Regioselective On-Surface Synthesis of [3]Triangulene Graphene Nanoribbons

The integration of low-energy states into bottom-up engineered graphene nanoribbons (GNRs) is a robust strategy for realizing materials with tailored electronic band structure for nanoelectronics. Low-energy zero-modes (ZMs) can be introduced into nanographenes (NGs) by creating an imbalance between the two sublattices of graphene. This phenomenon is exemplified by the family of [n]triangulenes (n ∈ N ). Here, we demonstrate the synthesis of [3]triangulene-GNRs, a regioregular one-dimensional (1D) chain of [3]triangulenes linked by five-membered rings. Hybridization between ZMs on adjacent [3]triangulenes leads to the emergence of a narrow band gap, Eg,exp ∼ 0.7 eV, and topological end states that are experimentally verified using scanning tunneling spectroscopy. Tight-binding and first-principles density functional theory calculations within the local density approximation corroborate our experimental observations. Our synthetic design takes advantage of a selective on-surface head-to-tail coupling of monomer building blocks enabling the regioselective synthesis of [3]triangulene-GNRs. Detailed ab initio theory provides insights into the mechanism of on-surface radical polymerization, revealing the pivotal role of Au-C bond formation/breakage in driving selectivity.

Dramatic Acceleration of the Hopf Cyclization on Gold(111): From Enediynes to Peri-Fused Diindenochrysene Graphene Nanoribbons

Hopf et al. reported the high-temperature 6π-electrocyclization of cis-hexa-1,3-diene-5-yne to benzene in 1969. Subsequent studies using this cyclization have been limited by its very high reaction barrier. Here, we show that the reaction barrier for two model systems, (E)-1,3,4,6-tetraphenyl-3-hexene-1,5-diyne (1a) and (E)-3,4-bis(4-iodophenyl)-1,6-diphenyl-3-hexene-1,5-diyne (1b), is decreased by nearly half on a Au(111) surface. We have used scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) to monitor the Hopf cyclization of enediynes 1a,b on Au(111). Enediyne 1a undergoes two sequential, quantitative Hopf cyclizations, first to naphthalene derivative 2, and finally to chrysene 3. Density functional theory (DFT) calculations reveal that a gold atom from the Au(111) surface is involved in all steps of this reaction and that it is crucial to lowering the reaction barrier. Our findings have important implications for the synthesis of novel graphene nanoribbons. Ullmann-like coupling of enediyne 1b at 20 °C on Au(111), followed by a series of Hopf cyclizations and aromatization reactions at higher temperatures, produces nanoribbons 12 and 13. These results show for the first time that graphene nanoribbons can be synthesized on a Au(111) surface using the Hopf cyclization mechanism.

Highly Regioselective Cyclodehydrogenation of Diphenylporphyrin on Metal Surfaces

Exploring the effect of porphin tautomerism on the regioselectivity of its derivatives is a big challenge, which is significant for the development and application of porphyrin drugs. In this work, we demonstrate the regioselectivity of 2H-diphenylporphyrin (H2-DPP) in the planarization reaction on Au(111) and Ag(111) substrates. H2-DPP monomer forms two configurations (anti- and syn-) via a dehydrogenation coupling, between which the yield of the anti-configuration exceeds 90%. Using high-resolution scanning tunneling microscopy, we visualize the reaction processes from the H2-DPP monomer to the final two planar products. Combined with DFT calculations of the potential reaction pathway and comparative experiments on Au(111) and Ag(111) substrates. Using M-DPP (M = Cu and Fe), we confirm that the regioselectivity of H2-DPP is derived from the reaction energy barrier during the cyclodehydrogenation reaction of different tautomers. This work reveals the regioselectivity mechanism of H2-DPP on the atomic scale, which holds great significance for understanding the chemical conversion process of organic macrocyclic molecules.

On-Surface Cross-Coupling Reactions

On-surface synthesis, as a bottom-up synthetic method, has been proven to be a powerful tool for atomically precise fabrication of low-dimensional carbon nanomaterials over the past 15 years. This method relies on covalent coupling reactions that occur on solid substrates such as metal or metal oxide surfaces under ultra-high-vacuum conditions, and the achievements with this method have greatly enriched fundamental science and technology. However, due to the complicated reactivity of organic groups, distinct diffusion of reactants and intermediates, and irreversibility of covalent bonds, achieving the high selectivity of covalent coupling reactions on surfaces remains a great challenge. As a result, only a few on-surface covalent coupling reactions, mainly involving dehalogenation and dehydrogenation homocoupling, are frequently used in the synthesis of low-dimensional carbon nanosystems. In this Perspective, we focus on the development and synthetic applications of on-surface cross-coupling reactions, mainly Ullmann, Sonogashira, Heck, and divergent cross-coupling reactions.

