Two-Dimensional Polyamide Networks with a Broad Pore Size

ARTICLE pubs.acs.org/JPCC

Two-Dimensional Polyamide Networks with a Broad Pore Size Distribution on the Ag(111) Surface Christoph H. Schmitz,* Julian Ikonomov, and Moritz Sokolowski Institut f€ur Physikalische und Theoretische Chemie, Universit€at Bonn, Wegelerstrasse 12, 53115 Bonn, Germany

bS Supporting Information ABSTRACT: Covalently bonded, two-dimensional polyamide networks were prepared by vacuum deposition polymerization under ultra-high-vacuum conditions on the Ag(111) surface. By a careful choice of the annealing conditions, ordered domains of the corresponding oligomers could be also achieved. The layers were characterized by scanning tunneling microscopy. We used the step-growth polymerization between a trifunctional acid chloride and a bifunctional amine, taking place directly on the surface. The resulting polymer networks are nonuniform and show an open porous structure with a broad distribution of pore sizes. Besides large pores of varying shape, also small, 8- to 12-membered polymer rings with a regular shape are formed. A detailed analysis of the pore size distribution gives insight into the influence of the kinetic preference and the role of conformational flexibility for the formation of disordered covalent networks.

1. INTRODUCTION Small organic molecules adsorbed on single-crystal surfaces have gained large interest as building blocks for two-dimensional self-organized structures. Supramolecular structures on surfaces are commonly based on noncovalent intermolecular forces.1,2 These architectures can be prepared with high structural conformity as the intermolecular bonding process is reversible,3 and thus, growth defects can heal or can be removed deliberately, for example, by prolonged annealing of the sample. However, due to the reversibility and the relative weakness of the intermolecular interactions, noncovalent supramolecular assemblies normally exhibit a limited thermal stability. Only recently, covalently bonded, two-dimensional molecular structures on surfaces have been introduced as an alternative with advanced stability.4
17 In all these studies, small organic building blocks have been deposited on a single-crystal surface, which acts as a planar template and promotes the reaction toward the covalent network directly on the surface. The concepts range from (thermally induced) homolytical cleavage of carbon halogen bonds, followed by C
C coupling reactions,5
8 or via carbene intermediates9 to (co-)adsorption of monomers with suitable functional groups, leading to the formation of imides,10,11 ureas,12 amides,13 imines,14,15 and boroxines.16 Due to their high thermal and chemical stability, covalent networks, which may also be addressed as two-dimensional polymers,18 are of great interest for applications, for example, as membranes or electronic devices.19,20 Typically, highly symmetric covalent arrangements based on the chosen monomers are proclaimed as a desired aim of the reaction on the surface. However, the lack of structural uniformity of such covalent networks is still a major drawback.21 Quite generally, it originates from the irreversible character of the intermolecular coupling r 2011 American Chemical Society

reactions, causing a propagation of structural defects through the entire network structure, in combination with a possible conformational flexibility of the monomers that are used. Hence, the important question must arise why the formed covalent networks nevertheless exhibit prevailing structural motifs and what are order-limiting factors in the specific cases. Here, we have investigated this question for the formation of a two-dimensional polyamide network prepared by vacuum deposition polymerization (VDP) on the Ag(111) surface. As monomers, the bifunctional amine p-phenylenediamine (PPD) and the trifunctional acid chloride trimesoyl chloride (TMC) were deposited on the single-crystal surface at room-temperature (Figure 1). The resulting layers were analyzed by scanning tunneling microscopy (STM). We note that thin films of this fully cross-linked polyamide gained by the reaction of PPD and TMC via interfacial polymerization in solution are used as reverse osmosis membranes in nanofiltration processes.22
26 Obviously, the performance of the membranes depends strongly on the molecular structure within the network. However, no detailed information about the structure exists as no method that directly proves the structure on a molecular scale is applicable in these amorphous layers. In a previous study, we have already demonstrated the formation of linear polyamide chains by an on-surface vacuum deposition polymerization via the same coupling reaction, namely, of PPD and the corresponding bifunctional acid chloride (terephthaloyl chloride, TPC) on Ag(111).13 In this case, the monomers are covalently bonded within one-dimensional polymer chains. Adjacent Received: December 9, 2010 Revised: February 7, 2011 Published: March 24, 2011 7270

