Two-Dimensional Crystallization of Carboxylated Benzene Oligomers

Jan 5, 2011 - Four analogues belonging to various point groups were studied. Comparison ... Two-dimensional (2D) crystallization has emerged as the me...
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Two-Dimensional Crystallization of Carboxylated Benzene Oligomers Christine N. Morrison, Seokhoon Ahn, Jennifer K. Schnobrich, and Adam J. Matzger* Department of Chemistry and the Macromolecular Science and Engineering Program, University of Michigan, Ann Arbor, Michigan 48109-1055, United States Received September 21, 2010. Revised Manuscript Received December 13, 2010 Various carboxylic acid substitution patterns on the 1,3,5-triphenylbenzene nucleus were explored, and their influence on the symmetry of the resulting two-dimensional (2D) crystal structures was assessed. The symmetry of 1,3,5-benzenetribenzoic acid (H3BTB) was reduced by modifying the substitution pattern of the arene and/or adding an additional carboxylic acid. Four analogues belonging to various point groups were studied. Comparison of the monolayers of the analogues to that of H3BTB shows that plane group symmetry and molecular symmetry are not correlated: H3BTB and its analogues exhibit the same plane group p2 at the heptanoic acid/graphite interface. The 2D crystal structure of the H3BTB analogues is more strongly controlled by the geometry of hydrogen-bonding interactions rather than molecular symmetry. Other significant observations in this study include porosity, uncommon hydrogenbonding motifs, and an unusually high number of inquivalent molecules (Z0 = 3) present in the 2D crystal of the lowest symmetry analogue. This research demonstrates that reduction of molecular symmetry based on geometric modification of noncovalent interactions allows for control over porosity of the 2D crystals (close-packed structures to nanoporous networks) without changing the core shape of the molecule.

Introduction Two-dimensional (2D) crystallization has emerged as the method of choice for creating nanopatterns with molecular-scale features. The self-assembly process giving rise to 2D crystals, much as in three-dimensional (3D) crystals, involves a delicate interplay of noncovalent interactions with the additional complication of substrate and solvent exerting some thermodynamic influence over structure. Scanning tunneling microscopy (STM) at the liquid/ solid interface offers high resolution and the ability to track dynamic phenomena with molecular-level detail, making it the core technique for structural investigation, although calorimetry1 and vibrational spectroscopy2 can provide additional insights.3-7 As in the case of molecular crystals, noncovalent interactions play a critical role in determining assembly outcome. Hydrogen bonding has been a particularly fruitful design element that, when used in concert with molecular geometry, opens the possibility of 2D crystal engineering. For example, 3-fold symmetric molecules can lead to the formation of 2D periodic patterns containing 6-fold rotation axes such as hexagonal chicken-wire structures or flower structures of 1,3,5-benzenetribenzoic acid (H3BTB) and trimesic acid (TMA), respectively.8,9 Reducing the symmetry of the molecules such that they can no longer be coincident with a symmetry element is expected to reduce network symmetry although systematic studies of this effect are lacking.

In this paper, 2D assemblies of reduced-symmetry carboxylated benzene oligomers are imaged wherein the hydrogen-bonding geometry is varied while maintaining the aromatic core. The series is based on H3BTB, a 3-fold symmetric molecule that has been scrutinized both in 2D and as a component in building highly porous 3D coordination polymers.10-15 The geometry of the hydrogen-bonding sites of the reduced-symmetry analogues was varied by changing the substitution pattern of the arene and/or by adding an additional acid group. During the structural investigation of various assemblies of the H3BTB analogues, new hydrogenbonding motifs and a structure possessing a high number of inequivalent (Z 0 = 3) molecules in the unit cell were observed.

Experimental Section Materials. Methyl 30 ,50 -dibromo-[1,10 -biphenyl]-4-carboxy-

late and dimethyl 50 -bromo-[1,10 :30 ,100 -terphenyl]-4,400 -dicarboxylate were prepared according to published procedures.16 Palladium tetrakis(triphenylphosphine) was purchased from Strem Chemicals. K 3PO4, 1,4-dioxane, and 1,3,5-tribromobenzene were purchased from Acros. Ethanol was purchased from Decon Laboratories, Inc. All other reagents were purchased from Fisher Scientific. All reagents and solvents were used as received.

