Combined Microscopy, Calorimetry and X-ray Scattering Study of Fluorinated Dimesogens

The material FDO11DFCB3 (compound 2 in this work) remains the only example of a liquid-crystalline material to exhibit a phase transition from the heliconical twist-bend phase into a lamellar smectic A mesophase, additionally this material exhibits a previously unidentified mesophase. We have prepared and characterised several homologues of this compound, with each material subjected to an in-depth analysis by optical microscopy, calorimetry and small angle X-ray scattering studies. Despite FDO11DFCB3 being similar in chemical structure to the novel materials presented herein its liquid-crystalline behaviour is rather different, indicating an unexpected sensitivity of the twist-bend phase to molecular structure

An experimental and computational study of calamitic and bimesogenic liquid crystals incorporating an optically active [2,2]-paracyclophane

Two liquid-crystalline materials containing an optically active (R)-4-hydroxy-[2,2]-paracyclophane group were prepared, one in which the chiral group is a bulky terminal unit and one in which it forms part of a terphenyl-like mesogenic unit. Both materials exhibit monotropic chiral nematic phases. Partial phase diagrams were constructed for mixtures of both materials with 5CB, allowing us to extrapolate pitch lengths and helical twisting power values (HTP) for each material. The HTP value of the material with a ‘locked’ paracyclophane is 70% higher than that of a ‘free’ paracyclophane and this is rationalised as being due to the reduction in conformational freedom of the former material relative to the later

The Dependency of Nematic and Twist-bend Mesophase Formation on Bend Angle

We have prepared and studied a family of cyanobiphenyl dimers with varying linking groups with a view to exploring how molecular structure dictates the stability of the nematic and twist-bend nematic mesophases. Using molecular modelling and 1D (1)H NOESY NMR spectroscopy, we determine the angle between the two aromatic core units for each dimer and find a strong dependency of the stability of both the nematic and twist-bend mesophases upon this angle, thereby satisfying earlier theoretical models

A Ten-Year Perspective on Twist-Bend Nematic Materials

The discovery of the twist-bend nematic phase (NTB) is a milestone within the field of liquid crystals. The NTB phase has a helical structure, with a repeat length of a few nanometres, and is therefore chiral, even when formed by achiral molecules. The discovery and rush to understand the rich physics of the NTB phase has provided a fresh impetus to the design and characterisation of dimeric and oligomeric liquid crystalline materials. Now, ten years after the discovery of the NTB phase, we review developments in this area, focusing on how molecular features relate to the incidence of this phase, noting the progression from simple symmetrical dimeric materials towards complex oligomers, non-covalently bonded supramolecular systems

In Silico Interactome of a Room-Temperature Ferroelectric Nematic Material

The ferroelectric nematic (NF) phase, characterised by the combination of orientational and polar order, offers unique properties that are challenging to replicate in other systems. Understanding the molecular structure requirements for generating the NF phase is crucial for the design of new materials with enhanced properties. This study investigates UUQU-4-N, a room-temperature NF material, using fully atomistic molecular dynamics simulations. UUQU-4-N does not spontaneously form an apolar nematic phase in silico, but exhibits a stable polar nematic configuration akin to the NF phase. The polar order remains significant and near saturation throughout the simulations. The study also examines the cylindrical pair correlation functions, providing insights into the preferred pairing modes and intermolecular interactions which we can then attribute to specific molecular features. We then simulate structural variants of UUQU-4-N, highlighting the potential for developing further examples of near-room-temperature ferroelectric nematic materials via the manipulation of the fluorination pattern, variations in terminal chain length, and replacement of the difluoromethyleneoxy linker

Experimental and Computational Study of a Liquid Crystalline Dimesogen Exhibiting Nematic, Twist-Bend Nematic, Intercalated Smectic, and Soft Crystalline Mesophases

