Doping-Mediated Energy-Level Engineering of M@Au12 Superatoms (M=Pd, Pt, Rh, Ir) for Efficient Photoluminescence and Photocatalysis

Unprecedented stacking-dependent piezoluminescence enhancement in atomically precise superatomic gold nanoclusters

Deciphering the structure-property relationship between cluster stacking and high-efficiency luminescence of metal nanoclusters is crucial for designing and synthesizing high-performance light-emitting materials and devices. Here, we successfully synthesized two polymorphic gold nanoclusters (Au8-C and Au8-P) and investigated their stacking-dependent piezoluminescence based on hydrostatic pressure. Under compression, Au8-C exhibits notable piezoluminescence enhancement. However, Au8-P presents monotonic piezoluminescence quenching. High-pressure structural characterizations confirm the existence of stacking-dependent anisotropic compression in Au8-C and Au8-P. Under high pressure, the columnar-stacked Au8-C shrinks faster along the a axis, increasing the aspect ratio (AR) of the fusiform Au8 core. However, the layered Au8-P is compressed faster along the c axis, reducing the AR and leading to a flatter Au8 core. High-pressure femtosecond transient absorption, time-resolved photoluminescence, and Raman spectra collaboratively confirm that differentiated anisotropic compression notably suppresses nonradiative loss caused by low-frequency vibrations of the Au8 core, which is responsible for the piezoluminescence enhancement in Au8-C.

Intensive near-infrared emitting Au7Cu10 nanoclusters for both energy and electron harvesting

Triplet excitons have gained increasing recognition as inherent characteristics of various nanomaterials. However, the practical application of triplet excitons is limited due to their confinement within the material where they originate, posing significant challenges to harnessing their potential. In this work, we report direct extraction of both triplet energy and electrons from newly synthesized atomically precise Au7Cu10 nanoclusters (NCs). These NCs exhibit intensive near-infrared (NIR) emission with a quantum yield of 31% at room temperature. They also display near-unity quantum yield of intersystem crossing (ISC) with strong spin-orbit coupling (SOC) up to 864 cm-1. Consequently, Au7Cu10 NCs can act as triplet sensitizers, facilitating efficient triplet-triplet annihilation and achieving upconverted emission with an efficiency of 18.4% in perylene. Furthermore, rapid electron injection from Au7Cu10 NCs in the triplet state to methyl viologen was clearly observed. This study represents the first direct extraction of both triplet energy and electrons from the same metal NCs, indicating their potential as molecular triplet energy and electron surrogates in optoelectronics, photocatalysis, and solar energy conversion.

A nearly perfect icosahedral Ir@Au12 superatom with superior photoluminescence obtained by ligand engineering

Heterometal doping and the introduction of surface ligands drastically alter the optical and photophysical properties of gold-based superatoms by modulating their electronic structures and the excited state dynamics. In this study, we investigate how the structures and the optical properties of an Ir@Au12 superatom capped by a diphosphine ligand, bis[benzo[b]phosphindole]ethane (bbpe), in which the rotation of the phenyl groups is prohibited, differ from those capped by the conventional diphosphine ligands, such as 1,2-bis(diphenylphosphino)ethane (dppe) and bis(diphenylphosphino)methane (dppm). The co-reduction of Ir(iii)- and Au(i)-precursors under mild reaction conditions yielded homoleptically capped [IrAu12(bbpe)6]3+ clusters (IrAu12-b) as the primary product. Single crystal X-ray diffraction analysis of IrAu12-b revealed the formation of a nearly perfect icosahedral Ir@Au12 superatomic core, in which the central Ir atom is equidistant from each vertex Au atom. The energy gap between occupied 1P and unoccupied 1D superatomic orbitals of IrAu12-b was larger than that of its dppm-capped counterpart, [IrAu12(dppm)6]3+ as evidenced by a blue shift (140 nm) of the photoluminescence (PL) wavelength and DFT calculations. IrAu12-b exhibited PL at 596 nm with a high quantum yield of 87% in deaerated CH2Cl2 due to the expanded 1P-1D energy gap and the restricted molecular motions of the bbpe ligands.

Halide-Directed Ligand Engineering Enables Expedient, Controlled and Divergent Syntheses of Diphosphine-Protected Au Nanoclusters

Despite substantial progress in ligand engineering, the efforts in the field of Au nanoclusters have been concentrated almost exclusively on organic ligands. Halides, the most typical auxiliary inorganic ligands widely present in Au clusters, remain virtually unexplored, particularly regarding their effects on cluster construction. Herein, diphosphine Ph2P(CH2)nPPh2 (Ln, n = 1-6) is chosen as the co-protecting organic ligands and a comparative analysis on the influential roles of halide ions (Cl-, Br-, I-) in guiding Au cluster synthesis is conducted. A simple yet efficient halide-directed synthetic approach has been developed and a series of Au nanoclusters, including the known [Au18(L1)6Br4]2+, [Au13(L2)5Cl2]3+ and [Au8(L3)4Cl2]2+ that however crystallized in new polymorphic forms, as well as the new reduction-active [Au18(L1)6Cl4]2+, luminescence-enhanced [Au14(L3)5Br4]2+ and core-isomeric [Au11(Ln)4X2]+ (n = 4-6; X = Cl, Br, I), are obtained in a more expedient and controllable manner. This work clearly demonstrates the non-negligible roles of halide ions in directing cluster synthesis, and provides an easier access to diverse diphosphine-protected Au nanoclusters. This approach, promising in gram-scale synthesis, is expected to further extend the ligand scope and holds promise for advancing the diversified syntheses of a broader range of ligand-protected metal nanoclusters.

Preparation Methods of Metal Nanoclusters

Metal nanoclusters, also known as ultrasmall nanoparticles, represent a promising class of nanomaterials due to their atomically precise characterizations and intriguing chemical-physical properties. The preparation is the cornerstone for advancing the nanocluster science, facilitating their structural determination, property investigation, and practical application. We have been devoted to exploring new and efficient approaches for the high-yield preparation of metal nanoclusters with customized structures and properties. We have proposed and developed four methodologies for the nanocluster preparation, including kinetic control, seeded growth, in situ two-phase ligand exchange, and metal exchange. More than 200 metal nanoclusters have been synthesized and structurally determined, laying the foundation for the elucidation of structure evolutions and structure-property correlations. In this concept, we emphasized our progress in proposing and developing the synthetic mythologies of metal nanoclusters. This Concept hopefully provides researchers attempting to study the preparation methods of metal nanoclusters with several feasible synthetic routes.

