Perylene-3,4:9,10-bis(dicarboximide) linked to [60]fullerene as a light-harvesting antenna

Article abstract of DOI:10.1016/j.tetlet.2005.04.132

New [60]fullerene-perylene-3,4:9,10-bis(dicarboximide) dyads 1 and 2 are described in the search of an energy transfer from the dye as a photoactive antenna to the fullerene moiety.

Full text of DOI:10.1016/j.tetlet.2005.04.132

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4599–4603
Perylene-3,4:9,10-bis(dicarboximide) linked to [60]fullerene as
a light-harvesting antenna
*
´ ˆ ´ ´
Jerome Baffreau, Lara Perrin, Stephanie Leroy-Lhez and Pietrick Hudhomme
´ ´ ´
Chimie Ingenierie Moleculaire et Materiaux d’Angers, UMR-CNRS 6200, E.R.T 15 Cellules Solaires Photovolta¨ıques Plastiques,
2 Bd Lavoisier, 49045 Angers, France
Received 15 April 2005; accepted 29 April 2005
Available online 23 May 2005
Abstract—New [60]fullerene–perylene-3,4:9,10-bis(dicarboximide) dyads 1 and 2 are described in the search of an energy transfer
from the dye as a photoactive antenna to the fullerene moiety.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Thanks to the remarkable electron-accepting properties
of C60 and the discovery of ultrafast photoinduced elec-
tron transfer (PET) from conjugated polymers to C60,1
considerable effort has been devoted to the development
of the organic photovoltaic devices based on the bulk
heterojunction concept.2 Until now, the photoactive
layer of these solar cells consists of an interpenetrating
network using a p-conjugated polymer acting as the
the search of an efficient PET, dyads involving C60
and another electron acceptor are much less known.8
The latter were described in order to essentially increase
the solubility for blending them with molecular or poly-
meric materials, or to improve the electron-accepting
ability of the system. But, very few dyads were designed
in order to increase C60 absorption capability of sun-
light. Consequently we were interested in the synthesis
of molecular systems associating a dye to C60, so that
the dye could act as an antenna by absorption of sun-
light with the aim of inducing an intramolecular energy
transfer to the fullerene. Perylene-3,4:9,10-bis(dicarbox-
imide) (PDI) was first chosen because of its well
known property of high absorption in the visible to
NIR region (e ꢀ 105 L molÀ1 cmÀ1).9 Here, we describe
the synthesis and preliminary electrochemical and
spectroscopic properties of flexible covalently linked
C60–PDI dyads 1 and 2, with the dye acting as a light-
harvesting antenna for their use in the fabrication of
efficient photovoltaic cells. In parallel to our work, the
synthesis and preliminary studies of a C60–PDI dyad
in which both entities are linked through a rigidified
spacer was reported.10 Another example of a C60–PDI
dyad was very recently described in order to form
thermally stable dyes.11
electron donor and
a functionalized [60]fullerene
([60]PCBM) as the electron acceptor. Based on this con-
cept, plastic solar cells3 with power conversion efficiency
around 3.5% were recently reported.4 Nevertheless,
further improvement seems limited by incomplete
absorption of the incident light because of a poor match
between the absorption spectrum of both polymer and
fullerene materials and the solar emission spectrum.
Particularly [60]PCBM, which represents at least 75%
of the active layer, presents a very low absorption
coefficient in the visible region. The principle of improv-
ing light absorption of fullerene derivatives and its rela-
tionship with the efficiency of photovoltaic cells was
recently demonstrated by either replacing [60]PCBM
by [70]PCBM5 or using a C60–azothiophene dyad.6
Another strategy consists in the search of an energy
transfer using dendrimers with a C60 core and peripheral
oligophenylenevinylene moieties.7
RR
Although a large number of dyads associating C60 to an
electron donating unit have been synthesized so far in
O
N
O
O
N
O
O
O
O
C5H11
O
O
O
RR
Keywords: Fullerene; Perylenediimide; Photovoltaics.
*
[60]PCBM
Corresponding author. Tel.: +33 241 73 50 94; fax: +33 241 73 54
05; e-mail: pietrick.hudhomme@univ-angers.fr
1 : R = Cl
2 : R = OPhtBu
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.04.132

