Thioether-Linked Liquid Crystal Trimers: Odd-Even Effects of Spacers and the Influence of Thioether Bonds on Phase Behavior.

We report the synthesis, phase-transition behavior, and mesophase structures of the first homologous series of thioether-linked liquid crystal (LC) trimers, 4,4'-bis[ω-(4-cyanobiphenyl-4'-ylthio)alkoxy]biphenyls (CBS n OBO n SCB with a wide range of spacer carbon numbers, n = 3-11). All CBS n OBO n SCB homologs exhibited LC phases. Interestingly, even- n and odd- n homologs showed monotropic layered smectic A (SmA) and pseudo-layered twist-bend nematic (N TB ) phases, respectively, below a nematic (N) phase. This alternate formation, which depends on spacer chain parity, is attributed to different average molecular shapes, which are associated with the relative orientations of the biphenyl moieties: linear and bent shapes for even- n and odd- n homologs, respectively. In addition, X-ray diffraction analysis indicated a strong cybotactic N phase tendency, with a triply intercalated structure. The phase-transition behavior and LC phase structures of thioether-linked CBS n OBO n SCB were compared with those of the all-ether-linked classic LC trimers CBO n OBO n OCB. Overall, thioether linkages endowed CBS n OBO n SCB with a monotropic LC tendency and lowered phase-transition temperatures, compared to those of CBO n OBO n OCB, for the same n . This is attributed to enhanced flexibility and bending (less molecular anisotropy) of the molecules, caused by the greater bond flexibility and smaller inner bond angles of the C-S-C bonds, compared to those of the C-O-C bonds.