On-surface Reactions Research Articles

Magnetism in Nonplanar Zigzag Edge Termini of Graphene Nanoribbons.

Graphene nanoribbons (GNRs) and nanographenes synthesized by on-surface reaction using tailor-made molecular precursors offer an ideal playground for a study of magnetism towards nano-spintronics. Although the zigzag edge of GNRs has been known to host magnetism, underlying metal substrates usually veil the edge-induced Kondo effect. Here, we report the on-surface synthesis of unprecedented, π-extended 7-armchair GNRs using 7-bromo-12-(10-bromoanthracen-9-yl)tetraphene as the precursor. Characterizations by scanning tunneling microscopy/spectroscopy revealed unique rearrangement reactions leading to pentagon- or pentagon/heptagon-incorporated, nonplanar zigzag termini, which demonstrated Kondo resonance even on bare Au(111). Density functional theory calculations indicate that the nonplanar structure significantly reduces the interaction between the zigzag terminus and the Au(111), leading to a recovery of the spin localization of the zigzag edge. Such distortion of planar GNR structures offers a degree of freedom to control the magnetism on metal substrates.

Self-Limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons.

Because of their theoretically predicted intriguing properties, it is interesting to embed periodic 585-ringed divacancies into graphene nanoribbons (GNRs), but it remains a great challenge. Here, we develop an on-surface cascade reaction from periodic hydrogenated divacancies to alternating 585-ringed divacancies and Ag atoms via intramolecular cyclodehydrogenation in a seven-carbon-wide armchair GNR on the Ag(111) surface. Combining scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy combined with first-principles calculations, we in-situ-monitor the evolution of the distinct structural and electronic properties in the reaction intermediates. The observation of embedded Ag atoms and further nudged elastic band calculations provide unambiguous evidence for Ag adatom-mediated C-H activation in the intramolecular cyclodehydrogenation pathway, where the strain-induced self-limiting effect contributes to the formation of the GNR superlattice with alternating 585-ringed divacancies and Ag atoms, which shows a band gap of about 1.4 eV. Our findings open an avenue to introducing periodic impurities of single metal atoms and nonhexagonal rings in on-surface synthesis, which may provide a novel route for multifunctional graphene nanostructures.

Tailoring On-Surface Molecular Reactions and Assembly through Hydrogen-Modified Synthesis: From Triarylamine Monomer to 2D Covalent Organic Framework.

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources─relying only on backfilled hydrogen gas and ion gauge filaments─and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br3)DTPA) as monomer precursors, we find that atomic hydrogen (H•) blocks aryl-aryl bond formation to such an extent that we suspect this reaction may be a factor in limiting the ultimate size of 2D COFs created through on-surface synthesis. Conversely, we show that control of the relative monomer and hydrogen fluxes can be used to produce large self-assembled islands of monomers, dimers, or macrocycle hexamers, which are of interest in their own right. On-surface synthesis of oligomers, from a single precursor, circumvents potential challenges with their protracted wet-chemical synthesis and with multiple deposition sources. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D COF (synthesized in the absence of atomic hydrogen) as the end point in an evolution of electronic structures from the monomer.

Metal Atoms Participate in the Self-Assembly and On-Surface Reaction Behaviors of 1,4-DBN on Ag(111) Surface.

Herein, using 1,4-dibromonaphthalene (1,4-DBN) as the precursor molecule and Ag(111) surface as the substrate, we have characterized the various coordination and covalent structures formed by 1,4-DBN by low-temperature scanning tunnelling microscopy. We observed that there are three ordered structures (phase I, II, III) and one metal-organic short-chain structure (phase IV) at high coverage, meanwhile a new type of chiral structure (phase V) is observed coexisting with phase II, III, IV at low coverage. Surprisingly, all these structures have surface Ag adatoms incorporated. In addition, the phase III should be formed by a dissymmetric dehalogenation reaction of 1,4-DBN. Furthermore, we showed that the Ullmann coupling and cyclodehydrogenation of 1,4-DBN to form the armchair-shaped graphene nanoribbons will occur after thermal annealing. Combining the experiment data and density functional theory simulations, our results show that the surface Ag adatoms play a critical role in both the self-assembly and the on-surface reaction.

