Heterochiral to Homochiral Transition in Pentahelicene 2D

Heterochiral to Homochiral Transition in Pentahelicene 2D Crystallization Induced by Second-Layer Nucleation Anaïs Mairena,† Laura Zoppi,‡ Johannes Seibel,† Alix F. Tröster,§ Konstantin Grenader,§ Manfred Parschau,† Andreas Terfort,§ and Karl-Heinz Ernst*,†,‡ Nanoscale Materials Science, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, CH-8600 Dübendorf, Switzerland ‡ Department of Chemistry, University of Zurich, CH-8057 Zürich, Switzerland § Institut für Anorganische und Analytische Chemie, Goethe-Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany †

S Supporting Information *

ABSTRACT: Gaining insight into molecular recognition at the molecular level, in particular, during nucleation of crystallites, is challenging and calls for studying well-defined model systems. Investigated by means of submolecular resolution scanning tunneling microscopy and theoretical molecular modeling, we report chiral recognition phenomena in the 2D crystallization of the helical chiral aromatic hydrocarbon pentahelicene on a Cu(111) surface. Homochiral, van der Waals bonded dimers constitute building blocks for self-assembly but form heterochiral as well as homochiral long-rangeordered structures. 2D racemate crystals, built up by homochiral dimers of both enantiomers, are observed at coverages close to a full monolayer. As soon as the coverage leads to second-layer nucleation, the dense racemate phase in the first layer disappears and a homochiral dimer conglomerate phase of lower 2D density appears. Our results show that, at the onset of second-layer nucleation, a local change of enantiomeric composition in the first layer occurs, causing the transition from a 2D racemate to a 2D conglomerate. KEYWORDS: chirality, scanning tunneling microscopy, helicenes, optical resolution, 2D crystallization enantiomer analogues.6 However, this empirical rule, mistakenly named after Otto Wallach,7 has been challenged several times.5,8,9 Because of their potential to serve in organic electronic devices, like optical sensors or spin filters,10,11 helicenes have attracted much interest recently.12−14 For better understanding of chiral intermolecular recognition and self-assembly, model studies with chiral molecules at surfaces have become very popular.15,16 In particular, the submolecular spatial resolution of scanning tunneling microscopy (STM) has contributed valuable insight.17 Here, we report the two-dimensional (2D) crystallization of racemic pentahelicene (rac-[5]H, C22H14, Figure 1), a polycyclic aromatic hydrocarbon built up by five [a,c]-annulated benzene rings, on a copper(111) surface. Already at very low coverages, formation of homochiral pairs is observed. These van der Waals (vdW) dimers serve as

I

n 1848, Louis Pasteur reported the optical resolution of ammonium sodium tartrate into a conglomerate of homochiral crystals, meaning that only tartrate anions of the same enantiomer aggregate into the same single crystal.1 His observation of opposite sense of optical activity of aqueous solutions of opposite-handed crystals (enantiomorphs) sparked the development of stereochemistry in the late 19th century.2 Chiral molecules crystallize either as a racemate with identical numbers of both enantiomers in the unit cell, as a conglomerate of homochiral crystals, or, in rare cases, as solid solution, that is, a random distribution of both enantiomers in the crystal.3 Crystallization has become the most important method in industry for separation of chiral molecules into their enantiomers.4 Yet, more than 160 years after Pasteur’s observation, chiral recognition in crystallization is not understood at all. In particular, it is still mysterious why most chiral compounds prefer heterochiral over homochiral intermolecular recognition.5 A first explanation was given by the German mineralogist Theodor Liebisch (1852−1922) in 1894 by reporting for eight of nine evaluated compounds a density for racemate crystals that was higher than that for the pure © 2017 American Chemical Society

Received: November 4, 2016 Accepted: January 6, 2017 Published: January 6, 2017 865

DOI: 10.1021/acsnano.6b07424 ACS Nano 2017, 11, 865−871

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(M) homochiral dimer, the upper parts overlap with the lower parts of a second molecule (Figure 2). Modeling of the electron density in such a dimer via extended Hückel theory confirms the observed STM contrast (Figure S2). Only homochiral vdW dimers are observed, which is in contrast to heptahelicene ([7]H) that forms only heterochiral vdW dimers on this surface.18 In order to evaluate the exact relative alignment of the molecules and their binding to the surface, we performed theoretical modeling of the adsorbate complexes based on Amber and MM+ force field approaches as well as density functional theory (DFT; see Methods section). Force field and DFT basically led to the same results in good agreement with the experimental observations. (A more detailed comparison of the results of the different approaches is given in the Supporting Information, Figures S3−S6.) That force field calculations work well for explaining self-assembly phenomena of helicenes on metal surfaces has been shown by us previously.19−24 The lowest binding energy configuration (Figure 3) shows a partial overlap of proximal and distal

