Nanowires of indigo and isoindigo-based molecules with thermally removable groups

Article abstract of DOI:10.1016/j.dyepig.2015.10.003

In this manuscript, indigo and isoindigo-based π-conjugated molecules with thermal removable tert-butoxycarbonyl (t-Boc) side groups were designed and synthesized. It was noted that the t-Boc side groups can be eliminated in nearly quantitative yields after thermal treatment at 200°C for 15 min, as confirmed by thermogravimetric analysis and Fourier transform infrared spectroscopy. From the thermal treated solution of isoindigo-based molecule DTIIC8C12 in the co-solvent of 1,2-dichlorobenzene/pyridine with volume ratio of 10/90, one-dimensional nanowires can be formed due to the hydrogen bonding assisted self-assembly. The afforded nanowires exhibited a moderate hole mobility of 1.3 × 10^-3 cm² V^-1 s^-1, as estimated from the organic field effect transistors. These observations illustrated that the utilization of thermal removable side chain functionalized conjugated polymers can be an effective strategy for developing conjugated polymers with impressive charge carrier transport.

Full text of DOI:10.1016/j.dyepig.2015.10.003

Dyes and Pigments 125 (2016) 54e63
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Nanowires of indigo and isoindigo-based molecules with thermally
removable groups
*
**
Chunchen Liu, Wenzhan Xu, Qifan Xue, Ping Cai, Lei Ying , Fei Huang , Yong Cao
Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 August 2015
Received in revised form
In this manuscript, indigo and isoindigo-based p-conjugated molecules with thermal removable tert-
butoxycarbonyl (t-Boc) side groups were designed and synthesized. It was noted that the t-Boc side
ꢀ
groups can be eliminated in nearly quantitative yields after thermal treatment at 200 C for 15 min, as
28 September 2015
confirmed by thermogravimetric analysis and Fourier transform infrared spectroscopy. From the thermal
treated solution of isoindigo-based molecule DTIIC8C12 in the co-solvent of 1,2-dichlorobenzene/pyri-
dine with volume ratio of 10/90, one-dimensional nanowires can be formed due to the hydrogen bonding
Accepted 1 October 2015
Available online 22 October 2015
assisted self-assembly. The afforded nanowires exhibited
a
moderate hole mobility of
ꢂ1 ꢂ1
V s , as estimated from the organic field effect transistors. These observations illus-
Keywords:
ꢂ3
2
1.3 ꢁ 10 cm
Conjugated molecules
tert-butoxycarbonyl
Thermal cleavage
Hydrogen bonding
Nanowires
trated that the utilization of thermal removable side chain functionalized conjugated polymers can be an
effective strategy for developing conjugated polymers with impressive charge carrier transport.
© 2015 Elsevier Ltd. All rights reserved.
Organic field effect transistors
1. Introduction
It has been well-established that the covalently linked tert-
butoxycarbonyl (t-Boc) substituents in the N atoms can impart
Over the past decades, much attention has been paid to organic
field-effect transistors (OFETs) due to their potential applications in
plastic electronics such as memory or storage devices [1e4]. The
development of novel conjugated organic semiconductors with
high carrier mobility, good ambient stability and low-cost pro-
cessability has led to significantly improved performances of OFETs.
Recently, various pigments such as diketopyrrolopyrrole (DPP)
semiconducting precursors excellent solubility, and the elimination
of the t-Boc unit can give rise to hydrogen bond, which can then
result in completely insoluble products due to the strong molecular
stacking [11e14]. Hydrogen bond can be utilized as the driving
force for self-assembly of molecules in solutions by inducing well-
organized nanostructures [15e18]. By utilizing hydrogen bond as
the driving force, self-assembled
p-conjugated small molecules
[
5e7], indigo [8] and isoindigo [9,10] have been utilized to
with highly ordered intermolecular packing motif have been
accessed [19e22].
construct -conjugated molecules as the semiconductors for OFETs.
p
These molecules exhibited close intermolecular stacking due to
their outstanding coplanar architectures. Of particular interests is
that these molecules can potentially form NH O]C hydrogen
In this manuscript, we developed two novel indigo and
isoindigo-based
p-conjugated molecules consisting of thermally
…
removable t-Boc side groups (Chart 1). It was worth mentioning
that the thermal cleavage of t-Boc groups was conducted in the
solutions, which allowed for the hydrogen bonding assisted in-situ
self-assembly to form one-dimensional nanowires (Chart 2). Uni-
form films can be obtained by spin-casting from such nanowires
solutions subsequently. Impressively, the obtained nanowires
bonding interaction owing to their unique lactam structure [7].
*
Corresponding author. Tel.: þ86 20 87114346; fax: þ86 20 87110606.
