Synthesis and characterization of benzo- and naphtho[2,1-b:3,4-b′] dithiophene-containing oligomers for photovoltaic applications

Article abstract of DOI:10.1039/c4tc00335g

Dicyanovinyl (DCV) end-capped oligomers 1-7 containing fused benzo[2,1-b:3,4-b′]dithiophene (BDT) and naphtho[2,1-b:3,4-b′] dithiophene (NDT) as central cores have been synthesized and characterized. These oligomers show excellent thermal stability due to the insertion of the central fused ring system thus allowing purification by gradient sublimation in high yield. With respect to the reference compound, non-fused tetramer DCV4T, the new oligomers showed hypsochromic shifts in absorption and emission spectra and larger band gaps in thin films. Due to high lying LUMO energy levels, they exhibit sufficient energy offset with respect to C₆₀ to allow for efficient charge transfer in organic solar cells. At the same time, low-lying HOMO energy levels result in high open-circuit voltages (V_OC). As a result, planar heterojunction (PHJ) solar cells derived from the novel oligomers and C₆₀ provide very high open circuit voltages (V_OC) of up to 1.21 V and power conversion efficiencies (PCEs) of up to 2.7%. Bulk-heterojunction (BHJ) devices comprising oligomers 1-4 and C₆₀ display a slightly lower V_OC of 1 V leading to efficiencies as high as 3.6%. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00335g

Journal of
Materials Chemistry C
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PAPER
Synthesis and characterization of benzo- and
naphtho[2,1-b:3,4-b0]dithiophene-containing
oligomers for photovoltaic applications†‡
Cite this: J. Mater. Chem. C, 2014, 2,
4879
Mirjam Lobert,a Amaresh Mishra,a Christian Uhrich,b Martin Pfeifferb
¨
a
*
and Peter Bauerle
¨
Dicyanovinyl (DCV) end-capped oligomers 1–7 containing fused benzo[2,1-b:3,4-b0]dithiophene (BDT) and
naphtho[2,1-b:3,4-b0]dithiophene (NDT) as central cores have been synthesized and characterized. These
oligomers show excellent thermal stability due to the insertion of the central fused ring system thus
allowing purification by gradient sublimation in high yield. With respect to the reference compound,
non-fused tetramer DCV4T, the new oligomers showed hypsochromic shifts in absorption and emission
spectra and larger band gaps in thin films. Due to high lying LUMO energy levels, they exhibit sufficient
energy offset with respect to C60 to allow for efficient charge transfer in organic solar cells. At the same
time, low-lying HOMO energy levels result in high open-circuit voltages (VOC). As a result, planar
heterojunction (PHJ) solar cells derived from the novel oligomers and C60 provide very high open circuit
voltages (VOC) of up to 1.21 V and power conversion efficiencies (PCEs) of up to 2.7%. Bulk-
heterojunction (BHJ) devices comprising oligomers 1–4 and C60 display a slightly lower VOC of 1 V
leading to efficiencies as high as 3.6%.
Received 19th February 2014
Accepted 21st March 2014
DOI: 10.1039/c4tc00335g
www.rsc.org/MaterialsC
molecular weight. Thus, they can be prepared with high purity
and reproducibility.9
Introduction
Further development of new p-conjugated molecules is
therefore of considerable interest in order to provide specially
tuned materials with appropriate physical properties. In this
context, special attention has recently been paid on planar
molecules comprising fused ring systems. Such molecules
combine excellent charge carrier mobilities in organic eld-
effect transistors (OFET) and high thermal stability at the same
time.10–12 Among them, benzo[2,1-b:3,4-b0]dithiophene (BDT)
and naphtho[2,1-b:3,4-b0]dithiophene (NDT) represent prom-
ising fused thiophene-based systems, which were included in
various copolymers and tested as donor materials in OSCs.13–19
It has been shown that in these systems annulation lowers the
HOMO energy level of the polymer, thus an improvement of air
stability and higher open circuit voltages (VOC) in OSCs were
Over the last few years, great progress has been made for
organic solar cells (OSC) based on structurally dened
p-conjugated oligomers, typically referred to as “small mole-
cules”, as electron donors and fullerene derivatives as electron
acceptors.1–3 The “driving force” for this rapid development
comes from the versatile design of appropriate p-type semi-
conducting oligomers and optimization of device fabrication
conditions in planar-heterojunction (PHJ) and bulk-hetero-
junction (BHJ) architectures. High power conversion efficien-
cies (PCEs) of 8–9% (ref. 4–6) and 10.1% (ref. 7) have very
recently been achieved for solution-processed oligomer BHJ
single junction cells and the rst oligomer/oligomer tandem
cell, respectively. Triple cells containing evaporated oligomers
reached record efficiencies of 12.0% at the beginning of last
year.8 Structurally dened oligomers comprise several inherent
advantages compared to conjugated polymers, such as
involving straightforward synthesis and purication processes
leading to well-dened molecular structures with dened
¨
shown. Mullen et al. recently reported that BDT-containing
polythiophenes had low-lying HOMO energy levels and favor-
able aggregation behavior resulting in higher eld-effect
mobilities in comparison to the polythiophene counterpart.20,21
OSCs of the BDT-polymer in combination with soluble fullerene
derivative PC71BM as the acceptor gave a PCE of 2.7%.13
To date, only one example of a BDT-containing p-conjugated
oligomer is reported. In comparison to the corresponding
bithiophene analogue introduction of fused rings resulted in a
hypsochromic shi of the longest wavelength absorption and a
lowering of the HOMO energy level which was attributed to the
reduction of the effective conjugation length because of the
aInstitute of Organic Chemistry II and Advanced Materials, University of Ulm,
Albert-Einstein-Allee 11, D-89081 Ulm, Germany. E-mail: peter.baeuerle@uni-ulm.de
bHeliatek GmbH, Treidlerstr. 3, 01139 Dresden, Germany
† In memoriam Michael Bendikov.
‡ Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00335g
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Chart 1 Series of benzo[2,1-b:3,4-b0]dithiophene-based oligomers
1–6 and naphtho[2,1-b:3,4-b0]dithiophene-based oligomer 7. The
molecular structure of DCV4T is shown for comparison.
Scheme 1 Synthesis of benzodithiophene containing oligomers 1–4.
(a) (1) n-BuLi, THF, ꢀ78 ꢁC, 1 h; (2) rt, 2 h; (3) Me3SnCl, THF, ꢀ78 ꢁC, 1 h;
(b) malononitrile, b-alanine, ethanol, reflux, 15 h; (c) malononitrile,
piperidine, DCE, reflux, 20 h; (d) Pd(PPh3)4, DMF, 80 ꢁC, 15 h.
bent structure of the BDT unit.22 Herein, we now present a series
of tetramers incorporating the BDT and NDT unit as a core
motif. We further studied the inuence of the molecular rigidity
on optoelectronic properties and energy levels of the frontier
orbitals and compared the data to quaterthiophene derivative
DCV4T as a reference which was reported previously23 (Chart 1).
Additionally, the inuence of different side chains and the
replacement of the terminal thiophenes by furans were inves-
tigated. Finally, a correlation of the structural features and
resulting properties with the solar cell performance was set up
in order to gain further insight into molecular structure–device
performance relationships.
Results and discussion
Synthesis of BDT-based and NDT-based oligomers
Benzodithiophene containing oligomers 1–4 were synthesized
in 81% to 90% yield by Pd0-catalyzed Stille-type cross-coupling
of DCV-capped terminal building blocks 10,24 12, 13,25 15,
and distannylated benzodithiophene BDT 9 (Scheme 1). 2-
[(5-Bromo-4-butylthien-2-yl)methylene]malononitrile 12 was
synthesized in 90% yield by a Knoevenagel condensation of 5-
bromo-4-butylthiophene-2-carbaldehyde 11 (ref. 26) and malo-
nonitrile in the presence of b-alanine as the catalyst. 2-[(5-Bro-
mofur-2-yl)methylene]malononitrile 15 was synthesized in 65%
yield by a Knoevenagel reaction of 5-bromofuran-2-carbalde-
Scheme 2 Synthesis of benzodithiophene-containing oligomer 5. (a)
(1) Triethylamine, DCM, 1 h, 0 ꢁC; (2) 2 h, rt; (b) (1) n-BuLi, THF, ꢀ78 ꢁC,
30 min; (2) 18, 45 min, ꢀ78 ꢁC; (c) (1) TiCl4, Zn, THF, 2 h, 0 ꢁC; (2) 20,
THF, 15 h, D; (3) THF, TBAF, 15 min, rt; (d) (1) n-BuLi, THF, ꢀ78 ꢁC,
30 min; (2) Me3SnCl, 2 h, ꢀ78 ꢁC; (e) Pd(PPh3)4, DMF, 80 ꢁC, 15 h.
hyde 14 and malononitrile using a catalytic amount of piperi- was synthesized as described in the literature.28 Firstly, diacetyl
dine as a base. Distannylation of BDT 8 (ref. 27) was carried out 20 was synthesized in 60% yield by lithiation of bithiophene 19
by lithiation using n-butyl lithium and subsequent quenching using n-BuLi followed by quenching with N-methoxy-N-methyl-
with trimethyltin chloride in 94% yield.
acetamide29 18. The second step was an intramolecular
Oligomers 5–7 were synthesized by Pd0-catalyzed Stille-type McMurry reaction using TiCl4 and elementary zinc followed by
cross-coupling reaction of bromo-derivative 10 with dis- deprotection of the trimethylsilyl (TMS) groups using tetra-n-
tannylated central units 22, 26, and 30 in yield between 83% and butylammonium uoride trihydrate (TBAF) to afford 4,5-dime-
96%. Synthesis of stannyl derivative 22 (Scheme 2) started with thylbenzo[2,1-b:3,4-b0]dithiophene 21 in a yield of 66% (for two
3,30-dibromo-5,50-bistrimethylsilyl-2,20-bithiophene 19, which
4880 | J. Mater. Chem. C, 2014, 2, 4879–4892
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steps). Finally, bithiophene 21 was reacted with n-BuLi and gave 29 in 53% yield (for two steps) which was stannylated using
trimethyltin chloride to give stannyl derivative 22 in 88% yield. n-BuLi and trimethylstannyl chloride to obtain NDT-derivative
Benzo[2,1-b:3,4-b0]dithiophene-4,5-dione 23 was readily 30 in a yield of 94%.
