A Thieno[2,3-b]pyridine-Flanked Diketopyrrolopyrrole Polymer as an n-Type Polymer Semiconductor for All-Polymer Solar Cells and Organic Field-Effect Transistors

Article abstract of DOI:10.1021/acs.macromol.7b00934

A novel fused heterocycle-flanked diketopyrrolopyrrole (DPP) monomer, thieno[2,3-b]pyridine diketopyrrolopyrrole (TPDPP), was designed and synthesized. When copolymerized with 3,4-difluorothiophene using Stille coupling polymerization, the new polymer pTPDPP-TF possesses a highly planar conjugated polymer backbone due to the fused thieno[2,3-b]pyridine flanking unit that effectively alleviates the steric hindrance with both the central DPP core and the 3,4-difluorothiophene repeat unit. This new polymer exhibits a high electron affinity (EA) of -4.1 eV and was successfully utilized as an n-type polymer semiconductor for applications in organic field-effect transistors (OFETs) and all polymer solar cells. A promising n-type charge carrier mobility of 0.1 cm² V^-1 s^-1 was obtained in bottom-contact, top-gate OFETs, and a power conversion efficiency (PCE) of 2.72% with a high open-circuit voltage (V_OC) of 1.04 V was achieved for all polymer solar cells using PTB7-Th as the polymer donor.

Full text of DOI:10.1021/acs.macromol.7b00934

Article
Cite This: Macromolecules XXXX, XXX, XXX−XXX
A Thieno[2,3‑b]pyridine-Flanked Diketopyrrolopyrrole Polymer as
an n‑Type Polymer Semiconductor for All-Polymer Solar Cells and
Organic Field-Effect Transistors
Hung-Yang Chen,*,† Mark Nikolka,‡ Andrew Wadsworth,† Wan Yue,†,∥ Ada Onwubiko,†
Mingfei Xiao,‡ Andrew J. P. White,† Derya Baran,†,§ Henning Sirringhaus,‡ and Iain McCulloch†,§
†Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K.
‡Cavendish Laboratory, University of Cambridge, J. J. Thompson Avenue, Cambridge CB3 0HE, U.K.
§King Abdullah University of Science and Technology (KAUST), KSC, Thuwal 23955-6900, Saudi Arabia
∥Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Material and Material
Engineering, Sun Yat-Sen University, Guangzhou 510275, China
S
* Supporting Information
ABSTRACT: A novel fused heterocycle-flanked diketopyrro-
lopyrrole (DPP) monomer, thieno[2,3-b]pyridine diketopyrro-
lopyrrole (TPDPP), was designed and synthesized. When
copolymerized with 3,4-difluorothiophene using Stille coupling
polymerization, the new polymer pTPDPP-TF possesses a
highly planar conjugated polymer backbone due to the fused
thieno[2,3-b]pyridine flanking unit that effectively alleviates
the steric hindrance with both the central DPP core and the
3,4-difluorothiophene repeat unit. This new polymer exhibits a
high electron affinity (EA) of −4.1 eV and was successfully
utilized as an n-type polymer semiconductor for applications in
organic field-effect transistors (OFETs) and all polymer solar
cells. A promising n-type charge carrier mobility of 0.1 cm2 V−1
s−1 was obtained in bottom-contact, top-gate OFETs, and a power conversion efficiency (PCE) of 2.72% with a high open-
circuit voltage (VOC) of 1.04 V was achieved for all polymer solar cells using PTB7-Th as the polymer donor.
Diketopyrrolopyrrole (DPP)-based small molecules and
polymers have shown excellent performance as p-type
semiconductors in OFETs and OSCs17 due to their high
structural planarity, small optical gap,18 and high absorption
coefficient.19 Impressive hole mobility of higher than 1 cm2
V−1 s−1 in OFETs20,21 and PCEs above 8% as donors in
OSCs20,22 have been reported in these DPP polymers. High
short-circuit current (JSC) of larger than 20 mA cm−2 in OSCs
using DPP polymer donors are also reported.20,23 The bicyclic
DPP core is usually flanked with electron-rich aryl units on
both sides at the 3- and 6-positions of DPP, such as phenyl,
furan, thiophene, selenophene, and thieno[3,2-b]-
thiophene.18,24,25 Although the bis-lactam core DPP unit is
electron-deficient, few examples are demonstrated to realize
DPP-based polymers as predominantly n-type polymer semi-
conductors.26 These examples utilize electron-deficient como-
nomers, such as benzothiadiazole,27 fluorinated phenylene,28
INTRODUCTION
■
Solution-processable polymer semiconductors are promising
candidates to realize next-generation lightweight and flexible
electronics, such as organic field-effect transistors (OFETs)
and organic solar cells (OSCs), via low cost and large area
printing. The past decade has witnessed a tremendous advance
in p-type polymer semiconductors with hole mobilities
surpassing 10 cm2 V−1 s−1 in OFETs1,2 and power conversion
efficiencies (PCEs) above 11% in OSCs.3,4 To achieve broad
applications of organic electronics such as organic comple-
mentary circuits and all polymer solar cells, congruent
performances for both p- and n-type polymer semiconductors
are desired. However, the progress in n-type polymer
semiconductor development is much slower in comparison
with its p-type counterparts with few polymers demonstrating
electron mobility higher than 5 cm2 V−1 s−1 in OFETs5,6 and
PCEs higher than 7% as acceptors in OSCs.7−11 Moreover,
most high performance n-type polymer semiconductors are
constructed by rylene diimide-based structure, such as perylene
diimide (PDI),11−13 naphthalene diimide (NDI),8,10,14 or
naphthodithiophene diimide (NDTI).15,16
Received: May 15, 2017
Revised: December 10, 2017
© XXXX American Chemical Society
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Scheme 1. Fused Heterocycle-Flanked DPP Polymers
Scheme 2. Synthetic Route of Thieno[2,3-b]pyridine−Diketopyrrolopyrrole (TPDPP) and Brominated Thieno[2,3-
a
b]pyridine−Diketopyrrolopyrrole (α- and β-Brominated TPDPP)
a
Reagents: (a) m-CPBA, CHCl3; (b) dimethylcarbamyl cloride, trimethylsilyl cyanide, CH3CN; (c) Na, cat. FeCl3, 2-methyl-2-butanol, diethyl
succinate; (d) 9-(bromomethyl)nonadecane, K2CO3, 18-crown-6, DMF; (e) NBS, DMF; (f) i. nBuLi/THF; ii. CBr4/THF; (g) dimethylcarbamyl
chloride, trimethylsilyl cyanide, CH2Cl2.
