Breaking Degeneracy of Tautomerization ... - ACS Publications

Breaking Degeneracy of TautomerizationMetastability from Days to Seconds Jens Kügel,*,† Aimee Sixta,†,§ Markus Böhme,† Andreas Krönlein,† and Matthias Bode†,‡ †

Physikalisches Institut and ‡Wilhelm Conrad Röntgen Center for Complex Material Systems (RCCM), Universität Würzburg, Am Hubland, 97074 Würzburg, Germany § University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: We present a detailed study of the tautomerization, that is, the switching of hydrogen protons, between different sites in the molecular frame of phthalocyanine (H2Pc) on a Ag(111) substrate by means of scanning tunneling microscopy (STM) and STM-based pump-and-sample techniques. Our data reveal that the symmetry mismatch between the substrate and the molecular frame lifts the energetic degeneracy of the two H2Pc tautomers. Their energy difference is so large that only one tautomer can be found in the ground state. Tip-induced tautomerization was triggered at sufficiently high bias voltages. The excited metastable H2Pc tautomer was found to exhibit a lifetime of at least several days, as derived from the fact that the molecule did not change back to the ground state within experimentally accessible time scales as long as noninvasive tunneling parameters were used to probe the state of the molecule. By the controlled removal of a hydrogen proton from the molecule, a four-level system was created. Pump-and-sample experiments reveal that the lifetime of the metastable positions amounts to seconds only. Current- and bias-dependent studies indicate that the presence of the STM tip modifies the potential barrier, thereby allowing for a controlled tuning of the metastable tautomer’s lifetime. KEYWORDS: tautomerization, phthalocyanine, metastability, scanning tunneling microscopy, pump-and-sample experiment

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between the four-fold symmetric molecular macrocycle and the symmetry of the underlying surface of the substrate. For example, it was demonstrated by adsorption on three-fold symmetric (111) surfaces that the molecule’s symmetry is reduced to two-fold with two opposing arms of the molecule bent closer to the surface.4 In this way, the degeneracy of orbitals can be lifted.7 Here, we present a detailed experimental study to investigate how symmetry reduction affects the behavior of the metal-free phthalocyanine molecule H2Pc. Instead of a metal ion, H2Pc carries two hydrogen protons in the center of the molecule. As

n the past decades, phthalocyanine molecules have been in the focus of research in a variety of fields ranging from molecular magnetism1 to biological sensors2 or photochemistry.3 These molecules consist of a four-fold symmetric molecular macrocycle and owe versatile applications due to their chemical stability and the fact that the electronic, optic, and magnetic properties can be tuned by the choice of the atoms in the center of the molecule. In addition to the chemical structure, the molecular environment is also of crucial importance for their potential implementation in devices. For example, interaction with the substrate can considerably affect and modify the molecular properties. Phthalocyanine molecules typically adsorb on substrates with the molecular plane parallel to the surface. It has been shown in various studies4−7 that the symmetry of phthalocyanine molecules is reduced in the case of a symmetry mismatch © 2016 American Chemical Society

Received: September 2, 2016 Accepted: November 12, 2016 Published: November 12, 2016 11058

DOI: 10.1021/acsnano.6b05924 ACS Nano 2016, 10, 11058−11065

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ACS Nano also demonstrated for similar molecules, the position of the two hydrogen protons in the center of the molecule can switch between different molecular sites, either spontaneously or as a result of an external stimulus, a process called tautomerization. For the specific molecule−substrate system studied here (i.e., H2Pc adsorbed on a Ag(111) surface), topographic images reveal that without manipulation only one out of two tautomers, A and B, can be observed experimentally, indicating that the symmetry mismatch between the substrate and the molecular frame breaks the energetic degeneracy between the two tautomers. This finding will be supported by biasdependent and spatially resolved studies of the electroninduced tautomerization, which show that the lifetime of the two tautomers is very unequal andwithin our measurement accuracyindependent of the exact tip position. As long as noninvasive scan parameters are used, the lifetime of the tautomer with the higher total energy, B, appears to be infinite. A different behavior is found for tautomers of the dehydrogenated molecule. As indicated by scanning tunneling microscopy (STM)-based pump-and-sample experiments, the lifetime of the metastable state, that is, when the hydrogen proton is positioned at the arm of the less stable tautomer, is reduced down to a few seconds. Current- and bias-dependent measurements indicate that the presence of the STM tip in close proximity to the molecule modifies the energetic barrier between the different tautomers.

