Correlating the Structure and Optical Absorption Properties of Au76

Article pubs.acs.org/JPCC

Correlating the Structure and Optical Absorption Properties of Au76(SR)44 Cluster Zhongyun Ma,† Pu Wang,† Guang Zhou,‡ Jian Tang,‡ Hengfeng Li,§ and Yong Pei*,† †

Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education and Hunan Key Laboratory for Computation and Simulation in Science and Engineering, Institute for Computational and Applied Mathematics, Xiangtan University, Hunan Province Xiangtan 411105, People’s Republic of China § School of Materials Science and Engineering, Central South University, Hunan Province Changsha 410083, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The atomic structure of recently synthesized thiolate-protected Au76 cluster is theoretically predicted via a simple structural rule summarized from the crystal structures of thiolateprotected Au44(SR)28, Au36(SR)24, and Au52(SR)32 clusters. We find that Au76(SR)44 (N = 7) and recently reported Au52(SR)32 (N = 4), Au44(SR)28 (N = 3), Au36(SR)24 (N = 2), and Au28(SR)20 (N = 1) belong to a family of homologous Au20+8N(SR)16+4N clusters whose Au cores follow a one-dimensional polytetrahedral growth pathway. The Au76(SR)44 cluster is predicted to contain an anisotropic facecentered-cubic (fcc) Au core, which can be viewed as combination of two helical tetrahedra Au4 chains and is remarkably different from the well-known spherical Au core in ligand-protected gold clusters in the size region of 1−2 nm. The intense near-infrared (NIR) absorption of Au76(SR)44 is attributed to the synergistic effect of anisotropic Au core structure and ligand protections. A plausible cluster-to-cluster transformation mechanism is further suggested.

1. INTRODUCTION The synthesis and functionalization of nanometer-scaled thiolate ligand protected gold Au−SR nanoclusters, denoted as Aun(SR)m, have aroused intensive research interest.1−10 In the past few years, a number of atomically precise Aun(SR)m clusters ranging from tens to hundreds of gold atoms have been successfully synthesized. Some magic stable thiolate-protected gold nanoclusters such as Au102(p-MBA)44 (p-MBA = pmercaptobenzoic acid, SPhCOOH) and Au25(SCH2CH2Ph)18− can be viewed as “superatoms”11 whose novel properties may be exploited for practical applications in catalysis, nanotechnology, or chemical biology. Determination of the structures of thiolate-protected gold nanoclusters is key to understanding size-dependent properties and evolution behaviors of Aun(SR)m nanoclusters. Recently, significant experimental advances have been made in the structure determination of several magic-sized Aun(SR)m clusters in the size region of about 0.5−2 nm. The atomic structures of Au133(SPh-t-Bu)52,12,13 Au130(SC12H25)50,14,15 Au102(p-MBA)44,16 Au68(3-MBA)31−34 (3-MBA = 3-mercaptobenzoic acid),17 Au52(TBBT)32 (TBBT = 4-tert-butylbenzenethiol),10 Au40(o-MBT)24 (o-MBT = mercaptobenzothiazole),18 Au38(SCH2CH2Ph)24,19 Au25(SCH2CH2Ph)18−,20,21 Au36(SPht- Bu ) 2 4 , 2 2 Au 3 0 S ( S - t- B u ) 1 8 , 2 3 Au 2 8 (S Ph- t- B u ) 2 0 , 2 4 Au24(SCH2Ph-t-Bu)20,25 Au24(SAdm)16,26 Au23(SPh)16−,27 Au20(SPh-t-Bu)16,28 and Au18(SC6H11)1429,30 were determined by single X-ray crystallography and powerful single-particle © XXXX American Chemical Society

