Synthesis and characterizations of red/near-IR absorbing A-D-A-D-A-type oligothiophenes containing thienothiadiazole and thienopyrazine central units

Article abstract of DOI:10.1039/c2jm13285k

A series of π-conjugated A-D-A-D-A-type oligothiophenes (1-3) comprising heterocyclic thieno[3,4-c][1,2,5]thiadiazole (TTDA) or thieno[3,4-b]pyrazine (TP) as the core and dicyanovinyl (DCV) or trifluoroacetyl (TFA) as terminal acceptor groups have been de

Full text of DOI:10.1039/c2jm13285k

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PAPER
Synthesis and characterizations of red/near-IR absorbing A–D–A–D–A-type
oligothiophenes containing thienothiadiazole and thienopyrazine central units
a
Simon Steinberger, Amaresh Mishra, Egon Reinold, Elena Mena-Osteritz, Heiko Muller,
a
a
a
a
*
€
b
Christian Uhrich, Martin Pfeiffer and Peter Bauerle
b
a
*
€
Received 13th July 2011, Accepted 23rd November 2011
DOI: 10.1039/c2jm13285k
A series of p-conjugated A–D–A–D–A-type oligothiophenes (1–3) comprising heterocyclic
thieno[3,4-c][1,2,5]thiadiazole (TTDA) or thieno[3,4-b]pyrazine (TP) as the core and dicyanovinyl
(DCV) or trifluoroacetyl (TFA) as terminal acceptor groups have been developed as red/near-IR
absorbers. The resulting oligomers were characterized by UV-Vis spectroscopy, cyclic voltammetry and
thermal analysis. The low energy absorption band of these oligomers was located at 620–707 nm in
solution and at 695–830 nm in thin films. Theoretical studies reveal that the insertion of the bicyclic
nonclassical thiophenes to the conjugated backbone imposed partial quinoidal character to the
resulting oligomers and thus reduced the band gap. HOMO–LUMO energies of 1–3 derived from
electrochemical measurements were found to be suitable for the use as donor material in combination
with fullerene-C60 as acceptor in planar heterojunction solar cells prepared by vacuum deposition. Such
devices incorporating these low band gap oligomers exhibited high FF values up to 0.60, and power
conversion efficiencies of up to 1.3% under air mass (AM) 1.5G illumination. External quantum
efficiency (EQE) spectra clearly showed the contribution of the low energy absorption to the overall
photocurrent.
bulk heterojunction (BHJ) solar cells prepared by co-evapora-
tion of a non-alkylated DCV5T as donor and C60 as acceptor.15
In order to cover an additional part of the solar emission
spectrum and thus harvest more incident photons, it is advan-
tageous to design compounds with strong absorptions in the
visible and near-infrared (NIR) region. Approaches towards
such low band gap materials are the use of strong donor–
acceptor push–pull systems or the incorporation of o-quinoidal
heterocycles in conjugated systems.19,20 In this respect, various
low band gap oligomers and polymers comprising proquinoid
acceptor units such as thienothiadiazole (TTDA)21–23 or thieno-
pyrazine (TP)22,24–26 have been reported.27–29 While TP proved to
be an excellent building block for the development of such
materials,26,30–40 the chemistry and application of nonclassical
bicyclic thieno[3,4-c][1,2,5]thiadiazole (TTDA) have not yet been
explored in detail.21–23,41 In 1969, Bower and Schlessinger first
reported the synthesis of a phenyl-capped TTDA unit by reaction
of 3,4-dibenzoyl-1,2,5-thiadiazole with phosphorus pentasulfide,
showing the longest absorption band at 558 nm in dichloro-
methane.42 Later in 1993, Tanaka and Yamashita synthesized
a thiophene-substituted TTDA unit which absorbed at 618 nm in
solution, resulting in a small band gap of 2.0 eV, being further
reduced to only 0.9 eV (lmax ¼ 934 nm) in the corresponding
polymer.21–23 The design of p-conjugated oligomers or polymers
containing electron-accepting fused-ring systems in the main
chain leads to a strongly enforced quinoidal character of the
ground state concomitant with a reduced band gap. Application
Introduction
During the last decade, the field of organic photovoltaics has
seen a remarkable development from syntheses to device
performance1–3 and provides bright prospects for the construc-
tion of low cost, lightweight and flexible energy sources. In
particular, intensive research on small molecule organic solar
cells (SMOSCs) has been achieved resulting in a remarkable
improvement of the device performance by judicious molecular
design, morphology control and device optimization.4–7
Currently, SMOSCs prepared by vacuum processing achieve
power conversion efficiencies (PCEs) of up to 8.3% using
dicyanovinylene (DCV)-substituted oligothiophenes (DCVnT)
as donors and fullerene-C60 as an acceptor.8 Various donor–
acceptor-substituted oligothiophenes have been developed as
light-harvesting p-type semiconductors due to their good thermal
stability which allows application of vacuum processing as an
effective method for controlling the nano-scale morphology of
thin films, thus improving the device performance.9–17 Planar
heterojunction (PHJ) solar cells based on DCV5T/C60 have been
reported with photovoltages as high as 1.0 V and PCEs of up to
4%.9,11,18 Very recently, the PCE has been improved to 5.2% in
aInstitute of Organic Chemistry II and Advanced Materials, University of
Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany. E-mail:
amaresh.mishra@uni-ulm.de; peter.baeuerle@uni-ulm.de
bheliatek GmbH, Treidlerstraße 3, 01139 Dresden, Germany
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of such narrow band gap polythiophene containing TTDA units
in combination with the fullerene derivative PC61BM in solution-
processed BHJ solar cells gave a very low PCE of only 0.1%.43
Very recently, Mikroyannidis et al. reported a PCE of 1.2% for
TTDA-containing oligomers as donors and PC61BM as
acceptor.44 Various TP containing polymers have been used as
donors in OSCs with PC61BM or PC71BM as acceptor, showing
a PCE of 0.5 to 1.5%.38,39,45–48 However, oligomers including
a TP-moiety for OSCs have not been reported.
hydrate in ethanol (88% yield) and due to its low stability it was
used without further purification. Reaction of amine 6 with thi-
onylaniline and trimethylsilyl chloride in pyridine afforded
trimeric thienothiadiazole 7 in a yield of 70%.
Bromination of terthiophene 7 using NBS followed by a Stille-
type cross-coupling reaction with 2-stannylthiophene afforded
TTDA-containing pentamer 9 in 72% yield (Scheme 1). Vilsme-
ier–Haack formylation of quinquethiophene 9 afforded bis-
aldehyde 10 in a yield of 33%. The use of excess formylating
reagent or prolonged reaction time did not improve the product
yield. Knoevenagel condensation of dialdehyde 10 with malo-
nonitrile and piperidine in 1,2-dichloroethane gave the target A–
D–A–D–A oligothiophene 1 in a yield of 87%.
The objective of this study is to shift the absorption of oligo-
thiophenes to the NIR region by insertion of TTDA or TP units
as additional electron-accepting core moieties into the main
chain. Dicyanovinyl (DCV) and trifluoroacetyl (TFA) groups are
applied as terminal acceptor groups in order to tune the thermal
and electro-optical properties and to assess useful information
about structure–property relationships of the resulting A–D–A–
D–A oligomers. Quantum chemical calculations will be used as
a guide for the analysis of the electronic spectra and to derive
molecular parameters such as ground-state geometry and the
electron density distribution. The application of these materials
in m-i-p (metal-intrinsic-p-doped)-type planar heterojunction
solar cells prepared by vacuum processing is described.
We further introduced TFA acceptor moieties at quinque-
thiophene 9 in order to study the influence of fluorine substitu-
ents on the thermal stability and device performance. TFA-
substituted TTDA-pentamer 2 was prepared in 65% yield by
reaction of quinquethiophene 9 with trifluoroacetic anhydride
under microwave irradiation (Scheme 1).
