Thieno[3,2-b]pyrrolo donor fused with benzothiadiazolo, benzoselenadiazolo and quinoxalino acceptors: Synthesis, characterization, and molecular properties

Article abstract of DOI:10.1021/ol202199v

Nitrogen-bridged donor-acceptor multifused dithienopyrrolobenzothiadiazole (DTPBT) and dibenzothiadiazolopyrrolothiophene (DBTPT) were successfully synthesized by intramolecular Cadogan annulation. The electron-deficient benzothiadiazole unit in DTPBT can be converted to benzoselenadiazole and quinoxaline moieties through reduction/cyclization to generate dithienopyrrolobenzoselenadiazole (DTPBSe) and dithienopyrroloquinoxaline (DTPQX), respectively. The nitrogen atoms function as the bridges for covalent planarization to induce intermolecular interaction and intramolecular charge transfer.

Full text of DOI:10.1021/ol202199v

ORGANIC
LETTERS
2011
Vol. 13, No. 20
5484–5487
Thieno[3,2-b]pyrrolo Donor Fused with
Benzothiadiazolo, Benzoselenadiazolo
and Quinoxalino Acceptors: Synthesis,
Characterization, and Molecular
Properties
Yen-Ju Cheng,*,† Chiu-Hsiang Chen,† Yu-Ju Ho,† Shu-Wei Chang,†,‡
Henryk A. Witek,†,‡ and Chain-Shu Hsu*,†
Department of Applied Chemistry, Institute of Molecular Science, National Chiao Tung
University 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan
yjcheng@mail.nctu.edu.tw; cshsu@mail.nctu.edu.tw
Received August 13, 2011
ABSTRACT
Nitrogen-bridged donorꢀacceptor multifused dithienopyrrolobenzothiadiazole (DTPBT) and dibenzothiadiazolopyrrolothiophene (DBTPT) were
successfully synthesized by intramolecular Cadogan annulation. The electron-deficient benzothiadiazole unit in DTPBT can be converted to
benzoselenadiazole and quinoxaline moieties through reduction/cyclization to generate dithienopyrrolobenzoselenadiazole (DTPBSe) and
dithienopyrroloquinoxaline (DTPQX), respectively. The nitrogen atoms function as the bridges for covalent planarization to induce intermolecular
interaction and intramolecular charge transfer.
In recent years, tremendous research effort has been
focused toward organic photovoltaics (OPVs) to realize
low-cost, lightweight, large-area, and flexible photovoltaic
devices.1 Development of novel conjugated polymers con-
tinues to play the most important role in boosting the high
efficiency of OPVs.2 The widely used strategy to produce a
low band gap (LBG) polymer is to polymerize an electron-
rich donor (D) monomer with an electron-deficient
acceptor (A) monomer along the conjugated polymer
backbone.3 In addition to the D-A strategy, forced planar-
ization by covalently fastening adjacent aromatic units in
the polymer backbone provides an effective way to reduce
the band gap and enhance the intrinsic charge mobility.4 In
this regard, extensive research effort has been focused on
the design and synthesis of coplanar electron-rich donors
having multifused aromatic or heteroaromatic structures.5
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† Department of Applied Chemistry.
‡ Institute of Molecular Science.
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10.1021/ol202199v
Published on Web 09/27/2011
2011 American Chemical Society

