Atom-by-Atom Dehalogenation of a Porphyrin Molecule Adsorbed on

Article pubs.acs.org/JPCC

Atom-by-Atom Dehalogenation of a Porphyrin Molecule Adsorbed on Ag(111) T. Kreuch, S. Meierott, N. Néel, W. J. D. Beenken, and J. Kröger* Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany ABSTRACT: Porphyrin molecules are ubiquitous and play an important role in chemical and biological processes. Their wide panoply of functions may be controlled by modifying their peripheral substituents. Electron injection from the tip of a scanning tunneling microscope into single 5,10,15,20-tetrakis(4-bromophenyl)-porphyrin-cobalt molecules adsorbed to Ag(111) was used to controllably remove Br atoms from the molecule periphery. Spectroscopy of the differential conductance, together with density functional calculations, assign spectral features to the spectroscopic signatures of molecular frontier orbitals. Different molecular orbitals are visible in the spectra of intact and dehalogenated molecules. These findings combine single-molecule chemistry with a characterization of each product’s electronic structure.

■

INTRODUCTION Porphyrins are organic tetrapyrrolic macrocycles with extended π conjugation. They readily form complexes with metals. Prominent naturally occurring porphyrin-based metal complexes are hemoglobin, myoglobin (Fe complexes), and chlorophyll (Mg complex). Moreover, metalloporphyrins exhibit a considerable variety of functionalities, e.g., they serve as electron donors and acceptors in gas sensors and organic semiconductor devices, as well as optical absorbers in dyes and light-harvesting chromophores.1 In part, this variety is due to different coordinations of the central metal atom. On surfaces modifications of the porphyrin−substrate interaction via the central metal atom have been induced by chemical stimuli.2−4 More recently, the attachment of NO to cobalt(II)− tetraphenyl-porphyrin (TPP−Co) on Ni(100) has been demonstrated to change the spin state of the molecule.5 Likewise, the specific modification of the peripheral substituents of a porphyrin molecule may tailor its properties and functions. Indeed, ligand engineering at the single-porphyrin level has been demonstrated to affect, e.g., the molecular magnetism.6−8 Therefore, atomically precise control over ligand reactions in a porphyrin molecule is highly desirable. The use of locally injected electrons and holes from the tip of a scanning tunneling microscope (STM) to induce singlemolecule translations, rotations, and reactions have been reported previously and are the subject of excellent review articles.9−12 In the majority of cases, the induced modifications have been imaged.9−12 For instance, a significant advance in visualizing bonds in single-molecule chemical reactions with an atomic force microscope has recently been published.13 A spectroscopic analysis of reaction products, however, remains scarce at the single-molecule level.14−18 In the present work, we show the stepwise, i.e., atom-byatom, dehalogenation of 5,10,15,20-tetrakis(4-bromophenyl)© XXXX American Chemical Society

porphyrin-cobalt (TBrPP−Co, Figure 1) on Ag(111) using tunneling electrons injected from the STM tip. The educt and

Figure 1. Top view of the calculated relaxed structure of the TBrPP− Co molecule in gasphase. The BrBr distance measured across the molecule center is 1.9 nm. The plane of the bromophenyl groups is rotated by 70° with respect to the porphyrin plane.

successive products were imaged with submolecular resolution. These images indicate that the relaxed structure of the intact molecule in vacuum is virtually retained upon adsorption. In addition, the electronic structure of the intact molecule and each reaction product was determined by spectroscopy of the Received: August 15, 2014 Revised: November 25, 2014

A

dx.doi.org/10.1021/jp508165c | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

comparable to the angle reported for free TPP.28 In line with adsorption configurations of other porphyrin molecules,18,29−32 it is likely that upon adsorption of TBrPP−Co on Ag(111) the molecule retains the rotation of its bromophenyl groups with respect to the porphyrin macrocycle. According to the freemolecule calculations this angle adopts values between 69° and 73° upon cleaving Br atoms. Atom-by-atom dehalogenation of TBrPP−Co on Ag(111) together with dI/dV spectroscopy of each product is presented in Figure 2. In Figures 2(a)−(e) STM images (top) and

differential conductance (dI/dV). Upon dehalogenation a broad occupied molecular resonance apparently is subject to a shift toward the Fermi energy. Quantum chemical calculations show that a variety of occupied frontier orbitals contribute to dI/dV spectra at negative bias voltages. These orbitals are mainly derived from Co d orbitals with different orientations and pyrrole π electron states. Dehalogenation induces changes in orbital energies and shapes. Spectra of intact and dehalogenated molecules thus reflect the occurrence of different orbitals, rather than the shift of a single molecular resonance.

