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Organic-based device performances have been rapidly improving in the last years, making them suitable for large-scale industrial applications, involving photo-voltaic cells, light emission systems and building of larger flexible electronics. In parallel, basic research has intensively focused on the chemical and physical properties of semiconducting π-conjugated organic molecules, as they appear to be promising for organic-based device construction. In particular, in controlling the charge injection on such devices, a predominant role is played by the molecule-substrate interaction. Charge transfer at the molecule-metal interface strongly affects the overall physical and magnetic properties of the system, and ultimately, the device performance.

On the perspective of possible technological applications, such as colorimetric gas sensors, organic spin-valves, field-effect transistors, etc., porphyrin represent a class of extremely versatile molecules, allowing for tailoring a variety of electronic, magnetic and conformational properties. In particular, supramolecular multi-porphyrin arrays are considered as functional components in nanodevices. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules (Ni-TPP) adsorbed on Cu(100).
NiTPP molecules tend to form well-ordered islands on the Cu(100) surface already at low coverages, in particular STM measurements reveal the presence of two rotational domains mirrored with respect to the [001] direction of Cu(100). These two domains are also commensurate with the substrate (see Fig. 1a). The molecule-substrate interaction strongly affects the molecular adsorption geometry and electronic structure. Indeed, hybrid functional DFT calculations suggest a significant charge transfer from Cu to the molecule, resulting in the occupation of the gas-phase LUMO/LUMO + 1 and LUMO + 3 molecular orbitals, accompanied by a back donation of charge from the molecule to the substrate. As a consequence of this strong interaction with the substrate, the porphyrin's macrocycle approaches the surface very closely (∽2 Å), forcing the phenyl ligands to bend upwards (Fig. 1b). Therefore, the STM contrast arises mainly from the electronic states of the phenyl rings preventing the STM tip to reliably probe the states related to the macrocycle.

This limitation can be overcome by molecular orbital tomography (MOT) which combines angle resolved photoelectron spectroscopy (ARPES) with DFT calculations. This approach gives a direct access to the molecular orbitals by looking at their signature in the angular distribution of the photoemitted electrons from the molecular film. 
MOT provides a relatively simple interpretation of µ-ARPES data, since the angle dependent photoemission intensity becomes proportional to the modulus squared of the Fourier transform calculated from the real space molecular orbital. On this respect, the photoelectron emission microscope (PEEM) installed at the beamline is an ideal set-up to measure µ-ARPES patterns within a single image acquisition.
Using MOT we have shown that the remarkable charge transfer takes place at NiTPP/Cu(100) interface and it leads to filling of the higher unoccupied orbitals up to LUMO+3, thereby confirming the DFT predictions.

Figure caption: (a) STM image including two Ni-TPP domains, labeled with A and B, respectively. STM image parameters: V_b = −1.5 V, I_t = 0.2 nA, image size 15 × 20 nm2, measured at 4.3 K. (b) . Proposed adsorption model for Ni-TPP/Cu(100), side view. (c) Valence band photoemission spectra of clean Cu(100) and Ni-TPP/Cu(100) acquired at 26 eV photon energy. (d) PDOS onto molecular orbitals for the Ni-TPP/Cu(100) system. The energy position of the corresponding gas-phase molecular orbitals, aligned with respect to the vacuum level, is indicated with colored bars on the top axis. (e) Comparison between μ-ARPES measured patterns (left) and the correspondent calculated |FT|² of the molecular orbitals (right).

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“Multi-orbital charge transfer at highly oriented organic/metal interfaces”,      G. Zamborlini, D. Lüftner, Zh. Feng, B. Kollmann, P. Puschnig, C. Dri, M. Panighel, G. Di Santo, A. Goldoni, G. Comelli, M. Jugovac, V. Feyer, C.M. Schneider,  Nature Communications, 8, 335 (2017),
doi: 10.1038/s41467-017-00402-0