Electronic Energy Transfer in Oriented Bilayer Films of Polysilanes

Abstract

The intermolecular energy transfer was studied in a bilayer film of polysilanes. The highly oriented films of poly(diethylsilylene) (PDES) were prepared by the mechanical deposition technique, which had been originally developed by Wittmann and co-workers. The poorly oriented layer was formed on the highly oriented PDES layer by spin-casting a solution of poly(methyloctadecylsilylene) (PMOdS). The bilayer films were characterized with polarized UV absorption and polarized fluorescence spectroscopy. It was shown that the light absorption in the PMOdS layer contributed to the fluorescence intensity of the PDES layer and that the electronic energy was transferred from the PMOdS layer to the PDES layer. Although the poorly oriented PMOdS layer absorbs both parallel-polarized light and perpendicular-polarized light, the highly oriented PDES layer emits only light polarized parallel to the orientation direction of the silicon main chains. Thus, the bilayer film has a function of rotating the polarization direction and of the isotropic-to-polarized light conversion. The fluorescence intensity of PDES was shown to increase with an increase in the thickness of the PMOdS layer, but the increase in the fluorescence intensity saturates above 500 nm. The saturation is caused by the limitation of the migration distance of excitons. The polarization of the excitation light influenced the emission intensity when the thickness of the PMOdS layer was thinner than 100 nm, suggesting that the excitons move between the sites having similar orientations of transition dipole moments. The possible mechanism of exciton migration was discussed in terms of the segment models, in which the molecular chains are separated into the ordered segments with various lengths.