Chalcogen Bridged Thieno- and Selenopheno[2,3- d:5,4- d′]bisthiazole and Their Diketopyrrolopyrrole Based Low-Bandgap Copolymers

Article abstract of DOI:10.1021/acs.macromol.8b00826

We report the synthesis and characterization of four novel small bandgap copolymers incorporating the electron-deficient thieno[2,3-d:5,4-d′]bisthiazole and selenopheno[2,3-d:5,4-d′]bisthiazole building blocks with a series of electron-deficient diketopyrrolopyrole units. The four resultant copolymers were synthesized via palladium Stille cross-coupling reaction, and their optical, thermal stability, electrochemical, and field-effect charge transport properties were investigated. All copolymers showed low optical bandgaps (1.53-1.56 eV); in addition, X-ray diffraction on solution-cast films revealed that the selenium-containing copolymers exhibit higher crystallinity compared to their thiophene counterparts.

Full text of DOI:10.1021/acs.macromol.8b00826

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Chalcogen Bridged Thieno- and
Selenopheno[2,3‑d:5,4‑d′]bisthiazole and Their
Diketopyrrolopyrrole Based Low-Bandgap Copolymers
Dhananjaya Patra,† Jaehyuk Lee,‡ Somnath Dey,† Jongbok Lee,§ Alexander J. Kalin,§
Anjaneyulu Putta,† Zhuping Fei,∥ Thomas McCarthy-Ward,∥ Hassan S. Bazzi,† Lei Fang,§
Martin Heeney,∥ Myung-Han Yoon,‡ and Mohammed Al-Hashimi*,†
†Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
‡Materials Science and Engineering, Gwangju Institute of Science and Technology, 123 Cheomdan-Gwagiro, Buk-gu, Gwangju
61005, South Korea
§Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77845-3255, United States
∥Department of Chemistry and Centre for Plastic Electronics, Imperial College London, Exhibition Road, London SW7 2AZ, U.K.
S
* Supporting Information
ABSTRACT: We report the synthesis and characterization of
four novel small bandgap copolymers incorporating the
electron-deficient thieno[2,3-d:5,4-d′]bisthiazole and
selenopheno[2,3-d:5,4-d′]bisthiazole building blocks with a
series of electron-deficient diketopyrrolopyrole units. The four
resultant copolymers were synthesized via palladium Stille
cross-coupling reaction, and their optical, thermal stability,
electrochemical, and field-effect charge transport properties
were investigated. All copolymers showed low optical
bandgaps (1.53−1.56 eV); in addition, X-ray diffraction on
solution-cast films revealed that the selenium-containing
copolymers exhibit higher crystallinity compared to their
thiophene counterparts.
bisthiazole unit.11−16 Most of the research to date on
conjugated copolymers based on bisthiazoles has been
INTRODUCTION
■
π-Conjugated polymers are interesting materials that are
extensively studied due to their tunable optoelectronic
properties, synthetic simplicity, and modularity which continue
dedicated to either the main-chain or side-chain engineering
of the electron-deficient unit.11,17−19 In addition, our research
group and others have reported the effects of introducing a
nitrogen atom to bridge the thiazole units to afford the fused
to provide a unique value for a host of applications including
organic field-effect transistors (OFETs),1 organic light-emitting
diodes (OLEDs),2,3 polymer solar cells (PSCs),4−6 photo-
diodes,7 and spintronics.8
building block pyrrole[2,3-d:5,4-d′]bisthiazole (PBTz) (Figure
1).20−23 We note here our gratitude to Rasmussen et al.23 for
highlighting that our earlier publications had used the incorrect
IUPAC name. The bridged architecture has significant effects
on the electronic and structural properties of the polymer main
chain. Thus, one way to further tune the electrochemical,
optical, and charge transport properties of the polymer is to
modify the bridging heteroatom in the heterocycle.
In recent years, several heteroatom-bridged bithio-
phene24−28 derivatives (Figure 1, DTP, DTT, DTG, DTS,
DTAs, and CPDT) have been comprehensively studied as
building blocks of conjugated polymers. Although the selenium
bridged bithiophene has not been reported to the best of our
The development of functional monomers via the
commonly used transition-metal-catalyzed polymerizations
such as Suzuki, Stille, and direct heteroarylation9 unlocks the
potential to design many new building blocks. Specifically,
careful modulation of the highest occupied and lowest
unoccupied molecular orbitals (HOMO/LUMO) is key to
engineering low-bandgap polymers. A common approach to
lowering the optical bandgap between the HOMO and LUMO
is using alternating electron-rich donors and electron-deficient
acceptors as building blocks whose frontier molecular orbital
energy levels can be easily tuned, ensuring effective charge
transport and, moreover, promoting intramolecular interaction
to enhance backbone ordering.10
Received: April 18, 2018
Revised: July 23, 2018
One building block of particular interest that has shown to
decrease the LUMO energy level is the electron-deficient
© XXXX American Chemical Society
A
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Figure 1. (a) Chemical structures of some bridge monomers in the literature and (b) monomers reported in this work and previous work.
