A Nanoscale View of Supramolecular Stereochemistry in Self

Monolayer at the Liquid/Solid Interface by a Supramolecular Chiral Auxiliary ..... Edwin C. Constable , H.-J. G?ntherodt , Catherine E. Housecroft...
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Langmuir 2004, 20, 9628-9635

A Nanoscale View of Supramolecular Stereochemistry in Self-Assembled Monolayers of Enantiomers and Racemates Wael Mamdouh,† Hiroshi Uji-i,† Andre´ Gesquie`re,† Steven De Feyter,*,† David B. Amabilino,*,‡ Mohamed M. S. Abdel-Mottaleb,† Jaume Veciana,‡ and Frans C. De Schryver† Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200-F, 3001, Leuven, Belgium, and Institut de Cie` ncia de Materials de Barcelona (CSIC), Campus Universitari, 08193 Bellaterra, Catalonia, Spain Received July 23, 2004 The effect that molecular chirality has on the formation of monolayer structures by pure enantiomers and their racemates at the liquid/solid interface has been investigated for two chiral compounds (1 and 2) which differ from each other by the presence or absence of an ester function in their respective molecular structures. 1 shows pseudoracemate formation when the achiral graphite support is exposed to a solution containing a racemate while 2 shows racemic conglomerate formation. This difference is rationalized in terms of the orientation of the pure enantiomers with respect to the graphite substrate and highlights the importance of molecular structure and its influence on balancing the interplay between molecular conformation and molecular packing on the surface. For 1, nonstoichiometric mixtures of both enantiomers have been investigated, and the results are discussed in the framework of the sergeant and soldiers principle. These results are important for the understanding and prediction of spontaneous resolution in monolayer systems.

Introduction The existence and induction of chirality are among the most intriguing and inspiring phenomena in chemistry. Many molecules are chiralsthe mirror forms are not superimposable by any rotation or translationseither intrinsically or transiently; i.e., they can adopt chiral conformations. Chirality is not only at play in individual molecules but also in the supramolecular systems of which they are a part. Many examples exist where molecular chirality is used to influence the supramolecular stereochemistry1 of containers,2 helical fibers,3 micelles,4 bilayers,5 and crystals.6 The study of transfer and expression of molecular chirality in monomolecular thin layers is still in its infancy. * Corresponding authors: Tel + 32 16 32 7921; Fax +32 16 32 7990; e-mail [email protected]. Tel +34 93 580 1853; Fax: +34 93 580 5729; e-mail: [email protected]. † Katholieke Universiteit Leuven. ‡ Institut de Cie ` ncia de Materials de Barcelona. (1) For general reviews, see: (a) Siegel, J. S. Supramolecular Stereochemistry; NATO ASI Series C473; Kluwer Academic: Dordrecht, 1995. (b) Lemieux, R. P. Acc. Chem. Res. 2001, 34, 845. (c) Yashima, E.; Maeda, K.; Nishimura, T. Chem.sEur. J. 2004, 10, 42. (d) Amabilino, D. B.; Veciana, J. In Encyclopaedia of Supramolecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker: New York, 2004. (2) (a) Castellano, R. K.; Kim, B. H.; Rebek, J., Jr. J. Am. Chem. Soc. 1997, 119, 12671. (b) Rivera, J. M.; Rebek, J., Jr. J. Am. Chem. Soc. 2000, 122, 7811. (3) (a) Sommerdijk, N. A. J. M.; Lambermon, M. H. L.; Feiters, M. C.; Nolte, R. J. M.; Zwaneburg, B. Chem. Commun. 1997, 1423. (b) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; MacKintosh, F. C. Nature (London) 1999, 399, 566. (c) Schenning, A. P. H. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavallini, M.; Cooper, H. J.; Derrick, P. J.; Feast, W. J.; Lazzaroni, R.; Lecie`re, Ph.; McDonell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem. Soc. 2002, 124, 1269. (d) Berthier, D.; Buffeteau, T.; Le´ger, J.-M.; Oda, R.; Huc, I. J. Am. Chem. Soc. 2002, 124, 13486. (e) van Gorp, J. J.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 14759. (4) (a) Belogi, G.; Croce, M.; Mancini, G. Langmuir 1997, 13, 2903. (b) Tickle, D.; George, A.; Jennings, K.; Camilleri, P.; Kirby, A. J. J. Chem. Soc., Perkin Trans. 2, 1998, 467. (c) Luchetti, L.; Mancini, G. Langmuir 2000, 16, 161. (d) Acιmιs¸ , M.; Ocak, C¸ .; O ¨ zacar, S¸ .; Go¨c¸ men, K. New J. Chem. 2002, 26, 427.

