Five-Vertex Lanthanide Coordination on Surfaces ... - ACS Publications

Article pubs.acs.org/JPCC

Five-Vertex Lanthanide Coordination on Surfaces: A Route to Sophisticated Nanoarchitectures and Tessellations José I. Urgel,† David Ecija,*,†,§ Willi Auwar̈ ter,*,† Anthoula C. Papageorgiou,† Ari P. Seitsonen,‡ Saranyan Vijayaraghavan,† Sushobhan Joshi,† Sybille Fischer,† Joachim Reichert,† and Johannes V. Barth*,† †

Physik Department E20, Technische Universität München, D-85748 Garching, Germany Institut für Chemie, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

‡

ABSTRACT: We report a combined scanning tunneling microscopy and density functional theory study on the formation of intricate networks involving a flexible 5-fold carbonitrile−lanthanide (cerium or gadolinium) coordination. By employing linear linkers equipped with terminal carbonitrile functional groups, and by tuning the local rare-earth to molecule stoichiometry, architectures evidencing high spatial complexity become manifested, including disordered islands of pentameric and nonameric supramolecules, a 2D hierarchical short-range orientational disordered network, and a 2D Archimedean snub square tessellation of the surface, which coexists with minority 2D Archimedean elongated triangular tiling motifs. The combination of both the intricate structural features and the unique properties of lanthanide elements prospects great potential in a variety of fields such as magnetism and catalysis. Furthermore, the expression of Archimedean tiling motifs based on 5-fold vertexes suggests a route to the design of self-assembled dodecagonal quasicrystals on surfaces.

1. INTRODUCTION During the last decades, supramolecular chemistry on surfaces has emerged as a versatile strategy to design surface-confined nanoarchitectures.1−3 Hereby, a delicate interplay between molecule−substrate and molecule−molecule interactions dictates the resulting structure. Protocols relying on metal-directed assembly provide frequently an excellent option given their advantageous performance with respect to the robustness and spatial regularity. 4,5 Accordingly, a varied portfolio of procedures was introduced exploiting the coordination capabilities of transition metals and their affinity to molecular species specially equipped with carbonitrile,6−8 pyridyl,9−12 or carboxylate functional groups.13−15 Only recently, we extended this approach to the family of lanthanides and showed the first designs based on 5-fold cerium−carbonitrile motifs.16 In this work, we present a scanning tunneling microscopy (STM) study of the advanced architectures created on a pristine silver surface by the coordination of lanthanide elements (cerium (Ce) or gadolinium (Gd)) with linear polyphenyl molecules equipped with carbonitrile terminal groups. We show that depending on the local lanthanide:molecule stoichiometry different phases emerge, which are based on a flexible 5-fold Ce (Gd)−carbonitrile coordination (cf. Figure 1). Hereby, three phases interfere: (i) a disordered phase of pentameric and nonameric supramolecules, (ii) a hierarchical supramolecular architecture based on dodecameric entities, presenting 2D short-range organizational disorder,11 and (iii) a snub square Archimedean tessellation.17 Remarkably, identical architectures are obtained by employing either cerium or gadolinium. Our STM studies are complemented by density © 2014 American Chemical Society

functional theory (DFT) simulations, which reveal a planarization of the molecular species upon adsorption on the surface, an increasing binding energy per molecule due to metal− organic coordination, and a minimal intramolecular structural change between purely organic and metal−organic architectures. In summary, our results highlight the potential of lanthanide coordination chemistry on surfaces to create sophisticated designs,18,19 anticipating novel and unconventional functionalities resulting from the combination of the intricate supramolecular patterns with the inherent properties of the lanthanide family.

