Analyzing and Tuning the Energetic Landscape of H2Pc

Article pubs.acs.org/JPCC

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

Analyzing and Tuning the Energetic Landscape of H2Pc Tautomerization Jens Kügel,*,† Lucas Klein,† Markus Leisegang,† and Matthias Bode†,‡ †

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

‡

S Supporting Information *

ABSTRACT: We present a detailed analysis of the energetic landscape of phthalocyanine (H2Pc) tautomerization on Ag(111), i.e., the switching of protons between different sites in the molecular frame, which is induced and measured by a low-temperature scanning tunneling microscope (STM). We demonstrate that tautomerization of this molecule is preferentially triggered by the excitation of the N−H stretching mode. Interestingly, a step-like increase of the tautomerization rate is observed at a bias voltage that corresponds to the second harmonic of this vibrational mode, which we attribute to the crossover from quantum tunneling through the tautomerization barrier to an excitation over the barrier. This hypothesis is supported by the analysis of four modified versions of H2Pc, produced by single silver atom manipulation and/or deprotonation of the pristine H2Pc molecule. Depending on the particular modification, the step-like increase varies strongly, spanning the entire range from further enhancement to almost complete disappearance. We interpret this behavior in terms of different tautomerization barrier heights which for most molecules lies between the first and second harmonics of the N−H stretching mode but which is strongly reduced well below the first harmonic for deprotonated H2Pc.

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porphycene molecules.3−6 However, this molecule exhibits a relatively narrow tautomerization hydrogen transfer barrier, such that quantum tunneling between the isomers is often observed.6−9 Furthermore, the height of the tautomerization barrier coincides with the energies of a plethora of molecular vibration modes,3 making the identification of the influence of individual modes very challenging.3 In contrast, phthalocyanine molecules (H2Pc) exhibit a much higher tautomerization barrier,10,11 such that contributions from low-lying in-plane and out-of-plane bending modes are strongly suppressed and tautomerization can exclusively be triggered via the N−H stretching mode. This makes H2Pc an ideal candidate to study details of inelastic tunneling-induced tautomerization. Here, we present a detailed analysis of the STM-induced tautomerization of H2Pc on Ag(111) and four manipulated versions of this molecule, which were created by adding single silver atoms to different locations of the molecular frame and/ or by removing one of the inner protons. Our data show that, indeed, the N−H stretching mode is responsible for triggering tautomerization. Interestingly, a strong increase in the electroninduced switching rate is observed at an electron energy corresponding to the second harmonic of this vibrational mode, which we attribute to a crossover from quantum tunneling

INTRODUCTION On the search for possible design pathways toward singlemolecule electronic switches, the tautomerization of molecules came in focus of research in the past decade.1,2 In this context, the proposal of altering the conductance of a molecular switch placed between two semiconducting or metallic electrodes through the reversible transition between two or more isomers appears highly interesting. In particular, local probe techniques, which allow for the real-space investigation of single molecules with submolecular resolution, have been extensively used to image and characterize the switching between isomers. Whereas the term tautomerization describes constitutional isomers of organic compounds whichat a given temperatureare in chemical equilibrium and, therefore, readily interconvert, the energy barrier between isomers is usually relatively high such that it cannot be frequently overcome by the available thermal energy. It has recently been shown, however, that the switching between different isomers can controllably be triggered by local STM-induced electronic excitation processes. In this method, inelastic contributions to the tunnel current that flows from the STM tip into the molecule or vice versa excite internal molecular vibration modes which in most cases involve the motion of protons and eventually enable the transition from one isomer through the hydrogen transfer barrier to another isomer. In the past, several studies have investigated STM-induced tautomerization processes, most of which were performed with © XXXX American Chemical Society

Received: October 25, 2017 Revised: November 23, 2017 Published: November 27, 2017 A

DOI: 10.1021/acs.jpcc.7b10564 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Topography of H2Pc molecules on Ag(111) (right panel: structural model of H2Pc). (b) Topography of the same area as in part a after movement of the molecules (yellow and red arrows in part a) and production of a single silver atom (lower part of the image). The right side shows the two isomers (1, 2) of H2Pc. The telegraph noise (c) measured at the position of the blue cross in part b shows two height levels, which correspond to isomers 1 and 2 (parameters: U = −0.84 V, I = 1.0 nA). (d) Topography of the same area as in part b after deprotonation of the central H2Pc. The right side shows the four isomers 3−6 of the HPc molecule. (e) Topography of the same area as in part d after moving the Ag atom into the center of the HPc molecule with the four different isomers 7−10 shown on the right side. (f) Telegraph noise (parameters: U = −1.05 V, I = 1.0 nA) taken at the green cross in part e showing four height levels which belong to the four isomers (7−10). Scan parameters for topographic images a, b, d, and e: U = −50 mV, I = 100 pA.