Figure 1. Space-filled molecular models of [5]H enantiomers.

building blocks for a long-range-ordered racemate structure appearing at coverages close to the saturated monolayer (ML, θ = 1.0). With further increasing coverage and onset of secondlayer nucleation, a transition into a conglomerate of homochiral 2D domains occurs. This first-layer racemate−conglomerate transition into a structure with substantial lower density is actually induced by second-layer nucleation.

RESULTS AND DISCUSSION Low-Coverage STM and Molecular Modeling. With increasing coverage of rac-[5]H, different modes of chiral organization are observed. At very low coverage, single molecules as well as homochiral pairs of [5]H are found on the surface. Figure 2 shows single (M)- and (P)-[5]H

Figure 3. Molecular modeling of [5]H dimers on Cu(111). (a) Side view on the optimized geometry of the DFT modeling calculations, showing that three proximal rings are almost parallel to the surface. (b) Full-space model based on vdW radii of the fully optimized lowest-energy configuration resulting from Amber force field calculations. (c) Ball-and-stick model of the dimer with the three proximal rings of both molecules located above different three-fold hollow sites.

Figure 2. STM image of [5]H molecules on Cu(111) at low coverage (15.3 × 15.3 nm2, I = 40 pA, U = 89 mV, T = 7 K). The three insets (2 × 2 nm2) show (from left to right) a single (M)-[5] H molecule, a M−M homochiral dimer, and a single (P)-[5]H molecule. Circular arrows indicate the sequence of topography from the upper part to the lower part of the molecules, thus revealing their absolute chirality. Besides undefined impurities and coadsorbed CO molecules (dark round protrusions), two other (P)- and one (M)-enantiomer are identified.

terminal rings of the two molecules in the dimer. The molecules are oriented such that the second proximal C6 ring is parallel to the surface plane and the two adjacent rings are bent down somewhat (Figure 3a). A similar footprint, that is, the three proximal rings oriented parallel to the surface, has been concluded from photoelectron diffraction studies of heptahelicene on Cu(111) and Cu(332).25 The perpendicular distance between the plane of the parallel second proximal ring and the z-coordinates of the Cu atoms in the topmost surface layer in DFT is 3.1 Å for the monomer as well as for the dimer (3.07 Å from MM+, 3.6 Å from Amber). This distance is identical to the one calculated for 5-aminohexahelicene on Cu(100).26 Dimer binding energies were calculated to be 5.7 kcal/mol (Amber) and 6.0 kcal/mol (DFT). The binding energy for the homochiral dimer is in the typical range of vdW interaction. The partial overlap of the terminal rings suggests π−π interactions as the binding mechanism for the dimer.27 It is larger than that calculated for π−π stacking interactions of benzene dimers,28 but here, all four terminal rings of both molecules are involved in the intermolecular interaction. The

enantiomers and a homochiral vdW dimer, in which both (M)-enantiomers are rotated by 180° with respect to each other. Single molecules appear in the STM as a round disk with a bright off-center protrusion corresponding to the location of their distal terminal ring, which also covers a proximal part of the molecule. We use proximal and distal here for the parts of a molecule closest toand farthest away fromthe surface, respectively. Such appearance allows determination of the sense of helicity (i.e., the absolute handedness of the chiral molecule; Figure 2). A clockwise sequence from the distal part via the middle part of the disk to the covered proximal part reveals the (P)-enantiomer, whereas a counterclockwise sequence identifies the (M)-enantiomer. (See Figure S1 for superpositions of molecular models with the STM contrast.) In the shown (M)− 866


