Conjugated ladder-type heteroacenes bearing pyrrole and thiophene ring units: Facile synthesis and characterization

Article abstract of DOI:10.1021/jo801898g

(Chemical Equation Presented) Ladder-type heteroacenes containing pyrrole and thiophene rings, dibenzo[b,b′]thieno[2,3-f:5,4-f′]-carbazoles (DBTCZ, 1), and diindolo[3,2-b:2′,3′-h]benzo[1],2-b:4,5-b′] bis[1]benzothiophene (DIBBBT, 2), were facilely synthes

Full text of DOI:10.1021/jo801898g

Conjugated Ladder-Type Heteroacenes Bearing Pyrrole and
Thiophene Ring Units: Facile Synthesis and Characterization
Peng Gao, Xinliang Feng, Xiaoyin Yang, Volker Enkelmann, Martin Baumgarten, and
Klaus Mu¨llen*
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany
muellen@mpip-mainz.mpg.de
ReceiVed August 27, 2008
Ladder-type heteroacenes containing pyrrole and thiophene rings, dibenzo[b,b′]thieno[2,3-f:5,4-f′]-
carbazoles (DBTCZ, 1), and diindolo[3,2-b:2′,3′-h]benzo[1,2-b:4,5-b′]bis[1]benzothiophene (DIBBBT,
2), were facilely synthesized through proper precursors (7, 11, and 18) respectively. The key step is a
triflic acid induced intramolecular electrophilic coupling reaction of corresponding aromatic methyl
sulfoxides with activated aromatic building blocks, which enables regioselective ring closure. Both
precursors (7 and 11) toward DBTCZ gave the symmetrical product but with solubilizing alkyl chains in
two different fashions. DIBBBT was also synthesized as the extended ladder-type heteroacene with defined
structure. These obtained heteroacenes are fully characterized (mass spectrometry, NMR, elemental
analysis), and their X-ray analysis and optical and electrochemical properties are reported.
Introduction
performance semiconducting materials is the development of
new ladder-type heteroacenes.
Fused acenes such as pentacene are attractive materials for
organic electronics. Both pentacene single crystals and thin
polycrystalline pentacene films exhibit charge mobilities above
1 cm2/V·s in organic field effect transistors (OFETs).1,2
However, pentacene suffers from insolubility and sensitivity to
visible light due to their high-lying HOMO levels and narrow
band gaps, which limit its practical applications under ambient
conditions.3 During the past years, a large number of new
heteroacenes as pentacene analogues have been developed and
tested as active semiconducting channels in OFETs, showing
dramatically improved stability and high performance close to
that of pentacene.4 This success could be attributed to their
coplanar structure and pronounced molecular assembly through
the intermolecular π-interactions and additive sulfur-sulfur
(S-S) interactions which are essential to achieve high charge
mobility.5 Therefore, one of the approaches toward high-
Compared with the tremendous amount of work focusing on
the synthesis of pentacene analogues, only a few examples of
further extended ladder-type molecules containing fused het-
erocycles (e.g., thiophene or/and pyrrole) have been reported,6-8
(4) (a) Tang, M. L.; Okamoto, T.; Bao, Z. J. Am. Chem. Soc. 2006, 128,
16002. (b) Du, C.; Guo, Y.; Liu, Y.; Qiu, W.; Zhang, H.; Gao, X.; Liu, Y.; Qi,
T.; Lu, K.; Yu, G. Chem. Mater. 2008, 20, 4188. (c) Laquindanum, J. G.; Katz,
H. E.; Lovinger, A. J. J. Am. Chem. Soc. 1998, 120, 664. (d) Gao, P.; Beckmann,
D.; Tsao, H. N.; Feng, X.; Enkelmann, V.; Pisula, W.; Mu¨llen, K. Chem.
Commun. 2008, 1548. (e) Wex, B.; Kaafarani, B. R.; Kirschbaum, K.; Neckers,
D. C. J. Org. Chem. 2005, 70, 4502. (f) Gao, J.; Li, R.; Li, L.; Meng, Q.; Jiang,
H.; Li, H.; Hu, W. AdV. Mater. 2007, 19, 3008. (g) Xiao, K.; Liu, Y.; Qi, T.;
Zhang, W.; Wang, F.; Gao, J.; Qiu, W.; Ma, Y.; Cui, G.; Chen, S.; Zhan, X.;
Yu, G.; Qin, J.; Hu, W.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 13281. (h) Ma,
Y.; Sun, Y.; Liu, Y.; Gao, J.; Chen, S.; Sun, X.; Qiu, W.; Yu, G.; Cui, G.; Hu,
W.; Zhu, D. J. Mater. Chem. 2005, 15, 4894. (i) Li, Y. N.; Wu, Y. L.; Gardner,
S.; Ong, B. S. AdV. Mater. 2005, 17, 849.
(5) (a) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey,
R.; Bre´das, J.-L. Chem. ReV. 2007, 107, 926. (b) Lemaur, V.; da Silva Filho,
D. A.; Coropceanu, V.; Lehmann, M.; Geerts, Y.; Piris, J.; Debije, M. G.; van
de Craats, A. M.; Senthilkumar, K.; Siebbeles, L. D. A.; Warman, J. M.; Bre´das,
J.-L.; Cornil, J. J. Am. Chem. Soc. 2004, 126, 3271. (c) Marseglia, E. A.; Grepioni,
F.; Tedesco, E.; Braga, D. Mol. Cryst. Liq. Cryst. 2000, 348, 137. (d) Barbarella,
G.; Zambianchi, M.; Bongini, A.; Antolini, L. AdV. Mater. 1993, 5, 834.
(1) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres, D. V.;
Theiss, S. D. Appl. Phys. Lett. 2003, 82, 3964.
