Coexistence of Racemic and Homochiral Two-Dimensional Lattices

Nov 20, 2008 - Dicarboxystilbene, a molecule that becomes chiral in the adsorbed state through the loss of its improper axis of rotation, forms long-r...
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NANO LETTERS

Coexistence of Racemic and Homochiral Two-Dimensional Lattices Formed by a Prochiral Molecule: Dicarboxystilbene on Cu(110)

2008 Vol. 8, No. 12 4162-4167

Rocı´o Corte´s,† Arantzazu Mascaraque,† Philipp Schmidt-Weber,‡ Hugo Dil,‡,| Thorsten U. Kampen,‡,§ and Karsten Horn*,‡ Departamento Fı´sica de Materiales, UniVersidad Complutense de Madrid, Madrid, Spain, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany Received June 4, 2008; Revised Manuscript Received October 29, 2008

ABSTRACT Dicarboxystilbene, a molecule that becomes chiral in the adsorbed state through the loss of its improper axis of rotation, forms long-range “handed” structures when adsorbed on Cu(110) as revealed by scanning tunnelling microscopy. We show that these structures are created from chiral “adsorption complex” building blocks, giving rise to a complete set of racemic and enantiomerically pure structural assemblies. We interpret the formation of these structures in terms of a balance between hydrogen bond mediated intermolecular interactions and the adsorbate-surface structural relationship and discuss the reasons for temperature-induced conversion from the metastable enantiomerically pure to the racemic structure.

Organic molecules on surfaces exhibit many interesting structural and physical properties,1-3 and processes of selfordering inducing a self-assembly of supramolecular structures may lead to functional surface nanostructures with applications in different fields.4 A wealth of studies demonstrates how surfaces may be used as platforms to guide the assembly of novel low-dimensional supramolecular “architectures” using a variety of molecular building blocks,5-7 which may be used in nanopatterning, sensing, and molecular recognition applications. A particularly interesting aspect of such assemblies relates to the observation of intriguing chiral phenomena, e.g., self-organization of chiral molecules leading to chiral amplification, and the possible use of chiral structures in functional materials8-13 and even in nonlinear optics.14 Moreover, in the specific case of so-called prochiral molecules that are achiral in the gas phase, but acquire a chiral character through the loss of symmetry elements in the adsorbed state, applications in enantioselective heterogeneous catalysis may be envisaged.15 Hence, enhancing our understanding of how molecules acquire a chiral character and how this process influences self-assembly on surfaces is of importance. On the surface, themutualinterplaybetweenintermolecularandadsorbate-substrate * Corresponding author: [email protected]. † Universidad Complutense de Madrid. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft. § Now at SPECS GmbH, Berlin. | Now at University of Zurich, Switzerland. 10.1021/nl801592c CCC: $40.75 Published on Web 11/20/2008

 2008 American Chemical Society

interactions influences the ordering on the surface and twodimensional pattern formation. As a result, specific molecular conformations are established or modified.16-21 Such processes may give rise to chiral self-assembly in two7,8 and one dimensions,9 even chiral switching.22 Among the class of molecules to which these ideas apply, stilbene and its derivatives are probably the simplest and thus most useful candidates as model systems in which to demonstrate adsorbate-induced chirality and self-assembly. The molecular frame of stilbene may assume a cis and a trans configuration, with the phenyl rings on the same or opposite sides of the central CdC double bond bridge, respectively.23 On surfaces, the trans configuration may exist in a left- and right-handed form in this two-dimensional environment. The molecule may thus acquire a planar chiral character. Apart from the stilbene molecule itself, its functionalized derivatives are of interest from the point of view of self-assembly,24 because of the differences in bonding to the surface induced by the end groups. Here we present a study of the adsorption of a functionalized stilbene derivative, i.e., 4,4′-stilbene dicarboxylic acid (hence DCSB) on Cu(110). Because of its simplicity, this system may be considered as a model for chirality induced in a molecule through the adsorption geometry. Cu(110) is a suitable substrate on account of its anisotropic surface mesh, suppressing the formation of rotational adsorbate domains. The formation of long-range ordered structures and the identification of the

