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,
The chemical bond between an adsorbed, laterally coordinated metal ion and a metal surface is affected by an additional axial ligand on the metal ion. This surface analogon of the trans effect was studied in detail using monolayers of various M(II)-tetraphenylporphyrins (MTTPs, M = Fe, Co, Zn) and their nitrosyl complexes on a Ag(111) surface. X-ray photoelectron spectroscopy (XPS) shows that the oxidation state of the Fe and Co (but not Zn) ions in the MTPP monolayers is reduced because of the interaction with the substrate. This partial reduction is accompanied by the appearance of new valence states in the UV photoelectron and scanning tunneling spectra (UPS and STS), revealing the covalent character of the ion-substrate bond. Subsequent coordination of nitric oxide (NO) to the metal ions (Fe, Co) reverses these surface-induced effects, resulting in an increase of the oxidation states and the disappearance of the new valence states. Removal of the NO ligands by thermal desorption restores the original spectroscopic features, indicating that the described processes are fully reversible. The NO coordination also changes the spin state and thus the magnetic properties of the metal ions. Density-functional theory (DFT) calculations on model systems provide structural and energetic data on the adsorbed molecules and the surface chemical bond. The calculations reveal that competition effects, similar to the trans effect, play a central role and lead to a mutual interference of the two axial ligands, NO and Ag, and their bonds to the metal center. These findings have important implications for sensor technology and catalysis using supported planar metal complexes, in which the activity of the metal center is sensitively influenced by the substrate.
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 present a combined multimethod experimental and theoretical study of the geometric and electronic properties of Co-tetraphenyl- porphyrin (Co-TPP) molecules adsorbed on a Ag(111) surface. Scanning tunneling microscopy (STM) topographs reveal that Co-TPP forms highly regular arrays with a square unit cell. Hereby, the Co-TPP molecules do not occupy a unique adsorption site on the Ag(111) atomic lattice. The central Co atom of the Co-TPP is found to reside predominantly above fcc and hcp hollow sites of the substrate, as determined from the photoelectron diffraction patterns. A strong adsorption-induced deformation of Co-TPP involving a saddle-shaped macrocycle is evidenced by high-resolution STM images and quantified by near-edge x-ray absorption fine-structure measurements. By scanning tunneling spectroscopy we resolved discrete molecular electronic states and mapped the pertaining spatial charge-density distribution. Specifically, we discuss the interaction of orbitals originating from the Co-metal center with the porphyrin macrocycle and show that the varying adsorption sites induce a modulation in the Co-TPP lowest unoccupied molecular orbital. These findings are corroborated by density-functional-theory calculations
We employed a de novo synthesized porphyrin module to construct one-dimensional (1D) Cu-coordinated polymers on Cu(111) and Ag(111) surfaces. The programmed geometry and functionality of the molecular module together with its conformational flexibility and substrate interaction yields sinuous metal-organic polymeric assemblies, based on an unusual two-fold Cu-pyridyl coordination motif. An analysis of scanning tunneling microscopy (STM) data reveals the occurrence of two enantiomers, resulting from the surface confinement that deconvolutes the module in 2D-chiral conformational isomers. The stereoisomers exhibit site-specific surface anchoring, from whence three discrete orientations are possible for each species. Their sequence and mutual arrangement determine direction and curvature of the metal-organic chains. The Cu-coordinated polymers are very similar on both Cu(111) and Ag(111), where their formation is induced by intrinsic and coevaporated adatoms, respectively, which indicates that the lateral bonding motif is predominantly independent of the substrate. In addition, molecular manipulation experiments show the collective motion of entire segments of the Cu-coordinated multi-porphyrin polymers.
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.
The templated synthesis of porphyrin dimers, oligomers, and tapes has recently attracted considerable interest. Here, we introduce a clean, temperature-induced covalent dehydrogenative coupling mechanism between unsubstituted free-base porphine units yielding dimers, trimers, and larger oligomers directly on a Ag(111) support under ultrahigh-vacuum conditions. Our multitechnique approach, including scanning tunneling microscopy, near-edge X-ray absorption fine structure and photoelectron spectroscopy complemented by theoretical modeling, allows a comprehensive characterization of the resulting nanostructures and sheds light on the coupling mechanism. We identify distinct coupling motifs and report a decrease of the electronic gap and a modification of the frontier orbitals directly associated with the formation of triply fused dimeric species. This new on-surface homocoupling protocol yields covalent porphyrin nanostructures addressable with submolecular resolution and provides prospective model systems towards the exploration of extended oligomers with tailored chemical and physical properties.
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.
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