Liangliang Cai

Dehalogenative Homocoupling of Terminal Alkynyl Bromides on Au(111): Incorporation of Acetylenic Scaffolding into Surface Nanostructures

On-surface C-C coupling reactions of molecular precursors with alkynyl functional groups demonstrate great potential for the controllable fabrication of low-dimensional carbon nanostructures/nanomaterials, such as carbyne, graphyne, and graphdiyne, which demand the incorporation of highly active sp-hybridized carbons. Recently, through a dehydrogenative homocoupling reaction of alkynes, the possibility was presented to fabricate surface nanostructures involving acetylenic linkages, while problems lie in the fact that different byproducts are inevitably formed when triggering the reactions at elevated temperatures. In this work, by delicately designing the molecular precursors with terminal alkynyl bromide, we introduce the dehalogenative homocoupling reactions on the surface. As a result, we successfully achieve the formation of dimer structures, one-dimensional molecular wires and two-dimensional molecular networks with acetylenic scaffoldings on an inert Au(111) surface, where the unexpected C-Au-C organometallic intermediates are also observed. This study further supplements the database of on-surface dehalogenative C-C coupling reactions, and more importantly, it provides us an alternative efficient way for incorporating the acetylenic scaffolding into low-dimensional surface nanostructures.

Dehydrogenative Homocoupling of Terminal Alkenes on Copper Surfaces: A Route to Dienes

Homocouplings of hydrocarbon groups including alkynyl (sp(1) ), alkyl (sp(3) ), and aryl (sp(2) ) have recently been investigated on surfaces with the interest of fabricating novel carbon nanostructures/nanomaterials and getting fundamental understanding. Investigated herein is the on-surface homocoupling of an alkenyl group which is the last elementary unit of hydrocarbons. Through real-space direct visualization (scanning tunneling microscopy imaging) and density functional theory calculations, the two terminal alkenyl groups were found to couple into a diene moiety on copper surfaces, and is contrary to the common dimerization products of alkenes in solution. Furthermore, detailed DFT-based transition-state searches were performed to unravel this new reaction pathway.

Bottom-Up Synthesis of Metalated Carbyne

Because of stability issues, carbyne, a one-dimensional chain of carbon atoms, has been much less investigated than other recent carbon allotropes such as graphene. Beyond that, metalation of such a linear carbon nanostructure with regularly distributed metal atoms is even more challenging. Here we report a successful on-surface synthesis of metalated carbyne chains by dehydrogenative coupling of ethyne molecules and copper atoms on a Cu(110) surface under ultrahigh-vacuum conditions. The length of the fabricated metalated carbyne chains was found to extend to the submicron scale (with the longest ones up to ∼120 nm). We expect that the herein-developed on-surface synthesis strategy for the efficient synthesis of organometallic carbon-based nanostructures will inspire more extensive experimental investigations of their physicochemical properties and explorations of their potential with respect to technological applications.

Formation of a G-Quartet-Fe Complex and Modulation of Electronic and Magnetic Properties of the Fe Center

Although the G-quartet structure has been extensively investigated due to its biological importance, the formation mechanism, in particular, the necessity of metal centers, of an isolated G-quartet on solid surfaces remains ambiguous. Here, by using scanning tunneling microscopy under well-controlled ultra-high-vacuum conditions and density functional theory calculations we have been able to clarify that besides the intraquartet hydrogen bonding a metal center is mandatory for the formation of an isolated G-quartet. Furthermore, by subtly perturbing the local coordination bonding schemes within the formed G-quartet complex via local nanoscale scanning tunneling microscopy manipulations, we succeed in modulating the d orbitals and the accompanying magnetic properties of the metal center. Our results demonstrate the feasibility of forming an isolated G-quartet complex on a solid surface and that the strategy of modulating electronic and magnetic properties of the metal center can be extended to other related systems such as molecular spintronics.

