Ullmann-type coupling of brominated tetrathienoanthracene on copper and silver

Construction of Two-Dimensional Organometallic Coordination Networks with Both Organic Kagome and Semiregular Metal Lattices on Au(111)

Two-dimensional metal-organic networks (2D MONs) having heterogeneous coordination nodes (HCNs) could exhibit excellent performance in catalysis and optoelectronics because of the unbalanced electron distribution of the coordinating metals. Therefore, the design and construction of 2D MONs with HCNs are highly desirable but remain challenging. Here, we report the construction of 2D organometallic coordination networks with an organic Kagome lattice and a semiregular metal lattice on Au(111) via the in situ formation of HCNs. Using a bifunctional precursor 1,4-dibromo-2,5-diisocyanobenzene, the coordination of isocyano with Au adatom on a room-temperature Au(111) yielded metal-organic coordination chains with isocyano-Au-isocyano nodes. In contrast, on a high-temperature Au(111), a selective debromination/coordination cascade reaction occurred, affording 2D organometallic coordination networks with phenyl-Au-isocyano nodes. By combining scanning tunneling microscopy and density functional theory calculations, we determined the structures of coordination products and the nature of coordination nodes, demonstrating a thermodynamically favorable pathway for forming the phenyl-Au-isocyano nodes.

Iodine passivation facilitates on-surface synthesis of robust regular conjugated two-dimensional organogold networks on Au(111)

Two-dimensional conjugated organogold networks with anthra-tetrathiophene repeat units are synthesized by thermally activated debrominative coupling of 2,5,9,12-tetrabromoanthra[1,2-b:4,3-b':5,6-b'':8,7-b''']tetrathiophene (TBATT) precursor molecules on Au(111) surfaces under ultra-high vacuum (UHV) conditions. Performing the reaction on iodine-passivated Au(111) surfaces promotes formation of highly regular structures, as revealed by scanning tunneling microscopy (STM). In contrast, coupling on bare Au(111) surfaces results in less regular networks due to the simultaneous expression of competing intermolecular binding motifs in the absence of error correction. The carbon-Au-carbon bonds confer remarkable robustness to the organogold networks, as evidenced by their high thermal stability. In addition, as suggested by density functional theory (DFT) calculations and underscored by scanning tunneling spectroscopy (STS), the organogold networks exhibit a small electronic band gap in the order of 1.0 eV due to their high π-conjugation.

Kinetic control over the chiral-selectivity in the formation of organometallic polymers on a Ag(110) surface

Methods to control chiral-selectivity in molecular reactions through external inputs are of importance, both from a fundamental and technological point of view. Here, the self-assembly of prochiral 6,12-dibromochrysene monomers on Ag(110) is studied using scanning tunneling microscopy. Deposition of the monomers on a substrate held at room temperature leads to the formation of 1D achiral organometallic polymers. When the monomers are instead deposited on a substrate held at 373 K, homochiral organometallic polymers consisting of either the left- or right-handed enantiomer are formed. Post-deposition annealing of room temperature deposited samples at >373 K does not transform the achiral 1D organometallic polymers into homochiral ones and thus, does not yield the same final structure as if depositing onto a substrate held at the same elevated temperature. Furthermore, annealing promotes neither the formation of 1D covalently-coupled polymers nor the formation of graphene nanoribbons. Our results identify substrate temperature as an important factor in on-surface chiral synthesis, thereby demonstrating the importance of considering kinetic effects and the decisive role they can play in structure formation.

Ullmann-Like Covalent Bond Coupling without Participation of Metal Atoms

Ullmann-like on-surface synthesis is one of the most appropriate approaches for the bottom-up fabrication of covalent organic nanostructures and many successes have been achieved. The Ullmann reaction requires the oxidative addition of a catalyst (a metal atom in most cases): the metal atom will insert into a carbon-halogen bond, forming organometallic intermediates, which are then reductively eliminated and form C-C covalent bonds. As a result, traditional Ullmann coupling involves reactions of multiple steps, making it difficult to control the final product. Moreover, forming the organometallic intermediates will potentially poison the metal surface catalytic reactivity. In the study, we used the 2D hBN, an atomically thin sp²-hybridized sheet with a large band gap, to protect the Rh(111) metal surface. It is an ideal 2D platform to decouple the molecular precursor from the Rh(111) surface while maintaining the reactivity of Rh(111). We realize an Ullmann-like coupling of a planar biphenylene-based molecule, i.e., 1,8-dibromobiphenylene (BPBr₂), on an hBN/Rh(111) surface with an ultrahigh selectivity of the biphenylene dimer product, containing 4-, 6-, and 8-membered rings. The reaction mechanism, including electron wave penetration and the template effect of the hBN, is elucidated by combining low-temperature scanning tunneling microscopy and density functional theory calculations. Our findings are expected to play an essential role regarding the high-yield fabrication of functional nanostructures for future information devices.

