Vibrational spectra of C60C8H8 and C70C8H8 in the rotor-stator and polymer phases
G. Klupp, F. Borondics, É. Kováts, Á. Pekker, G. Bényei, I. Jalsovszky, R. Hackl, S. Pekker, K. Kamarás
aa r X i v : . [ c ond - m a t . m t r l - s c i ] S e p Vibrational spectra of C · C H and C · C H inthe rotor-stator and polymer phases G. Klupp ∗† , F. Borondics † , ´E. Kov´ats † , ´A. Pekker † ,G. B´enyei ‡ , I. Jalsovszky ‡ , R. Hackl § , S. Pekker † , K. Kamar´as ∗† Research Institute for Solid State Physics and Optics,Hungarian Academy of Sciences,P.O. Box 49, H-1525 Budapest, Hungary;Department of Organic Chemistry, E¨otv¨os Lor´and University,Budapest, Hungary;Walther Meissner Institute, Bavarian Academy of Sciences and Humanities,85748 Garching, Federal Republic of GermanyOctober 31, 2018 ∗ Authors to whom correspondence should be addressed. Email: [email protected] (G.Klupp), [email protected] (K. Kamar´as) † Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences ‡ Department of Organic Chemistry, E¨otv¨os Lor´and University § Walther Meissner Institute, Bavarian Academy of Sciences and Humanities bstract C · C H and C · C H are prototypes of rotor-stator cocrystals. Wepresent infrared and Raman spectra of these materials and show how therotor-stator nature is reflected in their vibrational properties. We mea-sured the vibrational spectra of the polymer phases poly(C C H ) andpoly(C C H ) resulting from a solid state reaction occurring on heating.Based on the spectra we propose a connection pattern for the fullerene inpoly(C C H ), where the symmetry of the C molecule is D h . On illu-minating the C · C H cocrystal with green or blue light a photochemicalreaction was observed leading to a similar product to that of the thermalpolymerization. Keywords: fullerene, cubane, infrared spectroscopy, Raman spectroscopy2
Introduction
Fullerenes and cubane have recently been shown to form so called rotor-stator cocrystals. These cocrystals are different from both orientationally or-dered and plastic crystals, as one of their constituents (the fullerene) is rotatingand the other one (the cubane) is fixed in a well-defined orientation. In thecase of C · C H rotating C molecules form a face centered cubic lattice andstatic cubane molecules, occupying interstitial octahedral sites, serve as bear-ings between them. C · C H crystallizes in a face-centered cubic structureabove 375 K. At room temperature the rotation of C is somewhat restricted,which leads to a tetragonal distortion; the C molecule is able to rotate aroundits main axis which, in turn, precesses around the crystallographic c axis. Theformation of these structures is driven by the molecular recognition between theconcave surface of the cubane and the round surface of the fullerenes. On heating the fullerene-cubane compounds undergo a topochemical reac-tion. As the reaction product is insoluble in common solvents, it is mostlikely a copolymer of the fullerene with cubane. X-ray diffraction patternsof the annealed samples, measured at room temperature, show a large emerg-ing amorphous part and weakening reflections compatible with fcc structure.Compared to the original monomer phase the shift of these reflections indicateslattice expansion and their intensity quickly vanishes at high angles. Becauseof the parallel appearance of the amorphous contribution and disappearance ofcrystallinity we can assume that the amorphous phase retains the local cubicorder. Another observation which makes this assumption reasonable is that themorphology of the crystals does not change on heating. In this paper we present a detailed vibrational (infrared and Raman) char-acterization of the monomer and polymer phases of C · C H and C · C H .In the monomer phases, we can confirm the rotor-stator nature of the materi-als. Based on the spectra of the polymer phases, we deduce the symmetry ofthe majority of the fullerene units as D h , similar to the linear cycloadditionpolymers. This conclusion is consistent with a substantial presence of linearsegments in the copolymer.We published the infrared spectra of the monomer and polymer phases ofC · C H and C · C H earlier as supplementary material to Ref. 1. A thor-ough study of polymerization of poly(C C H ) at high temperature and highpressure has been performed by Iwasiewicz-Wabnig et al. , using x-ray diffrac-tion and Raman spectroscopy. Our results, obtained at ambient pressure on an-nealing, are complementary to that study, except that we observe a photopoly-merization reaction on illumination with green or blue light, which accounts forthe laser