Homo-coupling of terminal alkynes on a noble metal surface

Chemical Identification and Bond Control of π-Skeletons in a Coupling Reaction

Highly unsaturated π-rich carbon skeletons afford versatile tuning of structural and optoelectronic properties of low-dimensional carbon nanostructures. However, methods allowing more precise chemical identification and controllable integration of target sp-/sp²-carbon skeletons during synthesis are required. Here, using the coupling of terminal alkynes as a model system, we demonstrate a methodology to visualize and identify the generated π-skeletons at the single-chemical-bond level on the surface, thus enabling further precise bond control. The characteristic electronic features together with localized vibrational modes of the carbon skeletons are resolved in real space by a combination of scanning tunneling microscopy/spectroscopy (STM/STS) and tip-enhanced Raman spectroscopy (TERS). Our approach allows single-chemical-bond understanding of unsaturated carbon skeletons, which is crucial for generating low-dimensional carbon nanostructures and nanomaterials with atomic precision.

Challenges in the synthesis of corannulene-based non-planar nanographenes on Au(111) surfaces

The on-surface synthesis of non-planar nanographenes is a challenging task. Herein, with the aid of bond-resolving scanning tunneling microscopy (BRSTM) and density functional theory (DFT) calculations, we present a systematic study aiming at the fabrication of corannulene-based nanographenes via intramolecular cyclodehydrogenation on a Au(111) surface. The formation of non-planar targeted products is confirmed to be energetically unfavorable compared to the formation of planar/quasi-planar undesired competing monomer products. In addition, the activation of intermolecular coupling further inhibits the formation of the final targeted product. Although it was not possible to access the corannulene moiety by means of on-surface synthesis, partial cyclodehydrogenation of the molecular precursors was demonstrated.

Heterocyclic Ring-Opening of Nanographene on Au(111)

Cyclo-dehydrogenation is of importance to induce the planarization of molecules on noble surfaces upon annealing. In contrast to a number of successful syntheses of polycyclic aromatic hydrocarbons by forming carbon-carbon bonds, it is still rare to conduct conjugation and cleavage of carbon-nitrogen bonds in molecules. Here, we present a systematic transformation of the C-N bonds in11,11,12,12-tetraphenyl-1,4,5,8-tetraazaanthraquinodimethane as well as three other derivatives on Au(111). With bond-resolved scanning tunneling microscopy, we discovered novel the "heterocyclic segregation" reaction of one pyrazine ring with two nitrogen atoms to form two quinoline rings with one nitrogen each. Density functional theory calculations showed that the intramolecular ring-forming and -opening of N-heterocycles are strongly affected by the initial hydrogen-substrate interaction.

On-Surface Synthesis and Molecular Engineering of Carbon-Based Nanoarchitectures

On-surface synthesis via covalent coupling of adsorbed precursor molecules on metal surfaces has emerged as a promising strategy for the design and fabrication of novel organic nanoarchitectures with unique properties and potential applications in nanoelectronics, optoelectronics, spintronics, catalysis, etc. Surface-chemistry-driven molecular engineering (i.e., bond cleavage, linkage, and rearrangement) by means of thermal activation, light irradiation, and tip manipulation plays critical roles in various on-surface synthetic processes, as exemplified by the work from the Ernst group in a prior issue of ACS Nano. In this Perspective, we highlight recent advances in and discuss the outlook for on-surface syntheses and molecular engineering of carbon-based nanoarchitectures.

