Reversible Potential-Induced Switching of Alkyl Chain Aggregation in

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Reversible Potential-Induced Switching of Alkyl Chain Aggregation in Octyl-Triazatriangulenium Adlayers on Au(111) Sonja Lemke, Chi-Hao Chang,† Ulrich Jung,‡ and Olaf M. Magnussen* Institute for Experimental and Applied Physics, Kiel University, Leibnizstraße 19, 24118 Kiel, Germany ABSTRACT: In situ scanning tunneling microscopy and cyclic voltammetry studies of self-assembled octyl-triazatriangulenium monolayers on Au(111) electrode surfaces in 0.1 M HClO4 reveal a complex surface phase behavior, involving two fast, highly reversible transitions between different ordered adlayer phases: With decreasing potential, the preadsorbed (√19 × √19)R23.4° adlayer first is converted into a (7√3 × 7√3) and then into a (2√3 × 2√3)R30° phase, corresponding to a stepwise increase in the local packing density of the molecules. The (7√3 × 7√3) → (2√3 × 2√3)R30° transition is accompanied by a reorientation of the peripheral octyl chains from a more planar to a close-packed vertical arrangement. This reversible potential-induced switching between a homogeneous adlayer of small vertical extension and a Au surface partially covered by islands of a compact hydrocarbon layer is attributed to changes in the adsorbate charge state and associated changes in the intermolecular interactions.

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INTRODUCTION Self-assembled organic layers on solid surfacesa central component of modern nanosciencecan in general be divided into two main categories: The first are linear molecules, typically aliphatic hydrocarbons, with a terminal group that strongly binds to the surface. Prototypical examples of this class of adsorbates are alkanethiols on Au surfaces.1,2 Under equilibrium conditions, these adsorbates form close-packed monolayers in which the terminal groups bind to defined adsorption sites and the linear chains are oriented away from the surface at angles that optimize the intermolecular van der Waals interactions. Only at low coverages, for example, in the initial stages of growth, adlayers in which the chains adsorb planar on the surface are found.1,3 A second major category are self-assembled adlayers of planar polyaromatic organic molecules, such as porphyrins and phthalocyanines.4−8 Adlayers of these species are the first step required for the preparation of defined supramolecular architectures on solid surfaces and of relevance to molecular electronics, artificial light-harvesting systems, chemical and biochemical sensors and novel (electro-) catalysts.9,10 Typically, well-ordered adlayers with a planar adsorption geometry are found, driven by interactions of the molecules π-systems with the solid substrate. Equipping polyaromatic molecules with long aliphatic chains allowed the preparation of complex two-dimensional supramolecular arrangements on graphite and metal surfaces.11−13 In these structures, the planar adsorbed alkyl chains are oriented parallel and interdigitate with each other, promoting long-range order. Orientation of the alkyl chains away from the surface was only found in rare cases. For example, Katsonis et al. reported for meso-tetradodecyl-porhyrin at the Au(111)/n-tetradecane © XXXX American Chemical Society

interface that only one of the dodecyl chains was physisorbed on the surface, which was attributed to partial solvation.14 Organic adlayers have been studied extensively not only under ultrahigh vacuum (UHV) and ambient conditions, but also in electrochemical environment.4,15−19 The latter allows changing the molecules charge state in a defined way via the applied electrode potential, which can be utilized for model molecular electronic devices15 as well as for controlled modification of the adlayer structure. In studies of porphyrins and phthalocyanines, potential-dependent phase transitions in the molecular adlayer and changes in the adsorbate surface mobility were observed in several systems.16−19 For example, scanning tunneling microscopy (STM) studies of porphyrins on Au(111), Au(100), and iodine-covered Ag(111) electrode surfaces revealed a strong reversible influence of the potential on the molecular order within the adlayer.16,17,20 Borguet and coworkers showed for tetra-pyridyl-porphin adlayers on Au(111) in acidic media an interesting and complex potential dependence of the adlayer’s structure and chemical reactivity.18,19 Ordered monolayers were only observed negative of the potential of zero charge (pzc), that is, on the negatively charged surface, whereas at more positive potentials ordering of the adlayer was kinetically hindered by insufficient surface mobility. Redox driven structural surface phase transitions in adlayers of aromatic molecular cations were reported by the group of Broekmann, who investigated dibenzyl-viologen derivatives on Cl-covered Cu(100) electrode surfaces.21−23 Electrochemical reduction of these dications to the radical monocation resulted in a structural transition from a cavitand phase to a striped phase, which was attributed to enhanced Received: February 10, 2015

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Langmuir π−π-interactions, followed by an order−disorder transition at even more negative potentials.21 Here, we present electrochemical investigations of octyltriazatriangulenium (TATA) adlayers on Au(111). TATA is a less commonly studied heterocyclic, aromatic cation with three peripheral alkyl chains (Figure 1). It has recently received much

Figure 1. Schematic model of the octyl-triazatriangulenium (octylTATA) cation. Figure 2. Cyclic voltammogram of an octyl-TATA adlayer on Au(111) in 0.1 M HClO4 (scan rate 20 mV s−1). The first (black line) and second (red line) cycles after immersion into the electrolyte are shown.

interest as a platform for the preparation of structurally-defined functional molecular architectures on solid substrates.24 Specifically, attachment to its central carbon atom allows vertical mounting of freestanding photoswitchable, redox-active, or light-harvesting groups on the surfaces.24−27 The properties of these TATA-derivatives have been studied extensively by spectroscopic and (photo-) electrochemical methods.28−30 In this work, we describe in situ STM studies of Au(111) electrode surfaces, covered by a self-assembled monolayer of the bare TATA cation. Our studies reveal a complex but fully reversible surface phase behavior in these adlayers upon reduction of these molecules, involving two transitions between ordered monolayer phases. Clear evidence for substantial reorientation of the peripheral alkyl chains during these surface phase transitions is found, resulting in changes between a more planar adsorption to a close-packed arrangement as in thiol selfassembled monolayers.

