Using Systematic Comparisons of 2D and 3D Structures To Reveal

Article pubs.acs.org/JPCC

Using Systematic Comparisons of 2D and 3D Structures To Reveal Principles of Molecular Organization. Tetraesters of Linear Bisisophthalic Acids Hui Zhou, Thierry Maris, and James D. Wuest* Département de Chimie, Université de Montréal, Montréal, Québec H3C 3J7, Canada S Supporting Information *

ABSTRACT: Linear bisisophthalic acids 1 and 2 and analogous structures are known to be adsorbed on graphite to give nanopatterns that are programmed by the concerted effects of topology and hydrogen bonding. For comparison, we have now studied the corresponding tetraesters 3−7, which have similar topologies and affinities for graphite but cannot form strong intermolecular interactions. As a result, they fail to crystallize in 2D and 3D according to consistent patterns. The sharply contrasting behavior of the tetraacids and tetraesters provides compelling evidence for the hypothesis that molecular organization is best controlled in both 2D and 3D by using topology and strong directional interactions in tandem to control the relative orientation of neighbors. When topology and dominant intermolecular interactions are in harmony, then organization can be expected to follow reliable patterns within a related series of compounds, and structures in 2D and 3D can be designed to show high levels of homology.

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INTRODUCTION The structures and properties of individual molecules can often be predicted with confidence, but molecular organization and aggregate behavior in materials remain poorly understood.1 In recent years, much effort has been devoted to learning more about these collective phenomena.2 This effort has been driven by a desire to reveal fundamental principles of molecular association as well as to create useful new materials by design. Early efforts to understand and direct organization in ordered molecular materials focused on the topology of the components as a key determinant of packing.3 A major subsequent advance was the recognition that organization reflects both topology and intermolecular interactions and that order can be best controlled when proper topology acts in concert with strong directional interactions.4 In this way, neighboring molecules can be compelled to adopt specific relative orientations. This strategy can provide predictably ordered periodic nanostructures, built by self-assembly from molecular components with suitable topologies and an ability to engage in reliable intermolecular interactions. This approach is now used routinely for diverse purposes in science and technology, such as for engineering molecular crystals,4,5 for inducing controlled molecular adsorption on surfaces,6−12 for accomplishing other tasks requiring defined molecular positioning,13 and even for avoiding ordered states by intentionally choosing topologies and interactions that jointly conspire against the formation of extended periodic arrangements.6,7,14 The strategy of using topology and directional interactions in tandem to direct molecular organization is broadly effective, and its full scope is being actively explored. Further testing of the strategy is particularly important when assembly occurs in 2D on surfaces, where organization of the adlayer is controlled by the complex © 2012 American Chemical Society

interplay of molecular topology, interadsorbate interactions, and the effects of the underlying surface itself. Understanding the origins of preferred patterns of organization requires detailed studies in which the molecular structures of a series of related compounds are altered systematically, and the effect of these changes on assembly is probed. Ideally, such studies should assess both 2D and 3D organization, using scanning probe microscopy and X-ray diffraction as complementary sources of detailed structural information. Surprisingly, such integrated analyses are still rare in studies of molecular association,15 but they are becoming an important source of new insights. In earlier work, we reported the adsorption of linear bisisophthalic acids 1 and 2 and related tetracarboxylic acids on graphite,6 and similar compounds have subsequently been studied by many other groups.7−9,16 These compounds have closely related elongated topologies, essentially identical abilities to engage in intermolecular hydrogen bonding of COOH groups according to reliable patterns,17 and strong affinities for graphite, as estimated by DFT calculations6,18 and confirmed by scanning tunneling microscopy (STM), which revealed well-resolved images under various conditions.6−9 Depositing solutions of compounds 1 and 2 and analogues on the surface of highly oriented pyrolytic graphite (HOPG) typically yields adsorbed nanopatterns of two basic types, in which the relative positions of neighboring molecules are determined, as planned, by their linear geometry and their ability to engage in hydrogen bonding of COOH groups. These patterns can be represented by parallel structure I and Kagome structure II, in which the 1,3,5-trisubstituted phenyl groups of compounds 1 and 2 are shown as triangles, acetylenic connectors Received: January 2, 2012 Revised: May 29, 2012 Published: May 29, 2012 13052

dx.doi.org/10.1021/jp300029z | J. Phys. Chem. C 2012, 116, 13052−13062

The Journal of Physical Chemistry C

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are represented by solid lines joining vertices of the triangles, and cyclic hydrogen-bonded pairs of COOH groups are denoted by broken lines between vertices.

