text: In recent years many industrial efforts have focused on tuning the color of lights using polyfluorenes. It was found that by doping green or red emitting materials into polyfluorenes one could tune the color emitted by the polymers. Since polyfluorene homopolymers emit higher energy blue light, they can transfer energy via Förster resonance energy transfer (FRET) to lower energy emitters. In addition to doping, color of polyfluorenes can be tuned by copolymerizing the fluorene monomers with other low band gap monomers. Researchers at the Dow Chemical Company synthesized several fluorene-based copolymers by alternating copolymerization using 5,5-dibromo-2,2-bithiophene which showed yellow emission and 4,7-dibromo-2,1,3-benzothiadiazole, which showed green emission. Other copolymerizations are also suitable; researchers at IBM performed random copolymerization of fluorene with 3,9(10)-dibromoperylene,4,4-dibromo-R-cyanostilbene, and 1,4-bis(2-(4-bromophenyl)-1-cyanovinyl)-2-(2-ethylhexyl)-5-methoxybenzene. Only a small amount of the co-monomer, approximately 5%, was needed to tune the emission of the polyfluorene from blue to yellow. This example further illustrates that by introducing monomers that have a lower band gap than the fluorene monomer, one can tune the color that is emitted by the polymer.
  Substitution at the nine position with various moieties has also been examined as a means to control the color emitted by polyfluorene. In the past researchers have tried putting alkyl substituents on the ninth position, however it has been found that by putting bulkier groups, such as alkoxyphenyl groups, the polymers had enhanced blue emission stability and superior polymer light-emitting diode performance (compared to polymers which have alkyl substituents at the ninth position).