Reassessing Alkyne Coupling Reactions While Studying the Electronic Properties of Diverse Pyrene Linkages at Surfaces

The combination of alkyne and halogen functional groups in the same molecule allows for the possibility of many different reactions when utilized in on-surface synthesis. Here, we use a pyrene-based precursor with both functionalities to examine the preferential reaction pathway when it is heated on an Au(111) surface. Using high-resolution bond-resolving scanning tunneling microscopy, we identify multiple stable intermediates along the prevailing reaction pathway that initiate with a clearly dominant Glaser coupling, together with a multitude of other side products. Importantly, control experiments with reactants lacking the halogen functionalization reveal the Glaser coupling to be absent and instead show the prevalence of non-dehydrogenative head-to-head alkyne coupling. We perform scanning tunneling spectroscopy on a rich variety of the product structures obtained in these experiments, providing key insights into the strong dependence of their HOMO-LUMO gaps on the nature of the intramolecular coupling. A clear trend is found of a decreasing gap that is correlated with the conversion of triple bonds to double bonds via hydrogenation and to higher levels of cyclization, particularly with nonbenzenoid product structures. We rationalize each of the studied cases.

Cyclobenzoin Esters as Hosts for Thin Guests

Cyclotetrabenzoin esters can host terminal triple bonds of alkynes and nitriles in their cavities, as revealed by cocrystal structures of four such complexes. Within cyclotetrabenzoin cavities, π-clouds of triple bonds establish favorable and virtually equidistant interactions with the four aromatic walls of the cyclotetrabenzoin skeleton. Binding is selective for aliphatic nitriles and terminal alkynes, with their aromatic counterparts residing outside of the cyclotetrabenzoin cavity.

Copper(i) oxide nanoparticle-mediated C–C couplings for synthesis of polyphenylenediethynylenes: evidence for a homogeneous catalytic pathway

Polyphenylenediethynylenes have been synthesized using copper(i) oxide nanocatalysts under ligandless conditions, mild base, and atmospheric air as the oxidant in good yield and number average molecular weight.

Structural, Electronic, and Vibrational Properties of a Two-Dimensional Graphdiyne-like Carbon Nanonetwork Synthesized on Au(111): Implications for the Engineering of sp-sp2 Carbon Nanostructures

Graphdiyne, atomically thin two-dimensional (2D) carbon nanostructure based on sp-sp2 hybridization is an appealing system potentially showing outstanding mechanical and optoelectronic properties. Surface-catalyzed coupling of halogenated sp-carbon-based molecular precursors represents a promising bottom-up strategy to fabricate extended 2D carbon systems with engineered structure on metallic substrates. Here, we investigate the atomic-scale structure and electronic and vibrational properties of an extended graphdiyne-like sp-sp2 carbon nanonetwork grown on Au(111) by means of the on-surface synthesis. The formation of such a 2D nanonetwork at its different stages as a function of the annealing temperature after the deposition is monitored by scanning tunneling microscopy (STM), Raman spectroscopy, and combined with density functional theory (DFT) calculations. High-resolution STM imaging and the high sensitivity of Raman spectroscopy to the bond nature provide a unique strategy to unravel the atomic-scale properties of sp-sp2 carbon nanostructures. We show that hybridization between the 2D carbon nanonetwork and the underlying substrate states strongly affects its electronic and vibrational properties, modifying substantially the density of states and the Raman spectrum compared to the free standing system. This opens the way to the modulation of the electronic properties with significant prospects in future applications as active nanomaterials for catalysis, photoconversion, and carbon-based nanoelectronics.

Silver(I) Oxide-/DBU-Promoted Synthesis of Dihydrofuran Units through Allenyl Silver Formation

A formal [3+2] cyclization mediated by silver(I) oxide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is described herein. Through a broad variety of carbonyl compounds, this system can promote cyclization reactions with high yield (up to 85 %) and diastereoselectivity (up to 95:5) for a straightforward access to complex and congested dihydrofuran derivatives in one step under mild conditions. Based on DFT studies, the proposed mechanism would involve an allenyl silver intermediate.

