Surface-Assisted Assembly of Discrete Porphyrin-Based Cyclic

Nov 4, 2009 - Pharmaceutical Sciences, University of Trieste, Piazzale Europa 1, ... by creating bicomponent structures combining building blocks with...
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Surface-Assisted Assembly of Discrete Porphyrin-Based Cyclic Supramolecules Daniel Heim,† Knud Seufert,† Willi Auwa¨rter,*,† Claudia Aurisicchio,‡ Chiara Fabbro,§ Davide Bonifazi,*,‡,§ and Johannes V. Barth† †

Physik Department E20, Technische Universita¨t Mu¨nchen, D-85748 Garching, Germany, ‡ Department of Chemistry, University of Namur, Rue de Bruxelles 61, B-5000, Namur, Belgium, and § Department of Pharmaceutical Sciences, University of Trieste, Piazzale Europa 1, I-34127, Trieste, Italy ABSTRACT We employed de novo synthesized porphyrin modules to construct discrete cyclic supramolecular architectures supported on a copper surface. The programmed geometry and functionality of the molecular modules together with their conformational flexibility and substrate interaction yields symmetric discrete assemblies, including dimers and chains as well as three- to six-membered cyclic structures. The area of the molecular cavities is extended by creating bicomponent structures combining building blocks with different symmetry. KEYWORDS Porphyrin, supramolecules, scanning tunneling microscopy, metal-organic coordination, copper surface

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5,10 (cis-like) and 5,15 (trans-like) positions, respectively (Chart 1). The synthesis of pyridyl-substituted porphyrins 1-3 is outlined in Scheme 1. Mixed condensation25-27 of pyrroles with the appropriate aromatic aldehydes and selective meso-functionalization of preformed25,27 disubstituted porphyrin scaffolds are, by far, the most used synthetic approaches toward the synthesis of “asymmetrically”28-32 meso-substituted porphyrins. Due to the easy separation of our porphyrin derivatives, in this work we opted for the mixed condensation route. Therefore, precursor 4, bearing a free terminal alkyne has been obtained via statistical macrocyclization of pyrrole in the presence of aldehydes 5 and 6 in a 3:1 ratio (BF3, CHCl3) followed by oxidation (DDQ) and removal (K2CO3, MeOH) of the TMS protecting group.33 Monosubstituted derivative 1 was then prepared in good yield following a Sonogashira-type cross-coupling reaction34 with 4-iodopyridine in the presence of [PdCl2(PPh3)2] and CuI in (i-Pr)2NH (Scheme 1, path c). In parallel, the synthesis of the two dipyridyl-porphyrin modules 2 and 3 bearing two pyridyl groups at the 5,10 (cis-like) and 5,15 (trans-like) positions, respectively, has been carried out. In this case, following a modified procedure as that described in the literature,35 the statistical condensation (BF3, CHCl3) between 5 and 6 in a 1:1 ratio with 4 equivalents of pyrrole, gave rise to an isomeric mixture of disubstituted porphyrins. Metalation (Zn(OAc)2, MeOH) and removal (K2CO3, MeOH) of the TMS protecting group afforded an isomeric mixture containing porphyrin derivatives 7 and 8, which were isolated by extensive chromatography (Scheme 1, path f). Subsequently, terminal bisacetylene poprhyrins 7 and 8 reacted separately with 4-iodopyridine under Sonogashira-type reaction conditions34 leading to targeted molecules 2 and 3 (Scheme 1, paths g and h). Detailed information on the synthetic

