Formation of Water-Insoluble Polycyclic Aromatic Hydrocarbons Adlayers at Electrochemical Interface Using "Molecular Containers"
Authors: Yoshimoto Soichiro 
Published: December 2024

Carbon materials, such as graphene and graphene nanoribbons (GNRs), have been extensively studied for their application in electronic devices,electrocatalysis, and sensors. Polycyclic aromatic hydrocarbons (PAHs), so-called nanographenes, are excellent materials for the creation of two-dimensional (2D) nanosheets.1 Form the viewpoints of surface science and material chemistry, the formation of adlayers on electrode surfaces is challenging because PAHs without peripheral substituents are used as 2D molecular templates and applications in conductance enhancement and organic field-effect transistors.2 Small PAHs such as ovalene,3 circobiphenyl,4 and dicoronylene, which are structurally defined and expected to be excellent building blocks as 2D molecular patterns, tend to exhibit poor or no solubilities in any organic solvents and water.

To overcome this issue, we employed water-soluble micelle capsules consisting of V-shaped amphiphilic molecules (1).5 The micellar capsules act as “molecular containers” for the water-insoluble PAHs to deliver to the electrode surface (see Figure). Characteristic electrochemical behaviors were observed in 0.1 M H2SO4 in the presence of water-soluble capsules containing PAHs such as dicoronylene. Furthermore, under these conditions, PAHs are gradually released from the micelle capsules, resulting in the formation of a 2D adlayer of PAHs at the electrochemical interface. Finally, using electrochemical scanning tunneling microscopy (EC-STM), we demonstrate that our molecular containers based on water-soluble molecular capsules allow the facile preparation of 2D PAH adlayers, in addition to structurally controlling nanostructure formation on Au surfaces.In particular, dicoronylene molecules form a highly ordered adlayer with a c(4 x 8√3)rect adlattice.6 The highly ordered adlayers of ovalene and circobiphenyl molecules were also formed on the Au(111) electrode surface in the same manner. In addition, much larger PAHs such as C96H30, C150H42, and C222H42 were synthesized. These larger PAHs molecules were also soluble and stable in water, and were examined to form adlayers on the Au(111) electrode surface using micellar capsules. The C96H30 molecules clearly reveal the formation of a molecular adlayer as a triangle-shaped image. In case of C150H42 and C222H42, we did not observe highly ordered arrays on Au(111), but we could recognize each molecular shape with some additional spots, indicating that chlorides of counter anions of V-shaped amphiphilic molecules are also co-adsorbed on the Au(111), as the so-called “specific adsorption of anions.

Thus, “Molecular Containers” method using micelle capsules is useful for the preparation of highly ordered PAHs adlayers from aqueous electrolyte – electrode interface.

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Figure 1