The two benzene rings in each unit make polyfluorene an aromatic hydrocarbon, specifically conjugated polymer, and give it notable optical and electrical properties, such as efficient photoluminescence.

When spoken about as a class, polyfluorenes are derivatives of this polymer, obtained by replacing some of the hydrogen atoms by other chemical groups, and/or by substituting other monomers for some fluorene units.

They stand out among other luminescent conjugated polymers because the wavelength of their light output can be tuned through the entire visible spectrum by appropriate choice of the substituents.

Fluorene, the repeat unit in polyfluorene derivatives, was isolated from coal tar and discovered by Marcellin Berthelot prior to 1883.

Since it was an interesting chromophore researchers wanted to understand which parts of the molecule were chemically reactive, and how substituting these sites influenced the color.

[8][9] The optical properties (such as variable luminescence and visible light spectrum absorption) that accompany the extended conjugation in polymers of fluorene have become increasingly attractive for device applications.

[17] Since the degree of delocalization and the spatial location of the orbitals on the molecule is influenced by the electron donating (or withdrawing) character of its substituents, the band gap energy can be varied.

In OLEDs, polyfluorenes are desirable because they are the only family of conjugated polymers that can emit colors spanning the entire visible range with high efficiency and low operating voltage.

The second decomposition process results in aggregation leading to a red-shifted fluorescence, reduced intensity, exciton migration and relaxation through excimers.

Additionally, researchers have tried adding large substituents at the nine position of the fluorene in order to inhibit excimer and aggregate formation.

In opposition to this tendency, researchers have used tri-functional monomers to create highly branched polyfluorenes which do not aggregate due to the bulkiness of the substituents.

[25] This solution reduces the ease of processability of the material because branched polymers have increased chain entanglement and poor solubility.

Another problem commonly encountered by polyfluorenes is an observed broad green, parasitic emission which detracts from the color purity and efficiency needed for an OLED.

[19] The color of the molecules can be designed through synthetic control over the electron donating or withdrawing character of the substituents on fluorene or the comonomers in polyfluorene.

However, oxidative polymerization does produce soluble polymers (from side-chain containing monomers) which are more easily characterized with nuclear magnetic resonance.

Such routes have enabled excellent control over the properties of polyfluorenes; the fluorene-thiophene-benzothiadiazole copolymer shown above, with a band gap of 1.78 eV when the side chains are alkoxy,[11] appears blue because it is absorbing in the red wavelengths.

[13] The luminescent color of polyfluorenes can be changed, for example, (from blue to green-yellow) by adding functional groups which participate in excited state intramolecular proton transfer.

Copolymerization of fluorene with other monomers allows researchers to optimize the absorption and electronic energy levels as a means to increase the photovoltaic performance.

In polymer blend solar cells, the voltage produced by the device is determined by the difference between the electron donating polymer’s highest occupied molecular orbital (HOMO) energy level and the electron accepting molecules lowest unoccupied molecular orbital (LUMO) energy level.

[36] For instance by adding electronegative groups on the end of conjugated side chains, researchers lowered the HOMO of a polyfluorene copolymer to −5.30 eV and increased the voltage of a solar cell to 0.99 V.[36][37][38] Typical polymer solar cells utilize fullerene molecules as electron acceptors because of their low LUMO energy level (high electron affinity).