Tri-icosahedral Gold Nanocluster [Au37(PPh3)10(SC2H4Ph)10X2](+): Linear Assembly of Icosahedral Building Blocks

[Au9Ag6(CCR)10(DPPM)2Cl2](PPh4): a four-electron cluster with a bi-decahedral twisted metal core

The assembly of cluster units in a distinct manner can give rise to nanoclusters exhibiting unique geometrical structures and properties. Herein, we present a one-pot synthesis and structural characterization of a AuAg alloy cluster, [Au9Ag6(CCR)10(DPPM)2Cl2](PPh4), denoted as Au9Ag6 (where HCCR is 3,5-bis(trifluoromethyl)phenylacetylene, and DPPM is bis(diphenylphosphino)methane). Single-crystal X-ray diffraction data analysis reveals that Au9Ag6 features a distinctive Au7Ag6 bi-decahedral core, formed by a twisted assembly of two Au4Ag3 decahedra sharing one vertex. The Au4Ag3 building blocks are bridged by two gold atoms on opposite sides of the bi-decahedral core. The Au9Ag6 cluster is monoanionic and it is stabilized by two chloride, two DPPM and ten alkynyl ligands. This cluster represents the first instance of a cluster of clusters built upon decahedral units.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Diachronic evolvement from tetra-icosahedral to quasi-hexagonal close-packed bimetal clusters

Here we report a diachronic evolvement from tetra-icosahedral Au30Ag12(C[triple bond, length as m-dash]CR)24 to quasi-hcp (hexagonal close-packed) Au47Ag19(C[triple bond, length as m-dash]CR)32 via a one-step reduction, in which the size/structure conversion of the two clusters is not a typical Oswald growth process, but involves interface shrinking followed by core rearrangement and surface polymerization. Au30Ag12(C[triple bond, length as m-dash]CR)24 has an aesthetic Au18Ag8 kernel that is composed of four interpenetrating Au10Ag3 icosahedra, while Au47Ag19(C[triple bond, length as m-dash]CR)32 has a twisted Au19 core capped by a Au12Ag19 shell that are stacked in a layer-by-layer manner with a quasi-hcp pattern. The discovery of the two clusters not only provides further evidence for icosahedral clusters with longer excited-state lifetime compared to hcp-like clusters, but also discloses a double increase in catalytic reactivity for electrocatalytic oxidation of ethanol over quasi-hcp clusters in comparison with icosahedral clusters. This work provides the rationale for reversing the bottom-up growth process to remake bimetal clusters.

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.

Concomitant Near-Infrared Photothermy and Photoluminescence of Rod-Shaped Au52(PET)32 and Au66(PET)38 Synthesized Concurrently

Gold nanoclusters exhibiting concomitant photothermy (PT) and photoluminescence (PL) under near-infrared (NIR) light irradiation are rarely reported, and some fundamental issues remain unresolved for such materials. Herein, we concurrently synthesized two novel rod-shaped Au nanoclusters, Au52(PET)32 and Au66(PET)38 (PET = 2-phenylethanethiolate), and precisely revealed that their kernels were 4 × 4 × 6 and 5 × 4 × 6 face-centered cubic (fcc) structures, respectively, based on the numbers of Au layers in the [100], [010], and [001] directions. Following the structural growth mode from Au52(PET)32 to Au66(PET)38, we predicted six more novel nanoclusters. The concurrent synthesis provides rational comparison of the two nanoclusters on the stability, absorption, emission and photothermy, and reveals the aspect ratio-related properties. An interesting finding is that the two nanoclusters exhibit concomitant PT and PL under 785 nm light irradiation, and the PT and PL are in balance, which was explained by the qualitative evaluation of the radiative and non-radiative rates. The ligand effects on PT and PL were also investigated.

A Versatile Strategy for the Controlled Synthesis of Atomically Precise Palladium Nanoclusters

The study of the structures, applications, and structure-property relationships of atomically precise metal nanoclusters relies heavily on their controlled synthesis. Although great progress has been made in the controlled synthesis of Group 11 (Cu, Ag, Au) metal nanoclusters, the preparation of Pd nanoclusters remains a grand challenge. Herein, a new, simple, and versatile synthetic strategy for the controlled synthesis of Pd nanoclusters is reported with tailorable structures and functions. The synthesis strategy involves the controllable transformations of Pd4(CO)4(CH3COO)4 in air, allowing the discovery of a family of Pd nanoclusters with well-defined structure and high yield. For example, by treating the Pd4(CO)4(CH3COO)4 with 2,2-dipyridine ligands, two clusters of Pd4 and Pd10 whose metal framework describes the growth of vertex-sharing tetrahedra have been selectively isolated. Interestingly, chiral Pd4 nanoclusters can be gained by virtue of customized chiral pyridine-imine ligands, thus representing a pioneering example to shed light on the hierarchical chiral nanostructures of Pd. This synthetic methodology also tolerates a wide variety of ligands and affords phosphine-ligated Pd nanoclusters in a simple way. It is believed that the successful exploration of the synthetic strategy would simulate the research enthusiasm on both the synthesis and application of atomically precise Pd nanoclusters.

Staggered Assembly of a Dimeric Au13 Cluster: Impacts on Coupling of Geometric Isomerism

The specific states of aggregation of metal atoms in sub-nanometer-sized gold clusters are related to the different quantum confinement volumes of electrons, leading to novel optical and electronic properties. These volumes can be tuned by changing the relative positions of the gold atoms to generate isomers. Studying the isomeric gold core and the electron coupling between the basic units is fundamentally important for nanoelectronic devices and luminescence; however, appropriate cases are lacking. In this study, the structure of the first staggered di-superatomic Au25 -S was solved using single-crystal X-ray diffraction. The optical properties of Au25 -S were studied by comparing with eclipsed Au25 -E. From Au25 -E to Au25 -S, changes in the electronic structures occurred, resulting in significantly different optical absorptions originating from the coupling between the two Au13 modules. Au25 -S shows a longer electron decay lifetime of 307.7 ps before populating the lowest triplet emissive state, compared to 1.29 ps for Au25 -E. The experimental and theoretical results show that variations in the geometric isomerism lead to distinct photophysical processes owing to isomerism-dependent electronic coupling. This study offers new insights into the connection between the geometric isomerism of nanosized building blocks and the optical properties of their assemblies, opening new possibilities for constructing function-specific nanomaterials.

