Anti-Heavy-Atom Effect Observed in Near-Infrared Emissive Bimetallic Nanoclusters Au28Cu12X4Cl4 (X = Cl, Br, and I)

Metal nanoclusters with high photoluminescence quantum yield (PLQY) in the near-infrared region (NIR) have attracted considerable interest, and the elucidation of their structure-property relationships and luminescence mechanisms facilitates rational synthesis and practical applications. The direct reduction of alkynyl-gold precursors and copper salts leads to the formation of a series of face-centered cubic bimetallic nanoclusters Au₂₈Cu₁₂X₄Cl₄ (X = Cl, Br, and I), which exhibits strong NIR emission (∼850 nm) with PLQY being 14, 10, and 8% in the solution at room temperature, respectively. In contrast to the well-known heavy-atom effect, when the four chlorides in the Au₂₄Cu₄ core are replaced with bromides or iodides, the luminescence of Au₂₈Cu₁₂X₄Cl₄ is not enhanced but diminished instead. Excited-state dynamics studies reveal that heavier halogen atoms have a negligible effect on the intersystem crossing rate. Instead, their larger atomic sizes lead to an expansion of the cluster cores, which increases nonradiative transition rates and reduces PLQY. This is the first observation of an anti-heavy-atom effect in luminescent nanoclusters.