Vibrational Properties of Thiolate-Protected Gold Nanoclusters

Influence of Aliphatic versus Aromatic Ligand Passivation on Intersystem Crossing in Au25(SR)18. J Phys Chem A 2024. [PMID: 39197122 DOI: 10.1021/acs.jpca.4c04387] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The electronic relaxation dynamics of gold monolayer protected clusters (MPCs) are influenced by the hydrocarbon structure of thiolate protecting ligands. Here, we present ligand-dependent electronic relaxation for a series of Au25(SR)18- (SR = SC8H9, SC6H13, SC12H25) MPCs using femtosecond time-resolved transient absorption spectroscopy. Relaxation pathways included a ligand-independent femtosecond internal conversion and a competing ligand-dependent picosecond intersystem crossing process. Intersystem crossing was accelerated for the aliphatic (SC6H13, SC12H25) thiolate MPCs compared to the aromatic (SC8H9) thiolate MPCs. Additionally, a 1.2 THz quadrupolar acoustic mode and a 2.4 THz breathing acoustic mode was identified in each cluster, which indicated that differences in ligand structure did not result in significant structural changes to the metal core of the MPCs. Considering that the difference in relaxation rates did not result from ligand-induced core deformation, the accelerated intersystem crossing was attributed to greater electron-vibrational coupling to Au-S vibrational modes. The results suggested that the organometallic semiring was less rigid for the aliphatic thiolate MPCs due to reduced steric effects, and in turn, increases in nonradiative decay rates were observed. Overall, these results imply that the protecting ligand structure can be used to modify carrier relaxation in MPCs.

Triple-Phosphorescent Gold Nanoclusters Enabled by Isomerization of Terminal Thiouracils in the Surface Motifs

Metal nanoclusters (NCs) hold great promise for expressing multipeak emission based on their well-defined total structure with diverse luminescent centers. Herein, we report the surface motif-dictated triple phosphorescence of Au NCs with dynamic color turning. The deprotonation-triggered isomerization of terminal thiouracils can evolve into a mutual transformation among their hierarchical motifs, thus serving a multipeak-emission expression with good tailoring. More importantly, the underlying electron transfer is thoroughly identified by excluding the radiative and nonradiative energy transfer, where electrons flow from the first phosphorescent state to the last two ones. The findings shed light on finely tailing motifs at the molecular level to motivate studies on customizable luminescence characteristics of metal NCs.

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.

Observing atomically precise nanocluster aggregates in solution by mass photometry

We report the first mass photometric characterization of nanoaggregates of atomically precise nanoclusters (NCs) in solution. The differently-sized nanoaggregates of silver-gold alloy NCs, [Ag11-xAux(DPPB)5Cl5O2]2+ [x = 1-5 and DPPB = 1,4-bis(diphenylphosphino)butane], formed in solution, were examined by mass photometry (MP) with a protein calibration. In addition, we conducted MP studies of varying solvent composition to understand the structural evolution of nanoaggregates. The masses of nanoaggregates were correlated to structures of 15 to 50 nm diameter observed in cryo-electron microscopy.

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.

Temperature-Controlled Selective Formation of Silver Nanoclusters and Their Transformation to the Same Product

Herein, two atomically precise silver nanoclusters, Ag54 and Ag33, directed by inner anion templates (CrO4 2- and/or Cl-), are initially isolated as a mixed phase from identical reactants across a wide temperature range (20-80 °C). Interestingly, fine-tuning the reaction temperature can realize pure phase synthesis of the two nanoclusters; that is, a metastable Ag54 is kinetically formed at a low temperature (20 °C), whereas such a system is steered towards a thermodynamically stable Ag33 at a relatively high temperature (80 °C). Electrospray ionization mass spectrometry illustrates that the stability of Ag33 is superior to that of Ag54, which is further supported by density functional theory calculations. Importantly, the difference in structural stability can influence the pathway of 1,4-bis(pyrid-4-yl)benzene induced transformation reaction starting from Ag54 and Ag33. The former undergoes a dramatic breakage-reorganization process to form an Ag31 dimer (Ag31), while the same product can be also achieved from the latter following a noninvasive ligand exchange process. Both the Ag54 and Ag33 have the potential for further remote laser ignition applications. This work not only demonstrates how temperature controls the isolation of a specific phase, but also sheds light on the structural transformation pathway of nanoclusters with different stability.

Site-Recognition-Induced Structural and Photoluminescent Evolution of the Gold-Pincer Nanocluster

The induced structural transformation provides an efficient way to precisely modulate the fine structures and the corresponding performance of gold nanoclusters, thus constituting one of the important research topics in cluster chemistry. However, the driving forces and mechanisms of these processes are still ambiguous in many cases, limiting further applications. In this work, based on the unique coordination mode of the pincer ligand-stabilized gold nanocluster Au8(PNP)4, we revealed the site-recognition mechanism for induced transformations of gold nanoclusters. The "open nitrogen sites" on the surface of the nanocluster interact with different inducers including organic compounds and metals and trigger the conversion of Au8(PNP)4 to Au13 and Au9Ag4 nanoclusters, respectively. Control experiments verified the site-recognition mechanism, and the femtosecond and nanosecond transient absorption spectroscopy revealed the electronic and photoluminescent evolution accompanied by the structural transformation.