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by the direction of the faint tail (representing a lower part of molecules) pointing away from the bright protrusion in a dimer (see arrows in Figure 5, for example). When defects and disordered areas are ignored, a thorough evaluation of the submolecular STM contrast in both structures leads to the conclusion that all observed dimers are homochiral and essentially constitute the building blocks for both ordered phases. The striking motif of the honeycomb structure is a hexagon built by six dimers with three distinct orientations. This structure arises in mirror domains, that is, domains that can be brought to coincide only by reflection but not by rotation and/ or translation in the plane. With respect to the highly symmetric [110̅ ] direction of the substrate surface, the domains are tilted by ±14 ± 2° (Figure S8). A closer look reveals that both mirror domains are homochiral. That is, one type is built up only by (P)−(P) dimers and the other by (M)−(M) dimers (Figure 5). Hence, these domains are enantiomorphous not just by a mirror symmetry breaking alignment but also because of a chiral crystal basis, that is, at the molecular level. Therefore, the honeycomb phase constitutes a 2D conglomerate of homochiral planar crystallites. Captured during crystallization are here and there single molecules within a hexagon (Figure 5b,c). The evaluation of their handedness does not deliver conclusive results regarding any enantioselective relation between the single molecules and the surrounding honeycomb. Like the honeycomb phase, the checkerboard phase is built up by dimers, but each single domain contains homochiral dimers of both enantiomers (Figure 5g). The pairs of bright protrusions of every second dimer are inclined differently. This appearance is well explained by a quasi-parallel alignment of all homochiral dimers but with alternating opposite handedness of their monomers (Figure 6a). The periodicity of dimers in the shown model has been determined by superposition of a Cu(111) lattice on several STM images. The structure belongs to the p2gg plane group. Because one vector of the adlattice of the checkerboard structure is aligned parallel to the highsymmetry [11̅0] direction, mirror domains are actually absent. The three-fold symmetry of the substrate rather generates three rotational domains. The alternating succession of homochiral pairs with opposite handedness makes the checkerboard structure a racemate crystal. There is no example, either in 2D or in 3D, in which homochiral vdW dimers aggregate into a racemate crystal. A model for the (P)-[5]H domain of the honeycomb structure is shown in Figure 6. The long axes of the dimers are aligned parallel to all three equivalent highsymmetry ⟨110̅ ⟩ directions within a single domain. Therefore, rotational domains are not distinguishable, and only the two mirror domains are observed. Nucleation of the Second Layer. The 2D density of the honeycomb structure is substantially lower than that of the checkerboard structure (Table 1). The pure monolayer system [5]H/Cu(111) seems to follow the above-mentioned “Wallach’s rule”; indeed with increasing coverage, the surface is increasingly covered with the denser checkerboard structure. But why is this process reversed with further deposition of molecules? The honeycomb phase is essentially only observed when the coverage exceeds the value for the dense checkerboard phase, that is, only as the second layer forms. The ordered monolayer turns then from a racemate into a conglomerate of homochiral domains, with a coexistence of both phases at coverages between θ = 1.01 and θ = 1.03. So the entire range of the transition spans only within a few percent

binding energies calculated for heterochiral dimers are substantially lower (3.7 kcal/mol, i.e., the total energy for the dimer was higher, Figure S7). The force field calculations yield “on-top” sites for the C6 rings (Figure 3b). On Cu(111), however, aromatic rings are usually oriented above a three-fold hollow site.29−32 In the DFT calculations, the centers of the three proximal benzene rings of [5]H are indeed located approximately above surface hollow sites (Figure 3c). However, these are different for both molecules of a dimer: one resides above hexagonal close-packed hollow sites, the other above face-centered cubic hollow sites (Figure 3c). Long-Range-Ordered Structures. Due to the relative high mobility of monomers and dimers at 60 K, formation of long-range-ordered structures is only revealed close to monolayer (ML) saturation coverage (Figure 4). At θ = 0.97

Figure 4. STM images (80 × 80 nm2) of the first layer at different coverages of [5]H on Cu(111). (a) Checkerboard domains (green area) are observed at θ = 0.97. The black circle shows a small patch of the honeycomb domain (I = 20 pA, U = 1.78 V). (b) Only the checkerboard structure is observed at θ = 1 ML (I = 20 pA, U = 2.27 V). (c) At θ = 1.01 ML, both phases coexist (I = 35 pA, U = 3.273 V). (d) At higher coverages (1.03 ≤ θ < 2.0), only the honeycomb structure is observed in the first layer (θ = 1.03, I = 20 pA, U = 2.84 V).