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experiment, the sample had been immersed in the electrolyte under potential control at 0.6 V and the cyclic voltammogram (CV) started at the same potential by a sweep in negative direction. In both CVs, two pairs of current peaks are found. As we will show later, these current peaks are associated with phase transitions in the molecular adlayer. As indicated by the small potential shift between corresponding anodic and cathodic peaks, these transitions are highly reversible. While the peaks 2A/2C are very symmetric, peak 1A is noticeably broader than 1C, suggesting that the anodic process is slightly kinetically hindered. The peak positions are very stable in subsequent cycles. Differences between the first and following cycles are mainly found near the positive potential limit, especially in the height and width of peak 1C. According to STM studies under ambient conditions, self-assembly of octyl-TATA on Au(111) results in well-ordered (√19 × √19)R23.4° adlayers with a surface coverage of 0.053 monolayers (ML).24,26 Because of the very low solubility of octyl-TATA in aqueous solution, the same overall coverage is expected after sample immersion. Assuming a 1 e‑ transfer reaction complete reduction of the TATA cation would correspond to a charge density of 11.7 μC cm−2. The measured charge densities of the current peaks (1.8 μC cm−2 for 2A; 4.6−5.3 μC cm−2 for 1A) are significantly smaller, showing that all observed current peaks have to be related to double layer reorganization or partial electron transfer. Such behavior is well-known for phase transitions in organic adlayers.31 For example, partial charge transfer upon adsorption was observed in electrochemical studies of thiols on metal electrode surfaces.32 The decrease of the current density negative of 0.05 V is due to reduction of remaining trace amounts of oxygen in the STM cell. Process 2A/2C is accompanied by changes in the double layer capacity. Specifically, the capacity obtained from the voltammograms is 25 μC cm−2 negative of this peak, but 32−38 μC cm−2 in the potential regime between 2A/2C and 1A/1C. This 30− 50% change in capacity provides a first indication of a structural change in the TATA adlayer. After immersion of the Au sample at sample potentials positive of peaks 1A/1C, highly ordered octyl-TATA adlayers with hexagonal structure are visible in the STM experiments (Figure 3a). The lattice constant in this molecular adlayer is a = (12.6 ± 0.8) Å and the angle between different rotational domains is α = (11.8 ± 2.0)°. These results are in excellent

EXPERIMENTAL SECTION

The octyl-TATA molecules were synthesized as described in our previous paper.25 According to NMR and mass spectroscopy the purity of the substances is better than 99%. The adsorbate layers were prepared on Au(111) single crystals (MaTecK GmbH, Jülich, Germany), cleaned by flame annealing in a butane gas flame for 4− 5 min. After cooling down in air the Au(111) substrate was immersed into 30−85 μM solutions of octyl-TATA with BF4− counterions in ethanol (Merck, p.a.) for 20−45 min at room temperature. Afterward, the sample was rinsed with the pure solvent to remove excess TATA molecules. In all experiments, 0.1 M HClO4 was used as electrolyte, prepared from Milli-Q water (18.2 MΩ cm) and 70% HClO4 (Suprapure, Merck). For in situ STM measurements, a PicoPlus scanning tunneling microscope (Agilent, Inc.) was employed with a Pt wire counter electrode and a Pt quasi-reference electrode, calibrated against a saturated calomel electrode (SCE). All potentials in the paper are quoted versus SCE. STM tips were prepared by etching a 0.25 mm Wwire in a 2 M KOH solution and coating it with polypropylene. For image processing, the SPIP software (Image Metrology) was employed. Lateral drift of the STM measurements was corrected with a dedicated software. The electrochemical cell of the STM was also used for cyclic voltammetry measurements, which were performed with a Compactstat (Ivium) potentiostat. Here, the entire cell was kept in argon atmosphere (6.0) during the measurements.

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RESULTS Figure 2 shows the first (black) and second (red) cycle of voltammograms of an octyl-TATA adlayer on Au(111), recorded in 0.1 M HClO4 at a scan rate of 20 mV s−1 (subsequent cycles are identical to the second one). In this B

