meso-Dibenzoporphycene molecules adsorbed on the Ag(111) surface and on 2-monolayer-thick NaCl films were studied using submolecular resolution atomic force microscopy (AFM), scanning tunneling microscopy (STM), and first-principles calculations to clarify their stability and tautomerization behavior. We have found that the bonding of the molecules with the surface is determined by the interplay of different contributions, including the interaction of the π-aromatic orbitals of the benzene rings and the metal-coordination bond of the lone-pair electrons of the imine nitrogen atoms with the metal atoms (Ag, Na) on each substrate. The strength of the latter ultimately governs the molecular adsorption configuration and determines the nature and energy barriers for tautomerization. On Ag(111), the interaction of the imine nitrogen atoms with the Ag atoms deforms the macrocycle of porphycene, leading to a distinct AFM contrast that allows a clear identification of the molecule in its cis tautomeric form. In contrast, on NaCl films, the weaker interaction with the Na atoms leads to a flatter geometry and very similar adsorption configurations for the cis- and trans-forms, which cannot be distinguished in AFM experiments. Although weak, the dominant role of this local N-Na interaction, compared to the essentially nondirectional dispersive interactions, results in a new type of tautomerization process. In this case, the transfer of hydrogen atoms within the porphycene cavity is accompanied by a significant displacement of the whole molecule to a new site to reach a new minimum energy adsorption configuration. Our theoretical calculations indicate that this lateral translation, rather than the intramolecular H transfer, dominates the activation energy on NaCl. This novel tautomerization behavior, which we have identified on a rather inert ionic surface, might also be present on other weakly interacting substrates.
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