Ag(111) surface by a home-built evaporator using a filamentheated quartz crucible. Finally, the sample was moved into our home-built low-temperature STM (“pan-design” STM head12) which is directly attached to the cold plate of a liquid helium bath cryostat and protected from thermal radiation by two cooling shields connected to liquid helium and nitrogen tanks, respectively, leading to a temperature of roughly T ≈ 4.5 K. For the movement of the molecules, we used a bias voltage of U = 20 mV and a tunneling current of I = 15−50 nA.

between the different isomers to an excitation over the tautomerization hydrogen transfer barrier. This hypothesis is supported by similar experiments performed on related molecules, which were created by STM manipulation with the purpose of tuning the energetic landscape of the tautomerization barrier between the different isomers. Deprotonation to HPc results in the disappearance of the pronounced switching rate increase at the second harmonic, indicative of a reduced tautomerization barrier height, well below the energy of the N−H stretching mode. In contrast, by adding a silver atom to various positions of the molecular frame, the tautomerization barrier reaches values slightly below or even exceeding those of intact H2Pc.

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RESULTS AND DISCUSSION

STM Imaging of Molecules and Their Structural Isomers. Figure 1a presents a topographic image of several H2Pc molecules on Ag(111). The structural model of the H2Pc molecule (right panel) shows the two hydrogen atoms in the center of the molecular frame which bind to opposing arms. In constant-current STM images, these arms can be identified by their elongated form and their elevated topographic height (yellow color).11,13 To ensure that it is not affected by any defect or nearby molecule, any molecule of interest was moved to a defect-free Ag(111) surface area (yellow arrow). Other molecules which are located close to that molecule and might potentially influence the results were moved away (red arrow). Furthermore, one silver atom which will be used for molecular manipulation was produced by gently dipping our silver STM tip into the Ag(111) surface. Details of the silver atom production can be read elsewhere.14

EXPERIMENTAL METHODS

Prior to the first sample preparation, H2Pc molecules (Alfa Aesar) were degassed for more than 2 days under ultrahigh vacuum (UHV) conditions. For the measurement, we used a Ag tip, which was created by electrochemically etching a silver wire (Alfa Aesar, purity: 99.985%) with a 10% solution of ammonia. As a substrate for our experiments, we used a Ag(111) single crystal (MaTeck, 7 × 7 × 1 mm3, purity 99.999%), which was prepared by cycles of 10 min of Ar+ sputtering with an ion energy of 0.5 keV and subsequent annealing up to a temperature of 700 K for about 10 min. After the last annealing cycle, the sample was cooled to room temperature and H2Pc molecules were deposited on the B