The manifestation of chirality in these essentially twodimensional (2D) systems seems at first sight a contradiction in terminis. However, in many cases even the simple adsorption of a single molecule on a substrate leads already to the formation of a chiral entity, even if the molecule itself is achiral. Adsorption of a chiral molecule leads always to a chiral entity, and in many cases, when 2D crystals are formed, they appear to be chiral, for instance by the formation of an oblique unit cell. This is often true even for achiral molecules,7 but the 2D crystals of achiral molecules will lead to an equal amount of mirrorimage type domains in the absence of any discriminating influence. In almost all cases reported so far, enantiopure molecules lead to the exclusive formation of one of the possible 2D diastereomeric arrangements while the opposite enantiomer gives the mirror-image monolayer.8,9 In the rare case that enantiomers form the positional (5) (a) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (b) Fuhrhop, J.-H.; Bedurke, T.; Hahn, A.; Grund, S.; Gatzmann, J.; Riederer, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 350. (c) Sommerdijk, N. A. J. M.; Hoeks, T. H. L.; Synak, M.; Feiters, M. C.; Nolte, R. J. M.; Zwaneburg, B. J. Am. Chem. Soc. 1997, 119, 4338. (d) Kawasaki, T.; Tokuhiro, M.; Kimizuka, N.; Kunitake, T. J. Am. Chem. Soc. 2001, 123, 6792. (e) Nakagawa, H.; Kobori, Y.; Yoshida, M.; Yamada, K.-i. Chem. Commun. 2001, 2692. (6) (a) Harris, K. D. M. Chem. Soc. Rev. 1997, 26, 279-289. (b) Koshima, H. J. Mol. Struct. 2000, 552, 111. (c) Tanaka, K.; Fujimoto, D.; Oeser, T.; Irngartinger, H.; Toda, F. Chem. Commun. 2000, 413. (d) Gervais, C.; Beilles, S.; Cardinae¨l, P.; Petit, S.; Coquerel, G. J. Phys. Chem. B 2002, 106, 646. (e) Alshahateet, S. F.; Bishop, R.; Craig, D. C.; Scudder, M. L. Cryst. Eng. Commun. 2002, 8, 1. (7) See, for example: (a) Li, C.; Zeng, Q.; Wu, P.; Xu, S.; Wang, C.; Qiao, Y.; Wan, L.; Bai, C. J. Phys. Chem. B 2002, 106, 13262. (b) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318. (c) France, C. B.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 12712. (8) (a) Stevens, F.; Walba, D. M.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591 and references therein. (b) De Feyter, S.; Grim, P. C. M.; Rucker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. 1998, 37, 1223. (c) De Feyter, S.; Gesquie`re, A.; Grim, P. C. M.; De Schryver, F. C.; Valiyaveettil, S.; Meiners, C.; Sieffert, M.; Mu¨llen, K. Langmuir 1999, 15, 2817. (d) Giancarlo, L. G.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491 and references therein.

10.1021/la048141s CCC: $27.50 © 2004 American Chemical Society Published on Web 09/22/2004

Stereochemistry in Enantiomers and Racemates

Figure 1. Chemical structures of 1 and 2.

mirror-type arrangement, both 2D structures are not truly enantiomorphoussthey are quasi-enantiomorphous10s and in principle are not exactly equivalent in energy. An interesting case is the fate of a solution of a racemate on a substrate. On the basis of symmetry considerations, it should be easier to induce racemic conglomerateseach domain contains only one of the two enantiomerss formation rationally at surfaces than in three-dimensional (3D) systems.11 In addition to the reduced symmetry possibilities in the presence of a substrate, the balance between adsorbate-substrate interactions and adsorbateadsorbate interactions plays a role. The ability to form a racemic conglomerate will depend on the nature of the substrate. For example, watersin which the molecular orientation is invariably perpendicular in Langmuir filmssimposes certain symmetry conditions,12 while graphiteswhere the molecules usually physisorb parallel to the 6-fold symmetry surfacesimposes other more stringent ones.13 Indeed, almost all studies reported so far for self-assembled monolayers on graphitesthe case which is pertinent heresdemonstrate racemic conglomerate formation7,8 while in 3D crystals, for instance, racemic compoundsin which the two enantiomers are present in equal proportions in the unit cellsformation accounts for 90-95% of the cases.14 Gaining insight into factors leading to racemic conglomerate formation or racemic compound/ pseudoracemate15 formation is a main target of the work we report here. Previously, we reported the 2D and 3D ordering of the enantiomers and racemate of a chiral formamide 1 (Figure 1) as well as an achiral analogue at the liquid/solid interface.16 The ordering of the racemate with respect to the symmetry of the graphite substrate was shown to be distinctly different from the 2D crystals formed by the (9) (a) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samorı´, P.; Spada, G. P. Chem. Eur. J. 2000, 6, 3242. (b) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Lett. 1998, 1133. (c) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 7311. (d) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021. (e) Ohtani, B.; Shintani, A.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 6515. (f) Lorenzo, M. O.; Baddelley, C. J.; Muryn, C.; Raval, R. Nature (London) 2000, 404, 376. (g) Gesquie`re, A.; Jonkheijm, P.; Schenning, A. P. H. J.; Mena-Osteritz, E.; Ba¨uerle, P.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. J. Mater. Chem. 2003, 13, 2164. (h) Chen, Q.; Frankel, D. J.; Richardson, N. V. Surf. Sci. 2002, 497, 37. (i) Xu, Q.-M.; Wang, D.; Wan, L.-J.; Wang, C.; Bai, C.-L.; Feng, G.-Q.; Wang, M.-X. Angew. Chem. 2002, 113, 35583561. (j) Fasel, R.; Parschau, M.; Ernst, K.-H. Angew. Chem., Int. Ed. 2003, 42, 5178. (k) Ku¨hnle, A.; Linderoth, T. R.; Besenbacher, F. J. Am. Chem. Soc. 2003, 125, 14680. (10) Parks, D. C.; Clark, N. A.; Walba, D. M.; Beale, P. D. Phys. Rev. Lett. 1993, 70, 607. (11) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (12) Weissbuch, I.; Kuzmenko, I.; Berfeld, M.; Leiserowitz, L.; Lahav, M. J. Phys. Org. Chem. 2000, 13, 426 and references therein. (13) Pe´rez-Garcı´a, L.; Amabilino, D. B. Chem. Soc. Rev. 2002, 31, 342. (14) Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates, and Resolutions; Krieger Publishing Co.: Malabar, FL, 1994. (15) In a pseudoracemate, both enantiomers are present but with no single composition of the unit cell (see ref 14). (16) De Feyter, S.; Gesquie`re, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217.

