Design of a series of polythiophenes containing C60 groups: Synthesis and optical and electrochemical properties

Article abstract of DOI:10.1021/ma501788d

(Figure Presented) In order to develop polymers combining both acceptor and donor moieties, a series of alternating copolymers based on C₆₀ fullerene and oligothiophenes were synthesized. First, a diamino-functionalized fullerene was obtained by a three-step modification of 4,4′-diaminobenzophenone, followed by a Bamford - Stevens addition on C₆₀. A series of oligo-3-alkylthiophenes O3AT, with various polymer and alkyl chains lengths, were synthesized using the Grignard metathesis method and dicarbonylated by Vilsmeier formylation. Alternating copolymers [C₆₀-O3AT]_n were finally obtained by polycondensation, taking a specific care concerning synthesis conditions in order to optimize polymerization degrees. Nuclear magnetic resonance, infrared spectroscopy, gel permeation chromatography and thermogravimetric analysis were achieved in order to determine the structure of each copolymers. In addition, their functional properties (UV-vis absorption, photoluminescence, and electronic levels) were measured and related to the ascertained average compositions.

Full text of DOI:10.1021/ma501788d

Article
pubs.acs.org/Macromolecules
Design of a Series of Polythiophenes Containing C60 Groups:
Synthesis and Optical and Electrochemical Properties
Lara Perrin,*,†,‡ Mathilde Legros,†,‡ and Regis Mercier‡
́
†LEPMI, Univ. Savoie, F-73000 Chamber
́
y, France
‡LEPMI, CNRS, F-38000 Grenoble, France
ABSTRACT: In order to develop polymers combining both
acceptor and donor moieties, a series of alternating copolymers
based on C60 fullerene and oligothiophenes were synthesized.
First, a diamino-functionalized fullerene was obtained by a three-
step modification of 4,4′-diaminobenzophenone, followed by a
Bamford−Stevens addition on C60. A series of oligo-3-
alkylthiophenes O3AT, with various polymer and alkyl chains
lengths, were synthesized using the Grignard metathesis method
and dicarbonylated by Vilsmeier formylation. Alternating
copolymers [C60-O3AT]n were finally obtained by polyconden-
sation, taking a specific care concerning synthesis conditions in
order to optimize polymerization degrees. Nuclear magnetic resonance, infrared spectroscopy, gel permeation chromatography
and thermogravimetric analysis were achieved in order to determine the structure of each copolymers. In addition, their
functional properties (UV−vis absorption, photoluminescence, and electronic levels) were measured and related to the
ascertained average compositions.
between the following characteristics: (i) a large surface of
interface between the acceptor and donor phases to facilitate
the excitons separation, (ii) a size of domains close to the
length of excitons diffusion (3−20 nm13−15) in order to
minimize the loss of charges by recombination but large
enough to allow the excitons separation, and (iii) a thin active
layer thickness (∼100−200 nm) in order to optimize the
charge collection toward the electrodes.
Traditionally, morphology improvement can be realized
during or after the active layer deposition through several ways:
modification of the D/A ratio,16,17 change of the solvent,18 use
of processing additives,19−21 and vapor22,23 or thermal18,24
annealing treatments. In addition, the use of D−A copolymers
as compatibilizing agent offers another especially interesting
approach to improve or stabilize the morphology. As regards
this latter strategy, several recent works25−27 and reviews28−31
report new structures incorporated as ternary components in
photoactive layers with the aim of enhancing photovoltaic
conversion or stabilizing device performance by control of the
nanoscale morphology. Among materials that have demon-
strated success, most of them are composed of two types of
units with structures close to D and A active materials used.
Introduced in the D/A blend system, these structures could
find a place at the interphase. For instance, diblock27,32,33 or
brush34 copolymers have demonstrated their potential for
beneficial effect on morphology. However, the presence of
1. INTRODUCTION
First described in the 1970s,1−3 organic photovoltaic (OPV)
materials seem to be an interesting alternative to inorganic
materials. More specifically, new “plastic” photovoltaic devices
have a great potential for low cost, low weight, flexibility, and
large-size processability.4 Recent studies even light up their real
potential for short energy payback time.5 However, in order to
become a real alternative to the inorganic technology, several
challenges must be overcome including the low conversion
efficiency and the rapid degradation of performance.
The active layer of organic solar cells usually consists of a
blend of two materials: an electron-donor π-conjugated
polymer D and an electron-acceptor fullerene derivative A.
Bulk heterojunction devices based on poly(3-hexylthiophene)
(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) represent the most investigated system and can be
considered as a reference structure with efficiencies up to 3−
5%.6,7 Higher performances around 9−11%8−10 have already
been reached using new low-band-gap polymers and tandem
cell architectures.
The photovoltaic efficiencies and their long-term stability are,
among other factors like the photoactive materials choice and
device encapsulation, directly related to the morphology of the
active layer.11,12 Indeed, in a bulk heterojunction solar cell, the
low miscibility between the acceptor and donor materials does
not allow an optimization of the size of their domains.
Furthermore, an unfavorable evolution of the morphology can
occur after several running hours. In both cases, this results in a
nonoptimized separation of charges in the D/A interpenetrat-
ing network. It is thus necessary to find a good compromise
Received: August 29, 2014
Revised: December 19, 2014
© XXXX American Chemical Society
A
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insulating parts unavoidable for the design of the C60 block can
degrade the rate of recovered charges. Recently, the use of
oligothiophene units grafted or end-capped by C60 units35−38
also proved their aptitude to improve or stabilize OPV
performances. Considering these interesting results, we were
interested to develop longer patterns using alternating
oligothiophene/C60 copolymers (Figure 1). Indeed, synthesis
In order to synthesize the alternating [C60-O3AT]n
copolymers presented in Figure 1, the chosen synthesis
methodology was a polycondensation reaction involving a
diamino fullerene C60-NH2/NH2 and a dialdehyde oligothio-
phene O3AT-CHO/CHO. Thus, the first stage of the work was
dedicated to the synthesis of both acceptor and donor
difunctionalized monomers, each time completed by a careful
stage of model syntheses in order to optimize the final
condensation conditions. A series of copolymers were then
synthesized using various alkyl and oligothiophene chains
lengths. Their chemical structures were investigated using
nuclear magnetic resonance (NMR), infrared spectroscopy
(IR), gel permeation chromatography (GPC), and thermogra-
vimetric analysis (TGA) and correlated to functional properties
(UV−vis absorption, photoluminescence, and electronic
levels).
2. RESULTS AND DISCUSSION
Figure 1. General structure of the copolymer family [C60-O3AT]n.
2.1. Acceptor Monomer. 2.1.1. Synthesis of a Diamino-
Functionalized Fullerene: C60-NH2/NH2. Among possible
fullerene diamino functionalization methods described in the
literature,57−60 the chosen derivative was a diaminophenyl-
modified C60 (see compound 5 in Scheme 1). The structure of
this derivative will allow a π-conjugated extension of the linked
donor unit, and its synthesis is accessible in four steps (Scheme
1).60
In a first step, the amino groups of 4,4′-diaminobenzophe-
none were protected by amido conversion using trifluoroacetic
anhydride. This protected compound was then transformed in a
diazo derivative by condensation with p-tosylhydrazide. After
that, the deprotection of amino groups was realized in basic
media. In order to confirm the entire success of the
deprotection, the reaction was monitored by 19F NMR analysis
until extinction of the −CF3 characteristic signal at −76.21
ppm. Surprisingly, two different solid compounds could be then
isolated at this step: a white precipitate in the reaction mixture
and a yellow solid obtained after treatment of the aqueous
reaction media (solvent extraction and concentration by
evaporation). 13C NMR spectra of both compounds were
quite similar and compatible with compound 4, while 1H NMR
spectra were really different (absence of the N−NH− signal
for the precipitate and significant shifts for the aromatic and
amine signals). Complementary analyses were realized by mass
ways can be conducted in order to ensure minimal insulating
linker between the active moieties. And, as already reported in
the literature,39,40 alternating or random copolymers could have
a role to play as compatibilizing agents.
The present paper describes the synthesis method deployed
to prepare a large variety of new linear and soluble
oligothiophene/C60 alternating copolymers [C60-O3AT]n as
potential compatibilizing agents for P3HT/PCBM-based
devices. The most reported procedures to access fullerene
containing copolymers (alternating or random) are the
following: polymers with pendant C60 fullerenes,41−49 radical
or Prato-based additions of a monomer on C60 fullerene,50−55
and reaction of a bifunctional fullerene with another bifunc-
tional monomer.56 Among these possibilities, the latter strategy
was chosen according to the interesting possibility to control
both the minimization of the insulating linker and the absence
of fullerene poly adducts derivatives. The minimization of the
insulating linker is expected to ensure enhancement of the
charges transfer and transport during the photovoltaic process.
