Buckyballs

Delgado, Juan L.; Filippone, Salvatore; Giacalone, Francesco; Herranz, Ma Ángeles; Illescas, Beatriz; Pérez, Emilio M.; Martín, Nazario

doi:10.1007/128_2012_414

Juan L. Delgado¹⁷,
Salvatore Filippone¹⁸,
Francesco Giacalone¹⁹,
Ma Ángeles Herranz¹⁸,
Beatriz Illescas¹⁸,
Emilio M. Pérez¹⁷ &
…
Nazario Martín^18,17

Part of the book series: Topics in Current Chemistry ((TOPCURRCHEM,volume 350))

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Abstract

Buckyballs represent a new and fascinating molecular allotropic form of carbon that has received a lot of attention by the chemical community during the last two decades. The unabating interest on this singular family of highly strained carbon spheres has allowed the establishing of the fundamental chemical reactivity of these carbon cages and, therefore, a huge variety of fullerene derivatives involving [60] and [70]fullerenes, higher fullerenes, and endohedral fullerenes have been prepared. Much less is known, however, of the chemistry of the uncommon non-IPR fullerenes which currently represent a scientific curiosity and which could pave the way to a range of new fullerenes. In this review on buckyballs we have mainly focused on the most recent and novel covalent chemistry of fullerenes involving metal catalysis and asymmetric synthesis, as well as on some of the most significant advances in supramolecular chemistry, namely H-bonded fullerene assemblies and the search for efficient concave receptors for the convex surface of fullerenes. Furthermore, we have also described the recent advances in the macromolecular chemistry of fullerenes, that is, those polymer molecules endowed with fullerenes which have been classified according to their chemical structures. This review is completed with the study of endohedral fullerenes, a new family of fullerenes in which the carbon cage of the fullerene contains a metal, molecule, or metal complex in the inner cavity. The presence of these species affords new fullerenes with completely different properties and chemical reactivity, thus opening a new avenue in which a more precise control of the photophysical and redox properties of fullerenes is possible. The use of fullerenes for organic electronics, namely in photovoltaic applications and molecular wires, complements the study and highlights the interest in these carbon allotropes for realistic practical applications. We have pointed out the so-called non-IPR fullerenes – those that do not follow the isolated pentagon rule – as the most intriguing class of fullerenes which, up to now, have only shown the tip of the huge iceberg behind the examples reported in the literature. The number of possible non-IPR carbon cages is almost infinite and the near future will show us whether they will become a reality.

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Chemical Reactivity and Addition Pattern on C60 and C70

Fullerenes revisited

Article 01 February 2015

Keywords

1 Introduction

From the 118 elements which have been identified according to the periodic table, only 94 elements are naturally occurring on earth while the remainder are considered artificial elements. Among the natural elements, carbon is the only one providing the basic requirements for life. Its ability of hybridization of atomic orbitals to produce sp³, sp², and sp hybrid orbitals confers upon it the singular property of having a variety of allotropic forms. Surprisingly, it is less than three decades since only two allotropes of carbon, namely diamond – constituted by sp³ carbon atoms – and graphite – formed by sp² carbon atoms – were known by the scientific community. Both allotropes show a reticular structure with carbon atoms spreading infinitely through the three space directions.

A new scenario emerged in 1985 with the advent of fullerenes, the third and only molecular allotropic form of carbon, formed by highly symmetric closed cages of a well-defined number of carbon atoms [1]. Interestingly, fullerenes have been present on our planet from its very beginning as well as in outer space (the presence of fullerenes C₆₀ and C₇₀ has recently been detected by IR in huge amounts in a young planetary nebula (Tc 1). The fullerene content is around 1.5% of the carbon present in the nebula, roughly corresponding to the mass of three moons [2]).

Soon after the discovery of fullerenes, other important different forms of carbon were found, namely, in chronological order, multiwall [3] and singlewall [4, 5] carbon nanotubes, and most recently graphenes [6] which have provoked great excitement and expectation in the scientific community (Fig. 1). In the meantime, a wide variety of other less common nanoforms of carbon also emerged such as nanohorns, nanoonions, nanotori, nanobuds, peapods, etc., whose properties and chemical reactivity are less well known to date [7]. Furthermore, fullerenes have been skillfully combined with other elements allocated to their inner empty space, affording a large, singular, and promising family of so-called endohedral fullerenes – those containing an atom, molecule, or complex in their inner cavity – whose properties and chemical reactivity are strongly influenced by the elements inside the ball [8].

The increasing number of nanoforms of carbon gives rise to a first taxonomic problem. Should the aforementioned nanoforms of carbon be considered as different allotropes? Answering this question could be accomplished by considering the IUPAC definition of allotrope as “the different structural modifications of an element” (IUPAC Compendium of Chemical Terminology, 2nd edition, 1997). Although at a first glance they could be considered as different allotropes, the scientific community considers fullerenes as the third allotropic form of carbon in which the sp² carbon atoms are bonded to form spherical, tubular, or ellipsoid structures, thus gathering all the above forms within the same allotrope of “fullerenes.”

The above considerations do not, however, affect “buckyballs,” a nickname which refers only to the former fullerenes, that is, those constituted exclusively by closed cages of a precise number of carbon atoms, the most representative examples being C₆₀ and C₇₀ molecules, and the most abundant and easy to obtain fullerenes. In this chapter we will discuss the properties, chemical reactivity, and some of the most realistic applications of fullerenes, including endohedral fullerenes and those rare fullerenes which do not follow the “isolated pentagon rule” (see below).

1.1 Brief History of Fullerenes

The discovery of fullerenes represents one of the most recent examples of serendipity in chemistry. Fullerenes were discovered by Robert F. Curl, Sir Harold W. Kroto, and the late Richard E. Smalley in 1985 [1] during carbon nucleation studies under red giant star conditions. Only 11 years later, in 1996, these scientists were awarded the Nobel Prize in Chemistry for “the discovery of fullerenes” [9–11]. Just a year before, C₆₀ had been declared the molecule of the year by the scientific journal Science.

This important finding has resulted in a new field with a broad impact in science, thus provoking great excitement in the scientific community, ranging from chemistry, physics, and engineering to practical applications in materials science and bio-medical applications [12]. Actually, the impact of the new fullerenes goes beyond the scientific world and, since this molecule was first found in Texas, the State Parliament declared Buckminsterfullerene C₆₀ the molecule of Texas State in May 1997.

However, a major breakthrough in fullerene science occurred in 1990 when Wolfgang Krätschmer and Donald Huffman (two astrophysicists) prepared fullerene C₆₀ for the first time in multigram amounts [13] thus opening the fullerene world to chemical functionalization and, therefore, to the unlimited imagination of chemists for synthesizing new and sophisticated fullerene architectures. The importance of this achievement was pointed out by some of the Nobel laureate scientists. In Smalley’s own words: “Had there not been a method to make it in measurable amounts, it would not have had an impact.” Curl also recognized this scientific contribution, stating: “Huffman’s work took it from mass spectrometers to the laboratory. It must have been a close decision by the Nobel committee over who should get it.”

However, the history of fullerenes started many years before. Actually, the possible existence of the C₆₀ molecule was proposed 15 years before its discovery by Eiji Osawa from Kyoto University. Kroto who did not know this work at the time of discovery, in part due to its publication in Japanese, has given great credit to this Japanese scientist.

The idea of a closed carbon cage was initially proposed by David E.H. Jones in 1966 in an article written in the New Scientist under the name “Daedalus” [14]. He suggested that giant empty cages could be formed by distorting a planar net of hexagonal carbons (a vision of graphene?) by adding impurities. However he was never able to explain how it could be done. It is interesting to note, nevertheless, that in situ TEM experiments correlated with quantum chemical modeling have demonstrated that flat graphene sheets undergo a direct transformation to fullerene cages under 80-keV electron beam irradiation [15].

Some more romantic chemists have seen in Leonardo da Vinci’s work the first scientific “modeling” of the C₆₀ molecule in his famous illustration for the book by Luca Pacioli entitled “De Divina Proportione,” published in Venice in 1509. In this illustration the truncated icosahedron called “Ycocedron Abscisus Vacuus” by Leonardo is shown (Fig. 2).

1.2 General Remarks on the Chemical Reactivity of Fullerenes

Buckyballs constitute a family of closed cage carbon allotropes that contains 2 (10 + H) carbon atoms, where H is the number of hexagons, while the number of pentagons is always fixed at 12. In principle, an unlimited number of fullerene structures could result. However, the simplest and most abundant is C₆₀, which is formed by 60 carbon atoms –12 pentagons and 20 hexagons–, followed by C₇₀. C₆₀ has icosahedral symmetry and a diameter of 7.8 Å. An important structural motif of fullerenes is the so-called “isolated pentagon rule,” which means that all pentagons in the molecule must be isolated from other pentagons, since structures with fused pentagons are highly destabilized due to the increase in strain energy and resonance destabilization [16] (Fig. 3).

In contrast to diamond and graphite, which are sparingly soluble in organic solvents, fullerenes are soluble in some organic solvents. They undergo a variety of chemical reactions in solution to afford a huge number of fullerene derivatives which, in general, preserve the outstanding chemical, physical, and electrochemical properties of pristine fullerenes. The study of the chemical reactivity of fullerenes has experienced an unprecedented development during the last two decades and is expected to continue on this steep slope.

The singular 3D geometry of fullerenes containing 30 or more highly reactive double bonds constitutes a new scenario where a variety of different chemical reactions can be tested. The convex surface of fullerenes offers new possibilities for the study of new reactions and mechanisms under severe geometrical constraints on carbon atoms showing a singular sp^2.3 hybridization [17].

The contributions stemming from well-established and versatile protocols to functionalize fullerenes chemically have yielded a broad spectrum of intriguing, tailor-made fullerene derivatives. The remarkable properties of the latter are continuously under investigation and form the basis in the search for practical applications of fullerenes.

As mentioned above, the C₆₀ molecule is formed from 12 pentagons and 20 hexagons linked by single and double carbon–carbon bonds. The calculated bond distances reveal subtle differences between the [5,6]- and [6,6]-bonds with values of 1.45 and 1.38 Å, respectively. Due to the mixed character of 1,3,5-cyclohexatrienes and [5]radialenes, C₆₀ behaves as a highly strained electron-poor alkene. The chemical reactivity is mainly driven by strain relief and, therefore, addition reactions have been widely used [18]. Interestingly, although similar reactivity patterns have also been observed for higher fullerenes, chemical reactivity tends to decrease significantly with their size [19–21].

A variety of chemical reactions, namely nucleophilic additions, cycloaddition reactions, free radical additions, halogenations, hydroxylation, and metal transition complexations, have been reported for C₆₀. However, addition reactions, electron transfer reactions, and reactions involving the opening of the fullerene cage (molecular surgery) have been studied in more detail. It is worth mentioning the ease with which fullerenes are reduced by means of electron-rich chemical reagents as well as electrochemically. Their oxidation, however, is considerably more difficult to achieve. These experimental findings are in agreement with former theoretical calculations which predicted that C₆₀ has a low energy LUMO which is triply degenerated and, therefore, accepts up to six electrons in solution to form up to the hexaanion [22]. The theoretical predictions were later confirmed by electrochemical measurements recording from the monoanion to the hexaanion using a toluene/acetonitrile 5:1 by volume solvent mixture at −10°C [23].

For a wider and more detailed study of the basic reactivity of fullerenes, the reader is referred to the aforementioned monographs that comprehensively cover the properties and chemical reactivity of fullerenes [19–21].

2 New Covalent Chemistry of Fullerenes

Significant effort is still being devoted to the chemical modification of fullerenes. Even though most of them are based on the chemistry of electron-poor olefins, fullerene curved double bonds have given rise to a quite peculiar fullerene chemistry. Remarkable examples of this reactivity have been provided by the use of fuller-1,6-enynes, fullerene analogues of 1,6-enynes involving a highly reactive fullerene double bond as the “ene” moiety. Thus, fulleropyrrolidines 1 bearing a propargyl group on the C-2 of the pyrrolidine ring undergo an unusual thermal [2+2] cycloaddition reaction affording regioselectively a cyclobutene-fullerene derivative 2 (Scheme 1) [24]. A different change in chemoselectivity is observed when an internal alkyne is used in the fullerenynes. In that case, allenofullerene derivatives (3) are obtained as a result of a formal “ene” reaction where the alkyne moiety with the α CH group acts as an “ene” component, despite the unfavorable geometry (Scheme 1) [25].

