Fullerenes

Principles and Applications

By Fernando Langa, Jean-Francois Nierengarten

The Royal Society of Chemistry

Copyright © 2007 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-551-8

Contents

Chapter 1 Production, Isolation and Purification of Fullerenes Roger Taylor, Glenn A. Burley, 1,
Chapter 2 Basic Principles of the Chemical Reactivity of Fullerness Fernando Langa, Pilar de la Cruz, 15,
Chapter 3 Three Electrodes and a Cage: An Account of Electrochemical Research on C60, C70 and their Derivatives Maurizio Carano, Massimo Marcaccio, Francesco Paolucci, 51,
Chapter 4 Light-Induced Processes in Fullerene Multicomponent Systems Nicola Armaroli, Gianluca Accorsi, 79,
Chapter 5 Encapsulation of [60]Fullerne into Dendritic Materials to Facilitate their Nanoscopic Organization Jean-François Nierengarten, Nathalie Solladie, Robert Deschenaux, 127,
Chapter 6 Hydrogen Bonding Donor–Acceptor Carbon Nanostructures M. Ángeles Herranz, Francesco Giacalone, Luis Sánchez, Nazario Martín]TC1, 152,
Chapter 7 Fullerenes for Material Science Stéphane Campidelli, Aurelio Mateo-Alonso, Maurizio Prato, 191,
Chapter 8 Plastic Solar Cells Using Fullerene Derivatives in the Photoactive Layer Piétrick Hudhomme, Jack Cousseau, 221,
Chapter 9 Fullerene Modified Electrodes and Solar Cells Hiroshi Imahori, Tomokazu Umeyama, 266,
Chapter 10 Biological Applications of Fullerenes Alberto Bianco, Tatiana Da Ros, 301,
Chapter 11 Covalent and Non-Covalent Approaches Toward Multifunctional Carbon Nanotube Materials Vito Sgobba, G.M. Aminur Rahman, Christian Ehli, Dirk M. Guldi, 329,
Subject Index, 152,

CHAPTER 1

Production, Isolation and Purification of Fullerenes

ROGER TAYLOR AND GLENN A. BURLEY

Faculty of Chemistry and Pharmacy, Ludwig-Maximilians University, Munich, Germany

1.1 Introduction

Eight fullerenes have been obtained in significant quantities. These are [60-Ih]-, [70-D5h]-, [76-D2]-, [78-D3]-, [78-C2v(I)]-, [78-C2v(II)]-, [84-D2(IV)]-, [84-D2d(II)]-fullerenes and are depicted in Figure 1. Of the fullerene family, [60]fullerene and [70]fullerene are the major isomers obtained in 75 and 24%, respectively, via the arc-discharge method of Hufmann and Krätschmer. The remaining 1% constitutes a variety of higher order fullerenes ranging from C74 to beyond C100. The colours of the fullerene family vary according to their molecular weight and symmetry. Their colours in solution are magenta (C60), port-wine red (C70), brown (C76 and C78), and yellow-green (C84). A major obstacle in higher order fullerene research is confronted when investigating these fullerenes [Figure 1(c)–(h)]. A gradual increase in the size of fullerenes is accompanied by an increase in the number of isomers of the same symmetry, therefore making definitive assignment of the fullerene structure difficult. Coupled with the difficulty of separation and decreasing solubility of higher order fullerenes with increasing size, makes further studies of these larger fullerenes unattractive.

The numbering system adopted for fullerene assignment in this review is the IUPAC system that has been in place for over a decade. The Roman numerals used for subdividing fullerenes exhibiting the same symmetry are those given by Fowler and Manolopoulos.

1.2 Production

Three methods have been used to make fullerenes. These are

(i) The Hufmann–Krätschmer procedure involving arc-discharge between graphite rods in an atmosphere of helium.

(ii) Combustion of benzene in a deficiency of oxygen.

(iii) Condensation of polycyclic aromatic hydrocarbons through pyrolytic dehydrogenation or dehydrohalogenation.

