Synthesis and characterization of all-conjugated graft copolymers comprised of n-type or p-type backbones and poly(3-hexylthiophene) side chains

Article abstract of DOI:10.1021/ma400043s

All-conjugated graft copolymers containing poly(3-hexylthiophene) (P3HT) side chains and both of p-type and n-type backbones that are connected with all π-conjugated linkages were synthesized via a two-step method involving the Stille coupling reaction and Kumada catalyst-transfer polycondensation (KCTP). A series of naphthalene diimide copolymers with different compositions of 3-(4′-chloro-3′-tolyl)thiophene (CTT) units (PNDICTT) were designed as n-type backbones, while the poly(3-(4′-chloro-3′-tolyl)thiophene- alt-thiophene) (PCTT) was designed as a p-type backbone which were converted into n-type or p-type macroinitiators, and P3HT side chains were then in situ grafted from the macroinitiators via an externally initiated KCTP at room temperature. By using this newly developed two-step method for the synthesis of all-conjugated graft copolymers, the number of P3HT side chains in the graft copolymers can be simply controlled by varying the composition of the CTT units in PNDICTT. Meanwhile, the chain length of P3HT was controllable by varying the feed molar ratio of the thiophene monomer to CTT unit during the KCTP. The optical and electrochemical properties of the all-conjugated graft copolymers were investigated by UV-vis, cyclic voltammetry (CV), and organic field-effect transistor (OFET) measurements. Moreover, the differential scanning calorimetry (DSC) and grazing incident wide-angle X-ray scattering (GIWAXS) results revealed that there were two distinguished crystalline domains in the thin films of the graft copolymer. The morphology of the graft copolymers was first observed by transmission electron microscopy (TEM), in which there was a microphase separation between the PNDICTT and P3HT domains, and the P3HT domains showed partial nanofibril structures with a width of 10-20 nm.

Full text of DOI:10.1021/ma400043s

Article
pubs.acs.org/Macromolecules
Synthesis and Characterization of All-Conjugated Graft Copolymers
Comprised of n‑Type or p‑Type Backbones and Poly(3-
hexylthiophene) Side Chains
Jin Wang,†,∥ Chien Lu,‡,∥ Tetsunari Mizobe,† Mitsuru Ueda,† Wen-Chang Chen,*,‡
and Tomoya Higashihara*,†,§
†Department of Organic and Polymeric Materials, Graduated School of Science and Engineering, Tokyo Institute of Technology,
2-12-1-H120, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan
‡Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617
§Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: All-conjugated graft copolymers containing poly-
(3-hexylthiophene) (P3HT) side chains and both of p-type and
n-type backbones that are connected with all π-conjugated
linkages were synthesized via a two-step method involving the
Stille coupling reaction and Kumada catalyst-transfer poly-
condensation (KCTP). A series of naphthalene diimide
copolymers with different compositions of 3-(4′-chloro-3′-
tolyl)thiophene (CTT) units (PNDICTT) were designed as n-
type backbones, while the poly(3-(4′-chloro-3′-tolyl)thiophene-
alt-thiophene) (PCTT) was designed as a p-type backbone
which were converted into n-type or p-type macroinitiators, and
P3HT side chains were then in situ grafted from the macroinitiators via an externally initiated KCTP at room temperature. By
using this newly developed two-step method for the synthesis of all-conjugated graft copolymers, the number of P3HT side
chains in the graft copolymers can be simply controlled by varying the composition of the CTT units in PNDICTT. Meanwhile,
the chain length of P3HT was controllable by varying the feed molar ratio of the thiophene monomer to CTT unit during the
KCTP. The optical and electrochemical properties of the all-conjugated graft copolymers were investigated by UV−vis, cyclic
voltammetry (CV), and organic field-effect transistor (OFET) measurements. Moreover, the differential scanning calorimetry
(DSC) and grazing incident wide-angle X-ray scattering (GIWAXS) results revealed that there were two distinguished crystalline
domains in the thin films of the graft copolymer. The morphology of the graft copolymers was first observed by transmission
electron microscopy (TEM), in which there was a microphase separation between the PNDICTT and P3HT domains, and the
P3HT domains showed partial nanofibril structures with a width of 10−20 nm.
P3HT-b-poly(methyl acrylate),12 P3HT-b-poly(2-vinylpyri-
INTRODUCTION
■
dine),13 and PS-b-P3HT-b-PS.14 Moreover, by using the
Bulk-heterojunction (BHJ) photovoltaic devices, using the
blending of donor−acceptor conjugated polymers in their
sequential monomer addition technique in the quasi-living
Grignard metathesis (GRIM) polymerization, also known as
active layers based on a solution-processable fabrication, have
been widely investigated because of their promising advantages,
such as light weight, cost-effective manufacturing, and the
possibility of flexible large-area devices for future industrial
applications.1−5 Poly(3-hexylthiophene) (P3HT) is one of the
most studied polymers due to its high mobility, stability, and
relatively efficient light absorption in the visible range of the
solar spectrum.6,7 Recently, P3HT and related materials have
received further attention especially after the independent
discovery of the chain-growth polycondensation of the
regioregular P3HT with a well-defined molecular weight and
low polydispersity by McCullough et al.8,9 and Yokozawa et
al.10,11
the Kumada catalyst-transfer polycondensation (KCTP),15−17
a
variety of all-conjugated block copolymers including P3HT-b-
poly(dodecylthiophene),9 P3HT-b-poly(3-(2-(2-
methoxyethoxy)ethoxy)methylthiophene),18 P3HT-b-poly(3-
phenoxymethylthiophene),19 P3HT-b-poly(3-(2-ethylhexyl)-
thiophene),20 P3HT-b-poly(3-(2-(2-(2-methoxyethoxy)-
ethoxy)ethoxy)methylthiophene),21 poly(3-butylthiophene)-b-
poly(3-octylthiophene) (P3BT-b-P3OT),22 and P3BT-b-
P3HT23 have been prepared.
Received: January 7, 2013
Revised: February 15, 2013
Published: February 26, 2013
Previously, various block copolymers containing P3HT
segments were reported, such as P3HT-b-polystyrene (PS),12
© 2013 American Chemical Society
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Interestingly, the externally initiated KCTP has been
developed for synthesizing regioregular P3HTs with well-
defined α-end groups, such as ethynylene, alcohol, tosylate,
azide, amine, and carboxylic acid.24 By using hexaphenylben-
zene and V-shaped and Y-shaped core compounds as initiators,
6-arm25, 2- and 3-arm26 all-conjugated branched P3HTs were
also successfully prepared. Most recently, all-conjugated block
copolymers comprising both the n-type and p-type have been
reported, in which the Br-end-functional P3HT was first
prepared by KCTP, and then it was reacted with n-type
monomers or chain-end-functional n-type polymers via the
Stille coupling reaction.27−32 Nevertheless, to the best of our
knowledge, there have never been reports concerning the all-
conjugated P3HT graft polymers with a conjugated backbone
and controlled length of the P3HT side chains. Furthermore,
the preparation of the all-conjugated graft copolymers with
both n-type and p-type segments is a big challenge, especially
when aiming at the single-component organic photovoltaic
(OPV) application, in which the n-type and p-type domains
may possibly be phase-separated via ideal distributing lengths of
10−20 nm which correspond to the exciton diffusion length.
