An Inherently Chiral Au24 Framework with Double-Helical Hexagold Strands

Cocrystallization-driven stabilization of metastable nanoclusters: a case study of Pd1Au9

The structural determination of metastable nanoclusters remains challenging, which impedes the in-depth understanding of their structural evolution. Herein, based on a case study of Pd1Au9, we present a "cocrystallization-driven stabilization" approach to stabilize the metastable nanocluster and then determine its atomically precise structure. The [Pd1Au9(TFPP)7Br2]+ nanocluster is unstable in solution and would spontaneously convert to Pd2Au23(TFPP)10Br7. The introduction of Au11(TFPP)7Br3 nanocluster to the crystallization process of [Pd1Au9(TFPP)7Br2]+ gives rise to the cocrystallized Pd1Au9(TFPP)6Br3@Au11(TFPP)7Br3, although the composition of Pd1Au9 changes from [Pd1Au9(TFPP)7Br2]+ to Pd1Au9(TFPP)6Br3 among this cocrystallization. With this approach, the overall structure of the metastable Pd1Au9 has been determined. Owing to the very similar cluster size and surface ligand environment between Au11 and Pd1Au9, the obtained Pd1Au9@Au11 cocrystal exhibits almost the same cell parameters as those of the single crystalized Au11. Overall, the proposed "cocrystallization-driven stabilization" approach hopefully sheds light on the structural determination of more metastable nanoclusters.

Enhanced Surface Ligands Reactivity of Metal Clusters by Bulky Ligands for Controlling Optical and Chiral Properties

Surface ligands play critical roles in determining the surface properties of metal clusters. However, modulating the properties and controlling the surface structure of clusters through surface-capping-agent displacement is challenging. Herein, [Ag₁₄ (SPh(CF₃ )₂ )₁₂ (PPh₃ )₄ (DMF)₄ ] (Ag₁₄ -DMF; DMF=N,N-dimethylformamide), with weakly coordinated DMF ligands on surface silver sites, was synthesized by a mixed-ligands strategy. Owing to the high surface reactivity of Ag₁₄ -DMF, the surface ligands are labile, easily dissociated or exchanged by other ligands. Based on the enhanced surface reactivity, easy modulation of the optical properties of Ag₁₄ by reversible "on-off" DMF ligation was realized. When chiral amines were introduced to as-prepared products, all eight surface ligands were replaced by amines and the racemic Ag₁₄ clusters were converted to optically pure homochiral Ag₁₄ clusters as evidenced by circular dichroism (CD) activity and single-crystal X-ray diffraction (SCXRD). This work provides a new insight into modulation of the optical properties of metal clusters and atomically precise homochiral clusters for specific applications are obtained.

Alkynyl-Stabilized Superatomic Silver Clusters Showing Circularly Polarized Luminescence

We report a new enantiomeric pair of superatomic silver clusters, R/S-Ag₁₇, prepared from chiral alkynyl ligands. R-Ag₁₇ and S-Ag₁₇ possess C₃ symmetry and emit near-infrared (NIR) light with a quantum yield (QY) of 8.0% under ambient condition as well as NIR circularly polarized luminescence (CPL) as a result of the chirality of the excited states. Both experiments and theoretical calculations indicate for the first time that the CPL originates from transitions between superatomic 1P_z (along the C₃ axis) and 1S orbitals. This work opens a new avenue for CPL-active metal nanoclusters by utilizing chiral alkynyl ligands and enlightens the chirality transfer from chiral protecting ligands to superatomic states in metal clusters.

Controlling the Crystallographic Packing Modes of Pt1Ag28 Nanoclusters: Effects on the Optical Properties and Nitrogen Adsorption-Desorption Performances

We herein report the manipulation of the crystallographic packing modes of Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ nanoclusters by altering counterions as different polyoxometalates (POMs). Specifically, the Cl^- anion of the presynthesized Pt₁Ag₂₈ nanocluster was substituted by POM anions including [Mo₆O₁₉]^2-, [W₆O₁₉]^2-, or [PW₁₂O₄₀]^3-. The crystal lattices of these Pt₁Ag₂₈ nanoclusters with diverse anions showed distinct packing modes and thus manifested remarkably distinguishable crystalline-state optical properties and nitrogen adsorption-desorption performances. Overall, the combination of intercluster control in this work and intracluster control reported previously (the control over metal-ligand within the nanocluster framework) accomplished a more comprehensive manipulation over the M₂₉(SR)₁₈(PR'₃)₄ nanocluster system, which enables us to further grasp the structure-property correlations at the atomic level.