Another example of the intriguing behavior of fullerene double bonds has recently been reported by Bazan et al. in which fullerenes behave as a neutral carbon based Lewis acid [26]. Thus, when C₆₀ reacts with the N-heterocyclic carbene 4, that acts as a Lewis base, a thermally stable zwitterionic Lewis acid–base adduct 5 is formed. The bulk of the substituents of carbene species, along with the delocalization of its positive charge, prevent the expected cyclopropanation reaction and a C–C single bond, with a length of 1.506 Å is formed instead (Scheme 2).

The addition also occurs onto C₇₀ with the regioselective formation of a new bond between the carbene and the carbon atom C-25 of [70]fullerene [27].

Among the numerous methods for chemical functionalization of fullerenes reported during the last decade, some important trends have been outlined for achieving new properties or major control over factors like reactivity, selectivity, and compatibility with a wider range of functional groups.

2.1 New Reactions on Fullerenes Involving Metals

The nucleophilic addition of organometallic reagents, such as organolithium [28] or Grignard salts [29], is one of the classical methods for fullerene functionalization.[30–32]. More recently, the use of transition metals has expanded the arsenal of chemical tools, achieving new structures with better control of reactivity and selectivity. Thus, fullerenes’ double bonds, despite their electron-poor character, are able to act as the alkene component in a Pauson–Khand (PK) reaction. Therefore, when a 1,6-fullerenyne is treated with Co₂(CO)₈, highly efficient and regioselective intramolecular PK products showing three [33] (or five [34]) fused pentagonal rings on the same hexagon of the fullerene surface were formed (Scheme 3).

A further step has been the use of transition metal catalysis in fullerene chemistry as a smart alternative to avoid high loading of organometallic reagent and to achieve remarkable levels of reactivity and selectivity. An interesting example of this approach has been the arylation and alkenylation of fullerenes catalyzed by a rhodium complex reported by Itami [35]. Similarly to the reaction of organoboron compounds with electron-deficient alkenes and alkynes, rhodium (I) complexes catalyze the hydroarylation of C₆₀ (or C₇₀) with arylboronic acid in aqueous solution. The reaction proceeds with high regioselectivity and in a mono-addition selective manner (Scheme 4)

The use of [Rh(cod)₂]BF₄ gave rise to an optimal combination of good yield (61%) and excellent selectivity (>95%) showing an important effect of the counteranion of the rhodium complexes in sharp contrast with the reported example of conventional olefins. The authors claimed a catalytic cycle reaction where cationic Rh complex and water produce Rh–OH species. After transmetalation of the thus-formed Rh–OH with RB(OH)₂, the Rh–R species undergoes addition on the C₆₀ double bond. Finally, protonolysis of the formed fullerenyl Rh species affords the product R–C₆₀–H (11, 12) with regeneration of the cationic Rh species.

Shortly after, the same authors also developed a palladium(II) catalyst Pd(2-PyCH═NPh)(OCOC₆F₅)₂ for the hydroarylation of fullerene with boronic acids that, along with good catalytic activity (reaction generally occurs at room temperature), presents a bench stability in the solid state and efficiency under air conditions. Single crystal X-ray diffraction analysis confirmed unequivocally the addition of the aryl moiety and hydrogen in a 1,2-fashion at the α bond of C₇₀ with the phenyl group attached at the position close to the pole of the C₇₀ unit [36].

Analogously, Co-catalyzed hydroalkylation of C₆₀ with reactive alkyl bromides in the presence of Mn reductant and H₂O at ambient temperature gave the monoalkylated C₆₀ in good to high yields. The reaction probably occurs through a reduced Co(0 or I) complex that promotes generation of a radical (R^•) and the addition to C₆₀ [37].

An intriguing copper-catalyzed radical reaction that involves a formal C–H bond activation has been reported by Nakamura. The reaction efficiently couples an arylacetylene or enyne to a penta(aryl)[60]fullerene bromide in a formal [4+2] fashion to form a dihydronaphthalene ring fused to a fullerene sphere [38].

Palladium acetate catalyzes cycloaddition onto C₆₀ of a variety of anilides through a C–H bond activation, affording fulleroindolines (14) in a highly regioselective manner (Scheme 5) [39].

2.2 Asymmetric Catalysis on Fullerenes

Chirality is an important but undeveloped topic in fullerene science [40, 41]. Along with inherently chiral pristine fullerenes, optically active derivatives have been obtained by chiral induction from chiral starting reagents or after tedious and expensive HPLC separation from the racemic mixture [40].

The non-coordinating nature of fullerenes has hampered the use of asymmetric metal catalysis and, therefore, the employ of enantiopure fullerene derivatives has been limited to a few examples [42–44].

In this respect, a major breakthrough has been the chiral activation of a 1,3-dipole in the cycloaddition of N-metalated azomethine ylides onto C₆₀ (Scheme 6) [45]. By using catalytic amounts of transition metals and the suitable ligand, the cycloaddition of a series of α-iminoesters occurs under very mild conditions and good yields, affording pyrrolidinofullerenes (probably the most important class of fullerene derivatives) with complete control of diastereoselectivity.

More important, chiral complex formed by copper(II) acetate and (R)-Fesulphos led the cycloaddition toward the formation of (2S,5S)-cis pyrrolidinofullerenes, whereas the use of silver acetate and chiral (−)-BPE ligand switches the enantioselectivity toward the opposite (2R,5R)-cis pyrrolidinofullerene enantiomers.

Shortly after, complete control on the stereochemical outcome and a fully stereodivergent synthesis of all the possible stereoisomers of disubstituted fulleropyrrolidines was achieved [46]. The use of the complex Cu(II) triflate/(R) or (S)-DTBM segphos switches the diastereoselectivity towards both enantiomers of the unusual trans pyrrolidine with high enantiomeric excess. For this latter process, the authors invoke the presence of a stepwise mechanism that justifies a supra–antara formal [4+2] cycloaddition (Scheme 6).

The application of this methodology to higher fullerenes has also been achieved with excellent selectivity. Indeed, higher fullerenes are characterized by a more complex covalent chemistry due to the minor degree of symmetry. In C₇₀, the number of different 6,6 double bonds is increased compared to the single kind of 6,6 double bond in C₆₀ and, therefore, even higher control is necessary to face the distinct levels of selectivity encountered.

The very mild conditions used for this transformation allowed direction of cycloaddition of chiral N-metalated azomethine ylides toward the more reactive α double bond (C₂₅–C₈ according IUPAC nomenclature) with an almost complete site-selectivity. Despite the use of unsymmetrical dipoles, all the complexes used afforded the pyrrolidines bearing methoxycarbonyl group in the polar region of C₇₀ as major product with good levels of regio-selectivity (Fig. 4) [47].

Finally, for this regioisomer all four possible stereoisomer were obtained by the use of the suitable metal chiral complex with high values of diastereo- and enantio-selectivity [43–45].

Theoretical calculations (B3LYP/LANL2DZ) indicated a stepwise mechanism for this cycloaddition where the first step is critical for the stereochemical outcome. Furthermore, the high regioselectivity has been accounted for by the calculated nucleophilic and electrophilic Fukui indexes.

Chiral functionalization of endofullerenes represents one step further in the application of chiral metal catalysis. The first chiral endohedral metallofullerenes were recently prepared by using such methodology on a racemic mixture of a non-IPR metallofullerene derivative, La@C₇₂(C₆H₃Cl₂) [48]. This mono-functionalized metallofullerene was chosen due to the calculated energy level of the LUMO orbital being suitable to give rise to 1,3-dipolar cycloaddition. Despite this complexity, eight optically pure bis-adducts of La@C₇₂ (four from the clockwise starting material enantiomer and four from the anticlockwise enantiomer) were isolated using non-chiral HPLC. These endohedral fulleropyrrolidines resulted from the addition onto only 2 sites among 108 possible addition sites. For each site, two 2,5-disubstituted pyrrolidine regioisomers are formed with fixed (2S,5S) and optical purities as high as 98% (Scheme 7). All the isomers feature a strong Cotton effect that is dominated by the inherent chirality of the carbon core. The four isomers formed from the clockwise enantiomers exhibit circular dichroism (CD) spectra that are opposite in sign to those derived from the anticlockwise enantiomer.

2.3 Retro-Cycloaddition Reactions of Fullerene Cycloadducts

Among the many well-known exohedral reactions developed on the fullerene sphere, cycloaddition reactions have played a prominent role with applications in fields such as medicinal chemistry [49] and materials science [50]. The fullerene derivatives obtained through this functionalization method display, in general, an acceptable degree of stability; however, in the last few years a number of studies have reported efficient retro-cycloaddition methodologies for the most important fullerene cycloadducts. In this section we will describe the different retro-cycloaddition conditions for each type of fullerene derivative, and the applications of these methodologies to carry out protection–deprotection protocols that could selectively add or remove addends from fullerenes while leaving others unperturbed.

2.3.1 Retro-Diels–Alder Reaction

Fullerenes are excellent dienophiles that can undergo [4+2] cycloaddition reactions with different dienes such as anthracene or cyclopentadiene. This reaction is controlled by the properties of the dienes and can proceed at room temperature, at reflux or under microwave irradiation. The rate of Diels–Alder reaction is affected by the overall gain or loss of aromaticity of the dienophile (fullerene core) and/or the diene [51]. Most of these Diels–Alder adducts are thermally unstable and can undergo efficient retro-Diels–Alder upon mild heating [51]. Even though the regiochemistry of Diels–Alder additions is low, a few exceptions have been reported. In this regard, the reaction of C₆₀ with an excess of 2,3-dimethyl-1,3-butadiene at elevated temperatures yields a hexakis-adduct with T _h symmetry with an average effective selectivity >80% for each addition step [52].

In our research group we have investigated the Diels–Alder cycloaddition of anthracene derivatives bearing fused π-extended tetrathiafulvalenes (TTFs) to C₆₀ to yield thermally reversible donor–acceptor materials (Scheme 8) [53]. NMR and cyclic voltammetry experiments allowed the determination that the retro-cycloaddition process starts around 50°C and continues during the 50–80°C range. Thus, taking advantage of this finding, this donor–acceptor compound is able to act as an ON/OFF switch, using non-fluorescing Diels–Alder adducts of C₆₀ which, upon heating, revert to the starting materials.

2.3.2 Retro-Cyclopropanation Reaction

In the Bingel–Hirsch reaction, the deprotonation of an α-halomalonate leads to a nucleophilic anion which attacks the fullerene core to yield methanofullerenes [54]. Using this methodology a large variety of fullerene derivatives have been described and, in general, they are stable in air and under high thermal and oxidative conditions. However, these derivatives can efficiently undergo a retro-cyclopropanation reaction under reduction conditions.

The electrochemical retro-Bingel reaction of the (alkoxycarbonyl)methanofullerenes of C₆₀, C₇₀, C₇₆, and ent-C₇₆ at the second reduction potential by controlled potential electrolysis (CPE) has been reported by Echegoyen, Diederich and co-workers [55]. The retro-cyclopropanation by CPE is selective for the methano-addend, which must have at least one strong electron-withdrawing group. The presence of other groups such as cyclohexene, pyrrolidine, and benzocyclobutene rings fused to the [6,6]-bond were not affected by CPE [56]. This singularity offered a new and versatile protecting/deprotecting group strategy. The chemical retro-cyclopropanation of C₆₀ and C₇₀ mono-adducts was reported in 2000 by Diederich, Echegoyen and co-workers [57]. In this case, the methano-addend was removed from C₆₀ and C₇₀ after heating at reflux with amalgamated magnesium powder (10% HgBr₂) for 3 days, yielding 73% of C₆₀ and 63% of C₇₀, respectively. In the case of bis-adducts of C₆₀, the yield of recovered C₆₀ varied between 13% and 48% and no isomerization reactions were detected. These chemical conditions can selectively remove only the methano addends in the presence of other functional groups such as pyrrolidines, offering a versatile protecting/deprotecting group strategy (Scheme 9) [57].

2.3.3 Retro-1,3-Dipolar Cycloaddition Reactions

Among the 1,3-dipolar cycloaddition reactions, the addition of azomethine ylides is considered as one of the simplest and most efficient procedures for the functionalization of fullerenes [58]. Azomethine ylides are reactive intermediates that can be generated in several ways, although the decarboxylation of iminium salts, derived from condensation of α-amino acids with aldehydes or ketones, is the easiest and most general procedure commonly followed. Pyrrolidinofullerenes are stable compounds, although in the last few years a number of methodologies have been described to promote the retro-cycloaddition reaction of these derivatives to afford pristine C₆₀ fullerene.

Martín et al. [59] have recently described the thermally induced retro-cycloaddition of pyrrolidino[3,4:1,2]fullerenes. The authors studied the retro-cycloaddition process on a series of pyrrolidinofullerenes under a variety of experimental conditions. The best results were obtained heating the corresponding fulleropyrrolidine in the presence of a dipolarophile such as maleic anhydride and copper triflate. Under these conditions, the reaction led to the quantitative formation of the parent unsubstituted C₆₀ in all cases. This methodology was also effective in inducing the retrocycloaddition for the mono-adduct mixture of three isomers of [70]fulleropyrrolidine which afforded pristine C₇₀ in 95% yield.

