Entirely organic phosphorescent crystals are a new class of light-emitting material

Ann Arbor, MI--University of Michigan researcher Jinsang Kim and his colleagues have developed an entirely organic material that phosphoresces—a property that has previously been seen only in nonorganic compounds or organometallics. The researchers made metal-free organic crystals that phosphoresce in blue, green, yellow, or orange when triggered by UV light. By changing the materials' chemical composition, the researchers can change the central emission wavelength.¹

The new phosphors could improve upon those used in current organic light-emitting diodes (OLEDs) and solid-state lighting. Bright, low-power OLEDs are used in some small screens on cell phones or cameras. At this time, they aren't very practical for use in large displays because of material costs and manufacturing issues. And today's OLEDs aren't 100% organic; the organic materials used in them must be spiked with metal to get them to emit light.

55% quantum yield

"Purely organic materials haven't been able to generate meaningful phosphorescence emissions. We believe this is the first example of an organic that can compete with an organometallic in terms of brightness and color-tuning capability," said Kim. The new phosphors exhibit quantum yields of 55%. Conventional pure organic compounds have a quantum yield of essentially zero.

In Kim's phosphors, the light comes from aromatic carbonyls, molecules of oxygen and carbon that normally produce phosphorescence only weakly and under special circumstances such as extremely low temperatures. In the new materials, the aromatic carbonyls form strong bonds with halogens in the crystal, packing the molecules tightly. This suppresses vibration and heat energy losses as the excited electrons fall back to the ground state, leading to strong phosphorescence. This new method also offers an easier way to make high-energy blue organic phosphors, which are difficult to achieve with organometallics.

Seeking commercialization partners

"This is in the beginning stage, but we expect that it will not be long before our simple materials will be available commercially for device applications," Kim said. "And we expect they will bring a big change in the LED and solid-state lighting industries because our compounds are very cheap and easy to synthesize and tune the chemical structure to achieve different colors and properties."

Former doctoral student Kangwon Lee discovered the unique properties of these materials while developing a biosensor—a compound that detects biological molecules and can be used in medical testing and environmental monitoring. The phosphors have applications in this area as well. After Lee's discovery, Bolton developed the metal-free pure-organic phosphors.

The university is pursuing patent protection for the intellectual property, and is seeking commercialization partners to help bring the technology to market.

REFERENCE:

1. Onas Bolton et al., Nature Chemistry, published online 13 February 2011; doi:10.1038/nchem.984.