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silver atom seems to be quite strongly bound to the center of the molecule, as it stayed there even if the molecule was moved many nanometers away (see the Supporting Information). Furthermore, the symmetric appearance of the respective two isomers, which are oriented along one high symmetry axis (isomer 7/9 and 8/10), supports the assumption that the Ag atom is positioned in or very close to the center of the molecule. This Ag-atom-manipulated HPc molecule will be labeled HPc(Ag) hereafter. In analogy to HPc, the telegraph noise of HPc(Ag) now shows four different height levels, corresponding to the four possible isomers (see Figure 1f; data taken at the green cross in part e). The highest and lowest zsignal belong to protons positioned at arms oriented along the [11̅ 0] direction (8 and 10), whereas the proton is oriented along the [1̅1̅2] direction for the two intermediate heights (7 and 9). Since it will be important for the data analysis performed below, we would like to note that the labeling of all isomers 1−10 presented in Figure 1b, d, and e follows a common rule: in isomers labeled with an even number, the proton points in the [1̅10] direction, whereas it is oriented along the [1̅1̅2] direction for odd-numbered isomers. Analysis of the Telegraph Noise. To gain insight into the energetic landscape of tautomerization, we analyze the biasdependent telegraph noise of the three types of molecules, i.e., H2Pc, HPc, and HPc(Ag). In order to ensure comparability between the different molecules, we displaced the STM tip by the same distance Δ along the [1̅10] and the [1̅1̅2] direction with respect to the center of the molecule: Δ[11̅ 0] = 375 pm and Δ[1̅1̅2] = 50 pm. This position is marked by a blue/green cross in Figure 1b and e, respectively. The small displacement in the [1̅1̅2] direction was added in order to be able to distinguish all four states in the case of HPc and HPc(Ag) molecules. Figure 2a shows a comparison of the switching rates f of the three types of molecules for a wide energy range. To achieve a sufficient number of events and thereby a low statistical error in this and the following measurements, we analyzed data which were taken within a few minutes for the highest switching rates and up to 24 h for the lowest switching rates. The data summarized in Figure 2a reveal that the thresholds and all prominent features (marked by black dotted lines) are symmetrically positioned around the Fermi energy, indicating that the switching process is most likely triggered by inelastic tunneling electrons which excite vibrational modes and eventually lead to the tautomerization of the molecules. This explanation is in line with many other STM-triggered tautomerization studies.2,3,15 Deprotonation of H2Pc strongly reduces the threshold for tautomerization (compare black and blue data points in Figure 2a), which indicates that the potential barrier between the different isomers of HPc is lower than that for H2Pc. After inserting the Ag atom into the center of the deprotonated molecule, the threshold for the tautomerization of the intact H2Pc molecule is essentially recovered. It should be noted that also the tip can affect the tautomerization landscape, which was shown in our previous work.11 However, different tips never changed the switching rate by more than a factor of 2, which is negligible in comparison to the 5 orders of magnitude observed in the action spectra. In order to resolve the vibrational modes that trigger tautomerization, the energy thresholds and prominent features were determined at a higher density of data points (cf. Figure 2b) in the negative bias range marked by the gray area in Figure 2a. This bias polarity was chosen to reduce the influence of an unoccupied resonant electronic excitation16−18 on the switch-

Figure 1b shows the same area as that in part a after the before-mentioned manipulation steps. The single silver atom can be seen in the lower center of the topographic image. A detailed view of a single H2Pc molecule is presented in the right upper corner (1). By positioning the tip on top of the molecule (blue cross) and the application of an enhanced bias voltage (|U| > 0.415 V), tautomerization is triggered; i.e., the molecule frequently interconverts between two isomers, as also shown in previous studies.11,13 This tautomerization process is also sketched with red arrows in the molecular structure in Figure 1a. Depending on the molecular state at the moment a noninvasive bias voltage is restored, configuration 1 or the other isomer (2) with the two protons sitting on the adjacent arms can be found. To monitor tautomerization, the topographic height is recorded while injecting electrons at a constant tunneling current into the molecular frame with the tip positioned at a fixed x,y-position over the molecule. This leads to a two-stage telegraph noise in the z-height (cf. Figure 1c), corresponding to the two H2Pc isomers. A bistable adsorption position or different molecular orientations can be excluded, since the molecular frame remains unmodified. From this telegraph noise, the switching rate can be determined by dividing the number of switching events by the measurement time6 (see the Supporting Information for further details). Furthermore, we analyzed the proportion of time the molecule stays in the respective isomer, shown in the histogram in Figure 1c on the right. Most of the time, the molecule stays in the topographic lower state, which in this case corresponds to protons positioned along the [1̅1̅2] direction. This anisotropy is caused by the fact that the symmetry mismatch between the substrate and the molecular frame lifts the energetic degeneracy of the two isomers. For H2Pc/Ag(111), the protons preferentially bind to ligand arms pointing in the [1̅1̅2] direction (isomer 1) and can only rarely be found in the [11̅ 0] direction (isomer 2).11 In addition to pristine H2Pc, we also investigated two modified versions of the molecule to analyze and tune the energetic landscape of tautomerization. The first modification was achieved by removing one proton from the center of the molecule by the application of a bias pulse of U = 2.0−2.2 V for a few seconds with the tip staying on top of the molecule center. The removal of the proton is evidenced by a one-time step-like decrease of the z-height. The topography after this manipulation is shown in Figure 1d. This deprotonated molecule will be labeled HPc hereafter. By applying a bias voltage of |U| ≥ 70 mV, the remaining proton can now be switched between four different positions (3−6) which are shown on the right side of the image. Due to the before mentioned lifting of the degeneracy, the two isomers with the proton oriented in the [1̅10] direction (4 and 6) have a relatively short lifetime of a few seconds only.11 In contrast, the other two isomers (3 and 5) appear to be stable on the time scale of our experiments (typically 1 h) and do not show any tautomerization if probed with noninvasive parameters. In another manipulation procedure, we moved the silver atom with the STM tip into the molecule center (cf. white arrow in Figure 1e), resulting in an almost doubled topographic height. Nevertheless, by applying a bias voltage of |U| ≥ 0.4 V, the molecule can still be switched between four different isomers (7−10) (images on the right side of Figure 1e). In contrast to HPc, all four isomers were stable on the time scale of our experiments if probed with noninvasive parameters. The C


