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

Dynamic Evolution and Reversibility of a Single Au25 Nanocluster for the Oxygen Reduction Reaction

Ultrasmall metallic nanoclusters (NCs) protected by surface ligands represent the most promising catalytic materials; yet understanding the structure and catalytic activity of these NCs remains a challenge due to dynamic evolution of their active sites under reaction conditions. Herein, we employed a single-nanoparticle collision electrochemistry method for real-time monitoring of the dynamic electrocatalytic activity of a single fully ligand-protected Au25(PPh3)10(SC2H4Ph)5Cl22+ nanocluster (Au252+ NC) at a cavity carbon nanoelectrode toward the oxygen reduction reaction (ORR). Our experimental results and computational simulations indicated that the reversible depassivation and passivation of ligands on the surface of the Au252+ NC, combined with the dynamic conformation evolution of the Au259+ core, led to a characteristic current signal that involves "ON-OFF" switches and "ON" fluctuations during the ORR process of a single Au252+ NC. Our findings reinvent the new perception and comprehension of the structure-activity correlation of NCs at the atomic level.

"Visualizing" the partially reversible conversion of gold nanoclusters via the Au23(S-c-C6H11)17 intermediate

Transformation chemistry of atomically precise metal nanoclusters has emerged as a novel strategy for fundamental research on the structure-property correlations of nanomaterials. However, a thorough understanding of the transformation mechanism is indeed necessary to understand the structural growth patterns and corresponding property evolutions in nanoclusters. Herein, we present the ligand-exchange-induced transformation of the [Au23(SR)16]- (8e-) nanocluster to the [Au25(SR')18]- (8e-) nanocluster, through the Au23(SR)17 (6e-) intermediate species. Identification of this key intermediate through a partially reversible transformation helped in a detailed investigation into the transformation mechanism with atomic precision. Moreover, photophysical studies carried out on this Au23(SR)17 species, which only differs by a single ligand from that of the [Au23(SR)16]- nanocluster reveal the property evolutions at the slightest change in the nanocluster structure.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Accurate assembly of thiophene-bridged titanium-oxo clusters with photocatalytic amine oxidation activity

Designing and synthesizing well-defined crystalline catalysts for the photocatalytic oxidative coupling of amines to imines remains a great challenge. In this work, a crystalline dumbbell-shaped titanium oxo cluster, [Ti10O6(Thdc)(Dmg)2(iPrO)22] (Ti10, Thdc = 2,5-thiophenedicarboxylic acid, Dmg = dimethylglyoxime, iPrOH = isopropanol), was constructed through a facile one-pot solvothermal strategy and treated as a catalyst for the photocatalytic oxidative coupling of amines. In this structure, Thdc serves as the horizontal bar, while the {Ti5Dmg} layers on each side act as the weight plates. The molecular structure, light absorption, and photoelectrochemical properties of Ti10 were systematically investigated. Remarkably, the inclusion of the Thdc ligand, with the assistance of the Dmg ligand, broadens the light absorption spectrum of Ti10, extending it into the visible range. Furthermore, the effective enhancement of charge transfer within the Ti10 was achieved with the successful incorporation of the Thdc ligand, as opposed to PTC-211, where terephthalic acid replaces the Thdc ligand, while maintaining consistency in other aspects of Ti10. Building on this foundation, Ti10 was employed as a heterogeneous molecular photocatalyst for the catalytic oxidative coupling reaction of benzylamine (BA), demonstrating very high conversion activity and selectivity. Our study illustrates that the inclusion of ligands derived from Thdc enhances the efficiency of charge transfer in functionalized photocatalysts, significantly influencing the performance of photocatalytic organic conversion.

All-catecholate-stabilized black titanium-oxo clusters for efficient photothermal conversion

The controlled synthesis of titanium-oxo clusters (TOCs) completely stabilized by organic dye ligands with high stability and superior light absorption remains a significant challenge. In this study, we report the syntheses of three atomically precise catechol (Cat)-functionalized TOCs, [Ti2(Cat)2(OEgO)2(OEgOH)2] (Ti2), [Ti8O5(Cat)9(iPrO)4(iPrOH)2] (Ti8), and [Ti16O8(OH)8(Cat)20]·H2O·PhMe (Ti16), using a solvent-induced strategy (HOEgOH = ethylene glycol; iPrOH = isopropanol; PhMe = toluene). Interestingly, the TiO core of Ti16 is almost entirely enveloped by catechol ligands, making it the first all-catechol-protected high-nuclearity TOC. In contrast, Ti2 and Ti8 have four weakly coordinated ethylene glycol ligands and six weakly coordinated iPrOH ligands, respectively, in addition to the catechol ligands. Ti16 is visually evident in its distinctively black appearance, which belongs to black TOCs (B-TOCs) and exhibits an ultralow optical band gap. Furthermore, Ti16 displays exceptional stability in various media/environments, including exposure to air, solvents, and both acidic and alkaline aqueous solutions due to its comprehensive protection by catechol ligands and rich intra-cluster supramolecular interactions. Ti16 has superior photoelectric response qualities and photothermal conversion capabilities compared to Ti2 and Ti8 due to its ultralow optical band gap and remarkable stability. This discovery not only represents a huge step forward in the creation of all-catecholate-protected B-TOCs with ultralow optical band gaps and outstanding stability, but it also gives key valuable mechanistic insights into their photothermal/electric applications.

