Qinggang Tan

On-Surface Formation of One-Dimensional Polyphenylene through Bergman Cyclization

On-surface fabrication of covalently interlinked conjugated nanostructures has attracted significant attention, mainly because of the high stability and efficient electron transport ability of these structures. Here, from the interplay of scanning tunneling microscopy imaging and density functional theory calculations, we report for the first time on-surface formation of one-dimensional polyphenylene chains through Bergman cyclization followed by radical polymerization on Cu(110). The formed surface nanostructures were further corroborated by the results for the ex situ-synthesized molecular product after Bergman cyclization. These findings are of particular interest and importance for the construction of molecular electronic nanodevices on surfaces.

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.

Atomic-Scale Investigation on the Facilitation and Inhibition of Guanine Tautomerization at Au(111) Surface

Nucleobase tautomerization might induce mismatch of base pairing. Metals, involved in many important biophysical processes, have been theoretically proven to be capable of affecting tautomeric equilibria and stabilities of different nucleobase tautomers. However, direct real-space evidence on demonstrating different nucleobase tautomers and further revealing the effect of metals on their tautomerization at surfaces has not been reported to date. From the interplay of high-resolution STM imaging and DFT calculations, we show for the first time that tautomerization of guanine from G/9H to G/7H is facilitated on Au(111) by heating, whereas such tautomerization process is effectively inhibited by introducing Ni atoms due to its preferential coordination at the N7 site of G/9H. These findings may help to elucidate possible influence of metals on nucleobase tautomerization and provide from a molecular level some theoretical basis on metal-based drug design.

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.

Atomic-Scale Insight into Tautomeric Recognition, Separation, and Interconversion of Guanine Molecular Networks on Au(111)

Although tautomerization may directly affect the chemical or biological properties of molecules, real-space investigation on the tautomeric behaviors of organic molecules in a larger area of molecular networks has been scarcely reported. In this paper, we choose guanine (G) molecule as a model system. From the interplay of high-resolution scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations, we have successfully achieved the tautomeric recognition, separation, and interconversion of G molecular networks (formed by two tautomeric forms G/9H and G/7H) with the aid of NaCl on the Au(111) surface in ultrahigh vacuum (UHV) conditions. Our results may serve as a prototypical system to provide important insights into tautomerization related issues, which should be intriguing to biochemistry, pharmaceutics, and other related fields.

Controllable Scission and Seamless Stitching of Metal–Organic Clusters by STM Manipulation

Scanning tunneling microscopy (STM) manipulation techniques have proven to be a powerful method for advanced nanofabrication of artificial molecular architectures on surfaces. With increasing complexity of the studied systems, STM manipulations are then extended to more complicated structural motifs. Previously, the dissociation and construction of various motifs have been achieved, but only in a single direction. In this report, the controllable scission and seamless stitching of metal-organic clusters have been successfully achieved through STM manipulations. The system presented here includes two sorts of hierarchical interactions where coordination bonds hold the metal-organic elementary motifs while hydrogen bonds among elementary motifs are directly involved in bond breakage and re-formation. The key to making this reversible switching successful is the hydrogen bonding, which is comparatively facile to be broken for controllable scission, and, on the other hand, the directional characteristic of hydrogen bonding makes precise stitching feasible.

Atomic-scale structures and interactions between the guanine quartet and potassium

The atomic-scale identification of the G4K1 structural motif is achieved using an interplay of STM imaging and DFT calculations. Its high stability is found to be caused by the delicate balance between hydrogen bonding and metal-ligand interaction, which is of utmost relevance to model interactions of the G-quadruplex with cations in vivo.

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.

A self-assembled molecular nanostructure for trapping the native adatoms on Cu(110).

The native copper adatoms get trapped in a self-assembled molecular nanostructure which is mainly formed by the intermolecular van der Waals interactions, and two dominating specific binding modes between the adatoms and the molecules are revealed at the atomic scale by high-resolution STM imaging.

Controlling on-surface molecular diffusion behaviors by functionalizing the organic molecules with tert-butyl groups

We have performed the systematic studies on three structurally similar aromatic molecules with different functional groups on a Cu(110) surface and investigated their on-surface molecular diffusion behaviors by the interplay of scanning tunneling microscopy imaging and density functional theory calculations. We have found that the tert-butyl groups could significantly affect the molecular adsorption geometries and moreover the mobility of the molecules on the surface. These findings could give further insights into the understanding of diffusion behaviors of organic molecules specifically with tert-butyl groups on surfaces.