*Corresponding author. E-mail: [email protected].

(1) Groszek, A. J. Proc. R. Soc. London, A 1970, 314, 473–498. (2) McClelland, A. A.; Ahn, S.; Matzger, A. J.; Chen, Z. Langmuir 2009, 25, 12847–12850. (3) He, Y.; Ye, T.; Borguet, E. J. Phys. Chem. B 2002, 106, 11264–11271. (4) Zhou, H.; Dang, H.; Yi, J. H.; Nanci, A.; Rochefort, A.; Wuest, J. D. J. Am. Chem. Soc. 2007, 129, 13774–13775. (5) Scherer, L. J.; Merz, L.; Constable, E. C.; Housecroft, C. E.; Neuburger, M.; Hermann, B. A. J. Am. Chem. Soc. 2005, 127, 4033–4041. (6) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1999, 103, 5712–5715. (7) Ahn, S.; Matzger, A. J. J. Am. Chem. Soc. 2009, 131, 13826–13832. (8) Kampschulte, L.; Werblowsky, T. L.; Kishore, R. S. K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. J. Am. Chem. Soc. 2008, 130, 8502–8507. (9) Gutzler, R.; Sirtl, T.; Dienstmaier, J. F.; Mahata, K.; Heckl, W. M.; Schmittel, M.; Lackinger, M. J. Am. Chem. Soc. 2010, 132, 5084–5090.

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(10) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.-B.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523–527. (11) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. Angew. Chem., Int. Ed. 2008, 47, 677–680. (12) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2009, 131, 4184–4185. (13) Sumida, K.; Hill, M. R.; Horike, S.; Dailly, A.; Long, J. R. J. Am. Chem. Soc. 2009, 131, 15120–15121. (14) Klein, N.; Senkovska, I.; Gedrich, K.; Stoeck, U.; Henschel, A.; Mueller, U.; Kaskel, S. Angew. Chem., Int. Ed. 2009, 48, 9954–9957. (15) Wang, Z.; Tanabe, K. K.; Cohen, S. M. Inorg. Chem. 2009, 48, 296–306. (16) Schnobrich, J. K.; Lebel, O.; Cychosz, K. A.; Dailly, A.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2010, 132, 13941–13948. (17) Wang, X. -j.; Sun, X.; Zhang, L.; Xu, Y.; Krishnamurthy, D.; Senanayake, C. H. Org. Lett. 2006, 8, 305–307.

Published on Web 01/05/2011

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Figure 1. Molecules investigated in this study. Assigned symmetries treat carboxylic acids as carboxylates to be consistent with the resolution achievable in the STM images.

Preparation of 50 -(4-Carboxyphenyl)-[1,10 :30 ,100 -terphenyl]3,3 00 -dicarboxylic Acid (1). Methyl-30 ,50 -dibromo-[1,10 biphenyl]-4-carboxylate (1.03 g, 2.78 mmol), 3-(methoxycarbonyl)phenylboronic acid17 (1.10 g, 6.12 mmol), toluene (30 mL), ethanol (10 mL), and an aqueous Na2CO3 solution (2 M, 10 mL) were added into a round-bottomed flask equipped with a magnetic stir bar and a water-jacketed condenser. The resulting suspension was degassed for 1 h by sparging with N2 gas. Pd(PPh3)4 (0.130 g, 0.112 mmol) was added, and the suspension was heated to reflux overnight under N2 atmosphere. After cooling to room temperature, diethyl ether (30 mL) was added to the solution. The organic and aqueous layers were separated, and the aqueous layer was extracted with diethyl ether (3  30 mL). The organic layers were combined, dried over anhydrous Na2SO4, and filtered, and the solvent was removed under reduced pressure. The residue was washed with ethyl acetate, and a white solid was collected by filtration. The residue was dissolved in 1,4-dioxane (30 mL) and H2O (20 mL), and KOH (0.840 g, 15.0 mmol) was added to the solution. This suspension was heated to reflux overnight. After cooling to room temperature, the solution was filtered, and solvent was removed from the filtrate by evaporation. The solid was dissolved in H2O (50 mL). The residual solids were filtered off, and the filtrate was acidified with concentrated HCl. The target compound was collected by filtration, washed with H2O, and dried under vacuum to produce a white powder. Yield: 77.1%; mp: >300 °C. 1H NMR (500 MHz, DMSO-d6): δ 13.07 (br, 1H), 8.35 (s, 2H), 8.16 (d, J = 7.8 Hz, 2H), 8.03 (m, 9H), 7.65 (t, J = 7.8 Hz, 2H). 13C NMR (125 MHz, DMSO-d6): δ 167.3, 167.2, 143.9, 141.1, 140.8, 140.2, 131.8, 131.6, 130.0, 129.9, 129.3, 128.7, 127.8, 127.4, 125.3, 125.1. HRMS (EI) calcd for C27H18O6 (m/z): 438.1103; found: 438.1113. Compound 2 was synthesized according to a published procedure.16

