Rapid Synthesis of Thiophene-Based, Organic Dyes for Dye-Sensitized Solar Cells (DSSCs) by a One-Pot, Four-Component Coupling Approach

Article abstract of DOI:10.1002/chem.201500979

This one-pot, four-component coupling approach (Suzuki-Miyaura coupling/C-H direct arylation/Knoevenagel condensation) was developed for the rapid synthesis of thiophene-based organic dyes for dye-sensitized solar cells (DSSCs). Seven thiophene-based, org

Full text of DOI:10.1002/chem.201500979

DOI: 10.1002/chem.201500979
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&
Photovoltaics
Rapid Synthesis of Thiophene-Based, Organic Dyes for Dye-
Sensitized Solar Cells (DSSCs) by a One-Pot, Four-Component
Coupling Approach
Keisuke Matsumura,[a] Soichi Yoshizaki,[a] Masato. M. Maitani,[a] Yuji Wada,[a] Yuhei Ogomi,[b]
Shuzi Hayase,[b] Tatsuo Kaiho,[c] Shinichiro Fuse,*[a, d] Hiroshi Tanaka,[a] and Takashi Takahashi[e]
Abstract: This one-pot, four-component coupling approach
(Suzuki–Miyaura coupling/CÀH direct arylation/Knoevenagel
condensation) was developed for the rapid synthesis of thio-
phene-based organic dyes for dye-sensitized solar cells
(DSSCs). Seven thiophene-based, organic dyes of various
donor structures with/without the use of a 3,4-ethylenedi-
oxythiophene (EDOT) moiety were successfully synthesized
in good yields based on a readily available thiophene boron-
ic acid pinacol ester scaffold (one-pot, 3-step, 35–61%). Eval-
uation of the photovoltaic properties of the solar cells that
were prepared using the synthesized dyes revealed that the
introduction of an EDOT structure beside a cyanoacrylic acid
moiety improved the short-circuit current (Jsc) while decreas-
ing the fill factor (FF). The donor structure significantly influ-
enced the open-circuit voltage (Voc), the FF, and the power
conversion efficiency (PCE). The use of a n-hexyloxyphenyl
amine donor, and our originally developed, rigid, and non-
planar donor, both promoted good cell performance (h=
5.2–5.6%).
Introduction
Among the numerous examples of oligoheteroaromatic
rings, oligothiophenes make up a particularly important class
of compounds because they are valuable elements that are
used in organic electronics for applications such as solar cells
and field-effect transistors as well as for chemical probes in
chemical biology.[2]
The one-pot, transition-metal-catalyzed, multicomponent cou-
pling approach for oligoheteroaromatic compounds starting
from readily available heteroaromatic scaffolds is a powerful
method. With this approach, complex oligoheteroaromatic
structures can be readily constructed from simple starting ma-
terials in one step under ambient reaction conditions without
the necessity of purifying synthetic intermediates.[1]
Several one-pot, palladium-catalyzed, multicomponent cou-
pling approaches for the synthesis of oligothiophenes based
on readily available thiophene scaffolds have been reported in
the past decade (Scheme 1). The reported syntheses fall into
one of the following three categories: 1) the regioselective
coupling approach; 2) the site-selective coupling approach;
and 3) the chemoselective coupling approach. In 2007, Handy
and co-workers reported a regioselective, one-pot, Suzuki–
Miyaura (SM) coupling[3] reaction based on dibromothiophene
scaffold A (category 1).[4] The intrinsic difference in reactivity
between C4ÀBr and C5ÀBr bonds against a palladium catalyst
was used for C5-selective SM coupling. Dauban, Dodd, and co-
workers reported a site-selective, one-pot, SM coupling reac-
tion based on the readily available, symmetric dibromothio-
phene scaffold B in 2009 (category 2, upper scheme).[5] In addi-
tion, in 2011, Yamada and co-workers reported a site-selective,
one-pot, SM coupling reaction based on a thiophene-2,5-dibor-
onic acid scaffold (category 2, not shown).[6] The approach of
category 2[7] allows the use of readily available symmetric scaf-
folds with two identical reaction sites, but the selective activa-
tion of the desired reaction site is generally somewhat difficult
compared with that of category 1. There are fewer reports that
are based on the chemoselective approach (category 3) com-
pared with those of regio- and site-selective approaches (cate-
[a] K. Matsumura, S. Yoshizaki, Dr. M. M. Maitani, Prof. Y. Wada, Dr. S. Fuse,
Dr. H. Tanaka
Department of Applied Chemistry, Tokyo Institute of Technology
2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
[b] Dr. Y. Ogomi, Prof. S. Hayase
Graduate School of Life Science and Systems Engineering
Kyusyu Institute of Technology, Fukuoka 808-0196 (Japan)
[c] Dr. T. Kaiho
Kanto Natural Gas Development Co., Ltd.
