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Competition Between Dehydrogenative Organometallic Bonding and Covalent Coupling Reactions of an Unfunctionalized Porphyrin on Cu (111) Feifei Xiang, Anja Gemeinhardt, and M. Alexander Schneider ACS Nano, Just Accepted Manuscript • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Competition Between Dehydrogenative Organometallic Bonding and Covalent Coupling of an Unfunctionalized Porphyrin on Cu (111) Feifei Xiang, Anja Gemeinhardt, M. Alexander Schneider * Solid State Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Staudtstr. 7, 91058 Erlangen, Germany

KEYWORDS: scanning tunneling microscopy, on-surface synthesis, polymerization, molecular wire, metalation.

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ABSTRACT

We studied the formation of linked porphyrin oligomers from 5, 15-diphenylporphyrin (2H-DPP) by thermal, substrate-assisted organometallic and dehydrogenation coupling on Cu (111) by scanning tunneling microscopy. In the range of 300 K-620 K, we find three distinct stages, at 300 K, the intact 2H-DPP molecules self-assemble into linear structures held together by van der Waals forces. Increasing the substrate temperature, self-metalation and intramolecular ring closing reactions result in planar and isolated DPP species on the surface. By C-H cleavage porphyrin oligomers bonded by organometallic and covalent bonds between the modified DPP are formed. The amount of covalently bonded DPP oligomers increases strongly with annealing time and temperature and they become the dominant species at 570 K. In contrast, the number of organometallically bonded DPP oligomers increases moderately even up to 620 K indicating that in this case the organometallic bond is no precursor of the covalent bond.


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On-surface synthesis is a versatile approach for constructing stable molecular structures that can be used e.g. as nano templates1–7 and tunable conductors. 8–13 Different from traditional wet chemistry, on-surface synthesis proceeds on two-dimensional (2D) solid surfaces in ultra-high vacuum (UHV). This is a promising strategy for insoluble molecules or products that are much easier to synthesize via the 2D confinement effect.14-16 By means of various surface science methods as well as density functional theory (DFT), many on-surface synthesized systems were well studied including their adsorption properties, 17–19 reaction mechanisms, 20,21 and electronic properties22–26 of precursors, intermediates and reaction products. Conjugated porphyrin wires are prominent examples of conducting wires due to their unique electronic, optical and magnetic properties. There are mainly three ways to produce them: 27 first, via π-bridge linkers such as butadiyne, ethyne, and ethene. Second, by fusing a benzene ring between the macrocycle pyrrole subunits of porphyrin molecules. And third, by direct C-C bond formation between porphyrin marcocycles (via meso-meso, β-β or meso-β). The third type can be achieved by dehydrogenation coupling under UHV conditions without adding extra functional groups to the macrocycles of porphyrin molecules,28 and hence, is ideal for fabricating conjugate porphyrin wires since it leaves no byproducts to contaminate the surface.29 Doping metal ions into porphyrin wires may improve the conductivity and change its magnetic properties.30,31 This may be achieved by organometallic reactions introducing metal atoms either into the macrocycle or as a link between porphyrin molecules. In the latter case however, the factors influencing the co-existence of organometallic and dehydrogenated coupling reactions are still not very clear. Herein, we investigate the steps of thermally induced on-surface reactions of 2H-DPP on the Cu (111) surface by which porphyrin wires are formed. We use Scanning Tunneling Microscopy (STM) to determine the occurrence of various molecular configurations on the surface in the


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temperature ranging from 300 to 620 K. Our experiments reveal that – unlike the cases reported on other surfaces where only either the organometallic or covalent bonding motif has been observed – on Cu (111) both bonds coexist at temperatures in excess of 530 K. We observe a strong preference of dehydrogenative over organometallic intermolecular coupling at elevated annealing temperatures.


