From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13 Cu2 Nanoclusters

Structural Isomerism in Bimetallic Ag20Cu12 Nanoclusters

Structural isomers of atomically precise metal nanoclusters are highly sought after for investigating structure-property relationships in nanostructured materials. However, they are extremely rare, particularly those of alloys, primarily due to the challenges in their synthesis and structural characterization. Herein, for the first time, a pair of bimetallic isomeric AgCu nanoclusters has been controllably synthesized and structurally characterized. These two isomers share an identical molecular formula, Ag20Cu12(C≡CR)24 (denoted as Ag20Cu12-1 and Ag20Cu12-2; HC≡CR is 3,5-bis(trifluoromethyl)phenylacetylene). Single-crystal X-ray diffraction data analysis revealed that Ag20Cu12-1 possesses an Ag17Cu4 core composed of two interpenetrating hollow Ag11Cu2 structures. This core is stabilized by four different types of surface motifs: eight -C≡CR, one Cu(C≡CR)2, one Ag3Cu3(C≡CR)6, and two Cu2(C≡CR)4 units. Ag20Cu12-2 features a bitetrahedron Ag14 core, which is stabilized by three Ag2Cu4(C≡CR)8 units. Interestingly, Ag20Cu12-2 undergoes spontaneous transformation to Ag20Cu12-1 in the solution-state. Density functional theory calculations explain the electronic and optical properties and confirm the higher relative stability of Ag20Cu12-1 compared to Ag20Cu12-2. The controlled synthesis and structural isomerism of alloy nanoclusters presented in this work will stimulate and broaden research on nanoscale isomerism.

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.

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

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

Dithiocarbonate-Protected Au25 Nanorods of a Chiral D5 Configuration and NIR-II Phosphorescence

Innovative surface-protecting ligands are in constant demand due to their crucial role in shaping the configuration, property, and application of gold nanoclusters. Here, the unprecedented O-ethyl dithiocarbonate (DTX)-stabilized atomically precise gold nanoclusters, [Au25(PPh3)10(DTX)5Cl2]2+ (Au25DTX-Cl) and [Au25(PPh3)10(DTX)5Br2]2+ (Au25DTX-Br), were synthesized and structurally characterized. The introduction of bidentate DTX ligands not only endowed the gold nanocluster with unique staggered Au25 nanorod configurations but also generated the symmetry breaking from the D5d geometry of the Au25 kernels to the chiral D5 configuration of the Au25 molecules. The chirality of Au25 nanorods was notably revealed through single-crystal X-ray diffraction, and chiral separation was induced by employing chiral DTX ligands. The staggered configurations of Au25 nanorods, as opposed to eclipsed ones, were responsible for the large red shift in the emission wavelengths, giving rise to a promising near-infrared II (NIR-II, >1000 nm) phosphorescence. Furthermore, their performances in photocatalytic sulfide oxidation and electrocatalytic hydrogen evolution reactions have been examined, and it has been demonstrated that the outstanding catalytic activity of gold nanoclusters is highly related to their stability.

Tailoring the subshell and electronic structure of an atomically precise AuAg alloy nanocluster

Manipulating the atomic-level structure of the subshell of a nanocluster while preserving the inner and outer shell structure is challenging. We present the synthesis and molecular structure of an alkynyl-protected Au34Ag27 nanocluster, which exhibits distinct third shell atomic arrangement, electronic structure, and optical properties from those of the Au34Ag28 nanocluster.