HOPG, and we have concurrently used X-ray crystallography to examine the 3D organization of selected examples. Tetraesters 3−7 are close structural relatives of tetraacids 1 and 2, and previous computational studies have suggested that benzoic acids and their simple alkyl esters have similar affinities for graphite.18 Indeed, we have found that tetraesters 3−7 are adsorbed on HOPG, but their 2D organization is distinctly different from that of analogues 1 and 2. Our observations thereby underscore the critical role of strong directional interactions such as hydrogen bonds as determinants of molecular organization in 2D.

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EXPERIMENTAL SECTION Syntheses of Tetraesters 3−7. As previously reported,6 tetraesters 3 and 4a were prepared by Sonogashira coupling of diethyl 5-iodo-1,3-benzenedicarboxylate19 with diethyl 5-ethynyl-1,3-benzenedicarboxylate19 and 1,4-diethynylbenzene, respectively. Analogous methods were used to make esters 4b,c.20 Coupling of diethyl 5-iodo-1,3-benzenedicarboxylate19 with 4,4′diethynylbiphenyl,21 1,4-diethynylnaphthalene,22 and 9,10-diethynylanthracene22 under similar conditions gave tetraesters 5− 7, as described in detail below. Additional reagents needed for these syntheses were purchased from commercial sources and used without further purification. Synthesis of Diethyl 5,5′-[4,4′-(1,1′-Biphenyl)-2,1ethynediyl]bis(1,3-benzenedicarboxylate) (5). A mixture of dry THF (30 mL) and di-isopropylamine (15 mL) was degassed by bubbling dry N2 through it for 30 min. Diethyl 5iodo-1,3-benzenedicarboxylate (1.75 g, 5.03 mmol)19 was added, followed by bis(triphenylphosphine)palladium(II) dichloride (0.093 g, 0.13 mmol) and copper(I) iodide (0.052 g, 0.27 mmol). The mixture was stirred at room temperature for 15 min, and then a solution of 4,4′-diethynylbiphenyl (0.513 g, 2.54 mmol)21 in dry THF (10 mL) was added dropwise. The resulting mixture was stirred overnight at room temperature and was then filtered. Hexane was added to the filtrate to form a precipitate, which was then separated by filtration and washed successively with water and hexane to give tetraester 5 (0.800 g, 1.24 mmol, 49%) as a yellow solid: mp 153 °C; IR (KBr) 2980, 1726, 1242 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.64 (t, 4J = 1.6 Hz, 2H), 8.38 (d, 4J = 1.6 Hz, 4H), 7.64 (s, 8H), 4.44 (q, 3J = 8.0 Hz, 8H), 1.44 (t, 3J = 8.0 Hz, 12H); 13C NMR (100 MHz, CDCl3) δ 165.60, 140.84, 136.78, 132.70, 131.72, 130.48, 127.42, 124.57, 122.34, 91.33, 88.93, 62.01, 14.74; HRMS (ESI) calcd for C40H34O8 + H m/e 643.2326, found 643.2306. Synthesis of Diethyl 5,5′-(1,4-Napthalene-2,1ethynediyl)bis(1,3-benzenedicarboxylate) (6). A mixture of dry THF (30 mL) and di-isopropylamine (15 mL) was degassed by bubbling dry N2 through it for 30 min. Diethyl 5iodo-1,3-benzenedicarboxylate (2.10 g, 6.03 mmol)19 was added, followed by bis(triphenylphosphine)palladium(II) dichloride (0.120 g, 0.171 mmol) and copper(I) iodide (0.057 g, 0.30 mmol). The mixture was stirred at room temperature for 15 min, and then a solution of 1,4-diethynylnaphthalene (0.590 g, 3.35 mmol)22 in dry THF (5 mL) was added dropwise. The mixture was stirred overnight at room temperature. Removal of volatiles by evaporation under reduced pressure left a residue, which was then purified by flash chromatography (1:20 hexane/CHCl3) to provide tetraester 6 (1.63 g, 2.64 mmol, 88%) as a yellow solid: mp 141 °C; IR (KBr) 2981, 1723, 1188 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.66 (s, 2H), 8.46 (s, 4H), 8.46 (m, 2H), 7.77 (s, 2H), 7.70 (m, 2H), 4.45 (q, 3J = 7.2 Hz, 8H), 1.46 (t, 3J = 7.2 Hz, 12H); 13C NMR (100 MHz, CDCl3) δ 165.56, 136.81, 133.34, 131.81, 130.68, 130.47, 128.06, 126.98, 124.45, 121.72, 94.56,