and thienylcyanovinylene,29 to increase the electron affinity
(EA) and achieve n-type charge transport.
enhance intermolecular charge-carrier hopping. This approach
was used to design an n-type semiconducting polymer based
on the DPP core with an electron-deficient flanking unit. A
fused heterocycle thieno[2,3-b]pyridine was chosen as the
flanking unit (Scheme 1). The thieno[3,2-b]thiophene-flanked
DPP unit has less steric effects than phenyl-flanked units as the
five-membered ring of thiophene adjacent to the DPP core
does not possess alpha C−H groups which can exhibit
torsional twisting, thus increasing the dihedral angle between
the linked units. On the other hand, thieno[2,3-b]pyridine-
flanked DPP monomer also shows a suppressed steric
hindrance through the positioning of the less steric nitrogen
atom adjacent to the DPP core to improve the coplanarity.
Furthermore, thieno[2,3-b]pyridine is more electron deficient
than thieno[3,2-b]thiophene which has the benefit of
increasing the electron affinity. Besides, we choose thieno-
[2,3-b]pyridine as the flanking substituent rather than
thieno[3,2-b]pyridine is due to its ability to adopt a quinoid
resonance form, which is potential for smaller band gap and
better charge transport for the polymer chain. However, a
quinoid resonance structure cannot be formed when using
thieno[3,2-b]pyridine as the flanking substituent on the DPP
core. Additionally, compared with the pyridine-flanked DPP
polymer, PDBPyBT, which has a larger steric hindrance
between 3,5-positions of hydrogen atoms on the pyridine with
There are, however, very few examples reported with
modification by electron-deficient flanking substituents on
the DPP core. Replacement of thiophene by thiazole to
synthesize a thiazole-flanked DPP unit was successfully
employed as a building block for n-type polymer semi-
conductors. The polymer PDPP2TzT possesses an electron
mobility of 0.13 cm2 V−1 s−1 in OFET and was employed as an
acceptor in OSC with a PCE around 3%.30,31 Pyridine was also
chosen as the flanking substituent for the DPP core, which is
more electron deficient than the commonly used aryl units
(thiophene, furan, and benzene) and also possesses a smaller
steric hindrance between the N−CH2 of the alkyl chain and
the nitrogen atom at the ortho-position of pyridine.5 When
copolymerized with 2,2′-bithiophene, the polymer PDBPyBT
demonstrated ambipolar charge transport compared with its
analogous thiophene polymer, PDQT, which showed only p-
type charge transport.32
A DPP-based repeat unit bearing the fused heterocycle,
thieno[3,2-b]thiophene, has previously been demonstrated
with impressive performances as p-type semiconducting
polymers in both OFETs and OSCs.20,25,33,34 The fused
heterocycle substituent with increased intermolecular associa-
tion through extended polymer coplanarity was shown to
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Scheme 3. Synthesis of pTPDPP-TF
Figure 1. Chemical structure and single crystal structure (front and side view) of 2EH-TPDPP.
linking comonomers, the flanking thiophene on the thieno[2,3-
b]pyridine flanked DPP should have reduced backbone
twisting with linking comonomers compared with the
pyridine-flanked DPP monomer.5 As shown in Scheme 1, the
electron-deficient 3,4-difluorothiophene was chosen as the
comonomer to ensure a high coplanarity and high electron
affinity (EA) for the polymer.
vis absorption spectrum of the polymer obtained from the β-
position dibrominated TPDPP monomer, β-Br2TPDPP, and
distannylated thiophene by Stille polymerization having a
much higher energy absorption than expected (see Supporting
Information, Figure S7). As shown in Figure S7, the absorption
of the polymer pTPDPP-β-T shows a less bathochromic shift
compared with the monomer β-Br2TPDPP, which is not in
accordance with literature reported DPP polymers using
thiophene as the comonomer. This is likely due to cross-
conjugation and therefore reduced delocalization. Therefore, a
single crystal of 3-bromothieno[2,3-b]pyridine-6-carbonitrile 5
was prepared to confirm the position of the bromine atom on
the thieno[2,3-b]pyridine-6-carbonitrile. The absolute config-
uration of the single-crystal structure indeed shows the
bromination occurred at the β-position of 3, and crystal data
of compound 5 are shown in the Supporting Information
(Figure S4). A previous literature has also shown the
bromination of thieno[2,3-b]pyridine35 using liquid bromine
can lead to the β-position brominated product. A similar
situation also occurred in the case of benzo[b]thiophene.36
Because of the higher reactivity for the β-position of
thieno[2,3-b]pyridine-6-carbonitrile 3 under electrophilic
halogenation, we changed our strategy by using n-butyllithium
to deprotonate the α-position of thieno[2,3-b]pyridine 1. The
nucleophilic α-lithiated thieno[2,3-b]pyridine was then reacted
with the bromine source tetrabromomethane (CBr4) to afford
2-bromothieno[2,3-b]pyridine 7. The carbonitrile precursor, 2-
bromothieno[2,3-b]pyridine-6-carbonitrile 9, was obtained in
two steps analogous to those in route I, in which the structure
of compound 9 was also confirmed from its single crystal
(Figure S5). Finally, the α-position dibrominated TPDPP, α-
Br2TPDPP, was obtained from 9 as a deep red solid, which is
soluble in common organic solvents.
RESULTS AND DISCUSSION
■
Scheme 2 shows the synthetic routes of the brominated
thieno[2,3-b]pyridine−diketopyrrolopyrrole (TPDPP) mono-
mer and its derivatives. Initially, the thieno[2,3-b]pyridine core
was synthesized in one step from commercially available 2-
nitrothiophene according to the literature procedure.35 Beyond
our expectations, it was tortuous to obtain the TPDPP
monomer. First, we started our approach using route I
following reactions analogous to those commonly used in the
diaryl-DPP. The carbonitrile precursor in preparing DPP,
thieno[2,3-b]pyridine-6-carbonitrile 3, can be easily synthe-
sized from thieno[2,3-b]pyridine by two steps in moderate
yields. After obtaining the non-alkylated TPDPP 4 from 3
according the literature method, a long and branched 2-
octyldodecyl chain was anchored on the TPDPP core to obtain
C8C10-TPDPP to ensure a good solubility for the polymer.
However, the bromination of C8C10-TPDPP using N-
bromosuccinimde (NBS) or liquid bromine resulted in a
decomposition of the TPDPP core and unidentified side
products.
We therefore modified our synthetic route to introduce the
bromine atom at the earlier step as presented in route II.
Bromination of thieno[2,3-b]pyridine-6-carbonitrile 3 using N-
bromosuccinimide proceeded smoothly in DMF. Surprisingly,
it was found afterward that the bromination occurred at the β-
position of 3 to afford 5, with the consequence that the UV−
As shown in Scheme 3, the polymer pTPDPP-TF was then
synthesized from dibrominated TPDPP monomer, α-
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Figure 2. Left: normalized UV−vis absorption spectra of pTPDPP-TF in 1,2-dichlorobenzene and as thin film. Middle: cyclic voltammograms of
pTPDPP-TF thin film drop cast on a Pt working electrode (reference electrode: Ag/AgCl, counter electrode: Pt gauze, electrolyte solution: 0.1 M
TBAPF6 in anhydrous acetonitrile). Right: XRD diffractogram of the as cast film on Si wafer for pTPDPP-TF.
Figure 3. Dihedral angles of energy-minimized polymer backbone structure of pTPDPP-TF (front and side view).
Br2TPDPP, and 2,5-bis(trimethylstannyl)-3,4-difluorothio-
phene comonomer using palladium-catalyzed Stille coupling
polymerization. The crude polymer was purified by Soxhlet
extraction in the sequence of methanol, hexane, ethyl acetate,
and chloroform. The purified polymer, soluble in chloroform,
chlorobenzene, and 1,2-dichlorobenzene at room temperature,
has a weight-average molecular weight (Mw) of 125.3 kDa and
number-average molecular weight (Mn) of 24.4 kDa giving a
polydispersity (PDI) of 5.1, likely an overestimate due to
aggregating effects in solution.
To confirm the planarity of the new TPDPP monomer, a
model TPDPP small molecule anchored with two shorter 2-
ethylhexyl branched chains (2EH-TPDPP) was synthesized,
and the single crystal structure was easily obtained from slow
evaporation of a binary solvent system (chloroform and
methanol). Detailed crystallographic data are shown in Figure
S6. Figure 1 shows the single crystal structure of 2EH-TPDPP,
which exhibits a dihedral angle of 7.1° between the thieno[2,3-
b]pyridine ring and the DPP core. Although 2-pyridinyl-
substituted DPP block has been reported with a dihedral angle
between pyridine and the DPP core being 0° from computer
simulations,5 no applicable references of the pyridinyl-DPP
crystal structure are available to the best of our knowledge.
Moreover, the small dihedral angle of 2EH-TPDPP is similar
to thiophene-containing DPP molecules that have dihedral
angles ranging from 3° to 9° in several reported crystal
structures.37 This indicates that a highly coplanar structure and
π-electron delocalization could be achieved in the polymer
backbone when using TPDPP as the building block.
absorption of the thin film is 1.66 eV, which is significantly
larger than the thieno[3,2-b]thiophene-flanked DPP polymer
(1.38 eV).25 This can be attributed to a lower degree of
intramolecular donor−acceptor molecular orbital hybridization
among the polymer conjugation backbone of pTPDPP-TF.
Photoelectron spectroscopy in air (PESA) was utilized to
measure the ionization potential (IP) of pTPDPP-TF thin
films. A large IP of −5.8 eV was measured, and the electron
affinity (EA) was subsequently obtained as −4.1 eV by the
addition of its optical band gap. The EA of pTPDPP-TF is
significantly higher than that of thieno[3,2-b]thiophene-
flanked DPP polymer (−3.7 eV).25 This high EA value
would be beneficial in electron injection for n-type OFETs and
also to promote charge separation in donor/acceptor blends in
bulk-heterojunction OSCs. In considering the ability of
pTPDPP-TF to be utilized as an n-type semiconductor, cyclic
voltammetry (CV) was used to study its electron affinity and
the stability of its negatively charged polaron. As shown clearly
in Figure 2, a reversible reduction cycle was observed,
suggesting its excellent electron-accepting ability and reversi-
bility. The EA value of pTPDPP-TF calculated from the onset
reduction potential is −3.51 eV, which is higher lying than that
of PC71BM (−3.90 eV) measured in the same conditions (see
Supporting Information). An improved open-circuit voltage
(VOC) relative to PC71BM can be expected in the solar cell
when using pTPDPP-TF as the acceptor.
pTPDPP-TF adopts a lamellar type packing motif similar to
other DPP polymers,18,20 which can be evident from the (100)
and (200) diffraction peaks in its XRD diffractogram (Figure 2,
right). The first-order diffraction peak 2θ = 4.5° corresponds to
a d-spacing of 19.7 Å. Although pTPDPP-TF orients
predominantly edge-on relative to the substrate as evident
from the stronger intensity of (100) diffraction peak, we
assume the broad featureless diffractions from 2θ = 15° to 30°
As shown in Figure 2, the UV−vis absorption spectra of
pTPDPP-TF in 1,2-dichlorobenzene (1,2-DCB) and as thin
film exhibit almost the same absorption profile. This is
attributed to small polymer aggregates of pTPDPP-TF in 1,2-
DCB solution, showing two vibronic peaks at around 642 and
711 nm. The optical band gap calculated from the onset
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Figure 4. (a) Transistor transfer characteristics and gate-voltage (VG) dependence of saturation mobility for OFETs (L = 20 μm, W = 1 mm). (b)
n-Type output characteristics for pTPDPP-TF gate-voltage (VG).
Figure 5. (a) J−V curves of optimized PTB7-Th:pTPDPP-TF (1:1) solar cell. (b) EQE spectrum (line + symbol) of optimized PTB7-
Th:pTPDPP-TF (1:1) solar cell alongside normalized thin film absorption spectra (line) of PTB7-Th and pTPDPP-TF.
likely contain reflections from face-on crystallites and other
tilted chain orientations.38
exhibits ambipolar charge transport characteristics with more
pronounced n-type than p-type transport behavior was
observed. As evident from the gate-voltage (VG) dependence
of extracted charge carrier mobilities presented in Figure 4a,
we found that the maximum n-type field-effect mobility of
pTPDPP-TF was obtained as 0.1 cm2 V−1 s−1 with a
corresponding on−off ratio of 104, and the threshold voltage
for pTPDPP-TF OFET was 14 V. Fitting of the transfer
characteristics with an ideal MOSFET model as described
elsewhere39 results in a conservative mobility of 0.04 cm2 V−1
s−1. The output characteristics in Figure 4b do not exhibit the
typical S-shape associated with an injection barrier, suggesting
that the LUMO level is close enough to the electrode Fermi
level to provide good injection of electrons.
Polymer−polymer solar cells, using PTB7-Th (also known
as PBDTTT-EFT) as the donor and pTPDPP-TF as the
acceptor, were fabricated in an inverted device architecture of
glass/ITO/ZnO/PTB7-Th:pTPDPP-TF (1:1)/MoO3/Ag.
PTB7-Th has been reported to have an EA value of −3.66
eV and a IP value of −5.24 eV using ultraviolet photoelectron
spectroscopy (UPS);40 therefore, sufficient energetic offsets
between PTB7-Th and pTPDPP-TF were expected for charge
separation. The active layer solution was prepared in 1,2-
dichlorobenzene, without the use of additives. The best device
gave a power conversion efficiency (PCE) of 2.72% with a
short-circuit current density (JSC) of 6.57 mA cm−2. A low fill
In order to further probe the dihedral angles between
adjacent backbone units, a computational modeling study
using density functional theory (DFT) at the B3LYP 6-31G
level was used to predict energy-minimized polymer backbone
structure and study backbone planarity for pTPDPP-TF. As
shown in Figure 3, pTPDPP-TF exhibits close to coplanar
dihedral angles between the DPP unit and adjacent
substituents (thieno[2,3-b]pyridine). This is attributed to the
effective suppression of steric hindrance from the 2-pyridinyl
groups. However, there is more of a twist in the molecule, as
observed from the single crystal structure, than is predicted by
DFT. It is possible that the small twist may arise from the
longer alkyl chain (2-ethylhexyl) of the model compound than
the calculated structure (methyl). Furthermore, pTPDPP-TF is
almost completely coplanar along the backbone due to the
absence of steric hindrance between TPDPP monomer and
3,4-difluorothiophene monomers. This highly planar polymer
backbone is in agreement with its tendency of forming small
aggregates in solution due to strong π−π interactions between
polymer chains.
In Figure 4, representative transfer characteristics of OFET
for the investigated polymer pTPDPP-TF are shown. We find
that in our device architecture (bottom-contact, top-gate with
gold electrodes), after annealing at 100 °C, pTPDPP-TF
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72.0 mmol) was added dropwise under an argon atmosphere,
followed by slowly addition of trimethyl cyanide (6.9 mL, 54.0
mmol). The reaction mixture was heated to 100 °C. After being
stirred at 100 °C for 4 h, the reaction mixture was cooled to room
temperature, and then 10% K2CO3 aqueous solution was added to
quench the reaction. The layers were allowed to separate; the organic
layer was isolated and dried by MgSO4, filtered, and then
concentrated. The crude product was purified by column chromatog-
raphy with petroleum ether/ethyl acetate (9:1) as the eluent to give
compound 3 as a white solid (5.6 g, 97% yield). 1H NMR (400 MHz,
CDCl3) δ: 8.22 (d, J = 8.2 Hz, 1H), 7.85 (d, J = 6.0 Hz, 1H), 7.70 (d,
J = 8.2 Hz, 1H), 7.40 (d, J = 6.0 Hz, 1H). 13C NMR (101 MHz,
CDCl3) δ: 162.23, 134.72, 132.16, 131.54, 129.44, 123.37, 121.46,
117.55.
3,6-Bis(thieno[2,3-b]pyridin-6-yl)-2,5-dihydropyrrolo[3,4-c]-
pyrrole-1,4-dione (4). Sodium (0.69 g, 30.0 mmol) and iron(III)
chloride (30 mg) were added to 20 mL of 2-methyl-2-butanol, and
the mixture was heated to reflux until full consumption of the sodium
was observed (about 3 h). The solution was cooled to 85 °C, and
then thieno[2,3-b]pyridine-6-carbonitrile (3, 3.2 g, 20.0 mmol) was
added, followed by diethyl succinate (1.33 mL, 8.0 mmol). After
stirring for 2 h, the reaction was cooled to 50 °C, and methanol (25.0
mL) was added. The reaction was quenched by addition of glacial
acetic acid (8.0 mL) and refluxed for 15 min. The reaction mixture
was filtered and washed with water (2 × 50 mL), hot methanol (2 ×
50 mL), acetone (2 × 25 mL), and hexane (25 mL) to afford the
product as a dark purple solid (2.1 g, 52% yield), which was used
without further purification.
factor (FF) of 0.40 was obtained, suggesting poor phase
separation in the blend, resulting in significant charge
recombination.30 However, a high open-circuit voltage (VOC
)
of 1.04 V was obtained, which has a significant improvement
compared with the PCE10:PC71BM blend due to the higher
EA of pTPDPP-TF (according to our comparison of EA values
from cyclic voltammetry).40 The EQE of the best PTB7-
Th:pTPDPP-TF device is shown in Figure 5b, alongside the
normalized absorption spectra of pristine PTB7-Th and
pTPDPP-TF. It can be seen clearly that the EQE around
450 nm is mainly contributed from pTPDPP-TF due to its
stronger absorption in this spectral region. This is also
evidence of charge generation from “channel II”,41 where the
exciton was generated from the acceptor, followed by hole
transfer from acceptor to donor. The EQE in the range of
500−800 nm can be attributed to both PTB7-Th and
pTPDPP-TF due to their overlapping absorptions and the
stronger EQE intensity.
CONCLUSIONS
■
A new fused heterocycle-flanked diketopyrrolopyrrole (DPP)
monomer possessing a planar and electron-deficient structure
was designed and successfully synthesized. The fused thieno-
[2,3-b]pyridine flanking unit effectively alleviates the steric
hindrance both with the central DPP core and the
comonomers due to the less sterically nitrogen atom and
thiophene ring on the thieno[2,3-b]pyridine, respectively.
When copolymerized with electron-deficient 3,4-difluorothio-
phene, the resulting polymer pTPDPP-TF features a highly
planar conjugated backbone with a high EA and IP values of
−4.1 eV and −5.8 eV, respectively. A good and reversible
electron-accepting ability was also observed from cyclic
voltammetry. A promising n-type charge carrier mobility of
0.1 cm2 V−1 s−1 was obtained in bottom-contact, top-gate
OFETs. All polymer solar cells using PTB7-Th as the polymer
donor gave a power-conversion efficiency of 2.72% with a high
open-circuit voltage of 1.04 V. These results demonstrate that
the new DPP monomer TPDPP as a potential building block
for the design of n-type semiconducting polymers.
2,5-Bis(2-octyldodecyl)-3,6-bis(thieno[2,3-b]pyridin-6-yl)-
2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (C8C10-TPDPP).
To
a mixture of 3,6-bis(thieno[2,3-b]pyridin-6-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (4, 0.49 g, 1.22 mmol),
potassium carbonate (0.56 g, 4.03 mmol), and 18-crown-6 (6 mg)
in DMF (20 mL) was added 9-(bromomethyl)nonadecane (1.46 g,
4.03 mmol). The reaction mixture was stirred at 120 °C for overnight
and then cooled to room temperature. Chloroform (50 mL) and
water were added (100 mL), and the layers were separated. The
organic layer was washed with saturated brine, dried over MgSO4,
filtered, and then concentrated under vacuum. Column chromatog-
raphy on silica gel with petroleum ether/CH2Cl2 (4:1) as the eluent
followed by recrystallization from CH2Cl2 and MeOH afforded
1
C8C10-TPDPP as a deep red solid (0.35 g, 30% yield). H NMR
(400 MHz, CDCl3) δ: 9.10 (d, J = 8.5 Hz, 2H), 8.28 (d, J = 8.5 Hz,
2H), 7.72 (d, J = 6.0 Hz, 2H), 7.39 (d, J = 6.0 Hz, 2H), 4.46 (d, J =
7.2 Hz, 4H), 1.74−1.63 (m, 2H), 1.34−1.06 (m, 64H), 0.87 (dt, J =
8.6, 7.0 Hz, 12H). 13C NMR (101 MHz, CDCl3) δ: 162.72, 160.92,
145.48, 144.42, 133.12, 131.51, 130.10, 123.11, 121.77, 111.66, 46.44,
38.55, 31.93, 31.89, 31.59, 30.11, 29.66, 29.57, 29.52, 29.37, 29.31,
26.56, 22.70, 14.12.
EXPERIMENTAL SECTION
■
Materials. Thieno[2,3-b]pyridine35 and 2,5-bis(trimethylstannyl)-
3,4-difluorothiophene42 were synthesized according to literature
procedures. All other reagents and solvents were purchased from
Sigma-Aldrich, VWR, Fluorochem, and Acros and used as received.
Dry solvents for anhydrous reactions were purchased from Sigma-
Aldrich, and no further attempt at drying them was made. All
reactions were carried out under an inert argon atmosphere unless
otherwise stated.
3-Bromothieno[2,3-b]pyridine-6-carbonitrile (5). To a stirred
solution of thieno[2,3-b]pyridine-6-carbonitrile (3, 0.32 g, 2.0 mmol)
in anhydrous DMF was added N-bromosuccinimide (0.37 g, 2.1
mmol) at room temperature. The reaction mixture was stirred at
room temperature for overnight. The reaction mixture was poured
into water and then extracted with ethyl acetate for three times. The
organic layers were collected and then washed with water and brine,
dried over MgSO4, filtered, and then concentrated under vacuum.
Column chromatography on silica gel with petroleum ether/ethyl
acetate (9:1) as the eluent gave compound 5 as a white solid (0.25 g,
53% yield). 1H NMR (400 MHz, CDCl3) δ: 8.24 (d, J = 8.2 Hz, 1H),
7.85 (s, 1H), 7.81 (d, J = 8.2 Hz, 1H). 13C NMR (101 MHz, CDCl3)
δ: 160.36, 133.57, 131.53, 130.68, 129.13, 124.02, 117.03, 105.56.
3,6-Bis(3-bromothieno[2,3-b]pyridin-6-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (6). Sodium (86 mg,
3.75 mmol) and iron(III) chloride (3.5 mg) were added to 2.5 mL
of 2-methyl-2-butanol, and the mixture was heated to reflux until full
consumption of the sodium was observed (about 3 h). The solution
was cooled to 85 °C, and then 3-bromothieno[2,3-b]pyridine-6-
carbonitrile (5, 0.6 g, 2.5 mmol) was added, followed by diethyl
succinate (0.17 mL, 1.0 mmol). After stirring for 2 h, the reaction was
Thieno[2,3-b]pyridine 7-Oxide (2). A stirred solution of
thieno[2,3-b]pyridine (1, 5.4 g, 40.0 mmol) in chloroform (180
mL) at room temperature was treated with m-chloroperoxybenzoic
acid (m-CPBA 77%, 12.4 g, 71.9 mmol) in portions over a period of
30 min. After the reaction mixture was stirred for over 48 h, the
solution was washed with 10% aqueous NaOH solution and then
water. The organic phase was dried over MgSO4, filtered, and
concentrated under vacuum to give a white solid (5.83 g, 96% yield).
The crude product was used in the next step without any further
purification. 1H NMR (400 MHz, CDCl3) δ: 8.35 (d, J = 6.2 Hz, 1H),
7.77 (m, J = 8.0, 0.4 Hz, 1H), 7.60 (d, J = 5.7 Hz, 1H), 7.37−7.30 (m,
2H). 13C NMR (101 MHz, CDCl3) δ: 136.88, 134.23, 128.44, 123.08,
121.92, 121.37.
Thieno[2,3-b]pyridine-6-carbonitrile (3). To a stirred solution
of thieno[2,3-b]pyridine-7-oxide (2, 5.44 g, 36.0 mmol) in acetonitrile
(360 mL) at room temperature, dimethyl carbamyl chloride (6.7 mL,
F
DOI: 10.1021/acs.macromol.7b00934
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Macromolecules
Article
cooled to 50 °C, and methanol (3.0 mL) was added. The reaction was
quenched by addition of glacial acetic acid (1.0 mL) and refluxed for
15 min. The reaction mixture was filtered and washed with water (2 ×
20 mL), hot methanol (2 × 20 mL), acetone (2 × 10 mL), and
hexane (10 mL) to afford the product 6 as a dark purple solid (0.35 g,
50% yield), which was used without further purification.
consumption of the sodium was observed (about 3 h). The solution
was cooled to 85 °C, and then 2-bromothieno[2,3-b]pyridine-6-
carbonitrile 9 (3.0 g, 12.55 mmol) was added, followed by diethyl
succinate (0.85 mL, 5.02 mmol). After stirring for 2 h, the reaction
was cooled to 50 °C, and methanol (17 mL) was added. The reaction
was quenched by addition of glacial acetic acid (5 mL) and refluxed
for 15 min. The reaction mixture was filtered and washed with water
(2 × 50 mL), hot methanol (2 × 50 mL), acetone (2 × 25 mL), and
hexane (25 mL) to afford the product as a black solid (1.4 g, 40%
yield), which was used without further purification.
3,6-Bis(2-bromothieno[2,3-b]pyridin-6-yl)-2,5-bis(2-octyl-
dodecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (α-
Br2TPDPP). To a mixture of 3,6-bis(2-bromothieno[2,3-b]pyridin-
6-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione 10 (0.56 g, 1.0
mmol), potassium carbonate (0.42 g, 3.0 mmol), and 18-crown-6
(6 mg) in DMF (20 mL) was added 9-(bromomethyl)nonadecane
(1.31 g, 3.0 mmol). The reaction mixture was stirred at 120 °C for
overnight and then cooled to room temperature. Chloroform and
water were added, and the layers were separated. The organic layer
was washed with saturated brine, dried over MgSO4, filtered, and then
concentrated under vacuum. Column chromatography on silica gel
with petroleum ether/CH2Cl2 (4:1) as the eluent followed by
recrystallization from CH2Cl2 and MeOH afforded α-Br2TPDPP as a
deep red solid (112 mg, 10% yield). 1H NMR (400 MHz, CDCl3) δ:
9.09 (d, J = 8.5 Hz, 2H), 8.15 (d, J = 8.5 Hz, 2H), 7.42 (s, 2H), 4.41
(d, J = 7.3 Hz, 4H), 1.71−1.61 (m, 2H), 1.36−1.06 (m, 64H), 0.88
(q, J = 6.9 Hz, 12H). MS (MALDI-TOF): m/z calcd for
C60H88Br2N4O2S2 1121.32; found 1121.7 (M + 1).
