On the Impact of Geometrical Factors on Hot Electron-Induced

Article Cite This: J. Phys. Chem. C 2019, 123, 17056−17061

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On the Impact of Geometrical Factors on Hot Electron-Induced Tautomerization Jens Kügel,*,† Tim Zenger,† Markus Leisegang,† and Matthias Bode†,‡ †

Physikalisches Institut, Experimentelle Physik II, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Wilhelm Conrad Röntgen-Center for Complex Material Systems (RCCM), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany

‡

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S Supporting Information *

ABSTRACT: The ability to controllably switch molecules, for example, by the application of external stimuli, such as charge carriers or photons, has fascinated the scientific community since the advent of nanoscience. A prominent example is isomerization, that is, the transformation of a molecule with a given shape into another atomic arrangement without changing the chemical formula. This rearrangement often only involves a small subunit of the molecule, the active part, whereas the inactive part remains unchanged. Here, we present a systematic low-temperature scanning tunneling microscopy investigation of the influence the inactive part has on hot electron-induced tautomerization processes. In our study, we investigate crosslike molecules, namely, deprotonated phthalocyanine and naphthalocyanine molecules, which exhibit the same central active unit but possess molecule arms of different lengths. If deposited on a sixfold symmetric Ag(111) surface, the four molecule arms loose degeneracy, resulting in two pairs of opposing arms with different tautomerization efficiencies. Our data reveal that the electron yield changes significantly depending on the specific molecular arm the charge carriers reach after injection into the Ag(111) surface state. To partially separate the fraction of hot electrons reaching the different arms, we performed distance- and angle-dependent molecular nanoprobe measurements. The experimental results are interpreted within a very simple geometrical model assuming rectangular shaped molecular arms, which nicely explains the effects observed for phthalocyanine and naphthalocyanine molecules.

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INTRODUCTION Motivated by the ability to create machine-like parts on the nanometer scale, the design and analysis of molecules attracted considerable interest.1,2 Significant research was focused but not limited to the creation of molecular rotors,3,4 motors,5 and switches.6−10 All of these molecular machines have in common that they include an active part which provides the desired functionality. This can, for example, be the rotational motion of a molecular part11 or a tautomerization process,12−14 for example, the switching of protons between different bonding sites in the molecule. These active parts are typically embedded in a larger molecular frame which includes parts that are not needed for dynamical processes of the molecular function but lead to an enhanced spatial extension of the molecule. Most of the available research focuses on the understanding of the active parts. For example, it has been shown that the tautomerization of porphycene15 or phthalocyanine16 molecules is triggered by vibrational modes involving the motion of the switchable protons. By contrast, the parts not directly involved in the dynamical process have rarely been analyzed in detail. Here, we present an in-depth analysis focusing on the effect of the geometrical shape of cross-shaped molecules, namely, phthalocyanine and naphthalocyanine molecules, on hot electron-induced tautomerization mediated by a Ag(111) © 2019 American Chemical Society

surface. Our data reveal that the spatial extension of the molecule plays an important role for the electron yield of the tautomerization. The experiments were performed by injecting hot electrons in the Ag(111) surface in close proximity (typically a few nanometers) to each of the molecular arms. Because of the symmetry mismatch between the fourfold symmetric phthalocyanine macrocycle and the sixfold symmetric Ag(111) surface layer, we observe a strong variation of the electron yield for hot electrons reaching different arms of a singly deprotonated phthalocyanine molecule. The two arms oriented along the [21̅1̅] direction of the Ag(111) surface exhibit a higher electron yield, whereas the other two adjacent arms oriented along the [01̅1] direction have a lower efficiency to convert the energy of incoming hot electrons into protontransfer reactions. Angle- and distance-dependent measurements were used to partially separate the effect hot electrons have when they reach the molecular arm, which is closest to the injection point as compared to the other two adjacent arms. The data can be fitted by a surprisingly simple model which assumes rectangular-shaped arms with two different efficiencies to convert hot electrons into switching events. This model is verified by the usage of naphthalocyanine molecules, Received: June 12, 2019 Published: June 13, 2019 17056

