Synthesis and characterization of dioctyloxybenzo[1,2- b:4,3- b ′]dithiophene-containing copolymers for polymer solar cells

Article abstract of DOI:10.1021/ma201565f

A series of D-A copolymers containing dioctyloxybenzo[1,2-b:4,3-b′] dithiophene (BdT) donor unit and different acceptor unit of bithiazole (BTz), thiazolo[5,4-d]thiazole (TTz), dithienylylbenzothiadiazole (DTBT), and dithienyl-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), PBdT-BTz, PBdT-TTz, PBdT-DTBT, and PBdT-DPP, were synthesized for the application as donor materials in polymer solar cells (PSCs). Bandgaps and HOMO (the highest occupied molecular orbital) energy levels of the copolymers were tuned effectively by using different acceptor units from 2.01 and -5.16 eV for PBdT-BTz, to 1.91 and -5.19 eV for PBdT-TTz, 1.70 and -5.09 eV for PBdT-DTBT, and 1.41 and -5.01 eV for PBdT-DPP. The PSCs based on the blend of PBdT-TTz (with a lower HOMO of -5.19 eV) and PC₇₀BM (1:3 w/w) exhibited a power conversion efficiency of 3.40% with a J_sc of 5.50 mA/cm², a high V_oc of 0.90 V, and a high FF of 68.7%.

Full text of DOI:10.1021/ma201565f

ARTICLE
pubs.acs.org/Macromolecules
0
Synthesis and Characterization of Dioctyloxybenzo[1,2-b:4,3-b ]-
dithiophene-Containing Copolymers for Polymer Solar Cells
Maojie Zhang, Yeping Sun, Xia Guo, Chaohua Cui, Youjun He, and Yongfang Li*
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China
ABSTRACT: A series of DꢀA copolymers containing dio-
0
ctyloxybenzo[1,2-b:4,3-b ]dithiophene (BdT) donor unit and
different acceptor unit of bithiazole (BTz), thiazolo[5,4-d]-
thiazole (TTz), dithienylylbenzothiadiazole (DTBT), and
dithienyl-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), PBdT-BTz,
PBdT-TTz, PBdT-DTBT, and PBdT-DPP, were synthesized
for the application as donor materials in polymer solar cells
(
PSCs). Bandgaps and HOMO (the highest occupied molec-
ular orbital) energy levels of the copolymers were tuned
effectively by using different acceptor units from 2.01 and ꢀ5.16
eV for PBdT-BTz, to 1.91 and ꢀ5.19 eV for PBdT-TTz, 1.70 and ꢀ5.09 eV for PBdT-DTBT, and 1.41 and ꢀ5.01 eV for PBdT-
DPP. The PSCs based on the blend of PBdT-TTz (with a lower HOMO of ꢀ5.19 eV) and PC BM (1:3 w/w) exhibited a power
7
0
2
conversion efficiency of 3.40% with a J of 5.50 mA/cm , a high V of 0.90 V, and a high FF of 68.7%.
sc
oc
’
INTRODUCTION
In the past decade, polymer solar cells (PSCs) have attracted
copolymer, and the copolymer showed higher photovoltaic
24
performance. Actually, there is another isomer of BDT, benzo-
0
[
1,2-b:4,3-b ]dithiophene (BdT) (see Scheme 1 for the compar-
considerable attention as a new potential renewable resource
because of their advantages of low cost, easy fabrication, light
weight, and the capability to fabricate flexible large-area devices.
