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Herein we report the facile reductive O-triflylation and O-silylation of naphthalene diimide as a scalable one-step approach. The resulting highly reactive 1,3,6,8-tetratriflato-2,7-diazapyrene was prepared at the 3 gram scale and serves as a versatile platform for further functionalizations via Sonogashira cross coupling or nucleophilic aromatic substitution by piperidine or iodide leading to tetrasubstituted diazapyrenes with tunable optoelectronic properties. Fine tuning is demonstrated by means of absorption and emission maxima varying within a range of more than 100 nm. The different diazapyrenes are characterized by means of XRD diffraction, UV-Vis and photoluminescence spectroscopy as well as cyclic voltammetry and TD-DFT calculations. The applicability of so far unaccessible tetra-1-alkynyl diazapyrenes for organic molecular beam epitaxy (OMBE) is demonstrated by characterization of a sub-monolayer on Cu(111) using scanning tunneling microscopy (STM).

Metal complexes at surfaces and interfaces play an important role in many areas of modern technology, including catalysis, sensors, and organic electronics. An important aspect of these interfaces is the possible exchange of the metal center, because this reaction can drastically alter the properties of the metal complex and thus of the interface. Here, we demonstrate that such metal exchange reactions are indeed possible and can proceed already at moderate temperatures even in the absence of solvents. Specifically, we studied the redox transmetalation of a monolayer of lead(ii)-tetraphenylporphyrin (PbTPP) with copper from a Cu(111) surface under ultrahigh-vacuum (UHV) conditions using multiple surface-sensitive techniques. Temperature-dependent X-ray photoelectron spectroscopy (XPS) reveals that the Pb/Cu exchange starts already below 380 K and is complete at 600 K. The identity of the reaction product, CuTPP, is confirmed by mass spectrometric detection in a temperature-programmed reaction (TPR) experiment. Scanning tunneling microscopy (STM) sheds light on the adsorbate structure of PbTPP at 300 K and uncovers the structural changes accompanying the transmetalation and side-reactions of the phenyl substituents. Moreover, individual free Pb atoms are observed as a product of the metal exchange.

Hybrid systems of two-dimensional (2D) materials such as transition metal dichalcogenides (TMDCs) and organic semiconductors (OSCs) have become subject of great interest for future device architectures. Although OSC–TMDC hybrid systems have been used in first device demonstrations, the precise preparation of ultra-thin OSC films on TMDCs has not been addressed. Due to the weak van der Waals interaction between TMDCs and OSCs, this requires precise knowledge of the thermodynamics at hand. Here, we use temperature-programmed desorption (TPD) and Monte Carlo (MC) simulations of TPD traces to characterize the desorption kinetics of pentacene (PEN) and perfluoropentacene (PFP) on MoS2 as a model system for OSCs on TMDCs. We show that the monolayers of PEN and PFP are thermally stabilized compared to their multilayers, which allows preparation of nominal monolayers by selective desorption of multilayers. This stabilization is, however, caused by entropy due to a high molecular mobility rather than an enhanced molecule–substrate bond. Consequently, the nominal monolayers are not densely packed films. Molecular mobility can be suppressed in mixed monolayers of PEN and PFP that, due to intermolecular attraction, form highly ordered films as shown by scanning tunneling microscopy. Although this reduces the entropic stabilization, the intermolecular attraction further stabilizes mixed films.

Polymorphism of organic molecular materials plays a prominent role in organic (opto)electronic devices, where the structural properties of metal/organic interfaces determine performance-critical pa...

Ring/chain competition in oligomerization reactions represents a long‐standing topic of synthetic chemistry and was treated extensively for solution reactions but is not well‐understood for the two‐dimensional confinement of surface reactions. Here, the kinetic and thermodynamic principles of ring/chain competition in on‐surface synthesis are addressed by scanning tunneling microscopy, X‐ray photoelectron spectroscopy, and Monte Carlo simulations applied to azulene‐based organometallic oligomers on Cu(111). Analysis of experiments and simulations reveals how the ring/chain ratio can be controlled through variation of coverage and temperature. At room temperature, non‐equilibrium conditions prevail and kinetic control leads to preferential formation of the entropically favored chains. In contrast, high‐temperature equilibrium conditions are associated with thermodynamic control, resulting in increased yields of the energetically favored rings. The optimum conditions for ring formation include the lowest possible temperature within the regime of thermodynamic control and a low coverage. The general implications are discussed and compared to the solution case., The competition between macrocycle and chain formation in organometallic on‐surface polymerization can be steered by variation of coverage and temperature. The entropically favored chains are preferentially formed under non‐equilibrium conditions and high coverage, whereas the more stable macrocycles prevail when the on‐surface synthesis is performed in equilibrium at low coverages.

Graphene nanoribbons (GNRs) are of enormous research interest as a promising active component in electronic devices, for example, field-effect transistors (FET). The recently developed “bottom-up” ...

