Atomic‐Scale Visualization of the Stepwise Metal‐Mediated Dehalogenative Cycloaddition Reaction Pathways: Competition between Radicals and Organometallic Intermediates

Orbital-symmetry effects on magnetic exchange in open-shell nanographenes

Open-shell nanographenes appear as promising candidates for future applications in spintronics and quantum technologies. A critical aspect to realize this potential is to design and control the magnetic exchange. Here, we reveal the effects of frontier orbital symmetries on the magnetic coupling in diradical nanographenes through scanning probe microscope measurements and different levels of theoretical calculations. In these open-shell nanographenes, the exchange energy exhibits a remarkable variation between 20 and 160 meV. Theoretical calculations reveal that frontier orbital symmetries play a key role in affecting the magnetic coupling on such a large scale. Moreover, a triradical nanographene is demonstrated for investigating the magnetic interaction among three unpaired electrons with unequal magnetic exchange, in agreement with Heisenberg spin model calculations. Our results provide insights into both theoretical design and experimental realization of nanographene materials with different exchange interactions through tuning the orbital symmetry, potentially useful for realizing magnetically operable graphene-based nanomaterials.

On-Surface Synthesis of Unsaturated Hydrocarbon Chains through C-S Activation

We use an on-surface synthesis approach to drive the homocoupling reaction of a simple dithiophenyl-functionalized precursor on Cu(111). The C-S activation reaction is initiated at low annealing temperature and yields unsaturated hydrocarbon chains interconnected in a fully conjugated reticulated network. High-resolution atomic force microscopy imaging reveals the opening of the thiophenyl rings and the presence of trans- and cis-oligoacetylene chains as well as pentalene units. The chemical transformations were studied by C 1s and S 2p core level photoemission spectroscopy and supported by theoretical calculations. At higher annealing temperature, additional cyclization reactions take place, leading to the formation of small graphene flakes.

Dissociative electron attachment to hexachlorobenzene

Gas phase molecules of hexachlorobenzene (C 6 Cl 6 ) were investigated by means of Dissociative Electron Attachment Spectroscopy (DEAS). Three channels of molecular negative ions decay have been recorded: Cl- and Cl 2 - as well as electron detachment (t a ~250 ms at 343 K). All of these decay channels exhibit temperature dependence. The electron affinity was estimated within the framework of the simple Arrhenius model (EA=1.6-1.9 eV), as well as quantum-chemically at various levels (EA a =0.9-1.0 eV). The reasons for the discrepancy between the obtained estimates of the EA value and the results of previous works (EA from 1 to 2 eV) are discussed.

On-Surface Hydrogen/Deuterium Isotope Exchange in Polycyclic Aromatic Hydrocarbons

Hydrogen plays important roles in the on-surface synthesis of carbon-based materials in ultra-high vacuum. The complex interplay between hydrogen and surface-adsorbed polycyclic aromatic hydrocarbons (PAHs) is tracked by in situ time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with isotope labeling. In situ deuterium labeling of prototypical PAHs, coronene (CR) and 7-armchair graphene nanoribbons (GNRs), on Au(111) is achieved by annealing either in D₂ gas or in the vapor of perdeuterio-acenaphthene. By following the mass spectra of in situ deuterated CR mixed with hydrogen-CR, it is demonstrated that PAHs adsorbed at hot Au(111) surfaces continuously exchange hydrogen atoms. Also, D₂ present during the Ullmann coupling step leads to incorporation of deuterium and to shorter GNRs.

On-Surface Synthesis of Chlorinated Narrow Graphene Nanoribbon Organometallic Hybrids

Graphene nanoribbons (GNRs) and their derivatives attract growing attention due to their excellent electronic and magnetic properties as well as the fine-tuning of such properties that can be obtained by heteroatom substitution and/or edge morphology modification. Here, we introduce graphene nanoribbon derivatives-organometallic hybrids with gold atoms incorporated between the carbon skeleton and side Cl atoms. We show that narrow chlorinated 5-AGNROHs (armchair graphene nanoribbon organometallic hybrids) can be fabricated by on-surface polymerization with omission of the cyclodehydrogenation reaction by a proper choice of tailored molecular precursors. Finally, we describe a route to exchange chlorine atoms connected through gold atoms to the carbon skeleton by hydrogen atom treatment. This is achieved directly on the surface, resulting in perfect unsubstituted hydrogen-terminated GNRs. This will be beneficial in the molecule on-surface processing when the preparation of final unsubstituted hydrocarbon structure is desired.

Graphene-Like Covalent Organic Framework with a Wide Band Gap Synthesized On Surface via Stepwise Reactions

Developing graphene-like two-dimensional materials naturally possessing a band gap has sparked enormous interest. Thanks to the inherent wide band gap and high mobility in the 2D plane, covalent organic frameworks containing triazine rings (t-COFs) hold great promise in this regard, whilst the synthesis of single-layer t-COFs remains highly challenging. Herein, we present the fabrication of a well-defined graphene-like t-COF on Au(111). Instead of single/multiple-step single-type reactions commonly applied for on-surface synthesis, distinct stepwise on-surface reactions, including alkynyl cyclotrimerization, C-O bond cleavage, and C-H bond activation, are triggered on demand, leading to product evolution in a controlled step-by-step manner. Aside from the precise control in sophisticated on-surface synthesis, this work proposes a single-atomic-layer organic semiconductor with a wide band gap of 3.41 eV.