Labile Ligands Protected Cu₅₀ Nanoclusters with Tailorable Optical Limiting Effect

Atomically precise copper nanoclusters have recently attracted tremendous attention, as they often exhibit structures and properties distinct to their gold and silver counterparts. Although considerable progress has been made in the preparation and structure determination of copper nanoclusters, the introduction of those newly created materials in more underlying areas such as nonlinear optics is in its infancy. Herein, we report a new class of Cu-50 nanoclusters exhibiting a tunable optical limiting effect. The facile preparation prototype reported in this work allows the discovery of up to four clusters with the nearly identical metal framework, namely, Cu-50(CF3COO)(12)(3,5-diMe-PhS)(18)(PPh3)(4)H-2 (Cu-50-1), Cu-50(CF3COO)(10)(4-F-PhS)(20)(PPh3)(6)H-2 (Cu-50-2), Cu-50(CF3COO)(10)(PhS)(20)(PPh3)(6)H-2 (Cu-50-3), and Cu-50(PhCOO)(10)(4-F-PhS)(20)(PPh3)(6)H-2 (Cu-50-4). The molecular structure of the Cu-50 series, as revealed by X-ray single crystal analysis, describes the stabilization of Cu-44 core by an organometallic shell of thiolate-carboxylate-phosphine-copper. The combinatorial studies by electrospray ionization mass spectroscopy (ESI-MS), nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations suggest that the clusters contain 18 free valence electrons, making them probably the first library of copper superatoms featuring 18e. As revealed by the nanosecond Z-scan measurements results, the clusters exhibit typical optical limiting effects, indicating their promising applications in the field of nonlinear optics (NLO). More importantly, the labile surface motifs on Cu-50 allow us to tune their NLO properties. This work not only reports a new class of atomically precise copper nanoclusters with facile preparation process, clear structures, and labile surface motifs but also encourages research on the exploration of copper nanoclusters in more exciting areas.