On-Surface Synthesis and Characterization of a Cycloarene: C108 Graphene Ring.

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.

The Macrocycle versus Chain Competition in On-Surface Polymerization: Insights from Reactions of 1,3-Dibromoazulene on Cu(111).

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.

Nanoribbons with Nonalternant Topology from Fusion of Polyazulene: Carbon Allotropes beyond Graphene.

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.

Topology-Selective Ullmann Coupling on Metal Surfaces by Precursor Design and Adsorbate-Substrate Interaction: Towards the Control over Polymer versus Macrocycle Formation.

In polymerization reactions, controlling the formation of open-chain versus cyclic product topologies is necessary because of the different properties of polymer chains and macrocycles. Here, we report a topology-selective Ullmann coupling on metal surfaces with control of the ring/chain competition. The precursor employed is 4,4''-dibromo-ortho-terphenyl (DBOTP), which features a 60° bent feature and polymerizes into zigzag polyphenylene chains on both Au(111) and Ag(111) surface via Ullmann coupling. However, the cyclotrimerization of the precursor occurs only on Ag(111) but not Au(111). It is proposed that the cyclotrimerization reaction on Au(111) is blocked, because the necessary C-C coupling of two carbon radicals with different vertical heights is unfavored. Such height difference stems from the intrinsic steric repulsion between the two ortho-substituted phenyl groups. On Ag(111), the stronger adsorbate-substrate interaction reduces the extent of the tilting of the phenyl group, resulting in a smaller height difference of the carbon radicals and consequently in the increased probability for the formation of the cyclic trimer.

Precise Monoselective Aromatic C-H Bond Activation by Chemisorption of Meta-Aryne on a Metal Surface.

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.

Kinetic Strategies for the Formation of Graphyne Nanowires via Sonogashira Coupling on Ag(111).

The selection of a reaction pathway with high energy barrier in a multipath on-surface reaction system has been challenging. Herein, we report the successful control of the reaction system of 1,1'-biphenyl-4-bromo-4'-ethynyl (BPBE) on Ag(111), in which three coupling reactions (Glaser, Ullman, Sonogashira) are involved. Either graphdiyne (GDY) or graphyne (GY) nanowires can be formed by distinct kinetic strategies. As the energetically favorable pathway, the formation of a GDY nanowire is achieved by hierarchical activation of Glaser (with lowest energy barrier) and Ullman coupling of BPBE. On the other hand, the formation of a GY nanowire originates from the high selectivity of the high-barrier Sonogashira coupling, whose indispensable kinetic parameters are high surface temperature, low molecular coverage, and low precursor evaporation rate, as derived from a series of control experiments. This work achieves the fabrication of GY nanowires via on-surface Sonogashira coupling for the first time and reveals mechanistic control strategies for potential syntheses of other functional nanostructures via cross-couplings on surfaces.

Chiral Kagome Lattices from On-Surface Synthesized Molecules.

Kagome lattices have attracted much attention owing to their potential applications in spin-frustrated magnetism and host-guest chemistry. Examples toward the fabrication of 2D Kagome lattices reported previously have in common that the precursor molecules were typically deposited on the surface structurally intact with no chemical reactions accompanied. Herein, by using a combination of synchrotron radiation photoelectron spectroscopy (SRPES) and scanning tunneling microscopy (STM), we demonstrated the fabrication of two types of chiral Kagome lattices from on-surface synthesized organometallic compounds, which are known as intermediates of Glaser coupling on silver single crystal surfaces. These Kagome lattices are stabilized by the interplay of various intermolecular interactions, including Br⋅⋅⋅Br bonds, C-Br⋅⋅⋅π bonds and π-π stacking. The chiral transference and host-guest supramolecular structure in the novel Kagome lattices were also studied. Our studies may pave a new way to engineer complex supramolecular networks through on-surface reactions.

Highly Selective Synthesis of cis-Enediynes on a Ag(111) Surface.

