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Angewandte
Eine Zeitschrift der Gesellschaft Deutscher Chemiker
Chemie
www.angewandte.de
Akzeptierter Artikel
Titel: Self-Assembly of Chiral Au Clusters into Crystalline Nanocubes
of Exceptional Optical Activity
Autoren: Lin Shi, Lingyun Zhu, Jun Guo, Lijuan Zhang, Yanan Shi,
Yin Zhang, Ke Hou, Yonglong Zheng, Yanfei Zhu, Jiawei Lv,
Shaoqin Liu, and Zhiyong Tang
Dieser Beitrag wurde nach Begutachtung und Überarbeitung sofort als
"akzeptierter Artikel" (Accepted Article; AA) publiziert und kann unter
Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer
(DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der
endgültigen englischen Fassung erscheinen. Die endgültige englische
Fassung (Version of Record) wird ehestmöglich nach dem Redigieren
und einem Korrekturgang als Early-View-Beitrag erscheinen und kann
sich naturgemäß von der AA-Fassung unterscheiden. Leser sollten
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Für die AA-Fassung trägt der Autor die alleinige Verantwortung.
Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.201709827
Angew. Chem. 10.1002/ange.201709827
Link zur VoR: http://dx.doi.org/10.1002/anie.201709827
http://dx.doi.org/10.1002/ange.201709827
10.1002/ange.201709827
Angewandte Chemie
COMMUNICATION
Self-Assembly of Chiral Au Clusters into Crystalline Nanocubes
of Exceptional Optical Activity
Abstract: Self-assembly of inorganic nanoparticles into ordered
structures is of paramount importance in both science and
technology because of expected generation of new property through
collective behavior; however, such nanoparticle assemblies with
characteristics largely distinct from individual building blocks are
hardly acquired so far. Here we use atomically precise Au clusters to
fabricate ordered assemblies with emerging optical activity. Chiral
Au clusters with strong circular dichroism (CD) but free of circularly
polarized luminescence (CPL) are successfully synthesized and
organized into uniform body-centred cubic (BCC) packing
nanocubes. Once the ordered structure is formed, the CD intensity is
significantly enhanced along with appearance of remarkable CPL
response. Both experiment and theory calculation disclose that the
CPL originates from restricted intramolecular rotation and ordered
stacking pattern of chiral stabilizers, which are fastened in the
crystalline lattices.
Inorganic nanoparticle self-assembly not only offers a feasible
route to realize possible application of nanomaterials in macro
world, but also provides opportunity to produce new
physiochemical properties beyond their individual building blocks
through the collective behavior.[1] Generally, the prerequisite of
self-assembly is that both size and shape of the building blocks
must be highly uniform.[2] Such stringent requirement severely
limits scale application of nanoparticles via self-assembly
process, because different from molecular synthesis, to prepare
monodisperse nanoparticles needs accurate control over
nucleation and growth process that is time- and cost-consuming.
Another great challenge in the field of nanoparticle assembly is
that the reported assemblies seldom exhibit the emerging
property or function greatly different from individual
nanoparticles, and therefore the advantage of the ordered
assemblies as well as the necessity of self-assembly fabrication
are not highlighted.[3]
We expect that a specific type of nanoparticles, noble
metal clusters with the size of less than 2 nm,[4] would be
excellent candidates for assembly building blocks. Thanks to
their thermodynamic stability, varied types of noble metal
clusters with magic number of composed atoms have been
easily synthesized in a large quantity, allowing possible
[a]
[b]
L. Shi, Prof. S. Liu
School of Materials Science and Engineering, Center for Micro and
Nanotechnology, Harbin Institute of Technology
Harbin 150001 (P. R. China)
E-mail: shaoqinliu@hit.edu.cn
L. Shi, Dr. L. Zhu, J. Guo, L. Zhang, Y. Shi, Y. Zhang, K. Hou, Y.
Zheng, Y. Zhu, J. Lv, Prof. Z. Tang
CAS Key Laboratory of Nanosystem and Hierarchical Fabrication,
CAS Center for Excellence in Nanoscience, National Center for
Nanoscience and Technology
Beijing 100190 (P. R. China)
E-mail: zytang@nanoctr.cn
Supporting information for this article is given via a link at the end of
the document.
construction of the assemblies at atomic level.[5] However, until
now there are very rare reports on self-assembly of noble metal
clusters into ordered colloidal structures;[6] and more importantly,
no any new function is achieved with the assembled structures
compared with individual building blocks though the clusters
themselves possess many intriguing optical, electrical, magnetic
and catalytic property.