Symmetry Breaking Enhancing the Activity of Electrocatalytic CO2 Reduction on an Icosahedron-Kernel Cluster by Cu Atoms Regulation

Recently, CO2 hydrogenation had a new breakthrough resulting from the design of catalysts to effectively activate linear CO2 with symmetry-breaking sites. However, understanding the relationship between symmetry-breaking sites and catalytic activity at the atomic level is still a great challenge. In this study, a set of gold-copper alloy Au13 Cux (x=0-4) nanoclusters were used as research objects to show the symmetry-controlled breaking structure on the surface of nanoclusters with the help of manipulability of the Cu atoms. Among them, Au13 Cu3 nanocluster displays the highest degree of symmetry-breaking on its crystal structure compared with the other nanoclusters in the family. Where the three copper atoms occupying the surface of the icosahedral kernel unevenly with one copper atom is coordinately unsaturated (CuS2 motif relative to CuS3 motif). As expected, Au13 Cu3 has an excellent hydrogenation activity of CO2 , in which the current density is as high as 70 mA cm-2 (-0.97 V) and the maximum FECO reaches 99 % at -0.58 V. Through the combination of crystal structures and theoretical calculations, the excellent catalytic activity of Au13 Cu3 is revealed to be indeed closely related to its asymmetric structure.

Symmetry breaking of highly symmetrical nanoclusters for triggering highly optical activity

Developing new approaches to fulfill the enantioseparation of nanocluster racemates and construct cluster-based nanomaterials with optical activity remains highly desired in cluster science, because it is an essential prerequisite for fundamental research and extensive applications of these nanomaterials. We herein propose a strategy termed "active-site exposing and partly re-protecting" to trigger the symmetry breaking of highly symmetrical nanoclusters and to render cluster crystals optically active. The vertex PPh3 of the symmetrical Ag29(SSR)12(PPh3)4 (SSR = 1, 3-benzenedithiol) nanocluster was firstly dissociated in the presence of counterions with large steric hindrance, and then the exposed Ag active sites of the obtained Ag29(SSR)12 nanocluster were partly re-protected by Ag+, yielding an Ag29(SSR)12-Ag2 nanocluster with a symmetry-breaking construction. Ag29(SSR)12-Ag2 followed a chiral crystallization mode, and its crystal displayed strong optical activity, derived from CD and CPL characterizations. Overall, this work presents a new approach (i.e., active-site exposing and partly re-protecting) for the symmetry breaking of highly symmetrical nanoclusters, the enantioseparation of nanocluster racemates, and the achievement of highly optical activity.

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.

Atomically precise chiral silver clusters based on non-chiral ligands for acid/base stimulated luminescence response

Chiral metal nanoclusters synthesized by non-chiral ligands are usually in the form of racemates. Thus, resolving racemic compounds continues to be a great challenge. Herein, we report a case of the racemic compound hexanuclear silver cluster (Ag6-Rac) protected by the non-chiral sulfhydryl ligand sodium 1H-1,2,3-triazole-5-thiolate (SHTT) and 2,6-bis(diphenylphosphino)pyridine (dpppy). The homochiral clusters in Ag6-Rac are able to spontaneously crystallize and undergo chiral resolution to obtain a racemic conglomerate (Ag6-S/Ag6-R) by solvent-induced crystallization. Interestingly, the Ag6-Rac clusters exhibit strong luminescence in solid and solution, which can respond to trifluoroacetic acid (TFA) and reversible cycling over five times using diethylamine (DEA). This work provides a new research model for resolving racemic clusters and constructing stimulus-responsive clusters.

Bright near-infrared circularly polarized electrochemiluminescence from Au9Ag4 nanoclusters

Metal nanoclusters are excellent electrochemiluminescent luminophores owing to their rich electrochemical and optical properties. However, the optical activity of their electrochemiluminescence (ECL) is unknown. Herein, we achieved, for the first time, the integration of optical activity and ECL, i.e., circularly polarized electrochemiluminescence (CPECL), in a pair of chiral Au9Ag4 metal nanocluster enantiomers. Chiral ligand induction and alloying were employed to endow the racemic nanoclusters with chirality and photoelectrochemical reactivity. S-Au9Ag4 and R-Au9Ag4 exhibited chirality and bright-red emission (quantum yield = 4.2%) in the ground and excited states. The enantiomers showed mirror-imaged CPECL signals at 805 nm owing to their highly intense and stable ECL emission in the presence of tripropylamine as a co-reactant. The ECL dissymmetry factor of the enantiomers at 805 nm was calculated to be ±3 × 10-3, which is comparable with that obtained from their photoluminescence. The obtained nanocluster CPECL platform shows the discrimination of chiral 2-chloropropionic acid. The integration of optical activity and ECL in metal nanoclusters provides the opportunity to achieve enantiomer discrimination and local chirality detection with high sensitivity and contrast.

