Conjugated oligomers with thiophene and indole moieties: Synthesis, photoluminescence and electrochromic performances

Article abstract of DOI:10.1016/j.tetlet.2016.11.090

Two series of thiophene oligomers and terthiophene oligomers consisting of both thiophene and indole moieties have been synthesized. They have same excitation-dependent photoluminescence characteristics, but different bandgaps and absorption behaviors, which relates to the number and denseness of indoles in the conjugated oligomers and the length of alkyl chains on indole moiety due to varied the π-π stacking interaction of conjugated structures in the as-prepared oligomers. A simple electrochromic device based on such a conjugated oligomer displays a novel four-color electrochromism from red to yellow, green and puce with the increased potential and possesses good environmental and redox stability. Such conjugated oligomer also exhibits high sensitivity and selectivity for Zn²⁺detection.

Full text of DOI:10.1016/j.tetlet.2016.11.090

Accepted Manuscript
Conjugated oligomers with thiophene and indole moieties: Synthesis, photolu-
minescence and electrochromic performances
Ben Dong, Baoyan Li, Yi Cao, Xinlei Meng, Han Yan, Shusheng Ge, Yun Lu
PII:
S0040-4039(16)31571-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.090
TETL 48376
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 October 2016
15 November 2016
21 November 2016
Please cite this article as: Dong, B., Li, B., Cao, Y., Meng, X., Yan, H., Ge, S., Lu, Y., Conjugated oligomers with
thiophene and indole moieties: Synthesis, photoluminescence and electrochromic performances, Tetrahedron
Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.11.090
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Conjugated oligomers with thiophene and indole moieties:
Synthesis, photoluminescence and electrochromic performances
Ben Dong, Baoyan Li, Yi Cao, Xinlei Meng, Han Yan, Shusheng Ge, and Yun Lu*
Department of Polymer Science and Engineering, State Key Laboratory of
Coordination Chemistry, Collaborative Innovation Center of Chemistry for Life
Sciences, Key Laboratory of High Performance Polymer Materials and Technology
(Nanjing University), Ministry of Education, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, P. R. China, Email:
yunlu@nju.edu.cn, Phone: 86-25-89686423.
KEYWORDS: Conjugated oligomers, electrochromic behavior, polythiophene
derivatives, chem-sensor
Abstract
Two series of thiophene oligomers and terthiophene oligomers consisting of both
thiophene and indole moieties have been synthesized. They have same
excitation-dependent photoluminescence characteristics, but different bandgaps and
absorption behaviors, which relates to the number and denseness of indoles in the
conjugated oligomers and the length of alkyl chains on indole moiety due to varied
the π-π stacking interaction of conjugated structures in the as-prepared oligomers. A
simple electrochromic device based on such a conjugated oligomer displays a novel
four-color electrochromism from red to yellow, green and puce with the increased
potential and possesses good environmental and redox stability. Such conjugated
oligomer also exhibits high sensitivity and selectivity for Zn2+ detection.
Introduction
Conjugated polymers have attracted lots of scientific and technological research
interest during the past few decades because of their unique features such as fast
switching time,1 high optical contrasts,2 processability,3-6 and easy tuning of color
with alternations in the structure.7 Among them, polythiophene and its derivatives
1

