Orientationally Resolved Electron-Spin Relaxation and Coherence in Fullerene-Encapsulated Rare-Earth Dimers with a Single-Electron Metal–Metal Bond

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.