Herein, we target investigating self-assembly of chiral Au
clusters mainly because of the following two reasons. (1) With
respect to other nanoparticles, specific chirality could appear in
Au clusters originating from organic ligand, surface staple or
intrinsic Au atom arrangement, which gives rise to many
additional unique features.[7] (2) It is known that chirality and its
corresponding property are highly sensitive to surrounding
environment and spatial organization, so their assemblies might
generate the novel property distinct from individual chiral
clusters.[8] Based on above idea, we attempt to design and
synthesize the smallest chiral Au clusters, which optical activity
is supposed to be easily influenced after self-assembly into the
ordered structures.
(R)- or (S)-2,2'-bis(di-p-tolylphosphino)-1,1'-binaphthyl, (R)or (S)-Tol-BINAP, was adopted as chiral ligands to synthesize
Au clusters (Supporting Information part 1.2).[9] The single
crystal X-ray diffraction (SC-XRD) analysis reveals that the
products are Au3[(R)-Tol-BINAP]3Cl and Au3[(S)-Tol-BINAP]3Cl
clusters, respectively (Table S1 and S2),[10] and as-formed
crystal structure of both Au3[(R)-Tol-BINAP]3Cl and Au3[(S)-TolBINAP]3Cl clusters belongs to a cubic space group, I213, which
is chiral. In detail, three Au atoms in one cluster connect with
each other and form a regular triangle, in which the Au-Au bond
lengths are 2.676(2) Å (Figure 1 and Figure S1b). These Au-Au
bond lengths are well in the reported range of 2.572(2)-3.216(2)
Å, suggesting presence of the attractive aurophilic interaction
inside chiral Au clusters.[11] In addition, each Au atom binds with
a chiral bi-phosphine ligand through the P-Au-P bond with
unequal bond length of 2.380(7) Å and 2.397(8) Å. By simply
changing the chirality of ligand from R to S during synthesis, one
can easily achieve the cluster of opposite chirality (Figure 1).
Figure 1. Au3[(R)-Tol-BINAP]3Cl (a) and Au3[(S)-Tol-BINAP]3Cl (b) (Au,
yellow; P, orange; C, gray; for clarity, hydrogen and counter anion chloride are
omitted). The surface aromatic ligands arrange in either left-handed or righthanded stacking when viewed perpendicularly to Au3 plane.
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Lin Shi[a], [b], Lingyun Zhu[b], Jun Guo[b], Lijuan Zhang[b], Yanan Shi[b], Yin Zhang[b], Ke Hou[b], Yonglong
Zheng[b], Yanfei Zhu[b], Jiawei Lv[b], Shaoqin Liu*[a], Zhiyong Tang*[b]
10.1002/ange.201709827
Angewandte Chemie
The molecular formula of Au3[(R)-Tol-BINAP]3Cl cluster
was further verified by electrospray ionization mass
spectrometry (ESI-MS). Figure S2a shows an isotopically
resolved peak at m/z = 2626.71987 with a +1 charge,
corresponding to a species of the cluster minus one Cl- (calcd:
2626.68411 in Figure S2b). As for the Au3 core, its charge state
is determined by measuring the bonding energy of Au4f7/2 via Xray photoelectron spectroscopy (XPS). The fitting result
manifests that all the Au atoms in the cluster are same,
characteristic with the bonding energy of Au 4f7/2 at 84.4 eV and
Au 4f5/2 at 88.1 eV that locates in the middle of Au (0) and Au (I)
(Figure S3). In regard of the surrounding chiral ligand, its six
aromatic rings can be divided into three groups based on the
crystal structure (Figure 1): two outward naphthalene rings
(yellow double hexagons), two outward p-tolyl rings (yellow
hexagons) and two inward p-tolyl rings (red hexagon and hollow
hexagon). These aromatic rings form left- or right-handed
stacking on the surface of Au clusters based on the chirality of
ligands (Figure 1), endowing the capability to produce the strong
optical activity of Au clusters[12].
