Understanding the twist-bend nematic phase: the characterisation of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl)hexane (CB6OCB) and comparison with CB7CB

Abstract

The synthesis and characterisation of the nonsymmetric liquid crystal dimer, 1-(4-cyanobiphenyl-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl)hexane (CB6OCB) is reported. An enantiotropic nematic (N)-twist-bend nematic (NTB) phase transition is observed at 109 °C and a nematic-isotropic phase transition at 153 °C. The NTB phase assignment has been confirmed using polarised light microscopy, freeze fracture transmission electron microscopy (FFTEM), (2)H-NMR spectroscopy, and X-ray diffraction. The effective molecular length in both the NTB and N phases indicates a locally intercalated arrangement of the molecules, and the helicoidal pitch length in the NTB phase is estimated to be 8.9 nm. The surface anchoring properties of CB6OCB on a number of aligning layers is reported. A Landau model is applied to describe high-resolution heat capacity measurements in the vicinity of the NTB-N phase transition. Both the theory and heat capacity measurements agree with a very weak first-order phase transition. A complementary extended molecular field theory was found to be in suggestive accord with the (2)H-NMR studies of CB6OCB-d2, and those already known for CB7CB-d4. These include the reduced transition temperature, TNTBN/TNI, the order parameter of the mesogenic arms in the N phase close to the NTB-N transition, and the order parameter with respect to the helix axis which is related to the conical angle for the NTB phase.

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

“…Paterson et al. reported 1‐(4‐cyanobiphenyl‐4′‐yloxy)‐6‐(4‐cyanobiphenyl‐4′‐yl)hexane (CB6OCB—compound 6 in this work), finding that replacing one of the two methylene groups adjacent to the 4‐cyanobiphenyl mesogenic unit in CB7CB confers a modest increase in the N TB –N transition temperature, T NTB–N 46. Positioning a single ether link in the centre of CB7CB (to give an isomer of 6 /CB6OCB) lead to the finding that T NTB‐N drops dramatically relative to both parent materials.…”

Free volume, molecular grains, self-organisation, and anisotropic entropy: machining materials

“…It is also important to note that the organisation is only dependent on the bent architectures of the molecules and not to their conformational structures. Moreover, with any twisted organisation there is the possibility of two twists at right angles to one another, and in the case of the pitch length for the principal helical structure, for the materials investigated the results are all very similar, and in the 8-9 nm range, no matter what the details of the molecular aspect ratios are or the electrostatic properties [10,[18][19][20][21]. Overall, the results indicate that molecular topology determines the twist-bend phase stability and the pitch length.…”

Developments in liquid-crystalline dimers and oligomers

“…they occur below the melting point. Values for CB6OCB were taken from a forthcoming paper by Archbold et al [41]; however, these values are only marginally different to those reported by Paterson et al [33]. a If rapidly cooled, a monotropic, unidentified smectic phase (SmY) is observed at roughly 25°C for i14; however, the sample crystallises before this point during DSC study.…”

“…It has been observed that as the number of ether linkages increases, the scaled N TB transition temperature (T NTB-N /T N-Iso ) decreases irrespective of the actual values of T NTB-N . We therefore opted to reduce the number of ether-linking groups and prepare materials that can be thought of, in crude terms, as oligomeric analogues of CB6OCB, reported by Paterson et al [33], with our synthetic approach outlined in Scheme 3. We hypothesised that the reduction in the number of ether linkages would increase the scaled N TB transition temperature and therefore afford further examples of oligomeric twist-bend nematogens.…”

“…It has been observed that many functional groups -including the SF 5 groupcan be incorporated into the structure of a suitable dimer without loss of the N TB phase. Last, the twistbend nematic phase -in dimers and bimesogens at least -is found mostly for methylene-linked materials, although a number of other linking groups are known to yield materials that afford this mesophase [7,21,22,33,[44][45][46][47][48][49]. In the first part of this paper, we present the latest part of our study into the structureproperty relationships of bis ether-linked dimers and bimesogens that exhibit the twist-bend phase.…”