synthesized as described in the literature.30 In the rst step,
twofold esterication of 23 to acetylated derivative 24 was ach-
ieved in 67% yield by reaction with acetic anhydride, zinc, and
Thermal properties
Thermal stability of optoelectronic materials is of paramount
importance for the construction of organic solar cells when
prepared by vacuum-deposition techniques at higher tempera-
tures. Therefore, we investigated melting points and decom-
position temperatures of the BDT- and NDT-based target
molecules 1–7 by differential scanning calorimetry (DSC) and
thermo-gravimetric analysis (TGA) under an inert atmosphere
at a heating rate of 10 ꢁC minꢀ1 (Fig. 1, Table 1). BDTs 1–6
meltꢁed between 311 ꢁC and 458 ꢁC and do not decompose below
380 C. NDT 7 showed an even higher melting temperature of
473 ꢁC with a decomposition temperature of 480 ꢁC. The
excellent thermal stability of oligomers 1–7 was conrmed by
TGA measurements, also showing decomposition temperatures
triethylamine. This reaction was carried out according to the
synthesis of benzo[1,2-b:4,3-b0]dithiophene-4,5-diyldiacetate as
described in the literature.31 Then, a twofold etherication with
cesium carbonate and methyl iodide afforded dimethoxy-BDT
25 in 86% yield. Stannylation of 25 with n-BuLi and trimethyltin
chloride gave bis-stannylated 26 in 90% yield (Scheme 3).
Synthesis of naphtho[2,1-b:3,4-b0]bithiophene and its deriv-
atives is described in the literature. The fused ring system was
on one hand synthesized via a palladium-catalyzed coupling of
3,30-diiodo-2,20-bithiophene and an alkyne14 or via oxidative
cyclization on the other hand.32,33 We developed a new
synthesis route, which includes a twofold Stille cross-coupling
reaction of [3,3'-bis(trimethylstannyl)-2,2'-bithien-5,5'-diyl]-
bis(trimethylsilane) and 1,2-dibromobenzene 28 (Scheme 4).
Stannyl derivative 27 was obtained in a yield of 88% by
quenching the lithiated species of bithiophene 19 with trime-
thyltin chloride. The Stille reaction of 27 with 1,2-dibromo-
benzene 28 and in situ deprotection of the trimethylsilyl groups
ꢁ
(Td) with 5% weight loss above 380 C.
ꢁ
The melting temperature (Tm) of oligomer 1 is 30 C higher
than that of DCV4T pointing to stronger intermolecular inter-
actions due to the planarization of the BDT unit. The furan
Scheme 3 Synthesis of 4,5-dimethoxybenzo[2,1-b:3,4-b']dithiophene
containing oligomer 6. (a) Zn, triethylamine, DCM, acetic anhydride, 15 h,
rt; (b) Cs2CO3, MeI, acetonitrile, 80 ꢁC, 72 h; (c) (1) n-BuLi, THF, ꢀ78 ꢁC,
30 min; (2) Me3SnCl, 2 h, ꢀ78 ꢁC, 12 h, rt; (d) Pd(PPh3)4, DMF, 80 ꢁC, 15 h.
Scheme 4 Synthesis of naphtho[2,1-b:3,4-b']bithiophene-containing
oligomer 7. (a) (1) n-BuLi, THF, ꢀ78 ꢁC, 30 min; (2) Me3SnCl, 2 h, ꢀ78
ꢁC, 2 h, rt; (b) (1) Pd2(dba)3$CHCl3, HP(tBu)3BF4, DMF, 100 ꢁC, 2 days; (2) Fig. 1 (a) DSC curves for benzodithiophenes 1–4 measured under
TBAF, THF, 15 min, rt; (c) (1) n-BuLi, THF, ꢀ78 ꢁC, 30 min; (2) Me3SnCl, 2 argon flow at a heating rate of 10 ꢁC minꢀ1. (b) TGA curvesꢀf1or 1–4
h, ꢀ78 ꢁC; (d) Pd(PPh3)4, DMF, 80 ꢁC, 15 h.
measured under nitrogen flow at a heating rate of 10 ꢁC min
.
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Table 1 Thermal properties of oligomers 1–7 and DCV4T
a
a
b
Oligomer
Tm
Td
Td
DCV4T
320
350
311
—
386
458
404
473
424
431
383
407
411
463
410
480
411
430
382
405
412
440
400
472
1
2
3
4
5
6
7
a
Melting temperature (Tm) and decomposition temperature (Td)
b
determined from DSC. Decomposition temperature corresponding
to 5% weight loss in N2 determined by TGA.
containing oligomer 4 showed a 36 ꢁC higher melting point
further supporting the higher intermolecular interactions of
furan-containing oligomers compared to oligothiophenes.34
Butyl side chains decreased the melting temperature and
compound 2 melted at 311 ꢁC which is 39 ꢁC lower than that of
BDT 1. Oligomer 3 did not show any phase transition before
decomposition. The substituents at the ethylene bridge of the
BDT unit of oligomers 5–7 resulted in an increase of the melting
temperature (Fig. S1†). BDT 5–7 showed a strong exothermic
peak immediately aer melting. The planar NDT containing
ꢁ
oligomer 7 showed the highest melting temperature of 473 C.
The compounds were puried by gradient vacuum sublimation.
Fig. 2 (a) Representative absorption and fluorescence spectra of
benzodithiophene 1, 3, and 4 in dichloromethane at room tempera-
ture. (b) Thin film (30 nm) absorbance and fluorescence spectra of
benzodithiophene 1, 3, and 4. Thin films were prepared by vacuum
sublimation. The absorption and emission spectra of DCV4T are
included for comparison.
Optical spectroscopy
The optical properties of the BDT-based oligomers 1–7 were
determined by UV-vis and uorescence spectroscopy in
dichloromethane and in thin lms prepared by vacuum subli-
mation. All data are summarized in Table 2. Representative
solution and thin lm spectra are shown in Fig. 2 and S2.†
In solution, BDT containing oligomers 1–6 showed an
absorption maximum between 484 nm and 497 nm. Each
corresponds to the p–p* (HOMO–LUMO) transition of the
conjugated p-system. The absorption bands of oligomers 1–6
are hypsochromically shied compared to DCV4T due to
lowering of the HOMO energy level as a consequence of the
fused BDT unit. In contrast to DCV4T, BDT-oligomer 1 showed a
strong alternation of the absorption band with two distinct
vibronic transitions appearing at 489 nm and 506 nm. The
emission maximum of 1 is about 42 nm blue-shied in
comparison to DCV4T showing its maximum at 565 nm. Similar
vibronic splitting of the absorption and emission bands was
also observed for the furan-containing oligomer 4. The smaller
Stokes shi for 4 compared to 1 can be assigned to the higher
Table 2 Optical data of molecules 1–7 compared to DCV4T. The values in brackets represent shoulders
Solution
Film
3max
[L molꢀ1 cmꢀ1
Stokes shi
[cmꢀ1
]
lem [nm]
DEopt [eV]
labs [nm]
lem [nm]
DEoptb [eV]
a
Compound
labs [nm]
]
DCV4T (ref. 21)
518
489 (506)
485
487
484, 509
494
66 800
74 500
54 100
—
—
—
2922
1856
3612
3381
1628
3982
2373
2093
612 (651)
565 (588)
588
2.09
2.23
2.20
2.17
2.26
2.19
2.20
2.12
560
518
529
522
505
531
528
549
670
707
715
694
675
735
725
766
1.78
1.95
1.94
1.96
2.03
1.87
1.90
1.80
1
2
3
4
5
6
7
587
555 (582)
615 (650)
588
497 (516)
512 (537)
—
—
605 (648)
a
b
Estimated using the onset of the UV-Vis spectra in DCM. Estimated using the onset of the UV-Vis spectra of lms prepared by vacuum
sublimation.
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rigidity of the oligofurans in comparison to that of
oligothiophenes.34
Cyclic voltammetry
The redox properties of oligomers 1–4 have been measured in
0.1 M tetrachloroethane–TBAPF6 at 80 C. All data are summa-
The absorption and uorescence spectra of butyl- and
methyl-substituted oligomers 2 and 3 displayed structureless
bands due to the presence of alkyl side chains. The Stokes shi
of these two oligomers is larger compared to that of 1, 4, and
DCV4T, which is an indication for a higher twist between the
alkylated thiophenes and the BDT unit.
In comparison to oligomer 1, the insertion of electron
donating methyl and methoxy groups at the BDT units in 5 and
6 showed only a small inuence on the absorption behaviour.
On the other hand, the emission maxima of oligomers 5 and 6
are 50 nm and 23 nm red-shied compared to 1 resulting in a
larger Stokes shi of 3982 cmꢀ1 and 2373 cmꢀ1, respectively.
The larger Stokes shi further conrms the steric hinderance in
oligomers 5 and 6 indicating a signicant structural reorgani-
zation on photoexcitation.
ꢁ
rized in Table 3 and the DPV of oligomer 1 is shown as an
example in Fig. 3. Oligomer 1 showed two reversible one-elec-
tron oxidation waves at 0.99 V and 1.33 V indicating the
formation of stable radical cations and dications, respectively.