Figure 1. (a) Structural model of a freestanding H2Pc molecule. The hydrogen switching between two degenerated ground states is indicated by red arrows. The energy landscape of this tautomerization process is sketched under the molecule. (b) Topographic overview image of H2Pc on Ag(111). The inset shows an atomically resolved image of the substrate taken roughly 10 nm away from the overview scan frame. (c) Zoom-in images of the three molecules depicted in (b). The elongated arms, where the hydrogen protons are located, point to one of the ⟨211⟩ directions, which is indicated by white arrows. These three rotations belong to the same configuration A. (d) In configuration B, the elongated arms point to one of the ⟨011⟩ directions. Schematic sketches of the energy landscape of the two configuration are shown next to the molecules in (c) and (d). Scan parameters (b−d): U = 0.1 V, I = 1 nA. Inset of (b): (molecule terminated tip) U = 20 mV, I = 2 nA.

RESULTS AND DISCUSSION Topographic Analysis of H2Pc Molecules. Figure 1a shows a structural model of a H2Pc molecule. It consists of the phthalocyanine macrocycle with two hydrogen atoms bound to opposing nitrogen atoms in the center of the molecule. Strong electron donation from the hydrogen to the macrocycle has been reported8 such that only protons remain within the molecule. As long as the molecule’s symmetry is not broken, for example, by interaction with a substrate, the two hydrogen protons may occupy either pair of opposing nitrogen with equal probability. The transition between these two configurations, the so-called tautomerization process, is indicated by red arrows in the molecular frame in Figure 1a. The freestanding molecule exhibits two energetically degenerated configurations, A′ and B′, as characterized by two minima in the schematic energy landscape sketched in the bottom part of Figure 1a. As we will show below, the adsorption of the H2Pc molecule on a substrate, such as Ag(111), may significantly alter this energy landscape and may lead to a lifting of the A′−B′ degeneracy. A topographic image of H2Pc molecules deposited on Ag(111) is shown in Figure 1b. The two-fold symmetry of the molecule is clearly visible in the images, where two opposing molecular arms are slightly elongated and exhibit a larger apparent height, indicated by their yellow color, whereas the other two arms are shorter and have a heart-like shape. In fact, owing to the different possible combinations to align the molecule and substrate axes, three orientations of the organic macrocycle are observed and presented in detail in Figure 1c. As has been shown on H2Pc/Ag(111)9 and a similar system10 and will also be confirmed for the particular system under investigation here, the hydrogen protons are located at the nitrogen atoms of the elongated arms. These arms are oriented along [211] equivalent crystallographic directions, socalled ⟨211⟩ directions, as determined by means of the atomically resolved image (see inset of Figure 1b) taken roughly 10 nm away from the topographic overview. These

three molecules belong to the tautomer with minimal total energy, labeled A hereafter. Application of voltage pulses with enhanced bias (|U| ≥ 0.45 V) with the tip positioned over the molecule leads to a tautomerization process, that is, the switching of the hydrogen protons to the adjacent arms. This is evidenced by the 90° rotation of the elongated arms into ⟨011⟩ directions observed in Figure 1d, which shows exactly the same molecules already presented in Figure 1c. This second tautomer will be labeled B hereafter. To characterize the distribution of H2Pc molecules on Ag(111) with respect to their orientation and tautomer (i.e., A or B), a total of 1124 molecules were scanned at noninvasive bias voltage (data not shown). Our result shows that the rotational orientations are equally distributed, whereas a huge asymmetry exists regarding tautomers A and B. In fact, there was not a single molecule found to exhibit tautomer B. This finding demonstrates that the energetic degeneracy of the two tautomers expected for the freestanding H2Pc molecule is lifted by adsorption on the Ag(111) substrate. A sketch of the potential landscape is shown on the right side of Figure 1c,d. As will be shown in detail below, the energy difference between the two tautomers affects not only the adsorption behavior of 11059