ꢂ3
2
ꢂ1 ꢂ1
exhibited a hole mobility of 1.3 ꢁ 10 cm V
s , as evaluated by
*
* Corresponding author. Tel.: þ86 20 87114346; fax: þ86 20 87110606.
the filed effect transistors. To our knowledge, it is the first work to
E-mail addresses: msleiying@scut.edu.cn (L. Ying), msfhuang@scut.edu.cn
(
F. Huang).
successfully prepare the isoindigo-based nanowires for OFETs.
http://dx.doi.org/10.1016/j.dyepig.2015.10.003
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
55
Chart 1. Thermal cleavage of t-Boc groups of molecules.
Chart 2. Schematic diagram for the self-assembly of the thermo-cleaved molecules to obtain the one-dimensional nanowires.
2
. Results and discussion
were further confirmed by matrix-assisted laser desorption/ioni-
zation time of flight mass spectroscopy (MALDI-TOF). The ob-
tained mass were 1151.52 and 1151.53 for DTIC8C12 and
DTIIC8C12, respectively, which were lower than the theoretical
molecular weights of 1352.05. This can be understood as the
loss of two tert-butoxycarbonyl (t-Boc) groups during the
measurements.
2.1. Synthesis of DTIC8C12 and DTIIC8C12
The synthesis routes for molecules DTIC8C12 and DTIIC8C12 are
shown in Scheme 1. The iodinization of the commercially available
-bromoindole (1) by using iodine (1 equivalent) and potassium
6
iodide (1 equivalent) in the presence of sodium hydroxide (1
equivalent) in methanol gave compound 3-iodo-6-bromoindole
(2), which was used for the following reaction without purifica-
2.2. Thermal properties
tion. By treating compound 2 with silver acetate (2 equivalent) in
ꢀ
acetic acid at 90 C for 3 h, the target compound 6-bromo-3-
Thermal behaviors of these two molecules were evaluated by
differential scanning chromatography (DSC) and thermogravi-
metric analysis (TGA). TGA characteristics illustrate the two-step
thermal decomposition procedures of these two molecules. As
shown in Fig. 1a, the first decomposition at about 190 C can be
attributed to the elimination of t-Boc groups, which yielded two
acetoxyindole (3) was afforded. The hydrolysis of 3 in aqueous
ethanol containing sodium hydroxide at room temperature gave
,6 -dibromoindigo (4), which was then reacted with di-tert-butyl-
dicarbonate to give the corresponding t-Boc substituted interme-
diate compound 8. The intermediate compound 9 was synthesized
0
6
ꢀ
according to the similar procedure as that of compound 8 by using
2
equivalents of isobutene and CO [11,25]. The first weight loss ratio
0
6
,6 -dibromoisoindigo (7) [23,24] as the starting materials. The
calculated from the TGA properties was 14.4% and 14.7% for
DTIC8C12 and DTIIC8C12, respectively, which was nearly identical
to the theoretical weight loss value of 14.8% for the cleavage of two
t-Boc groups from these compounds, indicating the quantitatively
thermal cleavage of t-Boc side groups. The additional weight loss
following palladium catalyzed Stille cross-coupling reaction of 8
0
0
and
9 with tributyl(5 -(2-octyldodecyl)-[2,2 -bithiophen]-5-yl)
stannane (10) gave the target conjugated molecules of DTIC8C12
and DTIIC8C12.
ꢀ
The molecular structures of all monomers and intermediates
are confirmed by 1H and C nuclear magnetic resonance (NMR)
spectroscopy. Molecular structures of DTIC8C12 and DTIIC8C12
step occurred at about 380 C can be ascribed to the decomposition
13
of the molecular backbones, indicating the good thermal stability of
the molecular scaffold after the elimination of t-Boc groups.

56
C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
ꢀ
Scheme 1. Synthetic routes for molecules: (i) I
iv) CH COOH, HCl, reflux; (v) DMAP, di-tert butyl dicarbonate, CH
tetrahydrofuran, 80 C, 4 h.
2
, KI, NaOH, methanol, room temperature, 3 h; (ii) CH
3
COOAg, CH
3
COOH, 90 C, 3 h; (iii) NaOH, ethanol, room temperature, overnight;
0
0
(
3
2
Cl
2
, room temperature 6 h; (vi) tributyl(5 -(2-octyldodecyl)-[2,2 -bithiophen]-5-yl)stannane, Pd
2
(dba)
3
, P(o-Tol)
3
,
ꢀ
We also investigated the thermal properties of these molecules
by DSC characterization. The initial heating cycle up to 250 C was
scrutinizing the profiles of DTIC8C12 (Fig. 1b), one can observe
ꢀ
weaker endothermic and exothermic peaks in the high tempera-
ꢀ
performed to accomplish the thermal cleavage of t-Boc groups from
molecules and eliminate the thermal history of the molecules (see
Figure S1). The trace for the first heating cycle showed the first
ture region of about 250 C, which might be attributed to the for-
mation of an additional kind of crystalline structure after thermal
elimination of t-Boc groups in DTIC8C12. Nonetheless, these ob-
servations indicated strong crystallization tendency of these mol-
ecules after cleavage of t-Boc groups. Detailed thermal properties of
these compounds were summarized in Table 1.
