Low-cost synthesis and physical characterization of thieno[3,4-c]pyrrole-4, 6-dione-based polymers

Article abstract of DOI:10.1021/jo301512e

The improved synthesis of thieno[3,4-c]pyrrole-4,6-dione (TPD) monomers, including Gewald thiophene ring formation, a Sandmeyer-type reaction, and neat condensation with an amine, is presented. This protocol enables faster, cheaper, and more efficient preparation of TPD units in comparison to traditional methods. Furthermore, a series of TPD homo- and pseudohomopolymers bearing various alkyl chains was synthesized via a direct heteroarylation polymerization (DHAP) procedure. UV-visible absorption and powder X-ray diffraction measurements revealed the relationship between the ratio of branched to linear alkyl chains and the optoelectronic properties of the polymers as well as their packing in the solid state.

Full text of DOI:10.1021/jo301512e

Article
pubs.acs.org/joc
Low-Cost Synthesis and Physical Characterization of
Thieno[3,4‑c]pyrrole-4,6-dione-Based Polymers
Philippe Berrouard, Stephane Dufresne, Agnieszka Pron, Justine Veilleux, and Mario Leclerc*
́
́ ́ ́ ́
Canada Research Chair on Electroactive and Photoactive Polymers, Departement de Chimie, Universite Laval, Quebec City, Quebec,
Canada G1V 0A6
S
* Supporting Information
ABSTRACT: The improved synthesis of thieno[3,4-c]pyrrole-4,6-dione (TPD)
monomers, including Gewald thiophene ring formation, a Sandmeyer-type reaction,
and neat condensation with an amine, is presented. This protocol enables faster,
cheaper, and more efficient preparation of TPD units in comparison to traditional
methods. Furthermore, a series of TPD homo- and pseudohomopolymers bearing
various alkyl chains was synthesized via a direct heteroarylation polymerization
(DHAP) procedure. UV−visible absorption and powder X-ray diffraction measure-
ments revealed the relationship between the ratio of branched to linear alkyl chains
and the optoelectronic properties of the polymers as well as their packing in the solid
state.
difference in the optical properties as well as solid-state packing
between homopolymers bearing straight and branched alkyl
substituents.
INTRODUCTION
■
Polymer-based organic photovoltaics (OPV) have received
enormous attention over the past decade as an alternative
source of energy.1−6 In particular, thieno[3,4-c]pyrrole-4,6-
dione (TPD) emerged as a very promising unit.7−10 This
building block combines rigidity and good solubility as well as a
favorable 3D arrangement in the solid state. TPD-based
materials have a relatively low energy band gap which is caused
by the stabilization energy via formation of a quinoidal
thiophene-maleimide structure in their excited state.11
Up to now, most of the research on TPD-based materials
have concentrated on push−pull type copolymers.12−15 Indeed,
TPD homopolymers are very rare in the literature.16−19 The
first example was reported by Pomerantz et al.17 in 2003.
Another example of a TPD homopolymer with a molecular
weight of 2−3 kDa prepared by Ullmann coupling was reported
by Bjørnholm et al.18 Recently, Facchetti et al.16 synthesized a
homopolymer with an n-dodecyl alkyl chain by Yamamoto
polymerization. Although this approach is known to result in
higher molecular weight polymers in comparison to Ullmann
coupling, a Mn value of only 4.7 kDa was reported, which is in
good agreement with values reported by Bjørnholm. One can
notice that all of the previously synthesized TPD homopol-
ymers contained linear alkyl chains (n-C6H13, n-C8H17, n-
C12H25), which most likely limit the solubility of the materials
in comparison to branched chains. Recently, we prepared a
TPD homopolymer bearing 2-octyldodecyl and 2-hexyldecyl
side chains by the direct heteroarylation polymerization
(DHAP) method.19 These large and branched alkyl chains
allowed a better solubility of the material during polymerization
reactions. As a result, higher Mn values were achieved (23 kDa)
in comparison to previous data. Additionally, we noticed a great
In order to shed more light on the optical and electro-
chemical properties of TPD polymers, we synthesized a series
of homopolymers and pseudohomopolymers using the DHAP
approach. This method proved to be very effective for the
synthesis of TPD-based conjugated polymers.20,21 It allowed
the preparation of pseudohomopolymers consisting of a TPD
backbone, with alternating linear and branched alkyl chains,
which would be extremely difficult to synthesize using
conventional synthetic methods. Furthermore, the synthesis
of TPD monomers was redesigned to yield these units in a
cheaper and more efficient way. It is important to remember
here that, in principle, one of the major advantages of
semiconductor plastic materials over standard inorganic
materials is their potentially lower production cost. From this
perspective, it is important to prepare low-cost monomers and
polymers.