Unlocking the Photoluminescence and Photostability of Au11 Clusters through Pt-Mediated Band-Gap Engineering

Au nanoclusters often demonstrate useful optical properties such as visible/near-infrared photoluminescence, in addition to remarkable thermodynamic stability owing to their superatomic behavior. The smallest of the 8e- superatomic Au nanoclusters, Au11, has limited applications due to its lack of luminescence and relatively low stability. In this work, we investigate the introduction of a single Pt dopant to the center of a halide- and triphenylphosphine-ligated Au11 nanocluster, affording a cluster with a proposed molecular formula PtAu10(PPh3)7Br3. Electrochemical and spectroscopic analysis reveal an expansion of the HOMO-LUMO gap due to the Pt dopant, as well as relatively strong near-infrared (NIR) photoluminescence which is atypical for an M11 cluster (λmax = 700 nm, Φ = 1.88 %). The Pt dopant additionally boosted photostability; more than tenfold. Lastly, we demonstrate the application of the PtAu10 cluster's NIR photoluminescence in the detection of the nitroaromatic compound 2,4-dinitrotoluene, with a limit-of-detection of 9.52 μM (1.74 ppm). The notable ability of a single central Pt dopant to unlock photoluminescence in a non-luminescent nanocluster highlights the advantages of heterometal doping in the tuning of both the optical and thermodynamic properties of Au nanoclusters.

Metal Nanoclusters as Highly Efficient, Versatile Type-II Photoinitiators via Generating Radicals from Non-Conventional Hydrogen Donors

Development of highly efficient photocatalysis or photoinitiation systems that are applicable to various types of illumination source is critical to virtually all the light-driven applications. Here, we report a generalizable strategy to achieve highly sensitive, versatile photoinitiation systems based on the combination of metal nanoclusters with non-conventional hydrogen donors (co-initiators). Discovery of this type-II photoinitiation pathway in metal nanoclusters not only improves their two-photon initiation sensitivity by up to three-orders-of-magnitude, it further opens the door for metal nanoclusters to trigger the photopolymerization using the low-power UV light-emitting diodes. Different from molecular type-II photoinitiators, we found that the selection rules of hydrogen donors for metal nanoclusters are largely dependent on their ligand structures. More importantly, using electron paramagnetic resonance and mass spectroscopy, we for the first time demonstrate that the photoexcited metal nanoclusters can function as versatile hydrogen atom abstractors which generate various types of previously unreported thiyl and nitrogen-centered radicals. This finding indicates the broad opportunity of the future application of metal nanoclusters in light-driven organic synthesis and radical chemistry.

Enhancement of Photoluminescence Quantum Yield of Silver Clusters by Heavy Atom Effect

Many ligand-protected metal clusters exhibit phosphorescence at room temperature. However, strategies for improving their phosphorescence quantum yield, a critical parameter of performance, remain poorly developed. In contrast, fluorescent dyes are commonly modified by introducing heavy atoms, such as iodine (I), to enhance intersystem crossing in the excited state, thereby harnessing the heavy atom effect to increase phosphorescence efficiency. In this study, a pair of ligand-protected silver (Ag) clusters is successfully synthesized with internal cavities encapsulating anions (Xz -), namely sulfide ions (S2-) or iodide ions (I-), which significantly differ in atomic number each other. Single-crystal X-ray diffraction and nuclear magnetic resonance spectroscopy revealed that the resulting Ag clusters are composed of X@Ag54S20(thiolate)20(sulfonate)m, where (X, m) = (S, 12) or (I, 11). X-ray photoelectron spectroscopy revealed that the Ag atoms in these compounds exhibit a mixed-valence state. Furthermore, experiments on their photoluminescence revealed that a heavy central anion induced an internal heavy-atom effect similar to that observed in organic fluorescent dyes. As a result, the phosphorescence quantum yield became 16 times higher when S2- is replaced by I- as the central atom.

Atomically Engineered Trimetallic Nanoclusters Toward Enhanced Photoluminescence and Photoinitiation Activity

Precise doping is of vital significance for atomic engineering and the establishment of structure-property relationships in nanocluster (NC) chemistry. Herein, two novel trimetallic MAu18Cd3 (M = Pd/Pt) NCs that are derived from M-doped Au25 templates of MAu24 are reported, in which the central doping of M atom and the surface-motif doping of Cd atoms are concurrently achieved. Compared to the original templates, Cd-induced surface engineering enhances the rigidity of the structural framework and enlarges the HOMO-LUMO gaps of the MAu18Cd3, significantly improving photoluminescent efficiency by suppressing nonradiative relaxation. The critical role of the central M (Pd/Pt) dopant in photoluminescence, which regulates the rate of radiative decay of excited-state electrons, has also been substantiated. More notably, the doped case of PtAu18Cd3 exhibits excellent photoinitiation activity in 3D two-photon printing with a high resolution of ≈140 nm, which may be attributed to the prolonged excited state. Overall, this work provides a generalized routine for the precise synthesis of multi-metal NCs with concurrent enhancements in photoluminescence and photoinitiation activity, which is expected to stimulate further research for the design and preparation of multi-functional, multi-metal NCs.

Sequential adsorption of multiple CO molecules on Au10- and Au9Zn- triangular clusters: the crucial role of a single atomic impurity

In this paper, we find, using DFT calculations, that the patterns of sequential adsorption sites of up to six CO molecules on the planar Au10- and Au9Zn- clusters differ radically. Thus, CO prefers bridge (top) adsorption sites forming Au10(CO)n- compounds with n = 1-3 (n = 4-5), but top (bridge) sites occur for Au9Zn(CO)n- compounds when n = 1-4 (n = 5-6). These facts are distinguishable in the CO stretching spectra of both types of compounds. Severe distortions and broken Au-Au bonds appear in the morphology of pure compounds after CO adsorption, but only small distortions are seen in the doped compounds. Using the nudged elastic band (NEB) method, we find the reaction paths of CO adsorption for pure n = 3 and doped n = 5 compounds. The estimated reaction barrier of the pure compound is much higher than that of the doped one. We also find important differences in the electronic properties of these compounds as functions of the size n: adsorption energy, bond lengths, angles, Bader atomic charges, and HOMO-LUMO gaps. We argue that all these differences are due to a larger charge reorganization in the doped than in the pure gold compounds, because of the Zn atom's lower electronegativity than that of gold and carbon. Comparison with previous results for multiple adsorption of NO molecules sheds light on the characteristics of Au-CO and Au-NO bonds.

Unraveling the Stoichiometric Interactions and Synergism between Ligand-Protected Gold Nanoparticles and Proteins

Nanomaterials that engage in well-defined and tunable interactions with proteins are pivotal for the development of advanced applications. Achieving a precise molecular-level understanding of nano-bio interactions is essential for establishing these interactions. However, such an understanding remains challenging and elusive. Here, we identified stoichiometric interactions of water-soluble gold nanoparticles (Au NPs) with bovine serum albumin (BSA), unraveling their synergism in manipulating emission of nano-bio conjugates in the second near-infrared (NIR-II) regime. Using Au25(p-MBS)18 (p-MBS = para-mercaptobenzenesulfonic acid) as paradigm particles, we achieved precise binding of Au NPs to BSA with definitive molar ratios of 1:1 and 2:1, which is unambiguously evidenced by high-resolution mass spectrometry and transmission electron microscopy. Molecular dynamics simulations identified well-defined binding sites, mediated by electrostatic interactions and hydrogen bonds between the p-MBS moieties on the Au25(p-MBS)18 surface and BSA. Particularly, positively charged residues on BSA were found to be pivotal. By careful control of the molar ratio of Au25(p-MBS)18 to BSA, atomically precise [Au25(p-MBS)18]x-BSA conjugates (x = 1 or 2) could be formed. Through a comprehensive spectroscopy study, an electron transfer process and synergistic effect were manifested in the Au25(p-MBS)18-BSA conjugates, leading to drastically enhanced emission in the NIR-II window. This work offers insights into the precise engineering of nanomaterial-protein interactions and opens new avenues for the development of next-generation nano-bio conjugates for nanotheranostics.