Alkynyl-Protected Chiral Bimetallic Ag22 Cu7 Superatom with Multiple Chirality Origins

Understanding the origin of chirality in the nanostructured materials is essential for chiroptical and catalytic applications. Here we report a chiral AgCu superatomic cluster, [Ag₂₂ Cu₇ (C≡CR)₁₆ (PPh₃ )₅ Cl₆ ](PPh₄ ), Ag₂₂ Cu₇ , protected by an achiral alkynyl ligand (HC≡CR: 3,5-bis(trifluoromethyl)phenylacetylene). Its crystal structure comprises a rare interpenetrating biicosahedral Ag₁₇ Cu₂ core, which is stabilized by four different types of motifs: one Cu(C≡CR)₂ , four -C≡CR, two chlorides and one helical Ag₅ Cu₄ (C≡CR)₁₀ (PPh₃ )₅ Cl₄ . Structural analysis reveals that Ag₂₂ Cu₇ exhibits multiple chirality origins, including the metal core, the metal-ligand interface and the ligand layer. Furthermore, the circular dichroism spectra of R/S-Ag₂₂ Cu₇ are obtained by employing appropriate chiral molecules as optical enrichment agents. DFT calculations show that Ag₂₂ Cu₇ is an eight-electron superatom, confirm that the cluster is chirally active, and help to analyze the origins of the circular dichroism.

DFT Insights into the Variety in the Coordination Modes of the Equatorial Halides in [Au13 Ag12 (PR3 )10 X8 ]+ (X=Cl/Br) Clusters

The bonding character within metal nanoclusters represents an intriguing topic, shedding light on the inherent driving force for the packing preference in nanomaterials. Herein, density functional theory (DFT) calculations were conducted to investigate the correlation of the series of isomeric [Au₁₃ Ag₁₂ (PR₃ )₁₀ X₈ ]⁺ (X=Cl/Br) clusters, which are mainly differentiated by the coordination mode of the equatorial halides (μ₂ -, μ₃ - and μ₄ -) in the rod-like, bi-icosahedral framework. The theoretical simulation corroborates the variety in the configuration of the Au₁₃ Ag₁₂ clusters and elucidates the fast isomerization kinetics among the different configurations. The easy tautomerization and the variety in chloride binding modes correspond to a fluxionality character of the equatorial halides and are verified by the potential energy curve analysis. The structural flexibility of the central Au₃ Ag₁₀ block is the main driving force, while the relatively stronger Ag-X bonding interaction (compared to that of Au-X), and a sufficient number of halides are also requisite for the associating Ag-X tautomerizations.

Atomically Precise Enantiopure Bimetallic Janus Clusters

Asymmetric bimetallic Janus nanocrystals with a side-by-side interface have unique properties and important applications. However, understanding their fundamental issues, including their formation mechanism, interfacial linkage, and related properties, remains challenging, as does the preparation of enantiopure samples. Atomically precise Janus bimetal nanoclusters would unequivocally resolve these issues, yet they have not been realized. Here, based on Au and transition metals (Cu/Cd), and employing an S/P biligand strategy, we prepare and structurally resolve four Janus nanoclusters, including racemate 6e Au 8 /Cu 4 , 6e R -/ S-Au 8 /Cu 4 enantiomers, and 2e racemate Au 3 /Cd. Their interfacial linkage is unambiguously resolved at the atomic level, superatomic orbital splitting emerges, and unique molecule-like electronic transitions and chiroptical properties are present; more importantly, the dipolar distribution of bicomponents leads to a maximum dipole moment of up to 45 D, which drives the formation of 1D nanowires through self-assembly. This work provides a fundamental knowledge of intermetallic nanomaterials and an avenue for the synthesis of Janus nanoclusters.

