Switchable ${\mathrm{ErSc}}_2\mathrm{N}$ Rotor within a ${\mathrm{C}}_{80}$ Fullerene Cage: An Electron Paramagnetic Resonance and Photoluminescence Excitation Study

Motivated by the possibility of observing photoluminescence and electron paramagnetic resonance from the same species located within a fullerene molecule, we initiated an EPR study of ${\mathrm{Er}}^{3+}$ in ${\mathrm{ErSc}}_2\mathrm{N}@{\mathrm{C}}_{80}$. Two orientations of the ${\mathrm{ErSc}}_2\mathrm{N}$ rotor within the ${\mathrm{C}}_{80}$ fullerene are observed in EPR, consistent with earlier studies using photoluminescence excitation (PLE) spectroscopy. For some crystal field orientations, electron spin relaxation is driven by an Orbach process via the first excited electronic state of the $4I_{15/2}$ multiplet. We observe a change in the relative populations of the two ${\mathrm{ErSc}}_2\mathrm{N}$ configurations upon the application of 532 nm illumination, and are thus able to switch the majority cage symmetry. This photoisomerization, observable by both EPR and PLE, is metastable, lasting many hours at 20 K.

Figure 1

(a) X-band cw EPR spectrum of ${\mathrm{ErSc}}_2\mathrm{N}@{\mathrm{C}}_{80}$ in frozen toluene solution at 5 K, with accompanying simulation performed using EasySpin [23]. The low-intensity sharp features marked * derive mostly from hyperfine coupling to the ${}^{167}\mathrm{Er}$ nuclear spin (natural abundance 22.9%). Experimental parameters: microwave frequency, 9.396 GHz, microwave power, 63 mW; modulation amplitude, 0.3 mT; modulation frequency, 100 kHz. (b) A representation of the ${\mathrm{ErSc}}_2\mathrm{N}@{\mathrm{C}}_{80}$ molecule.

Figure 2

(a) Linewidth of the two highest intensity features of the cw EPR spectrum shown in Fig. 1. Their respective energy gaps $\Delta$ are obtained under the assumption that the features are homogeneously (lifetime) broadened by an Orbach mechanism [see Eq. (1)]. These gaps match the energy level diagram (obtained from PL measurements) shown in (b) for the two major configurations of the ${\mathrm{ErSc}}_2\mathrm{N}$ rotor [11].

Figure 3

cw EPR spectra taken at 5 K in the dark, and upon illumination with a 532 nm laser (inset expands the low-field region 45–75 mT). By pairing peaks according to whether their intensities increase or decrease upon illumination, we assign the features centered at approximately 51 and 220 mT to Conf 1, and those at 60 and 110 mT to Conf 2.

Figure 4

PLE spectra of ${\mathrm{ErSc}}_2\mathrm{N}@{\mathrm{C}}_{80}$ in frozen ${\mathrm{CS}}_2$ solution at 5 K, before (and after) having been illuminated at 532 nm for 20 minutes prior to data acquisition. The peak attributed to Conf 1 (1496 nm) weakens upon illumination while that attributed to Conf 2 (1499 nm) increases in intensity, consistent with what is observed in EPR (see Fig. 3).