Ligand-mediated core structure engineering and luminescence of metal nanoclusters: tuning emission from NIR-I to NIR-II

Abstract

Luminescent metal nanoclusters featuring near-infrared (NIR) emission hold promise for diverse applications. Although ligand engineering can modulate their NIR optical properties, separating the effects of a single ligand in dual-ligand systems remains challenging. In this study, we employed a controlled variable approach for preparing three structurally comparable silver-rich clusters, Ag₁₁Cu₃, Cu@Ag_14−xCu_x, and Ag₁₈. By maintaining a constant thiolate ligand and solely varying the phosphines, we elucidated the synergistic regulatory impacts of the ligands on the core configuration and excited-state dynamics. Structural and optical analyses reveal that the monodentate phosphine (TPP) stabilizes an Ag₆@Ag₅Cu₃ core of the Ag₁₁Cu₃ cluster, leading to NIR-I emission (720 nm) originating from mixed triplet state metal-centered and charge transfer transitions. In contrast, the bidentate phosphine (trans-dppe) facilitates the formation of a nested Ag₆@Ag₈@Ag₄ core of the Ag₁₈ cluster, triggering NIR-II emission (1424 nm) associated with highly delocalized core-based excited states. Moreover, the spatial influence of the chiral bidentate phosphine ((S,S)-BDPP) can induce core-structural distortion, which promotes nonradiative decay and results in luminescence quenching of the Cu@Ag_14−xCu_x cluster. This work clarifies the relationship between the structure and luminescence properties of ligand-mediated metal clusters, offering valuable insights for the design of tailored NIR optical nanomaterials.