Homo-coupling of terminal alkynes on a noble metal surface

Unconventional Band Structure via Combined Molecular Orbital and Lattice Symmetries in a Surface-Confined Metallated Graphdiyne Sheet

Graphyne (GY) and graphdiyne (GDY)-based monolayers represent the next generation 2D carbon-rich materials with tunable structures and properties surpassing those of graphene. However, the detection of band formation in atomically thin GY/GDY analogues has been challenging, as both long-range order and atomic precision have to be fulfilled in the system. The present work reports direct evidence of band formation in on-surface synthesized metallated Ag-GDY sheets with mesoscopic (≈1 µm) regularity. Employing scanning tunneling and angle-resolved photoemission spectroscopies, energy-dependent transitions of real-space electronic states above the Fermi level and formation of the valence band are respectively observed. Furthermore, density functional theory (DFT) calculations corroborate the observations and reveal that doubly degenerate frontier molecular orbitals on a honeycomb lattice give rise to flat, Dirac and Kagome bands close to the Fermi level. DFT modeling also indicates an intrinsic band gap for the pristine sheet material, which is retained for a bilayer with h-BN, whereas adsorption-induced in-gap electronic states evolve at the synthesis platform with Ag-GDY decorating the (111) facet of silver. These results illustrate the tremendous potential for engineering novel band structures via molecular orbital and lattice symmetries in atomically precise 2D carbon materials.

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.

Transmetalation in Surface-Confined Single-Layer Organometallic Networks with Alkynyl-Metal-Alkynyl Linkages

Transmetalation represents an appealing strategy toward fabricating and tuning functional metal-organic polymers and frameworks for diverse applications. In particular, building two-dimensional metal-organic and organometallic networks affords versatile nanoarchitectures of potential interest for nanodevices and quantum technology. The controlled replacement of embedded metal centers holds promise for exploring versatile material varieties by serial modification and different functionalization. Herein, we introduce a protocol for the modification of a single-layer carbon-metal-based organometallic network via transmetalation. By integrating external Cu atoms into the alkynyl-Ag organometallic network constructed with 1,3,5-triethynylbenzene precursors, we successfully realized in situ its highly regular alkynyl-Cu counterpart on the Ag(111) surface. While maintaining a similar lattice periodicity and pore morphology to the original alkynyl-Ag sheet, the Cu-based network exhibits increased thermal stability, guaranteeing improved robustness for practical implementation.

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.

100% selective cyclotrimerization of acetylene to benzene on Ag(111)

Benzene, a high-volume chemical, is produced from larger molecules by inefficient and environmentally harmful processes. Recent changes in hydrocarbon feedstocks from oil to gas motivate research into small molecule upgrading. For example, the cyclotrimerization of acetylene reaction has been demonstrated on Pd, Pd alloy, and Cu surfaces and catalysts, but they are not 100% selective to benzene. We discovered that acetylene can be converted to benzene with 100% selectivity on the Ag(111) surface. Our temperature programmed desorption experiments reveal a threshold acetylene surface coverage of ∼one monolayer, above which benzene is formed. Furthermore, additional layers of acetylene increase the amount of benzene produced while retaining 100% selectivity. Our scanning tunneling microscopy images show that acetylene prefers square packing on the Ag(111) surface at low coverages, which converts to hexagonal packing when acetylene multilayers are present. Within this denser layer, features consistent with the proposed C4 intermediates of the cyclotrimerization process are observed. Density functional theory calculations demonstrate that the barrier for forming the crucial C4 intermediate generally decreases as acetylene multilayers are formed because the multilayer interacts more strongly with the surface in the transition state than in the initial state. Given that acetylene desorbs from Ag(111) at ∼90 K, the C4 intermediate on the pathway to benzene must be formed below this temperature, implying that if Ag-based heterogeneous catalysts can be run at sufficiently high pressure and low enough temperature, efficient and selective trimerization of acetylene to benzene may be possible.

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 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.

