Dialkylthienosilole and N-alkyldithienopyrrole-based copolymers: Synthesis, characterization, and photophysical study

Article abstract of DOI:10.1002/poc.4063

We synthesized and characterized a set of D-π-A conjugated copolymers containing thiophene π-bridge. While benzothiadiazole serves as an acceptor (A) unit, the 4,4-dialkyldithieno[3,2-b:2′,3′-d]silole (DTSi) or N-alkyldithieno[3,2-b:2′,3′-d]pyrrole (DTP) act as a donor (D) unit. The copolymers were synthesized via the commonly Stille cross-coupling reaction and exhibited molecular weights of 18.6 to 31.3 kg/mol. The main structural differences among the copolymers are the type of donor moiety (DTSi or DTP) and the position of hexyl side chains on the thiophene π-bridge units between the D and A moieties. The ultimate goal of this work is to explore the effect of three structural factors that could control the photophysical properties of polymers in order to help in the rational design of polymers having specific properties used in optoelectronic devices. The physical properties include thermal stability, photophysical, and electrochemical properties. The structural factors are (a) the power of donor moiety, (b) the position of alkyl side chain on the thiophene π-bridge, and (c) the nature of the alkyl side chain. Also, we utilized the density functional theory calculations to calculate the geometric and electronic structures. A good agreement was remarked between the experimental and theoretical findings.

Full text of DOI:10.1002/poc.4063

Received: 24 October 2019
DOI: 10.1002/poc.4063
Revised: 18 January 2020
Accepted: 28 January 2020
R E S E A R C H A R T I C L E
Dialkylthienosilole and N-alkyldithienopyrrole-based
copolymers: Synthesis, characterization, and photophysical
study
1
2
3
Ashraf A. El-Shehawy
| Nabiha I. Abdo | Morad M. El-Hendawy
|
1
4
Abdul-Rahman I.A. Abdallah | Jae-Suk Lee
1
Department of Chemistry, Faculty of
Abstract
Science, Kafrelsheikh University, Egypt
2
We synthesized and characterized a set of D-π-A conjugated copolymers con-
taining thiophene π-bridge. While benzothiadiazole serves as an acceptor (A)
unit, the 4,4-dialkyldithieno[3,2-b:2 ,3 -d]silole (DTSi) or N-alkyldithieno[3,2--
Higher Institute of Engineering and
Technology, New Borg El-Arab City,
Egypt
0
0
3
Department of Chemistry, Faculty of
0
0
b:2 ,3 -d]pyrrole (DTP) act as a donor (D) unit. The copolymers were synthe-
sized via the commonly Stille cross-coupling reaction and exhibited molecular
weights of 18.6 to 31.3 kg/mol. The main structural differences among the
copolymers are the type of donor moiety (DTSi or DTP) and the position of
hexyl side chains on the thiophene π-bridge units between the D and
A moieties. The ultimate goal of this work is to explore the effect of three
structural factors that could control the photophysical properties of polymers
in order to help in the rational design of polymers having specific properties
used in optoelectronic devices. The physical properties include thermal stabil-
ity, photophysical, and electrochemical properties. The structural factors are
(a) the power of donor moiety, (b) the position of alkyl side chain on the thio-
phene π-bridge, and (c) the nature of the alkyl side chain. Also, we utilized the
density functional theory calculations to calculate the geometric and electronic
structures. A good agreement was remarked between the experimental and
theoretical findings.
Science, New Valley University, Egypt
4
School of Material Science and
Engineering, the Grubbs Center for
Polymers and Catalysis and Research
Institute for Solar and Sustainable Energy
(
RISE), Gwangju Institute of Science and
Technology (GIST), Gwangju, Republic of
Korea
Correspondence
A. A. El-Shehawy, Department of
Chemistry, Faculty of Science,
Kafrelsheikh University, Kafrelsheikh
3
3516, Egypt.
Email: elshehawy65@yahoo.com;
elshehawy@sci.kfs.edu.eg
J.-S. Lee, School of Material Science and
Engineering, the Grubbs Center for
Polymers and Catalysis and Research
Institute for Solar and Sustainable Energy
K E Y W O R D S
(
RISE), Gwangju Institute of Science and
D-π-A, DFT calculations, donor-acceptor polymers, DTSi and DTP, Stille cross-coupling
Technology (GIST), Gwangju 61005,
Republic of Korea.
Email: jslee@gist.ac.kr
Funding information
Kafrelsheikh University Research
Program; GIST Research Institute (GRI),
Grant/Award Number: GRI-2019; Science
and Technology Development Fund
(
STDF), Grant/Award Number: 7973
J Phys Org Chem. 2020;e4063.
wileyonlinelibrary.com/journal/poc
© 2020 John Wiley & Sons, Ltd.
1 of 17
https://doi.org/10.1002/poc.4063

2
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EL-SHEHAWY ET AL.
1
| INTRODUCTION
DTSi moieties via the commonly used palladium-
catalyzed Stille cross-coupling method. Besides, the posi-
tion of alkyl side chains at π-bridge is another essential
The design and realization of new π-conjugated organic
semiconductors have been attracted significant attention
from several research groups in the past decade.
structural factor that has recently been employed to con-
[
1]
[22]
trol the planarity of the D-π-A copolymers.
For exam-
Polycondensed benzenoids, polyacetylenes, and (oligo)
polyheteroaromatics represent prominent examples of
ple, the 4-position anchoring of alkyl chains on the
flanking thiophene units in di-2-thienyl-2,-
1,3-benzothiadiazole has negligible impact on the steric
[1,2]
such materials.
Among these, the silole-based poly-
[23]
mers outperformed owing to their high fluorescent effi-
hindrance relative to that of 3-position anchoring.
[3–5]
ciency, photostability, and chemical stability.
One
Inspired by these findings, in this work, the effect of mov-
ing the hexyl chains on thiophene units from the
3-position to the 4-position on the photophysical and
electrochemical properties of the copolymers was investi-
gated. Due to the similar structure between the examined
polymers and our previously studied polymer
privilege of silole-based polymer over the carbon-fused
analog is the enhancement of solid-state ordering as a
result of their stronger π-stacking interaction. This could
improve the charge transport, which is favorable to
[6]
obtain a higher JSC value.
20a, c, d
Rasmussen and co-workers pioneered the use of N-
PBTHT,
this respect.
a comparative study was performed in
0
alkyldithieno[3,2-b:2 ,3d]pyrrole (DTP) as a promising
[
5,7]
fused aromatic building block for electronic materials.
DTP has an utterly flat crystal structure, indicating good
π-conjugation across the fused rings. Upon polymeriza-
tion, poly(N-alkyldithieno[3,2-b:2 ,3 -d]pyrrole) (PDTPs)
2 | RESULTS AND DISCUSSION
0
0
exhibited excellent stability in their oxidized state, low
2.1 | Synthesis of comonomers and
polymers
[
8]
bandgaps, and efficient red fluorescence in solution.
The nitrogen atom in the DTP molecule enhances the
[
9]
electron-donating nature in the resulting polymer.
Figure 1 shows the chemical structure of four precursory
0
0
Meanwhile, inserting π-bridges to form a D-π-A structure
with extended conjugation is a widely spread method to
modulate the optoelectronic properties of the poly-
comonomers, 4,7-bis(5-bromo-3,3 /4,4 -hexylthiophene-
2-yl)benzo[c][2,1,3]thiadiazoles (HT-BT-HT; 1 and 3),
and their corresponding tributylstannyl derivatives
(2 and 4), as building blocks to construct the title copoly-
mers. These comonomers were synthesized via our
[10]
mers.
It has been demonstrated that light absorption,
energy levels, charge transport, and photovoltaic perfor-
mances can be efficiently adjusted by introducing differ-
20a
recently reported methods.
[11]
ent π-bridges.
The D-π-A copolymers incorporating
Scheme 1 shows the synthetic pathways for the prepa-
ration of another set of donors, synthesized by some
modifications of reporting procedures, which also used as
precursors for the synthesis of the target polymers.
DTP moieties are described as low bandgap donor mate-
rials, which were realized in organic photovoltaic and
[12–16]
organic field-effect transistor applications.
0
0
0
Benzothiadiazole-based copolymers with different kinds
of electron-rich and/or electron-deficient units showed
narrow bandgaps and exhibited high Power Conversion
Lithiating of 3,3 ,5,5 -tetrabromo-2,2 -bithiophene (6),
0
prepared by brominating 2,2 -bithiophene (5) with
bromine, with n-butyllithium (n-BuLi) selectively at the
3,3 -positions at 90 C followed by treating with chlo-
15
[17–19]
0
ꢀ
Efficiencies (PCEs).
We have recently reported the
0
synthesis of a series of alternating π-conjugated copolymers
based on 2,1,3-benzothiadiazole and different numbers of
hexylthiophene units via Pd-mediated Stille cross-coupling
rotrimethylsilane at the same temperature afforded 3,3 -
0
0
dibromo-5,5 -bis (trimethylsilyl)-2,2 -bithiophene (7) in
ꢀ
73% yield. On lithiating 7 with n-BuLi at 80 C followed
[20]
and C–H arylation methodologies.
The introduction of
by the reaction with dichlorodi-hexyl or ethylhexylsilane,
the corresponding 4,4-di-hexyl or ethylhexyl-5,5 -bis
0
the benzothiadiazole acceptor unit into polymer chains
showed a pronounced impact on each of the photophysical
and electrochemical properties.
In continuation to our interest towards the synthesis
of π-conjugated polymeric materials and small molecules
[20,21]
for optoelectronic applications,
we introduced here
a simple synthetic design to prepare a set of π-conjugated
0
0
copolymers based on comonomers 4,7-bis(3,3 /4,4 -
hexylthiophene-2-yl)benzo[c][2,1,3]thiadiazole (HT-BT-
HT) moieties along with other donors such as DTP and
FIGURE 1 Structures of HT-BT-HT comonomers

