Crankshafts: Using Simple, Flat C2h-Symmetric ... - ACS Publications

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Crankshafts: Using Simple, Flat C2h-Symmetric Molecules to Direct the Assembly of Chiral 2D Nanopatterns Hui Zhou and James D. Wuest* Département de Chimie, Université de Montréal, Montréal, Québec H3C 3J7 Canada ABSTRACT: Linear D2h-symmetric bisisophthalic acids 1 and 2 and related substances have well-defined flattened structures, high affinities for graphite, and strong abilities to engage in specific intermolecular interactions. Their adsorption produces characteristic nanopatterns that reveal how 2D molecular organization can be controlled by reliable interadsorbate interactions such as hydrogen bonds when properly oriented by molecular geometry. In addition, the behavior of these compounds shows how large-scale organization can be obstructed by programming molecules to associate strongly according to competing motifs that have similar stability and can coexist smoothly without creating significant defects. Analogous new bisisophthalic acids 3a and 4a have similar associative properties, and their unique C2hsymmetric crankshaft geometry gives them the added ability to probe the poorly understood effect of chirality on molecular organization. Their adsorption shows how nanopatterns composed predictably of a single enantiomer can be obtained by depositing molecules that can respect established rules of association only by accepting neighbors of the same configuration. In addition, an analysis of the adsorption of crankshaft compounds 3a and 4a and their derivatives by STM reveals directly on the molecular level how kinetics and thermodynamics compete to control the crystallization of chiral compounds. In such ways, detailed studies of the adsorption of properly designed compounds on surfaces are proving to be a powerful way to discover and test rules that broadly govern molecular organization in both 2D and 3D.

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INTRODUCTION It remains impossible to foresee in detail how complex flexible molecules will crystallize in 3D.1−4 In contrast, when molecules have well-defined topologies, recent advances have begun to allow accurate structural predictions,5−8 particularly when the topologies act in concert with strong intermolecular interactions to control the relative orientation of neighboring molecules.9 The strategy of using topology and specific interactions in tandem has proven to be very effective for predetermining how molecules are positioned in 3D,10−12 and it is also a powerful method for controlling the 2D organization of molecules physisorbed on surfaces.13−54 Compared to efforts to direct association in 3D, working in 2D is simplified by the effects of surface confinement, which typically require adsorbed molecules to adopt a limited set of suitably flattened conformations; at the same time, however, new forces are exerted on the adsorbates by the underlying surface, and molecular organization is no longer controlled by topology and interadsorbate interactions alone. Nevertheless, a growing body of evidence suggests that strong intermolecular interactions such as hydrogen bonds can be designed to have a major effect on 2D organization, leading to the creation of predictable molecular nanopatterns even when they are not commensurate with the underlying surface.13,34−36,55−57 By choosing such systems for study, crystal engineers and surface scientists can now examine subtle aspects of molecular organization in 2D without fearing that their observations and conclusions are unduly influenced by effects of the surface on which adsorption occurs. © 2012 American Chemical Society

Among the subtle factors that influence molecular organization is chirality. Predicting how racemic mixtures of chiral molecules will crystallize is complicated by the existence of multiple options, including the possibility that each crystal will contain both enantiomers in a 1:1 ratio (racemic crystal) or that each crystal will be composed of only one enantiomer (conglomerate). Statistical analyses of reported 3D structures have shown that racemic crystals are much more common than conglomerates,58,59 although the reasons for this preference are not fully understood. Analogous alternatives arise when molecules that are chiral in 2D are adsorbed on surfaces. Earlier studies of this phenomenon have suggested that racemic crystals are less predominant in 2D than in 3D,37−39,60−66 but no clear consensus about the generality of these data has emerged, and the reasons for significantly different behavior in 2D and 3D have not been fully delineated. Molecular chirality has widespread consequences in nature, and adsorption on surfaces to create chiral patterns may have been a critical step in the origin of life. As a result, it is important to examine 2D organization in systems where the molecular components are simple and the surface plays a relatively minor role, allowing the intrinsic effects of chirality to come to the forefront. Special Issue: Interfacial Nanoarchitectonics Received: September 11, 2012 Revised: October 20, 2012 Published: October 23, 2012 7229

dx.doi.org/10.1021/la303659c | Langmuir 2013, 29, 7229−7238

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Figure 1. Linear bisisophthalic acids 1 and 2 and representations of nanopatterns produced by their association.

