Superphotooxidant properties of a molecular manganese complex
It’s a major challenge in photochemistry to find photocatalysts with high reducing or oxidizing capabilities. So far, only a few transition metal complexes with Earth-abundant metal ions, such as chromium, iron, and cobalt, have reached excited state oxidants. These photocatalysts require high energy light for excitation and have not fully utilized their oxidizing power. Additionally, most of these catalysts contain precious and expensive metals. However, a team of researchers headed by Professor Katja Heinze from Johannes Gutenberg University Mainz (JGU) has developed a new molecular system based on the element manganese. Unlike precious metals, manganese is widely available and very cheap, being the third most abundant metal after iron and titanium.
Unique Properties of “Molecular Braunstein”
Professor Katja Heinze’s team has created a soluble manganese complex that absorbs visible light from blue to red, as well as parts of near-infrared light. This panchromatic absorption is similar to the dark color of the natural mineral Braunstein or manganese dioxide. Unlike the mineral, the new “molecular Braunstein” emits NIR-II light after being excited with visible or NIR-I light. This is a unique property for a manganese-based molecular system with an oxidation state of +IV and is essentially unprecedented even for noble metals, according to Professor Katja Heinze.
Beyond the NIR-II luminescence, the “molecular Braunstein” also demonstrates the ability to oxidize various organic substrates upon photoexcitation. This includes challenging aromatic molecules with high oxidation potentials such as naphthalene, toluene, and benzene.
Observation of Two Photoactive States using Ultrafast Spectroscopy
Ultrafast spectroscopic techniques showed that the “molecular Braunstein” has two different photoactive states: a short-lived but extremely oxidizing high-energy state, and a longer-lived moderately oxidizing lower-energy state. The former can attack solvent molecules close to the complex, while the latter can attack aromatic substrates after diffusional collision, demonstrating static and dynamic quenching of the excited states.
Quantum Chemical Calculations to Understand Unusual Photoprocesses
Quantum chemical calculations were used to model the involved excited states in the context of the spectroscopic results. The advanced calculations were made possible using the computing power of the supercomputers MOGON and ELWETRITSCH in Rhineland-Palatinate.
Looking to the future, Professor Katja Heinze hopes that the common and abundant metal manganese can be used to develop new light-driven reactions, which may replace the more costly ruthenium and iridium compounds. These efforts are part of the Light Controlled Reactivity of Metal Complexes priority program funded by the German Research Foundation.