Polymorphism in liquid crystals from star-shaped mesogens

Abstract

Star-shaped molecules, based on a phloroglucinol core with three oligobenzoate arms, are powerful mesogens that can be combined to columnar liquid crystalline materials and soft organic crystals. The structural properties of such modern materials exhibit polymorphism at two different length scales. On the molecular level, the mesogens are highly flexible molecules with a large variety of almost isoenergetic conformers of varying propensity towards regular stacking in the bulk phase. On the supramolecular level, polymorphic structures occur, which comprise different liquid crystalline phases with anisotropic columnar order and soft crystals with a more isotropic three-dimensional ordering. For the smallest mesogen, the present results are evidence that the two forms of polymorphism are linked. Columnar liquid crystals are formed from a low-symmetry, folded conformer, which already accounts for the nano-segregation of the planar aromatic centre and the flexible aliphatic chain parts of the mesogen, whereas the soft, more isotropic crystals are grown from the C-3-symmetric, star-shaped conformer of a derivative with shorter alkyl chains. Molecular dynamics simulations confirm the stability of each structure at room temperature.