Preparation of 50 -(4-Carboxyphenyl)-[1,10 :30 ,100 -terphenyl]3,4 00 -dicarboxylic Acid (3). Dimethyl 5 0 -bromo-[1,1 0 :

3 0 ,1 00 -terphenyl]-4,4 00 -dicarboxylate (0.160 g, 0.376 mmol), 3-(methoxycarbonyl)phenylboronic acid (0.075 g, 0.414 mmol), and K3PO4 (0.239 g, 1.13 mmol) were combined in 1,4-dioxane (30 mL) and sparged with N2 gas for 30 min. Pd(PPh3)4 (0.022 g, 0.019 mmol) was added, and the suspension was heated to reflux under N2 for 18 h. After cooling, the solvent was evaporated and the residue was taken up in CH2Cl2. The solution was filtered on a short silica plug using CH2Cl2 as eluent, and the solvent was removed by evaporation. The residue was suspended in 1,4dioxane (10 mL), and an aqueous KOH solution (9 M, 1 mL) was added. This suspension was heated to reflux for 12 h. The solvent was evaporated, and then the residue was dissolved in H2O (10 mL). The residual solids were filtered off, and the filtrate was acidified with concentrated HCl. The solid was collected by filtration and then dissolved in ethanol. The product was precipitated by adding H2O, obtained by filtration, and washed with H2O. The white solid obtained was dried under vacuum. Yield: 28.8%; mp: >300 °C. 1H NMR (400 MHz, DMSO-d6): δ 13.08 (br, 3H), 8.34 (s, 1H), 8.15 (d, J = 7.8 Hz, 1H), 8.05 (m, 12H), 7.65 (t, J = 7.8 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 167.3, 167.2, 143.8, 141.1, 140.7, 140.1, 131.8, 131.6, 130.0, 129.9, 129.4, Langmuir 2011, 27(3), 936–942

128.7, 127.8, 127.4, 125.5, 125.3. HRMS (EI) calcd for C27H18O6 (m/z): 438.1103; found: 438.1104.

