Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes

A nearly perfect icosahedral Ir@Au12 superatom with superior photoluminescence obtained by ligand engineering

Heterometal doping and the introduction of surface ligands drastically alter the optical and photophysical properties of gold-based superatoms by modulating their electronic structures and the excited state dynamics. In this study, we investigate how the structures and the optical properties of an Ir@Au12 superatom capped by a diphosphine ligand, bis[benzo[b]phosphindole]ethane (bbpe), in which the rotation of the phenyl groups is prohibited, differ from those capped by the conventional diphosphine ligands, such as 1,2-bis(diphenylphosphino)ethane (dppe) and bis(diphenylphosphino)methane (dppm). The co-reduction of Ir(iii)- and Au(i)-precursors under mild reaction conditions yielded homoleptically capped [IrAu12(bbpe)6]3+ clusters (IrAu12-b) as the primary product. Single crystal X-ray diffraction analysis of IrAu12-b revealed the formation of a nearly perfect icosahedral Ir@Au12 superatomic core, in which the central Ir atom is equidistant from each vertex Au atom. The energy gap between occupied 1P and unoccupied 1D superatomic orbitals of IrAu12-b was larger than that of its dppm-capped counterpart, [IrAu12(dppm)6]3+ as evidenced by a blue shift (140 nm) of the photoluminescence (PL) wavelength and DFT calculations. IrAu12-b exhibited PL at 596 nm with a high quantum yield of 87% in deaerated CH2Cl2 due to the expanded 1P-1D energy gap and the restricted molecular motions of the bbpe ligands.

Electrochemical dechlorination promotes syngas production in N-heterocyclic carbene protected Au13 nanoclusters

Surface ligands play an important role in dictating the structure and catalytic properties of metal nanoclusters. Recently, a novel class of Au clusters protected by N-heterocyclic carbenes (NHCs) and halogens has been synthesized; however, the dynamic stability of the Au-NHCs/Au-halogen interface in real electrochemical environments as well as the influence of the ligand layer on the catalytic process remains obscure. Herein, we combined first-principles simulations with experiments to investigate the metal-ligand interface interaction of the classical [Au13(NHCMe)9Cl3]2+ cluster and its unique potential to promote electrocatalytic CO2 reduction to syngas. Our simulations revealed the facile shedding of chlorine ligands from the surface of the Au13 core upon electrochemical biasing, and the more negative the applied potential, the faster the kinetics of the Au-Cl bond cleavage. By contrast, the Au-NHC interface is highly stable, indicating the greater stability of Au-C bonds over the Au-Cl bonds under electrochemical conditions. Interestingly, the exposed icosahedral Au in dechlorinated [Au13(NHCMe)9Cl2]3+ cluster is capable of efficiently catalyzing electrochemical CO2 reduction to generate CO and H2 with comparable barriers in a wide potential range, showcasing its strong potential for syngas formation. Our predictions are further corroborated by experimental electrochemical data, where X-ray photoelectron spectroscopy (XPS) verified halogen stripping under acid or neutral media, and the activated Au13 cluster demonstrated enhanced catalytic efficacy for syngas formation with a CO : H2 ratio of approximately 0.8 to 1.2 across a broad potential range of -0.50 to -1.20 V. This work reveals an exciting frontier in the understanding of ligand etching dynamics in NHC-protected metal nanoclusters, and particularly, the catalytic preference for syngas production is revealed for the first time in gold-based nanoclusters, which is distinctive from previously reported Au nanoclusters that mainly produce CO.

Remove the innermost atom of a magnetic multi-shell gold nanoparticle for near-unity conversion of CO2 to CO

Few reports on paramagnetic metal nanoparticles with atomic precision and their difficult tailoring retard the insightful investigation of metal nanoparticle paramagnetism. Herein, we introduced a thiol-iodine mixture ligand-protecting strategy to successfully synthesize multi-shell paramagnetic [Au127I4(TBBT)48 (I: iodine, TBBT: 4-tert-butylphenylthiolate)]. The innermost Au atom was successfully removed via thiol induction without altering the structure framework to produce diamagnetic Au126I4(TBBT)48 with local ligand arrangement changed (butterfly effect), which could be further transformed into paramagnetic [Au126I4(TBBT)48]+ via hydrogen peroxide oxidation. The spin populations of both paramagnetic nanoparticles are more densely distributed on surface iodine than sulfur. Diamagnetic Au126I4(TBBT)48 exhibited a Faradaic efficiency of ~100% at -0.57 volt during the electrocatalytic reduction of carbon dioxide to carbon monoxide, while paramagnetic Au127I4(TBBT)48 and [Au126I4(TBBT)48]+ exhibited the maximum Faradaic efficiency of 87% at -0.67 volt and 90% at -0.57 volt, respectively, indicating the spin-catalytic activity correlation.

Tailoring Atomically Precise Gold Nanoclusters for Boosting Selective Hydrogenation of Nitrostyrene with H2

Hydrogenation reactions represent some of the most extensively studied topics within the field of catalysis. A novel alkynyl and phosphine coprotected [Au34(PhC≡C)14(Ph3P)6](SO3CF3)2(1) nanocluster has been synthesized, and its structure was determined by single crystal X-ray diffraction (SCXRD). Density functional theory calculation shows that 1 features an 18-electron superatomic molecule character with a configuration of (1σ)2(1n)2(1π)2(2σ)2(1σ*)2(3σ)2(2n)2(3n)2(1π*),2 which is significantly different from previously reported 18-electron metal nanoclusters. In comparison with some gold nanoclusters with similar composition or size but different ligands or surface coordination structures, except for the ligand effect (different kinds of ligands), the surface coordination structure involving the Au(I) sites derived from the PhC≡C-Au-C≡CPh monomeric staple motif and the steric hindrance of PhC≡C and Ph3P on the surface of 1, and the special electronic structure play a critical role in ensuring the enhanced catalytic performance of 1/TiO2 toward the chemoselective hydrogenation of 4-nitrostyrene with H2. The turnover frequency (TOF) of 1322.5 h-1 and the turnover number (TON) of 23500 represent the highest values observed among the gold nanocluster-based catalysts toward the same reaction. It presents an example of tailoring the surface coordination structure to modulate the catalytic performance, and offers valuable insights for the rational design and synthesis of catalysts to trade off the catalytic activity and selectivity at the atomic level.