Another important finding was the use of C₆₀ as a dipolarophile. A mixture of a fullerene bis-adduct was heated to reflux in o-DCB in the presence of C₆₀ and HPLC analysis of the reaction confirmed the quantitative formation of the corresponding monoadduct (Fig. 5) [59]. This interesting result opened new avenues for improving the yields of mono-adduct formation from the usually undesired bis-cycloadducts obtained as byproducts in 1,3-dipolar cycloaddition reactions of azomethine ylides to C₆₀. This observation has recently been used as an indirect proof of the covalent attachment of pyrrolidine fragments to single-walled carbon nanotubes (SWCNT). The authors applied the same conditions described by Martín et al. [59] by heating a sample of pyrrolidino-SWCNT in the presence of C₆₀ to act as dipolarophile and the corresponding pyrrolidinofullerene compound was detected, thus confirming the efficient trapping of the thermally generated azomethine ylide [60].

Guryanov et al. [61] described an alternative protocol to achieve the retro-cycloaddition of pyrrolidinofullerenes. The authors reported the quantitative retro-cycloaddition of pyrrolidinofullerenes under microwave irradiation in an ionic liquid (1-methyl-3-n-octyl-imidazolium tetrafluorborate) without further additives. The combination of microwave irradiation in an ionic liquid offers the unique opportunity for very efficient flash-thermal activation in conjunction with a strong stabilization of ionic intermediates or reactants. In this case, the ionic liquid served as an ideal medium to solvate the incipient 1,3-dipole whose release in solution was likely assisted by electrostatic interactions with the complementary ions of the solvent. In agreement with the mechanism proposed by Filippone et al. [62], cycloreversion occurred through the formation of a reactive 1,3-dipolar intermediate which was expected to be stabilized by the ionic liquid environment.

Lukoyanova et al.[63] had reported an alternative way to induce the retro-cycloaddition of pyrrolidinofullerenes using electrochemical techniques. The authors induce the retro-process by controlled potential electrolysis (CPE) at an applied potential determined from cyclic voltammetry experiments.

In order to determine whether the experimental conditions previously used for the retro-cycloaddition of fulleropyrrolidines and fulleroisoxazolines [64] are suitable for 2-pyrazolinofullerenes, Delgado et al. [65] followed the above-mentioned protocol: excess of dipolarophile, as well as copper triflate to facilitate the retro-cycloaddition process. According to the experimental findings, C-aryl-N-aryl-2-pyrazolino[60]fullerenes do not undergo an efficient retro-cycloaddition process under a variety of experimental conditions, which reveals that these compounds are thermally stable fullerene derivatives. In contrast, the presence of an alkyl chain in the carbon atom of the pyrazole ring results in an easier cleavage of the 1,3-dipole, leading to pristine C₆₀ in good yields (72%). These results show the importance of thermal stability in order to prepare new C₆₀-based materials, as well as the nature of the substituents (alkyl or aryl) which has a strong influence on the thermal stability of the cycloadducts. In particular, this is a key issue in photovoltaic cells, where long exposure to sunlight results in drastic temperature increases of the photo- and electro-active materials [65].

3 Macromolecular Chemistry of Fullerenes

Soon after the first protocols for the chemical functionalization of fullerene were set up, the rush for its incorporation in polymeric backbones started [66]. The main aim was to combine the well established processability, ease of handling, and toughness of polymers with the rather unique pool of properties of fullerenes in order to achieve new materials with combined features or even with unprecedented properties. Obviously, the final scope of the preparation of such hybrids is their application in cutting edge technologies. Nevertheless, due to the incomplete disclosure of the chemistry of fullerene, the firsts attempts to achieve fullerene-polymers led to uncharacterisable or inutile materials often obtained by employing empirical synthetic methods. Fortunately, these problematics were overcome with the development of well established methodologies for the modification of fullerenes, and nowadays the macromolecular–fullerene hybrids can be designed and tailored at will and fully characterized. Thanks to the synthetic versatility of polymers, several examples of polyfullerenes have been reported to the date, which can be classified accordingly to their chemical structure or to their properties as well as to their applications [67–69]. Herein we use a “classical” classification in which each family is structurally homogeneous and few common protocols can be followed to prepare their corresponding members.

3.1 Classification and Synthetic Strategies

An easy way to order polyfullerenes is accordingly to their increasing chemical complexity and difficulty in preparing them (Fig. 6). The synthesis of macromolecular fullerenes may be as simple as the mixing of C₆₀ and a polymeryzing reactant, or may require several carefully controlled reaction steps leading to unprecedented superarchitectures.

Even simpler, in some ways, is the preparation of all-carbon fullerene polymers [70, 71], which are all those materials constituted exclusively by fullerene units covalently linked to each other without any additional linking groups or side groups. The members of this family are prepared by exposing pristine fullerene to a strong external stimulus such as visible light [72], pressure [73], and plasma irradiation [74], with no problems in getting the final structure. During the polymerization, [2+2] cycloaddition reactions between two double bonds of two neighboring C₆₀ molecules take place generating new cyclobutane rings [75].

Analogously, heterometallic polymers, a family of heteroatom-containing polymers, in which elements other than carbon are present in their structures, are obtained by means of charge-transfer polymerization mediated by metals [76] and also by electro-reduction in the case of fullerene epoxide [77, 78].

Another family of polyfullerenes that needs little chemical control is the cross-linked set. Their synthesis usually proceeds from tridimensional random and quick reactions involving several of the 30 equivalent double bonds of the fullerene cage. Nevertheless, some control over the addition reactions is required in order to avoid a dramatic intractability of the final products. Different synthetic strategies have been followed till now for their preparation including the reaction between fullerene (or a C₆₀-derivative) and a monomer or with the pending groups (or the end termini) of preformed polymers.

On the other hand, the incorporation of one or two fullerene units at the terminal positions of linear polymeric chains leads to the end-capped C₆₀-polymers. It is worth noting that the presence of C₆₀ moieties strongly influences both the molecular and the bulk behavior of the parent polymers as a result of the modification of their hydrophobicity. Two different synthetic strategies have been employed to prepare this class of polymers: the capping of a polymer chain with C₆₀ or the growth of a polymeric backbone from the surface of a fullerene moiety or a C₆₀-derivative, the so called “graft to” and “graft from” approaches. These protocols can also be easily used in the synthesis of the members of the star-shaped polymers, which are constituted by 2–12 long and flexible polymer chains covalently linked to a fullerene cage with topologies similar to that of sea-stars.

One of the less explored families of fullerene polymers is the main-chain one. Here, the C₆₀ spheres are directly allocated in the polymer backbone forming a necklace-type structure. Unfortunately, the double addition on the C₆₀ moieties results in the formation of complex regioisomeric mixtures (up to eight isomers) and also the formation of cross linking products by multiple additions can occur.

The preparation of these in-chain polymers can be carried out by direct reaction between the C₆₀ unit and a suitable symmetrically difunctionalized monomer or by means of polycondensation between a fullerene bisadduct (or a mixture) and a difunctionalized monomer. In contrast, side-chain polymers, sometime called on-chain or “charm-bracelet,” represent the most studied family of polyfullerenes and show the C₆₀ pending from the main polymer chain. A century of studies on polymers has been exploited in the binding of C₆₀ to all the “classic” families of polymers such as polystyrenes [79, 80], polyacrylates [81, 82], polyethers [83], polycarbonates [84], polysiloxanes [85], and polysaccharides [86] in the search for improved processability and enhanced properties, with a wide range of potential applications. In this family can also be included the “double-cable” polymers [87, 88], in which the π-conjugated semiconducting polymer (p-type cable) with electron-donating characteristics contains electron-accepting fullerene units covalently connected (n-type cable), with remarkable advantages for construction of photovoltaic devices. The synthesis of the members belonging to this family can be achieved by direct introduction of fullerene itself (or a C₆₀-derivative) into a preformed polymer, or by homo-/co-polymerization of a suitable C₆₀-derivative. Moreover, for the double-cable polymers, electropolymerization is also possible.

Finally, the most recent family of macromolecular fullerene is that composed by supramolecular polymers created through any type of self-assembly and via reversible interactions of one or more types of components. Interestingly, these reversible interactions can often allow supramolecular polymers to equilibrate thermally with their monomers or oligomers. There are several ways to obtain such supramolecular assemblies; among them several systems may be obtained by interactions between functionalized polymers and C₆₀ derivatives or fullerene itself or through the assembly of self-complementary C₆₀ derivatives. More recently, assemblies between ditopic concave guests and [60]fullerene by means of concave–convex complementary interactions have also been reported [89, 90].

3.2 Properties and Applications

Despite hundreds of examples of polyfullerenes having been reported over the last two decades, to date this class of smart material has not had a real application. However, the progress achieved year by year reveals new potential applications and improved properties with respect to early examples, or even with respect to the state of the art, as in the case of photovoltaic applications. In the present section, some of the most promising and recent applications for C₆₀-polymers will be shown. Since 1991 fullerene polymers have been studied as active materials in membranes both for gas separation [91, 92] and for proton exchange fuel cells [93], and also as active polymers in electroluminescent devices [77, 94, 95] and in non-volatile flash devices [96], among others. One of the most promising applications for polyfullerenes seems to be as an active layer in optical limiters. These are devices that strongly attenuate optical beams at high intensities while exhibiting higher transmittance at low intensities, which are used for protecting human eyes and optical sensors from intense laser beams. In this regard, C₆₀ is an excellent optical limiter operating in a reverse saturable absorption mode [97], and its derivatives may be considered as potent broadband optical limiters due to the broad coverage of the characteristic ground- and excited-state absorptions over a wide wavelength range [98, 99]. However, the poor solubility and processibility of fullerene has limited its use in optical limiting devices and for this reason, in order to overcome such a drawback, C₆₀ has been incorporated in polymeric matrices. In Fig. 7 some examples of fullerene-containing polymers used for optical limiting are collected.

In 1995 random solid fullerene-containing polystyrenes (PS) showed nonlinear optical (NLO) properties about five times greater than those of a C₆₀ solution [100, 101]. Later on, random poly(methyl methacrylate) [102] and linear PS-containing fullerene 15 (Fig. 7) [103] were found to be optical limiters. In 2000 the star shaped C₆₀ containing poly(1-phenyl-1-butyne) 16 turned out to be stable, and film-forming with a fullerene content of up to 9.1 wt%. NLO studies revealed that at 532 nm (Nd:YAG laser) 16 shows improved optical limiting performances compared to C₆₀ in solution at a linear transmission of 43% [104]. In the same year the side chain polyfullerene 17, prepared by Friedel–Crafts addition of C₆₀ and of a C₆₀-derivative to a polycarbonate, also displayed good optical limiting properties [105]. Interestingly, the polycarbonate endowed with the fullerene derivative behaved better than that functionalized with fullerene itself. Very recently the same synthetic protocol has been used in order to prepare fullerene-functionalized polysulfones 18 [106]. These materials show not only a very high thermal stability and glass transition temperatures depending on the C₆₀ content, but also optical limiting properties.

On the other hand, it is well known that fullerene may act as a scavenger. When excited in the ultraviolet region (340–400 nm) it generates reactive oxygen species (ROS) acting as an effective photosensitizer, also useful in the visible-light cleavage of DNA in the photodynamic cancer therapy (PCT) [107]. However, in order to be successfully employed C₆₀ needs to be transformed in a water-soluble derivative and, among other things, its incorporation in hydrophilic polymers proved to be an excellent opportunity. In this regard, different polyfullerenes have been tested as DNA cleavers such as the supramolecular polymer 19 [108] formed by fullerene units complexed within the upper rims of cyclodextrin dimers (Fig. 8). Cyclodextrins were also employed in order to afford water solubility to the main chain polymer 20, formed through nucleophilic polyaddition reaction between C₆₀ and the β-cyclodextrin-bis(p-aminophenyl) ether [109]. Once again, under visible light conditions 20 proved to be a highly efficient DNA-cleaving agent for oligonucleotides. More recently a photosensitizer with magnetic resonance imaging (MRI) activity has been achieved by linking polyethylene glycol to fullerene at one end and the diethylenetriaminepentaacetic acid Gd³⁺ complex at the other (21, Fig. 8) [110]. Intravenous injection of 21 into tumor bearing mice followed by irradiation showed a significant anti-tumor PCT effect which depended on the timing of light exposure that correlated with tumor accumulation as detected by the enhanced intensity of MRI signals. Finally, the photoactivity of star-shaped poly(ε-caprolactone)-C₆₀ 22 has been successfully proven in the benchmark transformation of 9,10-anthracene dipropionic acid into its endoperoxide [111]. In fact, large amounts of ¹O₂ have been obtained upon irradiation of 22 with visible light. These kinds of polymers are of interest especially because they are biodegradable, biocompatible, and non-toxic to living organisms.

