A simple route toward the synthesis of bisbenzothiadiazole derivatives

Article abstract of DOI:10.1021/ol8022837

(Chemical Equation Presented) A simple and efficient route toward the synthesis of 4,4′-bis(2,1,3-benzothiadiazole) and 7,7′-dibromo-4, 4′-bis(2,1,3-benzothiadiazole) has been developed. Oligomers were synthesized with bisbenzothiadiazole units either at the periphery or core, and each oligomer was characterized by X-ray crystallography. Both crystal structures display supramolecular interactions between the conjugated backbones, although the former, bearing two bisbenzothiadiazole units, has extended interactions within layers that engage all of the thiadiazole rings.

Full text of DOI:10.1021/ol8022837

ORGANIC
LETTERS
2008
Vol. 10, No. 24
5533-5536
A Simple Route toward the Synthesis of
Bisbenzothiadiazole Derivatives
Piyush Anant,† Nigel T. Lucas,‡ and Josemon Jacob*,†
Centre for Polymer Sciences and Engineering, Indian Institute of Technology, Hauz
Khas, New Delhi 110016, India, and School of Chemistry, The UniVersity of Sydney,
NSW 2006, Australia
jacob@polymers.iitd.ac.in
Received October 4, 2008
ABSTRACT
A simple and efficient route toward the synthesis of 4,4′-bis(2,1,3-benzothiadiazole) and 7,7′-dibromo-4,4′-bis(2,1,3-benzothiadiazole) has been
developed. Oligomers were synthesized with bisbenzothiadiazole units either at the periphery or core, and each oligomer was characterized
by X-ray crystallography. Both crystal structures display supramolecular interactions between the conjugated backbones, although the former,
bearing two bisbenzothiadiazole units, has extended interactions within layers that engage all of the thiadiazole rings.
2,1,3-Benzothiadiazole-based oligomers and polymers have
been widely studied in recent years as active materials in
various optoelectronic devices because of the electron-
accepting capability of the heterocyclic group and the
observed low band gap in polymers containing it.1-3
Thiadiazole-containing compounds show a tendency to form
well-ordered crystal structures due to strong intermolecular
interactions across heteroatom contacts and/or π-π interac-
tions.4 2,1,3-Benzothiadiazole derivatives are efficient fluo-
rophores, and polymers containing them have shown promise
for use in electroluminescent devices.5-7 Copolymerization
of benzothiadiazole with fluorene, thiophene, or other suitable
aromatics can be used as a means to tune the HOMO-LUMO
levels in the resulting polymers.8,9 However, there are
relatively few reports on the use of bisbenzothiadiazole based
materials, in part due to a lack of an efficient method for
their synthesis.10-12 Here we report a facile method for the
synthesis of 4,4′-bis(2,1,3-benzothiadiazole) and its bromi-
nated derivative 7,7′-dibromo-4,4′-bis(2,1,3-benzothiadiaz-
ole). The latter has been coupled with suitable alkylated
fluorene moieties to generate soluble conjugated molecules
with bisbenzothiadiazole units both at the core and the
periphery.
(6) Huang, J.; Niu, Y.; Yang, W.; Mo, Y.; Yuan, M.; Cao, Y.
Macromolecules 2002, 35, 6080–6082.
† Indian Institute of Technology.
(7) Bouffard, J.; Swager, T. M. Macromolecules 2008, 41, 5559–5562.
‡ The University of Sydney.
(8) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91,
(1) Dhanabalan, A.; van Duren, J. K. J.; van Hal, P. A.; van Dongen,
954–985
(9) Herguth, P.; Jiang, X.; Liu, M. S.; Jen, A. K. Y. Macromolecules
2002, 35, 6094–6100
(10) Akhtaruzzaman, M.; Tomura, M.; Nishida, J.; Yamashita, Y. J. Org.
Chem. 2004, 69, 2953–2958
(11) Akhtaruzzaman, M.; Tomura, M.; Nishida, J.; Yamashita, Y. Synth.
Met. 2003, 137, 873–874
(12) Suzuki, T.; Okubo, T.; Okada, A.; Yamashita, Y.; Miyashi, T.
Heterocycles 1993, 35, 395–406
.
J. L. J.; Janssen, R. A. J. AdV. Funct. Mater. 2001, 11, 255–262
(2) Zhang, M.; Tsao, H. N.; Pisula, W.; Yang, C.; Mishra, A. K.; Mullen,
K. J. Am. Chem. Soc. 2007, 129, 3472–3473
.
.
.
(3) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477–487
.
.
(4) Suzuki, T.; Fujii, H.; Yamashita, Y.; Kabuto, C.; Tanaka, S.;
Harasawa, M.; Mukai, T. J. Am. Chem. Soc. 1992, 114, 3034–3043.
(5) Raimundo, J. M.; Blanchard, P.; Brisset, H.; Akoudad, S.; Roncali,
.
J. Chem. Commun. 2000, 939–940
.
.
10.1021/ol8022837 CCC: $40.75
Published on Web 11/13/2008
2008 American Chemical Society