N-Alkyl Chain Induced Molecular Aggregation in Mononuclear Gold(I)-N-Heterocyclic Carbene Complexes for Blue Light Emitting Applications

The gold(I) N-heterocyclic carbene (NHC) molecule exhibits outstanding luminescent properties along with high thermal stability due to its strong carbene gold bond. Gold(I)-NHC complexes are known to emit green to yellow color, while blue emission is limited. Herewith, we report the photoluminescence and thermal stability correlation between blue emitting mononuclear gold(I)-NHC chloride complexes, N-(9-anthracenyl)-N'(n-alkyl)imidazol-2-ylidene gold(I) chloride (alkyl=n-butyl (1), n-pentyl (2), n-hexyl (3)). These complexes were synthesized by transmetallation route and characterized by FT-IR, NMR, TGA, and single crystal X-ray diffraction analysis. The solid-state study reveals a unique molecular packing due to hydrogen bonding, aurophilic interaction, and CH⋅⋅⋅π interactions in all 1-3. In addition, the π⋅⋅⋅π stacking has been found in 1. All these complexes show good thermal stability due to the strong metal-ligand bond strength along with crystal packing. In addition, 1-3 emits a blue colour in both solution and crystalline state. The quantum yield in the crystalline state was found to be higher for 1 (22.44 %) compared to 2 (9.90 %), and 3 (14.98 %). A similar high quantum yield for blue-emitting gold(I)-NHC chloride complex is rare. This promising quantum yield for 1 can be ascribed to the crystal packing effect. The theoretical calculations were carried out to understand the electronic and structural properties of 1-3. Computational studies revealed the origin of the luminescence behaviour of complexes. The blue light emitting thin film and LED are demonstrated using 1 exhibited the emission peak at the blue region with Commission Internationale de L'Eclairage (CIE) coordinates near the National Television Standards Committee (NTSC) value.

How Carbene Ligands Transform AuAg Alloy Nanoclusters for Electrocatalytic Urea Synthesis

Metal nanoclusters stabilized by N-heterocyclic carbene (NHC) ligands have attracted increasing interest for their special structures and diverse applications. However, developing synthetic strategies and extending the database of NHC-protected nanoclusters are still challenging tasks. In this work, a novel and rapid synthetic method is developed to prepare AuAg alloy nanocluster ligated by carbene based on the reactivity of nanoclusters. The rod-like carbene-capped bimetal nanocluster, [Au13Ag12(PPh3)8(BMIm)2I8]SbF6 (Au13Ag12 : BMIm), was achieved and characterized by a series of techniques. The alloy nanocluster consists of two vertex-sharing icosahedrons and carbene ligands, phosphine ligands, and I atoms. Interestingly, the introduced carbene ligands show strong coordination capabilities with Au, enhancing the interaction between metal core and ligands. To the best of our knowledge, the carbene-capped Au13Ag12 : BMIm nanocluster is the first of its kind to show higher thermostability and higher sensitivity to light compared with the homogeneously capped analogue nanocluster ([Au13Ag12(PPh3)10I8]SbF6). Density functional theory calculations attribute these properties to a unique delocalization of electrons within the frontier orbitals. Finally, the Au13Ag12 : BMIm anchored on NiFe-LDH exhibits remarkable electrocatalytic activity in the electrosynthesis of urea from NO3 - and CO2, achieving a urea production rate of 29.5 mmol gcat -1 h-1 with a Faradaic efficiency of 34 % at -0.5 V (vs. RHE).

A Stable Open-Shelled Au26 Nanocluster with Remarkable Performance in Selective Oxidation of Benzyl Alcohol

Open metal sites are crucial in catalysis. We have used a "loose coordination strategy" (LCS) to preorganize open metal sites in gold cluster catalysts. A gold nanocluster with composition of [Au26(3,4-Me2-Ph-form)9(iPr2-imy)3(Me2S)](BF4)2 (iPr2-imy=1,3-Diisopropylimidazolium tetrafluoroborate, 3,4-Me2-Ph-form=N,N'-Di(3,4-dimethyl-phenyl)formamidine) (Au26) has been obtained by one pot synthesis, i.e. the direct reduction of Me2SAuCl in the presence of N-heterocyclic carbenes and amidinate ligands. ESI-TOF-MS reveals that the Me2S ligand is detached from the cluster to form open sites. The accessibility of the exposed Au atoms has been confirmed quantitatively by luminescent titration with 2-naphthalenethiol. Surprisingly, Au26 has 15 valence electrons, and the presence of an unpaired electron is confirmed by superconducting quantum interference device (SQUID) and electron paramagnetic resonance (EPR). This open-shelled Au26 not only shows unexpected high stability but also exhibits excellent catalytic performance toward the selective oxidation of benzyl alcohol to benzaldehyde, achieving a remarkable turnover number up to 100670.

Ligand Effects on Luminescence of Atomically Precise Gold Nanoclusters

Great efforts have been made to establish the structure-property relationships of ligand-protected metal nanoclusters. An alkynyl-protected gold nanocluster Au28(tBuC6H4C≡C)20 (Au28-C2) was synthesized and structurally characterized. It has an isostructural metal kernel and a quite similar ligand shell to its counterpart Au28(tBuC6H4S)20 (Au28-S), which provides a valuable platform for studying ligand effects on the luminescence of clusters. The emission intensity of Au28-C2 in solution is ∼10-fold that of Au28-S. The only structural difference between these two clusters is the donor atom or group, i.e., C≡C for Au28-C2 versus S for Au28-S. Temperature-dependent experiments indicate that the two Au28 clusters have different thermodynamic behaviors and radiative processes. The study on the excited-state dynamics by transient absorption spectroscopy indicates that Au28-C2 has a larger triplet-state population than Au28-S. This work shows important ligand effects on the emission properties of metal nanoclusters, which provide new perspectives for the designed synthesis of functional metal nanoclusters.

