Synthesis of 3,7-diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b′] difurans: Functional building blocks for the design of new conjugated polymers

Article abstract of DOI:10.1039/c2cc34070d

3,7-Diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]difurans are efficiently prepared by an iodine-promoted double cyclization. This new heterocyclic core is readily modified by the attachment of alkyl chains for improved solubility. The use of these compo

Full text of DOI:10.1039/c2cc34070d

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Synthesis of 3,7-diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b0]difurans:
functional building blocks for the design of new conjugated polymersw
a
a
a
b
´
Brandon M. Kobilka, Anton V. Dubrovskiy, Monique D. Ewan, Aimee L. Tomlinson,
Richard C. Larock,a Sumit Chaudharyc and Malika Jeffries-EL*a
Received 7th June 2012, Accepted 20th July 2012
DOI: 10.1039/c2cc34070d
3,7-Diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b0]difurans are
efficiently prepared by an iodine-promoted double cyclization. This
new heterocyclic core is readily modified by the attachment of alkyl
chains for improved solubility. The use of these compounds for the
synthesis of new conjugated polymers is also reported.
Organic semiconductors are finding widespread use as replacements
for their inorganic counterparts in a range of applications, including
field effect transistors (OFET)s, light-emitting diodes (OLED)s, and
photovoltaic cells (OPVC)s.1 These materials offer advantages in
Scheme 1 Synthesis of 3,7-diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b0]-
difurans 2a and 2b and their subsequent alkynylation.
the form of facile device fabrication via solution-based techniques
and energy levels that can be tuned by chemical synthesis.
Tuning can be accomplished through the synthesis of materials with
alternating electron-donating and electron-accepting moieties.2
Among electron-donating building blocks, benzo[1,2-b:4,5-b0]-
dithiophene (BDT) is particularly promising, due to its planar
conjugated structure that facilitates p–p stacking, leading to
higher hole mobility.3 Bulk heterojunction photovoltaic cells
(BHJ-PVC)s using BDT copolymers as donors have achieved
power-conversion efficiencies (PCE)s up to 7.4%.3c
Our synthetic route to the BDFs is shown in Scheme 1. This
approach offers several benefits, including: (1) the use of easily
prepared starting materials and inexpensive catalysts; (2) the
enhanced solubility afforded by the alkyl chains on the flanking
thiophene rings; and (3) the opportunity to generate a variety of
new substituted BDFs from a common intermediate via cross-
coupling reactions. Compounds 1a and 1b have been prepared by
the Sonogashira cross-coupling of 1,4-dibromo-2,5-dimethoxy-
benzene and either 2-ethynyl-3-decyl-thiophene or 2-bromo-3-
decyl-5-ethynylthiophene (see the ESIw). The iodine-induced
double cyclization of compounds 1a and 1b afforded 3,7-
diiodo-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b0]difurans 2a and
2b in 70 and 82% yield respectively.7 Both regioisomers are
easily purified by recrystallization. The Sonogashira cross-
coupling reactions of 2a and 2b with 1-decyne afforded 3a
and 3b in 96% and 92% yield respectively. Crystals of 2a
suitable for X-ray analysis were obtained by recrystallization.
Detailed crystallographic data can be found in the ESI.w In
addition to confirming the identity of this new compound, the
X-ray analysis indicated that the BDF ring system is planar,
which is beneficial to promoting efficient p-stacking and
improving the charge transport of materials derived from this
intermediate. The torsion angles between the BDF and thiophene
rings are approximately 175.31.
Recently, furan-containing molecules have been explored for
the design of organic semiconductors.4 Furan is an attractive
alternative to thiophene, since it is isoelectronic, while possessing
a Dewar resonance energy of 18.0 kJ molꢀ1, which is less aromatic
than that of thiophene (27.2 kJ molꢀ1).5 Thus, replacing thiophene
with furan is expected to favor the formation of quinoid structures,
leading to a reduction in the band gap of the resulting materials.
Although benzo[1,2-b:4,5-b0]difurans (BDF)s are known in the
literature, the lack of methods for the synthesis of substituted
derivatives has prevented their widespread use.6 Encouraged by
some of our earlier studies on iodocyclization, we explored this
approach for synthesizing BDFs.7 Herein, we report the synthesis
of functional BDFs and their polymerization with isoindigo, an
electron-deficient moiety that has been used in several polymers
with high PCE, when used as donor materials in BHJ-PVCs.8
a Department of Chemistry, Iowa State University, Ames, IA 50011,
USA. E-mail: malikaj@iastate.edu; Fax: +1 515-294-0105
b Department of Chemistry, North Georgia College & State
University, Dalhonega, GA 30597, USA
The remaining synthetic steps to the desired polymers are
shown in Scheme 2. The hydrogenation of the alkynyl BDFs
3a and 3b afforded the alkylated derivatives 4a and 4b each in
B95% yield. Subsequent stannylation afforded 5a and 5b each
in B94% yield. The Stille cross-coupling polymerization of 5a
or 5b with 6,60-dibromo-N,N0-(2-octyldodecanyl)-isoindigo 69
c Department of Electrical Engineering, Iowa State University, Ames,
IA, USA
w Electronic supplementary information (ESI) available: Experimental
details, calculations, and spectroscopy. See DOI: 10.1039/c2cc34070d
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.

