Efficient synthesis of a regioregular oligothiophene photovoltaic dye molecule, MK-2, and related compounds: A cooperative hypervalent iodine and metal-catalyzed synthetic route

Article abstract of DOI:10.1002/chem.201203503

We have successfully established an efficient route to the core structure of donor-acceptor head-to-tail (H-T)-linked regioregular oligothiophenes, which includes the following key synthetic steps, that is, hypervalent iodine induced direct and regioselec

Full text of DOI:10.1002/chem.201203503

FULL PAPER
DOI: 10.1002/chem.201203503
Efficient Synthesis of a Regioregular Oligothiophene Photovoltaic Dye
Molecule, MK-2, and Related Compounds: A Cooperative Hypervalent
Iodine and Metal-Catalyzed Synthetic Route
Toshifumi Dohi, Nobutaka Yamaoka, Shota Nakamura, Kohei Sumida,
Koji Morimoto, and Yasuyuki Kita*[a]
Abstract: We have successfully established an efficient route to the core structure
of donor–acceptor head-to-tail (H–T)-linked regioregular oligothiophenes, which
includes the following key synthetic steps, that is, hypervalent iodine induced
À
Keywords: C C coupling · donor–
acceptor systems · dyes/sensitizers ·
direct and regioselective coupling of thiophenes and the use of the obtained bithio-
phenes as excellent coupling substrates for the Suzuki and Stille couplings. The
iodine · sulfur heterocycles
versatility of this new approach is highlighted in the dramatic improvement of the
yield (ca. 59% overall yield) of MK-2, a high-performance organic dye, for photo-
voltaic applications.
Introduction
The dye-sensitizing solar cells (DSSCs), which were first de-
veloped by the Grꢀtzel groupꢁs pioneering research in the
early 1990s,[1] meet a consensus of mankind in demanding
the clean production of renewable energy from sunlight for
global environmental conservation. Originally, the DSSCs
typically used ruthenium-based hybrid metal–organic dyes
to achieve high levels of solar-energy-to-electricity conver-
sion efficiency (h).[2] During the development of more effi-
cient DSSCs, optimizing the properties and cost of the dye
photosensitizers has been further required. Thus, systematic
molecular engineering that contributes to the multidiscipli-
nary fields of the photovoltaic material development has
been presented to date based on a wide variety of artificial
organic compounds with their successful use as dye sensitiz-
ers.[3]
One of the impressive research studies on such dye devel-
opments for solar cells has been made in this century by the
group of Hara and Koumura, who developed the oligothio-
phene-based MK dyes with the donor–acceptor moieties as
nonmetal sensitizers with a promising efficient photovoltaic
performance close to the ruthenium-based dyes for the
DSSCs.[4] For the molecular design, the regioregular long
alkyl chains at the oligothiophenes are very important to
suppress aggregation of the dye molecules and to increase
the molar extinction coefficient.[5] Alkyl substituents prevent
the approach of electron carriers to the TiO2 surface, which
decreases the reorganization energy of the dye. Therefore,
dyes with unsubstituted oligothiophene donor–acceptor link-
ages (type A) showed poorer h values than those with sub-
stituted oligothiophene linkages (type B) in photovoltaic ap-
plications.[6] The screening of the MK-type compounds has
proved that MK-2 containing the tetrameric oligothiophene
spacer showed the highest h value of 8.3%. The nonmetal
organic dyes have several advantages in view of the material
cost and green chemistry and the characteristics of the ex-
tinction coefficients and their structural variations. Since the
discovery of MK-2, an exhaustive number of artificial
donor–acceptor-type compounds tethered by the p-thio-
phene spacers have been investigated for understanding the
structure–activity relationship on the photovoltaic function
of the organic dye sensitizers with the challenge of achieving
a higher h value for the purpose of developing a more effi-
cient solar cell.[7,8]
[a] Dr. T. Dohi, N. Yamaoka, S. Nakamura, K. Sumida, K. Morimoto,
Prof. Dr. Y. Kita
The MK dyes are comprised of the donor and acceptor
units at the edges of the regioregular oligothiophene centers.
Despite the elegant molecular design of the MK dyes and
related compounds as photovoltaic materials, the regioselec-
tive construction of the oligothiophene structures and ex-
College of Pharmaceutical Sciences, Ritsumeikan University
1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577 (Japan)
Fax : (+81)77-561-5829
E-mail: kita@ph.ritsumei.ac.jp
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pansion of the p-moieties by the conventional cross-coupling
methods, such as the Suzuki and Stille reactions, usually re-
quired multiple steps,[9] thus making the preparation of
these dyes quite cumbersome. Hence, the synthesis of MK-2
in the original report consists of a total of 10 steps with an
overall 13% yield by starting from the commercially availa-
ble 3-hexylthiophene, in which the head-to-tail (H–T) thio-
phene linkage and donor carbazole were embedded stepwise
by repetition of the conventional Suzuki coupling.[4] The
MK-2 synthesis is illustrated in Scheme 1. During the syn-
the construction of the oligothiophenes that involve selec-
À
tive recognition of the thiophene C H bonds in the pres-
ence of a halogen functional group for further diversifica-
tion,[10] which has realized the concise synthesis of MK-2 in
a similar yield to that of the original report.[10b]
We have also been engaged in developing the oxidative
biaryl coupling reactions of thiophenes and another series of
heteroaromatic compounds for synthesizing the oligomers in
our continuous studies of hypervalent iodine chemistry.[11,12]
Thus, the coupling of electron-rich thiophenes would be in-
duced by the combination of a hypervalent iodine reagent
with a suitable Lewis acid.[11] After having been inspired by
the importance of the oligothiophene structures in many or-
ganic materials,[13] we have recently extended the strategy to
3-alkylthiophenes in a highly regioselective manner. As a
result, a novel direct route to the H–T bithiophenes from 3-
alkylthiophenes has been realized for the first time with re-
gioselectivity control by activation of the oxidant with bro-
motrimethylsilane.[14] Our direct approach from the mono-
mers has significant advantages as it can avoid the stoichio-
metric metalation or halogenation steps of the aromatics
that increases the synthetic steps,[15] which has stimulated
our regiocontrolled bithiophene synthesis by reducing the
difficulty in the synthesis of the oligothiophene-based dyes.
