Synthesis of a thiophene-fused isoindigo derivative: A potential building block for organic semiconductors

Article abstract of DOI:10.1016/j.tetlet.2013.12.076

Thiophene-fused isoindigo (TII) was synthesized from thieno[2,3-f]indol- 6(7H)-one in a one-pot reaction, in which the alkylation, oxidation and condensation were finished in one step. It exhibits better intramolecular charge transfer properties and higher reductive potential compared with isoindigo(II), as evidenced by its optical and electrochemical properties, which shows that it might be used as a building block for n-type or ambipolar OFET materials.

Full text of DOI:10.1016/j.tetlet.2013.12.076

Tetrahedron Letters 55 (2014) 1040–1044
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of a thiophene-fused isoindigo derivative: a potential
building block for organic semiconductors
Na Zhao a,b, Li Qiu a,b, Xiao Wang a, Zengjian An a, Xiaobo Wan a,
⇑
a CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy & Bioprocess Technology, Chinese Academy of Sciences, 189 Songling Road, Qingdao 266101, PR China
b University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Thiophene-fused isoindigo (TII) was synthesized from thieno[2,3-f]indol-6(7H)-one in a one-pot reaction,
in which the alkylation, oxidation and condensation were finished in one step. It exhibits better intramo-
lecular charge transfer properties and higher reductive potential compared with isoindigo(II), as evi-
denced by its optical and electrochemical properties, which shows that it might be used as a building
block for n-type or ambipolar OFET materials.
Received 14 October 2013
Revised 9 December 2013
Accepted 20 December 2013
Available online 29 December 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Isoindigo derivatives
Organic semiconductors
Oxidation
Introduction
H
O
O
N
O
H
N
Nitrogen-containing electron-deficient dyes, such as diketopyr-
rolopyrrole (DPP),1–4 indigo,5–8 and isoindigo(II)9–13 (as shown in
Scheme 1) have attracted an increasing attention as building
blocks for organic semiconductors (OSCs) recently. Among them,
isoindigo is becoming a popular building block for organic solar
cells14–17 and for organic field effect transistors (OFETs).10–13 High
charge carrier mobility was achieved for conjugated polymers
based on isoindigo derivatives. For instance, Pei et al. reported an
isoindigo-based conjugated polymer with an exceptionally high
NH
HN
N
H
N
H
O
O
O
2 Indigo
3 Isoindigo (II)
1 DPP
H
N
H
O
O
N
S
S
S
S
N
H
O
N
H
O
4 thienoisoindigo
hole mobility of 0.79 cm2 Vꢀ1 sꢀ1 9
3.62 cm2 Vꢀ1 sꢀ1 12
Bao et al. synthesized a siloxane-terminated
isoindigo-based conjugated polymer with a hole mobility as high
as 2.48 cm2 Vꢀ1 sꢀ1 10
, which was later improved to
5 thiophene-fused isoindigo (TII)
target of this work
.
Scheme 1. N-containing electron-deficient dyes used in organic semiconductors.
.
In many cases, isoindigo was directly used as the building block
for oligomers or polymers, and less attention was paid on the
manipulation on its core structure. Still, modification on isoindigo
core may have dramatic influence on its electric properties. Pei
et al. prepared ambipolar polymers based on fluorinated isoindi-
go,11 which not only maintained high hole mobility but also in-
creased the electron mobility considerably in OFET devices
fabricated in ambient conditions. They also reported chlorinated
isoindigo polymers with balanced charge carrier mobility.18 Ashraf
et al. reported the synthesis of a novel thienoisoindigo (Scheme 1,
compound 4) and its copolymer showed ambipolar nature with
There are many factors that influence the charge carrier mobil-
ity in polymeric OFETs. Generally speaking, the increase of the
effective conjugation length of a conjugated polymer leads to high-
er intra-chain charge carrier mobility.20 Good
p–p stacking, on the
other hand, leads to higher inter-chain mobility.21 Inter-chain
charge carrier mobility is the rate-limiting factor for carrier trans-
portation in polymeric OFETs. One possible way to improve the
mobility is to incorporate larger fused aromatic monomers into
conjugated polymers, which not only increases the effective conju-
both hole and electron mobility over 0.1 cm2 Vꢀ1 sꢀ1 19
.
gation length, but also provides stronger
p–p stacking. In fact,
many small molecular OFETs based on large fused acenes and hete-
roacenes were reported to have excellent charge carrier mobil-
ity.21–23 Therefore, we envisioned that using isoindigo fused with
⇑
Corresponding author. Tel./fax: +86 532 8066 2740.
