Electron Transfer to [60]Fullerene and Its o-Quinodimethane Adducts

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J. Phys. Chem. 1996, 100, 16232-16237

Electron Transfer to [60]Fullerene and Its o-Quinodimethane Adducts in Dimethyl Sulfoxide (EPR, Visible/Near-IR, and Electrochemical Study) Vlasta Brezova´ ,† Andreas Gu1 gel,‡ Peter Rapta,† and Andrej Stasˇko*,† Faculty of Chemical Technology, SloVak Technical UniVersity, Radlinske´ ho 9, SK-812 37 BratislaVa, SloVak Republic, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, D-55 128 Mainz, Germany ReceiVed: February 22, 1996; In Final Form: June 7, 1996X

The o-quinodimethane adducts 2-4 of [60]fullerene dissolved in dimethyl sulfoxide (DMSO) form the corresponding monoanion radicals with a yield of up to 20% if UV-irradiated under argon at 295 K. The radicals are presented in EPR by a single Lorentzian line with g(2-4) ) 2.0000 and a peak-to-peak width of pp ) 0.105 mT. Simultaneously, a characteristic monoanion band in the near-IR spectrum at 1006 nm appears to be accompanied by a decrease in original fullerene absorption in the UV region at 265 nm. Given alternating saturation with oxygen and argon followed by irradiation, the decay and regeneration process for the monoanion can be repeated many times. The reoxidation of monoanion is coupled with the formation of superoxide anion. Lowering the temperature from 380 to 260 K narrows the line from pp(380 K) ) 0.2026 mT to pp(260 K) ) 0.089 mT, then widens it abruptly at 240 K to a Lorentzian with pp = 0.3 mT, and then leaves it constant down to 100 K. This temperature behavior is reversible and no wide-line component is evident. [60]Fullerene 1 also forms a monoanion with the same g value as its derivatives 2-4, but with slightly smaller pp ) 0.09 mT and in considerably lower concentration due to its limited solubility in DMSO.

The formation of [60]fullerene anion radicals is the subject of intensive research.1 Various methods of generation such as cathodic reduction,2-4 reduction by alkali metals5 or organometallic6 and organic7 compounds, photochemical charge transfer in the homogeneous8,9 and heterogeneous10 systems, and radiolytic reduction10f,11 have been described in the literature. Their unambiguous identification was established by visible/ near-IR spectroscopy with characteristic bands of mono-, di-, tri-, and tetraanions.3a,f,4a,c,5a,c,d,6b,c,9a,g,10d Generally the method of choice for studying the anion radicals is EPR spectroscopy; however, we faced a difficult task in the assignment of the observed single lines to the individual ions.2-6,9g,10d The description of [60]fullerene monoanion with a wide line (up to a few millitesla) with a g value in the range of 1.997-2.000 is considered to be well-established.2,3a,b,4a,b Such EPR measurements have been carried out at low and also at room temperature with a time delay after the generation of radicals. Additionally, [60]fullerene monoanion has been identified by a characteristic band at 1075 nm in the near-IR region.3a,4a The narrow EPR lines, frequently superimposed on the wide lines,4c,5a are assumed to originate from impurities or from some consecutive unrelated products.3a Previously, by using photosensitized TiO2 suspension10d or by quenching the photoexcited [60]fullerene with Et3N donor,9g we succeeded in generating two radical products with narrow, well-characterized single EPR lines: (A) with gA ) 2.0000 and peak-to-peak-width ppA ) 0.09 mT and (B) (consecutive product of A) with gB ) 2.0006 and ppB ) 0.04 mT. The formation of single-line A is accompanied by the characteristic [60]fullerene monoanion band at 1075 nm in the near-IR spectrum.9g,10d The original assumption mentioned earlier that the narrow lines are not related to [60]fullerene monoanion no longer holds. The paper presented here on the investigations of [60]fullerene, and especially on its o-quinodimethane adducts 2-412 described in the experimental part, provides further unambiguous evidence for the connection of the narrow lines with [60]†

Slovak Technical University. Max-Planck-Institut fu¨r Polymerforschung. X Abstract published in AdVance ACS Abstracts, September 15, 1996. ‡

