Thiadiazole fused indolo[2,3-a]carbazoles as new building blocks for optoelectronic applications

Article abstract of DOI:10.1016/j.tetlet.2011.02.041

New indolo[2,3-a][1,2,5]thiadiazolo[3,4-c]carbazoles including functional groups (halide or hydroxyl groups) on the meta positions relative to the nitrogen atoms which are known to be the most conjugated, have been synthesized following two synthetic rout

Full text of DOI:10.1016/j.tetlet.2011.02.041

Tetrahedron Letters 52 (2011) 1811–1814
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Thiadiazole fused indolo[2,3-a]carbazoles as new building blocks
for optoelectronic applications
Laure Biniek a, , Ibrahim Bulut a, Patrick Lévêque b, Thomas Heiser b, Nicolas Leclerc a
⇑
a Laboratoire d0Ingénierie des Polymères pour les Hautes Technologies (LIPHT), Ecole Européenne de Chimie, Polymères et Matériaux (ECPM),
Université de Strasbourg (UdS), 25 rue Becquerel, 67087 Strasbourg, France
b Institut d0Electronique du Solide et des Systèmes, Université de Strasbourg, CNRS, 23 rue du Loess, 67037 Strasbourg, France
a r t i c l e i n f o
a b s t r a c t
Article history:
New indolo[2,3-a][1,2,5]thiadiazolo[3,4-c]carbazoles including functional groups (halide or hydroxyl
groups) on the meta positions relative to the nitrogen atoms which are known to be the most conjugated,
have been synthesized following two synthetic routes. The characterizations of their optical and electro-
chemical properties evidence their potential as new building blocks for optoelectronic materials.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 1 December 2010
Revised 3 February 2011
Accepted 8 February 2011
Available online 13 February 2011
Keywords:
Benzothiadiazole
Indolo[3,2-a]carbazole
Conducting fused-molecules
Cadogan
Optoelectronic
Great attention has been paid to organic semiconductors (OSCs)
over the past two decades. OSCs are very attractive materials for
optoelectronic applications as their properties are easily tunable,
their synthesis pathway is versatile, and the OSCs based-devices
can be flexible. The most investigated OSCs based-devices are
Organic Field Effect Transistors (OFETs),1 Organic Light Emitting
Diodes (OLEDs),2 and Organic Photovoltaic Cells (OPVs).3 Develop-
ment of conjugated materials relies on the synthesis of several
chemical molecules based on units such as thiophene, carbazole,
furan, and fluorene. In particular, the fused-ring heterocyclic aro-
matic compounds have been widely investigated. Their co-planar
structure allows a good molecular organization4 and a strong
strategies to answer to the absorption issue met in bulk hetero-
junction (BHJ) OPVs.7 Therefore, several interesting oligomers
and co-polymers have been synthesized following this alternation
approach and used with success in OSCs.8,9 Additionally, the facile
alkylation of the nitrogen atoms can lead to high solubility and
processability of indolocarbazole derivatives and therefore helps
to overcome a classical weakness of fused-ring-molecules based
materials.
However, since the molecules developed by Balaji et al. are
reactive-function free, electropolymerization led only to polyindo-
locarbazole with a conjugation pathway going through the nitro-
gen atoms (polymerization through the 3- and 9-positions, see
p
-stacking which are beneficial for high charge carrier mobilities.5
Balaji et al. have investigated a new indolo[2,3-a]carbazole
fused with a benzothiadiazole unit.6 In addition to the ladder-type
nature of this fused heterocyclic aromatic compound, this mole-
cule presents an alternation in the conjugated pathway of elec-
tron-rich moieties (indolo[2,3-a]carbazole) and electron-deficient
units (2,1,3-benzothiadiazole). The concept of alternating electron
donor and electron acceptor moieties is well known in the field of
low band-gap co-polymers. Indeed, the internal charge transfer
(ICT) from the electron-rich to the electron-poor units considerably
lowers the co-polymer band-gap. It is one of the most popular
S
N
N
8
4
7
5
9
3
10
2
X
X
N
N
6
12
1
11
1A, X = Cl
1B, X = OH
⇑
Corresponding author. Tel.: +33 390 242 786; fax: +33 390 242 716.
E-mail address: laure.biniek@etu.unistra.fr (L. Biniek).
Scheme 1. Structure of the synthesized molecules.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.041

