Surface-Activated Coupling Reactions Confined on a Surface

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.

Advances in the Dynamics of Adsorbate Diffusion on Metal Surfaces: Focus on Hydrogen and Oxygen

Adsorbates on metal surfaces are typically formed from the dissociative chemisorption of molecules occurring at gas-solid interfaces. These adsorbed species exhibit unique diffusion behaviors on metal surfaces, which are influenced by their translational energy. They play crucial roles in various fields, including heterogeneous catalysis and corrosion. This review examines recent theoretical advancements in understanding the diffusion dynamics of adsorbates on metal surfaces, with a specific emphasis on hydrogen and oxygen atoms. The diffusion processes of adsorbates on metal surfaces involve two energy transfer mechanisms: surface phonons and electron-hole pair excitations. This review also surveys new theoretical methods, including the characterization of the electron-hole pair excitation within electronic friction models, the acceleration of quantum chemistry calculations through machine learning, and the treatment of atomic nuclear motion from both quantum mechanical and classical perspectives. Furthermore, this review offers valuable insights into how energy transfer, nuclear quantum effects, supercell sizes, and the topography of potential energy surfaces impact the diffusion behavior of hydrogen and oxygen species on metal surfaces. Lastly, some preliminary research proposals are presented.

Construction of Two-Dimensional Organometallic Coordination Networks with Both Organic Kagome and Semiregular Metal Lattices on Au(111)

Two-dimensional metal-organic networks (2D MONs) having heterogeneous coordination nodes (HCNs) could exhibit excellent performance in catalysis and optoelectronics because of the unbalanced electron distribution of the coordinating metals. Therefore, the design and construction of 2D MONs with HCNs are highly desirable but remain challenging. Here, we report the construction of 2D organometallic coordination networks with an organic Kagome lattice and a semiregular metal lattice on Au(111) via the in situ formation of HCNs. Using a bifunctional precursor 1,4-dibromo-2,5-diisocyanobenzene, the coordination of isocyano with Au adatom on a room-temperature Au(111) yielded metal-organic coordination chains with isocyano-Au-isocyano nodes. In contrast, on a high-temperature Au(111), a selective debromination/coordination cascade reaction occurred, affording 2D organometallic coordination networks with phenyl-Au-isocyano nodes. By combining scanning tunneling microscopy and density functional theory calculations, we determined the structures of coordination products and the nature of coordination nodes, demonstrating a thermodynamically favorable pathway for forming the phenyl-Au-isocyano nodes.

On-Surface Covalent Synthesis of Carbon Nanomaterials by Harnessing Carbon gem-Polyhalides

The design of innovative carbon-based nanostructures stands at the forefront of both chemistry and materials science. In this context, π-conjugated compounds are of great interest due to their impact in a variety of fields, including optoelectronics, spintronics, energy storage, sensing and catalysis. Despite extensive research efforts, substantial knowledge gaps persist in the synthesis and characterization of new π-conjugated compounds with potential implications for science and technology. On-surface synthesis has emerged as a powerful discipline to overcome limitations associated with conventional solution chemistry methods, offering advanced tools to characterize the resulting nanomaterials. This review specifically highlights recent achievements in the utilization of molecular precursors incorporating carbon geminal (gem)-polyhalides as functional groups to guide the formation of π-conjugated 0D species, as well as 1D, quasi-1D π-conjugated polymers, and 2D nanoarchitectures. By delving into reaction pathways, novel structural designs, and the electronic, magnetic, and topological features of the resulting products, the review provides fundamental insights for a new generation of π-conjugated materials.

Possibilities and Limitations of Kinetic Studies in On-Surface Synthesis by Real Time X-ray Photoelectron Spectroscopy

The kinetics of coupling reactions on surfaces can be quantitatively studied in real time by X-ray Photoelectron Spectroscopy (XPS). From fitting experimental data, kinetic reaction parameters such as the rate constant's pre-exponential and activation energy can be deduced and compared to quantum chemical simulations. To elucidate the possibilities and limitations of this approach, we propose studies in which experimental data are first simulated and subsequently fitted. Knowing the exact kinetic parameters used in the simulation allows one to evaluate the accuracy of the fit result. Here, several experimental influences, such as the data point density and the addition of noise, are explored for a model reaction with first-order kinetics. The proposed procedure sheds light on the accuracy with which kinetic parameters can be derived and may also help in the design of future experiments.

