Heterochiral to Homochiral Transition in Pentahelicene 2D

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Heterochiral to Homochiral Transition in Pentahelicene 2D Crystallization Induced by Second-Layer Nucleation Anais Mairena, Laura Zoppi, Johannes Seibel, Alix F. Tröster, Konstantin Grenader, Manfred Parschau, Andreas Terfort, and Karl-Heinz Ernst ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b07424 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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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, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland. §

Department of Chemistry, University Zurich, CH-8057 Zürich

‡

Institut für Anorganische und Analytische Chemie, Goethe-Universität Frankfurt, Max-von-

Laue-Straße 7, 60438 Frankfurt, Germany

Corresponding Author * E-mail: [email protected]

KEYWORDS chirality, scanning tunneling microscopy, helicenes, optical resolution, 2D crystallization

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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 sub-molecular 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-range ordered structures. Close to a full monolayer 2D racemate crystals, built-up by homochiral dimers of both enantiomers, are observed. As soon as the coverage leads to 2nd layer nucleation, the dense racemate phase in the 1st layer disappears and a homochiral dimer conglomerate phase of lower 2D density appears. Our results show that, at the onset of 2nd layer nucleation, a local change of enantiomeric composition in the 1st layer occurs, causing the transition from a 2D racemate to a 2D conglomerate.

In 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 racemate with identical numbers of both enantiomers in the unit cell, as conglomerate of homochiral crystals or in rare cases as solid solution, i.e., a random distribution of both enantiomers in the crystal.3 Crystallization has become the most important method in industry for separation of chiral


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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 higher density for racemate crystals than for the pure 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 sub-molecular spatial resolution of scanning tunneling microscopy (STM) has contributed valuable insight.17 Here we report the twodimensional (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 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 2nd layer nucleation, a transition into a conglomerate of homochiral 2D domains occurs. This 1st layer racemate–conglomerate transition into a structure with substantial lower density is actually induced by 2nd layer nucleation.


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M

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

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 enantiomers and a homochiral vdW dimer, in which both (M)-enantiomers are rotated by 180° with respect to each other. Single molecules appear in STM as round disk with a bright offcenter 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, while a counterclockwise sequence identifies the (M)enantiomer. (See Figure S1 for superpositions of molecular models with the STM contrast.) In the shown (M)-(M) homochiral dimer, the upper parts overlap with the lower parts of a second molecule (Figure 2). Modeling of the electron density in such dimer via Extended Hückel theory confirms the observed STM contrast (Figure S2). Only homochiral vdW dimers are observed,


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which is in contrast to heptahelicene ([7]H) that forms only heterochiral vdW dimers on this surface.18

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. 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 Experimental section). Force field and DFT basically led to the same results in good agreement to the experimental observations. (A more detailed comparison of the results of the different approaches is given in the Supporting Information, Figure S3-S6.) That force field calculations work well for explaining


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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 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, i.e., 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 2nd proximal ring and the z-coordinates of the Cu atoms in the top most 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 binding mechanism for the dimer.27 It is larger than 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 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 threefold 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 hcp hollow sites, the other above fcc hollow sites (Figure 3c).


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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 threefold hollow sites. 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 (97% of one 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 one 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 one ML coverage.


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Figure 4. STM images (80 × 80 nm2) of the 1st 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). Both long-range ordered structures are build up by molecular pairs (Figure 5). The analysis of the absolute molecular handedness in the close-packed layers is not as straight forward as 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-range ordered phases follows the same pattern as for isolated dimers. Hence, the handedness of the molecules can still be determined 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). Ignoring defects and disordered areas, a thorough evaluation of the submolecular STM contrast in both


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structures leads to the conclusion that all observed dimers are homochiral and essentially constitute the building blocks for both ordered phases.

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 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. The striking motif of the honeycomb structure is a hexagon built by six dimers with three distinct orientations. This structure arises in mirror domains, i.e., domains that can be brought to coincide only by reflection but not by rotation and/or translation in the plane. With respect to the


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highly symmetric [1 1 0] 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, 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, i.e., 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 build 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 high-symmetry [1 1 0] direction, mirror domains are actually absent. The threefold 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 high-symmetry


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directions within a single domain. Therefore, rotational domains are not distinguishable and only the two mirror domains are observed.

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. Table 1. Structural parameters for the two structures of rac-[5]H on Cu(111) chiral master composition matrix33

structure honeycomb

homochiral

checkerboard

racemic

molec. / unit cell

area / molec.

15 3 3 12

6

177.4 Å2

8 4 0 11

4

123.9 Å2


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Nucleation of the 2nd layer. The 2D density of the honeycomb structure is substantially lower than the one of the checkerboard structure (Table 1). The pure monolayer system [5]H/Cu(111) seems to follow the abovementioned ‘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, i.e., only as the 2nd 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 1.03. So the entire range of the transition spans only within a few percent 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 1st layer is only induced by 2nd 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 decreases to 150 Å2, but it is still substantially larger than 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 2nd 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 [1 1 0] substrate direction is identical to that of the honeycomb phase, i.e., ±14°±2°. Moreover, the 2nd layer structure seems to have as well hexagonal symmetry. The STM resolution for the


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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 1st-layer undergoes a heterochiral – homochiral transition. For [7]H, the principle 1st-layer motif, pronounced racemic (M)-(P) zigzag rows, is still dominating, even at low 1st layer coverages caused by 2nd 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 nucleates before the 1st layer upon cooling, i.e. at higher temperature. This conclusion comes from the fact that coverages of few % above the closed-packed monolayer cause a substantial dilution of the first layer and the observed 2nd layer areas seem larger than the excess coverage beyond the closed-packed layer. Hence, this process includes transport of molecules from the 1st into the 2nd layer. However, in order to initiate double layer nucleation, it is required to have molecules already in the disordered 2nd layer, i.e., 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).


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Figure 7. STM images of 2nd layer islands. (a) Two mirror domains µ and μ 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 [11 0] 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°. 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 1st layer when 2nd layer nucleation


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and growth occurs. The 1st layer nucleation and growth must therefore occur just after the double-layer formation.

Figure 8. STM image of the initial stage of the transition from racemate to conglomerate due to 2nd 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 blue filled circles. Next to the area shown here is a more extended homochiral honeycomb domain (Figure S8). 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 2nd layer when the coverage exceeds that of the monolayer saturation. A homochiral


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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 in order 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. METHODS Pentahelicene synthesis. 2,2’-Dimethyl-1,1’-binaphthyl was first tetrabrominated with Nbromosuccinimide

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 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 ultra-high


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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 down to about 60 K and scanned using a variable temperatureSTM (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) FFT-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 HyperChemTM 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 ultra-soft

pseudo-potentials,

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 cut-off of 25 Ry 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 are used. During relaxations, the bottom two layers are fixed and all other atoms are allowed to relax unconstrained until the forces on each atom are less than 0.013 eV/Å. 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.


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Supporting Information Additional details about [5]H synthesis, STM contrast modeling, molecular modeling and an additional STM images. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES

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Heterochiral to Homochiral Transition in Pentahelicene 2D

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