Synthesis of the first tetracene-[60]fullerene dyad

Article abstract of DOI:10.1002/ejoc.200400506

We report on a multistep synthesis of the newly designed tetracene-[60]fullerene dyad involing a Bingel reaction between the tetracene and C₆₀ moieties. This strategy prevents the usual Diels-Alder reaction which would result in the loss of the

Full text of DOI:10.1002/ejoc.200400506

SHORT COMMUNICATION
Synthesis of the First Tetracene-[60]fullerene Dyad
[a]
[a]
´
Sebastien Taillemite and Denis Fichou*
Keywords: Fullerenes / Tetracene / Cycloaddition / Dyad
We report on a multistep synthesis of the newly designed
tetracene-[60]fullerene dyad involing a Bingel reaction be-
tween the tetracene and C60 moieties. This strategy prevents
the usual Diels−Alder reaction which would result in the loss
of the tetracene aromaticity in the target dyad.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
tors,[16,17] it is interesting to develop synthetic strategies to
obtain functionalized acenes.
The small reorganization energy of fullerenes in electron-
transfer reactions makes them particularly attractive as elec-
tron-accepting materials for energy conversion and stor-
age.[1] Blends of functionalized [60]fullerenes mixed to-
gether with conjugated polymers such as poly-p-phenyl-
enevinylene allow for the building of photovoltaic cells with
conversion efficiencies of up to 3.5%.[2Ϫ4] In these systems,
a fast photoinduced electron transfer from the polymer do-
nor moiety to the fullerene acceptor takes place, followed
by a slow charge recombination. Similarly, donor-linked
[60]fullerenes have been prepared in view of developing
solid state photovoltaic systems.[5Ϫ8] A number of poly-
cyclic aromatic hydrocarbons (PAHs) have also been coval-
ently linked as donor moieties to C60.[9Ϫ11] However, with
the exception of perylene (Eox ϭ 0.85 V vs. SCE in
MeCN),[11] they all exhibit energy transfer (instead of
charge transfer) under photoexcitation due to a very high
oxidation potential (Eox Ͼ 1.09 V).
We report here on the multistep synthesis of the newly
designed tetracene-[60]fullerene dyad 1 (Figure 1). The 3,5-
bis(dodecyloxy)benzylic group is introduced to ensure solu-
bility of the dyad in organic solvents.
Figure 1. Tetracene-[60]fullerene dyad 1
Surprisingly, higher acenes such as tetracene and penta-
cene possess electrochemical (Eox Ͻ 0.77 V) and photophys-
ical properties (1*E Ͻ 2.6 eV), which are consistent with the
required donor properties for photoinduced charge transfer.
In particular, in contrast to other PAHs, they possess low
oxidation potentials (Eox Ͻ 0.77 V), low singlet excited-
state energies (1*E Ͻ 2.6 eV), and a strong absorption in
the visible region. However, they probably have never been
used as donors in C60-donor dyads since they undergo
[4ϩ2] cycloadditions with the [6,6] bonds of C60.[12Ϫ15]
Since crystalline tetracene and pentacene possess excellent
hole-transport properties and are currently studied as or-
ganic semiconductors in devices such as field-effect transis-
Results and Discussion
Our strategy for the synthesis of the tetracene moiety is
based upon the [2ϩ2ϩ2] cycloaddition of propargylic al-
cohol with the 1,7-diyne 7. Preparation of the latter com-
pound was performed in three steps (Scheme 1).
Bromination of commercially available 2,3-dimethylnaph-
thalene quantitatively gave compound 5, which was then
converted into the iodinated derivative in 85% yield. Cop-
per coupling of acetylene Grignard with 6 led to 7 in 57%
yield. The tetracene skeleton 8 was then obtained by rho-
dium-catalyzed [2ϩ2ϩ2] cycloaddition in the presence of
20% of the Wilkinson catalyst in 87% yield .[18] This cycliza-
tion of a 1,7-diyne allowed the generation of two six-mem-
bered rings in a single step with high yields for such a semi-
intermolecular reaction.[19] The choice of rhodium as the
[a]
CEA-Saclay, LRC Nanostructures et Semi-Conducteurs
Organiques CEA-UMR 7611 CNRS-Paris 6, DRECAM/SPCSI,
91191 Gif-sur-Yvette, France
E-mail: fichou@drecam.cea.fr
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2004, 4981Ϫ4984
DOI: 10.1002/ejoc.200400506
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4981

