Adsorption of a Switchable Industrial Dye on Au (111) and Ag (111)

Article pubs.acs.org/JPCC

Adsorption of a Switchable Industrial Dye on Au(111) and Ag(111) K. Boom,† M. Müller,† F. Stein,† St. Ernst,‡ and K. Morgenstern*,§ †

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 2, 2015 | http://pubs.acs.org Publication Date (Web): July 24, 2015 | doi: 10.1021/acs.jpcc.5b04883

Abteilung für atomare und molekulare Strukturen (ATMOS), Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, D-30167 Hannover, Germany ‡ FEW Chemicals GmbH, Ortsteil Wolfen, Technikumstr. 1, D-06766 Bitterfeld, Germany § Lehrstuhl für physikalische Chemie I, Ruhr-Universität Bochum, D-44780 Bochum, Germany ABSTRACT: We investigate astraphloxine, an industrial dye, on two metal surfaces, Au(111) and Ag(111). Low-temperature scanning tunneling microscopy with submolecular resolution in comparison to semiempirical calculations reveal that only two of the nine possible conformers of this molecule are adsorbed. The two conformers adsorb via one of their indol groups, which serves as a platform that decouples the rest of the molecule from the surfaces. A change from one to the other conformer is demonstrated by injecting inelastic electrons from the tunneling tip selectively into individual molecules.

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without influencing the properties of its functional unit.23,24 Photoisomerisation is possible because of the decoupling of the photoactive unit from the surface. In general, the platform needs to be chosen in dependence of the surface used. For the successful photoisomerisation on Si(100), a phenyl group was used. Hydrocarbon rings are also attractive candidates for the platform approach on metal surfaces, because they have the tendency to adsorb parallel to the surface in order to maximize overlap of their π-system with metallic states. Cyanine dyes consist of two hydrocarbon end groups. The nitrogen atoms within the end groups are connected via a methin chain.25 The cyanine dyes absorb light in the visible range via the delocalized π-electron system due to their mesomerie. The wavelength corresponds to blue for the cyanines first mentioned in literature,26 giving this class its name. In general, it depends on their chain length. Today, cyanine dyes are used on an industrial level, for example, on DVD master disks. Here, we investigate the cyanine dye 2-[3-(1,3-dihydro-1,3,3trimethyl-2H-indol-2-ylidene)-propenyl]-1,3,3-trimethyl-3H-indoliumchlorid, commonly named astrophloxin. The thermodynamically stable cis-isomer is adsorbed on the surfaces of Au(111) and Ag(111) and investigated by low-temperature scanning tunneling microscopy. High-resolution imaging in connection with semiempirical gas phase calculations confirm a platform-like adsorption geometry of the molecule. The isomerisation from one to the other conformer is possible by inelastic electron tunneling (IET) manipulation.

INTRODUCTION Molecular electronics aims at performing logical operations based on molecular switches.1,2 Thereby, this branch of nanotechnology aims at using single molecules or small agglomerations of them as electronic compounds. Molecules that undergo a reversible isomerization reaction are an attractive class of molecules for this purpose.3−7 Photochromic molecules are preferred candidates within this class, because light can reversibly switch them between different states.8−10 In particular, azobenzene derivatives are studied extensively because of their robust photoisomerization in solution8−10 and on top of self-assembled monolayers.11,12 Azobenzene derivatives were also investigated on metal surfaces6,7 and in some cases with respect to photoisomerisation.13−18 The contact to a surface alters the photoisomerization properties substantially because of, for example, bond formation, steric hindrance, or electronic quenching. A rare exception is the weakly interacting Au(111) surface on which the isomerization of azobenzene derivatives was successful by electrons, electric fields, and light.19 In contrast, scanning tunneling microscopy (STM)-induced isomerization of an azobenzene derivative adsorbed on Cu(100) is irreversible,20 and on Cu(111) and Au(100) it is completely suppressed.5 A possibility to preserve the switching ability of adsorbed molecules is to decouple their functional unit from the surfaces. Different strategies were followed to meet this challenge, for example, functionalizing the molecule by spacer groups,13−15 using an insulating spacer layer,21 or trapping the molecule in its physisorption well.18 Furthermore, the so-called platform approach was successful on the even more reactive but technologically important Si(100) surface.22 In the platform approach, the molecule is anchored via a photoinactive group to the surface in a defined way © 2015 American Chemical Society

