Synthesis of diindolocarbazoles by Ullmann reaction: A rapid route to ladder oligo(p-aniline)s

Article abstract of DOI:10.1021/ol048543r

(Chemical Equation Presented) New and facile synthesis of symmetric diindolocarbazoles was developed using the copper-catalyzed Ullmann reaction. The key step is a double-intramolecular cyclization reaction realized on N-alkyl-3,6-dibromo-2,7-bis(2′aminophenyl)carbazole derivatives which offers the desired symmetric ladder oligo(p-aniline)s. Depending upon the nature of the side- and/or end-groups, well-defined thin films and/or semiladder polymers could be obtained. These electroactive ladder oligomers may have great potential in organic electronics.

Full text of DOI:10.1021/ol048543r

ORGANIC
LETTERS
2004
Vol. 6, No. 19
3413-3416
Synthesis of Diindolocarbazoles by
Ullmann Reaction: A Rapid Route to
Ladder Oligo(p-aniline)s
Salem Wakim, Jimmy Bouchard, Nicolas Blouin, Alexandre Michaud, and
Mario Leclerc*
Department of Chemistry, UniVersite´ LaVal, Quebec City, QC, Canada G1K 7P4
mario.leclerc@chm.ulaVal.ca
Received July 26, 2004
ABSTRACT
New and facile synthesis of symmetric diindolocarbazoles was developed using the copper-catalyzed Ullmann reaction. The key step is a
double-intramolecular cyclization reaction realized on N-alkyl-3,6-dibromo-2,7-bis(2′aminophenyl)carbazole derivatives which offers the desired
symmetric ladder oligo(p-aniline)s. Depending upon the nature of the side- and/or end-groups, well-defined thin films and/or semiladder polymers
could be obtained. These electroactive ladder oligomers may have great potential in organic electronics.
Organic semiconducting materials including polymers, oli-
gomers, and small molecules are a subject of high interest
as potential active materials in electronic devices.1 In the
particular area of p-type organic field-effect transistors
(OFETs), the best candidates are regioregular polythiophenes,2
oligothiophenes,3 oligofluorenes,4 oligo(p-phenylevinylene)s,5
oligo(2,6-anthrylene)s,6 and fused aromatic compounds such
as pentacene.7 Of all organic semiconducting materials
reported so far, the highest charge mobilities have been
recorded with pentacene (about 5 cm2 V-1 s-1 with on-off
current ratios up to 108 in some cases).7d,e Although pentacene
shows impressive performances, it is not clear whether this
molecule will ultimately be used in organic electronics.4,8
In fact, pentacene suffers from facile atmospheric degrada-
tion, insolubility, and sensitivity to visible light.9 Therefore,
novel organic semiconductors combining high environmental
(1) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R.
N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990,
347, 539. (b) Morin, J. F.; Beaupre´, S.; Leclerc, M.; Le´vesque, I.; D’Iorio,
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P. R. L. AdV. Mater. 2002, 14, 99. (d) Winder, C.; Sariciftci, N. S. J. Mater.
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Tokito, S. Angew. Chem., Int. Ed. 2003, 42, 1159.
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D. G. IEEE Electron DeVice Lett. 1997, 18, 87. (b) Lin, Y. Y.; Gundlach,
D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron DeVice Lett. 1997, 18,
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10.1021/ol048543r CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/25/2004