PSCs are composed of an active layer of the blend of a conjugated
polymer donor and a fullerene derivative acceptor, sandwiched
between an ITO positive electrode and a low workfunction
metal negative electrode. Regioregular poly(3-hexylthiophene)
ison of the three isomers), which could possess similar property
1ꢀ3
as BDT and BDP. From this consideration, here we synthesized a
0
derivative of benzo[1,2-b:4,3-b ]dithiophene (BdT) and used it
as the donor segment in DꢀA copolymers. In order to tune the
absorption and molecular energy levels to meet the requirements
of an ideal donor toward highly efficient photovoltaic perfor-
mance, we selected several acceptor units to copolymerize with
(
(
P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester
PCBM) are the most representative donor and acceptor mate-
0
0
0
BdT unit, such as 4, 4 -dihexyl-5,5 -di(thiophen-2-yl)-2,2 -bithia-
zole (BTz), 2,5-bis(3-hexylthiophen-2-yl)-thiazolo[5,4-d]thiazole
(TTz), 4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzothiadiazole
rials, respectively, and the power conversion efficiency (PCE) of
4
the PSCs based on P3HT/PCBM has reached over 4%. But
(
[
DTBT), and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo-
3,4-c]pyrrole-1,4-dione (DPP), and investigated the relationship
between their structures and properties.
P3HT has a relatively large band gap (∼2.0 eV) and only harvests
photons with wavelength shorter than 650 nm; in addition, its
relatively high HOMO (the highest occupied molecular orbital)
5
energy level (ꢀ4.76 eV) results in low open-circuit voltage (ca.
0
.6 V) of the PSCs with PCBM as acceptor. Therefore, the design
’
RESULTS AND DISCUSSION
and synthesis of new conjugated polymer donor materials with
broad absorption and lower HOMO energy level has been one of
the most important issues at present. Among many attempts,
alternating electron-rich (donor) and electron-deficient (acceptor)
Synthesis and Thermal Stability. The general synthetic routes
6
for the monomers and copolymers are outlined in Schemes 2
and 3. The alternative DꢀA copolymers were synthesized by
Stille coupling reaction. All the polymers have good solubility
in common organic solvents such as chloroform, toluene, and
chlorobenzene. Molecular weights and polydispersity indices
PDIs) of the polymers were determined by gel permeation
chromatography (GPC) analysis with a polystyrene standard
calibration, and the results are listed in Table 1.
copolymerization has been proved to be an effective strategy to
7
ꢀ16
tune the band gap and energy levels of polymers.
To date, the
PCE of PSCs based on the donorꢀacceptor (DꢀA) copolymers
1
4ꢀ16
has reached ∼6ꢀ7%.
(
0
Benzo[1,2-b:4,5-b ]dithiophene (BDT) has been widely used
as donor unit in high performance conjugated DꢀA copolymer
2,14a,16ꢀ23
photovoltaic materials for PSCs.
BDT unit possesses a
planar structure, and the copolymers containing BDT unit show
higher hole mobility. Recently, benzo[2,1-b:3,4-b ]dithiophene
Received: July 8, 2011
0
Revised:
August 19, 2011
(BDP), which is an isomer of BDT, was also used in photovoltaic
Published: September 08, 2011
r 2011 American Chemical Society
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|Macromolecules 2011, 44, 7625–7631

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Scheme 1. Molecular Structures of the Isomers of Benzodithiophene
a
Scheme 2. Synthetic Routes of the Monomers
a
Reagents and conditions: (i) oxalyl chloride, 1,2-dichloroethane, reflux; (ii) Zn, Ac
2
O, CH
2
Cl
2
; (iii) Cs
2
CO
3
, C
8
H
17I, CH
3
3
CN; (iv) NBS, CHCl /
HOAC; (v) n-BuLi, ꢀ78 °C, (CH
3 3
) SnCl, THF.
Thermal stability of the polymers was investigated with
thermogravimetric analysis (TGA), as shown in Figure 1. The
TGA analysis reveals that the onset temperatures with 5% weight
710 nm, indicating that there is a strong interchain interactions
and good πꢀπ stacking in solid-state polymer films.
