Covalent, Organometallic, and Halogen-Bonded Nanomeshes from

Article pubs.acs.org/JPCC

Covalent, Organometallic, and Halogen-Bonded Nanomeshes from Tetrabromo-Terphenyl by Surface-Assisted Synthesis on Cu(111) Qitang Fan,† Cici Wang,† Liming Liu,‡ Yong Han,† Jin Zhao,‡ Junfa Zhu,*,† Julian Kuttner,§ Gerhard Hilt,§ and J. Michael Gottfried*,§ †

National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230029, People’s Republic of China ‡ Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China § Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Str., 35032 Marburg, Germany S Supporting Information *

ABSTRACT: The selective temperature-controlled surface-assisted synthesis of covalent, organometallic, and halogen-bonded nanomeshes based on a 3,5,3″,5″-tetrabromo-para-terphenyl (TBrTP) precursor was studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (STM) in ultrahigh vacuum. Vapor deposition of TBrTP onto Cu(111) at 90 K leads to a highly ordered organic monolayer stabilized by Br···Br and Br···H intermolecular bonds between the intact T-type assembled TBrTP molecules, as confirmed by density functional theory (DFT) calculations. Annealing the monolayer to 300 K results in C−Br bond scission and the formation of C−Cu−C bonds, which link adjacent para-terphenyl fragments such that stable organometallic frameworks are formed. Pore sizes correlate with the number of enclosed adatoms (most likely Br atoms), which presumably play a size-determining role during the process of the pore formation. Larger islands of the organometallic framework are obtained by deposition of TBrTP onto the copper surface held at 460 K. A further increase in sample temperature to 570 K during deposition gives rise to the formation of covalent organic frameworks with pores of tetragonal and trigonal symmetry. The covalent nanostructures are not completely planar, but contain phenylene units which are tilted relative to the surface plane, most likely due to steric hindrance between the C−H bonds inside the pores. Comparison of the three different bonding regimes reveals that the degree of long-range order correlates inversely with the strength of the bonds between the building blocks.

■

hyperbenzene,19 which then can self-assemble in a reversible process to form large ordered structures. Due to the reversible nature of the assembly process, “wrong” structural elements, if present, can be expelled from the islands. Another possible approach is to establish the long-range order in the network using weaker bonds (such as van der Waals or coordinative bonds), which allow for a reversible bond scission and reformation.20−24 Once an ordered network is formed, an addition reaction step inducing covalent bond formation is performed. A reaction of interest in this context is the surfaceassisted Ullmann reaction for C−C coupling between haloarenes. This reaction starts with an initial surface-assisted scission of the carbon−halogen bond, followed by the reversible formation of intermediate carbon−metal−carbon bonds.25−29 The reversible nature of these bonds has been demonstrated previously.30 At higher temperatures, the metal atoms are released and carbon−carbon bonds are formed. Since the formation of carbon−carbon bonds thus passes through a

INTRODUCTION Research on carbon-based conjugated nanostructures has found tremendous interest during the past decade,1−4 especially in the context of graphene-related research.5−9 Graphene layers on metal surfaces can be synthesized from a wide range of carbon containing precursors, because graphene represents the global thermodynamic minimum of all possible planar, conjugated carbon allotropes.10−13 Other structures, which represent only local minima, are much more difficult to make and require specific precursors and a careful control of the reaction conditions. Because of these difficulties, most attempts to create “porous graphene” have resulted in rather small islands of the desired structure or in the formation of multiple structural motifs.9,14−16 The major reason for this problem is that the networks must be grown under conditions where the C−C bond formation is irreversible, because otherwise, graphene is formed. However, irreversible C−C bond formation also means that once a structural defect is formed, it cannot be removed by post-treatment such as annealing. As a possible solution, it has been suggested to use a hierarchical approach,2,3,17,18 by which precursors undergo surface-assisted reactions to form large conjugated hydrocarbons such as © 2014 American Chemical Society

Received: April 16, 2014 Revised: May 22, 2014 Published: May 22, 2014 13018

dx.doi.org/10.1021/jp5037475 | J. Phys. Chem. C 2014, 118, 13018−13025

The Journal of Physical Chemistry C

Article

Figure 1. Proposed surface-assisted Ullmann coupling reaction of TBrTP (3,5,3″, 5″-tetrabromo-para-terphenyl) 1 to covalent organic framework 3 via the organometallic framework intermediate 2.

