[60]Fullerene dimers

Abstract

The main types of fullerene dimers synthesized so far, including the early all-carbon fullerene dimers, other new carbon allotropes containing two fullerene units and the specially interesting fullerene dimers bridged through electroactive spacers, are reviewed. The different synthetic strategies employed have been divided into two main types, namely: (i) dimerization of C₆₀ itself or some C₆₀ derivatives and (ii) bifunctional cycloaddition reactions to C₆₀.

 These new systems are of interest since they can find applications in areas such as artificial photosynthesis, novel molecular electronic devices and in supramolecular chemistry.

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José L. Segura

Nazario Martín studied chemistry at the Universidad Complutense de Madrid (UCM), where he obtained his PhD under Professor C. Seoane in 1984. After a year working on X-ray contrast agents in a pharmaceutical company, he worked as a postdoctoral fellow (1987–1988) at the Institut für Organische Chemie der Universität Tübingen under Professor M. Hanack on electrically conducting organic materials. In 1994, he was a visiting Professor at the Institute for Polymers and Organic Solids (IPOS) at the University of California, Santa Barabara (UCSB) working with Professor F. Wudl on modified fullerenes. Since 1989, he has been Associate Professor of Organic Chemistry at the University Complutense (UCM). His research interests range over the chemistry of fullerenes and electroactive molecules with emphasis on the electrical and optical properties. He is also active in the asymmetric synthesis of heterocyclic systems.

Nazario Martín

José L. Segura received his education in chemistry at the Universidad Complutense de Madrid, where he obtained his diploma in 1989 and his PhD in 1994 under Professor N. Martín and Professor C. Seoane. In 1993 he visited the group of Professor W. Daily (Univesity of Pennsylvania) where he worked in the area of strained-ring organic syntheses. After post-doctoral stays in the group of Professor M. Hanack (University of Tübingen) working in the synthesis of electroluminescent conjugated polymers and in the group of Professor F. Wudl (University of California at Santa Barbara) working on the functionalization of C60 fullerene, Dr Segura joined the faculty in the Department of Organic Chemistry at the Universidad Complutense de Madrid.

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1 Introduction

Connecting fullerene cages has been a subject of interest since the observations of fullerene coalescence upon laser irradiation of C₆₀ films.¹ The experimental evidence for molecular coalescence of C₆₀ molecules, that is, formation of stable higher fullerenes which are multiples of the initial masses, is clearly manifested by measurements of the mass spectra where C₁₂₀ is the most abundant product. However, the shapes of the products from the coalescence reactions are not clearly known and it is not clear whether they are formed as large clusters or as dumbell-shaped (C₆₀)₂ dimers.¹ While the coalescence products are too complex to be analyzed, the seemingly simple addition polymers have not been well characterized either. Thus, as dimeric fullerenes have been proposed as essential subunits of the all-carbon fullerene polymers, it has been of great importance to clarify their detailed structure as well as their physical and chemical properties for a better understanding of the nature of fullerene polymers.² Whereas early efforts were mainly directed at the obtention of all-carbon fullerene dimers in the search for new carbon allotropes, with the development of fullerene chemistry it has been possible to obtain properly functionalized carbon spheres in order to synthesize a variety of the so called dumbbell type dimeric materials with different spacers between the fullerene units. In this article we present an overview of the main types of [60]fullerene dimers synthesized so far, putting particular emphasis on the different synthetic strategies followed for their obtention. These strategies will be divided into two main classes: (i) dimerization of either fullerene itself or a suitably functionalized fullerene and (ii) bifunctional cycloadditions to [60]fullerene.

On the other hand, the design of novel organofullerenes containing electroactive moieties constitutes a promising field due to the interesting optical and electronic properties they can display. Another approach to this field has been the study of fullerene dimers in which two fullerene spheres are attached either directly or are covalently connected through a molecular electroactive bridge. The electronic interactions between the C₆₀ units themselves as well as between the C₆₀ units and the electroactive spacer have been the subject of different studies and we will look especially at the electrochemical and photophysical properties of these redox active compounds.