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.

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.

Small symmetry-breaking triggering large chiroptical responses of Ag70 nanoclusters

The origins of the chiroptical activities of inorganic nanostructures have perplexed scientists, and deracemization of high-nuclearity metal nanoclusters (NCs) remains challenging. Here, we report a single-crystal structure of Rac-Ag₇₀ that contains enantiomeric pairs of 70-nuclearity silver clusters with 20 free valence electrons (Ag₇₀), and each of these clusters is a doubly truncated tetrahedron with pseudo-T symmetry. A deracemization method using a chiral metal precursor not only stabilizes Ag₇₀ in solution but also enables monitoring of the gradual enlargement of the electronic circular dichroism (CD) responses and anisotropy factor g_abs. The chiral crystals of R/S-Ag₇₀ in space group P2₁ containing a pseudo-T-symmetric enantiomeric NC show significant kernel-based and shell-based CD responses. The small symmetry breaking of T_d symmetry arising from local distortion of Ag−S motifs and rotation of the apical Ag₃ trigons results in large chiroptical responses. This work opens an avenue to construct chiral medium/large-sized NCs and nanoparticles, which are promising for asymmetric catalysis, nonlinear optics, chiral sensing, and biomedicine.

Having control over the chirality of metal nanoclusters is challenging. Here, the authors report the deracemization of silver nanoclusters and monitor the chiroptical responses.

An Insight, at the Atomic Level, into the Intramolecular Metallophilic Interaction in Nanoclusters

The intermolecular metallophilic interaction has been exploited to orderly aggregate nanocluster compounds into multidimensional assemblies, while the intramolecular metallophilic interaction was rarely reported. Herein, based on an Au13Cu2 nanocluster template,...

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.

Phosphorescent Cyclometalated Platinum(II) Enantiomers with Circularly Polarized Luminescence Properties and Their Assembly Behaviors

Platinum(II) complexes as supramolecular luminescent materials have received considerable attention due to their unique planar structures and fascinating photophysical properties. However, the molecular design of platinum(II) complexes with impressive circularly polarized luminescence properties still remains challenging and rarely explored. Herein, we reported a series of cyclometalated platinum(II) complexes with benzaldehyde and its derived imine-containing alkynyl ligands to investigate their phosphorescent, chiroptical, and self-assembly behaviors. An isodesmic growth mechanism is found for their temperature-dependent self-assembly process. The chiral sense of the enantiomers can be transferred from the chiral alkynyl ligands to the cyclometalated platinum(II) dipyridylbenzene N^C^N chromophore and further amplified through supramolecular assembly via intermolecular noncovalent interactions. Notably, distinctive phosphorescent properties and nanostructured morphologies have been found for enantiomers 4R and 4S. Their intriguing self-assembled nanostructures and phosphorescence behaviors are supported by crystal structure determination, ¹H NMR, emission, and UV-vis absorption spectroscopy, scanning electron microscopy, and X-ray powder diffraction studies.

Controllable synthesis and electrocatalytic applications of atomically precise gold nanoclusters

Nanoclusters are composed of metal atoms and ligands with sizes up to 2-3 nm. Due to their stability and unique structure, gold nanoclusters with precise atomic numbers have been widely studied. Until now, atomically precise gold nanoclusters have been synthesised by various methods. Common ones include the Brust-Schiffrin method and the size-focusing method. With more detailed research on gold nanoclusters, more novel methods have been adopted to synthesise atomically precise gold nanoclusters, such as anti-galvanic reduction, ligand-exchange reactions from metal nanoclusters, the seed growth method, and so on. Besides, the nanoclusters also have many unique properties in electrochemical catalyses, such as the ORR, OER, etc., which are helpful for the development of the energy and environment. In this review, the synthesis methods and electrochemical applications of atomically accurate gold nanoclusters in recent years are introduced.

