Chiral Polymorphism: A Scanning Tunneling Microscopy Study

Imaging on-surface hierarchical assembly of chiral supramolecular networks. Laerte L. Patera , Zhiyu Zou , Carlo Dri , Cristina Africh , Jascha Repp ,...
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Langmuir 2000, 16, 9887-9894

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Chiral Polymorphism: A Scanning Tunneling Microscopy Study Steven De Feyter,* Andre´ Gesquie`re, and Frans De Schryver* Katholieke Universiteit Leuven, Celestijnenlaan 200 F, B-3001 Heverlee, Belgium

Christian Meiners, Michel Sieffert, and Klaus Mu¨llen Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55021, Mainz, Germany Received May 19, 2000. In Final Form: September 6, 2000 Physisorbed monolayer films of a chiral terephthalic acid derivative have been imaged on highly oriented pyrolytic graphite (HOPG) at the solution-substrate interface using scanning tunneling microscopy. The molecule comprises a nonchiral aromatic moiety and a chiral handle. It is found to form several twodimensional polymorphs. The packing arrangement of the molecule is primarily directed by the aromatic part, whereas the chiral part may assume different conformations on the graphite surface. The highresolution data confirm that for all polymorphs molecular chirality is transferred to the two-dimensional adlayer structure in an enantiospecific way. This enantiospecificity is attributed to adsorbate-substrate interactions.

Introduction Chirality is an important and intriguing structural physical property. The building blocks of life are chiral; many drugs are chiral, and their effects are often enantiomer selective. Since Pasteur’s discovery that the threedimensional crystals of the salt of tartaric acid exist in two enantiomorphous forms,1 the influence of molecular chirality on the two- and three-dimensional ordering of molecules has attracted a lot of interest, including the study of spontaneous chiral segregation in two and three dimensions. In three dimensions, homochiral molecules crystallize in chiral space groups. It is also not uncommon for achiral molecules to crystallize in chiral space groups. Racemic mixtures not only crystallize into the pure enantiomeric forms but also crystallize as racemic compounds; that is, the unit cell contains both enantiomers. Recently, the spontaneous enantiomeric resolution in a fluid smectic phase of a racemate was demonstrated by Takanishi et al.2 In two dimensions, a vast number of experimental studies deal with the monolayer ordering of chiral amphiphilic molecules in Langmuir films at the air-water interface.3 With grazing incidence X-ray diffraction, it was possible to probe the structure of these films with high resolution, and this technique provided strong evidence for the spontaneous resolution in two dimensions for some chiral systems.4 Scanning probe techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM) allow the study of monolayers on solid supports in great detail. For instance, Eckhardt et al. reported the separation of chiral phases in an ordered monolayer of rigid, chiral amphiphiles, transferred to mica by AFM.5 Using the same technique, Viswanathan et al. reported that achiral fatty acid molecules can also spontaneously form chiral domains in a Langmuir-Blodgett layer on a mica substrate.6 With STM, atomic resolution can be readily achieved. Thus, this technique should be especially suited to gain (1) Pasteur, L. C. R. Hebd. Seances Acad. Sci. 1848, 26, 535-539. (2) Takanishi, Y.; Takezoe, H.; Suzuki, Y.; Kobayashi, I.; Yajima, T.; Terada, M.; Mikami, K. Angew. Chem., Int. Ed. Engl. 1999, 38, 23532356.

