12 research outputs found
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 (Au<sub>130</sub>(SR)<sub>50</sub>, Au<sub>144</sub>(SR)<sub>60</sub>, and Au<sub>500</sub>(SR)<sub>120</sub>). 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 [Au<sub>20</sub>(PP<sub>3</sub>)<sub>4</sub>]<sup>4+</sup>
The
recently solved crystal structure of the [Au<sub>20</sub>(PP<sub>3</sub>)<sub>4</sub>]ÂCl<sub>4</sub> cluster (PP<sub>3</sub>: trisÂ(2-(diphenylphophino)Âethyl)Âphosphine)
is examined using density functional theory (DFT). The Au<sub>20</sub> 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 [Au<sub>20</sub>(PP<sub>3</sub>)<sub>4</sub>]ÂCl<sub>4</sub> 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<sup>2</sup> 1P<sup>6</sup> 1D<sup>6</sup> 2S<sup>2</sup> is found. The
2S band is strongly stabilized, and the 1D states are nondegenerate
with a large gap. Ligand protection of the (Au<sub>20</sub>)<sup>4+</sup> 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 Au<sub>38</sub>(SR)<sub>24</sub>. We observe that the chiral arrangement of the protecting Au-SR
units in Au<sub>38</sub>(SR)<sub>24</sub> 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 Au<sub>38</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>24â2<i>x</i></sub>(<i>R</i>âBINAS)<sub><i>x</i></sub>
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>-1,1â˛-binaphthyl-2,2â˛-dithiol (BINAS) into the ligand
layer of <i>rac</i>-Au<sub>38</sub>(2-PET)<sub>24</sub> clusters
(2-PET: 2-phenylethylthiolate, SCH<sub>2</sub>CH<sub>2</sub>Ph) was
shown to be diastereoselective. In this contribution, we isolated
and characterized the diastereomeric reaction products of the first
exchange step, <i>A</i>-Au<sub>38</sub>(2-PET)<sub>22</sub>(<i>R</i>-BINAS)<sub>1</sub> and <i>C</i>-Au<sub>38</sub>(2-PET)<sub>22</sub>(<i>R</i>-BINAS)<sub>1</sub> (<i>A</i>/<i>C</i>, anticlockwise/clockwise)
and the second exchange product, <i>A</i>-Au<sub>38</sub>(2-PET)<sub>20</sub>(<i>R</i>-BINAS)<sub>2</sub>. The absorption
spectra show minor, but significant influence of the BINAS ligand.
Overall, the spectra are less defined as compared to Au<sub>38</sub>(2-PET)<sub>24</sub>, which is ascribed to symmetry breaking. The
CD spectra are similar to those of the parent Au<sub>38</sub>(2-PET)<sub>24</sub> 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 Au<sub>38</sub>(2-PET)<sub>24</sub>. 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 goldâsulfur interface
Racemization of a Chiral Nanoparticle Evidences the Flexibility of the GoldâThiolate Interface
Thiolate-protected gold nanoparticles and clusters combine
size-dependent
physical properties with the ability to introduce (bio)Âchemical 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 goldâsulfur
interface. Here we report the first study on racemization of an intrinsically
chiral gold nanocluster, Au<sub>38</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>24</sub>, which goes along with a drastic rearrangement of
its surface involving place exchange of several thiolates. This racemization
takes place at modest temperatures (40â80 °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 AuâS
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 β<sub>0</sub> of eight model clusters [Au<i><sub>m</sub></i>(SH)<sub><i>n</i></sub>]<sup>z</sup> (<i>m</i> = 18â38)
by means of density-functional theory. We used simplified ligands
âSH, which may lead to higher symmetries than in actual systems
(e.g., with âSCH2CH2Ph or âSPh ligands). A dependence
of the obtained values on the exchange-correlation functional during
geometry optimization was found. No correlation between cluster size
and β<sub>0</sub> was identified. Instead, the symmetry of the
clusters seems to dominate the NLO properties. Our survey predicts
strong NLO responses in the chiral Au<sub>38</sub>(SR)<sub>24</sub> cluster, whereas centrosymmetric structures such as the [Au<sub>25</sub>(SR)<sub>18</sub>)]<sup>â</sup> yield hyperpolarizabilities
close to zero. This is in line with recent experimental results obtained
by second-harmonic generation. The centrosymmetry of the Au<sub>25</sub> cluster is efficiently destroyed by ligand exchange, as demonstrated
by the inclusion of chiral, bidentate ligands (1,1â˛-binaphthyl-2,2â˛-dithiol,
1,1â˛-biphenyl-2,2â˛-dithiol) and two thiophenolate ligands.