constant current mode. Moderate filtering (Gaussian smooth, background subtraction) has been applied using the WSXM software.34 For the Gaussian smooth, a three-point decay distance was used in both directions. For Figure 2c as an example, this is equivalent to a decay distance of 0.75 Å. Related scale-dependent values result for the other images. The Cu(111) single crystal was purchased from MaTecK, Germany, with an alignment of better than 0.1° with respect to the nominal orientation. Preparation of a clean and structurally well controlled Cu(111) surface was achieved by cycles of bombardment with Ar+ ions (1500 eV) and annealing at 850 K. 3,5,3″,5″-Tetrabromo-para-terphenyl (TBrTP) 1 was synthesized via a palladium-catalyzed Suzuki-cross coupling reaction utilizing 1,4-diiodobenzene and 3,5-dibromophenyl pinacolboronic ester (see the Supporting Information for more details). TBrTP was vapor-deposited from a home-built Knudsen cell evaporator with a Ta crucible held at 430 K. STM images were recorded at a sample temperature of 300 K or at the indicated temperatures. The XPS experiments were performed with a VG MARK II spectrometer using an Mg Kα X-ray source (1253.6 eV). All binding energies were referenced to the Fermi edge of the clean Cu surface (Eb ≡ 0). Unless otherwise indicated, the photoelectrons were detected at an angle of 60° to the surface normal for increased surface sensitivity. The deposition rate of TBrTP was about 0.07 ML per minute (unless indicated otherwise). Coverages were derived from STM images. The coverage of 1 ML (monolayer) denotes a densely packed layer of orthogonal network-like structure formed at 90 K (as shown in Figure 2b). This coverage equals 0.037 TBrTP molecules per surface copper atom. Periodic density functional theory (DFT) calculations were performed with the Vienna ab initio simulation package

reversible intermediate state, it should in principle be possible to obtain well-ordered covalent structures, even though previous attempts in this direction were not particularly successful. In extension of our recent work on 4,4″-dibromometa-terphenyl, which has two linker positions and thus can only form rings or chains,19,30 we used here 3,5,3″,5″tetrabromo-para-terphenyl (TBrTP) with four linker positions to allow for the formation of two-dimensional organometallic and covalent networks. According to previous work, bromoarenes on Cu(111) are stable at low temperatures of up to 170 K.31 Between 170 and 240 K, surface-assisted scission of the C−Br bonds occurs and is followed by the formation of C−Cu−C bonds, which are stable up to at least 450 K.30,32 Above this temperature, the Cu atoms are released and C−C bonds are formed. For TBrTP, we therefore expect a temperature-controlled reaction sequence, as shown in Figure 1: TBrTP 1 should adsorb intact at low temperatures. Warming to 300 K is expected to induce C−Br bond scission and formation of C−Cu−C bonds, which should here lead to a two-dimensional organometallic network of the type 2. At 570 K, we expect formation of C−C bonded twodimensional covalent organic frameworks (COFs) 3. In the following, we will analyze the actual products formed under these conditions using scanning tunneling microscopy and Xray photoemission spectroscopy.

■

EXPERIMENTAL AND COMPUTATIONAL DETAILS The experiments were performed in a two-chamber UHV system, which has been described previously,33 at a background pressure below 10−10 mbar. The scanning tunneling microscope is a SPECS STM 150 Aarhus with SPECS 260 electronics. All voltages refer to the sample and the images were recorded in 13019



Article

appear as a bone-like feature and arrange on the surface in a Ttype pattern, such that the long axis of one molecule is perpendicular to the long axes of its next neighbors. The molecular dimensions with a center-to-center distance of 17.8 ± 0.2 Å between neighboring molecules with the same orientation (see the unit cell in Figure 2c) provide additional proof that the molecules are intact. The DFT-calculated differential charge density plot in Figure 2d, based on a gas-phase model (Figure S1 in the Supporting Information), show decreased electron density at the H atoms, while the Br atoms have both regions with increased and with reduced electron density, in agreement with previous work.37 In Figure 2d, possible halogen and hydrogen bond interactions are marked by dotted lines within one quartet node formed by four neighboring molecules. Note that the concept of halogen bonds is well-established in biological chemistry,38−42 but has only recently found attention in surface science.37,43−46 The effects of the halogen bonds on the supramolecular arrangement observed here is qualitatively very similar to those obtained by Yoon et al. for dibromoanthraquinones on Au(111).37 Organometallic Networks. After post-deposition annealing of the structure in Figure 2 to 300 K, the highly ordered molecular network is converted into the complex network structure shown in Figure 3b, which consists of a small, defectrich island of the organometallic framework 2 with paraterphenyl fragments connected by C−Cu−C bridges. The latter can be identified by their central protrusions, which have been observed in previous work and are related to Cu atoms in C− Cu−C bridges.19,28,30 The structural change is therefore the result of the scission of C−Br bonds and their replacement by C−Cu−C bonds. Similar C−Br bond scission processes of other bromoarenes on Cu(111) at 300 K have previously been reported.9,19,27,28 In addition, the reaction is proven by the Br 3p XP spectra discussed below. The dark features visible in Figure 3b are vacancy islands of monatomic depth. They have previously been observed and it has been proposed that the missing Cu atoms are incorporated into the C−Cu−C bonds.30 A different preparation route for the organometallic networks is the direct deposition of TBrTP onto Cu(111) at 300 K. This approach leads to a similar structure, as shown by Figure 3a. Comparison of Figure 3a and b shows that both preparation conditions result in small islands with dendritic shape. In order to obtain larger islands of the organometallic frameworks with a higher degree of long-range order, TBrTP was deposited onto Cu(111) held at 460 K (Figure 3c). The higher temperature of Cu(111) provides sufficient mobility and availability of the Cu adatoms, which also leads to the absence of vacancy islands. As we have shown previously, no C−C bond formation occurs at this temperature.30 For a closer inspection of the organometallic framework, a magnified view of the red-framed region in Figure 3c is displayed in Figure 3d. The framework contains long and short linkers, which are assigned to (ph)3 and (ph)-Cu-(ph) moieties, respectively. They can be differentiated on the basis of an apparent height profile clockwise along the blue line in Figure 3d. From the starting point (left, position of arrow in the blue line), the height profile curve (Figure 3e) shows peaks of alternating widths and heights, such that a smaller, but wider peak is followed by a taller, but narrower peak. Considering the molecular dimensions, the three wide peaks are associated with (ph)3 moieties (long linker), and the three tall peaks are associated with (ph)-Cu-(ph) moieties (short linker). According to previous reports, the (ph)3 moiety is longer (8.3 Å) than