information on chirality-related phenomena on surfaces. Indeed, STM experiments show that both chiral and achiral molecules self-assemble into chiral arrays. However, there is an important difference between achiral and chiral molecules. Achiral molecules form both lefthanded and right-handed monolayer structures.7,8 These structures are related to each other by a reflection in a mirror plane, are chiral, and are called enantiomorphous (3) (a) Du, X. Z.; Liang, Y. Q. Chem. Phys. Lett. 1999, 313, 565-568. (b) Xue, Q. B.; Yang, K. Z.; Xiao, C.; Zhang, Q. Z. Thin Solid Films 1999, 347, 263-271. (c) Kuzmenko, I.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1999, 121, 2657-2661. (d) Zhong, O. Y.; Xu, X. B.; Wu, C. X.; Iwamoto, M. Phys. Rev. E 1999, 59, 2105-2108. (e) Iwamoto, M.; Wu, C. X.; Zhong, O. Y. Phys. Rev. E 1999, 59, 586590. (f) Prus, P.; Pietraskiewicz, M.; Bilewicz, R. Supramol. Chem. 1998, 10, 17-25. (g) Scalas, E.; Brezesinski, G.; Kaganer, V. M.; Mo¨hwald, H. Phys. Rev. E 1998, 58, 2172-2178. (h) Yamagishi, A.; Sasa, N.; Taniguchi, M. Langmuir 1997, 13, 1689-1694. (i) Scalas, E.; Brezesinski, G.; Mo¨hwald, H.; Kaganer, V. M.; Bouwman, W. G.; Kjaer, K. Thin Solid Films 1996, 285, 56-61. (j) Rietz, R.; Rettig, W.; Brezesinski, G.; Bouwman, W. G.; Kjaer, K.; Mo¨hwald, H. Thin Solid Films 1996, 285, 211-215. (k) Vollhardt, D.; Emrich, G.; Gutberlet, T.; Fuhrhop, J. H. Langmuir 1996, 12, 5659-5663. (l) Parazak, D. P.; Uang, J. Y.-J.; Turner, B.; Stine, K. J. Langmuir 1994, 10, 3787-3793. (m) Selinger, J. V.; Wang, Z.-G.; Bruinsma, R. F.; Knobler, C. M. Phys. Rev. Lett. 1993, 70, 1139-1142. (n) Rietz, R.; Brezesinski, G.; Mo¨hwald, H. Ber. BunsenGes. Phys. Chem. 1993, 97, 1394-1399. (o) Andelman, D.; Orland, H. J. Am. Chem. Soc. 1993, 115, 12322-12329. (p) Dvolaitsky, M.; GuedeauBoudeville, M. A. Langmuir 1989, 5, 1200-1205. (q) Arnett, A. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131-138. (r) Tachibana, T.; Yoshizumi, T.; Hori, K. Bull. Chem. Soc. Jpn. 1979, 52, 34-41. (4) (a) Nassoy, P.; Goldman, M.; Bouloussa, O.; Rondelez, F. Phys. Rev. Lett. 1995, 75, 457-460. (b) Weissbuch, I.; Berfeld, M.; Bouman, W.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leisorowitz, L. J. Am. Chem. Soc. 1997, 119, 933-942. (c) Kuzmenko, I.; Weissbuch, I.; Gurovich, E.; Leiserowitz, L.; Lahav, M. Chirality 1998, 10, 415-424. (d) Lahav, M.; Leiserowitz, L. Angew. Chem., Int. Ed. Engl. 1999, 38, 2533-2536. (5) Eckhardt, C. J.; Peachey, N. M.; Swanson, D. R.; Takacs, J. M.; Khan, M. A.; Gong, X.; Kim, J.-H.; Wang, J.; Uphaus, R. A. Nature 1993, 362, 614-616. (6) Viswanathan, R.; Zasadzinski, J. A.; Schwartz, D. K. Nature 1994, 368, 440-443. (7) (a) Smith, D. P. E. J. Vac. Sci. Technol., B 1991, 9, 1119-1125. (b) Rabe, J. P.; Buchholz, S. Phys. Rev. Lett. 1991, 66, 2096-2099. (c) Sowerby, S. J.; Heckl, W. M.; Petersen, G. B. J. Mol. Evol. 1996, 43, 419-424. (d) Charra, F.; Cousty, J. Phys. Rev. Lett. 1998, 80, 16821685. (e) Claypool, C. L.; Faglioni, F.; Matzger, A. J.; Goddard, W. A., III; Lewis, N. S. J. Phys. Chem. B 1999, 103, 9690-9699. (f) Patrick, D. L.; Cee, V. J.; Morse, M. D.; Beebe, T. P. J. Phys. Chem. B 1999, 103, 8328-8336.

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the two-dimensional (2D) ordering of molecules in general and especially of chiral molecules on atomically flat substrates such as highly oriented pyrolytic graphite (HOPG). The main focus is on the observation of polymorphism and if or how this affects the enantiospecific monolayer formation. Molecular mechanics calculations aid in gaining more insight. Experimental Section

Figure 1. (A) Mirror image packing (enantiomorphous) patterns of an achiral compound (L-shaped). The dashed line in the middle represents a mirror plane. The achiral compound can form both mirror image packing patterns. (B) Mirror image packing patterns of a chiral compound which comprises a nonchiral moiety (L-shaped) and a chiral handle (circle). The dashed line in the middle represents a mirror plane. The symmetry operation implies that the species on both sides of the mirror plane have a different configuration. For instance, if the molecules at the left side of the mirror plane have S configuration, then the molecules at the right side must have R configuration, and vice versa. Therefore, in contrast to achiral compounds, a single enantiomer (S or R) is expected to form only one arrangement and not its reflection in a mirror plane. (C) Occasionally, a single enantiomer (S or R) might form quasienantiomorphous structures. These are 2D structures which are not truly enantiomorphous on a molecular scale because the molecules are present as only one enantiomer, but the unit cells might appear enantiomorphous. For instance, the nonchiral units may assume enantiomeric arrangements, forcing the chiral parts of the molecule to assume diastereoisomeric and not enantiomeric arrangements. The dotted line in the middle does not represent a specific symmetry operation. However, one can think of symmetry operations which could lead to the proposed arrangement (for example, a 2-fold axis of symmetry parallel to the substrate plane). The difference of the pattern in the circles reflects different orientations of the chiral handle.