■

EXPERIMENTAL AND THEORETICAL METHODS Experiments were performed with a homemade STM operated in ultrahigh vacuum with a base pressure of 10−8 Pa and at 4.5K and 77K. Clean Ag(111) surfaces were obtained after Ar+ bombardment and annealing. TBrPP−Co molecules were sublimated from a heated Ta crucible and adsorbed to Ag(111) at room temperature with submonolayer coverages. All STM images were acquired at constant current with the voltage applied to the sample. Constant-height dI/dV spectroscopy and constant-current dI/dV mapping was performed by modulating the sample voltage and measuring the current response with a lock-in amplifier. The geometric and electronic properties of the free TBrPP− Co molecule were determined by density functional calculations using the B3LYP functional19−21 and the 6-31G(2d,p) basis set22 as provided by the quantum chemistry package Gaussian09.23 According to previous work the Co dz2 orbital of intact TBrPP−Co is occupied by an unpaired electron, i.e., the molecule represents a spin doublet.24,25 Therefore, restricted and unrestricted open-shell calculations26 were performed for the intact TBrPP−Co molecule. Since the unrestricted calculations resulted in a considerable spin contamination (⟨S2⟩ = 0.7911) only the restricted open-shell calculations were further used for interpreting experimental data. Dehalogenation is modeled as a symmetric breaking of σ bonds between Br substituents and phenyl groups, which turns dehalogenated molecules into radicals. Due to the doublet spin state of intact TBrPP−Co singly and triply dehalogenated molecules represent singlet spin states, which were calculated as closeshell systems.27 To proof the suitability of close-shell calculations, restricted and unrestricted open-shell calculations were additionally performed. The obtained results are in agreement with close-shell calculations. The relaxed structures of intact TBrPP−Co as well as of dehalogenated molecules were determined by starting from the asymmetric conformer, for which electronic states are not degenerate.

Figure 2. (a)−(e) Constant-current STM images of an intact TBrPP− Co molecule on Ag(111) (a) and its stepwise dehalogenated products (b)−(e) (0.1 V, 0.5 nA, 2.9 × 2.9 nm2, 77 K). The gray scale ranges from 0 (black) to 135 pm (white) for all images. Dehalogenation is indicated by −nBr (n = 1···4) with n the number of removed Br atoms. The bottom row shows sketches of the imaged molecules. (f) Spectra of dI/dV acquired atop the center of intact TBrPP−Co molecules (black) and of the dehalogenated species (−1 Br, red; −2 Br, blue; −3 Br, green; −4 Br, gray). For clarity, spectra were vertically offset by 0.05 nS (−1 Br), 0.2 nS (−2 Br), 0.4 nS (−1 Br), and 0.5 nS (−4 Br). Before data acquisition the feedback loop had been disabled at −1.5 V and 0.5 nA. The double-headed arrow indicates the voltage range (−1 to 0 V) in which spectroscopic signatures of occupied molecular orbitals are visible.

sketches (bottom) of the molecule illustrate the stepwise Br detachment. The left STM image shows the intact molecule with the characteristic cross shape. Close inspection of STM images reveals that the angle between the two molecular axes deviates from 90° (vide infra). In addition, all bromophenyl groups appear identical in STM images. As discussed below, the four circular protrusions at the molecule periphery are assigned to the Br atoms. The successively dehalogenated products (−nBr, n = 1···4, n: number of removed Br atoms) were obtained by injecting tunneling electrons (3.3 V, 1 nA). A variety of bias voltages in the range of 2 to 4 V has been applied to the junction. The minimum voltage required for reproducible Br cleavage was 3.3 V, which is higher than the cleavage voltage reported previously for TBrPP−Co on Ag(111)33 and for TBrPP on Au(111).34 Several injection sites along a line between the center of the molecule and the top of the subsequently removed Br atom were suitable for single-atom dehalogenation. Occasionally, more than one Br atom was removed from the molecule upon the voltage pulse. For the intact and partly dehalogenated molecules dI/dV spectra were acquired atop the center of the molecule [Figure 2(f)]. Each spectrum shows broad features at negative and

■

RESULTS AND DISCUSSION The calculated relaxed structure of the intact molecule in vacuum is depicted in Figure 1. For this geometry, all forces on the nuclei were lower than 3 meV nm−1. Bromine atoms reside on the corners of a square with a diagonal of 1.9 nm. Bromophenyl groups are rotated by 70° out of the macrocycle plane. This rotation is due to a subtle interplay between two counteracting mechanisms. First, mesomerization forces the phenyl substituents to be coplanar with the porphyrin macrocycle. Second, the repulsion between H atoms in ortho position of the phenyl substituents and in β position of the pyrrole rings force the phenyl substituents to be perpendicular to the macrocycle plane. The resulting angle of 70° is B



Article

positive voltages. At negative voltages, which correspond to energies of occupied sample states a dominant peak at ∼−0.4 V is visible in spectra of intact molecules. Apparently, this peak shifts toward more positive voltages upon progressive dehalogenation. Extracted peak positions are summarized in Table 1. Additional weak peaks and shoulders to the main Table 1. Voltages (V) of the Main Peak Extracted from dI/ dV Spectra Acquired Atop Intact and Dehalogenated Moleculesa n

0

1

2

3

4

V (V)