Preparation of 50 -(3-Carboxyphenyl)-[1,10 :30 ,100 -terphenyl]3,3 00 -dicarboxylic Acid (4). 1,3,5-Tribromobenzene (1.31 g, 4.17 mmol), 3-(methoxycarbonyl)phenylboronic acid (3.00 g, 16.7 mmol), K3PO4 (8.12 g, 38.25 mmol), and 1,4-dioxane (75 mL) were added into a 250 mL round-bottomed flask equipped with a magnetic stirbar and a water-jacketed condenser. The resulting suspension was degassed for 15 min by sparging with N2 gas. Pd(PPh3)4 (0.862 g, 0.746 mmol) was added, and the suspension was heated to reflux overnight under N2 atmosphere. After cooling to room temperature, the solvent was removed under reduced pressure. The residue was dissolved in CH2Cl2 (40 mL). The organic layer was washed with 1 M NaOH (40 mL), H2O (40 mL), and brine (40 mL). The organic layer was dried over anhydrous Na2SO4 and filtered, and the solvent was removed under reduced pressure. The desired material was isolated by column chromatography on silica gel using CH2Cl2 as the eluent. The compound was then suspended in 1,4-dioxane (4 mL) and water (4 mL), and KOH (0.576 g, 10.3 mmol) was added. This suspension was heated to reflux overnight. After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was dissolved in H2O (20 mL). The residual solids were filtered off, and the filtrate was acidified with 1 N HCl. The crude product was collected by filtration and purified by recrystallization from hot ethanol. The target compound, a white powder, was collected by filtration and dried under vacuum. Yield: 36.7%; mp: >300 °C. 1 H NMR (400 MHz, DMSO-d6): δ 8.35 (s, 3H), 8.16 (d, J = 8.0 Hz, 3H), 8.00 (m, 6H), 7.65 (d, J = 8.0 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 167.2, 141.0, 140.1, 131.7, 131.5, 129.3, 128.6, 127.7, 124.9. HRMS (EI) calcd for C27H18O6 (m/z): 438.1103; found: 438.1108. Scanning Tunneling Microscopy. A Nanoscope E STM (Digital Instruments) was used for all imaging. Highly oriented pyrolytic graphite (HOPG) (SPI-1 grade, Structure Probe Inc.) was used as a substrate for monolayer formation. A heptanoic acid solution of the desired molecule was made, of which 1 μL was placed on freshly cleaved HOPG to obtain a self-assembled monolayer. The tips were made from Pt/Ir wire (20% Ir, 0.010 in. diameter, California Fine Wire) by mechanical cutting. STM imaging was performed using quasi-constant height mode under ambient conditions, and typical STM settings include 300 pA of current and 700-900 mV of bias voltage (sample positive). All images are unfiltered. The concentrations of the solutions used for STM experiments are as follows: 1.0 mM for 1, 0.10 mM for 2, 0.05 mM for 3, and 0.75 mM for 4. Computational Modeling. The packing structures apparent from the metrics and symmetry of the STM images were modeled using Materials Studio version 4.3 (Accelrys Software Inc.). Energy minimization was performed in Cerius2 version 4.2 (Accelrys Software Inc.) using a COMPASS force field because this method has been shown to yield reasonable energies for the relative stability of three-dimensional polymorphic packing arrangements in molecular crystals.18 The computed unit cell parameters (18) Mitchell-Koch, K. R.; Matzger, A. J. J. Pharm. Sci. 2008, 97, 2121–2129.

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Morrison et al. Table 1. Experimental and Computed Unit Cell Parametersa for All 2D Structures Observed experimental molecule

a (nm)

b (nm)

computed γ (deg)

a (nm) b (nm) γ (deg)

1 2.14 ( 0.02 1.68 ( 0.03 104.2 ( 0.5 2.13 1.67 104.4 2 1.67 ( 0.02 3.61 ( 0.07 105 ( 1 1.66 3.49 107.4 3-I 2.71 ( 0.06 1.64 ( 0.04 54.5 ( 0.6 2.73 1.65 54.0 3-II 3.10 ( 0.09 1.66 ( 0.04 96 ( 2 3.04 1.67 96.1 4 2.87 ( 0.04 4.36 ( 0.06 111.2 ( 0.4 2.88 4.31 116.8 a The computed unit cell parameters were obtained through molecular mechanics using a COMPASS force field.

Figure 2. Schematic representation of all assemblies observed at the heptanoic acid/HOPG interface. The point group of each molecule, the plane group of 2D crystals, and the numbers of molecules in the asymmetric unit (Z0 values) are indicated. were obtained through energy minimization performed in the absence of substrate because it has been shown in our previous reports that the perturbation of the periodic structure upon simulation of adsorption to the graphite slab was minimal.7,19,20

