A CO-functionalized atomic force microscope tip is used to locally probe local chemical reactivity and subtle structural relaxations of a single phthalocyanine molecule at different stages of pyrrolic-H abstraction. Spatially resolved vertical force spectroscopy unveils a variation of the maximum short-range attraction between CO and intramolecular sites, which is interpreted as a measure for the local chemical reactivity. In addition, the vertical position of the point of maximum attraction is observed to vary across the molecules. These changes follow the calculated adsorption heights of the probed molecular atoms.

An atomic force microscope is used to determine the attractive interaction at the verge of adding a Ag atom from the probe to a single free-base phthalocyanine molecule adsorbed on Ag(111). The experimentally extracted energy for the spontaneous atom transfer can be compared to the energy profile determined by density functional theory using the nudged-elastic-band method at a defined probe−sample distance.

Rubrene (C42H28) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C42H28 consistently reflect the progressive reduction of the molecule–substrate hybridization. Separate C42H28 clusters are observed on Pt(111) as well as broad molecular resonances. On Au(111) and graphene-covered Pt(111) compact molecular islands with similar unit cells of the superstructure characterize the adsorption phase. The highest occupied molecular orbital of C42H28 on Au(111) exhibits weak vibronic progression while unoccupied molecular resonances appear with a broad line shape. In contrast, vibronic subbands are present for both frontier orbitals of C42H28 on graphene. They are due to different molecular vibrational quanta with distinct Huang–Rhys factors.

Intentionally terminating scanning probes with a single atom or molecule belongs to a rapidly growing field in the quantum chemistry and physics at surfaces. However, the detailed understanding of the coupling between the probe and adsorbate is in its infancy. Here, an atomic force microscopy probe functionalized with a single CO molecule is approached with picometer control to two conformational isomers of Ag-phthalocyanine adsorbed on Ag(111). The isomer with the central Ag atom pointing to CO exhibits a complex evolution of the distance-dependent interaction, while the conformer with Ag bonded to the metal surface gives rise to a Lennard-Jones behavior. By virtue of spatially resolved force spectroscopy and the comparison with results obtained from microscope probes terminated with a single Ag atom, the mutual coupling of the protruding O atom of the tip and the Ag atom of the phthalocyanine molecule is identified as the cause for the unconventional variation of the force. Simulations of the entire junction within density functional theory unveil the presence of ample relaxations in the case of one conformer, which represents a rationale for the peculiar vertical-distance evolution of the interaction. The simulations highlight the role of physisorption, chemisorption, and unexpected junction distortions at the verge of bond formation in the interpretation of the distance-dependent force between two molecules.

No abstract