1.2.1 The Hufmann–Krätschmer Method

Hitherto this has been the most important method for fullerene production, and its introduction marked the real beginning of fullerene science. It is the preferred method because the only by-product is graphite, which if necessary, can be reformed into rods and recycled. The Hufmann–Krätschmer method involves arc-discharge between high-purity carbons rods of ca. 6 mm diameter in an atmosphere of 100–200 torr helium. Argon may also be used but is less effective. The temperature required for fullerene formation is ca. 2000°C, and obviously a small gap between the rods is necessary to prevent a fall in temperature. The need for this gap was shown at an early stage and was confirmed subsequently. The yield of fullerenes in the soot produced is approximately 5% and is higher when taken from the reactor at greater distances from the arc source, which implies that the initially formed fullerenes are subsequently degraded by UV irradiation.

Numerous ingenious variations in reactor design were introduced at an early stage, including, for example, a carousel and one with an autoloading device (Figure 2). The latter method, designed by Bezmelnitsyn and Eletskii is especially notable, using carbon strips 7×3.5×400 mm cut from a reactor moderator block. A stack of 24 of these (anode) are housed in a 450 mm long by 280 mm diameter water-cooled chamber equipped with a hinged door fitted with a O-ring seal. The cathode consists of slowly rotating 70 mm-diameter carbon wheel which passes a scraper to remove accumulated slag. The strips are gravity fed and the lowest strip is slowly wheel-driven into the cathode. When consumed it drops away exposing the next strip, and the process continues during 24 h to yield 100–200 g of fullerene-containing soot, accessed by opening the end door of the reactor.

1.2.2 The Combustion Process

Howard and co-workers discovered that combustion of benzene in a deficiency of oxygen resulted in the formation of both [60]- and [70]fullerenes. This continuous method has been developed to the extent that a purpose-built factory has been erected in Japan, capable of producing 5000 ton of fullerenes per year, but currently running at about one-tenth of that capacity. One envisages that this investment must be driven by the expectation or knowledge that large-scale applications of fullerene lie ahead.

1.2.3 Condensation of Polycyclic Aromatic Hydrocarbons through Pyrolytic Dehydrogenation or Dehydrohalogenation

These methods produce fullerenes but not in sufficient quantities for practical applications. Rather, they provide a means of deducing the mechanisms of fullerene synthesis. For example, C60 consists of six dehydronaphthalene moieties located at the octahedral sites, and pyrolysis of naphthalene does indeed produce C60, as does corannulene, (which has been detected as a precursor in the combustion process), 7,10-bis(2,20-dibromovinyl)fluoranthene, and 11,12- benzofluoranthene. The dehydrogenation involved is a high-energy process and dehydrohalogenation of precursors is more successful, a feature made use of in formation of C60 from a chloroaromatic precursor (Scheme 1). No other fullerene has yet been made in this way.

1.3 Isolation and Purification of Fullerenes

The fullerene-containing soot is extracted using a Soxhlet apparatus with either chloroform (slowest), toluene, or 1,2-dichlorobenzene (fastest, but removal of traces of solvent requires high vacuum). If carbon disulfide is used at any stage during the extraction/concentration, then it must be vigorously removed under vacuum, otherwise the fullerene will be contaminated with sulfur.

Purification of fullerenes was first carried by column chromatography using neutral alumina as the stationary phase with elution by hexane. This procedure was improved through the use of mixtures of alumina and other stationary phases and, e.g. toluene/hexane mixtures as eluents. However, [60]fullerene produced in this way is degraded slightly, and must be repeatedly washed with acetone to remove impurities. A method which renders all others obsolete involves passing a toluene solution of the fullerene mixture through a column packed with Elorit carbon (a grade used for making car battery cases). Pure C60 is rapidly eluted and 10 g h-1 can easily be purified in this way. C70 can be removed subsequently by elution with 1,2-dichlorobenzene, but the higher fullerenes are retained firmly by the carbon. Chromatographic separation of fullerenes is too costly for large-scale production, and is the reason that the price has not fallen in line with expectation. It is likely that other methods of purification will be needed, possibly through the use of multiple recrystallisations.