Chiroptical activity of Au13 clusters: experimental and theoretical understanding of the origin of helical charge movements

Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au13 clusters, we show that these surface ligands slightly twist the Au13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores.

Enriching Structural Diversity of Alkynyl-Protected Gold Nanoclusters with Chlorides

The synthesis and isolation of alkynyl/chloride-protected gold nanoclusters is described. Silica gel column chromatography is effective in isolating gold nanoclusters from the as-synthesized cluster mixture to give the clusters Na[Au₂₅ L₁₈ ] (Au₂₅ ), [HNEt₃ ]₃ [Au₆₇ L₃₂ Cl₄ ] (Au₆₇ ), [HNEt₃ ]₄ [Au₁₀₆ L₄₀ Cl₁₂ ] (Au₁₀₆ ), L=3,5-bis(trifluoromethyl)-phenylacetylide. Au₆₇ and Au₁₀₆ are new clusters; the structures were determined by X-ray single-crystal diffraction. Au₆₇ contains a distorted Au₁₈ Marks decahedron shelled by an irregular Au₃₂ and further protected with two V-shaped Au₂ L₃ , 13 linear AuL₂ staples and 4 chlorides. Au₆₇ is the first structurally determined 34e superatomic gold nanocluster. Au₁₀₆ is composed of 106 Au atoms co-protected by alkynyls and chlorides. It has a Au₇₉ kernel, like in Au₁₀₂ (p-MBA)₄₄ . The surface structure of Au₁₀₆ includes 20 linear Au-alkynyl staples, 5 Cl-Au-Cl and 2 Cl-Au motifs. These three gold nanoclusters show size-dependent electrochemical properties.

Controllable synthesis and formation mechanism study of homoleptic alkynyl-protected Au nanoclusters: recent advances, grand challenges, and great opportunities

In the past decade, atomically precise coinage metal nanoclusters have been a subject of major interest in nanoscience and nanotechnology because of their determined compositions and well-defined molecular structures, which are beneficial for establishing structure-property relationships. Recently ligand engineering has been extended to alkynyl molecules. Homoleptic alkynyl-protected Au nanoclusters (Au NCs) have emerged as a hotspot of research interest, mainly due to their unique optical properties, molecular configuration, and catalytic functionalities, and more importantly, they are used as a counterpart object for fundamental study to compare with the well-established thiolate Au NCs. In this review, we first summarize the recently reported various controllable synthetic strategies for atomically precise homoleptic-alkynyl-protected Au NCs, with particular emphasis on the ligand exchange method, direct reduction of the precursor, one-pot synthesis, and the synchronous nucleation and passivation strategy. After that, we switch our focus to the formation mechanism and structure evolution process of homoleptic alkynyl-protected Au NCs, where Au144(PA)60 and Au36(PA)24 (PA = phenylacetylide) are given as examples, along with the prediction of the possible formation mechanism of some other cluster molecules. In the end of this review, the outlook and perspective of this rapidly developing field including grand challenges and great opportunities are discussed. This review can stimulate more research efforts towards developing new synthetic strategies to enrich the limited examples and unravel the formation/growth mechanism of homoleptic alkynyl-protected Au NCs.

Assembly-Induced Strong Circularly Polarized Luminescence of Spirocyclic Chiral Silver(I) Clusters

Spirocyclic Ag₉ clusters, as a new form of intrinsically chiral metal clusters, were constructed through vertex-sharing of two in-situ-generated heteroaryl diide-centered metal rings. Such core-peripheral type clusters exhibit versatile photoluminescent and chiroptical behavior under different aggregation conditions. In contrast to a ligand-based fluorescence emission in a diluted solution of the clusters, a solvent polarity-caused assembly gives rise to new cluster-based phosphorous luminescence owing to radiative mode switching and aggregation-induced emission. Assembly of cluster enantiomers leads to micrometer-long helical nanofibers, whose handedness is determined by absolute configuration of individual spirocyclic clusters. Benefiting from exciton couplings of helical arrangements of chelating ligands at molecular and microscopic levels, the assembled film of cluster enantiomers exhibits circularly polarized luminescence with a high anisotropy factor (0.16).

