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

[Ag59(2,5-DCBT)32]3-: a new cluster and a precursor for three well-known clusters

We report the synthesis of a new silver cluster, [Ag₅₉(2,5-DCBT)₃₂]^3- (I) (2,5-DCBT: 2,5-dichlorobenzenethiol), which acts as a precursor for the synthesis of three well-known silver clusters, [Ag₄₄(2,4-DCBT/4-FTP)₃₀]^4- (II) (4-FTP: 4-fluorothiophenol and 2,4-DCBT: 2,4-dichlorobenzenethiol), [Ag₂₅(2,4-DMBT)₁₈]^- (III) (2,4-DMBT: 2,4-dimethylbenzenethiol) and [Ag₂₉(1,3-BDT)₁₂(PPh₃)₄]^3- (IV) (1,3-BDT: 1,3-benzenedithiol and PPh₃: triphenylphosphine). This newly synthesized silver cluster, I, is characterized using UV-vis absorption studies, high resolution electrospray ionization mass spectrometry (ESI MS) and other analytical tools. The optical absorption spectrum shows distinct features which are completely different from the previously reported silver clusters. We perform the rapid transformations of I to other well-known clusters II, III and IV by reaction with different thiols. The time-dependent UV-vis and ESI MS measurements reveal that I dissociates into distinct thiolate entities in the presence of thiols and the thiolates recombine to produce different clusters. The conversion mechanism is found to be quite different from the previous reports where it occurs through the initial formation of ligand exchanged products. Here, we also show the synthesis of a different cluster core, [Ag₄₄(2,4-DCBT)₃₀]^4- (IIa) using 2,4-DCBT, a structural isomer of 2,5-DCBT under the same synthetic conditions used for I. This observation demonstrates the effect of isomeric thiols on controlling the size of silver clusters. The conversion of one cluster to several other clusters under ambient conditions and the effect of ligand structure in silver cluster synthesis give new insights into the cluster chemistry.

Density functional investigation of the adsorption effects of PH3 and SH2 on the structure stability of the Au55 and Pt55 nanoclusters

Although several studies have been reported for Pt₅₅ and Au₅₅ nanoclusters, our atomistic understanding of the interplay between the adsorbate-surface interactions and the mechanisms that lead to the formation of the distorted reduced core (DRC) structures, instead of the icosahedron (ICO) structure in gas phase, is still far from satisfactory. Here, we report a density functional theory (DFT) investigation of the role of the adsorption effects of PH₃ (one lone pair of electrons) and SH₂ (two lone pairs) on the relative stability of the Pt₅₅ and Au₅₅ nanoclusters. In gas phase, we found that the DRC structures with 7 and 9 atoms in the core region are about 5.34 eV (Pt₅₅) and 2.20 eV (Au₅₅) lower in energy than the ICO model with I_h symmetry and 13 atoms in the core region. However, the stability of the ICO structure increases by increasing the number of adsorbed molecules from 1 to 18, in which both DRC and ICO structures are nearly degenerate in energy at the limit of 18 ligands, which can be explained as follows. In gas phase, there is a strong compression of the cationic core region by the anionic surface atoms induced by the attractive Coulomb interactions (core⁺-surface^-), and hence, the strain release is obtained by reducing the number of atoms in the cationic core region, which leads to the 55 atoms distorted reduced core structures. Thus, the Coulomb interactions between the core⁺ and surface^- contribute to break the symmetry in the ICO₅₅ structure. On the other hand, the addition of ligands on the anionic surface reduces the charge transfer between the core and surface, which contributes to decrease the Coulomb interactions and the strain on the core region of the ICO structure, and hence, it stabilizes a compact ICO structure. The same conclusion is obtained by adding van der Waals corrections to the plain DFT calculations. Similar results are obtained by the addition of steric effects, which are considered through the adsorption of triphenylphosphine (PPh₃) molecules on Au₅₅, in which the relative stability between ICO and DRC is the same as for PH₃ and SH₂. However, for Pt₅₅, we found an inversion of stability due to the PPh₃ ligand effects, where ICO has higher stability than DRC by 2.40 eV. Our insights are supported by several structural, electronic, and energetic analyses.