Synthesis of DCV-terminated thienopyrazine (TP) oligomer 3
is outlined in Scheme 2. Twofold thiophene-substituted thieno
[3,4-b]pyrazine 11 was obtained in 68% yield by condensation of
diaminoterthiophene 6 with benzile in methanol. Bromination of
11 using NBS in chloroform and acetic acid afforded trimer 12 in
85% yield. Boronic ester 14 as a coupling reagent was prepared in
87% yield by protection of thiophene-2-carbaldehyde as dioxo-
lane (13, 63% yield),50 followed by lithiation and subsequent
quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane at ꢀ78 ꢁC. Suzuki-type cross-coupling reaction of
dibromoterthiophene 12 and boronic ester 14 gave quinque-
thiophene 15 in 89% yield. The dioxolane groups were depro-
tected by using catalytic amount of p-toluenesulfonic acid in
THF and water affording dialdehyde 16 in 54%. Finally,
Knoevenagel condensation of pentamer 16 with malononitrile in
Results and discussion
Synthesis
A Stille-type cross-coupling reaction of stannylated diethylth-
iophene 4 (ref. 16) and 2,5-dibromo-3,4-dinitrothiophene49 using
Pd(PPh3)4 as catalyst in dry THF gave dinitroterthiophene 5 in
81% yield (Scheme 1). Reduction of the nitro groups in 5 by
SnCl2/EtOH or Pd/hydrogen was found to be difficult, resulting
in low conversion yields. Finally, transformation of 5 to 30,40-
diaminoterthiophene 6 was performed by Pd/C and hydrazine
Scheme 1 (a) Pd(PPh3)4, THF, reflux, 3 d, 81%; (b) hydrazine hydrate, Pd/C (10%), ethanol, rt, 1 h, 88%; (c) TMS-chloride, pyridine, rt, 1 h, 70%; (d)
NBS, DCM, 0 ꢁC to rt, 1 d, 92%; (e) 2-(tributylstannyl)thiophene, Pd2dba3.CHCl3, tri-2-furylphosphine, THF, reflux, 3 d, 72%; (f) DMF, POCl3, 1,2-
dichloroethane, reflux, 4 h, 32%; (g) malononitrile, piperidine, 1,2-dichloroethane, reflux, 1.5 h, 87%; and (h) trifluoroacetic anhydride, 1,2-dichloro-
ethane, microwave, 120 ꢁC, 4 h, 65%.
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Scheme 2 (a) Benzile, methanol, 60 ꢁC, 4 h, 68%; (b) NBS, acetic acid, chloroform, 2 h, rt, 85%; (c) 14, Pd2dba3.CHCl3, HP(tBu)3BF4, THF/water
(4 : 1), sodium phosphate, 100 ꢁC, 1 h, microwave, 89%; (d) p-toluenesulfonic acid, THF/water, rt, 24 h, 54%; (e) malonitrile, piperidine, 1,2-dichlo-
roethane, rt, 4–6 h, 66%; (f) p-toluenesulfonic acid, toluene, glycol, Dean–Stark apparatus, reflux, 3 h, 63%; and (g): (1) n-BuLi, THF, ꢀ78 ꢁC, 1 h; (2)
ITDB, THF, ꢀ78 ꢁC—rt, 1 h, 87%.
the presence of piperidine furnished DCV-substituted TP-olig-
omer 3 in 66% yield. All compounds were fully characterized by
different spectroscopic techniques and elemental analysis.
were not able to assign the follow-up products of 1 and 3 after
melting on the basis of NMR spectral analysis. Thus, we suspect
that the compounds reacted in the melt and underwent
decomposition.
Thermal properties
Spectroscopic studies
The thermal stability of organic materials is one of the important
parameters for device fabrication. Therefore, thermal properties
of 1–3 were determined by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) under an inert
atmosphere at a heating rate of 10 ꢁC minꢀ1 as depicted in Fig. 1.
TGA analysis revealed that the onset temperatureꢁs with 5%
weight loss (Td) of 1, 2 and 3 are 312, 324 and 365 C, respec-
tively. This result indicates that the thermal stability of the
oligomers could be critical for the vacuum processing during
device fabrication. DCV-substituted TTDA-derivative 1 showed
a moderate sublimation yield (ꢂ20%) since during the sublima-
tion process most of the material was decomposed. To overcome
this thermal instability, the DCV groups were replaced with TFA
units in TTDA-pentamer 2 to lower the sublimation temperature
and hence reduce thermal decomposition.51 The melting
temperatures (Tm) of 1–3 determined by DSC were 306, 240 and
Pentamer 9 incorporating a TTDA unit showed the lowest
energy absorption maximum at 666 nm with two additional
bands at 313 and 390 nm (Fig. 2). The higher energy absorption
bands could be correlated to the extended p-conjugative path in
the quinquethiophene backbone (lmax ¼ 384 nm). The insertion
ꢁ
341 C, respectively. In DSC, DCV-oligomers 1 and 3 showed
a strong exothermic peak immediately after melting, while no
such behavior was observed for TFA derivative 2. It has been
reported in the literature that the non-classical TTDA hetero-
cyclic unit undergoes an intermolecular Diels–Alder reaction
with various dienophiles at high temperature.42,52,53 However, we
Fig. 2 UV-Vis spectra of compounds 9, 10 and 16 measured in
dichloromethane solution.
Fig. 1 (a) TGA thermograms of oligomers 1–3 measured under N2. Conditions: N2 flow, 50 ml minꢀ1; heating rate, 10 ꢁC minꢀ1. (b) DSC trace of all
derivatives measured under Ar flow at a heating rate of 10 ꢁC minꢀ1
.
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of aldehyde units does not significantly affect the lowest energy
absorption band of 10 (669 nm), while, due to the electron
accepting character of the aldehyde groups and extension of the
conjugated p-system, a 57 nm red-shift was observed for the
higher energy absorption band of 10 (447 nm). Similarly, alde-
hyde functionalized TP oligomer 16 showed the lowest energy
absorption maximum at 573 nm with two additional bands at
395 and 338 nm. The lowest energy absorption maximum of 10 is
96 nm red-shifted compared to that of 16, suggesting that the
TTDA unit is a stronger electron acceptor than TP.
absorption transitions between 300 and 520 nm can be assigned
to multiple excitations arising from the combination of the D–A
chromophore in the main p-conjugated chain. In solution, the
optical band gaps (Egopt) for 1–3 estimated from the low energy
absorption onset were 1.47, 1.54 and 1.59 eV, respectively, which
are significantly lower than that of DCV5T (ꢂ2.0 eV).9,14,15
The absorption spectrum of oligomer 1 in thin films was
extended to over 950 nm with a maximum at 830 nm (Fig. 3b). In
addition, the second broad absorption band at 512 nm in the
solid state was more intense than in the solution spectrum (516
nm). However, the absorption maximum of TFA derivative 2 in
thin films was red-shifted by only 23 nm relative to that obtained
in solution. This smaller red-shift of the low energy absorption
band of 2 in thin films was rationalized with the weaker acceptor
strength of the terminal TFA units relative to that of the DCV
groups in oligomer 1. In thin films, TP-oligomer 3 showed an
absorption maximum at 700 nm with an onset extending beyond
850 nm. The strong bathochromic shifts of the low energy
absorption bands of 1 and 3 in thin films compared to the
solution spectra are likely due to strong intermolecular p–p-
stacking in the solid state. Here it should be noted that the strong
red-shift (Dl ¼ 130 nm) of the low energy absorption band of
oligomer 1 compared to that of 3 is due to the stronger acceptor
strength of the central TTDA unit compared to the TP moiety.
The optical band gaps of these oligomers in thin films are in the
range of 1.2 to 1.4 eV.
Fig. 3 depicts absorption spectra of oligomers 1–3 measured in
dichloromethane and in thin films and the data are summarized
in Table 1. Transformation of the aldehyde groups to dicyano-
vinyl (DCV) in 1 and 3 further red-shifted the lowest energy
absorption band to 707 nm for 1 and 620 nm for 3, ascribed to
the stronger electron accepting character of the DCV groups. On
the other hand, a smaller red-shift was observed for the tri-
fluoroacetyl-substituted pentamer 2 (lmax ¼ 675 nm), when
compared to oligomer 9 or dialdehyde 10. The absorption
spectra of 1–3 covered the entire visible spectral region extending
from 300 up to 850 nm. In comparison to DCV5T9,14 (lmax ¼ 507
nm, 3 ¼ 51 000 L molꢀ1 cmꢀ1), pentamers 1–3 incorporating
TTDA or TP units showed a strong red-shift of the longest
wavelength absorption band, while the molar extinction coeffi-
cients of 1–3 were lowered by a factor of nearly two. The lowest
energy absorption maximum of oligomer 1 was red-shifted by 32
nm relative to that of 2 and 88 nm to that of TP-based oligomer
3. The optical absorption data reveal that these oligomers are
characterized by a small HOMO–LUMO separation, which
could be the result of a significant degree of intramolecular
charge transfer from the thienyl donors to the central electron
accepting TTDA or TP unit. Furthermore, the high-energy
We noted light-induced photodegradation of thienopyrazine 3
and further studied this process. It has been demonstrated that
thienopyrazine-substituted compounds rapidly degrade when
exposed to UV light and air at the same time and was speculated
to the possible cleavage of C]N double bonds.54 Fig. 4 depicts
the UV-Vis absorption spectra of 3 in dichloromethane solution
Fig. 3 Absorption spectra of compounds 1–3 in (a) solution and in (b) solid state, 30 nm thin films prepared by vacuum evaporation.
