Coherent Vibrational Dynamics of [Au25(SR)18]- Nanoclusters?

Coherent vibrational dynamics can be observed in atomically precise gold nanoclusters using femtosecond time-resolved pump-probe spectroscopy. It can not only reveal the coupling between electrons and vibrations, but also reflect the mechanical and electronic properties of metal nanoclusters, which holds potential applications in biological sensing and mass detection. Here, we investigated the coherent vibrational dynamics of [Au₂₅(SR)₁₈]^- nanoclusters by ultrafast spectroscopy and revealed the origins of these coherent vibrations by analyzing their frequency, phase and probe wavelength distributions. Strong coherent oscillations with frequency of 40 cm^-1 and 80 cm^-1 can be reproduced in the excited state dynamics of [Au₂₅(SR)₁₈]^-, which should originate from acoustic vibrations of the Au₁₃ metal core. Phase analysis on the oscillations indicates that the 80 cm^-1 mode should arise from the frequency modulation of the electronic states while the 40 cm^-1 mode should originate from the amplitude modulation of the dynamic spectrum. Moreover, it is found that the vibration frequencies of [Au₂₅(SR)₁₈]^- obtained in pump-probe measurements are independent of the surface ligands so that they are intrinsic properties of the metal core. These results are of great value to understand the electron-vibration coupling of metal nanoclusters.     

One‐Pot Synthesis of Phenylmethanethiolate‐Protected Au20(SR)16 and Au24(SR)20 Nanoclusters and Insight into the Kinetic Control

We report two synthetic routes for concurrent formation of phenylmethanethiolate (‐SCH₂Ph)‐protected Au₂₀(SR)₁₆ and Au₂₄(SR)₂₄ nanoclusters in one‐pot by kinetic control. Unlike the previously reported methods for thiolate‐protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH₄ and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold–thiolate cluster synthesis. We demonstrate that the synthesis of ‐SCH₂Ph‐protected Au₂₀ and Au₂₄ nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH₄ under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH₄ under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au₂₀(SCH₂Ph)₁₆ and Au₂₄(SCH₂Ph)₂₀ clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH₄ solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH₂S‐protected Au₂₀ and Au₂₄ nanoclusters with the PhC₂H₄S‐protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH₂S‐capped nanoclusters when compared to the PhC₂H₄S‐capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.     

Tuning the Accessibility and Activity of Au25(SR)18 Nanocluster Catalysts through Ligand Engineering

In this work, the effects of thiolate ligands (‐SR, e.g., chain length and functional moiety) on the accessibility and catalytic activity of thiolate‐protected gold nanoclusters (e.g., Au₂₅(SR)₁₈) for 4‐nitrophenol hydrogenation is reported. The data suggest that Au₂₅(SR)₁₈ bearing a shorter alkyl chain shows a better accessibility to the substrates (shown by shorter induction time, t₀) and a higher catalytic activity (shown by higher apparent reaction rate constant, k_app). The functional moiety of the ligands is another determinant factor, which clearly suggests that ligand engineering of Au₂₅(SR)₁₈ would be an efficient platform for fine‐tuning its catalytic properties.     

The Structure and Optical Properties of the [Au18(SR)14] Nanocluster

Decreasing the core size is one of the best ways to study the evolution from Au^I complexes into Au nanoclusters. Toward this goal, we successfully synthesized the [Au₁₈(SC₆H₁₁)₁₄] nanocluster using the [Au₁₈(SG)₁₄] (SG=L ‐glutathione) nanocluster as the starting material to react with cyclohexylthiol, and determined the X‐ray structure of the cyclohexylthiol‐protected [Au₁₈(C₆H₁₁S)₁₄] nanocluster. The [Au₁₈(SR)₁₄] cluster has a Au₉ bi‐octahedral kernel (or inner core). This Au₉ inner core is built by two octahedral Au₆ cores sharing one triangular face. One transitional gold atom is found in the Au₉ core, which can also be considered as part of the Au₄(SR)₅ staple motif. These findings offer new insight in terms of understanding the evolution from [Au^I(SR)] complexes into Au nanoclusters.     

