Thiol ligand-induced transformation of Au38(SC2H4Ph)24 to Au36(SPh-t-Bu)24

Ligand exchange reactions on the chiral Au38 cluster: CD modulation caused by the modification of the ligand shell composition

Ligand exchange reactions have become a highly versatile post-synthetic strategy to accurately engineer the ligand shell of atomically precise noble metal nanoclusters. Modifying the chemical structure of the exchanging ligand with chromophore substituents or adding chiral centers allow direct functionalization of the cluster with desired properties. As such, post-functionalized gold nanoclusters with unique physicochemical properties find applications in optoelectronics, catalysis and biomedicine. Herein, we successfully carried out ligand exchange reactions between the chiral Au38(2-PET)24 cluster (both racemic and enantiopure forms) and the helically chiral but configurationally labile 2-thio[4]helicene ligand (TH4). The reaction products with a composition of Au38(2-PET)24-x(TH4)x were analyzed using UV-vis spectroscopy and MALDI mass spectrometry. It was found that up to ten 2-PET ligands can be replaced with the helicene ligand on the cluster surface according to MALDI analysis. Consequently, the UV-vis and CD spectra of the cluster have been strongly affected by the ligand exchange reaction. The intensities of the CD signals of Au38(2-PET)24-x(TH4)x were drastically reduced and red shifted with respect to the reference Au38(2-PET)24 cluster. Moreover, the appearance of the other enantiomer in the HPLC chromatogram revealed the partial racemization of the cluster. DFT calculations were performed and they support the experimental observations and show that the observed chiroptical changes in UV-vis and CD spectra are exchange-site dependent. The calculations also demonstrate that charge transfer (CT) transitions occur between the Au38 cluster and the helicene ligand. Thus the ligand is directly involved in these transitions and contributes to the electronic states comprising those transitions.

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.

Size Transformation of the Au22(SG)18 Nanocluster and Its Surface-Sensitive Kinetics

For many applications of well-defined gold nanoclusters, it is desirable to understand their structural evolution behavior under working conditions with molecular precision. Here we report the first systematic investigation of the size transformation products of the Au₂₂(SG)₁₈ nanocluster under representative working conditions and highlight the surface effect on the transformation kinetics. Under thermal and aerobic conditions, the consecutive and pH-dependent transformation from Au₂₂ to both well-defined clusters and small Au(I)SR species was identified by ESI-MS and UV-vis spectroscopy. By introducing a perturbation onto the Au₂₂ surface, significant changes in the activation parameters were determined from the kinetic study of the Au₂₂ transformation. This indicates the sensitivity of the nanocluster transformation pathway to the cluster surface. The systematic study of cluster transformation and the sensitivity of cluster transformation to the surface revealed herein has significant implications for future attempts to design gold nanoparticles with adaptation to the working environment and the regeneration of active nanoparticles.

Ligand exchange on Au38(SR)24: substituent site effects of aromatic thiols

Understanding the critical roles of ligands (e.g. thiolates, SR) in the formation of metal nanoclusters of specific sizes has long been an intriguing task since the report of ligand exchange-induced transformation of Au38(SR)24 into Au36(SR')24. Herein, we conduct a systematic study of ligand exchange on Au38(SC2H4Ph)24 with 21 incoming thiols and reveal that the size/structure preference is dependent on the substituent site. Specifically, ortho-substituted benzenethiols preserve the structure of Au38(SR)24, while para- or non-substituted benzenethiols cause its transformation into Au36(SR)24. Strong electron-donating or -withdrawing groups do not make a difference, but they will inhibit full ligand exchange. Moreover, the crystal structure of Au38(SR)24 (SR = 2,4-dimethylbenzenethiolate) exhibits distinctive ππ stacking and "anagostic" interactions (indicated by substantially short AuH distances). Theoretical calculations reveal the increased energies of frontier orbitals for aromatic ligand-protected Au38, indicating decreased electronic stability. However, this adverse effect could be compensated for by the AuH-C interactions, which improve the geometric stability when ortho-substituted benzenethiols are used. Overall, this work reveals the substituent site effects based on the Au38 model, and highlights the long-neglected "anagostic" interactions on the surface of Au-SR NCs which improve the structural stability.

