Dependency of the twist-bend nematic phase formation on the molecular shape of liquid crystal dimers: A view through the lens of DFT

Abstract

In view of the importance of molecular structural features, particularly the molecular curvature, that form the basis for the appearance and stabilization of the twist-bend nematic (Ntb) phase, the present work reports a computational study on a set of cyanobiphenyl dimers. In these dimers, the chemical nature of the linkage between the spacer and mesogenic units has been systematically varied by incorporating either methylene, ether, or thioether groups. Through computational modeling, we calculated the bend angle between the two mesogenic arms from all-trans conformations as well as Boltzmann-weighted average bend angle from conformational ensembles for each dimer. It was observed that the incidence of the Ntb phase is not only sensitive to the degree of molecular bend angle but also the conformational distribution. The present analysis recognized that the Ntb phase is only seen in liquid crystal dimers that have a sufficiently bent molecular structure. In addition, we have found a strong dependency of transitional properties upon these molecular factors. The changes in the phase transition temperatures appear to be a direct consequence of discrete changes in the molecular bend angle, as this is the major difference when comparing these dimers.