11964
J. Phys. Chem. B 2001, 105, 11964-11969
Thermal Stability and High Temperature Graphitization of Bisazafullerene (C59N)2 As Studied by IR and Raman Spectroscopy Matthias Krause,*,†,‡ Silvia Baes-Fischlmair,† Rudolf Pfeiffer,† Wolfgang Plank,† Thomas Pichler,†,‡ Hans Kuzmany,† Nikos Tagmatarchis,§ and Kosmas Prassides§ Institut fu¨ r Materialphysik, UniVersita¨ t Wien, Strudlhofgasse 4, A-1090 Wien, Austria, Institute for Solid State and Materials Research Dresden, Helmholtzstrasse 20, D-01171 Dresden, Germany, and School of Chemistry, Physics, and EnVironmental Science, UniVersity of Sussex, BN1 9QJ Brighton, U.K. ReceiVed: June 8, 2001; In Final Form: August 23, 2001
Thermal stability and high-temperature degradation of pristine bisazafullerene (C59N)2 were analyzed by infrared and Raman spectroscopy in the temperature range from 300 to 745 K. The spectra showed a reversible temperature dependence up to 550 K. For higher temperatures an irreversible degradation into disordered graphite was found. Approximately 55% of the material resisted a thermal treatment of 745 K for several hours. A mechanism involving a low stationary concentration of C59N• monomer radical species and a slow graphitization is proposed for the thermolysis of the material. The remarkable thermal stability of pristine bisazafullerene (C59N)2 is ascribed to the low stability of the C59N• monomer radical species, the high activation energy for graphitization, and the absence of other reaction partners under high vacuum conditions.
1. Introduction Due to their unique electronic properties fullerenes have attracted the interest of chemists, physicists, and material scientists since the first production of macroscopic C60 quantities by Kra¨tschmer et al. in 1990.1 In the solid state the most abundant empty fullerenes, C60, C70, and C84, are semiconductors. The electronic structure of fullerenes can be modified by doping with alkali and alkaline earth metals,2-4 substitution of one cage carbon by nitrogen,5,6 chemical derivatization,7,8 and encapsulation of endohedral atoms or clusters.9-13 So far, pristine C60 turned out as the most interesting material, as the stepwise filling of the F1u LUMO changes the electronic structure from semiconducting to metallic. The substitution of one C60 atom by nitrogen was expected to form a molecular metal in the early days of fullerene research, as there is one extra electron per cage as in AC60, where A is an alkali metal. However, both experimental work and quantum-chemical calculations have established a dimer as the most stable form of azafullerene, C59N, in which the two carbon cages are connected by a carbon-carbon single bond.6,14 The bridging carbons are sp3 hybridized and directly neighbored to the nitrogen atoms, which are oriented in a transoid conformation with respect to the intercage bond. The molecular symmetry is C2h. Quantum-chemical calculations predicted a HOMO with a strong localization of electron density on the nitrogen atoms and along the intercage bond. The LUMO on the other hand is mainly F1u derived and has a weak amplitude in the intercage bond.15 The absorption onset of bisazafullerene (C59N)2 is red shifted by 0.4 eV compared to C60. Whereas the preferred stability of the (C59N)2 dimer at room temperature is well established, there are only a few studies about its behavior at elevated temperatures, where dissociation * Corresponding author. E-mail:
[email protected]. † Universita ¨ t Wien. ‡ Institute for Solid State and Materials Research Dresden. § University of Sussex.
