Self-Assembled Supramolecular Complexes with âRods-in-Belt

Article pubs.acs.org/JPCC

Self-Assembled Supramolecular Complexes with “Rods-in-Belt” Architecture in the Light of Soft X‑rays Anna A. Makarova,†,‡ Elena V. Grachova,§ Dmitry V. Krupenya,§ Oleg Vilkov,†,‡ Alexander Fedorov,†,∥ Dmitry Usachov,† Alexander Generalov,†,‡ Igor O. Koshevoy,§,⊥ Sergey P. Tunik,§ Eckart Rühl,# Clemens Laubschat,‡ and Denis V. Vyalikh*,†,‡ †

Department of Physics, St. Petersburg State University, St. Petersburg 198504, Russian Federation Institut für Festkörperphysik, Technische Universität Dresden, D-01062 Dresden, Germany § Department of Chemistry, St. Petersburg State University, St. Petersburg 198504, Russian Federation ∥ Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, 01069 Dresden, Germany ⊥ Department of Chemistry, University of Eastern Finland, Joensuu 80101, Finland # Physikalische Chemie, Institut für Chemie und Biochemie, Freie Universität Berlin, 14195 Berlin, Germany ‡

ABSTRACT: One of the most important properties of the recently discovered “rods-in-belt” supramolecular complexes containing Au−Cu or Au−Ag cluster cores is the possibility of tuning their electronic and photophysical properties through modification of the ligand environment. This opens great perspectives for their applications in lightemitting devices and bioimaging. The high structural ordering and selfassembly properties of these unique objects may be used to design artificial nanostructures with complex topologies that could become ideal building blocks for next-generation electronics. Here we present a detailed experimental study of the electronic structure of “rods-in-belt” supramolecular complexes. Applying X-ray photoemission and absorption spectroscopy, we systematically unraveled the structure of their occupied and unoccupied electronic states near the Fermi level. The major contribution to the highest occupied molecular orbitals is made by the triple-bonded carbons hosted in the dialkynylgold “rods” and the copper (silver) atoms from the central cluster core of the heterometallic Au−Cu (Au−Ag) molecules. The lowest unoccupied molecular orbitals are located on the carbon skeleton of the complexes, including −CC− and −CC− aromatic orbitals. The onset of the valence band in the Au−Ag systems is ∼0.3 eV lower than that in the Au−Cu complexes, implying a slightly larger energy gap for the silver-based molecules. With increasing size, the complexes become more and more sensitive to X-ray damage.

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are held together by metallophilic interactions of the AuI centers with the heteroions (CuI and AgI) supplemented by π bonding of the ions with the alkynyl triple bonds. Additionally, the structure is stabilized by the {Au3(P−P)3} “belt”, where “P−P” denotes a diphosphine ligand with a variable-length spacer between the phosphorus atoms. The “belt” wraps about the central cluster core and holds it together by metallophilic and electrostatic interactions (see Figure 1). The network of metallophilic and π-bonding interactions proved to be extremely effective, making the self-assembly process of these highly structurally ordered molecules thermodynamically favorable in spite of the huge decrease in entropy during their formation. It is worth noting that the selfassembly process can be altered by changing the spacer length in the diphosphine ligand as well as the alkynyl/metal ratio.

INTRODUCTION Nowadays, the chemistry of d10 complexes from the copper subgroup (copper, gold, silver) is experiencing somewhat of a renaissance because of the spectacular advances in the synthesis of self-assembled polynuclear supramolecular complexes with unique architectures, topologies, and photophysical properties.1−5 The chemistry, structure, and important physical characteristics of these materials are dictated by metallophilic interactions in the multimetallic cluster core as well as by the coordination properties of the polydentate ligands that help to assemble the polynuclear complexes. Targeted variations of the polydentate ligand characteristics open perspectives for the design, control, and reliable tuning of the chemical composition, structure, and electronic properties of such supramolecular compounds. We recently discovered a family of d10-heterometallic AuI− CuI and AuI−AgI alkynyl−phosphine complexes displaying a very unusual “rods-in-belt” topology.6−11 Their architecture is based on dialkynylgold “rods” ([RCC−AuI−CCR]−) that © XXXX American Chemical Society