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M HClO435,36 and metastable reconstructed areas can be maintained underneath the self-assembled TATA monolayer,26,33 the characteristic double-row-like elements of the reconstruction can be occasionally observed in the STM images (see, e.g., Figure 5). From such measurements, angles between the close-packed rows of octyl-TATA adlayer and the [11̅0] direction of the Au(111) surface of γ1 = (10.3 ± 2.6)° and (30.0 ± 3.0)° were obtained. Furthermore, the orientation of this molecular adlattice relative to that of the (√19 × √19)R23.4° phase (see below) is rotated by angles of δ1 = (31.2 ± 2)° and (11.0 ± 2)°. Contrary to TATA adlayers under ambient conditions,33 these structural data could not be rationalized by a simple commensurate superstructure containing only one molecule per unit cell. For clarification of the adlayer structure we performed a detailed structural analysis of the adlattice spacing and orientation and compared these experimental data with a number of higher-order commensurate superstructures with nearest neighbor spacings between 10.5 and 11.8 Å. The experimentally measured spacing is close to one-third of 7√3 dAu (with dAu = 2.885 Å being the atomic spacing in the Au(111) surface). For this spacing two different superstructures exist, which only differ in their orientation relative to the Au lattice: a (7√3 × 7√3)R30°, in which the close-packed directions of the TATA adlayer are oriented along the [112]̅ direction of the Au substrate (Figure3d), and a (7√3 × 7√3)R8.2° structure, where the adlayer is rotated by γ1 = ± 8.2° relative to the Au(111) lattice (Figure 3f). In both arrangements, the superstructure contains nine octyl-TATA molecules per unit cell, corresponding to a nominal surface coverage of 0.061 ML, and the molecules within the adlayer occupy two different adsorption sites of the Au(111) substrate at a ratio of 1:2 (e.g., top and three-fold hollow sites, as in Figure 3). Detailed quantitative analysis of the angles α1 between >15 different domains as well as the rotation angles γ1 and δ1 relative to the lattice of the Au(111) substrate and the (√19 × √19)R23.4° TATA adlayer, respectively, strongly suggests that both (7√3 × 7√3)R30° and (7√3 × 7√3)R8.2° domains exist on the surface. All other superstructures considered in this analysis proved to be incompatible with the observed domain orientations. We therefore will in the following denote this ordered adlayer summarily as (7√3 × 7√3) phase. The parallel presence of close-related structural phases is not uncommon on organic adlayers37 and the parallel presence of adlayers that are aligned and rotated with respect to the substrate lattice can be justified also theoretically.38 The ordered (7√3 × 7√3) adlayer does not cover the entire Au surface in this potential regime, but coexists with smaller areas in which the adlayer appears to be disordered and highly mobile (Figure 4a, left-hand side of image). The latter manifests as rapid fluctuations in the position and shape of these disordered domains in successive STM images, as visible, for example, in Figure 4a and b, and is further supported by highresolution STM images of boundaries between (7√3 × 7√3) domains and disordered areas as shown in Figure 4f. Here, submolecular resolution of the octyl-TATA molecules was achieved in the inner parts of the (7√3 × 7√3) domain (see inset), suggesting resolution of the three benzene rings of the TATA molecule as individual maxima (the unequal intensity of these three rings may be caused by the tip shape). TATA molecules at the domain boundary between ordered and disordered areas appear blurred and their submolecular structure is not resolved, indicating fast dynamic fluctuations.

Figure 3. (a,c,e) High-resolution STM images (15 × 15 nm2, It = 30 pA, UBias = 450 mV) of octyl-TATA adlayers on Au(111) in 0.1 M HClO4 at (a) 0.58 V, (c) 0.38 V, and (e) 0.34 V. The observed adlayers can be described by (b) a (√19 × √19)R23.4° superstructure and a coexistence of a (d) (7√3 × 7√3)R8.2° and (f) (7√3 × 7√3)R30.0° superstructure phase, respectively.

agreement with the (√19 × √19)R23.4° commensurate superstructure (a = 12.57 Å, α = 13.2°) found under ambient conditions (Figure 3b).24,26,33 As already shown in our previous STM studies, under ambient conditions, the peripheral alkyl chains are highly mobile at room temperature and therefore cannot be resolved, a behavior known from STM studies of close-packed alkanethiols.1 In contrast, it was possible to image the flat lying octyl side chains of adsorbed TATA molecules on Au surfaces under UHV conditions by Hauptman et al.34 Indirect evidence for the presence of the alkyl side chains at room temperature comes from systematic studies of the influence of the chain length on the adlayer structure.33 Changing the potential into the regime between the current peaks 1 and 2 of the CV, a more densely packed hexagonally ordered adlayer with a lattice constant a1 = (11.3 ± 0.5) Å is formed (Figure 3c). Angles of α1 = (21.8 ± 2.0)° between different rotational domains of this new adlayer phase were found (Figure 3e). By comparison with the orientation of the Au(111) surface reconstruction, it was possible to determine the orientation of the hexagonal ordered adlayer with respect to the substrate surface. Because potential-induced lifting of the Au(111) reconstruction occurs only positive of 0.44 VSCE in 0.1 C