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Figure 1. Self-assembly of 2H-DPP on Cu (111) after deposition at RT. (a) large area STM image, linear self-assembly structures reflect the threefold symmetry of the substrate. U=-1.6V, I=0.35nA. (b) Highly resolved STM image showing that the bright protrusions on the chains are formed by the phenyl rings of two neighboring molecules. The axes connecting the phenyl groups of one molecule are indicated on the boxed wires. The circled molecule with grey color in the upper right has lost one of its two phenyl rings. U=-0.3V, I=0.35nA. (c) Model of the two enantiomers of the 2H-DPP self-assembly structure resulting from a DFT calculation (see


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supplementary section 5). (d) STM image and the approximated configuration of a single 2HDPP. RESULTS AND DISCUSSION Figure 1(a) shows a large-area STM image of the self-assembly of 2H-DPP on Cu (111) after the deposition at 300 K. 2H-DPP molecules form one-dimensional chain-like structures growing along the three equivalent ሾ112തሿ directions. In each growth direction, the self-assembled chains show two arrangements (marked by blue and green rectangular frames) having mirror symmetry to each other along ሾ112തሿ or equivalent directions. A single 2H-DPP molecule, like the one marked by a yellow circle in Figure 1(b), is imaged as an oval shape with a dim center, two bright protrusions along its major axis connecting to a “square doughnut”. These features are assigned to the phenyl rings and the porphyrin macrocycle respectively as has been established on other surfaces.32 Submolecular resolution of single 2H-DPP molecules and their self-assembly structure allows us to understand the chains’ packing style. The model derived from experiment and DFT calculations is shown in Figure 1(c). At 300 K, 2H-DPP molecules remain unmetalated and adsorb on bridge sites of the Cu surface.33,34 The DFT calculations also indicate that the 2HDPP molecules are packing head to tail with each other by van der Waals interaction between a phenyl ring and the macrocycle of neighboring molecules (see supplementary section 1). The appearance of a mirror symmetric configuration is due to the two hydrogen atoms inside the 2HDPP porphyrin core that can switch their positions to orthogonal iminic nitrogens.35 Some halfdecomposed 2H-DPP molecules (hd-2H-DPP) with only one phenyl ring (marked by grey circle in Figure 1(b)) are also found to coexist with intact 2H-DPP molecules. When they participate in the self-assembly they terminate the chains as expected for an intermolecular interaction via the phenyl rings. The frequency of occurrence of hd-2H-DPP does not change with substrate


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temperature, indicating that these species already lose one of their phenyl rings before they land on the Cu surface.


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Figure 2. Products of intramolecular dehydrogenation coupling at 460K. (a) STM image of reaction products. The species align to the threefold symmetry of the substrate. U=-1.40 V, I=0.35nA. (b) Highly resolved STM images of different planar DPP configurations found on the surface and the reaction scheme for intramolecular dehydrogenation coupling of 2H-DPP. (c) Frequency of occurrence of the planar DPP (pDPP) configurations on the surface. The error bar is given by the sampling error.


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Metalation and intramolecular dehydrogenation reactions of 2H-DPP are triggered at T≥ 380 K and finished at 460 K (Figure 2(a)). At the center of the DPP, a bright protrusion appears indicating that a Cu atom has entered the macrocycle and coordinates with the four nitrogen atoms.32 The two phenyl rings form C-C bonds with the macrocycle of a DPP molecule via a dehydrogenation coupling process. The intramolecular ring-closing reaction results in a new, flat DPP species on the surface with vanishing intermolecular van der Waals interaction (see supplementary section 1). As the result, the planar DPP molecules remain isolated entities on the surface. They occur as enantiomeric trans- (trans-1 and trans-2) and cis-isomers (Figure 2(b)). On Cu (111), we observe a ratio of trans- to cis- isomers of about 6:4 as shown in Figure 2(c). Similar intramolecular reactions of porphyrin molecules were also observed on the Ag (111) surface,36,37 but there the cis-isomers are greatly suppressed. Assuming sequential ring closing, the data on Cu (111) indicates that when the second phenyl forms the C-C bond the route towards a trans-configuration is favored with a probability of only 60%. We suggest that this is a consequence of the molecule-surface interaction that is stronger on Cu (111) than on Ag (111) and hence allows stabilization of the more geometrically distorted cis- configuration. These metalated, planar DPP species, designated as pDPP in the following, act as the fundamental building blocks for intermolecular surface reactions on the surface occurring at higher temperatures.


