A breakthrough in conjugated polymers was coming-out in the end of 1960’s – beginning of 70’s. When one considers the early days of conjugated polymer synthesis, the
Chapter II. A Concise Review: from Polyacetylene till Modern DA Polymers.
work on the oxidation of aniline, presumably forming polyaniline, and on polypyrrole are often referred to as landmark developments in the field. The discovery of polypyrrole147 and polyaniline148,149conductive films doped by halogens, organic charge-transfer salts such as those based on TTF:TCNQ150, metal-like inorganic polymer sulphur nitride (SN)x151
contributed greatly to the following finding. The breakthrough was realized by three awarders of Nobel Prize in Chemistry in 2000, who were Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa. In 1977, they accidentally discovered that insulating π-conjugated polyacetylene (PA) could become conductor with a conductivity up to 103 S/cm by conversion of cis-PA into trans-PA followed by iodine doping of the latter form106. The unexpected discovery has not only broken a traditional concept, according to which organic polymers were only regarded as the insulators, but also established a new field of conducting polymers, which also called as «Synthetic Metals»152. Unfortunately, PA is not processable and degrades in the presence of oxygen or water.
Since then many scientists have investigated more stable conjugated polymers. In the beginning of the 80’s, a lot of studies were devoted to polypyrroles, polythiophenes, and polyanilines produced by the means of electropolymerization. This method has the advantage of being able to prepare thin films of these infusible and insoluble conjugated polymers in one step. The insolubility was caused by the strong van-der-Waal’s attraction between the polymeric backbones. Relatively rigid planar system stuck in a π-stacking due to the multiple interactions, especially between doped sites of molecules. Large crystallites of polymer were not melt or solution-processable. However, the main goal was the development of polymeric materials that combine the electrical and optical properties of metals or semiconductors with the processing advantages and mechanical properties of conventional polymers146. To meet these requirements, further attempts to achieve much better processability led to the «second generation» of conductive polymers. First experiments showed that the presence of bulky substituents introduced solubility and processability but caused a twisting of the backbone giving poorly conjugated materials.
A breakthrough occurred in the midst of the 80’s with the syntheses of highly conjugated and processable poly(3-alkylthiophene)s153 by a simple one-step oxidation reaction. Five-membered thiophene rings exhibit a conformation with reduced steric hindrance developed by the presence of the solubilizing side-chains. Following the first studies on alkylthiophene)s, it became clear that the synthesis of well-defined poly(3-alkylthiophene)s would lead to a significant improvement in the performance of these polymeric materials154.
Chapter II. A Concise Review: from Polyacetylene till Modern DA Polymers.
Page 30 Figure S8. Chemical structure of the conjugated polymers. For polypyrrole oxidized (doped) and reduced forms are presented. For polyaniline partly oxidized most stable emeraldine form is presented.
TTF:TCNQ, as a well-studied charge-transfer complex is presented.
Despite all the prospects of highly conductive polymers, by the end of the 80’s, none of them had practical application. Up to 1990s research in organic electronics was focused on a relatively narrow number of structures. However, emerging of PEDOT (poly(3,4-ethylenedioxythiophene)), which was invented in 1988 by Bayer AG, changed the situation.
Several tricks were developed to make polymer soluble and highly conductive at the same time. Such a famous composition is PEDOT:PSS (Figure S2). Dopant (peroxides) was added on the stage of chemical synthesis of PEDOT upon oxidation. Additives of PSS (polystyrene sulfonate) which served as counter-ions, provided the solubility in water due to a polyelectrolyte effect. This is probably the best conducting polymer available in terms of conductivity, processability and stability61.
The next generation of benchmark materials was introduced in the beginning of 90’s by several research groups. The focus shifted from the synthesis of highly conducting polymers to the design of stable semiconducting polymers with tunable electronic and optical properties. This shift was mainly driven by the development of polymeric light emitting diodes and photovoltaic cells. The polymer electronics probably originates from the synthesis of PPV155 (polyphenylenevinylene) and its derivatives155,156 in the early 1990s. Group of Friend and Holmes invented a way to construct OLED with active PPV layer from solution-processed precursor. The next step was equally important when a soluble form of PPV,
Chapter II. A Concise Review: from Polyacetylene till Modern DA Polymers.
namely MEH-PPV (Figure S8), was synthesized by the group of Wudl and Heeger and applied to OPV devices157.
McCullough158 and Rieke159 independently reported the first preparations of well-defined regioregular poly(3-alkylthiophene)s by metal-catalyzed polymerization methods.
These relatively complicated polymerization procedures have been optimized and simplified, leading to the Grignard metathesis method (GRIM)160. For instance, this existing polymerization method eliminates the need for highly reactive metals and can even lead to well-defined block copolymers. Among all poly(3alkylthiophene)s investigated, regioregular poly(3-hexylthiophene) (rr-P3HT) shows the best electrical properties together with adequate processability145.
These discoveries were crucial and had a key role for the further design of soluble conjugated polymers. Synthetic chemistry appeared to be the main instrument for a new material. The emerging of polyfluorenes161 and, to a greater extent, polyalkylthiophenes54 expanded the possibilities of CPs potential applications. These materials were eligible and acceptable because, as one of the major advantages, they were soluble in organic solvents due to the presence of alkyl chains and, therefore, easily processable.
Further progress was dictated by achieving a better control over the bandgap of a polymer and efficient tuning of energy levels. It was difficult to reach required conditions in the so called «homopolymer» design concept. Homopolymer consists of a single aromatic/conjugated unit and properties of such polymer are determined by the intrinsic properties of the single aromatics. The «third» generation of benchmark materials are marked by the shifting from «homopolymer» to a «quinoid» and «donor-acceptor» (DA) approaches162. «Quinoid-approach» focuses on stabilizing the quinoid resonance structure. An aromatic form of the molecule is more energetically stable, however transformation of aromatic core into quinoid results in a smaller band gap13,162. In order to stabilize quinoid form and reduce aromaticity destruction, fusing of another aromatic ring was suggested163. The «DA-approach» allows adjusting energy levels of the polymer carefully selecting the donor and acceptor moieties for constructing the backbone. This approach offers an important advantage of individually tuning the band gap (this way is described in details in Chapter III, Results and Discussion part). Most of the conjugated polymers reported so far are based on these concepts3,13,162,164.