On-surface polymerization of 1,4-diethynylbenzene on Cu(111)

Scanning Tunneling Spectroscopy Investigation of Au-bis-acetylide Networks on Au(111): The Influence of Metal-Organic Hybridization

Graphdiyne (GDY) is an appealing two-dimensional carbon material, but the on-surface synthesis of a single layer remains challenging. Demetalation of well-crystalline metal acetylide networks, though in its infancy, provides a new avenue to on-surface synthesized GDY substructures. In spite of the synthetic efforts and theoretical concerns, there are few reports steeped in elaborate characterization of the electronic influence of metalation. In this context, we focused on the surface supported Au-bis-acetylide network, which underwent demetalation after further annealing to form hydrogen-substituted GDY. We made a comprehensive study on the geometric structure and electronic structure and the corresponding demetalized structure on Au(111) through STM, noncontact atomic force microscopy (nc-AFM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) simulations. The bandgap of the Au-bis-acetylide network on Au(111) is measured to be 2.7 eV, while the bandgap of a fully demetalized Au-bis-acetylide network is estimated to be about 4.1 eV. Our findings reveal that the intercalated Au adatoms are positioned closer to the metal surface compared with the organic skeletons, facilitating electronic hybridization between the surface state and unoccupied frontier molecular orbitals of organic components. This leads to an extended conjugation through Au-bis-acetylene bonds, resulting in a reduced bandgap.

On-surface synthesis of enetriynes

Belonging to the enyne family, enetriynes comprise a distinct electron-rich all-carbon bonding scheme. However, the lack of convenient synthesis protocols limits the associated application potential within, e.g., biochemistry and materials science. Herein we introduce a pathway for highly selective enetriyne formation via tetramerization of terminal alkynes on a Ag(100) surface. Taking advantage of a directing hydroxyl group, we steer molecular assembly and reaction processes on square lattices. Induced by O₂ exposure the terminal alkyne moieties deprotonate and organometallic bis-acetylide dimer arrays evolve. Upon subsequent thermal annealing tetrameric enetriyne-bridged compounds are generated in high yield, readily self-assembling into regular networks. We combine high-resolution scanning probe microscopy, X-ray photoelectron spectroscopy and density functional theory calculations to examine the structural features, bonding characteristics and the underlying reaction mechanism. Our study introduces an integrated strategy for the precise fabrication of functional enetriyne species, thus providing access to a distinct class of highly conjugated π-system compounds.

Highly Selective On-Surface Reactions of Aryl Propiolic Acids via Decarboxylative Coupling

Aryl propiolic acids are introduced as a new class of monomers in the field of on-surface chemistry to build up poly(arylenebutadiynylenes) through decarboxylative Glaser coupling. As compared to aryl alkynes that are routinely used in the on-surface Glaser coupling, it is found that the decarboxylative coupling occurs at slightly lower temperature and with excellent selectivity. Activation occurs through decarboxylation for the propiolic acids, whereas the classical Glaser coupling is achieved through alkyne CH activation, and this process shows poor selectivity. The efficiency of the decarboxylative coupling is documented by the successful polymerization of bis(propiolic acids) as monomers. It is also found that the new activation mode is compatible with aryl bromide functionalities, which allows the formation of unsymmetric metal-organic polymers on the surface by chemoselective sequential reactions. All transformations are analyzed by a scanning tunneling microscope and are further studied by density functional theory calculations.

