2,7-substituted fluorenone-based liquid crystal trimers: twist-bend nematic phase induced by outer thioether linkage
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2,7-Substituted fluorenone has a planar and slightly bent aromatic structure and is used as a building block for organic liquid crystals (LCs). Herein, we report for the first time the synthesis, phase-transition behaviour, and mesophase structure of fluorenone-incorporated LC trimers that exhibit an emerging helical twist-bend nematic (N<sub>TB</sub>) phase. The LC trimers possess a central 2,7-ether-substituted fluorenone and bilateral cyanobiphenyl groups linked via odd alkylene C<i><sub>n</sub></i>H<sub>2<i>n</i></sub> spacers (<i>n</i> = 7 or 9) with either ether or thioether outer linkages; these are abbreviated CBO<i>n</i>OFlnO<i>n</i>OCB and CBS<i>n</i>OFlnO<i>n</i>SCB, respectively. The CBO<i>n</i>OFlnO<i>n</i>OCB trimers having only ether linkages display only the conventional nematic (N) phase, whereas the outer thioether-linked CBS<i>n</i>OFlnO<i>n</i>SCB trimers form an N<sub>TB</sub> phase below the temperature range of the N phase. Furthermore, the phase-transition behaviour of these fluorenone-based LC trimers is compared with that of the corresponding all-biphenyl-based motif trimer analogues, CBO<i>n</i>OBO<i>n</i>OCB and CBS<i>n</i>OBO<i>n</i>SCB.