Chemical Synthesis of ~1 nm Multilevel Capacitor-like Particles with Atomic Precision

Can the chemically synthesized nanoparticles act as nanodevices or nanomachines? Herein, we demonstrated this feasibility. A novel nanocluster (ultrasmall nanoparticle) [Au44Cd20(m-MBT)40][N(C8H17)4]2 (Au44Cd20 in short, m-MBTH: m-methylbenzenethiol) obtained via developing a synthesis method has a cannula-like structure of the outer shell and an internal sleeve, revealed by single-crystal X-ray diffraction. Natural population analysis (NPA) charge calculations, charge carrier transport of Au44Cd20 (during which an intra-nanocluster anti-galvanic reaction was observed) after unneutral charging using NaBH4 as well as voltammetry proved the capacitor-like character of Au44Cd20. The subsidiary capacitor-like character of the outer shell of Au44Cd20 was further probed via NPA charge calculations and electrocatalytic reduction of CO2 to CO. Thus, this study predicts a new era of engineering metal nanoparticles for realizing atomically precise ultrasmall nanodevices and nanomachines.

Heterogeneous Catalysis of Molecular-Like Au8M(PPh3)8 n+ Clusters Cultivated in Mesoporous SBA-15

It is a dream of researchers to be able to tailor the catalytic performances by adjusting heterogeneous catalysts at the atomic level. Atomically precise metal clusters provide us with the possibility to achieve this challenge. Here, we design a push-and-pull synthesis strategy coupled with TiOx coating to prepare the heterogeneous catalysts denoted as TiOx/Au8M@SBA via cultivating atomically precise Au8M(PPh3)8 n+ (M=Pd, Pt or Au; n=2 for Pd/Pt and 3 for Au) clusters in mesoporous molecular sieve. The catalysts are made up of the three functional units, which include Au8M(PPh3)8 n+ clusters that can act as the active sites, the pore environment of the SBA-15 that can announce a catalysis show for the clusters with precise number of atoms maintained during the chemical reactions, and the TiOx coating that can further inhibit the migration of the clusters under reaction conditions. The selective hydrogenation of acetylene performed in the fixed-bed reactor taken, for example, we learn how the atom-by-atom tailoring of a heterogeneous catalyst can switch on elusive heterogeneous mechanisms with cluster catalysis. This work sheds light on the fundamental insight into catalysis origin of heterogeneous catalysts and achieves a distinguished level of detail for cluster catalysis.

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

Site-specific substitution in atomically precise carboranethiol-protected nanoclusters and concomitant changes in electronic properties

We report the synthesis of [Ag17(o1-CBT)12]3- abbreviated as Ag17, a stable 8e⁻ anionic cluster with a unique Ag@Ag12@Ag4 core-shell structure, where o1-CBT is ortho-carborane-1-thiol. By substituting Ag atoms with Au and/or Cu at specific sites we created isostructural clusters [AuAg16(o1-CBT)12]3- (AuAg16), [Ag13Cu4(o1-CBT)12]3- (Ag13Cu4) and [AuAg12Cu4(o1-CBT)12]3- (AuAg12Cu4). These substitutions make systematic modulation of their structural and electronic properties. We show that Au preferentially occupies the core, while Cu localizes in the tetrahedral shell, influencing stability and structural diversity of the clusters. The band gap expands systematically (2.09 eV for Ag17 to 2.28 eV for AuAg12Cu4), altering optical absorption and emission. Ultrafast optical measurements reveal longer excited-state lifetimes for Cu-containing clusters, highlighting the effect of heteroatom incorporation. These results demonstrate a tunable platform for designing nanoclusters with tailored electronic properties, with implications for optoelectronics and catalysis.

In situ QXAFS study of CO and H2 adsorption on Pt in [PtAu8(PPh3)8]-H[PMo12O40] solid

The adsorption behaviors of H2 and CO molecules in crown-motif [PtAu8(PPh3)8]-H[PMo12O40] (PtAu8-PMo12) solids were investigated by in situ quick-scan X-ray absorption fine structure (QXAFS) measurements with a time resolution of 0.1 s. The electronic state of Pt in PtAu8-PMo12 was drastically changed by the adsorption of H2 and CO molecules because of the formation of Pt-H2/Pt-CO interactions. H2 was adsorbed more rapidly (<0.5 s) on Pt than CO (∼2.5 s) and showed reversible adsorption/desorption behavior on Pt atoms in PtAu8-PMo12. The rapid adsorption of H2 is due to the fast diffusion of H2, which has a smaller kinetic diameter than CO, in the narrow channels between the closed voids in PtAu8-PMo12. Meanwhile, CO was irreversibly adsorbed on Pt, resulting in structural isomerization to the stable "chalice-motif" PtAu8, which was determined by XAFS analysis and density functional theory calculations. Structural isomerization was involved by pushing ligands aside to make space for CO adsorption as the void size near Pt in the crown-motif PtAu8-PMo12 was narrower than the kinetic diameter of CO.

Loading Lewis Acid/Base Pair on Metal Nanocluster for Catalytic Ugi Reaction

Constructing structurally robust and catalytically active metal nanoclusters for catalyzing multi-component reactions is an interesting while challenging task. Inspired by Lewis acid and Lewis base catalysis, we realized the combination of both Lewis acid and Lewis base sites on the surface of a stable gold nanocluster Au35Cd2. The catalytic potential of Au35Cd2 in four-component Ugi reaction was explored, demonstrating high activity and exceptional recyclability. In-depth mechanism studies indicate that the catalytic synergy of the Lewis acid/base pair is crucial for the high efficiency of Au35Cd2-catalyzed Ugi reaction. Bearing the stable structure, multiple activation sites and hierarchical chirality, Au35Cd2 is expected to display further interesting catalytic performance such as asymmetric catalysis.

Recent Advances in Understanding Triplet States in Metal Nanoclusters: Their Formation, Energy Transfer, and Applications in Photon Upconversion

Recent experimental findings, rapidly accumulating over the past few years, has revealed that in the electronic excited states of metal nanoclusters (MNCs) composed of noble metal atoms (e.g., Cu, Ag, or Au), triplet states are generated with remarkably high efficiency, exerting a pivotal influence over the photophysical properties of the MNCs, notably their photoluminescence characteristics. As a result, MNCs are increasingly recognized as promising luminescent nanomaterials that exhibit room-temperature phosphorescence and thermally activated delayed fluorescence. Furthermore, the significance of triplet-state-mediated energy transfer and charge transfer in intermolecular photophysical processes is gaining increasing recognition, particularly in the applications of MNCs as photosensitizers for singlet oxygen and organic molecular triplets. This Perspective focuses on recent advances in understanding of the formation and photophysics of triplet states in MNCs. Additionally, a brief overview is provided of a series of studies exploring the use of MNCs as triplet sensitizers for photon upconversion via triplet-triplet annihilation, and future prospects for this emerging application are discussed.