Tuning Dehalogenative Coupling of Br2Py on Bimetallic Templates

Considerable attention has been paid to on-surface Ullmann coupling during the past decade owing to the feasible synthesis of artificial nanostructures. While previous reports mainly concentrated on coupling reactions on single-metal-atom surfaces, herein we present the Ullmann coupling of 2,7-dibromopyrene (Br₂Py) on bimetallic surfaces, Bi-Ag(111) and Bi-Au(111), respectively, with scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). On the Bi-decorated Ag(111), self-assembly of intact Br₂Py is realized due to the reduced activity at the interface. Subsequent annealing promotes the dehalogenation of Br₂Py on Bi-Ag(111), while Bi adatoms do not bring any visible influence on coupling reactions. Furthermore, post-deposition of Bi onto preassembled nanostructures on Ag(111) immediately initiates the Ullmann coupling by inducing more Ag adatoms available on the surface, while stepwise annealing afterward leads to complete polymerization and formation of covalent chains with lateral displacement compared to that on the bare Ag(111), probably due to the space hindrance and confinement. For Bi-Au(111) with the modified reconstruction, higher-temperature annealing is required to trigger Ullmann coupling compared to that on Au(111). The exception is that the C-C coupling reaction remains impervious to Bi adatoms, and recovery of the Bi-Au reconstruction is realized after intensive annealing. In principle, bimetallic surfaces herein present intriguing behavior toward the controllable Ullmann coupling, and this report might provide different insights into the comprehensive atomistic elucidation of reaction mechanisms as well as the design of a new platform to effectively regulate Ullmann coupling.

Monte Carlo Simulations of the Metal-Directed Self-Assembly of Y-Shaped Positional Isomers

The rational fabrication of low-dimensional materials with a well-defined topology and functions is an incredibly important aspect of nanotechnology. In particular, the on-surface synthesis (OSS) methods based on the bottom-up approach enable a facile construction of sophisticated molecular architectures unattainable by traditional methods of wet chemistry. Among such supramolecular constructs, especially interesting are the surface-supported metal–organic networks (SMONs), composed of low-coordinated metal atoms and π-aromatic bridging linkers. In this work, the lattice Monte Carlo (MC) simulation technique was used to extract the chemical information encoded in a family of Y-shaped positional isomers co-adsorbed with trivalent metal atoms on a flat metallic surface with (111) geometry. Depending on the intramolecular distribution of active centers (within the simulated molecular bricks, we observed a metal-directed self-assembly of two-dimensional (2D) openwork patterns, aperiodic mosaics, and metal–organic ladders. The obtained theoretical findings could be especially relevant for the scanning tunneling microscopy (STM) experimentalists interested in a surface-assisted construction of complex nanomaterials stabilized by directional coordination bonds.

Two-Dimensional Polymers and Polymerizations

Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.

Steering Metal-Organic Network Structures through Conformations and Configurations on Surfaces

Molecular adsorption conformations and arrangement configurations on surfaces are important structural aspects of surface stereochemistry, but their roles in steering the structures of metal-organic networks (MONs) remain vague and unexplored. In this study, we constructed MONs by the coordination self-assembly of isocyanides on Cu(111) and Ag(111) surfaces and demonstrated that the MON structures can be steered by surface stereochemistry, including the adsorption conformations of the isocyanide molecules and the arrangement configurations of the coordination nodes and subunits. The coordination self-assembly of 1,4-phenylene diisocyanobenzene afforded a honeycomb MON consisting of 3-fold (isocyano)₃-Cu motifs on a Cu(111) surface. In contrast, geometrically different chevron-shaped 1,3-phenylene diisocyanobenzene (m-DICB) failed to generate a MON, which is ascribable to its standing conformation on the Cu(111) surface. However, m-DICB was adsorbed in a flat conformation on a Ag(111) surface, which has a larger lattice constant than a Cu(111) surface, and smoothly underwent coordination self-assembly to form a MON consisting of (isocyano)₃-Ag motifs. Interestingly, only C₃-Ag nodes with heterotactic configurations could grow into larger subunits; those subunits with heterotactic configurations further grew into Sierpiński triangle fractals (up to fourth order), while subunits with homotactic configurations afforded a triangular MON.

