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.

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