Twist-bend liquid crystal phases and molecular structure: the role of methoxybiphenyl

The synthesis and characterisation of the 4-[{[4-({6-[4-(4-methoxyphenyl)phenyl]hexyl}oxy)phenyl]methylidene}amino] phenyl 4-alkyloxybenzoates is reported. These are referred to using the acronym MeOB6OIBeOm in which m denotes the number of carbon atoms in the terminal alkyloxy chainand is varied from one to ten. All ten members exhibit an enantiotropic conventional nematic (N) phase. In addition, for m=1-9, the twist-bend nematic (NTB) phase was observed on cooling the N phase. The N-isotropic (I) and NTB-N transition temperatures decrease on increasing the length of the terminal chain and this is more pronounced for the former. This supports the view that the NTB-N transition is predominantly shape driven and this depends largely on the length and parity of the flexible spacer. The transitional behaviour of the MeOB6OIBeOm series is compared with that of the corresponding dimers based instead on a cyanobiphenyl fragment, the CB6OIBeOm series. The rich smectic polymorphism exhibited by the CB6OIBeOm series is extinguished for the MeOB6OIBeOm series. The CB6OIBeOm series shows higher values of the N-I transition temperature than the corresponding members of the MeOB6OIBeOm series whereas the values of the NTB-N transition temperatures are rather similar for corresponding members of the two series. This again suggests that the NTB-N transition is largely shape driven whereas the mixed core interaction plays a more distinct role in driving N phase formation. The promotion of smectic behaviour in the CB6OIBeOm series is attributed to the strong tendency of the cyanobiphenyl fragment to adopt anti-parallel associations.