At this point, we have commenced the synthesis of MK-2
and now suggest a more practical convergent synthetic route
to MK-2 in Scheme 2 based on the progressive use of the bi-
thiophene molecules directly obtained by the reaction of the
thiophene monomer with hypervalent iodine reagents.
Scheme 1. Original linear synthesis of MK-2.
thesis, all the aryl–aryl bonds in MK-2 were connected by
the Suzuki coupling, and the oligothiophene structure was
repeatedly elongated by introducing the thiophene mono-
mer employing the corresponding thienyl boronate as the
main component (linear synthesis). This scheme must deal
with a number of coupling precursors derived from the thio-
phene monomer, dimer, and trimer as intermediates, which
causes the large number of synthetic steps and would thus
restrict the production of MK-2 to between 6–27% overall
yields. To reduce the number of linear steps of the synthesis,
Mori et al. has recently proposed new coupling strategies for
We now report an efficient coupling route to the core
structure of the donor–acceptor regioregular oligothio-
phenes for photovoltaic applications by utilizing the unsym-
metrical bithiophenes, which can be effectively obtained by
our regioselective hypervalent iodine coupling technology,
as a crucial intermediate. Based on the cooperative use of
the hypervalent iodine and conventional metal-catalyzed
Scheme 2. Convergent synthesis of MK-2 utilizing bithiophene units.
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Regioregular Oligothiophene Photovoltaic Dye Molecules
FULL PAPER
couplings, the significance of this approach is highlighted by
the dramatic improvement in the yield of the MK-2 dye.
The highly efficient Suzuki coupling procedures utilizing the
bithiophene as the electrophile for the middle-stage intro-
duction of the donor carbazole moiety and a series of poten-
tially useful electron-donors have also been documented.
dants would typically produce symmetric homodimers as the
major product.[19]
Accordingly, we performed a gram-scale preparation of
the required H–T bithiophene unit 1 by the hypervalent
iodine oxidative coupling as the first step of the MK-2 syn-
thesis from the commercial 3-hexylthiophene (Scheme 4).
Results and Discussion
Efficient approach to the regioregular bithiophene units by
using the hypervalent iodine reagent: The unsymmetric bi-
thiophene structure, the focus of this study, is readily acces-
sible by using the hypervalent iodine oxidative coupling of
3-alkylthiophene.[16] The exclusive formation of the H–T
dimers of 3-alkylthiophene can be well explained by the re-
action mechanism shown in Scheme 3. To the hypervalent
Scheme 4. Synthesis of 3-hexylthiophene H–T-type dimer 1 and its func-
tionalized compounds 1’ and 1’’: a) 3-hexylthiophene, PhI(OH)OTs
(HTIB, 1 equiv), TMSBr (2 equiv) in HFIP, RT, 3 h; b) bithiophene 1, n-
butyllithium (1 equiv) in tetrahydrofuran (THF), À788C, 2 h; then molec-
ular bromine (1.05 equiv), 35 min; c) bithiophene 1, LDA (1 equiv) in
THF, À408C, 10 min.; then tri-n-butyltin chloride (1.05 equiv), À788C to
RT, 3 h.
With the standard procedure that uses HTIB and bromotri-
methylsilane, the desired H–T-linked coupling product 1 was
formed in a perfect regioselective manner (over 99%) in
preference to other regioisomers. After chromatography or
distillation, the pure bithiophene 1 was obtained in 78%
yield, which was then converted to the halogen electrophile
1’ and organometallic nucleophile 1’’ in yields of 90 and
91% by selective lithiation[20] followed by quenching with
molecular bromine (for halide 1’) and tri-n-butyltin chloride
(for 1’’), respectively, for the later-stage metal-catalyzed
cross-couplings. These bithiophenes 1, 1’, and 1’’ are stable
in a refrigerator for at least several months without special
storage conditions.
Scheme 3. Straightforward approach toward H–T bithophenes by hyper-
valent iodine induced regioselective coupling. R=electron-rich groups,
such as alkyl.
iodine reagent, the alkyl thiophenes are inherently reactive
at the most electron-rich 2-position of the thiophene ring,
rapidly yielding the corresponding thienyliodonium
as the sole product by the dehydrative condensation with
[hydroxy
(tosyloxy)iodo]benzene (HTIB, Koserꢁs reagent).[17]
ACHTUNGTRENUN(NG III) salts
ACHTUNGTRENNUNG
Interestingly, the formed iodonium salts, on the other hand,
can exclusively react at the other remote a-position of the
thiophene ring with external aromatic nucleophiles by suita-
ble activations.[18] The fluoroalcohols, that is, hexafluoroiso-
propanol (HFIP), can promote both the reaction processes
as the highly polar and non-nuleophilic protic solvent. The
two-reaction sequence formally enables the arylation at the
less reactive 5-position of the 3-alkylthiophenes. Based on
this unique reactivity by switching to a concept through the
iodonium salts for modulating the reactivity of the heteroar-
omatic compounds, the H–T-linked bithiophenes can be di-
rectly obtained in one pot from the 3-alkylthiophenes when
using the thiophenes themselves as the aromatic nucleo-
philes. The two-thiophene connection can selectively occur
at the 5-position of the thiophene ring of the iodonium salts
and the 2-position of the 3-alkylthiophenes, resulting in the
formation of the unsymmetrical bithiophene without pro-
ducing other possible regioisomers. This type of oxidative
coupling is quite unique for hypervalent iodine reagents, as
other oxidative coupling methods that use metal-based oxi-
Bithiophenes as the efficient coupling partner for donor
molecules: In previous reports,[4,10] the efficiency of the in-
stallation of the donor carbazole moiety to the thiophene
core of MK-2 has remained unsatisfactory, leading to the de-
creasing overall yields during the syntheses. In fact, the reac-
tion of the thiophene boronate with the bromocarbazole
during the early stage was reported in the original report[4a]
to only allow the Suzuki coupling in 46% yield for the in-
corporation of the donor aromatic to the thiophene mono-
mer [Eq. (1)]. Thereafter, the same research group modified
the coupling substrates for a better production yield of the
target compound, which afforded the same MK-2 intermedi-
ate in a moderate yield (74%) by the Kumada coupling
[Eq. (2)]. Later, Mori and co-workers applied their elegant
oxidative C–H arylation strategy of thiophenes to aryl io-
dides for introducing the carbazole group to the thiophene
[Eq. (3)] during a shorter synthetic route to MK-2 (nine
steps from 3-hexylthiophene), despite the lower overall
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Y. Kita et al.
