Adsorbate−Substrate-Mediated Growth of Oligopyridine Monolayers at

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Adsorbate-Substrate-Mediated Growth of Oligopyridine Monolayers at the Solid/Liquid Interface Christoph Meier, Katharina Landfester, and Ulrich Ziener* Institute of Organic Chemistry III (Macromolecular Chemistry and Organic Materials), Albert-Einstein-Allee 11, 89081 Ulm, Germany ReceiVed: September 23, 2008; ReVised Manuscript ReceiVed: NoVember 3, 2008

Oligopyridine molecules are highly efficient building blocks for the construction of two-dimensional nanopatterns by self-assembly processes at the highly oriented pyrolytic graphite (HOPG)-liquid interface based on intermolecular (weak) hydrogen bonds between the terminal pyridine moieties. By introducing additional phenylene groups the principal hydrogen bonding function is maintained but the adsorbate-substrate interactions are favored over the adsorbate-adsorbate interactions leading to more unified structures and a stronger orientation of the adlayer on the underlying HOPG. Thus, by tailoring the molecules, the selfassembly behavior can be fine-tuned and directed toward the desired pattern. Though the molecules are achiral, chiral polymorphic phases are found caused by asymmetric hydrogen bonding interactions. Introduction The fundamental understanding of molecular self-assembly on solid substrates at the solid-gas and solid-liquid interface is of great importance for the successful realization and implementation of next generation technologies based on the properties of single molecules and molecular assemblies. Within this context, self-assembled hydrogen bonded networks (HBN) of organic molecules on solid substrates have attracted considerable interest in the past years.1-4 Building blocks with functional subunits allow for a precise control of the monolayer structure through minimal constitutional variations of the molecular structure.5-8 In contrast to intermolecular interactions, no systematic investigations regarding the adsorbate-substrate influence on two-dimensional (2D) HBNs at the solid/liquid interface have been carried out so far. The interadsorbate and adsorbate-substrate interactions of long chain alkane derivatives are well understood in the meantime.9 For alkyl chains it was shown that the size of lamella increases with chain length.10 The variation of alkoxy side chain length and position affords a fine-tuning of the surface structures.11 Recently, a strong influence of the alkyl chain length on the molecular ordering was shown for dehydrobenzoannulene derivatives.12 Therein, the elongation of alkyl chains is correlated with an increase in the interadsorbate and adsorbate-substrate van der Waals interactions. In the case of tribenzoic acids, the symmetric extension of the molecular backbone with phenylene spacers leads to structures with similar morphologies, in which the elongation of the molecular backbone is expressed in the elongation of the honeycomb network periodicities.13 The frustrated 2D molecular crystallization of a tetrabenzoic acid derivative in contrast to its backbone shortened or extended analogue is explained with subtle differences in hydrogen bonding.14 Furthermore, it was stated that directional intermolecular interactions can be more important than adsorbatesubstrate interactions for the preferentially formed monolayer structure.15 Linear aromatic molecules, terminated by pyridyl groups, 1,4-bis(4-pyridyl)benzene and 1,4-bis(4-pyridyl)biphenyl, respectively, form chains on a Cu(001) surface in ultrahigh * Corresponding author.

