Glaser coupling at metal surfaces

Theoretical Investigation on the Initial Reaction Mechanism of Hexaethynylbenzene on Au(111) Surface

Graphyne has attracted considerable interest and attention since its successful synthesis, due to its enormous potential for applications in the fields of electronics, energy, catalysis, information technology, etc. Although various methods for synthesizing graphyne have been explored, single-layer graphynes have not been successfully developed. Hexaethynylbenzene (HEB) is considered an ideal precursor molecule because it can undergo Glaser coupling reactions between molecules to synthesize single layer graphdiyne on single crystal metal surfaces via on-surface reactions. Unfortunately, this method fails to achieve the expected results, and the underlying mechanism is not clear. In this work, we employed a combination of ab initio molecular dynamics (AIMD) and quantum mechanics (QM) methods to investigate the initial reaction mechanism of HEB molecules on a Au(111) surface. We revealed that HEB molecules undergo both intermolecular coupling and intramolecular cyclization on the Au(111) surface. The favorable pathways of these two types of reactions were then distinguished, confirming that the distance between the terminal carbon atoms of the ethynyl groups plays an important role in C-C coupling. The insights revealed from this work could facilitate the rational design of precursor molecules and deepen the understanding of the reaction processes.

Thickness-Driven Synthesis and Applications of Covalent Organic Framework Nanosheets

Covalent organic frameworks (COFs) are two-dimensional, crystalline porous framework materials with numerous scopes for tunability, such as porosity, functionality, stability and aspect ratio (thickness to length ratio). The manipulation of π-stacking in COFs results in truly 2D materials, namely covalent organic nanosheets (CONs), adds advantages in many applications. In this Minireview, we have discussed both top-down (COFs→CONs) and bottom-up (molecules→CONs) approaches with precise information on thickness and lateral growth. We have showcased the research progress on CONs in a few selected applications, such as batteries, catalysis, sensing and biomedical applications. This Minireview specifically highlights the reports where the authors compare the performance of CONs with COFs by demonstrating the impact of the thickness and lateral growth of the nanosheets. We have also provided the possible scope of exploration of CONs research in terms of inter-dimensional conversion, such as graphene to carbon nanotube and future technologies.

On-Surface Synthesis of Five-Membered Copper Metallacycles Using Terminal Alkynes

We present our studies on the adsorption, deprotonation, and reactions of 4,4″-diethynyl-1,1':4',1″-terphenyl on Cu(111) under ultrahigh-vacuum conditions using scanning tunneling microscopy combined with density functional theory calculations. Sequential annealing treatments induce deprotonation of pristine molecules followed by chemical reactions, resulting in branched nanostructures. Within the nanostructures, a previously unreported, double-spot linkage is observed. Our density functional theory calculations unravel that this linkage corresponds to a five-membered copper metallacycle.

Recent advances in organic molecule reactions on metal surfaces

Chemical reactions of organic molecules on metal surfaces have been intensively investigated in the past decades, where metals play the role of catalysts in many cases. In this review, first, we summarize recent works on spatial molecules, small H2O, O2, CO, CO2 molecules, and the molecules carrying silicon groups as the new trends of molecular candidates for on-surface chemistry applications. Then, we introduce spectroscopy and DFT study advances in on-surface reactions. Especially, in situ spectroscopy technologies, such as electron spectroscopy, force spectroscopy, X-ray photoemission spectroscopy, STM-induced luminescence, tip-enhanced Raman spectroscopy, temperature-programmed desorption spectroscopy, and infrared reflection adsorption spectroscopy, are important to confirm the occurrence of organic reactions and analyze the products. To understand the underlying mechanism, the DFT study provides detailed information about reaction pathways, conformational evolution, and organometallic intermediates. Usually, STM/nc-AFM topological images, in situ spectroscopy data, and DFT studies are combined to describe the mechanism behind on-surface organic reactions.

On-Surface Synthesis of Polyene-Linked Porphyrin Cooligomer

π-Conjugated molecules are viewed as fundamental components in forthcoming molecular nanoelectronics in which semiconducting functional units are linked to each other via metallic molecular wires. However, it is still challenging to construct such block cooligomers on the surface. Here, we present a synthesis of [18]-polyene-linked Zn-porphyrin cooligomers via a two-step reaction of the alkyl groups on Cu(111) and Cu(110). Nonyl groups (-C9H19) substituted at the 5,15-meso positions of Zn-porphyrin were first transformed to alkenyl groups (-C9H10) by dehydrogenation. Subsequently, homocoupling of the terminal -CH2 groups resulted in the formation of extended [18]-polyene-linked porphyrin cooligomers. The structures of the products at each reaction step were investigated by bond-resolved scanning tunneling microscopy at low temperatures. A combination of angle-resolved photoemission spectroscopy and density functional theory calculations revealed the metallic property of the all trans [18]-polyene linker on Cu(110). This finding may provide an approach to fabricate complex nanocarbon structures on the surface.

