UV/VIS AND IR SPECTROSCOPIC STUDIES ON MOLECULAR-ORIENTATION IN ULTRATHIN FILMS OF POLYTHIOPHENE MODEL COMPOUNDS

The characteristic differences shown by optical spectra of oligothiophenes (OTs) in films on silica surfaces and in solution are explained by the Molecular Exciton Model. Thus unsubstituted OTs in monolayers are arranged in a side-by-side manner, whereas substitution with terminal alkyl groups destroys this alignment, thereby reducing the pi-overlap of adjacent molecules. With the orientation of the transition moments within the molecular framework known from stretched sheet experiments polarized UV/Vis and IR spectra reveal that the long molecular axis of the unsubstituted OTs in monolayers on silica is directed perpendicularly to the surface, while alkyl substituted oligomers do not show any anisotropy. This influence of the alkyl groups and the reduced order in multilayers of unsubstituted OTs both suggest a decisive role of the interaction between surface and pi-electron-system for the molecular arrangement in thin films.