The low emission band at 2.2-2.3 eV in polyfluorene-based conjugated materials is studied by various spectroscopic methods on defined copolymers of 9-9'-difarnesyl-fluorene with 9-fluorenone, which can be seen as a model compound for degraded polyfluorenes. Absorption, electroluminescence, and photoluminescence in the film (temperature-dependent) and solution (room temperature) reveal the optical properties of this low-energy emission band emerging in polyfluorene-type polymers upon degradation. All the experimental evidence presented yield direct evidence against excimer or aggregate formation as the primary source of the low-energy emission band. Instead keto defect sites can be shown to be responsible for the emissive defect.