This induces significant hyperpolarizabilities, surpassing those of
intrinsically chiral clusters (e.g., Au<sub>38</sub>(SH)<sub>24</sub>). 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 [Au<sub>25</sub>(SR)<sub>18âx</sub>(SRâ˛)<sub><i>x</i></sub>]<sup>â</sup>, where SRⲠrepresents
a para-substituted thiophenolate derivative (SPh-4-X). We computed
the first hyperpolarizabilities, screening a broad range of substituents
X. For [Au<sub>25</sub>(SR)<sub>17</sub>(SRâ˛)<sub>1</sub>]<sup>â</sup> 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, âpush/pullâ 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 chemistâs toolbox
Electronic Structure and Optical Properties of the Thiolate-Protected Au<sub>28</sub>(SMe)<sub>20</sub> Cluster
The recently reported crystal structure
of the Au<sub>28</sub>(TBBT)<sub>20</sub> 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 Au<sub>3</sub>(SR)<sub>4</sub> binding motif. The cluster can be formulated as Au<sub>14</sub>(Au<sub>2</sub>(SR)<sub>3</sub>)<sub>4</sub>(Au<sub>3</sub>(SR)<sub>4</sub>)<sub>2</sub>. The electronic structure of the Au<sub>14</sub><sup>6+</sup> 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: Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub> and Au<sub>38</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>24</sub>. At a fundamental wavelength of 800
nm, the Au<sub>38</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>24</sub> cluster is active. In contrast, Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub> does not yield significant SHG signal. We
ascribe this to the center of inversion in the Au<sub>25</sub> cluster.
Measurements on chiral Au<sub>25</sub>(capt)<sub>18</sub> (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
Au<sub>36</sub>(SPh)<sub>24</sub> Nanomolecules: Xâray 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 Au<sub>36</sub>(SPh)<sub>24</sub> nanomolecule, which has a face-centered cubic (<i>fcc</i>) core similar to other nanomolecules such as Au<sub>36</sub>(SPh-tBu)<sub>24</sub> and Au<sub>36</sub>(SC<sub>5</sub>H<sub>9</sub>)<sub>24</sub> 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 âAu<sub>36</sub>(SPh)<sub>23</sub>â 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 Au<sub>36</sub>(SPh)<sub>24</sub>, 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 UVâvis spectroscopy,
cyclic voltammetry, and differential pulse voltammetry. Au<sub>36</sub>(SPh)<sub>24</sub> shows a broad electrochemical gap near 2 V, a
desirable optical gap of âź1.75 eV for dye-sensitized solar
cell applications, as well as appropriately positioned electrochemical
potentials for many electrocatalytic reactions
Au<sub>36</sub>(SPh)<sub>24</sub> Nanomolecules: Xâray 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 Au<sub>36</sub>(SPh)<sub>24</sub> nanomolecule, which has a face-centered cubic (<i>fcc</i>) core similar to other nanomolecules such as Au<sub>36</sub>(SPh-tBu)<sub>24</sub> and Au<sub>36</sub>(SC<sub>5</sub>H<sub>9</sub>)<sub>24</sub> 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 âAu<sub>36</sub>(SPh)<sub>23</sub>â 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 Au<sub>36</sub>(SPh)<sub>24</sub>, 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 UVâvis spectroscopy,
cyclic voltammetry, and differential pulse voltammetry. Au<sub>36</sub>(SPh)<sub>24</sub> shows a broad electrochemical gap near 2 V, a
desirable optical gap of âź1.75 eV for dye-sensitized solar
cell applications, as well as appropriately positioned electrochemical
potentials for many electrocatalytic reactions