Orientational ordering and chiral symmetry breaking in organic monolayers composed of disklike mesogenic molecules:...

Orientational ordering of disklike molecules on a flat surface is investigated using a molecular-statistical theory and Monte-Carlo simulations. The theory is based on the two-dimensional orientational order parameter for molecules with a threefold symmetry axis, and on a simple model interaction potential which has been derived taking into consideration only the symmetry of basic molecular structure. The theory reveals three different anisotropic phases. One of them exactly corresponds to the structure which has recently been observed experimentally in self-assembling monolayers of discotic mesogenic molecules on a pyrolitic graphite surface. This is a two-dimensional (2D) chiral anisotropic phase composed of nonchiral molecules. The phase consists of three sublattices with different orientational order. One sublattice is orientationally disordered, while the other two sublattices are ordered with the same scalar order parameter and different orientations of the ordering tensor. Both order parameters of the directions of ordering are determined self-consistently by minimizing the total free energy of the system. The detailed structure of this unusual phase is also confirmed by the results of Monte Carlo simulations based on the same model interaction potential. The results of the theory qualitatively explain existing experimental data and also shed some light on the origin of supramolecular structures observed in 3D columnar phases composed of similar molecules.