The Birth of New Luminescent Materials

Recent years have witnessed an enormous increase in the demand for energy as a result of industrial development and population growth. But at the same time, human beings are facing the consequence of over-exploitation of fossil fuels and global warming. At this critical time, the need for clean and sustainable energy sources is pressing.

Last year, researchers at the department of Chemistry of the University of Hong Kong successfully synthesized two new materials, Gold(III) Triphenylamine and NFBC, which can help to give out light in organic light-emitting diodes with less energy consumed.

According to Earth Policy Institute, an independent non-profit environmental organization based in Washington DC, about 19 percent of world electricity demand goes to lighting, and the carbon emissions generated by this sector equal roughly 70 percent of those produced by the global automobiles.

Professor Vivian Yam, the Chair Professor in chemistry at the HKU, and her team have been studying the synthesis of metal complexes that have light absorbing and emitting properties for years. The luminescent materials they created can be used in OLEDs, inside which there is a thin film of organic compounds that will give out light in response to an electric current.

In the world we live in, there are 112 known chemical elements, some of which are transition metal complexes, a kind of metal-containing compounds that can absorb UV-visible light. Because of their unique way of electronic arrangements, those metal complexes’ chromophores, parts of a molecule responsible for its color, can absorb certain wavelengths of light and present a light excitation state. In this way, complexes can display different colors.

Professor Yam adopted an innovative approach to create luminescent materials. “Unlike the conventional approach of performing chemical modification on the molecules, we use supramolecular control and assembly to tune the spectroscopic, excited state and structural properties of molecular materials,” she said.

The Process of Getting a Desired Complex

Researchers first followed the design of a rational synthetic route that included the raw materials and steps of reactions to obtain a desired complex. To conduct the reaction in a moisture-free and air-free condition, they heated the solution containing the desired product with an attached condenser to prevent reagents from escaping. The crude product was then dissolved in an organic solvent to separate the desired complex from other soluble impurities. Under a reduced pressure and increasing heating condition, the solvent was removed by evaporation, and after being re-crystallized the final product was obtained.

To identify the structure of the product, a test tube containing the desired compound was placed inside a brush pot-like machine, which was actually a strong external magnetic field. The machine was connected to a computer that showed a nuclear magnetic resonance spectrum of the compound. Through the spectrum, researchers could tell the structure of molecules by analyzing the resonant frequency of the nucleus.

Measurement for Luminescent Properties

After knowing the structure of the compound, researchers have to text them in different solutions to measure their luminescent properties.

“To perform chemical reaction does not mean that mix two reactants, a reaction must occur. From the color changes, you may sense whether or not the reaction has proceeded to completion or has been over-reacted, “ Yam said. “Chemistry research relies very much on your ability to observe the changes. ”

To begin with, liquid nitrogen was used to remove oxygen, as it often quenched phosphorescence produced by the compound. Then the same compounds they synthesized previously were dissolved in different solutions. Researchers used UV flashlight to shed light on the test tubes to make the liquid absorb it and give out phosphorescence, a property of being luminous after being exposed to light or radiation.

“It is the extent of the interaction between molecules of the compound we designed and the compounds in solutions during a non-covalent metal-metal reaction that gives rise to drastic colors and luminescence changes,“ Yam said.

Compared with other chemical compounds, organic compounds that incorporate carbon-metal bonds are often believed to have great potential in light-emitting area. Professor Kenneth Lo, studying the utilization of luminescent transition metal complexes as biomolecular probes and bioimaging reagents, at the City University of Hong Kong, said, “Many organometallic compounds can be used as luminescent materials, as they are stable, contain more color dyes and show intense emission with long emission lifetime.”

Applications in Energy Saving

The discovery of novel luminescent metal-based materials with controllable absorption and emission colors and the structure of their molecules can not only help to develop high efficient lighting devices like OLEDs, but can be used to improve new classes of solar-energy storage materials and luminescence sensors for biomedical applications.

Professor Lo, who worked under the supervision of Professor Yam for his PHD degree in 1990s, had successfully used luminescent mental complexes to diagnose and cure diseases. ”Luminescent transition metal complexes are attractive candidates to probe biomolecules and image live cells and animals,” he said.

With 28 patents in a decade, Professor Yam said her research was in the upstream stage. So far, they have not participated in specific products’ development. More applications of new luminescent mental complexes are waiting to be discovered.

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