Abstract
Molecular beam deposition of quaterthiophene oligomer on a single crystal of potassium acid phtalate yields thin films exhibiting a novel supramolecular organisation which has been determined by a combined study of X-ray diffraction and polarized optical transmission measurements. In this structure consisting of stacks of tilted H aggregates, a commensurate ratio between the KAP substrate and the overgrown oligothiophene layer (quasi-epitaxial growth) is attained. The samples are optically extremely anisotropic, with the projection of the molecular axis on the substrate surface perfectly aligned along the b crystallographic axis. Using these novel structural data, the optical properties are interpreted within the framework of the transition density approach.