Recent Advances in the Synthesis, Characterization, and Applications

Ind. Eng. Chem. Res. 2009, 48, 545–571

545

REVIEWS Recent Advances in the Synthesis, Characterization, and Applications of Fulleropyrrolidines Boris I. Kharisov,* Oxana V. Kharissova, Marco Jimenez Gomez, and Ubaldo Ortiz Mendez CIIDIT-UniVersidad Auto´noma de NueVo Leo´n, Monterrey, Me´xico

Recent publications on C60- and C70-fulleropyrrolidines are reviewed. Main attention is paid to a series of novel dyads and triads with porphyrins and their metal complexes, ferrocene derivatives, S-containing ligands, calixarenes, crown-ethers, polymers, and enzymes, as well as to relatively simple ligand systems. A certain attention is paid to fulleropyrrolidine polyadducts, multifullerenopyrrolidines, and functionalization of carbon nanotubes with pyrrolidine-containing moieties. Synthesis techniques, physicochemical methods to study the compounds obtained, and their main applications are examined. Representative examples for synthesis and applications are tabulated. Introduction 1

Buckminsterfullerene C60 was discovered in 1985, and the organic chemistry of fullerenes was successfully developed during the next 23 years. There were many successful methods of its functionalization, among which was the widely applied 1,3-dipolar cycloaddition of azomethine ylides. This method led to an important class of fullerene derivatives, pyrrolidino[3′,4′: 1,2][60]fullerenes (familiarly known as fulleropyrrolidines), intensively being developed mainly in the research groups of Prato and Martin. The main achievements in this area have been generalized in a series of monographs of Hirsch,2,3 Guldi,4 and other researchers,5-11 as well as in reviews; 12-20 additionally, synthetic methods and applications of fulleropyrrolidines are examined in a series of patents.21-31 Among this series of publications, we note an excellent recent review20 dedicated to new reactions in fullerene chemistry, including those of fuller1,6-enynes (involving the fulleropyrrolidine moiety) as new and versatile blocks in fullerene chemistry, and detailed discussion on retro-cycloaddition processes of fulleropyrrolidines. The simplest compound of fulleropyrrolidine type, N-methylfulleropyrrolidine, is shown in Figure 1. The classic synthetic procedure, leading to this fullerene derivative and its numerous analogues, is the 1,3-dipolar cycloaddition of azomethine ylides32 (reactive intermediates generated in situ by many different ways, in particular by decarboxylation of immonium salts, formed as a result of condensation of R-aminoacids with aldehydes) to C60 (Prato reaction). As a result of this powerful methodology for obtaining functionalized fullerene derivatives, the fulleropyrrolidines are formed, in which a pyrrolidine ring is fused to a junction between two six-membered rings of a fullerene sphere5 (Figure 1). When the reaction is carried out in the presence of large excesses of reagents, up to nine pyrrolidine rings can be introduced.15 The key features and strategies of this type of reaction are summarized in refs 5, 15, 17, and 33 in particular. Its main advantages are as follows: (a) the reactions lead to individual [6,6]-closed isomers, (b) majority of precursors are commercially available or could be easily prepared, and (c) two substituents can be simultaneously introduced into the pyrrolidine cycle. So, functionalization of the fullerene sphere on the Prato’s reaction basis occupies a leading place in the synthesis of fullerene derivatives to get new materials and potentially biologically active compounds. The physicochemical properties of the fulleropyrrolidines are expected to arise from the combination of the two molecular fragments, the fullerene and the pyrrolidine moiety. The * To whom correspondence should be addressed. E-mail: [email protected].