The synthesis of bisbenzothiadiazole 3 is shown in Scheme
1. 4-Bromo-2,1,3-benzothiadiazole 1 was prepared according
to literature.13 This was converted to the corresponding
boronate ester 2 in 62% yield using bis(pinacolato)diboron
in the presence of Pd(dppf)Cl2 as a catalyst. Standard Suzuki
coupling of 1 and 2 gave bisbenzothiadiazole 3 in 86%
isolated yield. This method is far superior to previously
reported nickel-based coupling methods in terms of both
yield and simplicity of purification.14 Bisbenzothiadiazole 3
was further brominated with molecular bromine by refluxing
it in the presence of aqueous HBr (47%) to afford 7,7′-
dibromo-4,4′-bis(2,1,3-benzothiadiazole) 4 in 50% yield.
Compounds 1, 2, and 4 are useful reagents for the synthesis
of well-defined bisbenzothiadiazole-based oligomers and
polymers, in particular the difunctional compound 4, which
holds great potential as a comonomer in the synthesis of low
band gap materials. Using the above reagents, two types of
oligomers were synthesized: one in which the benzothiadia-
zole units are introduced at the periphery and a second in
which a bisbenzothiadiazole unit constitutes the core.
diazole core (Scheme 3). Again, Suzuki coupling of 4 with
a 2-boronate ester of fluorene 9 gave 7,7′-di(9,9-dioctylfluo-
ren-2-yl)-4,4′-bis(2,1,3-benzothiadiazole) 10 in 50% yield.
This compound also shows very good solubility in common
organic solvents. All of the oligomers described above were
characterized by 1H and 13C NMR, elemental analysis, mass
spectrometry (MALDI-TOF), UV-vis absorption and emis-
sion spectroscopies, and in some cases, by X-ray crystal-
lography.
Scheme 2. Synthesis of
2,7-Di(bisbenzothiadiazolyl)-9,9-dioctylfluorene 8
Scheme 1. Synthesis of
7,7′-Dibromo-4,4′-bis(2,1,3-benzothiadiazole) 4
The synthetic approach toward the synthesis of oligomer
8 with two benzothiadiazole units at either side of a fluorene
moiety is shown in Scheme 2. Suzuki coupling of the
diboronate ester 5 with 1 gave 2,7-di(2,1,3-benzothiadiazol-
4-yl)-9,9-dioctylfluorene 6 in 78% yield. This was bromi-
nated with molecular bromine to generate 2,7-di(4-bromo-
2,1,3-benzothiadiazol-7-yl)-9,9-dioctyl-fluorene 7 in 71%
yield. Coupling of the boronic ester 2 with 7 under standard
conditions generated 2,7-di(bisbenzothiadiazolyl)-9,9-dio-
ctylfluorene 8 in 50% yield. This monomer shows good
solubility in common organic solvents and can be purified
by recrystallization.
To investigate their electronic properties, compounds 3,
6, 8, and 10 were characterized by UV-vis spectroscopy.
A plot of extinction coefficient versus wavelength for all the
above compounds as measured in THF is depicted in Figure
1. Bisbenzothiadiazole has an absorption maximum of 360
nm, whereas 2,1,3-benzothiadiazole absorbs at 306 nm.
When fluorene is end-capped at the 2,7-positions with
benzothiadiazole (6), the absorbance maximum is observed
at 393 nm. It should be noted that 9,9-dioctylfluorene absorbs
at 314 nm, and thus the introduction of two benzothiadiazole
units induces a red shift of 79 nm. When two benzothiadia-
zole units are introduced at both sides of the fluorene moiety,
the wavelength of maximum absorbance now increases to
425 nm. On the other hand, when bisbenzothiadiazole is end-
capped with fluorene units on either side, the absorbance
maximum is observed at 436 nm. The observed trend is in
accordance with what is expected; however, the extent of
The related molecule 10 was synthesized in which alky-
lated fluorene units are at the periphery of a bisbenzothia-
(13) Pilgram, K.; Zupan, M.; Skils, R. J. Heterocycl. Chem 1970, 7,
629–633.
(14) Fukushima, T.; Okazeri, N.; Miyashi, T.; Suzuki, K.; Yamashita,
Y.; Suzuki, T. Tetrahedron Lett. 1999, 40, 1175–1178.
5534
Org. Lett., Vol. 10, No. 24, 2008