Figure 2. Linear structure III, crankshaft isomer IV, bisisophthalic acids 3a and 4a, and corresponding tetraesters 3b and 4b.

by the simple expedient of changing how the spacers are joined to the core (Figure 2).70−72 We selected bisisophthalic acids 3a and 4a as specific examples of crankshaft architecture IV (Figure 2), prepared the compounds and their corresponding esters 3b and 4b by straightforward routes, and examined the adsorption of all four compounds on HOPG. The behavior of these compounds was expected to help reveal how molecular organization in 2D can be controlled when chirality, geometry, and strong interadsorbate interactions all exert important effects.37−43 A unique feature of this study is the opportunity to compare the 2D organization of prochiral C2h-symmetric compounds 3a,b and 4a,b with that of achiral D2h-symmetric analogues. Both series of compounds have similar geometries, interadsorbate interactions, and affinities for the underlying surface, thereby isolating chirality as the primary variable and allowing inherent preferences for forming racemic crystals and conglomerates in 2D to be compared without bias.

To probe the inherent preferences of molecules that are chiral in 2D, we elected to study a new family of compounds related to linear bisisophthalic acids 1 and 2 and their analogues (Figure 1).13−19 These compounds have elongated flattened topologies, virtually identical abilities to form intermolecular hydrogen bonds with COOH groups according to established patterns,67,68 and strong affinities for graphite, as estimated by DFT calculations13,69 and confirmed by scanning tunneling microscopy (STM), which has produced well-resolved images under various conditions.13−19 The adsorption of compounds 1 and 2 and analogues on highly oriented pyrolytic graphite (HOPG) typically generates characteristic nanopatterns in which adjacent molecules are positioned according to plan, as determined by their linear topology and the formation of predictable interadsorbate hydrogen bonds involving the COOH groups. The preferred organization can be represented by parallel structure I and Kagome structure II (Figure 1) in which the 1,3,5-trisubstituted phenyl groups of compounds 1 and 2 are shown as triangles, acetylenic linkers are represented by solid lines joining the vertices of the triangles, and cyclic hydrogen-bonded pairs of COOH groups are denoted by broken lines between vertices. Compounds 1 and 2 are members of a broad class of D2hsymmetric compounds represented by structure III (Figure 2) in which two sites of molecular recognition (gray circles) are linked by collinear spacers to a central molecular core (large gray rectangle). Structure III is achiral in 2D, but C2hsymmetric crankshaft isomer IV is prochiral and can be created

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RESULTS AND DISCUSSION Syntheses of Compounds 3a,b and 4a,b. Tetraesters 3b and 4b were prepared in 87 and 66% yields, respectively, by the Sonogashira coupling of diethyl 5-iodo-1,3-benzenedicarboxylate73 with 1,5-diethynylnaphthalene74 and 1,8-diethynylanthracene.75 Hydrolysis of the tetraesters gave corresponding tetraacids 3a and 4a in 81 and 97% yields, respectively. Two-Dimensional Crystallization of Tetraacid 3a. A freshly exposed surface of HOPG was treated with a droplet of a solution of tetraacid 3a in heptanoic acid, and the liquid− 7230


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solid interface was analyzed by STM. Representative images are shown in Figure 3, along with superimposed unit cells and

