Chiroptical Activity Enhancement via Structural Control: The Chiral Synthesis and Reversible Interconversion of Two Intrinsically Chiral Gold Nanoclusters

Liquid crystal-mediated self-assembly of copper nanoclusters with induced circular dichroism and amplified circularly polarized luminescence

Elucidating the mechanism of chiral transfer is key to regulating chiral expression and generalizing the structure-property relationship of chiral functional systems. However, it is still an important challenge to select novel building blocks to achieve chiral induction, chiral transfer and chiral modulation. Liquid crystals (LCs) can be considered as promising smart soft materials due to their responsiveness and adaptability. Confining chiral metal nanoclusters (NCs) in an achiral LC phase to construct chiral LCs provides an expanded strategy for the self-assembly of chiral metal NCs in different matrices. Herein, chiral glutathione-stabilized copper NCs (G-SH-Cu NCs)/polyoxyethylene tert-octylphenyl ether (TX-100) LCs were constructed and systematically investigated. The results showed that the introduction of G-SH-Cu NCs into TX-100 LCs induced the generation of supramolecular chirality. More interestingly, the circular dichroism (CD) handedness can be controlled by changing the amount of TX-100 or G-SH-Cu NCs; when the ratio of G-SH-Cu NCs and TX-100 was proportionally matched, the strength of the noncovalent interactions was sufficient to induce chiral inversion. Meanwhile, TX-100 LCs provide effective confinement of G-SH-Cu NCs, which improves the expression of asymmetry at the aggregation level and induces a 2-fold enhancement of the circularly polarized luminescence (CPL) signal. This work realizes the spatial amplification of chirality through dopants in LCs, which provides an effective method for accurately regulating the supramolecular chirality of metal NCs in the LC phase.

Chiral Separation of Copper Sulfide [S-Cu36] Nanocluster Using a Chiral Adaptive Counterion

This work presents a new strategy to achieve the growth of copper sulfide nanoclusters with high nuclearity. Through a phosphine-assisted C-S reductive cleavage approach, an intrinsically chiral [Cu4] cluster passes through a [S-Cu9] cluster and transforms into a higher-nuclearity [S-Cu36] cluster, which features a core-shell structure with a [Cu4]4+ core encapsulated by a chiral [Cu20S12] shell. Interestingly, the spiral arrangement of the bidental ligands on the surface of the [S-Cu36] cluster leads to the L-/R-enantiomeric configurations. Moreover, by utilization of [Na(THF)6]+ as a chiral adaptive counterion, [S-Cu36] can be interlocked separately, thus enabling the isolation of homochiral clusters. Theoretical calculation suggests that the configuration transition between two enantiomeric [Na(THF)6]+ species is favorable at room temperature, thereby promoting the cocrystallization of resulting chiral products. This study introduces a novel perspective on the synthesis of chiral copper sulfide nanoclusters and presents an innovative approach to achieving the chiral separation of nanoclusters.

Interplay of kernel shape and surface structure for NIR luminescence in atomically precise gold nanorods

It is challenging to attain strong near-infrared (NIR) emissive gold nanoclusters. Here we show a rod-shaped cluster with the composition of [Au28(p-MBT)14(Hdppa)3](SO3CF3)2 (1 for short, Hdppa is N,N-bis(diphenylphosphino)amine, p-MBT is 4-methylbenzenethiolate) has been synthesized. Single crystal X-ray structural analysis reveals that it has a rod-like face-centered cubic (fcc) Au22 kernel built from two interpenetrating bicapped cuboctahedral Au15 units. 1 features NIR luminescence with an emission maximum at 920 nm, and the photoluminescence quantum yield (PLQY) is 12%, which is 30-fold of [Au21(m-MBT)12(Hdppa)2]SO3CF3 (2, m-MBT is 3-methylbenzenethiolate) with a similar composition and 60-fold of Au30S(S‑t‑Bu)18 with a similar structure. time-dependent DFT(TDDFT)calculations reveal that the luminescence of 1 is associated with the Au22 kernel. The small Stokes shift of 1 indicates that it has a very small excited state structural distortion, leading to high radiative decay rate (kr) probability. The emission of cluster 1 is a mixture of phosphorescence and thermally activated delayed fluorescence(TADF), and the enhancement of the NIR emission is mainly due to the promotion of kr rather than the inhibition of knr. This work demonstrates that the metal kernel and the surface structure are both very important for cluster-based NIR luminescence materials.

Atomically Precise Chiral Metal Nanoclusters for Circularly Polarized Light Detection

Circularly polarized light (CPL) detection is of great significance in various applications such as drug identification, sensing and imaging. Atomically precise chiral metal nanoclusters with intense circular dichroism (CD) signals are promising candidates for CPL detection, which can further facilitate device miniaturization and integration. Herein, we report the preparation of a pair of optically active chiral silver nanoclusters [Ag7(R/S-DMA)2(dpppy)3] (BF4)3 (R/S-Ag7) for direct CPL detection. The crystal structure and molecular formula of R/S-Ag7 clusters are confirmed by single-crystal X-ray diffraction and high-resolution mass spectrometry. R/S-Ag7 clusters exhibit strong CD spectra and CPL both in solution and solid states. When used as the photoactive materials in photodetectors, R/S-Ag7 enables effective discrimination between left-handed circularly polarized and right-handed circularly polarized light at 520 nm with short response time, high responsivity and considerable discrimination ratio. This study is the first report on using atomically precise chiral metal nanoclusters for CPL detection.

