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Porphyrin Derivative-Protected Gold Cluster with a Pseudo-Tetrahedral Shape Daichi Eguchi, Masanori Sakamoto, Daisuke Tanaka, Yasuo Okamoto, and Toshiharu Teranishi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10972 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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The Journal of Physical Chemistry

Porphyrin Derivative-Protected Gold Cluster with a Pseudo-Tetrahedral Shape Daichi Eguchi,1 Masanori Sakamoto,2,3* Daisuke Tanaka,4 Yasuo Okamoto,1 Toshiharu Teranishi2*

1

Department of Chemistry, Graduate School of Science, Kyoto University, Gokasho,

Uji, Kyoto 611-0011, Japan

2

Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011,

Japan

3

Japan Science and Technology Agency (JST), Precursory Research for Embryonic

Science and Technology (PRESTO), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

4

Graduate School of Pure and Applied Science, University of Tsukuba, 1-1-1 Tennodai,

Tsukuba, Ibaraki 305-8571, Japan

1 Environment ACS Paragon Plus


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ABSTRACT

The shape of a nanomaterial plays an important role in controlling the packing arrangement and properties of the assembly that it forms. In the case of metal clusters, the shape of the metal core is veiled by a flexible organic ligand with a comparable size to that of the metal core. Here, we controlled the overall shape of ligand-protected gold clusters (AuCs) using a rigid, planar molecule as a ligand by following a simple geometrical relationship between an inscribed sphere and a circumscribed polyhedron. For the rigid, planar molecule, we synthesized a new porphyrin derivative that could strongly attach to the AuCs in a face-coordination fashion. As a result, 1.0±0.2 nm porphyrin derivative-protected AuCs with a pseudo-tetrahedral shape were successfully synthesized. The present results pave the way to a new concept for controlling the pseudosymmetry of ligand-protected metal clusters.


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INTRODUCTION

Ordered assemblies of inorganic nanoparticles (NPs) exhibit prominent physicochemical properties that are not observed in isolated NPs as a result of coupling and delocalization of wave functions of the NPs in the assembly.1-4 These properties are closely related to the packing arrangements of the NP building blocks, which, in turn, are determined by the shapes of the building blocks. In fact, unlike isotropically shaped NPs, assemblies formed from anisotropically shaped NPs have unique packing arrangements that reflect their symmetry.5 Because large anisotropically shaped NPs have clearly-defined flat crystal faces, hydrophobic and other interactions between the organic ligands on these faces are likely to occur. Assemblies of metal clusters < 2 nm in size, which can be defined by their molecular formulae, are also important research targets owing to the characteristic features resulting from their discrete electronic structures.6-7 For example, Roy and coworkers have reported that an assembly of cobalt chalcogenide and iron oxide clusters exhibits new optical properties derived from intervalence charge transfer.8 However, the metal cores in these clusters have complex shapes, such as icosahedral- or tetragonal rod-shapes, and the cores are completely veiled by flexible, bulky organic ligands that have a similar size to that of the metal cores.9-11 Therefore, there are few reports on the formation of metal cluster



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assemblies depending on the cluster shape. Thus, an alternative and versatile approach to controlling the overall shape of ligand-protected metal clusters is highly desirable. A simple geometrical relationship between an inscribed sphere and a circumscribed polyhedron inspired us to develop a novel method to control the shape of ligand-protected metal clusters. That is, when the size of a planar ligand molecule (or the face of a circumscribed polyhedron) is determined, the number of ligand molecules will vary depending on the size of the inscribed metal core. Following this relationship, we have already synthesized gold clusters (AuCs) face-coordinated by six and 14 porphyrin derivatives,12,13 in which the porphyrin derivatives were considered to be faces in a polyhedron, and the Au cores were inscribed to a pseudo-hexahedron and a cuboctahedron, respectively (Figure 1). The sizes of these inscribed AuCs were 1.2±0.1 and 1.9±0.3 nm for the pseudo-hexahedron and cuboctahedron, respectively; this size difference is reasonable if we consider the geometrical relationship. This result strongly indicates that the number of porphyrin derivatives is determined by the inscribed AuC size and that further reduction of the AuC size would give pseudo-tetrahedral and pseudolinear shapes with four and two coordinated porphyrin derivatives, respectively. In the present work, we demonstrate a further reduction of the AuC size to form pseudotetrahedral AuCs face-coordinated by four porphyrin derivatives following the above


