Analyzing the Influence of Substituents on Proton Tautomerization—A

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C: Physical Processes in Nanomaterials and Nanostructures

Analyzing the Influence of Substituents on Proton Tautomerization - a Comparison of tetra-tert-Butyl Phthalocyanine Isomers Markus Leisegang, Matthias Bode, and Jens Kügel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10758 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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The Journal of Physical Chemistry

Analyzing the Influence of Substituents on Proton Tautomerization — a Comparison of tetra-tert-Butyl Phthalocyanine Isomers Markus Leisegang,∗,† Matthias Bode,†,‡ and Jens Kügel∗,† †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 E-mail: [email protected]; [email protected]

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Abstract We present a comparative study of tetra-tert-butyl phthalocyanine (ttbPc) isomers on a Ag(111) surface by means of low temperature scanning tunneling microscopy to analyze the influence of substituents on the tautomerization, a proton transfer reaction, in single molecules. By comparing ttbPc with the well studied phthalocyanine (H2 Pc) molecule, we demonstrate a decoupling from the surface by the tert-butyl substituents. A comparison between the four ttbPc isomers, which naturally exist due to different bonding positions of the tert-butyl groups on the macrocycle, reveals a significant influence of the structural differences on their tautomerization behavior, as evidenced by a switching rate which varies by up to a factor of four between ttbPc isomers. These findings can be understood by an energetic landscape of the proton switch which links the binding distance of tert-butyl groups with the height of potential barriers. This model is supported by the analysis of two types of deprotonated ttbPc molecules with the molecular nanoprobe (MONA) technique.

Introduction Proton transfer reactions are one of the most fundamental reversible processes taking place in molecules. 1,2 Pioneered by the work of Liljeroth et al., 3 this so called tautomerization has been investigated on the single molecule level for the organic macrocyles of porphyrins, porphycene, and phthalocyanines on various conducting and insulating surfaces. 4–8 Some fundamental understanding of the physical processes that determine the proton transfer was achieved by the controlled modification of the environment of single molecules. For example, it was shown that depending on the distance of an intentionally placed single Cu adatom the energy difference of the two tautomers in a porphycene molecule can be tuned over a wide range and even be inverted. 9 Furthermore, it has been demonstrated that the potential landscape, in particular the tautomerization barrier, can strongly be modified by positioning the molecule on top of a single atom, resulting in a reduction of the threshold energy for 2


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tautomerization by almost an order of magnitude. 10 However, the extrinsic tuning of the potential landscape of single molecules through atomic manipulation puts high requirements on the surface quality and brings along the necessity of controlled nanomanipulation. By contrast, intrinsic modification of molecules, for example through substitution of chemical compounds, can be readily achieved in the test tube during synthesis. 11 This can change the tautomerization behavior without the need of an additional manipulation, as shown by a shift of the excitation thresholds by replacing hydrogen with deuterium in the center of porphycene. 5 Moreover, the adsorption and electronic structure of organic macrocycles can be tuned by different polyatomic substituents. The substitution of phenyl groups to a porphyrine molecule lead, for example, to the nonplanar adsorption on Au(111), 12 thereby affecting the coupling to the surface which also influences the energetic position of the LUMO. 13 In this work, we analyze the influence of substituents on the tautomerization behaviour of the different tetra-tert-butyl phthalocyanine (ttbPc) isomers which are compared to the well-studied pristine phthalocyanine (H2 Pc). 10 The three-dimensional substituents of ttbPc lead to an effective decoupling of the the planar phthalocyanine macrocycle from the surface, as evidenced by a tautomerization rate which is much lower than that of H2 Pc. Furthermore, significant differences in the tautomerization behavior of the different ttbPc isomers are observed. Namely, the switching rate varies by more than a factor of three depending on the precise binding position and orientation of the tert-butyl substituents. These differences can be understood in terms of a variation of the energetic landscape for tautomerization, where the barrier height is correlated to the binding distance between the tert-butyl groups. This explanation is corroborated by molecular nanoprobe (MONA) measurements on deprotonated ttbPc molecules.

