Noncontact Layer Stabilization of Azafullerene Radicals: Route toward High-Spin-Density Surfaces

We deposit azafullerene C₅₉N^• radicals in a vacuum on the Au(111) surface for layer thicknesses between 0.35 and 2.1 monolayers (ML). The layers are characterized using X-ray photoemission (XPS) and X-ray absorption fine structure (NEXAFS) spectroscopy, low-temperature scanning tunneling microscopy (STM), and by density functional calculations (DFT). The singly unoccupied C₅₉N orbital (SUMO) has been identified in the N 1s NEXAFS/XPS spectra of C₅₉N layers as a spectroscopic fingerprint of the molecular radical state. At low molecular coverages (up to 1 ML), films of monomeric C₅₉N are stabilized with the nonbonded carbon orbital neighboring the nitrogen oriented toward the Au substrate, whereas in-plane intermolecular coupling into diamagnetic (C₅₉N)₂ dimers takes over toward the completion of the second layer. By following the C₅₉N^• SUMO peak intensity with increasing molecular coverage, we identify an intermediate high-spin-density phase between 1 and 2 ML, where uncoupled C₅₉N^• monomers in the second layer with pronounced radical character are formed. We argue that the C₅₉N^• radical stabilization of this supramonolayer phase of monomers is achieved by suppressed coupling to the substrate. This results from molecular isolation on top of the passivating azafullerene contact layer, which can be explored for molecular radical state stabilization and positioning on solid substrates.