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7√3) domains decreased (Figure 4d, upper right corner), and after further increase of the potential to 0.4 V (Figure 4e) these domains completely disappear, resulting in complete restoration of the (√19 × √19)R23.4° adlayer on the entire surface. Such coexistence of these two different adlayer phases on the surface (as in Figure 4c,d) could only be observed in rare cases; usually this transition occurred instantaneously on the time scale of the STM experiments. In parallel to this structural change in the ordered adlayer, also the defect density substantially decreases. In particular, the disordered area in the lower left corner of Figure 4b completely disappears after the formation of the (√19 × √19)R23.4° phase in this surface area (the bright spots at the bottom edge of Figure 4d,e are monatomic Au islands which become visible in the imaged surface area due to lateral drift). Furthermore, point defects are only present in the (7√3 × 7√3) domains, but not in the (√19 × √19)R23.4° adlayer. These phenomena are fully consistent with the decrease in the local TATA packing density associated with the (7√3 × 7√3) → (√19 × √19)R23.4° transition as described before. Obviously, these transitions solely involve fully reversible lateral rearrangement of the TATA adlayer on the surface without any loss of the molecules into the electrolyte solution. The direct observations of the (7√3 × 7√3) → (√19 × √19)R23.4° transition in Figure 4 can be used for a rough estimation of the TATA coverage in the disordered areas, utilizing the full coverage by a perfect (√19 × √19)R23.4° adlayer at positive potentials and the conservation of the global TATA surface density during the transition. Taking into account that the disordered areas cover approximately 20% of the visible area in Figure 4c and correcting for the point defects in the (7√3 × 7√3) adlayer, the local coverage in the disordered areas is 0.024 ML. This is about 40% of the coverage in the ordered (7√3 × 7√3) phase and would readily explain the high mobility of the molecules in these regions. It is worthwhile mentioning that reproducible observation of a reversible (7√3 × 7√3) ↔ (√19 × √19)R23.4° transition was only possible in experiments, where the sample was immersed under potential control at potentials positive of peaks 1A/1C, that is, in the stability range of the (√19 × √19)R23.4°. For samples immersed at more negative potentials or under open circuit conditions, that is, in the potential range of the (7√3 × 7√3) phase (the open circuit potential is usually ≈0.3 V), the surface was often completely covered by a (7√3 × 7√3) phase that was stable against conversion into the (√19 × √19)R23.4° structure at positive potentials. This is surprising taking into account that the (√19 × √19)R23.4° is the phase present on the surface after the selfassembly process. Potentially, this behavior may be caused by a rapid transition of the TATA adlayer into a (7√3 × 7√3) phase during immersion under these conditions, made possible by the uptake of small amounts of additional TATA molecules (e.g., of residual physisorbed species on top of the (√19 × √19)R23.4° monolayer). Because of the low solubility of TATA in the solution, this would kinetically inhibit the potential-induced transition of the (7√3 × 7√3) into the low coverage (√19 × √19)R23.4° phase. An even more complex phase behavior is found for the second transition, associated with peaks 2A/2C in the voltammograms. Selected STM images from an experiment where the potential was gradually decreased from 0.24 to −0.01 V are shown in Figure 5. Prior to the potential change, four (7√3 × 7√3) domains and a disordered area are visible in the

Figure 4. (a−e) Series of STM images (60 × 80 nm2) recorded at time intervals of 65 s at (a,b) 0.3 V, (c,d) 0.35 V, and (e) 0.4 V, showing (a,b) temporal fluctuations in the (7√3 × 7√3) superstructure and (c−e) the phase transition to the (√19 × √19)R23.4° superstructure ((a−c) It = 20 pA, UBias = 330 mV, (d,e) It = 20 pA, UBias = 400 mV). Black arrows in the lower left corners indicate the scan direction. (f) High-resolution STM image of the edge of a (7√3 × 7√3) domain (11 × 15 nm2, It = 25 pA, UBias = 300 mV) at 0.25 V, indicating a high mobility of the molecules near the boundary. The inset shows an enlarged part of the inner, well-ordered adlayer domain, in which the model of the octyl-TATA molecule is superimposed on the submolecular resolution image.

Furthermore, the intensity of these species is lower than in the well-ordered areas, which can be attributed to rapid attachment/detachment of the molecules from the edge of the domain. In addition, well-defined holes within (7√3 × 7√3) domains with an apparent depth of (0.6 ± 0.1) Å are observed (example marked by white arrow in Figure 4a), which can be associated with single missing octyl-TATA molecules in the adlayer. As seen e.g. in Figure 4a and b, these point defects change their positions significantly in consecutive STM images, indicating again a high surface mobility of adsorbed octylTATA molecules. Both point defects and disordered areas can be attributed to the 0.008 ML higher local coverage of the (7√3 × 7√3) as compared to the (√19 × √19)R23.4° adlayer phase. As a consequence of this higher packing density, defects with a locally reduced coverage have to emerge on the surface during the (√19 × √19)R23.4° → (7√3 × 7√3) surface phase transition. Support for this comes from direct in situ STM observations of the reverse (7√3 × 7√3) → (√19 × √19)R23.4° transition, initiated by an increase in the electrode potential to 0.35 V directly before the recording of Figure 4c. This results in the rapid conversion of the (7√3 × 7√3) domains (denoted by I in Figure 4) to domains of the (√19 × √19)R23.4° phase (denoted by II). With time, the size of the remaining (7√3 × D

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indicates that this transition is inhibited by the presence of the ordered TATA adlayer and becomes possible only after dissolution of the (7√3 × 7√3) phase. Vice versa, this observation suggests a lower adlayer density in these areas, as also inferred in the studies of the (7√3 × 7√3) ↔ (√19 × √19)R23.4° transition. In parallel to the dissolution of the (7√3 × 7√3) phase, growth of islands starts in the disordered areas of the sample (Figure 5d, arrow). These islands have diameters of up to 5 nm and an apparent height of about 2 Å, i.e., are clearly higher than the maxima observed in the disordered areas at more positive potentials, which have heights ≤ 1 Å. This island growth occurs on slower time scales than the disordering of the (7√3 × 7√3) adlayer and still proceeds after complete disappearance of the ordered adlayer domains. However, it saturates after several minutes, when approximately 50% of the sample surface is covered by the islands (Figure 5f). This saturation at island coverages ≤50% is characteristic and was observed in a large number of experiments. Interestingly, island formation was not observed on reconstructed areas, which may be caused by a higher TATA surface mobility on the reconstructed Au surface (see below). Also this phase transition is highly reversible, as illustrated by the sequence of subsequent STM images in Figure 6. In this experiment an overview image was recorded at 0.20 V (Figure 6a), which shows two atomically smooth terraces of the Au substrate, separated by a monoatomically high step and covered by several Au monolayer islands (one of them marked by an x as positional reference). Terraces and Au islands are covered by a (7√3 × 7√3) TATA adlayer. In the subsequent image, the scan size was reduced in order to better resolve the adlayer structure (Figure 6b) and the potential was stepwise decreased to −0.10 V near the center of the image (marked by dotted line), resulting in disordering of the (7√3 × 7√3) phase within ≈30 s and the onset of island nucleation and growth (Figure 6b, upper half). This process proceed in the following two STM images (Figure 6c,d), measured at the same sample potential. Nucleation of the islands occurs predominantly at surface defects, such as the lower and upper edge of the Au steps, followed by slow lateral growth. Switching the potential back to 0.20 V in Figure 6e (dotted line) results in instantaneous disappearance of the islands, whereas the TATA adlayer still appears disordered. Approximately 10 s later, first short-range ordered molecular structures appear (marked by white arrow). Full recovery of the (7√3 × 7√3) adlayer phase occurs within ≈1 min after the potential step. As visible in the subsequent image (Figure 6f), the hexagonal order is restored on the entire surface area observed by STM. The (7√3 × 7√3) domain size and defect density before and after this potential excursion are similar. The morphology of the islands formed under the conditions of the experiment in Figure 6 is better defined compared to those shown in Figure 5, providing more in depth insight into the nature of this phase. The STM images in Figure 6b−d reveal two types of islands with dendritic and needlelike shape, respectively. The dendritic islands can reach overall lateral extensions of up to 70 nm, but the widths of the individual dendrite arms is typically ≤20 nm. Their apparent height is about 2 Å (see Figure 7b), which is clearly lower than the height of monatomic steps of the Au(111) substrate. Holes in these islands and fjords between the dendrite arms are not filled, even in the later stages of the growth process, indicating a low surface mobility of the TATA molecules within these