By copolymerization with appropriate acceptors, these
fascinating structures have produced a variety of pro-
mising D-A polymers that led to high-performance solar
cells. We envision that the electronic interactions be-
tween the donor and the acceptor units in a D-A
copolymer might be greatly enhanced, if the neighboring
electron-rich donor and electron-deficient acceptor
units along the polymer backbone are covalently locked
and conformationally rigidified into a coplanar inter-
fused D-A assembly.6 To our surprise, however, utiliza-
tion of a fused D-A polymer for OPVs has not been well
exploited presumably due to the lack of a useful syn-
thetic approach to prepare a suitable fused D-A mono-
mer precursor. Among a range of electron-deficient
heteroaromatic units, the benzothiadiazole (BT), ben-
zoselenadiazole (BSe), and quinoxaline (QX) units are of
particular interest due to their strong electron affinity
and simple planar structure.7 The benzothiadiazole unit
(acceptor) coupled with two electron-rich thienyl rings
(donor) to form a nonfused D-A-D 4,7-di(thiophen-2-
yl)benzothiadiazole (DTBT) unit has been the com-
monly used core structure in D-A polymers for OPVs.8
Inspired by the skeleton of the DTBT unit, we have
developed a fused D-A-D dithienopyrrolobenzothiadia-
zole (DTPBT) structure in which the 3-positions of the
two outer thiophenes (D) are covalently linked to the 5-
and 6-positions of the central benzothiadiazole core (A)
by a nitrogen bridge, forming two pyrrole rings em-
bedded in a pentacyclic structure (Scheme 1). Further-
more, a fused A-D-A type structure, dibenzothiadia-
zolopyrrolothiophene (DBTPT) with reversed structural
arrangement of BT and thiophene units in DTPBT, was
also synthesized (Scheme 2). More importantly, the struc-
tureofDTPBTcanserve asa useful precursortosynthesize
two other fused D-A-D structures, dithienopyrrolobenzo-
selenadiazole (DTPBSe) and dithienopyrroloquinoxaline
(DTPQX), with different acceptors. The nitrogen bridges
in the pentacyclic units also allow us to introduce aliphatic
side chains to promote sufficient solubility of the resultant
conjugated polymers.
Scheme 1 depicts the synthetic route of DTPBT.
Nitration of 4,7-dibromobenzothiadiazole 1 afforded
the 4,7-dibromo-5,6-dinitrobenzothiadiazole 2 which
was then reacted with 2-tributylstannyl thiophene by
Stille coupling to yield compound 3. Double intramole-
cular Cadogan reductive cyclization of 3 in the presence
of triethyl phosphate successfully furnished the fused
D-A-D structure 4 in 61% yield.9 N-Alkylation of 4 with
an excess amount of 1-bromododecane in the presence of
potassium hydroxide resulted in the formation of
DTPBT in 71% yield. Fortunately, NBS bromination
of DTPBT regioselectively occurred at the 5-position of
the thienyl moieties to yield the brominated monomer
Br-DTPBT which can be readily polymerized with a
variety of conjugated units to prepare polymers contain-
ing fused D-A segments.
Scheme 1. Synthesis of Fused D-A-D Pentacylic Structure
DTPBT
The synthesis of fused A-D-A DBTPT is shown in
Scheme 2. Suzuki coupling of compound 6 with 7 afforded
the compound 8. Double intramolecular Cadogan annula-
tion of 8 obtained the desired product 9 where the central
dipyrrolothiophene moiety was fused with two outer ben-
zothiadiazole units. N-Alkylation of 9 resulted in DBTPT
with two dodecyl side chains in a lower yield of 11%
probably due to the less nucleophilic nitrogens as a result
of two electron-withdrawing BT units.
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Scheme 2. Synthesis of Fused A-D-A DBTPT
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2005, 70, 5014. (b) Qi, T.; Guo, Y.; Liu, Y.; Xi, H.; Zhang, H.; Gao, X.;
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Org. Lett., Vol. 13, No. 20, 2011
5485

Scheme 3. Synthesis of Fused D-A-D DTPBSe and DTPQX
Figure 2. Crystal packing of Br-DTPBT along a-axis. Hydrogen
atoms are removed for clarity.
The thiadiazole ring in DTPBT was reduced by Zn/
AcOH to generate corresponding diamine intermediate 10
(Scheme 3). Without any purification, 10 was directly
reacted with selenium dioxide to produce DTPBSe in
30% yield. Meanwhile, double imine condensation of 10
with benzil led to the formation of DTPQX in 25% yield.
All the target molecules with aliphatic side chains exhibit
good solubility in common organic solvents such as hex-
ane, dichloromethane, acetone, chloroform, and THF. All
the compounds were fully characterized by using 1H and
13C NMR and mass spectrometry (see the Supporting
Information (SI)).
Figure 3. Absorption spectra (a) and emission spectra (b) of all
the fused DꢀA molecules in toluene.
data are summarized in Table 1. The nonfused DTBT
exhibited two characteristic bands in the absorption spec-
tra in toluene (Figure S1). The shorter wavelength absor-
bance at ∼310 nm comes from the πꢀπ * transition of the
pentacyclic units, while the lower energy band at ∼450 nm
is attributed to the intramolecular charge transfer (ICT)
between the thiophene units and the benzothiadiazole
segments. DTPBT displays well-defined vibronic struc-
tures in the πꢀπ* transition bands as a result of its
coplanar and rigid conformation. In addition to the two
typical bands similar to DTBT, an extra band at ∼368 nm
was observed in the fused DTPBT analogue. This band
could be ascribed to another charge transfer transition
from a short D-π-A structure in DTPBT. This indicates
thattheN-atom not onlyservesasabridgebut alsoinduces
a charge transfer due to the coplanar structure, leading to a
greater dipole moment in the excited state. Consistently,
Figure 1. Top view (top) and side view (bottom) of ORTEP
drawing of Br-DTPBT (50% probability for thermal ellipsoids).
The structure of Br-DTPBT was particularly analyzed
by single-crystal X-ray crystallography as shown in Figure
1 (side and top views). The dihedral angle between the
central phenylene and outer thiophene rings of Br-DTPBT
is 6.9°, indicating a coplanar π-conjugated framework due
to the nitrogen bridges. The crystal packing is also shown
in Figure 2. It is found that Br-DTPBT molecules are
packed as anticofacial dimers with a short intermolecular
˚
distance of 3.4 A, indicating strong πꢀπ interaction. Note
that the aliphatic side chains are also interdigitated in a
regular arrangement.
The absorption spectra of these fused molecules in
toluene are shown in Figure 3a, and the corresponding
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Org. Lett., Vol. 13, No. 20, 2011

