Dye molecules doped polythiophene (PT) films prepared by electrochemical doping were investigated by Fourier transform infrared spectroscopy (FTIR) reflection absorption spectroscopy (RAS) and transmission in order to investigate whether the presence of an electric field could control the ordering of the dye molecule assembly in the polymer. The present work focuses on organic dye molecules, which contain three benzene rings in plane with a high symmetry such as crystal violet (CV) and brilliant green (BG) as a dopant. In the case of CV doped PT by applying triangle-shaped negative voltage, the orderings of CV molecules were found by FTIR measurements. Since the ring stretching mode due to the componential benzene ring around 1585 cm(-1) was observed only in the FTIR transmission spectra while not observed in the spectrum of FTIR RAS spectra, this result shows that CV molecules are doped in the polymer so that the ring plane of CV is inclined to be parallel to the polymer film plane. In the case of CV doped PT samples by field-free casting method, the ring stretching modes were observed in both spectra of FTIR RAS and transmission, indicating that the dopant molecules were randomly oriented. (c) 2007 American Vacuum Society.