Au38(SPh)24: Au38 Protected with Aromatic Thiolate Ligands

Au₃₈(SR)₂₄ is one of the most extensively investigated gold nanomolecules along with Au₂₅(SR)₁₈ and Au₁₄₄(SR)₆₀. However, so far it has only been prepared using aliphatic-like ligands, where R = -SC₆H₁₃, -SC₁₂H₂₅ and -SCH₂CH₂Ph. Au₃₈(SCH₂CH₂Ph)₂₄ when reacted with HSPh undergoes core-size conversion to Au₃₆(SPh)₂₄, and existing literature suggests that Au₃₈(SPh)₂₄ cannot be synthesized. Here, contrary to prevailing knowledge, we demonstrate that Au₃₈(SPh)₂₄ can be prepared if the ligand exchanged conditions are optimized, under delicate conditions, without any formation of Au₃₆(SPh)₂₄. Conclusive evidence is presented in the form of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectra (ESI-MS) characterization, and optical spectra of Au₃₈(SPh)₂₄ in a solid glass form showing distinct differences from that of Au₃₈(S-aliphatic)₂₄. Theoretical analysis confirms experimental assignment of the optical spectrum and shows that the stability of Au₃₈(SPh)₂₄ is not negligible with respect to that of its aliphatic analogous, and contains a significant component of ligand-ligand attractive interactions. Thus, while Au₃₈(SPh)₂₄ is stable at RT, it converts to Au₃₆(SPh)₂₄ either on prolonged etching (longer than 2 hours) at RT or when etched at 80 °C.

Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate-Protected Metal Clusters to Synthesize Controlled Metal Clusters

It is now possible to accurately synthesize thiolate (SR)-protected gold clusters (Au_n (SR)_m ) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Au_n (SR)_m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.

Reversible conversion between phosphine protected Au6 and Au8 nanoclusters under oxidative/reductive conditions

In this study, we found that phosphine protected [Au₆(dppp)₄]²⁺ and [Au₈(dppp)₄Cl₂]²⁺ nanoclusters could be reversibly converted under oxidative/reductive conditions. This work not only provides new insights into the relationship between the [Au₆(dppp)₄]²⁺ and [Au₈(dppp)₄Cl₂]²⁺ nanoclusters, but also offers a novel method for controlling structural evolution of different Au nanoclusters.

Understanding energy transfer with luminescent gold nanoclusters: a promising new transduction modality for biorelated applications

AuNCs engage in energy transfer by a non-Förster process although many of the same photophysical requirements are needed.

Quantitatively Monitoring the Size-Focusing of Au Nanoclusters and Revealing What Promotes the Size Transformation from Au44(TBBT)28 to Au36(TBBT)24

"Size-focusing" is a well-recognized process and widely employed for the synthesis of atomically monodisperse metal nanoclusters. However, quantitatively monitoring the size-focusing of Au nanoclusters has not been achieved yet, and the in-depth understanding of the size focusing is far from completed. Herein, we introduce a facile, cheap, and powerful tool, preparative thin-layer chromatography (PTLC), to quantitatively track the size-focusing process, to reveal that mainly ∼3 nm nanoparticles promote the transformation from Au₄₄(TBBT)₂₈ to Au₃₆(TBBT)₂₄ (where TBBT is 4-tert-butylbenzenethiolate) and to improve the syntheses of Au₄₄(TBBT)₂₈ and Au₃₆(TBBT)₂₄. Our work further demonstrates the usefulness of PTLC in nanocluster research and advances one step toward understanding the "size-focusing" process of nanoclusters.