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a subtle shift to lower energy, which can be attributed to the removal of one proton. Interestingly, for the deprotonated molecule, also vibrational modes with lower energy contribute to the tautomerization. The mode at an energy EIPB matches very well with the in-plane bound mode.20 Furthermore, the data of Figure 2b reveal that the tautomerization of HPc can be induced with electrons with an energy down to E ≈ 70 meV. Obviously, further low-lying vibrational modes involving inplane or out-of-plane motion of the proton can also trigger tautomerization. A reasonable fit was not possible in this low energy range (70−140 meV), which is most likely based on the fact that many vibrational modes contribute to the tautomerization process in this energy interval.20,21 In the case of the HPc(Ag) molecule, modes other than the N−H stretching mode do not contribute to the tautomerization anymore, indicating that the potential barrier between the different isomers is increased by this manipulation. It should be noted that the absence of the in-plane and out-of-plane modes in the action spectrum is unlikely caused by a shift of electronic states, as resonant electronic excitations only result in a relatively small amplification of the tautomerization rate (cf. the influence of the HOMO at U = −1.3 V for the intact molecule). Furthermore, also a frequency shift of these modes is unlikely to be responsible for the disappearance of these modes in the action spectrum, as this effect should be relatively tiny, as can be seen by the small shift of roughly 2% in the case of the N−H stretching mode. After assigning the different features to specific vibrational modes, to understand the energetic landscape of the tautomerization, it needs to be clarified if the excitation in the different energy intervals is based on one-electron or multielectron processes. Therefore, current dependencies of the switching rate were analyzed at U = −0.5 V and U = −0.9 V for all three molecules and additionally at U = −0.2 V for the HPc molecule. These data are presented in Figure 2c. The data sets were fit with power laws, and the resulting exponents are summarized in Table 2. At low tunneling currents (I < 4 nA),

Figure 2. (a) Bias-dependent switching rate of H2Pc (black squares), HPc (blue triangles), and HPc(Ag) (red circles) molecules extracted from telegraph noise (tunneling current I = 1.0 nA; see text for details). (b) Bias-dependent switching rate in a narrower energy window (marked with a gray square in part a) (tunneling current I = 1.0 nA). (c) Current-dependent switching rate for the three types of molecules probed at different bias voltages. The data were fit with power laws (orange lines). For the data set of H2Pc at a bias voltage of U = −0.5 V, a crossover to a higher exponent is observed, which is fit with another power law (violet line).

Table 2. Exponent of the Power Law Fits Shown in Figure 2ca molecule

ing rate of the molecules. In fact, the LUMO is energetically located at E ≈ 0.7 eV,11 i.e., very close to the N−H stretching mode threshold, and thus could potentially affect a measurement at positive bias polarity. In contrast, the HOMO is located at an energy of E ≈ −1.3 eV,11 i.e., well above the energy interval probed in Figure 2b. The energies of the vibrational modes were extracted with the help of a fit function19 and are summarized in Table 1. The energy ES1 of the pristine H2Pc molecule is in very good agreement with the N−H stretching mode of the molecule,20,21 which is the highest energy mode involving the motion of the inner proton. For this reason, ES2, which is twice as large as ES1, can most likely be attributed to the second harmonic of this vibrational mode. This trend of ES1 and ES2 is also visible for HPc and HPc(Ag1). For HPc, there is

H2Pc HPc HPc(Ag)

molecule

EIPB (meV)

ES1 (meV)

ES2 (meV)