Figure 5. Series of STM images (65 × 65 nm2, It = 30 pA, UTip = −160 mVSCE) showing the second phase transition in the adlayer at (a,b) 0.24 V, (c) 0.19 V, (d) 0.14 V, (e) 0.09 V, and (f) −0.01 V. The scan direction in all images is from top to bottom, and the recording time is given in the images.

imaged surface area (Figure 5a). The bright island in the lower left corner of Figure 5a−c is present on the surface at all potentials and can be identified as a Au monolayer island on the basis of its height and the presence of the TATA adlayer on top. It provides a positional reference during the STM sequence. In addition, a single double-row of the Au(111) reconstruction beneath the (7√3 × 7√3) TATA adlayer (upper half of image, marked by white arrow) can be seen in Figure 5a. This surface morphology is stable at 0.24 V, apart from some fluctuations at the domain boundaries (example marked by arrow in Figure 5b). However, upon changing the potential to 0.19 V (Figure 5c), clear shrinking of the (7√3 × 7√3) domains and an increase of the disordered areas is observed. This transition mainly takes place at the domain boundaries. With decreasing potential, this lateral growth of the disordered areas at the expense of the (7√3 × 7√3) domains continuously proceeds (Figure 5d), until the complete disappearance of the ordered TATA adlayer at 0.09 V (Figure 5e). Furthermore, additional elements of the Au reconstruction emerge on the newly formed disordered surface areas (Figure 5d). On bare Au(111) surfaces, the potential-induced formation of the reconstruction starts at about 0.24 V in 0.1 M HClO4.35 The ≈100 mV shift toward more negative values E

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Figure 7. (a) High-resolution STM image of a (2√3 × 2√3)R30° island at −0.01 V (18 × 28 nm2, It = 20 pA, UBias = 200 mV) and (b) corresponding height profile at the position indicated by black line in (a). (c) Schematic model of the (2√3 × 2√3)R30° superstructure.

located at irregular positions and correspond to molecules with a similar arrangement as in the needlelike islands. The latter are often embedded in the dendritic islands in the later stages of the growth process. High-resolution images of the needles indicate a vertical modulation along these structures with a periodicity of a2, corresponding again to 2√3dAu. Hence, these needles correspond to 1D molecular structures with the same TATA packing density as in the dendritic islands. The higher packing density in the (2√3 × 2√3)R30° phase explains the incomplete coverage of the surface by islands. For a surface initially covered by a perfect (√19 × √19)R23.4° the maximum coverage by (2√3 × 2√3)R30° would be 63%. By similar arguments it can be excluded that the (2√3 × 2√3)R30° islands correspond to a bilayer phase. The latter would only allow a coverage of 32% of the surface, which is clearly lower than the experimentally observed island saturation coverage of ≈50%. On the other hand, the latter value is lower than the theoretical possible coverage of 63%, indicating a residual TATA coverage on the Au surface areas not covered by islands. This is in agreement with the STM observations, which show that in the TATA adlayer phase surrounding the islands individual TATA molecules and patches with short-range order are still visible. On the average these adsorbates have lateral distances of (13.0 ± 1.2) Å which is slightly larger than the intermolecular distances within the (√19 × √19)R23.4° superstructure. With increasing distance from the islands the adlayer appears increasingly blurred, indicating (as in Figure 4f) a higher TATA surface mobility. This indicates that, similar as at boundaries of (7√3 × 7√3) domains, TATA molecules of the low density disordered adlayer phase can attach to the dendritic islands and are temporarily stabilized by those. The average TATA coverage in this disordered phase, calculated for

Figure 6. Consecutive series of STM images, It = 20 pA, UBias = 200 mV, showing (a) the initial (7√3 × 7√3) adlayer at 0.20 V (160 × 160 nm2) and (b−f) images recorded during the (7√3 × 7√3) ↔ (2√3 × 2√3)R30° phase transition (100 × 100 nm2), initiated by a potential step to −0.10 V in (b) and a step back to 0.20 V in (e) (marked by dotted lines). Scan direction and recording times are marked in the images.

