Thiolate-Protected Au25 Superatoms as Building Blocks: Dimers and

Thiolate-Protected Au25 Superatoms as Building Blocks: Dimers and Crystals ... Publication Date (Web): April 27, 2010. Copyright © 2010 American Chem...
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J. Phys. Chem. C 2010, 114, 15986–15994

Thiolate-Protected Au25 Superatoms as Building Blocks: Dimers and Crystals† Jaakko Akola,‡,§ Katarzyna A. Kacprzak,‡ Olga Lopez-Acevedo,‡ Michael Walter,‡,| Henrik Gro¨nbeck,⊥ and Hannu Ha¨kkinen*,‡,# Department of Physics, Nanoscience Center, P.O. Box 35, UniVersity of JyVa¨skyla¨, FI-40014 JyVa¨skyla¨, Finland, Department of Physics, Tampere UniVersity of Technology, P.O. Box 692, FI-33101 Tampere, Finland, Department of Applied Physics and Competence Centre for Catalysis, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and Department of Chemistry, Nanoscience Center, P.O. Box 35, UniVersity of JyVa¨skyla¨, FI-40014 JyVa¨skyla¨, Finland ReceiVed: February 20, 2010; ReVised Manuscript ReceiVed: April 14, 2010

A particularly stable thiolate-protected gold nanocluster, Au25(SR)18, was structurally characterized from X-ray crystallography in 2008, and concomitantly its electronic and optical properties were analyzed via density functional theory. The robust geometry and a well-understood electronic structure of this cluster motivate explorations of properties of extended systems made out of Au25(SR)18 building blocks. As a first step in this direction, we analyze here structural, vibrational, electronic, and optical properties of the Au25 cluster anion as it was observed in the crystalline environment and predict properties of cluster dimers, where the Au25 units are linked together via an aromatic dithiolate linker. We show that properties of each Au25 unit of the dimer can be quite independently modified from the other by doping with a nonmagnetic (Pd) or magnetic (Mn) metal atom. We anticipate that material systems with interesting properties could be made from these building blocks, provided that a suitable chemistry for their controlled linking can be found. 1. Introduction There is a strong interest to design nanostructured materials with tunable mechanical, electronic, optical, magnetic, or chemical properties by using well-defined, robust nanoparticles as building blocks. The early advances in understanding physical and chemical properties of clusters composed of metal atoms, i.e., their stability in the context of the electron shell model,1–3 led to proposals that electronically closed shell metal clusters with large energy gaps between highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) could be used as such building blocks.4 In fact, one has adopted a practice to refer to these building blocks as “superatoms”.4–6 However, metal clusters, even the electronically most stable ones, are rather reactive as unprotected species. For instance, it was early computationally demonstrated that the “magic nature” of a closed-shell Na8 cluster depends greatly on the environment: the cluster retains its closed-shell electronic structure when interacting weakly with an ionic support, but loses it completely upon reaction with another Na8 in the gas phase.7 However, suitable chemistry can be devised usually to stabilize even the most reactive clusters; a rather extreme example from the recent literature is the stabilization of white phosphorus (P4 clusters) in air by enclosing the clusters into suitably designed supramolecular cages.8 † Part of the special issue “Protected Metallic Clusters, Quantum Wells and Metallic Nanocrystal Molecules”. * Corresponding author, [email protected]. ‡ Department of Physics, Nanoscience Center, University of Jyva¨skyla¨. § Department of Physics, Tampere University of Technology. | Present address: Freiburg Materials Research Center, University of Freiburg, D-79104 Freiburg i. Br., Germany. ⊥ Department of Applied Physics and Competence Centre for Catalysis, Chalmers University of Technology. # Department of Chemistry, Nanoscience Center, P.O. Box 35, University of Jyva¨skyla¨.

The seminal work by Brust and coauthors9 in 1994 on the preparation of thiol-stabilized nanometer-sized gold clusters opened an avenue to explore this class of systems. The great potential of this approach to generate air-stable gold clusters with varied but controllable core sizes and with a modifiable organic outer surface was immediately realized and utilized by several groups.10 It has been known since the late 1990s that a series of stable thiolate-stabilized clusters can be generated by a modified Brust method with gold core masses from ca. 5 to 29 kDa (20-25 Au atoms to 150 atoms).11 The work by Tsukuda and co-workers in 2005 revealed for the first time, that the precise molecular formula for a particularly stable cluster in the low-mass range of around 5-6 kDa is Au25(SR)18q (ref 12). Electrospray ionization mass spectrometry (ESI-MS) studies gave indications that while this particle survives many oxidation states (-1 e q e +2), the most prominent charged species of this particle is the anion.13 Pure solutions of Au25(SR)18 yield a characteristic optical absorption spectrum, which has enabled the reassignment of previously published results where this particle was incorrectly labeled as Au28(SR)16 (refs 11 and 14) or Au38(SR)24 (ref 15). The final breakthrough for the characterization of Au25(SR)18 was achieved in 2008 by two total structure determinations16,17 via X-ray crystallography for Au25(SEtPh)18- (SEtPh ) phenylethane-thiolate).Inthesingle-crystalunitcell,onecluster-counterion pair was detected, the counterion being the positive TOA+ (TOA ) tetraoctyl ammonium), which confirmed the anionic charge state of the cluster. The atomic structure of Au25(SR)18 was found to comprise an approximately icosahedral core of 13 Au atoms, protected by six Au2(SR)3 units in an octahedral arrangement, making Au-S contacts with all 12 Au atoms on the core surface. It is notable that independent density functional theory (DFT) calculations had serendipitously predicted correctly the positions of gold and sulfur atoms in the cluster.18 Further analysis of the electronic structure and the lowest optical