The synthesis of cycloarenes in solution is challenging because of their low solubility and the often hindered cyclodehydrogenation reaction of their nonplanar precursors. Using an alternative on-surface synthesis protocol, we achieved an unprecedented double-stranded hexagonal cycloarene containing 108 sp2 carbon atoms. Its synthesis is based on hierarchical Ullmann coupling and cyclodehydrogenation of a specially designed precursor on a Au(111) surface. The structure and other properties of the cycloarene are investigated by scanning tunneling microscopy/spectroscopy, atomic force microscopy, and density functional theory calculations.

openaire: EC/H2020/839242/EU//EMOF | openaire: EC/H2020/845060/EU//QMKPFM | openaire: EC/H2020/788185/EU//E-DESIGN The quest for planar sp2-hybridized carbon allotropes other than graphene, such as graphenylene and biphenylene networks, has stimulated substantial research efforts because of the materials’ predicted mechanical, electronic, and transport properties. However, their syntheses remain challenging given the lack of reliable protocols for generating nonhexagonal rings during the in-plane tiling of carbon atoms. We report the bottom-up growth of an ultraflat biphenylene network with periodically arranged four-, six-, and eight-membered rings of sp2-hybridized carbon atoms through an on-surface interpolymer dehydrofluorination (HF-zipping) reaction. The characterization of this biphenylene network by scanning probe methods reveals that it is metallic rather than a dielectric. We expect the interpolymer HF-zipping method to complement the toolbox for the synthesis of other nonbenzenoid carbon allotropes.

The quest for planar sp2-hybridized carbon allotropes other than graphene, such as graphenylene and biphenylene sheet, has stimulated substantial research efforts because of the materials' predicted unique mechanical, electronic and transport properties. However, their synthesis remains challenging due to the lack of reliable protocols for generating non-hexagonal rings during the in-plane tiling of carbon atoms. Here, we report the bottom-up growth of ultra-flat biphenylene sheet with periodically arranged four-, six-, and eight-membered rings of sp2 carbon atoms, via an unprecedented on-surface inter-polymer hydrogen fluoride (HF)-zipping reaction. The characterization of this biphenylene sheet by scanning probe methods solves the long-standing controversy over its dielectric or metallic nature. We expect the inter-polymer HF-zipping method to complement the toolbox for the synthesis of other nonbenzenoid carbon allotropes.

Metal complexes at surfaces and interfaces play a prominant role in many areas of modern technology, including catalysis, sensors, and organic electronics. An important aspect of these interfaces is the possible exchange of the metal center, because this reaction can drastically alter the properties of the metal complex and thus of the interface. Here, we demonstrate that such metal exchange reactions are indeed possible and can proceed already at moderate temperatures even in the absence of solvents. Specifically, we studied the redox transmetalation of a monolayer of lead(II)-tetraphenylporphyrin (PbTPP) with copper from a Cu(111) surface under ultrahigh-vacuum (UHV) conditions using multiple surface-sensitive techniques. Temperature dependent X-ray photoelectron spectroscopy (XPS) reveals that the Pb/Cu exchange starts already below 380 K and is complete at 600 K. The identity of the reaction product, CuTPP, is confirmed by mass spectrometric detection in a temperature-programmed reaction (TPR) experiment. Scanning tunneling microscopy (STM) sheds light on the adsorbate structure of PbTPP at 300 K and uncovers the structural changes accompanying the transmetalation and side-reactions of the phenyl substituents. Moreover, individual free Pb atoms are observed as a product of the metal exchange. Our study suggests that surfaces functionalized with metal complexes may consist of other species than intended under operation conditions, which often involve elevated temperatures.

ACS nano 14(11), 15766-15775 (2020). doi:10.1021/acsnano.0c06798, Published by American Chemical Society, Washington, DC

The chemoselective three-phase hydrogenation of an Ombrabulin intermediate based on a nitro-stilbene framework was used as a case study for the formation of the primary amine without affecting the C C double bond. Two catalysts were tested: Pt nanoparticles supported on ZnO (Pt/ZnO) and agglomerated nitrogen-doped carbon nanotubes (NCNTs). Both materials show a high selectivity at close to full conversion in terms of amine formation (>99%) without changing the configuration of the central stilbene C C double bond. The continuous hydrogenation process was first tested in lab scale reactors (2 mL and 6 mL) combined with a high pressure interdigital micromixer to ensure efficient three-phase contact. While the specific reactor performance (defined as productivity per unit of reactor volume) was 1.5-fold higher for the Pt/ZnO, the NCNT catalyst was found to be more suitable for flow operation due to a lower pressure drop over the catalytic packed bed. The catalytic reactor was then scaled up to 50 mL and operated in the EcoTrainer mobile modular platform for the hydrogenation of the Ombrabulin intermediate demonstrating a safe, flexible and sustainable fine chemical synthesis.