One quasi-reversible reduction wave was observed at ꢀ1.41 V
which is assigned to the reduction of the DCV end groups. As a
consequence of the fused BDT unit, the oxidation potentials of
oligomer 1 are shied to slightly higher potentials (DEox ¼ 0.15
V and 0.13 V) compared to DCV4T whereas no change was
observed for the reduction potential. The oxidation potentials of
oligomer 2 are very similar to that of 1. Oligomer 3 containing
four methyl groups showed slightly lower oxidation potentials
(DEox ¼ 0.05 V and ꢂ0.14 V) compared to 1 and 2. In contrast,
furan-containing oligomer 4 did not show a reversible oxidation
behavior and the waves were shied to lower potentials of 0.85 V
and 1.17 V. Similar behavior is also reported for oligofurans.34
The HOMO and LUMO energy levels of oligomers 1–4 were
calculated from the onset values of the rst oxidation and
reduction wave using the standard approximation that the Fc/
Fc+ HOMO energy level is ꢀ5.1 eV versus vacuum.35 The results
are presented in Fig. 4 and the data are compiled in Table 3. The
HOMO and LUMO levels of compounds 1 and 2 are identical
showing an electrochemical band gap of 2.19 eV. The LUMO
energy level of oligomer 3 is similar to 1 and 2 but showed a
The molar extinction coefficient of 1 was about 7700 L molꢀ1
cmꢀ1 higher than that of the parent DCV4T, while due to
molecular exibility the molar extinction coefficient is reduced
for oligomer 2. The molar extinction coefficient of oligomers 3–
7 could not be determined due to their poor solubility.
NDT-containing oligomer 7 showed a bathochromic shi of
the absorption band in comparison to BDT 1–6 due to the
higher electron-donating strength of the NDT unit. The
absorption maximum of 7 was located at 512 nm with a
shoulder at around 537 nm. The emission maximum of 7 was
strongly red-shied by Dl ¼ 40 nm compared to 1 resulting in a
slightly larger Stoke shi of 2093 cmꢀ1
.
In solution the optical energy gap of oligomers 1–6 is found
to be slightly larger (2.17–2.26 eV) compared to 2.09 eV for
DCV4T. While, NDT 7 showed a very similar energy gap of
2.12 eV compared to DCV4T.
Thin lm absorption and emission spectra of BDTs 1–7 are
signicantly broadened and bathochromically shied
compared to the solution spectra (Fig. 2 and S2†). This red-shi
and spectral broadening in thin lms suggest higher planarity
of the molecules in lms which induces a higher ordering and
an increase of the intermolecular interactions. The absorption
maxima of BDT oligomers 1–6 are localized between 505 nm
and 529 nm and the emission maxima between 675 nm and 735
nm. The absorption maximum of NDT 7 is further red-shied to
549 nm and the emission maximum to 766 nm. The optical
band gaps in thin lms are lowered by about 0.3 eV compared to
the gaps obtained from solution spectra.
Fig. 3 Representative DPV of benzodithiophene-containing oligomer
1 measured in TCE–TBAPF6 (0.1 M), c z 1 ꢃ 10ꢀ3 mol Lꢀ1, 80 ꢁC, scan
rate ¼ 100 mV sꢀ1
.
Table 3 Electrochemical data of benzodithiophene-containing oligomers 1–4 compared to DCV4Ta
HOMOb [eV]
LUMOb [eV]
DECV
c
Compound
Eox1 [V]
Eox2 [V]
Ered [V]
DCV4T (ref. 21)
0.84
0.99
1.00
0.94
0.85
1.20
1.33
1.30
1.19
1.17
ꢀ1.41
ꢀ1.41
ꢀ1.47
ꢀ1.48
ꢀ1.57
ꢀ5.85
ꢀ6.00
ꢀ5.98
ꢀ5.89
ꢀ5.81
ꢀ3.87
ꢀ3.81
ꢀ3.80
ꢀ3.80
ꢀ3.62
1.98
2.19
2.18
2.09
2.19
1
2
3
4
a
Measured in TCE–TBAPF6 (0.1 M), [compound] ¼ 10ꢀ3 mol Lꢀ1, 80 ꢁC, V ¼ 100 mV sꢀ1, vs. Fc+/Fc. b Calculated from the onset of the rst oxidation
and reduction wave, set Fc+/Fc EHOMO ¼ ꢀ5.1 eV. c Calculated from the difference between the values of Eonset,red1 and Eonset,ox1
.
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Consequently, the efficiency is not limited by charge carrier
transport, but rather by the exciton diffusion length. The stack
of the PHJ solar cells consisted of 15 nm C60 evaporated on an
ITO-coated glass substrate, followed by deposition of a 6 nm
donor layer of the respective oligomers 1–4, 5 nm of undoped
and 50 nm of p-doped 9,9-bis{4-[di-(p-biphenyl)aminophenyl]}
uorene (BPAPF) as a hole transport layer (HTL). The undoped
BPAPF layer was introduced to avoid direct contact between the
active layer and the p-doped BPAPF layer which otherwise could
lead to quenching of excitons by the dopants.39 As a p-dopant,
NDP9 (10 wt%) was used. Another 1 nm thick layer of NDP9 was
introduced between the HTL and the Au top contact to facilitate
charge extraction (Fig. 5).
The PHJ solar cell parameters are summarized in Table 4.
The current density–voltage (J–V) curves and the external
quantum efficiency (EQE) spectra of the PHJ-devices are shown
in Fig. 6. Oligomer 1 gave a PCE of 2.3%, with a short circuit
current density (JSC) of 4.7 mA cmꢀ2, a VOC of 1.09 V, and a FF of
44%. The HOMO energy levels of oligomers 1–3 are reduced by
0.04 to 0.15 eV compared to DCV4T. In PHJ cells, VOC scales
linearly with the energy difference of the LUMO of the acceptor
(C60) and the HOMO of the donor.40 Consequently, a very high
VOC of up to 1.21 V was obtained for oligomer 2, which is one of
the highest values ever reported for PHJ devices.41,42 The higher
VOC value of 2 was consistent with its deeper low-lying HOMO
energy level. On the other hand, the J–V-characteristics of the 2-
based device generated an s-kink due to an energetic barrier at
the interface to the HTL resulting in a low FF of 37% and an
efficiency of 1.5%. Even though, the HOMO energy level of
oligomer 1 is the lowest among all oligomers, it yields only a VOC
of 1.09 V. This could be explained by the formation of pinholes
in 6 nm thin lms of the oligomer leading to some close contact
between the C60 acceptor and BPAPF hole transporter, thus,
making a direct contact between the cathode and anode. Friend
and co-workers reported that these direct paths act as a shunt
Fig. 4 Representation of HOMO/LUMO energies of oligomers 1–4
and DCV4T derived from electrochemical data compared to C60. The
HOMO/LUMO energy levels of C60 are taken from the literature36 and
were determined from the onset of the first oxidation and reduction
waves measured in 0.1 M TBAPF6–tetrachloroethane.
slightly higher HOMO level, which, resulted in a lower electro-
chemical band gap of 2.09 eV. Compound 4 comprising furan
units showed HOMO and LUMO energy values which are about
0.2 eV higher than that of 1, thus an identical electrochemical
band gap of 2.19 eV. The redox properties of oligomers 5–7
could not be determined due to their low solubility. As can be
seen in Fig. 5 the low lying HOMO energy levels of these olig-
omers could be favorable for obtaining higher VOC in organic
solar cells using C60 as an acceptor. It is known that the VOC
depends on the difference between the HOMO energy level of
the donor and the LUMO energy level of the acceptor.
Preparation and characterization of vacuum-processed solar
cells
The DCV end-capped oligomers 1–4 were tested as electron donor
materials in PHJ and BHJ solar cells using fullerene C60 as an
electron acceptor. The cells were prepared by vacuum sublima-
tion following the m-i-p concept reported in the literature.37,38
resistance in parallel with the active layer of the device, result-
43
ing in the lowering of VOC
.
Oligomers 3 and 4 showed VOC
values of 1.07 V and 0.97 V which correspond well to the ener-
getic shi of the HOMO energy level with respect to the C60
acceptor. Furthermore, oligomers 3 and 4 based devices
reached FF values of 44% and 59%, JSCs of 4.3 mA cmꢀ2 and 3.4
mA cmꢀ2 and PCEs of 2.0 and 1.9%, respectively. The EQE
spectrum of 4 is signicantly shied to lower wavelengths
corroborated well with the thin lm absorption spectrum
(Fig. 6b).
Planar heterojunction solar cells
Planar heterojunction (PHJ) solar cells consist of a well-dened
planar interface between the donor and the acceptor layer.
PHJ solar cells prepared with oligomers 5 and 6 containing
methyl and methoxy-substituents at the BDT unit achieved
PCEs of 1.5% and 2.7%, respectively (Fig. S3†). The lower
performance of 5 compared to 1 is due to the reduction in JSC
,
VOC, and FF values. However, the efficiency of oligomer 6 is very
promising. In comparison to oligomer 1, higher JSC and VOC
values of 5.2 mA cmꢀ2 and 1.15 V have been measured.
The perpendicular extension of the ring fusion in NDT-
containing oligomer 7 led to a higher FF of 49% compared to
44% for BDT 1. The J–V characteristics of NDT 7 showed a poor
rectifying behavior in the dark, which is a hint for a poor lm
quality. A signicantly lower VOC of 0.90 V was measured for
Fig. 5 General device architecture of investigated PHJ solar cells.
4884 | J. Mater. Chem. C, 2014, 2, 4879–4892
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Table 4 Photovoltaic parameters of planar heterojunction solar cells containing DCV end-capped BDT-containing oligomers 1–6 and NDT-
derivative 7 compared to DCV4T. The active layer thickness is 6 nm
Compound
JSCa [mA cmꢀ2
]
VOC [V]
FF [%]
h [%]
Saturationb
Intensity [mW cmꢀ2
]
DCV4T
6.2
4.7
3.4
4.3
3.4
3.8
5.2
4.0
1.00
1.09
1.21
1.07
0.97
1.00
1.15
0.90
48
44
37
44
59
39
45
48
3.0
2.3
1.5
2.0
1.9
1.5
2.7
1.8
1.2
1.3
1.3
1.2
1.2
1.4
1.2
1.4
92
106
106
103
107
93
1
2
3
4
5
6
7
92
92
a
b
Measured with mask and corrected to 100 mW cmꢀ2
.
Dened as J(ꢀ1 V)/JSC
.
and a FF of 48%. The saturation values for all compounds are in
the range of 1.2 to 1.4. A saturation value close to unity, i.e., weak
voltage bias dependence of the current in the reverse direction, is
an indication for an efficient charge separation and extraction.
