Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au₂₅(SR)₁₈ Cluster

Au₂₅(SR)₁₈ (R = −CH₂–CH₂–Ph) is a molecule-like nanocluster displaying distinct electrochemical and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepared monodisperse Au₂₅(SR)₁₈⁰ and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its reduction or oxidation to a series of charge states, −2, −1, +1, +2, and +3. For each electrode process, we determined the standard heterogeneous electron-transfer (ET) rate constants and the reorganization energies. The latter points to a relatively large inner reorganization. Reduction to form Au₂₅(SR)₁₈^2– and oxidation to form Au₂₅(SR)₁₈²⁺ and Au₂₅(SR)₁₈³⁺ are chemically irreversible. The corresponding decay rate constants and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au₂₅ magnetism was addressed by comparing the continuous-wave electron paramagnetic resonance (cw-EPR) behaviors of radical Au₂₅(SR)₁₈⁰ and its oxidation product, Au₂₅(SR)₁₈⁺. As opposed to recent experimental and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and density of states of the −1, 0, and +1 charge states nicely reproduced the experimentally estimated dependence of the HOMO–LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: In particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochemical, EPR, and computational results thus provide a self-consistent picture of the properties of Au₂₅(SR)₁₈ as a function of its charge state and may furnish a methodology blueprint for understanding the redox and magnetic behaviors of similar molecule-like gold nanoclusters

Influence of Surface Structure on Single or Mixed Component Self-Assembled Monolayers via in Situ Spectroelectrochemical Fluorescence Imaging of the Complete Stereographic Triangle on a Single Crystal Au Bead Electrode

The use of a single crystal gold bead electrode is demonstrated for characterization of self-assembled monolayers (SAM)­s formed on the bead surface expressing a complete set of face centered cubic (fcc) surface structures represented by a stereographic projection. Simultaneous analysis of many crystallographic orientations was accomplished through the use of an in situ fluorescence microscopic imaging technique coupled with electrochemical measurements. SAMs were prepared from different classes of molecules, which were modified with a fluorescent tag enabling characterization of the influence of electrical potential and a direct comparison of the influence of surface structure on SAMs adsorbed onto low index, vicinal and chiral surfaces. The assembly of alkylthiol, Aib peptide and DNA SAMs are studied as a function of the electrical potential of the interface revealing how the organization of these SAMs depend on the surface crystallographic orientation, all in one measurement. This approach allows for a simultaneous determination of SAMs assembled onto an electrode surface onto which the whole fcc stereographic triangle can be mapped, revealing the influence of intermolecular interactions as well as the atomic arrangement of the substrate. Moreover, this method enables study of the influence of the Au surface atom arrangement on SAMs that were created and analyzed, both under identical conditions, something that can be challenging for the typical studies of this kind using individual gold single crystal electrodes. Also demonstrated is the analysis of a SAM containing two components prepared using thiol exchange. The two component SAM shows remarkable differences in the surface coverage, which strongly depends on the surface crystallography enabling estimates of the thiol exchange energetics. In addition, these electrode surfaces enable studies of molecular adsorption onto the symmetry related chiral surfaces since more than one stereographic triangle can be imaged at the same time. The ability to observe a SAM modified surface that contains many complete fcc stereographic triangles will facilitate the study of the single and multicomponent SAMs, identifying interesting surfaces for further analysis

Influence of Surface Structure on Single or Mixed Component Self-Assembled Monolayers via in Situ Spectroelectrochemical Fluorescence Imaging of the Complete Stereographic Triangle on a Single Crystal Au Bead Electrode

The use of a single crystal gold bead electrode is demonstrated for characterization of self-assembled monolayers (SAM)­s formed on the bead surface expressing a complete set of face centered cubic (fcc) surface structures represented by a stereographic projection. Simultaneous analysis of many crystallographic orientations was accomplished through the use of an in situ fluorescence microscopic imaging technique coupled with electrochemical measurements. SAMs were prepared from different classes of molecules, which were modified with a fluorescent tag enabling characterization of the influence of electrical potential and a direct comparison of the influence of surface structure on SAMs adsorbed onto low index, vicinal and chiral surfaces. The assembly of alkylthiol, Aib peptide and DNA SAMs are studied as a function of the electrical potential of the interface revealing how the organization of these SAMs depend on the surface crystallographic orientation, all in one measurement. This approach allows for a simultaneous determination of SAMs assembled onto an electrode surface onto which the whole fcc stereographic triangle can be mapped, revealing the influence of intermolecular interactions as well as the atomic arrangement of the substrate. Moreover, this method enables study of the influence of the Au surface atom arrangement on SAMs that were created and analyzed, both under identical conditions, something that can be challenging for the typical studies of this kind using individual gold single crystal electrodes. Also demonstrated is the analysis of a SAM containing two components prepared using thiol exchange. The two component SAM shows remarkable differences in the surface coverage, which strongly depends on the surface crystallography enabling estimates of the thiol exchange energetics. In addition, these electrode surfaces enable studies of molecular adsorption onto the symmetry related chiral surfaces since more than one stereographic triangle can be imaged at the same time. The ability to observe a SAM modified surface that contains many complete fcc stereographic triangles will facilitate the study of the single and multicomponent SAMs, identifying interesting surfaces for further analysis