(97% of 1 ML), islands of an ordered structure are observed. It has a “checkerboard” pattern appearance and dominates the monolayer at slightly higher coverage (Figure 4b). We assign this coverage as 1 ML (θ = 1.0) because it has the highest lateral density observed for the monolayer. For coverages above θ = 1.0, formation of a honeycomb structure is observed until the checkerboard patterns completely disappeared. With the single exception shown in Figure 4a, this “honeycomb” structure is only observed when the coverage exceeds the nominal 1 ML coverage. Both long-range-ordered structures are built up by molecular pairs (Figure 5). The analysis of the absolute molecular handedness in the close-packed layers is not as straightforward as that for isolated dimers at low temperature. That is, imaging lower parts of the molecules is affected by adjacent dimers. However, the STM appearance of dimers in the long-rangeordered phases follows the same pattern as for isolated dimers. Hence, the handedness of the molecules can still be determined 867


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Figure 5. (a) STM image showing the coexistence of the checkerboard and honeycomb structures (θ = 1.02 ML, 50 × 50 nm2, I = 35 pA, U = 2.5 V). (b,c) Magnified images of the two mirror domains of the honeycomb phase. Arrows indicate weak features representing lower parts of a molecule near the bright protrusion: (b) 5.8 × 5.8 nm2, I = 95 pA, U = 1.12 V; (c) 7.7 × 7.7 nm2, I = 35 pA, U = 2.50 V. (d,e) Same as (b,c) but with indications of the handedness of the molecules. Circles mark the bright protrusions; curved arrows mark the lower molecular part spiraling down. Single domains of the honeycomb structure are homochiral. (g,f) STM images of the checkerboard structure. (f) Within a single domain (green area), two different apparent alignments of the dimers are observed (16.6 × 16.6 nm2, I = 35 pA, U = 2.50 V). (g) Highresolution STM image (7.5 × 7.5 nm2, I = 35 pA, U = 2.5 V) showing the regular coexistence of homochiral dimers of opposite handedness.

Table 1. Structural Parameters for the Two Structures of rac[5]H on Cu(111) structure

chiral composition

master matrix33

molecules/ unit cell

area/ molecules (Å2)

honeycomb

homochiral

⎛ 15 3 ⎞ ⎜ ⎟ ⎝− 3 12 ⎠

6

177.4

checkerboard

racemic

⎛8 4 ⎞ ⎜ ⎟ ⎝ 0 11⎠

4

123.9

decreases to 150 Å2, but it is still substantially larger than that for the checkerboard phase. In order to form the honeycomb phase, the coverage must exceed that of the racemic checkerboard phase. At the nominal sub-monolayer coverage equivalent to the honeycomb phase (θ = 0.7), no ordered structures have been observed here at all. STM images of the second layer are shown in Figure 7. Like for the honeycomb phase, mirror domains are observed and the orientation of adlattice vectors of both domains with respect to the [11̅0] substrate direction is identical to that of the honeycomb phase (i.e., ±14 ± 2°). Moreover, the second-layer structure seems to have, as well, hexagonal symmetry. The STM resolution for the second-layer domains achieved here is not sufficient to allow conclusions on the chiral composition of the second layer. For [7]H on Cu(111), we have observed a similar effect. That is, upon exceeding the nominal monolayer coverage, the system turned from a 2D racemate into a 3D racemate, in which the two enantiomers alternate from layer to layer.41 What is different here, however, is that the first layer undergoes a heterochiral−homochiral transition. For [7]H, the principle first-layer motif, pronounced racemic (M)−(P) zigzag rows, is still dominating, even at low first-layer coverages caused by second-layer nucleation. The driving force for this transition from the racemate to conglomerate phase must be a chiral chemical potential, a term of the Gibbs free energy due to local enantioselective mass change in the first layer. At first, the double-layer phase

Figure 6. (a) Model and superposition of model and STM image (9.5 × 9.5 nm2, I = 35 pA, U = 2.50 V) for the checkerboard structure. The red and dark blue parts highlight the distal rings of (P)-[5]H and (M)-[5]H, respectively. The high-symmetry directions of the substrate surface are indicated. (b) Model and superposition of model and STM image (8.5 × 8.5 nm2, I = 30 pA, U = 2.30 V) for the (P)-[5]H domain of the honeycomb structure. The long axes of the dimers point into three symmetry-equivalent directions (indicated by three arrows with same origin). Unit cells are presented as rectangles and parallelograms.

