Synthesis and photovoltaic performance of pyrazinoquinoxaline containing conjugated thiophene-based dendrimers and polymers

Article abstract of DOI:10.1021/ma302404y

Pyrazinoquinoxaline-based building blocks were incorporated into both π-conjugated dendrimers and polymers. The dendrimers were synthesized using a convergent/divergent approach whereas the donor/acceptor copolymers were synthesized via Stille cross-coupling reactions. The structurally defined dendrimers and the π-conjugated polymers were investigated with respect to their optical and electronic properties as well as their performance in photovoltaic devices. Because of the presence of the electron-deficient pyrazinoquinoxaline moiety, the absorption spectra of the materials under investigation were red-shifted with respect to the all thiophene-containing materials. Power conversion efficiencies up to 1.7 and 0.8% were obtained from blends of second-generation dendrimers and polymers with PC₇₁BM, respectively.

Full text of DOI:10.1021/ma302404y

Article
pubs.acs.org/Macromolecules
Synthesis and Photovoltaic Performance of Pyrazinoquinoxaline
Containing Conjugated Thiophene-Based Dendrimers and Polymers
Gisela L. Schulz,*,‡ Michael Mastalerz,‡ Chang-Qi Ma,‡,§ Martijn Wienk,† Rene Janssen,†
́
and Peter Bauerle*,‡
̈
†Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
‡Institute of Organic Chemistry II and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
S
* Supporting Information
ABSTRACT: Pyrazinoquinoxaline-based building blocks were incorporated into both π-conjugated dendrimers and polymers.
The dendrimers were synthesized using a convergent/divergent approach whereas the donor/acceptor copolymers were
synthesized via Stille cross-coupling reactions. The structurally defined dendrimers and the π-conjugated polymers were
investigated with respect to their optical and electronic properties as well as their performance in photovoltaic devices. Because of
the presence of the electron-deficient pyrazinoquinoxaline moiety, the absorption spectra of the materials under investigation
were red-shifted with respect to the all thiophene-containing materials. Power conversion efficiencies up to 1.7 and 0.8% were
obtained from blends of second-generation dendrimers and polymers with PC71BM, respectively.
workers, in which a vinylene group was incorporated between
the phenyl and thiophene moieties. In this case the authors
were able to improve the PCEs to 3.0% when they blended the
star-shaped triphenylamine with [6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM, 1:2).7 Furthermore, Kopidakis and co-
workers increased the PCE of an X-shaped thiophene-based
dendrimer via core-functionalization upon addition of electron-
withdrawing cyano groups on the central benzene ring from
0.40 to 1.12%.8 Sun et al. reported on another X-shaped
oligothiophene consisting of a tetrasubstituted thiophene core,
that when blended with [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM), showed an overall PCE of 0.8%.9 Based on
work published by our group in 2008, wherein all-thiophene
dendrimers up to the fourth generation were described, PCEs
of up to 1.7% were reported for blends of a third-generation
dendrimer containing 42 thiophene units (42T) with
PC61BM.10,11 Other dendrimer work by our group involved
incorporating ruthenium(II) phthalocyanine complexes func-
tionalized with dendritic oligothiophenes in the axial position,
in bulk heterojunction solar cells. PCEs up to 1.7% were
reported with blends consisting of RuPcCO(Py-3T) and
INTRODUCTION
■
Over the past decades, there has been increasing interest in the
development of new materials for applications in organic bulk
heterojunction solar cells (BHJSC).1−3 The field has been
expanding rapidly with the number of new compounds being
produced at an increasingly faster rate.4,5 The materials
investigated to date can be classified into the following four
categories: conjugated polymers, dendrimers, oligomers, and
small dye molecules. This work reports on the synthesis and
structure−property relationships of two of the above classes of
materials, i.e., π-conjugated dendrimers and polymers, for use as
electron donors in fullerene-based BHJSCs.
Shape-persistent molecules such as conjugated dendrimers
are of interest for applications in photovoltaics because of their
defined structure, monodispersity, and the possibility to derive
structure−property relationships. Problems with respect to
reproducibility of results due to batch to batch differences, such
as in the case of polymers, are of less significance. Challenges in
the field of dendrimers lie in the synthesis and most frequently
in the purification of the often large molecules. Several groups
have reported on dendritic systems and their performance in
BHJSCs. Roncali et al. investigated triphenylamine-based
molecules end-capped with dicyanovinylene, yielding power
conversion efficiencies (PCEs) up to 1.85% in bilayer solar
cells.6 A very similar molecule was reported by Zhang and co-
Received: November 21, 2012
Revised: February 28, 2013
© XXXX American Chemical Society
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Scheme 1. Deprotection of First- and Second-Generation Dendrimers D1 and D3 To Yield D2 and D4
PC71BM.12 Oligothiophene dendrimers incorporating ethynyl
groups were also developed showing PCEs in the range of 0.4−
0.8%.13 Furthermore, dendritic oligothiophene−perylene bisi-
mide hybrid systems were synthesized and investigated with
respect to their optoelectronic properties.14
Additionally, Wang et al. synthesized a similar pyrazinoquinoxa-
line-co-thiophene using Stille cross-coupling and obtained a
PCEs up to 2.1% with PC71BM.19 Unver et al. also have
described the electropolymerization of pyrazinoquionoxaline
copolymers, wherein cyclic voltammetry and UV−vis−NIR
absorption were investigated.20 More recently, a series of
pyrazinoquinoxaline-co-indolocarbazole polymers were re-
ported by Peng et al. They measured PCEs up to 3.2% from
polymer:PC71BM blends.21 Herein, we report on the synthesis
and characterization of four pyrazinoquinoxaline-based den-
drimers and two donor/acceptor copolymers based on
thiophene and pyrazinoquinoxaline alternating repeat units.
Structure−property relationships and photovoltaic perform-
ances are discussed.
A recent report from Tam et al. on the substituent effect on
the electronic properties of pyrazino[2,3-g]quinoxaline mole-
cules demonstrated the potential for fine-tuning the acceptor
characteristics.15 With the aim of broadening the absorption
spectrum and thereby increasing the number of harvested
photons, we employed the same pyrazino[2,3-g]quinoxaline as
electron-deficient moiety in the core of the dendritic molecules.
