Interplay of hydrogen bonding and molecule-substrate interaction in self-assembled adlayer structures of a hydroxyphenyl-substituted porphyrin

The formation of hydrogen-bonded organic nano-structures and the role of the substrate lattice thereby were investigated by scanning tunneling microscopy. The self-organization of 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin (H2THPP) molecules leads to two molecular arrangements on Au(111). One of these is characterized by pair-wise hydrogen bonding between hydroxyl groups and a low packing density which enables a rotation of individual molecules in the structure. A different interaction with stronger chain-like hydrogen bonding and additional interactions of phenyl groups was observed for the second structure. The influence of the substrate on the epitaxial behavior is demonstrated by the adsorption of H2THPP on the highly anisotropic Ag(110) substrate. There, several balances between the occupation of favorable adsorption positions and the number of hydrogen bonds per molecule were found. The molecules form molecular chains on Ag(110) and also assemble into two-dimensional periodic arrangements of differently sized close-packed blocks similar to the second type of supramolecular ordering found on Au(111). Dispersion corrected Density Functional Theory calculations were applied to understand the adsorption and complex epitaxy of these molecules. It is shown that the azimuthal orientation of the saddle-shape deformed molecule plays an important role not only for the intermolecular but also for the molecule-substrate interaction.