N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters

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

Tiara-like Hexanuclear Nickel-Platinum Alloy Nanocluster

Tiara-like metal nanoclusters (TNCs) have attracted a great deal of attention because of their high stability and easy synthesis under atmospheric conditions as well as their high activity in various catalytic reactions. Alloying is one of the methods that can be used to control the physicochemical properties of nanoclusters, but few studies have reported on alloy TNCs. In this study, we synthesized alloy TNCs [NixPt6-x(PET)12, where x = 1-5 and PET = 2-phenylethanethiolate] consisting of thiolate, nickel (Ni), and platinum (Pt). We further evaluated the stability, geometric structure, and electronic structure by high-performance liquid chromatography and density functional theory calculations. The results revealed that NixPt6-x(PET)12 has a distorted structure and is therefore less stable than single-metal TNCs.

Parasitism in Metal Nanoclusters: A Case Study of (AuAg)25·(AuAg)27

Studying the interactions of atomically precise metal nanoclusters in their assembly systems is of great significance in the nanomaterial research field, which has attracted increasing interest in the last few decades. Herein, we report the cocrystallization of two oppositely charged atomically precise metal nanoclusters in one unit cell: [Au1Ag24(SR)18]- ((AuAg)25 for short) and [AuxAg27-x(Dppf)4(SR)9]2+ (x = 10-12; (AuAg)27 for short) with a 1:1 ratio. (AuAg)27 could maintain its structure in the presence of (AuAg)25, whether in the crystalline and the solution state, while the metastable (AuAg)27 component underwent a spontaneous transformation to (AuAg)16(Dppf)2(SR)8 after dissociating the (AuAg)25 component from this cocrystal, demonstrating the "parasitism" relationship of the (AuAg)27 component over (AuAg)25 in this dual-cluster system. This work enriches the family of cluster-based assemblies and elucidates the delicate relationship between nanoparticles of cocrystals.

Symmetry breaking of highly symmetrical nanoclusters for triggering highly optical activity

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

Reduction-Oxidation Cascade Strategy for Reforming a Au13-Kerneled Gold Thiolate Nanocluster

Gold nanoclusters protected by thiolate ligands are ideal models for investigating the structure-property correlation of nanomaterals. Introducing relatively weak coordinating ligands into gold thiolate nanoclusters and thus reforming their structures is beneficial for further releasing their activities. However, controlling the selectivity of the process is a challenging task. In this work, we report a cascade strategy for deeply and purposefully reforming the structures of gold thiolate nanoclusters, exemplified by a Au13-kerneled Au23 nanocluster. Specifically, weakly coordinated triphenylphosphine was utilized to reduce (activate) the surface of Au23, enabling its further structural reformation by the following oxidation step. A structurally distinctive Au20 nanocluster was obtained based on this reduction-oxidation cascade strategy. Mechanism studies reveal that both the reduction and oxidation steps and their working sequence are critical for the transformation. Theoretical and experimental results all indicate that the deep structural reformation results in the evolution of the electronic and photoluminescent properties of the gold thiolate nanocluster.

Photochemical synthesis of group 10 metal nanoclusters for electrocatalysis

Four group 10 metal nanoclusters, Ni10(4-MePhS)20, Ni11(PhS)22, Pd9(PhS)18 and Pd10(PhS)20 were synthesized from disulfides based on a photochemical reduction-oxidation cascade process, which proceeds via a different mechanism to that of the conventional two-step reduction process. The as-obtained nanoclusters possess oxidative resistance and structural robustness under different conditions. Their atomically precise structures are determined to be nickel or palladium rings in which the metal atoms are bridged by Ar-S groups. Their catalytic performance in oxygen reduction reaction was compared, and the ring size-dependent catalytic activity of the group 10 metal nanoclusters was revealed. This work provides an efficient route to atomically precise and structurally stable group 10 metal nanoclusters, and sheds light on their further applications in electrocatalysis.

Stepwise construction of Ag29 nanocluster-based hydrogen evolution electrocatalysts

Although several silver-based nanoclusters have been controllably prepared and structurally determined, their electrochemical catalytic performances have been relatively unexplored (or showed relatively weak ability towards electro-catalysis). In this work, we accomplished the step-by-step enhancement of the electrocatalytic hydrogen evolution reaction (HER) efficiency based on an Ag29 cluster template. A combination of atomically precise operations, including the kernel alloying, ligand engineering, and surface activation, was exploited to produce a highly efficient Pt1Ag28-BTT-Mn(10) nano-catalyst towards HER, derived from both experimental characterization and theoretical modelling. The precision characteristic of the Ag29-based cluster system enables us to understanding the correlations between nanocluster structures and HER performances at the atomic level. Overall, the findings of this work will hopefully provide more opportunities for the customization of new cluster-based nano-catalysts with enhanced electrocatalytic capacities.