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 polymerization reactions of dibrominated hexaphenylbenzene influenced by densely packed self-assembly

Controlled bottom-up fabrication of molecular nanostructures through on-surface reactions of tailor-made precursors is of scientific and technological interest. Recently, on-surface polymerization reactions influenced by precursor self-assembly have been reported. Thus, a fundamental understanding of the reaction process is a prerequisite for controlled formation. Herein, we report on the influence of molecular self-assembly of dibrominated hexaphenylbenzene (Br2-HPB) on the on-surface polymerization reactions on a Au(111) substrate. By using low-temperature scanning tunnelling microscopy (STM), we find that the polymerization of Br2-HPB proceeds while maintaining the long-range ordered self-assembly, despite a decrease in HPB-HPB distance due to debromination and successive covalent bonding of Br2-HPB. From the STM investigations of the polymerization process, we conclude that the polymerization of Br2-HPB is accompanied by molecular rotations to maintain the periodic array of the self-assembled structure, contrary to the conventional understanding of the polymerization of the self-assembled precursor layer.

On-Surface Synthesis of Silole and Disila-Cyclooctene Derivatives

The incorporation of Si atoms into organic compounds significantly increases a variety of functionality, facilitating further applications. Recently, on-surface synthesis was introduced into organosilicon chemistry as 1,4-disilabenzene bridged nanostructures were obtained via coupling between silicon atoms and brominated phenyl groups at the ortho position on Au(111). Here, we demonstrate a high generality of this strategy via syntheses of silole derivatives and nanoribbon structures with eight-membered sila-cyclic rings from dibrominated molecules at the bay and peri positions on Au(111), respectively. Their structures and electronic properties were investigated by a combination of scanning tunneling microscopy/spectroscopy and density functional theory calculations. This work demonstrates a great potential to deal with heavy group 14 elements in on-surface silicon chemistry.

Sterically Selective [3 + 3] Cycloaromatization in the On-Surface Synthesis of Nanographenes

Surface-catalyzed reactions have been used to synthesize carbon nanomaterials with atomically predefined structures. The recent discovery of a gold surface-catalyzed [3 + 3] cycloaromatization of isopropyl substituted arenes has enabled the on-surface synthesis of arylene-phenylene copolymers, where the surface activates the isopropyl substituents to form phenylene rings by intermolecular coupling. However, the resulting polymers suffered from undesired cross-linking when more than two molecules reacted at a single site. Here we show that such cross-links can be prevented through steric protection by attaching the isopropyl groups to larger arene cores. Upon thermal activation of isopropyl-substituted 8,9-dioxa-8a-borabenzo[fg]tetracene on Au(111), cycloaromatization is observed to occur exclusively between the two molecules. The cycloaromatization intermediate formed by the covalent linking of two molecules is prevented from reacting with further molecules by the wide benzotetracene core, resulting in highly selective one-to-one coupling. Our findings extend the versatility of the [3 + 3] cycloaromatization of isopropyl substituents and point toward steric protection as a powerful concept for suppressing competing reaction pathways in on-surface synthesis.

Light-Induced Increase of the Local Molecular Coverage on a Surface

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied. Using scanning tunneling microscopy and X-ray photoemission spectroscopy, we observe a substantial increase of the local molecular coverage with the sample illumination time at the center of the laser spot. We find that the interplay between thermally induced diffusion and the reduced mobility of reaction products steers the accumulation of material. Moreover, the debromination of the adsorbed species never progresses to completion within the experiment time, despite a long irradiation of many hours.