Table 1 UV-Vis data of title compounds 1–3 in dichloromethane solution ([c] ¼ 10ꢀ5 M)
Compound
labs/nm sol.
3a/L molꢀ1 cmꢀ1
Egopt/eV sol.
labs/nm film
Egopt/eV film
9
10
16
1
313, 390, 666
313, 447, 669
338, 395, 573
319, 362, 458 (sh), 516, 707
317, 408 (sh), 483, 675
358, 512 (sh), 620
506
14 400
20 600
16 100
29 000
30 000
31 000
51 100
1.52
1.52
1.73
1.47
1.54
1.59
2.04
512, 830
514, 698
700
1.20
1.39
1.39
1.76
2
3
DCV5Tb
569
a
b
The molar extinction coefficient values are given for the longest wavelength absorption band. Values taken from ref. 14.
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Fig. 4 Absorption spectra of compound 3 in (a) dichloromethane solution ([c] ¼ 10ꢀ5 mol$Lꢀ1) and in (b) solid state, 10 nm thin film prepared by
vacuum evaporation.
and in thin films recorded over time when exposed to air and
sunlight.
The introduction of the aldehyde groups in 10 resulted in a slight
positive shift of the oxidation potentials (DEox ¼ 90 and 130 mV)
when compared to 9 and the appearance of a second reduction
wave at ꢀ1.66 V related to the electron accepting aldehyde
groups. The replacement of the TTDA unit by TP in 16 has no
influence on the oxidation potential, while the reduction poten-
tials shifted to more negative values (DEox ¼ 250 and 100 mV).
This negative shift in the reduction potentials further supports
the weaker acceptor strength of TP when compared to the TTDA
unit.
When exposed to sunlight under ambient atmosphere, the
longest wavelength absorption of 3 at 620 nm rapidly decreased
in solution, while a new absorption band appeared with
a maximum at 420 nm. After 12 hours of irradiation this trans-
formation was completed (Fig. 4a). In the solid state and in the
absence of oxygen, no change in absorption was observed after
one hour of sunlight irradiation. After exposure to ambient
atmosphere, degradation was observed, yet in a significantly
lower rate than in solution. In addition, the change in the
absorption profile in the solid state is characterized by
a moderate decrease of the longest wavelength absorption
together with slight broadening of the absorption band in the
region from 350 to 500 nm and an additional absorption at
around 460 nm, which would correspond to the new band at 420
nm in solution (Fig. 4a). MALDI-TOF and 1H-NMR analyses of
3 after light exposure showed multiple peaks due to photo-
degradation which could not be unequivocally assigned to
possible follow-up products.
The investigated compounds 1–3 each showed two fully
reversible one-electron oxidation waves, which can be assigned to
the oxidation of the oligothiophene backbone. At cathodic
potentials, we observed a reversible wave between ꢀ1.2 and
ꢀ1.36 V (vs. Fc/Fc+) corresponding to a two-electron reduction
of the TTDA/TP units and a quasi-reversible reduction wave
between ꢀ1.86 and ꢀ2.08 V associated with the one-electron
reduction of the DCV and TFA groups, respectively. The results
indicate the formation of stable radical cations, dications, radical
anions, and dianions. Compared to DCV5T, a negative shift of
the oxidation waves and a positive shift of the reduction waves
were observed for these new oligomers.
Electrochemical characterization
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energies of all oligomers
were determined from the onset values of the first oxidation and
reduction waves using the standard approximation that the Fc/
Fc+ HOMO energy level is at ꢀ5.1 eV vs. vacuum.55 These data
are listed in Table 2. The low band gap materials 1–3 studied here
showed strong positive shifts of the reduction potentials and
Fig. 5 depicts cyclic voltammograms of oligomers 1–3 measured
in dichloromethane solution containing 0.1 M TBAPF6 as sup-
porting electrolyte. The electrochemical data are summarized in
Table 2. Pentamer 9 showed a reversible one-electron reduction
associated with the TTDA unit and two reversible one-electron
oxidation processes for the oligothiophene backbone (Fig. 5a).
Fig. 5 Cyclic voltammogram of TTDA derivatives 9, 10 and TP derivative 16 (a) and of pentamers 1, 2 and 3 (b) (1 ꢃ 10ꢀ3 mol$Lꢀ1) in DCM/TBAPF6
(0.1 M).
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Table 2 Summarized cyclovoltammetry data of title compounds 1–3 measured in DCM (HOMO/LUMO vs. Fc/Fc+ ¼ ꢀ5.1 eV)
vac
Compound
EꢁOx1/V
EꢁOx2/V
EꢁRed1/V
EꢁRed2/V
HOMO/eV
LUMO/eV
Eg/eV
9
10
16
1
0.23
0.32
0.34
0.35
0.39
0.36
0.59
0.56
0.69
0.69
0.73
0.78
0.71
0.88
ꢀ1.35
ꢀ1.37
ꢀ1.62
ꢀ1.21
ꢀ1.33
ꢀ1.36
ꢀ1.51
ꢀ5.24
ꢀ5.34
ꢀ5.38
ꢀ5.39
ꢀ5.41
ꢀ5.36
ꢀ5.62
ꢀ3.84
ꢀ3.83
ꢀ3.59
ꢀ3.94
ꢀ3.88
ꢀ3.83
ꢀ3.75
1.40
1.51
1.79
1.45
1.53
1.53
1.87
ꢀ1.66
ꢀ1.76
ꢀ1.91
ꢀ2.08
ꢀ1.86
ꢀ1.98
2
3
DCV5Ta
a
Values taken from ref. 14.
consequently lower LUMO energy levels compared to those of
DCV5T, demonstrating that the TTDA/TP units behave as
strong electron acceptors. It has been reported that upon
attachment of electron-withdrawing moieties at the terminal a-
positions of linear p-conjugated chains, the relative HOMO and
LUMO energies shift toward lower energy values with respect to
the unsubstituted system. The LUMO energies are more affected
than those of the HOMOs and thus the band gaps are low-
ered.15,56 However, the determined orbital energies for 1–3
revealed a different trend. The insertion of o-quinoidal TTDA/
TP units in the core of an oligomer stabilizes the LUMO, but
destabilizes the HOMO energy at the same time, thus reducing
the band gap to a large extent relative to DCV5T (Fig. 6). The
low-lying LUMO energies of these oligomers could lead to
problems in electron transfer to C60.
The ground-state geometry and the electron density distribu-
tion of the HOMO and LUMO of 1–3 have been fully optimized
using density functional theory (DFT) (B3LYP 6-31G*) and
semi-empirical methods (Austin Model 1, AM1). The MO
distributions of the frontier orbitals from the latter method are
shown in Fig. 7 including DCV5T for comparison. It has been
hypothesized that the band gap of a conjugated system can be
reduced either by incorporation of suitable donor and acceptor
units or by lowering the degree of bond-length alternation (BLA)
(defined as the difference between average lengths of C–C single
and double bonds in a conjugated path).58,59 It has also been
proposed that the introduction of appropriate quinoid structures
into the aromatic backbone can minimize the BLA and may
Fig. 7 Electron density distribution of the frontier orbitals in derivatives
1–3 and DCV5T obtained from quantum chemical calculations (carbon
atoms in black, fluorine in light blue, nitrogen in blue, oxygen in red,
sulfur in yellow and hydrogen atoms are omitted for clarity reasons).
reduce the band gap accordingly.58,60,61 The increasing double-
bond character between the repeating units of a conjugated
system can be accomplished by making the quinoidal structure
energetically more favorable.