Three-atom-wide gold quantum rods with periodic elongation and strongly polarized excitons

Atomically precise control over anisotropic nanoclusters constitutes a grand challenge in nanoscience. In this work, we report our success in achieving a periodic series of atomically precise gold quantum rods (abbrev. Au QRs) with unusual excitonic properties. These QRs possess hexagonal close-packed kernels with a constant three-atom diameter but increasing aspect ratios (ARs) from 6.3 to 18.7, all being protected by the same thiolate (SR) ligand. The kernels of the QRs are in a Au1-(Au3)n-Au1 configuration (where n is the number of Au3 layers) and follow a periodic elongation with a uniform Au18(SR)12 increment consisting of four Au3 layers. These Au QRs possess distinct HOMO-LUMO gaps (Eg = 0.6 to 1.3 eV) and exhibit strongly polarized excitonic transition along the longitudinal direction, resulting in very intense absorption in the near-infrared (800 to 1,700 nm). While excitons in gapped systems and plasmons in gapless systems are distinctly different types of excitations, the strongly polarized excitons in Au QRs surprisingly exhibit plasmon-like behaviors manifested in the shape-induced polarization, very intense absorption (~106 M-1 cm-1), and linear scaling relations with the AR, all of which resemble the behaviors of conventional metallic-state Au nanorods (i.e., gapless systems), but the QRs possess distinct gaps and very long excited-state lifetimes (10 to 2,122 ns), which hold promise in applications such as near-infrared solar energy utilization, hot carrier generation and transfer. The observation of plasmon-like behaviors from single-electron transitions in Au QRs elegantly bridges the distinct realms of single-electron and collective-electron excitations and may stimulate more research on excitonics and plasmonics.

Building Block Metal Nanocluster-Based Growth in 1D Direction

Metal nanoclusters with precisely modulated structures at the nanoscale give us the opportunity to synthesize and investigate 1D nanomaterials at the atomic level. Herein, it realizes selective 1D growth of building block nanocluster "Au13 Cd2 " into three structurally different nanoclusters: "hand-in-hand" (Au13 Cd2 )2 O, "head-to-head" Au25 , and "shoulder-to-shoulder" Au33 . Detailed studies further reveals the growth mechanism and the growth-related tunable properties. This work provides new hints for the predictable structural transformation of nanoclusters and atomically precise construction of 1D nanomaterials.

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.

Second-order superatoms: Au52-PAP featuring a three-dimensional cluster-of-clusters core

The recent characterization of Au52-PAP cluster can be viewed as a three-dimensional arrangement featuring four Au13 motifs. As a result, a new set of superatomic orbitals are built up from the superatomic shell of each constituent unit, denoted by 1S'21P'62S'21D'102P'61F'6 and, thus, referred to as a second-order superatomic shell structure. This favors the rationalization of larger species toward the formation of cluster-assembled materials of different sizes.

Towards Green Synthesis of Fluorescent Metal Nanoclusters

In the modern development of nanoscience and nanotechnology, metal nanoclusters have emerged as a foremost category of nanomaterials exhibiting remarkable biocompatibility and photo-stability having dramatically distinctive optical, electronic, and chemical properties. This review focuses on synthesizing fluorescent metal nanoclusters in a greener way to make them suitable for biological imaging and drug delivery application. The green methodology is the desired route for sustainable chemical production and should be utilized for any form of chemical synthesis including nanomaterials. It aims to eliminate harmful waste, uses non-toxic solvents, and employs energy-efficient processes for the synthesis. This article provides an overview of conventional synthesis methods, including stabilizing nanoclusters by small organic molecules in organic solvents. Then we focus on the improvement of properties, applications of green synthesized metal nanoclusters, challenges involved, and further advancement required in the direction of green synthesis of MNCs. There are plenty of problems for scientists to solve to make nanoclusters suitable for bio-applications, chemical sensing, and catalysis synthesized by green methods. Using bio-compatible and electron-rich ligands, understanding ligand-metal interfacial interactions, employing more energy-efficient processes, and utilizing bio-inspired templates for synthesis are some immediate problems worth solving in this field that requires continued efforts and interdisciplinary knowledge and collaboration.

Elucidation of the electronic structures of thiolate-protected gold nanoclusters by electrochemical measurements

Metal nanoclusters (NCs) with sizes of approximately 2 nm or less have different physical/chemical properties from those of the bulk metals owing to quantum size effects. Metal NCs, which can be size-controlled and heterometal doped at atomic accuracy, are expected to be the next generation of important materials, and new metal NCs are reported regularly. However, compared with conventional materials such as metal complexes and relatively large metal nanoparticles (>2 nm), these metal NCs are still underdeveloped in terms of evaluation and establishment of application methods. Electrochemical measurements are one of the most widely used methods for synthesis, application, and characterisation of metal NCs. This review summarizes the basic knowledge of the electrochemistry and experimental techniques, and provides examples of the reported electronic states of thiolate-protected gold NCs elucidated by electrochemical approaches. It is expected that this review will provide useful information for researchers starting to study metal NCs.

Motif-to-Core Nucleation in a Decahedral Evolution Pattern

The atomic precision of ultrasmall metal nanoclusters has opened the door to elucidating the structural evolution principles of metal nanomaterials at the molecular level. Here, we report a novel set of super-atomic Ag clusters, including [Ag19(TBBT)16(DPPP)4]+ (Ag19), [Ag22(DMAT)8(DPPM)4Cl8]2+ (Ag22), Ag26(SPh3,5-CF3)15(DPPF)4Cl5 (Ag26), and [Ag30(DMAT)12(DPPP)4Cl8]2+ (Ag30). The core structures of these clusters correspond to one decahedral Ag7, perpendicular bi-decahedrons, three-dimensional penta-decahedrons, and hexa-decahedrons, respectively. The Ag atoms in AgS2 blocks show a strong correlation with the decahedral cores: the five equatorial Ag atoms in the decahedral Ag7 core of Ag19 all adopt the AgS2 coordination, while the Ag atoms in AgS2 blocks of Ag22, Ag26, and Ag30 unexceptionally constitute additional decahedral structures with the core Ag atoms. Specifically, two and four core Ag atoms of Ag26 and Ag30 clusters occupy positions that highly resemble that of Ag (in AgS2 motifs) of Ag22. The strong structural correlation demonstrates the motif-to-core evolution of the surface Ag (on AgS2) to build extra-decahedral blocks. Density functional theory calculations indicate that the 2e, 4e, 6e, and 8e clusters (from Ag19 to Ag30) adopt 1S2, 1S21P2, 1S21P4, and 1S21P6 electron configurations, all of which feature excellent super-atomic characters.