electronic properties of the fulleropyrrolidines are typical of most C60 monoadducts.13 This review is devoted to the most recent achievements (2000-2008) in the area of fulleropyrrolidines including, in some cases, their metallocomplexes. A definite attention will be paid to the functionalized carbon nanotubes containing pyrrolidine groups. Representative examples of the synthesis of fulleropyridines are shown in Table 1 and those of applications are shown in Table 2. Discussion Porphyrin-Containing Fulleropyrrolidines. Fullerene derivatives, containing porphyrins and/or their metallocomplexes (generally those of Zn,34,35 Mg, or Ru), continue to be one of the hottest topics in fullerene organic chemistry because of their many applications (see below), in particular for solar energy conversion. Among a series of publications in this area, we note the development of a new fullerene building block (N-pyridylfulleropyrrolidine, Figure 2, left), capable of forming axially symmetric complexes with metalloporphyrins and designed with the potential for defined geometry and good electronic communication.36 This compound was prepared by the Prato reaction from N-pyridylglycine, C60, and paraformaldehyde as precursors in o-dichlorobenzene (31% yield), but compared to the fullerene ligand, wherein the pyridine is connected via an insulating sp3 carbon atom, it is uncertain whether the pyridine N atom of N-pyridylfulleropyrrolidine really communicates with the fullerene core. “Tail-on” and “tail-off” binding mechanisms are examined for an axial ligand coordination in porphyrin-fullerene dyads.37 The donor-acceptor proximity is controlled either by temperature variation or by an axial ligand replacement method (Figure 3). It is observed that in the “tail-off” form the charge-separation efficiency changes to some extent in comparison with the results obtained for the “tail-on” form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C60 moiety in the “tail-off” form. Spectroscopic, redox, and electron transfer reactions of a selfassembled donor-acceptor dyad formed by axial coordination of magnesium meso-tetraphenylporphyrin (MgTPP) and fulleropyrrolidine appended with an imidazole-coordinating ligand (C60Im) were investigated by a series of methods.38 Spectroscopic studies revealed the formation of a 1:1 C60Im:MgTPP supramolecular complex (Figure 4). Among many other porphyrin-containing fulleropyrrolidines,39,40 the most interesting recently synthesized, in our point of view, are as follows: (i) the supramolecular assembly containing an electron donor, (ii)

10.1021/ie800602j CCC: $40.75  2009 American Chemical Society Published on Web 12/19/2008

546 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

Figure 1. Classic synthesis of N-methylfulleropyrrolidine.

Figure 2. N-pyridylfulleropyrrolidine (left), its ZnTPP complex (middle), and C-pyridylfulleropyrrolidine (right).

Figure 4. Magnesium meso-tetraphenylporphyrin-fulleropyrrolidine complex.

zinc 5,10,15,20-meso-tetraferrocenylporphyrin and a pyridinesubstituted fulleropyrrolidine as an electron acceptor (Figure 5),41 (iii) supramolecular ferrocene-porphyrin-fullerene constructions, in which covalently linked ferrocene-porphyrin-crown ether compounds are self-assembled with alkylammonium cation functionalized fullerenes,42 (iv) multimodular systems, composed of three covalently linked triphenylamine, entities at the

meso position of the porphyrin ring, and one fulleropyrrolidine at the fourth meso position (Figure 6),43 and (v) mixed assembly metal-free porphyrin (5-(3′-(2′′-(3′′′- or 4′′′-pyridyl)fulleropyrrolidinyl-N)ethoxyphenyl)-10,15,20-triphenylporphyrin)-ferrocene-metal porphyrin H2P-C60Py-ZnP (Figure 7).44

Figure 3. Temperature variation (left) or an axial ligand replacement (right) study of the donor-acceptor proximity in the porphyrin-fullerene dyad.

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 547

Figure 5. Supramolecular assembly of zinc 5,10,15,20-meso-tetraferrocenylporphyrin and pyridine-substituted fulleropyrrolidine

Figure 6. Multimodular systems, composed of three entities of triphenylamine, porphyrin, and fullerene.

Figure 8. Fulleropyrrolidines with dihexyloxynaphthalenethiophene (left) and dihexyloxybenzene-thiophene (right) moieties.

Figure 9. Bithiophene-fulleropyrrolidine.

Figure 7. Supramolecular triads of the type (donor-1)-acceptor-(donor2) composed of free-base porphyrin, fullerene, and zinc porphyrin. Figure 10. Dimeric fulleropyrrolidine-(S-donor)-fulleropyrrolidine triads.

In comparison with a lot of reported porphyrin-containing fulleropyrrolidines during the past decade, only a few investigations are devoted to the fulleropyrrolidines, containing phthalocyanine and its analogues,45-48 having possible applications for solar energy conversion.49,50 Fulleropyrrolidines with Sulfur-Containing Groups. The synthesis of thiophene-containing fulleropyrrolidines, covalently linked to fluorescent conjugated systems bearing long alkyl chains (Figure 8), attached to the naphthalene or benzene moieties, has been carried out from suitably functionalized oligomers by Prato reaction; the obtained products were studied by cyclic voltammetry.51

Among other studied fulleropyrrolidines, containing thiophene moieties, we note bithiophene-fulleropyrrolidine (Figure 9, yield 49%)52 and dimeric fullerene-donor-fullerene triads, containing an electron-rich pyrrole ring53 (Figure 10) in the donor moiety (for other multifulleropyrrolidines, see later). A direct palladation as a pathway toward fullerene SCS-based ([C6H2(CH2SPh)2-2,6-R-4]-) organometallic complexes is reported in ref 54. C60-modified polythiophene copolymers are shown in the section dedicated to polymer-containing fulleropyrrolidines. Highly conjugated tetrathiafulvalene analogues with the p-quinodimethane structure covalently attached to C60 (Figure


Figure 11. C60-pyrrolidine-tetrathiafulvene system.