The local order in Figure 3c is of two basic types, which can be described as parallel and chevron patterns. The parallel pattern is obviously related to standard motif I, previously observed when achiral D2h-symmetric bisisophthalic acids are adsorbed on HOPG. However, the adsorption of prochiral C2hsymmetric analogues in parallel can generate a periodic array only when all molecules in the domain have the same configuration, as represented by motif V or its enantiomeric form (Figure 4). The observed homochiral pattern of adsorption is the logical result of combining a crankshaft geometry with a preference for parallel association directed by hydrogen bonding. The chevron pattern that appears in Figure 3 is unexpected but arises plausibly from association according to racemic motif VI. This motif is the fortuitous result of the particular molecular structure of tetraacid 3a, and similar patterns cannot be formed by linear D2h-symmetric analogues for geometric reasons nor can they arise from the adsorption of other crankshaft molecules unless the interadsorbate hydrogen bonds are substantially deformed. A careful examination of Figure 3c,d provides visual evidence for the crankshaft geometry of individual adsorbed molecules and for the racemic composition of the chevron phase. The unit cell parameters are measured to be a = 2.1 ± 0.1 nm, b = 2.1 ± 0.1 nm, and γ = 50 ± 1° for the parallel network and a = 2.1 ± 0.1 nm, b = 3.0 ± 0.1 nm, and γ = 87 ± 1° for the chevron network. Using models V and VI, with H···O distances chosen to be 2.05 Å, we estimate the unit cell parameters to be a = 1.99 nm, b = 2.15 nm, and γ = 50° for the parallel network and a = 2.19 nm, b = 3.05 nm, and γ = 90° for the chevron network. The close agreement between the empirical and theoretical values provides additional support for our structural assignments. The densities of surface coverage are 0.30 and 0.32 molecule/nm2 for the parallel and chevron phases, respectively, so the unexpected chevron motif may be formed in part because it allows slightly closer molecular packing. Both networks leave a significant fraction of the surface available for additional adsorption, and areas of dark contrast in STM images that appear to be empty may be temporarily occupied by solvent. The high degree of apparent openness is a predictable feature of the molecular structure of tetraacid 3a and related compounds, which cannot form networks that are both closely packed and optimally hydrogen-bonded. In such cases, the imperatives of strong intermolecular interactions often outweigh the advantages of close packing, in both 2D and 3D, even when the resulting structure must incorporate substantial interstices occupied by guests.10 Given the relatively large aromatic surface of tetraacid 3a and its correspondingly high affinity for HOPG, it is noteworthy that a few judiciously positioned hydrogen bonds can prevent the compound from achieving a higher density of coverage. This suggests that a small number of strong interadsorbate interactions can match or exceed the sum of all adsorbate−substrate interactions in importance, at least in the case of tetraacid 3a adsorbed on HOPG. An examination of Figure 3d reveals multiple sites where areas of local parallel organization (motif V or its enantiomer) merge smoothly with adjacent areas of chevron organization (motif VI). The sustained growth of a single phase is kinetically problematic because each new molecule recruited by an existing local array can be added in two distinct ways with essentially identical thermodynamic consequences, as assessed by the contribution of the new recruit to the overall heat of adsorption

Figure 3. STM images of the adsorption of tetraacid 3a on HOPG (deposition from heptanoic acid with Vbias = −1.5 V and Iset = 100 pA). (a) View of an area of 80 nm × 80 nm, with a region highlighted in blue. (b) Enlargement of the area highlighted in part a (40 nm × 40 nm). (c) Additional enlargement showing smoothly intersecting domains composed of parallel motif V (domain B) and chevron motif VI (domains A). (d) Further enlargement showing the measured unit cells for the two motifs, with molecular models superimposed to facilitate the interpretation of the image.

schematic models of the proposed molecular organization. The images are well-resolved, demonstrating that tetraacid 3a has a strong affinity for the surface despite the absence of long alkyl chains, which are often used to facilitate the adsorption of molecules on HOPG.76−78 Even though a high degree of organization is clearly visible in Figure 3a,b, the sustained growth of large periodic arrays is not observed (Figure 3c). A similarly striking reluctance to crystallize in 2D is exhibited by analogue 2 and related compounds,13,14 and it has been attributed to frustration caused by an inability to choose among closely related molecular arrangements. Specifically, frustration in 2D appears to arise when the alternatives have essentially identical heats of adsorption, densities of packing, and intermolecular interactions and when the competing networks can coexist and merge fluidly without necessarily introducing defects where molecules are missing, improperly oriented, or unable to form optimal hydrogen bonds with their neighbors. The relatively small number of flagrant defects may help to stabilize the disordered phase kinetically by removing sites of inherently high molecular mobility. The broad implications of frustrated 2D crystallization are still being explored, but the phenomenon has now been confirmed to occur in chemically diverse systems,13,14,79 and further study may spur advances in important areas of science and technology such as learning how to control local molecular organization in glasses or finding ways to make amorphous and polyamorphous molecular materials by design.80−82 7231