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Figure 3. Intermolecular reactions of planar DPP (pDPP). STM images acquired after heating the sample for 15 min to 530K (a), 570K (b) and 620K (c) respectively. (a) U=-1.46 V, I=0.35nA; (b) U=-1.5 V, I=0.35nA; (c) U=-1.45 V, I=0.35nA; (d) Height profile of the pDPP oligomer marked by the blue line in (b) from top to down; (e) length measurement of the pDPP oligomer marked by the green line in (c) from top to down. (d) and (e) include models illustrating the proposed configuration of the oligomers.

Intermolecular reactions between pDPP species start at 530 K (Figure 3(a)), when the first few oligomers are formed. Two different types of pDPP oligomers are found on the surface, one of which is linked by C-C covalent bonds (marked by a blue circle in Figure 3(a)) and the other is connected by C-Cu-C organometallic bonds (marked by a green circle in Figure 3(a)). The Cu


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link is imaged as a bright protrusion in STM which allows easy distinction from the C-C covalently bonded oligomers where the bond has the same contrast as other parts of the molecule (see also supplementary section 4). As the annealing temperature is increased, the number of oligomers increases, as well as the size of the oligomer (defined as the number of pDPP units per oligomer). At the elevated temperature, oligomers incorporating both types of links are found on the surface (Figure 3(b) marked by yellow circle) serving as a direct height and bond length comparison between C-Cu-C and C-C bonds in one oligomer. The height profile of the oligomer marked by the yellow circle in Figure 3 (b) is shown in Figure 3 (d), a Cu link is imaged 0.3Å higher than a C-C link. An analysis of the width of pDPP units with triple C-Cu-C and C-C bonds is shown in Figure 3(e) (the line cut is taken from Figure 3 (c)). The center to center distance between Cu linked pDPP unit is found to be 10.8 Å while that of C-C linked pDPP units is only 8.6 Å. Both of the values are consistent with the calculated and experimental values published previously,28,29,38 further confirming the two different ways of fusing DPP molecules on the Cu (111) surface. The covalently bonded oligomer is 4% shorter than the distance between ideal adsorption sites along the ሾ112തሿ direction of the substrate, while the organometallic oligomer is 3% shorter along a direction slightly off the ሾ112തሿ direction.

Therefore Cu (111) can accommodate both species equally well. It is apparent that the geometrical constraint exerted by the substrate is not strong enough to produce well-ordered oligomer structures. For that to occur, the variability of the binding sites between pDPP must be suppressed. Instead, many different coupling geometries between molecules are found (see Fig. 3(b), rectangular frame).


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Figure 4. The frequency of occurrence of Cu linked and C-C covalent bonded pDPP reaction sides. (a) The chemical structure of a (trans-)pDPP molecule. The hydrogen atoms are numbered and sorted into different categories highlighted by different colors. (b) Frequency of occurrence of pDPP-pDPP links with single, double, or triple C-Cu-C bonds as a function of annealing temperature. The different oligomer configurations are identified by their appearance in STM as shown in the diagram above. (c) as (b) but now comparing the frequency of occurrence of unreacted, C-C and total Cu pDPP-pDPP links as a function of annealing temperature.

A quantitative analysis of both, the organometallic and the dehydrogenation coupling reactions after applying different annealing temperatures for 15 minutes is shown in Figure 4. We found that the C-H bonds at different sites of the pDPP molecule show different chemical activities towards forming intermolecular bonds. Organometallic reactions only occur at 6-8 and 14-16 CH positions (Figure 4(a)). Dehydrogenation coupling between pDPP molecules occurs also mainly at these sites but there are few pDPP molecules that form intermolecular C-C bonds via their phenyl ring at sites 1-3 and 9-11. Therefore organometallic and dehydrogenation coupling