−0.41

−0.33

−0.25

−0.17

−0.13

a

n denotes the number of Br atoms detached from TBrPP−Co (n = 0 corresponds to the intact molecule). Figure 3. (a) Cross-sectional profiles of TBrPP−Co acquired along the directions indicated in (b). The profiles include the Co atom which appears as a central protrusion at ∼1.28 nm. The upper profile has been shifted by 20 pm for clarity. (b) Constant-current STM image of TBrPP−Co (0.1 V, 0.5 nA, 2.75 × 2.50 nm2). The dashed lines indicate the directions of the cross-sectional profiles in (a). (c)− (e) Suggestion for adsorption geometries of TBrPP−Co on Ag(111). The Ag(111) lattice is depicted as light blue circles. STM data of (b) are superimposed in (c) (blue). Three orientations of the long molecular axis are indicated by 0°, 60°, and 120°. Bromine atoms of the long axis adopt Ag(111) hollow sites without modification of the free-molecule structure (Figure 1). The second pair of Br atoms can likewise reside at hollow sites (indicated by short arrows) upon clockwise rotation of the relevant molecule axis. In all adsorption configurations, the central Co atom resides at a Ag(111) top site. In (c) the crystallographic [1̅10] direction is indicated.

feature are visible in the broad spectral range extending from ∼−1 V up to 0 V. These observations hint at the contribution of more than one molecular orbital to the spectra in this voltage range. Indeed, our calculations reveal several occupied molecular orbitals with compatible energies. The feature at positive voltages is assigned to the spectroscopic signature of the lowest unoccupied molecular orbital (LUMO), in agreement with findings for TBrPP−Co on Cu(111).7 Since the LUMO appears with a rather large width (>1 V) in dI/dV spectra a possible shift upon dehalogenation is difficult to discern. In particular, for n = 3 [green line in Figure 2(f)] and n = 4 [gray line in Figure 2(f)] only a faint LUMO-related dI/dV signal is observed at ∼1 V. Therefore, we concentrate on the evolution of the sharper occupied molecular resonances with dehalogenation. As a first step in the analysis, we attribute the four circular protrusions visible at the periphery of intact TBrPP−Co molecules in STM images to Br atoms. This assignment is based on the comparison of experimentally determined and calculated molecule dimensions. Cross-sectional profiles were acquired across the molecular center [Figure 3(a)]. Along the direction indicated by the red dashed line in Figure 3(b) the distance between the outermost maxima is ∼1.87 nm, which is comparable with the calculated distance of 1.9 nm. In addition, the long distance is in agreement with a previous report on TBrPP adsorbed on Au(111), where Br atoms likewise appear as circular protrusions in STM images.34 Along the second direction [green dashed line in Figure 3(b)] the molecule appears compressed with respect to the calculated dimension, i.e., the BrBr distance is ∼1.70 nm. Further, unlike in the relaxed vacuum structure of the molecule the axes of the adsorbed molecule enclose an angle of (86 ± 2)°, rather than 90°. These deviations may be understood from the adsorption geometry suggested in Figure 3(c)−(e). Bromine atoms of the long molecule axis adsorb to 3-fold coordinated Ag(111) hollow sites without changing the relaxed vacuum geometry along this direction. This geometry is achieved when the long axis encloses an angle of ∼21.1° with the indicated crystallographic [11̅ 0] direction.35 This angle is in agreement with experimental observations. Comparison of molecule orientations with the atomically resolved Ag(111) lattice shows that the angle between the long molecule axis and [1̅10] is (20 ± 2)°. The BrBr distance along this axis is ∼1.87 nm,35 which likewise matches experimental data. The second pair of Br atoms may likewise reside at hollow sites in their close vicinity [indicated by short arrows in Figure 3(c)−(e)] by clockwise rotation of the relevant molecule axis about the surface normal

through the Co center. The BrBr distance along this axis is ∼1.77 nm,35 which is slightly higher than the experimental distance. The angle enclosed by the two molecular axes is ∼88°,35 which is in accordance with the experimental observation. To see the accordance more clearly, experimental data has been added to Figure 3(c). Equivalent adsorption geometries occur when the long axis of the molecule is rotated by 60° [Figure 3(d)] and 120° [Figure 3(e)] with respect to the long axis shown in Figure 3(c). Consequently, three orientations of the distorted molecule are expected and observed in the experiments. In all suggested geometries, the central Co atom resides at a Ag(111) top site. For TBrPP−Co on Cu(111) a similar adsorption geometry, i.e., Br atoms adopting substrate hollow sites and the Co atom residing at Cu(111) top sites, was suggested.25 On Cu(111), due to a different lattice constant, all Br atoms adsorb to hollow sites and the molecular axes are perpendicular to each other. The cleaved Br atoms were not visible in STM images acquired at 77K, which is probably due to an enhanced Br diffusion. Diffusion processes were suppressed in experiments performed at 4.5 K. Figure 4(a) shows a constant-current STM image of a TBrPP−Co dimer, where two TBrPP−Co molecules are bonded via a CC bond between adjacent phenyl groups.34 Stepwise dehalogenation of the right molecule with resulting single Br atoms in the vicinity of the remaining phenyl groups is indicated by arrows in Figures 4(b)−(d). Before identifying the dI/dV features, we comment on planar and saddle conformations of TBrPP−Co that were previously reported on different surfaces.7,36,37 While for deposition of TBrPP−Co at room temperature STM images reveal a single adsorption species, deposition at 12 K (not shown) leads to C