wavelength dependence of the Raman spectra. Cubane was prepared following the method of Eaton and Cole. Cubane andthe fullerenes C and C were coprecipitated from toluene by adding isopropylalcohol or by evaporating the solvent to form C · C H and C · C H . − . Temperature-dependent measure-ments were conducted in a flow cryostat cooled by liquid nitrogen or helium withthe temperature adjustable between 20 and 600 K. The KBr pellet techniquehas the disadvantage that the index of refraction of the samples is generally inmismatch with that of the medium, therefore the lineshapes become asymmetric(Christiansen effect). However, the alternative of using organic oils as Nujol wasdiscarded because we wanted to identify as many new infrared lines as possible,without disturbing absorption from the medium.Raman microscopy data were acquired in backscattering geometry on pow-der samples either under ambient conditions or in an evacuated glass capillary.Spectra were taken with three lines (468 nm, 531 nm and 676 nm) of a Kr-ionlaser on a triple monochromator (Jobin-Yvon T64000). The laser power wascarefully adjusted not to cause polymerization or any other type of changes inthe samples. This was guaranteed with a power of 70-100 µ W focused to a spotof approximately 2 µ m diameter. The slit width was set at 300 or 400 µ m. Forthese small crystals (typically less than 10 µm ) the orientation of the principalaxes with respect to the polarization of the incident ( e i ) and the scattered ( e s )light could not be determined. However, in case of highly symmetric moleculesthe fully symmetric A g vibrations can easily be identified by comparing polar-ized ( e s k e i ) and depolarized ( e s ⊥ e i ) spectra. For simplicity we label theseby xx and xy , respectively. The Raman spectra taken with the 785 nm laser lineof a diode laser were collected by a Renishaw 1000 MB Raman spectrometer. The Raman and infrared spectra of C · C H in the rotor-stator phase areshown in Figs. 1, 2 and 3 and those of C · C H in Figs. 4 and 5. The frequen-cies of the observed vibrational peaks of C · C H are listed in Tables 1 and2, and those of C · C H in Tables 3 and 4. We compare these frequencies toexperimental data on cubane and C (Ref. 8) and calculated Raman and in-frared spectra of C , respectively. As expected for molecular cocrystals withthe lattice stabilized by van der Waals interaction only, the spectra are superpo-sitions of those of the constituents. As no crystal field splitting of the fullerenelines is observed, the infrared measurement confirms that the fullerene moleculesare rotating in the crystal. The cubane lines are not split either, proving thatthe crystal field around the cubane has the same point group, i.e. O h , as thatof the isolated molecule. In the Raman spectrum of the rotor-stator cocrys-tals taken with 785 nm excitation the fullerene lines are significantly strongerthan the cubane lines, most probably because of the enhanced Raman cross sec-tion caused by the conjugated bonds, similarly to what was found in fullereneclathrates. This effect renders the cubane lines almost unnoticeable. When4hanging the wavelength of the exciting laser to 531 nm, all of the cubane linesare lost (Fig. 2), because we approach resonant scattering in the fullerenes. C belongs to the icosahedral ( I h ) point group and consequently shows fourinfrared-active vibrational modes with T u symmetry. Out of its ten Raman-active modes, two belong to the A g and eight to the H g irreducible represen-tation. We could observe all of these modes in the spectrum of C · C H (the H g (1) mode can be seen in Fig. 2). C has D h symmetry and altogether 31IR active and 53 Raman active vibrational modes. The IR modes can be de-composed as 21 E ′ + 10 A ′′ , and the Raman modes as 12 A ′ + 22 E ′ + 19 E ′′ .Similarly to the case of pristine C , not all of these modes have sufficient in-tensity to be easily detected. Cubane belongs to the octahedral ( O h ) pointgroup. Its three infrared-active T u modes are clearly visible in the spectra ofthe C · C H and C · C H rotor-stator cocrystals. This cubane spectrum isindeed the closest to that of isolated cubane in a crystalline environment; solidcubane shows a more complicated spectrum because of the lower site symme-try. The eight Raman-active modes of cubane are classified as 2 A g + 2 E g +4 T g . Only three out of these eight appear in the C · C H spectrum takenwith the 785 nm laser and none in the spectra taken with the 531 nm laser,because of the aforementioned cross-section differences.In the C · C H cocrystal, the depolarization