Chemical Education Today
Chemistry in the News
Nobel Prizes, 2000 by John W. Moore all-cis-polyacetylene (copper colored)
Chemistry Organic polymers consist of extremely large molecules, and, like other molecular substances, they usually do not conduct electricity. Polyvinyl chloride and other polymers find many applications as electrical insulators—protecting us from electric currents, not carrying them. However, judicious choices of molecular structure and special processing have produced a few organic polymers whose conductivities approach those of metals. To honor the original discovery of electrically conductive polymers, the Royal Swedish Academy of Sciences has awarded the Nobel Prize in Chemistry for 2000 to Alan J. Heeger, University of California at Santa Barbara, Alan J. MacDiarmid, University of Pennsylvania, and Hideki Shirakawa, University of Tsukuba, Japan. The story of this discovery is a nice illustration of serendipity. Shirakawa was experimenting with polyacetylene, which contains alternating single and double carbon–carbon bonds along the polymer backbone, trying to find ways to use Ziegler–Natta catalysts to prepare well-defined polymer films. (Ziegler and Natta won the Nobel Prize in Chemistry in 1966 for their discovery of polymerization catalysts, and Natta was the first to prepare polyacetylene.) One of Shirakawa’s students mistakenly used a catalyst concentration 1000 times larger than was planned and obtained a polymer film that looked metallic—like aluminum foil. Shirakawa found that by varying reaction conditions and solvents, high catalyst concentrations could be used to prepare samples of polyacetylene in which all of the double bonds were cis and samples in which all of the double bonds were trans (Fig. 1). These formed copper-colored and silver-colored films, respectively, on the surface of the polymerization vessel. Though the polyacetylene films looked metallic, they did not conduct electricity nearly as well as a metal. A second chance encounter led to the discovery of how the films could be made to conduct. At a scientific meeting in Tokyo, MacDiarmid met Shirakawa, learned of the metallic-looking films, and invited Shirakawa to the University of Pennsylvania. MacDiarmid and Shirakawa treated polyacetylene film with iodine. The silvery film changed color to silvery black, and its other properties changed as well. Heeger, who was then in the physics department at the University of Pennsylvania, was called on to measure the film’s electrical conductivity, and he found that it had increased by 107 times! The iodine-treated polyacetylene had a conductivity of 3000 siemens per meter. Subsequently other forms were synthesized that had conductivities as high as 105 S m᎑1. This can be compared (see Fig. 2) with a nonconductive polymer such as teflon (10᎑16 S m᎑1) and metals such as silver or copper (108 S m᎑1). The three scientists reported their work in May 1977 (http://www.rsc.org/is/journals/current/chemcomm/nobel.htm), and that seminal paper was the basis for their Nobel Prize. Electric conductivity increases when polyacetylene is treated with iodine because iodine can oxidize the polymer molecule. Oxidation corresponds to removal of an electron, 8
all-trans-polyacetylene (silver colored)
Figure 1. Polyacetylene can have cis or trans structures around each double bond.
Figure 2. Electric conductivities of various materials. 3
I3−
/2 I2
a +
b +
c +
d +
e +
f Figure 3. Transport of charge within a polyacetylene molecule.