Trinuclear cationic silver nanoclusters based-on bis-(phosphine) ligands and stabilized by CF3SO3− anions

Three trinuclear cationic silver nanoclusters based-on bis-(phosphine) ligands and stabilized by CF₃SO₃⁻ anions, displayed excellent photocurrent responses and electrochemical properties.

Structural determination of a metastable Ag27 nanocluster and its transformations into Ag8 and Ag29 nanoclusters

The atomically precise structure of a metastable nanocluster, Ag27H11(SPhMe2)12(DPPM)6, was determined, and its transformations into size-reduction Ag8 and size-growth Ag29 nanoclusters have been mapped out.

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.

Hierarchical structural complexity in atomically precise nanocluster frameworks

The supramolecular chemistry of nanoclusters is a flourishing area of nano-research; however, the controllable assembly of cluster nano-building blocks in different arrays remains challenging. In this work, we report the hierarchical structural complexity of atomically precise nanoclusters in micrometric linear chains (1D array), grid networks (2D array) and superstructures (3D array). In the crystal lattice, the Ag29(SSR)12(PPh3)4 nanoclusters can be viewed as unassembled cluster dots (Ag29–0D). In the presence of Cs+ cations, the Ag29(SSR)12 nano-building blocks are selectively assembled into distinct arrays with different oxygen-carrying solvent molecules―Cs@Ag29(SSR)12(DMF)x as 1D linear chains (Ag29–1D), Cs@Ag29(SSR)12(NMP)x as 2D grid networks (Ag29–2D), and Cs@Ag29(SSR)12(TMS)x as 3D superstructures (Ag29–3D). Such self-assemblies of these Ag29(SSR)12 units have not only been observed in their crystalline state, but also in their amorphous state. Due to the diverse surface structures and crystalline packing modes, these Ag29-based assemblies manifest distinguishable optical absorptions and emissions in both solutions and crystallized films. Furthermore, the surface areas of the nanocluster crystals are evaluated, the maximum value of which occurs when the cluster nano-building blocks are assembled into 2D arrays (i.e. Ag29–2D). Overall, this work presents an exciting example of the hierarchical assembly of atomically precise nanoclusters by simply controlling the adsorbed molecules on the cluster surface.

Rendering hydrophobic nanoclusters water-soluble and biocompatible

Hydrophobic and hydrophilic nanoclusters embody complementary superiorities. The means to amalgamate these superiorities, i.e., the atomic precision of hydrophobic clusters and the water dissolvability of hydrophilic clusters, remains challenging. This work presents a versatile strategy to render hydrophobic nanoclusters water-soluble-the micellization of nanoclusters in the presence of solvent-conjoined Na+ cations-which overcomes the above major challenge. Specifically, although [Ag29(SSR)12(PPh3)4]3- nanoclusters are absolutely hydrophobic, they show good dissolvability in aqueous solution in the presence of solvent-conjoined Na+ cations (Na1(NMP)5 or Na3(DMF)12). Such cations act as both counterions of these nanoclusters and surface cosolvent of cluster-based micelles in the aqueous phase. A combination of DLS (dynamic light scattering) and aberration-corrected HAADF-STEM (high angle annular dark field detector scanning transmission electron microscopy) measurements unambiguously shows that the phase-transfer of hydrophobic Ag29 into water is triggered by the micellization of nanoclusters. Owing to the excellent water solubility and stability of [Ag29(SSR)12(PPh3)4]3-[Na1(NMP)5]3 + in H2O, its performance in cell staining has been evaluated. Furthermore, the general applicability of the micellization strategy has been verified. Overall, this work presents a convenient and efficient approach for the preparation of cluster-based, biocompatible nanomaterials.