2. METHODS The experiments were performed using two independent custom designed ultrahigh vacuum systems that hosted an Aarhus 150 STM (VT-STM, see www.specs.com) and a lowtemperature STM (LT-STM, see www.lt-stm.com), respectively. The base pressure was below 2 × 10−10 mbar in the LTSTM system and below 1 × 10−9 mbar in the variable temperature-STM system. Vbias is applied to the sample. The Ag(111) substrate was prepared by standard cycles of Ar+ sputtering (800 eV) and subsequent annealing to 723 K for 10 min. All STM images were taken in constant-current mode with electrochemically etched tungsten tips. Received: March 24, 2014 Revised: May 26, 2014 Published: June 10, 2014 12908

dx.doi.org/10.1021/jp502901z | J. Phys. Chem. C 2014, 118, 12908−12915

The Journal of Physical Chemistry C

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Figure 1. Five-vertex lanthanide supramolecular architectures on Ag(111). (a) High-resolution STM images of the self-assembly patterns of p-NCPh3-CN-p (linker 1, ball-and-stick model in left inset) and p-NC-Ph4-CN-p (linker 2, ball-and-stick model in right inset) on the Ag(111) surface, respectively (scale bar: 5 nm). (b) Sketch of the 5-fold terphenyl-4,4″-dicarbonitrile-Ce (1) and the quaterphenyl-4,4″-dicarbonitrile-Ce (2) pentameric units. C, N, H, and Ce atoms are depicted in green, violet, white, and red, respectively. (c) Large-scale STM image displaying the coexistence of the disordered phase α and the dodecameric phase β, obtained by mixing linker 1 and Ce with a local stoichiometric ratio (Ce: molecule) close to 1:4 (Vb = 0.2 V, I = 80 pA). (d) Large-scale STM image of the snub square Archimedean tessellation of the Ag(111) surface, achieved by mixing linker 2 and Ce with a local stoichiometric ratio (Ce: molecule) of ∼2:5 (Vb = 0.2 V, I = 70 pA). Black lines drawn to connect the Ce centers highlight the triangular and square tiles, which combine in a 3.3.4.3.4 fashion, i.e., the snub square Archimedean tiling. Occasionally, some of the squares appear filled by an additional molecule, not coordinated to the centers. (c, d) Scale bar: 10 nm. (e) Zoom-in of images c and d showing with high resolution the pentameric, nonameric, and dodecameric units and the snub square Archimedean tiling motif, respectively.

with an angle of 91.945° with eight molecules in the purely molecular phase.21 In the β structure, the DFT-optimized value of the lattice constant of Ag was used because laterally the structure is covalently bound and we indeed observed strain when the experimental lattice constant was employed. Vertically, the cell dimension was chosen as 35 Å except in the β phase where a value of 25.25 Å was applied. Only the Γ point was used to sample the surface Brillouin zone, and Fermi−Dirac broadening at 300 K was used to broaden the occupation numbers around the Fermi energy. As the exchange-correlation functional, we employed the revPBE22 generalized-gradient approximation, with long-range dispersion included using the DFT-D3 empirical correction.23 The Kohn−Sham equations were solved within the Gaussian plane wave (GPW) scheme.24 The details are similar to our recent investigations of h-BN adsorbed on a transition metal surface:25 The ionic cores were described using Goedecker− Teter−Hutter pseudopotentials,26 the density expanded in a plane wave basis set up to the cutoff energy of 900 Ry (in molecular phase 500 Ry), and the wave functions in the DZVP Gaussian basis set of the MOLOPT type;27 for Ce, we included 30 electrons in the valence and a [4432] basis set. The binding energies were calculated without relaxing the system upon the removal of the molecule in question, i.e., just removing the atomic coordinates of the molecule.