EL-SHEHAWY ET AL.
3 of 17
SCHEME 1 Synthetic pathways for the synthesis of DTSi and DTP derivatives
0
0
ꢀ
ꢀ
(
trimethylsilyl)dithieno[3,2-b:2 ,3 -d]silole (8a,b) was
BINAP/Toluene 110 C, RNH ; and (g) n-Bu-Li/−80 C,(-
2
ꢀ
obtained in 55% and 60% yields, respectively. 4,4-Di-hexyl
Bu) SnCl/−80 C to room temperature.
3
0
0
0
and/or
silole (9a,b) was finally obtained in high yields (94% and
1%, respectively) by brominating 8a,b with NBS in THF
ethylhexyl-5,5 -dibromo-dithieno[3,2-b:2 ,3 -d]
The HT-BT-HT comonomers are used as building
blocks for synthesizing a variety of low bandgap copol-
ymers (P1-P8) via the commonly used Pd-catalyzed
Still cross-coupling method as shown in (Scheme 2).
The polymerization reaction was catalyzed by Pd
(PPh3)4 under microwave reaction conditions in N,N-
dimethylformamide (DMF). Stille cross-coupling of
equimolar amounts of comonomer 2 with dibromo-
9
at room temperature.
Reagents and conditions: (a) Br , glac. AcOH/CHCl ,
Zn-dust/EtOH/glac.
c) (CH ) SiCl/THF, −90 C; (d) (R) SiCl /THF, -80 C;
2
3
reflux;
(
(
(b)
AcOH,
reflux;
ꢀ
ꢀ
3
3
2
2
ꢀ
e) NBS/THF, 0 C to room temp.; (f) t-NaOBu, Pd dba ,
2
3
SCHEME 2 General synthetic pathways for the synthesis of π-conjugated copolymers P1-P8

4
of 17
EL-SHEHAWY ET AL.
hexyl or ethylhexyl of DTSi (9a,b) afforded the
corresponding copolymers P1 and P2 in 86% and 85%
yields, respectively. Under the same reaction condi-
tions, copolymerization of comonomers 1 and 2 with
each of 13 or 12b afforded the copolymers P3 and P4
in 81% and 79% yields, respectively. Similarly, copoly-
mers P5 and P6 were prepared by Pd-catalyzed Stille
cross-coupling of dibromo-hexyl or ethylhexyl of DTSi
All copolymers have higher molecular weight distri-
bution with a number-average molecular weight (M ) rel-
n
ative to PBTHT (Table 1). Except for P8, all DTSi-based
polymers had a higher tendency to aggregate to form a
bigger size. The observed relatively lower M values of
n
copolymers P5-P8 compared with P1-P4 are probably
due to the relatively steric hindrance effect resulting from
0
the hexyl chains at the β,β -positions in the thiophene
(
9a,b) with comonomer 4. Finally, copolymerization of
rings with the alkyl chains in the DTSi or DTP units.
However, there should be a much less steric hindrance
effect in the Stille coupling reaction in the case of using
comonomers 1 or 2.
comonomers 3 and 4 with each of 13 or 12b afforded
the copolymers P7 and P8 in 88% and 83% yields,
respectively. For comparison, copolymer PBTHT was
prepared by our previously reported method via Stille
cross-coupling polymerization.
of discussion, sometimes, the 3,3 - and 4,4 -positions of
20a
For the convenience
0
0
2.2 | Thermal properties
hexyl groups on the thiophene π-bridge units in the
0
0
synthesized copolymers are named as α,α - and β,β -
positions, respectively (see Scheme 2).
The thermal stability of the copolymers was explored by
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC), under nitrogen atmosphere.
TGA of copolymers reveals that the residual weights of
copolymers P1, P3, P4, P7, and P8 are greater than 50%
After precipitation in methanol, the crude copolymers
were filtered off and washed extensively with methanol,
followed by Soxhlet extraction with methanol and ace-
tone successively to remove byproducts and oligomers.
Analysis of the polymers showed that the palladium cata-
lyst was wholly removed from the polymers. The chemi-
cal structures of the resulting copolymers were confirmed
ꢀ
when the temperature rises to 800 C (Figure 2). The poly-
mers exhibited good thermal stability with onset
decomposition temperatures (T , 95 wt% residues)
d
corresponding to a 5% weight loss in the range of 306.62
1
ꢀ
by H NMR spectroscopy. The findings showed that all
to 405.06 C (Table 1), indicative of high thermal stabili-
data are entirely consistent with the proposed structures.
On the other hand, all copolymers showed excellent solu-
bility at room temperature in common organic solvents,
such as chloroform, chlorobenzene, and dichlorobenzene
ties, which could be assigned to side-chain decomposition
[
24]
upon heating processes.
The three polymers P3, P4,
and P6 have the best thermal stability with onset decom-
position temperatures (T ) corresponding to a 5% weight
d
−
1
ꢀ
(>5 mg/mL ). The excellent solubility could be attrib-
loss at 400.68, 392.91, and 405.06 C, respectively.
uted to the presence of solubilizing side chains on thio-
phene moieties.
The glass transition temperature (T ) of copolymers
P1-P8 along with PBTHT data are also summarized in
g
a
TABLE 1 Characterization results of copolymers PBTHT and P1-P8
b
b
n
c
ꢀ
d
ꢀ
Tg, C)
e
Polymer
M
n
,kg/mol
PDI, M
2.10
2.29
1.64
1.72
1.75
1.37
1.61
1.68
1.55
w
/M
Yield %
87
T
d, C)
f
PBTHT
18.63
26.89
22.63
31.31
27.89
21.49
19.82
24.18
20.19
365.80
375.25
306.62
400.68
392.91
362.16
405.06
355.05
336.86
72.40
53.54
51.98
121.94
87.73
87.00
80.02
165.38
128.85
P1
P2
P3
P4
P5
P6
P7
P8
86
85
81
79
82
78
88
83
Abbreviation: TGA: thermogravimetric analysis.
a
All polymerizations were carried out using the Stille cross-coupling method in dry DMF in the presence of Pd (PPh
3
)
4
under microwave conditions.
b
3
Determined by gel permeation chromatography with polystyrene as standard and CHCl , as eluent.
c
Based on the weight of the polymer obtained after Soxhlet extraction and drying under vacuum.
d
ꢀ
Onset decomposition temperature (5% weight loss) measured by TGA under nitrogen atmosphere at a heating rate of 10 C/min.
e
ꢀ
Determined by DSC under the nitrogen atmosphere at a heating rate of 10 C/min.
f
20a
Data were taken from our previously published work.