could occur and the monomer might be stabilized. A preliminary Raman study indicated first the stability of the dimer in the temperature range from 100 to 490 K.16 Solid-state ESR measurements supported this result in that way, as no signal of nitrogen could be detected up to 500 K.17 However, three equally spaced lines appeared in the ESR spectrum at 520 K. They were attributed to the formation of monomeric C59N• radicals.17 Their concentration was very weak and increased from approximately 50 to 300 ppm in the temperature range from 520 to 740 K.17 Butcher and co-workers showed that (C59N)2 sublimes in monomeric form when it is strongly heated to about 825 K.18 However, these studies revealed only some aspects of the temperature influence on pristine (C59N)2. The low concentration indicates that the C59N• radical species are either a byproduct or an intermediate of the thermolysis. The response of the major fraction, the existence of competing degradation pathways and the relevance of evaporation in comparison to solid-state processes are still open questions. This paper focuses on the infrared and Raman response of pristine (C59N)2 during a high-temperature treatment up to 745 K. Whereas the Raman spectra are well understood by experiment and theory16 only a preliminary list of infrared lines is available.6 Based on a comparison to pristine C60 and hydroazafullerene C59HN we present the first detailed IR analysis of (C59N)2. A well-defined mode splitting, the activation of cage modes that are forbidden for C60 and strong dimer fingerprint lines between 800 and 850 cm-1 were observed. The spectra showed a reversible temperature dependence up to 550 K. For higher temperatures an irreversible degradation into disordered graphite was found. Approximately 55% of the material resisted a thermal treatment of 745 K for several hours. A mechanism involving a low stationary concentration of C59N• monomer radical species and a slow graphitization is proposed for the thermolysis of the material in the studied temperature range. The remarkable thermal stability of pristine bisazafullerene (C59N)2 compared to other fullerene dimers is ascribed to the low stability of the C59N• monomer radical species, the high
10.1021/jp012186+ CCC: $20.00 © 2001 American Chemical Society Published on Web 11/03/2001
IR and Raman Spectroscopy of Bisazafullerene
J. Phys. Chem. B, Vol. 105, No. 48, 2001 11965
TABLE 1: Frequencies (ν˜ ), Relative Infrared and Raman Intensities (IRel), and Tentative Assignments of Selected Modes of C60, C59HN and (C59N)2; Intensity Labels as Standard C60 IR20 ν˜ (cm-1)
Irel
526
vs
576
s
1183 1428
C59HN Raman20
IR
ν˜ (cm-1)
Irel
495
s
s s-m 1468 1419 1424
vs m m
1574
m
ν˜ (cm-1)
(C59N)2 Raman
Irel
490 523 529 568 574 579 611 825
vw vs vs m m m w-m vw
843 1127 1174 1186 1197 1414 1422 1443 1461 1414 1422 1443 1562 1574
vw m m m m m s m m m s m w w-m
ν˜ (cm-1) 493 525 530 568 614
1177 1187 1193 1417 1426 1440 1462 1417 1426 1440 1565 1572
Raman16
IR Irel s m sh vw
ν˜ (cm-1)
Irel
523 529 568 576 579
vs vs m m m
819 836 844
m m s
vw
w m w sh s m vs sh s m m m
1174 1186 1196 1416 1423 1443 1460 1416 1423 1443 1565 1574
m m m sh m m m sh m m w-m sh
ν˜ (cm-1)
Irel
493 525 530 570 576 582 623
s m sh w vw vw m
1172 1183 1190 1417 1425 1443 1462 1417 1425 1443 1565 1573
w w vw m s m vs m s m m m
related C60 mode/ assignment
}
}
} } } }
Ag(1) F1u(1) F1u(2) Gg (4) ν(C-N) + ν(C-C) ν(C-N) + ν(C-C) ν(C-N) + ν(C-C) δ(C-H) F1u(3) F1u(4), overlapped with Hg(7) Ag(2) Hg(7), overlapped with F1u(4) Hg(8)
activation energy for graphitization, and the absence of other reaction partners under high vacuum conditions. 2. Experimental Section The details of the synthesis of bisazafullerene (C59N)2 and hydroazafullerene C59HN were reported elsewhere.16,19 Briefly, starting from C60 and following the two-step literature procedure of Hummelen et al.,6 [60]-N-MEM ketolactam was isolated, degassed with argon, and treated for 15 min with an excess of p-toluenesulfonic acid at 500 K under argon. After the reaction mixture was cooled, flash column chromatography using silica gel with toluene as eluent afforded bisazafullerene (C59N)2 as the least polar fraction. The material was further purified by multiple recrystallizations from CS2/diethyl ether, and finally degassed under a dynamic vacuum at 350 K for 30 h. The sample