As regards the previous stage of fullerene difunctionalization to
consider due to the ability of C60 to accept up to six electrons,
this will undeniably help to obtain linear, un-cross-linked, and
soluble copolymers.
Scheme 1. Synthesis of the Acceptor Monomer C60-NH2/NH2
B
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Scheme 2. Synthesis of Two Model Triads Tm-C60-Tm
spectrometry (ESI-TOF), and the same major m/z signal,
compatible with [M + H]+ of 4, was observed for both
compounds. The additional presence of the [M + K]+ signal
only in the case of the white solid suggests that it corresponds
to a ionic salt form (K+, −−N−N). Furthermore, this ionic
form could be converted into the corresponding protonated
compound by passing through a silica gel column (known to be
weakly acid).
The last reaction step was the addition of the previous
hydrazone on C60 according to a 1,3-dipolar cycloaddition
mechanism using the Bamford−Stevens method to afford the
C60-NH2/NH2 derivative 5. The overall yield after the four
steps reaction was around 11%. 13C NMR analysis indicates the
formation of only the [6−6]-closed fullerene isomer.61 Indeed,
under high reaction temperature, the [5−6]-open isomer can
undergo conversion to the [6−6]-closed isomer which is more
thermodynamically stable.62 Even after a long drying period at
100 °C under vacuum, the final product always contains some
residual toluene. This residual toluene signal may have probably
1
Figure 2. H NMR spectra of the two model triads Tm-C60-Tm (400
MHz, CDCl3, zoom on aromatic signals, real integration values are
given in italics, the cut signal corresponds to the NMR solvent).
polymerization degree by modulation of the monomer/catalyst
ratio.66,67 This polymerization method is based on a Kumada
coupling that occurs under the action of a catalyst (Ni or Pd)
and involves a halogenated compound and an organometallic
initiator.
The aim was to study both the influence of the alkyl chain
and polymer chain lengths. Thus, three different alkylated
thiophene monomers (hexyl, octyl, and decyl) were initially
prepared. As presented in Scheme 3, the polymerization was
realized using 2-bromo-5-iodothiophene monomers (10) in
order to obtain regiospecific magnesium intermediates and
highly regioregular polythiophenes.65
As regards the synthesis of initial 2-bromo-5-iodothiophene
reagents, a specific care was taken in order to obtain high purity
compounds. In an initial approach, the iodation of 2-
bromothiophene derivatives was realized using N-iodosuccini-
mide.68 This synthesis method was not enough regioselective to
reach adequate purity. Therefore, a iodation using the
combination of iodine and iodobenzene diacetate65 was used.
After Kugelrohr distillation, the aimed compounds 10 were
obtained with purities greater than 98% (determined by gas
chromatography/mass spectrometry).
As shown in Scheme 3, the synthesis of dicarbonylated oligo-
3-alkylthiophenes was conducted in two main stages: the chain
growth polymerization and the formylation of end-chain units.
The polymerization was conducted according to a four-step
procedure. First, the thiophene monomer is activated in
position 5 by the action of tert-butylmagnesium chloride
Grignard reagent. Second, the chain growth occurs at room
temperature under the action of 1,3-bis(diphenylphosphino)-
propane nickel(II) chloride catalyst ([Ni(dppp)Cl2]). After 4 h,
the polymerization is quenched by addition of an HCl aqueous
60
1
misled previous literature H NMR interpretation.
2.1.2. Synthesis of Two Model Triads: Tm-C60-Tm. In order
to validate the reactivity of the low soluble C60-NH2/NH2
derivative in future amine/aldehyde condensations, two model
triads based on thiophene were synthesized. In addition, the 1H
NMR signature of these model triads will undeniably clarify
future copolymers spectra. Scheme 2 illustrates the general
procedure used for the synthesis of model triads.
Initially, 3-hexylthiophene-2-carbaldehyde (6) and 3,3′-
dihexyl-2,5′-bithiophene-2-carbaldehyde (7) reagents were
synthesized in the laboratory using procedures close to those
reported in the literature.63,64 Then, both aldehyde compounds
were involved in a condensation reaction with the diamino-
functionalized fullerene C60-NH2/NH2 affording respectively T-
C60-T (8) and T2-C60-T2 (9) triads with a 70% yield after
purification.
As shown in Figure 2, the structure of each triad was
1
confirmed by H NMR analysis. The characteristic signal of
Schiff bases (−CHN−) was identified at 8.6 ppm, thus
indicating the success of the condensation reaction. The two
doublets at 8.1 and 7.3 ppm, observed for both triads, are
attributed to the phenyl groups carried by the C60 fullerene
moiety. The characteristic signature of thiophenic protons can
also be observed in Figures 2a and 2b corresponding to
thiophene and bithiophene units, respectively.
2.2. Donor Monomer. 2.2.1. Synthesis of a Dicarbony-
lated Oligo-3-alkylthiophenes Series: O3AT-CHO/CHO. In
order to elaborate a series of regioregular oligo-3-alkylth-
iophenes with controlled length, the Grignard metathesis
(GRIM) method65 was chosen. This method can be carried
out at ambient temperature and allows easy control of the
C
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Scheme 3. Synthesis of a Series of Donor Macromonomers O3AT-CHO/CHO
solution in order to favor low dispersities.69 At this step, an
additional reduction of dispersities was achieved using Soxhlet
fractionation with methanol, petroleum ether, and chloroform.
Dispersities Đ measured by gel permeation chromatography
(solvent THF, 25 °C) were between 1.3 and 1.6 for all selected
oligothiophene fractions. Afterward, the oligothiophenes series
was debrominated by action of the lithium aluminum hydride
in order to prepare the formylation. The success of this step
was checked by MALDI-TOF mass spectrometry showing the
disappearance of m/z characteristic signals corresponding to
H/Br end groups in favor of m/z characteristic signals of H/H
end groups.
addition to O3AT-CHO/CHO characteristic m/z mass signals.
However, according to the good recovery of proton
integrations in the three new signals (a′, b′, and c′) compared
to missing initial signals (a and b) with a standard deviation
lower than 5%, it can be assumed that the degree of aldehyde
end-functionalization is about 95−100%.
The oligothiophene regioregularity RR can also be evaluated
1
by H NMR. The degree of RR is defined as the fraction of
monomers adopting a head-to-tail (HT) configuration, rather
than other arrangements. According to Barbarella et al.,73
identification of NMR shifts (see Figure 3), the regioregularities
of the oligothiophene series were estimated to be always higher
than 90%.
Four different oligothiophenes O3AT were synthesized
according to the preceding procedure. The choice was guided
by the two following objectives: (i) study of the impact of the
alkyl chain length with the synthesis of three oligo-3-
alkylthiophenes having close molecular weights and different
alkyl chains: hexyl, octyl, and decyl for 12a, 12b, and 12c,
respectively; (ii) study of the impact of the polymer chain
length with two oligo-3-hexylthiophenes having two different
molecular weights: 12a and 12a′ with a targeted polymerization
degree of 30 and 15 units, respectively.
Lastly, the series of oligothiophenes was difunctionalized by
aldehyde functions via the Vilsmeier−Haack reaction (Scheme
3). This reaction has already proved reliable results on
polythiophene difunctionalization in the literature.70,71 The
end-functionality was evaluated by both MALDI-TOF mass
spectrometry (m/z characteristic signals for CHO/CHO end
1
groups) and H NMR spectroscopy (Figure 3).
As shown in Figure 4, the GPC chromatograms of the four
synthesized O3AT oligomers indicate similar elution volumes
1
Figure 3. H NMR spectra of oligothiophenes (400 MHz, CDCl3,
zoom on the different end-chain characteristic signals): (a) O3HT-H/
H; (b) O3HT-CHO/CHO (also given the thiophene units arrange-
ment using H for head and T for tail).
Figure 4. GPC chromatograms of donor oligothiophene monomers
O3AT (conditions: THF, 25 °C).
As shown in Figure 3, the success of the dialdehyde
functionalization was confirmed by H NMR analysis. Indeed,
1
the CHO functionalization is evidenced by the presence of the
aldehyde signal at 10.01 ppm. Moreover, the disappearance or
shift of small signals attributed to aromatic and aliphatic end-
chain protons indicates the quite overall end-functionalization.
