Selenoether-Linked Liquid Crystal Trimers and the Twist-Bend Nematic Phase

Bent-shaped liquid crystal (LC) dimers, trimers, and oligomers are intriguing because of their unique liquid crystallinities, which have gained further impetus after the identification of the twist-bend nematic (NTB) phase in these molecules. LC trimers exhibiting the NTB phase still remain relatively rare compared to the predominant LC dimers. We report the first homologs of selenium-linked LC trimers, 4,4′-bis[ω-(4-cyanobiphenyl-4′-ylseleno)alkoxy]biphenyls (CBSenOBOnSeCB) with carbon numbers in the alkyl-chain spacers, n = 7 or 9). Polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD) measurements were performed to investigate the phase transition behavior and mesophase structures of the trimers. Both CBSenOBOnSeCB trimers exhibited nematic (N) and NTB phases. The XRD measurements revealed the presence of smectic A-like cybotactic clusters with a triply intercalated structure in the N and NTB phases. The LC phase transition temperatures of CBSenOBOnSeCB were lower than those of the already-known ether-linked CBOnOBOnOCB and thioether-linked CBSnOBOnSCB counterparts. This trend is ascribed to the enhanced molecular bending and molecular flexibility of CBSenOBOnSeCB, which are caused by the smaller bond angle and greater bond flexibility of C–Se–C compared to C–O–C and C–S–C. This study offers a new molecular design for multiply linked LC oligomers with heavier chalcogen atoms.