The supramolecular networks based on the Ce/Gd-ligand coordination motifs described in the manuscript were fabricated in a two-step process: (1) The molecular linkers p-NC-Ph3-CN-p (p-NC-Ph4-CNp) (ref 21) were deposited by organic molecular beam epitaxy (OMBE) from a quartz crucible held at T = 478 K (503 K) onto a clean Ag(111) crystal held at ∼300 K. (2a) For the Ce networks: Ce atoms were evaporated from a homemade water-cooled cell by resistively heating a W filament enclosing a Ce ball of high purity (99.9999%, MaTecK GmbH, 52428 Jülich, Germany) onto the sample held at ∼300 K. (2b) For the Gd networks: Gd atoms were deposited by means of electron beam evaporation onto the sample held at ∼300 K from an outgassed Gd rod (99.9%, MaTecK GmbH, 52428 Jülich, Germany). After the growth protocol, the scanning tunneling microscopy inspection reveals whether the deposition of lanthanide on the surface was adequate or, on the contrary, lower or higher than expected. The DFT calculations were performed using the QuickStep module20 within the CP2K program suite (http://www.CP2K. org/). The adsorbed structures were modeled with the slab approach, where four layers of substrate were used, of which the two uppermost were relaxed. Hexagonal cells with lateral dimensions of 46.27 and 43.03 Å were used in the α and β phases, respectively, and a monoclinic cell of 30.61 Å × 32.20 Å 12909



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3. RESULTS AND DISCUSSION The selected ditopic dicarbonitrile−polyphenyl species, p-NC(Ph)n-CN-p (n = 3 for terphenyl-4,4″-dicarbonitrile (linker 1) and n = 4 for quaterphenyl-4,4″-dicarbonitrile (linker 2); cf. Figure 1a, left and right panel, respectively), have been previously used on surfaces for the design of 2D metal− organic networks with transition metal centers, where 3- and 4fold coordination nodes prevail,6,7 and, recently, with cerium atoms, yielding a 5-fold coordination.16 The deposition of linker 1 and linker 2 on pristine Ag(111) results in the formation of a supramolecular chevron-like pattern and a rhombic porous network, respectively, as depicted in Figure 1a.21 Upon the incorporation of Ce or Gd atoms, the scenario is dramatically changed, and depending on the local lanthanide to molecular species stoichiometric ratio, the presence of different phases based on a flexible 5-fold Ce/ Gd-linker coordination is detected. In particular, in this manuscript, we inspect the supramolecular assemblies created on Ag(111) by depositing Ce and linker 1, Ce and linker 2, and Gd and linker 2. The data presented in the following was recorded at T ≈ 6 K if not stated otherwise. 3.1. Supramolecular Pentameric Units (Phase α). For small surface concentrations and a Ce or Gd to linker stoichiometric ratio around 1:5, individual pentameric units made of linker 1 and Ce (cf. Figure 2a,e), linker 2 and Ce (cf.

supramolecules and highlight the adequate balance between flexibility and strength of the lanthanide−linker coordination required to produce the advanced networks at higher Ce/ Gd:molecule stoichiometries (cf. below).28 An increment in the local molecular coverage and in the Ce/ Gd:molecule stoichiometry, keeping it below 1:4, entails the appearance of nonameric units (cf. Figure 1c,e), in which two Ce/Gd centers are joined by a linker and, simultaneously, each center is coordinated by four other linkers constituting altogether nonamers that coexist with the pentamers and form disordered islands of supramolecules. 3.2. 2D Hierarchical Short-Range Orientational Disordered Crystalline Network (Phase β). For a Ce/Gd:linker ratio of ∼1:4, a new phase emerges, termed phase β, giving rise to extended domains over entire terraces and coexisting with residues of phase α (cf. Figure 1c). Equivalent assemblies could be produced by the combination of linker 1 and Ce and linker 2 and Ce (or Gd) on Ag(111). These are based on a two-level hierarchy and exhibit minor differences among them, mostly attributed to the different length and registry of the linker employed. Phase β follows a two-level hierarchical design based on dodecameric units, which are distributed on the surface, forming a hexagonal network (cf. Figure 3b,c,e,f and Table 1 for structural details). A dodecamer comprises three Ce/Gd centers and 12 linkers, arranged in a fashion in which three linkers are coordinated at both ends with lanthanide atoms, forming a triangle, and the nine remaining linkers are just singly coordinated to the Ce/Gd centers. The stability of the dodecamer is based on a 5-fold Ce/Gd-linker coordination, whose projected lengths are shown in Table 1. The second level of hierarchy reveals the assembly of the dodecamers on the surface in a hexagonal fashion via CN···phenyl attractive interactions29 between adjacent dodecamers (cf. projected CN···H distances in Table 1). Importantly, Ce or Gd hierarchical dodecameric phases involving linker 2 are geometrically indistinguishable. Both dodecameric networks present two organizational domains designated domain β1 and domain β2, respectively, related by a 180° rotation. As depicted in Figure 3 for domain β2, the surface registry of the linkers is manifested by a distinct angle of the hexagonal network with respect to the close-packed ⟨1-10⟩ directions of the silver surface, corresponding to 10° for Ce and linker 1 and to 30° for the Ce or Gd and linker 2 cases, respectively. In addition, phase β features supramolecular organizational chirality, which results in the formation of two sets of mirror symmetric domains (cf. Figure 4). Furthermore, a careful experimental analysis of the internal structure of the dodecamers reveals a random variation of the opening angles between constituent linkers of the dodecameric units, i.e., a flexibility of the supramolecule is detected, thanks to the adaptiveness of the lanthanide coordination sphere. This variation is related to the CN···phenyl interaction of each supramolecule with its adjacent dodecamers, since the length of the linkers allows for two equivalent contacts between dodecameric supramolecules for any linker length, as depicted in Figure 5. This flexibility can be inspected by STM measurements, and in addition, it can be induced by applying voltage pulses of 1.2 V (Ce-linker 2) and of 1.5 V (Ce-linker 1) on the metal centers with the STM tip. Figure 5 exemplifies one transition in a Ce-linker 2 (a, b) and in a Ce-linker 1 (c, d) dodecameric contact, where a change in the opening angle of 11 and 15° is detected, respectively. Thus, we observe that,

