Anindita Das

Monoplatinum Doping of Gold Nanoclusters and Catalytic Application

We report single-atom doping of gold nanoclusters (NCs), and its drastic effects on the optical, electronic, and catalytic properties, using the 25-atom system as a model. In our synthetic approach, a mixture of Pt(1)Au(24)(SC(2)H(4)Ph)(18) and Au(25)(SC(2)H(4)Ph)(18) was produced via a size-focusing process, and then Pt(1)Au(24)(SC(2)H(4)Ph)(18) NCs were obtained by selective decomposition of Au(25)(SC(2)H(4)Ph)(18) in the mixture with concentrated H(2)O(2) followed by purification via size-exclusion chromatography. Experimental and theoretical analyses confirmed that Pt(1)Au(24)(SC(2)H(4)Ph)(18) possesses a Pt-centered icosahedral core capped by six Au(2)(SC(2)H(4)Ph)(3) staples. The Pt(1)Au(24)(SC(2)H(4)Ph)(18) cluster exhibits greatly enhanced stability and catalytic activity relative to Au(25)(SC(2)H(4)Ph)(18) but a smaller energy gap (E(g) ≈ 0.8 eV vs 1.3 eV for the homogold cluster).

A 200-fold Quantum Yield Boost in the Photoluminescence of Silver-Doped AgxAu25−xNanoclusters: The 13 th Silver Atom Matters

The rod-shaped Au25 nanocluster possesses a low photoluminescence quantum yield (QY=0.1%) and hence is not of practical use in bioimaging and related applications. Herein, we show that substituting silver atoms for gold in the 25-atom matrix can drastically enhance the photoluminescence. The obtained Ag(x)Au(25-x) (x=1-13) nanoclusters exhibit high quantum yield (QY=40.1%), which is in striking contrast with the normally weakly luminescent Ag(x)Au(25-x) species (x=1-12, QY=0.21%). X-ray crystallography further determines the substitution sites of Ag atoms in the Ag(x)Au(25-x) cluster through partial occupancy analysis, which provides further insight into the mechanism of photoluminescence enhancement.

Nonsuperatomic [Au23(SC6H11)16]− Nanocluster Featuring Bipyramidal Au15 Kernel and Trimeric Au3(SR)4 Motif

We report the X-ray structure of a cyclohexanethiolate-capped [Au23(SR)16](-) nanocluster (counterion: tetraoctylammonium, TOA(+)). The structure comprises a cuboctahedron-based bipyramidal Au15 kernel, which is protected by two staple-like trimeric Au3(SR)4 motifs, two monomeric Au(SR)2 and four plain bridging SR ligands. Electronic structure analysis reveals nonsuperatomic feature of [Au23(SR)16](-) and confirms the Au15 kernel and surface motifs. The Au15 kernel and trimeric staple motif are unprecedented and offer new insight in understanding the structure evolution of gold nanoclusters.

Total structure and optical properties of a phosphine/thiolate-protected Au24 nanocluster.

We report the synthesis and total structure determination of a Au(24) nanocluster protected by mixed ligands of phosphine and thiolate. Single crystal X-ray crystallography and electrospray ionization mass spectrometry (ESI-MS) unequivocally determined the cluster formula to be [Au(24)(PPh(3))(10)(SC(2)H(4)Ph)(5)X(2)](+), where X = Cl and/or Br. The structure consists of two incomplete (i.e., one vertex missing) icosahedral Au(12) units joined by five thiolate linkages. This structure shows interesting differences from the previously reported vertex-sharing biicosahedral [Au(25)(PPh(3))(10)(SC(2)H(4)Ph)(5)X(2)](2+) nanocluster protected by the same type and number of phosphine and thiolate ligands. The optical absorption spectrum of Au(24) nanocluster was theoretically reproduced and interpreted.

Structure Determination of [Au18(SR)14]

Unravelling the atomic structures of small gold clusters is the key to understanding the origin of metallic bonds and the nucleation of clusters from organometallic precursors. Herein we report the X-ray crystal structure of a charge-neutral [Au18(SC6H11)14] cluster. This structure exhibits an unprecedented bi-octahedral (or hexagonal close packing) Au9 kernel protected by staple-like motifs including one tetramer, one dimer, and three monomers. Until the present, the [Au18(SC6H11)14] cluster is the smallest crystallographically characterized gold cluster protected by thiolates and provides important insight into the structural evolution with size. Theoretical calculations indicate charge transfer from surface to kernel for the HOMO-LUMO transition.

Cyclopentanethiolato-protected Au36(SC5H9)24 nanocluster: crystal structure and implications for the steric and electronic effects of ligand.

