Manganese Phthalocyanine Derivatives Synthesized by On-Surface

Article pubs.acs.org/JPCC

Manganese Phthalocyanine Derivatives Synthesized by On-Surface Cyclotetramerization Marten Piantek,*,†,‡ David Serrate,‡,§ Maria Moro-Lagares,‡,§ Pedro Algarabel,†,‡ Jose I. Pascual,§,∥ and M. Ricardo Ibarra‡,§ †

Instituto de Ciencia de Materiales de Aragón, CSIC−Universidad de Zaragoza, 50009 Zaragoza, Spain Departamento Física Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain

‡

ABSTRACT: We report a two-step on-surface synthesis of Mn phthalocyanine molecules from four tetracyanobenzene (TCNB) monomers through coordinative bonding with manganese adatoms. In the first step, the coevaporation of TCNB and Mn atoms leads to a metal−organic network stabilized by a combination of coordination and hydrogen (H) bonds. In the second step, annealing the coordination networks above room temperature results in individual Mn phthalocyanine molecules. The results shown here demonstrate the viability of an on-surface chemical synthesis strategy involving the reduction of the organic reagents by metal ions to produce metal−organic molecules.

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terminated molecular precursors.11−14 We further demonstrate that the suggested surface-restricted cyclotetramerization of four organic building blocks around a transition metal atom appears only at elevated temperatures. The presented prototypical reaction results in manganese phthalocyanine (MnPc) monomers. The chemical pathway involves the concerted reaction of four TCNB monomers with a manganese adatom, demonstrating the viability of surface reactions from a chemical template of four coordinated organic constituents simultaneously.

INTRODUCTION On-surface chemical synthesis of organic species is a rapidly emerging tool in surface science for the in situ creation of large organic species on clean surfaces under ideal vacuum conditions.1,2 In this synthesis strategy, the surface of a metal serves as a support and catalyzer, steering the formation of strong chemical bonds between molecular precursors. Previous results demonstrated that on-surface synthesis can successfully create complex chemical entities such as macromolecules, extended polymeric chains, and two-dimensional covalent networks from small constituents acting as building blocks.3−6 The incorporation of magnetic atoms into such structures constitutes a promising approach toward the production of novel nanostructured metal−organic materials, combining high charge-carrier mobility with magnetism.7−9 Such self-assembled functional structures offer a huge potential for the application of molecular based materials in future information technologies, such as spintronics and quantum computation. However, suitable chemical pathways that encage a metal ion into a covalent cavity on surfaces are still missing, due to the complexity of the reaction process that has to comprise more than two reactants. Here we follow the idea of Abel et al.10 to synthesize phthalocyanine-based two-dimensional polymers by surfacerestricted cyclotetramerization of tetracyanobenzene (TCNB) molecules with transition metal atoms. From our experimental data it becomes clear that the coevaporation of Mn and TCNB molecules on an Ag(111) surface at room temperature leads, instead of to the afore-claimed polymerization, to the formation of two different metal−organic networks (MOCNs) involving Mn−CN coordination bonds similar to a variety of combinations of metallic atoms and nitrogen- and oxygen© 2014 American Chemical Society

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METHODS

All experiments have been performed under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM). The growth of the MOCNs was monitored during in situ coevaporation into a variable-temperature (VT) STM at room temperature (Aarhus, SPECS GmbH). High-resolution STM was conducted at 4.6 K (LT-STM Omicron GmbH). All STM topographies were measured with a set point of IT = 50 pA at UGap = −500 mV. We used a (111)-oriented Ag single crystal surface atomically cleaned and ordered by repeated cycles of Ar+ ion sputtering and annealing.15 Mn atoms and TCNB molecules were simultaneously evaporated onto the sample at the VT-STM stage from a commercial electron-beam metal evaporator and a homemade Knudsen cell at 380 K, respectively. Geometry optimization as well as the calculation of the molecular orbital structure of the gas phase TCNB Received: July 3, 2014 Published: July 7, 2014 17895

dx.doi.org/10.1021/jp506652j | J. Phys. Chem. C 2014, 118, 17895−17899

The Journal of Physical Chemistry C

Article

molecule was done at a DFT/B3LYP/6-31++G** level of theory.16

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FORMATION OF METAL−ORGANIC COORDINATION NETWORKS The coevaporation of Mn and TCNB at room temperature leads to well-ordered two-dimensional islands as shown in Figure 1a. The structure resembles a coarse-mesh network with

Figure 2. High-resolution STM topography at 4.6 K of the different network types on Ag(111) for (a, b) Mn:TCNB = 1/2 (α-phase) and (c, d) Mn:TCNB = 1/1 (β-phase). (e and f) Chemical structure of the α- and β-phase metal−organic coordination networks, respectively. (g) Surface plots of the frontier orbitals of TCNB in gas phase.