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Figure 4. Representations of homochiral parallel motif V and heterochiral chevron motif VI. Each motif can be considered to result from the side-byside association of hydrogen-bonded chains, shown in black and gray. Hydrogen bonds are represented by broken lines.

illustrates how crystallization can be thwarted by programming molecules to associate strongly according to competing motifs that have similar stability and can coexist smoothly over large areas. Tetraacid 3a is of special interest because it is chiral in 2D; moreover, its adsorption simultaneously produces homochiral and heterochiral patterns V−VI of very similar thermodynamic stability, making it possible to use compound 3a to clarify in molecular detail how kinetic factors help control crystallization. In particular, STM images show an apparent statistical bias favoring the growth of racemic phase VI, which may help explain why racemic crystals are more common than homochiral conglomerates in 3D. In addition, racemic motif VI is found to be denser than homochiral alternative V, which is consistent with a similar tendency in 3D crystallization known as Wallach’s rule.59 Such observations show how using STM to probe the details of adsorption on surfaces can provide a valuable new understanding of molecular organization in 3D. Two-Dimensional Crystallization of Tetraester 3a. To better define the role of chirality in controlling adsorption, we decided to compare the behavior of tetraacid 3a with that of tetraester 3b, which has a very similar topology and affinity for HOPG69 but no ability to form strong interadsorbate hydrogen bonds. A solution of tetraester 3b in heptanoic acid was applied to the surface of HOPG, and STM was used to examine the liquid−solid interface. Figure 5 provides typical images, along with superimposed unit cells and schematic models of the proposed molecular organization. The observation of wellresolved individual molecules confirms that tetraester 3b has a

of the array and to the total interadsorbate interactions. For sustained growth according to the same pattern, local domains composed of homochiral parallel motif V must recruit new molecules adsorbed in the same configuration, whereas heterochiral chevron motif VI can accommodate either 2D enantiomer. In theory, racemic motif VI therefore enjoys a kinetic bias, and experimental evidence can be adduced by statistical analyses of STM images. For example, Figure 3d shows approximately 40 individual molecules that are sufficiently well resolved to allow their long axes to be clearly identified, thereby allowing relative orientations to be categorized unambiguously as parallel or chevron. Together, these molecules enclose approximately 40 open interstitial spaces (regions of dark contrast), each bounded either by parallel motif V (or its enantiomer) or chevron motif VI. The two types of spaces are present in a ratio of approximately 7:3 in favor of racemic chevron motif VI, which reflects its inherent 2-fold statistical advantage and its slightly higher density of coverage. Analyses of other 13 nm × 13 nm areas of adsorption chosen at random confirm a general preference for local organization according to chevron motif VI. Despite its structural simplicity, tetraacid 3a is a powerful probe for examining the fundamentals of molecular organization. In particular, its behavior shows how geometry and strong interactions can operate in concert to position adjacent molecules in predictable ways, even when the resulting structure is not closely packed or commensurate with an underlying surface. In addition, the behavior of tetraacid 3a 7232


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Figure 6. Representation of homochiral square-grid motif VII. C− H···O interactions are represented by broken lines.

