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

Abstract

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.

Columnar Mesophases with 3D Order from New Functional Nonconventional Star‐Shaped Mesogens

“…Figure 3 shows pairs of folded, E-shaped mesogens 2b organized in helical aggregates, similar to the self-assembly recently suggested for the parent stars 1 [14] or different wedge-shaped mesogens.…”

“…3) building the mesophases. [4,14] At low temperatures, the number of reflections at the meridian increases, and they become less diffuse (Fig. 1A).…”

Star Mesogens (Hekates)—Tailor‐Made Molecules for Programming Supramolecular Functionality

“…1 H double quantum solid-state NMR experiments confirm the nanosegregation of aromatic and aliphatic building blocks and show also special proximities of aliphatic and aromatic hydrogen atoms, [50] which can be rationalised by an E-shaped conformer and the presence of aliphatic protons in helical grooves. Owing to this topology, the columns pack in orthorhombic structures.…”

“…The final structure has been thoroughly studied by X-ray, dilatometry, solid-state NMR spectroscopy, AFM and molecular modelling for the parent mesogen 1 b (Figures 5A and 7). [50] This compound shows a very slow transition from a monotropic hexagonal LC to a soft columnar crystal phase. Indeed undulation can be observed with high-resolution AFM.…”

“…They were proposed upon indirect evidence from various methods, such as powder Xray diffraction (size of the structural features in the mesophases), density measurements (dilatometry) or solid-state NMR techniques. [50] In principle, the oligobenzoates 1 may accommodate in a large number of molecular shapes (T-, Eor star-shapes) if only perfect s-cis and s-trans structures of each carbonyloxy group are considered. [46] However, due to packing and nanosegregation, the E-shaped conformers are most likely.…”