Scheme 1. Synthetic Route to Intermediates 1 and 2 and Dibromothieno[2,3-d:5,4-d′]bisthiazole (BTzS 8) and
Dibromoselenopheno[2,3-d:5,4-d′]bisthiazole (BTzSe 9) Building Blocks
Scheme 2. Synthetic Route to P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) Copolymers
by Stille Cross-Coupling Polymerization
knowledge, other reports suggest that replacing sulfur with the
heavier and less electronegative selenium atom leads to
stronger intermolecular interactions and a higher degree of
rigidity and as a result extends the absorption spectrum toward
the near-infrared region.29−32 Similarly we recently reported a
series of vinylene copolymers containing 3-dodecylthiophene,
selenophene, and tellurophene in which we found that the
inclusion of the heavier heteroatoms resulted in a significant
reduction in optical bandgap in comparison to the thiophene
counterpart, mainly as a result of stabilization of the polymer
LUMO.33 Inspired by these works, we were interested to
prepare heteroatom bridged bisthiazole monomers, which are
unknown to the best of our knowledge.
selenopheno[2,3-d:5,4-d′]bisthiazole BTzSe building blocks.
The newly designed bisthiazole moieties were copolymerized
with a series of electron-deficient diketopyrrolopyrole (DPP)
units having long and branched side chains on the two lactam
rings flanked by either bisthiophene (BT) or thieno[3,2-
b]thiophene (TT). The optical, physical, electrochemical, and
field-effect charge transport properties of the four resultant
copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-
BTz-Se), and P(DPPTT-BTz-Se) were characterized. All
copolymers showed low optical bandgaps (1.53−1.56 eV),
and the nature of the bridged heteroatom was shown to have a
subtle impact on the polymer ionization potential. X-ray
diffraction on solution-cast films suggested that the selenium-
containing copolymers P(DPPBT-BTz-Se) and P(DPPTT-
BTz-Se) exhibited higher crystallinity compared to their
Here we describe the first synthesis of the novel electron-
deficient thieno[2,3-d:5,4-d′]bisthiazole BTzS and
B
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Table 1. Molecular Weights and Optical and Electrochemical Properties of P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-
BTz-Se), and P(DPPTT-BTz-Se) Copolymers
λmax (nm)
a
a
b
c
d
d
e
f
polymers
Mw (kDa) Mn (kDa)
Đ
Td (°C)
solution
film
HOMO (eV) LUMO (eV) Egec (eV) Egopt (eV)
P(DPPBT-BTz-S)
P(DPPTT-BTz-S)
P(DPPBT-BTz-Se)
P(DPPTT-BTz-Se)
36.5
46.6
41.6
41.1
6.8
7.8
6.7
5.4
4.1
4.2
3.9
316
330
362
368
425, 654
426, 624
441, 683
427, 639
465, 656
430, 628
474, 686
435, 639
−5.15
−5.13
−5.11
−5.10
1.56
1.55
1.55
1.53
1.56
1.55
1.55
1.53
1.51
1.52
1.45
1.50
10.5
a
b
Determined by gel permeation chromatography (against polystyrene standards) in chlorobenzene at 80 °C. λmax in chloroform dilute solution.
c
d
+
Spin-coated from chloroform solution onto a glass surface.
E
HOMO/ELUMO = [−(Eonset − Eonset (FC/FC
+)) − 4.8] eV, where 4.8 eV is the
vs Ag/Ag
e
energy level of ferrocene below the vacuum level and the formal potential Eonset(FC/FC vs Ag/Ag+) is equal to 0.45 V. Electrochemical bandgap: Egec
=
+
f
Eox/onset − Ered/onset. Optical bandgap: Egopt = 1240/λedge
.
Figure 2. Normalized UV−vis absorption spectra of P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se): (a)
dilute solutions in CHCl3 and (b) thin films on quartz surfaces at room temperature.
thiophene counterparts P(DPPBT-BTz-S) and P(DPPTT-
BTz-S).
remove low molecular weight oligomers and catalyst residues.
The chloroform soluble fraction was concentrated in volume,
and the copolymers were collected by precipitation in
methanol to afford blue solids in moderate 45−58% yields.
The resultant low-Mw copolymers displayed good solubility in
chloroform at room temperature.
Number-average (Mn) and weight-average molecular weight
(Mw) were determined by GPC in chlorobenzene at 80 °C.
The copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-S), P-
(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) exhibited modest
Mn in the range 6.71−10.53 kDa, with Đ in the range 3.9−5.4,
as listed in Table 1. All copolymers have relatively low Mw and
are in the oligomer range. The relatively large polydispersity
indexes of the copolymers can be attributed to the aggregation
in solution at low temperatures. In a similar system (fused
thiophene−diketopyrrolopyrrole) it has been found that to
obtain nonaggregated polymers elevated temperatures of 200
°C are required.35
The relatively low molecular weights may result from the
difficulty in obtaining the trimethylstanyl monomers in high
purity. Such tin monomers have a tendency to decompose
upon silica during attempted purification. The commercially
available monomers 10 and 11 had a low purity. Both
monomers were oils, and our attempts at recrystallization were
not successful. We did attempt to purify by column
chromatography using ALOX, but this led to further loss of
the stannyl groups.
