Two-Dimensional Crystallization of Enantiopure and Racemic

Article pubs.acs.org/JPCC

Two-Dimensional Crystallization of Enantiopure and Racemic Heptahelicene on Ag(111) and Au(111) Johannes Seibel,† Manfred Parschau,† and Karl-Heinz Ernst*,†,‡ Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

† ‡

S Supporting Information *

ABSTRACT: Insight into recognition among helical molecules is highly relevant for understanding chiral separation or biomolecular activity. The two-dimensional selfassembly of enantiopure and racemic heptahelicene has been studied on the (111) surfaces of silver and gold by means of scanning tunneling microscopy. As found earlier for racemic heptahelicene on Cu(111), the racemate forms zigzag rows with alternating enantiomers. In contrast to Cu(111), no enantiomorphism in the form of oblique alignment of molecular adlattice vectors with respect to substrate lattice vectors is observed. That is, the zigzag rows run parallel to the high-symmetry directions of the substrate. Because of the larger interatomic distances, stress builds up in the racemic molecular layer on Ag(111), which leads to relaxation into different structures after every third or fourth zigzag row. For enantiopure (M)-heptahelicene, identical structures at monolayer saturation coverage are observed, showing a pronounced transmission of chirality via azimuthal alignment of adjacent molecules. conglomerate with a racemic phase.38 There are also studies of [11]anthrahelicene and hexathia[11]helicene on various substrates.39−42 A spontaneous 2D resolution for a nonfunctionalized helicene has been reported so far only for dibenzo[5]helicene on gold(111).18 Studying self-assembly of enantiopure [7]H on Cu(111) revealed a pronounced transmission of chirality from the molecule into mesoscopic structures in the monolayer.32 For aspects of self-assembly of chiral molecules, i.e., transmission of handedness and 2D resolution of enantiomers at surfaces, one might expect an influence of different substrates. In particular, different atomic distances of the substrate surface atoms and different strength of molecule−substrate interactions should influence the stereochemical recognition between molecule and the surface and therefore the balance between intermolecular lateral interaction and molecule−surface interactions. Such structural differences of the substrate surface should then be reflected in different crystalline structures of the molecular monolayer. To evaluate the influence of different substrates with the same symmetry, we studied the 2D self-assembly of enantiopure and racemic [7]H on the Ag(111) and Au(111) surfaces and compared the results to the previously studied [7] H on Cu(111).

1. INTRODUCTION Stereochemical recognition among chiral molecules plays an important role in their resolution into enantiomers during crystallization and for the performance of liquid crystal devices.1,2 Adsorption of chiral molecules onto single-crystalline metal surfaces became an excellent model system for understanding molecular chiral recognition in these complicated processes and for better insight into the mechanisms of heterogeneous enantioselective catalysis.3−5 In combination with the use of scanning tunneling microscopy (STM), this led to the discovery of new phenomena such as chirality switching,6−10 cooperative chiral amplification and symmetry breaking,11−15 diasteriomeric recognition among different chiral species,16−18 and enantiospecific molecular recognition at chiral kinks.19,20 As for chiral crystallization in general, one of the most important questions in two-dimensional (2D) systems is whether resolution into homochiral domains occurs for certain species on certain substrate surfaces or interfaces.21,22 Besides molecules containing an asymmetric carbon atom, like amino acids for example, an interesting model system for chiral recognition are monolayers built-up by carbohelicenes.23 Ortho-annulated, π-conjugated [n]helicenes have been studied in the past because their outstanding chiroptical properties make them promising candidates for new organic devices. In particular, the 2D self-assembly of heptahelicene ([7]H, C30H18, Figure 1a) and its derivatives has been studied previously on different substrates.24−37 On Cu(111), [7]H forms heterochiral 2D crystals with the M-P enantiomers arranged in zigzag rows,24 but its polar 6,13-dicyano-derivative undergoes lateral separation into a 2D conglomerate of homochiral domains.35 On the other hand, 5-amino[6]helicene shows at the Au(111)/liquid interface a coexistence of a 2D © XXXX American Chemical Society

Special Issue: John C. Hemminger Festschrift Received: May 12, 2014 Revised: June 11, 2014

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dx.doi.org/10.1021/jp504673g | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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HyperChemTM Realease 7.1 for Windows. The [7]H molecules were aligned with the proximal phenanthrene group parallel to the surface planes and the rest of the molecule spiraling away from the surface, as found for [7]H on Cu(111).31 The final images were rendered using POV-Ray Version 3.0.

