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Driving Forces Underlying the Formation of Chiral Domains of Fluorinated Diacids on Graphite S. N. Patole, C. J. Baddeley,* M. Schu¨ler, D. O’Hagan, and N. V. Richardson EaStCHEM School of Chemistry, UniVersity of St. Andrews, St. Andrews, Fife KY16 9ST, United Kingdom ReceiVed July 8, 2008. ReVised Manuscript ReceiVed NoVember 17, 2008 The role of intermolecular interactions, molecule-substrate interactions, and molecular chirality in the construction of 2-D surface architectures is the subject of much current interest. A racemic mixture of long chain hydrocarbons was synthesized with terminal carboxylic acid functionalities at each end and two amide linkages in the central region of the molecule on either side of two F-containing chiral centers. Using scanning tunnelling microscopy (STM), we have examined how the functionality of these molecules influences their self-assembly on a highly oriented pyrolytic graphite (HOPG) surface. The key factors determining the nature of ordered domains have been identified.
Introduction Establishing control over the formation of 2-D supramolecular networks1 at surfaces via H-bonding,2,3 ionic,4,5 or covalent interactions6-8 is a rapidly expanding area of research. An understanding of such assembly processes is important in applications such as the design of molecular electronic devices, (bio)molecular recognition, and corrosion inhibition.9 While a significant number of these investigations have been carried out under ultrahigh vacuum (UHV) conditions, there are many advantages to preparing 2-D arrays at the liquid-solid interface. Many molecular species are incompatible with UHV deposition for reasons of, for example, thermal instability. In addition, at the liquid-solid interface, dynamic exchange of adsorbed molecules with liquid-phase species can repair adlayer defects, and the choice of solvent can be tuned to optimize adlayer ordering.10 The self-assembly of molecular species at highly oriented pyrolytic graphite (HOPG) surfaces has been the subject of much recent attention.9–19 Long chain hydrocarbons have long been * To whom correspondence should be addressed. E-mail: cjb14@ st-and.ac.uk. (1) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (2) Swarbrick, J. C.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 6110. (3) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (4) Clair, S.; Pons, S.; Fabris, S.; Baroni, S.; Brune, H.; Kern, K.; Barth, J. V. J. Phys. Chem. B 2006, 110, 5627. (5) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000. (6) Weigelt, S.; Busse, C.; Bombis, C.; Knudsen, M. M.; Gothelf, K. V.; Strunskus, T.; Woll, C.; Dahlbom, M.; Hammer, B.; Laegsgaard, E.; Besenbacher, F.; Linderoth, T. R. Angew. Chem., Int. Ed. 2007, 46, 9227. (7) Matena, M.; Riehm, T.; Stohr, M.; Jung, T. A.; Gade, L. H. Angew. Chem., Int. Ed. 2008, 47, 2414. (8) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687. (9) Tao, F.; Cai, Y. G.; Bernasek, S. L. Langmuir 2005, 21, 1269. (10) De Feyter, S.; De Schryver, F. C. Angew. Chem., Int. Ed. 2005, 109, 4290. (11) De Feyter, S.; Gesquiere, A.; De Schryver, F.; Meiners, C.; Sieffert, M.; Mullen, K. Langmuir 2000, 16, 9887. (12) Lei, S. B.; Wang, C.; Yin, S. X.; Xu, Q. M.; Bai, C. L. Surf. Interface Anal. 2001, 32, 253. (13) Liu, D. J.; De Feyter, S.; Grim, P. C. M.; Vosch, T.; Grebel-Koehler, D.; Wiesler, U. M.; Berresheim, A. J.; Mullen, K.; De Schryver, F. C. Langmuir 2002, 18, 8223. (14) Takajo, D.; Nemoto, T.; Isoda, S. Jpn. J. Appl. Phys., Part 1 2004, 43, 4667. (15) 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.