RESULTS AND DISCUSSION
■
Synthesis of Monomers, Polymers, and Character-
ization. The general synthetic route of the monomers is
depicted in Scheme 1. Precursors 1 and 2 were synthesized
according to reported literature procedures with slight
modifications.34 The detailed synthesis of 4,4′-dibromo-2,2′-
bis(triisopropylsilyl)-5,5′-bithiazole (3) is reported in our
previously published work.16,20,23
The new comonomers 8 and 9 were synthesized in three
steps from monomer 3. Stille coupling of 3 with precursors 1
and 2 in toluene at 170 °C afforded compounds 4 and 5 in
moderate yields (35−36%). Subsequent deprotection of the
triisopropylsilyl (TIPS) groups using TBAF gave 6 and 7.
Treatment of compounds 6 and 7 with N-bromosuccinimide
(NBS) in DMF afforded 2,6-dibromothieno[2,3-d:5,4-d′]-
bisthiazole (8) and 2,6-dibromoselenopheno[2,3-d:5,4-d′]-
bisthiazole (9) as yellow solids in 52% and 56% yields
(Scheme 1).
As depicted in Scheme 2, P(DPPBT-BTz-S), P(DPPTT-
BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) copoly-
mers were synthesized via microwave-assisted Stille coupling
polymerization of dibrominated monomers 8 and 9 with the
corresponding distannylated DPPBT (10) and DPPTT (11)
in the presence of Pd(PPh3)4 and tri(o-tolyl)phosphine in
anhydrous chlorobenzene at 180 °C. Purification of the four
copolymers was performed by sequential Soxhlet extraction
with methanol, acetone, hexane, and dichloromethane to
Thermal Properties. The thermal stability of the
copolymers was evaluated by thermal gravimetric analysis
(TGA) and differential scanning calorimetry (DSC) depicted
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Figure 3. (a) Cyclic voltammograms of P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) as thin films (scan
rate: 50 mV s−1). (b) HOMO and LUMO energy levels of the copolymers.
in Figures S1 and S2. The TGA curves reveal that all the
copolymers are thermally stable with a degradation temper-
ature (Td) of 5% weight loss greater than 300 °C under a
nitrogen atmosphere at a heating rate of 10 °C/min (Figure
S1). Interestingly, copolymers containing the selenium
heteroatom resulted in slightly higher thermal stability than
the sulfur-containing counterparts. We note that the high
thermal stability should be adequate for their applications in
optoelectronic devices. The copolymers all showed a glass
transition temperature (Tg) over 250 °C calculated from the
DSC curve. However, the copolymers did not show any
endothermic or exothermic peaks (Figure S2), such as melting
or crystallization peaks when heated or cooled, perhaps as a
result of the extended backbone.
Optical Properties. The UV−vis absorption spectra of all
four copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-S), P-
(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) were recorded at
room temperature both in dilute chloroform solution and as
thin films spin-coated onto quartz substrates (Figure 2). The
maximum absorbance (λmax) in solution and solid state and the
optical bandgaps (Egopt) are summarized in Table 1. The
absorption spectra of all the copolymers in solution exhibited
two main absorption bands: one in the range 350−500 nm,
which is attributed to localized π−π* transitions, and a second
band having a vibronic shoulder in the range of 500−820 nm
in the long wavelength region. The substitution of the sulfur
heteroatom in P(DPPBT-BTz-S) and P(DPPTT-BTz-S) with
selenium in P(DPPBT-BTz-Se) and P(DPPTT-BTz-Se) leads
to a red-shift of λmax of 29 and 15 nm, respectively. All the four
copolymers exhibit a small red-shifted absorption in the thin
solid films (550−870 nm) compared to the chloroform
solution. The red-shift is likely a result of backbone
planarization in the solid state.
1.51, 1.52, 1.45, and 1.50 eV from the absorption onset as
shown in Table 1. As expected, the selenium-containing
copolymers exhibited smaller bandgaps than their sulfur
counterparts, which can be explained due to the decrease in
the aromaticity as the size of the heteroatom increases.36
Electrochemical Properties. The ionization and reduc-
tion potentials of the copolymers were determined using cyclic
voltammetry (CV) as depicted in Figure 3a. All four
copolymers underwent quasi-reversible oxidation in the
positive potential range and either quasi-reversible P-
(DPPBT-BTz-S) and P(DPPBT-BTz-Se) or irreversible
reduction in the negative potential range. The onset oxidation
potentials (Eox) of P(DPPBT-BTz-S), P(DPPTT-BTz-S),
P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) were found to
be 0.80, 0.78, 0.76, and 0.75 V vs Ag/Ag+, while the onset
reduction potentials (Ered) were −0.76, −0.77, −0.79, and
−0.78 V vs Ag/Ag+, respectively.
The LUMO energy levels, as calculated from reduction
onsets for the copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-
S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se), were very
similar at −3.59, −3.58, −3.56, and −3.57 eV, respectively.
The HOMO energy levels of the copolymers were calculated
from the onset oxidation potential and are −5.15, −5.13,
−5.11, and −5.10 eV as shown in Figure 3b and Table 1. The
calculated electrochemical bandgaps were approximately 1.56
eV for P(DPPBT-BTz-S), 1.55 eV for P(DPPTT-BTz-S), 1.55
eV for P(DPPBT-BTz-Se), and 1.53e V for P(DPPTT-BTz-
Se). The electrochemical bandgaps of the copolymers are well
matched with their optical bandgaps within the experimental
error.
Overall, it can be seen that replacing the heteroatom from
sulfur to selenium in the bridged bisthiazole unit only has a
subtle effect on the HOMO and LUMO energy levels of the
resultant copolymers. The subtle effect likely stems from the
relatively minor percentage component the heteroatom makes
to the overall conjugated polymer backbone. Nevertheless, the
trend observed for the HOMO and LUMO energy levels is in
agreement with other reports on selenium-containing copoly-
mers.37
X-ray Diffraction and Surface Morphology. To gain
insight into the film morphology and supramolecular
organization of the copolymers, X-ray diffraction (XRD)
measurements were performed on drop-cast polymer thin
For the thin films, comparison of the sulfur bridged
polymers P(DPPBT-BTz-S) and P(DPPTT-BTz-S) to the
selenium bridged P(DPPBT-BTz-Se) and P(DPPTT-BTz-Se)
shows a red-shift of λmax of 30 and 11 nm, in agreement with
the solution studies. The red-shift can be attributed to the
electronic effect of the heteroatom. Thus, increasing the size of
the heteroatom from sulfur to selenium is one way to red-shift
the copolymers to a longer wavelength. The optical bandgaps
(Egopt) of P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-
BTz-Se), and P(DPPTT-BTz-Se) thin film were estimated as
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Figure 4. (a) X-ray diffraction patterns and (b−e) AFM topography images of copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-
Se,) and P(DPPTT-BTz-Se) and AFM topography phase images of the copolymer films P(DPPBT-BTz-S) (b), P(DPPTT-BTz-S) (c), P(DPPBT-
BTz-Se) (d), and P(DPPTT-BTz-Se) (e) as cast films at 250 °C.
Figure 5. Transfer and output characteristics of OTFTs with P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se).
(a) A schematic of bottom gate-top contact OTFT structure. (b) Transfer characteristics and (c) output characteristics of P(DPPBT-BTz-S),
P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se). Black curves are drain current, and red curves are (drain current)1/2
.
films (Figure 4a). The four copolymers are all characterized by
a diffraction peaks around 2θ = 4.7°−4.8°, which is suggestive
of lamellar type ordering with an interchain distance of around
18−19 Å as well as broad diffraction peaks, which we assign to
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Table 2. Summary of the OFET Charge Carrier Mobilities in Hole for Copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-S),
P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se)
a
polymers
annealing temp (°C) channel length (μm) channel width (μm) μh,lin, (cm2 V−1 s−1
)
μh,sat, (cm2 V−1 s−1
)
Vth hole (V)
Ion/Ioff
P(DPPBT-BTz-S)
P(DPPTT-BTz-S)
P(DPPBT-BTz-Se)
P(DPPTT-BTz-Se)
250
250
250
250
50
50
50
50
500
500
500
500
2.1 × 10−3
6.2 × 10−3
1.7 × 10−4
5.1 × 10−3
4.3 × 10−3
9.5 × 10−3
3.8 × 10−4
1.9 × 10−2
−10
−10
−8.5
−3.5
2.5 × 104
3.0 × 104
1.0 × 104
1.0 × 101
a
Ion/Ioff refers to the on-to-off current at the max hole mobility.
π−π stacking. All polymers exhibit a similar π−π stacking
distance of 3.5 Å. In the case of the selenium-containing
copolymers P(DPPBT-BTz-Se) and P(DPPTT-BTz-Se), the
diffraction peaks showed slightly higher crystalline features
compared to the sulfur-containing copolymers P(DPPBT-BTz-
S) and P(DPPTT-BTz-S), which exhibited a second (200) and
third (300) order lamellar packing features (2θ values around
9.52° and 14.28°).
The surface morphology of the four copolymer thin films
was investigated using atomic force microscopy (AFM). Figure
4b−e illustrates the tapping-mode AFM images for P(DPPBT-
BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P-
(DPPTT-BTz-Se) prepared by spin-casting from chloroform
solution onto SiO2 substrates. All of the polymer films
exhibited finely aggregated surfaces, and when thermally
annealed at 250 °C, the surface roughness slightly increased.
The average root-mean-square (rms) roughness values after
annealing are 0.43, 0.49, 0.41, and 0.34 nm for the films of
P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se),
and P(DPPTT-BTz-Se), respectively. In addition, P(DPPTT-
BTz-Se) copolymer thin film had a slightly smaller rms
roughness value (0.34 nm) in the series, which formed
interconnected nanoscale granules, resulting in low surface
roughness.
Field-Effect Transport. The carrier mobilities were
studied using organic thin film transistors (OTFTs) with a
bottom-gate top-contact (BGTC) device configuration on p-
type silicon wafer substrate as the gate electrode and SiO2 (300
nm) layer as the gate dielectric (Figure 5a). A self-assembled
monolayer of hexamethyldisilazane (HMDS) was deposited on
SiO2 to improve the device performance by reducing the
trapping of charge in the interface between the dielectric layer
and the semiconductor layer as well as potentially aiding the
alignment of the polymers. Films were formed by spin-coating
followed by a two-step heat treatment. First, preannealing at
relatively low temperature (80 °C, 5 min) was performed to
remove any remaining solvent, and then postannealing at a
higher temperature (250 °C, 5 min) was performed to improve
the device performance by the alignment of the semi-
conducting polymeric layer.
Figure 5 shows the typical transfer and output characteristics
curves of the four copolymers, and the mobilities are
summarized in Table 2. All the copolymers exhibit p-type
characteristics in the HMDS-treated devices. The as-cast thin
film of semiconducting copolymers P(DPPBT-BTz-S), P-
(DPPTT-BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-
Se) exhibited saturation mobilities of 4.3 × 10−3, 9.5 × 10−3,
3.8 × 10−4, and 1.9 × 10−2 cm2 V−1 s−1, respectively, with
negligible hysteresis. We note that there is some leakage
current in the low-voltage region of the output characteristic.
This phenomenon occurs in an OTFT with the unpatterned
semiconducting layers and causes an increase in the off current
due to parasitic current from the OTFT channel to the gate
electrode edges.
Among the four copolymers P(DPPTT-BTz-Se) was found
to exhibit the highest hole mobility and P(DPPBT-BTz-Se)
the lowest hole mobility in OTFTs in comparison to the
others. Given the similar bandgap and thin-film crystallinity,
the highest and low mobilities of P(DPPTT-BTz-Se) and
P(DPPBT-BTz-Se) are likely due to their different molecular
weights, with the higher weight polymer exhibiting the best
performance. In addition, in both sets of copolymers the
DPPTT comonomers consistently gives higher performance
than the corresponding DPPBT copolymers, in agreement with
early studies.38 Moreover, the hole mobility of the selenium
copolymer with the DPPTT unit is double (1.9 × 10−2 cm2
V−1 s−1) that of the copolymer with DPPBT (3.8 × 10−4 cm2
V−1 s−1) moiety. Furthermore, the hole mobility of the
corresponding selenium bridged copolymer P(DPPTT-BTz-
Se) is improved by 1.5-fold over that of their corresponding
sulfur-based copolymer P(DPPTT-BTz-S).
CONCLUSION
■
In summary, we have synthesized four novel bisthiazole-DPP-
based copolymers P(DPPBT-BTz-S), P(DPPTT-BTz-S), P-
(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) via the Stille
coupling reaction and investigated the influence of changing
the chalcogen heteroatom from sulfur to selenium. The
copolymers were found to have small optical bandgaps and
good thermal stability. The influence of the bridging chalcogen
is found to be subtle, with a small reduction in optical bandgap
observed. All four copolymers exhibited p-type semiconducting
behavior in transistor devices, with a highest hole mobility of
1.9 × 10−2 cm2 V−1 s−1 observed for P(DPPTT-BTz-Se).
Further studies are currently underway to optimize the
properties and device characteristics for this class of conjugated
materials.
EXPERIMENTAL SECTION
■
General Procedures. All chemicals and solvents were purchased
in reagent grade from Aldrich and Solamer, except for Pd(PPh3)4,
which was obtained from Strem Chemicals Inc. Tetrahydrofuran
(THF) and toluene were distilled over Na/benzophenone; all
reagents were used as received. 4,4′-Dibromo-2,2′-bis-
(triisopropylsilyl)-5,5′-bithiazole1 (3) was synthesized as from a
reported procedure.16 2,5-Bis(2-octyldodecyl)-3,6-bis(5′-(trimethyl-
stannyl)-[2,2′-bithiophen]-5-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-
dione (10) and 2,5-bis(2-octyldodecyl)-3,6-bis(5-(trimethylstannyl)-
thieno[3,2-b]thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-
dione (11) were purchased from Solamer.
Measurements and Characterization. Nuclear magnetic
resonance (NMR) spectra were recorded on Bruker 400 MHz or
DRX500 NMR spectrometers at 22 °C, using chloroform-d as solvent
and the residual chloroform as the internal standard (7.26 ppm).
Flash chromatography (FC) was performed on silica gel (Merck
Kieselgel 60 F254 230−400 mesh) unless otherwise indicated. Thin
F
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layer chromatography (TLC) was performed on Merck aluminum-
backed plates precoated with silica (0.2 mm, 60 F254). Mass spectra
were obtained using gas chromatography−mass spectrometry
(GCMS, Shimadzu GCMS-QP2010). Elemental analyses were
performed on a PerkinElmer 2400 Series II CHNS/O analyzer. The
thermal stability of the polymers was evaluated by thermal gravimetric
analysis (TGA) and differential scanning calorimetry (DSC) analyses.