Figure 1. Room temperature STM images of DCSB adsorbed in the herringbone structure on Cu(110): (a) partly covered surface showing atomic resolution in the substrate alongside the DCSB structure (-1.79 V, -0.69 nA), with substrate azimuths as indicated. Inset enlarges Cu atoms of the same interface (-1 V, 2 nA). (b) large scale image showing perfect long-range order (1.25 V, 0.83 nA). (c) Close-up of the herringbone structure, (1.25 V, 0.83 nA). (d) Submolecular resolution of the DCSB structure (-1.683 V, -0.5 nA). Left- and right-handed DCSB molecules are indicated by the red and green outlines in (c) and (d). Schematic drawings on the right-hand side show right-and left-handed “surface enantiomers” of the DCSB molecule in the adsorbed state (see text for details). Size bar indicates 1 nm except in (b) where it indicates 10 nm.

adsorption sites occupied by the molecules were investigated using scanning tunnelling microscopy (STM). We use core level photoelectron spectroscopy to demonstrate that the carboxylic end groups of DCSB induce a specific chemisorptive bond through the oxygen atoms upon deprotonation. We find that the left- and right-handed adsorbed molecules form “handed” long-range ordered structures, both racemic and enantiomerically pure, which arise from a small set of specific adsorption sites. This system thus exhibits chiral resolution as a consequence of chiroselective interactions. The experiments were performed in two different ultrahigh vacuum (UHV) systems with base pressures of 7 × 10-11 mbar. The Cu(110) crystals were cleaned by sputtering with Ar+ (1,5 kV) and annealing at about 673 K, resulting in wellordered and clean surfaces. DCSB with a purity of 99% (Aldrich) was deposited from a Knudsen type evaporation cell at temperatures between 453 and 488 K. During deposition the substrates were either kept at room temperature or heated at 393 K. Photoemission spectroscopy was performed at the UE 56/2 PGM 1 beamline at the BESSY II synchrotron radiation facility using a Phoibos 100 hemispherical analyzer with a two-dimensional (2D) CCD detector (SPECS GmbH). The formation of molecular self-assembled structures was investigated using a SPECS 150 Aarhus variable temperature STM. Images were analyzed using WSxM software.25 The adsorption of DCSB on Cu(110) results in highly ordered long-range structures as shown in Figure 1. The molecules, which on a surface may assume a right- (dextro-, δ) and left-(laevo-, λ)-handed conformation23 (see models on the right-hand side of Figure 1), appear as oblong shapes of about 1.2 nm length in the STM images. They have a Nano Lett., Vol. 8, No. 12, 2008

Figure 2. Oxygen 1s core level photoemission spectra of DCSB adsorbed on Cu(110) in the mono- and multilayer regime, with a schematic drawing indicating the assignment of lines to different oxygen atoms in the molecule.

clear handedness, as revealed in the high-resolution images of Figure 1, panels c and d, and are therefore assigned to the δ and λ conformation of adsorbed DCSB. When deposited on the surface at elevated temperatures (∼395 K), they form a “herringbone structure”: the two differently shaped molecules arrange in long rows inside which the molecules, indicated by the red and green outlined shapes, lie at an angle of about 50° with respect to each other. Similar herringbone structures are quite common in adsorbed organic molecules on a variety of surfaces.26,27,6 We have never observed single, isolated molecules in the images; this is a strong indication that the molecules can readily diffuse across the surface in this phase at room temperature. STM images taken in quick succession show a continuous adsorption and desorption of molecules at step edges or domain boundaries. Even islands consisting of four molecules are found to diffuse across the surface and only larger islands appear to be stable. The appearance of the molecules in the STM images has a pronounced dependence upon bias voltage, with submolecular features related to tunneling out of specific occupied molecular orbitals28 apparent at negative bias (Figure 1d). These images show that one-half of the molecules, the δ type, are arranged at an angle of 20° to the [11j0] surface azimuth, while the other species (λ) has an angle of 20° to the [001] azimuth. The “handed” appearance of the molecules, apparent through protrusions at either end, is actually somewhat different for the δ- and λ-type, a fact that is discussed below. For a proper assignment of the STM features, it is important to examine the nature of the chemisorptive bond of DCSB on Cu(110). To reveal this information, we use oxygen 1s core level photoemission (Figure 2). After deposition of about one monolayer of DCSB at ∼393 K, a single O 1s line at a binding energy of 530.3 eV is found. In the multilayer, where molecules occur in an arrangement undisturbed by the presence of the metal surface, this line decreases in intensity, and two new lines at 531.5 and 532.8 4163