Chronoamperometric interrogation of an electrochemical aptamer-based sensor with tetrahedral DNA nanostructure pendulums for continuous biomarker measurements

Tetrahedral DNA nanostructure (TDN) is highly promising in developing electrochemical aptamer-based (E-AB) sensors for biomolecular detection, owing to its inherit programmability, spatial orientation and structural robustness. However, current interrogation strategies applied for TDN-based E-AB sensors, including enzyme-based amperometry, voltammetry, and electrochemical impedance spectroscopy, either require complicated probe design or suffer from limited applicability or selectivity. In this study, a TDN pendulum-empowered E-AB sensor interrogated by chronoamperometry for reagent-free and continuous monitoring of a blood clotting enzyme, thrombin, was developed. TDN pendulums with extended aptamer sequences at three vertices were immobilized on a gold electrode via a thiolated double-stranded DNA (dsDNA) at the fourth vertex, and their motion is modulated by the bonding of target thrombin to aptamers. We observed a significantly amplified signalling output on our sensor based on the TDN pendulum compared to E-AB sensors modified with linear pendulums. Moreover, our sensor achieved highly selective and rapidly responsive measurement of thrombin in both PBS and artificial urine, with a wide dynamic range from 1 pM to 10 nM. This study shows chronoamperometry-enabled continuous biomarker monitoring on a sub-second timescale with a drift-free baseline, demonstrating a novel approach to accurately detect molecular dynamics in real time.

Increased Selectivity in Photolytic Activation of Nanoassemblies Compared to Thermal Activation in On-Surface Ullmann Coupling

On-surface synthesis is a powerful method that has emerged recently to fabricate a large variety of atomically precise nanomaterials on surfaces based on polymerization. It is very successful for thermally activated reactions within the framework of heterogeneous catalysis. As a result, it often lacks selectivity. We propose to use selective activation of specific bonds as a crucial ingredient to synthesize desired molecules with high selectivity. In this approach, thermally nonaccessible products are expected to arise in photolytically activated on-surface reactions with high selectivity. We demonstrate for assembled 2,2'-dibromo biphenyl clusters on Cu(111) that the thermal and photolytic activations yield distinctly different products, combining submolecular resolution of individual product molecules in real-space imaging by scanning tunneling microscopy with chemical identification in X-ray photoelectron spectroscopy and supported by ab initio calculations. The photolytically activated Ullmann coupling of 2,2'-dibromo biphenyl is highly selective, with only one identified product. It starkly contrasts the thermal reaction, which yields various products because alternate pathways are activated at the reaction temperature. Our study extends on-surface synthesis to a directed formation of thermally inaccessible products by direct bond activation. It promises tailored reactions of nanomaterials within the framework of on-surface synthesis based on the photolytic activation of specific bonds.

Designing 2D stripe winding network through crown-ether intermediate Ullmann coupling on Cu(111) surface

Chemical synthesis typically yields the most thermodynamically stable ordered arrangement, a principle also governing surface synthesis on an atomically level two-dimensional (2D) surface, fostering the creation of structured 2D formations. The linear connection arising from energetically stable chemical bonding precludes the generation of a 2D random network comprised of one-dimensional (1D) convoluted stripes through on-surface synthesis. Nonetheless, we underscored that on-surface synthesis possesses the capability not solely to fashion a 2D ordered linear network but also to fabricate a winding 2D network employing a precursor with a soft ring and intermediate state bonding within the Ullmann reaction. Here, on-surface synthesis was exhibited on Cu(111) employing a 2D self-assembled monolayer array of 4,4',5,5'-tetrabromodibenzo[18]crown-6 ether (BrCR) precursors. These precursors were purposefully structured, with a crown ether ring at the core and Br atoms positioned at the head and tail ends, facilitating preferential connections along the elongated axis to foster a 1D stripe configuration. We illustrate how adjustments in the quantities of the intermediate state, serving as a primary linkage, can yield a labyrinthine, convoluted winding 2D network of stripes. The progression of growth, underlying mechanisms, and electronic structures were scrutinized using an ultrahigh vacuum low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) setup combined with density functional theory (DFT) calculations. This experimental evidence opens a novel functionality in leveraging on-surface synthesis for the formation of a 2D random network. This discovery holds promise as a pioneering constituent in the construction of a ring host supramolecule, augmenting its capability to ensnare guest atoms, molecules, or ions.

