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We present an efficient strategy for on-surface engineering of organic metal-free supramolecular complexes with long-term spin protection. By vacuum deposition of azafullerene (C 59 N • ) monomers on a pre-deposited template layer of [10]cycloparaphenylene ([10]CPP) nanohoops on Au(111) surface we exploit the molecular shape matching between the C 59 N • and [10]CPP for the azafullerene encapsulation with nanohoops in a guest-host complexation geometry. C 59 N • ⊂[10]CPP supramolecular complexes self-assemble into an extended two-dimensional hexagonal lattice yielding a high density network of stable spin-1/2 radicals. We find compelling evidence for electronic coupling between the guest C 59 N • and the host [10]CPP in supramolecular species. At the same time, [10]CPP effectively protects the radical state of encapsulated azafullerenes against dimerization and inhibits C 59 N • coupling to the Au substrate. Azafullerene encapsulation by nanohoops represents a viable realization of molecular spin protection while simultaneously demonstrating exceptional self-assembling properties by which large-scale 2D architectures of molecular spins can be realized., Competing Interests: Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

We deposit azafullerene C 59 N • 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 C 59 N orbital (SUMO) has been identified in the N 1s NEXAFS/XPS spectra of C 59 N layers as a spectroscopic fingerprint of the molecular radical state. At low molecular coverages (up to 1 ML), films of monomeric C 59 N are stabilized with the nonbonded carbon orbital neighboring the nitrogen oriented toward the Au substrate, whereas in-plane intermolecular coupling into diamagnetic (C 59 N) 2 dimers takes over toward the completion of the second layer. By following the C 59 N • SUMO peak intensity with increasing molecular coverage, we identify an intermediate high-spin-density phase between 1 and 2 ML, where uncoupled C 59 N • monomers in the second layer with pronounced radical character are formed. We argue that the C 59 N • 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.