Activity of Different AunSn+1 Staples in the Ligand Exchange of Au23(SR)16- with a Single Foreign Thiolate Ligand

Ligand exchange has been widely used to synthesize novel thiolated gold nanoclusters and to regulate their specific properties. Herein, density functional theory (DFT) calculations were conducted to investigate the kinetic profiles of the ligand exchange of the [Au23(SCy)16]- nanocluster with an aromatic thiolate (2-napthalenethiol). The three types of staple motifs (i.e., trimetallic Au3S4, monometallic AuS2, and the bridging thiolates) of the Au23 cluster precursor could be categorized into eight groups of S sites with different chemical environments. The ligand exchange of all of them occurs favorably via the SN1-like pathway, with one site starting with the Au-S dissociation and seven other sites starting with the H-transfer steps. By contrast, the SN2-like pathway (i.e., the synergistic SCy-to-SAr exchange prior to the H-transfer step) is unlikely in the target systems. Meanwhile, the Au-S bond on the capping Au atom of the bicapped icosahedral Au15 core is the most active one, while the S sites on Au3S4 (except for the one remote from the metallic core) are all competitive exchanging sites. The ligand exchange activity of the bridging thiolate and the remote S site on Au3S4 is significantly less reactive. The calculation results correlate with the multiple ligand exchange within only a few minutes and the preferential etching of the AuS2 staple with the foreign ligands reported in earlier experiments. The relative activity of different staples might be helpful in elucidating the inherent principles in the ligand exchange-induced size-evolution of metal nanoclusters.

Tuning Ligands Ratio Allows for Controlling Gold Nanocluster Conformation and Activating a Nonantimicrobial Thiol Fragrance for Effective Treatment of MRSA-Induced Keratitis

Bacterial keratitis is a serious ocular disease that affects millions of people worldwide each year, among which ≈25% are caused by Staphylococcus aureus. With the spread of bacterial resistance, refractory keratitis caused by methicillin-resistant S. aureus (MRSA) affects ≈120 000-190 000 people annually and is a significant cause of infectious blindness. Atomically precise gold nanoclusters (GNCs) recently emerged as promising antibacterial agents; although how the GNC structure and capping ligands control the antibacterial properties remains largely unexplored. In this study, by adjusting the ratio of a "bulky" thiol fragrance to a linear zwitterionic ligand, the GNC conformation is transformed from Au25 (SR)18 to Au23 (SR)16 species, simultaneously converting both inactive thiol ligands into potent antibacterial nanomaterials. Surprisingly, mixed-ligand capped Au23 (SR)16 GNCs exhibit superior antibacterial potency compared to their monoligand counterparts. The optimal GNC is highly potent against MRSA, showing >1024-fold lower minimum inhibitory concentration than the corresponding free ligands. Moreover, it displays excellent potency in treating MRSA-induced keratitis in mice with greatly accelerated corneal recovery (by approximately ninefold). Thus, this study establishes a feasible method to synthesize antibacterial GNCs by adjusting the ligand ratio to control GNC conformation and active non-antibacterial ligands, thereby greatly increasing the repertoires for combating multidrug-resistant bacterial infections.

Dicarboxylic Acids Induced Tandem Transformation of Silver Nanocluster

Structural transformation of metal nanoclusters (NCs) is of great ongoing interest regarding their synthesis, stability, and reactivity. Although sporadic examples of cluster transformations have been reported, neither the underlying transformation mechanism nor the intermediates are unambiguous. Herein, we have synthesized a flexible 54-nuclei silver cluster (Ag54) by combining soft (tBuC≡C-) and hard (nPrCOO-) ligands. The existence of weakly coordinated nPrCOO- enhances the reactivity of Ag54, thus facilitating the dicarboxylic acid to induce structural transformation. X-ray structural analyses reveal that Ag54 transforms to Ag28 cluster-based 2D networks (Ag28a and Ag28b) induced by H2suc (succinic acid) and H2glu (glutaric acid), whereas with H2pda (2,2'-(1,2-phenylene)diacetic acid), a discrete Ag28 cluster (Ag28c) is isolated. The key intermediate Ag17 that emerges during the self-dissociation of Ag54 was isolated by using cryogenic recrystallization and characterized by X-ray crystallography. The "tandem transformation" mechanism for the structure evolution from Ag54 to Ag28a is established by time-dependent electrospray ionization mass spectrometry (ESI-MS) and UV-vis spectroscopy. In addition, the catalytic activity in the 4-nitrophenol reduction follows the sequence Ag28c > Ag28b > Ag28a > Ag54 due to more bare silver sites on the surface of the Ag28 cluster unit. Our findings not only open new avenues to the synthesis of silver NCs but also shed light on a better understanding of the structural transformation mechanism from one cluster to another or cluster-based metal-organic networks induced by dicarboxylates.

