Luminescence and Electron Dynamics in Atomically Precise Nanoclusters with Eight Superatomic Electrons

Doping-mediated excited state dynamics of diphosphine-protected M@Au12 (M = Au, Ir) superatom nanoclusters

Doping heterometal atoms into ligand-protected gold superatom nanoclusters (Aun NCs) is proposed to further diversify their geometrical and electronic structures and enhance their photoluminescence properties, which is attributed to the mixing and effects between atoms. However, the fundamental principles that govern the optoelectronic properties of the doped Aun NCs remain elusive. Herein, we systematically explored two prototypical 8-electron Aun (n = 11 and 13) NCs with and without Ir dopant atoms using comprehensive ab initio calculations and real-time nonadiabatic molecular dynamics simulations. These doped Aun NCs maintain their parent geometrical structures and 8-electron superatomic configuration (1S21P6). Strong core-shell (Ir-Aun) electronic coupling significantly expands the energy gap, resulting in a weak nonadiabatic coupling matrix element, which in turn increases the carrier lifetime. This increase is mainly governed by the low-frequency vibration mode. We uncovered the relationship between electronic structures, electron-vibration, and carrier dynamics for these doped Aun NCs. These calculated results provide crucial insights for the atomically precise design of metal NCs with superior optoelectronic properties.

Vibration-Dependent Dual-Phosphorescent Cu4 Nanocluster with Remarkable Piezochromic Behavior

The dual emission (DE) characteristics of atomically precise copper nanoclusters (Cu NCs) are of significant theoretical and practical interest. Despite this, the underlying mechanism driving DE in Cu NCs remains elusive, primarily due to the complexities of excited state processes. Herein, a novel [Cu4(PPh3)4(C≡C-p-NH2C6H4)3]PF6 (Cu4) NC, shielded by alkynyl and exhibiting DE, was synthesized. Hydrostatic pressure was applied to Cu4, for the first time, to investigate the mechanism of DE. With increasing pressure, the higher-energy emission peak of Cu4 gradually disappeared, leaving the lower-energy emission peak as the dominant emission. Additionally, the Cu4 crystal exhibited notable piezochromism transitioning from cyan to orange. Angle-dispersive synchrotron X-ray diffraction results revealed that the reduced inter-cluster distances under pressure brought the peripheral ligands closer, leading to the formation of new C-H⋅⋅⋅N and N-H⋅⋅⋅N hydrogen bonds in Cu4. It is proposed that these strengthened hydrogen bond interactions limit the ligands' vibration, resulting in the vanishing of the higher-energy peak. In situ high-pressure Raman and vibrationally resolved emission spectra demonstrated that the benzene ring C=C stretching vibration is the structural source of the DE in Cu4.

Filling the gaps in icosahedral superatomic metal clusters

Chemically modified superatoms have emerged as promising candidates in the new periodic table, in which Au13 and its doped M n Au13- n have been widely studied. However, their important counterpart, Ag13 artificial element, has not yet been synthesized. In this work, we report the synthesis of Ag13 nanoclusters using strong chelating ability and rigid ligands, that fills the gaps in the icosahedral superatomic metal clusters. After further doping Ag13 template with different degrees of Au atoms, we gained insight into the evolution of their optical properties. Theoretical calculations show that the kernel metal doping can modulate the transition of the excited-state electronic structure, and the electron transfer process changes from local excitation (LE) to charge transfer (CT) to LE. This study not only enriches the families of artificial superatoms, but also contributes to the understanding of the electronic states of superatomic clusters.

Photoluminescent Characterization of Metal Nanoclusters: Basic Parameters, Methods, and Applications

Metal nanoclusters (MNCs) can be synthesized with atomically precise structures and molecule formulae due to the rapid development of nanocluster science in recent decades. The ultrasmall size range (normally < 2 nm) endows MNCs with plenty of molecular-like properties, among which photoluminescent properties have aroused extensive attention. Tracing the research and development processes of luminescent nanoclusters, various photoluminescent analysis and characterization methods play a significant role in elucidating luminescent mechanism and analyzing luminescent properties. In this review, it is aimed to systematically summarize the normally used photoluminescent characterizations in MNCs including basic parameters and methods, such as excitation/emission wavelength, quantum yield, and lifetime. For each key parameter, first its definition and meaning is introduced and then the relevant characterization methods including measuring principles and the revelation of luminescent properties from the collected data are discussed. Then, it is discussed in details how to explore the luminescent mechanism of MNCs and construct NC-based applications based on the measured data. By means of these characterization strategies, the luminescent properties of MNCs and NC-based designs can be explained quantitatively and qualitatively. Hence, this review is expected to provide clear guidance for researchers to characterize luminescent MNCs and better understand the luminescent mechanism from the measured results.

Construction of novel Ag(0)-containing silver nanoclusters by regulating auxiliary phosphine ligands

Controlled synthesis of metal clusters through minor changes in surface ligands holds significant interest because the corresponding entities serve as ideal models for investigating the ligand environment's stereochemical and electronic contributions that impact the corresponding structures and properties of metal clusters. In this work, we obtained two Ag(0)-containing nanoclusters (Ag17 and Ag32) with near-infrared emissions by regulating phosphine auxiliary ligands. Ag17 and Ag32 bear similar shells wherein Ag17 features a trigonal bipyramid Ag5 kernel while Ag32 has a bi-icosahedral interpenetrating an Ag20 kernel. Ag17 and Ag32 showed a near-infrared emission (NIR) of around 830 nm. Benefiting from the rigid structure, Ag17 displayed a more intense near-infrared emission than Ag32. This work provides new insight into the construction of novel superatomic silver nanoclusters by regulating phosphine ligands.