islands. The needlelike islands, which are a minority structure, have a well-defined characteristic width of about 2 nm and lengths of up to 20 nm. They are strictly oriented along the [11̅0] and 60° rotated directions, that is, parallel to the closepacked directions of the Au(111) surface lattice, and appear 0.1−0.8 Å higher as compared to the dendritic islands. Close inspection of the top of the dendritic islands reveals a weak hexagonally ordered molecular lattice with a nearest neighbor spacing of a2 = (9.7 ± 0.6) Å. As an example, a highresolution image of a dendrite island (obtained in a different experiment) is shown in Figure 7a. The close-packed directions in this adlattice are oriented along the [112̅] directions of the Au substrate. These structural parameters are in perfect agreement with a simple commensurate (2√3 × 2√3)R30° superstructure with a TATA coverage of 0.083 ML (Figure 7c). The lateral molecular arrangement within this adlayer is very similar as in the (7√3 × 7√3)R30° superstructure, but the inplane packing density is 36% higher and, contrary to the (7√3 × 7√3) phase, all TATA adsorbates reside on the same type of adsorption sites. In addition, maxima of lateral dimensions in the range of 1 nm and a height of up to 0.2 Å can be found on top of these islands (see, e.g., Figure 6c,d). These maxima are F

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Langmuir the saturation island coverage of 50% (and an initially perfect (√19 × √19)R23.4° adlayer), is 0.023 ML, i.e., very similar as found for the disordered areas coexisting with the (7√3 × 7√3) phase.

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DISCUSSION Our in situ STM and cyclic voltammetry studies reveal highly reversible structural phase transitions in adlayers of octyl-TATA adsorbates on Au(111) electrode surfaces in 0.1 M HClO4. Our observations show that in these experiments the adsorbates are strictly confined to the Au surface and transfer into the adjacent solution is negligible. Because TATA adsorption/desorption or contributions of the nonspecifically adsorbing electrolyte can be excluded, the observed peaks in the voltammograms can be clearly assigned to charge transfer between the molecular adlayer and the Au electrode, specifically TATA reduction/ oxidation. In the potential region positive of peaks 1A/1C, the molecules adopt the (√19 × √19)R23.4° superstructure, well-known for octyl-TATA and octyl-TATA derivatives under ambient conditions.24,28 This structure corresponds to the adlayer phase formed during the self-assembly process. Previous studies of these adlayers by X-ray photoelectron spectroscopy found no indications for coadsorption of the BF4− counterions and concluded that the TATA cations are at least partially discharged upon adsorption on the Au surface.28 The latter is in agreement with our electrochemical studies, which find that the total charge transfer in peaks 1A/1C and 2A/2C is significantly lower than required for reduction of a fully cationic (√19 × √19)R23.4° TATA adlayer. The majority of the charge transfer occurs in peaks 1A/1C and could account for ≈1/2 e− per adsorbed TATA molecule. Thus, the character of the TATA adsorbates changes in the surface phase transition associated with the reduction process 1C to a significantly more neutral state. Interestingly, the 1A/1C peak potential is close to the potential of zero charge (pzc) of the bare Au(111) surface, suggesting that the partially cationic state of the TATA molecules may be stabilized by the negative charge on the metal electrode. The partial discharge of the molecules negative of peaks 1A/ 1C should reduce repulsive electrostatic interactions between these molecules. This is in very good agreement with the observed change from the (√19 × √19)R23.4° to the 16% more densely packed (7√3 × 7√3) adlayer structure (schematically shown in Figure 8). In fact, the coexistence of the latter phase with disordered areas of significantly lower TATA surface density indicates that the effective lateral interactions between the TATA adsorbates in the potential regime negative of peaks 1A/1C are attractive. However, this attraction seems to be rather weak. The fluctuations observed at the (7√3 × 7√3) domain boundaries indicate rapid exchange of TATA molecules in the ordered (7√3 × 7√3) and in the low-coverage disordered phase, that is, local dynamic equilibrium between these adsorbate phases at room temperature. This suggests lateral interaction energies on the order of a few tens of meV, which could be explained by van der Waals interactions (e.g., between the peripheral octyl chains). In addition, also the balance between adsorbate−adsorbate and adsorbate−substrate interactions, which affects the structural order within adsorbate layers, seems to change in the (√19 × √19)R23.4° ↔ (7√3 × 7√3) transition. That this balance can be directly influenced by the sample potential has been shown in a large number of studies,39 including

Figure 8. Schematic model, illustrating the potential-driven phase transitions in octyl-TATA adlayers on Au(111) between the (√19 × √19)R23.4° (I), the (7√3 × 7√3) (II), and the (2√3 × 2√3)R30° (III) adlayer phase.