10.1021/jp1015438  2010 American Chemical Society Published on Web 04/27/2010

Thiolate-Protected Au25 Superatoms transitions17–19 revealed that the highest occupied and lowest unoccupied valence electron states of the gold core, derived from the Au(6s) states, are arranged in shells of dominant angular momenta, much as electrons in bare metal clusters.1,2 The six protective Au2(SR)3 units are formally anionic. Thus, these units localize 6 of the 14 s electrons of the (anionic) icosahedral core and leave 8 electrons in an 1S21P6 configuration in the “superatomic orbitals” of the metal core.20 Femtosecond spectroscopy has confirmed this theoretical picture.21 The current situation, where the atomic and electronic structure of the Au25(SR)18 cluster is well understood, creates an ideal platform for further computational work to investigate and predict properties of modified Au25 nanoparticles and nanoparticle systems. Doping of the metal core of a single Au25(SR)18 cluster by magnetic or nonmagnetic transition metal atoms has already been considered,22–26 partially motivated by the results from electrochemistry and mass spectroscopy of bimetal clusters MAu24(SR)18 (M ) Pd, Pt).27 In this work, we have investigated structural and electronic properties of the Au25 nanoparticle in various environments and with two different thiolates (SEtPh, and methyl thiolate, SMe): (i) as an anionic particle 1 Au25(SEtPh)18- in the crystal with the TOA+ counterion, as observed in ref 16; (ii) as an anion-counterion complex 2 [Au25(SEtPh)18-][TOA+] in vacuum; (iii) as a component in the following dimeric units, linked together by benzene dithiolate (BDT): 3 [Au25(SMe)17BDT(SMe)17Au25]-2, 4 [PdAu24(SMe)17-BDT(SMe)17Au25]-, 5 [MnAu24(SMe)17-BDT-(SMe)17Au25]-, and 6 MnAu24(SMe)17-BDT-(SMe)17Au24Mn. We characterize the electronic structure and vibrational properties of system 1 and discuss the vibrational properties of the Au25(SEtPh)18- cluster with respect to recent experiments.21,28,29 We show that system 3 behaves truly as a dimer of two eight-electron superatomic systems and that one part of the system can be modified independently from the other, such as doping by the Pd atom in 4. Investigation of systems 5 and 6 was motivated by recent theoretical suggestions that it should be possible to “shield” strong local magnetic moments of magnetic transition metal atoms M at the center of a doped cluster MAu24(SH)18 provided that the cluster retains the eight-electron “superatomic” valence configuration 1S21P6 (refs 25 and 26). Here, we report that the dimer 5 has magnetic (MnAu24) and nonmagnetic (Au25) superatomic parts and that the dimer 6 has a ferromagnetic state with an extremely high magnetic moment of ∼10 µB. 2. Methods The periodic DFT calculations for the crystalline geometry 1 were performed using the Car-Parrinello molecular dynamics (CPMD) program package.30 The generalized gradient-corrected approximation (GGA) of the exchange-correlation energy Exc by Perdew, Burke, and Ernzerhof (PBE)31 is used in this work generally, but we apply also other functionals (PBEsol, TPSS)32,33 that belong to the same family of analytical functionals. PBEsol is based on GGA and has the same form as PBE, but with parameters optimized for densely packed solids. TPSS is the most sophisticated among the three Exc functionals and the most expensive in terms of computations. In addition to density and its gradient, TPSS includes also Kohn-Sham orbital kinetic energy density (meta-GGA), and it has been observed to improve binding energies and bond distances in molecules.32,33 TheCPMDcalculationsappliedscalar-relativisticTroullier-Martins pseudopotentials34 for the electron-ion interaction including nonlinear core corrections for Au (5d106s1 valence), and the kinetic energy cutoff of the plane wave basis set is 70 Ry. The