A comprehensive review of atomically precise metal nanoclusters with emergent photophysical properties towards diverse applications

Atomically precise metal nanoclusters (MNCs) composed of a few to hundreds of metal atoms represent an emerging class of nanomaterials with a precise composition. With the size approaching the Fermi wavelength of electrons, their energy levels are well-separated, leading to molecule-like properties, like discrete single electronic transitions, tunable photoluminescence (PL), inherent structural anisotropy, and distinct redox behavior. Extensive synthetic efforts and electronic structure revelation have expanded applicability of MNCs in catalysis, optoelectronics, and biology. This review highlights the intriguing photophysical and electrochemical behaviors of MNCs and their regulatory parameters and applications. Initially, we present a brief discussion on the evolution of MNCs from gas-phase naked metal clusters to monolayer ligand-protected MNCs along with representative studies on their electronic structure. Due to their quantized molecular orbitals, they often exhibit PL, which can be regulated based on their capping ligands, number of atoms, crystal packing, presence of heterometal, and surrounding environment. Apart from PL, the relaxation pathways of MNCs on an ultrafast time scale have been extensively studied, which significantly differ from that of plasmonic metal nanoparticles. Moreover, their interaction with high-intensity light results in unique non-linear optical properties. The synergy between MNCs in a hierarchical self-assembled structure has been exploited to enhance their PL by precisely tuning their non-covalent interactions. Moreover, several NC-based hybrids have been designed to exhibit efficient electron or energy transfer in the photoexcited state. In the next section, we briefly focus on the redox behavior of NCs and facile electron transfer to suitable substrates, which result in enzyme-like catalytic activity. Utilizing these photophysical and electrochemical behaviors, NCs are widely employed in catalysis, optical sensing, and light-harvesting applications, which are also discussed in this review. In the final section, conclusions and open questions for the NC research community are included. This review will provide a comprehensive view of the emerging physicochemical properties of MNCs, thereby enabling an understanding for their precise modulation in future.

Theory-Directed Ligand-Shell Engineering of Ultrasmall Gold Clusters: Remarkable Effects of Ligand Arrangement on Optical Properties

Ligand-shell engineering of ultrasmall metal clusters is a burgeoning research field aiming to develop cluster-specific properties. However, predicting these properties prior to synthesis is challenging due to their high sensitivity to geometric and/or electronic variations in ultrasmall metal cores, hindering further exploration. In this study, we present a theory-directed ligand-shell design and significant red-shift in absorption of a prolate-shaped [Au8(diphosphine)4Cl2]2+ cluster by synthesizing and characterizing enantiopure octagold clusters bearing chiral BINAP-type ligands [BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl]. Crystallographic analysis reveals the predesigned ligand arrangement and twisted gold-core framework. The enantiomeric clusters show significant changes in both absorption and photoluminescence compared with a previous Au8 analogue and exhibit chiroptical signals. Furthermore, theoretical calculations visually unveil the atomic level origins of their optical and chiroptical absorption characteristics. This work not only highlights the effectiveness of ligand-shell engineering in creating unique photophysical properties but also offers a viable, theory-guided strategy for designing and functionalizing ligated metal clusters.

Impact of Ligand Structure on Biological Activity and Photophysical Properties of NHC-Protected Au13 Nanoclusters

N-heterocyclic carbene (NHC)-protected gold nanoclusters display high stability and high photoluminescence, making them well-suited for fluorescence imaging and photodynamic therapeutic applications. We report herein the synthesis of two bisNHC-protected Au13 nanoclusters with π-extended aromatic systems. Depending on the position of the π-extended aromatic system, changes to the structure of the ligand shell in the cluster are observed, with the ability to correlate increases in rigidity with increases in fluorescence quantum yield. Density functional theory analysis reveals that both synthesized Au13 nanoclusters are 8-electron superatoms but have distinct differences in the characteristics of the lowest unoccupied single-electron states. Qualitatively, this implies different mechanisms for excitations and their decay over the fundamental energy gap. Stability and photophysical studies were carried out to provide the emission lifetime and optical purity of the two clusters. Active intracellular uptake of the nanoclusters was confirmed in vitro using confocal microscopy in human epithelial carcinoma cells. Reactive oxygen species production was measured at 7% efficiency. The high cluster stability, photoluminescence quantum yields, and efficient cellular uptake in cancer cells suggest potential for these nanoclusters as highly efficient and tunable nanomedical platforms.

Diving into Unknown Waters: Water-Soluble Clickable Au13 Nanoclusters Protected with N-Heterocyclic Carbenes for Bio-Medical Applications

The use of gold nanoclusters in biomedical applications has been steadily increasing in recent years. However, water solubility is a key factor for these applications, and water-soluble gold nanoclusters are often difficult to isolate and susceptible to exchange or oxidation in vivo. Herein, we report the isolation of N-heterocyclic carbene (NHC)-protected atomically precise gold nanoclusters functionalized with triethylene glycol monomethyl ether groups. These clusters are highly luminescent and water soluble and are shown to be stable in biological media. Importantly, the core structure, stability, and high quantum yield of the nanoclusters were conserved after backbone modification. Depending on the nature of the halide group, clusters have high stability in simulated biofluids and resist attack by glutathione. In vivo studies show that no abnormal cellular morphology is introduced in the kidney, liver, or spleen of mice treated with [Au13(NHC)5Br2]Br3 nanoclusters protected by 1,8-dimethylnaphthyl-linked NHCs. This cluster has a blood elimination half-life of 0.68 h. Functionalization of the wingtip groups of the cluster with azide groups is demonstrated, and complete reaction of all 10 azide groups with strained alkynes is shown, highlighting the potential of these clusters in biological settings.

Synthesis, Structural Characterization, and Electronic Structure Analysis of F2-type Superatomic Molecules

The investigation of bonding interactions between superatoms continues to be a largely unexplored area of study. In this study, we present the synthesis and characterization of two F2-type superatomic molecules [Au2Ag25(C7H4NOS)13(DPPB)3] and [Au9Ag18(C5H4NS)11(DPPM)5]2+ (Au2Ag25 and Au9Ag18 for short, respectively). The overall structures were confirmed via X-ray crystallography, revealing the horizontal expansion of the biicosahedral Au2Ag21 yielding [Au2Ag25(C7H4NOS)13(DPPB)3] and vertical expansion of the biicosahedral Au8Ag15 yielding [Au9Ag18(C5H4NS)11(DPPM)5]2+. Furthermore, their electronic structures were elucidated through density functional theory (DFT) calculations. Spectroscopic analysis of electronic absorption characteristics, in conjunction with Tamm-Dancoff approximation DFT (TDA-DFT) calculations, revealed that the Au2Ag21(+9) and Au8Ag15(+9) cores were analogues of the F2 molecule in electronic configuration.

