C60-based dumbbells: connecting C60 cages through electroactive bridges

C60-based dumbbells: connecting C60 cages through electroactive bridges

Luis Sanchez, Ma Angeles Herranz and Nazario Martın*

Received 18th November 2004, Accepted 5th January 2005

First published as an Advance Article on the web 26th January 2005

DOI: 10.1039/b417580h

Recent examples of C60 dumbbells covalently linked by different electron-donor fragments

(porphyrins, p-conjugated oligomers, TTFs, etc.) are summarized and their main properties

highlighted. Although, similarly to the related C60–donor dyads, no significant electronic

interaction has been observed between the three electroactive components in the ground state,

namely two C60 units and the electron-donor fragment, the presence of a second C60 cage provides

enhanced photophysical properties to the final triads in comparison to the related dyads. These

findings suggest the presence of a cooperative effect. Some of these C60 dumbbell triads have been

applied in the construction of photovoltaic devices and the results obtained indicate that they can

play a very active role in the emerging field of optoelectronic devices.

*[email protected]

Luis Sanchez (left) is Associate Professor of Organic Chemistry atthe University Complutense of Madrid (UCM), Spain. Hereceived his Ph.D. in Organic Chemistry at the UniversityComplutense of Madrid in 1997, where he carried out the synthesisand studied the properties of C60–donor dyads and triads. From1999 to 2000 he worked as a postdoctoral researcher with ProfessorJan C. (Kees) Hummelen (University of Groningen, TheNetherlands) on the synthesis of supramolecular architecturesbased on C60 and their application in the preparation of organicsolar cells. In 2002, he was appointed as an associate professor atUCM. His current research interests are focussed on newsupramolecular C60-based ensembles especially in the study ofelectron transfer processes and photovoltaic applications.

Ma Angeles Herranz (center) obtained her Ph.D. in 2001 from theUniversity Complutense of Madrid (UCM) under Profs. NazarioMartın and Carlos Seoane, working on the synthesis and propertiesof donor–acceptor systems based on [60]fullerene and tetrathiaful-valene (TTF). Starting in 2001, she spent three years as apostdoctoral researcher with Prof. Luis Echegoyen, first at the

University of Miami (Florida, USA) and then atClemson University (South Carolina, USA),where she was engaged in a number of projects,including electrochemical and spectroelectro-chemical characterization of new TTF-baseddyads, and characterization of fullerene-derivedmaterials employing electrochemical, ESR, andNMR techniques. Currently she holds a positionas research associate at UCM, where she isfocussed on the chemistry of fullerenes andcarbon nanotubes and the study of their electronicproperties for applications in materials science.

Nazario Martın (right) studied chemistry at theUniversity Complutense of Madrid (UCM),where he obtained his Doctorate at theDepartment of Organic Chemistry in 1984.After spending a year working on X-ray contrastagents in a pharmaceutical company, he workedas postdoctoral fellow (1987–1988) with Prof.Michael Hanack at the Institut fur Organische

Chemie der Universitat Tubingen on electrically conducting organicmaterials. In 1994, he was a visiting Professor with Prof. FredWudl at the Institute for Polymers and Organic Solids (IPOS) atthe University of California, Santa Barbara (UCSB) working onthe chemistry of fullerenes. He is currently full professor of OrganicChemistry at University Complutense of Madrid (UCM). Hisresearch interests range over electroactive molecules with emphasison the covalent and supramolecular chemistry of fullerenes,electron donor tetrathiafulvalenes (TTFs) and p-conjugatedoligomers and dendrimers in the context of electron transferprocesses and photovoltaic applications. He is currently a memberof the International Advisory Editorial Board of Journal ofMaterials Chemistry, Regional Editor for Europe of the journalFullerenes, Nanotubes and Carbon Nanostructures, and GeneralEditor of the Spanish Royal Society of Chemistry. He haspublished over 250 peer reviewed papers in addition to several bookchapters and other scientific articles. He has been guest co-editor ofthe special issue ‘‘Functionalized Fullerene Materials’’ in theJournal of Materials Chemistry and is co-editor of four books onNew Materials and Fullerenes.