J. Phys. Chem. C, Vol. 114, No. 38, 2010 15987 minimization of the energy functional used the direct inversion of the iterative subspace (DIIS). The crystalline phase is simulated with periodic boundary conditions in a triclinic unit cell by using a single point (k ) 0) in the Brillouin zone. The structure was optimized in a fixed (experimental) unit cell, and the final forces between atoms were less than 0.01 eV/Å. The electronic density of states (eDOS) and its projections were computed with the Lanczos diagonalization scheme and a free energy functional (T ) 1000 K).35 The vibrational frequencies and eigenmodes (normal modes) were calculated by applying the method of finite differences with atomic displacements of 0.01 Å in each Cartesian direction. Systems 2-6 were studied by using the grid-based projectoraugmented wave (GPAW) code.36 All clusters were set in a sufficiently large supercell so that a vacuum region of at least 4 Å separated the outermost atoms from the box boundary. The structures were optimized with no symmetry constraints until residual forces between atoms were smaller than 0.05 eV/Å. The PBE functional was used for the exchange-correlation interaction.31 Au was treated at a scalar-relativistic level with 5d106s1 electrons in the valence. The Bader method was used for charge analysis.37 Optical response of system 2 was studied by using the linear-response formalism of the time-dependent DFT in GPAW.38 3. Analysis of the Crystal Structure of the [TOA+][Au25(SEtPh)18-] 3.1. Benchmarking the PBE, PBEsol, and TPSS Functionals against the Experimental Structure of the Cluster Anion-Counterion Complex in Bulk Crystal. Panels B and C of Figure 1 show the structure of 1, optimized by using the TPSS functional. The crystalline geometry involves packing of the cluster anion and TOA+ in the c-axis direction, whereas the EtPh side groups of the neighboring nanoparticle replica interact directly in the other two directions (for the triclinic crystal unit cell parameters, see the caption to Figure 1). The gas phase geometry (Figure 1A) was optimized starting from the coordinates of the crystalline sample after removal of TOA+. The resulting geometry turns out highly symmetric as the SEtPh side groups obey inversion symmetry. The apparent distortions of the oligomeric S-Au-S-Au-S units in the cluster anion crystal are visible also in the gas phase geometry. The effect is most probably reminiscent of the crystalline starting geometry, which itself is affected by the presence of TOA+ and crystal packing (from the crystal structure analysis, we conclude that a single TOA+ has steric interactions with three of the S-Au-S-Au-S units of a given Au25 nanoparticle, which means that in the close-packed crystal two neighboring TOA+ ions interact with all six gold-thiolate ligand units of a given Au25). Our previous results for the Au25(SMe)18- (ref 18) do not show the oligomer distortions, which indicates that the effect may be related to the specific orientation of the bulky side groups. We note that all the six S-Au-S-Au-S units are arranged symmetrically and without distortions in the reported crystal structure of the neutral Au25(SEtPh)18 particle.39 The structural parameters of system 1 are presented in Table 1. We have optimized the sample in a fixed triclinic unit cell starting from the experimental coordinates and unit cell volume. We used three different approximations for the exchangecorrelation energy, namely, PBE, PBEsol, and TPSS. The results show that all considered functionals overestimate Au-Au distances in the icosahedral core: by 1.2% for TPSS and 2.2% for PBE and PBEsol. The Au-AuI distances between the core and the ligand shell (sometimes the AuI-AuI interaction is

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Figure 1. (A) Geometry of the Au25(SEtPh)18- in the gas phase and (B) with the TOA+ counterion in the crystal. (C) The unit cell in (B), replicated three times along the c axis of the crystal. Note the alternation of the nanoparticle and TOA+. The triclinic unit cell parameters are a ) 16.111 Å, b ) 17.331 Å, and c ) 18.581 Å for the box sides, and R ) 106.27°, β ) 105.49°, and γ ) 90.96° for the angles.

TABLE 1: Structural Parameters of the Crystalline [TOA+][Au25(SEtPh)18-]a PBE

PBEsol

TPSS

experimental values (ref 16)

AuC-Au Au-Au Au-AuI Au-S AuI-S S-C AuC-N

2.835 (0.018) 2.983 (0.084) 3.226 (0.087) 2.407 (0.007) 2.337 (0.005) 1.841 (0.004) 8.008

Distances (Å) 2.834 (0.017) 2.981 (0.082) 3.228 (0.086) 2.406 (0.007) 2.334 (0.005) 1.835 (0.004) 8.006

2.808 (0.014) 2.954 (0.075) 3.144 (0.072) 2.413 (0.007) 2.342 (0.005) 1.848 (0.003) 8.003

2.774 (0.010) 2.918 (0.062) 3.161 (0.068) 2.371 (0.006) 2.311 (0.009) 1.855 (0.040) 8.025

