A-D-A-type oligothiophenes for small molecule organic solar cells: Extending the π-system by introduction of ring-locked double bonds

Article abstract of DOI:10.1002/adfm.201404210

A series of novel acceptor-donor-acceptor oligothiophenes terminally substituted with the 1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC) acceptor has been synthesized. Structural, thermal, optoelectronic, and photovoltaic properties of the π-extended DCCnT

Full text of DOI:10.1002/adfm.201404210

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A–D–A-Type Oligothiophenes for Small Molecule Organic
Solar Cells: Extending the π-System by Introduction of
Ring-Locked Double Bonds
Roland Fitzner, Elena Mena-Osteritz, Karsten Walzer, Martin Pfeiffer, and Peter Bäuerle*
reproducibility.[1–5] In addition to standard
A series of novel acceptor–donor–acceptor oligothiophenes terminally substi-
solution-processing, molecularly defined
tuted with the 1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC) acceptor has
materials allow for solvent-free processing
been synthesized. Structural, thermal, optoelectronic, and photovoltaic prop-
erties of the π-extended DCCnTs (n = 1–4) are characterized and contrasted
to the trends found for the series of parent dicyanovinyl (DCV)-substituted
oligothiophenes DCVnT. The optoelectronic properties reveal the influence of
the additional exocyclic, sterically fixed double bonds in trans-configuration
in the novel DCCnT derivatives. A close correspondence for derivatives with
equal number of double bonds, that is, DCCnTs and DCV(n + 1)Ts, is identi-
fied. Despite having the same energy gap, the energy levels of the frontier
orbitals, HOMO and LUMO, for the DCC-derivatives are raised and more
destabilized due to the aromatization energy of a thiophene ring versus two
exocyclic double bonds indicating improved donor and reduced acceptor
strength. DCC-terthiophenes give good photovoltaic performance as donor
materials in vacuum-processed solar cells (power conversion efficiencies
≤ 4.4%) clearly outperforming all comparable DCV4T derivatives.
by vacuum evaporation techniques.
Systematic structural optimization of the
oligomer donor materials allowed for an
increase in power conversion efficiencies
(PCE) to impressive values of close to 10%
for single junction small molecule organic
solar cells (SMOSC). Chen and co-workers
investigated soluble oligothiophene-based
acceptor–donor–acceptor
(A–D–A)-type
donor materials comprising central benzo-
dithiophene units and could improve PCE
to outstanding 9.95%.[1,6] The groups of
Bazan and Heeger developed highly effi-
cient D1–A–D2–A–D1-type co-oligomers
bearing a central dithienosilole unit leading
to PCEs of up to 9% in solution-processed
bulk heterojunction (BHJ) solar cells.[2,7]
Very recently, Holmes and co-workers
reported an efficiency of 7.9% for a vac-
uum-processed BHJ device using a triarylamine-benzothiadia-
zole-based D–A donor material,[8] which was introduced before
by Wong and co-workers reaching PCEs of 6.8%.[9] Very recently,
Cnops et al. published a vacuum-processed three layer device in a
cascade architecture combining sexithiophene as donor and two
subphthalocyanines as acceptors reaching a remarkable PCE of
8.4%.[10]
1. Introduction
Over the recent years, research in organic solar cells (OSC) has
attracted great attention fueled by the potential for a future large-
scale production of clean and inexpensive organic photovoltaic
devices onto flexible substrates using continuous roll-to-roll
processes. In OSCs, the interface between p- and n-type semi-
conducting organic materials is used to separate the strongly
bound electron–hole pairs, formed upon photoexcitation, into
free charge carriers. Besides the use of π-conjugated polymers
as photoactive components, π-conjugated oligomers, often
denoted as “small molecules,” are gaining increasing popularity
as they offer small batch-to-batch variations providing high
Our group explored the family of acceptor–donor–acceptor
(A–D–A)-type oligothiophenes representing an efficient and
well-investigated family of donor materials for vacuum-pro-
cessed SMOSC. In particular, 2,2′-dicyanovinyl (DCV) as
acceptor proofed to be ideal in order to achieve the optimal
optoelectronic properties in the series of DCVnTs (Scheme 1) .
In the first very promising solar cell application, butyl-substi-
tuted quinquethiophene DCV5T-Bu was deployed in SMOSC
devices yielding a PCE of 3.4%.[11] Meanwhile, this material
class has been extensively explored by the synthesis and char-
acterization of several series of DCVnT oligomers. For example,
by variation of the oligothiophene chain length, important
structure–property relationships have been derived correlating
the number of thiophene units in the conjugated backbone[12]
or the length of the alkyl side chains with optoelectronic prop-
erties and photovoltaic performance.[13] A correlation between
the packing of molecules in the solid state obtained from X-ray
data and photovoltaic performance was established in a series
R. Fitzner, Dr. E. Mena-Osteritz, Prof. P. Bäuerle
Institute of Organic Chemistry II and Advanced Materials
University of Ulm
Albert-Einstein-Allee 11, 89081 Ulm Germany
E-mail: peter.baeuerle@uni-ulm.de
Dr. K. Walzer, Dr. M. Pfeiffer
Heliatek GmbH
Treidlerstrasse 3, 01139 Dresden, Germany
R. Fitzner
Heliatek, GmbH, Sedanstraße 14, 89077 Ulm, Germany
DOI: 10.1002/adfm.201404210
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onset temperatures (Td) estimated from the onset of the exo-
thermic signals in the DSC traces (Figure 1) are compiled in
Table 1. Thermal degradation of the DCCnTs occurred between
344 °C for DCC1T and 375 °C for DCC3T-Me. This indicates a
high, but somewhat reduced thermal stability in comparison
to the alkyl free DCVnTs, which exhibited decomposition onset
temperatures of about 400 °C.[12c] The DCCnT derivatives up to
the trimers sublimed at temperatures between 250 and 280 °C
(p ≈ 10−6 mbar) yielding crystalline material of high purity in
almost quantitative yield. The sublimation of DCC4T occurred
at significantly higher temperatures around the melting point
and led to poor yields due to material decomposition.
Melting temperatures (Tm) of the DCCnTs (Table 1) were
determined from the onset of the endothermic signals in the
DSC traces. All DCCnT derivatives exhibited high melting
points between 290 and 333 °C which is an important require-
ment for efficient sublimation of the materials from the solid
phase. In order to compare and to illustrate trends in the
melting behavior of both oligothiophene series, in Figure 2
the melting temperatures of the DCCnTs and DCVnTs are
plotted versus the number of thiophene rings in the respective
oligomer.
Comparing oligomers with identical oligothiophene cores,
the DCC-substituted derivatives melted at higher tempera-
tures than their DCV-capped analogues. This indicates that
the two additional cyclohexene rings in the molecular struc-
ture of the DCCnTs, which increase the molecular weight
by 132 g mol−1 compared to the DCVnTs, induce additional
intermolecular interactions. The differences in melting points
between DCCnTs and DCVnTs (ΔTm in Table 1) diminished
with increasing oligomer length from ΔTm = 47 °C for the 1T
to 13 °C for the 4T derivatives. As the number of thiophene
units increases, the oligothiophene core dominates the molec-
ular structure and the intermolecular forces. As we have shown
previously for the DCVnT series, the DCCnT oligomers as well
exhibited an odd–even effect of the melting points with respect
to the number of thiophene units. Even-numbered DCVnTs
and DCCnTs melt at significantly higher temperatures com-
pared to their odd-numbered homologues. This trend may
originate from a variance in molecular dipoles caused by dif-
ferences in molecular symmetry between odd- and even-num-
bered oligomers.[10] The methylated oligomers DCC3T-Me,
DCV3T-Me, and DCV5T-Me exhibited higher melting tempera-
tures in comparison to the corresponding alkyl free derivatives
which indicates additional intermolecular interactions induced
by the methyl groups. Compared to their nonalkylated ana-
logues, DCC3T-Me and DCV5T-Me exhibited similar increases
in melting temperature (ΔTm = 15 °C and 14 °C, respectively),
while DCV3T-Me showed a larger enhancement in melting
point (ΔTm = 38 °C).