The purity of C60 that is described in many papers is wholly unrealistic, values of 99, 99.5, and even 99.9% being claimed without any justification. To begin with, there is no known way of determining the purity of a fullerene, since combustion is never complete. Also all C60 contains a significant amount of C120O to the extent that it can be recovered through the use of HPLC. Moreover, the presence of hydrocarbons can readily be detected by running an 1H NMR spectrum of a solution in CS2. It seems improbable that C60 of purity greater than 98% has been used in any chemical synthesis. Very pure C60 can of course be obtained through vacuum sublimation, but here there are substantial losses due to the reaction of the fullerene with the impurities at the high temperatures involved.

1.4 Formation and Stability

Fullerenes, especially the higher order members, degrade to an insoluble material on storage in air. The C2v(II) isomer of [78]fullerene is completely degraded after storage for 5 months. Strong heating of KBr discs of the products yield CO2 showing that they are oxygen containing. The instability of [C2v(II)]fullerene may account for the variable relative yield obtained by various research groups. Given the instability of fullerenes, the question arises as to why they are formed. Since fullerenes are thermodynamically less stable than graphite and oxidatively degrade on standing in air, why are they formed in preference to graphite in the first place? The assumption that elimination of dangling bonds is all-important maybe misleading, since graphite is able to accommodate huge numbers of dangling bonds yet remain stable because of the resonance stability arising from the sp2-hybridised carbon atoms in hexagonal arrays. It is the proportion of dangling bonds in the carbon fragment that is critical. The ratio of the number of dangling bonds to the number of carbon atoms n in a single-graphene sheet can be shown to be approximately 1/n. For 1 g of graphite (5 x 1022 atoms), the number of dangling bonds can be calculated to lie between 5.4 x 1011 and an upper limit of 5 x 1022 this latter being the hypothetical case of all-separated benzenoid rings. Notable therefore, freshly prepared pyrolytic carbon has spin density of 1018 g-1, even higher values being reported for pitch derived from pyrolysis of either anthracene or naphthalene, and these values are within the theoretical range.

The arc-discharge process of fullerene formation must in the first instance, produce small carbon fragments possessing dangling bonds. The preferred formation of fullerenes arises from the much higher Arrhenius A-factors (higher collision frequency) as a result of intramolecular processes. The intermolecular reaction will be favoured by high carbon vapour concentration, which may account for the variable yields of fullerenes with changes of the inert gas pressure in arc-discharge reactors. Ring closure in organic chemistry occurs faster the smaller the chains due to the more favourable A-factors. Similar factors may account for the roughly decreasing yields of fullerenes with increasing size, even though the thermodynamic stability increases in this direction due to the reduction in strain.

1.5 Fullerenes with Incarcerated Atoms: incarFullerenes (Endohedrals)

1.5.1 Nitrogen

i-NC60 had been prepared by heating [60]fullerene to ca. 450°C in a glow discharge reactor containing nitrogen. In the recovered fullerene fractions, the ratio of nitrogen-containing to empty molecules is approximately 10-5–10-6. The nitrogen does not bond to the inner surface of the cage because the orbital coefficients on the inside of the cage are small, and also because such bonding would result in increased strain: any carbon involved in bonding would have to move towards the cage centre, increasing the strain on the three other carbon atoms to which it is attached. The absence either of bonding or of charge transfer is shown by the three-fold degenerate EPR spectrum which is however altered slightly by the presence of addends on the cage. This arises because the distortion of the cage causes the three p-orbitals of the nitrogen to be no longer degenerate. The nitrogen-containing and empty molecules show no differences in reactivity, which is also consistent with the lack of bonding between the nitrogen and the cage (which has been described as a ‘chemical Faraday cage’).