Article

additional molecule−surface bonds may be formed, which in turn increase the binding energy. As a result, the molecule adopts the planar adsorption geometry. For TPP−Co on Ag(111) vibration modes involving the Co atom reveal energies above 24 meV,18 which lends support to our explanation. In a next step, occupied molecular resonances whose spectroscopic signatures are visible in dI/dV spectra [Figure 2(f)] will be identified. Above a single intact TBrPP−Co molecule [Figure 5(a)] spectra of dI/dV were acquired at the

Figure 5. (a) Constant-current STM image (0.15 V, 1 nA, 3.4 × 3.4 nm2) of a single TBrPP−Co on Ag(111). The gray scale ranges from 0 (black) to 134 pm (white). Colored dots indicate positions of dI/dV spectra presented in (b). (b) Spectra of dI/dV acquired above TBrPP−Co at the indicated sites in (a). The feedback loop had been disabled at 1.5 V and 0.5 nA for all spectra. Arrows indicate voltages at which maps of dI/dV in (c),(d) were acquired. (c),(d) Normalized constant-current dI/dV maps of TBrPP−Co shown in (a) acquired at −270 mV (c) and −500 mV (d) (1 nA, 8 mVrms, 5.75 kHz, 3 × 3 nm2). Sketches of the molecule are superimposed for clarity. (e) Constant-current STM image (0.1 V, 0.5 nA, 4 × 3.7 nm2) of a TBrPP−Co assembly containing a TBrPP molecule (horizontal arrow). The gray scale ranges from 0 (black) to 100 pm (white). (f) dI/dV spectrum acquired atop the center of the TBrPP molecule shown in (e). The feedback loop had been disabled at 1.5 V and 0.5 nA.

Figure 4. Constant-current STM images (0.1 V, 0.1 nA, 7 × 4 nm2, 4.5 K) of two TBrPP−Co molecules on Ag(111). The gray scale ranges from 0 (black) to 250 pm (white) in all images. (a) Dimer of TBrPP− Co molecules, which may be covalently bonded via a CC bond between adjacent phenyl groups.34 (b)−(d) Cleaved Br atoms (arrows) are visible in the vicinity of the remaining phenyl group. Each STM image shows two Br atoms in the upper left corner, which indicate the same scanned surface area.

indicated positions [colored dots in Figure 5(a)]. Along Co Br [Figure 5(b)] directions the peak at negative voltages is attenuated toward and even vanishes at the Br atoms [red line in Figure 5(b)]. A similar evolution of the occupied resonance has been observed in spectra recorded along Co−pyrrole directions, in which the spectroscopic signature is attenuated toward the periphery of the porphyrin macrocycle. These findings show that the molecular resonances contributing to the spectral features at negative voltages are confined to the central region of the macrocycle, rather than to the Co site alone. Therefore, we suggest that Co orbitals with different symmetries are involved, e.g., Co dz2, Co dxz, and Co dyz. For TBrPP−Co on Cu(111), the observed occupied resonance was attributed to a Co dz2 orbital alone, similar to findings for phthalocyanines with a Co center on different surfaces.6,38,39 To corroborate the picture of various molecular orbitals con-

two adsorption conformations. These adsorption geometries are similar to the planar and saddle conformations reported from TBrPP−Co on Cu(111), which was deposited at 100 K.7 In the planar form the central Co atom is close to the substrate surface and the Br atoms approximately reside at the corners of a square. The saddle shape of the molecule is characterized by a lifted Co atom with Br atoms located at the corners of a rectangle. In our case, i.e., after TBrPP−Co deposition at room temperature, only the planar conformation has been observed. The deposition temperature likely plays an important role in the presence of specific adsorption configurations. Porphyrin vibration modes that involve the central Co atom are excited at room temperature and let the molecule explore the energy landscape for different adsorption configurations. In the course of a vibration, the Co atom approaches the surface and D



Article

On the basis of these observations TBrPP−Co likely exhibits a rather weak coupling to Ag(111). Figure 6 shows calculated isosurfaces of occupied frontier orbitals, i.e., HOMO, HOMO-1, HOMO-2, and HOMO-3, of