On-surface synthesis of enetriynes

Belonging to the enyne family, enetriynes comprise a distinct electron-rich all-carbon bonding scheme. However, the lack of convenient synthesis protocols limits the associated application potential within, e.g., biochemistry and materials science. Herein we introduce a pathway for highly selective enetriyne formation via tetramerization of terminal alkynes on a Ag(100) surface. Taking advantage of a directing hydroxyl group, we steer molecular assembly and reaction processes on square lattices. Induced by O₂ exposure the terminal alkyne moieties deprotonate and organometallic bis-acetylide dimer arrays evolve. Upon subsequent thermal annealing tetrameric enetriyne-bridged compounds are generated in high yield, readily self-assembling into regular networks. We combine high-resolution scanning probe microscopy, X-ray photoelectron spectroscopy and density functional theory calculations to examine the structural features, bonding characteristics and the underlying reaction mechanism. Our study introduces an integrated strategy for the precise fabrication of functional enetriyne species, thus providing access to a distinct class of highly conjugated π-system compounds.

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

On-surface synthesis is a rapidly developing field involving chemical reactions on well-defined solid surfaces to access synthesis of low-dimensional organic nanostructures which cannot be achieved via traditional solution chemistry. On-surface reactions critically depend on a high degree of chemoselectivity in order to achieve an optimum balance between target structure and possible side products. Here, we demonstrate synthesis of graphene nanoribbons with a large unit cell based on steric hindrance-induced complete chemoselectivity as revealed by scanning probe microscopy measurements and density functional theory calculations. Our results disclose that combined molecule-substrate van der Waals interactions and intermolecular steric hindrance promote a selective aryl-aryl coupling, giving rise to high-quality uniform graphene nanostructures. The established coupling strategy has been used to synthesize two types of graphene nanoribbons with different edge topologies inducing a pronounced variation of the electronic energy gaps. The demonstrated chemoselectivity is representative for n-anthryl precursor molecules and may be further exploited to synthesize graphene nanoribbons with novel electronic, topological and magnetic properties with implications for electronic and spintronic applications.

Supplementary Information

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

Head-to-Tail Oligomerization by Silylene-Tethered Sonogashira Coupling on Ag(111)

On-surface synthesis is a powerful method for the fabrication of π-conjugated nanomaterials. Herein, we demonstrate chemoselective Sonogashira coupling between (trimethylsilyl)ethynyl and chlorophenyl groups in silylethynyl- and chloro-substituted partially fluorinated phenylene ethynylenes (SiCPFPEs) on Ag(111). The desilylative Sonogashira coupling occurred with high chemoselectivity up to 75 %, while the competing Ullmann and desilylative Glaser homocoupling reactions were suppressed. A combination of bond-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) and DFT calculations revealed that the oligomers were obtained by the formation of intermolecular silylene tethers (-Me₂ Si-) through CH₃ -Si bond activation at 130 °C and subsequent elimination of the tethers at an elevated temperature of 200 °C.

Post-Functionalization of Supramolecular Polymers on Surface and the Chiral Assembly-Induced Enantioselective Reaction

Although post-functionalization is extensively used to introduce diverse functional groups into supramolecular polymers (SPs) in solution, post-functionalization of SPs on surfaces still remains unexplored. Here we achieved the on-surface post-functionalization of two SPs derived from 5,10,15-tri-(4-pyridyl)-20-bromophenyl porphyrin (Br-TPyP) via cross-coupling reactions on Au(111). The ladder-shaped, Cu-coordinated SPs preformed from Br-TPyP were functionalized through Heck reaction with 4-vinyl-1,1'-biphenyl. In the absence of Cu, Br-TPyP formed chiral SPs as two enantiomers via self-assembly, which were functionalized via divergent cross-coupling reaction with 4-isocyano-1,1'-biphenyl (ICBP). Surprisingly, this reaction was discovered as an enantioselective on-surface reaction induced by the chirality of SPs. Mechanistic analysis and DFT calculations indicated that after debromination of Br-TPyP and the first addition of ICBP, only one attack direction of ICBP to the chiral SP intermediate is permissive in the second addition step due to the steric hindrance, which guaranteed the high enantioselectivity of the reaction.

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.

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.

