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

Abstract

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 ↔ ΦOB). The low-temperature phase transition was attributed to the transition between a highly ordered, oblique columnar (ΦOK) phase and the ΦOB 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 ΦOK phase were packed into an ordered mesophase having about 64% of the methylene carbon atoms in long trans segments at room temperature. In the ΦOB 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 ΦOK phase was proposed and qualitatively proven using a simulated structural diffraction.