EL-SHEHAWY ET AL.
5 of 17
FIGURE 2 Thermogravimetric analysis thermograms of copolymers P1-P8
Table 1, while their DSC curves are presented in
intermolecular interactions and the increasing interchain
regularity caused by introducing DTP to copolymer main
chains. The high thermal stability of the copolymers
could prevent the deformation of their morphology and
the degradation of their polymeric active layer under
applied electric fields. Thus, the synthesized polymers
might be considered as good candidates for device appli-
cations, especially for light-emitting diodes, organic thin-
film transistors, and solar cell applications.
Figure 3. The copolymer samples were heated up to
ꢀ
3
00 C, and the DSC data were obtained from the second
heating cycle. DSC analysis revealed that PBTHT, P1,
P2, P4, P5, and P6 are amorphous materials with a glass
transition temperature at 72.40, 53.48, 51.98, 87.73, 87.00,
ꢀ
and 80.02 C, respectively. The amorphous nature of the
0
P1, P2, P5, and P6 can be understood from the 4,4 -
dihexyl groups of the DTSi moieties protruding out of
the polymer backbone planes. The otherwise stated poly-
mers exhibited T values in the range of 121.94 to
g
ꢀ
1
65.38 C. Interestingly, PBTHT showed the lowest Tg
2.3 | Optical properties
value along all of the other polymers except for P1 and
P2. More interestingly, the Tg of the copolymers
increases with incorporating the DTP moiety in the
copolymer backbones than that of polymer containing
DTSi moiety (Table 1). This owes to the extra
s
The optical absorption spectra of copolymers P1-P8 in
chlorobenzene solutions and as thin films cast on glass
slides were extensively investigated (Figures 4 and 5).
The corresponding optoelectronic properties are summa-
op
g
rized in Table 2. The optical bandgaps (E ) of the copoly-
mers were deduced from their absorption onsets. Each
polymer is characterized by two major absorption peaks,
a feature that is commonly observed for alternating D-A
copolymers. The low-wavelength peak can be attributed
*
to a π-π transition, while the high-wavelength transition
is believed to be related to an intramolecular D-A charge
transfer.
Upon inspection of Figure 4, one can explore the
effect of three structural factors on the spectral features
of the examined polymers as the following:
1
. The power of donor moiety: The structure of PBTHT is
very similar to those of P5 and P7. They differ only in
the structure of the donor moiety. The replacement
one of benzothiadiazole moieties of PBTHT by DTP
or DTSi species has a dramatic impact on geometry,
spectral profile, and spectroscopic parameters (such as
op
FIGURE 3 Differential scanning calorimetry curves of
copolymers P1-P8
in λmax, λonset and Eg ), as shown in Figure 4A
and Table 2. The power of donor moieties,

6
of 17
EL-SHEHAWY ET AL.
FIGURE 4 Ultraviolet-visible spectra of the studied polymers P1-P8 in chlorobenzene
FIGURE 5 Ultraviolet-visible spectra of the studied copolymer thin films P1-P8 in comparison with PBTHT
benzothiadiazole, DTSi, and DTP was computed by
B3LYP/6-311+G(d,p) as their ionization energy, 9.67,
our density functional theory (DFT) results. The syn-
ergistic effect of both factors can control the ICT lead-
8.49, and 7.98 eV, respectively. Accordingly, the lower
ing to the findings. Also, the vast shift in the λ
ie,
onset,
op
the ionization potential moiety, the easier to remove
an electron from it and can be described as a stronger
donor group. This could explain the significant bat-
hochromic shift in λmax (~50 nm) upon the structural
replacement by benzothiadiazole moiety in PBTHT
copolymer by DTSi (P5) or DTP (P7). Strictly speak-
ing, this result is not attributed only to the increase of
donating ability but also because of increasing the
twisting angle φ1 (angle between the donor moiety
reached 109 nm, as well as the reduction in Eg was
observed. This shed light on the proper choice of
donor moiety to enhance the light-harvesting effi-
ciency. As an extension to the power of donor moiety,
the effect of replacement of silicon atom in the
tricyclic-ring by nitrogen one was studied. In the
inspection of Table 2 and Figure 4B, we can divide the
studied copolymers into two subsets, P1-P4 and
P5-P8, based on the position of the hexyl chain. Com-
pared with the P5-P8 subset, a relatively significant
ꢀ
and thiophene ring) from ~2 to ~ −25 as shown by

EL-SHEHAWY ET AL.
7 of 17
[a]
TABLE 2 Optical and electrochemical properties of polymers PBTHT and P1-P8
UV-vis absorption
[c]
Solution
Film
Cyclic Voltammetry
op
g
[b]
op
g
[b]
ec
Polymer
λ
max, nm
λ
onset, nm
E
, eV
λ
max, nm
λ
onset, nm
E
, eV
HOMO, eV
−5.66
LUMO, eV
−3.37
E , eV
g
[d]
PBTHT
P1
513
519
497
545
538
564
550
561
555
593
616
600
645
643
685
673
702
686
2.09
2.01
2.06
1.92
1.92
1.81
1.84
1.77
1.80
532
541
508
569
578
614
564
578
589
644
654
681
794
720
779
720
802
804
1.92
1.82
1.89
1.56
1.72
1.59
1.72
1.55
1.54
2.29
1.88
2.12
1.79
1.64
1.61
1.83
1.62
1.67
−5.36
−3.48
P2
−5.46
−3.34
P3
−5.16
−3.37
P4
−4.98
−3.34
P5
−4.97
−3.36
P6
−5.36
−3.53
P7
−4.91
−3.29
P8
−5.02
−3.35
Abbreviations: CV: cyclic voltammetry; UV-vis: ultraviolet-visible.
a
All data are for those polymers prepared by microwave polymerization reaction conditions.
b
Optical bandgap (E og p) was calculated from the intersection of the tangent on the low energetic edge of the absorption spectrum with the baseline.
c
The onset potentials are obtained from the intersection of the two tangents down at the rising current and the baseline changing current of the CV curves.
d
20a
Data were taken from our previously published work.
change in the spectral properties mainly λmax (26 and
1 nm) and λonset (29 and 43 nm) was observed in the
the ultraviolet-visible (UV-Vis) absorption spectra of
similar polymers.
[
22]
4
P1-P4 subset upon the replacement of DTP by DTSi
moiety. On the other hand, a limited effect was
observed in the P5-P8 subset on λmax (~3 and 5 nm)
and λonset (~17 and 13 nm) because of the
antiinterference effect of another structural factor.
Further details will be discussed in the next section.
We can conclude some points: (a) whatever the donor
moiety is, the movement of the position of hexyl side
0
0
chain from α,α - to the β,β -positions lead to a bat-
hochromic shift in λ and λonset as well as a reduction
max
op
in Eg . This is attributed to the stronger coupling of
electron-donating hexyl groups in β,β -positions together
0
with the donor moiety, which enhances the ICT; (b) the
stronger donating ability of DTP relative to DTSi led to
stronger ICT with the acceptor and thiophene bridge car-
2
. The positioning effect of hexyl side chains on thiophene
rings: When changing the hexyl side chains in the π-
0
0
0
bridge (thiophene unit) from the α,α - to the β,β -posi-
tions, the low-energy peak is significantly red-shifted,
while the high-energy peak remains almost
rying hexyl chain in β,β -positions. This highlights the
considerate selection of the position of the alkyl chain on
π-bridge because their proper choice is essential to
enhance the efficiency of light-harvesting.
0
unchanged. These results suggest that the β,β -
positions induce
a more pronounced coupling
between the D and A units, enhancing charge transfer
3. The nature of alkyl chains on the DTSi or DTP moie-
ties: As shown in Figure 4D, the replacement of hexyl
by ethylhexyl chain in DTP donor moiety, for exam-
ple, leaves almost constant absorption spectra for
polymers P3 and P4. However, a slight blue shift from
6 to 23 nm in the high-wavelength peak was observed
as a result of this structural change between each pair
of the following polymers P1 and P2, P3 and P4, P5
and P6, and P7 and P8. This shift may indicate a
larger twist between the neighboring rings induced by
the more bulky ethylhexyl side chains.
op
and reducing the E . This effect works more signifi-
g
cantly in the DTSi-based polymers, ie, P1, P2, P5, and
P6, compared with the DTP-based polymers
(Figure 4C). For instance, a significant bathochromic
shift in λmax, 45, and 53 nm, was observed ongoing
from P1 to P5 and from P2 to P6, respectively, as a
0
result of moving the alkyl side chains from α,α - to the
0
β,β -positions. On the other hand, a limited shift,
1
6 and 17 nm, was remarked ongoing from P3 to P7
and from P4 to P8, respectively. A considerable shift
op
in λonset (range: 43-73 nm) and reduction in Eg
(
range: 0.12-0.22 eV) were noticed as a result of this
Similarly, the absorption spectra of polymer thin films
are mostly affected by the alkyl side-chains positioning
structural change. A similar attitude was observed in