Metalated Graphyne-Based Networks as Two-Dimensional Materials: Crystallization, Topological Defects, Delocalized Electronic States, and Site-Specific Doping

Graphyne-based two-dimensional (2D) carbon allotropes feature extraordinary physical properties; however, their synthesis as crystalline single-layered materials has remained challenging. We report on the fabrication of large-area organometallic Ag-bis-acetylide networks and their structural and electronic properties on Ag(111) using low-temperature scanning tunneling microscopy combined with density functional theory (DFT) calculations. The metalated graphyne-based networks are robust at room temperature and assembled in a bottom-up approach via surface-assisted dehalogenative homocoupling of terminal alkynyl bromides. Large-area networks of several hundred nanometers with topological defects at domain boundaries are obtained due to the Ag-acetylide bonds' reversible nature. The thermodynamically controlled growth mechanism is explained through the direct observation of intermediates, which differ on Ag(111) and Au(111). Scanning tunneling spectroscopy resolved unoccupied states delocalized across the network. The energy of these states can be shifted locally by the attachment of a different number of Br atoms within the network. DFT revealed that free-standing metal-bis-acetylide networks are semimetals with a linear band dispersion around several high-symmetry points, which suggest the presence of Weyl points. These results demonstrate that the organometallic Ag-bis-acetylide networks feature the typical 2D material properties, which make them of great interest for fundamental studies and electronic materials in devices.

Intramolecular Coupling of Terminal Alkynes by Atom Manipulation

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

C-C Coupling Reactions for the Synthesis of Two-Dimensional Conjugated Polymers

Extension of conjugated polymers from 1D to 2D can not only significantly enhance the dissociation of charge and excitons, but also induce other advantages, such as high in-plane mechanical strength, large specific surface area and porosity, and more active centers. 2D conjugated polymers can be divided into C-C bonded 2D polymers based on C-C coupling reactions, and heteroatomic bonded 2D polymers based on reversible heteroatom coupling reactions. C-C bonded 2D polymers are generally more stable than heteroatomic bonded 2D polymers as the latter bonds are easily hydrolyzed. This Review mainly summarizes C-C coupling reactions that are suitable for synthesizing 2D conjugated polymers, and the properties of these 2D conjugated polymers are also introduced.

Artificial Carbon Graphdiyne: Status and Challenges in Nonlinear Photonic and Optoelectronic Applications

The creative integration of sp-hybridized carbon atoms into artificial carbon graphdiyne has led to graphdiyne with superior properties in terms of uniformly distributed pores, ambipolar carrier transport, natural bandgap, and broadband absorption. Consequently, graphdiyne, regarded as a promising carbon material, has garnered particular attention in light-matter interactions. Light-matter interactions play an important role in optical information technology and meet the increasing demand for various energy sources. Herein, the status and challenges in nonlinear photonic and optoelectronic applications of graphdiyne, which are still in the infancy stage, are summarized. Furthermore, the bottleneck and perspective of graphdiyne in these aspects are discussed. It is therefore anticipated that this review could promote the development of graphdiyne in photonic and optoelectronic fields.

Atomic-Scale Visualization of Stepwise Growth Mechanism of Metal-Alkynyl Networks on Surfaces

One of the most appealing topics in the study of metal-organic networks is the growth mechanism. However, its study is still considered a significant challenge. Herein, using scanning tunneling microscopy, the growth mechanisms of metal-alkynyl networks on Ag(111) and Au(111) surfaces were investigated at the atomic scale. During the reaction of 1,3,5-tris(chloroethynyl)benzene on Ag(111), honeycomb Ag-alkynyl networks formed at 393 K, and only short chain intermediates were observed. By contrast, the same precursor formed honeycomb Au-alkynyl networks on Au(111) at 503 K. Progression annealing led to a stepwise evolution process, in which the sequential activation of three Cl-alkynyl bonds led to the formation of dimers, zigzag chains, and novel chiral networks as the intermediates. Moreover, density functional theory calculations indicate that chlorine atoms are crucial in assisting the breakage of metal-alkynyl bonds to form Cl-metal-alkynyl, which guarantees the reversibility of the break/formation equilibration as the key to forming regular large-scale organometallic networks.

