J. Phys. Chem. 1996, 100, 4823-4827
4823
Electronic Communication in Fullerene Dimers. Electrochemical and Electron Paramagnetic Resonance Study of the Reduction of C120O Alan L. Balch,* David A. Costa, W. Ronald Fawcett,* and Krzysztof Winkler† Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed: October 25, 1995; In Final Form: December 14, 1995X
Cyclic voltammetry (cv) and differential pulse voltammetry (dpv) studies of C120O reveal that this bis(fullerene) undergoes six sequential, one-electron reductions to form the ions [C120O]n- where n is -1 to -6. The first two reductions of C120O occur at potentials that are close to that for the reduction of C60 to [C60]- and it appears that the two electrons are added successively to each of the two fullerene cages. Similarly, the second two reductions of C120O occur at potentials that are close to the reduction potential for the conversion of [C60]- to [C60]2-. Electron paramagnetic resonance spectra have been observed for both [C120O]2- and [C120O]4-. For [C120O]2- the EPR spectrum in o-dichlorobenzene at 77 K consists of a doublet-like feature (g ) 2.0016) that is surrounded by features characteristic of a triplet state (g ) 2.0030). It is concluded that for [C120O]2- the spectrum indicates that there is significant interaction between the two electrons that have been added to the fullerene and that the species is not a diradical.
Introduction The electrochemistry of fullerenes and their derivatives has received considerable attention, and it is well established that C60 undergoes six reversible, one-electron reduction processes.1 The addition of both organic and inorganic groups to the fullerene framework generally leads to negative shifts in the reversible redox potentials of the derivatized fullerene relative to C60 itself,2 and the magnitude of this cathodic shift increases as the number of addends increases.3,4 Recently, this laboratory has demonstrated that the reductive electrochemistry of the fullerene epoxide, C60O, is remarkable in that it undergoes polymerization upon reduction to form tough, redox-active films that adhere to a variety of electrode surfaces.5,6 A related electrochemical polymerization also occurs during the reduction of a dihydromethanofullerene.7 In this report, the study of the electrochemistry of fullerene oxides has been extended to include C120O, which has recently been reported to form from the thermal reaction of C60 with C60O.8,9 The proposed structure for C120O, which involves a direct C-C bond between fullerene units, is shown in Figure 1. Related structural elements might be present in the fullerene polymer obtained from C60O. This study was undertaken in part to see if C120O might function as another precursor to polymeric fullerene films. Previous studies of fullerene dimers with phenylene10 and acetylene4 spacers connecting methanofullerenes have revealed that the two fullerene cores act as noninteracting units11 in their electrochemical behavior. Similarly it has been shown that methanofullerenes incorporated into polyester and polyurethane polymers retain their redox characteristics but again are present as noninteracting units.12 The proximity of the two C60 units in C120O suggests that the fullerene units in this dimer-like molecule may be more strongly interacting. Results Electrochemical Studies. Cyclic voltammetry (cv), differential pulse voltammetry (dpv), and chronoamperometry (ca) were used to study the electrochemistry of C120O. In Figure 2 † On leave from the Institute of Chemistry, University of Warsaw, Bialystok Branch. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4823$12.00/0
Figure 1. Energy minimized structure for C120O.
the cv and dpv curves recorded in o-dichlorobenzene solutions of C60 and C120O are compared. Three reversible, one-electron reduction steps are observed for C60 in the potential range from 0 to -2500 mV. The electrochemistry of C120O in the same potential range is more complicated. In the cv trace, two asymmetric peaks (R1 and R2) are observed along with a clearly resolved pair of peaks R3′ and R3′′. Analysis of the dswv curves for this potential window indicates that R1 and R2 are each the result of the superposition of two processes with relatively close standard potentials. Further, the cv curves indicate that the separation in the peak potentials of these reduction pairs increases as more electrons are added to the system. To assess the potential of C120O as a precursor to electropolymerized fullerene films, multicyclic voltammetry experiments were performed. No polymer formation was observed in electrochemical studies in o-dichlorobenzene, and unlike the case of C60O,5 no polymer formation was observed for a toluene/ acetonitrile solution of C120O. It seems reasonable that the formation of C120O-type linkages may serve as the chainterminating step in polymerization initiated by C60O, but we conclude that C120O itself does not undergo polymerization. The cyclic voltammetric curves of C120O were deconvoluted according to the procedure proposed by Dalrymple-Alford et al.13 Deconvolution involved the semidifferentiation of current with respect to time (which is correlated with the electrode potential) and allowed the transformation of the nonsymmetrical linear scan voltammetry peaks into symmetrical peaks. This procedure gives better resolution of multicomponent signals and © 1996 American Chemical Society
4824 J. Phys. Chem., Vol. 100, No. 12, 1996
Balch et al. TABLE 1: Comparison of the Formal Redox Potentials (in mV) of C60 and C120O Reduction Processes in o-Dichlorobenzene C60
Figure 2. (a) Cyclic voltammogram of 0.35 mM C60 in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). (b) Cyclic voltammogram (solid line) and differential pulse voltammogram (broken line) of 0.19 mM C120O in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). Sweep rate, 100 mV/s (cv) and 20 mV/s (dswv); pulse amplitude, 50 mV; sample width, 17 ms; pulse width, 50 ms; pulse period, 200 ms; quiet time, 2 s.
C120O
R1
-864
R2
-1260
R3
-1718
R1′ R1′′ R2′ R2′′ R3′ R3′′
-836 -875 -1238 -1299 -1822 -1960
Figure 4. Convoluted cyclic voltammetric curve for 0.19 mM C120O in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). Sweep rate, 100 mV/s.
indicates that these electrode processes are electrochemically reversible in both C60 and C120O. The small differences in the potentials of anodic and cathodic peaks reflects the partial compensation of the solution resistance. The formal potentials for the C60 and C120O reduction processes are summarized in Table 1.
Figure 3. Deconvoluted cyclic voltammetric curves of (a) 0.35 mM C60 and (b) 0.19 mM C120O in o-dichlorobenzene with 0.1 M tetra(nbutyl)ammonium perchlorate as supporting electrolyte at a gold electrode (0.75-mm radius). Sweep rate, 100 mV/s.
higher sensitivity than dc voltammetry. Figure 3 shows the deconvoluted cv curves for both C60 and C120O. Because of the relatively low concentration of fullerenes in the solution, and the relatively high background current, it was necessary to subtract the background current prior to deconvolution. The close agreement between the peak current and potentials for the cathodic and anodic segments of the deconvoluted cv curves
To estimate the number of electrons involved in each of the reduction processes for C120O, the cv curves presented in Figure 2 were convoluted by use of the algorithm of Grenness and Oldham14 (see Figure 4). The convolution involved semiintegration of the faradic current with respect to time. The steady-state voltammetric curves are obtained as a result of this operation. For C60 the convolution yielded three waves of equal intensity which represent three one-electron reductions. For C120O the heights of first two waves are almost the same (the ratio of the height of the R2 to the R1 wave is 0.95), which indicates that the number of electrons transferred in each step is the same. The limiting currents for R3′ and R3′′ are almost the same, but each is about one-half the limiting current of the waves R1 and R2. Given the diffusion coefficient (D) and the reactant concentration (co), the number of electrons (n) involved in a given electrode process can be calculated from the limiting convoluted current (m1) on the basis of the following equation:15
m1 ) nFAD1/2co
(1)
The diffusion coefficient of C120O was determined experimentally by using chronoamperometry. For an electrode process
Electronic Communication in Fullerene Dimers
J. Phys. Chem., Vol. 100, No. 12, 1996 4825
Figure 5. Dependence of it1/2 on the square root of the time obtained on the analysis of the chronoamperometric curve of 0.19 mM C120O in o-dichlorobenzene with 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte at a mercury hemispheric ultramicroelectrode (49.6 ( 0.6 µm diameter).
controlled by spherical diffusion, the limiting current is described by the following equation:16
i ) nFAcoD{1/(πDt)1/2 + 1/ro}
(2)
where A is the surface area of the electrode, t is the electrolysis time, and ro is the radius of the electrode. It can be shown that a linear relation between it1/2 and t1/2 can be expected for an electrode process controlled by spherical diffusion. The requirement of spherical diffusion is satisfied by the use of ultramicroelectrodes. The linear relation between it1/2 and t1/2 obtained for C120O reduction at a mercury hemispherical ultramicroelectrode at -1200 mV is presented in Figure 5. Extrapolation of the straight line to it1/2 ) 0 allows one to determine the value of t* for use in the following equation:
D ) ro2/πt*
(3)
The time, t*, can also be calculated as the square root of the ratio of the intercept to the slope of the plot presented in Figure 5. The average value (nine experiments) for the diffusion coefficient of C120O in o-dichlorobenzene obtained in this way is (3.42 ( 0.23) × 10-6 cm2/s. On the basis of the experimentally obtained diffusion coefficient and the analytical C120O concentration, the number of electrons (n) associated with the R1 and R2 waves is 1.85 and 1.8, respectively. These values were confirmed by calculating n from the heights of the voltammetric peaks which produced values of 1.75 and 1.7 for processes R1 and R2, respectively. The lower values for n based on voltammetric peak heights reflect the fact that both voltammetric peaks are the result of the overlap of two reduction processes with similar formal redox potentials. Values of n of 1.1 and 1.3 have been obtained for the R3′ and R3′′ waves, respectively. These results suggest that the reduction processes R1, R2, and R3 each represent two individual electron transfers to the C120O molecule. The reduction potential of the second peak in each pair becomes increasingly separated from the first peak as one proceeds down the sequence R1, R2, R3. Electron Paramagnetic Resonance Data. EPR data have been obtained for C120O in o-dichlorobenzene solution with 0.1 M tetra(n-butyl)ammonium perchlorate as electrolyte after reduction at -1.30 V where [C120O]2- is formed and at -1.65 V where [C120O]4- is formed. A solution of [C120O]2- when
Figure 6. EPR spectrum of [C120O]2- in frozen o-dichlorobenzene solution at 77 K. A, entire spectrum; B, expansion to magnify the flanking triplet features.
frozen at 77 K gives the spectrum shown in Figure 6. The spectrum has a marked similarity to the spectrum observed for [C60]2-.17-19 An intense central feature at g ) 2.0016 with a peak to peak separation of 1.1 G is observed, which is due to an apparent spin doublet species. Clearly defined flanking features are characteristic of a spin triplet species20 with g ) 2.0030, and D ) 13 G. However, no half-field feature could be observed, but other cases of triplet species that lack halffield features in their EPR spectra are known.17 Moreover, simulation of the experimental spectrum indicates that, with the low D value, the half-field feature will have insufficient intensity to be observable. Like the case with [C60]2-, increasing the microwave power brings about a diminution in the intensity of the central feature due to its saturation. The EPR spectrum of a sample of [C120O]4- taken under conditions analogous to those used to obtain the spectrum of [C120O]2- is shown in Figure 7. The spectrum is grossly similar to that of [C120O]2-, but variations of g values and linewidths indicate that this spectrum is distinct from that of [C120O]2-. For [C120O]4- a central feature with g ) 2.0025 and a peak to peak width of 1.3 G is observed along with four features that appear to arise from a triplet state with g ) 2.0037. Another notable feature of this spectrum is the observation that it is only one-tenth as intense as that of [C120O]2-. The appearance of the EPR spectrum of [C120O]4- suggests that both apparent spin doublet and spin triplet features are present with the intense central line representing the apparent spin doublet state. Discussion The results indicate that C120O undergoes sequential reversible reductions. The closely spaced waves in R1 represent reduction to form [C120O]- and [C120O]2-, the waves in R2 involve reduction to form [C120O]3- and [C120O]4-, and the wellresolved waves in R3 and R3′ represent reduction to form [C120O]5- and [C120O]6-. It is noteworthy that the separation between the two components of each process increases in the order R1, R2, and R3. This increase in separation is likely to result from the effects of increasing Coulombic repulsion. The proximity of the reduction potentials for process R1 to the first reduction potential for C60 (to form [C60]-) suggests
4826 J. Phys. Chem., Vol. 100, No. 12, 1996 that the electrons are added first to one C60 moiety and then to the second C60 moiety within the C120O molecule. Similarly, the process R2 probably results in addition of an electron to one C60- unit and then to the second C60- unit of the C120O molecule. Previous studies of molecules containing two fullerene units connected by covalent links have not been able to resolve independent reduction waves for each component. In the case of p- and m-phenylene-linked methanofullerenes, three reversible, two-electron reduction processes were observed.10 For a diacetylene-linked methanofullerene, an initial irreversible twoelectron reduction was observed.4 This was followed by two waves that were ascribed to resolved one-electron reductions. However, because of the irreversibility of the preceding process, further study is necessary to identify the nature of the successive electrode reactions. The observation of some degree of resolution of individual one-electron steps in the electrochemistry of C120O indicates that we are dealing with an interacting pair of fullerene moieties. For noninteracting redox-active units, the electrochemistry is expected to consist of unresolved multielectron processes that occur at the potential expected for any one component.11 Such behavior has been observed for remotely linked ferrocenes.11 However, true electronic interaction can result in separation of individual one-electron steps as seen for directly linked ferrocenes21 and here for C120O. For fullerenes, however, it is remarkable that such a close linkage as shown in Figure 1 results in such small separation of individual singleelectron processes. The EPR spectrum for [C120O]2- (Figure 6) shows characteristics that suggest electronic interaction between the two fullerene moieties. If this species acted as a diradical with independent C60 cages, a spectrum similar to that of [C60]would be expected. At 77 K the EPR spectrum of [C60]consists of a single sharp line at g ) 1.997 and a linewidth of 5 G.17,22 In contrast [C120O]2- displays both doublet and triplet EPR features with higher g values. These features resemble those of [C60]2-.17-19 In particular the appearance of the triplet features indicate the existence of a state with significant interaction between the two electrons. If these are located primarily on the two separate fullerene cages, the most obvious means of communication would come about between π-π overlap in the region where the two cages are held closest. Experimental Section Reagents. C60 was purchased from MER Corp. (Tucson, AZ) and used without additional purification. C60O was prepared by a literature method.23 C120O was prepared by dissolving 10 mg of freshly prepared C60O and 100 mg of C60 in 25 mL of CS2 followed by evaporation of the carbon disulfide to yield a homogeneous mixture of C60 and C60O. The solid fullerene mixture was heated under vacuum at 200 °C for 12 h, dissolved in o-dichlorobenzene, and separated by HPLC using a Regis Buckyclutcher 1 column. The material used in this study produced only a single, symmetrical peak when subjected to analytical HPLC using either a Regis Buckyclutcher 1 or a Cosmosil Buckyprep column. Tetra(n-butyl)ammonium perchlorate (TBAP) (Sigma Chemical Co.) was dried under reduced pressure at 70 °C for 24 h. The o-dichlorobenzene (Aldrich Chemical Co., Inc.) was purified by distillation under argon atmosphere over calcium hydride. Apparatus. Electrochemical experiments were performed using the BAS 1234 system in a three-electrode cell. The working electrode was a gold wire (Bioanalytical Systems) with a diameter of 1.5 mm. In chronoamperometric experiments a hemispherical mercury ultramicroelectrode deposited electro-
Balch et al.
Figure 7. EPR spectrum of [C120O]4- in frozen o-dichlorobenzene solution at 77 K.
chemically on the gold disk with a 49.6 ( 0.6 µm diameter (Goodfellow Metal Ltd.) sealed into a soft glass capillary served as a working electrode. The diameter of the gold electrode was measured by optical microscopy. Before each experiment, the electrode was polished sequentially with fine carborundum paper and a 0.5-µm alumina slurry. Then the electrode was sonicated in order to remove traces of alumina from the gold surface, washed with water, and air dried. A silver wire immersed in 0.01 M silver perchlorate and 0.09 M TBAP in acetonitrile and separated from the working solution by a ceramic tip (Bioanalytical Systems) served as the reference electrode. All potentials were normalized and are reported with respect to the ferrocene/ ferrocinium redox potential, which was established by cyclic voltammetry after the addition of ferrocene to the solution under investigation. The counter electrode was a platinum tab with an area of ∼0.5 cm2. A large platinum electrode (1 cm2) was used as the working electrode in bulk electrolysis experiments. In these studies the counter electrode was immersed in a solution containing the supporting electrolyte, which was separated from the working solution by a ceramic tip. EPR experiments were performed on samples of C60 and C120O, which were prepared and transferred to EPR tubes in an inert atmosphere drybox and immediately frozen in liquid nitrogen. The X-band spectra wer recorded at 110 K on a Varian E4 spectrometer that was equipped with a V-4540 variable-temperature controller. For the spectra in Figures 6 and 7 the spectrometer frequency was 9.067 GHz and the modulation was 0.5 G. Acknowledgment. We thank the National Science Foundation (Grant CHE 9321257 to A.L.B.) and the Office of Naval Research (to W.R.F.) for support, Dow Corporation for a fellowship for D.A.C., and Dr. A. Ozarowski and Prof. A. H. Maki for helpful discussions. References and Notes (1) (a) Xie, Q.; Perez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992, 114, 3978. (b) Ohsawa, Y.; Saji, T. J. Chem. Soc, Chem. Commun. 1992, 781. (c) Fhou, F.; Jehoulet, C.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 11004. (2) (a) Koefod, R. S.; Hudgens, M. F.; Shapley, J. R. J. Am. Chem. Soc. 1991, 113, 8957. (b) Prato, M.; Suzuki, T.; Foroudian, H.; Khemani, K.; Wudl, F.; Leonetti, J.; Little, R. D.; White, T.; Rickborn, B.; Yamago, S.; Nakamura, E. J. Am. Chem. Soc. 1993, 115, 2594. (c) Suzuki, T.; Maruyama, Y.; Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1994, 116, 1359. (d) Evans, D. H.; Lerke, S. A. Recent AdVances in the Chemistry and Physics of Fullerenes and Related Materials; The Electrochemical Society: Pennington, NJ, 1994; p 1087.
Electronic Communication in Fullerene Dimers (3) (a) Suzuki, T.; Li, Q.; Khemani, K. C.; Wudl, F.; Almarsson, O ¨. Science 1991, 254, 1186. (b) Lerke, S. A.; Parkinson, B. A.; Evans, D. H.; Fagan, P. J. J. Am. Chem. Soc. 1992, 114, 7807. (c) Balch, A. L.; Cullison, B.; Fawcett, W. R.; Ginwalla, A.; Olmstead, M. M.; Winkler, K. J. Chem. Soc., Chem. Commun. 1995, 2287. (4) Boudon, C. Gisselbrecht, J.-P.; Gross, M.; Isaacs, L.; Anderson, H. L.; Faust, R.; Diederich, F. HelV. Chem. Acta 1995, 78, 1334. (5) Fedurco, M.; Costa, D. A.; Balch, A. L.; Fawcett, W. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 194. (6) Winkler, K.; Costa, D. A.; Balch, A. L.; Fawcett, W. R. J. Phys. Chem. 1995, 99, 17431. (7) Anderson, H. L.; Boudon, C.; Diederich, F.; Gisselbrecht, J.-P.; Gross, M.; Seiler, P. Angew Chem., Int. Ed. Engl. 1994, 33, 1628. (8) Lebedkin, S.; Ballenweg, S.; Gross, J.; Taylor, Kra¨tschmer, W. Tetrahedron Lett. 1995, 36, 4971. (9) Smith, A. B., III; Toyuyama, H.; Strongin, R. M.; Furst, G. T.; Romanow, W. J.; Chait, B. T.; Mirza, U. A.; Haller, I. J. Am. Chem. Soc. 1995, 117, 9359. (10) Suzuki, T.; Li, Q.; Khemani, K. C.; Wudl, F.; Almarson, O ¨ J. Am. Chem. Soc. 1992, 114, 7300. (11) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248. (12) Shi, S.; Khemani, K. C.; Li, Q. C.; Wudl, F. J. Am. Chem. Soc. 1992, 114, 10656. (13) Dalrymple-Alford, P.; Goto, M.; Oldham, K. B. J. Electroanal. Chem. 1977, 85, 1.
J. Phys. Chem., Vol. 100, No. 12, 1996 4827 (14) Grenness, M.; Oldham, K. B. Anal. Chem. 1972, 44, 1121. (15) Oldham, K. B. Anal. Chem. 1972, 44, 196. (16) Galus, Z. Fundamentals of Electrochemical Analysis, 2nd rev. ed.; Ellis Horwood: New York, 1994; p 190. (17) Dubois, D.; Jones, M. T.; Kadish, K. M. J. Am. Chem. Soc. 1992, 114, 6446. (18) Khaled, M. M.; Carlin, R. T.; Trulove, P. C.; Eaton, G. R.; Eaton, S. S. J. Am. Chem. Soc. 1994, 116, 3465. (19) 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. (20) Ayscough, P. B. Electron Spin Resonance in Chemistry; Methuen: London, 1967; p 401. (21) LeVanda, C.; Bechgaard, K.; Cowan, D. O.; Rausch, M. D. J. Am. Chem. Soc. 1977, 99, 2964. (22) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 4364. 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. Kato, T.; Kodama, T.; Oyama, M.; Okasaki, S.; Shida, T.; Nakagawa, T.; Matsui, Y.; Susuki, S.; Shiromaru, H.; Yamaguchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 186, 35. (23) Balch, A. L.; Costa, D. A.; Noll, B. N.; Olmstead, M. M. J. Am. Chem. Soc. 1995, 117, 8926.
JP953144M