3. RESULTS AND DISCUSSION 3.1. Racemic [7]H. Ordered structures were observed only at coverages close to a saturated monolayer. At lower coverages the mobility of the molecules, even at temperatures as low as 60 K, was too high to form 2D crystalline structures. Besides disordered areas close to step edges, the close-packed monolayer of rac-[7]H on Ag(111) is composed of large domains with zigzag rows as building blocks (Figure 1b). This similarity to the Cu(111) case, where zigzag rows have been shown to consist of alternating M-P enantiomers,25 and the clear dissimilarity to the enantiopure structure (see section 3.2.) allows the conclusion that a 2D racemic crystalline phase has been formed here as well. However, on Ag(111), the zigzag rows run along highly symmetric directions of the substrate. Hence, when any molecular structure is ignored, only rotational domains and no mirror domains are observed. Such observation is in contrast to the zigzag row structure of rac[7]H on Cu(111), where, because of opposite oblique tilt angles between the zigzag rows in each mirror domain and the highly symmetric substrate directions, mirror domains were present.24 Another striking difference to the Cu(111) surface is the observation that dark lines between the zigzag rows are observed in the domains (Figure 1b and Figure S1 of Supporting Information). A closer inspection of such a line reveals a discontinuity of the zigzag motif. That is, after every third or fourth zigzag row, the molecules are differently aligned within the rows, causing a mismatch between the molecules in the discontinuity and the zigzag rows (Figure 1b, inset). Figure 1c shows an STM image taken from such a discontinuity with higher magnification. Within a single zigzag row the alternating enantiomers are identified by intramolecular contrast. Each molecule has a three-lobe appearance. The different height within a molecule in a constant-current STM image is coded by brightness. Going from the brightest to the weakest lobe reveals the different height within the molecule and thus the absolute handedness. A clockwise sequence of brightest-to-weakest lobes reveals a P-enantiomer, a counterclockwise brightest-to-weakest sequence an M-enantiomer. The possibility of such assignment shows that the molecules must be more or less oriented with their helical axis perpendicular to the surface. Such orientation has previously been found for [7]H/Cu(111) by X-ray photoelectron diffraction.31 Black lines drawn through the two brightest lobes of a molecule highlight azimuthal molecular orientations in Figure 1c. They show that within a single zigzag row the enantiomers are identically aligned, but from row to row there are variations. There are at least four different relative M-P alignments observed near a discontinuity in Figure 1c. Note that for Cu(111) only at lower coverages, zigzag structures with relative alignments different than at those saturation coverage have been identified.24 Although this discontinuity effect excludes homogeneous structures, we briefly discuss possible alignments within the zigzag rows. Figure 2 shows models for zigzag rows on Cu(111) and Ag(111). The structure for Cu(111) presents the so-called ρ-phase or (4 −1, 5 8) structure (Figure 2a).24,44 (The (2 × 2)