Journey from Small-Molecule Diyne Structures to 2D Graphdiyne: Synthetic Strategies

Graphdiyne (GDY) exhibits unique characteristics of a highly conjugated π system, evenly distributed nanopores, and a direct band gap. This has encouraged multidisciplinary research groups to investigate its application in energy conversion and storage, catalysts, electronic devices, sensing, and separation. Herein, the achievements of synthetic strategies for preparing small-molecule diyne structures (GDY substructure), 1D nanoribbons, and 2D GDY are presented. These studies may help future investigations into the basic structure-related properties of GDY and synthetic methodology for the future developments of GDY-related 2D carbon materials.

Stability of radical-functionalized gold surfaces by self-assembly and on-surface chemistry

We have investigated the radical functionalization of gold surfaces with a derivative of the perchlorotriphenylmethyl (PTM) radical using two methods: by chemisorption from the radical solution and by on-surface chemical derivation from a precursor. We have investigated the obtained self-assembled monolayers by photon-energy dependent X-ray photoelectron spectroscopy. Our results show that the molecules were successfully anchored on the surfaces. We have used a robust method that can be applied to a variety of materials to assess the stability of the functionalized interface. The monolayers are characterized by air and X-ray beam stability unprecedented for films of organic radicals. Over very long X-ray beam exposure we observed a dynamic nature of the radical-Au complex. The results clearly indicate that (mono)layers of PTM radical derivatives have the necessary stability to withstand device applications.

On-Surface Synthesis of All-cis Standing Phenanthrene Polymers upon Selective C-H Bond Activation

On-surface synthesis has emerged as a powerful approach to the atomically precise fabrication of molecular architectures with potential applications in nanotechnology. However, it is challenging to synthesize molecular structures that can protrude from the surface such as polymer chains forming by the molecules with upright conformations, since most of the on-surface reaction products, particularly the conjugated structures, prefer to adsorb parallel on the surface to maximize the molecule-substrate interaction. Here, we show an up-standing phenanthrene polymer chain with an all-cis configuration obtained by on-surface synthesis upon highly selective C-H activation. Using bond-resolved nc-AFM imaging, the reaction route of polymers from an in-plane to an all-cis upright conformation is fully characterized, and the reaction mechanism is further revealed in combination with first principles calculations. Our results on this selective dehydrogenation induced upright-oriented polymer chains that will enrich the toolbox for the on-surface synthesis of novel molecular structures and may provide new insights on designing optimized precursors for preparing three-dimensional molecular frameworks through on-surface synthesis.

Organometallic polymers synthesized from prochiral molecules by a surface-assisted synthesis on Ag(111)

Organometallic polymers can be successfully synthesized on a Ag(111) surface via a surface-assisted synthesis by choosing prochiral 4,4'-dibromo-2,2'-bis(2-phenylethynyl)-1,1'-biphenyl (DBPB) molecules as the designed precursor. High-resolution scanning tunneling microscopy investigation reveals that prochiral molecules show chirality on a surface and can evolve into organometallic chains on the Ag(111) surface based on Ullmann coupling. Due to the special structural features of DBPB molecules, chiral selectivity will be lost in the organometallic polymers. This result may provide an important basis for selecting suitable precursors to fabricate chiral covalent nanostructures on a surface.

Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2] cycloadditions

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces.

Dissymmetric On-Surface Dehalogenation Reaction Steered by Preformed Self-Assembled Structure

Ullmann coupling of 4,4″-dibromo-p-terphenyl (DBTP) thermally catalyzed on a Ag(111) surface was studied by scanning tunneling microscopy. Detailed experimental measurement shows that the Ullmann coupling reaction pathways of DBTP molecules can be controlled by pre-self-assembly, and the dissymmetric dehalogenation reaction is realized. Moreover, self-assembly of the reactants in a rectangular network undergoes a dissymmetric debromination transfer to a newly observed rhombic network formed by organometallic dimers prior to the formation of longer symmetric organometallic intermediates on a Ag(111) surface, while the ladder assembled phase is more likely to induce the symmetric debromination reaction and converts into the symmetric organometallic intermediate. These findings help us to understand the essentials of the dissymmetric dehalogenation reaction that originated from a symmetric compound and pave new avenues for advancing the emerging field of on-surface synthesis.

