Elucidation of the structure-property relationship of p-type organic semiconductors through rapid library construction via a one-pot, Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1021/co500071x

The elucidation of the structure-property relationship is an important issue in the development of organic electronics. Combinatorial synthesis and the evaluation of systematically modified compounds is a powerful tool in the work of elucidating structure-property relationships. In this manuscript, D-π-A structure, 32 p-type organic semiconductors were rapidly synthesized via a one-pot, Suzuki-Miyaura coupling with subsequent Knoevenagel condensation. Evaluation of the solubility and photovoltaic properties of the prepared compounds revealed that the measured solubility was strongly correlated with the solubility parameter (SP), as reported by Fedors. In addition, the SPs were correlated with the J_sc of thin-film organic solar cells prepared using synthesized compounds. Among the evaluated photovoltaic properties of the solar cells, J_sc and V_oc had strong correlations with the photoconversion efficiency (PCE).

Full text of DOI:10.1021/co500071x

Research Article
pubs.acs.org/acscombsci
Elucidation of the Structure−Property Relationship of p‑Type
Organic Semiconductors through Rapid Library Construction via a
One-Pot, Suzuki−Miyaura Coupling Reaction
Shinichiro Fuse,*,† Keisuke Matsumura,† Atsushi Wakamiya,‡ Hisashi Masui,§ Hiroshi Tanaka,†
Susumu Yoshikawa,◊ and Takashi Takahashi§
†Department of Applied Chemistry, Tokyo Institute of Technology, Tokyo 152-8552, Japan
‡Institute for Chemical Research, Kyoto University, Kyoto 611-0011, Japan
§Yokohama College of Pharmacy, Kanagawa 245-0066, Japan
◊Institute of Advanced Energy, Kyoto University, Kyoto 611-0011, Japan
S
* Supporting Information
ABSTRACT: The elucidation of the structure−property
relationship is an important issue in the development of
organic electronics. Combinatorial synthesis and the evaluation
of systematically modified compounds is a powerful tool in the
work of elucidating structure−property relationships. In this
manuscript, D−π−A structure, 32 p-type organic semi-
conductors were rapidly synthesized via a one-pot, Suzuki−
Miyaura coupling with subsequent Knoevenagel condensation.
Evaluation of the solubility and photovoltaic properties of the
prepared compounds revealed that the measured solubility was
strongly correlated with the solubility parameter (SP), as
reported by Fedors. In addition, the SPs were correlated with
the Jsc of thin-film organic solar cells prepared using
synthesized compounds. Among the evaluated photovoltaic properties of the solar cells, Jsc and Voc had strong correlations
with the photoconversion efficiency (PCE).
KEYWORDS: p-type semiconductor, structure−property relationship, Suzuki−Miyaura coupling, one-pot
between a single molecule structure and the properties of
materials/devices.6,7
INTRODUCTION
■
Elucidating the relationship between the chemical structure of
single molecules and the properties of materials/devices is
important in the development of organic electronics.1 Recent
dramatic progress in theoretical calculation has made it possible
to estimate the properties of single molecules such as the
conformation, absorption spectra, and the electron distributions
of the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO).2 However, because
intermolecular interactions dramatically influence the proper-
ties of the materials/devices, a rational estimation of these
important interactions is not simple. Another method of
estimation is the use of structure−property information, which
is drawn from extensive past reports.3,4 However, the extraction
of precise structure−property information is also not a simple
process, because the procedure for the preparation and
evaluation of materials/devices varies depending on the
research group.5 The combinatorial synthesis of systematically
modified compounds, and the evaluation of the properties of
materials/devices using those synthesized compounds, have
been a powerful tool in the work of elucidating the relationship
Organic solar cells have garnered considerable attention due
to their simple fabrication procedures, low production cost, a
lightweight construction, and mechanical flexibility.3,4,8 Bulk
heterojunction solar cells now demonstrate remarkable achieve-
ments with power conversion efficiencies (PCEs) in excess of
10%.9 Although the best PCEs have been achieved with
polymer p-type organic semiconductors that consist of
alternating donor−acceptor (D−A) repeating structures,
small-molecule p-type counterparts have recently demonstrated
high PCEs. In 2012, Heeger and co-workers reported a
symmetric small-molecule with A−D−A structure that exerted
a high PCE of 6.7%.10 In 2013, Chen and co-workers reported a
small-molecule with A−D−A structure that exerted a
remarkable PCE of 8.1%.11 Wong and co-workers reported a
D−A−A type asymmetric small-molecule that also exerted a
high PCE of 6.8%,12 although this was still lower than the A−
D−A symmetric-structure. By comparison with corresponding
Received: May 1, 2014
© XXXX American Chemical Society
A
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ACS Combinatorial Science
Research Article
polymers, small-molecule p-type organic semiconductors have
predominant advantages such as well-defined molecular
structures, easier purification, and better batch-to-batch
reproducibility. For the development of novel small-molecule
p-type semiconductors with higher PCEs, a fundamental
elucidation of the structure−property relationship is important.