stability, processability and appropriate charge carrier mobil-
ity are still required. One potentially interesting approach is
the development of pentacene-like oligomers which would
show a similar coplanar structure and favorable packing
geometry together with improved stability and processability.
Synthetic ladder-type π-conjugated molecules having opti-
mized chemical, physical, and structural properties could be
excellent candidates for such improvements. Short linear
fused ring compounds such as benzodithiophene10 and
dithieno[3,2-b:2′,3′-d]thiophene11 have been explored as a
building block for the synthesis of a variety of planar
structures, especially dimers. However, only a few examples
of longer ladder conjugated oligomers, usually containing
fused-ring thiophenes, have been reported but without any
test of their performances in electronic devices.12 Substituted
anthradithiophenes were also prepared by Laquindanum et
al.9 This synthetic approach gives a mixture of syn and anti
isomers, but these molecules show better solubility and
solution stability when compared to pentacene. However, no
attempt was made to characterize or separate the isomers.
Dibenzothienobisbenzothiophene (DBTBT) was also syn-
thesized by Sirringhaus et al.13 via an intramolecular
coupling, but an inseparable mixture of different regio-
isomers was obtained. Poor FET performances are observed
when DBTBT films contain a mixture of different isomers.
These results prove that isomeric purity is of first importance
for achieving high charge-transport mobility.
solvents and stable under ambient conditions. X-ray analyses
of a single crystal of 5,11-dioctyl-6,12-dimethylindolo[3,2-
b]carbazole 1b showed a coplanar molecular structure with
an interesting π-stacking arrangement of the molecules. This
type of organization is very important to enhance charge
carrier mobility along the π-π stacking.1c,11,16
As we previously reported, Cadogan ring closure is not
regioselective, and the use of carbazole precursors with
methyl protective groups is necessary to obtain the desired
isomers.15 We report here a more rapid and regioselective
(or direct) synthetic approach for the synthesis of symmetric
diindolocarbazoles using the copper-catalyzed Ullmann reac-
tion for the intramolecular ring-closure reactions. Interest-
ingly, a diindolocarbazole with amphiphilic side chains was
also prepared. This kind of molecule has the potential to lead
to well-defined thin films through Langmuir-Blodgett
processing, which allows fine control of both the orientation
and the thickness of the film.17 Moreover, a 3,10-dichloro-
diindolocarbazole was synthesized for further development
of semiladder poly(3,10-diindolocarbazole)s.
The symmetric diindolocarbazoles were generated in a
four-step synthetic sequence starting from N-octyl-2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)carbazole 3.15
As shown in Scheme 1, a double Suzuki cross-coupling
reaction of compound 3 with 1-bromo-2-nitrobenzene or
1-bromo-4-chloro-2-nitrobenzene using palladium catalyst
gives compounds 415 and 5 in good yields. Interestingly, it
is possible to introduce regioselectively the bromine atoms
at the 3- and 6- positions on the carbazole unit of compounds
4 and 5 by using 2 equiv of N-bromosuccinimide in acetone.
Thus, compounds 6 and 7 were obtained after a simple
recrystallization in methanol, with excellent yields.
In this regard, we have recently reported14,15 a new class
of pentacene-like semiconducting organic materials (see
Figure 1). Symmetric indolocarbazole14 1 and diindolocar-
The nitro groups were then reduced to amine moieties with
the use of SnCl2 according to a procedure developed by
Bellamy et al.18 to lead to compounds 8 and 9 with good
yields. It is important to notice that peaks of some protons
1
of the H NMR spectrum and of some carbon atoms of the
13C NMR spectrum of compounds 6 to 9 were doubled. This
can be explained by a phenomenon of atropisomerism19 due
to a limited (hindered) rotation around the aryl-aryl bond
(between phenyl and carbazole moieties). To obtain the
trimer 10, we first attempted the palladium-catalyzed ami-
(11) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S.
C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc.
1998, 120, 2206.
(12) (a) Mazaki, Y.; Kobayashi, K. Tetrahedron Lett. 1989, 30, 3315.
(b) Wex, B.; Kaafarani, B. R.; Neckers, D. C. J. Org. Chem. 2004, 69,
2197. (c) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125,
13662.
(13) Sirringhaus. H.; Friend, R. H.; Wang, C.; Leuninger, J.; Mu¨llen, K.
J. Mater. Chem. 1999, 9, 2095.
Figure 1. Indolocarbazoles and diindolocarbazoles.
(14) Wakim, S.; Bouchard, J.; Simard, M.; Drolet N.; Tao, Y.; Leclerc,
M. Chem. Mater. 2004, published ASAP July 7, 2004, http://dx.doi.org/
10.1021/cm049786g.
(15) Bouchard, J.; Wakim, S.; Leclerc. M. J. Org. Chem. 2004, 69, 5705.
(16) (a) Anthony, J. E.; Brooks, J. S.; Eaton, D.; Parkin, S. R. J. Am.
Chem. Soc. 2001, 123, 9482. (b) Mas-Torrent M.; Durkut, M.; Hadley, P.;
Ribas, X.; Rovira, C. J. Am. Chem. Soc. 2004, 126, 984.
(17) (a) Xiao, K.; Liu, Y.; Huang, X.; Xu, Y.; Yu, G.; Zhu, D. J. Phys.
Chem. B 2003, 107, 9226. (b) Xu, G.; Bao, Z.; Groves, J. T. Langmuir
2002, 16, 1834.
bazole15 2 derivatives were indeed prepared by the Cadogan
ring-closure reaction using N-alkyl-substituted carbazole
precursors. These oligomers are soluble in common organic
(9) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem.
Soc. 1998, 120, 664. (b) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res.
2001, 34, 359.
(10) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A.
AdV. Mater. 1997, 9, 36.
(18) Bellamy, F. D.; Ou, K. Tetrahedron Lett. 1984, 25, 839.
(19) Cammidge, A. N.; Cre´py, K. V. L. J. Org. Chem. 2003, 68, 6832.
3414
Org. Lett., Vol. 6, No. 19, 2004

