Electronic Structure and Bistable Conformational Study of the Tetraphenylporphyrin Erbium(III) Acetylacetonate Complex on the CaF2/Si(100) Surface at Low Temperatures

The synthesis of tetraphenylporphyrin erbium(III) acetylacetonate (acac) complexes is realized and their properties studied at the nanoscale when adsorbed on a semi-insulating CaF2/Si(100) surface. Our findings reveal that the ErTPP-(acac) molecules can adsorb in two main on-site conformations. Following precisely located dI/dV measurements at various specific positions [phenyls, pyrroles, and Er-(acac)], the relative locations of the Er cation and the apical ligand (acac) can be deciphered for each observed conformation. Hence, one of the adsorbate conformations presents the acac ligand parallel to the porphyrin plane with the Er atom outside the macrocycle plane. The second conformation is related to what is known in the gas phase, where the acac ligand is oriented vertically on top of the Er atom. This work is combined with a theoretical investigation that uses density functional theory methods to bring into light details of the two observed conformations. Additional proofs of our discoveries are related to the vibrational excitations of ErTPP-(acac). A comparison with a theoretical estimation of the vibrational modes reveals how the electronic resonance near the valence-band edge of the insulting layer is suitable to distinguish between the two adsorbed conformations.