The performance of graphene-based (opto)electronic devices depends critically on the graphene/metal interface formed at the metal contacts. We show here that the interface properties may be controlled by topological defects, such as the pentagon–heptagon (5–7) pairs, because of their strongly enhanced bonding to the metal. To measure the bond energy and other key properties not accessible for the embedded defects, we use azulene as a molecular model for the 5–7 defect. Comparison to its isomer naphthalene, which represents the regular graphene structure, reveals that azulene interacts more strongly with a Pt(111) surface. Its adsorption energy, as measured by single-crystal adsorption calorimetry (SCAC), exceeds that of naphthalene by up to 116 kJ/mol (or up to 50%). Both isomers undergo hybridization of their frontier orbitals with metal states, as indicated by X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with molecular orbital (MO) projection analysis through dispersion-corrected, periodic density functional theory (DFT) calculations. Based on the NEXAFS/DFT analysis, the stronger bond of the 5–7 system is attributed to the different energetic response of its unoccupied frontier orbitals to adsorption. Adsorption-induced bond-length changes show substantial topology-related differences between the isomers. Electron transfer occurs in both directions through donation/back-donation, resulting in the partial occupation (deoccupation) of formerly unoccupied (occupied) orbitals, as revealed by periodic energy decomposition analysis (pEDA) for extended systems. Our model study shows that the topology of the π-electron system strongly affects its bonding to a transition metal and thus can be utilized to tailor interface properties.\ud \ud

Phthalocyanines possess unique optical and electronic properties and thus are widely used in (opto)electronic devices, coatings, photodynamic therapy, etc. Extension of their π-electron systems could produce molecular materials with red-shifted absorption for a broader range of applications. However, access to expanded phthalocyanine analogues with more than four isoindoline units is challenging due to the limited synthetic possibilities. Here, we report the controlled on-surface synthesis of a gadolinium-supernaphthalocyanine macrocycle and its open-chain counterpart poly(benzodiiminoisoindoline) on a silver surface from a naphthalene dicarbonitrile precursor. Their formation is controlled by the on-surface high-dilution principle and steered by different metal templates, i.e., gadolinium atoms and the bare silver surface, which also act as oligomerization catalysts. By using scanning tunneling microscopy, photoemission spectroscopy, and density functional theory calculations, the chemical structures along with the mechanical and electronic properties of these phthalocyanine analogues with extended π-conjugation are investigated in detail., Extending the π‐conjugation of phthalocyanine dyes, while synthetically challenging, has the potential to produce desirable new molecular materials. Here, the authors use a templated on‐surface approach to synthesize several extended phthalocyanine derivatives from the same building block, including a lanthanide superphthalocyanine and an open‐chain polycyanine.

Various two-dimensional (2D) carbon allotropes with nonalternant topologies, such as pentaheptites and phagraphene, have been proposed. Predictions indicate that these metastable carbon polymorphs, which contain odd-numbered rings, possess unusual (opto)electronic properties. However, none of these materials has been achieved experimentally due to synthetic challenges. In this work, by using on-surface synthesis, nanoribbons of the nonalternant graphene allotropes, phagraphene and tetra-penta-hepta(TPH)-graphene, have been obtained by dehydrogenative C-C coupling of 2,6-polyazulene chains. These chains were formed in a preceding reaction step via on-surface Ullmann coupling of 2,6-dibromoazulene. Low-temperature scanning probe microscopies with CO-functionalized tips and density functional theory calculations have been used to elucidate their structural properties. The proposed synthesis of nonalternant carbon nanoribbons from the fusion of synthetic line-defects may pave the way for large-area preparation of novel 2D carbon allotropes.

We report the on-surface formation and characterization of [30]-honeycombene, a cyclotriacontaphenylene, which consists of 30 phenyl rings (C180H120) and has a diameter of 4.0 nm. This shape-persistent, conjugated, and unsubstituted hexagonal hydrocarbon macrocycle was obtained by solvent-free synthesis on a silver (111) single-crystal surface, making solubility-enhancing alkyl side groups unnecessary. Side products include strained macrocycles with square, pentagonal, and heptagonal shape. The molecules were characterized by scanning tunneling microscopy and density functional theory (DFT) calculations. On the Ag(111) surface, the macrocycles act as molecular quantum corrals and lead to the confinement of surface-state electrons inside the central cavity. The energy of the confined surface state correlates with the size of the macrocycle and is well described by a particle-in-the-box model. Tunneling spectroscopy suggests conjugation within the planar rings and reveals influences of self-assembly on the ...

A carbon nanotube supported catalyst containing cobalt/cobalt oxide (Co/Co3O4) nanoparticles encapsulated within a shell of nitrogen-doped graphene layers (Co3O4/NGr@CNT) was prepared. It shows excellent chemoselectivity in the hydrogenation of 1-iodo-4-nitrobenzene, which contains an iodine substituent highly sensitive against hydrodehalogenation. In contrast to traditional activated charcoal-supported catalysts such as Pt-V/C or the closely related Vulcan carbon black supported Co3O4/NGr@C, the advantageous morphological properties of the CNT support allow for the application of the new Co3O4/NGr@CNT as a fixed bed catalyst in a continuous flow reactor. Under optimized conditions, no dehalogenation side products could be detected. This remarkable selectivity in combination with its mechanical stability under operation conditions render Co3O4/NGr@CNT a catalyst particularly relevant for application in continuous processes based on a packed bed reactor.