On-Surface Reaction of 1,4-Dibromo-2,5-Diiodobenzene on Au(111) and Ag(100)

The on-surface reaction processes of 1,4-dibromo-2,5-diiodobenzene (C6H2Br2I2) on Au(111) and Ag(100) were systematically investigated under ultra-high vacuum conditions by using scanning tunneling microscopy. Deiodination underwent at RT on both surfaces with the formation of organometallic intermediates connected by C–Au–C and C–Ag–C linkages. In particular, partial deiodination on Au(111) resulted in self-assembled ordered structures of organometallic trans-trimers, which were converted into longer organometallic chains after complete deiodination at 100 °C. On Ag(100), complete deiodination gave rise to disordered molecular clusters at RT and organometallic intermediates were formed via the debromination process after annealing to 150 °C. Further annealing to higher temperatures resulted in covalent C–C bonded polyphenylene chains and finally disordered graphene nanostructures via cyclodehydrogenation. Our study provides fundamental comprehension of temperature-selective on-surface dehalogenation reactions of multi-halogen-substituted precursors for constructing designer covalently bonded networks.

Bottom-Up Synthesis of Metalated Carbyne Ribbons via Elimination Reactions

Elimination reactions are one of the most important reactions in organic synthesis, especially in the formation of alkenes and alkynes. Herein, based on scanning tunneling microscopy, we report the bottom-up synthesis of one-dimensional carbyne-like nanostructures, metalated carbyne ribbons with the incorporation of Cu or Ag atoms, through α- and β-elimination reactions of tetrabromomethane and hexabromoethane on surfaces. Density functional theory calculations demonstrate a width-dependent band gap modulation within these ribbon structures, which is affected by interchain interactions. Moreover, mechanistic insights into the on-surface elimination reactions have also been provided in this study.

Insights into the Polymerization Reactions on Solid Surfaces Provided by Scanning Tunneling Microscopy.

Understanding the polymerization process at the molecular level is essential for the rational design and synthesis of polymers with controllable structures and properties. Scanning tunneling microscopy (STM) is one of the most important techniques to investigate the structures and reactions on conductive solid surfaces, and it has successfully been used to reveal the polymerization process on the surface at the molecular level in recent years. In this Perspective, after a brief introduction of on-surface polymerization reactions and STM, we focus on the applications of STM in the study of the processes and mechanism of on-surface polymerization, from one-dimensional to two-dimensional polymerization reactions. We conclude by a discussion of the challenges and perspectives on this topic.

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.

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 and spontaneous segregation of conjugated tetraphenylethylene macrocycles

Creating conjugated macrocycles has attracted extensive research interest because their unique chemical and physical properties, such as conformational flexibility, intrinsic inner cavities and aromaticity/antiaromaticity, make these systems appealing building blocks for functional supramolecular materials. Here, we report the synthesis of four-, six- and eight-membered tetraphenylethylene (TPE)-based macrocycles on Ag(111) via on-surface Ullmann coupling reactions. The as-synthesized macrocycles are spontaneously segregated on the surface and self-assemble as large-area two-dimensional mono-component supramolecular crystals, as characterized by scanning tunneling microscopy (STM). We propose that the synthesis benefits from the conformational flexibility of the TPE backbone in distinctive multi-step reaction pathways. This study opens up opportunities for exploring the photophysical properties of TPE-based macrocycles.

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

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

Influence of Molecular Configurations on the Desulfonylation Reactions on Metal Surfaces.

