Blacksburg, Va., April 7, 2002 -- Virginia Tech chemistry professor Harry W. Gibson has measured the constants that describe self-assembly in the creation of a supramolecular assembly potentially important to the processing of many novel materials.
The work will be presented at the 223nd national meeting of the American Chemical Society, April 7-11 in Orlando.
Pseudorotaxanes are chemical compounds that contain non-covalent linkages consisting of a cyclic unit penetrated by a linear species. Think of rings tossed on pegs.
For 15 years, with funding from the National Science Foundation (NSF) Materials Division, Gibson and his group have explored the possibilities of pseudorotaxanes and rotaxanes (counterparts in which the ends of the linear species are too bulky to allow ‘dethreading’ to occur) and of adding them to other molecules to make polyrotaxanes. Think of different kinds of yarn and string threaded through rings then knotted, or heated and melted, or snarled. Materials created have included indestructible polymers, reversible supramolecular polymers, and molecular transport mechanisms.
In his latest work, being published in the Journal of the American Chemical Society, he used a molecule with three arms (secondary ammonium ion) as a trifunctional building block and added a ring-shaped compound (crown ether) to each arm. The rings and pegs self assemble based on hydrogen bonding and Gibson has been able to measure the efficiency of each individual step of the process of putting one, two, and three rings on in succession.
Because the bond in these pseudorotaxanes is non-covalent -- not based on shared electrons between the molecules' atoms -- it can be reversed. In the case of the experimental compounds, a change of pH will allow the pegs (the guest compound) to fall out of the ring (the host compound). An application might be clean up of a waste stream using a host that will attract a specific guest chemical, for instance.
But Gibson has gone an additional step. He has attached dendron-shaped molecules to the ring. Dendrons are wedge-shaped molecules with many arms -- like a tree. The dendron attachment to the ring is covalent. The dendron adds additional functionality to the rotaxane. The many-armed dendron can be used to attach to other substances. If it is hydrophilic and the rotaxane is hydrophobic, the dendron allows the rotaxane to be water soluble for instance. With the dendron-linked rings, "Once one ring goes on, the other two do too," he says. "The self-assembly process becomes cooperative." It is like when oxygen binds to hemoglobin, he says. "All of the connection sites take up oxygen and the hemoglobin becomes fully loaded. We are beginning to mimic nature."
Gibson will present highlights from his students over the past 15 years and conclude with the research that resulted in measurements of the self-assembly process. The presentation is at 1:30 p.m. Wednesday, April 10, in Convention Center room 314A.
There will also be two posters by Zhongxin Ge, post doctoral fellow working with Gibson in the chemistry department at Virginia Tech.
A poster by Ge and Gibson entitled "Fullerene-Cored Dendrimer Self-Assemblies" describes the process for covalently bonding benzylammonium salts to a C60 fullerene -- a hollow molecule made up of 60 carbon atoms.
There is great interest in fullerenes, also called buckeyballs, for nanotechnology applications.
The achievement was based on the research described above with rotaxanes. "We used a host-guest system to add solubility," explains Gibson. "We can put up to 12 guest or host sites on a C60 molecule, allowing non-covalent modification via pseudorotaxane self-assembly."
The presence of the molecular recognition sites on the fullerene molecule allows it to be manipulated in two ways. First, the host/guest species are like handles that will allow the fullerene to be organized -- three dimensionally -- in the solid state. Second, by use of bulky partners the fullerene core can be isolated, which would prevent self-quenching when it is used in photovoltaic films, for instance.
The research is funded by Luna Technologies of Blacksburg, Va.
The second poster, by Ge, chemistry student Jason W. Jones, and Gibson, entitled "Non-Covalent Block Copolymers by Self-Assembly: New Polypsuedorotaxanes," describes the non-covalent synthesis of a strong, flexible new polymer from two cheap polymers.
Industry wants to use cheap polymers like styrene for a variety of plastics, such as pickup truck bed caps, but styrene is brittle. By covalent incorporation of a segment of a rubbery polymer, such as butadiene, the brittleness can be removed, producing an impact resistant material. Gibson believes that by placing a host species at one end of a styrene molecule and a guest species at the end of a rubber-like polymer such as butadiene self-assembly via pseudorotaxane formation will result in a material with properties similar to the commercial analog. The advantage of the self-assembly approach is that upon heating the non-covalent linkage is reversed, lowering the viscosity and permitting easier molding.
The research is funded by the NSF Materials Division.
The posters will be presented at 6 p.m. Sunday, April 7, in Hall C of the convention center.
Illustrations of the various structures can be found at www.chem.vt.edu/chem-dept/gibson/pages/recent_research_results.html
PR CONTACT: Susan Trulove (540) 231-5646 STrulove@vt.edu
Journal
Journal of the American Chemical Society