A PCE of 1.3% was obtained for the 40T pyrazinoquinoxaline
dendrimer (D3) with TMS protecting groups on the periphery
of the molecule.16 The observed increase in PCE from 1.0 to
1.3%, when going from the protected 42T to 40T
pyrazinoquinoxaline containing material, indicated the promis-
ing nature of this new series of thiophene dendrimers.11
Using the knowledge gained from these dendrimeric systems,
we then proceeded to design and synthesize conjugated
polymers based on structurally similar building blocks. To
date, there have been a few pyrazinoquinoxaline containing
polymers published in the literature. A phenyl-substituted
pyrazinoquinoxaline-co-fluorene polymer was reported by
Zhang et al. which displayed a PCE of 1.7% with PC61BM
and 2.3% with PC71BM.17 Shortly thereafter, Zoombelt et al.
described two pyrazinoquinoxaline-co-thiophene polymers
synthesized via Yamamoto cross-coupling. Of the two, the
best performing polymer showed a PCE of 0.37% and 0.72%
when blended with PC61BM and PC71BM, respectively.18
RESULTS
■
The pyrazinoquinoxaline-cored dendrimers D1 and D3 were
synthesized using a previously reported method via a 4-fold
Suzuki cross-coupling reaction between a tetra(bromothienyl)-
pyrazinoquinoxaline and a terthiophene (3T)- or 9T-dendritic
boronic ester to give the first- and second-generation dendritic
oligothiophenes (DOT), respectively (Scheme 1).16 With
respect to the removal of the TMS groups, we have previously
shown that it occurs quantitatively for the all-thiophene
dendrimers using tetrabutylammonium fluoride (TBAF).10
However, under the same conditions, we observed decom-
position of the pyrazinoquinoxaline-based dendrimers. Multiple
attempts were made to perform the deproctection, including
the use of trifluoroacetic acid in dichloromethane (CH2Cl2),
acetic acid (AcOH) in dichloromethane, trifluoroacetic acid in
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Scheme 2. Synthetic Route Used To Obtain P1
o-dichlorobenzene (ODCB), and simply neat acetic acid, all of
which were unsuccessful. In general, deprotection using low
boiling point solvents was also unsuccessful as higher
temperatures were found to be necessary. This was later
ascribed to the significantly reduced solubility of the product in
comparison to the starting material. Solubility of D2 and D4
increased in the following solvents: CH2Cl2 < tetrahydrofuran
(THF) < ODCB < 1,1,2,2-tetrachloroethane (TCE). An
insoluble black solid was obtained when the reaction was
carried out using trifluoroacetic acid. Finally, it was shown that
the TMS groups could be cleaved in the presence of acetic acid
and TCE after heating to 130 °C for 4 days. After work-up, the
obtained solid was washed with pentane and methanol and
then dried under vacuum to give the desired dendrimer in 95%
yield.
yield.22 The condensation of compound 1 with tetraamino-
benzene tetrahydrochloride was performed in n-butanol under
argon for 16 h and gave quinoxaline 2 in 85% yield. Compound
2 was further reacted with elemental bromine to yield the
dibromo derivative 3 in 95% yield after recrystallization.
Dibromopyrazinoquinoxaline (3) was further reacted with
tributylstannylthiophene in order to obtain dithienylquinoxa-
line (4) in 86% yield. The desired dibrominated monomer 5
was synthesized by reacting 4 with NBS, followed by
precipitation in methanol to give an overall yield of 85%.
Polymer P1 was successfully synthesized using Stille cross-
coupling; the crude product was precipitated in MeOH and
underwent successive Soxhlet extraction in MeOH, acetone,
hexane, and THF. The THF fraction was then reprecipitated in
methanol and dried under vacuum to yield P1 in 48% yield.
Molecular weight was estimated using gel permeation
chromatography versus polystyrene standards, resulting in Mn
= 6300 g/mol and Mw = 8900 g/mol, giving a PDI of 1.4. The
The synthetic route used to obtain copolymer P1 is shown in
Scheme 2. The dimethylpiperazine-2,3-dione was reacted with
the lithiated trifluorophenyl compound according to a known
literature procedure to produce the desired diketone 1 in 79%
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Scheme 3. Synthetic Route Used To Obtain P2
rather low molecular weight of P1 is attributed to its poor
solubility.
some optimization, the desired product 9 could be obtained by
first reaction of 8 with 2 equivs. of Br2 and 2 equivs. of
NaHCO3 in CHCl3 for 24 h at 80 °C, followed by cooling to
room temperature and addition of another 2 equivs. of Br2, 2
equivs. of NaHCO3, and further reaction at 50 °C for 6 h.
Dibromopyrazinoquinoxaline (9) was purified using silica gel
chromatography, followed by recrystallization from THF/
MeOH to yield bright orange crystals in 45% yield. From
5,10-dibromopyrazinoquinoxaline (9) and 2,5-bis-
(trimethylstannyl)bithiophene, polymerization was carried out
via a Stille cross-coupling reaction, as shown in Scheme 3. The
crude product was precipitated in MeOH and underwent
successive Soxhlet extraction in MeOH, acetone, hexane, and
THF. The THF fraction was then reprecipitated in methanol
and dried under vacuum to yield donor−acceptor polymer P2
in 95% yield. Molecular weights were determined using gel
permeation chromatography versus polystyrene standards,
resulting in Mn = 53 000 g/mol and Mw = 113 000 g/mol,
giving a PDI of 2.1.
Scheme 3 depicts the synthetic route followed to obtain
polymer P2. First, 1-bromo-4-(2′-ethylhexyl)benzene (6) was
synthesized using Negishi cross-coupling to yield a colorless
liquid in 36% yield. Compound 6 then underwent a metal−
halogen exchange reaction with n-butyllithium, followed by
quenching with dimethyl-piperazine-2,3-dione to produce 7, in
84% yield, as described in a similar reaction in the literature.22
The condensation of compound 7 with tetraaminobenzene
tetrahydrochloride was performed in acetic acid under argon for
24 h and gave the desired pyrazinoquinoxaline derivative, 8, in
74% yield.
As depicted in Scheme 2, the trifluoromethyl analogue of 8,
derivative 2, underwent bromination in the presence of 4
equivs. of elemental bromine, 4 equivs. of sodium hydrogen
carbonate, and chloroform in 95% yield after reacting for 4 h at
80 °C. This unfortunately was not the case for the analogous
reaction of 8 shown in Scheme 3. The bromination of 8 under
the same conditions was more sluggish and less selective. After
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Figure 1. (a) UV−vis spectra of first- and second-generation dendrimers, with and without TMS protecting groups measured in TCE. (b) UV−vis
spectra of D4 measured in TCE at various concentrations.