Figure 1. (a) Ball-and-stick molecular models for (M)- and (P)-[7]H. (b) STM image of rac-[7]H self-assembled on Ag(111) (200 × 200 nm2; U = 2.78 V; I = 11 pA). Large 2D domains with rows running parallel to the ⟨11̅0⟩ surface directions are observed. A solid yellow line (labeled RB) marks a rotational domain boundary on a single terrace. Near the step edge, labeled with an S, some disorder is observed. The inset (20 × 20 nm2; U = 1.23 V; I = 59 pA) shows that in the rows the molecules are aligned in a zigzag fashion and that different zigzag row structures are present (molecular pairs are marked with short black and green lines; the different orientations of their long axes in different rows are highlighted by dashed lines). Single molecules in the zigzag rows are circled. (c) STM image of the rac-[7] H zigzag rows on Ag(111) (10 × 10 nm2; U = −2.84 V; I = 10 pA). The handedness of the molecules is identified by intramolecular contrast. A clockwise sequence from bright to dark identifies a Penantiomer; a counterclockwise sequence the M-enantiomer. Molecules of adjacent zigzag rows have different relative M-P orientations, as indicated by lines and numbers. (d) STM image of rac-[7]H on Au(111) also showing the zigzag row motif (40 × 40 nm2; U = 2.78 V; I = 11 pA). White arrows point in the [11̅0] direction. A single molecule at the end of a zigzag row is circled.

2. EXPERIMENTAL SECTION Racemic [7]H was purchased from Chiracon GmbH, Luckenwalde, Germany. The enantioseparation of [7]H by high-performance liquid chromatography (HPLC) and metal substrate preparations have been described in more detail previously.30 Determination of the absolute sign of helicity has been performed by calculation of vibronic circular dichroic (VCD) and comparison to experimental VCD spectra taken of the pure enantiomers in solution.43 Racemic and enantiopure (M)-[7]H were deposited by sublimation in vacuo from a homemade evaporation cell held at 140 °C. The substrates were kept at room temperature during deposition and then slowly cooled to 60 K to allow 2D crystallization. The molecules were too mobile at room temperature to form ordered structures. All images were taken in constant current mode with a commercial variable-temperature scanning tunneling microscope (Omicron Nanotechnologies) at sample temperatures around 60 K under ultrahigh vacuum conditions (p ≈ 5 × 10−10 mbar). The given bias voltages refer to the sample; a positive value indicates tunneling from the tip into the sample. Molecular models were prepared with the B



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Figure 2. Molecular models for the zigzag rows of rac-[7]H. (a) Cu(111) with all molecules on identical adsorption sites. The zigzag row alignment direction (yellow line) clearly deviates from the [11̅0] direction of the substrate. (b) Same relative alignment as in panel a on Ag(111), but occupying variable adsites. (c) Imposing identical adsites causes substantial overlap between molecules (red ellipse). (d) Molecular model of M-P pair alignment into a zigzag row structure concluded from experiment. (e) Mirror image of the structure shown in panel d. (f) Alternation of mirrorrelated zigzag rows from structures in panels d and e incorporated into a single domain.

Although there is no oblique tilt with respect to the highly symmetric substrate directions, M-P zigzag rows themselves are enantiomorphous. That is, there must exist a mirror-related alignment (Figure 2e). A survey of different domains did not reveal any differences. Likewise, mirror-related M-P rows may coexist in a single domain (Figure 2f), which could explain the absence of enantiomorphism because of cancellation of the M-P enantiomorphism. Although submolecular resolution was achieved sometimes in our experiments (Figure 1c), we could not identify any M-P/M-P arrangement-based enantiomorphism, nor could we identify any mixed M-P/P-M domains. Although these model structures do not account for the variation in the orientations of molecules in adjacent parallel zigzag rows, they deliver a strong hint for the reason for this continuous deviation from a single alignment. In contrast to Cu(111), no structure with identical adsites can be constructed that is compatible with experimental results. The lateral molecular van der Waals interaction prevails and forces the molecules into their favorite close-packing. However, a mismatch with more or less favorite adsites induces a penalty that builds up with size. At a certain length scale the strain induced in the molecular layer by this substrate mismatch becomes too large to allow the favorite short-range structure. Consequently, relaxation occurs and new optimal alignments to the surface as well as to adjacent molecules are created. Although adsorption-induced stress in substrate and thin atomic layers has been studied for a long time, it is rarely observed in molecular layers.45 In the case of Au(111), zigzag rows are also observed (Figure 1d). As for Ag(111), they run along the ⟨110̅ ⟩ directions of the