Figure 2. STM images of the sample prepared by deposition of 0.63 ML TBrTP 1 onto Cu(111) held at 90 K; Imaging at 90 K. (a) Overview STM image of the as-prepared sample; tunneling parameters: U = −3.2 V, I = 0.04 nA. (b) Defect-free region of the molecular organic network structure of TBrTP. (c) Zoom-in STM image of (b) overlaid with a molecular model and the unit cell; U = −2.2 V, I = 0.08 nA. (d) Differential charge density with yellow and green representing increased and decreased electron density, respectively. Possible intermolecular interactions are shown as purple dotted lines (halogen bonds between Br atoms) and green dotted lines (hydrogen bonds between Br and H atoms).

(VASP)35 using the generalized gradient approximation (GGA) functional with the Perdew−Burke−Ernzerhof (PBE) exchange-correlation description.36 Plane-wave basis energy cutoffs of 400 eV were used to expand the valence electronic wave function with valence configuration of Br-4s24p5, C2s22p2, and H-1s1. The force convergence threshold for structure optimization was set to 0.02 eV/Å, and the total energy tolerance for the electronic self-consistent field (SCF) was converged to 1.0 × 10−5 eV. A slab model was adopted to simulate the TBrTP 2D organic network, the lattice parameters were optimized to 16.0 Å × 16.0 Å, which represents the minimum in total energy (see Figure S1 in the Supporting Information). The height of the unit cell was fixed at 15 Å to ensure the interaction perpendicular to 2D organic network plane being negligible. Only the γ-point was used to optimize geometry structures, and a k-point mesh set of 4 × 4 × 1 was adopted to operate SCF calculations.

■

RESULTS AND DISCUSSION Molecular Low-Temperature Phase of TBrTP. Figure 2a shows a large-scale STM image taken after deposition of 0.63 ML TBrTP onto Cu(111) held at 90 K. Under these conditions, the molecules stay intact, which is also confirmed by XPS, as discussed below. The image, taken at 90 K, shows that the surface is partially covered by an orthogonal networklike structure. Figure 2b shows an almost defect-free region of this network. Further structural details are revealed by the magnified STM image in Figure 2c, which is overlaid with a molecular model and a unit cell. Intact TBrTP molecules 13020



Article

Figure 4. Br 3p XP spectra of submonolayer TBrTP (0.77 ML, deposition rate = 0.07 ML/min) on Cu(111) directly after deposition at 90 K and after heating to the indicated temperatures. The spectra labeled 90 K (a) and 240 K (b) were taken at these temperatures, the other spectra (c−e) at 300 K.

(Figure 4a) shows the spectrum of a TBrTP submonolayer (0.77 ML) on Cu(111) at 90 K. The Br 3p3/2 binding energy (BE) of 184.2 eV is indicative of intact C−Br bonds, according to previous work.48,49 After annealing the submonolayer to 240 K (Figure 4b), the Br 3p3/2 peak of TBrTP is located at 182.2 eV. The chemical shift by 2.0 eV indicates a dramatic change in the chemical state of the Br atoms. The direction of the shift toward lower BE is consistent with an increased electron density at the Br atoms, as would be expected by the formation of a bromide-like species. This interpretation is in line with previous work.49 Annealing the sample to higher temperatures did not affect the Br 3p BE, as shown by the spectra c−e in Figure 4, but a slight reduction of the peak intensity was observed, which may be due to the partial desorption of Br. Pore Sizes. To obtain deeper insight into the factors that determine the pore size distribution, high resolution STM images resolving single atoms inside the pores were recorded (Figure 5a−d). Depending on the pore sizes, there are 1, 2, 3, or 4 protrusions enclosed. In agreement with previous work,13,25,26,28−30 these protrusions are assigned to Br atoms, although it cannot be completely excluded that the protrusions stem from Cu adatoms. Figure 5e−h display the proposed adsorption configuration of the four types of pores in Figure 5a−d. The molecular models suggest that the Br atoms are trapped inside in the pores and stabilized by Br···H bonds (marked as red dotted lines). Statistical analysis of a large ensemble of pores (in total, 680 pores, see Figure S2) shows that the number of enclosed adatoms is directly correlated to the size of the nanopores, that is, the pores are always filled with the maximum number of adatoms. This observation can be interpreted in two different ways: first, the adatoms may gain substantial energy by filling a pore and are mobile enough to occupy a pore after it was formed; second, the adatoms occupy the pore already during its formation. (This would be the case if the activation energy for an adatom hopping into a pore after it was formed is so high that this process is very slow.) In this second scenario, the fact that all pores contain the maximum number of adatoms suggests that the number of adatoms determine the pore size by the space they occupy. Intermediate Organometallic/Covalent Network. Deposition of TBrTP onto Cu(111) at 500 K leads to the coexistence of C−Cu−C bonded organometallic and C−C