The more significant shift (2.61 to 2.95 ppm) concerns the
triplet attributed to α-CH2 of end-chain thiophene units. All
other signals attributions are represented on Figures 3a and 3b,
in accordance with literature reported interpretations.54,70,72
Proton NMR analyses conducted on the dicarbonylated
oligo-3-alkylthiophenes series evidence a relative good end
difunctionalization of all macromononomers. According to
MALDI-TOF mass spectra, the presence of monofunctional-
ized derivatives O3AT-CHO/H can be sometimes detected in
for 12a, 12b, and 12c derivatives and lower value for 12a′. The
corresponding number-average molecular weights (Mn) are
presented in Figure 5. Nevertheless, one major issue in this
work was to find a convenient way to estimate the real
molecular weights of O3AT donor units. Indeed, the GPC
method used relies on a calibration using polystyrene standards
which implies a difference with the true molecular weight.74 (It
should be mentioned that the absolute molecular weight
determination GPC method using light scattering is not
suitable for thiophene-based polymers due to their lumines-
cence properties.)
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2.2.2. Synthesis of a Model Copolymers Series: [ODA-
O3AT]n. Scheme 4 presents the general synthesis route
executed with a series of ODA/O3AT weight ratio for each
selected O3AT monomers. The polycondensation was realized
in a mixture of THF/DMF (6/2), an aprotic solvent shared by
O3AT, ODA, and C60-NH2/NH2 monomers. To favor the
condensation mechanism, the reaction was thermally activated
and acid-catalyzed using p-toluenesulfonic acid (PTSA) in the
presence of calcium chloride in order to remove produced
water from the reaction media.
For the four selected O3AT monomers, a series of
polycondensation reactions were realized by varying the
ODA/O3AT weight ratio (minimum six ratios chosen between
GPC and MALDI-TOF number-average molecular weight
1
estimations). Figure 6 shows examples of H NMR profiles for
Figure 5. Molecular weights estimation of donor macromonomers
(O3AT) using different methods (lines are a guide to the eyes).
In order to optimize the final C60/O3AT polycondensation
reaction, it is necessary to know the molecular weight of each
monomer unit used with an adequate precision. Indeed, the
copolymer molecular weight will be directly linked to the NH2/
CHO functional units ratio (equivalent to the C60/O3AT
ratio), which should be as close as possible to 1.
Figure 5 presents the molecular weights estimation of O3AT
monomers using five different methods. First of all, four
traditional methods were compared: room temperature GPC,
high temperature GPC, proton NMR, and MALDI-TOF mass
spectrometry. For GPC measured values, a significant differ-
ence is observed between measurements realized either at 25
°C in tetrahydrofuran (THF) or at 150 °C in 1,2,4-
trichlorobenzene (TCB): diminution of a factor close to 1.5−
1.9 for long O3AT and 2.6 for short O3AT. According to the
literature,75 polythiophenes adopt a rodlike conformation in
solution leading to a GPC overestimation of molecular weights
when compared to polystyrene. This overestimation is less
important in TCB as the solubility and mobility are increased
Figure 6. 1H NMR spectra of different [ODA-(short O3HT)]n
synthesis batches (NMR conditions: 400 MHz, C2D2Cl4, 70 °C).
the crude reaction products (spectra are normalized on the
signal of thiophenic b protons). The low solubility of obtained
copolymers did not allow complete GPC analyses of crude
samples. Thus, the optimal polycondensation conditions were
determined according to NMR analyses. Indeed, it could be
considered that the ODA/O3AT ratio is close to the optimum
when the signal due to the Schiff base is maximal (see signal c in
Figure 6) and that signals due to initial monomers or end units
are minimal (protons a, c′, d′, e′, f, g and free ODA). As shown
in the example presented in Figure 6b, the selected ratio was
0.44 mmol of ODA per gram for the short O3HT (12a′), value
equivalent to 0.88 mmol of CHO functions. To illustrate the
success of the polymerization, values given by GPC analysis of
the chloroform soluble fraction of this copolymer were Mn =
17 500 and Đ = 3 (conditions: THF, 25 °C, PS equiv),
corresponding to an average repetition of 4−5 units.
1
by both solvent and temperature. H NMR estimations could
also result in an overestimation increasing with the molecular
weight because the calculation is realized using a very weak
end-chain NMR signal (see protons b and e in Figure 3), thus
giving a measurement uncertainty. As regards MALDI-TOF,
Mn values (obtained by the statistical average method) are
significantly lower than the others. This difference between
GPC and MALDI-TOF measurements was also observed by
Lui et al., with a stronger difference when Mn increase.76
However, these latter values should also be used with caution as
the desorption/ionization phenomena in mass spectrometry
could be more favorable to lower molecular weight chains.77,78
In order to determine more precisely the optimal quantity of
O3AT-CHO/CHO oligomer required to obtain a NH2/CHO
ratio close to 1, a model polycondensation series introducing a
substitution diamine was realized. The 4,4′-oxidianiline (ODA)
was chosen due its commercial availability and structure close
to the C60-NH2/NH2 acceptor monomer.
The number of aldehyde equivalents per gram of O3AT was
determined for the four selected oligothiophenes according to
the same NMR identification process, and the issues are
presented in Table 1. The estimated polymerization degrees of
Scheme 4. Synthesis of a Model Copolymers Series [ODA-O3AT]n
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Table 1. O3AT-CHO/CHO Characteristics Selected for the
[C60-O3AT]n Copolymers Syntheses
aldehyde equivalents (mmol)/g of oligothiophene
12a′: (HT)13
12a: (HT)28
12b: (OT)30
12c: (DT)25
0.880
0.424
0.344
0.352
O3AT monomer units are also indicated using the following
nomenclature: (AT)m with A the nature of the alkyl chain and
m the average number of thiophene T repeating units
determined using the model copolymerization with ODA. As
shown in Figure 5, the corresponding Mn values were
intermediate to other determination methods used and close
to high temperature GPC values.
2.3. Conception of a Series of Oligothiophene/
Fullerene Alternating Copolymers. 2.3.1. Synthesis and
Purification of the [C60-O3AT]n Copolymers. As shown in
Scheme 5, the synthesis route used for the C60/O3AT
copolymers was similar to the previous ODA/O3AT
copolymers synthesis. The C60/O3AT molar ratios introduced
were close to one, and the corresponding weight amounts were
calculated according to Table 1 using the same O3AT synthesis
batches as for the model ODA copolymers syntheses.
1
Figure 7. H NMR spectra of the different Soxhlet fractions for the
[C60-(DT)25]n synthesis batch: (a) chloroform extraction; (b) o-
dichlorobenzene extraction (NMR conditions: 400 MHz, C2D2Cl4, 70
°C).
Figure 8 shows the GPC chromatogram of the chloroform
soluble fraction of the [C60-(AT)13]n copolymer. When
In order to reduce the impact of the sterical hindrance
brought by fullerene moieties, a reaction time of 7 days was
applied. Each [C60-O3AT]n polymer synthesis was then
followed by a purification and fractionation using Soxhlet
extraction with methanol, hexane, chloroform, and o-dichlor-
obenzene, sequentially. For each synthesis, two fractions could
be isolated: CHCl3 (chloroform soluble) and o-DCB (o-
dichlorobenzene soluble). Each time, a non-negligible fraction
of insoluble material was also produced.
2.3.2. Determination of the Chemical Structure of [C60-
O3AT]n Copolymer Fractions. The success of the poly-
condensation reaction was confirmed by both IR and NMR
spectroscopies. The characteristic IR signal for an imine (CN
stretching vibration mode) was identified at 1650 cm−1, and the
NMR characteristic signal of Schiff bases (−CHN−) was
identified at 8.7 ppm.
Figure 8. GPC chromatograms of fractions soluble in CHCl3 for both
C60 and ODA copolymers based on (HT)13, compared to (HT)13 and
(HT)28 oligothiophene monomers (GPC conditions: THF, 25 °C).
compared to its ODA copolymer equivalent, it is obvious
that the polycondensation was less efficient and less
homogeneous when using the C60 derivative. Nevertheless,
considering that the GPC contribution of a C60 unit is weak,79
the comparison with (HT)13 and (HT)28 oligothiophene
monomers chromatograms allows to qualitatively assign the
three observed shoulders. The shoulder A can be composed of
C60-O3HT diads and C60-O3HT-C60 triads with possible
residual O3HT. The shoulder B probably consists in a
copolymer chain composed of two O3AT patterns with
As shown in the 1H NMR examples presented in Figure 7, all
expected signals were identified. Nevertheless, some signals due
to end-chain units were also represented, suggesting a weak
polymerization degree. In particular, the characteristic signal of
aldehyde functions (−CHO) was present in all fractions. In
addition, some fractions were also more or less rich in C60
characteristic end-units (generally o-DCB fractions were richer
than their CHCl3 counterparts; see Figure 7).
