SEMIEMPIRICAL CALCULATIONS AND SCANNING PROBE MICROSCOPY STUDIES OF POLYTHIOPHENE THIN-FILMS

Electrochemically prepared films of polythiophene were studied using t he technique of noncontact scanning probe microscopy. Images of the fi lm surfaces revealed many polymer strands atop the bulk film surface. The average diameter of these strands was 9-15 +/- 2 Angstrom, indicat ing a coil or helical structure. The substructure of the bulk film sur face was not resolvable, likely indicating a close-packed structure of (planar) polymer chains. Semiempirical AM1 calculations on isolated b ithiophene and terthiophene oligomers indicate minima at torsional ang les of 35 degrees and 153 degrees for bithiophene and combinations of these torsional angles for terthiophene. These calculations show the e nergetic feasibility of a nonplanar, syn-like thiophene molecular conf ormation on the film surface, where interactions with neighboring poly thiophene are minimized. Geometry-optimized AM1 calculations were also performed for hydrated bithiophene. We show that partial orientation of the polar water molecules in the neighborhood of the bithiophene ol igomer alters the energetics of torsion angle rotation for this molecu le. Torsion angle minima were found to be at 0 degrees and 180 degrees , with the relative potential barrier between these conformations redu ced with respect to the isolated case.