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trend reflects the energy difference of the two tautomers reported above. In order to obtain a better understanding of the physical process that leads to the STM-induced tautomerization of H2Pc molecules on Ag(111), we performed multiple measurements similar to the one presented in Figure 2a, whereby experimental parameters, such as the bias voltage, the tunneling current, or the tip−sample distance, were systematically varied. It should be noted that this and all experimental results presented in the remainder of the article were performed with the feedback loop turned on to excite the system with a constant rate of tunneling electrons. The parameters of the feedback loop are chosen such that the time for regulating the tip after a tautomerization event was roughly a factor of 100 shorter than the average lifetime for the highest switching rates. Thereby, the effect of the feedback loop on the experiment is essentially negligible. Figure 2b presents the bias-voltage-dependent tautomerization rate f, as defined by the number of tautomerization events divided by the total time. Data were recorded at the position of the blue cross in Figure 2a (set point current I = 1 nA). In order to achieve a sufficiently narrow standard deviation, the total time of the time trace for each particular bias voltage was increased from 1 min for the data with the highest switching rate up to 10 h for the data with the lowest switching rate. Thereby, we are able to analyze tautomerization rates f that span more than 4 decades, ranging from 10−3 Hz up to more than 10 Hz. The plot exhibits a behavior that is essentially symmetric around the Fermi level. Specifically, we find a tautomerization threshold of Uth ≈ ±0.45 V. Increasing the absolute value of the bias voltage beyond this value leads to a slightly sublinear apparent increase of f in a semilogarithmic plot until the slope strongly increases at Uinc ≈ ±0.85 V. The fact that the observed voltage dependence of the tautomerization rate f behaves symmetrically around the Fermi energy indicates that tautomerization is driven by inelastic processes.11,12 Small asymmetries in the bias polarity of the switching rate can be attributed to resonant electronic excitations13−16 due to the presence of molecular orbitals. For example, the HOMO of the H2Pc molecule is located at an energy of EHOMO ≈ −1.3 eV (cf. Supporting Information Figure S1), giving rise to the step-like increase of the switching rate at the corresponding bias voltage. The slightly higher switching rate at positive biases is likely attributed to the LUMO of the H2Pc molecule, which is centered around ELUMO ≈ 0.7 eV and strongly energetically broadened. To better understand the origin of the sudden jump of f at Uinc, we studied the current dependency at two selected bias voltages: below Uinc > U = −0.6 V and above Uinc < U = −0.9 V, presented as red and black data points in Figure 2c, respectively. Fitting both data sets with a power law up to a current of roughly I ≈ 5 nA results in an exponent of 0.99 ± 0.09 for U = −0.6 V (blue line) and an exponent of 1.04 ± 0.06 for the U = −0.9 V data (green line). An exponent close to 1 indicates that the electroninduced tautomerization is triggered by a single-electron process at both voltages. This result excludes that the jump in tautomerization rate at U = 0.85 V is caused by a crossover from a multielectron process to a one-electron process. Rather, we interpret the symmetric behavior around the Fermi level as evidence for two vibrational modes with excitation energies of roughly 0.45 and 0.85 eV, which are excited by tunneling electrons and induce tautomerization. This mechanism was already shown to drive tautomerization in a similar system, namely, porphycene molecules on Cu(110).17 To prove this

the H2Pc molecules but also the STM-induced tautomerization process. Tautomerization Analysis of the Pristine H 2 Pc Molecule. To analyze the tautomerization process, that is, the 90° rotation of the hydrogen protons, the tip was positioned on top of one arm pointing into a ⟨011⟩ direction, as marked with a blue cross in the topographic image in the right part of Figure 2a. After increasing the bias over a threshold