ratio ρ = φ xy φ xx (with φ ij theoscillator strength of an excitation at either xy or xx polarization; see section2) should be zero for the fullerene A g modes and for the H g modes. The A g modes were indeed found totally polarized, and the depolarization ratio was0.90 for the H g (1) and 0.71 for the H g (4) mode (see Fig. 2). In contrast thetotally symmetric modes of C should not vanish completely in the xy geometrybecause of its D h symmetry. This is what is found in the C · C H cocrystal.The modes that have lower depolarization ratios are labeled by A in Fig. 4.These modes correspond to the ones assigned to A ′ by Sun and Kert´esz. In contrast to the fullerenes, the frequencies of the cubane principal lines inthe rotor-stator crystals deviate from those of cubane in its pure solid form. Ifwe compare the vibrational frequencies for various environments of the cubanemolecule, a clear trend can be observed. The highest vibrational frequenciesoccur in the gas phase. In pure solid cubane or in solution the lines shiftto lower frequencies. Further downshift is found in C · C H and finally inC · C H . This trend is similar to that found in the vibrational frequenciesof molecules trapped in rare gas matrices and is caused by van der Waalsinteraction: the higher the polarizability of the environment, the lower thefrequency. The relatively large shifts in the solids reflect the high polarizabilityof the fullerenes. C H ) The spectra of C · C H change dramatically upon annealing to 470 K eitherin a furnace or in a heated cryostat in the IR spectrometer (Fig. 3). The Ramanand IR spectra of the annealed sample are plotted in Figs. 2 and 3, and thepeak positions listed in Table 1 and 2, respectively. Upon heating to 470 K an5rreversible reaction takes place. When annealing a few tens of mg sample in thefurnace, the first changes in the IR spectra appear after 40 minutes: C modessplit and new modes appear. Further annealing leads to the disappearance ofthe original C and cubane modes and increased intensity of the new peaks.The new features of the final reaction product in the IR spectrum are the same,irrespective of whether the annealing was done in a furnace or in situ in acryostat.In the Raman spectrum of the annealed C · C H the A g modes of C donot split, but the low energy, i.e. radial H g modes show at least a threefoldsplitting, best seen on the lone-standing H g (1) mode. In the IR spectrum theoriginal T u modes of the fullerene split into at least two lines, and new peaksappear between 700 and 800 cm − . The splitting and the new modes indicatethat the C molecule is distorted. However, the number of new lines is consid-erably less than would be expected if the cage opened. In contrast, the changein the cubane lines is striking. The original lines disappear completely, only aweak IR line at 2948 cm − indicates that there are still hydrocarbon groups inthe sample. We infer from the position of this line, which corresponds to the C-H stretching in saturated hydrocarbons, that the carbon atoms involved are sp hybridized. In the reaction, we have to account for all atoms since no mass losswas observed by thermogravimetry-mass spectrometry (TG-MS) up to 570 K. This suggests that the cubane transforms into a different constitutional isomerand covalently bonds to C , leading to a structural distortion. The reactionproduct is most probably a covalently bound copolymer, as the products areinsoluble in common solvents.Pristine cubane also isomerizes at 470 K, the same temperature where thepolymerization appears in C · C H . Hence, a straightforward assumption isthat the first step of the copolymerization reaction must be the decompositionof cubane. Pristine cubane can decompose into several products, e.g. cyclooc-tatetraene, bicyclooctatriene, styrene and dihydropentalene. As the first threeform known adducts with C , which we could not detect by either IR spec-troscopy or HPLC, we can exclude these as being the connecting units betweenthe fullerenes.In principle both fullerene-fullerene and fullerene-C H bonds can be realizedin the polymer. C H -C H bonds can be excluded, as the C H molecules arewell separated by the fullerene molecules. We can also exclude the possibilityof covalent fullerene-fullerene bonding because of the following experimentalobservations. There are two known bond types between fullerene molecules infullerene homopolymers. In neutral polymers the [2+2] cycloaddition leads toa cyclobutane-type ring with two single bonds between the buckyballs. ARaman peak at approximately 950 cm − is associated with