Rh-Sb Nanoclusters: Synthesis, Structure, and Electrochemical Studies of the Atomically Precise [Rh20Sb3(CO)36]3- and [Rh21Sb2(CO)38]5- Carbonyl Compounds

The reactivity of [Rh₇(CO)₁₆]^3– with SbCl₃ has been deeply investigated with the aim of finding a new approach to prepare atomically precise metalloid clusters. In particular, by varying the stoichiometric ratios, the reaction atmosphere (carbon monoxide or nitrogen), the solvent, and by working at room temperature and low pressure, we were able to prepare two large carbonyl clusters of nanometer size, namely, [Rh₂₀Sb₃(CO)₃₆]^3– and [Rh₂₁Sb₂(CO)₃₈]^5–. A third large species composed of 28 metal atoms was isolated, but its exact formulation in terms of metal stoichiometry could not be incontrovertibly confirmed. We also adopted an alternative approach to synthesize nanoclusters, by decomposing the already known [Rh₁₂Sb(CO)₂₇]^3– species with PPh₃, willing to generate unsaturated fragments that could condense to larger species. This strategy resulted in the formation of the lower-nuclearity [Rh₁₀Sb(CO)₂₁PPh₃]^3– heteroleptic cluster instead. All three new compounds were characterized by IR spectroscopy, and their molecular structures were fully established by single-crystal X-ray diffraction studies. These showed a distinct propensity for such clusters to adopt an icosahedral-based geometry. Their characterization was completed by ESI-MS and NMR studies. The electronic properties of the high-yield [Rh₂₁Sb₂(CO)₃₈]^5– cluster were studied through cyclic voltammetry and in situ infrared spectroelectrochemistry, and the obtained results indicate a multivalent nature.

The reactivity of [Rh₇(CO)₁₆]³⁻ with SbCl₃ has been deeply investigated as a new approach to prepare atomically precise metal nanoparticles. By varying the reaction conditions, we obtained three large carbonyl nanoclusters, [Rh₂₀Sb₃(CO)₃₆]³⁻, [Rh₂₁Sb₂(CO)₃₈]⁵⁻, and [Rh_28−xSb_x(CO)₄₄]⁶⁻, and the lower-nuclearity [Rh₁₀Sb(CO)₂₁PPh₃]³⁻ species. They have all been characterized through X-ray diffraction, IR spectroscopy, and other techniques based on their specific nature. Spectroelectrochemical studies on [Rh₂₁Sb₂(CO)₃₈]⁵⁻ unravelled its multivalent nature.

Size Evolution Dynamics of Gold Nanoclusters at an Atom-Precision Level: Ligand Exchange, Growth Mechanism, Electrochemical, and Photophysical Properties

Interpretation of size evolution is an essential part of nanocluster transformation processes for unraveling the mechanism at an atom-precision level. Here we report the transformation of a non-superatomic Au₂₃ to a superatomic Au₃₆ nanocluster via Au₂₈ cluster formation, activated by the bulky 4-tert-butylbenzenethiol ligand. Time-dependent matrix-assisted laser desorption ionization mass spectrometry data revealed that the conversion proceeds through ligand exchange followed by the size focusing method, ultimately leading to size growth. We also validated this transformation through time-dependent ultraviolet-visible data. Density functional theory calculations predicted that the kernel of the Au₂₈ cluster evolved through a linear combination of molecular orbitals of the fragment of 2e^- units (Au₄²⁺ and Au₃⁺) from the kernel of the Au₂₃ cluster. Periodic growth of gold cores through continuous growth of Au₄ tetrahedral unit leads to the formation of the Au₃₆ cluster from the Au₂₈ cluster. These results reinforce the plausibility of size evolution through the growth mechanism during the transformation process. Differential pulse voltammetry studies showed that the highest occupied molecular orbital-lowest unoccupied molecular orbital gap inversely varies with the kernel size of these clusters. Photophysical experiments support the molecular-like intersystem crossing rather than core-shell relaxation to these clusters. The trends of photoluminescence lifetime were found to be the reverse of those of the energy gap law. The increment of lifetimes for the larger cluster can be mainly due to the contribution of both hot carriers and band-edge carriers.

Post-synthetic functionalization and ligand exchange reactions in gold(i) phenylthiolate-based coordination polymers

Post-modification and ligand exchange reactions from 1D or 2D gold thiolate coordination polymers occur through a dissolution–recrystallization pathway.