"Sergeants-and-soldiers" principle in the synthesis of intrinsically chiral Au13 clusters

Surface ligand modification on atomically precise metal clusters is one of the important strategies not only to stabilize monodisperse clusters, but also to derive unique optical and electronic properties. The chiral coordination of surface ligands on clusters controls the global chirality in clusters giving rise to unambiguous optical activity including circular dichroism (CD) and circularly polarized luminescence (CPL). Stereoselective synthesis of intrinsically chiral clusters has been achieved by the use of chiral ligands stabilizing one-handed clusters selectively. Herein, we investigate the effect of chiral ligand coordination on the deracemization of intrinsically chiral Au13 superatom clusters. The mixture of achiral bis(diphenylphosphino)ethane (DPPE) and chiral (R,R)-1,2-bis[(2-methoxyphenyl)phenylphosphino]ethane (R-DIPAMP) led to the formation of a mixture of [Au13(R-DIPAMP)x(DPPE)5-xCl2]3+ clusters with varied x-values (x = 0-5). Reverse phase HPLC successfully fractionalized the mixture into solutions composed of single component Au13 clusters depending on the x-values. The separated Au13 clusters afforded similar optical activity to that of [Au13(R-DIPAMP)5Cl2]3+ in the CD study regardless of x-values (x = 1-4). The DFT calculation supports that the coordination of a single DIPAM ligand is enough to bias the twisting direction in the Au13 superatom with dictating the tortional orientation of other four DPPE ligands. The emergence of the "sergeants-and-soldiers principle", wherein a small number of chiral ligands determine the chiral orientation of other achiral ligands, is thus demonstrated in the synthesis of intrinsically chiral Au13 clusters.

Photosynthesis of Au8Cu6 nanocluster for photocatalysis in oxidative functionalization of alkynes

Ligand-protected metal nanoclusters provide an ideal platform for investigating photoredox catalysis. The central challenge is balancing their stability and catalytic activity. Here we show a photochemical reduction-oxidation cascade method for synthesizing an Au8Cu6 nanocluster, which features a robust structure and active surface. Photoredox catalytic activity of Au8Cu6 is developed for the functionalization of alkynes under oxidative conditions. Mechanism studies based on the precise structure reveal the catalytic process of the Au8Cu6 nanocluster. Oxidant-dependent selectivity of Au8Cu6 catalysis is developed for chemodivergent synthesis of mono- and di-functionalized products in high efficiency. The results will stimulate more research on metal nanocluster synthesis and catalysis.

Molecular Interactions in Atomically Precise Metal Nanoclusters

For nanochemistry, precise manipulation of nanoscale structures and the accompanying chemical properties at atomic precision is one of the greatest challenges today. The scientific community strives to develop and design customized nanomaterials, while molecular interactions often serve as key tools or probes for this atomically precise undertaking. In this Perspective, metal nanoclusters, especially gold nanoclusters, serve as a good platform for understanding such nanoscale interactions. These nanoclusters often have a core size of about 2 nm, a defined number of core metal atoms, and protecting ligands with known crystal structure. The atomically precise structure of metal nanoclusters allows us to discuss how the molecular interactions facilitate the systematic modification and functionalization of nanoclusters from their inner core, through the ligand shell, to the external assembly. Interestingly, the atomic packing structure of the nanocluster core can be affected by forces on the surface. After discussing the core structure, we examine various atomic-level strategies to enhance their photoluminescent quantum yield and improve nanoclusters' catalytic performance. Beyond the single cluster level, various attractive or repulsive molecular interactions have been employed to engineer the self-assembly behavior and thus packing morphology of metal nanoclusters. The methodological and fundamental insights systemized in this review should be useful for customizing the cluster structure and assembly patterns at the atomic level.

Au25 Cluster-Based Atomically Precise Coordination Frameworks and Emission Engineering through Lattice Symmetry

The atomically precise metal nanoclusters (NCs) have attracted significant attention due to their superatomic behavior originating from the quantum confinement effect. This behavior makes these materials suitable for various photoluminescence-based applications, including chemical sensing, bioimaging, and phototherapy, owing to their intriguing optical properties. Especially, the manipulation of inter- or intracluster interaction through cluster-assembled materials (CAMs) presents significant pathways for modifying the photophysical properties of NCs. Herein, two distinct CAMs, Au25-Zn-Hex and Au25-Zn-Rod, were synthesized via forming a coordination bond between [Au25(p-HMBA)18]- (p-H2MBA = 4-mercaptobenzoic acid) and Zn2+. Au25-Zn-Rod exhibited a 6-fold higher luminescence intensity in the near-infrared region compared to Au25-Zn-Hex, attributed to synergistic inter- and intracluster interactions that induce exciton delocalization and structure rigidification at the atomic scale. This study highlights the potential of diverse lattice symmetries in cluster-based frameworks for tuning the photophysical properties, contributing to a deeper understanding of the structure-property relationship in Au NCs.

Raising Near-Infrared Photoluminescence Quantum Yield of Au42 Quantum Rod to 50% in Solutions and 75% in Films

Highly emissive gold nanoclusters (NCs) in the near-infrared (NIR) region are of wide interest, but challenges arise from the excessive nonradiative dissipation. Here, we demonstrate an effective suppression of the motions of surface motifs on the Au42(PET)32 rod (PET = 2-phenylethanethiolate) by noncoordinative interactions with amide molecules and accordingly raise the NIR emission (875/1045 nm peaks) quantum yield (QY) from 18% to 50% in deaerated solution at room temperature, which is rare in Au NCs. Cryogenic photoluminescence measurements indicate that amide molecules effectively suppress the vibrations associated with the Au-S staple motifs on Au42 and also enhance the radiative relaxation, both of which lead to stronger emission. When Au42 NCs are embedded in a polystyrene film containing amide molecules, the PLQY is further boosted to 75%. This research not only produces a highly emissive material but also provides crucial insights for the rational design of NIR emitters and advances the potential of atomically precise Au NCs for diverse applications.

Synthesis of atomically precise Ag16 nanoclusters and investigating solvent-dependent ultrafast relaxation dynamics

In this article, the main focus is to employ a new synthetic strategy to prepare atomically precise Ag nanoclusters (NCs) and unveil the critical role played by the solvents in the excited state dynamics of Ag NCs. The compositional analysis confirms the formula of the nanoclusters as Ag16(PDT)8(PPh3)4 (Ag-PDT NCs). These NCs showed a sharp absorption band at 525 nm and a comparatively broad absorption band at 633 nm. The emission maximum was 630 nm with a quantum yield (QY) of 0.23%. Three-component relaxation dynamics was retrieved from global analysis and described as core relaxation (664 fs), core-to-surface state relaxation (500 ps), and ground state relaxation (>1 ns) for Ag NCs in the DCM solvent. The time constants are slightly higher at 1.25 ps, 624.25 ps, and >1 ns for Ag NCs in the DMF solvent because of the less effective charge separation. The high QY in DMF follows this low charge separation (0.23% vs. 0.63%). The straight-chain dithiol capping agent (with lower electron density than an electron-rich aromatic ring) is mainly responsible for this less effective charge separation. Finding the pivotal role of the solvent in NC chemistry will help to characterize it thoroughly and produce a strategy for precise applications in various fields.

