Resonance Enhancement of Nonlinear Optical Scattering in Monolayer-Protected Gold Clusters

Monolayer-protected metal clusters (MPCs) have recently gained significant research interest, since they are promising candidates for various applications in bioimaging and catalysis. Besides this, MPCs promise to aid in understanding the evolution of the metallic state from bottom-up principles. MPCs can be prepared with atomic precision, and their nonscalable properties (indicating molecule-like behavior) have been studied with a variety of techniques both theoretically and experimentally. Here, we present spectrally resolved second-order nonlinear optical scattering experiments on thiolate-protected gold clusters (Au₁₃₀(SR)₅₀, Au₁₄₄(SR)₆₀, and Au₅₀₀(SR)₁₂₀). The three clusters share common resonance enhancement around 490 nm, which is ascribed to an interband transition. This indicates emerging metal-like properties, and we tentatively assign the onset of metal-like behavior somewhere between 102 and 130 gold atoms

Electronic Structure and Optical Properties of the Intrinsically Chiral 16-Electron Superatom Complex [Au₂₀(PP₃)₄]⁴⁺

The recently solved crystal structure of the [Au₂₀(PP₃)₄]­Cl₄ cluster (PP₃: tris­(2-(diphenylphophino)­ethyl)­phosphine) is examined using density functional theory (DFT). The Au₂₀ core of the cluster is intrinsically chiral by the arrangement of the Au atoms. This is in contrast to the chirality of thiolate-protected gold clusters, in which the protecting Au-thiolate units are arranged in chiral patterns on achiral cores. We interpret the electronic structure of the [Au₂₀(PP₃)₄]­Cl₄ cluster in terms of the superatom complex model. The 16-electron cluster cannot be interpreted as a dimer of 8-electron clusters (which are magic). Instead, a superatomic electron configuration of 1S² 1P⁶ 1D⁶ 2S² is found. The 2S band is strongly stabilized, and the 1D states are nondegenerate with a large gap. Ligand protection of the (Au₂₀)⁴⁺ core leads to a significant increase of the HL-gap and thus stabilization. We also tested a charge of +II, which would give rise to an 18-electron superatom complex. Our results indicate that the 16-electron cluster is indeed more stable. We also investigate the optical properties of the cluster. The experimental absorption spectrum is well-reproduced by time-dependent DFT. Prominent transitions are analyzed by time-dependent density-functional perturbation theory. The intrinsic chirality of the cluster is compared to that of Au₃₈(SR)₂₄. We observe that the chiral arrangement of the protecting Au-SR units in Au₃₈(SR)₂₄ has very strong influence on the strength of the CD spectra, whereas phosphine protection in the title compound does not

Stabilization of Thiolate-Protected Gold Clusters Against Thermal Inversion: Diastereomeric Au₃₈(SCH₂CH₂Ph)_{24–2<i>x</i>}(<i>R</i>‑BINAS)_<i>x</i>

Intrinsically chiral thiolate-protected gold clusters were recently separated into their enantiomers, and their circular dichroism (CD) spectra were measured. Introduction of the chiral <i>R</i>-1,1′-binaphthyl-2,2′-dithiol (BINAS) into the ligand layer of <i>rac</i>-Au₃₈(2-PET)₂₄ clusters (2-PET: 2-phenylethylthiolate, SCH₂CH₂Ph) was shown to be diastereoselective. In this contribution, we isolated and characterized the diastereomeric reaction products of the first exchange step, <i>A</i>-Au₃₈(2-PET)₂₂(<i>R</i>-BINAS)₁ and <i>C</i>-Au₃₈(2-PET)₂₂(<i>R</i>-BINAS)₁ (<i>A</i>/<i>C</i>, anticlockwise/clockwise) and the second exchange product, <i>A</i>-Au₃₈(2-PET)₂₀(<i>R</i>-BINAS)₂. The absorption spectra show minor, but significant influence of the BINAS ligand. Overall, the spectra are less defined as compared to Au₃₈(2-PET)₂₄, which is ascribed to symmetry breaking. The CD spectra are similar to those of the parent Au₃₈(2-PET)₂₄ enantiomers, readily allowing the assignment of handedness of the ligand layer. Nevertheless, some characteristic differences are found between the diastereomers. The anisotropy factors are slightly lower after ligand exchange. The second exchange step seems to confirm the trend. Inversion experiments were performed and compared to the racemization of Au₃₈(2-PET)₂₄. It was found that the introduction of the BINAS ligand effectively stabilizes the cluster against inversion, which involves a rearrangement of the thiolates on the cluster surface. It therefore seems that introduction of the dithiol reduces the flexibility of the gold–sulfur interface