this bond. Thispeak is absent in the spectrum of poly(C C H ). The other possible bond typeis one single bond between two fullerene molecules. This bond leads to theappearance of a characteristic IR peak between 800-850 cm − . As this peak isalso absent we can rule out the fullerene-fullerene direct bond. There is stillanother observation which confirms this assumption. In fullerene polymers and in the dimer-oxide C O interball vibrational peaks appear in the6aman spectrum between 100-140 cm − . We measured the Raman spectrumdown to 20 cm − , but did not find any peaks below the split H g (1) mode.The reason for the absence of the interfullerene bonding comes from structuralconsiderations. The large interfullerene distance observed by x-ray diffraction does not allow the C molecules to approach each other close enough for areaction to occur between them.In the following we try to establish the connection pattern of the fullereneunit based on the infrared and Raman spectra. Since the IR and Raman spectraretain mutual exclusion (no lines are observed to appear simultaneously in both),the inversion center of the C balls must be preserved. This means that thepossible point groups of the C molecules are: I h , T h , S , D d , D d , D h , C h or C i . In Table 5 we show the evolution and splitting of the Raman active A g and H g and the IR active T u modes caused by symmetry reduction from I h to these point groups (correlation table). The C h and C i point groups canbe ruled out because the expected number of additionally activated peaks is too high to be reconciled with the observed data. A D h distortion couldin principle be positively identified as it leads to a threefold splitting of the T u modes, in contrast to the others; unfortunately, in this case our fits werenot sufficiently robust to distinguish between a three- or twofold splitting. I h or T h symmetry would not cause splittings, therefore these cannot be the onlypoint groups appearing; there must be units of reduced symmetry even if theconnection pattern of the fullerene units is not uniform throughout the wholepolymer.To draw the possible structures with the appropriate point groups we recallour assumption based on structural data that the local arrangement of themolecules does not change significantly on polymerization; thus the fullerenesmust still be surrounded octahedrally by cubanes. In addition, on polymer-ization the inversion center of the C molecule can be retained only if it isconnected to an even number of C H molecules. The connection patterns se-lected by this condition from the set of possible point groups are depicted inFig. 6. This subset contains T h , S , D d and D h .Three types of fullerene-C H connections appear in the possible structures.In the first case (pattern a, b and d in the second column of Fig. 6) the C H -fullerene connection involves two adjacent carbon atoms on the double bondof the C molecule connecting two hexagons, just as in the case of the high-pressure high-temperature (HPHT) C polymers. The difference is that whilein those polymers a cyclobutane ring is formed on polymerization, here both afour-center (cyclobutane) and a three-center (cyclopropane) ring is possible. Thesecond type of fullerene-C H connection (pattern c and e in the third column ofFig. 6.) is formed again by two atoms of C , but these lie on pentagon-hexagonbonds. It has been shown that such a connection pattern can only exist if theball is opened. As an opening was excluded based on IR results, pattern c and e can be eliminated. The last type of connection between a fullerene and aC H is a single bond (pattern f, g and h in the fourth column of Fig. 6).Next we subject these remaining structures to closer scrutiny. Pattern a wasobserved in the linear orthorhombic C polymer, and b in the two-dimensional7etragonal polymer. In these polymers and in the C dimer an empiricalrelation holds between the shift of the A g (2) mode and the number of bondson a single C ball: the shift is 5 cm − for every cycloaddition connection (i.e.two adjacent single bonds). The softening occurs because the bonds formedin the polymerization reaction originate from the π -bonds of the fullerene. Theshift of 10 cm − in poly(C C H ) fits perfectly to pattern a . As the half widthof the measured peak is 7 cm − , it is highly unlikely that pattern b or pristineC are present in poly(C C H ).We can rule out that each fullerene is connected to six cubanes. In thiscase, because of the stoichiometry, the C H molecule should also show sixfoldcoordination, which would lead to a steric tension