The solely motif-doped Au36-xAgx(SPh-tBu)24 (x = 1-8) nanoclusters: X-ray crystal structure and optical properties

We report the observation of new doping behavior in Au36-xAgx(SR)24 nanoclusters (NCs) with x = 1 to 8. The atomic arrangements of Au and Ag atoms are determined by X-ray crystallography. The new gold-silver bimetallic NCs share the same framework as that of the homogold counterpart, i.e. possessing an fcc-type Au28 kernel, four dimeric AuAg(SR)3 staple motifs and twelve simple bridging SR ligands. Interestingly, all the Ag dopants in the Au36-xAgx(SR)24 NCs are selectively incorporated into the surface motifs, which is in contrast to the previously reported Au-Ag alloy structures with the Ag dopants preferentially displacing the core gold atoms. This distinct doping behavior implies that the previous assignments of an fcc Au28 core with four dimers and 12 bridging thiolates for Au36(SR)24 are more justified than other assignments of core vs. surface motifs. The UV-Vis adsorption spectrum of Au36-xAgx(SR)24 is almost the same as that of Au36(SR)24, indicating that the Ag dopants in the motifs do not change the optical properties. The similar UV-Vis spectra are further confirmed by TD-DFT calculations. DFT also reveals that the energies of the HOMO and LUMO of the motif-doped AuAg alloy NC are comparable to those of the homogold Au36 NC, indicating that the electronic structure is not disturbed by the motif Ag dopants. Overall, this study reveals a new silver-doping mode in alloy NCs.

Isomerism in Au28(SR)20 Nanocluster and Stable Structures

Understanding the isomerism phenomenon at the nanoscale is a challenging task because of the prerequisites of precise composition and structural information on nanoparticles. Herein, we report the ligand-induced, thermally reversible isomerization between two thiolate-protected 28-gold-atom nanoclusters, i.e. Au28(S-c-C6H11)20 (where -c-C6H11 = cyclohexyl) and Au28(SPh-(t)Bu)20 (where -Ph-(t)Bu = 4-tert-butylphenyl). The intriguing ligand effect in dictating the stability of the two Au28(SR)20 structures is further investigated via dispersion-corrected density functional theory calculations.

Controlled synthesis of pure Au25(2-Nap)18 and Au36(2-Nap)24 nanoclusters from 2-(diphenylphosphino)pyridine protected Au nanoclusters

The controlled synthesis of pure Au₂₅(2-Nap)₁₈ and Au₃₆(2-Nap)₂₄ nanoclusters were realized via etching 2-(diphenylphosphino)pyride protected polydispersed Au nanoclusters with the mass of 1 kDa to 3 kDa at 80 °C and 50 °C, respectively.

The impact of endohedral atoms on the electronic and optical properties of Au25(SR)18 and Au38(SR)24

Analysis of the endohedral atom role allows to rationalize their versatility as nanomaterials.

High-resolution separation of thiolate-protected gold clusters by reversed-phase high-performance liquid chromatography

This perspective summarizes our work on high-resolution separation of thiolate-protected gold clusters using reversed-phase high-performance liquid chromatography, new findings obtained by those separation, and future prospects for this field.

Heavily doped Au25–xAgx(SC6H11)18− nanoclusters: silver goes from the core to the surface

The Au₂₅(SR)₁₈ nanocluster (where R = c-C₆H₁₁) can be heavily doped with silver through Ag(i)–thiolate complex induced size/structure transformation of Au₂₃(SR)₁₆⁻ into Au_25–xAg_x(SR)₁₈⁻.

X-Ray crystal structure, and optical and electrochemical properties of the Au15Ag3(SC6H11)14 nanocluster with a core-shell structure

We report the X-ray crystallographic structure of an 18-metal atom Au-Ag bimetallic nanocluster (NC) formulated as [Au15Ag3(SC6H11)14]. This NC consists of a Au6Ag3 bi-octahedral kernel, which is built up by two octahedral Au3Ag3 units through sharing one Ag3 triangular face. The [Au15Ag3(SC6H11)14] can be viewed as a core-shell structure with the doped Ag atoms as the core and Au atoms as the shell. Detailed analyses by UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements clearly show distinct differences in the electronic structure between [Au15Ag3(SC6H11)14] and the homometal [Au18(SC6H11)14] NC. This study contributes to the deep understanding on bimetallic nanoclusters.