Adatoms in the Surface-Confined Ullmann Coupling of Phenyl Groups

Despite the importance of the on-surface Ullmann coupling for synthesis of atomically precise carbon nanostructures, it is still unclear whether this reaction is catalyzed by surface atoms or adatoms. Here, the feasibility of the adatom creation and adatom-catalyzed Ullmann coupling of chloro-, bromo-, and iodobenzene on Cu(111), Ag(111), and Au(111) surfaces is examined using density functional theory modeling. The extraction of a metal atom is found to be greatly facilitated by the formation of strong phenyl-metal bonds, making the extraction energy barrier comparable to, and in the case of Ag(111) even lower than, that for the competing surface-catalyzed phenyl-phenyl bond formation. However, if the phenyl-adatom bonds are too strong, as on Cu(111) and Ag(111), they create an insurmountable barrier for the subsequent adatom-catalyzed C-C coupling. In contrast, Au adatoms do not bind phenyl groups strongly and can catalyze the C-C bond formation almost as efficiently as surface atoms.

Conformational evolution following the sequential molecular dehydrogenation of PMDI on a Cu(111) surface

Molecular spatial conformational evolution following the corresponding chemical reaction pathway at surfaces is important to understand and optimize chemical processes. Combining experimental and theoretical methods, the sequential N-H and C-H dehydrogenation of pyromellitic diimide (PMDI) on a Cu(111) surface are reported. STM experiments and atomistic modeling allow structural analysis at each well-defined reaction step. First, exclusively the aromatic N-H dehydrogenation of the imide group is observed. Subsequently, the C-H group at the benzene core of PMDI gets activated leading to a dehydrogenation reaction forming metalorganic species where Cu adatoms pronouncedly protruding from the surface are coordinated by one or two PMDI ligands at the surface. All reactions of PMDI induce conformational changes at the surface as confirmed by STM imaging and DFT simulations. Such conformational evolution in sequential N-H and C-H activation provides a detailed insight to understand molecular dehydrogenation processes at surfaces.

Spectroscopic Evidence of New Low-Dimensional Planar Carbon Allotropes Based on Biphenylene via On-Surface Ullmann Coupling

The bottom-up synthesis and preliminary characterizations of a new biphenylene-based 2D framework are presented. This new low-dimensional carbon allotrope potentially completes the many hypothesized carbon networks based on biphenylene.

Nano-sheets of two-dimensional polymers with dinuclear (arene)ruthenium nodes, synthesised at a liquid/liquid interface

We developed a new class of mono- or few-layered two-dimensional polymers based on dinuclear (arene)ruthenium nodes, obtained by combining the imine condensation with an interfacial chemistry process, and use a modified Langmuir-Schaefer method to transfer them onto solid surfaces. Robust nano-sheets of two-dimensional polymers including dinuclear complexes of heavy ruthenium atoms as nodes were synthesised. These nano-sheets, whose thickness is of a few tens of nanometers, were suspended onto solid porous membranes. Then, they were thoroughly characterised with a combination of local probes, including Raman spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy in imaging and diffraction mode.

Benzene Ring Knitting Achieved by Ambient-Temperature Dehalogenation via Mechanochemical Ullmann-Type Reductive Coupling

The current approaches capable of affording conjugated porous networks (CPNs) still rely on solution-based coupling reactions promoted by noble metal complexes or Lewis acids, on-surface polymerization conducted in ultrahigh-vacuum environment at very high temperatures (>200 °C), or mechanochemical Scholl-type reactions limited to electron-rich substrates. To develop simple and scalable approaches capable of making CPNs under neat and ambient conditions, herein, a novel and complementary method to the current oxidative Scholl coupling processes is demonstrated to afford CPNs via direct aromatic ring knitting promoted by mechanochemical Ullmann-type reactions. The key to this strategy lies in the dehalogenation of aromatic halides in the presence of Mg involving the formation of Grignard reagent intermediates. Products (Ph-CPN-1) obtained via direct CC bond formation between 1,2,4,5-tetrabromobenzene (TBB) monomer feature high surface areas together with mesoporous architecture. The versatility of this approach is confirmed by the successful construction of various CPNs via knitting of the corresponding aromatic rings (e.g., pyrene and triphenylene), and even highly crystalline graphite product was obtained. The CPNs exhibit good electrochemical performance as the anode material in lithium-ion batteries (LIBs). This approach expands the frontiers of CPN synthesis and provides new opportunities to their scalable applications.

