Low bandgap polymers based on silafluorene containing multifused heptacylic arenes for photovoltaic applications

Article abstract of DOI:10.1021/ma300839c

A series of donor-acceptor copolymers based on a new silafluorene containing multifused heptacylic arenes have been designed and synthesized in order to further modulate and optimize their electronic and optical properties. Polymer solar cells based on a blend of these polymers and PC₆₁BM exhibited high open circuit voltages of up to 0.86 V. Through simple and straightforward engineering of molecular structures, the devices based on the PSiFDCTBT:PC₆₁BM (1:3.5 in wt %) blend provided, on average, a V _oc of 0.86 V, a J_sc of 8.8 mA/cm², a FF of 56%, delivering a PCE of 4.2%.

Full text of DOI:10.1021/ma300839c

Article
pubs.acs.org/Macromolecules
Low Bandgap Polymers Based on Silafluorene Containing Multifused
Heptacylic Arenes for Photovoltaic Applications
Mingjian Yuan,† Pinyi Yang,† Matthew M. Durban,‡ and Christine K. Luscombe*,†
†Materials Science and Engineering Department, University of Washington, Seattle, Washington 98195-2120, United States
‡Department of Chemistry, University of Washington, Seattle, Washington 98195-1750, United States
S
* Supporting Information
ABSTRACT: A series of donor−acceptor copolymers based
on a new silafluorene containing multifused heptacylic arenes
have been designed and synthesized in order to further
modulate and optimize their electronic and optical properties.
Polymer solar cells based on a blend of these polymers and
PC61BM exhibited high open circuit voltages of up to 0.86 V.
Through simple and straightforward engineering of molecular
structures, the devices based on the PSiFDCTBT:PC61BM
(1:3.5 in wt %) blend provided, on average, a Voc of 0.86 V, a
Jsc of 8.8 mA/cm2, a FF of 56%, delivering a PCE of 4.2%.
In considering the above, we have designed and synthesized a
new variety of low bandgap copolymers that contain a
multifused thienyl−fluorene−thienyl subunit where the bridg-
ing carbon atom on the fluorine has been replaced with a
silicon atom. Forced planarization by covalently fastening
adjacent aromatic units within the polymer backbone helps to
strengthen parallel π-orbital interactions to extend the effective
conjugation length and facilitate π-electron delocalization,
providing an effective way to reduce the band gap. Moreover,
coplanar geometries and rigid structures can suppress rotational
disorder around interannular single bonds and lower the
reorganization energy, which can in turn enhance intrinsic
charge mobilities.8
In order to further modulate and optimize the electronic and
optical properties, exploration of different electron-deficient
units incorporated into multifused thienyl-fluorene-thienyl
based polymeric backbone is highly desirable. Benzothiadiazole
(BT) and thieno[3,4-c]pyrrole-4,6-dione (PD) units are widely
used electron-deficient units introduced in the D−A copoly-
mers due to their suitable electron affinity and easy synthesis.9
In addition, diketopyrrolopyrrole (DPP) has also emerged as a
useful acceptor unit because of its planar conjugated bicyclic
structure and strong electron-withdrawing nature of polar
amide group.10 On the basis of the modified multifused
thienyl−fluorene−thienyl as the core structure, we have
successfully synthesized three D−A copolymers, PSiFDCTPD,
PSiFDCTBT, and PSiFDCTDPP (Scheme 1). The synthesis,
characterization, and photovoltaic applications of these
polymers will be discussed.
