Doping Silver Increases the Au₃₈(SR)₂₄ Cluster Surface Flexibility

Multiple Ag atoms were doped inside Au₃₈­(SCH₂­CH₂­Ph)₂₄ nanoclusters using the metal exchange method for the first time for the synthesis of Ag_<i>x</i>­Au_{38–<i>x</i>}­(SCH₂­CH₂­Ph)₂₄. MALDI-TOF mass spectrometry revealed the time dependence of the synthesis. Cluster species with different numbers of Ag atoms (different <i>x</i> values) migrate differently on a chromatography (HPLC) column, which allows one to isolate cluster samples with a narrowed distribution of exchanged metal atoms. The enantiomers of selected Ag_<i>x</i>­Au_{38–<i>x</i>}­(SCH₂­CH₂­Ph)₂₄ samples (average <i>x</i> = 6.5 and 7.9) have been separated by HPLC. Doping changes the electronic structure, as is evidenced by the significantly different CD spectra. UV–vis spectra of the doped sample also show diminished features. The temperature required for complete racemization follows Au₃₈ > Ag_<i>x</i>Au_{38–<i>x</i>} (<i>x</i> = 6.5) > Ag_<i>x</i>Au_{38–<i>x</i>} (<i>x</i> = 7.9). To our surprise, the racemization of Ag_<i>x</i>­Au_{38–<i>x</i>}­(SCH₂­CH₂­Ph)₂₄ (<i>x</i> = 7.9) occurred even at 20 °C. Racemization involves a rearrangement of the staple motifs at the cluster surface. The results therefore show an increased flexibility of the cluster with increasing silver content. The weaker Ag–S bonds compared to Au–S are proposed to be at the origin of this observation. The experimentally determined activation energy for the racemization is ca. 21.5 kcal/mol (<i>x</i> = 6.5) and 19.5 kcal/mol (<i>x</i> = 7.9), compared to 29.5 kcal/mol for Au₃₈­(SCH₂­CH₂­Ph)₂₄, suggesting no complete metal–S bond breaking in the process

Adsorption of Gold and Silver Nanoparticles on Polyelectrolyte Layers and Growth of Polyelectrolyte Multilayers: An In Situ ATR-IR Study

Attenuated total reflection infrared (ATR-IR) spectroscopy is used to study the adsorption of gold and silver nanoparticles and the layer-by-layer (LBL) growth of polyelectrolyte multilayers on a Ge ATR crystal. The Ge ATR crystal is first functionalized using positively charged polyelectrolyte poly­(allylamine hydrochloride) (PAH). Then citrate-stabilized gold or silver nanoparticles are adsorbed onto the modified Ge ATR crystal. When gold or silver nanoparticles are adsorbed, a drastic increase of the water signal is observed which is attributed to an enhanced absorption of IR radiation near the nanoparticles. This enhancement was much larger for the silver nanoparticles (SNP). On top of the nanoparticles multilayers of oppositely charged polyelectrolytes PAH and poly­(sodium 4-styrenesulfonate) (PSS) were deposited, which allowed to study the enhancement of the IR signals as a function of the distance from the nanoparticles. Furthermore, adsorption of a thiol, <i>N</i>-acetyl-l-cysteine, on the nanoparticles confirmed the enhancement. In the case of SNP an absorbance signal of about 15% was observed, which is a factor of about 40 times larger compared to typical signals measure without nanoparticles

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

Racemization of Chiral Pd₂Au₃₆(SC₂H₄Ph)₂₄: Doping Increases the Flexibility of the Cluster Surface

Pd₂Au₃₆(SC₂H₄Ph)₂₄ clusters have been prepared, isolated and separated in their enantiomers. Compared to the parent Au₃₈(SC₂H₄Ph)₂₄ cluster, the doping leads to a significant change of the circular dichroism spectrum; however, the anisotropy factors are of similar magnitude in both cases. Isolation of the enantiomers allowed us to study the racemization of the chiral cluster, which reflects the flexibility of the ligand shell composed of staple motifs. The doping leads to a substantial lowering of the racemization temperature. The change in activation parameters due to the doping may be solely due to modification of the electronic structure

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

Convergent Synthesis, Resolution, and Chiroptical Properties of Dimethoxychromenoacridinium Ions

Cationic azaoxa[4]­helicenes can be prepared in a single step from a common xanthenium precursor by addition of nucleophilic amines under monitored conditions (160 °C, 2 min, MW). The (−)-(<i>M</i>) and (+)-(<i>P</i>) enantiomers can be separated by chiral stationary-phase chromatography. Determination of the absolute configuration and racemization barrier (Δ<i>G</i>^⧧ (433 K) 33.3 ± 1.3 kcal·mol^–1) was achieved by VCD and ECD spectroscopy, respectively