purity was better than 98%, as checked by X-ray diffraction measurements. For spectroscopic studies solutions of 250 µg of (C59N)2, C59HN, and C60 in 1 mL of toluene were drop-coated on gold-covered silicon substrates under ambient conditions. The resulting polycrystalline films were degassed and dried in high vacuum of e10-6 mbar at 375 K for 4 h. Subsequently, they were transferred into either the vacuum cell of the IR or the Raman spectrometer and kept in a high vacuum of e10-6 mbar. A home-built controller was applied for temperature-dependent infrared studies, whereas a Lakeshore model 330 was used for temperature control in Raman experiments. Infrared measurements were performed in a reflectionabsorption geometry on a FTIR spectrometer IFS 66v (Bruker, Germany) with a resolution of 0.5 cm-1. A total of 5000 infrared scans was taken for (C59N)2, C59HN, and C60 at 300 K. For temperature-dependent studies on (C59N)2, 500 scans were accumulated from 300 to 740 K each 12.5 K. The reversibility of the infrared response was analyzed by repeated measurements at a reference temperature of 400 K after the sample had been exposed to 460, 550, 670, and 740 K. Raman spectra were excited by 514 nm radiation of an argon/krypton laser model 2018 (Spectra Physics) using a line focus of 0.1 × 2.0 mm2
Figure 1. Midinfrared spectra of pristine C60, C59HN, and (C59N)2 at 300 K; the dimer fingerprint lines of (C59N)2 are indicated by a bracket, the splitting pattern of two F1u derived modes of (C59N)2 and two fingerprint modes of C59HN are marked by arrows.
area. A premonochromator and an adapted interference filter were applied to eliminate laser plasma lines. The Raman radiation was collected in a backscattering geometry by a triple spectrometer XY 500 (Dilor, France) equipped with a CCD detector. A spectral band-pass of 3 cm-1 and an integration time of 2 h were used for all Raman recordings. Spectra were taken each 25 K from 300 to 745 K after temperature stabilization for 30 min so that an almost identical time window was guaranteed for the Raman and the IR experiment. To check the reversibility, the Raman recording at 300 K was repeated after the temperature had been raised to 500, 650, and 745 K. 3. Results 3.1. Infrared Signatures of Bisazafullerene (C59N)2. To identify the characteristic infrared features of (C59N)2, we compare its response with that of C60 and of the hydroazafullerene monomer C59HN. The overall infrared spectra of the materials are displayed in Figure 1 and the most important frequency and intensity data are listed in Table 1. The spectrum of C60 is dominated by the four IR active F1u modes at 526,
11966 J. Phys. Chem. B, Vol. 105, No. 48, 2001 576, 1183, and 1428 cm-1. The few more weak lines are due to forbidden fundamentals that are activated by the distortion of the icosahedral symmetry in the crystal. Although C59HN and (C59N)2 have, in general, many more infrared modes than C60, those lines derived from the dipole-allowed C60 modes can be immediately identified due to their high intensity and a welldefined splitting due to the complete removal of degeneracy. Their frequencies and relative intensities agree within the accuracy of the experiment for both azafullerenes. A similar good agreement was found for the majority of the other infrared lines. Most of the F1u derived IR lines of C59HN and (C59N)2 have complementary Raman counterparts. Vice versa intense Raman lines as, e.g., the modes derived from the Ag(2), Hg(7), and Hg(8) mode of C60 appeared with medium intensity in the infrared spectra. Whereas this behavior of C59HN can be understood on the basis of the molecular Cs symmetry, for (C59N)2 the Raman and IR vibrations are mutually exclusive due to the center of symmetry, and each cage mode is expected to split into one Raman active in-phase and one IR active outof-phase molecular vibration with slightly different frequencies. As the observed frequencies coincide, a weak intercage coupling has to be concluded in (C59N)2 and the cage modes can be discussed on the basis of a single C59N cage with Cs local symmetry. Minor differences in the number of infrared lines are apparent in the range from 550 to 600 cm-1, and some of the modes between 700 and 800 cm-1 are more intense for C59HN than for (C59N)2. The most characteristic infrared features of (C59N)2 are the