percent excess above single-layer coverage. Coexistence of racemate and conglomerate as well as 2D homochiral racemate transitions within the monolayer have been reported previously for prochiral molecules,34−38 chiral tartaric acid,39 and helicenes.21,40 However, here, the racemate−conglomerate transition in the first layer is only induced by second-layer nucleation. The lateral density of the conglomerate phase is only 70% of the racemate phase (Table 1). Considering the single molecules in the hexagons, the value of area per molecule 868


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Figure 8. STM image of the initial stage of the transition from racemate to conglomerate due to second-layer nucleation (θ = 1.01, 21.4 × 21.4 nm2, U = 1.90 V, I = 30 pA). The examples for patches of the checkerboard structure are marked in green. Homochiral nuclei of the honeycomb are marked with semitransparent bluefilled circles. Next to the area shown here is a more extended homochiral honeycomb domain (Figure S10).

first-layer nucleation and growth must therefore occur just after the double-layer formation.

CONCLUSIONS Submolecular resolution of STM allows the discrimination of absolute configuration of single chiral molecules and thus the study of chiral recognition in molecular monolayers. The chiral crystallization of racemic pentahelicene on a Cu(111) surface shows, in the first layer, with increasing amount of molecules, a sharp transition from a racemate crystal structure to a conglomerate phase with homochiral domains. This transition is tightly connected to nucleation of the second layer when the coverage exceeds that of the monolayer saturation. A homochiral double-layer nucleation causes excess of the other enantiomer in the first layer, thus not allowing growth of the racemate phase. Building blocks of all extended crystal structures are homochiral vdW dimers, but the racemic phase contains dimers of both enantiomers whereas single domains of the conglomerate contain dimers of only one enantiomer. This observation is in contrast to the heptahelicene/Cu(111) system, in which a strong tendency for heterochiral dimers has been reported. Although STM is too slow to follow directly nucleation and growth, it does provide insight into fundamental processes of crystallization. The effect observed here may be used to tailor molecular structure at electrode interfaces of organic electronic devices. Small excess in coverage is sufficient to completely change the first-layer structure and therefore its properties.

Figure 7. STM images of second-layer islands. (a) Two mirror domains μ and μ̅ with different orientations are formed at coverages above θ = 1 (100 × 100 nm2, I = 20 pA, U = 4.00 V). (b,c) STM images of both mirror domains (30 × 30 nm2, I = 20 pA, U = 4.00 V). Their relative alignment with respect to the [11̅0] direction (dashed line and arrow) of the Cu(111) surface is shown by white and black lines. The angle between characteristic directions of both domains (white and black lines) is 28°.

nucleates before the first layer upon cooling, that is, at higher temperature. This conclusion comes from the fact that coverages of a few percent above the closed-packed monolayer cause a substantial dilution of the first layer and the observed second-layer areas seem larger than the excess coverage beyond the closed-packed layer. Hence, this process includes transport of molecules from the first into the second layer. However, in order to initiate double-layer nucleation, it is required to have molecules already in the disordered second layer, that is, a coverage above one monolayer (θ > 1 ML). As long as there are only molecules in the first layer, only first-layer nucleation and growth occurs. A homochiral double-layer phase will cause locally the enantiomeric excess in the first layer that is required in order to explain nucleation and growth of the homochiral phase. The second layer seems to be epitaxial to the homochiral honeycomb domain. Certain features of both structures, like voids, for example, have the same periodicity (Figure S9). Homochiral nuclei are observed already at θ = 1.01 (Figure 8). Due to the fact that the other enantiomer is lacking to some extent, the racemic checkerboard phase is not viable for further growth. The homochiral pairs are then arranged into hexagons that may serve as nuclei for the honeycomb domain (Figure S10). Because the first layer is overall racemic, this scenario requires also a limit on the mean free path of diffusion in the first layer when second-layer nucleation and growth occurs. The

METHODS Pentahelicene Synthesis. 2,2′-Dimethyl-1,1′-binaphthyl was first tetrabrominated with N-bromosuccinimide and dibenzoylperoxide. This tetrabromide was cyclized to 7,8-dibromopentahelicene by potassium tert-butoxide. The total yield over both steps was about 50%. For substitution of both bromine atoms with hydrogen atoms, we first followed the route of Goretta et al.,42 removing one bromine atom with zinc and acetic acid followed by treatment with butyl lithium and hydrolysis. Because this procedure did not lead to complete removal of the bromine atoms, the two steps were repeated to obtain the desired 869