Barth J, Lobo-Checa J, Corso M, Rogero C. Enhancing Haloarene Coupling Reaction Efficiency on an Oxide Surface by Metal Atom Addition

The bottom-up synthesis of carbon-based nanomaterials directly on semiconductor surfaces allows for the decoupling of their electronic and magnetic properties from the substrates. However, the typically reduced reactivity of such nonmetallic surfaces adversely affects the course of these reactions. Here, we achieve a high polymerization yield of halogenated polyphenyl molecular building blocks on the semiconducting TiO2(110) surface via concomitant surface decoration with cobalt atoms, which catalyze the Ullmann coupling reaction. Specifically, cobalt atoms trigger the debromination of 4,4″-dibromo-p-terphenyl molecules on TiO2(110) and mediate the formation of an intermediate organometallic phase already at room temperature (RT). As the debromination temperature is drastically reduced, homocoupling and polymerization readily proceed, preventing presursor desorption from the substrate and entailing a drastic increase of the poly-para-phenylene polymerization yield. The general efficacy of this mechanism is shown with an iodinated terphenyl derivative, which exhibits similar dehalogenation and reaction 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.

A Cu2(C6O6) metal-organic framework monolayer assembled on silicon carbide grown graphene exhibiting a metallic band structure

We report the self-assembly of a monolayer metal-organic framework of Cu-benzenehexol (BHO) on a graphene/SiC substrate assisted by in situ Cu-catalyzed deprotonation reactions. The density functional theory calculations reveal that the free-standing framework is a semiconductor with a band gap of 0.485 eV. Interestingly, upon adsorption on the substrate, the Fermi level is up-shifted to the conduction band of the free-standing framework due to the n-doped graphene on SiC, while the other band structure features are largely preserved. The metallic nature corroborates the scanning tunneling microscopy images acquired near the Fermi level. This work demonstrates that the graphene substrate, which interacts weakly with the framework, can be used to tune the Fermi level of the metal-organic framework.

Extending on-surface synthesis from 2D to 3D by cycloaddition with C60

As an efficient molecular engineering approach, on-surface synthesis (OSS) defines a special opportunity to investigate intermolecular coupling at the sub-molecular level and has delivered many appealing polymers. So far, all OSS is based on the lateral covalent bonding of molecular precursors within a single molecular layer; extending OSS from two to three dimensions is yet to be realized. Herein, we address this challenge by cycloaddition between C60 and an aromatic compound. The C60 layer is assembled on the well-defined molecular network, allowing appropriate molecular orbital hybridization. Upon thermal activation, covalent coupling perpendicular to the surface via [4 + 2] cycloaddition between C60 and the phenyl ring of the molecule is realized; the resultant adduct shows frozen orientation and distinct sub-molecular feature at room temperature and further enables lateral covalent bonding via [2 + 2] cycloaddition. This work unlocks an unconventional route for bottom-up precise synthesis of three-dimensional covalently-bonded organic architectures/devices on surfaces.

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.

C-C Coupling of Carbene Molecules on a Metal Surface in the Presence of Water

A novel surface-confined C-C coupling reaction involving two carbene molecules and a water molecule was studied by scanning tunneling microscopy in real space. Carbene fluorenylidene was generated from diazofluorene in the presence of water on a silver surface. While in the absence of water, fluorenylidene covalently binds to the surface to form a surface metal carbene, and water can effectively compete with the silver surface in reacting with the carbene. Water molecules in direct contact with fluorenylidene protonate the carbene to form the fluorenyl cation before the carbene can bind to the surface. In contrast, the surface metal carbene does not react with water. The fluorenyl cation is highly electrophilic and draws electrons from the metal surface to generate the fluorenyl radical which is mobile on the surface at cryogenic temperatures. The final step in this reaction sequence is the reaction of the radical with a remaining fluorenylidene molecule or with diazofluorene to produce the C-C coupling product. Both a water molecule and the metal surface are essential for the consecutive proton and electron transfer followed by C-C coupling. This C-C coupling reaction is unprecedented in solution chemistry.