Temperature-dependent luminescent copper nanoclusters with noncovalent interactions for determination of β-galactosidase activity

The synthesis of a novel bidentate ligand-protected copper nanocluster via a solid-state strategy is reported. Single-crystal X-ray diffraction analysis result reveals that the copper nanocluster features an octahedral core (Cu6) coordinated by six ligands. Noncovalent interactions (C-H…π and π…π) exist between the copper nanoclusters. The copper nanocluster displays luminescence even at 250 °C. The luminescence intensity is linearly correlated with temperature changes. The copper nanocluster can assemble into luminescent nanosheets whose emission is quenched by 4-nitrophenol. Spectroscopic analysis and theoretical calculations results demonstrate that the inner filter effect and electron transfer cause the above quenching effect. A probe based on luminescent nanosheets was constructed for β-galactosidase activity determination. The linearity range is 3.3-91.8 U·L-1, and the limit of detection is 0.45 U·L-1. This probe was also evaluated for determination of the β-galactosidase activity in human serum via spiking experiments. The recoveries ranged from 96.2% to 101.8%.

N-Heterocyclic carbene-stabilized gold-copper nanoclusters: synthesis, bonding and mechanochromism

N-Heterocyclic carbene (NHC) ligands have emerged as highly effective surface ligands for the protection and functionalization of metal nanoclusters (NCs). However, research on NHC-stabilized metal NCs, including their synthesis, structure, properties, and applications, is still in its early stages. In this study, we present the first gold-copper alloy cluster protected by both NHC and alkyne ligands, denoted as Au3Cu(iPrNHCiPr)(PA)4 (abbreviated as Au3Cu, where iPrNHCiPr is a bidentate N-heterocyclic carbene ligand with isopropyl as the N-substituent, and PA is a phenylethynyl group). The precise composition of Au3Cu was confirmed through the utilization of electrospray ionization mass spectrometry (ESI-MS), and its structure was determined via X-ray single-crystal diffraction. It is worth noting that although the Au3Cu clusters do not display substantial light emission when exposed to UV lamps, they are capable of emitting green fluorescence subsequent to undergoing mechanical milling (λem = 500 nm). Powder X-ray diffraction (PXRD) analysis reveals that this transition is attributed to a crystalline-amorphous transformation of the cluster crystals. These atomically precise alloy clusters are expected to serve as a model for further investigation into the principles of mechanical milling of metal clusters for discolouration.

LnIII/CuI Bimetallic Nanoclusters with Enhanced NIR-II Luminescence

Coinage-metal clusters with excellent luminescence properties have attracted considerable interest due to their intriguing structures and potential applications. However, achieving strong near-infrared (NIR) luminescence in these clusters is highly challenging. Here, we have successfully synthesized the first LnIII/CuI bimetallic clusters, formulated as [LnCu54O6Cl3(2-MeO-PhC≡C)36] (ClO4)6 (Ln = Yb for YbCu54, Er for ErCu54, and Gd for GdCu54). Single crystal X-ray diffraction showed that the LnCu54 clusters have a three-layered core-shell structure, consisting of (LnO6)@Cu18Cl3@Cu36 units protected by 36 2-MeO-PhC≡C- ligands. Notably, the YbCu54 cluster exhibits significant NIR-II luminescence at 986 nm with the solid quantum efficiency of 33.3%, the highest among Cu clusters with NIR-II emission. This work not only reports the first category of LnIII/CuI clusters but also presents a method to enhance NIR luminescence in coinage-metal clusters through the incorporation of LnIII ions.

Raising Near-Infrared Photoluminescence Quantum Yield of Au42 Quantum Rod to 50% in Solutions and 75% in Films

Highly emissive gold nanoclusters (NCs) in the near-infrared (NIR) region are of wide interest, but challenges arise from the excessive nonradiative dissipation. Here, we demonstrate an effective suppression of the motions of surface motifs on the Au42(PET)32 rod (PET = 2-phenylethanethiolate) by noncoordinative interactions with amide molecules and accordingly raise the NIR emission (875/1045 nm peaks) quantum yield (QY) from 18% to 50% in deaerated solution at room temperature, which is rare in Au NCs. Cryogenic photoluminescence measurements indicate that amide molecules effectively suppress the vibrations associated with the Au-S staple motifs on Au42 and also enhance the radiative relaxation, both of which lead to stronger emission. When Au42 NCs are embedded in a polystyrene film containing amide molecules, the PLQY is further boosted to 75%. This research not only produces a highly emissive material but also provides crucial insights for the rational design of NIR emitters and advances the potential of atomically precise Au NCs for diverse applications.

Synthesis and crystallization of a carboxylate functionalized N-heterocyclic carbene-based Au13 cluster with strong photo-luminescence

Here we report the synthesis and crystallization of a -COOH-capped N-heterocyclic carbene (NHC)-protected Au13 cluster. The single-crystal structure of the -COOH-capped NHC-Au13 cluster reveals a classic icosahedral core with one Au atom in its center. The icosahedral core is surrounded by five NHC ligands with pseudo C5 symmetry and exposed carboxyls in a pentagonal antiprism fashion. The detailed formula of the Au cluster was identified as Au13(bi-NHC carboxyl)5Cl2 (hereafter abbreviated as Au13-c). The density functional theory (DFT) calculations confirm that Au13-c is an electronically stable eight-electron super-atom cluster and elucidate its optical transitions in the UV-Vis range. The Au13-c cluster exhibits excellent thermal and chemical stability under bio-relevant conditions. Additionally, this cluster shows a strong red emission in DMF and H2O with an excellent quantum yield (QY) of 40% and 12.6%, respectively. The high QY of Au13-c enables its use in cell imaging on both cancer and noncancerous cells.