Brine Resources Research & Development Division
Chiba 297-8550 (Japan)
[d] Dr. S. Fuse
Present address: Chemical Resources Laboratory
Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku
Yokohama 226-8503 (Japan)
E-mail: sfuse@res.titech.ac.jp
[e] Prof. T. Takahashi
Yokohama College of Pharmacy, 601, Matano-cho
Totsuka-ku, Yokohama 245-0066 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500979. It contains synthetic, photo-
physical, and electrochemical procedures, and the characterization of the
synthesized compounds.
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solar cells that were prepared using the synthesized dyes, were
evaluated to elucidate the structure–property relationships.
Results and Discussion
For the development of a chemoselective, one-pot, multicom-
ponent coupling procedure, the precursor of a simple and typ-
ical D-p-A compound 1[10i,k,12] was selected as the target. We
considered two synthetic routes, A and B, for the synthesis of
1 from the readily available thiophene scaffold D (Scheme 2).
Scheme 1. One-pot, multi-component coupling approaches for the synthesis
of oligo-thiophenes. dba=dibenzylideneacetone; Pin=pinacol; SPhos: 2-di-
cyclohexylphosphino-2’,6’-dimethoxybiphenyl.
Scheme 2. Chemoselective, one-pot, three-component coupling approaches
to the synthesis of 1.
gories 1 and 2). As far as we could ascertain, only Staubitz and
co-workers, in 2012, have reported a chemoselective, one-pot,
Migita–Kosugi–Stille (MKS) coupling[8]/SM coupling approach
based on the unique thiophene scaffold C (category 3).[9] Al-
though several useful one-pot approaches have been reported,
as described above, the development of high-yielding and
mild one-pot approaches based on readily available thiophene
scaffolds continues to be important.
Both routes included two types (SM coupling or MKS coupling
and CÀH direct arylation) of coupling reactions. The order of
the introduction of building blocks 2 and 3 was switched be-
tween routes A and B. Route B was investigated in the present
study, because intermediate 5 retained more acidic (reactive)
protons compared with the corresponding intermediate 4 in
route A. This allowed the activation of the desired CÀH bond
under mild conditions. The key to success is how to conduct
the second CÀH direct arylation step in high yields in the pres-
ence of the remaining/generating compounds of the first cou-
pling step. This was not simple because the Pd catalysts are
generally somewhat susceptible to contaminants.
We have reported combinatorial library synthesis for drug
discovery and materials development using palladium-cata-
lyzed cross-coupling reactions.[10] In addition, we rapidly syn-
thesized thiophene-based organic compounds using a site-se-
lective, one-pot, SM coupling/SM coupling/Knoevenagel con-
densation approach (category 2, lower scheme).[10i] However,
undesired double SM coupling products were usually generat-
ed in the first SM coupling step. Based on these results, we de-
cided to investigate either a chemoselective, one-pot, SM cou-
pling or a MKS coupling, and the subsequent CÀH direct aryla-
tion[11] approach (category 3) based on a scaffold D. We antici-
pated that the selectivity in the first coupling reaction (SM or
MKS coupling) would be improved because CÀH bonds should
not be activated under the first coupling conditions. To further
improve synthetic efficiency, we also developed a one-pot,
four-component coupling approach for the rapid synthesis of
thiophene-based donor-p-bridge-acceptor (D-p-A) dyes. In ad-
dition, the photochemical and electrochemical properties of
synthesized dyes, as well as the photovoltaic properties of
The key CÀH direct arylation of 5 with 2 was examined in
accordance with a procedure established by Fagnou,[13] as
shown in Table 1. Three inorganic bases (K3PO4, K2CO3, and
Cs2CO3) were examined (Table 1, entries 1–3). The use of
Cs2CO3 afforded the best results. Solvent and temperature
were critical for the CÀH direct arylation (entries 3–6). The con-
dition of entry 6 (1,4-dioxane/reflux) afforded the desired cou-
pling product in the highest yield (74%), whereas the other
conditions (THF/reflux, toluene/reflux, DMA/1408C) all resulted
in lower yields (entries 3, 4, and 5). The use of other phosphine
ligands including PPh3 and [(Cy3P)H]BF4 (entries 7 and 8), or
the addition of 1 equivalent of water (entry 9) decreased the
yield. Under microwave irradiation conditions, the reaction
time was dramatically shortened (18 h to 45 min), although the
yield was slightly decreased (entry 10). Interestingly, a proce-
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Table 1. Examination of CÀH direct arylation for the synthesis of 1.