Figure 1. STM topography at room temperature after coevaporation of TCNB molecules and Mn atoms on an Ag(111) surface for different stoichiometric ratios: (a) lower and (b) higher Mn concentrations. α and β indicate the different occurring network phases. (c) and (d) show zooms into smaller areas of the networks with lower and higher Mn concentrations, respectively. Unit cell vectors are represented by the black arrows. Red rectangles and blue circles indicate the locations of organic ligands and metal atoms, respectively. Yellow circles mark the Mn atoms at the phase boundary.

formation of a polycyclic Pc-based network,10 we note that the size of the squared unit cell is also compatible with the expected dimensions of the Mn−TCNB metal−organic coordination network (MOCN) as given in Figure 2e. Remarkably, the organic ligands of the α-phase appear with a pronounced cloverleaf shape, inherent to the chemical structure of the TCNB molecule (cf. Figure 2b). The high-resolution STM topography given in Figure 2b resolves this structure as composed of four lobes connected to a central node. The lowest unoccupied molecular orbital (LUMO) of the gas phase TCNB (shown in Figure 2g) has also two nodal planes crossing through the central nodal point, and defining four orbital lobes at the central benzene rings that point toward the CN groups. Hence, we identify the ligands as intact TCNB molecules and attribute their internal contrast in STM images to the predominant role of this orbital in the tunneling conductance. Furthermore, since this molecular shape is resolved at negative sample bias, where the STM topography is only sensitive to the occupied density of states, the LUMO of the gas phase TCNB appears to be partially filled in the coordination network, evidencing electron transfer into the organic subunit, as was found for similar metal−cyano coordinated networks.17−19 The inspection of the β-phase at low temperature reveals further evidence of the formation of MOCNs on the surface at room temperature (see Figure 2c). Here, each ligand coordinates to four metal atoms in the rectangular unit cell, leading to a stoichiometric ratio Mn:TCNB of 1:1. The rectangular shape of this phase can only be attributed to the formation of an MOCN, in which the intact TCNB molecules bond to four Mn atoms as shown in Figure 2f. A closer look shows that the TCNB molecules in the β-phase (Figure 2d)

a squared unit cell a = 11.8 ± 0.5 Å (see Figure 1c) that was reported for Fe + TNCB coevaporation on Ag(111) with a stoichiometric ratio of Fe:TCNB = 1/2.10 Increasing the Mn:TCNB ratio for the metal and molecule coevaporation leads to the gradual growth of a different network structure, in response to the higher density of Mn atoms. This new phase (β-phase in Figure 1b) that appears as densely packed domains mixed with the coarse-mesh structure (α-phase) has not been reported before. The β-phase exhibits a rectangular unit cell (a = 10.5 ± 0.5 Å, b = 8.0 ± 0.5 Å) as indicated in Figure 1d. The structures of both phases appear oriented parallel to the crystallographic axis of the underlying Ag(111) substrate. The existence of preferential growth directions attests to a noticeable interaction between the surface and the adlayer. However, the influence of the hexagonal substrate is not sufficient to distort the 4-fold symmetry of the metal−organic assembly. In order to analyze the structure and bond character of the metal−organic networks, we conducted STM measurements at liquid-He temperature. Figure 2a shows a domain of the αphase together with a region of the pure (uncoordinated) TCNB phase (upper right area). In the metal−organic α-phase, each Mn atom can be visualized as a round feature and appears connecting four organic ligands in an orthogonal fashion, which leads to a squared unit cell. The positions of organic and metal components are indicated by red rectangles and blue circles, respectively. Although these domains were attributed to the 17896



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Figure 3. (a) Topography at 4.6 K of Mn−TCNB network on Ag(111) after annealing of the sample. The blue arrow indicates a Pc molecule created in the Mn−TCNB MOCN, and the red circle encloses four Pc molecules self-assembled around an Mn atom. (b) Topography at 4.6 K of MnOCyPc’s assembled into an island; black arrows indicate the unit cell. (c) Zoom into (a) with the smaller area with the suggested absorption structure. (d) Reaction scheme of the on-surface cyclotetramerization of MnOCyPc.