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Figure 1. Principle reaction scheme and an example for an ideal pore consisting of six PPD and six TMC monomers (“6 þ 6 pore”). The red arrows exemplify four of the total 24 rotatable bonds inside the pore.

chains form long-range ordered two-dimensional domains due to weak hydrogen bonds between the amide moieties of the polymer strands. The present study is an extension of this concept toward all covalently bonded, two-dimensional polymer layers. Figure 1 illustrates the basic coupling reaction. In addition, one possible resulting polyamide pore is depicted. Such a pore, formed by six PPD and six TMC monomer units (for short, “6 þ 6 pore”) can serve as a building unit for an extended, completely uniform hexagonal network and is thus often identified as an “ideal” pore in covalent networks consisting of 3-fold branching points (here, TMC) and linear linkers (here, PPD). This motif is of course inspired by supramolecular honeycomb networks, based on weak interactions. However, in the case of allcovalently bonded networks, such a pore is only ideal as long as there are uniform bond lengths and 120° bonding angles between all atoms in the whole structure. Even under these assumptions, the formation of 6-fold pores is not likely when using monomers with rotatable bonds. In Figure 1, the four rotatable bonds between the repetition units of the polymer are exemplified with red arrows. A rotation of the bond will change the linkage between adjacent monomeric building units from the shown trans conformation (for PPD, the amide moieties point to different sides of the long axis of the molecule) to a cis conformation. The statistical probability for an ideal pore can hence be estimated by counting all possible conformations of a single 6 þ 6 pore. Assuming that the amide moiety itself is always in a trans position (for simplicity reasons, this is, however, not necessarily the case), there are still 24 rotatable bonds between the amide moieties and the aromatic rings. To obtain the ideal pore, all bonds must be formed in the right orientation directly during the reaction as a flipping on the surface is presumably

hindered due to the size of the oligo- or polymers. Evidently, the probability for an ideal pore is very small. Figure 2 schematically illustrates different possibilities of the formation of 12-membered pores. In panel a, the “ideal” 6 þ 6 pore corresponding to the Lewis formula in Figure 1 is shown. Here, all rotatable bonds of the monomeric building units exhibit the same trans orientation. For completeness, we note that these all-trans pores are chiral on the surface. Alternatively, 6-fold pores can be formed in an all-cis conformation. The two different variations of this conformation are shown in panels b and c in Figure 2. However, as mentioned above, a uniform honeycomb network is only possible with all-trans pores (panel a). Honeycomb networks with an all-cis conformation are necessarily mixtures of both pore types (panels b and c). Thus, we only call the 6-fold pore in panel a an “ideal” pore. All other pore configurations described herein, which possess a shape of regular or nearly regular polygons (e.g., besides the nonideal 6 þ 6 pores, also 5 þ 5 and 6 þ 6 pores; see below), are, in contrast, called “regular” pores. Flipping of only one rotatable bond between the aromatic core and the amide moiety out of the all-cis or all-trans conformation prevents the pore from being closed and results in an irregular shape (panels e and f). Of course, the formation of 12-membered pores is also possible if the differing linkage motifs compensate each other, as illustrated in panel d. Note again that, in this illustration, we assume that only 120° bond angles are present between the atoms. Any deviations from this value will obviously lead to regularly or irregularly shaped pores of different sizes. As a consequence, the formation of closed pores, and, in particular, ideal pores, is very unlikely from simple statistical arguments. We will address this issue when thoroughly analyzing the structure of the obtained pores in the TMC
PPD network. 7271

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Figure 3. STM images taken directly after deposition of the monomers. (a) PPD monomers form ordered monolayer domains with varying structures. TMC monomers form disordered clusters with a multilayer thickness. (b) The formation of small oligomers (here, TMC(PPD)3 “stars” and (TMC)2(PPD)5 “bones”) does already take place directly at the domain boundaries. Tunneling parameters: (a) 40  30 nm2,
0.9 V, 30 pA; (b) 11  9 nm2,
0.7 V, 160 pA.