Exciting Bright and Dark Eigenmodes in Strongly Coupled Asymmetric Metallic Nanoparticle Arrays

The strong coupling between planar arrays of gold and silver nanoparticles mediated by a near-field interaction is investigated both theoretically and experimentally to provide an in-depth study of symmetry breaking in complex nanoparticle structures. The asymmetric composition allows to probe for bright and dark eigenmodes, in accordance with plasmon hybridization theory. The strong coupling could only be observed by separating the layers by a nanometric distance with monolayers of suitably chosen polymers. The bottom-up assembly of the nanoparticles as well as the stratified structures themselves gives rise to an extremely flexible system that, moreover, allows the facile variation of a number of important material parameters as well as the preparation of samples on large scales. This flexibility was used to modify the coupling distance between arrays, showing that both the positions and relative intensities of the resonances observed can be tuned with a high degree of precision. Our work renders research in the field of “plasmonic molecules” mature to the extent that it could be incorporated into functional optical devices

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

Preparation and Spectroscopic Properties of Monolayer-Protected Silver Nanoclusters

Silver nanoclusters protected by 2-phenylethanethiol (<b>1</b>), 4-fluorothiophenol (<b>2</b>), and l-glutathione (<b>3</b>) ligands were successfully synthesized. The optical properties of the prepared silver nanoclusters were studied. The absorption signal of Ag@SCH₂CH₂Ph in toluene can be found at 469 nm, and Ag@SPhF in THF shows two absorption bands at 395 and 462 nm. Ag@SG in water absorbs at 478 nm. Mie theory in combination with the Drude model clearly indicates the peaks in the spectra originate from plasmonic transitions. In addition, the damping constant as well as the dielectric constant of the surrounding medium was determined. In addition, the CD spectra of silver nanoclusters protected by the three ligands (<b>1</b>–<b>3</b>) were also studied. As expected, only the clusters of type <b>3</b> gave rise to chiroptical activity across the visible and near-ultraviolet regions. The location and strength of the optical activity suggest an electronic structure of the metal that is highly sensitive to the chiral environment imposed by the glutathione ligand. The morphology and size of the prepared nanoclusters were analyzed by using transmission electron microscopy (TEM). TEM analysis showed that the particles of all three types of silver clusters were small than 5 nm, with an average size of around 2 nm. The analysis of the FTIR spectra elucidated the structural properties of the ligands binding to the nanoclusters. By comparing the IR absorption spectra of pure ligands with those of the protected silver nanoclusters, the disappearance of the S–H vibrational band (2535–2564 cm^–1) in the protected silver nanoclusters confirmed the anchoring of ligands to the cluster surface through the sulfur atom. By elemental analysis and thermogravimetric analysis, the Ag/S ratio and, hence, the number of ligands surrounding a Ag atom could be determined

Structural Information on the Au–S Interface of Thiolate-Protected Gold Clusters: A Raman Spectroscopy Study

The Raman spectra of a series of monolayer-protected gold clusters were investigated with special emphasis on the Au–S modes below 400 cm^–1. These clusters contain monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold–thiolate staples in their surface. In particular, the Raman spectra of [Au₂₅(2-PET)₁₈]^0/–, Au₃₈(2-PET)₂₄, Au₄₀(2-PET)₂₄, and Au₁₄₄(2-PET)₆₀ (2-PET = 2-phenylethylthiol) were measured in order to study the influence of the cluster size and therefore the composition with respect to the monomeric and dimeric staples. Additionally, spectra of Au₂₅(2-PET)_{18–2<i>x</i>}(<i>S</i>-/<i>rac</i>-BINAS)_<i>x</i> (BINAS = 1,1′-binaphthyl-2,2′-dithiol), Au₂₅(CamS)₁₈ (CamS = 1<i>R</i>,4<i>S</i>-camphorthiol), and Au_<i>n</i>BINAS_<i>m</i> were measured to identify the influence of the thiolate ligand on the Au–S vibrations. The vibrational spectrum of Au₃₈(SCH₃)₂₄ was calculated which allows the assignment of bands to vibrational modes of the different staple motifs. The spectra are sensitive to the size of the cluster and the nature of the ligand. Au–S–C bending around 200 cm^–1 shifts to slightly higher wavenumbers for the dimeric as compared to the monomeric staples. Radial Au–S modes (250–325 cm^–1) seem to be sensitive toward the staple composition and the bulkiness of the ligand, having higher intensities for long staples and shifting to higher wavenumbers for sterically more demanding ligands. The introduction of only one BINAS dithiol has a dramatic influence on the Au–S vibrations because the molecule bridges two staples which changes their vibrational properties completely