Isomerization-induced enhancement of luminescence in Au28(SR)20 nanoclusters

Understanding the origin and structural basis of the photoluminescence (PL) phenomenon in thiolate-protected metal nanoclusters is of paramount importance for both fundamental science and practical applications. It remains a major challenge to correlate the PL properties with the atomic-level structure due to the complex interplay of the metal core (i.e. the inner kernel) and the exterior shell (i.e. surface Au(i)-thiolate staple motifs). Decoupling these two intertwined structural factors is critical in order to understand the PL origin. Herein, we utilize two Au₂₈(SR)₂₀ nanoclusters with different –R groups, which possess the same core but different shell structures and thus provide an ideal system for the PL study. We discover that the Au₂₈(CHT)₂₀ (CHT: cyclohexanethiolate) nanocluster exhibits a more than 15-fold higher PL quantum yield than the Au₂₈(TBBT)₂₀ nanocluster (TBBT: p-tert-butylbenzenethiolate). Such an enhancement is found to originate from the different structural arrangement of the staple motifs in the shell, which modifies the electron relaxation dynamics in the inner core to different extents for the two nanoclusters. The emergence of a long PL lifetime component in the more emissive Au₂₈(CHT)₂₀ nanocluster reveals that its PL is enhanced by suppressing the nonradiative pathway. The presence of long, interlocked staple motifs is further identified as a key structural parameter that favors the luminescence. Overall, this work offers structural insights into the PL origin in Au₂₈(SR)₂₀ nanoclusters and provides some guidelines for designing luminescent metal nanoclusters for sensing and optoelectronic applications.

Two Au₂₈(SR)₂₀ nanoclusters with an identical core but different shells exhibit a ∼15-fold difference in photoluminescence.     

Homoleptic Alkynyl‐Protected Gold Nanoclusters: Au44(PhC≡C)28 and Au36(PhC≡C)24

For the first time total structure determination of homoleptic alkynyl‐protected gold nanoclusters is reported. The nanoclusters are synthesized by direct reduction of PhC≡CAu, to give Au₄₄(PhC≡C)₂₈ and Au₃₆(PhC≡C)₂₄. The Au₄₄ and Au₃₆ nanoclusters have fcc‐type Au₃₆ and Au₂₈ kernels, respectively, as well as surrounding PhC≡C‐Au‐C₂(Ph)Au‐C≡CPh dimeric “staples” and simple PhC≡C bridges. The structures of Au₄₄(PhC≡C)₂₈ and Au₃₆(PhC≡C)₂₄ are similar to Au₄₄(SR)₂₈ and Au₃₆(SR)₂₄, but the UV/Vis spectra are different. The protecting ligands influence the electronic structures of nanoclusters significantly. The synthesis of these two alkynyl‐protected gold nanoclusters indicates that a series of gold nanoclusters in the general formula Au_x (RC≡C)_y as counterparts to Au_x (SR)_y can be expected.     

An Atomic‐Level Strategy for Unraveling Gold Nanocatalysis from the Perspective of Aun(SR)m Nanoclusters