Received: May 21, 2015 Revised: July 13, 2015 Published: July 15, 2015 17718

DOI: 10.1021/acs.jpcc.5b04883 J. Phys. Chem. C 2015, 119, 17718−17724

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The Journal of Physical Chemistry C


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EXPERIMENTAL METHODS STM measurements are performed with a low-temperature STM under ultrahigh vacuum (UHV) conditions (base pressure below 8 × 10−10 mbar; during sample preparation the pressure is reduced below 4 × 10−10 mbar by a cold trap). Both surfaces are cleaned by repeated cycles of Ne+-sputtering (3 × 10−5 mbar, 1−2 μA, 1.3 keV) and annealing. The Ag(111) surface is annealed at (900 ± 25) K. For final preparation, the sputtering and annealing times in the last three cycles are 45, 30, and 15 min each. For preparation of the Au(111) surface, we anneal the surface at (885 ± 10) K. On Au(111), the times are 30 min for annealing and sputtering during the first cycle and 30 min sputtering and 15 min annealing for the second cycle. Finally, the surface is flashed to 945 K. Astraphloxin is a crystalline salt with a melting temperature of (553 ± 5) K and a phase transition at (480 ± 5) K. A purity of 99.9% is determined by high-performance liquid chromatography. The pink powder is filled into a glass tube, which is connected via a leak valve to the UHV chamber. An aluminum foil protects the molecule from light. The glass tube is annealed by means of a heating band wrapped around it. For cleaning, the vapor above the molecules is pumped for some days, while keeping the tube at 433 K, that is, well below the transition temperature of astraphloxin. The cleanliness of the deposit is monitored during cleaning by quadrupole mass spectrometry of the vapor above the salt. The largest impurity is water. Consequently, we aim for a reduction of the water peak at mass 18 as compared to the molecule fragment at mass 50 during the cleaning procedure. Note that the molecules are dissociated at the hot filament of the mass spectrometer. Ex-situ NMR spectroscopy of the remaining powder confirms that the molecules in the powder do not degenerate during this cleaning procedure or the deposition procedure described below. The sample temperature during deposition is chosen to be just below the desorption temperature of the dye molecules to allow equilibration on the surface. This sample temperature is determined by thermal desorption spectrometry (TDS). For this aim, around 10 ML of the molecule are deposited on the surface held at ∼20 K. Molecules leaving the surface during heating are monitored by a quadrupole mass spectrometer. The heating rate is 1 K/s. The TDS shows a multilayer and a monolayer peak, well separated by approximately 10 K. On the basis of the monolayer TDS peaks, the deposition temperature is set to (83 ± 1) K for deposition on Ag(111) and to (113 ± 1) K for deposition on Au(111). For deposition, the glass tube temperature is adjusted to achieve a vapor pressure of 2.5−3.0 × 10−7 mbar. Note that this pressure is measured close to the powder. It is larger, by 3−4 orders of magnitude, than the one in front of the sample. Final coverages are thus much more precisely deduced from STM images. On Ag(111), (1.4 ± 0.2) × 10−2 molecules/nm2 are deposited at a deposition rate of (4.2 ± 0.1) × 10−4 ML/s. On Au(111), (2.2 ± 0.2) × 10−2 molecules/nm2 are deposited at a deposition rate of (7.2 ± 0.2) × 10−4 ML/s. The substrate is quenched to ∼20 K immediately after deposition and then transferred into the STM. STM images are recorded at 5 K in constant current mode. IET manipulation27 is performed by placing the tip above the molecule, deactivating the feedback loop, and increasing tunneling current and/or voltage for some seconds. For changes that occur on a seconds to fraction of seconds time scale, the desired voltage is established and the parameters are

kept at this voltage for some seconds. For changes in the ms range, the voltage is swept over the desired voltage range at a constant V/s slope. The current is recorded during the manipulation in both cases. Sudden changes of the current indicate a successful manipulation. A control image is recorded after reactivating the feedback loop by scanning the same spot of the surface with the same parameters as before the manipulation. Details are given in ref 28.