Thiolato-protected gold nanoclusters have acquired wide applications; however, on the fundamental science end there is still a lack of deep understanding of their high stability. Recent success in transforming the highly robust biicosahedral Au38(SCH2CH2Ph)24 nanocluster into an extremely stable tetrahedral Au36(SPh-(t)Bu)24 nanocluster raises an important question: Is the transformation due to the bulkiness effect of SPh-(t)Bu or the electronic conjugation effect of the aromatic ligand as opposed to the nonaromatic SCH2CH2Ph? Toward this goal, we report our success in the crystallization of a nonaromatic thiolato-protected Au36(SC5H9)24 nanocluster (where, SC5H9 = cyclopentanethiolato). Comparison of Au36(SC5H9)24 with the aromatic thiolato-protected Au36(SPh-(t)Bu)24 nanocluster rules out the thought that the face-centered cubic, tetrahedral structure of Au36(SPh-(t)Bu)24 is dictated by the aromatic ligand; it also reveals that the electronic conjugation effect in aromatic ligand makes the S-C bond shorter and stronger, and this affects the S-Au bonds, resulting in a larger separation between the staple motifs and the inner Au28 kernel. Overall, this work sheds some light on the major question of the specific roles of thiol ligand in determining the cluster size and structure.

Tri-icosahedral Gold Nanocluster [Au37(PPh3)10(SC2H4Ph)10X2](+): Linear Assembly of Icosahedral Building Blocks.

The [Au37(PPh3)10(SR)10X2](+) nanocluster (where SR = thiolate and X = Cl/Br) was theoretically predicted in 2007, but since then, there has been no experimental success in the synthesis and structure determination. Herein, we report a kinetically controlled, selective synthesis of [Au37(PPh3)10(SC2H4Ph)10X2](+) (counterion: Cl(-) or Br(-)) with its crystal structure characterized by X-ray crystallography. This nanocluster shows a rod-like structure assembled from three icosahedral Au13 units in a linear fashion, consistent with the earlier prediction. The optical absorption and the electrochemical and catalytic properties are investigated. The successful synthesis of this new nanocluster allows us to gain insight into the size, structure, and property evolution of gold nanoclusters that are based upon the assembly of icosahedral units (i.e., cluster of clusters). Some interesting trends are identified in the evolution from the monoicosahedral [Au13(PPh3)10X2](3+) to the bi-icosahedral [Au25(PPh3)10(SC2H4Ph)5X2](2+) and to the tri-icosahedral [Au37(PPh3)10(SC2H4Ph)10X2](+) nanocluster, which also points to the possibility of achieving even longer rod nanoclusters based upon assembly of icosahedral building blocks.

Comparison of the Catalytic Properties of 25-Atom Gold Nanospheres and Nanorods

The catalytic properties of two nanocluster catalysts with atomically precisely known structures, icosahedral two-shelled Au25(SC2H4Ph)18 nanospheres and biicosahedral Au25(PPh3)10(SC2H4Ph)5Cl2 nanorods, were compared. Their catalytic performance in the two reactions of the selective oxidation of styrene and chemoselective hydrogenation of α,β-unsaturated benzalacetone was investigated. The catalytic activities of icosahedral Au25(SC2H4Ph)18 nanospheres were superior to those of the bi-icosahedral Au25(PPh3)10(SC2H4Ph)5Cl2 nanorods for both reactions. The better catalytic performance of the Au25(SC2H4Ph)18 nanospheres can be attributed to their unique core-shell (Au13/Au12) geometric structure that has an open exterior atomic shell and to their electronic structure with an electron-rich Au13 core and an electron-deficient Au12 shell.

Highly efficient three-component coupling reaction catalysed by atomically precise ligand-protected Au38(SC2H4Ph)24 nanoclusters

The catalytic potential of atomically precise quantum-sized gold nanoclusters (Au38(SC2H4Ph)24) is explored for the three-component coupling of an aldehyde, an alkyne and an amine to synthesize propargylamines. A high catalytic efficiency with a very low loading (0.01 mol%) is achieved. Furthermore, the synergistic effect of the electron-deficient surface (i.e. Auδ+, 0 < δ+ < 1) and the electron-rich Au23 core of the ligand-protected nanoclusters is critical for this catalytic reaction.

Hydrogen‐Bond‐Regulated Distinct Functional‐Group Display at the Inner and Outer Wall of Vesicles

A unique supramolecular strategy enables the unidirectional assembly of two bola-shaped unsymmetric π-amphiphiles, NDI-1 and NDI-2, which feature a naphthalene-diimide chromophore connected to nonionic and anionic head groups on opposite arms. The amphiphiles differ only in the location of a hydrazide group, which is placed either on the nonionic or on the anionic arm of NDI-1 and NDI-2, respectively. The formation of hydrogen bonds between the hydrazides, which compensates for electrostatic and steric factors, promotes unidirectional alignment and the formation of monolayer vesicles. The zeta potentials and cation-assisted quantitative precipitation reveal negatively charged and nonionic outer surfaces for NDI-1 and NDI-2, respectively, indicating that hydrogen bonding also dictates the directionality of the monolayer curvature, ensuring that in both cases, the hydrazides remain at the inner wall to benefit from stronger hydrogen bonding where they are in closer proximity. This is reflected in their different abilities to inhibit α-chymotrypsin, which possesses a positively charged surface: NDI-1 induced an inhibition of 80% whereas hardly any inhibition was observed with NDI-2.