exploited as model systems in understanding assembly processes on graphite. Such molecules display a very high affinity for graphite9 and have been shown to adsorb on graphite with their carbon skeletons parallel to and aligned along high symmetry directions of the graphite surface. Long chain hydrocarbons were designed and synthesized with terminal carboxylic acid functionalities at each end and two amide linkages in the central region of the molecule. In between the two amide linkages are two chiral centers each containing a single C-F bond (Figure 1). There are a number of motivations for the present study. First, as will be explained below, the liquid phase structure of the central portion of the molecule was found to adopt a geometry which, if adopted on adsorption, would prevent the molecule from lying flat. We aim to understand the nature of any relaxation in this liquid-phase conformation upon adsorption. Second, we are interested in establishing control over 2-D assembly via intermolecular H-bonding interactions. Third, the molecules were synthesized as a racemic mixture of (R,R) and (S,S) species, so we are able to probe the influence of chirality on the ordered adlayers produced on graphite. Lastly, we are interested in the possibility to use F atoms as contrast agents in scanning tunneling microscopy (STM). Stabel et al. studied F-substituted stearic acid on graphite and identified F atoms as dark features in the images.20 Similarly, De Feyter and co-workers have observed a characteristic dark contrast for perfluorinated segments of alkylated isophthalic acids on graphite.21 The use of F as a contrast agent can greatly help in elucidating structural aspects of complex molecule adsorption at surfaces.
Experimental Section Synthesis of (R,R)- and (S,S)-Difluorodiacids. Methyl 11-aminoundecanoate hydrochloride (0.18 g, 0.72 mmol) was added to a solution of threo-2,3-difluorosuccinic acid22 (0.5 g, 0.3 mmol) in dimethylformamide (DMF) (5 cm3), which had been neutralized (16) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 7311. (17) Yablon, D. G.; Guo, J. S.; Knapp, D.; Fang, H. B.; Flynn, G. W. J. Phys. Chem. B 2001, 105, 4313. (18) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 173. (19) Tao, F.; Goswami, J.; Bernasek, S. L. J. Phys. Chem. B 2006, 110, 4199. (20) Stabel, A.; Dasaradhi, L.; Ohagan, D.; Rabe, J. P. Langmuir 1995, 11, 1427. (21) Abdel-Mottaleb, M. M. S.; De Feyter, S.; Sieffert, M.; Klapper, M.; Mullen, K.; De Schryver, F. C. Langmuir 2003, 19, 8256. (22) O’Hagan, D.; Rzepa, H. S.; Schuler, M.; Slawin, A. M. Z. Beilstein J. Org. Chem. 2006, 2.
10.1021/la802170f CCC: $40.75 2009 American Chemical Society Published on Web 12/17/2008
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Figure 1. Molecular structures of the (R,R)- and (S,S)-difluorodiacid compounds. The molecular length is ∼3.6 nm if the molecule adopts the planar geometry as shown.
prior to addition with N-methylmorpholine (0.15 cm3, 0.5 mmol). Hydroxybenzotriazole (HOBt) (0.9 g, 0.72 mmol) was added as a solution of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (0.19 g, 1.0 mmol) in CHCl3 (2 cm3) at 0 °C. The mixture was stirred at room temperature for 24 h, H2O (10 cm3) was added, and then the mixture was extracted into ethyl acetate (3 × 10 cm3). The combined extracts were washed with 1 N HCl (2 × 10 cm3) and then saturated NaHCO3 (2 × 10 cm3) and brine (10 cm3). The solvent was dried over MgSO4 and removed by evaporation. Purification over silica gel gave the dimethyl ester as a colorless gum (0.42 g, 90%) (found: M + Na+, 517.3525. C28H50N2O6F2Na requires 571.3535, -1.7 ppm). A solution of the dimethylester (0.4 g, 0.73 mmol) in acetone (5 cm3) and 1 N HCI (5 cm3) was heated under reflux for 48 h. The solvent was removed by evaporation, and recrystallization from hot acetone gave the racemic dicarboxylic acid 1 as white powder (0.35 g, 92%), mp 145-146 °C (found: C, 59.69; H, 9.14; N, 5.29. C26H46O6N2F2 requires C, 59.98; H, 8.90; 5.38 N%). νmax/cm-1 (KBr): 