The role of keto defect sites for the emission properties of polyfluorene-type materials

In this paper we report on the identification of a dominant degradation mechanism of blue light emitting polyfluorene (PFs) type polymers, which turns the desired blue emission color of these polymers into an undesired blue–green emission. As shown the low energy emission band at 2.2–2.3 eV can be identified as the emission from exciton and/or charge trapping keto defect sites (9-fluorenone sites), which in fact can be regarded as emission of a fluorescing guest effect incorporated into the π-conjugated PF backbone. We present spectroscopic evidence for the formation of keto defect sites in 9-monoalkylated polyfluorenes (PF-a) which are identified as the source of the strong low energy emission band. Furthermore, we show that the keto defects can be generated directly during polymer synthesis as shown for the PF-a or can be formed as main channel of an (photo-, thermal- or electro-)oxidative degradation process as shown for the 9-bisalkylated PF.