Electronic energy transfer in oriented bilayer films of polysilanes

The intermolecular energy transfer was studied in a bilayer film of polysil anes. The highly oriented films of poly(diethylsilylene) (PDES) were prepar ed by the mechanical deposition technique, which had been originally develo ped by Wittmann and co-workers. The poorly oriented layer was formed on the highly oriented PDES layer by spin-casting a solution of poly(methyloctade cylsilylene) (PMOdS). The bilayer films were characterized with polarized U V absorption and polarized fluorescence spectroscopy. It was shown that the light absorption in the PMOdS layer contributed to the fluorescence intens ity of the PDES layer and that the electronic energy was transferred from t he PMOdS layer to the PDES layer. Although the poorly oriented PMOdS layer absorbs both parallel-polarized light and perpendicular-polarized light, th e highly oriented PDES layer emits only light polarized parallel to the ori entation direction of the silicon main chains. Thus, the bilayer film has a function of rotating the polarization direction and of the isotropic-to-po larized light conversion. The fluorescence intensity of PDES was shown to i ncrease with an increase in the thickness of the PMOdS layer, but the incre ase in the fluorescence intensity saturates above 500 nm. The saturation is caused by the limitation of the migration distance of excitons. The polari zation of the excitation light influenced the emission intensity when the t hickness of the PMOdS layer was thinner than 100 nm, suggesting that the ex citons move between the sites having similar orientations of transition dip ole moments. The possible mechanism of exciton migration was discussed in t erms of the segment models, in which the molecular chains are separated int o the ordered segments with various lengths.