Understanding Ligand-Exchange Reactions on Thiolate-Protected Gold Clusters by Probing Isomer Distributions Using Reversed-Phase High-Performance Liquid Chromatography

Thiolate-protected gold clusters (Aun(SR)m) have attracted considerable attention as functional nanomaterials in a wide range of fields. A ligand-exchange reaction has long been used to functionalize these clusters. In this study, we separated products from a ligand-exchange reaction of phenylethanethiolate-protected Au24Pd clusters (Au24Pd(SC2H4Ph)18), in which Au25(SR)18 is doped with palladium, into each coordination isomer with high resolution by reversed-phase high-performance liquid chromatography. This success has enabled isomer distributions of the products to be quantitatively evaluated. We evaluated quantitatively the isomer distributions of products obtained by the reaction of Au24Pd(SC2H4Ph)18 with thiol, disulfide, or diselenide. The results revealed that the exchange reaction starts to occur preferentially at thiolates that are bound directly to the metal core (thiolates of a core site) in all reactions. Further study on the isomer-separated Au24Pd(SC2H4Ph)17(SC12H25) revealed that clusters vary the coordination isomer distribution in solution by the ligand-exchange reaction between clusters and that control of the coordination isomer distribution of the starting clusters enables control of the coordination isomer distribution of the products generated by ligand-exchange reactions between clusters. Au24Pd(SC2H4Ph)18 used in this study has a similar framework structure to Au25(SR)18, which is one of the most studied compounds in the Aun(SR)m clusters. Knowledge gained in this study is expected to enable further understanding of ligand-exchange reactions on Au25(SR)18 and other Aun(SR)m clusters.

Transformation Chemistry of Gold Nanoclusters: From One Stable Size to Another

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. This dream has first been realized in the case of gold nanoparticles. We previously discussed a size-focusing methodology for the syntheses of atomically precise gold nanoclusters protected by thiolate ligands (referred to as Aun(SR)m, where n and m represent the exact numbers of gold atoms and surface ligands). This methodology led to molecularly pure nanoclusters such as Au25(SR)18, Au38(SR)24, Au144(SR)60, and many others in recent work. In this Perspective article, we shall further discuss a new methodology for controlling the size and structure of nanoclusters through ligand-exchange-induced transformation of Aun(SR)m nanoclusters. Notable examples include the transformations of Au25(SR)18 to Au28(SR')20, Au38(SR)24 to Au36(SR')24, and Au144(SR)60 to Au133(SR')52. Total structures of the new nanoclusters have also been attained. The transformation processes are remarkable and resemble the organic transformation chemistry. We have also achieved mechanistic understanding on the transformation process, and a disproportionation mechanism has been for the first time identified. This new methodology (i.e., ligand-exchange-induced size/structure transformation, LEIST for short) has not only demonstrated the important role of thiolate ligand in the transformation chemistry of clusters but also paved the way for creating an expanded "library" of Aun(SR)m nanoclusters for exploration of their magic sizes, structures, properties, and applications.

Tuning the Magic Size of Atomically Precise Gold Nanoclusters via Isomeric Methylbenzenethiols

Toward controlling the magic sizes of atomically precise gold nanoclusters, herein we have devised a new strategy by exploring the para-, meta-, ortho-methylbenzenethiol (MBT) for successful preparation of pure Au130(p-MBT)50, Au104(m-MBT)41 and Au40(o-MBT)24 nanoclusters. The decreasing size sequence is in line with the increasing hindrance of the methyl group to the interfacial Au-S bond. That the subtle change of ligand structure can result in drastically different magic sizes under otherwise similar reaction conditions is indeed for the first time observed in the synthesis of thiolate-protected gold nanoclusters. These nanoclusters are highly stable as they are synthesized under harsh size-focusing conditions at 80-90 °C in the presence of excess thiol and air (i.e., without exclusion of oxygen).

