Photoisomerization of an Azobenzene on the Bi (111) Surface

Article pubs.acs.org/JPCC

Photoisomerization of an Azobenzene on the Bi(111) Surface Christopher Bronner,*,†,‡ Beate Priewisch,§ Karola Rück-Braun,§ and Petra Tegeder*,†,‡ †

Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany § Institut für Chemie, Technische Universität Berlin, Straβe des 17. Juni 115, 10623 Berlin, Germany ‡

S Supporting Information *

ABSTRACT: Modifying surface-bound molecular switches by adding side groups is an established concept for restoration of functionality which a molecule possesses in solution and which is often quenched upon adsorption. Instead of decoupling the photochromic unit from the substrate, we follow a different approach, namely treating the complete molecule− substrate system. We use photoelectron spectroscopies to determine the energetic positions of the frontier orbitals of di-m-cyanoazobenzene on Bi(111) and to elucidate the isomerization mechanism which is stimulated by a substrate-mediated electron transfer process.

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INTRODUCTION The ability of azobenzenes to perform a trans/cis isomerization which is accompanied by a large change in the geometrical structure is one reason why azobenzenes are key elements in several functionalized molecular systems.1,2 Furthermore, azobenzene adsorbed on various metal surfaces has become an important model system for the development of dispersion corrections in density functional theory.3,4 In solution, azobenzene can be isomerized from the stable trans to the metastable cis form with ultraviolet light while the back-reaction can be induced with visible light or thermally. In the case of the photoinduced reactions, the ground state barrier is overcome by electronic transitions and subsequent relaxation on the potential energy surface of the excited state.5 Adsorption of a molecular switch on a metal surface usually leads to a loss of this photoisomerization ability6−8 which makes it necessary to restore this functionality if one wants to build a surface-bound molecular device. In general, there are two approaches to pursue this goal: the first one is to minimize the interaction with the substrate as much as possible by geometrical or electronic decoupling of the (photochromic) molecule, e.g., in a self-assembled monolayer9−12 or by means of an isolating layer13−15 or bulky spacer groups.16−20 The second approach is to explicitly take the substrate and its influence on the molecular switch into account, which of course requires a detailed understanding of the mutual interaction when manipulating the system and its properties. Besides disadvantages such as steric effects or reduction of lifetimes of excited states, the presence of the substrate can also offer new excitation channels, e.g., via ionic resonances upon photoinduced charge transfer of electrons or holes to the adsorbate.21,22 In order to maintain its isomer© 2013 American Chemical Society

izaton ability in direct contact with a surface, the molecular switch must remain bistable in the adsorbed state23 and a sufficiently efficient7 reaction pathway must exist, leading from one form to the other. In this paper we follow the second approach, namely by modifying the azobenzene molecule and simultaneously carefully choosing an adequate substrate, the properties of which foster the photoisomerization of the adsorbate. For the free molecule, rotation around the central diazo (−NN−) bridge of azobenzene is a major pathway for isomerization;5 therefore, a lowering of the rotational barrier should make the switching more efficient. Adding cyano groups to both phenyl rings is thought to have such an effect on the rotational barrier in the anionic resonance of the resulting molecule di-mcyanoazobenzene (DMC, see Figure 2a).24 Furthermore, as will be shown below, the frontier orbitals of this modified molecule align with the band structure of the substrate in a way which allows for efficient optically induced electron transfer from the substrate to the adsorbate, thus creating such a negative ion resonance (NIR). As mentioned above, the choice of the substrate is an important aspect of the adsorbate−substrate system in its entirety. DMC has been investigated previously on the Au(111) surface where strong van der Waals interaction leads to a strong coupling of the phenyl rings to the substrate and where isomerization is not observed.25,26 On the other hand, on Cu(100), a trans/cis reaction can be induced in a tunneling junction which is unidirectional, i.e., irreversible due to the Received: October 29, 2013 Revised: December 3, 2013 Published: December 4, 2013 27031