Arylgold nanoclusters: Phenyl-stabilized Au44 with thermal-controlled NIR single/dual-channel phosphorescence

Arylation of gold holds paramount importance in the domain of organometallic chemistry; however, the exploration of arylgold nanoclusters remains in its infancy primarily due to the synthetic challenge. Here, we present a facile and effective arylation strategy to directly synthesize two arylgold nanoclusters (Au44a and Au44b), by using tetraarylborates, capable of transferring aryl fragments to metal centers. X-ray crystallography reveals that both Au44 nanoclusters contain an Au44 kernel co-protected by six aryl groups, two tetrahydrothiophene, and 16 alkynyl-ether ligands, the latter is generated in situ through Williamson ether reaction during the assembly processes. Notably, Au44 nanoclusters exhibit near-infrared (NIR) phosphorescence (λmax = 958 nm) and microsecond radiative relaxation at ambient condition, which is a thermal-controlled single/dual-channel phosphorescent emission revealed by temperature-dependent NIR, time-resolved emission, and femtosecond/nanosecond transition absorption spectra. This work represents a breakthrough in using aryl as protective ligands for the construction of gold nanoclusters, which is poised to have a transformative impact on organometallic nanoclusters.

Symmetry Breaking Enhancing the Activity of Electrocatalytic CO2 Reduction on an Icosahedron-Kernel Cluster by Cu Atoms Regulation

Recently, CO2 hydrogenation had a new breakthrough resulting from the design of catalysts to effectively activate linear CO2 with symmetry-breaking sites. However, understanding the relationship between symmetry-breaking sites and catalytic activity at the atomic level is still a great challenge. In this study, a set of gold-copper alloy Au13 Cux (x=0-4) nanoclusters were used as research objects to show the symmetry-controlled breaking structure on the surface of nanoclusters with the help of manipulability of the Cu atoms. Among them, Au13 Cu3 nanocluster displays the highest degree of symmetry-breaking on its crystal structure compared with the other nanoclusters in the family. Where the three copper atoms occupying the surface of the icosahedral kernel unevenly with one copper atom is coordinately unsaturated (CuS2 motif relative to CuS3 motif). As expected, Au13 Cu3 has an excellent hydrogenation activity of CO2 , in which the current density is as high as 70 mA cm-2 (-0.97 V) and the maximum FECO reaches 99 % at -0.58 V. Through the combination of crystal structures and theoretical calculations, the excellent catalytic activity of Au13 Cu3 is revealed to be indeed closely related to its asymmetric structure.

Biological Interaction and Imaging of Ultrasmall Gold Nanoparticles

The ultrasmall gold nanoparticles (AuNPs), serving as a bridge between small molecules and traditional inorganic nanoparticles, create significant opportunities to address many challenges in the health field. This review discusses the recent advances in the biological interactions and imaging of ultrasmall AuNPs. The challenges and the future development directions of the ultrasmall AuNPs are presented.

Dissecting the Triplet-State Properties and Intersystem Crossing Mechanism of the Ligand-Protected Au13 Superatom

Icosahedral Au13 nanoclusters are among the most typical superatoms and are of great interest as promising building blocks for nanocluster-assembled materials. Herein, the key parameters involved in the intersystem crossing (ISC) process of [Au13(dppe)5Cl2]3+ (Au13; dppe = 1,2-bis(diphenylphosphino)ethane) were characterized. Quenching experiments using aromatic compounds revealed that the T1 energy of Au13 is 1.63 eV. An integrative interpretation of our experimental results and the relevant literature uncovered important facts concerning the Au13 superatom: the ISC quantum yield is unity due to the ultrafast ISC (∼1012 s-1), the lowest absorption band includes contributions of direct singlet-triplet transitions, and there exists a large S1-T1 gap of 0.73 eV. To explain the efficient ISC, the El-Sayed rule was applied to the superatomic orbitals corresponding to the excited-state hole/electron distributions obtained from theoretical calculations. The strong spin-orbit coupling between the S1 and T2-T4 states offers a reasonable explanation for the ultrafast ISC.

Monoarsine-protected icosahedral cluster [Au13(AsPh3)8Cl4]+: comparative studies on ligand effect and surface reactivity with its stibine analogue

Ligand shells of gold nanoclusters play important roles in regulating their molecular and electronic structures. However, the similar but distinct impacts of the homologous analogues of the protecting ligands remain elusive. The C_2v symmetric monoarsine-protected cluster [Au₁₃(AsPh₃)₈Cl₄]⁺ (Au₁₃As₈) was facilely prepared by direct reduction of (Ph₃As)AuCl with NaBH₄. This cluster is isostructural with its previously reported stibine analogue [Au₁₃(SbPh₃)₈Cl₄]⁺ (Au₁₃Sb₈), enabling a comparative study between them. Au₁₃As₈ exhibits a blue-shifted electronic absorption band, and this is probably related to the stronger π-back donation interactions between the Au₁₃ core and AsPh₃ ligands, which destabilize its superatomic 1P and 1D orbitals. In comparison to the thermodynamically less stable Au₁₃Sb₈, Au₁₃As₈ achieves a better trade-off between catalytic stability and activity, as demonstrated by its excellent catalytic performance towards the aldehyde-alkyne-amine (A³) coupling reaction. Moreover, the ligand exchange reactions between Au₁₃As₈ with phosphines, as exemplified by PPh₃ and Ph₂P(CH₂)₂PPh₂, suggest that Au₁₃As₈ may be a good precursor cluster for further cluster preparation through the "cluster-to-cluster" route.