The UV−vis absorption spectra measured in TCE of the
first- and second-generation dendrimers, with and without
TMS groups, are displayed in Figure 1. Absorption in the high-
energy region of the spectrum (between 350 and 500 nm) is
attributed to the π−π* transition of the oligothiophene
branches. It follows that this intensity should increase with
longer oligothiophenes chains, i.e., upon increasing dendrimer
generation. The intensity of the charge transfer band observed
at longer wavelengths (∼575 nm), comparatively, does not
change its intensity upon increasing generation size; however,
the onset is shifted from 643 nm for D2 to 679 nm for D4. The
molar absorption coefficients of the pyrazinoquinoxaline
containing DOTs are in the range 55 900−188 000 L mol−1
cm−1. We observe an increase in extinction coefficient with
increasing generation size (86 500 vs 188 000 L mol−1 cm−1)
for D1 and D3 as well as a small blue-shift in the absorption
maximum from 410 to 400 nm, respectively. The second-
generation deprotected dendrimer, D4, had the tendency to
form aggregates in solution. This was observed in the broadness
of the 1H NMR peaks as well as in absorption spectra measured
for various concentrations (see Figure 1b and Supporting
Information). In Figure 1b, one can see the blue-shift of the
absorption maximum, from 395 to 373 nm, as the
concentration of D4 in TCE is reduced to 1.5 × 10−7 mol L−1.
The absorption spectra of thin films of the deprotected first-
and second-generation dendrimers, D2 and D4, spin-coated
from TCE, are displayed in Figure 2. Relative to measurements
performed in solution, the absorption maximum and onset of
the low-energy band of the D4 thin film displayed a red-shift of
6 nm (λmax,soln = 581 nm vs λmax,film = 587 nm) and 45 nm
(λonset,soln = 675 nm vs λonset,film = 720 nm), respectively. The
absorption spectrum of D2 does not return to the baseline at
longer wavelengths due to poor film-forming properties/
solubility limitations which result in light scattering. As seen
in the case for the measurements performed in solution, the
thin films of the pyrazinoquinoxaline-cored DOTs have
reduced absorption at 600 nm upon increasing dendrimer
size. This is explained by the relative increase in donor content
within dendrimer structure going from D2 to D4. Figure 2 also
displays the absorption profiles of the thin films of polymers P1
and P2. Relative to the dendrimers, a significant red-shift in the
charge transfer band is observed (λCT,D4 = 592 nm vs λCT,P1
=
870 vs λCT,P2 = 865 nm) due to the different connectivity of
pyrazinoquinoxaline unit to the thiophene moieties. In the case
of the dendrimers, the donor thiophene units are bound to
pyrazinoquinoxaline at the 2-, 3-, 7-, and 8-positions, whereas in
the polymers the thiophene units are connected through the 5-
and 10-positions. The latter is stabilized through the formation
a quinoid resonance structure via intramolecular charge transfer
resulting in the observed red-shift. The difference seen in the
intensity of the longer wavelength absorption maxima of P1
and P2 is attributed to enhanced π−π stacking in the film of P2
vs P1, likely due to the significantly higher molecular weight of
P2 over P1 (53 000 vs 6000 g/mol).
In addition, we also performed fluorescence experiments in
solvents with different polarity to determine the effect of the
incorporation of the electron-deficient pyrazinoquinoxaline
core on the dendritic molecules (see Figure 3). The emission
Figure 2. UV−vis spectra of D2, D4, P1, and P2 thin films. The
dendrimer films were spin-coated from TCE and the polymers from
CHCl3.
Figure 3. Fluorescence spectra of 42T, D1, and D3 in apolar toluene
and polar o-dichlorobenzene (ODCB). Excitation wavelength was 400
nm for all measurements.
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Figure 4. Energy level diagram (energy (eV) vs vacuum) showing the HOMO and LUMO levels of the dendrimers, polymers, and PCBM
derivatives.23−25
Figure 5. (a) J−V characteristics of D1, D2, D3, and D4 blended with PC61BM (1:3 ratio). (b) J−V characteristics of D3, D4, and P2 blended with
PC71BM (1:3 ratio).
Figure 6. EQE characteristics of D4 blended with PC61BM or PC71BM (1:3 ratio) (a), D2, D4, P1, and P2 blended with PC71BM (1:3 ratio) shown
in comparison to the absorption spectrum of P1 (b).
maxima of our reference all-thiophene dendrimer, 42T, in
apolar toluene and polar o-dichlorobenzene (ODCB) were
found to be similar at 580 and 589 nm, respectively,
demonstrating the minor effect of solvent polarity on the
excited state. For dendrimers D1 and D3, this however was not
the case. D1 in toluene displayed a maximum at 631 nm,
whereas the same dendrimer in ODCB showed an emission
maximum at 675 nm. It can therefore be concluded that this
excited state displays charge-transfer character. Furthermore,
we found a partial quenching of the fluorescence of D1 in
ODCB. On the other hand, for D3, we observe an emission
maximum of the CT band in toluene at 662 nm, whereas the
fluorescence is completely quenched in ODCB. From the
fluorescence experiments, we can conclude that the band at low
energies show a strong charge-transfer character, most probably
between the dendrimer arms (3T moieties in D1 and 5T
moieties in D3) and the pyrazinoquinoxaline core. It is likely
that such a situation is not ideal for efficient electron transfer
from the dendrimer core to a fullerene acceptor molecule in a
solar cell device. This will be discussed further in another
section.
Cyclic voltammetry (CV) was used to investigate the
electrochemical properties of the dendrimers16 and polymers
(see Figure S1). For example, the HOMO and LUMO energy
levels of P1 were calculated to be −5.3 and −4.0 eV,
respectively, from the onset of the first oxidation and reduction
wave. The energy levels of the TMS-protected DOTs, D1 and
D3, are represented in Figure 4. Because of the limited
solubility of the deprotected dendrimers, D2 and D4, CV could
not be performed. We have previously shown that the HOMO/
LUMO energy levels are not significantly altered upon removal
of trimethylsilyl groups, and it is therefore expected that levels
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Table 1. PV Device Results for the Pyrazinoquinoxaline Dendrimers and Polymers for Devices Made with PC61BM
D:A ratio
solvent
EQE
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
a
D1:PC61BM
D2:PC61BM
D3:PC61BM16
D4:PC61BM
42T:PC61BM11
1:3
1:3
1:3
1:3
1:3
CB
0.22
0.26
0.28
0.31
0.45
2.7
2.7
3.3
3.5
3.9
0.97
0.84
1.00
0.85
0.98
0.32
0.30
0.38
0.30
0.43
0.8
0.7
1.3
0.9
1.7
b
TCE
CB
TCE
CB
a
b
CB = chlorobenzene. TCE = tetrachloroethane.