α-phase on the surface after postannealing the sample above 150 °C. New cross-shaped species can be now observed, as highlighted in Figure 3a. The species show an explicit D4h symmetry and a bright center in topography images at negative bias voltages, allowing us to associate them with metal phthalocyanine (Pc) molecules.22 The formation pathways of the substituted phthalocyanine molecules can be sketched by observing molecules that are integrated into the metal−organic coordination network, such as the one indicated by a blue arrow in Figure 3a. There, the newly created Pc molecule replaces a Mn[TCNB]4 unit. The bright center of the molecule resides where the Mn atom was before, and the four wings of the Pc substitute the TCNB molecules. The resulting Pc monomer appears rotated with respect to the MOCN lattice, and its dimensions are slightly larger than those for unsubstituted MnPc’s (1.7 nm along the cross lines). This is most likely due to the survival of cyano end groups from the TCNB precursors at its ends. The manganese octacyanophthalocyanine (MnOCyPc) molecules can be created on the surface following the reaction pathway sketched in Figure 3d. Thermal excitation in the MOCN leads to an intermediate state with a change of the bond configuration of the coordinated cyano end groups. This process is supported by the initial electron transfer to the ligand, as attested to for the coordination networks above. The change in the bond configuration is then followed by the cyclotetramerization of the involved four TCNB units around the Mn atom.

also exhibit a similar cloverleaf-like contrast, as in the α-phase, at negative bias, hinting that charge transfer is also present in this case. The similar intramolecular shapes for both network phases attest to the presence of the same organic ligand, and confirm that both structures are Mn−TCNB coordination networks with just different concentrations of Mn atoms and packing density. An analogous example for the formation of both MOCN phases has been reported on the coevaporation of Mn + tetracyanoquinodimethane (TCNQ) on Ag(100).20 Interestingly, for the case of uncoordinated TCNB phase (upper right corner in Figure 2a), the intramolecular resolution is absent in the inspected range of applied bias. This allows us to associate the formation of a coordination bonding to Mn adatoms with the electron donation into the TCNB units, presumably leaving the Mn atom in an oxidized state. The reduction of the organic ligand is an indispensable initial step on the pathway of the complex cyclotetramerization reaction toward the Pc molecules.21 The α-phase can thereby be considered as a chemical template for the subsequent polymerization of the metal−organic network.

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ON-SURFACE CYCLOTETRAMERIZATION The possibility of synthesizing metal pthalocyanines from the cyclic reaction of TCNBs as proposed in ref 10 remains then as a challenge. We tested if such surface reactions could still be triggered by increasing the sample temperature after codeposition. A clear molecular transformation occurs for the 17897



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ACKNOWLEDGMENTS Financial support by the Spanish Ministry of Science through Plan Nacional de I+D+i grant MAT2010-19236, University of Zaragoza (JIUZ-2013-CIE-12), and the German Academic Exchange Program (DAAD) is acknowledged.