An overview on the current understanding of the photophysical properties of metal nanoclusters and their potential applications

Photophysics of atomically precise metal nanoclusters (MNCs) is an emerging area of research due to their potential applications in optoelectronics, photovoltaics, sensing, bio-imaging and catalysis. An overview of the recent advances in the photophysical properties of MNCs is presented in this review. To begin with, we illustrate general synthesis methodologies of MNCs using direct reduction, chemical etching, ligand exchange, metal exchange and intercluster reaction. Due to strong quantum confinement, the NCs possess unique electronic properties such as discrete optical absorption, intense photoluminescence (PL), molecular-like electron dynamics and non-linear optical behavior. Discussions have also been carried out to unveil the influence of the core size, nature of ligands, heteroatom doping, and surrounding environments on the optical absorption and photophysical properties of metal clusters. Recent findings reveal that the excited-state dynamics, nonlinear optical properties and aggregation induced emission of metal clusters offer exciting opportunities for potential applications. We discuss briefly about their versatile applications in optoelectronics, sensing, catalysis and bio-imaging. Finally, the future perspective of this research field is given.

Elucidating ligand effects in thiolate-protected metal clusters using Au24Pt(TBBT)18 as a model cluster

2-Phenylethanethiolate (PET) and 4-tert-butylbenzenethiolate (TBBT) are the most frequently used ligands in the study of thiolate (SR)-protected metal clusters. However, the effect of difference in the functional group between these ligands on the fundamental properties of the clusters has not been clarified. We synthesized [Au24Pt(TBBT)18]0, which has the same number of metal atoms, number of ligands, and framework structure as [Au24Pt(PET)18]0, by replacing ligands of [Au24Pt(PET)18]0 with TBBT. It was found that this ligand exchange is reversible unlike the case of other metal-core clusters. A comparison of the geometrical/electronic structure and stability of the clusters between [Au24Pt(PET)18]0 and [Au24Pt(TBBT)18]0 revealed three things with regard to the effect of ligand change from PET to TBBT on [Au24Pt(SR)18]0: (1) the induction of metal-core contraction and Au-S bond elongation, (2) no substantial effect on the HOMO-LUMO gap but a clear difference in optical absorption in the visible region, and (3) the decrease of stabilities against degradation in solution and under laser irradiation. By using these two clusters as model clusters, it is expected that the effects of the structural difference of ligand functional-groups on the physical properties and functions of clusters, such as catalytic ability and photoluminescence, would be clarified.

Light-Induced Size-Growth of Atomically Precise Nanoclusters

A photo-induced transformation from [Au₂₃(S-c-C₆)₁₆]^-(TOA)⁺ to Au₂₈(S-c-C₆)₂₀ nanocluster was first reported in this work. The [Au₂₃(S-c-C₆)₁₆]^-(TOA)⁺ nanocluster is first excited to [Au₂₃(S-c-C₆)₁₆]^•-(TOA)⁺ by photons with energy higher than its E_g (E_g = HOMO - LUMO energy gap), and then, the negatively charged [Au₂₃(S-c-C₆)₁₆]^•- nanocluster was oxidized to the neutral state by transfering one electron to O₂. The unstable neutral cluster [Au₂₃(S-c-C₆)₁₆]⁰ obtained was decomposed into smaller nanocluster and finally reassembled into the Au₂₈(S-c-C₆)₂₀ nanocluster. Time-dependent UV-vis, matrix-assisted laser desorption/ionization time of flight mass spectrometry, electron paramagnetic resonance, and electrospray ionization mass spectrometry characterizations were performed to monitor the nanocluster size transformation.

Identification of Intermediate Au22(SR)4(SR')14 Cluster on Ligand-Induced Transformation of Au25(SR)18 Nanocluster

We report the ligand-exchange-induced transformation from an icosahedral Au₂₅(SR)₁₈ cluster (where SR = 2-phenylethanethiol (PET)) to a bitetrahedral Au₂₂(SR)₄(SR')₁₄ cluster (where SR' = 4-tert-butylbenzenethiol (TBBT)). This partial exchange of the ligands was achieved by controlling the concentration of the incoming TBBT ligand. Being a bulky and aromatic ligand, TBBT can efficiently distort the atomic structure of the Au₂₅PET₁₈ cluster, resulting in Au₂₂(PET)₄(TBBT)₁₄, which is highly stable and considered to be an intermediate with a bitetrahedral structure. Time-dependent mass spectrometry and optical spectroscopy revealed the dissociation of the parent cluster and gave a deep insight on the ligand-exchange mechanism. Theoretical calculations and extended X-ray absorption fine structure studies confirm the formation of the Au₂₂ structure. Identifying the atomic structure of the intermediate species opens a new avenue to study the transformation of one cluster to another.

