Highly reducing or oxidizing photocatalysts are a fundamental challenge in photochemistry. To date, only a few transition metal complexes with Earth-abundant metal ions have transitioned to the excited state of oxidizers, including chromium, iron, and cobalt. All these photocatalysts require high-energy light for excitation, and their oxidation capacity is not yet fully utilized. Moreover, precious and therefore expensive metals are important components in many cases.

A research team led by Professor Katia Heinze from Johann Gutenberg University Mainz (JGU) has developed a new molecular system based on the element manganese. Unlike precious metals, manganese is the third most abundant metal after iron and titanium and is therefore widely available and very cheap. The study was published in the journal Nature Chemistry.

Unusual behavior of “Molecular Brownstein”

Professor Kata Heinze’s team has developed a soluble manganese complex that absorbs all visible light from blue to red, with wavelengths between 400 and 700 nanometers and including near-infrared light up to 850 nanometers. This panchromatic absorption of the complex resembles the dark color of the natural mineral Brownstein, or manganese dioxide.

Unlike the Brownstein mineral, the new “molecular Brownstein” emits NIR-II light with a wavelength of 1,435 nanometers or NIR-I light with a wavelength of 850 nanometers when excited with visible light. “This is an unusual observation for a molecular system based on manganese in the +IV oxidation state. Even in noble metals, the radiation in this energy region is unprecedented,” said Professor Katya Heinze.

Even more intriguing, in addition to the NIR-II luminescence from the molecular manganese system, is the observation that upon photoexcitation, “molecular Brownstein” can oxidize a variety of organic substrates. This includes extremely complex aromatic molecules with very high oxidation potential, such as naphthalene, toluene or benzene.

Dr. prepared the new complex and carried out all the photolysis experiments during his doctorate. “Even very stable solvents can be attacked by the superphotooxidator when excited by LED light,” said Nathan East. In the group of Professor Katya Haintze.

Observation of two photoactive states with ultrafast spectroscopy

Ultrafast spectroscopic techniques using laser pulses with sub-picosecond resolution revealed unusual excited-state reactivity and two distinct photoactive states: a very short-lived but highly oxidizing high-energy state and a long-lived, moderately oxidizing, low-energy state. The former can attack solvent molecules already in close proximity to the complex before light excitation, while the latter excited state persists long enough to attack aromatic substrates after a diffusion collision.

Dr. D., a senior researcher in Professor Kata Heinze’s group who specializes in time-resolved spectroscopy. “This is called static and dynamic quenching of excited states,” explained Robert Naumann.

Quantum chemical calculations to understand unusual photo processes

“A detailed picture of photo-induced processes emerged when we modeled excited states using quantum chemical calculations in light of spectroscopic results,” said Heinze.

Senior researcher in the group of Katia Heinze, who is actively involved in quantum research, said Dr. “These advanced and time-consuming calculations were only possible with the computing power of the MOGON and ELWETRITSCH supercomputers in Rhineland-Palatinate,” said Christoph Förster. . chemical research.

In the future, scientists will be able to develop complex new reactions controlled by light using the widely used metal manganese. This would not only replace the rarer, more expensive ruthenium and iridium compounds still most widely used today, but would also provide classes of reactivity and substrates not found in classical compounds.

“Thanks to our newly installed ultra-fast laser system, the computing power of high-performance supercomputers, and the creativity and skills of our PhD students, we will continue to develop more sustainable photochemistry,” the professor said. Katya Heintze.