Structure and Charge Transfer in Binary Ordered Monolayers of Two

Article pubs.acs.org/JPCC

Structure and Charge Transfer in Binary Ordered Monolayers of Two Sulfur-Containing Donor Molecules and TNAP on the Au(111) Surface Benjamin Fiedler,† Werner Reckien,§ Thomas Bredow,§ Johannes Beck,‡ and Moritz Sokolowski*,† †

Institut für Physikalische und Theoretische Chemie der Universität Bonn, Wegelerstraße 12, 53115 Bonn, Germany Institut für Anorganische Chemie der Universität Bonn, Gerhard-Domagk Straße 1, 53121 Bonn, Germany § Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie der Universität Bonn, Beringstraße 4, 53115 Bonn, Germany ‡

ABSTRACT: We report on ordered binary monolayer structures consisting of a sulfurcontaining π-conjugated molecule, namely, tetrabenzo thianthrene (TBTA) or tetrathiatetracene (TTT) as a donor, and tetracyano naphtho quinodimethane (TNAP) as an acceptor on the Au(111) surface. The investigations were performed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) and additional density functional theory (DFT) calculations for TTT/TNAP. Both pairs of molecules (TTT/TNAP and TBTA/ TNAP) form long-range ordered, commensurate structures on the Au(111) surface with a 1:1 stoichiometry. The structures consist of alternating rows containing only one type of molecule. The TNAP rows are stabilized by hydrogen bonds. Submolecular resolved STM images indicate a net charge transfer from the donor molecules to the TNAP molecules. The reconstruction of the Au(111) surface is modified upon the formation of the ordered binary structures, pointing to a significant surface−molecule interaction. For TTT/TNAP, the surface interaction leads to bonds of the S atoms to the Au atoms. For TBTA/TNAP, there exists an additional porous, TBTA-rich structure in which TBTA builds the framework.

1. INTRODUCTION In this paper we report on the structures of ordered monolayers on the Au(111) surface formed by a mixture of an organic electron donating molecule (donor) and an organic electron accepting molecule (acceptor). The donor molecule was either tetrathiatetracene (TTT) or tetrabenzo thianthrene (TBTA), while the acceptor molecule was tetracyano naphtho quinodimethane (TNAP). The molecules are illustrated in Figure 1. Ordered bulk phases of electron donating and accepting molecules are particularly interesting because due to charge transfer (CT) from the donor to the acceptor they can show properties that are completely different than those of the pure compounds and hence have been of strong interest in recent years. 1 One of the prototypic systems is given by tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ), which exhibits an anisotropic and metallic conductivity.2 The investigation of thin films3,4 and of ordered monolayers5,6 of donor and acceptor molecules on well-defined surfaces is a rather recent development. It is partially motivated by the interest in using organic CT materials in organic thinfilm devices. Besides that, there are fundamental questions of interest: (1) Is it possible to prepare ordered binary monolayers of donor and acceptor molecules on surfaces, and how do these comply with the respective bulk structures? (2) How is the CT possibly modified by the presence of the underlying surface? This latter question can be investigated in particular by scanning tunneling microscopy (STM) and spectroscopy © 2014 American Chemical Society

(STS), which allow monitoring local variations of the density of states. To date, several binary systems of organic donor and acceptor molecules have been studied on surfaces. Mostly, the chemically rather inert Au(111) surface was used as the substrate. On Au(111), mixed monolayers of TTF/TCNQ,5,7 TM(tetramethyl)TTF/TCNQ,8 and 6T(α-sexithiophene)/F4TCNQ9,10 were studied. Interestingly, TTF/TCNQ forms an ordered phase of alternating rows consisting of only one type of molecule. Furthermore, STS showed a Kondo resonance due to the radical character of the organic molecules in the monolayer and proved a CT of 0.6 electrons from TTF to TCNQ.5 A structure with alternating rows was also found for TMTTF/ TCNQ,8 while monolayers of 6T/F4-TCNQ were found to be disordered.9 From theoretical investigations a charge transfer of 0.4 electrons from 6T to F4-TCNQ was calculated.10 Hence, mixed 6T/F4-TCNQ and TTF/TCNQ monolayers show a comparable charge transfer but differ in their geometric structures. The disorder in 6T/F4-TCNQ might partly arise from the different sizes of the 6T and F4-TCNQ molecules, which may hinder the formation of an ordered 1:1 phase, as is present in the TTF/TCNQ monolayer. The layers of the pure constituents were either ordered, e.g., TCNQ,11 or disordered, e.g., TTF.12 We note that investigations on thin polycrystalline Received: July 30, 2013 Revised: November 8, 2013 Published: January 13, 2014 3035

dx.doi.org/10.1021/jp407579z | J. Phys. Chem. C 2014, 118, 3035−3048

The Journal of Physical Chemistry C

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et al.6 recently reported the preparation of an ordered mixed TTF/TNAP monolayer on Au(111). The structure consists of alternating rows of TTF and TNAP, similar to those in the TTF/TNCQ monolayer on the Au(111) surface.7 However, the CT for TTF/TNAP is stronger than that of TTF/TCNQ. As a result, the LUMO of TNAP is doubly occupied and no Kondo resonance is found,6 which is in contrast to TTF/ TCNQ.5 The structure of the monolayer of pure TNAP on Au(111) was also reported by Umbach and co-workers.6 Here we will report additional low-energy electron diffraction (LEED) data and a refined unit cell of the TNAP monolayer on Au(111). Sulfur-containing molecules are often used as donor molecules of larger size, e.g., 6T, which has been studied on different Au surfaces.18,19 Here we also used two sulfurcontaining molecules of a size comparable to that of TNAP, namely, tetrathiatetracene (TTT) and tetrabenzothianthrene (TBTA). Figure 1b,c shows their molecular structures. Both molecules are supposed to donate electrons to an acceptor, and we expect a partial depletion of the respective HOMO by CT. Hence, for both donor molecules, the HOMO and the HOMO−1 are shown in Figure 1. For TTT an ionization potential (IE) of 4.3 eV20 was reported, which is considerably lower than that of TTF (IE = 6.70 eV21). Concerning TBTA, we could not find data on ionization potentials in the literature. However, its parent molecule thianthrene can donate electrons and form radical cations that can oxidize TTF.22 Hence, we expect thianthrene and TBTA to be weaker electron donors than TTF and TTT. This is also compliant with the different structures of TBTA and TTT monolayers on Au(111) reported in a previous paper.23 Although TBTA and TTT both form commensurate, long-range ordered monolayers on the Au(111) surface, the interaction of these molecules with the substrate is different. TBTA is planarized upon adsorption on Au(111), and the Au(111) surface reconstruction is preserved. The TBTA monolayer structure is also commensurate with respect to the reconstructed Au(111) surface; however, contrary to TBTA, TTT lifts the Au(111) surface reconstruction, and the TTT monolayer structure is commensurate with respect to the unreconstructed Au(111) surface.