Impact of the Peripheral Ligand Layer on the Excited-State Deactivation Mechanism of Au38S2(S-Adm)20 and Au30(S-Adm)18 (S-Adm = Adamantanethiolate) Clusters

Gold nanoclusters are ideal fluorescent labels for biological imaging, disease diagnosis, and treatment. Understanding the origin of the photoluminescence phenomenon in ligand-protected gold nanoclusters is crucial for both basic science and practical applications. In this study, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to study the mechanism of excited state deactivation of Au38S2(S-Adm)20 and Au30(S-Adm)18 (S-Adm = adamantanethiolate) clusters, which have similar sizes and compositions. The computational results indicate that the differences in structural symmetry and peripheral ligand layer lead to quite different excited state deactivation mechanisms and excited state lifetimes in Au38S2(S-Adm)20 and Au30(S-Adm)18. Specifically, the μ3-S atoms and bridging thiolate (SR) in the ligand layer of Au38S2(S-Adm)20 significantly suppress the structural relaxation of ligand motifs, resulting in a prolonged excited state lifetime and higher quantum yield. For the Au30(S-Adm)18, due to the symmetry forbidden and large structural relaxation of the ligand shell, a rapid nonradiative transition process resulted. This study provides new insights into how the photoluminescence of ligand-protected gold nanoclusters is influenced by their structure and symmetry.

If the Crown Fits: Sterically Demanding N-Heterocyclic Carbene Promotes the Formation of Au8Pt Nanoclusters

While N-heterocyclic carbenes (NHCs) have recently been shown to be effective ligands for gold nanoclusters, very few examples of heterometallic clusters incorporating nongroup 11 metals are known. We present herein an Au-Pt NHC cluster featuring a crown-shaped [Au8Pt(NHC)8]2+ core, produced in high yield without the need for chromatographic purification. The method was largely independent of the substitution pattern of the NHC backbone; however, bulky wingtip groups were needed for clean conversion to the Au8Pt cluster. Clusters were characterized using single crystal X-ray diffraction, multinuclear nuclear magnetic resonance, electrospray ionization mass spectroscopy, and ultraviolet-visible spectroscopy, and electrochemical features of the cluster are also presented. A detailed analysis of the in-progress reaction mixture by ESI-MS supports the direct involvement of Au-H species as intermediates in cluster formation. These studies further demonstrate that NHC wingtip sterics play a key part in determining the nature of the initial cluster species, providing critical information for the generation of new NHC-stabilized nanoclusters.

Isomeric Effects of Au28(S-c-C6H11)20 Nanoclusters on Photoluminescence: Roles of Electron-Vibration Coupling and Higher Triplet State

The exploration of near-infrared photoluminescence (PL) from atomically precise nanoclusters is currently a prominent area of interest owing to its importance in both fundamental research and diverse applications. In this work, we investigate the near-infrared (NIR) photoluminescence mechanisms of two structural isomers of atomically precise gold nanoclusters of 28 atoms protected by cyclohexanethiolate (CHT) ligands, i.e., Au28i(CHT)20 and Au28ii(CHT)20. Based on their structures, analysis of 3O2 (triplet oxygen) quenching of the nanocluster triplet states, temperature-dependent photophysical studies, and theoretical calculations, we have elucidated the intricate processes governing the photoluminescence of these isomeric nanoclusters. For Au28i(CHT)20, its emission characteristics are identified as phosphorescence plus thermally activated delayed fluorescence (TADF) with a PL quantum yield (PLQY) of 0.3% in dichloromethane under ambient conditions. In contrast, the Au28ii(CHT)20 isomer exhibits exclusive phosphorescence with a PLQY of 3.7% in dichloromethane under ambient conditions. Theoretical simulations reveal a larger singlet (S1)-triplet (T1) gap in Au28ii than that in Au28i, and the higher T2 state plays a critical role in both isomers' photophysical processes. The insights derived from this investigation not only contribute to a more profound comprehension of the fundamental principles underlying the photoluminescence of atomically precise gold nanoclusters but also provide avenues for tailoring their optical properties for diverse applications.

Doping-mediated excited state dynamics of diphosphine-protected M@Au12 (M = Au, Ir) superatom nanoclusters

Doping heterometal atoms into ligand-protected gold superatom nanoclusters (Aun NCs) is proposed to further diversify their geometrical and electronic structures and enhance their photoluminescence properties, which is attributed to the mixing and effects between atoms. However, the fundamental principles that govern the optoelectronic properties of the doped Aun NCs remain elusive. Herein, we systematically explored two prototypical 8-electron Aun (n = 11 and 13) NCs with and without Ir dopant atoms using comprehensive ab initio calculations and real-time nonadiabatic molecular dynamics simulations. These doped Aun NCs maintain their parent geometrical structures and 8-electron superatomic configuration (1S21P6). Strong core-shell (Ir-Aun) electronic coupling significantly expands the energy gap, resulting in a weak nonadiabatic coupling matrix element, which in turn increases the carrier lifetime. This increase is mainly governed by the low-frequency vibration mode. We uncovered the relationship between electronic structures, electron-vibration, and carrier dynamics for these doped Aun NCs. These calculated results provide crucial insights for the atomically precise design of metal NCs with superior optoelectronic properties.

Assembly of anionic silver nanoclusters with controlled packing structures through site-specific ionic bridges

The assembly of metal nanoclusters (NCs) into crystalline lattice structures is of interest in the development of NC-based functional materials. Here we demonstrate that the assembled structures of tri-anionic tetrahedral symmetric [Ag29(BDT)12]3- (Ag29 NC, BDT: 1,3-benzenedithiol) NCs are controlled into a polyethylene-like zigzag chain and a "poly-ring-fused-cyclohexane"-like honeycomb arrangement through ionic interactions with alkali metal cations such as K+ and Cs+. The site-specific binding of alkali metal ions on the tetrahedrally arranged binding sites of Ag29 NCs successfully connects the adjacent NCs into various packing modes. The number and type of bridges between NCs determine the Ag29 NC packing structures, which are affected by the solvent species, enabling the transformation of packing modes in the single-crystalline state. The photoluminescence (PL) properties of the crystals responded to the packing modes of the NCs in terms of anisotropy and bridge linkage style inducing a varied degree of relaxation of the excited state depending on the relocation mobility of alkali metal ions in the crystals.