Figure 2. Individual supramolecular pentameric units based on a 5-fold Ce/Gd coordination to dicarbonitrile polyphenyl linkers 1 and 2 on Ag(111). (a) STM image of a Ce(linker 1)5 pentameric unit (Vb = 0.2 V, I = 50 pA). (b) STM image of a Ce(linker 2)5 supramolecule (Vb = 0.2 V, I = 100 pA). (c, d) STM images of a Gd(linker 2)5 pentamer displaying conformational flexibility before and after applying a voltage pulse with the STM tip (Vb = −0.02 V, I = 100 pA). Scale bar in parts a−d: 1 nm. (e−g) Atomistic models of parts a, b, c, and d, respectively, where C, N, H, Ce, and Gd atoms are depicted in green (maroon), violet, white, red, and brown, respectively. Maroon is used to highlight the C atoms of the linker that was manipulated by the tip.

Figure 2b,f), or linker 2 and Gd (cf. Figure 2c,d,g) are visualized in independent experiments. Molecules are imaged like rods, whereas the Ce (Gd) centers appear as bright protrusions. Typically, the pentamers are found isolated near steps or forming disordered patches. The 5-fold coordination node is promoted by the balance between the large size of the lanthanide atom compared to transition metals and their tendency to high coordination numbers, the surface confinement of the linkers, and the steric hindrance among the molecular species. Remarkably, the geometry of any pentameric species reflects a high degree of flexibility, exhibiting opening angles between two adjacent molecular linkers deviating from the ideal 72° pentagonal angle and ranging from 60 ± 5 to 85 ± 5°, as confirmed inspecting STM images before and after voltage pulses (cf. Figure 2c,d,g). Lateral manipulation experiments of the pentamers reveal the robustness of the 12910