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Experimental Methods The experiments were performed in a home-built low temperature STM at 4.5 K. The Ag(111) surface was prepared by cycles of 10 minutes Ar+ ion sputtering with an energy of 0.5 keV and subsequent annealing at 700 K for 20 minutes. ttbPc and H2 Pc molecules were deposited on the clean surface at room-temperature using a home-built evaporator with a filament-heated quartz crucible. All measurements were performed with a W tip, created by electrochemically etching a W wire with a 2 mol NaOH solution. For rotating and moving molecules on the surface, we applied a bias voltage of U = 20 mV and a tunneling current in the range of I = 15 − 50 nA.

Results and discussion ttbPc configurations and their isomers In Fig. 1(a) the structural model of a tetra-tert-butyl phthalocyanine (ttbPc) is shown. It consists of a phthalocyanine macrocycle with four tert-butyl groups attached to its benzene (a) 8 7

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Figure 1: (a) Structural model of ttbPc with numbers 1-8 marking the possible binding positions of tert-butyl (tb)/hydrogen on phthalocyanine (Pc) macrocycle. Red arrows symbolize the tautomerization of the inner two hydrogen atoms. (Inset) Rotated zoom in of a tb at number 4. (b) STM topography image showing four different ttbPc molecules, labeled D1, D2, C1, and C4. (c–f) Higher magnification scans comparing the two isomers (hydrogen tautomerization) of each of the four molecules presented in (b). The intra-molecular distances between butyl groups labeled s, m, and l are defined in (c). (g) Line profiles across opposing butyl groups in D1. Scan parameters: U = −50 mV, I = 100 pA

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rings. At each benzene ring one of the outer protons is substituted by a tert-butyl group. The possible binding positions of these tert-butyl groups are represented as numbers 18. To highlight the three-dimensionality of these substituents, the rotated tert-butyl at position number 4 is exemplarily shown at higher magnification in the inset. Taking all possible combinations of binding positions for the tert-butyl groups into account leads to 24 = 16 configurations for ttbPc. However, this number includes rotational and mirror symmetric equivalents, such that the theoretical number of configurations is reduced to four distinguishable isomers of ttbPc. 14 The topographic appearance of four representative molecules—one for each distinguishable ttbPc isomer—adsorbed on a Ag(111) surface is shown in Fig. 1(b). In each case the four tert-butyl groups appear as protrusions 15 (bright yellow color), the binding positions of which are marked by white numbers with respect to the model in Fig. 1(a). Starting with the top left molecule and proceeding clockwise, they will be named hereafter by their symmetry group as molecule D1 (identity plus one mirror axis), D2 (identity plus two mirror axes), C1 (identity), and C4 (identity and three rotations). Higher magnification images of these molecules are shown in Fig. 1(c)-(f), respectively. As exemplary sketched for the D1 molecule in Fig. 1(c), these four isomers can be distinguished based on a precise measurement of the inter–tert-butyl group distances. As can be seen in Fig. 1(a), the number of unsubstituted binding positions between two tert-butyl groups can amount to zero, one, and two, resulting in a short (s), intermediate (m), or long (l) inter–tert-butyl group distance, respectively. An analysis of the D1 molecule [cf. Fig. 1(c)] quantify these inter–tert-butyl group distances to l = (1.32±0.04) nm, m = (1.09±0.04) nm, and s = (0.87 ± 0.04) nm. Due to the two possible configurations for the two protons which are bound to opposing nitrogen atoms in the center of the molecule, 16 two additional isomeric states of each molecule (D1, D2, C1, C4) exist. The switching between these two states, the so-called tautomerization, is schematically sketched by red arrows in Fig. 1(a) and can be induced by

