Intermediate Silver Doping of Au25(SR)18: Variation of Electronic, Optical, and Chiroptical Properties along Au25-xAgx(SH)18- (x = 0-12) Stoichiometry from DFT Calculations.

The silver analogue of the prominent Au25(SR)18 nanocluster reveals the possibility of finding "gold"-like behavior despite their different nature, in addition to the common features among molecular AgNP. Herein, we explore the effect of successive additions of silver atoms reaching an intermediate Ag/Au doping ratio where the parent gold cluster exhibits properties from both elements. Our results show a more favorable situation as the Ag/Au ratio increases along the Au25-xAgx(SH)18- (x = 0-12) clusters, with structural distortions mainly centered at the ligand-protected shell. The calculated optical spectrum shows that from the Au19Ag6 species, a plasmon-like peak appears along species with a doping ratio above 25%, where all the silver atoms are located within the M12 icosahedron. In addition, the chiral properties were explored, showing mild optical activity from the calculated circular dichroism spectra due to the distorted ligand-shell avoiding a centrosymmetric structure. Thus, an intermediate doping ratio ascribed to a specific structural layer can recover inherent properties to both elements in the binary Au25-xAgx(SH)18- series, suggesting the possibility of having clusters with dual properties at a certain degree of element exchange. This can be useful for further exploration theoretically and synthetically toward different and larger-nuclearity clusters.

HPLC of Monolayer-Protected Gold Clusters with Baseline Separation.

The properties of ultrasmall metal nanoparticles (ca. 10-200 metal atoms), or monolayer-protected metal clusters (MPCs), drastically depend on their atomic structure. For systematic characterization and application, assessment of their purity is of high importance. Currently, the gold standard for purity control of MPCs is mass spectrometry (MS). Mass spectrometry, however, cannot always detect small impurities; MS of certain clusters, for example, ESI-TOF of Au40(SR)24, is not successful at all. We here present a simple reversed-phase HPLC method for purity control of a series of small alkanethiolate-protected gold clusters. The method allows the detection of small impurities with high sensitivity. Linear correlation between alkyl chain length of Au25(SC nH2 n+1)18 clusters ( n = 6, 8, 10, 12) and their retention time was noticed.

Enhancement of Nonlinear Optical Scattering by Gold Nanoparticles through Aggregation-Induced Plasmon Coupling in the Near-Infrared.

Gold nanoparticles (AuNPs) are regarded as promising building blocks in functional nanomaterials for sensing, drug delivery and catalysis. One remarkable property of these particles is the localized surface plasmon resonance (LSPR), which gives rise to augmented optical properties through local field enhancement. LSPR also influences the nonlinear optical properties of metal NPs (MNPs) making them potentially interesting candidates for fast, high resolution nonlinear optical imaging. In this work we characterize and discuss the wavelength dependence of the hyper-Rayleigh scattering (HRS) behavior of spherical gold nanoparticles (GNP) and gold nanorods (GNR) in solution, from 850 nm up to 1300 nm, covering the near-infrared (NIR) window relevant for deep tissue imaging. The high-resolution spectral data allows discriminating between HRS and two photon photoluminescence contributions. Upon particle aggregation, we measured very large enhancements (ca. 104 ) of the HRS intensity in the NIR, which is explained by considering aggregation-induced plasmon coupling effects and local field enhancement. These results indicate that purposely designed coupled nanostructures could prove advantageous for nonlinear optical imaging and biosensing applications.

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.

Chirality in thiolate-protected gold clusters.

