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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Chiral Discrimination and Manipulation of Individual Heptahelicene Molecules on Cu(001) by Noncontact Atomic Force Microscopy Akitoshi Shiotari,*,† Koichi Tanaka,† Takahiro Nakae,‡ Shigeki Mori,§ Tetsuo Okujima,∥ Hidemitsu Uno,∥ Hiroshi Sakaguchi,‡ and Yoshiaki Sugimoto† †

Department of Advanced Materials Science, The University of Tokyo, 277-8561 Kashiwa, Japan Institute of Advanced Energy, Kyoto University, 611-0011 Uji, Japan § Advanced Research Support Center, Ehime University, 790-8577 Matsuyama, Japan ∥ Graduate School of Science and Engineering, Ehime University, 790-8577 Matsuyama, Japan ‡

S Supporting Information *

ABSTRACT: The adsorption configurations of heptahelicene ([7]H) molecules on Cu(001) are investigated with noncontact atomic force microscopy (ncAFM) and scanning tunneling microscopy (STM). Because of the suppression of thermal diffusion at 5 K, racemic [7]H molecules exist as monomers, dimers, trimers, and tetramers on the surface. The terminal naphthaleno part of the molecule is attached horizontally to the substrate so that the two benzene ring centers are located at the hollow sites, whereas the other terminal is protruded toward a vacuum. A procedure for picking a [7]H molecule up from the surface (vertical manipulation) enables us to functionalize the tip apex to enhance the spatial resolution of ncAFM. The ncAFM images with the helicene tip clarify that whereas the tetramers are homochiral, the dimers and trimers are heterochiral. In contrast, homochiral dimers and trimers are unobservable probably because of the rapid formation of the stable homochiral tetramers. Thus, ncAFM imaging can identify the geometries and chiralities of the individual component molecules in a nondestructive manner, which would be an indispensable method to characterize complicated chiral aggregates at the single-molecule level.

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for study; indeed, thin films of helicene derivatives have great potential for application to optoelectric/spintronic devices.13−16 Moreover, single helicene molecules are expected to be utilized as electromechanical molecular machines. Very recently, Stetsovych et al.17 showed that a helicene derivative on a metal surface has a large converse piezoelectric effect. In particular, the adsorption structures of heptahelicene ([7]H; Figure 1a) on metal surfaces have been intensively studied, and in some cases, the helicities of the individual molecules were distinguishable by STM at the single-molecule level.18 For example, an STM study of racemic [7]H on Cu(111) at a nearly saturated coverage revealed that the adsorbates create characteristic heterochiral monolayers consisting of enantiomeric (M)-[7]H−(P)-[7]H pairs.19 Such a heterochiral pair has also been observed at lower coverage as an isolated dimer; the chirality of the component molecules was successfully identified by STM manipulation with which a dimer was decomposed into two isolated molecules.20 In contrast, racemic [7]H on Cu(001) forms homochiral

INTRODUCTION Chirality, mirror-image asymmetricity of molecular geometries, is a fundamental and considerable property of molecules, which is intimately associated with various chemical, physical, and biological phenomena.1,2 Adsorption of chiral molecules on surfaces is an effective way to fabricate low-dimensional chiral systems; various chiral and prochiral molecules adsorbed on metal surfaces can be self-assembled to yield well-ordered enantiomeric clusters and layers.3,4 The structures of chiral molecular assemblies are strictly dependent on not only the molecular species and surface atomic structures, but also the temperature, molecular coverage, and enantiomeric excess.5,6 Direct observation of the assemblies with scanning tunneling microscopy (STM) has been a powerful method to identify the adsorption configurations and chirality of single molecules on surfaces,7,8 whereas chiral discrimination of each molecule in assemblies is still a great challenge. Helicene, a polycyclic aromatic hydrocarbon with a springlike geometry, is one of the most typical chiral compounds.9−12 As shown in Figure 1a, the molecular geometry is represented by a left-handed (minus; M) or right-handed (plus; P) helix, which is responsible for the chirality. Helicene and its derivatives adsorbed on surfaces have been interesting subjects © XXXX American Chemical Society

