Electron spin relaxation of N@C60 in CS2

Abstract

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S ≥ 1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170K.

The effects of a pyrrolidine functional group on the magnetic properties of N@C60

“…While the effect on the faster T 2,o component falls within experimental error, the effect on T 2,i is clear and consistent with the additional relaxation mechanism of fluctuating hyperfine anisotropy. The effect of ESEEM on this outer hyperfine line [4] is negligible at these short relaxation times.…”

“…It has been shown that nuclear spins in the solvent environment provide a mechanism for electron spin relaxation [5]. To probe the effects of the functional group on spin relaxation, it is important to use a solvent in which there are no naturally abundant nuclear spins, such as carbon disulfide.…”

“…The relative amplitudes of the 'inner' and 'outer' contributions to the total measured echo signal are 2:3. In the case of an S = 3/2 spin system, a fluctuating ZFS term will lead to the following relaxation times [4,15]:…”

“…Relaxation in unmodified N@C 60 has been found to be driven by a vibrational Orbach mechanism [4]. If the fullerene is modified by a functional group in such a way as to introduce a zero-field-splitting (ZFS), the fluctuation of this ZFS due to molecular tumbling in liquid solution provides an additional relaxation pathway characterized by the magnitude of the ZFS, and the rotational correlation time of the molecule.…”

“…Such a biexponential decay was not observed in earlier studies on fullerene derivatives, reportedly due to limited signal to noise and a relatively fast rotational correlation time. The effect of nuclear spins in the solvent may also have complicated the analysis by adding additional relaxation pathways [5]. The observation of a biexponential decay is simplified by choosing a bulky functional group, such as pyrrolidine, to slow down the rotation further.…”