Received: May 6, 2013 Revised: May 23, 2013

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dx.doi.org/10.1021/jp404459k | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Table 1. List of the Complexes Studied

a

complexa

composition

metal core stoichiometry

Au−Cu−S Au−Cu−M Au−Cu−L Au−Ag−S Au−Ag−M Au−Ag−L

[Au6Cu2(C2Ph)6(PPh2C6H4PPh2)3][PF6]2 [Au9Cu6(C2Ph)12(PPh2(C6H4)2PPh2)3][PF6]3 [Au13Cu12(C2Ph)20(PPh2(C6H4)3PPh2)3][PF6]5 [Au6Ag3(C2Ph)7(PPh2C6H4PPh2)3][PF6]2 [Au9Ag6(C2Ph)12(PPh2(C6H4)2PPh2)3][PF6]3 [Au13Ag12(C2Ph)20(PPh2(C6H4)3PPh2)3][PF6]5

(Au(CCPh)2)3Cu2 (Au(CCPh)2)6Cu6 (Au(CCPh)2)10Cu12 (Au(CCPh)2)3Ag2 (Au(CCPh)2)6Ag6 (Au(CCPh)2)10Ag12

In the names of the complexes, “S”, “M”, and “L” denote the sizes of the metal cores (small, medium, and large, respectively).

Figure 1. Side and top views on the “rods-in-belt” Au−Cu−S system. Color legend: gold, yellow; copper, blue; phosphorus, red; fluorine, green; carbon, gray; hydrogen, light gray.

nature and energy of the occupied and unoccupied molecular states close to the Fermi level (EF) to be determined experimentally. We anticipate that the link to the electronic structure will help to provide a better understanding of the remarkable properties of the supramolecular complexes and to envisage pathways for the design of new families of such compounds with desired properties.

Thus, different metal core structures and huge and exotic supramolecular aggregates can be obtained.6−11 In turn, the structural diversity and easy modification of the electronic and steric characteristics of the alkynyl ligands allow systematic variation and tuning of the properties of the complexes to provide extremely attractive photophysical characteristics. A wide range of emission wavelengths, unusually effective phosphorescence (with quantum yields of up to 100% without substantial emission quenching by dioxygen), and very high two-photon absorption (TPA) cross sections have been observed.6,7 Naturally, these findings give strong motivation to gain deeper insight into the electronic structure of the complexes and, in particular, the structure of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), which are responsible for the photophysical characteristics of the chromophoric centers in the complexes. To describe these systems theoretically, one may apply the well-known schemes of density functional theory (DFT) calculations, although these can yield only a qualitative picture because of the large sizes and complicated structures of the molecules. The best of our knowledge, there have been no attempts to study experimentally the electronic properties of the AuI−CuI and AuI−AgI “rods-in-belt” complexes or similar polynuclear d10 complexes comprehensively. Herein we report the first high-resolution X-ray photoemission spectroscopy (XPS) and X-ray absorption investigation of the AuI−CuI and AuI−AgI supramolecular aggregates mentioned above, which allowed the

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SAMPLE PREPARATION AND EXPERIMENTAL DETAILS All of the heterometallic AuI−CuI and AuI−AgI supramolecular complexes were prepared according to well-documented procedures.6−9 High-performance liquid chromatography (HPLC)-grade solvents (Aldrich) were used as received. Immediately before the naturally oxidized Si wafers were coated, they were treated with acetone, ethanol, and deionized water in an ultrasonic bath and then dried at 100 °C for 2 h. Thin films of polycrystalline sheets were prepared on Si wafers in air by spin-coating of acetone and chloroform solutions (∼10−3 M concentration) followed by evacuation in the preparation chamber at 10−7 mbar for 2 h. The films of AuI− CuI and AuI−AgI supramolecular complexes prepared in this way were then used as samples for spectroscopic measurements. Spectroscopic measurements were performed at the Berliner Elektronenspeicherring für Synchrotron Strahlung (BESSY II) using radiation from the Russian−German beamline.12,13 This dipole-based beamline provides a moderate photon flux B



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Figure 2. Core-level P 2p spectra of (a) Au−Cu and (b) Au−Ag complexes. The lower-BE component reflects the electron emission from the P atoms in the “belt” diphosphine molecules, while higher-BE component demonstrates the contribution from the [PF6]− counterions.