Scheme 1. Synthesis of Symmetric Diindolocarbazoles 10 and 11
nation reaction pioneered by Hartwig20 and Buchwald.21
to offer the amphiphilic diindolocarbazole 15 in a 64%
isolated yield.
Different reaction conditions using Pd(dba)2,20b Pd(OAc)2,22a
22b
or Pd(PPh3)4 were tested in diluted conditions to avoid
All of these substituted diindolocarbazoles are soluble in
common organic solvents as THF, CH2Cl2, toluene, etc., and
their 1H NMR and 13C NMR analyses are in good agreement
with their chemical structures.
the intermolecular coupling, but we were unable to obtain
the desired product 10 via this type of reactions. To achieve
the double-intramolecular cyclization, copper-catalyzed Ul-
lmann reaction was then performed using conditions reported
by Field et al.23 Compounds 10 and 11 were successively
obtained with interesting isolated yields (57% and 68%,
respectively). By comparison with the reductive Cadogan
ring-closure reaction,15 this methodology is more straight-
forward and leads to the symmetric diindolocarbazoles in
higher yields. As we reported earlier, these compounds are
excellent intermediates to develop different kinds of diin-
dolocarbazoles depending upon the nature of the substituents.
To obtain totally symmetric diindolocarbazoles, compounds
10 and 11 were alkylated with 1-bromooctane in the presence
of sodium hydride as described in Scheme 2. Thus, com-
The UV-vis absorption and electrochemical properties of
compound 12 were also studied. The UV-vis absorption
spectrum of 12 in CH2Cl2 displays two absorption maxima
in the visible range, at 439 and 466 nm (Figure 2).
Scheme 2. Synthesis of Diindolocarbazoles 12, 13, and 15
Figure 2. UV-vis spectrum of compound 12.
Cyclovoltammetric measurements on of diindolocarbazole
12 in THF solution (with 0.1 M tetrabutylammonium
perchlorate) show that the oxidation at 0.67 V (vs SCE) and
the reduction at -2.22 V (vs SCE) are reversible (see Figure
(20) (a) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217.
(b) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-
Roman L. M. J. Org. Chem. 1999, 64, 5575.
(21) (a) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc.
1996, 118, 7215. (b) Ali, M. H.; Buchwald, S. L. J. Org. Chem. 2001, 66,
2560.
(22) (a) Mann, G.; Hartwig, J. F.; Driver, M. S.; Ferna´ndez-Rivas, C. J.
Am. Chem. Soc. 1998, 120, 827. (b) Lin, G.; Zhang A. Tetrahedron 2000,
56, 7163.
(23) Field, J. E.; Hill, T. J.; Venkataraman, D. J. Org. Chem. 2003, 68,
6071.
pounds 12 and 13 were isolated in excellent yields. Diin-
dolocarbazole 10 was also alkylated with bromotriethylene
glycol monomethyl ether 14 according to a similar procedure
Org. Lett., Vol. 6, No. 19, 2004
3415

3). Interestingly, the band gap calculated from electrochemi-
cal data (2.69 eV) is in excellent agreement with the one
obtained from UV-vis spectroscopy (2.59 eV).
ring-closure reaction. Starting from N-octyl-2,7-diboronic
ester carbazole 3, the symmetric diindolocarbazoles 10 and
11 were generated in four steps with overall yields of about
35%. This synthetic approach offers the opportunity to
prepare rapidly a large variety of diindolocarbazoles by the
appropriate choice of end-groups and side chains. For
instance, 3,10-dichlorodiiindolocarbazole 13 was prepared
as a precursor to semiladder poly(3,10-diindolocarbazole)s.
Amphiphilic diindolocarbazole 15 was also prepared. This
kind of molecule has the potential to lead to well-defined
thin films through Langmuir-Blodgett processing. Prelimi-
nary studies of the spectroscopic and electrochemical proper-
ties of the diindolocarbazole 12 are particularly promising
for the utilization of this class of materials in organic field-
effect transistors. For instance, hole mobility of 0.001 cm2
V-1 s-1 and Ion/Ioff ratio of 105 were already reported with
indolocarbazoles14 and the present longer conjugated oligo-
mers should lead to even better performances. All these
physical studies are currently in progress.
Acknowledgment. This work was supported by the
Canada Research Chair program and NSERC grants.
Figure 3. Cyclic voltammogram of compound 12.
Supporting Information Available: Detailed experi-
1
mental procedures and H and 13C NMR spectra for all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
In conclusion, we have described a new and straightfor-
ward approach for the synthesis of symmetric diindolocar-
bazoles via the copper-catalyzed Ullmann reaction as the
OL048543R
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Org. Lett., Vol. 6, No. 19, 2004