Figure 1. Structure formulas and relevant frontier orbitals of TNAP (a), TTT (b), and TBTA (c). For the electron accepting TNAP molecule, the LUMO and the HOMO are plotted, while for the electron donating TTT and TBTA molecules, the HOMO and the HOMO-1 are given. All orbitals were calculated by DFT. Note that for TBTA, a bended/folded configuration with the folding axis through the two sulfur atoms and a dihedral angle of 55° has been drawn.

films of TTF/TCNQ by STM3 and on bulk crystals of TTF/ TCNQ by AFM4 were also reported. For our experiments, we used either TTT or TBTA as a donor molecule and TNAP as an acceptor molecule (Figure 1). With respect to the smaller TTF and TCNQ molecules, these molecules have lower vapor pressures, which can be beneficial when thin films are prepared under vacuum conditions. Formally, TNAP can be derived from TCNQ by replacing the single quinone moiety by a quinoid naphthalene core. Figure 1a shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the TNAP molecule. From inverse photoemission spectroscopy measurements on multilayer films of TNAP on highly oriented pyrolythic graphite, an electron affinity (EA) of 4.70 eV was derived.13 Hence TNAP is expected to be a strong electron acceptor, comparable to TCNQ (EA = 4.23 eV).13 TNAP has successfully been used to synthesize CT complexes (CTCs) with TTF.14 The bulk structure of TTF/TNAP crystals14 is similar to that known for TTF/TCNQ crystals1 because segregated stacks of TTF and TNAP are formed, similar to the stacks in TTF/TCNQ crystals. The TNAP molecule has also been used for the synthesis of other CTCs with larger donor molecules like bis(methyl)-dithiapyrene15 and hexamethylenetetraselenafulvalene (HMTSF).16 In combination with alkali metals, TNAP forms saltlike compounds.17 Therefore, we consider TNAP to be a versatile electron acceptor. Concerning mixed monolayers with TNAP, Umbach

2. EXPERIMENTAL SECTION AND DFT CALCULATIONS 2.1. Experimental Section. The experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 mbar. The chamber contained a variable temperature STM (type UHV 300) manufactured by RHK Technology and a multichannel plate (MCP) LEED instrument purchased from OCI. An Omicron spot profile analysis (SPA)-LEED instrument was used for additional highresolution LEED profile measurements. This instrument was operated at electron beam currents (5−20 nA) higher than those of the MCP-LEED (∼1 nA). However, we did not observe any indications for electron-induced beam damage for both LEED instruments. The shown MCP LEED images have been corrected computationally for the distortion caused by the MCP geometry as well as for local defects on the phosphor screen. All reported STM images were recorded at room temperature (RT) in constant current mode using Pt/Ir tips. The bias voltages (Ub) refer to the sample. Positive (negative) Ub values hence correspond to tunneling into unoccupied (out of occupied) states of the sample. For all adsorbate systems reported in this paper we define the coverage of one monolayer 3036



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probably no adsorption of TNAP on the TTT monolayer or strong clustering of the TNAP occurs. For the disordered/gaslike phase of TTT, the deposition of 1 ML of TNAP on top of 0.20−0.40 ML of TTT led to some ordering of the TTT molecules by compression of these into ordered domains, which was visible from the appearance of a weak TTT LEED pattern. However, STM investigations (performed at RT) on this sample showed no molecular resolution, similar to the situation for this coverage of pure TTT on the Au(111) surface.23 Quite differently, deposition of TNAP on a closed monolayer of TBTA followed by an annealing step at 380−410 K yielded ordered binary phases. Alternatively, the TBTA precovered sample could also be heated to 380 K during the deposition of the TNAP. In this case the formation of binary phases occurred spontaneously. However, the order in these films was smaller than that obtained by using the standard method described above. Typically we found an excess of TBTA in these films leading to pure TBTA islands, clustered rows of TBTA, and formation of TBTA-rich phases. One of those TBTA-rich phases will be described in detail below (see Section 3.3.C Pore Structure of TBTA/TNAP ). It has a 2:1 TBTA-to-TNAP stoichiometry. Independent from the preparation method, annealing at temperatures above 460 K led to desorption of TNAP from the mixed monolayer, and the LEED pattern of pure TBTA monolayer was reobtained. Similarly, mixed TTT/ TNAP layers were found to decompose upon annealing at temperatures above 380 K. 2.2. DFT Calculations. In addition to experiments, we performed quantum-chemical calculations. First, we calculated the frontier orbitals of the free molecules that are relevant in the context of this work. These are displayed in Figure 1. These calculations were performed with the TURBOMOLE program package27 employing Kohn−Sham density functional theory (DFT) with the B3LYP functional28 and the TZVP29 basis set. The orbital pictures were created with the program Molden.30 Furthermore, we studied the adsorption of the binary ordered TTT/TNAP monolayer on the unreconstructed Au(111) surface with the plane-wave code VASP.31 We employed the PBE exchange−correlation functional32 in combination with the projector augmented wave method33 to account for the core electrons. A cutoff energy of 400 eV for the plane-wave valence basis and a Monkhorst−Pack integration setup with a 3 × 3 × 1 kpoint mesh was chosen. To account for the dispersion interaction we applied a recent implementation34 of Grimme’s dispersion correction (DFT-D3).35 The gold surface was modeled by a (4, 2,-3,6) supercell (see Section 3.2 Structure of the TTT/TNAP Layer) containing three Au layers. The Au atoms in the bottom layer were fixed to their perfect bulk positions during the structural optimization process. The 22 × √3 reconstruction of the Au(111) surface was not included in the DFT calculations. Although it is present for the real systems, we suppose that it has only a minor effect on the total energies because of its long range (63 Å) and that the conclusions from the DFT calculations remain valid. The net charge transfer was studied on the basis of a Bader analysis36,37 of the electron density (ρ). The electron densities and the electron density differences (Δρ) (see Figure 7), which were obtained from the DFT calculations in VASP, were analyzed with the program VESTA.38 Here, Δρ is defined as ρ of the bonded system TTT/TNAP/Au(111) minus ρ of the free TTT and TNAP

(1 ML) as the coverage corresponding to a closed, highly ordered layer of flat-lying molecules of the respective system. The Au(111) single-crystal was prepared by several cycles of Ar+-ion sputtering at 500 eV for 30 min and annealing at 900− 1000 K for 60 min. The LEED pattern of the clean Au(111) substrate showed the typical diffraction spots of the 22 × √3 reconstruction of the Au(111) surface, as shown in the inset of Figure 4 (below) and reported, for instance, in ref 24. All our experiments started from the reconstructed Au(111) surface. The details of the herringbone pattern formed by the prominent discommensuration lines (DLs) of the Au(111) reconstruction can be found, e.g., in ref 25. Because it was necessary to perform LEED at a very low electron energy E = 18.5 eV in order to observe the diffraction spots of the organic layer, we were not able to use the Au(111) substrate spots for k-space calibration of the LEED patterns. Hence, we used the diffraction spots of the commensurate TTT monolayer measured at the same electron energy for this purpose.23 TBTA and TTT were both purified by gradient vacuum sublimation; TNAP was used as purchased from TCI. All three substances were vapor deposited from homebuilt evaporators. Under UHV conditions with crucibles held at room temperature (RT), none of the three molecules had a vapor pressure above the detection limit of our quadrupole mass spectrometer. We considered different methods for preparing binary layers. Either both components can be deposited simultaneously or they can be deposited in sequence. Concerning the experimental effort, the simultaneous deposition has the drawback that it requires matching of the deposition rates of both components to obtain the correct stoichimetry. For sequential deposition, the actual deposition rates are less important because one can adjust the total amounts of the deposited materials by the respective deposition times. This facilitates the control of the stoichiometry, and we have preferentially used this method. However, for TTT and TNAP, the deposition sequence was found to be important, whereas it was less relevant for TBTA and TNAP (see below). During deposition, the sample was always held at RT and the ordering of the adsorbate layers occurred spontaneously. For the evaporation of TNAP, we used a crucible temperature of 448 K. For the evaporation of TBTA and TTT, we refer the reader to details given in ref 23. For the noted crucible temperature of TNAP, a deposition time of 2 min was necessary to achieve a closed monolayer with highly ordered domains of the structure reported in this paper. This structure appears to be the most dominant of TNAP on Au(111) and was also obtained by Umbach et al.6 However, deviations from the noted preparation conditions can lead to other ordered phases of TNAP on Au(111), which we observed and report in a forthcoming publication.26 For the binary films of TTT and TNAP, there was a strong influence of the deposition sequence. By primary deposition of 0.66 ML of TNAP and subsequent deposition of 0.50 ML of TTT or TBTA, it was possible to obtain ordered binary structures of TNAP/TTT and TNAP/TBTA. We consider this deposition sequence as the standard method for both systems for preparing highly ordered binary monolayers. For the inverted deposition sequence (TNAP on TTT), neither deposition of TNAP on a closed ordered monolayer of TTT nor deposition of TNAP on the disordered/gas-like phase of TTT23 led to ordered binary structures. For the closed TTT monolayer we could not observe any changes in the LEED pattern upon deposition of TNAP. Hence, either there is 3037



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molecules and minus ρ of the bare Au(111) surface. For this purpose the electron densities of the particular fragments were calculated at the geometry that was obtained for the bonded system.