Number-average (Mn) and weight-average (Mw) molecular weights
were determined by Agilent Technologies 1200 series GPC running
in chlorobenzene at 80 °C, using two PL mixed B columns in series,
and calibrated against narrow polydispersity polystyrene standards.
UV−visible (UV−vis) absorption spectra were recorded in dilute
chloroform solutions (10−6 M) on a PerkinElmer Lambda 950
spectrophotometer. Thin films for UV−vis measurements were spin-
coated on a glass substrate from chloroform solutions with a
concentration of 5 mg/mL. Cyclic voltammetry (CV) measurements
were performed using a BAS 100 electrochemical analyzer with a
standard three-electrode electrochemical cell in a 0.1 M of
tetrabutylammonium hexafluorophosphate (TBAPF6) solution (in
acetonitrile) at room temperature with a scanning rate of 100 mV/s.
During the CV measurements, the solutions were purged with
nitrogen for 30 s. In each case, a carbon working electrode coated
with a thin layer of copolymers, a platinum wire as the counter
electrode, and a silver wire as the quasi-reference electrode were used,
and the Ag/AgCl (3 M KCl) electrode served as a reference electrode
for all potentials quoted herein. The redox couple of the ferrocene/
ferrocenium ion (Fc/Fc+) was used as an external standard. The
corresponding highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels were calculated
reflux for 24 h. The pale-brown suspension was cooled to 0 °C,
tributyltin chloride (17.1 g, 52.5 mmol) was added, and the reaction
mixture was stirred for another 24 h. Toluene (100 mL) was then
added to the crude mixture. The residue was passed through a plug of
silica, and the solvent was removed. The residue was passed through a
second plug of silica containing potassium fluoride (8:2 ratio) to yield
the target product as a pale yellow oil (11.4 g, 68%). 1H NMR
(CDCl3): δ 1.58−1.54 (m, 12H), 1.37−1.30 (m, 14H), 1.14−1.10
(m, 10H), 0.94 (t, 18H, J = 7.2 Hz).
2,6-Bis(triisopropylsilyl)thieno[2,3-d:5,4-d′]bis(thiazole) (4). To a
20 mL sealed microwave vial, compound 1 (0.47 g, 0.78 mmol), 4,4′-
dibromo-2,2′-bis(triisopropylsilyl)-5,5′-bithiazole (3)16 (1.0 g, 1.57
mmol), Pd(PPh3)4 (0.72 mg, 4 mol %), and dry toluene (4 mL) were
added, and the reaction mixture was purged with argon for 20 min.
The mixture was then heated to 170 °C in an oil bath overnight. After
cooling the reaction mixture to room temperature, water (5 mL) was
added, and the layers were extracted using dichloromethane (3 × 25
mL). The combined organic fractions were dried over MgSO4, filtered
through a silica plug, and concentrated in vacuo. The crude product
was purified by flash chromatography on silica gel with hexanes/
dichloromethane (3:1) to afford a white solid (0.29 g, 36%). 1H NMR
(400 MHz, CDCl3): δ 1.56−1.49 (septet, 6H, J = 8.0 Hz), 1.18−1.17
(d, 36H). 13C NMR (100 MHz, CDCl3): δ 172.2, 160.2, 132.4, 129.9,
123.9, 18.5, 11.7. GC-MS calculated for [C24H42N2S3Si2] 510.96;
found 510.90. ESI-MS m/z calcd for C24H43N2Si2S3 (M + H)+
511.2127; found 511.2118.
2,6-Bis(triisopropylsilyl)selenopheno[2,3-d:5,4-d′]bis(thiazole)
(5). Compound 5 was synthesized according to the same procedure
for 4 with the respective monomer to afford a white solid (0.31 g,
36%). 1H NMR (400 MHz, CDCl3): δ 1.54−1.46 (septet, 6H, J = 8.0
Hz), 1.18−1.70 (m, 36H). 13C NMR (100 MHz, CDCl3): δ 171.7,
158.9, 125.2, 18.5, 11.8. GC-MS calculated for [C24H42N2S2SeSi2]
558.14; found 558.10.
onset
onset
using Eoxd
and Ered
for experiments in solid films of polymers,
which were performed by drop-casting films with the similar thickness
from THF solutions (ca. 5 mg/mL). The onset potentials were
determined from the intersections of two tangents drawn at the rising
currents and background currents of the CV measurements.