We have reported the construction of combinatorial libraries
based on palladium-catalyzed cross-coupling reactions.13−21
Herein, we wish to report the rapid synthesis of asymmetric
small-molecules (32 compounds) with D−π−A structures for
use as p-type semiconductors of organic thin film solar cells via
a one-pot, SM coupling with subsequent Knoevenagel
condensation. In addition, the solubility and photovoltaic
properties of prepared compounds were evaluated. The
relationships between the structures of p-type semiconductors,
and the solubility and photovoltaic properties are discussed.
Scheme 1. Synthesis of the Organic p-Type Semiconductor
6{1,1,1} Based on the One-Pot, SM Coupling−Knoevenagel
Condensation
RESULTS AND DISCUSSION
The D−π−A structure of organic p-type semiconductor I was
selected as a target compound (Figure 1). We planned to
■
Figure 1. One-pot approach for the synthesis of p-type semi-
conductors with D−π−A structures that was based on a Suzuki−
Miyaura coupling with subsequent Knoevenagel condensation.
couple a donor block III with the aromatic scaffold of II
because the C−Br bond of the first coupling product would be
less reactive compared with the corresponding C−Br bond of
the aromatic scaffold II, and, therefore, an undesirable
overreaction affording a D−π−D compound would be
suppressed. The one-pot Suzuki−Miyaura (SM) coupling
with IV afforded the desired coupling product V. The
subsequent Knoevenagel condensation afforded the desired
compound I.
For the development of a one-pot, SM coupling-Knoevenagel
condensation sequence, a simple and typical compound
6{1,1,1}22 was selected as the target (Scheme 1). The first
SM coupling reaction was carried out using a combination of 1
mol % Pd2(dba)3,23 2 mol % Xantphos,24 and Na2CO3 in THF/
H2O. After confirming the completion of the first SM coupling
reaction, 3, Pd2(dba)3, [(tBu3P)H]BF4,25 and Na2CO3 were
added to afford the desired coupling product 4{1,1} in a 67%
yield (one-pot, 2-step yield). The subsequent Knoevenagel
condensation of 4{1,1} with 5{1} proceeded smoothly to afford
the desired compound in a good yield.
Figure 2. Synthesis of D−π−A small-molecule, p-type organic
semiconductors via a one-pot, palladium-catalyzed coupling reaction
with subsequent Knoevenagel condensation.
electron transfer. On the other hand, generally highly planar
molecules are poorly soluble to organic solvents, which are used
for the film formation process in preparing organic solar cells.
For elucidation of the relationships between the structures of
small-molecules, and their solubility and photovoltaic proper-
ties, building blocks with different solubilities were used for the
library construction.