Scheme 1. Molecular structures of DCC- and DCV-capped oligothio-
phene series DCCnT and DCVnT.
of DCV-capped quaterthiophenes with systematic variation of
the alkyl substituents identifying methyl-substituted DCV4T-Me
as best-performing tetramer (PCE = 3.8%).[14] The optimization
of the positioning of methyl substituents along the conjugated
oligothiophene backbone in a series of quinquethiophenes led
to excellent PCE values of 8.3% in single junction and 9.7%
in multi junction cells, measured for the centrally methylated
derivative.[15]
As the next step on the road to optimized A–D–A-type oli-
gothiophene donor materials, we aspired to enlarge the con-
jugated π-system by introducing additional double bonds
between the oligothiophene core and the electron-withdrawing
DCV units. We endeavored to avoid structural cis/trans isom-
erism and to ensure thermal stability by integrating them
into a cyclohexene ring.[16] In this contribution, we report
on a novel series of A–D–A-type oligothiophenes bearing
1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC) units at both
α-termini (DCCnT). Structural, thermal, optoelectronic, and
photovoltaic properties were characterized and contrasted to
the trends found for the series of dicyanovinyl-substituted oli-
gothiophenes DCVnT, reported earlier (Scheme 1).
2. Results and Discussion
2.1. Synthesis
For the synthesis of the shorter DCCnT derivatives, bromocy-
clohexenone 3 was reacted with bisstannylated thiophene 1 or
bithiopene 2 in Pd-catalyzed Stille-type cross-coupling reactions
to afford diketo derivatives 4 and 5. Subsequent Knoevenagel
condensation with malonitrile using β-alanine as the catalyst
afforded target oligomers DCC1T and DCC2T in good yields
(Scheme 2).
The longer less soluble DCC3T and DCC4T oligomers were
synthesized by Stille-type cross-coupling reactions of DCC-
substituted bromothiophene 9 with stannyl derivatives 1, 2, or
3,4-dimethylthiophene 10 as central building blocks yielding
oligomers DCC3T, DCC4T, or DCC3T-Me, respectively. In order
to obtain building block 9, 2,5-dibromothiophene was mono-
lithiated with n-butyl lithium and quenched with methoxycy-
clohexenone 7 to yield ketone 8. Knoevenagel condensation
with malonitrile provided DCC-substituted bromothiophene
9 as terminal building block.
Maximum solubilities of DCCnTs and DCVnTs in dichlo-
romethane were determined up to the tetramers. The corre-
sponding values are compiled in Table 1 and plotted versus
the number of thiophene units in the oligothiophene back-
bone (Figure 2). Despite having higher melting points, the
DCCnTs displayed enhanced solubility in this solvent com-
pared to their DCV-substituted counterparts. Compared
to the DCV group, this might be a result of enhanced flex-
ibility and greater rotational freedom of the DCC acceptor
2.2. Thermal Properties and Solubility
Differential scanning calorimetry (DSC) was used to investigate
the thermal properties of all DCCnTs. The decomposition
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Scheme 2. Synthesis of DCCnT oligomers.
moiety around the bond to the oligothiophene core. In both
oligomer series, maximum solubilities showed an odd–even
effect with respect to the number of thiophene units, where
oligomers with an even number of thiophenes are less sol-
uble than those containing an odd number. This trend is the
reverse of that found for the melting point temperatures.
Methylated derivatives DCC3T-Me and DCV3T-Me were less
soluble in dichloromethane compared to the corresponding
alkyl free derivatives which is in accordance with the trend
observed in melting points. Compared to the extremely
insoluble alkyl free or methylated DCVnTs, the solubilizing
effect of the DCC acceptor moieties might be still not suf-
ficient to enable a reliable fabrication of organic solar cells by
solution-processing.
2.3. X-Ray Structure Analysis
Single crystals of DCC3T-Me were obtained by vacuum gra-
dient sublimation. X-ray structure analysis provided valu-
able information about molecular geometry and packing and
revealed notable similarities to the crystal structure of methyl-
ated DCV-capped quinquethiophene DCV5T-Me, which has the
same substitution pattern, the same number of rings, and was
reported earlier,[15a] including efficient π–π-stacking of the oli-
gothiophene cores. The molecular structures of both oligomers
will be compared, followed by an analysis of the crystal packing
and intermolecular interactions. The crystals of DCC3T-Me
and DCV5T-Me belong to the monoclinic space group C2/c
with two molecules in the unit cell. The molecular structures
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Figure 1. DSC traces of DCCnTs measured under argon flow at a heating
rate of 10 °C min−1
.
of both oligomers display C2 symmetry with the thiophene
rings in an all-anti-conformation and the DCC or DCV units
adopting syn-conformations with respect to the sulfur atoms of
the terminal thiophene rings (Figure 3). The terminal double
bonds of the acceptor units take very similar orientations with
respect to the long molecular axis forming angles of 118° and
114° for DCC3T-Me and DCV5T-Me, respectively. Both oli-
gomers displayed almost planar oligothiophene backbones,
whereas the inter-ring dihedral angles are particularly small for
DCC3T-Me (0.7°) compared to DCV5T-Me (5°, 7°). The DCC
cyclohexene rings in the DCC3T-Me structure adopt half-chair
conformation, in which the ring double bond is nearly coplanar
with the adjacent thiophene ring (2.4°). In both structures,
the dicyanomethylene moieties are significantly twisted out
of the oligothiophene plane by 24° and 8° for DCC3T-Me and
DCV5T-Me, respectively. This twist appears to be an effect of
the particular molecular packing behavior observed for both oli-
gomers, vide infra.
In Figure 4 the crystal packing of DCC3T-Me is presented
and compared to the very similar molecular arrangement
observed for DCV5T-Me.[15] A common feature is the antipar-
allel alignment of the molecules on their long molecular
axes, giving rise to the formation of ribbon-like structures
(Figure 4a). This arrangement is favored by electrostatic CH…
NC interactions between the terminal acceptor units of neigh-
boring molecules (blue lines in Figure 4a and Table 2). In both
crystal structures, adjacent molecular ribbons are offset from
Figure 2. Melting points (top) and maximum solubilities in dichlo-
romethane at room temperature (bottom) of DCCnTs and DCVnTs versus
the number of thiophene units in the oligothiophene backbone.
co-planarity by half the π–π-stacking distance (see side views in
Figure 4b). The ribbons stabilize by CH…NC quasi hydrogen-
bonding interactions between vinylic cyclohexenyl hydrogens
and cyano nitrogen atoms (red lines in Figure 4a, top).