upramolecular engineering on surfaces provides access to a multitude of nanoscale architectures, including clusters of distinct symmetry and size,1,2 onedimensionalgratings,3 orporoustwo-dimensionalnetworks.4-9 A variety of noncovalent interactions and bonding motifs has been explored,10-12 but to date pathways to realize discrete surface-confined cyclic structures are lacking, albeit regular arrays of metallacycles have been obtained by segregation of prefabricated entities at solid-liquid interfaces.13-16 A particular versatile class of molecular building blocks is provided by porphyrins, where meso-substituted derivatives have been employed to steer the formation of specific assemblies in solution,17-19 bulk materials,20 and on surfaces.2,21-23 Here we make use of three ingredients for the surface-assisted assembly of supramolecular cyclic architectures with distinct shape and size: (i) the geometry and chemical properties of the molecular building blocks providing functional end groups to promote directional intermolecular interactions via substrate-mediated metal-ligand bonding, (ii) their conformational flexibility24 bestowing a discrete set of bonding motifs, and (iii) molecule-surface interactions to guide the azimuthal orientation of the supramolecules. Synthesis of the Mono- and Disubstituted Pyridylporphyrin Derivatives 1-3. The self-assembling molecular modules studied in this work are based on a tetrapyrrolic core differently substituted at the four meso-positions: porphyrin module 1 features three 3,5-di-tert-butyl (t-Bu2Ph) substituents and one pyridyl function (Chart 1), while porphyrins 2 and 3 bear two t-Bu2Ph and two pyridyl substitutents at the

* Authors to whom correspondence should be addressed: davide.bonifazi@ fundp.ac.be and [email protected]. Received for review: 09/10/2009 Published on Web: 11/04/2009 © 2010 American Chemical Society

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CHART 1.

Schematic Structures of the Investigated Porphyrin Compounds

SCHEME 1.

Synthesis of Mono- and Disubstituted Pyridylporphyrins 1-3a

a Reagents and conditions: a, 5 (3 equiv), 6 (1 equiv), BF3·Et2O, DDQ, CHCl3, rt, 2 h; b, K2CO3, MeOH/THF, rt, 2 h; c, 4-iodopyridine, [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 16 h, 50%; d, 5 (2 equiv), 6 (2 equiv), BF3·Et2O, DDQ, CHCl3, rt, 2 h; e, K2CO3, MeOH/THF, rt, 2 h; f, Zn(OAc)2, CHCl3/MeOH, rt, 2 h, 7 23%, 8 34%; g, 4-iodopyridine, [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 16 h, 36%; h, 4-iodopyridine, [PdCl2(PPh3)2], CuI, (i-Pr)2NH, rt, 16 h, 48%.

protocols and spectroscopic characterizations are available as Supporting Information. © 2010 American Chemical Society

Scanning Tunneling Microscopy (STM) Observations. All STM experiments were performed in a custom-designed 123

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FIGURE 1. (a) Molecular structure of porphyrin 1 showing the three t-Bu2Ph groups and the pyridyl substituent. The molecular dimensions are extracted from PM3 calculations as implemented in the HyperChem38 molecular modeling package and are consistent with values39 reported in literature. (b) STM image of self-assembled dimers (1)2 on a Cu(111) surface (data acquired with I ) 0.12 nA, U ) -1 V). (c) Detailed view of a single (1)2 dimer exhibiting the three double lobes representing the t-Bu2Ph groups of each molecular module and the head-on aligned pyridyl substituents (I ) 0.4 nA, U ) -0.9 V). (d) The t-Bu2Ph groups follow the direction of the dense-packed Cu(111) atomic rows (nearest neighbor distance between atoms: 2.55 Å; I ) 2.9 nA, U ) -31 mV, low-pass filtered). The right panel summarizes a statistical analysis of the orientations of the dimers. (e) Molecular structure of porphyrin 2. (f) Coexistence of triangular and rhombic supramolecules after deposition of porphyrin 2 (I ) 0.1 nA, U ) -0.5 V). The arrows and lines mark the distinguishable azimuthal orientations of the supramolecules. (g-j) Detailed view and structural properties of supramolecular cycles (2)n (g, n ) 3; h, n ) 4; i, n ) 5; j, n ) 6). The two possible opening angles of the t-Bu2Ph groups are indicated (f, I ) 0.2 nA, U ) -0.22 V; g, I ) 0.1 nA, U ) -1 V; i, I ) 0.2 nA, U ) -1 V; j, I ) 0.1 nA, U ) 1 V).

