Qitang Fan

Surface-Assisted Formation, Assembly, and Dynamics of Planar Organometallic Macrocycles and Zigzag Shaped Polymer Chains with C–Cu–C Bonds

The formation, structure, and dynamics of planar organometallic macrocycles (meta-terphenyl-Cu)n and zigzag-shaped one-dimensional organometallic polymers on a Cu(111) surface were studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Vapor deposition of 4,4″-dibromo-meta-terphenyl (DMTP) onto Cu(111) at 300 K leads to C-Br bond scission and formation of C-Cu-C bonds, which connect neighboring meta-terphenyl fragments such that room-temperature stable macrocycles and zigzag chains are formed. The chains self-assemble to form islands, which are elongated in the direction of the chains. If DMTP is deposited onto Cu(111) held at 440 K, the island size is drastically increased (>200 × 200 nm(2)). STM sequences show the formation of ordered structures through reversible scission and reformation of the C-Cu-C bonds. The cyclic organometallic species such as the hexamer (meta-terphenyl-Cu)6 may represent intermediates in the surface-confined Ullmann synthesis of hydrocarbon macrocycles such as the recently discovered hyperbenzene.

Confined Synthesis of Organometallic Chains and Macrocycles by Cu–O Surface Templating

The bottom-up construction of low-dimensional macromolecular nanostructures directly on a surface is a promising approach for future application in molecular electronics and integrated circuit production. However, challenges still remain in controlling the formation of these nanostructures with predetermined patterns (such as linear or cyclic) or dimensions (such as the length of one-dimensional (1D) chains). Here, we demonstrate that a high degree of structural control can be achieved by employing a Cu(110)-(2×1)O nanotemplate for the confined synthesis of organometallic chains and macrocycles. This template contains ordered arrays of alternating stripes of Cu-O chains and bare Cu, the widths of which are controllable. Using scanning tunneling microscopy and low-energy electron diffraction, we show that well-defined, ordered 1D zigzag organometallic oligomeric chains with uniform lengths can be fabricated on the Cu stripes (width >5.6 nm) of the Cu(110)-(2×1)O surface. In addition, the lengths of the meta-terphenyl (MTP)-based chains can be adjusted by controlling the widths of the Cu stripes within a certain range. When reducing the widths of Cu stripes to a range of 2.6 to 5.6 nm, organometallic macrocycles including tetramer (MTP-Cu)4, hexamer (MTP-Cu)6, and octamer (MTP-Cu)8 species are formed due to the spatial confinement effect and attraction to the Cu-O chains. An overview of all formed organometallic macrocycles on the Cu stripes with different widths reveals that the origin of the formation of these macrocycles is the cis-configured organometallic dimer (MTP)2Cu3, which was observed on the extremely narrow Cu stripe with a width of 1.5 nm.

Tribromobenzene on Cu(111): Temperature-dependent formation of halogen-bonded, organometallic, and covalent nanostructures

The temperature-controlled surface-assisted synthesis of halogen bonded, organometallic, and covalent nanostructures based on 1,3,5-tribromo-benzene (TriBB) was studied with scanning tunneling microscopy and X-ray photoemission spectroscopy in ultrahigh vacuum. Vapor deposition of TriBB onto a Cu(111) surface held at 90 K leads to the formation of large domains of a honeycomb-like organic monolayer structure stabilized by triangular nodes with Br⋯Br intermolecular bonds. Upon annealing the organic monolayer to ∼140 K, a new hexagonal close-packed structure with intact TriBB molecules connected by Cu adatoms is formed. Further warming up the sample to 300 K gives rise to the scission of C-Br bonds and formation of C-Cu-C bonds between phenyl fragments such that stable dendritic organometallic networks are formed. Larger islands of organometallic networks are obtained by maintaining the temperature of Cu(111) at 420 K during deposition of TriBB. Simultaneously, large islands of Br atoms are formed around the organometallic networks. Annealing the more extended organometallic network (prepared at 420 K) to 520 K leads to the formation of a branched covalent organic framework (COF) which comprises structural elements of porous graphene and is surrounded by Br islands. These organometallic networks and COFs appear as small dendritic and branched domains, most likely due to the steric influence exerted by the Br islands.

Role of oxygen incorporation in electronic properties of rubrene films

The electronic properties of rubrene films exposed to oxygen have been studied by photoemission and x-ray absorption spectroscopies. Oxygen incorporation causes the highest occupied molecular orbital of rubrene to shift to a ∼1.0 eV deep level due to chemical bonds formed by the oxygen atoms with the carbon atoms within the tetracene backbone of rubrene molecules. Deformation of the molecular backbone induced by the C–O bonds destroys the delocalized nature of the molecular orbitals. The oxidized rubrene molecules in a single crystal are proposed to act as point defects that disturb the long-range periodicity and produce localized acceptor states.

Highly Selective Synthesis of cis-Enediynes on a Ag(111) Surface

Cis-enediyne-type compounds have received much attention as potent antitumor antibiotics. The conventional synthesis of cis-enediynes in solution typically involves multiple steps and various side reactions. For the first time, selective one-step synthesis of cis-enediyne from a single reactant is reported on a Ag(111) surface with a yield up to 90 %. High selectivity for the formation of cis-enediyne originates from the steric effect posed by weak intermolecular interactions, which protect the cis-enediyne from further reaction. A series of comparative experiments and DFT-based transition-state calculations support the findings. The described synthetic approach for directing reaction pathways on-surface may illuminate potential syntheses of other unstable organic compounds.

Chiral Kagome Lattices from On-surface Synthesized Molecules

Kagome lattices have attracted much attention owing to their potential applications in spin-frustrated magnetism and host-guest chemistry. Examples toward the fabrication of 2D Kagome lattices reported previously have in common that the precursor molecules were typically deposited on the surface structurally intact with no chemical reactions accompanied. Herein, by using a combination of synchrotron radiation photoelectron spectroscopy (SRPES) and scanning tunneling microscopy (STM), we demonstrated the fabrication of two types of chiral Kagome lattices from on-surface synthesized organometallic compounds, which are known as intermediates of Glaser coupling on silver single crystal surfaces. These Kagome lattices are stabilized by the interplay of various intermolecular interactions, including Br⋅⋅⋅Br bonds, C-Br⋅⋅⋅π bonds and π-π stacking. The chiral transference and host-guest supramolecular structure in the novel Kagome lattices were also studied. Our studies may pave a new way to engineer complex supramolecular networks through on-surface reactions.

On-Surface Ullmann Reaction for the Synthesis of Polymers and Macrocycles

Compared to organic synthesis in solution, the benefits of on-surface synthesis are apparent especially when it involves insoluble reactants or when in situ characterizations of products with large molecular weight are required. This article covers the on-surface synthesis of hydrocarbon polymers and macrocycles via an Ullmann type reaction of haloarenes on metal single-crystal surfaces. The related formation of stable organometallic reaction intermediates with carbon-metal-carbon bonds is also discussed. Regarding the on-surface synthesis of polymers, deposition onto hot metal surfaces (in contrast to post-annealing) as well as high diffusion rates of the monomers lead to the increase of their chain lengths. To obtain high yields of macrocycles, it is important to enable the ring closure at the stage of the organometallic species with reversible carbon-metal-carbon bonds and to apply high-dilution conditions. In contrast, the formation of macrocycles by cyclisation of polymer chains with only covalent bonds is of low probability due to the particular mechanism of the ring-closure process in two-dimensional confinement.