3. RESULTS AND DISCUSSION 3. 1. Structure of the TNAP Monolayer. Figure 2a shows an STM image of an ordered TNAP domain. The TNAP domains exceed 250 nm2, but do not overgrow step edges of the Au substrate. The single TNAP molecules are imaged as rectangular protrusions, without submolecular-sized features. The arrangement of the molecules appears to be very similar to a brick wall type arrangement. In Figure 2a the brightness (apparent height) of the individual TNAP molecules strongly depends on their adsorption site with respect to the underlying Au(111) surface reconstruction. The TNAP molecules located on the DLs are imaged brighter than those between the DLs. Hence, the reconstruction of the Au(111) surface below the TNAP monolayer can be recognized. In general, TNAP is a prochiral molecule (Figure 1a). In the case of a planar adsorption geometry on a surface, TNAP can adsorb in two different orientations on the surface. Both molecular configurations are mirror images of each other, and it is impossible to transform one into the other without desorption and readsorption. Hence, the adsorption process yields two enantiomeric forms of TNAP. However, their footprints are rather similar, and in the STM image shown in Figure 2a we cannot distinguish two different enantiomers. The corresponding LEED pattern in Figure 2b shows diffraction spots that seem to be elongated at first sight. However, by closer inspection it becomes obvious that each elongated spot consists of two spots, located close to each other. To explain these pairs of spots it is necessary that the vectors (b1, b2) of the TNAP unit cell differ in length by about 10%. In this aspect, the structure derived here differs from the brick wall structure suggested by Umbach et al.6 on the basis of STM images, where both unit cell vectors have the same length. However, our vectors are within the experimental error range of those of ref 6. We determined the unit cell by simulating the LEED pattern. Figure 2b shows the simulated LEED pattern on the right-hand side; it explains all experimentally observed LEED spots. As for the experimental LEED pattern, there are always pairs of spots which are due to the difference in lengths of b1 and b2. We find a unit cell (b1, b2) with the following superstructure matrix with respect to the unit cell of the unreconstructed Au(111) [(a1, a2), see Figure 2c] surface: ⎛ 3.60 0.97 ⎞⎛ a1 ⎞ ( b1 b2 ) = ⎜ ⎟⎜ ⎟ ⎝ 0.96 3.83 ⎠⎝ a 2 ⎠

Figure 2. (a) STM image of a monolayer of TNAP on the Au(111) surface (Ub = 2.25 V, It = 28 pA). The TNAP molecules are ordered in a brick wall type arrangement. The Au(111) surface reconstruction is preserved upon the adsorption. The white arrow indicates the direction of a domain boundary of the Au(111) reconstruction, which is overgrown by the TNAP layer. (b) MCP-LEED image (left) of an ordered monolayer of TNAP on the Au(111) surface (electron energy E = 18.5 eV) and corresponding simulation (right). There are three rotational domains and two corresponding mirror domains which are marked by different colors and symbols.(c) Real space model of TNAP on the Au(111) surface. TNAP adsorbs in a flat geometry, with a molecular arrangement close to a brick wall structure. Both enantiomers of the TNAP molecule are present in the domain. They are indicated by different colors. Because there is no specific registry to the reconstruction of the Au(111) surface, an unreconstructed Au(111) surface has been drawn for simplicity. The chosen adsorption site is arbitrary.

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The accuracy of the matrix elements is about ±0.05. A summary of all structural parameters of the unit cell of the TNAP monolayer is given in Table 1. We note that for the investigated coverages up to 1 ML the underlying Au(111) reconstruction is not lifted because the reconstruction can be observed in the STM images, and the respective LEED spots are still present, e.g., around the (0,0) spot as observed in SPALEED patterns (not shown here). However, we do not observe a preferential orientation of the TNAP domains with respect to the Au(111) reconstruction. For instance, in Figure 2a, a domain boundary between two domains of the Au(111) reconstruction (marked by the white arrow), where the direction of the DLs changes by 120°, is overgrown by the 3038



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Table 1. Structural Parameters of the Investigated Systemsa TNAP monolayer 3.60 0.98 0.96 3.88 present b1 = 9.3 ± 0.2 Å b2 = 10.0 ± 0.3 Å 90° ± 3° 16°

mixed layer of TTT and TNAP row structure −43 26 modified b1 = 9.9 ± 0.5 Å b2 = 22.7 ± 1.1 Å 109° ± 2° 30°

mixed layer of TBTA and TNAP row structure −41 82

mixed layer of TBTA and TNAP pore structure

modified b1 = 9.9 ± 0.5 Å b2 = 24.6 ± 1.2 Å 96° ± 2° 30°

n.d. b1 = 28.4 ± 2.5 Å b2 = 28.4 ± 2.5 Å 95° ± 7° n.d.

90 Å2

212 Å2

242 Å2

803 Å2

(

Au(111) reconstruction length of unit cell vectors angle between the unit cell vectors angle between b1 and [11̅ 0] direction area of the unit cell

)

(

)

(

)

a

All superstructure matrices are given with respect to unreconstructed Au(111). For the incommensurate TNAP monolayer, the accuracy of superstructure indices is about ±0.05. n.d.= not determined.

Figure 3. (a) STM image of the mixed monolayer of TTT/TNAP on the Au(111) surface. The main image was recorded for Ub = −142 mV and a tunneling current It = 25 pA. Here, submolecular resolution was achieved. The shape of the TNAP molecules (darker rows) is similar to that of the LUMO orbital. However, as illustrated, both enantiomers fit to the local contrast in the STM picture within error. Within the TTT rows (bright) no separated molecules can be distinguished, although submolecular-sized features are resolved. The green arrow marks a mirror domain boundary along a TTT row. The inset was recorded at Ub = −1.42 V and It = 25 pA. The bright lines consist of TTT molecules, while the darker lines consist of TNAP molecules. For the TNAP molecules, two different orientations of the long molecular axis with respect to the rows are observed. (b) Close up of (a), but recorded for an inverted bias voltage polarity (Ub = 142 mV, It = 23 pA). Under these conditions submolecular resolution was also achieved. Here, for both the TTT and TNAP rows, individual molecules can be distinguished, as indicated by the stick and ball models of the molecules. (c) MCP-LEED image (left) of an ordered monolayer of TTT/TNAP on Au(111) (E = 18.5 eV) and simulation (right). There are three rotational domains marked by different colors. The two corresponding mirror domains of each rotational domain coincide. (d) Real space model of TTT/TNAP on Au(111). An unreconstructed Au(111) surface has been drawn for simplicity. The chosen adsorption site is arbitrary. Both molecules adsorb in a flat-lying orientation. In the case of TNAP, both enantiomers (marked by different colors) can substitute for each other with minimal distortion in the structure.