S. Taillemite, D. Fichou
SHORT COMMUNICATION
Scheme 1. Reagents and conditions: (i) NBS, AIBN, CCl4, reflux (97%); (ii) NaI, acetone, reflux (85%); (iii) ethynylmagnesium bromide,
CuI, THF, 60 °C, (57%); (iv) propargylic alcohol, [RhCl(PPh3)2], toluene, reflux (87%)
catalyst [instead of CpCo(CO)2, for example] avoids the tri-
merization of both the 1,7-diyne and propargylic alcohol.
In addition, the use of a ruthenium-based catalyst, as re-
cently proposed by Yamamoto et al. for the efficient cycliza-
tion of 1,6-diynes, has not been adapted for use with 1,7-di-
ynes.[20]
N,NЈ-dicyclohexylcarbodiimide(DCC)-mediated esterifi-
cation of benzylic alcohol 8 with carboxylic acid 9 afforded
malonate 10. Compound 9 was introduced to ensure the
solubility of the dyad in organic solvents, and was synthe-
sized in 70% yield from 3,5-dihydroxybenzyl alcohol.[21] Ar-
omatization of dihydronaphthacene derivative 10 was per-
formed with DDQ to give the desired donor moiety in good
yield. Finally, the reaction of C60 with compound 11 under
Bingel conditions[22,23] afforded the target dyad 1 in 27%
yield. The Bingel reaction can be carried out at room tem-
perature thus avoiding the undesirable DielsϪAlder side-
reaction. After purification by flash chromatography, 1 was
isolated as a black powder. The molecular structure of 1
is unambiguously confirmed by H NMR, 13C NMR and
1
Scheme 2. Reagents and conditions: (i) DCC, DMAP, CH2Cl2, 0
MALDI-TOF MS measurements (m/z ϭ 1520.4). It should
be noted that we also synthesized the insoluble analog of 1,
i.e. a C60-tetracene dyad bearing a methyl group (R ϭ CH3)
instead of the 3,5-bis(dodecyloxy)benzylic group, with the
use of the same strategy. However, because of its insolu-
°C to room temperature (73%); (ii) DDQ, benzene, reflux (50%);
(iii) C60, I2, DBU, toluene, room temperature (27%)
1
bility, this compound was only characterized by mass spec- over, in the H NMR spectrum of 1, singlets that corre-
troscopy.
spond to the central rings of tetracene are deshielded by
The 1H NMR spectrum of 1 recorded in CDCl3 at 293 K 0.2 ppm relative to the tetracene derivative 11. In the case of
shows that all the signals are split. In particular, the two a [4ϩ2] C60-tetracene adduct, the protons of the remaining
ϪCH2OϪ groups located on each side of the linker appear central ring are shielded by 0.5 ppm.[11] Finally, in the
1
as four singlets in the δ ϭ 5.4Ϫ5.7 ppm range. Although it 5.0Ϫ6.0 ppm range of the H NMR spectrum, we only ob-
was recently observed for an anthracene-C60 dyad that was serve the signals for the two benzylic methylenes, and no
synthesized by the Bingel reaction,[14] an intramolecular signal that could be assigned to a DielsϪAlder adduct is
DielsϪAlder reaction between the tetracene moiety and the visible. The absence of a DielsϪAlder reaction can be attri-
fullerene can be ruled out here. Firstly, in the 13C NMR buted to the position of the malonate linker on carbon 2 of
spectrum of 1 in deuterated toluene, there is no signal for a tetracene (see Figure1), which prevents close overlap of the
primary carbon in the 15Ϫ100 ppm range and only three π-orbitals of the central rings of tetracene with those of the
signals for quaternary carbons at δ ϭ 67.5, 69.2, and [6,6] bonds of the fullerene. This assumption is substan-
69.3 ppm, which correspond to the cyclopropane ring. It is tiated by ab initio calculations at the HF/3Ϫ21G level
well-known that the 13C NMR signature of a DielsϪAlder (Gaussian 03).[24] In order to facilitate these calculations,
adduct between an acene and a fullerene is a resonance they have been performed without the introduction of the
around 50Ϫ60 ppm for the bridging primary carbon of the bulky solubilizing alkyl chains that were changed into a
acene moiety and a resonance around 70Ϫ80 ppm for the methyl group (dyads 12 and 13). The DielsϪAlder adduct
bridging quaternary carbon of the fullerene moiety. More- of dyad 12 is 5 kcal·molϪ1 less stable than dyad 12, whereas
4982
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2004, 4981Ϫ4984