(Figure 1A). Chiral molecules also form chiral monolayer structures but in an enantiospecific way; the monolayer structure of an enantiomer is the mirror image of the other enantiomer, and as such these structures are enantiomorphous (Figure 1B).9-13 However, the general validity of this statement might be questioned by the results obtained with a chiral secondary alcohol.14 Note that in some systems, STM allows the determination of the absolute configuration of chiral molecules.13,15 In this contribution, we present STM studies on a monochiral terephthalic acid derivative. This study is aimed at gaining more insight into the factors controlling (8) (a) Bohringer, M.; Morgenstern, K.; Schneider, W. D.; Berndt, R. Angew. Chem., Int. Ed. Engl. 1999, 38, 821-823. (b) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. Rev. Lett. 1999, 83, 324-327. (c) Bohringer, M.; Schneider, W. D.; Berndt, R. Angew. Chem., Int. Ed. 2000, 39, 792-795. (9) (a) Stevens, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900-901. (b) Walba, D. M.; Stevens, F.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591-597. (10) De Feyter, S.; Grim, P. C. M.; Rucker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. Engl. 1998, 37, 1223-1226. (11) De Feyter, S.; Gesquiere, A.; Grim, P. C. M.; De Schryver, F. C.; Valiyaveettil, S.; Meiners, C.; Sieffert, M.; Mullen, K. Langmuir 1999, 15, 2817-2822. (12) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Lett. 1998, 1133. (13) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 7311-7315. (14) Le Poulennec, C.; Cousty, J.; Xie, Z. X.; Mioskowski, C. Surf. Sci. 2000, 448, 93-100. (15) (a) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D.; Wolkow, R. A. Nature 1998, 392, 909-911. (b) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Zgierski, M. Z.; Wolkow, R. A. J. Am. Chem. Soc. 1999, 121, 4532-4533.

Synthesis. The R- and S-enantiomers of 2-eicosyloxy-5-(1methylheptyloxy)terephthalic acid are synthesized according to a Mitsunobu condensation of diethyl 2-eicosyloxy-5-hydroxyterephthalate with (S)- or (R)-2-octanol, respectively, in diethyl ether/diisopropyl azodicarboxylate (or DIAD)/triphenylphosphine with inversion of the configuration at the stereogenic center. The enantiomeric purity of the 2-octanol enantiomers is 99.5%. The diethyl ester is purified by column chromatography. The pure fraction and the mixed fraction were hydrolyzed quantitatively. The hydrolyzed mixed fraction yields the pure compound after preparative high-performance liquid chromatography (HPLC). Procedure for the Synthesis of Diethyl 5-Eicosyloxy5-hydroxyterephthalate. Diethyl 2,5-dihydroxyterephthalate (4 g, 15.7 mmol) is dissolved in 30 mL of absolute DMF, and NaH (0.63 g, 15.7 mmol) is added. This mixture was stirred for 30 min at 80 °C. To this mixture, eicosylbromide (5.69 g, 15.7 mmol) was added. The solvent was then evaporated, and the residue was mixed with water, followed by extraction with dichloromethane (DCM). The organic phase was dried over magnesium sulfate and evaporated. Crystallization of the oil reaction product in 75 mL of DCM led to an increased content of the product in the filtrate. The product (1.69 g, 20%) was isolated as a yellow solid after HPLC (THF/H2O 70/30). Mp 102.4 °C. 1H NMR (300 MHz, CDCl3, 25 °C): δ ) 0.85 (t, 3J(H,H) ) 6.5 Hz, 3H, CH3), 1.181.49 (m, 40H, CH2, OCH2CH3), 1.77 (m, 2H, ArOCH2CH2), 3.95 (t, 3J(H,H) ) 6.5 Hz, 2H, ArOCH2), 4.35 (m, 4H, OCH2CH3), 7.28 (s, 1H, ArH), 7.32 (s, 1H, ArH), 10.34 (s, 1H, ArOH). 13C NMR (75.5 MHz, CDCl3, 25 °C): δ ) 14.07, 14.20, 22.66, 25.97, 29.27, 29.33, 29.37, 29.58, 29.60, 29.64, 29.68, 31.90, 61.28, 61.84, 70.07, 113.84, 114.75, 119.72, 129.08, 150.11, 155.09, 165.56, 169.25. MS-FD: m/z ) 534.4 [M+]. Anal. Calcd for C32H54O6 (534.78): C, 71.87; H, 10.17. Found: C, 71.83; H, 10.39. Procedure for the Synthesis of Diethyl 2-Eicosyloxy5-((R)-1-methylheptyloxy)terephthalate. The synthesis of the S-enantiomer is analogous. Diethyl 2-eicosyloxy-5-hydroxyterephthalate (0.395 g, 7.39 mmol), (S)-2-octanol (99.5% Senantiomer) (0.0962 g, 7.39 mmol), and triphenylphosphine (0.1937 g, 7.39 mmol) are mixed in 20 mL of absolute diethyl ether. Diisopropyl azodicarboxylate (0.1572 g, 7.39 mmol) is added at -8 °C (ice/NaCl) during 15 min. The reaction mixture was stirred for 5 days at room temperature. After partial evaporation, NMR revealed that no reaction took place. Absolute diethyl ether (20 mL) was added to the partially evaporated solution. An equivalent of triphenylphosphine was added, followed by a second equivalent of diisopropyl azodicarboxylate while cooling. After 2 days, according to NMR no starting material was observed and the reaction mixture was concentrated through evaporation and purified by column chromatography on silica gel (PE/EE 12:1), which resulted in a pure colorless oil fraction of diethyl 2-eicosyloxy-5-((R)-1-methylheptyloxy)terephthalate (141 mg, 0.22 mmol) and some nonpure product fraction (215 mg). Comparison of the pure R-enantiomer and pure S-enantiomer with analytical HPLC on chiral Chiradex-Phase shows that no traces of the S-enantiomer in the R-product can be detected. This indicates that the Mitsunobu reaction takes place with a complete or nearly complete inversion of the configuration at the stereogenic center. [R]D22 ≈ -6° (c ) 0.81 in THF). 1H NMR (200 MHz, CDCl3, 25 °C): δ ) 0.85 (t, 3J(H,H) ) 6.2 Hz, 6H, CH3), 1.1-1.65 (m, 53H, OCH2CH3, CH2), 1.75 (m, 2H, ArOCH2CH2), 3.97 (t, 3J(H,H) ) 6.4 Hz, 2H, ArOCH2), 4.32 (m, 5H, ArOCH, OCH2CH3), 7.27 (s, 1H, ArH), 7.32 (s, 1H, ArH). 13C NMR (50.3 MHz, CDCl3, 25 °C): δ ) 13.99, 14.23, 19.52, 22.55, 22.65, 25.30, 25.97, 29.31, 29.66, 31.77, 31.89, 36.46, 61.12, 69.87, 76.491, 116.34, 118.84, 124.71, 126.32, 150.59, 151.78, 166.08, 166.30. MS-FD: m/z ) 646.4 [M+].

