Synthesis of Surface Covalent Organic Frameworks via Dimerization and Cyclotrimerization of Acetyls

On-surface synthesis of aligned functional nanoribbons monitored by scanning tunnelling microscopy and vibrational spectroscopy

In the blooming field of on-surface synthesis, molecular building blocks are designed to self-assemble and covalently couple directly on a well-defined surface, thus allowing the exploration of unusual reaction pathways and the production of specific compounds in mild conditions. Here we report on the creation of functionalized organic nanoribbons on the Ag(110) surface. C–H bond activation and homo-coupling of the precursors is achieved upon thermal activation. The anisotropic substrate acts as an efficient template fostering the alignment of the nanoribbons, up to the full monolayer regime. The length of the nanoribbons can be sequentially increased by controlling the annealing temperature, from dimers to a maximum length of about 10 nm, limited by epitaxial stress. The different structures are characterized by room-temperature scanning tunnelling microscopy. Distinct signatures of the covalent coupling are measured with high-resolution electron energy loss spectroscopy, as supported by density functional theory calculations.

On-surface synthesis, in which molecular units assemble and couple on a defined surface, can access rare reaction pathways and products. Here, the authors synthesize functionalized organic nanoribbons on the Ag(110) surface, and monitor the evolution of the covalent reactions by an unorthodox vibrational spectroscopy approach.

Template-controlled Sonogashira cross-coupling reactions on a Au(111) surface

We employed three metal-coordination structures as templates to control the Sonogashira cross-coupling reactions on a Au(111) surface.

Packing fractal Sierpiński triangles into one-dimensional crystals via a templating method

Crystalline structures with Sierpiński triangles as building blocks were constructedviaa templating method in ultra-high vacuum and studied by low-temperature scanning tunneling microscopy.

Dynamic covalent chemistry of imine polymers at the liquid/solid interface investigated by scanning tunneling microscopy

How a dynamic covalent library adapts to the presence of a surface is of fundamental importance for the design of adaptive materials. Our investigation demonstrates that the surface markedly rearrange the composition of the DCC libraries.

Construction of Sierpiński triangles with the coexistence of C60or MnPc molecules

Sierpiński triangles up to the fourth order were successfully prepared using 4,4′′-dicyano-1,1′:3′,1′′-terphenyl molecules and Fe or Co atoms with the coexistence of the third molecules (C₆₀or MnPc) on a reconstructed Au(111) substrate.

Interplay of Chemical and Electronic Structure on the Single-Molecule Level in 2D Polymerization

Single layers of covalently linked organic materials in the form of two-dimensional (2D) polymers constitute structures complementary to inorganic 2D materials. The electronic properties of 2D polymers may be manipulated through a deliberate choice of the organic precursors. Here we address the changes in electronic structure-from precursor molecule to oligomer-by scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy. For this purpose, we introduce the polymerization reaction of 1,3,5-tris(4-carboxyphenyl)benzene via decarboxylation on Cu(111), which is thoroughly characterized by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We present a comprehensive study of a contamination-free on-surface coupling scheme and study how dehydrogenation, decarboxylation, and polymerization affect the electronic structure on the molecular level.

Catalytic Dealkylation of Ethers to Alcohols on Metal Surfaces

On-surface synthesis has prompted much interest in recent years because it provides an alternative strategy for controlling chemical reactions and allows for the direct observation of reaction pathways. Herein, we combined scanning tunneling microscopy and density functional theory to provide extensive evidence for the conversion of alkoxybenzene-containing ethers into alcohols by means of surface synthesis. The reported dealkylation reactions are finely controlled by the annealing parameters, which govern the onset of successive alkyl chains dissociations. Moreover, density functional theory calculations elucidate the details of the reaction pathways, showing that dealkylation reactions are surface-assisted and very different from their homogeneous analogues in solution.

On-Surface Domino Reactions: Glaser Coupling and Dehydrogenative Coupling of a Biscarboxylic Acid To Form Polymeric Bisacylperoxides

Herein we report the on-surface oxidative homocoupling of 6,6'-(1,4-buta-1,3-diynyl)bis(2-naphthoic acid) (BDNA) via bisacylperoxide formation on different Au substrates. By using this unprecedented dehydrogenative polymerization of a biscarboxylic acid, linear poly-BDNA with a chain length of over 100 nm was prepared. It is shown that the monomer BDNA can be prepared in situ at the surface via on-surface Glaser coupling of 6-ethynyl-2-naphthoic acid (ENA). Under the Glaser coupling conditions, BDNA directly undergoes polymerization to give the polymeric peroxide (poly-BDNA) representing a first example of an on-surface domino reaction. It is shown that the reaction outcome varies as a function of surface topography (Au(111) or Au(100)) and also of the surface coverage, to give branched polymers, linear polymers, or 2D metal-organic networks.

Structural Variation in Surface-Supported Synthesis by Adjusting the Stoichiometric Ratio of the Reactants

Surface-supported coupling reactions between 1,3,5-tris(4-formylphenyl)benzene and aromatic amines have been investigated on Au(111) using scanning tunneling microscopy under ultra-high-vacuum conditions. Upon annealing to moderate temperatures, various products, involving the discrete oligomers and the surface covalent organic frameworks, are obtained through thermal-triggered on-surface chemical reactions. We conclude from the systematic experiments that the stoichiometric composition of the reactants is vital to the surface reaction products, which is rarely reported so far. With this knowledge, we have successfully prepared two-dimensional covalently bonded networks by optimizing the stoichiometric proportions of the reaction precursors.

Reaction mechanisms for on-surface synthesis of covalent nanostructures

In recent years, on-surface synthesis has become an increasingly popular strategy to form covalent nanostructures. The approach has great prospects for facilitating the manufacture of a range of fascinating materials with atomic precision. However, the on-surface reactions are enigmatic to control, currently restricting its bright perspectives and there is a great need to explore how the reactions are governed. The objective of this topical review is to summarize theoretical work that has focused on comprehending on-surface synthesis protocols through studies of reaction mechanisms.

Surface-Controlled Mono/Diselective ortho C-H Bond Activation

One of the most charming and challenging topics in organic chemistry is the selective C-H bond activation. The difficulty arises not only from the relatively large bond-dissociation enthalpy, but also from the poor reaction selectivity. In this work, Au(111) and Ag(111) surfaces were used to address ortho C-H functionalization and ortho-ortho couplings of phenol derivatives. More importantly, the competition between dehydrogenation and deoxygenation drove the diversity of reaction pathways of phenols on surfaces, that is, diselective ortho C-H bond activation on Au(111) surfaces and monoselective ortho C-H bond activation on Ag(111) surfaces. The mechanism of this unprecedented phenomenon was extensively explored by scanning tunneling microscopy, density function theory, and X-ray photoelectron spectroscopy. Our findings provide new pathways for surface-assisted organic synthesis via the mono/diselective C-H bond activation.

Preparation and engineering of oriented 2D covalent organic framework thin films

Hybrid metal/COF stack multilayers and patterned COF films were fabricated via the flexible combination of solvothermal deposition and compatible film processing techniques.

Growth of covalently bonded Sierpiński triangles up to the second generation

Growth of covalently bonded Sierpiński triangles (CB-STs) on metal surfaces was investigated by scanning tunneling microscopy (STM).