Thiadiazole fused indolo[2,3-a]carbazole based oligomers and polymer

Article abstract of DOI:10.1021/ol901806q

Indolo[2,3-a]carbazole-based heteroacenes containing thiadiazole units were synthesized. Compounds showed a sandwich herringbone packing In solid state with improved stability. The change In photophysical and electrochemical properties upon incorporation

Full text of DOI:10.1021/ol901806q

ORGANIC
LETTERS
2009
Vol. 11, No. 19
4450-4453
Thiadiazole Fused Indolo[2,3-a]carbazole
Based Oligomers and Polymer
Ganapathy Balaji, Wong Low Shim, Manoj Parameswaran, and
Suresh Valiyaveettil*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117 543
chmsV@nus.edu.sg
Received August 5, 2009
ABSTRACT
Indolo[2,3-a]carbazole-based heteroacenes containing thiadiazole units were synthesized. Compounds showed a sandwich herringbone packing
in solid state with improved stability. The change in photophysical and electrochemical properties upon incorporation of an acceptor moiety
(benzothiadazole) in a fused ring system was studied. The unsubstituted compound was electropolymerized to yield a stable polymer.
Over the years, development of conjugated materials for the
applications in organic electronics involves several common
groups such as oligothiophenes,1 oligofluorenes,2 polycyclic
aromatic compounds,3 and fused heterocyclic aromatic
compounds.4 In particular, fused-ring aromatic compounds
exhibit good π-stacking and highly ordered molecular
structure, which are beneficial for the efficient charge
transport. Among the reported fused-ring aromatic systems,
mobilities of ∼3 cm2 V-1 s-1 were obtained for pentacene.5
However, pentacene is vulnerable to photooxidation due to
a high-lying HOMO and a narrow band gap.6 In addition,
higher [n]acenes exhibit poor solubility in most of the
common solvents which limits the processability of such
materials.7 Hence, for electronic applications, molecules of
high planarity with optimum band gap, close packing, and
good solubility are desired. Among the conjugated systems,
fused heterocyclic aromatic compounds (heteroacenes) con-
taining nitrogen atom showed a diverse range of applications,
for example, in biological applications, fabrication of new
electronic devices, and anion sensing.8 In electronic applica-
tions, heteroacenes demonstrated improved properties of the
highly ordered acene molecules.9 Among the five indolo-
carbazole isomers,8 indolo[3,2-b]carbazole has been explored
extensively for electronic applications.10 Extensive studies
(1) (a) Barbarella, G.; Favaretto, L.; Zanelli, A.; Gigli, G.; Mazzeo, M.;
Anni, M.; Bongini, A. AdV. Funct. Mater. 2005, 15, 664. (b) Oliveira,
M. A. D.; Duarte, H.; Pernaut, J.; Almeida, W. B. D. J. Phys. Chem. A
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T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys. Chem. B 2003,
107, 5877.
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(b) Belletete, M.; Beaupre, S.; Bouchard, J.; Blondin, P.; Leclerc, M. J.
Phys. Chem. B, 2000, 104, 9118.
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Z.; Siegrist, T.; Kloc, C.; Chen, C. AdV. Mater. 2003, 15, 1090.
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P. L. T.; Wakim, S.; Blouin, N.; Simard, M.; Tessier, C.; Tao, Y.; Leclerc,
M. J. Am. Chem. Soc. 2007, 129, 9125. (c) Qi, T.; Guo, Y.; Liu, Y.; Xi,
H.; Zhang, H.; Gao, X.; Liu, Y.; Lu, K.; Ku, C.; Yu, G.; Zhu, D. Chem.
Commun. 2008, 6227.
(3) (a) Afzali, A.; Dimitrakopoulos, C. D.; Breen, T. L. J. Am. Chem.
Soc. 2002, 124, 8812. (b) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth,
I.; Kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mu¨llen, K. AdV. Mater.
2005, 17, 684.
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Hu, W.; Zhu, D. Appl. Phys. Lett. 2006, 88, 242113. (b) Sun, Y.; Ma, Y.;
Liu, Y.; Lin, Y.; Wang, Z.; Wang, Y.; Di, C.; Xiao, K.; Chen, X.; Qiu, W.;
Zhang, B.; Yu, G.; Hu, W.; Zhu, D. AdV. Funct. Mater. 2006, 16, 426. (c)
Wu, Y.; Li, Y.; Gardner, S.; Ong, B. S. J. Am. Chem. Soc. 2005, 127, 614.
(10) (a) Li, U.; Ong, B. S.; Wu, Y.; Liu, P. (Xerox Corp.). Application:
US 2007/0112167 A1, 2007. (b) Li, U.; Ong, B. S.; Wu, Y.; Liu, P. (Xerox
Corp.). Application: US 2007/0112172 A1, 2007.
10.1021/ol901806q CCC: $40.75
Published on Web 09/08/2009
 2009 American Chemical Society