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RESULTS AND DISCUSSION Astraphloxin (C25H29ClN2, atomic mass: 392.96416) consists of two indol groups that are connected via a methin chain of three carbon atoms (Figure 1a, inset). It absorbs light in the wavelength range between 470 to 570 nm (Figure 1a). The absorption maximum is at 545 nm. Astraphloxin is mesomeric

Figure 1. Astraphloxin. (a) Absorption spectrum, inset: molecule model. (b) Mesomeric forms. (c,d) Ball-and-stick model of all-trans isomer in ll-conformation: C in black, H in white, N in blue, (c) top view; dotted circles indicate groups that are expected to dominate the STM image, if the molecule was adsorbed parallel to the surface (d) side view; models are calculated semiempirically in gas phase using the parametric method 3 parametrization of MNDO for the Hamiltonian as implemented in Arguslab.29 17719


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STM image. In the ll-conformation, this resulted in a pair of protrusions at ∼0.4 nm distance (Figure 1c). Because of steric repulsion between the dimethyl groups, all conformations of the all-trans-isomer are energetically unfavorable with respect to the 10-cis-isomer. The stereoisomers of the 10-cis-isomer are of comparable energy but differ considerably in geometric shape (Figure 2). For each stereoisomer, one of the indol groups is expected to adsorb parallel to the surface, serving as the intended platform (Figure 2a,c,e). The images of the 10-cis-isomer will be dominated by the part of the molecule that is not parallel to the surface as marked in Figure 2b,d,f. On the basis of this reasoning, we expect the three stereoisomers to be imaged as single protrusions of different lateral size and with different positions of the most protruding part. In order to identify the adsorbed isomer, we now analyze the protrusions that are imaged after adsorption on both surfaces (Figure 3a,c). Note that the intrinsic surface steps are also


(Figure 1b) because its positive charge is delocalized between the two nitrogen atoms. In the salt used here, the charge is compensated by a chlorine ion. The ionic bond to the chlorine ion is not uniquely defined because of the delocalized charge on the molecule. We start by calculating the geometry of the isomers of astraphloxin in the gas phase in order to set the stage for the interpretation of the STM images. There exist three configurational isomers: all-trans, 10-cis, and 11-cis. There are three stereoisomers for each of these isomers in dependence of the relative orientations of the indol groups. We name them “ll”, “dl”, or “dd” in the following for the two dimethyl groups on the same (ll or dd) or opposite sides (dl or ld) of the methin chain, respectively (Figure 2). The conformation ll is shown for

Figure 2. Stereoisomers of 10-cis-astrophloxin calculated semiempirically in gas phase;29 for naming on left-hand side see text, C in black, H in white, N in blue: (a,c,e) side view, if adsorbed with one indol group parallel to the surface; numbers are distances between π-system parallel to the surface and highest point of molecule above the surface (b,d,f) top view; dotted circles mark positions of expected protrusions in STM images.

Figure 3. Astraphloxin on (a,b) Ag(111) and (c−e) Au(111.: (a) Overview image; types 1 and 1* of single protrusions are marked in yellow and red, respectively, 100 mV, 10 pA. (b) Histogram of cluster size, N0 = 214; 1* denotes fuzzy molecules; n.c. are noncountable clusters. (c) Overview image, −88.5 mV, 6.4 pA. (d) Detail image, −199 mV, 10 pA. (e) Histogram of cluster size, N0 = 1918.

the all-trans isomer in Figure 1c. We calculated all nine isomers but some are not discussed because they are energetically very unfavorable. These are the 11-cis-isomer as compared to the 10-cis-isomer and the dl- and dd-conformers of the all-transisomer as compared to its ll-conformer. Steric repulsion of the methyl groups disfavors the dl- and dd-conformers. The all-trans-isomer of astraphloxin is mainly planar apart from the dimethyl groups that point out-of-plane (Figure 1c,d). The aromatic system, and thus both indol groups, are expected to adsorb parallel to the surface. Such a parallel adsorption geometry is unfavorable for preserving a switching ability.22 Thereby, a distortion of the dimethyl groups could bring the aromatic system even closer to the surface for enhanced surface-molecule interaction. For instance, tert-butyl-azobenzene distorted its bulky spacer groups to bring the aromatic system closer to the surface.30 Regardless of the distortion, the dimethyl groups will be the highest points of the molecule above the surface. They are thus expected to dominate the