ultrahigh vacuum (UHV) system providing a base pressure below 1 × 10-10 mbar.36 The monocrystalline Cu(111) substrate was cleaned by repeated Ar+ sputtering cycles at an energy of 800 eV, followed by annealing at 730 K for ∼10 min. Subsequently, a submonolayer coverage of the porphyrin modules (1-3) was deposited by organic molecular beam epitaxy from thoroughly degassed quartz crucibles held at 680 K. During deposition, the Cu(111) surface was kept at 340 K and the pressure remained < 5 × 10-10 mbar. The STM images were acquired employing a low-temperature CreaTec-STM37 with the sample held at 6 K using electrochemically etched W tips. In the figure captions, U refers to the bias voltage applied to the sample. The first step in our study is the deposition of a submonolayer of porphyrin module 1 onto a Cu(111) surface under UHV. While very few single protrusions corresponding to individual porphyrin molecules are observed in the resulting STM images (cf. Figure 1b), the surface mainly hosts larger units characterized by two bright terminations and a dim central connection. Specifically, high-resolution images (Figure 1c) reveal that each assembly consists of two molecules © 2010 American Chemical Society

1 interconnected through their pyridyl substitutents and thus resulting in a dimeric (1)2 species. This structural assignment is based on the molecular dimensions and on previous STM work on 5,10,15,20-tetra(t-Bu2Ph)porphyrin molecules2,40 in which each double-lobed feature corresponds to the t-Bu2Ph groups. Besides the six bright t-Bu2Ph groups, the two pyridyl substitutents are clearly resolved. They align in an antiparallel head-on configuration, where the pyridyl groups face each other. Importantly, the combination of the 6-fold symmetry of the first Cu layer with the C2h symmetry of the porphyrin core in the gas phase, together with a considerable molecule-substrate interaction, leads to a conformational adaptation of the molecules upon adsorption. A rotation of the t-Bu groups results in an alignment of the double-lobed features with the dense-packed substrate directions. Accordingly, the double lobes representing the t-Bu2Ph substituents are not perpendicular to each other, but enclose an angle of either 60° or 120° (see Figure 1c, d). Nevertheless, a close inspection of Figure 1b reveals that the (1)2 dimer orientations, defined by the axis connecting the centers of the two constituting molecules, do not exclusively follow the Cu(111) 124

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dense-packed rows but are frequently rotated by an angle of (15° (see right panel in Figure 1d). These multiple orientations of the dimers relative to the underlying atomic lattice are induced by an apparent flexibility of the pyridyl legs allowing for some lateral bending (vide infra). The coupling mechanism of the dimers apparently induced by the pyridyl groups is mediated by copper atoms (vide infra), whereby the center-to-center distance in straight dimers amounts to ∼32 Å. To further explore the controlled aggregation of porphyrins guided by the their pyridyl substituents, we investigated a porphyrin module bearing two functional pyridyl moieties in a cis-like configuration and proceed to molecule 2 (cf. Figure 1e). The large-scale image in Figure 1f shows that different aggregates of well-defined shape and orientation coexist on the surface after deposition of building block 2. Triangular and rhombic architectures (Figure 1g, h) consisting of three and four molecular modules 2, (2)3 and (2)4, are clearly resolved. Again, the molecular modules are interacting via the head-on pyridyl functions as observed for dimers (1)2. While the trimeric species (2)3 dominate at low coverage, the tetrameric (2)4 assemblies prevail at higher surface coverages where additionally pentameric (Figure 1i) and hexameric (Figure 1j) assemblies occur. This trend indicates that the spontaneous self-assembly of the supramolecules is based on a statistical process related to the probability that a given number n (n ) 3, 4, 5, ...) of molecules meets while diffusing on the surface and is not solely induced by an energy difference favoring either triangular or rhombic architectures. Together with the pyridyl-to-pyridyl coupling, the conformational flexibility of the macrocyclic structure and thus its adaptability and interaction with the underlying surface governs the geometrical properties of the supramolecules. Notably, the different interactions between the porphyrin and the Cu(111) surface induce marked changes in the conformation of the central tetrapyrrolic core (Figure S1 in the Supporting Information) strongly affecting the opening angles of both t-Bu2Ph and pyridyl groups. This results in opening angles of the t-Bu2Ph groups of either 60° or 120°. In addition, the pyridyl substitutents can also laterally bend.41 Besides the intrinsic opening angle of 90° rarely observed on the surface, we determined opening angles of either 80 ( 5° or 100 ( 5° (Figure S1 in the Supporting Information). The small t-Bu2Ph angles favor the small pyridyl angle and vice versa. As a consequence, to assemble a supramolecular trimeric cycle by a nearly straight headon coupling of the pyridyl groups, the molecules have to adapt to the t-Bu2Ph opening angle of 60° and, as a result, symmetric trimeric species can only exist in two distinguishable azimuthal orientations on the surface, rotated by 180° with respect to each other (Figure S2 in the Supporting Information). This is visualized in Figure 1f, where trimeric supramolecules (2)3 are observed exclusively in two orientations with respect to the substrate atomic lattice. Exceptions © 2010 American Chemical Society

