Liquid crystal tetramers and spontaneous chirality: the heliconical twist-bend nematic phase

Abstract

The synthesis and characterisation of six groups of liquid crystal tetramers that vary in the nature of the flexible spacers is reported. Each group consists of four mesogenic units: two outer cyanobiphenyl fragments and two inner benzylideneaniline units. Three different types of spacers are used to connect these: ether linked, methylene linked, and an alkyloxy chain. In each group of tetramers, the outer spacers are held constant, and the central spacer is varied. All the tetramers reported exhibit a conventional nematic, N, phase. The nematic-isotropic transition temperature, TNI, and associated entropy change are dependent on the parity of the central spacer but not as strongly as seen in liquid crystal dimers, indicating that the four mesogenic units are not strongly orientationally correlated along the molecular length. In addition to the N phase, tetramers with two odd-membered outer spacers exhibit a twist-bend nematic, NTB, phase whereas those having two even-membered outer spacers show a smectic A phase, revealing that the length and parity of the spacer mostly determine phase behaviour. A compound with an odd-membered methylene linked central spacer is a single exception to this behaviour and this is attributed to the highly bent nature of the central spacer. A surprising inversion is seen in the sense of the odd-even effect associated with the NTB-N transition temperature on switching an ether linked to a methylene linked central spacer. This is attributed to differences in the interaction strength between the unlike mesogenic groups.