Two-dimensional self-assembled nanostructures of nucleobases and their related derivatives on Au(111)

Noncovalent wedging effect catalyzed the cis to syn transformation of a surface-adsorbed polymer backbone toward an unusual thermodynamically stable supramolecular product

The significant influence of noncovalent interactions on catalytic processes has been recently appreciated but is still in its infancy. In this report, it is found that wedging Me-PTCDI (small-molecule) between the alkyl chains of PffBT4T-2OD (polymer) and a graphite substrate can reduce the energy barrier of flipping over the surface-adsorbed alkylthiophene group from the cis to syn conformation, revealing the catalytic role of Me-PTCDI via a noncovalent wedging effect. The wedging of Me-PTCDI brings the interactions between the alkyl chains and substrate to a very weak level by lifting up the alkyl chains, which eliminates the major hindrance of the flipping process to one main factor: the torsion of the dihedral angles of the thiophene group. The Me-PTCDI/syn PffBT4T-2OD arrangement shows unusual stability compared to the cis one because the syn conformation allows the alkyl chains to construct dense lamella and facilitates interactions between Me-PTCDI and the syn PffBT4T-2OD backbones. The results are helpful for boosting the development of noncovalent catalysis and bottom-up fabrications toward devices functionalized at a molecular level.

Preparation and characterization of non-aromatic ether self-assemblies on a HOPG surface

On-surface self-assemblies of aromatic organic molecules have been widely investigated, but the characterization of analogous self-assemblies consisting of fully sp³-hybridized molecules remains challenging. The possible on-surface orientations of alkyl molecules not exclusively comprised of long alkyl chains are difficult to distinguish because of their inherently low symmetry and non-planar nature. Here, we present a detailed study of diamondoid ethers, structurally rigid and fully saturated molecules, which form uniform 2D monolayers on a highly oriented pyrolytic graphite (HOPG) surface. Using scanning tunneling microscopy, various computational tools, and x-ray structural analysis, we identified the most favorable on-surface orientations of these rigid ethers and accounted for the forces driving the self-organization process. The influence of the oxygen atom and London dispersion interactions were found to be responsible for the formation of the observed highly ordered 2D ether assemblies. Our findings provide insight into the on-surface properties and behavior of non-aromatic organic compounds and broaden our understanding of the phenomena characteristic of monolayers consisting of non-planar molecules.

The Effects of Potential and pH on the Adsorption of Guanine on the Au(111) Electrode

The adsorption of nucleobase at a gold electrode has been a model system to study the interaction between biomolecule and metal, which is relevant to the development of sensors and molecular electronics. The current study has employed in situ scanning tunneling microscopy (STM) and voltammetry to investigate the adsorption configuration and spatial structure of guanine (G) on a well-defined Au(111) electrode in perchloric acid (HClO₄) and neutral phosphate buffer solution (PBS) containing 50 μM G. Potential control had a profound effect on the adsorption of the G molecule on the Au(111) electrode. No adsorption of G was observed at a potential more negative than 0 V in HClO₄ and -0.2 V (versus Ag/AgCl) in PBS; shifting potential positively triggered a rapid adsorption of G to yield a well-ordered G array. Different spatial structures of G admolecules were imaged with STM in HClO₄ and PBS, suggesting that ions in the electrolyte were important in this adsorption event. Shifting potential positively caused a more compact G adlayer with molecules adopting a tilted orientation. Meanwhile, G molecules continued to deposit on the Au(111) electrode leading to a multilayer G film. These processes were reversible to the potential modulation. G admolecules on the Au(111) electrode could be irreversibly oxidized in 0.1 M PBS, which resulted in a prominent peak at 0.74 V in the voltammogram. This oxidation process could be used to analyze the G molecule in a sample.

A two-dimensional crystal formed by pentamers on Au(111)

A new type of two-dimensional crystal comprising supramolecular pentamers on Au(111) is studied using an ultra-high vacuum low-temperature scanning tunnelling microscope.