decorated by a line of protrusions. As these show a much smaller apparent height than the protrusions on the terraces, they are not included in the following statistics of molecular clusters. The line of protrusion might be chlorine atoms. On both surfaces, single protrusions dominate (Figure 3b,e). Some of the isolated protrusions have a fuzzy appearance as marked in red in Figure 3a, while others are stable under the tip (marked in yellow). We name them in the following type 1* and type 1, respectively. In all clusters the molecules are stable under the tip. On Ag(111) the dominant species is, at (45.3 ± 6.7) %, of type 1*. Molecules of type 1 at (12.1 ± 3.5) % are much less frequent, even less than double protrusions (dimers) at (15.4 ± 3.9) %. Trimers, tetramers, and hexamers are minority species 17720


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at (9.8 ± 3.1) %, (2.3 ± 1.5) %, and (2.8 ± 1.7) %, respectively. The number of pentamers is even smaller. On Au(111), single protrusions of type 1 dominate, at (75.8 ± 8.7) %. Type 1* protrusions at (1.6 ± 1.3) % are a minority species. Dimers and trimers at (14.8 ± 3.8) % and (6.8 ± 2.6) %, respectively, are more frequent. Only very few clusters are larger. On Au(111), single dots are mainly adsorbed on soliton lines, while double protrusions occupy predominantly elbow sites. The reduced tendency to form clusters is thus attributed to the elbows and soliton lines of the Au(111) reconstruction that are efficient heterogeneous nucleation centers. The large portion of single protrusions and clusters with an odd number of protrusions suggests that each protrusion corresponds to a single molecule. Nonetheless, we discuss the possibility that double protrusions correspond to all-transisomers. For this aim, we measure the distance between the two protrusions of a dimer (Figure 4a). The distance between

dimers is broadly distributed (Figure 4a), inconsistent with the defined distances of groups within a molecule. Distances up to 1.6 nm are observed, much larger than the total length of the all-trans-isomer. The major distance, at 0.9 nm, corresponds approximately to three atomic distances of the substrate. It suggests that a dimer consists of two molecules adsorbed in specific adsorption sites. However, the large spread shows that the adsorption site preference is not very strong. Our interpretation of two molecules forming a dimer is corroborated by the fact that dimers are easily separated into two monomers at manipulation voltages as low as 1 V, far below the dissociation energy of covalent bonds and at rather short pulse duration of 32 ms only. An example is shown in Figure 4b,c. The distance between the two protrusions is around 0.9 nm before the manipulation. The distance is increased to approximately 1.7 nm by injection of electrons at an energy of 1.5 eV in the upper protrusion. The new distance is beyond any distance observed after adsorption and any distance within the molecule, even in the all-trans-isomer (cf. Figure 1c). After having established that each protrusion corresponds to one molecule, we tackle the question whether or not the molecule is adsorbed in the desired platform approach. For this aim, we analyze apparent heights. Though STM largely underestimates real heights of adsorbed molecules, typical apparent heights of certain groups are well established. The typical height of methyl groups in STM are, for example, 150 pm in CH3SSCH3 on Au(111) at 30 to 50 mV and 50 to 100 pA.31 The protrusions here protrude by almost the double the amount on the same surface (Figure 5b). As apparent heights depend on voltage, we determine the apparent height in dependence of bias voltage between −3 V and +4 V (Figure 5c). The apparent height on Au(111) varies between 270 and 410 pm in this voltage range. The increase in the positive voltage range is thereby attributed to tunneling into the LUMO of the molecule. The largest apparent height of 410

Figure 4. Analysis of dimers. (a) Histogram of distance between protrusions of dimers; astraphloxin on Ag(111); insets: STM images for distance of 0.88 and 1.38 nm (both 100 mV, 10 pA). (b,c) Manipulation series showing dimer separation on Au(111) (−200 mV, 24 pA): (b) before manipulation with 1.5 V for 32 ms at cross, set point 0.1 nA (c) after manipulation.