Received: January 15, 2018 Revised: February 15, 2018 Published: February 21, 2018 A

DOI: 10.1021/acs.jpcc.8b00487 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

structures were not resolved, probably because moleculeterminated tips were not used. Recently, Stetsovych et al.29 studied a [7]helicene derivative on Au(111), and the structures and chiralities of enantiomeric products yielded by on-surface thermal reaction were successfully identified by high-resolution ncAFM imaging. This report strongly suggests that the chirality of individual helicene molecules can be clarified by ncAFM as well as STM, although high-resolution ncAFM imaging of intact (unreacted) helicene molecules has not been reported so far. In this study, we observe [7]H molecules and multimers on Cu(001) with low-temperature STM and ncAFM with Cu-, CO-, and [7]H-terminated tips. The coadsorption of CO molecules is helpful for determining the adsorption sites of [7] H molecules and multimers on the surface. High-resolution ncAFM images of the adsorbates with a CO-terminated tip enable us to discriminate the chiralities. Moreover, the chirality of individual helicene molecules can be recognized by a probe of a helicene molecule itself; a [7]H molecule can be picked up and attached to the tip apex in a controlled manner, which also works as an ncAFM tip to discriminate the chiralities of the [7] H multimers on the surface. Such nondestructive determination of the geometries and chiralities of component molecules in aggregates would provide crucial insights into the intermolecular interactions and the formation mechanism of the chiral aggregates.

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EXPERIMENTAL METHODS The STM and ncAFM experiments were conducted in an ultrahigh-vacuum chamber at 5 K (Omicron low-temperature STM/ AFM system). As a force sensor, a tuning fork with an etched tungsten tip was used in frequency-modulation mode30 (resonance frequency f 0 = 24 kHz, spring constant k0 ≈ 1.8 × 103 N/m, quality factor Q ≈ 2−5 × 104). For ncAFM images, the frequency shift Δf was measured in constant-height mode at a sample bias V = 0 mV and an oscillation amplitude A = 1 Å. STM images were acquired in constant-current mode. Δf(z) curves were recorded at V = 0 mV and A = 2 Å, and the corresponding force curves F(z) were calculated by the Sader formula.31 The origin of the tip height z is the set-point height determined by STM over the bare Cu surface at V = 200 mV and I = 20 pA. z < 0 means that the tip is closer to the sample than the set-point height. Single-crystalline Cu(001) was cleaned by repeated cycles of Ar+ sputtering and annealing. The probe tip was sometimes poked slightly into the bare Cu surface so that its apex was coated with Cu atoms (i.e., Cu-terminated tip). Racemic [7]H was thermally sublimated from alumina crucibles at ∼130 °C and deposited onto the clean surface at room temperature. As needed, the surface at ∼6 K was subsequently exposed to CO gas, and a CO admolecule was picked up to attach to the tip apex (CO-terminated tip).32 The geometric optimization of a [7]H molecule in free space was performed within the density functional theory (B3LYP; 6-311G**) using GAMESS code.33,34

Figure 1. (a) Schematic illustration of [7]H adsorbates on Cu(001): a (P)-[7]H monomer, an (M)-[7]H monomer, and an (M)-[7]H tetramer, labeled “P”, “M”, and “M4”, respectively. Black (white) spheres represent C (H) atoms. (b) Typical STM image of [7]H/ Cu(001) at 5 K, acquired with a Cu-terminated tip. (c) Magnified STM image of a (M)-[7]H tetramer. (d) STM image after the lateral manipulation of one (M)-[7]H molecule in the tetramer in (c), as shown by the yellow arrow, to form a monomer and a trimer. (e) STM image of a (M)-[7]H tetramer with coadsorbed CO molecules, acquired with a CO-terminated tip. White lines indicate the lattice of the topmost Cu atoms. (f) Scheme of the adsorption site of a (M)-[7] H tetramer on Cu(001). Blue spheres indicate six-membered rings attached to the substrate. The images in parts b−e were obtained with V = 200 mV and I = 20 pA.

