Construction of an Au12Cd2 nanocluster with circularly polarized luminescence by a metal- and ligand-exchange strategy.

Excellent luminescence properties and unique chiral structures enable nanoclusters to be a novel class of circularly polarized luminescence (CPL) materials, and their precise structures facilitate the clarification of structure-activity relationships. However, efficiently preparing nanoclusters with CPL properties is still a great challenge. In this work, the luminescent properties as well as the molecular symmetry were simultaneously manipulated to transform the centrosymmetric Au14Cd1 into a chiral Au12Cd2 nanocluster, which has CPL properties. In detail, Cd doping and chiral-ligand exchange were performed simultaneously on the Au14Cd1 nanocluster to realize its photoluminescence enhancement and chiral-framework construction by increasing the alloying degree which is defined as deep-alloying and chiral ligand induction at the same time, resulting in the formation of an Au12Cd2 nanocluster with CPL properties. Further investigations revealed an increased alloying degree in the structure-maintained M6 kernel of Au12Cd2, which results in a 15-fold enhancement in quantum yield.

[Au14(2-SAdm)9(Dppe)2]+: a gold nanocluster with a crystallization-induced emission enhancement phenomenon.

In this work, a gold nanocluster [Au14(2-SAdm)9(Dppe)2]+ was synthesized and structurally determined by X-ray crystallography. The crystals of this cluster exhibit a 50-fold enhancement in quantum yield (5.05% for crystals) compared with its solution. Crystallographic analysis reveals that the weak intermolecular interactions (C-H⋯π, π⋯π) can inhibit the molecular vibration and thus generate the crystallization-induced emission enhancement phenomenon.

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.

Motif-to-Core Nucleation in a Decahedral Evolution Pattern.

The atomic precision of ultrasmall metal nanoclusters has opened the door to elucidating the structural evolution principles of metal nanomaterials at the molecular level. Here, we report a novel set of super-atomic Ag clusters, including [Ag19(TBBT)16(DPPP)4]+ (Ag19), [Ag22(DMAT)8(DPPM)4Cl8]2+ (Ag22), Ag26(SPh3,5-CF3)15(DPPF)4Cl5 (Ag26), and [Ag30(DMAT)12(DPPP)4Cl8]2+ (Ag30). The core structures of these clusters correspond to one decahedral Ag7, perpendicular bi-decahedrons, three-dimensional penta-decahedrons, and hexa-decahedrons, respectively. The Ag atoms in AgS2 blocks show a strong correlation with the decahedral cores: the five equatorial Ag atoms in the decahedral Ag7 core of Ag19 all adopt the AgS2 coordination, while the Ag atoms in AgS2 blocks of Ag22, Ag26, and Ag30 unexceptionally constitute additional decahedral structures with the core Ag atoms. Specifically, two and four core Ag atoms of Ag26 and Ag30 clusters occupy positions that highly resemble that of Ag (in AgS2 motifs) of Ag22. The strong structural correlation demonstrates the motif-to-core evolution of the surface Ag (on AgS2) to build extra-decahedral blocks. Density functional theory calculations indicate that the 2e, 4e, 6e, and 8e clusters (from Ag19 to Ag30) adopt 1S2, 1S21P2, 1S21P4, and 1S21P6 electron configurations, all of which feature excellent super-atomic characters.

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

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

Cd Atom Goes into the Interior of Cluster Induced by Directional Consecutive Assembly of Tetrahedral Units on an Icosahedron Kernel.

"Core sliding" in metal nanoclusters drives the reconstruction of external structural units and provides an ideal platform for mapping their precise transformation mechanism and evolution pathway. However, observing the movement behavior of metal atoms in experiments is still challenging because of the uncertain stability of intermediates. In this work, a series of Au-Cd alloy nanoclusters with continuously assembled kernels (one icosahedral building block assembled with 0 to 3 tetrahedral units) were constructed. As the assembly continued, it eventually led to the Cd atom doping into the inner positions of the clusters. Importantly, the Cd doped into the interior of the cluster exhibits a different behavior than the surface or external Cd atoms (dispersion doping vs localized occupy), which provides experimental evidence of the sliding behavior in the nanocluster kernel. Furthermore, density functional theory (DFT) calculations reveal that this sliding behavior in the inner sites of nanoclusters is an energetically favorable process. In addition, these Au-Cd nanoclusters exhibit tunable optical properties with different assembly patterns in their kernels.

H-bond-induced luminescence enhancement in a Pt1Ag30 nanocluster and its application in methanol detection.