vacuum.16 It was shown that the molecule length affects the chain stability and structure, which was explained by the adjustment of the molecule length and the chain structure commensurability with the substrate. A dramatic change of the assembling characteristics is found in monolayers of trimesic acid (TMA) on HOPG upon introducing surface charge by application of a high voltage (4 kV) and thus tuning the adsorbate-substrate interactions electronically.17 Recently, we have reported that adsorption and self-assembly of the oligopyridine 2,4′-BTPsthe parent compound of one of the oligomers described in the present reportsyields identical network morphologies even on different substrates.18,19 On the other hand, on metallic substrates, the adsorbate-substrate interactions are considered to be stronger than on graphite. The network structures are stabilized through weak C-H · · · N hydrogen bonding interactions.20-23 In the first publication of this series8 it was shown that a slight positional variation of the nitrogen atoms in the peripheral pyridine rings of an oligopyridine strongly affects the intermolecular interactions and thus the 2D patterns on HOPG. To investigate the influence of the aromatic backbone on the self-assembly behavior of these oligopyridines, we have synthesized three phenylene extended analogues (Figure 1) and investigated their self-assembly behavior at the solid/liquid interface. Results and Discussion Synthesis. The synthesis via variation of the Kro¨hnke method was successful for two of the phenylene extended oligopyridines 2,4′-BTPP and 3,3′-BTPP. For the third constitutional isomer, 4,3′-BTPP, at first a dibromo-substituted precursor was also synthesized via variation of the Kro¨hnke method. This precursor was then transformed to the phenylene extended oligopyridine 4,3′-BTPP in a Suzuki coupling reaction (see the SI).24,25 Self-Assembly on HOPG. To the molecular backbone (dipyridylphenylpyrimidyl subunit) are attached pyridyl rings and phenylene-pyridyl moieties symmetrically on both sides. As shown earlier in the case of the parent oligopyridine derivatives, the terminal pyridyl rings provide functional groups for intermolecular hydrogen bonding motifs.8 For the phenyleneextended oligopyridines BTPP, the distance between the terminal

10.1021/jp808431t CCC: $40.75  2009 American Chemical Society Published on Web 01/02/2009

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Figure 1. Molecular sketch of the phenylene extended oligopyridines (top) and high-symmetry adsorption sites on graphite (bottom, carbon: cyan; hydrogen: gray; nitrogen: blue): (a) on top of substrate atoms, the least stable adsorption configuration; (b) carbon atoms of the pyrimidyl ring on the substrate ring centers; and (c) nitrogen atoms of the pyrimidyl ring on the substrate ring centers. Both are the most stable adsorption configurations and of similar energy.

Figure 2. STM images and models for the 2,4′-BTPP self-assembled at the TCB/graphite interface: (a) large scale STM image and the underlying graphite surface (inset) [the domain boundary is highlighted with a dotted line with a lateral shift d of neighbored molecule rows]; (b) magnified part of the monolayer; (c) schematic model of the oligopyridine monolayer; (d) adsorption model as derived from STM images as shown in panel a; and (e) hydrogen bonding motif in the monolayer.

pyridyl moieties is lengthened by phenylene spacers. The constitution of the terminal pyridyl moieties is identical with that of the earlier published oligopyridine derivatives. Therefore, hypothetical network structures with similar morphology can be constructed with the assumption that the self-assembly behavior is only governed by adsorbate-adsorbate interactions and is not affected by the interaction of the molecules with the graphite substrate. DFT calculations with the parent oligopyridines on two sheets of graphite revealed that the adsorption configuration with a nitrogen atom centered on top of a graphite ring center, following the AB graphene layer, is the energetically most stable adsorption configuration.26 This result is in accordance with earlier calculations of benzene on HOPG.27 With these results, in the most stable adsorption site the aromatic moieties are centered on a site with 3-fold symmetry above a substrate atom (Figure 1b,c). The energetic difference

between an oligopyridine nitrogen atom on top of a graphite carbon atom or an oligopyridine carbon atom on top of a graphite carbon atom is not known in the literature so far. For benzene the least stable adsorption site is with the carbon atoms on top of the graphite atoms destabilized by around 17% compared to the more stable adsorption site.27 From exemplary simple force field (mm+) calculations the energetic differences between the different adsorption sites for 3,3′-BTPP were estimated to at most 1% (see Figure S1 in the Supporting Information) but qualitatively in agreement with the findings for benzene, i.e., the on-top adsorption (Figure 1a) is the least stable configuration. The adsorption site with the aromatic moieties on top of a graphite bond is located between the two boundary adsorption configurations. 2,4′-BTPP. In the C2V-symmetric phenylene extended oligopyridine 2,4′-BTPP, the terminal pyridyl rings are attached to the molecular backbone in ortho and meta positions with