3318, 2918, 2850, 1698, 1665, 1557, 1470, 1436, 1289, 1119, 1946, 925, 823, 722, and 604. δH (DMSO): 8.4 (2H, m, NH), 5.3 (2H, AA′XX′, J ) 45.0 Hz, J ) -12.1 Hz, J ) 2.2 Hz, 2 × CHF), 3.1 (4H, m, NH-CH2), 2.2 (t, J ) 7.6 Hz, CH2COOH), 1.4 and 1.2 (32 H, m, 16 × CH2). δC (DMSO): 174.9 (s, COOH), 165.3 (m, CONH), 89.6 (dd, J ) 195.9 Hz, J ) 20.5 Hz, 2 × CHF), 38.8 (NHCH2), 34.0, 29.3, 29.3, 29.3, 29.1, 29.1, 28.9, 26.6, and 24.9 (s, CH2). δF (DMSO): -206.6 (2 F, AA′XX′, CHF). Monolayer Deposition. The HOPG sample was freshly cleaved and exposed to a saturated solution of the racemic mixture of (R,R)and (S,S)-difluorodiacid in phenyloctane under ambient conditions. STM Measurements. All STM measurements were carried out in air at room temperature using a Molecular Imaging Picoscan STM instrument. In all cases, tips were prepared mechanically by cutting a 0.25 mm Pt/Ir alloy (8:2, Goodfellow) wire. The data were collected in constant current mode using tunneling currents of 0.1 nA and a sample bias of typically +0.4 V.
Results Figure 2a shows a large scale image following the adsorption of a racemic mixture of (R,R)- and (S,S)-difluorodiacid onto HOPG from phenyloctane solution. Large rectangular domains of molecular features are observed. Two types of domain exist rotated by ∼8° with respect to each other. As expected by the symmetry of the graphite substrate, three rotationally equivalent domains of each type are observed. In Figure 2b, the domains are manifested by parallel bright lines separated by ∼5.5 nm. Equidistant from two parallel bright lines appears an additional parallel line exhibiting a less intense contrast. In some domains, lines can be observed to run perpendicular to the bright lines. In other domains, similar lines are observed to zigzag between the parallel bright lines. The separation of these features is ∼3.5 nm. In Figure 3a, similar parallel features are observed. Despite the tunneling conditions being identical to those of Figure 2, the features are now observed as parallel dark lines with a slightly less dark line splitting two intense features. We ascribe this change
Figure 2. STM images of the racemic mixture of (R,R)- and (S,S)-diacid adsorbed on HOPG from phenyl octane solution at 300 K. Tunneling parameters: +0.4 V and 0.1 nA. (a) (230 nm × 230 nm) Labeled are two domains rotated by 8° with respect to each other. (b) (110 nm × 110 nm).
to tip modification, most probably by pickup of a molecule onto the tip. It is now possible to resolve molecular features of length 33 ( 3 Å (consistent with the expected dimensions of the molecule in its linear form). Figure 3b shows a higher resolution image
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Figure 4. Schematic diagram of the central portion of a (S,S)-difluoro compound showing the anti-coplanar relationship between C-F bonds and the adjacent carbonyl functionality and the gauche relationship between the C-F bonds on adjacent carbon atoms.23
Figure 5. Assuming that the C-F bonds at the two chiral centers point away from the surface, there exists the possibility for (R,R) pairs or (S,S) pairs to form two intermolecular H-bonds. This is not the case for a (R,R)-(S,S) pair.
Figure 3. STM image (a) (47 nm × 47 nm) and (b) (18 nm × 18 nm) of the racemic mixture of (R,R)- and (S,S)-diacid adsorbed on HOPG from phenyl octane solution at 300 K. Tunneling parameters: +0.4 V and 0.1 nA. (c) Tip height profile taken from line shown in Figure 3b.
of the same domain. It is clear that the molecular features zigzag within the domains. Two types of junctions occur between molecular features: a “straight on” junction where two molecules are parallel to each other at the head-to-head junction and a “kinked” junction where two molecules are rotated by 120° with respect to each other at the head-to-head junction. The line profile presented in Figure 3c shows that the straight-on junctions are manifested by relatively large variations in apparent tip height (60-100 pm) compared to the kinked junctions (20-40 pm).
