There is hardly an aspect of our lives that is not touched by synthetic polymers. In the field of optics and electronics these materials are traditionally used in applications such as packaging, electrical insulators, and photoresists. However, new opportunities emerged with the discovery of conducting polymers (i.e., polyacetylene) in 1977.1 These novel conducting polymers have tremendous potential for innovation. After 20 years of progress, these unusual polymeric materials can now be used as transparent antistatic coatings, electromagnetic shielding, superconductors, modified electrodes, electrochromic windows, supercapacitors, transistors, light-emitting diodes, lasers, conducting photoresists, photovoltaic cells, biosensors, and so forth.2, 3 The significance of this class of polymers was recently highlighted by the awarding of the 2000 Nobel Prize in Chemistry to H. Shirakawa, A. G. MacDiarmid, and A. J. Heeger, the three scientists who pioneered this novel materials field. This attribution of the Nobel Prize also indirectly recognizes a fruitful collaboration between two chemists and one physicist. To comment on this issue of multidisciplinary work, the statement recently made by Prof. Dr. K. Müllen is appropriate: It is important for the development of conducting polymers that physicists be aware that a molecular formula does not become a real molecule only by being written on paper, but that simple facts of chemical feasibility, purity, and structure proof must be obeyed whereas a chemist must learn that a chemical compound, available in milligram quantities and characterized in dilute solution, is not a material.3
Processable Aromatic Polymers
As mentioned above, the first evidence of high electrical conductivities in synthetic polymers was obtained with polyacetylene [Fig. 1(A)]. Its delocalized electronic structure (alternation of single and double bonds) is responsible for the good intrachain and interchain mobility of the charge carriers (radical cations and dications) that are created upon doping (partial oxidation or reduction). This delocalized (conjugated) structure is also responsible for a strong absorption in the UV–visible range. Unfortunately, polyacetylene is difficult to process and unstable in the presence of oxygen.
The more stable, aromatic polymers were rapidly investigated chemists. The development of the electropolymerization technique to yield conducting polymers opened new avenues in this young research field. In the early 1980s a lot of studies were devoted to electropolymerized polypyrrole4 [Fig. 1(B)], polythiophene5 [Fig. 1(C)], polyaniline6 [Fig. 1(D)], polyfluorene7 [Fig. 1(E)], and so forth. Indeed, this technique has the advantage of being able to prepare thin films of these infusible and insoluble conjugated polymers in one step. However, the ultimate goal was still the development of polymeric materials that combine the electronic properties of metals and semiconductors with the processing advantages and mechanical properties of traditional polymers. The first studies on such processable and conducting polymers based on substituted polyacetylenes and polyanilines failed because the presence of bulky substituents forces a twisting of the backbone that leads to processable but poorly conjugated materials with reduced electrical properties.8 Nevertheless, some researchers (Elsenbaumer, Roncali, Garnier, Wudl, Kaeriyama, Sugimoto, etc.) were more successful with polythiophene derivatives and a major breakthrough occurred with the synthesis of processable and conducting (10–100 S/cm) poly(3-alkylthiophene)s [Fig. 1(F)].9 These reports were rapidly followed by the development of poly(2,5-disubstituted-1,4-phenylene)s10 [Fig. 1(G)], poly(2,5-disubstituted p-phenylenevinylene)s11 [Fig. 1(H)], and poly(9,9-dialkylfluorene)s12 [Fig. 1(I)].
Polyfluorene Derivatives and Light-Emitting Diodes
Among these polymers, poly(9,9-dialkylfluorene)s have recently received a lot of attention that can be attributed to another major issue: the possibility that they could be used to develop all plastic, full-color, light-emitting diodes. An important driving force for this research is the dream of building ultrathin and flexible screens for computers and televisions. The basic structure of a polymeric light-emitting diode consists of a positive hole-injecting electrode (usually transparent) with a high work function such as In-SnO2 (ITO) or a conducting polymer, a negative electron-injecting electrode with a low work function such as Al, In, Mg, or Ca, and the light-emitting polymer film sandwiched between these two electrodes (Fig. 2). In this layered structure the injected holes and electrons migrate across the polymer layer and combine to form excitons, which then decay with photon emission. Depending upon the chemical structure of the emissive polymer, different colors can be obtained. The first experiments were carried out by Burroughes et al.13 using poly(p-phenylenevinylene)s that emitted green light, but polyfluorene derivatives were quickly investigated in such electrooptical devices by Ohmori et al.14 Polyfluorene derivatives are a particularly suitable class of materials because they contain a rigid biphenyl unit (which leads to a large band gap with efficient blue emission), and the facile substitution at the remote C9 position provides the possibility of improving the solubility and processability of polymers without significantly increasing the steric interactions in the polymer backbone. However, these first experiments on diodes that emitted blue light were carried out with oxidatively prepared polyfluorene derivatives [Fig. 1(I)]. The nonspecific oxidation reaction produces some partially crosslinked materials, and NMR studies on the low molecular weight soluble fraction of these polymers showed some evidence of irregular couplings along the backbone.15
In parallel, and mainly on the basis of studies on polyacetylene16 and polythiophenes,17 it became quite clear that the synthesis of well-defined (hopefully defect-free) conjugated polymers should lead to a significant improvement in the performance of electroactive and photoactive conjugated polymers. Surprisingly, the importance of synthetic chemistry to materials science is often underestimated by chemists (and funding agencies); consequently, relatively few academic organic chemists (particularly in North America) are active in this important research field. Nevertheless, some chemists accepted the challenge; in attempts to bring more reliable synthetic procedures to the field of electronic materials, a variety of synthetic tools (Grignard, Stille, Yamamoto, Heck, and Suzuki couplings, etc.) were utilized that allowed significant advances in this research field. For instance, investigations by Pei and Yang on nickel-catalyzed Yamamoto couplings of 2,7-dibromo-9,9-disubstituted fluorenes [Fig. 3(A)] led to the synthesis of well-defined, highly conjugated, and processable poly[2,7-(9,9-dialkylfluorene)]s [Fig. 1(J)].18 This first report on regioregular poly[2,7-(9,9-dialkylfluorene)]s was rapidly followed by investigations on the polymerization of well-defined fluorene-containing conjugated polymers using palladium-catalyzed Suzuki coupling reactions between 2,7-dibromofluorene derivatives and 2,7-diboronylfluorene derivatives [Fig. 3(B)].19 The utilization of a phase transfer catalyst gives higher molecular weight (a number-average molecular weight of ca. 50,000 instead of ca. 15,000).19, 20
Figure 3. Schematics of the polymerization of fluorenes from (A) Yamamoto, (B) Suzuki, or (C) Stille coupling reactions.