Yamashita and co-workers reported that after the substitution
of the conjugated systems by terminal acceptor units56 or by
attaching nonclassical heterocyclic cores,23,62 the aromatic char-
acter of the backbone changes to a partially quinoid-like char-
acter in which the degree of BLA was significantly reduced. The
semiempirical analysis of the overall BLA for oligomers 1–3
revealed a strong reduction from aromatic quinquethiophene
ꢀ
ꢀ
ꢀ
(0.039 A) to DCV5T (0.032 A) to a value of 0.021 A for 1 and 2
and 0.023 for 3. Thus, the bicyclic nonclassical thiophene
imposes a partial quinoidal character to the conjugated back-
bone, leading to a more delocalized molecular structure. The
same trend is obtained from DFT calculations. The decrease of
BLA can be directly correlated to the influence of, on the one
hand, the TTDA/TP acceptor moiety and, on the other hand, the
terminal acceptor groups on the extended p-system of the
molecules and is the origin of the red-shifted absorption band
(vide supra). The electron density of the HOMO for all molecules
is distributed not only on the oligothiophene backbone, but also
on the central TTDA/TP-acceptor moiety, while that of the
LUMO is mainly localized on the central acceptor units and the
adjacent thiophene rings. The phenyl rings attached to pyrazine
Fig. 6 HOMO–LUMO energy levels of the investigated oligomers 1–3
deduced from electrochemical data and compared to C60. The HOMO
and LUMO energy values of C60 estimated from the onset of the
oxidation and reduction potentials are taken from ref. 57.
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Table 3 Characteristic photovoltaic data of m-i-p-type planar hetero-
junction solar cells of 1–3 in full sunlight (AM 1.5G, 100 mW cmꢀ2
in 3 are strongly twisted out of plane, hence do not contribute to
the frontier molecular orbital distribution. Although the delo-
calization of the exciton can be favored by the decrease in BLA in
1–3, the localization of the electron density on the central TTDA/
TP unit in the excited state (LUMO) could act as a trap for
excitons. This could further affect the device performance (vide
infra).
)
JSC/mA
cmꢀ2
Compound
VOC/mV
FF
h (%)
1
2
3
0.70
0.77
0.68
2.76
2.89
3.20
0.43
0.48
0.60
0.8
1.1
1.3
Solar cell performance
and low values of JSC and FF. It should be noted that the FF
value obtained with oligomer 3-based devices (FF ¼ 0.60) is
among the highest values reported for small molecule solar cells.
All the studied materials displayed as well moderate Voc values in
the range of 0.68 to 0.77 V. This is due to the fact that VOC is
limited by the difference between the HOMO energy level of the
donor and the LUMO energy level of the acceptor,63,64 restricting
the overall device efficiencies (h), which were determined to be
0.8, 1.1 and 1.3% for oligomers 1–3, respectively. To ensure an
efficient charge separation, it has been suggested that a minimum
LUMO(D)–LUMO(A) offset of about 0.3–0.4 eV is neces-
sary.38,64 The difference between the LUMO of these oligomers
and the LUMO of C60 is only about 0.27–0.16 eV (obtained from
Fig. 6), which indeed appears to be insufficient for efficient
electron transfer at the donor/acceptor interface from the excited
state of these low band gap oligomers. Therefore, only a small
fraction of the absorbed photons could be dissociated into charge
carriers that can reach the electrodes under influence of the
internal electric field. Fig. 8b shows the external quantum effi-
ciency (EQE) spectra of devices based on pentamers 1, 2 and 3.
Compounds 2 and 3 display broad EQE spectra covering from
400 up to 850 nm region, which is extended up to 900 nm for
oligomer 1. The result is also consistent with their absorption
spectrum in thin films, thus showing the contribution of the CT-
band to the EQE and JSC. The lower JSC values obtained with
these devices are also in good agreement with the lower molar
extinction coefficients of oligomers 1–3 and low EQEs over the
entire visible light region.
The HOMO/LUMO energy levels of some of these low band gap
oligomers are suitable for the application of these materials as
donors in organic solar cells when using C60 as electron acceptor.
As the compounds are stable in the solid state, m-i-p-type planar
heterojunction (PHJ) solar cells were fabricated by vacuum
evaporation. Fig. 8 shows J–V curves of SMOSCs using 6 nm
thick layers of 1–3 as donors and C60 as an acceptor. The device
structure with optimized layer sequences consisted of ITO/C60
(15 nm)/1–3 (6 nm)/Di-NPB (5 nm)/Di-NPB:NDP9 (40 nm, 5 wt
%)/ZnPc:NDP9 (10 nm, 3 wt%)/Au (50 nm). By using these
highly optimized device structures, it enables us to investigate the
effect of the donor/active layer on power conversion efficiencies.
In these device structures the hole transport layers (HTLs) have
to be chosen in order to match the energy levels of the active
layers with the Fermi level of the metal electrodes. By using p-
doped HTL N,N0-diphenyl-N,N0-bis(40-(N,N-bis(naphth-1-yl)-
amino)-biphenyl-4-yl)-benzidine (Di-NPB) we can adjust the
HOMO of the HTL to the HOMO of the donor materials
(oligomers 1–3), avoiding energetic barriers and assuring a good
hole transport (due to the doping). For p-doping, NPD9
(Novaled AG, Dresden) was used between the HTL and the Au
top electrode, to facilitate charge extraction. The intrinsic HTL
was introduced to avoid direct contact between the active layer
and the doped HTL that otherwise could lead to quenching of
excitons by the dopants. Furthermore, by using p-doped ZnPc
we assure a good ohmic contact to the Au top electrode. Thus, we
can really analyze the influence of the donor material at the C60
interface in a device that is highly reproducible and with ener-
getically adjusted HOMO levels. Table 3 presents the photo-
voltaic data of the corresponding devices.
Conclusions
All compounds showed moderate short-circuit current density
(JSC) values between 2.6 and 3.2 mA cmꢀ2. This could be ascribed
to the lack of transport of the photo-generated electrons and
holes at the D–A interface, resulting in significant recombination
We have developed novel conjugated A–D–A–D–A-type oligo-
thiophenes with thienothiadiazole/thienopyrazine as core and
dicyanovinylene/trifluoroacetyl as terminal acceptor units. The
Fig. 8 (a) J–V characteristics of m-i-p-type planar heterojunction solar cells under standard AM 1.5G conditions (100 mW cmꢀ2). (b) EQE spectra of
pentamers 1 and 3.
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obtained materials showed strong absorption down to the near
IR-region with narrow band gaps down to 1.5 eV in solution and
about 1.2 eV in thin films. All compounds showed broad
absorption bands ranging from 300 to 850 nm. Substitution of
dicyanovinyl units with electron withdrawing trifluoroacetyl
groups resulted in a highly stable compound, thus reducing the
thermal decomposition of the material during vacuum sublima-
tion. The insertion of TTDA/TP units reduces the bond length
alternation along the p-conjugated backbone and leads to
a more delocalized i.e. partially quinoidal character. The results
show that the lowering of the band gap in these A–D–A–D–A
systems with respect to the standard DCV5T comes not only
from the partial stabilization of the LUMO energy but also from
a similar destabilization of the HOMO energy. Despite their low
molar extinction coefficients, m-i-p-type photovoltaic devices
based on the new materials as donors with fullerene-C60 as the
acceptor showed a high fill factor of up to 0.60 and power
conversion efficiencies (PCE) in the range of 0.8 to 1.3%. The
values reported here represent one of the best performances
among low-band gap conjugated small molecules. The obtained
PCEs can be explained in part due to the lower molar extinction
coefficients of these oligomers and to the low-lying LUMO
energy levels of the donor, which provides an insufficient driving
force for electron transfer from the LUMO of the donor to the
LUMO of the acceptor. Increasing the molar absorptivity in the
low energy region by attaching suitable building blocks and at
the same time maintaining appropriate HOMO/LUMO energy
levels might be a possible way to obtain better JSC and efficiency
values, and work to this end is in progress in our laboratory.
AgCl reference electrode. All potentials were internally refer-
enced to the ferrocene/ferrocenium couple. Tetrahydrofurane
(Merck) was dried under reflux over sodium/benzophenone
(Merck) and dichloromethane (Merck) over calcium hydride
(Merck). All synthetic steps were carried out under argon
atmosphere. N-Bromo succinimide (NBS), NaOH and K2CO3
were purchased from Merck, Pd(PPh3)2Cl2, trifurylphosphine
(TFP) and nBuLi (1.6 mol Lꢀ1 in hexane) from Acros, tri-n-
butyltin chloride and tri-n-methyltin chloride from Aldrich.
Hyperchem (Austin Model 1) and Gaussian 09 (AM1 and
B3LYP/6-31G*) software were used to perform the geometry
optimization for all oligomers.
Solar cell fabrications. N,N0-Di(naphthalen-1-yl)-N,N0-
diphenyl-benzidine (Di-NPB) and zinc phthalocyanine (ZnPc),
doped with proprietary compounds NDP2 or NDP9 of Novaled
GmbH (Dresden), were used as p-doped hole transport layers.
