  The adsorption properties of free base
5,10,15,20-tetrakis(p-cyanophenyl)porphyrin (2H-TCNPP) on thin films of rock
salt (rs) CoO(100) on Au(111) was studied in ultra-high vacuum by a combination
of low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) and
density functional theory (DFT). Films of rs-CoO(100) on Au(111) are prepared
with excellent quality in a suitable thickness range. Particularly, we found
that films of only 1 nm thickness show a semiconducting energy gap of
$E_\mathrm{g}=(2.5\pm 0.2)\,\textrm{eV}$. Upon deposition at 300 K, 2H-TCNPP
adsorbs flat-lying and self-assembles in a long-range ordered superstructure
that is stable at 80 K. The adsorption geometry of the molecules on the surface
and within the self-assembly is analyzed by DFT. We find that the
self-assemblies are stabilized by hydrogen bridge bonding via the functional
cyano groups. Our STS data shows molecular states within the fundamental gap of
the CoO. By comparison with the calculated DOS we determine the energetic
positions of the frontier orbitals and find that the first three LUMO states
2H-TCNPP are located within the band gap, whereas the HOMO is shifted 1 eV
below the CoO conduction band edge. Upon annealing to 420 K the molecules
change their appearance in STM images and a new prominent electronic state
located at the center of the molecule is formed. We interpret this changed
configuration as Co-TCNPP created by self-metalation on the oxide surface.