Among the possible practical applications of fullerene-polymers, their use as electron acceptors in the active layer of organic photovoltaic devices seems to be one of the most realistic. Despite the disappointing results, in terms of efficiency, associated with the “double-cable” polymers, investigations of which were time consuming, the very latest few years are witnessing a new momentum in the use of polyfullerenes. This is probably due to improved technologies and methodologies that are allowing scientists to achieve efficiencies comparable to those obtained in bulk heterojunction cells based on molecular C₆₀-derivatives. The first improvement in organic solar cells performances has been obtained by using a new approach in which the glycidol ester of [6,6]-phenyl C₆₁butyric acid has been first prepolymerized in the presence of a Lewis acid as the initiator (23, Fig. 9) [112]. Second, after spin coating the prepolymer in a blend with P3HT (poly-3-hexyl thiophene), the ring-opening polymerization has been completed by heating the photovoltaic device which showed 2.0% conversion energy efficiency, probably due to the morphological stabilization of the active layers architecture. Soon after, a new approach was reported in which the amphiphilic diblock-polymer 24 carries both the C₆₀ units and P3HT fragments acting as a compatibilizer between PCBM and P3HT in the active layer [113]. When added to a blend of PCBM:P3HT at 17 wt% the photovoltaic device prepared displayed an efficiency of 2.8%, along with enhanced stability of the devices against destructive thermal phase segregation. This improvement has been accounted by for the higher control in the blend morphology of the active layer due to the presence of fragments of P3HT in the polymer backbone which act as compatibilizer between PCBM and P3HT. Analogously, rod-coil block copolymer 25 has been used at various concentrations as surfactant/compatibilizer for the active layer of bulk-heterojunction solar cells in blends with PCBM [114]. This approach resulted in 35% increase of the photocurrent efficiency, increasing from 2.6% to 3.5% when the copolymer was used at 5 wt%. Such enhancement has been ascribed to the improvement in the bicontinuous interpenetrating network due to the compatibilizing action of the copolymer, as also evidenced by AFM studies.

Finally, a revolutionary approach has recently been described in which the cross-linked C₆₀-polymer 26 is generated in situ allowing the subsequent deposition of the active layer to avoid interfacial erosion [115]. The inverted solar cell ITO/ZnO/26/P3HT:PCBM/PEDOT:PSS/Ag showed an outstanding device performance with a PCE of 4.4% and an improved cell lifetime with no need for encapsulation. The strength of this new approach is its wide and general application. In fact, changing PCBM with fullerene bis-adduct 27 as acceptor in an inverted bulk heterojunction cell with architecture ITO/ZnO/26/P3HT:27/PEDOT:PSS/Ag gave rise to the impressive value of 6.22% of efficiency, which retains 87% of the magnitude of its original PCE value after being exposed to ambient conditions for 21 days [116].

4 Supramolecular Chemistry of Fullerenes

The examples of covalent modification of fullerenes we have seen so far imply the saturation of at least one of the double bonds of the polyenic structure. This might be beneficial or detrimental for the application in mind. Alternatively, the chemist can choose to interact with the fullerenes or their derivatives in a supramolecular fashion, making use of weak noncovalent interactions. In the following we will very briefly review some of the most important advances in H-bonded fullerene assemblies as well as the host–guest chemistry of fullerenes, organized according to the main type of noncovalent interaction present in the associates.

4.1 H-Bonded Fullerene Assemblies

H-bonds are, perhaps, the best studied non-covalent interactions. Although they are weak interactions, with binding energies in the range of ~ 5 kcal/mol, hydrogen bonds are also selective and directional [117]. When molecules interact by forming two or more hydrogen bonds, secondary electrostatic interactions can give rise to dramatic differences in the stability of the supramolecular complexes. The combination of different non-covalent interactions, such as ionic, π–π interactions, etc., with hydrogen bonds allows one to modulate the affinity between the interacting molecules, giving rise to a wide spread of supramolecular architectures (Fig. 10).

The importance of hydrogen bonds in determining the geometry and, overall, the function of biomolecules such as DNA, RNA, proteins, tobacco mosaic virus, and so forth is well known. Another natural example comes from the photosynthetic apparatus, in which a highly ordered supramolecular array of electron-donors (chlorophylls) and electron-acceptors (quinones) harvests and converts sunlight into chemical potential energy through cascades of short-range electron transfer steps [118]. Inspired by the natural photosynthetic event, intramolecular photoinduced electron transfer processes have been thoroughly studied in different covalent and non-covalent systems formed by donor and acceptor electroactive moieties for their implementation in molecular electronic devices [119, 120]. In this context, fullerene C₆₀ has probably been the most studied electroactive entity owing to its unique electron-acceptor properties and low reorganization energy in electron transfer processes.

The first example of a supramolecular H-bonded architecture involving fullerene was a dimer with a pseudorotaxane-type structure (28, Fig. 10) reported by Diederich et al. [121]. For this dimer a K _a value of ~970 M⁻¹ was obtained. More robust supramolecular dimers of fullerene were obtained by employing a quadruple array of H-bonds based on 2-ureido-4-pyrimidone (UP) (29 and 30) [122, 123]. In these cases the DDAA H-bonds disposition gives rise to complementary pairs with affinities in the range of K _a ~ 10⁶ M⁻¹.

Leigh-type clipping reactions have allowed the preparation of several fullerene containing rotaxanes. The first bistable rotaxane containing a fullerene is depicted in Fig. 11 (31) [124]. Depending on the polarity of the solvent, the macrocycle resides preferentially over the glycylglicyne unit or over the alkyl chain. A slight modification of the structure of the thread allows the macrocycle to shuttle in the opposite direction, from the amide to the fullerene moiety [125].

This displacement of the macrocycle has been used to increase the stability of a fulleropyrrolidine N-oxide (32, Fig. 11) [126]. The formation of hydrogen bonds between the macrocycle and the N-oxide inhibits the deoxygenation reaction, thus enhancing the stability of the N-oxide derivative.

Remarkably, the shuttling of the macrocycle can also be stimulated electrochemically by the formation of the fullerene trianion [127]. This electrochemically driven molecular shuttling has recently been achieved at very low reduction potential by introducing positive charges in the macrocycle (33, Fig. 11). In the latter case, only one electron is needed to induce the operation of the shuttle [128].

The first reported donor–acceptor supramolecular dyad based on C₆₀ is a pseudorotaxane formed between the dibenzylammonium salt of a Bingel-type fullerene and the crown ether of a zinc phthalocyanine (34) [129]. This complex has an association constant of ~ 1.4 × 10⁴ M⁻¹ and, what is more important, an intracomplex photoinduced electron transfer process leads to a radical pair species (C₆₀ ^•−–ZnPc^•+) with a remarkable lifetime in the region of microseconds (Fig. 12).

Since that, a plethora of donor–acceptor dyads in which the acceptor unit is a fullerene C₆₀ have been studied. These studies have revealed that a strong electronic communication is found through the supramolecular ensembles. That is the case for dyad 35 (Fig. 13) [130], formed by a two point amidinium–carboxylate bonding motif which gives rise to an extraordinarily high affinity, with a K _a value up to 10⁷ M⁻¹ in toluene. The strong electronic coupling (36 cm⁻¹) facilitates the formation of a long-lived radical pair with a lifetime of ~1 μs in THF.

Dyads 36 and 37 employ a three point guanosine–cytidine couple to form fullerene based hybrids with a porphyrin or phthalocyanine respectively. While for the first dyad the formation of a charge separated state after irradiation with a long lifetime of ~2.02 μs was observed, the lifetime of the radical pair in the case of the zinc phthalocyanine dyad was only 3 ns [131, 132]. This difference has been ascribable to a pronounced coupling between the ZnPc and C₆₀ in 37, which is reflected in a large binding constant of 1.7 × 10⁷ M⁻¹ (vs a K _a of 5.1 × 10⁴ M⁻¹ found for 36).

The six-point Hamilton array has also been used to form highly stable dyads when interacting with a cyanuric acid moiety, with association constants usually in the range of 10⁴–10⁵ M⁻¹ for monotopic receptors [133] and even higher when a ditopic Hamilton receptor is employed [134]. In recent work, Hirsch et al. have employed this Hamilton receptor together with metal complexation to control the step-by-step assembly of the different components in triad 38 [135]. In this triad the perylenediimide (PDI) moiety acts as a light harvester unit and, after selective photoexcitation, an energy transfer to the porphyrin unit takes place, which has been corroborated by steady-state and time-resolved measurements. The energy transfer is followed by an electron transfer event from the porphyrin to the axially complexated fullerene moiety driving to the formation of a radical ion pair with a lifetime of 3.8 ns. This lifetime is longer than that of the corresponding dyad from the porphyrin and the fullerene.

Tetrathiafulvalene (TTF) and π-extended tetrathiafulvalene (exTTF) have also been used as the electron donor counterpart in D–A nanohybrids with fullerenes in supramolecular assemblies. In pseudorotaxane-type structures between exTTF and C₆₀, it has been found that the interaction between the donor moiety of exTTF and the acceptor unit of fullerene is stronger when a flexible spacer allows the involvement of the intramolecular interaction between the convex fullerene surface and the concave face of exTTF (Fig. 14) [136]. In this case, a binding constant of 1.58 × 10⁶ M⁻¹ in chlorobenzene was observed, more than two orders of magnitude higher than the K _a values obtained for related systems in which only the crown ether-ammonium salt motif can interact. For this dyad a remarkable anodic shift of ~ 100 mV of the oxidation potential of exTTF reflects the strong donor–acceptor interaction. Transient absorption spectroscopy showed the formation of a radical ion pair with a lifetime of 9.3 ps.

An appealing topic in molecular electronics is the use of supramolecular interactions to control the assembly of the electron donors and acceptors in order to obtain highly ordered supramolecular entities with specific functions. In this context, Bassani et al. reported the hierarchical self-assembly of a barbituric acid appended fullerene and a thiophene oligomer substituted with a melamine moiety to build a photovoltaic device. They found that the photocurrent is 2.5-fold greater in this device than in analogous ones constructed with fullerene C₆₀ and the oligomer without the H-bonding units, which is attributable to higher order at the molecular level [137].

Recently, trying to take advantage of the ability of supramolecular assembly to control the electronic interactions between the fullerene units, OFET devices were constructed with derivative 39, which combines the solubilizing 3,4-ditertbutylbenzene group with the barbituric acid motif [138]. The fabricated OFET devices showed a mobility approximately two orders of magnitude lower than the devices constructed with pristine fullerene, owing to the anisotropy of the electrical conductivity of the crystals of 39 (Fig. 15).

4.2 On Concave–Convex Interactions

We have just seen some examples of supramolecular associates of fullerene derivatives based on hydrogen bonding. All of these rely on the previous covalent modification of the fullerene to introduce adequate chemical groups. Pristine fullerenes, on the other hand, are unfunctionalized, approximately spherical polyenes. From the point of view of their supramolecular chemistry this means that very weak, non-directional dispersion interactions (mainly van der Waals and π−π) will account for the vast majority of the binding energy in host–guest complexes [139]. Since these forces depend directly on surface area, the shape complementarity between host and guest becomes critical. In this sense, distorted concave recognition motifs seem ideally suited for the association of the convex fullerenes. The importance of this shape complementarity is beautifully illustrated by the distortion from planarity observed in the solid state structure of some porphyrin–fullerene supramolecular complexes reported by Aida. In the associates the porphyrins adopt a non-planar concave conformation to maximize the positive interactions with the C₆₀ guest, even at the expense of some degree of conjugation [140]. Besides the optimization of dispersion interactions due to shape complementarity between concave hosts and convex fullerene guests, in 2006 Kawase and Kurata suggested that there might be an additional positive effect arising from the unsymmetrical nature of the π orbitals of the contorted molecules with respect to the convex (outer in the case of the fullerenes) and concave (inner in the case of the fullerene) sides [141]. They termed this “concave–convex interaction” (Fig. 16).

Despite this, most of the examples of receptors for fullerene reported to date rely on planar recognition motifs. Curved molecules are geometrically tensioned structures, with bond angles away from the preferred ones and consequently are not always an easy synthetic target. However, some concave molecules are synthetically accessible and have been employed as hosts for fullerenes. We will now overview some prominent examples.

