Anthracene and Tetracene on Au(111) - ACS Publications - American

Article pubs.acs.org/JPCC

Molecular Self-Assembly Driven by On-Surface Reduction: Anthracene and Tetracene on Au(111) Justus Krüger,† Frank Eisenhut,† Thomas Lehmann,† José M. Alonso,‡ Jörg Meyer,† Dmitry Skidin,† Robin Ohmann,†,# Dmitry A. Ryndyk,† Dolores Pérez,‡ Enrique Guitián,‡ Diego Peña,‡ Francesca Moresco,*,† and Gianaurelio Cuniberti†,§ †

Institute for Materials Science, Max Bergmann Center of Biomaterials, and Center for Advancing Electronics Dresden, TU Dresden, 01069 Dresden, Germany ‡ Centro de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, Santiago de Compostela 15782, Spain § Dresden Center for Computational Materials Science (DCMS), TU Dresden, 01069 Dresden, Germany S Supporting Information *

ABSTRACT: Epoxyacenes adsorbed on metal surfaces form acenes during thermally induced reduction in ultrahigh vacuum conditions. The incorporation of oxygen bridges into a hydrocarbon backbone leads to an enhanced stability of these molecular precursors under ambient condition; however, it has also a distinct influence on their adsorption and selfassembly on metal surfaces. Here, a low-temperature scanning tunneling microscopy (LTSTM) study of two different epoxyacenes on the Au(111) surface at submonolayer coverage is presented. Both molecules show self-assembly based on hydrogen bonding. While for the molecules with a single epoxy moiety nanostructures of three molecules are formed, extended molecular networks are achieved with two epoxy moieties and a slightly higher surface coverage. Upon annealing at 390 K, the molecules are reduced to the respective acene; however, both systems keep a similar assembled structure. The experimental STM images supported by theoretical calculations show that the self-assembly of the on-surface fabricated acenes is greatly influenced by the on-surface reaction and strongly differs from the adsorption pattern of directly deposited acenes, highlighting the importance of the cleaved oxygen in the self-assembly.

I. INTRODUCTION On-surface chemistry provides indispensable tools for the formation of extended molecular structures which cannot be prepared by standard solution-based methods due to limited solubility and chemical stability.1−4 In particular, the atomically precise and controllable synthesis of nanographene structures has been made possible via on-surface reactions of specifically designed precursors.5,6 By reporting on the formation of tetracene,7 we have added on-surface reduction of epoxyacenes as a novel reaction to the toolbox which allows the generation of polycyclic aromatic hydrocarbons with linearly fused benzene rings. This class of molecules is known as acenes and features compelling properties for applications in molecular electronics and spintronics.8 However, large acenes are highly reactive under ambient conditions and therefore call for novel preparation methods. Our recent work on the synthesis of hexacene9 and unprecedented decacene10 on Au(111) has demonstrated that the surface-assisted deoxygenation of specifically designed epoxyacenes is a synthetic method to access acenes beyond the boundaries set by stability, while the conversion of α-diketone precursors11 was also shown to be a promising surface strategy. High-symmetry acenes tend to form densely packed and well-structured films on coinage metal substrates;12−15 © 2017 American Chemical Society

however, the incorporation of epoxy groups breaks the symmetry and favors directed interaction. In this article, we focus on the role of this moiety at the terminal rings and aim to describe the distinct influence on the molecular self-assembly for two different epoxyacenes. First, we study the epoxyanthracene An1O (see Figure 1a) with an epoxy group at one terminal ring. This allows the detailed understanding of the adsorption geometry and modeling of the observed nanostructures. Once the system is comprehensively described, the experimental observations for the diepoxytetracenes Tn2O are addressed. Notably, the presence of two epoxy groups in either syn or anti-configuration (see Figure 1b) leads to a more complex system. Scanning tunneling microscopy (STM) based studies of the self-assembly and growth mode of molecules on metal surfaces are particularly suited to shed light on the influence of functional groups for intramolecular bonding and the molecule−substrate interactions.16,17 Received: June 22, 2017 Revised: August 16, 2017 Published: August 28, 2017 20353

DOI: 10.1021/acs.jpcc.7b06131 J. Phys. Chem. C 2017, 121, 20353−20358

Article

The Journal of Physical Chemistry C

Figure 1. Chemical structure, adsorption geometry as determined by DFT, and reduced form of the investigated molecules. (a) An1O molecule which is reduced to anthracene upon annealing. (b) The two diastereomers of Tn2O (syn and anti) and tetracene as a product of the on-surface deoxygenation.