Influence of ligands on the optical properties of rod-shaped Au25 nanoclusters

In this study, we successfully synthesized rod-shaped [Au25(PPh3)10(S-Adm)5Cl2]2+ nanoclusters using kinetic controls. The complete molecular structure was determined by single-crystal X-ray crystallography and electrospray ionization mass spectrometry. In comparison with the previously reported [Au25(PPh3)10(PET)5Cl2]2+ clusters, both nanoclusters have an icosahedral composition of Au13 linked by Au atoms that share a vertex, but [Au25(PPh3)10(S-Adm)5Cl2]2+ clusters appear elongated due to the rigidity of adamantane. We conducted ultraviolet-visible spectrophotometry (UV-vis) measurements of [Au25(PPh3)10(PET)5Cl2]2+ and [Au25(PPh3)10(S-Adm)5Cl2]2+ in dichloromethane solvent to elucidate the modulation of the cluster properties of different ligands. The lowest energy absorption peak of [Au25(PPh3)10(S-Adm)5Cl2]2+ shifted to lower energies compared to the [Au25(PPh3)10(PET)5Cl2]2+ clusters in UV-vis measurements. Temperature-dependent absorption measurements revealed that [Au25(PPh3)10(S-Adm)5Cl2]2+ clusters were less affected by temperature compared to [Au25(PPh3)10(PET)5Cl2]2+. This result is attributed to the exciton phonon coupling of [Au25(PPh3)10(S-Adm)5Cl2]2+ clusters being weaker than [Au25(PPh3)10(PET)5Cl2]2+ clusters. Furthermore, the absorption spectra of [Au25(PPh3)10(PET)5Cl2]2+ and [Au25(PPh3)10(S-Adm)5Cl2]2+ clusters were measured using different types of solutions, and it was found that the lowest energy absorption peaks of [Au25(PPh3)10(S-Adm)5Cl2]2+ were shifted and affected by the solution at room temperature, which suggested that the [Au25(PPh3)10(S-Adm)5Cl2]2+ clusters with solution hydrogen bonds also interacted strongly at room temperature. Theoretical calculations show that changes in ligands affect the differences in the molecular orbitals and structures of the clusters, which cause changes in the optical properties.

Monoarsine-protected icosahedral cluster [Au13(AsPh3)8Cl4]+: comparative studies on ligand effect and surface reactivity with its stibine analogue

Ligand shells of gold nanoclusters play important roles in regulating their molecular and electronic structures. However, the similar but distinct impacts of the homologous analogues of the protecting ligands remain elusive. The C_2v symmetric monoarsine-protected cluster [Au₁₃(AsPh₃)₈Cl₄]⁺ (Au₁₃As₈) was facilely prepared by direct reduction of (Ph₃As)AuCl with NaBH₄. This cluster is isostructural with its previously reported stibine analogue [Au₁₃(SbPh₃)₈Cl₄]⁺ (Au₁₃Sb₈), enabling a comparative study between them. Au₁₃As₈ exhibits a blue-shifted electronic absorption band, and this is probably related to the stronger π-back donation interactions between the Au₁₃ core and AsPh₃ ligands, which destabilize its superatomic 1P and 1D orbitals. In comparison to the thermodynamically less stable Au₁₃Sb₈, Au₁₃As₈ achieves a better trade-off between catalytic stability and activity, as demonstrated by its excellent catalytic performance towards the aldehyde-alkyne-amine (A³) coupling reaction. Moreover, the ligand exchange reactions between Au₁₃As₈ with phosphines, as exemplified by PPh₃ and Ph₂P(CH₂)₂PPh₂, suggest that Au₁₃As₈ may be a good precursor cluster for further cluster preparation through the "cluster-to-cluster" route.

Anion-templated silver nanoclusters: precise synthesis and geometric structure

Metal nanoclusters (NCs) are gaining much attention in nanoscale materials research because they exhibit size-specific physicochemical properties that are not observed in the corresponding bulk metals. Among them, silver (Ag) NCs can be precisely synthesized not only as pure Ag NCs but also as anion-templated Ag NCs. For anion-templated Ag NCs, we can expect the following capabilities: 1) size and shape control by regulating the central anion (anion template); 2) stabilization by adjusting the charge interaction between the central anion and surrounding Ag atoms; and 3) functionalization by selecting the type of central anion. In this review, we summarize the synthesis methods and influences of the central anion on the geometric structure of anion-templated Ag NCs, which include halide ions, chalcogenide ions, oxoanions, polyoxometalate, or hydride/deuteride as the central anion. This summary provides a reference for the current state of anion-templated Ag NCs, which may promote the development of anion-templated Ag NCs with novel geometric structures and physicochemical properties.

Unique Electron-Transfer-Mediated Electrochemiluminescence of AuPt Bimetallic Nanoclusters and the Application in Cancer Immunoassay

Noble Metal nanoclusters (NCs) are promising electrochemiluminescence (ECL) emitters due to their amazing optical properties and excellent biocompatibility. They have been widely used in the detection of ions, pollutant molecules, biomolecules, etc. Herein, we found that glutathione-capped AuPt bimetallic NCs (GSH-AuPt NCs) emitted strong anodic ECL signals with triethylamine as co-reactants which had no fluorescence (FL) response. Due to the synergistic effect of bimetallic structures, the ECL signals of AuPt NCs were 6.8 and 94 times higher than those of monometallic Au and Pt NCs, respectively. The electric and optical properties of GSH-AuPt NCs differed from those of Au and Pt NCs completely. An electron-transfer mediated ECL mechanism was proposed. The excited electrons may be neutralized by Pt(II) in GSH-Pt and GSH-AuPt NCs, resulting in the vanished FL. Furthermore, abundant TEA radicals formed on the anode contributed electrons to the highest unoccupied molecular orbital of GSH-Au_2.5Pt NCs and Pt(II), booming intense ECL signals. Because of the ligand effect and ensemble effect, bimetallic AuPt NCs exhibited much stronger ECL than GSH-Au NCs. A sandwich-type immunoassay for alpha fetoprotein (AFP) cancer biomarkers was fabricated with GSH-AuPt NCs as signal tags, which displayed a wide linear range from 0.01 to 1000 ng·mL^-1 and a limit of detection (LOD) down to 1.0 pg·mL^-1 at 3S/N. Compared to previous ECL AFP immunoassays, this method not only had a wider linear range but also a lower LOD. The recoveries of AFP in human serum were around 108%, providing a wonderful strategy for fast, sensitive, and accurate cancer diagnosis.

