C3-symmetrical supramolecular architectures:: Fibers and organic gels from discotic trisamides and trisureas

Hydrogen bonded C-symmetrical molecules that associate into supramolecular stacks are described. Structural mutation on these molecules has been performed to elucidate the contribution of the different secondary interactions (hydrogen bonding, pi-pi stacking) to the self-assembly of the disks into chiral stacks. Twelve C3-symmetrical molecules have been investigated, six of which contain three central amide functionalities (1a-f) and six of which contain three central urea groups (2a-f). Peripheral groups of the disks are "small", "medium", or "large", half of them being achiral and the other half being chiral, to enable investigation of the supramolecular architectures with CD spectroscopy. In all of the cases, elongated, helical stacks are formed in apolar solution, except for the "medium" amide disks 1c/d. The elongated stacks of the C-symmetrical disks form gels, which are visualized by AFM and SANS, and this confirms the directionality of the interactions. For the "large" urea disk, 2f, fibers with a length of up to 2 mum are observed. Temperature dependent and "sergeants-and-soldiers" CD measurements reveal that the urea stacks are much more rigid than the corresponding amide ones. In case of the "medium" urea disks, 2c/d, a true rigid rod, is formed. Where amide disks immediately reach their thermodynamic equilibrium, kinetic factors seem to govern urea aggregation. In a number of experiments aimed at reversibility with the urea stacks, hysteresis is observed, implying that these urea disks initially form a poorly defined stack, which subsequently transforms slowly into a well-defined, chiral architecture.