Platonic hexahedron composed of six organic faces with an inscribed Au cluster

Article abstract of DOI:10.1021/ja209634g

The structures of nanomaterials determine their individual properties and the suprastructures they can form. Introducing anisotropic shapes and/or interaction sites to isotropic nanoparticles has been proposed to extend the functionality and possible suprastructure motifs. Because of symmetric anisotropy, Platonic solids with regular polygon faces are one of the most promising nanoscale structures. Introduction of Platonic solid anisotropy to isotropic nanomaterials would expand the functionality and range of possible suprastructure motifs. Here, we demonstrate a novel strategy to obtain nano-Platonic solids through the face coordination of square porphyrins on an inscribed Au sphere with adequate size. The face coordination of the multidentate porphyrin derivatives, with four acetylthio groups facing the same direction, on the Au cluster encased the Au cluster in a Platonic hexahedron with six porphyrin faces. Transmission electron microscopy, mass spectrometry, elemental analysis, and scanning tunnelling microscopy were used to confirm the formation of the nano-Platonic hexahedron.

Full text of DOI:10.1021/ja209634g

Communication
pubs.acs.org/JACS
Platonic Hexahedron Composed of Six Organic Faces with an
Inscribed Au Cluster
Masanori Sakamoto,†,‡,¶ Daisuke Tanaka,†,‡ Hironori Tsunoyama,§ Tatsuya Tsukuda,§
Yoshihiro Minagawa,‡,∥ Yutaka Majima,‡,∥,⊥ and Toshiharu Teranishi*,†,‡,#
†Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan
‡CREST, Japan Science and Technology Agency (JST), Yokohama 226-8503, Japan
¶PRESTO, JST, Tsukuba 305-8571, Japan
§Section of Catalytic Assemblies, Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
∥Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
⊥Department of Printed Electronics Engineering, Sunchon National University, Sunchon 540-742, Korea
#Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan
S
* Supporting Information
anisotropy in shape and interaction is a key to regulation of
ABSTRACT: The structures of nanomaterials determine
their individual properties and the suprastructures they can
form. Introducing anisotropic shapes and/or interaction
sites to isotropic nanoparticles has been proposed to
extend the functionality and possible suprastructure motifs.
Because of symmetric anisotropy, Platonic solids with
regular polygon faces are one of the most promising
nanoscale structures. Introduction of Platonic solid
anisotropy to isotropic nanomaterials would expand the
functionality and range of possible suprastructure motifs.
Here, we demonstrate a novel strategy to obtain nano-
Platonic solids through the face coordination of square
porphyrins on an inscribed Au sphere with adequate size.
The face coordination of the multidentate porphyrin
derivatives, with four acetylthio groups facing the same
direction, on the Au cluster encased the Au cluster in a
Platonic hexahedron with six porphyrin faces. Trans-
mission electron microscopy, mass spectrometry, elemen-
tal analysis, and scanning tunnelling microscopy were used
to confirm the formation of the nano-Platonic hexahedron.
the possible arrangement of a suprastructure. Introducing
anisotropic shapes and/or interaction sites to isotropic
nanomaterials is essential for expansion of the functionality
and range of possible suprastructure motifs. Isotropic building
blocks with specific surface modifications, such as DNA linkers
and attractive patches, have been proposed for controllable
formation of suprastructures.3 Moreover, Stellacci et al.
demonstrated that a metal nanoparticle with two coordination
sites at polar positions can be linked to form nanoparticle
chains.4 However, these pathways are limited for assembly of
isotropic nanomaterials, and a novel strategy for this needs to
be developed. The encasement of an isotropic nanoparticle in a
Platonic solid is an alternative solution for design of
nanomaterials with specific and anisotropic surface function-
alities. This method could be extended to the controllable
formation of a nanoparticle suprastructure. This will allow
development of novel materials with new collective phenom-
ena. While this concept is simple in theory, the assembly of
regular polygonal molecules on a metal sphere is extremely
difficult in practice and has not been successful.