Within the TCNB networks, we only found MnOCyPc monomers. The larger size and rotation of a newly created monomer induces a notable distortion of the network structure in the surrounding, hindering further cyclotetramerization steps in the adjacencies. In turn, MnOCyPc formation is more efficient at the borders of the Mn−TCNB MOCNs. Beside the MnOCyPc monomers integrated in the MOCN, most of the MnOCyPc molecules appear assembled in small islands of up to 50 molecules (Figure 3b) with a square unit cell of a = 14.6 ± 0.5 Å which is typically found for metal-Pc’s.22 In the presence of the outer cyano groups one would expect a slightly bigger unit cell. However, the assembly is stabilized via hydrogen bonds involving the cyano moieties, as can be seen by the appended chemical structure in Figure 3c. The enhanced intermolecular interaction might shrink the unit cell toward values reported for nonfunctionalized metal-Pc’s. The presence of the surface facilitates the ordered assembly of the created Pc’s, in contrast with the formation of disordered films of Pc’s and their polymeric derivatives observed when transition metal atoms and gas phase TCNB molecules are mixed at larger temperatures.23,24 MnOCyPc islands like that shown in Figure 3b appear in the sample with a relative low frequency (about 1 island/μm2) and usually are decorated with additional features visible as bright protrusions. The low yield of the cyclotetramerization reaction is due to the existence of competing chemical processes, which give rise to additional adsorbate species. An extended study of the involved kinetics might lead to an optimization of the synthesis of phthalocyanine. An interesting perspective of the reaction scheme proposed here is that it may also apply if MnOCyPc’s were the molecular precursors. Figure 3a shows an assembly of four MnOCyPc monomers bound to a Mn atom in a windmill fashion (red circle). This configuration suggests that a second hierarchy of MOCN can be created on the surface, in which four MnOCyPc units undergo a further polymerization reaction. The result of this reaction would be an extended array of Pc molecules covalently bound on the surface.10,24 However, during the course of our experiments we have not found evidence of this poly-cyclotetramerization reaction. This might be attributed to a significantly lower efficiency of the Mn[TCNB]4 → MnOCyPc transformation. Nevertheless, we foresee that initiating the reaction using solely MnOCyPc’s as molecular precursors could be an efficient way of creating extended covalent assemblies of Pc’s. Besides the creation of polymeric metal−organic complexes, we would like to emphasize the potential of this type of chemical reaction to synthesize more exotic Pc-like molecules on the surface by replacing TCNB with adequate but more complex building blocks.

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REFERENCES

(1) Lackinger, M.; Heckl, W. M. A STM Perspective on Covalent Intermolecular Coupling Reactions on Surfaces. J. Phys. D: Appl. Phys. 2011, 44, 464011. (2) Méndez, J.; López, M. F.; Martín-Gago, J. A. On-Surface Synthesis of Cyclic Organic Molecules. Chem. Soc. Rev. 2011, 40, 4578. (3) 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. (4) Zwaneveld, N. A.; Pawlak, R.; Abel, M.; Catalin, D.; Gigmes, D.; Bertin, D.; Porte, L. Organized Formation of 2D Extended Covalent Organic Frameworks at Surfaces. J. Am. Chem. Soc. 2008, 130, 6678− 6679. (5) Lafferentz, L.; Eberhardt, V.; Dri, C.; Africh, C.; Comelli, G.; Esch, F.; Hecht, S.; Grill, L. Controlling On-Surface Polymerization by Hierarchical and Substrate-Directed Growth. Nat. Chem. 2012, 4, 215−220. (6) Lin, T.; Shang, X. S.; Adisoejoso, J.; Liu, P. N.; Lin, N. Steering On-Surface Polymerization with Metal-Directed Template. J. Am. Chem. Soc. 2013, 135, 3576−3582. (7) Zhou, J.; Sun, Q. Magnetism of Phthalocyanine-Based Organometallic Single Porous Sheet. J. Am. Chem. Soc. 2011, 133, 15113. (8) Li, L.-H.; Li, J.-Q.; Wu, L.-M. Polymeric Fused-Ring Type Iron Phthalocyanine Nanosheet and Its Derivative Ribbons and Tubes. J. Phys. Chem. C 2012, 116, 9235. (9) DiLullo, A.; Chang, S.-H.; Baadji, N.; Clark, K.; Klöckner, J.-P.; Prosenc, M.-H.; Sanvito, S.; Wiesendanger, R.; Hoffmann, G.; Hla, S.W. Molecular Kondo Chain. Nano Lett. 2012, 12, 3174. (10) Abel, M.; Clair, S.; Ourdjini, O.; Mossoyan, M.; Porte, L. Single Layer of Polymeric Fe-Phthalocyanine: An Organometallic Sheet on Metal and Thin Insulating Film. J. Am. Chem. Soc. 2011, 133, 1203. (11) Tait, S. L.; Wang, Y.; Costantini, G.; Lin, N.; Baraldi, A.; Esch, F.; Petaccia, L.; Lizzit, S.; Kern, K. Metal−Organic Coordination Interactions in Fe−Terephthalic Acid Networks on Cu(100). J. Am. Chem. Soc. 2008, 130, 2108−2113. (12) Kezilebieke, S.; Amokrane, A.; Boero, M.; Clair, S.; Abel, M.; Bucher, J.-P. Steric and Electronic Selectivity in the Synthesis of Fe1,2,4,5-Tetracyanobenzene (TCNB) Complexes on Au(111): From Topological Confinement to Bond Formation. Nano Res. 2014, 7, 888−897. (13) Abdurakhmanova, N.; Floris, A.; Tseng, T.-C.; Comisso, A.; Stepanow, S.; Kern, K. Stereoselectivity and Electrostatics in ChargeTransfer Mn- and Cs-TCNQ4 Networks on Ag(100). Nat. Chem. 2012, 3, 940. (14) Langner, A.; Tait, S. L.; Lin, N.; Chandrasekar, R.; Meded, V.; Fink, K.; Ruben, M.; Kern, K. Selective Coordination Bonding in Metallo-Supramolecular Systems on Surfaces. Angew. Chem., Int. Ed. 2012, 51, 4327−4331. (15) Obretenov, W.; Höpfner, M.; Lorenz, W. J.; Budevski, E.; Staikov, G.; Siegenthaler, H. Characterization of the Surface Structure of Silver Single Crystal Electrodes by Ex situ and In situ STM. Surf. Sci. 1992, 271, 191. (16) Chakraborty, S.; Wategaonkar, S. Spectroscopic Characterization of 1,2,4,5-Tetracyanobenzene. Chem. Phys. Lett. 2008, 460, 18− 22. (17) Giovanelli, L.; Savoyant, A.; Abel, M.; Maccherozzi, F.; Ksari, Y.; Koudia, M.; Hayn, R.; Choueikani, F.; Otero, E.; Ohresser, P.; Themlin, J.-M.; Dhesi, S. S.; Clair, S. Magnetic Coupling and SingleIon Anisotropy in Surface-Supported Mn-Based Metal−Organic Networks. J. Phys. Chem. C 2014, 118, 11738−11744. (18) Tseng, T.-C.; Lin, C.; Shi, X.; Tait, S.; Liu, X.; Starke, U.; Lin, N.; Zhang, R.; Minot, C.; Van Hove, M.; Cerdá, J.; Kern, K. Two-