Discussion The central section of the molecule is based on bis(amino acid amides) of 2,3-difluorosuccinic acid. Relevant information on the preferred geometry of this central section is provided by the solution and crystal structures of related molecules described by
O’Hagan and co-workers.22,23 In these crystal structures, a preference was found for the C-F bond to be syn-coplanar with the N-H bond and anti-coplanar to the amide carbonyl. In addition, the vicinal fluorine atoms prefer a gauche orientation. This molecular structure is shown schematically in Figure 4. A barrier to the rotation of the C-CO bonds of this moiety of around 33 kJmol-1 was reported.24 If a similar conformation of the central section of the molecule existed in the adsorbed layer, it would be impossible for the molecular units to lie flat on the HOPG surface. The dimensions of the long, straight features observed in STM suggest that, in binding to the HOPG surface, the central section no longer adopts its preferred geometry and that the molecule is essentially adsorbed parallel to the surface. A number of driving forces may combine to define the growth directions and periodicity of the adsorbed arrays. First, it has been shown that long alkyl chains tend to align along the closepacked directions of the graphite surface.10 Second, there exists the opportunity for intermolecular H-bonding between the -CO · · · NH- units on adjacent molecules and between the terminal -COOH functional groups. Assuming that the C-F bonds at the chiral centers prefer to point away from the surface, the (R,R) and (S,S) species would adopt geometries with respect to the surface as shown in Figure 5. The assumption regarding the orientation of the C-F bonds is supported by the conclusions of Yablon et al., who found that Br atoms in 2-bromohexadecanoic acid pointed away from the graphite surface.17 The geometry shown in Figure 5 retains the gauche conformation of the two C-F bonds within a molecule, but there is a loss of planarity of both -CO-CF- units in the molecule. (23) Schuler, M.; O’Hagan, D.; Slawin, A. M. Z. Chem. Commun. 2005, 4324. (24) Banks, J. W.; Batsanov, A. S.; Howard, J. A. K.; O’Hagan, D.; Rzepa, H. S.; Martin-Santamaria, S. J. Chem. Soc., Perkin Trans. 2 1999, 2409.
Formation of Chiral Domains of Fluorinated Diacids
Figure 6. (a) Head-to-head junctions between (R,R) pairs result in parallel molecular species. (b) A junction between an (R,R)-(S,S) pair is manifested by a rotation of ∼120° of the molecular backbones.
Density functional theory (DFT) calculations (B3LYP, 6-311 g) were carried out on (C2H5NHCOCHF)2 species (i.e., the central fragment of the diacid molecule shown schematically in Figure 4), allowing the free molecule to fully relax its structure. This calculation yielded planar NH-CO-CF units with the CF bond anti to the carbonyl group and syn to the NH functionality, that is, as expected on the basis of the crystal structure.23 In addition, the calculated FCCF dihedral angle was ∼60°, that is, also as expected. The above constraints can also be achieved by having a FCCF dihedral angle of -60°, that is, rotating the RHFC-CFHR bond by 120°. Constraining the molecule into this geometry was found to result in an energy ∼123 kJ mol-1 higher than that of the relaxed structure. This substantial energy difference is likely to be mainly due to the fact that the two O atoms of the carbonyl groups are forced into close proximity. We believe this is the underlying reason why the molecules are adsorbed in a linear geometry rather than exhibiting a bend about the central moiety. The calculation also showed that an all-trans, planar backbone, involving a loss of planarity at the -CO-CF- by requiring a dihedral angle of 120 degrees rather than 180 degrees, lies 76 kJ mol-1 above the ground-state conformation. Nevertheless we anticipate that this all-trans conformation is much more favorable at the graphite surface with an adsorption energy sufficient to offset this cost. Also in this conformation, the CdO bonds and N-H bonds are parallel to the surface and thereby optimized for intermolecular H-bonding. It is clear that two N-H · · · O hydrogen bonds can be formed between adjacent molecules of the same type while only one H-bond can be formed between neighboring (R,R) and (S,S) species. In addition, such a H-bond would cause a significant lateral offset between adjacent molecules. No such offset is observed in the STM images. We therefore conclude that the side-by-side stacking of molecular species overwhelmingly favors enantiomerically pure domains. We may next examine the end-to-end interactions of the molecular species. Assuming that the chain of methylene units adopts an all-trans conformation (i.e., as observed for stearic acid on graphite14,25), the stereochemistry of the F-containing chiral centers dictates that two (R,R) species can form dimers with the molecular axes parallel to each other (Figure 6a) while (25) Kislov, V. V.; Nevernov, I. E.; Panov, V. I. Dokl. Akad. Nauk SSSR 1990, 315, 91.