The building blocks were coupled using the developed one-
pot procedure in a parallel fashion. Building block 1{3} was
unstable, and attempts at purification using silica-gel column
chromatography failed. Therefore, lithiation, borylation, and
double SM coupling reaction were performed sequentially in a
one-pot. To a stirred solution of 2-(diphenylamino)thiophene
in THF, nBuLi was added dropwise at −78 °C. then iPrOBPin
was added to in-situ-generated unstable thiophene boronic acid
pinacol ester 1{3}. To a stirred solution of 1{3}, water, 2{1},
Pd2(dba)3, Xantphos, and Na2CO3 were added. After
Three donors 1{1−3}, 4 π-bridges 2{1−4}, and 3 acceptors
5{1−3}, were selected for the construction of the asymmetric-
structure, small-molecular library (Figure 2). Small-molecules
of p-type semiconductors should have a higher planarity that
would allow an ordered stacking for a smooth intermolecular
B
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ACS Combinatorial Science
Research Article
confirmation of the completion of the first SM coupling
reaction, 3, Pd2(dba)3, [(tBu3P)H]BF4, and Na2CO3 were
added. To our delight, the desired compound 4{3,1} was
obtained in a good yield. It should be noted that although the
2-(diphenylamino)thiophene structure is an attractive building
block in materials science,26−37 the introduction of this useful
building block via palladium-catalyzed cross-coupling reac-
tions28,29,31−37 was not simple because of instability of the
coupling precursors: either thienyl boronic acid or thienyl
stannane. Recently, Buckwald and co-workers reported an
elegant lithiation/borylation/SM coupling sequence of simple
thiophenes and aromatic rings in a microflow reactor.38 Our
developed one-pot, lithiation/borylation/SM coupling/SM
coupling sequence should be useful for introducing the 2-
(diphenylamino)thiophene building block.
Table 1. Yields of One-Pot, SM Coupling Reactions with
Subsequent Knoevenagel Condensations
Scheme 2. Synthesis of 4{3,1} Based on the One-Pot,
Borylation-Double SM Coupling Sequence
entry
1
2
product (yield 1)
5
product (yield 2)
a
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
1
1
1
2
2
2
3
3
3
4
4
4
1
1
1
2
2
2
3
3
3
4
4
4
1
1
1
2
2
2
3
3
3
4
4
4
4{1,1} (67%)
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
6{1,1,1} (87%)
6{1,1,2} (81%)
6{1,1,3} (74%)
6{1,2,1} (69%)
6{1,2,2} (58%)
2
3
a
4
4{1,2} (29%)
5
c
6
6{1,2,3} (−)
a
7
4{1,3} (55%)
6{1,3,1} (91%)
6{1,3,2} (83%)
6{1,3,3} (quant.)
6{1,4,1} (73%)
6{1,4,2} (50%)
6{1,4,3} (61%)
6{2,1,1} (72%)
6{2,1,2} (76%)
6{2,1,3} (84%)
6{2,2,1} (90%)
6{2,2,2} (95%)
6{2,2,3} (quant.)
6{2,3,1} (69%)
6{2,3,2} (67%)
6{2,3,3} (65%)
6{2,4,1} (67%)
6{2,4,2} (68%)
6{2,4,3} (75%)
6{3,1,1} (67%)
6{3,1,2} (77%)
8
9
a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
4{1,4} (23%)
a
4{2,1} (72%)
Thirty-two compounds in 36 trials were obtained in
satisfactory to good yields, as shown in Table 1. The obtained
compounds were purified by silica-gel column chromatography
with subsequent gel-permeation chromatography (GPC) or
recrystallization. The one-pot yields tended to decreased when
2{2} or 2{4} building blocks were employed. Four compounds,
6{1,2,3}, 6{3,1,3}, 6{3,2,2}, and 6{3,2,3} were not obtained as
pure products due to their poor solubility. Residual palladium
reportedly have detrimental effects for their PCEs.39 Therefore,
we measured the residual quantity of palladium in the prepared
samples using inductively coupled plasma mass spectrometry
(ICP-MS). We selected two compounds, 6{1,1,1} and 6{2,1,1},
that were purified by different methods, that is, either silica-gel
column chromatography/recrystallization or silica-gel column
chromatography/GPC. As a result, less than 1 ppb of the
residual palladium was detected. Thus, our synthesis and
purification procedure proved to be sufficient for providing the
p-type organic semiconductors library.
The solubilities of 9 randomly selected, solid compounds
against chlorobenzene were briefly measured (see Supporting
Information for details), and are shown as solubility scores in
Figure 3. The compounds with higher scores retained better
solubility against chlorobenzene. The solubility parameter (SP)
for each compound was calculated in accordance with the
Fedors group substitution method,40 SP = (ΔEv/V)0.5, where
ΔEv is the energy of vaporization, and V is the molar volume.