dx.doi.org/10.1021/jp4106663 | J. Phys. Chem. C 2013, 117, 27031−27038

The Journal of Physical Chemistry C

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them to the first monolayer. The saturation of two features in TPD could also result from a bilayer structure, but as reported below, the image potential state (IPS) of the bare surface decreases in intensity upon adsorption of DMC and, simultaneously, a new IPS of the adsorbate-covered surface arises. If a bilayer was formed, the IPS of the bare surface should be quenched already at a nominal coverage of 0.5 ML (i.e., when still applying the ML scale which assumes no bilayer growth) because then the entire surface would be covered with DMC. We can thus be sure that α2 also originates from the first layer. It is actually not uncommon for TPDs of azobenzene derivatives desorbing from coinage metal surfaces to exhibit a second monolayer feature which is usually ascribed to a compressed phase.30,31 All molecules are desorbed upon heating to 410 K after recording the TPD, as the two-photon photoemission (2PPE) spectrum after the TPD is identical to the one before deposition of the molecules. The absolute coverage can be determined by referencing the TPD integrals to that of a full monolayer. To that end, one needs to select one TPD which is used to normalize the TPD areas of the others. However, due to the finite number of available TPD measurements, one causes a systematic error in the coverage determination since the TPD chosen has a coverage lower than the actual full monolayer (because we choose one in which no multilayer peak is observed). Therefore, coverages determined by normalizing to this TPD area are generally too large which explains why e.g. the IPS of the bare surface is still observable in the spectrum with a nominal coverage of 1.0 ML. The third feature α3 rises indefinitely in intensity, and the peak form indicates zero-order desorption which is typical for desorption from the multilayer. Femtosecond laser pulses of tunable wavelength in the visible and ultraviolet (UV) regime were used in two-color 2PPE (2C2PPE), and UV pulses were used in one-color 2PPE (1C2PPE) experiments. Frequency-quadrupled pulses of the 800 nm fundamental of the same laser system were used for the UPS measurements (see e.g. refs 32 and 33 for details on 2PPE and refs 22, 34, and 35 for a detailed description of the experimental setup). The illumination experiments were carried out in alternating periods of illumination and photoemission measurements, both with the same laser beam but different intensities, because of the low cross section of the isomerization. Depending on the photon energy hν, three different types of measurement have to be distinguished: When the photon energy was larger than the work function Φ of the sample, UPS spectra were obtained. When Φ/2 < hν < Φ, the spectra are dominated by the 2PPE signal, and when hν < Φ/2, the three-photon signal was the most intense one. Since the only common feature in all three types of spectra, which is specific to the system investigated, is the low-energy cutoff of the spectra, i.e. Φ, this quantity was used as benchmark for the quantification of the photoinduced changes, i.e., the relative number of molecules in the cis-form. In order to irradiate a larger portion of the sample surface, we employed a KrF excimer laser which has a wavelength of 248 nm (hν = 5.0 eV). The laser operates in a pulsed mode with a repetition rate of 57 Hz and a pulse energy of 1.7 ± 0.4 mJ cm−2. During exposure of the sample to a large spot (diameter 5.8 mm) of this KrF laser, the work function was characterized by 2PPE using the visible output of the laser system described above at a photon energy of 2.44 eV.

strongly covalent character of the adsorbate−substrate interaction.27,28 These two cases demonstrate the strong influence of the substrate and accordingly the binding character, i.e., van der Waals and/or covalent binding, which has to be carefully balanced in order to ensure both bistability and switching efficiency. A balance of both van der Waals and covalent interactions can for example be expected in case of the semimetal bismuth which has proven to be a suitable substrate for functional molecules.29 In this paper we employ ultraviolet photoemission (UPS) and two-photon photoemission (2PPE) to study the electronic structure and especially the frontier orbitals of DMC on the Bi(111) surface which are important to identify potential excitation mechanisms in photoisomerization reactions at surfaces. A photoinduced reaction which we assign to a trans/cis isomerization can be followed by 2PPE, and a quantitative analysis of the photoisomerization cross section as a function of photon energy allows to determine the underlying mechanism which turns out to be mediated by the bismuth substrate.

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EXPERIMENTAL METHODS A Bi(111) single crystal, mounted in ultrahigh vacuum (UHV) on a flow cryostat equipped with resistive heating, was used as a substrate, and a clean surface was prepared by sputtering and annealing at 410 K for 10 min. Di-m-cyanoazobenzene (DMC) molecules were evaporated at 400 K onto the cooled surface (120 K). The resulting coverage was determined by temperature-programmed desorption (TPD, see below) using a quadrupole mass spectrometer. By heating to a temperature at which the higher layers desorb but not the first one (300 K, 1 min), a single monolayer (ML) can be produced which was then annealed at 240 K for 30 min to allow for ordering of the molecular layer. In order to characterize the DMC film on the Bi(111) surface after deposition, TPD was performed, in which the substrate is heated with a heating rate of β = 1 K s−1, and the desorbing molecules are analyzed with a quadrupole mass spectrometer (QMS) at a fragment mass of 102 amu, which corresponds to a phenyl ring with an attached cyano group (C6H4CN+). The integral of a resulting TPD curve is proportional to the initial coverage. Figure 1 shows a series of TPD curves. Three features are observed, two of which (labeled as α1 and α2) saturate at higher coverages. We assign