(2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99.
(3) Yamada, M.; Ikemoto, I.; Kuroda, H. Bull. Chem. Soc. Jpn. 1988, 61,
1057.
10.1021/jo801898g CCC: $40.75
Published on Web 11/11/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 9207–9213 9207

Gao et al.
FIGURE 1. Ladder-type π-conjugated heteroacenes with thiophene or pyrrole ring units.
SCHEME 1. Synthesis of DBTBT via Double Intramolecularly Electrophilic Coupling Reaction
presumably due to the difficulty in the establishment of efficient
and practical synthetic protocols. For example, the sulfur atom
bridged p-quaterphenyl dibenzothienobisbenzothiophene (DBT-
BT, Figure 1a) was synthesized as an inseparable mixture of
three different regioisomers due to the poor selectivity of acid-
induced electrophilic coupling reaction (Scheme 1).6 During the
same time, Bouchard et al. have tried several methods to get
symmetrical ladder oligo(p-aniline) (diindolocarbazole) as a fully
nitrogen atom bridged p-quaterphenyl (Figure 1b), and finally
they found that the intramolecular Ullmann reaction instead of
an unregioselective Cadogan ring closure was the most effective
pathway.7 In contrast, Sonntag et al. successfully synthesized
bisindenocarbzole via the Friedel-Crafts-type alkylation ring
closure which occurred exclusively at the 3 and 6 position of
the carbazole (Figure 1c).8
On the basis of the known examples described above, we
tried to further broaden the family of π-extended oligoacences,
especially with the combination of electron-rich thiophene and
pyrrole ring units in one molecular skeleton. Herein we report
a regioselective synthetic approach toward a new series of
dibenzo[b,b′]thieno[2,3-f:5,4-f′]-carbazole (DBTCZ 1, Figure 1d)
and its ring-extended derivative diindolo[3,2-b:2′,3′-h]benzo[1,2-
b:4,5-b′]bis[1]benzothiophene (DIBBBT 2, Figure 1e). Their
crystal structure and physicochemical properties are also
described.
Results and Discussion
Synthesis of Dibenzo[b,b′]thieno[2,3-f:5,4-f′]-carbazole
(DBTCZ). The carbazole skeleton is a modest nucleophile that
can be readily derivatized with a variety of electrophiles, and
the most reactive positions for electrophilic substitution are the
3 and 6 positions (“para” to the nitrogen atom). On the other
hand, triflic acid activated aromatic methyl sulfoxides are well-
known for use as an electrophile,9 and the intramolecular
electrophilic coupling reaction could introduce the thiophene
ring units easily. The problem of regioisomers met in synthesiz-
ing DBTBT raised the question: how to construct a useful
precursor molecule for DBTCZ 1?6 Here both precursors 7 and
11 seemed possible (Scheme 2). Through precursor 7, one could
get compound 1 with alkyl chains lying in one side of molecular
central rings, whereas precursor 11 allows the facile introduction
of two alkyl substituents in the long-axis of the molecular
skeleton. The total synthetic route toward 1 via these two
different precursors is therefore described in Scheme 3.
Obviously, the shortest synthetic route toward 1 is via the
synthesis of precursor 7. Starting from 2,7-dibromocarbazole,
compounds 5a-d were made by introducing different alkyl side
chains.10 Subsequently, compounds 5 were treated with n-BuLi
in hexane at -78 °C and quenched with 2-isopropoxy-4,4,5,5-
(9) Mancuso, A. J.; Swern, D. Synthesis 1981, 165.
(6) Sirringhaus, H.; Friend, R. H.; Wang, C.; Leuninger, J.; Mu¨llen, K. J.
Mater. Chem. 1999, 9, 2095.
(10) Dierschke, F.; Grimsdale, A. C.; Mu¨llen, K. Synthesis 2003, 16, 2470.
(11) Dierschke, F.; Grimsdale, A. C.; Mu¨llen, K. Macromol. Chem. Phys.
2004, 205, 1147.
(7) (a) Bouchard, J.; Wakim, S.; Leclerc, M. J. Org. Chem. 2004, 69, 5705.
(b) Wakim, S.; Bouchard, J.; Blouin, N.; Michaud, A.; Leclerc, M. Org. Lett.
2004, 6, 3413.
(12) (a) Haryono, A.; Yamamoto, K.; Shouji, E.; Tsuchida, E. Macromol-
ecules 1998, 31, 1202. (b) Leuninger, J.; Wang, C.; Soczka-Guth, T.; Enkelmann,
V.; Pakula, T.; Mu¨llen, K. Macromolecules 1998, 31, 1720.
(8) Sonntag, M.; Strohriegl, P. Tetrahedron 2006, 62, 8103.