2 Dimerization reactions of [60]fullerene and derivatives

C₁₂₀ is the most abundant product present in the mass spectra of coalescence experiments as well as in different polymerizations. Therefore, theoretical and experimental studies have been carried out in order to determine its ground-state configuration. These studies reveal that, in the neutral state, the energetically most favourable bonding between the two C₆₀ molecules is accomplished through a [2 + 2] cycloaddition, where the (6,6) short bonds between two hexagons on adjacent molecules are broken and converted into a four membered ring (1, Fig. 1)³ with D_2h symmetry.

Fig. 1

Therefore, 1 has been proposed as the smallest subunit of the all-carbon fullerene polymers produced by coalescence reactions on [60]fullerene. In contrast to the many theoretical studies carried out on this compound, only recently have efficient methods for its synthesis been reported.² Thus, the bulk synthesis of 1 was achieved by the solid state mechanochemical reaction of C₆₀ with KCN by using a high speed vibration milling (HSVM) technique (Scheme 1). Two different mechanisms have been proposed for this reaction: (i) dimerization via nucleophilic addition and (ii) dimerization via coupling through an electron-transfer step.³

Scheme 1

Dimer 1 is a dark brown powder which is barely soluble in most organic solvents. The solubility depends greatly on the purity: the purer the sample, the less soluble it is. It was found to dissociate quantitatively into C₆₀ when heated at 175 °C for 15 min and also to undergo slow photochemical dissociation into C₆₀ under exposure to room light. 0.8% of C₆₀ is produced when a powdery sample of 1 is subjected to a hydraulic pressure of 100 kg cm⁻² for 30 min. The cyclic voltammogram of 1 exhibits three reversible reduction waves which are almost identical to those of C₆₀ observed under the same conditions. A slight shoulder on the first reduction peak of 1 is the only remarkable difference to C₆₀, thus indicating that dimer 1 readily dissociates into two C₆₀ units immediately after C₁₂₀ acquires extra negative charge.³ The structure of 1 has been determined by X-ray crystallography for a single crystal grown in an o-dichlorobenzene (ODCB) solution.² The dimer is composed of two C₆₀ cages sharing a cyclobutane ring with the C₁₂₀ molecules arrayed in highly ordered layers, different from the face-centered cubic arrangement of C₆₀. The relative elongation of the inter-cage bonds (1.575(7) Å) compared with the ordinary C(sp³)–C(sp³) single bond may be responsible for the above mentioned ready thermal cleavage of 1.

The close similarity between the FT-IR spectrum of all-carbon poly[60]fullerenes and 1 has been used as evidence for the proposed [2 + 2] cycloaddition linking pattern in polyfullerenes.

Other extensively studied fullerene dimers are fullerene oxides such as C₁₂₀O. The synthesis of this type of compounds was stimulated by the discovery of the odd-numbered fullerene clusters⁴ and Taylor’s proposed mechanism for the generation of C₁₁₉via thermal decarbonylation of C₆₀O and addition of the resultant C₅₉ to C₆₀.⁵

In 1995 it was almost simultaneously reported by two independent groups^6,7 that thermolysis of C₆₀O with a [6,6] epoxide structure (2) in the presence of C₆₀ affords the corresponding oxygen-bridged C₆₀ dimer (4). By using the same experimental conditions, the methylene-bridged C₆₀ dimer (5) was also obtained by thermolysis of the [6,6] fullerene cyclopropane 3 and C₆₀ (Scheme 2).

Scheme 2

Very recently, Taylor and co-workers have shown that at ambient temperature and in the solid state C₆₀ degrades to C₁₂₀O (4) which is present in up to 1% concentration in the samples examined and they have been able to extract multimilligram quantities of C₁₂₀O (which crystallizes from carbon disulfide as black needles) from commercial C₆₀.⁸ The authors claim that C₆₀ undergoes oxidation to C₆₀O (2) on exposure to air, but given that the mono-oxide is strained and is predicted to be unstable relative to C₁₂₀O, it is captured by a further C₆₀ molecule in a [2 + 2] reaction to produce C₁₂₀O (4).

