Star-shaped Oligobenzoates: Non-Conventional Mesogens Forming Columnar Helical Mesophases

Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repaeting units have been synthesised. These non-conventional mesogens possess various columnar mesophases over a broad temperature range. The liquid crystal phases are characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and Solid State NMR spectroscopy. Beside a high temperature hexagonal columnar phase, the columnar structures transform to higher ordered, undulated columns in a body-centered orthorhombic unit cell at low temperatures. A model of E-shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low temperature phases. Space-filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and polar oligobenzoate scaffold are not perfectly separated for these star-shaped mesogens.