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Mapping Complex Chiral Adlayers: A Truly Random 2-D Solid Solution of (RS)-3-Pyrroline-2-Carboxylic Acid on Cu(110)† Matthew Forster,‡,§ Matthew S. Dyer,‡,§ Steve D. Barrett,‡,| Mats Persson,‡,§,⊥ and Rasmita Raval*,‡,§ Surface Science Research Centre and Departments of Chemistry and Physics, UniVersity of LiVerpool, LiVerpool, L69 3BX, United Kingdom, and Department of Applied Physics, Chalmers UniVersity of Technology, SE-412 96, Go¨teborg, Sweden ReceiVed: July 14, 2010
The characterization of chiral molecular adlayers remains a considerable challenge, ultimately requiring detailed knowledge of the molecular chirality, the chemical form, molecular orientation, and interaction with the surface for each molecule within the assembly. In this work, we show that the (4 × 2) organization of a racemic mixture of the amino acid derivative 3-pyrroline-2-carboxylic acid (PCA) on Cu(110) may be analyzed at the single-molecule level in which the molecular chirality and adsorption footprints of each molecule are identified. Such a detailed mapping reveals the surprising outcome that within the racemic (4 × 2) organization the adlayer does not form either the 2-D racemic compound or racemic conglomerate. Instead, both the molecular chirality and adsorption footprints are randomly distributed, creating a truly random solid solution in which both molecular chirality and footprint chirality are scrambled. 1. Introduction Functionalized interfaces created by the adsorption of chiral organic molecules on surfaces have received wide attention over the past decade because of their potential technological applications in enantioselective catalysis,1-5 sensors,6,7 and molecular electronics.8-10 Amino acids, in particular, have attracted much interest because of the central role they play in biomaterial applications and in driving the enantioselective hydrogenation of β-ketoesters on Ni catalysts.11-14 In such applications, the detailed nature of the amino acid/metal interface ultimately dictates performance. Previous studies of the adsorption of amino acids on metal surfaces demonstrate that a variety of complex nanoscale architectures may be created, ranging from 1-D chains to macroscopic 2-D arrays.15-40 The complete characterization of such surface assemblies has, however, proven to be challenging because it requires detailed knowledge of not only the molecular organization but also of the chirality, conformation, and adsorption footprint of each molecule within the overlayer. We have recently demonstrated that the highly organized (4 × 2) structure of the amino acid (S)-proline (Figure 1a (i)) on a Cu(110) surface may be characterized at the singlemolecule level15 in which the conformation and bonding points to the surface (adsorption footprint) of each molecule are ascertained. This analysis relies on the structural rigidity of proline, arising because of the presence of a pyrrolidine ring and the three-point interaction of the molecule with the surface. In this bonding motif, the anionic prolate species binds to the surface via the two oxygen atoms of the carboxylate group, bonding at on-top sites on adjacent copper atoms in a closepacked row, and the nitrogen atom of the amino group, bonding †
Part of the “Alfons Baiker Festschrift”. * Corresponding author. E-mail:
[email protected]. Surface Science Research Centre, University of Liverpool. § Department of Chemistry, University of Liverpool. | Department of Physics, University of Liverpool. ⊥ Chalmers University of Technology. ‡
Figure 1. (a) (i) Chemical structure of the amino-acid proline. (ii) High-resolution STM image of a (4 × 2) unit of (S)-proline on Cu(110). (iii) The two preferred molecular conformations (A1 and B1) shown from above, from the side, and in terms of the triangular adsorption footprint. (iv) Schematic representation of the heterochiral adsorption footprint arrangement of (S)-proline on Cu(110) (solid triangles, conformer A1; open triangles, conformer B1). (b) (i) Chemical structure of the proline derivative 3-pyrroline-2-carboxylic acid (PCA). (ii) Highresolution STM image of a (4 × 2) unit of (S)-PCA on Cu(110). (iii) Arrow points to a single preferred molecular conformation (B2) shown from above, from the side, and in terms of its triangular adsorption footprint. (iv) Schematic representation of the homochiral adsorption footprint arrangement of (S)-PCA on Cu(110) (open triangles, conformer B2).