E-mail address: wanxb@qibebt.ac.cn (X. Wan).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.12.076

N. Zhao et al. / Tetrahedron Letters 55 (2014) 1040–1044
1041
H
N
The Friedel–Crafts cyclization of the corresponding oxime occurred
exclusively at 4-position and dione 10 was obtained as the sole
product, since 4-position on the benzothiophene is more reactive
than 6-position (see Scheme 3).
H2N
O
O
H
N
S
S
S
O
6
7
8
O
S
S
H
N
Although the corresponding isoindigo derivative could be syn-
thesized from compound 10, the coplanarity of the isoindigo could
not be maintained due to the strong steric hindrance around the
formed double bond. So 4-position of compound 6 has to be
blocked to force the reaction to occur at 6-position. We first tried
to introduce bromine to block 4-position of compound 6. Although
NBS failed to brominate compound 6, double bromination was
achieved using HBr as the reagent to afford 11 in 95% yield. Desired
mono-brominated derivative could not be obtained even when the
amount of HBr was decreased. Since both 4- and 6-position of the
benzothiophene ring were blocked, this method could not be ap-
plied to synthesize the corresponding dione. Indeed, even the con-
version of 11 to the corresponding oxime 12 was impossible due to
the strong steric hindrance.
O
N
H
5 TII
Scheme 2. Proposed synthetic route towards thiophene-fused isoindigo.
aromatic/heteroaromatic rings as a building block in polymers
might further improve the carrier mobility of the resulting poly-
mers. Inspired by Ashraf’s work, we herein wish to report the syn-
thesis of
a novel thiophene-fused isoindigo (TII, Scheme 1,
compound 5), and the study on its optical and electrochemical
properties. The results show that it might be a promising novel
building block for OSCs.
To solve this problem, we turned our attention to chlorine atom,
which is smaller than bromine. To our delight, N-chloro-succini-
mide (NCS) was effective to introduce chlorine atom exclusively
at 4-position of 6 to give compound 13 in good yield, as shown
in Scheme 4. No chlorination took place at 2-position of 6, thus left
room for further modification (for example, bromination), which
will facilitate the incorporation of the final isoindigo derivative
into polymers. Treating compound 13 with chloral hydrate and
hydroxylamine hydrochloride gave the corresponding oxime 14
and further treatment with conc. sulfuric acid afforded 4-chloro-
5H-thieno[2,3-f]indole-6,7-dione (15) in good yield. Subsequently,
compound 15 was reduced with zinc and titanium tetrachloride to
give 4-chloro-5H-thieno[2,3-f]indol-6(7H)-one (16) in 85% yield.
Results and discussion
The original designed synthetic route was quite straight-for-
ward, and shown in Scheme 2. We speculated that thiophene-fused
dione 7 could be prepared from benzo[b]thiophen-5-amine 6.24 It
could then be reduced to afford compound 8. Subsequential con-
densation between 7 and 8 should give the desired TII.
Compound 6 was synthesized from commercially available
2-chloro-5-nitrobenzaldehyde in high yield according to the
literature.25 It was then converted to (E)-N-(benzo[b]thiophen-5-
yl)-2-(hydroxyimino)acetamide (9) in 60% yield. However, the rest
of route towards the targeted compound became quite tortuous.
O
O
CCl3CH(OH)2,HCl,
NH2OH.HCl, Na2SO4
H
H2N
H2SO4
90oC, 2h
78%
N
OH
N
NH
reflux, 2h
60%
S
O
S
6
9
S
10
HBr, DMSO
rt, 12h, 95%
Br
Br
H
N
OH
H2N
Br
N
O
S
S
Br
11
12
Scheme 3. Synthetic exploration of the precursor of thiophene-fused isoindigo.
