2,7-substituted fluorenone-based liquid crystal trimers: twist-bend nematic phase induced by outer thioether linkage

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 (NTB) phase. The LC trimers possess a central 2,7-ether-substituted fluorenone and bilateral cyanobiphenyl groups linked via odd alkylene CnH2n spacers (n = 7 or 9) with either ether or thioether outer linkages; these are abbreviated CBOnOFlnOnOCB and CBSnOFlnOnSCB, respectively. The CBOnOFlnOnOCB trimers having only ether linkages display only the conventional nematic (N) phase, whereas the outer thioether-linked CBSnOFlnOnSCB trimers form an NTB 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, CBOnOBOnOCB and CBSnOBOnSCB.