Characterization of Pt-doping effects on nanoparticle emission: a theoretical look at Au24Pt(SH)18 and Au24Pt(SC3H7)18

Developments in nanotechnology have made the creation of functionalized materials with atomic precision possible. Thiolate-protected gold nanoclusters, in particular, have become the focus of study in literature as they possess high stability and have tunable structure-property relationships. In addition to adjustments in properties due to differences in size and shape, heteroatom doping has become an exciting way to tune the properties of these systems by mixing different atomic d characters from transition metal atoms. Au₂₄Pt(SR)₁₈ clusters, notably, have shown incredible catalytic properties, but fall short in the field of photochemistry. The influence of the Pt dopant on the photoluminescence mechanism and excited state dynamics has been investigated by a few experimental groups, but the origin of the differences that arise due to doping has not been clarified thoroughly. In this paper, density functional theory methods are used to analyze the geometry, optical and photoluminescent properties of Au₂₄Pt(SR)₁₈ in comparison with those of [Au₂₅(SR)₁₈]^1-. Furthermore, as these clusters have shown slightly different geometric and optical properties for different ligands, the analysis is completed with both hydrogen and propyl ligands in order to ascertain the role of the passivating ligands.

Isomerism effects in relaxation dynamics of Au24(SR)16thiolate-protected gold nanoclusters

Understanding the excited state behavior of isomeric structures of thiolate-protected gold nanoclusters is still a challenging task. In this paper, based on grand unified model and ring model for describing thiolate-protected gold nanoclusters, we have predicted four isomers of Au₂₄(SR)₁₆nanoclusters. Density functional theory calculations show that the total energy of one of the predicted isomers is 0.1 eV lower in energy than previously crystallized isomer. The nonradiative relaxation dynamics simulations of Au₂₄(SH)₁₆isomers are performed to reveal the effects of structural isomerism on relaxation process of the lowest energy states, in which that most of the low-excited states consist of core states. In addition, crystallized isomer possesses the shorter e-h recombination time, whereas the most stable isomer has the longer recombination time, which may be attributed to the synergistic effect of nonadiabatic coupling and decoherence time. Our results could provide practical guidance to predict new gold nanoclusters for future experimental synthesis, and stimulate the exploration of atomic structures of same sized gold nanoclusters for photovoltaic and optoelectronic devices.

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.

Mechanistic insights into Ag+ induced size-growth from [Au6(DPPP)4]2+ to [Au7(DPPP)4]2+ clusters

The size conversion of atomically precise metal nanoclusters lays the foundation to elucidate the inherent structure-activity correlations on the nanometer scale. Herein, the mechanism of the Ag⁺-induced size growth from [Au₆(dppp)₄]²⁺ to [Au₇(dppp)₄]³⁺ (dppp is short for 1,3-bis(diphenylphosphino)propane) is studied via density functional theory (DFT) calculations. In the absence of extra Au sources, the one "Au⁺" addition was found to be regulated by the Ag⁺ doping induced Au-activation, i.e., the formation of formal Au(i) blocks via the Ag⁺ alloying processes. The Au(i) blocks could be extruded from the core structure in the formed Au-Ag alloy clusters, triggering a facile Au⁺ migration to the Au₆ precursor to form the Au₇ product. This study sheds light on the structural and stability changes of gold nanoclusters upon the addition of Ag⁺ and will hopefully benefit the development of more metal ion-induced size-conversion of metal nanoclusters.

Doping-Mediated Energy-Level Engineering of M@Au12 Superatoms (M=Pd, Pt, Rh, Ir) for Efficient Photoluminescence and Photocatalysis

We synthesized a series of MAu₁₂ (dppe)₅ Cl₂ (MAu₁₂ ; M=Au, Pd, Pt, Rh, or Ir; dppe=1,2-bis(diphenylphosphino)ethane), which have icosahedral M@Au₁₂ superatomic cores, and systematically investigated their electronic structures, photoluminescence (PL) and photocatalytic properties. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was expanded when doping an M element positioned at the lower left of the periodic table. The PL quantum yield was enhanced with an increase in the HOMO-LUMO gap and reached 0.46-0.67 for MAu₁₂ (M=Pt, Rh, or Ir) under deaerated conditions. The bright PLs from MAu₁₂ (M=Pt, Rh, or Ir) were assigned to phosphorescence based on quenching by O₂ . MAu₁₂ (M=Pt, Rh, or Ir) acted as a more efficient and stable photocatalyst than Au₁₃ for intramolecular [2+2] cycloaddition of bisenone via the oxidative quenching cycle. This study provides rational guides for designing photoluminescent and photocatalytic gold superatoms by the doping of heterometal elements.

