On the variation of cluster core characteristics by an endohedral atom. Shape variation in 8-ce [EAu4(PPh3)4]2+ (E = N, P, As, Sb) clusters

Atomically precise gold superatoms have attracted interest owing to their suitable use as building blocks for cluster-assembled materials, favoring ordered structures with advanced properties. In this sense, expanding their versatility is a relevant issue for controlling their properties and retaining a specific nuclearity. Interestingly, the reported structure for isoelectronic [Au4N(PPh3)4]+ and [Au4Sb(PPh3)4]+ clusters denotes two contrasting shapes featuring a tetrahedral and square pyramidal structure, respectively. Herein, we further explore the [Au4E(PPh3)4]+ (E = N, P, As, Sb) series in order to evaluate energetic and structural factors determining the overall shape. Our results show a favorable [Au4(PPh3)4]4+/E3- interaction energy, predicting particular patterns in their UV-vis spectrum. Thus, the use of dopant atoms is enabled to vary the core shape and, in turn, to modify the cluster properties, which serve as a structural control, in addition to ligand-based and size approaches.

On the halide aggregation into the [Au4(PPh3)4]4+ cluster core. Insights from structural, optical and interaction energy analysis in [(Ph3PAu)4X2]2+ and [(Ph3PAu)4X]3+ species (X = Cl-, Br-, I-)

The aggregation of halide atoms into gold clusters offers an interesting scenario for the development of novel metal-based cavities for anion recognition and sensing applications. Thus, further understanding of the different contributing terms leading to efficient cluster-halide aggregation is relevant to guide their synthetic design. In this report, we evaluate the formation of [(Ph3PAu)4X2]2+ and [(Ph3PAu)4X]3+ species (X = Cl-, Br-, I-) in terms of different energy contributions underlying the stabilization of the cluster-halide interaction, and the expected UV-vis absorption profiles as a result of the variation in frontier orbital arrangements. Our results denote that a non-planar Au4 core shape enables enhanced halide aggregation, which is similar for Cl-, Br-, and I-, in comparison to the hypothetical planar Au4 counterparts. The electrostatic nature of the interaction involves a decreasing ion-dipole term along with the series, and for iodine species, higher-order electrostatic contributions become more relevant. Hence, the obtained results help in gaining further understanding of the different stabilizing and destabilizing contributions to suitable cluster-based cavities for the incorporation of different monoatomic anions.

Counteranion-induced structural isomerization of phosphine-protected PdAu8 and PtAu8 clusters

Controlling the geometric structures of metal clusters through structural isomerization allows for tuning of their electronic state. In this study, we successfully synthesized butterfly-motif [PdAu₈(PPh₃)₈]²⁺ (PdAu8-B, B means butterfly-motif) and [PtAu₈(PPh₃)₈]²⁺ (PtAu8-B) by the structural isomerization from crown-motif [PdAu₈(PPh₃)₈]²⁺ (PdAu8-C, C means crown-motif) and [PtAu₈(PPh₃)₈]²⁺ (PtAu8-C), induced by association with anionic polyoxometalate, [Mo₆O₁₉]^2- (Mo6) respectively, whereas their structural isomerization was suppressed by the use of [NO₃]^- and [PMo₁₂O₄₀]^3- as counter anions. DR-UV-vis-NIR and XAFS analyses and density functional theory calculations revealed that the synthesized [PdAu₈(PPh₃)₈][Mo₆O₁₉] (PdAu8-Mo6) and [PtAu₈(PPh₃)₈][Mo₆O₁₉] (PtAu8-Mo6) had PdAu8-B and PtAu8-B respectively because PdAu8-Mo6 and PtAu8-Mo6 had bands in optical absorption at the longer wavelength region and different structural parameters characteristic of the butterfly-motif structure obtained by XAFS analysis. Single-crystal and powder X-ray diffraction analyses revealed that PdAu8-B and PtAu8-B were surrounded by six Mo6 with rock salt-type packing, which stabilizes the semi-stable butterfly-motif structure to overcome high activation energy for structural isomerization.

