Polyfluorene

Polyfluorene is a polymer with formula (C13H8)n, consisting of fluorene units linked in a linear chain — specifically, at carbon atoms 2 and 7 in the standard fluorene numbering. It can also be described as a chain of benzene rings linked in para positions (a polyparaphenylene) with an extra methylene bridge connecting every pair of rings. 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. These polymers are being investigated for possible use in light-emitting diodes, field-effect transistors, plastic solar cells, and other organic electronic applications. 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.

History Fluorene, the repeat unit in polyfluorene derivatives, was isolated from coal tar and discovered by Marcellin Berthelot prior to 1883. Its name originates from its interesting fluorescence (and not to fluorine, which is not one of its elements). Fluorene became the subject of chemical-structure related color variation (visible rather than luminescent), among other things, throughout the early to mid-20th century. 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. For instance, by adding various electron donating or electron accepting moieties to fluorene, and by reacting with bases, researchers were able to change the color of the molecule. The physical properties of the fluorene molecule were recognizably desirable for polymers; as early as the 1970s researchers began incorporating this moiety into polymers. For instance, because of fluorene’s rigid, planar shape a polymer containing fluorene was shown to exhibit enhanced thermo-mechanical stability. However, more promising was integrating the optoelectronic properties of fluorene into a polymer. Reports of the oxidative polymerization of fluorene (into a fully conjugated form) exist from at least 1972. However, it was not until after the highly publicized high conductivity of doped polyacetylene, presented in 1977 by Heeger, MacDiarmid and Shirakawa, that substantial interest in the electronic properties of conjugated polymers took off. As interest in conducting plastics grew, fluorene again found application. The aromatic nature of fluorene makes it an excellent candidate component of a conducting polymer because it can stabilize and conduct a charge; in the early 1980s fluorene was electropolymerized into conjugated polymer films with conductivities of 10−4 S cm−1. 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. Throughout the 1990s and into the 2000s, a number of devices such as organic light-emitting diodes (OLEDs), organic solar cells., organic thin film transistors, and biosensors have all taken advantage of the luminescent, electronic and absorptive properties of polyfluorenes.

Properties

Polyfluorenes are an important class of polymers which have the potential to act as both electroactive and photoactive materials. This in part due to the shape of fluorene. Fluorene is generally planar; p-orbital overlap at the linkage between its two benzene rings results in conjugation across the molecule. This in turn allows for a reduced band gap as the excited state molecular orbitals are delocalized. 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. This chemical control over the band gap directly influences the color of the molecule by limiting the energies of light which it absorbs. Interest in polyfluorene derivatives has increased because of their high photoluminescence quantum efficiency, high thermal stability, and their facile color tunability, obtained by introducing low-band-gap co-monomers. Research in this field has increased significantly due to its potential application in tuning organic light-emitting diodes (OLEDs). 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. Furthermore, polyfluorenes are relatively soluble in most solvents, making them ideal for general applications. Another important quality of polyfluorenes is their thermotropic liquid crystallinity which allows the polymers to align 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 liquid crystal displays (LCDs) because the synthesis of liquid crystal displays requires that the liquid-crystal molecules at the two glass surfaces of the cell be aligned 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 grooves are then generated in the polyamide layer and the liquid crystal in contact with the polyamide, the polyfluorene, can align in the rubbing direction. In addition to LCDs, polyfluorene can also be used to synthesize light-emitting diodes (LEDs). Polyfluorene has led to LEDs that can emit polarized light with polarization ratios of more than 20 and with brightness of 100 cd m−2. Even though this is impressive, it is not sufficient for general applications.

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