The observed nanopatterns can be interpreted as the logical result of molecular design based on the following general principles: (1) The molecules should be able to adopt flattened conformations with strong affinities for the underlying surface; (2) the adsorbates should incorporate properly oriented functional groups that engage in strong intermolecular interactions; and (3) the geometry and interactions of the adsorbates should operate in concert to place adjacent molecules in predetermined positions and to direct periodic tiling of the surface. Molecules designed according to these principles typically behave as expected in 2D; in addition, their 3D crystals often show impressive structural homology and can be considered to result from the stacking of sheets similar to those observed in 2D.15 Despite the evident logic of this approach and its success in many cases studied so far, rigorous testing is needed. Specifically, it is important to examine the behavior of analogues of tetraacids 1 and 2 that have similar topologies and affinities for graphite but lack an ability to form strong interadsorbate interactions. We have done this by analyzing the adsorption of tetraesters 3−7 on

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89.52, 62.08, 14.75; HRMS (ESI) calcd for C38H32O8 + H m/e 617.2170, found 617.2161. Synthesis of Diethyl 5,5′-(9,10-Anthracene-2,1ethynediyl)bis(1,3-benzenedicarboxylate) (7).10 A mixture of dry THF (30 mL) and di-isopropylamine (15 mL) was degassed by bubbling dry N2 through it for 30 min. Diethyl 5-iodo-1,3-benzenedicarboxylate (0.776 g, 2.23 mmol)19 was added, followed by bis(triphenylphosphine)palladium(II) dichloride (0.040 g, 0.057 mmol) and copper(I) iodide (0.030 g, 0.16 mmol). The mixture was stirred at room temperature for 15 min, and then a solution of 9,10-diethynylanthracene (0.228 g, 1.01 mmol)22 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. The solution was washed with water and then dried over anhydrous MgSO4. Removal of volatiles by evaporation under reduced pressure provided tetraester 78 (0.582 g, 0.873 mmol, 86%) as a red solid: mp >250 °C; IR (KBr) 2980, 1724, 1317, 1239, 1218 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.72 (m, 4H), 8.71 (t, 4J = 1.6 Hz, 2H), 8.59 (d, 4J = 1.6 Hz, 4H), 7.72 (m, 4H), 4.49 (q, 3J = 7.2 Hz, 8H), 1.48 (t, 3J = 7.2 Hz, 12H); 13C NMR (100 MHz, CDCl3) δ 165.25, 136.39, 132.20, 131.53, 130.35, 127.27, 127.17, 124.27, 118.22, 100.49, 88.09, 61.76, 14.39; HRMS (ESI) calcd for C42H34O8 + H m/e 667.2326, found 667.2318. Anal. Calcd for C42H34O8: C, 75.66; H, 5.14. Found: C, 75.64; H, 4.68. Studies of 2D 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 a saturated solution of tetraesters 3−7 in heptanoic acid (∼10−4 M) was applied. STM investigations were then carried out at the liquid−solid interface in the 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 a freeware (WSxM 5.0 Develop 1.2, Nanotec Electrónica S. L.).23 A smooth 3 × 3 matrix convolution filter was used to produce the final images. Images obtained from less concentrated solutions of tetraesters 3−7 (∼10−5 M) were essentially identical. Studies of 3D Crystallization by XRD. Crystals were obtained by allowing hexane to diffuse into solutions of tetraesters 3, 4a, 5, and 7 in CHCl3. Crystallographic data were collected at 100 K using a Bruker Microstar diffractometer with Cu Kα radiation. The structures were solved by direct methods using SHELXS-97 and refined with SHELXL-97.24 Nonhydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed in ideal positions and refined as riding atoms.

Figure 1. STM images of the adsorption of tetraester 3 on HOPG (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 100 pA). (a) View of an area of 40 nm × 40 nm. (b) Enlarged view of an area of 20 nm × 20 nm, showing the measured unit cell (white) and representations of hypothetical tetramer III (blue) added as visual aids to facilitate interpretation of the pattern of contrasts.

demonstrate that tetraester 3 has a strong affinity for the surface despite the absence of long alkyl chains, which are often used to facilitate adsorption of molecules on HOPG,25 and despite the inability of compound 3 to form a robust hydrogen-bonded 2D network. Parallel networks (structure I) and Kagome patterns (structure II) are formed locally by the corresponding tetraacid 1, but adlayers of tetraester 3 exhibit a completely different organization, as highlighted in Figure 1b. The observed pattern of adsorption suggests hypothetical structure III, in which neighboring molecules are linked by specific C−H···O interactions.26,27 This enforces an angle of 60° between the long molecular axes, as observed experimentally (Figure 1b). In addition, the unit cell parameters estimated for a pattern defined by motif III (a = 2.3 nm, b = 3.5 nm, and γ = 90°, based on the assumption that the H···O distances are 2.42 Å) are very close to the observed values. Although the particular mode of C−H···O association proposed in structure III is plausible, it has not been previously noted in crystallographic studies of diesters of isophthalic acid and related compounds.

