Study of electrochemical properties of pyrrolidinofullerenes by microelectrode voltammetry

Microchemical Journal 72 Ž2002. 115᎐122

Study of electrochemical properties of pyrrolidinofullerenes by microelectrode voltammetry Min Wei, Hongxia Luo, Nanqiang LiU , Sheng Zhang, Liangbing Gan Department of Chemistry, Peking Uni¨ ersity, Beijing 100871, PR China Received 7 March 2001; received in revised form 6 June 2001; accepted 8 June 2001

Abstract The electrochemical behavior of nine pyrrolidinofullerenes has been investigated by cyclic voltammetry on a gold microdisk electrode. Four reversible reduction peaks and two irreversible reduction peaks are observed for each fullerene derivative. The half-wave potentials of all pyrrolidinofullerenes are more negative than those of C 60 itself. The diffusion coefficient of these compounds is measured by their steady-state voltammograms. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical properties; Pyrrolidinofullerenes; Microelectrode voltammetry

1. Introduction The electrochemistry of fullerenes has been one of the most intensely studied aspects of fullerene chemistry. The electrochemical properties of C 60 w1᎐4x, several higher fullerenes w5,6x, and numerous fullerene derivatives have been reported w7᎐18x. The discovery has indicated that C 60 is electron deficient and can act as an electrophile, which led to follow-up studies on the reactions of C 60 with various nucleophiles and opened up ways for the derivatization of C 60 . U

Corresponding author. Fax: q86-10-62751708. E-mail address: [email protected] ŽN. Li..

Driven by visions of interesting new materials of specific electronic and optical properties, investigators tried to determine how the nature, geometry, structure and number of addends influence the electrochemical behavior of this fullerene. In 1991, Wudl’s group reported that C 60 can be derived with organic groups while maintaining its unique electronic properties w19x. In 1994, Suzuki et al. w20x reported a more comprehensive account of the effect of derivatization of C 60 on the electrochemical behavior. After that, Boudon et al. w21x and Guldi et al. w22x reported that reductions become increasingly more difficult and irreversible as the fullerene cage becomes increasingly functionalized. In this field, Echegoyen and

0026-265Xr02r$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 6 - 2 6 5 X Ž 0 1 . 0 0 0 9 6 - 0

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lidines and C 60 . Kutner et al. w31x reported the effects of alkyl chain length and protonation of the pyrrolidine nitrogen on the electrochemical behavior of 2-Ž n-alkyl.fulleropyrrolidines in solutions as well as in thin solid films. In this work, another nine types of new pyrrolidinofullerenes Ž1᎐9, Scheme 1. were synthesized, and their electrochemical properties were studied on an Au microdisk electrode, in order to learn more about the effect of derivatization of C 60 on the electrochemical behavior.

2. Experimental

Scheme 1. Pyrrolinofullerenes 1᎐9.

co-workers have also done much work w23᎐28x. They reported six cathodic waves of some fullerene derivatives by extending the potential window to more negative values w23x. This has been explained in terms of a stepwise loss of conjugation, which causes the LUMO to become increasingly higher in energy. Because most of the fullerene derivatives are only soluble in organic solvents whose resistance is high, the microelectrode is thus used to eliminate the IR drop effect due to the very low cell current. In our previous work w29x, five compounds of pyrrolidinofullerenes were studied by cyclic voltammetry, and their electroreduction peak potentials shifted negatively with respect to the potential of the corresponding C 60 cage. In 1998, Prato and co-workers w30x reported that a family of N-methylpyrrolidinium fullerene iodide salts shows enhanced electron-accepting properties with respect to both the parent fulleropyrro-