Progress in the self-assembly of porphyrin derivatives on surfaces: STM reveals

The latest progress in the assembly of porphyrin derivatives on solid surfaces.

On-Surface Cascade Reaction Based on Successive Debromination via Metal-Organic Coordination Template

Precise control over on-surface covalent reaction pathways is crucial for engineering organic nanostructures with the single-atom precision. Herein, we demonstrate a step-by-step control of an on-surface cascade covalent reaction based on a successive debromination templated by noncovalent metal-organic coordination motifs. The molecular precursor is predesigned with different reactive sites and functional ligands, allowing for both chemical and structural tuning during on-surface reactions. Through the Fe-terpyridine template effect, we are able to direct the reaction to proceed in a three-step cascade pathway and finally to achieve a porous polyarylene nanoribbon structure. The approach opens new opportunities for construction of on-surface organic nanostructures in a predictable manner.

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-surface synthesis of planar dendrimers via divergent cross-coupling reaction

Dendrimers are homostructural and highly branched macromolecules with unique dendritic effects and extensive use in multidisciplinary fields. Although thousands of dendrimers have been synthesized in solution, the on-surface synthetic protocol for planar dendrimers has never been explored, limiting the elucidation of the mechanism of dendritic effects at the single-molecule level. Herein, we describe an on-surface synthetic approach to planar dendrimers, in which exogenous palladium is used as a catalyst to address the divergent cross-coupling of aryl bromides with isocyanides. This reaction enables one aryl bromide to react with two isocyanides in sequential steps to generate the divergently grown product composed of a core and two branches with high selectivity and reactivity. Then, a dendron with four branches and dendrimers with eight or twelve branches in the outermost shell are synthesized on Au(111). This work opens the door for the on-surface synthesis of various planar dendrimers and relevant macromolecular systems.

Although many strategies exist to synthesize dendrimers in solution, the synthesis of planar dendrimers on a surface has proven challenging. Here, the authors produce planar dendrimers through a divergent on-surface cross-coupling reaction between one aryl bromide and two isocyanides, which enables the growth of branches from a single reactive site.

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.

Coordination-Controlled C-C Coupling Products via ortho-Site C-H Activation

The coordination-restricted ortho-site C-H bond activation and dehydrogenative homocoupling of 4,4'-(1,3-phenylene)dipyridine (1,3-BPyB) and 4,4'-(1,4-phenylene)dipyridine (1,4-BPyB) on different metal surfaces were studied by a combination of scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations. The coupling products on Cu(111) exhibited certain configurations subject to the spatial restriction of robust two-fold Cu-N coordination bonds. Compared to the V-shaped 1,3-BPyB, the straight backbone of 1,4-BPyB helped to further reduce the variety of reactive products. By utilizing the three-fold coordination of Fe atoms with 1,4-BPyB molecules on Au(111), a large-scale network containing single products was constructed. Our results offer a promising protocol for controllable on-surface synthesis with the aid of robust coordination interactions.

Selective on-surface covalent coupling based on metal-organic coordination template

Control over on-surface reaction pathways is crucial but challenging for the precise construction of conjugated nanostructures at the atomic level. Herein we demonstrate a selective on-surface covalent coupling reaction that is templated by metal-organic coordinative bonding, and achieve a porous nitrogen-doped carbon nanoribbon structure. In contrast to the inhomogeneous polymorphic structures resulting from the debrominated aryl-aryl coupling reaction on Au(111), the incorporation of an Fe-terpyridine (tpy) coordination motif into the on-surface reaction controls the molecular conformation, guides the reaction pathway, and finally yields pure organic sexipyridine-p-phenylene nanoribbons. Emergent molecular conformers and reaction products in the reaction pathways are revealed by scanning tunneling microscopy, density functional theory calculations and X-ray photoelectron spectroscopy, demonstrating the template effect of Fe-tpy coordination on the on-surface covalent coupling. Our approach opens an avenue for the rational design and synthesis of functional conjugated nanomaterials with atomic precision.

Synthesizing precise conjugated nanostructures on a surface requires fine control over the covalent reaction pathways. Here, the authors show that reversible coordinative bonds can be used to template on-surface C-C coupling reactions, guiding the formation of porous organic nanoribbons.

On-surface reactions of aryl chloride and porphyrin macrocycles via merging two reactive sites into a single precursor

The reaction of aryl chloride and porphyrin macrocycles, which are merged into a single precursor, has been achieved on Cu(111). Scanning tunneling microscopy analysis of the oligomer products showed that the adjacent porphyrin moieties linked mainly by the phenyl group with the porphyrin macrocycle.