Size Growth of Au4Cu4: From Increased Nucleation to Surface Capping

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

Guiding the High-Yield Synthesis of NHC-Ligated Gold Nanoclusters by 19F NMR Spectroscopy

Optimizing the synthesis of atomically precise metal nanoclusters by virtue of molecular tools is highly desirable but quite challenging. Herein we report how ¹⁹F NMR spectroscopy can be used to guide the high-yield synthesis of N-heterocyclic carbene (NHC)-stabilized gold nanoclusters. In spite of little difference, ¹⁹F NMR signals of fluoro-incorporated NHCs (^FNHC) are highly sensitive to the tiny change in their surrounding chemical environments with different N-substituents, metals, or anions, thus providing a convenient strategy to discriminate species in reaction mixtures. By using ¹⁹F NMR, we first disclosed that the one-pot reduction of ^FNHC-Au-X (X is halide) yields multiple compounds, including cluster compounds and also a large amount of highly stable [Au(^FNHC)₂]⁺ byproduct. The detailed quantitative ¹⁹F NMR analyses over the reductive synthesis of NHC-stabilized Au nanoclusters reveal that the formation of the di-NHC complex is deleterious to the high-yield synthesis of NHC-stabilized Au nanoclusters. With the understanding, the reaction kinetic was then slowed by controlling the reduction rate to achieve the high yield of a [Au₂₄(^FNHC)₁₄X₂H₃]³⁺ nanocluster with a unique structure. The strategy demonstrated in this work is expected to provide an effective tool to guide the high-yield synthesis of organic ligand-stabilized metal nanoclusters.

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

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

Highly Efficient Luminescence from Charge-Transfer Gold Nanoclusters Enabled by Lewis Acid

Understanding the complicated intramolecular charge transfer (ICT) behaviors of nanomaterials is crucial to the development of high-quality nanoluminophores for various applications. However, the ICT process in molecule-like metal nanoclusters has been rarely explored. Herein, a proton binding-induced enhanced ICT state is discovered in 6-aza-2-thiothymine-protected gold nanoclusters (ATT-AuNCs). Such an excited-state electron transfer process gives rise to the weakened and red-shifted photoluminescence of these nanoclusters. By the joint use of this newfound ICT mechanism and a restriction of intramolecular motion (RIM) strategy, a red shift in the emission maxima of 30 nm with 27.5-fold higher fluorescence quantum efficiency is achieved after introducing rare-earth scandium ion (Sc³⁺) into ATT-AuNCs. Furthermore, it is found that upon the addition of Sc³⁺, the photoinduced electron transfer (PET) rate from ATT-AuNCs to minocycline is largely accelerated by forming a donor-bridge-acceptor structure. This paper offers a simple method to modulate the luminescent properties of metal nanoclusters for the rational design of next-generation sensing platforms.

Co-reactant-Free Transformation in Atomically Precise Metal Nanoclusters

Transformation chemistry has advanced significantly in recent years as an excellent methodology for synthesizing new nanoclusters and functionalizing the existing ones. However, rational synthesis and fundamental understanding of the structural evolution among clusters have not yet been achieved in nanocluster science. A deeper understanding of the fundamental aspects of structure-property correlation is necessary for the employment of befitting nanoclusters for specific applications. Very recently, the transformation of nanoclusters without the use of conventional co-reactants has been brought to light. These co-reactant-less transformations are triggered by various conditions, such as pH, solvent, light, temperature, etc. In this perspective, we discuss how this unique method of transformation without any co-reactant benefits the basic understanding of growth patterns and the corresponding property evolution in nanoclusters.