Figure 1. TPD curves of DMC/Bi(111) for different initial coverages. 27032



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Figure 2. (a) Molecular structure of the di-m-cyanoazobenzene (DMC) molecule in both isomeric forms together with the respective calculated dipole moments24 and the isomerization reactions in solution. (b) UPS spectra for the bare Bi(111) surface and varying coverages of DMC. The four observed features are fitted as described in the text and the relative peak intensity changes of the fit components are shown in the inset. (c) 2C-2PPE spectra for the bare Bi(111) surface and for various DMC coverages. The spectrum at 0.8 ML exemplarily shows how the features are fitted. The spectra are displayed as a function of the final state energy EFinal − EF.

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the bare substrate and reaches a value of 4.05 ± 0.02 eV at a coverage of one monolayer. The 2PPE spectrum of the bare substrate shows three features, the most prominent of which, at a final state energy of 5.54 ± 0.04 eV, can be assigned to an unoccupied electronic state based on observation of the peak shift while varying the photon energy.32,33 Subtracting the photon energy hν1 from the final state energy, we obtain a binding energy of 3.59 ± 0.05 eV relative to EF. This state arises from the unoccupied image potential state (IPS) of the bare surface.34,36 When the DMC coverage is increased, this peak becomes less intense and a new peak gains intensity, which lies at a binding energy of 3.40 ± 0.05 eV (final state energy of 5.35 ± 0.04 eV). This adsorbateinduced state shows an effective mass of meff = 1.3 ± 0.3 in angle-resolved 2PPE, and the energetic difference to the IPS of Bi(111) is very similar to the work function difference between the bare and the DMC-covered surface. We therefore assign this state to the IPS of the adsorbate-covered surface. Note that neither of the two IPS shifts in energy as molecules are deposited on the surface but that the original peak loses intensity while the other gains intensity. Since image potential states are pinned to the vacuum level37 (which corresponds to the value of Φ) rather than to EF, this demonstrates that the work function does not decrease in the same fashion everywhere on the surface, but it indicates a submonolayer growth in islands of DMC where the local work function (to which the IPS is pinned) has a constant value independent of the overall coverage. The residual intensity of the IPS peak associated with the bare surface in the spectrum at 1ML DMC is due to a systematic overestimation of the coverage in the TPD measurements, as mentioned in the Experimental

RESULTS In order to investigate molecule-induced electronic states which are essential for the understanding of photoisomerization at surfaces, we utilized UPS and 2PPE to study occupied and unoccupied electronic states, respectively. The UPS spectra for various DMC coverages are shown in Figure 2b. In the case of the bare surface, three features A, B, and C are observed which must hence originate from the substrate and have been studied in detail before.34 The spectrum has been fitted with three Gaussian peak profiles on a linear background which was cut off by a Fermi function. With increasing coverage, an additional feature is observed at lower energies, and these spectra then have been fitted with an additional peak profile while keeping the position and width of the Bi features fixed and only varying their intensities. The inset in Figure 2b shows that the intensities of the substrate features decrease in the same fashion as DMC is deposited whereas the molecule-induced feature first rises in intensity and then decreases at higher coverages. We assign this feature to the highest occupied molecular orbital (HOMO), which in addition shows no dispersion in angle-resolved UPS experiments. Its energy as determined from the fits amounts to EHOMO = −0.98 ± 0.05 eV. To get insight also into the unoccupied electronic structure of the system, we used 2C-2PPE and again recorded spectra for the clean surface as well as for various DMC coverages (see Figure 2c). The spectra are displayed as a function of the final state energy EFinal of the photoelectron, referenced to the Fermi level EF. The binding energy can be obtained from this quantity by subtraction of the photon energy of one (unoccupied state) or both (occupied state) beams, respectively. With increasing coverage, the work function decreases from 4.23 ± 0.03 eV for 27033