RESULTS AND DISCUSSION
■
Monomer Synthesis. The most common pathway toward
TPD units, shown in Scheme 1a, involves formation of
thiophene anhydride from the corresponding 3,4-thiophenedi-
carboxylic acid (1) followed by a condensation with an amine.22
This methodology possesses two drawbacks: (i) although 3,4-
thiophenedicarboxylic acid is commercially available, it is a
relatively expensive starting material23 and (ii) the condensa-
tion reaction proceeds with moderate yield. The lower than
Received: July 20, 2012
© XXXX American Chemical Society
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bromo derivative 6, during the condensation of 4, iodine
elimination occurred, leading directly to 2. The mechanism of
this reaction is under investigation. By using our protocol
(Scheme 2), we were able to synthesize TPD units bearing
various alkyl chains, both linear and branched ones. Yields of
64−76% were achieved for condensation of linear (n-octyl) and
C2-branched (2-octyl-1-dodecyl) alkylamines with 1. When
C1-branched alkyl amines (heptadecan-9-amine) were used,
slightly lower yields were obtained, most probably due to steric
hindrance. Our new methodology, which involves (i) the
Gewald formation of thiophene, (ii) the Sandmeyer-type
reaction, and (iii) condensation with neat amine, enabled us
to prepare TPD units in a much faster, cheaper, and more
efficient way. The need for harsh conditions and toxic reagents
such as thionyl chloride was eliminated. Our new methodology
is a substantial improvement of the synthesis of TPD. We
believe that it will ensure wide use of TPD monomers.
Polymer Synthesis. Taking advantage of the recent
expertise in DHAP,31,32 which allowed synthesis of well-defined
conjugated polymers without the need for organometallic
reagents, we prepared a series of homo and pseudohomo TPD-
based polymers. As described in our previous work, polymer P1
can be prepared from the iodo-TPD derivative 8. As shown in
Scheme 3, we were able to prepare the same polymer from the
bromo derivative 7a. Reactions with aryl bromides are more
versatile than those with aryl iodides. Literature results show
that iodine can potentially poison the catalyst.33,34 Thus, by
using bromine instead of iodide derivatives, we eliminated the
need to use stoichiometric amounts of silver acetate. The
polymerization of 7a resulted in polymer P1* with similar Mn
values and comparable yield to the previously prepared polymer
P1.
Scheme 1. (a) Published Procedure for TPD and (b)
Example of Gewald Reaction
expected yield can be attributed to the harsh conditions
required (SOCl2 and/or high temperature),24 which cause
decomposition of the product during the reaction. To
overcome these problems, our research focused on developing
a new synthetic pathway for TPD monomers, which would
enable us to prepare the desired compounds in a simpler, less
expensive, and more efficient way.
In this regard, the Gewald reaction (Scheme 1b) has recently
gained a great deal of attention as a cheap and efficient method
for the preparation of the functionalized aminothiophenes
3.25−27 In our previous studies, we showed that it is possible to
convert aminothiophene into the iodothiophene derivative via a
standard Sandmeyer reaction.28 As shown in Scheme 2, by
Scheme 2. Synthesis of TPD Units
As shown in Scheme 3, we prepared various TPD
homopolymers bearing long and branched alkyl chains to
ensure good solubility. All polymers were synthesized using
Herrmann’s palladium35 and tris(o-methoxyphenyl)phosphine
as a catalytic system and potassium acetate as a base. The
polymerization reaction was carried out in THF at 120 °C.
Purification involved Soxhlet extractions with acetone, hexane,
and chloroform, followed by precipitation from methanol. Alkyl
chains branched both on C2 (P1 and P1*) and C1 (P2 and
P2*) were incorporated into polymers for comparison studies.
Polymers P1 and P2 are readily soluble in common organic
solvents: i.e., chloroform and toluene. Much higher molecular
weights (21−25 kDa) were obtained for P1 and P2 in
comparison to polymers with straight-chain alkyl substituents
(Mn up to 4.7 kDa).16−19 Our results clearly indicate that
branched alkyl chains are necessary to obtain high-molecular-
weight and processable polymers.