Aromatic C-H bond activation has attracted much attention due to its versatile applications in the synthesis of aryl-containing chemicals. The major challenge lies in the minimization of the activation barrier and maximization of the regioselectivity. Here, we report the highly selective activation of the central aromatic C-H bond in meta-aryne species anchored to a copper surface, which catalyzes the C-H bond dissociation. Two prototype molecules, i.e., 4',6'-dibromo- meta-terphenyl and 3',5'-dibromo- ortho-terphenyl, have been employed to perform C-C coupling reactions on Cu(111). The chemical structures of the resulting products have been clarified by a combination of scanning tunneling microscopy and noncontact atomic force microscopy. Both methods demonstrate a remarkable weakening of the targeted C-H bond. Density functional theory calculations reveal that this efficient C-H activation stems from the extraordinary chemisorption of the meta-aryne on the Cu(111) surface, resulting in the close proximity of the targeted C-H group to the Cu(111) surface and the absence of planarity of the phenyl ring. These effects lead to a lowering of the C-H dissociation barrier from 1.80 to 1.12 eV, in agreement with the experimental data.

Carbon-based nanostructures have attracted tremendous interest because of their versatile and tunable properties, which depend on the bonding type of the constituting carbon atoms. Graphene, as the most prominent representative of the π-conjugated carbon-based materials, consists entirely of sp(2)-hybridized carbon atoms and exhibits a zero band gap. Recently, countless efforts were made to open and tune the band gap of graphene for its applications in semiconductor devices. One promising method is periodic perforation, resulting in a graphene nanomesh (GNM), which opens the band gap while maintaining the exceptional transport properties. However, the typically employed lithographic approach for graphene perforation is difficult to control at the atomic level. The complementary bottom-up method using surface-assisted carbon-carbon (C-C) covalent coupling between organic molecules has opened up new possibilities for atomically precise fabrication of conjugated nanostructures like GNM and graphene nanoribbons (GNR), although with limited maturity. A general drawback of the bottom-up approach is that the desired structure usually does not represent the global thermodynamic minimum. It is therefore impossible to improve the long-range order by postannealing, because once the C-C bond formation becomes reversible, graphene as the thermodynamically most stable structure will be formed. This means that only carefully chosen precursors and reaction conditions can lead to the desired (non-graphene) material. One of the most popular and frequently used organic reactions for on-surface C-C coupling is the Ullmann reaction of aromatic halides. While experimentally simple to perform, the irreversibility of the C-C bond formation makes it a challenge to obtain long-range ordered nanostructures. With no postreaction structural improvement possible, the assembly process must be optimized to result in defect-free nanostructures during the initial reaction, requiring complete reaction of the precursors in the right positions. Incomplete connections typically result when mobile precursor monomers are blocked from reaching unsaturated reaction sites of the preformed nanostructures. For example, monomers may not be able to reach a randomly formed internal cavity of a two-dimensional (2D) nanostructure island due to steric hindrance in 2D confinement, leaving reaction sites in the internal cavity unsaturated. Wrong connections between precursor monomers, here defined as intermolecular C-C bonds forcing the monomer into a nonideal position within the structure, are usually irreversible and can induce further structural defects. The relative conformational flexibility of the monomer backbones permits connections between deformed monomers when they encounter strong steric hindrance. This, however, usually leads to heterogeneous structural motifs in the formed nanostructures. This Account reviews some of the latest developments regarding on-surface C-C coupling strategies toward the synthesis of carbon-based nanostructures by addressing the above-mentioned issues. The strategies include Ullmann coupling and other, "cleaner" alternative C-C coupling reactions like Glaser coupling, cyclo-dehydrogenation, and dehydrogenative coupling. The choice of substrate materials and precursor designs is crucial for optimizing substrate reactivity and precursor diffusion rates, and to reduce events of wrong linkage. Hierarchical polymerization is employed to steer the coupling route, which effectively improves the completeness of the reaction. Effects of byproducts on nanostructure formation is comprehended with both experimental and theoretical studies.

Structural transformations of supramolecular systems triggered by external stimuli maintain great potential for application in the fabrication of molecular storage devices. Using combined ultrahigh vacuum scanning tunneling microscopy, X-ray photoemission spectroscopy, and density functional theory calculations, we observed the surface adatom mediated structural transformation from 4,4''-dibromo- m-terphenyl (DMTP)-based halogen-bonded networks to DMTP-Cu(Ag) coordination networks on Cu(111) and Ag(111) at low temperatures. The halogen-bonded networks, which were formed on Cu(111) at 97 K and on Ag(111) at 93 K, consist of intact DMTP molecules stabilized by triple Br···Br bonds. The DMTP-Cu(Ag) coordination networks form on Cu(111) at 113 K and on Ag(111) at 103 K. They contain alternatingly arranged intact DMTP molecules and Cu(Ag) adatoms stabilized by weak C-Br···Cu(Ag) coordination bonds. Annealing the DMTP-Ag structure to 333 K leads to the initiation of C-Br bond scission. This observation suggests that the DMTP-Ag coordination network represents the intermediate phase ready for dehalogenation, which is the first step of the surface Ullmann reaction.