Scheme 5. Synthesis of the Alternating A−D Copolymers Series [C60-O3AT]n
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between 1 and 3 fullerene units. The shoulder C is made of
copolymer chains composed of three or more O3AT patterns.
The molecular weight estimation given by high temperature
GPC for the different [C60-O3AT]n copolymers fractions is
presented in Table 2. The molecular weights of their initial
fractions: O3AT-C60-O3AT and (ii) o-DCB Soxhlet fractions:
C60-O3AT-C60-O3AT-C60.
2.4. Optical Properties. 2.4.1. Tm-C60-Tm Model Triads.
Figure 10 presents the UV−vis absorption spectra in
Table 2. Molecular Weights of the Different [C60-O3AT]n
Soxhlet Fractions, with the Corresponding O3AT
Macromonomers Analyzed in the Same GPC Conditions
(Trichlorobenzene, 150 °C, PS equiv)
[C60-O3AT]n
copolymers
O3AT monomers
Mn
Đ
Mn
Đ
[C60-(HT)13]n
CHCl3 fraction
4200
2.2
2.6
3.5
2.4
2.1
3.6
1500
3.2
[C60-(HT)28]n
CHCl3 fraction
7600
4300
7300
7300
5900
5900
2.0
1.9
1.9
1.5
1.5
[C60-(OT)30]n
CHCl3fraction
11700
15800
10300
13300
[C60-(OT)30]n
o-DCB fraction
Figure 10. UV−vis absorption spectra of the two model triads Tm-C60-
Tm (1.5 × 10−4 M in chloroform). The inset highlights a very weak
characteristic absorption band of the fullerene moiety.
[C60-(DT)25]n
CHCl3 fraction
[C60-(DT)25]n
o-DCB fraction
chloroform of both synthesized model triads (8 and 9). The
fullerene pattern exhibits three absorption peaks: the character-
istic C60 moiety absorption at 280 and 330 nm80 and a weak
peak at 700 nm, specific for methanofullerene compounds.44 As
regards the contribution of the phenyl−thiophene π-conjugated
systems, the spectrum of T2-C60-T2 triad clearly reveals the
phenyl−bithiophene absorption band at around 375 nm. In the
case of the T-C60-T triad, the absorption contribution of the
less extended π-conjugated phenyl−thiophene system is
evidenced by a slight shoulder at 360 nm.
The UV−vis absorption spectra of these model triad systems
consist of a superposition of the spectra of each units separated
by a nonconjugated linkage. This is in accordance with other
similar systems described in the literature.81−84
O3AT building blocks, determined in the same analyses
conditions, are also presented for comparison. It can be thus
concluded that the average polymerization degree is probably
close to 2. This weak value can be a consequence of the
copolymers limited solubility in the reaction media. Moreover,
a high fraction of insoluble materials was always collected
meaning that higher polymerization degrees should lead to
insoluble copolymers.
Synthesized copolymers were also characterized by thermog-
ravimetric analysis (TGA). All copolymer fractions present a
degradation temperature close to 400 °C, like their initial
oligothiophene building blocks. As shown in Figure 9, the C60
2.4.2. [C60-O3AT]n Copolymers. Figure 11 presents the UV−
vis absorption spectra in o-dichlorobenzene of the different
[C60-O3AT]n copolymer fractions isolated. The spectra of all
samples consist of the superposition of the C60 fullerene (330
nm) and π-conjugated chain (about 460 nm for all O3AT
monomers and copolymers) absorptions, similarly to model
triads and other existing C60−polythiophene linked systems.85
As shown in Figures 11a and 11b, two different behaviors
were observed depending on the Soxhlet fraction of the
copolymers. For the considered samples, CHCl3 soluble
fractions show quite similar UV−vis traces, and no influence
of the alkyl/polymer chain lengths was detected on the
maximum and edge wavelengths. When considering o-DCB
soluble fractions, all samples also show quite similar UV−vis
traces with an important reduction of the O3AT/C60 optical
density ratio in comparison with CHCl3 soluble fractions. The
decrease of the mass extinction coefficient ε corresponding to
the oligothiophene moiety was quantified to be about 30%
between CHCl3 and o-DCB Soxhlet fractions (Table 3). This
observation is consistent with the previously considered average
compositions of copolymer fractions.
Figure 9. TGA thermograms of the different Soxhlet fractions for the
[C60-(DT)25]n copolymer compared to the (DT)25 initial oligothio-
phene monomer and to C60 (under a nitrogen atmosphere).
fullerene part does not undergo degradation until 700 °C.
Therefore, TGA thermograms can help to confirm the
composition of the copolymers fractions. Associated with
NMR and GPC previous results, it can be thus generally
concluded that the average composition of majority [C60-
O3AT]n chains should be the following: (i) CHCl3 Soxhlet
Figure 12 presents the comparison between optical
absorption spectra recorded from solution and solid film states
for the different Soxhlet fractions of one [C60-(AT)m]n
copolymer example. Corresponding data obtained for all
synthesized copolymers are summarized in Table 3. First of
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Figure 11. UV−vis absorption spectra of the [C60-(AT)m]n different Soxhlet fractions (0.1 mg/mL in o-dichlorobenzene).
(shoulders) can be observed in solid state films, which is
characteristic of the existence of π-stacking interactions.
Second, the absorption spectra in film of o-dichlorobenzene
Soxhlet fractions show a less pronounced bathochromic shift
than their corresponding chloroform Soxhlet fractions (differ-
ence around 15 nm for λmax and 5 nm for λonset). This means
that the organization is disturbed either by the presence of
more fullerene units or by the rigid imino linkage between
oligothiophene units. However, the optical band gaps Eogpt of all
[C60-(AT)m]n copolymers fractions are close to 1.90 eV like for
the initial (AT)m macromonomers (Table 3).
According to UV−vis absorption data, no significant
interaction in the ground state was observed between
oligothiophene and fullerene chromophores. On the other
hand, both chromophores markedly interact in the excited state
(Figure 13). Indeed, the copolymers emission spectra are
Table 3. Optical Properties of Synthesized Copolymers
a
b
solution
film
λonset Eogpt
d
ε
λmax
(nm)
λmax
Φcopo/Φoligo (nm) (nm) (eV)
c
copolymers
(L g−1 cm−1
)
[C60-(HT)13]n
CHCl3 fraction
40
42
27
46
31
50
31
459
463
461
462
461
461
457
0.50
0.51
0.39
0.66
0.56
0.88
0.74
513
518
503
521
509
520
507
644
648
646
652
650
650
643
1.93
1.91
1.92
1.90
1.91
1.91
1.93
[C60-(HT)28]n
CHCl3 fraction
[C60-(HT)28]n
o-DCB fraction
[C60-(OT)30]n
CHCl3 fraction
[C60-(OT)30]n
o-DCB fraction
[C60-(DT)25]n
CHCl3 fraction
[C60-(DT)25]n
o-DCB fraction
a
b
Measured on o-dichlorobenzene solutions. Observed with films
c
casted from o-dichlorobenzene solutions on quartz plates. The value
given for Φcopo/Φoligo corresponds to the fluorescence quantum yield
reduction observed for [C60-(AT)m]n copolymers compared to the
corresponding oligothiophene macromonomer measured in the same
d
conditions (λexcitation = 460 nm). The optical band gap was estimated
in film using the formula 1240/λonset
.
Figure 13. Fluorescence spectra of the [C60-(DT)25]n copolymer
different fractions compared to the initial (DT)25 oligothiophene
macromonomer (2 × 10−4 mg/mL in o-dichlorobenzene, λexcitation
460 nm).
=
characterized by a reduction of fluorescence when the
oligothiophene chromophore is excited (λexcitation = 460 nm,
λemission ∼ 590 nm for all copolymers and their initial
macromonomers). Analyses were conducted in o-dichloroben-
zene, using very diluted concentrations (∼2 × 10−4 mg/mL) in
order to minimize interchain transfers. Table 3 presents the
reduction factor of fluorescence quantum yields observed
between initial pristine O3AT units and O3AT units involved
in a [C60-O3AT]n copolymer (Φcopo/Φoligo). A stronger
quenching was observed for o-DCB Soxhlet fractions when
Figure 12. Comparison of solution and solid state UV−vis absorption
spectra for the different fractions of the [C60-(DT)25]n copolymer.
all, an important red-shift and enlargement of the main peak are
noticed on solid state spectra (difference of about 40−60 nm
for λmax and 90−100 nm for λonset). This phenomenon is
attributed to conformation changes from the solution to the
solid film state. In addition, a vibrational fine structure
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compared to their CHCl3 Soxhlet fractions counterparts, these
fractions being richer in C60 according to previously determined
average compositions.