The main difficulties in the preparation of polymers suitable
for organic electronics lie in balancing solubility in common
organic solvents and good packing in the solid state. Thus, in
order to better understand the structure−property and
structure−packing relationships of the TPD homopolymer
series, we designed and synthesized pseudohomo polymers
P3−P6. Our approach was based on the idea of using branched
chains to ensure processability and straight chains to sustain
good packing in the solid state. As shown in Scheme 3, we
synthesized a series of pseudohomopolymers with various ratios
of branched vs linear chains. Polymers P3 and P4, with a 1/1
ratio, were prepared by copolymerization of dibromo and
unsubstituted TPDs 2b/2a and 5c, respectively. This approach
ensured an alternating structure of the polymers. It should be
modifying the Sandmeyer protocol, we were able to achieve
monoiodo (4), monobromo (6), and unsubstituted (1)
thiophene-3,4-dicarboxylic acids/esters in moderate to good
yields. It is worth mentioning that deamination of 3 required
the utilization of t-BuONO in refluxing THF to achieve, after
hydrolysis, 1 in 66% yield.29,30 All attempts to carry out this
transformation in aqueous media failed. Additionally, we
discovered that neat condensation between alkylamine and
thiophenedicarboxylic acids/esters (4, 6, 1) at temperatures
above 200 °C results in the formation of TPD derivatives 2 and
7 as major products. Only 1 equiv of alkylamine was necessary
to perform this reaction. Simple filtration through a pad of silica
gave the desired products. Interestingly, in contrast to the
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a
Scheme 3. Polymer Synthesis
a
Conditions: (i) 2% Herrmann's Pd, 8% tris(o-methoxyphenyl)phosphine, 1 equiv of KOAc, THF [0.25 M]; (ii) the same as for (i) except 2 equiv
of KOAc.
Figure 1. Schematic representation of chains orientation in TPD homopolymers.
noted that the crystal structure of TPD dimers indicated
interactions between the carbonyl of the pyrroledione moiety
and the sulfur of the thiophene unit.28 As a result, two
neighboring TPD units adopt an anticoplanar conformation
(Figure 1). Reaction of dibromo-TPD dimer 9 bearing straight
octyl chains and 2b resulted in polymer P5 with a 2/1 ratio of
straight to branched alkyl chains. As shown in Figure 1, in the
case of P3 and P4, branched alkyl chains are on the same side
of the polymer backbone, whereas in the case of P5 they are
oriented in opposite directions. Additionally, the reaction of
monobrominated 7c and 7b resulted in the random polymer
P6 with a 3/1 ratio. The design limited the number of branched
alkyl chains which can potentially induce twisting of the
backbone. Regioregular polymers P3−P5 demonstrate well the
advantages of the DHAP method. Due to the difficulties in
preparation of organometallic derivatives of TPD, it was not
possible for us to synthesize these polymers via classical cross-
coupling polymerization procedures.
Properties of the Synthesized Polymers. Polymers P1−
P6 exhibit very good thermal stability. TGA analysis revealed
Tdec > 380 °C (Table 1). DSC analysis showed no glass
transitions between 50 and 350 °C. These results can be
explained by the rigid-rod nature of the conjugated backbones.
In order to gain information about the solid-state
organization of polymers, powder X-ray diffraction analyses
were performed (Figure 2). Bjørnholm et al. reported that TPD
homopolymers do not spontaneously crystallize and are mostly
amorphous.18 Peaks at low angle (27.8 Å) as well as at 2θ =
19.4° (4.6 Å) were recorded for their polymers. The earlier
peak corresponds to the lamellar repeating distance between
the polymer chains, whereas the later peak can be attributed to
the distance between alkyl chains and/or π-stacking. Polymers
P1, P2, and P6 show no peaks corresponding to π-stacking.
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Table 1. Physical and Thermal Properties
a
b
polymer
Mn (kDa)
PDI
n
(DP)
Tdec (°C)
P1
23
22
21
25
10
8
1.5
1.4
1.5
1.3
1.7
1.8
1.6
1.6
53 (53)
51 (51)
54 (54)
64 (32)
32 (16)
23 (11)
11 (5)
420
420
420
420
410
420
440
380
P1*
P2
P2*
P3
P4
P5
5
P6
7
24 (24)
a
b
Number of TPD units. Determined by TGA at 5% mass loss under
nitrogen.
Their absence can be explained by (i) the chirality of the side
chain in the case of P1, (ii) the bulkiness of the side chains in
the case of P2, and (iii) the statistical distribution of the
monomer with branched side chains in the structure of P6. On
the other hand, polymers P3−P5 exhibit a small peak at 2θ =
24.5° (3.6 Å) corresponding to π-stacking of the polymer
backbone. The additional organization was introduced by the
increased percentage of the linear side chain, which allowed a
better stacking of the polymer chains. Comparison of polymers
P3−P5 and a Bjørnholm homopolymer bearing a straight n-
dodecyl chain revealed that the distance attributed to π-stacking
is greater for P3−P5 and the intensity of the peak is weaker as a
result of the incorporation of the branched alkyl chains. Similar
results were observed in the case of regioregular polythiophenes
bearing linear and branched alkyl chains by Thompson et al.36
These results show that it is important to balance the
percentage of bulky substituents, which ensure solubility, and
straight chains, which provide organization within the material.
UV−vis absorption spectra of polymers P1−P6 in solution
and in thin films are shown in Figures 3 and 4, respectively.
One can easily notice (Table 2) a strong blue shift (42 nm) of
the absorption maximum of P2 (481 nm) in comparison to P1
(523 nm). This could be explained by the fact that, in order to
accommodate bulky 9-heptadecyl chains branched on C1 in P2,
it may be necessary to distort bonds of the pyrrolidone moiety.
As a result, the distance between the oxygen atom of
pyrrolidone unit and the sulfur atom of the next TPD unit
may decrease, causing twisting of adjacent TPD units. In the
case of P1, branching on C2, further from the backbone, allows
much more flexibility. A 16 nm bathochromic shift is observed
for P1, on comparison of absorption in solution and in the solid
state, indicating additional interchain interactions and further
planarization in the bulk. Additionally, the well-defined and
probably coplanar backbone of P1 results in pronounced
vibronic bands. Bulkiness of the chains in P2 prevents any
interactions in the solid state; thus, no shift was observed when
UV−vis absorption spectra were compared in solution and in
films. Substitution of each second branched chain by a linear
chain (P3) allows much more flexibility of the TPD unit; thus,
planarization of the backbone is possible. As a result, a
pronounced red shift (36 nm) of the absorption maximum is
observed in comparison to P2. When the ratio of the linear vs
branched chains increases, i.e. 2/1 and 3/1 for P5 and P6,
respectively, a further red shift of the absorption maxima is
observed.
Figure 2. Powder XRD spectra of polymers P1, P3, and P5.
corresponded to a HOMO energy level at −6.1 eV and a
LUMO energy level at −4.3 eV, respectively. The electro-
chemical band gap (1.8 eV) is in good agreement with the
optical band gap (1.9 eV) calculated from the onset of the
absorption spectra. It is interesting to note that TPD-based
homopolymers have almost the same band gap as regioregular
poly(3-alkylthiophenes), but with HOMO and LUMO energy
levels lower by about 1.0 eV. Incorporation of some of these
polymers into OFET devices did reveal electron mobility of
about 10−3 cm2 V−1 s−1 that could lead to interesting electron
acceptors in all-polymer solar cells.
In order to investigate the electrochemical properties of the
synthesized polymers, cyclic voltammetry measurements of
polymer films were performed. Oxidation at 1.5 V and
reduction at −0.4 V were estimated for polymer P1, which
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pseudohomopolymers with ratios of linear to branched alkyl
chains 1/1 and 1/2, polymers P3 and P5, respectively, combine
good solubility and good packing in the solid state.
Incorporation of the synthesized polymers into electrochemical
and electronic devices is in progress.
EXPERIMENTAL SECTION
■
General Experimental Methods: Instrumentation. 1H and 13
C
NMR spectra were recorded in deuterated chloroform or acetone
solution at 298 K. Number-average (Mn) and weight-average (Mw)
molecular weights were determined by size exclusion chromatography
(SEC) using styrene-DVB gel columns in TCB at 110 °C. For the
calibration curve, a series of monodisperse polystyrene standards was
used. Differential scanning calorimetry (DSC) analyses were calibrated
with ultrapure indium, at a scanning rate of 20 °C/min under a
nitrogen flow. UV−vis−near-IR absorption spectra were recorded
using a spectrophotometer and dropcast films on glass plates have
been used for solid-state measurements. Optical band gaps were
determined from the onset of the absorption band. Wide-angle X-ray
diffraction (WAXD) spectra were obtained by a X-ray diffractometer
using a graphite-monochromated copper radiation (Kα = 1.5418 Å).
The operation power was 40 kV, 20 mA, and the collimator is 0.8 mm
in diameter. The samples were inserted in 0.01 mm thin-walled glass
capillary tubes (1.0 mm diameter). Cyclic voltammograms (CV) were
recorded on a Solartron 1287 potentiostat using platinum wires as
working and counter electrodes at a scan rate of 50 mV/s. A thin
polymeric film was coated on the working electrode. The reference
electrode was Ag/Ag+ (0.01 M of AgNO3 in acetonitrile), and the
electrolyte was a 0.1 M solution of tetrabutylammonium tetrafluor-
oborate in dry acetonitrile. Under these conditions, the oxidation
potential of ferrocene was 0.09 V versus Ag/Ag+, whereas the
oxidation potential of ferrocene was 0.41 V versus SCE. The HOMO
and LUMO energy levels were determined from the oxidation and
reduction onsets (where the current differs from the baseline)
assuming that the SCE electrode is −4.7 eV from vacuum.
Figure 3. UV−visible absorption spectra in CHCl3 solution of P1−P6.