Here, we detail the construction of a Platonic solid from
square porphyrins and a Au sphere. A regular hexahedron was
formed by face coordination of square porphyrins on a suitable
metal cluster, instead of attaching the sides of the porphyrins to
each other (Figure 1). This approach relies on the face
coordination of porphyrins on a Au cluster with a similar size to
the porphyrin. For this purpose, we synthesized the following
multidentate macrocyclic porphyrin thioester derivatives:
tetrakis-5α,10α,15α,20α-(2-acetylthiomethylphenyl) porphyrin
(SC1P) and tetrakis-5α,10α,15α,20α-(2-acetylthioethylphenyl)
porphyrin (SC2P) (Figure 1a).5 The SCnP (n = 1 or 2) ligands
contain four acetylthio groups that face the same direction and
offer a quadridentate coordination site that cooperates with the
π-system of the macrocycle to bind Au clusters in a face-
coordination fashion (Figure 1b). The face coordination of
he structure of a nanomaterial is an important parameter
Tthat determines its individual characteristics and the
suprastructures that it can form through assembly.1 Because of
symmetric anisotropy, Platonic solids are one of the most
promising nanoscale structures. A Platonic solid is a convex
polyhedron with congruent regular polygon faces and identical
polyhedral angles. Regular tetrahedron, hexahedron, octahe-
dron, dodecahedron, and icosahedron are known to be Platonic
solids. Various nanoscale Platonic solids, from nanoparticles to
molecular cages, have attracted considerable attention because
of the potential to control their nanostructures and the
functionalities of both individual and assembled structures.2
An inscribed sphere inside a Platonic solid is tangent to the
center of each face. Deliberate adhesion of the face of regular
polygon on an isotropic sphere results in the formation of the
anisotropic Platonic solid. It should be emphasized that
Received: October 13, 2011
Published: December 21, 2011
© 2011 American Chemical Society
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larger ligand size. Transmission electron microscope images of
the SCnP−AuCs are shown in Figure 2b. Highly monodisperse
clusters with sizes of 1.1 0.2 and 1.2 0.1 nm were observed
for the SC1P− and SC2P−AuCs, respectively. It is noteworthy
that the formation of the monodisperse Au clusters was only
observed when α,α,α,α-isomers were employed as protecting
ligands. Using other conformational isomers resulted in the
formation of aggregates of polydisperse Au clusters, which were
derived from cross-linking of the Au clusters by the ligands with
four acetylthio groups facing in the separate direction. The face-
coordination of α,α,α,α-SCnP on the Au cluster strongly
protects the core and suppresses this cross-linking.
Fourier transform infrared spectroscopy provided further
evidence for face coordination of SCnP on the Au clusters.
Figure 2c shows the Fourier transform infrared spectra of SCnP
and the SCnP-AuCs. For the SCnP ligand, several strong peaks
were observed at 1690 and 990 cm−1, which can be attributed
to the carbonyl stretching vibration of the acetylthio group and
pyrrole ring breathing motion, respectively.6 Similar peaks were
also observed for the SCnP−AuCs, which indicates that the
SCnP ligand attaches to the Au clusters. Because the carbonyl
stretching vibration was observed for the SCnP−AuCs, it can be
concluded that the acetyl groups were not dissociated during
cluster formation. When the Au clusters were synthesized in the
similar manner using benzylthioacetate (SC1) as a protecting
ligand, tetraoctylammonium-protected Au clusters were formed
instead of SC1-protected Au clusters; tetraoctylammonium was
used to dissolve Au(III) ions in the organic phase (see SI). This
result suggests that the acetylthio group is not able to bind to
the Au surface as strongly as thiols or amines. Use of the
quadridentate acetylthio group of SCnP can overcome this and
stably bond to the Au surface.
