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

The twist-bend nematic (N-TB) 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 N-TB 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-N-TB and rare direct Iso-N-TB 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-N-TB phase transition. In this study, we report that CB3CB also shows the direct Iso-N-TB phase transition, whereas CB2OCB exhibits the typical Iso-N-N-TB phase sequence with decreasing temperature. The Iso-LC (Iso-N-TB or Iso-N) phase-transition temperatures upon cooling show the order CB2OCB (108 degrees C) > CB3CB (49 degrees C) > CB2SCB (43 degrees 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.