ꢀ
ꢀ
endothermic peak at 87 C for DTIC8C12 and 159 C for DTIIC8C12,
which can be attributed to the phase transition caused by the side
ꢀ
chains. The second endothermic peaks at 195 C for both molecules
could be attributed to the thermal decomposition of the t-Boc
groups as demonstrated in TGA characterization. In the second
heating cycle (Fig. 1b), the endothermic peaks at 195 C dis-
2.3. FT-IR spectroscopy
ꢀ
appeared, while one can observe the obvious melting point of
Fourier transform infrared (FT-IR) spectroscopy was utilized to
investigate the thermal cleavage of t-Boc groups (Fig. 2). The pris-
tine films were spin-coated from the chloroform solutions of
ꢀ
ꢀ
197 C and 213 C for DTIC8C12 and DTIIC8C12, respectively along
with the emergence of apparent crystallization characteristic. By
ꢀ
ꢂ1
.
Fig. 1. TGA (a) and DSC (b) characteristics of DTIC8C12 and DTIIC8C12 with heating rate of 10 C min

C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
57
Fig. 2. The FT-IR spectra for the pristine films (black curves) and after thermal treatment (red curves) for molecules DTIC8C12 (a) and DTIIC8C12 (b). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. UVeVis absorption profiles of DTIC8C12 (a) and DTIIC8C12 (b) in chloroform solution and as films.
ꢂ1
DTIC8C12 and DTIIC8C12 with concentration of 10 mg mL , which
were thermally annealed at 200 C for 15 min. As shown in Fig. 2,
2.4. Optical and electrochemical properties
ꢀ
one can clearly observe the characteristic stretching vibration band
Fig. 3 showed the UVeVis absorption spectra of DTIC8C12 and
DTIIC8C12 in chloroform solution and as thin films. In chloroform
solutions, both DTIC8C12 and DTIIC8C12 exhibited dual character-
istic absorption bands, wherein the short wavelength can be
ꢂ1
at 1700 cm of C]O corresponding to the t-Boc moiety, which
ꢀ
disappeared after thermal annealing the films at 200 C for 15 min.
These observations demonstrated that t-Boc groups could be
completely removed after thermal treatment.
attributed to the characteristic absorption of p-p* transition of
ꢀ
Fig. 4. Electrochemical properties of DTIC8C12 (a) and DTIIC8C12 (b), pristine (black curves) and annealed films at 200 C for 15 min, (red curves). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)

58
C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
Fig. 5. The change of dihedral angles of molecules before and after cleavage of t-Boc groups.
molecular backbones, while the long wavelength can be attributed
to the intramolecular charge transfer (ICT) effects between
electron-donating thiophene unit and electron-withdrawing in-
digo and isoindigo units [26,27]. In contrast, the absorption profiles
as thin films at low-energy band were extended with the appear-
ance of shoulder peaks, indicating that these compounds have
strong aggregation tendency in solid state. However, the films are
not uniform due to the potential crystallization after thermal
treatment, thereby we failed to record reliable absorption profiles
of these compounds. The optical band-gap as calculated from the
absorption onset in films was 1.64 eV for DTIIC8C12, which was
much smaller than that of 2.0 eV for DTIC8C12. All relevant ab-
sorption data were summarized in Table 2.
Cyclic voltammetry (CV) measurements were performed to
investigate the influence of the elimination of t-Boc groups on the
frontier molecular orbitals. The measurements were carried out
under an inert atmosphere by using tetra-n-butylammonium
hexafluorophosphate (n-Bu NPF , 0.1 M in acetonitrile) as the
4 6
supporting electrolyte with a ITO-coated glass working electrode, a
platinum wire counter electrode and a saturated calomel electrode
Fig. 6. Tapping-mode AFM images (10
ODCB/pyridine (33/67); (d) ODCB/pyridine (0/100).
m
m ꢁ 10
mm) of spin-coated thin films from thermo-cleaved DTIIC8C12 solutions: (a) ODCB/pyridine (100/0); (b) ODCB/pyridine (50/50); (c)

C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
59
Fig. 7. Tapping-mode AFM images of spin-coated DTIIC8C12-based nanowires films with different scanning size, (a) 10
m
m ꢁ 10
m
m and (b) 30
m
m ꢁ 30
m
m for nanowires self-
assembled in solvent of ODCB/pyridine (20/80); (c) 10
m
m ꢁ 10
mm and (d) 30
mm ꢁ 30
mm for nanowires self-assembled from solvent of ODCB/pyridine (10/90).
