Molecular Gold Nanocluster Au₁₅₆ Showing Metallic Electron Dynamics

The boundary between molecular and metallic gold nanoclusters is of special interest. The difficulty in obtaining atomically precise nanoclusters larger than 2 nm limits the determination of such a boundary. The synthesis and total structural determination of the largest all-alkynyl-protected gold nanocluster (Ph₄P)₆[Au₁₅₆(C≡CR)₆₀] (R = 4-CF₃C₆H₄-) (Au) are reported. It presents an ideal platform for studying the relationship between the structure and the metallic nature. Au has a rod shape with the length and width of the kernel being 2.38 and 2.04 nm, respectively. The cluster contains a concentric Au₁₂₆ core structure (Au₄₆@Au₅₀@Au₃₀) protected by 30 linear RC≡C-Au-C≡CR staple motifs. It is interesting that Au displays multiple excitonic peaks in the steady-state absorption spectrum (molecular) and pump-power-dependent excited-state dynamics as revealed in the transient absorption spectrum (metallic), which indicates that Au is a critical crossover cluster for the transition from molecular to metallic state. Au is the smallest-sized gold nanocluster showing metal-like electron dynamics, and it is recognized that the cluster shape is one of the important factors determining the molecular or metallic nature of a gold nanocluster.