Novel conjugated copolymers with dithienyl and cyclopentadithienyl substituted dicyanoethene acceptor blocks

Article abstract of DOI:10.1016/j.mencom.2019.09.028

Novel bifunctional acceptor monomer, 2-[bis(5-bromothiophen-2-yl)methylidene]malononitrile, is obtained in three steps in a total yield of 79%. The Stille cross-coupling between this monomer and 2,6-ditin derivative of 4,4-didecyl-4H-silolo[3,2-b: 4,5-b′]dithiophene afforded donor–acceptor polymer whose properties were compared to the analogue containing a planar cyclopenta[2,1-b: 3,4-b′]dithiophene acceptor block. The copolymers have low narrow optical band gaps and effectively absorb visible light, however the former possesses improved solubility and thermal stability as compared to the latter.

Full text of DOI:10.1016/j.mencom.2019.09.028

Mendeleev
Communications
Mendeleev Commun., 2019, 29, 561–563
Novel conjugated copolymers with dithienyl and cyclopentadithienyl
substituted dicyanoethene acceptor blocks
a
a,b
a
c
Fedor V. Drozdov,* Yuriy N. Luponosov,* Evgeniya A. Svidchenko, Svetlana M. Peregudova,
d
a,d
a,b
Petr V. Dmitryakov, Sergei N. Chvalun and Sergei A. Ponomarenko
a
N. S. Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 117393 Moscow,
Russian Federation. Fax: +7 495 335 9000; e-mail: drozdov@ispm.ru, luponosov@ispm.ru
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation
b
c
d
National Research Center ‘Kurchatov Institute’, 123182 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2019.09.028
ꢉ21C10
S
C10ꢉ21
S
ꢉ21C10
S
C10ꢉ21
S
Si
NC
CN
Si
Novel bifunctional acceptor monomer, 2-[bis(5-bromothio-
phen-2-yl)methylidene]malononitrile, is obtained in three steps
in a total yield of 79%. The Stille cross-coupling between this
monomer and 2,6-ditin derivative of 4,4-didecyl-4H-silolo-
S
S
S
S
n
n
1
1
0
0
0
0
.2
.0
.ꢂ
.ꢁ
.ꢀ
.2
1.2
1.0
0.ꢂ
0.ꢁ
0.ꢀ
0.2
0.0
NC
CN
[
3,2-b:4,5-b']dithiophene afforded donor–acceptor polymer
film
whose properties were compared to the analogue containing
a planar cyclopenta[2,1-b:3,4-b']dithiophene acceptor block.
The copolymers have low narrow optical band gaps and effec-
tively absorb visible light, however the former possesses improved
solubility and thermal stability as compared to the latter.
solution
solution
ꢅ.2ꢆ
film
2.ꢇꢆ
0.0
ꢅ.2ꢆ
2.2ꢆ
1.ꢇꢆ
1.2ꢆ
2.ꢇꢆ
EnergyꢈeV
2.2ꢆ
1.ꢇꢆ
1.2ꢆ
EnergyꢈeV
Organic p-conjugated compounds have attracted extensive attention
in various fields of organic electronics and photonics. Conjugated
compounds with D (donor) and A (acceptor) blocks, often linked
to each other by a p-bridge, would form the simplest D–p–A
afforded dibromide 2 in 93% yield. The target monomer, 2-[bis-
(5-bromothiophen-2-yl)methylidene]malononitrile 3, is new. Tuning
the Knoevenagel conditions (solvent, temperature and reaction
duration) provided the best (95%, GPC data) yield of dicyano-
ethene 3 upon stirring in pyridine at room temperature for 72 h
(reflux in pyridine led to a partially hydrolyzed product with amide
functionality).
1
structure. Some of more complex p-conjugated compounds, for
instance, low-molecular ones with the D–p–A and D–A–p–A
3
,4
structures, are used for Grätzel type photovoltaic cells. On the
other hand, compounds with the A–p–D–p–A structure can be
Thecyclopentadithiophene-basedmonomer7wassynthesized
from bithiophene derivative 4. The Knoevenagel reaction between
ketone 6 and malononitrile was carried out similarly to that for 3.
The full conversion of compound 6 into the target product was
achieved after 3 h of stirring at room temperature. The yield of pure
product was 78%, much higher as compared to the published
5–7
used both as a donor and acceptor component of bulk hetero-
8
junction organic solar cells. Compounds with the A–p–D–p–A
9
structure showed high luminescence efficiency. More complex
10
symmetric molecules with D–A–D–A–D and D–A–D–p–
1
1
12
D–A–D and non-symmetric D1–A1–A2–p–D2 structures
were used in photovoltaics. Various non-symmetric molecules of
D1–D2–p–A1–A2 or D1–A1–p–D2–A2 structure type can
3
1
synthesis. The shorter reaction time in comparison to the syn-
thesis of analogue 3 can apparently be explained by greater electo-
philicity of the keto group in compound 6. The bifunctional donor
1
3
serve as chromophores in nonlinear optical devices, as fluore-
scent chemosensors
active layer.
14–18
32
and as components of organic solar cell