(
SCE) as reference. The potential of the ferrocene/ferrocenium (Fc/
þ
Fc ) redox couple was measured as a standard. It is assumed that
E
HOMO ¼ ꢂeðEox þ 4:48ÞðeVÞ;
E
LUMO ¼ ꢂeðEred þ 4:48ÞðeVÞ
þ
the redox potential of Fc/Fc has an absolute energy level of ꢂ4.8 eV
where the Eox and Ered are the onset of the oxidation and redox
potential vs. SCE, respectively. The EHOMO of the pristine DTIC8C12
and DTIIC8C12 were determined to be ꢂ5.51 and ꢂ5.33 eV,
respectively. After the thermal cleavage of t-Boc groups, the EHOMO
of DTIC8C12 increased to ꢂ5.43 eV, which could be attributed to the
enhanced molecular coplanarity and elongated conjugation [7,25].
In contrast, the EHOMO of DTIIC8C12 decreased to ꢂ5.42 eV, which
to vaccum [28]. Under the same experimental conditions, the onset
þ
potential of Fc/Fc was measured to be 0.32 V with respect to the
SCE reference electrode. Therefore, the highest occupied molecular
orbital energy levels (EHOMO) and lowest unoccupied molecular
orbital energy levels (ELUMO) of the molecules were calculated ac-
cording to the following equation,
Fig. 8. UVevis absorption spectra of DTIIC8C12 in chloroform solution, as pristine
films and films of nanowires, (O/P: ODCB/pyridine).
Fig. 9. XRD patterns of initial DTIIC8C12 and nanowires self-assembled in different
solvents, (O/P: ODCB/pyridine).

60
C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
Fig. 10. OFET architecture (a), OFET output (b) and transfer characteristics (c) of DTIIC8C12 nanowires self-assembled in ODCB/pyridine ¼ 10/90.
ꢀ
ꢀ
can be correlated to the decreased electron-withdrawing proper-
ties of the isoindigo unit after the elimination of the electron-
deficient t-Boc groups. The ELUMO of pristine DTIC8C12 and
DTIIC8C12 were calculated to be ꢂ3.86 eV and ꢂ3.70 eV, both of
which slightly decreased to ꢂ3.92 eV and ꢂ3.83 eV, respectively
DTIC8C12 significantly decreased from 23.8 to 0.2 after the elimi-
nation of t-Boc groups. The formation of the nearly quasi-coplanar
conformation indicated that the steric hindrance originated from
the bulky t-Boc side groups can be significantly reduced after elimi-
nation.Itisworthnotingthattheimprovedmolecularplanaritymight
be beneficial for the ordered and closed intermolecular stacking, and
thereby theoretically leads to the improved charge carrier mobility.
For DTIIC8C12, the dihedral angle between two fused rings of iso-
(Fig. 4) (Table 3).
2.5. DFT electronic structure calculation
ꢀ ꢀ
indigo unit (4 ) reduced from 18.9 to 10.7 , which is much weaker
3
than that of DTIC8C12. It can be speculated that DTIC8C12 might
exhibit much stronger intermolecular aggregation after the elimina-
tion of t-Boc side groups than that of DTIIC8C12.
To get insight into the variation in molecular conformation of
molecules after thermal cleavage of t-Boc groups of DTIC8C12 and
DTIIC8C12, theoretical calculation was carried out by using density
functional theory (DFT) at the B3LYP/6-31G(d) basis with the
Gaussian 09 package [29]. The calculated torsional angles of mol-
ecules before and after the elimination of t-Boc groups were shown
in Fig. 5, with relevant data summarized in Table 4.
The calculated HOMO of DTIIC8C12 was delocalized in the mo-
lecular backbone, which slightly decreased from ꢂ4.97 eV
to ꢂ5.01 eV after the elimination of t-Boc groups. However, the
calculated HOMO of DTIC8C12 slightly increased from ꢂ5.25 eV
to ꢂ5.18 eV after the elimination of t-Boc groups. This is under-
standable, since the two indolinone moieties in the single indigo
unit were not ideally conjugated, thus the calculated HOMO of
DTIC8C12 was not delocalized in molecular backbone due to the
steric hindrance effect of t-Boc groups. In this respect, the enhanced
coplanarity of DTIC8C12 after the elimination of t-Boc groups can
facilitate the electron delocalization, which in turn led to an
increased HOMO. Besides, the calculated LUMOs mainly located in
It was realized that after the elimination of t-Boc groups, dihedral
angles between two thiophene units (4
between the thiophene units and indigo unit (4
obvious disparity for both DTIC8C12 and DTIIC8C12. However, the
1
, 4
5
) and dihedral angles
2
, 4 ) did not show
4
3
dihedral angle between two fused rings of indigo unit (4 ) for
Table 1
Thermal properties of molecules DTIC8C12 and DTIIC8C12.