Relationship between Structural Defects and Free Electrons in Icosahedral Nanoclusters

According to the classic superatom model, metal nanoclusters with a "magic number" of free valence electrons display high stability, manifesting as the closed-shell-dependent electronic robustness. The icosahedral nanobuilding blocks containing eight free electrons were the most common in constructing metal nanoclusters; however, the structure defect-dependent variations of the free electron count in icosahedral configurations are still far from thorough research. Here, we reported a hydride-containing [Pt2Ag15(SAdm)4(DPPOE)4H]2+ nanocluster with two largely defective Pt1Ag8 icosahedral cores. Together with previously reported complete or slightly defective icosahedra in metal nanoclusters, the largely defective Pt1Ag8 core provided important clues to reveal the evolutionary mode of structural defects and free electrons in icosahedral nanoclusters; the free electron count of icosahedron was reduced two-by-two (i.e., from 8e to 6e and then to 4e) accompanied by the structure defection. Overall, the work presented a novel Pt2Ag15 nanocluster with a largely defective core structure that enables an atomic-level understanding of the relationship between structural defects and free electrons in icosahedral nanoclusters.

If the Crown Fits: Sterically Demanding N-Heterocyclic Carbene Promotes the Formation of Au8Pt Nanoclusters

While N-heterocyclic carbenes (NHCs) have recently been shown to be effective ligands for gold nanoclusters, very few examples of heterometallic clusters incorporating nongroup 11 metals are known. We present herein an Au-Pt NHC cluster featuring a crown-shaped [Au8Pt(NHC)8]2+ core, produced in high yield without the need for chromatographic purification. The method was largely independent of the substitution pattern of the NHC backbone; however, bulky wingtip groups were needed for clean conversion to the Au8Pt cluster. Clusters were characterized using single crystal X-ray diffraction, multinuclear nuclear magnetic resonance, electrospray ionization mass spectroscopy, and ultraviolet-visible spectroscopy, and electrochemical features of the cluster are also presented. A detailed analysis of the in-progress reaction mixture by ESI-MS supports the direct involvement of Au-H species as intermediates in cluster formation. These studies further demonstrate that NHC wingtip sterics play a key part in determining the nature of the initial cluster species, providing critical information for the generation of new NHC-stabilized nanoclusters.

A single-gold-atom addition regulates sharp redshift in the fluorescence of atomically precise nanoclusters

The manipulation of emission peaks at the atomic level and the investigation of the fluorescent origin mechanism are important issues. In this study, a phosphine-mediated modification method was employed on Au36(TBBT)24 nanocluster to produce a new gold nanocluster Au37(TBBT)21(TPP)2. The structural comparison revealed that Au37(TBBT)21(TPP)2 has a structural framework similar to that of Au36(TBBT)24 except for the reconstruction of its surface motifs, the addition of one gold atom into the kernel, and local structural distortion. Interestingly, compared with Au36(TBBT)24, the emission peak of Au37(TBBT)21(TPP)2 is red-shifted into the NIR-II windows (972 nm vs. 1152 nm in CDCl3) with a quantum yield of 1.5%. Furthermore, the origin of the NIR-II fluorescence in Au37(TBBT)21(TPP)2 and the red-shift mechanism of the emission peak were explored by combining the crystal structure and DFT calculations. The results reveal that the insertion of the 37th gold atom into the core can increase the contribution of the gold atoms to the HOMO orbitals and change the origin of their fluorescence from local excitation (LE) to inter fragment charge transfer (IFCT).

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

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

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.

Chemical Flexibility of Atomically Precise Metal Clusters

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

Structure Effects of Ligands in Gold-Ligand Complexes for Controlled Formation of Gold Nanoclusters

Metal nanoclusters (NCs) are a special class of nanoparticles composed of a precise number of metal atoms and ligands. Because the proportion of ligands to metal atoms is high in metal NCs, the ligand type determines the physical properties of metal NCs. Furthermore, ligands presumably govern the entire formation process of the metal NCs. However, their roles in the synthesis, especially as factors in the uniformity of metal NCs, are not understood. It is because the synthetic procedure of metal NCs is highly convoluted. The synthesis is initiated by the formation of various metal-ligand complexes, which have different numbers of atoms and ligands, resulting in different coordinations of metal. Moreover, these complexes, as actual precursors to metal NCs, undergo sequential transformations into a series of intermediate NCs before the formation of the desired NCs. Thus, to resolve the complicated synthesis of metal NCs and achieve their uniformity, it is important to investigate the reactivity of the complexes. Herein, we utilize a combination of mass spectrometry, density functional theory, and electrochemical measurements to understand the ligand effects on the reactivity of AuI-thiolate complexes toward the reductive formation of Au NCs. We discover that the stability of the complexes can be increased by either van der Waals interactions induced by the long carbon chain of ligands or by non-thiol functional groups in the ligands, which additionally coordinate with AuI in the complexes. Such structural effects of thiol ligands determine the reduction reactivity of the complexes and the amount of NaBH4 required for the controlled synthesis of the Au NCs.

Two-Orders-of-Magnitude Enhancement of Photoinitiation Activity via a Simple Surface Engineering of Metal Nanoclusters

Development of high-performance photoinitiator is the key to enhance the printing speed, structure resolution and product quality in 3D laser printing. Here, to improve the printing efficiency of 3D laser nanoprinting, we investigate the underlying photochemistry of gold and silver nanocluster initiators under multiphoton laser excitation. Experimental results and DFT calculations reveal the high cleavage probability of the surface S-C bonds in gold and silver nanoclusters which generate multiple radicals. Based on this understanding, we design several alkyl-thiolated gold nanoclusters and achieve a more than two-orders-of-magnitude enhancement of photoinitiation activity, as well as a significant improvement in printing resolution and fabrication window. Overall, this work for the first time unveils the detailed radical formation pathways of gold and silver nanoclusters under multiphoton activation and substantially improves their photoinitiation sensitivity via surface engineering, which pushes the limit of the printing efficiency of 3D laser lithography.