tetramers (right side of Figure 1a) in which four molecules with the same helicity are aggregated.21 At a nearly saturated coverage, the tetramers dominantly construct large homochiral domains (i.e., racemic conglomerates) on the surface.21 Therefore, the formability and stability of the homochiral tetramers would play a critical role in the chiral selective growth of the monolayer. In addition to STM, noncontact atomic force microscopy (ncAFM) imaging has been widely used to characterize individual molecules on surfaces,22−24 because submolecularresolution images can be achieved by using a probe tip terminated by a molecule/atom, such as carbon monoxide (CO).25 Although ncAFM studies of helicene and its derivatives have been reported,26−28 the intramolecular

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RESULTS AND DISCUSSION Figure 1b shows typical images of [7]H molecules on Cu(001) at 5 K. The coverage is ∼0.03 monolayers (ML) relative to the saturation coverage (9.1 × 1013 molecules/cm2).21 At this temperature, isolated [7]H monomers were observed together with tetramers. According to the previous study with STM at B


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The Journal of Physical Chemistry C 40 K, the homochiral tetramers existed on the surface21 whereas isolated [7]H monomers were not reported. We confirmed the temperature dependence of the observable species; conducting STM at 78 K, we did not observe isolated [7]H monomers but rather observed tetramers on the surface. This indicates that [7]H molecules excluding tetramers diffuse rapidly above 40 K. However, the thermal diffusion is inhibited at 5 K, which can reasonably explain the difference of the observable species with the previous study.21 At 5 K, we also observed middle-size multimers, i.e., dimers and trimers, on Cu(001), which have not been reported previously (the details will be described later). Chirality of the tetramer is easily identified by the shape of the STM image (Figure 1b); the homochiral tetramer is observed as a four-blade-propeller-like protrusion, and the propeller orientation originates from the helicity of the composition molecules.21 The STM image of a monomer appears as a comma-shaped protrusion, as well as one component molecule of the tetramer. Therefore, the helicity of each isolated monomer can be determined as labeled in Figure 1b. The STM appearance and the chirality assignment for the monomers are quite similar to those for isolated [7]H molecules on Cu(111).20 Our assignment of the monomer is also supported by decomposition of the tetramer using STM manipulation as follows. After the tip was fixed over the bare Cu surface, the feedback loop was open, and the tip height was set closer (z = −0.5 Å). At the constant tip height, the tip was moved parallel with the surface along the yellow arrow in Figure 1c. One of the molecules constituting a (M)-[7]H tetramer was then dragged by the STM tip, while the residual molecules remained at the original locations (Figure 1d). The STM appearance of the decomposed (M)-[7]H molecule was consistent with that of an intact (M)-[7]H monomer assigned as above (Figure 1b). We next determined the adsorption site of the [7]H adsorbates with the help of coadsorbed CO molecules. Figure 1e shows an STM image of a (M)-[7]H tetramer surrounded by several CO molecules. It is well-known that CO is imaged as a round protrusion with STM with a CO-terminated tip32 and is adsorbed vertically at the atop site of Cu(001).35 The relative positions to CO molecules indicate that the center of the tetramer is located at the hollow site. The structure model of the (M)-[7]H tetramer on Cu(001) is shown in Figure 1f. The relative positions between the component [7]H molecules are determined by a reasonable packing model in the saturated monolayer,21 which is validated by AFM images as will be described later. For each [7]H molecule, the two terminal benzene rings are bonded flatly to the substrate so that their ring centers are located at the adjacent hollow sites (see the blue spheres in Figure 1f). This adsorption site is in disagreement with the previous tentative assignment in which the center of the terminal benzene ring was considered to be located just above the topmost Cu atom.18,36 In contrast, various PAHs, such as benzene,37,38 naphthalene,39 and pentacene,38,40,41 on Cu surfaces prefer the hollow-site adsorption of their benzene rings, which supports our determination. Figure 2a shows the adsorption site of an isolated monomer, which is the same as that of the component molecule of the tetramers. The location and orientation of the protruded part (i.e., the terminal benzene ring at the vacuum side) are uniquely determined by the location of the surface-attached part and the helicity of the molecule, and vice versa (Figure 2b). The