hexane increasing (Figure 2b), resulting from formation of strong
intermolecular interactions among chiral ligands upon cluster
assembly.[13] In detail, the negative CD peak at 239 nm red shifts
to 256 nm, and the original positive peak at 267 nm becomes
weak and finally disappears; while the negative peak at 288 nm
decreases and red shifts, accompanying with formation of new
negative peaks at 325 nm and 355 nm (Figure 2b). Most
importantly, the positive peak at around 366 nm of the maximum
gabs factor gradually diminishes and finally disappears (black
arrow in Figure 2b), whereas a new positive peak progressively
appears at 445 nm and enhances with the fraction of n-hexane
increasing (red arrow in Figure 2b).The maximum gabs value at
445 nm is up to 8.6 x 10-3 in 70% n-hexane (Figure S5b). When
the fraction of n-hexane exceeds 70%, the mixed solvent
becomes turbid and UV-Vis absorption intensity increases
substantially (Figure 3c), implying that large-sized aggregates
with strong light scattering are produced with too much
antisolvent (Figure S6). As a result, the maximum gabs factor is
decreased (4.5 x 10-3 for 80% n-hexane and 4.0 x 10-3 for 90%
n-hexane, respectively) (Figure 2c, d).
Figure 2. a, CD spectra of Au3[(R)-Tol-BINAP]3Cl and Au3[(S)-Tol-BINAP]3Cl
clusters in DCM. b-d, CD spectra (b), UV-Vis spectra (c) and corresponding
gabs factor (d) of Au3[(R)-Tol-BINAP]3Cl clusters in DCM with various n-hexane
content (cluster concentration: 5 x 10-5 M; optical path length: 1 mm).
Figure 3. a, PL spectra of Au3[(R)-Tol-BINAP]3Cl clusters in DCM with various
n-hexane content (inserts present digital photos of samples with 0 or 70% nhexane under irradiation of 365 nm UV light). b, Relative PL intensity against
composition of the mixed solvent. c, d, CPL spectra (c) and corresponding glum
factor (d) of Au3[(R)-Tol-BINAP]3Cl (blue curve) and Au3[(S)-Tol-BINAP]3Cl
(red curve) assemblies in 70% n-hexane. The dashed fluctuations in (c) are
the original collected data, while the solid curves are the smoothed ones
(cluster concentration: 5 x 10-5 M; optical path length: 1 mm).
The enantiomers of Au clusters that are well dispersed in
dichloromethane (DCM) exhibit intense CD response with an
excellent mirror image in the wavelength range of 220 nm - 500
nm (Figure 2a), and thus we select Au3[(R)-Tol-BINAP]3Cl
clusters as the representative for following study. The CD
spectrum of Au3[(R)-Tol-BINAP]3Cl possesses four distinct
peaks at 239 nm, 267 nm, 288 nm and 366 nm, respectively,
which is largely different with CD feature of pure chiral ligands
(Figure S4). Impressively, the maximum absorption anisotropy
factor (gabs factor) reaches 7.0 x 10-3 at 366 nm (Figure S5a),
which is record high among the reported noble metal clusters
(Table S3). Subsequent self-assembly of chiral Au clusters in
polar DCM was easily implemented through adding non-polar nhexane as antisolvent. Evidently, all the CD peaks show gradual
bathochromic shift and become broadened with the fraction of n-
A striking discovery in optical property is that upon selfassembly, the non-luminescent Au clusters progressively
become highly luminescent (Figure 3a and Figure S6). An
orange emission band centered at 583 nm appears for Au3[(R)Tol-BINAP]3Cl clusters when the fraction of n-hexane is
increased to 40%, and displays further enhancement with
increase of n-hexane fraction. When the fraction value of nhexane gets to 70%, the photoluminescence (PL) intensity
reaches the highest value with a quantum yield (QY) of 3.6%
(calibrated with luminescent Rhodamine 6G) (Figure 3b). The PL
intensity shows obvious decrease when the fraction value of nhexane exceeds 70%, likely due to formation of large-sized
This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/ange.201709827
Angewandte Chemie
aggregates with poor crystallinity.[14] The similar change is found
by analyzing their PL decay profiles. The dominant PL decay
time also follows the order: 50% n-hexane (0.59 μs)  60% nhexane (0.69 μs)  70% n-hexane (0.79 μs)  80% n-hexane
(0.75 μs)  90% n-hexane (0.68 μs) (Figure S7). The
microsecond PL decay time and the large Stokes shift of ~138
nm clearly suggest that the luminescence of clusters originates
from a triplet parentage, which is assigned to a triplet ligand-tometal charge transfer (3LMCT) excited state or a triplet ligand-tometal-metal charge transfer (3LMMCT) excited state.[15] It needs
to be stressed that based on the facts of 2.676(2) Å Au-Au bond
length and the mixed-valence of Au (0/I), the attractive aurophilic
interaction should considerably contribute to the PL property of
chiral Au clusters.[16] More intriguingly, these chiral Au cluster
assemblies show strong CPL response in the same wavelength
region as their PL peak centered at 583 nm (Figure 3c). It is
noted that the luminescence anisotropy factor (glum factor),
remains zero when the fraction of n-hexane is less than 40%,
followed by dramatic increase and subsequent slight decrease
with gradual increase of n-hexane content (Figures S8 and S9).
