Imprinting Directionality into Proton Transfer Reactions of an Achiral

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Imprinting Directionality into Proton Transfer Reactions of an Achiral Molecule Jens Kügel,*,† Markus Leisegang,† and Matthias Bode†,‡ †

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

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‡

S Supporting Information *

ABSTRACT: Directionality is key for the functionality of molecular machines, which is often achieved by built-in structural chiralities. Here, we present a scanning tunneling microscopy study of achiral H2Pc and HPc molecules that acquire chirality by adsorption onto a Ag(100) surface. The adsorption-geometry-induced chirality is caused by a −29° (+29°) rotation of the molecules with respect to the [011] substrate direction, resulting in tautomerization processes that preferentially occur in a clockwise (counterclockwise) direction. The directionality is found to be independent of the particular energy and location of charge carrier injection. In contrast to built-in structural chiralities that are fixed by the molecular structure, the direction of proton motion in HPc on Ag(100) can be inverted by a rotation of the molecule on the substrate. KEYWORDS: scanning tunneling microscopy, directionality, chirality, tautomerization, molecular nanoprobe, phthalocyanine

I

our results reveal a directionality that is essentially independent of the charge carrier energy. Furthermore, we demonstrate on a single HPc molecule that directionality can reversibly be inverted by the controlled rotation of the molecule.

n the past decades, tremendous progress has been achieved in the design, building, and analysis of molecular machines, which mimic the basic functionalities of their industrial counterparts on the scale of nanometers.1 This includes the invention of nanovehicles,2,3 delivery systems,4 motors,5,6 and many other nanomachines. Key for the functionality of all these machines is the existence of directional motion, a prerequisite of controlled processes. In most cases the unidirectional motion is a direct consequence of the structural chirality of the molecular units.7 However, with molecules having a fixed chirality also the direction of motion is determined and cannot be easily reversed. To circumvent this limitation, we used a phthalocyanine molecule (H2Pc), which itself is achiral. Due to adsorption on a Ag(100) surface, chirality is induced, which allows to imprint and control directionality in tautomerization, i.e., the proton transfer reaction in the central cavity of the molecule. Specifically we show that upon the controlled removal of one proton from the inner cavity by means of a scanning tunneling microscope tip, the switching of the remaining proton between the four N sites in the center of the molecule can be induced by inelastic electrons. The adsorptiongeometry-induced chirality of the molecule on the Ag(100) surface directly translates into a directionality of tautomerization where the proton prefers to rotate either in a clockwise or in a counterclockwise direction. Irrespective if the hot electrons are injected in the molecular frame or into substrate, © XXXX American Chemical Society

RESULTS AND DISCUSSION Topographic Analysis of the Molecules. In Figure 1(a) the topography of H2Pc molecules adsorbed on a Ag(100) surface is presented together with a structural model of the molecule (inset). The molecule consists of a molecular macrocycle with two protons bound to opposing nitrogen atoms in the central cavity. The position of these protons can be easily identified since the corresponding molecule arms exhibit a characteristic topographic elevation (yellow color), as was also shown on similar systems.8−10 Similar to the adsorption of other phthalocyanine molecules on the Ag(100) substrate,11−13 two orientations of the H2Pc molecule exist, r-H2Pc and l-H2Pc, which are both shown in the zoom-in image in Figure 1(b). Whereas r-H2Pc is rotated by +29° with respect to the [011] direction, l-H2Pc is rotated by −29°. A similar adsorption-geometry-induced chirality with almost the same rotational angle has been observed for CuPc on Received: June 27, 2018 Accepted: August 7, 2018 Published: August 7, 2018 A