Doping concentrations of Di-NPB and ZnPc were 2–5 wt%. C60
and ZnPc have been provided by Alfa Aesar and Di-NPB by
Sensient Imaging Technologies. Except the oligothiophenes, all
materials were cleaned at least twice by thermal gradient subli-
mation before evaporation. The organic layers and metal
contacts were thermally deposited in an ultra high vacuum
system (‘‘UFO’’, Bestec GmbH) at 10ꢀ8 to 10ꢀ7 mbar through
shadow masks onto semitransparent indium tin oxide (ITO)
coated glass substrates (Thin Film Devices Inc., sheet resistance <
30 U sqꢀ1) without breaking the vacuum. Ceramic or graphite
crucibles were used as sublimation sources (Creaphys GmbH,
Reinhardtsgrimma), temperatures were set by a Eurotherm
controller and film thicknesses were determined by quartz crystal
monitors (Leybold Inficon Inc.). Substrates were cleaned using
detergent, acetone, and ethanol. The active area of the solar cells
was 6.4 mm2. In order to precisely determine the solar cell effi-
ciencies, all three cells were measured with an aperture mask of
2.76 mm2. J–V characteristics were measured using a Source
measurement Unit (Keithley SMU 236) in an N2-filled glovebox
without exposing the samples to air. Illumination was provided
by a sun simulator SOL 1200 (Hoehnle) with an intensity of 127
mW cmꢀ2, which was calibrated using an outdoor reference cell
provided by the Fraunhofer Institute for Solar Energy Systems
(Freiburg). The mismatch between the spectrum of our sun
simulator and the AM1.5 spectrum was taken into account when
calculating the power conversion efficiencies of the solar cells.
External quantum efficiency (EQE) spectra were recorded using
a custom-made setup utilizing a xenon arc lamp, a mono-
chromator (Newport Cornerstone) and a lock-in amplifier (Signal
Recovery 7265 DSP). A silicon reference diode (Hamamatsu
S1337) was used to calibrate the setup. Organic films for spectral
analysis were vapor deposited onto quartz substrates under ultra
Experimental
Instrumentations. NMR spectra were recorded on a Bruker
AMX 500 (1H NMR: 500 MHz, 13C NMR: 125 MHz) or an
Avance 400 spectrometer (1H NMR: 400 MHz, 13C NMR: 100
MHz), unless mentioned otherwise at 25 ꢁC. Chemical shift
values (d) are expressed in parts per million using residual solvent
protons (1H NMR, dH ¼ 7.26 for CDCl3, dH ¼ 5.32 for CD2Cl2,
dH ¼ 5.93 for CD2Cl4; 13C NMR, dC ¼ 77.0 for CDCl3, dC ¼ 53.8
for CD2Cl2, dC ¼ 74.2 for CD2Cl4) as internal standard. The
splitting patterns are designated as follows:
s (singlet),
d (doublet), t (triplet) and m (multiplet). The assignments are Ar–
H (H-atoms of aromatic systems) and Th–H (H-atoms of thio-
phene rings). Melting points were determined using a Bu€chi B-
545 apparatus and were not corrected. Elemental analyses were
performed on an Elementar Vario EL (University of Ulm) and
a Carlo Erba 1104 (University of Stuttgart). Thin layer chro-
matography was carried out on aluminium 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. CI and EI mass spectra
were recorded on a Finnigan MAT SSQ-7000 or a Varian Saturn
2000 GC-MS, MALDI-TOF on a Bruker Daltonics Reflex III.
UV/Vis absorption spectra were recorded in 1 cm cuvettes with
Merck Uvasol grade solvents on a Perkin Elmer Lambda 19
spectrometer. Cyclic voltammetry experiments 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/
high vacuum with deposition rates of about 0.1 A sꢀ1. A
ꢀ
commercial Shimadzu UV-2101/3101 spectrophotometer was
used for absorption measurements. Luminescence spectra were
recorded using a Spex FluoroMAX with separated mono-
chromators for excitation and luminescence light.
3,4:300,400-Tetraethyl-3040-dinitro-2,20:50,200-terthiophene (5). To
a
solution of 2,5-dibromo-3,4-dinitrothiophene (10.0 g,
30.2 mmol) in absolute THF (200 ml) under argon atmosphere
were added tributyl-(3,4-diethyl-thien-2-yl)-stannane (4) (31.0 g,
2708 | J. Mater. Chem., 2012, 22, 2701–2712
This journal is ª The Royal Society of Chemistry 2012

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72.2 mmol) and Pd(Ph3)4 (346 mg, 0.30 mmol). After refluxing
for 3 h, a second portion of catalyst (346 mg) was added and the
reaction mixture was refluxed for another 19 h. The solvent was
evaporated to a large extent and the raw product was precipi-
tated by addition of n-hexane. Subsequent recrystallization from
ethanol, followed by column chromatography (SiO2, petroleum
(t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CDCl3, d ppm) 156.7, 144.8,
140.4, 128.9, 121.0, 112.9, 22.1, 21.3, 14.0, 13.8. MS (CI): m/z ¼
418 (M+, 51%); 419 (M+ + H, 100%). Elemental analysis:
C20H22N2S4: calc. C, 57.38; H, 5.30; N, 6.69%; found C, 57.43;
H, 5.34; N, 6.72%.
ꢁ
ether (40–65 C)/dichloromethane ¼ 3 : 2 v/v, Rf ¼ 0.25) affor-
4,6-Bis(5-bromo-3,4-diethylthien-2-yl)-thieno[3,4-c][1,2,5]thia-
diazole (8). To a solution of terthiophene 7 (654 mg, 1.56 mmol)
in DMF (49 ml) was added dropwise a solution of NBS (612 mg,
3.44 mmol) in DMF (33 ml) at 0 ꢁC. The cooling bath was
removed and the solution was kept stirring at room temperature
until completion of the reaction. The reaction was controlled by
TLC using dichloromethane : n-hexane (1 : 1) as eluent. After 5 h
of stirring, the mixture was dissolved in dichloromethane (150
ml) and washed with water (3 ꢃ 200 ml). The organic phase was
dried over sodium sulfate and the solvent was removed by
vacuum. The product was purified by column chromatography
(SiO2, dichloromethane 1 : 1 n-hexane) to obtained 8 (818 mg,
1.42 mmol) as deep blue crystals in a yield of 92%. M.p.: 130.5–
132 ꢁC. 1H NMR (CDCl3, d ppm) 2.93 (q, 3J ¼ 7.6 Hz, 4H, CH2),
ded 5 (11.08 g, 24.6 mmol) in 81.4% yield as yellow crystals. M.p.:
100–101 ꢁC. 1H NMR (CDCl3, TMS, 400 MHz): d [ppm] ¼ 7.17
3
3
(s, 2H, Th-H), 2.61 (q, J ¼ 7.5 Hz, 4H, CH2), 2.60 (q, J ¼ 7.5
Hz, 4H, CH2), 1.31 (t, 3J ¼ 7.5 Hz, 6H, CH3), 1.11 (t, 3J ¼ 7.5 Hz,
6H, CH3). 13C NMR (CDCl3, d ppm) 146.0, 144.5, 138.5, 135.2,
123.7, 120.5, 22.1, 21.1, 14.6, 13.6; MS (CI): m/z ¼ 450 (M+,
40%); 451 (M+ + H, 60%). Elemental analysis: C20H22N2O4S3:
calc. C, 53.31; H, 4.92; N, 6.22%; found C, 53.25; H, 4.89; N,
6.23%.
3,4,300,400-Tetraethyl-3040-diamino-2,20:500,200-terthiophene (6).
To a suspension of ground 3,4:300,400-tetraethyl-30,40-dinitro-
2,20:50,200-terthiophene 5 (0.90 g, 2.0 mmol) in ethanol (16 ml)
under argon atmosphere were added Pd/C (0.45 g, 10% Pd by
weight) and hydrazine monohydrate (10.0 g, 200 mmol). After
stirring for 2 h, the Pd/C catalyst was removed by filtration
through a Celite pad and washed with dichloromethane (caution:
dry catalyst is pyrophorous). The organic phase was washed with
aqueous ammonium chloride, saturated aqueous sodium bicar-
bonate solution and water. The organic phase was then dried
(MgSO4), filtered, and concentrated by a rotary evaporator. The
crude product was column chromatographed (SiO2, n-hexane/
THF ¼ 2 : 1 v/v; Rf ¼ 0.37) to yield 6 (691 mg, 1.77 mmol, 88.4%)
as a slightly brownish viscous liquid which was pure enough to
3
3
2.65 (q, J ¼ 7.6 Hz, 4H, CH2), 1.27 (t, J ¼ 7.5 Hz, 6H, CH3),
1.20 (t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CDCl3, d ppm) 156.6,
143.6, 143.2, 140.8, 139.9, 129.2, 22.3, 21.8, 14.3, 14.2. MS (CI):
m/z 573.7 [M]+ (calc. for C20H20Br2N2S4: 573.9). Elemental
analysis: calc. for C20H20Br2N2S4: C, 41.67; H, 3.50; N, 4.86%;
found: C, 41.86; H, 3.62; N, 4.84%.
