The development of a variety of nanoscale applications 1, 2 requires the fabrication and control of atomic [3][4][5] or molecular switches 6, 7 that can be reversibly operated by light 8 , a short range force 9, 10 , electric current 11,12 or some other external stimulus [13][14][15] . In order for such molecules to be used as electronic components, they should be directly Fig. 1d and f) at ambient temperatures, the process being called tautomerization 24,

Max-Planck Institut für Polymerforschung, Ackermannweg 10, Mainz, Germany.ABSTRACT Ultra-thin films of boron nitride (BN) have recently attracted considerable interest given their successful incorporation in graphene nanodevices and their use as spacer layers to

We investigated the surface bonding and ordering of free-base porphine (2H-P), the parent compound of all porphyrins, on a smooth noble metal support. Our multitechnique investigation reveals a surprisingly rich and complex behavior, including intramolecular proton switching, repulsive intermolecular interactions, and density-driven phase transformations. For small concentrations, molecular-level observations using low-temperature scanning tunneling microscopy clearly show the operation of repulsive interactions between 2H-P molecules in direct contact with the employed Ag(111) surface, preventing the formation of islands. An increase of the molecular coverage results in a continuous decrease of the average intermolecular distance, correlated with multiple phase transformations: the system evolves from an isotropic, gas-like configuration via a fluid-like phase to a crystalline structure, which finally gives way to a disordered layer. Herein, considerable site-specific molecule-substrate interactions, favoring an exclusive adsorption on bridge positions of the Ag(111) lattice, play an important role. Accordingly, the 2D assembly of 2H-P/Ag(111) layers is dictated by the balance between adsorption energy maximization while retaining a single adsorption site counteracted by the repulsive molecule-molecule interactions. The long-range repulsion is associated with a charge redistribution at the 2H-P/Ag(111) interface comprising a partial filling of the lowest unoccupied molecular orbital, resulting in long-range electrostatic interactions between the adsorbates. Indeed, 2H-P molecules in the second layer that are electronically only weakly coupled to the Ag substrate show no repulsive behavior, but form dense-packed islands.

Suitable templates to steer the formation of nanostructure arrays on surfaces are indispensable in nanoscience. Recently, atomically thin sp(2)-bonded layers such as graphene or boron nitride (BN) grown on metal supports have attracted considerable interest due to their potential geometric corrugation guiding the positioning of atoms, metallic clusters or molecules. Here, we demonstrate three specific functions of a geometrically smooth, but electronically corrugated, sp(2)/metal interface, namely, BN/Cu(111), qualifying it as a unique nanoscale template. As functional adsorbates we employed free-base porphine (2H-P), a prototype tetrapyrrole compound, and tetracyanoquinodimethane (TCNQ), a well-known electron acceptor. (i) The electronic moirons of the BN/Cu(111) interface trap both 2H-P and TCNQ, steering self-organized growth of arrays with extended molecular assemblies. (ii) We report an effective decoupling of the trapped molecules from the underlying metal support by the BN, which allows for a direct visualization of frontier orbitals by scanning tunneling microscopy (STM). (iii) The lateral molecular positioning in the superstructured surface determines the energetic level alignment; i.e., the energy of the frontier orbitals, and the electronic gap are tunable.

We report on the adsorption and self-metalation of a prototypic tetrapyrrole compound, the free-base porphine (2H-P), on the Cu(111) surface. Our multitechnique study combines scanning tunneling microscopy (STM) results with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) data whose interpretation is supported by density functional theory calculations. In the first layer in contact with the copper substrate the molecules adsorb coplanar with the surface as shown by angle-resolved NEXAFS measurements. The quenching of the first resonance in the magic angle spectra of both carbon and nitrogen regions indicates a substantial electron transfer from the substrate to the LUMO of the molecule. The stepwise annealing of a bilayer of 2H-P molecules sequentially transforms the XP and NEXAFS signatures of the nitrogen regions into those indicative of the coordinated nitrogen species of the metalated copper porphine (Cu-P), i.e., we observe a temperature-induced self-metalation of the system. Pre- and post-metalation species are clearly discriminable by STM, corroborating the spectroscopic results. Similar to the free-base porphine, the Cu-P adsorbs flat in the first layer without distortion of the macrocycle. Additionally, the electron transfer from the copper surface to the molecule is preserved upon metalation. This behavior contrasts the self-metalation of tetraphenylporphyrin (2H-TPP) on Cu(111), where both the molecular conformation and the interaction with the substrate are strongly affected by the metalation process.

