Directed metalation cascade to access highly functionalized thieno[2,3-f ]benzofuran and exploration as building blocks for organic electronics

Article abstract of DOI:10.1021/ol402787d

A tandem directed metalation has been successfully applied to the preparation of thieno[2,3-f]benzofuran-4,8-dione, providing an efficient and facile approach to symmetrically and unsymmetrically functionalize the thieno[2,3-f]benzofuran core at the 2,6 positions as well as to introduce the electron-withdrawing or -donating groups (EWG or EDG) at its 4,8 positions. The presence of various functional groups makes late-stage derivatization attainable.

Full text of DOI:10.1021/ol402787d

ORGANIC
LETTERS
2
013
Vol. 15, No. 21
586–5589
Directed Metalation Cascade
5
To Access Highly Functionalized
Thieno[2,3‑f]benzofuran and Exploration
as Building Blocks for Organic Electronics
†
,§
†,§
Yves Aeschi, Hui Li, Zhencai Cao, Songjie Chen, Anneliese Amacher, Nathalie Bieri,
‡
†
†
†
†
†
†
Bilal Ozen, Juerg Hauser, Silvio Decurtins, Songting Tan, and Shi-Xia Liu*
‡
,†
€
Departement f u€ r Chemie und Biochemie, Universit a€ t Bern, Freiestrasse 3,
CH-3012 Bern, Switzerland, and College of Chemistry, Xiangtan University,
Xiangtan 411105, P.R. China
liu@iac.unibe.ch
Received September 26, 2013
ABSTRACT
A tandem directed metalation has been successfully applied to the preparation of thieno[2,3-f]benzofuran-4,8-dione, providing an efficient and
facile approach to symmetrically and unsymmetrically functionalize the thieno[2,3-f]benzofuran core at the 2,6 positions as well as to introduce the
electron-withdrawing or -donating groups (EWG or EDG) at its 4,8 positions. The presence of various functional groups makes late-stage
derivatization attainable.
Benzodichalcogenophene (BDC) derivatives, in particu-
lar benzodifuran (BDF) and benzodithiophene (BDT), are
of prime interest as organic semiconductors due to their
fascinating features including structural symmetry and
planarity as well as rigid and π-extended conjugation
which can enhance electron delocalization and intermo-
field effect transistors (OFET), and organic light-emitting
7ꢀ15
diodes (OLED).
molecule BHJ solar cells show the hitherto reported high-
For example, BDT-based small-
8
est efficiency of 7.4%. The single micrometer ribbon
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1ꢀ6
lecular interactions to improve charge mobility.
They
(
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are most commonly used as π-electron donors for high-
performance organic electronics, such as bulk heterojunc-
tion (BHJ) and dye-sensitized solar cells (DSSC), organic
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3
350–3357.
(
10) Keller, S.; Yi, C.; Li, C.; Liu, S.-X.; Blum, C.; Frei, G.; Sereda,
†
Universit €a t Bern.
Xiangtan University.
These authors contributed equally to this work.
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0.1021/ol402787d r 2013 American Chemical Society
Published on Web 10/21/2013

