Interaction
of organic molecules with solid surfaces reflects the
local electronic structure of nonequivalent surface sites. The Si(111)-Sn-√3
× √3 surface with intrinsic defects offers dangling bonds
with various electron occupancies. In this study, we report on the
interaction of copper phthalocyanine with this surface studied with
scanning tunneling microscopy and spectroscopy. Sn atoms in the adlayer
contain unpaired electrons in their dangling bonds. The molecules
are found to adsorb on Sb and Si substitutional defects instead of
the homogenous Sn areas. The symmetry of the molecules is lowered
or lost after adsorption, and we find substantial differences in the
electronic structure of the molecules adsorbed on either type of defects.
Furthermore, the molecules preferentially adsorb on double Si substitutional
defects where they can switch between a static and a fuzzy state,
for both of which a corresponding bonding model is proposed. Our study
shows a pivotal role of substitutional defects in the reactivity of
the Si(111)-Sn-√3 × √3 surface for the copper phthalocyanine
molecules, which we attribute to local changes of electronic structure
and discuss it in terms of dangling bond state occupancy.