General Procedure for the Synthesis of Monomers and
Intermediates. 3-Ethyl 4-methyl 2-aminothiophene-3,4-dicarboxylate
(3), 3-ethyl 4-methyl 2-iodothiophene-3,4-dicarboxylate (4), 1,3-
dibromo-5-(2-octyldodecyl)thieno[3,4-c]pyrrole-4,6-dione (5a), 1,3-
dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (5c), 1-iodo-5-(2-
octyldodecyl)thieno[3,4]pyrrole-4,6-dione (8), and 3,3′-dibromo-
5,5′-dioctyl l,1′-bi(thieno[3,4-c]pyrrole)-4,4′,6,6′-tetrone (9) were
prepared via procedures reported in previous work.15,19,24,28
Preparation of 5-Alkylthieno[3,4-c]pyrrole-4,6-diones 2a−c from
4. In a small vial was placed 2 (1 equiv) and the desired alkylamine (1
equiv). The neat reaction mixture was gradually warmed to 200−280
°C in a graphite bath over 10 min. The product was purified through a
silica plug with dichloromethane. The resulting product was washed or
recrystallized with acetone and a few drops of water to give 2a−c
(32%) as white solids.
Preparation of 5-Alkylthieno[3,4-c]pyrrole-4,6-diones 2a−c from
7. In a round-bottom flask was placed 7a−5c (1 equiv) in 0.05 M
EtOH/AcOH (3/1) solution. Zinc powder (2 equiv) was added, and
the reaction mixture was refluxed for 3 h. Water was then added and
the precipitate filtered. The resulting product was washed or
recrystallized with a minimum amount of acetone and a small amount
of water to give the title compound (88%) as a white solid.
Figure 4. Solid-state UV−visible absorption spectra of P1−P6.
Table 2. Optical Properties
a
polymer
λmax (nm)
λmax (nm)
Eg opt (eV)
P1
523
523
481
481
518
531
523
536
539
539
481
481
527
542
533
544
1.9
1.9
2.2
2.2
1.9
1.8
1.8
1.8
P1*
P2
P2*
P3
P4
P5
P6
a
Measured in CHCl3 solution.
Preparation of 5-Alkylthieno[3,4-c]pyrrole-4,6-diones 2a−c from
1. This synthesis was performed as described for the preparation of
2a−c from 4.
CONCLUSION
■
In conclusion, we were able to improve the synthetic strategy
toward TPD monomers. Our approach including Gewald
thiophene ring formation and a Sandmeyer-type reaction
followed by a neat condensation with an amine enabled us to
prepare various TPD units in a cheaper and more efficient way
in comparison to traditional methods. Taking advantage of the
DHAP method, we synthesized various TPD-based homo- and
pseudohomopolymers. Their investigation by means of UV−vis
absorption and powder XRD measurements revealed that
2a: 1H NMR (400 MHz, CDCl3, ppm) δ 7.79 (s, 2H), 3.47 (d, 2H,
J = 7.2 Hz), 1.83 (m, 1H), 1.22 (m, 32H), 0.85 (t, 6H, J = 2.2 Hz); 13C
NMR (100 MHz, CDCl3, ppm) δ 163.1, 136.8, 125.6, 42.9, 37.0, 32.1,
32.1, 31.6, 31.6, 30.2, 30.2, 29.9, 29.9, 29.8, 29.8, 29.6, 29.5, 26.5, 26.5,
22.9, 22.9, 14.4, 14.4; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C26H44NO2S 434.3093, found 434.3104; mp 35−37 °C (uncorrected).
1
2b: H NMR (400 MHz, CDCl3, ppm) δ 7.78 (s, 2H), 4.10 (hept,
1H), 2.10−1.95 (m, 2H), 1.71−1.60 (m, 2H), 1.33−1.18 (m, 24H),
0.85 (t, 6H); 13C NMR (100 MHz, CDCl3, ppm) δ 163.3, 136.7,
E
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125.5, 52.9, 32.0, 29.7, 29.5, 29.5, 26.9, 22.9, 14.3; HRMS (ESI-TOF)
m/z [M + H]+ calcd for C23H38NO2S 392.2623, found 392.2618; mp
40−42 °C (uncorrected).
mixture was extracted with diethyl ether (10 × 75 mL) and washed
with saturated sodium bisulfite solution (5 × 100 mL) and water (3 ×
100 mL). The combined organic layer was dried with Na2SO4 and
evaporated to dryness. The crude product was chromatographed over
silica gel using methylene chloride as eluent to give a white solid (7.29
g, 25 mmol). This solid was diluted in 250 mL of NaOH (1 M), and
the solution was stirred at 80 °C in a one-necked flask overnight. The
solution was then acidified with HCl to pH 3 and extracted with
diethyl ether (10 × 150 mL). The combined organic layers were dried
with Na2SO4 and evaporated to dryness to give a white solid (6.12 g,
2c: 1H NMR (400 MHz, CDCl3, ppm) δ 7.81 (s, 2H), 3.61 (t, 2H, J
= 7.4 Hz), 1.63 (m, 2H), 1.24 (m, 10H), 0.86 (t, 3H, J = 7.0 Hz); 13C
NMR (100 MHz, CDCl3, ppm) δ 162.4, 136.7, 125.2, 37.9, 31.7, 29.2,
29.2, 28.4, 26.8, 22.4, 14.1; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C14H20NO2S 266.1215, found 266.1219; mp 120−122 °C (un-
corrected).
