The quasi-band-structure description of conjugated oligomers

We have investigated momentum dependent excitations in conjugated organic oligomer materials. Theoretical simulations based on the intermediate neglect of differential overlap (INDO) approach coupled to a single configuration interaction (SCI) technique considering the proper transition matrix elements for inelastic electron scattering are compared to experimental electron energy-loss spectroscopy (EELS) investigations. In this work we concentrate on sexiphenyl (6P), sexithienyl (6T) and beta-carotene. 6P and 6T are widely used in a number of electronic and optoelectronic devices and together with beta-carotene they can be regarded as model substances for the three big classes of conjugated polymer materials, namely phenylenes, thiophenes and acetylenes. Both experimental and theoretical results underline the validity of the so called quasi-band-structure model for the description of periodic molecules with only a finite number of repeat units. We also highlight the influence of chain orientation on EELS spectra and discuss the reasons for certain deviations between molecule-based simulations and the excitation processes occurring in the actual molecular crystals upon inelastic electron scattering.