In the electrochemical study of dimer 4 formed by two C₆₀ cages connected by a tetrahydrofuran ring, two reversible asymmetric peaks along with a resolved set of two reversible peaks can be observed. Each asymmetric peak corresponds to two closely spaced waves resulting from the superposition of two processes with very close reduction potentials. This electrochemical behaviour indicates that the two cages are not reduced simultaneously, thus suggesting that the two C₆₀ moieties are communicating, in contrast to what is observed for dimer 1 where the two C₆₀ units are connected by a cyclobutane ring.

Following thermal reactions of the C₆₀/C₆₀O/C₆₀O₂ system in the solid state, various other dimeric fullerene derivatives such as C₁₂₀O₂ (6) and C₁₁₉ (7) have been isolated and characterized. In C₁₂₀O₂ the links are composed of two furanoid rings sharing a C–C bond interconnecting with a central 4-membered ring.⁹ Here, in contrast to C₁₂₀O, the four-membered ring is thought to connect between single (6,5) bonds of the original C₆₀’s (Fig. 2).

Fig. 2

More recently, the thermolysis of C₁₂₀O has been performed in the presence of sulfur yielding the corresponding sulfur analogue C₁₂₀OS (8).¹⁰ This is the first sulfur containing dimeric [60]fullerene derivative to be synthesized and characterized by spectroscopic methods. The data obtained are consistent with the lowest-energy modeled structure in which two fullerene cages are linked via furan and thiophene bridging rings with a cis configuration of the 6,6 junctions (Fig. 3).¹⁰

Fig. 3

After early mass spectroscopic work proved that odd-numbered dimer-related species can be formed out of fullerene epoxides,⁵ it has proven possible to isolate C₁₁₉ (7). The ‘classical’ fullerenes contain only an even number of quasi-sp² carbon atoms in a closed-cage structure in which pentagons are completely surrounded by hexagons. However, McElvany et al. discovered the presence of odd-number clusters C₁₁₉, C₁₂₉ and C₁₃₉⁴ in the thermal desorption mass spectra of a toluene extract of comercial soot and in the spectra of the products of ozone–fullerene reactions. These species can be obtained by thermal decomposition of C₁₂₀O_x (x < 3). C₁₁₉ is oxygen sensitive and decomposes within hours under ambient conditions although under argon is stable in a refrigerator for weeks. C₁₁₉ is thought to consist of two partially opened cages connected by three sp³ carbons and two seven-membered rings (Fig. 4). Consequently, although it retains a high degree of topological analogy to the title dimers, the structural integrity of the individual cages is no longer maintained in C₁₁₉.

Fig. 4

As was previously stated, in the neutral case, the energetically most favourable bonding between the two C₆₀ molecules is accomplished through a [2 + 2] cycloaddition. Thus, the energy calculated for a single bonded (SB) isomer (9) formed by direct covalent bonding between two C₆₀ molecules is higher than that obtained for 1. With the addition of one electron to each C₆₀ molecule, however, different calculations¹¹ indicate that the relative energy ordering between the [2 + 2] and SB structure reverts. As a result, the SB dimer is predicted to be more stable than the [2 + 2] isomer in mono-anionic fullerides. Recently, X-ray results show that quenching the high-temperature face-centered cubic (fcc) phase of A₁C₆₀ (A = K, Rb) below 280 K produces a metastable orthorhombic structure in which the molecules exist as dimer pairs. The experimentally observed 9.34 Å center-of-mass distance in the charged dimer is in good agreement with the calculated value of 9.30 Å for the SB configuration with a C_2h symmetry (the two balls being in a trans position, Fig. 5a).¹¹ Interestingly, these dimers resulting from the monodoped fulleride structures (C₁₂₀²⁻) are isoelectronic with the azafullerene dimer (C₅₉N)₂ (10) which has been shown to be bonded by a single C–C intermolecular bond (Fig. 5b).¹²

Fig. 5

Some of the best studied fullerene dumbbell-shaped dimers are those with the general structure RC₆₀C₆₀R (11 R = H, halo, alkyl, fluoroalkyl) which are formed by the recombination of RC₆₀· radicals, which in turn can be prepared in situ by addition of R· to C₆₀.^13,14 (Fig. 6).