Universal inter-molecular radical transfer reactions on metal surfaces

On-surface synthesis provides tools to prepare low-dimensional supramolecular structures. Traditionally, reactive radicals are a class of single-electron species, serving as exceptional electron-withdrawing groups. On metal surfaces, however, such species are affected by conduction band screening effects that may even quench their unpaired electron characteristics. As a result, radicals are expected to be less active, and reactions catalyzed by surface-stabilized radicals are rarely reported. Herein, we describe a class of inter-molecular radical transfer reactions on metal surfaces. With the assistance of aryl halide precursors, the coupling of terminal alkynes is steered from non-dehydrogenated to dehydrogenated products, resulting in alkynyl-Ag-alkynyl bonds. Dehalogenated molecules are fully passivated by detached hydrogen atoms. The reaction mechanism is unraveled by various surface-sensitive technologies and density functional theory calculations. Moreover, we reveal the universality of this mechanism on metal surfaces. Our studies enrich the on-surface synthesis toolbox and develop a pathway for producing low-dimensional organic materials.

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.

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.

Multimodal Imaging of Pancreatic Cancer Microenvironment in Response to an Antiglycolytic Drug

Lipid metabolism and glycolysis play crucial roles in the progression and metastasis of cancer, and the use of 3-bromopyruvate (3-BP) as an antiglycolytic agent has shown promise in killing pancreatic cancer cells. However, developing an effective strategy to avoid chemoresistance requires the ability to probe the interaction of cancer drugs with complex tumor-associated microenvironments (TAMs). Unfortunately, no robust and multiplexed molecular imaging technology is currently available to analyze TAMs. In this study, the simultaneous profiling of three protein biomarkers using SERS nanotags and antibody-functionalized nanoparticles in a syngeneic mouse model of pancreatic cancer (PC) is demonstrated. This allows for comprehensive information about biomarkers and TAM alterations before and after treatment. These multimodal imaging techniques include surface-enhanced Raman spectroscopy (SERS), immunohistochemistry (IHC), polarized light microscopy, second harmonic generation (SHG) microscopy, fluorescence lifetime imaging microscopy (FLIM), and untargeted liquid chromatography and mass spectrometry (LC-MS) analysis. The study reveals the efficacy of 3-BP in treating pancreatic cancer and identifies drug treatment-induced lipid species remodeling and associated pathways through bioinformatics analysis.

On-surface synthesis of nitrogen-doped nanographene with an [18]annulene pore on Ag(111)

On-surface synthesis is of importance to fabricate low dimensional carbon-based nanomaterials with atomic precision. Here, we synthesize nitrogen-doped nanographene with an [18]annulene pore and its dimer through sequential reactions of debromination, aryl-aryl coupling, cyclodehydrogenation and C-N coupling on Ag(111) from 3,12-dibromo-7,8-diaza[5]helicene. The inner structures of the products were characterized with scanning tunneling microscopy with a CO terminated tip at low temperature. Furthermore, the first four unoccupied electronic states of the nanographene were investigated with a combination of scanning tunneling spectroscopy and theoretical calculations. Except for the LUMO + 2 state observed at +1.3 V, the electronic states at 500 mV, 750 mV and 1.9 V were attributed to the superatom molecular orbitals at the [18]annulene pore, which were significantly shifted towards the Fermi level due to the hybridization with the confined surface state.

On-Surface Two-Dimensional Polymerization: Advances, Challenges, and Prospects

Two-dimensional polymers (2DPs) are molecularly thin networks consisting of monomers covalently linked in at least two directions in the molecular plane. Because of the unique structural features and emergent physicochemical properties, 2DPs promise application potentials in catalysis, chemical sensing, and organic electronic devices. On-surface synthesis is of great interest to fabricate 2DPs with atomic precision, and the properties of the 2DPs can be characterized in situ through scanning probe techniques. In this Perspective, we first introduce the recent developments of on-surface 2D polymerization, including the design principle, the synthetic reactions, and the factors affecting the synthesis of 2DPs on surface. Then, we summarize some major challenges in this field, including the fabrication of high-quality 2DPs and the study of the intrinsic electronic properties of 2DPs, and we discuss some of the available solutions to address these issues.