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Figure 3. STM images and models for 3,3′-BTPP self-assembled at the TCB/graphite interface: (a) large scale STM image with two enantiomeric domains and the corresponding unit cells [the unit cell vectors A1 of enantiomorphic domains form an angle of 47° shown by the white lines (see text); the contour lines of eight molecules in each domain are overlaid with the STM image; the arrows in the upper right corner indicate the orientation of the underlying HOPG substrate]; (b) magnified detail of the monolayer; (c) schematic model with a cavity containing coadsorbed TCB molecules; (d) adsorption model featuring the molecular unit cell (solid) and the commensurate unit cell (red); and (e) hydrogen bonding motif in the self-assembled monolayer.

respect to the nitrogen atoms. Deposited from a concentrated solution, the molecules spontaneously self-assemble into a highly ordered monolayer. The submolecularly resolved STM image allows for the assignment of the bright image parts to the individual molecules, highlighted with molecular contours (Figure 2). The 2,4′-BTPP molecules are arranged in a linear structure with large domains. The molecules are rotated 180° toward each other, lying side-by-side, forming a row-like structure. The parameters of the molecular unit cell are A ) 1.81 ( 0.03 nm, B ) 4.13 ( 0.04 nm with ∠A,B ) 68 ( 2°. The rhombohedral unit cell contains two molecules with their C2 axis perpendicular to a primitive substrate vector. The unit cell vector B is oriented parallel to a main symmetry axis of the substrate, and the vector A is tilted 8 ( 2° to a main symmetry axis. The molecular packing is identical with the parallel chain structure (PCS) as observed under specific conditions for the parent oligopyridine of 2,4′-BTPP, namely 2,4′-BTP.8,19 The weak intermolecular interactions perpendicular to the row direction allow for the formation of domain boundaries between equally oriented domains, in which the two adjacent rows are shifted toward each other in row direction (Figure 2a). The shift is about 1.49 nm, corresponding to six times the graphite vector length. This reflection of the surface corrugation can be understood as a result of the significant adsorbate-substrate interaction. The monolayer is stabilized in row direction through intermolecular hydrogen bonds. As a result of the position of the nitrogen atoms in the terminal pyridyl rings, only weak van der Waals interactions between individual rows (in the A vector direction) contribute to the overall monolayer stabilization. The hydrogen bonding pattern is identical with the pattern found in the PCS of the original oligopyridine 2,4′-BTP.8,19 The intermolecular H · · · N distances in the B direction were estimated to 0.31 nm, resulting in a stabilization energy of ca. -10 kJ mol-1. For the PCS phase of the original oligopyridine 2,4′BTP we estimated the intermolecular H · · · N distance to 0.26 nm, resulting in a stabilization energy of ca. -13 kJ/mol. The elongation of the hydrogen bonding distance in the network of

the phenylene extended oligopyridine with respect to the distance in the network of the original compound points to a significant molecule surface corrugation. Compared to the vector length of 3.2 ( 0.2 nm in the network of the original compound, the lattice vector of 4.13 ( 0.04 nm in the network of the phenylene extended oligopyridine is elongated by 0.9 nm perpendicular to the C2 molecule axis (B vector direction). This experimental observation is a result of the additional phenylene spacers and the significant epitaxial relation of the oligopyridines with the substrate. On the basis of the STM data and the energetic considerations of different adsorption sites given above, a simplified adsorption model and hydrogen bonding pattern was deduced (Figure 2d). The commensurate unit cell corresponds to the molecular unit cell and is expressed in the matrix notation as:

() (

) ()

A 17 17 a ) · B 7 -1 b

with the primitive substrate lattice vectors a and b. The oligopyridine molecules are physisorbed on energetically equivalent adsorption sites. The adsorption geometry of an individual oligopyridine molecule corresponds to an energetically less stable adsorption site. The nitrogen atoms of the pyrimidyl ring are placed on top of the graphite layer carbon atoms. If the molecules are hypothetically placed on adsorption sites corresponding to the graphene AB-layer (the most stable adsorption configuration, see Figure 1), every second oligopyridine molecule is located in the on-top configuration, destabilizing the substrate-adsorbate interaction. Therefore, the adsorption site with the nitrogen atoms on the substrate carbon atoms should be energetically favored. 3,3′-BTPP. In contrast to 2,4′-BTPP, the terminal pyridyl rings in the phenylene extended oligopyridine 3,3′-BTPP are attached symmetrically in meta position with respect to the nitrogen atoms to the molecular backbone. Deposited on a freshly cleaved graphite surface from a concentrated TCB solution, the spontaneous self-assembly of the molecules results

