Raman Spectroscopic Fingerprints of Atomically Precise Ligand Protected Noble Metal Clusters: Au38 (PET)24 and Au38-x Agx (PET)24.

Distinct Raman spectroscopic signatures of the metal core of atomically precise, ligand-protected noble metal nanoclusters are reported using Au38 (PET)24 and Au38-x Agx (PET)24 (PET = 2-phenylethanethiolate, -SC2 H4 C6 H5 ) as model systems. The fingerprint Raman features (occurring <200 cm-1 ) of these clusters arise due to the vibrations involving metal atoms of their Au23 or Au23-x Agx cores. A distinct core breathing vibrational mode of the Au23 core has been observed at 90 cm-1 . Whereas the breathing mode shifts to higher frequencies with increasing Ag content of the cluster, the vibrational signatures due to the outer metal-ligand staple motifs (between 200 and 500 cm-1 ) do not shift significantly. DFT calculations furthermore reveal weak Raman bands at higher frequencies compared to the breathing mode, which are associated mostly with the rattling of two central gold atoms of the bi-icosahedral Au23 core. These vibrations are also observed in the experimental spectrum. The study indicates that low-frequency Raman spectra are a characteristic fingerprint of atomically precise clusters, just as electronic absorption spectroscopy, in contrast to the spectrum associated with the ligand shell, which is observed at higher frequencies.

Predictive optical photoabsorption of Ag24Au(DMBT)18 - via efficient TDDFT simulations.

We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.

Metal-Ligand Interface in the Chemical Reactions of Ligand-Protected Noble Metal Clusters.

We discuss the role of the metal-ligand (M-L) interfaces in the chemistry of ligand-protected, atomically precise noble metal clusters, a new and expanding family of nanosystems, in solution as well as in the gas phase. A few possible mechanisms by which the structure and dynamics of M-L interfaces could trigger intercluster exchange reactions are presented first. How interparticle chemistry can be a potential mechanism of Ostwald ripening, a well-known particle coarsening process, is also discussed. The reaction of Ag59(2,5-DCBT)32 (DCBT = dichlorobenzenethiol) with 2,4-DCBT leading to the formation of Ag44(2,4-DCBT)30 is presented, demonstrating the influence of the ligand structure in ligand-induced chemical transformations of clusters. We also discuss the structural isomerism of clusters such as Ag44(SR)30 (-SR = alkyl/aryl thiolate) in the gas phase wherein the occurrence of isomerism is attributed to the structural rearrangements in the M-L bonding network. Interfacial bonding between Au25(SR)18 clusters leading to the formation of cluster dimers and trimers is also discussed. Finally, we show that the desorption of phosphine and hydride ligands on a silver cluster, [Ag18(TPP)10H16]2+ (TPP = triphenylphosphine) in the gas phase, leads to the formation of a naked silver cluster of precise nuclearity, such as Ag17+. We demonstrate that the nature of the M-L interfaces, i.e., the oxidation state of metal atoms, structure of the ligand, M-L bonding network, and so forth, plays a key role in the chemical reactivity of clusters. The structure, dynamics, and chemical reactivity of nanosystems in general are to be explored together to obtain new insights into their emerging science.

Chiral Functionalization of an Atomically Precise Noble Metal Cluster: Insights into the Origin of Chirality and Photoluminescence.

We probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag24Au1(DMBT)18 (DMBT = 2,4-dimethylbenzenethiolate), and the origin of chirality in its chirally functionalized derivatives, Ag24Au1(R/S-BINAS)x(DMBT)18-2x, with x = 1-7 (R/S-BINAS = R/S-1,1'-[binaphthalene]-2,2'-dithiol), using chiroptical spectroscopic measurements and density functional theory (DFT) calculations. Combination of chiroptical and luminescence spectroscopies to understand the nature of electronic transitions has not been applied to such molecule-like metal clusters. In order to impart chirality to the achiral Ag24Au1(DMBT)18 cluster, the chiral ligand, R/S-BINAS, was incorporated into it. A series of clusters, Ag24Au1(R/S-BINAS)x(DMBT)18-2x, with x = 1-7, were synthesized. We demonstrate that the low-energy electronic transitions undergo an unexpected achiral to chiral and back to achiral transition from pure Ag24Au1(DMBT)18 to Ag24Au1(R/S-BINAS)x(DMBT)18-2x, by increasing the number of BINAS ligands. The UV/vis, luminescence, circular dichroism, and circularly polarized luminescence spectroscopic measurements, in conjunction with DFT calculations, suggest that the photoluminescence in Ag24Au1(DMBT)18 and its chirally functionalized derivatives originates from the transitions involving the whole Ag24Au1S18 framework and not merely from the icosahedral Ag12Au1 core. These results suggest that the chiroptical signatures and photoluminescence in these cluster systems cannot be solely attributed to any one of the structural components, that is, the metal core or the protecting metal-ligand oligomeric units, but rather to their interaction and that the ligand shell plays a crucial role. Our work demonstrates that chiroptical spectroscopic techniques such as circular dichroism and circularly polarized luminescence represent useful tools to understand the nature of electronic transitions in ligand-protected metal clusters and that this approach can be utilized for gaining deeper insights into the structure-property relationships of the electronic transitions of such molecule-like clusters.