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.

Chirality variation from self-assembly on Ullmann coupling for the DBCh adsorbate on Au(111) and Ag(111)

On-surface Ullmann coupling has been considered an appealing approach for the precise fabrication of carbon-based covalent nanostructures under solution-free conditions. However, chirality has seldom been discussed in Ullmann reactions. In this report, self-assembled two-dimensional chiral networks are initially constructed in a large area on Au(111) and Ag(111) after adsorption of the prochiral precursor, 6,12-dibromochrysene (DBCh). Self-assembled phases are then transformed into organometallic (OM) oligomers after debromination, preserving the chirality; in particular, the formation of scarcely reported OM species on Au(111) is discovered herein. With the aryl-aryl bonding induced after intensive annealing, covalent chains are fabricated via the cyclodehydrogenation between chrysene blocks, resulting in the formation of 8-armchair graphene nanoribbons with staggered valleys on both sides. Before chiral polymer chains are constructed by chrysene blocks, the high structural flexibility of OM intermediates on Ag(111) is also revealed during reactions, which is derived from the twofold coordination of Ag atoms and conformationally flexible metal-carbon bonding. Our report not only provides solid evidence of atomically precise fabrication of covalent nanostructures with a feasible bottom-up approach but also sheds insights into the comprehensive investigation of chirality variation from monomers to artificial architectures via surface coupling reactions.

Thioetherification of Br-Mercaptobiphenyl Molecules on Au(111)

Thioether polymers are fundamental for a variety of applications. Their synthesis is, however, more challenging than that of other metal-catalyzed reactions due to the reported detachment of the S atom during thermal activation. In this study, it has been demonstrated unambiguously that thermal annealing results in the thioetherification of the 4-bromo-4-mercaptobiphenyl molecule (Br-MBP) adsorbed on the surface of Au(111). Through complementary techniques, such as scanning tunneling microscopy, spectroscopy, and first-principle calculations, we have identified four reaction steps, involving sulfhydryl or bromine molecular functional groups and leading to the formation of intermolecular C-S bonds. To form the thioether polymer and to overcome the competitive formation of C-C bonds, two reaction steps, the dehalogenation, and dissociation of the S-Au bond, must occur simultaneously. We detail the electronic properties of the phenyl-sulfur bond and the polymer as a function of the ligand length. This result suggests a wider perspective of this chemical synthesis.

On-Surface Synthesis of Self-Assembled Covalently Linked Wavy Chains with Site-Selective Conformational Switching

Conformational arrangements in polymers on surfaces determine the overall shape as well as the potential properties. It is generally believed that conformational diversity leads to uncontrollable or disordered structures in on-surface synthesis. However, in this study, we obtain two well-ordered self-assembled covalently linked wavy chains with site-selective conformational switching via the Ullmann reaction of 1,2-bis(3-bromophenyl)ethane with multiple conformations on Ag(111). Two kinds of wavy chains exhibit distinct conformational arrangements, where chain I contains one repeating unit conformation of -cis-trans1-cis-trans1-cis-cis-trans1-, while the adjacent parallel parts in wavy chain II have two different conformational arrangements of -cis-cis-trans1- and -cis-cis-trans2-. Wavy chains coassemble with dissociated bromine atoms, suggesting that the Br···H-C interactions between Br atoms and molecular chains are crucial for the construction of ordered wavy chains. High-resolution scanning tunneling microscopy is employed to reveal the surface reaction process at the molecular scale. In depth growth mechanism analysis combined with density functional theory calculations unveils that the substrate also plays an important role in the fabrication of well-ordered wavy chains. The present work extends the surface reaction of conformational flexible precursors.

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.

