Hideki Shirakawa, co-discoverer of the field of conducting polymers, more commonly known as “synthetic metals,” was the chemist responsible in 1977 for the chemical and electrochemical doping of polyacetylene, (CH)x, the “prototype” conducting polymer. Hideki Shirakawa shared a Nobel Prize2000 in Chemistry with Alan J. Heeger and Alan G. MacDiarmid. The Royal Swedish Academy of Sciences awarded the prize to the three for the discovery and development of conductive polymers.
Hideki Shirakawa was born in Tokyo on August 20, 1936. After graduating from the Tokyo Institute of Technology with a degree in chemical engineering in 1961, he enrolled in the graduate program there and received his doctorate in engineering in 1966. He subsequently worked as an assistant at the Chemical Resources Laboratory at his alma mater until 1976, when he went to the University of Pennsylvania in the United States as a researcher.
Three years later he returned to Japan, joining the faculty of theUniversity of Tsukuba as an associate professor. In 1982 he became a professor, and in April 2000 he was appointed professor emeritus. In 1983 he received the Award of the Society of Polymer Science, Japan, for his research into polyacetylene.
Hideki Shirakawa, a 64-year-old professor emeritus at the University of Tsukuba, has been named the recipient of the Nobel Prize in Chemistry for 2000. The prize was presented jointly to Shirakawa and two U.S.scientists – Alan Heeger, 64, of the University of California at Santa Barbara and Alan MacDiarmid, 73, of the University of Pennsylvania – for their discovery and development of conductive polymers, or plastics that can transmit electric current. Shirakawa is the ninth Japanese to become a Nobel laureate and the first since Kenzaburo Oe, who won the prize for literature in 1994. He is the second Japanese to receive the chemistry award. The first was the late Ken’ichi Fukui, who won it in 1981.
At the presentation ceremony held on December 10, 2000, in Stockholm, Sweden, Chairman Bengt Norden of the Nobel Committee for Chemistry concluded his remarks with a short congratulatory message to Shirakawa in Japanese. For a moment Shirakawa seemed startled, but he quickly broke into a smile.
Hideki Shirakawa receiving his Nobel Prize from His Majesty the King. Prize Award Ceremony at the Stockholm Concert Hall 2000.
Prof. Shirakawa, at the Press Conference at the University Hall (Oct. 13, 2000)
How can plastic become conductive?
Plastics are polymers, molecules that form long chains, repeating themselves like pearls in a necklace. In becoming electrically conductive, a polymer has to imitate a metal, that is, its electrons need to be free to move and not bound to the atoms. The first condition for this is that the polymer consists of alternating single and double bonds, called conjugated double bonds. However, it is not enough to have conjugated double bonds. To become electrically conductive, the plastic has to be disturbed – either by removing electrons from (oxidation), or inserting them into (reduction), the material. The process is known as doping.
What Heeger, MacDiarmid and Shirakawa found was that a thin film of polyacetylene could be oxidised with iodine vapour, increasing itselectrical conductivity a billion times. This sensational finding was the result of their impressive work, but also of coincidences and accidental circumstances.
How polymer conductivity was revealed – and the importance of a coffee-break
The leading actor in this story is the hydrocarbon polyacetylene, a flat molecule with an angle of 120 degree between the bonds and hence existing in two different forms, the isomers cis-polyacetylene and trans-polyacetylene (the latter form illustrated below).
The story of conducting polymers began in the 1960s with attempts by Shirakawa, then working on his PhD under Sakuji Ikeda at the Tokyo Institute of Technology, to form polymers from acetylene using a so-called Ziegler–Natta catalyst to bind the molecules into long chains. Shirakawa wanted to elucidate the polymerization process for these triple-bonded molecules.
One day, through a miscommunication, a visiting scholar from Korea added 1000 times more catalyst than specified by Shirakawa, and a shiny, filmlike substance formed on the surface of the catalyst solution. Shirakawa was stimulated by this discovery. This product was far more intriguing than the rather uninteresting brown-black polyacetylene powder that was normally produced. The effect of the higher concentration of catalyst apparently was to produce more polymer chains.