with six of the eight C atomsof the hydrocarbon bound to a C molecule. Therefore structures d, f, g and h would automatically imply structure a to be present as well.According to our knowledge no fullerene compounds with the connectionpattern d, f, g and h have been thoroughly investigated by vibrational spec-troscopy so far. A similar well known structure only appears in the case ofpattern d : the two-dimensional rhombohedral C polymer has six pairs of σ -bonds on hexagon-hexagon bonds of the C molecule, although arranged ina different way. The rhombohedral polymer shows the A g (2) peak at 1406 cm − (Ref.28). We can expect a shift of similar magnitude in the case of pattern d , buta peak with such a shift was not observed. Another argument which confirmsthe absence of pattern d comes from the polarization dependence of the Ramanspectrum. If poly(C C H ) contained only fullerenes with T h symmetry, thenthe spectrum should show totally polarized modes, which is not the case. If, onthe other hand, it contained fullerenes with different connection patterns andpattern d were one of these, then the peaks should shift or at least change theirshape as we change the polarization. As this was not observed either, we canagain come to the conclusion that pattern d is not present in poly(C C H ).Up to this point we derived that poly(C C H ) definitely contains fullereneunits with connection pattern a , but the possibility of patterns f , g , and/or h cannot be unambigously excluded. If more connection patterns are present,then many newly activated modes should appear, which would lead to a veryrich spectrum, like e.g. that of the C photopolymer. This is in contradictionto the observed spectra. The presence of sixfold, besides twofold, coordinatedC would also mean that in the frequency region of the A g , H g and T u modeswe would have to see at least 2, 8 and 5 modes, respectively. Instead, we onlysee somewhat broader peaks as usual. The only remaining possibility would bethat all of the Raman and infrared modes of the sixfold coordinated C unitsbehave in a very similar way to those of the units with pattern a , which wouldlead to unobservable splitting. This is very unlikely since the fullerene-C H bonds in the two cases are different. Thus, based on our infrared and Ramanmeasurements we propose that poly(C C H ) consists of C H molecules andfullerene molecules connected according to pattern a .The twofold coordination of the fullerene unit means that the C H unit alsohas a coordination number of two leading to a structure consisting of chains.We cannot derive a definite assignment as to the structure of the cubane isomer8onnecting two fullerenes. One possible product, dihydropentalene, would leadto linear chains, but there are possibilities to introduce a 90 ◦ turn as well. Thesimultaneous appearance of the two would introduce disorder in all directions,leading to the cubic and amorphous crystal structure in accordance with x-raydiffraction. The variety in the connecting cubane isomers would also explainthe broadening of the vibrational lines.We can also relate the above conclusions to the structural data on C · C H polymerized at various temperatures and pressures. Iwasiewicz-Wabnig et al. found two different polymer structures depending on polymerization tempera-ture and pressure: a pseudo-cubic and a pseudo-orthorhombic one. They con-cluded from Raman spectroscopy that the two do not differ significantly on themolecular level, but the pseudo-orthorhombic form is more ordered since its for-mation occurs at pressures where the rotation of the fullerene balls is stericallyhindered. This leads us to believe that the D h symmetry, compatible with theorthorhombic crystal structure, is intrinsic to the polymer, and the pseudo-cubicallotrope results from a disordered arrangement of these molecular units. · C H We observed a reaction between the constituents on illumination at roomtemperature similar to that taking place on heating. After already 100 s oflaser illumination in the Raman microscope at both 531 nm and 468 nm, theintensity of the Raman peak at 1469 cm − decreases and a new line at 1459 cm − appears. The Raman spectrum obtained after about an hour of illumination bythe 531 nm laser is depicted in Fig 7. The new features in the spectrum coincidewith those of the polymer produced by annealing. However, as we will see later,the polymerization here is not triggered by laser-induced heating. Unfortunatelywe do not observe any cubane vibrations when exciting with the laser lines at531 nm and 468 nm, so we do not know whether