The DCV5T-Me crystal structure contains similar interac-
tions between cyano nitrogen atoms and thienyl hydrogen
atoms (red lines in Figure 4a, bottom).
Figure 4c depicts the molecular packing in the π–π-stacking
direction. Both oligomers form slipped π–π-stack assemblies
with slip angles of 41° or 57°, for DCC3T-Me and DCV5T-Me,
respectively. The π–π-stacking molecular distances of 3.46 Å in
DCC3T-Me are identical to the value obtained for DCV5T-Me
(3.46 Å). Thus, the introduction of the DCC rings in the DCC3T-
Me structure does not impair efficient π–π-interactions. Besides
that, CH…π intermolecular interactions involving methyl- or
cyclohexene hydrogens appear in both crystal structures (green
and orange lines in Figure 4c). As a measure for the overall
packing density, the Kitaigorodskii packing coefficients CK were
determined for both crystal structures by dividing the occu-
pied molecular van der Waals volume by the total volume of
the unit cell. Comparing DCC3T-Me and DCV5T-Me, the DCC-
substituted oligomer exhibited a somewhat larger coefficient
Table 1. Thermal properties and maximum solubilities of DCCnTs and DCVnTs in dichloromethane.
Oligomer
Tm
[°C]
Td (DSC)
[°C]
Solubility
Oligomer
Tm
[°C]
Solubility
ΔTm
[°C]
[mg mL−1
]
[mg mL−1
]
DCC1T
319
331
290
305
333
343
362
340
375
352
2.6
DCV1T
DCV2T
272
300
265
303
320
1.8
47
31
25
2
DCC2T
0.42
0.16
DCC3T
3.3
DCV3T
0.67
DCC3T-Me
DCC4T
3.0
DCV3T-Me
DCV4T
0.50
1.6 × 10−2
8.1 × 10−3
13
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compiled in Table 3. All DCCnTs showed
broad absorption profiles in the visible
region. The absorption bands exhibited
bathochromic shifts and increasing molar
absorptivity with growing number of thio-
phene units indicating efficient conjugation
throughout the π-system. The maximum
abs
absorption wavelengths λmax and the cor-
responding extinction coefficients εmax of
the DCCnTs coincide well with the values
determined for the DCVnT compounds
Figure 3. ORTEP diagrams (50% ellipsoids) of DCC3T-Me (left) and DCV5T-Me (right).
(0.709) than the DCV-capped derivative (0.674) indicating more
efficient 3D packing. In consideration of identical π–π-stacking
distances, this finding supports the closer molecular packing
observed between the ribbons in the DCC3T-Me crystal struc-
ture (7.54 Å vs 7.95 Å for DCV5T-Me, Figure 4a).
Figure 5 highlights the alignment of adjacent DCC units
in the DCC3T-Me crystal structure. Neighboring DCC groups
form a linear zipper-like network, which is brought about by
the two types of centrosymmetric interaction motifs (blue and
red lines) described above. The well polarizable vinyl-hydrogens
of the DCC moiety stabilize the arrangement by nonclassical
hydrogen bond interactions.
comprising one additional thiophene unit, respectively. The
bathochromic shift caused by the two additional cyclohexene
double bonds in the DCCnTs is therefore equivalent to adding
one extra thiophene unit in the DCVnT series. DCVnT spectra
showed increasing vibronic structure with decreasing number
of thiophene units. DCCnT π–π* absorption bands, in con-
trast, are structureless even for the shortest oligomer DCC1T.
Compared to the DCV group, this may be attributed to a lower
energy barrier of rotation of the DCC unit around the bond to
the oligothiophene backbone. The DCVnTs exhibit attractive
intramolecular interactions between the sulfur atoms of the
terminal thiophene rings and a DCV cyano group hindering
rotation of the acceptor groups.[12c] Owing to the reduced con-
jugation of the oligothiophene core in solution, methylated
DCC3T-Me displayed a slightly blue-shifted (Δλmax = 10 nm)
and broadened absorption band with reduced molecular absorp-
tivity compared to nonalkylated DCC3T. This is consistent with
the behavior of alkyl substituted DCVnT derivatives.
2.4. Optoelectronic Properties
Optical absorption spectra of the DCCnT series determined
in dichloromethane solution are presented in comparison to
the DCVnT spectra in Figure 6. The corresponding values are
Figure 4. View of the crystal packing of DCC3T-Me and DCV5T-Me in a) the molecular plane, b) end view of the ribbone-like structures, and c) view
perpendicular to the molecular plane showing the π–π-stacking of the molecules.
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Table 2. Distances corresponding to intermolecular close CH···NC con-
tacts in the crystal structures of DCC3T-Me and DCV5T-Me.
Oligomer
Close contacts (color code in Figure 8)
N1···H11A (blue)
d [Å]
2.673
2.598
2.553
2.709
2.589
d–dvdW [Å]
−0.077
−0.152
−0.197
−0.041
−0.161
DCC3T-Me
N2···H13 (red)
DCV5T-Me
N2···H10 (blue)
N2···H8 (blue)
N1···H7 (red)
Emission spectra of the DCCnTs and DCVnTs measured in
dichloromethane solution are depicted in Figure 7. The emis-
sion maxima shifted bathochromically upon elongation of
the π-system (Table 3). Unlike in the absorption spectra, the
emission bands of the shorter DCCnTs up to DCC3T showed
vibronic structure which indicates a higher degree of struc-
tural rigidity and planarity of the molecules in the electronically
excited state compared to the ground state. Compared to alkyl-
free DCC3T, the emission band of methyl-substituted DCC3T-
Me is less structured which may be due to increased rotational
disorder in the oligothiophene backbone. Within the DCVnT
series, a gradual increase in Stokes shift with increasing
number of thiophene units from 320 cm−1 for DCV1T to
1803 cm−1 DCV6T was determined. However, all nonalkylated
DCCnTs displayed similar and significant Stokes shifts between
1282 cm−1 for DCC2T and 1547 cm−1 for DCC4T suggesting
substantial differences in the electron distribution and molec-
ular dipole moment between the ground state and excited
state. Alkylated DCC3T-Me displayed the largest Stokes shift of
1883 cm−1 within the DCCnT series.
Figure 6. Absorption spectra of DCCnT (bottom) and DCVnT oligomers
(top) measured in dichloromethane at room temperature.
Figure 8 shows absorption and emission spectra of DCC3T
and DCC3T-Me in thin films of equal thickness fabricated by
vacuum-deposition. Compared to solution spectra, absorption
and emission bands are red-shifted and significantly broad-
ened in thin films for both oligomers indicating planarization
and favorable ordering of the molecules in the bulk. Absorp-
tion profiles of the terthiophene derivatives are very similar
with maximum optical densities of 0.46 and 0.45 at 544 and
547 nm for DCC3T and DCC3T-Me, respectively. Compared
series determined from the onset of absorption in solu-
tion spectra diminished from 2.33 eV for DCC1T to 2.03 eV
for DCC4T. In thin films, the optical gaps of the terthio-
opt
phene derivatives DCC3T and DCC3T-Me (Eg = 1.79 eV)
are reduced compared to the solution by 0.26 and 0.28 eV,
respectively, which is comparable to the shifts determined
for the DCVnT derivatives (ΔEg = 0.33–0.35 eV).[12c] Cyclic
voltammograms of the DCCnT oligomers were determined in
0.1 molar solutions of tetrabutyl ammonium hexafluorophos-
phate (TBAPF6) in dichloromethane and are displayed in
Figure 9a. The corresponding data are compiled in Table 3.