TNAP layer without a significant distortion. This is in agreement with a weak interaction of TNAP with the Au(111) surface, as derived by Kanai et al. from infrared reflection−absorption spectroscopy (IR-RAS) and ultraviolet photoelectron spectroscopy (UPS)39 and by Umbach et al. from STS.6

The area of this unit cell obtained from LEED corresponds to the footprint of one flat-lying TNAP molecule (Figure 2c). This aspect leads to the question whether the domains are enantiopure. We propose that that this is not the case and that the domains contain a statistical mixture of both enantiomers which are interchangeable because of the only small difference 3039



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as demonstrated in Figure 3a by the superposition of the LUMOs of both enantiomers with the STM image, this detail is beyond the resolution of the STM. Both enantiomers fit reasonably to the image; thus, we cannot distinguish between the two. However, within groups of molecules along the rows, the individual molecules are imaged with the identical STM contrast and brightness. We take this as an indication that the chirality of the TNAP molecules is the same within these groups of TNAP molecules. We therefore propose that enantiopure “clusters” of TNAP molecules form along the TNAP rows. Between these groups we observed single molecules with a smaller apparent height (darker). We speculate that these molecules are isolated molecules of the wrong chirality within an enantiopure cluster (point defects) or at the domain boundaries between two clusters of different chirality, which exhibit a slightly different electronic structure due to the neighborhood with a TNAP molecule of different chirality. Hence, although we cannot specify the absolute chirality, we have small indications for changes of the chirality. The above-noted different inclination angles of the long axes of TNAP molecules with respect to the rows (see Figure 3a and the inset) are not caused by different enantiomers. They are explained by a mirror domain boundary running along the TTT row which will be discussed in detail below together with our structure model. In Figure 3a, we marked the mirror domain boundary by a green arrow. Interestingly, the apparent height and the details of the submolecular contrast of the TTT molecules at the mirror domains boundary differ from those of TTT molecules not located at the mirror domain boundary. This effect strongly depends on Ub because the difference is visible in the main part of Figure 3a (Ub = −142 mV) but not in the inset (Ub = −1.42 V). Figure 3a reveals that two of the cyano groups of one TNAP molecule point to the quinoid cores of the two neighboring TNAP molecules. Hence, two H bonds are formed between neighboring TNAP molecules. The respective structure model of TTT/TNAP is shown in Figure 3d. The two H bonds are separated by a pair of two opposing H atoms. This arrangement of H bonds was also present in the pure TNAP monolayer (see Figure 2c). Hence, we conclude that the TNAP rows in the TTT/TNAP structure are stabilized by the same intermolecular H bonds as the pure TNAP structure. In our DFT calculations the H bonds are supported by a nonadiabatic interaction energy of −27 kJ/mol between the TNAP molecules in the TNAP rows of the TTT/TNAP mixed layer. The nonadiabatic interaction energy is the energy difference between the adsorbate monolayer without the underlying gold surface and the corresponding isolated (noninteracting) molecules. For this purpose all molecules are calculated within the geometry of the optimized monolayer. The calculated overall adsorption energy for the TTT/TNAP monolayer on the Au(111) surface is −594 kJ/mol. About 90% of the adsorption energy is related to dispersion interaction, which is a consequence of the planar adsorption geometry leading to a large footprint of the molecule. However, because these are less site-specific, the covalent interfacial bonds are still important for the details of the structure formation. In the inset of Figure 3a (Ub = −1.42 V), both the TTT and the TNAP molecules are imaged as separated entities, but without submolecular resolution. In contrast, at Ub = −142 mV (Figure 3a, main part), only the TNAP molecules are imaged as separated entities, while the TTT molecules within a row are imaged as one continuous feature. However, we still resolve

between their footprints. In fact, the formation of large enantiopure domains is also not very plausible because the adsorption occurs statistically and leads to a racemic mixture. Therefore, the formation of enantiopure domains would require a mechanism of chiral recognition between the TNAP enantiomers in combination with a rapid diffusion, which we consider to be less likely. In the structural model in Figure 2c, the enantiomers are marked by different colors. As noted above, the two enantiomers are interchangeable with very low distortions of the long-range order, which is consistent with the observation of a sharp LEED pattern. In Figure 2c, the molecular arrangement is similar to that of a brick wall structure. In an ideal brick wall structure, the displacement of the molecules in adjacent “layers” corresponds to half of the length of a molecule; hence, both unit cell vectors would have the same length, as suggested in ref 6. In our structure model, the small difference in the length of the unit cell vectors causes a displacement of the TNAP molecules in the adjacent brick wall layer by ±0.5 Å with respect to the position in an ideal brick wall structure. Presumably, intermolecular hydrogen bonds are important for the formation of the domains. As shown in Figure 2c, the four electronegative cyanogroups of a molecule point to toward H atoms of the electron-poor quinoid cores of four neighboring TNAP molecules in the two adjacent brick wall layers. Although the C−H bonds at the quinoid naphthalene core of TNAP may be not strongly polarized, we suppose that there are H bonds between the H atoms of the quinoid cores and the cyano groups of the neighboring TNAP molecules. Two different H bond binding motifs between adjacent molecules can be distinguished (see Figure 2c). In one case the two H bonds are located in close proximity to each other (see Figure 2c, marked in orange). In the other case, they are separated by a pair of two opposing hydrogen atoms (see Figure 2c, marked in blue). 3.2. Structure of the TTT/TNAP Layer. A. Alternating Row Structure of TTT/TNAP. Figure 3a shows an STM image of the molecular arrangement of a well-ordered binary monolayer of TTT and TNAP on the Au (111) surface. The inset shows a large-scale STM image. There are alternating bright and dark rows of TTT and TNAP molecules running from the left to the right side. Within the rows, the molecules are visible as separated entities. Within one row, all molecules are imaged with the same brightness; therefore, one row consists only of one type of molecule. Hence, we name this structure the “alternating row structure”. In Figure 3a, the TNAP molecules are imaged darker than the TTT molecules. While for the brighter TTT molecules only one molecular orientation is visible within one ordered domain, for the TNAP molecules two different orientations of the long molecular axis are found for different rows of the same domain (see inset of Figure 3a). This is also visible in the lower part of the main image of Figure 3a, where the molecular arrangement of TTT and TNAP is shown with submolecular resolution. For Figure 3a we used a bias voltage (Ub) of −142 mV; hence, only the filled orbitals close to the Fermi edge are imaged. In fact, the TNAP molecules are imaged with a contour that is very similar to the shape of the LUMO of the free molecule, which is illustrated in Figure 1c. This indicates an at least partial filling of the LUMO of TNAP by electron transfer. In general, it should be possible to differentiate between the two TNAP enantiomers by the inclination of the nodal planes of the LUMO with respect to the long molecular axis. However, 3040



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⎛ 2 1 ⎞⎛ a1 ⎞ ⎟⎜ ⎟ ( s1 s 2 ) = ⎜ ⎝ 0 22 ⎠⎝ a 2 ⎠

submolecular-sized protrusions along the TTT rows. Therefore, the resolution of the STM is principally sufficient to resolve individual TTT molecules. Instead of separated molecules, the STM records a more or less continuous electron density along the TTT rows when filled states are imaged. In Figure 3b, we further analyzed this effect using the same small absolute bias voltage of 142 mV, but with opposite, positive polarity; hence, empty states were probed. Now both TTT and TNAP are imaged with submolecular resolution, and both molecules are resolved as separated entities in very close proximity. Hence, the continuous electron density observed along the TTT rows at negative Ub could point to the presence of a delocalized filled electron state along the TTT rows. Figure 3c shows the LEED pattern of TTT/TNAP on the left-hand side. In comparison to the LEED pattern of the pure TNAP monolayer on Au(111), we observe an increased diffuse background. However, there are clearly separated spots in the LEED pattern. We found a unit cell (b1, b2) that explains all experimental LEED spots. This unit cell is commensurate with the unreconstructed Au(111) surface ⎛ 4 2 ⎞⎛ a1 ⎞ ⎟⎜ ⎟ ( b1 b2 ) = ⎜ ⎝ − 3 6 ⎠⎝ a 2 ⎠

(3)

The direction of the compression is along s2. We note that there is one pair of DLs parallel to s1 in each unit cell, separating areas with preferential fcc and hcp sites of Au atoms in the reconstructed top layer.25 The corresponding distance between the pairs of DLs is given by |s2| = 62.7 Å. In the STM images of the TTT/TNAP structure, we did not observe the DLs of the Au(111) reconstruction. The ordered TTT/TNAP domains are about 30−50 nm wide and often exceed the typical width (20−25 nm) of the stripe-shaped domains with equal orientation of the DLs of the Au(111) reconstruction. Together with the fact that the structure of TTT/TNAP is commensurate with the unreconstructed Au surface, this could indicate that the Au surface reconstruction is lifted upon adsorption. However, this is not the case. Instead, we propose that the prominent DLs of the reconstruction are obscured in the STM images because they are parallel to the alternating rows of TTT and TNAP. We conclude this from the fact that b1 and s1 are parallel, namely, b1 = 2 s1 (see eqs 2 and 3). The presence of the surface reconstruction could be derived directly from additional SPA-LEED scans which are displayed in Figure 4. As a consequence of the parallel orientation of b1

