Synthesis and properties of the anti and syn isomers of dibenzothieno[b,d]pyrrole

Article abstract of DOI:10.1039/b813683a

A novel type of anti and syn isomers of a pentacyclic compound consisting of two thiophene rings and one pyrrole ring were efficiently synthesized from benzo[b]thiophene, and the anti isomer exhibits better charge transport and OFET properties compared with the syn isomer and its N-hexyl substitution. The Royal Society of Chemistry.

Full text of DOI:10.1039/b813683a

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Synthesis and properties of the anti and syn isomers
of dibenzothieno[b,d]pyrrolewzy
Ting Qi, Yunlong Guo, Yunqi Liu,* Hongxia Xi, Hengjun Zhang,
ab
ab
a
ab
ab
ab
ab
ab
ab
a
a
Xike Gao, Ying Liu, Kun Lu, Chunyan Du, Giu Yu and Daoben Zhu
Received (in Cambridge, UK) 6th August 2008, Accepted 22nd September 2008
First published as an Advance Article on the web 14th October 2008
DOI: 10.1039/b813683a
A novel type of anti and syn isomers of a pentacyclic compound
consisting of two thiophene rings and one pyrrole ring were
efficiently synthesized from benzo[b]thiophene, and the anti
isomer exhibits better charge transport and OFET properties
compared with the syn isomer and its N-hexyl substitution.
the hexyl side chain is favorable for charge transport due to its
6
better self-organization, so we selected the hexyl group as a
substituent. According to the different effect of isomers and
alkyl substitution, we studied the relationship of their struc-
ture and properties, which is important for the improvement
of OFET materials.
The linearly fused acenes, such as pentacene, are fundamental
1
components of organic field-effect transistors (OFETs). New
Our strategy to construct the N-bridged ladder bibenzo[b]-
7
thiophene skeleton is based on the Cadogan reaction to
2
pentacene-like heteroacenes containing sulfur or nitrogen
3
realize the ultimate nitrogen cyclization. Facile and efficient
synthetic routes to anti-1, syn-1 and 7, their properties, and the
preliminary results of field-effect transistor (FET) character-
istics of their evaporated thin films are reported.
atoms have been developed to improve the solubility, stability
and co-facial packing in the solid state. Recently, for the
pentacyclic molecules containing both sulfur and nitrogen,
0
we first synthesized 5,6-disubstituted diindolo[3,2-b:4,5-b ]-
The synthetic routes to the isomers anti-1, syn-1 and 7 are
outlined in Scheme 1. Their starting material is benzo[b]thio-
phene which is commercially available. Notably, anti-1 was
obtained easily in three-step reaction sequences according to
Scheme 1. The homo-coupling reaction of benzo[b]thiophene
4
thiophenes (DITs). The results indicate that the S–S interac-
tions enhance the electronic transport between molecules.
Thus, in this paper, we designed and synthesized heteroacenes
containing two sulfur atoms and one nitrogen atom, increas-
ing the S : N ratio, which may further enhance intermolecular
interactions due to S–S contact. Meanwhile, we also synthe-
sized its isomers as anti (anti-1) and syn (syn-1) with respect to
the orientation of thiophene and pyrrole rings along the long
in the 2-position may take place in the presence of n-butyl-
8
at reflux in diethyl ether. The next mono-
lithium and CuCl
2
0
substituted nitration reaction of 2,2 -bibenzo[b]thiophene (2)
was carried out using 2 equiv. fuming nitric acid to produce
3-nitro compound 3 in high yield. Effective reductive cycliza-
tion of nitro 3 occurred when reflux in o-DCB using a slight
excess of triphenylphosphine. The pure product anti-1 was
obtained easily by column chromatography and precipitation
in 78% final yield. The synthesis of syn-1 was accomplished
in a similar four-step procedure (Scheme 1). Differently,
axis of the molecule. Research on the synthesis and properties
5
llen’s and
of isomers for OFETs is seldom reported. Only Mu
¨
2
g
Neckers’ groups recently studied the field-effect property of
isomers on the analogues of polyacenes having a benzo[b]thio-
phene repeat unit. In addition, we tried to synthesize their
alkyl substitutions compared with their parents. Commonly,
3
-bromobenzo[b]thiophene (4) was at first formed by bromi-
a
Beijing National Laboratory for Molecular Sciences, Key
nating with NBS directly to ensure the homo-coupling reac-
9
tion in the 3-position. Then, the reaction may take place in
Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing, 100190, China.