4,6-Bis(3,4-diethyl-2,20-bithien-5-yl)-thieno[3,4-c][1,2,5]thiadia-
zole (9). Dibromo-terthiophene 8 (817 mg, 1.42 mmol), tri-2-
furylphosphine (131.6 mg, 0.57 mmol) and Pd2(dba)3$CHCl3
(73.35 mg, 0.07 mmol) were dissolved in freshly dried THF (140
ml). After stirring at room temperature for 20 min, 2-(tributyl-
stannyl)-thiophene (1.85 g, 4.96 mmol) was added and the
1
use without further purification. H NMR (CDCl3, d ppm) 6.98
(s, 2H, Th-H), 3.47 (broad s, 4H, NH2), 2.61 (q, 3J ¼ 7.5 Hz, 4H,
3
3
ꢁ
CH2), 2.60 (q, J ¼ 7.5 Hz, 4H, CH2), 1.30 (t, J ¼ 7.5 Hz, 6H,
CH3), 1.10 (t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CDCl3, d ppm)
144.3, 144.5, 142.8, 134.5, 128.0, 120.0, 109.4, 22.3, 20.8, 15.0,
13.7. MS (CI): m/z ¼ 390 (M+, 55%); 391 (M+ + H, 100%).
Elemental analysis: C20H26N2S3: calc. C, 61.50; H, 6.71; N,
7.17%; found C, 61.60; H, 6.74; N, 7.06%.
mixture was heated up to 60 C. Stirring was continued at this
temperature for two days. After cooling, aqueous ammonium
chloride solution was added and the mixture was extracted with
dichloromethane (300 ml). The organic phase was dried over
sodium sulfate and the solvent was removed under reduced
pressure. The product was purified by column chromatography
(SiO2, DCM : n-hexane (1 : 1)) to obtain 9 (654 mg, 1.12 mmol)
ꢁ
1
4,6-Di-(3,4-diethylthien-2-yl)-thieno[3,4-c][1,2,5]thiadiazole (7).
a
stirred solution of 3,4:300,400-tetraethyl-30,40-diamino-
in 79% as deep green powder. M.p.: 196 C. H NMR (CDCl3,
d ppm) 7.35 (dd, J ¼ 5.1 Hz and 1.2 Hz, 2H, Th-Hb), 7.24 (dd, 3J
¼ 3.6 Hz and 1.1 Hz, 2H, Th-Hb), 7.11–7.08 (dd, J ¼ 5.1 Hz and
3.6 Hz, 2H, Th-Ha), 3.0 (q, 3J ¼ 7.5 Hz, 4H, CH2), 2.83 (q, 3J ¼
7.5 Hz, 4H, CH2), 1.33 (t, 3J ¼ 7.5 Hz, 6H, CH3), 1.27 (t, 3J ¼ 7.5
Hz, 6H, CH3). 13C NMR (CDCl3, d ppm) 141.5, 141.2, 135.9,
132.5, 130.2, 128.3, 127.5, 126.6, 126.1, 125.6, 99.9, 21.9, 15.3,
14.2; MS (MALDI): m/z 581.9 [M]+ (calc. for C28H26N2S6:
582.0). Elemental analysis: calc. for C28H26N2S6: C, 57.69; H,
4.50; N, 4.81%; found: C, 57.86; H, 4.69; N, 4.63%.
To
2,20:50,200-terthiophene 6 (1.53 g, 3.92 mmol) in dry pyridine (22
ml) under argon atmosphere was added freshly distillated N-
thionylaniline (1.09 g, 7.84 mmol, b.p. 77 ꢁC/15 mbar) via
a syringe. Then, trimethylsilyl chloride was added (2.98 g, 27.4
mmol), resulting in a dark blue color. After 2 h of stirring at
ambient temperature, dichloromethane (250 ml) was added and
the solution was washed with 1 N hydrochloric acid (2 ꢃ 150 ml)
and with water. The organic phase was dried over MgSO4 and
then the solvent was removed under vacuum, yielding the crude
product. The excess of N-thionylaniline was removed by
extraction with n-hexane (100 ml). The solid product was filtered
and recrystallized from n-hexane (300 ml) to give 7 (1.14 g, 2.73
mmol,) in a yield of 70% as a dark blue colored crystal. M.p.: 115
ꢁC. TLC: Rf ¼ 0.53 (n-hexane/THF ¼ 2 : 1 v/v), Rf ¼ 0.74
50,500-(Thieno[3,4-c][1,2,5]thiadiazol-4,6-diyl)bis(30,40-diethyl-
2,20-bithiophene-5-carbalde-hyde) (10). Phosphoryl chloride (0.78
ml, 8.50 mmol) and dimethylformamide (0.66 ml, 8.50 mmol)
were dissolved in 1,2-dichloroethane (8.5 ml) and stirred at room
temperature for two hours. Quinquethiophene 9 (110 mg, 0.19
mmol) was dissolved in 1,2-dichloroethane (6 ml) and the formed
Vilsmeier reagent was added. The mixture was stirred under
1
(dichloromethane/petroleum ether ¼ 2 : 1). H NMR (CDCl3,
d ppm) 7.07 (s, 2H, Th-H), 2.92 (q, 3J ¼ 7.5 Hz, 4H, CH2), 2.64
3
3
ꢁ
(q, J ¼ 7.5 Hz, 4H, CH2), 1.32 (t, J ¼ 7.5 Hz, 6H, CH3), 1.26
This journal is ª The Royal Society of Chemistry 2012
reflux (110–120 C) for 4 h. After cooling, dichloromethane (80
J. Mater. Chem., 2012, 22, 2701–2712 | 2709

View Article Online
ml) and aqueous solution of sodium hydrogen carbonate (240
ml) were added. Stirring was continued overnight at room
temperature. The organic phase was washed several times with
water, dried over sodium sulfate, filtered, and evaporated.
Column chromatography with dichloromethane and ethyl
acetate (9 : 1) as the mobile phase afforded compound 10 (40 mg,
0.06 mmol) in a yield of 33%. The corresponding mono-aldehyde
C32H24F6N2O2S6: C, 49.60; H, 3.12; N, 3.61%; found: C, 49.66; H,
3.08; N, 3.43%.
5,7-Bis(3,4-diethylthien-2-yl)-2,3-diphenylthieno[3,4-b]pyrazine
(11). 3,4,300,400-Tetraethyl-[2,20;50,200]terthiophen-30,40-diamine 6
(4.08 g, 10.4 mmol, 1.0 eq.) and benzile (2.43 g, 11.6 mmol, 1.1 eq.)
were dissolved in methanol (250 ml). The mixture was heated up to
70 ꢁC and stirred for 12 h under argon atmosphere. After cooling
down, the formed precipitate was filtered off, washed with a small
amount of methanol and dried under vacuum. Column chroma-
tography (silica, DCM : n-hexane ¼ 7 : 3) furnished product 11
(4.02 g, 7.1 mmol) as deep red powder in a yield of 68%. M.p.: 168–
169 ꢁC. 1H NMR (CDCl3, 400 MHz) d ppm: 7.58–7.54 (m, 4H, Ar-
H), 7.37–7.27 (m, 6H, Ar-H), 7.12 (ws, 2H, Th-H), 2.96 (q, 3J ¼ 7.6
Hz, 4H, –CH2), 2.65 (dq, J ¼ 7.5 Hz and 1.0 Hz, 4H, –CH2), 1.33
(t, 3J ¼ 7.4 Hz, 6H, CH3), 1.27 (t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR
(CDCl3, 100 MHz) d ppm: 152.3, 144.1, 141.2, 139.0, 137.8, 129.9,
128.7, 128.0, 127.8, 125.4, 121.8, 22.0, 21.3, 14.4, 13.8. MS
(MALDI-TOF): m/z 564.2 [M]+ (calc. for C34H32N2S3: 564.2).
Elemental analysis: calc. (%) for C34H32N2S3: C, 72.30; H, 5.71; N,
4.96; found: C, 72.10; H, 5.77; N, 4.83.