S0022-3654(96)00547-3 CCC: $12.00

fullerene monoanion (Chart 1). Upon the irradiation of dimethyl sulfoxide solutions of fullerenes 2-4, a stable single-line product described in EPR with pp ) 0.105 mT and g ) 2.0000 is found, showing the bands of monoanions in the region of 1006 nm in the near-IR spectra. Simultaneously, a decrease in UV absorption for [60]fullerene o-quinodimethane adducts in the region of 265 nm was found with the increased formation of fullerene anion under argon. Upon saturation of such a system with oxygen, the monoanion line in the EPR and near-IR spectra vanished, and it can be restored again after resaturation with argon and repeated irradiation. These results should stimulate the reconsideration of both narrow and wide EPR lines of monoanions, which probably reflect their various solvation states and the formation of associates. Experimental Section [60]Fullerene (1) in gold quality was purchased from Hoechst (Germany). Its adducts 2-4 were prepared as described in ref 12. Dimethyl sulfoxide (DMSO for gas chromatography) was obtained from Merck, and spin-trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) originated from Aldrich. Tetrabutylammonium perchlorate (TBAP) of electrochemical grade was obtained from Fluka. The saturated solutions of fullerenes 2-4 (0.1 mM 2 and 0.3 mM 3 and 4 fullerenes) and a suspension of 1 in DMSO solvent under argon were irradiated directly in the cavity of a Bruker 200D EPR spectrometer equipped with an Aspect 2000 computer, using a medium-pressure mercury lamp and cutting the wavelengths below 300 nm by means of a Pyrex window. Simultaneously, EPR spectra were measured in a flat cell at a temperature of 295 K and in the cylindrical probe tubes at lower tempertures down to 100 K, using a variable-temperature ER 4111 VT unit. The deaerated fullerene 2-4 solutions after bubbling with argon were irradiated, and the UV/vis/near-IR spectra were recorded using a Lambda 9 Perkin-Elmer UV/vis/ near-IR spectrometer. (Such investigations with [60]fullerene (1) were limited due to its low solubility in DMSO.) Then the solutions were continuously exposed to air or oxygen, and the monoanion decay in the near-IR region (1006 nm), as well as © 1996 American Chemical Society

Electron Transfer to [60]Fullerene and o-Quinodimethane Adducts

J. Phys. Chem., Vol. 100, No. 40, 1996 16233

Figure 1. Time evolution of EPR spectra observed under argon during a continuous irradiation, after stopping irradiation, and then exposure to oxygen in (a) suspension of [60]fullerene (1) and (b) 125 µM solution of fullerene 3 in dimethyl sulfoxide. In the second cycle the first experiment was repeated with the same solution after resaturation with argon.

CHART 1: Investigated [60]Fullerene 1 and Its Adducts 2-4

the simultaneous increase in the original fullerene concentration (265-365 nm), was monitored over time using a 1.0 or 0.1 cm thick cell. To investigate the process of fullerene reoxidation, after the EPR measurements, the irradiated fullerene 2-4 solutions were poured out from the EPR cell, exposed to air in the presence of

the DMPO spin trap (0.001 M solution), and then deaerated with argon. The solution thus obtained was placed in the EPR cell again and EPR spectra of the •DMPO-O2- adduct were recorded. An analogous experiment with fullerene 1 was carried out using electrochemically generated [60]monoanion, as described in more detail in the following.

16234 J. Phys. Chem., Vol. 100, No. 40, 1996

Brezova´ et al. TABLE 1: Half-wave Potentials (Volts), E1/2, of the First Reduction Step of [60]Fullerene (1), Its Substituted Derivatives 2-4, and [60]-o-Xylyl (Ws Reference Electrode and Ferrocene/Ferricenium Internal Potential Marker) along with the Shift in E1/2 Due to the Fullerene Substitution fullerene sample 1 1 1 1 2 3 4 [60]-o-xylyl

E1/2 Vs ref. electrode Fc/Fc+

E1/2 Vs Fc/Fc+

-0.34a -0.57b -0.34c -0.43d -0.50a -0.45a -0.495a -0.54d

-0.723 -0.775 -0.86 -0.90 -0.88 -0.84 -0.875 -1.010

0.38 0.205 0.52 0.47 0.38 0.38 0.38 0.47

E1/2(probe) E1/2([60])

sources

-0.16 -0.12 -0.16 -0.11

this work ref 20 ref 2e ref 14 this work this work this work ref 14

a Ag/AgCl wire immersed in DMSO containing 0.1 M TBAP. b Ag wire immersed in DMSO containing 0.1 M TBAP and 0.01 M AgNO3. c SCE immersed in pyridine containing 0.1 M TBAP Via fritted glass bridge. d SCE immersed in PhCN containing 0.1 M TBAP Via fritted glass bridge.

Figure 2. Cyclic voltammogram of 0.2 mM [60]fullerene adduct 3 in 0.1 M TBAP-DMSO solution (scan rate was 100 mV s-1; potential is referenced Vs Ag/AgCl reference electrode). Inset shows the EPR spectrum observed during in situ reduction at the potential of the first reduction peak.

To optimize the potentiostatic and amperostatic reduction of fullerenes in the EPR cavity, the cyclic voltammograms of the saturated fullerene in 0.1 M TBAP-DMSO solutions were taken by means of a PAR 270 instrument (Princeton Applied Research) using platinum working, counter, and Ag/AgCl reference electrodes. Here as well, three characteristic reduction peaks, as already described in the literature,2a,4f were found. The amperostatic and potentiostatic in situ reductions in the EPR spectrometer were carried out on a platinum net, using a Varian electrolytic EPR cell at the potential of the first cyclovoltammetric reduction peak. All experiments, if not otherwise stated, were carried out at 295 K. Results and Discussion Before we start a more detailed presentation of the results, it should be stated that all three substituted fullerenes 2-4 behaved practically identically and also showed the same EPR spectra. Therefore, the characteristic results found for the derivatives 2-4 are illustrated with those obtained for fullerene 3. [60]Fullerene (1) is less soluble in DMSO; consequently, its radical yields were considerably lower, but it exhibited behavior similar to that of its adducts 2-4. The corresponding small differences will be outlined in the following. Figure 1a shows the time evolution of a single EPR line with pp ) 0.095 mT and g ) 2.0000 observed during continuous irradiation of a [60]fullerene (1) suspension in DMSO. After a few minutes of irradiation, a stationary concentration was reached, and it decreased slowly during an additional few minutes after the irradiation was stopped. If such a solution was poured out from the EPR cell and exposed to air or oxygen for a few seconds, the EPR line vanished immediately. This experiment can be repeated numerous times, obtaining similar EPR behavior each time, as is illustrated in the lower part of Figure 1a, if the same solution is resaturated with argon and irradiated again. Under conditions similar to those described for [60]fullerene (1), its adducts 2-4 show a time behavior, as illustrated in Figure 1b using fullerene 3. They have the same g values as [60]fullerene (1) (2.0000) and their peak-to-peak widths are slightly higher (0.105 mT).

To prove that in the photochemical generation described the corresponding fullerene monoanion is generated, we completed these investigations with a cathodic reduction in the potential region of the first reduction step. The corresponding cyclic voltammogram, along with the EPR spectrum found at the potential of the first reduction peak (-420 mV Vs Ag/AgCl) using fullerene 3, appears in Figure 2. This confirms that the radical product with the narrow EPR line found in the photochemical investigations is the monoanion of fullerene 3. Similar results were obtained with fullerenes 2 and 4. As the solubility of [60]fullerene (1) in DMSO for the analogous investigations described earlier is very low, we carried out the following experiment. Platinum net was repeatedly dipped into the saturated [60]fullerene (1) solution in toluene and dried. In this way a film of [60]fullerene (1) was formed on the electrode. The electrode was put under argon into the EPR flat cell filled with 0.1 M TBAP in DMSO, and in situ potentiostatic reduction at the potential of the first reduction peak was performed directly in the cavity of the EPR spectrometer. A narrow line with g ) 2.0000 and pp ) 0.09 mT was found. Then the electrode was pulled out. The narrow EPR line was also found for the remaining homogeneous solution, ruling out the possibility that this signal observed originates from the solid fullerene 1. With this prepared solution of fullerene 1 monoanion a cyclic voltammogram was taken, starting the scan behind the potential of the first reduction peak (-0.6 V Vs Ag/AgCl) and scanning toward zero and then backward. The reversible oxidation (-0.295 V Vs Ag/AgCl) and the counter-reduction peaks (-0.39 V Vs Ag/AgCl) were found in this way. The corresponding cyclovoltammetric data found for fullerenes 1-4 are summarized in Table 1. As can be seen from the data presented, the potentials attributed to the substituents in fullerenes 2-4 are shifted by about 130 mV to more negative values. In Figure 3a-c we compare the experimental and simulated EPR spectra obtained in photochemical electron transfer using [60]fullerene (1) and its derivative 3 with those obtained in the cathodic reduction (at -420 mV Vs Ag/AgCl) of fullerene 3. All three spectra have narrow lines, the same g value (2.0000), and closely related peak-to-peak widths, pp(1) ) 0.095 mT and pp(3) ) 0.105 mT. The cathodic reduction here proves again that the photochemically generated radicals are fullerene monoanions. Further, the results obtained confirm that the asymmetrization associated with [60]fullerene sdubstitution is of only negligible influence on the line width of the monoanions.

Electron Transfer to [60]Fullerene and o-Quinodimethane Adducts

J. Phys. Chem., Vol. 100, No. 40, 1996 16235

Figure 3. Experimental and simulated EPR spectra obtained in DMSO during (a) irradiation of [60]fullerene (1) suspension, (b) irradiation of 125 µM solution of fullerene 3, (c) cathodic reduction of fullerene 3 at the potential of the first reduction peak (-420 mV Vs Ag/AgCl), and (d) after electron transfer from photogenerated fullerene 3 monoanion to oxygen in the presence of DMPO spin-trap (aH and aN are splitting constants in millitesla).

Unambiguous confirmation that the generated radicals are fullerene monoanions was also obtained in the UV/vis/near-IR investigations shown in Figure 4. In analogs to the EPR experiments, the solution of fullerene 3 in DMSO was irradiated for a few minutes and then the UV/vis/near-IR spectrum was measured. A characteristic band of the monoanion for [60]fullerene (1) at 1077 nm was hypsochromically shifted to 1006 nm for fullerene 3, as shown in Figure 4, and for fullerenes 2 and 4 we found 1003 and 1006 nm, respectively. A similar shift for “dihydrofullerene” monoanions was also reported by Anderson (1000 nm),9b Baumgarten (1006 nm),13 and Kadish (1006 nm).14 Although g values and peak-to-peak widths in the EPR spectra of monoanions described with narrow lines practically are not influenced upon substitution, the shift observed here in the vis/near-IR spectra is remarkable. As shown in Figure 4 the band at 1006 nm continuously vanished during exposure to air, and it could be regenerated again after resaturating the solution with argon followed by irradiation. Similar behavior was found for the monoanion line in EPR experiments described in Figure 1. Further, the inset in Figure 4 shows details from the UV region before irradiation (dotted line) and after irradiation during exposure to the air (full line).

The standard fullerene 3 band in the region of 265 nm decreased during the irradiation, and then its previous intensity was restored after exposure to air. We assume that the photoexcited fullerene is reversibly reduced to its monoanion by the DMSO solvent acting as an electron donor15 and then oxidized to the original fullerene by oxygen. The reduction and reoxidation are repeatedly reproducible with the same probe, as was the case in the experiments described earlier in the EPR investigations. More details on the mechanism of the oxidation of fullerene anion with oxygen were obtained by the spin-trapping technique. The probe, after irradiation characterized with the EPR spectrum of monoanion, was poured out from the EPR cell and exposed to air for a few seconds (or oxygen, with the same result) in the presence of a DMPO spin-trap and then deaerated with argon. An EPR spectrum characteristic for the •DMPO-O2adduct16 shown in Figure 3d was obtained, confirming the electron transfer from fullerene 3 monoanion to oxygen molecule. This is an unambiguous experimental confirmation of the mechanism frequently assumed in the literature.11 The same behavior was also found for fullerene 1. However, due to its low solubility in DMSO, the yield of [60]fullerene

16236 J. Phys. Chem., Vol. 100, No. 40, 1996

Figure 4. UV/vis/near-IR spectra observed from fullerene 3 solution in dimethyl sulfoxide under argon before irradiation (dotted line). The solution was irradiated for 100 s and then, after the irradiation was stopped, the decay of the monoanion band at 1006 nm over time was monitored during exposure to oxygen (solid line). The inset shows a simultaneous recovery of the original fullerene absorption in the region of 265-365 nm, indicating the reversibility of the reoxidation. After resaturation of the same solution with argon and reirradiation, the redox cycle can be reproduced.

monoanion in the irradiated suspension was low. Therefore, solution of fullerene 1 monoanion was prepared in the cathodic reduction as described earlier and then poured out and exposed to air in the presence of DMPO. A spectrum of •DMPO-O2adduct identical to that shown in Figure 3d was then observed, confirming the electron trnasfer from [60]fullerene monoanion to oxygen under the formation of superoxide anion. An exact quantitative evaluation of EPR spectra generally17 and UV/vis/near-IR in particular is rendered difficult due to some specific problems encountered here. We took 2,2,6,6tetramethylpiperidine N-oxide (TEMPO) as a reference, recorded its EPR spectra under the same conditions as described in the measurements with fullerenes, and evaluated the fullerene 3 monoanion concentration from its spectra shown in Figure 1b to 20 ( 7 µM. The molar extinction coefficients of the generated monoanions of fullerenes 2-4 in the near-IR region are not known. Therefore, we calibrated the UV spectra of fullerene 3 in the region of 265 nm at various concentrations and evaluated the decrease in concentration due to its conversion to the monoanion shown in Figure 4 to 18 ( 2 µM. This concentration change corresponds satisfactorily to the formation of the monoanion radical evaluated in EPR. The concentrations thus obtained represent 10-20% conversions of fullerene 3 to its monoanion. Figure 5 shows the changes in the peak-to-peak widths with changing temperature in the EPR spectrum of fullerene 3 monoanion. Upon decreasing the temperature from 380 to 260 K, the line width decreases from pp(380 K) ) 0.2026 mT to pp(260 K) ) 0.089 mT, and this temperature dependence is described with an exponential function: pp(T) ) -0.027(1 exp(0.0056T)) mT. Although the system is already solid at 290 K, no anisotropic contribution18 is evident; only a single line with Lorentzian shape is observable. Below 260 K the line width sharply increases, and below 220 K it remains approximately constant with pp ) 0.285 mT. Its changes in this temperature region are well-described with the following relation: pp(T) ) 0.286 - 8 × 10-15 exp(0.12T) mT. Again

Brezova´ et al.

Figure 5. Dependence of the peak-to-peak width (pp) on the temperature found in EPR spectra of fullerene 3 monoanion photogenerated in DMSO solution.

it is well-fit with Lorentzian shape, and no significant contribution of the wide-line component is evident up to 100 K. Raising and lowering of the temperature between 380 and 100 K produces changes in the line shape shown in Figure 5, which are readily reproducible. Recently, a paper was published by Eaton et al.19 reporting on [60]fullerene monoanion in DMSO-containing solution. The emphasis of this paper is on the broad signals that account, under the given experimental conditions, for more than 95% of the total signal area. However, an overlapped sharp line was also observed, whose shape shows little temperature dependence from 110 to 220 K. An analogous observation for fullerene 3 is reported here in Figure 5. It is noteworthy that the dominating wide-line contribution in ref 19 was found in ex situ EPR investigations, whereas exclusively narrow lines were observed in our in situ experiments. This documents relatively high sensitivity of [60]fullerene monoanion to various parameters, such as solvent polarity, type of counterions and presence of salts, temperature, time after generation, and degree of conversion. Acknowledgment. We thank Prof. P. Grier for helpful discussions and the Slovak Grant Agency and VolkswagenStiftung for financial support. References and Notes (1) (a) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. ReV. 1991, 91, 1213. (b) Hammond, G. S., Kuck, V. J., Eds. Fullerenes. Synthesis, Properties and Chemistry of Large Carbon Clusters; ACS Symposium Series 481; American Chemical Society: Washington, DC, 1992. (c) Kroto, H. W., Fischer, J. E., Cox, D. E., Eds. The Fullerenes; Pergamon Press: Oxford, UK, 1993. (d) Kadish, K. M., Ruoff, R. S., Eds. Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; The Electrochemical Society: Pennington, NJ, 1994. (e) Bernier, P., Bethune, D. S., Ching, L. Y., Ebbesen, T. W., Metzer, R. M., Mintmire, J. W., Eds. Science and Technology of Fullerene Materials; MRS Symposium Series 359; MRS: Pittsburgh, PA, 1995. (2) (a) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 4364. (b) Dubois, D.; Kadish, K. M.; Flanagan, S.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 7773. (c) Dubois, D.; Jones, M. T.; Kadish, K. M. J. Am. Chem. Soc. 1992, 114, 6446. (d) Koh, W.; Dubois, D.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1992, 96, 4163. (e) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1992, 96, 7137. (3) (a) Khaled, M. M.; Carlin, R. T.; Trulove, P. C.; Eaton, G. R.; Eaton, S. S. J. Am. Chem. Soc. 1994, 116, 3465. (b) Allemand, P.-M.;

Electron Transfer to [60]Fullerene and o-Quinodimethane Adducts Koch, A.; Wuld, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. Soc. 1991, 113, 1050. (c) Xie, Q.; PerezCordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992, 114, 3978. (d) Jehoulet, C.; Bard, A. J.; Wudl, F. J. Am. Chem. Soc. 1991, 113, 5456. (e) Zhou, F.; Yau, S.-L.; Jehoulet, C.; Laude, D. A., Jr.; Guan, Z.; Bard, A. J. J. Phys. Chem. 1992, 96, 7175. (f) Cliffel, D. E.; Bard, A. J. J. Phys. Chem,. 1994, 98, 8140. (g) Carlin, R. T.; Trulove, P. C.; Eaton, G. R.; Eaton, S. S. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-24, p 986. (4) (a) Kato, T.; Kodama, T.; Oyama, M.; Okazaki, S.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 186, 35. (b) Rataiczak, R. D.; Koh, W.; Subramanian, R.; Jones, M. T.; Kadish, K. M. Synth. Met. 1992, 56, 3137. (c) Greaney, M. A.; Gorun, S. M. J. Phys. Chem. 1991, 95, 7142. (d) Allemand, P.-M.; Srdanov, G.; Koch, A.; Khemani, K.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. Soc. 1991, 113, 2780. (e) Heinze, J.; Smie, A. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-24, p 1117. (f) Dunsch, L. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-24, p 1068. (5) (a) Schell-Sorokin, A. J.; Mehran, F.; Eaton, G. R.; Eaton, S. S.; Viehbeck, A.; O’Toole, T. R.; Brown, C. A. Chem. Phys. Lett. 1992, 195, 225. (b) Bhyrappa, P.; Paul, P.; Stinchcombe, J.; Boyd, P. D. W.; Reed, C. A. J. Am. Chem. Soc. 1993, 115, 11004. (c) Stinchcombe, J.; Pe´nicaud, A.; Bhyrappa, P.; Boyd, P. D. W.; Reed, C. A. J. Am. Chem. Soc. 1993, 115, 5212. (d) Baumgarten, M.; Gu¨gel, A.; Gherghel, L. AdV. Mater. 1993, 5, 458. (e) Boyd, P. D. W.; Bhyrappa, P.; Paul, P.; Stinchcombe, J.; Bolskar, R. D.; Sun, Y.; Reed, C. A. J. Am. Chem. Soc. 1995, 117, 2907. (6) (a) Boulas, P.; Subramanian, R.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Electrochem. Soc. 1994, 140, L130. (b) Subramanian, R.; Boulas, P.; Vijayashree, M. N.; D’Souza, F.; Jones, M. T.; Kadish, K. M. J. Chem. Soc., Chem. Commun. 1994, 1847. (c) Subramanian, R.; Boulas, P.; Vijayashree, M. N.; D’Souza, F.; Jones, M. T.; Kadish, K. M. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-24, p 779. (d) Friedrich, J.; Schweitzer, P.; Dinse, K.-P.; Rapta, P.; Stasˇko, A. Appl. Magn. Reson. 1994, 7, 415. (f) Dinse, K.-P.; Friedrich, J.; Steren, C. A.; van Willigen, H.; Rapta, P.; Stasˇko, A. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-24, p 1030. (7) (a) Pola, J.; Darwish, A. D.; Jackson, R. A.; Kroto, H. W.; Meidine, M. F.; Abdul-Sada, A. K.; Taylor, R.; Walton, D. R. M. Fullerene Sci. Technol. 1995, 3, 299. (b) Gol’dshleger, N. F.; Lobach, A. S.; Astakhova, A. S.; Kaplunov, M. G.; Kulikov, A. V.; Moravskii, A. P.; Roshupkina, O. S.; Shul’ga, Y. M. Russ. Chem. Bull. 1994, 43, 1081. (8) (a) Sension, R. J.; Szarka, A. Z.; Smith, G. R.; Hochstrasser, R. M. Chem. Phys. Lett. 1991, 185, 179. (b) Gevaert, M.; Kamat, P. V. J. Phys. Chem. 1992, 96, 9863. (c) Palit, D. K.; Ghosh, H. N.; Pal, H.; Sapre,

J. Phys. Chem., Vol. 100, No. 40, 1996 16237 A. V.; Mittal, J. P.; Seshadri, R.; Rao, C. N. R. Chem. Phys. Lett. 1992, 198, 113. (d) Biczo´k, L.; Linschitz, H.; Walter, R. I. Chem. Phys. Lett. 1992, 195, 339. (e) Biczo´k, L.; Linschitz, H.; Walter, R. I. Chem., Phys. Lett. 1994, 221, 188. (f) Biczo´k, L.; Linschitz, H. J. Phys. Chem. 1995, 99, 1843. (g) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 4093. (9) (a) Arbogast, J. W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc. 1992, 114, 2277. (b) Anderson, J. L.; An, Y.-Z.; Rubin, Y.; Foote, C. S. J. Am. Chem. Soc. 1994, 116, 9763. (c) Osaki, T.; Tai, Y.; Tazawa, M.; Tanemura, S.; Inukawa, K.; Ishiguro, K.; Sawaki, Y.; Saito, Y.; Shinohara, H.; Nagashima, H. Chem. Lett. 1993, 789. (d) Watanabe, A.; Ito, S. J. Phys. Chem. 1994, 98, 7736. (e) Schaffner, E.; Fischer, H. J. Phys. Chem. 1993, 97, 13149. (f) Dimitrijevic´, N. M.; Kamat, P. V. J. Phys. Chem. 1993, 97, 7623. (g) Brezova´, V.; Stasˇko, A.; Rapta, P.; Domschke, G.; Bartl, A.; Dunsch, L. J. Phys. Chem. 1995, 99, 16234. (h) Foote, C. S. Top. Curr. Chem. 1994, 169, 347. (10) (a) Kamat, P. V. J. Am. Chem. Soc. 1991, 113, 9705. (b) Kamat, P. V. Chem. ReV. 1993, 93, 267. (c) Kamat, P. V.; Bedja, I.; Hotchandani, S. J. Phys. Chem. 1994, 98, 9137. (d) Stasˇko, A.; Brezova´, V.; Biskupicˇ, S.; Dinse, K.-P.; Schweitzer, P.; Baumgarten, M. J. Phys. Chem. 1995, 99, 8782. (11) (a) Guldi, D. M.; Hungerbu¨hler, H.; Janata, E.; Asmus, K.-D. J. Phys. Chem. 1993, 97, 11258. (b) Guldi, D. M.; Hungerbu¨hler, H.; Asmus, K.-D. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-24, p 854. (c) Guldi, D. M.; Hungerbu¨hler, H.; Janata, E.; Asmus, K.-D. J. Chem. Soc., Chem. Commun. 1993, 84. (12) (a) Belik, P.; Gu¨gel, A.; Spickermann, J.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 78. (b) Belik, P.; Gu¨gel, A.; Kraus, A.; Spickermann, J.; Enkelmann, V.; Frank, G.; Mu¨llen, K. AdV. Mater. 1993, 5, 854. (c) Gu¨gel, A.; Kraus, A.; Spickermann, J.; Belik, P.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1994, 33, 559. (d) SÄ liwa, W. Fullerene Sci. Technol. 1995, 3, 243. (13) Gherghel, L.; Baumgarten, M. Synth. Met. 1995, 69, 1389. (14) Kadish, K. M.; Boulas, P. L.; Vijayashree, M. N.; Subramanian, R.; Gao, X.; Mead, S.; Tan, Z.; Jones, M. T. In Fullerenes: Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1995; Vol. 95-10, p 213. (15) Reichardt, C. SolVent Effects in Organic Chemistry; Verlag Chemie: Weinheim, Germany, 1979. (16) (a) Reszka, K.; Chignell, C. F. Free Radical Res. Commun. 1991, 14, 97. (b) Stasˇko, A.; Blazˇkova´, A.; Brezova´, V.; Breza, M.; Lapcˇik, L.; Lapcˇik, L., Jr. J. Photochem. Photobiol. A: Chem. 1993, 76, 159. (17) Yordanov, N. D. Appl. Magn. Reson. 1994, 6, 241. (18) Kobayashi, H.; Tomita, H.; Moriyama, H.; Kobayashi, A.; Watanabe, T. J. Am. Chem. Soc. 1994, 116, 3153. (19) Eaton, S. S.; Kee, A.; Konda, R.; Eaton, G. R.; Trulove, P. C.; Carlin, R. T. J. Phys. Chem. 1996, 100, 6910. (20) Trulove, P. C.; Carlin, R. T.; Eaton, G. R.; Eaton, S. S. J. Am. Chem. Soc. 1995, 117, 6265.

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