Elettra | MiNaC, ALOISA Caging molecular spins into 2D arrays: Protecting azafullerene radicals with nanohoop templates In the realm of quantum materials and nanostructured magnets, a central challenge lies in stabilizing and organizing organic radicals—molecules with unpaired electrons that offer tunable magnetic and electronic functionalities. When properly stabilized, these radicals represent the simplest molecular platform for encoding spin states, making them attractive candidates for quantum information processing or enable catalytic reactions on surfaces. However, their extreme reactivity typically leads to rapid loss of spin/radical character on surfaces due to dimerization or interactions with the substrate. Common stabilization strategies rely on synthesising organic radicals with either a highly delocalized lone electron (one example are Blatter radicals), or by sterically protecting the lone electron through bulky side groups. In this work we present a novel route to stabilization and protection of organic radicals by on-surface encapsulation. A particularly promising spin-active molecule is the azafullerene radical (C59N•), a heterofullerene in which one nitrogen atom replaces a carbon in the fullerene cage. The resulting electronic imbalance localizes an unpaired electron on a carbon neighboring the nitrogen site, making C59N• an attractive spin-½ system. Yet, in bulk, due to their high reactivity these radicals immediately form diamagnetic dimers (C59N)2. The monomeric C59N• can be recovered upon thermal or photo activation. The radicals’ short lifetime and high surface reactivity have until now hindered their use in surface-based spin architectures. Our work introduces a new strategy to overcome these limitations by encapsulating individual C59N• radicals in supramolecular nanotemplates composed of [10]cycloparaphenylene ([10]CPP) nanohoops. The nanohoops act as a sterical protection of the lone electron in the azafullerene frontier molecular orbital. These macrocyclic hosts stand out for their radially oriented π-conjugated frameworks, well-defined internal cavities, and ability to form close-packed monolayers on noble metal surfaces such as Au(111). Their radially π-conjugated ring and internal diameter closely match the size and shape of C59N•, making them particularly wellsuited for supramolecular guest-host complexation. [10]CPP molecular rings further self-assemble on Au(111) surface to form regular arrays of nanoscale cavities over large areas. The structure and electronic properties of C59N• [10]CPP complexes on Au(111) were investigated using polarizationdependent Near Edge X-ray Absorption Fine Structure (NEXAFS) and Resonant Photoemission Spectroscopy (RPES) at the ALOISA beamline, supported by Scanning Tunneling Microscopy (STM) and density functional theory (DFT) calculations. Fig. 1 (a) shows an STM image of a [10] CPP monolayer on Au(111), featuring a hexagonally packed array of flat-lying nanohoops that form a 2D template of molecular shape matched cavities for guest–host complexation of C59N monomers. We achieved a high-yield entrapment of C59N• radicals in the [10] CPP cavities forming a stable 2D array of spin protected units by ultra-high vacuum (UHV) deposition of C59N• radicals onto the pre-grown [10] CPP/Au(111) template layer at room temperature and followed by thermal annealing. Fig. 1 (b) shows an STM image of individual C59N monomers captured within the cavities of the [10]CPP template. We note, that the hexagonal packing of the nanohoop template is preserved upon deposition of C59N• monomers and formation of the C59N• [10]CPP complexes. The formation of a high density array of hexagonally packed encapsulated assemblies after thermal annealing to about 320 °C is shown in Figs. 1 (c, d). A particular case of a high resolution STM topographic image of a single complex of an encapsulated C59N• in a [10]CPP nanohoop showing the central azafullerene and the circular [10]CPP with a resolved 10-fold phenyl ring structure is shown in Fig. 1 (d). The radical character of the heterolayer is confirmed by nitrogen K-edge NEXAFS spectroscopy, which reveals the characteristic peak of the half-filled singly unoccupied molecular orbital (SUMO) at about 400.5 eV photon energy, as shown in Figs. 1 (e, f). The next prominent peak at about 401.6 eV photon energy is attributed to excitations to the lowest unoccupied molecular orbital (LUMO). Furthermore, the NEXAFS spectra in Figs. 1 (e, f) show that the intensity ratio of the SUMO and LUMO peak does not change upon thermal annealing from room temperature to 300 °C, confirming the excellent temporal and thermal stability of the supramolecular radical complexes. In summary, we have demonstrated that on-surface encapsulation of azafullerene radicals in [10]CPP nanotemplates on Au(111) substrate not only preserves the radical’s spin character by suppressing dimerization and preventing direct coupling to the metal substrate, but also guides the radicals into a highly ordered stable two-dimensional hexagonal arrays. Our combined approach reveals the strength of guest–host coupling, effective radical confinement, and excellent thermal stability of the engineered 2D arrays of stable radicals. The used nano-engineering method not only offers a robust route to fabricate large-scale, surface-supported networks of molecular spins, but also opens possibilities for a potential application in quantum computing and molecular spintronics or surface catalytic reactions. ∩ ∩ Elettra Highlights 2024–2025 116
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