1
was also isolated in a yield of 12%. M.p.: 251 ꢁC. H NMR
(CDCl3, d ppm) 9.91 (s, 2H, OHC), 7.74 (d, 3J ¼ 4.0 Hz, 2H, Th-
3
3
H), 7.37 (d, J ¼ 4.0 Hz, 2H, Th-H), 3.03 (q, J ¼ 7.5 Hz, 4H,
3
3
CH2), 2.91 (q, J ¼ 7.5 Hz, 4H, CH2), 1.35 (t, J ¼ 7.5 Hz, 6H,
CH3), 1.31 (t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CDCl3, d ppm)
206.5, 159.9, 157.1, 145.8, 143.6, 142.5, 142.1, 136.7, 131.5, 130.6,
126.4, 21.9, 21.2, 14.9, 14.0. MS (MALDI): m/z 637.7 [M]+ (calc.
for C30H26N2O2S6: 638.0). Elemental analysis: calc. for
C30H26N2O2S6: C, 56.40; H, 4.10; N, 4.38%; found: C, 56.66; H,
4.32; N, 4.11%.
2,20-(50,500-(Thieno[3,4-c][1,2,5]thiadiazol-4,6-diyl)bis(30,40-
diethyl-2,20-bithien-5,50-diyl))bis-(methan-1-yl-1-ylidene)dimalo-
nonitrile (1). Bis-aldehyde 10 (30 mg, 0.05 mmol), malononitrile
(40 mg, 0.61 mmol) and piperidine (8.0 mg, 0.09 mmol) were
dissolved in 1,2-dichloroethane (10 ml) and stirred at room
5,7-Bis(5-bromo-3,4-diethylthien-2-yl)-2,3-diphenylthieno[3,4-
b]pyrazine (12). Thienopyrazine 11 (888 mg, 1.57 mmol) was
dissolved under heat in chloroform (83 ml) and after cooling
down to 0 ꢁC conc. acetic acid (83 ml) was added. To this
solution, NBS (588 mg, 3.3 mmol, 2.1 eq.) was added in one
portion and the mixture was stirred in darkness at ambient
temperature for one hour. Then the mixture was extracted
carefully with a saturated solution of sodium hydrogen
carbonate and washed several times with water. The organic
phase was dried over sodium sulfate and the solvent was
removed under reduced pressure. Column chromatography
(silica, DCM : n-hexane ¼ 2 : 1) yielded the desired product 12
(965 mg, 1.34 mmol, 85%) as deep blue powder. M.p.: 198–200
ꢁC. 1H NMR (CDCl3, 400 MHz) d ppm: 7.58–7.53 (m, 4H,
ꢁ
temperature for 1 h. The mixture was then heated up to 70 C
and stirred again for another hour. The reaction was controlled
by TLC using dichloromethane and ethyl acetate (9 : 1) as the
mobile phase. After cooling down to room temperature, the
solid product was filtered, washed with n-hexane and dried
under vacuum. Column chromatography on silica using
dichloromethane as the solvent afforded 1 (30 mg, 0.04 mmol,
87%) as deep lilac powder. M.p.: 305 ꢁC. 1H NMR (CD2Cl4, 500
ꢁ
3
MHz, 100 C) d ppm: 7.71 (d, J ¼ 4.2 Hz, 2H, Th-H), 7.70 (s,
2H, Vinyl-H), 7.36 (d, 3J ¼ 4.2 Hz, 2H, Th-H), 3.01 (q, 3J ¼ 7.5
Hz, 4H, CH2), 2.90 (q, 3J ¼ 7.5 Hz, 4H, CH2), 1.33 (t, 3J ¼ 7.5
Hz, 6H, CH3), 1.29 (t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR
(CD2Cl4, 125 MHz, 100 ꢁC) d ppm: 157.50, 149.50, 148.16,
145.09, 143.01, 138.56, 134.53, 132.05, 131.14, 126.98, 120.49,
114.33, 113.57, 113.38, 21.94, 21.51, 14.65, 14.02. MS
(MALDI): m/z 733.7 [M]+ (calc. for C36H26N6S6: 734.1).
Elemental analysis: calc. for C36H26N6S6: C, 58.83; H, 3.57; N,
11.43%; found: C, 58.57; H, 3.58; N, 11.50%.
3
Phenyl), 7.40–7.30 (m, 6H, Phenyl), 2.98 (q, J ¼ 7.6 Hz, 4H,
3
CH2), 2.66 (q, 3J ¼ 7.6 Hz, 4H, CH2), 1.30 (t, J ¼ 7.5 Hz, 6H,
3
CH3), 1.19 (t, J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CDCl3, 100
MHz) d ppm: 152.79, 142.88, 140.76, 138.89, 137.97, 130.07,
129.03, 128.54, 128.13, 124.39, 112.74, 22.37, 21.77, 14.73,
14.19. MS (MALDI-TOF): 720.0 [M]+ (calc. for
C34H30Br2N2S3: 720.0). Elemental analysis: calc. (%) for
C34H30Br2N2S3: C, 56.51; H, 4.18; N, 3.88%; found: C, 56.49;
H, 4.21; N, 3.73.
1,10-(50,500-(Thieno[3,4-c][1,2,5]thiadiazol-4,6-diyl)bis(30,40-diethyl-
2,20-bithien-5,50-diyl))bis-(2,2,2-trifluoroethanone) (2). To a solution
of quinquethiophene 9 (300 mg, 0.51 mmol) in 1,2-dichloroethane
(32 ml) was added trifluoroacetic anhydride (21 ml). The mixture
was irradiated by microwave for four hours at 120 ꢁC. After
completion of the reaction, the solvent was removed under reduced
pressure and the product was purified by column chromatography
(SiO2) eluating with pure dichloromethane. Recrystallization, using
Soxhlet apparatus with 1,2-dichloroethane as solvent, afforded 2
(259 mg, 0.33 mmol, 65%) as dark brown powder. M.p.: 221 ꢁC. 1H
Thien-2-yl-[1,3]dioxolane (13). Thiophene-2-carbaldehyde
(15.5 g, 134 mmol), ethylene glycol (12.58 g, 203 mmol) and
a catalytic amount of p-toluenesulfonic acid (38 mg, 0.20 mmol)
were dissolved in toluene (50 ml). The solution was refluxed and
stirred for 2 hours at 160 ꢁC oil bath temperature and water was
continuously removed by a Dean–Stark apparatus. Reaction
control was performed by gas chromatography (GC), showing
that ethylene glycol was removed in parts with the formed water.
Therefore, more glycol was added dropwise, until complete
turnover was observed by GC. Then the reaction mixture was
washed with aqueous solution of sodium hydrogen carbonate
and water. The organic phase was separated, dried over sodium
sulfate and the solvent was removed under reduced pressure.
ꢁ
NMR (CD2Cl4, 500 MHz, 100 C) d ppm: 7.90–7.85 (m, 2H, Th-
H), 7.32 (d, 3J ¼ 4.2 Hz, 2H, Th-H), 2.99 (q, 3J ¼ 7.5 Hz, 4H, CH2),
2.88 (q, 3J ¼ 7.5 Hz, 4H, CH2), 1.31 (t, 3J ¼ 7.5 Hz, 6H, CH3), 1.28
(t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CD2Cl4, 125 MHz, 100 ꢁC)
d ppm: 157.5, 144.8, 142.9, 136.8, 134.9, 131.6, 131.1, 127.1, 120.5,
113.3, 21.9, 21.4, 14.8, 14.1. MS (MALDI-TOF): m/z 775.0 [M]+
(calc. for C32H24F6N2O2S6: 774.0). Elemental analysis: calc. (%) for
2710 | J. Mater. Chem., 2012, 22, 2701–2712
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Fractional distillation over Vigreux gave the product (13.1 g, 84
mmol) as colorless liquid in a yield of 63%. B.p.: 108–110 ꢁC (14
mbar). 1H NMR (CDCl3, 400 MHz) d ppm: 7.33 (dd, J ¼ 5.0 and
1.3 Hz, 1H, Th-H), 7.18 (ddd, J ¼ 3.5 Hz, 1.2 Hz and 0.7 Hz, 1H,
Th-H), 7.00 (dd, J ¼ 5.0 Hz and 3.5 Hz, 1H, Th-H), 6.11 (s, 1H,
OCH), 4.15–4.06 (m, 2H, CH2), 4.02–3.94 (m, 2H, CH2). 13C
NMR (CDCl3, 100 MHz) d ppm: 126.6, 126.3, 126.2, 100.3, 67.8,
66.8, 65.2. Analytical data are according to the literature.50
50,500-(2,3-Diphenylthieno[3,4-b]pyrazine-5,7-diyl)bis(30,40-
diethyl-2,20-bithiophene-5-carbalde-hyde) (16). Dioxolane 15
(88 mg, 0.1 mmol) was dissolved in THF (8 ml). Water (2 ml) and
a catalytic amount of p-toluenesulfonic acid (10 mg) were added.