Concerning positive potentials, one irreversible oxidation
peak was detected for the three functionalized fullerene
derivatives. The fullerene C60 is known to be very difficult to
be oxidized,88,89 and no electrochemical oxidation could be
observed with the experimental conditions for pristine C60.
Observed oxidation answers are thus attributed to linked
moieties, i.e., aminophenyl, iminophenylthiophene, or bithio-
phene with onset potentials at respectively +0.35, +0.66, and
+0.68 eV vs Fc/Fc+. The replacement of the thiophene unit by
a bithiophene shows no significant impact on the associated
HOMO level (∼−5.5 eV, see Table 4).
As regards negative potentials, three reversible reductions
waves were observed for the three functionalized fullerene
derivatives (pointed out by the letters B, C, and D in Figure
14). Theses reductions are attributed to the C60 moiety and are
very close to the corresponding ones of pristine C60 with a
slight negative shift of about 0.10 eV. This little change is due
to the more difficult reduction of the fullerene moiety after
functionalization (loss of one double bond).89 An additional
irreversible peak (pointed out by the letter A in Figure 14) was
also observed in the case of Tm-C60-Tm triads. This peak of
unknown origin has also been previously detected by Cravino
et al.90 in the case of a polythiophene with pendant C60
moieties. In both Tm-C60-Tm triads, this peak could be observed
on CV measurements realized either in solution or on films at
onset potentials around −0.90 and −0.50 eV vs Fc/Fc+ for
respectively solution and film analyses (Figures 14 and 15).
Furthermore, as shown in Figure 15, it was noticed that this
additional irreversible peak was observed only in the case of
previous scanning of positive potentials up to the region
corresponding to the oxidation peak related to moieties linked
to C60. Thus, it could be concluded that the reduction answer A
is related to the phenyl−thiophene pattern grafted on C60.
Considering the above-mentioned particularity of this peak, the
LUMO energy levels of triads were estimated using the
reduction onset of peak B (Table 4).
In the same condition of concentration, the O3AT
fluorescence quantum yields were not affected when mixed
with free fullerene in 1/1 or 1/2 molar ratio (the more soluble
PCBM derivative was taken in order to ensure complete
dissolution). This indicates that an efficient photoinduced
intramolecular transfer take place from the oligothiophene unit
to the fullerene unit in the case of copolymers, due to the fact
that they are closely linked. A similar phenomenon has already
been observed in the literature81,85−87 in the case of
oligothiophene−C60 diads or triads.
2.5. Electronic Energy Levels. 2.5.1. Tm-C60-Tm Model
Triads. Figure 14 presents the cyclic voltammograms (CV) of
Figure 14. Cyclic voltammograms of the C60 fullerene, the
intermediate derivative C60-NH2/NH2, and the two model triads T-
C60-T and T2-C60-T2 (10−4 M solutions in o-DCB/CH3CN: 95/5).
2.5.2. [C60-O3AT]n Copolymers. Table 4 presents CV data of
the different Soxhlet fractions of synthesized [C60-O3AT]n
copolymers; each time values related to the corresponding
both T-C60-T and T2-C60-T2 model triads compared to the
pristine C60 fullerene and to the diaminophenyl-functionalized
fullerene precursor C60-NH2/NH2.
Table 4. Electronical Properties (Reduction and Oxidation Potentials, LUMO and HOMO Energy Levels) of Synthesized
Compounds
a
a
OX
onset
b
b
c
RED
onset
compounds
C60 (pristine)
E
(V) vs Fc/Fc+
E
(V) vs Fc/Fc+
LUMO (eV)
HOMO (eV)
ΔECV (eV)
−0.97
−1.05
−1.05
−3.83
−3.75
−3.75
−3.75
−2.57
−2.61
−2.61
−2.75
−2.19
−2.69
−2.43
−2.32
−2.56
−2.52
1.86 (lit.91
)
C60-NH2/NH2 (5)
+0.35
+0.66
+0.68
+0.33
+0.43
+0.42
+0.44
+0.37
+0.52
+0.54
+0.26
+0.55
+0.52
−5.15
−5.46
−5.48
−5.13
−5.23
−5.22
−5.24
−5.17
−5.32
−5.34
−5.06
−5.35
−5.32
1.4
d
T1-C60-T1 (8)
1.7
d
T2-C60-T2 (9)
−1.05
1.7
(HT)13 (11a′)
[C60-(HT)13]n − CHCl3 fraction
(HT)28 (11a)
−2.23
−2.19
−2.19
−2.05
−2.61
−2.11
−2.37
−2.48
−2.24
−2.28
2.6
2.6
2.6
[C60-(HT)28]n − CHCl3 fraction
(OT)30 (11b)
2.5
3.0
[C60-(OT)30]n − CHCl3 fraction
[C60-(OT)30]n − o-DCB fraction
(DT)25 (11c)
2.6
2.9
2.7
[C60-(DT)25]n − CHCl3 fraction
[C60-(DT)25]n − o-DCB fraction
2.8
2.8
a
b
RED
OX
E
and E
are respectively the first onset reduction and oxidation potentials determined by CV. The LUMO and HOMO values were
onset
onset
calculated with respect to ferrocene (reference energy level = −4.8 eV below the vacuum level) according to the following equations:92,93 LUMO =
c
d
RED
onset
+
OX
onset
+
−[(E
vs Fc/Fc ) + 4.8] and HOMO = −[(E
vs Fc/Fc ) + 4.8]. ΔECV is the calculated LUMO−HOMO difference. Onset reduction
potential of peak B (see Figure 14).
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of solubility of higher polymerization degrees. According to
NMR, GPC, and TGA analyses, it could be considered that
their average compositions are O3AT-C60-O3AT for chloro-
form soluble fractions and C60-(O3AT-C60)2 for o-dichlor-
obenzene soluble fractions.
According to cyclic voltammetry measurements, [C60-
O3AT]n copolymers are less donor materials than their initial
corresponding O3AT macromonomers. However, the [C60-
O3AT]n copolymer family behave similarly to parent
constituents in terms of light absorption.
Because of the covalent attachment of the fullerene acceptor
to the conjugated donor polymer chain, which may enhance
charge transfer, this new [C60-O3AT]n copolymer family can be
an interesting candidate for different use in organic photo-
voltaics. The investigation of its potential, as morphology
compatibilizing or stabilizing agent, and as unique acceptor−
donor material, is currently under progress.
Figure 15. Differences in cyclic voltammograms according to the
scanned potential window (T2-C60-T2 model triad as example, casted
films on Pt working electrode, measured in CH3CN solution).
4. EXPERIMENTAL SECTION
4.1. General Methods. Nuclear magnetic resonance analyses were
performed using a Bruker Avance III/Ultrashield Plus 400 MHz
spectrometer equipped with a 5 mm BBFO broadband probe. 1H, 19F,
and 13C frequencies were respectively 400.13, 376.46, and 100.62
MHz. The spectra were calibrated using the solvent residual peaks, and
chemical shifts (δ) are expressed in parts per million (ppm). NMR
multiplicities are reported by using the following abbreviations: s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad.
Infrared spectra were recorded at room temperature in attenuated
total reflectance mode (ATR-FTIR) using a Nicolet 5700 FT-IR
spectrometer.
Mass spectrometry analyses were performed using electrospray
ionization time-of-flight (ESI-TOF) and matrix-assisted laser desorp-
tion ionization time-of-flight (MALDI-TOF) for molecules and
polymers, respectively. ESI-TOF spectra were carried out on positive
mode using an acceleration time-of-flight LCT WATERS mass
spectrometer equipped with an electrospray ionization source. Samples
were first dissolved in an adequate solvent and then diluted 80 times in
methanol. The exact mass calibration was realized by internal
calibration using o-phosphoric acid. For the MALDI-TOF experi-
ments, a Voyager DE-STR/Applied Biosystems spectrometer was
used. The polymer solutions were prepared in tetrahydrofuran (THF)
at a concentration of 10 g/L and then mixed with a matrix solution
(dithranol, 10 g/L in THF) in a 1/10 ratio. The final solutions were
deposited onto an INOX target plate and allowed to dry in air at room
temperature. The spectrometer was equipped with a nitrogen laser (λ
= 337 nm), and the acquisitions were carried out using the reflectron
positive ion mode.