E-mail: liuyq@iccas.ac.cn
Graduate School of Chinese Academy of Sciences, Beijing, 100190,
China
the presence of n-butyllithium and CuCl
2
at ꢀ78 1C in diethyl
1
ether, because benzo[b]thiophene is more active in the
0
b
1
3-position at low temperature. The final product syn-1
1
w Electronic supplementary information (ESI) available: Synthesis,
single-crystal X-ray analysis of anti-1 and 7, physicochemical proper-
ties, X-ray diffraction studies, AFM studies, device fabrication and
1
13
evaluation and H and C NMR spectra. CCDC 697266 and 697267.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b813683a
z Crystallographic data for anti-1: C16
9 2
H NS , M = 279.36, monoclinic,
˚
space group P2 /n, a = 9.916(2), b = 10.619(2), c = 23.425(5) A,
1
3
ꢀ3
˚
V = 2453.0(8) A , Z = 8, D
collected: 7940, independent reflections: 4328 (Rint = 0.0199), GOF =
.168, 343 parameters, R = 0.0623, wR = 0.0993 for all reflections.
CCDC 697266.
y Crystallographic data for 7: C22
space group P2 /c, a = 11.582(2), b = 8.1596(16), c = 19.770(4) A,
c
= 1.513 g cm , T = 173(2) K, reflections
1
1
2
H
2
21NS , M = 363.52, monoclinic,
˚
1
3
ꢀ3
˚
V = 1787.7(6) A , Z = 4, D
reflections collected: 12637, independent reflections: 4248 (Rint
c
= 1.351 g cm , T = 113(2) K,
=
=
0
0
.0717), GOF = 1.033, 227 parameters, R
.1106 for all reflections. CCDC 697267.
1 2
= 0.0567, wR
Scheme 1 Syntheses of anti-1 and syn-1.
This journal is ꢁc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6227–6229 | 6227

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0
was prepared by nitration and cyclization reactions from 3,3 -
bibenzo[b]thiophene (5) similar to the synthetic route of anti-1.
syn-1 was also purified easily by column chromatography and
precipitation in 51% final yield. The overall yield of anti-1 was
0
Scheme 2 The stable molecular structure syn-1 .
5
5%, and the overall yield of syn-1 was 27%. In addition,
anti-1 is easily alkylated in the presence of KOH to give
N-hexyl product 7, while the N-alkyl syn isomer cannot be
obtained under same conditions.
of parallel dimers to prevent p–p interactions between con-
jugated planes (ESIw). Although the mean S–S contact (3.44 A)
˚
of 7 is shorter than that of anti-1 (3.49 A), the crystal structure
˚
Unexpectedly, anti-1 and syn-1 exhibit good solubility in
common solvents. In particular, both of them can be dissolved
in ethanol, and the solubility of anti-1 is much better than that
of 7 does not exhibit p–p interactions that are more favorable
for electronic transport. Therefore, the anti isomer without
alkyl substituents has better charge transport structure. In the
˚
anti-1 crystal, short S–S contacts (3.38, 3.51, and 3.58 A) and
˚
C–C contacts (3.36 A) exist between parallel slipped dimers,
ꢀ1
of syn-1 at room temperature (over 20 mg ml for anti-1, and
ꢀ
1
under 2 mg ml
revealed that anti-1 has better thermal stability than syn-1,
for syn-1). Thermogravimetric analysis
˚
and short C–Hꢂ ꢂ ꢂp contacts (2.66B2.89 A) exist in a face-to-
with decomposition temperatures T of 286 1C for anti-1 and
d
edge manner between dimers, indicative of the two-dimen-
sional electronic structure of the crystal.
2
35 1C for syn-1. The decomposition temperature Td of
N-hexyl anti isomer 7 is 296 1C, a little higher than anti-1.
