Phase behaviors of classic liquid crystal dimers and trimers: Alternate induction of smectic and twist-bend nematic phases depending on spacer parity for liquid crystal trimers

Abstract

A new heliconical mesophase without apparent molecular positional ordering, which was discovered in achiral bent molecules [i.e., the twist-bend nematic (NTB) phase], has become a hot topic of research in the field of liquid crystals (LCs). In this study, a series of classic LC trimer homologs, namely 4,4′-bis[ω-(4-cyanobiphenyl-4′-yloxy)alkoxy]biphenyls (CBOnOBOnOCB; n = 4–11), was synthesized, and their phase transitions and mesophase structures were evaluated in detail, to reinvestigate their elusive monotropic mesophases. Polarized optical microscopy (POM) observations revealed that below the nematic (N) phases, even-n homologs exhibited the fan-shaped and polygonal textures of the layered smectic (Sm) phase, while odd-n homologs displayed the blocky and striped textures associated with the recently discovered heliconical NTB phase. X-ray diffraction analyses of CBOnOBOnOCB (n = 10 and 11) revealed that their mesophases exhibited the cybotactic N phase property, and suggested that the monotropic mesophase observed for odd CBO11OBO11OCB was the pseudo-layered heliconical NTB phase rather than the layered Sm phase. Therefore, it was suggested that the Sm and NTB phases were alternately induced, depending on the spacer parity of classic LC trimer homologs CBOnOBOnOCB. This could be ascribed to the steric interactions associated with different molecular geometries, which are largely governed by the imposed spacer parity, and possess parallelly and obliquely located mesogenic biphenyl moieties for the even-n and odd-n homologs, respectively. In addition, the phase-transition behavior of a homologous series of ether-linked cyanobiphenyl-based LC dimers (CBOnOCB; n = 3–12) was investigated. POM observations shows that odd-n CBOnOCB homologs (n = 5, 7, 9, and 11) exhibited the monotropic N–NTB phase transition, which was only observed in the small supercooled N domains. Even-n CBOnOCB homologs exhibited only the N phase and did not exhibit Sm phases below the temperature of the N phase upon cooling. Hence, alternate induction of the Sm and NTB phases depending on n parity was observed only in the LC trimer homologs.