Table 2. Photochemical and electrochemical properties of dyes 16–22.
[a]
Entry Dye lmax
e[b]
Optical
edge
EHOMO
[V][c]
ELUMO
[V][c]
[nm]
[LmolÀ1 cmÀ1
]
[nm]
(vs. NHE) (vs. NHE)
1
2
3
4
5
6
7
16 467[d]
17 503[d]
18 520
19 514
20 524
21 528
22 520
29000[d]
29000[d]
39000
26000
40000
21000
24000
580[d]
617[d]
623
599
625
1.32[d]
1.28[d]
À0.81[d]
À0.73[d]
À0.88
À0.77
À0.82
À0.82
À0.89
Entry
2
Ligand
Base
Solvent/T
Yield [%][a]
–
–
32
49
1.11
1
2
3
4
5
6
7
8
9[d]
10
11[f]
2a [(tBu3P)H]BF4 K3PO4
2a [(tBu3P)H]BF4 K2CO3
2a [(tBu3P)H]BF4 Cs2CO3 THF/reflux
2a [(tBu3P)H]BF4 Cs2CO3 toluene/reflux
2a [(tBu3P)H]BF4 Cs2CO3 DMA/1408C
2a [(tBu3P)H]BF4 Cs2CO3 1,4-dioxane/reflux
2a PPh3
2a [(Cy3P)H]BF4
2a [(tBu3P)H]BF4 Cs2CO3 1,4-dioxane/reflux
2a [(tBu3P)H]BF4 Cs2CO3 1,4-dioxane/1608C[e]
2b PPh3
THF/reflux
THF/reflux
1.30
1.16
1.14
1.11
[b]
630
619
<12
74
[a] Absorption maxima in DMF. [b] Molar absorption coefficients in DMF.
[c] EHOMO was determined by the cyclic voltammetry. EHOMOÀELUMO gap was
determined by the edge of the absorption spectra, as defined by the
wavelength where the absorbance revealed 1/10 of the peak top; that is,
EHOMOÀELUMO gap [eV] was calculated by 1240 [nm]/optical edge [nm].
ELUMO was thus calculated by the summation of the HOMO potential and
the EHOMOÀELUMO gap. [d] Data from reference [10k].
[c]
Cs2CO3 1,4-dioxane/reflux
Cs2CO3 1,4-dioxane/reflux
–
–
[c]
43
63
72
Ag2CO3 H2O/MeCN=9:1/808C
[a] Isolated yield. [b] The reaction was very slow. A large amount of sub-
strates were recovered. [c] The complex mixtures were obtained. [d] 1 equiv
of water was added. [e] The reaction was carried out under microwave irradi-
ation (200 W) for 45 min. [f] [Pd(dppf)Cl2]·CH2Cl2 was employed as a Pd cata-
lyst instead of Pd(OAc)2. PivOH was not added. DMA=N,N-dimethylaceta-
mide; dppf=1,1’-bis(diphenylphosphino)ferrocene; PivOH=pivalic acid.
tion conditions (Table 1, entry 6 vs. 9). Therefore, a MKS cou-
pling/CÀH direct arylation procedure was examined (Scheme 3,
procedure 1), because MKS coupling required no water. How-
ever, the desired product 1 was obtained in an unsatisfactory
yield (26%). We also examined an SM coupling/CÀH direct ary-
lation procedure (Scheme 3, procedure 2). The SM coupling of
the borate D-2 with 3 also required no water. However, again,
the yield of desired compound 1 was low (<24%). These re-
sults turned our attention to employ Greaney’s CÀH direct ary-
lation[14] using a H2O/MeCN biphasic system (proce-
dure 3). To our delight, procedure 3 afforded the de-
sired compound in a good yield (one-pot, 2-step,
68%). It should be noted that the developed one-
pot procedure 3 could be carried out at ambient
temperature.
dure established by Greaney[14] for the CÀH direct arylation of
2H-indazoles was also effective for the synthesis of 1 (entry 11).