Predicting Organometallic Intermediates in the Surface-Assisted Ullmann Coupling of Chrysene Isomers

On-surface polymerization of functional organic molecules has been recently recognized as a promising route to persistent low-dimensional structures with tailorable properties. In this contribution, using the coarse-grained Monte Carlo simulation method, we study the initial stage of the Ullmann coupling of doubly halogenated chrysene isomers adsorbed on a catalytically active (111) crystalline surface. To that end, we focus on the formation of labile metal-organic precursor structures preceding the covalent bonding of chrysene monomers. Four monomeric chrysene units with differently distributed halogen substituents were probed in the simulations, and the resulting precursor structures were compared and quantified. Moreover, the effect of (pro)chirality of chrysene tectons on the structure formation was elucidated by running separate simulations in enantiopure and racemic systems. The calculations showed that suitable manipulation of the halogen substitution pattern allows for the creation of diverse precursor architectures, ranging from straight and winded chains to cyclic oligomers with enantiopure, racemic, and nonracemic composition. The obtained findings can be helpful in developing synthetic strategies for covalent polymers with predefined architecture and functionality.

Graphdiyne Preparation and Application in Photocatalytic Hydrogen Evolution

Graphdiyne (GDY) is a new two-dimensional carbon network material composed of sp2 hybrid carbon and sp hybrid carbon conjugation. It has unique physical and chemical properties, such as high porosity, good electrical conductivity, high carrier mobility, adjustable band gap, and so on. The preparation of GDY and GDY derivatives by adjusting physical and chemical methods and changing monomers has become the key material in the fields of photocatalysis, energy storage, life science, and so on. In this paper, new methods for controllable growth of GDY are reviewed, including liquid phase chemical classical total synthesis, chemical vapor deposition, the interface method, the explosion method, and the mechanically driven ball milling method. FT-IR, Raman, NMR, and XAS are the main means to characterize the structure of GDY. Finally, the representative application of GDY in the field of photocatalytic hydrogen evolution is summarized, and its future development has been explored.

On-surface cyclization of vinyl groups on poly-para-phenylene involving an unusual pentagon to hexagon transformation

On-surface synthesis relies on carefully designed molecular precursors that are thermally activated to afford desired, covalently coupled architectures. Here, we study the intramolecular reactions of vinyl groups in a poly-para-phenylene-based model system and provide a comprehensive description of the reaction steps taking place on the Au(111) surface under ultrahigh vacuum conditions. We find that vinyl groups successfully cyclize with the phenylene rings in the ortho positions, forming a dimethyl-dihydroindenofluorene as the repeating unit, which can be further dehydrogenated to a dimethylene-dihydroindenofluorene structure. Interestingly, the obtained polymer can be transformed cleanly into thermodynamically stable polybenzo[k]tetraphene at higher temperature, involving a previously elusive pentagon-to-hexagon transformation via ring opening and rearrangement on a metal surface. Our insights into the reaction cascade unveil fundamental chemical processes involving vinyl groups on surfaces. Because the formation of specific products is highly temperature-dependent, this innovative approach offers a valuable tool for fabricating complex, low-dimensional nanostructures with high precision and yield.

Dramatic Acceleration of the Hopf Cyclization on Gold(111): From Enediynes to Peri-Fused Diindenochrysene Graphene Nanoribbons

Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.

Surface-confined alternating copolymerization with molecular precision by stoichiometric control

Keen desires for artificial mimicry of biological polymers and property improvement of synthesized ones have triggered intensive explorations for sequence-controlled copolymerization. However, conventional synthesis faces great challenges to achieve this goal due to the strict requirements on reaction kinetics of comonomer pairs and tedious synthetic processes. Here, sequence-controlled alternating copolymerization with molecular precision is realized on surface. The stoichiometric control serves as a thermodynamic strategy to steer the polymerization selectivity, which enables the selective alternating organometallic copolymerization via intermolecular metalation of 4,4"-dibromo-p-terphenyl (P-Br) and 2,5-diethynyl-1,4-bis(phenylethynyl)benzene (A-H) with Ag adatoms on Ag(111) at P-Br: A-H = 2, as verified by scanning tunneling microscopy and density functional theory studies. In contrast, homopolymerization yield increases as the stoichiometric ratio deviates from 2. The microscopic characterizations rationalize the mechanism, providing a delicate explanation of the stoichiometry-dependent polymerization. These findings pave a way to actualizing an efficient sequence control of copolymerization by surface chemistry.