tributing to the dI/dV signal at negative voltages, constantcurrent maps of dI/dV were acquired at bias voltages covered by the broad peak [indicated by arrows in Figure 5(b)]. Since constant-current maps of dI/dV cannot directly be interpreted as the spatial variation of the local density of states a normalization procedure was applied to the raw data.40,41 This procedure takes the varying tip−sample distance into account, which is inevitable in constant-current data acquisition. The normalized data represent an improved approximation to the local density of states40 and are shown in Figure 5(c),(d). At −270 mV [Figure 5(c)], the central region of the molecule dominates the dI/dV map. In particular, Br atoms and phenyl groups are hardly visible. The porphyrin macrocycle appears with a different pattern at −500 mV [Figure 5(d)]. Rather than a single antinode [Figure 5(c)] three antinodes are discernible that align along a direction spanning two opposite pyrrole groups. While one antinode is still present at the Co site the two additional antinodes are located at the pyrrole rings of the molecule. Further evidence for the participation of Co orbitals in spectra and maps of dI/dV is the absence of spectral features at negative bias voltages for TBrPP molecules with a missing Co center, which were occasionally observed. Constant-current STM images of these molecules appear with a depression at the molecule center [Figure 5(e), horizontal arrow]. A typical dI/ dV spectrum acquired atop the center of a TBrPP molecule is displayed in Figure 5(f). Obviously, the peak at negative voltages is missing while the unoccupied resonance is closer to zero voltage than observed from TBrPP−Co molecules. This result is in agreement with a previous report where the TBrPP−Co resonance at negative voltages disappeared upon exchanging the central Co atom by a Cu atom.7 To gain further insight into the electronic structure of TBrPP−Co, isosurfaces, i.e., surfaces of constant wave function magnitude, of a variety of occupied Kohn−Sham molecular orbitals42 were determined by density functional calculations. Since the calculations were performed for the isolated, i.e., gasphase, molecule the results represent an approximation to the electronic structure of the adsorbed molecule. However, evidence for a weak molecule−substrate interaction is available. We found that individual TBrPP−Co molecules were easily displaced laterally across the surface and vertically toward the tip in manipulation experiments. In addition, the rotated bromophenyl groups enable TBrPP−Co to adopt a lander-like adsorption geometry.43−45 As a consequence, the porphyrin macrocycle likely exhibits a weak interaction to Ag(111). Moreover, for a similar molecule on Ag(111), TPP−Co, a weak molecule−metal coupling was likewise suggested.30 By comparing TPP−Co orbital energies on different surfaces and considering adsorption-induced work function changes it was demonstrated that the orbital energies align with respect to the vacuum level, EV, rather than to the Fermi energy, EF, of the substrate. Alignment of molecular orbital energies with respect to EV (EF) reflects weak (strong) molecule−substrate coupling.46,47 The energy of the TBrPP−Co HOMO is −0.7 eV on Cu(111)7 and −0.2 eV on Au(111).36 Work function changes were not reported and, therefore, the energy level alignment of TBrPP−Co orbitals with respect to EV cannot unambiguously be proved. However, assuming similar work function changes induced by TBrPP−Co adsorption on Cu(111) and Au(111) the aforementioned orbital energies approximately reflect the work function difference of the two surfaces, 4.9 eV for Cu(111) and 5.3 eV on Au(111),48 and thus the alignment of the HOMO energy with respect to EV.

Figure 6. Isosurfaces of constant wave function magnitude (green, +0.02; red, −0.02) calculated for occupied frontier orbitals of intact TBrPP−Co (top row), singly (middle), and triply (bottom) dehalogenated molecules. Directions x, y are indicated in the top left panel. The z axis is oriented perpendicular to the macrocycle plane. Calculation details are exposed in the text.

intact TBrPP−Co (top row), singly (middle) and triply (bottom) dehalogenated molecules. The indicated energies are referred to EV = 0 eV. The HOMO isosurface of the intact molecule is characterized by strong contributions of a Co dyz orbital. Occupied orbitals at more negative energies exhibit appreciable electron density at the porphyrin macrocycle around the Co site, in particular close to but not on the bromophenyl groups (HOMO-1) and at the pyrolline subunits (HOMO-2, HOMO-3). These findings may explain experimental data to some extent. At low negative voltages normalized dI/dV maps show that the local density of states is particularly strong at the TBrPP−Co central region [Figure 5(c)]. Thus, it is likely that at low negative voltage the spectroscopic signature of the HOMO is observed in spectra and maps of dI/dV. In addition, contributions to the dI/dV map in Figure 5(c) may arise from the HOMO-1 and the HOMO-2 whose isosurfaces are located close to the periphery of the macrocycle. The isosurfaces of these orbitals are mainly derived from π orbitals located at the pyrrole groups. The calculations further reveal that the HOMO-3 is derived from a Co dxz and a pyrrole π orbital, which may partly be the origin to the three antinodes visible in dI/dV maps acquired at −500 mV [Figure 5(d)]. In the following, we compare calculated orbital energies with experimental data. With the exception of the HOMO of the intact TBrPP−Co molecule, the energies of the depicted E



Article

single molecular resonance toward EF. Rather, different molecular orbitals become visible in spectra of dI/dV [Figure 2(f)], which may be related to the changes in isosurfaces at the macrocycle central region (Figure 6). The increased charge density at dehalogenated bromophenyl groups is not compatible with experimental data. The STM images in Figure 2(a)−(e) do not show contrast at the sites of cleaved Br atoms. This observation may be interpreted in terms of molecule− substrate hybridization, where upon dehalogenation, the former CBr bond is replaced by a CAg bond. Similar findings were reported for dehydrogenated phthalocyanine molecules.6,16,51