Figure 3. (a) Adsorption site of DCSB in the λ and δ conformations as building blocks for the P and H long-range structures (see text). (b) STM image of an island of DCSB on Cu(110), which consists of a left-handed parquet PL phase (upper part) and a right-handed herringbone HD phase (-1 V, -0.93 nA, 200 K). (c) Schematic drawing of the arrangement of λ and δ molecules in each phase; color code of molecules as in (a), expect for those in the transition region which are shown in blue. The dashed lines indicate the λ-type species that retain their orientation but are shifted by one Cu row.

eV appear. We assign the latter to oxygen atoms in the CdO and CsOsH bonds of the undisturbed molecule, respectively. All oxygen atoms in the monolayer are thus in an identical chemical environment, which can only occur through deprotonation of the carboxylic group and bonding to the surface in a flat geometry, i.e., with the phenyl rings parallel to the surface. No bonds due to the other moieties in the free molecule exist in the monolayer, and all four oxygen atoms are bound to neighboring Cu atoms in the topmost layer as shown in the structural models in Figure 3. This is also the most likely adsorption site inferred from the STM images; the slight difference in appearance (Figure 1) hence is not related to a difference in bonding. Our photoemission results thus lend strong experimental support to previous assignments of the bonding mechanism based on infrared reflection-absorption and STM data.29-31 An important aspect of our study is the ability to image the molecules and the bare Cu surface with atomic resolution in the same image (Figure 1a). This fact, together with the information gained from images in which we observe molecules at step edges with their known structure, and the chemical bonding information from photoemission, permits a direct determination of the different adsorption sites with respect to the substrate atoms that the molecules assume in the herringbone phases. For each of the δ and λ-type molecules, two specific adsorption sites exist, shown schematically in Figure 3a: one (suffix 1) in which the DCSB molecule crosses five Cu atoms rows, with the oxygen atoms on either end displaced by one Cu-Cu distance, while the other (suffix 2), oriented roughly along the Cu rows, again 4164

with the oxygen atoms at either end displaced, here by one interchain distance. It is then clear that the herringbone phase shown in Figure 1 is formed by self-assembly from equal amounts of λ1- and δ2-type molecules. The λ2- and δ1-type molecules have adsorption sites which makes the “adsorption complexes” mirror images of one another, as is obvious from the models in Figure 3a. This fact is bound to be related to the specific bonding interaction of the double bond backbone with the geometry of the Cu(110) surface, i.e., its trenches of atoms. Moreover, the different arrangement of the phenyl rings on the Cu atom rows in the type 1 and 2 adsorption sites is the likely cause for the small differences in appearance of the molecules in the high-resolution STM images of Figure 1, panels c and d. These structural motifs can be used as building blocks on the basis of which we can predict further possible long-range ordered structures, in a combination different from that used to explain the STM images in Figure 1. Considering the arrangements of λ1 and δ2 adsorption complexes in the herringbone (H) structure, we can conjecture to find a mirror image of that structure made up of λ1 and δ2 complexes. This structure indeed exists, as can be seen in the lower half of Figure 3b. The two herringbone phases are mirror images of one another and, thus, have a handedness which we designate with subscripts L for (for laeVo, left-handed) and D (for dextro, the right-handed structure). The structure in Figure 1 then represents a left-handed herringbone, i.e., HL phase, while the one in the bottom part of Figure 3b is HD. The building blocks in Figure 3a may also lend themselves to an arrangement in an entirely different structure, built by self-assembly from only δ- or λ-type molecules. Using λ1and λ2-type molecules, this would result in four molecules forming what we term the “parquet” pattern (P) structure;32 here the molecules are roughly at a right angle to one another. In fact, this predicted structure is observed when depositing DCSB molecules at low temperature, as shown in the upper part of the island in Figure 3b. An important difference to the herringbone phases is that the parquet structure is metastable at room temperature and converts into the stable herringbone phase upon annealing. By careful control of this process, situations can be achieved where both herringbone and parquet phases coexist as shown in Figure 3b, where a left-handed PL phase occurs in the upper part of Figure 3b and a HD in the lower one; both are connected by a transition line of molecules (in blue). While in the herringbone phase, an oblique bonding between the molecules occurs; they arrange in an almost right-angle pattern in the parquet phase. On the basis of these DCSB adsorption complexes in Figure 3a, the schematic diagram in Figure 3c shows how the molecules arrange themselves in these phases. The excellent agreement with the underlying contours of the shapes taken from the STM images suggests that the adsorption site models in Figure 3a provide an excellent basis for the explanation of the way in which the structures form. The transition between the H and the P phase is of particular interest; here the λ species retain their orientation with respect to the copper atom rows in both structures but are displaced by one more Cu atom row as indicated by the dashed lines Nano Lett., Vol. 8, No. 12, 2008