The Polymers of Diethynylarenes-Is Selective Polymerization at One Acetylene Bond Possible? A Review

In this review, all available publications on the polymerization of all isomers of bifunctional diethynylarenes due to the opening of C≡C bonds were considered and analyzed. It has been shown that with the use of polymers of diethynylbenzene, heat-resistant and ablative materials, catalysts, sorbents, humidity sensors, and other materials can be obtained. Various catalytic systems and conditions of polymer synthesis are considered. For the convenience of comparison, the publications considered are grouped according to common features, including the types of initiating systems. Critical consideration is given to the features of the intramolecular structure of the synthesized polymers since it determines the entire complex of properties of this material and subsequent materials. Branched and/or insoluble polymers are formed as a result of solid-phase and liquid-phase homopolymerization. It is shown that the synthesis of a completely linear polymer was carried out for the first time by anionic polymerization. The review considers in sufficient detail publications from hard-to-reach sources, as well as publications that required a more thorough critical examination. The review does not consider the polymerization of diethynylarenes with substituted aromatic rings because of their steric restrictions; the diethynylarenes copolymers with complex intramolecular structure; and diethynylarenes polymers obtained by oxidative polycondensation.

Surface-assisted fabrication of low-dimensional carbon-based nanoarchitectures

On-surface synthesis, as an alternative to traditional in-solution synthesis, has become an emerging research field and attracted extensive attention over the past decade due to its ability to fabricate nanoarchitectures with exotic properties. Compared to wet chemistry, the on-surface synthesis conducted on atomically flat solid surfaces under ultrahigh vacuum exhibits unprecedented characteristics and advantages, opening novel reaction pathways for chemical synthesis. Various low-dimensional nanostructures have been fabricated on solid surfaces (mostly metal surfaces) based on this newly developed approach. This paper reviews the classic and latest works regarding carbon-based low-dimensional nanostructures since the arrival of on-surface synthesis era. These nanostructures are categorized into zero-, one- and two-dimensional classes and each class is composed of numerous sub-nanostructures. For certain specific nanostructures, comprehensive reports are given, including precursor design, substrate choice, synthetic strategies and so forth. We hope that our review will shed light on the fabrication of some significant nanostructures in this young and promising scientific area.

Unveiling Adatoms in On-Surface Reactions: Combining Scanning Probe Microscopy with van't Hoff Plots

Scanning probe microscopy has become an essential tool to not only study pristine surfaces but also on-surface reactions and molecular self-assembly. Nonetheless, due to inherent limitations, some atoms or (parts of) molecules are either not imaged or cannot be unambiguously identified. Herein, we discuss the arrangement of two different nonplanar molecular assemblies of para-hexaphenyl-dicarbonitrile (Ph₆(CN)₂) on Au(111) based on a combined theoretical and experimental approach. For deposition of Ph₆(CN)₂ on Au(111) kept at room temperature, a rhombic nanoporous network stabilized by a combination of hydrogen bonding and antiparallel dipolar coupling is formed. Annealing at 575 K resulted in an irreversible thermal transformation into a hexagonal nanoporous network stabilized by native gold adatoms. However, the Au adatoms could neither be unequivocally identified by scanning tunneling microscopy nor by noncontact atomic force microscopy. By combining van't Hoff plots derived from our scanning probe images with our density functional theory calculations, we were able to confirm the presence of the elusive Au adatoms in the hexagonal molecular network.

On-Surface Self-Assembled Structural Transformation Induced by Schiff Base Reaction and Hydrogen bonds

By utilizing scanning tunneling microscopy (STM), the self-assembled nanostructures of three characteristic aldehydes have been examined at the solution-solid interface. By introducing the active reactant 5-aminoisophthalic acid (5-AIPA), we succeeded in changing the self-assembled molecular structures through the condensation reaction and obtained the information on structural transformation in real time. The corresponding carboxyl conjugated derivatives were formed in situ and developed into the closely packed and ordered molecular architectures via hydrogen bonds at the solution-solid surface. The relevant simulations have been utilized to interpret the mechanisms of forming the nanostructures. The corresponding theoretical calculation is used to explain the reaction mechanism. Compared with the traditional ways, the on-surface condensation reaction in situ could not only provide a more convenient method for regulating the self-assembled architectures but also offer a promising strategy for building functional nanostructures and devices.

