Stepwise on-surface synthesis of nitrogen-doped porous carbon nanoribbons

Precise synthesis of carbon-based nanostructures with well-defined structural and chemical properties is of significance towards organic nanomaterials, but remains challenging. Herein, we report on a synthesis of nitrogen-doped porous carbon nanoribbons through a stepwise on-surface polymerization. Scanning tunneling microscopy revealed that the selectivity in molecular conformation, intermolecular debrominative aryl-aryl coupling and inter-chain dehydrogenative cross-coupling determined the well-defined topology and chemistry of the final products. Density functional theory calculations predict that the ribbons are semiconductors, and the band gap can be tuned by the width of the ribbons.

Adatoms in the Surface-Confined Ullmann Coupling of Phenyl Groups

Despite the importance of the on-surface Ullmann coupling for synthesis of atomically precise carbon nanostructures, it is still unclear whether this reaction is catalyzed by surface atoms or adatoms. Here, the feasibility of the adatom creation and adatom-catalyzed Ullmann coupling of chloro-, bromo-, and iodobenzene on Cu(111), Ag(111), and Au(111) surfaces is examined using density functional theory modeling. The extraction of a metal atom is found to be greatly facilitated by the formation of strong phenyl-metal bonds, making the extraction energy barrier comparable to, and in the case of Ag(111) even lower than, that for the competing surface-catalyzed phenyl-phenyl bond formation. However, if the phenyl-adatom bonds are too strong, as on Cu(111) and Ag(111), they create an insurmountable barrier for the subsequent adatom-catalyzed C-C coupling. In contrast, Au adatoms do not bind phenyl groups strongly and can catalyze the C-C bond formation almost as efficiently as surface atoms.

Structural Phase Transitions of Molecular Self-Assembly Driven by Nonbonded Metal Adatoms

The involvement of metal atoms in molecular assemblies has enriched the structural and functional diversity of two-dimensional supramolecular networks, where metal atoms are incorporated into the architecture via coordination or ionic bonding. Here we present a temperature-variable study of the self-assembly of the 1,3,5-tribromobenzene (TriBB) molecule on Cu(111) that reveals the involvement of nonbonded adatoms in the molecular matrix. By means of scanning tunneling microscopy and noncontact atomic force microscopy, we demonstrate the molecular-level details of a phase transition of TriBB assembly from the close-packed to porous honeycomb structures at 78 K. This is an unexpected transformation because the close-packed phase is thermodynamically favored in view of its higher molecular density and more intermolecular bonds as compared to the honeycomb lattice. A comprehensive density functional theory calculation suggests that Cu adatoms should be involved in the formation of the honeycomb network, where the Cu adatoms help stabilize the molecular assembly via enhanced van der Waals interactions between TriBB molecules and the underlying substrate. Both calculation and experimental results suggest no chemical bonding or direct charge transfer between the adatoms and the molecules, thus the electronic characteristics of the Cu adatoms trapped in the molecular confinement are close to the intrinsic ones on a clean metal surface and different from those in the traditional coordination-bonded framework. The nonbonded metal adatoms embedded self-assemblies may complement the metal-organic coordination system and can be used to tailor the chemical reactivity and electronic properties of supramolecular structures.

Synthesis and photophysics of new pyridyl end-capped 3D-dithia[3.3]paracyclophane-based Janus tectons: surface-confined self-assembly of their model pedestal on HOPG

Once synthesized, these new tectons demonstrated both ionic and coordination bonding. Surprisingly, P forms a quasi-square self-assembly independently of the underlying HOPG lattice.