N-Heterocyclic carbene-based C-centered Au(I)-Ag(I) clusters with intense phosphorescence and organelle-selective translocation in cells

Photoluminescent gold clusters are functionally variable chemical modules by ligand design. Chemical modification of protective ligands and introduction of different metals into the gold clusters lead to discover unique chemical and physical properties based on their significantly perturbed electronic structures. Here we report the synthesis of carbon-centered Au(I)-Ag(I) clusters with high phosphorescence quantum yields using N-heterocyclic carbene ligands. Specifically, a heterometallic cluster [(C)(Au^I-L)₆Ag^I₂]⁴⁺, where L denotes benzimidazolylidene-based carbene ligands featuring N-pyridyl substituents, shows a significantly high phosphorescence quantum yield (Φ = 0.88). Theoretical calculations suggest that the carbene ligands accelerate the radiative decay by affecting the spin-orbit coupling, and the benzimidazolylidene ligands further suppress the non-radiative pathway. Furthermore, these clusters with carbene ligands are taken up into cells, emit phosphorescence and translocate to a particular organelle. Such well-defined, highly phosphorescent C-centered Au(I)-Ag(I) clusters will enable ligand-specific, organelle-selective phosphorescence imaging and dynamic analysis of molecular distribution and translocation pathways in cells.

Photoluminescent gold clusters have unique chemical and physical properties based on their perturbed electronic structures. Here, the authors report the synthesis of carbon-centered Au(I)-Ag(I) clusters with high phosphorescence quantum yields using N-heterocyclic carbene ligands.

Enhanced Fluorescence with Tunable Color in Doped Diphosphine-Protected Gold Nanoclusters

Rational control of the luminescent properties of ligand-protected coinage metal clusters has long been pursued but remains challenging. Here we explore the crucial structural and electronic factors governing the fluorescence of a diphosphine-protected [Au₁₃(dppe)₅Cl₂]³⁺ cluster by time-dependent density functional theory calculations. By substituting the central Au atom with group 5 to group 11 transition metal atoms, the emission wavelength is adjustable from red to blue, accompanied by enhanced fluorescence intensity compared with the undoped cluster. The evolution of light-emitting behavior upon doping and the corresponding roles of the dopant, Au cage, ligands, and their interplay are interpreted at the electronic structure level. In particular, strong dopant-Au cage interaction associated with large electron-hole overlap on the dopant are is a key factor to endow large emission energy and intensity. These theoretical results provide vital guidance for designing atomically precise nanoclusters with visible fluorescence and high quantum yield for practical uses.

Anion-Directed Regulation of Structures and Luminescence of Heterometallic Clusters

Anions have been used to regulate the structures and luminescence of heterometallic clusters. Introducing ClO₄ ^- into orange-emissive, butterfly-like [(C)(Au-PPhpy₂ )₆ Ag₄ ](BF₄ )₆ (1, PPhpy₂ =bis(2-pyridyl)phenylphosphine) leads to the formation of red-emissive [(C)(Au-PPhpy₂ )₆ Ag₅ (ClO₄ )₃ ](ClO₄ )₄ (2) with a novel trigonal bipyramidal structure; employing PhCO₂ ^- gives yellow-emissive, hexagram-like [(C)(Au-PPhpy₂ )₆ Ag₆ (PhCO₂ )₃ ](BF₄ )₅ (3). Notably, 1 exhibits weak luminescence in CH₂ Cl₂ /CH₃ OH=1 : 1 (v : v) with a quantum yield (QY) of 0.05, whereas it was dramatically increased to 0.49 and 0.83 for 2 and 3, respectively. Theoretical calculation confirms that the involvement of anions in the electronic structures is responsible for the shifts of emission. The high QYs of 2 and 3 are attributed to the protection provided by ligands and anions. This work demonstrates that anions may serve as an extra designable factor beyond just counterions for functional metal clusters.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Ligand Shell Isomerization Induces Different Fluorescence Origins of Two Au28 Nanoclusters

Understanding the origin of the photoluminescence (PL) phenomenon in ligand-protected metal nanoclusters is of paramount importance in both fundamental science and practical applications. In this study, we have studied the origin of fluorescence emission of two thiolate-ligand-protected Au₂₈ clusters (Au₂₈(CHT)₂₀ and Au₂₈(TBBT)₂₀) by means of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Theoretical calculation results show that the ligand shell isomerization induces different ligand motif-to-metal core charge transfers (LMCT) of Au₂₈(TBBT)₂₀ and Au₂₈(CHT)₂₀. Moreover, in Au₂₈(CHT)₂₀, the emission process of S₂ → S₀ can compete favorably with the internal conversion of S₂ → S₁. Furthermore, the high quantum yield of Au₂₈(CHT)₂₀ is attributed to its high symmetric structure, which leads to less energy dissipation during the structural relaxation process.

A reasonable approach for the generation of hollow icosahedral kernels in metal nanoclusters

Although the hollow icosahedral M₁₂ kernel has been extensively observed in metal nanoclusters, its origin remains a mystery. Here we report a reasonable avenue for the generation of the hollow icosahedron: the kernel collapse from several small nano-building blocks to an integrated hollow icosahedron. On the basis of the Au alloying processes from Ag₂₈Cu₁₂(SR)₂₄ to the template-maintained Au_xAg_28-xCu₁₂(SR)₂₄ and then to the template-transformed Au₁₂Cu_yAg_32-y(SR)₃₀, the kernel evolution/collapse from “tetrahedral Ag₄ + 4^∗Ag₃” to “tetrahedral Au₄ + 4^∗M₃ (M = Au/Ag)” and then to “hollow icosahedral Au₁₂” is mapped out. Significantly, the “kernel collapse” from small-sized nano-building blocks to large-sized nanostructures not only unveils the formation of hollow icosahedral M₁₂ in this work, but also might be a very common approach in constructing metallic kernels of nanoclusters and nanoparticles (not limited to the M₁₂ structure).