Au-S-AuI S-AuI-S AuI-S-AuI Au-S-C AuI-S-C S-C-C S-S-S

88.84 (1.56) 173.07 (0.51) 100.89 (0.79) 106.71 (2.42) 103.50 (2.24) 112.02 (0.88) 107.27 (0.91)

Angles (deg) 88.97 (1.55) 173.19 (0.51) 101.02 (0.82) 106.88 (2.42) 103.62 (2.24) 112.10 (0.88) 107.28 (1.00)

84.79 (1.74) 170.95 (0.79) 100.47 (0.67) 106.43 (2.20) 103.09 (2.25) 111.50 (1.04) 109.01 (0.97)

86.85 (0.71) 172.75 (0.73) 101.08 (0.51) 106.52 (3.04) 103.17 (2.59) 111.56 (1.55) 107.80 (0.68)

∆(Au) ∆(S) ∆(C)

Displacement with Respect to Experimental 0.087 (0.029) 0.083 (0.027) 0.124 (0.040) 0.123 (0.041) 0.166 (0.095) 0.159 (0.094)

Reference Structure (Å) 0.065 (0.020) 0.090 (0.041) 0.148 (0.099)

a AuC refers to the central gold of the Au13 core and AuI to the oxidized gold in the ligand shell. Standard deviations are reported in parentheses. ∆(Au), ∆(S), and ∆(C) are the individual atomic displacements with respect to the experimental reference structure (ref 16). The geometries have been optimized by using three different exchange-correlation functionals in the experimental triclinic unit cell with periodic boundary conditions. We have deliberately given the structural data with four to five significant digits in order to facilitate comparison between the functionals.

termed “aurophilic interaction”) display interesting differences: PBE and PBEsol overestimate also these distances by 2.0%, while the TPSS underestimates it by only 0.5%. The Au-S bond lengths are closer to the experiment, overestimated by 1.0-1.8%

by all three functionals, thus here TPSS does not lead to any improvement. The S-C bond lengths are in good agreement with the experimental value for all the functionals. The gold cluster-counterion distance, measured from the central Au atom

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Figure 2. The density of electron states (eDOS) in the crystal [TOA+][Au25(SEtPh)18-], and its projections to atomic orbitals. Top and bottom panels show the weights on different elements, and the middle panel shows the Au band decomposed to 5d, 6s, and 6p states. The HOMO-LUMO gap is centered around the zero energy.

TABLE 2: HOMO-LUMO Gaps of Systems 1 and 2, Calculated with and without Structural Relaxations system

gap (eV)

(A) Au25(SMe)18- in gas phase (ref 18) (B) system 1, relaxed (C) Au25(SEtPh)18- extracted from the experimental crystal structure, unrelaxed in gas phase (D) [TOA+][Au25(SEtPh)18-] complex extracted from the experimental crystal structure, unrelaxed in gas phase (E) system (C), relaxed (F) system (D), relaxed

1.22 1.24 1.37 1.37 1.22 1.28

to the nitrogen center of TOA+, is reproduced accurately by all the functionals. Overall, the bond angles are very close to the experiment, and no functional can be judged to perform better than the others. The angles in the ligand shell (S-AuI-S and AuI-S-AuI) are within 2° from the experimental values, and the angles around the covalent bonds from sulfur to the EtPh group are reproduced even with higher accuracy. The significantly shorter Au-AuI distances from TPSS are reflected in the corresponding angles (Au-S-AuI and S-AuI-S). In addition to bond distances and angles, we compare our results with the experiment in Table 1 by presenting values for all atomic displacements (∆) with respect to the experimentally determined structure. Again, the results show the best agreement for the TPSS functional for all the elements. The deviations are the largest for carbons, which is not surprising taking into account that these atoms have also the largest experimental inaccuracy, and they involve the weak van der Waals interaction of the phenyl rings and TOA+. Especially, the counterion displays significant deviations, and exclusion of TOA+ from the analysis reduces the TPSS result for ∆(C) down to 0.129 Å. 3.2. Electronic Structure of the Cluster Anion-Counterion Complex in the Crystal and in the Gas Phase. The electronic density of states (eDOS) of system 1 is presented in Figure 2 together with local atomic projections on different elements. The values of the crystal band gap of 1 and the corresponding HOMO-LUMO gaps of the cluster anion and the cluster anion/ counterion complex in the gas phase are reported in Table 2. The crystalline structure has a band gap of 1.24 eV which is essentially the same as our previously reported value for the gas phase anion Au25(SMe)18- (1.22 eV, ref 18). Atom-centered

Figure 3. The density of states of the vibration modes (VDOS) in the [TOA+][Au25(SEtPh)18-] crystal, decomposed to mass-weighted components on the elements.