medium to strong lines at 819, 836, and 844 cm-1. These lines have two very weak counterparts in the infrared spectrum of C59HN, whereas this spectral range is empty for pristine C60, C60O, and C120.20,21 This comparison implies that the lines are due to the particular molecular structure of bisazafullerene (C59N)2 and not the result of a pure symmetry reduction, the existence of an intercage bond at all, or the presence of nitrogen alone. We tentatively assign the lines to (C-N) stretching vibrations that are coupled with the intercage (C-C) stretching mode and that gain intensity from the particular dimeric structure in (C59N)2. This situation could arise when the nitrogen atoms of both C59N cages vibrate in phase opposite to the carbons of the intercage single bond. Then the first derivative of the molecular dipole moment could be 2 times larger than for a single C59N cage. The infrared intensity, which is proportional to the square of the first derivative of the molecular dipole moment, would be enhanced by a factor of 4. This qualitative interpretation is supported by predictions of Andreoni et al.14 In conclusion, the strong modes between 800 and 850 cm-1 are the infrared fingerprints for the dimer bond in (C59N)2. Hydroazafullerene C59HN has two IR fingerprint lines at 611 and 1127 cm-1, as can be seen in Figure 1 and Table 1. The latter line has been ascribed to a C-H deformation mode.19,22 The former is very close to the Raman mode of (C59N)2 at 625 cm-1, which is derived from the Gg(4) mode of C60 and is tentatively assigned to a comparable mode of C59HN. 3.2. Temperature Effect on the Infrared and Raman Spectrum of (C59N)2. To analyze the thermal stability of bisazafullerene (C59N)2, the temperature dependence of its infrared and Raman spectrum was studied. The absorbance of the sample was chosen to be the intensity axis for the spectra representation in Figure 2, as it is directly proportional to the number of sample’s molecules. Only such parts of the spectrum that contain the strongest lines are displayed. From these spectra the integrated intensities for the three strongest F1u derived modes and the characteristic ν(C-N) + ν(C-C) dimer modes
Krause et al.
Figure 2. Temperature-dependent infrared spectra of (C59N)2; the arrow indicates the thermally induced new line around 1590 cm-1, the reference spectrum at 400 K denoted with “400 K, ref.” was recorded after thermal treatment.
Figure 3. Upper part: normalized infrared intensity of the F1u(1), F1u(2), F1u(3) derived and the ν(C-C) + (C-N) lines of (C59N)2 as a function of sample temperature. Lower part: normalized infrared intensity of the F1u(1), F1u(3) derived and the ν(C-C) + (C-N) lines of (C59N)2 at 400 K as a function of annealing temperature; i.e., the temperature the sample had been subjected to before.
were estimated. They were normalized with respect to the intensity at 300 K and are displayed as a function of temperature in the upper part of Figure 3. The lower part of this figure shows integrated intensities of selected lines for repeated recordings at 400 K after different stages of thermal treatment normalized to the initial intensity at this temperature. This allows us to distinguish between reversible and irreversible effects of the temperature on the infrared intensity. According to Figure 2 the vibrational signatures of the (C59N)2 dimer are dominant and qualitatively unaltered in the whole temperature range from 300 to 740 K and also in the 400 K reference spectrum. With increasing temperature, all lines were slightly softened, broadened, and lost intensity. The reference spectrum at 400 K shows that intensity loss and line broadening are irreversible results of the thermal treatment. Whereas a pure intensity loss could be due to evaporation, the line broadening is a first hint for a temperature induced degradation of the
IR and Raman Spectroscopy of Bisazafullerene
Figure 4. Upper part: Raman spectrum of (C59N)2 in the Ag(2) mode region at 300 K as a function of annealing temperature, i.e., the temperature the sample had been subjected to before. Lower part: difference spectra between 300 K Raman data of (C59N)2 after different stages of thermal treatment and initial Raman spectrum: 500 K (lower trace), 650 K (middle trace), 745 K (upper trace); The temperature induced new lines are marked by arrows.
