Optical Activity from Anisotropic-Nanocluster-Assembled Supercrystals in Achiral Crystallographic Point Groups.

Accomplishing optical activity in achiral materials has long been a challenge. Achiral nanomaterials that crystallize in achiral point groups are generally optically inactive. Herein we report the surprising observation of optical activity in several achiral point groups for supercrystals assembled from anisotropic metal nanoclusters with atomic precision. By analyzing multiple achiral nanoclusters with different molecular structures and symmetry space groups, we have identified that the molecular anisotropy of nanocluster entities and their asymmetric arrangement in point groups of supercrystals are the two key factors for the realization of optical activity in such supercrystals. We have further exploited the polarization effect of the nanocluster supercrystals as a polarization switch that can alter the polarized state of the linearly polarized light. Our findings have broadened the fundamental principles for producing nanomaterial-based optical activity and devices with polarization effects.

Au11Ag6 nanocluster: Controllable preparation, structural determination, and optical property investigation.

The structure/composition of nanoclusters has a decisive influence on their physicochemical properties. In this work, we obtained two different Au-Ag nanoclusters, [Au9Ag12(SAdm)4(dppm)6Cl6]3+ and Au11Ag6(dppm)4(SAdm)4(CN)4, via controlling the Au/Ag molar ratios by a one-pot synthetic approach. The structure of nanoclusters was confirmed and testified by single-crystal x-ray diffraction, electrospray ionization time-of-flight mass spectrometry, XPS, powder x-ray diffraction, and electron paramagnetic resonance. The Au11Ag6 nanocluster possessed a M13 core caped by four Au atoms and four dppm and four AdmS ligands. Interestingly, four CN are observed to locate at the equator of the M13 core. Both nanoclusters contain a similar icosahedral M13 core, whereas their surface structures are totally different. However, the Au11Ag6 nanocluster exhibits good stability and strong red photoluminescence in solution.

Symmetry breaking of highly symmetrical nanoclusters for triggering highly optical activity.

Developing new approaches to fulfill the enantioseparation of nanocluster racemates and construct cluster-based nanomaterials with optical activity remains highly desired in cluster science, because it is an essential prerequisite for fundamental research and extensive applications of these nanomaterials. We herein propose a strategy termed "active-site exposing and partly re-protecting" to trigger the symmetry breaking of highly symmetrical nanoclusters and to render cluster crystals optically active. The vertex PPh3 of the symmetrical Ag29(SSR)12(PPh3)4 (SSR = 1, 3-benzenedithiol) nanocluster was firstly dissociated in the presence of counterions with large steric hindrance, and then the exposed Ag active sites of the obtained Ag29(SSR)12 nanocluster were partly re-protected by Ag+, yielding an Ag29(SSR)12-Ag2 nanocluster with a symmetry-breaking construction. Ag29(SSR)12-Ag2 followed a chiral crystallization mode, and its crystal displayed strong optical activity, derived from CD and CPL characterizations. Overall, this work presents a new approach (i.e., active-site exposing and partly re-protecting) for the symmetry breaking of highly symmetrical nanoclusters, the enantioseparation of nanocluster racemates, and the achievement of highly optical activity.

An insight, at the atomic level, into the intramolecular metallophilic interaction in nanoclusters.

The intermolecular metallophilic interaction has been exploited to orderly aggregate nanocluster compounds into multidimensional assemblies, while the intramolecular metallophilic interaction was rarely reported. Herein, based on an Au13Cu2 nanocluster template, the presence of the intracluster Au-Cu metallophilic interaction was beneficial to achieving enhanced near-infrared emission intensity and thermal stability.

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.

Atomic structure of a seed-sized gold nanoprism.

The growth of nanoparticles along one or two directions leads to anisotropic nanoparticles, but the nucleation (i.e., the formation of small seeds of specific shape) has long been elusive. Here, we show the total structure of a seed-sized Au56 nanoprism, in which the side Au{100} facets are surrounded by bridging thiolates, whereas the top/bottom {111} facets are capped by phosphine ligands at the corners and Br- at the center. The bromide has been proved to be the key to effectively stabilize the Au{111} to fulfill a complete face-centered-cubic core. In femtosecond electron dynamics analysis, the non-evolution of transient absorption spectra of Au56 is similar to that of larger-sized gold nanoclusters (n > 100), which is ascribed to the completeness of the prismatic Au56 core and an effective electron relaxation pathway created by the stronger Au-Au bonds inside. This work provides some insights for the understanding of plasmonic nanoprism formation.

Reversible transformation between Au14Ag8 and Au14Ag4 nanoclusters.