transmission electron microscopy (SP-TEM). Based on the resolved cluster structures, a generic structural rule denoted as the “divide-and-protect” concept has been summarized. According to this concept,31,32 any Aun(SR)m nanoclusters can be divided into a symmetric gold core covered by interfacial staple motifs (i.e., −SR[AuSR]x−, x = 1, 2, 3, ...). Over the past few years, the “divide-and-protect” concept combined with density functional theory (DFT) computations has led to predictions of a number of cluster structures, including Au187(SR)68,33 Au144(SR)60,34 Au68(SR)34,35 Au67(SR)352−,36 Au44(SR)28,37 Au40(SR)24,38 Au38(SR)24,39,40 Au25(SR)18−,41 Au24(SR)20,42 Au20(SR)16,43 Au18(SR)14,44 Au15(SR)13,45,46 and Au12(SR)9+.47 Although tremendous research efforts have been made on the characterization of atomic structures, an important issue about the structural evolution of Aun(SR)m clusters in the size range 1−2 nm remained elusive. To date, the clusters containing gold atoms with n ≤ 52 and n ≥ 102 have been extensively explored by both experiment and theory. The unraveled cluster structures indicated that the structural pattern of gold cores and interfacial ligand shells is very sensitive to the number of gold atoms (n) and thiolate ligands (m), as well as the substituent group (−R) in the thiolate ligand.48 However, Received: April 26, 2016 Revised: June 8, 2016

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DOI: 10.1021/acs.jpcc.6b04212 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Isosurface and Occupancy Number of Each 4c−2e Bond in Au Core of Aun(SR)m Clusters with n ≤ 52a

a

The isovalue for plotting the electronic isosurface is 0.005.

monolayer protected gold nanoparticles. For ligand protected gold clusters with diameter less than 2 nm, they exhibit an optical onset corresponding to electronic transitions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the positions of feature adsorption peaks are generally less than 900 nm.60 The intense NIR adsorption of Au76(4-MEBA)44 implies a different kind of cluster structure with known ligand protected gold nanoclusters. By means of DFT calculations and optical absorption spectrum computations, a unique anisotropic face-centered-cubic Au50 core is predicted for the Au76(SR)44 cluster. We find Au76(SR)44 and recently reported Au52(SR)32, Au44(SR)28, Au36(SR)24, and Au28(SR)20 belong to a family of homologous Au20+8N(SR)16+4N clusters whose Au cores follow a one-dimensional polytetrahedral growth pathway. The intense NIR absorption properties of Au76(SR)44 are a result of the synergistic effect of anisotropic Au core structure and ligand protections.

very limited information is available about the magic stable size and atomic structures of Aun(SR)m clusters within the range 52 < n < 102. Recently, some magic stable gold nanoclusters including Au51(SC2H4Ph)32,39 Au64(SC6H11)32,50 Au67(SC2H4Ph)352−,36 Au68(3-MBA)31−34,29,30 Au68(SC2H4Ph)34,51 Au76(4-MEBA)44 (4-MEBA = 4-(2-mercaptoethyl)benzoic acid), 52 and Au99(SPh)4253 were reported. With the aid of the powerful experimental techniques and DFT computations, the atomic structures of Au67(SR)352−, Au68(SR)32, and Au68(SR)34 have been either resolved experimentally or predicted by theory. Azubel et al. have reported the atomic structure of a 68 Au atom cluster from SP-TEM combined with DFT calculation and absorption spectroscopy measurement.9 The SP-TEM and DFT calculations indicated the atomic positions of Au atoms in the Au68(SR)32 cluster adopted a face-centered-cubic (fcc)-like spherical framework.17,54 The reported atomic structures of Au67(SR)352−, Au68(SR)32, and Au68(SR)34 suggested a popular structural evolution pathway for the thiolate-protected gold nanocluster; i.e., Au25(SR)18−, Au67(SR)352−, Au68(SR)32, Au68(SR)34, Au130(SR)50, Au133(SR)52, and Au144(SR)60 created a succession of spherical structures which show a core−shell structural pattern. This kind of evolution fashion is consistent with the predictions made by Teo et al. from the summarization of crystal structures of ligand protected group 11 metal clusters.55,56 One-dimensional ligand protected gold clusters containing polyicosahedral Au cores were also unraveled experimentally or proposed theoretically recently.57−59 In this work, a novel anisotropic structure is predicted for ligand protected Au76 clusters. The synthesis of Au76(SR)44 (SR = 4-MEBA) was reported recently by Takano et al.52 An intriguing property of Au76(SR)44 is that it shows an intense near-infrared (NIR) adsorption peak around 1350 nm, which is much different from the optical absorption properties of known