Figure 2. (a) Typical time trace of the z-position of the scanner with the tip located on one arm of the molecule marked by a blue cross (time trace parameters: U = −0.9 V; I = 1.0 nA). Two levels can be seen indicating the tautomerization between the two configurations A and B. On the right side, the height histogram of the time trace is presented showing the ratio of the time spent at a certain high to the complete time. (b) Bias-dependent tautomerization rate taken at the blue cross marked in (a) with a current set point I = 1 nA. (c) Current dependency with a bias voltage of U = −0.6 V and U = −0.9 V, which is fit with a power law. For the first mentioned data set, two power law fits were used as the exponent is changing for higher current values. For most data points, uncertainty margins are smaller than the symbol size.

value, which is roughly at U = ±0.45 V, a telegraph noise in the z-signal appeared (cf. left panel of Figure 2a). The temporal evolution of the tip height shows switching between two discrete levels, representing the tautomerization process between tautomers A and B. The high state corresponds to the situation when a proton is located at the arm where the tip is located, whereas the low state represents the case where both protons are bound to the two adjacent arms. In the given example, the high and low state belong to tautomers B and A, respectively. On the right side of Figure 2a, we show a histogram of the tip height, which gives access to the proportion of time the H2Pc molecule stays in the respective state. For the sake of simplicity, we will refer to it as a “probability” in the remainder of this report. It can be clearly seen that tautomer A (low state) is strongly preferred. This 11060


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which is shown in Figure 3b. In the difference image, red regions indicate that the molecular height of tautomer A is higher than that of tautomer B and vice versa for the blue regions. In the black regions, both outside and within the molecule, tautomers A and B cannot be distinguished because of their almost equal apparent height. Although the triggering of tautomerization may still occur at these tip positions, our inability to determine the actual tautomer requires arbitrarily setting the tautomerization rate f to zero and the probability staying in the high state to 50% for further analysis. In Figure 3c, the spatial distribution of the tautomerization rate f is plotted. The data show that tautomerization can be induced over the complete organic macrocycle of the H2Pc molecule, an observation also made on similar systems11,20 such as naphthalocyanine molecules on bilayers of NaCl or RbI on Cu(111).10 In contrast to the last mentioned measurement, however, on H2Pc/Ag(111), we observe the highest tautomerization rate with the tip positioned over the center of the molecule. This difference might be attributed to the absence of a buffer layer in our experiments, which potentially inhibits direct tunneling into the center of the molecule. Figure 3d shows the spatially resolved probability of staying in the high state extracted from the time traces. We observe large variations ranging from 0.1 to 99%. It has to be stressed, however, that Figure 3d cannot be interpreted as a spatial distribution of the probability being tautomer A. This can be understood by looking at the difference image of Figure 3b. While the topographic height of tautomer B is higher than the height of A in the blue regions, the opposite is true for the red regions. In order to convert Figure 3d into a spatial map of the probability of tautomer A, we need to invert the data in the blue regions. The result is shown in Figure 3e. It reveals that tautomer A is strongly preferred, independently of the exact tip position over the molecule. This observation reflects that the two tautomers A and B are not energetically equivalent and fits very well to the fact that tautomer B was not observed in the ground state as mentioned before. A closer look to the results of Figure 3e reveals that there are some deviations from a completely homogeneous spatial distribution of the probability. Comparison of the data obtained with the tip positioned over the upper left and the lower right arm reveal that the probability of being tautomer A, that is, the proton being located at the other arm, is slightly higher than when the tip is positioned over the other two arms, that is, in the lower left and upper right corner of the scan range. This indicates that the tip is preferentially repelling the hydrogen protons and features the possibility of controlling tautomerization through the position of the tip. The previous analysis has shown that the intact molecule exhibits two tautomers, whereby one is energetically favored over the other one. Nevertheless, H2Pc molecules excited to the metastable tautomer B remained in this configuration as long as bias voltages with an absolute value below 0.4 V were used, indicating that the barrier between the two tautomers is too high and/or broad to be overcome by thermal excitation or tunneling. Tautomerization Analysis of the Dehydrogenated H2Pc Molecule. Many of the findings reported above change even qualitatively when one hydrogen proton is removed from the central region of the molecule. This so-called dehydrogenation was already used in the case of tetraphenyl porphyrin molecules adsorbed on Ag(111)11 to build a four-level tautomerization-based conductance switch. After one hydrogen proton was removed from the center of this molecule, which is

