Rare-earth dimers with single-electron metal-metal bonds feature strong hyperfine and exchange interactions between metal atoms and an unpaired valence electron. Their stabilization inside carbon cages opens the possibility for applications in quantum information processing and sensing but requires investigation of their unusual spin states and dynamics. Herein, we present a systematic EPR study of dimetallofullerenes M2@C80(CH2Ph) with M25+ dimers, whose composition varied from Y2 (S = 1/2) through YGd (S = 4) to Gd2 (S = 15/2). High-field W-band EPR spectroscopy facilitated the analysis of different molecular orientations and T1 anisotropy and allowed resolution of individual |mS⟩ → |mS + 1⟩ transitions in high-spin Gd dimetallofullerenes. For Y2@C80(CH2Ph), we demonstrated that fullerene isomerism strongly influences the molecular and spin dynamics of metal dimers. The pronounced difference in spin-lattice relaxation times T1 was found for the Ih and D5h cage isomers and rationalized by Raman spectroscopic study and calculations of spin-phonon couplings. Lateral metal modes, which play the main role in the spin-lattice relaxation of endohedral metallofullerenes, were found at higher frequencies in the D5h isomer, in line with its longer T1. The orientational dependence of spin decoherence was ascribed to the rotational motion of the Y2@C80(CH2Ph) molecules in glassy toluene. Substitution of Y with Gd accelerated spin-lattice relaxation and decoherence. A similar T1 and phase-memory relaxation time (Tm) of YGd@C80(CH2Ph) and Gd2@C80(CH2Ph) proved that the effect does not scale with the number of Gd atoms. While the Rabi frequency is constant across the whole spectrum in Y2@C80(CH2Ph), YGd@C80(CH2Ph) and Gd2@C80(CH2Ph) exhibit variations in the Rabi frequencies for their |mS⟩ → |mS + 1⟩ transitions.
Zalibera et al. (Tue,) studied this question.