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pure enantiomers, indicating that both enantiomers were locallyson the scale of hundreds of nm2sadsorbed as mixtures, in contrast to racemic conglomerate formation. This is one of the very rare occasions where, upon deposition on the graphite support, the racemate does not undergo spontaneous resolution into enantiomorphous domains.16 An intriguing question regarding this system is the fate of nonracemic mixtures, i.e., when there is an excess of one of the enantiomers. Will this give rise to more domains composed of the pure enantiomer than expected on statistical grounds alone as a result of a sergeants and soldiers effect, in which a small excess of one enantiomer induces a domain chirality of one-handedness across the whole sample?17 Or would the excess of one of the enantiomers induce separation and thus racemic conglomerate formation? In the first part of this paper, we provide a detailed analysis of the ordering and expression of molecular chirality of 1 at the liquid/solid interface, including the study of non-racemic mixtures. In the second part, we present and discuss the expression of molecular chirality of 2. The latter compound resembles 1, except that the ester function linking the two aromatic rings is absent, a feature which is expected to affect significantly the intermolecular interactions. In contrast to 1, the racemate of 2 leads to racemic conglomerate formation. Finally, we compare the monolayer properties of both compounds and discuss the origin of the racemic compound/pseudoracemate vis-a`-vis racemic conglomerate formation in these and related compounds. Experimental Section The compounds were prepared according to published procedures.18 Scanning tunneling microscopy (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. Prior to imaging, all compounds under investigation were dissolved in 1-phenyloctane or 1-heptanol (Aldrich, 99%) at a concentration of approximately 1 mM, and a drop of the solution was applied onto a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, grade ZYB, Advanced Ceramics Inc., Cleveland, OH). To get the monolayers to form, a high concentration of the molecule is necessary, typically 1 mM. In the original report on this compound16 we used 1-heptanol as the solvent, though similar results were obtained in 1-phenyloctane (data not shown). Then, the STM tip was immersed in the solution, and images were recorded at the liquid/solid interface. The STM images were acquired in the variable-current mode (constant height) under ambient conditions. The measured tunneling currents are converted into a gray scale: black (white) refers to a low (high) measured tunneling current. After the successful (17) (a) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.; Lifson, S. Science 1995, 268, 1860 and references therein. (b) Okamoto, N.; Mukaida, F.; Gu, H.; Nakamura, Y.; Sato, T.; Teramoto, A.; Green, M. M.; Andreola, C.; Peterson, N. C.; Lifson, S. Macromolecules 1996, 29, 2878. (c) Selinger, J. V.; Selinger, R. L. B. Phys. Rev. Lett. 1996, 76, 58. (d) Mu¨ller, M.; Zentel, R. Macromolecules 1996, 29, 1609. (e) Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119, 11345. (f) Stegemeyer, H.; Mainush, K. J. Naturwissenschaften 1971, 58, 599. (g) Saeva, F. D.; Sharpe, P. E.; Olin, G. R. J. Am. Chem. Soc. 1973, 95, 7656. (h) Gottarelli, G.; Samori, B.; Marzocchi, S. Tetrahedron Lett. 1975, 1981. (i) Yamada, K.; Takanishi, Y.; Ishikawa, K.; Takezoe, H.; Fukuda, A.; Osipov, M. A. Phys. Rev. E 1997, 56, R43-R46. (j) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W. Angew. Chem. 1997, 109, 2763; Angew. Chem., Int. Ed. Engl. 1997, 36, 2648. (18) (a) Amabilino, D. B.; Ramos, E.; Serrano, J. L.; Sierra, T.; Veciana, J. J. Am. Chem. Soc. 1998, 120, 9126. (b) Amabilino, D. B.; Ramos, E.; Serrano, J. L.; Sierra, T.; Veciana, J., submitted for publication.