Intramolecular Coupling of Terminal Alkynes by Atom Manipulation

Glaser-like coupling of terminal alkynes by thermal activation is extensively used in on-surface chemistry. Here we demonstrate an intramolecular version of this reaction performed by atom manipulation. We used voltage pulses from the tip to trigger a Glaser-like coupling between terminal alkyne carbons within a custom-synthesized precursor molecule adsorbed on bilayer NaCl on Cu(111). Different conformations of the precursor molecule and the product were characterized by molecular structure elucidation with atomic force microscopy and orbital density mapping with scanning tunneling microscopy, accompanied by density functional theory calculations. We revealed partially dehydrogenated intermediates, providing insight into the reaction pathway.

Near-Enantiopure Trimerization of 9-Ethynylphenanthrene on a Chiral Metal Surface

Enantioselectivity in heterogeneous catalysis strongly depends on the chirality transfer between catalyst surface and all reactants, intermediates, and the product along the reaction pathway. Herein we report the first enantioselective on-surface synthesis of molecular structures from an initial racemic mixture and without the need of enantiopure modifier molecules. The reaction consists of a trimerization via an unidentified bonding motif of prochiral 9-ethynylphenanthrene (9-EP) upon annealing to 500 K on the chiral Pd₃ -terminated PdGa{111} surfaces into essentially enantiopure, homochiral 9-EP propellers. The observed behavior strongly contrasts the reaction of 9-EP on the chiral Pd₁ -terminated PdGa{111} surfaces, where 9-EP monomers that are in nearly enantiopure configuration, dimerize without enantiomeric excess. Our findings demonstrate strong chiral recognition and a significant ensemble effect in the PdGa system, hence highlighting the huge potential of chiral intermetallic compounds for enantioselective synthesis and underlining the importance to control the catalytically active sites at the atomic level.

On-Surface Synthesis of Graphyne-Based Nanostructures

The successful synthesis of stacking graphdiynes has stimulated numerous fascinating applications. However, it still remains challenging to prepare atomically precise 2D graphdiyne and other graphyne-based structures. The development of on-surface synthesis has contributed to many secondary graphyne-based structures that are directive in fabricating extended graphyne networks. Herein, the recent progress concerning on-surface synthesis of graphyne-based nanostructures, especially atomically precise graphdiyne nanowires, is summarized.

Exploring Approaches for the Synthesis of Few-Layered Graphdiyne

Graphdiyne (GDY) is an emerging carbon allotrope in the graphyne (GY) family, demonstrating extensive potential applications in the fields of electronic devices, catalysis, electrochemical energy storage, and nonlinear optics. Synthesis of few-layered GDY is especially important for both electronic applications and structural characterization. This work critically summarizes the state-of-art of GDY and focuses on exploring approaches for few-layered GDY synthesis. The obstacles and challenges of GDY synthesis are also analyzed in detail. Recently developed synthetic methods are discussed such as i) the copper substrate-based method, ii) the chemical vapor deposition (CVD) method, iii) the interfacial construction method, and iv) the graphene-templated method. Throughout the discussion, the superiorities and limitations of different methods are analyzed comprehensively. These synthetic methods have provided considerable inspiration approaching synthesis of few-layered or single-layered GDY film. The work concludes with a perspective on promising research directions and remaining barriers for layer-controlled and morphology-controlled synthesis of GDY with higher crystalline quality.