Fig. 6

An important aspect of the chemistry of C₆₀ is its reactivity towards free radicals. Radicals add rapidly and multiply to the double bonds of C₆₀ to give products R_nC₆₀. A remarkable feature of the EPR spectra of the RC₆₀ adducts is the dramatic increase in intensity of the spectrum with increasing temperature, in contrast to the usual decrease in intensity associated with the Curie law. Temperature dependence of this type may be indicative of an equilibrium between a radical and its dimer. Thus, the dependence of signal intensity on temperature and dilution at a fixed temperature established that the radical adducts of C₆₀ exist in equilibrium with their dimers and that the dimer bond strength depends on the size of the group R.¹³

RC₆₀C₆₀R ⇌ 2RC₆₀ (1)

Substituents R are considered to be attached at ortho or para positions relative to the intercage bond, based on the EPR spectra of monomeric species RC₆₀· showing the unpaired electron localized within the six membered ring bearing the R group.¹³ C–C single bonds bearing large cage fragments at both ends provide interesting targets for studies of internal rotation and rotational isomerism. Thus, Osawa and coworkers have studied the rotation about the intercage bond of singly bonded RC₆₀C₆₀R (R = H, Me, and t-Bu attached at the ortho or para positions from pivot carbon atoms).¹⁴ They observed that rotamers with R groups situated close to each other as in the gauche form are predicted to be more stable than those with maximally separated R groups (trans) and rationalized the unexpected stability of the gauche-like conformation in terms of a lever effect peculiar to the dumbell shape of the molecule.¹⁴

An interesting example of an RC₆₀C₆₀R dimer, in which R is a morpholine group (11, R = morpholine) has been reported by Hirsch and co-workers.¹⁵ This first aminated fullerene dimer with a single inter-fullerene connection was isolated, together with dehydrogenated amino adducts with a preferred 1,4-addition pattern, in the reaction of [60]fullerene with an excess of morpholine in benzene in the presence of oxygen. The authors claimed that the fact that only dehydrogenated products were isolated shows that the initially formed intermediates such as aminated fullerene carbanions R₂NC₆₀⁻ or zwitterions R₂HN⁺C₆₀⁻ are oxidized by oxygen to form aminated radicals R₂NC₆₀· which may recombine through a dimerization process (Fig. 6).

A different approach towards the obtention of a new kind of carbon allotrope involves the combination of fullerene and acetylene units in the so called all carbon chemistry. Among the attractive intermediates on the route towards these new carbon allotropes are the cyclic polyynes (12, 13) depicted in Fig. 7 in which macrocyclic polyynes are ‘end-capped’ with methanofullerenes.¹⁶

Fig. 7

With this aim, Diederich and co-workers developed new synthetic routes to symmetrically and unsymmetrically substituted diethynylmethanofullerenes which have been used in the preparation of dimeric fullerene derivatives. Thus, monoprotected diethynylmethanofullerene (14) was homocoupled under Hay coupling conditions to the corresponding butadiynyl-linked dimeric methanofullerene (15, Scheme 3) which was isolated in 37% yield as black shiny crystals, reasonably soluble in cyclohexane, toluene and CS₂.

Scheme 3

In the cyclic voltammetry measurements performed on 15, a single reduction potential value (−0.99 V, in CH₂Cl₂vs. Fc/Fc⁺) assigned to the transfer of one electron to each of the two C₆₀ moieties and two very similar potentials (−1.43 and −1.56 V) for their second reduction steps were observed. These values are similar to those observed for the monomeric analogs, thus suggesting that both through-bond and through-space electronic interactions between the two fullerene moieties in 15 are not very efficient. This fact is in agreement with the UV/Vis data¹⁶ as well as with the X-ray structural data¹⁷ and indicates that the C(sp³) atoms in the two butadiynediyl-linked methano bridges of 15 act as insulators.

