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MATERIALS SCIENCE
Northwestern University chemistry professor Tobin J. Marks and coworkers have synthesized a set of perfluoroarene-modified thiophene oligomers that can be either p-type or n-type semiconductors, depending on the positions of the electron-deficient perfluoroarene building blocks [Angew. Chem. Int. Ed., 42, 3900 (2003)]. This is the first known family of compounds in which individual members can take on different conduction pathways, the researchers say. "The achievement should aid in the design of high-mobility n-type organic semiconductors," Marks adds. Because organic-based semiconductors can be made smaller and are more flexible than silicon-based materials, they are expected to lie at the heart of future generations of thin-film transistors, light-emitting diodes, and other electronic devices. These "plastic electronics" are generally fabricated by alternating layers of p-type (hole-transporting) and n-type (electron-transporting) materials. A number of p-type organic compounds have already been synthesized, but n-type organic compounds are more challenging to prepare and are still rare. The researchers made six-ring oligomers in which the two perfluoroarene rings are either in the middle of the chain, next to the ends of the chain, or in the terminal positions of the chain. Solutions and thin films of the compounds were characterized by a suite of analytical methods, including X-ray diffraction and cyclic voltammetry, and compared with unmodified sexithiophene (a chain of six thiophenes). The researchers also prepared thin-film transistors from the oligomers. The oligomers with perfluoroarenes in the inner positions of the chain are p-type semiconductors, they found--the same as sexithiophene. But the oligomer with perfluoroarene rings at the ends of the chain is an n-type semiconductor. Marks and coworkers attribute the differences in electronic behavior "to a subtle interplay of molecular stereoelectronic effects on molecular energy levels." These effects are modulated by differences in intermolecular packing of the oligomers that result from interactions between the electron-poor perfluoroarene rings and electron-rich thiophene rings, they note. "The synthesis and careful mapping of what happens as the number and position of fluorine atoms changes in the oligomer is nice work," notes Ananth Dodabalapur, a professor of electrical and computer engineering at the University of Texas, Austin. "The new compounds are useful since they are based on the well-known sexithiophene."
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