INTRODUCTION
■
Polymer solar cells (PSCs) have attracted increased attention
due to their potential application toward flexible, large-area, and
low-cost photovoltaic devices.1 Since the introduction of the
bulk heterojunction (BHJ) concept, significant efforts have
been put forth to improve the power conversion efficiency
(PCE) to meet the criterion of high performance photovoltaic
applications.2 Very recently, efficiencies higher than 8% have
been achieved by several groups, which shows great potential
for commercialization.3
To achieve highly efficient PSCs, conjugated polymers with
relatively low band gaps, high absorption-coefficients, balanced
HOMO and LUMO energy levels, good solubility, and
appropriate miscibility with fullerene derivatives in blended
active-layers are important prerequisites. Extensive π-conjuga-
tion of a rigid polymer backbone will facilitate intermolecular
interactions between polymer chains and increase the charge
mobility of the polymers.4 Recently, several ladder-type
copolymers have been investigated for achieving efficient
PSCs.5 Specifically, silole-containing semiconducting polymers
have been known to exhibit altered properties with respect to
their carbon analogues. A variety of functionalized silole-
containing semiconducting polymers have been reported to
show promising characteristics as materials for PSCs.6,7 The
silole(silacyclopentadiene) moiety has been investigated as a
system where the σ*-orbital of the silicon−carbon bond
interacts with the π*-orbital of the butadiene fragment resulting
in a lower lying LUMO.7a The research groups of Yang and
Brabec have independently shown that replacing the bridging
carbon atom in cyclopenta[2,1-b;3,4-b′]dithiophene containing
polymers with silicon helps to increase the crystallinity of the
polymers7b,c providing polymers that have relatively high charge
carrier mobilities.7d Studies have also indicated that these
silicon bridged polymers have good environmental stability.7e
Received: April 25, 2012
Revised: July 19, 2012
Published: July 27, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Route for the SiFDCT Monomer and the Corresponding Copolymers
Figure 1. Normalized absorption spectra of PSiFDCTPD, PSiFDCTBT, PSiFDCTDPP in chloform (a) and as a thin film (b).
give PSiFDCTPD (Mn = 16.3 kDa, PDI = 2.1), PSiFDCTBT
(Mn = 37.4 kDa, PDI = 2.5), and PSiFDCTDPP (Mn = 10.4
kDa, PDI = 1.6), respectively. The structures of the polymers
were determined by NMR spectroscopy. All polymers showed
excellent solubilities in common organic solvents, such as
chloroform, toluene and dichlorobenzene.
Optical Properties. As shown in Figure 1a, the absorption
spectra of the three polymers in dilute chloroform exhibited
two characteristic bands. Each polymer has an absorption band
located between 300 and 550 nm with a second broad
absorption from 500 to 800 nm. The shorter wavelength
absorption can be attributed to the π−π* transition of the
heptacylic units, while the lower energy band can be attributed
to the intramolecular charge transfer (ICT) between the
electron-rich and electron-deficient segments.11 Compared to
PSiFDCTPD, which shows absorption maxima at 535 and 562
nm in solution, PSiFDCTBT exhibited an absorption maximum
at 410 nm with a bathochromic shift of the ICT band to 646
nm. PSiFDCTDPP exhibited an absorption maximum at 430
nm with the ICT band at 714 nm. From Figure 1b, the
RESULTS AND DISCUSSION
Synthesis and Characterization. The synthetic route of
■
the SiFDCT monomer 5 is depicted in Scheme 1. Suzuki
coupling of 2,7-diboronic ester silafluorene 1 with ethyl 2-
bromothiophene-3-carboxylate afforded compound 3. Double
nucleophilic addition of freshly prepared 4-(n-pentyl)-
phenylmagnesium bromide to the ester groups of compound
3 led to the formation of tertiary benzylic alcohol compound 4,
which was subjected to intramolecular annulation through
Lewis acid-mediated Friedel−Crafts reaction to furnish
SiFDCT 5. A bulky aromatic substituent was used instead of
flexible aliphatic chains for solubilization for synthetic ease.5e
SiFDCT 5 was brominated in CHCl3 to yield compound 6 in
good yield. Treatment of compound 6 with t-butyl lithium
followed by quenching with trimethyltin chloride successfully
afforded the distannyl compound 7. Compound 7 was then
copolymerized with 1,3-dibromo-5-octylthieno[3,4-c]pyrrole-
4,6-dione (PD), 4,7-dibromo-2,1,3-benzothiadiazole (BT),
and 3,6-bis(5-bromothiophene-2-yl)-2,5-bis[(2-ethyl)hexyl]-
pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), by Stille coupling to
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Figure 2. Optimized conformations for the structure of PSiFDCTPD, PSiFDCTBT, PSiFDCTDPP top view (a) and side view (b). Wave functions
of the HOMO (c) and LUMO (d) orbitals of the corresponding polymers calculated at the level of B3LYP/6-31G (d,p).