2. COMPUTATIONAL METHOD AND DETAILS DFT and time-dependent DFT (TD-DFT) calculations are performed using the Amsterdam Density Functional (ADF 2010) software packages61 with the R group is simplified by either −CH3 (for energy evaluations) or H atom (for optical absorption calculations), all within the generalized gradient approximation with the Perdew−Burke−Ernzerhof (PBE) functional.62,63 The triple-ζ polarized (TZP) basis set with inclusion of scalar relativistic effect via zero-order regular approximation (ZORA) implemented in the ADF package is adopted. The TD-DFT calculations evaluate the lowest 1500 singlet-to-singlet excitation energies for Au76(SR)44 (R = H or CH3) and the lowest 300 singlet-to-singlet excitation energies for other Au20+8N(SR)16+4N clusters (R = H). The theoretical XB


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The Journal of Physical Chemistry C ray diffraction (XRD) patterns were calculated using the Debye formula: I (s ) =

∑∑ i

j≠i

Scheme 2. (a) Embedding Discovered Au Cores of Aun(SR)m Clusters into a Bulk Au Crystal Structure;a (b) General Structural View of Au20+8N(SR)16+4N Clusters, in Which −R Is Simplified by −CH3 Group)b

⎛ Bs 2 ⎞ sin(2πdij) cos θ exp⎜ − ⎟f f 1 + α cos 2θ ⎝ 2 ⎠ i j 2πdij

where s is the diffraction vector length, satisfying s = 2 sin θ/λ. The λ and α are determined by the experimental setup and are set to 0.105 196 7 nm and 1.01, respectively. B is the damping factor, which reflects the thermal vibrations, and was set to 0.03 nm2. The corresponding atomic numbers were used for the scattering factors f i. dij is the distance between atoms i and j. The adaptive natural density partitioning (AdNDP) analysis64 is used to explore the multicentered bonding properties of Au cores in order to give a reasonable division of core structure. For the AdNDP analysis, the PBE functional with LANL2DZ and 6-31G* basis set is used to do single energy calculations using the Au core extracted from the optimized cluster structures. The molecular orbitals are visualized via the VMD program.65 The AdNDP analysis is performed using the Multiwfn software package.66 The multicenter bond is visualized from the analysis of Kohn−Sham (KS) orbitals computed from the PBE/LANL2DZ/6-31G* method implemented in the Gaussian 03 package.67 The Born−Oppenheimer molecular dynamics (BOMD) simulations are performed based on a DFT method with a mixed Gaussian and plane-wave (GPW) basis.68 The PBE functional with the DZP-MOLOPT basis set69 is used to compute the exchange−correlation energy. The energy cutoff is set to be 80 Ry for the plane-wave wave functions. The interaction between the valence electrons and the atomic cores is accounted for using the Goedecker−Teter−Hutter (GTH) pseudopotential69,70 as implemented in the CP2K code.71 The constant-temperature and constant-volume ensemble (NVT) is adopted in all simulations.

a

The cluster structures surrounded by black frame are theoretical models. bPurple and yellow balls denote Au atoms in Au core and ligand shell. Green, red, and gray balls are C, O, and H atoms, respectively.

3. RESULTS AND DISCUSSION 3.1. Structural Prediction of Au76(SR)44. Au76(SR)44 shows an intense NIR absorption band around 1350 nm.60 According to the NIR absorption properties of one-dimensional ligand protected gold nanoclusters and gold nanorod, Takano et al.52 proposed three anisotropic Au core models (e.g., Au49, Au52, and Au56) for Au76(SR)44. At present, we give a new structure prediction of the cluster structure of Au76(SR)44 through investigating the structural fashions of known Au− SR clusters. The structure prediction of Au76(SR)44 is made based on a simple structural evolution rule summarized from several revealed thiolate-protected gold cluster structures. Scheme 1a collects known Au core structures of Aun(SR)m clusters. We find most Aun(SR)m clusters (n ≤ 52) can be classified into the category of polytetrahedral cluster, whose Au cores are constructed from multiple tetrahedral Au4 units fused together via vertex, edge, or face sharing. Herein, AdNDP analysis is performed for these Au cores. As shown in Scheme 1a, for most Au cores, the cluster valence electrons are localized in separated Au4 tetrahedra, which confirms the tetrahedral partition of cluster structures. It is well-known that tetrahedra are the natural part of the fcc structure. We therefore further compared the Au core structures of Aun(SR)m clusters (n ≤ 52) with bulk Au crystal. As shown in Scheme 2a, the experimentally resolved or theoretically predicted Au cores in Aun(SR)m clusters (n ≤ 52)

are indeed identical to a small group of Au atoms in bulk crystal with the exception of Au25(SR)18− and Au38(SR)24. Au25(SR)18− and Au38(SR)24 are made of an icosahedra Au13 or bi-icosahedra Au23 core, where the Au core atoms do not adopt a fcc-like packing. From Scheme 2a, an interesting evolution trend of possible elongation of Au core along the crystal [100] direction is clearly found from the Au core structures of Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32. We note recent experimental and theoretical studies have proposed a simple evolution fashion among Au 28 (SR) 20 , Au 36 (SR) 24 , and Au44(SR)28 clusters. That is, the Au36 and Au44 clusters can evolve from the Au28 by sequential addition of [Au8(SR)4] units: Au28(SR)20 + [Au8(SR)4] → Au36(SR)24 + [Au8(SR)4] → Au44(SR)28.37,72,73 By extending this relation, a series of homologous clusters can be envisioned, i.e., Au52(SR)32, Au60(SR)36, Au68(SR)40, Au76(SR)44, etc. These clusters, therefore, adopt a common structural formula of Au20+8N(SR)16+4N (N ≥ 1). Based on the cluster structures of Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32, a general structural scheme is proposed for Au20+8N(SR)16+4N cluster. From Scheme 2b, any Au20+8N(SR)16+4N cluster (N > 1) is composed of two head units and a body part. The head unit has a structural formula of Au18(SR)12, which is composed of four dimeric staple motifs (−SR−Au−SR−Au−SR−) and 10 kernel Au atoms. The body part has a structural formula of [Au8(SR)4]N−2 containing 2(N − 2) monomeric −SR−Au−SR− motifs and N−2 rectangular C


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The Journal of Physical Chemistry C