FIGURE 2. (a) Molecular structure of porphyrin 3. (b) STM image of the one-dimensional supramolecular assembly following deposition of 3 on Cu(111) (I ) 77 pA, U ) -0.1 V). (c) Schematic model of the (2·3·2)2 binary cyclic assembly shown in (e). The black dots represent Cu adatoms embedded in between the terminal pyridyl N atoms. (d) STM image showing the coexistence of homomolecular assemblies (2)3 and (2)4 with bicomponent rhombic (2·3·2)2 and (2·3)4 triangular (2·3)3 architectures formed upon mixing of the respective modules. (e, f) Detailed view of the bicomponent cyclic architectures: (e) (2·3·2)2 and (f) (2·3)4. (d, f, I ) 0.1 nA, U ) -1 V; e, I ) 0.2 nA, U ) -1 V).

constituted by triangles exhibiting t-Bu2Ph opening angles of 120° were rarely spotted. Their assembly follows the same empirical rules, but they exhibit a more complex, chiral structure. The area of the regular triangular cavities amounts to ∼6 nm2. Following these rules, the only possible geometry for symmetric and quadrangular supramolecules coupled by the pyridyl groups is a rhombic shape, where two opposing corner molecules exhibit a t-Bu2Ph angle of 60°, whereas the other pair has a t-Bu2Ph opening angle of 120°. Consequently, the rhombic (2)4 supramolecule exists exclusively in three distinguishable azimuthal orientation on the surface (compare Figure 1f and Figure S2 in the Supporting Information) and regular square or pentagonal structures cannot form. Over 95% of the observed quadrangular supramolecules (2)4 are regular rhombs exhibiting 60°/120° t-Bu2Ph opening angles and a cavity area of 10.5 nm2. For the pentagons and the hexagons (cf. Figure 2i,j) the area amounts to 15 and 19 nm2, respectively. To complete the characterization of the self-assembly of our basic building blocks, we proceed to the trans-like isomer 3 depicted in Figure 2a. The STM image reproduced in Figure 2b reveals that the positioning of the pyridyl groups (trans 125

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versus cis position) has a drastic impact on the resulting assemblies. As expected from the head-on coupling of the pyridyl groups, molecules 3 align in extended chains, whereby straight segments have a stride of 32.7 ( 0.5 Å (see Figure 2b). After characterizing the homomolecular supramolecular architectures and developing empirical rules to rationalize their diverse yet highly regular shapes, we now proceed to the construction of larger supramolecular cavities by mixing ditopic building blocks with different symmetry. In order to probe the recognition properties of the different molecular modules, we thus studied bicomponent assemblies. As an example, Figure 2d shows the nanoarchitectures evolving after subsequent deposition of 2 and 3 with a 3.6:1 ratio. Besides the homomolecular triangles (2)3 and rhombs (2)4 discussed above, unprecedently large cyclic architectures incorporating modules 3 are also clearly discernible. Figure 2e shows a cyclic assembly (2·3·2)2 with a 2:1 ratio of 2:3 and a cavity area of 20.6 nm2. Accordingly, when passing from assembly (2·3·2)2 to (2·3)4 (Figure 2f) the area of the cavities is markedly extended to 41.2 nm2. Again, the statistical assembly process inhibits a uniform size distribution of the supramolecules. To our knowledge this is the first observation of individual supramolecular cavities at surfaces formed by multiple molecular modules. An overview of possible assemblies originating from combinations of porphyrins 1, 2, and 3 is given in Figure S2 in the Supporting Information. In order to obtain further insight into the flexibility of both the porphyrin modules and the cyclic supramolecules as well as the strength of the intermolecular coupling, molecular manipulation experiments have been performed. To this end, the STM tip was approached to the surface by decreasing the tunneling resistance and laterally moved across the supramolecules in constant current mode. A similar procedure has been applied previously to translate individual porphyrins substituted by four identical t-Bu2Ph substitutents on a Cu(100) surface.41 Figure 3 compares states prior and after such manipulation steps. Panels a-c of Figure 3 indicate that the shape of (2)4 supramolecules can be completely changed without breaking the intermolecular bonds, evidencing a conformational deformation of the cyclic assembly without apparent disruption of the noncovalent intermolecular recognition site. The yellow color in Figure 3b originates from a superposition of the initial state (green) and the final state (red) and highlights regions molecules overlap prior to and after the manipulation step. Nevertheless, it is possible by STM manipulation to rupture the noncovalent interactions and to transfer molecules to the STM tip: Figure 3d shows an example where a transformation from a rhombic to a triangular assembly was induced by the extraction of one module. Finally, we detail the origins of the pyridyl coupling mechanism resulting in the appreciable intermolecular bond strength evidenced by the experiments. Our data reveal an © 2010 American Chemical Society

