Structural, Electronic, and Photophysical Insights into a Few Atom Copper–Sulfur Cluster in the Solid and Solution States

Coinage‐metal chalcogenide clusters are widely studied for their attractive photoluminescence properties. Copper chalcogenides are especially promising, but are often confined to solid‐state investigations due to their limited solution stability and the difficulty of synthesizing stable, well‐defined clusters. Here, we investigate copper–sulfur clusters incorporating a small number of Cu atoms to elucidate fundamental atomic interactions, ground‐ and excited‐state characteristics, and photophysical behavior in both solid and solution. We have synthesized the Cu ₆ (4,6‐dimethyl‐2‐mercaptopyrimidine) ₆ cluster in both neutral and charged states, Cu6 and Cu6‐2H ²⁺ , respectively, by selective ligand protonation. The molecular structures are determined using single‐crystal X‐ray diffraction, while Cu K‐edge X‐ray absorption spectroscopy is used to probe Cu electronic structure differences arising from the ligand modification. Steady‐state and pump‐probe optical spectroscopy is used to investigate photophysical properties, interpreted using density functional theory methods. Both clusters exhibit good stability in the solid state and in solution and show characteristic near‐infrared emission with microsecond lifetimes. Overall, the Cu ₆ S ₆ clusters display favorable charge–transfer characteristics and show potential for further use in driving photochemical transformations.