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recording of an image of the monolayer and an atomically resolved image of the graphite substrate at exactly the same location, recorded with identical scanning parameters, except for the sample bias which was lowered in order to visualize the graphite substrate, the sample was translated in order to find a domain that had another orientation guaranteeing that a different domain was probed. Each image was corrected for eventual scanner drift by using SPIP software with graphite as the calibration grid. A graphite image of the same area was recorded immediately after recording a monolayer image, using identical scan parameters, except for lowering the sample bias which allows visualizing the graphite substrate underneath the monolayer.

Results and Discussion Some representative scanning tunneling microscopy (STM) images of monolayers formed from enantiopure solutions of (R)-1 and the racemate (rac)-1 are shown in Figure 2. In the STM images the phenyl benzoate moieties of 1 show up as the bright structures corresponding to high tunneling current through the aromatic groups. The alkyl chains are located in between the rows of the phenyl benzoate groups, and sometimes their orientation can be discerned. In the STM images, the location and orientation of a number of phenyl benzoate groups are indicated by sticks. The molecules form chains which in turn form dimers. The X-ray data of their 3D crystals indicate hydrogen-bonding interactions along the chains and between them through the formamide groups.16 The molecular chirality of the pure enantiomers of compound 1 is expressed at the liquid/solid interface at two different levels: (1) at the level of the monolayer structure as expressed by the orientation of the phenyl benzoate moieties (bars in Figure 2) with respect to the tape normal ((R1) in Figure 3) and (2) at the level of the orientation of the adlayer with respect to the underlying graphite lattice as expressed by the direction of the tape axis of the monolayer with respect to the symmetry axes (red lines in Figure 2) of graphite ((θ1) in Figure 3). Note that the latter correlation can be made because each time the graphite lattice is recorded after imaging a monolayer under identical experimental conditions (location, scan size, scan speed, etc.), except for lowering the bias voltage which allows imaging of the graphite surface underneath the monolayer. Figure 3 summarizes possible diastereomeric outcomes which result from chirality at the two levels discussed above. The s and z prefixes refer to the shape of the phenyl benzoate dimer on graphite. The M(inus) and P(lus) indicators are analogous to the helical descriptors conventionally used in organic stereochemistry19 and reflect the sign of θ1 and R1 according to the color code in Figure 3. Note that the value of these angles can also be zero (see below). For instance, in all images which allow identification of the orientation of the phenyl benzoate unit with respect to the tape axis (Figure 2), the rotation direction is determined by the chirality of the compound: for (R)-1 (Figure 2A,B), the phenyl benzoate dimers are rotated anticlockwise to the tape normal (R1 < 0) while R1 > 0 for (S)-1 (not shown). At this level, the racemate was found to reflect characteristic features of both enantiomers; within the same domain, tapes of clockwise and anticlockwise rotated phenylbenzoate moieties coexist randomly (R1 > 0 and R1 < 0) (Figure 2C), characteristic of a pseudoracemate. In general, it is not at all straightforward to identify the orientation of the phenylbenzoate units in the STM (19) (a) Meurer, K. P.; Vo¨gtle, F. Top. Curr. Chem. 1985, 127, 1. (b) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley: New York, 1994.

Figure 2. STM image of (A) (R)-1 (12.5 × 12.5 nm2, 1.0 nA, -0.44 V, (B) (R)-1 (11.0 × 11.0 nm2, 1.0 nA, -0.52 V), and (C) (rac)-1 (13.6 × 13.6 nm2, 1.0 nA, -0.54 V) at the 1-heptanol/ graphite interface. Although A and B are both images formed by the same enantiomer, the monolayers differ in their registry with the graphite substrate. The normal to a graphite axis is indicated in red, and the images in A and B show clearly that the angle between the tape and lattice axis does not occur with a single value. The orientation of some of the phenyl benzoate groups is indicated by a white bar. Note that the orientation of the graphite substrate with respect to the monolayer could be identified by recording a “graphite” image immediately after registration of the monolayer image.

images because of variations in contrast. Thus, a statistical analysis is not feasible. Therefore, the orientation of the adlayer with respect to the symmetry axes of the substrate underneath (angle θ1) has been used to determine their chirality. The analysis principle used for evaluation of the orientation of the monolayers with respect to the substrate is illustrated in Figure 3. Figure 3A is an STM image of graphite, in which only half of the carbon atoms of the surface are “seen”.20 The “honeycomb” background (20) Atamny, F.; Spillecke, O.; Scho¨gl, R. Phys. Chem. Chem. Phys. 1999, 1, 4113.