Gold Fusion: From Au₂₅(SR)₁₈ to Au₃₈(SR)₂₄, the Most Unexpected Transformation of a Very Stable Nanocluster

The study of the molecular cluster Au₂₅(SR)₁₈ has provided a wealth of fundamental insights into the properties of clusters protected by thiolated ligands (SR). This is also because this cluster has been particularly stable under a number of experimental conditions. Very unexpectedly, we found that paramagnetic Au₂₅(SR)₁₈⁰ undergoes a spontaneous bimolecular fusion to form another benchmark gold nanocluster, Au₃₈(SR)₂₄. We tested this reaction with a series of Au₂₅ clusters. The fusion was confirmed and characterized by UV–vis absorption spectroscopy, ESI mass spectrometry, ¹H and ¹³C NMR spectroscopy, and electrochemistry. NMR evidences the presence of four types of ligand and, for the same proton type, double signals caused by the diastereotopicity arising from the chirality of the capping shell. This effect propagates up to the third carbon atom along the ligand chain. Electrochemistry provides a particularly convenient way to study the evolution process and determine the fusion rate constant, which decreases as the ligand length increases. No reaction is observed for the anionic clusters, whereas the radical nature of Au₂₅(SR)₁₈⁰ appears to play an important role. This transformation of a stable cluster into a larger stable cluster without addition of any co-reagent also features the bottom-up assembly of the Au₁₃ building block in solution. This very unexpected result could modify our view of the relative stability of molecular gold nanoclusters

Electron Transfer through 3D Monolayers on Au₂₅ Clusters

The monolayer protecting small gold nanoparticles (monolayer-protected clusters, MPCs) is generally represented as the 3D equivalent of 2D self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of MPCs in important applied areas, such as catalysis and nanomedicine, our knowledge of the structure of 3D SAMs in solution is still extremely limited. We prepared a large series of monodisperse Au₂₅(SC_<i>n</i>H_2<i>n</i>+1)₁₈ clusters (<i>n</i> = 2, 4, 6, 8, 10, 12, 14, 16, 18) and studied how electrons tunnel through these monolayers. Electron transfer results, nicely supported by ¹H NMR spectroscopy, IR absorption spectroscopy, and molecular dynamics results, show that there is a critical ligand length marking the transition between short ligands, which form a quite fluid monolayer structure, and longer alkyl chains, which self-organize into bundles. At variance with the truly protecting 2D SAMs, efficient electronic communication of the Au₂₅ core with the outer environment is thus possible even for long alkyl chains. These conclusions provide a different picture of how an ultrasmall gold core talks with the environment through/with its protecting but not-so-shielding monolayer

Gold Nanowired: A Linear (Au₂₅)_<i>n</i> Polymer from Au₂₅ Molecular Clusters

Au₂₅(SR)₁₈ has provided fundamental insights into the properties of clusters protected by monolayers of thiolated ligands (SR). Because of its ultrasmall core, 1 nm, Au₂₅(SR)₁₈ displays molecular behavior. We prepared a Au₂₅ cluster capped by <i>n</i>-butanethiolates (SBu), obtained its structure by single-crystal X-ray crystallography, and studied its properties both experimentally and theoretically. Whereas in solution Au₂₅(SBu)₁₈⁰ is a paramagnetic molecule, in the crystal it becomes a linear polymer of Au₂₅ clusters connected <i>via</i> single Au–Au bonds and stabilized by proper orientation of clusters and interdigitation of ligands. At low temperature, [Au₂₅(SBu)₁₈⁰]_<i>n</i> has a nonmagnetic ground state and can be described as a one-dimensional antiferromagnetic system. These findings provide a breakthrough into the properties and possible solid-state applications of molecular gold nanowires

Conformationally Constrained Functional Peptide Monolayers for the Controlled Display of Bioactive Carbohydrate Ligands

In this study, we employed thiolated peptides of the conformationally constrained, strongly helicogenic α-aminoisobutyric acid (Aib) residue to prepare self-assembled monolayers (SAMs) on gold surfaces. Electrochemistry and infrared reflection absorption spectroscopy support the formation of very well packed Aib-peptide SAMs. The immobilized peptides retain their helical structure, and the resulting SAMs are stabilized by a network of intermolecular H bonds involving the NH groups adjacent to the Au surface. Binary SAMs containing a synthetically defined glycosylated mannose-functionalized Aib-peptide as the second component display similar features, thereby providing reproducible substrates suitable for the controlled display of bioactive carbohydrate ligands. The efficiency of such Aib-based SAMs as a biomolecular recognition platform was evidenced by examining the mannose–concanavalin A interaction via surface plasmon resonance biosensing

Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X‑ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV–vis absorption spectroscopy features of similar Au₂₅ clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au₂₅(SEt)₁₈ were fully characterized by ¹H and ¹³C NMR spectroscopy, which confirmed the monolayer’s properties and the paramagnetism of neutral Au₂₅(SEt)₁₈⁰. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure’s approximations or omissions

Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X‑ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV–vis absorption spectroscopy features of similar Au₂₅ clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au₂₅(SEt)₁₈ were fully characterized by ¹H and ¹³C NMR spectroscopy, which confirmed the monolayer’s properties and the paramagnetism of neutral Au₂₅(SEt)₁₈⁰. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure’s approximations or omissions