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 M@C(CHPh) with M dimers, whose composition varied from Y = 1/2 through YGd = 4 to Gd = 15/2. High-field W-band EPR spectroscopy facilitated the analysis of different molecular orientations and anisotropy and allowed resolution of individual |⟩ → | + 1⟩ transitions in high-spin Gd dimetallofullerenes. For Y@C(CHPh), we demonstrated that fullerene isomerism strongly influences the molecular and spin dynamics of metal dimers. The pronounced difference in spin-lattice relaxation times was found for the and 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 isomer, in line with its longer . The orientational dependence of spin decoherence was ascribed to the rotational motion of the Y@C(CHPh) molecules in glassy toluene. Substitution of Y with Gd accelerated spin-lattice relaxation and decoherence. A similar and phase-memory relaxation time () of YGd@C(CHPh) and Gd@C(CHPh) proved that the effect does not scale with the number of Gd atoms. While the Rabi frequency is constant across the whole spectrum in Y@C(CHPh), YGd@C(CHPh) and Gd@C(CHPh) exhibit variations in the Rabi frequencies for their |⟩ → | + 1⟩ transitions.