Synthesis of a new ladder-type benzodi(cyclopentadithiophene) arene with forced planarization leading to an enhanced efficiency of organic photovoltaics

Article abstract of DOI:10.1021/cm3024082

We have developed a new heptacyclic benzodi(cyclopentadithiophene) (BDCPDT) unit, where 3,7-positions of the central benzo[1,2-b:4,5-b′]dithiophenes (BDT) subunit are covalently rigidified with 3-positons of the two external thiophenes by two carbon bridges, forming two external CPDT rings that share two thiophene rings with the central BDT core. The distannyl-BDCPDT building block was copolymerized with 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione (TPD) by Stille polymerization to afford a new alternating donor-acceptor copolymer PBDCPDT-TPD. The implementation of forced planarization greatly suppresses the interannular twisting to extend the effective conjugated length and preserve the interactions between the donor and acceptor segments. The device using the PBDCPDT-TPD/PC₇₁BM (1:3 in wt%) blend processed with dimethyl sulfoxide as an additive delivered a marked PCE of 6.6% which represents a significant enhancement compared to the device using the corresponding nonfused polymer analogue with a PCE of 0.2%.

Full text of DOI:10.1021/cm3024082

Article
pubs.acs.org/cm
Synthesis of a New Ladder-Type Benzodi(cyclopentadithiophene)
Arene with Forced Planarization Leading to an Enhanced Efficiency
of Organic Photovoltaics
Yung-Lung Chen, Chih-Yu Chang, Yen-Ju Cheng,* and Chain-Shu Hsu*
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan
S
* Supporting Information
ABSTRACT: We have developed a new heptacyclic benzodi-
(cyclopentadithiophene) (BDCPDT) unit, where 3,7-posi-
tions of the central benzo[1,2-b:4,5-b′]dithiophenes (BDT)
subunit are covalently rigidified with 3-positons of the two
external thiophenes by two carbon bridges, forming two
external CPDT rings that share two thiophene rings with the
central BDT core. The distannyl-BDCPDT building block was
copolymerized with 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-
dione (TPD) by Stille polymerization to afford a new
alternating donor−acceptor copolymer PBDCPDT-TPD.
The implementation of forced planarization greatly suppresses
the interannular twisting to extend the effective conjugated
length and preserve the interactions between the donor and
acceptor segments. The device using the PBDCPDT-TPD/PC71BM (1:3 in wt%) blend processed with dimethyl sulfoxide as an
additive delivered a marked PCE of 6.6% which represents a significant enhancement compared to the device using the
corresponding nonfused polymer analogue with a PCE of 0.2%.
KEYWORDS: forced planarization, conjugated polymers, solar cells, additive
interaction for efficient charge transport. Conjugated polymers
incorporating benzo[1,2-b:4,5-b′]dithiophenes (BDT) deriva-
tives have accomplished high-performance polymer solar cells.5
On the other hand, thieno[3,4-c]pyrrole-4,6-dione (TPD)
emerges as an unique acceptor because of its compact
thoiphene-based structure with a moderate electron-with-
drawing ability.6 In 2010, Leclerc et al. first reported the
synthesis of a poly(benzo[1,2-b:4,5-b′]dithiophenes-alt-thieno-
[3,4-c]pyrrole 4,6-dione) PBDT-TPD copolymer. The solar
cell with this polymer reached a high power conversion
efficiency of 5.5% (Scheme 1).7 Meanwhile, Jen et al.8 and
INTRODUCTION
■
Considerable progress has been made on the development of
organic materials for photovoltaic applications in recent years.1
Bulk heterojunction (BHJ) using an n-type and a p-type
semiconductor materials in the active layer is the most widely
adopted device architecture to ensure maximum internal
donor−acceptor (D−A) interfacial area for efficient charge
separation.2 To achieve high efficiency of PSCs, the most
critical challenge at the molecular level is to develop a p-type
conjugated polymer that can simultaneously possess sufficient
solubility for processability and miscibility with an n-type