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Figure 3. Phase β: 2D hierarchical short-range orientational disordered crystalline network, obtained from the Ln (Ce or Gd)-directed assembly of dicarbonitrile polyphenyl linkers on Ag(111) for a 1:4 (lanthanide:linker) stoichiometric ratio (the black lines in parts a−e mark the Ag(111) high symmetry axes). (a) Large-scale topographic STM image of the hierarchical network employing Ce and linker 1. (Vb = −0.2 V, I = 80 pA, scale bar = 20 nm). A fast Fourier transform is also depicted revealing the spatial periodicity of the tiling pattern (the white scale bar represents 0.045 Å−1). (b) Zoom-in of image a displaying the hierarchical network based on dodecameric units (Vb = 0.6 V, I = 95 pA, scale bar = 2 nm). (c) Atomistic model of part b, highlighting the dodecameric unit (violet lobes) and depicting the hexagonal network arrangement of the Ce centers described in the text. (d) Large-scale topographic STM image of the hierarchical network obtained with Gd and linker 2 (Vb = 0.7 V, I = 50 pA, scale bar = 20 nm). A fast Fourier transform is also depicted revealing the spatial periodicity of the tiling pattern (the white scale bar represents 0.045 Å−1). (e) Zoom-in of image d (Vb = 0.3 V, I = 69 pA, scale bar = 2 nm). (f) Atomistic model of part e, in which the orange lobes highlight the dodecameric unit. In parts c and f, C atoms of the linkers involved in a single coordination with Ce/Gd are depicted in green, whereas those connecting two Ce/Gd atoms are represented in maroon. N, H, Ce, and Gd atoms are depicted in violet, white, red, and brown, respectively.

Table 1. Structural Distances of the Hierarchical Dodecameric Assemblies Produced by the Deposition of Ce and Linker 1, Ce and Linker 2, and Gd and Linker 2 on Ag(111), Respectivelya

a

system

unit cell vector (Å)

CN−Ln (Å)

⟨CN···H⟩ (Å)

Ce and linker 1 Ce and linker 2 Gd and linker 2

42 ± 2 Å, DFT: 43.0 Å 50 ± 3 Å 50 ± 3 Å

2.2 ± 0.5 Å, DFT: 2.6 Å 2.8 ± 0.5 Å 2.6 ± 0.3 Å

2.3−3.5 Å, DFT: 2.3−3.0 Å 2.5−3.6 Å 2.3−3.6 Å

All experimental distances refer to projected lengths.

though the dodecameric assemblies show Ce (Gd) atoms periodically distributed on the surface, they present a random orientation within their supramolecular constituents, and the architecture, as a whole, cannot be considered a periodic network with strict translational symmetry but, more appropriately, should be regarded as a 2D short-range orientational crystalline network.11 3.3. Modeling of Five-Vertex Ce: p-NC-Ph3-CN-p Motif. In order to complement our experimental findings, DFT calculations involving p-NC-Ph3-CN-p species were carried out. We started by studying the purely molecular phase on the surface, and after the relaxation of the chevron supramolecular structure, the binding energy was 2.69 eV per molecule compared to the free molecules in the gas phase. The

average height of the carbon atoms in the molecules is 3.17 Å above the first layer of the substrate. Upon adsorption on the Ag(111) surface, the molecules become much more flat with the average phenyl dihedral angle being 0.9° and the phenyl rings oriented parallel with respect to the surface (for comparison in the gas phase molecule, this dihedral angle is 38.5°).30 The flattening of the molecule is in agreement with the small distortion energy of the molecule, 0.18 eV between the flat and optimized, rotated conformations in the gas phase. The final value of the average dihedral angle is a competition among (i) the van der Waals forces between the molecular species and the surface, which tend to flatten the molecule in order to make the phenyls approach the surface more before the Pauli repulsion limits the adsorption height; (ii) the steric 12911



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Figure 4. Phase β: Supramolecular organizational chirality. (a) Large-scale STM image displaying the coexistence of the four organizational chiral domains achieved by mixing linker 1 and Ce on Ag(111) with a local stoichiometric ratio (Ce: molecule) of ∼1:4 (Vb = −0.2 V, I = 80 pA, and image size = 957 × 957 Å2). Black lines mark the Ag(111) high symmetry axes. (b) High resolution STM images of the four organizational chiral domains. (c) Atomistic models of the dodecamers shown in part b. In order to facilitate the inspection of the mirror symmetry, the close-packed directions (shown in solid black lines) of the substrate are rotated to be aligned with the vertical.