studies of heterocyclic adsorbate layers.18 Detailed previous studies under ambient conditions have shown that selfassembled alkyl-TATA adlayers always adopt simple commensurate superstructures, in which the molecules occupy the same type of adsorption sites on the Au(111) surface, most likely top sites.33 This indicates a strong preference of the molecules for these specific sites. The (√19 × √19)R23.4° structure belongs to this class of adlayer structures, whereas the (7√3 × 7√3) superstructure requires parallel occupation of two different adsorption sites (e.g., on top and three-fold hollow sites, see Figure 3d). Apparently, the energetic costs associated with the latter are compensated by the gain in lateral interaction energy due to the higher packing in this adlayer structure. Nevertheless, both superstructures are in good agreement with the expected behavior, if one assumes that the TATA-substrate interaction is dominated by dispersion interactions between the aromatic system and the metal surface.33 In ordered benzene adlayers on Au(111), both three-fold hollow and on-top sites are occupied according to UHV-STM studies,40 in agreement with theoretical calculations, which report a weak preference for these sites.41 Reduction of the adlayer in peak 1C can occur via (kinetically unhindered) charge transfer and subsequent separation in denser ordered and less densely packed disordered domains. In the reverse (7√3 × 7√3) → (√19 × √19)R23.4° transition these local differences in coverage have to be equilibrated, which involves long-range mass transport of TATA adsorbates on the Au surface. This kinetic limitation may explain the larger width of peak 1A as compared to 1C as well as the differences between the first cycle, which corresponds to the reduction of a well-ordered self-assembled (√19 × √19)R23.4° adlayer, and the subsequent cycles. STM data on the second phase transition, occurring at peaks 2A/2C, clearly indicate a further, even stronger compression of the ordered TATA adlayer phase. The pronounced differences in apparent height relative to the (7√3 × 7√3) phase strongly suggest that the resulting (2√3 × 2√3)R30° structure not merely corresponds to a TATA adlayer with 36% higher packing density, but exhibits substantial, qualitative differences as compared to the adlayer phases at more positive potentials. Our results provide strong indications that these differences are associated with a reorientation of the peripheral octyl chains of the TATA molecules. This idea mainly originates in the high packing density of the alkyl units in the (2√3 × 2√3)R30° G

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Langmuir structure. Even in the more open (√19 × √19)R23.4° adlayer the space requirement of the octyl groups is too large for complete adsorption on the Au surface, requiring these units to partly reside above the TATA adlayer.28,33 More precisely, the surface area per alkyl group in this structure is 45 Å2, which is close to that in a planar octane monolayer.42 In the electrochemical environment, hydrophobic interactions with the solution should therefore enforce a planar arrangement of these chains on top of the TATA platforms (Figure 8, I,II). In the (2√3 × 2√3)R30° phase, each unit cell contains one TATA with three peripheral octyl chains, corresponding to an octyl surface coverage of 0.25 ML or area per octyl chain of 28.8 Å2, respectively. This coverage is only 25% lower than the packing density in the saturation coverage c(4 × 2) phase of alkane-thiol monolayers on Au(111).1 Similar as in the latter we expect the alkyl chains in (2√3 × 2√3)R30° TATA adlayers to exhibit a close-packed arrangement, in which these chains are tilted relative to the surface normal (Figure 8, III). The appearance of this phase in the STM images supports this model. The apparent height of the (2√3 × 2√3)R30° phase relative to the neighboring surface (≈2 Å) is similar to that found in systems where close-packed thiolate adlayers and planar adsorbates coexist on the surface, such as methylbiphenyl-ethanethiol coadsorbed in perylene-tetracarboxylic diimide melamine networks.43 Because of the aliphatic nature of the alkyl layer, this apparent height is much lower than its geometric height. The presence of a dense hydrocarbon layer on top of the TATA platforms is also in accordance with the much lower corrugation of the (2√3 × 2√3)R30° in the STM images as compared to the (7√3 × 7√3) and (√19 × √19)R23.4° adlayer, which can be rationalized by the larger tip-sample spacing in the (2√3 × 2√3)R30° covered areas. We presume that as for close-packed alkane-thiol monolayers1 the alkyl chains are mobile and disordered. They will consequently provide only an average (spatially uniform) contribution to the STM images (e.g., a change in the tunnelling barrier). The maxima in the STM image of the (2√3 × 2√3)R30° structure most probably correspond to the TATA platforms−again similar as in alkane-thiol SAMs, where the contrast stems from the sulfur anchor group.1 Incorporation of the aqueous electrolyte in the vertically extended closepacked hydrocarbon layer on top of the (2√3 × 2√3)R30° phase is highly unlikely due to hydrophobic interactions. Consequently, an increase in the spatial extension of the electrochemical double layer is expected, which is supported by the 30 to 50% reduction in the double layer capacity during the (7√3 × 7√3) → (2√3 × 2√3)R30° transition. The nature of the minority phase, that is, the needlelike islands, is still somewhat unclear. Based on the width, apparent height, and the intermolecular spacings of 2√3 dAu, these structures might correspond to a close-packed one-dimensional chain of TATA molecules with vertically extended alkyl chains. However, in such an arrangement all the hydrocarbon chains would be exposed to the electrolyte solution, leading to energetically unfavorable hydrophobic interactions. A 1D structure of this type hence should be less stable than a 2D island. It thus seems likely that the molecular structure within the needles differs from that in the (2√3 × 2√3)R30° phase, which is also supported by the height difference between needles and dendritic islands. One possible explanation, which could account for a stabilization of 1D molecular structures, would be a bilayer arrangement in the needles, caused by direct covalent bonding between the central carbon atoms of the