Figure 3. STM images recorded at 300 K after (a) deposition of 0.28 ML TBrTP onto Cu(111) at 300 K; tunneling parameters: U = −3.0 V, I = 0.02 nA. (b) Deposition of 0.91 ML TBrTP onto Cu(111) at 90 K, followed by postdeposition annealing to 300 K; U = −2.8 V, I = 0.01 nA. (c) Deposition of 0.56 ML TBrTP onto Cu(111) at 460 K; (d) Zoom-in STM image of the red-framed region in (c); U = −3.0 V, I = 0.04 nA. (e) Apparent height profile along the blue hexagon in (d) clockwise from the left. (f) Molecular model overlaid on (d). Gray circles represent carbon atoms; coppery, copper; white, hydrogen.

the (ph)-Cu-(ph) moiety (6.7 Å).27 The length of the short linker marked by the white double-head arrow in Figure 3d is 6.8 ± 0.2 Å, which agrees well with the previously reported length of the (ph)-Cu-(ph) moiety and thus supports our assignment. In addition, the circumference is 3 × 6.7 + 3 × 8.3 = 45.0 Å, as can be seen in Figure 3e, and thus is in perfect agreement with an alternating sequence of (ph)3 and (ph)-Cu(ph) moieties. Based on these considerations, the STM image was overlaid with a molecular model, as shown in Figure 3f. The organometallic framework encloses nanopores with different sizes, which result from different orientations of the (ph)3 units involved in the formation of the nanopores. Another reason for the presence of different pore sizes is the fact that the framework is trapped in a state far from thermodynamic equilibrium.47 X-ray Photoelectron Spectroscopy. C−Br bond scission as the initial step of the reaction between TBrTP and Cu(111) precedes the formation of C−Cu−C bonds. Evidence for the C−Br bond scission is provided by the Br 3p X-ray photoelectron spectra displayed in Figure 4. The black curve 13021



Article

Figure 5. High-resolution STM images of organometallic nanopores enclosing (a) 1, (b) 2, (c) 3, and (d) 4 adatoms (most likely Br atoms). (e−h) Indicate the molecular model of (a−d), respectively. All STM images were taken with the tunneling parameters U = −3.0 V and I = 0.01 nA. Red dotted lines indicate possible Br···H bonds. Gray circles represent carbon atoms; coppery, copper; white, hydrogen; red, bromine.

covalent bonded frameworks, as shown in Figure 6a. A close inspection of this transition regime reveals that some of the C− Cu−C bonds in the organometallic frameworks have already lost Cu atoms and turned into C−C covalent bonded moieties. In Figure 6b, showing a magnified view of the blue framed region in Figure 6a, C−Cu−C bonds can be distinguished through the apparent height profile of the structure, similar to Figure 3e, and by size considerations (see below). The covalently bonded moieties (marked by white circles in Figure 6b) consist of (ph)3 units connected via C−C covalent bonds and are embedded into regions with the organometallic framework. The green circles in Figure 6b mark some intact parts of the organometallic framework. The assignment of the white-circled moieties to C−C covalent bonded structures is further supported by the structural changes: For example, the triangle feature in the inset of Figure 6b clearly shows the absence of the (ph)-Cu-(ph) moiety which was visible in Figure 3d,f. The apparent height profile along the periphery of the triangle also shows smaller height variations compared to that of C−Cu−C bonded moieties. This point will be further discussed below. Features that could be related to (Cu)2-(Ph)3-(Cu)2 units were occasionally observed after deposition at 500 K, as marked by the green arrows in Figure 6c,d. As shown in the structural models (Figure 6e,f), the distance between two neighboring Cu atoms is 4.7 ± 0.1 Å, which is shorter than the distance between two Cu atoms in the Cu-ph-Cu structural element, 6.5 ± 0.1 Å (see Figure 6d). Notably, 4.7 Å equals the typical length of two Cu−Br bonds in Cu(I) complexes,50 suggesting that these Cu atoms are linked by a Br atom, as schematically shown in Figure 6e,f. The (Cu)2-(ph)3-(Cu)2 species are immobile at 300 K (see the STM time sequence in Figure S3), which supports the idea of linkage through Cu−Br−Cu bridges. As shown by Figure 6d,f (the latter is a molecular model corresponding to the red framed region in Figure 6d) six adatoms are enclosed in the pore formed by three (ph)3 units. The space required by these adatoms possibly prevents formation of a smaller pore with shorter C−Cu−C bridges (which would require formal elimination of CuBr from each bridge). The enclosed adatoms may contibute to the stability of the pore and thus also to the stability of the (Cu)2-(ph)3-(Cu)2 moieties.

