Phase behaviour and Janus hierarchical supramolecular structures based on asymmetric tapered bisamide

Abstract

A precisely defined molecular Janus compound based on asymmetric tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamide (abbreviated as C₂₂PhBAEO₃) was designed and synthesized, and its phase behavior was fully investigated. The C₂₂PhBAEO₃ compound possesses a rigid core with three aromatic rings connected with amide bonds which possess the ability to form hydrogen (H) bonds. Three hydrophobic alkyl flexible tails and three hydrophilic flexible methyl terminated triethylene glycol tails are located at the other end. Major phase transitions and their origins in C₂₂PhBAEO₃ were studied via DSC and 1D WAXD techniques. Its hierarchical supramolecular crystal structure was further identified through combined techniques of 2D WAXD and SAXS as well as SAED. Results based on computer simulations confirmed the structure determination. It was found that the C₂₂PhBAEO₃ possesses three phases through various thermal treatments including a micro-phase separated columnar liquid crystal (col.) phase, a metastable crystal I phase and a stable crystal II phase. Among them, the crystal II phase showed that the columnar structure possesses 3D inter-column order and highly crystalline alkyl tails with a long-range overall orientational order. Four C₂₂PhBAEO₃ molecules self-assembled into a phase-separated disc with an ellipsoidal shape having a C₂ symmetry along the disc normal. These discs then stacked on top of each other to generate a 1D asymmetric column through H-bonding, and further packed into a 3D long-range ordered monoclinic lattice. The unit cell parameters of this lattice were determined to be a = 5.08 nm, b = 2.41 nm, c = 0.98 nm, α = 90°, β = 90°, and γ = 70.5°. The alkyl chain tails crystallize within the hydrophobic layers and possess a relatively fixed orientation with respect to the column packing due to the selective interactions based on the hydrophobic/hydrophilic microphase separation. Both phase behaviour and unit cell structure showed significant difference compared with the symmetrically tapered counterparts. The results provided a new approach of fine-tuning not only in the Janus supramolecular structures but also in the formation pathway of the self-assembling process in order to meet the specific requirements for optical and biological applications.