Multiscale Plasma-Catalytic On-Surface Assembly

Controlled modification of surfaces is one of the key pursuits of the nanoscience and nanotechnology fields, allowing for the fabrication of bespoke materials with targeted functionalities. However, many surface modifications currently require painstakingly precise and/or energy intensive processing to implement, and are thus limited in scope and scale. Here, a concept which can enhance the capacity for control of surfaces is introduced: plasma-assisted nucleation and self-assembly at atomic to nanoscales, scalable at atmospheric pressures.

On-surface synthesis of graphyne nanowires through stepwise reactions

We have achieved on-surface synthesis of graphyne nanowires through stepwise reactions involving two different types of dehalogenative homocoupling reactions (i.e., C(sp³)–Br and C(sp²)–Br).

Research Progress on Thermal Conductivity of Graphdiyne Nanoribbons and its Defects: A Review

BACKGROUND

Graphdiyne has a unique pi-conjugated structure, perfect pore distribution and adjustable electronic properties of sp2, sp hybrid planar framework. Due to the presence of acetylenic bonds, it has more excellent properties compared to grapheme, such as a unique structure-dependent Dirac cone, abundant carbon bonds and a large bandgap. As one of the important raw materials for nanodevices, it is extremely important to study the thermal properties of graphdiyne nanoribbon.

OBJECTIVE

This paper mainly introduces and discusses recent academic research and patents on the preparation methods and thermal conductivity of graphdiyne nanoribbons. Besides, the applications in engineering and vacancy defects in the preparation process of graphdiyne are described.

METHODS

Firstly, taking thermal conductivity as an index, the thermal conductivity of graphdiyne with various vacancy defects is discussed from the aspects of length, defect location and defect type. In addition, the graphdiyne nanoribbons were laterally compared with the thermal conductivity of the graphene nanoribbons.

RESULTS

The thermal conductivity of graphdiyne with defects increases with the length and width, which is lower than the intrinsic graphdiyne. The thermal conductivity of the acetylene chain lacking one carbon atom is higher than the one lacking the benzene ring. Typically, the thermal conductivity is larger in armchair than that of zigzag in the same size. Moreover the thermal conductivity of nanoribbons with double vacancy defects is lower than those nanoribbons with single vacancy defects, which can also decrease with the increase of temperature and the number of acetylene chains. The thermal conductivity is not sensitive to shear strain.

CONCLUSION

Due to the unique structure and electronic characteristics, graphdiyne has provoked an extensive research interest in the field of nanoscience. Graphdiyne is considered as one of the most promising materials of next-generation electronic devices.

Hydrogen bond guided synthesis of close-packed one-dimensional graphdiyne on the Ag(111) surface

Aided by hydrogen bonding, alkyne and oxazole functionalized precursors undergo uniform self-assembly, which serves as a template for the fabrication of one-dimensional graphdiyne-like wires on the Ag(111) surface.

Multiscale Design of Graphyne-Based Materials for High-Performance Separation Membranes

By varying the number of acetylenic linkages connecting aromatic rings, a new family of atomically thin graph-n-yne materials can be designed and synthesized. Generating immense scientific interest due to its structural diversity and excellent physical properties, graph-n-yne has opened new avenues toward numerous promising engineering applications, especially for separation membranes with precise pore sizes. Having these tunable pore sizes in combination with their excellent mechanical strength to withstand high pressures, free-standing graph-n-yne is theoretically posited to be an outstanding membrane material for separating or purifying mixtures of either gases or liquids, rivaling or even dramatically exceeding the capabilities of current, state-of-art separation membranes. Computational modeling and simulations play an integral role in the bottom-up design and characterization of these graph-n-yne materials. Thus, here, the state of the art in modeling α-, β-, γ-, δ-, and 6,6,12-graphyne nanosheets for synthesizing graph-2-yne materials and 3D architectures thereof is discussed. Different synthesis methods are described and a broad overview of computational characterizations of graph-n-yne's electrical, chemical, and thermal properties is provided. Furthermore, a series of in-depth computational studies that delve into the specifics of graph-n-yne's mechanical strength and porosity, which confer superior performance for separation and desalination membranes, are reviewed.