The TD-DFT predicts a first major absorption peak (absorption peak a in Figure 2a) at ∼1123 nm, which is close to the experimental feature absorption peak (∼1350) nm. The noticeable absorption peaks at ∼680 nm and ∼580 nm in experimental spectra are also seen in the theoretical optical absorption curve as well (absorption peaks b and c in Figure 2a around 728 and 598 nm, respectively). The powder X-ray diffraction curve calculated from the predicted structure model is also compared with the experimental one. From Figure 2b, the theoretical powder X-ray diffraction curve matches very well with the experimental one. The relative stabilities of proposed structural models are further evaluated. Ab initio molecular dynamics simulations are first carried out to examine the thermodynamic stability of a proposed structure model. Two 8 ps BOMD simulations are performed at 323 and 373 K, respectively. From the snapshot structure of the BOMD trajectory displayed in Figure 3, the cluster structure suffers small structural distorsions at evaluated temperature, suggesting high stability of the predicted structure model. On the other hand, we also make energy evalulations of the predicted structure model with previously proposed ones. Takano et al.52 proposed three anisotropic Au core models. At present, we have constructed three anisotropic isomers based on the proposed Au cores. From Figure 4, the energy computations indicated the new structure model is more stable by 5.26−7.97 eV than that constructed from Au49, Au52, and Au56 cores. Because of the close atomic compositions of Au76(SR)44 and the recently reported nearly spherical Au67(SR)352− and Au68(SR)32 clusters, we have constructed a series of isomers containing spherical Au cores as well in order to give a rigorous confirmation of the relative stabilities of the proposed structure model. These spherical isomer structures are constructed from a Au55 core with either icosahedral, decahedral, or octahedral configuration. It is found that the spherical isomers are less stable by 5.46−6.56 eV than the lowest energy icosotropic one. Taking the optical adsorption properties, powder XRD simulation, and comparison of relative stabilities of various isomer structures together, we confirm the new structure model containing an anisotropic Au50 core is the best candidate for understanding the atomic structure and optical absorption properties of the Au76(SR)44 cluster. 3.2. Correlating the Structure and Optical Absorption Properties of Au76(SR)44. The discovery of anisotropic Au50 core in the Au76(SR)44 cluster raised several interesting questions relating to structure−property relations and evolution behaviors of thiolate-protected gold nanoclusters. An interesting issue is about the origin of NIR absorption bands. Jin et al. summarized the HOMO−LUMO gaps and optical adsorption properties of Au−SR nanoclusters.49 The optical absorption curve shows discrete absorption peaks at smaller cluster size and is featureless with the increase of the diameter of clusters. Moreover, thiolate-protected gold nanoclusters with size less than ∼2 nm generally show very weak NIR absorption. Takano et al.52 proposed that the NIR absorption band of the Au76(SR)44 cluster arises from the unique anisotropic structure. At present, we carried out a detailed analysis of optical absorption properties of the Au76(SH)44 cluster by means of TD-DFT calculations. From the analysis of dipole components (longitudinal and transversal) of each excitation electronic transition, we find the optical absorption of Au76(SH)44 is majorly contributed by longitudinal electronic

Au6 units. We note that the Au20+8N(SR)16+4N clusters can be divided into two categories according to the relative orientation of two head units (cf. Scheme 2b). The recently discovered clusters such as Au52(SR)32, Au44(SR)28, and Au36(SR)24 belong to two different structural categories with N = 4, 3, and 2, respectively. Au28(SR)20 (N = 1) is unique which contains only one head unit. From the right part of Scheme 2b, a common structural feature of Au20+8N(SR)16+4N clusters is that the Au cores are elongated along the crystal [100] direction and demonstrate fcc packing style. Of note, several fcc-type kernel structures for silver nanoclusters protected by thiolate or coprotected by thiolate and phosphine have been also reported to date, for instance, the complete fcc Ag14 core in [Ag 62 S 12 (SBu t ) 32 ] 2+ and the octahedral Ag 6 core in Ag14(SC6H3F2)12(PPh3)8 nanoclusters.74,75 However, it is not appropriate to simply apply the structure modes in Scheme 2b for Au20+8N(SR)16+4N clusters to predict the structures of thiolated-protected Ag nanoclusters with fcc packed cores because of the structure differences between them, including no staple motifs in the latter ones. The Au76(SR)44 cluster satisfies the structural formula of Au20+8N(SR)16+4N with N = 7. This inspires us to provide a theoretical prediction of the structure of the Au76(SR)44 cluster and hence explore its structure−property relations. Note that some very recent studies made a similar prediction of Au20+8N(SR)16+4N clusters.72,73 However, the structure−property relationships of the Au76(SR)44 cluster are not discussed. Figure 1 displays the proposed structure for the Au76(SR)44 cluster, which contains a Au50 core and 10 monomeric −SR−

Figure 1. Au−S framework and fcc Au kernel of Au76(SR)44 cluster. Purple and yellow balls denote Au atoms in Au core and ligand shell, respectively. Red balls are S atoms. The R group is not drawn for clarity.