Figure 5. Flexibility of phase β as probed by voltage pulses with the STM tip. (a, c) High-resolution STM images of the dodecameric phase created by Ce and linker 2 on Ag(111) (Vb = 0.2 V, I = 73 pA). (b) Maximized insets of parts a and c show the change of the lateral coordination CN···H between dodecameric units by applying voltage pulses of 1.2 V on top of the Ce centers (4.8 × 4.8 Å2). (d, f) High-resolution STM images of the dodecameric phase produced by Ce and linker 1 on Ag(111) (Vb = −0.2 V, I = 80 pA). (e) Maximized insets of parts d and f show the change of the lateral coordination CN···H between dodecameric units by applying voltage pulses of 1.5 V on top of the Ce centers (3.7 × 3.7 Å2). (a, c, d, f) Scale bar: 2 nm.

hindrance of the phenyl rings, which favors nonplanarity; and (iii) the resulting supramolecular packing on the surface, which due to intermolecular interactions interferes with the rotation of the phenyls. In this sense, for submonolayer p-NC-Ph4-CN-p species adsorbed on Ag(111), it was reported that the terminal benzonitrile groups are planar on the surface. It was suggested that the absence of packing constraints in the submonolayer regime, together with the lateral supramolecular hydrogen bonds, planarizes the terminal groups.30 Since the supramolecular chevron pattern formed by p-NC-Ph3-CN-p species on Ag(111) presents more CN···H bonds per molecule than the supramolecular assembly exhibited by p-NC-Ph4-CN-p linkers, it is reasonable to assume a higher planarization of the phenyl rings in the case of species 1 as compared with linker 2, which is in agreement with our simulations. Subsequently, we have inspected a pentameric supramolecule adsorbed on Ag(111), comprising five p-NC-Ph3-CN-p species coordinated to one Ce atom (phase α). Figure 6 depicts the relaxed geometry. The Ce atom is located 2.64 Å above the first

Figure 6. Ball-and-stick model derived by density-functional theory simulations of a supramolecular pentameric unit adsorbed on Ag(111) (phase α), which is based on a 5-fold Ce coordination to p-NC-Ph3CN-p species. (a) Top view of the bonding of the linker molecules to the Ce atom. The Ce−N distances are indicated in red. (b) Perspective view of part a, revealing the planarity of the five organic linkers connected to the central Ce atom. Ce, nitrogen, carbon, and hydrogen are depicted in red, violet, green, and white, whereas silver substrate atoms are shown in gray.

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Figure 7. Phase γ: Supramolecular snub square Archimedean tessellation of Ag(111) obtained by the assembly of dicarbonitrile polyphenyl linkers and Ce or Gd, following a lanthanide:linker stoichiometry of 2:5. (a) Large-scale topographic LT-STM image of the snub square Archimedean tessellation employing Ce and linker 2 (Vb = 2.4 V, I = 100 pA, TSTM = 6 K). (b) Large-scale STM image of the snub square Archimedean tiling obtained by using Ce and linker 1 (Vb = 2.4 V, I = 100 pA, TSTM = 173 K). (a, b) Scale bar: 5 nm. (c) STM image of the coexistence at 281 K of snub square Archimedean tiling motifs, produced by the coordination of Ce and linker 1, with diffusing species (Vb = 2.3 V, I = 100 pA, scale bar: 10 nm). (d) High resolution STM image of the snub square Archimedean tiling obtained by the self-assembly of Ce and linker 2 (Vb = 0.2 V, I = 100 pA). (e) High resolution STM image of the snub square Archimedean tessellation phase achieved by the coordination between Gd and linker 2 (Vb = 0.3 V, I = 69 pA). (d, e) Scale bar: 2 nm. (f) Atomistic model of part d superposed with a 3.3.4.3.4 sequence tiling motif. The different tile color represents the different orientation. Red circles are assigned to single Ce centers, and linkers are depicted in green. The unit cell of the network is defined by two squares and three triangles on each vertex. (a−f) Black or white stars represent the close-packed directions of Ag(111).

