Ž .Microchemical Journal 72 2002 115�122

Study of electrochemical properties ofpyrrolidinofullerenes by microelectrode voltammetry

Min Wei, Hongxia Luo, Nanqiang Li�, Sheng Zhang, Liangbing GanDepartment 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 goldmicrodisk electrode. Four reversible reduction peaks and two irreversible reduction peaks are observed for eachfullerene derivative. The half-wave potentials of all pyrrolidinofullerenes are more negative than those of C itself.60The diffusion coefficient of these compounds is measured by their steady-state voltammograms. � 2002 ElsevierScience B.V. All rights reserved.

Keywords: Electrochemical properties; Pyrrolidinofullerenes; Microelectrode voltammetry

1. Introduction

The electrochemistry of fullerenes has beenone of the most intensely studied aspects offullerene chemistry. The electrochemical proper-

� � � �ties of C 1�4 , several higher fullerenes 5,6 ,60and numerous fullerene derivatives have been

� �reported 7�18 . The discovery has indicated thatC is electron deficient and can act as an elec-60trophile, which led to follow-up studies on thereactions of C with various nucleophiles and60opened up ways for the derivatization of C .60

� Corresponding author. Fax: �86-10-62751708.Ž .E-mail address: imwx@chem.pku.edu.cn N. Li .

Driven by visions of interesting new materials ofspecific electronic and optical properties, investi-gators tried to determine how the nature, geome-try, structure and number of addends influencethe electrochemical behavior of this fullerene. In1991, Wudl’s group reported that C can be60

derived with organic groups while maintaining its� �unique electronic properties 19 . In 1994, Suzuki

� �et al. 20 reported a more comprehensive ac-count of the effect of derivatization of C on the60

electrochemical behavior. After that, Boudon et� � � �al. 21 and Guldi et al. 22 reported that reduc-

tions become increasingly more difficult and irre-versible as the fullerene cage becomes increas-ingly functionalized. In this field, Echegoyen and

0026-265X�02�$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 2 6 - 2 6 5 X 0 1 0 0 0 9 6 - 0

( )M. Wei et al. � Microchemical Journal 72 2002 115�122116

Scheme 1. Pyrrolinofullerenes 1�9.

� �co-workers have also done much work 23�28 .They reported six cathodic waves of somefullerene derivatives by extending the potential

� �window to more negative values 23 . This hasbeen explained in terms of a stepwise loss ofconjugation, which causes the LUMO to becomeincreasingly higher in energy.

Because most of the fullerene derivatives areonly soluble in organic solvents whose resistanceis high, the microelectrode is thus used to elimi-nate the IR drop effect due to the very low cell

� �current. In our previous work 29 , five com-pounds of pyrrolidinofullerenes were studied bycyclic voltammetry, and their electroreductionpeak potentials shifted negatively with respect tothe potential of the corresponding C cage. In60

� �1998, Prato and co-workers 30 reported that afamily of N-methylpyrrolidinium fullerene iodidesalts shows enhanced electron-accepting proper-ties with respect to both the parent fulleropyrro-

� �lidines and C . Kutner et al. 31 reported the60effects of alkyl chain length and protonation ofthe pyrrolidine nitrogen on the electrochemical

Ž .behavior of 2- n-alkyl fulleropyrrolidines in solu-tions as well as in thin solid films. In this work,another nine types of new pyrrolidinofullerenesŽ .1�9, Scheme 1 were synthesized, and their elec-trochemical properties were studied on an Aumicrodisk electrode, in order to learn more aboutthe effect of derivatization of C on the elec-60trochemical behavior.

2. Experimental

Cyclic voltammetric measurements were per-formed on an EG & G PAR 273 potentio-stat�galvanostat with Model 270 electrochemical

Žsoftware at higher potential scan rate �100 mV�1 .s ; and an EG&G PAR Model 174 polaro-

graphic analyzer with type 3086 x�y recorder atthe scan rate of 5 mV s�1. A conventional three-electrode electrochemical cell with a 24-�m di-ameter Au microdisk as the working electrode, aPt electrode as the counter electrode, and an Agwire coated with AgCl as a pseudo-referenceelectrode was employed. The potential was cali-brated with the ferrocene couple, which is re-ferred to as Fc�Fc�.