Probing the surface-active sites of metal nanoclusters with atomic precision: a case study of Au5Ag11

The determination of surface-active sites in metal nanoclusters is of great significance for the in-depth understanding of structural evolutions and physicochemical property mechanisms. In this work, the surface-active sites of the Au5Ag11(DMBT)8(DPPOE)2 cluster template towards metal-/ligand-exchange reactions were unambiguously identified at the atomic level. The active-site tailoring of this nanocluster gave rise to three derivative nanoclusters, Au5Ag9Cu2(DMBT)8(DPPOE)2, Au5Ag11(DMBT)6(DCBT)2(DPPOE)2, and Au5Ag11(DCBT)8(DPPOE)2. The single-crystal structural analysis revealed that all these M16 (M = Au/Ag/Cu) clusters exhibited almost the same framework. Besides, the surface-active site tailoring contributed to significant changes in optical absorptions and emissions of these metal nanoclusters. The findings in this work not only provide an in-depth understanding of the active-site tailoring of cluster surface structures but also develop an intriguing template that enables us to grasp the structure-property correlations at the atomic level.

Mechanistic Study of the Hydride Migration-Induced Reversible Isomerization in Au22(SR)15H Isomers

Unraveling the evolution mechanism of metal nanoclusters is of great importance in understanding the formation and evolution of metallic condensed matters. In this work, the specific evolution process between a pair of gold nanocluster (Au NC) isomers is completely revealed by introducing hydride ligands to simplify the research system. A hydride-containing Au NC, Au₂₂(SR)₁₅H, was synthesized by kinetic control, and the positions of the hydrides were then confirmed by combining X-ray diffraction, neutron diffraction, and DFT calculations. Importantly, a reversible structural isomerization was found to occur on this Au₂₂(SR)₁₅H. By combining the crystal structures and theoretical calculations, the focus was placed on the hydride-binding site, and a [Au-H] migration mechanism of this isomerization process is clearly shown. Furthermore, this [Au-H] migration mechanism is confirmed by the subsequent capture and structural determination of theoretically predicted intermediates. This work provides insight into the dynamic behavior of hydride ligands in nanoclusters and a strategy to study the evolution mechanism of nanoclusters by taking the hydride ligand as the breakthrough point.

Preorganized Nitrogen Sites for Au11 Amidation: A Generalizable Strategy toward Precision Functionalization of Metal Nanoclusters

Atomically precise metal nanoclusters have received tremendous attention due to their unique structures and properties. Although synthetic approaches to this kind of nanomaterial have been well developed, methods toward precision functionalization of the as-synthesized metal nanoclusters are extremely limited, hindering their interfacial modification and related performance improvement. Herein, an amidation strategy has been developed for the precision functionalization of the Au₁₁ nanocluster based on preorganized nitrogen sites. The nanocluster amidation did not change the number of gold atoms in the Au₁₁ kernel and their bonding mode to the surface ligands but slightly modified the arrangement of gold atoms with the introduction of functionality and chirality, thus representing a relatively mild method for the modification of metal nanoclusters. The stability and oxidation barrier of the Au₁₁ nanocluster are also improved accordingly. The method developed here would be a generalizable strategy for the precision functionalization of metal nanoclusters.

Key factors for connecting silver-based icosahedral superatoms by vertex sharing

Metal nanoclusters composed of noble elements such as gold (Au) or silver (Ag) are regarded as superatoms. In recent years, the understanding of the materials composed of superatoms, which are often called superatomic molecules, has gradually progressed for Au-based materials. However, there is still little information on Ag-based superatomic molecules. In the present study, we synthesise two di-superatomic molecules with Ag as the main constituent element and reveal the three essential conditions for the formation and isolation of a superatomic molecule comprising two Ag_13-xM_x structures (M = Ag or other metal; x = number of M) connected by vertex sharing. The effects of the central atom and the type of bridging halogen on the electronic structure of the resulting superatomic molecule are also clarified in detail. These findings are expected to provide clear design guidelines for the creation of superatomic molecules with various properties and functions.

Protein-Directed Au(0)-Rich Gold Nanoclusters as Ratiometric Luminescence Sensors for Auric Ions via Comproportionation-Induced Emission Enhancement

Gold nanoclusters (Au NCs) are widely used as fluorescent probes in biomedical sensing and imaging due to their versatile optical properties and low cytotoxicity. Surface engineering of gold nanoclusters (Au NCs) aims to design a surface with versatile physicochemical performances, but previous investigations have primarily focused on the acquisition of the "brightest" species. This has resulted in other types of Au NC being neglected. In the present study, our group prepared a series of Au NCs that were rich in surface Au(0), using the "aged" form of bovine serum albumin (BSA) via controlling the pH during synthesis. We found that slight increases of alkalinity during synthesis over that which produced Au NCs with the most intensive photoluminescence generated the "darkest" Au NCs, which exhibited the strongest absorption. These Au NCs included more Au atoms and had a higher Au(0) content. Furthermore, the addition of Au³⁺ quenched the emission of the "brightest" Au NCs, but increased that of the "darkest" Au NCs. The increased Au(I) proportion observed in the Au³⁺-treated "darkest" Au NCs resulted in a novel comproportionation-induced emission enhancement effect, which we utilized to construct a "turn-on" ratiometric sensor for toxic Au³⁺. The addition of Au³⁺ generated simultaneous, opposite effects on blue-emissive diTyr BSA residues and red-emissive Au NCs. After optimization, we successfully constructed ratiometric sensors for Au³⁺ with high sensitivity, selectivity, and accuracy. This study will inspire a new pathway to redesign the protein-framed Au NCs and analytical methodology via comproportionation chemistry.