transistor of a BDT derivative displayed high mobility up
2
ꢀ1 ꢀ1 15
to 0.81 cm V s . Since we developed a facile and
efficient synthetic route to fully functionalized BDF
Scheme 1. Synthetic Route to TBF Derivatives 1ꢀ4
9
,10
derivatives,
first BDF-based copolymers for BHJ solar cells,
which allowed us to further prepare the
1
1,12
an
increasing number of BDF-based polymers and small
conjugated molecules has appeared in the literature, with
1
3
a high efficiency up to 6.3%. In contrast to an extensive
investigation on BDT, thieno[2,3-f]benzofuran (TBF), as a
structural hybrid of BDF and BDT, has not yet been
explored, owing to the limited preparative accessibility
and the lack of an efficient and convenient synthetic
methodology. To date, despite successful and well-
established synthetic approaches to the BDF and BDT
9ꢀ17
derivatives,
synthesis of the key precursor thieno[2,3-f]benzofuran-
,8-dione (1, Scheme 1) with a poor yield of 24% under
there has been only one report on the
4
1
8
strict reaction conditions. Our keen interest in TBF-
based chromophores stems from the assumption that they
show remarkably different optoelectronic properties from
BDF and BDT derivatives by replacement of a thiophene
ring with a furan in the BDT core, as reported in the case of
1
9
thieno[2,3-b]thiophene. Finally, a synergetic effect of
thiophene and furan rings in the TBF molecules on device
performance is of great importance. As a consequence, we
have optimized the synthetic procedure to produce 1 with a
marked improvement of the reported yield, whereby var-
ious TBF-based building blocks bearing different func-
tional groups such as Br, I and boronic ester can readily be
prepared for late-stage derivatization via Suzuki, Sonogashira,
and Stille coupling reactions. As an example, a series of
TBF-based donorꢀacceptor alternating polymers have
been synthesized, fully characterized, and tested as BHJ
solar cell devices.
TIPS-acetylide followed by the subsequent deoxygena-
tion and aromatization with hydrochloric acid SnCl2
solution. Moreover, the corresponding BDF and BDT
analogues (IꢀIV, Figure 1) were synthesized according to
similar procedures, allowing a comparative study of their
electrochemical and photophysical properties.
As illustrated in Scheme 1, the key precursor 1 was ob-
tained by the condensation of thiophene-3-carboxaldehyde
with lithiated furan precursor. Lithiation with n-BuLi
instead of s-BuLi/TMEDA within a well-adjusted reac-
tion time turns out to be more efficient due to the
elimination of side reactions. Therefore, synthesis of 1
was accomplished under a much milder reaction condi-
tion with a much higher yield of 60% compared to the
1
8
literature. Compounds 2ꢀ4 were prepared to probe the
electronic effects engendered by the addition of electron-
donating (alkoxy) or electron-withdrawing (triisopro-
pylsilyl (TIPS)ethynyl) groups to the 4,8-positions of
the redox-active TBF chromophore. Reduction of 1 and
subsequent alkylation afforded 2 and 3 in 51% and 72%
yields, respectively. Compound 4 was readily prepared in
Figure 1. Chemical structures of BDF and BDT analogues IꢀIV.
Due to the inherent asymmetry of TBF derivatives, a
regioselective nucleophilic bromination of 2 can be accom-
plished via a lithiation followed by the addition of CBr4,
leading to the formation of two regioisomers 5 and 6 with a
molar ratio of 6:4 in total yield of 70% (Scheme 2). They
can be isolated simply by chromotography. A Vilsmeier
reaction has been further applied to introduce a functional
aldehyde group, thus yielding unsymmetrically functiona-
lized TBF derivatives 7 and 8 in good yields.
6
0% yield via a nucleophilic attack on 1 with a resulting
(
16) (a) Abdul-Aziz, M.; Auping, J. V.; Meador, M. A. J. Org. Chem.
995, 60, 1303–1308. (b) Hosseiny Davarani, S. S.; Fakhari, A. R.;
Fumani, N. S.; Kalate-bojdi, M. Electrochem. Commun. 2008, 10, 1765–
768. (c) Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li, J.-
1
1
H.; Yang, Y. Macromolecules 2008, 41, 6012–6018. (d) Beimling, P.;
Kossmehl, G. Chem. Ber 1986, 119, 3198–3203.
(
17) Takimiya, K.; Konda, Y.; Ebata, H.; Niihara, N.; Otsubo, T.
J. Org. Chem. 2005, 70, 10569–10571.
18) Watanabe, M.; Snieckus, V. J. Am. Chem. Soc. 1980, 102, 1457–
460.
19) Henssler, J. T.; Matzger, A. J. Org. Lett. 2009, 11, 3144–3147.
(
Further functionalization of 3 has been conducted
through the introduction of Br, I and boronic ester groups,
1
(
Org. Lett., Vol. 15, No. 21, 2013
5587