Efficient synthesis of Au₉₉(SR)₄₂ nanoclusters

We report a new synthetic protocol of Au99(SPh)42 nanoclusters with moderate efficiency (∼15% yield based on HAuCl4), via a combination of the ligand-exchange and "size-focusing" processes. The purity of the as-prepared gold nanoclusters is characterized by matrix-assisted laser desorption ionization mass spectrometry and size exclusion chromatography.

The ligand effect on the isomer stability of Au24(SR)20 clusters

A key challenge in nanocluster research in particular and nanoscience in general is structure prediction for known compositions. Usually a simple ligand such as a methyl group is used to replace complex ligands in structure prediction of ligand-protected nanoclusters. However, how ligands dictate the energy landscape of such a cluster remains unclear. Here we elucidate the role of the ligand effect on the isomer stability of Au24(SR)20 nanoclusters by computing the relative energy of two isomers (one from the experiment, denoted as the "J" isomer; the other is the best theoretical model, denoted as the "P" isomer) of Au24(SR)20 with dispersion-corrected density functional theory. We find that when R = -CH3, the two isomers are equally stable (within 0.13 eV), but for R = -CH2CH2Ph the P isomer is more stable by 1.6 eV and for R = -CH2Ph-(t)Bu the J isomer is more stable by 1.0 eV. Partition of the total energy into DFT and vdW contributions indicates that the higher stability of the P isomer in the case of R = -CH2CH2Ph stems from the stronger vdW interactions among -CH2CH2Ph groups, while the higher stability of the J isomer in the case of R = -CH2Ph-(t)Bu is due to its better capacity to respond to the steric effect of the larger -CH2Ph-(t)Bu groups. This finding confirms that the ligand plays a crucial role in dictating the isomer stability.

Atomically precise metal nanoclusters: stable sizes and optical properties

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. Breakthroughs have been made in the case of gold nanoparticles, at least for nanoparticles smaller than ∼3 nm in diameter. Such ultrasmall gold nanoparticles indeed exhibit fundamentally different properties from those of the plasmonic counterparts owing to the quantum size effects as well as the extremely high surface-to-volume ratio. These unique nanoparticles are often called nanoclusters to distinguish them from conventional plasmonic nanoparticles. Intense work carried out in the last few years has generated a library of stable sizes (or stable stoichiometries) of atomically precise gold nanoclusters, which are opening up new exciting opportunities for both fundamental research and technological applications. In this review, we have summarized the recent progress in the research of thiolate (SR)-protected gold nanoclusters with a focus on the reported stable sizes and their optical absorption spectra. The crystallization of nanoclusters still remains challenging; nevertheless, a few more structures have been achieved since the earlier successes in Au102(SR)44, Au25(SR)18 and Au38(SR)24 nanoclusters, and the newly reported structures include Au20(SR)16, Au24(SR)20, Au28(SR)20, Au30S(SR)18, and Au36(SR)24. Phosphine-protected gold and thiolate-protected silver nanoclusters are also briefly discussed in this review. The reported gold nanocluster sizes serve as the basis for investigating their size dependent properties as well as the development of applications in catalysis, sensing, biological labelling, optics, etc. Future efforts will continue to address what stable sizes are existent, and more importantly, what factors determine their stability. Structural determination and theoretical simulations will help to gain deep insight into the structure-property relationships.

Theoretical examination of solvent and R group dependence in gold thiolate nanoparticle synthesis

The reaction of phenylthiol with AuCl₄⁻ yields gold thiolate nanoparticle precursors in polar solvents.

Designing ligand-enhanced optical absorption of thiolated gold nanoclusters

By appropriately selecting the ligands, optical excitations of nanocrystal molecules can be amplified and delocalized over the whole compound.

Recent Progress in the Functionalization Methods of Thiolate-Protected Gold Clusters

Nanomaterials that exhibit both stability and functionality are currently considered to hold great promise as components of nanotechnology devices. Thiolate-protected gold clusters (Aun(SR)m) have long attracted attention as functional nanomaterials. Magic Aun(SR)m clusters are an especially stable group of thiolate-protected clusters that have particularly high potential as functional materials. Although numerous application experiments have been conducted for magic Aun(SR)m clusters, it is important that functionalization methods are also established to allow for effective utilization of these materials. The results of recent research on heteroatom doping and the use of other chalcogenide ligands strongly suggest that these strategies are promising as functionalization methods of magic Aun(SR)m clusters. In this Perspective, we focus on studies relating to three representative types of magic clusters-Au25(SR)18, Au38(SR)24, and Au144(SR)60-and discuss the recent progress and future issues.