A one-pot, three-component coupling was examined as
shown in Scheme 3. As described in the previous paragraph,
the water had an adverse effect on our best CÀH direct aryla-
To further improve the synthetic efficiency, a one-
pot, four-component coupling approach (SM cou-
pling/CÀH direct arylation/Knoevenagel condensa-
tion) was examined (Scheme 4). A solution of bro-
mothiophene 3, thienylboronic acid pinacol ester D-
3, Pd(OAc)2, [(tBu3P)H]BF4, and Na2CO3 in a mixed
solvent of H2O and MeCN (3:1) was stirred at 608C
under an argon atmosphere. After the mixture had
been cooled to room temperature, aryl iodide 2b,
[Pd(dppf)]Cl2·CH2Cl2, PPh3, Ag2CO3, and H2O were
added. After being stirred at 808C under an argon
atmosphere, the mixture was cooled to room tem-
perature, and tert-butyl cyanoacetate (8), piperidine,
and CH2Cl2 were added. It should be noted that the
use of tert-butyl ester improved the solubility of the
product and enabled ready purification.[10k] After
being stirred at room temperature under an argon
atmosphere, the mixture was poured into saturated
aq. NH4Cl. The following standard workup procedure
afforded the desired product 9 in a 46% yield.
To examine the scope of our developed synthetic
methodology, seven protected dyes, 9–15, contain-
ing different donor moieties with/without an EDOT
Scheme 3. Examination of one-pot, three-component coupling reactions for the synthe-
sis of 1.
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using trifluoroacetic acid (TFA) afforded the desired
dyes 16–22 in high yields.
The absorption spectra and electrochemical prop-
erties of the seven dyes, 16–22, were measured, as
shown in Table 2. The 8–47 nm redshift of the ab-
sorption maxima for EDOT containing dyes 19–21
compared with those of dyes 16–18 without the
EDOT was observed (Figure 2). This tendency was
consistent with our previous observation.[10h] The
HOMO and LUMO levels of the seven dyes met the
minimum requirement for a dye-sensitizer in DSSCs,
that is, EHOMO and ELUMO were located more positive
than the iodine/iodide redox potential (+0.4 V vs.
normal hydrogen electrode (NHE)) and more nega-
Scheme 4. Examination of the one-pot, four-component coupling for the synthesis of 9.
moiety[10h,15] were rapidly synthesized, as shown in Figure 1. All
the dyes were obtained in satisfactory to good yields (for the
synthetic details of the building blocks, see the Supporting In-
formation). The subsequent removal of a tert-butyl group
tive than the conduction band edge of TiO2 (À0.5 V vs.
NHE[16]).
The photovoltaic properties were evaluated for solar cells
that were prepared using the seven dyes, 16–22 (Table 3 and
Figure 3). Dyes 19–21 had an EDOT moiety and ex-
erted an equal or higher Jsc and IPCE compared with
dyes 16–18 that were without the EDOT moiety
(Table 3, entry 1 vs. 4, 2 vs. 5, and 3 vs. 6 and Fig-
ure 3c). This tendency was consistent with our previ-
ous observation.[10 h] However, interestingly, the best
dye was not found from dyes 19–21 that contained
the EDOT moiety (Table 3, entries 4–6). This was be-
cause the FF was decreased by introducing the EDOT
moiety (Table 3, entry 1 vs. 4, 2 vs. 5, and 3 vs. 6).