Results and Discussion H3BTB and TMA, highly symmetric molecules with 3-fold rotation axes, form several phases at the liquid/solid interface under various conditions and have been extensively studied by STM.8,9,21-25 Because of the presence of more sites for substitution, H3BTB was selected as a standard material for modification in preference to TMA. The symmetry of H3BTB was reduced by modifying functional group placement on the three outer benzene rings. These reduced-symmetry analogues are shown in Figure 1. To assign the symmetry of molecules and monolayers, the rotation of the outer benzene rings is considered in cases where a difference arises between coplanar conformations; the orientation of the carboxylic acid group is not considered because their hydrogen-bonding orientations in 2D assemblies cannot be determined by STM. As such, the assigned symmetries treat the (19) Plass, K. E.; Engle, K. M.; Matzger, A. J. J. Am. Chem. Soc. 2007, 129, 15211–15217. (20) Ahn, S.; Morrison, C. N.; Matzger, A. J. J. Am. Chem. Soc. 2009, 131, 7946–7947. (21) Lackinger, M.; Griessl, S.; Kampschulte, L.; Jamitzky, F.; Heckl, W. M. Small 2005, 1, 532–539. (22) Li, Z.; Han, B.; Wan, L. J.; Wandlowski, T. Langmuir 2005, 21, 6915–6928. (23) 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. (24) Lackinger, M.; Heckl, W. M. Langmuir 2009, 25, 11307–11321. (25) Nath, K. G.; Ivasenko, O.; MacLeod, J. M.; Miwa, J. A.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. J. Phys. Chem. C 2007, 111, 16996–17007.

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carboxylic acids as carboxylates. Because of the rotation of metasubstituted benzene rings in 1 and 4, these molecules can be represented in two different point groups (Figure 1). Through analysis of their assembled structures, the specific point group observed in the monolayer is assigned; the relationship between a specific molecular symmetry and an assembled structure is discussed below. To examine geometric and symmetry effects in isolation, all molecules were imaged in heptanoic acid because solvent identity is known to play a critical role in determining structure.26-32 For example, the oblique structure of H3BTB is observed using butanoic, pentanoic, hexanoic, and heptanoic acid, and the chicken-wire structure is observed in nonanoic acid and 1-phenyloctane.23 Assembled structures of H3BTB analogues are schematically represented in Figure 2. The molecular symmetry and the monolayer plane groups are assigned on the basis of their STM images. The number of molecules in the asymmetric unit (Z0 value) is also indicated. The H3BTB analogues formed five different assemblies: two nanoporous networks (void g1 nm), one small pore structure (void ∼0.9 nm), and one close-packed structure. Their unit cell parameters are shown in Table 1. Compound 1. Compound 1 has two meta-substituted benzene rings, affording the molecule C2v or Cs symmetry depending on the rotation of the benzene rings; however, the 2D crystal consists of molecules with only one of two C2v symmetric conformations. A plausible explanation for the absence of the other conformations of this molecule in the 2D crystal is that this would require a more porous structure.33 The assembled structure is close packed and contains exclusively 2-fold rotation axes, resulting in p2 symmetry (Figure 3). Because the intermolecular interactions are not visible in the STM image of this monolayer, an energyminimized computed model was constructed to provide the hydrogen-bonding structure as a complementary tool to STM (Figure 3b). The 2-fold rotation axes are formed from meta-meta hydrogen bonding, resulting in a close-packed structure, whereas para-para hydrogen bonding of H3BTB molecules causes the formation of a bowtie dimer and a nanoporous structure (26) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608– 6619. (27) Tahara, K.; Furukawa, S.; Uji-i, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613–16625. (28) Gutzler, R.; Lappe, S.; Mahata, K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. Chem. Commun. 2009, 680–682. (29) Lackinger, M.; Griessl, S.; Heckl, W. A.; Hietschold, M.; Flynn, G. W. Langmuir 2005, 21, 4984–4988. (30) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317–325. (31) Li, Y. B.; Ma, Z.; Qi, G. C.; Yang, Y. L.; Zeng, Q. D.; Fan, X. L.; Wang, C.; Huang, W. J. Phys. Chem. C 2008, 112, 8649–8653. (32) Ahn, S.; Matzger, A. J. J. Am. Chem. Soc. 2010, 132, 11364–11371. (33) The energies of three conformations are within a 0.3 kcal/mol window computed from the planar conformations at B3LYP/6-31G*, suggesting that a conformational bias is not the driving force for this selection.

