Self-assembled supra-molecular liquid crystalline behavior via hydrogen bonding in symmetric tapered bisamides

Recently, the concepts of supra-molecular mesophase structures and their dynamics have became one of the central research topics in self-assembled systems. To further understand the structural relationship between molecular and supra-molecular levels, symmetric tapered bisamide (1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido]benzene bisamide, C_nPhA (here n is the number of methylene units and methyl groups in the alkyl chains in the molecules) show supra-molecular liquid crystalline behavior induced by H-bonding. These molecules were designed and synthesized in Dr. Harris' group. The molecules consist of three aromatic rings and two amide groups alternatively linking the rigid-rod and their alkyl chains are tapered at the each end of the rod.; Two major phase transitions of C₁₄PhA bisamide were identified based on differential scanning calorimetry (DSC) experiments. Structural sensitive experiments such as one- and two-dimensional wide angle X-ray diffraction (WARD) and electron diffraction (ED) methods indicated that the high-temperature phase transition was between the isotropic melt and a low-ordered oblique columnar phase (Φ_OB). The low-temperature phase transition was between the Φ_OB phase and a high-ordered LC columnar (Φ _K) phase. The Φ_OB packing was a 2D structure, with parameters a = 3.34 nm, b = 4.62 nm, α = β = 90°, γ = 56.6°. The Φ_K phase was monoclinic with a = 3.28 nm, b = 3.87 nm, c = 0.97 nm, α = β = 90°, γ = 58.1°. The formation of these ordered supra-molecular structures were critically dependent upon formation of the basic building blocks (“columns”) constructed by C₁₄PhA bisamide. There were two factors in forming these supra-molecular columns: H-bonding formed via the bisamide mesogenic cores and micro-phase separation between the amide cores and alkyl chains. A detailed packing model for this “Φ_K” monoclinic unit cell was also proposed. It was suggested that within the column, two C14PhA bisamide molecules were face-to-face with the phenylenes in the amide cores and packed in edge-on scheme. Their rigid core axes were ∼±27.5° away from the column long axis. Every pair was rotated 90° from its neighboring pair along the column long axis, leading to an altered orientation of the rigid core axes in the neighboring C₁₄PhA bisamide cores.