In this study, the externally initiated KCTP method was
employed to synthesize a series of all-conjugated graft
copolymers with P3HT side chains and both p-type and n-
type backbones at ambient temperature via a one-pot grafting
from process for the first time. As presented in Schemes 1 and
2, both the n-type and p-type polymers with different numbers
of initiation sites (CTT units) for the KCTP were prepared via
the Stille coupling reaction, and then the P3HT side chains
were polymerized from the precursors via the externally
initiated KCTP. The structures, optical and electrochemical
properties, and crystalline and phase-separated morphologies of
these newly developed all-conjugated graft copolymers were
investigated in detail.
Scheme 1. Synthetic Process of (a) CTT Monomer, (b) n-
Type Macromoinitiator PNDICTT, and (c) p-Type
Macroinitiator PCTT
EXPERIMENTAL SECTION
■
Materials and Characterization. All reagents were purchased
from commercial sources and used without further purification unless
otherwise noted. N,N′-Bis(2-decyltetradecyl)-2,6-dibromonaphtha-
lene-1,4,5,8-bis(dicarboximide) (NDI-Br2) (monomer 3) and 2,5-
bis(trimethylstannyl)thiophene (monomer 4) were prepared accord-
ing to literature procedures.1,2
The samples were irradiated at a fixed incident angle on the order of
0.12° through a Huber diffractometer, and the GIWAXS patterns were
recorded with a 2-D image detector (Pilatus 300 K). GIWAXS
patterns were recorded with an X-ray energy of 12.39 keV (λ = 1 Å).
The samples for GIWAXS were prepared by drop-casting the polymer
solution on the Si/SiO2 substrate. TEM was performed using a JEOL
1230 operated at an acceleration voltage of 100 kV; the samples were
prepared from the polymer solution (10 mg/3 mL in chloroform) via
slow evaporation over 1 week. After drying, the films were annealed at
240 °C for 10 min and at 100 °C for 6 h.
Synthesis of 3-(4′-Chloro-3′-tolyl)thiophene (CTT) (1). The
mixture of 3-thiopheneboronic acid (0.281 g, 2.2 mmol), 2-chloro-5-
iodotoluene (0.505 g, 2.0 mmol), Pd(PPh3)4 (57.8 mg, 0.050 mmol),
toluene (20 mL), and 2 M K2CO3 aqueous solution (5 mL) was
stirred at 100 °C for 20 h under a nitrogen atmosphere. After cooling
to room temperature, the mixture was washed with water and the
organic layer was dried over MgSO4 and evaporated. The residue was
purified with column chromatography (hexane) to yield 1 as a white
powder (0.364 g, 87%). 1H NMR (300 MHz, DMSO-d6, ppm, 40 °C):
δ = 7.85−7.87 (H, q, Ar−H), 7.71−7.72 (H, d, J = 2.1 Hz, Ar−H),
7.61−7.64 (H, q, Ar−H), 7.53−7.57 (2H, m, Ar−H), 7.40−7.43 (H, d,
J = 3.6 Hz, Ar−H), 2.38 (3H, s, CH3). 13C NMR (300 MHz, DMSO-
d6, ppm, 40 °C): δ = 140.7, 136.2, 134.4, 132.2, 129.5, 129.1, 127.5,
126.5, 125.5, 121.7, and 19.9; mp 74.8−76.0 °C. Anal. Calcd for
C11H9ClS: C, 63.27%; H, 4.35%. Found: C, 63.21%; H, 4.31%.
Synthesis of 2,5-Dibromo-3-(4′-chloro-3′-tolyl)thiophene
(DBCTT) (2). N-Bromosuccinimide (0.427 g, 2.4 mmol) was added
to a stirred solution of 1 (0.200 g, 0.096 mmol) in 20 mL of CHCl3/
1H and 13C NMR spectra were recorded on a Bruker DPX (300
MHz) in DMSO-d6 (40 °C) or CDCl3 (25 °C) using tetramethylsilane
as the internal standard. Size exclusion chromatography (SEC)
measurements were carried out on a Jasco GULLIVER 1500 equipped
with a pump and an absorbance detector (UV, λ = 254 nm) using
polystyrene standards. THF was used as an eluent at a flow rate of 1.0
mL/min at 40 °C. UV−vis absorption spectra of the polymer solutions
in chloroform and films were recorded on a Jasco V-560 spectrometer
over a wavelength range of 200−1000 nm. Matrix-assisted laser
desorption ionization with time-of-flight (MALDI-TOF) mass spectra
were recorded on a Kratos Kompact MALDI instrument operated in a
linear detection mode to generate positive ion spectra using 2,2′:5′,2″-
terthiophene as the matrix and THF as a solvent. Elemental analyses
were performed on a Yanaco MT-6 CHN recorder elemental analysis
instrument. DSC analysis was performed on a Seiko EXSTAR 6000
DTA 6300 thermal analyzer at a TA Instruments Q-100 connected to
a cooling system at a heating rate of 10 °C min−1. CV was performed
with the use of a three-electrode cell in which ITO was used as a
working electrode, and the polymer film was coated on it in 0.5 × 0.7
cm2. A platinum wire was used as an auxiliary electrode. All cell
potentials were taken in a 0.1 mol/L acetonitrile solution of
tetrabutylammonium perchlorate at a scan rate of 0.1 V/s with the
use of a homemade Ag/AgCl, KCl(sat.) reference electrode. GIWAXS
experiments were conducted at the SPring-8 on beamline BL19B2.
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Scheme 2. Synthetic Schemes of (a) Model Reaction of KCTP Initiated by CTT, (b) All-Conjugated Grafting P3HT, and (c)
All-Conjugated Graft Copolymers Initiated by n-Type Precursor
CH3COOH (1:1 in volume) at room temperature under a nitrogen
atmosphere. The mixture was then stirred for 24 h and quenched by
water. It was washed with distilled water, the organic layer was dried
with Na2CO3 and filtrated, and the solvents were removed by rotary
evaporation. The residue was purified with column chromatography
precipitated solid was collected by filtration under reduced pressure.
Poly(3-(4′-chloro-3′-tolyl)thiophene-alt-thiophene) (PCTT) was
washed with methanol, and poly(N,N′-bis(2-decyltetradecyl)-1,4,5,8-
naphthalene diimide-alt-3-(4′-chloro-3′-tolyl)thiophene)
(PNDICTTxy: x and y indicate the feed molar ratio of 3 to 2) was
subjected to sequential Soxhlet extraction with methanol, acetone, and
chloroform. Finally, the polymer solution in chloroform was passed
through a silica gel column. The polymers are finally freeze-dried from
dehydrated benzene before use. For preparing PNDICTTxy series, the
feed molar ratio (x:y) of 3 to 2 is 9:1, 7:3, and 5:5, respectively. PCTT:
deep red solid (44.0 mg, 56%). 1H NMR (CDCl3, 300 MHz, ppm): δ
= 6.67−7.33 (6nH, m, Ar−H), 2.26−2.47 (3H, br, t, J = 31.5 Hz,
CH3). PNDICTT91: deep blue solid (284 mg, 86%). 1H NMR
(CDCl3, 300 MHz, ppm): δ = 8.97 (2xH, s, Ar−H), 7.45 (2xH, s, Ar−
H), 7.35 (6yH, s, Ar−H), 4.14 (4xH, br, N−CH2−C(C)2), 2.42−2.44
(3yH, d, J = 6.0 Hz, CH3 in CTT), 2.03 (2xH, br, C−CH(C)2), 1.19−
1.34 (80xH, br, −CH2−), 0.81−0.85(12xH, m, CH3). PNDICTT73:
1
(hexane) to yield 2 as a white solid (0.197 g, 56%). H NMR (300
MHz, DMSO-d6, ppm, 40 °C): δ = 7.52−7.53 (H, d, J = 2.1 Hz, Ar−
H), 7.48−7.51 (H, d, J = 8.4 Hz, Ar−H), 7.37−7.41 (H, q, Ar−H),
7.37 (H, s, Ar−H), 2.38 (3H, s, CH3). 13C NMR (300 MHz, DMSO-
d6, ppm, 40 °C): δ = 141.21, 136.15, 133.63, 132.61, 132.58, 131.52,
129.34, 127.85, 111.46, 108.05, and 19.89; mp 66.2−67.8 °C. Anal.
Calcd For C11H7ClBr2S: C, 36.06%; H, 1.93%. Found: C, 36.11%; H,
2.08%.
General Procedure for Polymerization of Precursors. To a
flask, 1 equiv of 2 and 1 equiv of monomer 4 were placed under argon
followed by adding 0.025 equiv of Pd(PPh3)2Cl2. The flask and its
contents were subjected to three pump/purge cycles with argon
followed by addition of anhydrous and degassed toluene (10 mL) via a
syringe. The reaction mixture was stirred at 90 °C for 2 days, followed
by adding 2-(tributylstannyl)thiophene and 2-bromothiophene in
sequence. After cooling to room temperature, the deeply colored
mixture was dripped into 100 mL of vigorously stirred methanol
(containing 5 mL of 12 M hydrochloric acid). After stirring for 3 h, the
1
blue solid (234 mg, 72%). H NMR (CDCl3, 300 MHz, ppm): δ =
8.84−8.98 (2xH, m, Ar−H), 7.44 (2xH, s, Ar−H), 7.34 (6yH, s, Ar−
H), 4.13 (4xH, br, N−CH2−C(C)2), 2.39−2.43 (3yH, d, J = 12.0 Hz,
CH3 in CTT), 2.01 (2xH, br, C−CH(C)2), 1.19−1.32 (80xH, br,
−CH2−), 0.81−0.85 (12xH, m, CH3). PNDICTT55: dark green solid
1
(209 mg, 88%). H NMR (CDCl3, 300 MHz, ppm): δ = 8.73−8.96
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Table 1. Compositions, Mns, PDIs, and Yields of PNDICTT and PCTT
molar ratio of 3:2
a
a
b
b
a
entry
feed
found
Mn (NMR) (kDa)
Mn (SEC) (kDa)
PDI (SEC) (kDa)
no. of CTT
yield (%)
PNDICTT91
PNDICTT73
PNDICTT55
PCTT
9:1
7:3
1:1
8.8:1
7.1:3
1.1:1
15.8
6.9
21.7
10.4
7.39
2.10
2.04
2.01
2.08
2.30
1.7
2.5
3.4
86
72
88
56
5.0
b
7.2
a
b
1
Calculated by H NMR. Determined by SEC using THF as eluent at 40 °C.
(2xH, m, Ar−H), 7.16−7.43 (2xH + 6yH, m, Ar−H), 4.13 (4xH, br,
N−CH2−C(C)2), 2.39−2.42 (3yH, d, J = 9.0 Hz, CH3 in CTT), 2.00
(2xH, br, C−CH(C)2), 1.19−1.32 (80xH, br, −CH2−), 0.81−0.85
(12xH, m, CH3).
extraction with methanol, acetone, and chloroform. The polymer
solution in chloroform was passed through silica gel column, followed
by freeze-drying from the dehydrated benzene solution. PNDICTT91-
1
g-P3HT: deep purple solid (282 mg, 95%). H NMR (CDCl3, 300
Model Reaction of P3HT Initiated by CTT via KCTP. KCTP
was carried out in a glovebox under a purified nitrogen atmosphere.
To a flask, 1 (6.3 mg, 0.030 mmol), Ni(COD)2 (8.3 mg, 0.030 mmol),
and PPh3 (23.6 mg, 0.090 mmol) were placed and dissolved in 2 mL of
toluene. The mixture was stirred for 24 h at room temperature, and the
color was changed from dark red to yellow. After adding dppp (24.7
mg, 0.060 mmol), the mixture was stirred for another 2 h, and the
mixture became light orange and transparent. In the meantime, 2,5-
dibromo-3-hexylthiophene (294 mg, 0.90 mmol) was dissolved in 40
mL of THF containing LiCl (38.0 mg, 0.90 mmol). Isopropylmagne-
sium chloride (0.495 mL, 0.99 mmol) was then added at room
temperature and stirred for 30 min. Then, the solution was transferred
into the 1/Ni mixture. After 5 h, 5 mL of 5 M HCl was added to
quench the polymerization. The reaction mixture was stirred for 15
min and poured into 300 mL of methanol/water (2:1 in volume)
mixture. The materials were isolated by filtration, washed with
methanol and acetone, and dried in vacuo (119.9 mg, 80%). 1H NMR
(CDCl3, 300 MHz, ppm): δ = 7.40−7.51 (6H, m, Ar−H in CTT),
6.98 (nH, s, Ar−H in P3HT), 2.78−2.83 (2nH, t, J = 7.5 Hz, Ar−
CH2−C−), 2.55 (3H, s, CH3 in CTT), 1.71 (2nH, m, Ar−C−CH2−
C−), 1.34−1.44 (6nH, m, Ar−C2−(CH2)3−C), 0.89−0.93 (3nH, t, J =
6.0 Hz, CH3 in P3HT).
MHz, ppm): δ = 8.97 (2xH, s, Ar−H), 7.45 (2xH−2yH, s, Ar−H),
7.35 (8yH, br, Ar−H), 6.98 (nH, s, Ar−H in P3HT), 4.13 (4xH, br,
N−CH2−C(C)2), 2.78−2.83 (2nH, t, J = 7.5 Hz, Ar−CH2−C−), 2.43
(3yH, br, CH3 in CTT), 2.02 (2xH, br, C−CH(C)2), 1.71 (2nH, m,
Ar−C−CH2−C−), 1.34−1.44 (6nH, m, Ar−C2−(CH2)3−C in
P3HT), 1.19−1.30 (80xH, br, −CH2− in PNDICTT), 0.91 (3nH,
br, CH3 in P3HT), 0.85 (12xH, m, CH3 PNDICTT). PNDICTT73-g-
1
P3HTa: deep purple solid (248 mg, 93%). H NMR (CDCl3, 300
MHz, ppm): δ = 8.95 (2xH, br, Ar−H), 7.35 (2xH+8yH, br, Ar−H),
6.98 (nH, s, Ar−H in P3HT), 4.12 (4xH, br, N−CH2−C(C)2), 2.78−
2.83 (2nH, t, J = 7.5 Hz, Ar−CH2−C−), 2.43 (3yH, br, CH3 in CTT),
2.0 (2xH, br, C−CH(C)2), 1.70 (2nH, br, Ar−C−CH2−C−), 1.34−
1.44 (6nH, m, Ar−C2−(CH2)3−C in P3HT), 1.22 (80xH, br, −CH2−
in PNDICTT), 0.85−0.93 (3nH +12xH, br, −CH3 in P3HT and
−CH3 in PNDICTT). PNDICTT55-g-P3HT: deep purple solid (241
mg, 90%). 1H NMR (CDCl3, 300 MHz, ppm): δ = 8.81−8.95 (2xH, d,
br, Ar−H), 7.34 (2xH+8yH, br, Ar−H), 6.98 (nH, s, Ar−H in P3HT),
4.10 (4xH, br, N−CH2−C(C)2), 2.78−2.83 (2nH, t, J = 7.5 Hz, Ar−
CH2−C−), 2.43 (3yH, br, CH3 in CTT), 2.02 (2xH, br, C−CH(C)2),
1.70 (2nH, m, Ar−C−CH2−C−), 1.33−1.43 (6nH, m, Ar−C2−
(CH2)3−C in P3HT), 1.21 (80xH, br, −CH2− in PNDICTT), 0.91
(3nH + 12xH, br, −CH3 in P3HT and −CH3 in PNDICTT).