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Hydrolysis to 2-Eicosyloxy-5-((R)-1-methylheptyloxy)terephthalic Acid ((R)-1). The synthesis of the S-enantiomer ((S)-1) is analogous. The hydrolysis of the ester was performed in a solution of sodium hydroxide (2 equiv) in ethanol and water (2:1 mixture) which was stirred for 6 h under reflux. After reaction, the solvent was evaporated and the residue was mixed with water. The white precipitate obtained after the addition of concentrated HCl was filtered, washed with water, and dried under high vacuum. Hydrolysis of the pure fraction yields the free acid (0.22 mmol (quantitative)). Hydrolysis of the nonpure 215 mg fraction yields the acid as a white powder after HPLC (THF/H2O 70:30) (60 mg, 0.1 mmol). The overall yield is 43%. Mp R-enantiomer and S-enantiomer: 102 °C. [R]D22 ≈ -6° (c ) 1 in THF) (for S-enantiomer, [R]D22 ≈ (6°). 1H NMR (200 MHz, CDCl3, 25 °C): δ ) 0.84(6) (t, 3J(H,H) ) 6.7 Hz, 3H, CH3), 0.85(2) (t, 3J(H,H) ) 6.9 Hz, 3H, CH3), 1.18-1.50 (m, 45H, CH2, CH3CH), 1.65-1.94 (m, 4H, ArOCH2CH2, CHCH2), 4.27 (t, 3J(H,H) ) 6.7 Hz, 2H, ArOCH2), 4.74 (m, 1H, ArOCH), 7.85 (s, 2H, ArH). 13C NMR (75.5 MHz, CDCl3, 25 °C): δ ) 13.96, 14.06, 19.57, 22.46, 22.65, 25.18, 25.75, 28.83, 29.00, 29.13, 29.32, 29.38, 29.48, 29.58, 29.62, 29.67, 31.55, 31.89, 36.15, 71.30, 79.04, 117.41, 119.03, 122.81, 123.66, 150.78, 151.67, 163.95, 164.09. MS-FD: m/z ) 589.7 [M+]. Anal. Calcd for C36H62O6 (590.88): C, 73.18; H, 10.58. Found: C, 73.19; H, 10.60. Scanning Tunneling Microscopy. Prior to imaging, all compounds under investigation were dissolved in 1-phenyloctane (Aldrich, 99%), and a drop of this solution was applied on a freshly cleaved surface of HOPG. The STM images were acquired in the variable current mode (constant height) under ambient conditions. In the STM images, white corresponds to the highest and black to the lowest measured tunneling current. STM experiments were performed using a Discoverer scanning tunneling microscope (Topometrix Inc., Santa Barbara, CA) along with an external pulse/function generator (model HP 8111 A), with negative sample bias. Tips were electrochemically etched from Pt/Ir wire (80%/20%, diameter 0.2 mm) in 2 N KOH/6 N NaCN solution in water. The experiments were repeated in several sessions using different tips to check for reproducibility and to avoid artifacts. All monolayer images were obtained at a tunneling current of 1.0 nA, and the bias voltage (tip positive) ranged from -0.3 to -0.9 V. After registration of a STM image of a monolayer structure, the underlying graphite surface was recorded at the same area by decreasing the bias voltage (-0.01 V to -0.1 V), serving as an in situ calibration. The images are not filtered unless stated otherwise.

