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We present a simplified molecular model of mesogenic dimers consisting of twoidentical uniaxial mesogenic cores separated by a fixed-length spacer andallowed to assume only two, statistically equivalent, conformations which arenon-planar and of opposite handedness. In the mean-field approximation, withadditive interactions among the mesogenic cores, the model yields up to threepositionally disordered phases, one isotropic and two nematic. The lowtemperature nematic phase ($\mathrm{N_X}$) has a local two-fold symmetry axiswhich is also a direction of molecular polar ordering and is tightly twistedabout a macroscopic phase axis. The onset of polar ordering generatesspontaneous chiral symmetry breaking, manifested primarily by the twisting ofthe polar director and the formation of chiral domains of opposite handedness.Within these domains the statistical balance between the two enantiomerconformations is shifted and the principal axes of the ordering tensors of themolecular segments twist at constant tilt angles with the helix axis. Keyexperimental results on the $\mathrm{N_X}$ phase of liquid crystalline dimersare discussed in the light of the theoretical predictions of the model, whichare also contrasted with the predictions of the twist-bend nematic model.

A Molecular Theory of the Nematic-Nematic Phase Transitions in Mesogenic Dimers