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Introduction

Polyfluorene

Identifiers
CAS Number	95270-88-5 Y
Properties
Chemical formula	(C13H10)n
Molar mass	Variable
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

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Polyfluorenes are important class of materials. They are relevant to both academic and industrial research because of their optical and electrical properties. Furthermore they are a prototypical conjugated polymer which can be used to discuss property tuning. Polylfuorenes are the only class of conjugated polymers which can emit light spanning the entire visible region.Polyfluorenes are primarily interesting because of the optoelectronic properties imbued by their chromophoric constituents and their extended conjugation. They are not a naturally occurring material, but rather, they are designed and synthesized for their applications. The design of polyfluorene derivatives relies on the character and properties of their monomers. Thus, the discovery and development of these polymeric repeat units has had a profound influence on the development of polyfluorenes.

Properties of Polyfluorenes

Figure 3. This is the photoluminescence of two very similarly structured polyfluorene derivatives. The one on the left has the structure in Figure 2, and the one on the right has the structure in Figure 4.

Figure 4. This polyfluorene derivative has a structure similar to that in Figure 2, except that the alcohol side chains can participate in ESIPT with the neighboring oxadiazole nitrogens causing a red-shifted emission. The meta-linkages in the backbone impart solubility in lieu of multiple side chains.

Polyfluorenes encompass an important class of polymers which have the potential to act as both electroactive and photoactive materials. This is due to the shape of fluorene. The fluorene monomer of polyfluorene is planar which enables p-orbital overlap at the linkage between its two benzene rings enabling conjugation across the molecule. This in turn allows for a reduced band gap due to the delocalized excited state molecular orbitals. Furthermore, since the degree of delocalization and the spatial location of the orbitals on the molecule is influenced by the electron donating character of its substituents, the band gap energy can be varied. This chemical control over the band gap directly dictates the color of the molecule by limiting the energies which it absorbs. Interest in polyfluorene derivatives has increased because of their high photoluminescence quantum efficiency, thermal stability, and also their facile color tunability, which can be obtained by introducing low-band-gap co-monomers. Research in this field has increased significantly because of their potential application to light emitting diodes because polyfluorenes are the only family of conjugated polymers that can emit colors spanning the entire visible range with high efficiency and low operating voltage. Furthermore, polyfluorenes are relatively soluble in most solvents, which makes them ideal for general applications. Another important quality of polyfluorenes is their thermotropic liquid crystallinity which allows the polymers to be used on rubbed polyimide layers. Thermotropic liquid crystallinity refers to the polymers ability to exhibit a phase transition into the liquid crystal phase as the temperature is changed. This is very important to the development of LCD’s (liquid crystal display) because the synthesis of liquid crystal displays entails that the liquid-crystal molecules at the two glass surfaces of the cell be aligned in parallel to the two polarizer foils. This can only be done by coating the inner-surfaces of the cell with a thin, transparent film of polyamide which is then rubbed with a velvet cloth. Microscopic groves are then generated in the polyamide layer and the liquid crystal in contact with the polyamide, which is polyfluorene, can align in the rubbing direction. In addition to LCDs, polyfluorene can also be used to synthesize LEDs. By using polyfluorene, LED’s have been synthesized that can emit polarized light with polarization ratios of more than 20 and with brightness of 100 cd/m2. Even though this is very impressive it is not sufficient for general applications.

Problems with Polyfluorenes

Even though they have many good qualities there are several issues that often arise when researchers are working with polyfluorenes which inhibits the use of polyfluorenes for many general applications. Polyfluorenes often show both excimer and aggregate formation upon thermal annealing or when current is passed through them. Excimer formation involves the formation of dimerized units of the polymer, which tend to emit light at lower energies than the polymer itself. As one would expect this hinders the use of polyfluorenes for most applications including in LEDs. When excimer or aggregate formation occurs this lowers the efficiency of the LEDs by decreasing the efficiency of charge carrier recombination. Excimer formation also causes a red shift in the emission spectrum. Researchers have attempted to eliminate excimer formation and enhance the efficiency of polyfluorenes by copolymerizing polyfluorene with anthracene and end-capping polyfluorenes with bulky groups which could sterically hinder excimer formation. Additionally, researchers have tried adding large substituents at the ninth position of the fluorine in order to inhibit excimer and aggregate formation. Furthermore, researchers have tried to improve LED’s by synthesizing fluorene-triarylamine copolymers and other multilayer devices that are based on polyfluorenes that can be cross-linked. These have been found to have high brightness and reasonable efficiencies. Polyfluorenes are often branched polymers which increases chain entanglement. Another problem commonly encountered by polyfluorenes is a commonly observed broad green, parasitic emission which detracts from the color purity and efficiency needed for an OLED. Initially attributed to excimer emission, this green emission has been shown to be due to the formation of ketone defects along the fluorene polymer backbone (oxidation of the 9 position on the monomer) at incompletely substituted 9 positions of the fluorene monomer. Routes to combat this involve ensuring full substitution of the monomer’s active site, or including aromatic substituents. These solutions may present sub-optimal structures (in terms of bulkiness) or may be synthetically difficult.

Industrial Uses of Polyfluorene

The fluorene molecule is most commonly linked at the 2 and 7 positions in polyfluorene derivatives. Also, the 9 position is typically where side chains are attached.

Fluorene-based polymers are of great interest to industrial researchers because of their ability to act as electro and photoactive materials. Like many conjugated polymers, researchers have always been interested in using polyfluorenes in light-emitting diodes, field-effect transistors, and plastic solar cells. As stated previously polyfluorenes gave high photoluminescence quantum yields, this along with their excellent solubility and the ability to control their properties by substituting different groups at the 9,9 position of the fluorene monomer has caused researchers to use polyfluorenes as blue light emitters in polymer light-emitting diodes. In recent years many industrial efforts have been focused on tuning color using polyfluorenes. It was found that by doping green or red emitting materials into polyfluorenes one could tune the color emitted by the polymers. In addition to doping, it was found that one could tune the color of polyfluorenes 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 emissions and 4,7-dibromo-2,1,3-benzothiadiazole, which showed green emissions. Other methods of copolymerization are also suitable, such as researchers at IBM used random copolymerization using 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. One can see 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 ninth position with various substituents has also been examined to control the color emitted 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 researchers found that 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.