9208 J. Org. Chem. Vol. 73, No. 23, 2008

Conjugated Ladder-Type Heteroacenes
SCHEME 2. Two Possible Precursor Structures 7 and 11
for the Synthesis of DBTCZ 1
Synthesis of Ring-Extended Diindolo[3,2-b:2′,3′-h]benzo[1,2-
b:4,5-b′]bis[1]benzothiophene. By following the similar strategy
as described above, we synthesized another precursor 18, which
allowed the final triflic acid induced ring closure toward further
extended but soluble oligoacene 2 (Scheme 4). The easily
available 2-bromo-7-chlorocarbazole13 was alkylated with an
ethyl group on the nitrogen atom to give 15, which was then
reacted with 1 equiv of n-BuLi at -78 °C. The monolithiated
species was then quenched with hexyl iodide to generate the
2-chloro-7-hexyl-9-ethylcarbazole (16). Monoboronic ester 17
from 16 was made in high yield by using a mixture of Pd2(dba)3
and 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl as
catalyst system.14 A double Suzuki cross-coupling reaction
between 17 and 14 using Pd(PPh3)4 as catalyst afforded
precursor 18 in 72% yield. Subsequent treatment of 18 with
trifluoromethanesulfonic acid at 0 °C for 72 h induced double
intramolecular ring closure. Later on, the intermediate was
refluxed in pyridine to afford target 2 in 95% yield as a yellow
tetramethyl-1,3,2-dioxaborolane to yield N-alkyl-2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolane-2-yl)carbazole (6a-d).11 Su-
zuki coupling of 6a-d with 1-bromo-2-(methylsulfinyl)benzene
(3) gave precursors 7a-d in good yields. The final ring-closure
reaction was crucial to determine the isomeric purity. Instead
of following exactly the procedure described in the synthesis
of DBTBT, precursors 7a-d were treated with trifluoromethane-
sulfonic acid in the presence of phosphorus pentoxide at 0 °C
and reacted for 72 h in the dark. Then the as-formed clear
mixture was poured into ice/water to give a yellowish powder
as precipitate, followed by filtration, drying, and reflux in
pyridine for 12 h. After normal workup, only one spot was
observed on thin layer chromatography (TLC) plates, which was
supposed to be the inseparable isomers or desired pure product
of DBTCZ. After column chromatography, the yellow powder
of DBTCZ was achieved in good yield (85-95%). The further
determination of isomeric purity by 1H NMR spectroscopy
(Figure 2a) and the single-crystal structure (Figure 3) indicated
that the desired ladder-type acene DBTCZ 1 was the sole
product. We ascribe this high regioselectivity to two reasons:
first, the 3 and 6 positions on carbazole are the most reactive
positions for electrophilic substitution, and second, the reactions
were all conducted near 0 °C since the regioselectivity of the
thermodynamically controlled reaction is strongly influenced by
reaction temperature and steric hindrance.12a,b
1
solid. The H NMR spectrum of 2 confirmed the pure product
formation and indicated the high regioselectivity of the elec-
trophilic ring closure reaction (Figure 2c).
Structure Proof of the Heteroacenes by 1H NMR
1
Spectroscopy. As depicted in Figure 2, the H NMR spectra
provide evidence for the regioselectivity of the ring-closure
reaction since all the typical singlets can be assigned to the
corresponding aromatic protons in the molecules. No isomers
could be discerned from the spectra. The signals of aromatic
protons 2, 3, and 5 of 1c (Figure 2a) shift to high field in the
case of 1e (aromatic protons 2, 3, and 4, Figure 2b), owing to
the electron-donating effects of the substituted alkyl chain. In
Figure 2c, all the aromatic proton signals were in good
agreement with the highly symmetrical structure of compound
2. The chemical shifts of the aromatic resonances for all the
compounds are independent of the concentration and temper-
ature, indicating that there is minimum or no aggregation in
solution.
X-ray Diffraction Study on the Single Crystals and the
Thin Film of 1c. Crystals of 1c suitable for single-crystal X-ray
diffraction (XRD) studies were grown from cold chloroform.
The skeleton of 1c is found to be planar and further confirms
the regioisomeric purity of the compound (Figure 3). Two
molecules form a pair with antiparallel structure, possibly due
to the steric effect of the substitution on N (Figure 3a). The
antiparallel pairs further stack in a tilted cofacial manner,
resulting in two-dimensional (2D) π-stacking lamella with
adjacent molecules partially overlapping each other (Figure 3b).
The angle R between two molecules in one antiparallel pair is
60°. The marked interplanar separation of 3.33 Å between the
adjacent molecules in one column indicates the presence of
π-interactions along the intermolecular stacking axis (Figure
3a). For 1a, 1b, 1d, 1e, and 2, however, no suitably sized crystals
for single-crystal X-ray analysis could be obtained from different
solvents.
Apart from the success in synthesizing 1a-d from precursors
7a-d, we followed the second route via precursor 11 toward
1e, which opened the opportunity to introduce two additional
alkyl chains in the molecular long-axis direction. A Suzuki
coupling reaction between 5a and 4 was carried out in a two-
phase system of toluene and aqueous potassium carbonate, with
Pd(PPh3)4 as catalyst, and gave 8 in good yield. Afterward, with
2 equiv of N-bromosuccinimide in acetic acid, it was possible
to regioselectively introduce two bromine groups at the 3 and
6 positions on the carbazole unit of compound 8. Compound 9
was obtained after a simple column chromatography and further
reacted with 2 equiv of n-BuLi to form the corresponding 3,6-
dilithiated species which was then quenched by dimethyl
disulfide to afford compound 10 in 70% isolated yield. Oxidation
of 10 with hydrogen peroxide in acetic acid gave precursor 11
in 72% yield. Finally, by following the same procedure as for
precursor 7, compound 1e was received as yellow flakes in 96%
yield. Compounds 1a-e were fully characterized by 1H NMR,
13C NMR spectroscopy, elemental analysis, and mass spec-
trometry (FD).
X-ray diffraction in reflection mode of the drop-cast thin film
from 1c was also carried out, with the result being illustrated
in Figure 4. The large number of higher order scattering
intensities in the diffractogram in Figure 4 point toward high
crystallinity of the drop-cast thin film. The appearance of only
(13) Zhang, M.; Yang, C.; Mishra, A. K.; Pisula, W.; Zhou, G.; Schmaltz,
B.; Baumgarten, M.; Mu¨llen, K. Chem. Commun. 2007, 1704.
(14) Billingsley, K. L.; Barder, T. E.; Buchwald, S. L. Angew. Chem., Int.
Ed. 2007, 46, 5359.
J. Org. Chem. Vol. 73, No. 23, 2008 9209

Gao et al.