material low band gap (LBG) for strong and broad absorption
spectrum to capture more solar photons and high hole mobility
for efficient charge transport. 4H-Cyclopenta[2,1-b:3,4-b′]-
dithiophene (CPDT) has attracted considerable research
interest because it can act as an excellent building block to
construct LBG polymers.3 Poly[cyclopentadithiophene)-alt-4,7-
(2,1,3-benzothiadiazole)] (PCPDTBT) represents one of the
early successful LBG polymers for the use in PSCs.4 The
abilities to lower HOMO−LUMO energy band gap and
enhance charge mobility are the advantageous features of the
coplanar CPDT moieties. The electron-rich benzo[1,2-b:4,5-
b′]dithiophene (BDT) derivatives, a central benzene ring fused
with two thiophenes, are an attractive building block due to its
symmetric, rigid, and coplanar structure. These structural
characteristics are crucial to induce strong intermolecular π−π
Frec
́
het et al.9 also independently reported the PBDT-TPD
derivatives with similar device results. In an attempt to further
optimize light-harvesting and charge transporting abilities, two
additional 3-octylthiophene units were introduced to the two
sides of the BDT cores in the backbone of PBDT-TPD.10
Unfortunately, the resulting polymer, poly(dithienyl BDT-alt-
TPD) (PDTBDT-TPD), only yielded an unexpectedly low
PCE of 0.2% (Scheme 1).10 The dramatic decay of the
performance might be attributed to the steric hindrance that
severely causes the interannular twisting along the conjugated
backbone of PDTBDT-TPD.11 A straightforward strategy to
Received: July 30, 2012
Revised: September 13, 2012
Published: September 13, 2012
© 2012 American Chemical Society
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Scheme 1. Chemical Structures of PBDT-TPD, PDTBDT-TPD, and PBDCPDT-TPD
Scheme 2. Synthetic Route of the Sn-BDCPDT Monomer Leading to the Targeted PBDCPDT-TPD copolymer
circumvent this structural deficiency is forced planarization by
covalently fastening adjacent aromatic units in the polymer
backbone, which strengthens the parallel p-orbital interactions
to elongate effective conjugation length and facilitate intrinsic
charge mobility.12
more planar and expanded conjugated structure. Furthermore,
four aliphatic side chains at the two bridging carbons can
readily provide sufficient solubility. As such, introducing
aliphatic side chains at 4, 8 positions of the BDT core of
BDCPDT is not necessary and can be thus omitted for better
synthetic economy.
In this research, we have designed a new heptacyclic
benzodi(cyclopentadithiophene) (BDCPDT) unit, where 3,7-
positions of the central BDT subunit are covalently bridged
with 3-positons of the two outer thiophenes by two carbon
atoms. The ladder-type BDCPDT structure contains two
external CPDT rings that share the two thiophene rings with
the central BDT core. The BDCPDT unit was copolymerized
with TPD unit to create a new alternating copolymer
PBDCPDT-TPD (Scheme 1). In fact, PDTBDT-TPD and
PBDCPDT-TPD have similar structural composition and
arrangement of the conjugated backbones. The major differ-
ence is that the BDCPDT unit in PBDCPDT-TPD is
covalently planarized, whereas the dithienyl-BDT segment in
PDTBDT-TPD is a nonfused open-chain counterpart. This
structural variation provides a good model to investigate the
planarization/rigidification effect. It is anticipated that this new
CPDT/BDT hybridized structure may inherit the intrinsic
advantages from the CPDT and BDT moieties leading to
promising electronic and optical properties attributable to a
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic route of
BDCPDT is depicted in Scheme 2. Stille coupling of 2,6-
trimethylstannyl BDT (1)13 with ethyl 2-bromothiophene-3-
carboxylate (2) yielded compound 3. Double nucleophilic
addition of the ester groups in 3 by (4-octyloxy)phenyl
magnesium bromide led to the formation of benzylic alcohols
in 4 which was subjected to intramolecular Friedel−Crafts
cyclization under acidic condition to successfully furnish the