absorption of all the polymers shifted toward longer wave-
lengths from solution to the solid state, indicating that the
planar structure of SiFDCT is capable of inducing strong
interchain interactions. The optical band-gaps (Egopt) deduced
from the absorption edges of the thin film spectra are in the
following order: PSiFDCTPD (1.88 eV) > PSiFDCTBT (1.76
eV) > PSiFDCTDPP (1.57 eV). The difference of their
absorption maxima as well as Egopt indicates that the acceptor
strength is in the order DPP > BT > DP.12 Note that the
intensities of the shorter wavelength bands of the polymer in
the solid state are apparently stronger than those in the solution
state, which suggests that the rigid and coplanar nonacyclic
units can enhance their light absorption ability in the solid state.
Theoretical Calculations. To further understand the effect
of planarization on the molecular structures and properties, we
performed theoretical calculations by density functional theory
at the level of B3LYP/6-31G(d). Two repeating units
SiFDCTPD, SiFDCTBT, and SiFDCTDPP were used as
simplified models for simulation of the corresponding three
copolymers. Methyl groups were used in the approximation of
long alkyl chains in order to reduce calculation time. The
optimized structures for PSiFDCTPD, PSiFDCTBT, and
PSiFDCTDPP are shown in Figure 2. All three polymers
show planar conformations through the entirety of the polymer
backbone. The dihedral angles between the acceptor moieties
and the neighboring SiFDCT fused-ring are ∼φ = 0°.
Electrochemical Properties. Cyclic voltammetry (CV)
was employed to investigate the electrochemical properties and
evaluate the HOMO and LUMO energy levels of the individual
polymers (Figure S1, Suuporting Information). The HOMO
and LUMO energy levels were calculated from the onset
oxidation potentials (Eoonxset) and onset reduction potentials
(Eroendset) vs Ag/Ag+, respectively, according to eqs 1 and 2.
HOMO = −(Eox + 4.75) (eV)
(1)
LUMO = − (Ered + 4.75) (eV)
(2)
The electrochemically determined band gaps were deduced
from the difference of the onset oxidation and reduction
potentials. The energy levels are summarized in Figure 3.
The frontier orbitals of the model compounds SiFDCTPD,
SiFDCTBT, and SiFDCTDPP are shown in Figure 2. The
calculated HOMO energy levels of SiFDCTPD, SiFDCTBT,
and SiFDCTDPP were −5.54, −5.72, and −5.68 eV,
respectively, which are in general agreement with the
experimentally determined values for the corresponding
polymers (see cyclic voltammetry measurements below) albeit
with a shift of 0.2−0.4 eV. For the corresponding alternating
dimers, the electron density of the LUMO was primarily
located on the electron-accepting unit (Figure 2d), whereas the
electron density of the HOMO was more evenly distributed
across both the donor and acceptor with the exception of
PSiFDCTDPP (Figure 2c). This redistribution of electron
density shows a pronounced intramolecular charge separation
between the donor and acceptor following excitation.
Figure 3. Energy level diagrams for PSiFDCTPD, PSiFDCTBT, and
PSiFDCTDPP.
All of the polymers showed one stable and reversible p-
doping and n-doping processes, which are important
prerequisites for p-type semiconductor materials. The
HOMO energy levels were estimated to be −5.31 eV for
PSiFDCTPD, −5.32 eV for PSiFDCTBT, and −5.26 eV for
PSiFDCTDPP, which are in an ideal range to ensure improved
air stability and greater attainable Voc in the final device. The
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Table 1. Photovoltaic and Hole-Mobility Characterization
mobility (cm2/(V·s))
blend
polymer
hole
electron
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
PSiFDCTPD
PSiFDCTBT
PSiFDCTDPP
1.0 × 10−8
1.0 × 10−8
1.1 × 10−8
2.5 × 10−6
5.4 × 10−5
4.6 × 10−7
1.2 × 10−8
4.8 × 10−8
3.1 × 10−8
0.84
0.86
0.79
7.3 0.1
8.8 0.2
44
56
1
1
2.7 0.1
4.2 0.1
3.78 0.02
49.5 0.3
1.48 0.02
Figure 4. (a) Absorption spectra of PSiFDCTPD, PSiFDCTBT, and PSiFDCTDPP pure polymer films. (b) Absorption spectra of
PSiFDCTPD:PC61BM, PSiFDCTBT:PC61BM, and PSiFDCTDPP:PC61BM blends films.
