Optical and structural properties of oligothiophene crystalline films

We have studied optical properties of oriented crystalline oligothiophene films on fused silica glasses. In crystalline films, the lowest absorption band of isolated molecules is split into a week lower-energy band and a strong higher-energy band. The absorption spectra depend on the polarization of oblique incident light. The dichroism of the absorption spectra and the enhancement of the higher-energy side of the absorption band are caused by the orientation of molecules, i.e., the formation of H aggregates in crystalline films. Luminescence appears near the week lower-energy hand of the absorption spectrum. The luminescence spectrum and dynamics depend on the thickness of the crystalline film. The correlation between optical properties and microstructures in oriented crystalline films is discussed.