Copolymers of alternating D–A class are actively
monomer 8 was synthesized as described. Using the monomers
19–22
obtained, copolymers P1 and P2 were synthesized via the Stille
cross-coupling reaction in a microwave reactor (GPC monitoring).
The crude copolymers were purified from inorganic impurities
used since they possess excellent film-forming properties and
flexibility as compared to small D–A molecules.2
3–26
Low band gap polymers derived from bithiophene bridged by
and then from oligomeric products. Their number-average (M ),
n
ketone, dicyanovinyl and other different acceptor functions were
weight-average(M )molecularweightsandindexofpolydispersity
(PDI) were measured by GPC using polystyrene standards (Table 1).
w
initially used as low bandgap semiconductors.2
7–30
In this work,
two new conjugated copolymers were obtained (Scheme 1). They
have the same donor unit, 4,4-didecyl-4H-silolo[3,2-b:4,5-b']dithio-
phene-2,6-diyl moiety, but different acceptor units with dicyano-
vinyl groups, namely, [di(2-thienyl)methylidene]propanedinitrile
Table 1 Molecular weights, polydispersity properties and solubility of the
copolymers obtained.
a
–3
Copolymer
MN
MW
PDI
Solubility /g dm
3
and (4H-cyclopenta[2,1-b:3,4-b']dithiophen-4-ylidene)propane-
dinitrile 7.
Symmetrical dithienyl ketone 1 was synthesized from 2-bromo-
P1
P2
a
25500
17000
42500
23300
1.67
1.37
9
7
thiophene by its lithiation followed by treatment with N,N-dimethyl-
carbamoyl chloride. The subsequent bromination of compound 1
Solubility was measured in o-dichlorobenzene as it is a convenient solvent
for fabrication of organic solar cells using the standard procedure.17
©
2019 Mendeleev Communications. Published by ELSEVIER B.V.
–
561 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2019, 29, 561–563
NC
CN
O
O
S
Br
i
S
S
ii
S
S
iii
95%
S
S
Br
Br
Br
Br
Br
8
9%
93%
1
2
3
NC
CN
S
Br
O
O
i
ii
iii
78%
S
Me3Si
SiMe3
86%
93%
S
Me3Si
SiMe3
Br
Br
Br
S
S
S
S
S
Br
4
5
6
7
H21C10
C10H21
Si
NC
CN
3
S
S
S
H21C10
S
S
H21C10
S
C10H21
n
Si
P1
C10H21
iv
Me3Sn
SnMe3
S
Si
8
7
S
S
S
n
NC
CN
P2
n
Scheme 1 Reagents and conditions: i, Bu Li, THF, –78°C, then Me NC(O)Cl; ii, NBS, DMF; iii, CH (CN) , pyridine; iv, Pd(PPh ) , toluene, reflux.
2
2
2
3 4
The average molecular weights and PDI of both copolymers are
dithiophene blocks exhibit lower stability than their analogues both
in air and in nitrogen atmosphere.
The electrochemical properties of the copolymers were
examined by cyclic voltammetry. From the data obtained, the
rather similar. The solubility of P1 in o-dichlorobenzene was
–3
found to be 9 g dm , which is a bit higher as compared to polymer
P2 having more rigid acceptor block.
The thermal stability of the copolymers was investigated by
thermogravimetric analysis (TGA) under nitrogen and in air. The
values of the HOMO and LUMO energies, as well as the E values
g
were calculated (Table 2). It was also shown that the electro-
EC
decomposition temperature calculated for 5% weight loss (T ) in
chemicalbandgapofcopolymerP2(E =1.48eV)issignificantly
d
g
EC
air was found to be 336 and 312°C for polymers P1 and P2, respec-
tively [Figure 1(a)]. The course of the curves has the same profile
in both cases and differs only above 500°C. Under nitrogen, Td
were somewhat higher, 350 and 388°C, respectively [Figure 1(b)]
and the course of the curves differed significantly. One can see that
the rate of destruction was much greater for P2. From the data
obtained, one can conclude that copolymers based on cyclopenta-
narrower than that of P1 (Eg = 1.98 eV).
1
0
.0
.ꢆ
ꢂaꢃ
0.ꢅ
0.ꢄ
0.2
0.0
1
.2
1.0
.ꢆ
0.ꢅ
oꢌt
g
E
0
1
.9 1.ꢆ 1.ꢊ 1.ꢅ 1.ꢉ
oꢌt
ꢈaꢉ
E
g
EnergyꢋeV
1
00
ꢂ0
0
0
0
.ꢄ solution in CꢇCl
ꢈ
.2
.0
film
ꢁ
ꢀ
2
0
0
0
0
ꢈ.ꢉ0 ꢈ.2ꢉ ꢈ.00 2.ꢊꢉ 2.ꢉ0 2.2ꢉ 2.00 1.ꢊꢉ 1.ꢉ0 1.2ꢉ
EnergyꢋeV
P1
P2
0.ꢅ
ꢂbꢃ
1
00
200
ꢃ00
ꢀ00
TꢆꢇC
ꢄ00
ꢁ00
ꢅ00
0.ꢄ
1
1
0
0
0
.2
.0
.ꢆ
.ꢅ
.ꢄ
0
0
.2
.0
ꢈbꢉ
1
00
ꢂ0
film
2
.00 1.ꢊꢉ 1.ꢉ0 1.2ꢉ
oꢌt
g
EnergyꢋeV
E
oꢌt
g
E
solution in oꢀdichlorobenꢁene
P1
P2
0.2
.0
ꢁ
ꢀ
0
0
0
ꢈ.ꢉ0 ꢈ.2ꢉ ꢈ.00 2.ꢊꢉ 2.ꢉ0 2.2ꢉ 2.00 1.ꢊꢉ 1.ꢉ0 1.2ꢉ
EnergyꢋeV
1
00
200
ꢃ00
ꢀ00
TꢆꢇC
ꢄ00
ꢁ00
ꢅ00
Figure 2 UV-VIS spectra of copolymers (a) P1 and (b) P2 obtained in
solutions and in thin films. Sidebars show methods of determination the Eg
using edges of the absorption spectra.
Figure 1 TGA curves of the copolymers measured in (a) air and (b) nitrogen.
–
562 –