Figure 2. Schematic illustration of the formation of polyamide pores. The bifunctional amide PPD is colored blue, the trifunctinal acid chloride TMC red. The split side groups of the monomers depict the two different possible bonding conformations due to the rotatability of the amine, or acid moiety, respectively. Molecules that are connected in a trans conformation are marked with solid areas; molecules in a cis conformation are marked with hatch-marked areas. (a) The ideal 6-fold pore consisting of six PPD and six TMC units analogue to Figure 1. All monomers show a uniform trans linkage. (b, c) Regular, 6-fold pores are also possible in an all-cis conformation. However, it is not possible to build up a honeycomb network with only one of these pores. (d) Differing cis/trans linkages may compensate each other. (e, f) Only one monomer with a differing linkage (here, a cis conformation in an otherwise all-trans pore) prevents the pore from being closed. (g) Extended honeycomb network of all-trans pores with a unit cell of the structure. For further details, see the text.

2. EXPERIMENTAL SECTION The experiments were carried out in an ultra-high-vacuum (UHV) chamber, equipped with a scanning tunneling microscope (purchased from RHK Technology), and a quadrupole mass spectrometer with a mass range of 1
200 amu (QMS, purchased from Pfeiffer). The base pressure of the chamber was typically 2  10
10 mbar. The Ag(111) crystal surface was prepared by repeated cycles of sputtering with argon ions with an energy of 800 eV and subsequent annealing at 770 K. The monomers PPD and TMC (purchased from Merck Chemicals, purity > 99%) were cleaned by gradient sublimation under vacuum at 390 K (PPD) and 380 K (TMC), yielding colorless powders. Both monomers were stored in separately pumped storage vessels, connected to the UHV chamber by two variable leak valves. The vapor pressure of the solid monomers at room temperature (RT) is sufficient to allow a dosing into the UHV system without heating of the substances. The deposition rates were controlled by the QMS. The recorded ion currents of the fragments m/z = 108 (PPD, Mþ), and m/z = 166 (TMC, Mþ
2COCl) were integrated over time and used to quantify the deposited amounts of the respective monomers. The factors of proportionality between the integrated ion currents and the coverage on the surface are, however, different for both monomers 7272

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due to the different ionization probabilities, fragmentation patterns, and sticking coefficients. The ideal stoichiometric ratio between both monomers in an ideal polymer network is TMC/PPD = 2:3. Experimentally, we achieved the best polymerization results with an ion current ratio in the range of TMC/PPD = 1:4, which is presumably related mainly to the small sticking coefficient of PPD. During the deposition process, the pressure was in the low 10
8 mbar range. The sample was held at RT. The details of subsequent annealing procedures are given below. The STM measurements were carried out at RT using a mechanically cut Pt/Ir tip. Given bias voltages refer to the sample. STM images were processed by plane correction and careful smoothening. With respect to the tunneling conditions, the structures were robust and a wide range of bias voltages (typically ( 0.5 to ( 3.0 V) and tunneling currents (up to the maximum current of the actual STM setup of 1 nA) could be used.