yield (<6%).[10a] They have also further improved the syn-
thetic steps to be more attractive by utilizing the controlled
Thus, the usability of the bithiophene 1’ as an electrophile
in cross-coupling was first investigated before carrying out
the synthesis of MK-2. We tested the Suzuki couplings for
the extensive range of organoboronic compounds in Table 1
derived from the different types of electron-rich aromatic
À
metalation of thiophene C H bonds by using the Knochel–
Hauser base (TMPMgCl·LiCl, TMPH=2,2,6,6-tetramethyl-
piperidine) or the combination of a Grignard reagent and a
catalytic amount of a secondary amine, followed by alterna-
tive cross-coupling with iodocarbazole, but the aryl–aryl
bond-forming step of the thiophene and carbazole [Eq. (4)]
is the most problematic in terms of the yield during the
entire process.[10b] All these reported cases have the synthet-
ic hurdle of introducing the donor moiety.
Table 1. Suzuki coupling of the bithiophene halide 1’ with carbazole
compounds and extended series of donor aromatics.[a]
Entry
Organoboron
Compound
Product,
Yield [%][b]
1
2, >99
2
3
2’, 96
2’’, 93
4
5
2’’’, >99
2’’’’, >99
[a] Reactions were carried out by using 1.1 equivalents of the organobo-
AHCTUNGERTGrNNUN on compound in the presence of 4 mol% palladium catalyst at reflux in
degassed toluene overnight. See details in the Experimental Section.
[b] Isolated yield after purification.
amines for the purpose of validating the reactivity of the bi-
thiophene halide 1’ as a universal module for the installation
of the donor moieties to the oligothiophene-based dyes.
Thus, the reaction of the bithiophene 1’ was examined by
using a 4 mol% loading of the palladium catalyst with only
a slight excess of the boronic acid in toluene at reflux in the
presence of the phase-transfer catalyst (Aliquat 336) and a
weak base (Table 1). Surprisingly, a series of electron-rich
carbazole[21] (Table 1, entries 1 and 2), indole[22] (Table 1,
entry 3), aromatic,[23] and aliphatic aryl amines[24] (Table 1,
entries 4 and 5) could be readily introduced to the bithio-
phene backbone in unbelievably high yields by the Suzuki
couplings by using the bithiophene halide 1 without any op-
timization of the reaction conditions. Even in these cases,
only
a slight excess of the organoboron compounds
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Regioregular Oligothiophene Photovoltaic Dye Molecules
FULL PAPER
(1.1 equiv) was required to completely convert the bithio-
phene molecule 1’.
Accordingly, the Suzuki coupling reaction of the bithio-
phene halide 1’ with the carbazole boronic acid derivative
was subsequently conducted to furnish the aryl–aryl bond
between the oligothiophene part and carbazole donor of
MK-2 (Scheme 5). We were very pleased to confirm the re-
Scheme 6. Completion of the synthesis of MK-2: a) carbazole–bithio-
phene coupling product 2, n-butyllithium (1.2 equiv) in THF, À788C,
30 min; then elemental iodine (2 equiv), 30 min; b) iodobithiophene 3,
stannyl bithiophene 1’’ (see Scheme 4, 1.5 equiv), cat. tetrakis(triphenyl-
phosphine)palladium (4 mol%) in degassed toluene, reflux, 12 h; c) quar-
terthiophene 4, phosphorus oxychloride in DMF, 08C, 30 min; then 708C
for 7 h; d) quaterthiophene aldehyde 5, cyanoacetic acid (2 equiv) in a
mixed solvent of acetonitrile, toluene, and piperidine, reflux, 4 h.
Scheme 5. High-yielding installation of donor carbazole by utilizing the
bithiophene halide 1’ as the electrophile for a Suzuki coupling reaction:
a) bithiophene halide 1, 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)carbazole (1.1 equiv), catalytic amount of tetrakis(triphenylphos-
ACHTUNGTRENNUNGphine)ACHTUNGTRENNUNGpalladium (4 mol%), a few drops of Aliquat 336, and 2m aq.
sodium carbonate in degassed toluene, reflux, 12 h.
the oxidative addition to be markedly slower during the
Stille coupling. The spectra of the obtained tetramer 4 are
in perfect agreement with an authentic sample from a previ-
ous study.[4,10]
markably high-yielding incorporation of the donor carbazole
under the stated reaction conditions. The use of 4 mol% of
the palladium catalyst was sufficient to maintain the reac-
tion rate with efficiencies similar to that in Table 1. It should
be noted that one of the important steps in the reported
MK-2 syntheses produces an unsatisfactory yield of the car-
bazole coupling.[4,10] The bithiophene 1’ thus seems to signifi-
cantly facilitate the metal-catalyzed coupling efficiency for
installing the donor carbazole moiety. On the other hand,
the use of a coupling combination of bithiophene boronic
acid and a carbazole halide would give rise to the same
product 2 in poorer yield under the same catalytic condi-
tions.[25,26]
Finally, the synthesis of MK-2 through this synthetic route
has been completed by attaching the end-capping acceptor,
that is, the vinyl cyanoacetic acid moiety, to the donor-carry-
ing tetramer 4 according to the original reports[4] by modi-
fied two-step procedures (steps 6 and 7, see the Experimen-
tal Section). Our short synthetic route to MK-2 involves a
total of seven steps and can provide the target compound in
a shorter reaction time and in a markedly higher yield (over
59% overall yield) than that of the original (13–26%)[4] and
later-reported approaches (ca. 27%)[10] when using the same
commercial material. The notable features of the present
strategy are characterized by the high yields of all the syn-
thetic steps, the benefits of which should come from the
smart preparation of the regioregular bithiophene 1 and suc-
cessful use of the coupling precursors 1’ and 1’’ during the
synthesis.