DCC1T and DCC2T showed a single reversible oxidation wave
within the accessible electrochemical window indicating the
formation of stable radical cations. For the longer oligomers
DCC3T, DCC3T-Me, and DCC4T two reversible one-electron
oxidation processes were detected corresponding to the genera-
tion of stable radical cations and dications. The first oxidation
potential is gradually shifted to lower values with the exten-
sion of the π-conjugated system from DCC1T (EOx1 = 1.24 V)
to DCC4T (EOx1 = 0.63 V) concomitant with the increased delo-
calization of the radical cations. All nonalkylated DCCnTs dis-
played reversible reduction signals, which are attributed to the
generation of stable radical anions localized on each acceptor
group. In comparison to the DCVnT oligomers, which exhibit
only quasi-reversible reduction waves, the DCC acceptor moiety
further stabilizes radical anionic species. In contrast to the
effect seen in the oxidation, the reduction potentials are less
em
to DCC3T (λmax = 736 nm), methylated DCC3T-Me showed
a slightly increased Stokes shift in the thin film with an emis-
sion maximum of 766 nm. The optical gaps of the DCCnT
Figure 5. Intermolecular CN···HC close contacts between adjacent DCC
groups in the crystal structure DCC3T-Me (side-view along the molecular
axis in inset).
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Table 3. Optoelectronic data of the DCCnT oligomers.
opt a)
c)
c)
CV
abs
em
Oligomer
Eg
Stokes
EOx
[V]
ERed
[V]
EHomo
[eV]
ELUMO
[eV]
Eg
λmax
[nm]
εmax
λmax
shiftb) [cm−1
]
[eV]
2.37
2.17
2.00
2.05
1.96
[L mol−1 cm−1
]
[nm]
587 (547)
595 (622)
624 (660)
651
[eV]
2.33
2.15
2.07
2.05
2.03
DCC1T
463
500
519
509
525
42 100
56 400
64 400
56 200
69 300
1517
1282
1518
1883
1547
1.24
−1.21
−1.29
−1.43
−1.47
−1.46
−6.31
−6.01
−5.71
−5.69
−5.64
−3.94
−3.84
−3.71
−3.64
−3.68
DCC2T
0.98
DCC3T
0.68, 1.08
0.63, 1.05
0.63, 0.93
DCC3T-Me
DCC4T
669
a)Estimated using the onset of absorption in solution spectra; b)Calculated as difference between the 0–0 vibronic transitions of absorption and emission spectra, deter-
mined by mathematical Gaussian deconvolution; c)Calculated from onset values of the first oxidation and reduction waves setting the ferrocene HOMO energy to −5.1 eV
versus vacuum.
affected by the enlargement of the π-system and ERed decreased
only from −1.21 V for DCC1T to −1.46 V for DCC4T because
the negative charges are rather localized on the electron-with-
drawing DCC acceptor moieties.
HOMO and LUMO frontier orbital energies were calculated
from the onset values of the first oxidation and reduction waves
setting the ferrocene HOMO energy to −5.1 eV versus vacuum
(Table 3). As a result of the increase in HOMO energies with
growing number of thiophene units, the electrochemical gaps
decreased from 2.37 eV for DCC1T to 1.96 eV for DCC4T. In
Figure 8. Absorbance and emission spectra of DCC3T and DCC3T-
Me in 30 nm thick films fabricated by vacuum-deposition onto quartz
substrates.
Figure 9b, frontier orbital energies of DCCnTs and DCVnTs are
plotted versus the number of double bonds in the oligomers.
Comparing both oligomer series, derivatives with equal num-
bers of double bonds showed very similar HOMO/LUMO
energies confirming the trend seen already for the maximum
absorption wavelengths λmaxabs. Regarding the potential of the
oligomers as donor materials in organic solar cells, the LUMO
energy levels of DCC3T (−3.71 eV), DCC3T-Me (−3.64 eV),
and DCC4T (−3.68 eV) should be sufficiently high to provide a
driving force for efficient electron transfer to the LUMO of the
acceptor C60 (≈−4.1 eV).
Density functional theory (DFT) calculations [m062x
(6–31g+(d))] have been performed on both series of A–D–A oli-
gothiophenes. Figure 10 displays the electronic distribution of
the frontier orbitals of DCC-3T and DCV4T as examples, which
comprise the same number of double bonds. The HOMO of
both derivatives shows a delocalization of the electron density
over the donor part up to the vinylic carbon adjacent to the
terminal dicyanomethylene groups (labeled by arrows). The
delocalization of the LUMO expands over the entire DCC3T
and DCV4T π-conjugated path. The LUMO+1 showed a pro-
nounced shift of the electronic density to the acceptor parts on
both derivatives. This virtual energetically high lying orbital
Figure 7. Emission spectra of DCCnT (bottom) and DCVnT oligomers
(top) measured in dichloromethane at room temperature.
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strength related to the DCV-derivatives. By keeping the acceptor
groups unaffected, the lengthening of the π-conjugated system
by two double bonds (DCV3T ð DCC3T) causes a significant
raising of the HOMO energy level by 0.38 eV compared to
the effect of the lengthening by one thiophene unit (DCV3T ð
DCV4T), which amounts to 0.24 eV. We can explain this dif-
ference (0.14 eV) by the more effective aromatic stabiliza-
tion in a thiophene ring with regard to two exocyclic double
bonds. In contrast to the nearly unaffected LUMO energy
levels in the DCV-series, the LUMO of DCC3T is raised by
0.16–0.19 eV, which can be ascribed as well to the loss of aro-
matization energy.
2.5. Photovoltaic Properties
To evaluate the photovoltaic properties of DCC3T and DCC3T-
Me as donor in combination with C60 as acceptor, planar het-
erojunction (PHJ) organic solar cells have been fabricated by
thermal vacuum deposition. For both devices, the following
layer sequence was employed: 15 nm C60 evaporated on
ITO-coated glass, 6 nm donor material, 10 nm of undoped
9,9-bis{4-[di-(p-biphenyl)aminophenyl]}fluorene (BPAPF), fol-
lowed by 45 nm BPAPF p-doped with NDP9 as hole transport
layer (HTL), 1 nm NDP9, and 100 nm aluminum as top elec-
trode. Figure 12 displays the J–V characteristics (Figure 12a)
and the external quantum efficiency (EQE) spectra (Figure 12b)
in comparison to the absorbance spectra of the donors in
vacuum-deposited neat films (Figure 12c). The corresponding
device parameters are summarized in Table 4. PHJ cells of
both, DCC3T and DCC3T-Me, showed excellent open circuit
voltages (VOC) of 1.00 V for DCC3T and an increased value of
1.05 V for DCC3T-Me which can be attributed to the low lying
HOMO energies of the donor oligomers. Similar short circuit
current densities (JSC) of 5.3 and 5.0 mA cm−2 were determined
for the DCC3T and DCC3T-Me devices, respectively. This is in
accordance with very similar EQE spectra measured for both
materials, which correlate well with thin film absorption pro-
files. The DCC3T-Me device showed an improved saturation
in current density of 1.08, defined as J( −1V )/JSC, and a con-
siderably increased fill factor (FF) of 65% compared to values
of 1.11 and 56%, respectively, for DCC3T devices. In total, the
DCC3T-Me PHJ device yielded PCE of 3.4% outperforming
the DCC3T cell with a PCE of 3.0%. We have as well seen
this trend for methylated A–D–A quinquethiophenes, which
gave an improved performance compared to all other substi-
tuted derivatives coming from special intermolecular interac-
tions and thus optimal packing of the oligomers in the solid
state.[15a] If we compare these results with PHJ solar cells made
from DCV4T, which comprises the same number of double
bonds and conjugation length as the novel DCC3T-derivatives,
the DCC-terthiophenes clearly surpass the quaterthiophene
DCV4T in all photovoltaic parameters, which showed a PCE of
only 1.2% under comparable conditions (Table 4). At that point
it is noteworthy, that typically terthiophenes such as DCV3T
and C60 did not lead to an efficient exciton separation into
charge carrier pairs at the interface, because of the low HOMO
offset between the two materials which leads to poor solar cell
performance.[12a]
Figure 9. a) Cyclic voltammograms of the DCCnT oligomer series meas-
ured in dichloromethane and TBAPF6 (0.1 M) as electrolyte, scan rates
were 100 mV s−1 and potentials are given versus Fc/Fc+. b) Frontier
orbital energies of DCCnTs and DCVnTs derived from electrochemical
data versus the number of double bonds in the oligomers.