1510 J. Phys. Chem. C, Vol. 113, No. 4, 2009 in a highly ordered monolayer. In the submolecularly resolved large-scale STM images the oligopyridine molecules are imaged with bright contrast, the position of individual molecules highlighted with contours (Figure 3a,b). The molecules are rotated 180° toward each other, forming dimers lying side-byside corresponding to the monolayer of 2,4′-BTPP (Figure 2). The molecular unit cell parameters are A1 ) 2.26 ( 0.13 nm, B1 ) 3.47 ( 0.16 nm, enclosing an angle of 88 ( 5°. Besides rotational domains, enantiomorphic domains are formed with an angle between the unit cell vector A1 of 47° (Figure 3a). In both enantiomorphic domains, the unit cell vector B1 is tilted 5.5 ( 0.5° with respect to a primitive substrate vector. In analogy to 2,4′-BTPP, 3,3′-BTPP subsequent imaging of the substrate lattice after imaging the monolayer revealed that the molecules are oriented with their C2 axis orthogonal to a main symmetry axis of the substrate, indicating an influence of the surface corrugation on the self-assembly behavior. On the basis of the STM data and the energetic considerations of different adsorption sites given above, a simplified adsorption model and hydrogen bonding pattern was deduced (Figure 3c-e). The corresponding hydrogen bonding network is in part identical to the hydrogen bonding network of its original compound 3,3′-BTP.8,23 Side-by-side neighbored oligopyridines are stabilized with pyridine dimer interactions as found in the densely packed polymorph of its parent compound, deposited from concentrated solution. From the molecular model, the intermolecular hydrogen bonding distance perpendicular to the C2 molecule axis was determined to ca. 0.31 nm, resulting in a stabilization energy of ca. -10 kJ · mol-1. The intermolecular H · · · N distances in the C2 axis direction were estimated to be ca. 0.42 nm, leading to a stabilization energy of -2.5 kJ · mol-1. In analogy to 2,4′-BTPP, 3,3′-BTPP shows much longer H · · · N distances than the van der Waals radii or the H · · · N distances found in the networks of the parent oligopyridines. Again, this can be explained with the snap in of the molecules on preferential adsorption sites. The monolayer offers voids of rhombohedral shape with a length of 1.8 nm and a width of 0.7 nm, bordered by the pyridyl-phenyl-subunits of the oligopyridines. The STM images do not show an evenly dark contrast in those voids but three gray dots. The void is large enough to accommodate three TCB solvent molecules (Figure 3c). The coadsorption of solvent molecules is the most likely reason for the observed contrast in the voids. Coadsorption of alkylated solvents such as 1-phenyloctane is considered quite often,12,28-33 whereas the coadsorption of small TCB molecules in a molecular monolayer is described only rarely.34 The orientation of the commensurate unit cell with respect to the substrate expressed in the matrix notation is:

( ) (

) ()

A2 19 7 a ) · B2 6 26 b

and for the enantiomeric phase:

( ) (

) ()

A2 a 7 19 ) · B2 b 26 6

with the primitive substrate lattice vectors a and b. The commensurate unit cell is rotated and enlarged compared to the molecular unit cell (Figure 3d). The commensurate unit cell contains six molecules on three different adsorption sites. Besides the energetically favored adsorption sites with the oligopyridine atoms in the center of a graphene ring, every third molecule is adsorbed with the oligopyridine atoms on top of the substrate atoms. Apparently, the ratio of two less stable