Unraveling the Solvent Regulation in the Heteroatom-Doped Endohedral Gold Clusters: A Theoretical Study on the Electronic Properties and O2 Activation

Liquid-phase synthesis of atomically precise nanoclusters has experienced rapid development recently, where polar solvents are indispensable in such a process. However, the regulation effect of solvents on the structural and electronic properties of different metal clusters and cluster assembly materials is still not well understood. Herein, a comprehensive density functional theory calculation has been performed to explore the solvation effect on heteroatom-doped endohedral gold clusters that always have remarkable stabilities and tunable electronic structures. The solvation free energy of the M@Au12 clusters (M = Cr, Mo, W, Co, Rh, Ir, Cu, Ag, and Au) was found to be related to the charge distribution of the central doped-atom M and the outer Au12 cage. Moreover, the aqueous solvent was observed to be able to increase the adsorption capacity of M@Au12 to O2 following the activation of O2 through the charge transfer from M@Au12 to O2, in which the transferred electrons occupy the π antibonding orbital of O2. In addition, the water solvent can also improve the hydrogenation reaction of O2 to form OOH over M@Au12, where the activation energy barrier for this process is very low with the participation of the solvent. Considering the importance of solvents in the liquid-phase synthesis of atomically precise clusters, these findings highlighted here could provide valuable theoretical guidance in potential applications of functional gold nanoclusters, especially in the liquid-phase cluster catalysis.

RhRu Bimetallic Oxide Cluster Catalysts for Cross-Dehydrogenative Coupling of Arenes and Carboxylic Acids

Noble-metal-based bimetallic oxide clusters are promising novel catalysts. In this study, we developed carbon-supported RhRu bimetallic oxide clusters (RhRuOx/C) with a mean diameter of 1.2 nm, which showed remarkable catalytic activity for the cross-dehydrogenative coupling (CDC) of arenes and carboxylic acids with O2 as the sole oxidant. RhRu bimetallic oxide cluster formation was confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and synchrotron X-ray absorption spectroscopy. Kinetic isotope and substituent effects indicated that arene C-H bond cleavage was the rate-determining step and proceeded via electrophilic concerted metalation-deprotonation mechanism, with a carboxylate as an internal base. Density functional theory calculations supported the proposed mechanism and indicated that the active center for C-H bond activation was Rh(V) rather than Rh(III), while Ru enhanced the electrophilicity of the Rh(V) site by decreasing the negative charge of the surrounding oxygen atoms. Electron-rich arenes showed relatively high reactivity for the RhRuOx/C-catalyzed CDC reaction, and both aliphatic and aromatic carboxylic acids were applicable to the reaction. The RhRuOx/C catalyst is promising for the CDC reaction of arenes and carboxylic acids to produce aryl esters. This work promotes the development of noble-metal-based bimetallic oxide clusters for C-H bond activation reactions.

Circular dichroism and circularly polarized luminescence of ligand-protected molecular metal clusters: insights into structure-chiroptical property relationships

Molecular noble metal clusters are an emerging class of circularly polarized luminescent (CPL) nanomaterials. Many of the ligand-protected metal clusters exhibit discrete electronic absorption bands, which are assigned to their structural components such as metal core, ligands and metal-ligand interfaces. This implies the suitability of the chiroptical spectroscopic approach to unravel the structure-chiroptical property relationships in molecular metal clusters. Due to the tremendous developments in computational methods for investigating chiroptical properties, along with circular dichroism (CD) and CPL spectroscopy, understanding of the structure-chiroptical properties of these clusters is rapidly progressing. This review discusses various strategies such as the use of chiral ligands, metal atom substitution, ligand exchange, co-crystallization with chiral ligands, etc., for inducing and enhancing the CPL of such metal clusters. This review demonstrates the potential of combined CD-CPL spectroscopic investigations and theoretical calculations to unravel the origins of photoluminescence and CPL activity of chiral metal clusters.

Synthesis of N2-Type Superatomic Molecules

Exploration of multiple bonds between superatoms remains an uncharted territory. In this study, we present the synthesis and characterization of N2-type superatomic molecules featuring triple bonds between two superatoms. The successful synthesis of M2Au17 (M = Pd, Pt) nanoclusters hinged upon the photoinduced fusion of MAu12 superatoms, achieved through sequential electron transfer and detachment of [AuPR3]+ species. Solid-state structures were confirmed via X-ray crystallography, while their electronic structures were elucidated through density functional theory (DFT) calculations. Analysis of electronic absorption properties, coupled with time-dependent DFT calculations, unveiled a symmetry-dependent electron transition nature between superatomic molecular orbitals, akin to that observed in conventional molecules.

NIR-II emissive anionic copper nanoclusters with intrinsic photoredox activity in single-electron transfer

Ultrasmall copper nanoclusters have recently emerged as promising photocatalysts for organic synthesis, owing to their exceptional light absorption ability and large surface areas for efficient interactions with substrates. Despite significant advances in cluster-based visible-light photocatalysis, the types of organic transformations that copper nanoclusters can catalyze remain limited to date. Herein, we report a structurally well-defined anionic Cu40 nanocluster that emits in the second near-infrared region (NIR-II, 1000-1700 nm) after photoexcitation and can conduct single-electron transfer with fluoroalkyl iodides without the need for external ligand activation. This photoredox-active copper nanocluster efficiently catalyzes the three-component radical couplings of alkenes, fluoroalkyl iodides, and trimethylsilyl cyanide under blue-LED irradiation at room temperature. A variety of fluorine-containing electrophiles and a cyanide nucleophile can be added onto an array of alkenes, including styrenes and aliphatic olefins. Our current work demonstrates the viability of using readily accessible metal nanoclusters to establish photocatalytic systems with a high degree of practicality and reaction complexity.

Photoluminescent Characterization of Metal Nanoclusters: Basic Parameters, Methods, and Applications

Metal nanoclusters (MNCs) can be synthesized with atomically precise structures and molecule formulae due to the rapid development of nanocluster science in recent decades. The ultrasmall size range (normally < 2 nm) endows MNCs with plenty of molecular-like properties, among which photoluminescent properties have aroused extensive attention. Tracing the research and development processes of luminescent nanoclusters, various photoluminescent analysis and characterization methods play a significant role in elucidating luminescent mechanism and analyzing luminescent properties. In this review, it is aimed to systematically summarize the normally used photoluminescent characterizations in MNCs including basic parameters and methods, such as excitation/emission wavelength, quantum yield, and lifetime. For each key parameter, first its definition and meaning is introduced and then the relevant characterization methods including measuring principles and the revelation of luminescent properties from the collected data are discussed. Then, it is discussed in details how to explore the luminescent mechanism of MNCs and construct NC-based applications based on the measured data. By means of these characterization strategies, the luminescent properties of MNCs and NC-based designs can be explained quantitatively and qualitatively. Hence, this review is expected to provide clear guidance for researchers to characterize luminescent MNCs and better understand the luminescent mechanism from the measured results.

Au36(SR)22 Nanocluster and a Periodic Pattern from Six to Fourteen Free Electrons in Core Size Evolution

An Au36(S-tBu)22 nanocluster (NC) is synthesized using the bulky tert-butyl thiol as the ligand. Single-crystal X-ray crystallography reveals that it has an Au25 core which evolves from the Au22 core in the previously reported Au30(S-tBu)18, and the Au25 core is protected by longer staple-like surface motifs. The new Au36 NC extends the members of the face-centered cubic structural evolution by adding an Au3 triangle and an Au4 tetrahedron unit. Additionally, it is found that Au36 emits near-infrared photoluminescence at 863 nm with a quantum yield (QY) of 4.3%, which is five times larger than that of Au30(S-tBu)18-the closest neighbor in the structural evolution pattern. The higher QY of Au36 is attributed to a larger radiative relaxation (kr), resulting from the structural effect. Finally, we find that the longer staple motifs lead to enhanced stability of Au36(S-tBu)22 relative to Au30(S-tBu)18.