Based on the significant observations, we consider that
the present coupling route to the regioregular oligothio-
phene compounds involving the bithiophenes as key inter-
mediates has significant merits as a guideline of being valu-
ACHTUNGTRENNUNGable not only to provide the efficient preparation of the
known high-performance synthetic dyes, for example, MK-2,
but also to influence future synthetic design of a wide array
of the related oligothiophene-based dyes.[27]
Conclusion
Completion of the MK-2 synthesis: In clear contrast to the
previous reports, our synthetic approach based on the cou-
pling of the bithiophene 1’ has, unexpectedly, realized the
smooth and perfect introduction of the carbazole group. For
the donor-installed thiophene dimer 2, iodination by selec-
tive a-lithiation of the terminal thiophene ring was then car-
ried out to produce the coupling precursor 3 for elongation
of the thiophene core (Scheme 6, step 4). The functionalized
dimer 3 obtained in 91% yield was successively subjected to
the standard Stille coupling conditions with the discretely
prepared bithiophene nucleophile 1’’ in Scheme 4, giving the
corresponding regioregular thiophene tetramer 4 in an
almost quantitative yield after heating at reflux for 12 h.
The use of the bromide instead of the iodide 3 decreased
the product yield and reaction efficiencies because it forces
We have described an efficient synthetic route to donor–ac-
ceptor-linked regioregular oligothiophenes, specifically, the
high-performance MK-2 dye, for photovoltaic applications
based on the cooperative use of the hypervalent iodine and
metal-catalyzed coupling technologies for the construction
of the p-expanded oligoarene architectures. All the steps to
the target molecules can proceed in high yields and the
overall yield for the MK-2 synthesis is dramatically im-
proved to approximately 59% in comparison to the known
reported sequences. The unsymmetrical bithiophene 1,
which is directly obtainable in good yield from 3-alkylthio-
phene by hypervalent iodine coupling with complete regio-
chemical control, serves as the key and excellent intermedi-
ate in this method. During the synthesis, we also noted that
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Y. Kita et al.
chloride (50 mL), and was dried over anhydrous sodium sulfate. The sol-
vent was removed under reduced pressure and the resulting residue was
purified by column chromatography on silica gel by using hexanes as the
eluent to give the regioselectively brominated H–T bithiophene 1’ (3.7 g,
9.0 mmol) in 90% yield.
5-Bromo-3,4’-dihexyl-2,2’-bithiophene (1’):[20a] Colorless oil; 1H NMR
(400 MHz, CDCl3): d=0.84–0.89 (m, 6H), 1.27–1.30 (m, 12H), 1.54–1.62
(m, 4H), 2.57 (t, 2H, J=8.4 Hz), 2.64 (t, 2H, J=8.4 Hz), 6.85 (s, 2H),
6.87 ppm (s, 1H).
the bithiophene unit 1’ clearly facilitated the metal-catalyzed
couplings for installing the donor moiety to the p-linked
oligoACHTUNGTRENNUNGthiophene systems, and thus the highly efficient Suzuki
coupling procedure utilizing the bithiophene electrophile 1’
toward a series of potentially useful electron donors was
demonstrated to contribute to the future study of the design
and access to the related regiocontrolled thiophene mole-
cules.
Step 2b: Preparation of organostannyl bithiophene compound 1’’ as a
Stille coupling precursor (Scheme 4): A solution of bithiophene 1 (2.47 g,
6.0 mmol) in anhydrous THF was cooled to À408C under a nitrogen at-
mosphere and treated with an equimolar amount of lithium diisopropyl-
Experimental Section
AHCTUNGERTGaNNUN mide (LDA) in THF, which was freshly prepared from distilled diisopro-
pylamine and n-butyllithium (in hexanes). The lithiation was conducted
for 10 min, and the solution was then treated at À788C with tri-n-butyltin
chloride (2.05 g, 6.3 mmol). The reaction mixture was allowed to warm to
room temperature over a 3 h period, then the solvent was evaporated to
dryness. The resulting oily residue was dissolved in hexanes and filtered
to remove solid lithium chloride byproduct to obtain compound 1’’ in ap-
proximately 91% yield. The organotin compound was used without fur-
ther purification for the synthetic step 5 in the synthesis of MK-2.
General: Melting point (m.p.) was measured by Stuart melting-point ap-
paratus SMP3 AC input 100V. 1H NMR (and 13C NMR) spectra were re-
corded by a JEOL JMN-400 spectrometer operating at 400 or 300 MHz
(100 or 75 MHz for 13C NMR spectra) in CDCl3 at 258C with tetra
methylsilane as an the internal standard. The data are reported as fol-
lows: chemical shift in ppm (d), integration, multiplicity (s=singlet, d=
doublet, t=triplet, q=quartet, m=multiplet), and coupling constant
(Hz). The IR spectra were obtained by using a Hitachi 270–50 spectrom-
eter. The HRMS were performed by the Elemental Analysis Section of
Osaka University.
5-Tributylstannyl-3,4’-dihexyl-2,2’-bithiophene (1’’): Brown oil; 1H NMR
(400 MHz, CDCl3): d=0.86–0.92 (m, 15H), 1.06–1.11 (m, 6H), 1.28–1.35
(m, 18H), 1.52–1.59 (m, 4H), 2.57 (t, 2H, J=8.4 Hz), 2.74 (t, 2H, J=
8.4 Hz), 6.83 (s, 1H), 6.90 (s, 1H), 6.92 ppm (s, 1H); IR (KBr): n˜ =2954,
2923, 2854, 1522, 1463, 1415, 1375, 1291, 1184, 1071, 1045, 1020, 962, 935,
Analytical TLC was performed on MERCK silica gel, grade 60 F254. The
spots and bands were detected by UV irradiation (254, 365 nm). Column
chromatography for isolation of the products was carried out on Merk
Sillica Gel 60 (230–400 mesh). Unless otherwise noted, all the chemicals
for the reactions and column chromatography in this study were obtained
from several commercial suppliers and used as received without further
purification.
913, 878, 835, 763, 747, 724, 686, 674, 649, 638, 619, 612, 601 cmÀ1
.
