Jellyfish protein proves promising light source

By Kimberly Patch, Technology Research News

If in several years you find yourself praising the screen readability and long battery life of your nifty new PDA, you may have the jellyfish to thank.

A group of researchers is working with a fluorescent protein, or chromophore found in jellyfish in order to create better materials for LEDs.

Most commercial LEDs use semiconductors like gallium arsenide and indium phosphide, which glow when electric current flows through them. Organic, or carbon based materials, however, are potentially easier and cheaper to manufacture.

The few used today in commercial applications, though, are not particularly efficient. They are generally used only in applications like cellphones and car stereo displays where display power use is not a large issue.

The jellyfish protein is one of many organic chemicals that researchers are trying to coax into forms that will enable cheaper, more efficient LEDs that span the full color spectrum.

The jellyfish protein is a cleverly put together molecule, said Mark Thompson, a chemistry professor at the University of Southern California. "The structure of the protein itself is a barrel like structure and the middle of the barrel is the emitter."

Because the part of the molecule that emits light is contained within the base molecule of the protein, changing the molecule in order to tune the color doesn't affect the emitter, said Thompson. "The thing that's neat about it is what were doing to tune color is just adding appendages to the outside the molecule. We can go through the whole visible spectrum with the same core -- the same central part of the molecule," he said.

This allows different aspects of the molecule, like the amount of energy needed to produce light or the amount of time the light lasts, to be changed, or tuned independently of the changes made to tune color, Thompson said.

Next, the researchers plan to subject the jellyfish chromophores to a process they have used before with other organic proteins. "We want to try to take our jellyfish chromophores, couple them with heavy metals -- anything in the bottom row of the periodic table is fair game -- and use the heavy metal to harvest [both] triplets and singlets in the device [in order to] get tremendously more light," said Thompson.

A singlet excited state involves one of a pair of electrons jumping to a higher orbital around an atom's nucleus, spinning in the opposite direction of the one that stays behind. In a triplet excited state, the separated pair of electrons spin in parallel.

These states produce light when an electron jumps back to a lower orbital, releasing energy in the form of a photon.

In the electroluminescence process one-fourth of the current passing through a device leads to a singlet excited state, or fluorescence, and the other three-fourths lead to triplets, which can potentially phosphoresce.

The trouble is, the triplet reaction does not produce light in the jellyfish protein, said Thompson.

"Triplets... cannot excite this molecule," he said.

This is because in the jellyfish protein, as in many organic molecules, electrons in the triplet excited state jump to another molecule rather than returning to the lower orbital and emitting a photon. "Most molecules in their triplet excited state... find some way to dump energy that is not making a photon," said Thompson.

Adding heavy metals to the chromophore molecules allows the molecule to produce light from both singlet and triplet reactions because the heavier atoms affect the spin of electrons, making it possible for the triplet excited state electrons to jump back by producing a photon, said Thompson.

It is difficult to say how useful the jellyfish protein may or may not turn out to be, said Lewis Rothberg, a chemistry professor at Rochester University. "It is a very long way from finding an emitter to making it transport charge and emit in the solid state for tens of thousands of hours. Any new class of compounds, of course, might turn out to be better than what is out there but a large number have been tried pretty thoroughly."

The strategy of incorporating heavy atoms is not a new one, but it has definite promise since it addresses the very real problem of the triplet state not producing light, Rothberg added.

If all goes well, the jellyfish chromophores could be ready to be used in LEDs "in potentially a couple of years. It just depends on whether we can get the metal incorporated or not," said Thompson.

Thompson's research colleagues were Y. You, Y. He, P. E. Burroughs, S. R. Forrest and N. A. Petasis. They published the research in the December 1, 2000 issue of Advanced Materials. The research was funded by Universal Display Corp., the Defense Advanced Research Projects Agency (DARPA) and the National Science Foundation (NSF).

Timeline:   2 years
Funding:   Corporate, Government
TRN Categories:  Semiconductors and Materials; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Fluorophores Related to the Green Fluorescent Protein and Their Use in Optoelectronic Devices," Advanced Materials, December 1, 2000


January 24, 2001

Page One

Light impresses atoms

Scattered signals boost capacity

Self-configuring robot mimics lifeforms

Nanotube kinks control current

Jellyfish protein proves promising light source


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