3,6-Bis(3-bromothieno[2,3-b]pyridin-6-yl)-2,5-bis(2-octyl-
dodecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (β-
Br2TPDPP). To a mixture of 3,6-bis(3-bromothieno[2,3-b]pyridin-
6-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (6, 0.56 g, 1.0
mmol), potassium carbonate (0.42 g, 3.0 mmol), and 18-crown-6
(6.0 mg) in DMF (20 mL) was added 9-(bromomethyl)nonadecane
(1.31 g, 3.0 mmol). The reaction mixture was stirred at 120 °C for
overnight and then cooled to room temperature. Chloroform and
water were added, and the layers were separated. The organic layer
was washed with saturated brine, dried over MgSO4, filtered, and then
concentrated under vacuum. Column chromatography on silica gel
with petroleum ether/CH2Cl2 (4:1) as the eluent followed by
recrystallization from CH2Cl2 and MeOH afforded β-Br2TPDPP as a
1
deep red solid (90 mg, 8% yield). H NMR (400 MHz, CDCl3) δ:
9.18 (d, J = 8.5 Hz, 2H), 8.29 (d, J = 8.5 Hz, 2H), 7.72 (s, 2H), 4.44
(d, J = 7.1 Hz, 4H), 1.71−1.61 (m, 2H), 1.40−1.07 (m, 64H), 0.88
(q, J = 7.0 Hz, 12H).
2-Bromothieno[2,3-b]pyridine (7). To a stirred solution of
thieno[2,3-b]pyridine (1, 1.5 g, 11.1 mmol) in anhydrous THF (30
mL) was added n-butyllithium (2.5 M, 4.9 mL, 12.2 mmol) dropwise
at −78 °C under an argon atmosphere. After the reaction mixture was
stirred for 1 h at −78 °C, a solution of tetrabromomethane (4.0 g,
12.2 mmol) in anhydrous THF (10 mL) was added dropwise. After
addition, the reaction mixture was allowed to warm to room
temperature and then stirred for overnight. The reaction was
quenched with 10% aqueous Na2S2O3 solution and then extracted
with ethyl acetate for three times. The organic phases were combined,
dried over MgSO4, filtered, and then concentrated. The crude product
was purified by silica gel column chromatography with petroleum/
ethyl acetate (4:1) as the eluent to give compound 7 as a white solid
(2.23 g, 94% yield). 1H NMR (400 MHz, CDCl3) δ: 8.60 (dd, J = 4.6,
1.5 Hz, 1H), 8.10 (dd, J = 8.0, 1.6 Hz, 1H), 7.56 (d, J = 6.0 Hz, 1H),
7.36−7.30 (m, 1H).
2,5-Bis(2-ethylhexyl)-3,6-bis(thieno[2,3-b]pyridin-6-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2EH-TPDPP). To a
mixture of 3,6-bis(thieno[2,3-b]pyridin-6-yl)-2,5-dihydropyrrolo[3,4-
c]pyrrole-1,4-dione 4 (0.8 g, 2.0 mmol), potassium carbonate (0.91 g,
6.6 mmol), and 18-crown-6 (12 mg) in DMF (40 mL) was added 2-
ethylhexyl bromide (1.2 mL, 6.6 mmol). The reaction mixture was
stirred at 120 °C for overnight and then cooled to room temperature.
Chloroform and water were added, and the layers were separated. The
organic layer was washed with saturated brine, dried over MgSO4,
filtered, and then concentrated under vacuum. Column chromatog-
raphy on silica gel with petroleum ether/CH2Cl2 (3:1) as the eluent
1
to afford 2EH-TPDPP as a deep red solid (0.68 g, 54% yield). H
2-Bromothieno[2,3-b]pyridine-7-oxide (8). A stirred solution
of 2-bromothieno[2,3-b]pyridine (7, 2.4 g, 11.1 mmol) in chloroform
(50 mL) at room temperature was treated with m-chloroperoxy-
benzoic acid (m-CPBA 77%, 3.11 g, 18.0 mmol) in portions over a
period of 30 min. After the reaction mixture was stirred for over 48 h,
the solution was washed with 10% aqueous NaOH solution and then
water. The organic phase was dried over MgSO4, filtered, and
concentrated under vacuum to give a light brown solid (1.94 g, 76%
yield). The crude product was used in the next step without any
further purification. 1H NMR (400 MHz, CDCl3) δ: 8.27 (dd, J = 6.3,
0.8 Hz, 1H), 7.63 (dd, J = 8.1, 0.9 Hz, 1H), 7.38 (s, 1H), 7.32 (dd, J =
8.1, 6.3 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ: 149.84, 136.93,
134.28, 125.64, 122.04, 120.89, 118.86.
NMR (400 MHz, CDCl3) δ: 9.13 (d, J = 8.5 Hz, 2H), 8.29 (d, J = 8.5
Hz, 2H), 7.72 (d, J = 6.0 Hz, 2H), 7.39 (d, J = 6.0 Hz, 2H), 4.56−
4.39 (m, 4H), 1.65 (q, J = 6.5 Hz, 2H), 1.45−1.12 (m, 16H), 0.95−
0.74 (m, 12H). 13C NMR (101 MHz, CDCl3) δ: 162.78, 160.96,
145.47, 144.39, 133.13, 131.57, 130.16, 123.14, 121.79, 111.72, 46.19,
39.91, 30.66, 28.81, 23.96, 23.16, 14.04, 10.86.
