Potential curves of two-molecule interactions in some smectic and ferroelectric liquid crystals

Abstract

The atom-atomic potential method is used to study the potential curve of pairwise intermolecular interactions in smectic crystals C [C5H11O-(OH)C6H3-CH=N-C6H4-C5H11 (A), C10H21O-C6H4-CH=CH-C6H4-OC10H21 (B)], ferroelectric smectic liquid crystals C* [C7H15O-C6H4-C6H4-COOCH2C*H(CH3)C2H5 (C), C8H17O-C6H4-C6H4-C2H4C*H(CH3)C2H5 (D)], and binary systems A+C and A+D. Both inter- and intramolecular parameters as well as their mutual effects are taken into consideration. The intermolecular interactions are shown to significantly affect the conformations of isolated molecules. For the CC, DD, and AD interactions, the antiparallel ⇅ packing of molecules is found to be most favorable. In the case of the AA and AC interactions, the ⇈ and ⇅ packings are equally favorable in terms of energy (energy difference Upair does not exceed 0.8 kcal/mole). Despite the structural similarity between the chiral molecules C and D, the presence of the highly polar groups -O- , OH, CH=N, and COO leads to pronounced changes in the position and orientation of molecules in their packing in the AC and AD pairs. The pair interaction energy Upair near the minimum point was studied as a function of the shift of molecules relative to each other along the long axis X and of the angle 9 between the long molecular axes. The curve of U(pair) vs the shift along X has a few deep local extrema. The possibility of formation of energetically favorable molecular associates (dimers) in the compounds is demonstrated. The associates are stabilized by dipole-dipole intermolecular interactions.