Larger values indicate that recombination losses and eld-
dependent dissociation of charge-carriers are present.44,45
Bulk heterojunction solar cells
BHJ solar cells with a photoactive layer thickness of 20 nm were
prepared using oligomers 1–4 as the donor and C60 as an
acceptor. The blend layer was prepared by co-evaporation of the
donor and the acceptor in a ratio of 1 : 1 by volume at a
substrate temperature of 70 ꢁC. This substrate temperature was
found to be optimal for quaterthiophene-based materials.25 It
has been shown that heating of the substrate should result in a
morphology, which provides better charge transport in the
active layer.39,46 All other layers were vacuum-deposited using
the following layer sequence: ITO-coated glass substrate, 15 nm
C
60, 20 nm photoactive blend layer, 5 nm of undoped and 50 nm
of p-doped 9,9-bis[4-(N,N-bis-biphenyl-4-ylamino)phenyl]-9H-
uorene (BPAPF) as the hole transport layer (HTL), 1 nm thick
NDP9 layer and the Au top electrode (Fig. 7). The data for the
BHJ solar cells are summarized in Table 5 and J–V characteris-
tics as well as EQE spectra of the solar cells are shown in Fig. 8.
Fig. 6 (a) J–V-characteristics of planar heterojunction solar cells with
DCV end-capped oligomers 1–4 and DCV4T as the donor material and
C
60 as an acceptor. (b) Corresponding EQE spectra of PHJ solar cells.
NDT 7. This could be explained by a poor packing of NDT 7 in
thin layers resulting in pinholes, which could lead to some close
contact between the C60 and BPAPF hole transporter and thus
reduces the VOC. Together with a JSC of 4.0 mA cmꢀ2 the device
generated a PCE of 1.8%.
In PHJ devices, reference compound DCV4T displayed the
highest performance among all oligomers. Oligomers 1–7
exhibited signicantly lower JSC compared to DCV4T which was
also reected in their EQE spectra. The DCV4T device generated a
PCE of 3.0% with a VOC of 1.0 V and a higher JSC of 6.2 mA cmꢀ2 Fig. 7 General device architecture of investigated BHJ solar cells.
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Paper
Table 5 Photovoltaic parameters of bulk-heterojunction solar cells containing DCV end-capped BDT-derivatives 1–4 and 6 compared to
DCV4T. The active layer thickness was 20 nm for all devices with an oligomer : C60 ratio of 1 : 1 by volume
Compound
JSCa [mA cmꢀ2
]
VOC [V]
FF [%]
h [%]
Saturationb
Intensity [mW cmꢀ2
]
DCV4T
7.4
6.0
4.0
5.6
4.6
5.4
0.96
1.00
1.01
0.99
1.00
0.98
46
60
52
57
61
50
3.2
3.6
2.1
3.2
2.8
2.6
1.24
1.12
1.18
1.12
1.14
1.10
102
100
99
100
103
99
1
2
3
4
6
a
b
Measured with mask and corrected to 100 mW cmꢀ2
.
Dened as J(ꢀ1 V)/JSC
.
most critical parameter in organic solar cells. Interestingly, the
BHJ devices with BDT derivatives 1–4 exhibited higher FFs (52%
to 61%), which are signicantly higher compared to devices of
reference DCV4T (46%). This suggests that the molecular pla-
narization in BDTs resulted in an improved charge carrier
transport in the donor–acceptor blend layers. Consequently, BHJ
cells prepared with BDTs 1–4 showed good efficiencies of 3.6%,
2.1%, 3.2%, and 2.8%. Additionally, the signicantly lower
saturation values of 1.12 to 1.18 for the BDT-based BHJ cells
compared to DCV4T (1.24) indicate a decrease of eld-dependent
recombination of photogenerated charge carriers. It is inter-
esting to note that the best cell in this series incorporating planar
BDT-oligomer 1 gave a 12.5% improvement in efficiency
compared to reference DCV4T (Table 5).
Oligomers 5 and 7 were not tested in BHJSCs due to their low
efficiencies in PHJ devices and high degree of decomposition
during sublimation. The BHJ device prepared with oligomer 6
gave a JSC of 5.4 mA cmꢀ2, a FF of 50%, and an efficiency of
2.6%. The lower performance of 6 is mainly ascribed to the
moderate ll factor.
Conclusions
We have synthesized and characterized a series of novel benzo-
[2,1-b:3,4-b0]dithiophene (BDT)- and naphtho[2,1-b:3,4-b0]-
dithiophene (NDT)-containing oligomers for applications in
organic solar cells. Gradient vacuum sublimation led to pure
organic semiconducting materials with high thermal stability.
Incorporation of the fused central BDT and NDT units resulted
in blue-shied absorption and emission spectra and in an
increase of the band gap compared to non-fused reference
DCV4T. PHJ and BHJ solar cells were fabricated by vacuum-
processing using these oligomers as the electron donor and C60
Fig. 8 (a) Representative J–V-characteristics of bulk-heterojunction
solar cells with DCV end-capped benzodithiophene 1 and DCV4T as
donor materials. (b) Corresponding EQE spectra of BHJ solar cells.
All BDT-containing oligomers gave high VOCs close to 1 V. This as the electron acceptor. PHJ solar cells constructed using all
was expected, since in BHJ solar cells the VOC is dominated by the oligomers displayed high VOCs and efficiencies in the range of
LUMO of the acceptor (C60) and the HOMO of the HTL (BPAPF).40 1.5–3.0% under full sun illumination. Despite a blue-shied
BHJ solar cells with oligomers 1–4 reached JSCs of 4.0 to 6.0 mA absorption and a larger band gap, the highest PCE of 3.6% was
cmꢀ2. Comparing the JSC achieved with all donor materials, the obtained in BHJ devices for BDT-oligomer 1 which is higher
BHJ cells followed the same trend as the PHJ solar cells. The compared to reference DCV4T. This improvement is mainly
development of the photocurrents correlates well with the EQE- ascribed to the large increase in the VOC and FF values. The
spectra. Oligomers 1–3 showed qualitatively similar spectra with current results clearly reveal that the structural concept pre-
a maximum at around 550 nm. The EQE spectrum of 4 is slightly sented here, i.e., integration of fused-ring systems, represents
blue-shied and corresponds well to the PHJ device and the thin an interesting direction in the design of novel photoactive
lm absorption spectra. The FF besides efficiency is one of the materials for solar cell applications.
4886 | J. Mater. Chem. C, 2014, 2, 4879–4892
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Journal of Materials Chemistry C
Photovoltaic characterization
J–V and EQE measurements were carried out in a glove box
under nitrogen atmosphere. J–V characteristics were
measured using a source-measure unit (Keithley SMU 2400) and
an AM 1.5G sun simulator (KHS Technical Lighting SC1200).
Experimental
Physical measurements and instrumentation
a
NMR spectra were recorded on a Bruker AMX 500 (1H NMR: 500
MHz, 13C NMR: 125 MHz) at 100 ꢁC or an Avance 400 spec-
trometer (1H NMR: 400 MHz, 13C NMR: 100 MHz) at 25 ꢁC
unless otherwise noted. Chemical shi values (d) are expressed
in parts per million using residual solvent protons (1H NMR, dH
The intensity was monitored with
a silicon photodiode
(Hamamatsu S1337) which was calibrated at Fraunhofer ISE.
The mismatch between the spectrum of the sun simulator and
the AM 1.5G spectrum was not taken into account. For well-
dened active solar cell areas, aperture masks (2.76 mm2) were
used. Simple EQE measurements were carried out using the sun
simulator in combination with color lters for monochromatic
illumination. The illumination intensities were measured with
a silicon reference diode (Hamamatsu S1337).
1
1
¼ 7.26 for CDCl3; H NMR, dH ¼ 5.92 for TCE; H NMR, dH
¼
5.32 for DCM; 13C NMR, dC ¼ 77.16 for CDCl3) as the internal
standard. The splitting patterns are designated as follows: s
(singlet), d (doublet), t (triplet), q (quartet), and m (multiplet).
The assignments are Th-H (thiophene protons), Fu-H (furan
protons), Ph-H (phenyl protons), and DCV-H (dicyanovinylene
protons). Melting points were determined using a Mettler
Toledo DSC 823e and were not corrected. Elemental analyses
were performed on an Elementar Vario EL. Thin layer chro-
matography was carried out on aluminum plates, pre-coated
with silica gel, Merck Si60 F254. Preparative column chroma-
tography was performed on glass columns packed with silica
gel, Merck Silica 60, particle size 40–43 mm. Electron impact (EI)
mass spectra were recorded on a Varian 3800, chemical ion-
isation (CI) mass spectra on a Finnigan MAT SSQ-7000 and
MALDI-TOF mass spectra on a Bruker Daltonics Reex III.
Optical measurements in solution were carried out in 1 cm
cuvettes with Merck Uvasol grade DCM. Absorption spectra
were recorded on a Perkin Elmer Lambda 19 spectrometer and
corrected uorescence spectra were recorded on a Perkin Elmer
LS 55 uorescence spectrometer. Cyclic voltammetry experi-
ments were performed with a computer-controlled Autolab
PGSTAT30 potentiostat in a three-electrode single-compartment
cell with a platinum working electrode, a platinum wire counter
electrode, and an Ag/AgCl reference electrode. All potentials
were internally referenced to the ferrocene/ferrocenium couple.
Materials
Tetrahydrofuran (THF, Merck) was dried under reux over
sodium/benzophenone (Merck). Dimethylformamide (DMF,
Merck) was dried under reux over phosphorous pentoxide
(Merck). Dichloromethane (DCM) and n-hexane were purchased
from Prolabo and distilled prior to use. All synthesis steps were
carried out under an argon atmosphere. n-Butyl lithium (1.6 N
in n-hexane) and Pd2(dba)3$CHCl3 were purchased from Acros.