Results Figure 2 is the chemical structure of the monochiral terephthalic acid 1. This compound is characterized by an achiral eicosyloxy side chain and a short chiral 2-octyloxy group (chiral handle). Figure 3A-C represents characteristic images of monolayers of the S-enantiomer of 1 ((S)-1) physisorbed from 1-phenyloctane on graphite. Bright or dark contrast features in the STM images reflect a high or low tunneling current, respectively. The images are characterized by rather large bright spots corresponding to the location of aromatic terephthalic acid groups, which are aligned in rows and define the lamella axis. The rows of smaller spots perpendicular to the lamella axis reflect the orientation of the extended eicosyloxy groups (∆L1) (3.1 ( 0.1 nm). According to Claypool et al., each of those spots corresponds to a hydrogen atom directed toward the tunneling tip. Two adjacent rows of those spots define an alkoxy chain.16 The relative positions of the spots (hydrogen atoms) also illustrate the orientation of the alkoxy chains; the plane defined by the carbon atoms of the eicosyloxy backbone is parallel to the graphite surface. The high quality of these nonfiltered STM images (16) Claypool, C. L.; Faglioni, F.; Goddard, W. A.; Lewis, N. S. J. Phys. Chem. B 1999, 103, 7077-7080.

Figure 2. Chemical structure of 2-eicosyloxy-5-(1-methylheptyloxy)terephthalic acid (1). The star indicates the location of the stereogenic center.

indicates that the eicosyloxy chains are immobilized effectively. The dimension of ∆L2 (1.6 ( 0.1 nm) is in agreement with the configuration of extended 2-octyloxy groups adsorbed on the graphite surface. Individual spots (hydrogen atoms) are hardly visible, which indicates a nonoptimal packing and increased dynamics. In some images, such as in Figure 3A,B, the orientation of individual 2-octyloxy groups is visible. They are slightly clockwise rotated with respect to the long axis of the eicosyloxy groups. However, in the majority of the images, the image resolution does not allow us to determine the orientation of the chiral side chains. Furthermore, the 2-octyloxy groups are not always adsorbed on the graphite surface, as found in the lower part of Figure 3B. The distance between adjacent rows of terephthalic acid groups (∆L3) is not large enough for the 2-octyloxy groups to lie flat on the graphite surface. This suggests that the 2-octyloxy groups point away from the graphite surface. Partial or complete desorption of parts of molecules is not uncommon and has been observed for other compounds, chiral and achiral, as well.11,17 The molecular model in Figure 3D reflects the two-dimensional ordering of the monolayer in Figure 3A. During the acquisition of the monolayer in Figure 4, the bias voltage was suddenly decreased and the underlying graphite surface was visualized. In most cases, the graphite substrate was imaged immediately after acquisition of the monolayer and compared to the monolayer image. In the lower part of the image, the so-called main graphite axes, or [012 h 0] direction of the graphite lattice (dotted lines), and the so-called reference axes, or [0010] direction of the graphite lattice (solid lines), are indicated. Such a reference axis (solid line) and the propagation direction of the lamella (dashed line) are indicated in the upper part of the image and are important for further analysis purposes (see below). The eicosyloxy chains are oriented parallel to a main graphite axis. This alignment is typical for alkylated organic molecules and suggests that the monolayer structure is at least partly influenced by adsorbate-substrate interactions. Some of the images contain a periodic contrast feature (moire´ pattern) which is attributed to the incommensurability of the alkyl chains with the graphite surface in the propagation direction of the lamella axis.18 One such period (∆M) is indicated in Figure 4B. The moire´ period contains four alkyl chains. On the basis of the presence of moire´ patterns which allows for a precise determination of structural parameters, the distance between eicosyloxy groups in those images was measured to be 0.479 nm. For cases in which the image contrast does not allow identifying the moire´ pattern, the images were analyzed in an alternative way, which leads to similar results but with (17) (a) Parks, D. C.; Clark, N. A.; Walba, D. M.; Beale, P. D. Phys. Rev. Lett. 1993, 70, 607-610. (b) Gorman, C. B.; Gorman, C. B.; Touzov, I.; Miller, R. Langmuir 1998, 14, 3052-3061. (c) Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2000, 16, 2326-2342. (18) Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Su¨ling, C. J. Phys. Chem. 1995, 99, 86908697.

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Figure 3. STM images of monolayers of (S)-1 physisorbed at the 1-phenyloctane/graphite interface. The molecules are arranged in lamellae. The orientation of a few molecules is indicated schematically. The circle indicates the location of a terephthalic acid group, and the short and long bars represent the 2-octyloxy and the eicosyloxy groups, respectively. ∆L1 determines the region occupied by the eicosyloxy groups and measures 3.1 ( 0.1 nm. ∆L2 (1.6 ( 0.1 nm) determines the width of the region formed by the 2-octyloxy regions when adsorbed on the graphite surface. ∆L3 indicates regions where the 2-octyloxy groups are rotated away from the graphite surface. (A) 10.5 × 10.5 nm2, θ ) -4.0°. The parallel lines running from the upper left to the lower right of the image are due to electronic noise. (B) 10.5 × 10.5 nm2, θ ) +3.5°. (C) 10.5 × 10.5 nm2, θ ) -11°. This image has been rotated to align the eicosyloxy groups in all the images in Figure 2 in the same way. (D) Molecular model representing the two-dimensional ordering in A. The conformation of 1 is based upon molecular mechanics calculations (MM+) of one molecule on graphite, thus neglecting nearest-neighbor interactions.