DOI: 10.1021/acs.jpcc.9b05602 J. Phys. Chem. C 2019, 123, 17056−17061

Article

The Journal of Physical Chemistry C

Figure 1. (a) Topographic image of H2Pc molecules on a Ag(111) surface. The inset shows an atomically resolved topograph of the Ag(111) surface. (b) Structural model of an H2Pc molecule. Red arrows show a tautomerization process, the switching of the two inner protons between adjacent nitrogen atoms. (c) Topographic image of the same location presented in (a) after movement and deprotonation of the H2Pc molecule. (d) Four isomeric states of an HPc molecule. Scan parameters: (a,c,d) U = −50 mV; I = 100 pA; inset of (a) U = 10 mV; I = 65.1 nA (moleculeterminated tip).

molecule is moved to a defect-free area prior to measurements, as indicated by the yellow arrow in Figure 1a. For this horizontal manipulation, a bias voltage of U = 20 mV and a tunneling current of I = 15−50 nA are used. Additionally, one of the inner protons of the molecule is removed by a voltage pulse (U ≈ 2.0−2.2 V; I = 1 nA), with the tip being positioned on top of the molecule. Deprotonation results in a reduced tautomerization barrier, thereby strongly enhancing the efficiency of hot electron-induced switching.16 Figure 1c shows the same area as in (a) after the aforementioned manipulation steps. The singly deprotonated H2Pc molecule, named HPc in the following, can be easily identified by its topographic appearance. After the removal of one inner proton, the binding position of the remaining proton is signaled by the larger apparent height of this arm. In total, the proton can be bound to any of the four arms. Consequently, four isomeric states of an HPc molecule exist, which are shown in Figure 1d. Because of symmetry mismatch between the surface and the molecular macrocycle, however, the degeneracy of these isomers is partially lifted. As a consequence, two isomers are metastable with a lifetime of a few seconds, whereas the other two isomers do not switch as long as noninvasive scan parameters (|U| ≤ 50 mV) are used for molecule state analysis.13 Our results show that the isomer with the proton bound to an arm pointing into ⟨21̅1̅⟩ directions (⟨01̅1⟩ directions) is stable (metastable). To analyze the effect of the geometrical shape of phthalocyanine molecules on hot charge carrier-induced tautomerization processes, we made use of the so-called molecular nanoprobe (MONA) technique.18,19 In short, it works as follows: first, the isomeric state of the HPc molecule is probed by a topographic scan of the molecule with noninvasive parameters (|U| ≤ 50 mV; I = 100 pA). Afterward, the tip is moved to an arbitrary point, for example, the bright yellow star in Figure 1c. At this position, bias and current are changed for a certain duration texc to excitation parameters to inject hot charge carriers into the Ag(111) surface. In our case, we used an excitation bias Uexc = 0.5 V for all MONA data presented in the rest of the study. Last, in order to analyze whether the isomeric state has changed because of excitations by hot electrons, the resulting isomeric state of the molecule is measured again by a topographic scan with noninvasive parameters. This procedure is repeated many times (≥5000)

derivatives of the phthalocyanine molecules, which are characterized by longer molecular arms. Our experiments clearly show that this leads to a higher fraction of hot electrons reaching the arms, which are transversely oriented with respect to the injection point, and results in a crossover in the electron yield for large tip−molecule distances.

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EXPERIMENTAL METHODS Naphthalocyanine (H2Nc) and phthalocyanine (H2Pc) molecules were purified by degassing in ultrahigh vacuum conditions for several hours. The Ag(111) surface was prepared by cycles of 10 min Ar+ ion sputtering at an energy of 0.5 keV and subsequent annealing at 700 K for 20 min. The molecules were deposited on the clean surface at room temperature by means of a home-built evaporator with a filament-heated quartz crucible. After preparation, the sample was immediately transferred into our home-built low-temperature scanning tunneling microscopy (STM) operated at a sample and tip temperature of T ≈ 4.5 K.