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Figure 3. (A) STM image of graphite. The main symmetry axes are indicated in yellow. The normal on the horizontal main symmetry axis is indicated in red. (B) Schematic representations of the orientation of “dimers” of 1 on graphite. The black lines in bold are the alkyl chains. The patterned rectangles reflect the phenyl benzoate moieties. The “normal” is indicated in red. Of the three possible “normals” related to the symmetry of graphite, that “normal” is selected as reference axis which makes the smallest angle with the tape axis. Experimentally, its orientation can be determined by recording an STM image of graphite as mentioned in the text. The dashed green line represents a lamella axis. θ1: direction of the tape axis with respect to the selected symmetry axes of graphite (the “normal”). R1: orientation of the phenyl benzoate moieties (dashed purple line) with respect to the tape normal (purple line). So the coordinate system for both angles is different. s and z prefixes refer to the shape of the phenyl benzoate dimer (these descriptors can actually be deduced from the others, but are included for clarity). M(inus) and P(lus) reflect the sign of θ1 and R1 according to the color code used. In case the value of an angle is zero, M or P is replaced by 0. So, for (B): (top left) θ1 > 0, R1 < 0; (top right) θ1 < 0, R1 > 0; (bottom left) θ1 > 0, R1 > 0; (bottom right) θ1 < 0, R1 < 0.

pattern in Figure 3B represents the upper layer of the graphite substrate (the images are not to scale). The main symmetry axes of the graphite surface are indicated in yellow in the STM image. The red line indicates the normal to the horizontal main symmetry axis (Figure 3A). This red line will be referred to as “the normal” further on in

the text. In general, to evaluate the orientation of the monolayer with respect to the surface, we evaluate the smallest angle (θ) between a graphite reference axis (a main graphite axis or normal axis) and the long tape axis. In the case of compound 1, the orientation of the long tape axis is evaluated with respect to the normal. A domain is

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Figure 4. Histograms of the angle θ1 observed for physisorbed monolayers formed of solutions of 1 composed of (A) 100R/0S, (B) 0R/100S, (C) rac., (D) 50R/50S, (E) 90R/10S, (F) 70R/30S, and (G) 40R/60S at the 1-heptanol/graphite interface.

assigned as positive if θ1 > 0 (clockwise rotation with respect to graphite reference axis) and vice versa for a counterclockwise rotation. If the tape axis runs parallel to the reference axis, θ1 ) 0. The histograms in Figure 4A-C, which represent the number of domains of which the tape axis is rotated a given angle with respect to the graphite’s reference axis, reflect this second level of expression of chirality for (R)1, (S)-1, and (rac)-1 mixture, respectively. From the histograms it is clear that in none of the three cases is the orientation of the tape axis with respect to the substrate’s symmetry completely random, although there is a considerable spread. (R)-1 has a strong tendency to form sPM domains (θ1 > 0) (such a domain is shown in Figure 2A) while (S)-1 forms zMP type domains (θ1 < 0). In addition, both enantiomers form a substantial fraction of domains for which the angle θ1 is close to zero (such a domain is shown for (R)-1 in Figure 2B): s0M type domains for (R)-1 and z0P type domains for (S)-1. In contrast to the enantiopure forms, the racemate forms exclusively domains with the angle θ1 close to zero. So, the patterns formed by the racemate are not a mere reflection of the adsorbate layers formed by the pure enantiomers. Therefore, no racemic conglomerate formation takes place.