larger error bars. The center-to-center distance between two adjacent terephthalic acid groups within the same row is 0.96 nm and does not differ significantly for the monolayers shown in Figure 3. Within experimental error, the intermolecular distance is identical to the values obtained for other chiral and achiral terephthalic acid derivatives studied in our group11 and matches the value found for three-dimensional (3D) crystals of terephthalic acid and its derivatives.19 This suggests that hydrogen bonding plays a significant role in the monolayer formation. The lamellae in Figure 3A-C differ in the orientation of their propagation direction with respect to the direction of the eicosyloxy groups. To illustrate this, the image in Figure 3C has been rotated in order to align the eicosyloxy groups in the same way as in Figure 3A,B. This suggests that different monolayer structures are formed. To characterize the monolayer structure, a structural parameter, that is, the angle θ between the propagation direction of the lamella axis and a graphite reference axis,

has been determined in the same way as indicated in Figure 4. In all cases, that particular graphite reference axis is selected which is perpendicular to the long axis of the eicosyloxy groups. The eicosyloxy groups are oriented parallel to a main graphite axis which allows the univocal characterization of the lamella structure in terms of its relation with the underlying graphite substrate. The angle θ for the two-dimensional structures in parts A, B, and C of Figure 3 measures -4.0°, +3.5°, and -11.0°, respectively. The absolute error on these values is about 1° for the small angles and about 2° for the larger angles.20 By way of illustration, images of domain boundaries of (S)-1 monolayers are shown. In Figure 5A, the value of the angle θ in the upper and lower parts of the image is -4° and +4°, respectively, whereas for the monolayer in Figure 5B, these values are -10° and -3.5°, respectively. For the (S)-1 adlayer in Figure 6, the white lines indicate the borders of the domains. The values of the angle θ in the domains I, II, III, and IV are +3.5°, -3.5°, +4.5°, and -11°, respectively.

(19) (a) Domenicano, M.; Schultz, G.; Hargittai, I.; Colapietro, M.; Portalone, G.; George, P. Struct. Chem. 1990, 1, 107. (b) Meiners, C.; Mu¨llen, K. Private communication.

(20) These error values reflect the uncertainty in the determination of the direction of the lamella axis and the graphite reference axis and also take into account uncertainties caused by possible image distortions.

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Figure 4. STM images of monolayers of (S)-1 physisorbed at the 1-phenyloctane/graphite interface. (A) 10.5 × 10.5 nm2. A sudden decrease of the bias voltage during scanning revealed the graphite surface underneath the monolayer. This procedure shows that the eicosyloxy groups are oriented parallel to one of the main axes of graphite. For illustration, the so-called main graphite axes (dotted lines) along the [012h 0] direction and the so-called reference axes (solid lines) along the [0010] direction, which are oriented perpendicular to the main axes, are indicated. In the upper part of the image, the propagation direction (dashed line) and the reference axis (solid line) which define the angle θ are indicated. (B) 8.3 × 8.3 nm2. In this combined image, the upper part of the image represents a monolayer. The lower part is graphite. The graphite surface has not been obtained at the same time but has been acquired just after collecting the monolayer image. For illustration purposes, part of this monolayer image has been replaced by the graphite image. The graphite image has been subjected to filtering for contrast enhancement. Bright eicosyloxy groups with identical contrast are indicated by white lines. The distance between two bars defines the contrast periodicity, also known as a moire´ period.

Figure 5. STM images of domain boundaries of monolayers of (S)-1 at the 1-phenyloctane/graphite interface. (A) 13.1 × 13.1 nm2. The values of the angle θ in the upper and lower parts are -4° and +4°, respectively. (B) 10.5 × 10.5 nm2. The values of the angle θ in the upper and lower parts are -10° and -3.5°, respectively.

The θ values have been determined for all domains composed of (S)-1, and a histogram of the values of θ is given in Figure 7A. The histogram can be subdivided in three parts. Two of those are characterized by negative angles. The mean values are approximately -3° and -11° to -12°. One is positive, and its mean value is approximately +3°. In a similar way, the two-dimensional ordering of the R-enantiomer was investigated. Figure 8 represents a few monolayer images of (R)-1. (R)-1 and (S)-1 form similar monolayer structures: the eicosyloxy chains are extended, interdigitated, and oriented along one of the main graphite axes (∆L1); the 2-octyloxy groups are extended and oriented parallel to the graphite surface (∆L2) or they protrude from the graphite surface (∆L3). Similar to (S)-1, the orientation of the 2-octyloxy groups is in most cases not well-defined. However, the details in Figure 8B,C suggest that the 2-octyloxy groups are rotated slightly counterclockwise with respect to the long axis of the eicosyloxy groups, in contrast to (S)-1. Figure 8D is a STM image of