Photocatalytic Terminal C-C Coupling Reaction Inside Water Soluble Nanocages

Developing methods that activate C-H bonds directly with high selectivity for C-C bond formation in complex organic synthesis has been a major chemistry challenge. Recently it has been shown that photoactivation of weakly polarized C-H bonds can be carried out inside a cationic water-soluble nanocage with visible light-mediated host-guest charge transfer (CT) chemistry. Using this novel photoredox activation paradigm, here we demonstrate C-C bond formation to photo-generate 1,3-diynes at room temperature in water from terminal aromatic alkynes for the first time. The formation of cavity-confined alkyne radical cation and the proton-removed neutral radical species highlight the unique C-C coupling step driven by supramolecular preorganization.

Highly Regioselective Cyclodehydrogenation of Diphenylporphyrin on Metal Surfaces

Exploring the effect of porphin tautomerism on the regioselectivity of its derivatives is a big challenge, which is significant for the development and application of porphyrin drugs. In this work, we demonstrate the regioselectivity of 2H-diphenylporphyrin (H2-DPP) in the planarization reaction on Au(111) and Ag(111) substrates. H2-DPP monomer forms two configurations (anti- and syn-) via a dehydrogenation coupling, between which the yield of the anti-configuration exceeds 90%. Using high-resolution scanning tunneling microscopy, we visualize the reaction processes from the H2-DPP monomer to the final two planar products. Combined with DFT calculations of the potential reaction pathway and comparative experiments on Au(111) and Ag(111) substrates. Using M-DPP (M = Cu and Fe), we confirm that the regioselectivity of H2-DPP is derived from the reaction energy barrier during the cyclodehydrogenation reaction of different tautomers. This work reveals the regioselectivity mechanism of H2-DPP on the atomic scale, which holds great significance for understanding the chemical conversion process of organic macrocyclic molecules.

Prospects of 2D graphdiynes and their applications in desalination and wastewater remediation

Water is an indispensable part of human life that affects health and food intake. Water pollution caused by rapid industrialization, agriculture, and other human activities affects humanity. Therefore, researchers are prudent and cautious regarding the use of novel materials and technologies for wastewater remediation. Graphdiyne (GDY), an emerging 2D nanomaterial, shows promise in this direction. Graphdiyne has a highly symmetrical π-conjugated structure consisting of uniformly distributed pores; hence, it is favorable for applications such as oil-water separation and organic-pollutant removal. The acetylenic linkage in GDY can strongly interact with metal ions, rendering GDY applicable to heavy-metal adsorption. In addition, GDY membranes that exhibit 100% salt rejection at certain pressures are potential candidates for wastewater treatment and water reuse via desalination. This review provides deep insights into the structure, properties, and synthesis methods of GDY, owing to which it is a unique, promising material. In the latter half of the article, various applications of GDY in desalination and wastewater treatment have been detailed. Finally, the prospects of these materials have been discussed succinctly.

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.

Engineering of Graphdiyne-Based Functional Coatings for the Protection of Arbitrary Shapes of Copper Substrates

Copper-based materials are very important for many application fields from marine industry to energy management and electronic devices. For most of these applications, the copper objects require long-term contact to a wet and salty environment, which leads to serious corrosion of copper. In this work, we report a thin graphdiyne layer directly grown on arbitrary shapes of copper objects at mild conditions, which could function as a protective coating for the copper substrates in artificial seawater with corrosion inhibition efficiency of ∼99.75%. To further improve the protective performance of the coating, the graphdiyne layer is fluorinated and followed by infusion with a fluorine-containing lubricant (i.e., perfluoropolyether). As a result, a slippery surface is obtained, which shows enhanced corrosion inhibition efficiency of ∼99.99% as well as excellent antibiofouling properties against microorganisms, such as protein and algae. Finally, the coatings are successfully applied in the protection of a commercial copper radiator from long-term attack of artificial seawater without disturbing its thermal conductivity. These results demonstrate the great potential of graphdiyne-based functional coatings for the protection of copper devices in aggressive environments.

Innovations in nanosynthesis: emerging techniques for precision, scalability, and spatial control in reactions of organic molecules on solid surfaces

The surface science-based approach to synthesising new organic materials on surfaces has gained considerable attention in recent years, owing to its success in facilitating the formation of novel 0D, 1D and 2D architectures. The primary mechanism used to date has been the catalytic transformation of small organic molecules through substrate-enabled reactions. In this Topical Review, we provide an overview of alternate approaches to controlling molecular reactions on surfaces. These approaches include light, electron and ion-initiated reactions, electrospray ionisation deposition-based techniques, collisions of neutral atoms and molecules, and superhydrogenation. We focus on the opportunities afforded by these alternative approaches, in particular where they may offer advantages in terms of selectivity, spatial control or scalability.

