Molecular structures of the novel luminescent manganese complexes. Image: Jakob Bilger.
Molecular structures of the novel luminescent manganese complexes. Image: Jakob Bilger.

Researchers at the University of Basel in Switzerland have reached an important milestone in their quest to produce more sustainable luminescent materials and catalysts for converting sunlight into other forms of energy. Using the cheap metal manganese, they have developed materials with properties that until now have primarily been found in noble metal compounds.

Smartphone screens and catalysts for artificial photosynthesis often contain very rare metals. Iridium, for instance, which is used in organic light-emitting diodes (OLEDs), is rarer than gold or platinum. Ruthenium, which is used in solar cells, is also one of the rarest stable elements. These metals are not only very expensive, by virtue of their scarcity, but also toxic in many compounds.

Now, a team led by Oliver Wenger and his doctoral student Patrick Herr at the University of Basel have for the first time succeeded in producing luminescent manganese complexes in which exposure to light causes the same reactions to take place as in compounds containing ruthenium or iridium. The advantage of using manganese is that the element is 900,000 times more abundant in the Earth’s crust than iridium, as well as being significantly less toxic and many times cheaper. The researchers report their findings in a paper in Nature Chemistry.

At present, the new manganese complexes perform worse than iridium compounds in terms of their luminous efficiency. But the light-driven reactions that are needed for artificial photosynthesis, such as energy- and electron-transfer reactions, take place at high speed in these complexes. This is due to their special structure, which leads to an immediate charge transfer from the manganese to its direct bonding partners on excitation with light. This design principle for complexes is already used in certain types of solar cells. Until now, however, these complexes have mostly featured noble metal compounds, and sometimes the less noble metal copper.

This is because the absorption of light energy normally causes greater distortion in complexes made with cheap metals than it does in noble metal compounds. As a result, the complexes begin to vibrate, and a large part of the absorbed light energy is lost. The researchers were able to suppress these distortions and vibrations by incorporating tailor-made molecular components into the complexes in order to force the manganese into a rigid environment. This design principle also increases the stability of the resulting compounds and their resistance to decomposition processes.

This is the first time anyone has succeeded in creating molecular complexes with manganese that can glow in solution at room temperature and that have these special reaction properties. “Patrick Herr and the involved postdocs really made a breakthrough in this respect – one that opens up new opportunities beyond the field of noble metals,” says Wenger.

In future research projects, Wenger and his group want to improve the luminescent properties of these new manganese complexes and anchor them to suitable semiconductor materials for use in solar cells. Other possible refinements include water-soluble variants that could potentially be used in place of ruthenium or iridium compounds in the photodynamic therapy used for treating cancer.

This story is adapted from material from the University of Basel, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.