Chunxue Yuan

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.

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.

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.

An acidic pH fluorescent probe based on Tröger's base

A novel pH fluorescent probe 2,8-(6H,12H-5,11-methanodibenzo[b,f]diazocineylene)-di(p-ethenyl-pyridine) (TBPP) incorporating an electron-donating amine moiety and electron-accepting pyridine group through Trogers base linker was designed and synthesized. TBPP exhibits an intramolecular charge transfer effect caused by the donor–acceptor interaction between its amine and pyridine units. Its emission can be reversibly switched between blue and dark states by protonation and deprotonation. Such behavior enables it to work as a turn-off fluorescent pH sensor in solution state. 1H NMR spectroscopy analysis suggests that the change in electron affinity of the pyridinyl unit upon protonation and deprotonation is responsible for such sensing processes.

On-Surface Synthesis of Adenine Oligomers via Ullmann Reaction

Despite the fact that DNA bases have been well-studied on surface, the on-surface synthesis of one-dimensional DNA analogs through in situ reactions is still an interesting topic to be investigated. Herein, from the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have delicately designed a halogenated derivative of adenine as precursor to realize the combination of DNA bases and Ullmann reaction, and then successfully synthesized adenine oligomers on Au(111) via Ullmann coupling. This model system provides a possible bottom-up strategy of fabricating adenine oligomers on surface, which may further give access to man-made DNA strands with multiple bases.

An unexpected dual-response pH probe based on acridine

A new pH fluorescent probe 2,8-bis(acridin-9-ylethynyl)-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (TBN), which has two acridine moieties attached to Trogers base, is a useful fluorescent probe for monitoring extreme acidic and alkaline pH. TBN displays an excellent pH dependent behavior and responds linearly to extreme conditions in the pH ranges of 1.4–3.4 and 12.5–15.0. TBN can represent a novel type of fluorescent probe with perfect emission properties in extreme acidic and alkaline conditions by utilizing only one functional group.

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.