HOMO and LUMO Energy Levels. The electrochemical cyclic
voltammetry was performed for determining the highest occu-
pied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) energy levels of the conjugated poly-
loss (T ) of PBdT-BTz, PBdT-TTz, PBdT-DTBT, and PBdT-
d
DPP are 348, 336, 350, and 255 °C, respectively. The slightly
lower T at 255 °C for PBdT-DPP could be due to the slightly
d
25
lower stability of DPP unit. Obviously, the thermal stability of the
copolymers PBdT-BTz, PBdT-TTz, and PBdT-DTBT is good
enough for the applications in optoelectronic devices.
mers. Figure 3 shows the cyclic voltammograms (CVs) of the
copolymer films on Pt disk electrode in 0.1 mol/L Bu NPF
4
6
acetonitrile solution. The onset reduction potentials (jred) of
Absorption Spectra. Figure 2 shows the ultravioletꢀvisible
PBdT-BTz, PBdT-TTz, PBdT-DTBT and PBdT-DPP are ꢀ1.89,
+
(
UVꢀvis) absorption spectra of the polymer dilute solutions in
ꢀ1.81,-1.61 and ꢀ1.35 V vs. Ag/Ag , respectively, while the
chloroform and films spin-coated on quartz substrates. For the
absorption spectra of the copolymer solutions shown in Figure 2a,
PBdT-DTBT and PBdT-DPP exhibited two major absorption
bands in the wavelength ranges of 300ꢀ500 and 500ꢀ900 nm.
The peak at longer wavelength could be attributed to the intra-
molecular charge transfer (ICT) transition, and that at shorter
wavelength comes from the πꢀπ* transition of the polymer
main chain. In contrast, copolymers PBdT-BTz and PBdT-TTz
have one absorption band in the range of 300ꢀ650 nm which is
in coincidence with the weak electron-deficient ability of bithia-
zole and thiazolothiazole units. Apparently, PBdT-DPP has a
much extended absorption with the absorption maximum at
onset oxidation potentials (jox) are 0.45, 0.48, 0.38, 0.33 V
+
vs Ag/Ag , respectively. From jox and jred of the polymers,
HOMO and LUMO energy levels as well as the energy
EC
bandgap (E
) of the polymers were calculated according
g
5,26
to the equations
HOMO ¼ ꢀeðjox þ 4:71Þ ðeVÞ;
LUMO ¼ ꢀeðj þ 4:71Þ ðeVÞ;
red
EC
Eg ¼ eðj ꢀ j Þ ðeVÞ
ox
red
+
where the units of jox and jred are V vs Ag/Ag . The results
of the electrochemical measurements are listed in Table 2. All
the copolymers exhibit relatively deep HOMO energy levels
6
50 nm, in agreement with the strong electron-deficient ability of
DPP unit. The absorption spectra of the four polymers in thin
films are shown in Figure 2b, and the optical properties are
summarized in Table 2. The shape of the peaks and the trend of
the absorption wavelength were generally similar to those of the
polymer solutions. The absorption maxima of the polymer films
all red-shifted in comparison with that in solutions, which is a
common phenomenon for the conjugated polymers owing to the
aggregation of the conjugated polymer main chains in the solid
films. In addition, PBdT-DPP exhibited a broad peak from 650 to
(<ꢀ5.0 eV) which will lead to higher open-circuit voltage (V )
oc
of the PSCs based on these copolymers as donor, since V of the
oc
PSCs is proportional to the difference between the LUMO of the
2
7
electron acceptor and the HOMO of the electron donor.
Hole Mobility. Hole mobility is another important parameter
for the conjugated polymer donor photovoltaic materials. Here
we measured the hole mobilities of the copolymers by space-
charge limit current (SCLC) method by using a device structure
of ITO/PEDOT:PSS/polymer/Au. For the hole-only devices,
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Scheme 3. Synthetic Routes of the Copolymers
Table 1. Molecular Weights and Thermal Properties of the
Polymers
a
a
n
a
b
polymers
M
w
M
PDI
T
d
(°C)
PBdT-BTz
PBdT-TTz
PBdT-DTBT
PBdT-DPP
31.3K
17.2K
19.1K
14.3K
20.2K
11.3K
11.2K
9.1K
1.55
1.52
1.70
1.57
348
336
350
255
a
n w
M , M , and PDI of the polymers were determined by GPC using
b
polystyrene standards in THF. The 5% weight loss temperatures under
an inert atmosphere.