Heptacene was generated by surface-assisted didecarbonylation of an α-diketone precursor on a Ag(111) surface. Monitoring of the surface reaction and characterization of the adsorbed heptacene was performed with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The surface-assisted formation of heptacene occurs around 460 K. Both the heptacene and the precursor molecules are oriented along the high-symmetry directions of the (111) surface and their molecular π systems face towards the substrate. The interaction with the Ag(111) substrate is not laterally uniform, but appears to be strongest on the central part of the molecule, in line with the expectations from Clar's rule. In the STM images, heptacene shows a dumbbell shape, which may correspond to the substantial out-of-plane deformations of heptacene on Ag(111). As revealed by DFT, the center of the molecule is closer to the surface than the outer parts. In addition, the inner rings are most affected by charge redistribution between surface and molecule. Heptacene acts as an acceptor and receives a negative charge of -0.6e from the Ag(111) surface. Since vacuum-sublimable α-diketone precursors for even larger acenes are available, the approach is promising for the on-surface synthesis of higher acene homologues such as octacene and nonacene.

Macrocycles have attracted much attention due to their specific "endless" topology, which results in extraordinary properties compared to related linear (open-chain) molecules. However, challenges still remain in their controlled synthesis with well-defined constitution and geometry. Here, we report the first successful application of the (pseudo-)high dilution method to the conditions of on-surface synthesis in ultrahigh vacuum (UHV). This approach leads to high yields (up to 84%) of cyclic hyperbenzene ([18]-honeycombene) via an Ullmann-type reaction from 4,4"-dibromo-meta-terphenyl (DMTP) as precursor on a Ag(111) surface. The mechanism of macrocycle formation was explored in detail using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). We propose that hyperbenzene (MTP)6 forms majorly by stepwise desilverization of an organometallic (MTP-Ag)6 macrocycle, which preforms via cyclisation of (MTP-Ag)6 chains under pseudo-high dilution condition. The high probability of cyclisation on the stage of the organometallic phase results from the reversibility of the C-Ag bond. The case is different from that in solution, in which cyclisation typically occurs on the stage of covalently bonded open-chain precursor. This difference in the cyclisation mechanism on a surface compared to that in solution stems mainly from the 2D confinement exerted by the surface template, which to a large extent prevents the flipping of chain segments necessary for cyclisation., 31 pages, 7 figures

The selective temperature-controlled surface-assisted synthesis of covalent, organometallic, and halogen-bonded nanomeshes based on a 3,5,3″,5″-tetrabromo-para-terphenyl (TBrTP) precursor was studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (STM) in ultrahigh vacuum. Vapor deposition of TBrTP onto Cu(111) at 90 K leads to a highly ordered organic monolayer stabilized by Br···Br and Br···H intermolecular bonds between the intact T-type assembled TBrTP molecules, as confirmed by density functional theory (DFT) calculations. Annealing the monolayer to 300 K results in C–Br bond scission and the formation of C–Cu–C bonds, which link adjacent para-terphenyl fragments such that stable organometallic frameworks are formed. Pore sizes correlate with the number of enclosed adatoms (most likely Br atoms), which presumably play a size-determining role during the process of the pore formation. Larger islands of the organometallic framework are obtained by deposition of TBrTP onto ...

The adsorption and reaction of 4,4″-dibromo-para-terphenyl (DBTP) and 1,3,5-tris(4-bromophenyl)benzene (TBB) on Cu(111) surface were studied with X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. In addition, complementary scanning tunneling microscopy (STM) data are presented. At submonolayer coverage, scission of C–Br bonds occurs between 170 and 240 K. The estimated activation energy for this process is considerably lower than the C–Br bond energy, indicating that bond scission is assisted by Cu atoms of the substrate. The remaining molecular backbones undergo linkage by C–Cu–C bonds to form organometallic oligomers. Annealing of these oligomers leads to the formation of C–C bonded covalent two-dimensional networks. Above monolayer coverage, complete C–Br cleavage requires higher temperature, confirming the role of the Cu surface in the reaction. The results provide insight into the C–Br bond scission as the initial s...

The influences of the substrate structure on the formation of one-dimensional organometallic and covalent oligomers on a Cu(110) surface were studied using scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS), and low energy electron diffraction (LEED) in ultrahigh vacuum (UHV). Vapor deposition of submonolayer 4,4''-dibromo-meta-terphenyl (DMTP) onto a Cu(110) surface at 300 K leads to scission of C-Br bonds and the formation of organometallic chains (cis/trans and all-trans) connected by C-Cu-C bonds. Larger islands (120 × 120 nm(2)) of all-trans zigzag organometallic chains as sole products were obtained by the deposition of DMTP onto Cu(110) held at 383 K. The domains are oriented along two directions with an angle of ±13° relative to the [0 0 1] direction due to the two-fold symmetry of the Cu(110) surface lattice. This study reveals at a sub-molecular level that the organometallic chains firstly lose copper atoms and then undergo C-C coupling into oligophenylene chains at a substrate temperature around 417 K. Annealing the large islands of organometallic chains at 458 K results in the formation of completely C-C covalently bonded zigzag oligophenylene chains. The zigzag angle of 125° slightly deviates from the ideal value of 120°. This is attributed to a stretching of the zigzag oligophenylene chains due to substrate template effects.