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Scheme 2 Modification and polymerization of the 2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b0]difurans.
Table 1 Reaction conditions and molecular weight data for PTBDFIDs
Polymera
PToutBDFID Pd(PPh3)4 82
Yieldb (%) Mw
Mw/Mn DPn
c
Catalyst
20 500 1.3
33 100 1.9
35 000 1.9
76 200 2.3
12
19
21
45
PToutBDFID Pd2(dba)3 86
Pd(PPh3)4 79
Pd2(dba)3 84
PTinBDFID
PTinBDFID
a
[Monomer] = 0.2 M in toluene, and Pd catalyst loading = 2 mol%.
c
Isolated yield. Molecular weight data were obtained by GPC (see ESI).
b
afforded the polymers PTinBDFID and PToutBDFID in excellent
yields after purification by Soxhlet extraction with methanol,
followed by acetone, to remove residual catalyst and low molecular
weight materials. Of the catalysts evaluated, Pd2(dba)3 gave the
best results (Table 1). The polymers are soluble in standard organic
solvents, such as THF and chloroform, at room temperature.
Monomer 5a consistently produced polymers with higher
molecular weights. Presumably, this is due to the reduced
steric hindrance at the 2- and 20-positions of the BDF moiety.
We anticipated that the differences in the regiochemistry of
the BDFs would result in differences in the optical spectra.
Compounds 2b, 3b, and 4b, with the alkyl substituents on the 4
and 40 positions of the thiophene rings, have less interaction
with the pseudo-peri iodine, alkyne, or alkane substituents,
and exhibit greater vibrational structure than analogs 2a–4a
(see ESIw). Arguably, this is due to the greater rigidity of the
overall aromatic chromophore, i.e. reduction of out-of-plane
rotation of the thiophene moieties. The dramatically different band
shapes between the members of each pair mean that comparison of
Fig. 1 UV-vis absorption of the polymers in solution and thin films.
Table 2 Electronic and optical properties of PTBDFIDs
lmax
Media (nm)
HOMOa LUMOb Eg
Eg
(eV)
opt c
EC d
Polymer
(eV)
(eV)
(eV)
PToutBDFID THF 399, 582
PToutBDFID Film 403, 612 ꢀ5.7
ꢀ3.8
ꢀ3.8
1.7
1.6
1.9
PTinBDFID THF 378, 600
PTinBDFID Film 415, 653 ꢀ5.7
1.9
a
c
ox
b
red
onset
HOMO = ꢀ(E
+ 5.1) (eV). LUMO = ꢀ(E
+ 5.1) (eV).
Estimated from the optical absorption edge. Onset of potentials
onset
d
(vs. Fc).
solid state, the lmax for the low energy band of PToutBDFID is
blue-shifted 41 nm relative to PTinBDFID and the difference in
the high energy band is only 12 nm. These results suggest that
PToutBDFID has a more twisted backbone than PTinBDFID.
The optimized geometries obtained for isoindigo–BDF oligomers
calculated using density functional theory also support the
notion that PToutBDFID has a more twisted structure (see ESIw).
The similarity of PToutBDFID’s solution and film spectra
indicates that the steric interaction between the out facing side
chain and the isoindigo group inhibits planarization.
lmax between ‘‘a’’ and ‘‘b’’ analogs is not particularly meaningful.
However, the leading edge of the onset of strong S1 absorption for
each ‘‘a–b’’ pair of compounds extrapolates to a very similar
wavelength, with each pair distinct from the other two. This reflects
the intrinsic electronic similarities between each pair of isomers.
As expected, the additional conjugation of the alkynyl
groups of 3a and 3b produces a red shift in the onset of
absorption and lmax of the S1 absorption band. Compared to
the alkyl substituents in compounds 4a and 4b, the iodo
substituents in 2a and 2b induce a red-shift – albeit smaller
than that of the alkyne – consistent with a reduction in
conjugation length and orbital overlap (see ESIw). The UV–vis
absorption spectra of PTinBDFID and PToutBDFID in
solution and in thin films are shown in Fig. 1 and the optical
and electronic properties are summarized in Table 2. Both
polymers exhibit two main absorption bands. The high energy
bands can be attributed to the p–p* transition, whereas the
low energy bands are due to intramolecular charge transfer
between the donor and acceptor units. In solution, the lmax of
PTinBDFID’s low energy band is red-shifted 18 nm relative to
PToutBDFID, whereas the lmax of PTinBDFID’s high energy
band is blue-shifted 21 nm relative to PToutBDFID. In the