Low-Temperature Layer-by-Layer Growth of Semiconducting Few-Layer γ-Graphyne to Exploit Robust Biocompatibility

The sp-hybridized carbon network in single- or few-layer γ-graphyne (γ-GY) has a polarized electron distribution, which can be crucial in overcoming biosafety issues. Here, we report the low-temperature synthesis, electronic properties, and amyloid fibril nanostructures of electrostatic few-layer γ-GY. ABC stacked γ-GY is synthesized by layer-by-layer growth on a catalytic copper surface, exhibiting intrinsic p-type semiconducting properties in few-layer γ-GY. Thickness-dependent electronic properties of γ-GY elucidate interlayer interactions by electron doping between electrostatic layers and layer stacking-involved modulation of the band gap. Electrostatic few-layer γ-GY induces high electronic sensitivity and intense interaction with amyloid beta (i.e., Aβ40) peptides assembling into elongated mature Aβ40 fibrils. Two-dimensional biocompatible nanostructures of Aβ40 fibrils/few-layer γ-GY enable excellent cell viability and high neuronal differentiation of living cells without external stimulation.

Toward Two-Dimensional Tessellation through Halogen Bonding between Molecules and On-Surface-Synthesized Covalent Multimers

The ability to engineer sophisticated two-dimensional tessellation organic nanoarchitectures based on triangular molecules and on-surface-synthesized covalent multimers is investigated using scanning tunneling microscopy. 1,3,5-Tris(3,5-dibromophenyl)benzene molecules are deposited on high-temperature Au(111) surfaces to trigger Ullmann coupling. The self-assembly into a semi-regular rhombitrihexagonal tiling superstructure not only depends on the synthesis of the required covalent building blocks but also depends on their ratio. The organic tessellation nanoarchitecture is achieved when the molecules are deposited on a Au(111) surface at 145 °C. This halogen-bonded structure is composed of triangular domains of intact molecules separated by rectangular rows of covalent dimers. The nearly hexagonal vertices are composed of covalent multimers. The experimental observations reveal that the perfect semi-regular rhombitrihexagonal tiling cannot be engineered because it requires, in addition to the dimers and intact molecules, the synthesis of covalent hexagons. This building block is only observed above 165 °C and does not coexist with the other required organic buildings blocks.

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.

The Intramolecular Povarov Tool in the Construction of Fused Nitrogen-Containing Heterocycles

Nitrogen heterocycles are part of the structure of natural products and agents with important biological activity, such as antiviral, antibiotic, and antitumor drugs. For this reason, heterocyclic compounds are one of today's most desirable synthetic targets and the Povarov reaction is a powerful synthetic tool for the construction of highly functionalized heterocyclic systems. This process involves an aromatic amine, a carbonyl compound, and an olefin or acetylene to give rise to the formation of a nitrogen-containing heterocycle. This review illustrates advances in the synthetic aspects of the intramolecular Povarov reaction for the construction of intricate nitrogen-containing polyheterocyclic compounds. This original review presents research done in this field, with references to important works by internationally relevant research groups on this current topic, covering the literature from 1992 to 2022. The intramolecular Povarov reactions are described here according to the key processes involved, using different combinations of aromatic or heteroaromatic amines, and aliphatic, aromatic, or heteroaromatic aldehydes. Some catalytic reactions promoted by transition metals are detailed, as well as the oxidative Povarov reaction and some asymmetric intramolecular Povarov processes.

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.

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.

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.

Unveiling the formation mechanism of the biphenylene network

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

On-surface 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.