projections onto atomic orbitals show that the Au contribution in 1 is predominant around the Fermi energy with some weight on sulfur. Previously, a projection of the Kohn-Sham orbitals onto angular momentum orbitals inside the Au13 core, centered at the cluster’s center-of-mass, revealed that the Au(6s) derived valence states in Au25(SMe)18- form an eight-electron system with a major shell closing between the 1P and 1D orbitals.18,20 The peaks around the band gap in Figure 2 correspond to 1P and 1D shells of the cluster. Furthermore, the atom-projected eDOS shows that the sd-hybridized Au valence band is mixed with the other elements. The region above -2 eV (but below HOMO) corresponds to localized orbitals that involve Au-S binding. Analysis of local charges on atoms, using the Bader method, reveals a total charge of -0.85 e on the cluster and the corresponding positive charge on the counterion. This finding confirms the expected anionic nature of the Au25 nanoparticle in the crystal. As noted in section 3.1, the PBE functional overestimates Au-Au distances on average by 2.2%; i.e., the volume of the metal core of the nanoparticle is overestimated and the valence electron density in the core is underestimated. A careful theoretical analysis on the model cluster Au25(SH)18- has already shown that this has a significant effect on the HOMO-LUMO gaps, optical absorption edge, and the structure of the optical absorption spectrum.19 To assess those effects in this work, we performed PBE calculations also for the Au25(SEtPh)18- anion and the anion-TOA+ complex in the nonrelaxed configuration extracted from the experimental crystal data and compared the calculated HOMO-LUMO gaps and optical absorption spectra to corresponding relaxed systems in gas phase (Table 2 and Supporting Information, Figure S-1). Table 2 shows that the HOMO-LUMO gap is significantly larger in the nonrelaxed crystal configuration for the cluster anion and the anion-counterion complex (1.37 eV for both) as compared to the relaxed systems (1.22 eV for the cluster anion and 1.28 eV for the anioncounterion complex). The larger HOMO-LUMO gap for the nonrelaxed structures shows up also as a higher optical absorption edge (Figure S-1 in Supporting Information). In the context of the superatom electron shell model, the larger energy gap can be understood by the destabilization effect of the D-symmetric LUMO states (and hence increased HOMO-LUMO transition energies) in the more compact metal core of the experimental nanoparticle structure. The first broad peak of the experimental optical absorption spectrum of Au25(SR)18- shows a typical low-energy shoulder,

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Figure 5. Structure of the relaxed dimer 3. Atomic colors as in Figure 4, and the methyl groups are shown by sticks. Figure 4. Typical breathing modes of the Au13 core (left) and the ligand shell (right). Note that several orthogonal modes of each type appear in these (narrow) frequency regions. Au is yellow, S is bright yellow, and AuI in the ligand shell is red.

regardless of the R-group.11–15 This peak shape is now considered as a fingerprint of this particle, and it can be (and has been) used to track back experimental data for reanalysis from nanoparticle samples that obviously have been those containing Au25 in a very pure form, even long before the composition and structure assignments were perfected (see, e.g., refs 11, 14, and 15). However, our previous calculation of the optical absorption spectrum for the predicted structure of the Au25(SMe)18-, that turned out to be almost perfectly in line with the experimentally detected structure, did not yield a clear asymmetric peak shape for the lowest transitions. As we show in Figure S-1 (Supporting Information), the asymmetric shape of the first peak is clearly visible in the calculated optical absorption spectrum of Au25(SEtPh)18-. Previous theoretical investigations by other groups19 have not addressed this feature in detail. We currently understand the asymmetric peak shape, as a consequenceofsymmetrybreakingofthecore-core(metal-metal) optical transitions by the ligands, as follows, by making an analogy to optical transitions in a many-electron atom. The electronic ground state of the Au25(SR)18- has a closed shell, i.e., it is of S symmetry (L ) 0) of a multielectron configuration in analogy with a spherical closed-shell atom. The superatomic frontier orbitals correspond to p and d orbitals of an ordinary atom. The lowest optical transitions across the HOMO-LUMO gap of the cluster are those that correspond to p-d single particle-single hole transitions creating 15 transitions that leave the excited system in P, D, or F symmetries of total orbital angular momentum (L can be 1, 2, or 3 as a result from coupling single particle states l ) 1 and l ) 2). For a perfectly spherical system, only S to P transitions (3 transitions out of 15) are allowed by the dipole selection rule ∆L ) (1. The symmetry of the Au-thiolate shell around the Au13 core in Au25(SR)18restricts the HOMO-LUMO transitions to six, since the lowest five empty single particle orbitals of the superatom (d-like single particle orbitals) are split to two manifolds LUMO and LUMO+1 (degeneracy 2 and 3 for lower and higher energy, respectively)bytheoctahedralarrangementoftheS-Au-S-Au-S units around the core. Consequently, out of the six single particle transitions that can contribute to the lowest absorption peak, three are always strong compared to the other three (since they are allowed by the dipole selection rule). For the other three to become visible, one needs to break the symmetry of the core states by the presence of larger R groups. The low-energy shoulder of the first absorption peak of Au25(SEtPh)18- (Figure S-1in Supporting Information) contains those three weak

