  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.