containing functional group in the heterocyclic ring are particularly attractive for
many special applications, including serving as active layers in electrochromic
devices, hole transport layers in field effect transistors, light-emitting diodes, chemo-
and bio-sensors.8-15 Up to now, numerous efforts have been devoted to the design and
synthesis of polythiophene derivatives with novel molecular structures.16, 17 For
instance,
a
polymer
electrochromic
device
based
on
poly(5,10-dihydrobenzo[f]thieno[3,4-b][1,4]dioxocine)
and
poly(4-(2,5-di(thiophen-2-yl)-1H pyrrole-1-yl) butane-1-amine) was constructed in a
sandwich configuration, presenting a blue or yellow color under the oxidation or
reduction state.18 The poly(octanoic acid 2-thiophen-3-yl-ethyl ester) was placed in
electrochromic device as an anodically coloring material, showing a nice color change
between yellow and light blue in a rather short time interval.19 In addition, the green
to
brown
color
changed
electrochromic
device
based
on
poly(2,3-bis(3-methoxyphenyl)-5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-qui
noxaline), poly(2,3-bis(3,4-bis(decyloxy)phenyl)-5,8-bis(2,3-dihydrothieno[3,4-b][1,
4]dioxin-5-yl)quinoxaline), and poly(2,3-bis(3,4-bis((2-octyldodecyl)oxy)phenyl)-5,
8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-yl)quinoxaline) was fabricated.20 These
previous studies clearly demonstrate that adjusting the structure of conjugated
polymer can not only effectively enhance the ease of processing, but also successfully
modify the electronic properties of the conjugated polymers, which encourage us to
explore more meaningful structures with novel properties for meeting the demands of
materials in multi-purpose applications.
On the other hand, polyindole and its derivatives are very important for material
structure design and engineering application because of their fairly good thermal
stability, high redox activity, slow degradation rate,21, 22 distinctive electrochromic
capabilities etc.21, 23 Recently, reports of polyindole and its derivatives as potential
materials for electrochromic devices have been published. For example, the
electrochromic devices incorporating the electropolymerized poly(indole-6-carboxylic
acid) film has been fabricated, showing a color switch between green and blue violet
under electric field.24 Poly(5-formylindole) was synthesized electrochemically by
2

direct anodic oxidation of 5-formylindole, and the prepared electrochromic device
achieved the color changes from yellow to green along with the increasing
potentials.25 Therefore, it is worth exploiting the different monomer structure obtained
via connecting the moieties of thiophene and indole and the possible properties of the
resulting polymers. However, to date, few studies have concerned the design of
conjugated polymers with thiophene and indole moieties while the multicolor emitting
and electrochromic behavior of the conjugated polymers or oligomers has been rarely
mentioned.26
In this work, we design and synthesize a series of monomers with thiophene and
indole moieties through direct C-C bonding (Scheme 1). Starting from these
monomers, the corresponding oligomers are prepared via a typical oxidative
polymerization in the presence of FeCl3. The electronic properties of these conjugated
oligomers including the oxidation potential and stability of the materials as well as the
effect of the varied structures of monomers with thiophene and indole moieties on the
optical and electrochemical property, photoluminescence(PL) intensity and
HOMO-LUMO gaps (ΔECV) of these resultant conjugated oligomers are investigated,
and the electrochromic property of the resultant oligomer used as an active material in
solid-state electrochromic device is discussed. Also, the as-prepared oligomer is
applied simply as a fluorescent sensor reagent for selectively detecting Zn2+.
3

Scheme 1. The chemical structures and synthetic approaches for M1-M6 and P1-P6.
Results and Discussion
Thiophenes and terthiophene have been under intensive investigation for their easy
polarizability in the thiophene ring and unique optical properties.27 More importantly,
they could be modified with a variety of functional groups to fine-tune their electronic
properties and utilized as electron donors for many organic conjugate systems.28, 29 In
our case, the thiophene and terthiophene are selected as core backbone in which
indole molecule with both benzene ring and pyrrole ring is introduced to improve the
color emitting and electrochromic behaviors of the conjugated oligomers. Also, due to
the solubility concerns, the H of NH in the indole has to be substituted with ethyl for
M2 and M4, and hexyl for M3 and M6 respectively. It is well known that
polythiophene derivatives are relatively stable, processable and have excellent optical
4