FIGURE 3. Molecular manipulation experiments with (2)4 assemblies. Between the initial (a, green) and final states (c, red) the STM tip was laterally moved at low tunneling resistance across the supramolecule following the path indicated by the arrow in (b). The experiments indicate a high flexibility of both the molecular building blocks and the supramolecular assemblies combined with stable intermolecular bonds and show that it is possible to induce a conformational change of the supramolecules without breaking them (a-c: imaging, I ) 0.1 nA, U ) -0.5 V; manipulation, I ) 10 nA, U ) -50 mV). (d) Transition of a rhombic to a triangular structure by extraction of one molecule by STM manipulation (imaging, I ) 0.1 nA, U ) -0.5 V; manipulation, I ) 8 nA, U ) -50 mV). The dark dots representing adsorbed gas molecules can be used as markers.

intermolecular distance of 32.7 ( 0.5 Å for straight 3 chain segments, where the distance measurement and calibration are the most precise. This value is consistent with the centerto-center spacing in the other aggregates. Subtracting the length of molecule 3 of 29.1 Å as determined from a PM3 optimization and preliminary crystallographic data of the molecular structure and in agreement with ref 39, yields an intermolecular N-N separation of 3.6 ( 0.5 Å between the two terminal pyridyl groups. From electrostatic considerations, a straight head-on coupling of the two opposite pyridyl groups is energetically highly unfavorable, as the terminating N atoms carry negative charge.42 Accordingly, on noble metal surfaces as Ag(111), the head-on coupling is not observed. Instead, the molecules form dense-packed islands where the pyridyl groups avoid each other even at low submonolayer coverages (see Figure S4 in the Supporting Information). Thus, the Cu surface plays a decisive role in promoting a head-on alignment of the pyridyl groups. In this context, the strong bonding interactions of pyridyl to Cu are well documented both on surfaces43-45 and for the solid state,46 where typical N-Cu bond distances range from 1.9 to 2.2 Å. These attractive interactions were reported to induce a rotation and bending of the terminal pyridyl groups of a porphyrin molecule toward the Cu(111) surface.44 Accordingly, a possible explanation of the head-on pyridyl coupling consistent with the determined N-N separation of 3.6 ( 0.5 Å would be that the two N atoms coordinate to a Cu surface atom by bending toward the substrate, possibly inducing a displacement of the Cu atom.47 However, this scheme is excluded because diffusion experiments show that the (1)2 dimers translate at 220 K without breaking (see Figure S5 in the Supporting Information). It is highly unlikely that two molecules of 1 would coherently form and break bonds to underlying Cu atoms embedded in the surface while 126