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Figure 3. (a) STM image (12  12 nm2) of the monolayer of 1 with overlaid molecular model and (b) the computed model of 2D crystal structure of 1.

Figure 4. (a) STM image (12  12 nm2) of the monolayer of 2 with overlaid molecular model and (b) the computed model of 2D crystal structure of 2.

(Figure 2).23 Note that symmetry elements of the p2 unit cell lie on the hydrogen-bonded homodimers (para-para or meta-meta hydrogen bonding) in both H3BTB and monolayers of 1. Because concentration is known to play a critical role in phase selection in 2D crystallization,8,9,32,34-36 dilution methods were employed in order to observe a porous phase of 1, but no other phases were observed. The 2D crystal structure of 1 makes a one-dimensionally aligned hydrogen-bonding network along one axis as indicated by the yellow arrows in Figure 3a. These one-dimensionally connected columns are stacked to satisfy close-packing without (34) Tahara, K.; Lei, S.; Mossinger, D.; Kozuma, H.; Inukai, K.; Van der Auweraer, M.; De Schryver, F. C.; Hoger, S.; Tobe, Y.; De Feyter, S. Chem. Commun. 2008, 3897–3899. (35) Tahara, K.; Okuhata, S.; Adisoejoso, J.; Lei, S.; Fujita, T.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2009, 131, 17583–17590. (36) Lei, S. B.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2008, 47, 2964–2968.

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specific interactions between columns. Because of the p2 symmetry of the unit cell, the monolayer is chiral. Indeed, two different enantiomorphous domains of the same phase were observed, and the column propagation directions in these domains were mirrorrelated to each other. Compound 2. Compound 2 has two para-substituted benzene rings and one benzene ring with two meta-substituted acid groups, affording the molecule C2v symmetry. The 2D crystal structure of 2 shows two different sized pores and exhibits p2 symmetry (Figure 4). The monolayer consists of bowtie dimers formed by para-para hydrogen bonding, resulting in a 2-fold rotation axis. These bowtie dimers are aligned in columns, causing oblique-shaped pores with an area of 2.12 nm2 to form. Columns are connected by satisfying the rest of the available hydrogen bonding, creating smaller, rectangular pores that have an area of 1.21 nm2. As observed in the monolayers of H3BTB and 1, symmetry elements exist in the monolayer of 2 on homodimers (both para-para and meta-meta hydrogen bonding). DOI: 10.1021/la103794j

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Figure 5. (a, c) STM images (12  12 nm2) of phase I and II of 3 with overlaid molecular model. (b, d) Computed model of 2D crystal structure of phases I and II of 3.

The 2D crystal structure of 2 closely resembles the oblique phase of H3BTB: both monolayers are chiral and consist of packed para-para dimers, causing the formation of large pores. However, in the oblique 2D structure of H3BTB the columns of para-para dimers are connected by an uncommon hydrogenbonding motif between oxygen atoms of the carboxylic acids and aromatic hydrogen atoms.23 As such, no smaller pores are formed, contrary to the 2D crystal of 2. Furthermore, the pores in the oblique 2D structure of H3BTB are elongated hexagons and have an area of 2.61 nm2, which is 23% larger than the large pores in the 2D structure of 2. Compound 3. Compound 3 has two para-substituted benzene rings and one meta-substituted benzene ring, resulting in Cs symmetry. Two different nanoporous structures were observed (Figure 5). The fact that the low-resolution STM images for 3 were always obtained suggests that there is a relatively high mobility in these 2D crystals. Both phases exhibit p2 symmetry and have a 2-fold rotation axis coincident with the para-para hydrogen-bonded dimers. In phase I, symmetry elements lie on the homodimers and do not, and indeed cannot occur, on the heterodimers. Similarly, symmetry elements exist on the homodimers of phase II (there are no heterodimers in phase II). In phase I, the bowtie dimer leads to the formation of a hydrogenbonded oblique structure that extends one-dimensionally. This is indicated by yellow arrows in Figure 5a. In contrast to the oblique phase of H3BTB, there is no specific interaction such as weak 940 DOI: 10.1021/la103794j