Au99(SPh)42 nanomolecules: aromatic thiolate ligand induced conversion of Au144(SCH2CH2Ph)60

A new aromatic thiolate protected gold nanomolecule Au99(SPh)42 has been synthesized by reacting the highly stable Au144(SCH2CH2Ph)60 with thiophenol, HSPh. The ubiquitous Au144(SR)60 is known for its high stability even at elevated temperature and in the presence of excess thiol. This report demonstrates for the first time the reactivity of the Au144(SCH2CH2Ph)60 with thiophenol to form a different 99-Au atom species. The resulting Au99(SPh)42 compound, however, is unreactive and highly stable in the presence of excess aromatic thiol. The molecular formula of the title compound is determined by high resolution electrospray mass spectrometry (ESI-MS) and confirmed by the preparation of the 99-atom nanomolecule using two ligands, namely, Au99(SPh)42 and Au99(SPh-OMe)42. This mass spectrometry study is an unprecedented advance in nanoparticle reaction monitoring, in studying the 144-atom to 99-atom size evolution at such high m/z (∼12k) and resolution. The optical and electrochemical properties of Au99(SPh)42 are reported. Other substituents on the phenyl group, HS-Ph-X, where X = -F, -CH3, -OCH3, also show the Au144 to Au99 core size conversion, suggesting minimal electronic effects for these substituents. Control experiments were conducted by reacting Au144(SCH2CH2Ph)60 with HS-(CH2)n-Ph (where n = 1 and 2), bulky ligands like adamantanethiol and cyclohexanethiol. It was observed that conversion of Au144 to Au99 occurs only when the phenyl group is directly attached to the thiol, suggesting that the formation of a 99-atom species is largely influenced by aromaticity of the ligand and less so on the bulkiness of the ligand.

Dynamic study on the transformation process of gold nanoclusters

In this paper, the transformation process from Au8 to Au25 nanoclusters (NCs) is investigated with steady state fluorescence spectroscopy and time-resolved fluorescence spectroscopy at various reaction temperatures and solvent diffusivities. Results demonstrate that Au8 NCs, protected by bovine serum albumin, transform into Au25 NCs under controlled pH values through an endothermic reaction with the activation energy of 74 kJ mol(-1). Meanwhile, the characteristic s-shaped curves describing the formation of Au25 NCs suggest this process involves a diffusion controlled growth mechanism.

Neat and complete: thiolate-ligand exchange on a silver molecular nanoparticle

Atomically precise thiolate-protected noble metal molecular nanoparticles are a promising class of model nanomaterials for catalysis, optoelectronics, and the bottom-up assembly of true molecular crystals. However, these applications have not fully materialized due to a lack of ligand exchange strategies that add functionality, but preserve the properties of these remarkable particles. Here we present a method for the rapid (<30 s) and complete thiolate-for-thiolate exchange of the highly sought after silver molecular nanoparticle [Ag44(SR)30](-4). Only by using this method were we able to preserve the precise nature of the particles and simultaneously replace the native ligands with ligands containing a variety of functional groups. Crucially, as a result of our method we were able to process the particles into smooth thin films, paving the way for their integration into solution-processed devices.

