Orientational Ordering and Intermolecular Interactions in the Rotor-Stator Compounds C60·C8H8 and C70·C8H8 Studied under Pressure

Abstract

Pressure-dependent transmittance measurements on C60·C8H8 and C70·C8H8 at room temperature were performed for pressures up to 10 GPa in the mid- and near-infrared frequency range, in order to monitor the vibrational modes of the fullerene and cubane molecules as a function of external pressure. Furthermore, the position of the absorption edge related to intramolecular electronic excitations on the fullerene molecules was studied with increasing pressure. For C60·C8H8, the anomaly at 0.5 GPa in the pressure dependence of the vibrational modes indicates a pressure-induced orientational ordering transition of the fullerene molecules and the anomaly at 1.3 GPa can be related to fullerene−cubane interaction. In C70·C8H8, the pressure-induced changes are more pronounced, with a splitting of the cubane modes at the pressures 0.8 and 1.75 GPa. This finding indicates stronger intermolecular interactions in C70·C8H8 compared to C60·C8H8, leading to larger distortions of cubane molecules. The energy position of the absorption edge in C60·C8H8 and C70·C8H8 shows a nonlinear decrease with increasing pressure. The zero-pressure value of the absorption edge in C60·C8H8 is larger than that of pristine C60 indicating that the cubane molecules reduce the overlap of the C60 molecular orbitals. In C70·C8H8, no shift of the optical absorption edge relative to C70 is found, in agreement with this compound being closer to a host−guest system than C60·C8H8.