Au−SR− and eight dimeric −SR−Au−SR−Au−SR− protection motifs. The structural formula of Au76(SR)44 can thus be written as Au50[Au(SR)2]10[Au2(SR)3]8. The geometric optimizations and frequency calculations (−R is simplified by the −CH3 group) confirmed the structure is a stable local minimum without imaginary vibration frequency. We note that the proposed structure model fits very well with the “divide-and-protect” concept. From Figure 1, the cluster model contains a highly symmetric Au core which shows near D2 point group symmetry. Meanwhile, the Au atoms (including ligand shell Au atoms) show a crystal fcc packing style. In order to confirm theoretical structure prediction, we have compared the TD-DFT computed optical absorption curve of the proposed structure model with the experimental curve, as the shape of the UV−vis absorption curve of the Aun(SR)m clusters is very sensitive to the composition and configuration.76−79 From Figure 2a, the simulated optical absorption curve of Au76(SR)44 (R is simplified by H atom) shows qualitatively good agreement with the experimental spectrum. D


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Figure 2. (a) (left) UV−vis optical absorption curve and transversal and longitudinal dipole moment transitions of Au76(SH)44 cluster computed by TD-DFT. (right) Electronic density diagram of HOMO and LUMO of Au76(SH)44. The isovalue used for plotting electronic density of HOMO and LUMO is 0.02. (b) Comparison of simulated powder X-ray diffraction curve with experimental one. The experimental optical adsorption and powder XRD curve is adopted from ref 52. (c) Simulated absorption curve of Au50 core in Au76(SH)44. Gauss broadening with a width at halfmaximum of 0.1 eV is used to fit the optical curve based on the computed excitation energies. (d) KS orbital energy levels and atomic orbital components of Au76(SR)44 cluster and its Au50 core. KS molecular orbitals ascribing to the excitation encompassed by the feature absorption peaks a and a′ of Au76(SR)44 cluster and its Au50 core are assigned.

compare the optical absorption properties of Au76(SR)44 with those of recently reported Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 clusters due to their homologous structures. From Figure 5, it is found that, with the increase of cluster aspect ratio from Au28 to Au52 cluster, the optical absorption peaks are red shifted in wavelength and a feature absorption peak near the NIR region appears in the Au52 cluster. For the Au76 cluster, the larger aspect ratio leads to much stronger longitudinal dipole moment transitions than the transversal transitions and the first absorption peak is red shifted to 1123 nm. These results suggest that the anisotropic structure is key

transitions, as shown in Figure 2a. The most intense transition encompassed by the first adsorption peak (a) is ascribed to a single excited configuration relative to the HOMO to LUMO transition and demonstrates obvious longitudinal electron transition character according to the electronic density diagram of HOMO and LUMO showing in Figure 2a. The strong longitudinal electronic transitions in the Au76(SR)44 cluster agree with recently theoretical studies of ultrathin linear gold superstructures with or without ligand protections.78−80 In order to obtain a better understanding of the origin of the NIR absorption properties of Au76 clusters, it is worthwhile to E


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Figure 3. Electronic energies and temperatures of BOMD simulations of Au76(SH)44 cluster: (a) 323 K and (b) 373 K. Snapshots of Au−S framework structure at the end of 8 ps simulations at different temperatures are displayed above. The H atom is not shown for clarity.