The electrochemical properties of the polymers have been
investigated by cyclic voltammetry (CV). Both polymers exhibit
measureable and reproducible oxidation and reduction processes.
The electrochemical band gaps of both are approximately 0.3 eV
higher than the optical band gaps (Egopt) determined via the
tangent lines on the absorption spectra. This difference can be
attributed to the electron injection barrier in the electrochemistry.10
The HOMO and LUMO values of both polymers are similar to
those reported previously for PBDT-OIO, a related terpolymer
of 6, thiophene and BDT (LUMO ꢀ3.91 eV and HOMO
ꢀ5.74 eV).11 Unfortunately, we cannot arrive at a conclusion
regarding the relative donor strength of BDF, as the BDT
group had two electron-donating alkoxy groups on the central
benzene ring. Although, the LUMO values are less than 0.3 eV
lower than those of the PC61BM acceptor, impeding charge
transfer, the HOMO levels of both polymers are deep enough
c
Chem. Commun.
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Table 3 Photovoltaic performance of P12 and P13 with PCBM
VOC (V) ISC (mA) JSC (mA cmꢀ2) FF (%) PCE (%)
optimizing the device performance in addition to developing
new materials based on BDFs.
Polymer
Support for this work has been provided by Iowa State
University (ISU) and the 3M Foundation. We thank Dr Kamel
Harrata and the ISU mass spectroscopy laboratory and Dr
Arkady Ellern and the ISU X-ray crystallography facility for
analyses. The PVCs were fabricated at ISU Institute for Physical
Research and Technology’s Microelectronics Research Center.
PinBDFID 0.7366 0.208
PoutBDFID 0.6410 0.164
1.66
1.306
48.6
36
0.590
0.301
Polymer films were prepared from solutions in o-DCB, 10 mg mLꢀ1
.
Notes and references
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Fig. 2 Current–voltage characteristics of polymer PVCs (left). Nor-
malized external quantum efficiency vs. wavelength curve of the PVCs
(right).
to ensure air stability, while providing good open-circuit
voltage (Voc).12
L. Yu, Adv. Mater., 2010, 22, E135; (d) A. Najari, S. Beaupre
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´
The performance of both polymers in BHJ-PVCs was
evaluated using PC61BM as the electron acceptor with a device
configuration of indium tin oxide (ITO)/poly(3,4-ethylene dioxy-
thiophene):polystyrene sulfonate (PEDOT:PSS)/polymer:PC61BM
(1 : 4, w/w)/LiF/Al. The active layer processing conditions
were chosen to yield a layer thickness less than 100 nm. In
general (for P3HT systems), thicker layers (B200 nm) are
better, because they absorb more light. However, since new
generation donor–acceptor polymer films do not have a long-
range order like P3HT, thicker layers tend to have increased
recombination due to hole traps, and thus lower efficiencies.13
The fabrication conditions and PVC parameters (fill factor (FF),
short-circuit current density (Jsc) and Voc) are summarized in
Table 3. The current–voltage (I–V) characteristics of our devices
are shown in Fig. 2.
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Overall, PTinBDFID PVCs performed better than PToutBD-
FID-based devices in all categories. This is most likely an effect
of the polymer’s planarity on morphology, and is currently
being evaluated further. Although the performance of these
devices is lower than other conjugated polymers, this is our first
attempt toward fabricating PVCs from these materials. We note
that the performance of most new systems can be dramatically
improved by the optimization of processing parameters.
In conclusion, we report the efficient synthesis of novel
electron-rich building blocks based on 2,6-di(thiophen-2-yl)-
benzo[1,2-b:4,5-b0]difurans and their use for the development
of donor–acceptor copolymers. The highlights of this work are
the overall high yields of the reactions and the versatility of the
synthetic approach. The energy levels of the new polymers
are suitable for their use as donor materials in BHJ-PVCs.
Preliminary device studies have shown good Voc and FF, but
low overall performance. We are currently working toward
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.