Figure 5. (a) J−V characteristics of ITO/PEDOT:PSS/polymer:PC61BM/Al under illumination of AM 1.5, 100 mW/cm2. (b) EQE characteristics
of the same devices.
LUMO energy levels are approximately located at −3.63 eV for
PSiFDCTPD and −3.71 eV for PSiFDCTBT, which are much
higher than the LUMO level of the PC61BM acceptor (−4.2
eV) to ensure energetically favorable electron transfer.
Supporting Information) were fabricated. The device data are
summarized in Table 1.
The hole mobilities of all three polymers prior to blending
with PC61BM were similar at around 1.0 × 10−8 cm2/(V s).
However, after blending with PC61BM, the hole mobilities of
the blended films exhibited an improvement in performance
several orders of magnitude higher than the pure films. This
phenomenon has been reported by others13 and is most likely
due to the intercalation of PC61BM between the polymer side
chains, which in turn inhibits coiling of the polymer chains. The
planarization of the polymer backbone is said to increase the
conjugation length, improves intermolecular interactions, and
improves charge mobilities. In Figure 4b, which shows the
absorption spectra of the blended films, we see a red-shift in the
absorption spectra compared to the pure polymer films (Figure
Photovoltaic and Hole-Mobility Characteristics. BHJ
photovoltaic cells were fabricated by spin-coating the blend
from chlorobenzene (CB) solutions at optimized poly-
mer:PC61BM (1:3.5) ratios using a ITO/PEDOT:PSS/
polymer:PC61BM/Al device configuration with an active layer
of approximately 100 nm. Device performances were measured
under a simulated AM 1.5 G illumination of 100 mW/cm2.
Additionally, in order to evaluate the hole and electron mobility
in the pure film and active-layer blend by space-charge limited
current (SCLC) theory, devices with appropriate configurations
(see Experimental Section for details; Figures S5−S7,
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4a) confirming the increased planarity of the polymers upon
blending. The hole mobilities of the blended films followed the
trend PSiFDCTDPP:PC61BM (4.6 × 10−7 cm2/(V s)) <
PSiFDCTPD:PC61BM (2.5 × 10−6 cm2/(V s)) <
PSiFDCTBT:PC61BM (5.4 × 10−5 cm2/(V s)), which may
indicate that the side chains on PSiFDCTBT are spaced in such
a way as to provide improved intercalation of PC61BM
compared to the side chains of PSiFDCTPD and
PSiFDCTDPP.14,15 Looking at the absorption spectra in Figure
4, PSiFDCTBT does indeed show the greatest red-shift when
blended with PC61BM indicating that it undergoes the greatest
degree of planarization upon blending in this particular series of
polymers.
The J-V and external quantum efficiency characteristics of the
devices are shown in Figure 5. High Voc values were obtained
from all devices. As the Voc can be linearly correlated with the
difference between the HOMO level of electron donor
(polymers) and the LUMO of electron acceptor (PC61BM),16
the Voc values matched the expected values calculated from the
energy levels of polymer materials.
In order to investigate the lower PCE of PL spectra of all
three polymers with and without PC61BM were taken (Figures
S2−S4, Supporting Information). Both PSiFDCTPD:PC61BM
and PSiFDCTBT:PC61BM blends exhibited over 90% PL
quenching, whereas only 20% PL quenching was observed in
blends consisting of PSiFDCTDPP:PC61BM showing that less
charge transfer is occurring in the latter devices. This is the
primary justification for that devices fabricated using
PSiFDCTDPP:PC61BM showing a much lower Jsc (3.8 mA/
cm2) and PCE (1.5%) than devices fabricated from the other
polymer:PC61BM blends.