On-Surface Activation of Trimethylsilyl-Terminated Alkynes on Coinage Metal Surfaces

The controlled attachment of protecting groups combined with the ability to selectively abstract them is central to organic synthesis. The trimethylsilyl (TMS) functional group is a popular protecting group in solution. However, insights on its activation behavior under ultra-high vacuum (UHV) and surface-confined conditions are scarce. Here we investigate a series of TMS-protected alkyne precursors via scanning tunneling microscopy (STM) regarding their compatibility with organic molecular beam epitaxy (OMBE) and their potential deprotection on various coinage metal surfaces. After in-situ evaporation on the substrates held in UHV at room temperature, we find that all molecules arrived and adsorbed as intact units forming ordered supramolecular aggregates stabilized by non-covalent interactions. Thus, TMS-functionalized alkyne precursors with weights up to 1100 atomic mass units are stable against OMBE evaporation in UHV. Furthermore, the TMS activation through thermal annealing is investigated with STM and X-ray photoelectron spectroscopy (XPS). We observe that deprotection starts to occur between 400 K and 500 K on the copper and gold surfaces, respectively. In contrast, on silver surfaces, the TMS-alkyne bond remains stable up to temperatures where molecular desorption sets in (≈600 K). Hence, TMS functional groups can be utilized as leaving groups on copper and gold surfaces while they serve as protecting groups on silver surfaces.

Surface Confined Synthesis of Hydroxy Functionalized Two-Dimensional Polymer: The Effect of the Position of Hydroxy Groups

We report on the on-surface synthesis of a series of two-dimensional polymers (2DPs) and macrocycles containing hydroxyl groups on a highly oriented pyrolytic graphite surface. The formed 2DPs and macrocycles were visualized through scanning tunneling microscopy. By varying the solvent and reaction temperature, structural evolution from oligomers to well-ordered 2DPs or discrete macrocycles was directly followed. In addition, we discovered that the reaction outcome can be steered from extended 2DPs to discrete macrocycles or catenular structures by exchanging the position of the hydroxyl and aldehyde group. These results indicate that the relative positions of hydroxyl and aldehyde groups on the biphenyl ring play a determining role in the control and selection of the final products of the surface-confined Schiff base coupling reaction.

On-surface synthesis of 2D COFs on Cu(111) via the formation of thermodynamically stable organometallic networks as the template

Template-directed polymerization is an effective approach used to afford regular 2D covalent organic frameworks (COFs), thus the regularity of the template is crucial for the quality of the resulting 2D COFs. For the Ullmann reactions on Cu(111), aryl iodides and bromides are activated at low temperature to form organometallic C-Cu-C structures, which lead to kinetic trapping and irregular organometallic networks. Therefore, the subsequent annealing step can only afford irregular 2D COFs. In this manuscript, the molecule 4,4''-dibromo-5'-(4-chlorophenyl)-1,1':3',1''-terphenyl incorporated two Br terminals and one Cl terminal has been used to demonstrate different reactivities of a C-Cl bond and a C-Br bond via the hierarchical activation of the C-Br bond and the C-Cl bond on Cu(111). At room temperature, zigzag, armchair, and ring-like organometallic chains formed due to the activation of the C-Br bond to generate a C-Cu-C structure while C-Cl remained intact, illustrating that the C-Cl bond is more stable than C-Br. Further annealing at 433 K activated the C-Cl bond to produce regular organometallic networks as the thermodynamic product. Using the simpler molecule 1,3,5-tris(4-chlorophenyl)benzene as the precursor, the self-assembly of the intact molecules was observed on Cu(111) at 300 K without activation of the C-Cl bond. After annealing at 433 K, similar thermodynamically stable organometallic networks formed directly, which were used as a template to generate regular 2D COFs upon further annealing at 510 K.

Electronic Coupling in π-Conjugated Molecule-Bridged Silicon Quantum Dot Clusters Synthesized by Sonogashira Cross-Coupling Reaction