11) were reported in ref 55. The molecular geometry of the formed compounds was calculated from semiempirical PM-3 calculations and reveals a highly distorted electron-donor moiety with a butterfly-type structure. A series of similar tetrathiafulvalene-containing C60-pyrrolidines,56 in particular dimeric compounds,57,58 are known, clearly showing that the tetrathiafulvalene moiety is one of the most used S-containing ligands in preparation of its fulleropyrrolidine dyads and triads (see also the section later dedicated to triads of ferrocene-containing fulleropyrrolidines, thiophenes, and tetrathiafulvene). Ferrocene-Containing Fulleropyrrolidines. Additionally to relatively simple C60-ferrocene-containing diads (Figure 12), intensively developed in the last decade of the 20th century,59 two types of the ferrocene-oligothiophene-fullerene triads (Figure 13, top), Fc-nT-C60 directly linking the ferrocene to the oligothiophene and Fc-tm-nT-C60 inserting a trimethylene spacer between the ferrocene and the oligothiophene, were synthesized60 to promote photoinduced charge separation for the oligothiophene-fullerene dyads (nT-C60). The physicochemical studies of Fc-nT-C60 indicate conjugation between the ferrocene and oligothiophene components and markedly quenched fluorescence of the oligothiophene in comparison to that observed for the dyads nT-C60. Authors stated that the additionally conjugated ferrocene evidently contributes to the stabilization of charge separation states, thus promoting intramolecular electron transfer; this fact is confirmed by the observation that the emission spectra of the nonconjugated triads Fc-tm-nT-C60 are essentially similar to the corresponding dyads nTC60. In

Figure 12. Fullerene-ferrocene diads.

another research,61 the ferrocene moiety is bound to the N-pyrrolidine atom through the CdO group, and tetrathiafulvalene is connected to the C-pyrrolidine atom (Figure 13, down). Among other combinations with ferrocene moieties, we note a series of molecular triads composed of ferrocene (through pyrrolidine C-binding), C60, and nitroaromatic entities (Npyrrolidine binding)R (R ) -CO-C6H3(3,5-NO2)2, -CH2C6H3(3,5-NO2)2, -C6H3(2,4-NO2)2, and -CO-C6H4(4-NO2)),62,63 photoactive fulleropyrrolidine-perylenetetracarboxylic diimideporphyrin triad and its zinc analogue,64 and much more.65-68 Physico-chemical properties of ferrocene-containing fulleropyrrolidines are intensively studied.69-71 Fulleropyrrolidine Bis- and Trisadducts. The synthesis, characterization, properties, and supramolecular organization of liquid-crystalline fullerene bisadducts and monoadduct (Figure 14) used as reference compound were reported in ref 72. The authors postulated that the mono- and bisadducts formed bilayered and monolayered smectic A phases, respectively. The obtained compounds are interesting, promising supramolecular materials as they retain the self-organizing behavior of liquid crystals and most of the properties of C60. A series of fulleropyrrolidine bisadducts (some of them are shown in Figure 15) with trans-1, trans-2, trans-3, trans-4, and equatorial bis-addition patterns were prepared from corresponding bis(benzaldehydes), sarcosine, and C60 by Prato reaction.73 Additionally, three chiral N-methylfulleropyrrolidine bisadducts were synthesized, isolated, and completely resolved into each enantiomer using a chiral HPLC column. These were then converted to the corresponding optically active, cationic C60bisadducts.74 Other bisadducts were reported in ref 75. Trisadducts are also known;76 they were obtained from C60 as starting material (Figure 16). Nine fulleropyrrolidine trisadducts, including three isomers, have been purified and unambiguously characterized. Additional information on fulleropyrrolidine bisadducts is given in ref 77. Enzyme-Containing Fulleropyrrolidines. N-(3-Maleimidopropionyl)-3,4-fulleropyrrolidine (Figure 17) was successfully attached to subtilisin through site-specific immobilization78 in a phosphate buffer containing mutant subtilisin. It was established that the nature of the immobilization support affects the catalytic properties of the enzyme. The interaction of fullerenes with biological molecules was also studied in ref 79. Polymer-Containing Fulleropyrrolidines. Polymer-containing fullerenes were discussed in a comprehensive review,80 where some attention was paid to polymers with fulleropyrrolidine moieties. Thus, a series of oligophenylenevinylene (OPV) dyads (Figure 18) and soluble double-cable copolythiophenes were prepared by multistep transformations from monomers or their precursors and characterized. It is noted that the copolymers contained from 7 to 14% of fullerene monomer. For the second


Figure 13. Ferrocene-oligothiophene-fullerene and ferrocene-tetrathiafulvalene-fullerene triads.

series of compounds, it was noted that photovoltaic devices incorporating these hybrids produced a photocurrent, showing that photoinduced electron transfer takes place. However, the efficiency of the devices is limited by the fact that photoinduced electron transfer from the OPV moiety to the C60 sphere must compete with an efficient energy transfer. Among other polymers containing fulleropyrrolidines, we note a series of new polymers containing porphyrin, poly(p-phenylenevinylene) and/or a pendant fullerene unit,81 in which the aggregation superstructures of three polymers (nanobriquetting, nanofiber, and hierarchical porous structures) were revealed. Other Intriguing Fulleropyrrolidines Liquid fullerenes based on fulleropyrrolidines were synthesized by refluxing the 2,4,6-tris-(alkyloxy)benzaldehyde with

N-methylglycine and C60 in dry toluene; their rheology was studied in ref 82. They (Figure 19) are electrochemically active and have a relatively large hole mobility. It is noted that the melting points dramatically decrease from n ) 8 to n ) 12, a fact that is correlated with the length of alkyl chains surrounding C60. Monoadducts of liquid-crystalline fulleropyrrolidines83,84 (their bisadducts were already discussed above), were obtained from mesomorphic aldehyde-based dendimers of first to fourth generation sarcosine or glycine and C60. Two different organizations were found depending upon the dendrimer generation: (1) the molecules are oriented in a head-to-tail fashion within the layers (for the second-generation dendrimers); (2) the mesogenic units are oriented above and below the dendritic core (for the third- and fourth generation dendrimers).


Figure 14. Fulleropyrrolidine monoadduct (reference compound, left) and liquid-crystalline fullerene bisadducts.

Carbon onions, one of the most intriguing forms of carbon allotropes consisting of large concentric (multishell) fullerenes,

were prepared by an arc discharge technique.85 An onion consists of concentric shells (Figure 20) and has diameters


Figure 15. Examples of fulleropyrrolidine bisadducts.

Figure 16. Synthesis of mono-, bis-, and trisfulleropyrrolidine adducts.

methodology which not only allows the isolation of giant fullerenes, but especially renders them soluble in organic solvents. The carbon onion was found to be soluble in many organic solvents, especially chloroform and dichloromethane, with a solubility of about 10 and 7 mg per 100 mL, respectively, and the solutions are stable for at least 2 weeks. It is emphasized that the functionalized onions are an example of soluble material that functions equally well in both the visible and the NIR regions.

Figure 17. N-(3-Maleimidopropionyl)-3,4-fulleropyrrolidine.

between 60 and 300 nm, while the space between the internal shells has a mean value of ca. 4 nm. The authors reported new

C60-Pyrrolidine-N-oxides, in which the tertiary amine is transformed into a quaternary amine bearing an oxygen atom (Figure 21),86 were prepared in moderate yields (20-40%) by oxidation of C60-pyrrolidines by a peracid (3-chloroperoxybenzoic acid). It is noted that the reaction is very selective, favoring the nitrogen atom of the pyrrolidine ring in preference to epoxidation of the fullerene cage; selective oxidation of the nitrogen atom was favored under dilute conditions, whereas concentrated solutions furnished mixtures of products. 15Nlabeled C60-pyrrolidine-N-oxide molecule was also synthesized and oxidized to the N-oxide to confirm the formation of the final product by 15N NMR. Solubility of fullerene-N-oxide molecules in organic solvents is different to their unoxidized counterparts. Stabilization of fulleropyrrolidine-N-oxides through intrarotaxane hydrogen bonding was examined in ref 87.


Figure 18. Oligophenylenevinylene dyads and soluble double-cable copolythiophenes.

Figure 19. Liquid fullerenes.