The origin of the hollow icosahedral M₁₂ kernel in metal nanoclusters is under debate. Here the authors demonstrate the Au alloying-induced kernel collapse from small-sized nano-building blocks as a viable approach for the generation of hollow icosahedral M₁₂ kernel in metal nanoclusters.

An insight, at the atomic level, into the polarization effect in controlling the morphology of metal nanoclusters

The polarization effect has been a powerful tool in controlling the morphology of metal nanoparticles. However, a precise investigation of the polarization effect has been a challenging pursuit for a long time, and little has been achieved for analysis at the atomic level. Here the atomic-level analysis of the polarization effect in controlling the morphologies of metal nanoclusters is reported. By simply regulating the counterions, the controllable transformation from Pt₁Ag₂₈(S-PhMe₂)_x(S-Adm)_18−x(PPh₃)₄ (x = 0–6, Pt1Ag28-2) to Pt₁Ag₂₄(S-PhMe₂)₁₈ (Pt1Ag24) with a spherical configuration or to Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ (Pt1Ag28-1) with a tetrahedral configuration has been accomplished. In addition, the spherical or tetrahedral configuration of the clusters could be reversibly transformed by re-regulating the proportion of counterions with opposite charges. More significantly, the configuration transformation rate has been meticulously manipulated by regulating the polarization effect of the ions on the parent nanoclusters. The observations in this paper provide an intriguing nanomodel that enables the polarization effect to be understood at the atomic level.

Based on the inter-conversion between Pt₁Ag₂₄(SR)₁₈ and Pt₁Ag₂₈(SR)₁₈(PPh₃)₄, an insight into the polarization effect in controlling the morphology of metal nanoparticles is presented.

Diarsine- vs diphosphine-protected Au13 clusters: Effect of subtle geometric differences on optical property and electronic structure

In the design of ligand-protected metal clusters, the choice of protecting ligands is a critical factor because they can profoundly affect the nuclearity, geometry, and electronic structures to afford a diverse range of cluster compounds. Here, we report the synthesis of two novel diarsine-protected Au₁₃ clusters ([Au₁₃L₅Cl₂]³⁺, L = diarsine) and compare these clusters with diphosphine analogs in terms of the core geometry and optical properties. In the crystal structure, the cluster bearing C3-bridged diarsines {[Au₁₃(dpap)₅Cl₂]³⁺, 3} had an apparently identical icosahedral Au₁₃ core to [Au₁₃(dppe)₅Cl₂]³⁺ (1) with C2-bridged diphosphines, but slight structural differences associated with the bridging unit of the ligands were found. Despite similar icosahedral Au₁₃ cores 1 and 3, their absorption and photoluminescence profiles were evidently different. Theoretical calculations revealed that the subtle deformation of the Au₁₃ icosahedron, rather than the coordinating atoms (As or P), notably influences the electronic structure to cause the difference in the absorption profiles.

High photoluminescence from self-assembled Ag2Cl2(dppe)2 clusters through metallophilic interactions

Ligand protected metal nanoclusters (NCs) are an emerging class of functional materials with intriguing photophysical and chemical properties. The size and molecular structure play an important role in endowing NCs with characteristic optical and electronic properties. Modulation of these properties through the chemical reactivity of NCs is largely unexplored. Here, we report on the synthesis of self-assembled Ag₂Cl₂(dppe)₂ clusters through the ligand-exchange-induced transformation of [Pt₂Ag₂₃Cl₇(PPh₃)₁₀] NCs [(dppe): 1,2-bis(diphenylphosphino)ethane; (PPh₃): triphenylphosphine]. The single crystal x-ray structure reveals that two Ag atoms are bridged by one dppe and two Cl ligands, forming a Ag₂Cl₂(dppe) cluster, which is subsequently self-assembled through dppe ligands to form [Ag₂Cl₂(dppe)₂]_n. Importantly, the Ag₂Cl₂(dppe)₂ cluster assembly exhibits high photoluminescence quantum yield: ∼18%, which is attributed to the metallophilic interactions and rigidification of the ligand shell. We hope that this work will motivate the exploitation of the chemical reactivity of NCs as a new path to attain cluster assemblies endowed with enhanced photophysical properties.

Toward quantitative electronic structure in small gold nanoclusters

Ligand-protected gold nanoclusters (AuNCs) feature a dense but finite electronic structure that can be rationalized using qualitative descriptions such as the well-known superatomic model and predicted using quantum chemical calculations. However, the lack of well-resolved experimental probes of a AuNC electronic structure has made the task of evaluating the accuracy of electronic structure descriptions challenging. We compare electronic absorption spectra computed using time-dependent density functional theory to recently collected high resolution experimental spectra of Au₉(PPh₃)₈ ³⁺ and Au₈(PPh₃)₇ ²⁺ AuNCs with strikingly similar features. After applying a simple scaling correction, the computed spectrum of Au₈(PPh₃)₇ ²⁺ yields a suitable match, allowing us to assign low-energy metal-metal transitions in the experimental spectrum. No similar match is obtained after following the same procedure for two previously reported isomers for Au₉(PPh₃)₈ ³⁺, suggesting either a deficiency in the calculations or the presence of an additional isomer. Instead, we propose assignments for Au₉(PPh₃)₈ ³⁺ based off of similarities Au₈(PPh₃)₇ ²⁺. We further model these clusters using a simple particle-in-a-box analysis for an asymmetrical ellipsoidal superatomic core, which allows us to reproduce the same transitions and extract an effective core size and shape that agrees well with that expected from crystal structures. This suggests that the superatomic model, which is typically employed to explain the qualitative features of nanocluster electronic structures, remains valid even for small AuNCs with highly aspherical cores.