π-Conjugated molecule-bridged silicon quantum dot (Si QD) clusters were first synthesized by Sonogashira cross-coupling reaction between 4-ethynylstyryl and octyl co-capped Si QDs (4-Es/Oct Si QDs) and 2,5-dibromo-3-hexylthiophene. The formation of Si QD clusters was confirmed by field emission transmission electron microscopy. The electronic coupling between the QDs in the Si QD cluster is significantly enhanced as compared with that for 4-Es/Oct Si QDs, which is verified from the red shift in ultraviolet-visible absorption and photoluminescence spectra of the Si QD cluster with the possibility of exciton transport, the increased charging effect found in the core-level photoemission spectra, the shift to lower binding energy of the valence band photoemission spectrum, and more decisively, the increase in electrical conductance of the Si QD cluster thin film. To investigate the physical origin of the temperature dependence of the electrical conductance, we have merged the microscopic viewpoint, Marcus theory, on the electron transfer (W) between the adjacent QDs, with macroscopic concepts, such as the conductance (G), mobility (μ), and diffusion coefficient (D). The effective reorganizational energies of charge transfer between the neighboring Si QDs in 4-Es/Oct Si QD and Si QD cluster thin films are estimated to be 170 and 140 meV, respectively, while the ratio of the effective electronic coupling of the latter to that of the former is determined to be 7.3:1.

Coordination-Controlled C-C Coupling Products via ortho-Site C-H Activation

The coordination-restricted ortho-site C-H bond activation and dehydrogenative homocoupling of 4,4'-(1,3-phenylene)dipyridine (1,3-BPyB) and 4,4'-(1,4-phenylene)dipyridine (1,4-BPyB) on different metal surfaces were studied by a combination of scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations. The coupling products on Cu(111) exhibited certain configurations subject to the spatial restriction of robust two-fold Cu-N coordination bonds. Compared to the V-shaped 1,3-BPyB, the straight backbone of 1,4-BPyB helped to further reduce the variety of reactive products. By utilizing the three-fold coordination of Fe atoms with 1,4-BPyB molecules on Au(111), a large-scale network containing single products was constructed. Our results offer a promising protocol for controllable on-surface synthesis with the aid of robust coordination interactions.

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.

Reversibility and intermediate steps as key tools for the growth of extended ordered polymers via on-surface synthesis

Surface-confined polymerization is a bottom-up strategy to create one- and two-dimensional covalent organic nanostructures with a π-conjugated backbone, which are suitable to be employed in real-life electronic devices, due to their high mechanical resistance and electronic charge transport efficiency. This strategy makes it possible to change the properties of the final nanostructures by a careful choice of the monomer architecture (i.e. of its constituent atoms and their spatial arrangement). Several chemical reactions have been proven to form low-dimensional polymers on surfaces, exploiting a variety of precursors in combination with metal (e.g. Cu, Ag, Au) and insulating (e.g. NaCl, CaCO₃) surfaces. One of the main challenges of such an approach is to obtain nanostructures with long-range order, to boost the conductance performances of these materials. Most of the exploited chemical reactions use irreversible coupling between the monomers and, as a consequence, the resulting structures often suffer from poor order and high defect density. This review focuses on the state-of-the-art surface-confined polymerization reactions, with particular attention paid to reversible coupling pathways and irreversible processes including intermediate states, which are key aspects to control to increase the order of the final nanostructure.

Competition between Dehydrogenative Organometallic Bonding and Covalent Coupling of an Unfunctionalized Porphyrin on Cu(111)

We studied the formation of linked porphyrin oligomers from 5,15-diphenylporphyrin (2H-DPP) by thermal, substrate-assisted organometallic and dehydrogenation coupling on Cu(111) by scanning tunneling microscopy. In the range of 300-620 K, we find three distinct stages, at 300 K, the intact 2H-DPP molecules self-assemble into linear structures held together by van der Waals forces. Increasing the substrate temperature, self-metalation and intramolecular ring-closing reactions result in planar and isolated DPP species on the surface. By C-H cleavage, porphyrin oligomers bonded by organometallic and covalent bonds between the modified DPP are formed. The amount of covalently bonded DPP oligomers increases strongly with annealing time and temperature, and they become the dominant species at 570 K. In contrast, the number of organometallically bonded DPP oligomers increases moderately even up to 620 K, indicating that in this case the organometallic bond is no precursor of the covalent bond.

