Star-Shaped Oligobenzoates: Non-conventional Mesogens Forming Columnar Helical Mesophases

• Published in: Chemistry : a European journal 2008-04, Vol.14 (12), p.3562-3576
• Main Authors: Lehmann, Matthias, Jahr, Michael, Donnio, Bertrand, Graf, Robert, Gemming, Sibylle, Popov, Igor
• Format: Article
• Language: English
• Subjects: liquid crystals; oligobenzoates; solid-state NMR spectroscopy; star-shaped mesogens; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200700922

Star‐shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non‐conventional mesogens form various columnar mesophases over a broad temperature range. The liquid‐crystal phases were characterised by optical microscopy, differential scanning calorimetry, X‐ray diffraction, dilatometry and solid‐state NMR spectroscopy. In addition to the high‐temperature hexagonal columnar phases, the columnar self‐assemblies undulate upon cooling and consequently form higher‐ordered body‐centred orthorhombic columnar 3D structures. 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 the polar oligobenzoate scaffold are not perfectly separated in these star‐shaped mesogens. Helices from folded stars: Star‐shaped oligobenzoates self‐assemble into hexagonal and orthorhombic columnar mesophases (see graphic). Folded conformers form columnar aggregates with a helical organisation by balancing nano‐segregation and space filling.