CCl3CH(OH)2,HCl,
NH2OH.HCl, Na2SO4
Cl
Cl
H
N
H2N
NCS, THF
0oC, 1h
71%
H2N
OH
N
reflux, 2h
63%
S
O
S
S
5
13
14
H
Cl
N
O
Cl
H
S
AcOH, HCl
reflux, 80%
H2SO4
N
O
80oC, 1h
70%
S
S
17 TII
N
O
Cl
O
H
15
Zn, TiCl4, THF
85%
C8H17
N
Cl
H
Cl
S
O
S
1-bromooctane, K2CO3
N
O
air, DMF, rt, 12h
70%
S
N
C8H17
O
Cl
16
18 C8-TII
Scheme 4. Synthetic route of thiophene-fused isoindigo.

1042
N. Zhao et al. / Tetrahedron Letters 55 (2014) 1040–1044
With compounds 15 and 16 in hand, the direct condensation
Currently, there are two major methodologies in isoindigo syn-
thesis. One involves the condensation reaction between the corre-
sponding indoline-2,3-dione and indolin-2-one, in which reflux in
acetic acid is general needed. The other routes involve dimeriza-
tion of the dione in one step, using either unpleasant Lawesson’s
reagent31 or expensive tris(diethylamino)phosphine30 as the cou-
pling reagent. Compared to current isoindigo synthesis methodol-
ogies, the beauty of this reaction is that the alkylation, oxidation
and condensation were completed in one mild step.
The optical and electrochemical properties of C8-TII were stud-
ied and compared with that of C8-BII and the alkylated isoindigo
C8-II. Their UV–vis absorption spectra are shown in Figure 1a. All
three compounds display broad absorption bands from 230 to
600 nm with similar band edges (Fig. 1a), indicating that they have
similar HOMO–LUMO band gap, and fusing a thiophene ring or a
benzene ring to the isoindigo core does not lead to a lower band
gap. The difference lies in the intensity of the absorption at long
wave length. A much more intensed absorption band with the peak
around 496 and 478 nm is observed for C8-TII and C8-BII, respec-
tively, compared to the case of C8-II at the same concentration (the
insertion in Fig. 1a). This absorption is assigned to the HOMO to
LUMO transition of isoindigo derivatives.32 This band shows an
intramolecular charge-transfer character, since HOMO of the isoin-
digo is much more delocalized throughout the whole molecule,
while the LUMO is more localized on the central rings. A strength-
ened absorption of C8-TII and C8-BII implies a strengthened intra-
molecular charge-transfer process33 that occurs in them. This
might be due to the aromatic/heteroaromatic rings fused onto
the isoindigo core in C8-TII and C8-BII.
was then tested in acetic acid. Thiophene-fused isoindigo 17 was
obtained as a red solid in 80% yield, but it is only sparsely soluble.
Unfortunately, all the attempts to alkylate compound 17 for better
solubility failed, which ended up with either no reaction (K2CO3 or
KOH in DMF at room temperature or 100 °C) due to its poor solu-
bility in those solvents or decomposition (NaH in THF, reflux). This
was disappointing since excellent solubility of the building block is
needed to synthesize solution-processable conducting polymers
for OFETs. Then alkylation before condensation might be an alter-
native way to construct soluble thiophene-fused isoindigo. Surpris-
ingly and also delightfully, we found that when treated with excess
K2CO3 in the presence of 1-bromooctane in DMF at room temper-
ature, compound 16 was directly converted into alkylated thio-
phene-fused isoindigo (C8-TII) in 70% yield in one step. The
possible mechanism might involve the alkylation of compound
16, and the oxidation of the alkylated compound by air under basic
conditions to afford the alkylated dione 15a.26–29 The alkylated
dione then underwent condensation reaction with unoxidized
16a to give C8-TII, as shown in Scheme 5. The aerobic oxidation
is the rate-determine step so that compound 15a generated in situ
would be consumed in time to afford the final product.
We also noticed that the condition for aerobic oxidation of 16a
could also be applied to the oxidation of its isomer 6H-thieno[3,2-
e]indol-7(8H)-one to afford dione 10 in 80% yield (SI). But 10 could
not react with the starting material to give the isomer of 18 due to
the steric hindrance. However, indolin-2-one could not be con-
verted to indoline-2,3-dione under the same conditions. It seemed
that fusing one electron-rich aromatic/heteroaromatic ring to
indolin-2-one was crucial for the oxidation to occur.
The electrochemical properties of the three compounds were
compared using cyclic voltammetry (CV), as shown in Figure 1b.
The energy level of LUMO of the isoindigo derivatives was
calculated from the reduction potential onsets, using
LUMO ¼ 4:44 eV þ Eorendset equation (referring to ferrocene standard).
The optical (UV–vis) and electrochemical properties of C8-II, C8-TII
and C8-BII are summarized in Table 1. For each of these isoindigo
derivatives two pairs of reduction–oxidation waves were observed.
Similar methodology was also applied to synthesize the ben-
zene-fused isoindigo (C8-BII), but with a low yield in the key step
(ca. 4%), as shown in Scheme 6. It was presumably due to the insta-
bility of compound 23 under basic conditions, since obvious
decomposition was observed during the reaction. Other method
30
such as coupling of alkylated 22 using P(NEt2)3 as the coupling
agent also end up with low yield (15%).
Cl
Cl
Cl
C8H17
C8H17
H
K2CO3
K2CO3
N
N
N
O
O
O
C8H17 Br
S
S
S
16a
16
O2
C8H17
Cl
S
Cl
Cl
N
O
C8H17
C8H17
S
- OH-
N
N
O
O
S
S
H
N
O
Cl
O
O
15a
O
C8H17
18 C8-TII
Scheme 5. Plausible mechanism of the one-pot formation of C8-TII.
CCl3CH(OH)2, HCl,
NH2OH.HCl, Na2SO4
Cl
Cl
H
N
NH2
H2SO4
80oC, 1h
73%
NCS, THF
0oC, 1h
93%
NH2
Cl
OH
N
reflux, 2h
63%
O
19
Cl
20
21
C8H17
N
Cl
O
H
N
H
N
C8H17 Br, K2CO3
Zn/TiCl4
THF, rt, 5 min
O
O
DMF, air, rt
4%
74%
N
O
O
Cl
C8H17
22
23
24 C8-BII
Scheme 6. Synthetic route of benzene-fused isoindigo.

N. Zhao et al. / Tetrahedron Letters 55 (2014) 1040–1044
1043
building block for n-type or ambipolar OFET materials, due to its
excellent electron-affinity, simplicity in synthesis and ease for fur-
ther modification. The modification of C8-TII and its copolymeriza-
tion study with electron-rich monomers are currently underway in
our laboratory.
(a)
C8 - II
C8H17
C8 - TII
C8 - BII
N
O
1.0
0.5
0.0
O
N
C8H17
Experimental section
General
400
500
600
C8-II
All glassware was thoroughly oven-dried before use. Chemicals
and solvents were either purchased from commercial suppliers or
purified by standard techniques. Thin-layer chromatography plates
were visualized by exposure to ultraviolet light and/or immersion
in a staining solution (phosphomolybdic acid) followed by heating
on a hot plate. Flash chromatography was carried out utilizing sil-
ica gel 200–300 mesh. Chemical shifts are given in d relative to tet-
ramethylsilane (TMS), the coupling constants J are given in Hz. The
spectra were recorded in CDCl3 or d6-DMSO as the solvent at room
temperature. Cyclic voltammograms (CV) were carried out in an
argon-saturated solution of 0.1 M of tetrabutylammonium hexa-
fluorophosphate (n-Bu4NPF6) in anhydrous chloroform, using a
three-electrode system (platinum as both the working electrode,
and counter electrode, Ag/AgCl as the reference electrode
300
400
500
600
Wavelength (nm)
(b)
0.0
-6
-5.0x10
-5
-1.0x10
-5
C8 - TII
C8 - BII
C8 - II
-1.5x10
calibrated against ferrocene) at a potential scan rate of 0.1 V sꢀ1
.
