Liquid Crystal Dimers Based on Seven-Membered Bridged Stilbene Exhibiting Twist-Bend Nematic Phases

We report the first examples of bent-shaped LC dimers based on a seven-membered bridged stilbene. We synthesized nonylene- and ether-linked cyano-terminated dimers (sC9-tCN and sOC7O-tCN, respectively) and a homologous series of nonylene-linked alkyl-terminated dimers (sC9-tCn) with alkyl carbon atoms n = 1–6. Polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction measurement were employed to investigate the phase-transition behavior and LC phase structures. sC9-tCN and sOC7O-tCN only exhibited a nematic (N) phase, whereas sC9-tCn (n = 1–5) formed both the NTB and N phases. sC9-tC5 additionally formed an unidentified X phase from the NTB phase and sC9-tC6 exhibited a smectic A phase from the N phase. The weak dispersion force and intermolecular affinity provided by the terminal alkyl chains are likely to be preferable to the large dipole–dipole interactions by the cyano termini for the NTB phase formation of the present dimers. The isotropic points of sC9-tCn showed an odd–even oscillation with n, whereas the N–NTB phase transition temperatures were comparable. Remarkably, the NTB stripe textures of sC9-tCn appeared perpendicular to the rubbing direction, and the N–NTB phase transitions exhibited their second-order nature. This study revealed the unique NTB phase properties of the 7-membered bridged stilbene-based LC dimers.