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the application of a voltage pulse (|U | > 0.415 V) with the STM tip positioned on top of the ttbPc molecule. The two isomeric states can be distinguished by a slight difference of their topographic appearance, as shown in left and right panels of Fig. 1(c)-(f) for the four different ttbPc molecules, respectively. One state (left panels) is characterized by a high topographic protrusion of the tert-butyl groups together with a deep depression in the center of the molecule, whereas the second state (right panels) shows a less pronounced corrugation across the molecule. These height differences are highlighted by line profiles taken across opposing tert-butyl groups of the two D1 tautomers, marked with green and purple lines in Fig. 1(c) and plotted in Fig. 1(g). Interestingly, the two isomeric states of molecules with mirror symmetry (D1, D2) do not have a mirror symmetric appearance. This symmetry break is most likely induced by the symmetry mismatch between the four-fold symmetry of the phthalocyanine macrocycle with respect to the six-fold symmetry of the substrate, similar to what was shown for H2 Pc on Ag(111). 6 This interpretation is also supported by the overall similarities between the four isomers with respect to the two isomeric states [cf. Fig. 1(c-f)]. The topographic contrast between the two isomeric states strongly varies across the molecule. For example, the green line shows a difference of δz = 33 pm at the central minimum whereas it changes by only δz = 9 pm at the right maximum. Even though the isomeric states have different topographic appearances, an unambiguous allocation of the states to specific positions of the two protons is not possible. While a one-to-one correspondence between the topographic elevation of the molecular arm and the position of the proton binding side is, for example, possible for phthalocyanine 6 and porphycene, 9 the tert-butyl groups of ttbPc inhibit a clear-cut topographic assignment. It should be noted that both isomeric states for all ttbPc isomers are stable on the time scale of our measurements as long as non-invasive scan parameters are used (U ≤ 0.4 V).

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Figure 2: (a) Topographic image of H2 Pc (white dashed circles) and ttbPc (left D1, right D2) on Ag(111). (b) Bias voltage-dependent switching rate ν for D2 ttbPc (black) and H2 Pc (red) measured with the STM tip positioned on top of each molecule [see yellow cross in the inset for D2 (left) and H2 Pc (right)]. (c,d) Sketch of the measurement process for external excitation of (c) D2 ttbPc and (d) H2 Pc with scan frame (yellow square), tip movement (yellow arrow) and injection point (yellow cross) (see main text for details). (e) Bias voltage-dependent switching probability ν of molecules in (c) and (d). Scan parameters for topographic images (a), (b), (d), and (e): U = −50 mV, I = 100 pA

Tautomerization of ttbPc and H2 Pc To analyze the influence of the tert-butyl groups on the tautomerization behavior of phthalocyanine molecules and their derivatives, ttbPc and H2 Pc 10 were co-deposited on Ag(111), as shown in a topographic scan in Fig. 2(a). Compared to the H2 Pc molecules (marked with white dashed circles), ttbPc has a higher topographic appearance, as can be seen for a D2 and a C1 molecule in the upper part of Fig. 2(a). This height difference can be explained by the three-dimensionality of the tert-butyl groups which on the one hand probably lift the ttbPc from the substrate and which on the other hand themselves give rise to an additional protrusion as compared to the flat H2 Pc structure. To compare the tautomerization behavior of ttbPc and H2 Pc molecules, the proton switching was first investigated with the STM tip positioned on top of the molecule. Since the tautomerization rate may depend on the precise position of charge carrier injection within the molecular frame, 6,17 we took great care in selecting the same charge injection point with respect to the center for both molecules [cf. yellow crosses in the inset in Fig. 2(b)]. A D2 ttbPc molecule was used for the comparative study, as the symmetry of this molecule allows