Photoluminescence of Doped Superatoms M@Au12 (M = Ru, Rh, Ir) Homoleptically Capped by (Ph2)PCH2P(Ph2): Efficient Room-Temperature Phosphorescence from Ru@Au12

A series of doped gold superatoms M@Au₁₂ (M = Ru, Rh, Ir) was synthesized by capping with the bidentate ligand (Ph₂)PCH₂P(Ph₂). A single-crystal X-ray diffraction analysis showed that all the M@Au₁₂ superatoms had icosahedral motifs with a significantly higher symmetry than that of the pure Au₁₃ counterpart due to different coordination geometries. The Ru@Au₁₂ superatom exhibited a room-temperature phosphorescence with the highest quantum yield of 0.37 in deaerated dichloromethane. Density functional theory calculations suggested that the efficient phosphorescence is ascribed to a rapid intersystem crossing due to the similarity between the singlet and triplet excited states in terms of structure and energy.

Cu2+ enhanced fluorescent Ag nanoclusters with tunable emission from red to yellow and the application for Ag+ sensing

In this work, the water-soluble fluorescent Ag nanoclusters (DPA@Ag NCs) were first prepared based on D-Penicillamine (DPA) as a stabilizer, however, the fluorescence quantum yield (QY) of DPA@Ag NCs was very low, then Cu²⁺ was employed to improve the fluorescence QY and the doped Ag nanoclusters with Cu²⁺ (DPA@Ag/Cu NCs) were obtained. The study showed that the QY increased fourfold and the emission of nanoclusters changed from red to yellow after addition of Cu²⁺. The reasonfor change of fluorescent properties wasattributed to the change of self-assembly structures caused by adding Cu²⁺ into reaction system, leading to the aggregation-induced emission enhancement (AIEE) effect and enhancing the band gap (Eg) between the HOMO and LUMO in nanoclusters. Subsequently, a fluorescent Ag⁺ sensor with high sensitivity and selectivity was established based on the DPA@Ag/Cu NCs as probes in aqueous solution. Experiments showed that the Ag⁺ could significantly quench the fluorescence of DPA@Ag/Cu NCs under experimental conditions, and there was a good linear relationship between the fluorescent intensity quenching value and Ag⁺ concentration in the range of 0.05-800 μM, and the limit of detection (LOD) was 0.03 μM (3σ/k). Meanwhile, most of common ions had no effect on the experimental results under the same conditions. In addition, the sensor was successfully applied on the detection of Ag⁺ in real water samples, and the recovery rate was 80.3-99.0%.

Alkynyl-Stabilized Superatomic Silver Clusters Showing Circularly Polarized Luminescence

We report a new enantiomeric pair of superatomic silver clusters, R/S-Ag₁₇, prepared from chiral alkynyl ligands. R-Ag₁₇ and S-Ag₁₇ possess C₃ symmetry and emit near-infrared (NIR) light with a quantum yield (QY) of 8.0% under ambient condition as well as NIR circularly polarized luminescence (CPL) as a result of the chirality of the excited states. Both experiments and theoretical calculations indicate for the first time that the CPL originates from transitions between superatomic 1P_z (along the C₃ axis) and 1S orbitals. This work opens a new avenue for CPL-active metal nanoclusters by utilizing chiral alkynyl ligands and enlightens the chirality transfer from chiral protecting ligands to superatomic states in metal clusters.

Optical Properties and Excited-State Dynamics of Atomically Precise Gold Nanoclusters

Understanding the excited-state dynamics of nanomaterials is essential to their applications in photoenergy storage and conversion. This review summarizes recent progress in the excited-state dynamics of atomically precise gold (Au) nanoclusters (NCs). We first discuss the electronic structure and typical relaxation pathways of Au NCs from subpicoseconds to microseconds. Unlike plasmonic Au nanoparticles, in which collective electron excitation dominates, Au NCs show single-electron transitions and molecule-like exciton dynamics. The size-, shape-, structure-, and composition-dependent dynamics in Au NCs are further discussed in detail. For small-sized Au NCs, strong quantum confinement effects give rise to relaxation dynamics that is significantly dependent on atomic packing, shape, and heteroatom doping. For relatively larger-sized Au NCs, strong size dependence can be observed in exciton and electron dynamics. We also discuss the origin of coherent oscillations and their roles in excited-state relaxation. Finally, we provide our perspective on future directions in this area.

Photoinduced charge-transfer in chromophore-labeled gold nanoclusters: quantum evidence of the critical role of ligands and vibronic couplings

The electron flow between a metallic aggregate and an organic molecule after excitation with light is a crucial step on which hybrid photovoltaic nanomaterials are based. So far, designing such devices with the help of theoretical approaches has been heavily limited by the computational cost of quantum dynamics models able to track the evolution of the excited states over time. In this article, we present the first application of the time-dependent density functional tight-binding (TD-DFTB) method for an experimental nanometer-sized gold-organic system consisting of a hexyl-protected Au₂₅ cluster labelled with a pyrene fluorophore, in which the fluorescence quenching of the pyrene is attributed to the electron transfer from the metallic cluster to the dye. The full quantum rationalization of the electron transfer is attained through quantum dynamics simulations, highlighting the crucial role of the protecting ligand shell in electron transfer, as well as the coupling with nuclear movement. This work paves the way towards the fast and accurate theoretical design of optoelectronic nanodevices.