28
SCLC is described by
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Figure 1. TGA plots of the copolymers with a heating rate of
2
3
J = ð9=8Þεε μ V expð0:89 V=E LÞ=L
ð1Þ
0
0
0
10 °C/min under an inert atmosphere.
where ε is the dielectric constant of the polymer, ε is the
0
permittivity of the vacuum, μ is the zero-field mobility, E is the
characteristic field, J is the current density, L is the thickness of
the films, and V = Vappl ꢀ V ; V is the applied potential, and
bi appl
Vbi is the built-in potential which results from the difference in
0
0
the work function of the anode and the cathode (in this device
3
2
0.5
structure, V = 0.2 V). Figure 4 shows the ln(JL /V ) vs (V/L)
bi
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ARTICLE
plots of the polymers measured based on the SCLC method.
According to eq 1, the hole mobilities obtained from the data in
voltage (V ) over 0.8 V, benefitted from the deep HOMO
energy levels of the polymers. In contrast, the device based on
oc
29
ꢀ
4
ꢀ3
ꢀ5
Figure 4 are 5.53 ꢁ 10 , 1.81 ꢁ 10 , 1.67 ꢁ 10 , and 1.66 ꢁ
PBdT-DPP demonstrated a low V of 0.71 V, which is consistent
oc
ꢀ
3
2
1
0
cm /(V s) for PBdT-BTz, PBdT-TTz, PBdT-DTBT, and
with its relatively higher HOMO level. The PSCs based on
PBdT-DTBT exihibited lower FF of 30.7% which could be
caused by its lower hole mobility. The power conversion
efficiency (PCE) of the PSC based on PBdT-BTz/PC BM
PBdT-DPP, respectively. Obviously, the hole mobilities of the
polymers except for PBdT-DTBT are good for the application as
photovoltaic donor materials in PSCs.
7
0
Photovoltaic Properties. To investigate and compare the
photovoltaic properties of the four copolymers, bulk heterojunc-
tion PSC devices with a configuration of ITO/PEDOT:PSS/
polymers:PC BM/Ca/Al were fabricated. Figure 5 shows the
(1:3 w/w) reached 2.81% with a short-circuit current (Jsc) of
5.20 mA/cm , a higher open-circuit voltage (Voc) of 0.88 V, and a
2
FF of 61.3%. The PSC based on PBdT-TTz/PC70BM (1:3 w/w)
2
exhibited an even higher PCE of 3.40% with a Jsc of 5.50 mA/cm ,
7
0
JꢀV curves of the PSCs under the illumination of AM1.5, 100
a Voc of 0.90 V, and a high FF of 68.7%. The high FF of the device
based on PBdT-TTz/PC70BM could be benefited from the
higher hole mobility of PBdT-TTz. But the FF (56.1%) of the
PSC based on PBdT-DPP/PC70BM is lower than that (61.3%)
of the device based on PBdT-BTz/PC BM although the hole
2
mW/cm , and the photovoltaic parameters of the PSCs are
summarized and listed in Table 3. The PSCs based on PBdT-
BTz, PBdT-TTz, or PBdT-DTBT all exhibited high open-circuit
70
ꢀ3
2
mobility (1.66 ꢁ 10 cm /(V s)) of PBdT-DPP is higher than
ꢀ
4
2
that (5.53 ꢁ 10 cm /(V s)) of PBdT-BTz. Probably, the lower
photocurrent influenced the fill factor of the device based on
PBdT-DPP/PC BM.
70
It should be mentioned that the weight ratios of the polymer/
PC BM are different for different polymers in the comparison of
70
their photovoltaic properties. Actually, we performed optimiza-
tion of the photovoltaic performance of the PSCs by changing
the weight ratios for each polymer, and we compared the
photovoltaic properties of the polymers with the optimized
weight ratios in Figure 5 and Table 3.
