Structure-property relationships in azobenzene-based twist-bend nematogens

The synthesis and characterisation of a new set of azobenzene-based non-symmetric liquid crystal dimers, the 1-(4-substitutedazobenzene-4ʹ-yloxy)-6-(4-methoxybiphenyl-4ʹ-yl)hexanes (MeOB6OABX), that exhibit the twist-bend nematic phase, N_TB, is described. The terminal substituents are methyl, methoxy, ethyl, butyl, butoxy, and nitrile. All six dimers exhibit both the N_TB and conventional nematic, N, phases. The identification of the N_TB phase was performed using polarised light microscopy and confirmed for binary mixtures with a standard twist-bend nematogen 1,7-bis-4-(4ʹ-cyanobiphenyl) heptane (CB7CB). The transitional behaviour of the MeOB6OABX dimers is compared with that of the corresponding ether-linked 1-(4-substitutedazobenzene-4ʹ-yloxy)-6-(4-methoxybiphenyl-4ʹ-yloxy)pentanes, MeOBO5OABX, all of which exhibit a conventional nematic phase. In addition, the nitrile-substituted MeOBO5OABCN shows the N_TB phase. The behaviour of these non-symmetric dimers is also compared to that of the corresponding symmetric dimers. Differences in the transitional properties between these sets of new materials are accounted for in terms of not only molecular shape but also other factors including the strength of the mixed mesogen interaction.