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Dipole Orientation Dependent Symmetry Reduction of Chloroaluminum Phthalocyanine on Cu(111) Tianchao Niu,† Miao Zhou,‡ Jialin Zhang,‡ Yuanping Feng,‡ and Wei Chen*,†,‡ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore

‡

S Supporting Information *

ABSTRACT: We demonstrate a dipole orientation dependent symmetry reduction of 4-fold symmetric chloroaluminum phthalocyanine (ClAlPc) molecules on a Cu(111) surface by combined low temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) calculations. Unexpected symmetry reduction from 4-fold (C4) to 2-fold (C2) was observed for Cl-down (dipole up) adsorbed ClAlPc, while molecules adopted Cl-up (dipole down) configuration reserved the C4 symmetry. DFT calculations indicated strong charge accumulation at the interface region between Cu surface and the Cl atom in Cl-down adsorbed ClAlPc due to the electron transfer from the bonded Cu atoms. This can result in charge redistribution within the phthalocyanine (Pc) macrocycle, and the formation of anionic Pc with an uptake of 1.3 e, which can be subjected to Jahn−Teller distortion. The inequivalent charge distribution onto the four lobes would be further enlarged due to the conformational distortion. The two down-bended lobes with more electrons interact stronger with the substrate and are much closer to the surface, leading to the C2 symmetry with one pair of up-bended lobes brighter and longer than their perpendicular counterparts for Cl-down adsorbed ClAlPc.

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INTRODUCTION Metal-phthalocyanines (MPcs),1 together with their derivatives, are active elements in the applications of heterogeneous catalysis,2 gas sensoring,3 organic photovoltaic,4 and molecular nanodevices,5 such as data storage,6 molecular switches,7 and molecular spin transistor.8 The physical properties and functionalities of these molecules adsorbed on metal surfaces are dramatically affected by the charge state of the macrocycle ligand,9,10 the central metal group,11−13 and the molecular conformation.14,15 Extensive investigations have been imposed on the correlation between the charge state and molecular conformations on various substrates.16,17 Illustrative examples include the alternated electronic and magnetic properties of transition-metal phthalocyanines (TM-Pcs) on metal surfaces due to the charge reorganization within the molecule, e.g., the additional spin on the Pc ligand of copper-phthalocyanine (CuPc) on Ag(100);18 manipulating Kondo temperature of single 5,10,15,20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) on the Cu(111) through conformation switching from saddle to planar shape, while the planar conformation exhibited extended Kondo states on the ligand induced by interfacial charge transfer;19 giant magnetoresistance (GMR) of the metal-free phthalocyanine (H2Pc) organic ligand on cobalt island induced by the spin-dependent hybridization of molecular Pc ligand and electrode orbitals;20 and switching on/off spin state on the Pc ligand of a single magnetic molecule (double-decker bis(phthalocyaninato)terbium(III), TbPc2) by © 2013 American Chemical Society

shifting the molecular frontier-orbital energy through tip pulse induced rotation of the upper ligand.21 It has been reported that the conductance of the tinphthalocyanine (SnPc)-down (Sn-atom pointing toward the substrate) was significantly higher than the SnPc-up (Sn-atom pointing toward the vacuum) molecule, in which 60% of the total current flows through the outmost C atoms and 40% through the Sn atom.22 Above features illustrate the critical role of the orbital coupling with the substrates, more specifically, coupling of the macrocycle of the conjugated ligand with the substrate. The molecular conformation and adsorption site can also affect the charge state of the ligand. Beside these, nonplanar phthalocyanines can adopt dipole up and down configurations after adsorption on the metal surface (Figure 1, up and down referring to the Cl atom pointing toward the vacuum and substrate, respectively) and hence the dipole orientation. This can influence the functionalities of the molecule and interfacial properties.23,24 However, the investigation on the structural and electronic properties of the nonplanar phthalocyanines,25 such as vanadyl-phthalocyanine (VOPc),26 chloroaluminum phthalocyanine (ClAlPc),27 chlorogallium phthalocyanine (GaClPc),24 and titanyl-phthalocyanine (TiOPc)28 is still very limited. Here, we present a Received: October 15, 2012 Revised: December 18, 2012 Published: January 2, 2013 1013

dx.doi.org/10.1021/jp310196k | J. Phys. Chem. C 2013, 117, 1013−1019

The Journal of Physical Chemistry C

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Figure 1. Molecular structure of chloroaluminum phthalocyanine (ClAlPc), the Cl atom protruding outside the molecular plane exhibits a permanent dipole moment of 3.7 D. Schematic representation of two configurations of ClAlPc on Cu(111), Cl-up (Cl atom pointing toward the vacuum, dipole-down) and Cl-down (Cl atom pointing toward the substrate, dipole-up).

