Sustainable chemical applications need to be able to employ renewable energy sources, renewable raw materials, and Earth-abundant elements. However, to date many techniques have only been possible with the use of expensive precious metals or rare earth metals, the extraction of which can have serious environmental impacts. An international team of researchers led by Professor Katja Heinze of Johannes Gutenberg University Mainz (JGU) has now achieved a breakthrough in the use of chromium, an abundant base metal which Heinze's group has been investigating for some time. The new findings show that chromium compounds can substitute expensive precious metals in photocatalysis. Their use can thus help reduce the impact of environmentally damaging processes. Photocatalysis is a technique currently used to synthesize pharmaceuticals or fine chemicals, for example.
Chromium compounds are 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 scarce while rare earth elements are only mined in a few countries, in China in particular. Furthermore, their extraction often involves considerable consumption of water, energy as well as chemicals. In some cases, such as gold mining, even 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 more plentiful in the Earth's crust than those of platinum and 100,000 times greater than those of iridium, meaning that it is available in sufficient quantities. "Unfortunately, the photophysical properties of abundant metals like chromium or iron are just not good enough to be useful in these technological applications, especially when it comes to the lifetime of their electronically excited states," explained Professor Katja Heinze of JGU's Department of Chemistry. It is only in the last few years that a significant progress in this regard has been made, with Heinze's team being one of the contributors. They were also involved in the development of so-called molecular rubies. These are soluble molecular compounds which possess exceptionally good luminescence characteristics, especially excellent photoluminescence quantum yields. Molecular rubies have already been used as molecular thermometers and pressure sensors.
Long lifetimes thanks to a concept from quantum chemistry
The international team of scientists from Mainz, Berlin, Kaiserslautern, Tübingen, and Montreal has achieved another breakthrough, managing to block all the typical decay channels of a molecular ruby's electronically excited state so efficiently that the excited state prevailed for an exceptionally long time. The concept extended the excited state lifetime of the synthesized molecular ruby in liquid solution at room temperature to a record level of 4.5 msec. "Our concept is based on the so-called Laporte's rule, which was developed by the physicist Otto Laporte who was born in Mainz in 1902," stated Steffen Treiling, who carried out the experiments in the Heinze group as part of his Master's thesis. Put simply, the high molecular symmetry of this specially designed molecular ruby has a positive effect on the lifetime of the excited state. "When you compare the lifetime of the electronically excited states with those of typical iron compounds, for example, we achieved an increase of around ten to twelve orders of magnitude,” explained Professor Christian Reber of the University of Montreal.
Despite the long excited state lifetime, the photoluminescence quantum yield of these molecular rubies can reach up to 8.2%. This means they could also be suitable for use in display and sensor technology. Moreover, the research team has also been able to demonstrate that the chromium compound exhibits photocatalytic properties similar to those of precious metal complexes. Consequently, scientists may be able to develop new light-driven reactions using the common metal chromium in the future instead of using the rare, more costly ruthenium and iridium compounds, which today are still the most frequently used. "Together with our partners at the German Federal Institute for Materials Research and Testing (BAM) in Berlin, TU Kaiserslautern, the University of Tübingen, and the University of Montreal we will continue to push on with our efforts to develop a more sustainable photochemistry," emphasized Professor Katja Heinze.
The group's results have been published in Angewandte Chemie, classified as a Very Important Paper (VIP) and are being awarded a title page.
The German Research Foundation (DFG), the Natural Sciences and Engineering Research Council of Canada, and the German Academic Exchange Service (DAAD) are funding this research. Part of the work was carried out using the MOGON supercomputer at Mainz University. In 2018, the German Research Foundation set up the priority program Light Controlled Reactivity of Metal Complexes (SPP 2102), coordinated by Professor Katja Heinze and involving more than 30 researchers working on 17 different projects in this highly topical research area.