Synthesis and magnetic properties of N@C60 derivatives

Readout and control of an endofullerene electronic spin

Atomic spins for quantum technologies need to be individually addressed and positioned with nanoscale precision. C₆₀ fullerene cages offer a robust packaging for atomic spins, while allowing in-situ physical positioning at the nanoscale. However, achieving single-spin level readout and control of endofullerenes has so far remained elusive. In this work, we demonstrate electron paramagnetic resonance on an encapsulated nitrogen spin (¹⁴N@C₆₀) within a C₆₀ matrix using a single near-surface nitrogen vacancy (NV) center in diamond at 4.7 K. Exploiting the strong magnetic dipolar interaction between the NV and endofullerene electronic spins, we demonstrate radio-frequency pulse controlled Rabi oscillations and measure spin-echos on an encapsulated spin. Modeling the results using second-order perturbation theory reveals an enhanced hyperfine interaction and zero-field splitting, possibly caused by surface adsorption on diamond. These results demonstrate the first step towards controlling single endofullerenes, and possibly building large-scale endofullerene quantum machines, which can be scaled using standard positioning or self-assembly methods.

Encasing a single atom within a fullerene (C₆₀) cage can create a robustly packaged single atomic spin system. Here, the authors perform electron paramagnetic resonance on a single encased spin using a diamond NV-center, demonstrating the first steps in controlling single spins in fullerene cages.

A porphyrin-centred fullerene tetramer containing an N@C60 substituent

An N@C₆₀-containing C₆₀ tetramer was synthesized by quadruple 1,3-dipolar cycloaddition (Prato) reaction. This molecule demonstrates the N@C₆₀ qubit's ability to form covalently linked arrays. Furthermore, it provides a promising scaffold with which to measure multiple qubit-qubit interactions; which must be well characterized for a functioning quantum information processing architecture.

Probing the Dipolar Coupling in a Heterospin Endohedral Fullerene-Phthalocyanine Dyad

Paramagnetic endohedral fullerenes and phthalocyanine (Pc) complexes are promising building blocks for molecular quantum information processing, for which tunable dipolar coupling is required. We have linked these two spin qubit candidates together and characterized the resulting electron paramagnetic resonance properties, including the spin dipolar coupling between the fullerene spin and the copper spin. Having interpreted the distance-dependent coupling strength quantitatively and further discussed the antiferromagnetic aggregation effect of the CuPc moieties, we demonstrate two ways of tuning the dipolar coupling in such dyad systems: changing the spacer group and adjusting the solution concentration.

Alignment of N@C60 derivatives in a liquid crystal matrix

The orientation of different N-substituted [N@C60]fulleropyrrolidine derivatives hosted in a nematic liquid crystal matrix has been studied by electron spin resonance spectroscopy. In this study, variations on the zero field splitting parameter of the guest species are employed as a means of determining their degree of orientation. Fulleropyrrolidines with more rigid N-substituents are preferentially oriented in the liquid crystal matrix, whereas those with less rigid substituents are almost randomly distributed. Additionally, the orientation of a C60 fullerene cage bearing a nitroxide radical has also been investigated in comparison.

A theoretical study of fullerene–ferrocene hybrids

Using density functional theory within the generalized gradient approximation, I analyzed the electronic structure of a C(60)-ferrocene hybrid [= C(60) (*) FeCp] around HOMO in comparison with that of ferrocene, where C(60) (*) and Cp denote C(60)(CH(3))(5) and a cyclopentadienyl ring. HOMO-LUMO gap is significantly smaller than that of ferrocene because of the intervention of pi(C(60) (*)) states below LUMO. In addition, geometrical and electronic structures of N@C(60) (*) FeCp are also investigated. I find that there are two isomers with the energy difference of 0.13 eV. In one of the two, the encased nitrogen atom is located at the center of the fullerene cage. The Fe atom is eta(5)-coordinated to both Cp and R*, where R* is a five-membered ring of C(60) (*) cage. On the other hand, the atom is coordinated to R* with eta(4)-hapticity, and the nitrogen atom is bonded to a carbon atom of the R* ring in the other isomer. Upon the isomerization between the two isomers, there occurs a partial transfer of spin density between the nitrogen and Fe atoms as well as the creation and breaking of a C-N bond.