Figure 2. Dipole alignment dependent symmetry reduction of ClAlPc on Cu(111). (A) The configurations of ClAlPc are denoted as U-I (Cl-up) with a bright spot in the center and D-I and D-II (Cl-down), which are symmetrically reduced. The molecular orientation with respect to the substrate lattice was imposed by arrows (Iset = 90 pA, Vtip= 1.6 V, 20 nm × 20 nm). (B) Line profile crossing the lobes of Cl-down (i) and Cl-up (ii) molecules as highlighted by yellow and orange squares in panel A. (C) High-resolution STM image of a single ClAlPc molecule with Cl-down configuration (Iset = 90 pA,Vtip = 0.8 V, 2.5 nm × 2.5 nm) and the corresponding line profile (D) taken across the neighboring lobes L1 and L2.

temperature. The molecular source was degassed for more than 24 h before the deposition. All the STM images were measured at liquid nitrogen temperature (77 K) using the constant current mode and an electrochemically etched tungsten (W) tip.30 The bias voltage was applied on the tip during the STM observation. DFT Calculations. The theoretical calculations were performed by using the Vienna ab initio Simulation Package (VASP) code based on density functional theory (DFT),31,32 with the projector augmented-wave (PAW) method and general gradient approximation (GGA) in Perdew−Burke− Ernzerhof (PBE) format with a kinetic energy cutoff of 400 eV.33,34 When calculating the ClAlPc molecule adsorption on Cu(111) surface, a supercell that consists of c(8 × 7) repeated Cu(111) slabs that were separated by a vacuum region of 20 Å was adopted. A 3 × 3 k-point sampling in the Brillouin zone was used in all calculations. We have used four-layer Cu atoms to model the surface, with the first two layers and the adsorbed molecules fully relaxed until the residual forces are smaller than 0.01 eV/Å. The van der Waals (vdW) interaction was also considered with the DFT-D2 method,35 which was found to be more appropriate for describing MPc molecule adsorption on metal surfaces.36 Charge transfer has been analyzed by using the Bader charge method.37

combined low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) investigation of the 4-fold symmetric C4v ClAlPc adsorbed on a Cu(111) surface. It shows an unexpected symmetry reduction for ClAlPc with Cl-down configuration, while the Cl-up adsorbed ClAlPc reserved the 4-fold symmetry on the 6-fold symmetric substrate. Bias-dependent STM images manifested the symmetry reduction is a geometric effect rather than a pure electronic origin. DFT calculations revealed that significant charge accumulation at the interface between the Cu surface and the Cl atom of Cl-down ClAlPc is ascribed to the charge donation from the Cu surface and results in an intake of 1.3 e for phthalocyanine lobes, which can be subjected to Jahn− Teller distortion. Furthermore, the inequivalent charge distribution onto the four lobes would be enlarged due to the conformational distortion. Two down-bended lobes with more electrons interact with the substrate stronger, and thus, these two lobes bend downward much closer to the substrate, giving rise to the C2 symmetry for Cl-down adsorbed ClAlPc.

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EXPERIMENTAL METHODS STM Experiments. STM measurements were performed in a custom built multichamber STM system with pressure better than 1.0 × 10−10 mbar, housing an Omicron LT-STM interfaced to a Nanonis controller.29 ClAlPc molecules were evaporated in ultrahigh vacuum (UHV) from a heated K-cell onto a sputter-annealed Cu(111) single crystal kept at room 1014



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Figure 3. Bias-dependent STM images of ClAlPc molecules with Cl-up and Cl-down configurations on Cu(111) surface, showing the symmetry conservation of ClAlPc-up (C4-up) and the symmetry reduction of Cl-down orientated ClAlPc (C2-down) (the set current for all the images is 90 pA).

Figure 4. Optimized geometric structures and adsorption sites of ClAlPc on Cu(111). (A) Top and side view of ClAlPc adopted U-I (left), D-I (middle), and D-II (right) configurations; the circles and triangles in the left panel highlight the pseudoequivalent atomic environment of aza-N and C atoms of the outer benzene rings. (B) Schematic showing the vertical distance between the atoms of distorted Cl-down ClAlPc and the Cu(111) surface.