can be stabilized experimentally by the presence of solvent and
therefore can be responsible for the charge transfer character of
the electronic transition.
The analysis of the calculated molecular orbital energies
(Table S5, Supporting Information) supports as well the trends
found in the electrochemical studies. In Figure 11 the experi-
mentally determined frontier orbital energies are displayed
elucidating the effect of the extrinsic double bonds in DCC3T:
Compared to the DCV-derivatives DCV3T and DCV4T, the
HOMO and the LUMO are lifted up in energy. However, for
the derivatives with equal numbers of double bonds (DCC3T
vs DCV4T), the band gap comes out nearly equal (2.0 eV and
1.98 eV), and both are as expected significantly smaller than
this of DCV3T (2.19 eV), which corresponds to the findings in
optical spectroscopy.
The destabilization of the HOMO and LUMO energies in
DCC3T denotes an improved donor and reduced acceptor
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the best one, which is methylated DCV4T-Me
giving a PCE of 3.8% at a Voc of 0.98 V, a Jsc
of 6.5 mA cm−2, and a fill factor of 59%.[14]
3. Conclusion
In summary, a series of novel A–D–A oli-
gothiophenes, from the mono- to the quar-
terthiophene, terminally substituted with
the DCC acceptor has been synthesized.
Structural, thermal, optoelectronic, and
photovoltaic properties of the π-extended
DCCnTs were characterized and contrasted
to the trends found for the series of dicy-
anovinyl (DCV)-substituted oligothiophenes
DCVnT. We were able to bring out the influ-
ence of additional exocyclic, sterically fixed
in trans-configuration, located between the
acceptor and donor units in such A–D–A oli-
gothiophenes. The direct comparison of the
two series showed that (1) the DCCnTs had
increased melting temperatures and signifi-
Figure 10. Representative electron density distribution of the frontier orbitals of DCC3T and
DCV4T obtained by DFT-calculations (carbons in black, sulphurs in yellow, nitrogens in blue,
and hydrogens in white).
DCC3T-Me as the best performing A–D–A oligomer in PHJs
was additionally tested in BHJ solar cells. In the BHJ device
stack, the neat donor layer employed in the PHJ setup was
replaced by a 20 nm thick blend layer of donor and acceptor fab-
ricated by co-evaporation of DCC3T-Me and C60 in a ratio of 2:1
by volume. During the deposition of the photoactive layer the
substrate was heated to 90 °C. Despite the VOC of the BHJ solar
cells was slightly reduced to 0.99 V and the fill factor dropped
to 53% compared to the PHJ device, a strongly enhanced JSC of
8.5 mA cm−2 was found and led to a further improved PCE of
4.4% (Table 4).
This is as well represented in the EQE data of DCC3T-
Me:C60-BHJ devices achieving maximum values of 60% @
500 nm compared to 35% in the PHJ devices, which can be
assigned to the stronger thin film absorption (Figure 12c).
Interestingly, this device performance of DCC-substituted ter-
thiophene DCC3T-Me excels those of all 4T derivatives even
cantly enhanced solubilities including an odd–even effect as a
function of the number of thiophene units due to molecular
symmetry, whereas the thermal stability is somewhat smaller.
(2) Great similarities in crystal packing were observed for
DCC3T-Me and DCV5T-Me, which have the same substitu-
tion pattern and the same number of rings including efficient
π–π-stacking of the oligothiophene cores. Both acceptor units,
DCC and DCV, govern the 2D-arrangement in the molecular
plane by multiple nonclassical hydrogen bonding. (3) The com-
parison of the optical properties of DCC- and DCV-capped oli-
gomers resulted in a close correspondence of the absorption
profiles for derivatives with equal number of double bonds, i.e.,
DCCnTs and DCV(n + 1)Ts. (4) Electrochemical and theoretical
investigations showed that despite having the same energy gap,
the HOMO/LUMO energy levels for the DCC-derivatives are
raised and more destabilized due to the aromatization energy
of a thiophene ring versus two exocyclic double bonds. This
result indicates that implementation of DCC-acceptor units
lead to improved donor and reduced acceptor strength in the
oligomers. (5) The incorporation of the DCC-terthiophenes
DCC3T and DCC3T-Me as donor materials in vacuum-pro-
cessed planar and bulk heterojunction solar cells resulted in
a good photovoltaic performance (PCE ≤ 4.4%), which clearly
outperformed all comparable DCV4T derivatives.
4. Experimental Section
Instruments and Measurements: NMR spectra were recorded on an
Avance 400 spectrometer (1H NMR: 400 MHz, 13C NMR: 100 MHz)
or a Bruker AMX 500 (1H NMR: 500 MHz, 13C NMR: 125 MHz), at
25 °C or 100 °C, respectively. Chemical shift values (δ) are expressed in
parts per million using residual solvent protons (1H NMR, δH = 7.26 for
CDCl3, δH = 2.50 for DMSO-d6, and δH = 5.93 for tetrachloroethane-d2;
13C NMR, δC = 77.0 for CDCl3, δC = 39.43 for DMSO-d6, and δC = 74.20
tetrachloroethane-d2) as internal standard. The splitting patterns are
designated as follows: s (singlet), d (doublet), and m (multiplet). The
Figure 11. Representation of HOMO/LUMO energies of DCC3T in com-
parison to DCV3T and DCV4T and to C60 derived from electrochemical
data. The HOMO/LUMO energy levels were determined from the onset
of the oxidation and reduction waves (vide supra). The energy values of
C60 are taken from the literature.[17]
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out in 1 cm cuvettes with Merck Uvasol grade solvents; absorption
spectra were recorded on a Perkin Elmer Lambda 19 spectrometer and
corrected fluorescence spectra were recorded on a Perkin ElmerLS 55
fluorescence spectrometer. Thin film absorption spectra were recorded
on a Shimadzu UV-2101/3101 UV–vis spectrometer. The thin film
emission spectra were recorded with an Edinburgh Instruments FSP920
fluorescence spectrometer. Cyclic voltammetry experiments were
performed with a computer-controlled Metrohm 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. X-ray diffraction data of DCC3T-Me
were collected on a Agilent SuperNova, Cu at zero, Atlas CCD using
graphite-monochromated Cu Kα radiation. Data collection strategy
was performed with the APEX2 software, data reduction, and cell
refinement with SAINT. DCC3T-Me: C32H24N4S3, Mr = 560.73, dark-green
platelet, 0.39 × 0.10 × 0.06 mm3, monoclinic, C2/c, a = 21.3166(12) Å,
b = 15.0156(7) Å, c = 9.1933(5) Å, β = 118.089(8)°, V = 2596.0(3) Å3,
Z = 4, µ = 2.850 mm−1, dx = 1.435 g cm−3, T = 150 K. 4754 reflections
collected (θmax = 73.723°) and merged to 2541 independent data (Rint
=
0.023); final R indices (I > 2σ(I)): R1 = 0.0402, wR2 = 0.0457. Detailed
crystallographic data for structure DCC3T-Me has been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication CCDC 1025975. Details on measurement and refinement are
given in the Supporting Information.
