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 (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 crystallization. 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. It is assumed that the difference in the mesogenic arm structures hardly influenced their molecular geometry or anisotropy in terms of entropy changes.