The solution was stirred for 12 h at rt. After that, reaction control
by TLC (100% DCM) showed complete turnover and the solvent
was removed under reduced pressure. The solution was then
extracted with DCM and water. The organic phase was dried
over Na2SO4 and the solvent was removed by rotary evapora-
tion. Bis-aldehyde 16 (43 mg, 0.05 mmol) was isolated by column
chromatography on silica (100% DCM) as deep green powder in
2-(5-[1,3]Dioxolan-2-yl-thien-2-yl)-4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolane (14). To a solution of dioxolane 13 (6.0 g, 38.4
mmol) in freshly dried THF (60 ml) was added slowly nBuLi
(25.2 ml of a 1.6 M solution in n-hexane, 40.3 mmol, 1.05 eq.) at
ꢀ78 ꢁC and under argon atmosphere. Stirring was continued for
a yield of 54%. M.p.: 248–250 C. 1H NMR (CDCl3, 400 MHz)
ꢁ
d ppm: 9.91 (s, 2H, OHC), 7.67 (d, 3J ¼ 4.0 Hz, 2H, Th-H), 7.62–
3
7.57 (m, 4H, Ar-H), 7.42–7.31 (m, 8H, Ar-H), 3.06 (q, J ¼ 7.5
ꢁ
3
3
15 minutes at ꢀ78 C. Then the cooling bath was removed and
Hz, 4H, CH2), 2.91 (q, J ¼ 7.5 Hz, 4H, CH2), 1.36 (t, J ¼ 7.5
Hz, 6H, CH3), 1.30 (t, 3J ¼ 7.5 Hz, 6H, CH3). 13C NMR (CDCl3,
100 MHz) d ppm: 182.60, 152.87, 146.49, 143.04, 142.90, 142.38,
138.80, 138.43, 136.86, 132.21, 130.10, 129.69, 129.19, 128.18,
126.22, 124.84, 21.89, 21.14, 15.06, 14.63. MS (HR
MALDI-TOF): 785.1459 [M + H]+ (calc. for C44H36N2O2S5:
784.1453).
the reaction mixture was stirred for 45 minutes. The mixture was
cooled again to ꢀ78 ꢁC and 2-isopropoxy-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (ITDB) (8.62 ml, 42.2 mmol, 1.1 eq.) was
added slowly. The cooling bath was removed and stirring was
continued for two hours. Finally the mixture was extracted with
diethyl ether and aqueous solution of bicarbonate. The organic
phase was separated, dried over sodium sulfate and the solvent
was removed under reduced pressure. The product (9.46 g, 33.5
mmol) crystallized as a wax like yellow solid in a yield of 87%.
The obtained material was pure enough for further syntheses. 1H
NMR (CDCl3, 400 MHz) d ppm: 7.51 (d, 3J ¼ 3.5 Hz, 1H, Th-H),
7.20 (dd, J ¼ 3.5 Hz and 0.5 Hz, 1H, Th-H), 6.17 (s, 1H, OCH),
4.15–4.06 (m, 2H, CH2), 4.06–3.97 (m, 2H, CH2), 1.33 (s, 12H,
CH3). 13C NMR (CDCl3, 100 MHz) d ppm: 149.2, 136.8, 127.1,
100.1, 84.1, 65.1, 24.7. MS (EI): m/z ¼ 282 ([M]+, 100), 195 (50),
124 (45).
2,20-(50,500-(2,3-Diphenylthieno[3,4-b]pyrazine-5,7-diyl)bis(30,40-
diethyl-2,20-bithien-50,5-diyl))-bis(methan-1-yl-1-ylidene)dimalo-
nonitrile (3). Bis-aldehyde 16 (482 mg, 0.61 mmol) was dissolved
in 1,2-dichloroethane (170 ml) and malononitrile (812 mg, 12.3
mmol, 20 eq.) was added at room temperature. Finally piperidine
(0.12 ml, 1.23 mmol) was added and the resulting solution was
stirred at room temperature for twelve hours, after which reac-
tion control by TLC (100% DCM) showed complete conversion.
After extracting the mixture with DCM, the organic phase was
washed with water, dried over Na2SO4 and freed from the solvent
under reduced pressure. To remove most of the side products,
column chromatography on silica with first pure DCM and later
a mixture of DCM and ethyl acetate (9 : 1) was applied. A second
column chromatography (silica, methanol : ethyl acetate ¼ 1 : 1)
was performed to eluate impurities, while the product remained
on the column head. The product was then extracted from silica
with pure DCM. Removal of the solvent under reduced pressure
furnished title compound 3 (31 mg, 0.04 mmol) as dark violet
powder in a yield of 66%. M.p.: 336 ꢁC. 1H NMR (CD2Cl4, 500
5,7-Bis(50-(1,3-dioxolan-2-yl)-3,4-diethyl-2,20-bithien-5-yl)-2,3-
diphenylthieno[3,4-b]pyrazine (15). Dibromo-terthiophene 12
(600 mg, 0.83 mmol), dioxoborolane 14 (773 mg, 2.74 mmol, 3.3
eq.), HP(tBu)3BF4 (77 mg, 0.27 mmol) and Pd2(dba)3$CHCl3 (69
mg, 0.08 mmol) were dissolved in freshly dried THF (50 ml).
Aqueous solution of K3PO4 (2 mol Lꢀ1, 4.25 g in 10 ml of water)
was added and the mixture was irradiated by microwave for 40
minutes at 90 ꢁC. TLC (silica, DCM : cyclohexane : triethyl-
amine ¼ 67 : 30 : 3) showed complete turnover and the mixture
was then extracted with water and DCM. The organic phase was
dried over Na2SO4 and freed from the solvent under reduced
pressure. Column chromatography (silica, DCM : cyclohexane :
triethylamine ¼ 67 : 30 : 3) furnished 15 (58 mg, 0.07 mmol) as
green powder (lilac in solution) in a yield of 89%. M.p.: 183–185
ꢁ
3
MHz, 100 C) d ppm: 7.73 (d, J ¼ 4.2 Hz, 2H, Th-H), 7.70 (s,
2H, Vinyl), 7.60–7.56 (m, 2H, Th-H), 7.39–7.29 (m, 8H, Phenyl),
3
3
3.06 (q, J ¼ 7.6 Hz, 4H, CH2), 2.92 (q, J ¼ 7.6 Hz, 4H, CH2),
1.35 (t, 3J ¼ 7.4 Hz, 6H, CH3), 1.30 (t, 3J ¼ 7.4 Hz, 6H, CH3). MS
(MALDI-TOF): m/z 880.4 [M]+ (calc. for C50H36N6S5: 880.2).
Elemental analysis: calc. (%) for C50H36N6S5: C, 68.15; H, 4.12;
N, 9.45%; found: C, 68.24; H, 4.22; N, 9.60%.
1
ꢁC. H NMR (CD2Cl2, 400 MHz) d ppm: 7.54–7.49 (m, 4H,
3
Phenyl), 7.35–7.25 (m, 6H, Phenyl), 7.10 (dd, J ¼ 3.7 Hz and
0.52 Hz, 2H, Th-H), 7.07 (s, 1H, Th-H), 7.06 (s, 1H, Th-H), 6.07
(s, 2H, C–H), 4.12–4.08 (m, 4H, O–CH2), 3.99–3.95 (m, 4H, O–
CH2), 2.98 (q, 3J ¼ 7.5 Hz, 4H, CH2–CH3), 2.80 (q, 3J ¼ 7.5 Hz,
4H, CH2–CH3), 1.27 (t, 3J ¼ 7.5 Hz, 6H, CH3), 1.22 (t, 3J ¼ 7.5
Hz, 6H, CH3). 13C NMR (CD2Cl2, d ppm) 152.94, 143.39, 142.42,
141.46, 139.52, 138.62, 137.53, 133.34, 130.36, 129.25, 128.40,
127.91, 127.06, 126.00, 125.32, 100.62, 65.71, 22.17, 21.26, 15.64,
14.96. MS (MALDI-TOF): m/z 872.3 [M]+ (calc. for
C48H44N2O4S5: 872.2). Elemental analysis: calc. (%) for
C48H44N2O4S5: C, 66.02; H, 5.08; N, 3.21%; found: C, 66.02; H,
5.19; N, 3.12.
Acknowledgements
We would like to thank the German Ministry of Education and
Research (BMBF) for financial support in the frame of project
OPEG 2010.
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