3. RESULTS AND DISCUSSION 3.1. Formation of Polymer Networks. Notably, the simultaneous deposition of both monomers on the Ag(111) surface at RT does not result in the formation of the desired polymer network. Instead, disordered TMC clusters with a height of more than one molecular layer that preferentially nucleate at step edges and partly ordered PPD monolayer domains in-between these are formed (Figure 3a). From experiments where the monomers were deposited individually on the surface, we know that TMC forms clusters at the step edges of the Ag(111) surface at RT. Annealing at 420 K leads to a dispersal of the clusters and a uniform, but disordered, distribution of the TMC molecules on the terraces. In contrast to that, pure PPD does not adsorb at RT on the Ag(111) surface at all. Adsorption of PPD at a temperature of 170 K leads to the formation of ordered islands with different structures depending on the coverage and the preparation conditions. The above-mentioned ordered domains in-between the TMC clusters for simultaneous deposition of TMC and PPD can be clearly identified as PPD domains from their structure (c.f. the Supporting Information). Each protrusion corresponds to one single PPD molecule. The PPD molecules form dimers that arrange in varying structures (center of Figure 3a) or arrange in molecular rows (left part of Figure 3a). Only at the domain boundaries of the TMC clusters can few small oligomers be found (Figure 3b). Mainly TMC(PPD)3 and (TMC)2(PPD)5 oligomers are formed (c.f. section 3.3). This shows that the polymerization, in principle, takes place at RT but is hindered by the limited interdiffusion of the monomers. The fact that, in contrast to the deposition of pure PPD at RT, domains of PPD are formed for simultaneous deposition of PPD and TMC is remarkable. We explain this by local interactions of PPD molecules with TMC molecules at the domain boundaries, or with small oligomers that have been formed at the phase boundaries, respectively. These interactions stabilize the PPD domains on the surface and thus prevent the desorption of the PPD molecules from the Ag(111) surface at RT that would otherwise occur, presumably via detachment of single PPD molecules from the domain boundaries. The formation of separated PPD and TMC domains is not caused by a segregation process of an intermixed phase of PPD and TMC molecules that has been initially formed during codeposition (this would contradict the observed limited polymerization at RT). Instead, the formation of separated PPD and TMC domains takes place directly during the codeposition. Experiments with a very low

Figure 4. STM images of the disordered polymer network after heating to 470 K. (a) The layer consists of polymer patches with different sizes that are interlaced. Polymer strands may partly overlap, which causes the small, bright protrusions. The network exhibits a broad distribution of different pore sizes. (b, c) Close-up images showing irregularly shaped pores of different sizes. (a) 50  50 nm2,
1.3 V, 0.74 nA; (b) 12  12 nm2,
1.3 V, 0.74 nA; (c) 12  12 nm2,
1.0 V, 280 pA.

TMC coverage show that, under these conditions, PPD does not adsorb on the surface. The adsorption behavior of PPD in this case is similar to the total absence of TMC. Thus, in the beginning of the codeposition of the monomers, only TMC adsorbs on the surface and clusters are formed due to diffusion. When a necessary minimal coverage of the surface is reached, which is, from our data, in the range of approximately 1/3 of a full TMC coverage, PPD adsorbs on the bare surface in the vicinity of the clusters, involving a reaction between the PPD and TMC monomers as noted above. We assume that, due to the higher dose of PPD, the remaining bare surface is rapidly covered with PPD and only occasionally further TMC molecules adsorb inside the PPD domains. This explains why oligomers are rarely found inside the PPD domains. In summary, at RT, TMC and PPD adsorb on Ag(111) in separated phases without intermixing and thus the polymerization takes place directly at the domain boundaries only. To initiate a noteworthy covalent coupling of the monomers toward the formation of extended networks, the above-described surface must be heated. As addressed in earlier studies,12,13 the fact that a strong progress of the reaction is observed after heating is attributed to the diffusion of the monomers on the surface, enabling matching pairs of functional groups to hit each other and react, rather than to the activation across an energetic barrier of the reaction. From XPS measurements on the one-dimensional 7273

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Figure 5. STM images (a
c) of regular 6 þ 6 (a), 5 þ 5 (b), and 4 þ 4 (c) pores inside the final polymer network after heating to 470 K along with regular all-trans molecular models of the respective pore (d
f). For simplicity, polymer strands attached to the pores are omitted in the models and all branching points are terminated by pristine acid chlorides. Especially, the different shape of the pore in (a) with respect to the model (d) is presumably due to the flexibility of the bond angles and a different conformation. Tunneling parameters: 6  6 nm2; (a)
1.0 V, 380 pA; (b) 1.3 V, 18 pA; (c)
1.0 V, 380 pA.

