Influence of Relativistic Effects on Assembled Structures of V-Shaped

Influence of Relativistic Effects on Assembled Structures of V‑Shaped Bispyridine Molecules on M(111) Surfaces Where M = Cu, Ag, Au Xue Zhang,†,∥ Na Li,†,∥ Hao Wang,† Chenyang Yuan,† Gaochen Gu,† Yajie Zhang,† Damian Nieckarz,‡ Paweł Szabelski,§ Shimin Hou,*,†,⊥ Boon K. Teo,† and Yongfeng Wang*,†,⊥ †

Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China Supramolecular Chemistry Laboratory, University of Warsaw, Biological and Chemical Research Centre, Zwirki i Wigury 101, 02-089 Warsaw, Poland § Department of Theoretical Chemistry, Maria-Curie Skłodowska University, Pl. M.C. Skłodowskiej 3, 20-031 Lublin, Poland ⊥ Peking University Information Technology Institute Tianjin Binhai, Tianjin 300450, China ‡

S Supporting Information *

ABSTRACT: The self-assembly behavior of a V-shaped bispyridine, 1,3-bi(4-pyridyl)benzene (BPyB), was studied by scanning tunneling microscopy on the (111) surfaces of Cu, Ag, and Au. BPyB molecules coordinately bonded with active Cu adatoms on Cu(111) in the form of complete polygonal rings at low coverages. On Ag(111), BPyB molecules aggregated into two-dimensional islands by relatively weak intermolecular hydrogen bonds. The coexistence of hydrogen bonds and coordination interaction was observed on the BPyB-covered Au(111) substrate. Density functional theory calculations of the metal− molecule binding energy and Monte Carlo simulations were performed to help understand the forming mechanism of molecular superstructures on the surfaces. In particular, the comprehensive orbital composition analysis interprets the observed metal−organic complexes and reveals the importance of relativistic effects for the extraordinary activity of gold adatoms. The relativistic effects cause the energy stability of the Au 6s atomic orbital and decrease the energy separation between the Au 6s and 5d orbitals. The enhanced sd hybridization strengthens the N−Au−N bond in BPyB−Au−BPyB complexes. KEYWORDS: relativistic effects, STM, surface chemistry, coordination chemistry, self-assembly coordinated structures and obtain fine control of the targeted MOSs, the interaction between molecules and single surface atoms needs to be thoroughly studied and fully understood. MOSs constructed on single-crystalline surfaces using native atoms are mainly stabilized by M−C, M−N, and M−O bonds. All Cu, Ag, and Au atoms can form MOSs with molecules through C−M−C linkages. This could be realized either through their reactions with terminal alkynes20−22 or during the process of Ullmann couplings.23−25 Metalizations of phthalocyanine (2HPc) through replacing H atoms and forming M−N coordination bonds were reported for both single Cu and Ag atoms on surfaces.26,27 In contrast, AuPc has not been prepared in this way. For cyano- and pyridyl-terminated molecules, they

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urface-assisted fabrication of nanostructures has continuously occupied the research hotspot in recent years for its great potential in functional molecular devices.1−4 The driving forces to assemble nanostructures on surfaces are of rich diversity. Relatively weak interactions such as van der Waals (vdW) forces and hydrogen bonds have been used to fabricate large-area and highly ordered structures.5−9 Robust metal−ligand coordination, covalent bonds, as well as strong ionic interaction are also powerful means in building stable and long-range-ordered nanoarchitectures.10−18 Among these supramolecular architectures, metal−organic structures (MOSs) have drawn much attention due to their potential applications in catalysis, chemical separation, and gas storage.19 Single-crystalline surfaces of copper, silver, and gold are usually used as supporting substrates to investigate surface reactions including the construction of MOSs.19 Similar to external deposited metals, surface native atoms can form MOSs with molecules via coordination interactions. To avoid mixed © 2017 American Chemical Society

