Self-assembled "Supra-molecular" Structures via Hydrogen Bonding and Aromatic/Aliphathic Microphase Separation on Different Length Scales in Symmetric-Tapered Bisamides

Self-assembled "supra-molecular" liquid-crystalline phases were investigated in one of a series of symmetrically tapered bisamides based on an amide core of 1,2-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene and three alkyl tails on each side of the core. One of this series of bisamides having 14 carbon atoms in each tail (abbreviated as C₁₄PhBA) served as an example in this study. Differential scanning calorimetry thermal diagrams showed two main phase transition processes in C₁₄PhBA. Wide-angle X-ray diffraction results revealed that the high-temperature phase transition is between the isotropic melt (I) and a 2D low-ordered oblique columnar phase (I ↔ Φ_OB). The low-temperature phase transition was attributed to the transition between a highly ordered, oblique columnar (Φ_OK) phase and the Φ_OB phase. Supra-molecular columns were directly visualized using transmission electron microscopy and atomic force microscopy. The formation of these supra-molecular structures is critically dependent upon the construction of building blocks ("columns") by C₁₄PhBA molecules. These building blocks developed via rigid amide core/alkyl tail micro-phase separation and the hydrogen (H) bond formation between the N-H and carbonyl groups among the cores. Spectroscopic studies showed that the columns were stabilized by intercore H-bonds oriented along the column long axis, and the alkyl tails in the Φ_OK phase were packed into an ordered mesophase having about 64% of the methylene carbon atoms in long trans segments at room temperature. In the Φ_OB phase, the alkyl tails adopt liquidlike disordered conformations. The high- and low-temperature phase transitions were thus mainly attributed to the sudden changes in H-bonding and alkyl tail conformation, respectively. A detailed packing model of the Φ_OK phase was proposed and qualitatively proven using a simulated structural diffraction.