Racemization of a Chiral Nanoparticle Evidences the Flexibility of the Gold–Thiolate Interface

Thiolate-protected gold nanoparticles and clusters combine size-dependent physical properties with the ability to introduce (bio)­chemical functionality within their ligand shell. The engineering of the latter with molecular precision is an important prerequisite for future applications. A key question in this respect concerns the flexibility of the gold–sulfur interface. Here we report the first study on racemization of an intrinsically chiral gold nanocluster, Au₃₈(SCH₂CH₂Ph)₂₄, which goes along with a drastic rearrangement of its surface involving place exchange of several thiolates. This racemization takes place at modest temperatures (40–80 °C) without significant decomposition. The experimentally determined activation energy for the inversion reaction is ca. 28 kcal/mol, which is surprisingly low considering the large rearrangement. The activation parameters furthermore indicate that the process occurs without complete Au–S bond breaking

Nonlinear Optical Properties of Thiolate-Protected Gold Clusters: A Theoretical Survey of the First Hyperpolarizabilities

A series of thiolate-protected gold clusters has been successfully crystallized in recent years, and on the basis of these crystal structures, we investigate the static first hyperpolarizabilities β₀ of eight model clusters [Au<i>_m</i>(SH)_<i>n</i>]^z (<i>m</i> = 18–38) by means of density-functional theory. We used simplified ligands −SH, which may lead to higher symmetries than in actual systems (e.g., with −SCH2CH2Ph or −SPh ligands). A dependence of the obtained values on the exchange-correlation functional during geometry optimization was found. No correlation between cluster size and β₀ was identified. Instead, the symmetry of the clusters seems to dominate the NLO properties. Our survey predicts strong NLO responses in the chiral Au₃₈(SR)₂₄ cluster, whereas centrosymmetric structures such as the [Au₂₅(SR)₁₈)]⁻ yield hyperpolarizabilities close to zero. This is in line with recent experimental results obtained by second-harmonic generation. The centrosymmetry of the Au₂₅ cluster is efficiently destroyed by ligand exchange, as demonstrated by the inclusion of chiral, bidentate ligands (1,1′-binaphthyl-2,2′-dithiol, 1,1′-biphenyl-2,2′-dithiol) and two thiophenolate ligands. This induces significant hyperpolarizabilities, surpassing those of intrinsically chiral clusters (e.g., Au₃₈(SH)₂₄). Our results are of significance for the use of monolayer-protected noble metal clusters as contrast agents in NLO imaging applications

Role of Donor and Acceptor Substituents on the Nonlinear Optical Properties of Gold Nanoclusters

In recent years, a large number of monolayer-protected clusters (MPCs) have been studied by means of single crystal structure characterization. A central aspect of research on MPCs is the correlation of their interesting optical properties with structural features and the formulation of a theoretical framework that allows interpretation of their unique properties. For this, superatom and jellium models have been proven successful. Little attention, however, has been paid to the influence of the protecting ligands. Here, we investigate the effect of changes in [Au₂₅(SR)_18‑x(SR′)_<i>x</i>]⁻, where SR′ represents a para-substituted thiophenolate derivative (SPh-4-X). We computed the first hyperpolarizabilities, screening a broad range of substituents X. For [Au₂₅(SR)₁₇(SR′)₁]⁻ clusters, significant first hyperpolarizabilities were calculated, spanning 2 orders of magnitude depending on X. A strong dependence on the electron-donating/withdrawing properties of the substituent was found for para-substituted thiophenol ligands. Protonation of amine substituents leads to a change from donor to acceptor substitution, leading to a record setting contrast for nonlinear optical proton sensing. Furthermore, “push/pull” systems were considered where both an acceptor and a donor ligand were positioned at opposite ends of the cluster. This induces significant increase of the first hyperpolarizability through charge transfer. Overall, our results indicate that the right choice of ligand can significantly impact the (nonlinear) optical properties of MPCs. This adds a new component to the cluster chemist’s toolbox

Electronic Structure and Optical Properties of the Thiolate-Protected Au₂₈(SMe)₂₀ Cluster

The recently reported crystal structure of the Au₂₈(TBBT)₂₀ cluster (TBBT: <i>p</i>-<i>tert</i>-butylbenzenethiolate) is analyzed with (time-dependent) density functional theory (TD-DFT). Bader charge analysis reveals a novel trimeric Au₃(SR)₄ binding motif. The cluster can be formulated as Au₁₄(Au₂(SR)₃)₄(Au₃(SR)₄)₂. The electronic structure of the Au₁₄⁶⁺ core and the ligand-protected cluster were analyzed, and their stability can be explained by formation of distorted eight-electron superatoms. Optical absorption and circular dichroism (CD) spectra were calculated and compared to the experiment. Assignment of handedness of the intrinsically chiral cluster is possible