II. METHODS STM experiments were performed using a custom-built instrument operating at a low temperature of T ≈ 5 K and ultrahigh vacuum (p ≈ 1 × 10−10 mbar) conditions. All STM measurements were acquired in constant-current mode with the bias voltage applied to the sample. A Au(111) single crystal was used as substrate and prepared by repeated cycles of sputtering (Ne+) and annealing (730 K). After this cleaning procedure, the molecules were evaporated from a Knudsen cell at a crucible temperature of 320 and 410 K for An1O and Tn2O, respectively. In both cases, the molecules were deposited on the clean Au(111) surface kept at room temperature. Temperature-induced experiments were carried out by annealing the sample at the respective temperature for 5 min. Density functional theory (DFT)18,19 was used to study adsorption and self-assembly by obtaining minimum energy configurations of individual molecules and trimers. We applied the Perdew−Burke−Ernzerhof exchange-correlation functional20 and valence double-ζ basis sets with Goedecker−Teter− Hutter pseudopotentials21 in the hybrid Gaussian and plane wave approach embedded in the Quickstep code of CP2K.22 The DFT-D3 method of Grimme23 accounted for dispersion correction, which adds a sum of pairwise interatomic C6R−6 correction terms to the Kohn−Sham energy. Furthermore, STM images were simulated based on the Tersoff-Hamann approach. The Au(111) surface was modeled by a periodic slab of six layers.

Figure 2. STM image of An1O molecules as deposited on Au(111) while keeping the surface at room temperature. (a) Overview STM image acquired at 0.5 V and 80 pA. Two mirror symmetric An1O trimers as well as isolated molecules are visible. (b) STM image (V = 0.1 V, I = 100 pA) of one isolated An1O molecule as well as one An1O trimer. (c) DFT-based modeling of the trimers. (d) Stabilization based on hydrogen bonds between oxygen and hydrogen as indicated by dashed lines.

used to prove that the relaxed structure of the An1O trimer corresponds to a local minimum in the potential energy surface. The calculated geometry, as presented in Figure 2c, suggests that the trimers are stabilized by three hydrogen bonds formed by intermolecular interactions between electronegative oxygen and hydrogen at the sp3-hybridized bridgehead carbon atom as shown in Figure 2d. The structure is confirmed by correctly reproducing intermolecular angles and coinciding simulated and experimental STM images. After studying the surface subsequent to the room-temperature deposition, we performed thermally induced experiments to investigate changes in the observed structures upon induced deoxygenation. Figure 3 shows the different adsorbed molecular species. Imaging the substrate after heating at 340 K allows the direct comparison between isolated An1O and anthracene molecules as the reduction was not yet achieved at a very high rate. An1O molecules were identified by their characteristic teardrop shape, while on-surface produced anthracenes appeared flat with a clearly reduced apparent height (see color scale in Figure 3a). Similar to the An1O molecules, anthracene molecules were found to be either isolated or in a trimer assembly. Figure 3a shows all prevailing motifs for this preparation in one close-up STM image. However, it is not evident how the structure is stabilized. Since the on-surface interaction between hydrocarbon molecules without any functional groups should not favor directed intermolecular forces, it is generally based on nondirected van der Waals forces which cannot explain the equiangled shape.24 DFT revealed a metastable trimer configuration of three anthracene molecules