Table 2. PV Device Results for the Pyrazinoquinoxaline Dendrimers and Polymers for Devices Made with PC71BM
D:A ratio
solvent
EQE
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
a
D1:PC71BM
D2:PC71BM
D3:PC71BM
D4:PC71BM
P1:PC71BM
P2:PC71BM
1:3
1:3
1:3
1:3
1:4
1:3
CB
0.34
0.35
0.35
0.41
0.12
0.10
4.8
4.6
4.9
5.6
0.8
2.2
0.91
0.73
0.98
0.87
0.73
0.54
0.33
0.32
0.35
0.32
0.38
0.63
1.4
1.1
1.7
1.6
0.2
0.8
b
TCE
CB
TCE
CB
CB
a
b
CB = chlorobenzene. TCE = tetrachloroethane.
for D2 and D4 are similar to those of D1 and D3.10 Upon
increasing generation size, it can be seen that the LUMO
energy level is somewhat stabilized (−3.8 vs −3.9 eV) and that
the HOMO energy level is to a larger extent destabilized (−5.7
vs −5.4 eV). P1 displayed the lowest LUMO energy level (−4.0
eV) due to the electron-withdrawing strength of the
trifluoromethyl groups relative to the weakly electron-donating
ethylhexyl chains located at the 2-, 3-, 7-, and 8-positions of the
pyrazinoquinoxaline moiety in P2 (−3.8 eV). The electro-
chemical experiments further confirmed that the polymers have
a smaller band gap than the dendrimers, supporting the results
of the absorption experiments, which is also attributed to the
different connectivities of the pyrazinoquinoxaline units to the
thiophene moieties.
of the dendrimers after deprotection, which leads to poor film
formation even when spin-coated from high temperatures.
Although the replacement of the central bithiophene core with
the pyrazinoquinoxaline unit resulted in a red-shift in the
absorption profile,11 which one would expect to result in
increased charge generation in the solar cell and therefore an
overall increase in PCE, this was not the case. The PV
characteristics of the analogous all-thiophene dendrimer
(42T)11 were included in Table 1 for comparison with the
40T-pyrazinoquinoxaline containing dendrimer (D4). Their
differences in solar cell performance can be in part explained by
the restricted rotational freedom of the planar pyrazinoquinoxa-
line core in comparison to the more flexible bithiophene core
present in the 42T dendrimer, which strongly influences the
solubility of the molecules. On the other hand, fluorescence
experiments (shown in Figure 3) indicated that the CT state
localizes charge in the core of the dendrimers in polar media,
which may also limit electron transfer to the fullerene acceptor.
Furthermore, in the J−V curves of blends of the donor−
acceptor dendrimers with PC61BM one can see that the VOC
and fill factor are reduced upon deprotection (see Figure 5 and
Table 1). D4 displays a slightly higher short-circuit current
density than D3 (3.5 vs 3.3 mA/cm2). This is also reflected in
the spectral response plot shown in Figure S2. In general, it can
be seen that PV performance of the dendrimers blended with
PC61BM and PC71BM display the same trends (see Tables 1
and 2). The main difference being that the magnitude of the
current produced by the photovoltaic device is increased in the
latter case due to absorption of PC71BM in the visible region of
the solar spectrum.26
The dendrimers and polymers were tested in solution-
processed bulk heterojunction solar cells with the following
device structure: ITO| PEDOT:PSS|donor:PCBM| LiF| Al. The
J−V and spectral response data are plotted in Figures 5 and 6,
respectively, and summarized in Tables 1 and 2.
The TMS-containing dendrimers D1 and D3 displayed good
solubility in common organic solvents such as CHCl3, THF,
and CB, whereas their TMS-free analogues D2 and D4 had
very low solubility in these solvents. It was found that
reasonable films for solar cells could only be made from hot
solutions, in this case, when spin-coated from TCE at 80 °C.
The first-generation TMS-protected dendrimers, D1, when
blended with PC61BM, displayed a short-circuit current density
(JSC) of 2.7 mA/cm2, an open-circuit voltage (VOC) of 0.97 V,
and a fill factor (FF) of 0.32, leading to an overall power
conversion efficiency of 0.8%. Devices based on D2:PC61BM
blends showed a JSC of 2.7 mA/cm2, which is similar to
dendrimer D1, while the VOC reduced to 0.84 V; the fill factor
also reduced slightly to 0.30, and the overall PCE was found to
be 0.7%.The second-generation TMS-protected dendrimer, D3,
displayed a JSC of 3.3 mA/cm2, a VOC of 1.00 V, and a fill factor
of 0.38, resulting in a PCE of 1.3% when blended with PC61BM.
In comparison, the deprotected D4 showed a small increase in
JSC to 3.5 mA/cm2, a reduced VOC (0.85 V), and a reduced fill
factor (0.30), resulting in a PCE of 0.9%. Unlike the previously
reported all-thiophene dendrimers, there is not a significant
increase in the PCE of the devices after the removal of the
trimethylsilyl groups. This is attributed to the reduced solubility
The two pyrazinoquinoxaline-based polymers were also
tested in photovoltaic devices with the same structure as
those for the dendrimers: ITO|PEDOT:PSS|donor:PCBM|LiF|
Al. In the case of P1, the LUMO energy level was estimated to
be equal to the LUMO of PC61BM (−4.0 eV), as shown in
Figure 4. Thus, for the polymer-based devices, we used
PC71BM as the electron acceptor due to its slightly lower
LUMO energy level of (−4.1 eV).25 For P1, the energy
difference between the LUMOs of the donor and acceptor was
calculated to be 0.1 eV, which turned out to be insufficient for
efficient electron transfer from the polymer to the PC71BM.