Alternatively, dimeric fullerene derivatives in which the acetylene spacers are directly attached to the fullerene core have been prepared. Diederich and co-workers have recently synthesized buta-1,3-diynediyl-linked bis(fullerenes) (17, 18) and have made attempts at their conversion into the corresponding dianion 19.¹⁸ Syntheses of these compounds were carried out by homocoupling of ethynylfullerenes 16, bearing base-stable substituents at their 2-position. In situ preparation of the Hay catalyst [CuCl–TMEDA–O₂] (TMEDA = N,N,N′,N′-tetramethylenedi amine) in PhCl in the presence of 16 followed by stirring for 24 h in dry air at room temperature afforded bisfullerenes 17. Removal of the tetrahydropyran-2-yl (THP) protecting groups in 17b gives the highly insoluble dumbbell 18 with hydroxymethylene groups on the 2,2′-positions of the buta-1,3-diynediyl-bridged carbon spheres. Attempted conversion of 18 to the all-carbon dianion 19 (C₁₂₄²⁻) via base-induced elimination was not successful¹⁸ (Scheme 4).

Scheme 4

As observed for the parent dumbbell fullerenes 15, the cyclic voltammetry measurements performed for 17 clearly indicate that the two C₆₀ moieties behave as two electrochemically independent units.¹⁸

Komatsu and co-workers have synthesized, in a very low yield, a fullerene dimer with an acetylene spacer (20), instead of the butadiyne linker present in 17, by reaction of dilithioacetylene with C₆₀ in toluene, followed by addition of dodecyl iodide at reflux temperature (Scheme 5).¹⁹

Scheme 5

As in the preceeding examples, there is no evidence for electronic interaction between both C₆₀ cages observable in the electronic spectra of 20. Cyclic voltammetry measurements performed on 20 show that fullerene moieties are simultaneously reduced, thus confirming the absence of electronic interaction between them.¹⁹

A recent and outstanding study on the dimerization of methanofullerenecarbene, C₆₀C: (24) and its addition to C₆₀ has led to the new carbon allotropes C₁₂₁ (25) and C₁₂₂ (26) (Scheme 6).²⁰

Scheme 6

Carbene intermediate 24, produced from the methanofullerene 21, was proposed as a precursor of C₁₂₁ and C₁₂₂ during pioneering studies by Osterodt and Vögtle, which were detected by mass spectrometry.²¹ In 1998, Strongin and co-workers reported the first isolation and characterization of C₁₂₂via the reaction of [60]fullerene with the atomic carbon precursor diazotetrazole 23.²² It was isolated as a black solid that formed a golden-yellow solution in toluene. It showed two intense characteristic fullerene absorptions at 210 and 260 nm, while no clear absorptions in the region from 400 to 700 nm were observed. Spectroscopic studies are consistent with a dimeric D_2h-symmetric structure. Dragoe and co-workers have also succeeded very recently in the syntheses of both C₁₂₁ and C₁₂₂.²⁰ When a mixture of C₆₀CBr₂ (21) and C₆₀ was heated at 450 °C in an argon atmosphere at a rate of 5 K min⁻¹ and then cooled to room temperature, a black powder, soluble in CS₂ and o-dichlorobenzene was obtained (the same product could be obtained by using C₆₀CHCO₂Et 22 instead of C₆₀CBr₂21 and a higher temperature, i.e. 550 °C). The mixture was chromatographed (GPC, toluene) and both C₁₂₁ and C₁₂₂ could be isolated. The amount of C₁₂₂ decreases when increasing the C₆₀/C₆₀CBr₂ ratio; the formation of C₁₂₂ may be suppressed when using more than a ten-fold excess of C₆₀. Spectroscopic data obtained for C₁₂₁ are in agreement with a C_2v symmetry for the molecule. Scanning tunneling microscopy (STM) measurements on samples of both C₁₂₁ and C₁₂₂ show similar images corresponding to dumbbell-like structures.²⁰

3 Bifunctional cycloadditions to [60]fullerene

A large variety of cycloadditions have been carried out with C₆₀ and the complete characterization of both monoadducts and multiple adducts greatly increased the knowledge of fullerene chemistry. [60]Fullerene is an excellent dienophile in cycloaddition reactions which reacts by a [6,6] double bond, thus avoiding the unfavourable [5,6] double bonds within the fullerene skeleton. Cycloadditions to C₆₀ have been well-studied, running from [2 + 1], [2 + 2], [3 + 2], [4 + 2] to [8 + 2], yielding the respective monocycloadducts which are easily isolated in moderate to good yields. In contrast, bisaddition leads to the formation of regioisomeric mixtures of bis-adducts which are usually very difficult to separate. Derivatization of C₆₀ with bifunctionalized molecules has been used as an alternative and efficient method for the preparation of fullerene dimeric materials.