O3AT macromonomer are also reported. As shown in the
example illustrated in Figure 16, the contribution of the
fullerene moiety was not or very weakly detected on
copolymers voltammograms. This is due to the small amount
of C60 units related to the number of thiophene patterns inside
O3AT units.
As a general trend, the oxidation and reduction of O3AT
units were both shifted respectively to higher and lower
potentials (in absolute value), when involved in copolymer
structures. This results in lower HOMO and LUMO energy
levels, while keeping closely similar gap values (ΔECV). As a
consequence, [C60-O3AT]n copolymers are less donor
materials than their initial corresponding O3AT macro-
monomers.
3. CONCLUSION
In this work, a series of fullerene/oligothiophene alternating
copolymers [C60-O3AT]n were successfully synthesized by
polycondensation. Several copolymers were designed using
various alkyl chain and oligothiophene lengths. Prior to each
synthesis, the operating conditions were first optimized using a
cautious process of model polycondensation.
Each fullerene/oligothiophene copolymer was fractionated
into two soluble fractions using a Soxhlet apparatus. The
molecular weights of these fractions were limited due to a lack
Figure 16. Cyclic voltammograms (right: oxidation; left: reduction) for the different fractions of [C60-(DT)25]n copolymer, compared to the initial
(DT)25 oligothiophene (casted films on Pt working electrode, measured in propylene carbonate solution).
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Gel permeation chromatography analyses were conducted in two
different conditions: at 25 °C in tetrahydrofuran and at 150 °C in
1,2,4-trichlorobenzene. The number-average (Mn) and weight-average
(Mw) molecular weights and dispersities (Đ = Mw/Mn) are given in
polystyrene equivalents (PS equiv). Room temperature GPC was
carried out using a tripe detection chromatograph equipped with an
Agilent 1200 Series isocratic pump, three Wyatt detectors (Viscostar at
25 °C, Optilab rEX at 658 nm−25 °C, Minidawn TREOS at 658 nm−
5.7°−90°−134.3°), and a two PLgel MIXED-D columns set.
Tetrahydrofuran at 25 °C was used as the mobile phase at a flow
rate of 0.7 mL/min, and samples solutions were filtered through a 0.45
μm PTFE filter. High temperature GPC was carried out on a
thermostated chromatograph equipped with a Waters GPCV2000
unity (pump, refractometric and viscometric detectors) and a three
PLgel Olexis Guard columns set. Trichlorobenzene (stabilized by 0.2
g/L of butylhydroxytoluol) at 150 °C was used as the mobile phase at
a flow rate of 1 mL/min, and samples solutions at 150 °C were
prepared with the help of a Polymer Laboratories PL-SP260 high
temperature sample preparation system during 2 h before to be filtered
through a 1 μm PTFE filter and injected.
Thermogravimetric analysis measurements were performed using a
METTLER TA2500 apparatus at a heating rate of 10 °C/min from 20
to 700 °C under a nitrogen atmosphere.
UV−visible spectra were recorded on a PerkinElmer Lambda 19
spectrometer. Solutions were as follow: 1.5 × 10−4 M in chloroform
for triads and 0.1 mg/mL in o-dichlorobenzene for polymers, inside a
0.2 cm path length quartz cell. For solid-state measurements, a
polymer solution (10 mg/mL in o-dichlorobenzene) was spin-coated
on quartz plates.
cooling, the excess of trifluoroacetic anhydride was neutralized by a
slow addition of methanol (30 mL) and water (30 mL). The resulting
solution was extracted with dichloromethane (3 × 50 mL), the organic
layer was dried over MgSO4, and the solvent was evaporated under
reduced pressure. The obtained solid was washed thoroughly with
petroleum ether. After filtration, the solid was dried at 60 °C under
1
vacuum to obtain 2 as white flakes; yield: 90%. H NMR (400 MHz,
3
acetone, δ, ppm): 10.53 (se, 2H, NH) ; 7.93 (d, J = 8.6 Hz, 4H,
aromatic) ; 7.86 (d, 3J = 8.6 Hz, 4H, aromatic). 19F NMR (376 MHz,
acetone, δ, ppm): −76.21 (s, CF3). 13C NMR (100 MHz, acetone, δ,
ppm): 194.19 (CO); 156.18 + 155.81 (2JCF = 37.4 Hz, CO);
141.13 (aromatic C); 135.47 (aromatic C); 131.85 (aromatic CH);
121.10 (aromatic CH); 118.24 + 115.38 (1JCF = 288.2 Hz, CF3). MS
(ESI): Calculated for [C17H10F6N2O3 + H]+: 405.0674. Found:
405.0668.
4,4′-(Trifluoroacetamido)benzophenone-p-tosylhydrazone (3).
In a round-bottom flask under nitrogen, a solution of 2 (17.00 g,
42.08 mmol) and p-tosylhydrazide (7.84 g, 42.08 mmol) in dry
toluene (200 mL) and absolute ethanol (17 mL) was stirred for 24 h
at reflux. After cooling, the obtained precipitate was collected by
filtration and washed thoroughly with petroleum ether. The solid was
dried at 60 °C under vacuum to obtain 3 as a fine white powder; yield:
70%. 1H NMR (400 MHz, MeOH, δ, ppm): 7.89 (d, 3J = 8.3 Hz, 2H,
3
3
aromatic); 7.85 (d, J = 8.6 Hz, 2H, aromatic); 7.63 (d, J = 8.8 Hz,
3
3
2H, aromatic); 7.44 (d, J = 8.3 Hz, 2H, aromatic); 7.39 (d, J = 8.8
Hz, 2H, aromatic); 7.30 (d, J = 8.6 Hz, 2H, aromatic); 2.47 (s, 3H,
3
CH3). 19F NMR (376 MHz, MeOH, δ, ppm): −76.20 (s, CF3);
−76.21 (s, CF3). 13C NMR (100 MHz, DMSO, δ, ppm): 157.14 +
156.77 (2JCF = 37.4 Hz, CO); 156.89 + 156.52 (2JCF = 37.4 Hz, C
O); 154.84 (CN); 145.50 (aromatic C); 139.15 (aromatic C);
137.31 (aromatic C); 135.94 (aromatic C); 132.19 (aromatic C);
130.96 (aromatic CH); 130.91 (aromatic C); 130.60 (aromatic CH);
129.50 (aromatic CH); 129.18 (aromatic CH); 122.66 (aromatic
CH); 121.58 (aromatic CH); 118.79 + 115.94 (1JCF = 288.2 Hz, CF3);
118.76 + 115.91 (1JCF = 288.2 Hz, CF3); 21.53 (CH3). MS (ESI):
Calculated for [C24H18F6N4O4 + H]+: 573.1031. Found: 573.1025.
4,4′-Aminobenzophenone-p-tosylhydrazone (4). The previous
compound 3 (12.87 g, 21.8 mmol) was dispersed in 100 mL of
methanol and 100 mL of an aqueous solution of potassium carbonate
(20% w/v) was added. After a few minutes of stirring, the entire
compound 3 was dissolved, and the reaction mixture was allowed to
react under stirring for 24 h at room temperature. The resulting
solution was then extracted with dichloromethane (3 × 70 mL), the
organic layer was dried over MgSO4, and the solvent was evaporated
under reduced pressure. The obtained beige solid (mixture of ionic
and protonated forms) was eluted through a rapid silica gel
chromatography (eluent: methanol, Rf = 0.70) to give 4 as a strong
yellow fine powder; yield: 90%. 1H NMR (400 MHz, DMSO, δ, ppm):
9.64 (s, 1H, NH); 7.78 (d, 3J = 8.3 Hz, 2H, aromatic); 7.40 (d, 3J = 8.3
Photoluminescence spectra were acquired on a Hitachi F-4500
fluorescence spectrophotometer using 1 cm path length quartz cells
and polymers solutions in o-dichlorobenzene (∼2 × 10−4 mg/mL).
The fluorescence intensity of studied samples was independent of
purging with inert gas so no precautions with regard to air were taken.