An atomic‐level strategy is devised to gain insight into the origin of nanogold catalysis by using atomically monodisperse Au_n(SR)_m nanoclusters as well‐defined catalysts for styrene oxidation. The Au_n(SR)_m nanoclusters are emerging as a new class of gold nanocatalyst to overcome the polydispersity of conventional nanoparticle catalysts. The unique atom‐packing structure and electronic properties of Au_n(SR)_m nanoclusters (<2 nm) are rationalized to be responsible for their extraordinary catalytic activity observed in styrene oxidation. An interesting finding is that quantum size effects of Au_n(SR)_m nanoclusters, rather than the higher specific surface area, play a major role in gold‐catalyzed selective oxidation of styrene. For example, Au₂₅(SR)₁₈ nanoclusters (≈1 nm) are found to be particularly efficient in activating O₂, which is a key step in styrene oxidation, and hence, the ultrasmall Au₂₅ catalyst exhibits higher activity than do larger sizes. This atomic‐level strategy has allowed us to obtain an important insight into some fundamental aspects of nanogold catalysis in styrene oxidation. The ultrasmall yet robust Au_n(SR)_m nanoclusters are particularly promising for studying the mechanistic aspects of nanogold catalysis and for future design of better catalysts with high activity and selectivity for certain chemical processes.     

Templated Atom‐Precise Galvanic Synthesis and Structure Elucidation of a [Ag24Au(SR)18]− Nanocluster

Synthesis of atom‐precise alloy nanoclusters with uniform composition is challenging when the alloying atoms are similar in size (for example, Ag and Au). A galvanic exchange strategy has been devised to produce a compositionally uniform [Ag₂₄Au(SR)₁₈]^? cluster (SR: thiolate) using a pure [Ag₂₅(SR)₁₈]^? cluster as a template. Conversely, the direct synthesis of Ag₂₄Au cluster leads to a mixture of [Ag_25?xAu_x(SR)₁₈]^?, x=1–8. Mass spectrometry and crystallography of [Ag₂₄Au(SR)₁₈]^? reveal the presence of the Au heteroatom at the Ag₂₅ center, forming Ag₂₄Au. The successful exchange of the central Ag of Ag₂₅ with Au causes perturbations in the Ag₂₅ crystal structure, which are reflected in the absorption, luminescence, and ambient stability of the particle. These properties are compared with those of Ag₂₅ and Ag₂₄Pd clusters with same ligand and structural framework, providing new insights into the modulation of cluster properties with dopants at the single‐atom level.     

Au10(TBBT)10: the Begining and the End of Aun(TBBT)m Nanoclusters

Gold(I) thiolate compounds (i.e. Au^I-SR) are important precursors for the synthesis of atomically precise Au_n(SR)_m nanoclusters. However, the nature of the Au^I-SR precursor remains elusive. Here, we report that the Au₁₀(TBBT)₁₀ complex is a universal precursor for the synthesis of Au_n(TBBT)_m nanoclusters (where TBBT=4-tertbutylbenzenethiol/thiolate). Interestingly, the Au₁₀(TBBT)₁₀ complex is also found to be re-generated through extended etching of the Au_n(SR)_m nanoclusters with excess of TBBT thiol and O₂. The formation of well-defined Au₁₀(TBBT)₁₀ complex, instead of polymeric Au^I-SR, is attributed to the bulkiness of the TBBT thiol. Through 1D and 2D NMR characterization, the structure of Au₁₀(TBBT)₁₀ is correlated with the previously reported X-ray structure, which contains two inter-penetrated Au₅(TBBT)₅ rings. The photophysical property of Au₁₀(TBBT)₁₀ complex is further probed by femtosecond transient absorption spectroscopy. The accessibility of the precise Au₁₀(TBBT)₁₀ precursor improves the efficiency of the synthesis of the Au_n(TBBT)_m nanoclusters and is expected to further facilitate excellent control and understanding of the reaction mechanisms of nanocluster synthesis.     