Exploring Ligand Effects on Structure, Bonding, and Photolytic Hydride Transfer of Cationic Gold(I) Bridging Hydride Complexes of Molybdocene and Tungstenocene

A diverse family of heterobimetallic bridging hydride adducts of the type [LAu(μ-H)2MCp2][X] (L = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, IPr; 1,3-bis(1-adamantyl)imidazole-2-ylidene, IAd; 1,3-bis(2,6-di-iso-propylphenyl)-5,5-dimethyl-4,6-diketopyrimidinyl-2-ylidene, DippDAC; triphenylphosphine, PPh3; 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl, tBuXPhos; X = SbF6-, BF4- or TfO-) was synthesized by reacting group VI metallocene dihydrides Cp2MH2 (Cp = cyclopentadienyl anion; M = Mo, W) with cationic gold(I) complexes [LAu(NCMe)][X]. Trimetallic [L'Au2(μ-H)2WCp2][X]2 and tetrametallic [L'Au2{(μ-H)2WCp2}2] [X]2 complexes (L' = rac-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene or bis(diphenylphosphinomethane)) were obtained by reacting digold [L'{Au(NCMe)}2][X]2 with Cp2WH2 in a 1:1 and a 1:2 stoichiometry. Accessing such a broad structural diversity allowed us to pinpoint roles played by the ancillary ligands and group VI metals on the bonding properties of this family of bridging hydrides. In particular, a clear effect of the ligand on the interaction energy and electronic structure was observed, with important implications on photolytic reactivity. UV or visible light irradiation, indeed, leads to the selective cleavage of the heterobimetallic Au(μ-H)2M arrangement and formation of molecular gold hydrides. The photolysis was found to be chromoselective (wavelength-dependent), which can be ascribed to different charge redistributions upon excitation to the first (Kasha's reactivity) and higher (anti-Kasha's reactivity) excited states.

Enantioselective Synthesis of Homochiral Hierarchical Nd8Fe3-Oxo Cluster from Racemic Nd9Fe2-Oxo Cluster

Enantioselective synthesis of homochiral rare earth clusters is still a great challenge. In this work, we developed an efficient "cluster to cluster" approach, that is, a pair of enantiomerical R/S-{Nd8Fe3}-oxo clusters were successfully obtained from the presynthesized racemic {Nd9Fe2}-oxo cluster. R/S-hydrobenzoin ligands trigger the transformation of the pristine clusters by an SN2-like mechanism. Compared to the pristine cluster with an achiral core, the new cluster exhibits hierarchical chirality, from ligand chirality to interface chirality, then to helix chirality, and finally to supramolecular double helix chirality. The spectral experiments monitored the transformation and confirmed distinctly structure-related optical activity. The enantiomeric pure cluster also exhibits a potential asymmetric catalytic activity.

Chiral Ligand-Concentration Mediating Asymmetric Transformations of Silver Nanoclusters: NIR-II Circularly Polarized Phosphorescence Lighting

Near infrared (NIR) emitter with circularly polarized phosphorescence (CPP), known as NIR CPP, has emerged as a key part in the research of cutting-edge luminescent materials. However, it remains a challenge to obtain nanoclusters with NIR CPP activity. Here, we propose an asymmetric transformation approach to efficiently synthesize two pairs of chiral silver nanoclusters (R/S-Ag29 and R/S-Ag16) using an achiral Ag10 nanocluster as starting material in the presence of different concentration chiral inducer (R/S)-1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (R/S-BNP). R/S-Ag29, formed in the low-concentration R/S-BNP, exhibits a unique kernel-shell structure consisting of a distorted Ag13 icosahedron and an integrated cage-like organometallic shell with a C3 symmetry, and possesses a superatomic 6-electron configuration (1S2|1P4). By contrast, R/S-Ag16, formed in the high-concentration R/S-BNP, features a sandwich-like pentagram with AgI-pure kernel. Profiting from the hierarchically chiral structures and superatomic kernel-dominated phosphorescence, R/S-Ag29 exhibits infrequent CPP activity in the second near-infrared (975 nm) region, being the first instance of NIR-II CPP observed among CPL-active metal nanoclusters. This study presents a new approach to reduce the difficulty of de novo synthesis for chiral silver nanomaterials, and facilitates the design of CPP-active superatomic nanoclusters in NIR region.

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.

Chiroptical Activity Amplification of Chiral Metal Nanoclusters via Surface/Interface Solidification

It remains a grand challenge to amplify the chiroptical activity of chiral metal nanoclusters (NCs) although it is desirable for fundamental research and practical application. Herein, we report a strategy of surface/interface solidification (SIS) for enhancing the chiroptical activity of gold NCs. Structural analysis of [Au19(2R,4R/2S,4S-BDPP)6Cl2]3+ (BDPP is 2,4-bis(diphenylphosphino)pentane) clusters reveals that one of the interfacial gold atoms is flexible between two sites and large space is present on the surface, thus hampering chirality transfer from surface chiral ligands to metal core and leading to low chiroptical activity. Following SIS by filling the flexible sites and replacing chlorides with thiolate ligands affords another pair of [Au20(2R,4R/2S,4S-BDPP)6(4-F-C6H4S)2]4+, which shows a more compact and organized structure and thus an almost 40-fold enhancement of chiroptical activity. This work not only provides an efficient approach for amplifying the chiroptical activity of metal nanoclusters but also highlights the significance of achiral components in shaping chiral nanostructures.