Wudl and co-workers reported in 1992 the first syntheses of dimers by reaction of bis(diazoderivatives).²³ Since then, different dumbbell-like fullerene dimers have been synthesized by using bisdienes in Diels–Alder cycloadditions. In 1993, Müllen and co-workers reported that bis-o-quinodimethane 29 (R = H), generated in situ from 1,2,4,5-tetrakis(bromomethyl)benzene (28 R = H), indeed captures two fullerene molecules to provide a highly insoluble bisadduct (30, n = 0, R = H, Scheme 7).²⁴ When flexible, solubilizing substituents were introduced in the bis-o-quinodimethane moiety (29 R = OHex) different oligomeric materials 30 (n = 0–5) could be detected by using MALDI-TOF mass spectrometry.²⁵ The dumbbell compound 30 (n = 0, R = OHex) was found to be soluble in halogenated benzenes and exists as a single regioisomer so that it was possible to characterize it by NMR spectroscopy.

Scheme 7

Paquette and co-workers have also used bisdienes to prepare dumbbell-like systems 32 in which a pair of C₆₀ units are linked through cyclohexa-1,4-dienyl ladders of different length (Scheme 8).²⁶ With control of the spacing of the spheres, it is expected to gain some knowledge of the possible operation of intramolecular communication between the fullerene cages. However, a restriction of exploitation of these systems is their striking insolubility.

Scheme 8

More recently, a solid state [4 + 2] cycloaddition of pentacene to C₆₀ using a high-speed vibration milling (HSVM) technique afforded a novel C₆₀ dimer (33) with the two C₆₀ cages assumed to be in the less sterically hindered anti-face arrangement.²⁷

Other selected fullerene dimers obtained from bifunctional cycloadditions are given in Fig. 9. Dimer 34 was obtained from the double 1,3-dipolar cycloaddition of [60]fullerene to cyanogen di-N-oxide (Fig. 9).²⁸ The solubility of 34 in a mixture of 1-chloronaphthalene–2-chloronaphthalene 9∶1 (v/v) allowed its characterization by recording ¹³C-NMR spectra. The cycloaddition occurs at the [6,6] ring junction of the fullerene and the number of signals clearly indicate a C_s symmetry of the molecule.

Fig. 8

Fig. 9

1,3-Dipolar cycloadditions of azomethine ylides to C₆₀ is a suitable and quite general procedure for the functionalization of [60]fullerene.²⁹ In 1996, we reported the first synthesis of the dimeric [60]fullerene 35 (Fig. 9) by using this type of cycloaddition reaction.³⁰ Thus, by reacting C₆₀ with sarcosine and the appropriate dialdehyde in refluxing toluene, 35 was isolated as a black solid showing an extremely low solubility in most organic solvents. Its UV-Vis spectrum shows the presence of a band at around 410 nm, in addition to the typical weak absorption band at 430 nm of dihydrofullerenes, thus confirming the [6,6]-closed character of the dimer. The cyclic voltammetry measurements indicate that there is no significant interaction between both fullerene cages of the dimer in the ground state, as has been previously observed for dyad 36.³⁰

4 Fullerene dimers connected through molecular electroactive bridges

As stated in the Introduction, the design of dyads bearing covalently linked electron donor moieties in close proximity to the C₆₀ surface is a valuable approach to artificial photosynthesis and novel molecular electronics. We have recently reviewed the wide variety of C₆₀ donor dyads in which the C₆₀ core is covalently attached to an electron donor unit as promising candidates for the search of electron-transfer properties.³¹ An intriguing and less studied approach consists of the synthesis of fullerene dimers in which the two fullerene spheres are connected through a molecular electroactive bridge. The electronic interactions between both C₆₀ units as well as the C₆₀ units and the electroactive spacer have been the subjects of different studies.