and electrical properties comparing to their corresponding thiophene monomers.30
Hence, starting from the above monomers, the corresponding oligomers are obtained.
As we expected, the PL quantum yield of the oligomers of P1- P6 are much higher
than those of the monomers of M1-M6, respectively (Table S1), thus only the
properties of the oligomers are discussed in the work. In the polymer chains, every
thiophene is attached with one indole group for P1-P3, and every three thiophene is
attached with one indole group for P4-P6. Note that since the structure of the
oligomers may be affected by the number and the denseness of the indole moieties in
the oligomer chains as well as the substituent on the N of indole during the
polymerization process, the oligomers probably contain varied structures with
different degrees of branching and ratios of heterocyclic connections. In such case, the
possible effects of the varied structures on the properties of the oligomers are the
issues worth studying.
The strategy used to prepare the monomers with thiophene and indole moieties is
introducing the halogen or boric acid group at position 3 or 4 of the thiophene ring
first, followed by forming C-C bond between thiophene and indole through Suzuki
coupling reaction. After the oxidative polymerization in the presence of FeCl3,
corresponding oligomers are obtained in the yield of 74% ~ 90% (Table 1). P1-P6
have different solubility in common organic solvents such as THF, CHCl3, CH2Cl2,
DMF and DMSO, depending on the length difference of the alkyl chains on indole in
oligomer chains. Not surprisingly, the oligomers P3 and P6 with hexyl chain possess
the best solubility. Figure 1 shows the FTIR spectra of the as-synthesized oligomers.
For P2, P3, P5 and P6, the peaks at about 2925 or 2930 and 2856 cm-1 are due to the
C-H stretching of the alkyl chain. For all samples, the peaks at about 3065 to 3090
cm-1, 1609 to 1618 cm-1 and 1458 to 1463 cm-1 are ascribed to the fundamental
vibrations of C=C-H and C=C on the skeletal structure of benzene ring.12 The peaks at
about 1362 to 1396 cm-1 are attributed to the characteristic vibrations of thiophene.26
The peaks at about 1173 to 1243 cm-1 belong to the C-N vibrations of indole.31 These
spectroscopic data suggest that the samples containing the moieties of benzene ring,
1
thiophene and indole have been achieved. With respect to H-NMR (Fig. S1), the
5

peaks at around 6.5 ppm attributed to protons of RN-HC=CH of the indole group in
the spectra of the M1-M6 and the peaks in the range of 7-8 ppm corresponding to the
protons on the thiophene ring of M1-M6 almost disappear in the spectra of P1-P6,
implying that for the M1-M6, three kinds of the possible oxidative polymerization
reactions (thiophene-thiophene, thiophene-indole and indole-indole) occur at the 2,5
position of the thiophene ring and 2,3 position of the indole group. The above
mentioned FTIR and NMR results indicate that the polymerization of M1-M6 has
been accomplished as expected. The proposed structure of the oligomers are
illustrated in Scheme 1．
Fig. 1 FTIR spectra of P1-P6.
Table 1 summarizes the polymerization results and thermal properties of P1-P6, in
which the average molecular weight and polydispersity index (PDI) of the P1-P6
measured by GPC are also showed, suggesting the successful achievement of the
oligomers. It is knowable from molecular design calculation that the oligomers of
P1-P3 have the same thiophene units and indole units as about 29, 25 and 22
respectively, while P4-P6 have thiophene units of 27, 39 and 30 and the
corresponding 1/3 indole units of 9, 13 and 10, respectively. Thermogravimetric
analysis (TGA) results (Table 1 and Fig. 2) show that the P3 and P6 possess better
thermal stability than the others under 400 oC, which may relate to their longer alkyl
side chains attached to the N of indole moiety. The mutual interactions of the side
chains make the P3 and P6 decompose at higher temperature, and also avoid the
o
significant weight loss in the temperature range of 261–264 C as P1, P2, P4 and P5
6