(C59N)2 material. The infrared spectra display only one weak new spectral signature, which points to the product of degradation: a broad shoulder of the highest energetic infrared line around 1590 cm-1. Although it is hard to decide at which temperature the shoulder starts to develop, it is clearly visible at least from 700 K on and in the 400 K reference spectrum. The quantitative evolution of the temperature dependence is shown in Figure 3. From 300 to 525 K the line intensities of the ν(C-N) + ν(C-C) lines and the F1u(2) and F1u(3) derived modes were constant. The F1u(1) derived line group intensity increased slightly in this range. At 550 K a first tendency to decreasing infrared intensities became apparent. In the temperature range from 575 to 740 K a significant overall intensity loss was observed. It was particularly pronounced for the ν(C-N) + ν(C-C) and the F1u(3) derived modes. From the lower part of Figure 3 the character of this intensity evolution is evident. After heating to 460 and 550 K the intensities for a subsequent measurement at 400 K were identical with the initial response at this temperature within the accuracy limit of (5%, which is ascribed to a reversible behavior in this temperature range. Temperatures >550 K caused a significant irreversible reduction of all line intensities compared to the initial intensity at 400 K. New lines attributable to fullerene structures were not observed. To analyze the temperature effect on the Raman spectrum, we have studied the frequency range from 1200 to 1700 cm-1, where (C59N)2 shows three strong line groups derived from the Ag(2), Hg(7), and Hg(8) modes of C60. In the upper part of Figure 4 the initial Raman spectrum at 300 K is compared to the roomtemperature response after the sample had been exposed to 500, 650, and 745 K. The onset of the Hg(8) line group is at 1600 cm-1, and the baseline is straight for the initial material. After thermal treatment the (C59N)2 signatures were still present, but in addition two broad lines at 1360 and 1590 cm-1 are apparent, which are marked by arrows. The spectral changes are better realized in the lower part of Figure 4, where the initial spectrum at 300 K was subtracted from the spectra taken at
J. Phys. Chem. B, Vol. 105, No. 48, 2001 11967
Figure 5. Temperature dependence of the frequency (upper part), intensity (middle part), and line width (lower part) of the Ag(2) derived (C59N)2 Raman line. The data at 650 K were measured twice, the data at 745 K three times; the four values at 300 K were obtained before thermal treatment, and after heating to 500, 650, and 745 K.
300 K after a thermal treatment at 500, 650, and 745 K, respectively. After the temperature had been raised to 500 K the difference spectrum was a straight line with a sharp feature around the Ag(2) position, which is due to a nonsignificant frequency shift from 1461.9 to 1461.6 cm-1. First spectral changes are to be seen after the material was subjected to 650 K: a significant intensity gain of all three (C59N)2 Raman lines and the appearance of a weak broad line at about 1590 cm-1. After further thermal treatment up to 745 K the broad new lines at 1590 and 1360 cm-1 had gained significantly more intensity. The frequencies of the Hg(7), Ag(2), and Hg(8) derived lines were identical with the initial response within the experimental accuracy. The detailed analysis for the temperature effect on the frequency, the intensity, and the line width of the Ag(2) derived mode is given in Figure 5. The frequency changed with a parabolic-like decreasing function, the intensity exhibited a slight and almost linear loss and the line width increased linearly with temperature. These results are in complete agreement with previous data for the temperature range from 100 to 490 K.16 After thermal treatment the relative intensities of the intrinsic (C59N)2 modes were by about 20% larger than before. This is remarkable as the new line at 1590 cm-1 which is strongly related to a transformation process of (C59N)2 is approximately half as intense as the Ag(2) mode derived line. 4. Discussion 4.1. Scenario for the High-Temperature Response of Crystalline Bisazafullerene (C59N)2. According to both the infrared and Raman data, after thermal treatment of pristine (C59N)2 up to a temperature of 745 K, two phases coexist in the solid state: (i) bisazafullerene (C59N)2 and (ii) a second species characterized by broad vibrational lines around 1360 and 1590 cm-1. The second species is a reaction product formed by thermal treatment of bisazafullerene. No structural phase transition of the (C59N)2 material as a whole was observed. Our infrared and Raman data provide no hint for the presence of other species within the detection limit of ca. 10%. This holds first for the monomeric azafullerenyl radical C59N•, whose extra
11968 J. Phys. Chem. B, Vol. 105, No. 48, 2001
Krause et al.