SCHEME 3. Synthesis of DBTCZ 1 through Triflic Acid Induced Ring-Closure Reactiona
a Reagents and conditions: (i) NaH, alkyl bromide, dry DMF, rt; (ii) THF, n-BuLi, -78 °C, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane; (iii)
toluene, 2 M K2CO3, 3, Pd(PPh3)4, 90 °C, 65%; (iv) a. CF3SO3H, rt, b. pyridine, reflux, 95%; (v) NBS, AcOH, CHCl3, 86%; (vi) THF, n-BuLi, -78 °C,
CH3SSCH3, 70%; (vii) AcOH, H2O2, 0 °C, 72%.
h00 reflections indicates an extraordinarily uniform arrangement
of the molecules toward the surface over a macroscopic area.
From the position of the 100 reflection, a d-spacing of 18.1 Å
was calculated, which is in good agreement with the molecular
length determined from the single-crystal data (17.2 Å). This
suggests that the molecules in the thin film are oriented with
their long molecular axes perpendicular to the substrate surface,
a phenomenon reported also for oligothiophenes as well as
anthradithiophenes.15
UV-Vis Absorption, Photoluminescence Spectra (PL), and
Electrochemical Properties of Ladder-Type π-Conjugated
Heteroacenes. UV absorption and photoluminescence spectra
(PL) of oligomers 1c and 2 are shown in Figure 5. In the UV
spectra, the strong absorptions at 338, 345, and 363 nm for 1c
as well as 366 and 387 nm for 2 are attributed to the ꢀ band of
π-π* transitions of the benzothienocarbazole backbones. On
the other hand, the longer wavelength and weaker absorption
observed at 400-450 nm for both compounds originates from
theRand/orpbandoftheπ-π*transition.16 TheHOMO-LUMO
energy gap of 1c of 2.81 eV can be evaluated from the
absorption edge (λ ) 442 nm), which is larger than that of
pentacene (1.77 eV)4a and diindolocarbazole (2.59 eV)7b and
smaller than that of bisindenocarbazole (3.2 eV).8 On the basis
of the absorption edge (λ ) 438 nm), the band gap of 2 is
calculated to be 2.83 eV. In the PL spectra, both compounds
exhibit weak blue fluorescence with the emission maximum at
439 nm. The small Stokes shift of 8 or 15 nm is similar to that
of bisindenocarbazoles due to the rigid planar structure.17 The
UV-vis absorption and photoluminescence spectra of other
derivatives of 1 do not show obvious differences compared to
1c.
To investigate the redox potentials of the new ladder-type
molecules, cyclic voltammetry (CV) measurements were per-
formed in dry CH2Cl2 (with 0.1 M tetrabutylammonium
hexafluorophosphate) by using Au as the working electrode and
Ag/Ag+ as the reference electrode. The cyclic voltammogram
of 1c revealed a quasi-reversible oxidation potential at Eox
onset
) 0.98 V (vs Ag/Ag+) (Figure S1a in the Supporting Informa-
tion). According to EHOMO ) -(Eox
+ 4.34) eV, a HOMO
onset
level of -5.2 eV for 1c is estimated, which is lower than that
of pentacene (-5.14 eV)4a and higher than that of bisindeno-
carbazole (-5.3 eV).8 This value is consistent with the HOMO
level calculated by using density functional theory (DFT) (-5.1
eV) (see the Supporting Information). Compounds 1a, 1b, 1d,
and 1e are also characterized by CV, indicating oxidation
behavior similar to that of 1c. However, due to the reduction,
part of the compound is out of the range of our experimental
setup, and we cannot observe the reduction in the CV data.
Therefore, there is no way to calculate the LUMO level by this
method. Taking into account an optical band gap of 2.81 eV
derived from the absorption onset of the UV-vis spectrum, the
LUMO value of 1c is empirically calculated to be -2.4 eV,
which may be an error compared with the real value of the
electron affinity since the optical band gap also includes the
(15) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem. Soc.
1998, 120, 664.
(16) Fogel, Y.; Kastler, M.; Wang, Z.; Andrienko, D.; Bodwell, G. J.; Mu¨llen,
K. J. Am. Chem. Soc. 2007, 129, 11743.
(17) Scherf, U. J. Mater. Chem. 1999, 9, 1853.
9210 J. Org. Chem. Vol. 73, No. 23, 2008

Conjugated Ladder-Type Heteroacenes
sulfinyl)benzene (3) (1.44 g, 6.56 mmol) was dissolved in 25 mL
of toluene. Then a 2 M K2CO3 solution (8 mL) was added. The
reaction mixture was degassed by three freeze/pump/thaw cycles
before 80 mg (6.92 × 10-5 mol) of Pd(PPh3)4 was added under
argon. The mixture was stirred for 24 h at 90 °C. The mixture was
then allowed to cool to room temperature, and the reaction mixture
was extracted three times with CH2Cl2 and dried with MgSO4. The
product was purified by silica chromatography with 2:1 THF/hexane
as the eluent, affording the pure product as colorless powder (1.1
g, 70% yield): 1H NMR (250 MHz, CD2Cl2) δ (ppm) 0.75 (t, 2H),
1.19 (m, 8H), 1.80 (m, 2H), 2.22 (s, 6H), 4.27 (t, 2H), 7.18 (dd,
2H), 7.40 (d, 2H, J ) 7.5 Hz), 7.42 (s, 2H), 7.50 (m, 4H), 8.04
(dd, 2H, J ) 7.5 Hz), 8.12 (d, 2H, J ) 7.5 Hz); 13C NMR (62.5
MHz, CD2Cl2) δ (ppm) 14.1, 21.0, 25.0, 27.5, 29.4, 32.0, 41.9,
110.1, 120.8, 121.2, 122.6, 123.7, 128.9, 130.9, 131.0, 136.3, 140.6,
141.4, 145.2; mp 95-98 °C; FD-MS m/z ) 527.20 (M+, 100.0%).
