Multi-Orbital Charge Transfer into Nonplanar Cycloarenes Revealed with CO-Functionalized STM Tips

On-surface synthesis enables the tunable preparation of diverse molecular systems with tailored properties. Recently, the highly selective synthesis of kekulene (>99%) on Cu(111) and isokekulene (92%) on Cu(110) from the same molecular precursor was demonstrated. Scanning tunneling microscopy (STM) with CO-functionalized tips can identify individual molecules based on their geometric structure at low coverage on Cu(110) but also reveals complex features arising from electronic contributions near the Fermi energy. Here, we investigate the origin of these features by simulating STM images based on a weighted sum of multiple molecular orbitals, for which we employ weights based on the calculated molecular-orbital projected density of states. This analysis provides direct experimental evidence for charge transfer from the surface into multiple formerly unoccupied molecular orbitals for single molecules of kekulene as well as isokekulene in its two nonplanar adsorption configurations. In comparison, the area-integrating photoemission orbital tomography technique confirms the charge transfer as well as the high selectivity for the formation of a full monolayer of predominantly isokekulene on Cu(110). Our STM-based approach is applicable to a wide range of adsorbed molecular systems and specifically also suited for strongly interacting surfaces, nonplanar molecules, and compounds accessible only in extremely low yields.