Spectroscopic studies on a series of rod-coil block copolymers with terfluorene as the rigid segment demonstrate that the main cause of color instability in fluorene oligomers and polymers is aggregate and/or excimer formation and not the presence alone of keto defects (fluorenone formation) along the molecular chain. Keto defects, when present, contribute to the appearance of the undesirable "green" emission band but are not the leading cause of color instability. Thus, the synthesis of materials where aggregation and/or interchain, intersegment interactions are inhibited is the key approach for the production of stable polymeric light-emitting devices (PLED's). The potential of this method is verified by the synthesis of photooxidative stable fluorene/styrene diblock copolymer blue emitters.