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a junction between a (R,R) and an (S,S) molecule would involve the molecular axes being at 120° to each other (Figure 6b). We believe such junctions account for the zigzag relationship between neighboring domains of molecular features. The angle between the dark lines at straight or kinked junctions and the molecular backbones is 60° or 120°. Taking this into account, it is not possible for linear CdO · · · HsN hydrogen bonds to exist between neighboring enantiomeric species as shown in Figure 6. We estimate an O · · · H-N bond angle of approximately 150°. It seems likely that this nonlinear H-bonding is caused by the fact that the combination of both H-bonding and the interactions between the molecular backbone and the graphite surface are required to be optimized in the formation of the lowest energy configuration. It has previously been reported that the -COOH functionality appears as a dark feature in STM images.26 Thus, we conclude that the parallel dark lines in the STM image correspond to the head-to-head H-bonding junctions between terminal carboxylic acid groups. The origins of the contrast variation observed for the straight and kinked junctions may be related to the relative ability of molecular species to optimize the H-bonding interactions in the two different types of terminal carboxylic acid junctions, since the overall energy of the dimer is influenced by other factors of comparable magnitude such as the interaction between the backbone and the graphite surface. As such, the strength of the H-bonding interactions at the dimer junction may be different for the straight versus the kinked junction giving rise to differences in the electron tunneling properties at the junction. Similar herringbone-like structures have been observed for 15-hydroxypentadecanoic acid on HOPG by Wintgens et al.18 Interestingly, the addition of one methylene unit (i.e., 16-hydroxyhexadecanoic acid) causes the loss of the herringbone structure. Similar odd-even effects have been reported by Tao et al.19 for cis-unsaturated carboxylic acids on graphite and by Fang et al.16 for the two enantiomers of 2-bromohexadecanoic acid on graphite. These “odd-even” effects are further support for the stability of the all-trans configuration of the carbon backbone in these structures. It is interesting to speculate as to whether a similar zigzag arrangement would be observed if the achiral, nonfluorinated analogue had been investigated. If the overall geometry of the nonfluorinated molecules were similar, the molecule, though achiral in the “gas phase”, would be able to adopt two mirror-equivalent adsorption geometries, giving it adsorption induced chirality. Since the intermolecular H-bonding between amide groups on adjacent molecules is thought to be an important driving force in the assembly of these molecules, it is likely that “homochiral” domains would be produced with a similar zigzag arrangement. However, one key role of fluorine appears to be in transferring structural information from one amide linkage to the other in a given molecule; that is, the desire for C-F bonds on adjacent C atoms to adopt the gauche conformation coupled with the desire for the CdO and C-F bonds to be approximately anticoplanar dictates that the CdO groups of the two amide linkages are on the opposite side of the molecule. In the absence of the two C-F groups, it is likely that this restriction would be lifted, giving the molecule additional degrees of freedom on the surface. If one assumes that, as has been shown before for similar molecular adsorbates on HOPG,10 the carbon backbone is aligned along the close-packed graphite directions, a further consequence of the head-to-head interactions is that chains of (R,R) species propagate at an angle of approximately 4° from the close-packed graphite direction while chains of (S,S) species propagate along mirror equivalent directions (Figure 2). Hence, if one domain is (26) Hibino, M.; Sumi, A.; Hatta, I. Jpn. J. Appl. Phys., Part 1 1995, 34, 3354.