(2)

Again, the error of the matrix elements is 0.05, and the parameters of the unit cell are given in Table 1. The simulation of the LEED pattern on the basis of the unit cell from above is displayed on the right-hand side of Figure 3c. From the findings from LEED and STM we obtain the hardsphere model presented in Figure 3d. The unit cell contains one molecule of each type. Within the TTT rows, the next neighbor molecules are in touch with each other at the positions of the sulfur atoms. Within the TNAP rows there are attractive interactions between adjacent TNAP molecules via H bonds as discussed above. We propose that, similar to the pure TNAP monolayer, both TNAP enantiomers are present in the structure because they can be exchanged without a significant distortion of the structure. The different orientations of the long axes of the TNAP molecules with respect to the rows (see Figure 3a) arise from mirror domain boundaries within the domain and not from different TNAP enantiomers. In Figure 3d such a mirror domain boundary is marked by a green arrow in the bottom right corner. The mirror plane is located on a TTT row and runs parallel to b1. The existence of these mirror planes is related to the commensurability of the adsorbate layer. In particular, the vector between identical possible adsorption sites for TTT along the TTT rows is 0.5 b1 (= (2, 1)). Therefore, it is possible to shift a TTT row by 0.5 b1 with respect to its position in the ideal structure without altering the adsorption sites of the TTT molecules. However, this shift implies a rearrangement of the TNAP molecules in the two adjacent TNAP rows that is identical to that obtained by a mirror operation. Hence, the two different orientations of the TNAP molecules are related to the degeneracy in the arrangement of the neighboring TTT rows. B. Relation to the Au(111) Reconstruction. We now turn to the relation of the TTT/TNAP structure with respect to the reconstruction of the underlying Au(111) surface. The rectangular unit cell of the 22 × √3 Au(111) surface reconstruction is described by the following matrix:

Figure 4. SPA-LEED line scans in the direction of the spots of the Au(111) surface reconstruction (i.e., [011̅ ]) measured on the bare Au(111) surface and TTT/TNA- and TBTA/TNAP-covered Au(111) surface (E = 68.5 eV). Different from other LEED data in this work, these scans were performed at liquid nitrogen temperature. The inset shows a corresponding two-dimensional SPA-LEED image of the bare Au(111) surface reconstruction showing the region around the (0,0) spot. The arrow indicates the direction of the line scans.

and s1 (see Figure 3d), the reciprocal lattice vectors b2* and s2* are also parallel (because b2* ⊥ b1 and s2* ⊥ s1) and are both oriented in the [011̅] direction. The SPA-LEED scans were hence taken across the (0,0) spot in the [01̅1] direction. The scan at the bottom of Figure 4 refers to the clean Au(111) surface and shows the strong (0,0) spot and three orders of satellite spots from the reconstruction. The second scan refers to the TTT/TNAP structure. Remarkable intensities and positions of the first- and second-order satellites of the 3041



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Figure 5. (a) STM image of the mixed monolayer of TBTA/TNAP on Au(111) (Ub = 1.50 V, It = 48 pA). The layer consists of alternating rows of only one type of molecule. (b) Close up of (a), (Ub = −16.5 mV, It = 26 pA). Both molecules are imaged with submolecular resolution. Note that the arrangement of the TNAP molecules within the rows is very similar to that in the TTT/TNAP monolayer. (c) Averaged STM height profiles of TBTA molecules in two different local environments. The tunnelling parameters were identical for all profiles (Ub = −16.5 mV, It = 26 pA). The scans were taken across the center of the molecule in the direction parallel to its long side as illustrated by the hardsphere model of TBTA. For TBTA molecules (blue colored line) within the mixed TBTA/TNAP layer, the dip at the center of the profile (corresponding to the positions of the central S atoms) is more pronounced than for TBTA molecules within the pure TBTA layer (blue colored line). We interpret this difference by a bended or planar conformation of the TBTA molecules in the TBTA/TNAP or pure TBTA layer, respectively. (d) MCP-LEED image (left) of an ordered monolayer of TBTA/TNAP on Au(111) (E = 18.5 eV) and simulation (right). There are three rotational domains marked by different colors. The two corresponding mirror domains of each rotational domain coincide. (e) Real space model of TBTA/TNAP on Au(111). An unreconstructed Au(111) has been drawn for simplicity. The adsorption sites are arbitrary. For TBTA, a bended conformation, as present in the bulk crystal, is assumed, whereas the TNAP molecules are planar. The arrangement of the TNAP molecules within the TNAP rows is very similar to that in TTT/TNAP. As illustrated by the different color (light and dark green) the two TNAP enantiomers can be interchanged without a large distortion of the lattice. 3042



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reconstruction (at 0.10 Å−1 and 0.20 Å−1) are preserved, which indicates that the reconstruction is maintained and that its periodicity does not change significantly upon the formation of the TTT/TNAP structure. The third-order satellite (at 0.31 Å−1) is however stronger and broadened with respect to that of the clean surface. This is due to an additional contribution from the spot at b2* of the TTT/TNAP structure which overlaps with the third-order satellite of the Au surface reconstruction. In detail, from eqs 2 and 3 we derive that b2* corresponds to k∥ = 2π/(7.50 × a), whereas 3 × s2* corresponds to k∥ = 2π/(22/ 3 × a) = 2π/(7.33 × a). Hence, b2* is smaller by only 2.3% with respect to 3 × s2*, and the two spots overlap. In conclusion, the surface reconstruction is preserved and an orientation of the TTT and TNAP rows parallel to the DLs is found. Three pairs of TTT/TNAP rows fit between two successive pairs of DLs, i.e., correspond to one periodicity of the reconstruction. However, we still have to explain how the commensurability of the TTT/TNAP layer with the unit cell of the unreconstructed Au surface complies with the presence of the surface reconstructions. It is most likely that the Au(111) surface reconstruction is very slightly modified by the adsorbate layer and becomes commensurate (in higher order) with the TTT/TNAP monolayer. In particular, the unit cell from eq 2 would be commensurate to the surface reconstruction if the vector s2 (0, 22) is changed to (−1, 22) (+2.3%). This would yield the transfer matrix:

Interestingly, this apparent height difference is smaller for clustered TBTA rows. This can be seen in detail in Figure 5c which compares the height profiles of TBTA molecules within the mixed TBTA/TNAP layer and within the pure TBTA layer. We interpret the different apparent heights of the phenanthrene moieties by two different molecular conformations of the TBTA, as has been discussed for the pure TBTA monolayer.23 We suppose that TBTA in the TBTA/TTT systems is bended, while it is planar in pure TBTA clusters or layers. This explains the different STM profiles illustrated in Figure 5c. However, the structural arrangement within the TBTA rows is the same, for both the clustered and the single TBTA rows; the long axes of the phenanthrene moieties point to the sulfur atoms of one of the neighboring TBTA molecules. The cyano groups of the TNAP molecules face the long side of the phenanthrene groups of the enclosing TBTA rows. For the alternating row structure (Figure 5a), the LEED pattern and the corresponding simulation are shown in Figure 5d. The parameters of the unit cell used for the simulation are given in Table 1. The unit cell is commensurate with the unreconstructed Au(111) surface and is described by the following superstructure matrix: ⎛ 4 2 ⎞⎛ a1 ⎞ ⎟⎜ ⎟ ( b1 b2 ) = ⎜ ⎝ − 1 8 ⎠⎝ a 2 ⎠

⎛ 0.5 0 ⎞⎛ b1 ⎞ ⎟⎜ ⎟ ( s1 s 2 ) = ⎜ ⎝ 2 3 ⎠⎜⎝ b2 ⎠⎟

(4)