DOI: 10.1021/acsnano.8b04868 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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positions of the remaining proton are exemplarily shown in Figure 1(c) for a r-HPc molecule. Similar to the pristine H2Pc the actual binding site of the proton can be identified by the bright yellow arm. Since only one arm shows an enhanced density of state it can be concluded that the proton is bound right at this molecular arm. The removal of one proton allows for the unanimous discrimination between switching events that occur in a clockwise (cw) or counterclockwise (ccw) fashion, as the final states for these two movements are different. For example, a proton initially bound to the upper arm would lead to the proton bound to the right arm in case of a clockwise movement, whereas in case of a counterclockwise movement the proton is expected to switch to the left arm. Figure 1(c) lists all possible clockwise (red arrows) and counterclockwise (blue) single-step transitions between the four states. Direct Charge Injection into Molecules. The tautomerization transitions between the four states presented in Figure 1(c) can not only be detected by scanning the entire molecule in its initial and final state, but also by positioning the tip at a suitable position over the molecule, e.g., at the green cross in the inset in Figure 2(a). When the bias is increased to |U| ≥ 70 mV at this position a telegraph noise in the tip height is observed, as exemplarily shown for a r-HPc molecule at U = 0.65 V in Figure 2(a). Four height levels can be distinguished, which correspond to the proton bound to the four possible molecular arms, which are labeled correspondingly in the topographic image in the inset of Figure 2(a). We can determine the electron yield by dividing number of switching events observed in the telegraph noise by the total number of electrons injected into the molecule. Figure 2(b) shows the bias-dependent electron yieldthe so-called action spectrum14,15for both molecular orientations, l-HPc and rHPc. To achieve a sufficient statistical accuracy we analyzed 80 s of telegraph noise for the highest switching rates and up to 10 h for the lowest switching rates. l-HPc and r-HPc molecules show an essentially identical behavior since the subtle recognizable deviations are insignificant. Similar to experiments performed on HPc/Ag(111),16 two pronounced features are visible in the data: First, a threshold bias at roughly 155 mV is required to obtain any significant nonzero electron yield. As the bias voltage is raised the electron yield increase slowly levels off until another steep increase occurs at about 400 meV. The latter feature marks the energetic onsets of the N−H stretching mode.16−18 Correspondingly, the first feature has been ascribed to the N−H in-plane bending mode,16−18 but the absence of a isotope-shift in porphycene indicated that the situation may be more complex.19 Whether the low-bias feature in Pc molecules is also caused by vibrational bands with negligible influence of proton movements, as proposed for porphycene on Cu(110),19 needs to be clarified in future experiments. It should be noted that both data sets were measured on the same molecule, which was rotated with the STM tip by applying a bias voltage of U = 20 mV and a set point current of I = 20−80 nA. These manipulation parameters of a very low bias voltage and a very high tunneling current strongly differ from the parameters used to measure the above shown action spectra. Therefore, we are convinced that the observed switching events cannot be explained by a transition between different molecule conformations or a rotation of the molecular frame, but are indeed caused by tautomerization of the central proton within the HPc molecule.

Figure 1. (a) Topography of H2Pc molecules adsorbed on a Ag(100) surface. The inset shows the structural model of the molecule. (b) Zoom-In image of a r-H2Pc and a l-H2Pc showing the two possible rotations of the molecule. The inset shows an atomically resolved image of the substrate. (c) Topographic images of the four states of a r-HPc molecule. Clockwise (cw) and counterclockwise (ccw) switching of the proton are marked by red and blue arrows, respectively. Scan parameters: (a−c) U = −50 mV, I = 100 pA. Inset of (b): (molecule terminated tip): U = 10 mV, I = 50 nA.

Ag(100).11 We would like to emphasize, however, that the central binding site of Cu does not allow for any tautomerization switching. In contrast, the off-center position of H protons in the Pc molecular frame allows for different energetically degenerate positions. As we will see below, the combination of a substrate-induced chirality and tautomerization switching leads to a directionality that can be inverted by molecular manipulation. By positioning the STM tip on top of a H2Pc molecule and applying a bias voltage of |U| ≥ 415 mV the tautomerization of the molecule, that is the switching of the inner protons to the adjacent molecular arms [cf. red arrows in Figure 1(a)], can be triggered, which is experimentally observed as a telegraph noise of the tip height (cf. Supporting Information). Since tautomerization processes are generally taking place on time scales much shorter than the temporal resolution of our STM, which is typically limited to ms time scales by the bandwidth of the preamplifier, only the initial and final state but no details of the transfer can be analyzed. Consequently, while we are able to measure the amount of switching events of the protons, we are not able to distinguish whether the two protons in a H2Pc molecule rotate clockwise or counterclockwise, as the initial and final state configurations for these two cases are identical. To circumvent this limitation, we removed one of the inner protons from the molecule by applying a voltage pulse of U = 2.1, ..., 2.3 V on top of the H2Pc. This creation of a HPc molecule is marked by a sudden increase in the tip height (cf. Supporting Information). The topography of all four possible B