159 ± 6

406 ± 8 392 ± 7 400 ± 26

816 ± 4 754 ± 35 800 ± 8

U = −0.5 V

U = −0.9 V

1.06 ± 0.06

1.01 ± 0.18/1.75 ± 0.15 1.02 ± 0.03 1.08 ± 0.09

1.14 ± 0.05 0.96 ± 0.07 1.15 ± 0.05

The first/second values of H2Pc at U = −0.5 V correspond to the orange/violet fit. a

all datairrespective of the particular moleculecan be fit over the entire energy range by an exponent which is close to unity, indicating that the excitations are based on one-electron processes. If the current is further increased (I > 4 nA), the data set of the H2Pc molecule probed at a bias voltage of U = −0.5 V shows a crossover to a higher exponent. This observation can be understood in terms of consecutively increasing the vibrational quanta of the N−H stretching mode:22−24 If the interval between successive electron-induced excitations becomes shorter than the lifetime of the stretching mode, a second electron can excite another vibrational quanta of the N−H stretching mode before the first quantum excited by a preceding tunneling event relaxes. Since this second harmonic of the stretching mode has a much higher efficiency to induce tautomerization (cf. Figure 2b), the excitation of this mode also

Table 1. Energies of the Vibrational Modes Extracted from the Fit Shown in Figure 2b

H2Pc HPc HPc(Ag)

U = −0.2 V

D


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Figure 3. Models for the energetic landscape of the tautomerization of H2Pc (a), HPc (b), HPc(Ag) (c), H2Pc(Ag-M) (d), and H2Pc(Ag-S) (e). The red parabola sketches the vibrational energies of the stretching mode. Blue solid lines mark possible transitions between isomers over the barrier and dashed blue lines quantum tunneling through the tautomerization barrier. The numbers under the energetic landscapes refer to the different isomers. Odd/even numbers refer to isomers, where the protons are bound to arms pointing in the [1̅1̅2]/[1̅10] direction, respectively.

only leads to a subtle increase in the switching rate, as can best be seen at the positive bias voltages U = 1.2, ..., 1.3 V in Figure 2a. If the tautomerization barrier height were somewhere between the second and third harmonics, we would expect a more substantial increase between νs = 2 and νs = 3 and a negligible quantum tunneling probability for the first harmonic (νs = 1). It shall be noted, however, that a negative bias polarity is not suitable for an analysis, as this energy range coincides with the HOMO, which may also affect the switching rate due to resonant electronic excitations. In the case of the HPc, the energy barrier between the different isomers is strongly reduced, as sketched in Figure 3b. Therefore, also other vibrational modes with lower energy (inplane and out-of-plane bound modes) can contribute to the tautomerization. Since the strong increase at the crossover from the first to the second harmonic of the stretching mode is absent for HPc, we conclude that already the energy of the first harmonic lies above the tautomerization barrier. By introducing the Ag atom to the center of the HPc molecule, the energy barrier is again enhanced (cf. Figure 3c). As a result, only the N−H stretching mode can trigger tautomerization. Similar to H2Pc, we observe a strong increase of the switching rate at the second harmonic of the stretching mode. Therefore, we conclude that the energetic height of the tautomerization barrier of the HPc(Ag) molecule is very similar to H2Pc, i.e., in between the first and second harmonics of this vibrational mode. Ag Atom Manipulation of H2Pc. If the proposed model of a crossover between quantum tunneling through the tautomerization barrier and excitation above this barrier is indeed true, the relative switching rate increase at the energy of the second harmonic should strongly depend on the actual barrier height. In the case of a very high potential barrier, i.e., with a barrier top just below the energy of the second harmonic but well above the first harmonic, a stronger crossover increase of the switching rate should be observed, as quantum tunneling is exponentially damped. In contrast, a barrier height that barely exceeds the first harmonic will lead to a much weaker increase at the second harmonic. To experimentally verify these two cases, Ag atoms were used to change the potential landscape of the intact H2Pc. As will become obvious in the following, moving one Ag atom under an energetically disfavored arm enhances the tautomerization barrier (see the Supporting Information for analysis of the adsorption site). This modification is labeled H2Pc(Ag-M) in Figure 4a and leads to the two isomers 11 and 12. Alternatively, we moved an Ag atom under one of the stable arms of the molecule [H2Pc(Ag-S)] in order to destabilize the stable position, the two isomers (13, 14) of which are shown in Figure 4b.