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Both enantiomers are incorporated randomly into the same domain, with tapes which appear to be rich in one enantiomer since tapes with an angle corresponding to the (R)- or (S)-enantiomers are seen (Figure 2C, in which tapes with positive and negative values of R1 are imaged with θ1 ) 0). A small deviation from strictly equimolar amounts of enantiomers could in principle have important effects on the chirality of the domains. Therefore, to investigate how the stereochemistry of the monolayers formed was affected, we mixed varying amounts of (R)-1 and (S)-1. The results for different compositions are presented in the histograms in Figure 4D-G for 50(R)/50(S), 90(R)/10(S), 70(R)/30(S), and 40(R)/60(S), respectively. The 50(R)/50(S) sample gives the same results as a synthetically prepared racemate, as it should. The absence of any effect of the history of preparation is because the samples are molecularly dissolved in 1-heptanol prior to deposition on the graphite. A quantitative analysis of the other histograms in Figure 4 is complicated by the fact that the distribution of θ1 values for the pure enantiomers shows a nonnegligible contribution (∼30%) at angles close to zero. For all nonequimolar conditions, pure enantiomer-type domains are formed in addition to pseudoracemate domains. However, no exact estimates of the relative ratio of pseudoracemate vs enantiopure domains can be provided. Nevertheless, there is a slight but clear tendency that the ratio of “enantiopure” vs pseudoracemate domains increases upon increasing the ratio between the different enantiomers. As expected, the ratio of (R)-domains vs (S)-domains follows qualitatively the differences in composition of the solution. The data do not indicate that the formation of the pseudoracemate domains would be hampered by a nonstoichiometric composition of the solution. An excess of one enantiomer will not drive the system to form exclusively enantiopure domains, so there is no classical “sergeants and soldiers” effect here.17 On the contrary, if anything, the pseudoracemate domains are more favored given the chance. To illustrate this point, the 90:10 mixture shows only one-fourth of its domains with an angle >5°, compared with around 70% in the enantiopure cases. Such a spread of the θ1 values for the pure enantiomers is not to be expected for alkylated compounds (they normally align their alkyl chains parallel to a main graphite symmetry axis, often leading to a single peaked histogram), implying either that 1 forms polymorphous monolayer structures (which requires different packing motifs) or the monolayer has more than one orientation with respect to a graphite symmetry axis. Within the accuracy of the analysis, unit cells parameters are independent of the θ1 values; however, polymorphism cannot be ruled out. Figure 5 shows domain boundaries for monolayers of (R)-1 with the symmetry axes of graphite superimposed. For the upper right domain in Figure 5A, θ1 ) 0 while for the lower left domain, θ1 ) +10°. The alkyl chains in the images appear to run parallel to one anothers their orientation can only be determined with an accuracy of ∼5°salthough there are variations in contrast as the tape axis direction changes. The angle between the rigid phenyl benzoate and the flexible 2-octyloxy chain can change, as is evident in the X-ray structure of the enantiopure compound.16 In this case, the balance between the phenyl benzoate interaction with the graphite substrate and that of the alkyl chains with graphite may change, leading to a change in angles and in contrast because of the changed conformation of the molecules. In Figure 5B, a similar observation is made. Given that the orientation of the long tape axes changes while the alkyl

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Figure 5. STM images of (R)-1 at the 1-heptanol/graphite interface: (A) (16.3 × 16.3 nm2, 1.0 nA, -0.45 V) and (B) (13.8 × 13.8 nm2, 1.0 nA, -0.48 V). The main graphite axes (yellow) and a normal (red) are superimposed on top of the STM image.

Figure 6. STM image of (A) (R)-2 (9 × 9 nm2, 0.8 nA, -0.344 V) and (B) (S)-2 (9 × 9 nm2, 0.85 nA, -0.366V) at the 1-phenyloctane/graphite interface. The unit cell is indicated in yellow.

chains remain almost parallel to a main graphite symmetry axis indicates polymorphism. To investigate factors that lead to racemic compound/ pseudoracemate or racemic conglomerate formation, the 2D molecular packing of 2 was investigated. 2 lacks the ester group which is anticipated to have an important effect on the intermolecular interactions and, consequently, monolayer formation. The data presented here were obtained at the 1-phenyloctane/graphite interface though similar results were obtained at the 1-heptanol/ graphite interface. Figures 6 and 7 represent highresolution images of physisorbed monolayers of (R)-2, (S)2, and (rac)-2 at the liquid/solid interface. The images are not determined by a characteristic contrast although regular dark and light patterns are formed. The brightest parts in the images can be assigned to the aromatic biphenyl groups.21 The molecules appear in parallel rows with an alternating light and dark contrast. The distance b between two adjacent points with identical contrast along the long axis of a row measures about 1.1 nm. The distance between two rows measured along the unit cell vector a is 2.2 nm. The angle γ formed by the unit cell vectors is about 78°. The averaged cell parameters for the monolayers formed by the enantiopure compounds and the racemate are identical within experimental error. On the basis of these unit cell parameters and the image contrast, no distinction can be made between monolayer structures formed by the enantiomers and those of the racemate, suggesting that racemic conglomerate formation takes place. The unit cell vector along a row (1.1 nm) is significantly larger than found for compound 1 (0.86 nm). The width of the rows indicates