Synthesis of PCTT-g-P3HT. To a flask containing PCTT (24.0
mg, 1.14 × 10−5 mol), Ni(COD)2 (22.0 mg, 8.02 × 10−5 mol), and
PPh3 (63.0 mg, 24.1 × 10−5 mol), 4 mL of toluene was added at
ambient temperature in a glovebox. The mixture was stirred for 24 h.
After adding dppp (66.0 mg, 0.16 mmol), a freshly prepared a 2-
bromo-5-chloromagnesio-3-hexylthiophene (5) (20 equiv to Ni-
(COD)2) solution in 40 mL of THF was added to the suspension.
The solution was then stirred for 5 h at room temperature. 5 mL of 5
M HCl was added to quench the polymerization. The reaction mixture
was stirred for 15 min and poured into 300 mL of methanol/water
(2:1 in volume). The materials were isolated by filtration and
subjected to sequential Soxhlet extraction with methanol, acetone, and
chloroform. The polymer solution in chloroform was passed through
silica gel column. After evaporation, the polymer was finally freeze-
dried from the dehydrated benzene solution. (252 mg, 88%). 1H NMR
(CDCl3, 300 MHz, ppm): δ = 7.41−7.71 (6xH, m, Ar−H in PCTT),
6.98 (nH, s, Ar−H in P3HT), 2.80 (2nH, br, Ar−CH2−C−), 2.37
(3xH, br, CH3 in PCTT), 1.71 (2nH, m, Ar−C−CH2−C−), 1.34−
1.44 (6nH, m, Ar−C2−(CH2)3−C), 0.91 (3nH, br, CH3 in P3HT).
Synthesis of PNDICTTxy-g-P3HT. The synthetic procedure of the
graft copolymers is the same as the model reaction. The typical
experiment of PNDICTT91-g-P3HT is as follows: To a flask presented
in a glovebox, PNDICTT91 (143 mg, 0.65 × 10−5 mol), Ni(COD)2
(4.2 mg, 1.54 × 10−5 mol), and PPh3 (12.1 mg, 4.62 × 10−5 mol) were
dissolved in 2 mL of toluene and stirred for 24 h; the color of the
mixture was dark purple. After addition of dppp (12.7 mg, 3.08 × 10−5
mol) and stirring for 2 h, the color of the mixture became blue. Then
freshly prepared 5 (64 equiv to Ni(COD)2) solution in 40 mL of THF
was added into the mixture, and the solution was stirred for 5 h at
room temperature. 5 mL of 5 M HCl was then added to quench the
polymerization. The reaction mixture was stirred for 15 min and
poured into 300 mL of methanol/water (2:1 in volume). The materials
were isolated by filtration and subjected to sequential Soxhlet
Fabrication and Characterization of Thin Film Transistors
(TFT). Highly doped n-type Si (100) wafers were used as substrates. A
300 nm SiO2 layer (capacitance per unit area C0 = 10 nF cm−1) as a
gate dielectric was thermally grown onto the Si substrates. These
wafers were cleaned in piranha solution, a 7:3 (v/v) mixture of H2SO4
and H2O2, rinsed with deionized water, and then dried by nitrogen.
The octadecyltrichlorosilane (OTS)-treated surfaces on SiO2/Si
substrates were obtained by the following procedure: a clean SiO2/
Si substrate was immersed into a 10 mM solution of octadecyltri-
chlorosilane in toluene at 80 °C for 2 h. Then the substrates were
rinsed with toluene and dried with a steam of nitrogen. FET devices
were deposited by spin-coating from chloroform/chlorbenzene (7−10
mg/mL) at a spin rate of 1000 rpm for 60 s and annealing at 140 °C
for 60 min. The top-contact source and drain electrodes were defined
by 100 nm thick gold through a regular shadow mask, and the channel
length (L) and width (W) were 50 and 1000 μm, respectively. FET
transfer and output characteristics were recorded in a nitrogen-filled
glovebox by using a Keithley 4200 semiconductor parametric analyzer.
RESULTS AND DISCUSSION
■
Preparation of n-Type (PNDICTTxy) and p-Type (PCTT)
Macroinitiators. Recently, the externally initiated KCTP has
been developed to synthesize various regioregular P3HTs with
specific initiators.33 Because of the living nature of the KCTP,
well-defined all-conjugated star-shaped P3HT25,26 and P3HT
brushes34,35 have been synthesized. Interestingly, Luscombe
and co-workers reported that the thiophene monomers were
successfully polymerized by the externally initiated KCTP from
o-chlorotoluene with a quantitative initiation efficiency.36
Therefore, it would be possible to prepare the all-conjugated
block copolymers containing a regioregular P3HT segment by
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Figure 1. MALDI-TOF mass spectrum of CTT-P3HT in model
reaction.
the externally initiated KCTP, in which a conjugated polymer
was first prepared with functional groups, such as Ar−Br or
Ar−Cl, at the chain end(s) that can subsequently be used as a
macroinitiator for the KCTP of thiophene monomers. More
interestingly, if the Ar−Br or Ar−Cl groups were introduced on
the backbone of a conjugated polymer, the all-conjugated graft
copolymers bearing P3HT side chains could be prepared.
Therefore, we proposed a two-step method to synthesize a
series of all-conjugated graft copolymers comprised of the n-
type or p-type backbone and P3HT side chains. As shown in
Schemes 1 and 2, a series of n-type naphthalene diimide
(NDI)-based and p-type thiophene-based backbone polymers
were synthesized via the Stille coupling reaction under the
similar conditions previously reported,37−39 followed by
grafting from the initiation sites via the KCTP.
1
Figure 2. H NMR spectra of (A) PCTT, (B) PCTT-g-P3HT, and
(C) SEC traces of PCTT and PCTT-g-P3HT.
First, 2,5-dibromo-3-(4′-chloro-3′-tolyl)thiophene
(DBCTT) (2) was designed as a key compound and
synthesized as illustrated in Scheme 1a. By varying the feed
molar ratio of 3 to 2, PNDICTTxy with different compositions
of CTT units were prepared via the Stille coupling reaction
(Scheme 1b). As there is an o-chlorotoluene group in the CTT
unit, the all-conjugated graft copolymers with P3HT side chains
could be prepared via the KCTP initiated by PNDICTT. Based
on the assumption of one P3HT side chain is initiated by one
CTT unit, the number of P3HT side chains can be controlled
by incorporating a different number of CTT units into the
PNDICTT. As a control experiment, PCTT, which has one
CTT group in each repeat unit, was prepared (Scheme 1c).
The yields, compositions, and molecular weights of the
products are summarized in Table 1.
Figure 3. 1H NMR spectra of PNDICTT73 and PNDICTT73-g-
P3HTb.
1
The structures of the products were confirmed by H NMR
(Figures S1−4 in Supporting Information), and the number-
Table 2. Yields and Mn Values of the All-Conjugated Graft Copolymers
c
Mn (SEC)
(kDa)
PDI
(SEC)
yield
(%)
a
b
b
entry
feed ratio 5:Ni cat. Mn (theory) (kDa) Mn (NMR) (kDa)
weight ratio P3HT:PNDI
PNDICTT91-g-P3HT
PNDICTT73-g-P3HTa
PNDICTT73-g-P3HTb
PNDICTT55-g-P3HT
PCTT-g-P3HT
64:1
25:1
60:1
25:1
20:1
26.4
11.2
16.9
9.2
31.2
16.8
26.4
15.0
35.7
27.6
11.5
18.8
12.1
33.7
1.44
1.52
1.33
1.40
2.20
0.92:1
1.5:1
4.5:1
2.7:1
95
93
87
90
89
25.9
a
b
c
1
Theoretical values based on 100% conversion. Calculated by H NMR results. Determined by SEC using THF as eluent.
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aromatic proton peak (8.96 ppm) from the NDI. Based on the
molar ratio of the CTT unit to NDI unit in PNDICTTxy, the
Mn of these n-type polymers were calculated by comparing the
integral area of the aromatic proton peak (7.44 ppm) from the
thiophene unit (adjacent to NDI) and that from the aromatic
proton peak (7.34 ppm) from the CTT and the chain ends.
1
The Mn of PCTT cannot be determined by H NMR as the
peaks of the chain end are overlapped with the aromatic
protons from the CTT units (Figure S4).
As can be seen in Table 1, the found molar ratios of the NDI
and CTT units in PNDICTTxy are almost the same as the feed
molar ratios, which indicates that the content of the CTT units
in the PNDICTTxy is controllable. Consequently, the number
of P3HT side chains in the graft copolymers could also be
predetermined. The Mn of PNDICTT91 determined by SEC is
around 22 kDa, which is similar to the reported results for
typical n-type polymers.37−39 However, with the increasing feed
molar ratio of 2 to 3, the Mn of the products gradually
decreased. For the PCTT, in which 100% of 2 was used
(without 3), only an Mn of ca. 2 kDa can be obtained due to the
poor solubility of PCTT, which precipitated out during the
polymerization. Thus, the decreasing Mn values of the
PNDICTT series with a higher content of the CTT unit
possibly resulted from a poorer solubility of the product
containing the higher content of the CTT unit. It is worth
pointing out that the Mn values of PNDICTTxy determined by
Figure 4. UV−vis spectra of all-conjugated graft copolymers and
PNDICTT91 in the film states.
Table 3. LUMO, HOMO, and Energy Band Gap Values of
PNDICTT91 and Graft Copolymers
a
b
c
d
LUMO
(eV)
HOMO
(eV)
Eegle
Eogpt
entry
(eV)
(eV)
PNDICTT91
−4.08
−2.58
−2.61
−5.89
−4.95
−4.92
1.81
2.37
2.31
1.74
1.89
1.89
PNDICTT91-g-P3HT
1
SEC are higher than that determined from the H NMR. The
PNDICTT73-g-
P3HTa
reason should be the overestimated of Mn values by SEC due to
the rigid-rod-like structure as well as aggregation of
PNDICTTxy, as have been reported elsewhere.40,41 Therefore,
the average number of CTT units in the PNDICTT series is
estimated based on the Mn values determined by 1H NMR and
the found molar ratio of CTT and NDI units. As illustrated in
Table 1, the average number of CTT unit in PNDICTT91 is
1.7, while that in PNDICTT55 is increased to 3.4.
PNDICTT55-g-P3HT
PCTT-g-P3HT
−2.72
−2.97
−4.78
−4.76
2.06
1.79
1.89
1.92
a
Calculated from the onset potential of reduction by cyclic
b
voltammetry. Calculated from the onset potential of oxidation by
c
cyclic voltammetry. Calculated from the equation Eegle = LUMO (eV)
d
− HOMO (eV) based on cyclic voltammetry. Calculated from the
onset wavelengths in UV−vis spectroscopy.
Model Reaction of KCTP Polymerization of P3HT
Initiated by CTT. The CTT units in the PNDICTT series,
which could potentially initiate the polymerization of P3HT via
KCTP, is not exactly the same as o-chlorotoluene and any other
reported initiators.33,36 Therefore, a model reaction was first
carried out using 1 as the initiator. As described in Scheme 2a, 1
equiv of Ni(COD)2 and 3 equiv of PPh3 were added to 1 equiv
of 1 in toluene, and then the mixture was stirred for 24 h. The
color of the solution changed from dark red to yellow upon the
addition of Ni(COD)2 and PPh3 similar to the literature,26,36
which indicated that the Ni complex had been formed by
incorporating Ni species into the CTT unit.24 Then, 2 equiv of
1,3-diphenylphosphinopropane (dppp) was added for the
ligand exchange, and the reaction mixture was allowed to
stand for 2 h. Subsequently, the thiophene monomer 5, which
was prepared in situ by the Grignard exchange reaction of 2,5-
dibromo-3-hexylthiophene and isopropylmagnesium chloride in
THF, was added to the initiator solution containing the 1/Ni
complex.
Figure 5. Cyclic voltammograms of (A) PCTT-g-P3HT, (B)
PNDICTT91, (C) PNDICTT91-g-P3HT, (D) PNDICTT73-g-P3HTa,
and (E) PNDICTT55-g-P3HT.
1
The H NMR spectra of the product shown in Figures S5
and S6 (Supporting Information) indicate that the P3HT chain
was initiated from the CTT group, as signals of the protons
from the aromatic rings (7.41−7.50 ppm) and the methyl
group (2.55 ppm) of CTT, and that from P3HT (6.98 ppm for
the proton at 4-position) can be finely ascribed. Moreover, the
signal of the methyl protons of the CTT group in the product
shows a clear downfield shift compared to that of 1. In addition,
the MALDI-TOF mass spectrum of the product presented in
average molecular weight (Mn) values of the products were
1
determined by SEC and H NMR. The contents of the CTT
(or molar ratio of CTT to NDI) in PNDICTTxy can be
determined by comparing the integral area of the methyl
proton peak (2.44 ppm) from the CTT and that of the
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a
Table 4. FET Characteristics of PNDICTT91 and All-Conjugated Graft Copolymers
b
nonannealed
annealed
c
c
entry
μ (cm2 V−1 s−1
)
Ion/Ioff
Vth
μ (cm2 V−1 s−1
)
Ion/Ioff
Vth
PNDICTT91
6.10 × 10−3
2.46 × 10−3
6.88 × 10−5
3.57 × 10−5
8.28 × 10−4
4.18 × 105
1.14 × 102
5.41 × 100
3.73 × 100
6.36 × 100
28.40
−2.07
47.55
1.73 × 10−3
1.88 × 10−3
5.17 × 10−4
2.60 × 10−5
1.20 × 10−4
4.19 × 107
1.85 × 103
8.40 × 100
2.63 × 100
23.51
−30.26
33.74
PCTT-g-P3HT
PNDICTT91-g-P3HT
PNDICTT73-g-P3HTa
PNDICTT55-g-P3HT
115.16
69.27
103.44
130.49
2.86 × 100
a
b
c
Devices were fabricated from the chloroform solutions with a concentration of 7.5 mg/mL. Annealed at 140 °C for 1 h. PNDICTT91 shows only
n-type charge mobility while the other samples show only p-type charge mobility.