Ag18(μ8-S)(p-TBBT)16(PPh3)8: symmetry breaking induced by the core to generate chirality

A chiral silver-sulfur nanocluster with a μ₈-S inside, Ag₁₈(μ₈-S)(p-TBBT)₁₆(PPh₃)₈, was successfully synthesized and characterized. This nanocluster is composed of a crown-like Ag₈S core and an Ag₁₀S₁₆P₈ meso-shell. The presence of the core induces symmetry breaking of the nanocluster from S₈ to C₄, and therefore endows the cluster with chirality.

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.

Oxometalate and phosphine ligand co-protected silver nanoclusters: Ag28(dppb)6(MO4)4 and Ag32(dppb)12(MO4)4(NO3)4

Thiols, alkynyls and phosphines are the most widely used organic ligands to attain atomically precise metal nanoclusters, while oxometalates as inorganic ligands have almost been neglected in this field. Here, we used oxometalates (e.g., MoO42- and WO42-) as protecting ligands along with phosphines, such as 1,4-bis(diphenylphosphino)butane (dppb), to design and synthesize a new class of silver nanoclusters including Ag28(dppb)6(MoO4)4, Ag28(dppb)6(WO4)4 and Ag32(dppb)12(MoO4)4(NO3)4. Each cluster consists of a two-shell Ag4@Ag24 core protected by 4 oxometalates. These clusters exhibit similar optical absorption and photoluminescence properties that are not dependent on surface ligands. Furthermore, the electronic structure analysis shows that the clusters are 20-electron "superatoms". This work demonstrates that oxometalates can play a key role in the formation of silver nanoclusters, and the effect of oxometalates should be considered in the design and synthesis of metal nanoclusters.

Stereospecific interactions between chiral inorganic nanomaterials and biological systems

Chirality is ubiquitous in nature and plays mysterious and essential roles in maintaining key biological and physiological processes.

Nanocluster growth via "graft-onto": effects on geometric structures and optical properties

The concept of “graft-onto” has been exploited to facilitate nanocluster growth from Pt₁Ag₂₈ to Pt₁Ag₃₁.

Atomically precise engineering on the nanocluster surface remains highly desirable for the fundamental understanding of how surface structures of a nanocluster contribute to its overall properties. In this paper, the concept of “graft-onto” has been exploited to facilitate nanocluster growth on surface structures. Specifically, the Ag₂(DPPM)Cl₂ complex is used for re-constructing the surface structure of Pt₁Ag₂₈(SR)₁₈(PPh₃)₄ (Pt₁Ag₂₈, SR = 1-adamantanethiolate) and producing a size-growth nanocluster – Pt₁Ag₃₁(SR)₁₆(DPPM)₃Cl₃ (Pt₁Ag₃₁). The grafting effect of Ag₂(DPPM)Cl₂ induces both direct changes on the surface structure (e.g., size growth, structural transformation, and surface rotation) and indirect changes on the kernel structure (from a fcc configuration to an icosahedral configuration). Remarkable differences have been observed by comparing optical properties between Pt₁Ag₂₈ and Pt₁Ag₃₁. Significantly, Pt₁Ag₃₁ exhibits high photo-luminescent intensity with a quantum yield of 29.3%, which is six times that of the Pt₁Ag₂₈. Overall, this work presents a new approach (i.e., graft-onto) for the precise dictation of nanocluster surface structures at the atomic level.

Gram-Scale Preparation of Stable Hydride M@Cu24 (M = Au/Cu) Nanoclusters

The instability of phosphine ligated copper hydride nanoclusters (CuH NCs) has largely limited their application in areas such as H₂ storage, CO₂ reduction, etc. In this work, the stability of CuH NCs was remarkably enhanced by improving their antioxidant capacity through two different approaches: (i) metal doping and (ii) ligand modification. Three NCs, AuCu₂₄H₂₂(PPh₃)₁₂, Cu₂₅H₂₂((p-FPh)₃P)₁₂, and AuCu₂₄H₂₂((p-FPh)₃P)₁₂, were controllably synthesized, and their structures were determined by single-crystal X-ray diffraction. The compositions of these NCs were further confirmed by electrospray ionization mass spectrometry and nuclear magnetic resonance. More importantly, we achieved gram-level production of M@Cu₂₄ (M = Cu/Au) NCs protected by electron-withdrawing ligands (p-FPh)₃P, which in turn proved their superior stability; such a large-scale preparation laid the foundation for future explorations of copper-rich NCs. This work hopes to shed light on large-scale generation of ultrastable Cu-based NCs.