polymer chains, there is evidence that the chlorine of the acid chloride is cleaved upon adsorption on the Ag(111) surface at room temperature.27 This is presumably also the case in the analogue twodimensional system considered here. The surface-induced cleavage of the chlorine is expected to have an influence on the coupling mechanism of the surface reaction. Possibly, it lowers the energetic barrier of the reaction with respect to that in solution. However, as we focus on the structural aspects of the resulting polymer network here, these details of the reaction kinetics will be the subject of a later publication.28 In the presented context, we conclude that a thermally activated intermixing of the separated TMC and PPD domains is mandatory for the polymer formation. The highest degree of polymerization can be obtained by heating the surface with the predeposited monomers to 470 K for about 10 min (Figure 4a). No residual monomers or small oligomers are then present on the surface any more, and only a large, covalent network of polyamide with an open porous structure is observed. For longer annealing times, no further changes are observed. The resulting polymer network shows a structure with randomly shaped pores of different sizes. Notably, not all TMC branching points are saturated with three connected polymer strands leading to extended pores that consist of more than 12 monomers (Figure 4b,c). Because of their irregular shape, most of these extended pores can be described as “collapsed” with the result that the enclosed bare surface within these pores (“pore size”) is smaller than the expected value for an uncollapsed pore with a regular shape. Furthermore, polymer strands connected to the branching points may be oriented to the inside of these irregularly shaped pores. The large variety of pore shapes in the

covalent network can be understood as a result of the random formation of linkages between the monomers that constitute a pore. Within the large variety of pore shapes, we, of course, find small pores with a regular shape, too. These are the hexagonal 6 þ 6 pore (Figure 5a), the pentagonal 5 þ 5 pore (Figure 5b), and the quadratic 4 þ 4 pore (Figure 5c). We note that it is not possible to unambiguously determine the structural conformation of the 6 þ 6 pores according to Figure 2 due to the limited resolution of the STM. Especially, the presence of 4 þ 4 pores demonstrates the conformational flexibility within the monomeric building units and emphasizes the fact that the 120° bond angle in the illustration (Figure 2) is only a strong oversimplification. For an optimized choice of initial amounts of monomers, the preparation can be carried out in such a way that the surface is fully covered by the resulting network. Nevertheless, the surface is not covered with one single polymer network, but rather with patches of the polymers with a broad distribution of sizes that are interlaced and may, in some cases, even overlap. Unreacted terminal functional groups of the polymer patches may also interact with amide moieties inside the network via hydrogen bonds at close distances. Especially, in large area STM scans on regions with a high material density, the distinction between covalent and noncovalent bonds is not unambiguous. Thus, a clear distinction between the different polymer patches is not always possible. Further annealing at elevated temperatures above 470 K has no significant influence on the appearance of the network or the median pore size but slowly decreases the coverage by desorption of smaller polymer patches. Nevertheless, the network shows a very high thermal stability. For example, starting from a full 7274

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Figure 6. Area-weighted distribution of pore sizes. Only preparations with an annealing temperature g 470 K were considered.

monolayer, there is still a coverage of 45% of the surface by intact polymer networks after the sample has been annealed at a temperature of 720 K for 5 min. We note that we did not find any evidence for degradation of the network, neither after thermal treatment nor after several days at room-temperature. 3.2. Analysis of the Pore Sizes. We have analyzed the pore sizes of such an polyamide network in detail using the “Grain Analysis” package in SPIP 4.8.4 (ImageMetrology) with the following assumptions: (i) the threshold that defines the border of a pore was set to half of the average apparent height of the polymer in relation to the bare Ag(111) surface; (ii) detected pores with a size smaller than 0.04 nm2 were ruled out as they are artifacts originating from polymer patches at direct contact or noise; and (iii) areas larger than 5 nm2 were considered as the bare surface and not as a pore. In total, over 27 000 pores were analyzed. The resulting histogram of the pore sizes is shown in Figure 6. In this figure, also the average sizes of the three regular pore types shown in Figure 5 are indicated, clearly showing that there is no preference towards these conformations. With the method described above, the 4 þ 4, 5 þ 5, and 6 þ 6 pores enclose a bare surface area of approximately 0.3, 0.9, and 1.5 nm2, respectively. The area-weighted histogram nicely shows the broad distribution of pores that enclose different areas. Notably, the area fraction decreases with increasing pore size, as can also be expected from statistical arguments. It is also interesting that most of the pores are larger than the ideal 6 þ 6 pore. However, note that the column in Figure 6 that includes, for example, the 4 þ 4 pores also includes larger pores that are partly filled or have an irregular shape, as described above. In particular, with regards to possible applications as membranes, molecular sieves, or coatings with a monatomic thickness, such a random distribution of pore sizes is, of course, not wanted. A selective adsorption of molecules with a distinct size on the bare surface inside the pores or a selective transfer of molecules