Sulfide Boosting Near-Unity Photoluminescence Quantum Yield of Silver Nanocluster

Silver nanoclusters have emerged as promising candidates for optoelectronic applications, but their room-temperature photoluminescence quantum yield (PLQY) is far from ideal to access cutting-edge device performance. Herein, two supertetrahedral silver nanoclusters with high PLQY in non-degassed solution at room temperature were constructed by interiorly supporting the core with multiple VO₄^3- and E^2- anions as structure-directing agents and exteriorly protecting the core with a rigid ligand shell of PhC≡C^- and Ph₂PE₂^- (E = S, Ag64-S; E = Se, Ag64-Se). Both clusters have similar outer Ag₅₈ tetrahedral cages and [Ag₆E₄@(VO₄)₄] cores, forming a pair of comparable clusters to decrypt the origin of such a high PLQY, particularly in Ag64-S, where the PLQY reached up to 97%. The stronger suppression effect of inner sulfides for nonradiative decay is critical to boost the PLQY to near unity. Transient absorption spectroscopy is employed to confirm the phosphorescence nature. The quadruple-capping assembly mechanism involving Ag₇ secondary building units on a Ag₃₆ truncated tetrahedron was also established by collision-induced dissociation studies. This work not only provides a strategy of core engineering for the controlled syntheses of silver nanoclusters with high PLQY but also deciphers the origin of a near-unity PLQY, which lays a foundation for fabricating highly phosphorescent silver nanoclusters in the future.

The mystery of Ph3PS revealed in magic-size Ag–S cluster nucleation

PPh3S was employed to direct the regulation of {Ag32S3} cluster by slowing down the kinetic process of nucleation. The process that Agn(CCBut)m and traces of water induces breakage of PS from [Ag2(Ph3PS)4]2+ to generate {Ag32S3} was established.

Surface Engineering Assisted Size and Structure Modulation of Gold Nanoclusters by Ionic Liquid Cations

Surface modification induced core size/structure change is a recent discovery in inorganic nanoparticles research, and has rarely been revealed at the molecular level. Here, we exemplify with atomically precise Au nanoclusters (NCs) that proper surface modification can selectively stabilize the desired Au⁰ core, conducive to the formation of size/structure-controlled Au NCs. Leveraging π-π enhanced ion-pairing interactions, ionic liquid (IL) cations are bonded to Au^I -thiolate complexes. The hydrophobic-hydrophobic interactions between IL cations subsequently provide a good mechanism to prolong the size of the Au^I -thiolate complexes, selectively producing small-sized Au NCs upon reduction. Through combined control over the structure and concentration of IL cations, pH and solvent polarity, we are able to produce atomically precise Au NCs with customizable size, atomic packing structure, and surface chemistry. This work also provides a facile means to integrate/synergize the materials functionalities of Au NCs and ILs, increasing their acceptance in diverse fields.

Gold Deassembly: From Au44(SPh-tBu)28 to Au36(SPh-tBu)24 Nanocluster through Dynamic Surface Structure Reconstruction

Molecular level understanding of the structural growth patterns and property evolution in nanoclusters (NCs) is crucial for the design and rational synthesis of clusters for specific properties and applications. In this regard, transformation has always been a versatile approach to achieve atomic precision with atomic purity and a deeper understanding of the growth mechanisms of noble metal NCs. To the latter end, we have demonstrated a structural transformation of Au₄₄(SPh-^tBu)₂₈ to Au₃₆(SPh-^tBu)₂₄ NC, which occurred through the deassembly of an Au₈(SPh-^tBu)₄ fragment. Kinetic studies conducted on the transformation showed that it follows zero-order kinetics with a low activation energy pathway. Theoretical studies demonstrated that this process happens via surface restructuring of the core-ligand interface, which was found to be the rate-determining step of this transformation. Based on this, a plausible mechanistic pathway for the transformation have been proposed which we envision, will provide useful insights into NC structure evolution.

Light-Activated Intercluster Conversion of an Atomically Precise Silver Nanocluster

Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron-carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag₄₂(CBDT)₁₅(TPP)₄]^2- (shortly Ag₄₂) using a ligand-exchange induced structural transformation reaction starting from [Ag₁₈H₁₆(TPP)₁₀]²⁺ (shortly Ag₁₈). The formation of Ag₄₂ was confirmed using UV-vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV-vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag₄₂ is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag₁₄(CBDT)₆(TPP)₆] (shortly Ag₁₄). Single crystal X-ray diffraction of Ag₁₄ exhibits Ag₈-Ag₆ core-shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag₁₄. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag₃₇(CBDT)₁₂(TPP)₄]^3- and [Ag₃₅(CBDT)₈(TPP)₄]^2- were formed after 8 h of light irradiation, and the second set comprised of [Ag₃₀(CBDT)₈(TPP)₄]^2-, [Ag₂₆(CBDT)₁₁(TPP)₄]^2-, and [Ag₂₆(CBDT)₇(TPP)₇]^2- were formed after 16 h of irradiation. After 24 h, the conversion to Ag₁₄ was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag₄₂ are responsible for light-activated transformation. Interestingly, Ag₄₂ showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 μs, indicating phosphorescence, while Ag₁₄ shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag₄₂ is shown to be responsible for the core transformation.

