Synthesis of 1,4-diaryl[60]fullerenes by bis-hydroarylation of C 60 and their use in solution-processable, thin-film organic photovoltaic cells

Article abstract of DOI:10.1016/j.tetlet.2011.01.031

An AlCl₃-mediated Friedel-Crafts reaction of arenes with C ₆₀ affords two-fold hydroarylated compounds, C₆₀Ar ₂H₂, which upon deprotonation with ^tBuOK and oxidation with CuBr·SMe₂

Full text of DOI:10.1016/j.tetlet.2011.01.031

Tetrahedron Letters 52 (2011) 2240–2242
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Tetrahedron Letters
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Synthesis of 1,4-diaryl[60]fullerenes by bis-hydroarylation of C60 and
their use in solution-processable, thin-film organic photovoltaic cells
Yutaka Matsuo a,b, , Ying Zhang a, Iwao Soga b,c, Yoshiharu Sato b,c, Eiichi Nakamura a,b,
⇑
⇑
a Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
b Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
c Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 14 January 2011
An AlCl3-mediated Friedel–Crafts reaction of arenes with C60 affords two-fold hydroarylated compounds,
C60Ar2H2, which upon deprotonation with tBuOK and oxidation with CuBrꢀSMe2 yield 1,4-diaryl[60]fuller-
enes, C60Ar2 (Ar = Ph, 4-Me-C6H4, 3,4-Me2-C6H3, and 4-Ph-C6H4) in good yield. A solution-processed, thin-
film organic photovoltaic device using C60(4-PhC6H4)2 as electron acceptor material showed a 2.3% power
conversion efficiency..
Dedicated to Professor Harry Wasserman on
the occasion of his 90th birthday
Ó 2011 Published by Elsevier Ltd.
Keywords:
Fullerene
Friedel–Crafts reaction
Hydroarylation
Organic photovoltaic cells
There is enormous interest in thin-film organic photovoltaic
(OPV) devices,1,2 in which fullerene derivatives3 serve as a key n-
type semiconductor component. Most of the reported studies use
a single compound, PCBM (phenyl C61-butyric acid methyl ester),4
which is a cyclopropane, that is, two C–C bonds next to each other,
and hence referred to as a 1,2-diadduct, or its congeners. We have
recently reported the synthesis of 1,4-dialkyl[60]fullerene deriva-
tives, SIMEFs (bis(silylmethyl)[60]fullerenes),5 and their applica-
tion as electron-acceptor materials for OPV devices.2 This 1,4-
adduct has a higher LUMO level than those of the 1,2-diadducts
such as PCBMs and gives a higher open-circuit voltage (VOC).
Although a few examples of the synthesis of 1,4-diaryl[60]fuller-
enes have already been reported,6 methods for the synthesis of
1,4-diadducts have been less explored than for their 1,2-counter-
parts.7 Herein, we report the synthesis of 1,4-diaryl[60]fullerene
derivatives (2a–d) in two steps—an AlCl3-mediated Friedel–Crafts
hydroarylation reaction8 followed by dehydrogenation (Table 1).
This two-step approach is unique among other routes to 1,4-diorg-
anofullerenes for its use of a diorganodihydro compound (1a–d) as
an intermediate, and it alleviates the use of expensive reagents—an
attractive character for a large-scale synthesis of fullerene deriva-
tives. The substituents on the aryl groups allow us to tune the
solubility and the physical properties of the molecules, and a solu-
tion-processed, thin-film organic photovoltaic device using 2d
showed 2.3% power conversion efficiency.
Through modification of the conditions that we previously
developed for the synthesis of triaryltrihydro[60]fullerenes,8 we
first synthesized diaryldihydro[60]fullerenes,
C60Ar2H2 (1a–d),
after which the product was dehydrogenated by sequential
treatment with tBuOK and CuBrꢀSMe2 to obtain the desired
diaryl[60]fullerenes 2a–d (Table 1).9 The reaction time and the
stoichiometry, in particular, that of water (1 equiv to fullerene),
Table 1
Synthesis of 1,4-diaryl[60]fullerenes by AlCl3-mediated bis-hydroarylation of C60
followed by oxidation
Ar
Ar
Ar
Ar
H
H
H2O (1.0 eq)
AlCl3 (5 eq)
arene (x eq)
1) t-BuOK (2.4 eq)
2) CuBr·SMe2 (5 eq)
C60
91–95%
1,2-Cl2C6H4
28–52%
1a-d
2a-d
Entry Arenes
x
Time (min) Yield of 1 (%) Yield of 2 (%)
1
2
3
Benzene (a)
Toluene (b)
3,4-Me2C6H4 (c) 10 120
15 270
10 60
28a
52
33
28
0
95
92
94
91
—
4
Biphenyl (d)
10 120
20 120
5b
Anisole
⇑
Corresponding authors. Tel.: +81 3 5841 4356; fax: +81 3 5800 6889.
E-mail addresses: matsuo@chem.s.u-tokyo.ac.jp (Y. Matsuo), nakamura@chem.
a
29% C60PhH was obtained.
The reaction was conducted at 80 °C.
b
s.u-tokyo.ac.jp (E. Nakamura).
0040-4039/$ - see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2011.01.031