III. RESULTS AND DISCUSSION An1O Molecules. The An1O molecules (Figure 1a) were deposited with a surface coverage of ∼0.2 ML onto the Au(111) surface kept at room temperature. STM images show that molecules on terraces are isolated or form triangularshaped nanoassemblies (with two different chiral enantiomeric motifs δ and λ) as presented in Figure 2a. The former can be recognized by a teardrop-shaped topography which corresponds to a nonplanar adsorption geometry. Density functional theory (DFT) calculations prove that single molecules preferably adsorb with the oxygens facing the surface, and two C−H groups are upright, giving rise to a distinctly enhanced apparent height at one side of the molecule as visible in the close-up image of Figure 2b. The triangular nanostructures are formed by self-assembly of three An1O molecules with nonplanar ends coming close to each other and the 3-fold symmetry of the structure being favored by the underlying Au(111) substrate. DFT calculations have been 20354


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hollow sites. The obtained radius coincides with the minimum energy periphery of the three oxygens (see Supporting Information for more details). After annealing the An1Ocovered sample at 390 K, no precursors were observed anymore but isolated anthracene molecules and An trimers only. Tn2O Molecules. Tn2O molecules were deposited on Au(111) with a slightly higher coverage of ∼0.6 ML; however, the principal experimental observations for the as-prepared surface were in line. First, single Tn2O molecules adsorb with a nonplanar geometry and were accordingly imaged as dumbbellshaped protrusions (see Figure 4). Note that Tn2O is a mixture

Figure 3. An1O-covered Au(111) surface after annealing at 340 K. (a) Close-up STM image (0.1 V, 80 pA) showing prevailing structures observed on the surface. By superimposing the molecular long axis of single molecules onto the observed trimers, the triangular nanostructures with clearly different molecular orientations are elucidated. (b) Calculated STM images of both An and An1O trimer assemblies. (c) Minimum energy configuration of an anthracene trimer with three cleaved oxygen adatoms close-by obtained by DFT calculations. (d) Calculated potential energy plotted over molecule to trimer center distance rMC as depicted in (c) with fixed oxygen positions. The minimum at 3 Å corresponds to the stabilization of the An trimer due to the cleaved oxygen adsorbates close-by.

Figure 4. STM images of Tn2O molecules as deposited onto the Au(111) surface. (a) STM image acquired at V = 0.1 V and I = 100 pA showing one isolated molecule as well as dimeric and trimeric patterns. (b) STM image (V = 1 V, I = 100 pA) showing regular hexagons. (c) Large-scale STM image (V = 1 V, I = 100 pA) of complex networks including regular honeycomb domains.

of syn and anti diastereomers based on the relative configuration of the oxygen atoms on the 1,4-epoxy moieties, and therefore it can appear in a symmetric or antisymmetric configuration on the surface. In contrast to An1O, where isolated nanostructures are formed, the Tn2O molecules build extended networks. This is attributed to the second oxygen atom, favoring directed interaction on both ends of the molecule. Therefore, different bonding motifs are observed. The dimeric features an off-center arrangement, and the trimer motif is equivalent to the triangular nanostructure observed for An1O. Further, we find hexagonal motifs, which are composed of six Tn2O molecules forming trimeric or dimeric patterns, with an inner hexagon of six Tn2O molecules. Figure 4b singles out two mirror symmetric examples where individual molecules are marked with white bars. Large-scale scans (Figure 4c) show that one can occasionally find honeycomb domains formed by several adjacent hexagons to establish an open-porous 2D network. This network can be interpreted as a Kagome lattice as will be discussed in more detail later. Hence, a simple, selective, and directed hydrogen bond interaction gives rise to a complex and intriguing self-assembled network which is not limited by the underlying surface reconstruction. The size of

