporating different mesogenic units and observe that those with a small aspect ratio are non mesogenic, whereas those with larger aspect ratios variously exhibit chiral nematic, TB, SmC and SmB phases.

Inexpensive, operationally simple flow reactor for rapid, chromatography free synthesis of siloxane based liquid crystals

We report on the hydrosilylation of terminal alkenes in flow using a “Karstedt-like” platinum-on-silica catalyst to afford liquid-crystalline materials.