Preparation of 1,3-Dibromo-5-(1-octylnonyl)thieno[3,4-c]-
pyrrole-4,6-dione (5b). To a mixture of 2b (0.7 g, 1.79 mmol) in
trifluoroacetic acid and sulfuric acid (9 mL/3 mL) was added NBS
(0.95 g, 5.36 mmol) in one portion. The resulting mixture was stirred
in the dark overnight. The mixture was extracted with chloroform, and
the organic phase was washed with an aqueous solution of KOH, dried
over MgSO4, and concentrated under vacuum. The product was
purified by column chromatography (silica gel) using hexane/DCM
(1/2) as an eluent. A pure sample was obtained as a low melting point
beige solid in 81% yield (0.79 g): 1H NMR (400 MHz, CDCl3, ppm) δ
4.06 (hept, 1H), 2.05−1.93 (m, 2H), 1.69−1.57 (m, 2H), 1.31−1.14
(m, 24H), 0.86 (t, 6H); 13C NMR (100 MHz, CDCl3, ppm) δ 160.94,
134.70, 113.01, 53.58, 32.39, 32.06, 29.65, 29.50, 29.46, 26.90, 22.88,
14.13; HRMS (ESI-TOF) m/z [M + H]+ calcd for C23H36Br2NO2S
548.0833, found 548.0838; mp 42−44 °C.
1
24 mmol, 37%): H NMR (400 MHz, acetone-d6, ppm) δ 8.28 (s,
1H); 13C NMR (100 MHz, acetone-d6, ppm) δ 164.0, 161.6, 136.3,
135.1, 132.8, 113.6; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C6H4BrO4S 250.9014, found 250.9013; mp 165−167 °C (uncor-
rected).
Preparation of Thiophene-3,4-dicarboxylic Acid (1). A solution of
3 (0.84 g, 3.7 mmol) in 75 mL of anhydrous THF was added dropwise
to a boiling solution of tert-butyl nitrite (0.40 g, 3.9 mmol) in 100 mL
of anhydrous THF under nitrogen. The mixture was refluxed for 3 h
before it was evaporated under reduced pressure. The remaining dark
brown oil was purified by chromatography on silica using 30% ethyl
acetate/70% hexanes as the eluent to give the product as an oil (0.53
g). This oil was diluted in 25 mL of NaOH (1 M), and the solution
was stirred at 80 °C in a one-necked flask overnight. The solution was
then acidified with HCl to pH 3 and extracted with diethyl ether (10 ×
150 mL). The combined organic layers were dried with Na2SO4 and
Preparation of 1-Bromo-5-(alkyl)thieno[3,4-c]pyrrole-4,6-diones
7a−c from 6. In a small vial was placed 6 (1 equiv) and the desired
alkylamine (1 equiv). The neat mixture was then heated to 260 °C
over 10 min. The product was purified through a silica plug with
dichloromethane. The resulting product was washed or recrystallized
with a minimum amount of acetone and a small amount of water to
give the title compound (64−76%) as an off-white solid/oil.
Preparation of 1-Bromo-5-(alkyl)thieno[3,4-c]pyrrole-4,6-diones
7a−c from 5. 5a−c (0.5 mmol) was placed in a 25 mL three-necked
flask with a condenser. A solution of ethanol (3.25 mL), acetic acid (1
mL), and 1 drop of HCl was added and then heated until dissolution
of the solid. Zinc powder (0.5 mmol) was then added, and the mixture
was refluxed for 1 h. The mixture was then filtered through fritted
glass, and the solvent was evaporated under vacuum. A silica column
was made using a gradient of 100% hexane to 100% chloroform to give
the pure products 7a−c (42%).
1
evaporated to dryness to give a white solid (0.43 g, yield 66%): H
NMR (400 MHz, acetone-d6) δ 12.44 (broad peak), 8.54 (s, 2H); 13
C
NMR (100 MHz, acetone-d6) δ 205.7, 164.5, 139.1 ppm; HRMS (ESI-
TOF) m/z [M + H]+ calcd for C6H5O4S 172.9909, found 172.9908;
mp 218−220 °C (uncorrected).