which is highly unfavorable compared to parallel alignment and unstable in low-temperature molecular dynamics. By considering possible intermolecular interactions which can be pertinent at metal surfaces25 and bearing in mind the induced surface reaction, it seems likely that this kind of assembly of three anthracene molecules is supported by the byproduct of the onsurface reduction, i.e., cleaved oxygen atoms. Starting from an An1O trimer, the molecules adsorb in such a way that all three oxygen atoms face the surface and are positioned at the center of the trimer. Upon thermally induced deoxygenation, the three cleaved oxygen atoms adsorb on a surface hollow site according to DFT calculation and remain presumably close to the initial adsorption position. Employed simulations clearly show that three oxygen adsorbates stabilize the trimer by providing hydrogen bond anchors on the surface, as depicted in Figure 3c. This effect has been quantified by the change in total potential energy while uniformly approaching three anthracene molecules toward a common center with and without oxygen adatoms (see Figure 3d). Whereas the energy steadily increases for the case without oxygens, a minimum appears at a radius of around rMC ∼ 3 Å after adding oxygens at three equidistant 20355


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scale scans (see Figure S1 in the Supporting Information) showed parts of the surface with well-ordered domains exceeding 100 nm in lateral dimensions. From the detailed image in Figure 5b it is apparent that single molecules within this regular lattice show three different adsorption orientations with the same 3-fold symmetry as the underlying hexagonal gold lattice. If the centers of single molecules are considered, the spatial distribution of molecules can be described by a Kagome lattice, as presented in Figure 6a, where two hexagons

well-ordered domains of this network is probably limited by the presence of two different diastereomers inducing disorder. In the field of surface science, there are many examples of supramolecular networks formed by intermolecular bonds between oxygen and hydrogen of neighboring molecules on coinage metal surfaces.26−29 Since the diffusion barrier is low on such surfaces, hydrogen bonds strongly dominate the molecular assembly despite the fact that the associated force is considered weak.25 While there are examples of hydrogenbonded assemblies which can be moved collectively by STM tip-induced voltage pulses,30 lateral manipulation with a tunneling resistance in the order of 106 Ω could be used to separate the self-assemblies of An1O and Tn2O molecules proving their noncovalent bonded character. Thermally induced experiments for Tn2O on Au(111) showed that an annealing temperature similar to the experiments with An1O was needed to trigger the on-surface reduction. After heating to 390 K (Figure 5), we did not

Figure 6. Supramolecular assembly of Tn and Tn2O on Au(111) described by Kagome lattices. (a) Close-up STM images (7.5 nm × 10.5 nm) of surface-produced tetracenes. The center position of single molecules (black bars) is marked by blue dots, while the blue areas correspond to the hexagons of the Kagome lattice. (b) Large-scale image (40 nm × 40 nm) on the same surface as in a. The inset shows the fast Fourier-transformed image of the marked region. (c) Close-up STM images (7.5 nm × 10.5 nm) of as-deposited Tn2O molecules with an equivalent Kagome structure. (d) Large-scale image (40 nm × 40 nm) on the same surface as in c. The inset shows the Fouriertransformed image of the marked region.

Figure 5. Tn molecules on Au(111) after annealing Tn2O-covered surfaces at 390 K. (a) STM image acquired at V = +0.1 V and I = 80 pA showing on-surface fabricated tetracene molecules. (b) STM image (1 V, 100 pA) recorded on another region of the same sample preparation showing ordered structures.

(marked in blue) and two triangles join at each vertex. In addition, the fast-Fourier transform of the regular pattern is presented in Figure 6b and shows an average periodicity for the hexagonal tiling of 2.1 nm. This kind of self-assembly of linear molecules has been reported on-surface for simple oligophenylenes functionalized with two nitrile groups,31,32 which are however clearly ditopic, meaning that they feature two distinct binding sites at either side. By contrast, no experimental observation of such an assembly for hydrocarbons without any functional group has been presented to our knowledge so far. Performing the control experiment and depositing tetracene directly onto a clean gold surface led to the observation that no Kagome lattice was found after room-temperature deposition as well as after annealing at 390 K (see Supporting Information Figures S2, S3, and S4). Therefore, it seems likely that the cleaved oxygen from the Tn2O precursors adsorbs on the surface as atomic oxygen and stabilizes the self-assembly of the tetracene molecules. That is supported by the fact that the