Phosphine-Stabilized Hidden Ground States in Gold Clusters Investigated via a Aun(PH3)m Database

Nanoclusters are promising materials for catalysis and sensing due to their large surface areas and unique electronic structures which can be tailored through composition, geometry, and chemistry. However, relationships correlating synthesis parameters directly to outcomes are limited. While previous computational studies have mapped the potential energy surface of specific systems of bare nanoclusters by generating and calculating the energies of reasonable structures, it is known that environmental ions and ligands crucially impact the final shape and size. In this work, phosphine-stabilized gold is considered as a test system and DFT calculations are performed for clusters with and without ligands, producing a database containing >10000 structures for Aun(PH3)m (n ≤ 12). We find that the ligation of phosphines affects the thermodynamic stability, bonding, and electronic structure of Au nanoclusters, specifically such that "hidden" ground state cluster geometries are stabilized that are dynamically unstable in the pure gold system. Further, the addition of phosphine introduces steric effects that induce a transition from planar to nonplanar structures at 4-5 Au atoms rather than up to 13-14 Au atoms, as previously predicted for bare clusters. This work highlights the importance of considering the ligand environment in the prediction of nanocluster morphology and functionality, which adds complexity as well as a rich opportunity for tunability.

Selective Semihydrogenation of Polarized Alkynes by a Gold Hydride Nanocluster

The hydridic hydrogen in nanogold catalysts has long been postulated as an important intermediate in hydrogenation reactions, but it has not been directly observed. Here, we report the synthesis of a new undecagold cluster with a bidentate phosphine ligand. The chelating effects of the bidentate ligand result in a more symmetric Au₁₁ core with two labile Cl^- ligands that can exchange with BH₄^-, leading to a novel undecagold hydride cluster. The new hydride cluster is discovered to readily undergo hydroauration reaction with alkynes containing electron-withdrawing groups, forming key gold-alkenyl semihydrogenation intermediates, which can be efficiently and selectively converted to Z-alkenes under acidic conditions. All key reaction intermediates are isolated and characterized, providing atomic-level insights into the active sites and mechanisms of semihydrogenation reactions catalyzed by gold-based nanomaterials. The hydridic hydrogen in the undecagold cluster is found to be the key to prevent over hydrogenation of alkenes to alkanes. The current study provides fundamental insights into hydrogenation chemistry enabled by gold-based nanomaterials and may lead to the development of efficient catalysts for selective semihydrogenation or functionalization of alkynes.

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.

Controlled Synthesis of Diphosphine-Protected Gold Cluster Cations Using Magnetron Sputtering Method

We demonstrated, for the first time, atomically precise synthesis of gold cluster cations by magnetron sputtering of a gold target onto a polyethylene glycol (PEG) solution of 1,3-bis(diphenylphosphino)propane (Ph₂PCH₂CH₂CH₂PPh₂, dppp). UV-vis absorption spectroscopy and electrospray ionization mass spectrometry revealed the formation of cationic species, such as [Au(dppp)_n]⁺ (n = 1, 2), [Au₂(dppp)_n]²⁺ (n = 3, 4), [Au₆(dppp)_n]²⁺ (n = 3, 4), and [Au₁₁(dppp)₅]³⁺. The formation of [Au(dppp)₂]⁺ was ascribed to ionization of Au(dppp)₂ by the reaction with PEG, based on its low ionization energy, theoretically predicted, mass spectrometric detection of deprotonated anions of PEG. We proposed that [Au(dppp)₂]⁺ cations thus formed are involved as key components in the formation of the cluster cations.

Mixed-diphosphine-protected chiral undecagold clusters Au11(S,S-DIOP)4(rac-/R-/S-BINAP): effect of the handedness of BINAP on their chiroptical responses

In this article, we report a preference of homochiral-type ligation of BINAP that produces SS-type ligand assembly onto the Au11 clusters protected by diphosphine S,S-DIOP. The Au11 clusters synthesized and isolated are Au11(S,S-DIOP)4(rac-/R-/S-BINAP), and their optical/chiroptical responses are characterized. Absorption spectra of these Au11 clusters are almost identical to each other, but their CD profiles are dependent on the handedness of BINAP. In Au11(S,S-DIOP)4(rac-BINAP), the yield of S-BINAP or R-BINAP coordination is roughly comparable, but we found a small but distinctive preference in the S-BINAP ligation; that is, homochiral-type (SS-type) ligand assembly formation. Quantum chemical calculations for simplified model clusters suggest equal contributions of S- and R-form BINAP coordination. The experimentally-observed preference of homochiral-type ligation can then be due to that of the whole ligand structures and assemblies involving interligand interactions. Chiral sorting and amplification processes through the assembly control of homochirality or heterochirality are of primary importance for the development of enantioselective reactions, so we anticipate this finding will contribute to further understanding of such processes based on various metal clusters with chiral ligands.

