Web Release Date: May 17,
Correlating Molecular Design to Microstructure in Thermally Convertible Oligothiophenes: The Effect of Branched versus Linear End Groups
and
Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Electrical Engineering and Computer Sciences and Department of Chemistry, University of California, Berkeley, California 94720, and Division of Materials Science, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received: January 31, 2006
In Final Form: April 3, 2006
Abstract:
The thin film microstructure development of functionalized oligothiophenes with branched, thermally removable
groups at each end of conjugated cores with five, six, and seven thiophene rings was monitored during their
thermal conversion from solution processible precursors to insoluble semiconductor products. The change in
end group character provides a comparison of branched vs linear end group functionalization in oligothiophenes.
Near edge X-ray absorption fine structure (NEXAFS) spectroscopy confirmed that branched 
-, 
-substitutions
of the precursors strongly influenced the packing of the conjugated core. The quinque- and sexithiophene
precursors oriented perpendicular to the substrate, whereas the septithiophene precursor oriented parallel to
the substrate, providing one of the first examples of length dependence in oligothiophene orientation. This
dependence may be due to a packing mismatch between the conjugated cores and the branched end groups.
The convertible septithiophene exhibits four distinct microstructures as it converts from precursor to product
that correlate strongly with its field-effect hole mobility in field-effect transistors. The extent of septithiophene
order and the surface-relative orientation of its ordered phases clearly influence field-effect transistor
performance.
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