Lateral Interfaces between Monolayer MoS2 Edges and Armchair Graphene Nanoribbons on Au(111)

The realization of electronic devices based on heterostructures of metallic, semiconducting, or insulating two-dimensional materials relies on the ability to form structurally coherent and clean interfaces between them, vertically or laterally. Lateral two-dimensional heterostructures that fuse together two different materials in a well-controlled manner have attracted recent attention, but the methods to form seamless interfaces between structurally dissimilar materials, such as graphene and transition-metal dichalcogenides (TMDCs), are still limited. Here, we investigate the structure of the lateral interfaces that arise between monolayer MoS₂ flakes on Au(111) and two families of armchair graphene nanoribbons (GNRs) created through on-surface assisted Ullmann coupling using regular organobromine precursors for GNR synthesis. We find that parallel alignment between the GNR armchair edge and MoS₂ leads to van der Waals bonded nanoribbons, whereas a perpendicular orientation is characterized by a single phenyl-group of the GNR covalently bonded to S on the edge. The edge-on bonding is facilitated by a hydrogen treatment of the MoS₂, and temperature control during growth is shown to influence the nanoribbon width and the yield of covalently attached nanoribbons. Interestingly, the temperatures needed to drive the intramolecular dehydrogenation during GNR formation are lowered significantly by the presence of MoS₂, which we attribute to enhanced hydrogen recombination at the MoS₂ edges. These results are a demonstration of a viable method to make laterally bonded graphene nanostructures to TMDCs to be used in further investigations of two-dimensional heterostructure junctions.

Designing 2D covalent networks with lattice Monte Carlo simulations: precursor self-assembly

Organic synthesis reactions in the adsorbed phase have been recently an intensively studied topic in heterogeneous catalysis and material engineering. One of such processes is the Ullmann coupling in which halogenated organic monomers are transformed into covalently bonded polymeric structures. In this work, we use the lattice Monte Carlo simulation method to study the on-surface self-assembly of organometallic precursor architectures comprising tetrasubstituted naphthalene building blocks with differently distributed halogen atoms. In the coarse grained approach adopted herein the molecules and metal atoms were modeled by discrete segments, two connected and one, respectively, placed on a triangular lattice representing a (111) metallic surface. Our simulations focused on the influence of the intramolecular distribution of the substituents on the morphology of the resulting superstructures. Special attention was paid to the molecules that create porous networks characterized by long-range order. Moreover, the structural analysis of the assemblies comprising prochiral building blocks was made by running simulations for the corresponding enantiopure and racemic adsorbed systems. The obtained results demonstrated the possibility of directing the on-surface self-assembly towards networks with controllable pore shape and size. These findings can be helpful in designing covalently bonded 2D superstructures with predefined architecture and functions.

The use of main-group elements to mimic catalytic behavior of transition metals I: reduction of dinitrogen to ammonia catalyzed by bis(Lewis base)borylenium diradicals

The use of boron (B) atoms as transition metal mimics opens the door to new research in catalytic chemistry. An emerging class of compounds, bis(Lewis base)borylenes with an electron-rich B(i) center, are potential metal-free catalysts for dinitrogen bonding and reduction. Here, the molecular geometry, electronic structure, and possible reaction mechanism of a series of bis(Lewis base)borylene-dinitrogen compounds corresponding to the nitrogen reduction reaction have been investigated by using density functional theory (DFT) calculations. Our DFT calculations show that these free borylene compounds possess radical features and have the capability to activate N2 molecules via an effective combination of π(B → N2), π(N2 → B), and σ(N2 → B) electron transfer processes. The possible reaction mechanisms for direct conversion of N2 into NH3 for these bis(Lewis base)borylene-dinitrogen compounds have been systematically investigated along distal and alternating paths. The calculated free energy profiles indicate that the limiting potential of a bis(phosphine)borylene-dinitrogen compound is comparable to that of metal-based catalysts, which is the most promising candidate for the reduction of N2 to NH3via the alternating mechanism among all compounds studied here. The electronic structure analysis shows that the B center plays the role of an electron donor and acceptor alternatively in the consecutive six protonation and reduction processes, and thus acts as the electron transfer medium.

Anisotropic Electron Conductance Driven by Reaction Byproducts on a Porous Network of Dibromobenzothiadiazole on Cu(110)

Efficiency in charge-transport is a fundamental but demanding prerequisite to allow better exploitation of molecular functionalities in organic electronics and energy-conversion systems. Here, we report on a mechanism that enables a one-dimensional conductance structure by connecting discrete molecular states at 2.1 eV through the pores of a metal-organic network on Cu(110). Two adjacent, periodic and isoenergetic contributions, namely a molecular resonance and the confined surface-state, add-up leading to anisotropic structures, as channels, observable in real-space conductance images. The adsorption configurations of Br atoms, inorganic byproduct of the redox-reacted 4,7-dibromobenzo[c]-1,2,5-thiadiazole (2Br-BTD) molecules on the copper surface, drive the confinement of the Cu surface state within the pores and critically control the channel continuity. Small displacements of the Br atoms change the local surface potential misaligning the energy levels. This work visualizes the effect of order-disorder transitions caused by the movement of single atoms in the electronic properties of two-dimensional organic networks.