N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters

This perspective highlights advances in the preparation and understanding of metal nanoclusters stabilized by organic ligands with a focus on N-heterocyclic carbenes (NHCs). We demonstrate the need for a clear understanding of the relationship between NHC properties and their resulting metal nanocluster structure and properties. We emphasize the importance of balancing nanocluster stability with the introduction of reactive sites for catalytic applications and the importance of a better understanding of how these clusters interact with their environments for effective use in biological applications. The impact of atom-scale simulations, development of atomic interaction potentials suitable for large-scale molecular dynamics simulations, and a deeper understanding of the mechanisms behind synthetic methods and physical properties (e.g., the bright fluorescence displayed by many clusters) are emphasized.

Chiral Hydride Cu18 Clusters Transform to Superatomic Cu15Ag4 Clusters: Circularly Polarized Luminescence Lighting

The manipulation of metal cluster enantiomers and their reconstruction remain challenging. Here, for the first time, we report an enantiomeric pair of hydride copper clusters [Cu18H(R/S-PEA)12](BF4)5 (R/S-Cu18H) made using designed chiral ligands. By manipulation of R/S-Cu18H with Ag+ ions, H- ions are released, leading to the reconstruction of 15 Cu atoms. Moreover, 4 Ag atoms replaced Cu atoms at the specific sites, resulting in the formation of homochiral [Cu15Ag4(R/S-PEA)12](BF4)5 (R/S-Cu15Ag4) with an isomorphic metal skeleton. This process was accompanied by a reduction reaction generating two free valence elections in the chiral alloying counterparts, which displayed orange emission. The solid-state R/S-Cu15Ag4 exhibited a photoluminescence quantum yield of 7.02% and excellent circularly polarized luminescence. The chiral transformations were resolved by single-crystal X-ray diffraction. The development of chiral copper hydride precursor-based metal clusters with chiroptical activities holds tremendous promise for advancing the field of optoelectronics and enabling new applications in lighting, displays, and beyond.

DppfCuBH4: new reducing agents for the synthesis of ferrocene-functionalized metal nanoclusters

A facile synthesis of atomically precise metal nanoclusters, especially those decorated with functional groups, is the prerequisite for finding applications in special fields and studying structure-and-property relationships. The exploration of simple and efficient synthetic prototypes for introducing functional ligands (such as ferrocene) into cluster moieties is thus of high interest. In this work, a type of reducing agent of dppfCuBH₄ (dppf is 1,1'-bis(diphenyphosphino)ferrocene) is introduced for the first time to prepare ferrocene-functionalized metal nanoclusters. Two new clusters of [Ag₂₅Cu₄(dppf)₆(3-F-PhCC)₁₂Cl₆]³⁺ (1) and [Ag₄(dppf)₅Cl₂]²⁺ (2) have been obtained from the simple synthetic method. The two compounds have been fully characterized by advanced techniques of electrospray ionization mass spectroscopy (ESI-MS), nuclear magnetic resonance (NMR), and ultraviolet-visible spectroscopy (UV-Vis). The total structure of the clusters, as determined by X-ray single-crystal diffraction, describes the Ag₁₃@Ag₁₂Cu₄(dppf)₆(3-F-PhCC)₁₂Cl₆ core-shell structure of 1 and [Ag₂Cl(dppf)₂]⁺-dppf-[Ag₂Cl(dppf)₂]⁺ polymeric structure of 2. This work opens the door to employing dppfCuBH₄ as a functional reducing agent to discover many underlying metal nanoclusters and even other nanomaterials which feature ferrocene-groups.

Asymmetric transformation of achiral gold nanoclusters with negative nonlinear dependence between chiroptical activity and enantiomeric excess

The investigation of chirality at the nanoscale is important to bridge the gap between molecular and macroscopic chirality. Atomically precise metal nanoclusters provide an ideal platform for this research, while their enantiopure preparation poses a challenge. Here, we describe an efficient approach to enantiopure metal nanoclusters via asymmetric transformation, that is, achiral Au₂₃(SC₆H₁₁)₁₆ nanoclusters are converted into chiral and enantiopure Au₂₄(L)₂(SC₆H₁₁)₁₆ nanoclusters by a chiral inducer phosphoramidite (L). Two enantiomers of Au₂₄(L)₂(SC₆H₁₁)₁₆ are obtained and the crystal structures reveal their hierarchical chirality, which originates from the two introduced chiral L molecules, the transformation-triggered asymmetric rearrangement of the staple motifs on the surface of the gold core, and the helical arrangement of nanocluster molecules. The construction of this type of enantiomerically pure nanoclusters is achieved based on the easy-to-synthesize and modular L. Lastly, the chirality-related chiroptical performance was investigated, revealing a negative nonlinear CD-ee dependence.