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for the most precise determination of the molecular center. We measured the bias voltagedependent switching rate ν—an action spectrum 18,19 —of the tautomerization in the range of U = (0.42 − 1.6) V [cf. Fig. 2(b)] by recording telegraph noise curves of the z-signal and dividing the number of observed switching events by the total measuring time. An exemplary telegraph noise of a D2 ttbPc molecule together with a histogram of the waiting times demonstrating its exponential distribution is shown in the supporting material. To provide sufficient statistical accuracy, total measurement times range from t = 8 min for the highest rate at U = 1.6 V up to t = 983 min for the lowest rate at U = 0.42 V. The data show qualitatively the same trends for both molecules: An onset of the switching rate at Uth1 ≈ 0.42 V, a steep increase at Uth2 ≈ 0.82 V, and a slight increase at Uth3 ≈ 1.2 V. To accurately determine the energetic thresholds of these features, the data were analyzed with a fit function 20 resulting in energies of ED2,th1 = eUD2,th1 = (402 ± 62) meV, ED2,th2 = (818 ± 4) meV, and ED2,th3 = (1229 ± 135) meV for the D2 isomer of ttbPc and EH2 Pc,th1 = (396 ± 23) meV, EH2 Pc,th2 = (830 ± 14) meV, and EH2 Pc,th3 = (1240 ± 115) meV for the H2 Pc molecule. These threshold energies are in very good agreement with the first, second, and third harmonic of the N–H stretching mode. 21,22 Kügel et al. 10 demonstrated for H2 Pc/Ag(111) that the energy of the first harmonic is below the tautomerization barrier leading to quantum tunneling of the inner protons. A crossover to an excitation over the barrier is observed for the second harmonic of the stretching mode, leading to the strong increase of the switching rate at U ≈ 0.82 V. The excitation of the third harmonic only slightly affects the switching rate, as the energy of the second harmonic was already over the tautomerization barrier. Since the action spectra acquired on both molecules are qualitative similar, we conclude that this model of the tautomerization landscape holds also true for the ttbPc molecule. However, there are some quantitative differences: Over the whole bias range, the switching rate of D2 ttbPc is lower than for the H2 Pc molecule which indicates a lower efficiency to excite vibrational modes by tunneling electrons 23 Additionally, the crossover between the first and the second harmonic of the stretching mode is marked by a stronger increase of

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the switching rate of the D2 ttbPc molecule as compared to H2 Pc, a difference, which is most likely linked to the particular shape and height of the tautomerization barrier. The attachment of tert-butyl groups often results in a decoupling of the molecules from the substrate, as was, e.g., shown for azobenzene molecules 24,25 . It has been demonstrated that this strongly enhances the capability of inducing isomerization of azobenzene molecules by light 24 or by the electric-field of an STM tip. 25 In our case, however, the comparison of ttbPc with H2 Pc reveals that the attachment of tert-butyl groups results in a reduced efficiency of the tunneling electrons to induce isomerization. Since the experiments in Refs. 24 and 25 not only used different excitation mechanisms but also different substrates, it is difficult to unanimously identify the relevant physical processes. For example, the observation of opposite trends might be caused by the fact that the excitation probability of tunneling electrons for the N–H stretching mode is lower than for photo-excited charge carriers, thereby resulting in a lower isomerziation probability. As we will show below, our remote tautomerization experiments confirm that a decoupling of the molecules from the substrate is indeed taking place. Action spectroscopy has the disadvantage that the tip placed directly over the molecule might influence the tautomerization due to proximity effects. 6 To overcome this issue we also triggered the tautomerization processes of both molecules remotely, a technique we coined molecular nanoprobe (MONA). 26 In this case the molecular excitation is triggered by hot electrons injected from the tip into the substrate at a distance of a few nanometer from the molecule. The method is schematically sketched in Fig. 2(c). First, the initial state of the molecule is probed by a topographic scan (U = 0.02 V, I = 0.1 nA). Then the tip is moved 5 nm away from the molecule to the injection point (yellow cross). At this position, bias and current are changed to excitation values for a certain duration t, followed by another scan of the molecule to probe its final state. This process is repeated at least 5000 times for each data point. If the isomeric state between two following scans has changed it will be counted as a switching event. The total number of switching events is divided by the number of