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RESULTS AND DISCUSSION 2D and 3D Crystallization of Tetraester 3. A freshly exposed surface of HOPG was treated with a solution of tetraester 3 in heptanoic acid, and the liquid−solid interface was analyzed by STM. Representative images are shown in Figure 1, along with a superimposed unit cell and a schematic model of the proposed molecular organization. The measured unit cell parameters are a = 2.4 ± 0.1 nm, b = 3.4 ± 0.1 nm, and γ = 90 ± 2°. The well-resolved images are noteworthy because they 13054



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Table 1. Crystallographic Data for Tetraesters 3, 4a, 5, and 7 crystal

3

4a

5·CHCl3

7·2CHCl3

formula crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z ρcalcd (g cm−3) T (K) μ (mm−1) R1, I > 2σ(I) (%) R1, all data (%) ωR2, I > 2σ(I) (%) ωR2, all data (%) parameters measured reflections independent reflections observed reflections, I > 2σ(I)

C26H26O8 triclinic P1̅ 9.3227(1) 10.6215(2) 13.2099(2) 76.492(1) 69.406(1) 78.765(2) 1181.44(3) 2 1.311 100 0.810 4.12 4.15 10.60 10.64 311 17836 4212 4117

C34H30O8 triclinic P1̅ 9.0092(2) 9.5914(2) 10.1026(1) 70.319(2) 66.113(1) 65.792(1) 712.59(2) 1 1.320 100 0.773 4.19 4.20 10.11 10.12 192 11005 2611 2544

C41H35Cl3O8 monoclinic P21 10.2725(2) 15.2009(3) 23.7310(4) 90 95.8960(10) 90 3686.02(12) 4 1.373 100 2.698 2.82 2.84 7.61 7.63 945 58949 12722 12610

C44H36Cl6O8 triclinic P1̅ 9.0194(7) 10.2710(12) 13.3188(11) 67.405(5) 73.348(3) 64.216(4) 1015.08(17) 1 1.481 100 4.3210 4.15 4.40 11.32 11.55 264 15440 3170 2928

Figure 2. Views of the structure of crystals of tetraester 3 grown from hexane/CHCl3. (a) View of part of a sheet, with carbon atoms shown in gray, hydrogen atoms in white, and oxygen atoms in red. Narrow lines between molecules represent C−H···O interactions with H···O distances slightly shorter than the sum of the van der Waals radii. (b) View of stacked sheets, with one layer highlighted in blue.

Crystallographic data are summarized in Table 1, and views of the structure are provided in Figure 2. Molecules of compound 3 adopt flattened conformations with approximate C2h symmetry (Figure 2a), which pack efficiently to give stacked sheets (Figure 2b). Crystallographic analysis thereby confirms that molecules of tetraester 3 can adopt a flattened conformation, align with the long axes parallel, and form closely packed sheets that are well suited for adsorption on graphite without substantial changes. Nevertheless, the observed organization of compound 3 is distinctly different in 2D and 3D. Differences presumably arise in part because tetraester 3 can assume multiple conformations of similar energy; moreover, the observed 2D and 3D structures are largely determined by diffuse interactions that do not require adjacent molecules to be positioned in specific ways. For these reasons, the organization of tetraester 3 in 2D and 3D is capricious and does not obey compelling qualitative guidelines, such as those governing the behavior of analogous

Adsorbed molecules of tetraester 3 also presumably engage in other contacts such as van der Waals interactions involving the ethyl groups, which may or may not lie in the plane of the adlayer. In model III, tetraester 3 adopts a planar C2v conformation in which each ester group favors a normal s-trans geometry. This particular conformation is achiral in 2D, but the proposed associative motif III generates a chiral structure and presumably leads to domains of opposite configuration. Conformers of compound 3 with alternative orientations of the COOEt groups are likely to have similar energies, but they may pack less efficiently in 2D or have weaker interactions with graphite. To provide complementary information about the preferred modes of self-association of tetraester 3, we grew crystals by allowing vapors of hexane to diffuse into a solution in CHCl3, and the structure was determined by X-ray crystallography. We were unable to find conditions that allowed us to compare 2D and 3D crystallization carried out in the same solvent. 13055