Programmable Metal Nanoclusters with Atomic Precision

With the recent establishment of atomically precise nanochemistry, capabilities toward programmable control over the nanoparticle size and structure are being developed. Advances in the synthesis of atomically precise nanoclusters (NCs, 1-3 nm) have been made in recent years, and more importantly, their total structures (core plus ligands) have been mapped out by X-ray crystallography. These ultrasmall Au nanoparticles exhibit strong quantum-confinement effect, manifested in their optical absorption properties. With the advantage of atomic precision, gold-thiolate nanoclusters (Au_n (SR)_m ) are revealed to contain an inner kernel, Au-S interface (motifs), and surface ligand (-R) shell. Programming the atomic packing into various crystallographic structures of the metal kernel can be achieved, which plays a significant role in determining the optical properties and the energy gap (E_g ) of NCs. When the size increases, a general trend is observed for NCs with fcc or decahedral kernels, whereas those NCs with icosahedral kernels deviate from the general trend by showing comparably smaller E_g . Comparisons are also made to further demonstrate the more decisive role of the kernel structure over surface motifs based on isomeric Au NCs and NC series with evolving kernel or motif structures. Finally, future perspectives are discussed.

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.

Reversible transformation between Au14Ag8 and Au14Ag4 nanoclusters

Although several approaches have been exploited to trigger the structural transformation of metal nanoclusters, most cases are assigned to the unidirectional conversion, while the reversible conversion of nanoclusters remains challenging. In this work, the reversible conversion between two Au-Ag alloy nanoclusters, Au₁₄Ag₈(Dppm)₆(CN)₄Cl₄ and Au₁₄Ag₄(Dppm)₆Cl₄, has been accomplished, which was tracked by UV-vis and ESI-MS spectroscopy. The condition of the nanocluster reversible conversion has been meticulously mapped out. Our results provide some new insights into the cluster transformation, which will benefit the future preparation of metalloid clusters with customized structures and properties.

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.

Symmetry Breaking of Atomically Precise Fullerene-like Metal Nanoclusters

Here we report a neutral fullerene-like core-shell homosilver Ag₁₃@Ag₂₀ nanocluster that is fully protected by an achiral bidentate thiolate ligand (9,12-dimercapto-1,2-closo-carborane, C₂B₁₀H₁₀S₂H₂), which crystallizes in centrosymmetric space group R3̅. Continuous Cu doping in the dodecahedral shell first induced symmetry breaking to generate chiral Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) containing two acetonitrile ligands in space group P2₁2₁2₁, and then produced symmetric all-thiolated Ag₁₃@Ag_20-nCu_n (20 ≥ n ≥ 13) in the higher space group Im3̅. The selectively copper-doped Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) cluster has its structure reorganized to a lower symmetry that shows chiroptical activity. Moreover, structural distortion of Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) further expanded in chiral R-/S-propylene oxide, which induced a more prominent core-based CD response. This work revealed a novel mechanism of chirality generation at the atomic level through asymmetric shell-doping of metal nanoclusters, which provides new insight into the origin of chirality in inorganic nanostructures.

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.

Multiple Ways Realizing Charge-State Transform in AuCu Bimetallic Nanoclusters with Atomic Precision