On-surface synthesis is a powerful methodology for the fabrication of low-dimensional functional materials. The precursor molecules usually anchor on different metal surfaces via similar configurations. The activation energies are therefore solely determined by the chemical activity of the respective metal surfaces. Here, we studied the influence of the detailed adsorption configuration on the activation energy on different metal surfaces. We systematically studied the desulfonylation homocoupling for a molecular precursor on Au(111) and Ag(111) and found that the activation energy is lower on inert Au(111) than on Ag(111). Combining scanning tunneling microscopy observations, synchrotron radiation photoemission spectroscopy measurements, and density functional theory calculations, we elucidate that the phenomenon arises from different molecule-substrate interactions. The molecular precursors anchor on Au(111) via Au-S interactions, which lead to weakening of the phenyl-S bonds. On the other hand, the molecular precursors anchor on Ag(111) via Ag-O interactions, resulting in the lifting of the S atoms. As a consequence, the activation barrier of the desulfonylation reactions is higher on Ag(111), although silver is generally more chemically active than gold. Our study not only reports a new type of on-surface chemical reaction but also clarifies the influence of detailed adsorption configurations on specific on-surface chemical reactions.

Intra- and Inter-Self-Assembly of Identical Supramolecules on Silver Surfaces

Self-assembly of identical organometallic supramolecules into ordered superstructures is of great interest in both chemical science and nanotechnology due to its potential to generate neoteric properties through collective effects. In this work, we demonstrate that large-scale self-organization of atomically precise organometallic supramolecules can be achieved through cascaded on-surface chemical reactions, by the combination of intra- and inter-supramolecular interactions. Supramolecules with defined size and shape are first built through intramolecular reaction and intermolecular metal coordination, followed by the formation of well-ordered two-dimensional arrays with the assistance of Br atoms by -C-H···Br interactions. The mechanism of this process has been investigated from the perspectives of thermodynamics and kinetics.

Radical-promoted room-temperature terminal alkyne activation on Au(111)

Exploring on-surface reactions of terminal alkyne derivatives are of primary importance for controlled synthesis of graphyne and graphdiyne-type materials. In this study, room-temperature (RT) adsorption of ethynyl-phenanthrene (EP) and ethynyl-iodophenanthrene (EIP) precursors on Au(111) were investigated by combined scanning tunneling microscopy and density functional theory (DFT) modeling. In contrast to inert adsorption of EP on Au(111), RT deposited EIP molecules are partially deiodinated, which gives rise to various kinds of reaction products. Surprisingly, polymeric chains with length up to ∼ 15 nm were identified. Our DFT modeling evidences that the alkyne activation and covalent coupling on Au(111) are more favored for the deiodinated precursors.

Nanopatterning Graphite Surface by Diazoniums Using Electrochemical Method

Molecular functionalization of graphitic surfaces with nanopatterned structures is regarded as one of the effective bottom-up techniques to tune their electronic properties towards electronics applications. Diazonium molecules have been often employed to covalently functionalize graphene and highly oriented pyrolytic graphite (HOPG) substrates. However, controlling the structure of the molecular adlayers is still challenging. In this contribution, we demonstrated an inconventional approach for covalent functionalization the HOPG surface by using mixture of 4-nitrobenzenediazonium (4-NBD) and 3,5-bis-tert-butylbenzenediazonium (3,5-TBD) molecules in which the former tends to polimezise and physisorb while the later chemically anchors on surface. The physisorbed features can be removed by washing with hot toluene and water. As a result, the HOPG surface is patterned in a quasi-periodic fashion. The efficiency of this development was verified by a combination of cyclic voltametry (CV) and atomic force microscopy (AFM) methods. This finding represents a convenient strategy for creating nanoconfined templates that might serve as nano-playgrounds for further supramolecular self-assembly and other on-surface reactions.

Kinetic control of self-assembly using a low-energy electron beam

Self-assembly and on-surface synthesis are vital strategies used for fabricating surface-confined 1D or 2D supramolecular nanoarchitectures with atomic precision. In many systems, the resulting structure is determined by the kinetics of the processes involved, i.e., reaction rate, on-surface diffusion, nucleation, and growth, all of which are typically governed by temperature. However, other external factors have been only scarcely harnessed to control the on-surface chemical reaction kinetics and self-assembly. Here, we show that a low-energy electron beam can be used to steer chemical reaction kinetics and induce the growth of molecular phases unattainable by thermal annealing. The electron beam provides a well-controlled means of promoting the elementary reaction step, i.e., deprotonation of carboxyl groups. The reaction rate increases with the increasing electron beam energy beyond the threshold energy of 6 eV. Our results offer the novel prospect of controlling self-assembly, enhancing the rate of reaction steps selectively, and thus altering the kinetic rate hierarchy.