2C) shows an obvious decrease in the elution time compared to
that of the pure PCTT, which also reveals the successful
transformation of PCTT to PCTT-g-P3HT. Furthermore, the
molecular weights of PCTT-g-P3HT determined by SEC and
1H NMR are 33.7 and 35.7 kDa, respectively (Table 2), which
are remarkably increased from 2 kDa of PCTT.
Preparation of All-Conjugated Graft Copolymers
Comprised of n-Type Backbone and P3HT Side Chains.
Based on the successful results of the model polymerization and
the synthesis of PCTT-g-P3HT, the all-conjugated graft
copolymers initiated by the n-type backbone PNDICTTxy
were synthesized by the externally initiated KCTP method as
illustrated in Scheme 2c. The compositions and PDI values of
the products are summarized in Table 2. The yields of all the
graft copolymers were relatively high (87−95%), indicating that
the thiophene monomer 5 was successfully initiated and
polymerized. The PDIs of all the products significantly
decreased (for instance, the PDI of PNDICTT91 is 2.08,
while that of PNDICTT91-g-P3HT decreased to 1.43) as
compared to their corresponding macroinitiators, possibly due
to the living nature of the KCTP of 5.
Figure 6. DSC curves of (A) CTT-P3HT, (B) PNDICTT91, and (C)
PNDICTT91-g-P3HT.
Figure 1 shows that all P3HT bears the CTT unit at the α-end
group and H or Br unit at the ω-end group, indicating a 100%
initiation efficiency from the CTT units. This result is crucial
for the successful preparation of the all-conjugated graft
copolymers in the following sections because the contami-
nation of the homo-P3HT byproduct should be absent in this
system. The Mn and PDI of the product determined by SEC are
1
The H NMR spectra of the graft copolymers and their
corresponding precursors are presented in Figures S7−S9. All
the peaks corresponding to the protons of the P3HT and
PNDICTT segments can be clearly assigned. Besides, the peak
of the aromatic protons in the NDI unit (a in Figures S7−S9)
was broadened and split into multimodal peaks, and that of the
methyl group in the CTT units (d) was strengthened with the
increase of the CTT unit in the PNDICTTxy series. The peaks
corresponding to the protons of the thiophene groups in
PNDICTTxy were clearly identified as e and f; however, in the
graft copolymers, these peaks were broadened and overlapped.
As shown in Figure 3, the peak indicated by a in PNDICTT73 is
assigned to the thiophene protons next to the NDI units, while
b corresponds to the protons of the aromatic rings in CTT as
well as that of the thiophene units next to the CTT unit. The
two peaks, a and b, were also identified in the other
PNDICTTxy series. Interestingly, the two peaks were
broadened and formed a large broad peak in the graft
copolymers, possibly due to the fact that P3HT was grafted
from the CTT units, considerably lowering the conformational
flexibility around the CTT units. This phenomenon is also seen
in the case of PCTT-g-P3HT. The SEC traces of PNDICTT91-
g-P3HT, PNDICTT73-g-P3HTa, and PNDICTT55-g-P3HT are
presented in Figure S10, all of which present sharp and nearly
unimodal peaks, although there is a small side peak on the
higher molecular weight side which might be resulted from
minor side reactions. All these results thus demonstrate that the
P3HT chains are successfully initiated from the n-type
backbones.
1
13 kDa and 1.4, respectively, while the Mn calculated by H
NMR (Figure S5) is 4.96 kDa, close to the theoretical value of
5.20 kDa, which indicates the relatively controlled manner of
the polycondensation process, as compared to similar
initiators.33
Synthesis of All-Conjugated Grafting P3HT via KCTP
Initiated by PCTT. PCTT was used as a macroinitiator to
synthesize the all-conjugated grafting P3HT (PCTT-g-P3HT).
The process (Scheme 2b) is the same as described in the model
reaction with the exception of using PCTT instead of 1. The
amount of the Ni(COD)2 catalyst was 1 equiv to the CTT unit
in the PCTT in order to suppress the exclusion of free Ni
species. 5 was then added to start the polymerization via the
grafting-from method. The yield and Mn values of the product
are presented in Table 2.
1
As can be seen in the H NMR spectrum of PCTT-g-P3HT
(Figure 2B), all the resonance signals from the P3HT and
PCTT protons can be identified. Moreover, the downfield shift
of the aromatic protons of the PCTT units in PCTT-g-P3HT
(indicated as i in Figure 2B) is observed by a comparison with
PCTT (Figure 2A). These results indicate that P3HT was
successfully initiated from the PCTT backbone, and the all-
conjugated grafting P3HT has been synthesized. To calculate
the Mn value of the product, the integral areas of the methyl
proton peak located at 2.44 ppm (a in Figure 2B) and that of
the aromatic proton peak of P3HT located at 6.98 ppm (b in
Figure 2B) are applied. The SEC curve of the product (Figure
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Figure 7. 2-D GIWAXS images of (a) PNDICTT91 and (b) PNDICTT91-g-P3HT after annealing at 150 °C for 30 min. Out-of-plane and in-plane
GIWAXS diffraction profiles of (a1 and a2) PNDICTT91 and (b1 and b2) PNDICTT91-g-P3HT, respectively.
1
It is noteworthy that the Mn values of the graft copolymers
determined from SEC (Table 2) only slightly increased when
compared to their precursors (except for PNDICTT73-g-
P3HTb, which showed a remarkable increase in Mn compared
to the precursor PNDICTT73). This may be due to the fact that
the PNDICTT series are aggregated in THF (eluent for SEC)
to provide overestimated Mn values. After grafting the P3HT
side chains, the aggregation of PNDICTT may be suppressed
due to the introduction of well-soluble P3HT chains. In
addition, the branched structure of a graft copolymer present a
smaller hydrodynamic volume than the linear analogue, so that
the Mn values of the graft polymer determined by SEC might be
underestimated. Indeed, as can be seen from Table 2, the Mn
values of all the graft copolymers determined by H NMR are
higher than those by SEC.
The number of P3HT side chains in these all-conjugated
graft copolymers prepared via the combination of the Stille
coupling reaction and KCTP can be predetermined by
controlling the feed molar ratio of 3 and 2 in the preparation
of PNDICTTxy. For instance, the average number of P3HT
side chains is 1.7 in PNDICTT91-g-P3HT, while that in
PNDICTT55-g-P3HT is 3.4. In addition, the chain length of
P3HT in these graft copolymers prepared by the proposed two-
step method can also be tailored by varying the feed molar ratio
of PNDICTTxy and 5 in the second step. As shown in Table 2,
PNDICTT73-g-P3HTa and PNDICTT73-g-P3HTb were syn-
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shoulder at ca. 580 nm, which is related to the absorption of the
PNDICTT backbone. The λmax values of the polymer in the
film state red-shift from 455 to 558 nm, which indicates a more
ordered morphology and extended π-conjugation length of
these graft copolymers in the film state.