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concept. The tetrahedron is an important building block in the fields of supramolecular chemistry and nanoscience.14-18 As mentioned above, we have succeeded in the synthesis of pseudo-hexahedral and pseudo-cuboctahedral AuCs using the porphyrin derivatives containing acetylthio groups, which can weekly coordinate to the AuC surface.12,13 To obtain the pseudo-tetrahedron, we designed a new porphyrin derivative containing disulfide ligands, which can coordinate more strongly to the AuC surface than can acetylthio groups,12,13,19 and used them to synthesize AuCs smaller than 1.2±0.1 nm by kinetic restriction of the AuC growth. The resulting AuCs were characterized by mass spectrometry, elemental analysis, transmission electron microscopy (TEM), and UV-vis absorption spectroscopy.



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Figure 1. Geometrical relationship between the sizes of inscribed metal clusters and circumscribed polyhedra. Only the zinc tetraphenylporphyrin moieties are shown for clarity.


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EXPERIMENTAL METHODS

General. 1H NMR spectra were measured on a JEOL JNM-ECP300 (1H: 300 MHz) and a JEOL ECA-600 (1H: 600 MHz) spectrometers. 1H (600 MHz), 13C (150 MHz), heteronuclear single quantum coherence (HSQC), and 1H–1H nuclear Overhauser effect correlation spectroscopy (NOESY) NMR spectra were recorded on a Bruker Avance III 600US Plus spectrometer. All NMR data were measured at 300 K and the chemical shifts (δ) are reported downfield from the internal standard tetramethylsilane (TMS, δ = 0.00 ppm) in CDCl3. For 1H and 13C assignments, the atom numbering of the obtained compounds was shown in Figure S1. The low temperature (-80 °C) for the AuCs synthesis was maintained by a constant temperature bath on UCR-150N (Techno Sigma). Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDITOF MS) was performed on a Bruker Autoflex Speed using trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as the matrix. Gel permeation chromatography-high performance liquid chromatography (GPC-HPLC) was performed on a LC-9225 NEXT (Japan Analytical Industry Co., Ltd.) with a JAIGEL-W253 column. The mobile phase was 50 mM LiBr in N,N-dimethylformamide (DMF) and the flow rate was 3.8 mL min−1. The chromatograms were collected by monitoring the wavelength of 430 nm. Inductively coupled plasma-atomic emission



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spectroscopy (ICP-AES) was performed on a Shimadzu ICPE-9000. UV-vis-near infrared (UV-vis-NIR) absorption spectra were recorded at room temperature on a Hitachi U-4100 spectrophotometer. Fluorescence and excitation spectra were recorded at room temperature on a Horiba Fluorolog-3. Absolute fluorescence quantum yields were determined with a Horiba Quanta-phi. Powder X-ray diffraction (XRD) patterns were recorded on a PANalytical X’Pert Pro MPD diffractometer with Cu Kα radiation. TEM images were recorded on a JEOL JEM-100 microscope, operated at an acceleration voltage of 100 kV. The histogram of the sizes of the Au cores was generated after measuring 1,000 particles. Materials. All reagents were used as received without further purification. Tetrakis5α,10α,15α,20α-(2-acetylthiophenyl)porphyrin

(SC2P)

was

synthesized

from

isochroman in three steps.12 Synthesis of 5α,10α,15α,20α-Tetrakis(2-acetylthiophenyl)porphyrinatozinc(II) (ZnSC2P). A methanol solution (3 mL) of zinc acetate (190 mg, 1.03 mmol) was added to a CH2Cl2 solution (30 mL) of SC2P (264 mg, 0.257 mmol). The mixed solution was refluxed for 150 min under an N2 atmosphere. After cooling, the solution was poured into deionized water and extracted with CH2Cl2. The organic phase was dried over Na2SO4. After removal of the solvent in vacuo, the residue was purified by column