The first example of an on-surface enzymatic reaction evaluated by paper spray mass spectrometry (PS-MS): Deacetylation of 1,2,3,4-tetra-O-acetyl-β-D-xylopyranose

Paper spray mass spectrometry (PS-MS) is a recent and innovative ambient ionization technique conveniently applied to investigate on-surface chemical reactions. The easy, fast, and efficient PS-MS technique has shown to be a convenient platform to investigate organic reactions, yielding promising results. Lipases are a group of enzymes responsible for esterification and hydrolysis reactions of organic analytes, such as carbohydrates and lipids. Hence, this work describes for the first time an on-surface enzymatic reaction monitored by PS-MS: the deacetylation of 1,2,3,4-tetra-O-acetyl-β-D-xylopyranose (1) induced by the enzyme Burkholderia cepacia lipase (BCL). The reaction was followed over 60 min when the sodium monodeacetylated product (including the respective sodium-bound dimer) and the oxonium intermediate were detected at high intensities. Besides, the influence of BCL concentration and the presence of water and hydrochloric acid were evaluated. Finally, the results described herein demonstrated the feasibility of applying PS-MS to evaluate and get information on on-surface enzymatic reactions, a field still underexplored. These results also open up a new and exciting perspective concerning studying other and perhaps more complex enzymatic reactions in a straightforward manner.

Steering the Reaction Pathways of Terminal Alkynes by Introducing Oxygen Species: From C-C Coupling to C-H Activation.

Selective regulation of chemical reactions is crucial in chemistry. Oxygen, as a key reagent in ubiquitous oxidative chemistry, exhibits great potential in regulating molecular assemblies, and more importantly, chemical reactions in molecular systems supported by metal surfaces. However, the unique catalytic performance and reaction mechanisms of oxygen species remain elusive, which are essential for understanding reaction selection and regulation. In this study, by a combination of scanning tunneling microscopy (STM) imaging/manipulations and density functional theory (DFT) calculations, we showed that the on-surface reaction pathways of terminal alkynes could be steered from C-C coupling to C-H activation with high selectivity by introducing O2 into the molecular system. The catalytic performance and reaction mechanisms of oxygen species were explored in the C-H activation processes, and both molecular O2 and atomic O could efficiently steer the reaction pathways. These results would provide a fundamental understanding of interfacial catalytic reaction processes.

Counting molecules: Python based scheme for automated enumeration and categorization of molecules in scanning tunneling microscopy images

Scanning tunneling and atomic force microscopies (STM/nc-AFM) are rapidly progressing to offer unprecedented spatial resolution of a diverse array of chemical species. In particular, they are employed to characterize on-surface chemical reactions by directly examining precursors and products. Chiral effects and self-assembled structures can also be investigated. This open source, modular, python based scheme automates the categorization of a variety of molecules present in medium sized (10$\times$10 to 100$\times$100 nm) scanned probe images.

Atomically Sharp Lateral Superlattice Heterojunctions Built‐In Nitrogen‐Doped Nanoporous Graphene

Nanometer scale lateral heterostructures with atomically sharp band discontinuities can be conceived as the 2D analogues of vertical Van der Waals heterostructures, where pristine properties of each component coexist with interfacial phenomena that result in a variety of exotic quantum phenomena. However, despite considerable advances in the fabrication of lateral heterostructures, controlling their covalent interfaces and band discontinuities with atomic precision, scaling down components and producing periodic, lattice-coherent superlattices still represent major challenges. Here, a synthetic strategy to fabricate nanometer scale, coherent lateral superlattice heterojunctions with atomically sharp band discontinuity is reported. By merging interdigitated arrays of different types of graphene nanoribbons by means of a novel on-surface reaction, superlattices of 1D, and chemically heterogeneous nanoporous junctions are obtained. The latter host subnanometer quantum dipoles and tunneling in-gap states, altogether expected to promote interfacial phenomena such as interribbon excitons or selective photocatalysis.