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Technologies. Topics in Current Chemistry; Springer: Berlin, 2007; Vol. 269, pp 1−313. (5) Brock, C. P.; Schweizer, W. B.; Dunitz, J. D. On the Validity of Wallach’s Rule: on the Density and Stability of Racemic Crystals Compared with Their Chiral Counterparts. J. Am. Chem. Soc. 1991, 113, 9811−9820. (6) Liebisch, T. Meeting of the Kö n igl. Gesellschaft der Wissenschaften zu Göttingen, 8, December, 1894. See: Wallach, O. Zur Kenntniss Der Terpene Und Der Ä therischen Oele 34. Liebigs Ann. Chem. 1895, 286, 119−143. (7) Ernst, K.-H. On the Validity to Call Wallach’s Rule Wallach’s Rule. Isr. J. Chem. 2016, DOI: 10.1002/ijch.201600029. (8) Dunitz, J. D.; Gavezzotti, A. Proteogenic Amino Acids: Chiral and Racemic Crystal Packings and Stabilities. J. Phys. Chem. B 2012, 116, 6740−6750. (9) Frišcǐ ć, T.; Fabian, L.; Burley, J. C.; Reid, D. G.; Duer, M. J.; Jones, W. Exploring the Relationship Between Cocrystal Stability and Symmetry: Is Wallach’s Rule Applicable to Multi-Component Solids? Chem. Commun. (Cambridge, U. K.) 2008, 1644−1646. (10) Yang, Y.; da Costa, R. C.; Fuchter, M. J.; Campbell, A. J. Circularly Polarized Light Detection by a Chiral organic Semiconductor Transistor. Nat. Photonics 2013, 7, 634−638. (11) Kiran, V.; Mathew, S. P.; Cohen, S. R.; Hernández Delgado, I.; Lacour, J.; Naaman, R. Helicenes-a New Class of Organic Spin Filter. Adv. Mater. 2016, 28, 1957−1962. (12) Shen, Y.; Chen, C.-F. Helicenes: Synthesis and Applications. Chem. Rev. 2012, 112, 1463−1535. (13) Gingras, M. One Hundred Years of Helicene Chemistry. Part 3: Applications and Properties of Carbohelicenes. Chem. Soc. Rev. 2013, 42, 1051−1095. (14) Ernst, K.-H. Stereochemical Recognition of Helicenes on Metal Surfaces. Acc. Chem. Res. 2016, 49, 1182−1190. (15) Raval, R. Chiral Expression From Molecular Assemblies at Metal Surfaces: Insights From Surface Science Techniques. Chem. Soc. Rev. 2009, 38, 707−721. (16) Ernst, K.-H. Molecular Chirality at Surfaces. Phys. Status Solidi B 2012, 249, 2057−2088. (17) De Feyter, S.; De Schryver, F. C. Two-Dimensional Supramolecular Self-Assembly Probed by Scanning Tunneling Microscopy. Chem. Soc. Rev. 2003, 32, 139−150. (18) Ernst, K.-H.; Baumann, S.; Lutz, C. P.; Seibel, J.; Zoppi, L.; Heinrich, A. J. Pasteur’s Experiment Performed at the Nanoscale: Manual Separation of Chiral Molecules, One by One. Nano Lett. 2015, 15, 5388−5392. (19) Fasel, R.; Parschau, M.; Ernst, K.-H. Amplification of Chirality in Two-Dimensional Enantiomorphous Lattices. Nature 2006, 439, 449− 452. (20) Seibel, J.; Allemann, O.; Siegel, J. S.; Ernst, K.-H. Chiral Conflict Among Different Helicenes Suppresses Formation of One Enantiomorph in 2D Crystallization. J. Am. Chem. Soc. 2013, 135, 7434−7437. (21) Seibel, J.; Parschau, M.; Ernst, K.-H. From Homochiral Clusters to Racemate Crystals: Viable Nuclei in 2D Chiral Crystallization. J. Am. Chem. Soc. 2015, 137, 7970−7973. (22) Seibel, J.; Parschau, M.; Ernst, K.-H. Two-Dimensional Crystallization of Enantiopure and Racemic Heptahelicene on Ag(111) and Au(111). J. Phys. Chem. C 2014, 118, 29135−29141. (23) Parschau, M.; Fasel, R.; Ernst, K.-H. Coverage and Enantiomeric Excess Dependent Enantiomorphism in Two-Dimensional Molecular Crystals. Cryst. Growth Des. 2008, 8, 1890−1896. (24) Fasel, R.; Parschau, M.; Ernst, K.-H. Chirality Transfer From Single Molecules Into Self-Assembled Monolayers. Angew. Chem., Int. Ed. 2003, 42, 5178−5181. (25) Fasel, R.; Cossy, A.; Ernst, K.-H.; Baumberger, F.; Greber, T.; Osterwalder, J. Orientation of Chiral Heptahelicene C30H18 On Copper Surfaces: an X-Ray Photoelectron Diffraction Study. J. Chem. Phys. 2001, 115, 1020−1027. (26) Ascolani, H.; van der Meijden, M. W.; Cristina, L. J.; Gayone, J. E.; Kellogg, R. M.; Fuhr, J. D.; Lingenfelder, M. X. Van Der Waals Interactions in the Self-Assembly of 5-Amino[6]Helicene on Cu(100)

