Raman Spectroscopy as a Probe of Thermally-Induced Electron Migration in a Superatomic Solid

Fullerene–cluster co-crystals are a versatile class of superatomic materials whose magnetic, optical, and transport properties can be precisely tailored through synthetic approaches such as ligand engineering and cluster design. In this work we incorporate an asymmetric chiral ligand to synthesize a novel fullerene-metal chalcogenide solid, [Co₆Se₈(PEt₂(S)-methylbutyl)₆]₄[C₆₀]₆ · 6C₅H₈, which hosts an intermolecular, discrete, valence tautomerism (VT) transition around 180 K. Temperature-dependent Raman spectroscopy reveals distinct changes in the A_g(2) “pentagonal pinch” mode of C₆₀, whose frequencies are highly sensitive to the charge state. As the sample temperature is cooled below 180 K, the spectral weight of the A_g(2) mode shifts between components corresponding to neutral and reduced C₆₀, establishing this mode as a clear vibrational marker of VT transition. We propose that this VT transition originates from thermally activated electron transfer from the cluster HOMO to the fullerene LUMO due to their close redox potentials. Circularly polarized Raman spectroscopy shows that phonon chirality is also modulated across the VT transition, suggesting coupling between electronic and vibrational properties in this chiral superatomic solid. Finally, this thermally driven charge transfer is further evidenced by a sharp quenching of fullerene photoluminescence above the transition temperature and corroborated by bond length changes measured via single-crystal X-ray diffraction.