Corannulene consists of five benzene rings fused into a central five-member ring. As could be expected considering its size – nearly identical to that of C₆₀ and thus too small to associate with it – chemical derivatization is necessary to enlarge the cavity of corannulene and observe binding. Following this strategy, several monotopic receptors have been reported [142].

A ditopic receptor, “buckycatcher” 40, was synthesized by Sygula et al. joining together two units of coronene via a rigid aromatic spacer [143], forming a tweezers-like host [144]. It forms stable complexes with C₆₀ (log K _a = 3.9, d ₈-toluene, room temperature),^{Footnote 1} in which the fullerene is included between the two coronene units, as demonstrated through X-ray diffraction studies on cocrystals of 40 and C₆₀ (Fig. 17).

Another very interesting family of curved aromatic hosts for fullerenes are the cyclic [n ]para-phenyl acetylenes (CPPAs, 41–44 in Fig. 18) reported by Kawase et al. in 1996 [145]. The diameter of the cavity of 41 is 1.33 nm as found in its solid state structure, a close to perfect fit for C₆₀. Consequently, the ability of 41 to associate with [60] fullerene was investigated through UV–vis titrations, the analysis of which afforded a binding constant of log K _a = 4.2 in benzene at room temperature [146]. In order to increase the depth of the cavity, and in turn increase interactions between host and fullerene, macrocycles 42–44 (Fig. 18) were synthesized [147]. In these carbon nanorings at least two of the p-phenylene moieties are substituted by naphthalene units. This structural optimization bore fruit, as all of the newly synthesized hosts form complexes of remarkable stability with both C₆₀ and C₇₀. In fact, the binding constants in benzene were log K _a > 5 as estimated through fluorescence quenching experiments.

Cycloparaphenylenes (CPPs, 45 in Fig. 18), in which the phenyl units are connected directly, have also been synthesized [148], and their association with fullerenes investigated. In particular, [10]CPP presents a concave cavity of 1.34 nm in diameter, ideally suited to associate with C₆₀. Indeed, it does so with a remarkable association constant of log K _a = 6.4 in toluene at room temperature [149]. These experiments have been followed up by the group of Jasti, who have reported the synthesis of gram quantities of [8]- and [10]cycloparaphenylenes, and the crystal structure of the [10]CPP·C₆₀ complex [150].

We have reported extensively on the ability of the curved electron donor exTTF to serve as a recognition motif for fullerenes. The geometric and electronic complementarity between the concave aromatic face of exTTF and the convex surface of the C₆₀ was exploited to build very simple tweezers-like receptors (46, in Fig. 19) which associate with C₆₀ with respectable binding constants in the order of log K _a = 3–4, in several solvents at room temperature [151, 152]. Thanks to their synthetic accessibility we could access a collection of hosts in which we could investigate the specific contribution of concave–convex interactions to the molecular recognition of C₆₀ (46–48 in Fig. 19) [153]. All three receptors bear the same number of aromatic rings and are approximately equal in size, so the contribution of π−π and dispersion interactions can be considered equivalent. Hosts 46 and 47 both feature concave recognition motifs, and differ only in their electronic character: while the exTTF moiety in 46 is an electron donor, the TCAQ units in 47 are electron acceptors. Meanwhile, the anthraquinone fragments in 48 have a similar electronic character to 47, but are completely flat. Unsurprisingly, 46 shows the highest association constant, due to the combination of electronic and geometric complementarity towards C₆₀. Remarkably, there is a noticeable difference between the binding affinities of 47 and 48 that can be attributed to concave–convex interactions. As we noted in our original communication, whether concave–convex interactions should be treated as a new kind of intermolecular force or just a particular case of preorganization is not straightforward. In any case, it is worth considering them for the few curved guests of interest, mainly fullerenes and carbon nanotubes [154].

We have also investigated the binding abilities of π-extended derivatives of tetrathiafulvalene with a truxene core (truxTTF), which feature up to three dithiole units connected covalently to the conjugated aromatic core. To accommodate the dithioles, the truxene moiety breaks down its planar structure and adopts an all-cis sphere-like geometry with the three dithiole rings protruding. The concave shape adopted by the truxene core perfectly mirrors the convex surface of fullerenes, indicating that van der Waals and concave–convex π−π interactions between them should be maximized. Indeed, the association of trux-TTF and fullerenes in solution was investigated by ¹H NMR titrations with C₆₀ and C₇₀ as guests affording binding constants of log K _a = 3.1 and 3.9 for C₆₀ and C₇₀ in CDCl₃/CS₂, respectively [155].

4.3 Tweezers and Macrocycles for the Molecular Recognition of Fullerenes

As we have seen in some of the examples described above, tweezer-like hosts have been a particularly popular design for the construction of receptors for fullerenes. This is so because tweezers are usually synthetically accessible, since it requires only connecting two recognizing units symmetrically via a spacer.

One of the earliest examples of molecular tweezers for C₆₀ was reported by the group of Fukazawa in 1998 [156]. They connected two units of calix[5]arene through a variety of rigid spacers, and obtained the best results for host 49, which showed a binding constant of log K _a = 4.9 in toluene at room temperature, a world record in complex stability at the time.

The positive interaction between porphyrins and fullerenes has often been exploited to construct this kind of receptor [157]. The first example of such porphyrin tweezers for fullerenes was reported by Boyd, Reed and co-workers over a decade ago [158]. They connected two porphyrin units appended with pyridine ligands through coordination of palladium. The structure of the tweezers 50 is shown in Fig. 20. The binding constant of this receptor towards C₆₀ was estimated to be log K _a = 3.7 in toluene-d ₈ at room temperature.

After this first example the same authors reported very similar receptors, in which the coordination link between the porphyrin units was substituted with covalent bonding through the amides of either isophthalic or terephthalic acid [159]. This resulted in a decrease in the association constant. Later, in collaboration with the group of Armaroli, they replaced the benzenedicarboxamide spacers with several calixarenes, reaching binding constants as high as log K _a = 5.4 and 6.4 for C₆₀ and C₇₀ respectively, both in toluene at room temperature [160].

A more sophisticated example of molecular tweezers for C₆₀ (51 in Fig. 21), with a mechanism to turn “on” and “off” their ability to bind the fullerene guest, was reported by Shinkai and co-workers [161]. As synthesized, each of the appending pyridines coordinates to the porphyrin metal, keeping the two porphyrins on opposite sides and preventing association of C₆₀. When an external palladium center is added, the conformation changes to bring both porphyrins to the same side of the molecule, allowing for association with fullerene, as shown in Fig. 21. In the “on” state, the binding constant was estimated to be log K _a = 3.7 in toluene/CH₂Cl₂ (50:1) at room temperature.

A distinctive advantage of the simplicity of the tweezers-like design is that it can easily be adapted to construct more elaborate supramolecular assemblies at relatively low synthetic cost. For instance, in our group, based on the exTTF tweezers 46, we have built both linear [162] and hyperbranched [163] supramolecular polymers [164], covalent dendrimers capable of associating several units of C₆₀ [165], and have extended the design to associate with and solubilize carbon nanotubes in aqueous solution.

On the negative side, tweezers are not well preorganized unless very rigid spacers are used to link the two recognizing units. This results in relatively modest association constants, in the case of fullerenes typically in the order of 10³–10⁴ M⁻¹. To obtain complexes of higher stability, a tried and tested strategy is to move from tweezers to macrocycles. Macrocycles are much better preorganized, and have better expressed binding sites, increasing the binding constants significantly. In return, they are more challenging synthetic targets, and the cavity of the host needs to be fine-tuned to match the size of the guest, or the preorganization will actually be detrimental to the binding event [166]. We will now see a few examples of macrocyclic hosts for fullerenes to illustrate these points.

In 1999, Aida, Saigo and co-workers published the first example of a bisporphyrin macrocyclic host for C₆₀ (52a in Fig. 22) [167], initiating what would become one of the largest and most successful families of receptors for fullerenes [140]. Through multiple structural variations they have been able to establish some clear structure–binding affinity relationships. For instance, it has become apparent that not only the length but also the flexibility of the links between the porphyrins is critical to the association event. The synthetic precursors to the alkane spacers are the corresponding alkynes, 52b. The macrocycles with these rigid spacers do not show any sign of binding towards C₆₀, despite theoretically having a cavity of the right size. In contrast, the macrocycles with flexible alkane spacers show remarkably high binding constants, including the world-record in complex stability, 52c, with an incredibly high log K _a = 8.1 toward C₆₀ in 1,2-dichlorobenzene (o-DCB) at room temperature [168]. This is a dramatic example of the price one has to pay for increased preorganization: it is often a make or break situation.

In the last few years, examples of macrocyclic hosts for fullerenes including more than one porphyrin recognition motif have been reported. Anderson’s group has described a rigid cyclic porphyrin trimer, 53, which associates with C₆₀ with log K _a = 6.2 [169]. The cavity of 53 is actually better suited to host higher fullerenes, so it shows a binding constant of log K _a = 8.2 for C₇₀ and log K _a > 9 for C₈₆, all in toluene at room temperature.

Osuka and co-workers have gone one step further, linking four porphyrin units to form what they called a “nanobarrel” (54 in Fig. 23) [170]. The solid state structure of 54 shows a rigid concave cavity of adequate size to associate with C₆₀. Despite this, the authors reported a binding constant of log K _a = 5.7 in toluene at room temperature, perhaps not as large as could be anticipated. This might be due to the use of Ni porphyrins, since previous studies have shown a decrease in the binding constant of approximately one order of magnitude from zinc to nickel in macrocyclic porphyrin dimers [171]. Alternatively it might be that the very rigid structure of 54 is not sufficiently flexible to optimize the C₆₀-porphyrin distances, again showing the adverse effects of an excess of preorganization.

Our group has also gone the distance from tweezers to macrocycles. We have recently synthesized a family of nine macrocyclic exTTF-based receptors, in which we have conserved the basic features of the tweezers design (two exTTF units linked through an aromatic spacer) and added alkene-terminated alkyl spacers, to perform ring-closing metathesis [172, 173]. We produced systematic variations of both the aromatic and the alkyl spacers, as shown in Fig. 24. The structural variation strategy proved to be successful, as among the family of hosts we found some of the best purely organic hosts for both C₆₀ and C₇₀. For instance, p-xylmac12 associates with C₆₀ with log K _a = 6.5 in chlorobenzene and 7.5 in benzonitrile, both at room temperature. Perhaps more interestingly, the synthesis of such a complete family of macrocyclic receptors showed that even very small variations in structure can lead to huge changes in binding abilities. For example, there is a difference of three orders of magnitude between the association constant of p-xylmac12 towards C₆₀ and that of p-xylmac10 (log K _a = 3.5 under the same experimental conditions). Besides changes in the stability constants, we even found variations in the stoichiometry of the associates. The smaller members of the family, p-xylmac10 and m-xylmac10, associate with C₇₀ forming both 1:1 and 2:1 host:guest complexes, while naphmac10, with only a slightly bigger cavity, forms exclusively 1:1 associates.

There are many other examples of tweezer-like and macrocyclic hosts for fullerenes, which we will not discuss in this chapter. The interested reader can refer to more comprehensive reviews [144, 166]. In this very brief account we intended to outline general considerations regarding the design principles for the construction of hosts for fullerenes, illustrating them with a few selected examples.

5 Endohedral Fullerenes: Improving Size, Shape, and Electronic Properties

Almost immediately after the discovery of the fullerene family, scientists wondered about the possibility of introducing different atoms and molecules into the carbon cages and speculated about the new properties that the hybrid materials could exhibit. This expectation, far from decreasing, is actually growing with the application of these materials in different fields such as medicine or molecular electronics [174]. The properties of endohedral fullerenes can easily be modified depending upon the species entrapped and the fullerene cages. In this sense, endohedral fullerenes are classified in two principal categories:

1. Metallofullerenes, which are those that contain elemental metals or their combined forms [175]. More specifically, this family includes classical metallofullerenes, M@C_2n or M₂@C_2n, with 60 ≤ 2n ≤ 88 [176], metallic carbides, M₂C₂@C_2n and M₃C₂@C_2n, where 68 ≤ 2n ≤ 92 [177], metallic nitrides, M₃N@C_2n, with 68 ≤ 2n ≤ 96 [178], and, more recently, metallic oxides, M₄O₂@C₈₀ [179]. These fullerenes are typically produced by laser-vaporization or arc discharge techniques of graphite-metal oxides composite materials in the atmosphere of certain gases [180].

2. Fullerenes encapsulating small molecules, such as noble gas atoms (helium, neon, argon, krypton, and even xenon have been introduced inside fullerene cages), which are obtained by treating the fullerene powder under forced conditions (650°C and 3,000 atm of noble gas), although the occupation level of the guest is as low as 0.1–1% [181]. In a second approach, molecules as water have been introduced in C₆₀ and C₇₀ cages by using organic reactions in the so-called “molecular surgery” [182]. This strategy consists in a series of steps which involve making an incision in the fullerene cage to form an opening on the surface, inserting the desired molecule through the opening, and finally closing the hole to reproduce the fullerene cage while retaining the guest species.