This is clearly shown in Figure 6, where the plot of the spectral
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fluorescence spectrometer of solutions with an optical density with less
than 0.1. Cyclic voltammetry experiments were performed with a
computer-controlled EG&G PAR 273 potentiostat and a three-
electrode single-compartment cell with a platinum working electrode,
a platinum wire counter electrode, and an Ag/AgCl reference
electrode. All potentials were internally referenced to the ferrocene/
ferrocenyl couple (−5.1 eV). Elemental analyses were performed on
an Elementar Vario EL. Plastic sheets precoated with silica gel, Merck
Si60 F254, were used for thin layer chromatography. Glass columns
packed with Merck Silica 60, mesh 0.063−0.2 μm, were used for
column chromatography. Size-exclusion chromatography was per-
formed on a glass column packed with biorad beads SX-1 swollen in
dichloromethane. Dichloromethane was used as the eluent. Molecular
weight determinations were performed using gel permeation
chromatography (HPLC pump from Dionex) calibrated against
polystyrene standards. Polymers were eluted with THF using a flow
rate of 1 mL/min and monitored with a UV−vis detector (Waters 486
at 254 nm). Solvents were purchased at ProLabo and distilled prior to
use. Commercially available 2,2′-thenyl 1 (Alfa Aesar, 98%) and
1,2,4,5-tetraaminobenzene tetrahydrochloride (Aldrich, techn.) were
used. Dendrimers D1 and D3 were prepared as described in the
literature.16
Device Fabrication. Photovoltaic devices were made by spin-
coating PEDOT:PSS (Clevios P, VP Al4083) onto precleaned,
patterned indium tin oxide (ITO) substrates (14 Ω/square) (Naranjo
Substrates). The photoactive layer (ca. 80 nm) was deposited by spin-
coating from of a mixed solution of dendrimers D1−D4 or P1−P2
with PC61BM or PC71BM (1:3 ratio w/w, β = 25 mg/mL in
chlorobenzene or β = 15 mg/mL in 1,1,2,2-tetrachloroethane (TCE)).
The active layers spin-coated from TCE were done from hot solutions
at 80 °C and substrates heated to 100 °C. PC61BM and PC71BM were
purchased from Solenne BV, Netherlands, 99% pure. The counter
electrode of LiF (1 nm) and aluminum (100 nm) was deposited by
vacuum evaporation at 3 × 10−7 mbar. The active areas of the cells
were 0.09 or 0.16 cm2. J−V characteristics were measured under ∼100
mW/cm2 white light from a tungsten−halogen lamp filtered by a
Schott GG385 UV filter and a Hoya LB120 daylight filter, using a
Keithley 2400 source meter. Short circuit currents under AM1.5G
conditions were obtained from the spectral response and convolution
with the solar spectrum.29 Spectral response was measured under
operation conditions using bias light from a 532 nm solid state laser
(Edmund Optics). Monochromatic light from a 50 W tungsten
halogen lamp (Philips focusline) in combination with monochromator
(Oriel, Cornerstone 130) was modulated with a mechanical chopper.
The response was recorded as the voltage over a 50 Ω resistance, using
a lock-in amplifier (Stanford Research Systems SR830). A calibrated Si
cell was used as reference. The device was kept behind a quartz
window in a nitrogen-filled container.
response is compared with the absorption profile of P1. Since
no current is generated where the polymer absorbs light (600−
1100 nm), it has been concluded that electron transfer from P1
to PC71BM does not take place. The small amount of current
that is generated in the photovoltaic cell is due to absorption by
the acceptor, PC71BM. The polymer plays a role in hole
transport but not in charge generation, leading to an inefficient
solar cell with PCE of 0.2% (Table 2).
Upon exchanging the electron-withdrawing CF3 group by the
weakly electron-donating 2-ethylhexyl side chains, the LUMO
energy level of P2 (−3.8 eV) increased relative to P1 (−4.0
eV). This increase results in an energy difference of 0.3 eV
between the LUMO of P2 and the LUMO of PC71BM, which
should be sufficient for electron transfer from P2 to the
PC71BM acceptor in the solar cell.27,28 P2 displayed a JSC of 2.2
mA/cm2, a VOC of 0.54 V, and a fill factor of 0.63, resulting in a
PCE of 0.8% when blended with PC71BM. Despite the high fill
factor, the PCE was found to be limited by a low short-circuit
current density. Figure 6 shows that P2 does give a
photoresponse up to the optical band gap at about 1.2 eV;
however, attempts to further improve the JSC by changing the
processing conditions such as the solvent have been so far
unsuccessful.
CONCLUSIONS
■
We report on the synthesis of pyrazinoquinoxaline-based
dendrimers and polymers as well as we compare their optical,
electrochemical, and photovoltaic properties. The different
connectivities of the pyrazinoquinoxaline units to the thiophene
moieties in the dendrimers (2,3,7,8-positions) and polymers
(5,10-positions) led to a change in optical and electrochemical
properties of these materials. This was apparent in the UV−vis
absorption, i.e., the large difference in band gaps (1.7 eV for D4
vs 1.2 eV for P2). Within the dendrimer series, we observe that
the PV performance unexpectedly decreases upon removal of
the solubilizing TMS groups. This finding was attributed to the
poor film forming ability of D2 and D4 as well as to the
electron-deficient core which can localize the charge transfer
state in the center of the dendrimers. PCEs decreased from 1.7
to 1.6% for D3 and D4 blends with PC71BM, respectively.
Replacement of electron-withdrawing CF3 groups on the
pyrazinoquinoxaline moiety (P1) with weakly donating ethyl-
hexyl chains (P2) raised the LUMO energy level from −4.0 to
−3.8 eV. This allowed for electron transfer from P2 to the
PC71BM acceptor in solar cell devices; however, the PCE of P2-
based photovoltaic devices remained modest at 0.8%.
Synthesis of Dendrimers and Polymers. 2,3,7,8-Tetrakis((5′-
(thien-2-yl)-2,2′:4′,2″-terthien-5-yl))-pyrazino[2,3-g]quinoxaline
(D2). 2,3,7,8-Tetrakis((5′-(5-trimethylsilylthien-2-yl)-5″-trimethylsil-
yl-2,2′:4′,2″-terthien-5-yl))-pyrazino[2,3-g]quinoxaline (D1) (72 mg,
0.035 mmol) was dissolved in tetrachloroethane (10 mL), to which 20
mL of acetic acid was added. The reaction mixture was heated to 130
°C for 4 days. Then the mixture was poured into water and extracted
with TCE. The organic layer was washed with water three times,
followed by washing with Na2CO3 solution and finally saturated NaCl
solution. The organic layer was dried over sodium sulfate. Because of
low solubility of the product, the filtrand was subjected to Soxhlet
extraction with TCE for 24 h, after which the solvent reduced in
volume. The obtained solid was washed with pentane (3 × 10 mL)
and methanol (3 × 10 mL) and dried under vacuum to give 50 mg
(95%) of D2 as a black solid. 1H NMR (C2D2Cl4, 500 MHz 100 °C):
δ (ppm) 8.78 (s, 2H, PQ-H), 7.56 (d, J1 = 1.18 Hz, 4H), 7.45 (s, 4H),
7.39−7.36 (dd, J1 = 1.17 Hz, J2 = 3.60 Hz, 8H), 7.27 (d, J1 = 1.18 Hz,
4H), 7.23 (d, 4H), 7.19 (d, 4H), 7.10−7.07 (m, 8H). MS (MALDI-
TOF, dithranol): m/z = 1494.0 [M+].
