The twist-bend nematic phase translational self-diffusion and biaxiality studied by H-1 nuclear magnetic resonance diffusometry

• Published in: Liquid crystals Vol. 44; no. 1; p. 204
• Main Authors: Cifelli, M., Domenici, V., Dvinskikh, Sergey, Luckhurst, G. R., Timimi, B. A.
• Format: Journal Article
• Language: English
• Published: 2017
• Subjects: local phase biaxiality; macroscopic rotational diffusion; molecular translational diffusion coefficients; motional averaging; residual quadrupolar tensors; Twist-bend nematic
• ISSN: 1366-5855; 0267-8292
• DOI: 10.1080/02678292.2016.1254292

Recently, there has been a surge of interest in mesogens exhibiting the twist-bend nematic (N-TB) phase that is shown to be chiral even though formed by effectively achiral molecules. Although it now seems to be clear that the N-TB phase in the bulk is formed by degenerate domains having opposite handedness, the presence of a supramolecular heliconical structure proposed in the Dozov model has been contradicted by the Hoffmann et al. model in which the heliconical arrangement is replaced by a polar nematic phase. The evidence in support of this is that the quadrupolar splitting tensor measured in various experiments is uniaxial and not biaxial as expected for the twist-bend nematic structure. In this debate, among other evidence, the molecular translational diffusion, and its magnitude with respect to that in the nematic phase above the N-TB phase, has also been invoked to eliminate or to confirm one model or the other. We attempt to resolve this issue by reporting the first measurements of the translational self-diffusion coefficients in the nematic and twist-bend nematic phases formed 1 '',7 ''-bis-4-(4'-cyanobiphenyl-4'-yl) heptane (CB7CB). Such measurements certainly appear to resolve the differences between the two models in favour of that for the classic twist-bend nematic phase. [GRAPHICS] .