Step 3: High-yielding Suzuki coupling utilizing the bithiophene halide 1’
(Scheme 5): Commercially available 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)carbazole (185 mg, 0.58 mmol) and tetrakis(triphenyl-
phosphine)palladium (23 mg, 0.02 mmol) in degassed toluene (2 mL), in-
cluding a few drops of Aliquat 336 and 2m aq. sodium carbonate solution
(0.33 mL), were added to a stirred mixture of the bithiophene halide 1’
(206 mg, 0.50 mmol). The solution was heated at reflux with vigorous stir-
ring for 12 h under a nitrogen atmosphere. The mixture was poured into
water (5 mL) and extracted with ethyl acetate. The organic extract was
successively washed with water (5 mL) and brine (5 mL). After drying
with anhydrous sodium sulfate, the solvent was evaporated and the resi-
due was purified by column chromatography on silica gel (eluent: hex-
anes/ethyl acetate 50:1) to give the pure coupling product (264 mg,
0.50 mmol) quantitatively.
9-Ethyl-3-[3,4’-dihexyl(2,2’-bithiophene)-5-yl]carbazole (2):[4] Orange oil;
1H NMR (400 MHz, CDCl3): d=0.86 (t, 3H, J=6.6 Hz), 0.91 (t, 3H, J=
6.6 Hz), 1.24–1.40 (m, 12H), 1.48 (t, 3H, J=7.1 Hz), 1.60–1.73 (m, 4H),
2.60 (t, 2H, J=7.8 Hz), 2.70 (t, 2H, J=7.8 Hz), 4.40 (q, 2H, J=7.1 Hz),
6.80 (s, 1H), 7.04 (d, 1H, J=1.3 Hz), 7.09 (s, 1H), 7.29–7.23 (m, 1H),
7.53–7.42 (m, 3H), 7.56 (dd, 1H, J=8.4, 1.8 Hz), 8.12 (d, 1H, J=7.7 Hz),
8.17 ppm (d, 1H, J=1.1 Hz); 13C NMR (100 MHz, CDCl3): d=13.8, 14.0,
14.1, 22.6 (ꢃ2), 28.8, 29.0, 29.2, 30.3, 30.5, 31.0, 31.6, 31.7, 37.6, 108.3,
108.6, 118.4, 119.0, 120.5, 121.1, 122.8, 123.0, 124.5, 125.0, 125.8, 125.9,
127.1, 135.0, 137.5, 137.9, 138.7, 139.2, 140.3, 144.0 ppm.
MK-2 synthesis
Step 1: Direct synthesis of H–T-linked bithiophene 1 by the hypervalent
iodine induced regioselective coupling (Scheme 4): A gram-scale prepara-
tion: In an open flask, HTIB (7.7 g, 20 mmol) and then bromotrimethylsi-
lane (5.4 mL, 40 mmol) were added to a stirred solution of 3-hexylthio-
phene (6.7 g, 40 mmol) in hexafluoroisopropanol (100 mL) at room tem-
perature; the color of the solution immediately changed to brown. After
stirring for 3 h, dichloromethane (300 mL) and saturated aq. sodium car-
bonate (200 mL) were successively added to the reaction mixture. The or-
ganic layer was then separated and the solvents were evaporated to dry-
ness. The residue was evaporated and subjected to column chromatogra-
phy on silica gel (eluent/hexanes) or bulb-to-bulb distillation under re-
duced pressure to give the regioselectively coupled H–T bithiophene 1
(5.3 g, 15.6 mmol) in 78% yield. The regiochemistry of the obtained bi-
thophene product 1 was confirmed by comparing it with an authentic
sample.
3,4’-Dihexyl-2,2’-bithiophene (1):[14] Pale-yellow oil; b.p. 1658C
(0.02 mmHg); 1H NMR (300 MHz, CDCl3): d=0.84–0.92 (m, 6H), 1.20–
1.35 (m, 12H), 1.52–1.70 (m, 4H), 2.63 (t, 2H, J=8.4 Hz), 2.74 (t, 2H,
J=8.4 Hz), 6.89–6.92 (m, 2H), 7.03 (s, 1H), 7.12 ppm (d, 1H, J=5.5 Hz);
13C NMR (75 MHz, CDCl3): d=14.1, 22.6, 29.0, 29.1, 29.2, 29.7, 30.4,
30.5, 30.7, 31.6, 31.7, 119.9, 123.4, 124.8, 127.3, 129.9, 130.9, 135.8, 139.3,
143.5 ppm.
Step 4: a-Iodination of the thiophene ring of carbazole–bithiophene cou-
pling product 2 (Scheme 6): A solution of n-butyllithium (1.57m in hex-
anes, 1.0 mL) was added dropwise at À788C under a nitrogen atmos-
phere to a stirred solution of carbazole–bithiophene coupling product 2
(686, 1.3 mmol) in THF (10 mL). After stirring for 30 min, elemental
iodine (660 mg, 2.6 mmol) was added in one portion and stirring was con-
tinued for a further 30 min. The reaction mixture was poured into water
(5 mL) and the organic phase was separated. The aqueous phase was ex-
tracted with diethyl ether (30 mL) twice. The combined organic layers
were dried over anhydrous sodium sulfate and concentrated in vacuo. Pu-
rification of the residue by column chromatography on silica gel by using
hexanes/ethyl acetate (50:1) as the eluent afforded the desired a-iodinat-
ed product 3 (771 mg, 1.18 mmol) in 91% yield.