Oxygen-Induced 1D to 2D Transformation of On-Surface Organometallic Structures

While surface-confined Ullmann-type coupling has been widely investigated for its potential to produce π-conjugated polymers with unique properties, the pathway of this reaction in the presence of adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″-dibromo-p-terphenyl or 1,4-dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host-guest chemistry.

Simulations of the 2D self-assembly of tripod-shaped building blocks

We introduce a molecular dynamics (MD) coarse-grained model for the description of tripod building blocks. This model has been used by us already for linear, V-shape, and tetratopic molecules. We wanted to further extend its possibilities to trifunctional molecules to prove its versatility. For the chosen systems we have also compared the MD results with Monte Carlo results on a triangular lattice. We have shown that the constraints present in the latter method can enforce the formation of completely different structures, not reproducible with off-lattice simulations. In addition to that, we have characterized the obtained structures regarding various parameters such as theoretical diffraction pattern and average association number.

Temperature-induced molecular reorganization on Au(111) driven by oligomeric defects

The formation of ordered molecular structures on surfaces is determined by the balance between molecule-molecule and molecule-substrate interactions. Whether the aggregation process is guided by non-covalent forces or on-surface reactions, a deeper understanding of these interactions is pivotal to formulating a priori predictions of the final structural features and the development of bottom-up fabrication protocols. Theoretical models of molecular systems corroborate the information gathered through experimental observations and help explain the thermodynamic factors that underpin on-surface phase transitions. Here, we report a scanning tunneling microscopy investigation of a tribromo-substituted heterotriangulene on the Au(111) surface, which initially forms an extended close-packed ordered structure stabilized by BrBr halogen bonds when deposited at room temperature. X-ray photoelectron spectroscopy reveals that annealing the self-assembled layer induces a fraction of the molecular precursors to partially dehalogenate that in turn leads to the formation of a less stable BrO non-covalent network which coexists with the short oligomers. Density functional theory (DFT) and Monte Carlo (MC) simulations illustrate how dimer moieties act as defects whose steric hindrance prevents the retention of the more stable configuration. A small number of dimers is sufficient to drive the molecular reorganization into a lower cohesive energy phase. Our study shows the importance of a combined DFT - MC approach to understand the evolution of molecular systems on substrates.

On-surface synthesis of one-type pore single-crystal porous covalent organic frameworks

Employing a 1,3,5-tris(4-bromophenyl)benzene precursor as a building block, we successfully fabricate large-scale, non-multihole and single-layer pCOFs on the Ag(111) surface in a controllable manner via the on-surface reaction. We reveal that two main factors, the heating rate and growth temperature, have a strong impact on the size and quality of the pCOFs by STM. Furthermore, the band gap of the pCOFs has been further measured to be approximately 3.01 eV.

Jahn-Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW₁₁O₃₉M(N₂)] ^n- (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N₂ molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N₂ moiety indicate that Mo^II-, Tc^II-, W^II-, Re^II-, and Os^II-POM complexes are significant for binding and activation of the inert N₂ molecule. The degree of the N₂ activation can be classified into the "moderately activated" category according to Tuczek's sense [ J. Comput. Chem. 2006 , 27 , 1278 ]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N₂ molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between σ_2s* orbital of N₂ and d _z² orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N₂ into NH₃ has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.

Surface-mediated assembly, polymerization and degradation of thiophene-based monomers

Ullmann coupling of halogenated aromatics is widely used in on-surface synthesis of two-dimensional (2D) polymers and graphene nanoribbons. It stands out among other reactions for regioselectively connecting aromatic monomers into 1D and 2D π-conjugated polymers, whose final structure and properties are determined by the initial building blocks. Thanks to their exceptional electronic properties, thiophene-containing monomers are frequently used for the synthesis of various conjugated materials. On the other hand, their use in on-surface polymerization is hampered by the possibility of ring opening when adsorbed on metal surfaces. In the present work, we mapped the temperature regime for these two competing reactions by investigating the adsorption of a thiophene-based prochiral molecule using scanning tunneling microscopy, X-ray photoelectron spectroscopy and density functional theory calculations. We followed the formation of organometallic (OM) networks, their evolution into covalent structures and the competition between C-C coupling and thiophene ring opening. The effect of surface reactivity was explored by comparing the adsorption on three (111) coinage metal substrates, namely Au, Ag and Cu. While outlining strategies to minimize the ring opening reaction, we found that the surface temperature during deposition is of paramount importance for the preparation of 2D OM networks, greatly enhancing the overall ordering of the product by depositing on hot Ag surface. Notably, the same protocol permits the creation of OM structures on the air-stable Au surface, thereby allowing the synthesis and application of 2D OM networks outside the ultra-high vacuum environment.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Recent Advances of Hierarchical and Sequential Growth of Macromolecular Organic Structures on Surface