Figure 6. STM images taken after deposition of 0.56 ML TBrTP molecules onto Cu(111) held at 500 K. (a) Overall STM image with tunneling parameters U = −3.6 V, I = 0.01 nA. (b) Zoom-in of the blue framed region in (a), U = −3.0 V, I = 0.01 nA. (c, d) Two sections of organometallic islands that possibly contain single (Cu)2(Ph)3-(Cu)2 species, U = −3.0 V, I = 0.02 nA. (e, f) Molecular models of the yellow and red framed regions in (c) and (d), respectively. See Figure 5 for the color scheme. The hypothetical Br atoms in the Cu− Br−Cu bridges are shown in pale red.

13022



Article

Covalent Networks. C−C bonded covalent organic frameworks (COFs) are formed when the Cu(111) sample is held at 570 K during deposition of TBrTP (Figure 7a). Besides

Formation of C−C bonds is confirmed by a closer inspection of the pore sizes. In Figure 7c, the corner-to-corner distance marked by a double-head arrow is 4.4 ± 0.3 Å. This is shorter than the 6.8 ± 0.2 Å measured above for the (ph)-Cu-(ph) moiety and agrees well with the literature value for the length of a (ph)2 moiety, 4.2 Å.27 We therefore conclude that the C− Cu−C bonds are converted into C−C bonds, as illustrated by the overlaid molecular model. Figure 7d displays an “8”-shaped species containing five (ph)3 moieties. The corner-to-corner distance of 4.5 ± 0.3 Å, as marked by the arrow, is almost identical to that in Figure 7c and thus demonstrates that the hexagons are also formed by (ph)3 moieties connected via C−C bonds rather than C−Cu−C bonds (compare Figure 3d for the C−Cu−C bonded counterpart). Figure 7e gives the apparent height profile clockwise along the blue line in the inset; a smaller variation of the height can be observed compared to the curve in Figure 3e for the organometallic hexagon in Figure 3d. This again confirms that the Cu atoms have been released from the C− Cu−C bonds and that C−C bonds have been formed. Figure 7c also reveals that the (ph)3 moieties have different appearent heights, which suggests that not all of them are coplanar. Specifically, in Figure 7c,f, the (ph)3 moieties marked by red arrows possess lower and more uniform apparent heights than those marked by green arrows. The protruding moieties (green arrows) are proposed to originate from the outof-plane rotation of the center para-phenylene unit. The reason for this rotation is most likely the relief of ring strain, which arises from the repulsion between neighboring H atoms inside the ring. In contrast, the para-phenylene units pointed by red arrows are oriented parallel to the surface. The protrusions (green arrows) show two weak maxima, which are assigned to the two H atoms of the rotated para-phenylene unit, as is illustrated by the molecular model superimposed in Figure 7c. This double-protrusion is also visible in the line scan in Figure 7g, where double-peaks appear around 5 and 30 Å distance. The flat area between 15 and 25 Å in Figure 7g is associated with a flat-lying phenylene unit. Similar protrusions have been reported previously for cyclohexaphenylene on Cu(111) and have been explained with phenylene moieties tilted out of the molecular plane to reduce steric repulsion.6 Further inspection of Figure 7b, which shows a magnified view of the green-framed region in Figure 7a, reveals a (√3 × √3) R30° structure with a hexagonal unit cell (marked in white) surrounding the patches of the covalent network. Since Br atoms are known to form such a structure on Cu(111),51 we assign this feature to close-packed islands of Br formed by C− Br bond scission. These Br islands may also limit the agglomeration of small COF patches to larger well-ordered areas.

Figure 7. (a) STM image taken after deposition of 0.56 ML TBrTP onto Cu(111) held at 570 K. (b) Magnified view of the green framed region in (a) with unit cell (white) and the Br adatom structure (marked by blue arrow). (c) A section of ordered covalent bonded organic network, partially overlaid with molecular model. Tunneling parameters: U = −3.0 V, I = 0.03 nA. (d) An “8”-shaped network fragment consisting of five (ph)3 fragments connected through C−C bonds. (e) Apparent height profile clockwise along the blue hexagon in inset cut out from (d). (f) A similar unit as in (d) with different adsorption configuration. (g) Apparent height profile clockwise along blue hexagon in inset cut out from (f). Note that the height axes in (e) and (g) are identical to those in Figure 3e to allow direct comparison.

