Unique [core+two] structure and optical property of a dodeca-ligated undecagold cluster : critical contribution of the exo gold atoms to the electronic structure

A novel dodeca-ligated undecagold cluster having a nonagold core plus two exo-attached gold atoms was synthesized. Unlike the conventional icosahedron-based "core-only" isomer, the [9+2] cluster showed an intense visible absorption band. Theoretical calculation showed the involvement of the exo gold atoms in the generation of the unique electronic structure

Facile synthesis and optical properties of magic-number Au13 clusters

Synthesis of molecular gold clusters through the post-synthetic scheme involving HCl-promoted nuclearity convergence was examined with various phosphine ligands. Systematic studies with a series of bis(diphenylphosphino) ligands (Ph2P-(CH2)m-PPh2) using electrospray ionization mass spectrometry (ESI-MS) and electronic absorption spectroscopy demonstrated that the use of dppp (m = 3), dppb (m = 4) and dpppe (m = 5) as the ligand resulted in the formation of [Au13P8Cl4]+ type clusters, whereas [Au13P10Cl2]^[3+] type cluster was formed with dppe (m = 2). The cluster species did not survive the HCl treatment step when monophosphines PPh3, PMe2Ph, and POct3 were employed, but [Au13(POct3)8Cl4]+ was isolated as a minor product in the NaBH4 reduction of Au(POct3)Cl in aqueous THF. Electronic absorption and photoluminescence studies of a series of Au13 clusters revealed that their optical properties are highly dependent on the phosphine/chloride composition ratio, but are far less so on the phosphine structure

Electronic Properties of [Core+<i>exo</i>]‑type Gold Clusters: Factors Affecting the Unique Optical Transitions

Unusual visible absorption properties of [core+<i>exo</i>]-type Au₆ (<b>1</b>), Au₈ (<b>2</b>), and Au₁₁ (<b>3</b>) clusters were studied from experimental and theoretical aspects, based on previously determined crystal structures. Unlike conventional core-only clusters having no <i>exo</i> gold atoms, these nonspherical clusters all showed an isolated visible absorption band in solution. Density functional theory (DFT) studies on corresponding nonphenyl models (<b>1</b>′–<b>3</b>′) revealed that they had similar electronic structures with discrete highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) bands. The theoretical spectra generated by time-dependent DFT (TD-DFT) calculations agreed well with the experimentally measured properties of <b>1</b>–<b>3</b>, allowing assignment of the characteristic visible bands to HOMO–LUMO transitions. The calculated HOMO–LUMO transition energies increased in the order Au₁₁ < Au₆ < Au₈, as was found experimentally. Frontier orbital analyses indicated that the HOMO and LUMO were both found in proximity to the terminal Au₃ triangles containing the <i>exo</i> gold atom, with the HOMO → LUMO transition occurring in the core → <i>exo</i> direction. The HOMO/LUMO distribution patterns of <b>1</b>′ and <b>3</b>′ were similar to each other but were markedly different from that of <b>2</b>′, which has longer core-to-<i>exo</i> distances. These findings showed that not only nuclearity (size) but also geometric structures have profound effects on electronic properties and optical transitions of the [core+<i>exo</i>]-type clusters

Cluster-pi electronic interaction in a superatomic Au-13 cluster bearing sigma-bonded acetylide ligands

An organometallic Au-13 cluster having two sigma-bonded acetylide ligands was synthesized and its structure was determined by X-ray crystallography. Absorption spectral studies indicated the presence of electronic coupling between the superatomic Au-13 core and the acetylide pi-orbitals, which was supported by theoretical considerations

Chiroptical activity of Au-13 clusters : experimental and theoretical understanding of the origin of helical charge movements

Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au-13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au-13 clusters, we show that these surface ligands slightly twist the Au-13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au-13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au-5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores

Terahertz Raman Spectroscopy of Ligand-Protected Au8 Clusters

For ligand-protected gold clusters, geometrical differences of gold cores and/or the presence of secondary gold core–ligand interactions influence their unique optical and electronic properties and can, in principle, be detected by spectral changes of gold core vibrations (phonon modes) in ultralow-frequency Raman spectroscopy. We report experimental and theoretical Raman spectra of Au8 clusters protected by phosphine ligands particularly in the “gold cluster fingerprint” region from 50 to 150 cm–¹ Raman shift (1.5 to 4.5 terahertz, THz). A characteristic core breathing mode observed at ca. 123 cm–¹ was sensitive to differences of core geometries. A new band was found at ca. 150 cm–¹, originating from a local strain on a polyhedral gold core caused by weak Au···π interactions. THz Raman spectroscopy can be utilized for metal nanoclusters to visualize core structural changes and Au···π interactions, which cannot be captured by single crystal X-ray analysis

Hexanuclear Platinum(II) Thiolate Macrocyclic Host: Charge-Transfer-Driven Inclusion of a Ag^I Ion Guest

The inclusion of a Ag^I ion by a hexanuclear platinum­(II) thiolate macrocycle in solution was demonstrated, and the inclusion structure was determined by X-ray crystallography. Unique host–guest intermetallic interactions driven by charge transfer were elucidated by optical absorption spectroscopy and theoretical calculations