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chromatography on silica gel using a mixed solution of CHCl3 and hexane (4:1 (v/v)) as the eluent. ZnSC2P was obtained in 85% yield. 1H NMR (300 MHz, CDCl3, δ): 1.33 (s, 12H, H1), 2.51–2.56 (m, 8H, H2), 2.66–2.71 (m, 8H, H3), 7.55 (t, J = 7.05 Hz, 4H, H6), 7.65 (d, J = 7.20 Hz, 4H, H4), 7.72 (t, J = 7.95 Hz, 4H, H5), 8.05 (d, J = 7.20 Hz, 4H, H7), 8.65 (s, 8H, H8); MS (MALDI): m/z: 1086.7 [M] +.

Synthesis

of

5α,10α-Bis(2-ethylthiophenyl)-15α,20α-bis(2ʹ -ethylthiophenyl)

porphyrinatozinc(II) (ZnSC2P-SS). A THF solution (15 mL) of methylamine (2.0 M, 30 mmol) was added to a THF solution (15 mL) of ZnSC2P (154 mg, 0.141 mmol). The mixed solution was stirred for 18 h at room temperature. After removal of the solvent, the resulting residue was washed with methanol to give ZnSC2P-SS in 97% yield. 1H NMR (600 MHz, CDCl3, δ): 1.41–1.45 (m, 4H, H1), 2.33–2.38 (m, 4H, H2), 2.67–2.72 (m, 4H, H3), 3.03–3.07 (m, 4H, H4), 7.49 (d, J = 7.80 Hz, 4H, H5), 7.65 (td, J = 7.50 and 1.20 Hz, 4H, H7), 7.72 (td, J = 7.65 and 1.60 Hz, 4H, H6), 8.41 (dd, J = 7.50 and 0.90 Hz, 4H, H8), 8.60 (s, 4H, H10), 8.61 (s, 4H, H9); 13C NMR (150 MHz, CDCl3, δ): 32.5 (C1), 38.6 (C2), 118.7 (C9), 124.6 (C6), 128.5 (C4), 128.6 (C5), 130.7 (C13), 131.4 (C11), 132.3 (C7), 141.7 (C3), 143.0 (C8), 149.9 (C10), 150.2 (C12); MS (MALDI): 914.6 [M]+.



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Synthesis of ZnSC2P-SS-Protected AuCs (ZnSC2P-SS/AuCs). An aqueous solution (10 mL) of hydrogen tetrachloroaurate(III) tetrahydrate (47.0 mg, 0.114 mmol) was added to a CH2Cl2 solution (15 mL) of tetraoctylammonium bromide (77.8 mg, 0.142 mmol). The mixed solution was stirred vigorously for 10 min at room temperature to transfer the Au3+ ions to the CH2Cl2 phase. After removal of the aqueous phase, a mixture of a CH2Cl2 solution (113 mL) of ZnSC2P-SS (20.4 mg, 22.3 μmol) and methanol (30 mL) was added to the CH2Cl2 solution of Au3+. The mixture was stirred for 60 min at room temperature, and then cooled at −80 °C for 30 min. A methanol solution (2 mL) of sodium borohydride (41.5 mg, 1.10 mmol) cooled at −80 °C was added to the solution of Au3+ and ZnSC2PSS to obtain the AuCs. This solution was stirred at −80 °C for 60 min, and then at room temperature for 60 min. The obtained solution was poured into deionized water and extracted with CH2Cl2. The organic phase was dried over Na2SO4. DMF (5 mL) was added to the solution, and the CH2Cl2 was evaporated under reduced pressure. The crude product was purified by GPC-HPLC to obtain the ZnSC2P-SS/AuCs. The yield of ZnSC2PSS/AuCs was 50% (atomic Au basis).


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RESULTS AND DISCUSSION

Synthesis and Characterization of ZnSC2P-SS. The ZnSC2P-SS ligand was synthesized in two steps from SC2P (Figure 2a). The structure of ZnSC2P-SS was assigned using NMR measurements, mass spectrometry, and single X-ray crystallographic analysis. The 1H NMR spectrum of ZnSC2P-SS in CDCl3 at room temperature showed two singlets at 8.60 and 8.61 ppm that were assigned to the proton in the β-position of the porphyrin ring (Figure S2). In the HSQC measurement, these proton signals were correlated with two different carbon signals at 130.7 and 131.4 ppm (Figure S4a). In general, the chemical environments of the protons in the β-position of a porphyrin ring are the same owing to the symmetry of the chemical structure, and the 1H NMR spectrum shows only one singlet. However, the HSQC analysis of ZnSC2P-SS indicates that there are two different chemical environments of the β-position of the porphyrin ring in ZnSC2P-SS. NOESY revealed a correlation between the proton signals at 3.02–3.07 and 8.61 ppm (Figure S4b). This correlation indicates the close proximity of the β-CH2 proton and the proton in the β-position of the porphyrin ring (β-CH2 denotes the position relative to the sulfur atom). It was thus determined that there were two different distances between the disulfide groups and the β-positions of the porphyrin ring in the chemical structure of ZnSC2P-SS, and that these induced different deshielding



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effects. Further, the molecular structure of ZnSC2P-SS was provided by X-ray crystallographic analysis of a single crystal of ZnSC2P-SS, which was obtained by the liquid–liquid diffusion method using benzene and methanol. The X-ray crystallographic analysis of ZnSC2P-SS indicated that the edge-to-edge distance of the porphyrin ring was 11 Å and the distance between the disulfide group and the porphyrin ring was 4.1 Å (Figure 2d, e). The two disulfide groups face in the same direction relative to the porphyrin ring, which gives ZnSC2P-SS a face-on configuration for tetradentate coordination to AuCs. The distances between the disulfide groups and the β-positions of the porphyrin ring (H9 or H10 in Figure S1) were 4.9 Å or 6.7 Å, which well agreed with the result of NOESY and induced different deshielding effects. The distance between the sulfur atom and the porphyrin ring in ZnSC2P-SS is shorter than that in SC2P (4.9 Å) (Figure 2b, c).13


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Figure 2. (a) Synthetic procedure for the preparation of ZnSC2P-SS and single X-ray crystal structures of SC2P and ZnSC2P-SS from (b, d) top view, and (c, e) side view. Hydrogen atoms have been omitted for clarity.

Formation of ZnSC2P-SS/AuCs. The ZnSC2P-SS/AuCs were synthesized by the reduction of Au3+ ions in the presence of ZnSC2P-SS (Au3+:ZnSC2P-SS = 1:0.2 (mol/mol)), and the crude product was purified by GPC-HPLC to remove byproducts and unreacted ZnSC2P-SS. The central peak of the AuCs in Figure S5a was collected for further characterization. To investigate the effect of the anchoring group of the porphyrin (disulfide or acetylthio), on the size of the AuCs, SC2P-protected AuCs (SC2P/AuCs) with



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a pseudo-hexagonal shape were also synthesized.12-13 Figure 3a shows chromatograms of ZnSC2P-SS/AuCs, SC2P/AuCs, ZnSC2P-SS, and SC2P recorded by monitoring the Soret band of the porphyrin derivatives. The retention time of the ZnSC2P-SS/AuCs was longer than that of the SC2P/AuCs, which indicates that the size of the ZnSC2P-SS/AuCs was smaller than that of the SC2P/AuCs. In addition, the Gaussian shape of the chromatogram indicates that the obtained ZnSC2P-SS/AuCs contained only a single component. Because the reaction conditions play an important role in controlling the size of metal clusters and NPs,20-23 we investigated the effect of the gold precursor/ZnSC2P-SS molar ratio on the AuC size. Figure S5b shows the GPC-HPLC chromatograms of crude samples synthesized at different gold precursor/ZnSC2P-SS molar ratios (1:1–1:0.2). The retention times of all of the products were almost the same. Tracy and coworkers have reported that bulky ligands, such as 1-adamantanethiol, tend to block the delivery of Au atoms to the Au cores, which results in Au NPs with smaller sizes.24 Because of the tetradentate coordination, it is considered that the ZnSC2P-SS ligands restrict further growth of the AuCs. We therefore concluded that the gold precursor/ZnSC2P-SS molar ratio only slightly affected the size of the AuCs. Figure 3b shows a TEM image of ZnSC2P-SS/AuCs. The histogram of the AuC sizes in Figure 3c was obtained by measuring 1,000 AuCs. The core size of the ZnSC2P-


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SS/AuCs was 1.0±0.2 nm, which is smaller than that of the SC2P/AuCs (1.2±0.1 nm).12 Therefore, the size of the AuCs estimated from the TEM measurement is in good agreement with the result obtained from the GPC-HPLC chromatograms. To confirm the configuration of ZnSC2P-SS on the AuCs, 1H NMR measurements of ZnSC2P-SS and ZnSC2P-SS/AuCs were carried out in CDCl3. As shown in Figure S6, the 1

H NMR spectrum of the ZnSC2P-SS/AuCs shows broadened signals, despite the use of

a 600 MHz spectrometer. Because the NMR signal of TMS (δ = 0.00 ppm) was properly tuned and matched, the broadened signals of the ZnSC2P-SS/AuCs are not the result of experimental error. There are many reports on the broadening of the NMR signals of organic ligands coordinated to AuCs and Au NPs,25-27 and this phenomenon can be explained by the difference in spin–spin relaxation (T2).27 It is widely recognized that the NMR signals of macromolecules are broadened because of their slow mobility in solution.28 The fact that the proton signals of free ZnSC2P-SS ligands are not observed in the NMR spectrum strongly indicates that the ZnSC2P-SS molecules are attached to the AuCs in a face-coordination fashion. Another important feature of the NMR spectrum was the downfield shift of the proton signals corresponding to ZnSC2P-SS coordinated to the AuCs. Häkkinen and coworkers have theoretically predicted a deshielding effect induced by the interaction between the AuC and the organic ligand.29 We therefore



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concluded that the line broadening and downfield shift were derived from spin–spin relaxation and deshielding effects.

Figure 3. (a) GPC-HPLC chromatograms of SC2P/AuCs, ZnSC2P-SS/AuCs, SC2P, and ZnSC2P-SS. (b) TEM image of ZnSC2P-SS/AuCs. (c) Core size distribution of ZnSC2PSS/AuCs. The core size of the ZnSC2P-SS/AuCs was 1.0±0.2 nm.

Chemical Composition of ZnSC2P-SS/AuCs. The chemical composition of the ZnSC2P-SS/AuCs was determined by MALDI-TOF MS in linear positive mode and ICPAES. As shown in Figure 4a, the sharp peak observed at approximately 15 kDa in the MALDI-TOF spectrum was assigned to a structure consisting of 58±4 Au atoms and four ZnSC2P-SS molecules. The line broadening in the MALDI-TOF MS spectrum of the ZnSC2P-SS/AuCs derives from laser-induced fragmentation during ionization.30-31 When the laser power was increased further, the broadening of the spectrum and the regular progression of Aun− became more prominent, as shown in Figure S7a, b. The regular


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progression of Aun– observed in Figure S7b suggests laser-induced decomposition of the Au core. The Au:Zn atomic ratio in the ZnSC2P-SS/AuCs was found to be 54:4 using ICP-AES, which agrees well with the MALDI-TOF MS result.

Figure 4. (a) MALDI-TOF spectrum and (b) powder XRD pattern of ZnSC2P-SS/AuCs. The black vertical bars in (b) indicate the peak positions corresponding to bulk Au.

Overall Structure of ZnSC2P-SS/AuCs. We measured powder XRD and absorption spectra to investigate the overall structure of the ZnSC2P-SS/AuCs. Powder XRD analysis



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was carried out to study the molecular structure of the ZnSC2P-SS/AuCs because it was difficult to form single crystals of these compounds. In general, the crystal structure of AuCs depends on the number of Au atoms, and the various structures give specific XRD patterns. As shown in Figure 4b, ZnSC2P-SS/AuCs gave a broad XRD pattern, and this was similar to that of Au67(SR)35, which is predicted to be a Marks-truncated decahedral (M-Dh) core.10, 32-33 Therefore, we speculate that the crystal structure of the ZnSC2PSS/AuCs is M-Dh. Our group has reported systematic studies on the interactions between the porphyrin πsystem and AuCs or Au NPs.12-13, 19 When the porphyrin ring approaches the Au surface, the π–metal orbital coupling induces the damping of the molar absorption coefficient and a bathochromic shift in the absorption peaks of the porphyrin derivative, which depends on the distance between the porphyrin ring and the AuC surface. Figure 5a and Figure S8 show the UV-vis-NIR absorption spectra of ZnSC2P-SS and ZnSC2P-SS/AuCs in DMF solution. The molar absorption coefficient of ZnSC2P-SS/AuCs was estimated using ICPAES. The peak positions of the Soret band in ZnSC2P-SS and ZnSC2P-SS/AuCs were 430 and 433 nm, and the molar absorption coefficients were 3.4 × 105 and 3.3 × 105 M−1 cm−1, respectively (Table 1). Comparison with the UV-vis-NIR spectrum of free ZnSC2PSS showed that the π–metal orbital coupling induced a bathochromic shift and a slight


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damping of the molar absorption coefficient. The bathochromic shift is derived from the mirror charge effect between the porphyrin ring and the AuC,13 and supports the face-on coordination of ZnSC2P-SS on the AuCs. It is noteworthy that the molar absorption coefficient of ZnSC2P-SS/AuCs is larger than that of SC2P on the pseudo-hexahedral AuCs (1.5 × 105 M−1 cm−1).12 This indicates that the distance between the porphyrin ring and the AuC in the ZnSC2P-SS/AuCs is longer than that in the SC2P/AuCs. When constructing a tetrahedral shape from four regular squares and an inscribed sphere, steric hindrance between neighboring squares (ZnSC2P-SS molecules) should be avoided. Owing to the flexibility of the methylene chains in ZnSC2P-SS, the porphyrin ring can be located away from the AuC surface to avoid steric hindrance. As a result, damping of the molar absorption coefficient of the ZnSC2P-SS/AuCs is suppressed because of weak hybridization between the π-orbital of the porphyrin ring and the metal orbitals. Figure 5b and Figure S9a, b show fluorescence and excitation spectra of the ZnSC2P-SS and the ZnSC2P-SS/AuCs in DMF solution, respectively. The absolute fluorescence quantum yields were estimated by using an integrated sphere (λex = 430 nm). The face-on coordinated porphyrin derivatives on AuCs formed the exciplex upon photoexcitation, resulting in the dramatic damping of fluorescence. The absolute fluorescence quantum yields of ZnSC2P-SS and ZnSC2P-SS/AuCs were 0.7 % and below the limitation of



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detection, respectively. From the comparison of fluorescence intensities between ZnSC2P-SS and ZnSC2P-SS/AuCs under the same absorbed photon numbers, the fluorescence quantum yield of ZnSC2P-SS/AuCs was roughly estimated to be < 10-3 %. These results indicate the face-on coordination of the ZnSC2P-SS on AuCs. ZnSC2PSS/AuC consists of an Au core with approximately 58 Au atoms and four facecoordinating ZnSC2P-SS molecules. Among all the possible structures, the pseudotetrahedral shape is the only shape that can satisfy the above conditions. Consequently, it seems reasonable to conclude that the ZnSC2P-SS/AuC has a pseudo-tetrahedral shape, as shown in Figure 5c. Efforts to form a single crystal of the ZnSC2P-SS/AuCs to support our results are underway.


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Figure 5. (a) UV-vis-NIR absorption spectra of ZnSC2P-SS (2.0 μM, black) and ZnSC2PSS/AuCs (2.0 μM of ZnSC2P-SS on AuCs, red) in DMF. The inset shows the magnified spectra at 400–460 nm. The molar concentration of ZnSC2P-SS/AuCs was estimated using ICP-AES. (b) Fluorescence spectra of ZnSC2P-SS (2.8 µM, black) and ZnSC2PSS/AuCs (2.6 µM of ZnSC2P-SS on AuCs, red) in DMF. The inset shows the magnified spectra of ZnSC2P-SS/AuCs at 575-725 nm. (c) Schematic illustration of the pseudo-



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tetrahedron constructed from four ZnSC2P-SS molecules and an inscribed AuC. Only the zinc tetraphenylporphyrin moieties are shown for clarity.

Table 1. Optical properties of ZnSC2P-SS/AuCs, ZnSC2P-SS, SC2P/AuCs, 13 and SC2P13 in DMF solution. Compounds

λmax (nm)

εa (× 105 M-1 cm-1)

FWHMb (nm)

ZnSC2P-SS/AuCs

433, 565, 604

3.3c

18

ZnSC2P-SS

430, 565, 602

3.4

10

SC2P/AuCs

418, 514, 542, 587, 644

1.5c

17

SC2P

418, 513, 545, 589, 644

3.9

12

a

Molar absorption coefficient of Soret band of porphyrin derivatives. bFull-width at half maximum at Soret band.


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CONCLUSIONS

In this study, we have developed a new method to control the shape (reduce the symmetry) of ligand-protected AuCs. We synthesized AuCs face-coordinated by four ZnSC2P-SS ligands with a core size of 1.0±0.2 nm. Structural assignment using MALDITOF MS, ICP-AES, and UV-vis-NIR absorption spectra indicated that the ZnSC2PSS/AuCs have a pseudo-tetrahedral shape. Thus, we have demonstrated that a simple geometrical relationship between an inscribed sphere and a circumscribed polyhedron can be applied to control the shape (symmetry) of metal clusters with a size of less than 2 nm. We believe that this concept will make it possible to synthesize ligand-protected metal clusters with various shapes and to construct further suprastructures from these cluster.



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ASSOCIATED CONTENT Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Additional figures and table of 1H, 13C, 1H–13C HSQC, and 1H–1H NOESY NMR spectra of ZnSC2P-SS; single crystal X-ray data of ZnSC2P-SS; GPC-HPLC chromatograms of crude products of ZnSC2P-SS/AuCs prepared under various conditions; 1H NMR spectrum of ZnSC2PSS/AuCs; MALDI-TOF-MS spectra of ZnSC2P-SS/AuCs; enlarged absorption spectra of ZnSC2PSS and ZnSC2P-SS/AuCs; and excitation and absorption spectra of ZnSC2P-SS and ZnSC2PSS/AuCs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Phone: +81-774-38-3120, Fax: +81-774-38-3121 Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grant Number JP16H06520 (Coordination Asymmetry) (T.T.), the Collaborative Research Project of the Institute of Chemical Research, Kyoto University (Grant 2016-74), and the Collaborative Research Project of the Laboratory for Materials and Structures, Tokyo Institute of Technology. The authors thank Prof. Masaharu Nakamura and Dr. Katuhiro Isozaki for their help with the X-ray crystallography, Ms.

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Kyoko Oonime for the 1H, 13C, 1H–13C HSQC, and 1H–1H NOESY NMR measurements, and Prof. Takashi Morii for the 300 and 600 MHz NMR measurements. Our thanks also go to Prof. Yutaka Majima, Dr. Yasuo Azuma, and Mr. Ouyang Chun for fruitful discussions on the structure.

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