Received: June 29, 2017 Accepted: July 20, 2017 Published: July 20, 2017 8511

DOI: 10.1021/acsnano.7b04559 ACS Nano 2017, 11, 8511−8518

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ACS Nano may form coordinated structures with Cu28−39 and Au35,40−42 but not Ag34,35,43,44 atoms on surfaces. Only Cu atoms formed surface-supported MOSs with deposited molecules through Cu−O bonds.45 In these reactions, Cu atoms are always reactive, and the activity of Ag and Au atoms depends on the types of reactions. Their different activities in forming surfacesupported MOSs were usually explained from the viewpoint of calculated bonding energy of the adsorbed molecule with the corresponding metal atom,34,42 without exploring the origin of the different bonding energies. It is known that the relativistic effects play an important role in chemistry for Au atoms and clusters.46−48 However, the mechanism for how the relativistic effects of single gold atoms influence the formation of MOSs on single-crystalline surfaces has never been demonstrated. Therefore, it is highly desirable to carry out comparative studies on the coordination of Cu, Ag, and Au atoms with the same molecule35 and elucidate the origin of their different coordinated energies. Here, the self-assembly of 1,3-bi(4-pyridyl)benzene (BPyB) on coinage metal M(111) surfaces, where M = Cu, Ag, and Au, and its binding to the respective adatoms was investigated by low-temperature scanning tunneling microscopy (LT-STM). The molecular particular 120° backbone favors the formation of rings and Sierpiński triangles.49−51 The particular combination of the pyridyl lone pair of electrons and the rigid phenyl rings offers an ideal opportunity to study the interplay between metal−ligand orbital bonding. BPyB molecules form densely packed patterns on the Ag(111) surface that are completely stabilized by hydrogen bonds, whereas BPyB molecules can coordinate with Cu adatoms on Cu(111) and Au adatoms on Au(111). The detailed orbital analysis based on density functional theory (DFT) calculations was carried out to understand bondings between the ligand and metal adatoms, highlighting the important role of the relativistic effects of gold.

Figure 1. (a) Top panel: schematic representations of the valence orbitals of isolated M atoms (M = Cu, Ag, Au) with (solid curves) and without (dashed curves) the relativistic effect. Bottom panel: LDOS contributed by the d orbitals of the metal adatom on the corresponding M(111) surface. (b) Relative energetics of the d and s atomic orbitals can significantly influence the morphology of assembled structures of BPyB on M(111) surfaces via formation of hydrogen bonds and/or metal−ligand coordination bonds.

hexagonal coordinated rings distributing over the entire surface, in which each Cu adatom bonded with two neighboring BPyB molecules forming a two-fold coordination motif (Figure 2a). This uniform distribution of small closed metal coordination rings is quite different from previously reported coordination systems formed by pyridyl groups and Cu adatoms, which exhibit connected open networks31−33 or one-dimensional chains.30 We presume that the V-shape of this molecule and the robust two-fold coordination with Cu adatoms contributed to the formation of closed rings. To verify that the BPyB molecules are connected by coordination with the Cu adatoms instead of relatively weak hydrogen bonds, we carried out repetitive lateral manipulation by moving the STM tip close to the sample surface (e.g., VB = 1 mV, It = 5 nA before the feedback was turned off), and the rings remained intact with only a slight lateral shift (see Figure S1 in the Supporting Information). The manipulations verified that BPyB molecules were coordination bonded and adsorbed on the substrate rather strongly. The center-to-center distance of adjacent BPyB molecules in the hexagonal ring was measured to be around 15.3 Å, close to the calculation value of 15.4 Å, further confirming the coordination nature of the bonding. Here, the “center” refers to the center of the central phenyl ring of BPyB. The high-resolution STM image and the corresponding molecular model are shown in Figure 2b,c. Moreover, variable-temperature experimental results revealed that the BPyB molecules bonded to Cu adatoms instead of surface atoms (Figure S4). Monte Carlo simulations reproduced the experimentally observed, most abundant, hexagonal structures (Figure S2). It means that the preferred formation of molecular rings instead of chains is kinetically favorable. Under the same deposition conditions, no coordinations between BPyB molecules and Ag adatoms were observed on Ag(111), similar to the previous results using cyano- and pyridyl-terminated molecules.34,35,43,44 Instead, BPyB molecules aggregated to form 2D islands, as shown in Figure 2d. An