that the alkyl chains are almost in line with the biphenyl axes and are fully extended. In some images such as in Figure 7A, the specific quasi-circular shape of individual phenyl rings was observed, although in condensed phase the phenyl rings of the biphenyl unit are usually tilted with respect to each other.22 In contrast to 1, we could not get compound 2 to form single crystals, but on the basis of the STM images, we have constructed a packing model (see Figure 7B). According to the analysis, 2 packs in head-to-head dimers which come together in a tail-to-tail fashion. The headto-head interactions are presumably favored through hydrogen bonds between the formyl moieties in the formamide groups, which are the only strong hydrogen bond forming moieties in the molecule. This interaction is seen in the 3D crystal structure of 1 and has other precedents.23 Note that for 1 also the ester function between the two phenyl rings was involved in the hydrogen bonding. This interaction is not possible in the case of 2, and presumably in part for this reason the molecules appear as “isolated” dimers. At the level of the monolayer structure, molecular chirality is hardly expressed, in contrast to compound 1, except for the oblique unit cells which are 2D chiral. For both enantiomers the alkyl chain follows the axis of the rigid part of the molecule, so there is no way to distinguish between the two.

(21) Lazzaroni, R.; Calderone, A.; Lambin, G.; Rabe, J. P.; Bre´das, J. L. Synth. Met. 1991, 41, 525.

(22) See, for example: (a) Hori, K.; Kurosaki, M.; Wu, H.; Itoh, K. Acta Crystallogr. C 1996, 52, 1751. (b) Kuribayashi, M.; Hori, K. Liq. Cryst. 1999, 26, 809. (c) Seo, N.; Hori, K. Liq. Cryst. 2001, 28, 77. (d) Na¨ther, C.; Jeβ, I.; Havlas, Z.; Bolte, M.; Nagel, N.; Nick, S. Solid State Sci. 2002, 4, 859. (e) Grein, F. J. Mol. Struct. (THEOCHEM) 2003, 624, 23. (f) Suzuki, T.; Saito, M.; Kawai, H.; Fujiwara, K.; Tsuji, T. Tetrahedron Lett. 2004, 45, 329.

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Figure 8. Histograms of the angle θ2 observed for physisorbed monolayers formed of 2: (A) 100R/0S, (B) 0R/100S, and (C) rac.

Taking into account the orientation of the monolayer with respect to the graphite symmetry, however, clear indications of chirality were found. The propagation direction of 2 along the unit cell vector b (dashed line in Figure 7c) is now evaluated with respect to a main symmetry axis. The histograms in Figure 8 reflect the spread and frequency of the angle θ2 for (R)-2, (S)-2, and the (rac)-2 mixture. The total number of observations equals the number of different domains probed. The data show that there is a spread in the angle θ2. For the (R)-enantiomer as well as for the (S)-enantiomer there is a bimodal distribution in the absolute values of the angle θ2, which peaks at ∼9° and ∼24°. For (R)-2, this angle is always positive, while for (S)-2, θ2 is negative. The racemate (rac)-2 gave equal contributions of positive as well as negative angles, similar to those absolute values found for the pure

enantiomers. This analysis supports the conclusions drawn based upon the image contrast and the unit cells and strongly suggests that the biphenyl formamides undergo spontaneous resolution at the liquid/solid interface, in contrast to 1. On the other hand, and in common with 1, there is a spread in θ2 which is related to the relative orientation of the alkyl chains and biphenyl groups on graphite. In those monolayers where θ2 is ∼24°, the alkyl chains run parallel with a main graphite axis and the molecular ordering is alkyl chain driven. If θ2 equals ∼9°, the alkyl chains are not parallel to a main graphite axis. The question arises why in the case of 1 the racemate shows pseudoracemate formationsthe histogram in Figure 4C shows a monomodal distribution peaked at θ1 ) 0swhile in the case of 2 a racemic conglomerate is formeds the histogram in Figure 8c reflects the contribution of both enantiomers. Clearly, both compounds show a spread in their orientation with respect to the symmetry axes of the graphite substrate. The type of spread seen in the orientation of the domains of these compounds has been seen before in other compounds in which the 2-octyl chain was incorporated into the adsorbate.24 It seems, then, that the flexibility of this group in terms of its indifference to orientation to the graphite is partially responsible for the racemic conglomerate formation by 1. On the other hand, and in partial contradiction, the first case of spontaneous resolution to be observed by STM was in a molecule incorporating this group.13 The shape and interactions that take place between the cores of the molecules are thus determining in the structural and stereochemical fate of the system. Here we have the unique case where very similar compounds, carrying an identical stereogenic center, show a pronounced difference both in their monolayer structures and in their expression of molecular chirality, at the level of the pure enantiomers and racemates. At the level of the monolayer structure, the phenyl benzoates form infinite tapes of stacked and hydrogenbonded dimers. Indeed, the 3D crystal structure of both the racemic compound and enantiopure crystals of 1 revealed that the molecules form chains in which the molecules are linked through strong N-H‚‚‚OdC hydro-

(23) Ribeiro-Claro, P. J. A.; Drew, M. G. B.; Fe´lix, V. Chem. Phys. Lett. 2002, 356, 318.