The bottom-up construction of low-dimensional macromolecular nanostructures directly on a surface is a promising approach for future application in molecular electronics and integrated circuit production. However, challenges still remain in controlling the formation of these nanostructures with predetermined patterns (such as linear or cyclic) or dimensions (such as the length of one-dimensional (1D) chains). Here, we demonstrate that a high degree of structural control can be achieved by employing a Cu(110)-(2×1)O nanotemplate for the confined synthesis of organometallic chains and macrocycles. This template contains ordered arrays of alternating stripes of Cu-O chains and bare Cu, the widths of which are controllable. Using scanning tunneling microscopy and low-energy electron diffraction, we show that well-defined, ordered 1D zigzag organometallic oligomeric chains with uniform lengths can be fabricated on the Cu stripes (width5.6 nm) of the Cu(110)-(2×1)O surface. In addition, the lengths of the meta-terphenyl (MTP)-based chains can be adjusted by controlling the widths of the Cu stripes within a certain range. When reducing the widths of Cu stripes to a range of 2.6 to 5.6 nm, organometallic macrocycles including tetramer (MTP-Cu)4, hexamer (MTP-Cu)6, and octamer (MTP-Cu)8 species are formed due to the spatial confinement effect and attraction to the Cu-O chains. An overview of all formed organometallic macrocycles on the Cu stripes with different widths reveals that the origin of the formation of these macrocycles is the cis-configured organometallic dimer (MTP)2Cu3, which was observed on the extremely narrow Cu stripe with a width of 1.5 nm.

We address the dynamic behavior and the surface chemical bond of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) around room temperature by variable-temperature scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) in ultrahigh vacuum. Between 280 and 345 K, the molecules predominantly display unidirectional diffusion along one of the three densely packed substrate ⟨110⟩ directions, which is attributed to a high site selectivity of the adsorbate−substrate bond. Above 305 K, the diffusion direction is found to change occasionally by ±120°. The activation barriers for the unidirectional diffusion and for rotation of the diffusion direction are determined to 0.71 ± 0.08 and 1.28 ± 0.12 eV, respectively. XPS shows that the iminic nitrogen atoms of 2HTPP interact strongly with the Cu surface. It is postulated that the local bonding situation is similar as in the initial complex (sitting-atop complex), which has previously been observed during the surface-confined in situ metalation of porph...

The formation of poly(p-phenylene-terephthalamide) (PPTA) by surface-assisted polymerization of terephthaloyl chloride (TPC) and p-phenylenediamine (PPD) on Ag(111) has been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the XPS data reveals that the polymer is formed in the (sub-) monolayer regime already at room temperature. Upon vacuum deposition of TPC onto the single-crystal surface, the carbon–chlorine bond is cleaved, and the polymerization proceeds via a p-phenylene dicarbonyl species, which is presumably surface stabilized. This contrasts the reaction mechanism in solution, where the chlorine atom is expelled after a nucleophilic attack by the amine group. Annealing of the sample leads to desorption of residual PPD monomers, lateral ordering of the polymer chains, and further progress of the polymerization that can be followed by XPS. The chain length distribution of the polyamide, as extracted from the XP spectra, is consistent with the typ...

Well dispersed and homogeneously distributed Pd nanoparticles within a silica matrix were synthesized by a two stage process: (1) intercalation of palladium tetrammine ions into the layered aluminium-free silicate Na ilerite followed by (2) a self-reduction of the complex through decomposition of the tetrammine ligands by calcination at 550 °C for 5 h under N2 or air. Samples calcined in N2 predominantly contained nanoparticles of metallic Pd, while calcination in air led largely to the formation of PdO, as confirmed by XPS and supported by XRD and TGA. Thus, the Pd/PdO loading in the silica can be selectively tuned through this method. The intercalates were characterized by standard methods such as XRD, TGA, ICP and TEM. TGA of the intercalates indicated that the stabilization of the palladium tetrammine complex was enhanced by intercalation into Na ilerite. Based on ICP analysis, the Pd loadings in the intercalates were estimated to be about 18 mass-%, which was also in agreement with the results from TGA. TEM revealed that the Pd/PdO nanoparticles were well dispersed in the silica matrix with an average particle size of approximately 8–10 nm after calcination in air.

An aromatic triphenylene molecule (2,3-diethyl-1,4-dimethoxytriphenylene; TPH) and its regioselectively Cr(CO)3-labeled complex (η6-(1,2,3,4,4a,12b)-tricarbonyl(2,3-diethyl-1,4-dimethoxytriphenylene)chromium(0); α-TPHC) were deposited on a Ag(111) surface by vapor deposition. The monolayers were investigated by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Both substances adsorb with the extended π-system parallel to the surface and form long-range ordered monolayers. The Cr(CO)3 group of the α-TPHC complex is oriented toward the vacuum. Although the footprints of both substances are similar, the additional Cr(CO)3 group on the α-TPHC leads to a lateral order (unit cell) that is significantly different from that of TPH.