hydrogen bonding between aromatic hydrogen atoms and carboxylic acids. Therefore, one-dimensionally connected columns are packed to reduce open space. The pores of phase I of 3 are very similar to those in the 2D crystal of 2: both are oblique and have similar size (phase I of the 2D crystal of 3 has pores that are 2.14 nm2). Phase II of 3 (Figure 5c) is characterized by two para-para interactions creating a 2-fold rotation axis. Contrary to the other crystals with para-para dimers, however, those in phase II of 3 are aligned diagonally. This is possibly due to a hydrogen bond between the meta-substituted carboxylic acid and carboxylic acids in the para-para dimer (Figure 7). The resulting rectangular pores have areas of 0.43 and 2.55 nm2. Compound 4. Compound 4 has three meta-substituted benzene rings and can therefore adopt either C3h or Cs symmetry. Although both of these point groups can be assembled, the energy-minimized computed model consisting of 4 with only Cs symmetry matches the STM images (Figure 6). A plausible explanation for the absence of C3h symmetry molecules in the 2D crystal is that this would require a more porous structure, whereas the 2D crystal structure of 4 has small pores. Observation of a nanoporous phase of 4 has not yet been achieved by dilution methods. The observed 2D structure of 4 possesses p2 symmetry. In contrast to the previous cases, no 2-fold rotation axis coincides with the homodimers. This is due primarily to meta-meta hydrogen bonding of chiral molecules as shown in Figure 6. It Langmuir 2011, 27(3), 936–942

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Figure 6. (a) STM image (12  12 nm2) of 4 with overlaid molecular model and (b) the computed model of 2D crystal structure of 4. (c) Schematic of computed model showing the chiral 4 molecules.

is known that even achiral molecules can exhibit chirality in 2D crystallization, giving rise to either racemic or separated enantiopure domains formed by each chirality.37-43 Although 3 can also be a 2D chiral molecule, the monolayer consists of molecules with the same handedness; in the 2D crystal of 4, both 2D enantiomers are observed but in a ratio of 2:1, giving rise to nonracemic domains containing both enantiomers (Figure 6c). In the model of the 2D crystal of 4, an uncommon hydrogenbonding motif is observed (indicated by the yellow circle in Figures 6b and 7b). Although these nonoptimum hydrogenbonding motifs observed in 2D crystals of 3 and 4 (Figure 7) are suggested by the fact that common hydrogen-bonding motifs cannot be used to construct the present assembly due to geometric (37) Elemans, J. A. A. W.; De Cat, I.; Xu, H.; De Feyter, S. Chem. Soc. Rev. 2009, 38, 722–736. (38) France, C. B.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 12712–12713. (39) De Feyter, S.; Gesquiere, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217–3218. (40) Weigelt, S.; Busse, C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K. V.; Besenbacher, F.; Linderoth, T. R. Nature Mater. 2006, 5, 112–117. (41) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 2000, 104, 7627–7635. (42) Wei, Y. H.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318–5322. (43) Tao, F.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 6233–6238. (44) Chantrapromma, S.; Fun, H.-K.; Razak, I. A.; Saewon, N.; Karalai, C.; Chantrapromma, K. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2000, 56, e598–e599. (45) Gowda, D. S. S.; Rudman, R. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1983, 39, 250–253.

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limitations, these motifs can be found in some 3D crystals.44,45 Furthermore, these 3D crystals also suggest a preferred orientation of the O-H in the carboxylic acid that does not make a dimer type hydrogen bonding. The O-H in this acid may make hydrogen bonds to CdO instead of O-H as shown in Figure 7. A high number of inequivalent molecules (Z0 = 3) is required in the model of the 2D crystal of 4.46 It has been suggested that Z0 > 1 is associated with strong noncovalent interactions in both 2D and 3D crystallization.47,48 This is well commensurate with the present case because the uncommon hydrogen-bonding motif plays a key role in causing the high Z0 value originating from the existence of both 2D enantiomers in the monolayer of 4. This observation suggests that the combination of strong noncovalent interactions and chirality can create complex features on the nanometer scale. Comparisons. The various 2D assemblies of the H3BTB analogues clearly show that geometry for noncovalent interactions plays a critical role in determining structure. In this study, no correlation was observed between molecular symmetry and monolayer symmetry: all assemblies exhibited p2 plane group (46) Only ∼0.6% of monolayers in the 2D Structural Database (2DSD) have such a high Z0 value. See: Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40, 287–293. (47) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042– 9053. (48) Anderson, K. M.; Goeta, A. E.; Hancock, K. S. B.; Steed, J. W. Chem. Commun. 2006, 2138–2140.