8
of 17
EL-SHEHAWY ET AL.
rather than by the chain chemical structure, as illustrated
in Figures 5A and 5B and Table 2. However, a significant
red-shift ongoing from the solutions to the solid films
was remarked. The spectral shift is generally attributed to
the intermolecular π-π stacking interactions between con-
jugated main chains in the solid-state. Changing alkyl
−4.91 to −5.66/−3.25 to −3.53 eV, respectively. Interest-
ingly, the ELUMO values of all polymers are higher than
that of PC BM (≈ –3.90 eV), which may guarantee the
61
efficient photoinduced charge separation between the
[
28]
polymer donor and PC BM.
61
As shown in Table 2, the estimated electrochemical
0
0
ec
g
side chains from the α,α - to the β,β -position leads, as
expected, to a significant reduction (~0.20 eV) of the poly-
mer energy bandgap. The resulting 1.54- and 1.55-eV
optical bandgap of both P7 and P8 is very close to the
bandgaps (E ) for all polymers are lower than that of
PBTHT. Incorporating different donor units in the poly-
mer chains has a direct impact on the EHOMO and ELUMO
of the resulting copolymers. Increasing the electron-
donating moieties in the polymer backbones lowers the
[
25]
optimal value for single-junction photovoltaic devices
[
29]
and makes these polymers highly promising for photovol-
taic applications. Interestingly, in the case of polymer
thin films P3, P5, P7, and P8, the shoulder results in cut-
off wavelengths were found to be of 794, 779, 802, and
bandgap and oxidation potentials.
Because the EHOMO
of copolymers P2, P3, and P8 are below the air oxidation
[
30]
threshold (approximately −5.27 eV),
these polymers
may show good stabilities toward air and oxygen. This is
one of the prerequisites for the polymer used in elec-
8
1
04 nm, respectively, corresponding to a bandgap of 1.56,
59, 1.55, and 154 eV. These strong broad absorptions are
[
30]
tronic device applications.
The estimated electrochem-
ec
ideal for PV applications due to favorable overlap with
the AM1.5 solar spectrum. It is worth mentioning that
the absorption spectra of all copolymer films with hexyl
ical bandgaps (E ) of all of the polymers were found to
g
be somewhat higher than the corresponding optical
op
bandgaps (Eg ), which might be originated from the
0
groups at β,β -positions (P2, P4, P6, and P8) in the thio-
interface energy barriers present between the polymer
26c, 31
phene π-bridge units are red-shifted than those
films and the electrode surfaces.
0
corresponding polymers with hexyl side chains α,α -
positions (P1, P3, P5, and P7) (see Figures 5A and 5B),
indicating a more planar structures due to reduced steric
hindrance in those polymers. More interestingly, copoly-
mers P3, P5, P7, and P8 have optical bandgaps of 1.56,
1
.59, 1.55, and 1.54 eV, respectively, which are close to
the ideal value of 1.5 eV for PSC application. This red-
shift might be attributed to the great co-planar conforma-
tion of adjacent thiophene rings induced by the hexyl
0
side chains at the β,β -positions on the thiophene rings in
the polymer backbone that increase the effective conjuga-
tion length. Similar behaviors were observed in the UV-
[22]
Vis absorption spectra of similar polymers.
2.4 | Electrochemical properties
The electrochemical properties of the synthesized copoly-
mers are measured by cyclic voltammetry (CV) in
CH CN solutions by using ferrocene as reference. The
3
CV curves of copolymers P1-P8 are presented in
Figure 6, and the corresponding data are listed in
Table 2. The highest occupied molecular orbital and the
lowest unoccupied molecular orbital energy levels
(
EHOMO and ELUMO, respectively) of the copolymers were
determined from their cast films on ITO glass substrates.
They were calculated according to the empirical formu-
17b, 26, 27
las
:EHOMO = −(E +4.40) eV and E
=
LUMO
ox
−
(E +4.40) eV, where E and E are the onset oxidation
re
ox
re
and reduction potentials of the polymers, respectively,
vs. SCE. ELUMO values of polymers lie in the range of
FIGURE 6 Cyclic voltammograms of copolymers P1-P8

EL-SHEHAWY ET AL.
9 of 17
FIGURE 7 Optimized structures of the studied comonomer
2
.5 | Theoretical study
adjacent subunits of the comonomers are listed in
Table 3.
[32]
DFT at wB97XD/6-31+G(d,p)
level using Gaussian
The findings showed that PBTHT is coplanar, where
[33]
ꢀ
0
9
was used to investigate the optimal geometric struc-
the torsion angles range from −0.04 to 1.92 . Upon
ture and the electronic configuration of PBTHT, P1, P3,
P5, and P7. Copolymers P2, P4, P6, and P8 gave very
similar results to those of P1, P3, P5, and P7, respec-
tively, because of the similar effect between ethylhexyl
and hexyl groups of DTSi or DTP units in polymer
chains, and therefore, the former polymers were not
modeled. The alkyl side chains were replaced by methyl
groups to reduce the computational load. It was reported
that this replacement does not affect the frontier orbitals
replacement of benzothiadiazole moiety of PBTHT by
DTP (P1) or DTSi species (P3), only a remarked increase
ꢀ
of φ by 25 and 30 , respectively, was observed. To verify
1
the validity of comonomers modeling as the correct
[34]
but affects the torsion angles to some extent.
The geo-
metrically optimized structures are shown in Figure 7,
and the torsion angles (φ φ , and φ ) between every two
1,
2
3
TABLE 3 Torsion angles in degrees along the conjugated
backbone of the optimized molecular geometries
Polymer
PBTHT
P1
φ
1
φ
2
φ
3
1.92
0.10
0.23
0.38
−
−23.49
−28.47
−65.31
−58.90
−
P3
−
P5
−9.60
−5.44
−14.04
−9.84
FIGURE 8 Experimental and simulated ultraviolet-visible
spectra of copolymer P7
P7

10 of 17
EL-SHEHAWY ET AL.
FIGURE 9 The contour plots of HOMO and LUMO orbitals of the studied compounds
representative for polymers, we simulated the UV-Vis
spectra using time-dependent density functional theory
The replacement of DTSi moiety by a stronger donor,
DTP as in P3 and P7, has a definite impact on the FMO
topology, where the HOMO is predominantly located on
the donor moiety, while the LUMO is primarily located
mainly on the acceptor moiety. As a result, polymer P3
and P7 had a clear charge separation when compared
with the other polymers. This is also supported by the
[32]
(TD-DFT) at wB97XD/6-31+G(d,p) level.
The experimental and simulated UV-Vis spectra of
P7 as representative examples are shown in Figure 8.
There is a good match between the positions of the
three peaks of both spectra, where the experimental
peaks located at 561, 425, and 342 nm, while the sim-
ulated ones located at very close values, 578, 403, and
3
24 nm. The area under a peak is not consistent with
the experimental one because of the difference in con-
ditions where our calculations are performed at zero
ꢀ
Kelvin against 298 for the experimental measure-
ments. Also, we used implicit solvation for simulation
compared with the explicit solvent effect in the real
condition.
The frontier molecular orbitals (FMO) of the tested
comonomers were shown in Figure 9. Three kinds of
observations were found as a result of the structural
changes. The HOMO and LUMO of PBTHT are distrib-
uted over the whole molecular selection because of the
absence of D-A character. On the other hand, the elec-
tron density of the HOMO electron is delocalized over
the molecular backbone of P1 and P5, while the LUMO
is primarily located on the acceptor moiety.
FIGURE 10 Experimental (cyclic voltammetry) and
calculated HOMO and LUMO energy levels

