Combination of a Manganese Salt and a Ligand Paves the Way for Photochemistry in Future Large‑Scale Applications

Reactions are usually driven by heat, but in recent years photochemistry has also achieved significant milestones. A research team led by Prof. Heinze has now developed a novel metal complex based on abundant and inexpensive manganese. Until now, photochemical transformations often required the catalysts ruthenium, osmium or iridium — all rare, expensive elements that are environmentally harmful to mine.

“This metal complex establishes a new benchmark in photochemistry: it combines a record‑breaking excited‑state lifetime with a straightforward synthesis,” explains Heinze. “Consequently, it provides a powerful and sustainable alternative to the noble‑metal complexes that have long dominated light‑driven chemistry.”

The findings were recently published in Nature Communications.

One‑Step Synthesis and Strong Absorption

Manganese is more than 100 000 times more abundant on Earth than the noble metal ruthenium, yet its use in photochemistry has been severely limited for two reasons: the labor‑intensive, multi‑step synthesis (often nine to ten steps) and the short lifetime of the excited state.

“The newly developed manganese complex overcomes both challenges,” reports Dr. Nathan East, former PhD student in Prof. Katja Heinze’s group, who carried out the original synthesis. The material is synthesized directly from commercially available starting materials — in a single synthetic step.

In addition to manganese, the researchers employ a ligand that allows the properties of the complex to be fine‑tuned.

“Mixing a colourless manganese salt with the colourless ligand in solution instantly produces a deep‑purple colour — just like ink. This is a very unusual colour for a manganese complex and told us that something unique was happening,” adds Sandra Kronenberger, who investigated this novel manganese complex as a doctoral student in the Heinze group at the Max Planck Graduate Center (MPGC).

The resulting manganese complex is not only visually striking; it also exhibits remarkable properties:

“Its light absorption is exceptionally strong, meaning the probability of capturing a photon is very high — the complex uses light extremely efficiently,” explains Dr. Christoph Förster, who supported the project with quantum‑chemical calculations.

Excited‑State Lifetime Exceeds the 190‑Nanosecond Mark

“The lifetime of the complex, 190 ns, is also remarkable. It is two orders of magnitude longer than any previously known complexes that contain common metals such as iron or manganese,” says lead scientist and spectroscopist Dr. Robert Naumann, who characterized the excited‑state dynamics by luminescence spectroscopy.

In photochemistry the catalyst —in this case the manganese complex — is excited by light. When it encounters another molecule by diffusion, it transfers an electron to that molecule. Because it can take nanoseconds for the reactants to find each other, the excited state must persist as long as possible.

“We were able to detect the initial product of the photoreaction — the electron transfer that occurred — and thereby prove that the complex behaves as intended,” summarizes Prof. Dr. Katja Heinze.

Outlook

This discovery expands the boundaries of sustainable photochemistry. Thanks to its scalable one‑step synthesis, efficient light absorption, robust photophysical behaviour, and long‑lived excited state, the new manganese‑based material paves the way for photoreactions in future large‑scale applications. It could become especially important for sustainable hydrogen production.