Thioether-linked liquid crystal dimers and trimers: The twist-bend nematic phase

Systematic synthesis of thioether-linked dimers and trimers was carried out to reveal molecular designs for inducing mesophases and twist-bend nematic (N-TB) phases. A five-fold approach based on molecular structural parameters including the terminal substituent, the position of the thioether bond, the nature of the bridge bond linking benzene rings in the mesogenic fragments, the nature of bonds at the linkage on the side oppose to the thioether linkage, and oligomeric effect was evaluated. Dimers with cyano groups at the 4,4'-positions in each mesogenic fragment of the thioether-based dimeric system were found to afford a large long-axis directional dipole moment in the fragments and exhibited potential for application as mesogens, with some molecules indicating the ability to form N-TB phase. The dimers containing the thioether bond as a part of the spacer and not at the linkage position to the mesogenic fragments deteriorate as twist-bend nematogens but behave as traditional nematogens. Also, imine-, azo- and triple-bond bridged mesogenic fragments lead the dimers to be twist-bend nematogenic. Notably, all asymmetric thioether-linked cyanobiphenyl dimers with different functional bonds at the linkage on the side oppose to the thioether linkage, such as methylene, ketone, and two-typed esters (C=OO and OC=O) can form NTB phases, with some of them retaining the phase upon cooling to room temperature, which highlights the utility of thioether linkage in the molecular design of twist-bend nematogens. In the trimer system (or oligomeric effect), a cyanobiphenyl-based trimer composed of interior ether and exterior thioether linkers formed an NTB phase. This paper, for the first time, not only reveals the systematic molecular design of thioether-linked dimers but also offers the prospect of the thioether-linked oligomers, for twist-bend nematic liquid crystals. (C) 2019 Elsevier B.V. All rights reserved.