Figure 4. Left: Two regions of the same STM image of the Cu(110) surface where the four different structural arrangements of DCSB, i.e., herringbone (H) and parquet pattern (P) phases, coexist. Right: schematic diagram of the arrangement of δ and λ DCSB molecules with respect to the Cu(110) substrate atoms in the H and P phases. Notice that PL and PD are enantiomerically pure while the two herringbone phases HL and HD are a racemic mixture. STM image taken at 200 K with -1.9 V and -0.38 nA.

in Figure 3c. This displacement permits the introduction of another λ-type species between the rows. The herringbone and parquet phases thus differ in one important point: while each domain of the herringbone structure is composed of equal amounts of left- and righthanded molecules, the parquet phase is enantiomerically pure, i.e., it consists only of one species (i.e., the λ type). The λ1and λ2-type species were employed to predict the PL phase; equally possible, however, is the formation of a mirrorimaged structure, built from δ1- and δ2-type species. This PD structure is thus made purely from the other enantiomer. Both PD and PL are mirror images of one another; i.e., they have a global handedness. Figure 4 shows that this predicted structure, i.e., the respective mirror image to the PL structure shown in Figure 3b, indeed exists. The data were recorded in one image, thereby excluding any errors in adsorption site assignment due to experimental effects; the thin white line indicates a region where a large part of the bare surface was cut from the image for illustrative reasons. Four islands occur, each with a different structure, labeled according to the scheme outlined above. A left-handed and a right-handed herringbone (HL and HD, respectively) exhibits the case where the chiral molecules arrange in two racemic yet handed surface structures. In contrast, the left- and right-handed parquet DCSB islands (PL and PD, respectively) are enantiomerically pure. The models on the right-hand side show the arrangement of δ- and λ-type DCSB molecules on the substrate lattice within each of the four long-range ordered structures. A closer inspection of those islands and their alignment with respect to the Cu(110) surface reveals that both left-handed phases (HL, PL) are described by one transformation matrix (4 -3, 3 4). The transformation matrix for the right-handed phases (HD, PD) is (4 3, -3 4). The fact that only two lattices are found which describe the chiral herringbone and parquet Nano Lett., Vol. 8, No. 12, 2008