Improved activity for the oxygen evolution reaction using a tiara-like thiolate-protected nickel nanocluster

Practical electrochemical water splitting and carbon-dioxide reduction are desirable for a sustainable energy society. In particular, facilitating the oxygen evolution reaction (OER, the reaction at the anode) will increase the efficiency of these reactions. Nickel (Ni) compounds are excellent OER catalysts under basic conditions, and atomically precise Ni clusters have been actively studied to understand their complex reaction mechanisms. In this study, we evaluated the geometric/electronic structure of tiara-like metal nanoclusters [Ni_n(PET)_2n; n = 4, 5, 6, where PET refers to phenylethanethiolate] with the same SR ligand. The geometric structure of Ni₅(SR)₁₀ was determined for the first time using single-crystal X-ray diffraction. Additionally, combined electrochemical measurements and X-ray absorption fine structure measurements revealed that Ni₅(SR)₁₀ easily forms an OER intermediate and therefore exhibits a high specific activity.

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

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

Enhanced charge transport across molecule-nanoparticle-molecule sandwiches

The electrical properties of large area molecular devices consisting of gold nanoparticles (GNPs) sandwiched between a double layer of alkanedithiol linkers have been examined. These devices have been fabricated by a facile bottom-up assembly in which an alkanedithiol monolayer is first self-assembled on an underlying gold substrate followed by nanoparticle adsorption and then finally assembly of the top alkanedithiol layer. These devices are then sandwiched between the bottom gold substrates and a top eGaIn probe contact and current-voltage (I-V) curves recorded. Devices have been fabricated with 1,5-pentanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol and 1,10-decanedithiol linkers. In all cases the electrical conductance of the double SAM junctions with GNPs is higher than the corresponding and much thinner single alkanedithiol SAM. Competing models for this enhanced conductance are discussed and it is suggested to have a topological origin arising from how the devices assemble or structure during the fabrication, which gives more efficient cross device electron transport pathways without the GNPs producing short circuits.

The Conformational Dynamics of the Ligands Determines the Electronic Circular Dichroism of the Chiral Au38(SC2H4Ph)24 Cluster

Effects of the conformational dynamics of 2-PET protective ligands on the electronic circular dichroism (ECD) of the chiral Au₃₈(SC₂H₄Ph)₂₄ cluster are investigated. We adopt a computational protocol in which ECD spectra are calculated via the first principle polTDDFT approach on a series of conformations extracted from MD simulations by using Essential Dynamics (ED) analysis, and then properly weighted to predict the final spectrum. We find that the experimental spectral features are well reproduced, whereas significant discrepancies arise when the spectrum is calculated using the experimental X-ray structure. This result unambiguously demonstrates the need to account for the conformational effects in the ECD modeling of chiral nanoclusters. The present procedure proved to be able of capturing the essential conformational features of the dynamic Au₃₈(SC₂H₄Ph)₂₄ system, opening the possibility to model the ECD of soluble chiral nanoclusters in a realistic way.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO₂ reduction reaction (CRR) is an effective strategy because it affords the conversion of CO₂ into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Cadmium-Doped and Pincer Ligand-Modified Gold Nanocluster for Catalytic KA2 Reaction

A gold nanocluster Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ (PNP=2,6-bis(diphenylphosphinomethyl)pyridine, SR=4-MeOPhS) consisting of an icosahedral Au₁₃ kernel, two Au₂ CdS₆ staple motifs, and two PNP pincer ligands has been designed, synthesized and well characterized. This cadmium and PNP pincer ligand co-modified gold nanocluster showed high catalytic efficiency in the KA² reaction, featuring high TON, mild reaction conditions, broad substrate scope as well as catalyst recyclability. Comparison of the catalytic performance between Au₁₇ Cd₂ (PNP)₂ (SR)₁₂ and the structurally similar single cadmium (or PNP) modified gold nanoclusters demonstrates that the co-existence of the cadmium and PNP on the surface is crucial for the high catalytic activity of the gold nanocluster. This work would be enlightening for developing efficient catalysts for cascade reactions and discovering the catalytic potential of metal nanoclusters in organic transformations.

Surface Site-Specific Replacement for Catalysis Selectivity Switching

Surface atom replacement in materials without other composition/structure changes is challenging but is important for fundamental scientific research and for practical applications. In particular, for nanoparticles including nanoclusters, surface metal site-specific replacement with atomic precision has not yet been achieved. In this study, we for the first time achieved surface site-specific antigalvanic replacement with the remaining composition/structure and surface replacement-dependent selectivity in the electrocatalytic reduction of CO₂. Density functional theory (DFT) calculations describe the catalysis selectivity switch induced by replacing Ag with Cu and explain why Cu replacement facilitates C₂ production. Also, CO₂ electroreduction to C₂ on well-defined metal nanoclusters is first reported in this study.

