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Supramolecular Assembly of Strongly Chemisorbed Sizeand Shape-Defined Chiral Clusters: S- and R-Alanine on Cu(110) Susan M. Barlow, Souheila Louafi, Delphine Le Roux, Jamie Williams, Christopher Muryn, Sam Haq, and Rasmita Raval* Surface Science Research Centre, University of Liverpool, Liverpool, L69 3BX, United Kingdom Received March 9, 2004. In Final Form: May 7, 2004
The bonding and self-assembly of a chirally organized monolayer of alanine on the Cu(110) surface has been investigated using reflection-absorption infrared spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). This multitechnique approach has enabled an in-depth understanding of the hierarchy of chirality transfer: from a single adsorbed molecule, to size-defined chiral clusters, and then to an overall chiral assembly. The data have indicated that the alanine is in its anionic form, bound to the copper surface through the oxygens of the ionized carboxylate group and the nitrogen of the neutral amino group. Importantly, the methyl group is held away from the surface, resulting in direct chirality transfer into the footprint of the adsorbed alanine molecules, with the local adsorption motif for S-alanine being the mirror image of that created for R-alanine. STM has shown that S-alanine molecules self-organize to form size-defined chiral clusters of six or eight molecules at the surface, interspersed with chiral channels of bare metal. Together, these clusters and channels further self-assemble into a chiral array with one unique chiral domain sustained across the entire surface. A similar chiral assembly, but with the mirror organization, has been observed for R-alanine. Structural models for the individual clusters are proposed, and in conjunction with LEED data, overall models for these chiral phases of both S- and R-alanine have been constructed. Overall, this adsorption system has been found to be both strongly chemisorbed and capable of extensive intermolecular H-bonding, causing stresses that lead not only to the chiral self-organization of molecules but also to a specific self-organization of the empty chiral channels and spaces that intersperse the structure which, in turn, chirally assemble across macroscopic length scales to give a surface with global organizational chirality.
* To whom correspondence should be addressed. Telephone: +44 151 794 6981. Fax: +44 151 794 3896. E-mail:
[email protected].
balance of molecule-metal and intermolecular interactions. These balances are only just being understood, with interesting insights already appearing for weakly adsorbed systems where intermolecular interactions dominate, leading to a range of two-dimensionally ordered chiral architectures.14-19 However, strongly adsorbed systems set a greater challenge in that both molecule-metal bonding and intermolecular interactions influence the array, potentially enabling greater variety and tunability of supramolecular organization.20 Here, we report on the manifestation of chirality for the amino acid alanine adsorbed on Cu(110), chosen as it is both strongly chemisorbed and capable of extensive intermolecular H-bonding. This combination should lead to a highly stressed system, and in this paper, we show how these stresses lead to the creation of a globally organized
(1) Raval, R. Nature 2003, 425, 463. (2) Orme, C. A.; Noy, A.; Wierzbicki, A.; McBride, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Nature 2001, 411, 775. (3) Switzer, J. A.; Kothari, H. M.; Poizot, P.; Nakanishi, S.; Bohannan, E. W. Nature 2003, 425, 490. (4) Bo¨hringer, M.; Morgenstern, K.; Schneider, W. D.; Berndt, R. Angew. Chem., Int. Ed. 1999, 38, 821. (5) Bo¨hringer, M.; Morgenstern, K.; Schneider, W. D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. Rev. Lett. 1999, 83, 324. (6) Ku¨hnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891. (7) Ku¨hnle, A.; Linderoth, T. R.; Besenbacher, F. J. Am. Chem. Soc. 2003, 125, 14680. (8) Barth, J. V.; Weckesser, J.; Cai, C. Z.; Gunter, P.; Burgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230. (9) Chen, Q.; Richardson, N. V. Nat. Mater. 2003, 2, 324. (10) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376.