Cis-enediyne-type compounds have received much attention as potent antitumor antibiotics. The conventional synthesis of cis-enediynes in solution typically involves multiple steps and various side reactions. For the first time, selective one-step synthesis of cis-enediyne from a single reactant is reported on a Ag(111) surface with a yield up to 90 %. High selectivity for the formation of cis-enediyne originates from the steric effect posed by weak intermolecular interactions, which protect the cis-enediyne from further reaction. A series of comparative experiments and DFT-based transition-state calculations support the findings. The described synthetic approach for directing reaction pathways on-surface may illuminate potential syntheses of other unstable organic compounds.

Surface-catalyzed C-C covalent coupling strategies toward the synthesis of low-dimensional carbon-based nanostructures.

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.

The role of the substrate structure in the on-surface synthesis of organometallic and covalent oligophenylene chains.

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.

Tribromobenzene on Cu(111): Temperature-dependent formation of halogen-bonded, organometallic, and covalent nanostructures.

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.

Surface-assisted organic synthesis of hyperbenzene nanotroughs.

A hexagonal macrocycle consisting of 18 phenylene units (hyperbenzene) was synthesized on a Cu(111) surface in ultrahigh vacuum by Ullmann coupling of six 4,4''-dibromo-m-terphenyl molecules. The large diameter of 21.3 Šand the ability to assemble in arrays makes hyperbenzene an interesting candidate for a nanotrough that could enclose metallic, semiconducting, or molecular quantum dots.

Steering On-Surface Reactions by Kinetic and Thermodynamic Strategies.

On-surface synthesis has emerged as a powerful tool to fabricate various functional low-dimensional nanostructures with atomic precision, thus becoming a promising platform for the preparation of next-generation semiconductive, magnetic, and topological nanodevices. With the aid of scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy, both the chemical structures and physical properties of the obtained products can be well characterized. A major challenge in this field is how to efficiently steer reaction pathways and improve the yield/quality of products. To address this problem, in recent years various kinetic and thermodynamic strategies have been successfully employed to control on-surface reactions. In this Perspective, we discuss these strategies in view of basic reaction steps on surfaces, including molecular adsorption, diffusion, and reaction. We hope this Perspective will help readers to deepen the understanding of the mechanisms of on-surface reactions and rationally design reaction procedures for the fabrication of high-quality functional nanomaterials on surfaces.

Unveiling the formation mechanism of the biphenylene network.

We have computationally studied the formation mechanism of the biphenylene network via the intermolecular HF zipping, as well as identified key intermediates experimentally, on the Au(111) surface. We elucidate that the zipping process consists of a series of defluorinations, dehydrogenations, and C-C coupling reactions. The Au substrate not only serves as the active site for defluorination and dehydrogenation, but also forms C-Au bonds that stabilize the defluorinated and dehydrogenated phenylene radicals, leading to "standing" benzyne groups. Despite that the C-C coupling between the "standing" benzyne groups is identified as the rate-limiting step, the limiting barrier can be reduced by the adjacent chemisorbed benzyne groups. The theoretically proposed mechanism is further supported by scanning tunneling microscopy experiments, in which the key intermediate state containing chemisorbed benzyne groups can be observed. This study provides a comprehensive understanding towards the on-surface intermolecular HF zipping, anticipated to be instructive for its future applications.

On-surface porphyrin transmetalation with Pb/Cu redox exchange.

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.

Biphenylene network: A nonbenzenoid carbon allotrope.

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.

Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization.

We revisit the question of kekulene's aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene's highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C-C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.

Template-controlled on-surface synthesis of a lanthanide supernaphthalocyanine and its open-chain polycyanine counterpart.

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.

Organometallic ring vs. chain formation beyond kinetic control: steering their equilibrium in two-dimensional confinement.

Control over the competition between an organometallic hexamer macrocycle and oligomer chains formed from the non-alternant aromatic 1,3-dibromoazulene (DBAz) precursor has been achieved in surface-assisted synthesis on a copper(111) surface. In contrast to kinetic reaction control via the high-dilution principle, the ring formation is achieved here by thermodynamic control, which is based on two-dimensional (2D) confinement and reversible bonds.

Surface Adatom Mediated Structural Transformation in Bromoarene Monolayers: Precursor Phases in Surface Ullmann Reaction.

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.