Luis Sanchez, Ma Angeles Herranz and Nazario Martın (left to right)

FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 1409–1421 | 1409

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Introduction

The absorption of sunlight by plants and bacteria initiates

photosynthesis: cascades of energy and electron transfer (ET

and eT) events, which eventually results in the conversion of

CO2 and H2O into glucose and energy (ATP). The transfer of

electrons is produced stepwise over a series of redox-active

cofactors thus spanning the cell membrane. In each step, a

fraction of energy is lost and the distance between charges of

different sign increases. It is important to note that all these

events are produced in a rigid matrix and, therefore, in well-

ordered media. The success of Nature in converting inexpen-

sive, non-polluting and inexhaustible sunlight into energy has

stimulated investigation of this natural process in order to

emulate it. The quest for biomimetic systems is a very active

field in science. Artificial photosynthetic models could reduce

the complicated natural mechanism to its basic elements,

leading to a better understanding of the photosynthetic

process. Additionally, these artificial systems have been widely

used in molecular-scale optoelectronics (especially in the field

of organic solar cells), photonics, sensors design, and other

areas of nanotechnology. Either to mimic photosynthesis or to

build optoelectronic devices, absorption of light, followed by

an electron transfer process, which leads to the corresponding

radical-pair species, is necessary. Therefore, the minimum

requirements for such biomimetic models must involve: i) the

presence of an electron donor (D) connected to an electron

acceptor (A); ii) the ability of one of those electroactive

components to absorb visible light, thus acting as an antenna,

and iii) an organizational principle that controls their

electronic interactions (and therefore the rates and yields of

electron transfer, eT).1

In the development of these photosynthetic models, the

spherical electron accepting [60]fullerene molecule plays a

crucial role. There are several important reasons that render

C60 as a very appealing candidate for the preparation of

biomimetic architectures: i) the easy chemical functionalization

of fullerenes, which has allowed the incorporation of most of

the organic functional groups covalently linked to the carbon

framework,2 ii) the delocalization of charges within the

spherical carbon cages3 and iii) the ability of C60 to accept

up to six electrons upon reduction (the first redox potential at

values similar to that of quinones).4

Although there are many strategies for the application of

the basic physical and chemical principles of photosynthesis

to artificial systems, the assembly of the two electroactive

units provides the necessary pathway that allows photo-

induced electron transfer to compete with the usual photo-

physical pathways that depopulate excited states. The most

studied approach for the interconnection of the two electro-

active fragments consists of the covalent binding of both

D and A moieties which has given rise to a plethora of C60–

donor dyads.5

The similarity of porphyrins (P) to the natural electron

donor centers has made this kind of chromophore the main

choice for the preparation of many C60–P dyads (Chart 1).6

Nevertheless, other classes of electron donor compounds

have also been used, for instance, organometallic complexes,

such as ferrocenes7 or Ru(II) bipyridine,8 carotenes,9 aniline

derivatives,10 p-conjugated oligomers11a–c and phthalocy-

anines,11d and tetrathiafulvalene derivatives12 (Chart 1).

Despite the supremacy of the covalent linkage between the

two electroactive chromophores over the non-covalent inter-

actions, the latter are also being used to obtain supramolecular

entities. The resulting architectures combine the fascinating

world of Supramolecular Chemistry with research in the field

of fullerenes.13

Most of these electroactive dyads undergo not only a fast

charge separation upon excitation but also a rapid, and less-

desirable, charge recombination (CR), thus decreasing the

lifetimes of the corresponding charge separated state to the

range of a few nanoseconds. This range is too low to carry out

any useful process involving radical pairs. To overcome this

problem the sequential assembly of several electroactive

fragments gives rise to triads, tetrads, pentads, etc., whose

long-distance radical pairs present remarkably larger lifetimes

(ranging up to milliseconds!) due to the cascade of charge

transfer steps.14 However, the synthesis of these sophisticated

systems is highly time consuming and, therefore, quite

expensive. A new approach able to enhance the eT and to

slow down the CR process is then necessary. In this regard, a

very elegant route to address such a challenge has been the

synthesis of C60 dimers15 connected through an electroactive

spacer. The resulting triads are easier to prepare, provide

larger lifetimes for the resulting charge-separated intermediates

and, additionally, the presence of two C60 fragments could

contribute to establish well-ordered solid phases, which is a

crucial factor in the field of optoelectronic devices research.

The aim of this article is twofold: i) to summarize the most

recent achievements regarding the synthesis and properties of

C60 dimers covalently bridged by electroactive spacers and,

ii) to compare, when it is possible, the properties of these

dumbbell triads with their analogous dyads, highlighting

the beneficial effect that a second C60 moiety has on their

photophysical properties.