heptacyclic arene BDCPDT. It should be noted that conven-
tional preparation of the tricyclic CPDT unit generally requires
tedious multistep synthesis.14 Due to the ring strain, the
formation of CPDT derivatives by using Friedel−Crafts
cyclization is also not successful with low chemical yield.15
Encouragingly, in our system, we are able to construct two
CPDT rings in BDCPDT with an excellent yield of 88%. To
preserve aromaticity of the central benzene ring, 3,7-positions
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Table 1. Summary of the Intrinsic Properties of the PBDCPDT-TPD
λmax (nm)
toluene
634
polymer
Mn (kDa)
PDI
3.06
Td (°C)
Egopt (eV) (film)
film
HOMO (eV)
LUMO (eV)
PBDCPDT-TPD
24.5
410
1.88
645
−5.36
−3.20
of the benzodithiophene core intrinsically have stronger
nucleophilicity, thereby facilitating intramolecular electrophilic
aromatic substitution. Afterward, the BDCPDT unit can be
lithiated by n-butyllithium followed by quenching with
trimethyltin chloride to afford distannyl Sn-BDCPDT in a
moderate yield of 62%. Sn-BDCPDT monomer was then
polymerized with 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione
acceptor by Stille coupling to afford a highly soluble
PBDCPDT-TPD (Mn = 24.5 kDa, PDI = 3.06) (Scheme 2,
Table 1).
in toluene solution (Table 1). Moreover, a vibronic shoulder in
its absorption spectrum reflecting rigid and coplanar character-
istics of BDCPDT was observed again (Figure 2). Although the
Thermal Properties. The thermal stability of the polymers
was analyzed by thermogravimetric analysis (TGA). It should
be noted that the fused PBDCPDT-TPD showed much higher
decomposition temperatures (Td) of 410 °C (Figure 1) than
Figure 2. Normalized absorption spectra of PBDCPDT-TPD in
toluene solution and in solid state.
profiles are essentially unchanged, the absorption spectrum of
PBDCPDT-TPD shifts toward longer wavelengths from the
solution state to the solid state, indicating that the planar
structure of BDCPDT is capable of inducing strong interchain
π−π* interactions.
Cyclic voltammetry (CV) was employed to examine the
electrochemical properties (Figure 3). PBDCPDT-TPD
showed stable and reversible p-doping/n-doping process in
the cathodic and anodic scans, and a deep-lying HOMO energy
level of −5.36 eV was estimated, which is at the ideal range to
Figure 1. Thermogravimetric analysis of PBDCPDT-TPD.
the nonfused PDTBDT-TPD with Td of 330 °C.10 It has been
proposed that dialkoxy groups at the 4, 8-positions of BDT
units are the relatively weak sites to undergo thermal
decomposition.16 Consistently, the absence of 4, 8-substituents
on the BDT core in our molecular design indeed improves the
thermal stability of PBDCPDT-TPD. Thermal properties of
the polymers were determined by differential scanning
calorimetry (DSC). PBDCPDT-TPD showed neither glass
transition temperatures (Tg) nor melting point, suggesting that
these polymers tend to form amorphous glasses.
Optical and Electrochemical Properties. In contrast to
PBDTTPD’s absorption spectrum showing a λmax at 614 nm
with vibronic structures, PDTBDT-TPD having two more
thiophene rings in the repeating unit exhibited a single broad
band with an unexpected blue-shifted λmax at 517 nm.10 The ca.
100 nm hypsochromic shift of the λmax implies that the
alkylthiophene rings cause steric hindrance to the central BDT
cores, giving rise to large twisting dihedral angles around the
interannular single bonds. Such disorder twisting conforma-
tions will in turn weaken the orbital interactions between the
electron-rich segments and electron-deficient TPD units.
However, with implementation of the forced planarization,
PBDCPDT-TPD does display much red-shifted λmax at 634 nm
Figure 3. Cyclic voltammogram of PBDCPDT-TPD in the thin film
at a scan rate of 100 mV/s.
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ensure better air stability and greater attainable Voc in the
device. The LUMO energy level was determined to be −3.2 eV.