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

A luminescent Ag8(DPPY)6(PhCC)6 cluster with a triangular superatomic Ag8 core

We have synthesized single crystals of a highly stable Ag8 nanocluster protected by six ligands of diphenyl-2-phosphinic pyridine (DPPY) plus six ligands of phenylacetylene (PhCC). This Ag8(DPPY)6(PhCC)6 cluster bears a triangular superatomic Ag8 core, with the vertex and edge Ag atoms (quasi-triangle Ag6) being protected by both P and N bidentate coordination of the six DPPY ligands; meanwhile, the six PhCC ligands via μ3-C coordination form coordination on the two central Ag atoms capped on both sides of the triangle facet. Apart from the well-organized coordination of the two ligands pertaining to the balanced interactions with the Ag8 core, this Ag8 nanocluster exhibits superatomic stability with two delocalized valence electrons (1S2||1P0), assuming that the six PhCC ligands fix 6 localized electrons from the Ag atoms. Interestingly, the Ag8(DPPY)6(PhCC)6 NCs display temperature-dependent dual emissions at 330 and 535 nm under deep ultraviolet excitation. TD-DFT calculations reproduced the experimental spectrum, shedding light on the nature of excitation states and metal-ligand interactions in such a superatomic metal cluster.

Gas-Phase Structures of [Au21(SR)14]- and [Au17(SR)10]- with Eight Electrons: Can They Support an Icosahedral Au13 Core?

A prototypical thiolate (RS)-protected gold cluster [Au25(SR)18]- has high stability due to specific geometric and electronic structures: an icosahedral (Ih) Au13 core with a closed electronic shell containing eight electrons is completely protected by six units of Au2(SR)3. Nevertheless, collisional excitation of [Au25(SR)18]- in a vacuum induces the sequential release of Au4(SR)4 to form [Au21(SR)14]- and [Au17(SR)10]- both containing eight electrons. To answer a naive question of whether these fragments bear an Ih Au13(8e) core, the geometrical structures of [Au21(SC3H7)14]- and [Au17(SC3H7)10]- in the gas phase were examined by the combination of anion photoelectron spectroscopy and density functional theory (DFT) calculation of simplified models of [Au21(SCH3)14]- and [Au17(SCH3)10]-. We concluded that [Au21(SC3H7)14]- retains a slightly distorted Ih Au13(8e) core, while [Au17(SC3H7)10]- has an amorphous Au13 core composed of triangular Au3, tetrahedral Au4, and prolate Au7 units. DFT calculations on putative species [Au19(SCH3)12]- and [Au18(SCH3)11]- suggested that the Ih Au13(8e) core undergoes dramatic structural deformation due to mechanical stress from μ2 ligation of only one RS.

Bright near-infrared emission from the Au39(SR)29 nanocluster

The synthesis of atomically precise gold nanoclusters with high photoluminescence quantum yield (PLQY) in the near-infrared (NIR) region and understanding their photoluminescence mechanism are crucial for both fundamental science and practical applications. Herein, we report a highly luminescent, molecularly pure Au39(PET)29 (PET = 2-phenylethanethiolate) nanocluster with PLQY of 19% in the NIR range (915 nm). Steady state and time-resolved PL analyses, as well as temperature-dependent PL measurements reveal the emission nature of Au39(PET)29, which consists of prompt fluorescence (weak), thermally activated delayed fluorescence (TADF), and phosphorescence (predominant). Furthermore, strong dipole-dipole interaction in the solid-state (e.g., Au39(PET)29 nanoclusters embedded in a polystyrene thin-film) is found to narrow the energy gap between the S1 and T1 states, which results in faster intersystem crossing and reverse intersystem crossing; thus, the ratio of TADF to phosphorescence varies and the total PLQY is increased to 32%. This highly luminescent nanocluster holds promise in imaging, sensing and optoelectronic applications.

Photoluminescence Quenching of Hydrophobic Ag29 Nanoclusters Caused by Molecular Decoupling during Aqueous Phase Transfer and EmissionRecovery through Supramolecular Recoupling

Exploiting emissive hydrophobic nanoclusters for hydrophilic applications remains a challenge because of photoluminescence (PL) quenching during phase transfer. In addition, the mechanism underlying PL quenching remains unclear. In this study, the PL-quenching mechanism was examined by analyzing the atomically precise structures and optical properties of a surface-engineered Ag29 nanocluster with an all-around-carboxyl-functionalized surface. Specifically, phase-transfer-triggered PL quenching was justified as molecular decoupling, which directed an unfixed cluster surface and weakened the radiative transition. Furthermore, emission recovery of the quenched nanoclusters was accomplished by using a supramolecular recoupling approach through the glutathione-addition-induced aggregation of cluster molecules, wherein the restriction of intracluster motion and intercluster rotation strengthened the radiative transition of the clusters. The results of this work offer a new perspective on structure-emission correlations for atomically precise nanoclusters and hopefully provide insight into the fabrication of highly emissive cluster-based nanomaterials for downstream hydrophilic applications.

Synthesis and investigation of a meta[6]cycloparaphenylene gold(I) N-heterocyclic carbene complex

Meta[n]cycloparaphenylenes (m[n]CPPs) as well as N-heterocyclic carbene (NHC) gold(I)-complexes are intriguing building blocks for material and life sciences due to their extraordinary structures resulting in unique photophysical properties. Herein, we report the combination of a m[6]CPP with a N-heterocyclic carbene serving as a ligand in a linear gold(I)-complex possessing the form [AuBr(NHC)]. Solid-state structures of both the precursor and the complex are presented and discussed. Moreover, we investigated the luminescence properties of both the imidazolium intermediate and the corresponding gold(I)-complex.

N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters

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

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.

Visible to NIR-II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near-infrared-I and -II wavelengths (NIR-I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure-PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near-infrared wavelengths, especially in the NIR-II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double-digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.

Tailoring the subshell and electronic structure of an atomically precise AuAg alloy nanocluster

Manipulating the atomic-level structure of the subshell of a nanocluster while preserving the inner and outer shell structure is challenging. We present the synthesis and molecular structure of an alkynyl-protected Au34Ag27 nanocluster, which exhibits distinct third shell atomic arrangement, electronic structure, and optical properties from those of the Au34Ag28 nanocluster.

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.