10.1021/jp1065314 2011 American Chemical Society Published on Web 09/16/2010
Mapping Complex Chiral Adlayers to a copper atom in the neighboring row. The position of the amino group relative to the carboxylate group determines the conformation. If the amino group lies to the left, then a lefthanded triangular adsorption footprint is described, and the pyrrolidine ring tilts significantly away from the surface (conformer A1, Figure 1a (iii)), imaging as a “bright” protrusion in scanning tunnelling microscopy (STM) (Figure 1a (ii)). If the amino group lies to the right, then a mirror-image righthanded triangular adsorption footprint is created and the pyrrolidine ring lies flat (conformer B1, Figure 1a (iii)) and images as a “faint” protrusion (Figure 1a (ii)). Therefore, as the molecule adopts different adsorption footprints, the ring reorients, leading to two distinct molecular conformations, causing distinct contrasts in STM images, which effectively act as a marker of local conformation and adsorption footprint. It should be noted here that the two preferred prolate conformers generate triangular adsorption footprints that are mirror images (Figure 1a (iii)) and because they are constrained in 2-D may therefore be considered chiral.20,21,23 Footprint chirality, however, should not be confused with molecular chirality, and a clear distinction of the various manifestations of chirality at surfaces is provided by Barlow et al.20,23 A molecule-by-molecule mapping of the enantiopure (S)-proline amino-acid assembly reveals that the (4 × 2) overlayer adheres to a strict heterochiral adsorption footprint pattern, where alternate molecular rows along the [001] direction adopt mirror-image footprints (Figure 1a (iv)).15 We also extended this type of analysis to the corresponding racemic (RS)-proline (4 × 2) assembly on Cu(110), where, in addition to discerning the conformation and adsorption footprint, the molecular chirality of each molecule was also identified.16 Interestingly, this reveals that the heterochiral adsorption footprint arrangement observed for the enantiopure (S)-proline adlayer persists; however, the distribution of molecular chirality within the racemic assembly is random, leading to a 2-D random solid solution,16 which is extremely rare in the solid state. Our studies of proline on Cu(110) highlighted the important role played by the adsorption footprint in dictating overlayer structure. This was further highlighted by our investigation of the effects of structural modifications on footprint preference.41 Therefore, simply by the inclusion of a double bond within the pyrrolidine ring, yielding 3-pyrroline-2-carboxylic acid (PCA), (Figure 1b (i)) it was demonstrated that the heterochiral adsorption footprint arrangement of proline is replaced by a homochiral footprint template within the (4 × 2) framework.41 Essentially, the interaction of the double bond with the surface promotes the stability of one particular conformer with a specific adsorption footprint (conformer B2, Figure 1b (iii)), in contrast with the two coexisting conformers favored by proline.15 The homochiral adsorption footprint pattern created by (S)-PCA on Cu(110) consists of a single adsorption footprint occupying two distinct orientations on the surface that are related by a 180° rotation, yielding two (S)-PCA rotamers (Figure 1b (iv)). In this work, we examine the adsorption of racemic (RS)PCA on a Cu(110) surface to understand how a racemic chiral overlayer behaves when the preferred adsorption footprints for each enantiomer is reduced from two to just one and whether such a singular footprint preference would drive the spontaneous resolution of enantiomers to yield a racemic conglomerate. To analyze such complex adlayers, we carried out a combined STM and periodic density functional theory (DFT) study to map the (RS)-PCA (4 × 2) overlayer.
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1181 2. Methods Section 2.1. STM Experiments. Experiments were conducted in an ultra high vacuum (UHV) chamber with a base pressure of 2 × 10-10 mbar. The Cu(110) surface was prepared by argon ion sputtering at 500 eV, followed by annealing to 800 K, and showed an average terrace size of 800 Å. Low-energy electron diffraction (LEED) was utilized to check the cleanliness of the sample with a sharp (1 × 1) pattern characteristic of clean Cu(110). (RS)-3-pyrroline-2-carboxylic acid (99%, Sigma Aldrich) was dosed from an electrically heated glass tube, separated from the main chamber by a gate valve and differentially pumped by a turbomolecular pump. The sample was thoroughly outgassed to ensure sample purity prior to dosing. In all experiments, the Cu(110) crystal was held at room temperature during dosing. STM images were recorded using a Specs Aarhus 150 STM operated in constant current mode with an electrochemically etched tungsten tip. The bias voltage was applied to the sample. 2.2. Periodic DFT Calculations. Calculations were carried out using the VASP periodic DFT package.42 Plane waves were used as a basis set with an energy cutoff of 400 eV. Valence electron-core interactions were included using the projector augmented wave method,43 and the generalized gradient approximation was used for the exchange-correlation functional.44 The Cu(110) surface was included as a six layer slab. Calculations of isolated PCA anions on this surface were carried out in a supercell corresponding to a (4 × 4) surface unit cell and a total height of 20.6 Å and were performed on a 4 × 3 × 1 k-point grid. Calculations of the (4 × 2) arrangements were carried out in a smaller supercell containing two anions in a (4 × 2) surface unit cell but with the same height and were performed on a 4 × 6 × 1 k-point grid. In every case, geometry relaxation was performed on the PCA anions and the top three copper layers until the force on every atom was smaller than 0.01 eV/Å. Corrections were made to the energy and electrostatic potential to compensate for periodic images of the dipole moment in the z direction. Despite the fact that the adsorbed PCA species is formally anionic, the calculations were performed in a cell without an overall charge. Charging occurs by transfer of electrons from the metal surface to the dehydrogenated PCA molecule, accompanied by a compensating screening charge within the metal substrate. Following geometry relaxation, we calculated the adsorption energy in the (4 × 4) supercell by comparing with the energies of a hydrogen atom adsorbed at the short bridge site, a PCA molecule in vacuum, and a clean copper surface calculated in the same supercell. We simulated constant current STM images in the TersoffHamann approximation45 by plotting contours of constant integrated local density of states (LDOS) from the Fermi energy to the bias potential. It is not possible to obtain absolute values of the tunnelling currents at a given tip-surface height using the Tersoff-Hamann approximation, so the LDOS plots in this study are shown with an average tip-surface distance of 7.5 Å. This is large enough to avoid the region in which there would be a chemical interaction between the tip and the sample, which is not included in this method. In addition, this tip-surface distance was utilized for the (S)-proline on Cu(110) study and yielded STM simulations that were representative of experiment.15 2.3. Image SXM Shape Analysis. The STM images were analyzed using routines in Image SXM.46 Molecules in the image that can be used as archetypes of the different conformers were first identified. The program then compares each feature
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Figure 2. (a) STM image showing the (4 × 2) organization of (RS)PCA on Cu(110) (170 × 178 Å2, It ) -0.53 nA, Vt ) -781 mV). (b) High-resolution STM image demonstrating PCA molecules imaging as a three-feature protrusion (37 × 37 Å2, It ) -0.53 nA, Vt ) -781 mV). (c) STM images of the individual (S)- and (R)-PCA molecules. (Dashed lines highlight the orientation of the triangular protrusion.)
(molecule) in the image to each of the four archetypes to identify the best match and assign a conformer to that molecule. The principal image processing and image analysis steps involve: (i) FFT filtering to calculate an FFT power spectrum, identify the lowest frequency component (LFC) of significant intensity, and remove from the image of all Fourier components