Open Nitrogen Site-Induced Kinetic Resolution and Catalysis of a Gold Nanocluster

The emerging metal nanocluster provides a platform for the investigation of structural features, unique properties, and structure-property correlation of nanomaterials at the atomic level. Construction of open sites on the surface of the metal nanocluster is a long-pursued but challenging goal. Herein, we realized the construction of "open organic sites" in a metal nanocluster for the first time. Specifically, we introduce the PNP (2,6-bis(diphenylphosphinomethyl)pyridine) pincer ligand in the synthesis of the gold nanocluster, enabling the construction of a structurally precise Au₈(PNP)₄ nanocluster. The rigidity and the unique bonding mode of PNP lead to open nitrogen sites on the surface of the Au₈(PNP)₄ nanocluster, which have been utilized as multifunctional sites in this work for efficient kinetic resolution and catalysis. The gold pincer nanocluster and the open nitrogen site-induced performance will be enlightening for the construction of multifunctional metal nanoclusters.

Structure and assembly of a hexanuclear AuNi bimetallic nanocluster

An Au₄Ni₂ nanocluster containing a square-planar [-PPh₂-Au-S-Au-]₂ ring and two nickel-pincer arms is reported here. Abundant intra- and inter-cluster noncovalent interactions promote the assembly of the nanocluster into a porous framework material. The assembly-dependent unique solubility and photoluminescence were also investigated.

Controlling the Nature of Photoluminescence of Emissive Metal Nanoclusters

Photoluminescence (PL) serves as one of the most attractive chemical-physical properties of metal nanoclusters. However, the control over the PL nature of metal nanoclusters as fluorescence or phosphorescence remains challenging. Basically, the PL nature control concerns the transition regulation of excited electrons in nanoclusters from their excited state to the ground state. Up to the present, some cases have been reported on adjusting the PL nature of emissive nanoclusters via different means, including the composition regulation, the isomerization, the aggregation, and the temperature variation. At the same time, theoretical calculations have been performed to thoroughly understand the PL nature transformation of these emissive nanoclusters in terms of their electronic structures and transition pathways. This Concept highlights and reviews the recent progress in controlling the PL nature of emissive nanoclusters as fluorescence or phosphorescence, which hopefully paves the way for fabricating novel nanoclusters or cluster-based nanomaterials with customized PL properties.

Mycosynthesis of Silica Nanoparticles Using Aspergillus niger: Control of Alternaria solani Causing Early Blight Disease, Induction of Innate Immunity and Reducing of Oxidative Stress in Eggplant

The threats to the life and production of crops are exacerbated by climate change and the misuse of chemical pesticides. This study was designed to evaluate the effectiveness of biosynthesized silica nanoparticles (SiO₂-NPs) as an alternative to pesticides against early blight disease of eggplant. Antifungal activity, disease index, photosynthetic pigments, osmolytes, oxidative stress, antioxidant enzymes activities were tested for potential tolerance of eggplant infected with Alternaria solani. Silica nanoparticles were successfully biosynthesized using Aspergillus niger through green and ecofriendly method. Results revealed that SiO₂-NPs exhibited promising antifungal activity against A. solani where MIC was 62.5 µg/mL, and inhibition growth at concentration 1000 µg/mL recorded 87.8%. The disease Index (DI) as a result of infection with A. solani reached 82.5%, and as a result, a severe decrease in stem and root length and number of leaves occurred, which led to a sharp decrease in the photosynthetic pigments. However, contents of free proline, total phenol and antioxidant enzymes activity were increased in infected plants. On the other hand, the treatment with SiO₂-NPs 100 ppm led to a great reduction in the disease Index (DI) by 25% and a high protection rate by 69.69%. A clear improvement in growth characteristics and a high content of chlorophyll and total carotenoids was also observed in the plants as a result of treatment with silica nanoparticles in (healthy and infected) plants. Interestingly, the noticeable rise in the content of infected and healthy plants of proline and phenols and an increase in the activity of super oxide dismutase (SOD) and polyphenol oxidase (PPO). It could be suggested that foliar application of SiO₂-NPs especially 100 ppm could be commercially used as antifungal and strong inducer of plant physiological immunity against early blight disease.

Tailoring the Electron-Phonon Interaction in Au25(SR)18 Nanoclusters via Ligand Engineering and Insight into Luminescence

Understanding the electron-phonon interaction in Au nanoclusters (NCs) is essential for enhancing and tuning their photoluminescence (PL) properties. Among all the methods, ligand engineering is the most straightforward and facile one to design Au NCs with the desired PL properties. However, a systematic understanding of the ligand effects toward electron-phonon interactions in Au NCs is still missing. Herein, we synthesized four Au₂₅(SR)₁₈^- NCs protected by different -SR ligands and carefully examined their temperature-dependent band-gap renormalization behavior. Data analysis by a Bose-Einstein two-oscillator model revealed a suppression of high-frequency optical phonons in aromatic-ligand-protected Au₂₅ NCs. Meanwhile, a low-frequency breathing mode and a quadrupolar mode are attributed as the main contributors to the phonon-assisted nonradiative relaxation pathway in aromatic-ligand-protected Au₂₅ NCs, which is in contrast with non-aromatic-ligand-protected Au₂₅ NCs, in which tangential and radial modes play the key roles. The PL measurements of the four Au₂₅ NCs showed that the suppression of optical phonons led to higher quantum yields in aromatic-ligand-protected Au₂₅ NCs. Cryogenic PL measurements provide insights into the nonradiative energy relaxation, which should be further investigated for a full understanding of the PL mechanism in Au₂₅ NCs.