Hydrogen bonding is an important type of interaction for constructing nanocluster assemblies. In this study, the role of hydrogen bonding interactions in regulating the fluorescence properties of nanoclusters is investigated. A [Pt1Ag30(SAdm)14(Bdpm)4Cl5]3+ (Pt1Ag30 for short) nanocluster containing hydrogen-accepting ligands is synthesized and its structure is determined. By introducing N-containing ligands into nanoclusters, hydrogen bonding interactions between nanoclusters and polar solvents can be established, which can result in a 35-fold enhancement in the fluorescence intensity (in MeOH vs. in DCM). A series of experiments are designed to demonstrate hydrogen bonding interactions between N atoms in the Pt1Ag30 cluster and H in the polar solvent and the results show that fluorescence enhancement is derived from the proton-coupled/uncoupled electron transfer between hydrogen bonds. Furthermore, this Pt1Ag30 is used for the naked-eye detection of MeOH on indicator paper.

Insights into mechanisms of diphosphine-mediated controlled surface construction on Au nanoclusters.

Unraveling the rules governing the size regulation of nanoclusters is of great importance not only in fundamental research, but also in practical applications because of the high structure-property correlation in nanoclusters. Diphosphine-mediated size tailoring is recognized as a powerful method for modulating the size, configuration, and properties of nanoclusters, but the role of diphosphines in these size-controlled processes is still poorly understood due to a lack of systematic studies. Herein, using Au23(SR)16- as the template for modification, the factors influencing the size-modulation of nanoclusters by diphosphines were systematically investigated. It is revealed that by controlling the length of the diphosphines (from shorter to longer), Au21(SR)12L2+ (L = diphosphine) and Au22(SR)14L can be produced. Moreover, introducing a rigid group into the diphosphines can twist the structural framework or lead to the formation of a new surface motif configuration in the nanoclusters, forming twisted Au22(SR)14L and Au25(SR)16L2+. The size regulation of these nanoclusters enables fine-tuning of the optical properties, including the absorption wavelengths and photoluminescence emission intensity, affording an avenue for precise control of the physicochemical properties of nanoclusters for practical applications.

Insight into the Effects of Chiral Diphosphine Ligands on the Structure and Optical Properties of the Au24Cd2 Nanocluster.

Introduction of chiral ligands has been regarded as an effective strategy to obtain nanoclusters with optical purity. However, how the chiral ligands work is still unclear due to the lack of structural comparison between racemic nanoclusters and the corresponding optically active ones. In this work, three structurally related Au24Cd2 nanoclusters, including one racemic and two homochiral nanoclusters, were synthesized, and their crystal structures were characterized using single-crystal X-ray crystallography (SC-XRD). Based on their crystal structures, the origin of the chirality in Au24Cd2 was found to be the twist of the kernel and the chiral arrangement of the metal-ligand surface. Au24Cd2 protected with chiral ligands exhibits a more twisted kernel than the racemic one. Therefore, the chirality of chiral diphosphine was found to transfer from the ligands to the metal-ligand interface and then to the metal core, inducing its distortion to produce enhanced chirality. In addition, the optical properties including optical absorption and circular dichroism of these structurally related Au24Cd2 nanoclusters were compared.

A double helical 4H assembly pattern with secondary hierarchical complexity in an Ag70 nanocluster crystal.

The hierarchical assemblies of well-defined structural nanoclusters can help to better understand those of biologically important molecules such as DNA and proteins. Herein, we disclose the synthesis and characterization of a new silver nanocluster, that is Ag70(SR)42(PPh3)5 (Ag70-TPP). Directed by the ligands, Ag70-TPP nanoclusters undergo self-hierarchical assembly into a highly space-efficient complex secondary structure of a double helical 4H (DH4H) close packing pattern. The chirality of Ag70-TPP, and the van der Waals forces interactions between the ligands are believed to drive its DH4H arrangement, and the observed interlocking of the phosphine ligands of adjacent Ag70-TPP nanoclusters also contributed. Overall, this work has yielded important and unprecedented insights into the internal structure and crystallographic arrangement of nanoclusters.

Unraveling the Nucleation Process from a Au(I)-SR Complex to Transition-Size Nanoclusters.

Atomically precise noble metal nanoclusters provide a critical benchmark for the fundamental research of the origin of condensed matter because they retain the original state of the metal bonds. Also, knowledge about the transition from organometallic complexes to a nanoclusters is important for understanding the structural evolution of the nanoclusters, particularly their nucleation mechanism. Herein, three transition-size gold nanoclusters are prepared via a controlled diphosphine-mediated top-down routine. Starting from small-size nanoclusters, three new nanoclusters including Au13(SAdm)8(L4)2(BPh4) (Au13), Au14(S-c-C6H11)10L4 (Au14), and Au16(S-c-C6H11)11LPh* (Au16) are obtained by controlled clipping on the surface and kernel of initial nanoclusters. Combining their atomically precise structures with DFT theoretical calculations, the overall atom-by-atom structural evolution process from Au12(SR)12 (0 e-) to Au18(SR)14 (4 e-) is mapped out. In addition, studies on their electronic structures show that the evolution from an organometallic complex to nanoclusters is accompanied by a dramatic decrease in the HOMO-LUMO gaps. Most importantly, the formation of the first Au-Au bond is captured in the "Au4S4 to Au5" nucleation process from Au12(SR)12 complex to the Au13 nanocluster. This work provides a deep insight into the origin of inner core in Au NCs and their structural transition relationship with metal complexes.