Metal Nanoclusters Stabilized by Selenol Ligands

The past decades have witnessed great advances in controllable synthesis, structure determination, and property investigation of metal nanoclusters. Selenolated nanoclusters, a special branch in the nanocluster family, have attracted great interest in these years. The electronegativity and atomic radius of selenium is different from sulfur, and thus the selenolated nanoclusters are anticipated to display different electronic/geometric structures and distinct chemical/physical properties relative to their thiolated analogues. This review covers the syntheses, structures, and properties of selenolated nanoclusters (including Au, Ag, Cu, and alloy nanoclusters). Ligand effects (between SeR and SR) on nanocluster properties, including optical absorption, stability, and electrochemical properties, are disclosed as well. At the end of the review, a scope for improvements and future perspectives of selenolated nanoclusters is highlighted. The review hopefully opens up new horizons for cluster scientists to synthesize more selenolated nanoclusters with novel structures and properties. This review is based on publications available up to May 2019.

Solvent-triggered reversible interconversion of all-nitrogen-donor-protected silver nanoclusters and their responsive optical properties

Surface organic ligands are critical in determining the formation and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, the amine ligand dipyridylamine is applied here as a protecting agent in the synthesis of atomically precise metal nanoclusters. We report two homoleptic amido-protected Ag nanoclusters as examples of all-nitrogen-donor-protected metal nanoclusters: [Ag₂₁(dpa)₁₂]SbF₆ (Ag₂₁) and [Ag₂₂(dpa)₁₂](SbF₆)₂ (Ag₂₂) (dpa = dipyridylamido). Single crystal X-ray structural analysis reveals that both clusters consist of a centered-icosahedron Ag₁₃ core wrapped by 12 dpa ligands. The flexible arrangement of the N donors in dpa facilitates the solvent-triggered reversible interconversion between Ag₂₁ and Ag₂₂ due to their very different solubility. The successful use of dpa in the synthesis of well-defined silver nanoclusters may motivate more studies on metal nanoclusters protected by amido type ligands.

Noble metal nanoclusters are commonly protected by thiolate, phosphine, or alkynyl ligands. Here, the authors synthesize two homoleptic amido-protected silver clusters, whose structures interconvert easily with changes of solvent due to the coordination flexibility and diverse binding modes of the nitrogen-donor ligands.

Rational construction of a library of M29 nanoclusters from monometallic to tetrametallic

Exploring intermetallic synergy has allowed a series of alloy nanoparticles with prominent chemical-physical properties to be produced. However, precise alloying based on a maintained template has long been a challenging pursuit, and little has been achieved for manipulation at the atomic level. Here, a nanosystem based on M₂₉(S-Adm)₁₈(PPh₃)₄ (where S-Adm is the adamantane mercaptan and M is Ag/Cu/Au/Pt/Pd) has been established, which leads to the atomically precise operation on each site in this M₂₉ template. Specifically, a library of 21 species of nanoclusters ranging from monometallic to tetrametallic constitutions has been successfully prepared step by step with in situ synthesis, target metal-exchange, and forced metal-exchange methods. More importantly, owing to the monodispersity of each nanocluster in this M₂₉ library, the synergetic effects on the optical properties and stability have been mapped out. This nanocluster methodology not only provides fundamental principles to produce alloy nanoclusters with multimetallic compositions and monodispersed dopants but also provides an intriguing nanomodel that enables us to grasp the intermetallic synergy at the atomic level.