Synthesis of the First Tetracene-[60]fullerene Dyad
SHORT COMMUNICATION
the DielsϪAlder adduct of dyad 13 is 27 kcal·molϪ1 more
stable than dyad 13 (see Figure 2).
Figure 3. Fluorescence emission spectra of tetracene 11 and dyad
1 in dichloromethane (λex ϭ 300 nm. [c] ϭ 9.5 ϫ 10Ϫ6 )
aration and efficient photovoltaic effect in the solid state.
However, recent studies show that the charge separation
lifetime of the phthalocyanine-[60]fullerene dyad may be
considerably longer in the solid state than in solution.[5]
We are currently investigating the photophysical properties
of 1.
Conclusion
In conclusion we have successfully synthesized the first
C60-tetracene dyad with the use of an original route that
includes a rhodium-catalyzed [2ϩ2ϩ2] cycloaddition and a
Bingel reaction. Compound 1 has been fully characterized,
which shows, in particular, that the C60 and tetracene moi-
eties do not undergo DielsϪAlder reaction and that 1 prob-
ably adopts a π-stacking conformation. We are presently
using this new approach to synthesize a series of substituted
C60-tetracene and C60-[N]phenylene analogs.[28]
Supporting Information (see also the footnote on the first
page of this article): The details of the synthesis and charac-
terization (1H and 13C NMR, FT-IR, elemental analysis) of
all compounds reported here are provided.
Figure 2. Tetracene-[60]fullerene dyads 12 and 13
1
The splitting of all H NMR signals of 1 suggests the
coexistence of two rotational isomers, as already reported
for C60-tetrathiafulvalene triads.[25] Besides, the substantial
low-field shift of δ ϭ 0.2Ϫ0.5 ppm for all tetracene protons
can be explained by a π-stacking conformation in which the
acene and the C60 moieties achieve close spatial proximity
due to van der Waals attraction. Both π-stacking and the
coexistence of conformers in 1 would be a common conse-
quence of the flexibility of the malonate linker. It should be
noted that intramolecular π-stacking has also been ob-
served in C60-porphyrin and C60-pyrene dyads.[26,27]
Acknowledgments
This study has been realized in the framework of the EURO-PSB
European Project (5th PCRD, N° ENK5ϪCT2002Ϫ00687). Au-
thors are grateful to C. Aubert and M. Malacria (University Paris
`
6) and to M-P. Teulade-Fichou (College de France, Paris) for fruit-
π-Stacking in dyad 1 is also supported by its structureless
UV/Vis absorption spectrum and the strong fluorescence
quenching of the tetracene moiety. A solution of 1 in tolu-
ene is a purpleϪred and its absorption spectrum consists of
a monotonous broad band extending over the 440Ϫ600 nm
range. Besides, under UV irradiation, the typical vibronic
feature of tetracene fluorescence in the 400Ϫ500 nm region
disappears (Figure 3). This indicates that a charge or energy
transfer (intra- or intermolecular) takes place in solution.
However, since no emission band is observed in the
650Ϫ900 nm range, it cannot be concluded that charge
transfer takes place in solution. If charge transfer effectively
occurs, π-stacking would yield excited states that have short
lifetimes, which would be inhibitory for further charge sep-
ful discussions.
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Received July 20, 2004
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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