transitions. The oscillator strengths of the corresponding transitions for Au25(SMe)18- are so low that the transitions do not show up in the scale of Figure S-1. 3.3. Vibrational Spectrum of the Cluster Anion-Counterion Crystal. The vibrational density of states (vDOS) is shown in Figure 3 together with mass-weighted normal mode projections onto different elements. Au atoms have been divided into two categories, Au and AuI, in order to indicate the differences in vibrational modes in the Au core and within the ligand-shell. The vDOS up to 1700 cm-1 is presented in Figure 3A where the carbon atoms in EtPh are predominant above 400 cm-1. The stretching and bending modes of C-C bonds are abundant, and the octyl groups of the TOA+ counterion contribute also. The Raman-active breathing mode of the phenyl ring in EtPh is located at 995 cm-1. Parts b and c of Figure 3 show narrower frequency ranges, where the former concentrates on the vibrations of Au and S at low frequencies, and the latter highlights the division of C-H bond stretching modes (aromatic vs aliphatic H) at high frequencies. The most interesting details can be found in the Au and S vibrations (Figure 3b). The projected vDOS shows highest density at 325 and 360 cm-1 for Au and AuI, respectively, corresponding to stretching modes with sulfur. The division into two classes reflects the observed difference in Au-S and AuI-S bond lengths (Table 1): the lower modes correspond to stretching of the bonds between sulfurs and core Au atoms while the higher modes correspond to AuI-S bonds in the ligand shell. The experimental Raman measurement of Au25(SEtPh)18exhibits peaks at about 110 and 290 cm-1 (ref 28). We observe corresponding radial breathing modes of the Au13 core around 110 cm-1 and the ligand shell around 300 cm-1. Some of these modes are visualized in Figure 4. Especially, the ligand shell breathing is an interesting feature as the measurements indicate that it is lowered by 24 cm-1 in a neutral Au25(SR)18 nanoparticle.28 This suggests that the oxidation process and the resulting change in the electronic structure of the nanoparticle softens these modes in the ligand shell. Other interesting details include S-Au-S bending modes at 205 and 230 cm-1 where the former correspondstobendingoftheterminalsulfursintheS-Au-S-Au-S plane and the latter involves simple transverse bending of the central S. The Au-Au stretching modes in the core are located at 150-180 cm-1. Here, we also wish to comment on the recent interesting results from femtosecond spectroscopy of Au25(SR)18 where excitations that span the few lowest electronic states were found to decay, after a rapid interconversion process, in a slow time scale (1-10 ps) coupling strongly to a collective vibration mode with a wavenumber of ca. 75-80 cm-1 (refs 21 and 29). This mechanism has been connected to interfacial modes between

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Figure 6. Electron density of states of dimer 3 (top) and 4 (bottom) and its projections onto spherical harmonics (L ) 0, 1, 2). The analysis is made with respect to the central atoms of the constituent clusters but with a radius that encloses the 13-atom metal core in each case. The vertical line (as also in Figures 7 and 8) denotes the HOMO energy of the dimer. Note that the contribution for L > 2 is large for states that lie deeper in energy from the HOMO-LUMO region. These states comprise the Au(5d) band. This applies also to Figures 7 and 8.

the Au13 core and the ligand-shell.21 Existence of the rather large HOMO-LUMO gap in Au25 enables the long lifetime of the electronic excitation, which can generate or reinforce the interfacial modes.29 In our analysis, we indeed find a maximum for vDOS around 80 cm-1 that involve all gold atoms in the particle, coupling core Au-Au and Au-AuI vibrations (Figure 3). Interestingly, these modes do not involve S and they are below the breathing modes of the Au13 core (that are around 110 cm-1, see above). Visual analysis of the modes around 80 cm-1 suggests that they are combinations of volume-conserving shear modes of the Au13 core and the 12 AuI atoms in the ligand shell. It is plausible that electronic coupling through Au(core)-AuI interactions is strong enough so that the excitation energy can be effectively transferred into the ligand shell through these modes. These modes should then be important also for near-infrared (NIR) luminescence.21,29 4. Dimeric Systems 3-6 We prepared the system 3 as a gas phase species by removing a methyl group from Au25(SMe)18-, reflecting the cluster through a mirror plane, and joining the two cluster anions by a C6H4 group. The relaxed structure is shown in Figure 5. Systems 4-6 were obtained by replacing (doping) the central Au atom(s) in 3 with either Pd or Mn and reoptimizing the structure in a given charge state. We find that the BDT moiety connecting the two Au25 nanoparticles does not disturb the electron shell structure of the individual parts. The HOMO-LUMO gap of 3, 1.25 eV, remains the same as for the isolated Au25(SMe)18-. Figure 6a