■

CONCLUSIONS We have demonstrated that 3,5,3″,5″-tetrabromo-para-terphenyl (TBrTP) is a versatile precursor for the surface-assisted fabrication of halogen-bonded, organometallic and covalent two-dimensional networks. TBrTP adsorbs on Cu(111) at 90 K without dissociation. The intact molecules form a well-ordered structures with a near-square unit-cell of 17.2 Å × 17.8 Å and a T-type arrangement, which is stabilized by Br···Br halogen bonds and Br···H hydrogen bonds. Annealing the intact TBrTP layer to 300 K or deposition of TBrTP at 300 K leads to the scission of C−Br bonds and the formation of organometallic networks with C−Cu−C bonds. The organometallic network

the dominating octaphenylene pores that are shown in Figure 1, the rather small islands also contain triangles and chains. The emergence of different structural motifs is attributed to the two possible orientations of the (ph)3 units during C−C bond formation, with 60° and 180° between long axes of two connected (ph)3 units, leading to the coexistence of pores with trigonal (60°) and tetragonal (180°) symmetry. 13023



Article

(4) Champness, N. R. Building with Molecules. Nat. Nanotechnol. 2007, 2, 671−672. (5) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; et al. Atomically Precise Bottom-up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470−473. (6) Treier, M.; Pignedoli, C. A.; Laino, T.; Rieger, R.; Muellen, K.; Passerone, D.; Fasel, R. Surface-Assisted Cyclodehydrogenation Provides a Synthetic Route Towards Easily Processable and Chemically Tailored Nanographenes. Nat. Chem. 2011, 3, 61−67. (7) Palma, C.-A.; Samori, P. Blueprinting Macromolecular Electronics. Nat. Chem. 2011, 3, 431−436. (8) Martin-Gago, J. A. Polycyclic Aromatics on-Surface Molecular Engineering. Nat. Chem. 2011, 3, 11−12. (9) Bieri, M.; Nguyen, M. T.; Groning, O.; Cai, J. M.; Treier, M.; AitMansour, K.; Ruffieux, P.; Pignedoli, C. A.; Passerone, D.; Kastler, M.; et al. Two-Dimensional Polymer Formation on Surfaces: Insight into the Roles of Precursor Mobility and Reactivity. J. Am. Chem. Soc. 2010, 132, 16669−16676. (10) de Heer, W. A.; Berger, C.; Wu, X.; First, P. N.; Conrad, E. H.; Li, X.; Li, T.; Sprinkle, M.; Hass, J.; Sadowski, M. L.; et al. Epitaxial Graphene. Solid State Commun. 2007, 143, 92−100. (11) Batzill, M. The Surface Science of Graphene: Metal Interfaces, Cvd Synthesis, Nanoribbons, Chemical Modifications, and Defects. Surf. Sci. Rep. 2012, 67, 83−115. (12) Gao, L.; Guest, J. R.; Guisinger, N. P. Epitaxial Graphene on Cu(111). Nano Lett. 2010, 10, 3512−3516. (13) Lipton-Duffin, J. A.; Ivasenko, O.; Perepichka, D. F.; Rosei, F. Synthesis of Polyphenylene Molecular Wires by Surface-Confined Polymerization. Small 2009, 5, 592−597. (14) Bieri, M.; Treier, M.; Cai, J.; Ait-Mansour, K.; Ruffieux, P.; Groening, O.; Groening, P.; Kastler, M.; Rieger, R.; Feng, X.; et al. Porous Graphenes: Two-Dimensional Polymer Synthesis with Atomic Precision. Chem. Commun. 2009, 6919−6921. (15) Blunt, M. O.; Russell, J. C.; Champness, N. R.; Beton, P. H. Templating Molecular Adsorption Using a Covalent Organic Framework. Chem. Commun. 2010, 46, 7157−7159. (16) Gutzler, R.; Walch, H.; Eder, G.; Kloft, S.; Heckl, W. M.; Lackinger, M. Surface Mediated Synthesis of 2d Covalent Organic Frameworks: 1,3,5-Tris(4-Bromophenyl)Benzene on Graphite(001), Cu(111), and Ag(110). Chem. Commun. 2009, 4456−4458. (17) Faury, T.; Clair, S.; Abel, M.; Dumur, F.; Gigmes, D.; Porte, L. Sequential Linking to Control Growth of a Surface Covalent Organic Framework. J. Phys. Chem. C 2012, 116, 4819−4823. (18) Lin, T.; Shang, X. S.; Adisoejoso, J.; Liu, P. N.; Lin, N. Steering on-Surface Polymerization with Metal-Directed Template. J. Am. Chem. Soc. 2013, 135, 3576−82. (19) Fan, Q.; Wang, C.; Han, Y.; Zhu, J.; Hieringer, W.; Kuttner, J.; Hilt, G.; Gottfried, J. M. Surface-Assisted Organic Synthesis of Hyperbenzene Nanotroughs. Angew. Chem., Int. Ed. 2013, 52, 4668− 72. (20) Bartels, L. Tailoring Molecular Layers at Metal Surfaces. Nat. Chem. 2010, 2, 87−95. (21) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671−679. (22) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Molecular and Supramolecular Networks on Surfaces: From Two-Dimensional Crystal Engineering to Reactivity. Angew. Chem., Int. Ed. 2009, 48, 7298−7332. (23) Li, Y.; Xiao, J.; Shubina, T. E.; Chen, M.; Shi, Z.; Schmid, M.; Steinrueck, H.-P.; Gottfried, J. M.; Lin, N. Coordination and Metalation Bifunctionality of Cu with 5,10,15,20-Tetra(4-Pyridyl)Porphyrin: Toward a Mixed-Valence Two-Dimensional Coordination Network. J. Am. Chem. Soc. 2012, 134, 6401−6408. (24) Buchner, F.; Flechtner, K.; Bai, Y.; Zillner, E.; Kellner, I.; Steinrueck, H.-P.; Marbach, H.; Gottfried, J. M. Coordination of Iron Atoms by Tetraphenylporphyrin Monolayers and Multilayers on Ag(111) and Formation of Iron-Tetraphenylporphyrin. J. Phys. Chem. C 2008, 112, 15458−15465.