(11) Lorenzo, M. O.; Haq, S.; Bertrams, T.; Murray, P.; Raval, R.; Baddeley, C. J. J. Phys. Chem. B 1999, 103, 10661. (12) Raval, R. Curr. Opin. Solid State Mater. Sci. 2003, 7, 67. (13) Raval, R. J. Phys.: Condens. Matter 2002, 14, 4119. (14) De Feyter, S.; Gesquiere, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217. (15) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139. (16) Barth, J. V.; Weckesser, J.; Lin, N.; Dmitriev, A.; Kern, K. Appl. Phys. A: Mater. Sci. Process. 2003, 76, 645. (17) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; De Vita, A.; Cai, C. Z.; Brune, H.; Gunter, P.; Kern, K. J. Am. Chem. Soc. 2002, 124, 7991. (18) Fasel, R.; Parschau, M.; Ernst, K.-H. Angew. Chem., Int. Ed. 2003, 42, 5178. (19) Bohringer, M.; Schneider, W. D.; Berndt, R. Angew. Chem., Int. Ed. 2000, 39, 792. (20) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201.
1. Introduction The supramolecular assembly of organic molecules at metal surfaces into organized chiral superstructures offers exciting opportunities in nonlinear optics, asymmetric catalysis, liquid crystals, and sensors and is thought to be a fundamental stepping-stone for creating templates for three-dimensional growth, for example, inorganic chiral catalysts and biomineralization.1-3 The length scales of these chiral superstructures can extend from local nanoscale chiral assemblies of just a few molecules4-7 to onedimensional strings8,9 and to crystalline-like architectures covering an entire macroscopic surface.10-20 The nature of this two-dimensional assembly is dependent on the
10.1021/la049391b CCC: $27.50 © 2004 American Chemical Society Published on Web 07/17/2004
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macroscopic array in which we observe not only the chiral self-organization of molecules but also a specific selforganization of the stress-induced chiral channels and spaces that intersperse the overall structure. 2. Experimental Section The Cu(110) crystal was cleaned by cycles of Ar+ ion sputtering, flashing, and annealing to 600 K. The surface ordering and cleanliness were monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). S- or R-alanine (99%) obtained from the Aldrich Chemical Co. was sublimed from a resistively heated glass tube, differentially pumped by a turbo molecular pump and separated from the main ultrahigh vacuum (UHV) chamber by a gate valve. The alanine was outgassed at ∼350 K and then heated to ∼370 K before exposure to the copper crystal, during which time the main chamber pressure typically rose to around 2 × 10-9 mbar. Experiments were carried out in two UHV chambers. The first chamber contained facilities for reflection-absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), LEED, AES, and sample cleaning and was interfaced to a Mattson 6020 Fourier transform infrared (FT-IR) spectrometer equipped with a liquid nitrogen cooled HgCdTe detector with a spectral range of 650-4000 cm-1. The second chamber was an Omicron Vakuum-physik system with facilities for scanning tunneling microscopy (STM), LEED, AES, and sample cleaning.
3. Results and Discussion When S-alanine is adsorbed on the Cu(110) surface under ultrahigh vacuum conditions, a variety of superstructures, discussed by us elsewhere,21 are formed with the most interesting, from the viewpoint of two-dimensional chirality, being the one created after annealing a saturated monolayer, deposited at room temperature, to between 400 and 430 K. A large-area STM image, Figure 1a, shows that this gives rise to a perfect two-dimensional chiral assembly, in which the amino acid adsorbates are aligned along a nonsymmetry direction. Only one unique chiral domain, that is nonsuperimposable on its mirror image, is sustained across the entire surface; i.e., the twodimensional chirality is expressed at the macroscopic length scale. A small-area STM image, Figure 1b, reveals an unexpected complexity in that the individual alanine molecules are self-assembled into defined chiral clusters (or packs) generally consisting of six or eight molecules. We attribute this very specific size-preference for chiral hexamer or octamer clusters to system stresses between molecule-molecule and molecule-metal bonding that ultimately fracture the assembly. What is of particular interest, here, is that these clusters plus their stress fractures now become synthons for a globally organized macroscopic array with a unique chiral arrangement of molecules interspersed with chiral channels and spaces. By combining the detailed IR spectroscopic information and the structural data from LEED and STM studies, we have been able to elucidate the details of the way the defined chiral clusters form and self-organize at the surface. The RAIR spectrum obtained from this phase, shown in Figure 2, is almost identical to that reported and discussed in detail by us22 for adsorption of this system at 300 K and shows vibrations arising from the COOcarboxylate and the NH2 amino groups, indicating the creation of the anionic alaninate species. Given the proximity of vibrations arising from the NH2 and NH3+ groups, the presence of the neutral amino nitrogen was (21) Barlow, S. M.; Louafi, S.; Le Roux, D.; Williams, J.; Murray, C.; Haq, S.; Raval, R. Surf. Sci., in preparation. (22) Williams, J.; Haq, S.; Raval, R. Surf. Sci. 1996, 368, 303.