The highest performing devices were found from solar cells
fabricated from PSiFDCTBT:PC61BM blends, with a PCE of
4.2% (Voc = 0.86 V, Jsc = 8.8 mA/cm2, FF = 56%). Devices
fabricated from PSiFDCTPD:PC61BM under the same
processing conditions only showed a PCE of 2.7%. From the
EQE spectra it can be seen that although
PSiFDCTPD:PC61BM achieved the highest maximum value
of approximately 65%, PSiFDCTBT:PC61BM showed a broader
profile and higher average EQE values. At short wavelengths,
the EQEs of PSiFDCTBT:PC61BM blends were about 2 to 3
times higher than the other two active layer blends. This may
be one reason why PSiFDCTBT:PC61BM devices showed a
higher overall PCE than PSiFDCTPD:PC61BM and
PSiFDCTDPP:PC61BM devices. Moreover, it has been shown
that achieving a balance between the hole and electron
mobilities in organic solar cells is one of the key issues in
obtaining high-performance devices.17 The similar electron and
hole mobilities in PSiFDCTBT:PC61BM (Table 1) may be
another reason for their improved device performance.
56%, delivering a PCE of 4.2%. The corresponding
PSiFDCTBT:PC61BM blend also showed a high hole mobility
of 5.4 × 10−5 cm2/(V s), leading to a high current density and
fill factor.
EXPERIMENTAL SECTION
■
General Measurement and Characterization. All chemicals
were purchased from Aldrich or VWR and used as received unless
otherwise specified. Compound 118 and the corresponding PD,9 BT,19
and DPP10 monomers were synthesized according to previous
1
literature procedures. H NMR and 13C NMR spectra were collected
on a Bruker Avance DPS-300 spectrometer. Mass spectrometry was
̈
performed using a Hewlett-Packard 5971A gas chromatograph and
Bruker Biflex III MALDI−TOF (both positive and negative ion
̈
reflector mode). The molecular weight of the polymers was measured
using Viscotek TDA 305 with polystyrene standards (room temper-
ature, THF as eluent). The absorption spectra were measured using a
Perkins-Elmer Lambda-9 spectrophotometer. Cyclic voltammetry of
the polymer films was conducted in acetonitrile with 0.1 M of
tetrabutylammonium hexafluorophosphate using a scan rate of 100 mV
s−1. ITO, Ag/AgCl, and Pt mesh were used as the working electrode,
reference electrode, and counter electrode, respectively.
Fabrication and Characterization of BHJ Devices. ITO/Glass
substrates were ultrasonically cleaned sequentially in detergent, water,
acetone, and isopropyl alcohol. The substrates were covered by a 30
nm layer of PEDOT:PSS by spin coating. After annealing in air at 140
°C for 10 min, the samples were cooled to room temperature.
Polymers were dissolved in CB and PC61BM was added to reach the
optimized ratio (1:3.5). The solutions were then heated at 90 °C and
stirred overnight. Prior to deposition, the solutions were filtered
through a 0.2 μm filter and the substrates were transferred into a
glovebox. The photoactive layer was then spin coated at different
speeds to get a thickness about 100 nm. The aluminum cathode (100
nm thick) was thermally evaporated through a shadow mask under
high vacuum about 4.0 × 10−7 Torr. Devices were then transferred
outside the glovebox to test in air using a Keithley 2400 source
measurement unit, and an Oriel xenon lamp (450 W) coupled with an
AM1.5 filter was used as the light source. The light intensity was
calibrated with a calibrated standard silicon solar cell with a KG5 filter
which is traced to the National Renewable Energy Laboratory and a
light intensity of 100 mW·cm−2 was used in all the measurements in
this study. Devices parameters were obtained by taking the average of
15 samples for PSiFDCTBT and 7−8 samples for PSiFDCTPD and
PSiFDCTDPP. Theoretical short circuit currents and mismatch factors
have been calculated and are shown in Table S1, Supporting
Information.