The error of the matrix elements is about 0.05. The unit cell derived from LEED is in good agreement with the findings from STM. The real space model is given in Figure 5e. The arrangement of the molecules within the unit cell was derived from the STM images. Notably, the arrangement of the TNAP molecules within the TNAP rows is identical to that found for TTT/ TNAP. Hence the “separated hydrogen bond motif” is also present here. For our structure model we assume that the TBTA molecules are nonplanar and have a bended conformation with a dihedral angle of 129°, as has been observed in the TBTA crystal.23 The phenanthrene groups are further from the surface than the sulfur-containing center of the molecule. This assumption is motivated by the size of the unit cell, the different apparent heights of the sulfur and phenanthrene parts of the molecule in the STM images, and the respective height profiles. Along the rows, the TBTA molecules are closely packed. Different from TTT in the TTT/ TNAP system, the long axis of the TBTA molecule is not perpendicular to the direction of the rows. Hence, there is no mirror plane in the direction of b1, and no mirror domain boundaries exist that could lead to different orientations of the TNAP molecules. As for the TTT/TNAP monolayer, we propose that the two TNAP enantiomers can be exchanged with each other without significant distortions of the structure, as is illustrated in Figure 5e. B. Relation to the Au(111) Reconstruction. The relation of the TBTA/TNAP structure to the reconstruction of the Au surface is rather analogous to that found for the TTT/TNAP structure. Again, b1 equals two times s1 and the orientation of the DLs is parallel to the rows of TBTA and TNAP. This could also be directly derived from STM images recorded for the only partially TBTA/TNAP-covered surface, where the DLs could be observed in the bare surface areas. However, similar to TTT/TNAP, the DLs could not be observed within the TBTA/TNAP domains by STM.

Such a modified reconstruction would be compliant with our experimental results within error. 3.3. Structure of the TBTA/TNAP Layer. A. Alternating Row Structure of TBTA/TNAP. For this system we found two different structures. We first report on the alternating row structure of TBTA/TNAP, which is very similar to that of TTT/TNAP. Figure 5a shows a large-scale STM image of this structure. In both types of rows, individual molecules can be resolved. There is only one orientation for both molecules throughout the entire domain. This alternating row structure of TBTA and TNAP was typically found for a slight stoichiometric excess of TNAP, similar to the situation found for TTT/TNAP. In contrast, for a slight stoichiometric excess of TBTA, we also found domains composed of several clustered TBTA rows, which were separated by few isolated TNAP rows. These domains are not completely disordered, but their longrange order is limited. In the close-up STM image (Figure 5b), submolecular resolution was achieved, which allows identifying the molecules within the rows. Similar to that for the TTT/TNAP layer, the STM contrast of the TNAP molecules corresponds well to the shape of the LUMO. Within the rows, the arrangement of the TNAP molecules is identical to that in the TTT/TNAP structure. The appearance of the TBTA molecules in Figure 5b cannot be identified with the HOMO or the HOMO-1 orbital (see Figure 1c). However, each TBTA molecule shows two elongated features, which represent the two phenanthrene moieties, and two almost round features between these, which correspond to the two central sulfur atoms. The apparent height of these features strongly depends on their local environment. For TBTA rows with two adjacent TNAP rows, the central sulfur atoms are imaged lower (darker) in comparison to the phenanthrene moieties (see Figure 5b.) 3043



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The corresponding SPA-LEED scan recorded in the [011̅ ] direction is displayed at the top of Figure 4. It shows the first three orders of satellite spots of the reconstruction. With respect to the positions of the spots on the clean surface, the spot positions from the TBTA/TNAP-covered surface are at about 10% smaller k∥ values. In addition, the third spot overlaps with the spot at b2* of the TBTA/TNAP structure. Hence, we find that the periodicity of the reconstruction has increased in this case. From the coincidence of the third-order satellite spot of the reconstruction and the spot at b2* of the TBTA/TNAP structure, we conclude that (as for TTT/TNAP) the average distance between the successive pairs of DLs corresponds to three pairs of TBTA/TNAP rows. This commensurability in combination with multiple scattering effects possibly explains why the second-order satellite spot is rather weak while the third-order satellite spot is rather strong again. The findings are compatible with a change of the vector s2 to (1, 26), which would yield a transfer matrix ⎛ 0.5 0 ⎞⎛ b1 ⎞ ⎟⎜ ⎟ ( s1 s 2 ) = ⎜ ⎝ 1 3 ⎠⎜⎝ b2 ⎠⎟

The modified periodicity is 73 ± 4 Å, which means an increase by 16% with respect to the clean surface. Hence, we derive a commensurability of the surface reconstruction and the TBTA/TNAP structure similar to that of TTT/TNAP. C. Pore Structure of TBTA/TNAP. For preparation conditions that yielded an excess of TBTA, we found structures which reminded us of a regular array of pores and which we therfore named “pore structures”. We found different pore structures; their domain sizes were usually only a few square nanometers. The pore structure shown in Figure 6a is the one we found most frequently and encountered with domains of the largest domain size. We never obtained a well-ordered film with this structure as the majority phase. Our real space model of the pore structure (Figure 6b) is derived from STM images. The stoichiometry of TBTA/TNAP is 2:1. The complex structural arrangement of the molecules can be understood from structural motifs known from the alternating row structure. Concerning the TBTA molecules, one can identify rows of molecules running parallel to b2 in Figure 6b. The rows are not straight; there is a kink after every three TBTA molecules, leading to groups of three TBTA molecules. As can be seen in Figure 6b, one group of three molecules is centered at each corner of the unit cell. The arrangement of neighboring TBTA molecules is identical to that found within in the TBTA rows in the alternating row structure (see Figure 5d). Within the groups, the neighboring TBTA molecules are shifted with respect to each other by b1/6 in the same direction; between two groups, the shift occurs in the opposite direction. This leads to an identical local coordination of all TBTA molecules within the rows with respect to their neighbors. However, between two TBTA rows there are single, additional TBTA molecules leading to the formation of a pore. The pore itself is filled by a pair of two TNAP molecules with an arrangement to each other that is identical to that found within the TNAP rows in the alternating row structure (see Figure 5b). Hence, the pair is again stabilized by H bonds. The orientation of the TNAP pair with respect to the rows of TBTA molecules running along b2 is very similar to that found for a row of TNAP molecules with respect to the TBTA molecules in the alternating row structure. In conclusion, the rather

Figure 6. (a) STM image of a porous network of TBTA molecules (Ub = −1.42 V, It = 25 pA). Each pore is filled with two TNAP molecules, leading to a 2:1 overall stoichiometry of TBTA/ TNAP. (b) Real space model of the porous structure TTT/TNAP on Au(111). Here, a planar TBTA molecule has been assumed.

complex structure of the pores can be understood by structural motifs from the alternating row structure, although a 2:1 TBTA/TNAP stoichiometry is present. However, we believe that, different from the 1:1 TBTA/TNAP phase, the TBTA molecule is planar in the pore structure, similar to the situation in the pure TBTA phase. We conclude this because, on the one hand, we did not find any experimental indications for a bending and, on the other hand, the nonplanar adsorption of TBTA in the 1:1 TBTA/TNAP phase appears to be related to the presence of TNAP, which has a significantly smaller stoichiometric value here and hence less impact on the TBTA. 3.4. Charge Transfer in the TTT/TNAP and TBTA/TNAP Layers. Given that the orbitals of the free molecules are meaningful also for the adsorbed species, we can principally derive indications for changes in the charge on the molecules upon adsorption from a comparison of the shape of the molecules as seen by STM with the shape of the respective molecular orbitals. For low negative values of Ub (Ub < 0 V), as we used for the high-resolution STM images, only filled molecular orbitals close to the Fermi level are expected to contribute to the tunneling current. In the case of TNAP we expect to see the partially filled LUMO. Indeed, the STM images of Figures 3a and 5b, which probe filled states, provide a molecular shape that corresponds quite well to the shape of the LUMO of the free molecule (see Figure 1). This is different from the situation found for the pure TNAP monolayer, where we could not resolve any submolecular features near the Fermi level. Thus, we deduce that partial filling of the LUMO occurs 3044