Recent progress in dichalcophosphate coinage metal clusters and superatoms

Atomically precise clusters of group 11 metals (Cu, Ag, and Au) attract considerable attention owing to their remarkable structure and fascinating properties. One of the unique subclasses of these clusters is based on dichalcophosphate ligands of [(RO)₂PE₂]^- type (E = S or Se, and R = alkyl). These ligands successfully stabilise the most diverse Cu, Ag, and Au clusters and superatoms, spanning from simple ones to amazing assemblies featuring unusual structural and bonding patterns. It is noteworthy that such complicated clusters are assembled directly from cheap and simple reagents, metal(I) salts and dichalcophosphate anions. This reaction, when performed in the presence of a hydride or other anion sources, or foreign metal ions, results in hydrido- or anion-templated homo- or heteronuclear structures. In this feature article, we survey the recent advances in this exciting field, highlighting the powerful synthetic capabilities of the system "a metal(I) salt - [(RO)₂PX₂]^- ligands - a templating anion or borohydride" as an inexhaustible platform for the creation of new atomically precise clusters, superatoms, and nanoalloys.

Regulating the Electronic Structure of Metal Nanoclusters by Longitudinal Single-Dithiolate Substitution

It is significant but challenging to understand the property evolution of metal nanoclusters by orientated regulation of the electronic structure. Previous research has demonstrated that the optical properties of metal nanoclusters with anisotropic structures are greatly impacted by their longitudinal electronic structure. However, the manipulation of optical properties of metal nanoclusters by regulating their electronic structure through longitudinal dithiolate substitutions has not yet been reported. In this study, we first achieved the longitudinal single-dithiolate replacement of metal nanoclusters and obtained two novel nanoclusters: Au₂₈(SPh-^tBu)₁₈(SCH₂SCH₂S) and Au₂₈(SPh-^tBu)₁₈(SCH₂CH₂CH₂S). Both experimental and theoretical results demonstrated the regulation of the electronic structure (dipole moment) in the z (longitudinal) and x directions, resulting in absorption redshift and photoluminescence (polarity) enhancement. These findings not only deepen the understanding of the property-electronic structure correlation of metal nanoclusters but also provide guidance for their subtle property tuning.

Cd Atom Goes into the Interior of Cluster Induced by Directional Consecutive Assembly of Tetrahedral Units on an Icosahedron Kernel

"Core sliding" in metal nanoclusters drives the reconstruction of external structural units and provides an ideal platform for mapping their precise transformation mechanism and evolution pathway. However, observing the movement behavior of metal atoms in experiments is still challenging because of the uncertain stability of intermediates. In this work, a series of Au-Cd alloy nanoclusters with continuously assembled kernels (one icosahedral building block assembled with 0 to 3 tetrahedral units) were constructed. As the assembly continued, it eventually led to the Cd atom doping into the inner positions of the clusters. Importantly, the Cd doped into the interior of the cluster exhibits a different behavior than the surface or external Cd atoms (dispersion doping vs localized occupy), which provides experimental evidence of the sliding behavior in the nanocluster kernel. Furthermore, density functional theory (DFT) calculations reveal that this sliding behavior in the inner sites of nanoclusters is an energetically favorable process. In addition, these Au-Cd nanoclusters exhibit tunable optical properties with different assembly patterns in their kernels.

Overall structure of Au12Ag60(S-c-C6H11)31Br9(Dppp)6: achieving a stronger assembly of icosahedral M13 units

Precise atomically assembled nanoclusters provide a great platform to elucidate the evolution of the assembly of building blocks. Herein, a large icosahedral (M₁₃)-based silver-gold alloy nanocluster [Au₁₂Ag₆₀(S-c-C₆H₁₁)₃₁Br₉(Dppp)₆]Br₂ (dppp = 1,3-bis(diphenylphosphino)propane) is reported. Structurally, Au₁₂Ag₆₀ consists of an Au₁₂Ag₄₀ kernel, which is viewed as the interpenetration of ten twisted complete icosahedrons (M₁₃) and two missing icosahedrons (M₁₂), and this is surrounded by a complex metal-organic shell. Benefiting from the extra doping of eight to twelve Au atoms, the octameric assembly was increased to a twelve-mer assembly. The time-dependent density functional theory (TDDFT) method with a Tamm-Dancoff approximation (TDA) was performed to investigate the difference in the optical properties of Au₁₂Ag₆₀ and Au₈Ag₅₇. The results indicate that the difference in the amount of Au atoms results in different optical properties. Furthermore, transient absorption spectroscopy (TA) was also performed, revealing that a twelve-mer assembly greatly enhances the excited-state lifetime. The [Au₁₂Ag₆₀(S-c-C₆H₁₁)₃₁Br₉(Dppp)₆]Br₂ alloy nanocluster has provided a breakthrough in the number of icosahedral M₁₃ assemblies, i.e., achieving a twelve-mer assembly, helping to elucidate the fusion growth of M₁₃-based assembled nanoclusters as well as their geometric/electronic structure correlations, which will promote further research on the assembly of M₁₃ nano-building blocks, especially on their optical properties.