Anal. Calcd for C32H33NO2S2: C, 72.83; H, 6.30. Found: C, 72.82;
H, 6.33.
Synthesis of Dibenzo[b,b′]thieno[2,3-f:5,4-f′]-N-hexylcarbazole
(DBTCZ) (1c). A 10 mL round-bottomed flask was filled with 2,7-
bis[2-(methylsulfinyl)phenyl]-N-hexylcarbazole (7c) (200 mg, 0.38
mmol), phosphorus pentoxide (28 mg, 0.2 mmol), and trifluo-
romethanesulfonic acid (6 mL). The mixture was stirred for 72 h
at room temperature to give a dark brown solution, which was then
poured into ice-water (100 mL). The yellow precipitate was
collected by suction filtration and dried under vacuum. The structure
of this compound, which was insoluble in apolar organic solvents,
was assumed to be the sulfonium salt. Demethylation of the solid
was achieved by refluxing in pyridine (30 mL) for 12 h. When the
suspension was cooled to room temperature, a large volume of
CH2Cl2 was added to extract the product. Dibenzo[b,b′]thieno[2,3-
f:5,4-f′]-N-hexylcarbazole (1c) was thus obtained as a yellow powder
after silica chromatography with CH2Cl2/hexane ) 1/9 as an eluent
1
FIGURE 2. Extended aromatic region of H NMR spectra of com-
pounds (a) 1c (700 MHz, 420 K, 1,1,2,2-tetrachloroethane-d2), (b) 1e
(250 MHz, 300 K, dichloromethane-d2), and (c) 2 (250 MHz, 300 K,
dichloromethane-d2).
1
(165 mg, 94% yield): H NMR (250 MHz, CD2Cl2) δ (ppm) 0.80
(t, 3H), 1.31 (m, 6H), 1.94 (m, 2H), 4.43 (t, 2H), 7.41 (m, 2H),
7.81 (d, 2H, J ) 7.5 Hz), 8.06 (s, 2H), 8.22 (d, 2H, J ) 7.5 Hz),
8.49 (s, 2H); 13C NMR (62.5 MHz, CD2Cl2) δ (ppm) 14.2, 23.0,
27.4, 28.9, 32.1, 101.0, 121.9, 123.3, 123.5, 124.6, 127.1, 130.7,
135.0, 136.1, 140.6, 140.9; mp 215-217 °C; FD-MS m/z ) 463.14
(M+, 100%). Anal. Calcd for C30H25NS2: C, 77.71; H, 5.43. Found:
C, 77.75; H, 5.36.
exciton binding energy. The redox behavior of compound 2
indicates an Eoxonset of ∼0.98 eV as well as a reduction wave at
the potential of 0.92 V (Figure S1b in the Supporting Informa-
tion). Therefore, a HOMO level similar to that of compound 1
could be calculated for compound 2.
Synthesis of 2,7-Bis(4-n-butyphenyl)-N-methylcarbazole (8). 2,7-
Dibromo-N-methylcarbazole 5a (1.5 g, 4.42 mmol) and 2.53 g (9.73
mmol) of 4 were dissolved in 25 mL of toluene. A 2 M K2CO3
solution (6 mL) and 0.1 g of trimethylbenzylammonium chloride
were added. The reaction mixture was degassed by three freeze/
thaw cycles before 100 mg (8.6 × 10-6 mol) of Pd(PPh3)4 was
added under argon. The mixture was stirred for 24 h at 90 °C. The
reaction mixture was extracted three times with CH2Cl2 and dried
with MgSO4. The product was purified by silica chromatography
with hexane as an eluent. Compound 8 (1.67 g, 85%, related to
5a) was obtained as a white solid: 1H NMR (250 MHz, CD2Cl2) δ
(ppm) 1.03 (t, 6H), 1.44 (m, 4H), 1.74 (m, 4H), 2.74 (s, 6H), 3.96
(s, 3H), 7.36 (d, 4H, J ) 8.1 Hz), 7.53 (dd, 2H, J ) 8.1 Hz), 7.67
(s, 2H), 7.72 (dd, J ) 8.1 Hz), 8.16 (d, 2H, J ) 8.1 Hz); 13C NMR
(62.5 MHz, CD2Cl2) δ (ppm) 14.2, 22.8, 29.5, 34.2, 35.7, 107.2,
118.9, 120.8, 121.9, 127.6, 129.3, 139.4, 139.7, 142.5, 142.6; mp
192-193 °C; FD-MS m/z ) 445.64 (M+, 100.0%). Anal. Calcd
for C33H35N: C, 88.94; H, 7.92. Found: C, 88.93; H, 7.91.
Synthesis of 3,6-Dibromo-2,7-bis(4-n-butyphenyl)-N-methylcar-
bazole (9). A 100 mL flask was charged with 1 g (2.24 mmol) of
8 and a 1:1 mixture of CH3Cl and acetic acid and cooled in an
ice-water bath. Then, 818.7 mg (4.6 mmol) of N-bromosuccinimide
(NBS) was added in several portions, and the mixture was stirred
overnight at room temperature in the dark. The mixture was
quenched with 15 mL of distilled water and extracted three times
with CH2Cl2. The combined organic fractions were dried over
magnesium sulfate, and the solvent was removed under reduce
pressure. The crude material was purified by silica chromatography
Conclusion
In conclusion, we have described a facile approach to the
synthesis of two new ladder-type π-conjugated heteroacenes
(DBTCZ and DIBBBT) with the inclusion of both thiophene
and pyrrole ring units from proper precursors, respectively.
Triflic acid induced electrophilic coupling reactions showed
surprisingly high regioselectivity on the 3,6 position of carbazole
and gave the pure products with overall yields of about 48%.
The second precursor toward DBTCZ also gave the desired
symmetrical product but with solublizing alkyl chain in different
fashion. Single-crystal studies demonstrated that 1c adopts a
shifted cofacial stacking, and the thin film XRD of 1c showed
a preferred specific orientation relative to the substrate surface.
DIBBBT is by far the longest ladder-type heteroacene with well-
defined structure. The spectroscopic and electrochemical char-
acterizations of 1 and 2 indicate their much lower HOMO energy
levels and larger band gaps compared with those of pentacene,
in spite of their much more extended π system. Further studies
on the applications of these materials to semiconducting
materials are in progress in our laboratory.
Experimental Section
Synthesis of 2,7-Bis[2-(methylsulfinyl)phenyl]-N-hexylcarba-
zole (7c). A mixture of 6c (1.5 g, 2.98 mmol), 2-bromo(methyl-
J. Org. Chem. Vol. 73, No. 23, 2008 9211

Gao et al.
FIGURE 3. Packing of 1c in solid state: (a) side view of the antiparallel column pair with the red dash line indicating the interplanar distance and
(b) top view of the shifted π-π stacking (hydrogen atoms were omitted for clarity).
SCHEME 4. Synthesis of DIBBBT (2) through Triflic Acid Induced Ring-Closure Reactiona
a Reagents and conditions: (i) NaH, ethyl bromide, dry DMF, rt; (ii) THF, n-BuLi, -78 °C, hexyl iodide, 65%; (iii) Pd2(dba)3, 2-dicyclohexylphosphino-
2′,4′,6′-triisopropylbiphenyl, bis(pinacolato)diboron, KOAc, 110 °C, 90%; (iv) toluene, 2 M K2CO3, 14, Pd(PPh3)4, 90 °C, 72%; (v) a. CF3SO3H, rt, b.
pyridine, reflux. 95%.
FIGURE 5. Normalized UV-vis absorption and PL spectra of
compound 1c (a) and 2 (b) (10-6 M).
FIGURE 4. X-ray diffractogram of the drop-cast thin film of 1c (40
100.0%). Anal. Calcd for C33H33Br2N: C, 65.68; H, 5.51. Found:
mg/mL 1,2-dichlorobenzene solution) on an untreated SiO2 substrate.
C, 65.67; H, 5.52.
The reflections are assigned by the Miller’s indices. Inset: schematic
illustration of the edge-on arrangement of 1c on the substrate; side
chains are omitted.
Synthesis of 3,6-Dimethylsulfide-2,7-bis(4-n-butyphenyl)-N-me-
thylcarbazole (10). 3,6-Dibromo-2,7-bis(4-n-butyphenyl)-N-meth-
ylcarbazole (9) (600 mg, 0.994 mmol) was dissolved in dry THF
(40 mL) and cooled to -78 °C. n-Butyllithium (1.6 M in hexane,
1.37 mL, 2.19 mmol) was added dropwise at this temperature. After
the addition was complete, the mixture was stirred for an additional
hour. Dimethyl disulfide (206 mg, 2.19 mmol) was added dropwise.
The cooling bath was removed, and the solution was stirred at room
temperature overnight. The crude material was purified by silica
chromatography with hexane/CH2Cl2 ) 10:1 as the eluent, and 10
was obtained as a light yellow solid (374 mg, 70% yield): 1H NMR
with hexane as an eluent to offer 1.16 g of the product as a white
1
solid (86% yield): H NMR (250 MHz, CD2Cl2) δ (ppm) 0.89 (t,
6H), 1.29 (m, 4H), 1.61 (m, 4H), 2.61 (t, 4H), 3.69 (s, 3H), 7.20
(d, 4H, J ) 8.1 Hz), 7.29 (s, 2H), 7.33 (d, 2H, J ) 8.1 Hz), 8.23
(s, 2H); 13C NMR (62.5 MHz, CD2Cl2) δ (ppm) 14.2, 22.9, 29.7,
34.1, 35.8, 111.8, 113.1, 122.4, 124.8, 128.3, 130.0, 139.7, 140.7,
141.3, 142.9; mp 203-205 °C; FD-MS m/z ) 603.43 (M+,
9212 J. Org. Chem. Vol. 73, No. 23, 2008

Conjugated Ladder-Type Heteroacenes
(250 MHz, CD2Cl2) δ (ppm) 0.89 (t, 6H), 1.33 (t, 4H), 1.58 (t,
4H), 2.35 (s, 6H), 2.62 (t, 4H), 3.71 (s, 3H), 7.19 (d, 4H, J ) 8.1
Hz), 7.22 (s, 2H), 7.35 (d, 2H, J ) 8.1 Hz), 8.00 (s, 2H); 13C NMR
(62.5 MHz, CD2Cl2) δ (ppm) 14.1, 18.2, 22.9, 29.6, 34.1, 35.8,
111.0, 120.1, 122.1, 127.2, 128.3, 129.9, 139.3, 140.4, 140.9, 142.6;
mp 208-213 °C; FD-MS m/z ) 537.82 (M+, 100.0%). Anal. Calcd
for C35H39NS2: C, 78.16; H, 7.31. Found: C, 78.158; H, 7.32.