statement, further experimental and/or theoretical studies have to be performed on this system. Taking a closer look at the current dependency of U = −0.6 V (cf. Figure 2c), a subtle change in the slope with crossover current of roughly 5 nA is observed. Beyond this current value, the exponent changes to 1.56 ± 0.12. This crossover usually appears when the time between two successive electron tunneling events becomes comparable with the vibrational relaxation times.18,19 The analysis presented here and later in the paper is mainly based on data taken at negative bias voltages. Probing with positive bias voltages leads to similar results, which can be seen in the Supporting Information Figures S2−S5. To determine the dependency of STM-induced tautomerization processes of H2Pc on the tip position, we recorded the telegraph noise during 162 s long time traces taken at each position of a 15 × 15 pixel grid laid over a 2.5 nm × 2.5 nm scan range. As can be seen in Figure 3a, which displays the average height of each time trace, a H2Pc molecule is located in the center of the scanned area. Since with our experimental procedure we can only distinguish between the high and the low state of the time trace but not between the two tautomers A and B (cf. left images of Figure 1c,d), a difference image of the two configurations was created for further data analysis,

Figure 3. (a) Topography of a 15 × 15 pixel grid (grid parameters: U = −0.8 V and I = 1 nA). Each pixel shows the average height of a 162 s time trace. (b) Difference image of the two configurations A and B. In the area of the red regions, the topographic height of configuration A is higher than that of B and vice versa for the blue regions. (c) Spatial mapping of the tautomerization rate, showing that tautomerization can be induced all over the molecular frame, with the highest efficiency in the center of the molecule. (d) Spatial distribution of the probability staying in the high state. (e) With the help of the spatial information on (b) and (d), the spatial distribution of the probability staying in the configuration A was extracted. 11061


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Figure 4. (a,b) Topographic images of a dehydrogenated H2Pc molecule with the remaining proton sitting at opposite positions within the molecule (scan parameters: U = −0.1 V; I = 1 nA). (c) Time trace of the tautomerization taken on the lower molecular arm slightly away from the high symmetry axis shows four distinguishable states, which belong to the hydrogen proton bound to one of the four molecular arms. (d) Bias-dependent tautomerization rate shows a reduced threshold of tautomerization compared to the pristine molecule. (e) Topography of a 15 × 15 pixel grid (grid parameters: U = −0.3 V; I = 4 nA). (f) Spatial distribution of the tautomerization rate indicates that the highest switching rate is at the center of the molecule. (g) Spatial distribution of the probability staying in the high or the low state extracted from the time traces of the grid measurement.

apparent height of which differs by about 0.1 pm only. Very fast topographic scans (not shown) reveal that these four states correspond to the hydrogen proton bound to the nitrogen atoms of the four different arms. The high state indicates that the hydrogen proton is located at the arm where the tip is actually positioned, whereas the hydrogen is bound to the opposing arm in the low state, as can be deduced from the topographic image in Figure 4b. The intermediate states correspond to the hydrogen proton bound to one of the adjacent arms. An analysis of numerous time traces showed that in over 99% of the tautomerization events, the hydrogen atom moved to an adjacent arm and not to an opposing arm. Nevertheless, a small number of the observed switching events (≈1%) appears to take place between two opposing arms without any detectable residence time at the intermediate adjacent arm. This may have two potential reasons: First, direct switching to the opposing arm may indeed be very rare due to a much higher energy barrier of this process. Alternatively, it may also be possible that there are indeed two consecutive tautomerization processes which occur in such a rapid succession thatgiven the limited time resolution (0.2 ms) of our experimental setupthey cannot be resolved as independent events. In Figure 4d, the bias-dependent tautomerization rate is plotted, which was extracted from time traces with the tip positioned at the blue cross in Figure 4b by counting all tautomerization events and dividing this number by the total probing time. Also for the dehydrogenated molecule, we find a threshold voltage that is symmetric around the Fermi energy, indicating an inelastic excitation mechanism. However, in this case, the threshold amounts to Uth ≈ ±0.1 V only, as compared to Uth ≈ ±0.45 V found for intact H2Pc molecules. This strong reduction is likely caused by the fact that only one hydrogen proton needs to be moved to an adjacent arm in the case of the dehydrogenated molecule, whereas two hydrogen have to be moved for the pristine H2Pc molecule.

