New patterns of twist-bend liquid crystal phase behaviour: the synthesis and characterisation of the 1-(4-cyanobiphenyl-4′-yl)-10-(4-alkylaniline-benzylidene-4′-oxy)decanes (CB10O·m)

Abstract

The synthesis and characterisation of the 1-(4-cyanobiphenyl-4′-yl)-10-(4-alkylanilinebenzylidene-4′-oxy)decanes (CB10O·m) are reported. This series shows a rich liquid crystal polymorphism including twist-bend nematic and smectic phases. All the homologues reported exhibit an enantiotropic conventional nematic phase. For the homologues with m ≤ 10, the local packing in the nematic phases and the layer spacing in the smectic phases indicates an intercalated arrangement of the molecules. An intercalated smectic C_A phase is observed if m/11 ≈ 0.5. Either side of this condition, the twist-bend nematic phase is observed, a novel pattern of behaviour for a series on increasing a terminal chain length. For longer chain lengths, m = 12, 14, 16 and 18, two twist-bend smectic C (SmC_TB) phases are observed, and the packing of the molecules is now of a bilayer-type. The higher temperature variant is termed SmC_TB-SH in which SH (single helix) refers to the presence of a short, distorted clock-type helix. In the lower temperature SmC_TB-DH phase, an additional longer helix is superimposed on the short one, and DH denotes double helix.