Step 2a: Selective bromination of the H–T bithiophene 1 (Scheme 4):
The H–T bithiophene 1 (3.34 g, 10.0 mmol) was added in a flame-dried
flask under a nitrogen atmosphere. After lowering the temperature to
À788C, n-butyllithium (2.5m solution in hexanes, 4.4 mL, 11.0 mmol) was
added dropwise over 1.5 h to the stirred solution, and then the resulting
solution was stirred for an additional 1 h. At the same temperature, a sol-
ution of bromine (1.68 g, 10.5 mmol) in dry tetrahydrofuran (THF, 5 mL)
was slowly added for about 15 min. The mixture was stirred for a further
20 min and a few drops of an aqueous methanolic solution of sodium
thioACHTUNGTRENNUNGsulfate were added at that temperature. The reaction mixture was
warmed to room temperature, poured into ice-cold water and extracted
with diethyl ether. The organic phase was washed successively with 3%
aq. sodium thiosulfate (30 mL), water (20 mL), and 10% aq. sodium
9-Ethyl-3-[3,4’-dihexyl-5’-iodo(2,2’-bithiophene)-5-yl]carbazole
(3):[10a]
Yellow sticky oil; 1H NMR (400 MHz, CDCl3): d=0.94–0.85 (m, 6H),
1.43–1.23 (m, 12H), 1.45 (t, 3H, J=7.1 Hz), 1.74–1.60 (m, 4H), 2.56 (t,
2072
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Chem. Eur. J. 2013, 19, 2067 – 2075

Regioregular Oligothiophene Photovoltaic Dye Molecules
FULL PAPER
2H, J=7.8 Hz), 2.75 (t, 2H, J=7.9 Hz), 4.38 (q, 2H, J=7.2 Hz), 6.82 (s,
1H), 7.17 (s, 1H), 7.27–7.25 (m, 1H), 7.40 (d, 1H, J=8.7 Hz), 7.42 (d,
1H, J=8.6 Hz), 7.49 (ddd, 1H, J=8.1, 7.0, 0.9 Hz), 7.70 (dd, 1H, J=8.4,
1.5 Hz), 8.14 (d, 1H, J=7.6 Hz), 8.29 ppm (d, 1H, J=1.5 Hz); 13C NMR
(100 MHz, CDCl3): d=13.9, 14.3, 22.7, 22.8, 29.1, 29.4, 29.7, 30.1, 30.8,
31.8, 31.9, 32.5, 37.7, 73.5, 108.7, 108.8, 117.6, 119.2, 120.7, 123.0, 123.5,
124.0, 124.9, 125.3, 126.0, 126.1, 128.5, 139.7, 140.5, 141.0, 141.4, 143.8,
147.6 ppm.
2-Cyano-3-[5’’’-(9-ethyl-9H-carbazol-3-yl)-3’,3’’,3’’’,4-tetrahexyl(2,2’:5’,2’’:
5’’,2’’’-quaterthiophene]-5-yl]-2-propenoic acid (MK-2):[4] Dark-red solid;
1H NMR (400 MHz, [D8]THF): d=0.90–0.94 (m, 12H), 1.41 (t, 3H, J=
7.2 Hz), 1.28–1.51 (m, 24H), 1.63–1.81 (m, 8H), 2.82–2.93 (m, 8H), 4.43
(q, 2H, J=7.2 Hz), 7.09 (s, 1H), 7.13 (s, 1H, s), 7.17 (t, 1H, J=7.2 Hz),
7.24 (s, 1H), 7.32 (s, 1H), 7.43 (ddd, 1H, J=8.0, 7.2, 0.8 Hz), 7.48–7.52
(m, 2H), 7.73 (d, 1H, J=8.5 Hz), 8.15 (d, 1H, J=7.7 Hz), 8.40 (s, 1H),
8.41 ppm (s, 1H); 13C NMR (100 MHz, [D8]THF): d=14.1, 14.4, 14.5
(ꢃ3), 23.5 (ꢃ2), 23.6 (ꢃ2), 29.1, 29.5, 29.9, 30.1, 30.2 (ꢃ2), 30.3, 30.4, 30.6,
30.8, 31.3, 31.4, 31.5, 32.2, 32.5, 32.7 (ꢃ2), 38.2, 98.1, 109.6, 109.8, 117.0,
117.9, 119.8, 121.2, 123.9, 124.4 (ꢃ2), 125.9, 126.0, 126.8, 128.2, 129.0,
129.1, 130.0, 130.1, 130.3, 130.7, 136.3, 137.1, 140.6, 141.5 (ꢃ2), 141.6,
143.4, 143.8, 144.6, 144.7, 155.3, 164.6 ppm.
Step 5: Stille-type cross-coupling of the two bithiophene units 3 and 1’’,
leading to the tetrameric core of MK-2 (Scheme 6): A mixture of 5-tribu-
tylstannyl-3,4’-dihexyl-2,2’-bithiophene 1’’ (372 mg, 0.60 mmol), 9-ethyl-3-
[3,4’-dihexyl-5’-iodo(2,2’-bithiophene)-5-yl]carbazole
0.40 mmol), and tetrakis(triphenylphosphine)palladium
3
(260 mg,
(18.6 mg,
The overall yield of the MK-2 synthesis is over 59% when starting from
commercially available 3-hexylthiophene.
0.016 mmol) in degassed toluene (4 mL) was heated at reflux for 12 h.
After cooling, a solution of tetrabutylammonium chloride in THF (1m,
0.4 mL) was added and the solvents were evaporated under reduced pres-
sure. The crude coupling product was purified by column chromatogra-
phy on silica gel (hexanes/ethyl acetate 50:1) to give the quaterthiophene
4 (339 mg, 0.399 mmol) in 99% yield.
Efficient Suzuki coupling of the bithophene halide 1’ with an extended
series of organoboron compounds (Table 1): The Suzuki-coupling was ex-
amined by using the bithiophene halide 1’ (0.25 mmol) with 1.1 equiva-
lents of organoboron compounds (0.275 mmol) in the same procedure to
that described in the Experimental Section, step 3. The pure products (2’:
138 mg, 0.24 mmol, 96%; 2’’: 108 mg, 0.23 mmol, 93%; 2’’’: 145 mg,
0.25 mmol, >99%; 2’’’’: 124 mg, 0.25 mmol, >99%) were obtained by
column chromatography on silica gel (2’: hexanes/ethyl acetate 50:1; 2’’:
hexanes/ethyl acetate 50:1; 2’’’: hexanes/ethyl acetate 50:1; 2’’’’: hexanes/
ethyl acetate 10:1).