Figure 5. Apparent height. (a) STM image of astraphloxin on Au(111), −199 mV, 10 pA. (b) Line scans as indicated in (a). (c) Apparent height h in dependence of bias voltage V on Au(111); different symbols for different molecules (d) histogram of apparent heights with respect to the surface value for (d1) Au(111) and (d2) Ag(111); dashed vertical lines mark minimum between the two apparent height maxima. 17721


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pm at (2.9 ± 0.2) V is still a lower limit of the real height of the most protruding atoms above the surface because of the low conductivity of molecules. Note that the absolute height with respect to the face-centered cubic (fcc) surface varies in dependence of adsorption site on Au(111), being for example, larger on soliton lines than on fcc domains. The assignment of the protrusions to cis-isomers based on their apparent height is corroborated by the manipulation experiment presented in Figure 6. Electrons are injected into

protrusion has changed to a double protrusion of a lower apparent height of 90 pm, only. The distance between the two protrusions is at 0.5 nm in the range expected for a trans-isomer (cf. Figure 1c). We conclude that nonplanar cis-isomers are adsorbed. The large scale image around the manipulated trimer region shows that the high manipulation voltage leads to indirect, nonlocal manipulation (Figure 6c,d). In particular, single molecules as far away as approximately 15 nm are induced to diffusion. Also, the third molecule of the trimer is moved to the upper right in the image, several nanometers away from its original position. Note that two distinct heights exist on Au(111) at approximately 250 and 300 pm at −199 mV (Figure 5b). A statistic for both surfaces is shown in Figure 5d. The absolute heights differ for molecules on the two surfaces, but two distinct heights are observed on both. This suggests two different conformers of the molecule. Interestingly, the larger apparent height dominates on Au(111), while the smaller apparent height dominates on Ag(111). Next, we assign the conformations to the protrusions of different apparent heights. This is possible using a so-called modified tip for imaging.32,33 Such a tip enhances the resolution due to molecules transferred to the tip. Some higher-resolved images are shown in Figure 7, top row. These reveal two qualitatively similar shapes of two different lateral sizes, consistent with the two different apparent heights observed above (Figure 5d). Both shapes consist of different height levels. The top level is thereby off-center of the bottom level. We superimposed the models of the semiempirically calculated molecules in the orientations shown in Figure 2 to the highresolution images. The lateral size of the ll-conformer is not consistent with the images. We propose that the larger protrusion corresponds to the dd-conformer and the smaller one to the dl-conformer, based on a better correspondence of the models to the protrusions (Figure 7, bottom row). Regardless of this assignments, two conformers of the cisisomer exist after adsorption on the metal surface. Both adsorb

Figure 6. Cis−trans isomerization. (a) Before manipulation with 2.5 V for 2.92 s into molecule marked by arrow, current set point 20 nA, tunneling parameters: 18.6 mV, 78 pA. (b) After manipulation. (c,d) Large scale images around manipulated trimer: (c) before manipulation, 177 mV, 5 pA and (d) after manipulation, 97.6 mV, 37 pA.

the LUMO of one of the three molecules of a trimer (marked by an arrow). After around 0.16 s, a sudden drop in tunneling current indicates a change of the molecule. Indeed, the single

Figure 7. High-resolution images of dimers of astraphloxin on Au(111) with modified tip in false colors (top) and superimposed with model from Figure 2 (bottom): (a) −199 mV, 13 pA, (b) −53.9 mV, 50 pA,and (c) −200 mV, 10 pA. 17722