Synthesis of 3,6-Dimethylsulfoxide-2,7-bis(4-n-butyphenyl)-N-
methylcarbazole (11). 3,6-Dimethylsulfide-2,7-bis(4-n-butyphenyl)-
N-methylcarbazole (10) (300 mg, 0.558 mmol) was dissolved in a
1:1 mixture of glacial acetic acid and chloroform and cooled with
an ice bath until the solvent was about to freeze. Hydrogen peroxide
(35%, 109 mg, 1.13 mmol) was added slowly. The cooling bath
was removed, and the mixture was stirred at room temperature for
12 h. Acetic acid was removed by vacuum evaporation, and CH2Cl2
was added to the residue. The organic fraction was washed with
saturated NaHCO3 solution and dried over MgSO4. The product
was purified by silica chromatography with THF/hexane (3:1) as
the eluent, affording the pure product as a diastereomeric mixture
of white solid (228 mg, 72% yield): 1H NMR (250 MHz, CD2Cl2;
signals of diastereomer A are marked with ′, those of diastereomer
B with ′′) δ (ppm) 0.88 (t, 6H), 1.30 (m, 4H), 1.58 (m, 4H), 2.32
(s′, 3H), 2.34 (s′′, 3H), 2.61 (t, 4H), 3.81 (s, 3H), 7.22 (d, 4H, J )
8.1 Hz), 7.31 (s, 2H), 7.33 (d, 4H, J ) 8.1 Hz), 8.83 (s′, 1H), 8.84
(s′′, 1H); 13C NMR (62.5 MHz, CD2Cl2) δ (ppm) 14.15, 22.81,
30.08, 34.01, 35.72, 42.89(′), 42.98(′′), 111.10(′), 111.14(′′),
117.23(′), 117.28(′′), 122.51(′), 122.56(′′), 129.05(′), 129.08(′′),
129.87(′), 129.89(′′), 136.13(′), 136.16(′′), 136.41, 138.49(′),
138.58(′′), 143.27(′), 143.30(′′), 143.66(′), 143.67(′′); mp 230-235
°C; FD-MS m/z ) 569.82 (M+, 100.0%). Anal. Calcd for
C35H39NO2S2: C, 73.77; H, 6.90. Found: C, 76.78; H, 6.91.
Synthesis of 6,6′-Dibutyl dibenzo[b,b′]thieno[2,3-f:5,4-f′]-N-me-
thylcarbazole (1e). Following the same method as 1c, 6,6′-dibutyl
dibenzo[b,b′]thieno[2,3-f:5,4-f′]-N-methylcarbazole was obtained as
yellow powder after silica chromatography with CH2Cl2/hexane )
1/9 as an eluent (90 mg, 96% yield): 1H NMR (250 MHz, CD2Cl2)
δ (ppm) 0.95 (t, 6H), 1.42 (m, 4H), 1.69 (m, 4H), 2.77 (t, 4H),
3.98 (s, 3H), 7.30 (dd, 2H, J ) 8.1 Hz), 7.66 (s, 2H), 8.04 (s, 2H),
8.14 (d, 2H), 8.49 (s, 2H); 13C NMR (62.5 MHz, CD2Cl2) δ (ppm)
14.1, 22.8, 39.9, 34.2, 36.2, 100.4, 114.2, 121.5, 122.7, 123.1, 125.6,
130.6, 133.9, 135.0, 140.8, 141.5, 142.6; mp 256-263 °C; FD-
MS m/z ) 505.74 (M+, 100%). Anal. Calcd for C33H31NS2: C,
78.37; H, 6.18. Found: C, 78.36; H, 6.17.
Synthesis of 2-Chloro-7-hexyl-N-ethylcarbazole (16). 2-Chloro-
7-bromo-9-ethylcarbazole (15) (1.5 g, 4.86 mmol) was dissolved
in dry THF (40 mL) and cooled to -78 °C. n-Butyllithium solution
(1.6 M in hexane, 3.34 mL, 5.35 mmol) was added dropwise at
this temperature. After the addition was complete, the mixture was
stirred for an additional hour. Hexyl iodide (0.71 g, 5.35 mmol)
was added dropwise. The cooling bath was removed, and the
solution was stirred at room temperature overnight. The crude
material was purified by silica chromatography with hexane as the
eluent, and 16 was obtained as a off-white solid (1.06 g, 70% yield):
1H NMR (300 MHz, CD2Cl2) δ (ppm) 0.81 (t, 3H), 1.24 (m, 6H),
1.31 (t, 3H), 1.62 (m, 2H), 1.98 (2.71), 4.20 (m, 2H), 6.68 (dd,
1H, J ) 8.1 Hz), 7.02 (dd, 1H, J ) 8.1 Hz), 7.14 (s, 1H), 7.29 (dd,
1H, J ) 8.1 Hz), 7.83 (dd, 2H, J ) 8.1 Hz); 13C NMR (75.5 MHz,
CD2Cl2) δ (ppm) 13.9, 14.3, 23.1, 29.5, 32.1, 32.5, 37.1, 38.0, 108.7,
108.9, 119.3, 120.3, 120.6, 120.7, 121.2, 122.0, 131.0, 141.0, 141.1,
142.1; mp 156-163 °C; FD-MS m/z ) 313.86 (M+, 100%). Anal.
Calcd for C20H24ClN: C, 76.53; H, 7.71. Found: C, 78.52; H, 7.72.
Synthesis of 2-Hexyl-N-ethyl-7-(4,4,5,5-tetramethyl-1,3,2-diox-
aborolan-2-yl)-carbazole (17). An oven-dried Schlenk tube was
charged with Pd2dba3 (27.6 mg, 0.03 mmol), 2-dicyclohexylphos-
phino-2′,4′,6′-triisopropylbiphenyl (57.6 mg, 0.12 mmol), bis(pina-
colato)diboron (2.28 g, 9 mmol), 16 (942 mg, 3 mmol), and KOAc
(883.2 mg, 9 mmol). The Schlenk tube was capped with a rubber
septum and then evacuated and backfilled with argon (this sequence
was carried out three times). 1,4-Dioxane (10 mL) was added via
a syringe, through a septum. The reaction mixture was heated to
110 °C and reacted overnight. At this point, the reaction mixture
was allowed to cool to room temperature. The reaction solution
was then filtered through a thin pad of Celite (eluting with ethyl
acetate), and the eluent was concentrated under reduced pressure.