The supporting electrolyte, tetra-n-butylam-�Ž . �monium hexafluorophosphate n-Bu N PF was4 6

purchased from Sigma. Toluene and acetonitrilewere distilled from P O prior to use. All other2 5reagents were of analytical grade. All measure-ments were carried out under nitrogen in a mixed

Ž .solution of toluene and acetonitrile 4:1, v�vcontaining 0.1 M n-Bu NPF . The concentration4 6of the substrate was 1 mM. The temperature wascontrolled by an ice-salt bath.

The synthesis and characterization of some ofthese pyrrolidinofullerenes have been published32. The others will be submitted in the nearfuture. The following describes the characteriza-tion of compound 7 as an example.

1 Ž . ŽH-NMR 200 Hz, CDCl : � 1.30�1.80 m,3. Ž . Ž .10H , 1.80�2.00 m, 2H , 2.32 t, 2H, J�7.2 Hz ,Ž . Ž . Ž .2.82 t, 2H, J�7.2 Hz , 3.68 s, 3H 3.96 s, 6H ,Ž . 13 Ž .6.58 s, 2H . C-NMR 100 MHz, CDCl : �3

( )M. Wei et al. � Microchemical Journal 72 2002 115�122 117

Ž . Ž . Ž .174.19 CO , 172.41 CO , 168.99 COO , 153.56,150.17, 147.51, 146.41, 146.37, 146.15, 146.11,

Ž .145.74, 145.63, 145.55, 145.49 broad , 145.37,145.31, 144.46, 144.41, 144.25, 143.20, 143.13,

Ž142.74, 142.67, 142.27, 142.10, 142.06, 141.83 very.broad , 140.21, 139.62, 137.42, 134.09, 71.19

Ž 3. Ž . Ž . Ž .2C,SP , 70.20 2CH , 52.92 OMe , 51.40 OMe ,Ž . Ž . Ž .34.34 CH , 34.04 CH , 29.27 CH , 29.212 2 2

Ž . Ž . Ž . Ž .CH , 29.12 CH , 29.08 CH , 24.98 CH ,2 2 2 2Ž . Ž .24.90 CH . FT-IR microscope : 2947, 2924,2

2850, 1741, 1677, 1458, 1433, 1398, 1268, 1202,1172, 1056, 751, 578, 526, 480 cm�1. MALDI-TOFŽ . Ž � . Ž . Ž .m�z : 1078 42%, M �1 , 880 17% , 820 51% ,

Ž � .720 100%, C . UV-Vis: 255, 313, 431 nm.60Ž .Anal.Calcd. for C H O N� H O : C%84.37;77 27 7 2

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 C and compounds601�9

The cyclic voltammograms of C and com-60pounds 1�9 were obtained at the scan rate of 1 V

�1 Ž .s Fig. 1 . A mixed solvent of toluene and

Ž .acetonitrile 4:1, v�v was used throughout themeasurements because the solubility of pyrrolidi-nofullerenes is relatively high in this mixed sol-vent and a wide potential window can be ob-tained. Fig. 1 shows the cyclic voltammetric analy-

Ž .sis of compound 1 in toluene�MeCN 4:1, v�vsolvent at different reversion potentials. Fig. 1a�cdisplays the first two, three, and four well-behaved, reversible reduction peaks, respectively.Fig. 1d corresponds to a cyclic voltammogramwhere the potential scan was switched at a slightlymore negative value, and it shows the appearanceof a much smaller fifth peak without correspond-ing anodic peak. The anodic peaks correspondingto the first four redox processes remained un-changed even after scanning through the fifthcathodic peak. When the scan goes even morenegative, Fig. 1e, another reduction peak isobserved. However, after scanning this peak, thepeak currents of the four anodic peaks decreasegreatly. The results above indicate that compound1 exhibits four reversible reduction peaks and twoirreversible reduction peaks. To some extent, theresults are in agreement with Wudl and co-

� �workers 14 and the work of Echegoyen and� �co-workers 23 .

The CVs of C and compounds 2�9 are dis-60

Ž . Ž .Fig. 1. Cyclic voltammograms of compound 1 in toluene�MeCN 4:1 v�v containing 0.1 M n-Bu N PF at different reversion4 6Ž . Ž . Ž . Ž . Ž . �1potentials: a �1.9 V; b �2.3 V; c �2.7 V; d �3.1 V; e �3.5 V. Scan rate: 1 V s . Temperature: 0�C.