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Unfortunately, poly[2,7-(9,9-dialkylfluorene)]s and poly[2,7-(9-alkylfluorene)]s generally show some excimer emission (the formation of dimerized units in the excited state that emit at lower energies) in the solid state that affects the color emission and the lifetime of light-emitting devices.18, 21 This formation of excimers is surprising because the remote substituents at the C9 position should limit interchain interactions. Despite some X-ray studies on monomers22 and polymers,23 these structural effects still deserve further investigation because, in my point of view, the conformation of the polyfluorene backbone and its 3-dimensional structure are not yet well established. As a matter of fact, it worth noting that we recently discovered that, using similar film preparation conditions, poly[2,7-(N-alkylcarbazole)]s [Fig. 1(K)] do not show any evidence of excimer formation.24 In the latter case the substituents are parallel to the plane of the repeat unit and should not inhibit strong interchain interactions. Extensive X-ray analyses on polyfluorenes and polycarbazoles should shed some light on this issue.
Nevertheless, as is usual in polymer chemistry, the preparation of random copolymers and alternating copolymers perturbs short- and long-range organization in the materials. A variety of copolymers derived from 2,7-fluorenes were thus recently reported19, 20, 25 (Fig. 4) and tested in different electrooptical devices. Most copolymers were obtained from a Yamamoto [Fig. 3(A)], Suzuki [Fig. 3(B)], or Stille [Fig. 3(C)] coupling reaction. In most cases excimer formation was suppressed and a fairly good correspondence was observed between solution and solid-state fluorescence spectra (usually, a slight redshift is observed in the solid state that is due to an extended delocalization length), as well as between solid-state fluorescence and electroluminescence spectra. In addition, the development of such copolymers permitted the preparation of a variety of fluorene-containing copolymers that emit colors spanning the entire visible range (red, green, blue). In many cases luminance between 100 and 10,000 cd/m2 was obtained at only few volts. Clearly, polyfluorenes are seen as one of the most promising classes of electroluminescent polymers. However, as is the case for all polymer-based light-emitting diodes, the problem of stability remains and hinders their industrial applications. Once again the role of polymer chemistry is still very important because the presence of catalytic residues and reactive end groups are among the major factors that limit the stability of these electrooptical devices.26
Thermochromism and Solvatochromism
Optical and electrochemical sensors have become another important research topic for conjugated polymers. For instance, some neutral conjugated polymers can undergo striking color changes upon exposure to various external stimuli. These optical changes are generally related to a conformational transition of the polymer backbone from a planar to nonplanar form, which is triggered by adequately functionalized and field-responsive side chains. In addition to optical transitions induced by heating (thermochromism) or solvent quality changes (solvatochromism), novel phenomena have been generated including the detection of ions (ionochromism), UV radiation (dual photochromism), and molecular recognition of chemical or biological moieties (affinity chromism).27
As shown in Figure 5, some polyfluorene derivatives exhibit interesting thermochromic and solvatochromic properties.28 The fluorescence is also affected by these modifications. However, up to now it has been difficult to distinguish between a conformational transition of the backbone and simple interchain (excitonic) interactions. A careful investigation of well-defined oligomers in different environments could determine the relative contribution of conformational versus excitonic effects. As with polydiacetylenes, polysilanes, and polythiophenes,27 appropriate functionalization of the C9 position in polyfluorenes could lead to a variety of specific chemical and biochemical sensors.
Figure 5. (A) Solid-state temperature-dependent and (B) solvent-dependent (v/v) UV–visible absorption spectra of poly[2,7-(9,9-dioctylfluorene)].
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Base Doping in Acidic Polyfluorenes
Finally, polyfluorenes can also show unusual electrical properties. In undergraduate courses of chemistry we learn that molecules made exclusively of carbon and hydrogen atoms (i.e., without any heteroatoms) are not usually strongly acidic. There are a few exceptions; fluorene is one of them because the resulting carbanion is greatly stabilized by resonance. Moreover, it is known that 9-monosubstituted fluorene derivatives bearing a strong electron-withdrawing substituent (ketone, benzoyl, cyano, etc.) exhibit a pKa lower than water, making the fluorenyl anion stable in air. Starting from this knowledge, we reported the first synthesis of well-defined acidic poly(2,7-fluorene) derivatives.19, 29 Upon deprotonation (base doping, see Fig. 6), these stable polyanions showed interesting electrical conductivities. We were looking for n-type conducting materials because acid-doped polyanilines are well known as p-type conductors, but we presumably obtained an excellent ionic conductor (ca. 10−2 S/cm).29 An extensive characterization of the electrical properties of various acidic polyfluorenes should lead to a better understanding of the nature of the charge carriers in these polymeric anions and possibly to a rational design of polyfluorenes with optimized (ionic or electronic) electrical conductivities.