EXPERIMENTAL SECTION
■
Materials and Measurements. NMR spectra were recorded on a
Bruker AMX 500 (1H NMR: 500 MHz; 13C NMR 125 MHz) or a
Bruker Avance 400 (1H NMR: 400 MHz; 13C NMR 100 MHz),
usually at 298 K, unless otherwise mentioned. Chemical shift values
(δ) are given in ppm and were calibrated on residual nondeuterated
1
solvent peaks (CDCl3: H NMR: 7.26 ppm, 13C NMR: 77.0 ppm;
1
1
C2D2Cl4: H NMR: 6.00 ppm, 13C NMR: 74.0 ppm; CD2Cl2: H
NMR: 5.32 ppm, 13C NMR: 53.5 ppm; THF-d8: 1H NMR: 3.58 ppm,
13C NMR: 67.7 ppm) as internal standard. EI and CI mass
spectroscopy were performed on a Finnigan MAT SSQ-7000 or a
Varian Saturn 2000 GCMS. MALDI-TOF spectra were recorded on a
Bruker Daltonics Reflex III using dithranol or DCTB (trans-2-[3-(4-
tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile) as matri-
ces. UV−vis absorption spectroscopy was carried out on a
PerkinElmer Lambda 19 using Merck Uvasol grade solvents. The
corrected fluorescence spectra were recorded on a PerkinElmer LS 55
2,3,7,8-Tetrakis-[5′-(thien-2-yl)-2,2′:4′,2″-terthien-5-yl))-5⁗-
(thien-2-yl)-2,2′:4′,2″:5″,2‴:4‴,2⁗-quinquethien-5-yl]pyrazino[2,3-g]-
quinoxaline (D4). 2,3,7,8-Tetrakis-[5′(5′-(5-trimethylsilylthien-2-yl)-
H
dx.doi.org/10.1021/ma302404y | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
5″-trimethylsilyl-2,2′:4′,2″-terthien-5-yl))-5⁗-(5-trimethylsilylthien-2-
yl)-5⁗-(trimethylsilyl)-2,2′:4′,2″:5″,2‴:4‴,2⁗-quinquethien-5-yl]-
pyrazino[2,3-g]quinoxaline (D3) (80 mg, 0.017 mmol) was dissolved
in tetrachloroethane (10 mL, to which 20 mL of acetic acid was added.
The reaction mixture was heated to 130 °C for 3 days. Then the
mixture was poured into water and extracted with TCE. The organic
layer was washed with water three times, followed by washing with
Na2CO3 solution and finally saturated NaCl solution. The organic
layer was dried over sodium sulfate. Because of low solubility of the
product, the filtrand was subjected to Soxhlet extraction with TCE for
24 h, after which the solvent reduced in volume. The obtained solid
was washed with pentane (3 × 10 mL) and methanol (3 × 10 mL) and
5 , 1 0 - ( B i s ( 5 - b r o m o t h i e n - 2 - y l ) - 2 , 3 , 7 , 8 - t e t r a ( 4 -
trifluoromethylphenyl)pyrazino[2,3-g]quinoxaline (5). 4 (0.265 g, 0.29
mmol) was dissolved in 10 mL of THF, to which N-bromosuccinimide
(0.112 g, 0.63 mmol) was added. The reaction was stirred at room
temperature for 24 h, after which it was precipitated in methanol,
collected using a centrifuge, and dried under vacuum to yield a green
solid (0.265 g, 85%). 1H NMR (THF-d8, 400 MHz): δ (ppm) 8.49 (d,
2H), 7.98 (d, 8H), 7.83 (d, 8H), 7.35 (d, 2H). 13C NMR (THF-d8,
100 MHz): δ (ppm) 153.07, 144.92, 137.63, 137.13, 136.77, 132.29,
130.74, 129.15, 126.60, 124.12, 120.30. MS (MALDI-TOF, DCTB):
m/z = 1080.1 [M+].
Polymer (P1). 2,5″-Distannyl-3,4,3″,4″-tetrabutylterthiophene (111
mg, 0.138 mmol) was added to a dry 25 mL two-necked flask, which
was evacuated and purged with argon (2×) before addition of 5 (150
mg, 0.138 mmol). A mixture of toluene and DMF (4:1, 10 mL) was
added to the flask followed by further degassing using argon. After 10
min, Pd(PPh3)4 (7 mg, 0.007 mmol) was added to the reaction
mixture followed by further degassing for another 15 min. The
reaction was heated to 120 °C and allowed to react for 24 h. Upon
cooling the crude product was precipitated in methanol and
underwent successive Soxhlet extraction in methanol, acetone, hexane,
and THF. The THF fraction was then reprecipitated in methanol and
1
dried under vacuum to give 57 mg (95%) of D4 as a black solid. H
NMR (C2D2Cl4, 500 MHz 100 °C): δ (ppm) 8.78 (s, 2H, PQ-H),
7.58 (d, J1 = 1.18 Hz, 4H), 7.47 (s, 4H), 7.36−7.31 (m, 16H), 7.29 (s,
4H), 7.27 (s, 4H), 7.22−7.18 (m, 28H), 7.13 (d, 8H), 7.06−7.03 (m,
16H). MS (MALDI-TOF, DCTB): m/z = 3462.0 [M+].
4,4′-Bis(trifluoromethyl)benzyl (1). To a solution of 4-bromoben-
zotrifluoride (3 g, 13.3 mmol) in 12.5 mL of dry THF was added 5.08
mL of n-BuLi (2.5 M in hexanes, 12.7 mmol) using a syringe at −78
°C. After stirring at −78 °C for 1 h, the mixture was transferred to a
suspension of 1,4-dimethylpiperazine-2,3-dione (0.86 g, 6.06 mmol) in
12.5 mL of dry THF dropwise via a cannula under argon at −78 °C.