( )M. Wei et al. � Microchemical Journal 72 2002 115�122118

played in Fig. 2. For C , six successive step60reductions on the microdisk electrode were mea-sured. The first five steps are reversible reduc-tions, while the sixth one seems to be a EC�process. The results are in agreement with the

� �literature 33 . The electrochemical behavior ofcompounds 2�9 is similar to that of compound 1,that is, only four successive reversible reductionpeaks were obtained under the same conditions,while the fifth and sixth reduction peaks are irre-versible and almost imperceptible. The resultsindicate that the reduction ability of compounds1�9 is lower than that of C .60

Reduction potentials of C and pyrrolidino-60fullerenes 1�9 are listed in Table 1 relative to the

Ž �.internal ferrocene�ferricinium Fc�Fc couple.The CV curves for compounds 1�9 show that

reduction of all the derivatives occurs at morenegative potentials in comparison with unsubsti-tuted C , which is expected on the basis of60saturation of a double bond in C . On the other60hand, the potential values for these first fourpeaks differ slightly from one compound to theother, reflecting the expected differences in elec-tronic properties of the attached groups. A 0.04-Vnegative shift relative to C was observed for60compounds 1 and 2. With the increase of thecarbon chains, the reduction potentials of com-pound 3�7 become more negative, that is, �1.03V. The ending groups of compound 3�7 are dif-ferent from each other, but it seems that theyshow the same electronegativity. This is becausefield effects cause very little difference in this

Žsituation when the distance is far enough in a

Ž . Ž .Fig. 2. Cyclic voltammograms of C and compounds 2�9 in toluene�MeCN 4:1 v�v containing 0.1 M n-Bu N PF . Scan rate: 160 4 6V s�1 . Temperature: 0�C.

( )M. Wei et al. � Microchemical Journal 72 2002 115�122 119

Table 1�Ž . Ž .E values V, vs. Fc�Fc of C and compounds 1�9 in 4:1 mixed toluene and acetonitrile containing 0.1 M n-Bu N PF at the1�2 60 4 6

�1scan rate of 1 V s

0�1� 1��2� 2��3� 3��4�E E E E1�2 1�2 1�2 1�2

C �0.98 �1.41 �1.89 �2.37601 �1.02 �1.41 �1.99 �2.442 �1.02 �1.42 �2.00 �2.453 �1.03 �1.43 �1.99 �2.464 �1.03 �1.44 �2.01 �2.475 �1.03 �1.43 �2.00 �2.476 �1.03 �1.43 �1.99 �2.467 �1.03 �1.44 �2.00 �2.488 �1.04 �1.45 �2.02 �2.499 �1.05 �1.46 �2.03 �2.51

Ž cat an .Errors are estimated at �5 mV. E � E �E �2.1�2 peak peak

.bond four bonds away or more . When two estergroups attached to the pyrrole ring of compound3 are substituted by two hydrogen atoms, thereduction potential of compound 9 becomesslightly more negative. This is reasonable, forester is an electron-withdrawing group which leads

to a stronger electronegativity. A similar resultwas observed between compound 2 and 8.

According to the discussion above, it can beseen that the reduction potentials depend on the

� �electronegativity of the attached groups 20 . Thatis, the stronger the electronegativity, the more

Ž . Ž .Fig. 3. Steady-state voltammograms of C and compounds 1�8 in toluene�MeCN 4:1 v�v containing 0.1 M n-Bu N PF . Scan60 4 6rate: 5 mV s�1 . Temperature: 20�C.

( )M. Wei et al. � Microchemical Journal 72 2002 115�122120

positive the E value and the easier the reduc-1�2tion process.

The half-wave potentials remained unchangedfor these pyrrolidinofullerenes as the scan ratechanges from 0.05 to 20 V s�1. The first four-peakcurrent i is proportional to �1�2 indicates thatpthey are reversible diffusive waves. However, theshape of the fifth and the sixth peaks was dis-torted when increasing the scan rate, and thedependence between the peak current and thescan rate can not be determined.