Insight into the Stoichiometry via P 2p Core-Level Spectra. To explore the stoichiometry of the materials obtained from the freshly prepared AuI−CuI and AuI−AgI “rods-in-belt” complexes and trace their possible modifications due to X-ray damage effects, the samples were examined by XPS (see Sample Preparation and Experimental Details). The number of phosphorus atoms located in the “belt” of the molecule and the [PF6]− counterions is well-defined by the stoichoiometry of the system. Therefore, we decided to analyze the P 2p core-level spectra first. The spectrum of each sample was recorded at the beginning of the experiments, and the set of spectra are shown in Figure 2. They exhibit two doublets, which are shifted relative to each other by 4.2 eV. The doublet structure reflects spin−orbit interactions in the 2p shell of the phosphorus atoms, which result in different final states for the system with J = 3/2 and J = 1/2. The value of the spin−orbit splitting derived from deconvolution of the PE spectra (see Figure 2) is ∼0.9 eV. It is well-known that the binding energies (BEs) of core levels are extremely sensitive to the chemical nature of the molecule and the bonding characteristics of the corresponding atoms. This is due to the role that the valence electrons play in shielding the core electrons. In the [PF6]− counterion, the valence electrons of the phosphorus atoms are substantially shifted toward the electronegative F atoms, leaving behind a poorly screened core at the P site. This results in a decrease in the kinetic energy of the 2p photoelectrons ejected from the phosphorus atoms of the counterion compared with the kinetic energy of those emitted from the phosphorus atoms seated in the “belt” of molecule. It should be noted that the detected BE of the 2p3/2 component at 136.1 eV is in good agreement with that found for the [PF6]− anions in other supramolecular heterometallic aggregates.17 In complexes containing σ-coordinated phosphorus atoms with a considerable metal-to-ligand π back-donation, the BE of the P 2p core electrons remains close to that in the free ligand (∼131.1 eV).18 In the samples studied here, however, the detected BE of the P 2p core electrons appeared to be

distributed continuously over a wide photon energy range from 30 to 1500 eV and is therefore particularly suitable for radiation-sensitive and fragile materials.14−16 Photoemission (PE) spectra were acquired with a hemispherical Phoibos 150 electron energy analyzer (SPECS GmbH) for high-energyresolution PE experiments. Soft X-ray absorption spectra were recorded in total-electron-yield mode. All of the measurements were carried out at room temperature.

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EXPERIMENTAL RESULTS AND DISCUSSION Chemical Composition and Structure of the “Rods-inBelt” Aggregates. To investigate the properties of the “rodsin-belt” complexes systematically, we used two series of alkynyl−phosphine supramolecular aggregates6−9 with very similar architectures in which the molecular size and in particular the size of metal core were steadily increased in the corresponding AuI−CuI and AuI−AgI molecular aggregates. Specifically, the [AuxMy(C2Ph)2x]y−x alkynyl cluster core (M = Cu, Ag) was varied in size from 5 metal atoms (x = 3, y = 2) to 22 metal atoms (x = 10, y = 12) (Table 1) and “wrapped” with the appropriately sized [Au3(PPh2(C6H4)nPPh2)3]3+ belt (n = 1−3).6−8 The central core fragment consists of slightly twisted [PhC2AuC2Ph]− “rods” held together by the M−Au and C C−M π bonding, as exemplified by the structure of the smallest complex, [Au6Cu2(C2Ph)6(PPh2C6H4PPh2)3][PF6]2 (Figure 1). Additional stabilization of the metal core is provided by the wrapping belt, which is bound to the core by electrostatic and aurophilic interactions. It is worth noting that the cluster core composition (amounts and ratio of the metal ions) in these complexes is solely defined by the length of bidentate phosphine belt, except for the smallest Au−Ag cluster. This complex still keeps the “rods-in-belt” structural motif, but the central [Au3Ag2(C2Ph)6] fragment is wrapped with an openloop tetrametallic diphosphinealkynyl [Au(C2Ph)Au2Ag(PPh2C6H4PPh2)3]3+ “belt” anchored to the central part by two Au−Au bonds and one Au−Ag bond.9 C



