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Organic phosphors glow in jewel tones

Photonics Spectra
Apr 2011
ANN ARBOR, Mich. – A new class of pure organic compounds that glow in jewel tones could lead to cheaper, more efficient and more flexible display screens, among other applications.

Researchers at the University of Michigan developed a class of material that shines with phosphorescence, a property previously seen only in nonorganic compounds and organometallics. The metal-free organic crystals are white in visible light and radiate blue, green, yellow and orange when triggered by ultraviolet light. The color emitted can be manipulated by changing the chemical composition.


Organic phosphors developed at the University of Michigan could one day lead to cheaper organic LEDs. Images courtesy of Marcin Szczepanski, College of Engineering, University of Michigan.


The phosphors could improve current organic LED (OLED) and solid-state lighting, the scientists said. Used mostly in small screens on cell phones or cameras, the OLEDs are not yet practical for use in larger displays because of materials cost and manufacturing difficulties.

OLEDs currently in use are not 100 percent organic, meaning that they are not made entirely of carbon compounds. In fact, the organic materials used in them must be spiked with metal in order for them to glow. The scientists noted that purely organic materials cannot yet generate a meaningful phosphorescence emission. However, they believe that their compound may be the first example of an organic that can compete with organometallics in terms of brightness and color-tuning capabilities.

The light in the researchers’ phosphors comes from molecules of oxygen and carbon called “aromatic carbonyls” – compounds that produce phosphorescence, but weakly and under special circumstances, like extremely low temperatures. These phosphors form strong halogen bonds with halogen in the crystals, packing the molecules tightly together. Vibration and heat energy losses are suppressed as the excited electrons fall back to the ground state, creating strong phosphorescence.


Pure organic phosphorescent crystals glow yellow and green when triggered by ultraviolet light.


The method provides an easier way to make high-energy blue organic phosphors, which scientists have found to be difficult to produce with organometallics. In addition to their efficiency, the new compounds are cheaper to produce because they do not require any precious metals.

The university is in pursuit of patent protection for the intellectual property and is seeking commercialization partners to help bring the technology to market.

Partly funded by the National Science Foundation and the National Research Foundation of Korea, the work was published online in Nature Chemistry on Feb. 13, 2011 (doi: 10.1038/nchem.984).


GLOSSARY
halogen
Any of the five elements astatine, chlorine, fluorine, bromine and iodine, grouped because their chemical properties are similar.
phosphorescence
Luminescence is the emission of light from a source that is delayed by more than 10-8 s following excitation.
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