The case of H₂O@C₆₀ is quite remarkable, since single-crystal X-ray analysis of the complex [H₂O@C₆₀⋅(NiOEP)₂] (OEP = octaethylporphyrin) reveals that, in contrast to many metallofullerenes where the metal often adopts an off-center location and does not move freely, the O atom is located at the center of C₆₀, with the O–H bonds pointing towards the Ni atoms. In addition, H₂O@C₆₀ and the empty fullerene can be separated quite easily by HPLC on a pyrenylated stationary phase, in stark contrast to the case of noble gas atoms or H₂ endohedrals. This easy access to a non-hydrogen-bonded H₂O molecule inside the apolar fullerene cage allows the investigation of the properties of the isolated H₂O molecule as well as the modification of the exohedral chemical reactivity of a unique wet C₆₀.

From the two categories of endohedral fullerenes mentioned above, the electronic properties of metallofullerenes are particularly promising considering that they are featured by a charge transfer from the encapsulated metal atoms to the carbon cage, forming a non-dissociating salt that consists of metal cation(s) encapsulated in a fulleride anion [183]. This electron-transfer was regarded to stabilize not only the encapsulated species but also the fullerene cage that can sometimes be otherwise unstable in the empty form. A striking example of this ionic model was the isolation of the cationic endohedral metallofullerene Li⁺@C₆₀ (Fig. 25) [184], which can only be stabilized significantly in ambient conditions when it co-exists with an appropriate counteranion. For example, the crystal structure of the salt [Li⁺@C₆₀](PF₆)⁻ exhibits a strong interaction between Li⁺, residing inside the C₆₀ cage, and PF₆ ⁻ on the outside, the interaction being shown to occur through the six-membered rings [185].

In the following sections we are going to concentrate on mono- and divalent metallofullerenes and metallic nitrides, since these metallofullerenes have been extensively investigated and their chemical reactivity reasonably explored.

5.1 Metallofullerenes and TNT Endofullerenes

Smalley and co-workers demonstrated in 1991 that a family of lanthanum containing fullerenes were produced under a modified Krätschmer–Huffman reactor and that extraction with toluene yielded mostly La@C₈₂ (Fig. 26), which was the first endohedral fullerene to be isolated [186]. La@C₈₂ has an electronic state best described as [La]³⁺[C₈₂]³⁻ with an open-shell electronic structure that is a consequence of a three-electron transfer from lanthanum to C₈₂ [187]. The resulting electron spin of La@C₈₂ imposes a unique chemical reactivity comparable to radical species, inducing magnetism on the molecular scale, or an enhanced electron conductivity [188, 189].

Since the isolation and characterization of the first mono-endohedral metallofullerene, La@C₈₂, many other classical M@C_2n (M = Sc, Y, La, Ce, Gd, etc.) have been obtained, with a C_2v-C₈₂ most abundant cage [176]. In all these fullerene cages filled with a single metal, the metal is not in the center of the cage but tends to coordinate with the cage carbons, being situated under a hexagonal ring along the C ₂ axis. As a result, the distribution of charge density is highly anisotropic over the surface, with electrophiles and nucleophiles selectively attacking the two different regions [190].

In the case of the endohedral fullerenes containing two metal, the M₂@C₈₀ (M = La, Ce, etc.) cage is typically obtained, with the two isomers I _h and D _5h as the most abundant [176]. In the case of these endohedral metallofullerenes, not only is the metal–cage interaction important but also the metal–metal interaction is crucial for the positioning and moving of the metal atoms. In the case of M₂@I _h-C₈₀ structures, it has been demonstrated how the metal atoms circulate three-dimensionally [191], in contrast to M₂@D _5h-C₈₀ species, where the metallic atoms circulate two-dimensionally along a band of ten contiguous hexagons inside the D _5h-C₈₀ cage [192].

However, for a long period of time the development of the chemistry of endohedral metallofullerenes was impeded by the relatively low yields in which they were produced. An important breakthrough in this chemistry occurred in 1999, when Dorn and co-workers reported the production of trimetallic nitride clusters with high yields [193]. In the trimetallic nitride template (TNT) method, packed graphite rods (metal oxide/carbon/catalyst) are burned in the presence of a dynamic flow of He/N₂ and afforded macroscopic quantities of materials such as Sc₃N@C₈₀, with yields that exceed those of the third most abundant (next to C₆₀ and C₇₀) empty cage C₈₄, produced under normal conditions.

The isolation of Sc₃N@C₈₀ in macroscopic quantities has facilitated the study of its physical structure and chemical reactivity [180]. From the seven possible constitutional isomers for C₈₀ satisfying the isolated pentagon rule (IPR), interestingly only the two least stable empty isomers with I _h and D _5h symmetries are the ones that predominate when they are filled with metallic nitride clusters, the I _h isomer certainly being the most abundant [194]. When considering the electronic structure of I _h-C₈₀ it is possible to rationalize this observation. It is characterized by the presence of an energetically low-lying fourfold degenerate LUMO, which enables this fullerene to accept up to six electrons, and the electronic distribution may be represented by the ionic model: (Sc₃N)⁶⁺@(C₈₀)⁶⁻ [195].

Considering the TNT method, endohedrals of the type M₃N@I _h-C₈₀ have been prepared for very different metals ( M = Sc, Y, La, Ce, Nd, Gd, Tb, Dy, Ho, Er, etc.) and, in general, the metal cluster rotates freely inside the fullerene, with the M₃N unit adopting a planar geometry, except for the case of Gd₃N@I _h-C₈₀ (Fig. 26), where the nitride ion is out of the plane of the three gadolinium ions [196].

Upon increasing the size of the encapsulated metal, the yield of cluster fullerenes usually decreases, and a distribution of molecules is obtained with larger metal ions favoring larger cages. For example, with gadolinium six cages are formed: Gd₃N@C₇₈, Gd₃N@C₈₀, Gd₃N@C₈₂, Gd₃N@C₈₄, Gd₃N@C₈₆, and Gd₃N@C₈₈ [197], whereas for the larger lanthanum only the formation of three very large cages, La₃N@C₈₈, La₃N@C₉₂, and La₃N@C₉₆, is observed [198]. In addition, Echegoyen and co-workers found a remarkable influence of the cage structure on the electrochemistry of the Gd₃N@C_2n family [197]. The cage size does not seem to affect significantly the reduction potential of these compounds, which displayed very similar first reduction potentials, but the oxidation potentials shift from +0.58 V vs Fc⁺/Fc in Gd₃N@C₈₀ to +0.06 V vs Fc⁺/Fc in Gd₃N@C₈₈, which suggests that the HOMO of the TNT-endofullerenes is probably cage-centered.

5.2 Chemical Reactivity of Endohedral Fullerenes

The chemical functionalization of endohedral metallofullerenes is essential to generate materials easy to process for multiple potential applications. Initial experiments on the functionalization of endohedral fullerenes demonstrated a high reactivity and the formation of multiple adducts or regioisomeric mixtures [199]. However, a remarkable regioselectivity has been observed in a few cases depending on the encapsulated cluster, metal species, carbon cage size or symmetry.

More specifically, the I _h isomer of the C₈₀ carbon cage presents high symmetry, with only two possible [1,2] addition sites: bonds between two hexagonal rings ([6,6]-junctions) and those between pentagonal and hexagonal rings ([5,6]-junctions) [200]. Several reactions have already been reported considering these C₈₀ cages: carbene additions [201], radical trifluoromethylations [202], or free radical additions [203], even though cycloaddition reactions are the most effective way to generate covalent derivatives of endohedral metallofullerenes to date.

[4+2]Cycloadditions. The Diels–Alder monoadduct resulting from heating an excess of 6,7-dimethoxyisochroman-3-one and Sc₃N@C₈₀ in trichlorobenzene (Fig. 27) was the first isolated organic derivative of a TNT endofullerene [204]. NMR experiments served to identify the regioisomer obtained as the one localized in a [5,6] ring junction of the I _h isomer, which was later corroborated by solving the X-ray structure of the Diels–Alder monoadduct [205]. TNT endofullerenes have a much lower reactivity than empty fullerenes and classical endohedral metallofullerenes and Dorn and co-workers used this selective reactivity to purify TNT endohedral metallofullerenes directly from as-prepared soots in a single facile step in Diels–Alder reactions with a cyclopentadiene-functionalized resin [206].

[3+2]Cycloadditions. So far these have been the processes more intensively investigated, in particular 1,3-dipolar cycloaddition reactions of azomethyne ylides to yield fulleropyrrolidines (Fig. 27) [200, 207], disilirane additions [208], and more recently cycloadditions with epoxides such as tetracyanoethylene oxide [209], or azides to form azafulleroids after nitrogen extrusion [210].

Disilirane additions were used to control the motion of the encapsulated Ce atoms inside the C₈₀ cage of Ce₂@C₈₀. In this compound, the free random motion of two Ce atoms is regulated under a hexagonal ring on the equator by the electron donation from the silyl groups, exohedrically introduced, to the C₈₀ cage [211]. The motion of La atoms encapsulated inside fullerenes has also been controlled exohedrically, by the addition positions, in pyrrolidine adducts of La₂@C₈₀. Two different compounds are obtained: in the [5,6]-adduct the two La atoms rotate rather freely, whereas in the [6,6]-adduct the metallic atoms are in fixed positions inside the cage [212].

The regioselectivity of 1,3-dipolar cycloaddition reactions of in situ generated azomethine ylides can also be controlled by the trimetallic nitride cluster. In this sense, Echegoyen et al. demonstrated that when the fullerene inner metal cluster was Sc₃N, the only product detected was the adduct at a [5,6]-junction (product of thermodynamic control). On the other hand, the 1,3-dipolar cycloaddition reaction occurred initially at a [6,6]-junction for the Y₃N@C₈₀, and underwent rearrangement to the thermodynamically more stable Y₃N@C₈₀ [5,6]-monoadducts upon heating [213]. These experimental results, as well as the computational studies by Poblet and Echegoyen, seem to indicate that, after thermalization of the kinetically favored product, a pirouette-kind of mechanism gives rise to the [5,6]-monoadduct that is thermodynamically preferred [214]. The rate of this rearrangement depends on the internal cluster and on the pyrrolidine addend.

Pyrrolidine adducts have been used as the organic addend to connect electron-acceptor Sc₃N@C₈₀, Y₃N@C₈₀, or La₂@C₈₀ units to powerful donors such as ferrocene [215] or π-extended tetrathiafulvalene derivatives (exTTFs) [216] in the preparation of electron–donor–acceptor (D–A) systems that, upon photoexcitation, yield radical ion pair states with remarkable lifetimes.

[2+2]Cycloadditions. The reaction of benzyne, generated from isoamyl nitrite and anthranilic acid, with Sc₃N@I _h-C₈₀ was successfully carried out recently to afford both [5,6]- and [6,6]-monoadducts (Fig. 27) [217]. Interestingly, when the reaction was carried out with 2-amino-4,5-diisopropoxybenzoic acid instead of anthranilic acid and under an aerobic atmosphere, in addition to the expected [2+2] benzyne adducts, oxygenation of the [5,6] regioisomer produces an intriguing third product, which is an open-cage metallofullerene. Under an inert atmosphere, the reaction gave only the expected [2+2] adducts [218].

[2+1]Cycloadditions. The investigation of the Bingel reaction on endohedral metallofullerenes demonstrates the remarkable regioselective control exerted by the encapsulated species. The [2+1] cycloaddition of bromodiethylmalonate (the Bingel–Hirsch reaction) in the presence of the non-nucleophilic base 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) produced extremely stable derivatives with Y₃N@C₈₀ and Er₃N@C₈₀ [219], while Sc₃N@C₈₀ did not react under the same experimental conditions. Under these conditions, the cyclopropanation of Sc₃N@C₇₈ with diethyl bromomalonate produced only one monoadduct and one dominant symmetric bisadduct. The high regioselectivity in the second addition is supported by the highest LUMO surface electron density value for the reactive bond, which corresponds to the kinetically preferred site for a second nucleophilic attack [220].

Another useful [2+1] cycloaddition process is the addition of diazocompounds, generated in situ from tosylhydrazones in basic media, to fullerenes. This reaction has been used for the preparation of relevant endohedral metallofullerene adducts in the context of organic photovoltaics, such as endohedral metallofullerene (Sc₃N@C₈₀, Lu₃N@C₈₀, or Ce₂@C₈₀) PCBM-based derivatives [221] or the first D–A dyad where an endohedral metallofullerene, Lu₃N@C₈₀, acts as an electron donor upon photoexcitation. In particular, when connected to perylenebisimide units [222].

Electrosynthetic routes have also been recently explored for the preparation of endohedral metallofullerene adducts that might not be accessible by conventional synthetic routes. In this sense, the different nucleophilicity observed by TNT dianions [223] and trianions [224] seems to be particularly relevant for the preparation of Sc₃@C₈₀ derivatives.

Surprisingly, the non-covalent functionalization of endohedral fullerenes has been scarcely investigated, only a few examples about the complexation with calixarenes and crown or thiacrown ethers are known, and size matching was found to be critical to the stability of the resulting complexes [176]. The most promising example is the formation of stable radical ion pairs of N-substituted p-phenylenediamine with La@C₈₂ and La₂@C₈₀ [188, 225]. Such spin-site exchange processes are reversible in solution and are readily controllable by changing the temperature and the solvent (Fig. 28).

6 Fullerenes for Organic Electronics

As mentioned above, a huge number of fullerene derivatives have been prepared, many of which have been tested in so-called organic molecular electronics. Their remarkable electron-accepting ability and low reorganization energy combined with solubility in organic solvents and outstanding photophysical properties have made fullerenes a most appealing system to be used in the preparation of electronic devices such as organic photovoltaics and the study of molecular wires, which are discussed in detail in the following sections.

6.1 Fullerenes for Organic Photovoltaics

Global dependence on fossil fuels is a key issue with important consequences in the world today. A reasonable alternative to overcome this need is the use of renewable energy sources, like solar energy, which could, in principle, fulfil our energy requirements with environmentally clean procedures and low prices. Actually, the energy received from the Sun, calculated to be 120,000 TW (5% ultraviolet, 43% visible, and 52% infrared), surpasses by several thousandfold that consumed by the planet. [226].

Photovoltaic (PV) solar cells are currently a hot topic in science and since the original silicon-based device was prepared by Chapin in 1954 exhibiting an efficiency around 6% [227], different semiconducting materials (inorganic, organic, molecular, polymeric, hybrids, quantum dots, etc.) have been used for transforming sunlight into chemical energy. Among them, photo- and electro-active organic materials are promising due to key advantages such as the possibility of processing directly from solution, thus affording lighter, cheaper, and flexible all-organic PV devices. The most widely used configuration of polymer solar cells is based on the use of a fullerene derivative as the acceptor component. Indeed, fullerenes have been demonstrated to be the ideal acceptor because of their singular electronic and geometrical properties and the ability of their chemically functionalized derivatives to form a bicontinuous phase network with π-conjugated polymers acting as electron conducting (n type) material (Fig. 29).

A great variety of chemically modified fullerenes were initially synthesized for blending with semiconducting polymers and to prepare photovoltaic devices. These fullerene derivatives were covalently linked to different chemical species such as electron acceptors, electron donors, π-conjugated oligomers, etc. [119] (Fig. 30). However, in general, the obtained blends resulted in PV devices exhibiting low energy conversion efficiencies [228].

The best known and most widely used fullerene derivative as acceptor for PV devices is [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM, 55) [229]. Since its first reported application in solar cells [230], it has been by far the most widely used fullerene, being considered as a benchmark material for testing new devices. This initial report inspired the synthesis of many other PCBM analogues [231] in an attempt to increase the efficiencies of the cells by improving the stability or PV parameters such as the open circuit voltage (V _oc) by raising the LUMO energies of the fullerene acceptor.

In this regard, only small shifts (<100 meV) of the LUMO level have been obtained by attaching a single substituent on the fullerene sphere, even by using electron-donating groups. In contrast, significantly higher V _oc values have been achieved through the polyaddition of organic addends to the fullerene cage (~100 mV raising the LUMO per saturated double bond). Recently, an externally verified power-conversion efficiency of 4.5% has been reported by Hummelen et al. employing a regioisomeric mixture of PCBM bisadducts as a result of an enhanced open-circuit voltage, while maintaining a high short-circuit current (J _sc) and fill factor (FF) values [232].

The cyclopropanation reaction to superior fullerenes to form PCBM analogues is more complex than for C₆₀. Indeed, the less symmetry and the presence of more than one reactive double bond are often responsible for the formation of regioisomeric mixtures. Nevertheless, the loss of symmetry of C₇₀ induces a stronger absorption, even in the visible region. As a result, PC₇₁BM [233] is considered a suitable candidate for more efficient polymer solar devices. Moreover, such devices displayed the highest verified efficiency determined so far in a BHJ solar cell, with an internal quantum efficiency approaching 100% [234]. Analogously, PC₈₄BM [235] has been obtained as a mixture of three major isomers. The stronger electron affinity and the diminished solubility gave rise, however, to poor power conversion efficiencies.

Although the PCBMs are the acceptors that guarantee best performances, it does not mean that they are necessarily the optimum fullerene derivatives. Therefore, a variety of other fullerene derivatives [236] have been synthesized in order to improve the device efficiency or to achieve a better understanding of the dependence of the cell parameters on the structure of the acceptor.

One of the most promising modified fullerene prepared so far is the diphenylmethanofullerene (DPM12, 56) prepared by Martín et al. [237] endowed with two alkyl chains which improve dramatically the solubility of the acceptor in the blend to reach efficiencies in the region of 3% (Fig. 31). Although the LUMO energy level for DPM12 is the same as that for PCBM, an increase in the V _oc of 100 mV for the DPM12 over PCBM has been observed [238]. This is currently an important issue for improving the design of future fullerene-based acceptors.

Other fullerene derivatives such as the dihydronaphthylfullerene benzyl alcohol benzoic ester (57) described by Fréchet et al. [239] have also shown their ability to produce efficient PV devices (PCE up to 4.5%).

Hou and Li reported the preparation of a bis-adduct fullerene derivative 58 formed by two indene units covalently connected to the C₆₀ sphere [240]. Interestingly, the presence of two aryl groups improves the visible absorption compared to the parent PCBM, as well as its solubility (>90 mg/mL in chloroform) and the LUMO energy level, which is 0.17 eV higher than PCBM. Photovoltaic devices formed with P3HT as semiconducting polymer revealed PCE values of 5.44% under illumination of AM1.5, 100 mW/cm², thus surpassing PCBM which afforded an efficiency of 3.88% under the same experimental conditions.

Although some of the fullerene derivatives prepared to date exhibit outstanding performances in PV devices, the synthesis of new fullerene derivatives with stronger visible and NIR absorption and higher LUMO energy levels than PCBM is currently a challenge in organic photovoltaics.

6.2 Fullerenes for Molecular Wires

Donor–bridge–acceptor (DBA) systems, in which the bridge mediates the transport of charge between the donor and the acceptor, provide good models to study the electron transfer processes at the molecular level. In these systems the rate of charge transfer (CT) is a combination of a strongly distance-dependent tunneling mechanism (or superexchange) and a weakly distance-dependent incoherent transport (or hopping).

The attenuation factor, β, is the parameter usually employed to describe the quality of a system as molecular wire, so that the lower the β value the longer the distance that the charge can be moved efficiently. The β parameter is characteristic for the decay of CT rate constant, k _ET, as a function of distance, R _DA:

$$ {k_{\mathrm{ ET}}}={k_0}{{\mathrm{ e}}^{{-\beta {{\mathrm{ R}}_{\mathrm{ DA}}}}}} $$

Typical β values can vary from 1.0–1.4 Å⁻¹ for proteins to 0.001–0.06 Å⁻¹ for highly conjugated bridges [241]. Other parameters exert an impact on the rate of charge transfer, in particular the underlying driving forces (−ΔG°), the corresponding reorganization energies (λ), and the electronic couplings (V) between the donor and acceptor moieties. Therefore β value depends not only on the bridge but rather on the wire system as a whole, that is, the DBA system, whether the D and A termini are molecular units or metallic contacts.

The unique electronic properties of fullerenes have prompted the study of the molecular wire behavior of different molecular bridges connected to fullerene C₆₀ as the electron acceptor and different electron donor fragments [242, 243]. The study of these systems with various bridge lengths allows the measurement of distance-dependent charge separation (CS) and charge recombination (CR) rates and, therefore, the determination of the corresponding β values.

It is well documented that molecular wires that exhibit para-conjugation facilitate charges to be transferred over larger distances and hence show lower β values related to non-conjugated bridges. Very fast electron transfer for both CS and CR has been reported for compound 59, with two acetylene bonds connecting the C₆₀ unit with the β-position of ZnP (k _CS > 1 × 10¹¹ s⁻¹, k _CR = 2.5 × 10¹⁰ s⁻¹) [244]. In contrast, for compounds 60, in which the polyacetylene bridge is connected through a phenyl ring in the meso position of the porphyrin, CS and CR are in the regions of 7.5 ± 2.4 × 10⁹ s⁻¹ and 1.6 ± 0.2 × 10⁶ s⁻¹, with a β value of 0.06 Å⁻¹ in PhCN [245]. This difference in the rate of electron transfer is due, at least in part, to more favorable orbital interactions between ZnP and C₆₀ in 59 and suggests that the phenyl ring retards the electron transfer processes (Fig. 32 ).

When comparing these systems with a ZnP–p-phenylenebutadiynilenes–C₆₀ series (61), the k _CS and k _CR values decrease drastically as the distances increase from 22 to 40 Å (k _CS range from 1.0 × 10¹⁰ s⁻¹ to 1.1 × 10⁸ s⁻¹ and k _CR range from 2.1 × 10⁶ s⁻¹ to 4.6 × 10⁵ s⁻¹) giving a quite large attenuation factor of 0.25 Å⁻¹ in PhCN [246]. This β value suggests that the phenyl ring inserted in the polyalkyne bridge acts as a resistor for electron transfer, giving rise to a less effective superexchange mechanism.

For the β-substituted ZnP–oligophenyleneethynylene–C₆₀ systems 62 (ZnP–oPPE–C₆₀), a strong increase in the lifetime of the charge separated state with increasing distance between the donor and acceptor moieties is observed, which implies a through-bond mechanism where the bridge plays an important role. A damping factor of 0.11 ± 0.05 Å⁻¹ has been determined for these compounds [247]. This value is lower than the β value reported for similar systems bearing exTTF as donor moiety (exTTF-oPPE-C₆₀), for which an attenuation factor of 0.21 Å⁻¹ was determined. This difference has been accounted for by a much more uniform distribution of the local electron affinity in ZnP–oPPE–C₆₀ 62 due to higher electron density of ZnP in comparison to exTTF. These results underline the dependence of the β value and hence the wire-like behavior on the whole DBA system and not only on the linker.

An extraordinarily small attenuation factor of 0.01 ± 0.005 Å⁻¹ was determined for compounds in which the donor exTTF and the acceptor C₆₀ are connected through a p-phenylenevinylene oligomer 63 (exTTF–oPPV–C₆₀) [248, 249]. This low β value has been explained attending to the para-conjugation of the bridge with the donor exTTF and the homogeneous distribution of the local electron affinity throughout the whole bridge. A remarkable value of ~5.5 cm⁻¹ for the coupling constant (V) was determined for these systems, unusually strong over a distance of 40 Å from the electron donor to the electron acceptor. Analogous systems in which the donor moiety has been substituted by a porphyrin 66 (ZnP–oPPV–C₆₀) give rise to a slightly higher β value (0.03 ± 0.005 Å⁻¹) due probably to a loss of conjugation between the donor ZnP and the linker [250] (Fig. 33 ).

The systematic study of the wire-like behavior in systems in which ZnP/C₆₀ conjugates are connected through a para-cyclophane–oPPV (pCp–oPPV) bridge has shown a β value of 0.039 ± 0.001 Å⁻¹. The inhomogeneous and weaker π-conjugation of pCp–oPPV related to oPPV accounts for the approximately four times greater damping factor [251].

The comparison of the charge transfer characteristics of oligofluorenes (oFl) 64 vs oligofluorenevinylenes (oFV) 65 shows that it is possible to tune the electronic properties by slight structural alterations of the compounds. Specifically, β values of 0.075 Å⁻¹ are obtained for the oFV wires, lower than the value determined for oFl (0.09 Å⁻¹) [252]. The vinylene groups improve the π-conjugation of the wire in such a way that charge injection into the bridge is favored, facilitating CS and CR processes [253].

When oligothiophenes (nT) are employed as bridges, distances up to 55.7 Å have been reached between D and A. For these H₂P–nT–C₆₀ systems, a dependency of the β value on solvent polarity was observed (0.11 Å⁻¹ in o-dichlorobenzene and 0.03 Å⁻¹ in benzonitrile) [254, 255]. Due to the electron-donating ability of the polythiophene oligomers, these bridges participate more actively in the CS/CR processes, which take place almost via the electron-hopping mechanism.