General Procedure for the Polymer Synthesis. P1 (H-A-I). 1-
Iodo-5-(2-octyldodecyl)thieno[3,4-c]pyrrole-4,6-dione (8; 100.00 mg,
0.18 mmol), trans-bis(μ-acetato)bis[o-(di-o-tolylphosphino)benzyl]-
dipalladium(II) (3.34 mg, 2 mol %), tris(o-methoxyphenyl)phosphine
(5.04 mg, 8 mol %), cesium carbonate (58.64 mg, 0.18 mmol), and
silver acetate (29.97 mg, 0.18 mmol) were added in a Biotage
microwave vial (size 2−5 mL) with a magnetic stirring bar. The vial
was sealed with a cap and then purged with nitrogen to remove the
oxygen. A 0.7 mL portion of THF was added, and the reaction mixture
was heated with an oil bath at 120 °C (reaction under pressure). After
22 h, the reaction mixture was cooled and the corresponding 5-
alkylthieno[3,4-c]pyrrole-4,6-dione was added as a capping agent (50
mg in 1 mL). The solution was heated again at 120 °C for 1 h to
complete the end-capping procedure. After an additional 1 h of
reaction, the whole mixture was cooled to room temperature and
poured into 500 mL of cold methanol. The precipitate was filtered.
Soxhlet extractions with acetone followed with chloroform were done.
The solvent was reduced to about 10 mL, and the mixture was poured
into cold methanol. The precipitate was filtered. P1 was achieved in
79% yield of soluble fraction in CHCl3 (Mn of 23 kDa).
1
7a: product obtained as a pale yellow oil; H NMR (400 MHz,
CDCl3, ppm) δ 7.72 (s, 1H), 3.49 (d, 2H, J = 7.2 Hz), 1.63 (m, 1H),
1.24 (m, 32H), 0.87 (t, 6H, J = 12.3 Hz); 13C NMR (100 MHz,
CDCl3, ppm) δ 161.8, 136.9, 134.4, 126.8, 113.6, 43.1, 37.1, 32.16,
32.14, 31.7, 30.2, 29.94 (two peaks overlap), 29.88 (two peaks
overlap), 29.84 (two peaks overlap), 29.79 (two peaks overlap), 29.59
(two peaks overlap), 29.53, 26.50, 22.93, 14.38; HRMS (ESI-TOF) m/
z [M + H]+ calcd for C26H43BrNO2S 512.2198, found 512.2220.
1
7b: product obtained as a pale yellow oil; H NMR (400 MHz,
CDCl3, ppm) δ 7.70 (s, 1H), 4.09 (m, 1H), 1.65 (m, 2H), 2.00 (m,
2H), 1.23 (m, 24H), 0.87 (t, 6H, J = 13.6 Hz); 13C NMR (100 MHz,
CDCl3, ppm) δ 162.2, 161.8, 136.7, 134.1, 126.7, 126.6, 113.4, 32.16,
32.4, 32.0, 31.8, 31.1, 29.66 (two peaks overlap), 29.50 (two peaks
overlap), 29.44 (two peaks overlap), 29.40 (two peaks overlap), 26.88,
22.8, 14.34; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C23H37BrNO2S 470.1728, found 434.1740.
P1* and P2 (H-A-Br). Monomer (7a or 7b; 0.18 mmol), trans-
bis(μ-acetato)bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II) (3.34
mg, 2 mol %), tris(o-methoxyphenyl)phosphine (5.04 mg, 8 mol %),
and potassium acetate (17.67 mg, 0.18 mmol) were added in a Biotage
microwave vial (size 2−5 mL) with a magnetic stirring bar. The vial
was sealed with a cap and then purged with nitrogen to remove the
oxygen. A 1 mL portion of THF was added, and the reaction mixture
was heated with an oil bath at 120 °C (reaction under pressure). After
22 h, the reaction mixture was cooled and the corresponding 5-
alkylthieno[3,4-c]pyrrole-4,6-dione was added as a capping agent (50
mg in 1 mL). The solution was heated again at 120 °C for 1 h to
complete the end-capping procedure. After an additional 1 h of
reaction, the whole mixture was cooled to room temperature and
poured into 500 mL of cold methanol. The precipitate was filtered.
Soxhlet extractions with acetone followed by chloroform were done.
The solvent was reduced to about 10 mL, and the mixture was poured
into cold methanol. The precipitate was filtered. P1* and P2 were
obtained in 75−85% yields of soluble fraction in CHCl3 (Mn of 22 for
P1* and 22 kDa for P2).
7c: product obtained as a white solid; 1H NMR (400 MHz, CDCl3,
ppm) δ 7.72 (s, 1H), 3.60 (t, 2H, J = 7.3 Hz), 1.63 (m, 2H), 1.29 (m,
10H), 0.84 (t, 3H, J = 7.2 Hz); 13C NMR (100 MHz, CDCl3, ppm) δ
161.9, 161.6, 137.0, 134.5, 126.8, 113.6, 38.9, 32.0, 29.4, 29.4, 28.6,
27.1, 22.9, 14.3; HRMS (ESI-TOF) m/z [M + H]+ calcd for
C14H19BrNO2S 344.0320, found 344.0324; mp 86−88 °C (un-
corrected).
