Long Spin Coherence Times on C59N-C60 Heterodimer Radicals Entrapped in Cycloparaphenylene Rings

Abstract

We investigate the effect of introducing C60 to (C59N)2 and the molecular ring, [10]cycloparaphenylene ([10]CPP), using electron paramagnetic resonance (EPR) measurements supported by density functional theory (DFT) calculations. Incorporating C60 into the system results in the formation of novel stable [10]CPP ⊃ C59N-C60• ⊂ [10]CPP encapsulated heterodimer radicals whose spin is localized on C60 and manifests in EPR measurements as a signal at g = 2.0022 without any discernable hyperfine structure. This signal has an exceptionally long spin coherence lifetime of 440 μs at room temperature, far longer than any of the radical fullerene species reported in the literature and over twice that of the C59N• ⊂ [10]CPP radical. The radicals are long-lived, with EPR signal still strong over a year after thermal activation. The [10]CPP ⊃ C59N-C60• ⊂ [10]CPP oligomer is more stable than C59N• ⊂ [10]CPP radicals and becomes the predominant species at room temperature after annealing. Its formation is thermally activated with an experimental activation energy of only 0.189 eV, as compared to 0.485 eV for the pure azafullerene-[10]CPP case. The [10]CPP ⊃ C59N-C60• ⊂ [10]CPP radicals discovered here could be used to bridge C59N• ⊂ [10]CPPs acting as qubits, providing effective coupling between them.