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Figure 3: (a) Topographic height of the four ttbPc isomers (grid: 15 × 15 pixel). (b) Switching rate for the four isomers (grids as in (a)). The number of switching events is colorcoded as indicated by the scale bar. (c) Simplified structural model for the four isomers with distances s, m, and l between adjacent tert-butyl substituents(yellow). (d) Model for potential landscape. Red circles represent the two opposing protons, arrows the tautomerization events. Parameters for the grids in (a) and (b): U = 0.95 V I = 1 nA

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repetitions minus one, giving the switching probability η. To minimize the influence of the environment, we placed the two interrogated molecules at exactly the same surface spot, as shown in Fig. 2(c) for D2 and Fig. 2(d) for H2 Pc. The bias dependency of the switching probability η is plotted for both species in Fig. 2(e). Below an energy of 900 meV the efficiency of electrons for tautomerization is too low to detect a significant switching probability within an experimentally accessible measurement time. When applying voltages U > 0.9 V a finite switching probability η was detected for both molecules. In either case η increases with bias voltage, but the rate is much higher for H2 Pc than for ttbPc. This difference can be explained by a decoupling of the ttbPc molecule from the substrate. Due to the bulky tert-butyl groups the distance of the phthalocyanine macrocycle is likely enlarged. This leads to a lower amount of hot electrons which are capable to reach the macrocycle and excite the isomerization.

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Comparative study of ttbPc groups As described above, all four ttbPc isomers (D1, D2, C1, C4) have exactly one tert-butyl substituent at each of their benzene groups. In general, we can expect that the much larger size of tert-butyl—as compared to the H atom it replaces—lifts the center of the ttbPc molecules, thereby leading to an effective decoupling from the surface. We have to keep in mind, however, that the tert-butyl substituents in the D1, D2, C1, and C4 isomers of ttbPc are differently arranged which may also result in variations of tautomerization behavior. To analyze these potential differences of the tautomerization behavior, one ttbPc molecule of each type was investigated by means of spatially resolved measurements. For this purpose, a grid of 15 × 15 pixel was placed over the molecules. On each pixel the telegraph noise was detected for 83 s. From these data sets the average height and the switching rate were calculated, which are shown in Fig. 3(a) and (b), respectively. The color code in (b) marks low switching rate with dark colors and high switching rates with bright colors, going from zero to the maximum switching rate indicated at the scale bars. The patterns observed in Fig. 3(b) for the ttbPc isomers D1, D2, C1, C4 are qualitatively very similar, thereby indicating that the general processes that lead to tautomerization are identical: The switching rate is highest in the center of each molecule and decreasing when moving the STM tip, i.e., the charge carrier injection point, away from the center. Two local maxima appear on the diagonal from top left to the bottom right. Comparisons with the topographic images presented in Fig. 3(a) reveal that they are located at the positions of the butyl groups. In contrast, only a slight increase of the switching rate can be observed on the diagonal from bottom left to the top right. This asymmetry might be explained by very tiny height differences between the isomers in the latter direction which themselves are induced by the symmetry mismatch between the substrate and phthalocyanine macrocycle and likely prevents us from detecting all switching events. In contrast to the very similar spatial distribution of the switching rate its absolute values vary significantly between the isomers, as can be seen by comparing the maximum rate given 11