phases is a direct consequence of the low number of different conformations (adsorption complex structures) assumed by adsorbed DCSB on Cu(110). This case is, to the best of our knowledge, the first example where all possible handed longrange structures, both racemic and enantiomerically pure, induced by self-assembly of a prochiral molecule upon acquisition of a chiral character on the surface, are experimentally observed. How does the specific surface geometry of Cu(110) and its interaction with the adsorbed DCSB molecules lead to the formation of the long-range handed structures? To put it differently, we need to understand why the δ2-type molecule does not adsorb equally well in a manner parallel to λ2, crossing one of the two Cu atom rows in the “left” direction (Figure 3a), forming a new structure. The answer lies in the manner in which the CdC backbone, which is bent in a mirror image fashion in the λ and δ species, and the phenyl rings interact with the substrate copper atom rows. If the δ1-type molecules were to lie parallel to the λ1 ones, the oxygen atoms at either end would have less overlap between neighboring Cu atoms. This reasoning applies equally well to the λ1 and δ1 molecules bridging several Cu atoms rows. From modeling adsorbate complexes in a manner similar to that in Figure 3c it is also likely that the overlap between the phenyl rings and the substrate atoms is more favorable in the structures shown in Figure 3c. Emerging from those four building blocks, λ1,2 and δ1,2, other assemblies on the surface (e.g., square ones) might well be possible, while we detect only the herringbone (H) and parquet (P) phases mentioned. The reason for the preferential formation of these phases is likely to be related to the delicate balance between the various contributions to adlayer ordering, i.e., the influence of specific adsorption geometries and the intermolecular interaction, through hydrogen bonding. Information on such intermolecular 4165

Figure 5. DCSB adsorbed on Cu(110) in the herringbone structure (recorded at a bias of -1.1 V I ) 1 nA). Ball-and-stick-models represent the molecular geometry. Extra intensity inducing an anisotropy at the carboxylic end groups along the H bridge bonds between adjacent molecules are clearly visible.

interaction may be inferred indirectly through distortions in the molecules in high-resolution STM images such as shown in Figure 5. The oblong shapes of the molecules exhibit a small kink in the center which can be directly assigned to the CdC bridge bond. The larger round shapes are representations of DCSBs’ two phenyl groups. The shapes of the molecules appear distorted in the direction of adjacent ones. The distortions involve the phenyl rings on either side of the CdC backbone, pointing toward the carboxylic end group of an adjacent one, suggesting an electrostatic interaction between the hydrogen atom on the phenyl ring and the oxygen atom. While hydrogen bonds generally involve an OsHsO bridge, several studies of self-assembled supramolecular structures have invoked CsHsO and NsHsO hydrogen bonds, for example, in NTCDI on Ag/Si(111),33 OPV on highly oriented pyrolytic graphite,34 and PVBA and PEBA on Ag(111).35 The latter study gives a bonding distance of about 2.5 Å for the NsHsO bond, and 2.8 Å for the OsHsC bond. From an analysis of images such as those shown in Figure 5, we obtain a bonding distance of about 2.5 Å in both the herringbone and parquet phases; note that we reach the limits of such determinations in these room temperature STM data, which in any case are based on the assumption that the molecules are undistorted in the adsorbed phase. The structural motifs discussed above are thus due to both an influence of the substrate lattice geometry and the hydrogen bonds between neighboring molecules. As pointed out above, the parquet structure is metastable at room temperature and converts into the stable herringbone structure upon annealing. This raises the question as to the causes of this phase transformation, for which related cases exist in the literature.36-38 In the bulk phase, many examples are known in which racemic crystals coexist or are thermodynamically preferred over enantiopure crys4166