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the r-HPc molecule. To quantify this behavior we defined the directionality η=

Ncw − Nccw Ncw + Nccw

(1)

whereby Ncw (Nccw) is the number of switches in a clockwise (counterclockwise) direction. A directionality of η = 100% (η = −100%) corresponds to a proton that exclusively switches in the clockwise (counterclockwise) direction, whereas a value of η ≈ 0% marks the absence of any net directionality where the same amount of switches in both directions occur on average. Figure 2(c) shows a plot of the bias-dependent directionality for both types of molecules. The data were extracted from the same measurements used to analyze the electron yield. The directionality observed for r-HPc and the l-HPc molecules is clearly different. Whereas a negative directionality is observed for r-HPc, it is positive for l-HPc, indicating that the molecule preferably switches by moving the proton in the counterclockwise and clockwise direction, respectively. Interestingly, the value of the directionality is essentially constant in the probed energy range. The blue lines in Figure 2(c) represent the weighted arithmetic average with average values of η = (−30.2 ± 0.6)% for l-HPc and η = (30.9 ± 0.6%) for r-HPc. This result of a constant directionality can be understood in terms of a ratchet mechanism.1,20,21 As sketched in Figure 2(d), the excitation of vibrational modes leads to the population of energy levels well above the tautomerization barrier. For the unperturbed molecule, the probability for a clockwise and a counterclockwise switch during the proton relaxation process is the same, as previously observed on singly deprotonated TPP molecules on Ag(111).10 Due to interaction with the substrate, however, rotational degeneracy is broken and chirality is induced in the molecule. This resulting modification of the tautomerization barrier leads to a reaction path that is no longer rotationally degenerate but exhibits a pronounced chirality [cf. Figure 2(d)]. Consequently, when the proton relaxes to the ground state in the case of the chiral HPc molecule, the probability for a proton transfer reaction in clockwise and counterclockwise direction are different. Analysis of the Directionality with Hot Electrons. The experiments we described so far reveal that electrons injected with the STM tip directly into the molecular frame of a HPc molecule induce a directional motion of the inner proton, the rotational sense of which depends on the orientation of the molecule with respect to the surface. It has been shown, however, that even an STM tip brought in close vicinity to a molecule can strongly affect the landscape of tautomerization.9,22,23 To investigate whether the tip is responsible for or contributes to the observed directionality, we made use of the molecular nanoprobing (MONA) technique,24,25 which in short works as follows: First, the topography of the molecule is analyzed with noninvasive parameters (U = −50 mV, I = 100 pA), either by a topographic scan of the molecule24,25 oras done in the followingby measuring the height of all four molecular arms (see Supporting Information). As previously described in the context of Figure 1(c) the molecular arm to which the proton is bound appears highest. Second, the tip is moved a certain distance d away from the center of the molecule, e.g., to the bright yellow cross in Figure 3(a). At this position, the bias voltage U and set point current I are changed for a duration t to excitation parameters, whereby a certain amount of hot electrons (holes for negative bias voltages) are

Figure 2. (a) Telegraph noise taken on the arm of a r-HPc (cf. green cross in the inset) (telegraph noiseparameters: U = 0.65 V; I = 50 pA). Four different height levels can be distinguished, corresponding to the proton bound to the four arms labeled in the inset. The amount of clockwise (cw) and counterclockwise (ccw) switching events can be extracted from the telegraph noise. (b) Bias-dependent electron yield extracted from data sets of telegraph noises taken on a r-HPc (black squares) and a l-HPc (red circles) (scan parameters: I = 500 pA for U ≤ 0.35 V; I = 50 pA for U > 0.35 V). (c) Directionality data for a r-HPc (black squares) and a lHPc (red circles)extracted from the same data sets used to analyze the electron yield (b). (d) Sketch of the energetic tautomerization landscape of an unperturbed (left) and chiral (right) HPc molecule. The straight black lines represent the energy level of excited vibrational modes.