Phosphinous Acid-Phosphinito Tetra-Icosahedral Au52 Nanoclusters for Electrocatalytic Oxygen Reduction

While the formation of superatomic nanoclusters by the three-dimensional assembly of icosahedral units was predicted in 1987, the synthesis and structural determination of such clusters have proven to be incredibly challenging. Herein, we employ a mixed-ligand strategy to prepare phosphinous acid-phosphinito gold nanocluster Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ with a tetra-icosahedral kernel. Unlike expected, each icosahedral Au₁₃ unit shares one vertex gold atom with two adjacent units, resulting in a "puckered" ring shape with a nuclearity of 48 in the kernel. The phosphinous acid-phosphinito ligand set, which consists of two phosphinous acids and one phosphinito motif, has strong intramolecular hydrogen bonds; the π-π stacking interactions between the phosphorus- and sulfur-based ligands provide additional stabilization to the kernel. Highly stable Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ serves as an effective electrocatalyst in the oxygen reduction reaction. Density functional theory calculations suggest that the phosphinous acid-phosphinito ligands provide the most active sites in the electrochemical catalysis, with O* formation being the rate-determining step.

Supervalence Bonding in Bi-icosahedral Cores of [M1Au37(SC2H4Ph)24]- (M = Pd and Pt): Fusion-Mediated Synthesis and Anion Photoelectron Spectroscopy

Au₃₈(PET)₂₄ (PET = SC₂H₄Ph) is known to have a bi-icosahedral Au₂₃ core consisting of two Au₁₃ icosahedrons by sharing three Au atoms. Previous theoretical studies based on a supervalence bond (SVB) model have demonstrated that the bonding scheme in the Au₂₃ core is similar to that in the F₂ molecule. The SVB model predicted that the electron configuration of the Au₂₃ core with 14 valence electrons is expressed as (1Σ)²(1Σ*)²(1Π)⁴(2Σ)²(1Π*)⁴ where each orbital is created by the bonding and antibonding interactions between the 1S and 1P superatomic orbitals of the icosahedral Au₁₃ units. Therefore, the bi-icosahedral Au₂₃ can be viewed as a di-superatomic molecule. To validate the SVB model, we herein conducted anion photoelectron spectroscopy (PES) on [M₁Au₃₇(PET)₂₄]^- (M = Pd and Pt), which are isoelectronic and isostructural with Au₃₈(PET)₂₄. To this end, the neutral precursors [M₁Au₃₇(PET)₂₄]⁰ were first synthesized by fusion reactions between hydride-doped clusters [HAu₉(PPh₃)₈]²⁺ and [M₁Au₂₄(PET)₁₈]^-. The formation of bi-icosahedral M₁Au₂₂ cores with open electronic structure in [M₁Au₃₇(PET)₂₄]⁰ was confirmed by single-crystal X-ray diffraction analysis and electron paramagnetic resonance measurement. Then, the target anions [M₁Au₃₇(PET)₂₄]^- were obtained by reducing [M₁Au₃₇(PET)₂₄]⁰ with NaBH₄, and isoelectronicity with [Au₃₈(PET)₂₄]⁰ was confirmed by optical spectroscopy and density functional theory calculations. Finally, anion PES on [M₁Au₃₇(PET)₂₄]^- observed two distinctive peaks as predicted by the SVB model: one from the nearly degenerate 1Π* orbitals and the other from the nearly degenarate 1Π and 2Σ orbitals.

Horizontal expansion of biicosahedral M13-based nanoclusters: resolving decades-long questions

For metal nanoclusters with the "cluster of clusters" intramolecular evolution pattern, most efforts have been made towards the vertical superposition of icosahedral nanobuilding blocks (e.g., from mono-icosahedral Au₁₃ to bi-icosahedral Au₂₅ and tri-icosahedral Au₃₇), while the horizontal expansion of these rod-shaped multi-icosahedral aggregates was largely neglected. We herein report the horizontal expansion of the biicosahedral M₂₅ cluster framework, yielding an [Au₁₉Ag₁₂(S-Adm)₆(DPPM)₆Cl₇]²⁺ nanocluster that contains an Au₁₃Ag₁₂ kernel and six Au₁(DPPM)₁(S-Adm)₁ peripheral wings. The structural determination of [Au₁₉Ag₁₂(S-Adm)₆(DPPM)₆Cl₇]²⁺ resolved a decades-long question towards rod-shaped multi-icosahedral aggregates: how to load bidentate phosphine and bulky thiol ligands onto the nanocluster framework? The structural comparison between [Au₁₉Ag₁₂(S-Adm)₆(DPPM)₆Cl₇]²⁺ and previously reported [Au₁₃Ag₁₂(PPh₃)₁₀Cl₈]²⁺ or [Au₁₃Ag₁₂(SR)₅(PPh₃)₁₀Cl₂]²⁺ rationalized the unique packing of Au₁(DPPM)₁(S-Adm)₁ motif structures on the surface of the former nanocluster. Overall, this work presents the horizontal expansion of rod-shaped multi-icosahedral nanoclusters, which provides new insights into the preparation of novel icosahedron-based aggregates with both vertically and horizontally growing extensions.

Atomically Precise Au42 Nanorods with Longitudinal Excitons for an Intense Photothermal Effect

Metallic-state gold nanorods are well known to exhibit strong longitudinal plasmon excitations in the near-infrared region (NIR) suitable for photothermal conversion. However, when the size decreases below ∼2 nm, Au nanostructures become nonmetallic, and whether the longitudinal excitation in plasmonic nanorods can be inherited is unknown. Here, we report atomically precise rod-shaped Au₄₂(SCH₂Ph)₃₂ with a hexagonal-close-packed Au₂₀ kernel of aspect ratio as high as 6.2, which exhibits an intense absorption at 815 nm with a high molar absorption coefficient of 1.4 × 10⁵ M^-1 cm^-1. Compared to other rod-shaped nanoclusters, Au₄₂ possesses a much more effective photothermal conversion with a large temperature increase of ∼27 °C within 5 min (λ_ex = 808 nm, 1 W cm^-2) at an ultralow concentration of 50 μg mL^-1 in toluene. Density functional theory calculations show that the NIR transition is mainly along the long axis of the Au₂₀ kernel in Au₄₂, i.e., a longitudinal excitonic oscillation, akin to the longitudinal plasmon in metallic-state nanorods. Transient absorption spectroscopy reveals that the fast decay in Au₄₂ is similar to that of shorter-aspect-ratio nanorods but is followed by an additional slow decay with a long lifetime of 2400 ns for the Au₄₂ nanorod. This work provides the first case that an intense longitudinal excitation is obtained in molecular-like nanorods, which can be used as photothermal converters and hold potential in biomedical therapy, photoacoustic imaging, and photocatalysis.