occupied frontier orbitals are compatible with experimental observations. Considering the work function of Ag(111), i.e., 4.7 eV,48 the energies of the HOMO-1, HOMO-2, and HOMO-3 fall into the broad range of ∼−1 to ∼0 eV with respect to EF, which matches the voltage range covered by occupied states [Figure 2(f)]. Since the experiments were performed at the single-molecule level with low local coverages, work function changes may be neglected. For benzene on Cu(111) work function changes on the order of 0.1 eV were induced by a single molecule,49 which are appreciably lower than work function changes of up to 1 eV induced by molecule coverages in the monolayer range.49,30,50 Therefore, we assign the peaks and shoulders in dI/dV spectra at negative voltages [Figure 2(f)] to the spectroscopic signatures of the aforementioned frontier orbitals. The spectra show that the signatures of the different molecular orbitals exhibit different magnitudes. This observation may be traced to their different spatial decay into vacuum,24 which determines the overlap with the orbital of the tip apex atom. In addition, some isosurfaces (Figure 6) show strong contributions at the Co site, e.g., HOMO and HOMO-3 of the intact molecule, while other orbitals (HOMO-1, HOMO-2 of intact TBrPP−Co) reveal a depletion. Since dI/dV spectra were acquired atop the Co site, the signatures of orbitals with appreciable charge density at Co are visible in the spectra while others appear with lower magnitude. Before analyzing the changes in the molecular electronic structure, we comment on the HOMO energy of the intact molecule, which deviates from experimental findings. A possible explanation for the discrepancy is the charge transfer between the molecule and the metal surface. For TBrPP−Co on Cu(111) a charge transfer from the metal to the molecule was proposed,25 in which the Co dz2 orbital becomes doubly occupied. Indeed, in close-shell calculations for TBrPP−Co with two electrons occupying the Co dz2 orbital, i.e., for the [TBrPP−Co]− anion, we observed that the HOMO energy decreases below −4 eV. Therefore, charge transfer to the molecule leads to an energy decrease and the visibility of the HOMO signature in dI/dV spectra at negative voltages. Upon stepwise Br cleavage from the free molecule the calculations reveal that radical states located at the dehalogenated phenyl group contribute significantly to some of the occupied frontier orbitals (Figure 6, middle and bottom row). For singly dehalogenated molecules (Figure 6, middle row) the HOMO as well as the HOMO-3 show a Co d orbital rotated by 45° with respect to the HOMO and HOMO-3 of the intact molecule (Figure 6, top). The HOMO-1 and HOMO-2 of -1 Br molecules are essentially unaltered except for a slight increase of charge density at the Co site. The latter may be rationalized in terms of the missing inductive effect of the detached Br atoms. For triply dehalogenated molecules (Figure 6, bottom row) the HOMO is a pure radical state with the electron wave function located exclusively at the radical phenyl groups. Therefore, the HOMO-1 becomes equivalent to the HOMO of intact TBrPP−Co with additional charge density at the dehalogenated phenyl group. Analogously, the HOMO-2 (HOMO-3) of -3 Br molecules is equivalent to the HOMO-1 (HOMO-2) of intact TBrPP−Co and singly dehalogenated molecules. According to Figure 6, the energies of equivalent molecular orbitals change by less than 30 meV due to Br detachment. In addition, a general trend toward lower energies upon dehalogenation is not observed. Therefore, stepwise Br detachment from TBrPP−Co does not induce a shift of a

■

CONCLUSIONS The atom-by-atom dehalogenation of TBrPP−Co on Ag(111) has been achieved by local electron injection from a STM tip. The structure and electronic configuration of each reaction product has been unraveled by STM images with submolecular resolution and spectroscopy of dI/dV. Several occupied frontier orbitals with similar energies give rise to spectroscopic signatures in dI/dV spectra. These orbitals are mainly derived from Co d resonances and pyrrole π states. Upon progressive dehalogenation, the occupied frontier orbitals are subject to changes in their isosurfaces. The concomitant modification of the spatial charge distribution at and in the vicinity of the molecule center leads to signatures with different weights in dI/ dV spectra acquired atop the Co atom.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Khadis, K. M., Smith, K. M., Guilard, R., Eds. The Porphyrin Handbook; Academic Press: New York, 2003; Vol. 1−20. (2) Williams, F. J.; Owain, O. P. H.; Vaughan, K. J.; Knox, N. B.; Lambert, R. M. First observation of capping/uncapping by a ligand of a Zn porphyrin adsorbed on Ag(100). Chem. Commun. 2004, 1688− 1689. (3) Vaughan, O. P. H.; Williams, F. J.; Bampos, N.; Lambert, R. M. A chemically switchable molecular pinwheel. Angew. Chem., Int. Ed. 2006, 45, 3779−3781. (4) Flechtner, K.; Kretschmann, A.; Steinrück, H.-P.; Gottfried, J. M. NO-induced reversible switching of the electronic interaction between a porphyrin-coordinated cobalt ion and a silver surface. J. Am. Chem. Soc. 2007, 129, 12110−12111. (5) Wäckerlin, C.; Chylarecka, D.; Kleibert, A.; Müller, K.; Iacovita, C.; Nolting, F.; Jung, T. A.; Ballav, N. Controlling spins in adsorbed molecules by a chemical switch. Nat. Commun. 2010, 1, 61−67. (6) Zhao, A.; Li, Q.; Chen, L.; Xiang, X.; Wang, W.; Pan, S.; Wang, B.; Xiao, X.; Yang, J.; Hou, J. G.; et al. Controlling the Kondo effect of an adsorbed magnetic ion through its chemical bonding. Science 2005, 309, 1542−1544. (7) Iancu, V.; Deshpande, A.; Hla, S.-W. Manipulating Kondo temperature via single molecule switching. Nano Lett. 2006, 6, 820− 823. (8) Heinrich, B. W.; Ahmadi, G.; Müler, V. L.; Braun, L.; Pascual, J. I.; Franke, K. J. Change of the magnetic coupling of a metal−organic complex with the substrate by a stepwise ligand reaction. Nano Lett. 2013, 13, 4840−4843. (9) Ho, W. Single-molecule chemistry. J. Chem. Phys. 2002, 117, 11033−11061. F