Figure 5. STM images of the adsorption of tetraester 3b on HOPG (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 100 pA). (a) View of an area of 43 nm × 43 nm showing multiple intersecting domains, with one area of intersection highlighted in blue. (b) Enlarged view (20 nm × 20 nm) of the area highlighted in part a, showing two adjacent domains A and B. (c) Further enlargement of domain A, showing the measured unit cell with molecular models superimposed to facilitate the interpretation of the image. (d) Similar enlargement of domain B.

square-grid organization of related compounds on various surfaces.40−43,83 The perpendicular orientation and crankshaft geometry of adsorbed molecules of tetraester 3b are evident in enlarged Figure 5c,d. Additional supporting evidence for proposed motif VII is provided by an estimation of the unit cell parameters (a = b = 2.09 nm and γ = 90° based on the assumption that the H···O distance is 2.42 Å), which are very similar to the measured values. As expected, homochiral domains of opposite configuration are formed with equal probability (Figure 5c,d). Together, these observations show that it is possible to create 2D analogues of 3D crystalline conglomerates by design, using prochiral molecules that can obey established patterns of association only by accepting neighbors of the same configuration. Two-Dimensional Crystallization of Tetraacid 4a. For additional information, we turned to tetraacid 4a, an analogue of compound 3a with an anthracene core. The surface of HOPG was exposed to a droplet of a solution of tetraacid 4a in heptanoic acid, and the liquid−solid interface was analyzed by STM, which yielded the representative images shown in Figure 7. Well-resolved patterns are observed, with unit cell parameters of a = 1.8 ± 0.1 nm, b = 1.7 ± 0.1 nm, and γ = 70 ± 1°. These values are inconsistent with the adoption of hydrogen-bonded parallel, chevron, or Kagome networks of the type formed by related bisisophthalic acids. Instead, the large molecular surface of compound 4a appears to require a more closely packed arrangement in which the density of coverage is increased at the expense of optimal hydrogen bonding. The observed organization appears to result from the formation of homochiral motif VIII (or its enantiomer), which is maintained by a network of O−H···O hydrogen bonds and C−H···O interactions (Figure 8). The unit cell parameters expected for this structure (a = 1.82 nm, b = 1.76 nm, and γ = 63° based on a H···O distance of 2.42 Å) are in good agreement with the experimental values. Motif VIII can be described as the result of

strong affinity for the surface even though it lacks features typically used to promote adsorption on HOPG, such as long alkyl chains or functional groups that favor the formation of networks held together by strong interadsorbate interactions.76−78 Adsorption shows a clear preference for the formation of a square-grid pattern,40−43 and the measured unit cell parameters are a = b = 2.2 ± 0.1 nm and γ = 90 ± 1°. Figure 5a reveals multiple intersecting domains with no consistent relative orientation, suggesting that the underlying surface may not enforce a commensurate relationship. Closely analogous square-grid patterns are typically formed when tetraesters derived from tetraacid 2 and related D2hsymmetric tetraacids are deposited on HOPG.83 This suggests that the square-grid motif has significant generality and is able to meet the challenge of accommodating prochiral tetraester 3b. Specifically, we propose that molecules of compound 3b adopt a planar C2h conformation, assume a perpendicular orientation with respect to neighbors, and form a set of characteristic interadsorbate C−H···O interactions to produce homochiral square-grid motif VII or its enantiomer (Figure 6).84,85 As in the case of tetraacid 3a, a homochiral phase results logically from the deposition of molecules that have a crankshaft geometry and an innate preference for a particular pattern of association that can accommodate only a single enantiomer. The network of intermolecular C−H···O interactions in proposed motif VII is plausible, but no analogous association has been noted in 3D crystal structures of isophthalate esters or related compounds. However, similar C−H···O interactions have been postulated to explain the 7233