Theabsorption bandsshow ared-shiftinthe orderofIII, 4,
and IV; in particular, the lower energy transition is shifted
significantly more than the higher energy transitions. Also
of importance is the increase in intensity of the lower
energy band upon moving to heavier chalcogen atoms,
possibly indicating increasing charge transfer character.
These observations hold true for the alkoxy-substituted
BDCs (Supporting Information), indicating that the elec-
tronegativity of the heteroatom (O and S) plays a funda-
Scheme 2. Synthetic Route to Unsymmetrically Functionalized
TBF Derivatives 5ꢀ8
1
7,19,20
mental role in the absorption maxima location.
Introduction of an TIPS-ethynyl substituent instead of
alkoxy leads to a significant red-shift (33ꢀ41 nm) due to
the extension of the conjugated BDC system and the
electron withdrawing effect of ethynyl groups. For in-
stance, chromophore 4 displays a lowest energy absorption
maximum at 366 nm, which is red-shifted by 34 nm
compared to that of 3. In comparison with the analogous
9
,10,17
as shown in Scheme 3. Bromination/iodination via double
deprotonation in the presence of n-BuLi followed by
treatment with CBr or elemental iodine gave the corre-
4
sponding products 9 and 10 in yields of 70% and 30%.
Similarly, the precursor 11 for the polymers was obtained
in 54% yield by double deprotonation of 3 and subsequent
nucleophilic attack on 4,4,5,5-tetramethyl-2-isopropoxy-
compounds,
the lowest energy absorption band cor-
responds to the πꢀπ* transition dominated by one elec-
tron excitation from HOMO to LUMO. Therefore, the
corresponding optical energy gaps are estimated to be in a
range from 3.1 to 3.6 eV based on onsets of the lowest
energy absorption bands (Table 1).
1
,3,2-dioxoborolane. Three alternating DꢀA copolymers
were synthesized via Suzuki coupling of 11 with diffe-
rent acceptors (isoindigo, 4,7-dithienylbenzothiadiazole
or 3-vinylthienyldithienylbenzothiadiazole).
All these new TBF derivatives have been fully character-
ized; a single-crystal structure has been resolved in the case
of 4 (Supporting Information).
Scheme 3. Synthetic Route to Symmetrically Functionalized
TBF Derivatives 9ꢀ11 as Well as Polymers 12aꢀc
Figure 2. Optical spectra of TIPS-ethynyl-substituted BDCs III
(blue curve), 4 (green curve) and IV (red curve) in dichloro-
methane.
Electrochemical properties of TBF derivatives 3 and 4 as
well as the analogues IꢀIV were investigated by cyclic
voltammetry (Table 1 and Supporting Information). Both
4
and III undergo one irreversible oxidation at 0.97 and
.06 V respectively, whereas IV shows one reversible
1
oxidation at 0.90 V. In contrast, all the alkoxy-substituted
BDCs undergo one reversible and one irreversible oxida-
tion. The first oxidation potentials of alkoxyl-substituted
BDCs are cathodically shifted by at least 0.50 V compared
to the corresponding TIPS-ethynyl-substituted BDCs.
These results can be attributed to the electron-donating
effect of the alkoxy groups on destabilization of the
HOMO. In other words, introduction of ethynyl substitu-
ents at the 4,8-positions of the BDC core has proven to be
The photophysical properties of TBF derivatives 3 and 4
along with the analogues IꢀIV were investigated by
electronic absorption spectroscopy in dichloromethane
(
Figure 2, Table 1, and Supporting Information). For the
TIPS-ethynyl-substituted BDCs, optical spectra are simi-
lar in shape, and all of them strongly absorb up to 400 nm.
(20) Bryant, J. J.; Lindner, B. D.; Bunz, U. H. J. Org. Chem. 2013, 78,
1038–1044.
5
588
Org. Lett., Vol. 15, No. 21, 2013