conjugation between neighboring benzothiadiazole units and
fluorene units will largely depend on the twist angle. The
above data clearly suggest that the corresponding polymers
have potential as low band gap materials, an application of
these oligomers that is currently being investigated.
Scheme 3. Synthesis of
7,7′-Di(9,9-dioctylfluoren-2-yl)-4,4′-bis(2,1,3-benzothiadiazole)
10
Figure 1. Normalized absorbance and emission spectra (the
observed maxima shown in brackets) of the various oligomers in
THF.
48.4°, respectively; however, the monocation of the first (as
a PF6 salt)12 is close to planar (3.6°/4.6°). The crystal packing
of 8 provides an indication of how it may organize within
the thin films typically used in optoelectronic devices. Tightly
packed sheets of 8, stacked along the a-axis, display
The photoluminescence spectra for the various oligomers
were recorded in THF and are depicted in Figure 1.
Bisbenzothiadiazole has an emission maximum at 442 nm.
The fluorene-bisbenzothiadiazole based oligomers emit in
the green region of the spectrum. When fluorene is end-
capped at the 2,7-positions with bisbenzothiadiazole (8), the
emission maximum is observed at 525 nm. However, when
bisbenzothiadiazole is end-capped with fluorene units on
either side, the emission maximum is observed at 531 nm.
The observed Stokes shifts for compounds 3, 6, 8, and 10
are 82, 105, 100, and 95 nm, respectively, which suggests
that there is a large twist between benzothiadiazole units
themselves as well as between benzothiadiazole and neigh-
boring aromatic units. This has further been probed by X-ray
crystallographic studies, which in addition to confirming
atomic connectivity provide insight into molecular conforma-
tion and solid state organization.
Slow diffusion of methanol into a solution of 8 in
dichloromethane yielded single crystals suitable for X-ray
diffraction structure analysis (Figure 2). The monomer 8
crystallizes with two molecules in the asymmetric unit,
related through a pseudoinversion center and thus having
similar geometries. The unsaturated core has a nonplanar
conformation, with twist angles of 43.8-54.0° within the
bisbenzothiadiazole units (anti to each other), which in turn
are 36.16-41.6° with respect to the central fluorene. Three
crystal structures containing a 4,4′-bis(2,1,3-benzothiadiazole)
have been previously reported, all symmetrically substituted
in the 7,7′ positions. 4,4′-Bis(7-(1-pyrrolidinyl)-2,1,3-ben-
zothiadiazole)12 and 4,4′-bis(7-iodo-2,1,3-benzothiadiazole)15
each have a nonplanar core with twist angles of 43.4° and
Figure 2. Crystal structure of 8. (a) ORTEP perspective of one of
two molecules in the asymmetric unit. (b) Packing as viewed down
the a-axis, indicating significant intermolecular interactions (only
the first carbons of the octyl chains are shown).
significant intermolecular contacts (Figure 2b). Pairs of short
S···N contacts (A) (3.088-3.237 Å) between each terminal
benzothiadiazole and its neighbor give rise to ribbons running
along the c-axis. These ribbons combine through π-stacking
(B) of inner benzothiadiazole groups, and C-H···arene
interactions (C) between the electron-poor benzothiadiazole
(C-H) and electron-rich fluorene. Close S···N intermolecular
contacts in crystals have been previously described and
(15) Tomura, M.; Akhtaruzzaman, M.; Suzuki, K.; Yamashita, U. Acta
Crystallogr. 2002, C58, o373–o375.
Org. Lett., Vol. 10, No. 24, 2008
5535