On-Surface Synthesis and Evolution of Self-Assembled Poly(p-phenylene) Chains on Ag(111): A Joint Experimental and Theoretical Study

The growth of controlled 1D carbon-based nanostructures on metal surfaces is a multistep process whose path, activation energies, and intermediate metastable states strongly depend on the employed substrate. Whereas this process has been extensively studied on gold, less work has been dedicated to silver surfaces, which have a rather different catalytic activity. In this work, we present an experimental and theoretical investigation of the growth of poly-p-phenylene (PPP) chains and subsequent narrow graphene ribbons starting from 4,4″-dibromo-p-terphenyl molecular precursors deposited at the silver surface. By combing scanning tunneling microscopy (STM) imaging and density functional theory (DFT) simulations, we describe the molecular morphology and organization at different steps of the growth process and we discuss the stability and conversion of the encountered species on the basis of calculated thermodynamic quantities. Unlike the case of gold, at the debromination step we observe the appearance of organometallic molecules and chains, which can be explained by their negative formation energy in the presence of a silver adatom reservoir. At the dehydrogenation temperature, the persistence of intercalated Br atoms hinders the formation of well-structured graphene ribbons, which are instead observed on gold, leading only to a partial lateral coupling of the PPP chains. We numerically derive very different activation energies for Br desorption from the Ag and Au surfaces, thereby confirming the importance of this process in defining the kinetics of the formation of molecular chains and graphene ribbons on different metal surfaces.

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.

Evolution of Br⋯Br contacts in enantioselective molecular recognition during chiral 2D crystallization

Halogen-mediated interactions play an important role in molecular recognition and crystallization in many chemical and biological systems, whereas their effect on homochiral versus heterochiral recognition and crystallization has rarely been explored. Here we demonstrate the evolution of Br⋯Br contacts in chiral recognition during 2D crystallization. On Ag(100), type I contacts prevail at low coverage and lead to homochiral recognition and the formation of 2D conglomerates; whereas type II contacts mediating heterochiral recognition are suppressed at medium coverage and appear in the racemates induced by structural transitions at high coverage. On Ag(111), type I contacts dominate the 2D crystallization and generate 2D conglomerates exclusively. DFT calculations suggest that the energy difference between type I and type II contacts is reversed upon adsorption due to the substrate induced mismatch energy penalty. This result provides fundamental understanding of halogen-mediated interactions in molecular recognition and crystallization on surface.

Halogen-mediated interactions control molecular recognition in many chemical and biological systems. Here, the authors demonstrate two types of Br⋯Br contacts and their importance in chiral on-surface crystallization.

Interfacial electric fields catalyze Ullmann coupling reactions on gold surfaces

The electric fields created at solid-liquid interfaces are important in heterogeneous catalysis. Here we describe the Ullmann coupling of aryl iodides on rough gold surfaces, which we monitor in situ using the scanning tunneling microscope-based break junction (STM-BJ) and ex situ using mass spectrometry and fluorescence spectroscopy. We find that this Ullmann coupling reaction occurs only on rough gold surfaces in polar solvents, the latter of which implicates interfacial electric fields. These experimental observations are supported by density functional theory calculations that elucidate the roles of surface roughness and local electric fields on the reaction. More broadly, this touchstone study offers a facile method to access and probe in real time an increasingly prominent yet incompletely understood mode of catalysis.

Carbon-based nanostructures as a versatile platform for tunableπ-magnetism

Emergence ofπ-magnetism in open-shell nanographenes has been theoretically predicted decades ago but their experimental characterization was elusive due to the strong chemical reactivity that makes their synthesis and stabilization difficult. In recent years, on-surface synthesis under vacuum conditions has provided unprecedented opportunities for atomically precise engineering of nanographenes, which in combination with scanning probe techniques have led to a substantial progress in our capabilities to realize localized electron spin states and to control electron spin interactions at the atomic scale. Here we review the essential concepts and the remarkable advances in the last few years, and outline the versatility of carbon-basedπ-magnetic materials as an interesting platform for applications in spintronics and quantum technologies.

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

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

Substrate-Modulated Synthesis of Metal-Organic Hybrids by Tunable Multiple Aryl-Metal Bonds

Assembly of semiconducting organic molecules with multiple aryl-metal covalent bonds into stable one- and two-dimensional (1D and 2D) metal-organic frameworks represents a promising route to the integration of single-molecule electronics in terms of structural robustness and charge transport efficiency. Although various metastable organometallic frameworks have been constructed by the extensive use of single aryl-metal bonds, it remains a great challenge to embed multiple aryl-metal bonds into these structures due to inadequate knowledge of harnessing such complex bonding motifs. Here, we demonstrate the substrate-modulated synthesis of 1D and 2D metal-organic hybrids (MOHs) with the organic building blocks (perylene) interlinked solely with multiple aryl-metal bonds via the stepwise thermal dehalogenation of 3,4,9,10-tetrabromo-1,6,7,12-tetrachloroperylene and subsequent metal-organic connection on metal surfaces. More importantly, the conversion from 1D to 2D MOHs is completely impeded on Au(111) but dominant on Ag(111). We comprehensively study the distinct reaction pathways on the two surfaces by visually tracking the structural evolution of the MOHs with high-resolution scanning tunneling and noncontact atomic force microscopy, supported by first-principles density functional theory calculations. The substrate-dependent structural control of the MOHs is attributed to the variation of the M-X (M = Au, Ag; X = C, Cl) bond strength regulated by the nature of the metal species.