tals.39,40 It has been argued that the conformational entropy plays an important role in preferring racemic over enantiopure phases (Wallach’s rule 41), but this has been rejected on general grounds.42 For the case at hand, the notion that the racemic herringbone phase has a higher entropy and is thus preferred if the adsorption enthalpy for both phases is similar (which is not known) is questionable since we have assigned different adsorption sites to the λ and δ species, and thus an entropy gain in going from the parquet to the herringbone phase would not occur. In the related case of the rodlike PVBA molecule on Cu(100), Vidal and co-workers32 find that a similarly enantiomerically pure parquet phase occurs at coverages below a critical value, beyond which the molecules arrange in a racemic layer. A similar effect was found by Bo¨hringer et al.43 for nitronaphtalene on Au(111), where a racemic structure gradually evolves with coverage. While Bo¨hringer et al. relate their observations to different adsorption sites on the complex reconstructed Au(111) surface, the phase transition in PVBA on the much simpler square Cu(100) surface is ascribed to packing constraints at high coverage overriding repulsive intermolecular interactions. Our results for DCSB on Cu(110) show a packing that is only slightly larger (∼10%) in the herringbone phase. We were not able to examine the phase transition as a function of coverage since even at room temperature the parquet structure is a minority phase. The fact that a slight annealing (∼400 K) even at low coverage results in the transformation of the P into the H phase rules out a packing density influence as in the case of PVBA. The structures of alanine and glycine, which also may acquire a chiral character upon adsorption, on Cu(100) and Cu(110) were studied through density functional theory calculations by Rankin and Sholl.38 For alanine, they find that racemic and enantiomerically pure adlayers have essentially the same energy, and spontaneous segregation of a racemic structure into enantiomerically pure domains as suggested by some experimental reports44 can be ruled out. The calculations were carried out for dense adlayers, however, such that a possible influence of surface coverage could not be investigated. The above argumentation excludes explanations for the occurrence of phase transitions in chiral adsorbate structures based on entropy, packing density, or phase segregation energy and thus requires further experimental and theoretical study. In summary, we find that dicarboxystilbene molecules adsorb on Cu(110) surface via bonding of all four oxygen atoms of the deprotonated carboxylic end groups to nextneighbor Cu atoms. In the two-dimensional environment of the surface, the molecules become chiral and induce a different handedness (levo and dextro) in self-assembled long-range structures of the H and P type. The enantiomerically pure domains of the P phase consist only of one DCSB enantiomer, while the domains of the H phase are a racemic mixture. The formation of the four observed HL, HD, PL, and PD structures is explained through a delicate balance between the adsorbate-substrate relationship and the intermolecular hydrogen bond-mediated interaction. Although the four Nano Lett., Vol. 8, No. 12, 2008

possible phases can be obtained in the same surface at room temperature, the metastable P phases transform into H upon increasing the temperature. Further studies are needed to shed light on the delicate balance between the stability of the observed chiral structures. Acknowledgment. A.M. gratefully acknowledges support by the Max-Planck-Society. R.C. is indebted to “Comunidad de Madrid” and “Fondo Social Europeo”. Part of this work was financed through project MOL-VIC through the European Science Foundation EUROCORES Programme SONS (MAT2002-11975-E). References (1) Lehn, J. M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (2) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (3) Forrest, S. R. Chem. ReV. 1997, 97, 1793. (4) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (5) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (6) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Lin, N.; Deng, X.; Cai, C.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229. (7) Barth, J.; Weckesser, J.; Lin, N.; Dmitriev, A.; Kern, K. Appl. Phys. 2003, A 76, 645. (8) Ortega Lorenzo, M.; Baddeley, C. J.; Muran, C.; Raval, R. Nature 2000, 404, 376. (9) Bo¨hringer, M.; Schneider, W. D.; Berndt, R. Angew. Chem., Int. Ed. 2000, 39, 792. (10) Ku¨hnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891. (11) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 1655. Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2004, 126, 14234. (12) Romer, S.; Behzadi, B.; Fasel, R.; Ernst, K. H. Chem. Eur. J. 2005, 11, 4149. (13) See the reviews in Supramolecular Chirality; Topics in Current Chemistry; Springer-Verlag: Berlin/Heidelberg,2006; p 265. (14) Verbiest, T.; Elsholst, S. V.; Kauranen, M.; Hellemans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.; Persoons, A. Science 2008, 282, 913. (15) Izumi, Y. AdV. Catal. 1983, 32, 215. (16) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K. Nature 1997, 386, 696. (17) Moresco, F.; Meyer, G.; Rieder, K.-H.; Tang, H.; Groudon, A.; Joachim, C. Phys. ReV. Lett. 2001, 86, 672. (18) Loppacher, Ch.; Guggisberg, M.; Pfeiffer, O.; Meyer, E.; Bammerlin, M.; Lu¨thi, R.; Schlittler, R.; Gimzewski, J. K.; Tang, H.; Joachim, C. Phys. ReV. Lett. 2003, 90, 066107. (19) Qiu, X. H.; Nazin, G. V.; Ho, W. Phys. ReV. Lett. 2004, 93, 196806.

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