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wandering large distances without drifting apart. On the other hand, it is well-known that appreciable Cu adatom densities exist on Cu surfaces even at moderate temperatures, enabling stabilization of supramolecular aggregates by metal-ligand bonding between pyridyl (or other) groups and Cu adatoms.16,21,45,48 The observed N-N distance of 3.6 ( 0.5 Å would yield a N-Cu bond length of 3.6 Å/2 ) 1.8 Å, which is slightly shorter than the values reported above, but consistent with calculated values for a pyridylCu-pyridyl complex.21 However, this number is a lower limit, as it only considers the projection of the pyridyl-Cu bond length onto the surface plane. Geometric effects as the separation of the terminal N atom from the surface dictated by the molecular backbone or a lateral offset of the coupling adatom can sensitively increase the real pyridyl-Cu distance. For example, assuming a typical vertical position of an adatom 2 Å above the surface and a characteristic separation of an aromatic backbone from the surface plane of 3 Å,49 the N-Cu bond length would increase to 2.1 Å, a value in full agreement with literature. The fact that metal atoms coordinating organic ligands are hardly seen in STM images because of electronic effects is well documented in the literature.50,51 The 2-fold pyridyl-Cu-pyridyl coordination48 motif suggested here indicates that the terminal pyridyl rings are rotated toward the surface plane. As a consequence, steric repulsions between H atoms of neighboring pyridyl rings prevents the 3-fold coordination motif observed for related metal-carbonitrile bidimensional metal-organic networks6 (cf. Figure 2c). The same mechanism disfavors pyridyl-pyridyl bonding angles that deviate strongly from the head-on configuration of 180° present in the regular (2)3 and (2)4 structures. Accordingly, the lowest observed angle in supramolecules assembled from intact modules amounts to 160° as measured in the regular (2)6 structures. This excludes for example (2)2 dimers or (2)6 hexagons exhibiting t-Bu2Ph opening angles of 60° (see Figure S2 in the Supporting Information). In conclusion, we synthesized novel porphyrin derivates exhibiting either one or two functional pyridyl recognition groups, the latter being cis-like and trans-like isomers. The design of these molecular modules promotes the spontaneous self-assembly of individual cyclic supramolecules on a Cu(111) surface. All supramolecular architectures are stabilized by attractive pyridyl-pyridyl interactions, mediated by Cu surface atoms. The strength of the intermolecular interactions allows for conformational changes of the entire assemblies by STM manipulation. The self-assembly of both the mono and the translike porphyrins results in distinct supramolecular dimers and chains, respectively. In contrast, the molecular symmetry in combination with the molecular flexibility and surface interactions explains the multiformity of the supermolecules formed by the cis species, expressed in open three- to six-membered cyclic assemblies. Furthermore, upon combination of the different ditopic porphyrin © 2010 American Chemical Society

building blocks, bicomponent cyclic supramolecules of unprecedented extended size could be engineered. Thus our study explores design criteria for the surface engineering of supramolecular cyclic compounds and reveals an intricate interplay of conformational adaptation and self-assembly to be generally considered when complex and flexible species are employed. The fabricated large cavities bear promises as hosts for molecular guest species,7,11 whereas the constituting functional porphyrin modules can serve as platforms for the axial coordination of additional ligands52 or as catalytic centers for activating organic reactions.20 Acknowledgment. Work supported by the Munich Center for Advanced Photonics (MAP) and by the European Union through the Marie-Curie Initial Training Network “FINELUMEN”, Grant Agreement PITN-GA-2008-215399, the University of Trieste, the University of Namur, the Belgian National Research Foundation (FRS-FNRS, through the contracts No. 2.4.625.08 F, 2.4550.09 F, and F.4505.10), the “Loterie Nationale”, and the Re´gion Wallonne through the “SOLWATT” program (Contract No. 850551). Dr. A. Llanes-Pallas is gratefully acknowledged for help with the organic synthesis. Note Added in Proof. Related work has been reported recently: Fendt, L-A.; Stöhr, M.; Wintjes, N.; Enache, M.; Jung, T. A.; Diederich, F. Chem. Eur. J. 2009, DOI 10.1002/ chem.200901502. Supporting Information Available. Schematic illustration of the molecular conformation and the empirical rules describing the molecular assembly, overview of the symmetric supramolecular aggregates emerging from mixing building blocks 1, 2, and 3, STM data highlighting the packing of building block 1 on Ag(111), and image sequence showing the diffusion of (1)2 dimers on Cu(111). This information is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1)

Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. Rev. Lett. 1999, 83, 324. (2) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619–621. (3) Schiffrin, A.; Riemann, A.; Auwa¨rter, W.; Pennec, Y.; WeberBargioni, A.; Cvetko, D.; Cossaro, A.; Morgante, A.; Barth, J. V. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 5279–5284. (4) Schull, G.; Douillard, L.; Fiorini-Debuisschert, C.; Charra, F.; Mathevet, F.; Kreher, D.; Attias, A.-J. Nano Lett. 2006, 6, 1360– 1363. (5) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X.; Cai, C.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229–233. (6) Schlickum, U.; Decker, R.; Klappenberger, F.; Zoppellaro, G.; Klyatskaya, S.; Ruben, M.; Silanes, I.; Arnau, A.; Kern, K.; Brune, H.; Barth, J. V. Nano Lett. 2007, 7, 3813–3817. (7) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–1031. (8) Kudernac, T.; Lei, S.; Elemans, J. A. A. W.; De Feyter, S. Chem. Soc. Rev. 2009, 38, 402–421. (9) Bonifazi, D.; Mohnani, S.; Llanes-Pallas, A. Chem.sEur. J. 2009, 15, 7004–7025. (10) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679.

127

DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128

(11) Barth, J. V. Annu. Rev. Phys. Chem. 2007, 58, 375–407. (12) Treier, M.; Ruffieux, P.; Gro¨ning, P.; Xiao, S.; Nuckolls, C.; Fasel, R. Chem. Commun. 2008, 4555–4557. (13) Li, S.-S.; Northrop, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. Acc. Chem. Res. 2009, 42, 249–259. (14) Safarowsky, C.; Merz, L.; Rang, A.; Broekmann, P.; Hermann, B. A.; Schalley, C. A. Angew. Chem., Int. Ed. 2004, 43. (15) Jeong, K. S.; Kim, S. Y.; Shin, U.-S.; Kogej, M.; Hai, N. T. M.; Broekmann, P.; Jeong, N.; Kirchner, B.; Reiher, M.; Schalley, C. A. J. Am. Chem. Soc. 2005, 127, 17672–17685. (16) Lin, N.; Stepanov, S.; Ruben, M.; Barth, J. V. Top. Curr. Chem. 2009, 287, 1–44. (17) Drain, C. M.; Varotto, A.; Radivojevic, I. Chem. Rev. 2009, 47, 1630–1658. (18) Tsuda, A.; Nakamura, T.; Sakamoto, S.; Yamaguchi, K.; Osuka, A. Angew. Chem., Int. Ed. 2002, 41, 2817–2821. (19) Tsuda, A.; Hu, H.; Tanaka, R.; Aida, T. Angew. Chem., Int. Ed. 2005, 44, 4884–4888. (20) Piot, L.; Bonifazi, D.; Samori, P. Adv. Funct. Mater. 2007, 17, 3689– 3693. (21) Eichberger, M.; Marschall, M.; Reichert, J.; Weber-Bargioni, A.; Auwa¨rter, W.; Wang, R. L. C.; Kreuzer, H. J.; Pennec, Y.; Schiffrin, A.; Barth, J. V. Nano Lett. 2008, 8, 4608–4613. (22) Bonifazi, D.; Kiebele, A.; Sto¨hr, M.; Cheng, F.; Jung, T.; Diederich, F.; Spillmann, H. Adv. Funct. Mater. 2007, 17, 1051–1062. (23) Wintjes, N.; Hornung, J.; Lobo-Checa, J.; Voigt, T.; Samuely, T.; Thilgen, C.; Sto¨hr, M.; Diederich, F.; Jung, T. Chem.sEur. J. 2008, 14, 5794–5802. (24) Yokoyama, T.; Kamikado, T.; Yokoyama, S.; Mashiko, S. J. Chem. Phys. 2004, 121, 11993–11997. (25) Lindsey, J. S.; Prathapan, S.; Johnson, T. E.; Wagner, R. W. Tetrahedron 1994, 50, 8941–8968. (26) Osuka, A.; Tanabe, N.; Nakajima, S.; Maruyama, K. J. Chem. Soc., Perkin Trans. 2 1996, 199–203. (27) Felber, B.; Diederich, F. Helv. Chim. Acta 2005, 88, 120–153. (28) Rao, P. D.; Littler, B. J.; Geier, G. R.; Lindsey, J. S. J. Org. Chem. 2000, 65, 1084–1092. (29) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7323–7344. (30) Blake, I. M.; Anderson, H. L.; Beljonne, D.; Bredas, J. L.; Clegg, W. J. Am. Chem. Soc. 1998, 120, 10764–10765. (31) Burrell, A. K.; Officer, D. L.; Plieger, P. G.; Reid, D. C. W. Chem. Rev. 2001, 101, 2751–2796. (32) The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978.