Cyclic voltammetric measurements were performed on an EG & G PAR 273 potentiostatrgalvanostat with Model 270 electrochemical software at higher potential scan rate Ž) 100 mV sy1 .; and an EG &G PAR Model 174 polarographic analyzer with type 3086 x᎐y recorder at the scan rate of 5 mV sy1 . A conventional threeelectrode electrochemical cell with a 24-␮m diameter Au microdisk as the working electrode, a Pt electrode as the counter electrode, and an Ag wire coated with AgCl as a pseudo-reference electrode was employed. The potential was calibrated with the ferrocene couple, which is referred to as FcrFcq. The supporting electrolyte, tetra-n-butylammonium hexafluorophosphate wŽ n-Bu 4 N.PF6 x was purchased from Sigma. Toluene and acetonitrile were distilled from P2 O5 prior to use. All other reagents were of analytical grade. All measurements were carried out under nitrogen in a mixed solution of toluene and acetonitrile Ž4:1, vrv. containing 0.1 M n-Bu 4 NPF6 . The concentration of the substrate was 1 mM. The temperature was controlled by an ice-salt bath. The synthesis and characterization of some of these pyrrolidinofullerenes have been published 32. The others will be submitted in the near future. The following describes the characterization of compound 7 as an example. 1 H-NMR Ž200 Hz, CDCl 3 .: ␦ 1.30᎐1.80 Žm, 10H., 1.80᎐2.00 Žm, 2H., 2.32 Žt, 2H, J s 7.2 Hz., 2.82 Žt, 2H, J s 7.2 Hz., 3.68 Žs, 3H. 3.96 Žs, 6H., 6.58 Žs, 2H.. 13 C-NMR Ž100 MHz, CDCl 3 .: ␦

M. Wei et al. r Microchemical Journal 72 (2002) 115᎐122

174.19 ŽCO., 172.41 ŽCO., 168.99 ŽCOO., 153.56, 150.17, 147.51, 146.41, 146.37, 146.15, 146.11, 145.74, 145.63, 145.55, 145.49 Žbroad., 145.37, 145.31, 144.46, 144.41, 144.25, 143.20, 143.13, 142.74, 142.67, 142.27, 142.10, 142.06, 141.83 Žvery broad., 140.21, 139.62, 137.42, 134.09, 71.19 Ž2C,SP 3 ., 70.20 Ž2CH., 52.92 ŽOMe., 51.40 ŽOMe., 34.34 ŽCH 2 ., 34.04 ŽCH 2 ., 29.27 ŽCH 2 ., 29.21 ŽCH 2 ., 29.12 ŽCH 2 ., 29.08 ŽCH 2 ., 24.98 ŽCH 2 ., 24.90 ŽCH 2 .. FT-IR Žmicroscope.: 2947, 2924, 2850, 1741, 1677, 1458, 1433, 1398, 1268, 1202, 1172, 1056, 751, 578, 526, 480 cmy1 . MALDI-TOF Ž mrz .: 1078 Ž42%, Mqq 1., 880 Ž17%., 820 Ž51%., . 720 Ž100%, Cq 60 . UV-Vis: 255, 313, 431 nm. Anal.Calcd. for C 77 H 27 O 7 N⭈ ŽH 2 O.: C%84.37; H%2.67; N%1.28; found: C%84.09; H%2.52; N%1.21.

3. Results and discussion 3.1. Cyclic ¨ oltammograms of C6 0 and compounds 1᎐9 The cyclic voltammograms of C 60 and compounds 1᎐9 were obtained at the scan rate of 1 V sy1 ŽFig. 1.. A mixed solvent of toluene and

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acetonitrile Ž4:1, vrv. was used throughout the measurements because the solubility of pyrrolidinofullerenes is relatively high in this mixed solvent and a wide potential window can be obtained. Fig. 1 shows the cyclic voltammetric analysis of compound 1 in toluenerMeCN Ž4:1, vrv. solvent at different reversion potentials. Fig. 1a᎐c displays the first two, three, and four wellbehaved, reversible reduction peaks, respectively. Fig. 1d corresponds to a cyclic voltammogram where the potential scan was switched at a slightly more negative value, and it shows the appearance of a much smaller fifth peak without corresponding anodic peak. The anodic peaks corresponding to the first four redox processes remained unchanged even after scanning through the fifth cathodic peak. When the scan goes even more negative, Fig. 1e, another reduction peak is observed. However, after scanning this peak, the peak currents of the four anodic peaks decrease greatly. The results above indicate that compound 1 exhibits four reversible reduction peaks and two irreversible reduction peaks. To some extent, the results are in agreement with Wudl and coworkers w14x and the work of Echegoyen and co-workers w23x. The CVs of C 60 and compounds 2᎐9 are dis-

Fig. 1. Cyclic voltammograms of compound 1 in toluenerMeCN Ž4:1 vrv. containing 0.1 M Ž n-Bu 4 N.PF6 at different reversion potentials: Ža. y1.9 V; Žb. y2.3 V; Žc. y2.7 V; Žd. y3.1 V; Že. y3.5 V. Scan rate: 1 V sy1 . Temperature: 0⬚C.