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The situation for the TTT molecules is more complex. The appearance of TTT in the STM images in Figure 3a and in the plot of ρ in Figure 7b can be less easily related to the HOMO of the free TTT molecule. This can be understood from the DFT calculations, which demonstrate that the TTT molecules are strongly distorted because of the formation of covalent bonds between the S atoms and the Au surface. Because the four S atoms of the TTT are at different relative positions with respect to the underlying closest Au atoms, this leads to an outof-plane distortion of the TTT molecule and a concomitant change in its electronic structure. The formation of the S−Au covalent bond can be seen in Figure 7c,d, which shows vertical sections of ρ and Δρ going through the Au−S bond of the S atom of the TTT that is located on top of an Au atom. Conclusively, the geometric and also the electronic structure of TTT are significantly modified by the interaction with the Au surface. The structure of the TTT molecules in the TTT/ TNAP/Au layer hence differs from that of TTT cations in bulk CT complexes, which are planar.40 The strong interaction of TTT and the Au(111) surface is consistent with the observation that the STM images at low negative values of Ub do not fit to the HOMO of the free TTT molecule as seen in Figure 3a. However, at small positive values of Ub (Ub > 0 V), the submolecular contrast of TTT molecules is reasonably close to the shape of the HOMO of the free TTT molecule. This indicates a partial depopulation of the HOMO of TTT, which can qualitatively also be seen in the Δρ map in Figure 7b from the blue areas under the TTT molecule. In summary, both STM data and the DFT calculations show charge accumulation on the TNAP and depletion on the TTT molecule which is more distorted because of bonds to the Au surface. The electron density maps in Figure 7a,c also show that H bonds are formed between TTT and TNAP. In the ρ map of Figure 7a, H bonds can be identified by the overlapping electron density between the H atoms of the TTT and the N atoms of the TNAP. A section through such a H bond (white dotted line) is given in Figure 7c. This section also shows an interaction of the N atom with the Au surface. However, in the corresponding section of the Δρ plot in Figure 7d it is visible that the induced changes in ρ at the N−Au interface are smaller than those related to the H−N bond formation. We have not performed DFT calculations for TBTA/TNAP/ Au(111); however, we believe that the situation is rather similar, with a few exceptions discussed below. In contrast to that of TTT, the geometry of TBTA strongly depends on the charge on the molecule. For a free positively charged (cationic) TBTA molecule, an almost planar configuration is energetically preferred, while the neutral TBTA molecule has a bended configuration as derived from DFT calculations.23 For the pure TBTA layer on Au(111), a planar configuration of TBTA is derived from our STM data. Strikingly, for the alternating row structure where a partially positively charged TBTA is supposed, we have indications for a bended TBTA molecule (see Figure 5c). This is particularly obvious from the different apparent heights of the sulfur and the phenanthrene moieties. The different heights are clearly related to interactions of TBTA with TNAP molecules because this effect is present only in the mixed layer of 1:1 stoichiometry. The smaller space requirement of the bended TBTA molecule also fits better to the size of the unit cell of the alternating row structure. Thus, we suppose that the observed differences in the apparent heights are indeed caused by a bended (nonplanar)

for TNAP interacting with TTT or TBTA on Au(111), whereas this is not the case for pure TNAP on Au(111).6 Figure 7 shows the calculated electron density ρ and the electron density difference Δρ of TTT/TNAP/Au(111). Very

Figure 7. (a) Section of the DFT-calculated electron density ρ at the height of the carbon atoms of the organic molecules above the Au(111) surface. Blue (red) color means low (high) ρ. White color indicates an electron density of less than 0.01 e/Å3. Note the significant electron density between CN groups of TNAP molecules and the H atoms of neighboring TNAP molecules, which indicate the formation of H bonds. (b) Section of the electron density difference Δρ at the height of the π-system of TNAP above the molecular layer toward the vacuum. Blue (red) color means loss (gain) of ρ, while green indicates no change in Δρ. The red areas on the TNAP molecules resemble the shape of the LUMO of the free molecule, thus indicating the CT to the TNAP. (c) Section of the electron density ρ perpendicular to the surface along the white dotted line in (a). The molecule on the left-hand side is the TTT molecule in the lower left corner of (a). The color code is the same as in (a). The section shows the Au−S bond of the S atom of the TTT molecule that is located on top of an underlying Au atom and the H bond between a H atom of TTT and a N atom the neighboring TNAP molecule. In addition, a slight rumpling of the Au(111) surface can be seen: the Au atom of the Au−S bond is pulled up, while the Au atoms below the tetracene core of TTT are pushed down. The dotted black lines are guide lines to the eye to facilitate the location of the atoms. (d) Plot of electron density difference (Δρ) in the same section as in (c). The color code is the same as in (b). Δρ confirms the strong localized covalent Au−S bond and the polarization charge below the N atom of the TNAP molecule. Note that this N atom is also involved in the H bond to the TTT molecule on its left.

clearly, the LUMO of the free TNAP molecule is reproduced in the Δρ map in Figure 7b by the red areas, in agreement with the finding from STM. A Bader analysis performed on TTT/ TNAP/Au(111) showed a positive charge of 0.65e on the TTT molecules, while the TNAP molecules gained 0.70e. The small difference (0.05e) is attributed to a small positive charging of the Au(111) surface, which will be discussed below. We point out that our DFT calculation was based on plane waves. Nevertheless, the LUMO-like shape of Δρ on the TNAP molecules was obtained as a direct result (see Figure 7b). This demonstrates that the LUMO of the free molecule is preserved upon adsorption to a good extent. 3045



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similar to that in the pure TBTA monolayer.23 However, a remaining difference is that TBTA is planarized in the pure monolayer, whereas it is bended in the mixed structure. 4.2. Surface-Mediated Charge Transfer. An important remaining question concerns the role of the Au(111) surface for the intermolecular CT, in particular in comparison to the CT in the respective bulk crystals. Unfortunately, to our knowledge, no bulk CT complexes of TTT/TNAP or TBTA/ TNAP have been reported in the literature. To date, our experiments to grow mixed crystals of TTT/TNAP or TBTA/ TNAP by cosublimation were not successful.41 Hence, no bulk phases are available for comparison. In general, three effects of the Au(111) surface have to be discussed for the CT. First, the bonding Au(111) surface induces a flat-lying orientation of the molecules and a lateral ordering that is commensurate to the surface and its reconstruction. Evidently, the resulting surface-induced structure will be important for the CT. The second effect is a possible screening of the charges on the molecules by polarization charges in the metal. The third effect, which is important for TTT in particular, is the change in the electronic structure of the molecules due to the surface bonding, limiting comparisons with gas-phase molecules and their respective orbitals to a certain level. We discuss these effects for TTT/TNAP using Figure 7. As noted above, the four S atoms of the TTT molecule occupy different adsorption sites and hence interact differently with the Au(111) surface. In particular, one S atom is located on top of a Au atom and has the shortest S−Au distance of all four S atoms of the TTT molecule. Thus, a significant covalent bond between this S atom and the respective Au atom underneath forms as seen in the section of ρ through the Au−S bond in Figure 7c. In Figure 7d, the covalent nature of this bond is visible by the charge accumulation (red area) in the middle on the axis between the S and Au atom. Both atoms redistribute or donate charge to form the covalent bond. In addition to the covalent bond, polarization charges are induced in the Au(111) surface, which possibly lead to the local variation of Δρ in the Au surface in the vicinity of the Au−S bond (Figure 7d). Compared to the S atom of the TTT molecule, the polarization effects of the surface due to the N atom of the TNAP molecule are smaller. Only a small positive (blue) polarization charge in the Au surface due to the strong accumulation of negative (red) charge on the N atom is seen in Figure 7c. The distribution of this local polarization charge can be distinguished from the covalent bond between the S atom of TTT and the underlying Au atom in Figure 7c. Between the N and the Au atom we find no charge accumulation, while on the Au−S axis a significant charge accumulation is found. One can expect that the formation of the polarization charges lowers the energy of the charged TTT/TNAP layer on Au(111) with respect to a free unsupported layer of the same structure. Finally, we mention the distortion of the TTT molecules induced by the bonding to the surface. This effect might also enhance the CT because it lowers the interaction of the disulfide groups with the conjugated π-system of the tetracene core of the TTT and hence supports their oxidization. To further quantify the role of the surface for the CT, we have made DFT calculations for different (artificial) models. In the first model, a free, unsupported TTT/TNAP layer with a fixed structure that is the same as that of TTT/TNAP supported by the Au(111) surface was calculated. Hence, this model included the distortions but not the covalent bonding