1
(oC)
2
(oC)
T
m
(oC) Weight lossa (%) Weight lossb (%)
Molecules
DTIC8C12
T
d
T
d
Table 2
192
382
385
194
213
14.8
14.8
14.4
14.7
Photophysical properties of molecules DTIC8C12 and DTIIC8C12.
DTIIC8C12 186
1
: Temperature of the first decomposition; T2
opt
T
d
d
: temperature of the second decom-
Molecules
lmax (nm) Solution
lmax (nm) films
E
g
(eV)
position; T
m
: melting point of molecules after cleavage of t-Boc groups.
Theoretical weight loss of t-Boc side groups.
DTIC8C12
DTIIC8C12
365, 496
359, 616
369, 489
384, 696
2.00
1.64
a
b
Experimental weight loss of t-Boc side groups.

C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
61
Table 3
2.7. Photophysical properties and XRD of DTIIC8C12-based
nanowires
Electrochemical properties of DTIC8C12 and DTIIC8C12.
Molecules
DTIC8C12
Conditions
E
ox (V)
E
red (V)
E
HOMO (eV)
ELUMO (eV)
Fig. 8 showed the UVevis absorption of DTIIC8C12-based
nanowires films. It was realized that the absorption profiles of
such nanowires films obviously hypsochromic shifted with respect
to that of the pristine DTIIC8C12 films, which might be ascribed to
the formation of intrinsic H-aggregation [15]. Such aggregation
would be beneficial for the charge transportation in the nanowires
films [30e32]. X-ray diffraction (XRD) was performed to investigate
the difference in molecular packing between the pristine
Pristine
Annealed
Pristine
1.03
0.95
0.85
0.94
ꢂ0.62
ꢂ0.56
ꢂ0.78
ꢂ0.65
ꢂ5.51
ꢂ5.43
ꢂ5.33
ꢂ5.42
ꢂ3.86
ꢂ3.92
ꢂ3.70
ꢂ3.83
DTIIC8C12
Annealed
Table 4
Dihedral angles of molecules before and after thermal cleavage of t-Boc groups by
DFT calculations.
DTIIC8C12 film and the self-assembled nanowires film. As shown in
Molecules
Conditions
Dihedral angles (deg)
ꢀ
Fig. 9, the peak intensity at 2
q
z 51.2 obviously increased in the
4
1
4
2
4
3
4
4
4
5
self-assembled nanowire film that was obtained from ODCB/pyri-
dine co-solvent systems with volume ratio of 20/80 and 10/90. The
enhanced peak intensity indicated more ordered and closer inter-
molecular stacking in the self-assembled nanowires, which can
facilitate the charge transporting in the spin-coated films [2,33,34].
DTIC8C12
DTIIC8C12
Before cleavage
After cleavage
Before cleavage
After cleavage
16.6
16.1
14.9
15.6
23.7
22.3
17.8
20.4
23.8
0.2
18.9
10.7
21.2
23.3
18.3
21.2
15.9
16.9
14.7
15.9
2.8. Organic field effect transistor performance
the electron-deficient indigo and isoindigo moieties. The LUMOs
slightly increased from ꢂ2.79 eV to ꢂ2.76 eV, and from ꢂ2.99 eV
to ꢂ2.80 eV for DTIC8C12 and DTIIC8C12, respectively. The calcu-
lated HOMOs and LUMOs were shown in Table S1, which were in
good agreement with the observed trends in CV measurements.
Bottom gate, top contact organic field effect transistors (OFETs)
with the architecture of Si/SiO
PMMA)/DTIIC8C12 nanowires/Ag were fabricated to investigate
the charge carrier mobility of the self-assembled nanowires films.