9-Ethyl-3-[3,4’,4’’,4’’’-tetrahexyl(2,2’:5’,2’’:5’’,2’’’-quaterthiophene)-5-yl]car-
bazole (4):[4] Orange oil; 1H NMR (400 MHz, CDCl3): d=0.89–0.95 (m,
12H), 1.30–1.44 (m, 24H), 1.46 (t, 3H, J=7.2 Hz), 1.62–1.80 (m, 8H),
2.63 (t, 2H, J=7.8 Hz), 2.75–2.87 (m, 6H), 4.38 (q, 2H, J=7.2 Hz), 6.91
(d, 1H, J=1.3 Hz), 6.99 (s, 1H), 7.00 (d, 1H, J=1.3 Hz), 7.02 (s, 1H),
7.21 (s, 1H), 7.27 (ddd, 1H, J=7.5, 7.0, 1.0 Hz), 7.43–7.39 (m, 2H), 7.50
(ddd, 1H, J=8.2, 7.0, 1.2 Hz), 7.73 (dd, 1H, J=8.5, 1.8 Hz), 8.15 (d, 1H,
J=7.5 Hz), 8.32 ppm (d, 1H, J=1.8 Hz); 13C NMR (100 MHz, CDCl3):
d=13.8, 14.1 (ꢃ3), 14.1 (ꢃ2), 22.6 (ꢃ3), 22.7, 29.0, 29.2, 29.3, 29.4, 29.5,
29.7, 30.4, 30.5 (ꢃ3), 30.6, 31.7 (ꢃ5), 37.5, 108.6, 108.7 (ꢃ2), 119.0, 119.9,
120.5, 122.8, 123.3, 123.7, 125.0, 125.2, 125.9, 127.0, 128.1, 128.2, 128.8,
129.9, 130.8, 133.6, 134.3, 135.5, 139.4, 139.5, 139.6, 140.3, 140.4, 143.1,
143.6 ppm.
3-[3,4’-Dihexyl(2,2’-bithiophene)-5-yl]-9-phenylcarbazole (2’): Yellow
sticky oil; 1H NMR (400 MHz, CDCl3): d=0.88–0.91 (m, 6H), 1.32–1.40
(m, 12H), 1.60–1.73 (m, 4H), 2.62 (t, 2H, J=8.4 Hz), 2.79 (t, 2H, J=
8.4 Hz), 6.88 (s, 1H), 7.00 (d, 1H, J=1.2 Hz), 7.19 (s, 1H), 7.30–7.39 (m,
1H), 7.40–7.47 (m, 4H), 7.55–7.65 (m, 5H), 8.17 (d, 1H, J=8.0 Hz),
8.33 ppm (d, 1H, J=1.6 Hz); 13C NMR (100 MHz, CDCl3): d=14.1, 22.6
(ꢃ2), 29.0, 29.3, 29.5, 30.4, 30.5, 30.7, 31.7, 109.9, 110.1, 117.3, 119.6,
120.2, 120.4, 123.3, 123.8, 124.1, 125.1, 126.3, 126.5, 126.9, 127.0, 127.6,
129.5, 129.9, 136.1, 137.5, 140.3, 140.4, 141.4, 142.7, 143.6 ppm; IR (KBr):
n˜ =3050, 2924, 2854, 1599, 1501, 1454, 1362, 1330, 1233, 1175, 1027, 912,
836, 806, 744, 696, 682, 667, 659, 640, 622 cmÀ1; HRMS (FAB): m/z: calcd
for C38H41NS2: 575.2860 [M]+; found: 575.2689.
Step 6: Improved procedure for the a-formylation of the thiophene ring
of tetramer 4 (Scheme 6): The Vilsmeier reagent was added to an ice-
cold solution of quaterthiophene 4 (270 mg, 0.31 mmol) in dry DMF
(2 mL), which was prepared in situ with phosphorus oxychloride
(0.12 mL) in DMF (0.6 mL) at 08C for 30 min. Then, the mixture was
stirred at 708C for 7 h. After cooling, the reaction mixture was quenched
with 10% aq. sodium acetate (35 mL) and extracted with ethyl acetate
(20 mL) three times. The combined organic layers were washed with
water (20 mL) and brine (20 mL), dried over anhydrous sodium sulfate
and evaporated under reduced pressure. The crude product was purified
by column chromatography on silica gel (hexanes/ethyl acetate 15:1) to
give the aldehyde 5 (272 mg, 0.31 mmol) in 99% yield.
3-[3,4’-Dihexyl(2,2’-bithiophene)-5-yl]-N-methylindole (2’’): Light-yellow
sticky oil; 1H NMR (400 MHz, CDCl3): d=0.88–0.91 (m, 6H), 1.32–1.41
(m, 12H), 1.62–1.69 (m, 4H), 2.61 (t, 2H, J=8.4 Hz), 2.76 (t, 2H, J=
8.4 Hz), 3.78 (s, 3H), 6.49 (d, 1H, J=2.4 Hz), 6.86 (s, 1H), 6.97 (d, 1H,
J=1.6 Hz), 7.04 (d, 1H, J=3.2 Hz), 7.11 (s, 1H), 7.29 (d, 1H, J=8.4 Hz),
7.47 (dd, 1H, J=8.4, 1.6 Hz), 7.85 ppm (d, 2H, J=1.2 Hz); 13C NMR
(100 MHz, CDCl3): d=14.1, 22.6 (ꢃ2), 29.0, 29.3, 29.5, 30.4, 30.5, 30.6,
31.7, 32.9, 11.3, 109.5, 118.0, 119.4, 120.1, 124.8, 125.9, 126.7, 128.8, 129.1,
129.6, 136.3, 136.4, 140.1, 143.5 ppm (ꢃ2); IR (KBr): n˜ =3100, 2925, 2854,
1616, 1537, 1467, 1420, 1377, 1332, 1297, 1247, 1155, 1106, 1080, 1010,
913, 832, 796, 740, 718, 682, 675, 641, 629, 611 cmÀ1; HRMS (FAB): m/z:
calcd for C29H37NS2: 463.2367 [M]+; found: 463.2386.