TD-DFT at 312, 349, and 454 nm, respectively, agreeing
well with their experimental λmax wavelengths (313, 368,
and 437 nm in toluene). A similar calculation of DBTPT is
also conducted and shown in the SI.
Table 1. Optical and Electrochemical Properties of Fused D-A
Materials
Stokes
λabs
λem
shift
HOMO a LUMO b Eg
opt
Compd
(nm)
(nm) (nm)
(eV)
(eV)
(eV)
DTPBT
313, 368, 528
437
91
111
125
66
ꢀ5.36
ꢀ2.84
2.52
DTPBSe 318, 369, 574
ꢀ5.31
ꢀ5.30
ꢀ5.46
ꢀ2.96
ꢀ2.73
ꢀ2.91
2.35
2.57
2.55
463
DTPQX
DBTPT
307, 332, 527
402
325, 383, 514
448
a The HOMO energy levels were obtained from the equation HOMO =
þ 4.8) eV. b LUMO levels were obtained
onset
onset
ꢀ(Eox
ꢀ E(ferrocene)
opt
.
from the equation LUMO = HOMO þ Eg
DTBT showed little solvatochromism effect, whereas
N-bridged DTPBT exhibited a more pronounced bath-
ochromic shift with the increasing solvent polarity.
DTPBSe and DTPQX also showed similar N-induced
absorption characteristics. However, compared to DTPBT,
DTPBSe exhibited bathochromic-shifted peaks, whereas
DTPQX showed a hypsochromic-shifted spectrum, indi-
cating that the accepting strength is in the order BSe > BT
> QX. The stronger accepting ability of benzoselenodia-
zole is associated with the smaller ionization potential and
more polarizable character of selenium.10 In comparison
with DTPBT, DBTPT having reversed arrangement of
thiophene and BT units exhibited a very similar absorption
profile. However, the lower-energy transition band in
DBTPT is more red-shifted and intense due to the higher
content of the BT units. The emission λmax of DTPBT,
DTPQX, and DBTPT are in the green-yellow region with
Figure 4. Frontier molecular orbital plots of DTPBT calculated
with TD-DFT at the B3LYP level.
The electrochemical properties have been evaluated by
cyclic voltammetry (Figure S3). The HOMO energy level
was estimated to be ꢀ5.36, ꢀ5.31, ꢀ5.30, ꢀ5.46 eV,
respectively (Table 1). It is envisaged that the low-
lying HOMO energy levels will be beneficial for oxida-
tive stability and open-circuit voltages of photovoltaic
devices.
The LUMO energy levels are deducted from their HOMO
energy levels and optical band gaps. The LUMO energy
levels were approximately estimated by subtracting the band
gap values from the corresponding HOMO levels
In summary, we have developed two fused donorꢀ
acceptor pentacyclic structures DTPBT and DBTPT by
double intramolecular Cadogan cyclization. The ben-
zothiadiazole unit in DTPBT can be further modified to
benzoselenadiazole and quinoxaline moieties through
reduction/cyclization to yield DTPBSe and DTPQX,
respectively. The nitrogen atoms can not only function as
the bridge for covalent planarization to induce πꢀπ inter-
action but also facilitate intramolecular charge transfer for
better light harvesting. Incorporation of these fused D-A
frameworks into conjugated polymers are highly promis-
ing and currently underway in our laboratory. Those
results will be reported in due course.
λmax at ∼514ꢀ528 nm, while the λmax of DTPBSe is in the
orange-red region with λmax at ∼574 nm in toluene
(Figure 3b). The emission spectra also showed a strong
dependence on the polarity of the solvent (Figure S2). All
the molecules showed a large Stokes shift between their
corresponding absorption and emission spectra. Such a
Stokes shift and solvent-dependent emission indicate the
effective intramolecular charge transfer in the excited
states between the thieno[3,2-b]pyrrole units and corre-
sponding acceptor units.
To gain insight into the excited states of DTPBT,
theoretical calculations have been performed using time-
dependent density functional theory (TD-DFT); for tech-
nical details, see SI. The recorded absorption spectra of
both compounds can be well interpreted using frontier
molecular orbitals shown in Figure 4. For DTPBT, the
three lowest, optically active excited states, roughly char-
acterized as the HOMOfLUMOþ1, HOMOꢀ1fLU-
MO, and HOMOfLUMO transitions, are located by
Acknowledgment. This work is supported by the
National Science Council and “ATP program” of the
National Chiao Tung University and Ministry of Educa-
tion, Taiwan.
Supporting Information Available. Detailed synthesis,
absorption and emission spectra, X-ray crystallographic
data of DTPBT, theoretical calculation of Br-DTPBT,
CV measurements, and NMR spectra. This material is
available free of charge via the Internet at http://pubs.
acs.org.
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