Ultra-Fast Preparation of Large-Area Graphdiyne-Based Membranes via Alkynylated Surface-Modification for Nanofiltration

Graphdiynes (GDYs), two-dimensional graphene-like carbon systems, are considered as potential advanced membrane material due to their unique physicochemical features. Nevertheless, the scale-up of integrated GDY membranes is technologically challenging, and most studies remain at the theoretical stage. Herein, we report a simple and efficient alkynylated surface-mediated strategy to prepare hydrogen-substituted graphdiyne (HsGDY) membranes on commercial alumina tubes. Surface alkynylation initiates an accelerated surface-confined coupling reaction in the presence of a copper catalyst and facilitates the nanoscale epitaxial lateral growth of HsGDY. A continuous and ultra-thin HsGDY membrane (∼100 nm) can be produced within 15 min. The resulting membranes exhibit outstanding molecular sieving together with excellent water permeances (ca. 1100 L m^-2  h^-1 MPa^-1 ), and show a long-term durability in cross-flow nanofiltration, owing to the superhydrophilic surface and hydrophobic pore walls.

Fast water transport and molecular sieving through ultrathin ordered conjugated-polymer-framework membranes

The development of membranes that block solutes while allowing rapid water transport is of great importance. The microstructure of the membrane needs to be rationally designed at the molecular level to achieve precise molecular sieving and high water flux simultaneously. We report the design and fabrication of ultrathin, ordered conjugated-polymer-framework (CPF) films with thicknesses down to 1 nm via chemical vapour deposition and their performance as separation membranes. Our CPF membranes inherently have regular rhombic sub-nanometre (10.3 × 3.7 Å) channels, unlike membranes made of carbon nanotubes or graphene, whose separation performance depends on the alignment or stacking of materials. The optimized membrane exhibited a high water/NaCl selectivity of ∼6,900 and water permeance of ∼112 mol m^-2 h^-1 bar^-1, and salt rejection >99.5% in high-salinity mixed-ion separations driven by osmotic pressure. Molecular dynamics simulations revealed that water molecules quickly and collectively pass through the membrane by forming a continuous three-dimensional network within the hydrophobic channels. The advent of ordered CPF provides a route towards developing carbon-based membranes for precise molecular separation.

Converting n-Alkanol to Conjugated Polyenal on Cu(110) Surface at Mild Temperature

Achieving C(sp³)-H activation at a mild temperature is of great importance from both scientific and technologic points of view. Herein, on the basis of the on-surface synthesis strategy, we report the significant reduction of the C(sp³)-H activation barrier, which results in the full C(sp³)-H to C(sp²)-H transformation of n-alkanol (octacosan-1-ol) at a mild temperature as low as 350 K on the Cu(110) surface, yielding the conjugated polyenal (octacosa-tridecaenal) as the final product. The reaction mechanism is revealed by the combined scanning tunneling microscope, density functional theory, and synchrotron radiation photoemission spectroscopy.

2D graphdiyne: an emerging carbon material

As a new member of carbon allotropes, graphdiyne (GDY) has the characteristics of being one-atom-thick with two-dimensional layers comprising sp and sp² hybridized carbon atoms, and represents a trend in the development of carbon materials. Its unique chemical and electronic structures give GDY many unique and fascinating properties such as rich chemical bonds, highly conjugated and super-large π structures, infinitely distributed pores and high inhomogeneity of charge distribution. GDY has entered a period of rapid development, especially with the significant emergence of fundamental research and applied research achievements over the past five years. As one of the frontiers of chemistry and materials science, graphdiyne was listed in the Top 10 research areas in the 2020 Research Frontiers report and was jointly released in the Top 10 in the world by Clarivate and the Chinese Academy of Sciences. The research results have shown the great potential of GDY in the applications of energy, catalysis, environmental science, electronic devices, detectors, biomedicine and therapy, etc. Scientists are eager to explore and fully reveal the new properties, discover new scientific concepts and phenomena, discover the new conversion modes and mechanisms of GDY in photoelectricity, energy, and catalysis, etc., and build the important scientific value of new conversion devices. This review covers research on the foundation and application of GDY, such as the controlled preparation of new methods of GDY and GDY-based materials, studies on new mechanisms and properties in chemistry and physics, and the foundation and applications in energy, catalysis, photoelectric and 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.