Thiolate-protected nanoclusters with different charge states usually show similar structure frameworks but different electronic configurations, which are proved to dramatically affect their properties such as magnetism, photoluminescence, and catalytic activity. Until now, few nanoclusters with alterable charge states have been reported and only some of them are structurally solved, limiting the in-depth studies on their interesting properties. Here, a new AuCu alloy nanocluster [Au₁₈ Cu₃₂ (SPhCl)₃₆ ]^2- (HSPhCl = 4-chlorophenylthiophenol) is synthesized and structurally solved by X-ray crystallography. Interestingly, it is found that this nanocluster can be reduced to another nanocluster with a different charge state, that is, [Au₁₈ Cu₃₂ (SPhCl)₃₆ ]^3- . This change in charge states is clearly proved by X-ray crystallography, electrospray ionization mass spectrometry, thermogravimetric analysis, and electron paramagnetic resonance. Furthermore, several redox methods are carried out to realize the reversible interconversion between these two nanoclusters, including electrochemical redox, introduction of H₂ O₂ /NaBH₄ , and oxidation with silica under air atmosphere. This work offers new insight into the transform progress of charge states with AuCu alloy nanoclusters which contributes to the understanding of the relationship between electronic structure and properties of nanoclusters and further development of AuCu nanoclusters with excellent performance.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)_x L_y ]^z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)_x L_y ]^z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)_x L_y ]^z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)_x L_y ]^z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

Chiral Inversion and Conservation of Clusters: A Case Study of Racemic Ag32Cu12 Nanocluster

Chiral metal nanoclusters have been widely reported, but their separation and optical stabilization remain challenging. We used a deracemization strategy to accomplish the enantioseparation of a racemic mixture of [Ag₃₂Cu₁₂(CH₃COO)₁₂(SAdm)₁₂(P(CH₃OPh)₃)₄] (M₄₄) in a yield exceeding 50%, forming two optically active [Ag₃₂Cu₁₂(R/S-Cl(CH₃)CHCOO)₁₂(SAdm)₁₂(P(CH₃OPh)₃)₄] (R/S-M₄₄') enantiomers. The optical activity of these products was conserved after exchange of the chiral carboxyl ligands with achiral ligand (Br^-), to give two additional optically active nanoclusters R/S-[Ag₂₈Cu₁₆Br₁₂(SAdm)₁₂(P(CH₃OPh)₃)₄] (R/S(Br)-M₄₄). The crystal structures of the above nanoclusters were determined by single-crystal X-ray crystallography. Based on these structures, the chiral transformation and conservation are mapped out.

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.

New Routes for Multicomponent Atomically Precise Metal Nanoclusters

Atomically precise metal nanoclusters (NCs), protected by a monolayer of ligands, are regarded as potential building blocks for advanced technologies. They are considered as intermediates between the atomic/molecular regime and the bulk. Incorporation of foreign metals in NCs enhances several of their properties such as catalytic activity, luminescence, and so on; hence, it is of high importance for tuning their properties and broadening the scope of applications. In most of the cases, enhancement in specific properties was observed upon alloying due to the synergistic effect. In the past several years, many alloy clusters have been synthesized, which show a tremendous change in the properties than their monometallic analogs. However, controlling the synthesis and tuning the structures of alloy NCs with atomic precision are major challenges. Various synthetic methodologies have been developed so far for the controlled synthesis of alloy NCs. In this perspective, we have highlighted those diverse synthetic routes to prepare alloys, which include co-reduction, galvanic reduction, antigalvanic reduction, metal deposition, ligand exchange, intercluster reaction, and reaction of NCs with bulk metals. Advancement in synthetic procedures will help in the preparation of alloy NCs with the desired structure and composition. Future perceptions concerning the progress of alloy nanocluster science are also provided.

Two pairs of chiral lanthanide–oxo clusters Ln14 induced by amino acid derivatives

Two pairs of chiral lanthanide–oxo clusters l-/d-Ln14 (Ln = Y/Dy) have been obtained under the action of anion template. The solid-state circular dichroism (CD) spectra of l-Y14/d-Y14 and l-Dy14/d-Dy14 displayed mirror symmetry effects.