Figure 6 shows the external quantum efficiency (EQE) curve
of the PSC based on PBdT-TTz:PC BM. It can be seen that the
70
EQE curve covers a broad wavelength range from 300 to 700 nm
with the maximum EQE value of 43% at ca. 475 nm. The integral
Figure 3. Cyclic voltammograms of copolymers film on a platinum
Figure 2. Absorption spetra of the copolymers: (a) in chloroform
solutions; (b) in solid films.
plate electrode measured in 0.1 mol/L Bu
a scan rate of 100 mV/s.
4
NPF
6
acetonitrile solutions at
Table 2. Optical and Electrochemical Properties of the Polymers
UVꢀvis absorption spectra
cyclic voltammetry
n-doping
solution
film
p-doping
opt
g
EC
g
polymers
λmax (nm)
λmax (nm)
λonset (nm)
E
(eV)
jox/HOMO (V)/(eV)
jred/LUMO (V)/(eV)
E
(eV)
PBdT-BTz
PBdT-TTz
PBdT-DTBT
PBdT-DPP
475
485
532
650
481
520
560
710
615
650
733
880
2.01
0.45/ꢀ5.16
0.48/ꢀ5.19
0.38/ꢀ5.09
0.33/ꢀ5.01
ꢀ1.89/ꢀ2.82
ꢀ1.81/ꢀ2.90
ꢀ1.61/ꢀ3.10
ꢀ1.35/ꢀ3.36
2.34
1.91
1.70
1.41
2.29
1.99
1.68
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3
2
0.5
Figure 6. EQE of PSC based on PBdT-TTz: PC70BM (1:3, w/w).
Figure 4. Plot of ln(JL /V ) vs (V/L) of the polymers for the
measurement of hole mobility by the SCLC method.
’
CONCLUSION
We have sythesized a series of copolymers containing diocty-
0
loxybenzo[1,2-b:4,3-b ]dithiophene (BdT) units, PBdT-BTz,
PBdT-TTz, PBdT-DTBT, and PBdT-DPP, by the Pd-catalyzed
Stille-coupling method. Band gaps of the copolymers were
modulated from 2.01 to 1.41 eV, and their HOMO energy levels
were tuned from ꢀ5.01 to ꢀ5.19 eV by using different acceptor
units. The PSCs based on the blend of PBdT-TTz (with HOMO
level of ꢀ5.19 eV) and PC BM (1:3 w/w) exhibited a PCE of
70
2
3
.40% with a J of 5.50 mA/cm , a high V of 0.90 V, and a
sc oc
higher FF of 68.7%. The results indicate that the BdT unit could
be also a good donor unit in composing the DꢀA copolymers for
the application in high performance PSCs.
’
EXPERIMENTAL SECTION
Materials. All chemicals and solvents were reagent grades and
purchased from Aldrich, Alfa Aesar, and TCI Chemical Co. Toluene,
tetrahydrofuran, and diethyl ether were distilled to keep anhydrous
before use. All of the other chemicals were used as received.
Figure 5. JꢀV curves of the PSCs based on polymers: PC70BM under
2
the illumination of AM 1.5, 100 mW/cm .
Measurements and Characterization. All new compounds
1
were characterized by H NMR spectroscopy performed on a Bruker
Table 3. Photovoltaic Performances of the PSCs Based
on Polymer:PC BM under the Illumination of AM1.5,
1
DMX-400 spectrometer. For the H NMR measurements, CDCl
3
was
70
used as the solvent. Chemical shifts in the NMR spectra were reported in
2
1
00 mW/cm
ppm relative to the singlet at 7.26 ppm for CDCl . The molecular weight
3
of the polymers was measured by gel permeation chromatography
(GPC), and polystyrene was used as a standard. Thermogravimetric
analysis (TGA) was performed on a Perkin-Elmer TGA-7. Mass spectra
were obtained with a Shimadzu QP2010 spectrometer. UVꢀvis absorp-
tion spectra were obtained on a Hitachi U-3010 spectrometer. Cyclic
voltammetry was performed on a Zahner IM6e electrochemical work-
station with a three-electrode system in a solution of 0.1 M Bu NPF in
blend ratio
V
oc
J
sc
FF
2
polymers
with PC70BM
(V)
(mA/cm )
(%)
PCE (%)
PBdT-BTz
PBdT-TTz
PBdT-DTBT
PBdT-DPP
1:3
1:3
1:1
1:2
0.88
0.90
0.80
0.71
5.20
5.50
4.58
3.28
61.3
68.7
30.7
56.1
2.81
3.40
1.13
1.31
4
6
acetonitrile at a scan rate of 100 mV/s. The polymer films were coated
2
on a Pt plate electrode (1.0 cm ) by dipping the electrode into the
corresponding solutions and then drying. A Pt wire was used as the
counter electrode, and Ag/Ag was used as the reference electrode.