A multi-responsive Au NCs@PMLE/Ca2+ antitumor hydrogel formed in situ on the interior/surface of tumors for PT imaging-guided synergistic PTT/O2-enhanced PDT effects

At present, although phototherapy and related imaging have proven to be promising cancer diagnosis and treatment strategies, the free diffusion of photosensitizers into normal tissues can cause side effects, and the efficiency of photodynamic therapy (PDT) can also be limited by the tumor hypoxic microenvironment. Herein, we designed and prepared a new cancer nanoplatform containing Au nanoclusters (NCs)@Premna microphylla leaf extract (PMLE) with both responsiveness to near-infrared (NIR) laser irradiation and tumor microenvironment (TME) by facile redox and coordination reactions. Then, the Au NCs@PMLE/Ca²⁺ hydrogel was constructed in situ inside and on the surface of tumors for locoregional antitumor activity under 808 nm laser irradiation. The Au NCs@PMLE nanoplatform showed distinguished performance in killing cancer cells and alleviating tumor hypoxia by enhancing the temperature of the tumor sites and producing reactive oxygen species (ROS) under NIR irradiation as well as catalyzing hydrogen peroxide (H₂O₂) decomposition in TME for oxygen (O₂) generation via catalase in PMLE. The ultra-small size of about 3 nm of the Au NCs in this nanoplatform was obtained using the biological molecules present in PMLE as reductants and coordination agents simultaneously, which also demonstrated the outstanding capability of photothermal (PT) imaging and photothermal therapy (PTT) towards tumors. Furthermore, the Au NCs@PMLE/Ca²⁺ hydrogel formed in situ through natural PMLE and intrinsic Ca²⁺ in TME could not only improve the biocompatibility of the nanoplatform and stability of Au NCs but was also highly concentrated around the tumor thus enhancing the therapeutic efficiency and inhibiting its migration to normal tissues, decreasing the side effects. The results of the experiments confirmed that the Au NCs@PMLE/Ca²⁺ hydrogel possessed PT imaging-guided NIR laser/TME-responsive synergetic cancer PTT/O₂-enhanced PDT and remarkable locoregional antitumor effect for cancer therapy. This work may open a new versatile route for multi-responsive localized cancer therapeutic nanoplatforms.

Fabrication of a family of atomically precise silver nanoclusters via dual-level kinetic control

The controllable preparation of metal nanoclusters in high yield is an essential prerequisite for their fundamental research and extensive application. Here a synthetic approach termed "dual-level kinetic control" was developed to fabricate a family of new silver nanoclusters. The introduction of secondary ligands was first exploited to retard the reduction rate and accomplish the first-level kinetic control. And the cooling of the reaction was performed to further slow the reduction down and accomplish the second-level kinetic control. A family of atomically precise silver nanoclusters (including [Ag₂₅(SR)₁₈]^-, [Ag₃₄(SR)₁₈(DPPP)₃Cl₄]²⁺, [Ag₃₆(SR)₂₆S₄]²⁺, [Ag₃₇(SR)₂₅Cl₁]⁺, and [Ag₅₂(SR)₂₈Cl₄]²⁺) were controllably prepared and structurally determined. The developed "dual-level kinetic control" hopefully acts as a powerful synthetic tool to manufacture more nanoclusters with unprecedented compositions, structures, and properties.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.

Looking at platinum carbonyl nanoclusters as superatoms

Although the chemistry of carbonyl-protected platinum nanoclusters is well established, their bonding mode remains poorly understood. In most of them, the average Pt oxidation state is zero or slightly negative, leading to the apparent average configuration 5d¹⁰ 6s^ε (ε = 0 or very small) and the apparent conclusion that metal-metal bonding cannot arise from the completely filled 5d shell nor from the empty (or almost empty) 6s orbitals. However, DFT calculations show in fact that in these species the actual average configuration is 5d^10-x 6s^x, which provides to the whole cluster a significant total number of 6s electrons that ensures metal-metal bonding. This ("excited") average configuration is to be related to that of coinage metals in ligated group 11 nanoclusters (nd¹⁰ (n + 1)s^x). Calculations show that metal-metal bonding in most of these platinum nanoclusters can be rationalized within the concepts of superatoms and supermolecules, in a similar way as for group 11 nanoclusters. The "excited" 5d^10-x 6s^x configuration results from a level crossing between 5d combinations and 6s combinations, the former transferring their electrons to the latter. This level crossing, which does not exist in the bare Pt_n clusters, is induced by the ligand shell, the role of which being thus not innocent with respect to metal-metal bonding.

Atomic structure of a seed-sized gold nanoprism

The growth of nanoparticles along one or two directions leads to anisotropic nanoparticles, but the nucleation (i.e., the formation of small seeds of specific shape) has long been elusive. Here, we show the total structure of a seed-sized Au₅₆ nanoprism, in which the side Au{100} facets are surrounded by bridging thiolates, whereas the top/bottom {111} facets are capped by phosphine ligands at the corners and Br⁻ at the center. The bromide has been proved to be the key to effectively stabilize the Au{111} to fulfill a complete face-centered-cubic core. In femtosecond electron dynamics analysis, the non-evolution of transient absorption spectra of Au₅₆ is similar to that of larger-sized gold nanoclusters (n > 100), which is ascribed to the completeness of the prismatic Au₅₆ core and an effective electron relaxation pathway created by the stronger Au-Au bonds inside. This work provides some insights for the understanding of plasmonic nanoprism formation.

The formation pathway of shape-anisotropic nanoparticles is difficult to characterize and not well understood. The authors synthesize a prismatic-shaped Au₅₆ nanocluster as possible seed of a prismatic nanoparticle and characterize the structure and ligand bonding motifs, providing insight into the formation and surface protection mechanisms.

