Different liquid crystalline phases with long-range orientational but not positional order, so-called nematic phases, are scarce. New nematic phases are rarely discovered, and such an event inevitably generates much interest. Here, we describe a transition from a uniaxial to a novel nematic phase characterized by a periodic splay modulation of the director. In this new nematic phase, defect structures not present in the uniaxial nematic phase are observed, which indicates that the new phase has lower symmetry than the ordinary nematic phase. The phase transition is weakly first order, with a significant pretransitional behavior, which manifests as strong splay fluctuations. When approaching the phase transition, the splay nematic constant is unusually low and goes towards zero. Analogously to the transition from the uniaxial nematic to the twist-bend nematic phase, this transition is driven by instability towards splay orientational deformation, resulting in a periodically splayed structure. And, similarly, a Landau-de Gennes type of phenomenological theory can be used to describe the phase transition. The modulated splay phase is biaxial and antiferroelectric.

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. Herein, we present our initial studies into the structure–property relationships that underpin the occurrence of the lower‐temperature nematic phase, and report several new materials that exhibit this same transformation. 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 by using small‐angle X‐ray scattering. We observed 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, structures of which are presumably degenerate.

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The nematic twist-bend phase (NTB) was, until recently, only observed for polar mesogenic dimers, trimers or bent-core compounds. In this article, we report a comprehensive study on novel apolar materials that also exhibit NTB phases. The NTB phase was observed for materials containing phenyl, cyclohexyl or bicyclooctyl rings in their rigid-core units. However, for materials with long (>C7) terminal chains or mesogenic core units comprising three ring units, the NTB phase was not observed and instead the materials exhibited smectic phases. One compound was found to exhibit a transition from the NTB phase to an anticlinic smectic C phase; this is the first example of this polymorphism. Incorporation of lateral substitution with respect to the central core unit led to reductions in transition temperatures; however, the NTB phase was still found to occur. Conversely, utilising branched terminal groups rendered the materials non-mesogenic. Overall, it appears that it is the gross molecular topology that drives the incidence of the NTB phase rather than simple dipolar considerations. Furthermore, dimers lacking any polar groups, which were prepared to test this hypothesis, were found to be non mesogenic, indicating that at the extremes of polarity these effects can dominate over topology.

A novel, highly polar rod-like liquid crystal was found to exhibit two distinct nematic mesophases (N and N X ).When studied by microscopy and X-ray scattering experiments, and under applied electric fields, the nematic phases are practically identical. However, calorimetry experiments refute the possibility of an intervening smectic mesophase, and the transformation between the nematic phases was associated with a weak thermal event. Analysis of measured dielectric data, along with molecular properties obtained from DFT calculations, applying the Maier-Meier relationship allowed for the degree of antiparallel pairing of dipoles in both nematic phases to be quantified. Based on the results, we conclude that the onset of the lower temperature phase is driven by the formation of antiparallel molecular associations.

Ferroelectric ordering in liquids is a fundamental question of physics. Here, we show that ferroelectric ordering of the molecules causes formation of recently reported splay nematic liquidcrystalline phase. As shown by dielectric spectroscopy, the transition between the uniaxial and the splay nematic phase has the characteristics of a ferroelectric phase transition, which drives an orientational ferroelastic transition via flexoelectric coupling. The polarity of the splay phase was proven by second harmonic generation (SHG) imaging, which additionally allowed for determination of the splay modulation period to be of the order of 5 -10 microns, also confirmed by polarized optical microscopy. The observations can be quantitatively described by a Landau-de Gennes type of macroscopic theory. SI A. Material RM734 was synthesized according to Ref.[1] and additionally purified as described in Ref. [2]. As determined by means of differential scanning calorimetry phase transition temperatures have been determined to be: isotropic to nematic TIN= 187.9 °C, nematic to second nematic transition at TNNs = 132.7 °C, and a melting point at Tm = 139.8 °C. We used the Gaussian G09e01 suite of programs [3] to determine the B3LYP/6-31G(d) minimized geometry of RM734 shown in Fig.1. The molecular dipole moment calculated by the B3LYP/6-31G(d) level of DFT is of 11.3748D and oriented almost along the molecular long axis.

In this article, we probe the formation of liquid crystal and soft crystal phases as a consequence of minimising the free volume of the system either through design engineering of molecular shape or through deformation of molecular architecture. Following this concept, a number of realisations were made, for example, smectic A phases with variable layer spacing, smectic C phases without layer shrinkage, lattices of free space and fibres in N TB phases.Prologue: At the start of my thesis research in 1974, the knowledge of the structures of smectic liquid crystals was somewhat limited. Even though his thought predated the discovery of ferreoelectric behaviour in liquid crystals, George Gray had the idea that smectic C (SmC) liquid crystals might underpin potential new display applications. As I launched into synthesising new SmC variants, I started to think about why molecules should tilt over in their layers, and then onwards to question why do certain materials exhibit different forms of smectic B (SmB) phases, which was before their classification as hexatic and crystal B phases, etc. In those days I became very much interested in how the molecules pack together in condensed phases. With protractors, compasses and graph paper, the Cartesian coordinates of the atoms of a variety of molecular structures in their all trans forms were located, and their mass axes were determined for their structures by computer methods using ticker-tape which used to shoot out of the machine in volumes to yield just one result -the minimum moment of inertia. Flipping the structures about their inertia axes by 180°yielded the surface contours and the rotational volumes of the molecules. Packing them together in ways to minimise the free volume gave snapshot pictures of the local structures of various mesophases.[1] Over the passing years, I discarded this methodology for the following reasons: (1) the molecules in liquid crystal phases are rotationally disordered and are in dynamic fluctuation as demonstrated by neutron scattering studies [2,3]; (2) X-ray data showed that the molecules pack close enough together that they interpenetrate each other's independent rotational volumes, meaning they share each other's space [4,5]; (3) there were no ways to simulate the interpenetration of space, and what was free space and what was not; (4) there were only a few families of materials that could provide full data sets on structure versus phase formation; to this day there are still relatively few full homologous series of materials being reported; and lastly (5) there was no discussion of how to determine the energy cost of not fully packing space with molecules in order to minimise the remaining free volume. However, with the advent of research exploring liquid crystal materials of unconventional structure and new modelling methods such as density functional theory (DFT) simulations becoming available, our work again began to explore the steric packing arrangements of the molecules in condensed phases. [6][7][8][9][10] Then recently, one o...

In this article we report several unsymmetrical phenyl-benzoate bimesogens that exhibit the twist-bend nematic phase and present further examples of oligomeric systems that display this unusual state of matter.