TABLE 2: Number of Intercage Bonds, Intercage Stretching Mode Frequencies, Intercage Valence Force Constants, and Transformation Temperatures for Various Fullerene Dimers fullerene dimer
number of intercage bonds
intercage stretching frequency (cm-1)
intercage stretching force constant (N cm-1)
transformation temperature (K)
(C59N)2 (C60-)2 C60 photodimer C120O C119 C120O2
1 1 2 2 3 4
8216 8825 9626 9920 11729 11720
1.43 1.64 1.95 2.08 2.90 2.90
g520 27024,25 41026 47327,28 g57329 not reported
electron should cause a downshift of the Ag(2) derived mode similar to A1C60 by about 5-7 cm-1. Neither during nor after the temperature treatment has a significant downshift of this line been observed. Also, the formation of C60 on the order of g5% can be excluded, as this would give rise to a strongly resonance enhanced Raman line around 1468 cm-1. The high-temperature transformation product of (C59N)2 has only two vibrational lines at about 1360 and 1590 cm-1, both of which are Raman active, the latter being infrared active too. It is straightforward to assign these features to disordered graphite,23 which is formed by degradation of the fullerene network as a whole. To develop a scenario for the thermally induced processes in pristine (C59N)2, we first of all have to address why the infrared data imply a transformation of about 45% of the initial (C59N)2 amount whereas the degradation of the material is not manifested by an intensity loss in the Raman spectrum. For nonabsorbing materials the Raman intensity is directly proportional to the number of scattering molecules. Resonance Raman scattering intensities are further dependent on the reabsorption of the scattered light by the sample itself. It is a well-known fact that the highest scattering intensity for absorbing materials is, in general, not observed for pristine samples but in a diluted state. Thus, an increasing dissolution of nontransformed (C59N)2 within the disordered graphite forming at high temperatures provides a qualitative explanation for the slightly increasing intensities of this line. Moreover, the layer thickness effect on the infrared and resonance Raman intensity is different. For the infrared intensity a linear relationship is valid, while for resonance Raman the penetration depth is on the order of the laser wavelength. As the film thickness of our samples is more than 100 times larger, scattering volume and resonance Raman intensity would remain constant until the material is almost completely evaporated. Thus the observation of bisazafullerene Raman lines confirms qualitatively the presence of initial material after thermal treatment. The quantitative relationship, i.e., the amount of nontransformed (C59N)2 is reflected by the infrared data. In the study of Simon et al.17 the three line ESR spectrum from monomeric C59N• radical species appeared at 520 K. This is very close to 550 K, where the infrared intensity of bisazafullerene (C59N)2 started to decrease. Due to the lower detection limit of ESR spectroscopy, it seems justified to define the temperature of 520 K as the upper stability limit of bisazafullerene (C59N)2. The lack of C59N• radical signatures in the vibrational spectra can be explained by the higher detection limit of infrared and Raman spectroscopy and confirms the proposed low stationary concentration of monomeric C59N• radicals in the solid. As other fullerene species were not found, neither in the infrared nor in the Raman spectrum, we conclude that disordered graphite, whose vibrational signatures were established for temperatures g650 K, is the major product of the high-temperature treatment of bisazafullerene (C59N)2. For two reasons it is tempting to attribute also the decreasing infrared
intensity in the temperature range from 575 to 650 K to graphitization: the loss on the order of 10-30% (see Figure 3) is too large to be due to monomeric C59N• radicals and fullerene evaporation can be excluded for temperatures of 520 K bisazafullerene (C59N)2 starts to dissociate into monomeric C59N• species, whose stationary concentration in the solid is in the ppm range. (iii) Once the monomeric C59N• units are formed, they degrade into disordered graphite, which is the final transformation product of the bisazafullerene (C59N)2 thermolysis in the solid state. The graphitization process proceeds slowly as approximately 55% of the material resisted a thermal treatment of 745 K for several hours. (iv) At temperatures g750 K graphitization should compete with sublimation of C59N• monomers, as reported.18 