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

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 (CnH2n) 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 (NTB) phase, while all the non-symmetric (CN)AzoSnSCB dimers exhibited the NTB phase. Notably, the NTB phase observed for non-symmetric (CN)AzoS7SCB could be supercooled to room temperature, leading to a glassy NTB 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 NTB phases supercooled to room temperature. Such prominent stability of the NTB 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.