(24) De Feyter, S.; Gesquie`re, A.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Mu¨llen, K. Langmuir 2000, 16, 9887.

Figure 7. (A) STM image of (rac)-2 at the 1-phenyloctane/ graphite interface (7 × 7 nm2, 0.6 nA, -0.556 V). (B) A molecular model of a (S)-2 type domain. (C) Schematic representation of the orientation of “dimers” of 2 on graphite, in the case of a (R)-enantiomer. The graphite symmetry axis is indicated in yellow. The dashed black line represents a lamella axis.

Stereochemistry in Enantiomers and Racemates

gen bonds as well as other weaker hydrogen bonds. These chains unite through head-to-head C-H‚‚‚OdC hydrogen bonds between formyl groups. These interactions combine to generate supramolecular tapes. 2 forms isolated dimers in a checkerboard-type pattern. Because of the absence of the ester group in 2, this compound lacks obviously a directional interaction between adjacent dimers, giving rise, as a result of a subtle interplay between adsorbateadsorbate and adsorbate-substrate interactions, to a checkerboard-type pattern. It is noteworthy that in 2 the 2-octyloxy chain follows the axis of the rigid part of the molecule, while in 1 it is bent. Yet both compounds form chiral polymorphous structures. The stereochemical fate of the systems is strongly influenced by the flexibility in the orientation of the 2-octyl group with respect to the graphite lattice and leads to a range of θ values. Though both compounds show polymorphism, each of the different polymorphs is homochiral: for a given phase, an enantiomer forms only one of the two possible mirror-images like structures. As mentioned above, the specific values and spread of the θ values are a result of subtle balance between adsorbateadsorbate and adsorbate-substrate interactions. However, the major difference between both compounds is that in the case of 1 both enantiomers have a phase with θ ) 0, a behavior not observed for 2. A possible reason for the monomodal distribution peaked at θ1 ) 0 observed for the racemate of 1 is that the ordering of the molecules in that case is alkyl chain driven. In other words, as both enantiomers show a common θ1 ) 0 type ordering, bringing both enantiomers together in one domain (with θ1 ) 0) should be energetically allowed and entropically favored. In absence of specific favorable interactions between both enantiomers of compound 2, forming a racemic compound will not be promoted. In the case of 1, the exclusive formation of θ1 ) 0 domains for the racemate and the histogram for nonracemic mixtures actually suggest that there is a preferred interaction between both enantiomers, as opposed to the formation of homochiral domains. The detailed origin of this effect is still under investigation, but an analogous behavior has been observed in 3D.16 Summary Both chiral compounds 1 and 2 form ordered polymorphous monolayer structures at the liquid-solid interface. Intermolecular hydrogen bonding plays an important role

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in the ordering of the adsorbates on graphite. While the phenyl benzoates form infinite tapes of stacked and hydrogen-bonded dimers, the biphenyl formamides form isolated dimers in a checkerboard-type pattern. Their molecular chirality is expressed at the level of the monolayer structure and/or their orientation with respect to the graphite lattice. The histograms of θ show a multimodal distribution for the pure enantiomers. As expected, the enantiomers differ from each other only in the sign of θ. Both compounds show a distinct difference in the behavior of their racemates. In the case of 1, the histogram peaked at θ1 equals zero, suggesting a pseudoracemate formation. The histogram of the racemate of 2 reflected the equal contribution of the respective enantiomers, indicating racemic conglomerate formation. In the case of 1, both enantiomers showed a common value for θ1 (θ1 ) 0), while such behavior was not observed for 2. This behavior is believed to be crucial for the formation of racemic compound/pseudoracemate or racemic conglomerate formation. No sergeant and soldiers effect was observed for non-racemic mixtures of 1. The spontaneous resolution of compounds in 3D is more often than not unpredictable. While there are groups of compounds which sometimes show spontaneous resolution, it is very difficult to rationalize why some compounds show the phenomenon and others do not. In 2D, we have shown that it may be possible to have a deeper understanding in the factors leading to racemic conglomerate or racemic compound formation through the insight given by the scanning tunneling microscope: the importance of molecular structure and its influence on balancing the interplay between molecular conformation and molecular packing on surface have been demonstrated. To our knowledge, this is the first time that, for structurally related compounds, a comparison has been presented between pseudoracemate and racemic conglomerate forming racemates. Acknowledgment. The authors thank the Federal Science Policy, through IUAP-V-03, the Fund for Scientific Research-Flanders (FWO), the DGI, Spain (Project No. BQU 2003-00760), DGR, Catalonia (Project 2001SGR00362), and COST (working group D19/004/01). W.M. and H.U. are grateful to KULeuven. H.U. is financed via an Interdisciplinary Research Program. S.D.F. is a Postdoctoral Fellow of FWO. LA048141S