Tri- and Tetra-superatomic Molecules in Ligand-Protected Face-Fused Icosahedral (M@Au12)n (M = Au, Pt, Ir, and Os, and n = 3 and 4) Clusters

Cluster assembling has been one of the hottest topics in nanochemistry. In certain ligand-protected gold clusters, bi-icosahedral cores assembled from Au₁₃ superatoms were found to be analogues of diatomic molecules F₂, N₂, and singlet O₂, respectively, in electronic shells, depending upon the super valence bond (SVB) model. However, challenges still remain for extending the scale in cluster assembling via the SVB model. In this work, ligand-protected tri- and tetra-superatomic clusters composed of icosahedral M@Au₁₂ (M = Au, Pt, Ir, and Os) units are theoretically predicted. These clusters are stable with reasonable highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps and proven to be analogues of simple triatomic (Cl₃^-, OCl₂, O₃, and CO₂) and tetra-atomic (N≡C-C≡N, and Cl-C≡C-Cl) molecules in both geometric and electronic structures. Moreover, a stable cluster-assembling gold nanowire is predicted following the same rules. This work provides effective electronic rules for cluster assembling on a larger scale and gives references for their experimental synthesis.

Evolution from superatomic Au24Ag20 monomers into molecular-like Au43Ag38 dimeric nanoclusters

Hierarchical assembly of nanoparticles has been attracting wide interest, as advanced functionalities can be achieved. However, the ability to manipulate structural evolution of artificial nanoparticles into assemblies with atomic precision has been largely unsuccessful. Here we report the evolution from monomeric Au₂₄Au₂₀ into dimeric Au₄₃Ag₃₈ nanoclusters: Au₄₃Ag₃₈ inherits the kernel frameworks from parent Au₂₄Ag₂₀ but exhibits distinct surface motifs; Au₂₄Ag₂₀ is racemic, while Au₄₃Ag₃₈ is mesomeric. Importantly, the evolution from monomers to dimers opens up exciting opportunities exploring currently unknown properties of monomeric and dimeric alloy nanoclusters. The Au₂₄Ag₂₀ clusters show superatomic electronic configurations, while Au₄₃Ag₃₈ clusters have molecular-like characteristics. Furthermore, monomeric Au₂₄Ag₂₀ catalysts readily outperform dimeric Au₄₃Ag₃₈ catalysts in the catalytic reduction of CO₂.

The work shows the evolution from monomeric Au₂₄Au₂₀ into dimeric Au₄₃Ag₃₈ nanoclusters and provides exciting opportunities for atomic manufacturing on metal nanoclusters to construct structures and functionality.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

Crystal Structure and Optical Properties of a Chiral Mixed Thiolate/Stibine-Protected Au18 Cluster

We report the first example of a chiral mixed thiolate/stibine-protected gold cluster, formulated as Au₁₈(S-Adm)₈(SbPh₃)₄Br₂ (where S-Adm = 1-adamantanethiolate). Single crystal X-ray crystallography reveals the origin of chirality in the cluster to be the introduction of the rotating arrangement of Au₂(S-Adm)₃ and Au(S-Adm)₂ staple motifs on an achiral Au₁₃ core and the subsequent capping of the remaining gold atoms by SbPh₃ and Br^- ligands. Interestingly, the structure and properties of this new Au₁₈ cluster are found to be different from other reported achiral Au₁₈ clusters and the only other stibine-protected [Au₁₃(SbPh₃)₈Cl₄]⁺ cluster. Detailed analyses on the geometric and electronic structures of the new cluster are carried out to gain insights into its optical properties as well as reactivity and stability of such mixed monolayer-protected clusters.

Photo-Induced Cluster-to-Cluster Transformation of [Au37-xAgx(PPh3)13Cl10]3+ into [Au25-yAgy(PPh3)10Cl8]+: Fragmentation of a Trimer of 8-Electron Superatoms by Light

We present the photoinduced size/structure transformation of [Au_37-xAg_x(PPh₃)₁₃Cl₁₀]³⁺ (M37) into [Au_25-yAg_y(PPh₃)₁₀Cl₈]⁺ (M25) cluster. Single-crystal X-ray diffraction revealed that M37 has a tri-icosahedron M₃₆ metal core assembled via the fusion of three Au₇Ag₆ icosahedrons in a cyclic fashion and that the M₃₆ core is further protected by phosphine and chloride ligands. The M37 cluster is found to be highly sensitive toward ambient light, and the M37 → M25 transition is observed with 530 nm irradiation, monitored by time-dependent UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and femtosecond transient absorption spectroscopy. Linear-response time-dependent DFT calculations indicated that the strong absorption of the M37 cluster close to 500 nm induces an antibonding-type configuration in the induced electron density within the plane of the three 8-electron systems, possibly promoting dissociation of one of the 8-electron superatoms. This theoretical result supports the experimental observation of the sensitivity of the M37 → M25 transition to 530 nm irradiation.

Programmable Metal Nanoclusters with Atomic Precision

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 smaller E_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.

Isolation of the Au145(SR)60X compound (R = n-butyl, n-pentyl; X = Br, Cl): novel gold nanoclusters that exhibit properties subtly distinct from the ubiquitous icosahedral Au144(SR)60 compound

We report the identification and quantitative isolation of Au₁₄₅(SR)₆₀X (R = n-butyl, n-pentyl; X = halide) along with elucidation of key properties as compared to the corresponding ubiquitous chiral-icosahedral Au₁₄₄(SR)₆₀ cluster known to have a central vacancy. The stoichiometries were assessed by electrospray mass spectrometry (ESI-MS) at isotopic resolution, and induced dissociation patterns indicate the 'extra' (Au,Br) atoms are strongly bound components of these structures. Voltammetric and spectroscopic characterization reveals Au₁₄₅(SR)₆₀X behaviors that are qualitatively similar to yet fascinatingly distinct from those of Au₁₄₄(SR)₆₀. (¹H,¹³C)-NMR spectra clearly show how both Au₁₄₅(SR)₆₀X and Au₁₄₄(SR)₆₀ are capped by 12 distinct ligand types of 5-fold equivalence, as was recently established for Au₁₄₄(SR)₆₀ capped by shorter ligands, demonstrating that this novel cluster shares the same chiral-icosahedral motif. Intriguingly, Au₁₄₅(SR)₆₀X is strongly near-IR luminescent, whereas under comparable conditions Au₁₄₄(SR)₆₀ barely emits. The photoluminescence pattern of Au₁₄₅(SR)₆₀X is very similar to that observed for Au₂₅(SR)₁₈, which contains the Au₁₃ core. The combined results are interpreted as consistent with neutral Au₁₄₅(SR)₆₀X as a diamagnetic species, electronically and structurally similar to the corresponding Au₁₄₄(SR)₆₀ compounds.

