A Long-Lived Azafullerenyl Radical Stabilized by Supramolecular Shielding with a [10]Cycloparaphenylene

Abstract

A major handicap towards the exploitation of radicals is their inherent instability. In the paramagnetic azafullerenyl radical C₅₉ N^. , the unpaired electron is strongly localized next to the nitrogen atom, which induces dimerization to diamagnetic bis(azafullerene), (C₅₉ N)₂ . Conventional stabilization by introducing steric hindrance around the radical is inapplicable here because of the concave fullerene geometry. Instead, we developed an innovative radical shielding approach based on supramolecular complexation, exploiting the protection offered by a [10]cycloparaphenylene ([10]CPP) nanobelt encircling the C₅₉ N^. radical. Photoinduced radical generation is increased by a factor of 300. The EPR signal showing characteristic ¹⁴ N hyperfine splitting of C₅₉ N^. ⊂ [10]CPP was traced even after several weeks, which corresponds to a lifetime increase of >10⁸ . The proposed approach can be generalized by tuning the diameter of the employed nanobelts, opening new avenues for the design and exploitation of radical fullerenes.

Keywords: [10]cycloparaphenylene; azafullerenes; host-guest complexes; long-lived radicals; photoinduced radical generation.

Figure 1

a) The [10]CPP host and the C₅₉N^. guest species. b) Room‐temperature X‐band EPR spectrum of C₅₉N^.⊂[10]CPP in 1‐chloronaphthalene as formed upon irradiation at 532 nm (open circles). The solid red line is a fit of the experimental spectrum to a model that assumes slow isotropic rotation of C₅₉N^.⊂[10]CPP, yielding a rotational diffusion correlation time of τ _corr=3.8 ns. Parameters used in the fit: The eigenvalues of the g‐factor tensor are g_xx=2.0010, g_yy=1.9993, and g_zz=2.0042, while the ¹⁴N hyperfine tensor eigenvalues are A_xx=1.6 MHz, A_yy=15.9 MHz, and A_zz=14.9 MHz. The three arrows on top of the spectrum indicate the main ¹⁴N hyperfine splitting of the EPR spectrum, whereas the two weaker signals, probably originating from the additional hyperfine splitting with ¹³C in its natural abundance, that flank the main triplet of lines are marked with *.

Figure 2

a) Structures of C₅₉N^. and C₅₉N^.⊂[10]CPP radicals. b) The X‐band EPR spectrum of bare C₅₉N^. in 1‐chloronaphthalene solution. c) Comparison of the solution X‐band EPR spectra of C₅₉N^.⊂[10]CPP (red) and C₅₉N^. (blue). All measurements were conducted at room temperature in degassed 1‐chloronaphthalene with samples possessing equal concentrations (2.3 mm). d) Time dependence of the X‐band EPR signal of C₅₉N^.⊂[10]CPP in 1‐chloronaphthalene after the illumination at 532 nm has been switched off. The solid blue line is a fit to an exponential time decay yielding the characteristic decay of 100 min. Inset: Comparison of spectra recorded during illumination (red), 60 min after switching off the light (cyan), and 120 min after switching off the light (black).

Figure 3

Top: DFT‐calculated relative energies for the different species; finite width bars indicate energy ranges dependent on the relative orientation of C₅₉N^. and [10]CPP in the radical complex. Bottom: Illustration of the light‐induced generation of the C₅₉N^.⊂[10]CPP radical complex and the decay pathway. Inset: The EPR signal of long‐lived C₅₉N^.⊂[10]CPP.

Figure 4

DFT relative energy barriers calculated using the nudged elastic band method to separate a) (C₅₉N)₂ in the absence of [10]CPP rings, b) (C₅₉N)₂⊂[10]CPP, and c) [10]CPP⊃(C₅₉N)₂⊂[10]CPP. Black and red lines/points indicate system spins of 0 μ_B and 2 μ_B, respectively, that is, after spin flipping of one electron.