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.

Pyridinic Nitrogen Modification for Selective Acetylenic Homocoupling on Au(111)

On-surface acetylenic homocoupling has been proposed to construct carbon nanostructures featuring sp hybridization. However, the efficiency of linear acetylenic coupling is far from satisfactory, often resulting in undesired enyne products or cyclotrimerization products due to the lack of strategies to enhance chemical selectivity. Herein, we inspect the acetylenic homocoupling reaction of polarized terminal alkynes (TAs) on Au(111) with bond-resolved scanning probe microscopy. The replacement of benzene with pyridine moieties significantly prohibits the cyclotrimerization pathway and facilitates the linear coupling to produce well-aligned N-doped graphdiyne nanowires. Combined with density functional theory calculations, we reveal that the pyridinic nitrogen modification substantially differentiates the coupling motifs at the initial C-C coupling stage (head-to-head vs head-to-tail), which is decisive for the preference of linear coupling over cyclotrimerization.

Construction of Nitrogen-abundant Graphyne Scaffolds via Mechanochemistry-Promoted Cross-Linking of Aromatic Nitriles with Carbide Toward Enhanced Energy Storage

The 2D graphyne-related scaffolds linked by carbon-carbon triple bonds have demonstrated promising applications in the field of catalysis and energy storage due to their unique features including high conductivity, permanent porosity, and electron-rich properties. However, the construction of related scaffolds is still mainly limited to the cross-linking of CaC₂ with multiple substituted aromatic halogens and there is still a lack of efficient methodology capable of introducing high-concentration heteroatoms within the architectures. The development of alternative and facile synthesis procedures to afford nitrogen-abundant graphyne materials is highly desirable yet challenging in the field of energy storage, particularly via the facile mechanochemical procedure under neat and ambient conditions. Herein, graphyne materials with abundant nitrogen-containing species (nitrogen content of 6.9-29.3 wt.%), tunable surface areas (43-865 m²  g^-1 ), and hierarchical porosity are produced via the mechanochemistry-driven pathway by deploying highly electron-deficient multiple substituted aromatic nitriles as the precursors, which can undergo cross-linking reaction with CaC₂ to afford the desired nitrogen-doped graphyne scaffolds efficiently. Unique structural features of the as-synthesized materials contributed to promising performance in supercapacitor-related applications, delivering high capacitance of 254.5 F g^-1 at 5 mV s^-1 , attractive rate performance, and good long-term stability.

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.

On-surface synthesis of non-benzenoid conjugated polymers by selective atomic rearrangement of ethynylarenes

Here, we report a new on-surface synthetic strategy to precisely introduce five-membered units into conjugated polymers from specifically designed precursor molecules that give rise to low-bandgap fulvalene-bridged bisanthene polymers. The selective formation of non-benzenoid units is finely controlled by the annealing parameters, which govern the initiation of atomic rearrangements that efficiently transform previously formed diethynyl bridges into fulvalene moieties. The atomically precise structures and electronic properties have been unmistakably characterized by STM, nc-AFM, and STS and the results are supported by DFT theoretical calculations. Interestingly, the fulvalene-bridged bisanthene polymers exhibit experimental narrow frontier electronic gaps of 1.2 eV on Au(111) with fully conjugated units. This on-surface synthetic strategy can potentially be extended to other conjugated polymers to tune their optoelectronic properties by integrating five-membered rings at precise sites.

Molecular insight into on-surface chemistry of an organometallic polymer

A molecular investigation of Cu-elimination and subsequent C-C coupling of DCTP (4,4''-dichloro-1,1':3',1''-terphenyl)-Cu organometallic (OM) polymers on Cu(111) is conducted by scanning tunneling microscopy and spectroscopy, revealing that the Cu adatoms embedded in the DCTP-Cu chains are located at the hollow and bridge sites on the Cu(111) surface. The difference in the catalytic activities of these surface sites leads to stepwise elimination of Cu adatoms in the OM chains. Moreover, the interchain interaction plays an important role in the Cu-elimination process of the DCTP-Cu chains as well. The interchain steric hindrance, on the one hand, induces the formation of Cu-eliminated intermediates that are scarcely observed in other Ullmann coupling systems, and on the other hand, promotes the cooperative Cu-elimination and C-C coupling of the OM segments in neighboring chains. These findings demonstrate the key role of the molecule-substrate and intermolecular interactions in mediating the reaction processes of the extended molecular systems on the surface.