observe any precursors but the reduced form of the molecules− tetracene. Similar to the anthracene formation, a successful reaction became apparent by a distinct topography of single molecules. Figure 5a shows that tetracenes appeared as elongated featureless protrusions if imaged at a bias close to the Fermi level. Furthermore, the STM measurements indicated that isolated molecules had the tendency to easily be manipulated by the tip during scanning. One can recognize several imaging artifacts in Figure 5a corresponding to tetracene molecules that suddenly jumped to a different surface position during scanning and hence appear longer or just as fragments. This high surface mobility at cryogenic temperatures was occasionally observed for the on-surface produced anthracene as well as reported in the literature12 for tetracene on Ag(111) at 8 K. By contrast, molecules in supramolecular assemblies were found to be less prone to STM tip-induced movement and could always be imaged in a stable manner. Notably, large20356


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Diego Peña: 0000-0003-3814-589X Francesca Moresco: 0000-0001-9607-8715 Gianaurelio Cuniberti: 0000-0002-6574-7848

minimum annealing temperature to fully reduce the An1O and Tn2O molecules is just 390 K, while previous studies of chemisorbed atomic oxygen on Au(111) have reported that a larger sample temperature of about 500 to 550 K was needed to detect its desorption as O2.33,34 We note that this regular lattice of tetracenes is connected to the self-assembly of precursor molecules and persists even after the thermally induced reduction. Accordingly, the open-porous networks occasionally observed on the Tn2O-covered surface (Figure 4c) and first introduced as hexagonal assembly can also be viewed as Kagome lattice by marking the position of single molecules instead of trimer centers. The result is shown in Figure 6c and 6d and illustrates that the ordered self-assembly of the Tn2O molecules as well as of their reduced form can be described in an equivalent way. Thus, the ditopic character of the precursors with two preferred intermolecular hydrogen binding sites lays the basis for the regular pattern for the tetracenes after on-surface reduction.

Present Address #

Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919−1 Tancha, Onna-son, Okinawa, 904-0495 Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the ICT-FET European Union Integrated Project PAMS (Agreement No. 610446). Support by the German Excellence Initiative via the Cluster of Excellence EXC1056 “Center for Advancing Electronics Dresden” (cfaed), the International Helmholtz Research School “Nanonet” and the Agencia Estatal de Investigación (MAT2016-78293-C6-3-R and CTQ2016-78157-R), the Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016-2019, ED431G/09), and the European Regional Development Fund (ERDF) is gratefully acknowledged. We further thank the Center for Information Services and High Performance Computing (ZIH) at TU Dresden for computational resources.

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CONCLUSION In summary, we have studied nanostructures and networks formed by epoxyacenes on Au(111) after room-temperature deposition as well as annealing experiments. In the case of anthracene derivative An1O with a single terminal epoxy group, trimer nanostructures are favorable assemblies due to the presence of intermolecular hydrogen bonds, as confirmed by DFT calculations. Notably, these trimer structures persist even after the oxygen is cleaved by means of thermally induced onsurface reduction due to their stabilization by oxygen adsorbates. The experimental observations for the tetracene derivatives Tn2O with two terminal epoxy groups show a clear equivalence. First, the stabilization of trimeric patterns is rediscovered and can be identified as a principal building block for well-ordered hexagonal domains. Second, STM images after annealing at 390 K showed the reduced form (tetracene) as the only molecules on the surface. Surprisingly, we occasionally observed also a Kagome lattice formed by tetracenes which one would not expect for symmetric hydrocarbons without functional groups. Bearing in mind the induced on-surface reaction, it seems likely that this ordered structure is carried over from the self-organization of the epoxyacenes and is further stabilized by the cleaved atomic oxygen to form a bicomponent architechture.35 Our results are an important step to address the structure of the organic−metal interface correctly and provide insights on the reaction mechanism of the onsurface reduction of organic molecules.

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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.7b06131. Details on the synthesis of epoxyacene An1O and diepoxyacenes Tn2O, additional STM data, as well as supplements on the DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robin Ohmann: 0000-0003-2483-1282 Dolores Pérez: 0000-0003-0877-5938 20357


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