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2D crystallization of crankshaft compound 4a is analogous to the formation of homochiral conglomerates in 3D. Two-Dimensional Crystallization of Tetraester 4b. To probe the ability of anthracene units to favor close molecular packing, we examined the behavior of tetraester 4b. A droplet of a solution in heptanoic acid was applied to a freshly exposed surface of HOPG, the liquid−solid interface was imaged by STM, and typical results are shown in Figure 9. Closely packed parallel patterns were found to be adsorbed over large areas (Figure 9a), and they proved to consist of homochiral tapes in which molecules are packed closely side-by-side (Figure 9b). The measured unit cell parameters are a = 1.1 ± 0.1 nm, b = 2.4 ± 0.1 nm, and γ = 85 ± 1°. Structure IX provides a detailed view of the proposed organization (Figure 10), which is maintained in part by a network of C−H···O interactions similar to those formed when an analogous D2h-symmetric anthracene tetraester is adsorbed on HOPG.83 The unit cell parameters expected for motif IX (a = 0.99 nm, b = 2.38 nm, and γ = 86° based on a H···O distance of 2.42 Å) agree well with the empirical values. Domains are composed uniquely of a single enantiomer of tetraester 4b, and its 2D crystallization is analogous to the formation of conglomerates in 3D. Once again, a homochiral pattern of 2D organization arises logically from the deposition of crankshaft molecules that favor a mode of association in which neighbors must have the same configuration.

Figure 7. STM images of the adsorption of tetraacid 4a on HOPG (deposition from heptanoic acid with Vbias = −1.5 V and Iset = 100 pA). (a) View of an area of 36 nm × 36 nm. (b) Enlarged view of an area of 18 nm × 18 nm. (c) Further enlargement, with the measured unit cell and molecular models superimposed to facilitate interpretation.

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CONCLUSIONS

Linear D2h-symmetric bisisophthalic acids 1 and 2 and related substances have been featured in a series of recent studies of molecular organization.13−19,83 These compounds are characterized by well-defined flattened structures, high affinities for surfaces, and strong abilities to engage in specific intermolecular interactions. Their adsorption on graphite produces character-

deforming a standard parallel network (Figure 4) by sliding adjacent hydrogen-bonded chains and bringing them closer together, thereby replacing interchain O−H···O hydrogen bonds by C−H···O interactions. Again, the preferred mode of

Figure 8. Representation of homochiral motif VIII. Key intermolecular interactions are represented by broken lines. 7234


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Figure 9. STM images of the adsorption of tetraester 4b on HOPG (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 100 pA). (a) View of an area of 80 nm × 80 nm showing parallel tapes. (b) Enlarged view (10 nm × 10 nm) showing homochiral tapes in detail, with the measured unit cell and molecular models superimposed to facilitate the interpretation of the image.

respect established rules of association only by accepting neighbors of the same configuration. In addition, analyses of adsorbed crankshaft compounds by STM reveal directly at the molecular level how kinetics and thermodynamics compete to control the crystallization of chiral compounds. In such ways, detailed studies of the adsorption of properly designed compounds on surfaces are proving to be a powerful way to discover and test basic rules of molecular organization in both 2D and 3D.

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EXPERIMENTAL SECTION

Syntheses of Compounds 3a,b and 4a,b: General Notes. Unless otherwise indicated, reagents needed for syntheses were purchased from commercial sources and used without further purification. Anhydrous oxygen-free solvents were obtained by passing them through columns packed with activated alumina and supported copper catalyst. Synthesis of Diethyl 5,5′-(1,5-Naphthalene-2,1-ethynediyl)bis(1,3-benzenedicarboxylate) (3b). A mixture of dry THF (20 mL) and diisopropylamine (10 mL) was degassed by bubbling dry N2 through it for 30 min. Diethyl 5-iodo-1,3-benzenedicarboxylate (0.898 g, 2.58 mmol)73 was added, followed by bis(triphenylphosphine)palladium(II) dichloride (0.048 g, 0.068 mmol) and copper(I) iodide (0.032 g, 0.17 mmol). The mixture was stirred at room temperature for 15 min, and then a solution of 1,5-diethynylnaphthalene (0.190 g, 1.08 mmol)74 in dry THF (10 mL) was added dropwise. The resulting mixture was stirred overnight at room temperature, and the precipitated solid was then separated by filtration and washed successively with water and acetone to give tetraester 3b (0.580 g, ̀ 0.941 mmol, 87%) as a yellow solid: mp 190 °C; IR (KBr) 2978, 1727, 1240 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.67 (t, 4J = 1.2 Hz, 2H), 8.50 (d, 3J = 8.0 Hz, 2H), 8.47 (d, 4J = 1.2 Hz, 4H), 7.87 (d, 3J = 8.0 Hz, 2H), 7.63 (t, 3J = 8.0 Hz, 2H), 4.46 (q, 3J = 7.2 Hz, 8H), 1.46 (t, 3J = 7.2 Hz, 12H); 13C NMR (100 MHz, CDCl3) δ 165.61, 136.81, 133.43, 131.92, 131.80, 130.61, 127.87, 126.80, 124.56, 121.15, 93.19, 89.39, 62.08, 14.76; HRMS (ESI) calcd for C38H32O8 + H m/e 617.2170, found 617.2165. Synthesis of 5,5′-(1,5-Naphthalene-2,1-ethynediyl)bis(1,3benzenedicarboxylic acid) (3a). Tetraester 3b (0.312 g, 0.506 mmol), methanol (5 mL), and solid potassium hydroxide (0.256 g, 4.56 mmol) were added to a 15 mL pressure tube containing a stirring bar. The tube was closed, and the mixture was stirred and held at 80