Ligand Design in Ligand-Protected Gold Nanoclusters

The design of surface ligands is crucial for ligand-protected gold nanoclusters (Au NCs). Besides providing good protection for Au NCs, the surface ligands also play the following two important roles: i) as the outermost layer of Au NCs, the ligands will directly interact with the exterior environment (e.g., solvents, molecules and cells) influencing Au NCs in various applications; and ii) the interfacial chemistry between ligands and gold atoms can determine the structures, as well as the physical and chemical properties of Au NCs. A delicate ligand design in Au NCs (or other metal NCs) needs to consider the covalent bonds between ligands and gold atoms (e.g., gold-sulfur (Au-S) and gold-phosphorus (Au-P) bond), the physics forces between ligands (e.g., hydrophobic and van der Waals forces), and the ionic forces between the functional groups of ligands (e.g., carboxylic (COOH) and amine group (NH₂ )); which form the underlying chemistry and discussion focus of this review article. Here, detailed discussions on the effects of surface ligands (e.g., thiolate, phosphine, and alkynyl ligands; or hydrophobic and hydrophilic ligands) on the synthesis, structures, and properties of Au NCs; highlighting the design principles in the surface engineering of Au NCs for diverse emerging applications, are provided.

Photoluminescence enhancement by controllable aggregation and polymerization of octanuclear gold clusters

Aggregation-induced luminescence behaviors of polynuclear metal clusters are intriguing but still mysterious to date. Herein, we synthesize a series of Au8 clusters with different peripheral ligands to investigate their different supramolecular assembly and distinguishable aggregation-induced luminescent behaviors in solution upon the variation of concentration and solvent polarity. Complex 1a is surrounded by two tetraphenyl ethylene (TPE) moieties, whose dense stacking engenders strong aggregation in solution. Furthermore, the incorporation of the TPE-decorated gold clusters into a polymer backbone promotes molecular restriction within a constrained polymer micelle and thus facilitates efficient emission enhancement of gold clusters. In contrast, the other two gold clusters with small ethynylbenzene and chloride as peripheral ligands exhibit low emission efficiency. The contrastive study showcases how the peripheral ligand arrangements influence assemblies of metal clusters and potentiates such effects in the rational design of cluster-based luminescent materials.

Toward the Formation of N-Heterocyclic-Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with Their Phosphine-Protected Analogues from Density Functional Theory Calculations

The structure and bonding of a series of selected phosphine-protected gold clusters (Au_n-P) of nuclearity varying from n = 6 to 13 were investigated by density functional theory (DFT) calculations and compared to those of the hypothetical homologues in which phosphines were replaced by N-heterocyclic carbene (NHC) analogues (Au_n-C). Both the Au_n-P and Au_n-C series exhibit similar stabilities and structural features, except for n = 6, where some differences are noted. The NHC ligands are found to be even slightly more strongly bonded to the gold core (by a few kilocalories per mole per ligand) than phosphines. Investigation of the optical properties of both series using time-dependent DFT calculations indicates similarities in the nature and energies of the UV-vis optical transitions and, consequently, relatively similar shapes of the simulated spectra, with a general blue-shift tendency when going from Au_n-P to Au_n-C. The fluorescence behavior observed experimentally for some of the Au_n-P species is expected to occur also for their Au_n-C analogues, which can be extended to other carbene-ligand-protected nanoclusters. Our results show that it should be possible to stabilize gold clusters with NHC ligands, in relation to the seminal Au₁₃-ligand-protected core, offering novel building blocks for the design of nanostructured materials with various properties.

Halogen effects on the electronic and optical properties of Au13 nanoclusters

We report an experimental and theoretical investigation of the electronic and optical properties of a series of icosahedral Au₁₃ nanoclusters, protected using different halogen ligands (Cl, Br, and I), as well as 1,2-bis(diphenylphosphino)ethane (dppe) ligands. All three clusters are comprised of the same Au₁₃ kernel with two halogens coordinated to the poles of the icosahedral cluster along with five dppe ligands. UV-vis absorption spectra indicate a systematic red shift from Cl to Br to I, as well as a sudden enhancement of the second excitonic peak for the I-coordinated cluster. Density functional theory (DFT) calculations suggest that all clusters possess a wide HOMO-LUMO energy gap of ∼1.79 eV and are used to assign the first two excitonic bands. Frontier orbital analyses reveal several HOMO → LUMO transitions involving halogen-to-metal charge transfers. For the I-coordinated cluster, more complicated I-to-metal charge transfers give rise to different excitation features observed experimentally. The current findings show that halogen ligands play important roles in the electronic structures of gold clusters and can be utilized to tune the optical properties of the clusters.

Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters

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

Isomerization in Alkynyl-Protected Gold Nanoclusters

We report the controlled synthesis and structures of two isomeric gold nanoclusters, whose compositions are determined to be Au₂₃(C≡CBu^t)₁₅ (denoted as Au₂₃-1 and Au₂₃-2) by single-crystal X-ray diffraction and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. This is the first time isomerism is discovered in alkynyl-protected gold nanoclusters. The metal-to-ligand ratios in these two clusters are different from known Au_n(SR)_m systems and have not been observed in the Au_x(C≡CPh)_y family. This pair of isomers exhibits different optical properties, although they have similar structures and identical components. For both Au₂₃ clusters, time-dependent density functional theory calculations revealed the frontier orbitals highest occupied molecular orbital (HOMO)-1, HOMO, and lowest unoccupied molecular orbital (LUMO) are mainly constructed from the Au₁₅ kernel and V-shaped alkynyl-gold motifs. The HOMO → LUMO transition of Au₂₃-1 is optically forbidden, whereas it is allowed in Au₂₃-2. It is also found that Au₂₃-2 cluster can be transformed to Au₂₃-1 spontaneously under ambient conditions. This work offers further insight into the synthesis and isomerism of all-alkynyl-protected gold nanoclusters and will stimulate more investigation of isomeric metal nanoclusters.

The positional isomerism in bimetal nanoclusters

Enriching the variety of isomerism in the nanocluster field is exciting but challenging.

Phosphine-Ligated Gold Clusters with Core+ exo Geometries: Unique Properties and Interactions at the Ligand-Cluster Interface

Over recent years, research on the structures and properties of ligand-protected gold cluster molecules has gained significant interest. The crystal structure information accumulated to date has revealed the structural preference to adopt closed polyhedral geometries, but the use of multidentate ligands sometimes leads to the formation of exceptional structures. This Account describes results of our studies on diphosphine-coordinated [core+ exo]-type gold clusters featuring extra gold atoms outside the polyhedral cores, highlighting (1) their distinct optical properties due to the unique electronic structures generated by the exo gold atoms and (2) electronic/attractive ligand-cluster interactions that cause definite perturbation effects on the cluster properties. Subnanometer gold clusters with [core+ exo]-type geometries (nuclearity = 6, 7, 8, and 11) commonly displayed single absorption bands in the visible region, which are distinct in patterns from those of conventional polyhedral-only homologues. Theoretical studies demonstrated that the exo gold atoms are critically involved in the generation of unique electronic structures characterized by the HOMO-LUMO transitions with dominant oscillator strengths, leading to the appearance of the isolated absorption bands. On the basis of the frontier orbital distributions, the HOMO and LUMO were shown to be localized around the polyhedral cores and exo gold atoms, respectively. Therefore, the HOMO-LUMO transitions responsible for the visible absorptions occur in the core → exo direction. The HOMO-LUMO gap energies showed no clear trends with respect to the nuclearity (size), indicating that the individual geometric features of the inorganic framework primarily govern the clusters' electronic structures and properties. Systematic studies using octagold clusters bearing various anionic coligands revealed that electronic or attractive interactions between the gold framework and ligand functionalities, such as π-electron systems and heteroatoms, cause substantial perturbations of the wavelength of the visible absorption band due to the HOMO-LUMO transitions. Especially, significant red shifts were observed as a result of the electronic coupling with specific π-resonance contributors. It was also found that the orientation of aromatic rings around the inorganic framework is a factor that affects the cluster photoluminescence. These findings demonstrate the utility of the ligand moieties surrounding the gold frameworks for fine-tuning of the optical properties. During these studies, unusual but definite attractive interactions between the gold framework and C-H groups of the diphosphine ligand were found in the hexagold clusters. On the basis of careful crystallographic and NMR analyses, these interactions were deemed as a certain kind of M···H hydrogen bonds, which critically affect the maintenance of the cluster framework. Such unique interaction activities are likely due to the valence electrons in the gold framework, which serve as the hydrogen-bond acceptor for the unfunctionalized C-H groups. Overall, these observations imply the uniqueness of the ligand-cluster interface associated with the partially oxidized gold entities, which may expand the scope of ligand-protected clusters toward various applications.

