Structure–property relationships in twist-bend nematogens: the influence of terminal groups

The synthesis and characterisation of a range of non-symmetric liquid crystal dimers designed to exhibit the twist-bend nematic phase is reported. Beginning with 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-cyanobiphenyl-4′-yl) hexane, each nitrile group is exchanged systematically for a methoxy group. The behaviour of these dimers is interpreted in terms of their bent shape being the predominant driving force for the formation of the twist-bend nematic phase, and the small differences between the twist-bend nematic–nematic transition temperatures are attributed to the differences between the interaction strength parameters of the mesogenic units. The 4-alkyloxyphenyl 4-[6-(4′-cyanobiphenyl-4-yl)hexyloxy]benzoates with ethyl, butyl, hexyl and octyl chains show the twist-bend nematic phase, whereas the corresponding 4-alkyloxyphenyl 4-[5-(4′-cyanobiphenyl-4-yloxy)pentyloxy]benzoates do not. This difference in behaviour is attributed to the more bent structure of the former. Increasing the terminal chain length initially decreases the twist-bend nematic–nematic transition temperature and this suggests that the chain disrupts the interactions between the mesogenic units. Subsequent increases in chain length have a smaller effect suggesting that the chain can be accommodated within an intercalated arrangement. The transitional behaviour of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-butyloxybiphenyl-4′-yl) hexane is compared to that of 1-(4-cyanobiphenyl-4′-yloxy)-6-(4-((S)-2-methyl)butyloxybiphenyl-4′-yl) hexane, and it is shown that chain branching strongly destabilises the twist-bend nematic phase. This is attributed to difficulties associated with packing the dimers.