No abstract

Intermediate states are elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces.

The bonds in metal organic networks on surfaces govern the resulting geometry as well as the electronic properties. Here, we study the nature of these bonds by forming phenazine-copper complexes on a copper surface by means of atomic manipulation. The structures are characterized by a combination of scanning probe microscopy and density functional theory calculations. We observed an increase of the molecule-substrate distance upon covalent bond formation and an out-of-plane geometry that is in direct contradiction with the common expectation that these networks are steered by coordination bonds. Instead, we find that a complex energy balance of hybridization with the substrate, inhomogeneous Pauli repulsion, and elastic deformation drives the phenazine-copper interaction. Most remarkably, this attractive interaction is not driven by electron acceptor properties of copper but is of completely different donation/back-donation mechanism between molecular π-like orbitals and sp-like metal states. Our findings show that the nature of bonds between constituents adsorbed on surfaces does not have to follow the common categories.

Intramolecular structural relaxations occurring upon electron transfer are crucial in determining the rate of redox reactions. Here, we demonstrate that subangstrom structural changes occurring upon singleelectron charging can be quantified by means of atomically resolved atomic force microscopy (AFM) for the case of single copper(II)phthalocyanine (CuPc) molecules deposited on an ultrathin NaCl film. Imaging the molecule in distinct charge states (neutral and anionic) reveals characteristic differences in the AFM contrast. In comparison to density functional theory simulations these changes in contrast can be directly related to relaxations of the molecule's geometric structure upon charging. The dominant contribution arises from a nonhomogeneous vertical relaxation of the molecule, caused by a change in the electrostatic interaction with the surface.

No abstract