The fabrication of macromolecular organic structures on surfaces is one major concern in materials science. Nanoribbons, linear polymers, and porous nanostructures have gained a lot of interest due to their possible applications ranging from nanotemplates, catalysis, optoelectronics, sensors, or data storage. During decades, supramolecular chemistry has constituted an unavoidable approach for the design of well-organized structures on surfaces displaying a long-range order. Following these initial works, an important milestone has been established with the formation of covalent bonds between molecules. Resulting from this unprecedented approach, various nanostructures of improved thermal and chemical stability compared to those obtained by supramolecular chemistry and displaying unique and unprecedented properties have been developed. However, a major challenge exists: the growth control is very delicate and a thorough understanding of the complex mechanisms governing the on-surface chemistry is still needed. Recently, a new approach consisting in elaborating macromolecular structures by combining consecutive steps has been identified as a promising strategy to elaborate organic structures on surface. By designing precursors with a preprogrammed sequence of reactivity, a hierarchical or a sequential growth of 1D and 2D structures can be realized. In this review, the different reaction combinations used for the design of 1D and 2D structures are reported. To date, eight different sequences of reactions have been examined since 2008, evidencing the intense research activity existing in this field.

High-Yield Formation of Graphdiyne Macrocycles through On-Surface Assembling and Coupling Reaction

Rationally designed halogenated hydrocarbons are widely used building blocks to fabricate covalent-bonded carbon nanostructures on surfaces through a reaction pathway involving generation and dissociation of organometallic intermediates and irreversible covalent bond formation. Here, we provide a comprehensive picture of the on-surface-assisted homocoupling reaction of 1,3-bis(2-bromoethynyl)benzene on Au(111), aiming for the synthesis of graphdiyne nanostructures. Submolecular resolution scanning tunneling microscopy and noncontact atomic force microscopy observations identify the organometallic intermediates and their self-assemblies formed in the dehalogenation process. The demetallization of the organometallic intermediates at increased temperatures produces butadiyne moieties that spontaneously formed two different covalent structures ( i.e., graphdiyne zigzag chains and macrocycles), whose ratio was found to depend on the initial coverage of organometallic intermediates. At the optimal condition, the stepwise demetallization and cyclization led to a high-yield production of graphdiyne macrocycles up to 95%. Statistical analysis and theoretical calculations suggested that the favored formation of macrocycles resulted from the complex interplay between thermodynamic and kinetic processes involving the organometallic bonded intermediates and the covalently bonded butadiyne moieties.

Tuning the conductance of a molecular wire by the interplay of donor and acceptor units

We investigate the conductance of optimized donor-acceptor-donor molecular wires obtained by on-surface synthesis on the Au(111) surface. A careful balance between acceptors and donors is achieved using a diketopyrrolopyrrole acceptor and two thiophene donors per unit along the wire. Scanning tunneling microscopy imaging, spectroscopy, and conductance measurements done by pulling a single molecular wire at one end are presented. We show that the conductance of the obtained wires is among the highest reported so far in a tunneling transport regime, with an inverse decay length of 0.17 Å-1. Using complex band structure calculations, different donor and acceptor groups are discussed, showing how a balanced combination of donor and acceptor units along the wire can further minimize the decay of the tunneling current with length.

On-surface self-assembly of tetratopic molecular building blocks

Self-assembly of functional molecules on solid substrates has recently attracted special attention as a versatile method for the fabrication of low dimensional nanostructures with tailorable properties. In this contribution, using theoretical modeling, we demonstrate how the architecture of 2D molecular assemblies can be predicted based on the individual properties of elementary building blocks at play. To that end a model star-shaped tetratopic molecule is used and its self-assembly on a (111) surface is simulated using the lattice Monte Carlo method. Several test cases are studied in which the molecule bears terminal arm centers providing interactions with differently encoded directionality. Our theoretical results show that manipulation of the interaction directions can be an effective way to direct the self-assembly towards extended periodic superstructures (2D crystals) as well as to create assemblies characterized by a lower degree of order, including glassy overlayers and quasi one-dimensional molecular connections. The obtained structures are described and classified with respect to their main geometric parameters. A small library of the tetratopic molecules and the corresponding superstructures is provided to categorize the structure-property relationship in the modeled systems. The results of our simulations can be helpful to 2D crystal engineering and surface-confined polymerization techniques as they give hints on how to functionalize tetrapod organic building blocks which would be able to create superstructures with predefined spatial organization and range of order.