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immediately: (i) How does the degradation of the system occur? (ii) What is the nature of the chemical transformations in which the fluorine and phosphorus atoms take part and why do the total numbers of P and F atoms decrease in the course of the experiments? (iii) Why is the degradation of the counterions not accompanied by a similarly strong effect for the metal-containing cationic part of the Au−Ag−L supramolecular aggregates? The following scenario for the irradiation-induced changes in this Au−Ag−L system could possibly answer these questions: The [PF6]− counterion can be decomposed according to the process [PF6]− → PF5 + F−, creating a volatile PF5 molecule. The PF5 can leave the system, whereas the remaining F− ions must be kept in the sample to compensate for the positive charge of the metal-containing aggregate. Interestingly, similar behavior was observed in solution during the synthesis of giant Au−Ag “rods-in-belt” clusters.10 To balance the unusually high positive charges of the central {Au(C2Ph)2}9Ag12 and {Au(C2Ph)2}18Ag30 cluster cores, the systems “grab” chloride ions from the CH2Cl2 solvent. This leads to stable polynuclear aggregates with the chloride ions seated inside the cluster core, thus substantially diminishing electrostatic repulsion within it. An analogous process does not occur in the smaller congeners based on shorter diphosphine ligands because the heterometallic core is more densely packed and there is less room available for the insertion of halide ions. The proposed hypothesis involving [PF6]− degradation to afford free fluoride ions in the systems under study is also supported by the well-known catalytic activity of nanosized gold and silver systems.19,20 Metal Core Properties. The natures and properties of the metal-to-metal and metal-to-ligand bonding in heterometallic clusters of this type have remained a puzzling and widely discussed issue. Further insight into this problem can be based on core-level Au 4f spectra. Analysis of relative XPS data that are available for a variety of homonuclear complexes and clusters containing covalently bound gold21−24 clearly shows that an increase in the size of the metal core results in a systematic shift of the Au 4f photoemission lines toward lower BEs. The noticeable sharpness of the reported peaks and a BE

substantially larger (131.9 eV). This implies rather weak backdonation from the Au atoms to the phosphine ligands in the belt. To evaluate the stoichiometry of the studied complexes, we determined the Pbelt/Pcounterion PE intensity ratios and compared them to the values expected from the chemical composition of the molecules. The results are summarized in Table 2. For the Table 2. Comparison of XPS- and Stoichiometry-Derived Pbelt/Pcounterion Ratios Pbelt /Pcounerion

a

complex

spectroscopic result

expected from stoichiometry

Au−Cu−S Au−Cu−M Au−Cu−L Au−Ag−S Au−Ag−M Au−Ag−L

2.74 1.82 1.43 2.95 2.04 1.74a

3.0 2.0 1.2 3.0 2.0 1.2

First scan measurement.

small- and medium-sized complexes, good agreement between the XPS-derived data and the results obtained on the basis of Xray crystallographic measurements was observed. However, for the large complexes of both series, appreciable differences between the values derived from the XPS spectroscopic data and those expected from the stoichiometry of the complexes were found. This was probably caused by fast decomposition of the [PF6]− ions under X-ray irradiation (vide infra). X-ray Damage. To explore this observation further, we followed the evolution of the P 2p and F 1s XPS spectra of the Au−Ag−L complex as a function of X-ray beam exposure time. The results are depicted in Figure 3. There is a gradual drop in the intensity of all of the PE signals with increasing scan number (i.e., irradiation time). A closer look at the P 2p spectra clearly shows that the counterions are more affected by the Xray irradiation than the cluster molecules. This suggests that although the [PF6]− ions are lost more quickly, the studied supramolecular aggregates seem to be able to survive longer under exposure to X-rays. Several questions therefore arise

Figure 3. Core-level (a) P 2p and (b) F 1s spectra for the Au−Ag−L complex taken in a time-dependent fashion, emphasizing the alteration produced by soft X-rays. D



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very close to that of bulk gold points to increasing metallicity of the cluster cores in such coordination compounds.18,25,26 Interestingly, this was not observed for the studied Au−Cu and Au−Ag supramolecules, despite the large size of the cluster cores. In Figure 4 we compare the Au 4f XPS spectra for the Au− Cu family with that for a bulk gold reference. No notable

Table 3. Average Au−Au and Au−M Distances (in Å) for Gold Cluster Compounds

a

Figure 4. Comparison of the core-level Au 4f spectrum of a bulk gold sample with the spectra obtained for the Au−Cu family, where the size of the metal core varied from 8 atoms for the S system to 25 atoms for L system and the number of gold atoms changed from 6 to 13, respectively.

compounda

Au−Au

[Au6L6]2+ [Au13L10X2]3+ [Au9L8]3+ [Au11L7X3] [Au11L10] Au−Cu−S Au−Ag−S

3.0325 2.8714 2.7414 2.8773 2.8697 3.1064 3.5709

Au−M

ref

2.8530 2.9401

27 28 29 30 26 7 9

L = neutral ligand (e.g., phosphine); X = halide ion.