Electrochemical Properties of Graft Copolymers.
Cyclic voltammetry experiments were performed in order to
determine the HOMO and LUMO energy levels of the all-
conjugated graft copolymers. Figure 5 shows the cyclic
voltammograms of all the graft copolymers as well as
PNDICTT91, and the associated numerical results are
summarized in Table 3. PCTT-g-P3HT (Figure 5A) possesses
a HOMO energy level of −4.76 eV. The n-type PNDICTT91
shows the expected low-lying HOMO energy level of −5.89 eV.
The HOMO energy levels of the graft copolymers based on the
PNDICTT backbones ranged from −4.78 to −4.95 eV, which
are lower than that of PCTT-g-P3HT based on a p-type
backbone. Moreover, with the increasing amount of
PNDICTT, the HOMO values of the graft copolymers are
reduced accordingly. These results indicate that the electro-
chemical properties of the graft copolymers can be controlled
by varying the weight ratios of the PNDICTT backbone and
P3HT side chains.
FETs and Charge Transport Properties. The effects of
the solvents and thermal annealing on the charge transporting
characteristics were investigated using field effect transistor
(FET) devices. The FET mobility was calculated in the
saturation regime by using the plot of the square root of the
drain-to-source current (Ids)1/2 versus the gate voltage (Vg).
The correlation between the Ids and other parameters uses the
equation42
Figure 8. TEM images of film morphology of (A) PNDICTT91-g-
P3HT and (B) PNDICTT73-g-P3HTa, annealed at 240 °C for 10 min
and at 100 °C for another 6 h under nitrogen.
thesized by using the same PNDICTT73 batch as the
macroinitiator. By increasing the ratio of [5]/PNDICTT73
from 25 to 60 equiv, the molecular weight of the corresponding
graft polymer increased from 11.5 kDa (PNDICTT73-g-
P3HTa) to 18.8 kDa (PNDICTT73-g-P3HTb). It should be
noted that the characterizations in the following sections will
focus on PNDICTT73-g-P3HTa rather than PNDICTT73-g-
P3HTb in the case of PNDICTT73-g-P3HT series. All the
results confirmed that the all-conjugated graft polymers were,
for the first time, successfully synthesized via a two-step method
involving the Stille coupling reaction and KCTP. In addition, it
was found that not only the number of P3HT side chains but
also the chain length of each P3HT was controllable.
Optical Properties of Graft Copolymers. The optical
properties of PNDICTTxy-g-P3HT and PCTT-g-P3HT were
investigated by measuring their absorption spectra in the thin
film state. The UV−vis spectra of the graft polymer are shown
in Figure 4, and the energy band gaps (Eogpt) determined by
UV−vis spectroscopy are presented in Table 3. As can be seen,
the λmax values of the graft copolymers are in the range 524−
558 nm. Furthermore, all the graft copolymers revealed a
shoulder at 607 nm, which is related to a vibronic absorption of
the P3HT segment. However, the shoulder of the graft
copolymers with the PNDICTT backbone would be related to
both the vibronic absorption of P3HT as well as the absorption
of the backbone because PNDICTT shows a strong absorption
around 607 nm. This might be the reason for the weaker
shoulder of PCTT-g-P3HT as compared to the PNDICTTxy-g-
P3HT at 607 nm. Because of the similar structure of these graft
copolymers, the Eogpt values of the graft copolymers were nearly
equal, around 1.9 eV, as shown in Table 3. Figure 4 also
presents the spectrum of the blend film of PNDICTT73 and
P3HT with the similar weight ratio to PNDICTT73-g-P3HT
(1:1.5 w/w). Different from the graft copolymers, the blend
system exhibits a clear optical feature of PNDICTT domains,
where the absorption at ca. 370 nm is stronger than that of graft
copolymers, and the onset absorption is red-shifted to 760 nm.
In the case of graft copolymers, the optical property of
PNDICTT backbone is not obvious, possibly due to the fact
that the PNDICTT backbone was covered by P3HT side
chains.
WCiμ
2L
Ids =
(V − V)2
g
t
where Ids is the drain-to-source current in the saturated region,
W and L are the channel width and length, respectively, μ is the
field-effect mobility, Ci is the capacitance per unit area of the
insulation layer, and Vg and Vth are the gate and threshold
voltages, respectively. The charge mobility values and Ion/Ioff
ratios of the devices fabricated from chloroform are presented
in Table 4, while those of the devices fabricated from
chloroform/chlorobenzene (95:5) are given in Table S1 of
the Supporting Information. Representative transfer character-
istics of the OFETs are given in Figure S12. PNDICTT91 shows
a moderate electron mobility of 6.10 × 10−3 cm2 V−1 s−1 and
relatively high Ion/Ioff ratio on the order of 105 and a high value
of 4.19 × 107 after annealing at 140 °C for 1 h. The all-
conjugated grafting P3HT, PCTT-g-P3HT, which has a p-type
backbone, shows a moderate hole mobility in the scale of 10−3
cm2 V−1 s−1 both before and after annealing. The Ion/Ioff ratio of
PCTT-g-P3HT increased more than 16 times after annealing.
The graft copolymers based on PNDICTT were expected to
show both n-type and p-type charge transfer characteristics as
they contain both n-type and p-type blocks. However, all the
graft copolymers show only the p-type charge transfer
characteristics. In addition, these graft copolymers present
very low Ion/Ioff ratios in the order of 100. These unexpected
values of the Ion/Ioff ratio would possibly result from the all-
conjugated structure, in which the n-type backbone and P3HT
side chains are connected by π-conjugated linkage. Therefore,
the quenching of the generated charges may significantly occur
in these systems to show only ohmic behavior in the FET
characteristics. To confirm this effect, an FET device was
For a comparison, the optical properties of a selected graft
copolymer, the n-type precursor, and linear P3HT prepared in
the model reaction were investigated in CHCl3, as shown in
Figure S11. As can be seen clearly, the spectrum of
PNDICTT91-g-P3HT shows the λmax value at ca. 455 nm,
corresponding to the absorption of the P3HT segment, and a
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fabricated based on the physical blends of PCTT-g-P3HT and
PNDICTT91 under the same conditions and device structure
(Figure S13). Though both PCTT-g-P3HT and PNDICTT91
show a relatively good charge mobility and Ion/Ioff ratios, those
of the device based on the blend are extremely low, as shown in
Table S2; the Ion/Ioff ratio is 3.8 and the hole mobility is 4.74 ×
10−7 cm2 V−1 s−1, which indicate that the charge of PCTT-g-
P3HT was significantly quenched by PNDICTT91. After
annealing, the Ion/Ioff ratio increased more than 24 times,
which indicated that a large phase separation may occur, so that
the quenching of the generated charges is suppressed. In
addition, the blend system also shows an n-type charge transfer
characteristic. The Ion/Ioff ratios of all the graft copolymers,
however, were not improved at all after annealing, probably due
to the microphase-separated structures between the p-type and
n-type domains, resulting in the frequent recombination of the
separated charges.
and P3HT side chains, in which the P3HT crystalline domains
align in the edge-on rich structure and the PNDICTT
crystalline domains are isotropically dispersed.