cubane isomerizes the sameway as in the thermal polymerization process; we can only deduce that theconnection pattern of the fullerene is identical.The gradual evolution of the new spectral pattern around the A g (2) modeduring illumination is illustrated in Fig. 8. We fitted the spectra with threeLorentzians: one for the A g (2) mode of the monomer, one for the A g (2) modeof the polymer and one for the H g (7) mode of the polymer. From the obtainedintegrated intensity values the intensity of the polymer A g (2) peak normalizedto the total intensity of the two A g (2) peaks was calculated. We repeated theprocedure for three exciting laser wavelengths: 531 nm, 468 nm and 676 nm (seeFig. 9). We found that longer-wavelength laser lines (676 nm or 785 nm) did notinduce the reaction, therefore the effect of laser heating can be excluded. Thewavelength dependence is analogous to that in C , where photopolymerizationtakes place on illumination. Based on these analogies, we classify the reactionas photo-copolymerization with excitation of C as the first step. (We notethat the photochemical reaction is also the reason why the accumulation timefor the spectrum of the C · C H cocrystal taken at 531 nm (Fig. 2) had to beshorter than for that taken at 785 nm (Fig. 1), which accounts for the poorer9tatistics of the former spectrum.) C H ) In C · C H a similar irreversible change as in C · C H takes place onheating to 470 K. We show the Raman and IR spectra of the reaction productin Figs. 4 and 5 and list the center frequencies of the peaks in Table 3 and 4,along with the assignments of C modes by Stratmann et al . The reactionleads to the disappearance of the cubane peaks from both the IR and Ramanspectra, and a new peak appears at 2946 cm − in the IR spectrum. At the sametime the IR lines of the fullerene split, but the splitting is much less than inthe C analogue. The Raman lines only broaden, probably due to unresolvedsplitting.We found that below 800 cm − the splitting is twofold in the case of doublydegenerate E ′ modes. Above 800 cm − no clear splitting can be seen, butthe lines become somewhat smeared out. From the apparent twofold splittingof the low frequency E ′ modes the loss of the fivefold axis can be concluded,corresponding to the point group of C being C v or one of its subgroups.The changes in the IR spectra of C · C H on annealing reveal a reactionin which the cubane structure changes completely. The resulting hydrocarbonbonds to C , whose cage distorts, but remains intact. As the reaction product isinsoluble in common solvents, it must indeed be a polymer. At this stage of theresearch we cannot say anything more about the structure of this polymer, whichis partly due to the scarcity of sound spectroscopic results on C derivativesand partly due to the more complicated structure of C . The IR and Raman spectra of C · C H and C · C H were measured bothin their rotor-stator and in their polymer phases. The rotor-stator nature of thecocrystals directly manifests itself in the spectra being simple superpositionsof those of the constituents. Hence, van der Waals forces are the exclusiveinteraction between the static cubane and rotating fullerene molecules. Theslightly lower frequency of the cubane lines can be explained on the basis of thehighly polarizable environment of the cubane molecules in these structures.In the IR and Raman spectra of the polymer phases the fullerene lines aresplit and new lines appear, corresponding to a symmetry lowering of the fullerenemolecules whilst their cage remains intact. As the cubane lines change dramat-ically during the polymerization, we conclude that the cubane isomerizes toanother constitutional isomer, which binds to the fullerenes. According to thevibrational spectra no C -C bonding occurs. The comparison of structuraland spectroscopic results allows us to identify linear chains connected via theapical cubane as the most probable polymerization pattern in poly(C C H ),with possibly another cubane isomer introducing occasional 90 ◦ turns in thechains. 10inally, we found a photochemical reaction in C · C H under illuminationwith green or blue light. The symmetry of the fullerene molecules in the productturns out to be the same as that in the thermopolymer. We gratefully acknowledge valuable discussions with G. Oszl´anyi and G.Bortel about x-ray diffraction measurements. This work was supported bythe Hungarian National Research Fund under Grant Nos. OTKA T 049338and T046700, and by the Alexander-von-Humboldt Foundation through theResearch Partnership Program 3 - Fokoop - DEU/1009755.