Figure 1. (a) Strategy for assembling the platonic solid from
porphyrins and a Au cluster. Chemical structures of SCnP are also
shown. (b) Schematic illustration of the coordination fashion of SC1P
on the Au cluster. Hydrogen atoms are omitted for clarity. (c)
Schematic illustration of the Platonic hexahedron constructed with six
porphyrin faces and an inscribed Au sphere. Only the tetraphenylpor-
phyrin moieties are shown for clarity.
SCnP on the Au cluster results in the formation of the regular
hexahedron with congruent porphyrin faces, as shown in Figure
1c.
The Au clusters coordinated by the SCnP (SCnP−AuCs)
were synthesized by reduction of Au(III) ions in CH2Cl2/
CH3OH at 200 K in the presence of SCnP (see the Supporting
Information (SI) for detail). The obtained SCnP−AuCs were
isolated by gel permeation chromatography (GPC). Figure 2a
The absorption spectra of SCnP and SCnP−AuCs provided
insights into the coordination fashion of SCnP. As shown in
Figure 3, the Soret bands of SCnP on the Au clusters were
Figure 3. UV−vis-NIR spectra for SCnP and SCnP−AuCs in DMF.
The extinction coefficients of SCnP on SCnP−AuCs were estimated
from inductively coupled plasma atomic emission spectroscopy data.
broad, and the peak position for SC1P was red-shifted
compared to the free ligand. Furthermore, on coordination to
the Au clusters, the extinction coefficients of SC1P and SC2P
were reduced up to 9% (from 3.9 × 105 to 3.5 × 104 cm−1 M−1)
and 36% (from 3.9 × 105 to 1.5 × 105 cm−1 M−1), respectively.
From X-ray crystallography, the distance between the
porphyrin ring and sulfur atoms was determined to be 3.45
and 4.85 Å for SC1P and SC2P, respectively (see SI). This
indicates that the spectral shift and the reduction of the
extinction coefficient depend on the distance between the
porphyrin ring and the Au cluster. Similar spectral changes were
observed for SC1P-coordinated Au nanoparticles (approx-
imately 10 nm in size) in an earlier study,5 and the reason
for this is currently under investigation. Possible explanations
Figure 2. (a) Typical chromatograms of SCnP−AuCs and SCnP. (b)
TEM images of SCnP−AuCs. (c) FT-IR spectra of SCnP and SCnP−
AuCs.
shows typical GPC chromatograms of the SCnP−AuCs. Each
SCnP−AuC showed only one peak with a highly reproducible
retention time. The peak shapes of SCnP−AuCs in the GPC
chromatograms had Gaussian distributions, which indicates that
the obtained SCnP−AuCs consist of single components. The
retention time of the SC2P−AuCs was slightly less than that of
the SC1P−AuCs, which suggests that the SC2P−AuCs are only
slightly larger in volume than the SC1P−AuCs because of the
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for this change are (i) exciton coupling between the porphyrin
moieties7 and/or (ii) electronic interaction between the π-
system of the porphyrin and the Au clusters. Although the
suggested spatial arrangement of the porphyrins (i.e.,
hexahedron) indicates the existence of the excitonic interaction,
the phenomena would not strongly depend on the distance
between the porphyrin ring and the Au cluster. Thus, we
consider that both phenomena work together and lead to the
dramatic spectral changes. To construct the Platonic hexahe-
dron, the center of each face should contact the inscribed
sphere. The interaction between the porphyrin and the Au
cluster is favorable for contact of the Au cluster with the
porphyrin ring and leads to formation of a Platonic hexahedron.