RESULTS AND DISCUSSION Clean single-crystalline Cu, Ag, and Au surfaces were used in view of their abundant supply of metal adatoms needed for metal−ligand interactions. As seen in the schematics of Figure 1a, the atomic orbitals of an isolated Au atom are significantly influenced by relativistic effects, leading to the contraction and the resulting rearrangement of the energetics of the orbitals. The local density of states (LDOS) contributed by the 5d atomic orbitals of the Au adatom on the Au(111) surface is much nearer to the Fermi level than that contributed by the 4d atomic orbitals of the Ag adatom on the Ag(111) surface, which significantly modifies the chemical activity of gold adatoms. BPyB molecules on metal surfaces can form MOSs through coordination bonds or supramolecular networks via hydrogen bonds (Figure 1b), making this system an ideal candidate to study metal−ligand interaction. It is expected that the orbital anomaly of gold induced by the relativistic effect may give rise to significant differences in metal−ligand bonding in comparison to silver and copper. Experimentally, BPyB molecules were deposited on the Cu(111) surface with the substrate maintained at room temperature. Unlike that of thiol and hydroxyl groups,45,52 the coordination between N and Cu adatoms can occur without postannealing, due to the high reactivity of Cu adatoms toward the nitrogen ligands with lone pairs.36−38 The Cu adatoms were not deposited but instead released from steps, kinks, vacancies, and other surface defects, as reported by previous work.39,53,54 At low coverage, the BPyB molecules preferred to form 8512


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Figure 2. Comparison of representative assembled structures of BPyB molecules on M(111) where M = Cu (first row), Ag (second row), and Au (third row). (a) Typical hexagonal coordinated rings on Cu(111). (b) High-resolution image of a hexagonal coordinated ring. (c) Molecular model of (b). (d) 2D island aggregated by BPyB molecules on Ag(111); the inset shows the unit cell marked in cyan. (e) Enlarged image of the square in (d) showing the basic unit of H-bonded tetramers. (f) Molecular model of (e) depicting the C−H···N hydrogen bonds (dashed lines). (g) Overview of the mixed nanostructures of BPyB molecules assembled on Au(111). (h) Typical ring composed of coordination bonds and H-bonds between molecules on Au(111). (i) Molecular model of (h). All arrows in these figures refer to [110̅ ] of the substrate. Scanning parameters: (a) constant-current mode, VB = 30 mV, It = 60 pA; (b) constant-height mode, VB = 10 mV, It = 70 pA; (d) constant-current mode, VB = 2 V, It = 30 pA; inset, constant-height mode, VB = 10 mV, It = 90 pA; (e) constant-height mode, VB = 10 mV, It = 90 pA; (g) constant-current mode, VB = 1 V, It = 50 pA; (h) constant-height mode, VB = 10 mV, It = 100 pA.