Controlling the Crystallographic Packing Modes of Pt1Ag28 Nanoclusters: Effects on the Optical Properties and Nitrogen Adsorption-Desorption Performances

We herein report the manipulation of the crystallographic packing modes of Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ nanoclusters by altering counterions as different polyoxometalates (POMs). Specifically, the Cl^- anion of the presynthesized Pt₁Ag₂₈ nanocluster was substituted by POM anions including [Mo₆O₁₉]^2-, [W₆O₁₉]^2-, or [PW₁₂O₄₀]^3-. The crystal lattices of these Pt₁Ag₂₈ nanoclusters with diverse anions showed distinct packing modes and thus manifested remarkably distinguishable crystalline-state optical properties and nitrogen adsorption-desorption performances. Overall, the combination of intercluster control in this work and intracluster control reported previously (the control over metal-ligand within the nanocluster framework) accomplished a more comprehensive manipulation over the M₂₉(SR)₁₈(PR'₃)₄ nanocluster system, which enables us to further grasp the structure-property correlations at the atomic level.

Observation of Core Phonon in Electron-Phonon Coupling in Au25 Nanoclusters

Temperature-dependent optical properties are of paramount importance for fundamentally understanding the electron-phonon interactions and phonon modes in atomically precise nanocluster materials. In this work, low-temperature optical absorption spectra of the icosahedral [Au₂₅(SR)₁₈]^- nanocluster are measured from room temperature down to liquid helium temperature by adopting a thin-film-based technique. The thin-film measurement is further compared with results from the previous solution-based method. Interestingly, the previously missing core phonon is revealed by a quantitative analysis of the film data through peak deconvolution and fitting of the temperature trend with a theoretical model. The two lowest-energy absorption peaks (at 1.6 and 1.8 eV) of Au₂₅ are determined to couple with the staple-shell phonon (average energy ∼350 cm^-1) in the solution state, but in the solid state these electronic transitions couple with the core phonon (average energy ∼180 cm^-1). The suppression of the staple-shell phonon in the solid state is attributed to the intracluster and cluster-matrix interactions.

[AuAg26(SR)18S]- Nanocluster: Open Shell Structure and High Faradaic Efficiency in Electrochemical Reduction of CO2 to CO

For atomically precise metal nanoclusters, distinctive molecular architectures and promising applications are urgently required to be intensively explored. Herein, we have first reported the open shell structure of the [AuAg₂₆(S-Adm)₁₈S]^- nanocluster and its application in the electrochemical reduction of CO₂. The X-ray crystal structure of the AuAg₂₆ nanocluster is composed of a AuAg₁₂ icosahedron kernel and a Ag₁₄(SR)₁₈S open shell. The shell includes a Ag₆(SR)₃S, a Ag₅(SR)₆, and three Ag(SR)₃ motifs. It is the first time twisty propeller-like Ag₅(SR)₆ and trefoil-like Ag₆(SR)₃S motifs in metal nanoclusters have been observed. Due to the novel open shell configuration of Ag₁₄(SR)₁₈S, four triangular facets of the kernel are exposed. The AuAg₂₆ nanocluster shows excellent catalytic activity in the electrochemical reduction of CO₂ to CO. The Faradaic efficiency of CO is up to 98.4% under -0.97 V. The electrochemical in situ infrared study and DFT calculations demonstrate that the open shell structure of the AuAg₂₆ nanocluster is beneficial to the forming of intermediate COOH* in the electrochemical reduction of CO₂ to CO.

Gas-phase preparation and the stability of superatomic Nb11O15. Phys Chem Chem Phys 2021;23:15766-15773. [PMID: 34286767 DOI: 10.1039/d1cp02128a]

We report a study of the reactions of pure metal clusters Nbn- with dioxygen in the gas phase. It is found that the presence of low-concentration dioxygen reactants results in oxygen-addition products, whereas sufficient high-concentration dioxygen enables oxygen-etching reactions giving rise to molecular niobium oxides. Interestingly, in the presence of a suitable gas flow rate of an intermediate dioxygen concentration, a highly selective product Nb11O15- shows up in the mass spectra. Utilizing density functional theory (DFT) calculations, we have discussed the reactivities of Nbn- (3 ≤ n ≤ 14) clusters with oxygen, and unveiled the reasonable stability of Nb11O15- pertaining to its unique geometric structure with a D5h Nb@Nb10 core fully protected by 15 bridge-oxygen atoms. The oxygen-passivated Nb@Nb10O15- cluster exhibits a large HOMO-LUMO gap (1.46 eV) and effective multicenter bonds with remarkable superatom orbitals for all the 26 valence electrons of the Nb@Nb10 core corresponding to well-staggered energy levels. We illustrate the superatomic features in the Nb@Nb10 metallic core for which the adaptive natural density partitioning (AdNDP) analysis unveils thirteen 11c-2e bonds. Among them, one of the 11c-2e bonds accounts for the superatomic S orbital, three bonds correspond to superatomic P orbitals, another five display vivid D orbital characteristics, and the remaining four 11c-2e bonds are assigned to F orbital features. In addition, the net atomic charge of the center Nb atom is as high as -0.804 |e| rendering core-shell electrostatic interactions and the shielding effect of the Nb10O15 shell.