4600
J. Baffreau et al. / Tetrahedron Letters 46 (2005) 4599–4603
The synthesis of dyad 1, incorporating an unsymmetri-
cal tetrasubstituted PDI at the bay region, was
performed using an efficient methodology. Instead of
applying methods reported to reach 3,4:9,10-perylene-
tetracarboxylic monoanhydride monoimide derivatives,12
we achieved the direct condensation of 1-pentylamine
and 2-aminoethanol in a stoichiometric ratio on 1,6,
7,12-tetrachloroperylene tetracarboxylic dianhydride as
the starting material. After separation and purification,
symmetrical compounds 3 and 5 were obtained in 31%
and 34% yields, respectively, and the unsymmetrical
monoalcohol 4 in 28% yield.
results from the formation of C602À–PDI2À species.
We should note that a two-electron process was ob-
served for the dyad unsubstituted at the bay region
ÀÅ
resulting from the simultaneous generation of C60
–
PDIÀÅ species.10 These differences are in agreement with
an enhancement of the electron affinity for the PDI
substituted by four chlorine atoms at the bay region,
the electron-withdrawing inductive effect of the chlorine
atoms stabilizing the anion radical PDIÀÅ, this minimiz-
ing their possible electron-donating mesomeric effect.
The oxidation of the C60 moiety is observed at
+1.73 V as an irreversible process, whereas the oxidation
Cl Cl
Cl Cl
Cl Cl
O
N
O
O
N
O
O
O
N
O
O
N
O
O
N
O
HO
OH
OH
C5H11
N C5H11
O
C5H11
Cl Cl
Cl Cl
Cl Cl
4
5
3
The subsequent nucleophilic substitution of chlorine
atoms was carried out according to an approach firstly
developed by Seybold and co-workers.13 Using com-
pound 4 and 4-tert-butylphenol in the presence of
K2CO3, tert-butylphenoxyPDI 6 was synthesized in mild
conditions in 87% yield. Unsymmetrical malonate deriv-
atives 7 and 8 were obtained from monoalcohols 4 and 6
using ethyl malonyl chloride in 92% and 91% yields,
respectively. Cyclopropanation of C60 was carried out
by reaction of malonate 7 or 8 with iodine and DBU14
affording methano[60]fullerene dyads 1 and 2 in 41%
and 46% yields, respectively, after column chromatogra-
phy (SiO2, CH2Cl2/EtOAc 4/1 for 1 and CH2Cl2 for 2).
Reference compound 9 was prepared according to Bin-
gelÕs methodology starting from a-bromoethyl malon-
ate.15 Thermogravimetric analysis (TGA) proved the
thermal stability of dyads 1 and 2 with their decomposi-
tion starting at 340 ꢁC.
of the PDI moiety is not observed in the experimental
conditions.
The CV of dyad 2 (Fig. 1B) shows four reversible reduc-
tion waves at E1/2 red1 = À0.57 V, E1/2 red2 = À0.71 V,
E1/2 red3 = À0.82 V and E1/2 red4 = À0.93 V (vs AgCl/Ag)
(Table 1). The first one-electron process is assigned to
the formation of the anion radical of the C60 moiety
(C60ÀÅ–PDI). This was followed by the one-electron
reduction processes of the PDI moiety leading to
C60ÀÅ–PDIÀÅ then C60ÀÅ–PDI2À species. The last one-
electron reduction wave arises from the generation of
the C602À–PDI2À species. Reduction potentials of the
PDI moiety are consequently shifted to more negative
values with the substitution of chlorine atoms by tert-
butylphenoxy groups. This can be explained by the elec-
tron-donating mesomeric effect of phenoxy groups with
less consideration for their possible electron-withdraw-
tBu
tBu
O
O
R
R
O
O
O
O
O
N
O
O
N
O
O
O
O
C5H11
N
C5H11
N
OH
O
O
O
O
O
O O
R
R
7
8
R = Cl
R = OPhtBu
9
6
tBu
tBu
The cyclic voltammogram (CV) of dyad 1 (Fig. 1A)
shows three reversible reduction waves at E1/2 red1
ing inductive effect. Consequently, this is the first exam-
ple of a C60–PDI dyad tailored so that the first reduction
unambiguously occurs on the C60 moiety. This consti-
tutes an important parameter since C60 plays the role
of charge carrier in photovoltaic cells. The first revers-
ible oxidation wave at +1.34 V is attributed to the oxida-
tion of the PDI moiety leading to C60–PDI+Å species.
The second reversible oxidation of the PDI moiety is ob-
served at +1.72 V. The oxidation of the C60 moiety
should occur also at this same potential, as an irrevers-
ible process (Fig. 1B). Comparison of these different val-
=
À0.31 V, E1/2 red2 = À0.55 V and E1/2 red3 = À0.93 V (vs
AgCl/Ag). By comparison with those of compounds 3
and 9 (Table 1), the first one-electron process is assigned
to the formation of the anion radical of the PDI moiety
(C60–PDIÀÅ). The following two-electron process corre-
sponds to the generation of the C60ÀÅ–PDI2À species
suggesting that the first reduction process of fullerene
and the second reduction process of PDI are overlap-
ping. The one-electron wave at more negative potential