1.5.2 Noble Gases

Helium, neon, argon, krypton, and xenon can be incarcerated in fullerenes through the application of high temperature (620°C) and pressure (ca. 40,000 psi). The incorporation fractions for [60]fullerene and [70]fullerene are approximately: He, 0.1%; Ne, 0.2%; Ar, 0.3%; Kr, 0.3%; Xe, 0.008%. The noble gases are released from the fullerenes by heating to 1000°C. Since, the energy required even for helium to pass through a hexagon is calculated to be ca. 200 kcal mol-1, noble gas insertion requires rupturing and reformation of the fullerene cage. Evidently noble gas insertion is not an entirely successful procedure since approximately 50% of the fullerene is lost during the incorporation procedure.

The presence of 3He can be monitored by 3He NMR, and the spectrum is a probe for the magnetic-shielding environment inside the fullerene cavity, in turn reflecting ring currents and hence the aromaticity of the fullerene. Thus, the more aromatic a fullerene, the more upfield should be the signal, hence [70]fullerene (-28.8 ppm) is indicated to be more aromatic than [60]fullerene (-6.3 ppm).

1.5.3 Hydrogen

Hydrogen has been incarcerated into C60 by purely chemical means, involving the creation of an eight-membered orifice-containing oxygen, nitrogen, sulfur, and carbon atoms. This was large enough to permit ingress of a hydrogen molecule, which was followed by thermal loss of the addends to give pure i-H2C60. This was successfully converted into the C120 dimer, by means of high-speed mechanical milling.

1.5.4 Other Atoms

A wide variety of metallic atoms have been incarcerated in a range of fullerenes, with up to four atoms accommodated in some cages. Notably, the cages that are present in low yields when empty are those which produce the most stable derivatives, C74 and C82 in particular. The very low yield of formation of these compounds (many have been detected sprectroscopically only), and the sensitivity of many of them to air means that their isolation, where achieved, is very time-consuming. Nevertheless, preparation of substantial quantities of i-LaC82 (27.4 mg) and i-GdC82 (12.0 mg) have now been described. The interest in the incarfullerenes has been focussed on the location of the atoms within the cages, their molecular motions, and electronic consequences of their presence.

The incarcerated metal transfers electrons to the fullerene cage thereby altering the properties of the latter. Incarfullerenes can be regarded as a ‘superatom’ in having a positively and a negatively charged cage. For example, in the case of i-LaC82, three electrons are transferred, and the electronic structure can thus be represented as i-La3+C823-. The incarfullerenes have longer HPLC retention times than their empty-cage analogues (due probably to stronger coordination of the more electron-rich cage with the stationary phase), and the retention time increases with size of the incarcerated atom. If an incarcerated element can exhibit two different oxidation states (e.g. Sm, Eu, Tm and Yb) then the incarfullerenes in which the element is in the 2+ oxidation state elute with shorter retention times than those in which the element is in the 3+ oxidation state, which follows from the greater charge transfer to the cage in the latter compounds, making them more polar.

Due to the cages being more electron-rich than those of empty fullerenes, they are less soluble in non-polar solvents, but conversely can be selectively separated by polar solvents such as aniline, pyridine, and dimethylformamide. The band gaps are also of the order of 0.2 eV compared to 1.6 eV for [60]fullerene, and the substantially reduced stabilities rules out characterisation of structures by EI mass spectrosopy since fragmentation occurs, and limits studies to laboratories that are equipped with soft-ionisation techniques. The range of fullerenes that have incarcerated atoms is shown in Table 1.

Isomers of incarfullerenes have been isolated by HPLC. Examples are the four isomers of i-CaC8272 and three isomers of i-TmC82 (each is very air stable, but they have quite different electronic properties). Three isomers of i– Sc2C84 have been separated similarly, and 13C NMR shows the main one to be the [D2d(II)] isomer, the symmetry of the spectrum showing that the positions of the scandium atoms are equivalent within the NMR timescale; the other isomers are Cs(II) and C2v(III). The equivalence of the scandium atoms is confirmed by 45Sc NMR which shows only one line. In both i-LaC82 and i-YC82 the metal atoms lie off-centre, so these molecules should be dipolar.

(Continues…)Excerpted from Fullerenes by Fernando Langa, Jean-Francois Nierengarten. Copyright © 2007 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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