Ligand-Protected Au55 with a Novel Structure and Remarkable CO2 Electroreduction Performance

A Au₅₅ nanocluster with the composition of [Au₅₅ (p-MBT)₂₄ (Ph₃ P)₆ ](SbF₆ )₃ (p-MBT=4-methylbenzenethiolate) is synthesized via direct reduction of gold-phosphine and gold-thiolate precursors. Single-crystal X-ray diffraction reveals that this Au₅₅ nanocluster features a face-centered cubic (fcc) Au₅₅ kernel, different from the well-known two-shell cuboctahedral arrangement in Au₅₅ (Ph₃ P)₁₂ Cl₆ . The Au₅₅ cluster shows a wide optical absorption band with optical energy gap (E_g =1.28 eV). It is found that the exclusion of chloride is crucial for the formation of the title cluster, otherwise rod-like [Au₂₅ (SR)₅ (PPh₃ )₁₀ Cl₂ ]²⁺ is obtained. The strategy to run synthetic reaction in the absence of halide leads to new members of phosphine/thiolate co-protected metal nanoclusters. The Au₅₅ nanocluster exhibits high catalytic activity and selectivity for electrochemical reduction of CO₂ to CO; the Faradaic efficiency (FE) reaches 94.1 % at -0.6 V vs. reversible hydrogen electrode (RHE).

The doping engineering and crystal structure of rod-like Au8Ag17 nanoclusters

Alloy nanoclusters protected by ligands were widely studied due to the synergistic effect of metal atoms, and they exhibit enhanced properties in different fields, such as bio-imaging and catalysis. Herein, we obtained Au₈Ag₁₇(PPh₃)₁₀Cl₁₀ nanoclusters via one-step simple synthesis. The atomically precise crystal structure was determined by x-ray crystallography. It is found that the rod-like Au₈Ag₁₇ nanoclusters were composed of two Au₄Ag₉ icosahedrons via sharing the same Ag atom. Two Au atoms occupy the center of icosahedrons, and the other six Au atoms are all at the neck sites. Four kinds of Cl-Ag connecting modes were observed in Au₈Ag₁₇ nanoclusters. Moreover, the ultraviolet-visible absorption spectrum shows that the prominent absorption peaks of Au₈Ag₁₇ nanoclusters are at ∼395 and 483 nm. This work provides a feasible strategy to synthesize alloy nanoclusters with precise composition via doping engineering.

Rod-Shaped Silver Supercluster Unveiling Strong Electron Coupling between Substituent Icosahedral Units

The first linear silver supercluster based on icosahedral Ag₁₃ units has been constructed via bridging of dpa ligands: Ag₆₁(dpa)₂₇(SbF₆)₄ (Hdpa = dipyridylamine) (Ag₆₁). Single-crystal X-ray diffraction reveals that this rod-shaped cluster consists of four vertex-sharing Ag₁₃ icosahedra in a linear arrangement. This Ag₆₁ cluster represents the longest one-dimensional metal nanocluster with a resolved structure. Unprecedented electron coupling develops between their constituent Ag₁₃ units. Theoretical studies disclose that the stabilities of the two superclusters are dictated by a strong interaction between the Ag₁₃ units as well as the ligand effect of the dpa-Ag motifs. The quantum size effect accounts for the significant enhancement of the metal-related absorptions and the red shift at the near-infrared region as the length of the cluster increases. This work sheds light on the evolution of one-dimensional materials and an understanding of the electronic communication between the constituent clusters.

Hydrogen Evolution Electrocatalyst Design: Turning Inert Gold into Active Catalyst by Atomically Precise Nanochemistry

Electrocatalytic hydrogen evolution reaction (HER) holds promise in the renewable clean energy scheme. Crystalline Au and Ag are, however, poor in catalyzing HER, and the ligands on colloidal nanoparticles are generally another disadvantage. Herein, we report a thiolate (SR)-protected Au₃₆Ag₂(SR)₁₈ nanocluster with low coverage of ligands and a core composed of three icosahedral (I_h) units for catalyzing HER efficiently. This trimeric structure, together with the monomeric I_h Au₂₅(SR)₁₈^- and dimeric I_h Au₃₈(SR)₂₄, constitutes a unique series, providing an opportunity for revealing the correlation between the catalytic properties and the catalyst's structure. The Au₃₆Ag₂(SR)₁₈ surprisingly exhibits high catalytic activity at lower overpotentials for HER due to its low ligand-to-metal ratio, low-coordinated Au atoms and unfilled superatomic orbitals. The current density of Au₃₆Ag₂(SR)₁₈ at -0.3 V vs RHE is 3.8 and 5.1 times that of Au₂₅(SR)₁₈^- and Au₃₈(SR)₂₄, respectively. Density functional theory (DFT) calculations reveal lower hydrogen binding energy and higher electron affinity of Au₃₆Ag₂(SR)₁₈ for an energetically feasible HER pathway. Our findings provide a new strategy for constructing highly active catalysts from inert metals by pursuing atomically precise nanoclusters and controlling their geometrical and electronic structures.

[Ag23Pd2(PPh3)10Cl7]0: A new family of synthesizable bi-icosahedral superatomic molecules

Icosahedral noble-metal 13-atom nanoclusters (NCs) can form connected structures, which can be regarded as superatomic molecules, by vertex sharing. However, there have been very few reports on the superatomic molecules formed using silver (Ag) as the base element. In this study, we synthesized [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ (Pd = palladium, PPh₃ = triphenylphosphine, Cl = chloride), in which two icosahedral 13-atom NCs are connected, and elucidated its geometric and electronic structures to clarify what type of superatomic molecules can be synthesized. The results revealed that [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ is a synthesizable superatomic molecule. Single crystal x-ray diffraction analysis showed that the metal-metal distances in and between the icosahedral structures of [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ are slightly shorter than those of previously reported [Ag₂₃Pt₂(PPh₃)₁₀Cl₇]⁰, whereas the metal-PPh₃ distances are slightly longer. On the basis of several experiments and density functional theory calculations, we concluded that [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ and previously reported [Ag₂₃Pt₂(PPh₃)₁₀Cl₇]⁰ are more stable than [Ag₂₅(PPh₃)₁₀Cl₇]²⁺ because of their stronger superatomic frameworks (metal cores). These findings are expected to lead to clear design guidelines for creation of new superatomic molecules.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.