Dielectric Study of Liquid Crystal Dimers: Probing the Orientational Order and Molecular Interactions in Nematic and Twist-Bend Nematic Phases

Dielectric spectroscopyin frequencies that range from 10 Hz to1 GHz has been used to study the molecular orientational dynamicsof the two types of dimers that form the twist-bend nematic phaseover a wide range of temperatures for both nematic and twist-bendnematic phases. The symmetrical and asymmetrical liquid crystal dimerswith the cyanobiphenyl mesogenic groups were investigated. The resultswere analyzed within the framework of the molecular theory of dielectricpermittivity for nematogens. The two molecular processes can be assignedto the reorientation of the monomeric unit: the high frequency oneto the precessional rotation of the longitudinal components of thecyanobiphenyl groups (CN) and the second (low frequency) to the end-over-endrotation of the CN dipole around the short molecular axis. The precessionmode, which is determined by the local order and is almost unaffectedby the phase transition from the nematic to the twist-bend phase,while the end-over-end rotation clearly slowed down at the transition,as it is affected by the growth of the intermolecular interactions.The latter corresponds well to the fact revealed by IR spectroscopyabout the longitudinal correlation of the molecular dipoles.