Cu Doping-Induced Transformation from [Ag62 S12 (SBut )32 ]2+ to [Ag62-x Cux S12 (SBut )32 ]4+ Nanocluster.

The change in the valence state of nanocluster can induce remarkable changes in the properties and structure. However, achieving the valence state changes in nanoclusters is still a challenge. In this work, we use Cu2+ as dopant to "oxidize" [Ag62 S12 (SBut )32 ]2+ (4 free electrons) to obtain the new nanocluster: [Ag62-x Cux S12 (SBut )32 ]4+ with 2 free electrons. As revealed by its structure, the [Ag62-x Cux S12 (SBut )32 ]4+ (x=10∼21) has a similar structure to that of [Ag62 S12 (SBut )32 ]2+ precursor and all the Cu atoms occupy the surface site of nanocluster. It's worth noting that with the Cu atoms doping, the [Ag62-x Cux S12 (SBut )32 ]4+ nanocluster is more stable than [Ag62 S12 (SBut )32 ]2+ at higher temperature and in electrochemical cycle. This result has laid a foundation for the subsequent application and exploration. Overall, this work reveals crystals structure of a new Ag-Cu nanocluster and offers a new insight into the electron reduction/oxidation of nanocluster.

Chiral Inversion and Conservation of Clusters: A Case Study of Racemic Ag32Cu12 Nanocluster.

Chiral metal nanoclusters have been widely reported, but their separation and optical stabilization remain challenging. We used a deracemization strategy to accomplish the enantioseparation of a racemic mixture of [Ag32Cu12(CH3COO)12(SAdm)12(P(CH3OPh)3)4] (M44) in a yield exceeding 50%, forming two optically active [Ag32Cu12(R/S-Cl(CH3)CHCOO)12(SAdm)12(P(CH3OPh)3)4] (R/S-M44') enantiomers. The optical activity of these products was conserved after exchange of the chiral carboxyl ligands with achiral ligand (Br-), to give two additional optically active nanoclusters R/S-[Ag28Cu16Br12(SAdm)12(P(CH3OPh)3)4] (R/S(Br)-M44). The crystal structures of the above nanoclusters were determined by single-crystal X-ray crystallography. Based on these structures, the chiral transformation and conservation are mapped out.

Surface engineering of linearly fused Au13 units using diphosphine and Cd doping.

Herein, surface engineering was delicately performed to assemble two new Au-Cd alloy nanoclusters, including [Cd2Au17(S-c-C6H11)12(DPPP)2](BPh4) and Cd2Au29(TBBT)17(DPPF)2. Both the Au13 (in Cd2Au17) and Au25 (in Cd2Au29) cores were covered by two identical Au2Cd(SR)6 motifs and two diphosphine ligands. In addition, their optical properties were explored to give clues on the kernel- and surface-dependent electronic structures.

Multiple Ways Realizing Charge-State Transform in AuCu Bimetallic Nanoclusters with Atomic Precision.

Thiolate-protected nanoclusters with different charge states usually show similar structure frameworks but different electronic configurations, which are proved to dramatically affect their properties such as magnetism, photoluminescence, and catalytic activity. Until now, few nanoclusters with alterable charge states have been reported and only some of them are structurally solved, limiting the in-depth studies on their interesting properties. Here, a new AuCu alloy nanocluster [Au18 Cu32 (SPhCl)36 ]2- (HSPhCl = 4-chlorophenylthiophenol) is synthesized and structurally solved by X-ray crystallography. Interestingly, it is found that this nanocluster can be reduced to another nanocluster with a different charge state, that is, [Au18 Cu32 (SPhCl)36 ]3- . This change in charge states is clearly proved by X-ray crystallography, electrospray ionization mass spectrometry, thermogravimetric analysis, and electron paramagnetic resonance. Furthermore, several redox methods are carried out to realize the reversible interconversion between these two nanoclusters, including electrochemical redox, introduction of H2 O2 /NaBH4 , and oxidation with silica under air atmosphere. This work offers new insight into the transform progress of charge states with AuCu alloy nanoclusters which contributes to the understanding of the relationship between electronic structure and properties of nanoclusters and further development of AuCu nanoclusters with excellent performance.

Structure determination of a metastable Au22(SAdm)16 nanocluster and its spontaneous transformation into Au21(SAdm)15.