Manipulation of C-C coupling pathways using different annealing procedures

From scanning tunnelling microscopy and density functional theory calculations, we demonstrate that different annealing mechanisms could modulate distinct reaction pathways, where in a stepwise annealing procedure the detached Br atoms may reduce the activation barrier of CH activation resulting in hierarchical cross dehydrogenative coupling, while in a one-step annealing procedure only Ullmann coupling products are observed.

Dielectric behavior of curcuminoid polymorphs on different substrates by direct soft vacuum deposition

Our work examines the structural-electronic correlation of a new curcuminoid, AlkCCMoid, as a dielectric material on different substrates. For this purpose, we show a homemade sublimation method that allows the direct deposition of molecules on any type of matrix. The electronic properties of AlkCCMoid have been evaluated by measurements on single crystals, microcrystalline powder, and sublimated samples, respectively. GIWAXS studies on surfaces and XRD studies on powder have revealed the existence of polymorphs and the effect that substrates have on curcuminoid organization. We describe the dielectric nature of our system and identify how different polymorphs can affect electronic parameters such as permittivity, all corroborated by DFT calculations.

Scalable Synthesis and Characterization of Multilayer γ-Graphyne, New Carbon Crystals with a Small Direct Band Gap

γ-Graphyne is the most symmetric sp²/sp¹ allotrope of carbon, which can be viewed as graphene uniformly expanded through the insertion of two-carbon acetylenic units between all the aromatic rings. To date, synthesis of bulk γ-graphyne has remained a challenge. We here report the synthesis of multilayer γ-graphyne through crystallization-assisted irreversible cross-coupling polymerization. A comprehensive characterization of this new carbon phase is described, including synchrotron powder X-ray diffraction, electron diffraction, lateral force microscopy, Raman spectroscopy, infrared spectroscopy, and cyclic voltammetry. Experiments indicate that γ-graphyne is a 0.48 eV band gap semiconductor, with a hexagonal a-axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å, which is consistent with theoretical predictions. The observed crystal structure has an aperiodic sheet stacking. The material is thermally stable up to 240 °C but undergoes transformation at higher temperatures. While conventional 2D polymerization and reticular chemistry rely on error correction through reversibility, we demonstrate that a periodic covalent lattice can be synthesized under purely kinetic control. The reported methodology is scalable and inspires extension to other allotropes of the graphyne family.

On-Surface Synthesis of Unsaturated Hydrocarbon Chains through C-S Activation

We use an on-surface synthesis approach to drive the homocoupling reaction of a simple dithiophenyl-functionalized precursor on Cu(111). The C-S activation reaction is initiated at low annealing temperature and yields unsaturated hydrocarbon chains interconnected in a fully conjugated reticulated network. High-resolution atomic force microscopy imaging reveals the opening of the thiophenyl rings and the presence of trans- and cis-oligoacetylene chains as well as pentalene units. The chemical transformations were studied by C 1s and S 2p core level photoemission spectroscopy and supported by theoretical calculations. At higher annealing temperature, additional cyclization reactions take place, leading to the formation of small graphene flakes.

Desilylative Coupling Involving C(sp2)-Si Bond Cleavage on Metal Surfaces

Desilylative coupling involving C-Si bond cleavage has emerged as one of the most important synthetic strategies for carbon-carbon/heteroatom bond formation in solution chemistry. However, in on-surface chemistry, C-Si bond cleavage remains a synthetic challenge. Here, we report the implementation of C(sp²)-Si bond cleavage and subsequent C-C bond formation on metal surfaces. The combination of scanning tunneling microscopy and density functional theory calculation successfully reveals that the incorporation of the C-Br group on the arylsilanes is critical to the success of this desilylative coupling reaction on metal surfaces. Our study represents a promising approach for the removal of protecting silyl groups in on-surface chemistry.

Tandem Desulfurization/C-C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer

Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C-C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C-C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.

On-Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction.

