Ligand-protected Gold Clusters Research Articles

Symmetry breaking in ligand-protected gold clusters probed by nonlinear optics

The first hyperpolarizabilities of [Au25(SR)18](-1/0) and Au38(SR)24 clusters were determined by Hyper-Rayleigh Scattering. A strong dependence on the molecular symmetry was observed, and we explore two strategies to destroy the center of inversion in [Au25(SR)18](-1/0), protection by chiral ligands and alloying of the cluster with silver. This may open new avenues to applications of Au : SR clusters in second-order nonlinear optics.

Geometric Quantification of Chirality in Ligand-Protected Metal Clusters

Chirality has been found as a relevant property of nanomaterials, including ligand-protected metal clusters and nanorods. This property not only is crucial in nanotechnology developments related to asymmetric catalysis and chiroptical phenomena but also generates fundamental questions on the existence of chirality at the nanoscale. In fact, X-ray total structure determination, electron diffraction studies, NMR, and circular dichroism spectroscopies as well as theoretical calculations performed on gold clusters protected with thiolate or phosphine ligands have confirmed the existence of chiral structures in the size range of 18–144 Au atoms. In this work, we realize a comparative analysis of the degree or amount of chirality existing in chiral ligand-protected gold clusters (LPGC), through a geometric quantification, using the Hausdorff chirality measure (HCM). The calculated HCM values provide a quantitative framework to compare, classify, and gain insight into the origin of chirality. Interestingly, thes...

Ab initio phonon‐coupled nonadiabatic relaxation dynamics of [Au25(SH)18]− clusters

Nanoscale gold clusters exhibit unique electronic and optical properties. Here, we study the excited state dynamics and electron–phonon coupling of [Au25(SH)18]– nanoclusters by using quantum‐classical molecular dynamics combined with time‐dependent density‐functional theory. Our result reproduces the experimental measurements and provides further insights into the relaxation dynamics of ligand protected gold clusters. We examine the ultrafast electronic dynamics induced by specific phonon modes. The roles of the Au13 core and Au12 ring in the optical process are investigated. We find that the LUMO–HOMO transition is dominated by AuAu stretching of the Au13 core, whereas the LUMO + 1 − HOMO transition is induced by breathing vibrations of the Au core as well as AuS stretching. We clarify the interplay between electron–phonon couplings and specific energy‐transfer processes in [Au25(SH)18]– nanoclusters.

Charge Redistribution Effects on the UV–Vis Spectra of Small Ligated Gold Clusters: a Computational Study

We analyze and compare the UV–visible absorption profiles, computed at the TD-DFT level, of several thiolate, chloride, and phosphine ligand protected gold clusters with Au13, Au25, and Au28 metallic cores. We show by investigating several model clusters and experimentally reported ones that the charge redistribution between the gold core and the ligand shell can be controlled by the electron acceptor or electron donor character of the ligands and their spatial distribution around the metallic core. A correlation between the increase of electronic charge in the gold core and a shift of the optical absorption band toward lower transition energies is highlighted. The identified correlation between the partial charge on the metallic gold core and the position of the lowest electronic transition is robust and remains valid even for small clusters with little structural or electronic similarity.

Quantum size effects in ambient CO oxidation catalysed by ligand-protected gold clusters

Finely dispersed nanometre-scale gold particles are known to catalyse several oxidation reactions in aerobic, ambient conditions. The catalytic activity has been explained by various complementary mechanisms, including support effects, particle-size-dependent metal-insulator transition, charging effects, frontier orbital interactions and geometric fluxionality. We show, by considering a series of robust and structurally well-characterized ligand-protected gold clusters with diameters between 1.2 and 2.4nm, that electronic quantum size effects, particularly the magnitude of the so-called HOMO-LUMO energy gap, has a decisive role in binding oxygen to the nano-catalyst in an activated form. This can lead to the oxidation reaction 2CO+O(2)→2CO(2) with low activation barriers. Binding of dioxygen is significant only for the smallest particles with a metal core diameter clearly below 2nm. Our results suggest a potentially viable route to practical applications using ligand-protected gold clusters for green chemistry.

A unified view of ligand-protected gold clusters as superatom complexes.

Synthesis, characterization, and functionalization of self-assembled, ligand-stabilized gold nanoparticles are long-standing issues in the chemistry of nanomaterials. Factors driving the thermodynamic stability of well documented discrete sizes are largely unknown. Herein, we provide a unified view of principles that underlie the stability of particles protected by thiolate (SR) or phosphine and halide (PR(3), X) ligands. The picture has emerged from analysis of large-scale density functional theory calculations of structurally characterized compounds, namely Au(102)(SR)(44), Au(39)(PR(3))(14)X(6)(-), Au(11)(PR(3))(7)X(3), and Au(13)(PR(3))(10)X(2)(3+), where X is either a halogen or a thiolate. Attributable to a compact, symmetric core and complete steric protection, each compound has a filled spherical electronic shell and a major energy gap to unoccupied states. Consequently, the exceptional stability is best described by a "noble-gas superatom" analogy. The explanatory power of this concept is shown by its application to many monomeric and oligomeric compounds of precisely known composition and structure, and its predictive power is indicated through suggestions offered for a series of anomalously stable cluster compositions which are still awaiting a precise structure determination.

Atomic and electronic structure of gold clusters: understanding flakes, cages and superatoms from simple concepts

Atomic structure and electronic structure are intimately interrelated properties of nanoclusters and nanoparticles, defining their stability, electronic, optical and chemical properties, in other words, their usability as potential components for nanoscale devices. This tutorial review attempts to describe the development in understanding the structures of bare and ligand-protected gold clusters over the past decade, based on selected density-functional-theory calculations. This review should be of interest both to newcomers in the field and to an interdisciplinary community of researchers working in synthesis, characterization and utilization of ligand-protected gold clusters.