Electronic spectra of self-organized oligothiophene films with ''standing'' and ''lying'' molecular units

The molecular orientations of vapor-deposited alpha-quinque- and alpha-sexithiophene films are investigated with angular resolved polarized electronic absorption spectroscopy. On stretched polyethylene substrates, the film constituting molecules are ''lying'' with their long axes parallel to the surface and to the stretching direction, forming a biaxially anisotropic layer. On fused silica, the molecules are ''standing'' perpendicular to the surface, even at him thicknesses of d = 100 nm, forming a uniaxially anisotropic layer. The distinct differences in the absorption spectra of the two film types are modeled with the Fresnel equations. Within this model, the strong spectral blue-shifts observed in H-aggregates with standing oscillators, are not a consequence of excitonic coupling but of the optical anisotropy of the film.