shows the electron density of states of 3 and the analysis of major angular momentum character of the frontier orbitals.18,20 The HOMO consists of a set of 12 single-particle states (accounting for spin) in a narrow energy range (0.1 eV). LUMO and LUMO+1 manifolds consist of 8 and 12 single-particle states, respectively, accommodating 20 electrons in total. This electronic structure can be understood via the angular momentum analysis of the states with respect to the Au13 core centers of the dimer. The HOMO manifold, 12 states, appears as a set of P-symmetric states in both cores, and the near-degeneracy of these states (0.1 eV) signals a negligible interaction between the superatoms. Similarly, LUMO and LUMO+1 manifolds can be understood by considering the known electronic structure of a single Au25(SMe)18-: the 10 D-symmetric states of Au25(SMe)18- are split into sets of 4 (LUMO) and 6 (LUMO+1) due to the octahedral ligand field.18 Consequently, those states constitute the 8- and 12-fold sets in the dimer. Recent experimental27 and theoretical studies22–26 have considered doped MAu24(SR)18q particles with nonmagnetic or magnetic transition metal atoms M. While the present conclusion based on a comparison of results from electrochemistry, optical spectroscopy,27 and DFT calculations23 of PdAu24(SR)18q seem to indicate that a mixture of particles with various doping sites (i.e., replacing Au in the ligand, on the surface of the Au13 core, and at the center of the core) is formed under experimental conditions, the center-doped systems are the most attractive for systematic theoretical investigations. Therefore, we have here considered dimeric systems 4-6 with nonmagnetic (Pd) and magnetic (Mn) dopants.

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Figure 7. Same as in Figure 6, but for the dimer 5. The bottom panel shows the spin states separately for the MnAu24 part. The peaks with the filled colors show the contributions from the atomic orbitals of the Mn atom. R and β label majority and minority spins, respectively.

Figure 8. Same as in Figure 6, but for the dimer 6. The electronic structure of the two MnAu24 units is identical, so only one is shown. The peaks with the filled colors show the contributions from the atomic orbitals from the Mn atom. R and β label majority and minority spins, respectively.

Figure 6b shows the electronic density of states of system 4, a singly negatively charged [PdAu24(SMe)17-BDT(SMe)17Au25]- dimer. We have previously shown that the PdAu24(SR)18q cluster has a closed eight-electron superatomic shell structure when q ) -2, i.e., the Pd atom retains the atomic 4d10s0 configuration and acts as a zerovalent dopant. System 4

could then be understood as a dimer of “six-electron” and “eightelectron” superatoms. This also suggests that two holes in the superatomic P-shell should be localized in the PdAu24 part. Figure 6 (bottom) indeed shows that while the superatomic P states of the Au25- are not affected (they constitute the HOMO manifold of the dimer), the P states of the PdAu24 part are split

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Figure 9. Spin density of the ferromagnetic dimer 6 with a magnetic moment of 9.9 µB. The spins are localized at the two Mn atoms, and the spherical spin density is a signature of the half-filled Mn 3d shell with the maximal magnetization, effected by Hund’s rule.