Control of single-ligand chemistry on thiolated Au25 nanoclusters

Diverse methods have been developed to tailor the number of metal atoms in metal nanoclusters, but control of surface ligand number at a given cluster size is rare. Here we demonstrate that reversible addition and elimination of a single surface thiolate ligand (-SR) on gold nanoclusters can be realized, opening the door to precision ligand engineering on atomically precise nanoclusters. We find that oxidative etching of [Au₂₅SR₁₈]⁻ nanoclusters adds an excess thiolate ligand and generates a new species, [Au₂₅SR₁₉]⁰. The addition reaction can be reversed by CO reduction of [Au₂₅SR₁₉]⁰, leading back to [Au₂₅SR₁₈]⁻ and eliminating precisely one surface ligand. Intriguingly, we show that the ligand shell of Au₂₅ nanoclusters becomes more fragile and rigid after ligand addition. This reversible addition/elimination reaction of a single surface ligand on gold nanoclusters shows potential to precisely control the number of surface ligands and to explore new ligand space at each nuclearity.

Diverse methods have been developed to tailor the number of metal atoms in metal nanoclusters, but controlling the number of surface ligands is rare. Here, the authors realize reversible addition and elimination of a single thiolate ligand on a gold nanocluster.

A pair of 2D chiral Ag(i) enantiomers with dual chiral elements: syntheses, structures, and photoluminescent and chiroptical properties

In this work, two new enantiopure bis-monodentate N-donor chiral ligands, namely (-)/(+)-2-(4'-pyridyl)-4,5-pinene-pyridine (L R /L S ), have been designed and synthesized. Using L R and L S as bridging ligands to react with AgClO4, a pair of novel 2D chiral Ag(i) enantiomers formulated as [Ag2(L R )2(ClO4)2] n (R-1) and [Ag2(L S )2(ClO4)2] n (S-1) were isolated and characterized. In R-1 and S-1, each Ag(i) ion is bonded by two N atoms from two different chiral L R or L S ligands, leading to the formation of 1D right- or left-handed -L-Ag(i)-L- helical chains. Moreover, two adjacent helical chains are further doubly linked by two monodentate ClO4 - anions through weak Ag-O contacts to form 2D network structures, in which dual chiral elements, i.e., center chirality and helical chirality coexist. Interestingly, each free ligand L R /L S and R-1/S-1 enantiomers show very different ECD spectra in the solid state and in solution, which are correlated to the intermolecular interactions and molecular structures in each state, respectively. Notably, as a representative, R-1 exhibits intense room temperature photoluminescence both in the solid state and in solution with different emission features and mechanisms, while it also shows more intense emission than that of free ligand L R . In particular, R-1 and S-1 represent the first examples of 2D Ag(i) chiral coordination polymers (CCPs) supported by ClO4 - anions, possessing dual chiral elements.

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.

Composition-Dependent Antimicrobial Ability of Full-Spectrum AuxAg25-x Alloy Nanoclusters

Alloying is an efficient chemistry to diversify the properties of metal nanoparticles; however, the atomic-level understandings of the composition-dependent physicochemical properties and their related biological performance are presently lacking. Here, we developed a full spectrum of alloy metal nanoclusters (NCs), Au_xAg_25-x(MHA)₁₈ (MHA = 6-mercaptohexanoic acid) with x = 0-25, and investigated their composition-dependent antimicrobial performance. Interestingly, we observed a U-shape antimicrobial behavior of Au_xAg_25-x(MHA)₁₈ NCs, where the alloy NCs showed decreased antimicrobial ability instead of the common trend of increasing. Detailed atomic-level characterizations of the AuAg NCs suggest that the decreased performance of alloy NCs is due to their enhanced stability after alloying, which can deactivate their capability in generating reactive oxygen species (ROS) that can kill the bacteria. More interestingly, the transition point of the antimicrobial performance was only obtained with our full-spectrum Au_xAg_25-x(MHA)₁₈ NCs, which indicates the importance of exploring the composition-dependent properties and application performance in a full-spectrum composition range. A library of full-spectrum alloy NCs also provides a good platform to investigate other composition-dependent physicochemical and biological properties of metal NCs.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.