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.

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.

Graphdiyne: from Preparation to Biomedical Applications

Graphdiyne(GDY) is a kind of two-dimensional carbon nanomaterial with specific configurations of sp and sp 2 carbon atoms. The key progress in the preparation and application of GDY is bringing carbon materials to a brand-new level. Here, the various properties and structures of GDY are introduced, including the existing strategies for the preparation and modification of GDY. In particular, GDY has gradually emerged in the field of life sciences with its unique properties and performance, therefore, the development of biomedical applications of GDY is further summarized. Finally, the challenges of GDY toward future biomedical applications are discussed.

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.

On-surface synthesis of 2D COFs via molecular assembly directed photocycloadditions: a first-principles investigation

Over the past decades, the rational synthesis of two-dimensional covalent organic framework (2D COFs) monolayer via on-surface chemistry has been widely explored. Herein, we propose the [2 + 2] photocycloaddition as a novel strategy for large-scale fabrication of COFs from theoretical perspective. Thanks to the symmetry forbidden of thermal [2 + 2] cycloaddition, the molecular precursors carrying vinyl groups will not chemically interact with each other during thermal annealing, which is essential to achieve molecular assembly. The subsequent photocycloaddition of these precursors may produce large-scale 2D COFs at low temperatures, in which the symmetry of molecular assembly remains unchanged. Our results show that 2D COFs can be produced via [2 + 2] photocycloadditions directed from self-assembled precursors, in which alkylbenzene molecules with vinyl groups on side chains exhibit appropriate intermolecular distances. By performing high-throughput calculations, several promising molecular precursors are proposed to achieve large-scale 2D COFs. This work provides an applicable strategy for the large-scale synthesis of 2D carbon materials.

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.

Developments in Synthesis and Potential Electronic and Magnetic Applications of Pristine and Doped Graphynes

Doping and its consequences on the electronic features, optoelectronic features, and magnetism of graphynes (GYs) are reviewed in this work. First, synthetic strategies that consider numerous chemically and dimensionally different structures are discussed. Simultaneous or subsequent doping with heteroatoms, controlling dimensions, applying strain, and applying external electric fields can serve as effective ways to modulate the band structure of these new sp2/sp allotropes of carbon. The fundamental band gap is crucially dependent on morphology, with low dimensional GYs displaying a broader band gap than their bulk counterparts. Accurately chosen precursors and synthesis conditions ensure complete control of the morphological, electronic, and physicochemical properties of resulting GY sheets as well as the distribution of dopants deposited on GY surfaces. The uniform and quantitative inclusion of non-metallic (B, Cl, N, O, or P) and metallic (Fe, Co, or Ni) elements into graphyne derivatives were theoretically and experimentally studied, which improved their electronic and magnetic properties as row systems or in heterojunction. The effect of heteroatoms associated with metallic impurities on the magnetic properties of GYs was investigated. Finally, the flexibility of doped GYs' electronic and magnetic features recommends them for new electronic and optoelectronic applications.

On-Surface Decarboxylation Coupling Facilitated by Lock-to-Unlock Variation of Molecules upon the Reaction

On-surface synthesis (OSS) involving relatively high energy barriers remains challenging due to a typical dilemma: firm molecular anchor is required to prevent molecular desorption upon the reaction, whereas sufficient lateral mobility is crucial for subsequent coupling and assembly. By locking the molecular precursors on the substrate then unlocking them during the reaction, we present a strategy to address this challenge. High-yield synthesis based on well-defined decarboxylation, intermediate transition, and hexamerization is demonstrated, resulting in an extended and ordered network exclusively composed of the newly synthesized macrocyclic compound. Thanks to the steric hindrance of its maleimide group, we attain a preferential selection of the coupling. This work unlocks a promising path to enrich the reaction types and improve the coupling selectivity hence the structual homogeneity of the final product for OSS.

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.