Ultrasmall Coinage Metal Nanoclusters as Promising Theranostic Probes for Biomedical Applications

Ultrasmall coinage metal nanoclusters (NCs, <3 nm) have emerged as a novel class of theranostic probes due to their atomically precise size and engineered physicochemical properties. The rapid advances in the design and applications of metal NC-based theranostic probes are made possible by the atomic-level engineering of metal NCs. This Perspective article examines (i) how the functions of metal NCs are engineered for theranostic applications, (ii) how a metal NC-based theranostic probe is designed and how its physicochemical properties affect the theranostic performance, and (iii) how metal NCs are used to diagnose and treat various diseases. We first summarize the tailored properties of metal NCs for theranostic applications in terms of biocompatibility and tumor targeting. We focus our discussion on the theranostic applications of metal NCs in bioimaging-directed disease diagnosis, photoinduced disease therapy, nanomedicine, drug delivery, and optical urinalysis. Lastly, an outlook on the challenges and opportunities in the future development of metal NCs for theranostic applications is provided.

Size Growth of Au4Cu4: From Increased Nucleation to Surface Capping

The size conversion of atomically precise metal nanoclusters is fundamental for elucidating structure-property correlations. In this study, copper salt (CuCl)-induced size growth from [Au₄Cu₄(Dppm)₂(SAdm)₅]⁺ (abbreviated as [Au₄Cu₄S₅]⁺) to [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ (abbreviated as [Au₄Cu₆S₄Cl₃]⁺) (SAdmH = 1-adamantane mercaptan, Dppm = bis-(diphenylphosphino)methane) was investigated via experiments and density functional theory calculations. The [Au₄Cu₄S₅]⁺ adopts a defective pentagonal bipyramid core structure with surface cavities, which could be easily filled with the sterically less hindered CuCl and CuSCy (i.e., core growth) (HSCy = cyclohexanethiol) but not the bulky CuSAdm. As long as the Au₄Cu₅ framework is formed, ligand exchange or size growth occurs easily. However, owing to the compact pentagonal bipyramid core structure, the latter growth mode occurs only for the surface-capped [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ structure (i.e., surface-capped size growth). A preliminary mechanistic study with density functional theory (DFT) calculations indicated that the overall conversion occurred via CuCl addition, core tautomerization, Cl migration, the second [CuCl] addition, and [CuCl]-[CuSR] exchange steps. And the [Au₄Cu₆(Dppm)₂(SAdm)₄Cl₃]⁺ alloy nanocluster exhibits aggregation-induced emission (AIE) with an absolute luminescence quantum yield of 18.01% in the solid state. This work sheds light on the structural transformation of Au-Cu alloy nanoclusters induced by Cu(I) and contributes to the knowledge base of metal-ion-induced size conversion of metal nanoclusters.

Chiral copper-hydride nanoclusters: synthesis, structure, and assembly

An effective strategy is developed to synthesize a novel and stable layered Cu nanocluster using a one-pot reduction method. The cluster, with a molecular formula of [Cu₁₄(^tBuS)₃(PPh₃)₇H₁₀]BF₄ which has been unambiguously characterized by single crystal X-ray diffraction analysis, exhibits different structures from previously reported analogues with core-shell geometries. In the absence of chiral ligands, the cluster displays intrinsic chirality owing to the non-covalent ligand-ligand interactions (e.g., C-H⋯Cu interactions and C-H⋯π interactions) to lock the central copper core. The interlacing of chiral-cluster enantiomers forms a large cavity, which lays the foundation for a series of potential applications such as drug filling and gas adsorption. Moreover, the C-H⋯H-C interactions of phenyl groups between different cluster moieties promote the formation of a dextral helix and realization of the self-assembly of nanostructures.

Ligand engineering of Au44 nanoclusters for NIR-II luminescent and photoacoustic imaging-guided cancer photothermal therapy

A theranostic probe was developed by conjugating NIR-II emitting Au44MBA26 nanoclusters with photothermal Cy7 molecules via click chemistry, achieving NIR-II luminescent and photoacoustic imaging-guided cancer photothermal therapy.

Structural Engineering toward Gold Nanocluster Catalysis

Atomically precise gold nanoclusters provide great opportunities to explore the relationship between the structure and properties of nanogold catalysts. A nanocluster consists of a metal core and a surface ligand shell, and both the core and shell have significant effects on the catalytic properties. Thanks to their precise structures, the active metal site of the clusters can be readily identified and the effects of ligands on catalysis can be disclosed. In this Minireview, we summarize recent advances in catalytic research of gold nanoclusters, emphasizing four strategies for constructing open metal sites, including by post-treatment, the bulky ligands strategy, the surface geometric mismatch method, and heteroatom doping procedures. We also discuss the effects of ligands on the catalytic activity, selectivity, and stability of gold cluster catalysts. Finally, we present future challenges relating to gold cluster catalysis.