of the EQE is in good agreement with the J value measured from
+
sc
the JꢀV curve.
Device Fabrication and Characterization of Polymer Solar
Cells. Polymer solar cells (PSCs) were fabricated with ITO glass as a
positive electrode, Ca/Al as a negative electrode, and the blend film of
the polymer/PC BM between them as a photosensitive layer. The ITO
The photovoltaic properties of the DꢀA copolymers reveal
that TTz is a good acceptor unit to copolymerize with BdT donor
unit, and the PSC based on PBdT-TTz:PC BM demonstrated
70
7
0
high Voc of 0.90 V and a high FF of 68.7%. Interestingly, in
comparison with PBdT-TTz, the copolymer of TTz with BDT
glass was precleaned and modified by a thin layer of PEDOT:PSS which
was spin-cast from a PEDOT:PSS aqueous solution (Clevious P VP AI
4083 H. C. Stark, Germany) on the ITO substrate, and the thickness of
the PEDOT:PSS layer is about 60 nm. The photosensitive layer was
prepared by spin-coating a blend solution of polymers and PC BM in
3
0
showed lower V of 0.76ꢀ0.77 V and a lower FF of 51ꢀ56.7%,
oc
indicating that BdT is a supervior donor unit than BDT when
copolymerizing with TTz acceptor unit for the application as
polymer donor materials in PSCs.
70
o-dichlorobenzene on the ITO/PEDOT:PSS electrode. Then the Ca/Al
7
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ARTICLE
cathode was deposited on the polymer layer by vacuum evaporation
The cooling bath was removed, and the reactant was stirred at ambient
temperature for 2 h. Then, it was poured into 200 mL of cool water and
extracted by ether three times. The organic layer was washed by water
ꢀ5
under 5 ꢁ 10 Pa. The thickness of the photosensitive layer is ca. 100 nm
measured on an Ambios Tech. XP-2 profilometer. The effective area of one
2
cell is 4 mm . The currentꢀvoltage (IꢀV) measurement of the devices was
4
two times and then dried by anhydrous MgSO . After removing solvent
conducted on a computer-controlled Keithley 236 source measure unit.
A xenon lamp with AM1.5 filter was used as the white light source, and the
under vacuum, the residue was used in next step for the copolymeriza-
1
tion without further purification. H NMR (CDCl , 400 MHz), δ
3
2
optical power at the sample was 100 mW/cm .
(ppm): 7.72 (s, 2H), 4.25 (t, 4H), 1.87ꢀ1.26 (m, 24H), 0.87 (m,
1
3
Synthesis. The synthetic routes of the monomer and polymers are
3
6H), 0.52(s, 18H). C NMR (CDCl , 100 MHz), δ (ppm): 141.9,
31
32 33
34
shown in Schemes 2 and 3. The compounds 7, 8, 9, and 10 were
synthesized according to the procedure in the literature. The detailed
synthetic processes are as follows.
139.0, 138.2, 132.4, 130.2, 77.6, 77.2, 76.9, 73.7, 32.1, 30.7, 29.7, 29.6,
26.4, 22.9, 14.4, ꢀ8.0.