Nonlinear Optical Properties of Thiolate-Protected Gold Clusters

Thiolate-protected gold clusters are promising candidates for imaging applications due to their interesting, size-dependent properties. Their high stability and the ability to functionalize the clusters with biocompatible ligands render the clusters interesting for various imaging techniques such as fluorescence microscopy or second-harmonic generation microscopy. The latter nonlinear optical effect has not yet been observed on this type of ultrasmall nanoparticle. We hereby present second- and third-harmonic generation and multiphoton fluorescence of two thiolate-protected gold clusters: Au₂₅(SCH₂CH₂Ph)₁₈ and Au₃₈(SCH₂CH₂Ph)₂₄. At a fundamental wavelength of 800 nm, the Au₃₈(SCH₂CH₂Ph)₂₄ cluster is active. In contrast, Au₂₅(SCH₂CH₂Ph)₁₈ does not yield significant SHG signal. We ascribe this to the center of inversion in the Au₂₅ cluster. Measurements on chiral Au₂₅(capt)₁₈ (capt: captopril) gave an SHG response, supporting this interpretation. We also observed third-harmonic generation at a fundamental wavelength of 1200 nm. At 800 and 1100 nm, the clusters decompose after short illumination time but are stable at illumination at 1200 nm. This may be exploited in combined deep tissue imaging and photothermal heating for theranostics applications

Au₃₆(SPh)₂₄ Nanomolecules: X‑ray Crystal Structure, Optical Spectroscopy, Electrochemistry, and Theoretical Analysis

The physicochemical properties of gold:thiolate nanomolecules depend on their crystal structure and the capping ligands. The effects of protecting ligands on the crystal structure of the nanomolecules are of high interest in this area of research. Here we report the crystal structure of an all aromatic thiophenolate-capped Au₃₆(SPh)₂₄ nanomolecule, which has a face-centered cubic (<i>fcc</i>) core similar to other nanomolecules such as Au₃₆(SPh-tBu)₂₄ and Au₃₆(SC₅H₉)₂₄ with the same number of gold atoms and ligands. The results support the idea that a stable core remains intact even when the capping ligand is varied. We also correct our earlier assignment of “Au₃₆(SPh)₂₃” which was determined based on MALDI mass spectrometry which is more prone to fragmentation than ESI mass spectrometry. We show that ESI mass spectrometry gives the correct assignment of Au₃₆(SPh)₂₄, supporting the X-ray crystal structure. The electronic structure of the title compound was computed at different levels of theory (PBE, LDA, and LB94) using the coordinates extracted from the single crystal X-ray diffraction data. The optical and electrochemical properties were determined from experimental data using UV–vis spectroscopy, cyclic voltammetry, and differential pulse voltammetry. Au₃₆(SPh)₂₄ shows a broad electrochemical gap near 2 V, a desirable optical gap of ∼1.75 eV for dye-sensitized solar cell applications, as well as appropriately positioned electrochemical potentials for many electrocatalytic reactions

Au₃₆(SPh)₂₄ Nanomolecules: X‑ray Crystal Structure, Optical Spectroscopy, Electrochemistry, and Theoretical Analysis

The physicochemical properties of gold:thiolate nanomolecules depend on their crystal structure and the capping ligands. The effects of protecting ligands on the crystal structure of the nanomolecules are of high interest in this area of research. Here we report the crystal structure of an all aromatic thiophenolate-capped Au₃₆(SPh)₂₄ nanomolecule, which has a face-centered cubic (<i>fcc</i>) core similar to other nanomolecules such as Au₃₆(SPh-tBu)₂₄ and Au₃₆(SC₅H₉)₂₄ with the same number of gold atoms and ligands. The results support the idea that a stable core remains intact even when the capping ligand is varied. We also correct our earlier assignment of “Au₃₆(SPh)₂₃” which was determined based on MALDI mass spectrometry which is more prone to fragmentation than ESI mass spectrometry. We show that ESI mass spectrometry gives the correct assignment of Au₃₆(SPh)₂₄, supporting the X-ray crystal structure. The electronic structure of the title compound was computed at different levels of theory (PBE, LDA, and LB94) using the coordinates extracted from the single crystal X-ray diffraction data. The optical and electrochemical properties were determined from experimental data using UV–vis spectroscopy, cyclic voltammetry, and differential pulse voltammetry. Au₃₆(SPh)₂₄ shows a broad electrochemical gap near 2 V, a desirable optical gap of ∼1.75 eV for dye-sensitized solar cell applications, as well as appropriately positioned electrochemical potentials for many electrocatalytic reactions