Meier et al. adsorption sites and four most stable adsorption sites seems to be favored for the observed monolayer packing. In addition, we cannot exclude that the adsorbed molecules further minimize the adsorption energy with a lateral relaxation, leading to intermediate adsorption sites. The oligopyridine 3,3′-BTPP may adsorb in different rotational conformers. The preferentially adsorbed conformation cannot be deduced directly from the STM images. The conformation of the pyridyl-pyridyl-subunits at the interface is unambiguous as the N,N-transoid conformation is energetically favored.35-38 The energetic differences of the three coplanar phenyl-pyridyl rotational conformers were calculated with Gaussian03 and the 6-31G(d,p) basis set in the gas phase. At the interface, two symmetric conformations named SymI and SymII and one asymmetric conformation Asym can be recognized (Figure 4). In the C2V-symmetric conformation SymI, all nitrogen atoms in the terminal pyridyl moieties point in the same direction. For the phenylene extended oligopyridine, this is the energetically most stable conformation with the highest dipole moment of 2.17 D though the energetic difference to the other conformational isomers is rather small (0.19 and 0.30 kJ · mol-1, respectively). In the other C2V-symmetric conformation SymII, the two terminal pyridyl rings attached to the phenylene spacer are rotated 180° compared to the SymI conformation. This conformation possesses a dipole moment of 0.38 D and is destabilized by 0.30 kJ · mol-1 relative to SymI. In the C1symmetric conformation Asym, both symmetric conformations SymI and SymII are combined. In this conformer, only one phenylene-attached terminal pyridyl ring is rotated 180°, affecting prochirality of this conformer. The Asym conformer is destabilized by 0.19 kJ · mol-1 with a dipole moment of 1.20 D. The energy difference to SymII is rather small (0.11 kJ · mol-1). Considering solely the small relative energetic differences determined in the gas phase, it is not possible to identify the conformational isomer preferentially adsorbed at the solvent-graphite interface. On the other hand it is striking that the dipole moment of TCB (1.26 D39) is very similar to the corresponding value for conformer Asym (1.20 D). In a solvent with a given dipole moment, the conformer with a similar dipole moment is considered to be the most stable one, thus favoring conformer Asym over SymI and SymII. In addition we assume that the C1-symmetric conformer Asym is preferred because of the optimization of the intermolecular hydrogen bonds in the monolayer with this conformation. The STM images show two enantiomorphic phases, covering the substrate in a racemic ratio (Figure 3a). As derived from the theoretical calculations, the preferentially adsorbed coplanar configuration isomer from TCB is the prochiral Asym conformer. Apparently, the resulting monolayer is built up from prochiral oligopyridines, which is common for prochiral (here a more precise expression for achiral) molecules.32,40-43 Upon adsorption, the oligopyridines form two energetically equivalent face-on mirror symmetric configurations on the surface, each with different preferential hydrogen bonding directions. Therefore, enantiomorphic discrimination occurs, leading to homochiral domains in which molecular interactions and packing density are optimized.44 The homochiral domains cover the same amount of the surface, resulting in a macroscopically achiral surface. The A1 vectors of both enantiomorphic domains with different molecule orientation enclose an angle of 47 ( 2° (Figure 3a). In the commensurate adsorption model, the angle R between domains with the same rotational orientation of the molecules was determined to be 107° (Figure 4b). The hexagonal substrate favors additionally angles of 47° and 167°

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Figure 4. (a) Schematic sketches of the three rotational conformers of 3,3′-BTPP with their relative energies Erel and their dipole moment as received from ab initio calculations. (b) Adsorption model of both enantiomeric phases consisting of the conformer Asym. The angle R depicts the relative orientation of the A1 vectors between both enantiomeric phases. The mirror plane is perpendicular to a primitive substrate vector.

