Steering on-surface reactions with self-assembly strategy

Revealing the Influence of Molecular Chemisorption Direction on the Reaction Selectivity of Dehalogenative Coupling on Au(111): Polymerization versus Cyclization

The control of reaction selectivity is of great interest in chemistry and depends crucially on the revelation of key influencing factors. Based on well-defined molecule-substrate model systems, various influencing factors have been elucidated, focusing primarily on the molecular precursors and the underlying substrates themselves, while interfacial properties have recently been shown to be essential as well. However, the influence of molecular chemisorption direction on reaction selectivity, as a subtle interplay between molecules and underlying substrates, remains elusive. In this work, by a combination of scanning tunneling microscopy imaging and density functional theory calculations, we report the influence of molecular chemisorption direction on the reaction selectivity of two types of dehalogenative coupling on Au(111), i.e., polymerization and cyclization, at the atomic level. The diffusion step of a reactive dehalogenated intermediate in two different chemisorption directions was theoretically revealed to be the key to determining the corresponding reaction selectivity. Our results highlight the important role of molecular chemisorption directions in regulating the on-surface dehalogenative coupling reaction pathways and products, which provides fundamental insights into the control of reaction selectivity by exploiting some subtle interfacial parameters in on-surface reactions for the fabrication of target low-dimensional carbon nanostructures.

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.

Anchoring and Reacting On-Surface to Achieve Programmability

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

Topology Selectivity in On-Surface Dehydrogenative Coupling Reaction: Dendritic Structure versus Porous Graphene Nanoribbon

Selective control on the topology of low-dimensional covalent organic nanostructures in on-surface synthesis has been challenging. Herein, with combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we report a successful topology-selective coupling reaction on the Cu(111) surface by tuning the thermal annealing procedure. The precursor employed is 1,3,5-tris(2-bromophenyl)benzene (TBPB), for which Ullmann coupling is impeded due to the intermolecular steric hindrance. Instead, its chemisorption on the Cu(111) substrate has triggered the ortho C-H bond activation and the following dehydrogenative coupling at room temperature (RT). In the slow annealing experimental procedure, the monomers have been preorganized by their self-assembly at RT, which enhances the formation of dendritic structures upon further annealing. However, the chaotic chirality of dimeric products (obtained at RT) and hindrance from dense molecular island make the fabrication of high-quality porous two-dimensional nanostructures difficult. In sharp contrast, direct deposition of TBPB molecules on a hot surface led to the formation of ordered porous graphene nanoribbons and nanoflakes, which is confirmed to be the energetically favorable reaction pathway through density functional theory-based thermodynamic calculations and control experiments. This work demonstrates that different thermal treatments could have a significant influence on the topology of covalent products in on-surface synthesis and presents an example of the negative effect of molecular self-assembly to the ordered covalent nanostructures.

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.

Two-Sidedness of Surface Reaction Mediation

A heterogeneous catalytic process involves many surface elementary steps that affect the overall catalytic performance in one way or another. In general, a high-performance heterogeneous catalyst should meet the main criteria: excellent catalytic activity and high selectivity toward target products. Using surface science techniques, the two-sidedness of the surface reaction mediations can be explored, from the perspectives of the surface and the molecule manipulations. The surface manipulation refers to a reaction that is mediated by composition and structure of the substrate as well as surface species, while the molecular manipulation relates to a reaction that is mediated by the reacting molecule via the precursor selection, environmental control, or external excitation. The best catalytic system should consist of the most efficient catalyst and the best suitable reacting molecule, in addition to its economic benefit and environmental amity. Recent research progress in surface reaction mediation is outlined, and its two-sidedness is governed by the Arrhenius equation. This should shed new light on the connection between basic theory and surface reaction mediation strategies. To conclude, challenges and possible opportunities are elaborated for efficient surface reaction mediations.

On Controllability and Applicability of Surface Molecular Self-Assemblies

Molecular self-assembly (MSA) refers to spontaneous arrangement of molecular building blocks into ordered structures governed by weak interactions. Due to its high versatility and reversibility, MSA has been widely employed as a robust bottom-up approach to fabricating low-dimensional functional nanostructures, which are used in various applications in nanoscience and technology. To date, tremendous effort has been devoted to constructing various MSAs at surfaces, ranging from self-assembled monolayers and two-dimensional (2D) nanoporous networks to complex 2D quasicrystals and Sierpiński triangle fractals. However, precise control of the assembled structures and efficient achievement of their full applicability remain two major challenges in the MSA field. As another widely employed bottom-up approach to fabricating nanostructures, on-surface reaction (OSR) refers to a reaction that occurs on the surface and is two-dimensionally confined. OSR offers the possibility to synthesize compounds that may not be feasibly achieved in solution chemistry. Compared with MSA based on weak intermolecular interactions, OSR-based structures possess high thermal and chemical stabilities due to internal strong covalent bonds. In this Account, we briefly overview recent achievements of MSAs on single crystal metal surfaces with a focus on their controllability and applicability in tweaking the properties of the molecular building blocks involved. Emphasis will be particularly placed upon mediation of OSRs with the MSA strategy. To explore surface MSAs, on the one hand, scanning tunneling microscopy and spectroscopy have been routinely employed as the experimental tools to probe the intermolecular interactions as well as geometric and electronic structures of the assemblies at the atomic and molecular levels. On the other hand, density functional theory and molecular dynamics have been theoretically applied to model and calculate the assembling systems, furthering our understanding of the experimental results. In principle, MSA is primarily balanced by molecule-molecule and molecule-substrate interactions under vacuum conditions. In terms of the assembling methodologies, people have been attempting to achieve rational design, accurate prediction, and controllable construction of assembled molecular nanostructures, namely, tentative design of specific backbones and functional groups of the molecular building blocks, and careful control of the assembling parameters including substrate lattice, temperature, coverage, and external environment as well. An obvious goal for the development of these methodologies lies in the ultimate applications of these MSAs. MSA can retrospectively affect the properties of the assembling molecules. For instance, self-assembled structures not only can serve as secondary templates to host guest molecules but also can stabilize surface metal adatoms. In fact, the electronics, magnetism, and optics of MSAs have been successfully explored. In surface chemistry, the MSA strategy can be further applied to mediate OSRs in at least three aspects: tweaking reaction selectivity, changing reaction pathway, and restricting reaction site. The governing principle lies in that the self-assembled molecules are confined in the assemblies so that the pre-exponential factors and the energy barriers in the Arrhenius equation of the involved reactions could be substantially varied because the subtle reaction mechanisms may change upon assembling. In this sense, the MSA strategy can be efficiently exploited to tune the properties of the assembling molecules and mediate OSRs in surface chemistry.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

The Orientation of Silver Surfaces Drives the Reactivity and the Selectivity in Homo-Coupling Reactions

Original reaction pathways can be explored in the on-surface synthesis approach where small aromatic precursors are confined to the surface of single crystal metals. The bis-indanedione molecule reacted with itself on silver surfaces in different ways, through a Knoevenagel reaction or an oxidative coupling, leading to the formation of a variety of new molecular compounds and covalently-linked 1D or 2D networks. Noteworthy, original reaction products were obtained that cannot be synthesized in traditional solvent-based chemistry. The lowest activation temperature for the homo-coupling reactions was found on the Ag(111) surface. The Ag(110) was highly selective in terms of coupling reaction type, while on Ag(100) the temperature could finely control the selectivity. The on-surface synthesis approach is shown here to be particularly efficient to produce original compounds in mild conditions, using activation temperatures as low as 200 °C. The different structures were characterized by scanning tunnelling microscopy (STM) together with X-ray photoelectron emission spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS).