Comparison of the efficiency of the laser photolysis of CGO and CT0 fullerenes in solution

Transcription

1 23 December 1994 Chemical Physics Letters 23 1 ( 1994) Comparison of the efficiency of the laser photolysis of CGO and CT0 fullerenes in solution Libor Juha a, V&a Hamplovi a, Pave1 Kub& b, Emmanuel Koudoumas, Stelios Couris a of Received 3 1 August 1994; in final form 10 October 1994 The effkiencies of the decomposition of two of the commonest fullerenes, C6,, and CT,,, induced in solution by irradiation with an XeCl (,I= 308 nm) or ArF (d= 193 nm) excimer laser are compared. It was found that when a solution containing a mixture of C& and C& fullerenes in n-hexane is irradiated with an XeCl laser, the ratio of the concentrations of the und~omposed clusters C&C,, increases with irradiation time. Thus, CT0 is decomposed more efficiently than CGO under these conditions. However, experiments carried out under other conditions ( ArF excimer laser, toluene solutions, bubbling of solutions with an inert gas or oxygen) did not exhibit this effect. 1. Introduction The photochemistry of fullerenes in solution was recently characterized by Taylor [ 1 ] as a field with poor reproducibility of results. Taylor [ 1,2] described the results of the irradiation of hexane solutions of CGO with a UV discharge lamp where four hours irradiation led to the complete decomposition of C60, while in another experiment under comparable conditions, twenty four hours irradiation did not produce any changes in the sample. The actual photolysis mechanism has not yet been satisfactorily explained; nonetheless there is agreement that small molecules (impurities in solvent and in the fullerene substance, low-molecular products of the photolysis of fullerenes, the solvent itself and dissolved gases, etc. ) play a significant role in the photolysis of fullerenes in solution [ 11. From a practical point of view it is important that, provided the photolysis is carried out with excimer lasers rather than with discharge lamps, the reproducibility becomes quite good [ 31. Based on that fact, we used ArF ( 193 nm) and X&l (308 nm) excimer lasers in order to compare the efficiency of the photolysis of two of the most common fullerenes, C6,, and C&, in solution. It has to be emphasized this is not straightfo~ard, since previously published works on that subject are in conflict between them. ( 1) It is known that Cd0 is thermodynamically less stable than CT0 [ The energy per carbon atom, based on the ab initio SCF-HF calculation of Scuseria [ 51 is 0.95 (or 0.92) kcal mol- less in CT0 than in 0. (2) Dunlap et al. [ 71 concluded that higher fullerenes, beginning with C7*, will be less easily oxidized /94/% Elsevier Science B.V. All rights reserved SsDI (94)01252-O

2 L. Juha et al. /Chemical Physics Letters 231(1994) than CeO because of the smaller curvature of the molecular surface. (3) In contrast, the stability of isolated clusters of C6,, irradiated by an XeCl excimer laser was found [ 101 to be clearly higher than the stability of CT0 under the same conditions. The authors [ lo] concluded that this is in accordance with structural factors which, however, they did not specify. The experiments described here were carried out to compare the stability of CeO and C,, in solution irradiated with an XeCl or ArF excimer laser at power densities of x 5 MW cm Experimental Fullerenes solutions were prepared from a C6,, and CT0 mixture purchased from Aldrich, the mixture containing a small amount of C6,,0 and higher fullerenes. Spectrophotometric grade n-hexane (Merck) and toluene either p.a. (Lachema) or HPLC grade (Labscan) were used as solvents. The concentration of the solutions for the majority component, i.e. CsO, is given for each experiment. The ratio of C,,,/C,,, in these solutions was determined by liquid chromatography and was found to equal about 3 : 1. In all experiments, 5 ml of solution were irradiated in cylindrical quartz cuvettes of 2.0 cm length by a certain number of either ArF or XeCl laser pulses (Lambda Physik LPXlOO and EMG 201MC). A 30 cm focal length quartz lens was used to focus the beam into the cuvette and the power density was measured at the entrance window of the cuvette. The irradiated samples were analyzed by highpressure liquid chromatography on a column with C 18 reverse phase, a mixture of toluene and methanol in a ratio of 1: 1 as the mobile phase and photometric detection at 330 nm. Three HPLC analyses were carried out on each sample and subsequent calculations were carried out using the average of the determined values. To check the degree of decomposition of CbO, the UV-VIS absorption spectra of each solution were measured before and after irradiation. Some of the samples were bubbled prior to irradiation with an inert gas (helium or argon) for one hour to remove oxygen from the solution or with oxygen for half an hour, to increase the oxygen concentration in the solution. 3. Results and discussion Figs. 1 and 2 depict the liquid chromatograms of an n-hexane solution of CeO and CT0 (2 x 10e5 M CeO) irradiated in air by 1000 and pulses from an XeCl excimer laser (the energy of a 30 ns long pulse was equal to 170 mj, with a power density at the entrance window of the cuvette of 5.4 MW cmw2). The comparison shows that the ratio C60/C70 increases in favour of C6,, after prolonged irradiation. The development of this ratio with increasing number of pulses irradiating the solution is depicted in Fig. 3. The total amount of ChO in solution does not change up to about 3000 pulses, during which this laser photolysis exhibits an induction period, followed by rapid decomposition of both fullerenes; after irradiation with 5000, and pulses, the original amount of C6,, in solution is reduced to 72%, 34O/o and 16%, respectively. It can thus be concluded that the ratio of the concentration of undecomposed clusters &/ C,,, increases with increasing irradiation time, i.e. C,,, U i kllv1 i 8001 LOO I -!I nc70 Fig. 1. Liquid chromatogram of an n-hexane solution of ChO and CT0 irradiated with 1000 pulses from an XeCl excimer laser. 200 Fig. 2. Liquid chromatogram of an n-hexane solution of C6,, and CT0 irradiated with pulses from an XeCl excimer laser.

3 316 L. Juha et al. /ChemicalPhysics Letters 231(1994) I I I Number of laser pulses Fig. 3. Dependence of the ratio of the peaks of Cc0 and CT0 in liquid chromatograms of n-hexane solutions irradiated by an XeCl excimer laser. The full point corresponds to this ratio in the initial unirradiated solution. is decomposed with greater efficiency under the given conditions than ChO. Two additional series of experiments were carried out. The first was designed to determine whether a similar effect can be observed during the irradiation of identical solutions with an ArF laser under otherwise comparable conditions. The results are given in Table 1, from which it follows that a similar change in the C60/C70 ratio to that observed in the previous experiment did not occur. This difference in the variation of the C,&C& ratio at 193 nm is a consequence of the lesser stability of CsO and cannot be fully understood if we consider only the difference in the relative absorbances of Cho and C,,,. Taking into account the published spectra [ , the CT0 absorbance is greater than that of C& for both wavelengths, the difference being more pronounced at 193 nm. Consequently, the change in the C6Q/C70 ratio should be greater in systems irradiated by an ArF extimer laser than in those irradiated by an XeCl laser. However, our experimental results clearly demonstrate that the opposite is true. This could be explained considering the significant role of the molecules produced from fullerene decomposition upon laser photolysis. The development of the shape of the UV absorption spectra of the solutions during the irradiation shows that these products absorb strongly at 193 nm and can subsequently react with the fullerenes. This mechanism could be responsible for the lesser stability of the isolated CbO and CT0 clusters at 193 nm. The effect of small molecules present in solution on the effkiency of the photolysis of C& and CT0 was studied in a second series of experiments. Since the solutions were bubbled with various gases, it was necessary to use a less volatile solvent than n-hexane. The experiments whose results are given in Table 2 were carried out using toluene solutions of mixtures of Cho and CT0 ( 1.6 x 10m5 M C&), The first five experiments of this table were performed in p.a. grade toluene, while for the rest HPLC grade toluene was used. As can be seen, in the absence of oxygen, the purity Table 1 The ratios of the areas of the peaks of Cso and CT0 in liquid ~hromato~ams of n-hexane solutions irradiated with an ArF excimer laser. R is the fraction of undecomposed C6,, related to the original amount in sotution Number of pulses of ArF laser R (%) Power density (MW cn+) Pulse energy fmjf c60/ c70 ratio _ a a In these experiments, the laser beam was introduced from above into the cuvette which was turned so that, at the site of contact of the beam with solution, there was a l-2 mm space between this surface and the entrance window of the cuvette. Thus, the solution is not irradiated through the contact of the solution with the surface of the entrance window of the cuvette.