1812
L. Biniek et al. / Tetrahedron Letters 52 (2011) 1811–1814
Scheme 1). Leclerc et al. showed recently by using a systematic
investigation of carbazole- and indolocarbazole-based molecules
that a longer conjugation length could be obtained in the case of
the para-phenylene-like conjugation pathway (polymerization or
extension of the conjugation through the 2- and 10-positions) than
in the case of the conjugation through the nitrogen atoms.10 Unfor-
tunately, the reactivity in indolocarbazole or carbazole is decreas-
ing from para positions (positions 3 and 9) to ortho and meta
positions (positions 1–11 and 2–10, respectively) of the nitrogen
atom. The meta-positions cannot be directly functionalized by
standard electrophilic aromatic substitution. Therefore a synthetic
strategy starting from an initial phenyl derivative including a func-
tional group in the 2- and 10-positions has to be used.
In this Letter, we report two different routes allowing the syn-
thesis of new indolo[2,3-a][1,2,5]thiadiazolo[3,4-c]carbazole
including functional groups (halide or hydroxyl groups) in most
conjugated 2- and 10-positions (Scheme 1).
Both routes are based on the cyclization of nitrobiaryl following
a Cadogan reaction procedure.11 Those ring-closures were achieved
via reductive deoxygenation of the nitro groups using a slight ex-
cess of triethylphosphite.
sium t-butoxide as the base and 18-crown-6 (crown ether) as the
phase transfer catalyst in dry THF. Compound 1A has been isolated
after purification by column chromatography.13 Despite these con-
ditions, the alkylation yield remains very low (ꢀ10%), in particular
in comparison to the yields described by Balaji et al. This could be
due to the deactivation of secondary amines in compound 5, which
in turn results from the presence of electronegative chlorine atoms
and an electron acceptor benzothiadiazole moiety. However, the
recovered monoalkyl and alkyl-free fractions could be used again
in order to increase significantly the final yield. The 2-ethylhexyl
chains give a very high solubility to the indolo[2,3-a]carbazole
1A in common organic solvents.
Despite the limited amount of synthesis steps (three), route A
has been shown to be inefficient especially due to the low yield
of the Suzuki cross-coupling reaction. In addition, the 2,1,3-benzo-
thiadiazole-4,7-bis(boronic acid pinacol ester) compound is rela-
tively expensive. We therefore investigated the alternative route
B (Scheme 3) based on the functionalization of 2,7-dibromo-
2,1,3-benzothiadiazole with nitro groups on 5- and 6-positions.
In this route, the methoxy function is a precursor of the triflate
group which is an excellent leaving group used in cross-coupling
reactions. We thus decided to start from the aniline unit as outer
phenyl ring. The 4,7-dibromo-5,6-dinitro-2,1,3-benthiadiazole
has been synthesized following a procedure reported in the litera-
ture.14 Stannylation of bromoanisole gave compound 7 in a high
yield. A Stille cross-coupling reaction has been carried out between
compound 8 and an excess of compound 7. The crude product has
been washed thoroughly with cyclohexane to give in good yield
(61%) compound 9, which is an orange solid.15 The following steps
are similar to the one used in route A. Cadogan cyclisations in tri-
ethylphosphite followed by a dialkylation with 2-ethylhexyl bro-
mide gave compound 11 in a moderate yield. It could be noted
that, as expected, the replacement of chlorine atom by electron do-
nor methoxy groups improved the efficiency of the N-alkylation of
indolocarbazole 10 with a significantly increased yield (37% com-
pared to 10% in the dichloro-indolocarbazole derivative 5). Finally,
the standard and quasi-quantitative deprotection reaction of
methoxy groups using BBr3 in methylene chloride gave the func-
tionalized dialcohol indolocarbazole derivative 1B.16 These hydro-
xyl groups are precursors of triflate groups that can undergo Stille
or Suzuki reactions.
Scheme 2 displays the synthesis route A, based on the approach
used recently by Balaji et al.6
In this route, the nitro groups are carried by the outer phenyl
moieties. However, in order to include a functional group in the
2- and 10-positions, we decided to start from the commercially
available 2-nitro-4-chloro-bromobenzene in which one halogen
(the bromine atom) will be used to achieve a cross-coupling reac-
tion and the other one (the chlorine atom) will be preserved as
functional group. Due to the lower reactivity of the chlorine atom
in comparison to the bromine atom during cross-coupling reaction,
a high regioselectivity could be expected. Thus, the Suzuki conden-
sation of commercially available compounds 2 and 3 leads to the
synthesis of bis-adduct compound 4 in 34% yield. The low reaction
yield, despite an excess of compound 2, could be due to the poor
purity of initial 2,1,3-benzothiadiazole-4,7-bis(boronic acid pina-
col ester) together with the very low solubility in a THF/water mix-
ture of the mono-adduct intermediate which could precipitate and
therefore suspend the reaction. Purification of the product was
accomplished by washing thoroughly the crude solid in cyclohex-
ane. Subsequent Cadogan ring-closure in triethylphosphite gave
the first thiadiazole fused indolo[3,2-a]carbazole compound 5 in
57% yield, similar to the one published by Balaji et al.12 Several
alkylation conditions have been tested. Finally, the dialkyl-product
was prepared by the reaction of 2-ethylhexyl bromide with potas-
Both routes allowed the synthesis of the targeted indolocarbaz-
ole. However, while the route A is shorter and starts from commer-
cially available compounds, it is significantly less efficient than
route B with an overall yield of 2%. Route B, with two additional
S
S
N
N
N
N
O
a
O
Cl
Br
NO2
+
Cl
Cl
B
B
O
O
NO2
O2N
2
3
4
b
S
S
N
N
N
N
c
Cl
Cl
N
N
Cl
Cl
N
N
H
H
5
1A
Scheme 2. Route A for the synthesis of thiadiazole fused indolocarbazole. Reagents and conditions: (a) Pd(PPh3)4, K2CO3 aq 2 M, THF, reflux, 24 h, 34%; (b) P(OEt)3, o-DCB,
reflux, 24 h, 57%; (c) t-BuOK, bromo-2-ethylhexyl, 18-crown-6, dry THF, reflux, 42 h, 12%.