low proportion of coordinated bonds. When we annealed the substrate at 350 K for 10 min, the proportion of N−Au−N coordinated bonds was significantly increased (Figure S4). This further indicated that Au adatoms were involved in these twofold coordination structures because the density of Au adatoms changes with the surface temperature. Moreover, the herringbone reconstruction is intact upon the formation of molecular rings and chains, suggesting that gold adatoms result from the lattice gas formed by the continuous attachment/ detachment of atoms from the step edges.40,55 In fact, there are many ring-like structures observed on Cu(111) and Au(111) in addition to those displayed in Figure 2b,h. For Cu(111), various ring structures, including pentagonal, hexagonal, heptagonal, and octagonal rings, have been found at low coverage (Figure 3a). Figure 3b shows the molecular models of these rings on the Cu(111) surface. The relative abundances are estimated to be 3:87:3:1 for five-, six-, seven-, and eight-membered rings, indicating that the hexagonal ring is the most stable structure at low coverages. Similar to the previous result,56 the abundance of hexagons on Cu(111) is due to the structure matching between BPyB and Cu(111). Following the increase of the coverage of BPyB molecules on Cu(111), more complex ring-like structures and chains were observed (Figure S5). Some were deformed six-, seven-, and eight-membered rings; others were more complex polygonal rings that are consistent with the three-fold symmetry of the

amplified image of the basic cluster unit is shown in Figure 2e. A molecular model of the tetramer is portrayed in Figure 2f, with the hydrogen bonds highlighted by dashed lines. Each terminal N atom of the BPyB molecule is bonded to the H atom on the arm of a neighboring molecule. These cyclicarrayed hydrogen bonds gave rise to the growth of densely packed 2D islands that were formed by the periodic arrangement of the homotactic tetramers. As we can see in the inset of Figure 2d, the unit cell contains four BPyB molecules, and its dimensions are a = b = 2.11 nm and θ = 90°. We tried to trigger coordination by annealing the substrate at higher temperatures; however, at approximately 330 K, BPyB molecules started to desorb from the Ag(111) surface, and thus our attempt of thermal activation did not succeed. On Au(111), molecular rings and chains (Figure 2g) adsorbed at the most reactive elbow sites of the herringbone reconstruction. Figure 2h shows one magnified ring-like structure with the corresponding molecular model given in Figure 2i, which is stabilized by Au-coordinated and C−H···N hydrogen bonds. The coexistence of N−Au−N coordination and hydrogen bonds can be further evidenced by the fact that hydrogen-bonded nodes can be easily destroyed by tip manipulation, as described above (Figure S3). Unlike the reported Au-coordinated systems that require thermal activation at high temperature,41 the N−Au−N coordination process here took place at room temperature, but with a relative 8513


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Figure 3. (a) Various coordinated structures including pentagonal, hexagonal, heptagonal, and octagonal rings on Cu(111). (b) Corresponding molecular models of these coordinated rings on Cu(111). (c) Periodic arrangement of hydrogen bonded tetramers in the 2D molecular island on Ag(111). (d) Corresponding model of hydrogen bonded tetramers in (c). (e) Various mixed rings formed by coordinated bonds with a short length and hydrogen bonds with a longer length on Au(111). (f) Corresponding molecular models of these mixed structures on Au(111). Scanning condition: (a) constant-height mode, VB = 10 mV, It = 70 pA; (c) constant-height mode, VB = 10 mV, It = 90 pA; (e) constant-height mode, VB = 10 mV, It = 100 pA for rings and constant-current mode, VB = 10 mV, It = 100 pA for the chain. All arrows indicate [11̅0] direction.

weak Ag−BPyB interaction. For Au(111), in addition to four-, five-, and six-membered rings, irregular chains were also observed in relative abundance (Figure 3e). Figure 3f provides the molecular models for the observed structures depicted in Figure 3e. In contrast to the BPyB−Cu rings, the BPyB−Au rings have breaks due to the presence of coordinated (N−Au− N) and hydrogen-bonded linkages between adjacent BPyB

underlying substrate. The common feature of these nanostructures is that they are all formed by the coordination of BPyB molecules with Cu adatoms. For Ag(111), only 2D densely packed islands were formed by the periodic arrangement of the homotactic tetramers connected exclusively through hydrogen bonds, as shown in Figure 3c. The molecular bonding model is depicted in Figure 3d. The pure H-bonded network reflects a 8514