Although several approaches have been exploited to trigger the structural transformation of metal nanoclusters, most cases are assigned to the unidirectional conversion, while the reversible conversion of nanoclusters remains challenging. In this work, the reversible conversion between two Au-Ag alloy nanoclusters, Au14Ag8(Dppm)6(CN)4Cl4 and Au14Ag4(Dppm)6Cl4, has been accomplished, which was tracked by UV-vis and ESI-MS spectroscopy. The condition of the nanocluster reversible conversion has been meticulously mapped out. Our results provide some new insights into the cluster transformation, which will benefit the future preparation of metalloid clusters with customized structures and properties.

The alloying-induced electrical conductivity of metal-chalcogenolate nanowires.

Alloying is one of the most effective strategies to change the properties of inorganic-organic hybrid materials, but there are few reports of the alloying of one-dimensional nanowires with precise atomic structure due to the difficulties in obtaining the single crystals of nanowires themselves. Herein, we describe the synthesis and characterization of an alloyed one-dimensional Ag-Cu nanowire [Ag2.5Cu1.5(S-Adm)4]n. Compared with the unalloyed [Ag4(S-Adm)4]n, our novel alloyed nanowire exhibits good conductivity, and its resistivity (as a powder) was determined to be 107 Ω m by impedance analysis-consistent with that of a semiconductor. Accordingly, based on these properties combined with its excellent thermal stability and high-yielding, gram-scale synthesis, [Ag2.5Cu1.5(S-Adm)4]n is proposed for electronic-device applications.

Atomically Precise Copper Cluster with Intensely Near-Infrared Luminescence and Its Mechanism.

In this work, a new Cu(I) cluster is synthesized and structurally characterized: [Cu11(TBBT)9(PPh3)6](SbF6)2 (where TBBT = 4-tert-butylbenzenethiol). This Cu(I) cluster exhibits good stability and a bright-red emission both in solution (685 nm) and in the solid state (675 nm) with a large Stokes shift (∼280 nm) under ambient conditions. Its absolute quantum yield is 0.22 in the solid state. Temperature-dependent emissions and theoretical calculations suggest that the origin of this cluster luminescence mainly results from a mixture of the metal-ligand charge transfer and the cluster-centered triplet excited states. This work not only opens new opportunities for functional applications of copper clusters but also sheds light on the structure-luminescence relationship.

Atomically resolved Au52Cu72(SR)55 nanoalloy reveals Marks decahedron truncation and Penrose tiling surface.

Gold-copper alloys have rich forms. Here we report an atomically resolved [Au52Cu72(p-MBT)55]+Cl- nanoalloy (p-MBT = SPh-p-CH3). This nanoalloy exhibits unusual structural patterns. First, two Cu atoms are located in the inner 7-atom decahedral kernel (M7, M = Au/Cu). The M7 kernel is then enclosed by a second shell of homogold (Au47), giving rise to a two-shelled M54 (i.e. Au52Cu2) full decahedron. A comparison of the non-truncated M54 decahedron with the truncated homogold Au49 kernel in similar-sized gold nanoparticles provides for the first time an explanation for Marks decahedron truncation. Second, a Cu70(SR)55 exterior cage resembling a 3D Penrose tiling protects the M54 decahedral kernel. Compared to the discrete staple motifs in gold:thiolate nanoparticles, the Cu-thiolate surface of Au52Cu72 forms an extended cage. The Cu-SR Penrose tiling retains the M54 kernel's high symmetry (D5h). Third, interparticle interactions in the assembly are closely related to the symmetry of the particle, and a "quadruple-gear-like" interlocking pattern is observed.

Sub-nanometer Cu(i) clusters: coordination-modulated (Se vs. S) atom-packing mode and emission.

Herein, the ligands' effect in Cu(i) clusters was initially explored. The results demonstrate that the Se atom possesses more coordination modes with Cu (µ2, µ3, µ4, µ6) than S, which significantly modulates the atom-packing mode of Cu(i) clusters. Importantly, this also endows these clusters with different temperature-dependent luminescent behaviours.

Insights into the effect of surface coordination on the structure and properties of Au13Cu2 nanoclusters.

Comparable systems are of great significance for understanding the structure-property relationship. Herein, a new Au13Cu2 nanocluster protected by phenylethanethiol (PET) and triphenylphosphine (TPP) is synthesized and structurally determined, including an icosahedral Au13 and two CuS3 configurations. Based on previous work, a comparable system was formed-only the surface coordination of Cu atoms changes from Cu-N to Cu-S, which results in a tremendous change in the optical properties. Based on this, the effect of the coordination mode on the structure and optical properties was primarily investigated in both experiment and theory. And the results demonstrate that changing the coordination mode from Cu-N to Cu-S has a significant effect on the electronic structure. This work will offer new insights into ligand engineering.