Figure 5. NEXAFS C K-edge spectrum of the Au−Cu−S system. The inset shows the core-level C 1s spectrum (hν = 400 eV).

∼285.4 eV, (C) ∼287.5 eV, (D) ∼288.8 eV, and (E) ∼293.5 eV. They can be assigned to the following transitions: (A) 1s → π*(CCaromatic), (B) 1s → π*(CC), (C) 1s → π*high energy + σ*C−H, (D) 1s → π*high energy, and (E) 1s → σ*aromatic.31,32 The lowest-lying states forming the LUMO seem to originate mainly from the π*(CC) and π*(CCaromatic) orbitals, in agreement with theoretical predictions.7,9 The Figure 5 inset shows the C 1s XPS spectrum, in which the component at BE = 285 eV is dominant. Assuming a similar screening of the remaining core holes in the X-ray absorption and photoemission processes in these systems, we may conclude that the EF is closely pinned to the bottom of the LUMO.14,33,34 Valence Band States. Valence band (VB) photoemission was used to characterize the occupied electronic states below EF. In contrast to the chemically selective NEXAFS spectra, VB photoemission spectra contain contributions from all structural fragments of the studied material. Therefore, their interpretation is more challenging. To identify the contributions from metal core atoms and the nature of the highest-lying states, the VB spectra were taken with a set of photon energies from 110 to 240 eV. This allowed the contributions from different states to be separated because of their different photon-energydependent cross sections (CSs). Figure 6a shows that the photoionization CSs for the Au 5d and Cu 3d states (normalized to the total CS) evolve with photon energy in the discussed region.35,36 Obviously, at ∼180 eV there is a clear minimum in the Au 5d CS curve. Hence, at this photon energy, the electron emission from the Au 5d states will be rather small relative to the contributions by other valence states, in particular the Cu 3d states.

differences in the line shapes and energy positions of the Au 4f peaks among the three Au−Cu systems can be seen, although the number of gold atoms changed from 6 to 13 and the total number of metal atoms increased from 8 to 25. These observations indicate that no metallic phase properties arise upon metal core growth. Hence, the character of the bonding in the heteronuclear metal core is different in an essential way from that in homonuclear clusters with covalent bonding between the metal atoms. This is an important indication that the formation of the cluster core in the Au−Cu and Au−Ag systems may be considered as a self-assembly process governed by metallophilic interactions between the d10 ions without an appreciable contribution of metallic bond character. This hypothesis is also supported by the significant differences between the metal−metal bond distances in the covalent metal cluster complexes and the studied supramolecular complexes. The bond lengths in the metal clusters are substantially shorter than those in the Au−Cu−S and Au−Ag−S supramolecular metallophilic complexes (Table 3). Electronic Structure Near the Fermi Energy. Unoccupied Electron States. To get experimental access to the unoccupied electron states of the supramolecular complexes, we used the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The structural features of the complexes under study imply that the most informative spectra can be obtained looking at excitations of the C 1s core level into unoccupied 2p-derived states. The C K-edge spectrum of the Au−Cu−S complex is shown in Figure 5. At least five spectral features can be distinguished: (A) 285.0 eV, (B) E



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composed of contributions from the CuI from the central cluster core and related triple-bonded carbons. A further proof of the Cu 3d contributions to the HOMO arises from comparison of the VB PE spectra with increasing cluster size (Figure 7a). The relative number of CuI ions grows

Figure 6. (a) Photoionization cross-section (CS) for the Au 5d and Cu 3d states, normalized to the total CS.36 (b) Valence-band PE spectra for the Au−Cu−S complex over the photon energy range from 110 to 240 eV.

Figure 7. (a) Evolution of the VB PE spectra with increasing size of the central cluster core in the Au−Cu complexes. (b) Comparison of the VB PE spectra of the smallest cluster core systems in the Au−Cu and Au−Ag families.