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses §

D.S., M.M.-S., J.I.P., M.R.I.: Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon, Universdad de Zaragoza, 50018 Zaragoza, Spain. ∥ J.I.P: CIC nanoGUNE, 20018 Donostia-San Sebastian, Spain, and Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain. Notes

The authors declare no competing financial interest. 17898



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Dimensional Metal-Organic Coordination Networks of Mn-7,7,8,8Tetracyanoquinodimethane Assembled on Cu(100): Structural, Electronic, and Magnetic Properties. Phys. Rev. B 2009, 80, 155458. (19) Shi, X. Q.; Lin, C.; Minot, C.; Tseng, T.-C.; Tait, S. L.; Lin, N.; Zhang, R. Q.; Kern, K.; Cerdá, J. I.; Van Hove, M. A. Structural Analysis and Electronic Properties of Negatively Charged TCNQ: 2D Networks of (TCNQ) 2 Mn Assembled on Cu(100). J. Phys. Chem. C 2010, 114, 17197−17204. (20) Tseng, T.-C.; Abdurakhmanova, N.; Stepanow, S.; Kern, K. Hierarchical Assembly and Reticulation of Two-Dimensional Mn- and Ni−TCNQx (x = 1, 2, 4) Coordination Structures on a Metal Surface. J. Phys. Chem. C 2011, 115, 10211. (21) Rao, P. S.; Sathyanarayana, D. N.; Jeevananda, T. Conducting Polymers: Synthesis and Electrical Conductivity. In Advanced Functional Molecules and Polymers, 1st ed.; Nalwa, H. S., Ed.; Taylor & Francis Group: Amsterdam, 2001; Vol. 3, p 112. (22) Jiang, Y.-H.; Liu, L.-W.; Yang, K.; Xiao, W.-D.; Gao, H.-J. SelfAssembly and Growth of Manganese Phthalocyanine on an Au(111) Surface. Chin. Phys. B 2011, 20, 096401. (23) Wöhrle, D.; Marose, U.; Knoop, R. Polymeric Phthalocyanines and their Precursors, 8. Synthesis and Analytical Characterization of Polymers from 1,2,4,5-Benzenetetracarbonitrile. Makromol. Chem. 1985, 186, 2209. (24) Yanagi, H.; Ueda, Y.; Ashida, M. Characterization of Monomeric and Polymeric (Octacyanophthalocyaninato) Metals in Thin-Films. Bull. Chem. Soc. Jpn. 1988, 61, 2313.

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Manganese Phthalocyanine Derivatives Synthesized by On-Surface

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