Figure 2. (a) Scheme of the adsorption site of a (P)-[7]H monomer on Cu(001). Blue spheres indicate six-membered rings attached to the substrate. (b) Space-filling model of a (P)-[7]H monomer on Cu(001). (c) STM image (V = 100 mV and I = 20 pA) and (d−f) ncAFM images [z = (d) +3.4, (e) +2.4, and (f) +1.4 Å] of a (P)-[7]H monomer acquired with a CO-terminated tip. Cyan spheres indicate the projected locations of atoms at the protruded region of the monomer.

geometrical information on the protruded part can be clarified by STM and ncAFM images. The intramolecular structure in the STM appearance of [7]H is somewhat emphasized by using a CO-terminated tip (Figure 2c), which is a typical feature of the tip functionalization.40,42−44 The image shape reflects the electronic states mainly of the protruded part (the cyan spheres in Figure 2b,c).21 The tip functionalization also has a further advantage that ncAFM images reflect the atomic structures within the adsorbate molecules. Figure 2d−f shows a series of the ncAFM images acquired at several tip−sample distances. At a large distance, the protruded part of [7]H is imaged as a dark (i.e., high |Δf |) region due to an attractive interaction between the CO-terminal tip and the adsorbate (Figure 2d). At a closer tip height to the adsorbate, a bright (i.e., low |Δf |) region appears, indicating that a repulsive force is predominant above the most protruded atoms (the bold cyan spheres in Figure 2e), whereas an attractive force between the other atoms in the molecule and the CO tip yields a dark halo in the image. At an additional close distance, some of the atoms in the second protruded benzene ring also feel the repulsive force to the CO, yielding a bright extended shape in the ncAFM image (Figure 2f; see also Figures S1 and S2 in the Supporting Information). In contrast to ncAFM images of planar hydrocarbon molecules, which clearly reflect carbon skeletons,25,45 in the case of nonplanar molecules on surfaces, only a portion of the atoms that protrude toward the vacuum are generally resolved.46−49 Some previous studies demonstrated that slightly tilted carbon skeletons can be imaged by specific scanning methods other than constant-height mode;50,51 however, we were unable to visualize the whole atomic structure of [7]H because (i) the protruded benzene rings of this molecule are steeply inclined from the surface plane and (ii) this molecule was easily displaced by the tip at a short tip−sample distance (see Figure 1c,d). In addition to the “lateral” manipulation of [7]H (Figure 1c,d), we can also perform “vertical” manipulation, that is, picking a [7]H molecule up from the surface. Figure 3a,b shows C


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“sharpened” tip probably causes a weaker van der Waals interaction with the surface. At z < −2 Å, the force between the [7]H tip and the Cu surface is weakened due to the short-range repulsive force (green curve in Figure 3d). This implies that the [7]H tip is rigid and stable even at the short tip−sample distance, in contrast to the [7]H molecules on the surface which can be manipulated. In fact, backward manipulation, i.e., putting the attached [7]H molecule down to the surface, rarely succeeded because of the strong interaction between the Cu tip apex and the attached molecule. For high-resolution ncAFM imaging, a target molecule on a surface can itself be used for the tip termination; for example, the intramolecular structure of a pentacene admolecule can be visualized with a pentacene-terminated tip.25 Because of the stable configuration, the [7]H-terminated tip is expected to be useful as a probe to obtain ncAFM images. For reference, we first used a CO-terminated tip to confirm the structure of two kinds of [7]H tetramers by using STM (Figure 4a,b) and