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(33) Zeitouny, J.; Aurisicchio, C.; Bonifazi, D.; De Zorzi, R.; Geremia, S.; Bonini, M.; Palma, C. A.; Samori, P.; Listorti, A.; Belbakra, A.; Armaroli, N. J. Mater. Chem. 2009, 19, 4715–4724. (34) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467–4470. (35) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538– 11545. (36) Auwa¨rter, W.; Schiffrin, A.; Weber-Bargioni, A.; Pennec, Y.; Riemann, A.; Barth, J. V. Int. J. Nanotechnol. 2008, 5, 1171–1193. (37) CreaTec, D-74391 Erligheim, Germany . (38) HYPERCHEM; Hypercube Inc.: 1115 NW 4th St, Gainesville, FL 32601. (39) Wintjes, N.; Bonifazi, D.; Cheng, F.; Kiebele, A.; Sto¨hr, M.; Jung, T.; Spillmann, H.; Diederich, F. Angew. Chem., Int. Ed. 2007, 46, 4089–4092. (40) Moresco, F.; Meyer, G.; Rieder, K.-H.; Ping, J.; Tang, H.; Joachim, C. Surf. Sci. 2002, 499, 94–102. (41) Jung, T. A.; Schlittler, R. R.; Gimewski, J. K.; Tang, H.; Joachim, C. Science 1996, 271, 181–184. (42) Wang, Y.; Ge, X.; Schull, G.; Berndt, R.; , C.; Bornholdt; Koehler, F.; Herges, R J. Am. Chem. Soc. 2008, 130, 4218–4219. (43) Wu, D. Y.; Hayashi, M.; Chang, C. H.; Liang, K. K.; Lin, S. H. J. Chem. Phys. 2003, 118, 4073–4085. (44) Auwa¨rter, W.; Klappenberger, F.; Weber-Bargioni, A.; Schiffrin, A.; Strunskus, T.; Wo¨ll, C.; Pennec, Y.; Riemann, A.; Barth, J. V. J. Am. Chem. Soc. 2007, 129, 11279–11285. (45) Klappenberger, F.; Weber-Bargioni, A.; Auwa¨rter, W.; Marschall, M.; Schiffrin, A.; Barth, J. V. J. Chem. Phys. 2008, 129, 214702. (46) Dietrich-Buchecker, C. O.; Guilhem, J.; Pascard, C.; Sauvage, J.P. Angew. Chem., Int. Ed. 1990, 29, 1154–1156. (47) Bedwani, S.; Wegner, D.; Crommie, M. F.; Rochefort, A. Phys. Rev. Lett. 2008, 101, 216105. (48) Tait, S. L.; Langner, A.; Lin, N.; Stepanow, S.; Rajadurai, C.; Ruben, M.; Kern, K. J. Phys. Chem. C 2007, 111, 10982–10987. (49) Stadler, C.; Hansen, S.; Kro¨ger, I.; Kumpf, C.; Umbach, E. Nat. Phys. 2009, 5, 153–158. (50) Classen, T.; Fratesi, G.; Constantini, G.; Fabris, S.; Stadler, F. L.; Kim, C.; de Gironcoli, S.; Baroni, S.; Kern, K. Angew. Chem., Int. Ed. 2005, 44, 6142–6145. Kühne, D.; Klappenberger, F.; Decker, R.; Schlickum, U.; Brune, H.; Klyatskaya, S.; Ruben, M.; Barth, J. V. J. Am. Chem. Soc. 2009, 131, 3881–3883. (51) Breitruck, A.; Hoster, H. E.; Meier, C.; Ziener, U.; Behm, R. J. Surf. Sci. 2007, 601, 4200–4205. (52) Williams, F. J.; Vaughan, O. P. H.; Knox, K. J.; Bambos, N.; Lambert, R. M. Chem. Commun. 2004, 1688.

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DOI: 10.1021/nl9029994 | Nano Lett. 2010, 10, 122-128