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adsorption consists of equal numbers of motif IV and its enantiomer.

tetraacid 1. In addition, it is important to note that the observed 2D and 3D structures of tetraester 3 reflect organization in different solvents. Comparing the behaviors of tetraester 3 and tetraacid 1 provides a valuable understanding of molecular organization in general. Both compounds have similar topologies and affinities for graphite, but in the case of tetraester 3, predictable organization and close homology of 2D and 3D structures are not enforced by strong directional interactions. Under these circumstances, subtle differences in adsorbate−surface interactions are likely to play an important role in determining what 2D pattern is favored. 2D and 3D Crystallization of Tetraester 4a. To test these hypotheses, we examined the behavior of elongated tetraester 4a. A droplet of a solution of tetraester 4a in heptanoic acid was deposited on HOPG, and the liquid−solid interface was analyzed by STM. Typical images are presented in Figure 3, along with a

In principle, the Kagome network favored by adsorbed tetraacid 2 allows extensive alignment with the 3-fold symmetry of the underlying graphite, but the square grid adopted by analogous tetraester 4a is less well matched. To probe the possible effects of incommensurate adsorption,29 we compared the affinities of compounds 2 and 4a directly by treating HOPG with a solution of a 1:1 mixture in heptanoic acid. Unexpectedly, STM images showed that square grids assembled from tetraester 4a are the only nanopatterns observed (Figure 4). This striking

Figure 3. STM images of the adsorption of tetraester 4a 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. (b) Enlarged view showing the measured unit cell (white) and a representation of hypothetical tetramer IV (blue) to facilitate interpretation of the pattern of contrasts.

superimposed unit cell and a schematic model of the hypothetical molecular organization. The unit cell parameters are a = b = 2.1 ± 0.1 nm and γ = 90 ± 2°. Again, the wellresolved images confirm that compound 4a, like analogous tetraester 3, is strongly adsorbed, despite the absence of long alkyl chains and interadsorbate hydrogen bonds. A Kagome pattern (structure II) is formed by adsorption of the corresponding tetraacid 2, but a distinctive square grid is favored by tetraester 4a (Figure 3b). Structure IV suggests that the square grid arises because tetraester 4a assumes a planar D2h conformation, and adjacent molecules are linked by a set of characteristic C−H···O interactions. These interactions require an angle of 90° between the long molecular axes, as found experimentally (Figure 3b). In addition, the unit cell parameters estimated for the pattern derived from structure IV (a = b = 2.0 nm and γ = 90°, based on the assumption that the H···O distances are 2.42 Å) are closely similar to the measured values. The proposed pattern of intermolecular C−H···O interactions is reasonable, but no analogous motif has been previously observed in 3D crystal structures of diesters of isophthalic acid or related compounds. However, square-grid patterns arising from the adsorption of similar linear molecules on various surfaces have been rationalized by related C−H···O interactions.28 Tetrameric structure IV is chiral in 2D, but the overall pattern of

Figure 4. STM image of the result of exposing HOPG to a 1:1 mixture of tetraester 4a and analogous tetraacid 2 (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 50 pA), showing five intersecting domains A−E.

preference presumably arises from a combination of two factors: (1) Tetraester 4a differs from tetraacid 2 by the insertion of four CH2CH2 units, which presumably increase the molar heat of adsorption; and (2) tetraacid 2 engages in strong directional interactions that favor the formation of an open network, whereas no similar forces divert tetraester 4a from maximizing its heat of adsorption by packing densely. The behavior of tetraester 4a confirms that extensive wellordered adlayers can be formed by simple molecules that lack features present in compounds traditionally used for adsorption on graphite, such as long alkyl chains or functional groups that 13056



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Figure 5. Views of the structure of crystals of tetraester 4a grown from hexane/CHCl3. (a) View of part of a sheet, with carbon atoms shown in gray, hydrogen atoms in white, and oxygen atoms in red. Key C−H···O interactions are represented by narrow lines. (b) View of stacked sheets, with one layer highlighted in blue.