in three, where the two filled states hold four electrons and the two P-holes are above HOMO. The two unoccupied P states of PdAu24 are separated by a rather small energy gap (0.2 eV) from the HOMO manifold. Figure S-2 (Supporting Information) shows their localization in the PdAu24 part. Consequently, the dimer should be rather electroaffine to fill those states and gain the configuration corresponding to a dimer of eight-electron superatoms. Finally, we considered doping the superatom dimer with one or two Mn atoms (systems 5 and 6). Previous theoretical work25,26 has shown that Hund’s rule maximizing the spin multiplicity of nd-metal atoms in vacuum is not seriously violated if those metals are used as dopants at the center of the thiolate-protected Au25 particle. Here we chose to study the interaction of the magnetic and nonmagnetic parts (system 5) and a ferromagnetic configuration of the system 6. Figure 7 shows the electronic structure of the singly doped dimer and verifies that the frontier orbitals (P-type HOMO and D-type manifold of empty states) of the nonmagnetic Au25- part are not disturbed by MnAu24. The energy separation of these states remains close to that of the HOMO-LUMO gap of Au25(SMe)18-. In the MnAu24 part, new (empty) states appear just above the HOMO manifold of the dimer, separated by less than 0.1 eV from the HOMO. Analysis of different spins, and the local analysis around the Mn atom confirms that these states are the empty β-spin 3d states of Mn, while the occupied R-spin 3d states are found in a range of 2.0-2.5 eV below HOMO, strongly hybridized with the Au 5d band. The calculated magnetic moment for dimer 5 is 4.6 µB, and the moment is concentrated on the Mn atom. Qualitatively, the electronic structure of the MnAu24 part of the dimer remains very much the same as was recently shown for a single MnAu24(SH)18 cluster.25 However, the existence of just a small energy gap separating the empty Mn 3d from the HOMO manifold implies that the singly doped dimer is rather electro-affine (the calculated adiabatic electron affinity is 1.8 eV) and that the added electron will occupy the Mn 3d state, which leads to a decrease of the magnetic moment to 4.0 µB. The electronic structure of the doubly doped dimer 6 is analyzed in Figure 8 in a ferromagnetic configuration of the spins. Here, the HOMO-LUMO gap has increased to ∼0.5 eV; i.e., the dimer is electronically more stabilized with respect to the singly doped dimer 5. The calculated magnetic moment of 6 is 9.9 µB which is basically the same as the ideal value 10 µB. The moment is localized in both Mn atoms, as shown in Figure 9. The empty β-spin Mn 3d states are in the energy gap of the superatomic P and D shells, which means that charging of the dimer with extra electrons would tend to decrease the magnetic moment.

In this work, structural, electronic, optical, and vibrational properties of the crystal made out of Au25(SEtPh)18- clusters were studied. The structural details of the protecting gold-thiolate layer due to close packing with TOA+ counterions in the crystal were discussed. Important vibrational modes were identified and discussed in connection with recent femtochemistry experiments (refs 21, 28, and 29). It was also shown that dimeric systems of all-gold Au25 and doped Au25 clusters retain their individual electron shell structures. The special “core-shell” or “divideand-protect”40 structural motif connected with the superatomic electronic structure seems to provide a robust framework for (quite independent) chemical modifications of the core or ligand shell. Ligand-exchange reactions have already been extensively explored on this nanoparticle experimentally.41 Through this kind of chemistry, it might be possible to build extended systems of chains or ordered networks42 that could have interesting and useful electronic, optical, and magnetic properties as novel type nanomaterials. Work is in progress now to characterize one-, two-, and three-dimensional systems made out of Au25 “building blocks” with tunable magnetic and optical properties. Acknowledgment. We thank R. L. Whetten, C. M. Aikens, and M. Nissinen for useful discussions and R.W. Murray for providing the coordinates of the crystal structure of the [Au25(SEtPh)18-][TOA+] system (ref 16). This research is funded by the Academy of Finland. The computational resources were provided by the CSCsthe Finnish IT Center for Science in Espoo, C3SE in Go¨teborg, and Forschungszentrum Ju¨lich (EU DEISA program). Supporting Information Available: Calculated optical spectra of 1 and 2 (Figure S-1) and further analysis of electronic structure of 4 (Figure S-2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Knight, W. D.; Clemenger, K.; deHeer, W. A.; Saunders, W. A.; Chou, M. Y.; Cohen, M. L. Phys. ReV. Lett. 1984, 52, 2141. (2) deHeer, W. A. ReV. Mod. Phys. 1993, 65, 611. Brack, M. ReV. Mod. Phys. 1993, 65, 677. (3) Leuchtner, R. E.; Harms, A. C.; Castleman, A. W., Jr. J. Chem. Phys. 1989, 91, 2753. (4) Khanna, S. N.; Jena, P. Phys. ReV. Lett. 1992, 69, 1664–1667. Khanna, S. N.; Jena, P. Phys. ReV. B 1995, 51, 13705. (5) Ball, P. New Scientist 2005, 30–33. (6) Reveles, J. U.; Khanna, S. N.; Roach, P. J.; Castleman, A. W. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18405–18410. Bergeron, D. E.; Roach, P. J.; Castleman, A. W., Jr.; Jones, N. O.; Khanna, S. N. Science 2005, 307, 231–235. Castleman, A. W.; Khanna, S. N. J. Phys. Chem. C 2009, 113, 2664. (7) Ha¨kkinen, H.; Manninen, M. Phys. ReV. Lett. 1996, 76, 1599. Ha¨kkinen, H.; Manninen, M. J. Chem. Phys. 1996, 105, 10565. (8) Mal, P.; Breiner, B.; Rissanen, K.; Nitschke, J. R. Science 2009, 324, 1697. (9) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–2. (10) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428. Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27–36. Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293–346. Brust, M.; Kiely, C. J. Colloids Surf., A 2002, 202, 175–186. Sardart, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Langmuir 2009, 25, 13840. (11) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630. Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (12) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (13) (a) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322. (b) Tracy, J. B.; Crowe,

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