TEM Observation. The microphase separation between the
P3HT side chains and PNDICTT backbone was further
investigated by TEM observations. Figure 8 exhibits the TEM
images of the thin films of PNDICTT91-g-P3HT and
PNDICTT73-g-P3HTa. It can be seen clearly that P3HT
forms a partial nanofibril structure (dark region) in the range
10−20 nm. Moreover, with the increasing amount of P3HT,
the area corresponding to the P3HT domains also shows a
significant increase (from A to B in Figure 8). It should be
mentioned that no large phase separation was observed or
aggregations in the entire images, which suggests that
microphase separation occurred between P3HT and
PDNICTT segments in the graft copolymers due to the
chemical linkage between the PNDICTT and P3HT chains.
The properties of the FET devices based on the graft
copolymers fabricated from chloroform/cholorobenzene are
also shown in Table S1. There is a slight increase in the charge
mobility of the graft copolymers with the elevated boiling point
of the mixing solvents; however, the Ion/Ioff ratio was not
improved. The Ion/Ioff ratio is expected to be improved by
introducing a nonconjugated linkage between the p- and n-type
segments, reducing the recombination of generated charges.
The synthesis and FET device characterization of newly
designed such block copolymers with p- and n-type segments
via the nonconjugated linkage at the junction point are now
under investigation.
Crystalline Morphology. Figure 6 shows the DSC curves
of the linear P3HT (A), PNDICTT91 (B), and PNDICTT91-g-
P3HT (C). As can be seen, both the linear P3HT (synthesized
in the model reaction) and PNDICTT91 are crystalline, with a
relatively high melting temperature (Tm) at ca. 233 and 224 °C,
respectively. The curve of the selected graft copolymer
(PNDICTT91-g-P3HT) exhibits two melting peaks at 174
and 210 °C, which could be attributed to the melting of crystals
formed by the P3HT side chains and PNDICTT91 backbone,
respectively. This result indicates that P3HT and the backbone
of the graft copolymer were phase separated and formed two
distinct crystalline domains. Similar results have been reported
for the all-conjugated block copolymers.31 The crystalline
structures of the graft copolymer were further confirmed by the
GIWAXS characterization.
As an example, the crystalline morphology in the thin films of
PNDICTT91 and PNDICTT91-g-P3HT was investigated by
GIWAXS. Figure 7 shows the 2D WAXS images of the thin film
and the diffraction profiles of the out-of-plane and in-plane
directions. As can be seen in Figures 7a, 7-a1, and 7-a2, there is
a series of strong diffraction signals (PNDICTT (100) at qxy =
0.248 (2.5 nm)) in both the out-of-plane and in-plane
directions of the PNDICTT91 thin films. Regarding the
PNDICTT91-g-P3HT thin film, the strong P3HT (h00)
diffractions at qz = 0.381 (1.6 nm) and 0.769 (0.82 nm) were
observed in the out-of-plane direction (Figure 7b1) in addition
to the diffraction of PNDICTT (100) at qz = 0.273 (2.3 nm).
In the in-plane direction profile, there are two distinct P3HT
(100) and PNDICTT (100) diffractions at qxy = 0.381 (1.6
nm) and 0.273 (2.3 nm), respectively (Figure 7b2). The P3HT
(010) diffraction was also observed at qxy = 1.675 (0.37 nm) in
the in-plane direction. These results indicate that the
PNDICTT91-g-P3HT thin film is composed of two distinct
crystalline domains corresponding to the PNDICTT backbone
CONCLUSION
■
A series of all-conjugated graft copolymers comprised of an n-
type or p-type backbone and P3HT side chains have been
synthesized via a newly developed two-step method. In the first
step, a series of n-type conjugated polymers, PNDICTTxy, with
different compositions of the CTT units were prepared via the
Stille coupling polycondensation. In the second step, P3HT
chains were grafted from the CTT group of PNDICTTxy via an
externally initiated KCTP using PNDICTTxy as macroinitiators.
The proposed method could produce the first examples of all-
conjugated graft copolymers comprising both n-type and p-type
semiconductors. The advantage of the method is that the
number of P3HT sides chains as well as the chain length of
P3HT in the graft copolymers are controllable. The UV−vis
spectra and CV results of the graft copolymers indicated that
the HOMO values decreased with the increasing number of
P3HT side chains, although the band gap values of all the graft
copolymers were almost the same. OFET devices based on the
graft copolymers were fabricated, and the results indicated that
all the graft copolymers showed a p-type characteristic with a
moderate hole mobility on the order of 10−4 cm2 V−1 s−1. DSC
curves revealed that there were two distinguished crystalline
domains in the graft copolymers corresponding to the
PNDICTT and P3HT segments. Moreover, the GIWAXS
results confirmed that the PNDICTT91-g-P3HT thin film
possesses two different crystalline domains of edge-on P3HT
domains and isotropically dispersed PNDICTT domains. The
TEM images of the thin film of the graft copolymers revealed a
microphase separation between the P3HT and PNDICTT
segments, in which the P3HT domains show a partial
nanofibril-like morphology with a width of 10−20 nm.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra of PNDICTT series, PCTT, CTT-P3HT,
CTT, and graft copolymers; SEC traces of graft copolymers.
UV−vis absorption of the product in chloroform; FET results
of the devices. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thigashihara@polymer.titech.ac.jp (T.H.); chenwc@
ntu.edu.tw (W.-C.C.).
1792
dx.doi.org/10.1021/ma400043s | Macromolecules 2013, 46, 1783−1793

Macromolecules
Article
(24) Smeets, A.; Willot, P.; Winter, J. D.; Gerbaux, P.; Verbiest, T.;
Koeckelberghs, G. Macromolecules 2011, 44, 6017−6025.
(25) Senkovskyy, V.; Beryozkina, T.; Bocharova, V.; Tkachov, R.;
Komber, H.; Lederer, A.; Stamm, M.; Severin, N.; Rabe, J.; Kiriy, A.
Macromol. Symp. 2010, 291−292, 17−25.
Author Contributions
∥J.W. and C.L. contributed equally.
Notes
The authors declare no competing financial interest.
(26) Yuan, M.; Okamoto, K.; Bronstein, H. A.; Luscombe, C. K. ASC
Macro Lett. 2012, 1, 392−395.
ACKNOWLEDGMENTS
■
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This work is supported by the Japan Science and Technology
Agency (JST), PRESTO program (JY 220176). GIWAXS
experiments were performed at the BL19B2 of SPring-8 with
the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2012B1728). We thank Prof.
Itaru Osaka (Hiroshima University) and Dr. Tomoyuki
Koganezawa (Japan Synchrotron Radiation Research Institute
(JASRI)) for operating the GIWAXS experiments. The
technical support of Mr. Ryohei Kikuchi, Material Analysis
O-okayama Center, Tokyo Institute of Technology, for the
TEM operation is also gratefully acknowledged. Work at NTU
was supported by National Science Council of Taiwan (Grant
NSC 101-2823-E-002-014-MY2).
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