Phase behaviour of ester-linked cyanobiphenyl dimers and fluorinated analogues: the direct isotropic to twist-bend nematic phase transition

ABSTRACT

Typically, the twist-bend nematic (N_TB) phase appears below the temperature of the conventional nematic (N) phase. The direct formation of the N_TB phase from the isotropic (Iso) phase is significantly rare. In this study, we discovered that certain ester-linked bent-shaped dimers exhibit a direct Iso–N_TB phase transition. We synthesised two homologous series of ester-linked cyanobiphenyl-based dimers (COOn) and their fluorinated analogues (F-COOn) containing different numbers of carbon atoms (n) in the alkyl spacer chains (n = 3, 5, 7, 9, 11, and 13). The phase transitions of these dimers were investigated using polarised optical microscopy and differential scanning calorimetry. The long-spacer COOn (n = 7, 9, 11, and 13) exhibited the usual Iso–N–N_TB phase sequence, whereas the short-spacer COOn (n = 3 and 5) showed the direct Iso–N_TB phase transition. Lateral fluorination significantly reduced the phase-transition temperatures for F-COOn compared to COOn. The short-spacer F-COOn (n = 3 and 5) did not exhibit any liquid crystal phases. The F-COOn with longer spacers (n = 9, 11, and 13) displayed the Iso–N–N_TB sequence, whereas F-COO7 showed the direct Iso–N_TB transition. Additionally, we analysed a single-crystal structure of F-COO7 to investigate its molecular conformation and superimposition.

GRAPHICAL ABSTRACT

KEYWORDS:

• Twist-bend nematic phase
• isotropic to twist-bend nematic phase transition
• liquid crystal dimer
• ester linkage
• fluorination

Acknowledgments

The authors acknowledge funding from the Japan Society for the Promotion of Science (KAKENHI grant number 20K15351) and from Toyohashi University of Technology.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2241039

Additional information

Funding

This research was financially supported by the Japan Society for the Promotion of Science [KAKENHI grant number 20K15351] and Toyohashi University of Technology.