a domain boundary. The values of θ in the left and right domains are +12° and +3°, respectively. To compare the chirality of the monolayers, the angle θ was determined for all (R)-1 domains, and the histogram is shown in Figure 7B. The angle θ adopts several values, and the histogram is characterized by three main regions. The mean values of θ are -3°, +3°, and +12°. Again, this suggests the formation of three major domain types. Note that the histograms of (S)-1 and (R)-1 form approximately each other’s mirror images. Both histograms are characterized by the same absolute θ values, but θ differs in sign for the large values. Discussion In a previous contribution, we reported the influence of chirality on the two-dimensional ordering of a symmetrical terephthalic acid derivative, which bears two identical stereogenic centers.11 The S-enantiomer was found to form well-defined enantiomorphous domains, and the angle θ

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Figure 6. STM image containing several domain boundaries of (S)-1 physisorbed at the 1-phenyloctane/graphite interface. 20 × 20 nm2. The white lines define the several domain areas. These domains are polymorphous and are characterized by different values for the angle θ. The values of the angle θ in the domains I, II, III, and IV are +3.5°, -3.5°, +4.5°, and -11°, respectively. The orientation of the 2-octyloxy groups can be distinguished only in the upper two domains. They appear to be rotated a few degrees clockwise with respect to the normal on the lamella axis.

Figure 7. Histograms reflecting the number of observations as a function of the value of the angle θ: (A) S-enantiomer and (B) R-enantiomer.

was centered around -3.7°. The orientation of the Renantiomer was identical but for the value of θ which was found to be +3.7°. In contrast, both enantiomers of 1 form three different domain types, as expressed by the three regions in the respective histograms. Surprisingly, both

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enantiomers take both positive and negative θ values for the small angle (∼3°). As stated in the Introduction and in Figure 1, one would expect that an enantiomer forms only one of both enantiomorphous structures whereas the other enantiomer forms the mirror image packing pattern, as is likely to be the case for the structure characterized by the absolute value centered around 12°. These contradictory observations put the concept of the uniqueness of enantiospecific transfer of chiral information on the molecular level to the monolayer structure into question. How can one account for these seemingly contradictory results? Does this compound form only two types of adlayers, (|θ| ≈ 3°) and (|θ| ≈ 11°), of which one (|θ| ≈ 3°) does not express the chirality of the individual molecules, or does this compound form three types of adlayers of which two are characterized by the same absolute value of θ? In the latter case, the domains characterized by θ ≈ +3° and θ ≈ -3° for the same enantiomer would be polymorphous and the same absolute value for θ would be fortuitous. One should note that whether the 2-octyloxy groups are in contact with the graphite substrate or are pointed away from it does not play a critical role in defining the chirality of the monolayer. The fact that the 2-octyloxy groups are rotated away from the graphite surface, such as shown in Figures 3B and 8A, could indeed leed to a loss of chiral structural information as the stereogenic centers are not in contact with the graphite substrate. In such a situation, the chiral molecules might behave as achiral ones. However, domains never exist exclusively of structures of the latter type and, as such, the chirality of those domains is governed by those other molecules which have their 2-octyloxy groups in contact with the graphite surface. In an extreme case, one might predict that the orientation of one chiral molecule is sufficient to control the chiral ordering of the surrounding molecules within the same domain. A solution to the seemingly contradictory results mentioned above can be found in the following. In some cases, it was possible to determine the orientation of individual 2-octyloxy groups. The best examples were found for (S)-1 in Figure 3A,B and domains I and II in Figure 6. In these images, the absolute value of θ is ∼3°. Although these domains differ with respect to the sign of θ, in both images the 2-octyloxy groups are rotated slightly clockwise with respect to the long axis of the eicosyloxy groups, irrespective of the sign of θ. In the present case, the nonchiral moieties assume enantiomeric arrangements, forcing the chiral parts of the molecule to assume diastereoisomeric and not enantiomeric arrangements (Figure 1C). This is an example of conformational polymorphism, a phenomenon well documented in 3D crystals.21 In agreement with symmetry considerations, the similar absolute values of θ for both positive and negative domains must be fortuitous and for the same enantiomer, domains with positive and negative values for θ cannot be truly enantiomorphous but are polymorphous. Indeed, for symmetry reasons it is impossible to generate two monolayer structures which are truly enantiomorphous and which contain only molecules of the same enantiomer type. In some cases, however, images of monolayers composed of the same enantiomer might appear enantiomorphous but they cannot be enantiomorphous on a molecular scale. Those structures must therefore be polymorphous and differ in energy.22 As expected, both enantiomers give true mirror image packing (enantiomorphous) patterns on the graphite (21) Bernstein, J.; Hagler, A. T. J. Am. Chem. Soc. 1978, 100, 673.