Shirakawa subsequently devised a procedure to synthesize large quantities of polyacetylene film and also to control the proportions of two isomeric forms known as cis- and trans-polyacetylene. The silvery film was trans-polyacetylene, and the corresponding reaction at another temperature gave a copper-coloured film instead. The latter film appeared to consist of almost pure cis-polyacetylene. This way of varying temperature and concentration of catalyst was to become decisive for the development ahead. The metallic appearance of the product caused chemists to see if it had metallic properties. In its original form it wasn’t a very good conductor.
In another part of the world, chemist MacDiarmid and physisist Heeger were experimenting with a metallic-looking film of the inorganic polymer sulphur nitride, (SN)x. MacDiarmid referred to this at a seminar in Tokyo. Here the story could have come to a sudden end, had not Shirakawa and MacDiarmid happened to meet, accidentally, during a coffee-break.
When MacDiarmid heard about Shirakawa’s discovery of an organic polymer that also gleamed like silver, he invited Shirakawa to theUniversity of Pennsylvania in Philadelphia. They set about modifying polyacetylene by oxidation with iodine vapour. Shirakawa knew that the optical properties changed in the oxidation process and MacDiarmid suggested that they ask Heeger to have a look at the films. One of Heeger’s students measured the conductivity of the iodine-doped trans-polyacetylene and – eureka! The conductivity had increased ten million times!
In the summer of 1977, Heeger, MacDiarmid, Shirakawa, and co-workers, published their discovery in the article “Synthesis of electrically conducting organic polymers: Halogen derivatives of polyacetylene (CH)n” in The Journal of Chemical Society, Chemical Communications. The discovery was considered a major breakthrough. Since then the field has grown immensely, and also given rise to many new and excitingapplications.
Doping – for better molecule performance
What exactly happened in the polyacetylene films? When we compare some common compounds with regard to conductivity, we see that the conductivities of the polymers vary considerably. Doped polyacetylene is, e.g., comparable to good conductors such as copper and silver, whereas in its original form it is a semiconductor. A metal wire conducts electric current because the electrons in the metal are free to move. How then do we explain the conductivity of the doped polymers?
When describing polymer molecules we distinguish between (sigma) bonds and (pi) bonds. The bonds are fixed and immobile. They form the covalent bonds between the carbon atoms. The electrons in a conjugated double bond system are also relatively localised, though not as strongly bound as the electrons. Before a current can flow along the molecule one or more electrons have to be removed or inserted. If an electrical field is then applied, the electrons constituting the bonds can move rapidly along the molecule chain. The conductivity of the plastic material, which consists of many polymer chains, will be limited by the fact that the electrons have to “jump” from one molecule to the next. Hence, the chains have to be well packed in ordered rows.
As mentioned earlier, there are two types of doping, oxidation or reduction. In the case of polyacetylene the reactions are written like this:
Oxidation with halogen (p-doping): [CH]n + 3x/2 I2 –> [CH]nx+ + x I3-
Reduction with alkali metal (n-doping): [CH]n + x Na –> [CH]nx- + x Na+
The doped polymer is a salt. However, it is not the iodide or sodium ions that move to create the current, but the electrons from the conjugated double bonds. Furthermore, if a strong enough electrical field is applied, the iodide and sodium ions can move either towards or away from the polymer. This means that the direction of the doping reaction can be controlled and the conductive polymer can easily be switched on or off.
Polarons – doped carbon chains
In the first of the above reactions, oxidation, the iodine molecule attracts an electron from the polyacetylene chain and becomes I3- . The polyacetylene molecule, now positively charged, is termed a radical cation, or polaron. The lonely electron of the double bond, from which an electron was removed, can move easily. As a consequence, the double bond successively moves along the molecule.
The positive charge, on the other hand, is fixed by electrostatic attraction to the iodide ion, which does not move so readily. If the polyacetylene chain is heavily oxidised, polarons condense pair-wise into so-called solitons. These solitons are then responsible, in complicated ways, for the transport of charges along the polymer chains, as well as from chain to chain on a macroscopic scale.