Free Valence Electron Centralization Strategy for Preparing Ultrastable Nanoclusters and Their Catalytic Application

Metal nanoclusters have attracted extensive interests owing to their atomically precise structures as well as intriguing properties. However, silver nanoclusters are not as stable as their gold counterparts, impeding the practical applications of Ag nanoclusters. In this work, a strategy of free valence electron centralization was exploited to render parent Ag nanoclusters highly stable. The stability of Ag₂₉(SSR)₁₂(PPh₃)₄ (SSR: benzene-1,3-dithiol) was controllably enhanced by stepwisely alloying the Ag₂₉ nanocluster to Ag₁₇Cu₁₂(SSR)₁₂(PPh₃)₄ and Au₁Ag₁₆Cu₁₂(SSR)₁₂(PPh₃)₄. Specifically, the trimetallic Au₁Ag₁₆Cu₁₂ is ultrastable even at 175 °C, which is close to the nanocluster decomposition temperature. The structures of Ag₁₇Cu₁₂ and Au₁Ag₁₆Cu₁₂ nanoclusters are determined by single-crystal X-ray diffraction. Furthermore, a combination of X-ray photoelectron spectroscopy measurements and density functional theory calculations demonstrates that the enhanced stability is induced by the centralization of the free valence electrons to the interior of the nanocluster. More importantly, the Au₁Ag₁₆Cu₁₂ enables the multicomponent A³ coupling reaction at high temperatures, which remarkably shortens the catalytic reaction time from ∼5 h to 3 min. Overall, this work presents a strategy for enhancing the thermal stability of nanoclusters via centralizing the free valence electrons to the nanocluster kernels.

Reversible nanocluster structure transformation between face-centered cubic and icosahedral isomers

The reversible transformation between a FCC and icosahedral configuration has been achieved at the atomic level, based on Pt₁Ag₂₈ nanocluster isomers.

Structural transformations between isomers of nanoclusters provide a platform to tune their properties and understand the fundamental science due to their intimate structure–property correlation. Herein, we demonstrate a reversible transformation between the face-centered cubic (FCC) and icosahedral isomers of Pt₁Ag₂₈ nanoclusters accomplished in the ligand-exchange processes. Ligand-exchange of 1-adamantanethiolate protected Pt₁Ag₂₈ by cyclohexanethiolate could transform the FCC kernel to the icosahedral isomer. Interestingly, the icosahedral Pt₁Ag₂₈ could be reversibly transformed to the FCC configuration when the cyclohexanethiolate ligand is replaced again by 1-adamantanethiolate. A combination of UV-vis absorption, mass spectrometry, photo-luminescence and X-ray absorption fine structure unambiguously identifies that the FCC-to-icosahedral structure transformation of Pt₁Ag₂₈ involves two distinct stages: (i) ligand-exchange induced outmost motif transformation and (ii) abrupt innermost kernel transformation. As a result of this structural transformation, the emission wavelength of Pt₁Ag₂₈ red-shifts from 672 to 720 nm, and the HOMO–LUMO energy gap reduces from 1.86 to 1.74 eV. This work presents the first example of nanocluster isomers with inter-switching configurations, and will provide new insights into manipulating the properties of nanoclusters through controllably tuning their structures.

Cations Controlling the Chiral Assembly of Luminescent Atomically Precise Copper(I) Clusters

Chiral assembly and asymmetric synthesis are critically important for the generation of chiral metal clusters with chiroptical activities. Here, a racemic mixture of [K(CH₃ OH)₂ (18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (1⋅CH₃ OH) in the chiral space group was prepared, in which the chiral red-emissive anionic [Cu₅ (S^t Bu)₆ ]^- cluster was arranged along a twofold screw axis. Interestingly, the release of the coordinated CH₃ OH of the cationic units turned the chiral 1⋅CH₃ OH crystal into a mesomeric crystal [K(18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (1), which has a centrosymmetric space group, by adding symmetry elements of glide and mirror planes through both disordered [Cu₅ (S^t Bu)₆ ]^- units. The switchable chiral/achiral rearrangement of [Cu₅ (S^t Bu)₆ ]^- clusters along with the capture/release of CH₃ OH were concomitant with an intense increase/decrease in luminescence. We also used cationic chiral amino alcohols to induce the chiral assembly of a pair of enantiomers, [d/l-valinol(18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (d/l-Cu_5V ), which display impressive circularly polarized luminescence (CPL) in contrast to the CPL-silent racemic mixture of 1⋅CH₃ OH and mesomeric 1.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

