Stability and templating studies of bi-molecular structures on metallic and semiconductor surfaces

Perdigao, Luis M.A.1; Saywell, Alex1; Smith, Nicholas1; Staniec, Paul A.1; Fontes, Giselle N.2; Goretzki, Gudrun1; Champness, Neil R.1; Beton, Peter H.1 1United Kingdom; 2Brazil

Hydrogen bonded networks can be used to create supramolecular templates. We have previously demonstrated that perylene tetracarboxlyic diimide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (melamine) may form a hexagonal network on two different subtrates, Au(111) and Ag terminated Si(111) [Nature 2003, 424, 1029; Phys. Rev. B 2006, 73, 195423]. The structure is stabilised by a triple hydrogen bond between constituent molecules and its pores can be used as molecular traps. The formation of these structures is demonstrated with a scanning tunnelling microscope operating at room temperature in ultrahigh-vacuum conditions. Here we report a new arrangement that PTCDI and melamine may form on Au(111) substrate, composed of double rows of PTCDI and melamine chains with connecting PTCDI molecules between the chains. The smaller pores formed can also be used to capture molecules such as dimers of C₆₀ and decanethiol. Its resistance to exposure to air is also demonstrated. We also explored the possibility of introducing functionality into the pores of the hexagonal structures using PTCDI-derivatives with chemical active side-groups attached to the perylene core. We observe that Br₂PTCDI forms close-packed rows on both Au(111) and Ag-Si(111) surfaces, with the addition of melamine stimulating the formation of mixed-species hexagonal networks, similar to the behaviour exhibited by PTCDI. By substituting the bromine with the larger and more reactive dipropylthioether group an alternative network arrangement is formed. Dipropylthioether-PTCDI self-assembles on Ag-Si(111) to produce a mono-species hexagonal network, as well as the expected close-packed row phase. The introduction of melamine again gives rise to mixed-species hexagonal and row phases. From these studies we propose models as to the arrangement and composition of the observed structures, supported by density-functional calculations. In summary, a new robust bimolecular arrangement has been found, indicating a promising route for template studies and applications. We also found that hydrogen bonded networks can be created using functionalised PTCDI derivatives. The addition of functional groups can cause significant, and unexpected, changes to the arrangements observed.

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