Synthesis of pTPDPP-TF. To a microwave vial was added 3,6-
bis(2-bromothieno[2,3-b]pyridin-6-yl)-2,5-bis(2-octyldodecyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (α-Br2TPDPP, 51.5 mg, 0.045
mmol, 1.0 equiv), 2,5-bis(trimethylstannyl)-3,4-difluorothiophene
(20.1 mg, 0.045 mmol, 1.0 equiv), P(oTol)3 (1.23 mg, 4.05 μmol,
0.09 equiv), and Pd2(dba)3 (0.91 mg, 0.99 μmol, 0.022 equiv).
Following further degassing the microwave vial, degassed chlor-
obenzene (4.5 mL) was added. The microwave vial was sealed, and
the reaction mixture was heated at 130 °C for 72 h. After cooling to
room temperature, the reaction mixture was poured into vigorously
stirring methanol, and the resulting polymeric precipitate was filtered.
The polymer was purified by Soxhlet extraction in a sequence of
methanol, hexane, ethyl acetate, and finally chloroform. The
chloroform fraction was concentrated by rotary evaporation,
suspended in methanol, and filtered to afford the polymer
pTPDPP-TF as a dark green flake (40 mg, 82%). GPC
(chlorobenzene, 80 °C): Mn = 24.4 kDa, Mw = 125.3 kDa, PDI = 5.1.
2-Bromothieno[2,3-b]pyridine-6-carbonitrile (9). To a stirred
solution of 2-bromothieno[2,3-b]pyridine-7-oxide (8, 3.0 g, 13.04
mmol) in dichloromethane at room temperature, dimethyl carbamyl
chloride (1.4 mL, 15.13 mmol) was added dropwise under an argon
atmosphere, followed by slowly addition of trimethyl cyanide (1.9 mL,
15.13 mmol). After the reaction mixture was stirred for 14 days at
room temperature, 10% K2CO3 aqueous solution was added to
quench the reaction. The layers were allowed to separate; the organic
layer was isolated and dried by MgSO4, filtered, and then
concentrated. The crude product was purified by column chromatog-
raphy with petroleum ether/ethyl acetate (9:1) as the eluent to give
1
compound 9 as a white solid (1.78 g, 57% yield). H NMR (400
ASSOCIATED CONTENT
MHz, CDCl3) δ: 8.09 (d, J = 8.2 Hz, 1H), 7.67 (d, J = 8.2 Hz, 1H),
7.44 (s, 1H).
3,6-Bis(2-bromothieno[2,3-b]pyridin-6-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (10). Sodium (0.43 g,
18.7 mmol) and iron(III) chloride (17 mg) were added to 12.5 mL of
2-methyl-2-butanol, and the mixture was heated to reflux until full
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b00934.
G
DOI: 10.1021/acs.macromol.7b00934
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Method and instruments, experimental details for
Article
Enabled by Naphthodiperylenetetraimide-Based Polymer Acceptor.
Adv. Mater. 2017, 29, 1700309.
fabrication and characterization of OFET and OSC
devices, NMR spectra of intermediates and monomers,
DSC curve of the polymer, and crystallographic data
(PDF)
(12) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.;
Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. A High-Mobility Electron-
Transport Polymer with Broad Absorption and Its Use in Field-Effect
Transistors and All-Polymer Solar Cells. J. Am. Chem. Soc. 2007, 129,
7246−7247.
(13) Zhou, E.; Cong, J.; Wei, Q.; Tajima, K.; Yang, C.; Hashimoto,
K. All-Polymer Solar Cells From Perylene Diimide Based Copoly-
mers: Material Design and Phase Separation Control. Angew. Chem.,
Int. Ed. 2011, 50, 2799−2803.
(14) Zhou, E.; Cong, J.; Hashimoto, K.; Tajima, K. Control of
Miscibility and Aggregation via the Material Design and Coating
Process for High-Performance Polymer Blend Solar Cells. Adv. Mater.
2013, 25, 6991−6996.
(15) Zhou, E.; Nakano, M.; Izawa, S.; Cong, J.; Osaka, I.; Takimiya,
K.; Tajima, K. All-Polymer Solar Cell with High Near-Infrared
Response Based on a Naphthodithiophene Diimide (NDTI)
Copolymer. ACS Macro Lett. 2014, 3, 872−875.
AUTHOR INFORMATION
Corresponding Author
*E-mail: h.chen@imperial.ac.uk (H.-Y.C.).
ORCID
Hung-Yang Chen: 0000-0001-8426-4254
Andrew Wadsworth: 0000-0002-9050-0599
Iain McCulloch: 0000-0002-6340-7217
■
Notes
The authors declare no competing financial interest.
(16) Yang, J.; Xiao, B.; Tajima, K.; Nakano, M.; Takimiya, K.; Tang,
A.; Zhou, E. Comparison Among Perylene Diimide (PDI),
Naphthalene Diimide (NDI), and Naphthodithiophene Diimide
(NDTI) Based N-Type Polymers for All-Polymer Solar Cells.
Macromolecules 2017, 50, 3179−3185.
ACKNOWLEDGMENTS
■
The authors thank KAUST for financial support and
acknowledge EC FP7 Project SC2 (610115) EC H2020
(643791), and EPSRC Projects EP/G037515/1 and EP/
M005143/1. Mingfei Xiao thanks the Cambridge Overseas
Trust and Chinese Scholarship Council for PhD funding.
(17) Tang, A.; Zhan, C.; Yao, J.; Zhou, E. Design of
Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-
Solar-Cell Applications. Adv. Mater. 2017, 29, 1600013.
(18) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Recent Advances in
the Development of Semiconducting DPP-Containing Polymers for
Transistor Applications. Adv. Mater. 2013, 25, 1859−1880.
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DOI: 10.1021/acs.macromol.7b00934
Macromolecules XXXX, XXX, XXX−XXX