We confirmed the chemical compositions of the SCnP-AuCs
by matrix-assisted laser desorption/ionization time-of-flight
(MALDI TOF) mass spectrometry. Figure 4a shows the
intensity of AunSm− (m ≥ 1) was much weaker than that of Aun−
(see Figure 4b, inset). This feature is in sharp contrast to the
laser desorption/ionization mass spectra of thiol-protected
AuCs8 but similar to those of polyvinylpyrrolidone-stabilized
AuCs.9 This result indicates that the Au−S bonds in the SC2P−
AuCs are much weaker than those in the thiol-protected Au
clusters. The Au cores of the SCnP−AuCs are stabilized by
multiple weak interactions, which is similar to the Au clusters
protected by polyvinylpyrrolidone.
To evaluate the SCnP−AuCs as a whole, we attempted to
determine the number of SCnP on a single Au cluster using
inductively coupled plasma atomic emission spectroscopy. The
Au to sulfur atom amount-of-substance ratios were 74:26 and
73:27 for SC1P− and SC2P−AuCs, respectively, which
indicates that 6 1 SCnP was attached on a single Au cluster.
Since the GPC chromatograms indicated that the SC1P−AuC
and SC2P−AuC had similar volumes, both clusters would show
similar geometry with the same number of porphyrins. The
most thermodynamically stable structure should have the
lowest surface energy by complete coverage of the Au cluster
surface. A hexahedron of six SCnP ligands is likely to completely
cover the Au cluster surface.
The scanning tunnelling microscopy (STM) provided further
information on the structure of SCnP−Au. The STM images of
the SC1P−AuCs are shown in Figure 5a.10 Generally, the tip
Figure 4. (a) MALDI-TOF mass spectra of SC2P−AuCs at various
laser powers. (b) MALDI-TOF mass spectra of SC2P−AuCs at high
laser power. Regular progressions of the mass spectra, which
correspond to Au and sulfur atoms, were observed. The inset shows
−
the expanded image of progressions of Aun− and AunSm (m ≥ 1).
negative-ion MALDI TOF mass spectra of the SC2P−AuCs
recorded at various laser powers. At a minimum laser power, a
broad (unresolved) peak was observed at approximately 19
kDa. A new peak appeared at approximately 13 kDa at a higher
laser power, and its relative intensity to that of the 19 kDa peak
increased with increasing laser power. These results indicate
that the peaks at approximately 19 and 13 kDa can be assigned
to the parent SC2P−AuCs and their fragments, respectively.
Since the mass difference between the two peaks (approx-
imately 6 kDa) corresponds to the molecular weight of six
SC2P ligands (6132 Da), the fragment peak at 13 kDa could be
assigned to the Au core composed of 66 5 Au atoms. The
mass spectral analysis suggests that the composition of SC2P−
AuCs is Au∼66(SC2P)∼6, which is in agreement with that
estimated from transmission electron microscopy and ele-
mental analysis. For the SC1P−AuCs, the mass peak for the Au
core was observed at approximately 13 kDa at a higher laser
power, whereas the mass peak assigned to the parent SC1P−
AuCs was not observed even at a lower laser power. Stronger
interaction of the Au core with SC1P than with SC2P, suggested
by the absorption spectra, may be responsible for this
phenomenon, although this needs to be clarified. The number
of Au atoms for the SC1P−AuCs was estimated to be 65 10.
These numbers of Au atoms constructing the AuCs are
different from the magic numbers that are known to thiol-
protected or gas phase AuCs, suggesting that the SCnP−AuCs
are formed through the kinetic stabilization by specific SCnP.
When the laser power was increased further, the progression
of Aun− became more prominent in the MALDI TOF mass
spectra of the SC2P−AuCs (Figure 4b). Interestingly, the
Figure 5. (a) A three-dimensional STM image of SC1P−AuCs
adsorbed on the Au(111)/mica surface at room temperature. The
scanning area was 85 × 85 nm2. The STM image is analyzed using
WSxM.13 (b−d) Three-dimensional STM images of SC1P−AuCs
adsorbed on the Au(111) surface after erasing manipulation of
nanoclusters 1, 2, and 3 in (a). (e) The height profiles before (black
line) and after erasing (red line). The erasing procedure was
conducted as follows: the STM tip was positioned on the target
SC1P−AuC numbered in (a−c); a sample bias voltage of 3 V was
applied to the substrate with a set point current of 50 pA for about 1
min; and the sample bias was decreased to 1.5 V and the STM images
were observed at a set point current of 8 pA. The erasing of SC1P−
AuC typically occurred within 5−10 s.