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RESULTS AND DISCUSSION

eV larger than that of the ClAlPc with Cl-up configuration on Cu(111), so both the Cl-up and Cl-down configurations were energetically favorable (Supporting Information). The molecular orientation with respect to the substrate lattice was imposed by the red arrow. The lobes of the ClAlPc with Cl-down configuration, which are orientated along the close-packed ⟨110⟩ directions, appeared brighter than the perpendicular pair of lobes (D-I). Besides this, the molecular axes of configuration D-II are rotated by 15° with respect to configuration D-I and the substrate crystallographic axis. Figure 2B shows the line profiles crossing the four lobes of the Cldown (i) and Cl-up (ii) adsorbed ClAlPc as highlighted by yellow and orange squares, respectively, in Figure 2A. Under the given scanning condition (Vtip = 2.0 V, Iset = 100 pA), the Cl-up ClAlPc (U−I) reserved the 4-fold symmetry with the

STM images (Figure 2A) show two distinct configurations of ClAlPc (molecular structure, Figure 1, dipole moment = 3.7 D)25 molecules on Cu(111) at 0.02 monolayer (ML, referring to a closely packed layer of flat-lying ClAlPc molecules fully covered the Cu(111) surface with their molecular π-plane parallel to the substrate). One type of the configurations exhibits four equally pronounced lobes centered with a bright spot (Cl-up, Figure 1).38 Another configuration leads to a symmetry reduction with one pair of aligned lobes brighter and longer than the perpendicular pair of lobes, and nonbright spot in the center, referring to Cl-down configuration (Figure 1). On the basis of our DFT calculations (see experimental methods), the adsorption energy of Cl-down adsorbed ClAlPc was 0.01 1015



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Figure 5. Side and top views of the isosurface of the charge-density redistribution of ClAlPc with Cl-up (A) and distorted Cl-down (B) configurations on Cu(111) surface, which illustrate how the charge density changes upon the adsorption of ClAlPc to Cu(111) with different configuration. Blue and yellow colors indicate electron depletion and accumulation, respectively (isosurface value, 0.002 e/Å3).

substrate can also contribute to the reduced symmetry under certain tip bias.42 In order to elucidate the origin of the C4 to C2 symmetry reduction for the dipole ClAlPc, i.e., whether it is geometric distortion or due to the pure electronic effects such as different conductance channels under selected bias44 and/or asymmetry charge transfer.42 Bias-dependent STM images show that ClAlPc with Cl-up configuration conserved the 4-fold symmetry, while Cl-down ClAlPc appeared 2-fold symmetry in the range of −2.5 to 2.5 V (Figure 3 and Supporting Information Figure S1). Differing from the above-mentioned symmetry reduction of planar CuPc, FePc, and CoPc on Cu(111), where the strong coupling between the Pc ring and Cu state can be expected to drive the deformation,41 the observed STM topography of ClAlPc on Cu(111) was unexpected. In the case of symmetry reduced Cl-down ClAlPc, the Pc lobes located further away from the surface than the planar Pc because the Cl-atom lifted the molecular plane up. Moreover, the Cl-up adsorbed ClAlPc, which conserved original C4 symmetry was also unexpected due to the incommensurability between the 4-fold symmetry of the molecule and 6-fold symmetry of the substrate.

four lobes at identical length and height as shown in the line profiles crossing the U−I (ii, Figure 2B). However, the apparent height and length of the lobes of Cl-down ClAlPc varies from each other obviously as depicted in the line profiles (i, Figure 2B). One pair of Pc lobes appears brighter and longer than the perpendicular pair. High-resolution STM image of one representative ClAlPc with Cl-down configuration (Figure 2C) and the corresponding line profile (Figure 2D) crossing the neighboring lobes manifested that the ClAlPc molecules with Cl-down configuration were subjected to symmetry reduction from C4 to C2. Symmetry reduction of single phthalocyanine molecules on metal surfaces have been previously observed like CoPc,39 FePc,40 and CuPc41 on Cu(111), CuPc on Ag(100),42 and SnPc36 and H2Pc43 on Ag(111). Such symmetry reduction or breaking can be attributed to geometric and/or electronic effects. The incommensurability of the 4-fold symmetric phthalocyanine and 6-fold symmetric metal surface (Ag(111) and Cu(111)) can result in different atomic environments underneath the pair of Pc lobes and hence the deformation of molecule along the perpendicular axes such as the C4 to C2 reduction of CoPc on Cu(111).39 However, the asymmetric charge transfer between the Pc lobes of CuPc and Ag(100) 1016