钯掺杂的25-原子和38-原子金纳米团簇

In this work, we describe two synthetic procedures for preparing palladium doped 25-atom nanoclusters (referred to as Pd₁Au₂₄(SR)₁₈, where ―SR represents thiolate, R=C₂H₄Ph). Pure Pd₁Au₂₄(SC₂H₄Ph)₁₈ nanoclusters are isolated by solvent extraction and size exclusion chromatography. Mass spectrometry and optical spectroscopy analyses demonstrate that the Pd₁Au₂₄(SC₂H₄Ph)₁₈ nanocluster adopts the same core-shell structure as that of the homogold Au₂₅(SC₂H₄Ph)₁₈ nanocluster, that is, a Pd- or Au-centered icosahedron surrounded by six Au₂(SR)₃ “staple”-like motifs. Similar doping behavior has also been observed in 38-atom M₃₈(SR)₂₄ (M: metal) nanoclusters, indicating the unique behavior of Pd dopant being preferentially located in the icosahedral center. The catalytic activity of Pd₁Au₂₄(SC₂H₄Ph)₁₈ has also been evaluated for the selective hydrogenation of α,β-unsaturated ketone (e.g., benzalacetone) to α,β- unsaturated alcohol, and a 42% conversion of benzalacetone is attained.     

Ag2Au50(PET)36 Nanocluster: Dimeric Assembly of Au25(PET)18 Enabled by Silver Atoms

The assembly of atomically precise metal nanoclusters offers exciting opportunities to gain fundamental insights into the hierarchical assembly of nanoparticles. However, it is still challenging to control the assembly of individual nanoclusters at a molecular or atomic level. Herein, we report the dimeric assembly of Au₂₅(PET)₁₈ (PET=2‐phenylethanethiol), where two Au₂₅(PET)₁₈ monomers are bridged together by two Ag atoms to form the Ag₂Au₅₀(PET)₃₆ dimer. The Ag₂Au₅₀(PET)₃₆ dimer is a unique mesomer, which has not been found in any other chiral metal nanoclusters. Furthermore, the Ag₂Au₅₀(PET)₃₆ dimer is distinct from the Au₂₅(PET)₁₈ monomer in its optical, electronic, and catalytic properties. This study is expected to provide a feasible strategy to precisely modulate the assembly of metal nanoclusters with controllable structures and properties.     

Fluorescent Gold Nanoclusters with Interlocked Staples and a Fully Thiolate‐Bound Kernel

The structural features that render gold nanoclusters intrinsically fluorescent are currently not well understood. To address this issue, highly fluorescent gold nanoclusters have to be synthesized, and their structures must be determined. We herein report the synthesis of three fluorescent Au₂₄(SR)₂₀ nanoclusters (R=C₂H₄Ph, CH₂Ph, or CH₂C₆H₄^tBu). According to UV/Vis/NIR, differential pulse voltammetry (DPV), and X‐ray absorption fine structure (XAFS) analysis, these three nanoclusters adopt similar structures that feature a bi‐tetrahedral Au₈ kernel protected by four tetrameric Au₄(SR)₅ motifs. At least two structural features are responsible for the unusual fluorescence of the Au₂₄(SR)₂₀ nanoclusters: Two pairs of interlocked Au₄(SR)₅ staples reduce the vibration loss, and the interactions between the kernel and the thiolate motifs enhance electron transfer from the ligand to the kernel moiety through the Au?S bonds, thereby enhancing the fluorescence. This work provides some clarification of the structure–fluorescence relationship of such clusters.     

The Structure–Property Correlations in the Isomerism of Au21(SR)15 Nanoclusters by Density Functional Theory Study

Isomerism of atomically precise noble metal nanoclusters provides an excellent platform to investigate the structure–property correlations of metal nanomaterials. In this study, we performed density functional theory (DFT) and time‐dependent (TD‐DFT) calculations on two Au₂₁(SR)₁₅ nanoclusters, one with a hexagonal closed packed core (denoted as Au₂₁ ^hcp ), and the other one with a face‐centered cubic core (denoted as Au₂₁ ^fcc ). The structural and electronic analysis on the typical Au–Au and Au–S bond distances, bond orders, composition of the frontier orbitals and the origin of optical absorptions shed light on the inherent correlations between these two clusters.     