2:5, we detect the appearance of phase γ, representing a fully reticulated network. These architectures present all linkers with both terminal carbonitriles coordinated to a Ce/Gd center and, thus, are only stabilized by 5-fold Ce/Gd−carbonitrile bonds. A reticulated network giving rise to phase γ was observed while employing linker 1 and Ce (cf. Figure 7b,c), linker 2 and Ce (cf. Figure 7a,d), and linker 2 and Gd (cf. Figure 4e). Similar to the dodecameric case, γ phases produced using linker 2 and Ce or Gd are structurally identical. A minority of centers (less than 15%) present a different appearance, which is attributed to axial ligation by residual gas contamination, since they could be modified by applying voltage pulses with the tunneling tip and, in addition, they obey the supramolecular lattice order. We observe that the Ce/Gd centers are 5-fold vertexes connected to adjacent centers by the linkers, following a design scheme that can be interpreted as a surface tessellation based on triangles and squares, with each center sharing three triangles and two squares according to a 3.3.4.3.4 pattern, i.e., a snub square Archimedean tessellation. Thus, the reticulated network is stabilized by a flexible 5-fold Ce/Gd-NC coordination (cf. the atomistic model for linker 2 in Figure 7f), presenting opening angles between adjacent linkers of 60 or 90° (strongly deviating from the 72° expected for a regular pentameric coordination) and exhibiting Ce/Gd−NC bond lengths of 2.4 ± 0.5 Å for linker 1 and Ce, 2.7 ± 0.5 Å for linker 2 and Ce, and 2.6 ± 0.3 Å for linker 2 and Gd architectures. Regarding the thermal stability of the snub square Archimedean tessellation, an inspection close to room temperature of the surface decorated by linker 1 and Ce in the network stoichiometry shows the coexistence of a 2D “sea” of mobile adsorbates (evidenced by streaking and the higher apparent height of the area outside the empty molecular pores circled) with some snub square Archimedean tessellation motifs

substrate layer. The average Ce−N distance is 2.58 Å, in good agreement with our experimental values, and the C−N bond length (1.17 Å) of the cyano group bound to the Ce has hardly increased (Δd < 0.01 Å) due to the bonding. The molecules are again flatter than in the gas phase, with an average dihedral angle of about 10°, their height from the substrate is almost unaltered at 3.14 Å, and the molecular backbone lies parallel to the surface. The binding energy of the linker molecules in this structure is 3.26 eV, thus considerably higher than in the molecular phase: the Ce−linker bond strength exceeds the molecule−molecule interaction in the molecule-only phase, and the linker−substrate interaction is similar in both cases. Finally, we have simulated the hierarchic supramolecular architecture (phase β) created by p-NC-Ph3-CN-p species and Ce adsorbed on Ag(111). Our results reveal that the structure around the Ce−linker bonds is only slightly different, with the largest effect being the average height of the Ce atoms above the substrate, now 2.75 Å, i.e., 0.11 Å higher than in phase α. The Ce−N bond lengths are similar, here 2.55 Å, and the C−N bonds again are little changed from the gas phase. The binding energies of the singly and doubly coordinated molecular species are 3.37 and 4.07 eV, respectively. The average height of the linker molecules from the substrate is 3.14 Å, only 0.03 Å closer than in phase α. The average dihedral angle is approximately 7°. Thus, we conclude that, even if the linker molecules bind relatively strong to the central Ce atom, the intramolecular structure is only slightly changed between the purely molecular phase, phase α, and phase β from the gas phase molecule, with the main difference being the nearly planar geometry of the molecules once adsorbed on the surface. 3.4. Supramolecular Snub Square Archimedean Tessellation (Phase γ). Next, for local Ce:linker 1, Ce:linker 2, and Gd:linker 2 stoichiometric ratios equal or higher than 12913



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Figure 8. Phase γ: Organizational domains of the snub square tessellation of Ag(111). (a−c) STM images of the three supramolecular orientational domains of phase γ, obtained by depositing Ce and linker 2 at a local Ce:molecule stoichiometry of ∼2:5 (Vb = 0.2 V, I = 80 pA, 136 × 136 Å2).