J. Baffreau et al. / Tetrahedron Letters 46 (2005) 4599–4603
4601
ues for dyads 1 and 2 with reference compounds 3, 8 and
9 suggest that no significant interaction takes place be-
tween both electroactive moieties in the ground state.
8
6
A
4
The absorption and steady-state fluorescence spectra of
dyads 1 and 2 in toluene are shown in Figure 2 and se-
lected data are collected in Table 2. The UV–vis absorp-
tion spectra of 1 and 2 match the profile obtained by
superimposition of the spectra of corresponding refer-
ence compounds (compounds 3 and 9 for 1, compounds
6 and 9 for 2), indicating the absence of significant
2
0
-2
-4
-6
-8
12
10
8
6x104
B
A
5x104
4x104
3x104
2x104
1x104
0
6
4
2
0
-2
-4
-6
350 400
450 500
550 600
650 700
750
1500 -1200 -900 -600 -300
0
300 600 900 1200 1500 1800
Wavelength (nm)
Tension (mV)
4x104
3x104
2x104
B
Figure 1. Deconvoluted cyclic voltammograms of dyads 1 (A) and 2
(B), respectively. Experimental conditions are detailed in Table 1.
Table 1. Redox potentials (V) of dyads 1 and 2 and reference
compounds 3, 8 and 9
Compound E1/2 red4 E1/2 red3 E1/2 red2 E1/2 red1 E1/2 ox1 E1/2 ox2
1x104
0
1
2
3
8
9
—
À0.93
À0.55a À0.31
À0.71 À0.57
À0.54 À0.34
À0.83 À0.69
À0.94 À0.55
+1.73b
—
À0.93 À0.82
+1.34 +1.72c
d
—
—
—
—
—
—
—
—
+1.32 +1.66
+1.71b
350 400 450 500 550 600
Wavelength (nm)
650 700 750
—
Values recorded in a CH2Cl2 solution using Bu4NPF6 0.1 M as the
supporting electrolyte, AgCl/Ag as the reference, platinum wires as
counter and working electrodes. Scan rate: 100 mV/s.
a Two-electron process.
Figure 2. (A) Absorption and fluorescence emission (kexc = 521 nm)
spectra of 1 (bold line), absorption spectra of 9 (dotted line) and 3 (full
line) in toluene at 298 K (c < 10À6 M). (B) Absorption spectra of 2
(bold line) and fluorescence emission spectrum of 2 (dashed line,
b Irreversible process.
c Irreversible plus one-electron reversible process.
d Not measurable in experimental conditions.
k
exc = 400 nm) and 6 (full line, kexc = 400 nm), in toluene at 298 K
(c < 10À6 M).
Table 2. Selected photophysical data in toluene
e (L molÀ1 cmÀ1
)
kmax (nm)
Ufd
a
b
Compound
kmax (nm)
1
2
3
6
9
524
579
29,800
36,800
33,900
42,000
549, 696
602, 698
548
3.3 · 10À3
3.4 · 10À3
522
575
1
601
697
1
689 (492, 428)c
612 (2070, 2280)c
5 · 10À4
a Absorption maxima.
b Fluorescence emission maxima.
c Other absorption bands and their corresponding epsilon values.
d Determined using cresyl violet as a standard (Uf = 0.54 at 20 ꢁC in methanol).