On the basis of the reasoning presented in connection with Figure 1(c) we can also determine how often the proton switches in a clockwise or counterclockwise direction, as demonstrated for the telegraph noise in Figure 2(a). The first switching event shows a transfer from arm 1 to arm 3 (1 → 3), i.e., a clockwise movement. All of the following transitions (3 → 1; 1 → 2; 2 → 4; 4 → 3; 3 → 1; 1 → 2) are transition in the counterclockwise direction. It appears already from this short time trace that the counterclockwise direction is preferred for C


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shows trends that are very similar to the results obtained when injecting the charge carriers directly into the molecular frame. In both cases we find a negative directionality for r-HPc and a positive directionality for l-HPc. Even quantitatively the data are very similar, with an average value of η = (−27.5 ± 1.3)% for the r-HPc and η = (28.8 ± 1.3)% for the l-HPc for the hot electron-induced directionality. In case of direct charge injection into the molecular frame, the directionality was independent of the bias voltage. To check whether similar trends are observed for hot charge carriers injected into the substrate, we performed biasdependent MONA measurements. As the results presented in Figure 3(b) confirmed that the directionality is independent of the direction of charge injection, only one tip position was used for each of the following measurements. These MONA measurements allow for the determination of the electron yield [cf. Figure 3(c)], which is calculated by dividing the number of switching events by the total charge injected into the substrate. Overall, the data for r-HPc and l-HPc are very similar; in both cases the electron yield increases with the energy of the hot electrons. It turns out that not only hot electrons but also hot holes can trigger the tautomerization, as revealed by the data point at U = −1.5 V in Figure 3(c). The switching efficiency of these holes is more than 1 order of magnitude lower as compared to hot electrons of the same energy, possibly due to their more efficient relaxation processes or a less efficient excitation of vibrational modes in the molecule. Similar to direct injection experiments, the MONA experiments show a directionality that is essentially independent of the bias voltage, as shown in Figure 3(d). We find average values of η = (−28.8 ± 1.1)% for the r-HPc and η = (28.2 ± 1.4)% for the l-HPc. Directionality Inversion by Single-Molecule Manipulation. The direct as well as the substrate-mediated excitation of the tautomerization experiments have shown that the inner proton of r-HPc preferably switches in a counterclockwise direction whereas l-HPc is characterized by a switching processes that more often occur in a clockwise direction. For any implementation in future devices it will be of crucial importance that the directionality can be reversibly changed. To experimentally investigate if the controlled reversal of the directionality is possible for the given molecule−substrate configuration, we investigated the switching behavior and tautomerization directionality of a single molecule that initially was in the r-HPc configuration, as shown in Figure 4(a). After the first analysis [see data points labeled first r-HPc in Figure 4(a)], the molecule was rotated with the STM tip to change it to a l-HPc molecule [cf. Figure 4(b)] and its directionality was again analyzed by the MONA technique. Even though the experiments presented in Figure 3 show that the directionality is not affected by the propagation direction of the hot electrons, we analyzed the directionality not only from one single injection point but instead varied the tip position between the three points marked by yellow crosses in Figure 4(a,b). In total, the molecule was rotated 5 times between the two configurations. The resulting series of directionality is shown in Figure 4(c). It clearly demonstrates that the preferred rotational sense of the inner proton can be reversibly altered by rotating the molecule back and forward between the two configurations.