Figure 6b shows four different VB PE spectra of the Au− Cu−S system taken at photon energies from 110 to 240 eV. Three regions can be distinguished in the spectra: (i) For BE > 12 eV, the spectral intensities change monotonically with photon energy and can be attributed to electron emission from the extended C 2s states.37 (ii) For BEs between 6 and 12 eV, the PE intensity is notably suppressed at a photon energy of 180 eV, pointing to a considerable contribution from Au 5d states. (iii) The intensity changes in the asymmetric feature near the HOMO at BE ≈ 4 eV are in good agreement with the behavior of the Cu 3d CS curve (Figure 6a). This indicates that the highest occupied states contain a major contribution from Cu 3d-derived states. On the other hand, it is well-known that systems containing aromatic carbons show a significant C 2p contribution to the VB structure at BE ≈ 4 eV, too.38−40 However, the geometry of the systems under study does not allow hybridization of Cu 3d-derived states and the C 2p states in the aromatic structures because they are spatially wellseparated and overlap weakly or not at all. Essential contributions from Au 5d states to the HOMO are unlikely because, as we have seen, they possess higher BEs. It should be noted that each CuI is linked to the carbon skeleton and coordinated to CC triple bonds hosted in “rods”. Therefore, it is most plausible that the HOMO structure is mainly

drastically with increasing cluster core size (Cu/Au = 2/6, 6/9, 12/13). At the same time, the intensity of the PE signal at BE = 4 eV becomes considerably enhanced for larger clusters, in line with our assignment of it to the Cu 3d states. It is noteworthy that the onset of the VB is not affected by the cluster size. However, it does change with the composition of the cluster core. In Figure 7b, the VB PE spectra of the Au−Cu and Au− Ag complexes are compared. Clearly, the Ag 4d states are lower in energy than the Cu 3d states and overlap with the Au 5d states. This does not exclude an admixture of the Ag 4d states to the HOMO, but such a contribution could hardly be as strong as the contributions from Cu 3d states to the HOMO in the Au−Cu systems. Furthermore, the onset of the VB in the Au−Ag complexes is shifted to higher BE by ∼0.3 eV relative to the Cu congeners. If on the basis of our NEXAFS data it is assumed that in both families the LUMO is located just above EF, this implies that the HOMO−LUMO gap is slightly larger in the Au−Ag complexes than in the Au−Cu systems. This agrees with the optical absorption properties of these materials. For instance, Au−Ag−S is colored bright lemon yellow, whereas Au−Cu−S has a saturated orange color. The energy difference between these two colors is ∼0.16 eV. In a simplistic F



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picture of the optical absorption, the HOMO−LUMO gaps for the two complexes should be of similar size, which is in good agreement with recent experimental and theoretical findings.6−9

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CONCLUSION The electronic structure and chemical composition of selfassembled AuI−CuI and AuI−AgI supramolecular complexes as well as changes of the latter under soft X-ray irradiation have been successfully revealed using XPS and NEXAFS techniques. When the P 2p and F 1s XPS spectra were recorded in a timedependent manner (i.e., as a function of X-ray exposure), a high sensitivity of the larger complexes to X-ray damage was found. A chemical model to describe the X-ray damage process that agrees well with previously obtained results has been suggested. Analysis of the Au 4f spectra implied that in contrast to covalently bonded polynuclear clusters, the “rods-in-belt” complexes studied here can be considered as a package of mononuclear complexes aggregated by efficient metallophilic interactions without a substantial contribution from covalent bonding. By exploiting the photon-energy dependence of the photoionization cross sections of states from different elements, we were able to disentangle the valence band structure of the complexes. Despite the fact that the spectral shapes look rather similar, the electronic states near the HOMO revealed notable differences. In particular, the HOMOs were estimated to lie at BE ≈ 2.2 eV for the Au−Cu complexes, while those for Au−Ag systems lie slightly deeper at BE ≈ 2.5 eV. Finally, it has been demonstrated that the HOMO of Au−Cu complexes consists of Cu 3d states that are hybridized with the π(CC) states. For the Au−Ag aggregates, the HOMO consists of a combination of Ag 4d and π(CC) states in which the 4d admixture is weaker than the corresponding 3d contribution in the Au−Cu systems and the contribution from the carbon atoms dominates. The X-ray absorption data suggested that Cderived orbitals of both π*(CC) and π*(CCaromatic) character contribute to the LUMO.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Helmholtz Zentrum Berlin für Materialien und Energie within the bilateral Russian−German Laboratory Program. A.A.M. acknowledges support from the Erasmus Mundus Action 2 Programme of the European Union and DFG Grant LA655/13-2. The authors also greatly appreciate the financial support from St. Petersburg State University through research grants 12.37.132.2011 and 12.0.109.2010 and from the Russian Foundation for Basic Research through grants 11-03-00974 and 13-02-01070. E.R. acknowledges financial support by Graduiertenkolleg 1582, TP A1. The authors acknowledge fruitful discussions with Kurt Kummer.

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