Aryl Radical Geometry Determines Nanographene Formation on Au(111)

The Ullmann coupling has been used extensively as a synthetic tool for the formation of C-C bonds on surfaces. Thus far, most syntheses made use of aryl bromides or aryl iodides. We investigated the applicability of an aryl chloride in the bottom-up assembly of graphene nanoribbons. Specifically, the reactions of 10,10'-dichloro-9,9'-bianthryl (DCBA) on Au(111) were studied. Using atomic resolution non-contact AFM, the structure of various coupling products and intermediates were resolved, allowing us to reveal the important role of the geometry of the intermediate aryl radicals in the formation mechanism. For the aryl chloride, cyclodehydrogenation occurs before dehalogenation and polymerization. Due to their geometry, the planar bisanthene radicals display a different coupling behavior compared to the staggered bianthryl radicals formed when aryl bromides are used. This results in oligo- and polybisanthenes with predominantly fluoranthene-type connections.

Remote functionalization in surface-assisted dehalogenation by conformational mechanics: organometallic self-assembly of 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl on Ag(111)

Even though the surface-assisted dehalogenative coupling constitutes the most abundant protocol in on-surface synthesis, its full potential will only become visible if selectivity issues with polybrominated precursors are comprehensively understood, opening new venues for both organometallic self-assembly and on-surface polymerization. Using the 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl (Br4F6BP) at Ag(111), we demonstrate a remote site-selective functionalization at room temperature and a marked temperature difference in double- vs. quadruple activation, both phenomena caused by conformational mechanical effects of the precursor-surface ensemble. The submolecularly resolved structural characterization was achieved by Scanning Tunneling Microscopy, the chemical state was quantitatively assessed by X-ray Photoelectron Spectroscopy, and the analysis of the experimental signatures was supported through first-principles Density-Functional Theory calculations. The non-planarity of the various structures at the surface was specifically probed by additional Near Edge X-ray Absorption Fine Structure experiments. Upon progressive heating, Br4F6BP on Ag(111) shows the following unprecedented phenomena: (1) formation of regular organometallic 1D chains via remote site-selective 3,5'-didebromination; (2) a marked temperature difference in double- vs. quadruple activation; (3) an organometallic self-assembly based on reversibility of C-Ag-C linkages with a thus far unknown polymorphism affording both hexagonal and rectangular 2D networks; (4) extraordinary thermal stability of the organometallic networks. Controlled covalent coupling at the previously Br-functionalized sites was not achieved for the Br4F6BP precursor, in contrast to the comparatively studied non-fluorinated analogue.

α-Diazo Ketones in On-Surface Chemistry

Polymerization of a biphenyl bis α-diazo ketone on Cu(111) and Au(111) surfaces to provide furandiyl bridged poly-para-phenylenes is reported. Polymerization on Cu(111) occurs via initial N₂ fragmentation leading to Cu-biscarbene complexes at room temperature as polymeric organometallic structure. At 135 °C, carbene coupling affords polymeric α,β-unsaturated 1,4-diketones, while analogous alkene formation on the Au(111) surface occurs at room temperature. Further temperature increase leads to deoxygenative cyclization of the 1,4-diketone moieties to provide alternating furandiyl biphenyl copolymers on Cu(111) (165 °C) and Au(111) (240 °C) surfaces. This work shows a new approach to generate Cu-biscarbene intermediates on surfaces, opening the pathway for the controlled generation of biphenyl copolymers.

On-surface synthesis of polyethylenedioxythiophene

On-surface synthesis of conjugated polymers is made challenging by the need to promote the desired reaction while preventing or minimizing unwanted ancillary reactions that compromise the product integrity. We perform a comprehensive study of the reactions of 2,5-dichloro-3,4-ethylenedioxythiophene on coinage metal surfaces, and demonstrate that only on Ag(111) can we obtain a planar polymer product, polyethylenedioxythiophene (PEDOT).