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ACS Nano ligands. Note that molecular coordinated structures in the disordered chain (Figure 3e) are also two-fold bonded, which can be clearly identified in high-voltage STM images. As shown in Figure S6, the coordination-bonded BPyB molecules appear much brighter than others, and three-fold structures are stabilized through hydrogen bonds. In order to clarify the formation mechanism of BPyB superstructures on the coinage metal M(111) surfaces, we first perform DFT calculations to investigate the binding characteristics of BPyB molecules with individual Cu, Ag, and Au atoms. Our calculation results show that one Cu, Ag, or Au atom binds two BPyB molecules with an N−M−N bond angle approaching 180° (Figure S7). The optimized M−N distances and the calculated binding energies are listed in Table 1. As we can see, Table 1. Calculated Metal−Ligand Binding Energies and Optimized N−M Distances, As Modeled by BPyB−M−BPyB (M = Cu, Ag, Au) without and with the Corresponding M(111) Surface parameter Eb/eV d(N−M)/Å

BPyB−M−BPyB (adsorbed on M(111))

BPyB−M−BPyB Cu

Ag

Au

Cu

Ag

Au

1.94 1.92

0.87 2.25

1.47 2.10

1.68 1.99

0.75 2.42

1.13 2.34

Figure 4. (a) Frontier molecular orbitals of an isolated BPyB molecule. (b) Partial orbital interaction diagram for BPyB−M− BPyB complexes (M = Cu, Ag, Au) in the gas phase. In each MO diagram, the energy levels of BPyB are shown on the left and the nd and (n + 1)s atomic orbitals of the metal atom are on the right. Spin-up and spin-down orbitals are drawn separately (top and bottom panels, respectively).

the binding energies for two BPyB molecules bonding with single Cu and Au atoms are rather large, 1.94 eV for Cu and 1.47 eV for Au; in contrast, the binding energy (0.87 eV) of two BPyB molecules connected to a single Ag atom is much smaller, and the Ag−N distance of 2.25 Å is also longer than the bond lengths of Cu−N (1.92 Å) and Au−N (2.10 Å). Taking the metal substrates into account, the same trend is still preserved; quantitatively, the M−N distances become a little longer and the binding energies are slightly decreased in comparison to the cases of individual metal atoms. As a result, BPyB molecules can form stable coordinated nanostructures with only Cu and Au adatoms on the corresponding M(111) surfaces due to their high binding energies. This is consistent with the above experimental findings. It should be noted that the relativistic effects play a vital role in the binding characteristics of BPyB molecules with gold, which are included through the pseudopotential. If the relativistic effects are neglected in the generation of the pseudopotentials of Au and Ag, the binding energy of two BPyB molecules bonding with a gold atom is significantly reduced to 0.66 eV, slightly less than that (0.67 eV) with a silver atom. To get a deep insight of the calculated BPyB−M−BPyB energies, the orbital composition and charge transfer between the BPyB molecules and the metal atoms were analyzed. Figure 4 presents the orbital interaction diagram of two BPyB molecules with individual Cu, Ag, and Au atoms. The HOMO−2 orbital of the BPyB molecule is σ-type, which accommodates the pyridyl lone pair of electrons; in contrast, the LUMO of the BPyB molecule is π-type and delocalized along the entire molecule. Here, HOMO and LUMO are, respectively, the acronyms of the highest occupied and lowest unoccupied molecule orbitals. Due to the symmetry matching, the Cu 4s and 3dz2 atomic orbitals hybridize with the HOMO− 2 orbitals of two BPyB molecules, forming occupied molecular orbitals of the BPyB−Cu−BPyB metal complex. Because the Cu 4s atomic orbital is much higher than the occupied molecular orbitals of BPyB molecules, the Cu atom donates