Horizontal expansion of biicosahedral M13-based nanoclusters: resolving decades-long questions

For metal nanoclusters with the "cluster of clusters" intramolecular evolution pattern, most efforts have been made towards the vertical superposition of icosahedral nanobuilding blocks (e.g., from mono-icosahedral Au₁₃ to bi-icosahedral Au₂₅ and tri-icosahedral Au₃₇), while the horizontal expansion of these rod-shaped multi-icosahedral aggregates was largely neglected. We herein report the horizontal expansion of the biicosahedral M₂₅ cluster framework, yielding an [Au₁₉Ag₁₂(S-Adm)₆(DPPM)₆Cl₇]²⁺ nanocluster that contains an Au₁₃Ag₁₂ kernel and six Au₁(DPPM)₁(S-Adm)₁ peripheral wings. The structural determination of [Au₁₉Ag₁₂(S-Adm)₆(DPPM)₆Cl₇]²⁺ resolved a decades-long question towards rod-shaped multi-icosahedral aggregates: how to load bidentate phosphine and bulky thiol ligands onto the nanocluster framework? The structural comparison between [Au₁₉Ag₁₂(S-Adm)₆(DPPM)₆Cl₇]²⁺ and previously reported [Au₁₃Ag₁₂(PPh₃)₁₀Cl₈]²⁺ or [Au₁₃Ag₁₂(SR)₅(PPh₃)₁₀Cl₂]²⁺ rationalized the unique packing of Au₁(DPPM)₁(S-Adm)₁ motif structures on the surface of the former nanocluster. Overall, this work presents the horizontal expansion of rod-shaped multi-icosahedral nanoclusters, which provides new insights into the preparation of novel icosahedron-based aggregates with both vertically and horizontally growing extensions.

Near-Infrared Dual Emission from the Au42(SR)32 Nanocluster and Tailoring of Intersystem Crossing

This work presents the synthesis and intriguing photoluminescence of the Au₄₂(PET)₃₂ (PET = 2-phenylethanethiolate) nanocluster (NC). The Au₄₂(PET)₃₂ NC exhibits dual emission at 875 and 1040 nm, which are revealed to be fluorescence and phosphorescence, respectively. The emission quantum yield (QY) of Au₄₂(PET)₃₂ in dichloromethane is 11.9% at room temperature in air, which is quite rare for thiolate-protected Au NCs. When Au₄₂(PET)₃₂ NCs are embedded in polystyrene films (solid state), the fluorescence was dramatically suppressed while the phosphorescence was significantly enhanced. This divergent behavior is explained by dipolar interaction-induced enhancement of intersystem crossing from singlet to triplet excited state.

Ligand-Shell Engineering of a Au28 Nanocluster Boosts Electrocatalytic CO2 Reduction

Subtle tailoring of gold nanoclusters (NCs) could significantly change their physicochemical properties. However, direct comparison of the catalytic performance of gold NCs with identical metal cores but distinct ligand shells is rarely elucidated. In this work, a novel gold NC, Au₂₈ (C₂ B₁₀ H₁₁ S)₁₂ (tht)₄ Cl₄ (Au₂₈ -S), was isolated by a facile self-reducing synthesis. Au₂₈ -S adopts an identical Au₂₈ metal framework to that of the reported alkynyl-protected Au₂₈ -C. The different protective layers lead to distinctions in their electronic structure and optical properties. Furthermore, Au₂₈ -S shows better catalytic activity for the electrochemical reduction of CO₂ to CO. Theoretical calculations identified the active sites and shed light on the catalytic mechanism to elucidate the different catalytic performances. This work provides an ideal platform to study the protective layer-activity relationship of gold NCs, and may also provide guidance in the design of metal NC-based catalysts.

Crystal structure of bulky-ligand-protected Au24(S-C4H9)16

Atomically precise thiolate-protected gold nanomolecules have attracted interest due to their distinct electronic and chemical properties. The structure of these nanomolecules is important for understanding their peculiar properties. Here, we report the X-ray crystal structure of a 24-atom gold nanomolecule protected by 16 tert-butylthiolate ligands. The composition of Au24(S-C4H9)16 {poly[hexadecakis(μ-tert-butylthiolato)tetracosagold]} was confirmed by X-ray crystallography and electrospray ionization mass spectrometry (ESI–MS). The nanomolecule was synthesized in a one-phase synthesis and crystallized from a hexane–ethanol layered solution. The X-ray structure confirms the 16-atom core protected by two monomeric and two trimeric staples with four bridging ligands. The Au24(S-C4H9)16 cluster follows the shell-closing magic number of 8.

