Isomeric trimesogens exhibiting modulated nematic mesophases

Modulated nematic phases, in which the constituent molecules possess some form of superstructure in addition to the average orientational organization and lack positional order present in classical nematic phases, are of current fascination in liquid crystal research. Most of the recent focus has been on the twist-bend nematic phase. This phase exhibits phenomena that are of fundamental importance to science; the spontaneous formation of a helical pitch of nanometre scale in a fluid and the spontaneous breaking of mirror symmetry leading to the emergence of chiral domains in an achiral system. In this communication we report on the synthesis of two liquid-crystalline trimesogens whose structures are an extension of the known dimeric material CB6OCB. One material (A63) exhibits a nematic phase with a twist-bend modulation (NTB phase), whereas a structurally isomeric material (B63) exhibits an unusual nematic like mesophase which we believe is a candidate for being a new form of modulated nematic. Whereas the properties of dimeric LCs are somewhat indifferent to chemical composition, the stark difference in behaviour between the two materials herein shows that oligomers may exhibit an unexpected sensitivity to molecular structure

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

Does Topology Dictate the Incidence of the Twist-Bend Phase? : Insights Gained from Novel Unsymmetrical Bimesogens

We prepared a significant number of unsymmetrical liquid-crystalline dimers that exhibit the twist-bend nematic phase; a state of matter that exhibits spontaneous breaking of mirror symmetry and, for some materials, a microsecond electrooptic response. A number of novel unsymmetrical bimesogens were synthesized and in comparing their thermal behaviour to previous literature examples, we have uncovered an unexpected relationship between the thermal stability of the nematic and NTB phases. This relationship demonstrates that molecular shape dictates the incidence of this fascinating phase of matter and leads us to speculate as to the existence of “twist-bend nematic phases” on length scales beyond those of the molecule

Rational Design of Rod-Like Liquid Crystals Exhibiting two Nematic Phases.

Recently a polar, rod-like liquid-crystalline material was reported to exhibit two distinct nematic mesophases (termed N and NX¬) separated by a weakly first order transition. In this work we present our initial studies into the structure-property relationships that underpin the occurrence of the lower temperature nematic phase, and in doing so report several new materials that exhibit this same transformation. In doing so we have prepared material with significantly enhanced temperature ranges, allowing us to perform a detailed study of both the upper- and lower- temperature nematic phases using small angle X-ray scattering. We observe a continuous change in d-spacing rather than a sharp change at the phase transition, a result consistent with a transition between two nematic phases whose structures are presumably degenerate

The Shape of Things to Come: : The Formation of Modulated Nematic Mesophases at Various Length Scales

The twist-bend nematic phase is a recently discovered liquid-crystalline phase that exhibits macroscopic chirality even when formed from achiral materials, and as such presents a unique testbed for studies concerning the spontaneous breaking of mirror symmetry in soft matter. It is primarily exhibited by materials whose molecular structure is composed of two rigid aromatic units (such as biphenyl connected by a flexible spacer). The local structure of the NTB phase is nematic-like - with molecules having an average orientational order but no positional order - with a nanoscale helix where the pitch is of the order of several nanometres. A helix is chiral, and so the bulk NTB phase - in the absence of a biasing chiral environment - spontaneously separates into macroscopic domains of opposite handedness. After discussing the structure of this mesophase and its elucidation this concept article presents the molecular factors that determine its incidence. The apparent dependency primarily on molecular shape and bend-angle rather than particular functional group combinations manifests in this mesophase being exhibited on length scales far beyond those of simple liquid-crystalline dimers, not only in oligomers and polymers but in aqueous suspensions of micron sized helical particles

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