() R ( 60°) between A1 of enantiomorphic domains with different rotational orientations. Therefore, the experimentally observed angle of 47 ( 2° supports the assumption of an epitaxial relation of the adsorbed phenylene extended oligopyridines with the substrate. The influence of the adsorbate-substrate interactions on the 2D pattern is further supported by simple force field calculations (see Figure S1 in the Supporting Information). Comparing the adsorption energy of the parent oligopyridine 3,3′-BTP with the phenylene extended analogue 3,3′-BTPP reveals a 30% increase for 3,3′-BTPP (280 kJ mol-1 vs 364 kJ mol-1). On the other side, the difference of the intermolecular interactions between both compounds should be negligible because the additional van der Waals interactions of the extra phenylene units with the pyridine moieties contribute only around 5% to the total intermolecular interactions (0.59 kJ mol-1 vs around 10.1 kJ mol-1) for the weak hydrogen bonding interactions.8,45 These assumptions differ from results in the literature where fundamental changes in 2D self-assembly of hydrogen bonded networksscaused by subtle changes in the geometry of an alkyl substituentswere ascribed to different intermolecular van der Waals interactions. There, the adsorbate-substrate (Au(111)) interaction was considered negligible.46 4,3′-BTPP. In the phenylene extended oligopyridine 4,3′BTPP, the terminal pyridyl rings are attached to the molecular backbone in para and meta positions with respect to the position of the nitrogen atoms. Deposited from a concentrated solution onto graphite, the molecules spontaneously self-assemble into a highly ordered monolayer. The large scale STM image shows three different phases, indicated as A, A′, and B (Figure 5a).

The phases A and A′ are highly ordered, whereas in phase B no long-range order can be identified. The submolecular resolution allows for the identification of individual molecules in the monolayer. The molecular arrangement is similar to that found for 3,3′-BTPP. The molecules are rotated 180° toward each other and are lying side-by-side, forming a dimeric monolayer subunit (Figure 5b,c). In contrast to 3,3′-BTPP, no intermolecular interactions stabilize the side-by-side dimer arrangement as a result of the different nitrogen position in the terminal pyridyl rings. The parameters of the unit cell are A1 ) 2.22 ( 0.01 nm and B1 ) 3.58 ( 0.04 nm enclosing an angle of 91 ( 1°. At the interface, two enantiomorphic domains A and A′ are formed (Figure 5a) covering an equal surface area. The angle between the B1 unit cell vectors of both domains A and A′ is 79 ( 1° (Figure 5a), whereas the molecules are equally oriented in both domains with respect to the surface lattice. The angle between the monolayer vector B1 and a primitive substrate vector was determined to be 53 ( 1°. As observed for the other phenylene extended oligopyridines 2,4′-BTPP and 3,3′-BTPP, the 4,3′-BTPP molecules are oriented with their C2 axis perpendicular to a primitive substrate lattice vector. In analogy to 2,4′-BTPP and 3,3′-BTPP, too, a simplified adsorption model and hydrogen bonding patternsbased on the STM results and the energetic considerations of different adsorption sites given aboveswas deduced for 4,3′-BTPP for the highly ordered phases A and A′, respectively (Figure 5c-e). The hydrogen bonding pattern in the direction of the molecular C2 axis between the phenyl-pyridyl units is similar to the 3,3′BTPP monolayer as a result of the identical conformation of one pair of terminal pyridyl rings. The intermolecular H · · · N

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Figure 5. STM images and models for 4,3′-BTPP self-assembled at the TCB-graphite interface: (a) large scale STM image with three different domains A, A′, and B, and overlaid contour lines of single molecules [the white lines with an angle of 79° represent the relative orientation of the A1 vectors of the unit cells of enantiomorphic domains; the arrows in the upper right corner indicate the orientation of the underlying HOPG substrate]; (b) magnified part of the monolayer in panel a (phase A) with a 4,3′-BTPP tetramer highlighted with molecule contours; (c) schematic molecular model of the monolayer in panel b; (d) adsorption model as derived from panel a; and (e) hydrogen bonding motif for the monolayer phase A.

Figure 6. (a, b) STM image time sequence highlighting the monolayer growth of the oligopyridine 4,3′-BTPP with the metastable “2D-liquid” phase M. (c) Adsorption model of both enantiomeric phases. The mirror plane is perpendicular to a primitive substrate vector.