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played in Fig. 2. For C 60 , six successive step reductions on the microdisk electrode were measured. The first five steps are reversible reductions, while the sixth one seems to be a EC⬘ process. The results are in agreement with the literature w33x. The electrochemical behavior of compounds 2᎐9 is similar to that of compound 1, that is, only four successive reversible reduction peaks were obtained under the same conditions, while the fifth and sixth reduction peaks are irreversible and almost imperceptible. The results indicate that the reduction ability of compounds 1᎐9 is lower than that of C 60 . Reduction potentials of C 60 and pyrrolidinofullerenes 1᎐9 are listed in Table 1 relative to the internal ferrocenerferricinium ŽFcrFcq. couple. The CV curves for compounds 1᎐9 show that

reduction of all the derivatives occurs at more negative potentials in comparison with unsubstituted C 60 , which is expected on the basis of saturation of a double bond in C 60 . On the other hand, the potential values for these first four peaks differ slightly from one compound to the other, reflecting the expected differences in electronic properties of the attached groups. A 0.04-V negative shift relative to C 60 was observed for compounds 1 and 2. With the increase of the carbon chains, the reduction potentials of compound 3᎐7 become more negative, that is, y1.03 V. The ending groups of compound 3᎐7 are different from each other, but it seems that they show the same electronegativity. This is because field effects cause very little difference in this situation when the distance is far enough Žin a

Fig. 2. Cyclic voltammograms of C 60 and compounds 2᎐9 in toluenerMeCN Ž4:1 vrv. containing 0.1 M Ž n-Bu 4 N.PF6 . Scan rate: 1 V sy1 . Temperature: 0⬚C.

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Table 1 E1r 2 values ŽV, vs. FcrFcq . of C 60 and compounds 1᎐9 in 4:1 mixed toluene and acetonitrile containing 0.1 M Ž n-Bu 4 N.PF6 at the scan rate of 1 V sy1

C60 1 2 3 4 5 6 7 8 9

E1r 2 0r 1y

E1r21yr2y

E1r22yr3y

E1r23yr4y

y0.98 y1.02 y1.02 y1.03 y1.03 y1.03 y1.03 y1.03 y1.04 y1.05

y1.41 y1.41 y1.42 y1.43 y1.44 y1.43 y1.43 y1.44 y1.45 y1.46

y1.89 y1.99 y2.00 y1.99 y2.01 y2.00 y1.99 y2.00 y2.02 y2.03

y2.37 y2.44 y2.45 y2.46 y2.47 y2.47 y2.46 y2.48 y2.49 y2.51

cat an . Errors are estimated at "5 mV. E1r 2 s Ž Epeak q Epeak r2.

bond four bonds away or more.. When two ester groups attached to the pyrrole ring of compound 3 are substituted by two hydrogen atoms, the reduction potential of compound 9 becomes slightly more negative. This is reasonable, for ester is an electron-withdrawing group which leads

to a stronger electronegativity. A similar result was observed between compound 2 and 8. According to the discussion above, it can be seen that the reduction potentials depend on the electronegativity of the attached groups w20x. That is, the stronger the electronegativity, the more

Fig. 3. Steady-state voltammograms of C 60 and compounds 1᎐8 in toluenerMeCN Ž4:1 vrv. containing 0.1 M Ž n-Bu 4 N.PF6 . Scan rate: 5 mV sy1 . Temperature: 20⬚C.

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positive the E1r2 value and the easier the reduction process. The half-wave potentials remained unchanged for these pyrrolidinofullerenes as the scan rate changes from 0.05 to 20 V sy1 . The first four-peak current i p is proportional to ␯ 1r2 indicates that they are reversible diffusive waves. However, the shape of the fifth and the sixth peaks was distorted when increasing the scan rate, and the dependence between the peak current and the scan rate can not be determined. 3.2. The steady-state ¨ oltammograms for C6 0 and compounds 1᎐8 The steady-state voltammograms of C 60 and compounds 1᎐8 were measured at 20⬚C at the scan rate of 5 mV sy1 , as shown in Fig. 3. It can be seen from Fig. 3 that the steady-state currents

of C 60 and these compounds were the shape of steps, not peaks. Moreover, the similar steadystate currents were obtained at the same scan rate at different temperature. The steady-state current Ž i L . can be expressed by w34x i L s 4 nFDrc

Ž1.