Y. Matsuo et al. / Tetrahedron Letters 52 (2011) 2240–2242
2241
Table 2
were crucial to maximize the yield of the Friedel–Crafts reaction.
Reduction potentials for C60, PCBM, and 1,4-diaryl[60]fullerenesa
The presence of excess water reduced the selectivity of the reaction
and gave a complex mixture of inseparable products. With careful
control of these variables in accordance with the reactivity of the
arenes, the double hydroarylation reaction with benzene, toluene,
o-xylene, and biphenyl was achieved to obtain the desired diaryldi-
LUMO levelb (eV)
red
1=2
E
vs. Fc/Fc+
Ar
E1 (V)
E2 (V)
E3 (V)
C60
PCBM
2b
2c
2d
ꢁ1.10
ꢁ1.18
ꢁ1.16
ꢁ1.17
ꢁ1.13
ꢁ1.49
ꢁ1.55
ꢁ1.55
ꢁ1.55
ꢁ1.54
ꢁ1.94
ꢁ2.05
ꢁ2.03
ꢁ2.03
ꢁ2.01
ꢁ3.70
ꢁ3.62
ꢁ3.64
ꢁ3.63
ꢁ3.67
hydro[60]fullerenes,
C60Ph2H2 (1a), C60(4-MeC6H4)2H2 (1b),
C60(3,4-Me2C6H3)2H2 (1c), and C60(4-PhC6H4)2H2 (1d) (Table 1).
We obtained no trace of the desired product in the reaction with
anisole, probably because of deactivation of the Lewis acid by
anisole.
a
Potential in volts versus a ferrocene/ferrocenium couple was measured with
cyclic voltammetry in 1,2-Cl2C6H4 solution containing Bu4NþPF6ꢁ (0.1 M) as a sup-
porting electrolyte at 25 °C with a scan rate of 0.1 V/s. Glassy-carbon, platinum
wire, and Ag/Ag+ electrodes were used as working, counter, and reference elec-
trodes, respectively.
The bis-hydroarylated fullerene derivatives 1 were then dehy-
drogenated in one step to the desired diaryl[60]fullerenes 2. After
considerable experimentation, we found that a rather exotic com-
b
The values of LUMO level were estimated using the following equation: LUMO
t
bination of a base and an oxidant, BuOK and CuBrꢀSMe2, afforded
level = ꢁ(4.8 + Er1e=d21) eV. See Ref. 11.
t
the desired product in an excellent yield (Table 1). Thus, BuOK
was the most suitable base among a variety of bases including
KH, which was equally effective but more difficult to handle. CuBr
or air, albeit less effectively, served as an oxidant in the second
step. APCI-mass spectra indicated the molecular formulas of
The diaryl[60]fullerene 2 undergoes reversible three-electron
reduction as shown in Figure 2 at potentials characteristic of
reduction of fullerene, indicating the formation of a monoanion,
a dianion, and a trianion through reduction of the fullerene moi-
ety. The reduction potentials and LUMO level are summarized in
Table 2. The diaryl adduct 2 showed similar reductive potentials
C
60Ar2 for these compounds, and the UV–vis spectra allowed us
to distinguish between 1,2- and 1,4-diadducts,10 because this class
of compounds is known to exhibit a weak, but characteristic, broad
absorption peak around 450 nm in the UV–vis spectra, as illus-
trated in Figure 1.
to PCBM, because both of them possess a 58p-electron system.
The symmetric diaryl[60]fullerene adducts 2 showed low solu-
bility in common organic solvents; however, bearing a substituent
on the phenyl rings, compounds 2b–d showed moderate solubility
in 1,2-Cl2C6H4 (0.2–0.4 wt %), which made it possible to use in a
solution process for the fabrication of a thin-film organic photovol-