The dynamics of benzene on Cu(111): a combined helium spin echo and dispersion-corrected DFT study into the diffusion of physisorbed aromatics on metal surfaces

We use helium spin-echo spectroscopy (HeSE) to investigate the dynamics of the diffusion of benzene adsorbed on Cu(111). The results of these measurements show that benzene moves on the surface through an activated jump-diffusion process between the adsorption sites on a Bravais lattice. Density Functional Theory (DFT) calculations with van der Waals (vdW) corrections help us understand that the molecule diffuses by jumping through non-degenerate hollow sites. The results of the calculations shed light on the nature of the binding interaction between this prototypical aromatic molecule and the metallic surface. The highly accurate HeSE experimental data provide a quantitatively stringent benchmark for the vdW correction schemes applied to the DFT calculations and we compare the performances of several dispersion interaction schemes.

Chemical Vapor Deposition Growth of Linked Carbon Monolayers with Acetylenic Scaffoldings on Silver Foil

Graphdiyne analogs, linked carbon monolayers with acetylenic scaffoldings, are fabricated by adopting low-temperature chemical vapor deposition which provides a route for the synthesis of two-dimensional carbon materials via molecular building blocks. The electrical conductivity of the as-grown films can reach up to 6.72 S cm^-1 . Moreover, the films show potential as promising substrates for fluorescence suppressing and Raman advancement.

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.

Frontiers of supramolecular chemistry at solid surfaces

Supramolecular chemistry on solid surfaces represents an exciting field of research that continues to develop in new and unexpected directions.

Simultaneous construction of two linkages for the on-surface synthesis of imine-boroxine hybrid covalent organic frameworks

The orthogonality between the Schiff base reaction and the boronic acid dehydration reaction is explored during the on-surface synthesis process. By activating the above two reactions in one-step and employing asymmetrical substituted monomers and the 3-fold symmetric monomer 1,3,5-tris(4-aminophenyl)benzene (TAPB), highly ordered imine-boroxine hybrid single-layered covalent organic frameworks (sCOFs) have been successfully constructed on HOPG by a gas-solid interface reaction method and characterized by scanning tunnelling microscopy (STM). In particular, the reaction between the meta-substituted monomer and TAPB generates sCOFB with a windmill structure, which is the first sCOF with surface chirality so far reported. The demonstration of the one-step synthesis of multiple linkages to form sCOFs can further enlarge the sCOF family and expand the design routes for functional 2D organic nanomaterials.

Dehalogenative Homocoupling of Terminal Alkynyl Bromides on Au(111): Incorporation of Acetylenic Scaffolding into Surface Nanostructures

On-surface C-C coupling reactions of molecular precursors with alkynyl functional groups demonstrate great potential for the controllable fabrication of low-dimensional carbon nanostructures/nanomaterials, such as carbyne, graphyne, and graphdiyne, which demand the incorporation of highly active sp-hybridized carbons. Recently, through a dehydrogenative homocoupling reaction of alkynes, the possibility was presented to fabricate surface nanostructures involving acetylenic linkages, while problems lie in the fact that different byproducts are inevitably formed when triggering the reactions at elevated temperatures. In this work, by delicately designing the molecular precursors with terminal alkynyl bromide, we introduce the dehalogenative homocoupling reactions on the surface. As a result, we successfully achieve the formation of dimer structures, one-dimensional molecular wires and two-dimensional molecular networks with acetylenic scaffoldings on an inert Au(111) surface, where the unexpected C-Au-C organometallic intermediates are also observed. This study further supplements the database of on-surface dehalogenative C-C coupling reactions, and more importantly, it provides us an alternative efficient way for incorporating the acetylenic scaffolding into low-dimensional surface nanostructures.