attributed to electrostatic interaction between dipoles and/or
weak intermolecular bonding involving a partial rehydrid-
ization at sulfur with d-orbital participation.4 The octyl chains
of 8 lie in the plane of and at the interface between adjoining
layers (Figure SI1, Supporting Information).
Crystals of 10 grown by slow diffusion of methanol into
a chloroform solution were also subjected to X-ray diffraction
analysis. Oliogomer 10 crystallizes in a centrosymmetric
space group, in this case with an inversion center lying on
the bond between the benzothiadiazoles (Figure 3). A twist
angle of 37.7° between the benzothiadiazoles (anti) and of
30.1° between the fluorenyl and bisbenzothiadiazole core
results in an overall rotation from end to end of nearly 98°.
In contrast to the layers of 8, the molecules of 10 pack into
ordered double columns along the c-axis (Figures SI2 and
SI3, Supporting Information), surrounded by a sheath of octyl
chains with significant disorder. No S···N contacts are
observed, but short edge-to-face contacts between S and the
neighboring fluorene ring (3.38 Å) (D; Figure 3b) and
fluorenyl C-Hs and a benzothiadiazole ring (E) are expected
to contribute the stability of the columns. The lower ratio of
benzothiadiazole to octyl chain in 10 (1:2) compared with 8
(2:1) provides fewer opportunities for favorable S···N interac-
tions, resulting in a lower density packing.
In conclusion, a simple and efficient route toward the
synthesis of 7,7′-dibromo-4,4′-bis(2,1,3-benzothiadiazole) has
been developed. Studies on oligomers containing this unit
suggest that its incorporation into polymers could potentially
lead to new classes of low band gap materials. Crystal-
lographic studies on the newly synthesized oligomers
containing bisbenzothiadiazole and dialkylfluorenyl moieties
highlight the types of supramolecular interactions that drive
their packing and provides evidence for rationalizing
structure-property relationships.
Figure 3. Crystal structure of 10. (a) ORTEP perspective of the
symmetry expanded asymmetric unit. (b) Packing as viewed down
the b-axis, showing significant intermolecular interactions (only the
first carbons of the octyl chains are shown).
Acknowledgment. The authors acknowledge the Depart-
ment of Science and Technology, New Delhi, India for a
generous research grant. P.A. acknowledges a research
fellowship from the Council for Scientific and Industrial
Research (CSIR), New Delhi, India. The Australian Research
Council is also thanked for financial support and the award
of an Australian Postdoctoral Fellowship to N.T.L.
Supporting Information Available: Synthetic and crys-
tallographic experimental procedures, full spectroscopic data
for all new compounds and additional crystal structure
packing diagrams, including CIF files. This material is
available free of charge via the Internet at http://pubs.acs.org.
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