consists of nanopores with varying sizes. Each nanopore encloses adatoms, most likely, Br atoms, whose number is directly correlated to (and possibly determines) the size of the pore. Deposition of TBrTP at 500 K results in the formation of larger islands of networks in which the (Ph)3 units are linked by C−Cu−C bonds and C−C bonds, that is, these networks represent a transition regime to the fully C−C bonded covalent organic frameworks (COFs) obtained at higher temperatures. These COFs are formed during the deposition of TBrTP onto Cu(111) at 570 K and consist of small islands with various structural motifs, resulting from the two possible relative orientations of C−C connected (ph)3 moieties (60 and 180°). The C−C bonded (ph)3 moieties are not all coplanar but contain phenylene units that are tilted relative to the surface plane, most likely due to steric hindrance between the C−H bonds inside the pores. The systematic study of three different bonding regimes with increasing bond energies, hydrogen/ halogen bonds, organometallic bonds, and covalent bonds, illustrates how the degree of long-range order decreases when the bond strength between the subunits increases. The formation of ordered organometallic and covalent structures is most likely also affected by adsorbed Br atoms, which appear to be a limiting factor for the successful surface Ullmann synthesis of expanded covalent networks.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedure for the synthesis of 3,5,3″,5″tetrabromo-para-terphenyl, structural model for the molecular layer, statistics of the number of the Br atoms enclosed by pores, and sequences of STM images. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 0086-551-63602064. *E-mail: [email protected]. Tel.: +496421 2822-541. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS J.F.Z. acknowledges the financial support from the National Basic Research Program of China (2010CB923302, 2013CB834605), the National Natural Science Foundation of China (Grant No. 21173200), and the Specialized Research Fund for the Doctoral Program of Higher Education of Ministry of Education (Grant No. 20113402110029). J.M.G. thanks the Chinese Academy of Sciences for a Visiting Professorship for Senior International Scientists (Grant No. 2011T2J33).

■

REFERENCES

(1) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nano-Architectures by Covalent Assembly of Molecular Building Blocks. Nat. Nanotechnol. 2007, 2, 687−691. (2) Champness, N. R. Surface Chemistry Making the Right Connections. Nat. Chem. 2012, 4, 149−150. (3) Lafferentz, L.; Eberhardt, V.; Dri, C.; Africh, C.; Comelli, G.; Esch, F.; Hecht, S.; Grill, L. Controlling on-Surface Polymerization by Hierarchical and Substrate-Directed Growth. Nat. Chem. 2012, 4, 215−220. 13024



Article

Dimensional Porous Networks on Ag(111). J. Phys. Chem. C 2013, 117, 302−306. (45) Gutzler, R.; Fu, C. Y.; Dadvand, A.; Hua, Y.; MacLeod, J. M.; Rosei, F.; Perepichka, D. F. Halogen Bonds in 2d Supramolecular SelfAssembly of Organic Semiconductors. Nanoscale 2012, 4, 5965−5971. (46) Abel, M.; Oison, V.; Koudia, M.; Maurel, C.; Katan, C.; Porte, L. Designing a New Two-Dimensional Molecular Layout by Hydrogen Bonding. ChemPhysChem 2006, 7, 82−85. (47) Whitelam, S.; Tamblyn, I.; Haxton, T. K.; Wieland, M. B.; Champness, N. R.; Garrahan, J. P.; Beton, P. H. Common Physical Framework Explains Phase Behavior and Dynamics of Atomic, Molecular, and Polymeric Network Formers. Phys. Rev. X 2014, 4, 011044. (48) Srivastava, S.; Badrinarayanan, S.; Mukhedkar, A. J. X-ray Photoelectron-Spectra of Metal-Complexes of Substituted 2,4Pentanediones. Polyhedron 1985, 4, 409−414. (49) Chen, M.; Xiao, J.; Steinrück, H.-P.; Wang, S.; Wang, W.; Lin, N.; Hieringer, W.; Gottfried, J. M. Combined Photoemission and Scanning Tunneling Microscopy Study of the Surface-Assisted Ullmann Coupling Reaction. J. Phys. Chem. C 2014, 118, 6820−6830. (50) Bowmaker, G. A.; Skelton, B. W.; White, A. H.; Healy, P. C. Synthesis and Characterization of the Novel Copper(I) Anion Cu(Pph3)Br2. J. Chem. Soc., Dalton Trans. 1988, 2825−2830. (51) Nanayakkara, S. U.; Sykes, E. C. H.; Fernandez-Torres, L. C.; Blake, M. M.; Weiss, P. S. Long-Range Electronic Interactions at a High Temperature: Bromine Adatom Islands on Cu(111). Phys. Rev. Lett. 2007, 98, 206108.