The crude material so obtained was purified via flash chromatog-
raphy on silica gel with hexane/EtOAc ) 10:1 as the eluent to
give 17 as light yellow oil: 1H NMR (250 MHz, CD2Cl2) δ (ppm)
0.84 (t, 3H), 1.34 (m, 12H), 1.37 (m, 6H), 1.63 (t, 3H), 1.68 (t,
2H), 2.77 (t, 2H), 4.34 (m, 2H), 7.02 (dd, 1H, J ) 8.1 Hz), 7.20 (s,
1H), 7.56 (d, 1H, J ) 8.1 Hz), 7.82 (s, 1H), 7.94 (d, 1H, J ) 8.1
Hz), 8.00 (d, 1H, J ) 8.1 Hz); 13C NMR (62.5 MHz, CD2Cl2) δ
(ppm) 12.3, 12.4, 21.1, 23.3, 27.7, 28.3, 30.3, 30.6, 35.3, 35.9, 82.2,
106.7, 113.3, 117.7, 118.3, 118.9, 119.0, 123.3, 123.9, 138.1, 139.4,
140.5; FD-MS m/z ) 405.38 (M+, 100.0%).
Synthesis of 7,7′-(2,5-Bis(methylsulfinyl)-1,4-phenylene)bis(2-
hexyl-N-ethylcarbazole) (18). Compound 6 (486 mg, 1.2 mmol)
and 1,4-dibromo-2,5-bis(methylsulfinyl)benzene (7) (100 mg, 0.28
mmol) were dissolved in 10 mL of toluene. A 2 M K2CO3 solution
(3 mL) was added. The reaction mixture was degassed by three
freeze/pump/thaw cycles before 15 mg (1.3 × 10-5 mol) of
Pd(PPh3)4 was added under argon. The mixture was stirred for 24 h
at 90 °C. The mixture was then allowed to cool to room temperature
and extracted three times with CH2Cl2 and dried with MgSO4. The
product was purified by silica chromatography with THF/hexane
(3:1) as the eluent, affording the pure product as colorless powder
1
(152 mg, 72% yield): H NMR (250 MHz, CD2Cl2) δ (ppm) 0.83
(t, 6H), 1.29 (m, 18H), 1.59 (m, 4H), 2.27 (s, 6H), 2.76 (t, 4H),
4.33 (m, 4H), 7.05 (d, 2H, J ) 7.5 Hz), 7.22 (s, 2H), 7.26 (d, 2H,
J ) 7.5 Hz), 7.48 (s, 2H), 7.96 (s, 2H), 8.09 (d, 2H, J ) 7.5 Hz),
8.12 (s, 2H); 13C NMR (62.5 MHz, CD2Cl2) δ (ppm) 14.3, 23.1,
29.5, 32.2, 32.5, 37.1, 38.0, 41.8, 108.8, 109.5, 120.1, 120.6, 120.7,
120.9, 123.7, 126.1, 134.1, 140.5, 140.8, 141.5, 142.5, 147.9; mp
198-201 °C; FD-MS m/z ) 757.20 (M+, 100.0%). Anal. Calcd
for C48H56N2O2S2: C, 76.15; H, 7.46. Found: C, 76.25; H, 7.40.
Synthesis of Diindolo[3,2-b:2′,3′-h]benzo[1,2-b:4,5-b′]bis[1]benzo-
thiophene (2). A 10 mL round-bottomed flask was filled with 7,7′-
(2,5-bis(methylsulfinyl)-1,4-phenylene)bis(2-hexyl-N-ethylcarba-
zole) (2a) (120 mg, 0.16 mmol), phosphorus pentoxide (1.2 mg,
0.008 mmol), and trifluoromethanesulfonic acid (3 mL). The
mixture was stirred for 72 h at room temperature to give a dark
brown solution, which was then poured into ice-water (100 mL).
The yellow precipitate was collected by suction filtration and dried
under vacuum. After refluxing the yellow powder in pyridine (30
mL) for 12 h, the suspension was cooled to room temperature and
a large volume of CH2Cl2 was added to extract the product.
Diindolo[3,2-b:2′,3′-h]benzo[1,2-b:4,5-b′]bis[1]benzothiophene (2)
was thus obtained as a yellow powder by silica chromatography
1
with hexane as the eluent (104 mg, 95%): H NMR (250 MHz,
CD2Cl2) δ (ppm) 0.83 (t, 6H), 1.29 (m, 18H), 1.59 (m, 4H), 2.76
(t, 4H), 4.33 (m, 4H), 7.03 (d, 2H, J ) 7.5 Hz), 7.17 (s, 2H), 7.97
(d, 2H, J ) 7.5 Hz), 8.07 (s, 2H), 8.40 (s, 2H), 8.61 (s, 2H); FD-
MS m/z ) 693.02 (M+, 100%).
Acknowledgment. We thank Dirk Beckmann, Nok Tsao, and
Dr. Wojciech Pisula for helpful discussions. Financial support
from Max Planck Society through the program ENERCHEM,
European Commission Project NAIMO (NMP4-CT-2004-
500355), and the German Science Foundation (Korean-German
IRTG) is gratefully acknowledged.
Supporting Information Available: Detailed experimental
procedures for 3, 4, 12, 13, and 14, spectroscopic data, and
summerized 1H and 13C spectra data for all new compounds, as
well as crystallographic data of 1c presented in CIF. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO801898G
J. Org. Chem. Vol. 73, No. 23, 2008 9213