Structure Determination of the Cl-Enriched [Ag52(SAdm)31Cl13]2+ Nanocluster

Cl atoms can serve as the innermost core, the peripheral ligand, or the counterions of metal nanoclusters. Herein, we report the structural determination a Cl-enriched [Ag₅₂(SAdm)₃₁Cl₁₃]²⁺. The ratio of Cl to AdmSH is quite high compared to those of other nanoclusters. Structurally, nine Cl atoms, existing at the interlayer of the inner kernel and the surface motif, serve as the bridging ligands to sustain the robustness of the whole structure. Interestingly, four Cl atoms on the motif structure can be substituted by Br. This work allows us to clear the regulation of Cl ligands in the structural construction of metal nanoclusters.

Atomic-Scale Tailoring and Molecular-Level Tracking of Oxygen-Containing Tungsten Single-Atom Catalysts with Enhanced Singlet Oxygen Generation

The local coordination structure of metal atoms in single-atom catalysts (SACs) greatly influences their catalytic performance. And for most SACs, single metal atoms were anchored on carbon materials with N or C coordination. However, the rational design of oxygen-containing SACs and analyzing its structure-performance relationship remain challenging. Herein, we used amino-rich compounds to tailor the metatungstate and fix the W atoms and finally obtained the oxygen-containing W-SACs. The structural evolution of tungsten and its coordination atoms were tracked by electrospray ionization high-definition mass spectrometry. Furthermore, aberration-corrected transmission electron microscopy, X-ray absorption fine-structure spectroscopy, and first-principles calculation results revealed that different from the traditional SACs, the WO₂N₂ moiety (W coordinated with two O atoms and two N atoms) may be the favored structure for W species. This special structure promoted the energy transfer for enhancing singlet oxygen generation. This work presents an efficient way to prepare more high-efficiency SACs by atomic-scale tailoring and structural evolution tracking at the molecular level.

Exploration of Formation and Size-Evolution Pathways of Thiolate-Gold Nanoclusters in the CO-Directed [Au25 (SR)18 ]- Synthesis

An intermolecular association and decarboxylation mechanism is proposed to understand the experimental evidence of the stepwise 2e^- hopping in the reductant-assisted thiolate-gold cluster synthesis. Based on the newly proposed intermolecular reaction mechanism, a total of 19 molecular-like reaction equations are deduced to account for the bottom-up formation of 2e^- -8e^- gold nanoclusters in the CO-directed [Au₂₅ (SR)₁₈ ]^- synthesis. With these established reaction equations, atomic pathways of three prototype cluster-size evolution reactions are comprehensively explored in the course of [Au₂₅ (SR)₁₈ ]^- synthesis, namely, the conversion of 0e^- homoleptic Au^(I) -SR complexes to the 2e^- intermediate Au₁₅ (SR)₁₃ cluster, the size-evolution of 2e^- Au₁₅ (SR)₁₃ cluster to the 4e^- -8e^- cluster (stepwise 2e^- -hopping), and the isoelectronic addition reaction of [Au₂₃ (SR)₁₆ ]^- to the [Au₂₅ (SR)₁₈ ]^- . The studies reveal that the CO can combine with the Au(I)-complex to form [Au_x (SR)_x -COOH]^- species in the alkaline condition, which acts as the active precursors in the 2e^- hopping cluster-size evolution process. Lastly, as a conceptual extension of the mechanistic studies of the CO-reduction system, a similar intermolecular reaction mechanism is proposed for the 2e^- reduction in the conventional "NaBH₄ reduction" system.

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.

Revealing the etching process of water-soluble Au25 nanoclusters at the molecular level

Etching (often considered as decomposition) is one of the key considerations in the synthesis, storage, and application of metal nanoparticles. However, the underlying chemistry of their etching process still remains elusive. Here, we use real-time electrospray ionization mass spectrometry to study the reaction dynamics and size/structure evolution of all the stable intermediates during the etching of water-soluble thiolate-protected gold nanoclusters (Au NCs), which reveal an unusual "recombination" process in the oxidative reaction environment after the initial decomposition process. Interestingly, the sizes of NC species grow larger and their ligand-to-metal ratios become higher during this recombination process, which are distinctly different from that observed in the reductive growth of Au NCs (e.g., lower ligand-to-metal ratios with increasing sizes). The etching chemistry revealed in this study provides molecular-level understandings on how metal nanoparticles transform under the oxidative reaction environment, providing efficient synthetic strategies for new NC species through the etching reactions.

