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Photoluminescence of ultrasmall, atomically precise gold nanoclusters constitutes an area of significant interest in recent years for both fundamental research and biological applications. However, the exploration of near-infrared photoluminescence of gold nanoclusters is still in its infancy due to the limitations of synthetic methods and characterization techniques. Herein, the photoluminescence properties of an Au₃₈(PET)₂₆(PET = 2-phenylethanethiolate) nanocluster are investigated in detail. The Au₃₈(PET)₂₆exhibits an emission peak at 865 nm, which is revealed to be a mix of fluorescence, thermally activated delayed fluorescence, and phosphorescence via the combined analyses of time-resolved and temperature-dependent photoluminescence measurements. The quantum yield of Au₃₈(PET)₂₆is determined to be 1.8% at room temperature under ambient conditions, which increases to above 90% by suppressing the non-radiative relaxation pathway at a cryogenic temperature (80 K). Overall, the results of this work discover the coexistence of three radiative processes in thiolate-protected Au nanoclusters and will pave the way for understanding the intriguing photoluminescence properties of gold nanoclusters in future studies.

 

With the recent establishment of atomically precise nanochemistry, capabilities toward programmable control over the nanoparticle size and structure are being developed. Advances in the synthesis of atomically precise nanoclusters (NCs, 1–3 nm) have been made in recent years, and more importantly, their total structures (core plus ligands) have been mapped out by X‐ray crystallography. These ultrasmall Au nanoparticles exhibit strong quantum‐confinement effect, manifested in their optical absorption properties. With the advantage of atomic precision, gold‐thiolate nanoclusters (Au_n(SR)_m) are revealed to contain an inner kernel, Au–S interface (motifs), and surface ligand (‐R) shell. Programming the atomic packing into various crystallographic structures of the metal kernel can be achieved, which plays a significant role in determining the optical properties and the energy gap (E_g) of NCs. When the size increases, a general trend is observed for NCs with fcc or decahedral kernels, whereas those NCs with icosahedral kernels deviate from the general trend by showing comparably smallerE_g. Comparisons are also made to further demonstrate the more decisive role of the kernel structure over surface motifs based on isomeric Au NCs and NC series with evolving kernel or motif structures. Finally, future perspectives are discussed.

 

Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand‐protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic–inorganic interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal–ligand surface bonding‐induced chirality for the nanoclusters is proposed. The different bonding modes of M–X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au–thiolate NCs with Au–phosphine ones further reveals the important roles of surface bonding. Compared to the Au–thiolate NCs, the Ag/Cu/Cd–thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high‐performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed.

 

The metallic bond is arguably the most intriguing one among the three types of chemical bonds, and the resultant plasmon excitation ( e.g. in gold nanoparticles) has garnered wide interest. Recent progress in nanochemistry has led to success in obtaining atomically precise nanoclusters (NCs) of hundreds of atoms per core. In this work, thiolate-protected Au 279 (SR) 84 and Au 333 (SR) 79 NCs, both in the nascent metallic state are investigated by cryogenic optical spectroscopy down to 2.5 K. At room temperature, both NCs exhibit distinct plasmon resonances, albeit the NCs possess a gap (estimated 0.02–0.03 eV, comparable to thermal energy). Interestingly, we observe no effect on plasmons with the transition from the metallic state at r.t. to the insulating state at cryogenic temperatures (down to 2.5 K), indicating a nonthermal origin for electron-gas formation. The electronic screening-induced birth of metallic state/bonding is discussed. The obtained insights offer deeper understanding of the nascent metallic state and covalent-to-metallic bonding evolution, as well as plasmon birth from concerted excitonic transitions.