Molecule-like and lattice vibrations in metal clusters

We report distinct molecule-like and lattice (breathing) vibrational signatures of atomically precise, ligand-protected metal clusters using low-temperature Raman spectroscopy. Our measurements provide fingerprint Raman spectra of a series of noble metal clusters, namely, Au25(SR)18, Ag25(SR)18, Ag24Au1(SR)18, Ag29(S2R)12 and Ag44(SR)30 (-SR = alkyl/arylthiolate, -S2R = dithiolate). Distinct, well-defined, low-frequency Raman bands of these clusters result from the vibrations of their metal cores whereas the higher-frequency bands reflect the structure of the metal-ligand interface. We observe a distinct breathing vibrational mode for each of these clusters. Detailed analyses of the bands are presented in the light of DFT calculations. These vibrational signatures change systematically when the metal atoms and/or the ligands are changed. Most importantly, our results show that the physical, lattice dynamics model alone cannot completely describe the vibrational properties of ligand-protected metal clusters. We show that low-frequency Raman spectroscopy is a powerful tool to understand the vibrational dynamics of atomically precise, molecule-like particles of other materials such as molecular nanocarbons, quantum dots, and perovskites.

Interactions of Metal Nanoclusters with Light: Fundamentals and Applications

The interactions of materials with light determine their applications in various fields. In the past decade, ultrasmall metal nanoclusters (NCs) have emerged as a promising class of optical materials due to their unique molecular-like properties. Herein, the basic principles of optical absorption and photoluminescence of metal NCs, their interactions with polarized light, and light-induced chemical reactions, are discussed, highlighting the roles of the core and protecting ligands/motifs of metal NCs in their interactions with light. The metal core and protecting ligands/motifs determine the electronic structures of metal NCs, which are closely related to their optical properties. In addition, the protecting ligands/motifs of metal NCs contribute to their photoluminescence and chiral origin, further promoting the interactions of metal NCs with light through various pathways. The fundamentals of light-NC interactions provide guidance for the design of metal NCs in optical applications, which are discussed in the second part. In the last section, some strategies are proposed to further understand light-NC interactions, highlighting the challenges and opportunities. It is hoped that this work will stimulate more research on the optical properties of metal NCs and their applications in various fields.

Selective formation of [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 by controlling ligand-exchange reaction

To use atomically precise metal nanoclusters (NCs) in various application fields, it is essential to establish size-selective synthesis methods for the metal NCs. Studies on thiolate (SR)-protected gold NCs (Au n (SR) m NCs) revealed that the atomically precise Au n (SR) m NC, which has a different chemical composition from the precursor, can be synthesized size-selectively by inducing transformation in the framework structure of the metal NCs by a ligand-exchange reaction. In this study, we selected the reaction of [Au25(SC2H4Ph)18]- (SC2H4Ph = 2-phenylethanethiolate) with 4-tert-butylbenzenethiol ( t BuPhSH) as a model ligand-exchange reaction and attempted to obtain new metal NCs by changing the amount of thiol, the central atom of the precursor NCs, or the reaction time from previous studies. The results demonstrated that [Au23(SPh t Bu)17]0, [Au26Pd(SPh t Bu)20]0 (Pd = palladium) and [Au24Pt(SC2H4Ph)7(SPh t Bu)11]0 (Pt = platinum) were successfully synthesized in a high proportion. To best of our knowledge, no report exists on the selective synthesis of these three metal NCs. The results of this study show that a larger variety of metal NCs could be synthesized size-selectively than at present if the ligand-exchange reaction is conducted while changing the reaction conditions and/or the central atoms of the precursor metal NCs from previous studies.

Atomically precise structures of Pt2(S-Adam)4(PPh3)2 complexes and catalytic application in propane dehydrogenation

As a bridge between single metal atoms and metal nanoclusters, atomically precise metal complexes are of great significance for controlled synthesis and catalytic applications at the atomic level. Herein, novel Pt₂(S-Adam)₄(PPh₃)₂ complexes were prepared via the conventional synthetic methods of metal nanoclusters. The atomically precise crystal structures of the binuclear Pt complexes with three kinds of packing modes in a unit cell were determined by X-ray crystallography. The two Pt atoms are bridged by two S atoms of thiolates, constructing a rhombus on a plane. Moreover, the ultraviolet visible absorption spectra of Pt₂(S-Adam)₄(PPh₃)₂ complexes show an apparent absorption peak centered at 454 nm. Furthermore, the Pt complexes were used as precursors to prepare catalysts for non-oxidative propane dehydrogenation. The as-prepared Pt-based catalysts with a particle size of approximately 1 nm demonstrated a propane conversion of about 18% and significantly enhanced selectivity for propylene, up to 93%. Our work will be beneficial to the basic understanding of platinum complexes, as well as the improvement of the catalytic dehydrogenation of propane.

Surface environment complication makes Ag29 nanoclusters more robust and leads to their unique packing in the supracrystal lattice

Silver nanoclusters have received unprecedented attention in cluster science owing to their promising functionalities and intriguing physical/chemical properties. However, essential instability significantly impedes their extensive applications. We herein propose a strategy termed “surface environment complication” to endow Ag₂₉ nanoclusters with high robustness. The Ag₂₉(S-Adm)₁₈(PPh₃)₄ nanocluster with monodentate PPh₃ ligands was extremely unstable and uncrystallizable. By substituting PPh₃ with bidentate PPh₂py with dual coordination sites (i.e., P and N), the Ag₂₉ cluster framework was twisted because of the generation of N–Ag interactions, and three NO₃ ligands were further anchored onto the nanocluster surface, yielding a new Ag₂₉(S-Adm)₁₅(NO₃)₃(PPh₂py)₄ nanocluster with high stability. The metal-control or ligand-control effects on stabilizing the Ag₂₉ nanocluster were further evaluated. Besides, Ag₂₉(S-Adm)₁₅(NO₃)₃(PPh₂py)₄ followed a unique packing mode in the supracrystal lattice with several intercluster channels, which has yet been observed in other M₂₉ cluster crystals. Overall, this work presents a new approach (i.e., surface environment complication) for tailoring the surface environment and improving the stability of metal nanoclusters.