4 L. Juha et al. /Chemical Physics Letters 231(1994) Table 2 The ratios of the areas of the peaks of C, and C,,, in liquid chromatograms of toluene solutions irradiated with an XeCl excimer laser. R is the fraction of und~mposed C, related to the original amount in solution Number of pulses of XeCl laser R (%) Pulse energy (mj) Power density (MW cm- ) Go/Go ratio air air HE He oooa Ar a Ar B He a air 4.1 In these experiments, the solutions were prepared using toluene of higher purity, i.e. HPLC grade. of the solvent plays a significant role on the photolysis efficiency for both fullerenes. In the solvent with higher impu~ty content (p.a. ) the laser photolysis of the fullerenes is more efficient than in the purer solvent (HPLC grade). This could be explained assuming that one (ArF laser) or two (XeCl laser) photons of laser radiation can open the structure of the fullerene cluster, as predicted theoretically by Musty and Scuseria [ 141. In an isolated molecule, this incision could repair itself (indicated [ 15 ] by the high stability of Cc0 held in a cyclotron resonance trap to radiation by an ArF laser at power densities which do not permit multiphoton decomposition of C,,+C,, + C, type); nonetheless, in solution it can be attacked by any suitable molecules that are present, initiating an irreversible transformation of the fullerene clusters. A significant effect of oxygen on the stability of C& and C,O clusters was found (Table 2 ), This effect can be explained also by the calculations of Murry and Scuseria [ 14 ] which demonstrate the low-energy formation of an incision only for fullerenes in the triplet state. It is known that oxygen is an excellent quencher of these states. In the absence of oxygen in the system, the lifetime of the triplet states of C6,, and CT0 is significantly longer than in the presence of oxygen and the above mechanism could become predominant, However, these ideas do not explain why the residual C6,#& ratio did not change to a greater degree in all the experiments. It must be taken into consideration that these are toluene solutions and that Taylor found [ 1 ] that the photochemical stability of fullerenes in benzene (i.e. a solvent similar to toluene) is higher than in n-hexane. If he carried out his expe~ments using a discharge lamp with radiation in a wide spectra range, then this fact could be relatively easily explained by the absorption of the shorter wave radiation in benzene and thus a lower amount of energy deposited in the clusters in benzene solutions. However, the experimental conditions are not given in Taylor s short note [ 11. However, in our experiments the laser radiation is not directly absorbed in the solvent, so that the differences in behaviour of the solutions in various solvents cannot be explained in this way. A more elaborate treatment of this phenomenon will probably become possible after completion of calculations estimating the probability of the formation of the abovedescribed incision in CeO and CT0 and after more detailed elucidation of the role of small molecules, including solvent molecules, in the photolysis of fullerenes in solution. 4. Conclusions We found in the irradiation of a mixture of C6,, and C,, in n-hexane with and XeCl excimer laser, that the ratio of the concentrations of undecomposed C&/C& increases with increasing irradiation time. It thus follows that CT0 is less stable under these conditions than