The tessellation of the Euclidean plane by regular polygons has been contemplated since ancient times and presents intriguing aspects embracing mathematics, art, and crystallography. Significant efforts were devoted to engineer specific 2D interfacial tessellations at the molecular level, but periodic patterns with distinct five-vertex motifs remained elusive. Here, we report a direct scanning tunneling microscopy investigation on the ceriumdirected assembly of linear polyphenyl molecular linkers with terminal carbonitrile groups on a smooth Ag(111) noble-metal surface. We demonstrate the spontaneous formation of fivefold Celigand coordination motifs, which are planar and flexible, such that vertices connecting simultaneously trigonal and square polygons can be expressed. By tuning the concentration and the stoichiometric ratio of rare-earth metal centers to ligands, a hierarchic assembly with dodecameric units and a surface-confined metalorganic coordination network yielding the semiregular Archimedean snub square tiling could be fabricated.T he tiling of surfaces is relevant for pure art (1), mathematics (2, 3), material physics (4), and molecular science (5). Johannes Kepler's pertaining, rigorous analysis revealed four centuries ago that in the Euclidean plane 11 tessellations based on symmetric polygonal units exist (6): three consist of a specific polygon (so-called regular tilings with squares, triangles, or hexagons, respectively), whereas eight require the combination of two or more different polygons (named semiregular or Archimedean tilings from triangles, squares, hexagons, octagons, and dodecagons).Manifestations of regular tessellations at the atomic and molecular level are ubiquitous, including crystalline planes and surfaces of elemental or molecular crystals, and honeycomb structures encountered, e.g., for graphene sheets, strain relief and supramolecular lattices. In addition, the family of semiregular Archimedean tilings features intriguing characteristics. They may represent geometrically frustrated magnets (7) or provide novel routes for constructing photonic crystals (8). However, with the exception of the frequently realized trihexagonal tiling (also known as the Kagomé lattice) (9-15), the other semiregular Archimedean tiling patterns remain largely unexplored.Three of the semiregular Archimedean tilings correspond to five-vertex configurations (Fig. 1 A-C): the snub hexagonal tiling (four triangles and one hexagon at each vertex, labeled 3.3.3.3.6), the elongated triangular tiling (three triangles and two squares join at each vertex in a 3.3.3.4.4 sequence), and the snub square tiling (three triangles and two squares at each vertex; labeled 3.3.4.3.4). They have been identified in bulk materials, such as layered crystalline structures of complex metallic alloys (4, 16-18), supramolecular dendritic liquids (19), liquid crystals (20), special star-branched polymers (21, 22), and binary nanoparticle superlattices (23). Moreover, recent experiments with colloids at a quasicrystalline substra...

The formation of 2D surface-confined supramolecular porous networks is scientifically and technologically appealing, notably for hosting guest species and confinement phenomena. In this study, we report a scanning tunneling microscopy (STM) study of the self-assembly of a tripod molecule specifically equipped with pyridyl functional groups to steer a simultaneous expression of lateral pyridyl-pyridyl interactions and Cu-pyridyl coordination bonds. The assembly protocols yield a new class of porous open assemblies, the formation of which is driven by multiple interactions. The tripod forms a purely porous organic network on Ag(111), phase α, in which the presence of the pyridyl groups is crucial for porosity, as confirmed by molecular dynamics and Monte Carlo simulations. Additional deposition of Cu dramatically alters this scenario. For submonolayer coverage, three different porous phases coexist (i.e., β, γ, and δ). Phases β and γ are chiral and exhibit a simultaneous expression of lateral pyridyl-pyridyl interactions and twofold Cu-pyridyl linkages, whereas phase δ is just stabilized by twofold Cu-pyridyl bonds. An increase in the lateral molecular coverage results in a rise in molecular pressure, which leads to the formation of a new porous phase (ε), only coexisting with phase α and stabilized by a simultaneous expression of lateral pyridyl-pyridyl interactions and threefold Cu-pyridyl bonds. Our results will open new avenues to create complex porous networks on surfaces by exploiting components specifically designed for molecular recognition through multiple interactions.

Studies of complex condensed matter systems have led to the discovery of materials of unexpected spatial organization as glasses, glassy crystals, quasicrystals, and protein and virus crystals. Here, we present two-dimensional (2D) short-range disordered molecular crystalline networks, which, regarding spatial organization, can be considered as surface analogues of 3D glassy crystals. In particular, the deposition of a flexible molecular module on Cu(111) gives rise to distinct phases whose characteristics have been examined in real space by scanning tunneling microscopy: a 2D short-range distortional disordered crystalline network and a 2D short-range orientational disordered crystalline network, respectively. Both phases exhibit a random arrangement of nanopores that are stabilized by the simultaneous presence of metal-organic and pyridyl-pyridyl interactions. The 2D short-range distortional disordered crystalline network displayed intriguing flexibility, as probed by the STM tip that modifies the pore shape, a prerequisite for adaptive behavior in host-guest processes.