We initially tried to fabricate the nanowires films on the Si/SiO
2
/poly(methyl methacrylate)
(
2
wafer that was treated with a self-assemble monolayer of octade-
cyltrichlorosilane (OTS); however, uniform films cannot be formed
on the surface of such OTS-treated substrates. Hence, PMMA was
used to modify the gate dielectric layer since uniform nanowires
can be obtained [35,36]. Semiconducting layer was spin-coated
from nanowires ODCB/pyridine (volume ratio of 10/90) solution
2
.6. AFM morphology of nanowires
Atomic force microscopy (AFM) was used to investigate the
morphology of the self-assembled nanowires. As shown in Fig. 6a,
desired nanowires of DTIIC8C12 could be obtained in the solvent of
o-dichlorobenzene (ODCB). However, the obtained solution of
nanowires was in gel-like state, which cannot be processed to give
uniform films by spin-coating method (inset in Fig. 6a). In order to
attain homogeneous solution that can be spin-coated to give uni-
form films for the fabrication of the semiconducting layer for
organic field effect transistors, different ratios of pyridine was
added to attenuate the intermolecular hydrogen bonding interac-
tion. It was noted that when 50% or 67% of pyridine was added,
although the homogeneous solution can be obtained, the nano-
wires cannot be formed (Fig. 6b, c). It was also noted that the
nanowire structures cannot be formed in the pure pyridine solution
ꢂ1
with concentration of 10 mg mL . Herein Ag instead of Au is used
as the electrode, since it is more cost-effective and can be easily
deposited on the top of the prefabricated semiconducting layer
films. We note that Ag electrode is less efficient for hole injection
than Au electrode, however, it can effectively reveal the charge
carrier mobility of the nanowires films [32,33]. Fig. 10 illustrated
the output and transfer characteristics of OFETs based on nano-
wires self-assembled from solution of ODCB/pyridine (volume ratio
of 10/90). The measurement was carried out by sweeping the gate
voltage (V
G
) from ꢂ50 to 10 V under a source-drain voltage (V
D
)
of ꢂ50 V. Hole mobility of nanowires self-assembled in ODCB/
pyridine (volume ratio of 10/90) solution was measured to be
ꢂ3
2
ꢂ1 ꢂ1
5
(Fig. 6d). However, homogeneous solutions along with desired
1.3 ꢁ 10 cm V
s , with the current on/off ratio of about 10 .
nanowires can be simultaneously obtained by increasing the pyri-
dine ratio to 80% and 90% in the co-solvent systems with ODCB. As
shown in Fig. 7, well-defined one-dimensional nanowires with
width in the range of 150e200 nm were observed. We noted that
the slightly different ratios of pyridine in the co-solvent systems
can have pronounced impact in the interpenetrating networks of
nanowires. By increasing the volume ratios of pyridine from 80%
This result indicated that in-situ self-assembly can be an effective
and low-cost method to acquire desired charge carrier mobility for
isoindigo-based molecules. To our knowledge, it is the first work to
successfully prepare the isoindigo-based nanowires for OFETs.
However, we failed characterize the OFET mobility of pristine
DTIIC8C12 films and nanowires films in solution of ODCB/pyridine
with volume ratio of 20/80, because uniform films cannot formed
on the surface of PMMA interlayer.
(Fig. 7a) to 90% (Fig. 7c), more closely entangled nanowires can be
observed along with the slightly increased width of nanowires.
Similar strategy was also utilized for the self-assembly of
nanowires based on DTIC8C12. However, serious aggregation was
observed, which was insensitive to the ratio of pyridine (Figure S2).
The fact can be attributed to the exceptionally strong intermolec-
ular stacking tendency owing to the quasi-coplanar conformation
after elimination of t-Boc side groups, as predicted by DFT calcu-
lation. Thus, further investigation about the thermal treatment of
DTIC8C12 solutions cannot be accomplished at the current stage.
Nonetheless, these observations highlighted that DIC8C12 and
DTIIC8C12 exhibit significantly different molecular stacking in so-
lution despite of very similar molecular structures.
3. Conclusion
In summary, we developed two novel indigo and isoindigo-
based
p-conjugated molecules DTIC8C12 and DTIIC8C12 consist-
ing of thermal removable tert-butoxycarbonyl (t-Boc) side groups.
It was realized that the solubilizing effect of t-Boc side groups can
impart the two resultant molecules good solubility in common
organic solvents. The t-Boc groups can be eliminated from these
molecules by thermal treatment in a quantitative yield, as
confirmed by the thermal gravimetric analysis and Fourier trans-
form infrared spectroscopy. The molecular coplanarity of the

62
C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
resulted compounds can be significantly enhanced after the elim-
ination of t-Boc side groups. DTIIC8C12 can self-assembled into
one-dimensional nanowires in solution of ODCB/pyridine with
volume ratio of 10/90, which exhibited a hole mobility of
600 MHz): 8.25 (s, 1H), 7.62 (d, 1H), 7.36 (d, 1H), 1.61 (s, 9H); 13C
NMR (CDCl3, 600 MHz): 182.39, 149.49, 149.41, 131.08, 127.77,
125.01, 121.67, 120.19, 85.25, 28.06.
General procedure for preparing DTIC8C12 and DTIIC8C12.
