Supramolecular multiporphyrin arrays are considered ideal model systems for the study of energy transfer mechanisms and of natural photosynthesis. A reliable and rigorous method is needed, in order to characterize the electronic and structural properties of porphyrins at the interface, where the molecule-substrate interactions play a fundamental role. We investigate the adsorption behavior of nickel tetraphenyl porphyrin (NiTPP) molecules on the Cu(100) surface by applying a comprehensive multi-technique approach, photoemission electron microscopy (PEEM), photoemission tomography (PT) and STM, complemented density functional theory (DFT) calculations.
The adsorbed NiTPP arrange in two different geometrical configurations with different orientation respect to  crystal direction. By combining STM with DFT calculations, we demonstrate that the contrast, in STM images, arises mainly from the phenyl peripheral groups, which are tilted upwards. This adsorption configuration prevents the macrocycle, where frontier orbitals are localized, to be resolved by the STM tip.
For this reason, STM cannot be used for mapping the charge distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). Therefore, as a complementary technique, we exploited the capability of our PEEM to directly image a wide reciprocal space in one single shoot. The angle-resolved photoemission data (ARPES) were then compared to DFT calculations, within the PT framework. The molecular orbitals exhibit peculiar features which can be used to unambiguously identify them in the valence band spectra. This is particularly useful to determine the molecule-substrate interaction and the charge transfer phenomena between adsorbed molecules and metal surfaces. The comparison between experiment and theory shows that the former gas-phase LUMO+3 becomes occupied upon adsorption on the metal surface. This unexpected result suggests that a multi-technique approach is mandatory in order to obtain a consistent picture of the adsorption behavior and electronic properties of the molecular system.