Photovoltaic and OFET Characteristics. Bulk hetero-
junction photovoltaic cells were fabricated on the basis of ITO/
PEDOT:PSS/PBDCPDT-TPD:PC71BM/Ca/Al configuration,
and their performances were measured under a simulated AM
1.5 G illumination of 100 mW/cm2. The characterization data
are summarized in Table 2, and the J−V curves of these
700 nm of light illumination, which is highly associated with
PBDCPDT-TPD’s improved absorption ability (Figure 4). To
gain more insight into the performance enhancement upon the
introduction of DMSO additive, the morphology of the blend
films was investigated by atomic force microscope (AFM)
(Figure 5). Compared to the pristine film, the additive-
processed PBDCPDT-TPD:PC71BM film indeed showed
larger surface roughness presumably due to the higher degree
of PBDCPDT-TPD aggregation, resulting in more favorable
morphological phase separation for efficient charge separation
and transport. The morphological changes induced by the
DMSO additive were consistent with the hole mobility (uh)
estimated from space-charge-limited current (SCLC) and field-
effect transistor (FET) measurements, as shown in Table 3.
The SCLC hole mobility of the additive-processed device
reached 2.5 × 10−3 cm2 V−1 s−1, which is more than 1 order of
magnitude higher than that of the nonadditive device (4.0 ×
10−4 cm2 V−1 s−1). In addition, a slightly higher FET hole
mobility (6.8 × 10−2 cm2 V−1 s−1) was also observed in the
additive-processed device as compared with that of the
nonadditive device (5.6 × 10−2 cm2 V−1 s−1) (Figure 6).
Furthermore, by changing the hole-extraction layer from
PEDOT:PSS to a layer of MoO3 in device C,19 all the
parameters were further improved, leading to a highest
efficiency of 6.6% with a Voc of 0.87 V, a Jsc of 12.21 cm2
V−1 s−1, and a FF of 62.2%. It has been known that the work
function of MoO3 is located at −5.4 eV.19 The enhancement
may be ascribed to the close energy alignment between the
work function of MoO3 and the HOMO energy level of
PBDCPDT-TPD, thereby facilitating charge transporting at the
interface.
Table 2. Photovoltaic Performances of the Devices
copolymers
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
a
PDTBDT-TPD
0.66
0.85
0.85
0.87
1.2
26
0.2
5.1
6.1
6.6
b
c
PBDCPDT-TPD
PBDCPDT-TPD
PBDCPDT-TPD
11.71
11.95
51.7
60.4
62.2
d
12.21
a
b
Data reported from ref 10. Device A: ITO/PEDOT:PSS/
c
PBDCPDT-TPD:PC71BM (1:3 in wt %)/Ca/Al. Device B: ITO/
PEDOT:PSS/PBDCPDT-TPD:PC71BM (1:3 in wt %) with DMSO/
d
Ca/Al. Device C: ITO/MoO3/PBDCPDT-TPD:PC71BM (1:3 in wt
%) with DMSO/Ca/Al.
polymers are shown in Figure 4. The device A using
PBDCPDT-TPD:PC71BM (1:3 in wt %) showed a high PCE
of 5.1% with a Voc of 0.85 V, a Jsc of 11.71 cm2 V−1 s−1, and a FF
of 51.7%. In sharp contrast, the device incorporating nonfused
PDTBDT-TPD counterpart only achieved a low PCE of 0.2%
with a Voc of 0.66 V, a Jsc of 1.2 cm2 V−1 s−1, and a FF of 26%.10
Clearly, twisting of the conjugated backbone of PDTBDT-
TPD severely affects the charge transporting properties.
Nanoscale morphology of the active layer can be controlled
and optimized by judicial choice of the processing additives.17
We envision that polar dimethylsulfoxide (DMSO) can serve as
a good alternative for a binary polymer:PC71BM blending
system.18 First, the boiling point of DMSO is higher than that
of the host solvent of ortho-dichlorobenzene (ODCB). Second,
DMSO is a good solvent for the PC71BM component but a
relatively poor solvent for the PBDCPDT-TPD polymer.
