The relationship between molecular structure and the incidence of the NTB phase

In this work, we present the first part of a study into the relationship between molecular structure and the occurrence of the twist-bend nematic phase' (N-TB). Given the large amount of chemical space that might reasonably be expected to give rise to the N-TB phase, this paper is only concerned with methylene-linked bimesogens bearing polar terminal groups based on the initial work of George Gray on cyanobiphenyls. As with other studies, we find that the N-TB phase is observed only for materials that contain an odd number of methylene units in the spacer chain. It also appears that, in a given series of materials, there is a weak negative correlation between the dipole moment of the individual mesogenic units and the thermal stability of the N-TB phase. Furthermore, we find that increasing the length-breadth ratio of the individual mesogenic units also provides a significant increase in the thermal stability of the N-TB phase. The electrooptic behaviour of two materials, one with a terminal nitrile unit and one with an isothiocyanate group, was investigated. The N-TB phase of the NCS-terminated material can be switched with a large applied voltage (20Vm(-1)); however, the analogous nitrile-terminated material showed no electrooptic response under these conditions. Either the threshold voltage to switching is simply lower for isothiocyanate materials than nitriles or that there is more than one phase currently identified as the twist-bend nematic.