transformation matrix links the adsorbate lattice vectors (b1, b2) to the substrate lattice vectors (a1, a2) via b1 = m11a1 + m12a2 and b2 = m21a1 + m22a2, is written here in the form (m11 m12, m21 m22). For phases with mirror domains we use the “master matrix” rules for naming the structure.44) It was actually identified by molecular modeling calculations.25 The enantiomorphism of the mirror domains is constituted by an opposite oblique tilt angle with respect to the underlying substrate lattice, which in turn results from two enantiomorphous relative alignments of the two enantiomers in the lattice. Because of the relative alignment on the surface, an M-P pair can be considered as a single chiral entity. Assuming the same relative alignment with strong lateral overlap for [7]H on Ag(111) and the row alignment along the [110̅ ] direction, the molecules will not occupy identical adsorption sites (Figure 2b). Such structure would have (4 0, 3 7) periodicity with respect to the substrate lattice. Enforcing identical adsites for the molecules in an M-P zigzag structure leads either to unrealistically large overlap of adjacent molecules (Figure 2c) or to very large intermolecular distances (not shown). From the accurate comparison of STM images of the clean Ag(111) surface with atomic resolution (Figure S2 of Supporting Information) and the molecular layer, a (4 0, 1 7) periodicity is determined for the zigzag structure between the discontinuities (Figure 2d). The areal density of this layer is then calculated to be 1.04 nm2/molecule, exactly the same value as determined for [7]H/Cu(111). The correct unit cell of this structure, however, would need to be at least four times larger along the ⟨112̅⟩ directions, i.e., perpendicular to [11̅0], in order to include the discontinuity. C



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shown by molecular modeling calculations for Cu(111) that this three-lobe appearance in STM is actually the result of a different azimuthal alignment of the molecules in a quasihexagonal packing. That is, the three molecules in the unit cell are rotated by 120° with respect to each other. Because the upper part is imaged as a bright lobe, this 120°-alignment causes the 3-lobe appearance. Interestingly, 5-amino[6]helicene shows basically the same structure on Au(111) after adsorption from the liquid phase.38 Enantiomorphism is expressed, as it so often is for molecular layers of chiral molecules, as oblique tilt angle of the adsorbate lattice with respect to a highly symmetric substrate direction. For all three (111) surfaces, the 3-structure of (M)-[7]H shows an identical, clockwise tilt away from [11̅0]. For Ag(111) and Au(111), the short unit cell vector has a tilt angle of 10° ± 2° with respect to the close-packed [110̅ ] surface direction (Table 1). At 90% of the monolayer saturation coverage, a different

substrate. Mirror-related alignments based on oblique tilt angles of the adlattice with respect to the substrate lattice are therefore not identified. Although the interaction with the substrate is expected to be weaker than that for Ag and Cu, a poor longrange order is observed (Figure S3 of Supporting Information). Considering identical cooling rates as for Cu and Ag, we do not attribute this poor order to kinetic effects. It can be rather seen as involving discontinuities to a larger extent. That means that commensurability would impose an even larger penalty into a layer. Because the atomic distances are not different to the Ag(111) surface, the Au(111) surface reconstruction may play here an additional role.46 In particular, structures built up by noncovalent bonded M-P pairs could be affected because a realignment of the pair is easily enforced. 3.2. Enantiopure [7]H. While the M-P zigzag motif is pervasive in the structures observed for racemic [7]H, it is the so-called 3-structure that represents the saturated enantiopure monolayer (θ = 1.0) on the (111) surfaces of Cu, Ag, and Au. Figure 3 shows this 3-fold symmetric structure observed for the M-enantiomer of [7]H on Ag(111) and Au(111). Each bright dot in the STM images represents a single molecule, as indicated by a circle in Figure 3a. Hence, the three-lobe appearance dominating the saturated monolayers on both surfaces represents a three-molecule structure. It has been