C60 dumbbells endowed with electroactive bridges

The general strategy for the synthesis of C60–donor–C60 triads

consists of a twofold cycloaddition of the corresponding

bifunctional starting material to [60]fullerene. The symmetry

of the precursors needed for the corresponding triads usually

implies less-demanding synthetic efforts than the related dyads,

since the synthetic protocols to obtain organic compounds

bearing two symmetric functional groups are well established.

In the Introduction the most commonly used electron donor

fragments in this field were summarized. They have also

been employed to obtain the respective C60–D–C60 systems.

Therefore, in this article, we will highlight the main recent

achievements involving these appealing C60-based structures.

C60–piperazine–C60 dumbbell triad

Our research group reported one of the first examples of such

dumbbells in 1996.16 In this case, the electron donor fragment

is a piperazine, which formally can be considered as a double

dimethylaniline moiety (1). The triad was obtained by a 1,3-

dipolar cycloaddition between the azomethine ylide, generated

in situ from the piperazine dialdehyde and N-methylglicine,

1410 | J. Mater. Chem., 2005, 15, 1409–1421 This journal is � The Royal Society of Chemistry 2005

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and one of the double bonds of C60. The presence of the two

C60 moieties led to a highly insoluble material whose cyclic

voltammogram showed the presence of the two electroactive

units. These electrochemical measurements also demonstrated

the absence of electronic interaction between the aniline-type

donor and the [60]fullerene moiety (Fig. 1).

C60–porphyrin–C60 dumbbell triads

Porphyrins (P) have also been used to connect two C60 units,

thus forming the corresponding C60–P–C60 triads. Diederich

et al. reported one of the first examples in 1998.17 The

synthesis of triad 3 (Scheme 1) was carried out by macro-

cyclization of the porphyrin-tethered bismalonates (2) follow-

ing a Bingel synthetic protocol and long dodecyloxy chains

were introduced in order to increase the solubility of these

triads. However, as occurs for triad 1, the P moiety does not

exert any remarkable electronic effect over the C60 cages and

vice versa in the ground state.

Whereas the photophysical properties of C60–P dyads have

been thoroughly investigated, the same does not occur for the

related triads.6 One of the few examples including photo-

physical details was reported by Imahori et al. in 1998.18 In

this communication, a comparative study was reported for

fulleropyrrolidines bearing a porphyrin fragment (Chart 2).

Although not many details are provided in the article, the

authors state that either for dyad 4a or for triad 4b, strong

quenching of the fluorescence intensities is observed in com-

parison to the separated redox chromophores. Interestingly,

this quenching is more noticeable for triad 4b than for dyad 4a.

Fig. 1 C60–piperazine–C60 triad (1) and its cyclic voltammogram.

Chart 1 Some representative electron donor moieties assembled with C60 to obtain dyads.


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To the best of our knowledge, no further photophysical data

have been reported for triad 4b. Nevertheless the changes in the

fluorescence intensities reveals a beneficial effect of a second

fullerene unit on the photophysical properties of such systems.

In 2000, Diederich et al. described the synthesis and a com-

parative study of the properties of a cyclophane-type C60–P

dyad (5a) as well as a C60–P–C60 dumbbell triad (5b).19 The

synthesis of such molecules was accomplished by following a

Bingel strategy, in a similar fashion to that mentioned for triad

3. As expected, the redox properties of dyad 5a and triad 5b

showed no mutual interactions between the electroactive

fragments in the ground state, despite their close spatial

proximity. However, the steady-state and time-resolved

absorption as well as fluorescence studies demonstrated the

formation of a charge-transfer state for both systems.

Remarkably, both molecules presented no substantial changes

in their photophysical properties. The authors claim that

these findings can be accounted for by considering a face-to-

face fullerene–porphyrin interaction for both 5a and 5b. In

the case of 5b, this implies the preference of conformational

change in such a way that conformation II is preferred over

conformation I (Chart 3).

One of the most recent examples of C60–P–C60 dumbbell

systems was described by Diederich et al. who reported the

synthesis as well as the redox and photophysical properties of

a triply fused C60–diporphyrin–C60 ensemble (8). As in

compounds 3 and 5, the synthetic strategy consisted of a

Bingel reaction between the corresponding bismalonate bear-

ing the triply fused diporphyrin system (7), which is, in turn,

obtained in a multistep synthetic procedure from the diaryl-P 6

(Scheme 2).20

The target triad 8 presented exceptional and very rich redox

properties. Thus, the DPV measurements showed nine rever-

sible redox events, which involve 15 electrons (Fig. 2). On the

oxidation side, four waves appear involving one electron each

and corresponding to the triply fused porphyrin moiety.