Recently, supramolecular ZnP/C₆₀ hybrids connected by a Hamilton receptor and employing different spacers as oPPE, oPPV, p-ethynylene, or fluorene have been systematically studied [256]. These studies demonstrate that the electronic communication between the donor and acceptor moieties is governed by the charge transfer properties of the conjugated spacers. Thus, spacers with low β values facilitate the charge transfer along the supramolecular bridge. The study of the dependence of the rate constants of CS and CR on the D–A distances allowed the calculation of the attenuation factor for the first time in a hydrogen-bonding-mediated electron transfer, rendering a value of 0.11 Å⁻¹, which is similar to that obtained for other covalent systems (Fig. 34 ).

7 The Future: Non-IPR Fullerenes

As stated above, C₆₀ is by far the most abundant and common of all fullerenes. The question is why, among the many possible cages that can be formed with carbon atoms, the one containing 60 atoms is favored? Furthermore, since all fullerenes Cn are constituted by hexagons (n ≥ 20 with the exception of n = 22) and pentagons (12 for all fullerene cages, which are responsible for the curved geometry), why among the 1,812 possible isomers for 60 carbon atoms was only the icosahedral symmetry I _h-C₆₀ molecule (soccer-ball shape) formed?

These intriguing questions were answered by Kroto, who proposed that the local strain increases with the number of bonds shared by two pentagons (pentalene units), thus affording less-stable molecules. This rule was coined as the “isolated pentagon rule” (IPR), stating that all pentagons must be surrounded by hexagons, thus forming the corannulene moiety [16]. The resonance destabilization that results from the adjacent pentagons (8π electrons which do not satisfy the Hückel rule) and reduction of the π-orbital overlapping because of cage curvature explains the lower stability of non-IPR fullerenes [257]. A head-to-tail exclusion rule has also been proposed to explain the higher stability of fullerenes obeying the IPR rule [258].

For a precise number of carbon atoms forming a cage, the number of non-IPR fullerene isomers is very much larger than the IPR ones. Furthermore, in addition to doubly fused pentagons found in non-IPR fullerenes, triple directly fused pentagons and more recently triple sequentially fused pentagons have been reported [259]. Therefore there is great interest in the search for the huge number of expected non-IPR fullerenes whose chemical reactivity and properties should be different from those known for IPR fullerenes [260].

In order to achieve non-IPR fullerenes, two different strategies have been developed to increase their stability, namely through endohedral and exohedral derivatization [261]. In both approaches the key issue to stabilize non-IPR fullerenes focuses on how to release or decrease the strains of fused pentagons.

The first endohedral strategy involves encaging a metal cluster inside the fullerene cage. The bending strain on fused pentagons is significantly decreased because of the strong interactions between the fused pentagons and the metal cluster.

Endohedral fullerenes have been known from the earliest times of fullerenes (see above). Although theoretical studies predicted in the early 1990s that elusive non-IPR fullerenes could be stabilized by the presence of clusters encapsulated in the fullerene cage, the first non-IPR fullerenes, namely Sc₂@C₆₆ [262] and Sc₃N@C₆₈ [263], were obtained in 2000. It is important to note that carbon cages in endohedral fullerenes are different from those observed in empty fullerenes. Therefore, favorable electronic interactions between the encapsulated species with the carbon cage are required for the stabilization of the resulting endohedral fullerene.

This method brings about the productions of other different non-IPR metallofullerenes, such as trimetallic nitride fullerenes (i.e., Tb₃N@C₈₄ [264], Sc₃N@C₇₀ [265], and Y₃N@C₇₈ [266] etc.), metal carbide endofullerene Sc₂C₂@C₆₈ [267], and metal cyanide endofullerene Sc₃NC@C₇₈ [268].

A simple and qualitative rule to predict the stability of a given endohedral is based on the calculated HOMO–LUMO gap for the resulting “ionic” endofullerene. This energy gap can be roughly calculated from the (LUMO-3)–(LUMO-4) gap determined for the neutral cage, thus predicting the most stable IPR and non-IPR endofullerenes [269].

Remarkably, non-IPR endohedral metallofullerenes (fullerenes containing one or more metal atoms in the inner cavity) show a strong coordination of the metal atoms to the fused pentagons, similar to that observed for a variety of organometallic species in which the concave face of the pentalene unit is coordinated to the metal atom [270]. In contrast, IPR endofullerenes show a motion for the encapsulated metals or clusters in the inner cavity. In some cases involving endofullerenes endowed with only one metal atom, the metal generally coordinates with the cage and motion is difficult.

On the other hand, the second strategy based on exohedral derivatization has afforded a variety of non-IPR derivatives based on the remarkable reactivity of the fused pentagons, thus changing the carbon bond hybridization and releasing the bending strains. The first small fullerene ^#271C₅₀ (the Fowler–Manolopoulos nomenclature to differentiate isomers is specified by symmetry and/or by spiral algorithm) was trapped and stabilized by chlorine atoms as ^#271C₅₀Cl₁₀ in 2004 [271]. Since then the fullerene cage has been exohedrically functionalized by introducing hydrogen or chloride atoms on the cage surface, to produce a variety of non-IPR fullerene derivatives, such as C₆₄H₄ [272], C₇₁H₂ [273], C₇₈Cl₈ [274], etc.

This stabilization of the resulting non-IPR fullerene derivatives has been accounted for by the “strain-relief principle” resulting from the rehybridization from sp² to sp³ carbon atoms, as well as by the “local aromaticity principle,” which involves maintaining the local aromaticity of the un-derivatized sp² carbon skeleton that remains after the derivatization process. Based on both principles, it has been possible to predict the stability of a variety of exohedrically functionalized non-IPR fullerenes.

Particular attention has been devoted to the structure of fullerene C₇₂, a so-called “missing fullerene” because empty C₇₂, IPR or non-IPR isomer, had never been isolated. However, along with further studies, recently several C₇₂-based species have been successfully prepared and characterized, namely La₂@D₂(10611)-C₇₂ [275], Ce₂@D₂(10611)-C₇₂ [276], La@C₂(10612)-C₇₂ [277], and Sc₂S@Cs(10528)-C₇₂ [278].

Experimental results have recently reported, for the first time, the higher stability of a non-IPR fullerene compared to its related IPR isomer for C₇₂Cl₄ [279, 280] (Fig. 35). These new results violate the “universal” IPR rule for fullerenes, but confirm the valuable “strain-release” and “local aromaticity” principles that have been so useful to predict the stability of a wide variety of fullerene derivatives. The IPR rule is therefore valid for pristine fullerenes, whereas for fullerene derivatives additional factors emerge that could eventually force a non-IPR cage to be the most stable one. These new results pave the way to the advent of a huge family of, so far, unknown non-IPR fullerenes whose number would be almost infinite. These thermodynamically less stable carbon cages should exhibit significant differences compared to those IPR obeying fullerenes, thus enhancing the properties and applications of these molecular allotropes of carbon. The scientific community should be ready for the advent of the unprecedented non-IPR fullerenes, the fullerenes for the near future!

8 Summary, Conclusions, Outlook

In the above sections we have presented some of the important achievements in the past few years with buckyballs from the viewpoint of a synthetic chemist and with an eye on the most remarkable properties of these molecules of interest for practical purposes, namely in organic electronics.

Fullerenes form a broad family of carbon nanostructures which, in principle, must obey the IPR rule as a criteria of stability. The so-called hollow fullerenes undergo a very rich covalent chemistry affording a wide variety of fullerene derivatives. In this review we have mainly focussed on those reactions carried out involving metals as catalysts, which have significantly enhanced the scope of derivatives prepared so far. Furthermore, in our group we reported the detailed retro-cycloaddition reaction of well-known fullerenopyrrolidines in a quantitative manner, thus providing a new protection–deprotection protocol in fullerene chemistry. This interesting reaction was later extended to other fullerene cycloadducts.

An important topic is combining the intriguing properties of fullerenes with those of highly versatile polymers, affording the new interdisciplinary field of “fullerene polymers.” This field has been reviewed with the most outstanding and recent advances and applications according to the rational structural classification given in the text.

The convex surface of fullerenes is an ideal scenario to interact with the concave surface of different organic molecules. Therefore the supramolecular chemistry of fullerenes has been presented with special emphasis on H-bonding interactions and π,π concave–convex interactions in which our group has been mainly engaged. This singular approach has been complemented with the most significant examples in the search for fullerene receptors from other authors.

Just a few years have been enough to show that the carbon family is much larger than initially thought and many other forms of carbon, sometimes encapsulating atoms, molecules, or clusters in their inner cavity (endohedral fullerenes), have been produced. Although the number of endohedral fullerenes is becoming larger and larger and new chemical species are placed at the inner cavity of fullerenes, the most frequent and studied endofullerenes have been presented according to their properties and chemical reactivity, thus complementing the related hollow fullerenes.

The applications of fullerenes is an open question which has been mainly focussed on the use of fullerenes for organic electronics, namely with their use in organic photovoltaics which, no doubt, represents the most realistic application of fullerenes, as well as their use in the study of molecular wires. Both topics give an idea of the interest of these carbon allotropes in the emerging fields of nanoscience and nanotechnology.

Finally, we have stressed the future of fullerenes on those which are nowadays considered an almost scientific curiosity, the non-IPR fullerenes. Since they are thermodynamically less stable than the corresponding fullerenes obeying the IPR rule, their syntheses as endo- or exohedrically functionalized species is currently a synthetic challenge and some of the most outstanding examples have been presented. Since the potential number is huge compared with those known so far, the history and development of fullerenes might just be in its infancy. The future will tell us how these and other fullerene species still to come will impact new technologies based on these new carbon nanoforms.

Notes

1.
All through the article we will report binding constants as logarithms, and without an error interval, for simplicity. The reader can refer to the original publications for these data.

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IMDEA-Nanoscience, Campus de Cantoblanco, 28049, Madrid, Spain
Juan L. Delgado, Emilio M. Pérez & Nazario Martín
Facultad de Química, Departamento de Química Orgánica, Universidad Complutense, 28040, Madrid, Spain
Salvatore Filippone, Ma Ángeles Herranz, Beatriz Illescas & Nazario Martín
Dipartimento di Scienzie e Tecnologie Biologiche, Chimiche e Farmaceutiche, Universitá degli Studi di Palermo, Palermo, Italy
Francesco Giacalone

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Juan L. Delgado
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Salvatore Filippone
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Francesco Giacalone
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Ma Ángeles Herranz
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Beatriz Illescas
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Emilio M. Pérez
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Nazario Martín
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School of Pharmaceutical Science and Technology, Tianjin University, Tianjin, China
Jay S. Siegel
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
Yao-Ting Wu

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Delgado, J.L. et al. (2013). Buckyballs. In: Siegel, J., Wu, YT. (eds) Polyarenes II. Topics in Current Chemistry, vol 350. Springer, Cham. https://doi.org/10.1007/128_2012_414

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DOI: https://doi.org/10.1007/128_2012_414
Published: 29 March 2013
Publisher Name: Springer, Cham
Print ISBN: 978-3-319-07301-9
Online ISBN: 978-3-319-07302-6
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Buckyballs

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Chemical Reactivity and Addition Pattern on C60 and C70

Chemical Reactivity and Addition Pattern on C60 and C70

Fullerenes revisited

Keywords

1 Introduction

1.1 Brief History of Fullerenes

1.2 General Remarks on the Chemical Reactivity of Fullerenes

2 New Covalent Chemistry of Fullerenes

2.1 New Reactions on Fullerenes Involving Metals

2.2 Asymmetric Catalysis on Fullerenes

2.3 Retro-Cycloaddition Reactions of Fullerene Cycloadducts

2.3.1 Retro-Diels–Alder Reaction

2.3.2 Retro-Cyclopropanation Reaction

2.3.3 Retro-1,3-Dipolar Cycloaddition Reactions

3 Macromolecular Chemistry of Fullerenes

3.1 Classification and Synthetic Strategies

3.2 Properties and Applications

4 Supramolecular Chemistry of Fullerenes

4.1 H-Bonded Fullerene Assemblies

4.2 On Concave–Convex Interactions

4.3 Tweezers and Macrocycles for the Molecular Recognition of Fullerenes

5 Endohedral Fullerenes: Improving Size, Shape, and Electronic Properties

5.1 Metallofullerenes and TNT Endofullerenes

5.2 Chemical Reactivity of Endohedral Fullerenes

6 Fullerenes for Organic Electronics

6.1 Fullerenes for Organic Photovoltaics

6.2 Fullerenes for Molecular Wires

7 The Future: Non-IPR Fullerenes

8 Summary, Conclusions, Outlook

Notes

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