which produces a radical cation site in the polymer and a triiodide ion, I3᎑. Once an electron has been removed from the alternating double and single bonds along the polymer chain, charge can be transferred by rearrangement of electrons as shown in Figure 3. Such migration of electric charge corresponds to an electric current within the molecule. Because the positive site in the molecule is attracted by negatively charged triiodide ions, which cannot move readily, it is easier for the electric current to flow if there are many triiodide ions scattered among the polymer molecules. In a sense the positive site can be “handed off ” from one negative ion to the next. Therefore, doping the polyacetylene with relatively large proportions of iodine is necessary to get high continued on page 14
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
Chemical Education Today
Chemistry in the News continued from page 8 +
by a million times or more. Unlike inorganic semiconductors based on silicon, organic polymers are flexible and lightweight. Rolling up a computer display made from conductive polymeric material and sticking it in a pocket is Figure 4. Transfer of charge from one polyacetylene molecule to another. not as fantastic an idea as it might seem. conductivity. (For a computer animation of this process, go From its beginning, the field of conductive polymers has to http://www.nobel.se/announcement/2000/cheminfoen.html.) involved chemists, physicists, and other scientists and engiBecause a polyacetylene molecule is much shorter than neers working in teams to solve problems. The Nobel selecthe dimensions of a polyacetylene film, transfer of charge tion committee is more than justified in praising interdiscialong a single polymer chain is not enough. Electric conducplinary cooperation in this research. The possibilities for revotivity requires that charge also be transferred from one polylutionary changes in how electronic devices are made and used mer molecule to another. This can happen if an unpaired elecare also quite evident. You can certainly expect to see more tron on one chain jumps to a positively charged site on anabout these interesting products of chemistry in the news and other. Once it is associated with another polymer chain, the in future issues of this Journal. positive site can migrate along the second chain, transfer to Physics a third, and eventually travel a long distance. The figure shows how such an intermolecular charge transfer can occur. This year’s Nobel Prize in physics breaks with tradition The Royal Swedish Academy of Sciences stated that it by honoring three scientists whose work has obvious commerhas given this award because of “the important scientific pocial applications. Jack S. Kilby, a retired engineer from Texas sition that the field has achieved and the consequences in terms Instruments, received a half share of the prize for his invenof practical applications and of interdisciplinary development tion of the integrated circuit, the device whose refinement has between chemistry and physics.” The discovery that polymade possible desktop computers and a broad range of elecacetylene could be made to conduct electricity has resulted in tronic communications devices. The other half of the prize synthesis of other conductive polymers, semiconducting polywent to Zhores I. Alferov, director of the A. F. Ioffe Physicomers, light-emitting polymer systems, organic field-effect tranTechnical Institute of St. Petersburg, and Herbert Kroemer, sistors, and organic photovoltaic devices. The properties of University of California, Santa Barbara. They independently polyacetylene are not good enough for it to have important developed electronic components called heterostructures that applications, but a number of related conductive polymers are made solid-state lasers practical and inexpensive. Such lasers beginning to be used commercially. Some examples are are important components of compact-disc players, barcode readers, and fiber-optic devices. They also contribute to the • Polyaniline—used as an electrical conductor and for operation of cellular phones and to satellite communications. electromagnetic shielding of electronic circuits. Integrated circuits and the devices made possible by • Poly(ethylenedioxythiophene)—used as an antistatic heterostructures are the basis for the vast improvement in comcoating material on photographic emulsions to prevent munications technology that resulted in the Internet. +
electric sparks from exposing the film.
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Poly(phenylene vinylidene) derivatives—candidates for use in electroluminescent displays for mobile telephones.
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Poly(dialkylfluorene) derivatives—used in full-color video matrix displays.
(For more on polyaniline, see “Polyaniline—A Conducting Polymer: Electrochemical Synthesis and Electrochromic Properties” by Bradford Charles Sherman, William B. Euler, and R. Ren Forcé, J. Chem. Educ. 1994, 71, A94–A96.) Computer technology, integrated circuits, and other products of solid-state physics are based on silicon and variations introduced into pure silicon by doping with other elements. The crystals used are far larger than molecular scale and must be connected to large electrodes to form diodes, transistors, and other components. Organic conductive polymers offer the opportunity for much greater and more finely tuned variability in the properties of such devices, and in principle the devices may be as small as the molecules themselves. The discovery of conductive polymers was the beginning of a series of developments that may achieve a hundredfold reduction in the dimensions of electronic circuits. This in turn could increase the speed and memory capacity of computers 14
Physiology or Medicine The three biologists who shared this year’s Nobel Prize in Physiology or Medicine all made important contributions to understanding the brain at the molecular level. They are Arvid Carlsson of the University of Gothenberg, Sweden, Eric R. Kandel of Columbia University, and Paul Greengard of Rockefeller University. Carlsson discovered that dopamine was an important neurotransmitter in the region of the brain that controls movement. This discovery has led to the use of L-dopa as the principal treatment for Parkinson’s disease. Kandel’s studies of neurons in the Aplysia sea slug led him to conclude that learning involves modifying the strengths of the connections between nerve cells, and he worked out many of the biochemical changes that accompany formation of memories. Greengard found that phosphorylation, the same kind of reaction that stores up chemical energy, mediates important signaling systems in the brain. Much remains to be learned about the many chemical mechanisms that contribute to thinking and memory, but this year’s prize winners have contributed greatly to our understanding. John W. Moore is editor of this Journal.
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