On-surface synthesis of triangulene trimers via dehydration reaction

Triangulene and its homologues are of considerable interest for molecular spintronics due to their high-spin ground states as well as the potential for constructing high spin frameworks. Realizing triangulene-based high-spin system on surface is challenging but of particular importance for understanding π-electron magnetism. Here, we report two approaches to generate triangulene trimers on Au(111) by using surface-assisted dehydration and alkyne trimerization, respectively. We find that the developed dehydration reaction shows much higher chemoselectivity thus resulting in significant promotion of product yield compared to that using alkyne trimerization approach, through cutting the side reaction path. Combined with spin-polarized density functional theory calculations, scanning tunneling spectroscopy measurements identify the septuple (S = 3) high-spin ground state and quantify the collective ferromagnetic interaction among three triangulene units. Our results demonstrate the approaches to fabricate high-quality triangulene-based high spin systems and understand their magnetic interactions, which are essential for realizing carbon-based spintronic devices.

Anchoring and Reacting On-Surface to Achieve Programmability

On-surface synthesis has developed into a modern method to fabricate low-dimensional molecular nanostructures with atomic precision. It impresses the chemistry community mostly via its simplicity, selectivity, and programmability during the synthesis. However, an insufficient mechanistic understanding of on-surface reactions and the discriminations in methodologies block it out from the conventional cognition of reaction and catalysis, which inhibits the extensive implication of on-surface synthesis. In this Perspective, we summarize the empirical paradigms of conceptually appealing programmability in on-surface synthesis. We endeavor to deliver the message that the impressive programmability is related to chemical heterogeneity which can also be coded at the molecular level and deciphered by the catalytic surfaces in varying chemical environments as specific chemical selectivity. With the assistance of structure-sensitive techniques, it is possible to recognize the chemical heterogeneity on surfaces to provide insight into the programmable on-surface construction of molecular nanoarchitectures and to reshape the correlation between the mechanistic understanding in on-surface synthesis and conventional chemistry.

Steering on-surface reactions through molecular steric hindrance and molecule-substrate van der Waals interactions

On-surface synthesis is a rapidly developing field involving chemical reactions on well-defined solid surfaces to access synthesis of low-dimensional organic nanostructures which cannot be achieved via traditional solution chemistry. On-surface reactions critically depend on a high degree of chemoselectivity in order to achieve an optimum balance between target structure and possible side products. Here, we demonstrate synthesis of graphene nanoribbons with a large unit cell based on steric hindrance-induced complete chemoselectivity as revealed by scanning probe microscopy measurements and density functional theory calculations. Our results disclose that combined molecule-substrate van der Waals interactions and intermolecular steric hindrance promote a selective aryl-aryl coupling, giving rise to high-quality uniform graphene nanostructures. The established coupling strategy has been used to synthesize two types of graphene nanoribbons with different edge topologies inducing a pronounced variation of the electronic energy gaps. The demonstrated chemoselectivity is representative for n-anthryl precursor molecules and may be further exploited to synthesize graphene nanoribbons with novel electronic, topological and magnetic properties with implications for electronic and spintronic applications.

Supplementary Information

The online version contains supplementary material available at 10.1007/s44214-022-00023-9.

On-surface synthesis of ethers through dehydrative coupling of hydroxymethyl substituents

On-surface synthesis has been a subject of intensive research during the last decade. Various chemical reactions have been developed on surfaces to prepare compounds and carbon nanostructures, most of which...

On-Surface Synthesis of sp-Carbon Nanostructures

The on-surface synthesis of carbon nanostructures has attracted tremendous attention owing to their unique properties and numerous applications in various fields. With the extensive development of scanning tunneling microscope (STM) and noncontact atomic force microscope (nc-AFM), the on-surface fabricated nanostructures so far can be characterized on atomic and even single-bond level. Therefore, various novel low-dimensional carbon nanostructures, challenging to traditional solution chemistry, have been widely studied on surfaces, such as polycyclic aromatic hydrocarbons, graphene nanoribbons, nanoporous graphene, and graphyne/graphdiyne-like nanostructures. In particular, nanostructures containing sp-hybridized carbons are of great advantage for their structural linearity and small steric demands as well as intriguing electronic and mechanical properties. Herein, the recent developments of low-dimensional sp-carbon nanostructures fabricated on surfaces will be summarized and discussed.

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.

On-Surface Synthesis of Porphyrin-Complex Multi-Block Co-Oligomers by Defluorinative Coupling

On-surface chemical reaction has become a very powerful technique to synthesize nanostructures by linking small molecules in the bottom-up approach. Given the fact that most reactants are simultaneously activated at certain temperatures, a sequential reaction in a controlled way has remained challenging. Here, we present an on-surface synthesis of multi-block co-oligomers from trifluoromethyl (CF₃ ) substituted porphyrin metal complexes. The oligomerization on Au(111) is demonstrated with a combination of bond-resolved scanning probe microscopy and density functional theory (DFT) calculations. Even after the first oligomerization of single monomer unit, the termini of the oligomer keep the CF₃ group, which can be used as a reactant for further coupling in a sequential order. Consequently, copper, cobalt, and palladium complexes of bisanthracene-fused porphyrin oligomers were linked with each other in a designed order.