4. CONCLUSIONS In summary, we have presented the rich versatility of 5-fold coordination chemistry on surfaces, based on lanthanide− carbonitrile contacts, to produce 2D advanced architectures with intricate designs, including disordered patterns (phase α), hierarchical short-range orientational disordered crystalline networks (phase β), and snub square Archimedean tessellations (phase γ) coexisting with elongated triangular Archimedean tiling motifs. The potential of these supramolecular designs in different fields is significant, including heterogeneous catalysis via the axial activity of the lanthanides (all phases), molecular recognition by structural adaptation (phase β), and novel photonic crystals37 or frustrated magnets38 predicted for the snub square Archimedean tessellations (phase γ). Furthermore, taking into account that the architectures exhibited in this work were identical for both Ce and Gd cases, the incorporation of the properties of different lanthanides39 into a known architecture seems feasible, just by selecting the appropriate rare-earth material to be involved in the coordination.

(cf. Figure 7c). This hints to the potential of the lanthanide coordination on surfaces to create room-temperature stable architectures, provided a proper engineering of the linking functional groups. Moreover, by systematic studies, the melting and condensation behavior of the tilings can be followed and analyzed. Within the distinct tilings, six different molecular orientations with respect to the substrate appear. In addition, three different organizational domains extending over a maximum area of 300 Å × 300 Å exist, each one related by a 60° rotation, as shown in Figure 8. In addition, occasionally, a different pattern could be detected, on the basis of three triangles and two squares joining in the 5-fold vertexes and arranged according to a 3.3.3.4.4 scheme (cf. Figure 9), which is identified as an

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +34-912998855. E-mail: [email protected]. *Phone: +49-8928912399. E-mail: [email protected]. *Phone: +49-8928912608. E-mail: [email protected].

Figure 9. Supramolecular Archimedean tiling motifs of phase γ on Ag(111), achieved by employing Ce and linker 2 following a local Ce:molecule stoichiometry of ∼2:5. (a) STM image of the snub square Archimedean tessellation (Vb = 0.2 V, I = 70 pA). Scale bar: 5 nm. (b) STM image of elongated triangle tiling motifs (Vb = 0.2 V, I = 50 pA). Scale bar: 2 nm.

Present Address §

IMDEA Nanoscience, 28049, Madrid, Spain.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

elongated triangular Archimedean tiling motif. Recently, the existence of a five-vertex (3.3.3.4.4) Archimedean tiling in a two-dimensional colloidal monolayer on a quasicrystalline surface31 or a supramolecular assembly based on pentameric building blocks, in which the chemistry of side groups can control the interaction of pentamers and influence the formation of a quasicrystalline structure,32 has been revealed. In addition, in materials such as binary nanoparticle superlattices33 and polymeric star systems,34 the presence of a fivevertex (3.3.4.3.4) Archimedean tessellation phase is accompanied by the existence of a quasicrystalline phase of dodecagonal symmetry. Thus, a link between five-vertex Archimedean tilings and quasicrystallinity has been established, and the quest for quasicrystalline phases in materials exhibiting five-vertex Archimedean tessellations has started.35,36 In this sense, we tentatively suggest the possibility of producing 2D supramolecular quasicrystals by exploiting 5-fold lanthanide coordination chemistry on surfaces.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the European Research Council Advanced Grant MolArt (Grant 247299), the German Research Foundation (Deutsche Forschungsgemeinschaft) through Grant BA 3395/2-1, and the Technische Universität München-Institute for Advanced Study. The DFT calculations were performed at CSCS (Centro Svizzero di Calcolo Scientifico) as a part of the project s425 and at the Informatikdienste of University of Zurich. We thank Prof. Mario Ruben and Dr. Svetlana Klyatskaya for the synthesis of the molecular compounds. We are grateful to Prof. Paul J. Steinhardt, Dr. Florian Klappenberger, and Dr. Francesco Allegretti for fruitful discussions. A.P.S. thanks Dr. Matthias Krack and Dr. Konstanze Hahn for help with the Ce pseudopotential and basis set. 12914



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