3.2. The steady-state �oltammograms for C and60compounds 1�8

The steady-state voltammograms of C and60compounds 1�8 were measured at 20�C at thescan rate of 5 mV s�1, as shown in Fig. 3. It canbe seen from Fig. 3 that the steady-state currents

of C and these compounds were the shape of60steps, not peaks. Moreover, the similar steady-state currents were obtained at the same scanrate at different temperature.

Ž .The steady-state current i can be expressedL� �by 34

Ž .i �4nFDrc 1L

Where n is the number of electrons involved inthe electrode reaction, F is the Faraday constant,D is the diffusion coefficient, r is the radius ofthe disk electrode, and c is the bulk concentra-tion of the reaction.

� �The diffusion coefficient is given by 35

0 � � Ž .D�D exp �E �RT 2al

Where D0 is the diffusion coefficient at T�� ,

Fig. 4. Plots of i vs. 1�T of C and compounds 1�8.L 60

( )M. Wei et al. � Microchemical Journal 72 2002 115�122 121

Table 2Ž .The calculated diffusion coefficient of C and compounds 1�8 in 4:1 mixed toluene and acetonitrile containing 0.1 M n-Bu N PF60 4 6

0 Ž . Ž . Ž .E D D 0�C D 20�C D 25�Cal�1 2 �1 2 �1 2 �1Ž . Ž . Ž . Ž .kJ mol cm s cm s cm s

�3 �6 �6 �5C 13.10 2.01�10 6.27�10 9.30�10 1.02�1060�4 �6 �6 �61 9.89 3.77�10 4.84�10 6.51�10 6.97�10�3 �6 �6 �62 12.65 1.31�10 4.98�10 7.28�10 7.04�10�4 �6 �6 �63 10.73 4.60�10 4.07�10 5.61�10 6.05�10�3 �6 �6 �64 13.14 1.42�10 4.35�10 6.45�10 7.06�10�3 �6 �6 �65 13.46 1.73�10 4.60�10 6.89�10 7.56�10�4 �6 �6 �66 11.41 7.65�10 5.03�10 7.08�10 7.66�10�3 �6 �6 �67 13.14 1.44�10 4.40�10 6.53�10 7.15�10�4 �6 �6 �68 11.48 6.35�10 4.05�10 5.71�10 6.18�10

E is the diffusion activity energy, R is the gasalconstant, and T is the thermodynamic tempera-

Ž . Ž .ture. Substituting Eq. 2 into Eq. 1 , yields:

0 Ž .ln i � ln4nF D rc�E �RT 3L al

The plot of ln i as a function of 1�T isLtherefore linear. The value of E can be obtainedalfrom the slope of the straight line, and the valueof D0 can be obtained from the intercept on they-axis. The steady-state voltammograms wererecorded at different temperatures, and i valuesLwere measured from the first reduction steps ofvarious compounds. Fig. 4 shows the plots of ln iLvs. 1�T of C and compounds 1�8, in which60compound 9 is not included, because it is poorlysoluble in the mixed solvent, the accurate concen-tration can not be determined.

The calculated diffusion coefficient of C and60compounds 1�8 are listed in Table 2. The diffu-sion coefficient of C determined in this work is60not the same as the value in the literature� �2,33,36 , this might be due to the different sol-vents which have a great effect on diffusion co-

� �efficient 2 . As shown in Table 2, the D value ofeach pyrrolidinofullerene is smaller than that ofC , showing that the diffusion coefficient de-60creases when C is substituted with other groups.60According to the equation of Stokes�Einstein,

� �D�kT�6� r 37 , the diffusion coefficient is in-versely proportional to the radius of the moleculeor the ion. This can be easily understood becausea larger molecule leads to lower transnationalmobility.

4. Conclusion

The electrochemical behavior of nine pyrrolidi-nofullerenes has been investigated by cyclic vol-tammetry on a gold microdisk electrode. Fourreversible reduction peaks and two irreversiblereduction peaks are observed for each fullerenederivatives. The half-wave potentials of all pyrro-lidinofullerenes are more negative than that ofC itself. The ability of accepting electrons of60pyrrolidinofullerenes is influenced by the elec-tronegativity of the attached groups. The diffu-sion coefficient of these compounds measured bytheir steady-state voltammograms is smaller thanthat of C .60

Acknowledgements

This project was supported by the NationalŽNatural Science Foundation of China No.

.29835110 .

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