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Figure 8. STM image of a monolayer of the R-enantiomer at the 1-phenyloctane/graphite interface. (A) 12.9 × 12.9 nm2. (B) 5.4 × 5.4 nm2. Close-up of part of the image in A. (C) 5.4 × 5.4 nm2. High-resolution image of the 2-octyloxy groups. This image suggests that the 2-octyloxy groups are rotated counterclockwise with respect to the normal on the lamella axis. (D) 15.8 × 15.8 nm2. Image of a domain boundary. The values of the angle θ in the left and right parts are -10° and -3.5°, respectively.

surface and they have to be of identical energy. The enantiomorphous monolayers formed by achiral molecules are also of identical energy.22 Polymorphism and quasi-enantiomorphism have also been reported for some liquid-crystalline compounds.9b,23 The orientation of the molecules in the different domain structures is not yet resolved. The contrast of the long alkyl chains, as expressed by the appearance of the hydrogen atoms, indicates that the plane formed by the carbon atoms in the eicosyloxy chain is parallel to the graphite substrate,16 independent of the value of θ. The orientation of the short 2-octyloxy groups is less welldefined. This results in a less favorable interaction of the 2-octyloxy group with the graphite substrate, and as such (22) Enantiomorphous monolayers formed by an achiral compound can be transformed into each other by reflection through a mirror plane perpendicular to the substrate. Therefore, both enantiomorphous monolayers have the same energy. This holds also for the mirror image packing patterns of the two enantiomers of a chiral compound (Figure 1A,B). On the other hand, the unit cell of the 2D monolayer ordering formed by an enantiopure chiral compound can be converted into the enantiomorphous unit cell by, for example, rotation about a C2 axis in the plane of the substrate. Although this symmetry operation leads to enantiomorphous unit cells, the monolayer structures are not enantiomorphous (Figure 1C). For instance, in one case the methyl group on the stereogenic center would be directed toward the supernatant solution whereas after the 2-fold rotation, this methyl group would be directed to the graphite substrate, resulting in different interaction energies. (23) Parks, D. C.; Clark, N. A.; Walba, D. M.; Beale, P. D. Phys. Rev. Lett. 1993, 70, 607-610.

this short group is more apt to dynamics. Molecular mechanics calculations (MM+) shed some light on the possible conformations a molecule can take on graphite. These calculations suggest that the bulky stereogenic center influences the conformation of the short alkoxy chain in such a way that its orientation is not parallel to that of the eicosyloxy groups but makes a small angle, as observed in the experiments. Concerning the angle between the 2-octyloxy group and the eicosyloxy group, the lowest energy conformation of this molecule on graphite as predicted by MM+ is in good agreement with experiment.24 The MM+ calculations also support the hypothesis that the interaction of the stereogenic center with the graphite surface is responsible for the enantiospecific monolayer formation. For instance, it is energetically unfavorable when the methyl group on the stereogenic center is turned toward the graphite surface. This is supported by experiments on the chiral terephthalic acid derivative reported in ref 11. For this compound, it was possible to determine the location and orientation of (24) For the MM+ calculations, one S-enantiomer molecule has been modeled on a sufficiently large piece of graphite. Several initial start conformations were sampled. The molecular structure corresponding to the lowest energy conformation was used as the building unit to construct the molecular model given in Figure 3D. The eicosyloxy group was oriented parallel to the main graphite axes. The calculations predict a small clockwise rotation of the 2-octyloxy group with respect to the eicosyloxy chain of 6°. No intermolecular interactions were considered in the calculation.

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the stereogenic groups as they appeared as protrusions in the topography images, which confirmed that the methyl group on the stereogenic center is directed away from the graphite surface. Although we are not able to explain the very details of the molecular ordering for the different polymorphs in the system presented, it is most likely that the interaction of the stereogenic center with the underlying graphite substrate dominates the enantiospecific monolayer formation. The distance between the terephthalic acid groups indicates that hydrogen bonding connects the terephthalic acid groups. However, the polymorphism also suggests that the hydrogen bonding pattern between the terephthalic acid groups might differ for the different domain types. For instance, it is possible that only one hydrogen bond is formed instead of two. Flexibility of the arylO-C bond could also contribute to the polymorphism. The very details of the exact ordering of the different polymorphous 2D structures are yet unknown and are still under investigation. High-level calculations are needed to unravel the subtleties of intermolecular and adsorbatesubstrate interactions and to determine their relative contribution in directing the 2D ordering.

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Conclusions The chiral compound under investigation forms wellordered two-dimensional adlayers at the liquid/graphite interface. The packing arrangement of the molecule is primarily directed by the aromatic part and the long aliphatic part, whereas the chiral part may assume different conformations on the graphite surface. Several polymorphs are formed. Enantiomers form enantiomorphous monolayers, and enantiospecific monolayer formation is maintained even when polymorphous domains are formed. Acknowledgment. The authors gratefully acknowledge the FWO, the Flemish Ministry of Education for support through GOA/1/96, the EC through the TMR Sisitomas and TMR Marie Curie, the VW Stiftung, and the support of DWTC (Belgium) through IUAP-IV-11. The European Science Foundation through SMARTON is thanked for financial support. S.D.F. is a postdoctoral fellow of the Fund for Scientific Research - Flanders. A.G. thanks the IWT for a predoctoral scholarship. LA000693X