Intramolecular processes such as electron and energy transfer have been observed in different C₆₀–porphyrin dyads. Very recently, triads C₆₀–porphyrin–C₆₀37, 38, and 39 (Fig. 10) have been synthesized, and their electronic properties studied.

Fig. 10

Triads 37 and 38 have been synthesized by using Bingel macrocyclization of porphyrin tethered bismalonates³² while 39 has been obtained by 1,3-dipolar cycloaddition from the corresponding formylporphyrins, N-methylglycine and C₆₀ in toluene.³³38 is obtained as a mixture of two conformers in slow equilibrium, as shown by a variable temperature NMR study and represents the first example of a bis(meta-substituted phenyl)porphyrin for which the barrier to free rotation is high enough at room temperature to be able to distinguish the cis and trans isomers. On the other hand, because of the unsymmetrical carbon of the pyrrolidine units, two stereoisomers corresponding to the meso and racemic isomers could be isolated for 39. A strong quenching of the flurescence emission of porphyrin is observed for the three triads (37-39). From the electrochemical measurements, it can be stated that the mutual electronic effects exerted by the fullerene on the porphyrin and vice versa in 39 are relatively small and, thus, the ground-state donor–acceptor interactions are hardly measurable by electrochemical procedures.³²

Other electroactive bridges used to connect fullerene moieties are bipyridine (bpy) ligands and the strong electron donor tetrathiafulvalene (Fig. 11). Very recently, the synthesis of the bis-C₆₀–bpy derivative 40 has been reported.³⁴ Functionalization of C₆₀ by using the Bingel reaction was followed by a multistep synthetic pathway to yield the fullerene dimer 40 as a soluble material in which the methanofullerene moieties are separated from the bipyridine ligand by polyethoxy spacers. The subsequent reaction of 40 with [Ru(bpy)₂Cl₂] yields the ruthenium(II) complex [Ru(40)bpy₂][PF₆]₂ (41). The electronic absorption spectrum of this triad displays absorptions assigned to the ruthenium complex subunit and the fullerene moieties. The observed data suggest the absence of charge transfer between both units.

Fig. 11

Fullerenes covalently attached to the strong electron donor tetrathiafulvalene (TTF) moieties are very attractive systems for the preparation of photovoltaic devices. The TTF moiety forms, upon oxidation, thermodynamically very stable radical-cation and dication species due to the aromatic character of the 1,3-dithiolium cations. This gain in aromaticity represents a new concept in stabilization of the charge-separated state in electroactive dyads. Although different attempts have been made to synthesize C₆₀–TTF–C₆₀ triads, to the best of our knowlege, only one example is reported in the literature.³⁵ Delhaès and co-workers have prepared three molecular assemblies in which a TTF derivative is covalently linked to two (42, Fig. 11) or even four C₆₀ molecules through flexible rings. UV-Vis spectroscopy as well as cyclic voltammetry measurements indicate that no interaction between the different donor and acceptor systems of the supermolecules can be detected in the ground state. Nevertheless, these systems are promising candidates for further photophysical studies in the search for improved optoelectronic devices.

The photoinduced electron transfer from conducting polymers derived from polyphenylenevinylenes as well as polythiophenes to C₆₀ has been used to prepare improved polymeric photovoltaic cells with high efficiency. These polymer–C₆₀ systems represent an outstanding and realistic application of fullerenes. While the forward electron transfer reaction occurs in a subpicosecond time scale, the recombination of the photogenerated electrons on fullerenes and the holes in the polymer is slow (μs). The study of photoinduced electron transfer in molecular dyads and triads of conjugated oligomers covalently linked to fullerenes has been proposed in order to investigate whether this large difference in electron transfer rates is an intrisic property of the structures involved. To the best of our knowledge, only two examples of conjugated oligomers end-capped with two C₆₀ moieties (43, 44, Fig. 12) have been reported and both have been prepared by 1,3-dipolar cycloaddition from the corresponding diformyl oligomers, N-methylglycineand C₆₀.^36,37 We have synthesized molecular triads of oligo-2,6-naphthylenevinylenes end-capped with [60]fullerene 43³⁶ and observed that the strong fluorescence of the conjugated oligomer is strongly quenched by the fullerene moiety in the triad. Similarly to the porphyrin analogues, the electrochemical study of 43 reveals that no interaction takes place between the conjugated oligomer and the fullerene moiety in the ground state. It is important to note that oligomers are well-defined molecules which enable a relationship to be established between the structural effects and electronic properties of the related polymers and, therefore, the C₆₀-oligomer-C₆₀ systems deserve to be studied in more depth.