engage in strong interadsorbate interactions. Moreover, full alignment with the symmetry axes of graphite is not a prerequisite for adsorption. Indeed, inspection of Figure 4 reveals multiple domains with widely varying orientations relative to the underlying surface. To further probe the self-association of tetraester 4a, we obtained crystals by allowing vapors of hexane to diffuse into a solution in CHCl3, and we determined the structure by X-ray crystallography. Crystallographic data are provided in Table 1, and views of the structure are shown in Figure 5. Molecules of compound 4a adopt a flattened conformation with approximate D2h symmetry (Figure 5a), unlike the alternative D2h arrangement proposed in 2D model IV. Close packing, directed in part by C−H···O interactions (H···O distance = 2.388 Å), produces a structure composed of stacked sheets (Figure 5b). The preferred conformation can be described as bolaamphiphilic, with all four alkyl groups maximally extended along the long molecular axis and well separated from the aromatic core. The resulting combination of a high aspect ratio and segregated aliphatic and aromatic regions presumably favors the observed crystalline arrangement, in which parallel molecules are aligned end-to-end with interdigitated alkyl groups. As in the case of tetraester 3, molecules of elongated analogue 4a crystallize in 3D in a flattened conformation, align with their long axes parallel, and pack closely to form sheets that appear to be highly suitable for adsorption on graphite. Again, however, tetraester 4a favors widely different arrangements in 2D and 3D, presumably because numerous conformations of similar energy are accessible, and the interadsorbate interactions are too weak and poorly directional to impose a consistent pattern of association. The absence of dominant intermolecular interactions has major consequences, as illustrated by the radically different relative orientations of tetraester 4a in 2D (perpendicular) and 3D (parallel). 2D and 3D Crystallization of Tetraester 5. To extend our analysis, we studied the association of tetraester 5, an even longer analogue of compounds 3 and 4a. A solution of compound 5 in heptanoic acid was deposited on HOPG, and the liquid−solid interface was imaged by STM (Figure 6). Unlike tetraesters 3 and 4a, which form remarkably well-ordered nanopatterns, adsorbed analogue 5 showed no extended periodicity, although conspicuous square grids were observed locally (Figure 6a). Figure 6b shows one of these regions in detail, and it includes a superimposed unit cell (a = b = 2.5 ± 0.1 nm and γ = 90 ± 2°) and a representation of the proposed molecular arrangement. Longer delays before recording images did not change the overall appearance substantially. Adsorbed molecules cover large areas and are individually well resolved (Figure 6a), but many do not show regular orientations with respect to their neighbors.

Figure 6. STM images of the adsorption of tetraester 5 on HOPG (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 50 pA). (a) View of an area of 39 nm × 39 nm, with a small region of square-grid organization highlighted in blue. (b) Enlargement of the highlighted area in (a), showing the measured unit cell (white) and a representation of the hypothetical perpendicular arrangement of molecules (blue) to facilitate interpretation of the pattern of contrasts.

Elongated tetraester 5 is presumably more strongly adsorbed on HOPG than shorter analogues 3 and 4a but does not readily form an extensive periodic array, possibly because many patterns of closely similar energy compete for space on the surface. The square grid highlighted in Figure 6b appears to arise because molecules of tetraester 5 adopt a planar D2h conformation and associate to form a structure analogous to motif IV formed by tetraester 4a, in which perpendicular neighbors engage in multiple C−H···O interactions. The estimated unit cell parameters (a = b = 2.4 nm and γ = 90°, based on the assumption that the H···O distances are 2.42 Å) are close to the observed values. Although tetraester 5 was reluctant to form extended periodic arrays in 2D (Figure 6a), we found that it could be crystallized in 3D by allowing vapors of hexane to diffuse into a solution in CHCl3, as in the cases of analogues 3 and 4a. The structure was determined by X-ray crystallography, and the composition proved to be 5·CHCl3. Table 1 summarizes additional crystallographic data, and Figure 7 provides views of the structure. Under essentially identical conditions, tetraesters 3 and 4a formed solvent-free crystals, but extended analogue 5 yielded crystals in which approximately 14% of the volume is available to guests.30,31 This difference may reflect the inherently nonplanar biphenyl core of compound 5, which disfavors the formation of closely packed stacks of sheets analogous to those produced by tetraesters 3 and 4a. Indeed, two distinct twisted conformers of low symmetry are present in the unit cell of crystals of compound 5, with biphenyl torsional angles of approximately 25° and 35°. As shown in Figure 7a, the two conformers can be considered to form pairs held together by 13057



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Figure 7. Views of the structure of crystals of tetraester 5·CHCl3 grown from hexane/CHCl3. (a) View of a pair of nonequivalent twisted conformers, shown in red and blue, with key C−H···O interactions represented by narrow lines. (b) View along the a axis showing the cross sections of parallel columnar bundles composed of red/blue pairs. Channels between the bundles contain included molecules of CHCl3, which are omitted for clarity.