Over recent years, research on thiolate-protected gold clusters Au(m)(SR)n has gained significant interest. Milestones were the successful determination of a series of crystal structures (Au102(SR)44, Au25(SR)18, Au38(SR)24, Au36(SR)24, and Au28(SR)20). For Au102(SR)44, Au38(SR)24, and Au28(SR)20, intrinsic chirality was found. Strong Cotton effects (circular dichroism, CD) of gold clusters protected by chiral ligands have been reported a long time ago, indicating the transfer of chiral information from the ligand into the cluster core. Our lab has done extensive studies on chiral thiolate-protected gold clusters, including those protected with chiral ligands. We demonstrated that vibrational circular dichroism can serve as a useful tool for the determination of conformation of the ligand on the surface of the cluster. The first reports on crystal structures of Au102(SR)44 and Au38(SR)24 revealed the intrinsic chirality of these clusters. Their chirality mainly arises from the arrangement of the ligands on the surface of the cluster cores. As achiral ligands are used to stabilize the clusters, racemic mixtures are obtained. However, the separation of the enantiomers by HPLC was demonstrated which enabled the measurement of their CD spectra. Thermally induced inversion allows determination of the activation parameters for their racemization. The inversion demonstrates that the gold-thiolate interface is anything but fixed; in contrast, it is rather flexible. This result is of fundamental interest and needs to be considered in future applications. A second line of our research is the selective introduction of chiral, bidentate ligands into the ligand layer of intrinsically chiral gold clusters. The ligand exchange reaction is highly diastereoselective. The bidentate ligand connects two of the protecting units on the cluster surface and thus effectively stabilizes the cluster against thermally induced inversion. A minor (but significant) influence of chiral ligands to the CD spectra of the clusters is observed. The studied system represents the first example of an intrinsically chiral gold cluster with a defined number of exchanged ligands, full control over their regio- and stereochemistry. The methodology allows for the selective preparation of mixed-ligand cluster compounds and a thorough investigation of the influence of single ligands on the cluster's properties. Overall, the method enables even more detailed tailoring of properties. Still, central questions remain unanswered: (1) Is intrinsic chirality a ubiquitous feature of thiolate-protected gold clusters? (2) How does chirality transfer work? (3) What are the applications for chiral thiolate-protected gold clusters? In this Account, we summarize the main findings on chirality in thiolate-protected gold cluster of the past half decade. Emphasis is put on intrinsically chiral clusters and their structures, optical activity, and reactivity.

Chiral phase transfer and enantioenrichment of thiolate-protected Au102 clusters.

The Au102(p-MBA)44 cluster (p-MBA: para-mercaptobenzoic acid) is observed as a chiral compound comprised of achiral components in its single-crystal structure. So far the enantiomers observed in the crystal structure are not isolated, nor is the circular dichroism spectrum known. A chiral phase transfer method is presented which allows partial resolution of the enantiomers by the use of a chiral ammonium bromide, (-)-1R,2S-N-dodecyl-N-methylephedrinium bromide ((-)-DMEBr). At sufficiently low concentration of (-)-DMEBr, the phase transfer from water to chloroform is incomplete. Both the aqueous and organic phases show optical activity of near mirror image relationship. Differences in the spectra are ascribed to the formation of diastereomeric salts. At high concentrations of (-)-DMEBr, full phase transfer is observed. The organic phase, however, still displays optical activity. We assume that one of the diastereomers has very strong optical activity, which overrules the cancelation of the spectra with opposite sign. Comparison with computations further corroborates the experimental data and allows a provisional assignment of handedness of each fraction.

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 1S(2) 1P(6) 1D(6) 2S(2) 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.

The fate of Au25(SR)18 clusters upon ligand exchange with binaphthyl-dithiol: interstaple binding vs. decomposition.

The ligand exchange reaction between monodisperse Au25(2-PET)18 (2-PET: 2-phenylethylthiolate) clusters and 1,1'-binaphthyl-2,2'-dithiol (BINAS) was long thought to induce decomposition of the cluster (Si et al., J. Phys. Chem. C, 2009). We repeated the experiment and analyzed the reaction products using MALDI-TOF mass spectrometry. The spectra clearly indicate successful ligand exchange, bidentate binding of the BINAS ligand and intact Au25 clusters. The reaction products are identified as Au25(2-PET)18-2x(BINAS)x (x = 1-4) for a 24 h reaction with a 50-fold molar excess of BINAS. Two likely binding motifs are discussed. Analysis of atomic distances in both the cluster and the free ligand indicates interstaple binding connecting the central sulfur atom of the protecting (SRAu)2SR with the outer sulfur atom of a second unit. The results presented have implications on the binding position of BINAS in Au38(SR)24-2x(BINAS)x clusters.

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: p-tert-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 Au14(6+) 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.

Chiroptical properties of intrinsically chiral thiolate-protected gold clusters.