Hole-Only Devices. To investigate the hole mobility of polymer
films, we chose specific top and bottom contact electrodes in order to
measure the charge mobilities within our films (pure film, ITO/
PEDOT:PSS/polymer/Al; blends films, electron mobility; ITO/Al/
polymer:PC61BM/Al; hole mobility, ITO/PEDOT:PSS/poly-
mer:PC61BM/Pd). The hole mobilities were calculated according to
space charge limited current theory (SCLC). The J−V curves were
fitted according to the following equation:
2
⎛
⎞
⎟
9
8
V
L ⎠
V
L
J
=
ε0εrμ exp 0.891γ
CONCLUSION
⎜
e(h)
e(h)
3
e(h)
■
⎝
We have successfully designed and synthesized a ladder-type
multifused thienyl-fluorene-thienyl unit with rigid and coplanar
backbone. The distannyl SiFDCT building block was
copolymerized with electron-deficient thieno[3,4-c]pyrrole-
4,6-dione (PD), benzothiadiazole (BT), and dithienyldiketo-
pyrrolopyrrole (DPP) units by Stille polymerization to afford
three alternating donor−acceptor copolymers, PSiFDCTPD,
PSiFDCTBT and PSiFDCTDPP respectively. Through simple
and straightforward engineering of molecular structures, the
device based on the PSiFDCTBT:PC61BM (1:3.5) blend
performed a Voc of 0.86 V, a Jsc of 8.8 mA/cm2, and a FF of
Here Jε(η) is the electron (hole) current, μe(h) the zero-field mobility of
the electrons (holes), γe(h) the field activation factor, ε0 the permittivity
of free space, εr the relative permittivity of the material, and L the
thickness of the active layer. The current was measured by Keithley
2400 source measurement unit. Polymer mobility was gained from the
best fitted device.
Synthesis of Compound 3. To a 50 mL two-neck flask was
introduced compound 1 (2.54 g, 3.86 mmol), ethyl 2-bromothio-
phene-3-carboxylate (2.35 g, 10 mmol), Pd(PPh3)4 (230 mg, 0.2
mmol, K2CO3 (2.76 g, 20 mmol), and Aliquat 336 (200 mg, 0.5
mmol) in a solution of degassed toluene (30 mL), and degassed H2O
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(5 mL). The mixture was heated to 90 °C under nitrogen overnight.
The resulting solution was extracted with ethyl acetate and washed
with brine. The combined organic layer was dried over Na2SO4. After
removal of the solvent under reduced pressure, the residue was
purified by column chromatography on silica gel (hexane/ethyl
acetate, v/v, 25:1) to give compound 3 as a sticky liquid (2.18 g, 79%).
1H NMR (300 MHz, CDCl3): 7.81 (d, 2H), 7.48 (d, 2H), 7.53−7.46
(m, 4H), 7.22 (d, 2H), 4.21 (m, 4H), 1.98 (m, 4H), 1.21−1.11 (t,
30H), 0.88 (t, 6H). 13C NMR (75 MHz, CDCl3): 158.29, 151.03,
150.86, 140.45, 138.75, 134.62, 130.05, 129.47, 125.21, 124.97, 120.59,
56.32, 42.76, 34.61, 31.77, 30.20, 30.16, 25.36, 23.21, 14.52, 14.26. MS
(MALDI−TOF): m/z (C42H54O4S2Si) calcd, 714.3; found, 713.8.
Synthesis of Compound 4. To a solution of 4-pentyl-1-
bromobenzene (2.73 g, 12 mmol) in anhydrous THF under nitrogen
was added n-BuLi (4.8 mL, 12 mmol, 2.5 M) dropwise at −78 °C. The
resulting solution was stirred for 1 h at this temperature. Then a
solution of compound 3 (1.43 g, 2 mmol) in anhydrous THF was
added slowly to the above solution. After the addition, the resulting
mixture was heated at reflux overnight. The resulting mixture was
extracted with ethyl acetate and washed with brine. The combined
organic layer was dried over Na2SO4. After removal of the solvent
under reduced pressure, the residue was purified by column
chromatography on silica gel (chloroform) to give compound 4 as a
anhydrous Na2SO4, the solvent was removed under reduced pressure.
The residue was washed with cold methanol and then dried under high
vacuum overnight to give compound 7 as a white solid (416 mg, 92%).