Electrochemical experiments were carried out with a Biologic SP-300
potentiostat. The cyclic voltammetry was performed in a three-
electrode cell equipped with platinum working electrode (2.01 mm2),
platinum counter electrode, and Ag/Ag+ reference electrode (silver
wire in an acetonitrile solution of AgNO3 (0.01 M) and TBAHFP (0.1
M)). Experiments were conducted in anhydrous and nitrogen-
saturated 0.1 M tetrabutylammonium hexafluorophosphate
(TBAHFP) solutions; either on compounds solutions (10−4 M for
triads) or on films (polymer diluted solutions dropped and evaporated
on the working electrode). Electrolyte solutions and scan rate
conditions used were o-dichlorobenzene/acetonitrile: 95/5 at 100
mV s−1 for solutions and acetonitrile or propylene carbonate at 20 mV
s−1 for films. The measured potentials were calibrated versus the
Fc/Fc+
ferrocene/ferrocenium couple (E
) measured in the same
1/2
3
3
experimental conditions.
Hz, 2H, aromatic); 6.95 (d, J = 8.6 Hz, 2H, aromatic); 6.85 (d, J =
8.4 Hz, 2H, aromatic); 6.61 (d, 3J = 8.4 Hz, 2H, aromatic); 6.43 (d, 3J
= 8.6 Hz, 2H, aromatic); 5.45 (s, 2H, NH2); 5.42 (s, 2H, NH2); 2.39
(s, 3H, CH3). 13C NMR (100 MHz, DMSO, δ, ppm): 156.74 (C
N); 150.25 (C−NH2); 149.59 (C−NH2); 142.92 (aromatic C);
136.27 (aromatic C); 130.08 (aromatic CH); 129.18 (aromatic CH);
129.00 (aromatic CH); 127.83 (aromatic CH); 129.21 (aromatic C);
119.53 (aromatic C); 113.17 (aromatic CH); 112.86 (aromatic CH);
21.03 (CH3). MS (ESI): Calculated for [C20H20N4O2S + H]+: 381.1.
Found: 381.1.
4.2. Synthetic Details. C60 fullerene (99.5% purity) was
purchased from SES Research, and PCBM (99.5% purity) from
Nano-C. The aldehyde compounds 6 and 7 were synthesized using
procedures close to those reported in the literature.63,64 The 2-bromo-
3-alkyl-5-iodothiopene derivatives (10a, 10b, 10c) were obtained
according to a described procedure.65 Other reagents were purchased
from Sigma-Aldrich, Acros, or Fluka. All organic solvents used were of
analytical or HPLC grade. Anhydrous diethyl ether, THF, and toluene
were distilled over sodium/benzophenone prior to use. Dimethylfor-
mamide (DMF) was distilled over BaO and stored over molecular
sieves.
All manipulations were performed under a dry nitrogen atmosphere
using either magnetic (for molecules or oligomers) or mechanical (for
copolymers) stirring. Purifications using column chromatography were
performed using silica gel (60 Å, 70−200 μm).
4,4′-(Trifluoroacetamido)benzophenone (2). In a round-bottom
flask under nitrogen, trifluoroacetic anhydride (66 mL, 478 mmol) and
pyridine (∼0.1 mL) were slowly added under stirring to a solution of
4,4′-diaminobenzophenone (1) (10.15 g, 47.8 mmol) in dry THF
(250 mL). The mixture solution was stirred for 4 h at reflux. After
[6,6]-Bis(4-aminophenyl)methanofullerene (5). In a round-bottom
flask under nitrogen, the previous compound 4 (2.00 g, 5.05 mmol)
was dissolved in 100 mL of anhydrous pyridine, and 27.30 mg (5.05
mmol) of sodium methoxide was added all at once. The mixture
solution was stirred for 15 min at room temperature. Then, a solution
of fullerene C60 (2.55 g, 3.54 mmol) in 250 mL of o-dichlorobenzene
(previously filled by nitrogen using three freeze−pump−thaw cycles)
was added to the reaction medium. This reaction mixture was stirred
for 18 h at reflux in the dark. After cooling, the solvents were
evaporated under reduced pressure (under a fume cupboard). The
obtained solid was purified by silica gel chromatography to give
K
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residual C60 (eluent: toluene, Rf = 1) and compound 5 (eluent:
toluene/ethyl acetate 8/2, Rf = 0.28). The final product was dried at
100 °C under vacuum during 48 h to give 5 as a fine brown powder
always containing 5−10% of residual toluene (1H NMR determi-
nation); yield: 20%. 1H NMR (400 MHz, DMSO/CF3COOD, δ,
ppm): 8.56 (d, J = 8.4 Hz, 2H, aromatic); 7.52 (d, J = 8.4 Hz, 2H,
aromatic). 13C NMR (100 MHz, DMSO/CF3COOD, δ, ppm):
148.45; 145.92; 144.99; 144.93; 144.45; 144.27; 144.11; 143.61;
142.77; 142.67; 141.91; 141.79; 140.44; 138.75 (aromatic C); 137.30;
132.70 (aromatic CH); 131.88 (aromatic C); 124.02 (aromatic CH);
78.85 (C60 sp3 C); 56.48 (bridge C). MS (ESI): Calculated for
[C73H12N2]•+: 916.1000. Found: 916.0990.
reaction was allowed to stir for 12 h. The reaction was quenched by
adding 1 mL of water, 1 mL of NaOH aqueous solution (15 wt %),
and 3 mL of water. The reaction mixture was then diluted in 300 mL
of chloroform, and a white precipitate was eliminated by filtration. The
organic layer was separated, washed with water, dried over MgSO4,
and concentrated under reduced pressure. The obtained blackish-
green solid was dried overnight under reduced pressure to give O3AT
H/H terminated (11); yield: 90−95%.
3
3
To convert O3AT-H/H into the dialdehyde-terminated oligothio-
phene, the Vilsmeier reagent was first formed in situ. In a dried round-
bottom flask under nitrogen, POCl3 (1 mL, 10.7 mmol) was added
dropwise to anhydrous DMF (1 mL, 12.9 mmol) at room temperature.
The solution is allowed to stir for 3 h until getting a red complex
typical of the Vilsmeier reagent. In another dried round-bottom flask
under nitrogen, the previous O3AT-H/H was dissolved (at 60 °C if
necessary) in 100 mL of dichloromethane (dried on molecular sieve),
and the Vilsmeier reagent was transferred dropwise at room
temperature. The reaction mixture was stirred for 5 days at 60 °C.
Then, after cooling to room temperature, 100 mL of sodium acetate
saturated aqueous solution was added. The reaction mixture was
allowed to stir for 6−24 h until getting a fluorescent solution. The
resulting solution was extracted with chloroform (3 × 50 mL), washed
with water, and dried over MgSO4. The polymer was collected by
solvent concentration under reduced pressure, precipitation in
methanol, and purification using Soxhlet extraction with methanol
and chloroform, sequentially. The concentrated chloroform fraction
was then collected and again precipitated in methanol before to be
dried 24 h at 60 °C under reduced pressure to obtain O3AT CHO/
CHO terminated (12) as blackish-green powder; yield: 90−97%.
General Procedure for the Synthesis of Model Triads Tn-C60-Tn. In
a dried round-bottom flask under nitrogen, compound 5 (99.22 mg,
0.11 mmol) and an excess of aldehyde (6 or 7, 0.97 mmol) were
introduced in 30 mL of m-cresol. The reaction mixture was stirred for
24 h at 60 °C. At the end of the reaction, the m-cresol and excess of
aldehyde were eliminated using a Kugelrohr distillation apparatus (100
°C, 0.67 mbar). The obtained solid was washed thoroughly with
methanol, collected by filtration, and purified by extraction using
chloroform. The solution was concentrated under reduced pressure,
and the final product dried at 60 °C under vacuum during 24 h to give
the desired triad.
1
8. Fine brown powder, yield: 70%. H NMR (400 MHz, CDCl3, δ,
ppm): 8.67 (s, 2H, Schiff base); 8.13 (d, 3J = 8.3 Hz, 4H, phenyl); 7.42
(d, 3J = 5 Hz, 2H, thiophene); 7.31 (d, 3J = 8.3 Hz, 4H, phenyl); 6.94
3
3
(d, J = 5 Hz, 2H, thiophene); 2.84 (t, J = 7.6 Hz, 4H, αCH2); 1.65
(q, 3J = 7.4 Hz, 4H, βCH2); 1.41−1.27 (m, 12H, aliph CH2); 0.88 (t,
6H, aliph CH3). 13C NMR (100 MHz, C2D2Cl4, δ, ppm): 152.17
(Schiff base); 151.21; 148.44; 147.95; 145.18; 144.97; 144.90; 144.49;
144.45; 144.06; 143.62; 142.86; 142.78; 142.11; 142.08; 141.91;
140.65; 137.99; 136.18; 135.97; 131.72; 130.11; 121.37; 78.92 (C60 sp3
C); 77.32 (C60 sp3 C); 57.19 (bridge C); 31.51; 31.17; 28.94; 28.51;
22.52; 14.09.