LUMO ← HOMO − 9 (transition weight 51%) and LUMO + 9 ← HOMO − 1 (transition weight 12%) are identified. The orbital transitions involved in the excitation encompassed by the feature adsorption peaks of a and a′ are displayed in Figure 2d. The much different optical adsorption properties of the bare Au core and the ligand-protected one highlighted the role of thiolate ligand for the NIR absorption of the Au76(SR)44 cluster. Interestingly, Xie et al. reported recently an abnormal optical absorption at about 780 and 980 nm for bithiolateprotected Au25 nanoclusters, in which the negatively charged thiolate ligands were collocated with positively or neutrally charged thiolate ligands. The origin of these abnormal absorptions was ascribed to the formed charge anisotropy on the surface of Au nanoclusters, suggesting that the anisotropy of gold nanoclusters could also be introduced by the ligand shell in addition to the anisotropic Au core structure.81,82 More attention thus deserved to be paid in future research. Together with the above discussions, we conclude that the intense NIR absorption properties of the Au76(SR)44 cluster are attributed to the synergistic effect of anisotropic Au core structure (with large aspect ratio) and ligand passivations. The chiral properties of Au76(SH)44 is also examined by TD-DFT computations of circular dichroism (CD) spectra. From Figure S1 in the Supporting Information, both the Au core and the ligand shell in the Au76(SR)44 cluster demonstrate mirror symmetry. The calculated CD spectrum confirms Au76(SR)44 is a chiral cluster, which has strong chiral responses in the excitation wavelength range of 550−950 nm. Finally, it was known that group 11 metal clusters tend to form partial or complete icosahedral structures, which was termed “icosahedricity”.55,56 In the present study of the structure of the Au76(SR)44 cluster, the anisotropic fcc packed Au core suggests a different kind of growth fashion. By analyzing the Au−Au bond lengths, we find the Au50 core of the Au76 cluster can be viewed as a dimer of Au25 subunits. Each Au25 subunit is composed of a string of vertex-shared tetrahedra Au4. In the Au50 core, two polytetrahedral Au25 subunits are antisymmetrically bonded, forming a double helical configuration. From Figure S2 in the Supporting Information, within the Au25 subunit, Au−Au bond lengths are in range 2.78−2.92 Å. The Au−Au bond lengths between two Au25 subunits are in the range 2.95−3.12 Å. In recent studies, Cheng et al. proposed a “superatom network” (SAN) concept to explain the structure and stability of the Au20(SR)16 cluster.83 It was known that the tetrahedral Au42+ cluster can be viewed as a 2e superatom. In the present

Figure 4. Relative energies (in eV) and Au−S frameworks of isomers of Au76(SH)44. The isomers 2−4 are constructed from the Au core structures proposed by Takano et al. (cf. ref 52). The spherical isomers 5−11 are built from nearly icosahedral, octahedral, and decahedral Au55 cores. H atoms are omitted for clarity.

for generating the NIR absorption properties of the Au76(SR)44 cluster. However, we note that the anisotropic cluster structure is not the sole reason that induces the NIR absorption property of Au76(SR)44. Figure 2c plots the optical absorption curve of the Au50 core of the Au76(SR)44 cluster. In comparison to the ligand protected cluster, we find that when the ligand shell is removed, the NIR adsorption peak vanishes. From Figure 2c, the TDDFT calculations indicated a weak adsorption peak (a′) at ∼880 nm for the bare Au50 core. The analysis of the involved orbitals ascribing to the excitation encompassed by the feature adsorption peaks a and a′ in Figure 2a and Figure 2c, respectively, indicates two different orbital transition modes. For the Au76(SR)44 cluster, an sp ← sp intraband transition is ascribed to the excitations related to the feature adsorption peak a, which is contributed exclusively by the LUMO ← HOMO transition. For the bare Au50 core, the molecular orbital transitions related to adsorption peak a′ is composed of multiexcitation components. Two major transitions involving F


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Figure 5. Simulated optical absorption curves and transversal and longitudinal dipole moment transitions of Au28(SR)20, Au36(SR)24, Au44(SR)28, Au52(SR)32, and Au76(SR)44 clusters with different aspect ratios. Gauss broadening with a width at half-maximum of 0.1 eV is used to fit the optical curve based on the computed excitation energies by TD-DFT method.

case, the Au5018+ core in the Au76(SR)44 cluster would be composed of 16 2e superatoms. From Figure 6, 16 Au42+ are found from the AdNDP analysis. We note the double-helical Au42+ superatom network was found in Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 as well.10,28 In view of the Au42+