The donor moiety in the dyes had a significant in-
fluence on the cell performance of the DSSCs. Dyes
17 and 20 had an ethylenedioxyphenyl amine donor
and exerted the lowest values for PCE and Voc
(Table 3, entries 2 and 5). The observed low Voc can
be ascribed to a large dark current (Figure 3b). It
should be noted that dyes 17 and 20 caused a larger
dark current than dyes 16 and 19, although dyes 17
and 20 had a sterically larger donor moiety than dyes
16 and 19. Based on these results, the ethylenedioxy-
phenyl amine donor might have had a detrimental
effect on the increase in the dark current. Dyes 16
and 19 had a triphenyl amine donor and exerted the
highest Voc (Table 3, entries 1 and 4). However, the
observed cell performance was not the highest be-
cause of a low FF. On the other hand, dyes 18 and
21, which contained
a n-hexyloxyphenyl amine
donor (Table 3, entries 3 and 6), and dye 22, which
contained our originally developed, rigid, and non-
planar donor (Table 3, entry 7), exerted a good PCE
(ꢀ5.2%), Voc (ꢀ0.68 V), and FF (ꢀ0.64).
Conclusion
We successfully developed a one-pot, four-compo-
nent coupling approach (SM coupling/CÀH direct ary-
lation/Knoevenagel condensation) based on a readily
available thiophene boronic acid pinacol ester scaf-
fold. Seven dyes were successfully synthesized in
Figure 1. Synthesis of 7 thiophene-based organic dyes 16–22 by a one-pot, four-compo-
nent coupling approach.
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Figure 2. Absorption spectra of synthesized dyes 16–22 in CH2Cl2.
Table 3. Photovoltaic properties of solar cells prepared using dyes 16–
22.
[a]
[a]
Entry
Dye
h
[a] [%]
FF[a]
Voc [V]
Jsc [mAcmÀ1
]
1
2
3
4
5
6
7
16
17
18
19
20
21
22
5.0Æ0.5[b] 0.57Æ0.06[b] 0.71Æ0.01[c] 12.6Æ0.1[c]
1.7Æ0.1
5.6Æ0.3
4.9Æ0.1
4.3Æ0.2
5.2Æ0.3
5.2Æ0.0
0.67Æ0.01
0.68Æ0.01
0.53Æ0.01
0.57Æ0.04
0.64Æ0.03
0.69Æ0.01
0.57Æ0.01
0.70Æ0.01
0.72Æ0.01
0.63Æ0.00
0.68Æ0.01
0.70Æ0.00
4.4Æ0.2
11.7Æ0.4
12.9Æ0.1
11.9Æ0.4
12.0Æ0.3
10.7Æ0.1
[a] Average values from two or three independent experiments. FF=fill
factor; Voc =open-circuit voltage; Jsc =short-circuit current. [b] Data from
ref. [10k].
Figure 3. Photocurrent-voltage curves obtained with DSSCs based on 16–22
under standard AM 1.5 (1 sun) solar conditions (a) and in the dark (b). c) Inci-
dent photon-to-current conversion efficiency (IPCE) spectra for DSSCs based
~
~
&
*
*
^
on 16–22. c: 16, : 17, : 18, : 19, : 20, : 21, : 22.
good yields (one-pot, 3-step, 35–61%) under mild reaction
conditions. Evaluation of photovoltaic properties of the solar
cells that were prepared using the synthesized dyes revealed
that the introduction of an EDOT structure beside a cyanoacrylic
acid moiety improved the Jsc, but it decreased the FF. The
donor structure significantly influenced the Voc, the FF, and the
PCE. An ethylenedioxyphenyl amine donor exerted low cell
performance due to a large dark current. On the other hand,
both an n-hexyloxyphenyl amine donor and our originally de-
veloped, rigid, and nonplanar donor resulted in good cell per-
formance. The developed synthetic methodology and struc-
ture–property relationships would be a valuable aid for rapid
materials development.
Morita, Tokyo Institute of Technology, for his valuable sugges-
tions.
Keywords: CÀH activation · cross-coupling · dyes/pigments ·
dye-sensitized solar cells · one-pot reactions
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Experimental Section
Experimental details are given in the Supporting Information.
Acknowledgements
The authors thank Dr. S. Sugiyama, Tokyo Institute of Technolo-
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mura, Kyusyu Institute of Technology, for the preparation and
evaluation of the DSSCs. The authors also thank Mr. Taiki
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Received: March 11, 2015
Published online on May 28, 2015
Chem. Eur. J. 2015, 21, 9742 – 9747
www.chemeurj.org
9747
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