Regarding the reduction side, the first wave (21.0 V) is

partially overlapped and involves three electrons: two due to

both C60 fragments and the other one corresponding to the

reduction of the porphyrin. In the second wave (21.35 V), the

second reduction of both fullerene cages and that of the P

moiety can be observed and therefore, corresponds to a net

Scheme 1 Regioselective synthesis of the highly soluble Bingel-type C60–P–C60 triad (3).

Chart 2 Fulleropyrrolidines C60–porphyrin dyad 4a and triad 4b.


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process of three electrons. The third reduction wave (21.85 V)

only corresponds to two electrons as a consequence of the

third one electron reduction of both C60 units and, finally, in

the fourth wave (22.28 V) another three electrons are involved

corresponding to the final reduction of the P and the fourth

reduction process of both fullerenes.

Surprisingly, the absorption spectrum of compound 8 does

not fit with the sum of the separated components of the triad

and, although the spectral changes are smaller than those

observed for other conjugates with face-to-face alignment of

porphyrin and C60 moieties,20 this fact can be accounted for

by weak electronic interchromophoric interactions. In the

Chart 3 a) C60-P parachute dyad 5a; b) conformational changes in triad 5b.

Scheme 2 Synthesis of the triple fused C60–P–C60 triad (8).

Fig. 2 DPV voltammogram of dumbbell-type triad 8. Reprinted from ref. 20 with permission from Wiley-VCH.


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fluorescence experiments a drastic quenching is observed

which has been ascribed to an energy transfer (ET) event

since the excitation of the fullerene moieties is followed by the

sensitisation of the singlet state of the P. Therefore, despite the

good electron donor character of the triply fused P systems, no

electron transfer process was observed.

C60–oligomer–C60 dumbbell triads

Organic solar cells constituted by semiconducting polymer/

fullerene bulk heterojunctions have provided high efficiencies

for the conversion of sunlight into power. However, one of the

main drawbacks for this kind of device is spontaneous phase

separation.21 To overcome this problem, one of the strategies

used consisted of the covalent attachment of p-conjugated

oligomers of different nature [oligo-phenylenevinylenes

(OPVs); oligo-thienylenevinylenes (OTVs); oligo-phenyl-

eneethynylenes (OPEs); etc.] to the C60 surface.11c In this

regard, Nierengarten et al.22a described in 2000 the first

example of this molecular approach to photovoltaic devices

and, since then, an important number of entities of this type

have been reported so far. The first one involving a dumbbell

triad was a series of C60–oligothiophene–C60 (9) ensembles

reported by Janssen in 2000 (Chart 4, a).22b More recently,

this research group has also described the synthesis and the

photophysical studies of a new series of dumbbell triads in

which the donor moiety is a thiophene–pyrrole–thiophene

unit (10) (Chart 4, b).23 The synthetic procedure followed

to obtain these triads was similar to that mentioned for

compound 1 (Fig. 1).

For both architectures the photophysical studies show the

formation of a radical pair whose lifetime [in the range of

subnanoseconds] strongly depends on the solvent polarity.

However, photoexcitation of films of triads 9b (n = 2) and 9c

(n = 3) led to a charge-separated state in the range of

milliseconds in an intermolecular fashion. These values are

significantly higher in comparison to those observed for

radical ions in solution, thus suggesting an ordering effect in

the solid state.

Otsubo et al. have also used oligothiophenes (nT) to bind

two [60]fullerenes. These authors have reported not only the

synthesis of these so-called C60–nT–C60 triads (13) (Scheme 3),

but also have prepared the corresponding PV devices using this

kind of dumbbell compounds as single component.24a Whereas

the PV cells Al/13/Au corresponding to 13a and 13b showed

no photocurrent upon irradiation with 10 mW cm22 mono-

chromatic light, the sandwich devices for 13c and 13d

presented efficiency values of 0.50% and 0.65%, respectively.

It is worth mentioning that the same authors had also

reported the synthesis and preparation of organic solar cells by

using dyads C60–nT (14) in which only a C60 fragment is

connected to the oligothiophene unit.24b

The energy conversion efficiency of Al/14/Au strongly

depends on the side that the cell is irradiated, being twofold

when the irradiation is on the Al side (0.32%) instead of on the

Au side (Chart 5). These findings suggest that the nT moieties

act as an efficient network for the hole transport in the film.