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RESULTS AND DISCUSSION Topography, Manipulation, and Measurement Method. In Figure 1a, the topographic image of a phthalocyanine molecule (H2Pc) on a Ag(111) surface is shown together with the structural model of the molecule in Figure 1b. The molecule consists of an organic macrocycle with a crosslike shape. The arms of the molecule are oriented along the highsymmetry crystallographic directions of the Ag(111) surface. Two of the arms point into one of the ⟨21̅1̅⟩ axes, whereas the other two are oriented along one of the ⟨01̅1⟩ axes, as can be identified by the atomically resolved surface in the inset of Figure 1a. In the center of an H2Pc molecule, two protons are bound to opposing nitrogen atoms (cf. Figure 1b). The binding position of these protons is characterized by an elevated appearance (bright yellow color) in the topographic image. As demonstrated in earlier publications, the tautomerization process can be triggered by hot charge carriers injected directly either into the molecular frame13,16,17 or into the surface and mediated by the substrate,18−20 which leads to a switching of the proton binding position to the adjacent nitrogen atoms (cf. red arrows in Figure 1b). To ensure that the molecule of interest is not adsorbed on a defect or influenced by any molecule or defect nearby, the 17057


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surface states sensitively depend on the shape of the cluster at the tip apex. A more detailed analysis based on a comparison of dI/dU data and the electron yield can be found in the Supporting Information. These data show that the higher the proportion of hot electrons tunneling into bulk states, the lower the electron yield. We are convinced that this behavior represents the lower probability of bulk electrons of reaching the molecule (decay scales ∝ d−2) as compared to surface-state electrons (∝d−1). Within the individual data sets, two main trends can be observed: first, the electron yield of the α = 0° (α = 90°) measurement is the same as the α = 180° (α = 270°) within the statistical error. Second, the electron yield obtained at α = 0°/180° is higher than at α = 90°/270°. For a better visualization of these two trends, all data sets were normalized to the α = 0° measurement and an average value of each direction was calculated, the result of which is shown in Figure 2c. By following the laws of error propagation, we included the error bar of the α = 0° direction to the error bars of the other directions. The data set shows that injection close to a metastable arm reduces the electron yield to roughly 75% of the corresponding value for the stable arm. The physical origin of this behavior can be deduced by analyzing the residence time of the isomeric states, which describes the amount of consecutive measurements the molecule stays in the same isomeric state. Details on how the residence time is calculated can be found in the Supporting Information. For the sake of clarity, all five data sets were normalized to the α = 0° measurement and an average value of each direction was calculated, as shown in Figure 2d. In agreement with the trend already observed for the electron yield, we also obtain a normalized residence time which is identical for α = 0° and α = 180° within the measurement error. In contrast, significant differences are observed between α = 0°/180° on one and α = 90°/270° on the other side: if hot electrons are injected close to a metastable arm (α = 90°/ 270°), the residence time of the metastable isomers is reduced and the residence time of the stable isomers is increased in comparison to an injection close to the stable arm (α = 0°/ 180°). In other words, the residence time of the stable (metastable) isomeric state is lowest if hot electrons are injected close to a stable (metastable) arm. This trend demonstrates that vibrational modes of the proton and thus tautomerization are more efficiently excited if the proton is bound on the axis of electron incidence. With the help of these findings, the differences in electron yield can be explained: if hot electrons are injected close to a metastable arm, the residence time of the protons on the metastable arm is reduced. As the lifetime of these metastable states is only in the range of a few seconds because of symmetry mismatch between the surface and organic macrocycle,13 a further reduction of the lifetime will only slightly affect the electron yield. In contrast, the stable isomers do not switch as long as noninvasive scan parameters are used. Thus, a reduction of the residence time of the stable state will lead to a stronger enhancement of the electron yield. It should be noted that the different efficiencies of the molecular arms should be in principle also detectable if the electrons are injected directly into the molecule. However, the proximity of the tip to the molecular frame energetically shifts the tautomerization landscape, which drastically affects the efficiencies.14,16 Rotational Test of HPc/Ag(111). To verify that the differences in electron yield are indeed related to the direction