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Methods section. At a DMC coverage of 1 ML, this feature should be quenched. In an energetic region closer to the secondary edge, the 2PPE spectrum of the clean surface shows three other features which originate from unoccupied p-bands.34 While the spectral intensity of bands generally represents the density of states at the corresponding energy, the observed features can been fitted with three Gaussian peak profiles on a linear background. With increasing coverage, they all lose intensity and, similar to the HOMO in the UPS spectra, a new feature arises which has a binding energy of 0.77 ± 0.05 eV relative to EF (final state energy of 4.67 ± 0.03 eV). Therefore, all spectra where DMC is adsorbed on the surface are fitted with an additional Gaussian peak, as is demonstrated exemplarily in Figure 2c for the spectrum with 0.8 ML coverage (details on the fitting procedure are described in the Supporting Information). We assign this feature to the lowest unoccupied molecular orbital (LUMO). In addition, it does not show a dispersion in angleresolved measurements (see Supporting Information). Upon illumination with light in the UV regime, the spectra of the DMC-covered surface change as a function of time, i.e., with increasing photon dose. Figure 3 exemplarily demonstrates this behavior for a photon energy of 3.90 eV. Note that the spectra discussed here are 1C-2PPE spectra, in contrast to the ones in Figure 2c. There are no significant photoinduced changes in the energetic region between 4.9 and 6.7 eV, and unlike in the two-color experiments shown above, the LUMO which can be observed in the 1C spectra in a region between 4.4 and 4.8 eV cannot clearly be separated from a feature of the Bi substrate, namely an unoccupied p-band. The photoinduced changes only occur at relatively high photon doses whereas the doses applied for recording the photoemission spectra discussed above are negligible. The photon dose varies for the different photon energies but only within an order of magnitude. As an example, in the experiment shown in Figure 3, a total dose of (1.7 ± 0.2) × 1024 cm−2 was applied in the entire illumination experiment which compares to only (2.0 ± 0.5) × 1021 cm−2 for a single spectrum. In Figure 3a, one can observe an increase of the work function with increasing photon dose until a saturation is reached at about 4.30 eV. Thus, the work function increases by ΔΦ = 250 meV. This is a rather large change compared to the case of tetra-tert-butylazobenzene (TBA) on Au(111). There, the work function change is 50 meV in a similar experiment.30 Assuming the photoinduced reaction to be an isomerization, the work function change might be due to a larger dipole moment of cis-DMC compared to cis-TBA24 or because of different cis/trans ratios in the photostationary states, or both. The fact that the work function increases would be consistent with the electrophilic cyano groups pointing away from the surface in the cis-form. There are furthermore photoinduced changes in the spectra in the energetic region close to the Fermi edge (see Figure 3b), i.e., in the vicinity of the IPS. With increasing light exposure, the intensity of the IPS decreases and the spectral weight shifts to higher energies, similar to the spectral changes in TBA on Au(111).22 Because of the noise level in this measurement, one cannot discriminate whether (i) the IPS loses intensity and a new peak simultaneously arises at higher energies or (ii) the IPS continuously shifts to higher energies while losing intensity. The second effect would be expected assuming that switching of a single molecule is independent of the state of the

Figure 3. (a) Changes of the secondary edge in a 1C-2PPE experiment as a function of photon dose (the spectra have been normalized to a region of the spectra where no light-induced changes are observed). The mean photon fluence in this experiment was f = 5.9 × 1020 cm−2 s−1, and the total applied dose (number of photons per unit area) was d = 1.7 × 1024 cm−2. In the region between 4.4 and 4.8 eV, a spectral feature originating partly from the LUMO and partly from substrate bands is observed. (b) In the same data set as in (a), at higher energies and close to the Fermi edge, changes in intensity are also observed in the region of the IPS (as indicated by arrows).

neighboring molecules, i.e., a random distribution of the cis molecules in the monolayer. Along with these pronounced photoinduced changes in the 2PPE spectra, a shift of the frontier orbitals (i.e., HOMO and LUMO) would be expected when the molecule is isomerized from trans to cis. This effect has been observed for instance in TBA/Au(111).38 In the present system we were not able to observe a shifting of photoemission features originating from the HOMO or LUMO since both peaks are rather broad. Addressing the question on the origin of the photoinduced reaction that is clearly observed in this experiment, one could discuss different processes: a rotation of a single phenyl ring of adsorbed trans-DMC as observed on the Au(111) surface25 can be excluded since one would not expect significant changes of the work function in this case. Another possibility would be laser-induced desorption, which however can also be ruled out 27034