Monomer 8 or monomer 9 (0.5 g, 0.81 mmol) and monomer 10
ꢂ3
2
ꢂ1 ꢂ1
1
4
4
.3 ꢁ 10 cm V
s
as evaluated by the field effect transistors.
(
1.42 g, 1.94 mmol) were added into a three-necked, over-dried
100 mL round bottom flask, which was purged with argon for
0 min. Tris(dibenzylideneacetone)dipalladium(0) (9 mg), and
. Experimental section
3
.1. Materials
tri(o-tolyl)phosphine (18 mg) were then added, after which 30 mL
ꢀ
anhydrous THF was added. The mixture solutionwas heated to 80 C
All reagents and solvents, unless otherwise specified, were ob-
for 4 h. The reaction solution was allowed to cool down to room
temperature and then was poured into 100 mL of water and
extracted three times with dichloromethane. The combined organic
solution was dried over anhydrous magnesium sulfate. After the
solvent was removed under reduced pressure, the raw product was
purified with a column chromatography on silica gel using hexane:
tained from Aldrich and Alfa-Aesar Chemical Co. and were used as
received. Solvent of tetrahydrofuran was distilled from sodium/
0
0
benzophone. 6,6 -Dibromoisoindigo (7) [23,24] and tributyl(5 -(2-
0
octyldodecyl)-[2,2 -bithiophen]-5-yl)stannane (10) [37] were pre-
pared according to the reported procedures. 6,6 -Dibromoindigo
0
(
4) was synthesized according to the modified method of the
dichloromethane (1:2) as eluent to obtain final product.
1
literature [8]. All the related compounds were synthesized as the
following procedures as below.
DTIC8C12. (794.7 mg, 72.9%). H NMR (CDCl
3
, 600 MHz): 8.29 (s,
1H), 7.76 (d, 1H), 7.42 (d, 2H), 7.12 (d, 1H), 7.07 (d, 1H), 6.70 (d, 1H),
6
-Bromo-3-iodoindole (2). To a solution of 6-bromoindole (1)
2.75 (d, 2H), 1.67 (s, 9H), 1.31 (s, 1H), 1.31e1.27 (m, 32H), 0.89 (m,
6H); C NMR (CDCl , 600 MHz): 182.14, 149.97, 149.63, 145.11,
3
13
(
200 mg, 1.02 mmol) and sodium hydroxide (41 mg, 1.02 mmol) in
methanol (10 mL) were added iodine (259 mg, 1.02 mmol) and an
aqueous solution (2 mL) of potassium iodide (169 mg, 1.02 mmol).
After the mixture was stirred at room temperature for 3 h, water
was added. The resulting precipitate was collected by filtration,
washed with water, and dried to obtain product 6-bromo-3-
iodoindole (2), which was used for the following reaction without
purification because of its lability.
141.48, 141.04, 140.07, 134.45, 126.29, 126.12, 124.14, 124.03, 112.88,
84.58, 39.99, 34.64, 33.20, 31.94, 31.93, 29.95, 29.69, 29.63, 29.38,
29.34, 28.19, 26.62, 22.70, 14.13. MS (MALDI-TOF): calcd for
þ
C
82
H
114
N
2
O
6
S
4
[M] , 1352.05; found, 1151.52.
1
3
DTIIC8C12. (734.5 mg, 67.4%). H NMR (CDCl , 600 MHz): 8.92
(d, 1H), 8.03 (s, 1H), 7.35 (d, 2H), 7.08 (d, 1H), 7.03 (d, 1H), 6.66 (d,
1H), 2.73 (d, 2H), 1.73 (s, 9H), 1.63 (s, 1H), 1.30e1.27 (m, 32H), 0.89
13
3
-Acetoxy-6-bromoindole (3). Silver acetate (341 mg,
3
(m, 6H); C NMR (CDCl , 600 MHz): 166.24, 148.68, 144.90, 141.79,
2
.04 mmol) was added to a solution of monomer 2 in acetic acid
141.55, 139.38, 138.07, 134.65, 130.24,129.55, 126.08, 125.63, 124.09,
123.84, 121.22, 120.53, 110.63, 84.82, 39.99, 34.64, 33.20, 31.94,
ꢀ
(
8 mL). After stirring for 3 h at 90 C, the mixture was cooled to
room temperature and filtered. The filtrate was evaporated to
dryness under reduced pressure. The residue was chromato-
31.93, 29.97, 29.69, 29.67, 29.63, 29.38, 29.35, 28.26, 26.61, 22.71,
þ
14.13. MS (MALDI-TOF): calcd for C82
found, 1151.53.