to reduce the statistical deviations. Finally, an electron yield was extracted from a data set by dividing the amount of hot electron-induced switches by the total amount of hot charge carriers injected into the substrate. MONA of HPc/Ag(111). As shown previously, the symmetry mismatch between the substrate and HPc organic macrocycle lifts the isomer degeneracy, thereby giving rise to two metastable and two stable isomeric states.13 To test whether this symmetry mismatch also influences the efficiency of hot electron-induced tautomerization processes, we performed MONA measurements along the four directions of the molecular arms. The injection points of these measurements are positioned on a circle with a radius d = 4.55 nm around the center of the molecule and marked by yellow stars in Figure 1c and in the schematic presentation in Figure 2a. For the angle α = 0° or 180°, the injection point is

Figure 2. (a) Schematic representation of the measurement setup: the injection points of hot electrons have a distance d = 4.55 nm from the center of the HPc molecule (orange cross) and are oriented along the four arms of the molecule. (b) Electron yield of five series of measurements (MONA parameter: Uexc = 0.5 V, Iexc = 2.0 nA, texc = 2 s). (c) Normalized averaged electron yield calculated by normalizing the data of (b) to the data points at α = 0°. (d) Normalized residence time of the stable and metastable isomeric states. Details on the calculation can be found in the Supporting Information.

closest to an arm of the stable isomeric state, whereas α = 90° or 270° describes the situation when the injection point is closest to an arm of the metastable isomeric states. For the sake of readability, the arm of a stable (metastable) isomeric state will be named the stable (metastable) arm hereafter. In Figure 2b, MONA data of five series of measurements are presented. With the exception of the last two data sets (labeled 4th and 5th), the measurement series were recorded with different tips and on different locations of the Ag(111) surface. Apparently, the electron yield between the different data sets varies strongly. This can be explained by the fact that the ratio of hot electrons which tunnel either into the bulk or into the Ag(111) surface state strongly depends on the particular tip. One possible explanation is that the orbital character of the tip states and therefore the tunneling matrix elements to bulk and 17058


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orientation of the tip to these arms is responsible for the observed electron yield, it should be mirror symmetric to the directions of the arms. Because of the rotation of the molecule by 120°, this mirror symmetry expresses itself in a mirror symmetry to the angle bisector of the two rotations (β = 60°), which is reflected in the data. In contrast, if not the relative orientation to the arms of the molecule but the tip position on the Ag(111) surface was important for the tautomerization, one would expect that the angle-dependent electron yield is only slightly influenced by the rotation of the HPc molecule. This scenario would appear if the asymmetry is given by an anisotropic transport of the hot electrons in the surface, as in such a case the absolute position of the injection point to the molecule and not the relative orientations to the arms of the HPc is responsible for the observed differences in electron yield. This theory is in disagreement with the observed mirror symmetric data and is unlikely to appear because of an isotropic surface state. Distance- and Angle-Dependent MONA: HPc/ Ag(111). The analysis presented so far has shown that the injection of hot electrons close to a stable arm leads to a higher electron yield as compared to hot electrons, which are injected close to a metastable arm. Nevertheless, one would expect that hot electrons, which are injected close to the stable arm, also have for geometrical reasons a certain probability to reach one of the two adjacent metastable arms. Consequently, there should always be a mixture of hot electrons reaching stable and metastable arms. In order toat least partiallydisentangle these contributions, we performed distance- and angledependent MONA measurements as schematically shown in Figure 4a. Three angles were used for these measurements,

of the metastable and stable arms and not affected by the substrate or the tip (cf. Figure 2b,c), we performed test measurements by rotating the molecule as presented in Figure 3a. First, an HPc molecule (rot. 1) was analyzed by MONA