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

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

Intercluster exchanges leading to hydride-centered bimetallic clusters: a multi-NMR, X-ray crystallographic, and DFT study

Encouraged by the successful syntheses of alloy nanoclusters (or nanoparticles) via intercluster (or interparticle) reactions, herein we apply this methodology to prepare a series of bimetallic hydride clusters. Mixing of two clusters, [Ag₇(H){E₂P(OⁱPr)₂}₆] (E = S, 1; Se, 3) and [Cu₇(H){E₂P(OⁱPr)₂}₆] (E = S, 2; Se, 4), yields two series of hydride-centered bimetallic clusters, [Cu_xAg_7-x(H){E₂P(OⁱPr)₂}₆] (x = 0-7; E = S, 5; Se, 6). Their compositions are fully characterized by positive-mode ESI-MS spectrometry, multi-NMR spectroscopy, and the structures of [Cu₆Ag(H){S₂P(OⁱPr)₂}₆] (5a) and [CuAg₆(H){Se₂P(OⁱPr)₂}₆] (6a) by single crystal X-ray diffraction. The presence of individual compounds in solution is the result of a (dynamic) chemical equilibrium primarily driven by metal exchanges. In fact, the process of inter-cluster exchange of 1 and 2 leading to hydride-centered bimetallic clusters 5 can be monitored by concentration-dependent ³¹P NMR spectroscopy of which the higher concentration of 1 in the reaction, the closer to its resonance will be the distribution, in accord with Le Chatelier's principle. The dynamic equilibrium is further confirmed by 2D exchange spectroscopy that reveals a stepwise process involving one metal exchange at a time. DFT calculations on a model series of clusters 6 show that silver prefers occupying the inner tetrahedral positions, while copper favors capping positions, in full agreement with the crystal structure of 5a and 6a.

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

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

Controlling ultrasmall gold nanoparticles with atomic precision

Gold nanoparticles are probably the nanoparticles that have been best studied for the longest time due to their stability, physicochemical properties and applications. Controlling gold nanoparticles with atomic precision is of significance for subsequent research on their structures, properties and applications, which is a dream that has been pursued for many years since ruby gold was first obtained by Faraday in 1857. Fortunately, this dream has recently been partially realized for some ultrasmall gold nanoparticles (nanoclusters). However, rationally designing and synthesizing gold nanoparticles with atomic precision are still distant goals, and this challenge might rely primarily on rich atomically precise gold nanoparticle libraries and the in-depth understanding of metal nanoparticle chemistry. Herein, we review general synthesis strategies and some facile synthesis methods, with an emphasis on the controlling parameters determined from well-documented results, which might have important implications for future nanoparticle synthesis with atomic precision and facilitate related research and applications.

The synthesis strategy, methods and parameters for atomically precise gold nanoclusters were reviewed, and future outlook was also proposed.

Synergistic Effect of Bridging Thiolate and Hub Atoms for the Aromaticity Driven Symmetry Breaking in Atomically Precise Gold Nanocluster

The symmetry of atomically precise nanoclusters is influenced by the specific geometry of the kernel and the arrangement of staple motifs. To understanding the role of ligand and its effect on the breaking of symmetry during ligand exchange transformation, it is necessary to have a mechanism of transformation in an atomically precise manner. Herein, we report the structural transformation from bipyramidal kernel to icosahedral kernel via ligand exchange. The transformation of [Au₂₃(CHT)₁₆]^- to [Au₂₅(2-NPT)₁₈]^- through ligand (aromatic) exchange revealed two important principles. First, the combined effort of experimental and theoretical study on structural analysis elucidated the mechanism of this structural transformation where "bridging thiolate" and "hub" gold atoms play a crucial role. Second, we have found that the higher crystal symmetry of the Au₂₃ cluster is broken to lower crystal symmetry during the ligand exchange process. This showed that during ligand exchange, the hub atoms and μ₃-S atoms get distorted and contributed to the ligand-staple motif formation. These phenomena specified that the ligand effects might be the pivotal factor to impose lower symmetry of the crystal system in the product clusters.

Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis, self-assembly, and applications

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of "metallic molecules". Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)-SR core-shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M-S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

Atom-by-Atom Evolution of the Same Ligand-Protected Au21, Au22, Au22Cd1, and Au24 Nanocluster Series

Atom-by-atom manipulation on metal nanoclusters (NCs) has long been desired, as the resulting series of NCs can provide insightful understanding of how a single atom affects the structure and properties as well as the evolution with size. Here, we report crystallizations of Au22(SAdm)16 and Au22Cd1(SAdm)16 (SAdm = adamantanethiolate) which link up with Au21(SAdm)15 and Au24(SAdm)16 NCs and form an atom-by-atom evolving series protected by the same ligand. Structurally, Au22(SAdm)16 has an Au3(SAdm)4 surface motif which is longer than the Au2(SAdm)3 on Au21(SAdm)15, whereas Au22Cd1(SAdm)16 lacks one staple Au atom compared to Au24(SAdm)16 and thus the surface structure is reconstructed. A single Cd atom triggers the structural transition from Au22 with a 10-atom bioctahedral kernel to Au22Cd1 with a 13-atom cuboctahedral kernel, and correspondingly, the optical properties are dramatically changed. The photoexcited carrier lifetime demonstrates that the optical properties and excited state relaxation are highly sensitive at the single atom level. By contrast, little change in both ionization potential and electron affinity is found in this series of NCs by theoretical calculations, indicating the electronic properties are independent of adding a single atom in this series. The work provides a paradigm that the NCs with continuous metal atom numbers are accessible and crystallizable when meticulously designed, and the optical properties are more affected at the single atom level than the electronic properties.

