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.
Highlights
CB7CB is an example of a liquid crystal dimer comprising molecules containing two mesogenic units linked by a flexible spacer.[8,9]
The para axes of the two cyanobiphenyl mesogenic units in an odd-membered dimer are inclined with respect to each other when the spacer is in the all-trans conformation, and this bent molecular shape which exists for most other conformers is thought to be a prerequisite for the formation of the NTB phase
In an attempt to clarify the relationships between the formation of the NTB phase and molecular structure, and in particular to the molecular curvature, here we describe the transitional properties of a nonsymmetric liquid crystal dimer, 1-(4-cyanobiphenyl-40-yloxy)-6-(4-cyanobiphenyl-40-yl)hexane, and refer to this using the acronym CB6OCB
Summary
The recent discovery of a fifth nematic phase, the twist-bend nematic phase, NTB, has understandably caused considerable excitement. A limited number of other liquid crystal dimers have been reported to show the NTB phase.[10,11,12,13,14,15,16,17,18,19] Additional examples of twist-bend nematogens include two liquid crystal trimers,[20,21] and a rigid bent-core liquid crystal.[22] Given this small set of known twist-bend nematogens, we have yet to develop and understand the empirical relationships linking molecular structure to the formation of the NTB phase Even at this early stage, it does appear clear that a bent molecular shape, most often achieved by using methylene linkages between the spacer and mesogenic units, is a prerequisite for the observation of the NTB phase,[23] and this view is supported by predictions made using a generalised Maier–Saupe theory modified to take into account the bent molecular architecture by assuming a V-shaped particle, and the twist-bend modulation of the director.[24] This predicts that the NTB–N transition temperature is sensitive to the molecular bend angle, defined as the angle between the mesogenic arms, and that the NTB phase is only seen for a limited range of bend angles.
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