convolution effect makes the lateral size larger than the
expected planar dimension,11 but accurate height information
can be obtained. The mean tip height of the bright spots in the
STM image (i.e., SC1P−AuCs) was about 2.2 nm. This agrees
with the SC1P−AuC height of 2.2 nm, which is equal to the 1.0
nm core diameter plus two times the 0.6 nm size of SC1P.
Another important result was that most SC1P−AuCs had
similar heights, with a mean and standard deviation of 2.2 and
0.4 nm, respectively. This indicates that the SC1P−AuCs have
very similar structures even if different faces of the SC1P−AuCs
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attach to the substrate. Consequently, it is obvious that only a
hexahedron satisfies these conditions among the possible
structures that can be constructed from 6 1 porphyrin faces.
Remarkably, the SC1P−AuCs could be erased one by one by
applying a sample bias voltage of 3 V (Figures 5a−d). It should
be noted that the quality of the STM images for the SC1P−
AuCs did not degrade even after multiple erasing processes.
The erasing mechanism is not clear but possibly includes
multistep reductive dissociation or heat decomposition.12 This
phenomenon strongly supports that the structures observed on
STM image are distinct SC1P−AuCs. The STM images were
observed even after the multiple erasing processes. This
behavior of the SC1P-AuCs could be applied to single cluster
write-once read-many memory.
In this study, we developed a new strategy for introducing
anisotropy to isotropic nanomaterials by constructing nano-
Platonic solids. Nanohexahedra were successfully synthesized
from six porphyrin derivatives around a Au cluster. These
hexahedra are regarded as novel “artificial atoms” with
anisotropically interactive faces. We believe that this approach
using spherical nanoparticles is a novel self-assembly technique,
because the formation of Platonic solid is automatically
determined by the relationship between the diameter of the
inscribed sphere and shapes/sizes of the polygonal faces. The
present strategy could be expanded to other Platonic solids.
Controlled fabrication of suprastructures from the clusters
could be possible by assembling the nanohexahedra through
coordination on the metal center of the metalloporphyrin
derivatives. Furthermore, because of low tunnel resistance from
the thin layer of protecting ligands and distance-dependent
interaction between the porphyrin and Au cluster, the present
hexahedra could function as Coulombic islands in single
electron transistors. Research on single electron transistors
using these hexahedra is now in progress.
Education, Science and Technology of Korea (R31-10022)
(Y.M.).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis of SCnP, SCnP−AuCs, SC1, and SC1-protected AuCs.
Characterization of SC1-protected AuCs. Sample preparation
and instrumental setup for STM imaging. Single crystal X-ray
crystallography of SCnP. The complete list of authors for
reference 2c. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
teranisi@scl.kyoto-u.ac.jp
ACKNOWLEDGMENTS
■
The authors thank Prof. A. Sekiguchi and Dr. M. Ichinohe for
their help with X-ray crystallography. The authors are grateful
to the Chemical Analysis Center, University of Tsukuba, for
elemental analysis data. This study was partially supported by a
KAKENHI Grant-Aid for Scientific Research A (no. 23245028)
(T.Teranishi) and a Grant-in-Aid for Scientific Research on
Innovative Areas (grant no. 20108011, π-Space) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT), Japan (Y.M.), by the Global COE Program of
“Photonics Integration-Core Electronics” MEXT (Y.M.), by the
Collaborative Research Project of Materials and Structures
Laboratory, Tokyo Institute of Technology, and by the World
Class University (WCU) Program through the Ministry of
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