Conformational effects on optical charge transfer in the emeraldine base form of polyaniline from electroabsorption measurements and semiempirical calculations

The electroabsorption (Stark) spectrum of the emeraldine base form of polya niline (EB) in a polymethyl methacrylate (PMMA) matrix at room temperature is presented. For the lowest-energy (1.98 eV) band, the absolute value of t he change in dipole moment on excitation (\ Delta<(<mu>)over right arrow>\) is 6.1 D and the trace of the change in polarizability (Tr <(<Delta>alpha )over left right arrow>) is 200 Angstrom (3). Similar values were obtained for the corresponding electronic transition of a five-ring oligomer of EB t hat is symmetrically terminated with phenyl rings, also in a PMMA matrix. I n order to understand the origin of the substantial value of \ Delta<(<mu>) over right arrow>\ measured for these nominally symmetric species, a series of calculations were performed in which the geometries of model oligomers were varied in a systematic fashion and resulting effects on the values of \ Delta<(<mu>)over right arrow>\ were determined. Numerous low-energy confo rmers were identified by this method having substantial values of \ Delta<( <mu>)over right arrow>\, suggesting that the large \ Delta<(<mu>)over right arrow>\ measured experimentally results from conformational heterogeneity. Our findings suggest that Stark spectroscopy is a useful probe of structur al disorder in this technologically important polymer in the solid state. ( C) 2001 American Institute of Physics.