Selenium-linked cyanobiphenyl-based liquid crystal dimers: the effects of chalcogen linkage and spacer length on the twist-bend nematic phase.

We synthesised selenium-linked liquid crystal dimers and examined their phase behaviour to determine the effects of chalcogen linkage and spacer length on the formation of a twist-bend nematic (NTB) phase. Two new homologous series of nonsymmetric selenoether- and ether- or thioether-linked cyanobiphenyl-based dimers (CBSenOCB and CBSenSCB, respectively) comprising an alkyl spacer chain with a carbon number (n) of 3, 5, 6, 7, 9, and 11 were developed. We also synthesised n = 6 and 11 of previously reported symmetrically selenoether-linked CBSenSeCB homologues. Conventional nematic (N) and NTB phases were observed for nearly all odd-n CBSenSeCB, CBSenOCB, and CBSenSCB dimers. The phase transition behaviours of the three homologous series were compared with those of the known symmetric ether-linked CBOnOCB, thioether-linked CBSnSCB, and asymmetric thioether- and ether-linked CBSnOCB dimers. Replacing ether with thioether or selenoether resulted in lower LC phase-transition temperatures, particularly for selenoether. Single-crystal structural analyses revealed that CBS3SCB and CBSe3SeCB exhibited folded U-shaped molecular shapes that were bent more than the V-shapes of CBOnOCB (n = 3 and 5) and CBS5SCB. In addition, thioether and selenoether linkages rendered the molecular forms of odd-spacer dimers in the crystals more bent. [ABSTRACT FROM AUTHOR]