Therefore, the presence of a small amount of DMSO may
induce a closer intermolecular packing of PBDCPDT-TPD.
Encouragingly, by adding 5 vol % DMSO into ODCB as the
processing additive in device B, the PCE was improved to 6.1%
with a Voc of 0.85, a Jsc of 11.95 cm2 V−1 s−1, and a FF of 60.4%.
Compared to the nonadditive device, the device processed with
DMSO additive exhibited a pronounced enhancement of
incident photon to current efficiency (IPCE) from 570 to
CONCLUSIONS
■
We have developed a new ladder-type BDCPDT unit
integrating the well-known CPDT and BDT subunits into a
coplanar conjugated entity. By taking advantage of the stronger
nucleophilicity at the 3,7-positions of the BDT unit, BDCPDT
arene can be successfully synthesized with a high yield through
a Friedel−Crafts cyclization. Due to the coplanar, symmetrical,
and elongated conjugated structure of BDCPDT, strong
donor−acceptor interactions along the conjugated backbone
can be preserved, leading to more red-shifted and broader
absorption spectrum and better charge mobility. Under the
optimal device conditions using DMSO as the additive and
MoO3 as the hole-selective layer, the device using PBDCPDT-
Figure 4. Current density−voltage characteristics (left) and IPCE spectra (right) of the devices. Device A: ITO/PEDOT:PSS/PBDCPDT-
TPD:PC71BM (1:3 in wt %)/Ca/Al); device B: ITO/PEDOT:PSS/PBDCPDT-TPD:PC71BM (1:3 in wt %) with DMSO/Ca/Al); device C: ITO/
MoO3/PBDCPDT-TPD:PC71BM (1:3 in wt %) with DMSO/Ca/Al).
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Figure 5. AFM tapping mode height images of the surface of PBDCPDT-TPD/PC71BM (left) and PBDCPDT-TPD/PC71BM with 5 vol % DMSO
(right) (1.0 μm × 1.0 μm).
thermally evaporated MoO3 as the hole-extraction layer. After
annealing in air at 150 °C during 30 min, the samples were cooled
down to rt. The active layer (90 nm thick) consisting of the blend of
PBDCPDT-TPD and PC71BM (Nano C) with a weight ratio of 1:3
(with or without 5 vol % DMSO) was then spin coated from ortho-
dichlorobenzene (ODCB) solvent. After annealing the active layer at
150 °C for 10 min, the cathode made of calcium (25 nm thick) and
aluminum (100 nm thick) was evaporated through a shadow mask
under high vacuum (<10−6 Torr). Each device is constituted of four
pixels defined by an active area of 0.04 cm2. The current density−
voltage (J−V) characteristics were measured under simulated air mass
1.5 global (AM 1.5G) irradiation of 100 mW cm−2.
Hole-Only Devices. In order to investigate the respective hole
mobility of the different copolymer films, unipolar devices have been
prepared following the same procedure except that the active layer is
made of pure PBDCPDT-TPD and the Ca/Al cathode is replaced by
evaporated gold (40 nm). The hole mobilities were calculated
according to space charge limited current theory (SCLC). The J−V
curves were fitted according to the following equation:
Table 3. SCLC and FET Hole Mobilities of the Device
Processed with and without DMSO Additive
condition
uh (cm2 V−1 s−1
)
no additive, SCLC
with additive, SCLC
no additive, FET
with additive, FET
4.0 × 10−4
2.5 × 10−3
5.6 × 10−2
6.8 × 10−2
TPD:PC71BM (1:3 in wt %) has delivered a marked PCE of
6.6%, which is one of the best performances among the BDT-
based or TPD-based copolymers. This efficiency dramatically
outperforms the device using the corresponding nonfused
PDTBDT-TPD as the donor material.
EXPERIMENTAL SECTION
■
All chemicals are purchased from Aldrich or Acros and used as
1
received unless otherwise specified. H and 13C NMR spectra were
V2
L3
measured using a Varian 300 MHz instrument spectrometer.