The Ligand-Exchange Reactions of Rod-Like Au25-n Mn (M=Au, Ag, Cu, Pd, Pt) Nanoclusters with Cysteine - A Density Functional Theory Study

The atomic precision of ultrasmall noble-metal nanoclusters (NMNs) is fundamental for elucidating structure-property relationships and probing their practical applications. So far, the atomic structure of NMNs protected by organic ligands has been widely elucidated, whereas the precise atomic structure of NMNs protected by water-soluble ligands (such as peptides and nucleic acid), has been rarely reported. With the concept of "precision to precision", density functional theory (DFT) calculations were performed to probe the thermodynamic plausibility and inherent determinants for synthesizing atomically precise, water-soluble NMNs via the framework-maintained two-phase ligand-exchange method. A series of rod-like Au_25-n M_n (M=Au, Ag, Cu, Pd, Pt) NMNs with the same framework but varied ligands and metal compositions was chosen as the modeling reactants, and cysteine was used as the modeling water-soluble ligand. It was found that the acidity of the reaction remarkably affects the thermodynamic facility of the ligand exchange reactions. Ligand effects (structural distortion and acidity) dominate the overall thermodynamic facility of the ligand-exchange reaction, while the number and type of doped metal atom(s) has little influence.

Metal Nanoclusters: New Paradigm in Catalysis for Water Splitting, Solar and Chemical Energy Conversion

A sustainable future demands innovative breakthroughs in science and technology today, especially in the energy sector. Earth-abundant resources can be explored and used to develop renewable and sustainable resources of energy to meet the ever-increasing global energy demand. Efficient solar-powered conversion systems exploiting inexpensive and robust catalytic materials for the photo- and photo-electro-catalytic water splitting, photovoltaic cells, fuel cells, and usage of waste products (such as CO₂ ) as chemical fuels are appealing solutions. Many electrocatalysts and nanomaterials have been extensively studied in this regard. Low overpotentials, catalytic stability, and accessibility remain major challenges. Metal nanoclusters (NCs, ≤3 nm) with dimensions between molecule and nanoparticles (NPs) are innovative materials in catalysis. They behave like a "superatom" with exciting size- and facet-dependent properties and dynamic intrinsic characteristics. Being an emerging field in recent scientific endeavors, metal NCs are believed to replace the natural photosystem II for the generation of green electrons in a viable way to facilitate the challenging catalytic processes in energy-conversion schemes. This Review aims to discuss metal NCs in terms of their unique physicochemical properties, possible synthetic approaches by wet chemistry, and various applications (mostly recent advances in the electrochemical and photo-electrochemical water splitting cycle and the oxygen reduction reaction in fuel cells). Moreover, the significant role that MNCs play in dye-sensitized solar cells and nanoarrays as a light-harvesting antenna, the electrochemical reduction of CO₂ into fuels, and concluding remarks about the present and future perspectives of MNCs in the frontiers of surface science are also critically reviewed.

Chiral Inversion of Thiolate-Protected Gold Nanoclusters via Core Reconstruction without Breaking a Au-S Bond

On the basis of density functional theory computations of the well-known chiral Au₃₈(SR)₂₄ nanocluster and its Pd- and Ag-doped derivatives, we propose here a mechanism for chiral inversion that does not require the breaking of a metal–sulfur bond at the metal–ligand interface but features a collective rotation of the gold core. The calculated energy barriers for this mechanism for Au₃₈ and Pd-doped Au₃₈ are in the range of 1–1.5 eV, significantly lower than barriers involving the breakage of Au–S bonds (2.5 eV). For Ag-doped Au₃₈, barriers for both mechanisms are similar (1.3–1.5 eV). Inversion barriers for a larger chiral Au₁₄₄(SR)₆₀ are much higher (2.5−2.8 eV). Our computed barriers are in good agreement with racemization barriers estimated from existing experiments for bare and doped Au₃₈. These results highlight the sensitivity of chiral inversion to the size, structure, and metal composition of the metal core and sensitivity to the detailed structure of the metal–thiolate interface. Our work also predicts that enantiopure Au₁₄₄(SR)₆₀ clusters would be promising materials for applications requiring high resistance to chiral inversion.