Multifulleropyrrolidines with three, four (Figure 22, up), and five functionalized fulleropyrrolidine moieties were prepared88 starting from 2,5-dimethoxycarbonyl[60]-fulleropyrro-

Figure 20. Carbon onions.

lidine as a starting precursor. The reaction time is 5 weeks for the hexakisfullerene. Additionally,89 another compound (Figure


Figure 21. C60-pyrrolidine-N-oxides.

22, bottom), a photoswitchable fullerene dimer, one-half of which comprised the endohedral N@C60, was prepared by refluxing a mixture of 4,4′-azobenzaldehyde, C60, and Nmethylglycine (sarcosine) in toluene for 2 days under a nitrogen atmosphere (yield 95%). Irradiation by ultraviolet and visible light has been used to switch between the trans and cis isomers of both the C60- and N@C60-based dimers. Other dimers having tetrathiafulvalene and analogous S-donors are described above in the section, devoted to S-containing fulleropyrrolidines. Simple fulleropyrrolidines bearing long aliphatic chains {-C6H2-3,4,5-((OCH2)15CH3)390,91 and -C6H2-3,4,5-((OCH2)19CH3)392}, showing applications as superhydrophobic surfaces, were prepared from the corresponding benzaldehydes, Nmethylglycine, and C60 in refluxing dry toluene. These robust

Figure 22. Examples of multifullerenopyrrolidines.

Figure 23. A C60-pyrrolidine derivative with a hydrophobic-hydrophilic-hydrophobic structure.

Figure 24. Cyclobutane-containing fulleropyrrolidines.

Figure 25. Calixarene-containing fulleropyrrolidines.


Figure 27. Bis(2,2′-bipyridine)(1′,5′-dihydro-3′-methyl-2′-(4-(4′-methyl-2,2′bipyridinyl))-2′H-[5,6]fullereno(C60-Ih)[1,9]pyrrole)ruthenium-bis-(hexafluorophosphate).

Figure 26. Nitrobenzene- and m-dinitrobenzene-containing fulleropyrrolidines.

and durable artificial nanocarbon superhydrophobic surfaces exhibit fractal morphology on both the nanometer and the micrometer scale. The superhydrophobic films on their basis are stable toward polar organic solvents and acidic/basic media and heating. Hierarchical organization of molecular components into macroscopic objects provides a solution for fabrication of low friction, low adhesion, and nonwetting surfaces for micro/ nanoelectromechanical systems or self-cleaning surfaces. SEM and TEM images of the compounds above formed from 1,4dioxane solutions showed flower-shaped supramolecular assemblies (for a detailed recent review on other nanoflowers of inorganic, organic, and coordination compounds see ref 93). A C60-pyrrolidine derivative with a hydrophobic-hydrophilichydrophobic structure (2-{3,4-di{2-[2-(2-decyloxyethoxy)-

Figure 29. Formation of three C70-containing pyrrolidine isomers.

Figure 28. Synthesis of fullerene derivative possessing both pyridyl and 4-imidazolylphenyl chelating groups.

ethoxy]ethoxy}}phenyl-3,4-fulleropyrrolidine, DTPF) (Figure 23) has been synthesized and characterized.94 It is noted this compound could form stable nanospheres by simply injecting its THF solution into water

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 555 Table 1. Representative Examples of the Synthesis of Fulleropyrrolidines

and then removing THF by purging gaseous nitrogen in sequence. Moreover, nanoassemblies of DTPF nanospheres and gold nanoparticles were obtained through in situ photoreduction of aqueous HAuCl4 in the presence of DTPF nanospheres.95 According to the authors opinion, the interaction between the positively charged nitrogen atom and the gold nanoparticles is the main driving force for the formation

of these nanoassemblies; the elaborated synthetic technique could lead to a wide variety of supernanostructures that show extraordinary optoelectronic properties. Among a lot of other reported fulleropyrrolidine derivatives, prepared and characterized in 2000-2008, we note those containing such moieties as diastereomerically pure function-


Figure 35. Functionalized CNTs.