4.2. Origins of the Thermal Stability of Bisazafullerene (C59N)2 in the Solid State. The fact that a fullerene dimer with an intercage single bond is stable up to e520 K (e247 °C) is remarkable. The isoelectronic and isostructural (C60)22- could be stabilized at temperatures from 160 to 260 K in the presence of Rb metal.24,25 Above this range an orthorhombic polymer was formed. Photopolymerized C60 monomerizes at about 410 K.26 C120O decays into C60 and less than 10% of C120O2 and C119 above 473 K.27,28 From these two dimeric materials the thermal stability is best characterized for C119, which degraded slowly to so far unknown products above 573 K.29 The stability of bisazafullerene (C59N)2 is hence comparable to C119, the most stable dimeric fullerene known so far. This high stability is not to rationalize in terms of the intercage bond strength. According to Eisler et al.30 fullerene dimers can be treated as superdiatomic oscillators with the reduced mass µred and the vibrational wavenumber ν˜ XY. The intercage valence force constant fXY, which is directly related to the intercage bond strength is given by
fXY ) (2π)2c02µredν˜ XY2
(1)
In Table 2 we have listed the number of intercage bonds, the wavenumbers of the intercage stretching modes, the intercage valence force constants and the transformation temperatures for various fullerene dimers. Whereas a good qualitative correlation between intercage bond strength and dimer stability exists for (C60-)2, the C60 photodimer, C120O, C120O2, and C119, bisazafullerene (C59N)2 represents a distinct exception. Its intercage bond is weaker than that of (C60-)2 but the thermal stability is comparable to that of C119. We attribute the high thermal stability of bisazafullerene (C59N)2 to the low stability of the monomer C59N• species and kinetic reasons. The C60 photodimer, C120O, and C120O2 dis-
IR and Raman Spectroscopy of Bisazafullerene sociate into closed shell fullerene structures. The dissociation energy is partly compensated by the formation of molecular oxygen or by the transformation of cage single bonds into double bonds. The dissociation of bisazafullerene (C59N)2 leads to two C59N• radicals that are open shell systems and whose calculated (C-N) bonds are significantly longer than the (6,5) bonds in C60.22 This process is energetically unfavorable compared to the dissociation of the other fullerene dimers, and the chemical equilibrium lies on the educt’s side until the temperature is high enough to enable degradation into disordered graphite or sublimation. The behavior of pristine (C59N)2 differs significantly from the situation in solution, where the C59N• radicals can be trapped and subsequently stabilized by suitable radical sources.31 5. Summary Thermal stability and high-temperature degradation of crystalline bisazafullerene (C59N)2 were analyzed by infrared and Raman spectroscopy in the temperature range from 300 to 745 K. The spectra showed a reversible temperature dependence up to 550 K. For higher temperatures an irreversible degradation into disordered graphite was found. Approximately 55% of the material resisted a thermal treatment of 745 K for several hours. The formation of significant amounts of C59N• monomer species or other fullerene structures could be excluded. No structural phase transition of the (C59N)2 material as a whole was observed. A mechanism with a low stationary concentration of C59N• monomer radical species and a slow graphitization was proposed for thermolysis of the material. Despite the weak intercage bond, pristine bisazafullerene (C59N)2 belongs to the most stable fullerene dimers known so far. This behavior was ascribed to the low stability of the C59N• monomer radical species, the high activation energy for graphitization, and the absence of other reaction partners under high vacuum conditions. Acknowledgment. We acknowledge financial support from the European Union, TMR project ERBFMRX-CT97-0155, and the Austrian Academy of Sciences for an APART fellowship (T.P.). N.T. thanks the European Union for a Marie-Curie fellowship. References and Notes (1) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulus, K.; Huffman, D. R. Nature 1990, 347, 354. (2) Haddon, R. C.; Hebard, A. F.; Rosseinsky, M. J.; Murphy, D. W.; Duclos, S. J.; Lyons, K. B.; Miller, B.; Rosamilia, J. M.; Flemming, R. M.; Kortan, A. R.; Glarum, S. H.; Makhija, A. V.; Muller, A. J.; Eick, R. H.; Zahurak, S. M.; Tycko, R.; Dabbagh, G.; Thiel, F. A. Nature 1991, 350, 320.
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