Characterization of chemically modified gold and silver clusters in gas phase

Atomically precise Au and Ag clusters protected by organic ligands can be viewed as chemically modified Au/Ag superatoms and have attracted interest as promising building units of functional materials and ideal platforms for studying the size-dependent evolution of structures and properties. Their structures, stability, and physicochemical properties have been characterized in solution and solid (or crystalline) phases by various methods conventionally used in materials science. However, novel and complementary information on their intrinsic stability and structures can be obtained by applying a variety of gas-phase methods, including mass spectrometry, ion mobility mass spectrometry, collision- or surface-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation mass spectrometry, to the chemically modified Au/Ag superatoms isolated in the gas phase. This perspective describes our recent efforts in the gas-phase studies on chemically synthesized Au/Ag superatoms.

Two-photon absorption and photoluminescence of colloidal gold nanoparticles and nanoclusters

This review provides a comprehensive description of nonlinear optical (NLO) properties of gold nanoparticles, which can be used in biological applications. The main focus is placed on two-photon absorption (2PA) and two-photon excited photoluminescence (2PEL) - the processes crucial for multiphoton microscopy, which allows deeper imaging of the material and causes less damage to the biological samples in comparison to conventional (one-photon) microscopy. We present the basics of 2PA measurement techniques and a summary of recent achievements in the understanding of multiphoton excitation and the resulting photoluminescence in gold nanoparticles, both plasmonic ones and small nanoclusters with molecule-like properties. The examples of 2PA applications in bioimaging are also presented, with a comment on future challenges and applications.

Transformation from [Au25(SCH2CH2CH2CH3)18]0 to Au28(SCH2CH(CH3)Ph)21 gold nanoclusters: gentle conditions is enough

Herein we report the transformation of [Au₂₅(SR)₁₈]⁰ into Au₂₈(SR)₂₁ induced by ligand exchange reaction under mild conditions.

Tailoring the photoluminescence of atomically precise nanoclusters

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

Engineering Functional Metal Materials at the Atomic Level

With continuous research efforts devoted into synthesis and characterization chemistry of functional nanomaterials in the past decades, the development of metal materials is stepping into a new era, where atom-by-atom customization of property-dictating structural attributes is expected. Herein, the state-of-the-art modulation of functional metal nanomaterials at the atomic level, by size- and structure-controlled synthesis of thiolate-protected metal (e.g., Au and Ag) nanoclusters (NCs), is exemplified. Metal NCs are ultrasmall (<3 nm) particles with hierarchical primary, secondary, and tertiary structures, reminiscent of natural proteins or enzymes. Given the proven dependence of their physicochemical properties on their size and structure, documented synthetic methodologies delivering NCs with atomic-level monodispersity and tailorable size and structural attributes at individual hierarchical levels are categorized and discussed. Such assured atomic-level modulation could confer metal NCs with novel application opportunities in diverse fields, which are also exemplified by their size- and structure-dictated catalytic and biomedical performance. The precise synthesis and application chemistry developed based on the hierarchical structure scheme of metal NCs could increase the acceptance of metal NCs as a new family of functional materials.

Interface Engineering of Gold Nanoclusters for CO Oxidation Catalysis

Catalysts based on atomically precise gold nanoclusters serve as an ideal model to relate the catalytic activity to the geometrical and electronic structures as well as the ligand effect. Herein, we investigate three series of ligand (thiolate)-protected gold nanoclusters, including Au₃₈(SR)₂₄, Au₃₆(SR')₂₄, and Au₂₅(SR″)₁₈, with a focus on their interface effects using carbon monoxide (CO) oxidation as a probe reaction. The first comparison is within each series, which reveals the same trend for the three series that, rather than the bulkiness of carbon tails as commonly thought, the steric hindrance of ligands at the interface between the thiolate, Au, and CeO₂ inhibits CO adsorption onto Au sites and hence adversely affects the activity of CO oxidation. The second comparison is between the sets Au₃₈(SR)₂₄ and Au₃₆(SR')₂₄ of nearly the same size, which reveals that the Au₃₆(SR')₂₄ nanoclusters (with face centered cubic structure) are not sensitive to thermal pretreatment conditions, whereas the Au₃₈(SR)₂₄ catalysts (icosahedral structure) are and an optimum activity is observed at a pretreatment temperature of 150 °C. Overall, the atomically precise Au _n(SR) _m nanoclusters have revealed unprecedented details on the catalytic interface and atomic structure effects. It is hoped that such insights will benefit the ultimate goal of catalysis in future design of enzymelike catalysts for environmentally friendly green catalysis.