þ
Table 1. Lowest Energy Absorption Maxima λ , Redox Potentials (V versus Fc/Fc ), HOMO, and LUMO Energy Levels and
max
opt
Optical Band Gaps E
g
of BDC Derivatives
a
ox1
1/2
ox2
1/2
opt
c
d
λ
max (nm)
E
(V)
E
(V)
E
g
(eV)
HOMO (eV)
LUMO (eV)
IV
4
384
366
349
351
332
308
0.90
3.1
3.3
3.4
3.3
3.4
3.6
ꢀ5.58
ꢀ5.70
ꢀ5.76
ꢀ5.18
ꢀ5.08
ꢀ5.18
ꢀ2.48
ꢀ2.40
ꢀ2.36
ꢀ1.88
ꢀ1.68
ꢀ1.58
b
0.97
1.06
0.40
0.41
0.43
b
III
II
3
b
0.86
0.95
1.00
b
b
I
a
ꢀ5
2 2
Measured in CH Cl solution (10 M) with 0.1 M tetra-n- butylammonium hexafluorophosphate. Irreversible peaks. Estimated from the onset
opt
b
c
d
of oxidation peak. Calculated from the HOMO levels and optical energy band gap, using HOMO þ E
g
.
2
(100 mW/cm ). Compared to the analogous BDT- and
effective in lowering both LUMO and HOMO energy
levels, consistent with the above-mentioned UVꢀvis spec-
troscopic results. From the onset of the oxidation peaks,
the HOMO levels of these chromophores are roughly
estimated under the premise that the energy level of
2
1
BDF-basedpolymer solarcells, the first example of TBF-
based photovoltaics shows a relatively poor performance,
very probably because of their low hole mobilities
ꢀ
5
ꢀ6
2
ꢀ1 ꢀ1
(10 ꢀ10 cm V s , see the Supporting Information)
and molecular weights. Consequently, in the ongoing
study we focus on the systematic modification of the
alternating DꢀA planar copolymer architectures to finely
tune absorption coefficients, band gaps, charge mobilities,
molecular weights, as well as the blend film morphologies
which critically influence the device performance.
þ
Fc/Fc is 4.8 eV below the vacuum level (Table 1). For
TIPS-ethynyl-substituted BDCs, the HOMO level elevates
in the same order of BDF < TBF < BDT due to lower
ionization potential of the heavier chalcogen atom. Of
interest are alkoxy-substituted BDCs with a similar
HOMO energy level. It appears that the chalcogen atom
effect on the electronic properties of these chromophores
somehow depends on the nature of substituents at 4,
In conclusion, we present a polyfunctionalization syn-
thetic strategy to access a family of TBF-based chromo-
phores. Various functional groups are thus introduced to
the TBF core allowing late-stage derivatization. Our in-
vestigation of the chalcogen-property relationship in these
compounds reveals a lowering of the band gap on going
from oxygen to sulfur. The nature of substituents at the
4,8-positions of the BDC core plays a crucial role in
tuning the electronic properties of the chromophores.
As a consequence, the present work gives insight into
the structureꢀproperty relationship of TBF-type semi-
conductors and paves a way to develop a manifold of
extended π-conjugated TBF molecules and polymers
with high structural diversity for high performance
device fabrication.
8
-positions of the BDC core.
TBF derivatives are electron-rich and planar π-systems
showing tunable energy levels, intrinsic optical properties,
and excellent solubility. Thus, they appear very promising
for application in solution-processed organic electronic
devices. As a starting point, three well-defined alternating
DꢀA polymers 12aꢀc bearing TBF chromophores were
synthesized (Scheme 3) and applied in BHJ polymer solar
cells. All of them exhibit broad and strong absorptions
ranging from 250 to 750 nm, and ambipolar redox proper-
ties (Supporting Information) with HOMO levels ranging
from ꢀ5.34 to ꢀ5.15 eV and LUMO levels from ꢀ3.44
to ꢀ2.78 eV, leading to narrow optical band gaps of
1
.66ꢀ2.08 eV. As expected, their band gaps as well as their
molecular electronic energy levels are readily tuned by
copolymerizing the TBF core with different π-conjugated
electron-withdrawing units. Bulk heterojunction solar cell
devices are fabricated using the photoactive blend layer of
the polymer and PCBM ([6,6]-phenyl-C -butyric acid
Acknowledgment. Financial support for this research
by the Swiss National Science Foundation (grant no.
200021-147143) and the Sino Swiss Science and Technol-
ogy Cooperation is gratefully acknowledged.
6
1
methyl ester). A preliminary study has revealed that the
devices based on polymer 12a show power conversion
efficiencies (PCE) up to 1.08% under AM 1.5 illumination
Supporting Information Available. Experimental pro-
cedure and characterization data for all new molecules
1
and polymers, copies of their H NMR spectra, UVꢀvis
spectra of I, 3 and II, single-crystal structure of 4, as well
as detailed information on device fabrication and char-
acterization. This material is available free of charge via
the Internet at http://pubs.acs.org.
(
21) (a) Kim, H. G.; Jo, S. B.; Shim, C.; Lee, J.; Shin, J.; Cho, E. C.;
Ihn, S.-G.; Choi, Y. S.; Kim, Y.; Cho, K. J. Mater. Chem. 2012, 22,
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1
M.; Li, Y.; Hou, J. Chem. Commun. 2012, 48, 3318–3320. (c) Chen, X.;
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The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 21, 2013
5589