December 2004 — 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.
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
researchers carried out the experiments in a low temperature
scanning tunneling microscope (STM) under ultrahigh
vacuum by exposing a close-packed copper surface to
p-diiodobenzene molecules. On the surface, the molecules
dissociate into phenylene (cyclic C6H4) reaction intermediates
and two iodine atoms. The positions of these phenylene
molecules are observed by STM. Extended structures
self-assembled as long chains on the surface. While
individual phenylene molecules remain mobile, molecules
in the chains did not move under the imaging conditions.
They were able to extract molecules from the chains,
however, by applying voltage pulses to the STM tip.
This suggests that the chains are not covalently bound
together, but instead are held by electronic reactions
between molecules that are mediated by the surface.
These chains extended across atomic steps on the copper
surface where the level of the surface drops by one
atom, resembling a stair step. This is a region in
which electronic perturbations would be expected to
disrupt the continuity. "It amazed us that these
extended structures could cross step boundaries,"
Weiss says. "These monomers have not yet formed
covalent chemical bonds, which would link them together
as a large molecule, but they are aligned and their
interaction is much stronger than any previously observed."
and McCarty were surprised to find that although the
protopolymer is 'ready' to form a molecule, individual
units can still be manipulated and even pulled out
of the chain. The protopolymer chains were stationary
on the copper surface, but short chains on a phenylene-coated
copper surface could be moved with the STM tip. The
existence of this bonding state could potentially
have significant implications for supramolecular design.
These intermolecular interactions could be used to
place compounds together like a jigsaw puzzle into
complex structures based on the choice of assembly
units and substrate surfaces--one more step toward
the molecular design and engineering of new nanostructured
research was funded, in part, by the National Science
Foundation, the Defense Advanced Research Projects
Agency (DARPA), and the Office of Naval Research.
Paul Weiss: email@example.com, (+1) 814-865-3693
Barbara Kennedy (PIO): firstname.lastname@example.org, (+1) 814-863-4682