Insights into mechanisms of diphosphine-mediated controlled surface construction on Au nanoclusters

Unraveling the rules governing the size regulation of nanoclusters is of great importance not only in fundamental research, but also in practical applications because of the high structure-property correlation in nanoclusters. Diphosphine-mediated size tailoring is recognized as a powerful method for modulating the size, configuration, and properties of nanoclusters, but the role of diphosphines in these size-controlled processes is still poorly understood due to a lack of systematic studies. Herein, using Au₂₃(SR)₁₆^- as the template for modification, the factors influencing the size-modulation of nanoclusters by diphosphines were systematically investigated. It is revealed that by controlling the length of the diphosphines (from shorter to longer), Au₂₁(SR)₁₂L₂⁺ (L = diphosphine) and Au₂₂(SR)₁₄L can be produced. Moreover, introducing a rigid group into the diphosphines can twist the structural framework or lead to the formation of a new surface motif configuration in the nanoclusters, forming twisted Au₂₂(SR)₁₄L and Au₂₅(SR)₁₆L₂⁺. The size regulation of these nanoclusters enables fine-tuning of the optical properties, including the absorption wavelengths and photoluminescence emission intensity, affording an avenue for precise control of the physicochemical properties of nanoclusters for practical applications.

Depletion Driven Assembly of Ultrasmall Metal Nanoclusters: From Kinetically Arrested Assemblies to Thermodynamically Stable, Spherical Superclusters

Nanoscale assembly of ultrasmall metal nanoclusters (MNCs) by means of molecular forces has proven to be a powerful strategy to engineer their molecule-like properties in multiscale dimensions. By leveraging depletion attraction as the guiding force, herein, we demonstrate the formation of kinetically trapped NCs assemblies with enhanced photoluminescence (PL) and excited state lifetimes and extend the principle to cluster impregnated cationic nanogels, nonluminescent Au(I)-thiolate complexes, and weakly luminescent CuNCs. We further demonstrate a thermal energy driven kinetic barrier breaking process to isolate these assemblies. These isolated assemblies are thermodynamically stable, built from a strong network among several discrete, ultrasmall AuNCs and exhibit several unusual properties such as high stability in various pH, strong PL, microsecond lifetimes, large Stocks shifts, and higher accumulation in the lysosome of cancer cells. We anticipate our strategy may find wider use in creating a large variety of MNC-based assemblies with many unforeseen arrangements, properties, and applications.

Robust Enantiomeric Two-Dimensional Assembly of Atomically Precise Silver Clusters

The facile syntheses of enantiomeric atomically precise silver clusters starting from achiral ligands remain a substantial challenge to explore. In this work, a pair of atomically precise enantiomers of R/S-[Ag₁₇Cl(ⁱPrS)₉S(CH₃COO)₅H₂O] (R/S-Ag₁₇, ⁱPrS = isopropanethiolate) clusters have been synthesized using a viable solvothermal approach. The chirality of the resulting enantiomeric R/S-Ag₁₇ clusters is attributed to the asymmetric arrangement of surface achiral ligands. Both R/S-Ag₁₇ enantiomers could form the two-dimensional (2D) assemblies via intercluster interactions of basic building blocks containing Ag₁₆S₈ moieties, ⁱPrS-Ag motifs, and S^2- linkers. Such a small ligand-induced 2D assembly greatly contributes to the enhancement of thermal stability and photocatalytic activity of R/S-Ag₁₇ clusters, providing possibilities for exploring robust coinage cluster-based assembly with attractive catalytic properties.

Bicarbonate insertion triggered self-assembly of chiral octa-gold nanoclusters into helical superstructures in the crystalline state

Constructing atomically precise helical superstructures of high order is an extensively pursued subject for unique aesthetic features and underlying applications. However, the construction of cluster-based helixes of well-defined architectures comes with a huge challenge owing to their intrinsic complexity in geometric structures and synthetic processes. Herein, we report a pair of unique P- and M-single stranded helical superstructures spontaneously assembled from R- and S-Au8c individual nanoclusters, respectively, upon selecting chiral BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthalene) and hydrophilic o-H2MBA (o-mercaptobenzoic acid) as protective ligands to induce chirality and facilitate the formation of helixes. Structural analysis reveals that the chirality of the Au8c individual nanoclusters is derived from the homochiral ligands and the inherently chiral Au8 metallic kernel, which was further corroborated by experimental and computational investigations. More importantly, driven by the O-H⋯O interactions between (HCO3 -)2 dimers and achiral o-HMBA- ligands, R/S-Au8c individual nanoclusters can assemble into helical superstructures in a highly ordered crystal packing. Electrospray ionization (ESI) and collision-induced dissociation (CID) mass spectrometry of Au8c confirm the hydrogen-bonded dimer of Au8c individual nanoclusters in solution, illustrating that the insertion of (HCO3 -)2 dimers plays a crucial role in the assembly of helical superstructures in the crystalline state. This work not only demonstrates an effective strategy to construct cluster-based helical superstructures at the atomic level, but also provides visual and reliable experimental evidence for understanding the formation mechanism of helical superstructures.

