Sustainable chemical applications must be able to employ renewable energy sources, renewable raw materials and earth-abundant elements. However, many techniques to date have only been made possible by the use of expensive precious metals or rare earth metals, the extraction of which can have serious environmental impacts. A team of researchers including Professor Katja Heinz and Professor Christoph Kerzig of the Johannes Gutenberg University Mainz (JGU) as well as Dr. Ute Rech-Ganger of the German Bundesanstalt für Materialforschung und-Prüfung (BAM) has now made a breakthrough in its use . Chromium, an abundant base metal, has been investigating for some time by the Heinz group.
New findings suggest that chromium compounds, also known as molecular rubies, could replace costly precious metals in photon upconversion. Photon shear (UC) is a process in which the successive absorption of two low-energy photons produces one high-energy photon. In principle this high energy photon could be employed to expand the use of low energy sunlight in solar cells or photochemical reactions that would otherwise require UV light for activation. Thus the use of molecular rubies can help reduce the impact of environmentally harmful processes such as mining of precious metals or rare earth elements, and expand photochemistry to more sustainable processes.
Chromium compounds as a promising alternative
Most photochemical and photophysical applications such as phosphorescent organic light-emitting diodes, dye-sensitized solar cells or light-driven chemical reactions use precious metals such as gold, platinum, ruthenium, iridium or rare earth metals. However, precious metals are expensive because they are rare whereas rare earth elements are mined only in a few countries, most notably in China. In addition, their extraction often involves considerable consumption of water, energy and chemicals. In some cases, such as in gold mining, highly toxic substances such as cyanide or mercury are employed.
On the other hand, resources of the metal chromium, which gets its name from the Ancient Greek word for color, are 10,000 times greater than that of platinum and 100,000 times greater than that of iridium in the Earth’s crust, meaning that it is available in sufficient quantities. In. “Unfortunately, the photophysical properties of abundant metals such as chromium or iron are not sufficient to be useful in technical applications, especially when it comes to the lifetimes and energies of their electronically excited states,” said JGU’s Department of Chemistry. Professor Katja Heinz explained. , A significant progress in this regard has only been made in the last few years, with Heinz’s team being one of the main contributors. He was also involved in the development of the so-called atomic ruby. These are soluble molecular compounds with exceptionally good excited state characteristics. Molecular rubies have already been used as molecular optical thermometers and pressure sensors.
Direct observation of energy transfer processes thanks to new large-scale laser device
The team of scientists from Mainz and Berlin has now made another breakthrough. “In this process, we observed a novel mechanism and understood in detail the high efficiency of the new chromium compounds,” said Professor Christoph Kerzig. Scientists recently managed to directly observe the unusual energy transfer pathway using a laser setup set up in the Kerzig group. This so-called laser flash photolysis technique allowed them to detect all intermediates that are important for the shear mechanism. Furthermore, quantitative laser experiments established the absence of inherent energy loss channels and side reactions, which lay the basis for efficient applications of this unexplored method to transfer and convert solar energy with chromium compounds.
As a result, scientists may in the future be able to develop new light-powered reactions using the common metal chromium, rather than using the rarer, more expensive ruthenium and iridium compounds, which are still used most often today. “Together with our partners at BAM in Berlin and other universities we will continue to move forward with our efforts to develop a more sustainable photochemistry,” emphasized Professor Katja Heinz.
group results. have been published in Angwente Chemi, classified as a hot pepper. The German Research Foundation (DFG) and the Chemical Industry Fund are funding this research. In 2018, the German Research Foundation established the priority program Light Controlled Reactivity of Metal Complexes (SPP2102), coordinated by Professor Katja Heinz, with a second funding period starting in 2022.
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