Twist–Bend Nematic Phase Behavior of Cyanobiphenyl-Based Dimers with Propane, Ethoxy, and Ethylthio Spacers

The twist–bend nematic (NTB) phase is a liquid crystal (LC) phase with a heliconical structure that typically forms below the temperature of the conventional nematic (N) phase. By contrast, the direct transition between the NTB and isotropic (Iso) phases without the intermediation of the N phase rarely occurs. Herein, we demonstrate the effects of linkage type (i.e., methylene, ether, and thioether) on the typical Iso–N–NTB and rare direct Iso–NTB phase-transition behaviors of cyanobiphenyl (CB) dimers CB3CB, CB2OCB, and CB2SCB bearing three-atom-based propane, ethoxy, and ethylthio spacers, respectively. In our previous study, CB2SCB exhibited the monotropic direct Iso–NTB phase transition. In this study, we report that CB3CB also shows the direct Iso–NTB phase transition, whereas CB2OCB exhibits the typical Iso–N–NTB phase sequence with decreasing temperature. The Iso–LC (Iso–NTB or Iso–N) phase-transition temperatures upon cooling show the order CB2OCB (108 °C) > CB3CB (49 °C) > CB2SCB (43 °C). The thioether-linked CB2SCB is vitrifiable, whereas CB3CB and CB2OCB exhibit strong crystallization tendencies. The phase-transition behaviors are also discussed in terms of the three bent homologous series with different oligomethylene spacers n: CBnCB, CBnOCB, and CBnSCB.