on indolo[2,3-a]carbazole-based compounds are illustrated
in biological applications such as protein kinase inhibitor,11
antimicrobial agents,12 and as a ligand for aromatic hydro-
carbon receptors (AhR).13 Indolo[2,3-a]carbazole and its
derivatives are also efficient sensors for anionic guest species,
Scheme 1
.
Synthesis of Thiadiazole-Fused Indolocarbazole
Derivatives
14
-
-
-
such as F-, Cl-, PhCO2 , H2PO4 , and HSO4 . However,
the material aspects of this isomer of indolocarbazole are
poorly explored. In this regard, thiadazole (acceptor moiety)-
fused indolo[2,3-a]carbazole (donor moiety)-based new
heteroacenes were synthesized. Their photophysical and
electrochemical properties along with their packing in the
crystal lattice were studied. Structures of the synthesized
compounds are depicted in Figure 1.
NMR and high-resolution mass spectrometry (see the Sup-
porting Information). Among the synthesized molecules, 5a
was analyzed by single-crystal X-ray diffraction techniques.
A solution containing 5a in ethyl acetate was evaporated
slowly at room temperature to yield single crystals of 5a
(Figure 2).
Figure 1. Structure of synthesized thiadiazole-fused indolocarba-
zole-based molecules.
Scheme 1 displays the synthesis route towards the newly
synthesized indolocarbazole derivatives. Benzothiadiazole (1)
was treated with 2.4-fold excess of bromine under acidic
conditions to give 4,7-dibromo-2,1,3-benzothiadiazole (2) in
good yield, 87%.15 Suzuki coupling of compound 2 was
carried out with 2-nitrophenylboronic acid (2-NBA) to get
the compound 3. Purification of the product was ac-
complished by washing the crude solid with hexane to
achieve quantitative yield of compound 3. Compound 4 was
obtained via the reported procedure of reductive Cadogan
ring-closure reaction,16 where compound 3 in 1,2-dichlo-
robenzene was refluxed with triethyl phosphite for 24 h.
Target molecule 4 in 51% yield was isolated after purification
by column chromatography. A few derivatives of compound
4 with various N-alkyl substituents (i.e., CH3, C6H13, and
CH2Ph) were prepared by alkylation with the corresponding
alkyl halides (5a-c).
Figure 2. Thermal ellipsoid plot of 5a (A). Packing diagram of 5a
along a-axis. Hydrogen atoms removed for clarity (B).
Compound 5a exhibits sandwiched herringbone type
packing with molecules existing as anti cofacial dimers with
a herringbone angle of 89°. This arrangement facilitates two-
dimensional electronic transport, which is advantageous for
better charge carrier mobility.17 In addition, a short inter-
molecular distance of 3.32 Å in the dimer indicates strong
π-π interactions. Such close contact is good as compared
to the intermolecular distance (3.45 Å) reported for planar
indolo[3,2-b]carbazole derivatives.18 Short N-S contact of
3.2 Å was observed between the edge-to-face interacting 5a
molecules, leading to a close packed structure.
Longer alkyl chain increased the solubility of these
indolocarbazoles in common organic solvents such as ethyl
acetate, chloroform, dichloromethane, THF, and acetone. The
synthesized compounds were characterized using 1H and 13
C
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Takahashi, M.; Murakata, C.; Sato, A. K.; Kaneko, M. Biochem. Biophys.
Res. Commun. 1987, 142, 436.
The UV-vis absorption and emission spectra of the
oligomers in solution (tetrahydrofuran) are shown in Figure
3. All spectra display multiple maxima in the UV region,
featuring typical fused-ring systems with more defined
(12) (a) Sezaki, M.; Sasaki, T.; Nakazawa, T.; Takeda, U.; Iwata, M.;
Watanabe, T. J. Antibiot. 1985, 38, 1437. (b) Yasuzawa, T.; Iida, T.;
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6, 1751.
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2007, 9, 4499.
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Leclerc, M. Chem. Mater. 2004, 16, 4386. (b) Liu, H.; Tao, X.; Yang, J.;
Yan, Y.; Ren, Y.; Zhao, H.; Xin, Q.; Yu, W.; Jiang, M. Cryst. Growth
Des. 2008, 8, 259.
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Quina, F. H.; Goncalves, R. S.; Dupont, J. Eur. J. Org. Chem. 2006, 4924.
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Org. Lett., Vol. 11, No. 19, 2009
4451