Isomerism in Au-Ag Alloy Nanoclusters: Structure Determination and Enantioseparation of [Au9Ag12(SR)4(dppm)6X6]3

Revealing structural isomerism in a nanocluster remains significant but challenging. Herein, we have obtained a pair of structural isomers, [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C and [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-Ac [dppm = bis(diphenyphosphino)methane; HSR = 1-adamantanethiol/ tert-butylmercaptan; X = Br/Cl; C stands for one of the structural isomers being chiral; Ac stands for another being achiral], that show different structures as well as different chiralities. These structures are determined by single-crystal X-ray diffraction and further confirmed by high-resolution electrospray ionization mass spectrometry. On the basis of the isomeric structures, the most important finding is the different arrangements of the Au₅Ag₈@Au₄ metal core, leading to changes in the overall shape of the cluster, which is responsible for structural isomerism. Meanwhile, the two enantiomers of [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C are separated by high-performance liquid chromatography. Our work will contribute to a deeper understanding of the structural isomerism in noble-metal nanoclusters and enrich the chiral nanocluster.

Diphosphine-protected ultrasmall gold nanoclusters: opened icosahedral Au13 and heart-shaped Au8 clusters

Due to distinctive quantum confinement effects, ultrasmall gold nanoparticles usually exhibit interesting electronic structure and molecular-like properties. However, the lack of atomically-precise structural information makes the understanding of them almost impossible, such as understanding the relationships between their compositions and unique properties. Herein, by reducing a diphosphine AuI precursor (Au2(dppm)2Cl2; dppm = Ph2PCH2PPh2) with or without a S2- releasing reagent, we enriched our knowledge of the members in the families of Au13 and Au8 by the structural determinations of two new dppm-protected gold nanoclusters, [Au13(dppm)6]5+ (SD/Au1) and [Au8(dppm)4S2]2+ (SD/Au2), respectively. Within SD/Au1, the Au13 kernel significantly deviates from the ideal Ih icosahedron by the elongation of three surface Au-Au bonds to ∼3.5 Å, giving it C3 symmetry, whereas SD/Au2 has a novel heart-shaped C2 symmetric Au8S2 core (central Au4 tetrahedron + two Au2S units) protected by four μ2-dppm ligands in the outer shell. Of note, SD/Au1 represents a rare Au13 nanocluster with an opened icosahedral geometry, and SD/Au2 shows a new edge-shared "core + 4exo" structure type that has never been observed before. The electronic structures and optical absorption spectra of these systems are correlated with time-dependent density functional theory (TDDFT) calculations. Based on the spherical jellium model, the stability of the Au13 and Au8 nanoclusters can be ascribed to 8- and 2-electron superatoms with 1S21P6 and 1S2 configurations, respectively. Interestingly, the cluster SD/Au2 exhibits bright yellow luminescence with an emission maximum at 591 nm that slightly hypsochromically shifts to 581 nm upon cooling to 93 K. Our findings not only enrich the family of diphosphine-protected ultrasmall gold nanoclusters, but also demonstrate the rich variations of gold kernels during the transformation from a simple AuI precursor to Au nanoclusters.

Cluster-to-cluster transformation among Au6, Au8 and Au11 nanoclusters

This manuscript demonstrates the cluster-to-cluster transformations among three gold nanoclusters, which were all monitored and corroborated using UV-Vis spectroscopy and ESI-MS.

Reversible conversion between phosphine protected Au6 and Au8 nanoclusters under oxidative/reductive conditions

In this study, we found that phosphine protected [Au₆(dppp)₄]²⁺ and [Au₈(dppp)₄Cl₂]²⁺ nanoclusters could be reversibly converted under oxidative/reductive conditions. This work not only provides new insights into the relationship between the [Au₆(dppp)₄]²⁺ and [Au₈(dppp)₄Cl₂]²⁺ nanoclusters, but also offers a novel method for controlling structural evolution of different Au nanoclusters.