Highly efficient electrogenerated chemiluminescence of Au38 nanoclusters

An investigation of mechanisms for the near-infrared (NIR) electrogenerated chemiluminescence/electrochemiluminescence (ECL) of Au38(SC2H4Ph)24 (Au38, SC2H4Ph = 2-phenylethanethiol) nanoclusters both in annihilation and coreactant paths is reported. Essentially, no ECL emission was produced in the annihilation route over the potential range of the accessible redox states of Au38, because of the short lifetime and/or low reactivity of the electrogenerated Au38 intermediates necessary for ECL. Highly efficient light emission with a nominal peak wavelength of 930 nm in the NIR region was observed in the anodic region upon addition of tri-n-propylamine (TPrA) as the coreactant. The ECL mechanisms were elucidated by means of ECL-potential curves and spooling ECL spectroscopy. It was discovered that the Au38(+*) (and also Au38(3+*)) were electrogenerated as the major excited species in the light emission processes. Benzoyl peroxide was also used as a coreactant in the cathodic potential range from which benzoate radicals, with a high oxidizing power, were formed. These radicals accepted electrons from the electrogenerated Au38(2-) HOMO, resulting in the Au38(-*) excited state that emitted light at 930 nm. The photoluminescence of the various Au38 charge states, namely, Au38(2-), Au38(-), Au38(0), Au38(+), Au38(2+), and Au38(4+), electrogenerated in situ, indicated no significant difference in the emission peak wavelength. This information allowed a careful mapping of the relevant ECL mechanisms. It was found that the ECL efficiency could reach an efficiency of 3.5 times as high as that of the Ru(bpy)3(2+)/TPrA system.

Thiolate-protected Au38(SR)24 nanocluster: size-focusing synthesis, structure determination, intrinsic chirality, and beyond

Thiolate-protected Au nanoclusters with core diameters smaller than 2 nm have captured considerable attention in recent years due to their diverse applications ranging from biological labeling to photovoltaics and catalysis. This new class of nanomaterials exhibits discrete electronic structure and molecular-like properties, such as HOMO-LUMO electronic transition, intrinsic magnetism, chiroptical properties, and enhanced catalytic properties. This review focuses on the research into thiolate-protected Au38(SR)24 – one of the most representative nanoclusters, including its identification, size-focusing synthesis, structure determination, and intrinsic chirality. The properties of two size-adjacent Au nanoclusters [Au40(SR)24 and Au36(SR)24] are also discussed. The experimental and theoretical methodologies developed in studies of the Au38(SR)24 model nanocluster open up new opportunities in the synthesis and properties investigation of other atomically precise Aun(SR)m nanoclusters.

Synthesis of a Au44(SR)28 nanocluster: structure prediction and evolution from Au28(SR)20, Au36(SR)24 to Au44(SR)28

We report the synthesis of a Au44(SR)28 nanocluster (SR = 4-tert-butylbenzenethiolate). Based on the structural rules learned from the known Au28(SR)20 and Au36(SR)24 structures, we propose a plausible structure for Au44(SR)28, which is predicted to comprise a six-interpenetrating cuboctahedral Au36 kernel protected by four dimeric staples and sixteen bridging thiolates, i.e. Au36[Au2(SR)3]4(SR)16.

Unique Bonding Properties of the Au36(SR)24 Nanocluster with FCC-Like Core

The recent discovery on the total structure of Au36(SR)24, which was converted from biicosahedral Au38(SR)24, represents a surprising finding of a face-centered cubic (FCC)-like core structure in small gold-thiolate nanoclusters. Prior to this finding, the FCC feature was only expected for larger (nano)crystalline gold. Herein, we report results on the unique bonding properties of Au36(SR)24 that are associated with its FCC-like core structure. Temperature-dependent X-ray absorption spectroscopy (XAS) measurements at the Au L3-edge, in association with ab initio calculations, show that the local structure and electronic behavior of Au36(SR)24 are of more molecule-like nature, whereas its icosahedral counterparts such as Au38(SR)24 and Au25(SR)18 are more metal-like. Moreover, site-specific S K-edge XAS studies indicate that the bridging motif for Au36(SR)24 has different bonding behavior from the staple motif from Au38(SR)24. Our findings highlight the important role of "pseudo"-Au4 units within the FCC-like Au28 core in interpreting the bonding properties of Au36(SR)24 and suggest that FCC-like structure in gold thiolate nanoclusters should be treated differently from its bulk counterpart.