Figure 1. STM images showing the chiral phase of S-alanine adsorbed on Cu(110) obtained after annealing the roomtemperature saturated surface to 403 K. (a) Large-area image showing the organized chiral surface (400 Å × 400 Å, V ) -2.08 V, I ) 1.97 nA). (b) Small-area STM image showing size-selected clusters of six or eight molecules (100 Å × 100 Å, V ) -2.08 V, I ) 1.97 nA.)
further confirmed from X-ray photoelectron spectroscopy (XPS) data21 that showed a N 1s peak characteristic of a neutral amino nitrogen. Thus, the adsorbed alanine molecules are in their anionic state and adsorb intact, retaining their essential chiral molecular structure. The hydrogen atoms released by the deprotonation of the carboxylic acid are believed to undergo recombinative desorption as hydrogen molecules, in a manner similar to that observed for other copper surfaces.23 Importantly, the growth mode for the adlayer revealed by the RAIRS data, combined with the application of the strict dipole selection rule that governs this technique, indicates that two differently oriented anionic species actually coexist at the surface as shown within Figure 2 and discussed in detail in our earlier work.22 One species is bound to the copper surface through both oxygens of the ionized carboxylate group and the nitrogen of the neutral amino group (hereafter referred to as the µ3 species), and the other is bonded to the copper through the amino group and only one of the carboxylate oxygen atoms (referred to as the µ2 species). The adsorption geometry of the µ3 species can be deduced from detailed structural studies on the simplest nonchiral amino acid, (23) Tabatabaei, J.; Sakakini, B. H.; Watson, M. J.; Waugh, K. C. Catal. Lett. 1999, 59, 143.
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Figure 2. RAIRS spectrum obtained after adsorption of S-alanine on Cu(110) at room temperature and annealing to 403 K. Previous work (ref 22) has shown that this spectrum arises from the coexistence of two species: the µ3 species which only exhibits the symmetric carboxylate stretch and the µ2 species that only displays the asymmetric carboxylate stretch. The adsorption geometries of the two species are as indicated.
glycine, on Cu(110), where the two oxygen atoms of the carboxylate group bond in equivalent near-on-top positions to two adjacent copper atoms in the close-packed 〈110〉 direction and the nitrogen of the amino group bonds almost atop to a copper atom in the next close-packed row.24-27 Note that there is a mismatch of molecular and surface atomic dimensions, so that the bonding oxygen and nitrogen groups are displaced by 0.80 ( 0.07 Å and 0.24 ( 0.10 Å, respectively, away from their preferred on-top positions. In addition, for both alanine species, the RAIRS indicates that the methyl group is held away from the surface. This results in direct chirality transfer into the footprint of the adsorbed S-alanine molecule which adopts a right-handed kink in the CCN backbone, when viewed from above. Therefore, the local chiral adsorption motif possesses a double chiral nature, arising from the chiral molecular structure and the chiral adsorption footprint. The small-area STM image in Figure 1b shows bright protrusions with a diameter of 3.75 Å, which is consistent with the size of an individual alaninate molecule. Thus the STM images in Figure 1 can be seen to arise from packs of molecules composed of either six or eight individual alanine molecules aligned broadly parallel to the nonsymmetric [1 h 12] direction. Thus, this adlayer displays two major effects: size selection and chiral organization. Adjacent molecules within the packs in the 〈110〉 direction cover a distance of 7.5 Å across, which is consistent with occupation over three close-packed copper atoms. Given (24) Booth, N. A.; Woodruff, D. P.; Schaff, O.; Giessel, T.; Lindsay, R.; Baumgartel, P.; Bradshaw, A. M. Surf. Sci. 1998, 397, 258. (25) Hasselstrom, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Samant, M. G.; Stohr, J. Surf. Sci. 1998, 407, 221. (26) Nyberg, M.; Hasselstrom, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2000, 112, 5420. (27) Nyberg, M.; Odelius, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2003, 119, 12577.