General Procedure for the Synthesis of Copolymers. These four poly-
mers were synthesized with the same procedure of coupling dibromide
compounds with bis(trialkylstannyl)-substituted compounds. 0.5 mmol
of dibromide compound, 0.5 mmol of bis(trialkylstannyl)-substituted
compound, and dry toluene (12 mL) were added to a 50 mL double-
neck round-bottom flask. The reaction container was purged with argon
0
35
Benzo[1,2-b:4,3-b ]dithiophene-7,8-quinone (2) . Oxalyl chloride
0
(
1
6.0 mL, 69 mmol) was added to a flask containing 3,3 -bithienyl (20.0 g,
20 mmol) dissolved in 1,2-dichloroethane (350 mL), and the mixture,
under a CaCl drying tube open to the atmosphere, was refluxed for
days. More oxalyl chloride (6.0 mL, 69 mmol) was added, and reflux
was continued for another 5 days; after that, the mixture was cooled to
2
5
for 20 min to remove O , and then Pd (PPh ) (15 mg) was added. After
2 3 4
2
5 °C and left to stand overnight. Then the reaction mixture was filtered,
another flushing with argon for 20 min, the reactant was heated to reflux
for 14 h. The reactant was cooled down to room temperature, poured
into MeOH (200 mL), and then filtered through a Soxhlet thimble,
which was then subjected to Soxhlet extraction with methanol, hexane,
and chloroform. Polymer was recovered from the chloroform fraction by
rotary evaporation as solid. The polymer was purified by chromatogra-
phy on silica gel with chloroform as the eluent, and the polymer solution
was concentrated and was poured into MeOH. Then the precipitates
were collected and dried under vacuum overnight.
and the filter cake was triturated thoroughly with hexanes and warm
ethanol. Red solid 2 (18 g) was obtained. Yield: 68%. GC-MS: m/z =
1
220. H NMR (CDCl
3
, 400 MHz), δ (ppm): 7.82 (d, 2H), 7.29 (d, 2H).
0
35
7
,8-Diacetoxybenzo[1,2-b:4,3-b ]dithiophene (3) . Dry CH
2 2
Cl
(300 mL) was added by cannula to a flask containing 2 (5.6 g,
2
2
5 mmol) and Zn dust (16.6 g, 250 mmol). Acetic anhydride (24 mL,
50 mmol) and triethylamine (53 mL, 380 mmol) were sequentially
syringed in. The mixture was stirred for 4 h at 25 °C and then filtered
through Celite. The filtrate was washed with water, 1 M HCl, saturated
aqueous NaHCO
PBdT-BTz. 225 mg. Yield: 47%. GPC: M
= 31.3K; M
, 400 MHz), δ (ppm): 7.75ꢀ7.55 (br, 6H),
4.25 (br, 4H), 2.85 (br, 4H), 1.82ꢀ0.87 (m, 52H). Anal. Calcd for
53 69 2 2 6
C H N O S (%): C, 66.41; H, 7.26; N, 2.92. Found (%): C, 65.32; H,
= 20.2K; M /
w
n w
1
, and again with water. The organic layer was dried
M = 1.51. H NMR (CDCl
n
3
3
(Na SO ), filtered, treated with activated charcoal, and filtered again
2
4
through Celite. Removal of solvent under reduced pressure and further
drying under high vacuum yielded 6.34 g of 3, an off-white solid. Yield:
7.15; N, 2.73.
PBdT-TTz. 192 mg. Yield: 42%. GPC: M
M /M = 1.52. H NMR (CDCl
w n 3
1
83%. GC-MS: m/z = 306. H NMR (CDCl
3
, 400 MHz), δ (ppm): 7.62
w
= 17.2K; M
, 400 MHz), δ (ppm): 7.71ꢀ6.85
(br, 4H), 4.25 (br, 4H), 2.95 (br, 4H), 1.84ꢀ0.87 (m, 52H). Anal. Calcd
for C50 (%): C, 65.46; H, 7.03; N, 3.05. Found (%): C,
n
= 11.3 K;
1
(
d, 2H), 7.51 (d, 2H), 2.41 (s, 6H). Yield: 68%
0
35
7
,8-Dioctyloxybenzo[1,2-b:4,3-b ]dithiophene (4) . A flask contain-
ing 3 (6.34 g, 20.7 mmol) and Cs
2
CO
3
(16 g, 103 mmol) was fitted with
H N O S
64 2 2 6
a reflux condenser. Acetonitrile (300 mL) was added through a cannula,
and 1-iodoocane (33 mL, 103 mmol) was syringed in. The mixture was
stirred and refluxed for 3 days; after that, it was cooled to 25 °C. The
solvent was removed under reduced pressure, and the remaining solids
65.37; H, 6.89; N, 2.88.