Differential scanning calorimetery (DSC) was measured on a TA
Q200 Instrument, and thermogravimetric analysis (TGA) was
recorded on a Perkin-Elmer Pyris under nitrogen atmosphere at a
heating rate of 10 °C/min. Absorption spectra were collected on a
HP8453 UV−vis spectrophotometer. The molecular weights of
polymers were measured by the GPC method on a Viscotek
VE2001GPC, and polystyrene was used as the standard (THF as
the eluent). The electrochemical cyclic voltammetry (CV) was
conducted on a Autolab ADC 164. A glassy carbon electrode coated
with a thin polymer film was used as the working electrode and
standard calomel electrode as the reference electrode, while 0.1 M
tetrabutylammonium tetrafluoroborate (Bu4NBF4) in acetonitrile was
the electrolyte. CV curves were calibrated using ferrocence as the
standard, whose oxidation potential is set at −4.8 eV with respect to
zero vacuum level. The HOMO energy levels were obtained from the
equation HOMO = −e(Eoxonset − E(ferrocene)onset + 4.8) eV. The LUMO
levels of polymer were obtained from the equation LUMO =
9
J = εμ
8
where ε is the dielectric permittivity of the PBDCPDT-TPD, μ is the
hole mobility, and L is the film thickness (distance between the two
electrodes)
FET Device Fabrication. An n-type heavily doped Si wafer with a
SiO2 layer of 300 nm and a capacitance of 11 nF cm−2 was used as the
gate electrode and dielectric layer. Thin films (40−60 nm in thickness)
of PBDCPDT-TPD were deposited on octadecantrichlorosilane
(OTS)-treated SiO2/Si substrates by spin-coating their o-dichlor-
obenzene solutions (6 mg mL−1). Then, the thin films were annealed
at 150 °C for 10 min. Gold source and drain contacts (60 nm thick)
were deposited by vacuum evaporation on the active layer through a
shadow mask, affording a bottom-gate, top-contact device config-
uration. Electrical measurements of OTFT devices were carried out at
room temperature in air using a 4156C (Agilent Technologies). The
field-effect mobility was calculated in the saturation regime by using
the equation IDS = (μWCi/2L)(VG − VT)2, where IDS is the drain-
source current, μ is the field-effect mobility, W is the channel width
(3500 μm), L is the channel length (60 μm), Ci is the capacitance per
unit area of the gate dielectric layer, and VG is the gate voltage.
Synthesis of Compound 3. To a 100 mL round-bottom flask was
introduced 1 (2 g, 3.88 mmol), ethyl 2-bromothiophene-3-carboxylate
2 (2.81 g, 9.31 mmol), Pd(PPh3)4 (0.22 g, 0.19 mmol), and degassed
onset
onset
−e(Ered
− E(ferrocene)
+ 4.8) eV. Surface topography was
investigated using Veeco Nanoscope 3100 AFM and standard tips
(type Tap 300; L, 135 m; FREQ, 300 MHz; k, 40 N/m).
Solar Cell Device Fabrication. ITO/Glass substrates were
ultrasonically cleaned sequentially in detergent, water, acetone, and
isopropyl alcohol. Then, the substrates were covered by either a 30 nm
PEDOT:PSS (Clevios 4083 provided by H. C. Stark) or a 10 nm
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Figure 6. Output characteristics of p-type OFET devices of PBDCPDT-TPD (a, b) and PBDCPDT-TPD with 5 vol % DMSO (c, d).
toluene (40 mL). The mixture was heated to reflux under nitrogen for
16 h. After removal of toluene under reduced pressure, the residue was
purified by column chromatography on silica gel (hexane/ethyl
acetate, v/v, 10/1) and then recrystallized from THF to give an orange
solid 3 (1.34 g, 69%). 1H NMR (CDCl3, 300 MHz, ppm): δ 1.31 (t, J
= 7.2 Hz, 6 H), 4.33 (q, J = 7.2 Hz, 4H), 7.29 (d, J = 5.4 Hz, 2 H), 7.55
(d, J = 5.4 Hz, 2 H), 7.75 (s, 2 H), 8.21 (s, 2 H). 13C NMR (CDCl3,
75 MHz, ppm): δ 14.4, 61.1, 116.9, 125.1, 129.4, 130.9, 135.1, 138.1,
138.3, 142.7, 158.0, 163.2. MS (EI, C24H18O4S4): calcd, 498.66; found,
498.