EL-SHEHAWY ET AL.
11 of 17
TABLE 4 Excited-state vertical transition energies (Cal λmax, nm), oscillator strengths (f, arbitrary units), and main electronic
configuration of the first excited state and their percentage contribution (C%) of the polymers, as calculated for the monomers using TD-DFT
at the B3LYP/6-31+G* level of theory
Polymer
PBTHT
Cal λmax, nm
f, au
1.06
0.13
0.26
1.07
0.56
0.28
0.93
0.86
0.38
0.94
0.17
0.77
0.94
0.60
0.31
Main configuration
H ! L
C%
Exp λmax, nm
522
0.96
0.90
0.86
0.94
0.79
0.81
0.94
0.86
0.60
0.92
0.64
0.77
0.92
0.68
0.84
513
---
4
37
33
H ! L+1
H ! L+2
H ! L
H ! L+1
H-1 ! L
H ! L
H ! L+1
H-3 ! L
H ! L
H-1 ! L
H ! L+1
H ! L
3
---
P1
P3
P5
P7
541
519
---
4
13
80
3
319
545
419
314
564
425
325
561
425
342
561
3
68
99
2
528
3
98
72
3
578
4
3
03
24
H-1! L
H ! L+1
Abbreviation: TD-DFT: time-dependent density functional theory.
clear intramolecular charge transfer seen in the absor-
bance spectra, as described earlier.
predominant component of the molecular orbitals
involved in the pertinent transitions, one can see that the
lowest singlet S !S excited state with high-oscillator
The predicted average HOMO and LUMO energy
levels are −6.07 and −3.58 eV giving an average bandgap
of 2.49 eV. This bandgap is higher to some extent than
the crossponding experimentally determined by optical
technique (1.99 eV), the CV (1.91 eV). The calculated
FMO energies are consistently underestimated compared
with the experimental values (Figure 10). This could pro-
duce overestimated bandgaps compared with those of the
experimental values. The systematic difference between
both results are originated from differences in conditions
between experiments and calculations with respect to,
e.g., finite temperature and explicit solvent effects, as well
as the shortcomings in standard hybrid DFT functionals.
TD-DFT calculations were performed to gain insights
into the excited state properties of the polymers. The cal-
culated excited-state vertical transition energies by nano-
meter, oscillator strengths, and transition electronic
configurations are given in Table 4.
0
1
strength (between 0.93 and 1.86) corresponds predomi-
nantly to HOMO!LUMO transition. Other single excita-
tions that contribute to the total excited state wave
function involve mainly HOMO!LUMO+1 and HOM-
O!LUMO+2 transitions.
2.6 | Conclusions
A series of low bandgap conjugated copolymers based on
0 0
comonomers
4,7-bis(3,3 /4,4 -hexylthiophene-2-yl)ben-
zo[c][2,1,3]thiadiazoles (HT-BT-HT) with two different
donor units were synthesized and fully characterized.
The nature and position of alkyl side-chain on the thio-
phene bridge were also deeply investigated. Due to the
similar structure between the examined polymers and
the well-studied polymer PBTHT, a comparative study
was performed in this respect. Based on the experimental
and theoretical findings, we have concluded the follow-
ing: (a) All copolymers showed good solubility in most
organic solvents. (b) Compared with PBTHT, all copoly-
mers exhibited high thermal stability. (c) The position of
alkyl-side chain on thiophene could significantly lead to
variation in the photophysical and electrochemical prop-
erties of the copolymers. (d) Interestingly, incorporating
DTP within the polymer backbone extends the
It can be seen that the calculations well reproduce
experimental absorption peaks. For all polymer systems,
the S !S transition is primarily a HOMO!LUMO tran-
0
1
sition (92-96%). All the copolymers show three optical
transitions, one in the range of 320 to 377 nm, arising
*
from a delocalized π!π transition in the main chain,
and the other two peaks occur in the range between
3
48 to 437 and 435 to 578 nm, originating from a charge
transfer state in the D–A segment. After examining the

12 of 17
EL-SHEHAWY ET AL.
absorption beyond 804 nm and creates an optical
bandgap that is much smaller than any of the other poly-
mers. (e) The findings shed light on the role of the posi-
tion of alkyl side chain on the thiophene bridge and the
power of donor moiety on the improvement of the light-
harvesting properties of the title copolymers as seen from
the remarked reduction of both optical and electrochemi-
cal bandgap and the significant bathochromic shifts in
A Pt wire and silver/silver chloride [Ag in 0.1 M KCl]
were used as the counter and reference electrodes,
respectively. The polymer films for electrochemical mea-
surements were spin-coated from
a
polymer-
chlorobenzene solution on ITO glass slides, approxi-
mately 10 mg/mL. The gel permeation chromatographic
analysis was carried with a Shimadzu (LC-20A Promi-
nence Series) instrument. Chloroform was used as a car-
ꢀ
λonset and λmax
.
rier solvent (flow rate: 1 mL/min, at 30 C), and
calibration curves were made with standard polystyrene
samples. Microwave-assisted polymerizations were per-
CEM
3
3
| EXPERIMENTAL SECTION
.1 | Reagents and materials
formed in a focused microwave synthesis system
(
Discover S-Class System). The low-temperature reactions
were essentially performed in a low-temperature bath
PSL1810-SHANGHA EYELA). A syringe pump KD Sci-
(
Unless otherwise noted, all manipulations and reactions
involving air-sensitive reagents were performed under a
dry oxygen-free nitrogen atmosphere. All reagents and
solvents were obtained from commercial sources and
dried using standard procedures. Thin-layer chromatog-
raphy (TLC) was used for monitoring all reactions for
entific (KDS-100) was used for delivering accurate and
precise amounts of reagents during the dropwise addition
processes.
3.3 | Synthesis of comonomers
0
0
0
completion. 3,3 -5,5 -Tetrabromo-2,2 -bithiophene (6),
0
0
0
0
3
,3 -dibromo-2,2 -bithiophene (10) were prepared
3.3.1 | Synthesis of 3,3 -dibromo-5,5 -bis
[35] 1
13
0
18c
according to the literature procedures.
H- and C-
(trimethylsilyl)-2,2 -bithiophene (7)
NMR spectra of the synthesized comonomers and poly-
mers are provided in the supporting information.
0
0
0
3,3 -5,5 -Tetrabromo-2,2 -bithiophene (6, 4.78 g, 10 mmol)
was dissolved in 150-mL dry THF, and the solution was
ꢀ
cooled down to −90 C by a methanol/liquid nitrogen
3
.2 | Instrumentation and methods
bath. Then, 8-mL n-butyllithium solution in hexane
(
2.5mol/L) was added drowsily throughout 1 hour after
1
13
H and C NMR spectra were measured on a Varian
the addition of n-butyllithium; the reactant was stirred
for 30 minutes. Subsequently, chlorotrimethylsilane (2.7
g, 25 mmol) was added in one portion, and the tempera-
ture of the reactant was raised to ambient temperature by
removal cooling bath. Then, the reactant was poured into
water and extracted by diethyl ether several times. The
volatiles were removed under vacuum. The residue was
purified by silica gel chromatography using hexane as
1
13
spectrometer (400 MHz for H and 100 MHz for C) in
CDCl at 25 C with TMS as the internal standard, and
chemical shifts were recorded in ppm units. The coupling
constants (J) are given in Hz. Flash column chromatogra-
phy was performed with Merck silica gel 60 (particle size
ꢀ
3
2
30-400 mesh ASTM). Neutralized silica gel was prepared
by adding triethylamine to the silica gel in the same elu-
ent used for column chromatography. Analytical TLC
was conducted with Merck 0.25-mm 60F silica gel pre-
coated aluminum plates and UV-254 fluorescent indica-
tors. The UV-Vis absorption spectra were obtained using
a Varian Cary UV-Vis-NIR-5000 spectrophotometer on
the pure polymer samples. The thermal degradation tem-
perature was measured using TGA (TGA-TA instrument
Q-50) under a nitrogen atmosphere. DSC was performed
on a TA instrument (DSC-TA instrument Q-20) under a
1
eluent to give a white solid of 7 (3.42 g, 73%). H NMR
(CDCl , 400 MHz, δ/ppm): 7.15 (s, 2H, 2X CH-CBr), 0.33
3
13
(s, 18H, 2X -Si (CH ) ). C NMR (CDCl , 100 MHz, δ/
3
3
3
ppm): 143.23, 137.35, 134.22, 113.25, 0. C H Br S Si
14
20 2 2
2
(468.42): calcd. C 35.90, H 4.30, Br 34.12, S 13.69, Si,
11.99; found C 36.12, H 4.39, Br 33.96, S 13.72.
0
3.3.2 | Synthesis of 4,4-dihexyl-5,5 -bis
ꢀ
0
0
nitrogen atmosphere at a heating rate of 10 C/min. CV
(trimethylsilyl)-dithieno[3,2-b:2 ,3 -d]silole
18c
measurements were performed on B-class solar simula-
tor: Potentiostate/Galvanostate (SP-150 OMA company).
The supporting electrolyte was tetrabutylammonium
hexafluorophosphate (TBAPF ) in acetonitrile (0.1 M) at
a scan rate of 50 mV/s . A three-electrode cell was used.
(8)
Compound 7 (5 mmol) and 40-mL THF were put into
ꢀ
a flask and cooled down to −80 C by a low-
6
−
1
temperature bath. Then, n-butyllithium solution in