Mechanistic insights into Ag+ induced size-growth from [Au6(DPPP)4]2+ to [Au7(DPPP)4]2+ clusters

The size conversion of atomically precise metal nanoclusters lays the foundation to elucidate the inherent structure-activity correlations on the nanometer scale. Herein, the mechanism of the Ag+-induced size growth from [Au6(dppp)4]2+ to [Au7(dppp)4]3+ (dppp is short for 1,3-bis(diphenylphosphino)propane) is studied via density functional theory (DFT) calculations. In the absence of extra Au sources, the one "Au+" addition was found to be regulated by the Ag+ doping induced Au-activation, i.e., the formation of formal Au(i) blocks via the Ag+ alloying processes. The Au(i) blocks could be extruded from the core structure in the formed Au-Ag alloy clusters, triggering a facile Au+ migration to the Au6 precursor to form the Au7 product. This study sheds light on the structural and stability changes of gold nanoclusters upon the addition of Ag+ and will hopefully benefit the development of more metal ion-induced size-conversion of metal nanoclusters.

Aggregation-Induced Emission of Gold Nanoclusters by Ionic Liquids for White Light-Emitting Diode and Multiple-Ion Probe Applications

White light-emitting diodes (WLEDs) require stable luminophores with bright emission, a large Stokes shift, and good processability. Herein, we fabricate WLEDs on the basis of synergetic aggregation-induced emission (AIE) of Au nanoclusters (NCs) induced by ionic liquids. Au NCs with orange emission at 615 nm are electrostatically bonded to 1-ethyl-3-methyl-imidazolium acetate (EmimOAc). The electrostatic bonding not only introduces complementary blue emission (from EmimOAc) into Au NCs but also enhances cluster emission via an intercluster AIE mechanism. Overall white emission of Au NC-EmimOAc composites can be further improved by their encapsulation into nanogels templated by cationic chitosan. Color conversion films constructed from these nanogels manifest ultrabright white emission on a commercial GaN ultraviolet LED chip, featuring a CIE 1931 color coordinate of (0.366, 0.368) and greatly enhanced mechanical strength. By properly selecting ionic liquids, we are also able to achieve multiple-ion detection of Cu²⁺, Sn²⁺, and Fe³⁺.

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.

Insight into the Effects of Chiral Diphosphine Ligands on the Structure and Optical Properties of the Au24Cd2 Nanocluster

Introduction of chiral ligands has been regarded as an effective strategy to obtain nanoclusters with optical purity. However, how the chiral ligands work is still unclear due to the lack of structural comparison between racemic nanoclusters and the corresponding optically active ones. In this work, three structurally related Au₂₄Cd₂ nanoclusters, including one racemic and two homochiral nanoclusters, were synthesized, and their crystal structures were characterized using single-crystal X-ray crystallography (SC-XRD). Based on their crystal structures, the origin of the chirality in Au₂₄Cd₂ was found to be the twist of the kernel and the chiral arrangement of the metal-ligand surface. Au₂₄Cd₂ protected with chiral ligands exhibits a more twisted kernel than the racemic one. Therefore, the chirality of chiral diphosphine was found to transfer from the ligands to the metal-ligand interface and then to the metal core, inducing its distortion to produce enhanced chirality. In addition, the optical properties including optical absorption and circular dichroism of these structurally related Au₂₄Cd₂ nanoclusters were compared.

Structural Control and Chiroptical Response in Intrinsically Tetra- and Pentanuclear Chiral Gold Clusters

Controlling the synthesis of chiral metal clusters in the aspects of nuclearity number, metal-metal interaction, and spatial arrangement of metal atoms is crucial for establishing the correlation of detailed structural factors with chiroptical activity. Herein, a series of enantiopure gold complexes with nuclearity numbers ranging from 2 to 5 were constructed and structurally characterized. On the basis of the annulation reaction between two aurated μ₂-imido nucleophilic units with various aldehydes, we finely adjusted the metal-metal interaction and torsion angles of a characteristic tetranuclear metal cluster by introducing different substituents into the resulting imidazolidine dianionic chiral skeleton. Further structural investigations, contrast experiments, and time-dependent density functional theory calculations confirmed that the chiroptical response of the acquired asymmetric metal clusters was mainly affected by the geometrically twisted arrangement of metal atoms. Finally, the tetranuclear gold cluster compound with the shortest intermetallic interaction and the largest torsion angle of a Au₄ core showed the highest absorption anisotropy factor up to 2.2 × 10^-3. In addition, the correlation of structural factors with the stability of chiral gold clusters was thoroughly evaluated by monitoring the CD, UV-vis, and NMR spectra at elevated temperatures. Insight into the relationship between the structural factors with the chiroptical property and stability of chiral gold clusters in this work will help us to design and achieve more stable chiral metal clusters and stimulate their practical applications in chiroptical functional materials.

Synthesis and Characterization of Enantiopure Chiral Bis NHC-Stabilized Edge-Shared Au10 Nanocluster with Unique Prolate Shape

Herein we report the first chiral Au₁₀ nanoclusters stabilized by chiral bis N-heterocyclic carbene (bisNHC) ligands. ESI-MS and single-crystal X-ray crystallography confirmed the molecular formula to be [Au₁₀(bisNHC)₄Br₂](O₂CCF₃)₂. The chiral Au₁₀ nanocluster adopts a linear edge-shared tetrahedral geometry with a prolate shape. DFT calculations provide insight into the electronic structure, optical absorption, and circular dichroism (CD) characteristics of this unique Au₁₀ nanocluster. CD spectra demonstrate chirality transfer from the chiral bisNHC ligand to the inner Au₁₀ nanocluster core. Examination of ESI-MS and UV-vis spectra show that cluster [Au₉(bisNHC)₄Br]Br₂ is formed initially and then transformed into the Au₁₀ nanocluster in solution.