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Figure 7. Growth of extended chains of (R,R) species occurs along a mirror equivalent surface direction to chains of (S,S) species. The two directions are rotated by ∼8° with respect to each other.
nucleated by the adsorption of a (R,R) molecule along a closepacked graphite direction, that domain will grow at an angle of approximately 8° with respect to the equivalent domain nucleated by the adsorption of an (S,S) molecule along the same closepacked graphite direction. Similar behavior was reported by Fang et al.16 for (R)- and (S)-2-bromohexadecanoic acid on graphite. It is important to note that this would be the case either if each island were exclusively made up of one enantiomeric type, if the island consists of alternating zigzags of (R,R) and (S,S), or if there is an uneven mixture of zigzags in an island. Figure 7 displays the different types of domains present on the surface. A number of earlier studies have reported that the replacement of a single (or multiple) C-H bond by a C-F bond results in a change in the image contrast with F atoms or perfluorinated alkyl chains imaging as depressions.21,27 This enabled the pinpointing of F substituents in the molecular species.20 In this study, we found no evidence for the two C-F-containing chiral centers appearing darker than the remainder of the molecule. It has also been reported that N-H · · · OdC< hydrogen bonds can appear as brighter features in the STM images.28 Since the central unit of the molecules under investigation contains both C-F bonds and two amide linkages and since we were unable to obtain atomically resolved images of the central portion of the molecule, it may be that the two opposing effects cancel each other. Since we can identify the edges of islands on the graphite surface, we are able to unequivocally conclude that the dark lines between zigzag molecular features correspond to the -COOH dimers. The dark lines are found to alternate in intensity. Under apparently identical imaging conditions (i.e., current, bias, Pt/Ir tip), the lines associated with the -COOH dimers can appear bright in some images and dark in others. Presumably, (27) Stabel, A.; Dasaradhi, L.; O’Hagan, D.; Rabe, J. P. Langmuir 1995, 11, 1427. (28) De Feyter, S.; Larsson, M.; Gesquiere, A.; Verheyen, H.; Louwet, F.; Groenendaal, B.; van Esch, J.; Feringa, B. L.; De Schryver, F. ChemPhysChem 2002, 3, 966.
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this must reflect the presence or absence of atomic/molecular species at the tip apex during scanning. Perpendicular to the dimer junctions, the parallel, or sometimes zigzagging, bright features observed in Figure 2 are likely to be caused by Moire´ structures. The intermolecular distances perpendicular to the alkyl chains are likely to be controlled by optimizing H-bonding interactions. There is likely to be a mismatch between the graphite lattice and the molecular periodicity, which will result in a periodic variation of the adsorption site over a distance of approximately eight or nine molecular separations. The brighter features produced by these Moire´ structures can either be perpendicular to the carboxylic acid dimers in the case of straight-on molecular junctions or zigzag between the rows of dimer bonds if the interactions between head groups are kinked.
Conclusions The liquid phase geometry adopted by the central moiety of the (R,R)- and (S,S)-diacid molecules is not adopted when the molecule is adsorbed on the graphite surface. The energy gained by interaction with the graphite surface more than compensates the deviation from the liquid phase geometry. The molecules adopt a linear geometry dictated by the desire of the neighboring C-F bonds to adopt a gauche orientation coupled with the fact that the linear geometry corresponds to the lower in energy of the two possible gauche orientations (the second would give a bent configuration). An all-trans conformation at the surface requires sacrificing the CF-CO anti-coplanar configuration but allows the -CO · · · NH- anticoplanar arrangement to be maintained as well as the gauche arrangement of C-F bonds. The ability to produce two intermolecular H-bonds favors the production of homochiral domains of molecules packed sideto-side with C-F bonds pointing away from the surface. Head-to-head interactions involving the formation of -COOH dimers result in straight-on junctions (homochiral) or kinkedjunctions (heterochiral). The stacking direction of (R,R) molecules is at 4° relative to the high symmetry direction of the underlying graphite, while the (S,S) molecules stack at an angle of -4°. This results in the observation of domains rotated by 8° with respect to each other. No evidence was found in this system for any STM contrast enhancement induced by fluorine substitution. Acknowledgment. We acknowledge funding from the Engineering and Physical Sciences Research Council (GR/T18585/01). LA802170F