Structural Variation in Surface-Supported Synthesis by Adjusting the Stoichiometric Ratio of the Reactants

Surface-supported coupling reactions between 1,3,5-tris(4-formylphenyl)benzene and aromatic amines have been investigated on Au(111) using scanning tunneling microscopy under ultra-high-vacuum conditions. Upon annealing to moderate temperatures, various products, involving the discrete oligomers and the surface covalent organic frameworks, are obtained through thermal-triggered on-surface chemical reactions. We conclude from the systematic experiments that the stoichiometric composition of the reactants is vital to the surface reaction products, which is rarely reported so far. With this knowledge, we have successfully prepared two-dimensional covalently bonded networks by optimizing the stoichiometric proportions of the reaction precursors.

The stereoselective synthesis of dienes through dehalogenative homocoupling of terminal alkenyl bromides on Cu(110)

We have successfully achieved the stereoselective synthesis of a specific cis-diene moiety through a dehalogenative homocoupling of alkenyl bromides on the Cu(110) surface, where the formation of a cis-form organometallic intermediate is the key to such a stereoselectivity as determined by DFT calculations.

Surface-Guided Formation of an Organocobalt Complex

Organocobalt complexes represent a versatile tool in organic synthesis as they are important intermediates in Pauson-Khand, Friedel-Crafts, and Nicholas reactions. Herein, a single-molecule-level investigation addressing the formation of an organocobalt complex at a solid-vacuum interface is reported. Deposition of 4,4'-(ethyne-1,2-diyl)dibenzonitrile and Co atoms on the Ag(111) surface followed by annealing resulted in genuine complexes in which single Co atoms laterally coordinated to two carbonitrile groups undergo organometallic bonding with the internal alkyne moiety of adjacent molecules. Alternative complexation scenarios involving fragmentation of the precursor were ruled out by complementary X-ray photoelectron spectroscopy. According to density functional theory analysis, the complexation with the alkyne moiety follows the Dewar-Chatt-Duncanson model for a two-electron-donor ligand where an alkyne-to-Co donation occurs together with a strong metal-to-alkyne back-donation.

Reaction mechanisms for on-surface synthesis of covalent nanostructures

In recent years, on-surface synthesis has become an increasingly popular strategy to form covalent nanostructures. The approach has great prospects for facilitating the manufacture of a range of fascinating materials with atomic precision. However, the on-surface reactions are enigmatic to control, currently restricting its bright perspectives and there is a great need to explore how the reactions are governed. The objective of this topical review is to summarize theoretical work that has focused on comprehending on-surface synthesis protocols through studies of reaction mechanisms.

Surface-Controlled Mono/Diselective ortho C-H Bond Activation

One of the most charming and challenging topics in organic chemistry is the selective C-H bond activation. The difficulty arises not only from the relatively large bond-dissociation enthalpy, but also from the poor reaction selectivity. In this work, Au(111) and Ag(111) surfaces were used to address ortho C-H functionalization and ortho-ortho couplings of phenol derivatives. More importantly, the competition between dehydrogenation and deoxygenation drove the diversity of reaction pathways of phenols on surfaces, that is, diselective ortho C-H bond activation on Au(111) surfaces and monoselective ortho C-H bond activation on Ag(111) surfaces. The mechanism of this unprecedented phenomenon was extensively explored by scanning tunneling microscopy, density function theory, and X-ray photoelectron spectroscopy. Our findings provide new pathways for surface-assisted organic synthesis via the mono/diselective C-H bond activation.

Comparing Ullmann Coupling on Noble Metal Surfaces: On-Surface Polymerization of 1,3,6,8-Tetrabromopyrene on Cu(111) and Au(111)

The on-surface polymerization of 1,3,6,8-tetrabromopyrene (Br4 Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br4 Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C-Cu-C bonds. After annealing at 473 K, the C-Cu-C bonds were converted to covalent C-C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self-assembled two-dimensional (2D) patterns stabilized by both Br-Br halogen and Br-H hydrogen bonds were observed upon deposition of Br4 Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C-Br bonds and the formation of disordered metal-coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br4 Py on the different substrates.

