Synthesis and self-association of an imine-containing m-phenylene ethynylene macrocycle

The purpose of this study was to test the suitability of the imine bond as a structural unit within the backbone of phenylene ethynylene macrocycles and oligomers by determining the ability of m-phenylene ethynylene macrocycle 1 to form π-stacked aggregates in both solution and the solid state. Macrocycle 1, with two imine bonds, was synthesized in high yield from diamine 4 and dialdehyde 5. The imine-forming macrocyclization step was carried out under a variety of conditions, with the best yield obtained simply by refluxing the reactants in methanol. The self-association behavior of 1 in various solvents was probed by ¹H NMR. The association constants (K_E) in acetoned₆ and tetrahydrofuran-d₈ were determined by fitting the concentration-dependent chemical shifts with indefinite self-association models. The results showed that solvophobically driven intermolecular π-π stacking could be preserved in the imine-containing m-phenylene ethynylene macrocycles. Interestingly, in acetone macrocycle 1 exhibited a stronger tendency to form a dimer rather than higher aggregates. We postulate that this behavior may be due to electrostatic attraction between dipolar imine groups. The solid-state packing of 1 was studied by wide- and small-angle X-ray powder diffraction (WAXD and SAXD). Bragg reflections of 1 were consistent with a hexagonal packing motif similar to our previous studies on m-phenylene ethynylene macrocycles that formed columnar liquid crystal phases.