alized cyclobutanes (Figure 24),96 calixarenes (Figure 25),97 coumarin,98 nitrobenzene and m-dinitrobenzene (Figure 26),99 dec-9-ynyl -(CH2)8CtCH,100 2,2′-bipyridine (Figure 27),101 biologically active 1,4-dihydropyridines,102 imidazolium,103 the first fullerene derivative possessing both pyridyl and 4-imidazolylphenyl chelating groups (Figure 28),104 and various alkyl-,105 phenyl-,106 and nitroso-107 groups, etc. A definite attention is paid to fullerene-containing rotaxanes,108,109 sugars,110 and crown-ethers.111,112 C70-Pyrrolidines. In comparison with a large amount of reported C60-pyrrolidines, there are a few examples of related C70 compounds. Thus, the reaction of C70 with paraformaldehyde and/or N-methylglycine under high-speed vibration milling (HSVM) solvent-free conditions was found to give three positional isomers of [70]fulleropyrrolidines in 41% yield with a ratio of 47:36:16 (Figures 29-34).113 When C70 reacts only with N-methylglycine in the same conditions, only two monoadducts are formed with a total yield of 23%. (C80-pyrrolidinesee example in Table 1). Carbon Nanotubes, Functionalized with Pyrrolidines. Prato described15,119 the functionalization of carbon nanotubes (CNTs) on the basis of similar techniques for fullerenes (1,3-

dipolar cycloaddition of azomethine ylides), examined above. The technique for CNTs (either single-walled or multiwalled) solubilizing was developed on the basis of their interaction in DMF with an excess of aldehyde and R-amino acid. This way, a large number of pyrrolidine rings fused to the carbon-carbon bonds of CNTs are produced. Representative examples of typical functionalized water-soluble CTNs (f-NTs), in particular containing ferrocene moieties, are shown in Figure 35; they can serve as intermediate synthons to covalently attach peptides for future biological studies. Similar f-NTs with OCH3 ending groups have solubility as high as 50 g L-1 through the attachment of pyrrolidines to the external wall of NT. High filling of single-wall nanotubes (SWCNTs) with the typical exohedrally functionalized fullerene derivative of C60 N-methyl-3,4-fulleropyrrolidine C60-C3NH7 (without excessive defects or sidewall functionalization as a result of this treatment) at the temperature of refluxing hexane is described in ref 120. It was confirmed that bundles of SWCNTs are highly filled with the fulleropyrrolidine and form the (C60-C3NH7)n peapods. Applied Techniques to Study Fulleropyrrolidines. The techniques to study synthesized fulleropyrrolidines are generally the same in the major part of reports. Nevertheless, we would like to pay special attention to some of them in order to show a variety of routes in obtaining experimental data. Thus, X-ray photoemission spectroscopy study was carried out for fulleropyrrolidine and neutral or positively charged pyrrolidine derivatives121 in order to determine the effects of the C60-cage on the pyrrolidine nitrogen. It was shown that the charge transfer from the carbon pyrrolidine ring to the C60-cages is observed and this charge redistribution influences not only the carbon atoms but also the nitrogen. The spectral characteristics of main excited states of the N-methylfulleropyrrolidine (Figure 1) and its several analogues with ferrocene moieties C60-Fc were obtained.122 It was noted that excited states of fullerenes and fullerene derivatives are easily generated from their ground states, due to their wide absorption window ranging from the UV to the near-IR. Among other similar investigations, we note the study of low-temperature vibronic spectra of two fulleropyrrolidines (1-methyl-3,4-fulleropyrrolidine and 1-methyl-2(4-pyridine)-3,4fulleropyrrolidine) embedded in a crystalline toluene matrix,123 and the interpretation of continuous wave electron spin resonance (CWESR)124 and EPR spectra of fulleropyrrolidine bisadducts with nitroxide addends.125,126 Unusual luminescence of hexapyrrolidine derivatives of C60 with Th and novel D3symmetry was reported in ref 127. Electrochemical characteristics of various synthesized fulleropyrrolidine derivatives have been extensively studied in a majority of reports.128 Thus, the cyclic voltammetric study of a series of bisfulleropyrrolidines and bisfulleropyrrolidinium ions is presented in ref 129. The eight possible stereoisomers of each series were investigated allowing the observation of up to four

Figure 36. Fulleropyrrolidines, used as models for determination of acid-base properties.


Figure 37. Four possible regioisomers from a 1,3-dipolar cycloaddition of azomethine ylides to C60.

Figure 38. 1-(4-, 5-, and 6-selenenyl derivatives-3-formyl-phenyl)pyrrolidinofullerenes.

Figure 39. Some fulleropyrrolidiniums used for Nafion modification.

and five subsequent reversible reductions, respectively. Some isomers have a rather unexpectedly wide potential difference between the second and third reductions. Experimental and theoretical investigations of acid-base properties of fulleropyrrolidines were carried out130 on the

example of 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(nalkyl)fulleropyrrolidines (Figure 36) in aqueous micellar media of a sodium dodecyl sulfate surfactant by using ab initio 3-21G(*) methods. A 0 < pKa

Recent Advances in the Synthesis, Characterization, and Applications

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