distance was estimated to be 0.41 nm, resulting in a stabilization energy of -2.5 kJ · mol-1. The intermolecular H · · · N distance between the pyridyl subunits in the direction of the C2 axis was estimated to be 0.29 nm, resulting in a stabilization energy of -11 kJ · mol-1. Similar to the other phenylene extended oligopyridines, the H · · · N distances are much longer than those found in the network of the parent compound or than the van der Waals distance, indicating an influence of the surface corrugation to the monolayer periodicity. The molecular monolayer offers rectangular shaped voids, bordered by the pyridyl-phenyl subunits of the oligopyridines, exhibiting a length of 1.8 nm and a width of 0.6 nm. In shape and length, the void is similar to those present in the 3,3′-BTPP monolayer, whereas their width is shortened by 0.1 nm as a result of the different hydrogen bonding pattern in the direction of the molecular C2 axis. Though it is not clearly visible from the STM images (Figure 5a,b) coadsorbed solvent molecules in the voids are expected to be present, too. It has to be noted that the contrast in the voids in the STM image of 4,3′-BTPP (Figure 5) is more diffuse than that in the case of 3,3′-BTPP, indicating that the (solvent) molecules in Figure 5 are fluctuating within the voids because of the less fitting geometry. Hence, it is assumed that the solvent molecules adsorb to fill the space but that they do not significantly contribute to the stabilization of the packing motif.

After deposition from a very diluted solution, ordered structures could only be observed after evaporation of most of the solvent (Figure 6). Induced by the slow evaporation rate of TCB, the slow self-assembly allowed for the observation of a metastable phase M without long-range order which was found in coexistence with the ordered phase A (Figure 6). In phase M, the oligopyridine molecules are physisorbed but their interfacial concentration is in a range in which desorption is impeded through adsorbate-substrate and weak and nondirectional intermolecular interactions. Thus, the phase can be described as a metastable “2D-liquid” phase. With proceeding solvent evaporation, the oligopyridine interfacial concentration increases, resulting in shorter molecule-molecule distances and in more directed and tighter intermolecular interactions, until the complete monolayer is accomplished (Figure 6b). The orientation of the commensurate unit cell, characterized by the lattice vectors A2 and B2, with respect to the substrate lattice expressed in matrix notation is:

( ) (

) ()

A2 a 8 20 ) · B2 b 24 0

and for the enantiomeric phase:

Growth of Oligopyridine Monolayers

( ) (

) ()

A2 20 8 a ) · B2 0 24 b

with primitive substrate lattice vectors a and b. The commensurate unit cell, containing six oligopyridine molecules, is rotated and enlarged compared to the molecular unit cell. Four oligopyridine molecules are adsorbed on the most stable adsorption sites with their atoms centered in the substrate carbon rings, whereas two molecules are adsorbed on the less stable sites with the molecule atoms on top of the substrate atoms. In analogy to 3,3′-BTPP, this configuration seems to be the most stable adsorption configuration for the monolayer packing of 4,3′-BTPP. As is visible from the Cs-symmetric unit cell, the phenylene extended oligopyridine 4,3′-BTPP physisorbs into two mirror symmetric phases. The hydrogen bonding model features the molecule conformation with optimized intermolecular hydrogen bonds. Therefore, in both enantiomorphic phases the C2V molecule symmetry is preserved. As a result, the molecules are achiral at the interface.8 The appearance of homochiral domains is a result of the asymmetric hydrogen bonding interactions perpendicular to the molecular C2 axis. The preference of distinct adsorption sites and the intermolecular interactions drive the oligopyridine molecules into a Cs symmetric unit cell with apparently no vertical mirror plane. Both resulting networks are energetically equivalent. The orientation of the mirror plane between both enantiomorphic domains is given by the orientation of a primitive substrate lattice vector. The B1 lattice vectors of both domains enclose an angle of 79° (Figure 6a). The orientation of the molecules with respect to the substrate is identical in both domains. Therefore, both phases are of identical rotational domains. The angle between the enantiomeric phases in different rotational domains was determined from the adsorption model to be 139° (Figure 5c). On the hexagonal graphite substrate, additional angles of 19° and 79° can be realized, supporting the proposed adsorption model. In analogy to 2,4′-BTPP and 3,3′-BTPP, the experimental observation and their agreement with the adsorption model indicate a significant epitaxial relationship of the 4,3′-BTPP molecules with the substrate. Summary In conclusion, we have investigated the self-assembly behavior of phenylene extended oligopyridine derivatives at the solid-liquid interface of TCB and graphite. The phenylene extended oligopyridine molecules exhibit the same position of the nitrogen atoms in the terminal pyridyl rings as the previously investigated parent compounds.8 Compared to the original oligopyridines, the resulting network structures of the phenylene extended oligopyridines are mainly conducted by the interactions of the adsorbed molecules with the substrate. In the present case, the balance of interadsorbate and adsorbate-substrate interactions is shifted toward the latter one. With more aromatic moieties and the same number of hydrogen bonding acceptors, the molecules prefer to optimize the interactions with the substrate rather than the interadsorbate interactions, which is energetically more advantageous. In all phenylene extended oligopyridine networks, the H · · · N distances are elongated with respect to the equilibrium distance or the van der Waals distance. This can be explained with a snap-in of the molecules on preferential adsorption sites. The H · · · N distance elongation is energetically favored, as a shortening of the hydrogen bonds would destabilize the monolayer through the creation of intermolecular van der Waals repulsion (Pauli repulsion). The