Liquid crystalline dimers and dimesogens have attracted significant attention due to their tendency to exhibit twist-bend modulated nematic (NTB) phases. While the features that give rise to NTB phase formation are now somewhat understood, a comparable structure–property relationship governing the formation of layered (smectic) phases from the NTB phase is absent. In this present work, we find that by selecting mesogenic units with differing polarities and aspect ratios and selecting an appropriately bent central spacer we obtain a material that exhibits both NTB and intercalated smectic phases. The higher temperature smectic phase is assigned as SmCA based on its optical textures and X-ray scattering patterns. A detailed study of the lower temperature smectic ‘’X’’ phase by optical microscopy and SAXS/WAXS demonstrates this phase to be smectic, with an in-plane orthorhombic or monoclinic packing and long (>100 nm) out of plane correlation lengths. This phase, which has been observed in a handful of materials to date, is a soft-crystal phase with an anticlinic layer organisation. We suggest that mismatching the polarities, conjugation and aspect ratios of mesogenic units is a useful method for generating smectic forming dimesogens

Raman scattering studies of order parameters in liquid crystalline dimers exhibiting the nematic and twist-bend nematic phases

Polarized Raman Spectroscopy (PRS) is used to quantify the orientational order in the conventional (N) and twist-bend (NTB) nematic phases of a homologous series of liquid crystalline dimers. The dimers investigated have 7, 8, 9 and 11 methylene groups connecting two cyanobiphenyl mesogens and data for 4-pentyl-4'-cyanobiphenyl (5CB) and 4-octyl-4'-cyanobiphenyl (8CB) are included for comparison. Simulated and measured Raman spectra for the materials are compared. PRS is used to determine both (P2) and (P4) order parameters across the nematic temperature range and immediately below the NTB–N phase transition using a model that takes into account the molecular bend of the odd dimers, which is described in detail. In the nematic phase, the odd dimers are found to exhibit rather low order parameters with hP2i taking values between 0.3 and 0.5 and (P4) about 0.25. In contrast, the even dimer shows extremely high values of the order parameters with (P2) taking values between 0.7 and 0.8 and (P4) between 0.4 and 0.45. For the odd dimers, the values of (P2) in the NTB phase are similar to those of the N phase, whereas (P4) jumps by approximately 5–10% and then decreases with temperature. On comparing the experimental data with the theoretical predictions, we find reasonable qualitative agreement for all materials with molecular field theory. The odd dimers, however, show higher (P4) values than obtained from theoretical models, a factor attributed to the neglect of molecular flexibility and biaxiality in the PRS analysis

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

The twist-bend phase (NTB) is most commonly observed in materials with a gross-bent shape: dimers; bent-cores; bent-oligomers. We had suggested previously that the bend-angle of such systems effectively dictates the relative thermal stability of the NTB phase. However, our earlier paper relied on the use of a single energy-minimum conformer and so failed to capture any information about flexibility and conformational distribution. In the present work, we revisit our hypothesis and examine a second set of dimers with varying linking groups and spacer composition. We have improved on our earlier work by studying the conformational landscape of each material, allowing average bend-angles to be determined as well as the conformer distribution. We observe that the stability of the NTB phase exhibits a strong dependence not only on the Boltzmann-weighted average bend-angle (rather than just a static conformer), but also on the distribution of conformers. To a lesser extent, the flexibility of the spacer appears important. Ultimately, this work satisfies both theoretical treatments and our initial experimental study and demonstrates the importance of molecular bend to the NTB phase

On the molecular origins of the ferroelectric splay nematic phase

Nematic liquid crystals have been known for more than a century, but it was not until the 60s–70s that, with the development of room temperature nematics, they became widely used in applications. Polar nematic phases have been long-time predicted, but have only been experimentally realized recently. Synthesis of materials with nematic polar ordering at room temperature is certainly challenging and requires a deep understanding of its formation mechanisms, presently lacking. Here, we compare two materials of similar chemical structure and demonstrate that just a subtle change in the molecular structure enables denser packing of the molecules when they exhibit polar order, which shows that reduction of excluded volume is in the origin of the polar nematic phase. Additionally, we propose that molecular dynamics simulations are potent tools for molecular design in order to predict, identify and design materials showing the polar nematic phase and its precursor nematic phases