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.

Gold Nanoclusters for NIR-II Fluorescence Imaging of Bones

Fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) holds great promise for deep tissue visualization. Development of novel clinical translatable NIR-II probes is crucial for realizing the medical applications of NIR-II fluorescence imaging. Herein, the glutathione-capped gold nanoclusters (AuNCs, specifically Au₂₅ (SG)₁₈ ) demonstrate highly efficient binding capability to hydroxyapatite in vitro for the first time. Further in vivo NIR-II fluorescence imaging of AuNCs indicate that they accumulate in bone tissues with high contrast and signal-background ratio. AuNCs are also mainly and quickly excreted from body through renal system, showing excellent ribs and thoracic vertebra imaging because of no background signal in liver and spleen. The deep tissue penetration capability and high resolution of AuNCs in NIR-II imaging render their great potential for fluorescence-guided surgery like spinal pedicle screw implantation. Overall, AuNCs are highly promising and clinical translatable NIR-II imaging probe for visualizing bone and bone related abnormalities.

Structural Oscillation Revealed in Gold Nanoparticles

Oscillation is an intriguing phenomenon in nature. However, structural oscillation has not yet been found in semiconducting nanoparticles, primarily due to the difficulty of structural resolution at the atomic level. The emergence of gold nanoclusters (ultrasmall nanoparticles) has provided an excellent opportunity to address some challenging issues in the nanoparticle field. Herein, two Au₂₈(CHT)₂₀ (CHT: cyclohexanethiolate) structural isomers (Au_28i and Au_28ii for short) were concurrently synthesized by employing a quasi-antigalvanic method, and they could be reversibly transformed into each other for at least 10 cycles, driven by dissolution and crystallization processes. The transformation from Au_28ii to Au_28i is solvent-dielectric-constant-dependent, with a notable deuteration effect from dichloromethane. The markedly different photoluminescence values of these two isomers not only have important implications for the structure-property correlations but also have potential applications in converting, sensing, etc.

Highly fluorescent few atoms silver nanoclusters with strong photocatalytic activity synthesized by ultrashort light pulses

While there are conventional chemical synthesis methods to generate metal nanoclusters (NCs), many of them are adversely affected by the unavoidable contamination of the nanoproduct solution, resulting in aggregation, background noise in analytical chemistry, toxicity, and deactivation of the catalyst. In this work, physical method of ultrafast laser ablation as a "green" synthesis approach together with mechanical centrifugation to obtain silver NCs, simplifying widely the chemical synthesis requirements, is proposed. Remarkably, compared with conventional methods for synthesizing Ag NCs, this new approach starts with a colloid that contains nanosized particles as well as smaller species, managing to obtain colloids with few atoms NCs by centrifugation. Those colloids were analyzed by fluorescence spectroscopy observing UV bands corresponding with HOMO-LUMO cluster transitions. Besides, independent HRTEM measurements were made confirming the presence of few atoms Ag NCs, as well as small NPs in different formation stages. Equally important, photocatalytic efficiency of the obtained NCs was studied through degradation of Methylene Blue (MB) when it was mixed with as-prepared or highly centrifuged colloid, showing an enhanced photocatalytic efficiency of 79% as compared to 57% for pure MB after 180 min of illumination. Consequently, this work contributes to establishing a simple approach to synthesize highly fluorescent and photocatalytic NCs.

Solvent-mediated assembly of atom-precise gold-silver nanoclusters to semiconducting one-dimensional materials

Bottom-up design of functional device components based on nanometer-sized building blocks relies on accurate control of their self-assembly behavior. Atom-precise metal nanoclusters are well-characterizable building blocks for designing tunable nanomaterials, but it has been challenging to achieve directed assembly to macroscopic functional cluster-based materials with highly anisotropic properties. Here, we discover a solvent-mediated assembly of 34-atom intermetallic gold-silver clusters protected by 20 1-ethynyladamantanes into 1D polymers with Ag-Au-Ag bonds between neighboring clusters as shown directly by the atomic structure from single-crystal X-ray diffraction analysis. Density functional theory calculations predict that the single crystals of cluster polymers have a band gap of about 1.3 eV. Field-effect transistors fabricated with single crystals of cluster polymers feature highly anisotropic p-type semiconductor properties with ≈1800-fold conductivity in the direction of the polymer as compared to cross directions, hole mobility of ≈0.02 cm² V^-1 s^-1, and an ON/OFF ratio up to ≈4000. This performance holds promise for further design of functional cluster-based materials with highly anisotropic semiconducting properties.