PBdT-DTBT. 204 mg. Yield: 45%. GPC: M
w
= 19.1K; M
, 400 MHz), δ (ppm): 7.20ꢀ6.95 (m,
n
= 11.2K;
1
M
/M
w
= 1.70. H NMR (CDCl
n
3
6H), 4.25 (br, 4H), 2.63 (m, 4H), 1.81ꢀ0.90 (m, 52H). Anal. Calcd for
were partitioned between CH
2
Cl
2
and water. The organic layer was
52 66 2 2 5
C H N O S (%): C, 68.53; H, 7.30; N, 3.07. Found (%): C, 67.32; H,
washed with 1 M HCl and with water, dried (Na SO ), and filtered. The
solvent was removed, and the product was triturated thoroughly with
7.15; N, 2.97.
PBdT-DPP. 250 mg. Yield: 50%. GPC: M
M /M = 1.57. H NMR (CDCl
w n 3
2
4
= 14.3K; M
, 400 MHz), δ (ppm): 7.72ꢀ6.99
(br, 6H), 4.25 (br, 4H), 3.01 (br, 4H), 2.01ꢀ0.87 (m, 60H). Anal. Calcd
for C57 (%): C, 69.68; H, 7.90; N, 2.85. Found (%): C,
= 9.1K;
w
n
1
cold ethanol and dried under high vacuum, yielding 8.3 g of 5, off-white,
1
waxy solid. Yield: 90%. GC-MS: m/z = 446. H NMR (CDCl
3
, 400
MHz), δ (ppm): 7.62 (d, 2H), 7.45 (d, 2H), 4.22 (t, 4H), 1.84 (quint,
H N O S
77 2 4 4
4
H), 1.54 (quint, 4H), 1.20ꢀ1.40 (m, 16H), 0.87 (m, 6H).
68.56; H, 7.69; N, 2.68.
0
2
,5-Dibromo-7,8-diacetoxybenzo[1,2-b:4,3-b ]dithiophene (5). Com-
pound 4 (4.46 g, 10.00 mmol) was dissolved in a mixture of chloroform
100 mL) and acetic acid (100 mL). NBS (3.56 g, 20 mmol) was then
added to the solution, which was then stirred for 10 h in the dark. The
product was poured into water (200 mL); the solution was extracted
with chloroform (100 mL ꢁ 3) and dried by magnesium sulfate. The
solvent was removed under reduced pressure, and the crude product
’ AUTHOR INFORMATION
(
Corresponding Author
*
E-mail: liyf@iccas.ac.cn.
’
ACKNOWLEDGMENT
was purified by column chromatography (silica gel, petroleum) to afford 5
1
This work was supported by NSFC (Nos. 20874106,
0821120293, 50933003, and 21021091), The Ministry of
(
5.2 g), an off-white solid. Yield: 86%. H NMR (CDCl
3
, 400 MHz), δ
2
(
ppm): 7.49 (s, 2H), 4.25 (t, 4H), 1.87ꢀ1.26 (m, 24H), 0.87 (m, 6H).
0
Science and Technology of China, and Chinese Academy of
Sciences.
2
,5-Bis(trimethyltin)-7,8-diacetoxybenzo[1,2-b:4,3-b ]dithiophene
(
6). Compound 5 (3.02 g, 5 mmol) and 100 mL of THF were added into
a flask under an inert atmosphere. The solution was cooled down to
78 °C by a liquid nitrogenꢀacetone bath, and 4.4 mL of n-butyllithium
11 mmol, 2.5 M in n-hexane) was added dropwise. After being stirred at
ꢀ
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