Figure 7. (a) STM image of the TMC(PPD)3 “star” structure formed after heating to 370 K. Tunneling parameters: 10  10 nm2, 0.6 V, 28 pA. (b) Corresponding model. For further details, see the text.

through the pores of a polymeric membrane (molecular sieve) cannot be achieved for such a network. Nevertheless, these experiments show why, in the above-mentioned applications, so far, thick layers are mandatory, when using two-dimensional polymer layers. Namely, by stacking of several polymer layers, the maximal and the average pore sizes are decreased to the desired value. As a consequence, also the flow through the film may be significantly lowered. As mentioned above, we observe regular 6 þ 6 as well as 5 þ 5 and 4 þ 4 pores within the covalent network. The fraction of these regularly shaped pores is very small, and within these, there is no significant preference of a distinct size in the sense of a “magic” pore size. No kinetic preference toward the formation of regular or even ideal 6 þ 6 pores can be observed. Thus, the 7275

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these oligomers embedded in the PPD domains in direct vicinity of the TMC clusters. After heating the sample at 370 K for 5 min, only few small TMC clusters can be found remaining on the surface. A large number of stars, bones, and also small fragments of a 2D network are present. At this point, the stars may form small, ordered domains themselves (Figure 7a). These domains have a size of up to 20  20 nm2. A model of the structure is shown in Figure 7b. Each molecule has six next-nearest neighbors. All molecules have the same orientation with their outer amine groups nested inbetween the amine groups of neighboring molecules. We have identified the lattice parameters of the superstructure as a = b = 1.79 ( 0.05 nm and γ = 119° ( 2°. Because of the 3-fold symmetry of the star molecule as well as the structural motif, a hexagonal unit cell is possible and probable within the margins of the experimental error. Furthermore, taking into account the orientation with respect to the underlying Ag(111) surface, which has been deduced from atomically resolved STM images of the bare surface prior to adsorption of the molecules, the results suggest a fully commensurate structure with the matrix notation 7
5

Figure 8. Oligomers of different sizes after heating to 370 K. The objects with a larger height are presumably residual TMC clusters. Tunneling parameters: (a) 10  7 nm2, 0.6 V, 29 pA; (b) 10  7 nm2, 0.6 V, 80 pA.

on-surface reaction toward the covalent network here results in a disordered structure because neither the irregular pores nor a mixture of regular pores is capable of forming a highly symmetric, long-range ordered network. 3.3. Oligomers. Besides an annealing of the predeposited monomers at 470 K with the result of a fully interlinked polymer network, it is also possible to limit the surface reaction to the formation of oligomers with different sizes by careful choice of annealing temperatures. The limited progress of the polymerization reaction is directly attributed to the limited diffusion and intermixing of the monomers on the surface. We performed additional studies of the resulting layers after annealing at different temperatures below 470 K, which nicely show the progress of the polymerization from single oligomers to the formation of network patches. Starting at 340 K, the TMC clusters remain partly intact and only a small amount of TMC diffuses into the PPD domains. The concentration of TMC molecules within the PPD domains is highest in the vicinity of the domain boundaries. Nevertheless, there is always an excess of the amine as every entering TMC molecule is directly surrounded by PPD. Thus, we only find PPD-terminated oligomers. The smallest possible oligomers of this kind are TMC(PPD)3 (“stars”, c.f. Figure 7a), which consist of one TMC branching point surrounded by three covalently bonded PPD molecules, and (TMC)2(PPD)5 (“bones”, Figure 8a), which consist of two TMC molecules as branching points, one PPD molecule as a linker in-between, and four peripheral PPD molecules to saturate the acid moieties of the TMC. We find small amounts of

5 2

!