DOI: 10.1021/la103794j

941

Article

Morrison et al.

Furthermore, 2D chiral molecules are created by rotating metasubstituted benzene rings or flipping the molecule 180° in 2D crystallization. The compatibility of the geometry and the noncovalent interaction of the 2D enantiomers allow molecules in the monolayer of 4 to form more complex features. This is in contrast to the comparatively simple structures of the 2D crystal of 3, which contain molecules of the same handedness. A variety of assemblies from H3BTB analogues arises not only due to geometry of molecules but also through various hydrogenbonding motifs. In the present case, uncommon hydrogen-bonding motifs were observed in the 3-II and 4 assemblies. These motifs were used to satisfy close packing-a default behavior in 2D crystallization.46 Regardless of the particular hydrogenbonding motif or molecular geometry, all molecules in this study including H3BTB produce chiral monolayers due to the p2 symmetry of the unit cell.

Conclusion

Figure 7. Hypothesized hydrogen-bonding motif observed in the 2D crystal of (a) 3-II and (b) 4.

symmetry whereas molecular symmetry varied from D3h to Cs. Supporting this observation is the formation of 2-fold rotation axes created by either para-para or meta-meta hydrogen bonding without any correlation to molecular symmetry. The 2-fold rotation axes on heterodimers were not, and indeed cannot, be observed. This result reveals that dimer structures created by hydrogen bonding play a more critical role in determining monolayer structure than molecular symmetry. To illustrate, whenever para-para hydrogen bonding creates a 2-fold rotation axis, the corresponding assembly is nanoporous as shown in the 2D crystals of H3BTB, 2, and 3. The shape of the bowtie dimer formed by the para-para interaction and additional hydrogen bonds inhibits the formation of close-packed structures. In contrast, the close-packed structures that maximize intermolecular interactions have a 2-fold rotation axis created by meta-meta hydrogen bonding, as shown in the 2D crystals of 1 and 4.

942 DOI: 10.1021/la103794j

The dominant force in controlling 2D crystallization of the reduced symmetry H3BTB analogues is the geometry of noncovalent interactions and in particular hydrogen bonding. It was found that symmetry elements, C2 axes in the case of p2 symmetry, lie on homodimers (para-para or meta-meta hydrogen bonding) but cannot lie on heterodimers (meta-para hydrogen bonding). The one exception to this observation is the 2D crystal of 4, in which symmetry elements do not lie on any dimer. Also, because of the triangular shape of the molecules, monolayers containing para-para dimers are all porous, and the absence of para-para dimers leads to close-packed structures. As such, it is possible to manipulate the porosity of the 2D crystals by modifying the number and/or placement of functional groups on the benzene rings. Notably, uncommon hydrogen bonding motifs in the 2D crystals of 3-II and 4 are present to satisfy the drive toward close-packing, and the 2D crystal of 4 exhibits an unusually high number of inequivalent molecules. The concept of reducing symmetry by changing sites for noncovalent interactions may have similar importance in 2D crystallization as in 3D because it acts as another variable for controlling assembly. As realized in this study, the plane group symmetry need not vary to have considerable structural diversity, although research is ongoing to determine whether this is inherent to 2D crystallization of reduced-symmetry analogues or specific to H3BTB analogues. Acknowledgment. This work was supported by the National Science Foundation (CHE-0957591). Supporting Information Available: 1H NMR spectra of compounds 1, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2011, 27(3), 936–942