References and Notes (1) Pekker, S.; Kov´ats, ´E.; Oszl´anyi, G.; B´enyei, G.; Bortel, G.; Jalsovszky, I.;Jakab, E.; Borondics, F.; Kamar´as, K.; Bokor, M.; Kriza, G.; Tompa, K.;Faigel, G..
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ABLE 1:
Raman frequencies of the C · C H cocrystal and poly(C C H )copolymer, and assignment of the cocrystal peaks. C stands for cubane andF for fullerene peaks.C · C H poly(C C H ) ν * (cm − ) assignment ν * (cm − )271 F, H g (1) 255272314428 F, H g (2) 429451495 F, A g (1) 486524560708 F, H g (3) 711732752770 F, H g (4) 774904 C, E g A g E g H g (5)1248 F, H g (6)1423 F, H g (7) 14261469 F, A g (2) 14591576 F, H g (8) 15663008 C, A g ABLE 2:
Infrared frequencies of the C · C H cocrystal and poly(C C H )copolymer, and assignment of the cocrystal peaks. C stands for cubane andF for fullerene peaks.C · C H poly(C C H ) ν * (cm − ) assignment ν * (cm − )527 F, T u (1) 526551561577 F, T u (2) 574705723742768857 C, T u T u (3) 11811224 C, T u T u (4) 142414582976 C, T u ABLE 3:
Raman frequencies of the C · C H cocrystal and their assignmentaccording to Ref. 9. All peaks are fullerene peaks. The peaks of poly(C C H )have essentially the same center frequencies. ν * (cm − ) assignment
259 A ′
397 A ′
411 E ′′
454 A ′
507 E ′
568 A ′
701 A ′
713 E ′′
737 E ′′
769 E ′ ′ ′ ′ ′ ′′ ′ ′′ ′′ ′ ′ ′′ ′ ABLE 4:
Infrared frequencies of the C · C H cocrystal and poly(C C H ),and the assignment of the former according to Ref.10. C stands for cubanepeaks, F for fullerene peaks.C · C H poly(C C H ) ν * (cm − ) assignment ν * (cm − )535 F, E ′ ′′ ′ ′ ′ ′ T u ′ ′′ ′′ T u ′ ′′ ′ ′ T u ABLE 5:
Correlation tables for the A g , H g , and T u representations of I h ,for the subgroups of I h containing inversion. R denotes Raman, IR infraredactive modes. I h A g (R) H g (R) T u (IR) T h A g (R) T g (R) + E g (R) T u (IR) S A g (R) A g (R) + 2 E g (R) A u (IR) + E u (IR) D d A g (R) A g (R) + E g (R) + E g (R) A u (IR) + E u (IR) D d A g (R) A g (R) + 2 E g (R) A u (IR) + E u (IR) D h A g (R) 2 A g (R)+ B u (IR)+ B u (IR)+ B u (IR)+ B g (R)+ B g (R)+ B g (R) C h A g (R) 3 A g (R)+2 B g (R) A u (IR)+2 B u (IR) C i A g (R) 5 A g (R) 3 A u (IR)17
00 800 1200 16000.05.0x10
400 800 1200 1600 2900 31000.05.0x10 CC A C FF A FFFFFF A F C .C H
785 nm xxxy I ( c oun t s ) Raman shift (cm -1 ) C A xxxy Figure 1.