-5
-2.0x10
Typical Procedure for the Synthesis of alkylated thiophene-
fused isoindigo C8-TII (18)
-2.0
-1.5
-1.0
-0.5
0.0
Potential (V vs Ag/AgCl)
Figure 1. (a) UV–vis absorption spectra of synthesized C8-II, C8-TII and C8-BII in
CH2Cl2 solution at the same concentration (10ꢀ5 M); (b) cyclic voltammogram of
C8-II, C8-TII and C8-BII in 0.1 M n-Bu4NPF6 chloroform solution.
To a suspension of 4-chloro-5H-thieno[2,3-f]indol-6(7H)-one
(112 mg, 0.50 mmol, 1.0 equiv) and fresh-dried potassium carbon-
ate (0.35 g, 2.5 mmol, 5.0 equiv) in anhydrous DMF (5.0 mL) at 0 °C,
1-bromooctane (190 lL, 1.1 mmol, 2.2 equiv) was injected through
a septum under argon. Then adequate amount of dry air was intro-
duced into the system. The mixture was stirred for 12 h at room
temperature and then poured into water (50 mL). The organic
phase was extracted by CH2Cl2, washed with brine and dried over
MgSO4. After removal of the solvent under reduced pressure, the
deep-red solids were purified by silica chromatography, eluting
with (CH2Cl2/Petroleum ether = 1:3) to give C8-TII (117 mg, 70%).
mp 249–250 °C. 1H NMR (CDCl3, 600 MHz) d: 9.74 (s, 2H), 7.66
(d, J = 5.40 Hz, 2H), 7.52 (d, J = 5.40 Hz, 2H), 4.29 (t, 4H), 1.81 (t,
4H), 1.46–1.29 (t, 20H), 0.89 (t, 6H). 13C NMR (CDCl3, 150 MHz)
d: 168.17, 141.61, 136.66, 134.30, 132.50, 131.65, 123.10, 123.08,
122.70, 108.23, 42.15, 31.81, 29.53, 29.35, 29.24, 26.80, 22.65,
14.11. HRMS (ESI): calcd for [C36H40Cl2N2O2S2 + H]+: 667.1987,
found: 667.1979.
Table 1
Optical (UV–vis) properties and electrochemical data of C8-II, C8-TII and C8-BII
onset
UV–vis kmax
(nm)
EgOptical
(eV)
LUMOa
(eV)
HOMO
(eV)
E
red
(eV)
C8-II
C8-TII
C8-BII
497
496
478
2.02
2.16
2.21
ꢀ0.8
ꢀ3.64
ꢀ3.81
ꢀ3.82
ꢀ5.66
ꢀ5.97
ꢀ6.03
ꢀ0.63
ꢀ0.62
a
Calculated from the reduction onset of CV.
Both redox pairs of C8-TII and C8-BII are reversible, while only the
first pair of C8-II is reversible, indicating a better reversibility of
the isoindigo derivatives with more fused aromatic/hetero-aro-
matic rings. Furthermore, the onset potential of reduction of C8-
TII and C8-BII is ꢀ0.63 and ꢀ0.62 eV respectively, which is higher
than that for C8-II (ꢀ0.8 eV), indicating that the introduction of
fused thiophene and benzene ring onto the isoindigo lowers the
energy level of the LUMO of isoindigo, making them better candi-
dates for n-type OSCs due to better electron affinity.
Acknowledgments
We appreciate the financial support from NSFC (21174157) and
‘100 Talents’ program from Chinese Academy of Sciences.
Supplementary data
Supplementary data (experimental details, copies of the 1H
NMR and 13C NMR spectra for all key intermediates and final
products) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2013.12.076.
Conclusions
In conclusion, novel thiophene-fused isoindigo C8-TII was syn-
thesized via a new facile method, and its properties were com-
pared with benzene-fused isoindigo C8-BII and the parent
isoindigo C8-II. Our investigations have shown that the intramo-
lecular charge-transfer process is greatly enhanced and the electro-
chemical redox reversibility is improved by fusion of one more
heteroaromatic/aromatic ring to the isoindigo core, which would
be a plus for better OFET materials. Especially, C8-TII is a promising
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