Figure 3. (a) Topography of the HPc molecule (rot. 1) together with four injection points (yellow stars) for the MONA measurements. The green contour shows the orientation of the HPc molecule after rotation (rot. 2). Solid lines mark the direction of the stable arm, and dashed lines indicate the direction of the metastable arms. The colors of these lines correspond to the two rotations. (b) Electron yield of the MONA measurements performed at the four injection points shown in (a) for the two rotations of the HPc molecule. (MONA parameter: Uexc = 0.5 V, Iexc = 2.0 nA, texc = 2 s.)

measurements performed at four excitation points, all of which have the same distance d = 4.55 nm to the center of the molecule. For an easier comparison, the angle β of the injection point is defined with respect to the [21̅1̅] direction of the Ag(111) surface in contrast to α, which was defined with respect to the stable arm of an HPc molecule. Two excitation points are oriented along the molecular arms of the rot. 1 molecule, namely, the metastable arm (β = 30°) and the stable arm (β = 120°). The other two excitation points (β = 0° and β = 90°) are rotated by 30° with respect to the aforementioned directions. The electron yield for all four injection points is shown in Figure 3b. In line with the aforementioned results, the electron yield of rot. 1 along the stable arm (β = 120°) is higher as compared to the metastable arm (β = 30°). For the other two directions, the electron yield is further reduced, most likely because the injection points are not oriented along the arms. A more detailed analysis of this behavior will be given in the next section. After these four MONA measurements, the same molecule was rotated by 120° (rot. 2), which is indicated by the green contour in Figure 3a. In this way, we ensure that we measure the same molecule with the same tip and at the same location but with a different orientation of the metastable and stable arms. One of the stable (metastable) arms is now oriented along the β = 0° (β = 90°) direction. The MONA data for this rotation are also included in Figure 3b. Again, the highest electron yield is given for an electron injection closest to the stable arm, which in fact was the direction of the lowest electron yield for the first rotation. Within statistical error, all data points of the two rotations are mirror symmetric with respect to the β = 60° angle. This behavior can be expected if the electron yield depends on the relative orientation of the injection point with respect to the stable and metastable arms, which can be explained as follows: the HPc molecules are oriented with the arms pointing in the high-symmetry direction of the Ag(111) surface. Therefore, if the relative

Figure 4. (a) Schematic model of the angle- and distance-dependent MONA measurements. The injection points are marked by yellow stars and are oriented in the 0° (along the stable arm), the 90° (along the metastable arm), and the angle bisector (45°) direction. (b) Angle- and distance-dependent electron yield of the three directions. The data were fitted with a simple model (colored lines) which assumes rectangular-shaped arms of the molecule with an effective length a, an effective width w, and different quantum efficiencies for the stable (blue) and metastable (red) arms as shown in (a). (MONA parameter: Uexc = 0.5 V, Iexc = 2.0 nA, texc = 1−3 s.)

namely, the α = 0° direction along the stable arm, the α = 90° direction along the metastable arm, and the angle bisector (α = 45°) of the other two directions. Furthermore, we systematically varied the distance between the injection point and center of the HPc molecule from d = 2 nm to d = 7.5 nm. The resulting electron yields of these measurements are shown in Figure 4b. In general, all data sets closely follow a power law with an exponent −1. For better visualization of this trend, two gray dashed lines serve as a reference for the slope of the d−1 power law. This result indicates that tautomerization is mainly 17059


