First observation of extended chains of molecules that exhibit a strong interaction without forming chemical bonds
Protopolyphenylene is composed of molecules lined up for reaction and held in place by the copper substrate surface and intermolecular interactions. The image of a 270-angstrom (27-nanometer) square area of the surface was recorded with a scanning tunneling microscope at 77 K in ultrahigh vacuum. The ridged ring structure is the protopolymer; the angled lines are one-atom-high steps of the copper substrate.
A new chemical state, designated a "protopolymer," has been observed by Penn State researchers in chains of phenylene molecules on a crystalline copper surface at low temperature. Protopolymers form when monomers, small molecules that link together chemically to form long chains, align and interact without forming chemical bonds. The novel structures were discovered by Paul S. Weiss, professor of chemistry and physics at Penn State and Gregory S. McCarty, a graduate student at time of discovery and now a research assistant professor of engineering science and mechanics. While surface-mediated pairing and other interactions have previously been seen on metal surfaces, this is the first observation of extended chains of molecules that exhibit a strong interaction without forming chemical bonds. This type of alignment could be used to control growth and assembly of molecules and for manipulation of nanostructured materials, which are assembled on an atomic or molecular scale. Nanostructured materials often exhibit very different properties from those made by conventional techniques. A paper describing the research results, titled "Formation and Manipulation of Protopolymer Chains," will be published in the Journal of the American Chemical Society on 15 December 2004.
Weiss points out that in substrate-mediated interactions, those in which the surface participates in the electronic interactions between molecules, the surface itself acts as a catalyst, holding molecules in place and enabling them to align for reaction. "If we use substrate-mediated interactions to direct the arrangement of monomers prior to chemical bonding, we may be able to build atomically precise structures," says Weiss. "The key is to understand how the electronic functions of the molecule-surface interaction drive reactions and how they can be used to enhance chemical selectivity."
Barbara K. Kennedy | EurekAlert!
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