The SPs of the compounds were calculated by dividing the
compound structures into different fragments, and the
corresponding values of vaporization energy (Δei) and molar
volumes (Δvi) for each of the fragments were obtained from a
previous report.40 The calculated SPs were compared to the
a
4{2,2} (50%)
a
4{2,3} (74%)
a
4{2,4} (40%)
b
4{3,1} (41%)
c
6{3,1,3} (−)
b
4{3,2} (26%)
6{3,2,1} (85%)
c
6{3,2,2} (−)
6{3,2,3} (−)
c
b
4{3,3} (52%)
6{3,3,1} (quant.)
6{3,3,2} (quant.)
6{3,3,3} (97%)
6{3,4,1} (quant.)
6{3,4,2} (75%)
6{3,4,3} (84%)
b
4{3,4} (19%)
a
b
c
One-pot, 2-step yield. One-pot, 3-step yield. Pure compounds were
not obtained because of their poor solubility.
measured solubility scores. The compounds with SPs that were
similar to that of chlorobenzene (SP = 10.4) were speculated to
retain good solubility against chlorobenzene. The result shown
in Figure 3 clearly shows that the measured solubility scores
correlated well (R2 = 0.83) with the calculated SPs. It was found
C
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ACS Combinatorial Science
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Figure 3. Correlations between measured solubility scores and
calculated solubility parameters.
that the SP should be less than 13.5 for the preparation of a
sufficient concentration of solutions for the active layers of solar
cells.
Organic solar cells were prepared using 10 randomly selected
compounds, which fulfilled the requirement of SPs (<13.5) in
accordance with standard procedures,41,42 but did not optimize
the fabrication of solar cells. This was because the aim of this
study was not to identify the best-performing p-type semi-
conductor, but rather to elucidate the structure−property
relationships. The measured photovoltaic properties of
prepared cells are shown along with photoabsorbance proper-
ties (film), HOMO and LUMO levels (film), and SPs in Table
2. The calculated LUMO levels of synthesized compounds were
sufficiently higher than those of PCBMs (−4.3 eV), and, thus,
an electron transfer from a p-type semiconductor to an n-type
semiconductor would occur.
The correlations between all the shown parameters and PCE
were examined (Figure 4). As a result, Jsc and Voc correlated
well (R2 > 0.6) with PCE. It was noteworthy that the SPs had a
fairly strong correlation with the Jsc, (R2 = 0.57), and the
compounds with higher SPs tended to exert a higher Jsc. These
obtained results indicated that the compounds retaining an SP
of around 13.5 should have a high Jsc. Actually, compounds
6{1,1,1} (SP = 13.1), and 6{1,4,1} (SP = 13.2), had SPs that
approximated 13.5, and both of these exerted a high Jsc (4.30,
Figure 4. Correlations between (a) PCE vs Voc, (b) PCE vs Jsc, and,
(c) Jsc vs SP.
4.05 mAcm−2, respectively). The compound 6{1,4,1} exerted
the highest PCE of 0.68%.