A strategy of “surface environment complication” has been exploited to endow Ag₂₉ nanoclusters with high robustness and a unique packing mode in the supracrystal lattice.

Development and Functionalization of Visible-Light-Driven Water-Splitting Photocatalysts

With global warming and the depletion of fossil resources, our fossil fuel-dependent society is expected to shift to one that instead uses hydrogen (H2) as a clean and renewable energy. To realize this, the photocatalytic water-splitting reaction, which produces H2 from water and solar energy through photocatalysis, has attracted much attention. However, for practical use, the functionality of water-splitting photocatalysts must be further improved to efficiently absorb visible (Vis) light, which accounts for the majority of sunlight. Considering the mechanism of water-splitting photocatalysis, researchers in the various fields must be employed in this type of study to achieve this. However, for researchers in fields other than catalytic chemistry, ceramic (semiconductor) materials chemistry, and electrochemistry to participate in this field, new reviews that summarize previous reports on water-splitting photocatalysis seem to be needed. Therefore, in this review, we summarize recent studies on the development and functionalization of Vis-light-driven water-splitting photocatalysts. Through this summary, we aim to share current technology and future challenges with readers in the various fields and help expedite the practical application of Vis-light-driven water-splitting photocatalysts.

Silver Cluster Interactions with Tyrosine: Towards Amino Acid Detection

Tyrosine (Tyr) is involved in the synthesis of neurotransmitters, catecholamines, thyroid hormones, etc. Multiple pathologies are associated with impaired Tyr metabolism. Silver nanoclusters (Ag NCs) can be applied for colorimetric, fluorescent, and surface-enhanced Raman spectroscopy (SERS) detection of Tyr. However, one should understand the theoretical basics of interactions between Tyr and Ag NCs. Thereby, we calculated the binding energy (Eb) between Tyr and Agnq (n = 1-8; q = 0-2) NCs using the density functional theory (DFT) to find the most stable complexes. Since Ag NCs are synthesized on Tyr in an aqueous solution at pH 12.5, we studied Tyr-1, semiquinone (SemiQ-1), and Tyr-2. Ag32+ and Ag5+ had the highest Eb. The absorption spectrum of Tyr-2 significantly red-shifts with the attachment of Ag32+, which is prospective for colorimetric Tyr detection. Ag32+ interacts with all functional groups of SemiQ-1 (phenolate, amino group, and carboxylate), which makes detection of Tyr possible due to band emergence at 1324 cm-1 in the vibrational spectrum. The ground state charge transfer between Ag and carboxylate determines the band emergence at 1661 cm-1 in the Raman spectrum of the SemiQ-1-Ag32+ complex. Thus, the prospects of Tyr detection using silver nanoclusters were demonstrated.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

Systematics of vibrational properties of Au nanoparticles: a Molecular Dynamics approach

This paper reports the results of a Molecular Dynamics (MD) study of the vibrational properties of spherical Au nanoparticles with a number of atoms (N) varying in the range 1985...

Effects of protecting groups on luminescent metal nanoclusters: spectroscopic signatures and applications

Luminescent metal nanoclusters (NCs) have been established as next-generation fluorophores. Their biocompatible and non-toxic nature, along with excellent chemical- and photo-stability, enables them to find applications in multi-disciplinary areas. However, preparing NCs which are stable is always challenging, primarily owing to their small size and propensity to self-aggregate. In this review, we highlight a holistic approach as to how ligands and templates can monitor the stability of NCs, tune their spectroscopic signatures, and alter their applications. The role of small molecules of a large ligand in the preparation of NCs and their associated limitations are also discussed. We have summarized how these NCs can be utilized in sensing several metal ions, pH, viscosity and temperature of many systems which have biological relevance. Additionally, these luminescent metal NCs find usage in cell-imaging, discriminating between cancerous and non-cancerous cell lines and also targeting specific organelles within the cellular environment.

Full-Color Tunable Circularly Polarized Luminescence Induced by the Crystal Defect from the Co-assembly of Chiral Silver(I) Clusters and Dyes

Four pairs of defective crystals exhibiting full-color emission and circularly polarized luminescence (CPL) with high luminescence dissymmetry factor (g_lum) values (∼3 × 10^-3) were successfully obtained by doping dye molecules into the chiral crystalline metal cluster-based matrixes. The dye molecules function as defect inducers and confer fluorescence on the crystals. Studies reveal that electrostatic interactions provide the main impetus in generating defective crystals, and the restricted effect of chiral space and the weak interactions in defect crystal enable the efficient chiral transfer from the intrinsically chiral host silver(I) clusters to achiral luminescent dopants and finally induce them to emit bright CPL. This defect engineering strategy opens a new way to versatile functions for crystalline cluster-based materials.