Chiral thiolate-protected gold clusters of atomic precision have gained increasing interest in recent years due to their potential use in catalysis, sensing or bioapplications. While the protection of gold clusters with chiral ligands is a rather trivial task, it was found that the clusters can bear intrinsically chiral features, most obvious in the arrangement of the protecting ligands on the surface of the cluster. Recent efforts showed the separation of the enantiomers of such intrinsically chiral gold clusters. This technique can be used for the prediction of chirality in structurally unknown clusters. Activation barriers for the racemization of Au38(SR)24 were determined. As this involves a huge rearrangement of the ligands, the flexibility of the gold-thiolate interface is demonstrated. Furthermore, the ligand exchange reactions between intrinsically chiral clusters and bidentate chiral thiols were studied. A limited, regioselective exchange was found. Most importantly, the reaction is diastereoselective and allows tailoring of gold clusters that are protected with a defined layer of ligands.

In situ reaction monitoring reveals a diastereoselective ligand exchange reaction between the intrinsically chiral Au38(SR)24 and chiral thiols.

The ligand exchange reaction between racemic Au(38)(2-PET)(24) (2-PET = 2-phenylethylthiolate) clusters and enantiopure 1,1'-binaphthyl-2,2'-dithiol (BINAS) was monitored in situ using a chiral high-performance liquid chromatography approach. In the first exchange step, a clear preference of R-BINAS for the left-handed enantiomer of Au(38)(2-PET)(24) is observed (about 4 times faster than reaction with the right-handed enantiomer). The second exchange step is much slower than the first step. BINAS substitution deactivates the cluster for further exchange, which is attributed to (stereo)electronic effects. The results constitute the first example of a ligand exchange reaction in a thiolate-protected gold cluster with directed enrichment of a defined species in the product mixture. This may open new possibilities for the design of nanomaterials with tailored properties.

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.

Au40(SR)24 cluster as a chiral dimer of 8-electron superatoms: structure and optical properties.

We predict and analyze density-functional theory (DFT)-based structures for the recently isolated Au(40)(SR)(24) cluster. Combining structural information extracted from ligand-exchange reactions, circular dichroism and transmission electron microscopy leads us to propose two families of low-energy structures that have a chiral Au-S framework on the surface. These families have a common geometrical motif where a nonchiral Au(26) bi-icosahedral cluster core is protected by 6 RS-Au-SR and 4 RS-Au-SR-Au-SR oligomeric units, analogously to the "Divide and Protect" motif of known clusters Au(25)(SR)(18)(-/0), Au(38)(SR)(24) and Au(102)(SR)(44). The strongly prolate shape of the proposed Au(26) core is supported by transmission electron microscopy. Density-of-state-analysis shows that the electronic structure of Au(40)(SR)(24) can be interpreted in terms of a dimer of two 8-electron superatoms, where the 8 shell electrons are localized at the two icosahedral halves of the metal core. The calculated optical and chiroptical characteristics of the optimal chiral structure are in a fair agreement with the reported data for Au(40)(SR)(24).

The wavelength-dependent non-linear absorption and refraction of Au25 and Au38 monolayer-protected clusters.

In the past decade, the structural and electronic properties of monolayer-protected metal clusters, which can be produced size-selected in macroscopic amounts, have received a lot of attention. Their great potential for optical applications has been identified. In the high intensity regime, monolayer-protected metal clusters show pronounced nonlinear absorption and refraction. Naturally, these phenomena are wavelength-dependent, however, such dependence is largely unexplored. Here, we quantify the wavelength-dependent non-linear optical absorption and refraction cross sections of atomically precise Au25(DDT)18 and Au38(DDT)24 clusters, using the z-scan technique in combination with a tunable nanosecond laser source. Qualitatively different non-linear optical phenomena were found to take place at different excitation wavelengths (two-photon and excited-state absorption, intensity saturation and non-linear refraction). Both clusters have high nonlinear absorption cross sections at 532 nm, and present a (local) maximum at 640 nm, together with a maximum in the absorption saturation. The nonlinear refraction is always negative for Au25(DDT)18, while it changes sign for Au38(DDT)24. Depending on the wavelength, the underlying mechanism of the nonlinear absorption effects is two-photon absorption or excited state absorption. The obtained very high nonlinear cross sections, on the order of 107-109 GM, demonstrate the great potential of those clusters as nonlinear absorption or refraction materials in optical applications.

Size exclusion chromatography for semipreparative scale separation of Au38(SR)24 and Au40(SR)24 and larger clusters.