Thieno[2,3-d:5,4-d′]bis(thiazole) (6). To a solution of 4 (1.9 g,
3.71 mmol) in dry THF (10 mL) was added TBAF (14.87 mL, 14.87
mmol, 1 M THF). The reaction mixture was stirred at room
temperature for 4 h, after which water was added and the residue was
extracted using chloroform (3 × 15 mL). The combined organic
fraction was washed with brine and water, dried over MgSO4, and
concentrated under reduced pressure to afford compound 6 as a pale
Transistor Fabrication and Characterization. Organic thin
film transistors (OTFTs) were fabricated as bottom gate-top contact
configuration. SiO2 (300 nm)/P2+−Si wafer substrate (Taewon
Scientific, South Korea) was cleaned by deionized water, acetone,
and isopropanol. Then hexamethyldisilazane (HMDS) was coated by
placing clean silicon substrates in a chamber saturated with HMDS
vapor for 24 h at normal temperature and pressure (reference). The
HMDS coated substrates were placed on a hot plate at 130 °C for 30
min to remove unreacted HMDS molecules (reference). The
substrates were spin-coated by semiconductor solutions with
chloroform (5 mg/mL) as follows in a N2 atmosphere: (1) 1000
rpm for 5 s, (2) 5000 rpm for 30 s, and (3) 1000 rpm for 5 s. Then
the coated substrates were annealed on a hot plate at 80 °C for 5 min
and at 250 °C for 30 min sequentially. Au source and drain electrodes
(thickness: 40 nm; channel length and width: 50 and 500 μm) were
deposited using a thermal evaporator, and electrical performance of
the OTFTs was characterized by using a Keithley 4200-SCS
(Keithley, USA) with a probe station (MS-Tech, South Korea).
Synthesis of Monomers. 1,1,1,3,3,3-Hexabutyldistannathiane
(1).34 A solution of tributyltin chloride (28.5 g, 87.4 mmol) in THF
(174 mL) was added to a solution of sodium sulfide nonahydrate
(Na2S·9H2O) (10.5 g, 175 mmol) in deionized water (34.8 mL).
Some additional deionized water (17.4 mL) was used to transfer the
remaining Na2S·9H2O to the flask. The reaction mixture was stirred at
85 °C for 6 h. The reaction mixture was allowed to cool to room
temperature, and the organic layer was evaporated under vacuum.
The residue was extracted with Et2O, dried over anhydrous
magnesium sulfate, and evaporated to give a colorless oil (21.8 g,
81%) and was used without further purification. 1H NMR (CDCl3): δ
1.58−1.54 (m, 12H), 1.36−1.31 (m, 12H), 1.09−1.05 (m, 12H), 0.91
(t, 18H, J = 7.2 Hz).
1
yellow solid (0.50 g, 70%). H NMR (400 MHz, CDCl3): δ 8.91 (s,
2H).13C NMR (100 MHz, CDCl3): δ 157.6, 153.1, 121.4. GC-MS
calculated for [C6H2N2S3] 197.92; found 197.90. ESI-MS m/z calcd
for C6H3N2S3 (M + H)+ 198.94; found 198.94.
Selenopheno[2,3-d:5,4-d′]bis(thiazole) (7). Compound 7 was
synthesized according to the same procedure for 6 to yield a yellow
solid (0.71 g, 60%). 1H NMR (400 MHz, CDCl3): δ 8.87 (2H, s).13C
NMR (100 MHz, CDCl3): δ 156.59, 152.73, 122.66. GC-MS
calculated for [C6H2N2S2Se] 245.88; found 245.90.
2,6-Dibromothieno[2,3-d:5,4-d′]bis(thiazole) (8). To a solution of
compound 6 (0.263 g, 1.33 mmol) in dry DMF (30 mL) was added
NBS (1.20 g, 6.60 mmol) in one portion. The reaction mixture was
stirred at 65 °C for 4 h. It was cooled to RT, water was then added,
and the reaction mixture was extracted with chloroform (3 × 50 mL).
The combined organic layers were washed with brine and water, dried
over MgSO4, and filtered, and the solvent was removed in vacuo.
Recrystallization of the crude product from methanol afforded a
compound 8 as a yellow solid (0.26 g, 56%). 13C NMR (100 MHz,
CDCl3): δ 153.9, 135.8, 123.9. GC-MS calculated for [C6N2S3Br2]
353.75; found 353.70.
2,6-Dibromoselenopheno[2,3-d:5,4-d′]bis(thiazole) (9). The
same procedure as 8 was followed to synthesize compound 9. The
crude product was recrystallized from a mixture of chloroform and
methanol to afford a yellow solid 9 (0.38 g, 52%). 13C NMR (100
MHz, CDCl3): δ 153.5, 135.2, 125.3. GC-MS calculated for
[C6N2S2SeBr2] 401.70; found 401.70.
General Synthetic Procedure of P(DPPBT-BTz-S), P(DPPTT-
BTz-S), P(DPPBT-BTz-Se), and P(DPPTT-BTz-Se) by Stille Cross-
Coupling Reaction. In a microwave vial, equimolar amounts of the
dibromo derivative 8 (0.16 mmol) and the appropriate DPP-based
1,1,1,3,3,3-Hexabutyldistannaselenane (2).34 To selenium pow-
der (2.07 g, 26.2 mmol), sodium metal pieces (1.21 g, 52.4 mmol)
and naphthalene (0.6 g, 4.7 mmol) were added under an argon
atmosphere, and the reaction mixture was degassed for 10 min. Dry
THF (250 mL) was added, and the reaction mixture was heated to
G
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monomer 10 (0.16 mmol) were dissolved in anhydrous chlor-
obenzene (2 mL) followed by addition of tetrakis(triphenyl-
phosphine)palladium(0) (Pd(PPh3)4) (2 mol %, 1.85 mg) and
tri(o-tolyl)phosphine (4 mol %, 1.95 mg). The resultant mixture was
degassed for 15 min under argon and securely sealed. The
polymerization was performed at 180 °C for 30 min in a microwave
reactor. The reaction mixture was precipitated into a mixture of
methanol (100 mL) and concentrated hydrochloric acid (2 mL) and
stirred for 1 h at room temperature. The polymeric precipitate was
then filtered into a cellulose thimble and was purified by Soxhlet
extraction with methanol (24 h), acetone (24 h), and hexane (24 h)
sequentially. The remaining polymer was dissolved in chloroform and
precipitated into methanol, filtered, and dried under vacuum to afford
the desired blue polymer.