similar to H2Pc, the remaining proton could switch between four sites. To remove one hydrogen proton, Auwärter and co-workers11 applied a bias voltage of roughly U = +2 V to the center of the molecule. In our case, a similar positive bias voltage of U = +2.0−2.2 V was needed to remove the hydrogen proton from the center of a H 2Pc molecule. We found that this dehydrogenation is strongly polarity-dependent, as no dehydrogenation was observed for negative bias voltages of up to −3.0 V. It shall be noted that to also remove the second proton an even higher bias of U ≈ +2.5 V is needed. The topography of a singly dehydrogenated molecule is shown in Figure 4a. Obviously, upon dehydrogenation, just one of the elongated arms with an enhanced apparent height remains visible, thereby confirming the above-mentioned hypothesis that the hydrogen protons of H2Pc molecules are located at the nitrogen atom of the elongated arm. This finding is also in agreement with observations of other studies.9,10 The remaining hydrogen proton of the dehydrogenated molecule is located at an arm oriented along the ⟨211⟩ direction, that is, of the former tautomer A. Application of a bias voltage |U| ≥ 0.1 V can be used to switch the hydrogen proton to another arm. Imaging the topography afterwarda measurement which usually takes roughly 5 minreveals that the hydrogen proton moved to the opposing arm, as shown in Figure 4b. To analyze the tautomerization of the dehydrogenated molecule in detail, we employed experimental techniques very similar to those already mentioned above for H2Pc molecules; that is, bias-dependent time traces of the tautomerization were recorded with the tip parked at a particular location, for example, at the blue cross in Figure 4b, which is slightly off the high-symmetry axis of the molecular macrocycle. A typical time trace taken at this position is shown in Figure 4c. It shows not only two but four discrete states, that is, a high state, a low state, and two intermediate states, the relative 11062