Mendeleev Commun., 2019, 29, 561–563
Table 2 UV-visible spectroscopy and cyclic voltammetry data of the copolymers.
Absorption in solution Absorption in film
CV data
Copolymer
lmax/nm
lonset/nm
E
go pt/eV
lmax/nm
lonset/nm
E
go pt/eV
Eox/V HOMO/eV
Ered/V LUMO/eV
E
gE C/eV
P1
P2
580
510
700
900
1.8
1.4
610
510
760
900
1.6
1.5
1.19
0.88
–5.59
–5.28
–0.79 –3.61
–0.60 –3.80
2.0
1.5
The absorption spectra of both copolymers were measured in
8 W. Wang, P. Shen, X. Dong, C. Weng, G. Wang, H. Bin, J. Zhang, Z.-G.
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dilute solutions (10 m) in o-dichlorobenzene and in thin films
obtained by depositing the copolymers on quartz substrates from
solutions using spin coating (Figure 2). Based on the optical
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opt
absorption data, the band gap (Eg ) was also estimated and the
opt
results are summarized in Table 2. The Eg was determined from
the intersection with the abscissa axis tangent to the long-wave
shoulder of the absorption band (Figure 2, inset). The Eg values
2016, 18, 16950.
opt
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1
EC
and 1.5 eV, respectively) and coincide well with E (1.5 eV). For
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In conclusion, the synthesis of two dicyanovinyl acceptor
monomer blocks has been accomplished. New 2-[bis(5-bromo-
thiophen-2-yl)methylidene]malononitrile 3 unit was prepared in
three steps in a total yield of 79%. Both blocks are suitable for
further polymerization with different donor units. Two new donor–
acceptor alternating copolymers, P1 and P2, were synthesized using
the Stille cross-coupling reaction. The comparison of polymers
properties demonstrates that P1 with a novel acceptor block has higher
molecular weight, better solubility and higher thermal stability.
Thus, compound 3 looks more promising acceptor unit in terms of
simplicity of its synthesis. Its copolymers seem to have good
prospects to be used in organic photovoltaics.
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The reported study was funded by the Russian Foundation for
Basic Research according to research project no. 18-33-20224.
The work was performed in the framework of leading science
school NSh-5698.2018.3. NMR and UV-VIS spectra were recorded
using the equipment of Collaborative Access Center ‘Center for
Polymer Research’ of N. S. Enikolopov Institute of Synthetic
Polymeric Materials of the Russian Academy of Sciences with
the financial support from the Ministry of Science and Higher
Education of the Russian Federation.
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