Biomimetic crystallization for long-pursued -COOH-functionalized gold nanocluster with near-infrared phosphorescence

As an interdisciplinary product, water-soluble gold nanoclusters (AuNCs) stabilized by ligands containing carboxyl (-COOH) group have garnered significant attention from synthetic chemists and biologists due to their immense potential for biomedical applications. However, revealing the crystallographic structures of -COOH-functionalized AuNCs remains a bottleneck. Herein, we successfully applied the salting-out method to obtain a series of high-quality single crystals of -COOH-functionalized Au₂₅ nanoclusters and revealed their crystallographic structures. Particularly, K₃Au₂₅(2-Hmna)₉(mna)₆]^- (Au25a) protected by 2-mercaptonicotinic acid features an unprecedented tetrameric Au₄(SR^S)₃(SR^S,N)₂ staple motifs surrounding the icosahedral Au₁₃ kernel, breaking the traditional perception on the structure of Au₂₅(SR)₁₈. Au25a exhibits a distinct near-infrared emission at 970 nm with long lifetime of 8690 ns, which have been studied by transient absorption spectroscopy and time-dependent density functional theory. This work compensates for the research gap in the experimental structure of -COOH-functionalized AuNCs and opens up a new avenue to explore their structure-property correlations.