did.32 Interestingly, such thermal behavior is quite opposite from the general point that
the longer alkyl chains cause the decreased thermal decomposition temperature due to
the plasticizing effect. This implies that, in our case, the longer alkyl chain on indole
unit in P3 and P6 strengthens the π−π stacking interactions between molecules
because they can induce large motional freedom of the main backbone. [41] Besides,
in the case of P1, P2, P4 and P5 without or only with short alkyl chain on indole unit,
the thiophene oligomer series possess preferable thermal properties than the
terthiophene oligomer counterparts, suggesting that the increase of indole moieties
plays an important role in achieving the good thermal stability.
Table 1 Molecular weight and thermal properties of oligomers.
a
b
Sample Yield(%)
Mw
Mn
d
T c (oC)
P1
P2
P3
P4
P5
P6
90
85
74
79
82
80
8481
7645
7163
4464
6501
6453
5688
5540
6169
3533
4898
4697
1.49
1.36
1.16
1.26
1.32
1.37
244
263
334
254
265
383
a Weight-average molecular weight. b Number-average molecular weight. c The
temperature of 5% weight-loss was chosen as the onset point of decomposition of the
oligomers using TGA under N2.
7

Fig. 2 TGA thermograms of P1- P6 in a nitrogen atmosphere (heating rate: 10
◦C/min).
The UV-vis normalized spectra of P1-P6 (Fig. 3) show two absorption peaks at 230
and 260 nm, corresponding to typical π-π* transitions of the benzenoid rings and π-π*
transition of pyrrole ring or thiophene ring respectively.33 From Fig. 3, we notice three
experiment phenomena for the peak near 260 nm. Firstly, regardless of thiophene
oligomers or terthiophene oligomers, the peak intensity all are enhanced along with
the extended alkyl chains on indole unit, further verifying that the longer alkyl chains
enhance the π-π stacking interaction between molecules and thus cause stronger
absorption.34 Secondly, the peak intensity of thiophene oligomers are higher than
those of terthiophene oligomers, demonstrating that the higher denseness of the indole
moieties in the conjugated oligomers results in higher degree of intramolecular charge
transfer and thus enhances the absorption intensity obviously.35 Thirdly, the peak near
260 nm, for P1 and P4, suffers slight red-shift compared with P2, P3 and P5, P6,
suggesting that there probably is special synergy interaction provided by thiophene
and indole moieties without alkyl attached in this system leading to more effective
electron transfer in oligomers.36 The above results indicate that the varied structures
resulted from the different number and denseness of indoles in the conjugated
oligomers and the changed length of alkyl chains on indole moiety affect the π-π
stacking interaction of conjugated structures in the as-prepared oligomers and cause
the difference of absorption behaviors.
8

Fig. 3 The UV-vis normalization absorption of P1-P6 at 230 nm in THF (1.0×10-5 mol
L-1).
Excellent optical properties are the most important features of conjugated polymers
which stimulates research and development of organic conjugated polymers in various
applications. The as-prepared P1-P6 exhibit an excitation wavelengths-dependent
emission, that is, the increase of their excitation wavelength result in the red shift of
their emission peak, and the intensities of the emission peak increase firstly and then
decrease with the increase of excitation wavelengths (Fig. 4). These spectral
appearances may be ascribed to the existence of different chromophores such as C–N
bonds afforded by indole moiety and C–S bond resulted from the thiophene moiety in
the oligomers. Such chromophores formed by interrupting the π-electron systems of
the conjugated oligomers at irregular intervals topological or chemical defects may
alter the electronic structures and/or generate additional low-energy band-gaps.32, 37, 38
Moreover, two series of conjugated oligomers exhibit a spectral split with increasing
excitation wavelength. In the case of thiophene oligomers, the spectra of P1 and P2
present an obvious emission peak at about 480 and 520 nm respectively with one or
two inconspicuous left-shoulder peaks under every excitation wavelength, which may
result from the interchain interaction of the thiophene and indole moiety. As to P3,
when the excitation wavelength extends to 350 nm the original single peak red shifts
9

