On the twist-bend nematic phase formed directly from the isotropic phase

Abstract

The intriguing twist-bend nematic (N_TB) phase is formed, primarily, by liquid crystal dimers having odd spacers. Typically, the phase is preceded by a nematic (N) phase via a weak first-order transition. Our aim is to obtain dimers where the N_TB phase is formed directly from the isotropic (I) phase via a strong first-order phase transition. The analogy between such behaviour and that of the smectic A (SmA)–N–I sequence suggests that this new dimer will require a short spacer. This expectation is consistent with the prediction of a molecular field theory, since the decrease in the spacer length results in an increase in the molecular curvature. A vector of odd dimers based on benzoyloxybenzylidene mesogenic groups with terminal ethoxy groups has been synthesised with spacers composed of odd numbers of methylene groups. Spacers having 5, 7, 9 and 11 methylene groups are found to possess the conventional phase sequence N_TB–N–I; surprisingly, for the propane spacer, the N_TB phase is formed directly from the I phase. The properties of these dimers have been studied with care to ensure that the identification of the N_TB phase is reliable.

Graphical Abstract

Keywords:

• Twist-bend nematic
• helicoidal structure
• first-order phase transitions
• rope-like texture
• liquid crystal dimers
• ²H NMR spectroscopy
• prochiral splitting
• molecular field theory

Acknowledgements

We are grateful to a number of colleagues who have helped us with this project. They are Mr Herbert Zimmermann of the Max-Plank-Institut für Medizinische Forschung, Heidelberg for the sample of CB7CB-d₄; Professor Corrie Imrie of the University of Aberdeen for allowing us to measure optical textures of certain dimers; Professor Ghazwan Faisal of the University of Duhok for assistance with the calculations of the chemical structure and dimensions of the dimers and Mr Kevin Adlem of Merck Chemicals for allowing us to use their DSC equipment.

Additional information

Funding

The Ganesha X-ray scattering apparatus used for this research was purchased under an EPSRC Grant ‘Atoms to Applications’ [grant ref. EP/K035746/1]. We wish to thank Zakho University for the award of a split-site PhD grant to Dr Alya Dawood.