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1513 presented results are of importance for a detailed understanding of the self-assembly of aromatic molecules on surfaces and interfaces. They show that the adsorbate-substrate interactions at the solid-liquid interface may affect the self-assembly behavior as predicted from the possible directional interactions, provided by functional subunits. These results are of high importance for controlling the periodicities or size of monolayer features, generated by the self-assembly process through the design of the molecular backbone structure in a molecular engineering approach. Experimental Section STM Investigations. The STM measurements were performed at the liquid/solid interface under ambient conditions with a low-current RHK 1000 STM system. All three compounds were dissolved in the appropriate solvents (see text) to give concentrated solutions (2 × 10-4 to 3 × 10-4 mol · L-1). Generally, after examination of the quality of the mechanically edged Pt/Ir tip (80/20) through atomic resolution of the freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, SPI-grade 3, SPI), a drop of the concentrated solution was applied to the surface with the tip in tunnel contact. All STM images were recorded in constant current mode with a negative sample bias. The presented images were checked for reproducibility in several sessions by using different tips and were derived from raw data without being subject to any manipulation or image processing except slope compensation. The distances and angles were determined by using an internal calibration. Acknowledgment. We thank the German Science Foundation (“Deutsche Forschungsgemeinschaft”) for financial support within the framework of the Research Center 569 (“Sonderforschungsbereich”) at the University of Ulm. Supporting Information Available: Synthetic details of the employed compounds and theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K.; Angew. Chem. 2000, 112, 1285–1288; Angew. Chem., Int. Ed. 2000, 39, 1230–1234. (2) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–1031. (3) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679. (4) Madueno, R.; Raisanen, M. T.; Silien, C.; Buck, M. Nature 2008, 454, 618–621. (5) Xu, B.; Yin, S. X.; Wang, C.; Zeng, Q. D.; Qiu, X. H.; Bai, C. L. Surf. Interface Anal. 2001, 32, 245–247. (6) De Feyter, S.; Gesquie`re, A.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Nano Lett. 2003, 3, 1485–1488. (7) Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652–13655. (8) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015–21027. (9) Yang, T.; Berber, S.; Liu, J.-F.; Miller, G. P.; Tomanek, D. J. Chem. Phys. 2008, 128, 1247091-8. (10) Claypool, C. L.; Faglioni, F.; Goddard, W. A.; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978–5995. (11) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem. Eur. J. 2006, 12, 3847–3857. (12) Tahara, K.; Johnson, C. A.; Fujita, T.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Haley, M. M.; Tobe, Y. Langmuir 2007, 23, 10190– 10197. (13) Kampschulte, L.; Lackinger, M.; Maier, A.-K.; Kishore, R. S. K.; Griessl, S.; Schmittel, M.; Heckl, W. M. J. Phys. Chem. B 2006, 110, 10829– 10836. (14) Zhou, H.; Dang, H.; Yi, J. H.; Nanci, A.; Rochefort, A.; Wuest, J. D. J. Am. Chem. Soc. 2007, 129, 13774–13775. (15) Dang, H.; Maris, T.; Yi, J. H.; Rosei, F.; Nanci, A.; Wuest, J. D. Langmuir 2007, 23, 11980–11985.

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