However, fullerenes do not interact with each other to promote

electron transport. Interestingly, the presence of a second C60

cage in triads 13 somehow enhances the interactions among

the fullerene units creating a more ordered network and,

therefore increasing the power efficiencies.

Chart 4 C60 dumbbells bridged by electroactive heterocyclic spacers.

Scheme 3 Synthesis of C60–nT–C60 triads 13a–d.


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In our research group, we have described a series of new

oligomeric frameworks covalently linked to two fullerene

moieties (15a–d, Chart 6), which were prepared by Prato’s

procedure (Fig. 1).25 In particular, different arylene units

ranging from naphthalene and benzene to thiophene with

diverse electrochemical and excited-state properties have

been incorporated into p-conjugated backbones (Chart 6).26

The different nature of the oligomeric spacers has

allowed either the absorption wavelength of the chromophore

or the oxidation potential of the oligomeric donor moiety to

be tuned.

The photophysical measurements demonstrated that both

singlet–singlet energy transfer as well as intramolecular

electron transfer take place and are in competition with

each other. Interestingly, the comparison between the

lifetime of the corresponding charge-separated states for

dyads 1626b (t = 1.37 ns) with their analogous triads 15d

(t = 1.84 ms) clearly indicates that the incorporation of a

second fullerene unit generates a beneficial effect in terms of

stabilizing the product and enhancing the yield of charge

separation.

Another type of oligomeric electroactive spacer used to

prepare fullerene dumbbells is the oligo(phenyleneethynylenes)

(OPEs). In this regard, Tour et al. have recently described

the synthesis and the redox properties of a series of OPEs

end-capped with two fullerenes (18; Scheme 4).27 The synthetic

procedure followed by Tour et al. consisted of the addition of

the corresponding lithium acetylide by following the condi-

tions previously reported by Komatsu et al.28 The redox

studies carried out for compound 18 indicate that no electronic

Chart 6 Oligomeric triads (15) and dyad 16.

Chart 5 PV cells prepared from dyad 14. The efficiency of the cell

varies with the illumination side.


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communication is observed between the two fullerenes and the

OPE unit since only a set of quasi-reversible reduction waves

appears in the cyclic voltammogram. Although no further data

about photophysical studies were given in this article, the

authors detected a fluorescence quenching of compound 18 in

comparison with its OPE precursor 17.

We have also used OPEs, in which the oligomer length has

been systematically increased from the monomer to the

heptamer, to connect two C60 cages (21). The synthesis of

target C60–OPE–C60 triads was carried out by following a

1,3-dipolar cycloaddition protocol starting from the arylal-

kynyl precursor 19 (Chart 7).29 As occurs for triads 18,

C60 dumbbells 21 showed amphoteric redox behaviour, due to

the presence of both donor (OPEs) and acceptor (C60)

moieties, but without appreciable electronic interaction

between them.

Excited state studies carried out by fluorescence showed a

dramatic and instantaneous deactivation in the 500–700 nm

range which corresponds to the OPE fragment. Transient

absorption spectroscopy experiments demonstrated an effi-

cient intramolecular energy transfer from the photoexcited

phenylene-acetylenes to the covalently linked fullerene,

although a weak competitive electron transfer could also

operate in the deactivation process. Consequently, it can be

assumed that upon the completion of the singlet–singlet energy

transfer, the fullerene singlet excited state can only decay to

the triplet excited state and finally to the singlet ground state

(see Chart 7).

The related C60–OPE dyads have been studied by

Nierengarten et al.30 Preliminary results on the photophysical

properties of these dyads showed a strong quenching of

the luminescence of the OPE by the fullerene moiety, indicative

of the occurrence of an intramolecular decay process.

Unfortunately, the lack of data regarding the lifetime of

the charge separated state of these dyads prevents a

reliable comparison with their corresponding C60–OPE–C60

dumbbells 21.

C60–TTF–C60 dumbbell triads

All the above donor units present a common feature: they all

lose totally or partially their aromatic character upon the

oxidation process that accompanies the formation of the

charge separated state. In this regard, tetrathiafulvalenes

(TTFs), in their one-electron oxidized form (namely, the 1,3-

dithiolium cation), display a (4n + 2) aromatic character, in

contrast to the non-aromatic ground state. In view of electron-

transfer processes, this gain of aromaticity is an important

requisite, assisting in the stabilization of charge separated

radical pairs. Others and we have reported the synthesis as well

as the photophysical properties of dyads12 and triads31 based

on C60 and TTF units.