and the lattice parameters a = b = 1.804 nm and γ = 120°. The plane symmetry group of the hexagonal structure is p3 with 3-fold centers of rotation as the only symmetry elements. From experimental results, there is no evidence for the exact position of the molecules with respect to the substrate. Nevertheless, as shown in Figure 7b, it is possible to position all molecules with their oxygen atoms on top of silver substrate atoms. Such a position of the oxygen atoms has been calculated as most favorable adsorption position in the chemically alike imide moiety of the diimide PTCDI on Ag(111).29 The star molecules have the possibility to act as both hydrogen-bond donors via their peripheral amine groups or the N
H part of the amide moiety and hydrogen acceptors via the oxygen atom of the amide moiety. In our proposed structure, the orientation of the molecules would, in principle, allow hydrogen bonds from the amide group of one molecule to the amine groups of neighboring molecules and vice versa. Namely, a slight bending of the amine carrying peripheral phenyl rings or a small rotation of the molecule would align the amine groups as hydrogen-bond donors with the oxygen atom of the amide moiety as a hydrogen-bond acceptor. However, the large distances between the molecules make such hydrogen bonds improbable, because these would have a total length (N
H 3 3 3 O) of approximately 5 Å. Thus, we interpret the observed registry with the Ag(111) surface as an indication for a significant adsorbate
substrate interaction. Possibly, surface-mediated repulsive intermolecular interactions overrule the possible energy gain related to the formation of shorter hydrogen bonds upon compression of the structure. A smaller, incommensurate lattice constant of the superstructure would also preclude the above-noted on-top positions of all oxygen atoms. These findings are consistent with our previous observations on the linear polyamide, where the hydrogen bonds between adjacent chains are elongated in comparison with the crystal structure in favor of a commensurate adsorption geometry of the polymer chains.13 However, in both cases, the very long hydrogen bonds may still have an aligning character. 7276

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The Journal of Physical Chemistry C The next largest oligomers, the bones, are partly incorporated in the star domains, preferentially at the domain boundaries (Figure 8a). Closer to the remaining clusters, where the TMC ratio is highest, small fragments of a 2D network are found (Figure 8a,b). The fragments consist of up to 20 monomers. Besides the stars, no other oligomers form ordered domains. They are intermixed and form disordered domains in which the larger oligomeric fragments are interlaced. Note that, at this point of the reaction progress, no pores are formed by amide formation. All fragments are oligoamides without closed-ring structures. Nevertheless, small pores are formed by multiple fragments that are interlaced at close distances by van der Waals interactions or hydrogen bonds. In the context of building networks with regularly shaped pores, the step toward the closing of the ring—the creation of the pore itself—is probably the most critical step. The shape of the larger fragments without closed pores (e.g., see Figure 8b) appears to be suited for the formation of an ordered honeycomb network. However, a closer inspection reveals that these fragments do not possess quite the regular conformation (bond length, angle) that is necessary for a regular ring closing. As mentioned above, already small deviations from the ideal shape are decisive. As for the closed pores, the determination of the exact conformation within the fragments is not unambiguously possible due to the accuracy of the STM data. Further annealing in the temperature range between 370 and 420 K for at least 5 min leads to an ongoing intermixing of the monomers and oligomers and thus to an ongoing reaction toward extended two-dimensional networks. After this annealing step, polymers with closed pores can be observed for the first time. Nevertheless, the surface is predominantly covered with oligomers, and only small areas of the porous polymer network exist. When the sample is annealed at the final temperature of 470 K, the amount of oligomers decreases further in favor of polymer patches with closed covalent pores. As a consequence, the median pore size of the layer increases. The progress of the polymerization is mainly attributed to the reaction of oligomers that have been formed in previous annealing steps to more extended polymer patches. We conclude that the described oligomers are formed as intermediates in the one-step annealing described in section 3.1 as well. 3.4. Final Discussion. We have shown for the case of a twodimensional polyamide network that conformational flexibility of the building blocks and hence of the covalent links and a lacking kinetic preference lead to a variation in the pore structure and thus to a nonuniform network. This is directly attributed to monomers without structural rigidity allowing a number of different structural conformations of the adsorbed species. In studies published in the literature so far,10
16 including the present, in which partly flexible monomers have been used or in which different conformations of newly formed bonds may occur, the resulting networks are nonuniform or even only small oligomeric fragments have been achieved. As an example, Weigelt et al.14 have shown that, for the formation of a polyimine on Au(111), there is a preference for the formation of four- to sixmembered rings, at least at the investigated low coverages. The polymer is based on 1,6-diaminohexane. However, due to the flexible alkyl chains and a nonuniform interlinking, this does also not result in the formation of regular polymer domains as the pores possess a varying shape. In this aspect, the reported system bears similarities to the one reported here, although we use rigid monomers, thus eliminating the influence of flexible alkyl chains.

