Orientational Ordering and Intermolecular Interactions in the Rotor-Stator Compounds C-60 center dot C8H8 and C-70 center dot C8H8 Studied under Pressure

Thirunavukkuarasu, K. and Kuntscher, C. A. and Nagy, B. J. and Klupp, Gyöngyi and Kamarás, Katalin and Kováts, Éva and Pekker, Sándor (2008) Orientational Ordering and Intermolecular Interactions in the Rotor-Stator Compounds C-60 center dot C8H8 and C-70 center dot C8H8 Studied under Pressure. JOURNAL OF PHYSICAL CHEMISTRY C - NANOMATERIALS AND INTERFACES, 112 (45). pp. 17525-17532. ISSN 1932-7447

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Abstract

Pressure-dependent transmittance measurements on C-60 center dot C8H8 and C-70 center dot 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 C-60 center dot 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 C-70 center dot 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 C-70 center dot C8H8 compared to C-60 center dot C8H8, leading to larger distortions of cubane molecules. The energy position of the absorption edge in C-60 center dot C8H8 and C-70 center dot C8H8 shows a nonlinear decrease with increasing pressure. The zero-pressure value of the absorption edge in C-60 center dot C8H8 is larger than that of pristine C-60 indicating that the cubane molecules reduce the overlap of the C-60 molecular orbitals. In C-70 center dot C8H8, no shift of the optical absorption edge relative to C-70 is found, in agreement with this compound being closer to a host-guest system than C-60 center dot C8H8.

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