Piperidine, triethylamine, acetic anhydride, titanium tetra-
chloride, b-alanine, acetonitrile, zinc, tetra-n-butylammonium
uoride, methyl iodine and chlorobenzene were purchased
from Merck. Malononitrile, o-dichlorobenzene, trimethyltin
chloride and 1,2-dibromo-benzene 28 were purchased from
Aldrich. HP(tBu)3BF4, cesium carbonate, 5-bromo-2-furaldehyde
14 and N,O-dimethylhydroxylamine hydrochloride 17 were
purchased from Aa (Aesar). Tetrakis(triphenyl-phosphine)
palladium (Pd(PPh3)4) was synthesized according to a literature-
known process.47
Benzo[2,1-b:3,4-b0]dithiophene 8,27 2-[(5-bromothien-2-yl)-
methylene]malononitrile 10,24 5-bromo-4-butylthiophene-2-car-
baldehyde 11,26 2-[(5-bromo-3,4-dimethylthien-2-yl)-methylene]-
malononitrile 13,25 (3,30-dibromo-[2,20-bithien]-5,50-diyl)bis-
(trimethylsilane) 19, and benzo[2,1-b:3,4-b0]dithiophene-4,5-
dione 23 (ref. 30) were synthesized as described in the literature.
Thin lm and device fabrication
Thin lms and heterojunction solar cell devices were prepared by
thermal vapor deposition in an ultra-high vacuum at a base pres-
sure of 10ꢀ7 mbar on the substrate at room temperature. Thin lms
for absorption and emission measurements were prepared on
quartz substrates and solar cells on tin-doped indium oxide (ITO)
coated glass (Thin Film Devices, USA, sheet resistance of 30 U sqꢀ1).
Layer thicknesses were determined during evaporation by using
Synthesis
2,7-Bis(trimethylstannyl)benzo[2,1-b:3,4-b0]dithiophene (9).
quartz crystal monitors calibrated for the respective material. The Benzo[2,1-b:3,4-b0]dithiophene 8 (353 mg, 1.86 mmol) was dis-
thin lms prepared for absorption and emission measurements are solved in 6.3 mL of dry THF. The solution was degassed three
ꢁ
approximately 30 nm thick. The thin lm absorption spectra were times and was cooled to ꢀ78 C. n-BuLi (2.56 mL, 4.08 mmol,
recorded on a Shimadzu UV-2101/3101 UV-vis spectrometer. The 1.6 M in hexane) was added over 7 minutes whereupon a grey
thin lm emission spectra were recorded with an Edinburgh suspension was formed. The suspension was stirred for 1 h at
Instruments FSP920 uorescence spectrometer. Bulk-heterojunc- ꢀ78 ꢁC and 2.5 h at room temperature. Aer cooling to ꢀ78 ꢁC
tion solar cells were prepared layer by layer without breaking the again, trimethylstannyl chloride (814 mg, 4.08 mmol) in 2.0 mL
vacuum. The layer structure of the bulk-heterojunction solar cells is THF was added in one portion. The mixture was stirred for 1 h
as follows: ITO; 15 nm C60; 20 nm blend layer of respective ben- at ꢀ78 ꢁC and at room temperature overnight. A light yellow
zodithiophenes and C60 (ratio of 1 : 1 by volume) prepared by co- solution was formed. 35 mL of n-hexane were added. The white
evaporation deposited on the heated substrate (70 ꢁC); 5 nm BPAPF; suspension was washed with 3 ꢃ 50 mL water, dried over
50 nm BPAPF doped with NDP9 (purchased from Novaled AG Na2SO4, and the solvent was removed. Aer drying in a high
Germany, 10 wt%); 1 nm NDP9; 50 nm gold.
vacuum, product 9 (938 mg, 1.75 mmol, purity: 96% (1H-NMR))
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1
1,10-[5,50-Bis(trimethylsilyl)-[2,20-bithien]-3,30-diyl]diethanone
(20). (3,30-Dibromo-[2,20-bithien]-5,50-diyl)bis(trimethylsilane) 19
(2.0 g, 4.2 mmol) was dissolved in 44 mL dry THF and cooled to
ꢀ78 ꢁC. n-BuLi (5.8 mL, 9.32 mmol, 1.6 M in hexane) was added
within 15 minutes. The mixture was stirred for 45 minutes at
ꢀ78 ꢁC and then N-methoxy-N-methylacetamide 18 (961 mg, 9.32
mmol) was added. The reaction mixture was allowed to stir for 2
h at ꢀ78 ꢁC and was warmed to room temperature overnight. The
reaction was quenched with 50 mL saturated NaHCO3 solution.
The product was extracted with DCM, the organic solution was
dried over Na2SO4, ltered off, and the solvent was removed. The
crude product was puried by column chromatography (ash
silica; DCM) to obtain pure 20 (1.10 g, 2.79 mmol; 66%) as a
slightly yellow solid. M.p. 84.0–85.2 ꢁC, 1H NMR (400 MHz,
CDCl3): d ¼ 7.57 (s, 2H, Th-H), 3.28 (s, 6H, CH3), 0.35 (s, 18H, Si–
CH3), 13C NMR (100 MHz, CDCl3): d ¼ 193.84, 144.04, 142.55,
141.52, 135.75, 29.32, 0.12. MS (EI): m/z ¼ 394 [M]+, 352 [M ꢀ
(CH3)3]+, elemental analysis: calc. (%) for C18H26O2S2Si2: C 54.77;
H 6.64; S 16.25; found: C 54.92; H 6.71, S 16.10.
was obtained as a white solid. H NMR (400 MHz, CDCl3): d ¼
7.73 (s, 1H, Th-H), 7.50 (s, 1H, Ph-H), 0.45 (s, 9H, Sn–CH3). 13
C
NMR (100 MHz, CD2Cl2): d ¼ 138.24, 138.11, 138.02, 132.93,
120.01, ꢀ8.10. MS (EI): m/z ¼ 514 [M ꢀ H]+, 502 [M ꢀ CH3]+. The
product was used without further purication and analytical
data were in agreement with the literature.48
2-[(5-Bromo-4-butylthien-2-yl)methylene]malononitrile (12).
5-Bromo-4-butylthiophene-2-carbaldehyde 11 (250 mg, 1.01
mmol), malononitrile (134 mg, 2.02 mmol), and b-alanine (4.5
mg, 57 mmol) were dissolved in 10 mL ethanol and heated to
reux for 15 h. The reaction mixture was cooled to room
temperature and 20 mL diethyl ether was added. The reaction
mixture was washed with water (6 ꢃ 20 mL) and dried over
Na2SO4. The removal of the solvent yielded 335 mg of a brown
solid, which was puried by column chromatography (silica; n-
hexane–DCM 1 : 1) and pure product 12 (302 mg, 1.03 mmol,
90%) was obtained as an orange solid. 1H NMR (400 MHz,
CDCl3): d ¼ 7.68 (s, 1H, DCV-H), 7.42 (s, 1H, Th-H), 2.65–2.57
(m, 2H, –CH2–), 1.63–1.52 (m, 2H, –CH2–), 1.41–1.32 (m, 2H,
–CH2–), 0.94 (t, 3J ¼ 7.3 Hz, 4H, –CH3). 13C NMR (100 MHz,
CDCl3): d ¼ 150.15, 144.83, 138.96, 134.96, 124.33, 113.90,
31.66, 29.01, 22.34, 13.93. MS (EI): m/z ¼ 296 [M]+, 253 [M ꢀ
C3H7]+. Elemental analysis: calc. (%) for C12H11BrN2S: C 48.82;
H 3.76; N 9.49; S 10.86; found: C 49.06; H 3.77; N 9.51; S 10.67.
2-[(5-Bromofuran-2-yl)methylene]malononitrile (15). 5-Bro-
mofuran-2-carbaldehyde 14 (1.00 g, 5.71 mmol) and malono-
nitrile (0.76 g, 11.4 mmol) were suspended in 40 mL
dichloroethane and degassed. Piperidine (30.0 mL, 0.57 mmol)
was added and degassed again. The solution was heated to
reux for 20 h. The reaction mixture was cooled to room
temperature and the solvent was removed. The residue was
recrystallized from ethanol and pure product 15 (828 mg, 3.71
mmol, 65%) was obtained as a brown crystalline solid. M.p.:
155.1–159.1 ꢁC, 1H NMR (400 MHz, CDCl3): d ¼ 7.41 (s, 1H,
DCV-H), 7.35 (d, 1H, 3J ¼ 3.8 Hz, furan-H4), 6.67 (d, 1H, 3J ¼ 3.8
Hz, furan-H3). 13C NMR (100 MHz, CDCl3): d ¼ 149.88, 141.67,
132.64, 124.84, 113.64, 112.45, 116.81, 77.98. MS (EI): m/z ¼ 224
[M]+. Elemental analysis: calc. (%) for C8H3BrN2O: C 43.08; H
1.36; N 12.56; found: C 43.35; H, 1.43, N: 12.63.
N-Methoxy-N-methylacetamide (18). N,O-Dimethylhydroxyl-
amine hydrochloride 17 (12.0 g, 0.12 mol) hydrochloride was
suspended under nitrogen in 120 mL DCM and cooled to 0 ꢁC.
Triethylamine (35.8 mL, 0.26 mol) and acetyl chloride 16 (9.22
mL, 0.13 mol) were successively added within 45 min, and the
temperature reached 20 C despite ice cooling. Stirring at 0 C
was continued for 1 h and stirring without cooling was
continued for 2 h. 120 mL of water was added to the suspension
and the organic phase was washed with 2 ꢃ 50 mL 1 N HCl, and
with 50 mL sat. NaCl, dried over Na2SO4, and the solvent was
removed. Distillation under reduced pressure gave a colorless
oil of 18 (5.6 g, 50 mmol, 44%). B.p. 50 C (18 mbar), H NMR
(400 MHz, CDCl3): d ¼ 3.60 (s, 3H, O–CH3), 3.17 (s, 3H, N–CH3),
2.12 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): d ¼ 171.74, 60.93,
31.75, 19.62. MS (EI): m/z ¼ 103 [M]+, elemental analysis: calc.
(%) for C4H9NO2: C 46.59; H 8.80; N 13.58; found: C 46.67; H,
8.76, N: 13.58. Analytical data were in accordance with the
literature.29,49
4,5-Dimethylbenzo[2,1-b:3,4-b0]dithiophene (21). 90 mL dry
THF was cooled to ꢀ10 ꢁC and TiCl4 (1.4 mL, 10 mmol) was
added dropwise. Zinc powder (1.6 g, 20 mmol) was added in a
ꢁ
nitrogen atmosphere and the mixture stirred for 2 h at 0 C. A
solution of 1,10-[5,50-bis(trimethylsilyl)-[2,20-bithien]-3,30-diyl]
diethanone 20 (500 mg, 1.27 mmol) in 60 mL dry THF was
added very slowly under reux. Aer addition, reux was
continued for further 12 h. The mixture was cooled to 0 ꢁC and
aqueous 5 wt% NaHCO3 was successively added. THF was
removed under reduced pressure and the mixture was extracted
with DCM. The extract was dried over Na2SO4 and ltered off.
Aer evaporation of the solvent a yellow solid was obtained.
This was dissolved in 20 mL THF and TBAF (1.6 g, 10 mmol) was
added as a solid. Aer stirring for 15 min at room temperature
the reaction was quenched with 25 mL water and THF was
removed. The product was extracted with 3 ꢃ 30 mL DCM, dried
over Na2SO4, and ltered off. The crude product was puried by
column chromatography (ash silica; n-hexane) and white
crystals of 21 (170 mg, 0.78 mmol; 66%) were obtained as a pure
1
ꢁ
product. M.p. 114.6–115.6 C, H NMR (400 MHz, CDCl3): d ¼
7.49 (d, 3J ¼ 5.5 Hz, 2H, Th-H), 7.39 (d, 3J ¼ 5.4 Hz, 2H, Th-H),
2.64 (s, 6H, CH3), 13C NMR (100 MHz, CDCl3): d ¼ 137.95,
130.98, 126.99, 123.67, 123.58, 16.46, MS (EI): m/z ¼ 220 [M +
2H]+; 205 [M ꢀ CH3]+, elemental analysis: calc. (%) for C12H10S2:
C 66.01; H 4.62; S 29.37; found: C 65.95; H 4.74; S 29.19.
2,7-Bis(trimethylstannyl)-4,5-dimethylbenzo[2,1-b:3,4-b']-
dithiophene (22). 4,5-Dimethylbenzo[2,1-b:3,4-b0]dithio-
phene 21 (120 mg, 0.55 mmol) was dissolved in 2.3 mL dry
THF and was degassed three times. The solution was cooled
ꢁ
ꢁ
ꢁ
to ꢀ78 C and n-BuLi (0.76 mL, 1.21 mmol, 1.6 M in hexane)
was added over a few minutes whereupon a white suspension
was formed. The suspension was stirred for 1 h at ꢀ78 ꢁC and
2.5 h at room temperature. Aer cooling to ꢀ78 ꢁC trime-
thylstannyl chloride (241 mg, 1.21 mmol) in 1.5 mL THF
was added in one portion. The mixture was stirred for 1 h
at ꢀ78 ꢁC and at room temperature overnight. 35 mL of
n-hexane were added. The white suspension was washed with
1
ꢁ
4888 | J. Mater. Chem. C, 2014, 2, 4879–4892
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Journal of Materials Chemistry C
3 ꢃ 50 mL water, dried over Na2SO4, and the solvent was hexane) was added over 20 minutes whereupon a grey suspen-
removed in a vacuum. A white solid of 22 was obtained, sion was formed. The suspension was stirred for 1.5 h at ꢀ78 ꢁC
ꢁ
which was dried in a high vacuum. The product (280 mg, 0.48 and 1.5 h at room temperature. Aer cooling to ꢀ78 C again,
mmol, 88%) was used in the next step without further puri- trimethylstannyl chloride (632 mg, 3.20 mmol) in 1.0 mL THF
cation. 1H NMR (400 MHz, CDCl3): d ¼ 7.53 (s with 13C- was added in one portion. The mixture was stirred for 1 h at
1
satellites, JC–H ¼ 28.5 Hz, 2H, Th-H), 2.65 (s, 6H, CH3), 0.45 ꢀ78 ꢁC and at room temperature overnight. A light yellow
2
(s with Sn-satellites, 2JSn119–H ¼ 57.6 Hz, JSn117–H ¼ 55.2 Hz, solution was formed and 35 mL of n-hexane were added. The
18H, Sn–CH3), 13C NMR (100 MHz, CDCl3): d ¼ 138.50, white suspension was washed with 3 ꢃ 50 mL water, dried over
137.10, 135.31, 131.53, 126.09, 16.51, ꢀ8.07.
Na2SO4 and the solvent was removed in a vacuum. The crude
Benzo[2,1-b:3,4-b']dithiene-4,5-diyl diacetate (24). 15 mL of product was dried in high vacuum to obtain 780 mg (1.24 mmol,
dry DCM were added to a tube containing benzo[2,1-b:3,4-b0]- 92%) of a white solid of 26, which was used in the next
1
dithiophene-4,5-dione 23 (269 mg, 1.22 mmol) and Zn dust step without further purication. H NMR (400 MHz, CDCl3):
(0.84 g, 12.8 mmol). Acetic anhydride (1.21 mL, 12.8 mmol) and d ¼ 7.56 (s with 13C-satellites, JC–H ¼ 28.2 Hz, 2H, Th-H), 4.06
1
triethylamine (2.5 mL, 18 mmol) were syringed in. The mixture (s with 13C-satellites, 6H, OCH3), 0.45 (s with Sn-satellites,
was stirred at 25 ꢁC overnight and then ltered through Celite. 2JSn119–H ¼ 57.8 Hz, 2JSn117–H ¼ 55.2 Hz, 18H, Sn–CH3), 13C NMR
The ltrate was washed with 50 mL brine, 50 mL 2 M HCl and (100 MHz, CDCl3): d ¼ 143.48, 138.74, 134.66, 129.47, 61.74 (s),
with 2 ꢃ 50 mL saturated NaHCO3. The organic layer was dried ꢀ8.06 (s).
over Na2SO4, ltered, treated with activated charcoal, and
[3,30-Bis(trimethylstannyl)-[2,20-bithien]-5,50-diyl]bis(trimethyl-
ltered again two times through Celite. Removal of the solvent silane) (27). 3,30-Dibromo-5,50-bis(trimethylsilyl)-2,20-bithiophene
under reduced pressure and further drying under high vacuum 19 (1.03 g, 2.20 mmol) was dissolved in 7.5 mL dry THF and
ꢁ
led to a slightly red crystalline solid. Pure product 24 was degassed ve times. The solution was cooled to ꢀ78 C and n-
obtained (210 mg, 0.69 mmol, 57%) by recrystallization from BuLi (3.01 mL, 4.82 mmol, 1.6 M in hexane) was added over eight
1
ꢁ
ꢁ
ethanol. M.p. 205.1–206.5 C, H NMR (400 MHz, CDCl3): d ¼ minutes. The solution was stirred for 30 minutes at ꢀ78 C and
7.44 (d, 3J ¼ 5.5 Hz, 2H, Th-H), 7.29 (d, 3J ¼ 5.5 Hz, 2H, TH-H), was quenched with trimethylstannyl chloride (961 mg, 4.82
2.44 (s, 6H, –CH3), 13C NMR (100 MHz, CDCl3): d ¼ 168.39, mmol) in 2.0 mL THF. The mixture was stirred for 2 h at ꢀ78 ꢁC
126.10, 121.37, 77.48, 76.84, 20.69, MS (GC): m/z ¼ 306 [M]+, and overnight at room temperature. 15 mL of n-hexane were
elemental analysis: calc. (%) for C14H10O4S2: C 54.89; H 3.29; S added and the organic solution was washed with 4 ꢃ 50 mL
20.93; found: C 54.76; H 3.40; S 21.11. Analytical data were in water, dried over Na2SO4, and the solvent was removed in a
accordance with the literature.50
vacuum. The crude product was dried in high vacuum. Recrys-
4,5-Dimethoxybenzo[2,1-b:3,4-b0]dithiophene (25). Benzo tallization from ethanol led to pure 27 (280 mg, 0.48 mmol, 88%)
[2,1-b:3,4-b0]dithien-4,5-diyl diacetate 24 (400 mg, 1.31 mmol), as a slightly yellow crystalline solid. M.p. 103.9–104.6 ꢁC, 1H NMR
1
cesium carbonate (2.13 g, 6.53 mmol), and methyl iodide (0.4 (400 MHz, CDCl3): d ¼ 7.16 (s with C-satellites, JC–H ¼ 14.6 Hz,
mL, 6.5 mmol) were dissolved in 20 mL acetonitrile. The reac- 2H, Th-H), 0.34 (s with Si-satellites, 2JSi–H ¼ 6.7 Hz, 18H, Si–CH3),
tion mixture was stirred at 75 ꢁC. Aer three days, GC/MS 0.11 (s with Sn-satellites, 2JSn119–H ¼ 56.7 Hz, 2JSn117–H ¼ 54.0 Hz,
analysis showed the presence of intermediate. Therefore, 18H, Sn–CH3), 13C NMR (100 MHz, CDCl3): d ¼ 148.96, 142.32,
methyl iodide (0.4 mL, 6.5 mmol) and cesium carbonate (2.13 g, 141.22, 140.35, 0.25 (s with Si-satellites, 1JSi–C ¼ 50.1 Hz), ꢀ8.04 (s
6.53 mmol) were added and the mixture was heated to 75 ꢁC for with Sn-satellites, 1JSn119–C ¼ 362.1 Hz, 1JSn117–C ¼ 346.1 Hz), MS
further 24 h. GC/MS analysis showed full conversion; the (EI): m/z ¼ 622 [M ꢀ CH3]+, elemental analysis: calc. (%) for
mixture was cooled to room temperature and acetonitrile was C20H38S2Si2Sn2: C 37.76; H 6.02; S 10.08; found: C 37.90; H 6.30; S
removed by rotary evaporation. The residue was partitioned 10.17.