131.8, 132.1, 136.9, 137.7, 138.1, 140.0, 146.9, 158.6. MS (FAB,
C76H94O6S4): calcd, 1231.82; found, 1230.
Synthesis of BDCPDT. To a solution of 4 (1 g, 0.8 mmol) in
octane (100 mL) was added acetic acid (10 mL) and sulfuric acid (1
mL, 19 mmol) slowly. The resulting solution was stirred for 4 h at 65
°C. After removal of the octane under reduced pressure, the residue
was extracted by sodium carbonate solution (50 mL × 3) and diethyl
ether (50 mL × 2) . Then, the crude products were purified by column
chromatography on silica gel (hexane/ethyl acetate, v/v, 100/1) to
1
give a yellow oil BDCPDT (0.85 g, 88%). H NMR (CDCl3, 300
Synthesis of Compound 4. A Grignard reagent was prepared by
the following procedure. To a suspension of magnesium turnings (0.36
g, 15.0 mmol) and 3−4 drops of 1,2-dibromoethane in dry THF (10
mL) was slowly added 1-bromo-4-(octyloxy)benzene (4.02 g, 15.0
mmol) dropwise, and the mixture was stirred for 1 h. To a solution of
3 (0.5 g, 1 mmol) in dry THF (40 mL) under nitrogen was added 4-
(octyloxy)benzene 1-magnesium bromide (5 mL, 5 mmol) dropwise
at room temperature. The resulting mixture was heated at reflux for 16
h. The reaction solution was extracted with diethyl ether (50 mL × 3)
and ammonium chloride solution (150 mL). The combined organic
layer was dried over MgSO4. After removal of the solvent under
reduced pressure, the residue was purified by column chromatography
on silica gel (hexane/ethyl acetate, v/v, 10/1) to give a brown oil 4
MHz, ppm): δ 0.88 (t, J = 6.9 Hz, 12 H), 1.28−1.45 (m, 40 H), 1.71−
1.76 (m, 8 H), 3.88 (t, J = 6.6 Hz, 8 H), 6.77 (d, J = 8.7 Hz, 8 H), 7.07
(d, J = 4.8 Hz, 2 H), 7.17 (d, J = 8.7 Hz, 8 H), 7.25 (d, J = 4.8 Hz, 2
H), 7.88 (s, 2 H).13C NMR (CDCl3, 75 MHz, ppm): 14.4, 22.9, 26.3,
29.5, 29.6, 32.1, 61.9, 68.1, 114.6, 116.2, 123.0, 127.2, 129.5, 131.2,
134.0, 136.7, 137.4, 141.8, 149.6, 158.4, 162.1. MS (FAB,
C76H90O4S4): calcd, 1195.79; found, 1195.