Reactivity and Lability Modulated by a Valence Electron Moving in and out of 25-Atom Gold Nanoclusters

The emergence of atomically precise metal nanoclusters with unique electronic structures provides access to currently inaccessible catalytic challenges at the single-electron level. We investigate the catalytic behavior of gold Au₂₅(SR)₁₈ nanoclusters by monitoring an incoming and outgoing free valence electron of Au 6s¹. Distinct performances are revealed: Au₂₅(SR)₁₈⁻ is generated upon donation of an electron to neutral Au₂₅(SR)₁₈⁰ and this is associated with a loss in reactivity, whereas Au₂₅(SR)₁₈⁺ is generated from dislodgment of an electron from neutral Au₂₅(SR)₁₈⁰ with a loss in stability. The reactivity diversity of the three Au₂₅(SR)₁₈ clusters stems from different affinities with reactants and the extent of intramolecular charge migration during the reactions, which are closely associated with the valence occupancies of the clusters varied by one electron. The stability difference in the three clusters is attributed to their different equilibria, which are established between the AuSR dissociation and polymerization influenced by one electron.     

精确控制合成Au36(SR)24金原子簇(SR = SPh,SC6H4CH3, SCH(CH3)Ph,SC10H7)

近十几年来,具有原子精确的金原子簇(Au_nL_m)逐渐发展成一种新型可靠的金纳米材料。在本研究中,报道一种简单实用合成脂肪或者芳香巯基保护Au₃₆(SR)₂₄金原子簇的方法。通过“尺寸聚焦”方法,成功地获得Au₃₆(SCH(CH₃)Ph)₂₄,Au₃₆(SC₆H₄CH₃)₂₄,Au₃₆(SPh)₂₄及Au₃₆(SC₁₀H₇)₂₄等金原子簇。这些原子簇通过UV-Vis光谱,电喷雾(ESI)和基质辅助激光解析飞行时间(MALDI)质谱以及TGA等表征进行了进一步的确定。同时发现在UV-Vis光谱中,芳香巯基保护Au₃₆(SR)₂₄金原子簇发生了明显的红移现象;例如与Au₃₆(SCH(CH₃)Ph)₂₄原子簇相比,萘巯基保护的Au₃₆(SC₁₀H₇)₂₄原子簇在570 nm左右的吸收峰发生了13 nm的位移。     

The Au25(pMBA)17Diglyme Cluster

A modification of Au₂₅(pMBA)₁₈ that incorporates one diglyme ligand as a direct synthetic product is reported. Notably the expected statistical production of clusters containing other ligand stoichiometries is not observed. This Au₂₅(pMBA)₁₇diglyme product is characterized by electrospray ionization mass spectrometry (ESI-MS) and optical spectroscopy. Thiolate for thiolate ligand exchange proceeds on this cluster, whereas thiolate for diglyme ligand exchange does not.     

Selective Hydrogenation of CO2 Dictated by Isomers in Au28(SR)20 Nanoclusters: Which One is Better?

It is a challenge to make clear how isomerism in a heterogeneous catalyst induces distinct differences in catalytic properties, as attainment of the structural isomerism in a conventional catalyst is difficult. By successfully identifying the isomerism in the atomically precise Au nanoclusters, an exciting opportunity for unravelling catalysis of isomeric catalysts is opened up. Herein, we report that the isomerism in the Au₂₈(SR)₂₀ nanoclusters with different surface atom arrangements can indeed render different catalytic behaviors in the selective hydrogenation of CO₂. We anticipate that our studies will serve as a starting point for fundamental investigations about how to control the catalytic activity and selectivity by the isomerism-induced catalysis.     

