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INTRAMOLECULAR OR INTERMOLECULAR
Experiments untangle ins and outs of conjugated polymers' interactions
Conjugated polymers are playing a key role in the emerging world of molecular electronics. When doped, for example, they can behave like metals or semiconductors--an area of research that claimed the 2000 Nobel Prize in Chemistry.
But polymers are almost always amorphous, full of all kinds of disorder, and therefore difficult to characterize. In probing the electronic properties of, say, a light-emitting diode, it's been a challenge to separate the roles of individual polymer chains from the contributions of interpolymer interactions.
By carefully designing conjugated polymers, however, chemists at MIT now have found a way to tease apart the intra- and the inter-polymer aspects. Chemistry professor Timothy M. Swager and former graduate student Jinsang Kim--currently a postdoc at Caltech--control the precise orientations of individual polymers in a single layer and follow how their properties shift in response to induced conformational changes [Nature, 411, 1030 (2001)].
"People have been trying to figure out what the role of conjugation is in these polymers in the absence of strong aggregation of the chains," Swager tells C&EN. "But you never could get beyond the fact that, once you have the polymers nice and flat in a thin film, they tend to want to stick together."
Kim and Swager have finessed that problem by constructing polymers with specifically tailored characteristics. Depending on the particular building blocks the researchers choose, the polymers line up as isolated chains without any interpolymer aggregation, as aggregated assemblies, or can be coaxed to switch back and forth between isolation and aggregation.
"In trying to understand the light-emitting properties of a solid polymer film, it is intuitive that there are intramolecular and intermolecular contributions," notes Jeffrey S. Moore, professor of chemistry and materials science and engineering at the University of Illinois, Urbana-Champaign. "Deconvoluting these two components, as Swager has done, is important for our understanding. Possibly, mastering control at this level will allow properties to be fine-tuned."
University of Tokyo chemistry professor Takuzo Aida, another researcher in the field, agrees that Swager and Kim's strategy allows manipulation of the inter- and intramolecular aspects of conjugated polymers. "This is a very smart approach, and to my knowledge, the first such demonstration," Aida says.
The MIT researchers use poly(p-phenylene-ethynylene) compounds in their experiments. The four different building blocks they designed differ in the choice of substituents on the phenyl rings. In one case, the ring carries hydrophobic groups positioned so that the phenyl ring prefers to lie flat at an air-water interface. In this "face-on" orientation, -orbital interactions between phenyl groups in adjacent chains are impossible, so each polymer molecule remains isolated.
Another building block's substituents direct its phenyl group to orient itself so that it is perpendicular to the interface. The ring's -orbitals then project toward those of adjacent chains, so the polymers interact and aggregate. Two other building blocks are designed to switch from a flat to a perpendicular orientation when the polymer is squeezed under pressure. Molecular modeling combined with detailed observations of the polymer systems as they undergo pressure-induced phase transitions allow Kim and Swager to separate inter- from intrapolymer effects.
Swager and coworkers plan to apply their strategy to conducting polymers and to the design of sensors--a major focus of the group's research.
|GRACE UNDER PRESSURE Polymers 14 are constructed from various combinations of building blocks AD. Polymer 1 prefers to lie flat--"face-on"--at an air-water interface, an orientation that isolates each polymer chain from its neighbors. Polymer 2 also adopts the face-on conformation at normal pressure, but building block C rotates under pressure into what MIT researchers call the "zipper" organization. Polymer 3 switches from a zipper to an "edge-on" orientation under pressure, which polymer 4 adopts under all conditions. Edge-on conformations allow interpolymer aggregation.
ADAPTED BY PERMISSION FROM NATURE 2001
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Copyright © 2001 American Chemical Society