Figure 3. (a) Topography of a l-HPc molecule showing 12 injection points (yellow stars), positioned d = 5 nm away from the center of the molecule (scan parameters: U = −50 mV, I = 100 pA). (b) Directionality that was analyzed by the MONA technique (see main text for details), with the hot electrons injected at the position marked in (a) (MONA parameters: U = 1.4 V; I = 2 nA; t = 1 s; d = 5 nm). (c) Bias-dependent electron yield, calculated by dividing the number of switching events measured by the MONA technique divided by the charge injected into the substrate (MONA parameters: I = 2 nA; t = 1 s; d = 5 nm). (d) Directionality extracted from the data sets presented in (c).

injected into the substrate. These charge carriers propagate within the substrate and some of them reach the molecule where they can potentially trigger the tautomerization of the molecule.24,26 Finally, the topography of the molecule is analyzed with noninvasive parameters again, to check whether the proton switched to another arm. In the case of a switching event, it was furthermore analyzed whether the proton transfer happened in a clockwise or counterclockwise direction. This procedure was repeated approximately 5000 times for each data point for an appropriate statistics. To analyze whether the directionality is also present if the charge carriers are not directly injected into the molecule and if any correlation between the direction of charge injection and directionality of the proton transfer is observable, the MONA method was used to inject charge carriers at 12 positions that are equidistant (d = 5 nm) from but in different directions with respect to the molecule. These injection points are marked by yellow stars in Figure 3(a). Eight of the 12 directions are oriented along the molecular arms of the r-HPc and l-HPc molecule, whereas the other directions are orientated along high symmetry directions of the substrate (⟨01̅1⟩ and ⟨011⟩ directions), i.e., symmetrically with respect to the arms of both, the r-HPc and the l-HPc molecule. The results of this measurement are presented in Figure 3(b). As a reference (0°) we define the injection point that is displaced along the [01̅1] direction with respect to the molecule and marked by the bright yellow star in Figure 3(a). It turns out that within the statistical accuracy the directionality is independent of the direction of the charge injection, as can be seen by the average values represented by the blue lines in Figure 3(b). The analysis of hot electron-induced directional switching processes

CONCLUSIONS In conclusion, we could demonstrate that the molecule− substrate interaction leads to directionality in proton transfer D


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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.

