Orientational Ordering and Intermolecular Interactions in the Rotor-Stator Compounds C 60 · C 8 H 8 and C 70 · C 8 H 8 Studied under Pressure K. Thirunavukkuarasu, † C. A. Kuntscher,* ,† B. J. Nagy, ‡ I. Jalsovszky, ‡ G. Klupp, § K. Kamara´s, § E ´ . Kova´ts, § and S. Pekker § Experimentalphysik II, UniVersita¨t Augsburg, D-86159 Augsburg, Germany, Department of Organic Chemistry, Eo¨tVo¨s Lora´nd UniVersity, Budapest, Hungary, and Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, Budapest, Hungary ReceiVed: July 9, 2008 Pressure-dependent transmittance measurements on C 60 · C 8 H 8 and C 70 · C 8 H 8 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 · C 8 H 8 , 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 · C 8 H 8 , 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 · C 8 H 8 compared to C 60 · C 8 H 8 , leading to larger distortions of cubane molecules. The energy position of the absorption edge in C 60 · C 8 H 8 and C 70 · C 8 H 8 shows a nonlinear decrease with increasing pressure. The zero-pressure value of the absorption edge in C 60 · C 8 H 8 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 · C 8 H 8 , 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 · C 8 H 8 . 1. Introduction The fullerene-cubane molecular crystals C 60 · C 8 H 8 and C 70 · C 8 H 8 , which were synthesized only recently, are the first members of the rotor-stator crystal family. 1 At room tempera- ture, C 60 · C 8 H 8 crystallizes in a face-centered cubic (fcc) structure, while C 70 · C 8 H 8 forms a body-centered tetragonal (bct) crystal. Static cubane molecules occupy the octahedral voids (elongated octahedron in case of C 70 · C 8 H 8 ) and act as bearings for the rotating fullerene molecules. Although there exists only a weak van der Waals interaction between the fullerene and cubane molecules, they form high-symmetry molecular crystals due to molecular recognition between the convex surface of fullerene and the concave surface of cubane. These novel rotor- stator cocrystals constitute a new family of fullerene derivatives with special intermolecular interactions between the constituents. On cooling at ambient pressure, C 60 · C 8 H 8 undergoes an orientational ordering transition at 140 K, where the rotational degrees of freedom of the fullerene molecules become restricted. 1,2 This transition is accompanied by a change in the crystal structure of C 60 · C 8 H 8 from fcc to orthorhombic. C 70 · C 8 H 8 undergoes two orientational ordering transitions upon cooling from above room temperature. During the first transition at 390 K, the crystal structure changes from fcc to bct: at room temperature, the free rotation of the C 70 molecules in C 70 · C 8 H 8 is therefore restricted, the basic motion of C 70 being a uniaxial rotation around the long molecular axis (C 5 ). In addition, the long axis precesses around the c axis of the tetragonal unit cell. On further cooling below 150 K, the precession of the long axis stops, while the uniaxial rotation of the fullerene molecules persists. This temperature-dependent orientational ordering in C 70 · C 8 H 8 is accompanied by a structural phase transition from tetragonal to monoclinic. The cubane molecules are orienta- tionally ordered and remain static in both C 60 · C 8 H 8 and C 70 · C 8 H 8 . The orientational ordering temperature in these rotor-stator compounds is much lower than in any other fullerene derivative, which was attributed to the expanded lattice and the special rotor-stator interaction facilitating the smooth rotation of the fullerenes. 2,3 Consequently, in fullerene-cubane molecular crystals pressure-induced phenomena are expected to occur at higher pressure compared to pristine C 60 and C 70 and other fullerene-derived compounds. Recently, the pressure-induced phenomena in C 60 · C 8 H 8 were investigated by pressure-depend- ent transmittance measurements in the infrared frequency range. 4 It was found that the orientational ordering in C 60 · C 8 H 8 indeed occurs at a pressure of about 0.8 GPa, which is significantly higher than for pristine C 60 . Furthermore, high-pressure X-ray diffraction and Raman spectroscopic studies were performed on C 60 · C 8 H 8 addressing the issue of pressure-induced polym- erization of this compound. 5,6 In these studies, the occurrence of a structural phase transition from cubic to orthorhombic, which is accompanied by the orientational ordering of the C 60 molecules in C 60 · C 8 H 8 , was observed at around 0.5 GPa. Due to the enlarged interfullerene distances in C 60 · C 8 H 8 and C 70 · C 8 H 8 and the molecular recognition between fullerene and cubane molecules, the fullerene-cubane interaction plays a significant role in addition to the interfullerene interaction. Pressure-dependent spectroscopic investigations enable us to * Corresponding author. E-mail: christine.kuntscher@physik.uni-augs- burg.de. † Universita¨t Augsburg. ‡ Eo¨tvo¨s Lora´nd University. § Research Institute for Solid State Physics and Optics. J. Phys. Chem. C 2008, 112, 17525–17532 17525 10.1021/jp806051s CCC: $40.75  2008 American Chemical Society Published on Web 10/17/2008