Table 1. Structural Parameters of Racemic and Enantiopure [7]H Adsorbate Systems Observed on Au(111) and Ag(111) as Well as Previously Reported for Cu(111). For Additional Structures Observed on Cu(111) and Their Parameters, See Reference 24 adsorbate system

matrix

rac-[7]H/Ag(111) rac-[7]H/Au(111) rac-[7]H/Cu(111) (M)-[7]H/Ag(111) (M)-[7]H/Au(111) (M)-[7]H/Au(111) (M)-[7]H/Cu(111)

⎛4 ⎜ ⎝1 − ⎛4 ⎜ ⎝5 ⎛6 ⎜ ⎝1

0⎞ ⎟ 7⎠

− 1⎞ ⎟ 8⎠ − 1⎞ ⎟ 7⎠

⎛ 6 − 1⎞ ⎟ ⎜ ⎝1 7 ⎠ ⎛6 2 ⎞ ⎜ ⎟ ⎝ 1 10 ⎠ ⎛ 8 2⎞ ⎜ ⎟ ⎝− 2 6 ⎠

molecules per unit cell

covered area per molecule

0°

2

1.04 nm2

0° 10.9°

− 2

− 1.04 nm2

10°

3

1.04 nm2

10°

3

1.04 nm2

5°

4

1.10 nm2

13.3°

3

0.98 nm2

tilt angle to [11̅0]

structure is observed on Au(111) (Figure 3c). Here, the molecules form clusters of four molecules, which are rotated by 90° with respect to each other (Figure S4 of Supporting Information). At somewhat higher coverage the 4-clusters coexist with the 3-cluster motif (Figure 3d). Theoretically, the ordered structure of the 4-cluster motif amounts to 95% of the saturated monolayer. All enantiopure structures show a pronounced long-range order (Figures S5 and S6 of Supporting Information). For the 3-structure on gold, more rotational domain boundaries are observed than on silver. This observation is attributed to the herringbone reconstruction, providing more nucleation sites due to the elbows of the reconstruction patterns. The herringbone reconstruction is not lifted upon adsorption and can be imaged through the molecular layer (Figure S6c of Supporting Information). The matrix notation of the 3structure adlattice on Ag(111) as well as on Au(111) is (6 −1, 1 7). In both structures surface area occupied by a single molecule is 1.04 nm2, which is an extension of about 6% compared to the 0.98 nm2 surface area of a molecule on Cu(111). The 4-structure on Au(111) has a (6 −2, 1 10) unit

Figure 3. Triplets and quadruplets in structures of enantiopure [7]H lattices on Ag(111) and Au(111). (a) STM image (20 × 20 nm2; U = 2.90 V; I = 21 pA) of (M)-[7]H on Ag(111) showing three-molecule clusters as packing motif in the saturated monolayer (θ = 1.0). A single molecule in a triplet is marked with a white circle. (b) The same triplet motif is observed in the saturated monolayer of (M)-[7]H on Au(111) (20 × 20 nm2; U = −2.90 V; I = 43 pA). Green lines highlight the tilt of the adlattice with respect to the [110̅ ] direction (arrow) of the substrate. (c) STM image (20 × 20 nm2; U = 2.73 V; I = 26 pA) of (M)-[7]H on Au(111) at a coverage of θ = 0.95, showing a different structure with four-molecule clusters as packing motif. A single molecule in a quadruplet is marked with a black circle. d) At intermediate coverage between θ = 0.95 and θ = 1.0, (M)-[7]H forms a nonordered layer on Au(111), where three- (yellow ellipse) and four- (green ellipse) molecule clusters coexist (40 × 40 nm2; U = −2.73 V; I = 35 pA). D