Table 1: Switching rates f for two bias voltages measured on top of the five molecules Molecule H2 Pc C4 D1 C1 D2

f (0.74 V)/s−1 f (0.95 V)/s−1 0.062 ± 0.003 3.44 ± 0.11 0.0027 ± 0.0003 0.55 ± 0.01 0.0043 ± 0.0004 0.63 ± 0.01 0.0067 ± 0.0004 0.73 ± 0.01 0.0100 ± 0.0005 1.40 ± 0.02

at each scale bar in Fig. 3(b). The highest rates of the molecules are in ascending order given by fC4 = 1.95 s−1 , fD1 = 2.17 s−1 , fC1 = 2.75 s−1 , and fD2 = 4.19 s−1 . Comparing these values with the switching rate obtained for H2 Pc before we can conclude that the switching rate for all ttbPc molecules is indeed lowered. This finding is in agreement with our expectation that the presence of tert-butyl groups lifts and decouples the molecular macrocycle from the substrate. Beyond this general finding, the large variation of switching rates observed between the four ttbPc isomers (D1, D2, C1, C4) shows that the specific ordering of the tert-butyl groups impacts the tautomerization. In order to investigate this behavior more closely, we measured the switching rate for the four ttbPc isomers at U = 0.74 V for at least t = 580 min and at U = 0.95 V for at least t = 50 min (I = 1 nA in both experimental runs). Similar to the procedures described before the data were obtained at comparable excitation points by moving the STM tip by a given distance from the center to the bottom right arm of each molecule. The resulting rates for the four ttbPc isomers and for H2 Pc are summarized in Table 1. In agreement with the grid measurements the rates are increasing in the same molecular order at both measurement voltages. These differences can be understood in a simple model taking account the different distances of the tert-butyl groups. Fig. 3(c) shows schematic models of the four types of molecules. We assume that the only relevant parameters are the intra-molecular distances s, m, and l between neighboring tert-butyl groups. In case of a long distance l between two tert-butyl groups, a stronger bending of the macrocycle between these groups can emerge,

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thereby reducing the distance between the bonding sites of the inner proton at the supporting arms and thus the tautomerization barrier. Furthermore, the tert-butyl groups are furthest from these two arms potentially leading to a reduced influence and thus a lower tautomerization barrier. Consequently, the lowest barrier height is given for the longest distance l, it slightly increases at an intermediate distance m, and reaches the largest barrier height for the short distance s. Under these assumptions, the tautomerization landscape of the four ttbPc configurations reduces to the simple sketch shown in Fig. 3(d). In each panel the red dumbbell represent the two protons which always bind to opposing N atoms in the center of the ttbPc molecule. If a switching is induced, these two protons move in the energy landscape either to the left (violet arrows) or to the right (red arrows) [cf. Figure 3(d)], representing a clockwise or a counterclockwise transition, respectively. With the exception of the C4 molecule, where every intra-molecular barrier is of intermediate height m, the symmetry of the ttbPc molecules lifts degeneracy of these two paths. In the case of the D2 molecule, for example, two large barriers need to be overcome for a transition to the left, whereas only two small barriers exist for a transition to the right. Consequently, proton transfers will preferentially occur over the smallest barriers. A qualitative comparison of the barrier heights of the different ttbPc molecules reveal that the switching rates are in line with this model. D2 has the highest switching rate, as a proton transition is possible over two small tautomerization barriers (s + s). The second highest rate is expected for C1 molecules, where one small and one medium (s + m) has to be overcome for the tautomerization process. The lowest rates are observed for the D1 and C4 molecule, which are described by the highest barriers (s + l and m + m).

Directional switching in deprotonated ttbPc In the case of pristine ttbPc, tautomerization involves the transition of two protons such that we inevitably probe two tautomerization barriers at the same time. This impedes a 13



more detailed analysis of the tautomerization energy landscape. To overcome this limitation we removed one of the inner protons. This deprotonation can be achieved by placing the tip on top of the molecule center and the application of a voltage pulse of U = 2.1...2.3 V at I = 1 nA until a step-like change in the z-height occurs. 16,17 With only one proton left for tautomerization we are now capable of probing single barriers in ttbPc, thereby allowing for a much more detailed investigation of the potential. In the following we compare the switching rates of two deprotonated ttbPc isomers, C4 and a D2. Structural models of these molecules are shown in Fig. 4(a) and (b), respectively. Topographic data of the four isomeric states of a deprotonated D2 molecule are shown in the supporting material. In contrast to an intact ttbPc molecule, where we were not able to unambiguously determine the binding sites of the central hydrogen atoms, the remaining proton in a deprotonated ttbPc molecule can easily be localized, as it leads to an elevated protrusion of the corresponding molecular arm. The measurements were performed by external excitation using the MONA technique, as described in Sec. 2, to minimize an influence of the tip on the directionality of the switching. Additionally, to ensure that there is no significant internal shielding effect caused by the different orientation of the tert–butyl groups, we measured the tautomerization rates from two tip positions which are at the same distance (a)