EL-SHEHAWY ET AL.
13 of 17
hexane (2.5mol/L) was added dropwise in 5 minutes,
and the reactant was stirred for another 30 minutes at
that temperature. Subsequently, dichlorodihexylsilane
H 6.29, Br 27.72, S 11.12, Si 4.87; found C 49.83, H 6.31,
Br 27.54, S 11.18.
(
6 mmol) was added in one portion, and the cooling
bath was removed, and the reactant was stirred for
hours under ambient temperature. Then, the reactant
3.3.4 | Synthesis of N-alkyldithieno[3,2-
0
0
18f
2
b:2 ,3 -d]pyrrole (11a,b)
was poured into water and extracted by diethyl ether
several times. The volatiles were removed under vac-
uum. The residue was purified by silica gel chromatog-
A solution of 10 (3.09 mmol), t-NaOBu (7.4 mmol),
Pd dba (0.077 mmol) and BINAP (0.31 mmol) in dry tol-
2
3
raphy using hexane as eluent to give a colorless oil of
uene (10 mL) was purged with nitrogen for 20 minutes.
Alkylamine (3.09 mmol) was added via a syringe, and the
mixture was refluxed under nitrogen for 7 hours. After
cooling to room temperature, water was added to the
solution, and the reaction mixture was extracted by
diethyl ether. After the organic phases were dried over
Na SO , the solvents were removed using a rotary evapo-
rator. The crude product was purified by column chroma-
tography, and the desired products of 11a,b were
obtained in a 92% and 90% yield, respectively.
1
8
7
2
0
in a 55% yield. H NMR (CDCl , 400 MHz, δ/ppm):
.12 (s, 2H, 2X CH-Si-), 1.42-1.40 (t, J = 8.00 Hz, 4H,
3
X Si-CH -), 1.23-1.20 (m, 16H, 2X Si-CH -(CH ) ),
2
2
2 4
.9-0.85 (m, 6H, 2X CH ), 0.33 (s, 18H, 2X -Si (CH ) ).
3
3 3
1
3
C NMR (CDCl , 100 MHz, δ/ppm): 154.31, 143.72,
3
1
0
40.81, 136.46, 32.37, 31.28, 24.07, 22.46, 13.99, 11.7,
2
4
. C H S Si (507.03): calcd. C 61.59, H 9.14, S 12.65,
26
46 2
3
Si 16.62; found C 61.51, H 9.19, S12.55.
0
0
0
3
.3.3 | Synthesis of 4,4-dialkyl-5,5 -
N-Hexyldithieno[3,2-b:2 ,3 -d]pyrrole (11a)
0
0
1
dibromo-dithieno[3,2-b:2 ,3 -d]silole
H NMR (CDCl , 400 MHz, δ/ppm): 7.09-7.08 (d, J = 4.00
Hz, 2H, CH-C-N), 6.98-6.97 (d, J = 4.00 Hz, 2H, CH-C-S),
.16-4.12 (t, J = 16.00 Hz, 2H, N-CH -), 1.84-1.80 (t, J =
3
1
8c
(9a,b)
4
2
Compound 8a,b (2.56 mmol) was dissolved in 20-mL
THF, and N-bromosuccinamide (5.12 mmol) was added
in one portion at 0 C. The reactant was stirred at
16.00 Hz, 2H, N-CH -CH -), 1.27-1.24 (m, 6H, N-CH -
2
2
2
3
1
CH -(CH ) -), 0.86-0.84 (t, J = 8.00 Hz, 3H, -CH ).
C
2
2 3
3
ꢀ
NMR (CDCl , 100 MHz, δ/ppm): 144.87, 122.67, 114.53,
3
ambient temperature for 4 hours and then extracted by
diethyl ether. The volatiles was removed under vac-
uum, and the residue was purified by silica gel chro-
matography using hexane as eluent to give a sticky
yellowish green oil of 9a,b in a 94% and 91% yield,
respectively.
110.90, 47.34, 31.36, 30.28, 26.60, 22.45, 14.16, 13.97.
C H NS (263.42): calcd. C 63.83, H 6.50, N 5.32, S
14
17
2
24.35; found C 64.01, H 6.62, N 5.25, S 24.44.
0
0
N-2-Ethylhexyldithieno[3,2-b:2 ,3 -d]pyrrole (11b)
1
H NMR (CDCl , 400 MHz, δ/ppm): 7.13-7.12 (d, J = 4.00
3
Hz, 2H, CH-C-N), 7.00-6.99 (d, J = 4.00 Hz, 2H, CH-C-S),
0
0
0
4
,4-Dihexyl-5,5 -dibromo-dithieno[3,2-b:2 ,3 -d]silole
4.05-4.02 (t, J = 12.00 Hz, 2H, N-CH ), 1.98 (t, 1H, N-
2
(
9a)
CH -CH-), 1.35 (m, 8H, N-CH -CH (CH )-(CH ) ),
2
2
2
2 3
1
13
H NMR (CDCl , 400 MHz, δ/ppm): 6.98 (s, 2H, 2X CH-
0.95-0.92 (t, 6H, 2X CH3). C NMR (CDCl , 100 MHz, δ/
3
3
Si-), 1.35-1.22 (m, 16H, 2X Si-(CH ) ), 0.86-0.83 (m, 10H,
ppm): 145.06, 122.40, 114.31, 110.87, 40.22, 30.41, 30.11,
2
4
13
2
1
1
5
3
X -CH -CH ). C NMR (CDCl , 100 MHz, δ/ppm):
48.85, 140.91, 132.10, 111.36, 32.80, 31.35, 23.95, 22.53,
28.45, 25.41, 22.79, 13.86, 10.47. C H NS (291.47):
2
3
3
16 21
2
calcd. C 65.93, H 7.26, N 4.81, S 22.00; found C 65.85, H
7.40, N 4.72, S 21.89.
4.08, 11.58. C H Br S Si (520.46): calcd. C 46.15, H
20
28 2 2
.42, Br 30.71, S 12.32, Si 5.40; found C 46.49, H 5.52, Br
0.28, S 12.41.
3
.3.5 | Synthesis of 2,6-dibromo-N-
0
0
0
0
0
4
,4-Bis(2-ethylhexyl)-5,5 -dibromo-dithieno[3,2-b:2 ,3 -d]
alkyldithieno[3,2-b:2 ,3 -d]pyrrole (12a,b)
silole (9b)
1
H NMR (CDCl , 400 MHz, δ/ppm): 6.97 (s, 2H, 2X CH-
Compound 11a,b (3.49 mmol) was dissolved in THF
(50 mL), and N-bromosuccinamide (NBS) (8.37 mmol)
was added in one portion. The reactant was stirred at
room temperature for 4 hours and then extracted by
diethyl ether. The volatiles were removed under vacuum,
and the combined organic layers were dried over Na SO .
3
Si-), 1.39-1.35 (m, 2H, 2X Si-CH -CH-), 1.25-1.13 (m, 16H,
2
0
CH -CH ). C NMR (CDCl , 100 MHz, δ/ppm): 148.56,
1
2
X Si-CH -CH-(CH ) ), 0.95-0.90 (m, 4H, 2X CH-CH -),
2 2 3 2
.83-0.78 (m, 6H, 2X CH-CH -CH ), 0.76-0.74 (m, 6H, 2X
2
3
13
2
3
3
41.95, 132.27, 111.31, 35.78, 35.56, 28.84, 28.80, 22.93,
2
4
1
7.45, 14.09, 10.75. C H Br S Si (576.57): calcd. C 50.00,
The residue was purified by column chromatography,
24
36 2 2