Notably, syn-1 shows different character from anti-1, which is
typical of N–H structure. Their mass spectra displayed that
anti-1 forms the strongest molecular ion peak, while syn-1
forms the strongest fractional ion peak when a hydrogen atom
leaves (ESIw), so syn-1 may have the active hydrogen at the
carbon near to the nitrogen. In addition, anti-1 exhibits a
The UV-Vis absorbance and emission spectra of anti-1
display red-shifts relative to those of syn-1 either in solution
or in the solid state (vacuum-deposited films), and their solid-
state spectra are red-shifted with respect to solution (Table 1
and ESIw). Meanwhile, at longer wavelengths, the solid-state
spectrum of the thin film of anti-1 shows two new peaks at 465
and 495 nm, which are attributed to intermolecular interac-
ꢀ1
strong IR band at 3414 cm corresponding to N–H stretch-
ꢀ
13
tions in the microcrystalline film and possibly related to the
1
ing, but syn-1 exhibits no bands at 3500B3300 cm and a
new band at 2920 cm , characteristic of C–H stretching of
p–p stacking observed in the crystal. The absorbance and
emission spectra of alkyl substituted anti isomer 7 are similar
to the unsubstituted anti-1 in solution. However, in the solid
state, the emission maximum of 7 exhibits an obvious red shift
ꢀ1
1
2
saturated carbons (ESIw). Therefore, we speculate that syn-1
more easily forms the stable molecular structure syn-1 as
0
1
shown in Scheme 2. In the H-NMR, d = 5.63 ppm line for
syn-1 in contrast to 11.94 ppm in anti-1 may also verify the
structure (ESIw). As aforementioned, syn-1 cannot perform
the N-alkylated reaction, which confirms this hypothesis
further.
Crystals of anti-1 and 7 suitable for single-crystal X-ray
diffraction (XRD) studies were grown from toluene and
dicholomethane, respectively. Both anti-1 and 7 have dimers
formed by the adjacent two molecules with anti conformation
to minimize the dipolar interactions (Fig. 1b and d). Differ-
˚
ently, the strong p–p interactions (3.41 A for the largest plane-
to-plane distance) form piled dimers for anti-1, while the
˚
strong S–S contacts (3.56, 3.21, and 3.56 A) and interactions
between alkyl chains form flatly spread dimers along the short
molecular axis for 7. Evidently, anti-1 assumes the so-called
sandwiched herringbone structure in which its dimers are
packed tightly in the crystal (Fig. 1a). Dimers for 7, nearly
in the same plane also construct a herringbone packing pattern
Fig. 1 Crystal structures of anti-1 (a,b) and 7 (c,d), (a) hydrogen
(
Fig. 1c), but the substituted alkyl chains insert in the middle
atoms removed for clarity, (c) alkyl chains removed for clarity.
Table 1 Photophysical and electrochemical data of anti-1, syn-1 and 7
d
Solution
Film
a
b
c
e
f
b
c
Compd
l
abs /nm
l
lum /nm
E
g
/eV
E
ox /V
E
HOMO /eV
labs/nm
l
lum /nm
g
E /eV
anti-1
syn-1
7
327, 340
286, 299, 307
329, 345
355, 365
356
357, 369
3.51
3.71
3.48
1.02
1.53
1.15
ꢀ5.30
ꢀ5.84
ꢀ5.38
343, 355
315
337, 356
402, 465, 495
366
445
3.23
3.37
3.26
a
ꢀ5
b
c
Measured in a dilute CH
2
Cl
2
solution (2 ꢃ 10 M). Excited at the second absorption peak. Estimated from the onset of absorption (E
g
=
d
e
ꢀ3 ꢀ1
4 6 2 2
240/lonset). Films were vacuum deposited (50 nm, quartz substrate). Performed in Bu NPF –CH Cl solution (10 M), v = 100 mV s . Calculated
onset
f
1
using the empirical equation: HOMO = ꢀ(4.44 + Eox
228 | Chem. Commun., 2008, 6227–6229
).