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Figure 6. (A) Chemical structure of neutral ClAlPc, monoanion [ClAlPc]−, and diainion [ClAlPc]2−. In the neutral form, the bonds B1, B2, and azaN4−C1 in ClAlPc are double. The intake of at least one electron changes these bonds from double to single and enhances the conformational freedom of the molecule. (B) Molecular orbital energy scheme of neutral and [ClAlPc]2− molecular ions. The neutral ClAlPc has C4v symmetry with a doubly degenerate eg level, upon the intake of electron into the LUMO, the eg level split into b2g and b3g to lower the energy. (C) HOMO and LUMO of neutral and dianionic ClAlPc in the gas-phase. (D) Summarizing the bond length variation of the ClAlPc at the neutral and charged state (unit, Å).

Nonsignificant distortion of the Cl-up ClAlPc was mainly ascribed to the pseudoequivalent atomic environment underneath the aza-N atoms and four outer benzene rings. In the case of Cl-down ClAlPc, significant structural distortion of the lobes can be discerned from the optimized model (Figure 4A, D-I and D-II configurations in the middle and right panels, respectively). The Cl-atom preferred to reside at the hollow site and form covalent bonds with the nearest three Cu atoms. The azimuth orientation of these two energetically favorable configurations corresponds to a 30° and 15° deviation of the lobe axis from the [011̅] direction of the Cu(111) surface. Nonpseudo, or equivalent atomic, environment can be found for both the aza-N atoms and the Pc lobes due to the Cl-atom residing at the hollow site. The bottom schematics (Figure 4B) displayed the vertical distance between the atoms of the Cl-down adsorbed ClAlPc and the Cu surface, showing the distinct structural distortion after the adsorption. The Cl atom located 2.14 Å above the metal surface, and the Cl−Al bond was enlarged to 2.29 Å. As seen from Figure 4B, the outermost C atom of down-bended Pc lobes was 3.87 Å away from the surface, while this value for upbended lobes was 4.98 Å, implying that the observed symmetry reduction was a geometric effect instead of a pure electronic origin. To get a better understanding of the mechanism of the symmetry-reduced Cl-down configuration and the symmetryconserved Cl-up configuration, we plotted the charge difference of Cl-up and Cl-down orientated ClAlPc on Cu surface as

To unravel the unexpected symmetry reduction of ClAlPcdown as well as the symmetry conservation of ClAlPc-up molecules adsorbed on Cu(111), we performed first-principles calculations based on DFT (see experimental methods). In the gas phase, Cl-atom protruding outside the molecular plane results in the C4v symmetry, and the molecule should appear as a four-lobe feature in STM images.38 Optimized energetic favorable adsorption sites and orientation of the U-I, D-I, and D-II ClAlPc were depicted in Figure 4A. We find that the Cl-up adsorbed ClAlPc preferred to reside on the top site with one molecular axes rotated 16° with respect to the [11̅0] direction of Cu(111). Scrutinizing the location of the aza-N atoms (inner N bonded with Al) and the outside benzene rings of the four lobes, a pseudoequivalent atomic environment for the perpendicular lobes and the aza-N atoms can be proposed as shown in the left panel of Figure 4A. As demonstrated for CuPc on Ag(100), the driving force for the stability of planar configuration was the aza-N bonding with the nearest Ag atoms, so a favorable conformation is 32° deviation of Culigand axis from the [001] direction of Ag(100) surface.42 In analogy to the case of Cl-up adsorbed ClAlPc on Cu(111), the stable orientation is also determined by the interaction between aza-N and Cu atoms as well as the π orbitals of Pc lobes coupling with the Cu d states. The four aza-N atoms located the same distance of 3.75 Å away from the nearest Cu atom underneath, and the carbon triangles of the four outer benzene rings were also identically positioned above the Cu-triangle and rotated by 16° (highlighted in the left panel of Figure 4A). 1017



