5064
J. Phys. Chem. B 2000, 104, 5064-5071
C60 and C70 Solvates Studied by Raman Spectroscopy A. Talyzin* and U. Jansson Department of Inorganic Chemistry, Ångstro¨ m Laboratory, Box 538, SE-751 21, Uppsala, Sweden ReceiVed: October 13, 1999; In Final Form: March 9, 2000
The formation and phase transformations of C60 and C70 solvates with benzene, toluene, and n-hexane have been studied in-solution by Raman spectroscopy for the first time. It was found that C60 and C70 solvates exhibit Raman spectra which are quite distinct from pure fullerenes and very similar to spectra from fullerenesulfur compounds. Two different solvates are found for the C60-toluene system in good agreement with recent DSC studies. Upon heating, all three solvates decompose to pure fcc C60 and all new peaks and shifts of peaks typical for the solvates disappear. No changes on Raman spectra of C60 and C70 solvates were found upon cooling to 78 K with the exception of the C70-benzene solvate which showed a broad phase transition around 263 K. Some possible explanations for the changes in Raman spectra are discussed.
1. Introduction It is known that C60 forms solvates with a number of different organic solvents.1 Many of these solvates are unstable at room temperature and lose solvent very rapidly immediately after taking them out of solution. Nevertheless structures of many C60 solvates have been successfully solved at low temperatures. These studies show that each of the solvents produces its own kind of phase with weak van der Waals bonding and different symmetries ranging from triclinic to cubic. A review on known structures of C60 solvates has been given by Ceolin et al.1 An interesting property of the solvates was discovered by studying the temperature dependence of the C60 solubility. Ruoff et al.2 reported that this temperature dependence has a maximum near room temperature for several solvents. Smith et al.3 showed that this anomalous behavior of the solubility can be explained thermodynamically. According to their model, two phases can exist in equilibrium with solution; one of them with a positive enthalpy of dissolution, another with a negative. These phases can be either both solvated with different amounts of solvent, or one of them can be pure C60. Usually at low temperature the solvate is stable in the solution but at some temperature a phase transition between the two phases occurs. The solvated crystal melts and another phase forms such as pure C60 or C60 with a smaller amount of solvent. This phase transition gives a change in the slope of the temperature dependence of solubility. Differential scanning calorimetric experiments made for a number of solvents4-6 showed that such transitions can be seen as a maximum in the temperature dependence of the heat capacity. This model is confirmed by the results of direct observations of the phase transition C60-C60•4C6H6 in benzene.7 The C60•4C6H6 solvate is stable in equilibrium with a saturated solution at temperatures below 313 K while the pure C60 fcc phase is stable above this temperature. Crystals of the benzene solvate can be easily obtained from C60 powder by spontaneous recrystallization of a C60 powder in equilibrium with a solution cooled to 278-280 K. These crystals remain stable upon heating to 313 K where a phase transition to pure fcc C60 occurs. The phase transition has been observed by optical microscopy and X-ray powder diffraction.7 Despite the fact that many of the solvates were synthesized a relatively long time ago, many of their properties, especially
near room temperature, have yet to be investigated. For example, IR and Raman spectroscopy studies of C60-benzene, C60toluene, and some other solvates have been performed out of solution.8,9 These data, however, probably do not originate from solvates because a significant degradation of the C60•4C6H6 structure occurs within a few minutes of taking the samples out of solution.7 This shows that experiments at room temperature can only be performed on samples which are covered with solution. This technique was used by He et al.10 for their NMR studies of the C60-benzene solvate which confirmed the weak van der Waals nature of bonding in this compound. Also they addressed the question of a possible difference between structures of C60-benzene determined at low temperature and room temperature. The optical activity of the room-temperature solvate described by He et al.10 is not compatible with a centrosymmetric low-temperature structure of C60•4C6H6. This may be investigated more generally since most of the solvate structures were determined at low temperatures and some phase transitions may exist below room temperature and below the freezing point of the solvent. Unfortunately, calorimetry data reported by several groups describe only a temperature range where the solvents exist as liquids. Even less information is now available for the C70 phases and solvates formed by crystallization from different solutions. The first remarkable difference from the C60 solvates is the absence of maxima in the solubility curves of C70 for studied solvents.11 The second important difference is the stability of C70 solvates out of solution. The C70-toluene solvate was first described by Agafonov et al.12 and studied later by Takahashi13 with high-resolution transmission electron microscopy and found to exhibit a orthorhombic structure with the Amm2 space group. Our experiments made by X-ray powder diffraction on insolution samples showed the existence of very similar structures for benzene and hexane solvates. Large crystals of benzene solvate also were found to be stable in air for several months and decomposed only after heating to 420 K.14 The dielectric properties of these crystals were studied by Sherman et al.15 and showed two anomalies which suggest phase transitions at 150 K and 275 K. It is clear that a variety of interesting phenomena such as phase transitions of C60 and C70 in solution and below the
10.1021/jp993658b CCC: $19.00 © 2000 American Chemical Society Published on Web 05/05/2000
C60 and C70 Solvates Studied by Raman Spectroscopy
J. Phys. Chem. B, Vol. 104, No. 21, 2000 5065
freezing points of the solvents require more detailed study. In this paper we use Raman spectroscopy for studying in-solution solvates of C60 and C70 with benzene, toluene, and hexane. One of the main advantages with this method is a very small illumination area. Unlike calorimetry and X-ray powder diffraction which give an average information over many crystals (so far calorimetry experiments have been performed only for powder due to problems with the growth of large crystals), we are able to focus the laser beam on individual small crystals and record Raman spectra for crystals with visual differences in morphology. This may be very useful, for example, when studying the C60-toluene system where probably two different solvates exist at the same temperature.16 Also in this study, Raman data obtained from the solvates are compared with spectra from sulfur-fullerenes reported by us recently17 since both of these groups of compounds have a similar kind of weak van der Waals bonding. It was also found that a weak van der Waals bonding between the crown-shaped S8 rings and C60 or C70 molecules produce certain changes in the Raman spectra of the fullerenes and it is possible that similar effects may exist in other groups of van der Waals fullerene solids. If this is true Raman spectroscopy may be a very efficient tool for studying phase transitions in many fullerene-solvent systems.
process without the formation of new crystals.14 This process takes several hours for hexane and 10-20 min for benzene and toluene. The rate of solvate formation which was studied by X-ray powder diffraction was found to depend on the solubility of fullerene in the solvent. The solubility of C70 in hexane at room temperature is approximately one hundred less (0.01 g/L) than benzene and toluene. Reference C60 and C70 spectra were obtained by sublimation of fullerene powder which resulted in the formation of a bulk polycrystalline sample which was analyzed out of vacuum. These spectra appeared to be the same as the spectra of C60 and C70 taken from unsublimed microcrystals studied in air. Raman spectra were recorded by Renishaw Raman 2000 spectrometer using a 780 nm excitation wavelength with a resolution of 2 cm-1. The temperature-dependent measurements were completed with the help of a heating/cooling stage down to 78 K. Precautions were taken to avoid laser-induced photopolymerization during the measurements. Only a low laser power (