L. Biniek et al. / Tetrahedron Letters 52 (2011) 1811–1814
1813
a
MeO
Sn(Me)3
MeO
Br
7
6
S
N
N
b
Br
O2N
Br
NO2
8
S
S
N
N
N
N
c
MeO
OMe
MeO
OMe
N
N
O2N
NO2
H
H
10
9
d
S
S
N
N
N
N
e
OH
HO
MeO
OMe
N
N
N
N
1B
11
Scheme 3. Route B for the synthesis of thiadiazole fused indolocarbazole. Reagents and conditions: (a) n-BuLi, dry THF, SnMe3Cl, ꢁ78 °C?RT; (b) Pd(PPh3)4, THF, reflux, 24 h,
73%; (c) P(OEt)3, o-DCB, reflux, 24 h, 61%; (d) t-BuOK, bromo-2-ethylhexyl, 18-crown-6, dry THF, reflux, 42 h, 37%; (e) BBr3, dry CH2Cl2, reflux, 12 h, 94%.
steps, has been easier to perform and more efficient. The overall
yield of route B (4 steps) is 16%.
The UV–vis absorption spectra of the indolo[2,3-a][1,2,5]thi-
adiazolo[3,4-c]carbazoles in solution are shown in Figure 1. Both
compounds show similar absorption spectra that are typical of pla-
nar conjugated systems,17 with several maxima.
Successive absorption maxima of both derivatives are centered
around 255, 320, and 380 nm with an edge of absorption at 455
and 490 nm for 1A and 1B, respectively. The longer conjugation
in the case of 1B, with a fourth maximum around 440 nm, is due
to the electron donating nature of the hydroxyl group.
peaks are observable at 1.25 V and ꢁ1.65 V, respectively. HOMO
and LUMO levels could be estimated from the oxidation and reduc-
tion onsets18 to 5.49 eV and 3.05 eV, respectively. As expected, the
HOMO level is in agreement with the value calculated by Balaji
et al. on their molecules (HOMO level of 5.49 eV and 5.53 eV for
the derivatives with methyl and hexyl chains as alkyl chains,
respectively). The difference between the HOMO and LUMO levels
leads to an electrochemical band-gap of 2.44 eV. In addition, the
low-lying HOMO level highlights the good air stability of our lad-
der unit.
In conclusion, we described the synthesis of indolo[2,3-
a][1,2,5]thiadiazolo[3,4-c]carbazoles functionalized, by halide or
hydroxyl groups, in 2- and 10-positions following two different
routes. Both compounds have shown interesting absorption
properties (UV–visible spectroscopy) as well as well-positioned
HOMO and LUMO levels (cyclic voltammetry) for optoelectronic
applications. In addition, due to the functionalization in the
‘meta-positions’ (2- and 10-positions) of the nitrogen atoms, these
thiadiazole-fused indolocarbazoles can be further extended or
polymerized by condensation reactions, such as Suzuki or Stille
reactions. Therefore, these new building blocks appear as good
candidates for the design and synthesis of new optoelectronic olig-
omers and polymers.
Finally, cyclic voltammetry has been performed on compound
1A in solution in dry methylene chloride. Oxidation and reduction
Acknowledgment
The authors acknowledge the financial support from the Région
Alsace.
References and notes
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220
270
320
370
420
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Wavelength (nm)
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Figure 1. UV–vis absorption of indolocarbazoles 1A (continuous line) and 1B (black
square) in solution with chloroform was used as the solvent.