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Nevertheless, structural conformity of the covalent network is an important issue and has already been achieved in few examples. We will briefly discuss and compare these; hence, the involved coupling reactions corroborate our conclusion concerning the important conformational rigidity for the formation of ordered networks. Covalently interlinked networks with a regular structure have been prepared, for instance, on the basis of homolytic cleavage of carbon
halogen bonds.5,6 In these studies, the carbon atom is part of a rigid extended π system and the new carbon
carbon bond is formed directly between two of these aromatic systems. For molecules with a symmetric aromatic backbone, such as porphyrins,5 there is, in consequence, only one possible conformation of the resulting network knots after the reaction. Furthermore, a slight bending of the new bond is hindered by sterical constraints between the neighboring R-H atoms of the aromatic ring, which also supports a rigid, linear arrangement. Although networks based on boroxine junctions presented by Abel and co-workers16 do not possess a long-range order, there is a strong preference of 5- or 6-fold rings with a regular shape. Here, only the bond between the aromatic core of the monomers and the reactive functional group is rotatable, which has no detrimental influence on the structural uniformity as the monomers are symmetric. The resulting boroxine junction is a rigid six-membered ring with only one possible conformation. Thus, the formation of a covalent network with long-range order is only hindered by the formation of rings with a different number of members, which is directly attributed to a bending of bonds. Hence, we conclude that, for the formation of ordered, porous networks, the aspect of a preordering of the monomers before the reaction is initiated is less important. The major point is to limit the possible conformations of links between the network knots—ideally to only one. The formation of various possible pores based on different conformations, as exemplified in Figure 2, hinders the formation of a uniform network structure. Concerning the more versatile bimolecular systems, such as polyamides, the future challenge will be the choice of tailormade monomeric units, in which the structure of the network is determined by either conformational rigidity of the monomers and/or sterical constrains.

4. CONCLUSIONS We have prepared monolayers of two-dimensionally interlinked oligo- and polyamides by on-surface step-growth polymerization. We find that, although the reaction, in principle, takes place at room-temperature, heating of the sample is necessary to induce a diffusion process, which leads to a intermixing of the monomers and finally to the chemical reaction. The smallest possible oligoamides, the TMC(PPD)3 “stars”, form small, ordered domains of a commensurate structure themselves. Small oligomeric fragments of the 2D network partly show a uniform structure. Nevertheless, the resulting extended covalent network is disordered with a broad distribution of pore sizes. Pores with a regular shape consist of 4 þ 4, 5 þ 5, and 6 þ 6 monomers, while the majority of the pores consist of more monomers and has an irregular shape. The polymer network layer exhibits a high thermal stability and shows no evidence of degradation within several days at room temperature. The results show that the reaction of amides and acid chlorides on the Ag(111) surface is a versatile and fundamental approach toward the formation of covalent networks with a user-defined appearance. The choice of the monomeric building blocks defines 7277

dx.doi.org/10.1021/jp111723k |J. Phys. Chem. C 2011, 115, 7270–7278

The Journal of Physical Chemistry C the properties of the resulting network. For example, the distribution of different pore sizes should depend on the size of the monomers used when substituting the small PPD against a larger, rodlike diamine. We have discussed the influence of conformational flexibility of the building blocks and the resulting network on the ordering, concluding that the choice of rigid building blocks is mandatory for the formation of long-range ordered covalent networks.

’ ASSOCIATED CONTENT

bS

Supporting Information. STM images of PPD and TMC on Ag(111) after individual deposition of the monomers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

ARTICLE

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Corresponding Author

*Tel: þ49 228 73-2520. Fax: þ49 228 73-2551. E-mail: [email protected].

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