Where n is the number of electrons involved in the electrode reaction, F is the Faraday constant, D is the diffusion coefficient, r is the radius of the disk electrode, and c is the bulk concentration of the reaction. The diffusion coefficient is given by w35x D s D 0 exp w yEal rRT x

Ž2.

Where D 0 is the diffusion coefficient at T ªA ,

Fig. 4. Plots of i L vs. 1rT of C 60 and compounds 1᎐8.

M. Wei et al. r Microchemical Journal 72 (2002) 115᎐122

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Table 2 The calculated diffusion coefficient of C 60 and compounds 1᎐8 in 4:1 mixed toluene and acetonitrile containing 0.1 M Ž n-Bu 4 N.PF6

C60 1 2 3 4 5 6 7 8

Eal ŽkJ moly1 .

D0

D Ž0⬚C. Žcm2 sy1 .

D Ž20⬚C. Žcm2 sy1 .

D Ž25⬚C. Žcm2 sy1 .

13.10 9.89 12.65 10.73 13.14 13.46 11.41 13.14 11.48

2.01= 10y3 3.77= 10y4 1.31= 10y3 4.60= 10y4 1.42= 10y3 1.73= 10y3 7.65= 10y4 1.44= 10y3 6.35= 10y4

6.27= 10y6 4.84= 10y6 4.98= 10y6 4.07= 10y6 4.35= 10y6 4.60= 10y6 5.03= 10y6 4.40= 10y6 4.05= 10y6

9.30= 10y6 6.51= 10y6 7.28= 10y6 5.61= 10y6 6.45= 10y6 6.89 = 10y6 7.08= 10y6 6.53= 10y6 5.71= 10y6

1.02= 10y5 6.97= 10y6 7.04= 10y6 6.05= 10y6 7.06= 10y6 7.56= 10y6 7.66= 10y6 7.15= 10y6 6.18= 10y6

Eal is the diffusion activity energy, R is the gas constant, and T is the thermodynamic temperature. Substituting Eq. Ž2. into Eq. Ž1., yields: ln i L s ln4 nF D 0 rc y EalrRT

Ž3.

The plot of ln i L as a function of 1rT is therefore linear. The value of Eal can be obtained from the slope of the straight line, and the value of D 0 can be obtained from the intercept on the y-axis. The steady-state voltammograms were recorded at different temperatures, and i L values were measured from the first reduction steps of various compounds. Fig. 4 shows the plots of ln i L vs. 1rT of C 60 and compounds 1᎐8, in which compound 9 is not included, because it is poorly soluble in the mixed solvent, the accurate concentration can not be determined. The calculated diffusion coefficient of C 60 and compounds 1᎐8 are listed in Table 2. The diffusion coefficient of C 60 determined in this work is not the same as the value in the literature w2,33,36x, this might be due to the different solvents which have a great effect on diffusion coefficient w2x. As shown in Table 2, the D value of each pyrrolidinofullerene is smaller than that of C 60 , showing that the diffusion coefficient decreases when C 60 is substituted with other groups. According to the equation of Stokes᎐Einstein, D s kTr6␲ r ␩ w37x, the diffusion coefficient is inversely proportional to the radius of the molecule or the ion. This can be easily understood because a larger molecule leads to lower transnational mobility.

4. Conclusion The electrochemical behavior of nine pyrrolidinofullerenes has been investigated by cyclic voltammetry on a gold microdisk electrode. Four reversible reduction peaks and two irreversible reduction peaks are observed for each fullerene derivatives. The half-wave potentials of all pyrrolidinofullerenes are more negative than that of C 60 itself. The ability of accepting electrons of pyrrolidinofullerenes is influenced by the electronegativity of the attached groups. The diffusion coefficient of these compounds measured by their steady-state voltammograms is smaller than that of C 60 .

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