The maximum glum factor of about ± 7 x 10-3 is acquired in 70%
n-hexane (Figure 3d).
Figure 4. a, Scanning electron microscope (SEM) image of Au3[(R)-TolBINAP]3Cl cluster assemblies in DCM with 70% n-hexane. The insert is side
view of a nanocube. b, PXRD pattern of Au3[(R)-Tol-BINAP]3Cl cluster
assemblies (black curve) and simulated pattern according to the crystal
structure (red lines). c, d, TEM image (c) and corresponding SAED patterns
(d) of self-assembled nanocubes.
What is the reason responsible for the significant change in
both CD and CPL activity upon tuning n-hexane fraction in the
mixture solvent? To answer this question, we firstly
characterized the morphology and structure of as-assembled
products. As revealed by dynamic light scattering (DLS)
measurement, Au3[(R)-Tol-BINAP]3Cl clusters are well dispersed
in the solvent with n-hexane fraction of less than 40%; whereas
the cluster aggregates appear when n-hexane fraction exceeds
40% and their sizes gradually grow from hundred nanometers to
several micrometers along with increase of poor solvent fraction
(Figure S10). Transmission electron microscopy (TEM) imaging
and corresponding selected area electron diffraction (SAED)
survey further indicate that the cluster aggregates are irregular
and amorphous when the fraction of n-hexane is less than 60%,
while well-defined nanocubes of sharp edges start to appear in
the mixed solvent of 60% hexane (Figure S11). Significantly,
cluster assemblies become very uniform when the fraction of nhexane reaches 70%, which exhibit a cubic morphology with an
average edge length of 366 nm (Figure 4a, c and Figure S12).
With further increase of the fraction of n-hexane, the grain sizes
of Au cluster aggregates instead decrease and their shapes turn
to be disparity (Figure S13), which are caused by too quick
aggregation of Au clusters in poor solvent.[17] We then focused
the structure investigation on uniform nanocubes obtained in
70% hexane (Figure 4). Notably, all the diffraction peaks in
powder X-ray diffraction (PXRD) curve of nanocubes are well
assigned to the simulated BCC packing pattern based on the
single crystal structure of Au3[(R)-Tol-BINAP]3Cl clusters (Figure
4b). Such an ordered BCC packing structure is also verified by
SAED observation on single nanocube (Figure 4c, d), where
sharp diffraction spots corresponding to (-200) and (020)
reflections are clearly discerned along [001] direction. The
calculated interplanar spacing along (-200) from SAED (1.41
nm) is very close to the value acquired by SC-XRD (1.44 nm),
which further confirms the crystalline BCC packing pattern inside
nanocubes.
To understand the influence of cluster assembly on the
optical activity, theory calculation on individual cluster was firstly
carried out. It deserves mentioning that the simulated UV-Vis
absorption and CD spectra of the enantiomers match very well
with the measured ones regardless of the peak sign or the peak
position, demonstrating the validity of our calculation (Figure
S14a). The HOMO of Au3[(R)-Tol-BINAP]3Cl and Au3[(S)-TolBINAP]3Cl clusters mainly lies on the Au and P atoms, while the
transition-related doubly degenerate LUMO and LUMO+1 mostly
locate on the naphthalene rings of (R)- or (S)-Tol-BINAP ligands
(Figure S14b). All the excited states are composed by metal-toligand charge transfer (MLCT) and ligand-to-ligand charge
transfer (LLCT), so the conformation of chiral ligands on Au
cluster surfaces is crucial in determining their optical activity (for
detailed analysis, see Figure S14-S25).