Dual Catalysis of Gold Nanoclusters: Photocatalytic Cross-Dehydrogenative Coupling by Cooperation of Superatomic Core and Molecularly Modified Staples

Thiolate-protected gold nanoclusters (AuNCs) have attracted significant attention as nano-catalysts, revealing a superatomic core and gold-thiolate staples as distinct structural units. Here, we demonstrate the unprecedented dual catalytic activity of thiolate-protected [Au25 (SR)18 ]- nanoclusters, involving both photosensitized 1 O2 generation by the Au13 superatomic core and catalytic carbon-carbon bond formation facilitated by Au2 (SR)3 staples. This synergistic combination of two different catalytic units enables efficient cross-dehydrogenative coupling of terminal alkynes and tertiary aliphatic amines to afford propargylamines in high yields of up to 93 %. Mixed-ligand AuNCs bearing both thiolate and alkynyl ligands revealed the intermediacy of the alkynyl-exchanged AuNCs toward both photosensitization and C-C bond-forming catalytic cycles. Density functional theory calculations also supported the intermediacy of the alkynyl-exchanged AuNCs. Thus, the use of ligand-protected metal nanoclusters has enabled the development of an exceptional multifunctional catalyst, wherein distinct nanocluster components facilitate cooperative photo- and chemo-catalysis.

Effect of Specific Heavy Doping of Silver Atoms into the Icosahedral Au13 on Electronic Structure and Catalytic Performance

The exploration of specific heavy doping of silver atoms into icosahedral Au13 clusters and their electronic structures and properties has been somewhat limited. Herein, we report two heavily Ag doped nanoclusters, [Au7Ag6(C7H4NOS)4(Dppf)3Cl]0 and [Au7Ag6(C7H4NOS)3(Dppf)3Cl](SbF6) (Au7Ag6-0 and Au7Ag6-1, respectively) [C7H4NOSH = 2-mercaptobenzoxazole, and Dppf = 1,1'-bis(diphenylphosphino)ferrocene]. The electronic structures and superatomic orbitals of nanoclusters were determined by density functional theory (DFT) calculations, and the energy degeneracy of the superatomic orbitals of Au7Ag6-1 is higher than that of Au7Ag6-0. Transient absorption spectroscopy was performed, revealing that Au7Ag6-0 significantly extends the excited-state lifetime. Both nanoclusters were supported on activated carbon for the oxygen reduction reaction. DFT calculations confirm that the catalytic activities mainly stem from the carbon atom of ferrocene rather than the iron atom. This study not only sheds light on the preparation of icosahedral alloy clusters but also provides insights into the regulation of icosahedral superatomic structure and electrocatalytic properties.

Thermal Stability and Electronic Properties of N-Heterocyclic Carbene-Protected Au13 Nanocluster and Phosphine-Protected Analogues

Despite significant advances in manufacturing atomically precise gold nanoclusters protected by various ligands, there is a limited understanding of the thermal stability dynamics and electronic properties of ligand effects. We conducted ab initio molecular dynamics (AIMD) simulations on the well-characterized [Au13(NHCMe)9Cl3]2+ nanocluster and its counterpart [Au13(PMe3)9Cl3]2+ cluster to evaluate the thermal stability induced by N-heterocyclic carbene (NHC) and phosphine ligands. The result shows that under vacuum conditions, [Au13(PMe3)9Cl3]2+ is more stable than [Au13(NHCMe)9Cl3]2+, and both lead to metal nucleation decomposition, breaking into the Au12 fragment and L-Au-Cl (L = NHCMe or PMe3) complexes eventually. The optical and electronic properties of these two clusters change significantly due to ligand alteration. Furthermore, we have designed a novel [Au13(NHCMe)(PMe3)8Cl3]2+ cluster coprotected by NHC and phosphine ligands, displaying higher thermal stability than the homoligand protected [Au13(NHCMe)9Cl3]2+ and [Au13(PMe3)9Cl3]2+. Our hypothetical species are an interesting model for nanostructured materials, facilitating the experimental exploration of cluster synthesis and catalytic applications.

One-step preparation of Pt/Ag nanoclusters for CO2 transformation

Structurally precise metal nanoclusters with a facile synthetic process and high catalytic performance have been long pursued. These atomically precise nanocatalysts are regarded as model systems to study structure-performance relationships, surface coordination chemistry, and the reaction mechanism of heterogeneous metal catalysts. Nevertheless, the research on silver-based nanoclusters for driving chemical transformations is sluggish in comparison to gold counterparts. Herein, we report the one-step synthesis of Pt/Ag alloy nanoclusters of [PtAg9(C18H12Br3P)7Cl3](C18H12Br3P), which are highly active in catalysing cycloaddition reactions of CO2 and epoxides. The cluster was obtained in a rather simple way with the reduction of silver and platinum salts in the presence of ligands in one pot. The molecular structure of the titled cluster describes the protection of the Pt-centred Ag9 crown by the shell of phosphine ligands and halides. Its electronic structure, as revealed by density function theoretical calculations, adopts a superatomic geometry with 1S21P6 configuration. Interestingly, the cluster displays high activity in the formation of cyclic carbonates from CO2 under mind conditions.

Insights into the synthesis of NHC-stabilized Au nanoclusters through real-time reaction monitoring

Atomically precise gold nanoclusters (AuNCs) are interesting nanomaterials with potential applications in catalysis, bioimaging and optoelectronics. Their compositions and properties are commonly evaluated by various analytical techniques, including UV-vis spectroscopy, NMR spectroscopy, ESI mass spectrometry, and single-crystal X-ray diffraction. While these techniques have provided detailed insights into the structure and properties of nanoclusters, synthetic methods still suffer from a lack of in situ and real-time reaction monitoring methodologies. This limits insight into the mechanism of formation of AuNCs and hinders attempts at optimization. We have demonstrated the utility of HPLC-MS as a monitoring methodology in the synthesis of two NHC-protected gold nanoclusters: [Au13(NHC)9Cl3]2+ and [Au24(NHC)14Cl2H3]3+. Herein we show that HPLC coupled with mass spectrometry and 13C NMR spectroscopy of labelled derivatives enables new insight into critical reaction dynamics of AuNCs synthesis and rapid reaction optimization.