Atomic-level separation of thiolate-protected metal clusters

Fine metal clusters have attracted much attention from the viewpoints of both basic and applied science for many years because of their unique physical/chemical properties and functions, which differ from those of bulk metals. Among these materials, thiolate (SR)-protected gold clusters (Au_n(SR)_m clusters) have been the most studied metal clusters since 2000 because of their ease of synthesis and handling. However, in the early 2000s, it was not easy to isolate these metal clusters. Therefore, high-resolution separation methods were explored, and several atomic-level separation methods, including polyacrylamide gel electrophoresis (PAGE), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC), were successively established. These techniques have made it possible to isolate a series of Au_n(SR)_m clusters, and much knowledge has been obtained on the correlation between the chemical composition and fundamental properties such as the stability, electronic structure, and physical properties of Au_n(SR)_m clusters. In addition, these high-resolution separation techniques are now also frequently used to evaluate the distribution of the product and to track the reaction process. In this way, high-resolution separation techniques have played an essential role in the study of Au_n(SR)_m clusters. However, only a few reviews have focused on this work. This review focuses on PAGE, HPLC, and TLC separation techniques, which offer high resolution and repeatability, and summarizes previous studies on the high-resolution separation of Au_n(SR)_m and related clusters with the purpose of promoting a better understanding of the features and the utility of these techniques.

Effect of subtle changes of isomeric ligands on the synthesis of atomically precise water-soluble gold nanoclusters

The subtle structural change of hydrophilic ligands on the size control of metal nanoclusters (NCs) is unclear but critically important for fundamental understanding. Herein, we report our findings that subtle changes of isomeric ligands lead to a dramatic difference in the size of water-soluble Au NCs. By using isomeric para-mercaptobenzoic acid (p-MBA), m-MBA, and o-MBA as model ligands, it was found that both the steric hindrance and the electronic effect of isomeric ligands significantly influences the size of Au NCs, resulting in the formation of different sized Au44(p-MBA)26 NCs, Au25(m-MBA)18 NCs, and Au37/43(o-MBA)22/26 NCs. Besides this, by collocating any two of the isomeric MBAs as ligand pairs to compare their protecting capability for Au NCs, the protecting abilities of such ligands were found to follow the trend: m-MBA > o-MBA > p-MBA. In addition, the growth process of Au44(p-/o-MBA)26 NCs from Au(i)-MBA complexes in the NaBH4 reduction system was also monitored by real-time UV-vis absorption spectroscopy and ESI mass spectrometry, which complies with the 2e- hopping growth principle, indicating the universal applicability of this principle in the synthesis of thiolated metal NCs. This study provides a fundamental understanding of the effect of ligands' steric hindrance and electronic factors on the size control of water-soluble metal NCs and sheds light on the formation of metal NCs in the NaBH4 reduction system.

Unlocking the catalytic activity of an eight-atom gold cluster with a Pd atom

It remains elusive as to how exactly the site-specific atom in a catalyst can induce a chemical reaction mainly due to the observed catalytic performance from an ensemble average of all active atoms in the catalyst. In this work, we have reported the catalytic properties of four metal clusters (namely, Au₈Pd, Au₉, Au₂₄Pd and Au₂₅) for the oxidation of benzyl alcohol. It was found that the Pd atom in the Au₈Pd cluster is likely to be a key to catalyze the oxidation reaction, in which the Pd atom can provide an active site to adsorb and activate O₂. Our calculation study suggests that the high catalytic activity of the Au₈Pd cluster is due to the unique ability of Au₈Pd to mediate the electrons and holes of the adsorbates. This work provides a feasible strategy to enable highly efficient chemical processes via precisely doping foreign atoms into catalysts with atomic precision.

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.

Steric and Electrostatic Control of the pH-Regulated Interconversion of Au16(SR)12 and Au18(SR)14 (SR: Deprotonated Captopril)

An understanding of the response of nanomaterials to specific environmental parameters is an essential prerequisite for their practical use, especially in living systems. Herein, we disclose the preparation of a water-soluble nanocluster Au₁₆(SR)₁₂ (SR denotes deprotonated captopril) and its characterization by a combination of theoretical (e.g., density functional theory calculations) and experimental (UV-vis, electrospray ionization mass spectrometry, etc.) methods. Interestingly, Au₁₆(SR)₁₂ was found to convert to Au₁₈(SR)₁₄ under acidic conditions, while the reverse conversion from Au₁₈(SR)₁₄ to Au₁₆(SR)₁₂ occurred upon the addition of base. A mechanistic investigation determined this pH regulation to originate from the distinct steric and electrostatic properties of these two clusters. This study is the first to report the susceptibility of Au₁₈(SR)₁₄ and Au₁₆(SR)₁₂ to pH, and the distinct pH stability unambiguously reveals the importance of size-tracking of nanomaterials in living systems for future clinical applications.