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compete at the pDPP macrocycle sides (under orange shadow in Figure 4(a)). The sites 4, 5, 12, and 13 do not participate in forming intermolecular bonds because of steric hindrance. In the following, we count and relate the number of formed links between molecules at the macrocycle sides to the maximum number of links that can occur in a set of N ≈ 500 molecules (see Experimental Methods). Figure 4 (b) presents the occurrence of organometallic bonds of different orders (1, 2, 3Cu) between pDPP units as a function of annealing temperature. At 530 K, 4±1% of all possible links are formed by a single organometallic bond (the majority of the molecules remain unreacted), the percentage of singly Cu linked pDPP species reduces to 2±1% and 0% after annealing at 570 K and 620 K respectively. At the same time, the numbers of doubly Cu linked (2Cu) and triply Cu linked (3Cu) pDPP species start to increase and outnumber the singly Cu linked species. The relative number of organometallic pDPP-pDPP links of any order gradually increases from 530 K to 620 K reaching 8% of all possible links at 620 K, indicating that the creation of additional C-Cu-C bonds between pDPP molecules requires thermal activation. In the studies of S. Haq, et al.29 and F. Hanke, et al.39 such an activation barrier was not observed for Cu-DPP on Cu (110). Organometallic bond formation via cleaving C-X (X=halogen, mainly Br) has also been observed between prophyrins on noble metal surfaces.1,40–43 Here, less energy is required for breaking the C-X bonds20,44 and hence the organometallic bond is found at lower annealing temperatures. However, it was observed that the C-Cu-C bonds between a variety of molecules on Cu (111) will dissolve and form C-C bonds at temperatures below 500 K.45,46 Here in our case, the Cu linked species remain on the surface up to 620 K without breaking. The evolution of the relative total numbers of unreacted, Cu linked and C-C covalently bonded pDPP species as a function of annealing temperature is shown in Figure 4(c). At 460K no


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intermolecular bonds are formed yet. Both the organometallic reaction and the dehydrogenation coupling start at the temperature interval up to 530K. At that temperature, the dominating species are isolated pDPP still, and only 6±1% of all possible links are formed, the number of Cu linked pDPP molecules is slightly larger than that of C-C bonded. The amount of covalently C-C bonded pDPP species is greatly boosted from 2±1% to 71±3% in the temperature interval between 530 K and 570 K. They become the dominant species on the surface, while 25±3% of the pDPP macrocycle sides remain unreacted indicating that the porphyrin ribbons are relatively short. At elevated temperatures of up to 620K, the number of unreacted pDPP macrocycle sides drastically drops to 7±1% and the formation of covalent pDPP-pDPP links reaches 86±3%, illustrating that much longer and covalently bonded porphyrin tapes are synthesized on the surface while organometallic bonds contribute only about 8%. Therefore, the covalent bonded pDPP oligomers become the dominant species as the annealing temperature increases.


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Table 1. Yield of Cu linked and C-C covalently linked DPP species at different thermal treatments at 570 K. Sample 1: 2H-DPP molecules were deposited on the surface at 570 K (deposition time 5 min). Sample 2: 2H-DPP molecules were deposited for 5 min on the surface kept at 570 K, then the deposition was stopped and the sample kept at 570 K for another 10 min. Sample 3: 2H-DPP molecules were deposited on the surface kept at RT and subsequently heated to 570 K for 15 min. The values in the table are given in percentage of the total number of possible intermolecular links.

This led us to investigate if the amount of Cu linked oligomers can be influenced by changing the experimental approach. First, we simply extended the annealing time of the sample at 530 K from 15 min to 1 hour. However, the amount of Cu linked pDPP stayed relatively constant at 3±1%. Therefore, extending the annealing time does not increase the formation of Cu linked pDPP molecules at that temperature. As an alternative approach, we deposited 2H-DPP molecules directly onto a substrate held at 570K. The deposition time was around 5 min in order to have the same molecular coverage as before (Sample 1, Table 1). Sample 2 was prepared by depositing the 2H-DPP on the substrate kept at 570 K and thereafter the sample held at the same temperature for another 10 mins. For comparison we quote sample 3 (data also shown in Fig. 3) where the molecules were deposited at 300 K and annealed for 15 mins at 570 K (Sample 3, Table 1). The percentage of organometallic pDPP links is the same for all three samples within