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.

Abiotic Formation of Amide Bond via Surface-Supported Direct Carboxyl-Amine Coupling

Amide bond formation is one of the most important reactions in biochemistry, notably being of crucial importance for the origin of life. Herein, we combine scanning tunneling microscopy and X-ray photoelectron spectroscopy studies to provide evidence for thermally activated abiotic formation of amide bonds between adsorbed precursors through direct carboxyl-amine coupling under ultrahigh vacuum conditions by means of on-surface synthesis. Complementary insights from temperature-programmed desorption measurements and density functional theory calculations reveal the competition between cross-coupling amide formation and decarboxylation reactions on the Au(111) surface. Furthermore, we demonstrate the critical influence of the employed metal support: whereas on Au(111) the coupling readily occurs, different reaction scenarios prevail on Ag(111) and Cu(111). The systematic experiments signal that archetypical bio-related molecules can be abiotically synthesized in clean environments without water or oxygen.

Direct growth of crystalline triazine-based graphdiyne using surface-assisted deprotection-polymerisation

Graphdiyne polymers have interesting electronic properties due to their π-conjugated structure and modular composition. Most of the known synthetic pathways for graphdiyne polymers yield amorphous solids because the irreversible formation of carbon–carbon bonds proceeds under kinetic control and because of defects introduced by the inherent chemical lability of terminal alkyne bonds in the monomers. Here, we present a one-pot surface-assisted deprotection/polymerisation protocol for the synthesis of crystalline graphdiynes over a copper surface starting with stable trimethylsilylated alkyne monomers. In comparison to conventional polymerisation protocols, our method yields large-area crystalline thin graphdiyne films and, at the same time, minimises detrimental effects on the monomers like oxidation or cyclotrimerisation side reactions typically associated with terminal alkynes. A detailed study of the reaction mechanism reveals that the deprotection and polymerisation of the monomer is promoted by Cu(ii) oxide/hydroxide species on the as-received copper surface. These findings pave the way for the scalable synthesis of crystalline graphdiyne-based materials as cohesive thin films.

We present a one-pot deprotection/polymerisation protocol for the synthesis of crystalline graphdiynes on top of a copper surface starting with stable trimethylsilylated alkyne monomers.

On-surface synthesis of organocopper metallacycles through activation of inner diacetylene moieties

The design of organometallic complexes is at the heart of modern organic chemistry and catalysis. Recently, on-surface synthesis has emerged as a disruptive paradigm to design previously precluded compounds and nanomaterials. Despite these advances, the field of organometallic chemistry on surfaces is still at its infancy. Here, we introduce a protocol to activate the inner diacetylene moieties of a molecular precursor by copper surface adatoms affording the formation of unprecedented organocopper metallacycles on Cu(111). The chemical structure of the resulting complexes is characterized by scanning probe microscopy and X-ray photoelectron spectroscopy, being complemented by density functional theory calculations and scanning probe microscopy simulations. Our results pave avenues to the engineering of organometallic compounds and steer the development of polyyne chemistry on surfaces.

The diacetylene skeletons of DNBD precursors are attacked on Cu(111) by copper adatoms resulting in the synthesis of organocopper metallacycles.

Head-to-Tail Oligomerization by Silylene-Tethered Sonogashira Coupling on Ag(111)

On-surface synthesis is a powerful method for the fabrication of π-conjugated nanomaterials. Herein, we demonstrate chemoselective Sonogashira coupling between (trimethylsilyl)ethynyl and chlorophenyl groups in silylethynyl- and chloro-substituted partially fluorinated phenylene ethynylenes (SiCPFPEs) on Ag(111). The desilylative Sonogashira coupling occurred with high chemoselectivity up to 75 %, while the competing Ullmann and desilylative Glaser homocoupling reactions were suppressed. A combination of bond-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) and DFT calculations revealed that the oligomers were obtained by the formation of intermolecular silylene tethers (-Me₂ Si-) through CH₃ -Si bond activation at 130 °C and subsequent elimination of the tethers at an elevated temperature of 200 °C.