Mar. 23, 1999 Virginia Tech chemistry graduate student Nori Yamaguchi and professor Harry W. Gibson are using the basic, familiar tool of hydrogen bonding to allow self-assembly to create large aggregate structures for the creation of fibers or for transport of target molecules. In presentations at the 217th American Chemical Society national meeting in Anaheim March 21-26, the chemists will describe how, using the same components -- crown ethers as hosts and secondary ammonium ions as guests, they have created two different, novel, and reversible structures based on hydrogen-bonding.
In one process, a linear supramolecular pseudorotaxane polymer is formed that can be drawn into fibers. The structure is the threading of one molecular component through another to form a linear aggregate that can be undone at the molecular level, using heat or pH.
Yamaguchi and Gibson were the first to demonstrate the linear array. The work was first published in Angewandte Chemie just this year. (Angew. Chem. Int. Ed. 1999, 38, No. 1/2: "Formation of Supramolecular Polymers from Homoditopic Molecules Containing Secondary Ammonium Ions and Crown Ether Moieties," by Yamaguchi and Gibson.)
The second arrangement of the same components resulted in dendritic pseudorotaxanes. Benzyl ether dendrons (wedge shaped molecules) with crown ether hosts at the "focal point" assemble in layers on a three-armed ammonium salt to form a macromolecular aggregate.
The huge supramolecule takes three days to form. Again the process was first described in Angewandte Chemie. (Angew. Chem. Int. Ed. 1998, 37, No. 23: "Dendritic Pseudorotaxanes," by Yamaguchi, Lesley M. Hamilton, and Gibson.) The result is "kind of a glob," says Gibson. Within the glob are pockets of a specific size that can be used to trap target molecules, which can then be transported and released by reversing the construction of the dendrimer. Gibson explains that the structures are sensitive to pH and synthesis can be reversed by exposure to base.
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