On-surface synthesis on a bulk insulator surface

On-surface synthesis has rapidly emerged as a most promising approach to prepare functional molecular structures directly on a support surface. Compared to solution synthesis, performing chemical reactions on a surface offers several exciting new options: due to the absence of a solvent, reactions can be envisioned that are otherwise not feasible due to the insolubility of the reaction product. Perhaps even more important, the confinement to a two-dimensional surface might enable reaction pathways that are not accessible otherwise. Consequently, on-surface synthesis has attracted great attention in the last decade, with an impressive number of classical reactions transferred to a surface as well as new reactions demonstrated that have no classical analogue. So far, the majority of the work has been carried out on conducting surfaces. However, when aiming for electronic decoupling of the resulting structures, e.g. for the use in future molecular electronic devices, non-conducting surfaces are highly desired. Here, we review the current status of on-surface reactions demonstrated on the (10.4) surface of the bulk insulator calcite. Besides thermally induced C-C coupling of halogen-substituted aryls, photochemically induced [2  +  2] cycloaddition has been proven possible on this surface. Moreover, experimental evidence exists for coupling of terminal alkynes as well as diacetylene polymerization. While imaging of the resulting structures with dynamic atomic force microscopy provides a direct means of reaction verification, the detailed reaction pathway often remains unclear. Especially in cases where the presence of metal atoms is known to catalyze the corresponding solution chemistry reaction (e.g. in the case of the Ullmann reaction), disclosing the precise reaction pathway is of importance to understand and generalize on-surface reactivity on a bulk insulator surface. To this end, density-functional theory calculations have proven to provide atomic-scale insights that have greatly contributed to unravelling the details of on-surface synthesis on a bulk insulator surface.

Controlling selectivity in the Ullmann reaction on Cu(111)

Using a surface science approach, the selectivity in the Ullmann cross-coupling of aryl halides on Cu(111) has been understood and controlled. The binding strength of the reactants and repulsion between them dictates which organometallic intermediates form, and hence the product distribution. Cross coupling can be maximized at low reactant concentrations.

Synthesis of armchair graphene nanoribbons from the 10,10'-dibromo-9,9'-bianthracene molecules on Ag(111): the role of organometallic intermediates

We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules on Ag(111) with the focus on the role of the organometallic (OM) intermediates. It is demonstrated that DBBA molecules on Ag(111) are partially debrominated at room temperature and lose all bromine atoms at elevated temperatures. Similar to DBBA on Cu(111), debrominated molecules form OM chains on Ag(111). Nevertheless, in contrast with the Cu(111) substrate, formation of polyanthracene chains from OM intermediates via an Ullmann-type reaction is feasible on Ag(111). Cleavage of C–Ag bonds occurs before the thermal threshold for the surface-catalyzed activation of C–H bonds on Ag(111) is reached, while on Cu(111) activation of C–H bonds occurs in parallel with the cleavage of the stronger C–Cu bonds. Consequently, while OM intermediates obstruct the Ullmann reaction between DBBA molecules on the Cu(111) substrate, they are required for the formation of polyanthracene chains on Ag(111). If the Ullmann-type reaction on Ag(111) is inhibited, heating of the OM chains produces nanographenes instead. Heating of the polyanthracene chains produces 7-AGNRs, while heating of nanographenes causes the formation of the disordered structures with the possible admixture of short GNRs.

Stabilizing surface Ag adatoms into tunable single atom arrays by terminal alkyne assembly

Ordered two-dimensional arrays of silver adatoms with tunable metal atom density stabilized by 1,4-diethynyl-2,5-dimethylbenzene, a terminal alkyne, were prepared on Ag(111) and scrutinized by scanning tunneling microscopy and density functional theory calculations. Stabilization of the adatom arrays was attributed to the substrate-mediated electron localizations of the Ag adatom and terminal alkynyl in the molecule.

Synthesis of corrugated C-based nanostructures by Br-corannulene oligomerization

The structure and electronic properties of carbon-based nanostructures obtained by metal surface assisted synthesis is highly dependent on the nature of the precursor molecule.

(Metallo)porphyrins for potential materials science applications

The bottom-up approach to replace existing devices by molecular-based systems is a subject that attracts permanently increasing interest. Molecular-based devices offer not only to miniaturize the device further, but also to benefit from advanced functionalities of deposited molecules. Furthermore, the molecules itself can be tailored to allow via their self-assembly the potential fabrication of devices with an application potential, which is still unforeseeable at this time. Herein, we review efforts to use discrete (metallo)porphyrins for the formation of (sub)monolayers by surface-confined polymerization, of monolayers formed by supramolecular recognition and of thin films formed by sublimation techniques. Selected physical properties of these systems are reported as well. The application potential of those ensembles of (metallo)porphyrins in materials science is discussed.