Figure 3. (a) STM image of an (M)-[7]H monomer (indicated by a yellow arrow) together with an (M)-[7]H tetramer on Cu(001), acquired with a Cu-terminated tip (V = 200 mV and I = 20 pA). (b) STM image after vertical manipulation of the monomer, yielding a helicene-terminated tip. A yellow circle represents the original position of the monomer. Insets of parts a and b are the schematic illustrations of the process for picking up the monomer. (c) Frequency shift curves for the picking-up process. A solid red (dashed green) curve represents Δf(z) recorded over the monomer in part a as the tip approached (retracted from) the substrate. As a reference, the Δf(z) curve recorded over the bare Cu surface is also shown as a black dotted curve. (d) Force curves recorded by the Cu- or [7]H-terminated tips tCu/helicene over the bare surface or [7]H adsorbate sCu/helicene.

STM images before and after moving a Cu-terminal tip closer to a (M)-[7]H monomer (yellow arrow in Figure 3a). By the sequence of the tip movement, the monomer vanished and the appearance of the residual tetramer was modified (right bottom in Figure 3a,b), suggesting the monomer is picked up from the surface and attached at the tip apex. Figure 3c shows Δf signals during the manipulation procedure. As the tip height decreases, Δf drops down significantly (red curve in Figure 3c) due to the strong attractive force between the tip and the adsorbate (red curve in Figure 3d) relative to the force between the tip and the bare Cu surface (black curve). Eventually, at a certain distance (z ≈ − 0.8 Å), the Δf signal suddenly jumped because the tip− sample interaction was significantly changed. After the pickingup procedure, we measured a further Δf(z) curve with the modified tip over the identical point (yellow circle in Figure 3b) and converted it into an F(z) curve (green curve in Figure 3d). The distinct difference between the red and green curves in Figure 3d suggests that the bonding structure of [7]H at the tip apex is quite different from the adsorbate configuration on the surface. We also show other examples of the vertical manipulation in the Supporting Information (Figure S3), indicating a similar tendency. According to previous studies on [7]H−metal complexes, a [7]H molecule can envelop a metal atom in the manner of tweezers.52,53 Referring to the complexes, therefore, we propose that a groove of the [7]H molecule strongly grips the apical Cu atom of the tip (inset of Figure 3b) as presumed previously.20 According to the force curves in Figure 3d, at any z, the attractive force (probably mainly van der Waals force) between the Cu surface and the [7]H tip is less than that of the Cu tip and the [7]H adsorbate. This is in line with the tip structure model (inset of Figure 3b); at the apex of the [7]H tip, an upright benzene ring protruded toward the surface. The

Figure 4. (a, b) STM images of (M)- and (P)-[7]H tetramers, respectively, with a CO-terminated tip (V = 200 mV and I = 20 pA). (c, d) NcAFM images of the (M)- and (P)-[7]H tetramers, respectively, with the CO-terminated tip (z = 2.1 Å). (e, f) NcAFM images of (M)- and (P)-[7]H tetramers, respectively, with the [7]Hterminated tip (z = (e) 3.2 and (f) 3.1 Å). (g, h) Space-filling models of (M)- and (P)-[7]H tetramers, respectively, on Cu(001). Red and cyan spheres indicate the projected locations of atoms at the protruded region of the (M)- and (P)-[7]H tetramers, respectively.

ncAFM (Figure 4c,d; see also Figure S2 in the Supporting Information). These images show the intramolecular structures of the component molecules and clarify the chirality. As in the case of a monomer (Figure 2e), bright and dark portions of the ncAFM images (Figure 4c,d) correspond to the location of the first and second most protruded benzene rings, respectively. Therefore, we determined the geometries of the tetramers as shown in Figure 4g,h; the resulting geometries were in good agreement with the structure of the building block in the saturated monolayer21 with the exception of the adsorption sites (Figure 1f). Next, we observed (M)- and (P)-[7]H tetramers with the [7]H-terminated tip that was fabricated by the vertical manipulation shown in Figure 3. Figure 4e,f shows the corresponding ncAFM images, which are quite similar to that with the CO tip. Thus, the helicene tip can distinguish the chirality of individual helicene adsorbates. Compared with the entire C4-symmetric AFM images with the CO tip, however, the appearances of the four component molecules are slightly asymmetric in the image with the [7]H tip. This is because the apical atomic structure of the [7]H tip is more complicated than that of the CO tip (see the inset of Figure 3b), which D