pentahelicene in 62% yield. Due to the low activation energy for racemization (∼23 kcal/mol) and the used method of deposition (sublimation), separation of enantiomers was not attempted.43−45 Scanning Tunneling Microscopy. The single Cu(111) crystal surface was cleaned in vacuo by Ar+ ion bombardment followed by annealing at 573 K. [5]H was evaporated under ultrahigh vacuum condition (p < 5 × 10−8 Pa) from an effusion cell held at 363 K onto the clean copper crystal kept at room temperature. The coverage was adjusted by different evaporation times. The sample surface was then cooled to about 60 K and scanned using a variable-temperature STM (Omicron Nanotechnology) in constant current mode. Low-coverage STM measurements (Figure 2) were performed with a home-built instrument at 7 K. STM images were flattened and (if necessary) fast Fourier transform filtered to remove the noise using WSxM 5.0. Theoretical Methods. Surface modeling was, in part, realized using MM+ and Amber force field geometry optimization calculations of HyperChem 8.0 on a four-layer slab of Cu(111) with periodic boundary conditions. For dimer modeling, the two single molecules either two enantiomers or a homochiral pairwere placed individually at arbitrary positions on the surface of the slab. The initial positions had no influence on the finally relaxed geometry. DFT calculations were performed using an optB86-vdW functional,46,47 which accounts for dispersion effects and ultrasoft pseudopotentials, as implemented in the Quantum Espresso package (http://www.quantum-espresso. org/). The single-particle electronic wave functions and charge densities were expanded in a plane-wave basis set, up to an energy cutoff of 25 and 250 Ry, respectively. A periodic four-atom-layer slab with 27 Å of vacuum and a 2 × 2 × 1 Monkhorst−Pack k-point mesh were used. During relaxations, the bottom two layers were fixed and all other atoms were allowed to relax unconstrained until the forces on each atom were less than 0.013 eV/Å.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07424. Additional details about [5]H synthesis, STM contrast modeling, molecular modeling, and additional STM images (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Karl-Heinz Ernst: 0000-0002-2077-4922 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the Swiss National Science Foundation (Supramolecular Chiral Films) is gratefully acknowledged. We thank the University Zurich Special Priority Program LightChEC for support, and Jack Dunitz for fruitful discussions. REFERENCES (1) Pasteur, L. Recherches Sur Les Relations Qui Peuvent Exister Entre La Forme Cristalline: La Composition Chimique Et Les Sens De La Polarisation Rotatoire. Ann. Chim. Phys. 1848, 24, 442−459. (2) Ramberg, P. J. Chemical Structure, Spatial Arrangement. The Early History of Stereochemistry 1874−1914; Ashgate Pub.: Burlington, 2003. (3) Jacques, J.; Collet, A.; Wilen, S. H. Enantiomers, Racemates, and Resolutions; Krieger Pub. Co.: Malabar, FL, 1994. (4) Coquerel, G.; Tamura, R.; Yoshioka, R.; Faigl, F.; Kellogg, R. M.; Sakai, K.; Sakurai, R.; Murakami, H. Novel Optical Resolution 870


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Heterochiral to Homochiral Transition in Pentahelicene 2D

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