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ACS Nano To determine the tip position dependence of tautomerization in dehydrogenated molecules, a 15 × 15 pixel grid of the central region was measured. The measurement procedures employed are the same as those already described for the pristine molecule. A topographic image of this measurement is shown in Figure 4e. The spatial distribution of the tautomerization rate (cf. Figure 4f) indicates that also in the case of the dehydrogenated molecule the highest switching rate is observed in the center of the molecule. For each particular tip position we additionally summed up the probability of staying in the high and the low state, since both probabilities add up to the probability that the hydrogen proton is located on arms pointing in the same high-symmetry axis. The data of this analysis are presented in Figure 4g. With the tip positioned at the lower left or upper right arm (cf. Figure 4g), the probability is very high that the hydrogen proton is oriented in this direction, which belongs to the former tautomer A. When the tip is moved to the other two arms, the probability is very low, indicating that the hydrogen proton spends less time on the arms of the former tautomer B. In total, the spatial distribution of STM-induced tautomerization processes in the pristine and the dehydrogenated H2Pc molecule are very similar. In both cases, the hydrogen protons strongly favor a binding site in the arms which are oriented along ⟨211⟩ directions (former tautomer A; stable state) as compared to arms pointing in ⟨011⟩ directions (former tautomer B; metastable state). Nevertheless, there is a significant difference between the intact and the dehydrogenated molecule concerning the lifetime of tautomer B: Whilewithin the duration of a typical STM measurement (up to 2 days)it was infinite for pristine H2Pc, the lifetime turned out to be much shorter in the case of the dehydrogenated molecule. In order to analyze the lifetime of this metastable state, we employed a pump-and-sample measurement technique, which is schematically shown in Figure 5a. The tip was first positioned on the molecular arm, where the lifetime should be investigated. Note that the feedback loop remains active during the entire experiment. To pump the hydrogen to this arm, the bias voltage is changed to U = −0.5 V. After a certain time, tautomerization takes place such that the hydrogen atom is now located at the arm where the tip is positioned. As soon as this tautomerization was detected by the characteristic retraction of the tip beyond a threshold z-height, the bias voltage was immediately reduced to probing parameters and the temporal evolution of the tip height was recorded. A subsequent jump indicates that the hydrogen moved away from the tip position to another arm (cf. Figure 5a). The time tp between the hydrogen proton moving to and away from the probed arm was extracted from the data. The measurement was repeated over 1000 times for each set of parameters. Histograms for three different probe voltages are shown in Figure 5b. These data sets were fitted with an exponential decay [exp(−t/τ)] to determine the lifetimes τ. For the data sets from −0.6 V ≤ U ≤ − 0.4 V, a continuous measurement instead of the pump-and-sample technique was used to extract the tp, as the excitation happens very fast for these voltages. Figure 5c shows the bias dependence of the lifetime τ obtained with the STM tips positioned on inequivalent arms, thereby probing both the stable and the metastable state. The current in this measurement was set to I = 1 nA. For both states, comparably short lifetimes well below 1 s are found at

Figure 5. (a) Schematic sketch of the pump-and-sample technique used to investigate the lifetime of the tautomers. With a bias voltage of U = −0.5 V, the hydrogen is pumped to the arm, indicated by an increase in the height, which is used as a threshold. Afterward, the time tp of the hydrogen proton staying on this arm is measured. (b) Histogram of three different bias voltages (I = 1.0 nA), which were fit with an exponential decay laws to determine the lifetimes. Fitting the three data sets leads to the following lifetimes: tp(U = −0.1 V) = (2.71 ± 0.10) s, tp(U = −0.3 V) = (1.62 ± 0.10) s, and tp(U = −0.5 V) = (0.19 ± 0.01) s. (c) Bias-dependent lifetime τ shows an increased lifetime of the stable state with decreasing electron energy, whereas it is almost constant in the low-energy regime of the metastable state. (d) Current dependency of the metastable state lifetime for different bias voltages. For U = −0.05 V, the lifetime is almost constant, whereas for U = −0.5 V, it follows a power law with an exponent of −1 (indicated by the blue line). (e) Height dependency of the lifetime of the metastable state shows an increasing lifetime with an enlargement of the tip−molecule distance. The height is regulated by the applied current set point.

low voltages, namely, U = −0.6 and −0.5 V. By decreasing the absolute value of the bias, thus reducing the energy of the tunneling electrons, the lifetime of the stable state continuously increases up to about 45 s at U ≈ −0.20 V. Decreasing the tunneling energy further leads to even longer lifetimes. For bias voltages of U ≤ −0.05 V, proton switching was not observed at all. Due to the very small number of switching events observed and the resulting very large statistical uncertainty, a reliable quantification of the lifetime is virtually impossible such that we decided to omit those data from the chart in Figure 5c. This trend of a lifetime which increases with decreasing absolute value of the bias could already be expected from the data of the bias-dependent tautomerization rate presented in Figure 4d. A different behavior is observed when the lifetime of the metastable state is probed (purple data points in Figure 5c). Going from U = −0.3 V to U = −0.01 V, the lifetime is slightly increased to U = −0.1 V. However, increasing the bias voltage further beyond that value leads to a decreasing lifetime. This behavior demonstrates that the hydrogen positioned on the 11063


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show that the lifetime can be tuned by about 25% by adjusting the tip−sample interaction.