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5a), the three protrusions seem to connect in an “L” shape with separations of ∼10 and ∼9 Å. This difference in the separations implies that the dimer and trimer have different packing structures from the tetramer; however, the helicities of the component molecules in dimers and trimers are hardly identifiable from the STM images. Next we observed the multimers by ncAFM with a [7]Hterminated tip (Figure 5b). We confirmed the tip quality by imaging both (M)- and (P)-[7]H tetramers so that we could readily discriminate between them (see Figure 4e,f). On the basis of the ncAFM images of the tetramers, the appearances of the individual component molecules could be discriminated, as shown by the red and blue enclosures in Figure 5b. Because there are eight combinations for the locations and orientations of the bright and dark parts in the ncAFM image, the asymmetric enclosure corresponds uniquely to (M)- or (P)-[7] H with one of four possible orientations. Thus, the helicities and orientations of the component [7]H molecules in the multimers are successfully assigned as shown in Figure 5b. All of the dimers we observed consist of one (M)-[7]H and one (P)-[7]H. Such heterochiral dimers were also observed on Cu(111).20 Here we focus on the three dimers labeled in Figure 5a: dimers α, β, and γ. As shown in Figure 5b, the left- and right-side molecules in α are assigned as (P)- and (M)-[7]H molecules, respectively. The adsorption structure is shown in the upper panel of Figure 5c,d. Although the two molecules are located in close proximity so that they overlap as much as possible, the separation is larger than in the homochiral tetramer. We note that the relative position and orientation of the helicene pair are strikingly similar to those of heterochiral pairs in racemic [7]H monolayers on Cu(111)19 and Ag(100).54 In dimer β, the left and right molecules are assigned as (M)- and (P)-[7]H molecules, respectively (bottom panel of Figure 5c,d). We easily find that α and β have mirror symmetry with respect to the [100] direction, which is represented by a green line in Figure 5c,d. Two kinds of orientations in these dimers can be explained by the fact that the heterochiral dimers show enantiomerism on the surface (purple and orange arrows). For example, dimer γ has the same chirality as dimer β, because γ is identical to 180°-rotated β (orange arrow in Figure 5b). In other words, α is an enantiomer of β and γ. The trimers shown in Figure 5b also include both (M)- and (P)-[7]H molecules, while no homochiral trimer was observed. Figure 5e,f shows the geometric structure of the trimer labeled δ (Figure 5a). In this trimer, the homochiral-pair part (i.e., the two adjacent (M)-[7]H molecules in Figure 5e,f) has the same stacking structure as the two adjacent molecules in the tetramer (see the right side of Figures 4h and 1f). In contrast, the heterochiral-pair part (for trimer δ, (P)- and (M)-[7]H molecules in Figure 5e,f) has the same geometry as the dimer (Figure 5c,d). It is noteworthy that neither a homochiral dimer nor a homochiral trimer was observed on the surface in spite of the predominant formation of the homochiral tetramers. As shown in Figure 1d, a homochiral trimer can be formed by STM manipulation at 5 K, but it is not an intrinsic species derived thermally. Above 40 K, all [7]H molecules but tetramers are rapidly diffused on the surface,21 and therefore, homochiral dimers and trimers are expected to readily encounter additional monomers with the same chirality to yield a homochiral tetramer. Once the tetramer forms, it is strongly attached to the substrate and is rarely decomposed because of the strong

probably causes multiple interactions between the atoms of the tip-attached molecule and the atoms on the surface. Nevertheless, the force between the topmost atoms of the tip and surface is dominant and leads to the high spatial resolution. We also confirmed that the appearances are not affected by the chirality of the [7]H molecule attached at the tip apex but rather by the configuration of just the topmost atoms (see Figure S4 in the Supporting Information). Using a [7]H-terminated tip, we demonstrate the chiral discrimination of various [7]H multimers on Cu(001) without coadsorption of CO molecules on the surface. As the coverage increases, the [7]H molecules are aggregated to form predominantly multimers. Figure 5a shows an STM image of