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behavior. Toward this goal, we used naphthalocyanine molecules, which possess a central structure which is identical to phthalocyanine but exhibit longer arms because of an additional benzene ring. In Figure 5a, the topographic image of

triggered by hot electrons mediated by the Ag surface and not by the bulk, as we would expect a d−2 power law in the latter case. This result also agrees with interference experiments performed on the same molecule−substrate system. 19 Furthermore, the presence of only small deviations from the d−1 power law indicates that the phase coherence length should be much larger than the length scale probed in our measurements, which is in agreement with a phase coherence length LΦ ≈ 32 nm determined by an analysis of Friedel oscillations.21 However, a detailed analysis also reveals some differences between the directions: over the whole distance range, we measure a higher electron yield for the α = 0° direction than for the α = 90° direction. This observation again demonstrates that an enhanced number of switching events are observed when hot electrons are injected close to the stable arm. Nevertheless, this enhancement effect is not constant but dependent on the injection point−molecule distance. With increasing distance, the effect is reduced, as indicated by the steeper slope of the black data points (0° direction). This trend can be rationalized by straightforward geometrical reasons: if the injection point is very close to a particular molecular arm, a very high proportion of hot electrons reaches this particular arm because of an increased field of view (cf. bright blue dotted lines in Figure 4a), thereby reducing the fraction of hot electrons reaching the two adjacent arms. In contrast, the field of view for the closest arm decreases and for the two adjacent arms increases if the injection point is further away from the arm (dark blue dotted lines). Consequently, the fraction of hot electrons that can reach the other two adjacent arms becomes larger as the distance of the hot carrier injection point is increased, whereas the fraction of hot electrons reaching the closest arm is reduced. In case the STM tip is located along a stable arm, this increased ratio of hot electrons reaching the metastable arm will result in a reduction of the electron yield because the metastable arms are rather inefficient in triggering the tautomerization. On the basis of these considerations, we developed a very simple geometrical model to fit the experimental data, as schematically shown in Figure 4a: each of the molecular arms was approximated by a rectangular shape with an effective length a and an effective width w. The stable (blue) and metastable (red) arms are assigned individual quantum efficiencies to describe their different probabilities of triggering hot electron-induced tautomerization. The result of this fit is represented by solid colored lines in Figure 4b. Despite the simplicity of the model, the agreement with the 0° and 90° data points is very good. Only the 45° data set shows significant deviations from this fit. This discrepancy is most likely caused by hot electrons which reach the arms of the molecule under a flat angle, possibly resulting in a lower transmission probability through the potential wall of the molecule. Comparison with Naphthalocyanine. So far, we could demonstrate that the angular- and distance-dependent hot electron-induced tautomerization of phthalocyanine on Ag(111) can be modeled by four rectangular-shaped molecule arms, whereby two of these arms exhibit a higher efficiency to convert the energy of the hot electrons into switching events. To verify this simple model, it would be useful to increase the arm length of the phthalocyanine molecules. Such a modification of the molecule shape should result in a predictable change of the geometry-dependent tautomerization

Figure 5. (a) Topographic image of HPc and HNc molecules together with their structural model (scan parameter: U = −50 mV; I = 100 pA). (b) Model to demonstrate the effect of elongated molecular arms. (c) Normalized electron yield of an HNc molecule excited along the direction of the molecular arms. The setup of the measurement is schematically shown in Figure 2a (MONA parameters: Uexc = 0.5 V; Iexc = 2.5 nA; texc = 3 s; d = 4.55 nm). (d) Angle- and distance-dependent MONA measurement of an HNc molecule. The setup of the measurement is schematically shown in Figure 4a (MONA parameters: Uexc = 0.5 V; Iexc = 7.5 nA; texc = 0.44−1.65 s; d = 4.55 nm).