Solventless Formation of G-Quartet Complexes Based on Alkali and Alkaline Earth Salts on Au(111)

Template cations have been extensively employed in the formation, stabilization and regulation of structural polymorphism of G-quadruplex structures in vitro. However, the direct addition of salts onto solid surfaces, especially under ultra-high-vacuum (UHV) conditions, to explore the feasibility and universality of the formation of G-quartet complexes in a solventless environment has not been reported. By combining UHV-STM imaging and DFT calculations, we have shown that three different G-quartet-M (M: Na/K/Ca) complexes can be obtained on Au(111) using alkali and alkaline earth salts as reactants. We have also identified the driving forces (intra-quartet hydrogen bonding and electrostatic ionic bonding) for the formation of these complexes and quantified the interactions involved. Our results demonstrate a novel route to fabricate G-quartet-related complexes on solid surfaces, providing an alternative feasible way to bring metal elements to surfaces for constructing metal-organic systems.

Competition between Hydrogen Bonds and Coordination Bonds Steered by the Surface Molecular Coverage

In addition to the choices of metal atoms/molecular linkers and surfaces, several crucial parameters, including surface temperature, molecular stoichiometric ratio, electrical stimulation, concentration, and solvent effect for liquid/solid interfaces, have been demonstrated to play key roles in the formation of on-surface self-assembled supramolecular architectures. Moreover, self-assembled structural transformations frequently occur in response to a delicate control over those parameters, which, in most cases, involve either conversions from relatively weak interactions to stronger ones (e.g., hydrogen bonds to coordination bonds) or transformations between the comparable interactions (e.g., different coordination binding modes or hydrogen bonding configurations). However, intermolecular bond conversions from relatively strong coordination bonds to weak hydrogen bonds were rarely reported. Moreover, to our knowledge, a reversible conversion between hydrogen bonds and coordination bonds has not been demonstrated before. Herein, we have demonstrated a facile strategy for the regulation of stepwise intermolecular bond conversions from the metal-organic coordination bond (Cu-N) to the weak hydrogen bond (CH···N) by increasing the surface molecular coverage. From the DFT calculations we quantify that the loss in intermolecular interaction energy is compensated by the increased molecular adsorption energy at higher molecular coverage. Moreover, we achieved a reversible conversion from the weak hydrogen bond to the coordination bond by decreasing the surface molecular coverage.

Dehydrogenative Homocoupling of Alkyl Chains on Cu(110).

Through the interplay of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, the stepwise dehydrogenative homocoupling of alkyl chains on Cu(110) is demonstrated, proceeding from the intact chain, via the dehydrogenative intermediates, to the formation of the divers final coupling products.

On‐Surface Formation of Cumulene by Dehalogenative Homocoupling of Alkenyl gem‐Dibromides

The on-surface activation of carbon-halogen groups is an efficient route to produce radicals for constructing various hydrocarbons and carbon nanostructures. To date, the employed halide precursors have only one halogen attached to a carbon atom. It is thus of interest to study the effect of attaching more than one halogen atom to a carbon atom with the aim of producing multiple unpaired electrons. By introducing an alkenyl gem-dibromide, cumulene products were fabricated on a Au(111) surface by dehalogenative homocoupling reactions. The reaction products and pathways were unambiguously characterized by a combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy measurements together with density functional calculations. This study further supplements the database of on-surface synthesis strategies and provides a facile manner for incorporation of more complicated carbon scaffolds into surface nanostructures.

Dehydrogenative and Dehalogenative Homocoupling Reactions of C–X Groups on Metal Surfaces

Surface-assisted synthesis involving hydrocarbons has aroused great attention due to its remarkable potential in constructing novel carbon nanostructures. Particularly, C–C coupling between reactants by cleaving the pre-defined C–X groups (X stands for hydrogen and halogens) followed by forming new carbon-carbon bonds, represents one of the best choices for controllable fabrication of advanced carbon nanostructures. In this chapter, we reviewed the recent achievements of the on-surface reactions of C–X groups activations and C–C couplings, where different carbon species including alkynyl (sp1), alkenyl (sp2), aryl (sp2) and alkyl (sp3) groups are studied.

Self-assembled nanostructures of a di-carbonitrile molecule on copper single-crystal surfaces

Through high-resolution UHV-STM imaging and DFT calculations, we find that a di-carbonitrile molecule unexpectedly prefers to form a hydrogen-bonded nanostructure rather than coordination bonded ones on both Cu(110) and Cu(100) at room temperature. These findings highlight the importance of the surface symmetry and molecule–surface interactions in controllable fabrication of metal–ligand coordination nanostructures.