On-Surface Synthesis of Graphyne-Based Nanostructures

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

The Accelerating World of Graphdiynes

Graphdiyne (GDY), a 2D allotrope of graphene, is first synthesized in 2010 and has attracted attention as a new low-dimensional carbon material. This work surveys the literature on GDYs. The history of GDYs is summarized, including their relationship with 2D graphyne carbons and yearly publication trends. GDY is a molecule-based nanosheet woven from a molecular monomer, hexaethynylbenzene; thus, it is synthesized by bottom-up approaches, which allow rich variation via monomer design. The GDY family and the synthetic procedures are also described. Highly developed π-conjugated electronic structures are common important features in GDY and graphene; however, the coexistence of sp and sp² carbons differentiates GDY from graphene. This difference gives rise to unique physical properties, such as high conductivity and large carrier mobility. Next, the theoretical and experimental studies of these properties are described in detail. A wide variety of applications are proposed for GDYs, including electrocatalysts and energy devices, which exploit the carbon-rich nature, porous framework, and expanded π-electron system of these compounds. Finally, potential uses are discussed.

Exploring Approaches for the Synthesis of Few-Layered Graphdiyne

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

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

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

Photochemistry Highlights on On-Surface Synthesis

On-surface chemistry is a promising way to achieve the bottom-up construction of covalently-bonded molecular precursors into extended atomically-precise polymers adsorbed on surfaces. These polymers exhibit unprecedented physical or chemical properties which are of great interest for various potential applications. These nanostructures were mainly obtained in ultra-high vacuum (UHV) on noble metal single-crystal surfaces by thermal annealing as stimulus to provoke the polymerization with a catalytic role of the surface adatoms. Nevertheless, photons are also a powerful source of energy to induce the formation of covalent architectures, even if it is less-used on surfaces than in solution. In this minireview, we discuss the photo-induced on-surface polymerization from the basic mechanisms of photochemistry to the formation of extended polymers on different kinds of surfaces, which are characterized by scanning probe microscopies.

Reaction selectivity of homochiral versus heterochiral intermolecular reactions of prochiral terminal alkynes on surfaces

Controlling selectivity between homochiral and heterochiral reaction pathways on surfaces remains a great challenge. Here, competing reactions of a prochiral alkyne on Ag(111): two-dimensional (2D) homochiral Glaser coupling and heterochiral cross-coupling with a Bergman cyclization step have been examined. We demonstrate control strategies in steering the reactions between the homochiral and heterochiral pathways by tuning the precursor substituents and the kinetic parameters, as confirmed by high-resolution scanning probe microscopy (SPM). Control experiments and density functional theory (DFT) calculations reveal that the template effect of organometallic chains obtained under specific kinetic conditions enhances Glaser coupling between homochiral molecules. In contrast, for the reaction of free monomers, the kinetically favorable reaction pathway is the cross-coupling between two heterochiral molecules (one of them involving cyclization). This work demonstrates the application of kinetic control to steer chiral organic coupling pathways at surfaces.

Controlling selectivity between homochiral and heterochiral reaction pathways on surfaces is intriguing but challenging. Here, the authors demonstrate strategies in steering the reactions of prochiral terminal alkynes between the homochiral and heterochiral pathways by tuning the precursor substituents and the kinetic parameters.

Cleaning up after the Party: Removing the Byproducts of On-Surface Ullmann Coupling

Ullmann coupling is one of the most frequently employed methodologies for producing π-conjugated surface-confined polymers. One unfortunate side product of the reaction is the creation of metal halide islands formed from liberated halogen atoms. Following the coupling reaction, these halide islands can account for a large proportion of the substrate surface area and thus inhibit domain growth and effectively poison the catalyst. Here, we describe an efficient and reliable methodology for removing the halogen byproduct at room temperature by exposure to a beam of atomic hydrogen; this action removes the halogen atoms in a matter of minutes, with minimal impact to the polymer structure. We also find that it is possible under certain circumstances to preserve the pre-exposure epitaxy after removal of the halogen. This finding provides a convenient and straightforward technique for addressing the most often-cited drawback of the on-surface Ullman coupling methodology and provides access to a previously inaccessible parameter space for these types of experiments.

One-Dimensional Double Wires and Two-Dimensional Mobile Grids: Cobalt/Bipyridine Coordination Networks at the Solid/Liquid Interface

Various architectures have been generated and observed by STM at a solid/liquid interface resulting from an in situ chemical reaction between the bipyridine terminal groups of a ditopic ligand and Co(II) ions. Large monodomains of one-dimensional (1D) double wires are formed by Co(II)/ligand coordination, with polymer lengths as long as 150 nm. The polymers are organized as parallel wires 8 nm apart, and the voids between wires are occupied by solvent molecules. Two-dimensional (2D) grids, showing high surface mobility, coexist with the wires. The wires are formed from linear chain motifs where each cobalt center is bonded to two bipyridines. 2D grids are generated from a bifurcation node where one cobalt bonds to three bipyridines. Surface reconstruction of the grids and of the 1D wires was observed under the STM tip. As an exciting result, analysis of these movements strongly indicates surface reactions at the solid/liquid interface.

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

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

On-Surface Synthesis of One-Dimensional Carbon-Based Nanostructures via C-X and C-H Activation Reactions

The past decades have witnessed the emergence of low-dimensional carbon-based nanostructures owing to their unique properties and various subsequent applications. It is of fundamental importance to explore ways to achieve atomically precise fabrication of these interesting structures. The newly developed on-surface synthesis approach provides an efficient strategy for this challenging issue, demonstrating the potential of atomically precise preparation of low-dimensional nanostructures. Up to now, the formation of various surface nanostructures, especially carbon-based ones, such as graphene nanoribbons (GNRs), kinds of organic (organometallic) chains and films, have been achieved via on-surface synthesis strategy, in which in-depth understanding of the reaction mechanism has also been explored. This review article will provide a general overview on the formation of one-dimensional carbon-based nanostructures via on-surface synthesis method. In this review, only a part of the on-surface chemical reactions (specifically, C-X (X=Cl, Br, I) and C-H activation reactions) under ultra-high vacuum conditions will be covered.

Stepwise on-surface dissymmetric reaction to construct binodal organometallic network

Dissymmetric reactions, which enable differentiated functionalization of equivalent sites within one molecule, have many potential applications in synthetic chemistry and materials science, but they are very challenging to achieve. Here, the dissymmetric reaction of 1,4-dibromo-2,5-diethynylbenzene (2Br-DEB) on Ag(111) is realized by using a stepwise activation strategy, leading to an ordered two-dimensional organometallic network containing both alkynyl-silver-alkynyl and alkynyl-silver-phenyl nodes. Scanning tunneling microscopy and density functional theory calculations are employed to explore the stepwise conversion of 2Br-DEB, which starts from the H-passivation of one Br-substituted site at 300 K in accompaniment with an intermolecular reaction to form one-dimensional organometallic chains containing alkynyl-silver-alkynyl nodes. Afterwards, the other equivalent Br-substituted site undergoes metalation reaction at 320-450 K, resulting in transformation of the chains into the binodal networks. These findings exemplify the achievement of the dissymmetric reaction and its practical application for controlled fabrications of complicated yet ordered nanostructures on a surface.