increases the exponent of the current dependency. As will become clear in the next section, the absence of a crossover for all other molecules and/or bias voltages can most likely be attributed to a much lower vibrational lifetime due to higher vibrational damping [in the case of the HPc(Ag)] or to the fact that the excitation of higher harmonics only leads to small changes in the vibrational rate (for HPc at U = −0.5 V and U = −0.9 V for all molecules). Models of the Energetic Landscapes. To understand the energetic landscape of tautomerization, it is helpful to shortly summarize the main features of the bias- and currentdependent switching rate. It strongly increases for H2Pc and HPc(Ag) when raising the electron energy from the excitation of the first harmonic (νs = 1) above the second harmonic (νs = 2) of the stretching mode. Since the data of Figure 2c unambiguously prove a one-electron process, we can exclude a crossover to a multielectron process. Furthermore, this strong increase is absent in the case of the HPc molecule, which only exhibits a very small increase of the switching rate at an electron energy corresponding to the second harmonic of the stretching mode. All of these features can be explained within a model, the key feature of which is a crossover from quantum tunneling through the tautomerization barrier to an excitation over the barrier. The energetic landscape of this model is qualitatively sketched in Figure 3a−c for H2Pc, HPc, and HPc(Ag), respectively. For the sake of simplicity, the tautomerization energy landscape (black curve) considered in Figure 3 shows a single tautomerization barrier between the different isomers only, equivalent to a simultaneous single-step transfer process of the protons. We would like to emphasize, however, thaton the basis of our datawe cannot exclude a two-step process with a metastable intermediate state where two protons bond to adjacent arms, as calculated for the tautomerization of porphycene.6 Since our data show no hint of any intermediate state at the available experimental time resolution (≈1 ms), we can only conclude that the second part of a two-step process if it existsmust occur at very short time scales. Since it is unclear whether the N−H stretching mode couples directly or via intermode coupling to the tautomerization process,25 this mode is visualized by an additional parabolic potential (red; cf. Figure 3). In the case of the pristine H2Pc molecule, the energy of the first harmonic of the stretching mode (νs = 1) lies just below the tautomerization barrier (Figure 3a), such that tunneling through this barrier is already possible (dashed blue arrow). If the energy of the second harmonic (νs = 2) of this mode is reached, the switching rate strongly increases, as this energy level lies well above the potential barrier. Correspondingly, increasing the energy further to the third harmonic (νs = 3) of the stretching mode E


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H2Pc, the protons of H2Pc(Ag-S) stay less time on the stable arm. Thus, the results support the before mentioned enhancement and attenuation of the tautomerization barrier between the two isomers. Since absolute values of the tautomerization rate might also reflect the influence of the Ag atom itself, e.g., by changing the excitation yield, rather than the changes of the tautomerization barrier with respect to the vibrational energies, we analyzed the crossover of the switching rate from the first to the second harmonic of the stretching mode by dividing the value of the switching rate f at U = (−0.8 − Δ) V above the threshold through the switching rate at U = (−0.8 + Δ) V below the threshold. Δ was chosen to be 0.2 V. For smaller values of Δ, the quotient is affected by the different steepness of the action spectroscopy, which arises due to the vibrational damping of the added silver atoms, especially in the case of the HPc(Ag) molecule. The quotient for all five investigated molecules is summarized in Table 3. The higher this quotient, the higher the Table 3. Quotient of the Switching Rates to Analyze the Crossover from the First to the Second Harmonic of the Stretching Mode f(−1.0 V)/f(−0.6 V) and the Second to the Third Harmonic f(1.4 V)/f(1.0 V) Figure 4. (a) Topography of two isomers (11, 12) of the H2Pc(Ag-M) molecule, created by moving a silver atom on the metastable arm. (b) Topography of two isomers (13, 14) of the H2Pc(Ag-S) molecule, created by moving a silver atom on the stable arm. (c) Bias-dependent switching rate of the H2Pc, H2Pc(Ag-M), and H2Pc(Ag-S) molecules (tunneling current I = 1.0 nA). (d) Bias-dependent proportion of time the proton stays on the stable arm (isomer 1/11/13) or metastable arm (isomer 2/12/14) of the H2Pc/H2Pc(Ag-M)/H2Pc(Ag-S) molecule extracted from the same data set presented in part c. Scan parameters for the topographic images a and b: U = −50 mV, I = 100 pA.

molecule H2Pc HPc HPc(Ag) H2Pc(Ag-M) H2Pc(Ag-S)

f(−1.0 V)/f(−0.6 V)

f(1.4 V)/f(1.0 V)