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

Diphosphine-Protected IrAu12 Superatom with Open Site(s): Synthesis and Programmed Stepwise Assembly

One or two phenylacetylide (PA) ligand(s) were successfully removed from the IrAu12 superatomic core of [IrAu12(dppe)5(PA)2]+ (dppe=1,2-bis(diphenylphosphino)ethane) by reaction with controlled amounts of tetrafluoroboric acid. Optical and nuclear magnetic resonance spectroscopies and density functional theory calculations revealed the formation of open Au site(s) on the IrAu12 core of [IrAu12(dppe)5(PA)1]2+ and [IrAu12(dppe)5]3+ with the remaining structure intact. Isocyanide was efficiently trapped at the open electrophilic site on [IrAu12(dppe)5(PA)1]2+, whereas a dimer or trimer of the IrAu12 superatoms was formed using diisocyanide as a linker. These results open the door to designed assembly of chemically modified metal superatoms.

Bright near-infrared emission from the Au39(SR)29 nanocluster

The synthesis of atomically precise gold nanoclusters with high photoluminescence quantum yield (PLQY) in the near-infrared (NIR) region and understanding their photoluminescence mechanism are crucial for both fundamental science and practical applications. Herein, we report a highly luminescent, molecularly pure Au39(PET)29 (PET = 2-phenylethanethiolate) nanocluster with PLQY of 19% in the NIR range (915 nm). Steady state and time-resolved PL analyses, as well as temperature-dependent PL measurements reveal the emission nature of Au39(PET)29, which consists of prompt fluorescence (weak), thermally activated delayed fluorescence (TADF), and phosphorescence (predominant). Furthermore, strong dipole-dipole interaction in the solid-state (e.g., Au39(PET)29 nanoclusters embedded in a polystyrene thin-film) is found to narrow the energy gap between the S1 and T1 states, which results in faster intersystem crossing and reverse intersystem crossing; thus, the ratio of TADF to phosphorescence varies and the total PLQY is increased to 32%. This highly luminescent nanocluster holds promise in imaging, sensing and optoelectronic applications.

Unraveling a Concerted Proton-Coupled Electron Transfer Pathway in Atomically Precise Gold Nanoclusters

Metal nanoclusters (MNCs) represent a promising class of materials for catalytic carbon dioxide and proton reduction as well as dihydrogen oxidation. In such reactions, multiple proton-coupled electron transfer (PCET) processes are typically involved, and the current understanding of PCET mechanisms in MNCs has primarily focused on the sequential transfer mode. However, a concerted transfer pathway, i.e., concerted electron-proton transfer (CEPT), despite its potential for a higher catalytic rate and lower reaction barrier, still lacks comprehensive elucidation. Herein, we introduce an experimental paradigm to test the feasibility of the CEPT process in MNCs, by employing Au18(SR)14 (SR denotes thiolate ligand), Au22(SR)18, and Au25(SR)18- as model clusters. Detailed investigations indicate that the photoinduced PCET reactions in the designed system proceed via an CEPT pathway. Furthermore, the rate constants of gold nanoclusters (AuNCs) have been found to be correlated with both the size of the cluster and the flexibility of the Au-S framework. This newly identified PCET behavior in AuNCs is prominently different from that observed in semiconductor quantum dots and plasmonic metal nanoparticles. Our findings are of crucial importance for unveiling the catalytic mechanisms of quantum-confined metal nanomaterials and for the future rational design of more efficient catalysts.

Atomically precise Au and Ag nanoclusters doped with a single atom as model alloy catalysts

Gold and silver nanoclusters (NCs) composed of <200 atoms are novel catalysts because their catalytic properties differ significantly from those of the corresponding bulk surface and can be dramatically tuned by the size (number of atoms). Doping with other metals is a promising approach for improving the catalytic performance of Au and Ag NCs. However, elucidation of the origin of the doping effects and optimization of the catalytic performance are hampered by the technical challenge of controlling the number and location of the dopants. In this regard, atomically precise Au or Ag (Au/Ag) NCs protected by ligands or polymers have recently emerged as an ideal platform because they allow regioselective substitution of single Au/Ag constituent atoms while retaining the size and morphology of the NC. Heterogeneous Au/Ag NC catalysts doped with a single atom can also be prepared by controlled calcination of ligand-protected NCs on solid supports. Comparison of thermal catalysis, electrocatalysis, and photocatalysis between the single-atom-doped and undoped Au/Ag NCs has revealed that the single-atom doping effect can be attributed to an electronic or geometric origin, depending on the dopant element and position. This minireview summarizes the recent progress of the synthesis and catalytic application of single-atom-doped, atomically precise Au/Ag NC catalysts and provides future prospects for the rational development of active and selective metal NC catalysts.

Synthesis of RhH-doped Au-Ag alloy nanoclusters and dopant evolution

Doping atomically precise metal nanoclusters (NCs) with heterometals is a powerful method for tuning the physicochemical properties of the original NCs at the atomic level. While the heterometals incorporated into metal NCs are limited to group 10-12 metals with closed d-shells, the doping of open d-shell metals remains largely unexplored. Herein, we report the synthesis of Rh-doped Au-Ag alloy NCs by a metal-exchange reaction of [RhHAg24(SPhMe2)18]2- NCs with an Au-thiolate complex. Combined experimental and theoretical structural studies revealed that the synthesized product is a dianionic [RhHAuxAg24-x(SPhMe2)18]2- NC (x = 8-12), consisting of RhH dopant, Au-rich kernel, and Ag-thiolate staple motifs, with the superatomic 8-electron configuration (1S21P6). Under aerobic conditions, the synthesized NCs underwent kernel evolution to generate a 6-electron [RhAuxAg24-x(SPhMe2)18]1- NC (1S21P4), which was initiated by the desorption of hydride from the kernel. Structural analysis of the [RhHAuxAg24-x(SPhMe2)18]2- NC suggests that the kernel evolution is induced by the change in chemical bonds surrounding the hydride in the Au-rich kernel.