-5.53, and -5.57 below vacuum level were estimated for
the compounds 4, 5a, 5b, and 5c, respectively (Table 1). It
can be noted that the synthesized compounds have low-lying
HOMO as compared to pentacene (EHOMO ) -4.6 eV)6 and
indolo[3,2-b]carbazoles (EHOMO ) -5.12 eV).4c
Scheme 2
.
Synthesis of Thiadiazole-Fused Indolocarbazole
Polymer
Figure 3. Normalized absorption spectra (A) and emission spectra
(B) of thiadiazole fused indolocarbazole derivatives in THF.
electronic states.19 The absorption maximum of alkyl-
substituted derivatives (5a-c) is red-shifted as compared to
the parent molecule 4. Leclerc et al. reported similar
phenomena in their work on carbazole-based ladder com-
pounds.19 Emission maximum of all derivatives are in the
blue-green region with λmax centered around 475 nm.
Interestingly all derivatives exhibit large Stoke′s shifts which
are unexpected for fused ring systems (Table 1). The
wavelength of emission also showed a strong dependence
on solvent (Figure S1, Supporting Information). Such a large
Stoke’s shift and solvent-dependent emission indicates
the effective intramolecular charge transfer (ICT) in the
excited state between the thiadiazole unit and indolocarbazole
unit.20 It is observed that the substituents on the N-atom have
a marginal effect on the optical properties of these com-
pounds. The fluorescene quantum yield of heteroacenes was
recorded in THF with quinine sulfate in 0.1 M H2SO4 as
standard. The fluorescence quantum yields (ΦF) of all
compounds are relatively low. Such low ΦF values relative
to indolo[3,2-b]carbazole may be due to an increase in the
nonradiative intersystem crossing caused by the heavy atom
(sulfur). The optical band gap estimated from the onset of
UV-vis absorption for these derivatives are within the
range of ca. 2.7-2.8 eV. Some of the reported indolo[3,2-
b]carbazole-based molecules show optical band gaps between
ca. 2.86 and 2.95 eV.21
Electropolymerization of compound 4 was carried out in
acetonitrile containing 0.1 M TBAPF6 as supporting elec-
trolyte at a scan rate of 100 mV/s. Polymer was synthesized
electrochemically over the ITO surface (Scheme 2) by
repetitive scanning of anodic potential in the range of 0.2 to
1.5 V. The growth of polymer is indicated by a gradual
increase in current during the potential cycles (Figure 4A).
The polymerization occurred smoothly and resulted in a
yellow-brown colored, uniform deposit of polymer on the
working electrode. The deposited polymer film was dedoped
electrochemically and washed with acetonitrile to remove
monomer and supporting electrolyte. To the best of our
knowledge, compound 4 is the first indolocarbazole-based
material that undergoes electropolymerization.
The IR spectrum (KBr disk) of the polymer showed a
strong peak at 3433 cm-1 corresponding to N-H stretching
vibration (Figure S3, Supporting Information). The other
peaks at 2915 cm-1 and 1630 cm-1 corresponds to aromatic
C-H stretching and aromatic CdC stretching, respectively.
The possible structure of the polymer is represented in
Scheme 2. The absorption spectrum (Figure S4, Supporting
Information) of the polymer film showed a broad absorption
peak characteristic of a polymeric system with an optical
band gap of 2.4 eV, which is less than the corresponding
monomer 4 (2.8 eV).
Cyclic voltammetry (CV) was performed to investigate
the redox properties of the new ladder molecules in dry
dichloromethane (Figure S2, Supporting Information). Oxi-
dation peaks of 4, 5a, 5b, and 5c were found to be 1.25,
1.3, 1.34, and 1.35 V. All alkyl-substituted derivatives
showed higher oxidation potential than the parent compound
4. HOMO energy levels of ladder compounds are estimated
by the oxidation onset using the relationship EHOMO
)
-(Eox + 4.38) eV.22 HOMO levels of -5.39, -5.49,
onset
(19) (a) Belletete, M.; Blouin, N.; Boudreault, P. T.; Leclerc, M.;
Durocher, G. J. Phys. Chem. A 2006, 110, 13696. (b) Belletete, M.; Wakim,
S.; Leclerc, M.; Durocher, G. THEOCHEM 2006, 760, 147.
(20) Neto, B. A. D.; Lopes, A. S. A.; Ebeling, G.; Gonc¸alves, R. S.;
Costa, V. E. U.; Quina, F. H.; Dupont, J. Tetrahedron 2005, 61, 10975.
(21) Kawaguchi, K.; Nakano, K.; Nozaki, K. J. Org. Chem. 2007, 72,
5119.
Figure 4. Electropolymerization of 4 from acetonitrile solution at
a scan rate of 100 mV/s. Inset picture: Linear relationship of increase
in current with number scans (A). Cyclic voltammogram of poly4
(B).
(22) (a) Li, Y.; Ding, J.; Day, M.; Tao, Y.; Lu, J.; D’iorio, M. Chem.
Mater. 2004, 16, 2165. (b) Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.;
Einerhand, R. E. F. Synth. Met. 1997, 87, 53. (c) Cui, Y.; Zhang, X.; Jenkhe,
S. A. Macromolecules 1999, 32, 3824.
4452
Org. Lett., Vol. 11, No. 19, 2009