Preparation of 2-Bromothiophene-3,4-dicarboxylic Acid (6). 3-
Ethyl 4-methyl 2-aminothiophene-3,4-dicarboxylate (3; 15.00 g, 65
mmol) in HBr (10% aqueous, 300 mL) was stirred at room
temperature for 20 min. The solution was cooled to 0 °C, and
NaNO2 (6.78 g, 130.88 mmol) was added to the solution. The mixture
was allowed to react for 30 min. Then CuBr (28.12 g, 196 mmol) was
added in small portions. The solution was heated at 80 °C for 1 h. The
F
dx.doi.org/10.1021/jo301512e | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
P2*−P5 (H-A-H + Br-B-Br). Monomer A (0.25 mmol), monomer B
(0.25 mmol), trans-bis(μ-acetato)bis[o-(di-o-tolylphosphino)benzyl]-
dipalladium(II) (2 mol %), tris(o-methoxyphenyl)phosphine (8 mol
%), and potassium acetate (0.50 mmol) were added in a Biotage
microwave vial (size 2−5 mL) with a magnetic stirring bar. The vial
was sealed with a cap and then purged with nitrogen to remove the
oxygen. A 1 mL portion of THF was added, and the reaction mixture
was heated with an oil bath at 120 °C (reaction under pressure). After
22 h, the reaction mixture was cooled and the corresponding 5-
alkylthieno[3,4-c]pyrrole-4,6-dione was added as a capping agent (50
mg in 1 mL). The solution was heated again at 120 °C for 1 h to
complete the end-capping procedure. After an additional 1 h of
reaction, the whole mixture was cooled to room temperature and
poured into 500 mL of cold methanol. The precipitate was filtered.
Soxhlet extractions with acetone followed by hexanes removed
catalytic residues and low-molecular-weight materials. Polymers were
then extracted with chloroform. The solvent was reduced to about 10
mL, and the mixture was poured into cold methanol. The precipitate
was filtered. P2*−P4 were obtained in 60−75% yields of soluble
fraction in CHCl3 and 55% for P5 in CHCl3 and o-DCB (Mn of 25 for
P2*, 10 for P3, 8 for P4, and 5 kDa for P5).
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mmol), tris(o-methoxyphenyl)phosphine (0.08 mmol), and potassium
acetate (1.00 mmol) were added in a Biotage microwave vial (size 2−5
mL) with a magnetic stirring bar. The vial was sealed with a cap and
then purged with nitrogen to remove the oxygen. A 2 mL portion of
THF was added, and the reaction mixture was heated with an oil bath
at 120 °C (reaction under pressure). After 22 h, the reaction mixture
was cooled and the corresponding 5-alkylthieno[3,4-c]pyrrole-4,6-
dione was added as a capping agent (50 mg in 1 mL). The solution
was heated again at 120 °C for 1 h to complete the end-capping
procedure. After an additional 1 h of reaction, the whole mixture was
cooled to room temperature and poured into 500 mL of cold
methanol. The precipitate was filtered. Soxhlet extractions with
acetone followed by hexanes removed catalytic residues and low-
molecular-weight materials. Polymers were then extracted with
chloroform. The solvent was reduced to about 10 mL, and the
mixture was poured into cold methanol. The precipitate was filtered.
P6 was obtained in 39% yield of soluble fraction in CHCl3 and o-DCB
(Mn of 7 kDa).
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ASSOCIATED CONTENT
* Supporting Information
■
S
(24) Najari, A.; Beaupre,
́
S.; Berrouard, P.; Zou, Y.; Pouliot, J.-R.;
Lepage-Perusse, C.; Leclerc, M. Adv. Funct. Mater. 2011, 21, 718.
́
Figures giving NMR spectra of all compounds as well as UV−
visible absorption spectra and powder XRD of all polymers.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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17, 1166.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mario.leclerc@chm.ulaval.ca.
Notes
■
(28) Berrouard, P.; Grenier, F.; Pouliot, J.-R.; Gagnon, E.; Tessier,
C.; Leclerc, M. Org. Lett. 2011, 13, 38.
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2012, 51, 3520.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge financial support from the Natural Sciences
and Engineering Research Council of Canada (NSERC). S.D.
■
thanks the Fonds queb
technologies (FQRNT) for a postdoctoral fellowship. Acknowl-
edgment is made to Simon Rondeau and Prof. Jean-Francois
́ ́
ecois de la recherche sur la nature et les
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dx.doi.org/10.1021/jo301512e | J. Org. Chem. XXXX, XXX, XXX−XXX