How ligand coordination and superatomic-states accommodate the structure and property of a metal cluster: Cu4(dppy)4Cl2 vs. Cu21(dppy)10 with altered photoluminescence

We have synthesized two copper nanoclusters (NCs) with a protection of the same ligand diphenylphosphino-2-pyridine (C17H14NP, dppy for short), formulated as Cu4(dppy)4Cl2 and Cu21(dppy)10, respectively. The former one bears a distorted tetrahedron Cu4 core with its six edges fully protected by chlorine and dppy ligands, while the latter presents a symmetric Cu21 core on which ten dppy molecules function as monolayer protection via well-organized monodentate or bidentate coordination. Interestingly, the Cu4(dppy)4Cl2 cluster exhibits a strong yellow emission at ∼577 nm, while Cu21(dppy)10 displays dual emissions in purple (∼368 nm) and green (∼516 nm) regions respectively. In combination with TD-DFT calculations, we demonstrate the origin of altered emissions and unique stability of the two copper nanoclusters pertaining to the ligand coordination and metallic superatomic states.