produced locally (Figure 8a), but their geometry is strikingly irregular (Figure 8b), presumably because the naphthalene cores of adjacent molecules of compound 6 can have diverse relative orientations, as shown in Figure 9.32 For this reason, we did not compare the 2D and 3D organization of compound 6 in greater detail. 2D and 3D Crystallization of Tetraester 7. Tetraesters 4a and 6 have closely similar molecular structures, are essentially identical in length, and are consistently adsorbed on HOPG as square grids held together in part by characteristic C−H···O interactions. By analogy, anthracene derivative 7 should follow the same pattern, and its enhanced aromatic character should increase its affinity for HOPG. Compound 7 was dissolved in heptanoic acid; a droplet was placed on HOPG; and the liquid−solid interface was analyzed by STM (Figure 10). Well-resolved patterns are formed, as expected, but the adsorbed molecules are arranged in rows, not square grids. The observed unit cell parameters are a = 1.2 ± 0.1 nm, b = 2.5 ± 0.1 nm, and γ = 78 ± 2°. As shown in Figure 10a and other STM images of adsorbed tetraester 7,32 rows in adjacent domains typically intersect at angles that are multiples of 60°, suggesting that the underlying surface must be properly aligned. The observed pattern appears to arise from association according to tape motif V, in which adjacent molecules are aligned with their long axes parallel and engage in multiple C−H···O

various contacts, including two specific C−H···O interactions (H···O distances = 2.591 and 2.762 Å). The pairs then stack along the a axis to form columnar bundles held together in part by additional C−H···O interactions, and the columns pack in parallel, leaving intervening channels for the inclusion of CHCl3 (Figure 7b). In both 2D and 3D, the organization of elongated tetraester 5 is inherently inefficient and undirected. Adsorption occurs readily; however, extended periodic arrays are not formed, and square grids of the type produced by shorter tetraester 4a are only observed locally, possibly because the density of coverage decreases with molecular length. In addition, the inclusion of guests in 3D suggests that molecules of tetraester 5 by themselves cannot readily find a closely packed alternative. Again, the absence of consistent patterns of organization in 2D and 3D can be attributed in part to the inability of tetraester 5 to engage in well-defined intermolecular interactions. 2D Crystallization of Tetraester 6. Further insight was provided by analysis of the association of tetraester 6, a close relative of compounds 3, 4a, and 5. A droplet of a solution of compound 6 in heptanoic acid was deposited on HOPG, and STM was used to image the liquid−solid interface (Figure 8).

Figure 8. STM images of the adsorption of tetraester 6 on HOPG (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 50 pA). (a) View of an area of 40 nm × 40 nm showing multiple small squaregrid domains. (b) Detailed view of the irregular geometry of a single domain.

interactions. Molecules in proposed motif V adopt planar C2h conformations and are therefore chiral in 2D. Adjacent rows consisting of molecules of the same configuration pack closely to

Like tetraester 5, analogue 6 is well adsorbed, yet it does not readily form extended periodic domains. Again, square grids are 13058



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Figure 9. Representation of the proposed 2D association of tetraester 6, suggesting how different relative orientations of the naphthalene core can lead to the formation of irregular square grids.

construct individual domains, and enantiomorphous domains occur with equal probability. Unit cell parameters estimated for the proposed nanopattern (a = 1.2 nm, b = 2.4 nm, and γ = 78°, based on the assumption that the H···O distances are 2.42 Å) are closely similar to the observed values. Moreover, a related network of C−H···O interactions appears in the 3D structure of analogous tetraester 4a, as shown in Figure 5. Crystallization of tetraester 7 was induced by allowing vapors of hexane to diffuse into a solution in CHCl3, as in the cases of compounds 3, 4a, and 5. The structure was determined by X-ray crystallography, and the composition was found to be 7·2CHCl3. Additional crystallographic data appear in Table 1, and views of the structure are presented in Figure 11. An essentially identical structure, obtained using crystals grown from CHCl3 by evaporation, was recently reported but not analyzed in detail.8 Like close analogue 4a, tetraester 7 crystallizes as stacked sheets of molecules arranged with their long axes parallel (Figure 11a). Individual molecules adopt flattened conformations of approximate C2h symmetry, as favored in 2D, but no intermolecular aryl C−H···O interactions are present. 2D Crystallization of Mixtures of Tetraesters 3, 4a, 5, 6, and 7. Tetraesters 3, 4a, 5, 6, and 7 are all adsorbed readily on

Figure 10. STM images of the adsorption of tetraester 7 on HOPG (deposition from heptanoic acid, with Vbias = −1.5 V and Iset = 50 pA). (a) View of an area of 40 nm × 40 nm showing molecules arranged in rows in two intersecting domains. (b) Detailed view showing the measured unit cell (white) with a superimposed model of molecules adsorbed in rows.