The reaction mixture was allowed to gradually come to room
temperature. After 3 days, the reaction was hydrolyzed with 100 mL of
10% HCl and extracted with CH2Cl2. The organic layer was dried over
sodium sulfate and reduced in volume. The product was purified by
column chromatography (silica gel, petroleum ether:CH2Cl2 5:1 v/v)
1
dried under vacuum to yield P1 (97 mg) in 48% yield. H NMR
(THF-d8, 400 MHz): δ (ppm) 8.75 (broad, 2H), 8.07 (broad, 8H),
7.84 (broad, 8H), 7.50 (broad, 2H), 7.28 (broad, 2H), 2.92 (broad,
8H), 1.56−1.29 (m, 16H), 0.89 (m, 6H). Mn = 6300 g/mol, Mw
8900 g/mol, and PDI = 1.4.
=
1-Bromo-4-(2′-ethylhexyl)benzene (6). To a suspension of Mg
(0.936 g, 0.038 mol) in dry diethyl ether (5 mL), 1-bromo-2-
ethylhexane (7.5 g, 0.039 mol) dissolved in 30 mL of dry diethyl ether
was added dropwise at 50 °C. The reaction was allowed to react for 12
h and was then transferred to a dropping funnel attached to a three-
necked flask containing ZnBr2 (8.74 g, 0.039 mol) in 20 mL of dry
THF. The reaction mixture was cooled to 0 °C, and the Grignard
reagent was added dropwise. Instantaneously, a white precipitate was
formed, and an additional 25 mL of dry diethyl ether was added so the
slurry could be stirred. 1-Bromo-4-iodobenzene (10.98 g, 0.039 mol)
was dissolved in 60 mL of dry diethyl ether and added dropwise to the
reaction mixture via the dropping funnel. Pd(dppf)Cl2 (420 mg, 0.51
mmol) then added to the mixture, followed by heating to 55 °C for 16
h, at which point the remainder of the catalyst was added (210 mg,
0.26 mmol) and the reaction was allowed to proceed at 55 °C for 24 h.
Upon cooling to room temperature, 20 mL of HCl (1 M) was added
to the reaction mixture. More diethyl ether was added, and the organic
phase was washed with water (3×) and NaCl solution (1×) and finally
dried over sodium sulfate. The solvent was reduced in volume, and
crude product was purified using multiple sublimations (12 mbar, 60
°C), isolating a white solid which was identified as the 1-bromo-4-
iodobenzene starting material. The remaining liquid was purified using
column chromatography (silica gel, petroleum ether) to yield a
1
to give a yellow solid (1.65 g, 79%). H NMR (CDCl3, 400 MHz): δ
(ppm) 3.53 (s, 4H), 3.03 (s, 6H).
2,3,7,8-Tetra(4′-trifluoromethylphenyl)pyrazino[2,3-g]quinoxaline
(2). 1 (0.5 g, 1.44 mmol) and 1,2,4,5-benzenetetraamine tetrahydro-
chloride (0.205 g, 0.72 mmol) were dissolved in 20 mL of degassed n-
butanol. The reaction mixture was heated to 120 °C for 16 h. After
cooling to room temperature, the mixture was poured into 100 mL of
water and extracted with CH2Cl2. The organic layer was washed with
water (2×), Na2CO3 solution (1×), and NaCl solution (1×) and
finally dried over sodium sulfate. The solvent was removed and
purification was done using column chromatography (silica gel,
petroleum ether:CH2Cl2 1:1) to yield an orange solid (0.46 g, 85%).
1H NMR (CDCl3, 400 MHz): δ (ppm) 9.09 (s, 2H), 7.75 (d, 8H),
7.71 (d, 8H).
5,10-Dibromo-2,3,7,8-tetra(4′-trifluoromethylphenyl)pyrazino[2,3-
g]quinoxaline (3). Pyrazinoquinoxaline 2 (154 mg, 0.2 mmol) was
combined with NaHCO3 (68 mg, 0.81 mmol) in 5 mL of CHCl3. The
solution was cooled to 0 °C, and Br2 (130 mg, 0.81 mmol) was added.
The reaction flask was sealed and heated to 80 °C for 4 h. After
cooling, the reaction mixture was put directly on a silica gel column
(petroleum ether:EA 1:0.05). The product was further purified by
recrystallization from a mixture of THF/pentane to obtain fluffy
orange crystals (173 mg, 95%). 1H NMR (CDCl3, 400 MHz): δ
(ppm) 7.90 (d, 8H), 7.74 (d, 8H). 13C NMR (THF-d8, 100 MHz): δ
(ppm) 155.07, 142.56, 139.71, 132.03, 126.35, 126.31, 123.88. MS
(MALDI-TOF, dithranol): m/z = 838.3 [M+−Br], 915.2 [M+].
5,10-Dithienyl-2,3,7,8-tetra(4′-trifluoromethylphenyl)pyrazino[2,3-
g]quinoxaline (4). 3 (0.6 g, 0.65 mmol) was combined with 2-
tributylstannylthiophene in 20 mL of dry DMF. The reaction mixture
was degassed, followed by addition of Pd(PPh3)2Cl2 (23 mg, 0.033
mmol). The reaction was heated to 70 °C and allowed to react 24 h.
Upon cooling the mixture was poured into water and extracted with
CH2Cl2. The organic phase was washed with water (2×), Na2CO3
solution (1×), and NaCl solution (1×) and finally dried over sodium
sulfate. The solvent was reduced in volume, and product was
precipitated in methanol to yield a blue solid (0.52 g, 86%). 1H
NMR (CDCl3, 400 MHz): δ (ppm) 8.37 (d, 2H), 7.91 (d, 8H), 7.80
(d, 2H), 7.71 (d, 8H), 7.40 (dd, 2H). 13C NMR (THF-d8, 125 MHz):
δ (ppm) 152.51, 143.23, 137.87, 135.80, 135.13, 132.16, 131.71,
130.48, 127.18, 126.46, 124.37. MS (MALDI-TOF, DCTB): m/z =
922.2 [M+].