Halogen and structure sensitivity of halobenzene adsorption on copper surfaces

The adsorption orientation of molecules on surfaces influences their reactivity, but it is still challenging to tailor the interactions that govern their orientation. Here, we investigate how the substituent and the surface structure alter the adsorption orientation of halogenated benzene molecules from parallel to tilted relative to the surface plane. The deviation of the parallel orientation of bromo-, chloro-, and fluorobenzene molecules adsorbed on Cu(111) and Cu(110) surfaces is determined, utilising the surface selection rule in reflection-absorption infrared spectroscopy. On Cu(111), all three halogenated molecules are adsorbed with their molecular plane almost parallel to the surface at low coverages. However, they are tilted at higher coverages; yet, the threshold coverages differ. On Cu(110), merely bromo- and chlorobenzene follow this trend, albeit with a lower threshold for both. In contrast, fluorobenzene molecules are tilted already at low coverages. The substantial influence of the halogen atom and the surface structure on the adsorption orientation, resulting from an interplay of molecule-molecule and molecule-surface interactions, is highly relevant for reactivity confined to two dimensions.

Molecular-Level Insights on Reactive Arrangement in On-Surface Photocatalytic Coupling Reactions Using Tip-Enhanced Raman Spectroscopy

Plasmon-enhanced photocatalytic coupling reactions have been used as model systems in surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) research for decades. However, the role of reactive arrangement on efficiency of these model reactions has remained largely unknown to date often leading to conflicting interpretations of experimental results. Herein, we use an interdisciplinary toolbox of nanoscale TERS imaging in combination with molecular-resolution ambient scanning tunnelling microscopy (STM) and density functional theory (DFT) modeling to investigate the role of reactive arrangement in photocatalytic coupling of 4-nitrobenzenethiol (4-NTP) to p,p'-dimercaptoazobisbenzene on single-crystal and polycrystalline Au surfaces for the first time. TERS imaging with 3 nm resolution clearly revealed a significantly higher catalytic efficiency inside a kinetically driven disordered phase of the 4-NTP adlayer on Au compared to the thermodynamically stable ordered phase. Furthermore, molecular level details of the self-assembled structures in the disordered and ordered phases obtained using ambient high-resolution STM enabled an unambiguous structure-reactivity correlation of photocatalytic coupling. Finally, quantitative mechanistic insights obtained from DFT modeling based on the accurate parameters determined from STM imaging emphatically confirmed that a combination of steric hindrance effect and energetic barrier leads to a lower reaction efficiency in the ordered phase of the 4-NTP adlayer. This fundamental study establishes the first direct structure-reactivity correlation in photocatalytic coupling and highlights the critical role of reactive arrangement in the efficiency of on-surface coupling reactions in heterogeneous catalysis at large.

Adatoms in the Surface-Confined Ullmann Coupling of Phenyl Groups

Despite the importance of the on-surface Ullmann coupling for synthesis of atomically precise carbon nanostructures, it is still unclear whether this reaction is catalyzed by surface atoms or adatoms. Here, the feasibility of the adatom creation and adatom-catalyzed Ullmann coupling of chloro-, bromo-, and iodobenzene on Cu(111), Ag(111), and Au(111) surfaces is examined using density functional theory modeling. The extraction of a metal atom is found to be greatly facilitated by the formation of strong phenyl-metal bonds, making the extraction energy barrier comparable to, and in the case of Ag(111) even lower than, that for the competing surface-catalyzed phenyl-phenyl bond formation. However, if the phenyl-adatom bonds are too strong, as on Cu(111) and Ag(111), they create an insurmountable barrier for the subsequent adatom-catalyzed C-C coupling. In contrast, Au adatoms do not bind phenyl groups strongly and can catalyze the C-C bond formation almost as efficiently as surface atoms.