Figure 10. Representation of homochiral motif IX. C−H···O interactions are represented by broken lines.

istic nanopatterns that reveal how 2D molecular organization can be controlled by reliable interadsorbate interactions such as hydrogen bonds, when properly oriented by molecular geometry. The patterns typically have no preferred alignment with the underlying surface, showing that organization is controlled primarily by the molecular structure of the adsorbates themselves. In addition, the behavior of these compounds reveals how large-scale organization can be obstructed by programming molecules to associate strongly according to competing motifs that have similar stability and can coexist smoothly without creating significant defects. Analogous new bisisophthalic acids 3a and 4a have similar associative properties, and their unique C2h-symmetric crankshaft geometry gives them the added ability to probe the poorly understood effect of chirality on molecular organization. The adsorption of these crankshaft compounds and their derivatives shows how nanopatterns composed predictably of a single enantiomer can be obtained by depositing molecules that can 7235


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°C for 3 h. Water (10 mL) was then added to dissolve solids, and the resulting solution was acidified to pH 1 to 2 with concentrated aqueous HCl (10 N). The precipitated solid was separated by filtration and washed successively with water and dichloromethane to give pure tetraacid 3a (0.208 g, 0.412 mmol, 81%) as a pale-yellow solid: mp >250 °C; IR (KBr) 3100 (br), 1713, 1191 cm−1; 1H NMR (400 MHz, DMSO-d6) δ 13.6 (bs, 4H), 8.53 (d, 3J = 8.0 Hz, 2H), 8.49 (s, 2H), 8.41 (s, 4H), 8.02 (d, 3J = 8.0 Hz, 2H), 7.77 (t, 3J = 8.0 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ 166.75, 136.60, 133.14, 132.97, 132.81, 130.84, 128.20, 128.05, 124.11, 120.80, 93.77, 89.31; HRMS (ESI) calcd for C30H16O8 H m/e 503.0772, found 503.0776. Synthesis of Diethyl 5,5′-(1,8-anthracene-2,1-ethynediyl)bis(1,3-benzenedicarboxylate) (4b). A mixture of dry THF (30 mL) and diisopropylamine (15 mL) was degassed by bubbling dry N2 through it for 30 min. Diethyl 5-iodo-1,3-benzenedicarboxylate (0.873 g, 2.51 mmol)73 was added, followed by bis(triphenylphosphine)palladium(II) dichloride (0.044 g, 0.063 mmol) and copper(I) iodide (0.043 g, 0.23 mmol). The mixture was stirred at room temperature for 15 min, and then a solution of 1,8-diethynylanthracene (0.258 g, 1.14 mmol)75 in dry THF (5 mL) was added dropwise. The mixture was stirred overnight at room temperature. The resulting solid was separated by filtration and dissolved in chloroform, and the organic phase was washed with water and dried over anhydrous MgSO4. The removal of volatiles by evaporation under reduced pressure left a residue of tetraester 4b (0.501 g, 0.751 mmol, 66%) as a bright-yellow solid: mp 220 °C; IR (KBr) 2982, 1727, 1241 cm−1; 1H NMR (400 MHz, CDCl3) δ 9.01 (s, 2H), 8.70 (s, 2H), 8.55 (s, 4H), 8.19 (d, 3J = 8.0 Hz, 2H), 