1H NMR (300 MHz, CDCl3): 7.48 (s, 2H), 7.31 (s, 2H), 7.17 (m,
8H), 7.11 (m, 8H), 6.97 (s, 2H), 1.96−1.60 (m, 4H), 1.50−0.75 (m,
74H), 0.39 (s, 18H). 13C NMR (75 MHz, CDCl3): 155.68, 154.01,
153.64, 149.23, 143.52, 140.16, 138.75, 135.35, 129.13, 128.43, 125.14,
122.35, 119.37, 112.14, 76.23, 42.12, 35.63, 34.28, 32.38, 32.17, 31.59,
30.42, 30.21, 29.58, 28.74, 26.21, 25.84, 25.13, 23.08, 17.13 16.20. MS
(MALDI−TOF): m/z (C88H118S2SiSn2) calcd, 1506.7; found, 1507.4.
General Synthetic Procedure for PSiFDCTPD, PSiFDCTBT,
and PSiFDCTDPP by Stille Coupling Reaction. All of the
polymers were prepared by a similar procedure. To a Schlenk flask
was introduced compound 7 (753.5 mg, 0.5 mmol), corresponding
acceptor monomer (0.5 mmol), and anhydrous chlorobenzene (4
mL). The solution was flushed with nitrogen for 10 min, and then a
catalytic amount of tris(dibenzylideneacetone) dipalladium(0) (8.6
mg, 3 mol %) and tri(o-tolyl)phosphine (22.9 mg, 15 mol %) was
added into the solution. After the resulting flask was degassed thrice
via a freeze−pump−thaw cycle, the reactants were heated up to 100
°C for 48 h. Then, the reaction was cooled to room temperature and
added into methanol dropwise. The precipitate was collected by
filtration and washed by Soxhlet extraction with methanol, acetone,
hexane, and chloroform. The chloroform fraction was then
concentrated and precipitated into methanol. The solid was filtered
and dried under vacuum for 1 day.
1
sticky liquid (1.41 g, 58%). H NMR (300 MHz, CDCl3): 7.46 (d,
2H), 7.23 (d, 2H), 7.19 (m, 8H), 7.12 (d, 2H), 7.08 (m, 8H), 7.02 (s,
2H), 6.79 (d, 2H), 3.02 (s, 2H), 1.98−1.67 (m, 4H), 1.57−0.80 (m,
74H). 13C NMR (75 MHz, CDCl3): 154.13, 150.62, 145.23, 139.87,
139.45, 138.21, 132.92, 132.12, 129.85, 128.56, 125.02, 121.78, 120.15,
113.53, 74.32, 36.21, 32.48, 32.14, 30.55, 29.93, 29.51, 28.76, 25.26,
24.77, 24.39, 22.64, 15.36, 12.85. MS (MALDI−TOF): m/z
(C82H106O2S2Si) calcd, 1214.7; found, 1213.6.
1
Polymer PSiFDCTPD. Red solid; yield 73%. H NMR (300 MHz,
CDCl3): 8.01 (br, 2H), 7.41 (m, 2H), 7.26 (m, 8H), 7.08(m, 8H),
6.74 (m, 2H), 3.09 (br, 4H), 2.87 (br, 2H), 2.08−0.72 (m, 93H). Mn =
16.3K; PDI = 2.1; Mw = 34.2K.
Polymer PSiFDCTBT. Red-blue solid; yield 82%. 1H NMR (300
MHz, CDCl3): 8.08 (br, 2H), 7.75 (br, 2H), 7.52 (m, 2H), 7.19 (m,
8H), 7.10 (m, 8H), 6.87 (m, 2H), 3.14 (br, 4H), 2.03−0.71 (m, 78H).
Mn = 37.4K; PDI = 2.5; Mw = 93.5K.
Synthesis of Compound 5. To a solution of compound 4 (2.5 g,
2 mmol) in acetic acid (150 mL) was added concentrated H2SO4 (3
mL) in one portion. The resulting mixture was refluxed for 4 h and
then was extracted with ethyl acetate and washed with brine. The
combined organic layer was dried over Na2SO4. After removal of the
solvent under reduced pressure, the residue was purified by silica gel
chromatograph with hexane as the eluent to give a yellow oil product
Polymer PSiFDCTDPP. Green solid; yield 65%. 1H NMR (300
MHz, CDCl3): 8.23 (br, 2H), 7.61 (br, 4H), 7.36 (m, 2H), 7.22 (m,
8H), 7.14 (m, 8H), 6.81 (m, 2H), 3.25 (br, 4H), 2.95 (br, 4H), 2.12−
0.83 (m, 104H). Mn = 10.4K; PDI = 1.6; Mw = 16.6K.