arm of the former tautomer B seems to be unstable, as evidenced by the spontaneous transition to the arm of the former tautomer A even at very low bias voltages. To further investigate the metastability of the hydrogen proton bound to the metastable arm, the current dependence of the lifetime was investigated for various voltages. The data are shown in a double-logarithmic plot in Figure 5d. As indicated by the blue line, the data set of U = −0.5 V (red points) follows a power law behavior with an exponent of −1 for high currents (I > 300 pA), as expected for a one-electron process. For tunneling currents of I < 300 pA, however, the observed lifetime values are shorter than anticipated. This may be caused by the fact that the decay of the metastable state not only is triggered by electrons but also happens spontaneously. This explanation is confirmed by experimental data measured at low bias voltage, that is, U = −0.05 V (green points). For this bias voltage, the lifetime is almost independent of the current set point, which indicates that the process is not driven by electrons in this bias regime. The measurement performed at U = −0.4 V (pink points) marks the transition region between the two extreme cases discussed previously. As the lifetime is almost independent of the current at low bias voltages, we performed some further analysis of the data obtained in this regime as presented in Figure 5e. The plot shows the lifetime for three different bias voltages in dependency of the relative height. Practically, the height was regulated by the set point of the current, which was between I = 0.1 nA and I = 3.2 nA. Focusing on the data marked by a blue square, we can see three data points with very similar lifetimes. As these points are taken roughly at the same tip−molecule distance but at very different bias voltages, we expect that the electric field significantly deviates between these three data points. Consequently, the observed lifetime differences in the low-bias regime cannot be explained by field-induced effects. The lifetime slightly increases when the tip is moved vertically away from the molecule (cf. Figure 5e). This trend could be explained by an influence of the tip on the energy barrier between the stable and the metastable state. It appears conceivable that the presence of the tip in very close proximity to the molecule may reduce the height and/or the width of the energy barrier for tautomerization, thereby increasing the tunneling probability of the hydrogen proton to an adjacent arm. A quantum tunneling mechanism in the low-bias regime for the dehydrogenated H2Pc molecule would also be in line with the fact that back-tunneling from the hydrogen proton to the metastable state was not observed (cf. Figure 5c), as the energy of this state is higher.

EXPERIMENTAL METHODS H2Pc (Alfa Aesar) molecules were degassed in ultrahigh vacuum (UHV) environment for more than 2 days. The Ag(111) crystal was prepared in a UHV chamber with a base pressure of p ≤ 5 × 10−11 mbar by cycles of Ar+ sputtering with an energy of 0.5 keV and subsequent annealing up to approximately 700 K. After the final annealing cycle, the sample was cooled to room temperature (lower sample temperatures for molecule deposition are not available in our UHV system) and H2Pc molecules were evaporated on the surface from a resistively heated crucible. Afterward, the sample was transferred to our home-built STM operating at a temperature of 4.5 K. Topographic images and time traces of the tip height were measured in the constant-current mode with the bias voltage applied to the sample.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05924. Scanning tunneling spectroscopy and dI/dU maps of a H2Pc molecule; tautomerization data taken at positive bias voltages: current dependency, spatially resolved tautomerization of an intact and a dehydrogenated H2Pc molecule, and the bias-dependent lifetime of a dehydrogenated H2Pc molecule (PDF)

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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CONCLUSION In conclusion, the symmetry mismatch between the six-fold symmetry of the Ag(111) substrate and the four-fold symmetry of the H2Pc macrocycle breaks the degeneracy of the two hydrogen protons bound to the arms pointing in the ⟨211⟩ (tautomer A) and the ⟨011⟩ (tautomer B). Therefore, the adsorption behavior is affected, leading to the exclusive appearance of molecules in tautomer A on the substrate. Furthermore, the energy difference between the two tautomers also affects the electron-induced tautomerization, as indicated by the much longer time the excited molecule spends in tautomer A as compared to tautomer B. At noninvasive tunneling parameters, the metastable tautomer B has a very long lifetime, which is reduced to seconds when removing one of the hydrogen protons from the central region of H2Pc. We 11064


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Breaking Degeneracy of Tautomerization ... - ACS Publications

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