a deprotonated phthalocyanine (HPc) molecule and a deprotonated naphthalocyanine (HNc) together with their structural models is shown, clearly demonstrating the enlargement of the molecular arms. In the following, we will perform a comparison of fundamental geometrical properties of HPc and HNc molecules. For the sake of simplicity, we keep distance between the hot carrier injection point and the molecule center constant. As schematically represented in Figure 5b, the hot electron current which impacts the molecule arm directly facing the injection point will only increase by a minuscule amount. In contrast, the amount of hot electrons reaching the two adjacent arms which are oriented transversely with respect to the hot carrier injection direction will substantially increase. Consequently, in the case of HNc, we may be able to realize a situation where more hot electrons are adsorbed at the two transversely oriented arms than at the arm which is closest to the injection point. To experimentally verify this effect, we performed MONA experiments with an HNc molecule. The results of an angle-dependent analysis along the molecular arms (d = 4.55 nm) and a more systematic distance- and angledependent data set are presented in Figure 5c,d, respectively. The first measurement series (cf. Figure 5c) demonstrates that in the case of HNc molecules, a higher electron yield is achieved if hot electrons are closest to the metastable arms (90°- and 270°-direction). This inversion is in agreement with the aforementioned assumption that a larger proportion of hot 17060


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an Array of Anchored Single-Molecule Rotors on Gold Surfaces. Phys. Rev. Lett. 2008, 101, 197209. (5) van Delden, R. A.; Ter Wiel, M. K. J.; Pollard, M. M.; Vicario, J.; Koumura, N.; Feringa, B. L. Unidirectional Molecular Motor on a Gold Surface. Nature 2005, 437, 1337. (6) Wang, Y.; Kröger, J.; Berndt, R.; Hofer, W. A. Pushing and Pulling a Sn Ion through an Adsorbed Phthalocyanine Molecule. J. Am. Chem. Soc. 2009, 131, 3639−3643. (7) Choi, B.-Y.; Kahng, S.-J.; Kim, S.; Kim, H.; Kim, H. W.; Song, Y. J.; Ihm, J.; Kuk, Y. Conformational Molecular Switch of the Azobenzene Molecule: A Scanning Tunneling Microscopy Study. Phys. Rev. Lett. 2006, 96, 156106. (8) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fréchet, J. M. J.; Trauner, D.; Louie, S. G.; et al. Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface. Phys. Rev. Lett. 2007, 99, 038301. (9) Liljeroth, P.; Repp, J.; Meyer, G. Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules. Science 2007, 317, 1203−1206. (10) Jaekel, S.; Richter, A.; Lindner, R.; Bechstein, R.; Nacci, C.; Hecht, S.; Kühnle, A.; Grill, L. Reversible and Efficient Light-Induced Molecular Switching on an Insulator Surface. ACS Nano 2018, 12, 1821−1828. (11) Moresco, F.; Meyer, G.; Rieder, K.-H.; Tang, H.; Gourdon, A.; Joachim, C. Conformational Changes of Single Molecules Induced by Scanning Tunneling Microscopy Manipulation: A Route to Molecular Switching. Phys. Rev. Lett. 2001, 86, 672−675. (12) Auwä r ter, W.; Seufert, K.; Bischoff, F.; Ecija, D.; Vijayaraghavan, S.; Joshi, S.; Klappenberger, F.; Samudrala, N.; Barth, J. V. A Surface-Anchored Molecular Four-Level Conductance Switch Based on Single Proton Transfer. Nat. Nanotechnol. 2012, 7, 41−46. (13) Kügel, J.; Sixta, A.; Böhme, M.; Krönlein, A.; Bode, M. Breaking Degeneracy of Tautomerization-Metastability from Days to Seconds. ACS Nano 2016, 10, 11058−11065. (14) Kumagai, T.; Hanke, F.; Gawinkowski, S.; Sharp, J.; Kotsis, K.; Waluk, J.; Persson, M.; Grill, L. Controlling Intramolecular Hydrogen Transfer in a Porphycene Molecule with Single Atoms or Molecules Located Nearby. Nat. Chem. 2014, 6, 41−46. (15) Kumagai, T.; Hanke, F.; Gawinkowski, S.; Sharp, J.; Kotsis, K.; Waluk, J.; Persson, M.; Grill, L. Thermally and Vibrationally Induced Tautomerization of Single Porphycene Molecules on a Cu(110) Surface. Phys. Rev. Lett. 2013, 111, 246101. (16) Kügel, J.; Klein, L.; Leisegang, M.; Bode, M. Analyzing and Tuning the Energetic Landscape of H2Pc Tautomerization. J. Phys. Chem. C 2017, 121, 28204−28210. (17) Sperl, A.; Kröger, J.; Berndt, R. Controlled Metalation of a Single Adsorbed Phthalocyanine. Angew. Chem., Int. Ed. 2011, 50, 5294−5297. (18) Kügel, J.; Leisegang, M.; Böhme, M.; Krönlein, A.; Sixta, A.; Bode, M. Remote Single-Molecule Switching: Identification and Nanoengineering of Hot Electron-Induced Tautomerization. Nano Lett. 2017, 17, 5106−5112. (19) Leisegang, M.; Kügel, J.; Klein, L.; Bode, M. Analyzing the Wave Nature of Hot Electrons with a Molecular Nanoprobe. Nano Lett. 2018, 18, 2165−2171. (20) Kügel, J.; Leisegang, M.; Bode, M. Imprinting Directionality into Proton Transfer Reactions of an Achiral Molecule. ACS Nano 2018, 12, 8733−8738. (21) Braun, K.-F.; Rieder, K.-H. Engineering Electronic Lifetimes in Artificial Atomic Structures. Phys. Rev. Lett. 2002, 88, 096801.