Synthesis of Sn-BDCPDT. To a solution of BDCPDT (0.45 g,
0.38 mmol) in dry THF (15 mL) was added a 2.5 M solution of n-
BuLi in hexane (0.4 mL, 1 mmol) dropwise at −78 °C. After stirring at
−78 °C for 30 min and at RT for 30 min, a 1.0 M solution of
chlorotrimethylstannane in THF (1.14 mL, 1.14 mmol) was
introduced by syringe to the solution at −78 °C. The mixture
solution was quenched with water and extracted with diethyl ether (50
mL × 2) and water (50 mL). The combined organic layer was dried
over MgSO4. After removal of the solvent under reduced pressure, the
residue was purified by recrystallizing from hexane to give a yellow
solid Sn-BDCPDT (382 mg, 62%). 1H NMR (CDCl3, 300 MHz,
ppm): δ 0.36 (s, 18 H), 0.88 (t, J = 6 Hz, 12 H), 1.28−1.43 (m, 40 H),
1
(1.15 g, 93%). H NMR (CDCl3, 300 MHz, ppm): δ 0.89 (t, J = 6.3
Hz, 12 H), 1.30−1.46 (m, 40 H), 1.75−1.80 (m, 8 H), 3.44 (s, 2 H),
3.93 (t, J = 6.6 Hz, 8 H), 6.45 (d, J = 5.4 Hz, 2 H), 6.82 (d, J = 7.8 Hz,
8 H), 6.88 (s, 2 H), 7.158 (d, J = 7.8 Hz, 8 H), 7.162 (d, J = 5.4 Hz, 2
H), 7.87 (s, 2 H). 13C NMR (CDCl3, 75 MHz, ppm): δ 14.4, 23.0,
26.3, 29.6, 29.7, 32.0, 68.2, 80.4, 114.0, 116.6, 124.4, 124.5, 129.1,
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1.70−1.74 (m, 8 H), 3.88 (t, J = 6.6 Hz, 8 H), 6.76 (d, J = 9 Hz, 8 H),
7.07 (s, 2 H), 7.17 (d, J = 9 Hz, 8 H), 7.82 (s, 2 H). 13C NMR
(CDCl3, 75 MHz, ppm): δ −7.74, 14.3, 22.9, 26.3, 29.45, 29.51, 29.6,
32.0, 61.3, 68.1, 114.5, 116.1, 129.6, 130.2, 131.3, 134.3, 137.4, 141.1,
141.7, 142.5, 149.4, 158.3, 164.0. MS (FAB, C82H106O4S4Sn2): calcd,
1521.4; found, 1521.
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Synthesis of PBDCPDT-TPD via Microwave-Assisted Stille
Coupling Polymerization. To a 50 mL round-bottom flask was
introduced Sn-BDCPDT (180 mg, 0.118 mmol), 1,3-dibromo-
thieno[3,4-c]pyrrole-4,6-dione (50 mg, 0.118 mmol), Pd2(dba)3 (4.3
mg, 0.0047 mmol), tri(o-tolyl)phosphine (11.5 mg, 0.037 mmol), and
dry chlorobenzene (8 mL). The mixture was then degassed by
bubbling nitrogen for 10 min at room temperature. The round-bottom
flask was placed into the microwave reactor and reacted for 50 min
under 270 W. To end-cap the resulting polymer, tributyl(thiophen-2-
yl)stannane (22.3 mg, 0.059 mmol) was added to the mixture solution
and reacted for 10 min under 270 W. Finally, 2-bromothiophene (9.61
mg, 0.063 mmol) was added to the mixture solution and reacted for 10
min under 270 W. The solution was added into methanol dropwise.
The precipitate was collected by filtration and washed by Soxhlet
extraction with acetone and hexane sequentially for three days. The
polymer was dissolved in hot THF, and the residual Pd catalyst and Sn
metal in THF solution was removed by Pd-thiol gel and Pd-TAAcOH
(Silicycle Inc.). After filtration and removal of the solvent, the polymer
was redissolved in THF again and added into methanol to precipitate.
The purified polymer was collected by filtration and dried under
vacuum for 1 day to give a dark-purple fiber-like solid (140 mg, 81%,
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1
Mn = 24500, PDI = 3.06). H NMR (CDCl3, 300 MHz): δ 0.86 (br,
12 H), 1.26−1.38 (br, 40 H), 1.72 (br, 8 H), 3.88 (br, 8 H), 6.78 (br, 8
H), 7.18 (br, 8 H), 7.87 (br, 4 H).
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ASSOCIATED CONTENT
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́
S
(11) Zhou, H.; Yang, L.; Xiao, S.; Liu, S.; You, W. Macromolecules
* Supporting Information
1H and 13C NMR spectra of the new compounds and
PBDCPDT-TPD polymer. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: yjcheng@mail.nctu.edu.tw (Y.-J.C.); cshsu@mail.nctu.
edu.tw (C.-S.H.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Science Council and
“ATP program” of the National Chiao Tung University and
Ministry of Education, Taiwan.
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