that the RAIRS data show the coexistence of two differently oriented alanine species and tying this in with the STM information, we propose a molecular model for the hexamers, shown in Figure 3a. Here, each pair of molecules within the cluster is made up of one µ3 and one µ2 alanine species. The individual alanine molecules within this sizedefined cluster are held together with a network of hydrogen bonds between the N-H and O groups and the C-H and O groups, similar to those exhibited by solid crystals of alanine. Knowing the orientations of the µ2 and µ3 species from the RAIRS data, we can see that these supramolecular interactions are propagated at two different levels with respect to the surface plane. The N-H‚‚‚O interactions all involve bonded groups close to the surface, whereas the C-H‚‚‚O interactions occur between nonbonded groups further from the surface and form a second tier of interaction. Thus, lateral N-H‚‚‚O hydrogen bonds along the “long” axis of the cluster and transversal C-H‚‚‚O hydrogen bonds weave the cluster together. Both of these interaction directions are along nonsymmetric directions of the surface, as a direct consequence of the chiral molecular footprint of the adsorbate which restricts the placement of the neighboring molecule to a chiral position in order to optimize intermolecular interactions. A question that arises is, why do the molecules form the clusters at all? We propose that the stress of maintaining both optimum adsorption sites for the alanine molecules and maximizing the intermolecular interactions becomes too great once a critical cluster size is reached, leading to a fracture in the assembly. There are two possible scenarios for this fracture: either intermolecular interactions progressively force subsequent pairs of molecules further away from their optimum bonding positions, until adsorption is no longer favorable and a new cluster is nucleated; or the strength of the chemisorption bond forces
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Figure 3. Cluster model for packs of six molecules of S-alanine adsorbed on the Cu(110) surface. (a) The unreconstructed surface. (b) The pseudo-(100) reconstructed surface.
surface reconstruction of the copper atoms in order tooptimize adsorption geometry.28,29 For example, a pseudo(100) surface reconstruction of the fcc(110) surface29 would allow the alanine nitrogen and oxygen atoms to adopt the favored on-top sites as illustrated in Figure 3b. This, in turn, would lead to stresses in the metal-metal bonding, culminating in strain breaks. In either case, it can be concluded that the formation of the defined clusters is a consequence of the interplay between the strong chemisorption of the alanine molecules to the metal surface and the strong H-bonding interactions between the molecules. The model detailed above shows that not only the local molecule adsorption motif but also each size-defined cluster and its associated strain breaks project a chiral footprint at the surface. Furthermore, from the STM data, it is clear that the self-assembly now proceeds to another stage in which the clusters and the interspersed spaces, in turn, self-assemble into a perfect extended twodimensional chiral organization at the surface. In this respect, our system deviates significantly from other reports of chiral cluster formation in which no higher superstructures are observed once the initial cluster is formed.4-7 The organizational integrity of this second hierarchy of self-assembly is seen with LEED, which is sensitive to the long-range order of the interface. Figure 4a displays the LEED pattern associated with this phase, with the real space unit cell described by the matrix (2 -2, 5 3) as indicated in the model of Figure 6a. The size of the unit cell (8.8 Å × 16.7 Å) suggests that each defined cluster and its surrounding space act as the repeat unit (28) Zhao, X. Y.; Zhao, R. G.; Yang, W. S. Langmuir 2000, 169, 812. (29) Woodruff, D. P. J. Phys.: Condens. Matter 1994, 6, 6067.
for scattering. Specifically, measurement from the STM images shows that this LEED unit cell works for a hexamer cluster as shown in Figure 6a,c for either a true (110) surface or a pseudo-(100) reconstructed surface. We note that the cluster-to-cluster alignment imaged by STM, Figure 1b, in which molecules along one strand of the cluster line up with molecules on the opposite strand of the adjoining cluster is rather better modeled by adsorption on the reconstructed surface than on the unreconstructed surface. We have seen some evidence for another LEED structure with a slightly larger unit cell which may work with octamer clusters, but we have not been able to reliably capture this pattern. Nevertheless, the most noteworthy aspect of this long-range order is that the sizedefined chiral clusters self-assemble into a defined chiral array with channels of bare metal left between the chiral clusters that are themselves chiral. This suggests that the stresses that induce them must also be chiral, in a manner reminiscent of that reported for individual molecules at surfaces.30,31 It is not clear at present what drives this overall chiral assembly with intercluster distances being too large to sustain H-bonding interactions. Therefore, we propose that this assembly is mediated via through-metal effects. We have also investigated the effect of inverting the chirality of the adsorbed alanine on the supramolecular organization of this phase. RAIRS data for R-alanine show that again, the µ2 and µ3 species are present. However, due to the need to keep the methyl group away from the surface, the CCN backbone of the molecule now has a (30) Humblot, V.; Haq, S.; Muryn, C.; Hofer, W. A.; Raval, R. J. Am. Chem. Soc. 2002, 124, 503. (31) Hofer, W. A.; Humblot, V.; Raval, R. Surf. Sci. 2004, 554, 141.