(25) Di Giovannantonio, M. E. G. M.; Lipton-Duffin, J.; Meunier, V.; Cardenas, L.; Fagot Revurat, Y.; Cossaro, A.; Verdini, A.; Perepichka, D. F.; Rosei, F.; Contini, G. Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization. ACS Nano 2013, 7, 9. (26) Park, J.; Kim, K. Y.; Chung, K.-H.; Yoon, J. K.; Kim, H.; Han, S.; Kahng, S.-J. Interchain Interactions Mediated by Br Adsorbates in Arrays of Metal-Organic Hybrid Chains on Ag(111). J. Phys. Chem. C 2011, 115, 14834−14838. (27) Walch, H.; Gutzler, R.; Sirtl, T.; Eder, G.; Lackinger, M. Material- and Orientation-Dependent Reactivity for Heterogeneously Catalyzed Carbon−Bromine Bond Homolysis. J. Phys. Chem. C 2010, 114, 12604−12609. (28) Wang, W. H.; Shi, X. Q.; Wang, S. Y.; Van Hove, M. A.; Lin, N. Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction. J. Am. Chem. Soc. 2011, 133, 13264−13267. (29) Chung, K.-H.; Koo, B.-G.; Kim, H.; Yoon, J. K.; Kim, J.-H.; Kwon, Y.-K.; Kahng, S.-J. Electronic Structures of One-Dimensional Metal-Molecule Hybrid Chains Studied Using Scanning Tunneling Microscopy and Density Functional Theory. Phys. Chem. Chem. Phys. 2012, 14, 7304−7308. (30) Fan, Q.; Wang, C.; Han, Y.; Zhu, J.; Kuttner, J.; Hilt, G.; Gottfried, J. M. Surface-Assisted Formation, Assembly, and Dynamics of Planar Organometallic Macrocycles and Zigzag Shaped Polymer Chains with C−Cu−C Bonds. ACS Nano 2014, 8, 709−718. (31) Xi, M.; Bent, B. E. Mechanisms of the Ullmann Coupling Reaction in Adsorbed Monolayers. J. Am. Chem. Soc. 1993, 115, 7426− 7433. (32) Villagomez, C. J.; Sasaki, T.; Tour, J. M.; Grill, L. Bottom-up Assembly of Molecular Wagons on a Surface. J. Am. Chem. Soc. 2010, 132, 16848−16854. (33) Chen, M.; Feng, X.; Zhang, L.; Ju, H.; Xu, Q.; Zhu, J.; Gottfried, J. M.; Ibrahim, K.; Qian, H.; Wang, J. Direct Synthesis of Nickel(II) Tetraphenylporphyrin and Its Interaction with a Au(111) Surface: A Comprehensive Study. J. Phys. Chem. C 2010, 114, 9908−9916. (34) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. Wsxm: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (35) Kresse, G.; Hafner, J. Vienna Ab Initio Simulation Package (Vasp), Version 5.2.; Institut für Materialphysik, Universität Wien: Vienna, 1991, www.vasp.at. (36) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (37) Yoon, J. K.; Son, W.-j.; Chung, K.-H.; Kim, H.; Han, S.; Kahng, S.-J. Visualizing Halogen Bonds in Planar Supramolecular Systems. J. Phys. Chem. C 2011, 115, 2297−2301. (38) Hassel, O. Structural Aspects of Interatomic Charge-Transfer Bonding. Science 1970, 170, 497−502. (39) Desiraju, G. R.; Parthasarathy, R. The Nature of Halogen··· Halogen Interactions: Are Short Halogen Contacts Due to Specific Attractive Forces or Due to Close Packing of Nonspherical Atoms. J. Am. Chem. Soc. 1989, 111, 8725−8726. (40) Metrangolo, P.; Resnati, G. Halogen Bonding: A Paradigm in Supramolecular Chemistry. Chem.−Eur. J. 2001, 7, 2511−2519. (41) Voth, A. R.; Hays, F. A.; Ho, P. S. Directing Macromolecular Conformation through Halogen Bonds. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6188−6193. (42) Politzer, P.; Murray, J. S.; Clark, T. Halogen Bonding and Other [Sigma]-Hole Interactions: A Perspective. Phys. Chem. Chem. Phys. 2013, 15, 11178−11189. (43) Chung, K.-H.; Park, J.; Kim, K. Y.; Yoon, J. K.; Kim, H.; Han, S.; Kahng, S.-J. Polymorphic Porous Supramolecular Networks Mediated by Halogen Bonds on Ag(111). Chem. Commun. 2011, 47, 11492− 11494. (44) Chung, K.-H.; Kim, H.; Jang, W. J.; Yoon, J. K.; Kahng, S.-J.; Lee, J.; Han, S. Molecular Multistate Systems Formed in Two13025


Covalent, Organometallic, and Halogen-Bonded Nanomeshes from

Recommend Documents