between water and dichloromethane. The organic phase was
Naphtho[2,1-b:3,4-b0]dithiophene (29). 3,30-Di(trimethyl-
washed with 2 ꢃ 100 mL 1 M HCl and then with 100 mL water, stannyl)-5,50-bis(trimethylsilyl)-2,20-bithiophene 27 (752 mg,
dried over N2SO4 and ltered off. Evaporation of the solvent led 1.14 mmol) was dissolved in 11.5 mL DMF and the solution was
to a brown oil. The crude product was puried by column degassed. 1,2-Dibromobenzene 28 (140 mL, 1.14 mmol),
chromatography (ash-silica; n-hexane–DCM 1 : 1). Pure Pd2(dba)3$CHCl3 (60 mg, 58 mmol), and HP(tBu)3BF4 (67.0 mg,
product 25 (282 mg, 1.13 mmol, 86%) was obtained as a white 231 mmol) were added and the mixture was degassed again. The
solid. M.p. 41.0–41.6 ꢁC, 1H NMR (400 MHz, CDCl3): d ¼ 7.52 (d, mixture was heated to 120 ꢁC for 2 days. The mixture was cooled
3
3J ¼ 5.4 Hz, 2H, Th-H), 7.37 (d, J ¼ 5.3 Hz, 2H, Th-H), 4.06 (s, to room temperature, quenched with water, and the product
6H, O–CH3), 13C NMR (100 MHz, CDCl3): d ¼ 144.18, 133.87, was extracted with DCM. The organic solution was washed with
129.52, 124.54, 121.91, 61.78, MS (GC): m/z ¼ 250 [M]+, 2 ꢃ 50 mL sat. NaHCO3, 2 ꢃ 50 mL brine, and with water, dried
elemental analysis: calc. (%) for C12H10O2S2: C 57.57; H 4.03; S over Na2SO4, and ltered off. Evaporation of the solvent under
25.62; found: C 57.75; H 4.13; S 25.83.
reduced pressure led to a slightly yellow oil. The oil was redis-
2,7-Bis(trimethylstannyl)-4,5-dimethoxybenzo[2,1-b:3,4-b']- solved in 10 mL THF and TBAF (0.8 g, 2.5 mmol) was added.
dithiophene (26). 4,5-Dimethoxybenzo[2,1-b:3,4-b0]dithiophene Aer stirring for 15 minutes at room temperature the reaction
25 (345 mg, 1.28 mmol) was dissolved in 4.7 mL dry THF and mixture was quenched with 10 mL of saturated NaHCO3 solu-
was degassed three times. The slightly yellow solution was tion. The product was extracted with 3 ꢃ 30 mL DCM. The
cooled to ꢀ78 ꢁC and n-BuLi (2.0 mL, 3.2 mmol, 1.6 M in organic solution was washed with 2 ꢃ 30 mL saturated NaHCO3
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and water, dried over Na2SO4, and ltered off. Evaporation of n-hexane. Aer drying, oligomer 2 (188 mg, 0.34 mmol, 87%)
the solvent under reduced pressure led to a slightly yellow oil. was obtained as a deep purple solid. The product was further
The crude product was puried by column chromatography puried by column chromatography (ash silica, DCM). M.p.
1
ꢁ
(ash silica; n-hexane–ethyl acetate 50 : 1) and a white crystal- 311 C (DSC). H NMR (500 MHz, DMSO-d6): d ¼ 8.51 (s, 2H,
line solid of 29 (145 mg, 0.60 mmol; 53%) was obtained as a DCV-H), 8.00 (s, 2H, Th-H), 7.95 (s, 2H, Th-H), 7.90 (s, 2H, Ph-H),
pure product. M.p. 166.5–167.6 ꢁC. 1H NMR (400 MHz, CDCl3): 2.98–2.94 (m, 4H, –CH2–), 1.76–1.64 (m, 4H, –CH2–), 1.47–1.40
3
8.42–8.36 (m, 2H, Ph-H), 8.01 (d, J ¼ 5.3 Hz, 2H, Th-H), 7.67– (m, 4H, –CH2–), 0.94 (t, 3J ¼ 7.3 Hz, 6H, –CH3). 13C NMR:
7.60 (m, 2H, Ph-H), 7.52 (d, 3J ¼ 5.3 Hz, 2H, Th-H). 13C NMR (100 solubility not sufficient. MS (MALDI-TOF): m/z ¼ 618.0 [M+]
MHz, CDCl3): d ¼ 134.49, 132.19, 127.77, 125.94, 124.50, 123.93, (calc. for C34H26N4S4: 618.10). Elemental analysis: calc. (%) for
122.96. MS (EI): m/z ¼ 240 [M]+. Elemental analysis: calc. (%) for C34H26N4S4: C, 65.99; H, 4.23; N, 9.05; S, 20.73; found: C, 65.72;
C
14H8S2: C, 69.96; H, 3.36; S, 26.68; found: C, 70.00; H, 3.49; S, H, 4.26; N, 8.97; S, 20.76.
26.40. Analytical data were in agreement with the literature.51
2,20-{[5,50-(Benzo[2,1-b:3,4-b0]dithien-2,7-diyl)bis(3,4-dime-
2,9-Bis(trimethylstannyl)naphtho[2,1-b:3,4-b0]dithiophene thylthien-5,2-diyl)]bis-(methan-1-yl-1-ylidene)}dimalononitrile
(30). Naphtho[2,1-b:3,4-b0]dithiophene 29 (191 mg, 0.79 mmol) (3). A mixture of 2,7-bis(trimethylstannyl)benzo[2,1-b:3,4-b0]-
was dissolved in 4.5 mL of dry THF. The solution was degassed dithiophene 9 (390 mg, 0.73 mmol), 2-[(5-bromo-3,4-dimethly-
three times and was cooled to ꢀ78 ꢁC. n-BuLi (1.10 mL, 1.75 thien-2-yl)methylene]malononitrile 13 (457 mg, 1.71 mmol) and
mmol, 1.6 M in hexane) was added slowly whereupon a grey Pd(PPh3)4 (68 mg, 59 mmol) in 18 mL dry DMF was heated to
suspension was formed. The suspension was stirred for 1.5 h at 80 ꢁC for 15 h. Aer cooling, the resulting precipitate was
ꢀ78 ꢁC and 2.5 h at room temperature. Aer cooling to ꢀ78 ꢁC ltered off and washed several times with methanol and n-
again, trimethylstannyl chloride (396 mg, 1.99 mmol) in 1.1 mL hexane. Aer drying, oligomer 3 (283 mg, 0.60 mmol, 81%) was
THF was added in one portion. The mixture was stirred for 1.5 h obtained as a deep red solid. The product was further puried
at ꢀ78 ꢁC and at room temperature overnight. A light yellow by Soxhlet-extraction with chlorobenzene. M.p. could not be
1
solution was formed. 35 mL of n-hexane were added. The white determined due to the decomposition of 3 before melting. H
suspension was washed with 3 ꢃ 50 mL water, dried over NMR (500 MHz, TCE-d2): d ¼ 7.96 (s, 2H, DCV-H), 7.90 (s, 2H,
Na2SO4, and the solvent was removed. Aer drying in high Th-H), 7.68 (s, 2H, Ph-H), 2.45 (s, 2H, –CH3), 2.35 (s, 2H, –CH3).
vacuum, product 30 (purity: 92% (GC) 457 mg, 0.74 mmol, 94%) 13C NMR: solubility not sufficient. MS (MALDI-TOF): m/z ¼
was obtained as a brownish oil. 1H NMR (400 MHz, CDCl3): d ¼ 562.0 [M+] (calc. for C30H18N4S4: 562.04). Elemental analysis:
8.51–8.47 (m, 2H, Ph-H), 7.12 (s with C-satellites, 1JC–H ¼ 27.35 calc. (%) for C30H18N4S4: C, 64.03; H, 3.22; N, 9.96; S, 22.79;
Hz, 2H, Th-H), 7.67–7.63 (m, 2H, Ph-H), 0.56 (s with Sn-satel- found: C, 64.13; H, 3.25; N, 10.14; S, 22.54.
2
2
lites, JSn119–H ¼ 57.62 Hz, JSn117–H ¼ 55.22 Hz, 18H, Sn–CH3),
the product was used without further purication.
2,20-{[5,50-(Benzo[2,1-b:3,4-b0]dithien-2,7-diyl)bis(furan-5,2-
diyl)]bis(methan-1-yl-1-ylidene)}dimalononitrile (4). A mixture
2,20-{[5,50-(Benzo[2,1-b:3,4-b0]dithien-2,7-diyl)bis(thien-5,2- of 2,7-bis(trimethylstannyl)benzo[2,1-b:3,4-b0]dithiophene
9
diyl)]bis(methan-1-yl-1-yl-idene)}dimalononitrile (1). A mixture (355 mg, 0.67 mmol), 2-((5-bromofuran-2-yl)methylene)malo-
of 2,7-bis(trimethylstannyl)benzo[2,1-b:3,4-b0]dithiophene
nonitrile 15 (350 mg, 1.57 mmol) and Pd(PPh3)4 (42 mg, 36
9
(380 mg, 0.69 mmol), 2-[(5-bromothien-2-yl)methylene]malo- mmol) in 17 mL DMF was heated to 80 ꢁC for 15 h. Aer cooling,
nonitrile 10 (381 mg, 1.60 mmol), and Pd(PPh3)4 (72 mg, 62 the resulting precipitate was ltered off and washed several
mmol) in 18 mL dry DMF was heated to 80 ꢁC for 12 h. Aer times with water, methanol, acetone and n-hexane. Aer drying,
cooling, the resulting precipitate was ltered off and washed oligomer 4 (286 mg, 0.60 mmol, 90%) was obtained as a deep
several times with methanol and n-hexane. Aer drying, olig- purple solid. The product was further puried by Soxhlet-
omer 1 (283 mg, 0.60 mmol, 81%) was obtained as a deep red extraction with chlorobenzene. M.p. 386 ꢁC. 1H NMR (500 MHz,
solid. The product was further puried by gradient vacuum DMSO-d6): d ¼ 8.18 (s, 2H, DCV-H), 8.15 (s, 2H, Th-H), 7.99 (s,
1
3
sublimation. M.p. 350 C. H NMR (500 MHz, DMSO-d6): d ¼ 2H, Ph-H), 7.60 (d, 3J ¼ 3.9 Hz, 2H, Fu-H), 7.43 (d, J ¼ 3.9 Hz,
ꢁ
8.59 (s, 2H, DCV-H), 8.13 (s, 2H, Th-H), 7.98 (d, 3J ¼ 4.1 Hz, 2H, 2H, Fu-H). 13C NMR: solubility not sufficient. MS (CI): m/z ¼ 475
Th-H), 7.95 (s, 2H, Ph-H), 7.75 (d, J ¼ 4.1 Hz, 2H, Th-H). 13C [M + H]+(calc. for C26H10N4O4S2: 474.02). Elemental analysis:
3
NMR (100 MHz, CDCl3): d ¼ 151.51, 146.10, 140.76, 138.34, calc. (%) for C26H10N4O2S4: C, 65.81; H, 2.12; N, 11.81; S, 13.51;
134.38, 133.32, 132.02, 126.56, 124.84, 121.89, 113.72, 113.10, found: C, 65.72; H, 2.20; N, 11.69; S, 13.33.