Toward the Formation of N-Heterocyclic-Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with Their Phosphine-Protected Analogues from Density Functional Theory Calculations

The structure and bonding of a series of selected phosphine-protected gold clusters (Au_n-P) of nuclearity varying from n = 6 to 13 were investigated by density functional theory (DFT) calculations and compared to those of the hypothetical homologues in which phosphines were replaced by N-heterocyclic carbene (NHC) analogues (Au_n-C). Both the Au_n-P and Au_n-C series exhibit similar stabilities and structural features, except for n = 6, where some differences are noted. The NHC ligands are found to be even slightly more strongly bonded to the gold core (by a few kilocalories per mole per ligand) than phosphines. Investigation of the optical properties of both series using time-dependent DFT calculations indicates similarities in the nature and energies of the UV-vis optical transitions and, consequently, relatively similar shapes of the simulated spectra, with a general blue-shift tendency when going from Au_n-P to Au_n-C. The fluorescence behavior observed experimentally for some of the Au_n-P species is expected to occur also for their Au_n-C analogues, which can be extended to other carbene-ligand-protected nanoclusters. Our results show that it should be possible to stabilize gold clusters with NHC ligands, in relation to the seminal Au₁₃-ligand-protected core, offering novel building blocks for the design of nanostructured materials with various properties.

Halogen effects on the electronic and optical properties of Au13 nanoclusters

We report an experimental and theoretical investigation of the electronic and optical properties of a series of icosahedral Au₁₃ nanoclusters, protected using different halogen ligands (Cl, Br, and I), as well as 1,2-bis(diphenylphosphino)ethane (dppe) ligands. All three clusters are comprised of the same Au₁₃ kernel with two halogens coordinated to the poles of the icosahedral cluster along with five dppe ligands. UV-vis absorption spectra indicate a systematic red shift from Cl to Br to I, as well as a sudden enhancement of the second excitonic peak for the I-coordinated cluster. Density functional theory (DFT) calculations suggest that all clusters possess a wide HOMO-LUMO energy gap of ∼1.79 eV and are used to assign the first two excitonic bands. Frontier orbital analyses reveal several HOMO → LUMO transitions involving halogen-to-metal charge transfers. For the I-coordinated cluster, more complicated I-to-metal charge transfers give rise to different excitation features observed experimentally. The current findings show that halogen ligands play important roles in the electronic structures of gold clusters and can be utilized to tune the optical properties of the clusters.

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.

Giant Emission Enhancement of Solid-State Gold Nanoclusters by Surface Engineering

Ligand-induced surface restructuring with heteroatomic doping is used to precisely modify the surface of a prototypical [Au₂₅ (SR¹ )₁₈ ]^- cluster (1) while maintaining its icosahedral Au₁₃ core for the synthesis of a new bimetallic [Au₁₉ Cd₃ (SR² )₁₈ ]^- cluster (2). Single-crystal X-ray diffraction studies reveal that six bidentate Au₂ (SR¹ )₃ motifs (L2) attached to the Au₁₃ core of 1 were replaced by three quadridentate Au₂ Cd(SR² )₆ motifs (L4) to create a bimetallic cluster 2. Experimental and theoretical results demonstrate a stronger electronic interaction between the surface motifs (Au₂ Cd(SR² )₆ ) and the Au₁₃ core, attributed to a more compact cluster structure and a larger energy gap of 2 compared to that of 1. These factors dramatically enhance the photoluminescence quantum efficiency and lifetime of crystal of the cluster 2. This work provides a new route for the design of a wide range of bimetallic/alloy metal nanoclusters with superior optoelectronic properties and functionality.

High Precision Electronic Spectroscopy of Ligand-Protected Gold Nanoclusters: Effects of Composition, Environment, and Ligand Chemistry

Atomically precise gold nanoclusters (AuNCs) are a class of nanomaterials valued for their electronic properties and diverse structural features. While the advent of X-ray crystallography of AuNCs has revealed their geometric structures with high precision, detailed electronic structure analysis is challenged by environmental, compositional, and thermal averaging effects present in electronic spectra of typical samples. To circumvent these challenges, we have adapted mass spectrometer-based electronic absorption spectroscopy techniques to acquire high-resolution electronic spectra of atomically precisely defined nanoclusters separated from a synthetic mixture. Here we discuss recent results using this approach to link the surface chemistry of triphenylphosphine-protected AuNCs to their electronic structure and expand on key elements of the experiment and the link between these gas-phase measurements and solution-phase behavior of AuNCs. Chemically derivatized Au₈(P(p-X-Ph)₃)₇²⁺ and Au₉(P(p-X-Ph)₃)₈³⁺ clusters, where X = -H, -CH₃, or -OCH₃, are used to derive systematic trends in the response of the electronic spectrum to the electron-donating character of the ligand shell. We find a linear relationship between the substituent Hammett parameter σ_p and the transition energy between both sets of clusters' highest occupied and lowest unoccupied molecular orbitals, a transition that is localized in the metal core within the limits of the superatomic model. The similarity of the mass-selective and solution-phase UV/vis spectra of Au₉(PPh₃)₈³⁺ indicates that the interpretation of these experiments is transferable to the condensed phase. He and N₂ environments are introduced to a series of isovalent clusters as a subtle probe of discrete environmental effects over electronic structure. Strikingly, select bands in the UV/vis spectrum respond strongly to the identity of the environment, which we interpret as a state-selective indicator of interfacially relevant electronic transitions. Physically predictable trends such as these will aid in building molecular design principles necessary for the development of novel materials based on nanoclusters.