Article

of a single molecule by plasmon-enhanced Raman scattering. Nature 2013, 498, 82−86. (30) Auwärter, W.; Seufert, K.; Klappenberger, F.; Reichert, J.; Weber-Bargioni, A.; Verdini, A.; Cvetko, D.; Dell’Angela, M.; Floreano, L.; Cossaro, A.; et al. Site-specific electronic and geometric interface structure of Co−tetraphenyl-porphyrin layers on Ag (111). Phys. Rev. B 2010, 81, 245403. (31) Kim, H.; Chang, Y. H.; Lee, S.-H.; Kim, Y.-H.; Kahng, S.-J. Switching and sensing spin states of Co−porphyrin in bimolecular reactions on Au(111) using scanning tunneling microscopy. ACS Nano 2013, 7, 9312−9317. (32) Weber-Bargioni, A.; Auwärter, W.; Klappenberger, F.; Reichert, J.; Lefrancois, S.; Strunskus, T.; Wöll, C.; Schiffrin, A.; Pennec, Y.; Barth, J. V. Visualizing the frontier orbitals of a conformationally adapted metalloporphyrin. ChemPhysChem 2008, 9, 89−94. (33) Deshpande, A.; Braun, K. F.; Hla, S.-W. Determination of chemical specific atomic interaction with scanning tunneling microscope. Appl. Phys. Lett. 2011, 99, 221902. (34) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nano-architectures by covalent assembly of molecular building blocks. Nat. Nanotechnol. 2007, 2, 687−691. (35) The angle between the molecular axes may be calculated from 120° − α − β with α = cos−1(3(3/31)1/2) and β = cos−1[3√3/ (2√7)]. The BrBr distance along the long (short) molecule axis is 2b(31/3)1/2 (4b(7/3)1/2) with b = 0.289 nm the Ag(111) lattice constant. (36) Kim, H.; Son, W.; Jang, W. J.; Yoon, J. K.; Han, S.; Kahng, S.-J. Mapping the electronic structures of a metalloporphyrin molecule on Au(111) by scanning tunneling microscopy and spectroscopy. Phys. Rev. B 2009, 80, 245402. (37) Li, Q.; Yamazaki, S.; Eguchi, T.; Kim, H.; Kahng, S.-J.; Jia, J.-F.; Xue, Q.-K.; Hasegawa, Y. Initial adsorption and Kondo resonance of 5,10,15,20-tetrakis(4-bromophenyl)porphyrin−Co molecules on Ag/ Si(111) surface studied by low-temperature scanning tunneling microscopy/spectroscopy. Jpn. J. Appl. Phys. 2009, 48, 08JB01. (38) Takada, M.; Tada, H. Low temperature scanning tunneling microscopy of phthalocyanine multilayers on Au(111) surfaces. Chem. Phys. Lett. 2004, 392, 265−269. (39) Kröger, J.; Jensen, H.; Néel, N.; Berndt, R. Self-organization of cobalt-phthalocyanine on a vicinal gold surface revealed by scanning tunnelling microscopy. Surf. Sci. 2007, 601, 4180−4184. (40) Ziegler, M.; Néel, N.; Sperl, A.; Kröger, J.; Berndt, R. Local density of states from constant-current tunneling spectra. Phys. Rev. B 2009, 80, 125402. (41) Normalization was performed by using constant-current (1nA) STM data acquired at −270mV [Figure 5(c)] and at −500mV [Figure 5(d)] together with a tip−sample distance of 0.5 nm and a work function of 4.7 eV. (42) Kohn, W.; Sham, L. J. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140, A1133−A1138. (43) Rosei, F.; Schunack, M.; Jiang, P.; Gourdon, A.; Lægsgaard, E.; Stensggard, I.; Joachim, C.; Besenbacher, F. Organic molecules acting as templates on metal surfaces. Science 2002, 296, 328−331. (44) Kuntze, J.; Berndt, R.; Jiang, P.; Tang, H.; Gourdon, A.; Joachim, C. Conformations of a molecular wire adsorbed on a metal surface. Phys. Rev. B 2002, 65, 233405. (45) Moresco, F.; Gourdon, A. Scanning tunneling microscopy experiments on single molecular landers. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 8809−8814. (46) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Energy level alignment and interfacial electronic structures at organic/metal and organic/ organic interfaces. Adv. Mater. 1999, 11, 605−625. (47) Alkauskas, A.; Ramoino, L.; Schintke, S.; von Arx, M.; Baratoff, A.; Güntherodt, H.-J.; Jung, T. A. Energy level alignment at metaloctaethylporphyrin interfaces. J. Phys. Chem. B 2005, 109, 23558− 23563. (48) Skriver, H. L.; Rosengaard, N. M. Surface energy and work function of elemental metals. Phys. Rev. B 1992, 46, 7157−7168.