Recent Advances in Ligand Engineering for Gold Nanocluster Catalysis: Ligand Library, Ligand Effects and Strategies

Advances in new ligands in the last decade facilitated in-depth studies on the property-relationship of gold nanoclusters and promoted the rational synthesis and related applications of such materials. Currently, more and more new ligands are being explored; thus, the ligand library of AuNCs is being expanded fast, which also enables investigation of ligand effects of AuNCs via direct comparison of different ligating shell with the identical gold core. It is now widely accepted that ligands influence the properties of AuNCs enormously including stability, catalysis, photoluminescence among others. These studies inspired ligand engineering of AuNCs. One of the goals for ligand engineering is to develop ligated AuNC catalysts in which the ligands are able to exert big-enough influence on electronic and steric control over catalysis as in a transition-metal or an enzyme system. Although increasing attention is paid to the further expansion of ligand library, the investigation of design principles and strategies regarding ligands are still in their infant stage. This review summarizes the ligands for AuNC synthesis, the ligand effects on stability and catalysis, and recently developed strategies in promoting AuNC catalytic performance via ligand manipulation.

DppfCuBH4: new reducing agents for the synthesis of ferrocene-functionalized metal nanoclusters

A facile synthesis of atomically precise metal nanoclusters, especially those decorated with functional groups, is the prerequisite for finding applications in special fields and studying structure-and-property relationships. The exploration of simple and efficient synthetic prototypes for introducing functional ligands (such as ferrocene) into cluster moieties is thus of high interest. In this work, a type of reducing agent of dppfCuBH₄ (dppf is 1,1'-bis(diphenyphosphino)ferrocene) is introduced for the first time to prepare ferrocene-functionalized metal nanoclusters. Two new clusters of [Ag₂₅Cu₄(dppf)₆(3-F-PhCC)₁₂Cl₆]³⁺ (1) and [Ag₄(dppf)₅Cl₂]²⁺ (2) have been obtained from the simple synthetic method. The two compounds have been fully characterized by advanced techniques of electrospray ionization mass spectroscopy (ESI-MS), nuclear magnetic resonance (NMR), and ultraviolet-visible spectroscopy (UV-Vis). The total structure of the clusters, as determined by X-ray single-crystal diffraction, describes the Ag₁₃@Ag₁₂Cu₄(dppf)₆(3-F-PhCC)₁₂Cl₆ core-shell structure of 1 and [Ag₂Cl(dppf)₂]⁺-dppf-[Ag₂Cl(dppf)₂]⁺ polymeric structure of 2. This work opens the door to employing dppfCuBH₄ as a functional reducing agent to discover many underlying metal nanoclusters and even other nanomaterials which feature ferrocene-groups.

Carbene-stabilized enantiopure heterometallic clusters featuring EQE of 20.8% in circularly-polarized OLED

Bright and efficient chiral coinage metal clusters show promise for use in emerging circularly polarized light-emitting materials and diodes. To date, highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs) with enantiopure metal clusters have not been reported. Herein, through rational design of a multidentate chiral N-heterocyclic carbene (NHC) ligand and a modular building strategy, we synthesize a series of enantiopure Au(I)-Cu(I) clusters with exceptional stability. Modulation of the ligands stabilize the chiral excited states of clusters to allow thermally activated delayed fluorescence, resulting in the highest orange-red photoluminescence quantum yields over 93.0% in the solid state, which is accompanied by circularly polarized luminescence. Based on the solution process, a prototypical orange-red CP-OLED with a considerably high external quantum efficiency of 20.8% is prepared. These results demonstrate the extensive designability of chiral NHC ligands to stabilize polymetallic clusters for high performance in chiroptical applications.

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.

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

Generating sustainable fuel from sunlight plays an important role in meeting the energy demands of the modern age. Herein, we report two-coordinate carbene-metal-amide (cMa, M = Cu(I) and Au(I)) complexes that can be used as sensitizers to promote the light-driven reduction of water to hydrogen. The cMa complexes studied here absorb visible photons (ε_vis > 10³ M^-1 cm^-1), maintain long excited-state lifetimes (τ ∼ 0.2-1 μs), and perform stable photoinduced charge transfer to a target substrate with high photoreducing potential (E^+/* up to -2.33 V vs Fc^+/0 based on a Rehm-Weller analysis). We pair these coinage metal complexes with a cobalt-glyoxime electrocatalyst to photocatalytically generate hydrogen and compare the performance of the copper- and gold-based cMa complexes. We also find that the two-coordinate complexes herein can perform photodriven hydrogen production from water without the addition of the cobalt-glyoxime electrocatalyst. In this "catalyst-free" system, the cMa sensitizer partially decomposes to give metal nanoparticles that catalyze water reduction. This work identifies two-coordinate coinage metal complexes as promising abundant metal, solar fuel photosensitizers that offer exceptional tunability and photoredox properties.

Size-Controlled Synthesis of Luminescent Few-Atom Silver Clusters via Electron Transfer in Isostructural Redox-Active Porous Ionic Crystals

Ag clusters with a controlled number of atoms have received significant interest because they show size-dependent catalytic, optical, electronic, or magnetic properties. However, the synthesis of size-controlled, ligand-free, and air-stable Ag clusters with high yields has not been well-established. Herein, it is shown that isostructural porous ionic crystals (PICs) with redox-active polyoxometalates (POMs) can be used to synthesize Ag clusters via electron transfer from POMs to Ag⁺ . Ag clusters with average numbers of three, four, or six atoms emitting blue, green, or red colors, respectively, are formed and stabilized in the PICs under ambient conditions without any protecting ligands. The cluster size solely correlates with the degree of electron transfer, which is controlled by the reduction time and types of ions or elements of the PICs. Thus, advantages have been taken of POMs as electron sources and PICs as scaffolds to demonstrate a convenient method to obtain few-atom Ag clusters.

Secondary hierarchical complexity in double-stranded cluster helicates covered by NNNNN type pincer ligands

We designed and synthesized a new tripyridine dipyrrolide pincer ligand, which could be doubly deprotonated to provide five-nitrogen-donor sites and then utilized to prepare a subnanometric chiral silver cluster. The cluster belongs to an S₄ point group and shows a double-stranded helicate. DFT calculations were performed to analyze the electronic structure of the cluster. Interestingly, through hierarchical intercluster interactions, the cluster helicates evolve into complex secondary structures including a right-handed helix and a folded sheet, both of which are reminiscent of secondary structures of proteins, i.e., an α-helix and an antiparallel β-sheet.