Quantum Chemical Calculations: Density functional theory was
employed with the hybrid functionals B3LYP and the basis set 6–31G*
from the NWChem package.[18]
Thin Film and Device Fabrication: Thin films and planar heterojunction
solar cell devices were prepared by thermal vapor deposition in ultra-
high vacuum at a base pressure of 10−7 mbar onto the substrate at room
temperature. Thin films for absorption and emission measurements
were prepared on quartz substrates; solar cells on tin-doped indium
oxide (ITO) coated glass (Thin Film Devices, USA, sheet resistance of
30 Ω sq−1). Layer thicknesses were determined during evaporation by
using quartz crystal monitors calibrated for the respective material.
The thin films prepared for absorption and emission measurements
are approximately 30 nm thick. The bulk-heterojunction solar cell was
prepared layer by layer without breaking the vacuum using the following
layer structure: ITO; 15 nm C60; 20 nm blend layer of DCC3T-Me and
C60 (ratio 2:1 by volume) prepared by co-evaporation on the heated
substrate (90 °C), 10 nm 9,9-bis{4-[di-(p-biphenyl)aminophenyl]}
fluorene (BPAPF), 50 nm BPAPF p-doped with NDP9 (purchased from
Novaled AG Germany, 10 wt%), 1 nm NDP9, 100 nm aluminum.
Photovoltaic Characterization: J–V and EQE measurements were
carried out in a glove box with 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). The intensity
was monitored with a silicon photodiode (Hamamatsu S1337), which
was calibrated at Fraunhofer ISE, Freiburg, Germany. The mismatch
between the spectrum of the sun simulator and the solar AM 1.5G
spectrum was taken into account for the calculation of current density.
For well-defined 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 filters for monochromatic
illumination. The illumination intensities were measured with a silicon
reference diode (Hamamatsu S1337).
Figure 12. a) J–V characteristics of PHJ and BHJ solar cells containing novel
DCC derivatives DCC3T and DCC3T-Me as donor material; b) corresponding
external quantum efficiency spectra c) compared to absorbance spectra of
thin films (30 nm) fabricated by vacuum-deposition onto quartz substrates.
Reagents and Chemicals: 2,5-Bis(trimethylstannyl)thiophene 1,[19]
5,5′-bis(trimethylstannyl)-2,2′-bithiophene 2,[20,21] 3-bromocyclohex-2-enone
3,[22–24] 3-methoxy-2-cyclohex-2-enon 7,[25] and 2,5-bis(trimethylstannyl)-
3,4-dimethylthiophen 10[26] were prepared according to published literature
assignments are ThH (thiophene protons), MeH (methyl protons),
and VH (vinyl protons). GC-MS (EI) mass spectra were recorded on a
Varian Saturn 2000, CI mass spectra on a Finnigan MAT SSQ-7000, and
MALDI-TOF mass spectra on a Bruker Daltonic Reflex III. Differential
scanning calorimetric measurements (DSC) were performed on a
Mettler Toledo DSC823e under argon atmosphere at a heating rate
of 10 °C min−1. Melting points were determined using a Büchi B-545
apparatus and were not corrected. Elemental analyses were performed
on an Elementar Vario EL. Preparative column chromatography was
performed on glass columns packed with silica gel (Merck Silica 60,
particle size 40--43 µm). Optical solution measurements were carried
procedures. 2,5-Dibromothiophene
6 was purchased from Aldrich.
Malonitrile and β-alanine were purchased from Merck. n-Butyl lithium
(1.6 M solution in hexanes) was purchased from Acros. All synthetic steps
were carried out under argon atmosphere.
3,3′-(Thien-2,5-diyl)bis(cyclohex-2-enone) 4:
(trimethylstannyl)thiophene 1 (200 mg, 488 µmol), 3-bromocyclohex-
2-enone (179 mg, 1.03 mmol), and tetrakis(triphenylphosphine)
A
mixture of 2,5-bis-
3
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Table 4. Photovoltaic characteristics of planar (PHJ) solar cell devices containing DCC3T and DCC3T-Me as donor component and C60 as acceptor in
comparison to DCV4T and of bulk heterojunction (BHJ) cells with DCC3T-Me.
Donor oligomer
Device type
VOC
[V]
JSC
[mA cm−2
FF
[%]
PCE
[%]
Saturationa)
Intensity
[mW cm−2
]
]
DCC3T
PHJ
PHJ
PHJ
BHJ
1.00
1.05
0.97
0.99
5.3
5.0
2.9
8.5
56
65
42
53
3.0
3.4
1.2
4.4
1.11
1.08
1.30
1.09
93
92
DCC3T-Me
DCV4Tb)
100
87
DCC3T-Me
a)Saturation defined as J( −1 V)/JSC, i.e., voltage bias dependence of the current in reverse direction; b)Data taken from the literature.[12c]
stirred under reflux for 6 days. The reaction mixture was allowed to
cool to room temperature, the precipitate was filtered off and washed
thoroughly with water. After concentration of the filtrate in vacuo to
about half the volume and cooling to room temperature, the resulting
precipitate was collected by filtration and washed with water to afford
a second crop of 9 (overall: 4.40 g, 14.4 mmol, 93% yield) as an orange
crystalline solid. Mp 195.5–197 °C;1H NMR (CDCl3, δ ppm) 7.23
(d, 1H, J = 3.9 Hz, ThH), 7.10 (d, 1H, J = 3.9 Hz, ThH), 7.01 (s, 1H,
VH), 2.80 (t, 2H, J = 6.4 Hz), 2.74 (t, 1H, J = 5.9 Hz), 2.00 (p, 1H, J =
6.1 Hz).13C NMR (CDCl3, δ ppm) 168.85, 149.24, 143.88, 131.71, 128.64,
118.57, 118.25, 113.28, 112.46, 78.34, 29.06, 27.73, 21.11. MS (EI) m/z:
306 [M+H+]; Elemental analysis for C13H9BrN2S: calculated. C, 51.16; H,
2.97; N, 9.18; S, 10.51%; found: C, 51.38; H, 3.04; N, 9.15; S, 10.54%.