HOPG under similar conditions to give well-resolved nanopatterns. By allowing equimolar mixtures of pairs of the compounds to compete for limited amounts of surface, we were able to compare their affinities for HOPG directly. Not 13059



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Figure 11. Views of the structure of crystals of tetraester 7·2CHCl3 grown from hexane/CHCl3. (a) View of part of a sheet, with carbon atoms shown in gray, hydrogen atoms in white, and oxygen atoms in red. Narrow lines between molecules represent C−H···O interactions with H···O distances slightly shorter than the sum of the van der Waals radii. (b) View of stacked sheets, with one layer highlighted in blue.

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CONCLUSIONS Linear bisisophthalic acids 1 and 2 and analogous structures associate predictably in ways directed by the concerted effects of topology and hydrogen bonding,6−9,16 whereas the corresponding tetraesters 3−7 fail to crystallize in 2D and 3D according to consistent patterns. The sharply contrasting behavior of the tetraacids and tetraesters cannot be attributed to major differences in molecular geometry, nor can it be ascribed to highly dissimilar affinities for HOPG, which strongly binds all the compounds studied. Our observations provide compelling evidence for the hypothesis that molecular organization is best controlled in both 2D and 3D by using topology and strong directional interactions in tandem to control the relative orientation of neighbors. When topology and dominant intermolecular interactions are in harmony, then organization can be expected to follow consistent patterns within related series of compounds, and structures in 2D and 3D can be designed to show high levels of homology.

surprising, competitions between the smallest tetraester (compound 3) and the largest (anthracene 7) were won overwhelmingly by tetraester 7, which appeared to be the only compound adsorbed.32 Similarly, simultaneous treatment of HOPG with tetraesters 3 and 4a led exclusively to patterns derived from the larger compound 4a.32 However, smaller compounds that pack with particular efficiency or engage in especially favorable patterns of intermolecular C−H···O interactions appear to be able to defeat larger opponents. For example, exposure of HOPG to equimolar mixtures of tetraesters 4a and 7 yielded domains derived from both compounds,32 with those produced by smaller compound 4a predominating, possibly because the favored motif IV offers an attractive combination of close packing and a high density of intermolecular C−H···O interactions. 2D Crystallization of Tetraesters 4b,c. Compounds 3, 4a, 5, 6, and 7 are suitably chosen for a systematic comparison of molecular organization in 2D and 3D because they have identical chemical functionality (diethyl isophthalate groups), linked by variable linear spacers of closely similar character. Nevertheless, seemingly minor structural alterations in this series of compounds can be seen to have profound and unexpected effects on their collective behavior. We were particularly intrigued by the surprising ability of tetraester 4a to compete effectively with tetraacid 2 and anthracene 7 for adsorption on HOPG, and we decided to probe the origin of its special affinity in greater detail. Close examination of hypothetical motif IV, in which a perpendicular arrangement of molecules is harmoniously reinforced by a network of C−H···O interactions, reveals that optimal packing can only be achieved if the peripheral ester groups fully occupy the surface not required for adsorbing the molecular cores. The notably effective adsorption of compound 4a suggests that ethyl groups fit motif IV well, whereas similar esters with slightly larger or smaller groups may not be accommodated as easily. To test this possibility, we exposed HOPG to tetramethyl ester 4b and tetrapropyl ester 4c under standard conditions. In the case of tetraester 4b, square grids were the only pattern observed, but they occurred as scattered domains not exceeding 100 nm2 in area; in the case of tetraester 4c, no well-defined patterns were formed at all.32 The variable behavior of closely similar compounds 4a−c underscores the difficulty of predicting and controlling molecular organization when only weak intermolecular forces are present.

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ASSOCIATED CONTENT

S Supporting Information *

Additional crystallographic details (including thermal atomic displacement ellipsoid plots and tables of structural data in CIF format) and supplementary STM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 Ji-Hyun Yi and Prof. Antonio Nanci for helping us obtain STM images.

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REFERENCES

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of the unit cell and Vg is the guest-accessible volume as calculated by PLATON. (31) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University: Utrecht, Netherlands, 2001. van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194−201. (32) See the Supporting Information for further details.

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Using Systematic Comparisons of 2D and 3D Structures To Reveal

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