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Figure 4: (a) Structural model of deprotonated C4 ttbPc. Tautomerization paths are marked with light and dark blue arrows, excitation directions are shown in yellow. (b) Structural model of deprotonated D2 ttbPc with tautomerization paths marked with red and orange arrows. (c) Relative switching rate of deprotonated C4 (light/dark blue) and deprotonated D2 (orange/red) obtained with the MONA technique for two excitation angles.

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from the respective molecule but located in different directions. They are defined by their angles α1 = 0◦ and α2 = 90◦ with respect to the symmetry axes of the phythalocyanine macrocycle and marked with yellow dashed lines in Fig. 4(a). Based on the model of the tautomerization landscape [Fig. 3(d)], due to its four-fold rotational symmetry the same switching rate can be expected for all proton transfer directions in the C4 molecule. In contrast, two paths with different switching rates, indicated by red and orange arrows in Fig. 4(b), should exist for the D2 molecule due to its two-fold symmetry. The switching rates along these two paths in the D2 molecule are compared to the switching rates marked with light and dark blue arrows for C4 in Fig. 4(a). For each molecule at least 5000 excitations were measured at an excitation voltage U = 0.5 V and a duration of t = 2 s, followed by a topographic scan of the molecule to extract its isomeric state. These data can be used to calculate the relative switching rates of the different paths by dividing the number of observed switches along a given paths by the total number of switching events. The result is plotted in Fig. 4(c). First we would like to emphasize that there is no significant difference between the data points for 0◦ and 90◦ . Obviously, there is no significant shielding or enhancement due to the differently positioned tert-butyl groups. In the case of the C4 molecule both paths have very similar relative switching rates of around 50% indicating the absence of a preferred path for the proton switch. In contrast, the two switching channels within the D2 molecule differ from each other by 34%, i.e., the probability of a proton switching on the red path with the long inter–tert–butyl distance l is twice as high as for the orange one with the short distance s. The homogeneous behavior of the dehydrogenated C4 and the difference in the switching rate for the paths within the D2 molecule are in very good agreement with our model for the potential landscape presented in Fig. 3(d). They show that the binding orientation and consequently the distance between the butyl groups have a strong influence on the potential landscape for the switching behavior of the inner hydrogen atom.

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Conclusion In summary, we could demonstrate that the tert-butyl substituents bound to a phthalocyanine macrocycle change the tautomerization behavior significantly by decoupling the molecule from the surface as compared to the pristine H2 Pc. Furthermore, our data show that the four ttbPc isomers on Ag(111) exhibit different switching behaviors. A model for the energetic landscape which links the height of the potential barriers for the proton switching with the intra-molecular bond length between adjacent tert-butyl substituents is able to qualitatively explain the observed variation of tautomerization rates. This model is also corroborated by a path-dependent switching rate analysis performed on deprotonated ttbPc molecules by MONA measurements. Our results demonstrate, that substituents can have a significant influence on the tautomerization behavior of molecules, which is relevant for basic research as well as for the application of proton transfer reactions in molecular switches.

Supporting Information Supporting Information Available: Telegraph noise and waiting time analysis, topography of deprotonated D2 molecule This material is available free of charge via the Internet at http://pubs.acs.org.

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High Low Switching Rate

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Analyzing the Influence of Substituents on Proton Tautomerization—A

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