The in situ metalation of monolayers and multilayers of 2H-tetraphenylporphyrin (2HTPP) with Fe atoms on Ag(111) was studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). This surface-confined coordination reaction results in the formation of adsorbed iron(II)-tetraphenylporphyrin (FeTPP). It is demonstrated that metalation of 2HTPP is achieved either by depositing iron atoms onto a monolayer of 2HTPP at room temperature or, alternatively, by depositing 2HTPP onto a Ag(111) surface with predeposited iron. The latter route requires elevated temperatures, indicating that this reaction includes at least one step with an activation barrier. In addition, it is demonstrated that vapor deposited Fe atoms also react with multilayers of the porphyrin at room temperature under the formation of FeTPP.

A Li4Ti5O12 (LTO) film was coated as buffer layer onto a LiNi0.5Mn1.5O4 (LNMO) high-voltage cathode, and after cycling of the cathode in a battery electrolyte, the LTO film was investigated by means of synchrotron radiation based hard X-ray photoelectron spectroscopy (HAXPES). By tuning the photon energy between 2 keV and 6 keV, we obtained non-destructive depth profiles of the coating material with probing depths ranging from 6 nm to 20 nm. The coating was found to be covered by a few nanometers thin surface layer resulting from electrolyte decomposition. This layer consisted predominantly of organic polymers as well as metal fluorides and fluorophosphates. A positive influence of the Li4Ti5O12 coating with regard to the size and stability of the surface layer was found. The coating itself consisted of a uniform mixture of Li(I), Ti(IV), Ni(II) and Mn(IV) oxides that most likely adopted a spinel structure by forming a solid solution of the two spinels LiNi0.5Mn1.5O4 and Li4Ti5O12 with Li, Mn, Ni and Ti cations mixing on the spinel octahedral sites. The diffusion of Ni and Mn ions into the Li4Ti5O12 lattice occurred during the heat treatment when preparing the cathode. The doping of Li4Ti5O12 with the open d-shell ions Ni(2+) (d(8)) and Mn(4+) (d(3)) should increase the electronic conductivity of the coating significantly, as was found in previous studies. The complex signal structure of the Ti 2p, Ni 2p and Mn 2p core levels provides insight into the chemical nature of the transition metal ions.

ConspectusCarbon-based nanostructures have attracted tremendous interest because of their versatile and tunable properties, which depend on the bonding type of the constituting carbon atoms. Graphene, as the most prominent representative of the π-conjugated carbon-based materials, consists entirely of sp2-hybridized carbon atoms and exhibits a zero band gap. Recently, countless efforts were made to open and tune the band gap of graphene for its applications in semiconductor devices. One promising method is periodic perforation, resulting in a graphene nanomesh (GNM), which opens the band gap while maintaining the exceptional transport properties. However, the typically employed lithographic approach for graphene perforation is difficult to control at the atomic level. The complementary bottom-up method using surface-assisted carbon–carbon (C–C) covalent coupling between organic molecules has opened up new possibilities for atomically precise fabrication of conjugated nanostructures like GNM and graphene n...

The temperature-controlled surface-assisted synthesis of halogen bonded, organometallic, and covalent nanostructures based on 1,3,5-tribromo-benzene (TriBB) was studied with scanning tunneling microscopy and X-ray photoemission spectroscopy in ultrahigh vacuum. Vapor deposition of TriBB onto a Cu(111) surface held at 90 K leads to the formation of large domains of a honeycomb-like organic monolayer structure stabilized by triangular nodes with Br⋯Br intermolecular bonds. Upon annealing the organic monolayer to ∼140 K, a new hexagonal close-packed structure with intact TriBB molecules connected by Cu adatoms is formed. Further warming up the sample to 300 K gives rise to the scission of C–Br bonds and formation of C–Cu–C bonds between phenyl fragments such that stable dendritic organometallic networks are formed. Larger islands of organometallic networks are obtained by maintaining the temperature of Cu(111) at 420 K during deposition of TriBB. Simultaneously, large islands of Br atoms are formed around the organometallic networks. Annealing the more extended organometallic network (prepared at 420 K) to 520 K leads to the formation of a branched covalent organic framework (COF) which comprises structural elements of porous graphene and is surrounded by Br islands. These organometallic networks and COFs appear as small dendritic and branched domains, most likely due to the steric influence exerted by the Br islands.