TATA platforms. Such dimerization upon reduction has been found for the structurally similar trioxatriangulenium cations.44 The space requirements for the peripheral alkyl groups of such dimers could probably be more easily fulfilled in a linear than in a 2D arrangement. Because of the low surface density of the needles, experimental studies that would allow testing of this hypothesis are difficult, however. The domains of the high-density adlayer phase with upright oriented alky-chains coexist with areas in which the TATA density is lower and the peripheral chains have a more planar arrangement. This morphology resembles those found in STM observations of the self-assembly or oxidative desorption of alkanethiols with comparable chain lengths, where a similar coexistence of the close-packed c(4 × 2) phase and the lowdensity “striped” phase, that is, upright and in-plane oriented alkyl chains, was observed.3,45 In general, the average surface coverage of the octyl chains of 0.158 ML is in the so-called “intermediate” coverage regime, where for alkanethiols on Au(111) phase coexistence of close-packed and planar adsorbate structures is generally found.1 The driving force for the (7√3 × 7√3) ↔ (2√3 × 2√3)R30° transition seems to be the redox process 2A/2C. Obviously, this leads to a change toward more attractive intermolecular interactions. Comparison with the case of the (uncharged) thiolates indicates that for neutral species with intermediate hydrocarbon chain lengths the formation of closepacked domains, in which the van der Waals interactions between the alkyl groups are maximized, is energetically favored. At more positive potentials, where the TATA adlayer is in its (partly) oxidized state, these attractive interactions compete with the repulsive electrostatic interactions. Apparently, the resulting effective interactions prohibit intermolecular spacings of 2√3 dAu in this regime, leading to a reduced surface density of the alkyl chains, which inhibits a close-packed upright arrangement of the latter. In contrast to the fast and highly reversible redox process itself, the actual growth of the (2√3 × 2√3)R30° islands occurs on a much slower time scale, which can be associated with the self-assembly of the octyl chain layer. This results in an even stronger lateral intermolecular binding between the TATA adsorbates via van der Waals interactions between the alkyl groups. The latter is indicated by the low TATA surface mobility within and along the (2√3 × 2√3)R30° islands. The latter manifests in the high defect density and dendritic island shape. Our STM data suggest that the (2√3 × 2√3)R30° island growth proceeds by stabilization of TATA adsorbates of the low-coverage disordered phase at the island edges, followed by an upward tilt of the alkyl groups and their attachment to the self-assembled dense monolayer phase. The slow kinetic of this process again resembles the well-known behavior of alkanethiol self-assembly, which occurs on much longer time scales as the thiolate adsorption process.1 The nearly instantaneous decay of the (2√3 × 2√3)R30° islands during the oxidation process 2A is surprising. It suggest that the concomitant change in the intermolecular interactions is so strong that the close-packed TATA structure becomes highly energetically unfavorable, driving the rapid disassembly of this phase.

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CONCLUSIONS The adsorbate system studied in this work, octyl-TATA molecules on Au(111) electrode surfaces, exhibits novel, interesting properties, not found previously in organic adlayers. H

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Specifically, our in situ STM investigations provide clear support for a potential-controlled aggregation of the peripheral octyl chains into a close-packed, upright oriented hydrocarbon layer on top of the molecular platforms. Because the average coverage is constant, this transition involves changes from a surface that is homogeneous covered by a (√19 × √19)R23.4° adlayer to an intermediate, denser (7√3 × 7√3) phase (coexisting with disordered areas) and, finally, to an adlayer exhibiting strong nanoscale heterogeneity. In the latter state, found at potentials ≤0.1 VSCE, nanoscale islands of the close-packed (2√3 × 2√3)R30° structure coexist with a low-density phase of disordered, mobile adsorbates; in the (√19 × √19)R23.4° regime at ≥0.4 VSCE, the molecules are uniformly spaced and the alkane chains mostly exhibit a planar arrangement. By changing the potential fast and fully reversible switching between these two very different adlayer states is possible. While potential-induced phase transitions involving reorientation from a planar to a more upright geometry are well documented,31 our system differs in an important aspect from those of previously studies. In the latter cases, the molecules were typically present in the electrolyte and the transitions involved the adsorption/desorption of species. These changes in molecular coverage are associated with changes in the free energy of adsorption and the overall packing density which (at least partly) can drive the phase transition. In contrast, the phase transition in the TATA adlayer has to be genuinely related to a change in the effective lateral interactions between the adsorbates and may be considered as a potential-induced “wetting/dewetting” of the surface-attached alkyl chains. These results have several interesting implications. First, similar phase transitions may occur for other irreversibly adsorbed molecules carrying hydrocarbon chains, for example, for porphyrins, phthalocyanins, or other polyaromatic compounds with peripheral alkyl chains. Previous STM studies of such adsorbate systems under ambient or vacuum conditions typically found a planar adsorption of the alkyl groups on the surface, often resulting in 2D supramolecular organization.11 The structure of these systems in electrochemical environment is largely unknown, however. According to our data, major (potential-dependent) reorganization may occur, which may be relevant for applications of these adlayers. Most likely, these phenomena depend on the length of the hydrocarbon chains, the type, size, and charge state of the aromatic platform, and the resulting (average) surface density of the hydrocarbon chains. The role of these parameters will have to be assessed in future studies. Second, a rather different electrochemical behavior of the functionalized electrode is expected for the two different adlayer states. While the homogeneous (√19 × √19)R23.4° and (7√3 × 7√3) adlayer phases should effectively block the direct adsorption of larger adsorbates on the Au substrate, the latter should be possible in the low coverage areas coexisting with the (2√3 × 2√3)R30° phase, enabling chemical interactions such as electrocatalytic reactions. Because of the sharply defined potential and high reversibility of the (7√3 × 7√3) ↔ (2√3 × 2√3)R30° transition, this may allow switching (within a narrow potential regime) back and forth between an unreactive electrode state and a state exhibiting local reactive areas. This might be interesting, for example, for controlling (bio)electrochemical reactivity of surfaces.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses †

C.-H.C.:Agricode, Taman Mount Austin, 81100 Johor Bahru, Johor, Malaysia. ‡ U.J.: Department of Chemistry, University of Illinois, UrbanaChampaign, Urbana, Illinois 61801, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding by the Deutsche Forschungsgemeinschaft (SFB 677) is gratefully acknowledged. We thank S. Ulrich and R. Herges for providing the octyl-TATA compound.

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REFERENCES

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