6
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relative to anti-1, which is probably due to the extra electron
physicochemical properties revealed that the anti isomer
(anti-1) has stronger intermolecular interactions than the syn
isomer (syn-1), and the anti has the better charge transport
structure than the alkyl substitution. The HOMO energy level
and photostability of anti-1 are more suitable to apply in an
FET device, and it can easily form an even film. The organic
thin-film transistor of anti-1 is fabricated and characteristic of
1
donation from the alkyl groups. In addition, through drop
4
coating films, we found that anti-1 can form an even and
ꢀ1
consecutive film from a toluene or ethanol solution (1 mg mL ),
observing sharp absorbance band edges, but syn-1 and 7 are
easier to aggregate and crystallize (ESIw). This property of
good film formation is seldom found in the rigid molecules
without long alkyl substitution. It is a feature of anti-1 that
could be applied in solution processing. This work will be
studied in depth.
2
ꢀ1 ꢀ1
p-type in the air. Mobility up to 0.012 cm V
s and on/off
5
ratio up to 10 were achieved under ambient conditions, and
the device is very stable in air. Further optimization of these
transistors is underway.
The photostability of their vacuum-deposited thin films was
studied by monitoring changes in the absorption of their thin
films deposited on quartz under ambient light for nearly three
months. syn-1 and 7 are quite unstable, the absorption decay-
ing to almost zero within a few days, while anti-1 is very stable
with no change of absorption (ESIw).
The present research was financially supported by the
National Natural Science Foundation (60671047, 50673093,
60736004, 20721061), the Major State Basic Research Develop-
ment Program (2006CB806203, 2006CB932103) and the
Chinese Academy of Sciences.
Cyclic voltammetry of anti-1 and 7 showed a reversible
oxidation peak at E1/2 = +0.95 V and +1.06 V, respectively,
indicating the good stability of the anti-1 and 7 radicals, while
syn-1 showed an irreversible oxidation peak at Eox = +1.53 V
Notes and references
1
J. E. Anthony, Chem. Rev., 2006, 106, 5028.
(
important factor in achieving high mobility, so the anti
Ag/AgCl as reference, ESIw). The stability of the radical is an
2 (a) K. Xiao, Y. Liu, T. Qi, W. Zhang, F. Wang, J. Gao, W. Qiu,
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1
5
+
isomer is superior to the syn. Given the Fc/Fc couple was
(
b) J. G. Laquindanum, H. E. Katz and A. J. Lovinger, J. Am.
1
6
used as the internal standard, HOMO levels of anti-1, syn-1
and 7 were estimated by using oxidation onsets ꢀ5.30, ꢀ5.84
and ꢀ5.38 eV from vacuum, respectively. It should be noted
that the HOMO level of anti-1 is closest to the work function
of Au electrode (5.1 eV) in devices. Furthermore, the optical
band gap of syn-1 is too large to benefit charge transport.
Thin-film transistors of anti-1, syn-1 and 7 were fabricated
by vacuum evaporation in a top-contact configuration using
Au as source and drain electrodes (W/L = 3 mm/50 mm). The
devices for anti-1 showed typical p-channel FET properties
under ambient conditions, but the devices for syn-1 and 7
showed no FET properties. The phenomenon indicates that
the anti isomer anti-1 is more favorable for charge transport
than the syn and the alkyl substitution under the same condi-
tions. Field-effect mobility (mFET) evaluated from the satura-
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2
ꢀ1 ꢀ1
tion regime for anti-1 were of the order of 10 cm V s ,
5
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conductor under ambient conditions (ESIw).
7
7
2
9
1
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In summary, we have established an efficient method to
synthesize the syn (syn-1) and anti (anti-1) isomers of dibenzo-
thieno[b,d]pyrrole and the hexyl substituted anti isomer (7),
using the final reductive cyclization. Detailed studies of their
1
1
Table 2 FET characteristics of devices fabricated on OTS-treated
Si/SiO substrates for anti-1, syn-1 and 7
a
2
1
2 ꢀ1 ꢀ1
Mobility/cm V s
Compd
On/off ratio
Vth/V
15 S. T. Bromley, M. Mas-Torrent, P. Hadley and C. Rovira, J. Am.
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5
anti-1
syn-1
7
0.012
nd
nd
10
ꢀ20.8
nd
nd
1
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Estimated from the saturation regime. Not determined.
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This journal is ꢁc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 6227–6229 | 6229