Premium
The Molecular Basis of Self‐Assembly of Dendron–Rod–Coils into One‐Dimensional Nanostructures
Author(s) -
Zubarev Eugene R.,
Sone Eli D.,
Stupp Samuel I.
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200600619
Subject(s) - dendrimer , supramolecular chemistry , small angle x ray scattering , molecule , stacking , non covalent interactions , crystallography , self assembly , chemical physics , hydrogen bond , covalent bond , chemistry , liquid crystal , nanostructure , materials science , nanotechnology , crystal structure , scattering , polymer chemistry , organic chemistry , physics , optoelectronics , optics
We describe here a comprehensive study of solution and solid‐state properties of self‐assembling triblock molecules composed of a hydrophilic dendron covalently linked to an aromatic rigid rod segment, which is in turn connected to a hydrophobic flexible 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 irreversible and do not melt at elevated temperatures, indicating high stability as a result of strong noncovalent interactions among DRC molecules. NMR experiments show that the strong interactions 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 demonstrate 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, aromatic π–π stacking and hydrogen bonding. Most importantly, the crystal structure of the rod–dendron compound shows that hydrogen bonding not only drives the formation of head‐to‐head cyclic structures, but also generates multiple linkages between them along the stacking direction. The cyclic structures are tetrameric in nature and stack into ribbonlike objects. We believe that DRC molecules utilize the same arrangement of hydrogen bonds and stacking of aromatic blocks observed in the crystals, explaining the exceptional stability of the nanostructures in extremely dilute solutions as well the thermal stability of the gels they form. This study provides mechanistic insights on self‐assembly of triblock molecules, and unveils general strategies to create well‐defined one‐dimensional supramolecular objects.