In 2000, we reported the photophysical properties of

triad 24, which, in turn, was obtained by an esterification

procedure starting from fullerene carboxylic acid 22

and dialcohol 23 in the presence of DCC and DMAP as

activating agents (Scheme 5). The presence of the triethyl-

eneglycol tail in the dumbbell provides high solubility to

compound 24.31

As in the case of the examples summarized above, the cyclic

voltammetry measurements of compound 24 reveal no notable

interaction between the chromophores in the ground state.

Nevertheless, photophysical data indicate that in the excited

state an intramolecular electron transfer, evolving from the

TTF donor to the singlet state of C60, prevails, yielding a long-

lived charge separated radical pair (1–2 ns in DMF). Although

we must take into account that the substitution patterns of the

C60 derivatives are a bit different, the photophysical features

of the triad 24 notably differ from that of the C60–TTF dyad

(25) (Chart 8).12e In the case of the C60–TTF dyad 25 the

deactivation pathways form the corresponding short-lived

C60?2-TTF?+ radical pair (1–2 ns), which, subsequently,

is deactivated to the triplet-state of C60 followed by an

intermolecular charge separation process affording the corres-

ponding intermolecular radical pair, namely (C60?2–TTF) +

(C60–TTF?+) (Chart 8). Therefore, the presence of a second

[60]fullerene unit in triad 24 not only changes drastically the

mechanism of deactivation upon excitation but also increases

the lifetime of the intramolecular radical pair. Again, a notable

stabilization effect can be observed as a consequence of the

presence of two C60 units in the same molecule.

Despite the interest of this kind of structure in the

development of optoelectronic devices, to the best of our

knowledge, just another couple of examples of C60–TTF–C60

ensembles have been reported in the literature (see 26 and 27

in Chart 9). In 1997, Delhaes et al. reported the synthesis

of several TTF derivatives covalently attached to two [60]full-

erene moieties (26) and even to four C60 units by means of

an esterification reaction.32 As expected, the cyclic voltam-

metric measurements of such C60–TTF assemblies presented

four reduction waves corresponding to the C60 cages and

two oxidation waves corresponding to the TTF core. Similarly

to triads 24, the redox potential values demonstrated no, or

very weak, electronic interactions between the two redox

chromophores.

Hudhomme et al. have also described a C60–TTF–C60

dumbbell assembly (27) that was prepared by a two-fold

Scheme 4 Synthesis of fullerene dumbbell 18 by lithium acetylide

addition to C60.


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Diels–Alder cycloaddition in o-dichlorobenzene (ODCB) as

solvent and in the dark.33 Despite the insolubility of such a C60

dumbbell both the redox properties and the ESR spectra were

determined. The cyclic voltammogram of compound 27 in

ODCB exhibited two oxidation waves (+0.58 and +1.12 V)

corresponding to the TTF fragment together with the first

three reduction waves of the fullerenes (20.65, 21.02 and

21.56 V, respectively). The redox potentials obtained indicate

the lack of electronic interaction in the ground state. The ESR

spectra provided definitive information about the electronic

structure of triad 27 since the signals corresponding to the

radical cation as well as that of the radical anion were

unequivocally assigned upon controlled oxidation and reduc-

tion, respectively.

More recently, we have carried out the preparation of new

C60–TTF–C60 dumbbell structures where a higher degree of

stabilisation of the oxidised donor via adding aromatic arenes

to the heteroaromatic rings of TTF has been accomplished (see

Scheme 6).34 Addition of the latter enhances the aromatic

stabilisation energy. In the oxidised form, the p-conjugation

is extended over the entire anthracene backbone and as a

net result, the dicationic species is completely aromatic. The

reaction of this kind of p-extended TTFs (exTTFs) endowed

with two malonate moieties (28) by using the Bingel cyclo-

propanation chemical approach yields triad 29 together with

the intramolecular bisadduct and the corresponding dyad.34

The electrochemical data reveal that both exTTF and C60

electroactive moieties do not interact in the ground state.

Unfortunately, the CV studies of triad 29 showed the presence

of intramolecular bisadducts and although the formation

of these derivatives is not clear at present, formation

under reductive electrochemical conditions involving a retro-

cyclopropanation process is suggested and triad 29 is not

electrochemically stable.35

Chart 7 Synthesis and deactivation pathways of C60–OPE–C60 triads 21.


