Structural insights into the twist-bend nematic phase from the integration of ²H-NMR data and modelling: CB7CB and CB6OCB as case studies

ABSTRACT

The appearance of some special features in ²H-NMR spectra, which could not be explained by the conventional nematic order, was crucial for the discovery of the twist-bend nematic (N_TB) phase. Such features reflect the order and symmetry of the local environment of a probe and in principle their analysis can provide quantitative insights into the structural parameters of the phase. We have developed a methodology for the analysis of experimental data, different from the approaches that are normally used for two main reasons: (i) the structural features of the N_TB phase, specifically its local polarity, biaxiality and chirality, are consistently taken into account; (ii) the molecular features are introduced in a detailed way, through molecular modelling. We have applied this methodology to analyse ²H-NMR quadrupolar splittings for two representative systems, CB7CB and CB6OCB. The integration of theory, computational modelling and experiment allows us to extract quantitative information from the experimental data and to predict various properties, even not directly probed by measurements. We discuss how the molecular differences between CB7CB and CB6OCB translate into a different phase behaviour and we address issues concerning the distinguishing features of the N_TB phase, such as the definition of a local tilted director.

Graphical Abstract

KEYWORDS:

• Twist-bend nematic
• ²H-NMR
• molecular modelling
• theory-experiment integration
• molecular bend angle
• heliconical structure
• liquid crystal dimers
• conical angle

Acknowledgements

It gives us considerable pleasure to dedicate this article as some small part of the Festschrift to mark this special occasion for Professor Claudio Zannoni. His knowledge and enthusiasm for the field of liquid crystals are well known and quite exceptional. They have certainly contributed and will no doubt continue to contribute to this fascinating area of soft matter in so many ways. We have certainly been stimulated and driven by just a few of Claudio’s innovative ideas.

Disclosure statement

No potential conflict of interest was reported by the authors.