Revolutionizing Chemistry with the ‘Molecular Braunstein’: A Manganese Complex that Absorbs Visible Light
A team of scientists led by Professor Katja Heinze has made a breakthrough in the field of light-driven reactions with the design of a soluble manganese complex. This unique compound, named ‘molecular Braunstein’ after the dark color of Braunstein or manganese dioxide, exhibits panchromatic absorption of visible light and parts of near-infrared light. When photoexcited, the complex emits NIR-II light and is capable of oxidizing various organic substrates. The research has been published in Nature Chemistry and is part of the Light Controlled Reactivity of Metal Complexes priority program funded by the German Research Foundation.
Unveiling the ‘Molecular Braunstein’
The manganese complex, also known as Mn dgpy 2 4, is based on Earth-abundant manganese and the tridentate 2,6-diguanidylpyridine ligand dgpy. Upon excitation with low energy near-infrared light, the complex evolves to a luminescent doublet ligand-to-metal charge transfer (2LMCT) excited state. This state has the capability to oxidize naphthalene to its radical cation and enables the oxidation of nitriles and benzene using Earth-abundant elements and low-energy light.
Furthermore, the ‘molecular Braunstein’ can absorb all visible light from blue to red, i.e., in a wavelength of 400 to 700 nanometers, and parts of the near-infrared light up to 850 nanometers. This panchromatic absorption allows the complex to oxidize various organic substrates, including extremely challenging aromatic molecules with very high oxidation potentials such as naphthalene, toluene, or benzene.
Observing the Unusual Excited-State Reactivity
Ultrafast spectroscopic techniques have revealed an unusual excited-state reactivity and two different photoactive states in the ‘molecular Braunstein’. These include a very short-lived but extremely oxidizing high-energy state and a longer-lived moderately oxidizing lower-energy state. Quantum chemical calculations were deployed to model the involved excited states, leading to a comprehensive understanding of the photoinduced processes.
Transformative Implications of the Research
Other studies on manganese complexes have shown their potential in various fields. For instance, a study on the transformation of chlorobenzene by soluble Mn(III), a product of the reductive dissolution of colloidal MnO2 by naturally-occurring organic acids, found that Mn(III) complexes could potentially be used to remove chlorobenzene from natural environments. Another study highlighted that low concentrations of Mn NPs promote radish growth and enhance root elongation and seed vigor, suggesting the potential of manganese-based NPs as nanofertilizers.
The development of the ‘molecular Braunstein’ is a significant leap forward, especially in the field of photooxidations. The use of manganese, an abundant element, in place of rare and costly compounds, not only makes the process more sustainable but also broadens the range of reactions that can be driven by light. The research has opened up a myriad of possibilities for further exploration, promising a new era of light-driven reactions in chemistry.