It is of great interest to investigate the evolution pattern of gold nanoclusters (Au NCs) due to its significance in understanding the growth mechanism and origin of Au NCs. Capture of metastable cluster intermediates is an effective way to meet this demand since they provide valuable information for understanding the conversion pathway of Au NCs. However, it is still challenging to obtain metastable Au NCs, especially thiol-protected Au NCs, and solve their structures. In this work, a metastable thiol-protected gold nanocluster, Au22(SAdm)16 (Au22), was synthesized and its structure was determined by single crystal X-ray diffraction. Au22 shows a close structure-evolution correlation with Au21(SAdm)15 (Au21). The symmetric Au10 kernel of Au21 is twisted by the insertion of an additional Au-SR unit on the motif during its structure evolution into Au22. The distortion in structures results in significantly distinguishing absorption and emission spectra between Au22 and Au21. Noteworthily, the structure correlation between Au22 and Au21 was also found experimentally that Au22 can spontaneously transform into Au21 due to the metastability of Au22 in solution. This size conversion process was monitored by time-dependent UV-vis spectroscopy and ESI-MS. Furthermore, the solvent effect on the size conversion process was also investigated. This transformation from Au22 to Au21 provides a unique platform for studies on the evolution pattern of gold nanoclusters at the single atom level.

Pressure-Induced Optical Transitions in Metal Nanoclusters.

Currently, a comprehensive understanding of the relationship between atomic structures and optical properties of ultrasmall metal nanoclusters with diameters between 1 and 3 nm is lacking. To address this challenge, it is necessary to develop tools for perturbing the atomic structure and modulating the optical properties of metal nanoclusters beyond what can be achieved using synthetic chemistry. Here, we present a systematic high-pressure study on a series of atomically precise ligand-protected metal nanoclusters. A diamond anvil cell is used as a high-pressure chamber to gradually compress the metal nanoclusters, while their optical properties are monitored in situ. Our experimental results show that the photoluminescence (PL) of these nanoclusters is enhanced by up to 2 orders of magnitude at pressures up to 7 GPa. The absorption onset red-shifts with increasing pressure up to ∼12 GPa. Density functional theory calculations reveal that the red-shift arises because of narrowing of the spacing between discrete energy levels of the cluster due to delocalization of the core electrons to the carbon ligands. The pressure-induced PL enhancement is ascribed to (i) the enhancement of the near-band-edge transition strength, (ii) suppression of the nonradiative vibrations, and (iii) hindrance of the excited-state structural distortions. Overall, our results demonstrate that high pressure is an effective tool for modulating the optical properties and improving the luminescence brightness of metal nanoclusters. The insights into structure-property relations obtained here also contribute to the rational design of metal nanoclusters for various optical applications.

A novel geometric structure of a nanocluster with an irregular kernel: Ag30Cu14(TPP)4(SR)28.

Here, we report a bi-ligand protected bimetallic nanocluster Ag30Cu14(TPP)4(SR)28. It is composed of an Ag27 kernel and Ag3Cu14(TPP)4(SR)28 shell. Typically, the metal atoms are arranged irregularly. Both the core and shell exhibit the characteristics of C2-symmetry.

All-thiolate-stabilized Ag42 nanocluster with a tetrahedral kernel and its transformation to an Ag61 nanocluster with a bi-tetrahedral kernel.

A new silver nanocluster, formulated as Ag42(SBut)24, was prepared by a NaSbF6-mediated two-phase ligand exchange method. The total structure adopts a quasi-tetrahedral symmetry with a three-layered tetrahedral (Td) Ag10 kernel. Furthermore, with the ligand-etching method, Ag42(SBut)24 can be transformed into Ag61(SC6H11)40Cl, whose kernel structure corresponds to a face-fused bi-tetrahedral Ag14.

Structural distortion and electron redistribution in dual-emitting gold nanoclusters.

Deciphering the complicated excited-state process is critical for the development of luminescent materials with controllable emissions in different applications. Here we report the emergence of a photo-induced structural distortion accompanied by an electron redistribution in a series of gold nanoclusters. Such unexpected slow process of excited-state transformation results in near-infrared dual emission with extended photoluminescent lifetime. We demonstrate that this dual emission exhibits highly sensitive and ratiometric response to solvent polarity, viscosity, temperature and pressure. Thus, a versatile luminescent nano-sensor for multiple environmental parameters is developed based on this strategy. Furthermore, we fully unravel the atomic-scale structural origin of this unexpected excited-state transformation, and demonstrate control over the transition dynamics by tailoring the bi-tetrahedral core structures of gold nanoclusters. Overall, this work provides a substantial advance in the excited-state physical chemistry of luminescent nanoclusters and a general strategy for the rational design of next-generation nano-probes, sensors and switches.