Size exclusion chromatography (SEC) on a semipreparative scale (10 mg and more) was used to size-select ultrasmall gold nanoclusters (<2 nm) from polydisperse mixtures. In particular, the ubiquitous byproducts of the etching process toward Au(38)(SR)(24) (SR, thiolate) clusters were separated and gained in high monodispersity (based on mass spectrometry). The isolated fractions were characterized by UV-vis spectroscopy, MALDI mass spectrometry, HPLC, and electron microscopy. Most notably, the separation of Au(38)(SR)(24) and Au(40)(SR)(24) clusters is demonstrated.

Ligand exchange reactions on Au(38) and Au(40) clusters: a combined circular dichroism and mass spectrometry study.

The thiolate-for-thiolate ligand exchange reaction between the stable Au(38)(2-PET)(24) and Au(40)(2-PET)(24) (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1'-binaphthyl-2,2'-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au(25)(2-PET)(18)](-) clusters. Maximum anisotropy factors of 6.6 × 10(-4) are found after 150 h of reaction time. During the reaction, a varying ratio between Au(38) and Au(40) clusters is found, which significantly differs from the starting material. As compared to Au(38), Au(40) is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au(38) and Au(40) clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS.

Tailoring atomic layer growth at the liquid-metal interface.

Engineering atomic structures at metal surfaces represents an important step in the development of novel nanomaterials and nanodevices, but relies predominantly on atomic/molecular beam epitaxy under ultrahigh vacuum conditions, where controlling the deposition processes remains challenging. By using solution-borne nanosized gold clusters as a precursor, here we develop a wet deposition protocol to the fabrication of atomically flat gold nanoislands, so as to utilize the dynamic exchange of surface-active molecules at the liquid-metal interface for manipulating the growth kinetics of ultrathin metallic nanostructures. While remarkable shape and size selection of gold nanoislands is observed, our experimental and theoretical investigations provide compelling evidences that organic adsorbates can impart a bias to the island orientation by preferred adsorption and alignment and intervene in the assembly and disassembly of adatom islands by complexing with Au adatoms. This approach offers a simple solution to regulate atomic layer growth of metals at ambient conditions.

Isolation of atomically precise mixed ligand shell PdAu24 clusters.

Exposure of PdAu24(2-PET)18 (2-PET: 2-phenylethylthiolate) to BINAS (1,1-binaphthyl-2,2-dithiol) leads to species of composition PdAu24(2-PET)18-2x(BINAS)x due to ligand exchange reactions. The BINAS adsorbs in a specific mode that bridges the apex and one core site of two adjacent S(R)-Au-S(R)-Au-S(R) units. Species with different compositions of the ligand shell can be separated by HPLC. Furthermore, site isomers can be separated. For the cluster with exactly one BINAS in its ligand shell only one isomer is expected due to the symmetry of the cluster, which is confirmed by High-Performance Liquid Chromatography (HPLC). Addition of a second BINAS to the ligand shell leads to several isomers. In total six distinguishable isomers are possible for PdAu24(2-PET)14(BINAS)2 including two pairs of enantiomers concerning the adsorption pattern. At least four distinctive isomers are separated by HPLC. Calculations indicate that one of the six possibilities is energetically disfavoured. Interestingly, diastereomers, which have an enantiomeric relationship concerning the adsorption pattern of chiral BINAS, have significantly different stabilities. The relative intensity of the observed peaks in the HPLC does not reflect the statistical weight of the different isomers. This shows, as supported by the calculations, that the first adsorbed BINAS molecule influences the adsorption of the second incoming BINAS ligand. In addition, experiments with the corresponding Pt doped gold cluster reveal qualitatively the same behaviour, however with slightly different relative abundances of the corresponding isomers. This finding points towards the influence of electronic effects on the isomer distribution. Even for clusters containing more than two BINAS ligands a limited number of isomers were found, which is in contrast to the corresponding situation for monothiols, where the number of possible isomers is much larger.

Symmetry breaking in ligand-protected gold clusters probed by nonlinear optics.

The first hyperpolarizabilities of [Au25(SR)18](-1/0) and Au38(SR)24 clusters were determined by Hyper-Rayleigh Scattering. A strong dependence on the molecular symmetry was observed, and we explore two strategies to destroy the center of inversion in [Au25(SR)18](-1/0), protection by chiral ligands and alloying of the cluster with silver. This may open new avenues to applications of Au : SR clusters in second-order nonlinear optics.