The redox reaction of superoxide (KO2) with highly charged iron porphyrins (Fe(P4+), Fe(P8+), and Fe(P8-)) has been investigated in the ionic liquids (IL) [EMIM][Tf2N] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [EMIM][B(CN)4] (1-ethyl-3-methylimidazolium tetracyanoborate) by using time-resolved UV/vis stopped-flow, electrochemistry, cryospray mass spectrometry, EPR, and XPS measurements. Stable KO2 solutions in [EMIM][Tf2N] can be prepared up to a 15 mM concentration and are characterized by a signal in EPR spectrum at g = 2.0039 and by the 1215 cm(-1) stretching vibration in the resonance Raman spectrum. While the negatively charged iron porphyrin Fe(P8-) does not react with superoxide in IL, Fe(P4+) and Fe(P8+) do react in a two-step process (first a reduction of the Fe(III) to the Fe(II) form, followed by the binding of superoxide to Fe(II)). In the reaction with KO2, Fe(P4+) and Fe(P8+) show similar rate constants (e.g., in the case of Fe(P4+): k1 = 18.6 ± 0.5 M(-1) s(-1) for the first reaction step, and k2 = 2.8 ± 0.1 M(-1) s(-1) for the second reaction step). Notably, these rate constants are four to five orders of magnitude lower in [EMIM][Tf2N] than in conventional solvents such as DMSO. The influence of the ionic liquid is also apparent during electrochemical experiments, where the redox potentials for the corresponding Fe(III)/Fe(II) couples are much more negative in [EMIM][Tf2N] than in DMSO. This modified redox and kinetic behavior of the positively charged iron porphyrins results from their interactions with the anions of the ionic liquid, while the nucleophilicity of the superoxide is reduced by its interactions with the cations of the ionic liquid. A negligible vapor pressure of [EMIM][B(CN)4] and a sufficient enrichment of Fe(P8+) in a close proximity to the surface enabled XPS measurements as a case study for monitoring direct changes in the electronic structure of the metal centers during redox processes in solution and at liquid/solid interfaces.

The formation, structure, and dynamics of planar organometallic macrocycles (meta-terphenyl-Cu)n and zigzag-shaped one-dimensional organometallic polymers on a Cu(111) surface were studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 4,4″-dibromo-meta-terphenyl (DMTP) onto Cu(111) at 300 K leads to C-Br bond scission and formation of C-Cu-C bonds, which connect neighboring meta-terphenyl fragments such that room-temperature stable macrocycles and zigzag chains are formed. The chains self-assemble to form islands, which are elongated in the direction of the chains. If DMTP is deposited onto Cu(111) held at 440 K, the island size is drastically increased (200 × 200 nm(2)). STM sequences show the formation of ordered structures through reversible scission and reformation of the C-Cu-C bonds. The cyclic organometallic species such as the hexamer (meta-terphenyl-Cu)6 may represent intermediates in the surface-confined Ullmann synthesis of hydrocarbon macrocycles such as the recently discovered hyperbenzene.

Graphene, a truly two-dimensional and fully π-conjugated honeycomb carbon network, is currently evolving into the most promising successor to silicon in micro- and nanoelectronic applications. However, its wider application is impeded by the difficulties in opening a bandgap in its gapless band-structure, as well as the lack of processability in the resultant intrinscially insoluble material. Covalent chemical modification of the π-electron system is capable of addressing both of these issues through the introduction of variable chemical decoration. Although there has been significant research activity in the field of functionalized graphene, most work to date has focused on the use of graphene oxide. In this Article, we report on the first wet chemical bulk functionalization route beginning with pristine graphite that does not require initial oxidative damage of the graphene basal planes. Through effective reductive activation, covalent functionalization of the charged graphene is achieved by organic diazonium salts. Functionalization was observed spectroscopically, and successfully prevents reaggregation while providing solubility in common organic media.

We studied the interaction of a highly ordered array of Co-tetraphenylporphyrin (CoTPP) with NO on Ag(111) by in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Upon NO exposure, the initially quadratically ordered CoTPP layer reorganizes, showing a wealth of highly ordered NO+CoTPP coadsorbate phases with increasing size of the unit cell, interpreted as due to attractive lateral dipole−dipole interactions between the two species. The findings not only suggest a novel approach to control the arrangement of adsorbed porphyrins in particular but also should generally be considered in the production of functional layers from large organic molecules under ambient conditions or after exposure to small electronegative molecules.

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.

The adsorption of cobalt (II) octaethylporphyrin (CoOEP) and 2H-octaethylporphyrin (2HOEP) on Ag(111) was investigated with scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS/UPS), in order to achieve a detailed mechanistic understanding of the surface chemical bond of coordinated metal ions. Previous studies of related systems, especially cobalt (II) tetraphenylporphyrin (CoTPP) on Ag(111), have revealed adsorption-induced changes of the oxidation state of the Co ion and the appearance of a new valence state. These effects were attributed to a covalent interaction of the Co ion with the silver substrate. However, recent studies show that the porphyrin ligand of adsorbed CoTPP undergoes a pronounced saddle-shape distortion, which could alter the electronic structure and thus provide an alternative explanation for the new valence state previously attributed to the formation of a surface coordinative bond. With the octaethylporphyrins investigated here, which were found to adsorb in a flat, undistorted conformation on Ag(111), the effects of geometric distortion can be separated from those of the electronic interaction with the substrate. The CoOEP monolayer gives rise to an adsorption-induced shift of the Co 2p signal (−1.9 eV relative to the multilayer), a new valence state at 0.6 eV below the Fermi energy, and a work-function shift of −0.84 eV (2HOEP: −0.44 eV) relative to the clean surface. Comparison with data for the distorted CoTPP confirms the existence of a covalent ion–surface interaction that is insensitive to the conformation of the ligand.