electrons is adsorbed at the two transversely oriented arms in comparison to the arm which is closest to the injection point. Our model is further supported by the angle- and distancedependent measurement presented in Figure 5d, whichas compared to the results of Figure 4bshows an inversion at large distance d between the injection point and the molecule. At close distance, however, a crossing of the electron yield is observed, an effect which is also in line with the simple geometrical model: if the injection of the hot electrons is very close to a molecular arm, less electrons are able to reach the two adjacent arms because of a decreased field of view.

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CONCLUSIONS In summary, we have shown that the arms of crosslike molecules, namely, phthalocyanine and naphthalocyanine molecules, play a crucial role in the efficiency to trigger the tautomerization of these molecules. Two opposing arms of the molecules are characterized by a low electron yield for incoming hot electrons, whereas the other two arms have a higher value. Consequently, the length and width of these arms strongly affect the total electron yield, which was confirmed by angle- and distance-dependent MONA measurements of a singly deprotonated phthalocyanine and naphthalocyanine molecule. Our results suggest that it is not only important to focus on the active parts of molecular machines but also to take into consideration the spatial expansion of these molecules.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b05602. Comparison of dI/dU signal and electron yield; calculations of the residence times; and details on the fit function (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jens Kügel: 0000-0002-9260-0972 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) − Project-ID 258499086 − SFB 1170 (A02) and by the Dresden−Würzburg Center for Topological Quantum Matter Research (ct.qmat).

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REFERENCES

(1) Erbas-Cakmak, S.; Leigh, D. A.; McTernan, C. T.; Nussbaumer, A. L. Artificial Molecular Machines. Chem. Rev. 2015, 115, 10081− 10206. (2) Abendroth, J. M.; Bushuyev, O. S.; Weiss, P. S.; Barrett, C. J. Controlling Motion at the Nanoscale: Rise of the Molecular Machines. ACS Nano 2015, 9, 7746−7768. (3) Zhang, Y.; Kersell, H.; Stefak, R.; Echeverria, J.; Iancu, V.; Perera, U. G. E.; Li, Y.; Deshpande, A.; Braun, K.-F.; Joachim, C.; et al. Simultaneous and Coordinated Rotational Switching of All Molecular Rotors in a Network. Nat. Nanotechnol. 2016, 11, 706. (4) Gao, L.; Liu, Q.; Zhang, Y. Y.; Jiang, N.; Zhang, H. G.; Cheng, Z. H.; Qiu, W. F.; Du, S. X.; Liu, Y. Q.; Hofer, W. A.; et al. Constructing 17061


On the Impact of Geometrical Factors on Hot Electron-Induced

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