14 of 17
EL-SHEHAWY ET AL.
and the desired products 12a,b were obtained in a 75%
and 70% yield, respectively.
31.43, 30.39, 29.10, 27.27, 26.70, 22.53, 14.01, 13.69, 10.88.
C H NS Sn (841.51): calcd. C 54.24, H, 8.26, N 1.66, S
38
69
2
2
7.62, Sn 28.21; found C 54.39, H 8.31, N 1.70, S 7.52.
0 0
,6-Dibromo-N-hexyldithieno[3,2-b:2 ,3 -d]pyrrole (12a)
2
1
H NMR (CDCl , 400 MHz, δ/ppm): 7.02 (s, 2H, CH-N),
3
4
.09-4.05 (t, J = 16.00 Hz, 2H, N-CH -), 1.81-1.78 (t, J =
3.4 | Synthesis of polymers
2
1
2.00 Hz 2H, N-CH -CH -), 1.28-1.25 (m, 6H, N-CH -
2
2
2
13
CH -(CH ) ), 0.88-0.85 (t, J = 12.00 Hz, 3H, CH ).
NMR (CDCl , 100 MHz, δ/ppm): 141.42, 114.21, 109.70,
4
C
3.4.1 | General procedure for
microwave-assisted Stille cross-coupling
polymerization
2
2 3
3
3
7.55, 31.34, 30.30, 26.56, 22.46, 13.97. C H Br NS
14 15 2 2
(421.21): calcd. C 39.92, H 3.59, Br 37.94, N 3.33, S, 15.23;
found C 39.81, H 3.64, Br 37.90, N 3.40, S 15.20.
Equimolar amounts (0.5 mmol) from the desired dibromo
and di (tributylstannyl) derivatives were dissolved in dry
DMF and degassed with N2 atmosphere for at least
0 0
,6-Dibromo-N-2-ethylhexyldithieno[3,2-b:2 ,3 -d]pyrrole
2
(
12b)
30 minutes followed by adding Pd (PPh) (5 mol% relative
4
1
H NMR (CDCl , 400 MHz, δ/ppm): 6.92 (s, 2H, CH-N)
.84-3.80 (t, J = 16.00 Hz, 2H, N-CH -), 1.80 (t, 1H, N-
to Br) and degassed again with nitrogen for 30 minutes.
The screw-capped glass tube was then irradiated by
microwave under the following conditions: 5 minutes at
3
3
2
CH -CH-), 1.25-1.21 (m, 8H, N-CH -CH (CH )-(CH ) ),
2
2
2
2 3
13
ꢀ
ꢀ
ꢀ
0
1
3
.86-0.82 (t, J = 16.00 Hz, 6H, 2X CH3). C NMR (CDCl3,
00 MHz, δ/ppm): 141.58, 114.42, 114.16, 109.51, 40.43,
0.40, 28.48, 23.84, 22.87, 13.93, 10.54. C H Br NS
2
100 C, 5 minutes at 120 C, and 30 minutes at 150 C. The
end-capping process was performed in separated two
steps: A solution of phenylboronic acid pinacol ester
(5 mol%) in 0.5-mL DMF was first added, followed by
irradiating the reaction mixture by microwave under the
16
19
2
(449.27): calcd. C 42.77, H 4.26, Br 35.57, N 3.12, S 14.27;
found C 42.71, H 4.33, Br 35.63, N 3.02, S 14.39.
ꢀ
following condition: 2 minutes at 100 C, 2 minutes at
ꢀ
ꢀ
1
20 C, and 5 minutes at 150 C. The same process was
3
.3.6 | Synthesis of 2,6-di (tributyltin)-N-
repeated by adding a solution of bromobenzene (5 mol%)
in 0.5-mL DMF. After the second end-capping process,
the screw capped glass tube was allowed to return to
room temperature, and the reaction mixture was poured
into methanol. The crude copolymer was collected by fil-
tration and washed consecutively with MeOH. The resid-
ual solid was loaded into an extraction thimble and
washed successively with MeOH (24 h) followed by ace-
tone (24 h).
0
0
24f
hexyldithieno[3,2-b:2 ,3 -d]pyrrole (13)
By the modification of the reported method: Under nitro-
gen atmosphere, compound 11a (1.22 g, 4.63 mmol) was
dissolved in dry THF (50 mL), was cooled to −80 C with
a cooling reactor-acetone bath and n-butyllithium (1.6 M
in hexane, 8.6 mL, 18.5 mmol), and was added dropwise
over 1 hour. The reaction mixture was stirred at −80 C
ꢀ
ꢀ
for 1 hour and then brought to room temperature. The
0
0
stirring was continued for 20 minutes, and the reaction
Poly[(4,4-di (hexyl)dithieno[3,2-b:2 ,3 -d]silole-2,6-diyl)-
ꢀ
mixture was cooled again to −80 C. A solution of
alt-(4,7-bis(3-hexylthiophen-2-yl) benzo[c][2,1,3]
tributylstannyl chloride (6.02 g, 18.5 mmol) in dry THF
thiadiazole)-5,5-diyl] (P1)
1
(7 mL) was added dropwise over 30 minutes. The mixture
H NMR (CDCl , 400 MHz, δ/ppm): 7.69 (br. s, 2H, 2X
3
was slowly brought to room temperature and stirred
another 2 hours. Subsequently, water was added to the
reaction mixture, and the aqueous phase extracted with
diethyl ether twice. The combined organic layers were
dried over Na SO . Afterward, the column chromatogra-
CH (Ph)), 7.22 (s, 2H, 2X CH-C-Si), 7.19 (s, 2H, 2X CH-C-
hexyl), 2.7-2.68 (br. m, 4H, 2X C-CH -hexyl), 1.75-1.56
2
(br. m, 4H, 2X C-CH -CH -), 1.50-1.20 (br. m, 32H, 2X Si
2
2
(CH ) + 2X (CH ) ), 0.87-0.83 (br. m, 12H, 2X CH
3
2
5
2 3
(DTSi) + 2X CH (HT)). (C H N S Si) (827.38) : calcd.
2
4
3
46 58 2 5
n
n
phy technique was utilized to purify the crude product to
C 66.78, H 7.07, N 3.39, S 19.38, Si 3.39; found C 66.91, H
7.01, N 3.44, S 19.29.
obtain the desired product 13 as a yellow oil (3.35 g,
1
8
6%). H NMR (CDCl , 400 MHz, δ/ppm): 6.95 (s, 2H,
3
0
0
CH-N), 4.20-4.17 (t, J = 12.00 Hz, 2H, N-CH -), 1.90 (t,
Poly[(4,4-bis(2-ethylhexyl)dithieno[3,2-b:2 ,3 -d]silole-
2
2
1
1
2
4
H, N-CH -CH -), 1.59-1.50 (m, 12H, 6X Sn-CH ),
2,6-diyl)-alt-(4,7-bis(3-hexylthiophen-2-yl)benzo[c][2,1,3]
2
2
2
.36-1.32 (m, 18H, 6X Sn-CH -CH - + (CH ) -hexyl),
thiadiazole)-5,5-diyl] (P2)
2
2
2 3
1
.14-1.10 (m, 12H, 6X Sn (CH ) -CH -), 0.91-0.88 (m,
H NMR (CDCl , 400 MHz, δ/ppm): 7.65 (br. s, 2H, 2X
2
2
2
3
1
3
1H, 2X (CH ) -butyl + CH -hexyl). C NMR (CDCl3,
CH (Ph)), 7.17 (s, 2H, 2X CH-C-Si), 7.15 (s, 2H, 2X CH-C-
3
3
3
00 MHz, δ/ppm): 147.90, 134.66, 120.13, 118.02, 47.34,
hexyl), 2.59-2.56 (br. m, 4H, 2X C-CH -hexyl), 1.65-1.55
2