A computational study of bulk porous two-dimensional polymers related to graphyne

We use density functional theory methods with periodic boundary conditions to investigate the stacking arrangements of the bulk 2D polymers multilayer porous graphyne, the analog in which the triple bonds are substituted by double bonds and the related carbon allotrope multilayer graphyne.

Surface-Activated Coupling Reactions Confined on a Surface

Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas-solid or liquid-solid). Recently, the emerging field of "surface-confined coupling reactions" has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid-vacuum or a solid-liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known "textbook" coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl-aryl (Ullmann-type) coupling and alkyne-alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.

On-surface synthesis of carbon-based scaffolds and nanomaterials using terminal alkynes

The covalent linking of acetylene compounds is an important synthetic tool to control carbon-carbon bond formation and has been extensively studied for more than a century. Notably, Glaser coupling and subsequently developed refined procedures present an important route for the fabrication of distinct carbon-based scaffolds incorporating units with both sp(2)- and sp-hybridizations, such as carbyne chains, or two-dimensional (2D) graphyne or graphdiyne networks. However, the realization of the envisioned regular low-dimensional compounds and nanoarchitectures poses formidable challenges when following conventional synthesis protocols in solution, which we briefly overview. Now, recent developments in on-surface synthesis establish novel means for the construction of tailored covalent nanostructures under ultrahigh vacuum conditions. Here we focus on the exploration of pathways utilizing interfacial synthesis with terminal alkynes toward the atomically precise fabrication of low-dimensional carbon-rich scaffolds and nanomaterials. We review direct, molecular-level investigations, mainly relying on scanning probe microscopy, providing atomistic insights into thermally activated reaction schemes, their special pathways and products. Using custom-made molecular units, the employed homocoupling, cyclotrimerization, cycloaddition, and radical cyclization processes indeed yield distinct compounds, extended oligomers or 2D networks. Detailed insights into surface interactions such as bonding sites or conformational adaptation, and specific reaction mechanisms, including hierarchic pathways, were gained by sophisticated density functional theory calculations, complemented by X-ray spectroscopy measurements. For the fabrication of regular nanostructures and architectures, it is moreover imperative to cope with spurious side reactions, frequently resulting in chemical diversity. Accordingly, we highlight measures for increasing chemo- and regioselectivity by smart precursor design, substrate templating, and external stimuli. The ensuing preorganization of functional groups and control of side reactions increases product yields markedly. Finally, the electronic band structures of selected cases of novel low-dimensional hydrocarbon materials accessible with the monomers employed to date are discussed with a specific focus on their differences to theoretically established graphyne- and graphdiyne-related scaffolds. The presented methodology and gained insights herald further advancements in the field, heading toward novel molecular compounds, low-dimensional nanostructures, and coherently reticulated polymeric layers, eventually presenting well-defined arrangements with specific carbon-carbon bond sequencing and electronic characteristics. The functional properties of these or other foreseeable scaffolds and architectures bear significant prospects for a wide range of applications, for example, in nanoelectronics, photonics, or carbon-based technologies.

Lattice-Directed Formation of Covalent and Organometallic Molecular Wires by Terminal Alkynes on Ag Surfaces

Surface reactions of 2,5-diethynyl-1,4-bis(phenylethynyl)benzene on Ag(111), Ag(110), and Ag(100) were systematically explored and scrutinized by scanning tunneling microscopy, molecular mechanics simulations, and density functional theory calculations. On Ag(111), Glaser coupling reaction became dominant, yielding one-dimensional molecular wires formed by covalent bonds. On Ag(110) and Ag(100), however, the terminal alkynes reacted with surface metal atoms, leading to one-dimensional organometallic nanostructures. Detailed experimental and theoretical analyses revealed that such a lattice dependence of the terminal alkyne reaction at surfaces originated from the matching degree between the periodicities of the produced molecular wires and the substrate lattice structures.