Atomic‐Level Alloying and De‐alloying in Doped Gold Nanoparticles

Atomically precise alloying and de‐alloying processes for the formation of Ag–Au and Cu–Au nanoparticles of 25‐metal‐atom composition (referred to as Ag_xAu_25?x(SR)₁₈ and Cu_xAu_25?x(SR)₁₈, in which R=CH₂CH₂Ph) are reported. The identities of the particles were determined by matrix‐assisted laser desorption ionization mass spectroscopy (MALDI‐MS). Their structures were probed by fragmentation analysis in MALDI‐MS and comparison with the icosahedral structure of the homogold Au₂₅(SR)₁₈ nanoparticles (an icosahedral Au₁₃ core protected by a shell of Au₁₂(SR)₁₈). The Cu and Ag atoms were found to preferentially occupy the 13‐atom icosahedral sites, instead of the exterior shell. The number of Ag atoms in Ag_xAu_25?x(SR)₁₈ (x=0–8) was dependent on the molar ratio of Ag^I/Au^III precursors in the synthesis, whereas the number of Cu atoms in Cu_xAu_25?x(SR)₁₈ (x=0–4) was independent of the molar ratio of Cu^II/Au^III precursors applied. Interestingly, the Cu_xAu_25?x(SR)₁₈ nanoparticles show a spontaneous de‐alloying process over time, and the initially formed Cu_xAu_25?x(SR)₁₈ nanoparticles were converted to pure Au₂₅(SR)₁₈. This de‐alloying process was not observed in the case of alloyed Ag_xAu_25?x(SR)₁₈ nanoparticles. This contrast can be attributed to the stability difference between Cu_xAu_25?x(SR)₁₈ and Ag_xAu_25?x(SR)₁₈ nanoparticles. These alloyed nanoparticles are promising candidates for applications such as catalysis.     

Atomically Tailored Gold Nanoclusters for Catalytic Application

Recent advances in the synthetic chemistry of atomically precise metal nanoclusters (NCs) have significantly broadened the accessible sizes and structures. Such particles are well defined and have intriguing properties, thus, they are attractive for catalysis. Especially, those NCs with identical size but different core (or surface) structure provide unique opportunities that allow the specific role of the core and the surface to be mapped out without complication by the size effect. Herein, we summarize recent work with isomeric Au_n NCs protected by ligands and isostructural NCs but with different surface ligands. The highlighted work includes catalysis by spherical and rod‐shaped Au₂₅ (with different ligands), quasi‐isomeric Au₂₈(SR)₂₀ with different R groups, structural isomers of Au₃₈(SR)₂₄ (with identical R) and Au₃₈S₂(SR)₂₀ with body‐centred cubic (bcc) structure, and isostructural [Au₃₈L₂₀(PPh₃)₄]²⁺ (different L). These isomeric and/or isostructural NCs have provided valuable insights into the respective roles of the kernel, surface staples, and the type of ligands on catalysis. Future studies will lead to fundamental advances and development of tailor‐made catalysts.     

Understanding and controlling the efficiency of Au24M(SR)18 nanoclusters as singlet-oxygen photosensitizers

Singlet oxygen, ¹O₂, can be generated by molecules that upon photoexcitation enable the ³O₂ → ¹O₂ transition. We used a series of atomically precise Au₂₄M(SR)₁₈ clusters, with different R groups and doping metal atoms M. Upon nanosecond photoexcitation of the cluster, ¹O₂ was efficiently generated. Detection was carried out by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The resulting TREPR transient yielded the ¹O₂ lifetime as a function of the nature of the cluster. We found that: these clusters indeed generate ¹O₂ by forming a triplet state; a more positive oxidation potential of the molecular cluster corresponds to a longer ¹O₂ lifetime; proper design of the cluster yields results analogous to those of a well-known reference photosensitizer, although more effectively. Comprehensive kinetic analysis provided important insights into the mechanism and driving-force dependence of the quenching of ¹O₂ by gold nanoclusters. Understanding on a molecular basis why these molecules may perform so well in ¹O₂ photosensitization is instrumental to controlling their performance.

Atomically precise Au₂₄M(SR)₁₈ clusters were used as singlet-oxygen photosensitizers. Comprehensive kinetic analysis provided insights into the mechanism and driving-force dependence of the quenching of ¹O₂ by gold nanoclusters.