Asymmetric Twisting of C-Centered Octahedral Gold(I) Clusters by Chiral N-Heterocyclic Carbene Ligation

Asymmetric induction of metal clusters by ligation of chiral ligands is intriguing in terms of the mechanism of chirality transfer and the stability of the resulting chiral structure. Here we report the asymmetric induction of C-centered hexagold(I) CAu^I₆ clusters into an asymmetrically twisted structure through monodentate, chiral benzimidazolylidene-based N-heterocyclic carbene (NHC) ligands. X-ray diffraction analysis revealed that the NHC-ligated CAu^I₆ cluster was diastereoselectively twisted with directionally selective, bond length expansion, and contraction of the Au···Au contacts and that the original cluster with high symmetry was transformed into an optically pure, asymmetric CAu^I₆ cluster with C₁ symmetry. Moreover, the circular dichroism spectroscopy and the time-dependent density functional theory calculation confirmed that the asymmetrically twisted CAu^I₆ structure was maintained even in solution. Such asymmetric induction of configurationally stable metal clusters would greatly expand the molecular design possibilities of asymmetric catalysts and chiroptical materials by utilizing library chiral NHC ligands.

Achiral copper clusters helically confined in self-assembled chiral nanotubes emitting circularly polarized phosphorescence

Achiral Cu5− cluster coassembled with a chiral amphiphile to afford helical nanotubes, where the Cu5− cluster was confined within the nanotubes, forming helical arrangement with emerged chiroptical activities (CD and CPP), with a large gabs up to 0.018.

Toward Controlled Syntheses of Diphosphine-Protected Homochiral Gold Nanoclusters through Precursor Engineering

Controllable syntheses of Au nanoclusters (NCs) with different nuclearities are of great significance due to the kernel-dependent physicochemical properties. Herein, two pairs of enantiomeric Au NCs [Au₁₉(R/S-BINAP)₄(PhC≡C)Cl₄] (SD/Au19) and [Au₁₁(R/S-BINAP)₄(PhC≡C)₂]·Cl (SD/Au11), both with atropos (rigid axial chirality) diphosphine BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthalene) as the predominant organic ligands, were controllably synthesized through precursor engineering. The former was obtained by direct reduction of HAuCl₄·4H₂O, while the latter was obtained by reduction of [Au(SMe₂)Cl] instead. Intriguingly, the kernel of SD/Au19 contains an Au₇ pentagonal bipyramid capped by two boat-like Au₆ rings, which represents another type of Au₁₉ kernel, making SD/Au19 a good candidate for comparative study with other Au₁₉ NCs to get more insight into the distinct structural evolution of phosphine-protected Au NCs. Despite the previous chiroptical studies on some other chiral undecagold NCs, the successful attainment of the X-ray crystal structures for SD/Au11 not only provides a step forward toward better correlating the chiroptical activities with their structural details but also reveals that even the auxiliary protecting ligands also play a nontrivial role in tuning the geometrical structures of the metal NCs. The chiroptical activities of both SD/Au19 and SD/Au11 were found to originate from the chiral ligands and core distortions; the extended π-electron systems in the BINAP ligands have proved to positively contribute to the electronic absorptions and thus disturb the corresponding circular dichroism (CD) responses.

Molecular Gold Nanocluster Au156 Showing Metallic Electron Dynamics

The boundary between molecular and metallic gold nanoclusters is of special interest. The difficulty in obtaining atomically precise nanoclusters larger than 2 nm limits the determination of such a boundary. The synthesis and total structural determination of the largest all-alkynyl-protected gold nanocluster (Ph₄P)₆[Au₁₅₆(C≡CR)₆₀] (R = 4-CF₃C₆H₄-) (Au₁₅₆) are reported. It presents an ideal platform for studying the relationship between the structure and the metallic nature. Au₁₅₆ has a rod shape with the length and width of the kernel being 2.38 and 2.04 nm, respectively. The cluster contains a concentric Au₁₂₆ core structure (Au₄₆@Au₅₀@Au₃₀) protected by 30 linear RC≡C-Au-C≡CR staple motifs. It is interesting that Au₁₅₆ displays multiple excitonic peaks in the steady-state absorption spectrum (molecular) and pump-power-dependent excited-state dynamics as revealed in the transient absorption spectrum (metallic), which indicates that Au₁₅₆ is a critical crossover cluster for the transition from molecular to metallic state. Au₁₅₆ is the smallest-sized gold nanocluster showing metal-like electron dynamics, and it is recognized that the cluster shape is one of the important factors determining the molecular or metallic nature of a gold nanocluster.

Fabrication of a chiral luminescent hydrogel from gold nanoclusters via molecular recognition

A novel supramolecular chiral hydrogel with enhanced emission was obtained by the co-assembly of achiral thiobarbituric acid-modified gold nanoclusters (TBA-AuNCs) with chiral histidine molecules. Chirality transfer from histidine to the supramolecular hydrogels was achieved through the π-π stacking and intermolecular H-bonding based on molecular recognition. This work gives a creative strategy for the building of chiral nanocluster-based materials.