Light assisted synthesis of poly-para-phenylene on Ag(001)

A detailed study of poly-para-phenylene (PPP) obtained by light-assisted on-surface-synthesis (OSS) on Ag(100) was carried out by scanning tunneling microscopy and spectroscopy together with density functional theory calculations. The use of light in combination with heat allows to lower by 50 K annealing temperature the each stage of the Ullmann coupling. Debromination of the 4,4″ dibromo-p-terphenyl precursors was thus realized at 300 K, the formation of the first oligomers from the organometallic intermediate by silver bridging atom release at 423 K and PPP by complete elimination of the silver at 473 K. This approach to lower the reaction temperature permits to enhance the Ag(100) surface reactivity to become comparable to that of Cu(111). The underlying mechanism of light effect was proposed to occur via surface mediated excitation, with the creation of photoexcited electrons known as hot electrons correlated with surface plasmon excitation. This original pathway combining both light and heat provides an additional parameter to control OSS by separating the precursor activation stage from the diffusion.

Order from a Mess: The Growth of 5-Armchair Graphene Nanoribbons

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.

Self-assembly of a strapped linear porphyrin oligomer on HOPG

Polymeric structures based on porphyrin units exhibit a range of complex properties, such as nanoscale charge transport and quantum interference effects, and have the potential to act as biomimetic materials for light-harvesting and catalysis. These functionalities are based upon the characteristics of the porphyrin monomers, but are also emergent properties of the extended polymer system. Incorporation of these properties within solid-state devices requires transfer of the polymers to a supporting substrate, and may require a high-degree of lateral order. Here we show that highly ordered self-assembled structures can be formed via a simple solution deposition protocol; for a strapped linear porphyrin oligomer adsorbed on a highly oriented pyrolytic graphite (HOPG) substrate. Two distinct molecule–molecule interactions are observed to drive the formation of two molecular phases (‘Interdigitated’ and ‘Bridge-stabilised’) characterised by scanning tunnelling microscopy, providing information on the unit cell dimensions and self-assembled structure. The concentration dependence of these phases is investigated, and we conclude that the bridge-stabilised phase is a thermodynamically stable structure at room temperature.

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.

Power discontinuity and shift of the energy onset of a molecular de-bromination reaction induced by hot-electron tunneling

Understanding the mechanism of molecular dissociation under applied bias is a fundamental requirement to progress in (electro)-catalysis as well as in (opto)-electronics. The working conditions of a molecular-based device and the stability of chemical bonds can be addressed in metal-organic junctions by injecting electrons in tunneling conditions. Here, we have correlated the energy of de-bromination of an aryl group with its density of states in a self-assembled dimeric structure of 4'-bromo-4-mercaptobiphenyl adsorbed on a Au(111) surface. We have observed that the electron-energy range where the molecule is chemically stable can be extended, shifting the bias threshold for the rupture of the -C-Br bond continuously from about 2.4 to 4.4 V by changing the electron current. Correspondingly, the power needed for the dissociation drops sharply at 3.6 V, identifying different reaction regimes and the contribution of different molecular resonance states.

On-surface photopolymerization of two-dimensional polymers ordered on the mesoscale

The use of solid supports and ultra-high vacuum conditions for the synthesis of two-dimensional polymers is attractive, as it can enable thorough characterization, often with submolecular resolution, and prevent contamination. However, most on-surface polymerizations are thermally activated, which often leads to high defect densities and relatively small domain sizes. Here, we have obtained a porous two-dimensional polymer that is ordered on the mesoscale by the two-staged topochemical photopolymerization of fluorinated anthracene triptycene (fantrip) monomers on alkane-passivated graphite surfaces under ultra-high vacuum. First, the fantrip monomers self-assemble into highly ordered monolayer structures, where all anthracene moieties adopt a suitable arrangement for photopolymerization. Irradiation with violet light then induces complete covalent crosslinking by [4+4] photocycloaddition to form a two-dimensional polymer, while fully preserving the long-range order of the self-assembled structure. The extent of the polymerization is confirmed by local infrared spectroscopy and scanning tunnelling microscopy characterization, in agreement with density functional theory calculations, which also gives mechanistic insights.

