  On-surface metal porphyrins can undergo electronic and conformational changes
that play a crucial role in determining the chemical reactivity of the
molecular layer. Therefore, understanding those properties is pivotal for the
design and implementation of organic-based devices. Here, by means of
photoemission orbital tomography supported by density functional theory
calculations, we investigate the electronic and geometrical structure of two
metallated tetraphenyl porphyrins (MTPPs), namely ZnTPP and NiTPP, adsorbed on
the oxygen-passivated Fe(100)-p(1x1)O surface. Both molecules weakly interact
with the surface as no charge transfer is observed. In the case of ZnTPP our
data correspond to those of moderately distorted molecules, while NiTPP
exhibits a severe saddle-shape deformation. From additional experiments on
NiTPP multilayer films, we conclude that this distortion is a consequence of
the interaction with the substrate, as the NiTPP macrocycle of the second layer
turns out to be flat. We further find that distortions in the MTPP macrocycle
are accompanied by an increasing energy gap between the highest occupied
molecular orbitals (HOMO and HOMO-1). Our results demonstrate that
photoemission orbital tomography can simultaneously probe the energy level
alignment, the azimuthal orientation, and the adsorption geometry of complex
aromatic molecules even in the multilayer regime.