5 318 &,. This could be simply explained from the fact that the absorbance of ChO at 308 nm is somewhat greater than that for C,,,. However, a change in the C&/&, ratio with increasing irradiation time did not occur in subsequent experiments with an ArF laser, in spite of the fact that the difference in the differences of absorbances of C,, and CT0 at 193 nm is at least comparable with that at 308 nm. Experiments with toluene solutions of ChO and C70 bubbled with various gases prior to irradiation with an XeCI excimer laser yielded results indicating almost identical efficiency of photolysis of CGO and C,,. Nonetheless, these experiments demonstrated that a significant role in the photolysis of fullerenes in solution could be played by the opening of the fullerene cages in their triplet excited states and subsequent interaction of these incisions with small molecules present in solution (impurities in the solvent, products of the photolysis of fullerenes, the solvent itself, etc.). Thus, to the three groups of findings indicating different photochemical activities of CGO and CTO, a fourth is maybe added, that can be formulated in terms of the following questions. What are the probabilities of opening of the C& and C& structures in their triplet excited states? Are the affinities of small molecules the same for these damaged CeO and C,, structures? In addition, it should be emphasized that the expected greater reactivity of Cso with oxygen, mentioned in Section 1, would mean that, in the absence of oxygen (samples bubbled with helium or argon), the increase in the C60/C70 ratio should be greater than for samples irradiated under the same conditions in the presence of oxygen. However, this was not confirmed in our experiments. This could indicate that the photo-oxidation of fullerenes does not predominate as the primary process in their photolysis, i.e. that the assumption of Dunlap et al. [7] is not justified. Our results did not completely confirm the finding of Wun et al. f IO] that isolated CeO clus- ters are more stable during irradiation with an XeCl excimer laser than CT0 clusters, and rather indicate a high specificity of photolysis of fullerenes in solution. LJ gratefully acknowledges support from the UV Laser Facility operated in FORTH-IESL within the Large Installation Plan of the European Union and the program of Cooperation in Science and Technology with Central and Eastern European Countries. References [ 1 ] R. Taylor, Phil. Trans. Roy. Sot. A 343 (1993) 101. [2] R. Taylor, J.P. Parsons, A.G. Avent, S.P. Rannard, T.J. Dennis, J.P. Hare, H.W. Kroto and D.R.M. Walton, Nature 351 (1991) 277. [3] L. Juha, J. K&a, L. Ldska, V. Hamplov~, L. Soukup, P. Engst and P. Kubat, Appl. Phys. B 57 (1993) 83. [4] H. Schwarz, Angew. Chem. Intern. Ed. Engl. 32 (1993) [ 51 GE. Scuseria, Chem. Phys. Letters 180 ( 1991) [ 6 J D. Bakowies and W. Thiel, J. Am. Chem. Sot. 113 ( 1991) [ 71 B.I. Dunlap, D.W. Brenner, J.W. Mintmire, R.C. Mowrey andc.t. White, J. Phys. Chem. 95 (1991) [X] R.M. Rudzin ski, Z. Slanina, M. Togasi, E. Osawa and T. Tizuka, Thermochim. Acta 125 ( 1988) 155. [9]M.D. Newton and R.E. Stanton, J. Am. Chem. Sot. 108 (1986) [ IO] P. Wurz, K.R. Lykke, M.J. Pellin and D.M. Gruen, J. Appl. Phys. 70 (1991) [ 111 J.B. Howard, J.T. McKinnon, Y. Makarovsky, A.L. Lafleur and M.E. Johnson, Nature 352 ( 1991) 139. [ 121 J.P. Hare, H.W. Kroto and R. Taylor, Chem. Phys. Letters 177 (1991) 394. [ 131 M. Diack, R.L. Hettich, R.N. Compton and G. Guiochon, Anal. Chem. 64 (1992) [ 14) R.L. Murry and G.E. Scuseria, Science 263 (1994) 791. [ 151 F.D. Weiss, J.L. Elkind, S.C. O Brien, R.F. Curl and R.E. Smalley, J. Am. Chem. Sot. 110 ( 1988) 4464.