Synthesis on inert surfaces

Extraordinarily robust extended covalent organic nanostructures with unprecedented structures and intriguing chemical and electronic properties are currently synthesized on metal surfaces. Envisaged electronic applications, for instance in field effect transistors or sensors, however, demand insulating supports. To obviate the need for a cumbersome post-synthetic transfer from the metal growth surface to the target substrate, synthesis directly on inert surfaces is highly desirable. Albeit reversible polycondensations are broadly established on inert graphite surfaces, carbon-carbon (C-C) coupling remains mostly elusive. Thermally activated coupling on weakly interacting supports suffers from the "desorption problem", that is the premature desorption of reactants due to increased reaction barriers, which becomes even worse on inert surfaces due to diminished desorption barriers. Consequently, C-C coupling on inert surfaces requires new paradigms. We propose either photochemical coupling or activation of monomers prior to deposition as possible alternatives, discuss the current state-of-the-art and identify future challenges.

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.

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.

Steering Effect of Bromine on Intermolecular Dehydrogenation Coupling of Poly(p-phenylene) on Cu(111)

Among the multitudinous methodologies to steer on-surface reactions, less attention has been paid to the effect of externally introduced halogen atoms. Herein, highly selective trans-dehydrogenation coupling at the specific meta-C-H site of two poly(p-phenylene) molecules, p-quaterphenyl (Ph₄) and p-quinquephenyl (Ph₅), is achieved on Cu(111) by externally introduced bromine atoms. Scanning tunneling microscopy/spectroscopy experiments reveal that the formed molecular assembly structure at a stoichiometric ratio of 4:1 for Br to Ph₄ or 5:1 for Br to Ph₅ can efficiently promote the reactive collision probability to trigger the trans-coupling reaction at the meta-C-H site between two neighboring Ph₄ or Ph₅ molecules, leading to an increase in the coupling selectivity. Such Br atoms can also affect the electronic structure and adsorption stability of the reacting molecules. It is conceptually demonstrated that externally introduced halogen atoms, which can provide an adjustable halogen-to-precursor stoichiometry, can be employed to efficiently steer on-surface reactions.

On-Surface Hydrogenation of Buckybowls: From Curved Aromatic Molecules to Planar Non-Kekulé Aromatic Hydrocarbons

Functionalization of surfaces with derivatives of Buckminsterfullerene fragment molecules seems to be a promising approach toward bottom-up fabrication of carbon nanotube modified electrode surfaces. The modification of a Cu(100) surface with molecules of the buckybowl pentaindenocorannulene has been studied by means of scanning tunneling microscopy, carbon monoxide-modified noncontact atomic force microscopy, time-of-flight secondary mass spectrometry, and quantum chemical calculations. Two different adsorbate modes are identified, in which the majority is oriented such that the bowl cavity points away from the surface and the convex side is partially immersed into a four-atom vacancy in the Cu(100) surface. A minority is oriented such that the convex side points away from the surface with the five benzo tabs oriented basically parallel to the surface. Thermal annealing leads to hydrogenation and planarization of the molecules in two steps under specific C-C bond cleavage. The benzo tabs of the convex side up species serve as a hydrogen source. The final product has an open-shell electron structure that is quenched on the surface.

On-surface isostructural transformation from a hydrogen-bonded network to a coordination network for tuning the pore size and guest recognition

Rational manipulation of supramolecular structures on surfaces is of great importance and challenging. We show that imidazole-based hydrogen-bonded networks on a metal surface can transform into an isostructural coordination network for facile tuning of the pore size and guest recognition behaviours. Deposition of triangular-shaped benzotrisimidazole (H3btim) molecules on Au(111)/Ag(111) surfaces gives honeycomb networks linked by double N-H⋯N hydrogen bonds. While the H3btim hydrogen-bonded networks on Au(111) evaporate above 453 K, those on Ag(111) transform into isostructural [Ag3(btim)] coordination networks based on double N-Ag-N bonds at 423 K, by virtue of the unconventional metal-acid replacement reaction (Ag reduces H+). The transformation expands the pore diameter of the honeycomb networks from 3.8 Å to 6.9 Å, giving remarkably different host-guest recognition behaviours for fullerene and ferrocene molecules based on the size compatibility mechanism.