superatom networks shown in Figure 6, an evolution trend is found that, in each cluster growth step from Au28 to Au76, two Au4 tetrahedra are grown in the Au core and the valence electron of cluster increases 4e concurrently. We term this kind of Au core evolution as a one-dimensional polytetrahedral growth pathway. According to this pathway, a series of linear homologous Au20+8N(SR)16+4N clusters can be derived, which have a general structure scheme as described in Scheme 2b. Based on the structural fashions of Au28(SR)20, Au36(SR)24, Au44(SR)28, Au52(SR)32, and Au76(SR)44 clusters, a cluster-tocluster growth mechanism is further proposed. As illustrated in Scheme 3, a larger size cluster may grow from a smaller one through incorporating two Au(0) atoms and two [Au3(SR)2] species at one ending of the smaller cluster. Taking the cluster growth from Au28 to Au36 as an example, two Au(0) atoms (i.e., Au1 and Au2) first insert into the Au−S bonds at one end of the Au28 cluster. In this step (1 → 2), two Au(I) atoms in the trimeric motifs collapse into the Au core and two dangling SR groups (i.e., S1 and S2) are generated. After that, the dangling

Figure 6. Evolution of double-helical Au4 tetrahedra network within Au cores of Au28(SR)20, Au36(SR)24, Au44(SR)28, Au52(SR)32, and Au76(SR)44. Polyhedron denotes the Au4 tetrahedron unit. ON, occupancy number of each 4c−2e bond. G


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Scheme 3. (a) Schematic Illustration of Cluster-to-Cluster Growth Mechanism of Au20+8N(SR)16+4N Clusters, Using Au28(SR)20 and Au36(SR)24 as Examples; (b) Cluster-to-Cluster Formation Energy from Au28(SR)16 and Au76(SR)44

Au50 core protected by eight dimeric and ten monomeric staple motifs. The unique NIR absorption properties of the Au76(SR)44 cluster can be explained from its anisotropic Au core structure and protection ligand shell. The larger aspect ratio of the Au76 cluster leads to enhanced longitudinal orbital transitions, and the ligand passivation results in the red shift of the absorption peak in wavelength of the cluster. Finally, a possible growth route of the Au76(SR)44 cluster from smaller clusters is suggested via a CO-directed reduction reaction. The formation of Au76(SR)44 is energetically favorable.

SR groups and the one ending of the dimeric [Au2(SR)3] motif mutually exchange their linked Au sites. Two [Au3(SR)2] species then cap to the cluster to finish the growth. According to recent studies of reduction−growth of Au25 nanocluster via CO-directed synthesis, the cleavage of Au(I)−SR species by CO-directed reduction is a key step for providing Au(0) atoms as well as other ligand motifs that are needed for cluster growth.84,85 We therefore proposed a possible one-step reaction for the sequential growth of Au20+8N(SR)16+4N clusters, which is written as eq 1.

■

Au 20 + 8N (SR)16 + 4N + 2Au4 (SR)4 + 2CO + 8HO− −

→ Au 20 + 8(N + 1)(SR)16 + 4(N + 1) + 4SR + 2CO3 + 4H 2O

ASSOCIATED CONTENT

S Supporting Information *

2−

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04212. Simulated CD spectra, Au−Au bond lengths, and coordinates of Au76(SH)44 (PDF)

(1)

From Scheme 3a, the reduction of Au4(SR)4 oligomers provides the sources of Au(0) atoms and the Au−SR−Au−SR− Au units that are needed during the cluster growth. The reaction energies of proposed growth routes from Au28 to Au76 clusters via the CO-directed reduction reactions are computed by DFT and are displayed in Scheme 3b. From Scheme 3b, the growth of clusters from Au28 to Au76 is a thermodynamically favorable process. The total energy release in each growth step is about −6.18 to −6.39 eV according to the growth mechanism described in eq 1. According to the computed formation energy curve, we can make a qualitative comparison of the relative stabilities of various clusters. It can be found that the formation of Au76(SR)44 (N = 7) cluster from the neighboring Au68(SR)40 (N = 6) leads to a large formation energy of −6.24 eV. This implies the Au76(SR)44 cluster may possess relatively higher thermodynamic stabilities.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-731-56239780. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21373176, 21422305, 21503182) and the Scientific Research Fund of Hunan Provincial Education Department (13A100).

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4. CONCLUSION In summary, the structure of the recently synthesized Au76(SR)44 cluster is theoretically predicted through investigating the structural evolution tendency of reported Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 clusters. The structure of the Au76(SR)44 cluster is best described by an anisotropic

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