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Figure 4. LEED patterns obtained for the chiral phases of S- and R-alanine on Cu(110). (a) S-alanine LEED (LEED energy, 52 eV; annealing temperature, 395 K). (b) R-alanine LEED (LEED energy, 54 eV; annealing temperature, 423 K).
Figure 5. STM images obtained for the chiral phases of S- and R-alanine on Cu(110). (c) S-alanine STM (130 Å × 130 Å; V ) -2.08 V; I ) 1.97 nA; annealing temperature, 403 K). (d) R-alanine STM (130 Å × 130 Å; V ) -2.08 V; I ) 1.04 nA; annealing temperature. 413 K).
left-handed kink when viewed from above, so the local chiral adsorption motif is the mirror image of that of S-alanine. Therefore, all supramolecular interactions should now manifest in the mirror arrangement. This indeed does occur, and STM data show the same defined hexamer or octamer chiral clusters but with self-assembly in the mirror [11 h 2] direction to give an overall chiral surface which is the mirror image of that created by S-alanine, illustrated in Figure 5. Furthermore, the ordered surface structure seen by LEED, shown in Figure 4b, is the mirror image of that obtained from S-alanine with the real space unit cell of (5 -3, 2 2) shown in Figure 6b. Again the LEED data are consistent with the clusters of six molecules acting as the repeat scattering unit, with the overall models of the R-alanine surface formed by the hexamers given in Figure 6b,d.
4. Conclusion Both S- and R-alanine are capable of forming surfaces which possess organizational chirality20 with a singlehandedness that is transmitted from the molecules to nanoscale clusters and then propogated across macroscopic length scales. An unusual feature of these systems is the size definition of the chiral clusters and their further self-assembly into a defined chiral array. Crucially, the stresses in the system also induce a regular array of chiral nanosized channels of bare metal interspersed between the chiral clusters. These channels fuse the dual advantage of reactivity with ultimate selectivity and are thus ripe for exploitation for molecular docking, molecular recognition, and stereocontrol of molecular reactions, for example, in heterogeneous enantioselective catalysis. The challenge for the future is to design metal-adsorbate
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Figure 6. Real space models and unit cells obtained for the chiral phases of S-alanine and R-alanine. (a) S-alanine on Cu(110) with the overlayer unit cell vectors ao, bo related to the substrate vectors as, bs by:
( ) (
)( )
( ) (
)( )
ao 2 -2 as ) bo bs 5 3 (b) R-alanine on Cu(110) with the overlayer unit cell vectors ao, bo related to the substrate vectors as, bs by: ao 5 -3 as ) bo bs 2 2 (c) S-alanine on the pseudo-(100) reconstructed Cu(110) surface with the overlayer unit cell the same as on the unreconstructed surface. (d) R-alanine on the pseudo-(100) reconstructed Cu(110) surface with the overlayer unit cell the same as on the unreconstructed surface.
systems with sufficient control of the relative strengths of the intermolecular and molecule-metal interactions so that the shape and size of the chiral spaces can be tuned for specific applications. However, this promise can only be delivered with continued development of experimental techniques and theoretical calculations30-34 that are capable of providing such insights at the nanoscale. (32) Barbosa, L. A. M. M.; Sautet, P. J. Am. Chem. Soc. 2001, 123, 6639.
Acknowledgment. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC), Biotechnology and Biosciences Research Council (BBSRC), and the University of Liverpool. LA049391B (33) Rankin, R. B.; Sholl, D. S. Surf. Sci. 2004, 548, 301. (34) Sljivancanin, Z.; Gothelf, K. V.; Hammer, B. J. Am. Chem. Soc. 2002, 124, 14789.