1
colorless liquid (3.7 g, 36%). H NMR (CDCl3, 400 MHz): δ (ppm)
7.37 (d, 2H), 7.02 (d, 2H), 2.49 (d, 2H), 1.54 (broad, 1H), 1.26 (m,
8H), 0.86 (m, 6H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 140.81,
131.08, 130.91, 119.19, 40.99, 39.46, 32.19, 28.77, 26.28, 22.99, 14.12,
10.73. GC-MS: m/z 268 [M]+.
1,2-Bis(4-(2′-ethylhexyl)phenyl)-1,2-ethanedione (7). 1-Bromo-4-
(2′-ethylhexyl)benzene (6) (3.5 g, 13 mmol) was added to a two-
necked flask. After the addition of THF (12.5 mL), n-butyllithium
(4.96 mL, 2.5 M, 12.4 mmol) was added dropwise at −78 °C. To a
suspension of piperazine in 12.5 mL of dry THF, the lithiated reactant
was added dropwise with a canule over several minutes at −78 °C. The
reaction mixture was then slowly allowed to come to room
temperature. The next day, 100 mL of HCl (10% v/v) was added
to the reaction mixture, and the product was extracted with CH2Cl2.
The organic phase was washed with water (2×) and NaCl solution
(1×) and finally dried over sodium sulfate. The solvent was reduced in
volume to yield an orange liquid, which was further purified using
column chromatography (silica gel, petroleum ether:CH2Cl2, 2:1).
1
The product was collected as a yellow oil (2,15 g, 84%). H NMR
(CDCl3, 400 MHz): δ (ppm) 7.88 (d, 4H), 7.29 (d, 4H), 2.60 (d,
I
dx.doi.org/10.1021/ma302404y | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
4H), 1.61 (s, 2H), 1.27 (m, 16H), 0.87 (t, 12H). 13C NMR (CDCl3,
100 MHz): δ (ppm) 194.53, 150.20, 130.76, 129.89, 129.78, 40.98,
40.42, 32.27, 28.75, 25.37, 22.96, 14.11, 10.70. MS (CI): m/z = 934.0
[M]. Elemental analysis calcd for C30H42O2: C: 82.90%, H: 9.74%.
Found: C: 82.76%, H: 9.59%.
Present Address
§Chang-Qi Ma: Printable Electronic Research Center, Suzhou
Institute of Nano-Tech and Nano-Bionics, Chinese Academy of
Sciences, 398 Ruoshui Road SEID SIP, Suzhou 215123, P. R.
China.
2,3,7,8-Tetra(4′-ethyl-2″-hexylphenyl)pyrazino[2,3-g]quinoxaline
(8). 7 (0.250 g, 0.57 mmol) and 1,2,4,5-benzenetetraamine
tetrahydrochloride (82 mg, 0.29 mmol) were dissolved in 20 mL of
degassed acetic acid. The reaction mixture was heated to 125 °C for 24
h. After cooling to room temperature, the mixture was poured into 100
mL of water and extracted with diethyl ether. The organic layer was
washed with water (2×), Na2CO3 solution (1×), and NaCl solution
(1×) and finally dried over sodium sulfate. The solvent was removed,
and purification was done using column chromatography (silica gel,
petroleum ether:CH2Cl2 1:0 → 1:1) to yield a yellow solid (0.40 g,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Dutch Polymer Institute (program no. 661) for
financial support as well as Mike Wendel (University of Ulm)
for performing the polymer molecular weight determinations.
1
74%). H NMR (THF-d8, 400 MHz): δ (ppm) 8.86 (s, 2H), 7.60 (d,
REFERENCES
■
8H), 7.21 (d, 8H), 2.63 (d, 8H), 1.67 (m, 4H), 1.36 (m, 32H), 0.95 (t,
24H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 155.36, 143.48, 140.31,
136.15, 129.72, 129.08, 128.40, 41.09, 39.99, 32.30, 28.84, 25.46,
23.03, 14.16, 10.84. MS (MALDI-TOF, dithranol): m/z = 936.0 [M+].
Elemental analysis calcd. for C66H86N4: C: 84.74%, H: 9.27%, N:
5.99%. Found: C: 84.83%, H: 9.38%, N: 5.71%.
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g]quinoxaline (9). Pyrazinoquinoxaline 8 (250 mg, 0.27 mmol) was
combined with NaHCO3 (45 mg, 0.53 mmol) in 5 mL of CHCl3. The
solution was cooled to 0 °C, and Br2 (85 mg, 0.53 mmol) was added.
The reaction flask was sealed and heated to 80 °C for 24 h. After
cooling, an additional 1 equiv of Br2 (42 mg) and NaHCO3 (23 mg)
was added to the reaction flask and allowed to further react at 50 °C
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4H), 1.33 (m, 32H), 0.92 (t, 24H). 13C NMR (CDCl3 100 MHz): δ
(ppm) 156.21, 145.10, 139.56, 137.12, 131.39, 130.06, 125.07, 42.39,
41.08, 33.60, 30.06, 28.69, 24.24, 14.82, 11.48. (MALDI-TOF,
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72.32%, H: 7.66%, N: 5.03%.
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Polymer P2. 2,5″-Distannylbithiophene (18 mg, 0.036 mmol) was
added to a dry 25 mL two-necked flask, which was evacuated and
purged with argon (2×) before addition of 9 (40 mg, 0.036 mmol).
Toluene (2.5 mL) was added to the flask followed by further degassing
using argon. After 10 min, Pd2(dba)3 (1.7 mg, 0.002 mmol) and P(o-
tolyl)3 (3.4 mg, 0.011) were added to the reaction mixture followed by
further degassing for another 15 min. The reaction was heated to 100
°C and allowed to react for 60 h. Upon cooling the crude product was
precipitated in methanol and underwent successive Soxhlet extraction
in methanol, acetone, hexane, and THF. The THF fraction was then
reprecipitated in methanol and dried under vacuum to yield P1 (39
mg) in 95% yield. 1H NMR (THF-d8 400 MHz): δ (ppm) 8.81 (broad
s, 2H), 7.86 (broad s, 8H), 7.66 (broad s, 2H), 7.22 (broad s, 8H),
2.58 (broad s, 8H), 1.63 (broad s, 4H), 1.28 (broad s, 32H), 0.85
(broad s, 24H). Mn = 53 000 g/mol, Mw = 113 000 g/mol, and PDI =
2.1.
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*E-mail gisela.schulz@uni-ulm.de (G.L.S.), peter.baeuerle@
uni-ulm.de (P.B.); Fax (+49) 731-502-2840.
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