We describe here a compre-hensive study of solution and solid-state properties of self-assembling tri-block molecules composed of a hydro-philic dendron covalently linked to an aromatic rigid rod segment,which is in turn connected to a hydrophobic flexi-ble coil.These dendron-rod-coil (DRC) molecules form well-defined supramolecular structures that possess a ribbonlike morphology as revealed by transmission-electron and atomic-force microscopy.In a large variety of aprotic solvents,the DRC ribbons create stable networks that form gels at concentrations as low as 0.2% by weight DRC.The gels are thermally ir-reversible and do not melt at elevated temperatures,indicating high stability as a result of strong noncovalent inter-actions among DRC molecules.NMR experiments show that the strong inter-actions leading to aggregation involve mainly the dendron and rodlike blocks,whereas oligoisoprene coil segments remain solvated after gelation.Small-angle X-ray scattering (SAXS) profiles of different DRC molecules demon-strate an excellent correlation between the degree-of-order in the solid-state and the stability of gels.Studies on two series of analogous molecules suggest that self-assembly is very sensitive to subtle structural changes and requires the presence of at least four hydroxyl groups in the dendron,two biphenyl units in the rod,and a coil segment with a size comparable to that of the rodlike block.A detailed analysis of crystal structures of model compounds revealed the formation of stable one-dimensional structures that involve two types of noncovalent interactions,aro-matic pi-pi stacking and hydrogen bonding.Most importantly,the crystal structure of the rod-dendron com-pound shows that hydrogen bonding not only drives the formation of head-to-head cyclic structures,but also gen-erates multiple linkages between them along the stacking direction.The cyclic structures are tetrameric in nature and stack into ribbonlike objects.We be-lieve that DRC molecules utilize the same arrangement of hydrogen bonds and stacking of aromatic blocks ob-served in the crystals,explaining the exceptional stability of the nanostruc-tures in extremely dilute solutions as well the thermal stability of the gels they form.This study provides mecha-nistic insights on self-assembly of tri-block molecules,and unveils general strategies to create well-defined one-di-mensional supramolecular objects.
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