and simultaneously a new emission peak appears. The original peak red shifts with an
enhanced peak intensity with the increase of excitation wavelength and finally locates
at 563 nm. This red shift of emission peaks for P3 is probably ascribed to the long
alkyl chain provide higher degree intramolecular charge transfer.35 While all the
spectra of terhiophene oligomers (P4-P6) present two peaks at the range of 400-450
nm and 470-550 nm respectively under every excitation wavelength, and the intensity
of the former is much higher than that of the latter. These phenomena may be
attributed to the different level of interchain interactions and varied energy of different
37
chromophores for these conjugated oligomers. To obtain a further insight into their
photophysical properties, we explored the PL emission and excitation behaviors of
P1-P6 in THF solution. Under optimal excitation wavelength (Fig. 5A), the emission
peak intensities of the thiophene oligomers are higher than those of terthiophene
oligomers (Fig. 5B), suggesting again that the enhanced indole denseness afford
stronger π-π stacking interactions between the thiophene and indole moieties in this
system.34 In addition, the maximum emission wavelengths of thiophene oligomers of
P1, P2 and P3 are at about 485, 516 and 533 nm respectively, while those of the
terthiophene oligomers of P4, P5 and P6 shift to 416, 419 and 420 nm respectively,
demonstrating that the emission wavelength of these oligomers could be tuned
through different electrons distribution which result from the different denseness of
indole moiety and alkyl length on indole in the oligomer.39 These large shifts are
probably due to the changing of their HOMO and LUMO energy levels, which are
affected by the indole units and the length of alkyl chain.40
10

Fig. 4 (A)-(F) Excitation-dependent PL spectra for P1-P6 in THF (1.0×10-5 mol L-1).
Fig. 5 The PL excitation (A) and emission (B) spectra of P1-P6 in THF (1.0×10-5 mol
L-1).
Cyclic voltametry (CV) is employed to understanding the redox behavior of the
oligomers of P1-P6 and estimating their HOMO and LUMO energy levels (Fig. S3).
From the electrochemical data summarized in Table 2, it can be seen that the
reduction potentials increase slightly from -0.51 eV to -0.41 eV for thiophene
oligomers and from -0.74 eV to -0.48 eV for terthiophene oligomers, but show a clear
trend in that they gradually become more positive. This is attributed to the alkyl chain
substituted on indole moiety getting longer and more electrons dispersing throughout
the backbone of the oligomers. The HOMO energy levers are determined to be -5.78
eV, -5.76 eV and -5.72 eV for P1, P2 and P3 respectively, and -5.98 eV, -5.87 eV and
-5.85 eV for P4, P5 and P6 respectively, corresponding to the varied structures of
11

these conjugated oligomers. Furthermore, the determined electrochemical
HOMO–LUMO gaps (ΔECV) are in the range from 1.88 eV to 1.73 eV for thiophene
oligomers and 2.32 eV to 1.93 eV for terthiophene oligomers, indicating that the band
gap could be tuned effectively by the incorporation of the different length of alkyl
chain and the denseness of indole groups. According to these experimental results, we
supposed that the enhanced intramolecular or/and intermolecular π-π stacking
interactions of thiophene oligomers come from the increased indole denseness, and
that the C-N bond resulting from the indole moiety and the C-S bond resulting from
the thiophene moiety could effectively participate in the extension of conjugation
structure. A comparison of the thiophene oligomers and terthiophene oligomers
reveals that increase of the indole denseness can lead to more pronounced π-π
stacking interactions between neighboring thiophene and indole in the oligomer
backbones, and accordingly to lower the band gap.41
Table 2 Electrochemical data of P1-P6. a
Entry
P1
Eoxb (V) Ered c (V) HOMO (eV) LUMO (eV) Eg d (eV)
1.38
1.36
1.32
1.58
1.47
1.45
-0.51
-0.45
-0.41
-0.74
-0.64
-0.48
-5.78
-5.76
-5.72
-5.98
-5.87
-5.85
-3.89
-3.95
-3.99
-3.66
-3.76
-3.92
1.88
1.81
1.73
2.32
2.11
1.93
P2
P3
P4
P5
P6
a
The ferrocene couple (Fc+/Fc) with a potential of 0.44 V was used as the internal reference and
b
under our experimental conditions, E(Fc+/Fc) vs. Ag/AgCl. Onset oxidation potential vs.
c
d
Ag/AgCl. Onset reduction potential vs. Ag/AgCl. Band gaps derived from the difference
between onset potentials of oxidation and reduction.
According to our experiment results, the performance of thiophene oligomer series
are superior to those of terthiophene oligomer series, which may be ascribed to that
the indole groups with larger number densify the π-π stacking in the oligomer and
enhance the backbone interchain interactions.42 The high solubility, good thermal
12