0.55 e− to the two BPyB molecules, resulting in two strong N− Cu bonds. In contrast, the bonding mechanism of the Au atom with two BPyB molecules is different, which can be described in terms of ligand-to-metal donation and metal-to-ligand π-backdonation. With the help of the 5dz2 atomic orbital, the Au 6s atomic orbital dominates the hybridization with the HOMO−2 orbitals of two BPyB molecules. Due to the large relativistic effect, the energy separation between the Au 6s atomic orbital and the BPyB HOMO−2 orbital is rather small, and thus electron donation occurs from two BPyB molecules to the Au atom. At the same time, orbital mixing also occurs for the LUMO of two BPyB molecules and the occupied Au 5dxz atomic orbital forming unoccupied molecular orbitals of the BPyB−Au−BPyB metal complex, which enables the π-backdonation from the Au atom to the BPyB molecules. As a result, the Au atom accepts 0.12 e− from two BPyB molecues, and two medium Au−N coordination bonds are formed between the Au atom and the two BPyB molecules. Different from the Cu and Au atoms, the energy separation between the Ag 5s and 4d atomic orbitals is much larger, and thus the Ag atomic orbitals cannot hybridize efficiently with frontier molecular orbitals of the BPyB molecules to form stable metal complexes. Considering that the metal substrate has only a minor influence on the binding energies between the BPyB molecules and the metal Cu or Ag adatoms, our orbital analysis without including substrates is reasonable. The clear calculated energy decrease of BPyB−Au−BPyB on Au(111) is due to the overestimated Au− Au(111) interaction because the herringbone reconstruction is ignored in the calculation.35 In this research, the relativistic effect induced orbital matching between Au and BPyB interprets the formation of 8515


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exchange-correlation functional B3LYP is used in conjunction with the LanL2DZ basis set for metal atoms (Au, Ag, and Cu)60 and the 6311+G(d,p) basis set for all nonmetal atoms (C, H and N). The extended charge decomposition analysis method is employed to analyze charge transfer between the metal atoms and BPyB molecules.61,62 More calculation details are given in the Supporting Information.

the BPyB−Au−BPyB complex. The 6s contraction and 5d expansion of Au orbitals enhances the donation from the pyridyl lone pair of electrons to the Au atom. The demonstrated mechanism can be used to explain the fomation of previously reported surface structures formed between Au atoms and cyano- or pyridyl-terminated molecules. For some other cases, the relativistic effect reduced the activity of Au atoms in forming the surface supported MOSs. As a consequence of the 6s contraction, the first ionization energy of Au is much larger than that of Ag and Cu. Therefore, the single Au atom is less reactive than Ag and Cu atoms in oxidation−reduction reactions. For example, AuPc cannot be formed through the reaction between Au and 2HPc. In contrast, the Cu and Ag atoms can replace H atoms and bond strongly to N atoms in 2HPc to form CuPc and AgPc.26,27

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04559. Details of tip manipulations, various molecular structures on Au(111) and Cu(111), variable-temperature experiments on Au(111) and Cu(111), Monte Carlo simulations, and DFT calculations (PDF)

CONCLUSIONS At low coverages, BPyB on Cu(111) gave rise to complete (BPyB−Cu)n rings (where n = 5,6,7,8) held together by BPyB− Cu linkages with linear N−Cu−N bonds. The hexagonal rings are most abundant owing to the strong Cu−N bonds and a near perfect match between the hexagonal (BPyB−Cu)6 ring and the three-fold hollow sites of the Cu(111) surface. Our DFT calculations showed that BPyB−Ag or BPyB−Ag−BPyB were much weaker than the Cu analogues such that only relatively weak hydrogen bonds are formed between BPyB molecules on Ag(111). A totally different picture evolved in the case of BPyB adsorbed on Au(111). Here, distorted polygonal rings held together by a combination of BPyB−Au linkages with linear N−Au−N bonds and hydrogen bonds between adjacent BPyB molecules were observed, along with chain structures. Our DFT calculations showed that BPyB−Au or BPyB−Au−BPyB were somewhat weaker than the Cu analogues but much stronger than the Ag analogues. The orbital composition analysis well-interpreted the different activity of Cu, Au, and Ag atoms in forming BPyB−M−BPyB complexes. In particular, the relativistic effects cause the energy stabilization of the Au 6s orbital and also decrease the energy separation between the Au 6s and 5d orbitals, which enhance the ligand-to-metal donation and metal-to-ligand π-backdonation, resulting in medium Au−N bonds in the BPyB− Au−BPyB linkage. Our findings point to future prospects of designing and building various unusual 2D assemblies of bipyridine ligands on metal surfaces.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Na Li: 0000-0001-6924-8387 Paweł Szabelski: 0000-0002-3543-9430 Yongfeng Wang: 0000-0002-8171-3189 Author Contributions ∥