(10) Lorente, N.; Rurali, R.; Tang, H. Single-molecule manipulation and chemistry with the STM. J. Phys.: Condens. Matter 2005, 17, S1049−S1074. (11) Hla, S.-W. Scanning tunneling microscopy single atom/ molecule manipulation and its application to nanoscience and technology. J. Vac. Sci. Technol. B 2005, 23, 1351−1360. (12) Morgenstern, K.; Lorente, N.; Rieder, K.-H. Controlled manipulation of single atoms and small molecules using the scanning tunnelling microscope. Phys. Status Solidi B 2013, 250, 1671−1751. (13) de Oteyza, D. G.; Gorman, P.; Chen, Y.-C.; Wickenburg, S.; Riss, A.; Mowbray, D. J.; Etkin, G.; Pedramrazi, Z.; Tsai, H.-Z.; Rubio, A.; et al. Direct imaging of covalent bond structure in single-molecule chemical reactions. Science 2013, 340, 1434−1437. (14) Baadji, N.; Kuck, S.; Brede, J.; Hoffmann, G.; Wiesendanger, R.; Sanvito, S. Controlled sequential dehydrogenation of single molecules by scanning tunneling microscopy. Phys. Rev. B 2010, 82, 115447. (15) Li, Z.; Li, B.; Yang, J.; Hou, J. G. Single-molecule chemistry of metal phthalocyanine on noble metal surfaces. Acc. Chem. Res. 2010, 43, 954−962. (16) Sperl, A.; Kröger, J.; Berndt, R. Controlled metalation of a single adsorbed phthalocyanine. Angew. Chem., Int. Ed. 2011, 50, 5294−5297. (17) Sperl, A.; Kröger, J.; Berndt, R. Demetalation of a single organometallic complex. J. Am. Chem. Soc. 2011, 133, 11007−11009. (18) Burema, S. R.; Seufert, K.; Auwärter, W.; Barth, J. V.; Bocquet, M.-L. Demetalation of a single organometallic complex. ACS Nano 2013, 7, 5273−5281. (19) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (20) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785−789. (21) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results obtained with the correlation energy density functionals of Becke and Lee, Yang and Parr. Chem. Phys. Lett. 1989, 157, 200−206. (22) Binning, R. C.; Curtiss, L. A. Compact contracted basis sets for third-row atoms: GaKr. J. Comput. Chem. 1990, 11, 1206−1216. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (24) da Silva, L. G. G. V. D.; Tiago, M. L.; Ulloa, S. E.; Reboredo, F. A.; Dagotto, E. Many-body electronic structure and Kondo properties of cobalt-porphyrin molecules. Phys. Rev. B 2009, 80, 155443. (25) Perera, U. G. E.; Kulik, H. J.; Iancu, V.; da Silva, L. G. G. V. D.; Ulloa, S. E.; Marzari, N.; Hla, S.-W. Spatially extended Kondo state in magnetic molecules induced by interfacial charge transfer. Phys. Rev. Lett. 2010, 105, 106601. (26) Pople, J. A.; Nesbet, R. K. Self-consistent orbitals for radicals. J. Chem. Phys. 1954, 22, 571−572. (27) Roothaan, C. C. J. New developments in molecular orbital theory. Rev. Mod. Phys. 1951, 23, 69−89. (28) Rosa, A.; Ricciardi, G.; Baerends, E. J. Synergism of porphyrincore saddling and twisting of meso-aryl substituents. J. Phys. Chem. A 2006, 110, 5180−5190. (29) Zhang, R.; Zhang, Y.; Dong, Z. C.; Jiang, S.; Zhang, C.; Chen, L. G.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y.; et al. Chemical mapping G



Article

(49) Dougherty, D. B.; Maksymovych, P.; Lee, J.; Yates, J. T., Jr. Local spectroscopy of image-potential-derived states: from single molecules to monolayers of benzene on Cu(111). Phys. Rev. Lett. 2006, 97, 236806. (50) Schmid, M.; Zirzlmeier, J.; Steinrück, H.-P.; Gottfried, J. M. Interfacial interactions of iron(II) tetrapyrrole complexes on Au(111). J. Phys. Chem. C 2011, 115, 17028−17035. (51) Wang, Y. F.; Kröger, J.; Berndt, R.; Vázquez, H.; Brandbyge, M.; Paulsson, M. Atomic-scale control of electron transport through single molecules. Phys. Rev. Lett. 2010, 104, 176802.

H


Atom-by-Atom Dehalogenation of a Porphyrin Molecule Adsorbed on

Recommend Documents