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the error margins, indicating that the formation of organometallic bonds saturates at 570 K in less than 5 minutes. In contrast, the formation of C-C covalent bonds between pDPP requires more time, notwithstanding that the main factor affecting the formation of C-C covalent bonded pDPP oligomers is the annealing temperature rather than annealing time. When depositing at RT and annealing the sample at 530 K the number of covalently bonded pDPP species is enhanced from 2±1% (15 mins annealing, Fig. 4(c)) to 13±3% (60 mins annealing, not shown), while it reaches 40±3% when heating the sample to 570K for only 5 mins and 71±3% for 15 mins (Table1). We also investigated the potential influence of molecular coverage. Three samples with 0.17 ML, 0.35 ML (Sample 3 in Table 1) and 0.55 ML were prepared at room temperature and then heated to 570 K for 15 min respectively. Compared to the analysis above increasing the molecular coverage by 60% does not change the ratio between the reaction products. However, for the sample with low molecular coverage (0.17 ML), the relative occurrence of Cu links increased to 14%, which is nearly three times larger than that of the higher coverage samples. Concomitantly, the relative number of covalent links reduces to 57±3%, but covalently bonded oligomers are still the dominant reaction product on the surface. Similar experiments have been conducted by other groups with porphyrins on Ag (111)28,32 and 2H-DPP on Cu (110). 29 On the Cu (110) surface, phenyl dehydrogenative ring closing of 2HDPP molecules was not observed and only purely Cu linked porphyrin chains which grow along the [001] direction are formed. The oligomer geometry perfectly matches the surface lattice. Along the orthogonal direction the Cu link becomes less favorable suppressing e.g. 2D growth of Cu-linked supramolecular porphine structures.39 In contrast, on Ag (111) 2H-DPP molecules go through an intramolecular dehydrogenation and ring closure but only at relative high temperatures of 530 K leaving the molecules still as monomers.36 Experiments with porphine


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molecules on Ag(111) show that the bare macrocycle almost exclusively fuses covalently beginning at 570 K. Considering the geometry of the Ag(111) substrate and the larger C-Ag-C bond length43 compared to C-Cu-C we also find less than 3% mismatch for both, the formation of organometallic or covalent oligomers along the [110] direction of Ag(111). Different molecular systems also show the similar intermolecular reaction selection on the substrates with different lattice parameters. 2,5-diethynyl-1,4-bis(phenylethynyl)-Benzene forms C-C bonded chains on Ag (111) while it only forms organometallic chains on (110) and (100) surfaces due to the very strong lattice matching of organometallic chain periodicity and surface lattice of 110 and 100 surfaces.47 Combining the results of these studies we see that only in the case of very strong geometrical bias as in the case of the Cu (110) surface can the outcome of intermolecular coupling be steered by substrate structure. If the bias is not strong like in the present case on Cu (111) (see section 3 in Supporting Information) organometallic and covalent intermolecular bonding occurs once the temperature is high enough. We argue that since the number of organometallic bonds within a set of molecules increases monotonously with temperature independent of the strong increase of the number of covalently linked species between 530 K and 570 K and also independent of annealing time at 570 K, the organometallic bond is no precursor of the covalent bond on Cu (111). The amount of molecules with organometallic bonds between them represents some thermal equilibrium of a low-rate reaction. The limiting factor for organometallic bond formation cannot be the availability of adatoms nor the probability that two (dehydrogenated) molecules meet. The adatom density should increase drastically with temperature and a reduced probability of two molecules meeting would also affect the covalent bonding. From our analysis of the order of the organometallic bonds, i.e. molecules linked by


