Thioether-linked azobenzene-based liquid crystal dimers exhibiting the twist-bend nematic phase over a wide temperature range

ABSTRACT

Two homologous series of sulfur-containing thioether-linked 4-cyanoazobenzene-based liquid crystal (LC) dimers were developed: a symmetric series based on 4-cyanoazobenzene arms [(CN)AzoSnSAzo(CN)] and a non-symmetric series based on 4-cyanoazobenzene and 4-cyanobiphenyl arms [(CN)AzoSnSCB]. They contained different flexible oligomethylene spacers (C_nH_2n) consisting of odd numbers of carbon atoms (n = 3, 5, 7, 9, and 11). The symmetric (CN)AzoSnSAzo(CN) dimers with n = 5, 7, 9, and 11 showed an elusive monotropic twist–bend nematic (N_TB) phase, while all the non-symmetric (CN)AzoSnSCB dimers exhibited the N_TB phase. Notably, the N_TB phase observed for non-symmetric (CN)AzoS7SCB could be supercooled to room temperature, leading to a glassy N_TB phase. (CN)AzoS9SCB showed a similar tendency accompanied by partial crystallisation. These are the first reported examples of azobenzene-based LC dimers exhibiting a broad temperature range of N_TB phases supercooled to room temperature. Such prominent stability of the N_TB phase is due to the supercooling effect assisted by the synergy of molecular bend arising from thioether linkages and non-symmetric mesogenic arms. The phase-transition temperatures were overall higher for symmetric (CN)AzoSnSAzo(CN) than for non-symmetric (CN)AzoSnSCB. The difference in the mesogenic arm structures hardly influenced their molecular geometry or anisotropy in terms of entropy changes.

GRAPHICAL ABSTRACT

KEYWORDS:

• Liquid crystal dimers
• room temperature twist–bend nematic phase
• azobenzene
• sulfur
• thioether linkage

Acknowledgments

We are grateful to financial support from the Japan Society for the Promotion of Science KAKENHI grant nos. 17K14493 and 20K15351, and research grants from the Naito Foundation, the Toukai Foundation for Technology, and Toyohashi University of Technology. We would like to thank Prof. Masatoshi Tokita (Tokyo Institute of Technology) for use of the instruments for XRD measurements, and Ms. Tsugumi Shiokawa and Dr. Hiroko Tada in the Division of Instrumental Analysis (Okayama University) for the mass measurements.

Disclosure statement

The authors declare no conflict of interest.

Supplementary material

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