2,2′-[(Thien-2,5-diyl)bis(cyclohex-2-en-3-yl-1-ylidene)]dimalonitrile
palladium(0) (28 mg, 24 µmol) in degassed DMF (3 mL) was stirred at 80 °C
for 3 h. Upon cooling to −20 °C, the resulting precipitate was filtered off and
washed thoroughly with water. The filtrate was poured into dichloromethane
(50 mL), washed with water (3 × 50 mL), dried over MgSO4, filtered, and
concentrated under reduced pressure. Recrystallization of the combined
solids from n-hexane/dichloromethane provided compound 4 (117 mg,
430 µmol, 88% yield) as yellow needles. Mp 149 °C (DSC);1H NMR
(DMSO-d6, δ ppm) 7.69 (s, 2H, ThH), 6.34 (s, 2H, VH), 2.78 (t, 4H,
J = 5.7 Hz), 2.38 (t, 4H, J = 6.6 Hz), 2.06–2.00 (m, 4H). 13C NMR (DMSO-
d6, δ ppm) 198.12, 151.56, 144.28, 129.56, 122.70, 36.75, 27.02, 21.87. MS
(EI) m/z: 272 [M+]; Elemental analysis for C16H16O2S: calculated. C, 70.56;
H, 5.92; S, 11.77%; found: C, 70.41; H, 5.75; S, 11.58%.
3,3′-[(2,2′-Bithien)-5,5′-diyl]bis(cyclohex-2-enone) 5:
5,5′-bis(trimethylstannyl)-2,2′-bithiophene (200 mg, 406 µmol),
3-bromocyclohex-2-enone (156 mg, 894 µmol), and tetrakis-
A
mixture of
2
DCC1T:
A solution consisting of 3,3′-(thien-2,5-diyl)bis(cyclohex-2-
3
enone) 4 (390 mg, 1.43 mmol), malonitrile (570 mg, 8.59 mmol), and
β-alanine (7 mg, 0.08 mmol) in a mixture of dichloroethane (5 mL) and
ethanol (5 mL) was stirred under reflux for 7 days. The reaction mixture
cooled to −18 °C, the precipitate was filtered off and recrystallized from
dichloromethane to yield DCC1T (427 mg, 1.16 mmol, 81% yield) as an
red crystalline solid. Mp 319 °C (DSC);1H NMR (tetrachloroethane-d2,
100 °C, δ ppm) 7.45 (s, 2H, ThH), 7.16 (s, 2H, VH), 2.80–2.74 (m,
8H), 1.01–1.96 (m, 4H).13C NMR (tetrachloroethane-d2, 100 °C, δ
ppm) 168.29, 149.26, 146.40, 129.18, 120.56, 113.18, 112.48, 80.02,
29.34, 28.69, 21.51. MS (CI) m/z: 369 [M+H+]; Elemental analysis for
C22H16N4S: calculated. C, 71.72; H, 4.38; N, 15.21; S, 8.70%; found: C,
71.98; H, 4.43; N, 15.26; S, 8.83%.
(triphenylphosphine)palladium(0) (24 mg, 20 µmol) in degassed DMF
(5 mL) was stirred at 80 °C for 3 h. Upon cooling to −20 °C, the resulting
precipitate was filtered off and thoroughly washed with methanol. The
filtrate was poured into dichloromethane (50 mL), washed with water
(3 × 50 mL), dried over MgSO4, filtered, and concentrated under reduced
pressure. Recrystallization of the combined solids from n-hexane/
dichloromethane provided diketone 5 (127 mg, 358 µmol, 88% yield)
as orange needles. Mp 214 °C (DSC);1H NMR (DMSO-d6, 100 °C, δ
ppm) 7.57 (d, 2H, J = 4.0 Hz, ThH), 7.44 (d, 2H, J = 4.0 Hz, ThH), 6.27
(s, 2H, VH), 2.79 (t, 4H, J = 5.5 Hz), 2.39 (t, 4H, J = 6.6 Hz), 2.10–2.05
(m, 4H). 13C NMR (DMSO-d6, 100 °C, δ ppm) 196.91, 150.88, 141,34,
137,87, 128,77, 125.71, 121,66, 36.27, 26.80, 21.43. MS (CI) m/z: 355
[M+H+]; Elemental analysis for C20H18O2S2: calculated. C, 67.77; H, 5.12;
S, 18.09%; found: C, 67.73; H, 5.28; S, 18.25%.
2,2′-[(2,2′-Bithien-5,5′-diyl)bis(cyclohex-2-en-3-yl-1-ylidene)]
dimalononitrile DCC2T: A suspension consisting of 3,3′-(2,2′-bithien-
5,5′-diyl)bis(cyclohex-2-enone)
5 (730 mg, 2.06 mmol), malonitrile
3-(5-Bromothien-2-yl)cyclohex-2-enone 8: To
a
solution of
(680 mg, 10.3 mmol), and β-alanine (10 mg, 0.11 mmol) in a mixture of
dichloroethane (10 mL) and ethanol (10 mL) was stirred under reflux for
21 days. The reaction mixture was allowed to cool to room temperature,
the precipitate was filtered off and recrystallized from dichloroethane to
yield DCC2T (691 mg, 1.53 mmol, 74% yield) as a dark red solid. Mp
331 °C (DSC);1H NMR (tetrachloroethane-d2, 100 °C, δ ppm) 7.40 (d,
2H, J = 4.0 Hz, ThH), 7.10 (s, 2H, VH), 2.79–2.75 (m, 8H), 2.01–1.96
(m, 4H).13C NMR was not possible due to low solubility. MS (CI) m/z:
451 [M+H+]; Elemental analysis for C26H18N4S2: calculated. C, 69.31; H,
4.03; N, 12.43; S, 14.23%; found: C, 69.50; H, 3.96; N, 12.38; S, 14.22%.
2,2′-[(2,2′:5′,2′′-Terthien-5,5′′-diyl)bis(cyclohex-2-en-3-yl-1-ylidene)]
dimalonitrile DCC-3T: A mixture of 2,5-bis(trimethylstannyl)thiophene 1
(767 mg, 1.87 mmol), 2-[3-(5-bromothien-2-yl)-cyclohex-2-en-1-ylidene]
malonitrile 9 (1.20 g, 3.93 mmol), and tetrakis(triphenylphosphine)
palladium(0) (108 mg, 94 µmol) in degassed DMF (30 mL) was stirred
at 80 °C for 30 h. Upon cooling, the resulting precipitate was filtered
off and washed several times with methanol and n-hexane. After drying,
DCC3T (783 mg, 1.47 mmol, 79% yield) was obtained as a dark purple
solid. Mp 290 °C;1H NMR (tetrachloroethane-d2, 100 °C, δ ppm) 7.36
(d, 2H, J = 4.0 Hz, ThH), 7.20 (s, 2H, ThH), 7.16 (d, 2H, J = 4.0 Hz,
ThH), 6.99 (s, 2H, VH), 2.74–2.71 (m, 8H), 1.96–1.90 (m, 4H).13C NMR
(tetrachloroethane-d2, 100 °C, δ ppm) 168.99, 150.23, 141.91, 141.66,
137.17, 130.06, 126.73, 125.75, 118.90, 113.76, 113.04, 77.88, 29.41,
2,5-dibromothiophene 6 (10.07 g, 25.0 mmol) in diethyl ether (75 mL),
stirred at −78 °C, n-butyl lithium (1.6 M solution in hexanes, 15.6 mL,
25.0 mmol) was added dropwise over 15 min. After stirring at −78 °C for
10 min, a solution of 3-methoxy-2-cyclohex-2-enon 7 (3.16 g, 25.0 mmol)
in diethyl ether (30 mL) was added and stirring was continued for 15 min.