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Figure 8. IET manipulation between two conformations on Au(111) by ramping the voltage to 3 V in 18 ms; 320 mV, 5.3 pA. (a) Before manipulation above molecules marked by arrows in (a). (b) After manipulation. (c) Line scans across dimers along lines shown in panels (a) and (b) in the same color. (3) Choi, B.-Y.; Kahng, S.-J.; Kim, S.; Kim, H.; Song, Y.; Ihm, J.; Kuk, Y. Conformational Molecular Switch of the Azobenzene Molecule: A Scanning Tunneling Microscopy Study. Phys. Rev. Lett. 2006, 96, 156106−156110. (4) Henzl, J.; Mehlhorn, M.; Gawronski, H.; Rieder, K.-H.; Morgenstern, K. Reversible Cis-Trans Isomerization of a Single Azobenzene Molecule. Angew. Chem., Int. Ed. 2006, 45, 603−6. (5) Alemani, M.; Peters, M.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L. Electric Field-Induced Isomerization of Azobenzene by STM. J. Am. Chem. Soc. 2006, 128, 14446−14447. (6) Wolf, M.; Oppen, F. Elementary processes in molecular switches at surfaces. Appl. Phys. A 2008, 241−364. (7) Morgenstern, K. Switching Individual Molecules by Light and Electrons: From Isomerisation to Chirality Flip. Prog. Surf. Sci. 2011, 86, 115−161. (8) Tamai, N.; Miyasaka, H. Ultrafast Dynamics of Photochromic Systems. Chem. Rev. 2000, 100, 1875−1890. (9) Behrens, P.; Glaue, A. M.; Oellrich, O. Host-Guest Systems Based on Nanoporous Crystals; Laeri, F., Schüth, F., Simon, U., Wark, M., Eds.; Wiley-VCH: Weinheim, 2003; pp 121−144. (10) Chang, C.-W.; Lu, Y.-C.; Wang, T.-T.; Diau, E. W.-G. J. Am. Chem. Soc. 2004, 126, 10109−10118. (11) Ichimura, K.; Oh, S. K.; Nakagawa, M. Light-Driven Motion of Liquids on a Photoresponsive Surface. Science 2000, 288, 1624−1626. (12) Kumar, A. S.; Ye, T.; Takami, T.; Yu, B.-C.; Flatt, A. K.; Tour, J. M.; Weiss, P. S. Reversible Photo-Switching of Single Azobenzene Molecules in Controlled Nanoscale Environments. Nano Lett. 2008, 8, 1644−1648. (13) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Frechet, J. M. J.; Trauner, D.; et al. Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface. Phys. Rev. Lett. 2007, 99, 038301− 038305. (14) Hagen, S.; Kate, P.; Leyssner, F.; Nandi, D.; Wolf, M.; Tegeder, P. Excitation Mechanism in the Photoisomerization of a SurfaceBound Azobenzene Derivative: Role of the Metallic Substrate. J. Chem. Phys. 2008, 129, 164102−164110. (15) Comstock, M. J.; Levy, N.; Cho, J.; Berbil-Bautista, L.; Crommie, M. F.; Poulsen, D. A.; Frechet, J. M. J. Measuring Reversible Photomechanical Switching Rates for a Molecule at a Surface. Appl. Phys. Lett. 2008, 92, 123107. (16) Levy, N.; Comstock, M.; Cho, J.; Berbil-Bautista, L.; Kirakosian, A.; Lauterwasser, F.; Poulsen, D.; Fréchet, J.; Crommie, M. SelfPatterned Molecular Photoswitching in Nanoscale Surface Assemblies. Nano Lett. 2009, 9, 935−939. (17) Henzl, J.; Puschnig, P.; Ambrosch-Draxl, C.; Schaate, A.; Ufer, B.; Behrens, P.; Morgenstern, K. Photoisomerization for a Molecular Switch in Contact with a Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 035410. (18) Bazarnik, M.; Henzl, J.; Czajka, R.; Morgenstern, K. Light Driven Reactions of Single Physisorbed Azobenzenes. Chem. Commun. 2011, 47, 7764−7766.

in the desired platform approach with one indol group parallel to the metal surface and the other group close to perpendicular. In images of lower resolution, the two isomers differ in their apparent height (cf. Figure 5d). The more stable dl-conformer dominates on Au(111) and the dd-conformer on Ag(111). As the ratio of conformers should be identical in the gas phase before adsorption, their conformation is changed differently in dependence of the surface, on which they adsorb. We finally show that we are able to selectively change molecules between the two conformations. In Figure 8, all molecules are initially in dd conformation. After IET manipulation of two molecules of two different dimers, these are selectively changed to dl conformation. External triggering of the conformational change is thus possible. This experiment allows us to propose that the fuzzy appearance of some of the monomers reflects a rapid change between the two conformers. The clustering stabilizes the conformers and the conformational change demands an external trigger, here energetic electrons.

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CONCLUSIONS In conclusion, we analyzed the adsorption of an industrial dye, astraphloxin, on two coinage metals by STM. One part of the molecule adsorbs parallel to the surface and thus acts as a platform. Above this platform, the molecule adopts two conformers of the 10-cis-isomer. A change from one to the other conformer is possible. Our study opens up a new class of molecules that might be used as possible switches on surfaces that usually quench isomerization.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the German Science Foundation (DFG) through contract MO 960/4-1 and by the Cluster of Excellence RESOLV (EXC 1089).

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REFERENCES

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Adsorption of a Switchable Industrial Dye on Au (111) and Ag (111)

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