Precision at the nanoscale: on the structure and property evolution of gold nanoclusters

Chemists are often regarded as “architects”, who are capable of building up complex molecular structures in the ultrasmall-dimensional world. However, compared with organic chemistry, nanochemistry – which deals with nanoparticles in the size range from 1 to 100 nm – is less precise in terms of synthesis, composition, and structure. Such an imprecise nature of nanochemistry has impeded an in-depth understanding as well as rational control of structures and properties of nanomaterials. Motivated by this, thiolate-protected gold nanoclusters (denoted as Au n (SR) m ) have recently emerged as a paradigm of atomically precise nanomaterials, in which all the nanoparticles are identical to each other with the same number of core atoms (n) and surface ligands (m) as well as the atomic arrangement. In this review, we provide a demonstration of how the precise nature of Au n (SR) m nanoclusters allows one to understand, decipher and discover some important, enigmatic and intriguing issues and phenomena in nanoscience, including (i) a precise nanoscale transformation reaction induced by surface ligand exchange, (ii) the total structures of crystalline metal phases and the self-assembled surface monolayers, (iii) the periodicities and quantum confinement in nanoclusters and (iv) the emergence of hierarchical complexity in the entire nanoparticle system. We expect that such an in-depth understanding will eventually lead to the rational design and precise engineering of complex architectures at the nanoscale.

Ligand Structure Determines Nanoparticles' Atomic Structure, Metal-Ligand Interface and Properties

The nature of the ligands dictates the composition, molecular formulae, atomic structure and the physical properties of thiolate protected gold nanomolecules, Aun(SR)m. In this review, we describe the ligand effect for three classes of thiols namely, aliphatic, AL or aliphatic-like, aromatic, AR, or bulky, BU thiol ligands. The ligand effect is demonstrated using three experimental setups namely: (1) The nanomolecule series obtained by direct synthesis using AL, AR, and BU ligands; (2) Molecular conversion and interconversion between Au38(S-AL)24, Au36(S-AR)24, and Au30(S-BU)18 nanomolecules; and (3) Synthesis of Au38, Au36, and Au30 nanomolecules from one precursor Aun(S-glutathione)m upon reacting with AL, AR, and BU ligands. These nanomolecules possess unique geometric core structure, metal-ligand staple interface, optical and electrochemical properties. The results unequivocally demonstrate that the ligand structure determines the nanomolecules' atomic structure, metal-ligand interface and properties. The direct synthesis approach reveals that AL, AR, and BU ligands form nanomolecules with unique atomic structure and composition. Similarly, the nature of the ligand plays a pivotal role and has a significant impact on the passivated systems such as metal nanoparticles, quantum dots, magnetic nanoparticles and self-assembled monolayers (SAMs). Computational analysis demonstrates and predicts the thermodynamic stability of gold nanomolecules and the importance of ligand-ligand interactions that clearly stands out as a determining factor, especially for species with AL ligands such as Au38(S-AL)24.

Gold Fusion: From Au25(SR)18 to Au38(SR)24, the Most Unexpected Transformation of a Very Stable Nanocluster

The study of the molecular cluster Au₂₅(SR)₁₈ has provided a wealth of fundamental insights into the properties of clusters protected by thiolated ligands (SR). This is also because this cluster has been particularly stable under a number of experimental conditions. Very unexpectedly, we found that paramagnetic Au₂₅(SR)₁₈⁰ undergoes a spontaneous bimolecular fusion to form another benchmark gold nanocluster, Au₃₈(SR)₂₄. We tested this reaction with a series of Au₂₅ clusters. The fusion was confirmed and characterized by UV-vis absorption spectroscopy, ESI mass spectrometry, ¹H and ¹³C NMR spectroscopy, and electrochemistry. NMR evidences the presence of four types of ligand and, for the same proton type, double signals caused by the diastereotopicity arising from the chirality of the capping shell. This effect propagates up to the third carbon atom along the ligand chain. Electrochemistry provides a particularly convenient way to study the evolution process and determine the fusion rate constant, which decreases as the ligand length increases. No reaction is observed for the anionic clusters, whereas the radical nature of Au₂₅(SR)₁₈⁰ appears to play an important role. This transformation of a stable cluster into a larger stable cluster without addition of any co-reagent also features the bottom-up assembly of the Au₁₃ building block in solution. This very unexpected result could modify our view of the relative stability of molecular gold nanoclusters.