taic device. We therefore examined the LUMO levels of compounds
2b–d.
However, the present diaryl system allows us to tune the LUMO
level by tuning the electronic properties of the aryl group. Thus,
the alkyl groups (4Me and 3,4-Me2) in compounds 2b and 2c
raised the LUMO level higher than that of 2d (phenyl). This
observation makes the diaryl[60]fullerene derivatives very
attractive candidates as n-type semiconductors in solar cell
research.
Finally, we evaluated the capacity of the diaryl[60]fullerene as
an acceptor molecule in the p-i-n solution-processed OPV devices
that we developed recently (tetrabenzoporphyrin (BP) as donor).2
The active layer of the device was fabricated by spin coating to ob-
tain the structure: indium tin oxide (ITO)/PEDOT:PSS/BP/BP:2d/2d/
bathocuproine (BCP)/Al. The pertinent photovoltaic properties are
the following: VOC = 0.63 V, JSC = 7.7 mA/cm2, FF = 0.47, power con-
version efficiency = 2.3%. The active layer of this OPV device exclu-
sively utilizes aromatic compounds (i.e., BP and 2d), which will
result in a long life.
In summary, we have developed a concise synthesis of 1,4-
diaryl[60]fullerene adducts 2 by a unique combination of a Fri-
edel–Crafts reaction and a Cu(I) mediated dehydrogenation. The
solubility and the photophysical and electrochemical properties
of these compounds suggested their utility in OPV devices, and this
expectation was confirmed by the fabrication of a device that per-
forms with 2.3% power conversion efficiency.
Figure 1. UV–vis spectra of C60 and 2d in 1,2-dichlorobenzene solution.
Acknowledgments
This work was partially supported by KAKENHI (#22000008),
‘Chemistry Innovation through Cooperation of Science and Engi-
neering’ GCOE program of the MEXT, Japan, The University of To-
kyo Special Scholarship for International Students and Strategic
Promotion of Innovative Research and Development from Japan
Science and Technology Agency.
Supplementary data
Supplementary data (synthetic procedures, NMR spectra, and
device fabrication procedures) associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.
2011.01.031.
Figure 2. Cyclic voltammogram of 2d. Measurement was performed in a 0.4 mM
1,2-Cl2C6H4 solution containing Bu4NþPF6ꢁ (0.1 M) as a supporting electrolyte at
25 °C with a scan rate of 0.1 V/s.

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Y. Matsuo et al. / Tetrahedron Letters 52 (2011) 2240–2242
8. Iwashita, A.; Matsuo, Y.; Nakamura, E. Angew. Chem., Int. Ed. 2007, 46, 3513.
9. Synthesis of 1,4-bis(biphenyl)[60]fullerene (2d): ODCB (20 mL) containing water
(1.8 L, 0.01 mmol) was added to mixture of C60 (72.1 mg, 0.1 mmol),
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aluminium(III) chloride (66.7 mg, 0.5 mmol), and biphenyl (154.2 mg,
1.0 mmol). The reaction mixture was stirred for 2 h at rt, before water (1 mL)
was added. The dark-brown reaction mixture was diluted with CS2 and filtered
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a
l
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