Excited Electronic States of Metallic Atoms Rely on their Sulfuric Neighbors

Scientists discover that bond covalency is an important property of excited states in molecules containing metal-sulfur bonds.

Strong interactions between neighboring metal and sulfur atoms are found to influence the properties of electronic excited states created by light absorption, as shown by X-ray spectroscopy methods at the Stanford Synchrotron Radiation Lightsource. — Image courtesy of Greg Stewart, SLAC National Accelerator Laboratory

The Science

Metal sulfur coordination complexes are combinations of transition metals (one of 28 metallic elements, including iron and other common metals) surrounded by molecules or atoms that contain sulfur. These complexes are important to many biological processes. For example, they are part of the so-called trace elements essential to life. Researchers also use these complexes in non-biological catalysis and other reactions. To learn more about how metal-sulfur coordination complexes interact with light, scientists tuned a spectroscopic technique called Resonant Inelastic X-ray Scattering (RIXS). RIXS is a method that uses scientific X-ray light sources to measure the energy of photons scattered from the sample; it was applied here to probe the same electronic states generated by visible light absorption by the metal sulfur complexes. These measurements reveal the covalent character of the electronic excited states of the complexes. A covalent bond is formed by sharing electrons between two or more atoms. Before adopting RIXS, scientists could not determine the contributions of sulfur to these processes.

The Impact

Metal-sulfur complexes play a large role in chemical reactions in some biological enzymes and artificial compounds. In this research, scientists directly monitored the states of the electrons in the outer shells of these complexes. This allowed them to measure the covalency of the metal-sulfur bonds in the excited states formed by light absorption. Covalent bonds are important because they let atoms share electrons and help determine the reactivity of a molecule. The experimental data agreed with theoretical predictions that were developed using novel numerical simulation methods. The excited state bond covalency is a good way to understand how metal-sulfur coordination complexes interact with light. This makes this measurement useful for predicting light-driven chemical processes in these compounds.

Summary

Metal-ligand covalency is a measure of local charge distribution, which affects the ground state chemistry of molecular electrocatalysts. However, the influence of covalency on electronic excited states has not been investigated in detail, in part because those states were not directly accessible to measurements.

In this work, researchers used a Resonant Inelastic X-Ray Scattering (RIXS) protocol tuned to probe the S 1s3p spectrum for metallo dithiolene complexes containing either copper or nickel atoms at the metallic center. The researchers then compared the RIXS spectrum to theoretical predictions based on novel time-dependent density functional theory to extract the degree of covalency in the valence excited states. Strong covalency was found to modify the charge distribution properties of those states compared to other non-sulfuric metal complexes. These results present metal-ligand covalency as a viable design principle for systematically tuning charge transfer character and energy of the ligand field excited states that dominate the photophysics of first row transition metal complexes.

Contact

Amy A. Cordones
SLAC National Accelerator Laboratory
[email protected]

Funding

This work was partially supported by the Department of Energy Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division through the Early Career Research Program.