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cell with an angle of 5° ± 2° between the short unit cell vector and the close-packed [11̅0] surface direction. The packing density is with 1.10 nm2 per molecule approximately 5% lower compared to that of the 3-structure on Au(111) (Table 1). Because the interatomic distances of the Ag and Au surfaces are basically identical at d = 288 pm, the 3-structure on Ag(111) and Au(111) are also identical. This structure is less dense than the one observed for Cu(111) (d = 255 pm),32 but from the contrast the same relative arrangement must be assumed (Figure 4). The way the molecules are azimuthally

grid. A step closer on the grid imposes too strong a repulsion, while the step farther away allows too little attraction. Such variation could be the cause for a different result for racemate crystallization, namely, a conglomerate of homochiral domains. However, it is quite remarkable that the M-P zigzag motif is found on all three (111) surfaces and that only the long-range order is affected by the substrate metal. Hence, we can also conclude that the heterochiral M-P interaction must be substantially larger than the homochiral interaction. On the basis of the fact of higher packing density of the enantiopure 3structure with respect to the M-P zigzag structure on Cu(111), a mixing entropy penalty in the racemic nonordered layer during deposition at room temperature was brought forward to explain the observation that a racemic crystal was established instead of the more stable conglomerate.24 For Ag and for Au, the heterochiral structure is at the same density as the enantiopure structures, but no favorite adsites are identified. The heterochiral interaction not only dominates over any homochiral possibilities of lateral arrangements but also is actually so strong that it suppresses the adsorption site grid scenario with favorite adsites. At least on Ag(111) such grid limitation is “ignored” by the racemate and M-P pairs form at as close a distance as possible.

4. CONCLUSIONS The self-assembly of racemic and enantiopure [7]H has been studied on Ag(111) and Au(111). Similar adsorption motifs and 2D crystal structures as observed for Cu(111) have been found. Pairs of enantiomers form racemic zigzag rows that are aligned parallel to the highly symmetric ⟨11̅0⟩ surface directions, while on Cu(111) such rows are tilted away from these directions. Because of mismatch to the Ag(111) surface periodicity, the zigzag row structure builds up stress, leading to relaxation into different molecular alignments in every fourth or fifth row. The saturated monolayer of enantiopure (M)-[7]H forms almost identical structures on all (111) surfaces with a slightly higher packing density on Cu(111). While in the enantiopure structures all molecules prefer identical adsorption sites, the stronger heterochiral recognition forces the molecules into variable sites on Ag(111) and Au(111). Extending the study from Cu to Au and Ag with different interatomic distances therefore revealed new insight and interesting new aspects of 2D molecular self-assembly at surfaces.

Figure 4. Models for a 3-fold symmetric molecular cluster and the unit cells of (M)-[7]H on Cu(111) and Ag(111). (a) 3-fold symmetric molecular cluster on Cu(111). (b) Model for the (8 2, −2 6) unit cell on Cu(111). (c) 3-fold symmetric molecular cluster on Ag(111). (d) Model for the (6 −1, 1 7) unit cell on Ag(111). In all models the molecules occupy identical adsorption sites, which results in different packing densities for Ag(111) and Cu(111).

aligned reflects a so-called frustration lattice and has been observed also for 3D crystals of isotactic polypropylene,47 which shows that 2D model systems are indeed valuable for studying principles of crystallization. However, it is quite remarkable that this close-packing strategy still works at somewhat larger distances and especially on a given adsorption grid with different distances of potentially favored adsorption sites. It is interesting to see that basically the same density of 1.04 nm2 per molecule for all saturation structures is obtained, no matter if racemate or pure enantiomer 2D-crystallize (Table 1). Only for the densest enantiopure structure on Cu(111) does the combination of binding to favorite adsites and special frustration-alignment allow a denser phase. Moreover, the comparison of racemate crystallization and enantiopure 2D crystals reveals another surprising aspect: For the racemic structure, the lateral interaction and therefore the zigzag motif prevails over favorite adsorption sites. That is, the molecules “ignore” potential best binding sites in order to get the densest self-assembly. However, for the pure enantiomers, it is just the opposite. There the molecules favor certain sites even at lower density. Such observation reflects an important principle of crystallization on surfaces: structures are built on an adsorption

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ASSOCIATED CONTENT

S Supporting Information *

STM images of racemic and enantiopure structures (largescale), clean Ag(111) surface, and the racemic Ag(111) zigzag row structure; a model for the Au(111) 4-structure. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +41 59 765 43 63. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Swiss National Science Foundation (project Supramolecular Chiral Films, 200020/144339) is gratefully acknowledged. E



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