Table 1. Photophysical and Electrochemical Properties of Thiadiazole-Fused Indolocarbazoles
a
b
compd
λabs (nm)
λem (nm)
Stoke’s shift (nm)
ΦF
Eg (eV)
Ec
(V)
HOMOd (eV)
LUMOe (eV)
oxid
4
5a
5b
5c
368, 309, 296, 247
387, 311, 298, 249
387, 311, 298, 249
386, 311, 298, 250
469
472
473
481
101
85
86
8
10
10
9
2.8
1.25
1.3
1.34
1.35
-5.39
-5.49
-5.33
-5.57
-5.29
-2.59
-2.74
-2.79
-2.85
-3.14f
2.75
2.74
2.72
2.4
95
poly4
a The sample was excited at its respective absorption maximum. b Optical band gaps estimated from the onset position of absorption spectra. c Versus
Ag/AgCl electrode in 0.1 M Bu4NPF6 in acetonitrile, Pt as working electrode with a scan rate of 100 mV s-1 d Calculated using the relationship EHOMO
. )
+ 4.38). e Derived from optical band gap and HOMO. f Derived from onset of reduction potential.
onset
-(Eoxi
The cyclic voltammogram of the polymer showed sharp
oxidation peak with the oxidation onset potentials at 1.05 V
(Figure 4B). Upon scanning in negative potential, the
quasireversible reduction peak is observed at -2 V. The
small reduction peaks around -0.7 V may correspond to
the reduction of charged species created by irreversible
oxidation process. The electrochemical band gap of the
polymer is found to be 2.2 eV, which corresponds well with
the optical band gap (2.4 eV). The lower oxidation potential
of polymers compared to monomer 4 (1.25 V) and reduction
in band gap are consistent with extended π-conjugation in
the polymer. The electrochemical properties of the polymer
are also summarized in Table 1.
Figure 5. Tapping mode AFM image of electrodeposited polymer
film.
The surface morphology of the poly4 was studied using
atomic force microscopy (AFM). AFM image showed a
uniform deposition of polymer over ITO surface with rms
roughness of ∼10-20 nm (Figure 5). AFM images of
visually smooth polymer film reveal the globular feature of
100-200 nm diameter, which are regular in shape and size.
The morphology is similar to those of other reported
electrodeposited polymers.23
In conclusion, thiadiazole-incorporated indolo[2,3-a]car-
bazole derivatives are synthesized and characterized. Thia-
diazole fusion leads to a close packing of indolocarbazole
systems in the crystal lattice and also decrease of both the
HOMO and LUMO energy levels of the system. The
decrease in HOMO energy leads to the oxidative stability;
therefore, the strategy of thiadiazole fusion can be utilized
to stabilize unstable fused ring systems. The unsubstituted
compound was electropolymerized to give the first elec-
tropolymerized polyindolocarbazole. The device fabrication
and characterization of these compounds for opto-electronic
application is currently in progress in our laboratory.
Acknowledgment. We thank the Agency for Science,
Technology and Research (A*STAR), National University
of Singapore (NUS) for financial support and Miss Tan Geok
Kheng and Mrs. Sajini Vadukumpully of the Department of
Chemistry, NUS, for the X-ray crystal diffraction and AFM
studies.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data, 1H and 13C NMR spectra, cyclic
voltammograms of all compounds and crystallographic
information file (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
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7893. (b) Taranekar, P.; Baba, A.; Fulghum, T. M.; Advincula, R.
Macromolecules 2005, 38, 3679. (c) Fulghum, T.; Karim, S. M. A.; Baba,
A.; Taranekar, P.; Nakai, T.; Masuda, T.; Advincula, R. C. Macromolecules
2006, 39, 1467.
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4453