CONCLUSION
■
A total of 32 organic p-type semiconductors with D−π−A
structures were rapidly synthesized via our originally developed,
one-pot, SM coupling with subsequent Knoevenagel con-
densation. Measurement of the remaining palladium in the
synthesized compounds using ICP-MS revealed that our
synthesis and purification procedure was sufficient for the
removal of palladium. An evaluation of the solubility and the
photovoltaic properties of the prepared compounds was
Table 2. Photoabsorption and Photovoltaic Properties of 10 Randomly Selected Compounds (SPs < 13.5)
a
b
c
c
c
entry compound EHOMO [eV] ELUMO [eV] λmax [nm] optical edge [nm] ε [L mol−1 cm−1
]
SP
Jsc [mAc m−2
]
Voc [V]
FF
PCE [%]
1
2
6{1,1,1}
6{1,4,1}
6{2,1,1}
6{2,1,2}
6{2,2,1}
6{2,2,2}
6{2,3,2}
6{2,4,1}
6{2,4,2}
6{3,3,1}
−5.45
−5.45
−5.47
−5.11
−5.31
−5.34
−5.33
−5.13
−5.12
−5.87
−3.40
−3.46
−3.53
−3.05
−3.38
−3.27
−3.24
−3.24
−3.10
−3.94
518
545
545
507
548
510
504
570
531
528
605
624
639
601
644
599
592
655
614
644
36900
47800
35100
30500
36000
38100
34300
47000
45100
26600
13.1
13.2
11.7
12.1
12.0
12.4
11.6
11.8
12.2
12.5
4.30
4.05
1.16
1.61
3.27
1.10
0.95
2.03
1.58
1.84
0.23
0.46
0.12
0.32
0.60
0.02
0.25
0.50
0.16
0.31
0.31
0.37
0.27
0.32
0.34
0.25
0.32
0.35
0.29
0.35
0.31
0.68
0.04
0.16
0.66
0.01
0.08
0.35
0.07
0.20
3
4
5
6
7
8
9
10
a
b
HOMO levels of the compounds were measured using photoelectron spectroscopy in air (PESA) for thin solid films on glass. The EHOMO−ELUMO
gap was determined by the edge of the absorption spectra, as defined by a wavelength where the absorbance revealed 1/10 at its peak top; that is, the
EHOMO−ELUMO gap [eV] was calculated by 1240 [nm]/optical edge [nm]. ELUMO was thus calculated by the summation of the HOMO potential and
the EHOMO−ELUMO gap. Absorption spectra of thin solid films spin-cast from chlorobenzene solutions.
c
D
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Fedors. In addition, the SP value had a fairly strong correlation
with the Jsc of thin-film organic solar cells prepared using
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ASSOCIATED CONTENT
■
S
* Supporting Information
Analytical data (1H, 13C NMR, and absorption spectra) for the
compounds and the details of the measurement of the solubility
score and the remaining palladium are provided. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: +81-3-5734-2111. Fax: +81-3-5734-2884. E-mail:
sfuse@apc.titech.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Ms. Yoko Otsuka, Center for Advanced Materials
Analysis, Tokyo Institute of Technology for the measurement
of the remaining palladium in the synthesized compounds. We
also thank JST, CREST for financial support.
ABBREVIATIONS
■
(20) Fuse, S.; Sugiyama, S.; Maitani, M. M.; Wada, Y.; Ogomi, Y.;
Hayase, S.; Katoh, R.; Kaiho, T.; Takahashi, T. Elucidating the
structure-property relationship of donor-π-acceptor dyes for DSSCs
through rapid library synthesis via a one-pot procedure. Chem. Eur. J.
2014, 20, 10685.
dba, dibenzylideneacetone; Xantphos, 9,9-dimethyl-4,5-bis-
(diphenylphosphino)xanthene; THF, tetrahydrofuran; Pin,
pinacol; Jsc, short-circuit current; Voc, open-circuit voltage; FF,
fill factor
(21) Fuse, S.; Matsumura, K.; Fujita, Y.; Sugimoto, H.; Takahashi, T.
Development of dual targeting inhibitors against aggregations of
amyloid-β and tau protein. Eur. J. Med. Chem. 2014, 85, 228.
(22) Gupta, A.; Ali, A.; Bilic, A.; Gao, M.; Hegedus, K.; Singh, B.;
Watkins, S. E.; Wilson, G. J.; Bach, U.; Evans, R. A. Absorption
enhancement of oligothiophene dyes through the use of a
cyanopyridone acceptor group in solution-processed organic solar
cells. Chem. Commun. 2012, 48, 1889.
(23) Takahashi, Y.; Ito, T.; Sakai, S.; Ishii, Y. A novel palladium(0)
complex; bis(dibenzylideneacetone)palladium(0). J. Chem. Soc. D:
Chem. Commun. 1970, 1065.
(24) Kranenburg, M.; Vanderburgt, Y. E. M.; Kamer, P. C. J.;
Vanleeuwen, P.; Goubitz, K.; Fraanje, J. New diphosphine ligands
based on heterocyclic aromatics inducing very high regioselectivity in
rhodium-catalyzed hydroformylationEffect of the bite angle.
Organometallics 1995, 14, 3081.
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