Fig. 12

Finally, with the aim of incorporating fullerenes into multicomponent molecular systems in order to provide access to the construction of two-and three-dimensional supramolecular architectures with defined geometries, some dimeric fullerenes have been prepared. Thus, taking advantage of the Pt(II)-directed self-assembly of multidentate ligands, a tetracationic cyclophane 45 containing two fullerene moieties has been recently synthesized as a tetrakis-triflate salt (Fig. 13).³⁸ The X-ray analysis of this structure indicates the almost quantitative nature of the self-assembly process. The use of this strategy paves the way for building up complex supramolecular fullerene arrays.

Fig. 13

On the other hand, a copper(I)-complexed rotaxane (46, Fig. 14) bearing two fullerene stoppers has been prepared in an interesting extension of the use of electro- and/or photoactive species in the preparation of multicomponent molecular systems displaying electronic and photochemical properties.³⁹ Rotaxane 46 was prepared by using the above studied procedure of oxidative coupling reaction of terminal alkynes.¹⁶ Electrochemical studies carried out on 46 indicate a strong effect of the fullerene moiety leading to a significant anodic shift of the Cu^I/II redox potential (+0.865 V vs. SCE in CH₂Cl₂) as compared to other related complexes (ΔE° ≈ 0.2 V). In contrast, the metal center does not seem to change the redox properties of the fullerene units which show the first reduction potential at around −0.6 V. Luminescence measurements show that emission from the metal-to-ligand charge-transfer triplet state of the copper(I) complex 46 is quenched by the two C₆₀ units as stoppers.

Fig. 14

5 Summary and conclusions

In this article, we have presented selected new classes of dimeric fullerenes in which the carbon spheres are connected in a variety of ways.

Early efforts in this area were mainly directed to the obtention of all-carbon fullerene dimers and polymers in the search for new carbon allotropes by using physical methods such as laser irradiation or high pressure transformations. However, the products obtained were too complex to be analyzed and many of the isolated products could not be properly characterized.

With the development of covalent fullerene chemistry, new molecular carbon allotropes containing two fullerene cages such as C₁₁₉, C₁₂₀, C₁₂₁ and C₁₂₂ could be prepared by means of different synthetic procedures. Spectroscopic and crystallographic studies have allowed the identification of their structures which has been of great importance in understanding the nature of all-carbon fullerene polymers.

The major advantage of the synthetic pathways toward dimeric fullerenes is the possibility of introducing functional groups which allows for the obtention of new types of soluble dumbbell fullerene dimers with very different structures and properties. The different synthetic strategies employed have been divided into two main types, namely: (i) dimerization of C₆₀ itself or some C₆₀ derivatives and (ii) bifunctional cycloaddition reactions to C₆₀.

Depending upon the spacer between the fullerene cages, different degrees of interaction among them, either in the ground or in the excited states, have been observed by using different electrochemical and photophysical methods. Specially interesting are fullerene dimers connected via molecular electroactive bridges as they can find applications in areas such as artificial photosynthesis and novel molecular electronic devices.

Finally, with the aim of incorporating fullerenes into multicomponent molecular systems in order to provide access to the construction of two- and three-dimensional supramolecular architectures with defined geometries, some dimeric fullerenes have been prepared.

In conclusion, C₆₀ dimers represent a new class of carbon allotropes and new modified fullerenes whose spheres can be either directly linked or connected through a functionalized spacer. Both supermolecules are promising candidates for the still demanding application of fullerenes.

Acknowledgements

We thank the DGICYT of Spain (Project PB95-0428-CO2) for financial support.

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