Energy transfer to dopant molecules in polyfluorene films
Polyfluorene is a promising material for applications such as polymeric light-emitting diodes. Upon optical or electrical excitation it exhibits efficient blue fluorescence and is easily made into thin films by spin-coating from solution. Incorporating suitable dopant molecules in these films changes the emission colour. The energy transfer processes taking place have been investigated here by steady-state and time resolved fluorescence measurements. To assist in the analysis of these measurements, the optical constants of polyfluorene were found by ellipsometry. Both unaligned and aligned films exhibit uniaxial anisotropy. Unaligned films have their optical axis normal to the film surface and are optically negative. Aligned films have their optical axis oriented in plane, parallel to the rubbing direction and are optically positive. In aligned films, light polarized in the alignment direction experiences absorption twice as high as light incident on unaligned films. Temperature dependent steady-state measurements were made on polyfluorene films doped with tetraphenyl poiphyrin. Energy transfer in such systems is usually described in terms of Forster transfer but it is shown here that exciton migration must also occur. A new model is developed to include this process. Diffusion lengths of 11 +1- 2 nm at low temperature and 20 +/- 2 nm at room temperature are found. Energy transfer was also investigated by fluorescence polarization anisotropy. It is shown that even at low temperature; excitons migrate far enough to depolarize the fluorescence. However, the emission is not completely depolarized and it is suggested that some excitons are trapped immediately after excitation. Polarized luminescence from dicyanomethylene in aligned polyfluorene films is observed. These molecules align themselves at least partially with the polymer chains. Although the polarization ratio not sufficient for use in a device, this is a viable method of achieving polarized fluorescence with a range of colours.