A grand unified model for liganded gold clusters

A grand unified model (GUM) is developed to achieve fundamental understanding of rich structures of all 71 liganded gold clusters reported to date. Inspired by the quark model by which composite particles (for example, protons and neutrons) are formed by combining three quarks (or flavours), here gold atoms are assigned three 'flavours' (namely, bottom, middle and top) to represent three possible valence states. The 'composite particles' in GUM are categorized into two groups: variants of triangular elementary block Au₃(2e) and tetrahedral elementary block Au₄(2e), all satisfying the duet rule (2e) of the valence shell, akin to the octet rule in general chemistry. The elementary blocks, when packed together, form the cores of liganded gold clusters. With the GUM, structures of 71 liganded gold clusters and their growth mechanism can be deciphered altogether. Although GUM is a predictive heuristic and may not be necessarily reflective of the actual electronic structure, several highly stable liganded gold clusters are predicted, thereby offering GUM-guided synthesis of liganded gold clusters by design.

Ligand-Based Toolboxes for Tuning of the Optical Properties of Subnanometer Gold Clusters

Recent advances in the crystal structure determination of ligand-protected metal clusters have revealed that their electronic structures and optical features are essentially governed by the nuclearity and geometries of the inorganic frameworks. In this Perspective, we point out the definite effects of the exterior ligand moieties on the properties of small gold clusters. On the basis of systematic experimental studies on the optical properties of Au₈ and Au₁₃ clusters with various anionic ligands, it was shown that not only the "through-bond" electronic effects of coordinating atoms but also the nonbonding interaction with neighboring heteroatoms and the electronic coupling with π-systems cause substantial perturbations. We also suggest that the steric rigidity of the ligand environments affects their photoluminescence efficiencies. These findings imply the feasibility of the facile modulation of the cluster properties through the appropriate choice of ligand modules, which may lead to the evolution of novel cluster-based materials with unique properties and functions.

Polymorphism of Phosphine-Protected Gold Nanoclusters: Synthesis and Characterization of a New 22-Gold-Atom Cluster

A new Au22 nanocluster, protected by bis(2-diphenyl-phosphino)ethyl ether (dppee or C28 H28 OP2 ) ligand, has been synthsized and purified with high yield. Electrospray mass spectrometry shows that the new cluster has a formula of Au22 (dppee)7 , containing 22 gold atoms and seven dppee ligands. The cluster is found to be stable as a solid, but metastable in solution. The new cluster has been characterized by UV-Vis-NIR absorption spectroscopy, collision-induced dissociation, and (31) P-NMR. The properties of the new cluster have been compared with the previous Au22 (dppo)6 nanocluster (dppo = 1,8-bis(diphenyl-phosphino)octane or C32 H36 P2 ), which contains two fused Au11 units. All the experimental data indicate that the new Au22 (dppee)7 cluster is different from the previously known Au22 (dppo)6 cluster and represents a new Au22 core, which contains most likely one Au11 motif with several Au2 (dppee) or Au(dppee) units. The Au22 (dppee)7 cluster provides a new example of the ligand effects on the nuclearity and structural polymorphism of phosphine-protected atom-precise gold nanoclusters.

Facile modulation of optical properties of octagold clusters through the control of ligand-mediated interactions

Organic ligand environments substantially affect the optical properties of gold clusters through electronic and steric interactions, offering versatile tools to tune cluster properties.

The Magic Au60 Nanocluster: A New Cluster-Assembled Material with Five Au13 Building Blocks

Herein, we report the synthesis and atomic structure of the cluster-assembled [Au60Se2(Ph3P)10(SeR)15](+) material. Five icosahedral Au13 building blocks from a closed gold ring with Au-Se-Au linkages. Interestingly, two Se atoms (without the phenyl tail) locate in the center of the cluster, stabilized by the Se-(Au)5 interactions. The ring-like nanocluster is unprecedented in previous experimental and theoretical studies of gold nanocluster structures. In addition, our optical and electrochemical studies show that the electronic properties of the icosahedral Au13 units still remain unchanged in the penta-twinned Au60 nanocluster, and this new material might be a promising in optical limiting material. This work offers a basis for deep understanding on controlling the cluster-assembled materials for tailoring their functionalities.