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likely origin of the observed reaction is photoisomerization, i.e., a trans−cis reaction. Observation of a photoinduced back-reaction with visible light, again yielding an all-trans monolayer, could be expected based on the reversible photoisomerization of azobenzene in solution. We have indeed used light in the visible spectrum (between 1.9 and 2.5 eV) but could not observe such a backreaction. A number of effects can prevent a cis molecule from photoisomerization to the trans form. (i) For example, the visible light might also induce a trans−cis reaction, and thus, a photostationary state would eventually be reached which might be characterized by a similar trans/cis ratio. (ii) The alignment of the cis-LUMO with the Bi bands might not allow for a charge transfer reaction (see below), and a direct intramolecular excitation may be inefficient due to fast quenching of excited states by energy transfer to the substrate. As discussed above, the work function is the most suitable benchmark for quantification of the photoisomerization. Figure 5a shows the work function extracted from the data set discussed above at a photon energy of 3.90 eV as a function of the applied photon dose d. The data are nicely described by a monoexponential function, Φ(d) = Φ(0) + ΔΦ exp(−σd). Here, σ denotes the effective cross section which in this experiment is σ = (2.1 ± 0.2) × 10−24 cm2. For comparison, the cross section for TBA/Au(111) is on the order of 10−22 cm2 21 or 10−23 cm2.39 For a different azobenzene derivative adsorbed on a Cu(111) surface, a UV-induced intramolecular ππ* excitation is reported to trigger a trans/cis isomerization with an effective cross section of 6 × 10−20 cm2.40 Photoisomerization in the present experiment therefore seems to be a rather inefficient process. In order to elucidate the excitation mechanism for the photoisomerization, we investigated the changes in the photoemission spectra as a function of the excitation energy using our tunable laser source and repeating the above experiment for different photon energies. In the UV regime, we always observed a reaction with similar spectral properties, particularly an increase of the work function. Upon illumination with visible light, on the other hand, no changes were observed in the spectra. Figure 5b shows the resulting photon energy dependence of the cross section as well as upper limits for the cross section in the visible regime which we can infer from the photon doses that have been applied in these cases. For comparison, the here determined HOMO−LUMO gap and the maximum of the absorption band for the ππ* transition (which triggers isomerization in solution) and the nπ* transition of DMC as determined in solution are shown. We observe a (stronger than) exponential dependence of the cross section on the photon energy over the entire UV range accessible in this experiment. We want to stress that this contradicts the observed process being thermally induced by the laser beam because such a heating effect should exhibit a linear dependence on the photon energy. Furthermore, neither at the HOMO− LUMO gap nor at the energies of the two direct intramolecular transitions do we observe a resonance from which we conclude that the mechanism leading to photoisomerization of DMC on the surface is different than in solution. Besides intramolecular excitations (as in the free molecule), another class of excitation processes, namely substratemediated ones, could drive the observed reaction. A direct photoinduced electronic excitation from the HOMO of DMC to the unoccupied p-bands of Bi on the one hand or an excitation from the occupied p-bands to the LUMO on the

because the spectra are not evolving toward the spectrum of the bare Bi surface. Besides the aforementioned possibilities, the photoinduced changes in the spectra could result from fragmentation of DMC on the surface. In order to exclude such beam damage effects, we can compare TPD curves recorded after a standardized preparation procedure which yields a DMC coverage of 1.0 ML (as described above) and subsequent illumination experiments: in most illumination experiments, the diameter of the irradiated area (which usually is on the order of 50−100 μm) is negligible compared to the total surface area of the sample (10 mm diameter), and consequently the TPD which stems from molecules desorbing from the entire surface is not influenced by the illumination. Figure 4 shows seven such TPD traces

Figure 4. Influence of sample irradiation on posterior TPD experiments. The main graph shows eight different TPD curves recorded under identical conditions except that in case of the red TPD the sample had been irradiated prior to the desorption experiment. The inset shows saturation of the work function with increasing photon dose. Integration of the TPD curves yields the DMC coverage (shown on the right) which is not significantly reduced upon illumination.