Hierarchical multi-shell 66-nuclei silver nanoclusters trapping subvalent Ag6 kernels

Hierarchical nano structures are hard to fabircate. Here, we present three novel hierarchical multi-shell 66-nuclei silver nanoclusters, trapping ultrasmall Ag64+ nano-fragments by nine MoO42- ions. This Ag6@(MoO4)9 core is further wrapped by an outer Ag60 shell. The Ag6 kernel evolves from reduction involving DMF solvent. Carboxylate ligands are very important in the modulation of the polygon patterns on the Ag60 shell.

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.

Toward Solution Syntheses of the Tetrahedral Au20 Pyramid and Atomically Precise Gold Nanoclusters with Uncoordinated Sites

A long-standing objective of cluster science is to discover highly stable clusters and to use them as models for catalysts and building blocks for cluster-assembled materials. The discovery of catalytic properties of gold nanoparticles (AuNPs) has stimulated wide interests in gaseous size-selected gold clusters. Ligand-protected AuNPs have also been extensively investigated to probe their size-dependent catalytic and optical properties. However, the need to remove ligands can introduce uncertainties in both the structures and sizes of ligand-protected AuNPs for catalytic applications. Ideal model catalysts should be atomically precise AuNPs with well-defined structures and uncoordinated surface sites as in situ active centers. The tetrahedral ( T_d) Au₂₀ pyramidal cluster, discovered to be highly stable in the gas phase, provided a unique opportunity for such an ideal model system. The T_d-Au₂₀ consists of four Au(111) faces with all its atoms on the surface. Bulk synthesis of T_d-Au₂₀ with appropriate ligands would allow its catalytic and optical properties to be investigated and harnessed. The different types of its surface atoms would allow site-specific chemistry to be exploited. It was hypothesized that if the four corner atoms of T_d-Au₂₀ were coordinated by ligands the cluster would still contain 16 uncoordinated surface sites as potential in situ catalytically active centers. Phosphine ligands were deemed to be suitable for the synthesis of T_d-Au₂₀ to maintain the integrity of its pyramidal structure. Triphenyl-phosphine-protected T_d-Au₂₀ was first observed in solution, and its stability was confirmed both experimentally and theoretically. To enhance the synthetic yield, bidentate diphosphine ligands [(Ph)₂P(CH₂) _nP(Ph)₂ or L ⁿ] with different chain lengths were explored. It was hypothesized that diphosphine ligands with the right chain length might preferentially coordinate to the T_d-Au₂₀. Promising evidence was initially obtained by the formation of the undecagold by the L³ ligand. When the L⁸ diphosphine ligand was used, a remarkable Au₂₂ nanocluster with eight uncoordinated Au sites, Au₂₂(L⁸)₆, was synthesized. With a tetraphosphine-ligand (PP₃), a new Au₂₀ nanocluster, [Au₂₀(PP₃)₄]Cl₄, was isolated with high yield. The crystal structure of the new Au₂₀ core did not reveal the expected pyramid but rather an intrinsically chiral gold core. The surface of the new chiral-Au₂₀ was fully coordinated, and it was found to be highly stable chemically. The Au₂₂(L⁸)₆ nanocluster represents the first and only gold core with uncoordinated gold atoms, providing potentially eight in situ catalytically active sites. The Au₂₂ nanoclusters dispersed on oxide supports were found to catalyze CO oxidation and activate H₂ without ligand removal. With further understanding about the formation mechanisms of gold nanoclusters in solution, it is conceivable that T_d-Au₂₀ can be eventually synthesized, allowing its novel catalytic and optical properties to be explored. More excitingly, it is possible that a whole family of new atomically precise gold nanoclusters can be created with different phosphine ligands.

An alkynyl-protected Au40 nanocluster featuring PhCC–Au–P^P motifs

An alkynyl-protected gold nanocluster [Au₄₀(PhCC)₂₀(dppm)₄](SbF₆)₄ (dppm = bis(diphenylphosphino)methane) (1) has been synthesized.