Wataru Kurashige

Isolation, structure, and stability of a dodecanethiolate-protected Pd1Au24 cluster

A dodecanethiolate-protected Pd(1)Au(24)(SC(12)H(25))(18) cluster, which is a mono-Pd-doped cluster of the well understood magic gold cluster Au(25)(SR)(18), was isolated in high purity using solvent fractionation and high-performance liquid chromatography (HPLC) after the preparation of dodecanethiolate-protected palladium-gold bimetal clusters. The cluster thus isolated was identified as the neutral [Pd(1)Au(24)(SC(12)H(25))(18)](0) from the retention time in reverse phase columns and by elemental analyses. The LDI mass spectrum of [Pd(1)Au(24)(SC(12)H(25))(18)](0) indicates that [Pd(1)Au(24)(SC(12)H(25))(18)](0) adopts a similar framework structure to Au(25)(SR)(18), in which an icosahedral Au(13) core is protected by six [-S-Au-S-Au-S-] oligomers. The optical absorption spectrum of [Pd(1)Au(24)(SC(12)H(25))(18)](0) exhibits peaks at approximately 690 and approximately 620 nm, which is consistent with calculated results on [Pd(1)@Au(24)(SC(1)H(3))(18)](0) in which the central gold atom of Au(25)(SC(1)H(3))(18) is replaced with Pd. These results strongly indicate that the isolated [Pd(1)Au(24)(SC(12)H(25))(18)](0) has a core-shell [Pd(1)@Au(24)(SC(12)H(25))(18)](0) structure in which the central Pd atom is surrounded by a frame of Au(24)(SC(12)H(25))(18). Experiments on the stability of the cluster showed that Pd(1)@Au(24)(SC(12)H(25))(18) is more stable against degradation in solution and laser dissociation than Au(25)(SC(12)H(25))(18). These results indicate that the doping of a central atom is a powerful method to increase the stability beyond the Au(25)(SR)(18) cluster.

A Critical Size for Emergence of Nonbulk Electronic and Geometric Structures in Dodecanethiolate-Protected Au Clusters

We report on how the transition from the bulk structure to the cluster-specific structure occurs in n-dodecanethiolate-protected gold clusters, Au(n)(SC12)m. To elucidate this transition, we isolated a series of Au(n)(SC12)m in the n range from 38 to ∼520, containing five newly identified or newly isolated clusters, Au104(SC12)45, Au(∼226)(SC12)(∼76), Au(∼253)(SC12)(∼90), Au(∼356)(SC12)(∼112), and Au(∼520)(SC12)(∼130), using reverse-phase high-performance liquid chromatography. Low-temperature optical absorption spectroscopy, powder X-ray diffractometry, and density functional theory (DFT) calculations revealed that the Au cores of Au144(SC12)60 and smaller clusters have molecular-like electronic structures and non-fcc geometric structures, whereas the structures of the Au cores of larger clusters resemble those of the bulk gold. A new structure model is proposed for Au104(SC12)45 based on combined approach between experiments and DFT calculations.

Ligand-Induced Stability of Gold Nanoclusters: Thiolate versus Selenolate

Thiolate-protected gold nanoclusters have attracted considerable attention as building blocks for new functional materials and have been extensively researched. Some studies have reported that changing the ligand of these gold nanoclusters from thiolate to selenolate increases cluster stability. To confirm this, in this study, we compare the stabilities of precisely synthesized [Au25(SC8H17)18](-) and [Au25(SeC8H17)18](-) against degradation in solution, thermal dissolution, and laser fragmentation. The results demonstrate that changing the ligand from thiolate to selenolate increases cluster stability in reactions involving dissociation of the gold-ligand bond but reduces cluster stability in reactions involving intramolecular dissociation of the ligand. These results reveal that using selenolate ligands makes it possible to produce gold clusters that are more stable against degradation in solution than thiolate-protected gold nanoclusters.

Recent Progress in the Functionalization Methods of Thiolate-Protected Gold Clusters

Nanomaterials that exhibit both stability and functionality are currently considered to hold great promise as components of nanotechnology devices. Thiolate-protected gold clusters (Aun(SR)m) have long attracted attention as functional nanomaterials. Magic Aun(SR)m clusters are an especially stable group of thiolate-protected clusters that have particularly high potential as functional materials. Although numerous application experiments have been conducted for magic Aun(SR)m clusters, it is important that functionalization methods are also established to allow for effective utilization of these materials. The results of recent research on heteroatom doping and the use of other chalcogenide ligands strongly suggest that these strategies are promising as functionalization methods of magic Aun(SR)m clusters. In this Perspective, we focus on studies relating to three representative types of magic clusters-Au25(SR)18, Au38(SR)24, and Au144(SR)60-and discuss the recent progress and future issues.

Isolation and structural characterization of an octaneselenolate-protected Au25 cluster.