(shown in gray) which do not differ significantly in shape or area, i.e. DMC coverage, as shown in the right panel of Figure 4. In contrast, the red trace in Figure 4 shows a TPD curve recorded after illumination of a relatively large area of the surface (beam diameter was 5.8 mm corresponding to 33% of the sample surface). This irradiation experiment was performed with a KrF excimer laser (for details, see Experimental Methods) and continued until saturation was reached as demonstrated in the inset of Figure 4. As can be seen, this illumination of a substantial part of the sample area did not lead to any changes in the TPD curve: the total area is still within the fluctuation of coverages determined in all TPD measurements, and we would particularly like to emphasize that the shape of the TPD curve is unchanged. The fact that the TPD spectrum is unaffected by the photoinduced process demonstrates that if a dissociation reaction occurs, then the fragments are not smaller than the detected one which is the cyanosubstituted phenyl ring (C6H4CN+) and the unchanged peak shape shows that the binding energy of the desorbing species is not altered. The latter is a clear counterindication for the occurrence of a photoinduced dissociation since the binding energy of the possible fragments should be different compared to that of the entire molecule, and accordingly the shape of the TPD curve should differ. We can thus conclude that the most 27035



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Figure 6. Overview over the electronic structure of the DMC/Bi(111) system and a schematic illustration of the proposed photoexcitation mechanism via the creation of a negative ion resonance.

DMC, creating a negative ion resonance which stimulates the isomerization, possibly assisted by an activation barrier in the anionic molecule which is lowered compared to the neutral one due to the cyano substituents at the phenyl rings.24 The overlap of the LUMO and the Bi p-bands is similar to that of the HOMO of TBA with the d-bands of Au. It has been shown that in the latter system this overlap plays an important role for photoisomerization efficiency:22 on the one hand, the overlap needs to be sufficient to allow for efficient charge transfer, but on the other hand, electron (hole) tunneling into the LUMO (HOMO) is most efficient when the orbital is aligned with the low (high) energy edge of the bands where the electrons (holes) accumulate upon carrier−carrier scattering. Because of the HOMO lying well within the occupied p-band range of Bi, we propose the isomerization to be induced via electron transfer to the LUMO. This mechanism would be in analogy to the one responsible for the photoisomerization of TBA/ Au(111)21,22 where light creates holes in the Au d-bands which relax to the top of the d-band, followed by tunneling into the HOMO of TBA, creating a positive ion resonance.

Figure 5. (a) Work function change extracted from the data set shown in Figure 3 as a function of the applied photon dose. The data are fitted with a monoexponential function yielding the effective cross section σ. (b) Cross sections (open circles) and upper cross section limits (gray bars) for different photon energies. The nπ* and ππ* transitions as observed in solution are shown as well as the HOMO− LUMO gap of the adsorbed molecule (dashed lines). The dotted line indicating the cross-section behavior is to guide the eye.

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other hand could lead to an isomerization via an ionic resonance of the adsorbate. However, considering the width of both the occupied and the unoccupied p-bands, one can exclude these processes due to the wide range of photon energies leading to the reaction. The energetic difference between the HOMO and the upper unoccupied p-band edge is around 4.4 eV; the corresponding value for a negative ion resonance would be 3.8 eV (see Figure 6). We can therefore rule out these two processes based on the observed increase well beyond these photon energies. Instead, we propose that the wide range of photon energies triggering the reaction is indicative of a different substratemediated process in which electrons are excited from the occupied to the unoccupied p-bands in the substrate and subsequently relax as hot electrons in the unoccupied p-bands (see Figure 6). The strong, monotonous increase of the cross section with photon energy is typical for photoinduced reactions of adsorbates induced by hot electron transfer.41,42 In our case, these hot electrons can tunnel into the LUMO of

CONCLUSION In conclusion, we utilized photoemission spectroscopies to determine the binding energies of the frontier orbitals of DMC adsorbed on Bi(111) and found a partial overlap of the LUMO with the unoccupied p-bands. Upon illumination of DMC in the first monolayer, we observed a photoinduced trans/cis reaction which is driven by an electronic excitation in the bismuth substrate while the direct intramolecular excitation channel which is responsible for isomerization in solution is quenched on the surface. The fact that this mechanism critically relies on the photochromic properties of the substrate, the alignment of the molecular orbitals with respect to the substrate bands, and the reduction of the rotational barrier in the ionic molecule by adding cyano groups all contribute to the switching ability of this system demonstrates that understanding of the complete adsorbate−substrate system is crucial for the conservation of functionality. 27036



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ASSOCIATED CONTENT

S Supporting Information *

All fit components for the 2C-2PPE spectra are presented separately for every spectrum; furthermore, coverage-dependent 1C-2PPE spectra are shown which also contain a nondispersing feature originating from the LUMO. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.B.). *E-mail: [email protected] (P.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Judith Specht, who assisted with the illumination experiments in the framework of her B.Sc. thesis. This project was funded by the German Research Foundation (DFG) through collaborative research center SFB658.

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REFERENCES

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Photoisomerization of an Azobenzene on the Bi (111) Surface

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