ACKNOWLEDGMENTS This research was supported by DFG (through SFB 1170 “ToCoTronics”; project A02). REFERENCES (1) Erbas-Cakmak, S.; Leigh, D. A.; McTernan, C. T.; Nussbaumer, A. L. Artificial Molecular Machines. Chem. Rev. 2015, 115, 10081− 10206. (2) Kudernac, T.; Ruangsupapichat, N.; Parschau, M.; Maciá, B.; Katsonis, N.; Harutyunyan, S. R.; Ernst, K.-H.; Feringa, B. L. Electrically Driven Directional Motion of a Four-Wheeled Molecule on a Metal Surface. Nature 2011, 479, 208. (3) Saywell, A.; Bakker, A.; Mielke, J.; Kumagai, T.; Wolf, M.; García-López, V.; Chiang, P.-T.; Tour, J. M.; Grill, L. Light-Induced Translation of Motorized Molecules on a Surface. ACS Nano 2016, 10, 10945−10952. (4) Berna, J.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.; Pérez, E. M.; Rudolf, P.; Teobaldi, G.; Zerbetto, F. Macroscopic Transport by Synthetic Molecular Machines. Nat. Mater. 2005, 4, 704. (5) Koumura, N.; Zijlstra, R. W.; van Delden, R. A.; Harada, N.; Feringa, B. L. Light-driven Monodirectional Molecular Rotor. Nature 1999, 401, 152. (6) Greb, L.; Lehn, J.-M. Light-Driven Molecular Motors: Imines as Four-Step or Two-Step Unidirectional Rotors. J. Am. Chem. Soc. 2014, 136, 13114−13117. (7) P, M. C.; Burkhard, K. Chemistry in MotionUnidirectional Rotating Molecular Motors. Angew. Chem., Int. Ed. 2004, 43, 1622− 1624. (8) Sperl, A.; Kröger, J.; Berndt, R. Controlled Metalation of a Single Adsorbed Phthalocyanine. Angew. Chem., Int. Ed. 2011, 50, 5294− 5297. (9) Liljeroth, P.; Repp, J.; Meyer, G. Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules. Science 2007, 317, 1203−1206. (10) Auwä r ter, W.; Seufert, K.; Bischoff, F.; Ecija, D.; Vijayaraghavan, S.; Joshi, S.; Klappenberger, F.; Samudrala, N.; Barth, J. V. A Surface-Anchored Molecular Four-Level Conductance Switch Based on Single Proton Transfer. Nat. Nanotechnol. 2012, 7, 41−46. (11) Mugarza, A.; Lorente, N.; Ordejón, P.; Krull, C.; Stepanow, S.; Bocquet, M.-L.; Fraxedas, J.; Ceballos, G.; Gambardella, P. Orbital Specific Chirality and Homochiral Self-Assembly of Achiral Molecules Induced by Charge Transfer and Spontaneous Symmetry Breaking. Phys. Rev. Lett. 2010, 105, 115702. (12) Mugarza, A.; Robles, R.; Krull, C.; Korytár, R.; Lorente, N.; Gambardella, P. Electronic and Magnetic Properties of MoleculeMetal Interfaces: Transition-Metal Phthalocyanines Adsorbed on Ag(100). Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 155437. (13) Kügel, J.; Karolak, M.; Senkpiel, J.; Hsu, P.-J.; Sangiovanni, G.; Bode, M. Relevance of Hybridization and Filling of 3d Orbitals for the Kondo Effect in Transition Metal Phthalocyanines. Nano Lett. 2014, 14, 3895−3902. (14) Kim, Y.; Motobayashi, K.; Frederiksen, T.; Ueba, H.; Kawai, M. Action Spectroscopy for Single-Molecule Reactions − Experiments and Theory. Prog. Surf. Sci. 2015, 90, 85−143. (15) Sainoo, Y.; Kim, Y.; Okawa, T.; Komeda, T.; Shigekawa, H.; Kawai, M. Excitation of Molecular Vibrational Modes with Inelastic Scanning Tunneling Microscopy Processes: Examination through

Figure 4. (a) Topography of a r-HPc molecule. (b) Topography of the same area as presented in (a) after rotating the molecule to create a l-HPc molecule. (c) Directionality of the same molecule, which was rotated in total 5 times (MONA parameters: U = 1.4 V; I = 2 nA; t = 1 s; d = 5 nm). The injection points (Pos. 1−3) of the MONA measurements are marked by yellow crosses in (a,b). Scan parameters: (a,b) U = −50 mV; I = 100 pA.

processes of singly deprotonated H2Pc molecules. The proton in the center of a l-HPc molecules is characterized by a movement mainly in a clockwise direction, whereas the proton of the r-HPc is more frequently switching in the counterclockwise direction. The rotational sense of the proton motion is found to be largely independent of the charge carrier injection point, i.e., whether the STM tip is positioned directly above the molecular frame or into the bare substrate in close vicinity to the molecule. Furthermore, we could demonstrate that the rotational sense can be repeatedly reversed by rotating a HPc molecule with the STM tip.

EXPERIMENTAL METHODS The STM measurements were performed with a tungsten tip, which was electrochemically edged with a 2 molar solution of sodium hydroxide. H2Pc molecules (Alfa Aesar) were purified by degassing under ultrahigh vacuum conditions for more than 2 days. The Ag(100) surface was prepared by cycles of Ar+ sputtering with an ion energy of 0.5 keV and subsequent annealing up to 650 K. After the last annealing cycle, the sample was cooled to room temperature and the H2Pc molecules were evaporated on the surface. Afterward, the sample was immediately transferred in our home-built low-temperature scanning tunneling microscope working at a temperature of T ≈ 4.5 K.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04868. Telegraph noise of H2Pc tautomerization; Deprotonation of H2Pc; Measurement protocol of MONA (PDF) E


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Imprinting Directionality into Proton Transfer Reactions of an Achiral

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