Engineering 2D spin networks by on-surface encapsulation of azafullerene radicals in nanotemplates

Abstract

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 (C₅₉N^•) 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 C₅₉N^• and [10]CPP for the azafullerene encapsulation with nanohoops in a guest-host complexation geometry. C₅₉N^•⊂[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 C₅₉N^• 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 C₅₉N^• 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.

Fig. 1. C₅₉N^•⊂[10]CPP on-surface encapsulation and supramolecular ordering in 2D.

a Nitrogen and carbon 1s XPS of [10]CPP/Au(111) (i), and of as-deposited C₅₉N^• on top of [10]CPP layer on Au (ii), followed by thermal annealing to (iii) 300 °C (1 min), (iv) 340 °C (4 min), and (v) 340 °C (10 min). Spectral decomposition shows azafullerene components of the thick layer (red), encapsulated C₅₉N^•⊂[10]CPP (blue), and [10]CPP (gray). b N1s and C1s XPS of C₅₉N monolayer on Au(111) (vi) and of as-deposited [10]CPP on C₅₉N/Au (vii) followed by thermal annealing to 300 °C (viii). Spectral decomposition shows C1s and N1s peaks of the C₅₉N/Au layer (green) and for the [10]CPP (gray). Dotted vertical lines at energies 400.6 eV (284.9 eV) and 400 eV (284.3 eV) indicate the binding energy of C₅₉N nitrogen (carbon) core levels in the multilayer and monolayer on Au(111), respectively. The measured binding energies are consistent with those reported in ref. . In the cartoons C₅₉N (blue circles with a dot), [10]CPP rings (gray ellipses) and Au(111) surface (dark yellow thick line) are schematically presented. The two thick arrows indicate the direction of increasing temperature upon annealing. Lines between data points serve as a guide to the eye. c Stick representation of [10]CPP and C₅₉N^• molecules and of the C₅₉N^•⊂[10]CPP complex. STM topographic images of [10]CPP/Au(111) layer d and C₅₉N^• deposition on [10]CPP/Au at RT (e) followed by thermal annealing to 220 °C (f) and 320 °C (g). Large areas of hexagonally packed C₅₉N^•⊂[10]CPP islands are observed after annealing to 320 °C (g). Each annealing step was performed by sample transfer into the preparation chamber (no vacuum breaking), then the sample was brought back into the STM scanning stage. Therefore, images shown in (e–g) do not show the same microlocation. The apparent height is represented by a false color map (black-blue-yellow).

Fig. 2. The fine structure of C₅₉N^•⊂[10]CPP supramolecular complex.

a STM image of C₅₉N/[10]CPP/Au(111) layer deposited at RT (top) shown with apparent height profile (bottom) along the depicted white line; b STM image and height profile for the same sample, annealed to 220 °C and c annealed further to 320 °C. d High resolution topographic image of the C₅₉N^•⊂[10]CPP supramolecular assembly captured at a terrace edge between C₅₉N/Au island and [10]CPP/Au island. The apparent height is represented by a false color map (black-blue-yellow).

Fig. 3. The electronic coupling between the molecular orbitals of host [10]CPP and guest C₅₉N^•.

Carbon K-edge NEXAFS: a of [10]CPP/Au(111) layer (red curve) and C₅₉N/Au(111) layer (blue filled curve), b of C₅₉N/[10]CPP heterolayer on Au(111) at RT and c of C₅₉N^•⊂[10]CPP supra-molecular complex layer on Au(111) after annealing to 340 °C. All spectra were taken with photon polarization in transverse magnetic geometry (p-polarization), i.e., electric field vector perpendicular to the Au substrate surface (see cartoon on top). Vertical dashed lines serve as guides to the eye indicating the energy shifts of C₅₉N (in cartoons depicted as blue circles with a dot) and [10]CPP (gray ellipses) supramolecular orbital levels from those of respective pristine layers with post-deposition annealing.

Fig. 4. The charge transfer dynamics between the coupled [10]CPP and C₅₉N molecules.

Reverse-order deposition of [10]CPP/C₅₉N/Au heterolayer. a STM topographic image of [10]CPP island on-top of C₅₉N layer displaying CPP-C₅₉N interface isolated from the Au(111). The apparent height is represented by a false color map (black-red-yellow). b Carbon K-edge RPES map of [10]CPP adlayer on C₅₉N/Au is shown in a 2D color map of intensity versus photon energy (vertical axis) and electron binding energy (BE, horizontal axis). The NEXAFS signal across the same photon energy range is shown aside on the top axis (yellow markers). Intensity quenching of the participant signal (blue curve) relative to its benchmark signal from the [10]CPP multilayer (red curve, for full RPES map see Fig. S4a) is detailed in the inset. c Schematic energy level diagram of the core hole (CH) decay following the core→lowest unoccupied molecular orbital (LUMO) excitation is shown for isolated (upper) and coupled (bottom) [10]CPP. Ultrafast delocalization of the core excited electron in the LUMO_CPP is depicted as charge transfer from the [10]CPP→C₅₉N, leading to the observed quenching of the participant Auger decay channel. Dark yellow filled circles represent electrons, empty (white) circles represent holes. The core-excited electron is shown as a red filled circle. Different possible electron transitions are indicated with arrows.

Fig. 5. Radical protection and stability of C₅₉N^•⊂[10]CPP assemblies.

a Nitrogen K-edge NEXAFS of a layer of C₅₉N monomers on Au at RT, and b after annealing to 240 °C whereupon a layer of (C₅₉N)₂ dimers is formed. c Nitrogen K-edge NEXAFS of encapsulated C₅₉N^•⊂[10]CPP layer at RT, and d after annealing to 300 °C. Black lines with markers are spectra represented in “magic angle” polarization geometry, and red/blue curves are spectra taken with photon in transverse magnetic/transverse electric polarization (p-pol/s-pol), respectively. The spectral fits of the singly unoccupied molecular orbital (SUMO, dark blue) and lowest unoccupied molecular orbital (LUMO, light blue) peaks are also shown. Thick arrows indicate the annealing step. In the cartoons C₅₉N (blue circles with a dot), [10]CPP rings (gray ellipses) and Au(111) surface (dark yellow thick line) are schematically presented.