1
11. General O3AT-H/H H NMR profile (400 MHz, CDCl3, δ,
ppm, minor peaks in italics): 6.98 (br s, arom H); 6.90 (br s, end arom
H); 2.80 (br t, αCH2); 2.61 (t, end αCH2); 1.75−1.60 (br m, βCH2);
1.50−1.20 (br m, CH2); 1.00−0.80 (br m, CH3).
12. General O3AT-CHO/CHO 1H NMR profile (400 MHz,
CDCl3, δ, ppm, minor peaks in italics): 10.01 + 9.98 (2s, CHO cis +
trans); 7.05 (br s, end arom H); 6.98 (br s, arom H); 2.95 (t, end
αCH2); 2.80 (br t, αCH2); 1.75−1.60 (br m, βCH2); 1.50−1.20 (br m,
CH2); 1.00−0.80 (br m, CH3).
1
9. Fine brown powder, yield: 70%. H NMR (400 MHz, CDCl3, δ,
ppm): 8.65 (s, 2H, Schiff base); 8.13 (d, 3J = 8.3 Hz, 4H, phenyl); 7.33
(d, 3J = 8.3 Hz, 4H, phenyl); 7.20 (d, 3J = 5.2 Hz, 2H, thiophene); 6.99
(s, 2H, thiophene); 6.94 (d, 3J = 5.2 Hz, 2H, thiophene); 2.83 (t, 3J =
7.7 Hz, 8H, αCH2); 1.71−1.61 (m, 8H, βCH2); 1.44−1.27 (m, 24H,
aliph CH2); 0.95−0.82 (m, 12H, aliph CH3). 13C NMR (100 MHz,
CDCl3, δ, ppm): 151.98 (Schiff base); 151.77; 148.73; 148.40; 145.52;
145.34; 145.17; 144.87; 144.79; 144.45; 144.00; 143.25; 143.18;
143.09; 142.40; 142.27; 141.07; 141.03; 138.42; 136.47; 135.74;
131.90; 130.68; 130.56; 128.35; 124.67; 121.68; 79.33 (C60 sp3 C);
77.87 (C60 sp3 C); 57.70 (bridge C); 31.84; 31.79; 31.41; 30.62; 29.62;
29.35; 29.20; 28.78; 22.76; 22.74; 14.23.
General Procedure for the Synthesis of Model Copolymers [ODA-
O3AT]n: Study of the Optimal ODA/O3AT Ratio. In a dried glass
reactor equipped with mechanical stirring, 250 mg of O3AT-CHO/
CHO (12) was dissolved in 6 mL of anhydrous THF. 2 mg of PTSA
(catalyst) and a spatula of CaCl2 (internal drying agent) were added to
the solution. In another flask, a solution of 4,4′-oxidianiline ODA (2 g)
dissolved in 50 mL of anhydrous DMF was prepared. Then, a series of
polycondensation was realized by introducing into the reactor various
quantities of the prepared ODA solution (from 0.1 to 1 mL) with an
additional volume of DMF in order to keep a 6/2 THF/DMF ratio.
The reaction mixture was stirred for 24 h at 55 °C. At the end of the
General Procedure for the Synthesis of Dicarbonylated
Oligothiophenes. In a dried round-bottom flask under nitrogen, a
3-alkyl-5-iodothiopene derivative (10a-b-c, 34.44 mmol) was dissolved
in 100 mL of fresh distilled THF and cooled down to 0 °C. Then, a 1.6
M tert-butylmagnesium chloride solution in THF (14.5 mL, 35 mmol)
was added dropwise, and the reaction mixture was stirred for 2h at 0
°C. A quantity of 1,3-bis(diphenylphosphino)propane nickel(II)
chloride [Ni(dppp)Cl2] was then added in accordance to the desired
degree of polymerization. The reaction mixture was stirred for 4 h at
room temperature. The resulting solution was diluted by addition of
200 mL of anhydrous THF and quenched with 20 mL of a 2 M
hydrochloric acid aqueous solution. After 30 min of stirring, the
organic layer was extracted with chloroform (3 × 100 mL), washed
with water (2 × 100 mL), and dried over MgSO4. The polymer was
collected by solvent evaporation under reduced pressure and purified
using Soxhlet extraction with methanol, petroleum ether, and
chloroform, sequentially. The concentrated chloroform fraction was
collected and dried overnight under reduced pressure to obtain the
corresponding oligo-3-alkylthiophene O3AT terminated either by H
or Br; yield: 70−80%.
1
reaction, a sample of the crude reaction mixture was taken for H
1
NMR analysis. For the optimal ODA/O3AT ratio, determined by H
NMR, the remaining solution was concentrated under reduced
pressure and purified using Soxhlet extraction with methanol,
petroleum ether, and chloroform, sequentially. The concentrated
chloroform fraction was then collected to obtain [ODA-O3AT]n
copolymer (13) as a blackish-green solid.
13. General [ODA-O3AT]n 1H NMR profile (400 MHz, C2D2Cl4,
70 °C, δ, ppm): 8.65 (br s, Schiff base); 7.30 (br d, phenyl); 7.10 (br d,
phenyl); 7.03 (br s, thiophene); 2.85 (m, αCH2); 1.85−1.65 (br m,
βCH2); 1.60−1.25 (br m, CH2); 1.05−0.85 (br m, CH3); possible
minor peaks corresponding to end-chain units are not detailed (see
Figure 10).
General Procedure for the Synthesis of Copolymers [C60-O3AT]n.
In a dried glass reactor equipped with mechanical stirring, 250 mg of
O3AT-CHO/CHO (12) was dissolved in 12 mL of anhydrous THF. 2
mg of PTSA (catalyst) and a spatula of CaCl2 (internal drying agent)
were added to the solution. In another flask, compound 5 (C60-NH2/
NH2, 0.11 mmol for 12a, 0.05 mmol for 12a′, 0.04 mmol for 12b and
12c) was dissolved in 4 mL of anhydrous DMF. This solution was
degassed by nitrogen bubbling and introduced into the reactor,
To convert the previous O3AT-H/Br into a fully proton terminated
oligothiophene, the polymer was dissolved in 100 mL of anhydrous
THF (at 60 °C if necessary) under nitrogen. Then, 10 mL of a 1.6 M
LiAlH4 solution in THF was added at room temperature, and the
L
DOI: 10.1021/ma501788d
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previously protected from light. The reaction mixture was stirred for 7
days at 55 °C in the dark. At the end of the reaction, the solution was
concentrated under reduced pressure and purified using Soxhlet
extraction with methanol, petroleum ether, chloroform, and o-
dichlorobenzene, sequentially. The concentrated chloroform and o-
dichlorobenzene fractions were then collected and dried 24 h under
reduced pressure to obtain two blackish-green fractions of the [C60-
O3AT]n copolymer (14).
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail lara.perrin@univ-savoie.fr (L.P.).
Present Address
R.M.: CNRS, UMR 5223, Ingenierie des Materiaux Polymeres,
́ ́ ̀
F-69622 Villeurbanne Cedex, France.
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Chen, W.-C. Macromolecules 2010, 43 (14), 6085−6091.
The authors declare no competing financial interest.
́ ̂
(27) Gernigon, V.; Leveque, P.; Brochon, C.; Audinot, J.-N.; Leclerc,
N.; Bechara, R.; Richard, F.; Heiser, T.; Hadziioannou, G. Eur. Phys. J.:
Appl. Phys. 2011, 56 (03), 34107/1−6.
ACKNOWLEDGMENTS
■
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(29) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Chem. Rev.
2013, 113 (5), 3734−3765.
The authors thank Prof. Lionel Flandin for fruitful discussions
and Vincent Martin for GPC characterizations. The authors
also thank Dr. Martial Billon from INAC/SPrAM/CREAB
(UMR 5819 CEA-CNRS-UJF) for useful discussions on cyclic
voltammetry measurements and Dr. Noella Lemaitre from
CEA/LITEN/INES for availability of fluorescence spectrosco-
py measurements. The Institut National de l’Energie Solaire
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(INES) and the financial support from Assemblee
Savoie, Centre National de la Recherche Scientifique, and
Commissariat a l’energie atomique et aux energies alternatives
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des Pays de
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DOI: 10.1021/ma501788d
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