1
compound 5 (630 mg, 26%). H NMR (300 MHz, CDCl3): 7.51 (s,
2H), 7.38 (s, 2H), 7.21 (d, 2H), 7.14 (m, 8H), 7.09 (m, 8H), 6.88 (d,
2H), 1.96−1.69 (m, 4H), 1.60−0.80 (m, 74H). 13C NMR (75 MHz,
CDCl3): 156.34, 155.07, 152.46, 149.53, 142.12, 138.76, 137.23,
134.68, 129.94, 128.04, 125.01, 120.45, 117.39, 114.05, 75.16, 37.01,
33.21, 33.04, 31.39, 30.18, 30.01, 29.34, 25.97, 25.01, 24.84, 22.21,
16.24, 15.72. MS (MALDI−TOF): m/z (C82H102S2Si) calcd, 1178.7;
found, 1177.1.
ASSOCIATED CONTENT
* Supporting Information
■
S
Cyclic voltammograms of the corresponding polymer, fluo-
rescent emission spectra of the corresponding polymer and
blend, hole/electron mobility fitting plots for the polymers and
1
blends, H NMR spectra of the copolymers, and table of
Synthesis of Compound 6. Compound 5 (500 mg, 0.42 mmol)
was dissolved in chloroform (15 mL), and then NBS (167 mg, 0.92
mmol) was added into the solution at 0 °C. The resulting mixture was
stirred at room temperature for another 2 h. Then the solution was
poured onto a sodium carbonate solution (2 M) and extracted with
chloroform. The organic phase was dried over anhydrous Na2SO4, the
solvent was removed under reduced pressure, and the residue was
purified by silica gel chromatograph with hexane as the eluent to get
mismatch factors. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: luscombe@u.washington.edu.
■
Present Address
1
compound 6 as a yellow solid (482 mg, 86%). H NMR (300 MHz,
†Materials Science and Engineering Department, University of
Washington, Seattle, Washington 98195−2120
CDCl3): 7.53 (s, 2H), 7.39 (s, 2H), 7.19 (m, 8H), 7.12 (m, 8H), 7.06
(s, 2H), 1.94−1.62 (m, 4H), 1.60−0.75 (m, 74H). 13C NMR (75
MHz, CDCl3): 157.25, 155.46, 153.01, 150.11, 144.29, 139.31, 138.07,
134.47, 128.72, 127.96, 124.31, 121.25, 118.26, 114.67, 75.31, 38.53,
34.07, 33.26, 31.35, 31.09, 30.78, 30.01, 26.45, 25.72, 24.68, 22.53,
16.85, 15.31. MS (MALDI−TOF): m/z (C82H100Br2S2Si) calcd,
1334.5; found, 1333.2.
Synthesis of Compound 7. Compound 6 (400 mg, 3 mmol) and
anhydrous THF were added to a flask under nitrogen atmosphere and
cooled to −78 °C. Subsequently, n-butyllithium (3 mL, 7.5 mmol, 2.5
M) was added dropwise into the solution. After stirring at −78 °C for
1 h, trimethyltin chloride (7.5 mL, 7.5 mmol, 1 M) was added into the
solution in one portion. The resulting solution was stirred at room
temperature for another 1 h. The solution was poured onto water and
extracted with diethyl ether twice. The organic phase was dried over
Author Contributions
The manuscript was written through the contributions of all
authors. All authors have given approval to the final version of
the manuscript. .
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSF (STC-MDITR DMR
0120967, SOLAR Award DMR 1035196, EFRI-SEED 1038165,
and CAREER Award DMR 0747489).
5939
dx.doi.org/10.1021/ma300839c | Macromolecules 2012, 45, 5934−5940

Macromolecules
Article
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