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Enantiopure C60–binaphthyl–C60 triads

Finally, we would like to remark upon the contribution of our

research group to the synthesis of enantiomerically pure C60

triads. It is known that fulleropyrrolidines generated by 1,3-

dipolar cycloaddition of azomethine ylides to C60, like

compounds 1, 4, 9, etc., present a stereogenic centre (marked

with an asterisk in compound 1, Fig. 1). However, to the best

of our knowledge, no fulleropyrrolidine stereoisomers have

been yet isolated, this type of C60 derivative always being

isolated as a racemic mixture. Thus, the synthesis of

enantiopure fullerenes is a topic that has received scarce

attention. Only a few examples of C60 chiral monoadducts

have been reported in the literature.36 With the challenge of

obtaining a chiral triad in mind, we decide to employ the

enantiopure 1,19-binaphthyl core, which is being used as an

important building block for the construction of complex

systems in areas such as asymmetric catalysis, nonlinear

optics and photoluminescence.37 Two different chiral C60–

binaphthyl–C60 triads were achieved starting from enantiopure

dialdehyde (R)-30. C60 dimer (R)-32 was obtained by a

subsequent reduction and esterification reactions from (R)-30

(Scheme 7). (R)-34 was synthesized by following a Bamford–

Stevens protocol from tosylhydrazone (R)-33 and treatment

with sodium methoxyde, pyridine and C60 (Scheme 7).38

The inclusion of this triad within this special kind of

electroactive dumbbell C60 derivative can be unequivocally

accounted for by considering their redox properties. In the

cyclic voltammograms of compounds (R)-32 and (R)-34 four

reduction waves appear corresponding to both C60 cores.

Additionally, on the oxidation side, an irreversible wave

corresponding to the alkoxynaphthalene moiety is observed

at 1.42 and 1.44 V, respectively. Work is currently in

progress to determine the photophysical properties of these

new chiral systems.

Summary and conclusions

We have reviewed in this article the most recent examples of

C60 dumbbells. Using different electron-donor moieties such as

porphyrins, p-conjugated oligomers or TTFs, the connection

between the two fullerene cages has been accomplished. The

electrochemical properties of most of C60-dumbbell triads con-

sidered indicate the lack of electronic interaction between the

Scheme 5 Synthesis of C60–TTF–C60 dumbbell 24.

Chart 8 C60–TTF dyad 25 and its deactivation pathways.

Chart 9 C60–TTF–C60 dumbbell triads.


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three electroactive moieties, namely two C60 units and

the corresponding electron-donor fragment. Additionally,

the photophysical properties of these systems have been

summarized. When possible, the photophysical features for

the radical pairs formed by photoinduced electron transfer

processes have been provided. In general, these data reveal

highly stabilized charge-separated states. On the other hand,

several of these systems have already been tested as the active

layer in photovoltaic devices shedding promising results.

The comparison of the properties of the C60–donor–C60

entities with their analogous dyads has been highlighted. In

principle, the presence of a second fullerene unit seems to

produce a stabilizing electronic or structural effect on the

photogenerated ion radical pair, although the contribution of

this second unit is unclear at present.

In conclusion, C60 dumbbells bridged by an electron-donor

fragment constitute a special class of systems within the huge

family of fullerene derivatives whose properties, together with

their synthetically less-demanding cost, can open new expecta-

tions in the application of fullerenes in the field of opto-

electronic devices. In this regard, more research on the

synthesis of new systems and study of their photophysical

properties is crucial in order to clarify the observed beneficial

effect of a second fullerene core.

Acknowledgements

The authors wish to express their gratitude to the MCYT

of Spain (Project BQU2002-00855) for generous financial

support. M.A.H. acknowledges the MCYT of Spain for a

research contract (Programa ‘‘Ramon y Cajal’’).

Luis Sanchez, Ma Angeles Herranz and Nazario Martın*Departamento de Quımica Organica, Facultad de Ciencias Quımicas,Universidad Complutense, E-28040 Madrid, Spain.E-mail: [email protected]

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Scheme 7 1,19-Binaphthyl-based enantiopure C60 dimers.

Scheme 6 Synthesis of the Bingel-type C60–exTTF–C60 triad 30.


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C60-based dumbbells: connecting C60 cages through electroactive bridges

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