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single, double, or triple bonds, we find that the single C-Cu-C bond is unstable at higher temperatures which might the reason behind the low rate with which the C-Cu-C bonds form. Another reason that limits the formation of Cu linked oligomers on the surface could be the configurations of the pDPP. As we discussed before, two types of pDPP, cis-pDPP and transpDPP, are formed during the intramolecular ring-closing reaction at 460K. The bare macrocycle reaction sides in these two configurations show slightly different chemical reactivity in organometallic coupling reactions. About 75% of copper linked pDPP units are in cis configuration. And of the cis-pDPP units 80% are linked via inner reaction sides (as defined in Figure 4(a)). As a result, most of the Cu linked species are either dimers or the outer side of a cis-pDPP is covalently linked to another molecule. Despite this, the majority of cis molecules form covalent links at high annealing temperatures when no strong selectivity with respect to the reaction sites is observed once the activation energy is available. CONCLUSION In summary, intermolecular organometallic and dehydrogenative coupling reactions of 2HDPP molecules are observed to compete with each other on the Cu (111) surface. 2H-DPP molecules first transfer into Cu metalated planar DPP species via surface self-metalation and intramolecular dehydrogenative ring closing. Both the organometallic and the covalent intermolecular coupling start at 530 K. The covalent coupling is boosted when the annealing temperature is increased to 570 K. As a consequence, C-C covalent bonded DPP oligomers become the dominant species on the surface. In contrast, the number of organometallically bonded DPP species increase only slightly with annealing temperature, at 620 K, the highest temperature applied, 8% of the intermolecular bonds are organometallic but only molecules with double or triple C-Cu-C bonds are observed. This points to a considerable stability of the triple


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organometallic bond comparable to the covalent intermolecular bond. Since the occurrence of organometallically bonded molecules is unaffected by the strong increase of covalently bonded species and unaffected by annealing time which lets the number of covalently bonded species increase, it can be excluded that organometallically bonded species act as precursors of covalently bonded oligomers. EXPERIMENTAL METHODS The experiments were carried out in a home-built ultra-high vacuum scanning tunneling microscopy with a base pressure of approximately 4×10-11 mbar. A single-crystal Cu (111) (SPL) was cleaned by several cycles of standard Ne+ bombarding at 1×10-4 mbar and subsequently annealed to 810 K for 5 mins. 5, 15-diphenylporphyrin (2H-DPP) (PorphyChem SAS, purity: 98%) was deposited from a Knudsen cell (510 K) on the clean Cu (111) surface held at 300K and followed by annealing steps. The deposition rate of the 2H-DPP is ~0.07 ML/min. After that, the samples were transferred into the STM chamber and cooled down to 90 K. All STM images were acquired by chemical etched tungsten tips under constant current mode with the bias voltage applied to the sample. The STM images were analyzed using the WSXM software.48 The statistics (shown in Figure 4 and Table 1) of different pDPP reaction products were obtained by the following method: Organometallic links can only occur at 6-8 and 14-16 carbons shown in Figure 4(a). The same is essentially true for the covalent links, only very few are formed C-C bonds at the phenyl ring sites. Since these and the order of C-C bonds between the macrocycles are not easy to identify we do not discriminate between the different types of C-C bonds. The probability of a certain bond formation is related to the total number of maximal possible bonds in a set of molecules. Practically, the relative occurrence of the various links is given as the ratio of the macrocycle sides engaged in a specific link to the total number of


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available sides (i.e. two per molecule). So if all molecules formed covalently bonded dimers that would result in 50% C-C links and 50% “unreacted” species, i.e. links not formed. This is equivalent to arranging all molecules in a circle and counting the links occurring. The margin of errors is given by sampling error from our statistics. We chose a confidence level of 95%, and hence the sampling error is calculated as: ∆p=1.96ට

௣(ଵି௣) ௡

,

Where p is the percentage of different link categories (1Cu, 2Cu, 3Cu, C-C and unreacted) with respect to the total number of pDPP reaction sides, n is the total number of pDPP reaction sides in one sample. For each statistical statement we counted about 500 molecules, i.e. n ≈ 1000.

SUPPORTING INFORMATION The supporting information contain: DFT calculations of molecular interaction energies, material to support the identification of molecular configurations by STM, a detailed discussion of the geometrical matching between molecular oligomers and the surface lattice, and a method section for the DFT calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail address: [email protected] The authors declare no competing financial interest.


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Funding sources This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the DFG Research Unit 1878 “funCOS”.

ACKNOWLEDGEMENTS F. Xiang thanks Martin Gurrath from the Department of Chemistry and Pharmacy, FriedrichAlexander-Universität Erlangen-Nürnberg for fruitful discussions.

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