Molecular curvature, specific intermolecular interactions and the twist-bend nematic phase: the synthesis and characterisation of the 1-(4-cyanobiphenyl-4′-yl)-6-(4-alkylanilinebenzylidene-4′-oxy)hexanes (CB6O.m)

Abstract

The syntheses and characterisation of the first ten homologues of the 1-(4-cyanobiphenyl-4′-yl)-6-(4-alkylanilinebenzylidene-4′-oxy)hexanes (CB6O.m) are reported. All ten members of the series exhibit an enantiotropic nematic, N, phase, and a monotropic twist-bend nematic, N_TB, phase. Only CB6O.10 shows a smectic phase. The assignment of both nematic phases was confirmed using X-ray diffraction. For short chain lengths (m = 1–6) the local packing in both nematic phases is an intercalated arrangement, for intermediate chain lengths a frustrated local structure is seen and for the longest chain length, a bilayer arrangement is observed. This change in the local structure on increasing m has no apparent effect on the stability of either nematic phase, and T_NTBN and T_NI show a regular dependence on m. Specifically, T_NTBN and T_NI decrease on increasing m and superimposed upon this is a weak odd–even effect in which the odd members show the higher values. T_NI decreases more rapidly than T_NTBN on increasing m such that the ratio T_NTBN/T_NI increases. The lower temperature liquid crystal phase shown by 1-(4-cyanobiphenyl-4′-yloxy)-5-(4-butylanilinebenzylidene-4′-oxy)pentane (CBO5O.4) is reassigned as a twist-bend nematic phase. The transitional properties of the CB6O.m, CB6O.Om and CBO5O.m series are compared.