Pure organic compounds that give gems their brilliant colors could lead to the development of cheap, flexible and more efficient screen displays, as an example of one of the possible applications for them.
The scientist Jinsang Kim from the University of Michigan, and his team of fellow researchers, have developed a new family of luminescent materials together with phosphorescence (phosphorescence, Wikipedia) – a property previously seen in inorganic or organometallic compounds.
The researchers prepared metal-free organic crystals that are colored white in visible light, and which glow blue, green, yellow and orange when exposed to ultraviolet radiation. By changing the chemical composition of the material, the researchers are able to prepare crystals that emit different colors. The new emitting materials could improve the organic light emitting diodes (OLEDs) that exist today in the commercial market as well as other light devices. Such diodes are used in tiny displays found in mobile phones and cameras. Today, they are not yet suitable for practical use in larger displays due to material and manufacturing cost considerations. Today's organic diodes are not completely organic, meaning they are not complex From carbon compounds only, the organic materials used in these diodes are mixed together with metal to obtain the desired property.
"Completely organic materials failed to cause significant luminescent emission. We believe that this is the first ever example of an organic material that is able to be competitive relative to organometallic materials in regards to brightness and the ability to tune the color," said the lead researcher, Professor of Materials Science and Engineering, Chemical Engineering and biomedical engineering. The research findings were published in the scientific journal Nature chemistry.
The innovative luminescent material exhibits a "quantum efficiency" of 55 percent. Quantum efficiency, a measure of the efficiency and brightness of a material, refers to the amount of energy that the electron dissipates once light, instead of heat, fades from an excited energy level to the ground level. Present purely organic compounds have essentially zero quantum efficiencies.
In the researchers' innovative material, the light emerges from molecules of oxygen and carbon known as "aromatic carbonyls", compounds that produce phosphorescence, but it is weak and only occurs under special conditions such as extremely low temperatures. The fact that distinguishes the new materials lies in the fact that the aromatic carbonyls form strong halogen bonds with other halogens present in the crystal for a strong connection. This unique arrangement reduces heat and vibrational energy losses as the excited electrons fall back down to the ground state, creating a strong spin. The new method provides an easier way to prepare organic materials that glow in blue, which are much more difficult to prepare with organometallic materials.
Organic light-emitting diodes are easier and cheaper to make than their inorganic counterparts, which are mainly composed of ceramic materials. However, existing organic light-emitting diodes still contain precious metals. The new compounds could lower costs even further because they do not require the use of precious metals. They consist mainly of the relatively cheap elements carbon, oxygen, chlorine and bromine.
"This is indeed the beginning of the road, but we predict that the day will not be far away when our innovative and simple materials will be commercially available for device applications, and will allow great progress in the light-emitting diode industry thanks to the low costs of the materials and production, as well as due to the easy ability to obtain desired properties and colors," he notes the researcher