1814
L. Biniek et al. / Tetrahedron Letters 52 (2011) 1811–1814
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1.89 (t, 3J = 6.1 Hz, 2H), 0.97 (m, 6H), 0.76 (m, 10H), 0.57 (m, 12H). 13C NMR
(75 MHz, CDCl3, d, ppm): 149.09, 141.66, 133.16, 130.59, 124.15, 123.07,
122.84, 112.46, 111.91, 52.35, 38.50, 29.59, 27.52, 23.17, 22.65, 13.64, 9.95.
Anal. Calcd for C34H40Cl2N4S: C, 67.20; H, 6.63; Cl, 11.67; N, 9.22; S, 5.28.
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15. Compound 9. To a degassed solution of compound 7 (660 mg, 2.43 mmol) and 8
(310 mg, 0.81 mmol) in THF (15 mL), catalytic amount of Pd(PPh3)4 was added.
The solution was then refluxed for 24 h. Then the reaction mixture was cooled
down to RT and filtered. The precipitate was then washed thoroughly with
cyclohexane to give 259 mg of compound 9 as an orange powder. 1H NMR
(300 MHz, DMSO d6, d, ppm): 7.57 (d, 3J = 8.7 Hz, 4H), 7.19 (d, 3J = 8.7 Hz, 4H),
3.36 (s, 6H). 13C NMR (75 MHz, DMSO d6, d, ppm): 160.57, 152.86, 141.13,
130.70, 128.31, 122.65, 114.36, 55.36.
16. Compound 1B. To a solution of compound 11 (80 mg, 0.134 mmol) in dry
dichloromethane (10 mL), BBr3 1 M in hexane (0.5 mL, 0.5 mmol) was added
dropwise. The solution was then refluxed for 12 h. After cooling to RT, the
reaction was quenched with methanol and the organic phase was washed with
water. After the solvent evaporation under vacuum, the crude product was
purified on flash column chromatography (90/10:cyclohexane/ethyl acetate as
the eluent) to give 68 mg of compound 1B as an orange oil. 1H NMR (300 MHz,
CDCl3, d, ppm): 8.56 (d, 3J = 8.3 Hz, 2H), 7.09 (d, 4J = 1.6 Hz, 2H), 7.01 (dd,
3J = 8.3 Hz, 4J = 1.4 Hz, 2H), 5.38 (s, 2H), 4.54 (d, 3J = 7.3 Hz, 4H), 1.95 (t,
3J = 6.0 Hz, 2H), 0.97 (m, 6H), 0.77 (m, 10H), 0.56 (m, 12H). 3C NMR (75 MHz,
CDCl3, d, ppm): 153.62, 149.12, 142.48, 133.00, 122.78, 120.14, 111.55, 111.28,
98.69, 52.14, 38.06, 30.18, 27.62, 23.13, 22.67, 13.66, 9.93. Anal. Calcd for
9. Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.;
Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photon. 2009, 3, 297–303.
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12. Compound 5. A solution of compound 4 (0.2 g, 0.45 mmol) in triethylphosphite
(5 mL) and o-DCB (2 mL) was refluxed for 24 h. After cooling to RT, solvents
were removed by distillation under vacuum and the crude product was
purified on column chromatography (60/40:cyclohexane/ethyle acetate as
eluent) to give 98 mg of compound 5. 1H NMR (300 MHz, DMSO d6, d, ppm):
12.03 (s, 2H), 8.45 (d, 3J = 8.5 Hz, 2H), 8.02 (d, 4J = 1.9 Hz, 2H), 7.44 (dd,
3J = 8.5 Hz, 4J = 1.9 Hz, 2H). 13C NMR (75 MHz, DMSO d6, d, ppm): 149.21,
139.17, 130.22, 129.88, 123.06, 122.67, 122.59, 113.50, 108.86.
13. Compound 1A. To
a solution of compound 5 (90 mg, 0.23 mmol), t-BuOK
(65 mg, 0.58 mmol) and 18-crown-6 crown ether (6.2 mg, 2.3 ꢂ 10ꢁ5 mol) in
dry THF (7 mL) was added bromo-2-ethylhexyl (104 lL, 0.58 mmol). The
solution was the refluxed for 42 h. After cooling to RT, the reaction was
quenched with water, extracted with ethyl acetate and the organic phase
was washed with water and dried over sodium sulphate. After solvent
evaporation, the crude product was purified on column chromatography (90/
10:cyclohexane/ethyl acetate as eluent) to give 17 mg of compound 1A as an
orange oil. 1H NMR (300 MHz, CDCl3, d, ppm): 8.65 (d, 3J = 8.4 Hz, 2H), 7.61 (d,
4J = 1.5 Hz, 2H), 7.44 (dd, 3J = 8.4 Hz, 4J = 1.7 Hz, 2H), 4.59 (d, 3J = 7.5 Hz, 4H),
C
34H42N4O2S: C, 71.54; H, 7.42; N, 9.82; O, 5.61; S, 5.62. Found: C, 71.48; H,
7.46; N, 9.83; O, 5.59; S, 5.64.
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