Preparation of multi-coloured different sized fluorescent gold clusters from blue to NIR, structural analysis of the blue emitting Au7 cluster, and cell-imaging by the NIR gold cluster

Blue, green, orange-red, red and NIR emitting gold quantum clusters have been prepared in aqueous media by using a bioactive peptide glutathione (reduced) at physiological pH. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) analyses show that the core structure sizes of the five different gold clusters are Au7 (blue), Au16 (green), Au19 (orange-red), Au21 (red) and Au22 (NIR). The photo-stability and pH-stability of these quantum clusters have been measured, and these are photo-stable against continuous UV irradiation for a few hours. They also exhibit moderate to good pH-stability within the pH range of 5-12.5. A computational study reveals the organisation of gold atoms in the thiolate-protected blue quantum cluster and its several structural parameters, including the mode of interaction of ligand molecules with Au atoms in the Au7 cluster. Interestingly, it has been found that NIR emitting gold quantum cluster can easily be internalized into the adenocarcinomic human alveolar basal epithelial cell line (A549 cell line). Moreover, a MTT assay indicates that our NIR emitting gold quantum cluster show very low cytotoxicy to A549 cancer cells.

Ligand-induced change of the crystal structure and enhanced stability of the Au11 nanocluster

Compared with the Au₁₁(PPh₃)₇Cl₃ and [Au₁₁(PPh₃)₈Cl₂]Cl, [Au₁₁(PPh₂(CH₂)₅Ph₂P)₄(SePh)₂]⁺ exhibits some structural differences and shows significantly enhanced stability in storage and thiol etching.

Cluster–π electronic interaction in a superatomic Au13 cluster bearing σ-bonded acetylide ligands

Electronic coupling between a Au₁₃ superatom and CC π-units was experimentally evidenced by the absorption spectrum. The existence of such a cluster–π electronic interaction was supported by theoretical calculations.

Structural and bonding patterns in gold clusters

The structures of phosphine and thiolato- clusters of gold are summarised and a theoretical model which unites the broad structural properties is presented.

Crystal structure of Au₂₅(SePh)₁₈ nanoclusters and insights into their electronic, optical and catalytic properties

The crystal structure of selenolate-capped Au25(SePh)18(-) nanoclusters has been unambiguously determined for the first time, and provides a solid basis for a deeper understanding of the structure-property relationships. The selenolate-capped Au25 cluster shows noticeable differences from the previously reported Au25(SCH2CH2Ph)18(-) counterpart, albeit both share the icosahedral Au13 core and semi-ring Au2(SeR)3 or Au2(SR)3 motifs. Distinct differences in the electronic structure and optical, catalytic and electrochemical properties are revealed by the coupling experiments with density functional theory (TD-DFT) calculations. Overall, the successful determination of the Au25(SePh)18(-) structure removes any ambiguity about its structure, and comparison with the thiolated Au25 counterpart helps us to further understand how the ligands affect the properties of the nanocluster.

[Au7](3+): a missing link in the four-electron gold cluster family

Ligand-stabilized ultrasmall gold clusters offer a library of diverse geometrical and electronic structures. Among them, clusters with four valence electrons form an exceptional but interesting family because of their unique geometrical structures and optical properties. Here, we report a novel diphosphine-ligated four-electron Au7 cluster (2). In good agreement with previous theoretical predictions, 2 has a "core+one" structure to exhibit a prolate shape. The absorption spectrum showed an isolated band, similar to the spectra of Au6 and Au8 clusters with "core+two" structures. TD-DFT studies demonstrated that the attachment of only one gold atom to a polyhedral core is sufficient to generate unique electronic structures and characteristic absorptions. The present result fills the missing link between Au6 and Au8 in the four-electron cluster family, showing that the HOMO-LUMO gap increases with increasing nuclearity in the case of the tetrahedron-based "core+exo" clusters.

Crystal structure and electronic properties of a thiolate-protected Au24 nanocluster

Solving the total structures of gold nanoclusters is of critical importance for understanding their electronic, optical and catalytic properties. Herein, we report the X-ray structure of a charge-neutral Au24(SCH2Ph-(t)Bu)20 nanocluster. This structure features a bi-tetrahedral Au8 kernel protected by four tetrameric staple-like motifs. Electronic structure analysis is further carried out and the optical absorption spectrum is interpreted. The Au24(SCH2Ph-(t)Bu)20, Au23(S-c-C6H11)16 and Au25(SCH2CH2Ph)18 nanoclusters constitute the first crystallographically characterized "trio".