We report the isolation and structural characterization of an octaneselenolate-protected Au(25) cluster ([Au(25)(SeC(8)H(17))(18)](-)). Isolated [Au(25)(SeC(8)H(17))(18)](-) was characterized by various analytical techniques. The results strongly suggest that [Au(25)(SeC(8)H(17))(18)](-) possesses a similar geometric structure to the well-studied thiolate (RS)-protected Au(25) cluster ([Au(25)(SR)(18)](-)) and that the charge transfer between the metal atoms and ligands in [Au(25)(SeC(8)H(17))(18)](-) is lower than that in [Au(25)(SR)(18)](-). To the best of our knowledge, this is the first report of the isolation of a selenolate-protected gold cluster. [Au(25)(SeC(8)H(17))(18)](-) is an ideal compound for determining how changing the ligand from thiolate to selenolate affects the fundamental properties of a cluster.

Hierarchy of bond stiffnesses within icosahedral-based gold clusters protected by thiolates

Unique thermal properties of metal clusters are believed to originate from the hierarchy of the bonding. However, an atomic-level understanding of how the bond stiffnesses are affected by the atomic packing of a metal cluster and the interfacial structure with the surrounding environment has not been attained to date. Here we elucidate the hierarchy in the bond stiffness in thiolate-protected, icosahedral-based gold clusters Au25(SC2H4Ph)18, Au38(SC2H4Ph)24 and Au144(SC2H4Ph)60 by analysing Au L3-edge extended X-ray absorption fine structure data. The Au–Au bonds have different stiffnesses depending on their lengths. The long Au–Au bonds, which are more flexible than those in the bulk metal, are located at the icosahedral-based gold core surface. The short Au–Au bonds, which are stiffer than those in the bulk metal, are mainly distributed along the radial direction and form a cyclic structural backbone with the rigid Au–SR oligomers.

Ag44(SeR)30: A Hollow Cage Silver Cluster with Selenolate Protection

Selenolate protected, stable and atomically precise, hollow silver cluster was synthesized using solid state as well as solution state routes. The optical absorption spectrum shows multiple and sharp features similar to the thiolated Ag44 cluster, Ag44(SR)30 whose experimental structure was reported recently. High-resolution electrospray ionization mass spectrometry (HRESI MS) shows well-defined molecular ion features with two, three, and four ions with isotopic resolution, due to Ag44(SePh)30. Additional characterization with diverse tools confirmed the composition. The closed-shell 18 electron superatom electronic structure, analogous to Ag44(SR)30 stabilizes the dodecahedral cage with a large HOMO-LUMO gap of 0.71 eV. The time-dependent density functional theory (TDDFT) prediction of the optical absorption spectrum, assuming the Ag44(SR)30 structure, matches the experimental data, confirming the structure.

Au25 Clusters Containing Unoxidized Tellurolates in the Ligand Shell

We report herein the synthesis and characterization of Au25 clusters containing tellurolates (TePh) in the ligand shell ([Au25(TePh)n(SC8H17)18-n](-); n = 1-18). [Au25(TePh)n(SC8H17)18-n](-) clusters were synthesized by reacting [Au25(SC8H17)18](-) with diphenyl ditelluride ((PhTe)2) in solution. Characterization of the products by mass spectrometry and X-ray absorption fine structure analysis revealed that the tellurolates in [Au25(TePh)n(SC8H17)18-n](-), unlike those in tellurolate-protected gold nanoparticles, were not oxidized. Various experiments on the products and theoretical calculations on related clusters revealed that protection by the tellurolates distorts (expands) the central Au13 core and decreases the HOMO-LUMO gap of the Au25 clusters.

Tuning the electronic structure of thiolate-protected 25-atom clusters by co-substitution with metals having different preferential sites

Trimetallic Au24-xAgxPd and tetrametallic Au24-x-yAgxCuyPd clusters were synthesized by the subsequential metal exchange reactions of dodecanethiolate-protected Au24Pd clusters. EXAFS measurements revealed that Pd, Ag, and Cu dopants preferentially occupy the center and edge sites of the core, and staple sites, respectively. Spectroscopic and theoretical studies demonstrated that the synergistic effects of multiple substitutions on the electronic structures are additive in nature.

Cosensitization Properties of Glutathione-Protected Au25 Cluster on Ruthenium Dye-Sensitized TiO2 Photoelectrode

Cosensitization by glutathione-protected Au25 clusters on Ru complex, N719-sensitized TiO2 photoelectrodes is demonstrated. Glutathione-protected Au25 clusters showed no significant changes in properties after adsorption onto TiO2 particles, as confirmed by optical absorption spectroscopy, transmission electron microscopy, and laser desorption/ionization mass spectrometry. Adsorption property of the glutathione-protected Au25 clusters depends on the pH, which affects the incident photon-to-current conversion efficiency (IPCE) of the TiO2 photoelectrode containing Au25 clusters. When pH 7. The IPCE of a TiO2 photoelectrode sensitized by both glutathione-protected Au25 clusters and N719 was increased compared with photoelectrodes containing either glutathione-protected Au25 clusters or N719, which suggests that glutathione-protected Au25 clusters act as a coadsorbent for N719 on TiO2 photoelectrodes. This is also supported by the results that the IPCE of N719-sensitized TiO2 photoelectrodes increased upon addition of glutathione. Furthermore, cosensitization by glutathione-protected Au25 clusters on N719-sensitized TiO2 photoelectrodes allows that wavelength of photoelectric conversion was extended to the near infrared (NIR) region. These results suggest that glutathione-protected Au25 clusters act not only as a coadsorbent to increase IPCE but also as an NIR-active sensitizer.