The solution was warmed to 0 °C, stirred for 3 h, and subsequently
allowed to warm to room temperature and stirred for 16 h. The reaction
was quenched with brine (300 mL) and the product was extracted with
diethyl ether (3 × 150 mL). The combined organic extracts were dried
over MgSO4, filtered, and the solvents were removed in vacuo. Column
chromatography (silica gel, DCM) and subsequent recrystallization from
n-hexane yielded ketone 8 (4.91 g, 19.1 mmol, 76% yield) as a colourless
crystalline solid. Mp 90.5–92 °C;1H NMR (CDCl3, δ ppm) 7.12 (d, 1H,
J = 4.0 Hz, ThH), 7.05 (d, 1H, J = 3.9 Hz, ThH), 6.29 (s, 1H, VH), 2.70
(t, 2H, J = 6.0 Hz), 2.45 (t, 2H, J = 6.6 Hz), 2.16–2.09 (m, 2H).13C NMR
(CDCl3, δ ppm) 199.08, 151.19, 143.97, 131.20, 127.57, 122.80, 116.38,
37.18, 27.35, 22.29. MS (EI) m/z: 258 [M+H+]; Elemental analysis for
C10H9BrOS: calculated. C, 46.71; H, 3.53; S, 12.47%; found: C, 46.97;
H, 3.59; S, 12.48%.
2-[3-(5-Bromothien-2-yl)cyclohex-2-en-1-ylidene]malonitrile 9: A solution
consistingof3-(5-bromothien-2-yl)cyclohex-2-enone8(4.00g,15.6mmol),
malonitrile (3.08 g, 46.7 mmol), and β-alanine (83 mg, 0.93 mmol)
in a mixture of dichloroethane (100 mL) and ethanol (100 mL) was
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Adv. Funct. Mater. 2015,
DOI: 10.1002/adfm.201404210
2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
11

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28.33, 21.58. MALDI-TOF MS m/z: 532 [M+]; Elemental analysis for
C30H20N4S3: calculated. C, 67.64; H, 3.78; N, 10.52; S, 18.06%; found: C,
67.70; H, 3.79; N, 10.30; S, 17.89%.
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8484; b) B. Kan, Q. Zhang, M. Li, W. Ni, G. Long, Y. Wang, X. Yang,
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P.-H. Wang, Y.-H. Liu, K.-T. Wong, J. Wen, D. J. Miller, S. B. Darling,
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2,2′-[(2,2′:5′,2′′:5′′,2′′′-Quaterthien-5,5′′′-diyl)bis(cyclohex-2-en-3-yl-1-
ylidene)]dimalonitrile DCC4T: A mixture of 5,5′-bis(trimethylstannyl)-
2,2′-bithiophene (1.00 mg, 2.03 mmol), 2-[3-(5-bromothien-2-yl)
cyclohex-2-en-1-ylidene]malonitrile
9
(1.30 g, 4.27 mmol), and
tetrakis(triphenylphosphine)palladium(0) (117 mg, 10 µmol) in degassed
DMF (40 mL) was stirred at 80 °C for 16 h. Upon cooling, the
resulting precipitate was filtered off and washed several times with
methanol and n-hexane. The crude product was extracted in a Soxhlet
extractor with chlorobenzene for 6 days. The extract was cooled to
room temperature and the precipitate was filtered off to yield DCC4T
(819 mg, 1.33 mmol, 66% yield) as a black solid. Mp 333 °C (DSC);1H
NMR (tetrachloroethane-d2, 100 °C, δ ppm) 7.39 (d, 2H, J = 4.0 Hz,
ThH), 7.21 (d, 2H, J = 3.9 Hz, ThH), 7.18 (d, 2H, J = 4.0 Hz, ThH), 7.15
(d, 2H, J = 3.9 Hz, ThH), 7.07 (s, 2H, VH), 2.78–2.75 (m, 8H), 2.00–1.95
(m, 4H).13C NMR was not possible due to low solubility. MALDI-TOF
MS m/z: 614 [M+]; Elemental analysis for C34H22N4S4: calculated. C,
66.42; H, 3.61; N, 9.11; S, 20.86%; found: C, 66.59; H, 3.51; N, 8.92; S,
20.60%.
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E. Reinold, P. Bäuerle, Adv. Mater. 2006, 18, 2872.
2,2′-{[3′,4′-Dimethyl-(2,2′:5′,2′′-terthien-5,5′′-diyl)]bis(cyclohex-
2-en-3-yl-1-ylidene)}dimalonitrile DCC3T-Me:
A mixture of 2,5-bis-
[12] a) C. Uhrich, R. Schüppel, A. Petrich, M. Pfeiffer, K. Leo, E. Brier,
P. Kilickiran, P. Bäuerle, Adv. Funct. Mater. 2007, 1, 2991;
b) R. Schüppel, K. Schmidt, C. Uhrich, K. Schulze, D. Wynands,
J. L. Bredas, E. Brier, E. Reinold, H. B. Bu, P. Bäuerle, B. Männig,
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E. Reinold, A. Mishra, E. Mena-Osteritz, H. Ziehlke, C. Körner,
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M. Weil, C. Körner, H. Ziehlke, C. Elschner, K. Leo, M. Riede,
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105, 063306.
(trimethylstannyl)-3,4-dimethylthiophene 10 (400 mg, 913 µmol),
2-[3-(5-bromothien-2-yl)cyclohex-2-en-1-ylidene]malonitrile 9 (586 mg,
1.92 mmol), and tetrakis(triphenylphosphine)palladium(0) (46 mg,
40 µmol) in degassed DMF (15 mL) was stirred at 80 °C for 16 h. Upon
cooling, the resulting precipitate was filtered off and washed several
times with methanol and n-hexane. After drying, DCC3T-Me (443 mg,
787 µmol, 86% yield) was obtained as a dark purple crystalline solid. Mp
305 °C (DSC);1H NMR (tetrachloroethane-d2, 100 °C, δ ppm) 7.42 (d,
2H, J = 4.0 Hz, ThH), 7.17 (d, 2H, J = 4.1 Hz, ThH), 7.02 (s, 2H, VH),
2.78–2.72 (m, 8H), 2.33 (s, 6H, MeH), 1.97–1.91 (m, 4H). 13C NMR
(tetrachloroethane-d2, 100 °C, δ ppm) 169.75, 150.91, 141.96, 141.68,
137.91, 130.67, 130.16, 127.56, 118.38, 114.23, 113.47, 76.95, 29.51,
28.29, 21.56, 15.30. MALDI-TOF MS m/z: 560 [M+]; Elemental analysis
for C32H24N4S3: calculated. C, 68.54; H, 4.31; N, 9.99; S, 17.15%; found:
C, 68.51; H, 4.18; N, 9.98; S, 16.99%.
Supporting Information
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Supporting Information is available from the Wiley Online Library or
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Acknowledgements
The authors would like to thank the German Research Foundation (DFG)
for financial support in the framework of special program SPP 1355. The
authors thank Novaled GmbH for providing NDP9.
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Received: November 27, 2014
Revised: January 17, 2015
Published online:
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©
12 wileyonlinelibrary.com
2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2015,
DOI: 10.1002/adfm.201404210