X.Z. and N.L. contributed equally to this paper.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 21522301, 21373020, 21403008, 61321001, 21433011, 61271050, and 61671021), the Ministry of Science and Technology (Nos. 2014CB239302, 2013CB933404, and 2017YFA0205003). REFERENCES (1) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Patterning Self-Assembled Monolayers. Prog. Surf. Sci. 2004, 75, 1−68. (2) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (3) Bartels, L. Tailoring Molecular Layers at Metal Surfaces. Nat. Chem. 2010, 2, 87−95. (4) Otero, R.; Gallego, J. M.; de Parga, A. L. V.; Martín, N.; Miranda, R. Molecular Self-Assembly at Solid Surfaces. Adv. Mater. 2011, 23, 5148−5176. (5) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Controlling Molecular Deposition and Layer Structure with Supramolecular Surface Assemblies. Nature 2003, 424, 1029− 1031. (6) Schlickum, U.; Decker, R.; Klappenberger, F.; Zoppellaro, G.; Klyatskaya, S.; Auwärter, W.; Neppl, S.; Kern, K.; Brune, H.; Ruben, M.; et al. Chiral Kagomé Lattice from Simple Ditopic Molecular Bricks. J. Am. Chem. Soc. 2008, 130, 11778−11782. (7) Chen, W.; Li, H.; Huang, H.; Fu, Y.; Zhang, H. L.; Ma, J.; Wee, A. T. S. Two-Dimensional Pentacene: 3,4,9,10-Perylenetetracarboxylic Dianhydride Supramolecular Chiral Networks on Ag (111). J. Am. Chem. Soc. 2008, 130, 12285−12289. (8) Stöhr, M.; Boz, S.; Schär, M.; Nguyen, M.-T.; Pignedoli, C. A.; Passerone, D.; Schweizer, W. B.; Thilgen, C.; Jung, T. A.; Diederich, F. Self-Assembly and Two-Dimensional Spontaneous Resolution of Cyano-Functionalized [7]Helicenes on Cu(111). Angew. Chem., Int. Ed. 2011, 50, 9982−9986.

METHODS Experiments were performed in an ultrahigh vacuum LT-STM system (Unisoku) equipped with standard surface preparation facilities. Clean Cu(111), Ag(111), and Au(111) were prepared by cycles of sputtering and annealing. The commercial BPyB molecules were deposited onto the metal substrates from a tantalum crucible with the deposition rate from 0.2 to 0.003 ML/min. The substrates were kept at room temperature during molecular deposition. Chamber vacuum was maintained at 10−10 Torr during molecular deposition and subsequent imaging. All STM images were acquired using a sharpened Pt/Ir tip at 4−5 K. The bias refers to the voltage applied to the sample. DFT calculations on the metal−ligand interaction were carried out with the SIESTA code. In order to account for the vdW interactions, the optB88-vdW functional was adopted.57 The atomic cores were described by the improved Troullier-Martins pseudopotentials,58 and the wave functions of the valence electrons were expanded in terms of a double-ζ plus polarization (DZP) basis set. The orbital composition analysis was performed using the natural atomic orbital method implemented in the Gaussian09 software package.59 A hybrid 8516


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