5’’’-(9-Ethyl-9H-carbazol-3-yl)-3’,3’’,3’’’,4-tetrahexyl(2,2’:5’,2’’:5’’,2’’’-quar-
terthiophene)-5-carboxaldehyde (5):[4] Dark-orange oil; 1H NMR
(400 MHz, CDCl3): d=0.95–0.88 (m, 12H), 1.29–1.42 (m, 24H), 1.46 (t,
3H, J=7.2 Hz), 1.68–1.80 (m, 8H), 2.79–2.86 (m, 6H), 2.95 (t, 2H, J=
7.8 Hz), 4.38 (q, 2H, J=7.2 Hz), 7.01 (s, 1H), 7.02 (s, 1H), 7.06 (s, 1H),
7.21 (s, 1H), 7.27 (ddd, 1H, J=7.9, 7.1, 0.9 Hz), 7.39–7.43 (m, 2H), 7.50
(ddd, 1H, J=8.2, 7.1, 1.1 Hz), 7.72 (dd, 1H, J=8.5, 1.8 Hz), 8.14 (d, 1H,
J=7.9 Hz), 8.31 (d, 1H, J=1.8 Hz), 10.02 ppm (s, 1H); 13C NMR
(100 MHz, CDCl3): d=13.7, 14.0 (ꢃ2), 14.1 (ꢃ2), 22.5, 22.6 (ꢃ3), 28.3,
28.9, 29.2 (ꢃ2), 29.3, 29.5, 29.7, 29.8, 30.1, 30.3, 30.4, 31.3, 31.5, 31.6 (ꢃ2),
31.7, 37.5, 108.5, 108.6, 117.3, 119.0, 120.4, 122.7, 123.2, 123.6, 124.9,
125.0, 125.9, 127.7, 128.0, 128.4 (ꢃ2), 129.1, 129.2, 135.1, 135.9, 136.0,
139.4, 140.3, 140.4, 140.6, 142.4, 143.3, 145.1, 153.2, 181.4 ppm.
4-[3,4’-Dihexyl(2,2’-bithiophene)-5-yl]-N,N-diphenylbenzenamine
(2’’’):
Yellow sticky oil; 1H NMR (400 MHz, CDCl3): d=0.85–0.89 (m, 6H),
1.28–1.36 (m, 12H), 1.59–1.65 (m, 4H), 2.58 (t, 2H, J=8.4 Hz), 2.72 (t,
2H, J=8.4 Hz), 6.85 (s, 1H), 6.94 (d, 1H, J=1.2 Hz), 6.99–7.10 (m, 8H),
7.22–7.26 (m, 5H), 7.41 ppm (d, 2H, J=8.4 Hz); 13C NMR (100 MHz,
CDCl3): d=14.1, 22.6, 29.0, 29.2, 29.4, 30.4, 30.5, 30.6, 31.7, 119.7, 123.0,
123.7, 124.5, 125.1, 126.3, 126.9, 128.3, 129.3, 129.8, 136.0, 140.2, 141.4,
143.6, 147.2, 147.5 ppm; IR (KBr): n˜ =3031, 2926, 2855, 1591, 1507, 1493,
1327, 1280, 1219, 1177, 853, 822, 775, 739, 696, 675, 663, 647, 625 cmÀ1
;
HRMS (EI): m/z: calcd for C38H43NS2 [FAB]+: 577.2837; found:
Step 7:[4] Completion of the synthesis of MK-2 (Scheme 6): A mixture of
577.2851.
aldehyde
5
(211 mg, 0.24 mmol) with cyanoacetic acid (40 mg,
0.48 mmol) in a mixed solvent of dry acetonitrile (5 mL) and toluene
(2 mL) was heated at reflux in the presence of piperidine (1 mL) for 4 h.
After cooling, the reaction mixture was diluted with dichloromethane
(30 mL), and the organic layer was washed with water (10 mL) and brine
(10 mL), dried over anhydrous sodium sulfate, and evaporated under re-
duced pressure. The crude product was purified by column chromatogra-
phy on silica gel (chloroform/ethanol) to give the dye, MK-2 (216 mg,
0.23 mmol), in 95% yield.
4-[3,4’-Dihexyl(2,2’-bithiophene)-5-yl]phenylmorpholine (2’’’’): Light-
yellow sticky oil; 1H NMR (400 MHz, CDCl3): d=0.87–0.90 (m, 6H),
1.27–1.39 (m, 12H), 1.61–1.66 (m, 4H), 2.59 (t, 2H, J=8.4 Hz), 2.72 (t,
2H, J=8.4 Hz), 3.17 (t, 4H, J=4.8 Hz), 3.85 (t, 4H, J=4.8 Hz), 6.85 (d,
1H, J=1.2 Hz), 6.88 (d, 2H, J=8.4 Hz), 6.94 (d, 1H, J=1.2 Hz), 7.01 (s,
1H), 7.48 ppm (d, 2H, J=8.4 Hz); 13C NMR (100 MHz, CDCl3): d=14.1,
14.2, 22.6, 29.0, 29.3, 29.4, 30.4, 30.5, 30.6, 31.7, 49.0, 66.8, 115.6, 119.6,
124.6, 125.9, 126.4, 126.8, 129.2, 136.1, 140.2, 141.7, 143.5, 150.5 ppm; IR
Chem. Eur. J. 2013, 19, 2067 – 2075
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2073

Y. Kita et al.
(KBr): n˜ =3029, 2926, 2854, 1607, 1514, 1448, 1378, 1335, 1228, 1123,
1070, 1051, 930, 857, 821, 740, 648, 626, 610 cmÀ1; HRMS (FAB): m/z:
calcd for C30H41ONS2: 495.2630 [M]+; found: 495.2647.
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (A)
and Encouragement of Young Scientists (A) from JSPS, a Grant-in-Aid
for Scientific Research on Innovative Areas “Advanced Molecular Trans-
formations by Organocatalysts” from MEXT, and Ritsumeikan Global
Innovation Research Organization (R-GIRO) project. T.D. is grateful to
financial support from the Asahi Glass Foundation and the Industrial
Technology Research Grant Program from the NEDO of Japan. N.Y. is
grateful for research fellowship from JSPS for young scientists.
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Chem. Eur. J. 2013, 19, 2067 – 2075

Regioregular Oligothiophene Photovoltaic Dye Molecules
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Received: October 2, 2012
Published online: December 23, 2012
Chem. Eur. J. 2013, 19, 2067 – 2075
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2075