The extremely enhanced optical activity via self-assembly
of the chiral Au clusters can be understood based on crystal
structure analysis. In the nanocube with the ordered chiral I213
packing structure, each Au cluster contacted with six nearby Au
clusters, which are divided into two groups as the cluster
assembly possesses a local chiral C3 symmetry (Supporting
Movie). Three adjacent Au clusters form CH/π interactions with
the central one via both outward naphthalene ring pairs (Figure
S26) and the inward p-tolyl ring pairs (Figure S27). Meanwhile,
the other three adjacent Au clusters form CH/π interactions with
the central one through an outward naphthalene rings and an
inward p-tolyl rings (Figure S28). Note that the attraction energy
of the CH/π interactions is in the range of -1.5 to -2.5 kcal/mol,[18]
which is much larger than the molecular thermal energy at room
temperature (0.57 kcal/mol). Once the Au clusters are
This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/ange.201709827
Angewandte Chemie
COMMUNICATION
Acknowledgements
The authors acknowledge financial support from National Key
Basic Research Program of China (2014CB931801 and
2016YFA0200700, Z.Y.T.), National Natural Science Foundation
of China (21475029 and 91427302, Z.Y.T.), Frontier Science
Key Project of the Chinese Academy of Sciences (QYZDJ-SSWSLH038, Z.Y.T.), Instrument Developing Project of the Chinese
Academy of Sciences (YZ201311, Z.Y.T.), CAS-CSIRO
Cooperative Research Program (GJHZ1503, Z.Y.T.), "Strategic
Priority Research Program" of Chinese Academy of Sciences
(XDA09040100, Z.Y.T.) and K.C.Wong Education Foundation.
Keywords: self-assembly • gold • nanocluster • chirality •
circularly polarized luminescence
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This article is protected by copyright. All rights reserved.
Accepted Manuscript
assembled, the strong intermolecular CH/π interactions would
largely restrict the intramolecular rotation of the inward p-tolyl
rings, which is confirmed by temperature- or concentrationdependent 1H nuclear magnetic resonance (NMR) spectroscopy
(Figures S29-S33). Subsequently, ordered left- or right-handed
stacking patterns are formed on the surface of R/S-Au3 clusters
(red arrows in Figure 1,), which are responsible for
enhancement of CD intensity. Such strong intermolecular CH/π
interactions between chiral ligands further result in red shift of
the peaks in both UV-Vis absorption and CD spectra (Figure 3b
and 3c), which is reasonable considering that all the peaks
originating from MLCT or LLCT process are largely contributed
by chiral ligands (Figure 5 and Tables S4 and S5). More
importantly, the restricted intramolecular rotation of inward ptolyl rings efficiently blocks the non-radiative relaxation channel
of the excited state and populates its radiative decay pathway,[19]
which finally facilitates generation of PL and CPL responses
from 3LMCT or 3LMMCT excited state. The corresponding
luminescence mechanism is as follows: the singlet state formed
via MLCT or LLCT process relaxes to triplet state through fast
intersystem crossing with aid of the large spin-orbit coupling of
heavy gold atoms[20], followed by PL and CPL generation via
3
LMCT or 3LMMCT process. Noteworthily, as for the exceptional
case of nanocubes with ordered BCC packing structure (Figure
4), every inward p-tolyl rings on the Au cluster surfaces are fully
fixed via intermolecular CH/π interactions, and thus both PL
intensity and glum factor reach the highest values (Figure 3a and
3b).
In summary, chiral Au3 clusters of the smallest size and the
record-high optical absorption activity have been successfully
synthesized and used as building blocks for spontaneous
organization into nanocubes of well-defined BCC arrangement.
Thanks to the strong intermolecular CH/π interactions, the
rotation of chiral ligands are drastically restricted, and therefore
the optical absorption activity of as-assembled products is redshifted and further enhanced. More excitingly, distinct from
individual clusters free of luminescence, the chiral Au cluster
assemblies become highly emissive and the strongest CPL
response is acquired with the ordered structure. It is highly
expected that design and application of clusters, which possess
accurate atomic structure and specific optical, magnetic or
catalytic property, will open the new era in the self-assembly
field beyond conventional molecules and nanoparticles.
10.1002/ange.201709827
Angewandte Chemie
COMMUNICATION
Lin Shi, Lingyun Zhu, Jun Guo, Lijuan
Zhang, Yanan Shi, Yin Zhang, Ke Hou,
Yonglong Zheng, Yanfei Zhu, Jiawei Lv,
Shaoqin Liu*, Zhiyong Tang*
Page No. – Page No.
Self-Assembly of Chiral Au Clusters
into Crystalline Nanocubes of
Exceptional Optical Activity
Accepted Manuscript
Chiral Au3 clusters that possess the
remarkable CD anisotropy factor are
successfully synthesized. This CD
anisotropy factor is further enhanced
upon cluster self-assembly. Distinct
from individual clusters free of
luminescence, the chiral Au cluster
assemblies become highly emissive
and the strongest CPL activity is
achieved with the ordered bodycentred cubic structure.
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