7.87 (d, 3J = 8.0 Hz, 2H), 7.54 (t, 3J = 8.0 Hz, 2H), 4.50 (q, 3J = 7.2 Hz, 8H), 1.49 (t, 3J = 7.2 Hz, 12H); 13C NMR (100 MHz, CDCl3) δ 165.28, 136.60, 136.52, 131.69, 131.45, 131.40, 131.19, 130.22, 125.78, 125.30, 124.33, 120.23, 92.87, 89.38, 61.70, 14.37; HRMS (ESI) calcd for C42H34O8 + H m/e 667.2326, found 667.2308. Synthesis of 5,5′-(1,8-Anthracene-2,1-ethynediyl)bis(1,3benzenedicarboxylic acid) (4a). Tetraester 4b (0.200 g, 0.300 mmol), methanol (4 mL), dichloromethane (2 mL), and solid potassium hydroxide (0.084 g, 1.5 mmol) were added to a 15 mL pressure tube containing a stirring bar. The tube was closed, and the mixture was stirred and held at 80 °C overnight. Water (10 mL) was then added to dissolve the solids, and the resulting solution was acidified to pH 1 to 2 with concentrated aqueous HCl (10 N). The precipitated solid was separated by filtration and washed successively with water and dichloromethane to give tetraacid 4a (0.162 g, 0.292 mmol, 97%) as a light-yellow solid: mp >250 °C; IR (KBr) 3100 (br), 1717, 1202 cm−1; 1H NMR (400 MHz, DMSO-d6) δ 13.6 (bs, 4H), 9.14 (s, 2H), 8.51 (d, 4J = 1.6 Hz, 4H), 8.50 (t, 4J = 1.6 Hz, 2H), 8.46 (d, 3J = 8.0 Hz, 2H), 8.02 (d, 3J = 8.0 Hz, 2H), 7.64 (t, 3J = 8.0 Hz, 2H); 13C NMR (100 MHz, DMSO-d6) δ 166.83, 136.73, 132.91, 132.81, 132.14, 131.39, 131.23, 130.78, 126.50, 126.27, 124.33, 120.10, 93.76, 89.76; HRMS (ESI) calcd for C34H18O8−H m/e 553.0929, found 553.0935. Studies of Two-Dimensional Crystallization by STM. All STM experiments were performed at room temperature (20−25 °C) using a JEOL-5200 SPM instrument equipped with a narrow scanner. Platinum/iridium STM tips were mechanically cut from wire (Pt/Ir, 80%/20%, diameter = 0.25 mm). In typical experiments, the freshly cleaved basal surface of HOPG (Structure Probe, Inc., SPI-1 grade) was first imaged to determine the quality of the Pt/Ir tip and the smoothness of the graphite surface. Once this was determined, a droplet (∼1 μL) of solutions of compounds 3a,b and 4a,b in heptanoic acid (∼10−4 M) was applied. STM investigations were then carried out at the liquid−solid interface in constant-current mode. STM imaging was performed by changing the tunneling parameters (voltage applied to the tip and the average tunneling current). Raw STM images were processed using a JEOL software package (WinSPM Data Processing System, version 2.15, R. B. Leane, JEOL Ltd.) and freeware (WSxM 5.0 Develop 1.2, Nanotec Electrónica S. L.).86 A smooth 3 × 3 matrix convolution filter was used to produce the final images.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministère de l’Éducation du Québec, the Canada Foundation for Innovation, the Canada Research Chairs Program, and Université de Montréal for financial support. In addition, we thank NanoQuébec for supporting infrastructure used to study nanostructured molecular materials (IMC/MMN), and we are grateful to JiHyun Yi and Prof. Antonio Nanci for helping us obtain STM images.

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