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from C4 to C2. Using a combination of LT-STM and DFT calculation, we find that the bonding of the Cl atom of Cl-down ClAlPc on the hollow site of Cu(111) leads to a modification of the charge state of the Pc lobes with the intake of 1.3 e, resulting in Jahn−Teller distortion of anionic Pc lobes. The enlarged inequivalent charge redistribution onto the four lobes gives rise to higher electron density of the down-bended lobes and hence facilitating the strong π-orbital coupling of the downbended lobes with the d state of Cu, leading to the C2 symmetry with one pair of up-bended lobes brighter and longer than the perpendicular ones. The results shown here exemplify the critical role of charge transfer across the functional anchoring point/metal interface on the molecular conformational variation and the charge state of Pc ligand, which can influence the interfacial electronic and magnetic properties of the molecule and can also contribute active sites for photochemical or electrochemical reactions.

shown in Figure 5. First, the optimized Cl-up ClAlPc shows a gain of electrons on the benzene rings of Pc and on the aza-N atoms. The charge distribution on the four lobes, in particular, the benzene ring, was almost identical. Such pseudoequal charge distribution conserved the 4-fold symmetry of Cl-up ClAlPc. However, for the Cl-down ClAlPc, significant charge transfer can be detected between the Cl and bonded Cu atoms (Supporting Information Figure S2, left); the charge depletion between the chloride and aluminum atom results in a decrease of the bond strength of Cl−Al, which can be confirmed from the calculated bond dilation of Cl−Al bond length from 2.20 to 2.29 Å. Bader charge analysis37 of the charge distribution for the Cl-down configuration of ClAlPc on the Cu surface shows that 1.3 e were populated to the Pc lobes. This is consisted with the previously reported electron buffer role of the Pc macrocycles played in metal phthalocyanine.1 The top view of the charge difference plot shows an in-equivalent charge distribution in the four lobes (Figure 5B). The two downbended lobes displayed significantly more electrons at the outside of the benzene ring, while the up-bended lobes were equally distributed among the whole benzene ring. A model can be proposed based on the change of structural and electronic properties of phthalocyanine after the charge redistribution on the Pc macrocycles. In contrast to transition metal phthalocyanine (TMPc), where the empty metal d orbitals lie between the HOMO−LUMO gap, the Cl−Al derived states lie deeper than the eg state of the Pc ring.45 As a consequence, adding one electron to the ClAlPc can not cause filling the Cl−Al derived state but can move to the eg state of the Pc ring. The LUMO of Pc was doubly degenerated; after the intake of 1.3 e onto the Pc macrocycles, the eg level can split into b2g and b3g, and hence, the anionic phthalocyanine lobes can be subjected to Jahn−Teller distortion.46,47 The Al−N1 distance is longer than Al−N2 (Figure 6D). Figure 6A schematically depicted the structural variation of ClAlPc after the intake of electrons based on the measured bond change of neutral and negatively charged ClAlPc (Figure 6D). Distinct change can be found for the dilation of aza-N−C1, the bridge N−C1, the C4−C5 of the outmost benzene ring, and the shortened C1−C2 bond in the pyrrolic ring. This can also be further confirmed from the variation of the LUMO of anionic Pc at the aza-N and bridge N (Figure 6C). The LUMO of the free ClAlPc was asymmetric on the pairs of perpendicular aromatic lobes. The inequivalent charge redistribution of the perpendicular lobes would be further enlarged due to the conformational distortion. The intake of electrons and the structural variations of the phthalocyanine anion were schematically shown in Figure 6A. After intake of one electron, the bonds B1 and B2 (the bridged NC bond) as well as the aza-N−C bond changed from double to single, which enhanced the conformation freedom of the molecule.48 The 1.3 electrons were unequally distributed onto the four lobes, which mainly spread over two lobes through the aza-N atoms (Figure 6A), so the coupling between the highly charged lobes with the Cu surface would be much stronger than another pair of lobes, and thus, the two lobes were further down-bended, giving rise to a symmetry reduction from C4 to C2.

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ASSOCIATED CONTENT

S Supporting Information *

Bias-dependent STM images showing the symmetry of the ClAlPc molecules with both Cl-up and Cl-down configurations. Charge density contour plots of downward bended ClAlPc showing the Cu−Cl bond and the differences between the two perpendicular lobes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 65-6516 2921. Fax: 65-6777 6126. E-mail: phycw@nus. edu.sg. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the Singapore ARF grant R143-000-505-112 and NUS YIA grant of R-143-000-452-101. REFERENCES

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CONCLUSIONS In conclusion, we have demonstrated the dipole orientation dependent symmetry reduction of ClAlPc on the Cu(111) surface. ClAlPc adopted Cl-up configuration reserved C4 symmetry; while Cl-down adsorbed ClAlPc reduced symmetry 1018



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