75.51. MS (CI): m/z ¼ 506 [M]+ (calcd for C26H10N4S4: 505.98).
2,20-{[5,50-(4,5-Dimethylbenzo[2,1-b:3,4-b0]dithien-2,7-diyl)-
Elemental analysis: calc. (%) for C26H10N4S4: C, 61.64; H, 1.99; bis(thien-5,2-diyl)]-bis(methan-1-yl-1-yl-idene)]}dimalononitrile
N, 11.06; S, 25.31; found: C, 61.77; H, 2.18; N, 11.15; S, 25.36.
(5). A mixture of 4,5-dimethylbenzo[2,1-b:3,4-b0]dithien-2,7-diyl-
2,20-{[5,50-(Benzo[2,1-b:3,4-b0]dithien-2,7-diyl)bis(4-butylth- bis(trimethylstannane) 22 (275 mg, 0.51 mmol), 2-[(5-bromo-
ien-5,2-diyl)]bis(methan-1-yl-1-yl-idene)}dimalononitrile (2). A thien-2-yl)methylene]malononitrile 10 (278 mg, 1.16 mmol),
mixture of 2,7-bis(trimethylstannyl)benzo[2,1-b:3,4-b0]dithio- and Pd(PPh3)4 (53 mg, 46 mmol) in 15 mL DMF was heated to
phene 9 (214 mg, 0.39 mmol), 2-((5-bromo-4-butylthien-2-yl) 90 ꢁC for 24 h. Aer cooling, the resulting precipitate was
methylene)malononitrile 12 (252 mg, 0.85 mmol) and Pd(PPh3)4 ltered off and washed several times with methanol, acetone
(44 mg, 38 mmol) in 10 mL dry DMF was heated to 80 ꢁC for 15 h. and n-hexane. Aer drying, oligomer 5 (252 mg, 0.47 mmol,
Aer cooling, the resulting precipitate was ltered off and 93%) was obtained as a black solid. The product was further
washed several times with water, methanol, acetone and puried by gradient vacuum sublimation. NMR spectra cannot
4890 | J. Mater. Chem. C, 2014, 2, 4879–4892
This journal is © The Royal Society of Chemistry 2014

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Paper
Journal of Materials Chemistry C
be recorded due to poor solubility. M.p. 458 ꢁC. MS (MALDI-
5 J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu,
Z. Li, G. He, C. Li, B. Kan, M. Li and Y. Chen, J. Am. Chem.
Soc., 2013, 135, 8484.
6 V. Gupta, A. K. K. Kyaw, D. H. Wang, S. Chand, G. C. Bazan
and A. J. Heeger, Sci. Rep, 2013, 3, 1965.
1
TOF): m/z ¼ 534.1 [M+] (calc. for C28H14N4S4: 534.01). H NMR
and 13C NMR: solubility not sufficient. Elemental analysis: calc.
(%) for C28H14N4S4: C, 62.90; H, 2.64; N, 10.48; S, 23.99; found:
C, 63.13; H, 2.78; N, 10.39; S, 24.17.
2,20-{[5,50-(4,5-Dimethoxybenzo[2,1-b:3,4-b0]dithien-2,7-diyl)-
bis(thien-5,2-diyl)]-bis(methan-1-yl-1-yl-idene)}dimalononitrile
(6). A mixture of 2,7-bis(trimethylstannyl)-4,5-dimethoxybenzo
[2,1-b:3,4-b']dithiophene 26 (750 mg, 0.1.18 mmol), 2-[(5-bro-
mothien-2-yl)methylene]malononitrile 10 (651 mg, 2.72 mmol)
and Pd(PPh3)4 (124 mg, 107 mol) in 37 mL DMF was heated to
7 Y. Liu, C.-C. Chen, Z. Hong, J. Gao, Y. Yang, H. Zhou, L. Dou,
G. Li and Y. Yang, Sci. Rep, 2013, 3, 3356.
8 http://www.heliatek.com and http://www.uni-ulm/nawi/
oc2.de, accessed February 18th, 2014.
9 J. E. Coughlin, Z. B. Henson, G. C. Welch and G. C. Bazan,
Acc. Chem. Res., 2014, 47, 257.
90 ꢁC for 15 h. Aer cooling, the resulting precipitate was 10 W. Wu, Y. Liu and D. Zhu, Chem. Soc. Rev., 2010, 39, 1489.
ltered off and washed several times with methanol, acetone 11 K. Takimiya, S. Shinamura, I. Osaka and E. Miyazaki, Adv.
and n-hexane. Aer drying, oligomer 6 (558 mg, 0.98 mmol,
Mater., 2011, 23, 4347.
´
83%) was obtained as a dark purple solid. The product was 12 A. R. Murphy and J. M. J. Frechet, Chem. Rev., 2007, 107,
ꢁ
further puried by gradient vacuum sublimation. M.p. 404 C.
1066.
1H NMR (500 MHz, DMSO-d6): d ¼ 8.67 (s, 2H, DCV-H), 8.10 13 M. Liu, R. Rieger, C. Li, H. Menges, M. Kastler,
(s, 2H, Th-H), 8.03 (d, 3J ¼ 3.9 Hz, 2H, Th-H), 7.89 (d, 3J ¼ 4.2 Hz,
M. Baumgarten and K. Mullen, ChemSusChem, 2010, 3, 106.
¨
2H, Th-H), 4.13 (s, 6H, –O–CH3). 13C NMR: solubility not suffi- 14 S. Xiao, H. Zhou and W. You, Macromolecules, 2008, 41, 5688.
cient. MS (MALDI-TOF): m/z
C
C
¼
566.3 [M+] (calc. for 15 M. Yuan, A. H. Rice and C. K. Luscombe, J. Polym. Sci., Part A:
28H14N4O2S4: 566.0). Elemental analysis: calc. (%) for Polym. Chem., 2011, 49, 701.
28H14N4O2S4: C, 59.34; H, 2.49; N, 9.89; S, 23.63; found: C, 16 S. Xiao, A. C. Stuart, S. Liu and W. You, ACS Appl. Mater.
Interfaces, 2009, 1, 1613.
2,20-{[5,50-(Naphtho[2,1-b:3,4-b0]dithien-2,9-diyl)bis(thien- 17 H. Zhou, L. Yang, S. Stoneking and W. You, ACS Appl. Mater.
5,2-diyl)]bis(methan-1-yl-1-yl-idene)}dimalononitrile (7). Interfaces, 2010, 2, 1377.
mixture of 2,9-bis(trimethylstannyl)naphtho[2,1-b:3,4-b0]dithio- 18 H. Zhou, L. Yang, S. C. Price, K. J. Knight and W. You, Angew.
phene 30 (420 mg, 0.68 mmol), 2-[(5-bromothien-2-yl)methy- Chem., Int. Ed., 2010, 49, 7992.
lene]malononitrile 10 (376 mg, 1.57 mmol) and Pd(PPh3)4 (40 19 L. Yang, J. R. Tumbleston, H. Zhou, H. Ade and W. You,
mg, 34 mmol) in 30 mL DMF was heated to 90 ꢁC for 24 h. Aer
Energy Environ. Sci., 2013, 6, 316.
cooling, the resulting precipitate was ltered off and washed 20 R. Rieger, D. Beckmann, W. Pisula, W. Steffen, M. Kastler
59.36; H, 2.54; N, 9.77; S, 23.49.
A
¨
and K. Mullen, Adv. Mater., 2010, 22, 83.
several times with methanol, acetone, n-hexane and DCM. Aer
drying, oligomer 7 (363 mg, 0.65 mmol, 96%) was obtained as a 21 R. Rieger, D. Beckmann, A. Mavrinskiy, M. Kastler and
¨
K. Mullen, Chem. Mater., 2010, 22, 5314.
black solid. The product was further puried by gradient
vacuum sublimation. NMR spectra could not be recorded due to 22 Y. Didane, G. H. Mehl, A. Kumagai, N. Yoshimoto, C. Videlot-
poor solubility. M.p. 473 ꢁC. MS (MALDI-TOF): m/z ¼ 556.2 [M+]
Ackermann and H. Brisset, J. Am. Chem. Soc., 2008, 130,
17681.
(calc. for C30H12N4S4: 555.99). 1H NMR and 13C NMR: solubility
not sufficient. Elemental analysis: calc. (%) for C30H12N4S4: C, 23 R. Fitzner, E. Reinold, A. Mishra, E. Mena-Osteritz,
¨
H. Ziehlke, C. Korner, K. Leo, M. Riede, M. Weil,
64.72; H, 2.17; N, 10.06; S, 23.04; found: C, 64.84; H, 2.28; N,
9.96; S, 22.93.
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24 T. Qi, Y. Liu, W. Qiu, H. Zhang, X. Gao, Y. Liu, K. Lu, C. Du,
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Acknowledgements
¨
25 R. Fitzner, C. Elschner, M. Weil, C. Uhrich, C. Korner,
We would like to thank the German Ministry of Education and
Research (BMBF) for nancial support in the frame of project
OPEG 2010.
M. Riede, K. Leo, M. Pfeiffer, E. Reinold, E. Mena-Osteritz
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