Room temperature Raman spectra of the C · C H cocrystal. Thediode laser was operated at the line indicated. Spectra taken with the incidentand scattered light polarizations parallel and perpendicular are labelled by xx and xy , respectively. Cubane modes are denoted by C, fullerene modes by F.Totally symmetric modes are marked by superscript A.18
00 800 1200 16004567881012 400 800 1200 1600 poly(C C H )531 nmxxxy I n t en s i t y ( c oun t s / s ) Raman shift (cm -1 ) C .C H
531 nmxyxx
Figure 2.
Raman spectra of C · C H at room temperature before annealing(monomer) and after annealing at 470 K (polymer). The Kr + laser line and thepolarizations are indicated. The spectra are vertically shifted for clarity.19
00 800 1000 1200 14000.40.50.60.70.80.91.0 600 800 1000 1200 1400 2800 31000.40.50.6 2800 3100 poly(C C H ) C .C H CC FFFF T r an s m i ss i on Wavenumber (cm -1 ) poly(C C H )C .C H III I C
Figure 3.
Infrared spectra of C · C H before (cocrystal) and after annealingat 470 K (copolymer). C stands for cubane modes, F for fullerene modes, andI for impurity. The spectra are vertically shifted for clarity. The changes in thespectra show that annealing leads to the polymerization of the sample.20 F F A FF A F A FFFFFF A F A F A FFF A F A FF A FF A F A C .C H
531 nmxyxx I n t en s i t y ( c oun t s / s ) poly(C C H )531 nm xxxy Raman shift (cm -1 ) Figure 4.
Room temperature Raman spectra of C · C H cocrystal andcopolymer. The Kr + laser line and the polarizations are indicated. The spec-tra are vertically shifted for clarity. Totally symmetric modes are denoted bysuperscript A. Fullerene peaks are marked by F, no cubane peaks were found.21
00 800 1000 1200 1400600 800 1000 1200 1400 2800 31002800 3100 poly(C C H ) C .C H FF FFCFFCFFFFFF T r an s m i ss i on ( a r b . un i t s ) Wavenumber (cm -1 ) poly(C C H )C .C H C Figure 5.
Infrared spectra of C · C H before and after annealing at 470 K(cocrystal and copolymer phase, respectively). C: cubane peaks, F: fullerenepeaks. The asymmetric line shape is due to the Christiansen effect.22 igure 6.
Possible connection patterns of the fullerene in poly(C C H ). Thefirst column shows the arrangement of C H molecules (white spheres) whichconnect to a C ball (grey sphere). In the next columns, the carbon atoms offullerene origin are colored blue, those of cubane origin by red. We assumed inthis scheme that the connection is four-centered, including two atoms of cubaneorigin. The point group of the fullerene unit is indicated.23
00 400 600 800 1400 16005101520 200 400 600 800 1400 1600 laser illuminatedpolymerized by heatingcocrystalC .C H , 531nm laser I ( c oun t s / s ) Raman shift (cm -1 ) Figure 7.
The Raman spectrum of poly(C C H ) after photochemical re-action compared to the spectrum of the cocrystal and the spectrum of thecopolymer obtained by annealing. 24
400 1450 1500
531 nm .C H I ( c oun t s / s ) Raman shift (cm -1 ) Figure 8.
The change of the Raman spectrum of C · C H on illuminationby the 531 nm laser. The time (in hours:minutes:seconds) of the illumination isindicated on the right hand side. 25 I ( c opo l y m e r) /I ( c opo l y m e r + c o c r ys t a l )( A g ( )) t (s) Figure 9.
The fractional intensity of the poly(C C H ) A gg