Broad Chiroptical Activity from Ultraviolet to Short-Wave Infrared by Chirality Transfer from Molecular to Micrometer Scale

Chiral nanomaterials provide a rich platform for versatile applications. Tuning the wavelength of polarization rotation maxima in the broad range including short-wave infrared (SWIR) is a promising candidate for infrared neural stimulation, imaging, and nanothermometry. However, the majority of previously developed chiral nanomaterials reveal the optical activity in a relatively shorter wavelength range (ultraviolet-visible, UV-vis), not in SWIR. Here, we demonstrate a versatile method to synthesize chiral copper sulfides using cysteine, as the stabilizer, and transferring the chirality from molecular- to the microscale through self-assembly. The assembled structures show broad chiroptical activity in the UV-vis-NIR-SWIR region (200-2500 nm). Importantly, we can tune the chiroptical activity by simply changing the reaction conditions. This approach can be extended to materials platforms for developing next-generation optical devices, metamaterials, telecommunications, and asymmetric catalysts.

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).

Gold Clusters: From the Dispute on a Gold Chair to the Golden Future of Nanostructures

The present work opens with an acknowledgement to the research activity performed by Luciana Naldini while affiliated at the Università degli Studi di Sassari (Italy), in particular towards gold complexes and clusters, as a tribute to her outstanding figure in a time and a society where being a woman in science was rather difficult, hoping her achievements could be of inspiration to young female chemists in pursuing their careers against the many hurdles they may encounter. Naldini’s findings will be a key to introduce the most recent results in this field, showing how the chemistry of gold compounds has changed throughout the years, to reach levels of complexity and elegance that were once unimagined. The study of gold complexes and clusters with various phosphine ligands was Naldini’s main field of research because of the potential application of these species in diverse research areas including electronics, catalysis, and medicine. As the conclusion of a vital period of study, here we report Naldini’s last results on a hexanuclear cationic gold cluster, [(PPh₃)₆Au₆(OH)₂]²⁺, having a chair conformation, and on the assumption, supported by experimental data, that it comprises two hydroxyl groups. This contribution, within the fascinating field of inorganic chemistry, provides the intuition of how a simple electron counting may lead to predictable species of yet unknown molecular architectures and formulation, nowadays suggesting interesting opportunities to tune the electronic structures of similar and higher nuclearity species thanks to new spectroscopic and analytical approaches and software facilities. After several decades since Naldini’s exceptional work, the chemistry of the gold cluster has reached a considerable degree of complexity, dealing with new, single-atom precise, materials possessing interesting physico-chemical properties, such as luminescence, chirality, or paramagnetic behavior. Here we will describe some of the most significant contributions.

Revealing the chirality origin and homochirality crystallization of Ag14 nanocluster at the molecular level

Although chirality is an ever-present characteristic in biology and some artificial molecules, controlling the chirality and demystifying the chirality origin of complex assemblies remain challenging. Herein, we report two homochiral Ag₁₄ nanoclusters with inherent chirality originated from identical rotation of six square faces on a Ag₈ cube driven by intra-cluster π···π stacking interaction between pntp⁻ (Hpntp = p-nitrothiophenol) ligands. The spontaneous resolution of the racemic (SD/rac-Ag14a) to homochiral nanoclusters (SD/L-Ag14 and SD/R-Ag14) can be realized by re-crystallizing SD/rac-Ag14a in acetonitrile, which promotes the homochiral crystallization in solid state by forming C–H···O/N hydrogen bonds with nitro oxygen atoms in pntp⁻ or aromatic hydrogen atoms in dpph (dpph = 1,6-bis(diphenylphosphino)hexane) on Ag₁₄ nanocluster. This work not only provides strategic guidance for the syntheses of chiral silver nanoclusters in an all-achiral environment, but also deciphers the origin of chirality at molecular level by identifying the special effects of intra- and inter-cluster supramolecular interactions.

The preparation of chiral monolayer-protected metal clusters is interesting for their potential applications in a variety of fields, including catalysis. Here, the authors synthesize chiral Ag₁₄ nanoclusters in an all-achiral environment, and decipher the origin of chirality at the molecular level; the solvent choice is key to achieve homochiral crystallization.

Tertiary Chiral Nanostructures from C-H⋅⋅⋅F Directed Assembly of Chiroptical Superatoms

We report the synthesis and structure of tertiary chiral nanostructures with 100 % optical purity. A novel synthetic strategy, using chiral reducing agent, R and S-BINAPCuBH₄ (BINAP is 2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl), is developed to access to atomically precise, intrinsically chiral [Au₇ Ag₆ Cu₂ (R- or S-BINAP)₃ (SCH₂ Ph)₆ ]SbF₆ nanoclusters in one-pot synthesis. The clusters represent the first tri-metallic superatoms with inherent chirality and fair stability. Both metal distribution (primary) and ligand arrangement (secondary) of the enantiomers exhibited perfect mirror images, and unprecedentedly, the self-assembly driven by the C-H⋅⋅⋅F interaction between the phenyl groups of the superatom moieties and SbF₆ ^- anions induced the formation of bio-mimic left- and right-handed helices, achieving the tertiary chiral nanostructures. DFT calculations revealed the connections between the molecular details and chiral optical activity.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.