± ± ± ± ±

3.05 ± 0.13

322 3.46 38.1 526 98.6

20 0.15 0.2 133 4.9

7.05 ± 0.15 3.48 ± 0.15 2.86 ± 0.12

increase of the switching rate at the crossover from the first to the second harmonic and the higher should be the tautomerization barrier. Obviously, the three H2Pc molecules follow the proposed trends. The quotient for the H2Pc(Ag-M) is clearly higher as compared to the H2Pc, which is in agreement with the enhancement of the tautomerization barrier. In contrast, the quotient for the H2Pc(Ag-S) is lower than that for pristine H2Pc, which is in line with the before mentioned lowering of the barrier. The models for the energetic landscape of H2Pc(Ag-M) and H2Pc(Ag-S) can be seen in Figure 3d and e, respectively. Interestingly, the quotient of the deprotonated molecule (HPc) is 2 orders of magnitude lower than the ones of the intact H2Pc molecules, indicating the absence of a crossover from quantum tunneling to excitation over the barrier. The recovery of the tautomerization barrier by the movement of a Ag atom into the center of the deprotonated molecule is also visible in the value of the quotient, which is increased by 1 order of magnitude for the HPc(Ag) in comparison to the HPc molecule. With the same technique, also the crossover from the second to the third harmonic of the stretching mode can be analyzed, by calculating the quotient of f(1.4 V)/f(1.0 V) (see right column of Table 3). With the exception of HPc(Ag), all quotients are very similar in magnitude and comparable to the HPc crossover from the first to the second harmonic. The higher value of HPc(Ag) is caused by the strong broadening of the action spectrum, probably due to vibrational damping induced by the Ag atom, rendering our analysis method meaningless in this case. In conclusion, the transition from the second to the third harmonic of the stretching mode for H2Pc, HPC(Ag), H2Pc(Ag-M), and H2Pc(Ag-S) as well as the transition from the first to the second harmonic for HPc is

The bias-dependent switching rates of these artificially modified molecules are shown in Figure 4c. Obviously, the switching rate of the H2Pc(Ag-M) molecule is strongly reduced in the probed bias range, which is in line with an enhancement of the tautomerization barrier from isomer 11 to isomer 12. In the case of the H2Pc(Ag-S) molecule, the differences compared to H2Pc are less pronounced. For low bias voltages (|U| < 0.9 V), the switching rate of the H2Pc(Ag-S) molecule is higher than that for the H2Pc molecule, whereas at higher bias voltages (|U| > 0.9 V) the switching rate is almost the same for both molecules. To further investigate the influence of the atom manipulation, we analyzed the bias-dependent proportion of time of the different isomers (Figure 4d). In the case of a degenerate ground state of two isomers and the absence of any tip influence, the proportion of time of both isomers should be the same (50%). In our case, the two isomers are energetically inequivalent11 and also the tip influences the tautomerization potential. Both results favor the same isomer (proton bound to stable arm). With increasing bias voltage, this effect is reduced, as the energy difference between the isomers has to be compared to the energy of the excitation. Whenever the energy of the tunneling electrons exceeds another vibrational mode or a higher harmonic, a strong change in the proportion of time is observed. Silver atom manipulation changes the proportion of time (cf. Figure 4d) and thus the energy of the isomers with respect to the tautomerization barrier height. In the case of H2Pc(Ag-M), the protons stay essentially all of the time on the stable arm (isomer 11). In contrast, in comparison to pristine F


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not based on a crossover between quantum tunneling and an excitation over the tautomerization barrier. Instead, the switching slightly increases due to the opening of another excitation channel. This result complements the picture of a crossover between the first and second harmonics of this vibrational mode for H2Pc, HPC(Ag), H2Pc(Ag-M), and H2Pc(Ag-S) molecules.

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CONCLUSION In summary, we could demonstrate that the tautomerization of H2Pc triggered by the excitation of the N−H stretching mode shows a crossover from quantum tunneling through the barrier to an excitation over the barrier, if the energy is increased from the first to the second harmonic of this vibrational mode. Furthermore, we could demonstrate that single silver atoms can be used to modify the tautomerization barrier, thereby allowing tuning of the stability of the two isomers and altering their switching rate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10564. Analysis of the switching rate, atom manipulation, and analysis of manipulated molecules (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 We acknowledge fruitful discussions with Bernd Engels (Universität Würzburg). REFERENCES

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