EL-SHEHAWY ET AL.
15 of 17
(
br. m, 4H, 2X C-CH -CH -), 1.34-1.32 (br. s, 2H, 2X Si-
2H, 2X Si-CH -CH-), 1.28-1.16 (br. m, 32H, 2X Si-CH -
2
2
2
2
CH -CH-), 1.25-1.08 (br. m, 32H, 2X Si-CH -CH (CH )-
CH (CH )-(CH ) + 2X (CH ) ), 0.81-0.76 (br. m, 18H, 2X
2 2 3 2 3
2
2
2
(
CH ) + 2X (CH ) ), 0.74-0.70 (br. m, 18H, 2X (CH -CH
(CH -CH + CH (CH -CH )) (DTSi) + 2X CH (HT)).
2 3 2 3 3
2
3
2 3
2
3
+
CH (CH -CH )) (DTSi)
+
2X CH3 (HT)).
(C H N S Si) (883.48) : calcd. C 67.97, H 7.53, N 3.17,
50 66 2 5 n n
2
3
(
C H N S Si) (883.48) : calcd. C 67.97, H 7.53, N 3.17,
S 18.15, Si 3.18; found C 67.84, H 7.63, N 3.09, S 18.07.
50
66 2 5
n
n
S 18.15, Si 3.18; found C 68.09, H 7.49, N 3.03, S 18.22.
0
0
Poly[(N-hexyl-dithieno[3,2-b:2 ,3 -d]pyrrole-2,6-diyl)-alt-
0
0
Poly[(N-hexyl-dithieno[3,2-b:2 ,3 -d]pyrrole-2,6-diyl)-alt-
4,7-bis(3-hexylthiophen-2-yl)benzo[c][2,1,3]
(4,7-bis(4-hexylthiophen-2-yl) benzo[c][2,1,3]
(
thiadiazole)-5,5-diyl] (P7)
1
thiadiazole)-5,5-diyl] (P3)
H NMR (CDCl , 400 MHz, δ/ppm): 8.00 (br. s, 2H, 2X
CH (Ph)), 7.87 (s, 2H, 2X CH-C-Ph), 7.12 (s, 2H, 2X CH-
3
1
H NMR (CDCl , 400 MHz, δ/ppm): 7.69 (br. s, 2H, 2X
3
CH (Ph)), 7.21 (s, 2H, 2X CH-C-N), 7.15 (s, 2H, 2X CH-C-
C-N), 4.20 (br. s, 2H, N-CH ), 3.02-2.92 (br. s, 4H, 2X C-
2
hexyl), 4.18 (br. s, 2H, N-CH ), 2.71-2.69 (br. m, 4H, 2X
CH ), 2.15 (br. m, 2H, N-CH -CH -), 1.82-1.79 (br. m, 4H,
2
2
2
2
C-CH ), 1.90-1.85 (br. m, 2H, N-CH -CH -), 1.71-1.69
C-CH -CH -), 1.33-1.24 (br. m, 18H, -(CH ) -CH (DTP)
2 2 2 3 3
2
2
2
(
br. m, 4H, C-CH -CH -), 1.33-1.25 (br. m, 18H, -(CH ) -
+ 2X -(CH ) -CH (HT)), 0.90-0.80 (br. m, 9H, CH (DTP)
2 3 3 3
2
2
2 3
CH (DTP) + 2X -(CH ) -CH (HT)), 0.87-0.84 (br. m, 9H,
+ 2X CH (HT)). (C H N S ) (728.13) : calcd. C 65.98,
3 40 45 3 5 n n
3
2 3
3
CH (DTP) + 2X CH (HT)). (C H N S ) (728.13)n:
H 6.23, N 5.77, S 22.02; found C 66.07, H 6.39, N 5.82, S
21.89.
3
3
40 45 3 5 n
calcd. C 65.98, H 6.23, N 5.77, S 22.02; found C 65.82, H
.41, N 5.86, S 21.89.
6
0
0
Poly[(N-2-ethylhexyl-dithieno[3,2-b:2 ,3 -d]pyrrole-
0
0
Poly[(N-2-ethylhexyl-dithieno[3,2-b:2 ,3 -d]pyrrole-
,6-diyl)-alt-(4,7-bis(3-hexylthiophen-2-yl)benzo[c][2,1,3]
2,6-diyl)-alt-(4,7-bis(4-hexylthiophen-2-yl)benzo[c][2,1,3]
1
2
thiadiazole)-5,5-diyl] (P8). H NMR (CDCl , 400 MHz, δ/
3
thiadiazole)-5,5-diyl] (P4)
ppm): 7.99 (br. s, 2H, 2X CH (Ph)), 7.83 (s, 2H, 2X CH-C-
Ph), 7.12 (s, 2H, 2X CH-C-N), 4.07 (br. s, 2H, N-CH2),
2.92 (br. s, 4H, 2X C-CH ), 1.99 (br. s, 1H, N-CH -CH-),
1
H NMR (CDCl , 400 MHz, δ/ppm): 7.69-7.65 (br. s, 2H,
3
2
X CH (Ph)), 7.21 (s, 2H, 2X CH-C-N), 7.15 (s, 2H, 2X
2
2
CH-C-hexyl), 4.04 (br. s, 2H, N-CH ), 2.71-2.65 (br. s, 4H,
1.79-1.69 (br. m, 4H, C-CH -CH -), 1.37-1.25 (br. m, 20H,
2
2
2
2
4
X C-CH ), 1.99 (br. s, 1H, N-CH -CH-), 1.75-1.65 (br. m,
-CH (CH )-(CH ) -CH (DTP) + 2X -(CH ) -CH (HT)),
2 2 3 3 2 3 3
0.92-0.86 (br. m, 12H, (-CH -CH + CH (CH -CH ) (DTP)
2 3 2 3
2
2
H, C-CH -CH -), 1.32-1.25 (br. m, 20H, -CH (CH )-
2
2
2
(
CH ) -CH (DTP) + 2X -(CH ) -CH (HT)), 0.92-0.83
+ 2X CH (HT)). (C H N S ) (756.18) : calcd. C 66.71,
3 42 49 3 5 n n
2
3
3
2 3
3
(
br. m, 12H, (-CH -CH + CH (CH -CH ) (DTP) + 2X
H 6.53, N 5.56, S 21.20; found C 66.89, H 6.51, N 5.63, S
21.11.
2
3
2
3
CH (HT)). (C H N S ) (756.18) : calcd. C 66.71, H
3
42 49 3 5 n
n
6
2
.53, N 5.56, S 21.20; found C 66.63, H 6.61, N 5.60, S
1.02.
ACKNOWLEDGMENT
This work was financially supported by the Science and
Technology Development Fund (STDF) (Egypt) through
the STDF-NRG Project-ID Project Number: 7973. This
work was also supported by the GIST Research Institute
(GRI). El-Shehawy A. would also like to acknowledge the
generous support from Kafrelsheikh University Research
Program.
0
0
Poly[(4,4-di (hexyl)dithieno[3,2-b:2 ,3 -d]silole-2,6-diyl)-
alt-(4,7-bis(4-hexylthiophen-2-yl) benzo[c][2,1,3]
thiadiazole)-5,5-diyl] (P5)
1
H NMR (CDCl , 400 MHz, δ/ppm): 7.98 (s, 2H, 2X CH
Ph)), 7.83 (s, 2H, 2X CH-C-Ph), 7.23 (s, 2H, 2X CH-C-Si),
.69 (br. m, 4H, 2X C-CH -hexyl), 1.85-1.65 (br. m, 4H,
X C-CH -CH -), 1.36-1.28 (br. m, 32H, 2X Si (CH ) +
X (CH ) ), 092-0.88 (br. m, 12H, 2X CH (DTSi) + 2X
CH (HT)). (C H N S Si) (827.38) : calcd. C 66.78, H
.07, N 3.39, S 19.38, Si 3.39; found C 66.89, H 7.19, N
.30, S 19.22.
3
(
2
2
2
2
2
2
2 5
2
3
3
ORCID
3
46 58 2 5
n
n
Ashraf A. El-Shehawy https://orcid.org/0000-0002-
7
3
0863-9701
0
0
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