4602
J. Baffreau et al. / Tetrahedron Letters 46 (2005) 4599–4603
interaction in the ground state between PDI and C60
moieties, in agreement with electrochemical results.
The absorption spectrum of 9 is very similar to that of
other C60 malonate derivatives16 and shows the charac-
teristic band of [6–6] substituted C60 at 428 nm. The low-
est singlet transition is observed at 689 nm. Compound 3
exhibits the three typical PDI absorption peaks at 431,
488 and 522 nm. A bathochromic effect is noticed for
compound 6 (absorption peaks at 448, 533 and
574 nm) as a result of the electron-donating effect of
phenoxy groups.
estimated to be 2.04 eV according to a DG value of
À0.28 eV, calculated using the Rehm–Weller equa-
tion)17 lies above the fullerene singlet excited state. Fur-
ther experiments are currently underway in order to
estimate the rate of energy transfer in the system.
In the case of dyad 2, evidencing the energy transfer
from PDI to C60 is more complex. Indeed, emission
from PDI moiety is red-shifted compared to dyad 1
(602 and 544 nm, respectively). Therefore, emission
bands characteristic of the PDI and fullerene moieties
are overlapping. Whatever, by subtracting the contribu-
tion of PDI from emission spectrum of 2, the fullerene
emission is still observed, even when PDI moiety is selec-
tively excited at wavelength above 520 nm (Fig. 3).
These results, in addition to energy levels of both units
(2.11 eV for 6 and 1.79 eV for 9), give evidence of a sing-
let–singlet energy transfer from PDI to C60. Competi-
tion with oxidative electron transfer process can be
ruled out for the same reasons underlined for 1 (energy
level of charge separated state for 2 was estimated to be
1.91 eV according to a DG value of À0.20 eV, calculated
using the Rehm–Weller equation).18
Apart from a fluorescence emission centred on the PDI
moiety that occurs in the 500–650 nm range, a second
emission band, characteristic of the methanofullerene
moiety, is observed in the 670–750 nm region for both
dyads. Furthermore, there is a dramatic decrease (ca.
99% and 98%, respectively) of the PDI moiety quantum
yield in dyads 1 and 2 compared to the one of com-
pounds 3 and 6, respectively. This implies a strong
quenching of the PDI fluorescence by the fullerene moi-
ety. This process must be intramolecular as the quantum
yield of neither 3 nor 6 is affected in a mixture of PDI
reference compound (3 or 6) and 9 in the same concen-
tration as in the dyads.
In conclusion, the synthesis and spectroscopic studies of
new C60–perylene-3,4:9,10-bis(dicarboximide) dyads are
described. It was clearly demonstrated that the position
of the first reduction potential and consequently the
behaviour towards C60 can be fine tuned by molecular
engineering around the perylenediimide core. In order
to demonstrate the role of a light-harvesting antenna
grafted onto C60 in the efficiency of solar cells, blends
of polymer with C60–PDI dyads are currently underway.
Such C60-antenna dyads that absorb strongly in the vis-
ible range could be good candidates for the development
of efficient photovoltaic devices.
In the case of dyad 1, the relative weight of each band
(centred at 544 and 697 nm) in the steady state lumines-
cence spectrum depends on the excitation wavelength.
By selectively exciting the PDI moiety in dyad 1, at
1
wavelength above 520 nm, an emission from C60*–
PDI was still recorded, giving evidence of a singlet–sing-
let energy transfer from the PDI moiety to C60 moiety.
This is in agreement with the energy levels of both units
(2.32 eV for 3 and 1.79 eV for 9) and was also confirmed
by the excitation spectrum recorded at 700 nm. Indeed,
this latter matches the absorption profile of 3 through-
out the UV–vis spectral region. These results are consis-
tent with a quantitative energy transfer according to the
different quantum yield values calculated. Competition
with electron transfer process can be ruled out as the
corresponding charge separated state (energy level was
Acknowledgements
This work was supported by grants from the Conseil
´ ´
´
General du Maine et Loire (for J.B.) and the Region
Pays de la Loire-CNRS-TotalFinaElf (for L.P.). The
authors acknowledge ADEME and CEA through the
CSPVP ÔCellules Solaires Photovolta¨ıques PlastiquesÕ
research programme. We thank BASF-AG, Ludwig-
shafen, for providing 1,6,7,12-tetrachloroperylene tetra-
carboxylic dianhydride and Dr. D. Bassani (LCOO
´
Universite Bordeaux 1) for fruitful discussions.
Supplementary data
Spectroscopic data of dyads 1 and 2. Supplementary
data associated with this article can be found, in the on-
line version at doi:10.1016/j.tetlet.2005.04.132.
References and notes
650
675
700
725
750
775
Wavelength (nm)
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at 298 K (c < 10À6 M).

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˚
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