Figure 5. (a) STM image of [7]H/Cu(001) at high coverage with a Cu-terminated tip (V = 500 mV and I = 10 pA). (b) NcAFM images of the molecules shown in part a with a [7]H-terminated tip (z = 3.1 Å). The STM image in part a is superimposed by the ncAFM images of the smaller areas. Red and blue enclosures represent rough outlines of the STM images for (M)- and (P)-[7]H molecules, respectively. Purple and orange arrows indicate the directivity of the “M → P” and “P → M” dimers, respectively. (c) Space-filling models of [7]H dimers on Cu(001). The upper (bottom) dimer corresponds to α (β) labeled in part a. (d) Scheme of the adsorption site of the dimers in part c. Red and blue spheres indicate six-membered rings of (M)- and (P)-[7]H molecules, respectively, attached to the substrate. (e) Space-filling model of a [7]H trimer corresponding to δ in part a. (f) Scheme of the adsorption site of the dimers in part e.

[7]H/Cu(001) at a coverage of 0.24 monolayers. At this coverage, 46%, 26%, and 27% of molecules exist as dimers, trimers, and tetramers, respectively, whereas only 1% of molecules are isolated on the surface. Multimers larger than tetramers have never been observed, which is in good agreement with the fact that the tetramers are stabilized by effective steric packing between the four homochiral molecules with specific orientations.21 In the STM image, the dimers seem to have two kinds of orientations; for example, the orientation of the dimer labeled α in Figure 5 is not identical to that of the dimers labeled β and γ. A dimer is imaged as a twin protrusion with a separation of ∼10 Å, which is larger than the distance between the nearest neighboring protrusions in the tetramer (∼8 Å). According to the STM image of a trimer (δ in Figure E


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attractive interactions between the sterically overlapped molecules. We suggest that heterochiral dimers and trimers are metastable and require some activation energy to remove the component molecule with the opposite chirality, which retards the formation of a homochiral tetramer. In contrast, at a much higher coverage near the saturated conditions, interactions between multimers may induce the formation of homochiral layers consisting of tetramers.21

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CONCLUSIONS With STM and ncAFM, we observed [7]H monomers, dimers, trimers, and tetramers on Cu(001) and discriminated the chiralities and orientations of individual molecules. A [7]H molecule can be picked up from the surface to strongly attach to the Cu tip apex. The [7]H-terminated tip is utilized as a probe for high-resolution AFM imaging as well as a COterminated tip. With the helicene tip, we can discriminate the chiralities of [7]H multimers; the dimers and trimers are heterochiral whereas the tetramers are homochiral. Therefore, we suggest that homochiral dimers and trimers have a very short lifetime due to the rapid formation of homochiral tetramers, whereas heterochiral dimers and trimers are metastable and observable at a low coverage.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00487. Tip-height dependence of the AFM images of the [7]H monomer and tetramer with a CO-terminated tip, other examples of the vertical manipulation of [7]H, and AFM images with (M)- and (P)-[7]H tips (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 (0)4 71363997. ORCID

Akitoshi Shiotari: 0000-0002-8059-3752 Takahiro Nakae: 0000-0002-7653-432X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the “Joint Usage/Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE29B-21).” The authors acknowledge the support by JSPS KAKENHI Grants JP16H00959, JP25110003, JP16H00967, JP17H05154, JP17K19024, JP16K17893, JP16K13680, JP16H00933, and JP15H03566. Y.S. acknowledges the support of the Iketani Science and Technology Foundation, the Noguchi Institute, the Japan Association for Chemical Innovation, the Murata Science Foundation, and the Hattori Hokokai Foundation.

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