configuration of the TBTA and are not caused by electronic effects alone. Interestingly, this conclusion appears to be contradictory to the planar configuration of the TBTA molecule expected for a positively charged molecule. This contradiction can likely be explained by the interaction of the TBTA with the Au(111) surface that is modified indirectly by the TNAP. The effect likely occurs on the phenanthrene groups because the TNAP withdraws electron density from these groups. As a consequence, the bonding of the phenanthrene groups to the surface is weakened and the groups fold upward. Hence, the arguments for conformation of TBTA molecules in the gas phase have to be modified on the surface. The filling of the LUMO of TNAP seen in the STM images and the change of the appearance of the TBTA molecule in the alternating row structure are hence in agreement with a CT from the TBTA to TNAP on Au(111).

4. FINAL DISCUSSION 4.1. Structural Considerations. We suppose that the H bonds between next neighbor TNAP molecules play the most important role in the arrangement of the TNAP molecules to each other in all three systems and in particular induce the formation of linear TNAP rows, which are a central motif in all three structures. Hence, the donor molecules play only a secondary role for the arrangement of the TNAP molecules. This conclusion is supported by the structure of TTF/TNAP on Au(111), in which very similar TNAP rows are formed.6 Furthermore, it is consistent with the findings for TCNQ, which can be considered as the smaller homologue of TNAP. The monolayer structure of pure TCNQ on Au(111) was also explained by a combination of intermolecular H bonds and an only weak template effect of the Au(111) surface.11 The Au(111) surface reconstruction was also found to be preserved, and STS revealed the absence of a significant CT between TCNQ and Au(111),11 analogous to our situation for TNAP/ Au(111). In the mixed monolayer of TTF/TCNQ on Au(111), the molecules arrange in rows,5,7 again analogous to our TTT/ TNAP and TBTA/TNAP structures. Therefore, rows of separated donors and acceptors seem to be very typical for mixed layers of acceptors that are capable forming H bonds between themselves. However, the mixed (sub-) monolayer of 6T and F4-TCNQ was found to be disordered.10 We attribute this to the fluorination of the acceptor, which increases its acceptor strength but excludes H bonds between the F4-TCNQ molecules. Thus, the Coulomb interactions which result from the CT between the 6T and F4-TCNQ molecules are presumably relevant in this system. Because 6T and F4TCNQ are of rather different size, the formation of an ordered structure is more difficult and a disordered mixed phase forms. In comparison to the TNAP, the donor molecules interact stronger with the Au(111) surface. For TTT, this is directly visible in the electron density maps of the mixed binary phase, where the bond of the S atoms to the Au(111) surface could be identified (see Figure 7c). The stronger surface interaction is also evident from the fact that TTT and TBTA form commensurate monolayer structures on the Au(111) surface and that TTT lifts the Au(111) surface reconstruction.23 However, the arrangement of the TTT molecules within the rows of TTT/TNAP differs considerably from the arrangement in the pure TTT monolayer because the opposing arrangement of the sulfur groups present in TTT/TNAP is missing in the pure TTT monolayer.23 For TBTA, the arrangement of the TBTA molecules within the rows of TBTA/TNAP is quite 3046



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bonding of the TTT and the TBTA molecule on the Au(111) surface can be also concluded from the change in the Au surface reconstruction induced by the adsorbate. The CT from TTT to TNAP on Au(111) has to be understood mainly as a superposition of the individual CT of the donors and acceptors with the Au surface. It has to be seen as a mainly surfacemediated CT, but not as a surface-enhanced CT, because the same order of CT is present in the unsupported layer. Finally, the comparisons with the CT in corresponding bulk CT compounds are of limited meaning because of the different packing of the molecules and because the sulfur-containing donors are significantly modified by the surface bonding.

(which causes the distortion) of the TTT molecule to the Au(111) surface. We found a CT from TTT to TNAP of 0.6e. Therefore, the CT from the TTT without and with the presence of the Au(111) surface (0.65e) is very similar and we cannot speak of a significant enhancement of the CT due to the Au(111) surface. However, it is evident that because of the chemical modification of the TTT related to its chemical bonding to the Au surface, the chemical details of the CT are not the same for the supported and the free TTT/TNAP layers; hence, the comparison is limited. For the unsupported TTT/TNAP layer, we estimated the nonadiabatic energy gain of the positive TTT rows and the negative TNAP rows to be −76 kJ/mol by nondispersive interactions and −28 kJ/mol by dispersive forces, which results in an overall nonadiabatic interaction of 104 kJ/mol. In a second and third model, the TTT and TNAP molecules, respectively, were replaced by a vacuum, while the remaining TNAP and TTT molecules, respectively, were fixed to the adsorption sites and geometries of the binary TTT/TNAP/ Au(111) layer. From these models the CT between the TNAP/ TTT molecules to the Au(111) surface was estimated by a Bader analysis. We found CT in opposite directions, i.e., from TTT (0.45e) to the Au(111) surface, and to TNAP (−0.48e) from the Au(111) surface, but with almost the same absolute values. These numbers for the charge transfer correspond to 69% of the values obtained by Bader analysis for the binary (supported) layer (0.65e/0.68e). This is remarkable because it indicates that the charging of the molecules on the Au(111) surface does not require the presence of the second molecule. It is primarily an effect of the interactions of the molecules with the Au surface on their own. The amount of the CT appears to be only slightly reinforced by the presence of the second molecule. Therefore, we conclude that the CT is mediated and modified, but not enhanced significantly, by the Au(111) surface. The observed charge donation of the Au(111) surface to TNAP appears to contradict the reported absence of CT in the TNAP monolayer on Au(111) by Umbach et al.6 However, in our artificial model structure of the TNAP layer, the TNAP rows were separated by empty space. Thus, the Coulomb repulsion between the negatively charged TNAP molecules is smaller than that in the close-packed TNAP monolayer investigated by Umbach et al. This may explain the difference in the experimental finding on the monolayer. In summary, this elucidates that the presence of the second molecule may have an indirect effect on the charge transfer to the metal by spacing the acceptors and donors laterally apart.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 0049 228 73 2507. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Gertrud Dittmann for the synthesis of the TTT and Petra Krieger-Beck for the synthesis of TBTA. We acknowledge a critical reading of the manuscript by Q. Guo. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through research center 813: “Chemistry at spin centers”.

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REFERENCES

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5. SUMMARY The donor molecules TTT and TBTA and the acceptor molecule TNAP form ordered, mixed monolayers on Au(111) which consist of alternating linear rows of the two molecules with a 1:1 stoichiometry. Intermolecular H bonds between the TNAP molecules are the most important motif for the formation of these structures. From the submolecular STM contrast we have indications for a CT from the donors to TNAP on Au(111). For TTT/TNAP/Au(111), DFT calculations yield charges on TTT and TNAP of 0.65e/−0.70e. Whereas the TNAP molecule is weakly bound to the Au(111) surface and the LUMO of the Au-bonded molecule is similar to the former LUMO of the free TNAP molecule, TTT forms significant bonds to the Au(111) surface which distort the molecule and change its electronic structure. The stronger 3047



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Structure and Charge Transfer in Binary Ordered Monolayers of Two

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