Single-ligand exchange on an Au-Cu bimetal nanocluster and mechanism

An Au-Cu bimetallic nanocluster co-capped by selenolate and phosphine is obtained and its X-ray structure shows an icosahedral Au13 kernel surrounded by three CuSe2PPh2Py motifs and one CuSe3 motif, formulated as [Au13Cu4(PPh2Py)3(SePh)9]. Interestingly, a single-ligand exchange process is observed in the growth reaction, in which an [Au13Cu4(PPh2Py)4(SePh)8]+ intermediate is first formed, but a prolonged reaction leads to one PPh2Py ligand being selectively replaced by a PhSe-ligand. DFT simulations reveal that both steric hindrance and bond dissociation energy have great effects on the single-ligand exchange reaction as well as the thermodynamics, which help to understand the mechanism of the ligand exchange. Temperature-dependent UV-vis absorption and photoluminescence (PL) properties of the Au-Cu nanocluster imply that the optical properties are mainly contributed by the metal core. Femtosecond time-resolved pump-probe analysis maps out further details of the PL process.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Revealing isoelectronic size conversion dynamics of metal nanoclusters by a noncrystallization approach

Atom-by-atom engineering of nanomaterials requires atomic-level knowledge of the size evolution mechanism of nanoparticles, which remains one of the greatest mysteries in nanochemistry. Here we reveal atomic-level dynamics of size evolution reaction of molecular-like nanoparticles, i.e., nanoclusters (NCs) by delicate mass spectrometry (MS) analyses. The model size-conversion reaction is [Au₂₃(SR)₁₆]⁻ → [Au₂₅(SR)₁₈]⁻ (SR = thiolate ligand). We demonstrate that such isoelectronic (valence electron count is 8 in both NCs) size-conversion occurs by a surface-motif-exchange-induced symmetry-breaking core structure transformation mechanism, surfacing as a definitive reaction of [Au₂₃(SR)₁₆]⁻ + 2 [Au₂(SR)₃]⁻ → [Au₂₅(SR)₁₈]⁻ + 2 [Au(SR)₂]⁻. The detailed tandem MS analyses further suggest the bond susceptibility hierarchies in feed and final Au NCs, shedding mechanistic light on cluster reaction dynamics at atomic level. The MS-based mechanistic approach developed in this study also opens a complementary avenue to X-ray crystallography to reveal size evolution kinetics and dynamics.

How metal nanoclusters evolve in size is poorly understood, particularly at the atomic level. Here, the authors use mass spectrometry to study the size conversion dynamics between two isoelectronic gold nanoclusters with atomic resolution, revealing that the growth reaction proceeds through a distinct balanced equation.

Connections Between Theory and Experiment for Gold and Silver Nanoclusters

Ligand-stabilized gold and silver nanoparticles are of tremendous current interest in sensing, catalysis, and energy applications. Experimental and theoretical studies have closely interacted to elucidate properties such as the geometric and electronic structures of these fascinating systems. In this review, the interplay between theory and experiment is described; areas such as optical absorption and doping, where the theory-experiment connections are well established, are discussed in detail; and the current status of these connections in newer fields of study, such as luminescence, transient absorption, and the effects of solvent and the surrounding environment, are highlighted. Close communication between theory and experiment has been extremely valuable for developing an understanding of these nanocluster systems in the past decade and will undoubtedly continue to play a major role in future years.

X-ray crystal structure and doping mechanism of bimetallic nanocluster Au36−xCux(m-MBT)24(x= 1–3)

Combined experimental and theoretical methods have been used to explore the doping preference of Cu atoms in novel Au_36−xCu_x(m-MBT)₂₄(x= 1–3) nanoclusters.

Dynamic Nature of Thiolate Monolayer in Au25(SR)18 Nanoclusters

Thiolate monolayer, protecting a gold nanocluster, is responsible for its chemical behavior and interaction with the environment. Understanding the parameters that influence the stability and reactivity of the monolayer will enable its precise and controlled functionalization. Here we present a protocol for the investigation of the monolayer reactivity in Au₂₅(SR)₁₈ based on MALDI mass spectrometry and NMR spectroscopy. Thiol exchange reaction between cluster and thiol molecules has been investigated showing how this reaction is affected by several factors (stability of the thiols in solution, the affinity of the sulfur to the gold cluster, intermolecular interactions within the ligand layer, etc.). Furthermore, intercluster thiol exchange has been clarified to occur during collisions between particles without thiol release to the solution. In this reaction, the stability of the thiols in solution and the affinity of the sulfur to the gold for the two thiols do not affect the equilibrium position because for both thiols one S-Au bond is broken and one is formed within the cycle. Importantly, the rate of direct thiol exchange between clusters is comparable to that of the ligand exchange with free thiols. However, the thermodynamic driving force of the two reactions is different, since only the latter involves free thiol species.

