Molecular structure and the twist-bend nematic phase: the role of spacer length in liquid crystal dimers

The liquid crystal dimers, the 1-(4-substitutedazobenzene-4′-yloxy)-4-(4-cyanobiphenyl-4′-yl)butanes (CB4OABX), are reported in which the terminal substituent is either a methyl, methoxy, butyl, butyloxy, cyano or nitro group. The butyloxy spacer endows these dimers with the required molecular curvature to exhibit the twist-bend nematic phase in addition to showing the conventional nematic phase. Their transitional properties are compared to those of the corresponding dimers with either a pentyloxy or hexyloxy spacer. As expected, the even-membered pentyloxy-based dimers show the highest nematic–isotropic transition temperature, T_NI, and exhibit smectic behaviour. These observations are attributed to their linear molecular shapes. The values of both the twist-bend nematic–nematic transition temperature, T_NTBN, and T_NI increase on passing from the butyloxy to hexyloxy spacer, but the change in T_NI is greater than that in T_NTBN. Thus, the ratio T_NTBN is greater for the shorter spacer reinforcing the view that molecular curvature drives the formation of the N_TB phase relative to the N phase. By comparison, the melting point decreases on passing from the butyloxy to hexyloxy spacer. Thus, increasing molecular curvature simultaneously increases both the melting point and N_TB phase stability and this highlights the design challenge in obtaining dimers that exhibit enantiotropic N_TB–I transitions.