P(DPPBT-BTz-S), P(DPPTT-BTz-S), P(DPPBT-BTz-Se), and
P(DPPTT-BTz-Se) were synthesized according to the general
procedure using the respective monomers. The data for each polymer
are as follows:
P(DPPBT-BTz-S) (97 mg, chloroform fraction, 51%). GPC: Mw:
36.5 kDa; Đ: 5.4. UV−vis: λmax (dilute chloroform solution) 425, 654
nm; λmax (film): 465, 656 nm.
P(DPPTT-BTz-S) (109 mg, chloroform fraction, 58%). GPC: Mw:
31.8 kDa; Đ: 4.1. UV−vis: λmax (dilute chloroform solution) 426, 621
nm; λmax (film): 430, 628 nm.
P(DPPBT-BTz-Se) (99 mg, chloroform, fraction, 48%). GPC: Mw:
28.2 kDa; Đ: 4.2. UV−vis: λmax (dilute chloroform solution) 441, 683
nm; λmax (film): 474, 686 nm.
P(DPPTT-BTz-Se) (89 mg, chloroform, fraction, 45%). GPC: Mw:
41.4 kDa; Đ: 3.9. UV−vis: λmax (dilute chloroform solution) 413, 630
nm; λmax (film): 435, 639 nm.
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ASSOCIATED CONTENT
* Supporting Information
(13) Paek, S.; Lee, J.; Lim, H. S.; Lim, J.; Lee, J. Y.; Lee, C. The
influence of electron-deficient comonomer on chain alignment and
OTFT characteristics of polythiophenes. Synth. Met. 2010, 160,
2273−2280.
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Janssen, R. A. J. Polymer Solar Cells with Diketopyrrolopyrrole
Conjugated Polymers as the Electron Donor and Electron Acceptor.
Adv. Mater. 2014, 26, 3304.
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.8b00826.
Experimental details, characterizations, and NMR
spectra (PDF)
(15) Fu, B. Y.; Wang, C. Y.; Rose, B. D.; Jiang, Y. D.; Chang, M.;
Chu, P. H.; Yuan, Z. B.; Fuentes-Hernandez, C.; Kippelen, B.; Bredas,
J. L.; Collard, D. M.; Reichmanis, E. Molecular Engineering of
Nonhalogenated Solution-Processable Bithiazole-Based Electron-
Transport Polymeric Semiconductors. Chem. Mater. 2015, 27,
2928−2937.
(16) Al-Hashimi, M.; Labram, J. G.; Watkins, S.; Motevalli, M.;
Anthopoulos, T. D.; Heeney, M. Synthesis and Characterization of
Fused Pyrrolo[3,2-d:4,5-d ’]bisthiazole-Containing Polymers. Org.
Lett. 2010, 12, 5478−5481.
(17) Shi, Q. Q.; Fan, H. J.; Liu, Y.; Chen, J. M.; Ma, L. C.; Hu, W.
P.; Shuai, Z. G.; Li, Y. F.; Zhan, X. W. Side Chain Engineering of
Copolymers Based on Bithiazole and Benzodithiophene for Enhanced
Photovoltaic Performance. Macromolecules 2011, 44, 4230−4240.
(18) Guo, B.; Li, W. B.; Guo, X.; Meng, X. Y.; Ma, W.; Zhang, M. J.;
Li, Y. F. A novel wide bandgap conjugated polymer (2.0 eV) based on
bithiazole for high efficiency polymer solar cells. Nano Energy 2017,
34, 556−561.
(19) Lee, J.; Chung, J. W.; Jang, J.; Kim, D. H.; Park, J. I.; Lee, E.;
Lee, B. L.; Kim, J. Y.; Jung, J. Y.; Park, J. S.; Koo, B.; Jin, Y. W.; Kim,
D. H. Influence of Alkyl Side Chain on the Crystallinity and Trap
Density of States in Thiophene and Thiazole Semiconducting
Copolymer Based Inkjet-Printed Field-Effect Transistors. Chem.
Mater. 2013, 25, 1927−1934.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mohammed.al-hashimi@qatar.tamu.edu (M.A.-H.).
ORCID
Dhananjaya Patra: 0000-0002-2471-5057
Jongbok Lee: 0000-0002-0086-0938
Lei Fang: 0000-0003-4757-5664
Martin Heeney: 0000-0001-6879-5020
Mohammed Al-Hashimi: 0000-0001-6015-2178
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge support for this work from
the Qatar National Research Fund project NPRP 8-245-1-059.
■
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