H
114
N
2
O
S
6 4
[M] , 1352.05;
graphed on silica gel with dichloromethane as eluent to afford
1
product 3 (yield: 254 mg, 63.0%). H NMR (CDCl
3
, 600 MHz): 7.97 (s,
1
H), 7.69 (d, 1H), 7.33 (d, 1H), 7.29 (m, 1H), 7.17 (m, 1H), 2.36 (s, 9H);
5. Measurement and characterization
13
C NMR (CDCl
20.14, 114.64, 113.17, 112.90, 20.93.
3
, 600 MHz): 168.65, 131.70, 129.82, 125.78, 121.61,
1
1H and 13C NMR were characterized with Bruker-300 spec-
trometer operating at 600 and 75 MHz in deuterated chloroform
solution at 298 K. Chemical shifts were recorded as
with the internal standard of tetramethylsilane (TMS). Mass spectra
(MALDI-TOF) were carried out on an instrument of MS Autoflex III
Smartbean. Differential scanning calorimetry (DSC) was performed
on a Netzsch DSC 204 under nitrogen flow at heating and cooling
0
6
,6 -Dibromoindigo (4). To a solution of 3 (53.7 mg, 0.211 mmol)
in ethanol (5 mL) was added aqueous 1 M sodium hydroxide
10 mL). After the mixture was stirred at room temperature for 2 h,
d values (ppm)
(
water was added. The resulting precipitate was collected by filtra-
tion, washed with water, and dried to give product as a purple solid.
The product was used directly in the next step without purification
due to its poor solubility in common solvents.
ꢀ
ꢂ1
rates of 10 C min . Thermogravimetric analyses (TGA) were per-
formed on a Netzsch TG 209 under nitrogen at a heating rate of
General procedure for preparing monomer 8 and monomer
. In a three-necked, oven-dried 100 mL round bottom flask, (1.26 g,
mmol) of monomer 1 or monomer 4 was dissolved in 30 mL
ꢀ
ꢂ1
9
3
10 C min . FT-IR spectra were observed on a NEXUS 670 instru-
ment. UVevis absorption spectra were recorded on a HP 8453
spectrophotometer. Cyclic voltammetry (CV) was performed on a
CHI600D electrochemical workstation with a working electrode of
ITO-coated glass and a Pt wire counter electrode at a scanning rate
dichloromethane and the resulting solution was purged with argon
for 20 min. Dimethylaminopyridine (DMAP) (37 mg, 0.3 mmol) was
added and the reaction mixture was stirred for 30 min under argon
at room temperature. Di-tert-butyl-dicarbonate (1.44 g, 6.6 mmol)
was then added in one portion and the mixture was stirred at room
temperature for 24 h. Then the reaction mixture was filtered to
obtain a reddish solid, which was further washed with several
portions of methanol. The crude product was purified by flash
chromatography using dichloromethane as eluent and the solvent
was removed in vacuo to give the pure product.
ꢂ1
of 50 mV s against the reference electrode of saturated calomel
electrode (SCE) with a nitrogen saturated anhydrous solution of
tetra-n-butylammonium hexafluorophosphate in acetonitrile
ꢂ1
(0.1 mol L ). Atomic force microscopy (AFM) measurements were
carried out using a Digital Instrumental DI Multimode Nanoscope
III in a taping mode. X-ray diffraction (XRD) measurements were
carried out using German Bruker 2 instrument.
0
0
0
(
E)-Di-tert-butyl-6,6 -dibromo-3,3 -dioxo-[2,2 -biindolinyli-
0
dene]-1,1 -dicarboxylate (8). (yield: 1.44 g, 77.3%). 1H NMR (CDCl3,
5.1. Self-assembly for nanowires
6
00 MHz): 8.25 (s, 1H), 7.62 (d, 1H), 7.36 (d, 1H), 1.61 (s, 9H); 13C
NMR (CDCl3, 600 MHz): 182.39, 149.49, 149.41, 131.08, 127.77,
25.01, 121.67, 120.19, 85.25, 28.06.
Nanowires (Nws) were attempted to be prepared via in-situ self-
1
assembly approaches. To acquire the desired nanowires,
0
0
0
ꢂ1
(
E)-Di-tert-butyl-6,6 -dibromo-2,2 -dioxo-[3,3 -biindolinyli-
10 mg mL
of molecules was dissolved in mixed solvent of
0
dene]-1,1 -dicarboxylate (9). (yield: 1.64 g, 88.1%). 1H NMR (CDCl3,
ODCB/pyridine (100/0, 50/50, 33/67, 20/80, 10/90, v/v) under

C. Liu et al. / Dyes and Pigments 125 (2016) 54e63
63
vigorous stirring at room temperature overnight. The solution was
then heated at 200 C under stirring for 15 min to complete the
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[
1
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.10.003.