Triplet properties and intersystem crossing mechanism of PtAg28 nanocluster sensitizers achieving low threshold and efficient photon upconversion

Ligand-protected metal nanoclusters have emerged as a promising platform for providing sensitizers for triplet-triplet annihilation upconversion (TTA-UC). Herein, we report [PtAg28(BDT)12]4- (PtAg28; BDT = 1,3-benzenedithiolate) as a sensitizer enabling TTA-UC at low excitation intensities. PtAg28 exhibits a long-lived triplet state (approximately 7 μs) generated with a 100% intersystem crossing (ISC) quantum yield. The mechanism driving this efficient ISC was unveiled with the aid of theoretical calculations. Specifically, the S1-T1 ISC reveals a small spin-orbit coupling (SOC) matrix element, attributed to their similar electron configuration. In contrast, the T2 state, which is energetically close to S1, features a hole distribution derived from the Py superatomic orbital of the icosahedral Pt@Ag12 core. This distribution enables direct SOC based on the orbital angular momentum change from the S1 state with a Pz-derived hole distribution. Consequently, the efficient ISC was rationalized by the S1 → T2 → T1 pathway. The T1 state possesses a metal core-to-surface metal charge transfer character, facilitating triplet energy transfer and conferring superior sensitization ability. Leveraging these characteristics, the combination of PtAg28 sensitizer with a 9,10-diphenylanthracene annihilator/emitter attained an extremely low UC threshold of 0.81 mW cm-2 at 532 nm excitation, along with efficient green-to-blue TTA-UC with an internal quantum yield (ΦUCg) of 12.2% (50% maximum). This results in a pseudo-first-order TTA process with strong UC emission under 1-sun conditions.

Revisiting ultrasmall phosphine-stabilized rhodium-doped gold clusters AunRh (n = 5, 6, 7, 8): geometric, electronic, and vibrational properties

Incorporation of other transition metals in Au nanoclusters has been thriving recently due to its effect on their electronic and photophysical properties. Here, the ultrasmall phosphine-stabilized Rh-doped gold clusters AunRh (n = 5, 6, 7, 8), with metal core structures represented as fragments of a rhodium-centered icosahedron, are considered. The geometric and electronic properties of these nanoclusters are revisited and analyzed using density functional theory (DFT). Moreover, infrared spectra are simulated to identify the effects of Rh doping on the clusters through vibrational properties. Peaks are assigned to breathing-like normal modes for all AuRh clusters except for Au8Rh, likely due to the presence of bound Cl ligands. Unlike their pure gold core counterparts, the % motions of both Au and Rh atoms are lower in the mixed metal clusters, suggesting more restrained metal cores by rhodium, which could result in other novel physical and chemical properties not hitherto discovered.

Visible to NIR-II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near-infrared-I and -II wavelengths (NIR-I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure-PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near-infrared wavelengths, especially in the NIR-II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double-digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.

Dual Catalysis of Gold Nanoclusters: Photocatalytic Cross-Dehydrogenative Coupling by Cooperation of Superatomic Core and Molecularly Modified Staples

Thiolate-protected gold nanoclusters (AuNCs) have attracted significant attention as nano-catalysts, revealing a superatomic core and gold-thiolate staples as distinct structural units. Here, we demonstrate the unprecedented dual catalytic activity of thiolate-protected [Au25 (SR)18 ]- nanoclusters, involving both photosensitized 1 O2 generation by the Au13 superatomic core and catalytic carbon-carbon bond formation facilitated by Au2 (SR)3 staples. This synergistic combination of two different catalytic units enables efficient cross-dehydrogenative coupling of terminal alkynes and tertiary aliphatic amines to afford propargylamines in high yields of up to 93 %. Mixed-ligand AuNCs bearing both thiolate and alkynyl ligands revealed the intermediacy of the alkynyl-exchanged AuNCs toward both photosensitization and C-C bond-forming catalytic cycles. Density functional theory calculations also supported the intermediacy of the alkynyl-exchanged AuNCs. Thus, the use of ligand-protected metal nanoclusters has enabled the development of an exceptional multifunctional catalyst, wherein distinct nanocluster components facilitate cooperative photo- and chemo-catalysis.

Effect of Specific Heavy Doping of Silver Atoms into the Icosahedral Au13 on Electronic Structure and Catalytic Performance

The exploration of specific heavy doping of silver atoms into icosahedral Au13 clusters and their electronic structures and properties has been somewhat limited. Herein, we report two heavily Ag doped nanoclusters, [Au7Ag6(C7H4NOS)4(Dppf)3Cl]0 and [Au7Ag6(C7H4NOS)3(Dppf)3Cl](SbF6) (Au7Ag6-0 and Au7Ag6-1, respectively) [C7H4NOSH = 2-mercaptobenzoxazole, and Dppf = 1,1'-bis(diphenylphosphino)ferrocene]. The electronic structures and superatomic orbitals of nanoclusters were determined by density functional theory (DFT) calculations, and the energy degeneracy of the superatomic orbitals of Au7Ag6-1 is higher than that of Au7Ag6-0. Transient absorption spectroscopy was performed, revealing that Au7Ag6-0 significantly extends the excited-state lifetime. Both nanoclusters were supported on activated carbon for the oxygen reduction reaction. DFT calculations confirm that the catalytic activities mainly stem from the carbon atom of ferrocene rather than the iron atom. This study not only sheds light on the preparation of icosahedral alloy clusters but also provides insights into the regulation of icosahedral superatomic structure and electrocatalytic properties.

Constructing perfect cubic Ag-Cu alloyed nanoclusters through selective elimination of phosphine ligands

The aspiration of chemists has always been to design and achieve control over nanoparticle morphology at the atomic level. Here, we report a synthesis strategy and crystal structure of a perfect cubic Ag-Cu alloyed nanocluster, [Ag55Cu8I12(S-C6H32,4(CH3)2)24][(PPh4)] (Ag55Cu8I12 for short). The structure of this cluster was determined by single-crystal X-ray diffraction (SCXRD) and further validated by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), Energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and 1H and 31P nuclear magnetic resonance (NMR). The surface deviation of the cube was measured to be 0.291 Å, making it the flattest known cube to date.

Tailoring Carbon Tails of Ligands on Au52(SR)32 Nanoclusters Enhances the Near-Infrared Photoluminescence Quantum Yield from 3.8 to 18.3

One of the important factors that determine the photoluminescence (PL) properties of gold nanoclusters pertain to the surface. In this study, four Au52(SR)32 nanoclusters that feature a series of aromatic thiolate ligands (-SR) with different bulkiness at the para-position are synthesized and investigated. The near-infrared (NIR) photoluminescence (peaks at 900-940 nm) quantum yield (QY) is largely enhanced with a decrease in the ligand's para-bulkiness. Specifically, the Au52(SR)32 capped with the least bulky p-methylbenzenethiolate (p-MBT) exhibits the highest PLQY (18.3% at room temperature in non-degassed dichloromethane), while Au52 with the bulkiest tert-butylbenzenethiolate (TBBT) only gives 3.8%. The large enhancement of QY with fewer methyl groups on the ligands implies a nonradiative decay via the multiphonon process mediated by C-H bonds. Furthermore, single-crystal X-ray diffraction (SCXRD) comparison of Au52(p-MBT)32 and Au52(TBBT)32 reveals that fewer methyl groups at the para-position lead to a stronger interligand π···π stacking on the Au52 core, thus restricting ligand vibrations and rotations. The emission nature is identified to be phosphorescence and thermally activated delayed fluorescence (TADF) based on the PL lifetime, 3O2 quenching, and temperature-dependent PL and absorption studies. The 1O2 generation efficiencies for the four Au52(SR)32 NCs follow the same trend as the observed PL performance. Overall, the highly NIR-luminescent Au52(p-MBT)32 nanocluster and the revealed mechanisms are expected to find future applications.