Trapping the Light Fantastic

Trapping the Light Fantastic

by Dr. John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period May- Jun 97	Rank Last Period Mar- Apr 97
1	Q. Pei, et al., "Polymer light-emitting electrochemical cells," Science, 269(5227):1086-8, 25 August 1995. [UNIAX Corp., Santa Barbara, CA] *RQ748	20	†
2	N.C. Greenham, et al., "Measurement of absolute photoluminescence quantum efficiencies in conjugated polymers," Chem. Phys. Lett., 241(1,2):89-96, 14 July 1995. [U. Cambridge, U.K.; Philips Res. Labs, Eindhoven, Netherlands] *RK762	17	†
3	J.J.M. Halls, et al., "Efficient photodiodes from interpenetrating polymer networks," Nature, 376(6540):498-500, 10 August 1995. [U. Cambridge, U.K.] *RN622	14	†
4	A. Shevchenko, et al., "Mass spectrometric sequencing of proteins from silver-stained polyacrylamide gels," Analyt. Chem., 68(5):850-8, 1 March 1996. [European Molec. Biol. Lab., Heidelberg, Germany] *TX873	13	†
5	Q. Pei, et al., "Polymer light-emitting electrochemical cells: In situ formation of a light-emitting p–n junction," J. Amer. Chem. Soc., 118(16):3922-9, 24 April 1996. [UNIAX Corp., Santa Barbara, CA] *UG695	12	†
6	A.P. Scott, L. Radom, "Harmonic vibrational frequencies: an evaluation of Hartree-Fock, Mřller-Plesset, quadratic configuration interaction, density functional theory, and semiempirical scale factors," J. Phys. Chem., 100(41):16502-13, 10 October 1996. [Australian Natl. U., Canberra] *VL579	11	†
7	B.A. Dressman, L.A. Spangle, S.W. Kaldor, "Solid phase synthesis of hydantoins using a carbamate linker and a novel cyclization/cleavage step," Tetrahedron Lett., 37(7):937-40, 12 February 1996. [Lilly Res. Labs., Indianapolis, IN] *TV157	9	†
8	T. Lenzer, et al., "Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene," J. Chem. Phys., 103(2):626-41, 8 July 1995. [U. Göttingen, Germany; U. Sydney, Australia; U. Melbourne, Australia] *RH105	9	†
9	E.J. Moran, et al., "Radio frequency tag encoded combinatorial library method for the discovery of tripeptide-substituted cinnamic acid inhibitors of the protein tyrosine phosphatase PTP1B," J. Amer. Chem. Soc., 117(43):10787-8, 1 November 1995. [Ontogen Corp., Carlsbad, CA; U. Calif., Los Angeles] *TD231	8	†
10	R.P.N. Veregin, et al., "The pivotal role of excess nitroxide radical in living free radical polymerizations with narrow polydispersity," Macromolecules, 29(8):2746-54, 8 April 1996. [Xerox Res. Ctr. Canada, Mississauga, Ont.] *UD693	8	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

Light-emitting polymers were first observed in 1990 at the University of Cambridge, U.K., by the group led by Richard Friend. Friend's group, based in physics, and a group in chemistry, led by Andrew Holmes, were collaborating to research PPV, short for poly(1,4-phenylene vinylene). This polymer has the ability to conduct electric current, a rare property for any organic material. What made it even rarer was that as it conducted it emitted light. The teams reported this amazing electroluminescence in Nature (see J.H. Burroughes, et al., 347:539-41, 1990).

The paper attracted a lot of attention, and it was speculated that the material would find use in all kinds of ways, from pocket televisions to luminescent wallpaper. However, the first devices only lasted a few days, due to the breakdown of the electrode-polymer interface and to the toll of the high voltages needed, which caused the polymer to disintegrate. But interest had been kindled and work soon began in laboratories around the world to overcome these problems.

This time the three most-cited papers are devoted to PPV-based polymers and how this has been achieved. Paper #1 reports a remarkable light-emitting electrochemical cell (LEC) made at the UNIAX Corporation of Santa Barbara, California the work of Alan Heeger's group. His LEC device consists of a solubilized form of PPV (with alkoxy groups attached) blended with an ion-transport polymer, poly(ethylene oxide), and with aluminum electrodes that were deposited on the surface. With a dash of lithium salt to promote conductivity, Heeger has produced a device that emits light within seconds, uses lower voltages, and is still operative after one year's storage with its original efficiency.

Light is given out as negative electrons from the cathode migrate through the cell and meet positive holes migrating from the anode. When this happens they form so-called excitons, and as the electrons drop into the holes energy is released as light.

Paper #3, which appeared a few days before #1, is from Friend's group at Cambridge and reports a similar electron-hole operation in an equally remarkable device, called a photodiode. In this, light is used to generate the excitons, which then separate into electrons and holes, which move in opposite directions to the electrodes there-by producing an electric current. Such devices are seen as suitable for solar panels and photodetectors in medical imaging. The Cambridge team used the same PPV polymer as Heeger, in this case blended with about 10% of cyano-PPV. It was deposited on a glass substrate, annealed at 100ş C for several hours, and then the aluminum electrodes were deposited on it.

Paper #2, also coauthored by Friend, is of more academic interest because it reports the measurement of photoluminescence efficiencies in solid films of several substituted PPVs. Photoluminescence efficiency in excess of 0.4 was recorded for the cyano-PPV used in #3. The paper confirms excitons really are the main product of photoexcitation in PPV.

Quite by chance I met Friend and one of his colleagues, Neil Greenham, at a one-day conference on photovoltaic devices held at my own institution, Imperial College, London, in July. I asked them why their work was attracting such attention.

"These two papers indicate that the potential performance to be got from polymers is better than previously supposed and by at least a factor of 10," said Friend. "Paper #2 reports polymers that are amongst the most fluorescent materials known. For light-emitting diodes, it is efficiency that is all-important and these polymers are capable of providing it. Efficiences have got so high that it is now possible to make optically pumped lasers."

Friend's company, Cambridge Display Technology, is developing LEDs based on PPV and employs 25 people. He has already licensed the technology to the electronics giant Philips and the leading chemical company Hoechst. If you want to know more, consult their recent papers in Nature (see N. Tessler, et al., 382[6593]:695-7, 1996) and Physical Review Letters (see G.J. Denton, et al., 78[4]:733-6, 1997).

Dr. John Emsley is Science Writer in Residence at the
Department of Chemistry, University of Cambridge, U.K.