Two-Way Transformation between fcc- and Nonfcc-Structured Gold Nanoclusters

Precisely tuning the structure of nanomaterials, especially in a two-way style, is challenging but of great importance for regulating properties and for practical applications. The structural transformation from nonfcc to fcc (face center cubic) in gold nanoclusters has been recently reported; however, the reverse process, that is, the structural transformation from fcc to nonfcc, not to mention the two-way structural transformation between fcc and nonfcc, remains unknown. We developed a novel synthesis method, successfully fulfilled the two-way structure transformation, and studied the stability of gold nanoclusters with different structures. Additionally, a novel gold nanocluster was synthesized and structurally resolved by single-crystal X-ray crystallography. This work has important implications for structure and property tuning of gold nanoclusters and might open up some new potential applications for gold nanoclusters.

Synthesis of Au38(SCH2CH2Ph)24, Au36(SPh-tBu)24, and Au30(S-tBu)18 Nanomolecules from a Common Precursor Mixture

Phenylethanethiol protected nanomolecules such as Au₂₅, Au₃₈, and Au₁₄₄ are widely studied by a broad range of scientists in the community, owing primarily to the availability of simple synthetic protocols. However, synthetic methods are not available for other ligands, such as aromatic thiol and bulky ligands, impeding progress. Here we report the facile synthesis of three distinct nanomolecules, Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-tBu)₂₄, and Au₃₀(S-tBu)₁₈, exclusively, starting from a common Au_n(glutathione)_m (where n and m are number of gold atoms and glutathiolate ligands) starting material upon reaction with HSCH₂CH₂Ph, HSPh-tBu, and HStBu, respectively. The systematic synthetic approach involves two steps: (i) synthesis of kinetically controlled Au_n(glutathione)_m crude nanocluster mixture with 1:4 gold to thiol molar ratio and (ii) thermochemical treatment of the purified nanocluster mixture with excess thiols to obtain thermodynamically stable nanomolecules. Thermochemical reactions with physicochemically different ligands formed highly monodispersed, exclusively three different core-size nanomolecules, suggesting a ligand induced core-size conversion and structural transformation. The purpose of this work is to make available a facile and simple synthetic method for the preparation of Au₃₈(SCH₂CH₂Ph)₂₄, Au₃₆(SPh-tBu)₂₄, and Au₃₀(S-tBu)₁₈, to nonspecialists and the broader scientific community. The central idea of simple synthetic method was demonstrated with other ligand systems such as cyclopentanethiol (HSC₅H₉), cyclohexanethiol(HSC₆H₁₁), para-methylbenzenethiol(pMBT), 1-pentanethiol(HSC₅H₁₁), 1-hexanethiol(HSC₆H₁₃), where Au₃₆(SC₅H₉)₂₄, Au₃₆(SC₆H₁₁)₂₄, Au₃₆(pMBT)₂₄, Au₃₈(SC₅H₁₁)₂₄, and Au₃₈(SC₆H₁₃)₂₄ were obtained, respectively.

Synthesis of Aromatic Thiolate-Protected Gold Nanomolecules by Core Conversion: The Case of Au36(SPh-tBu)24

Ultrasmall nanomolecules (<2 nm) such as Au₂₅(SCH₂CH₂Ph)₁₈, Au₃₈(SCH₂CH₂Ph)₂₄, and Au₁₄₄(SCH₂CH₂Ph)₆₀ are well studied and can be prepared using established synthetic procedures. No such synthetic protocols that result in high yield products from commercially available starting materials exist for Au₃₆(SPh-X)₂₄. Here, we report a synthetic procedure for the large-scale synthesis of highly stable Au₃₆(SPh-X)₂₄ with a yield of ∼42%. Au₃₆(SPh-X)₂₄ was conveniently synthesized by using tert-butylbenzenethiol (HSPh-tBu, TBBT) as the ligand, giving a more stable product with better shelf life and higher yield than previously reported for making Au₃₆(SPh)₂₄ from thiophenol (PhSH). The choice of thiol, solvent, and reaction conditions were modified for the optimization of the synthetic procedure. The purposes of this work are to (1) optimize the existing procedure to obtain stable product with better yield, (2) develop a scalable synthetic procedure, (3) demonstrate the superior stability of Au₃₆(SPh-tBu)₂₄ when compared to Au₃₆(SPh)₂₄, and (4) demonstrate the reproducibility and robustness of the optimized synthetic procedure.