property, smaller bandgap, higher PL quantum yield of P3 exactly meet the
requirements as electrochromic materials in potential applications. Most of the
polythiophene derivatives generally present a red or blue color in the reduction or
oxidation state.19, 43, 44 In the case of polyindole, its electrochromism only shows the
color changes from yellow to green upon changes from reduction to oxidation state.25
By contrast, the electrochromic device based on P3 displays a novel electrochromic
behavior, that is, shows red, yellow, and green color at 0 V, 1.2 V, and 2.8 V,
respectively, and finally exhibits puce color at 3.8 V, as shown in Fig. 6. Further
oxidation caused a change in the color from puce to black, resulting in the loss of
electroactivity. Such a device possesses both electrochromic behaviors of
polythiophene and polyindole, and simultaneously shows a fast color switch in about
3 s, verifying the high coloring efficiency of the individual layer device. Also, the
reversibility test result obtained under the multiple cyclic voltammograms scanning
from 0 to 3.0 V with a scan rate of 100 mV s-1 indicates that this electrochromic
device still maintains 96% of its electroactivity after of 100 cycles (Fig. 7), thus
possesses good environmental and redox stability.
Fig. 6 The electrochromism of P3
13

Fig 7. Cyclic voltammograms of the device obtained at 1 and 100 cycles with a scan
rate of 100 mV s-1.
As a fluorescent material, P3 was also used for detection of metal ions. As the
different metal ions were added to the THF solution of P3 respectively, a remarked
fluorescence enhancing effect was observed for Zn2+, while a limited fluorescence
quenching occurred for others such as Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Cu2+, Fe2+,
Mn2+, Fe3+, Al3+ and Pb2+ under the same condition (Fig. S4A). It can be deduced that
the Zn2+ ion with a closed-shell d10 configuration could coordinate with N and S of the
P3 leading to the planarization of conjugation structure in the polymer chain as well
as perturbation of the electronic system,45, 46 thus leading to the selective enhancement
of emission.47 A fluorescence emission titration experiment was performed with the
gradual addition of Zn2+, and most enhanced intensity was observed upon the addition
of 100 ppm Zn2+ (Fig. S4B), meaning that, as an efficient chem-sensor for selective
detection of Zn2+, P3 possesses a relatively large measurement range and high
detection accuracy.
Conclusion
A series of monomers with thiophene and indole moieties and their corresponding
oligomers of P1-P6 have been synthesized and characterized by NMR, GPC, FTIR
and TGA measurements. The information of structure and formation for these
oligomers and the contribution of the increase of indole moieties to their thermal
stability are given. The UV and CV results verify that the number and denseness of
indoles in the conjugated oligomers and the length of alkyl chains on indole moiety
can affect the π-π stacking interaction of conjugated structures in the as-prepared
14

oligomers and lead to the difference in bandgaps and absorption behaviors. This
provides us with a valuable approach to predictably tune the Frontier molecular
orbital energy levels and guide the synthesis of fluorescent emissive materials. The
electrochromic device on the basis of the oligomer of P3 exhibits a four-color
electrochromic property and good environmental and redox stability, suggesting its
potentiality for application as a novel electrochromic material. Moreover, the
oligomer of P3 also can be appropriately applied as selective fluorescence sensor for
the detection of Zn2+ because of its high sensitivity and selectivity.
Materials and methods
Materials
All reagents and chemicals are analytical grade and used as received without further
purification unless otherwise noted. 5-Bromoindole, 2,3,5-tribromothiophene,
1-bromohexane, bromoethane, 2-bromothiophene, trimethylamine were purchased
from Aladdin Chemical Co., China., tetrakis(triphenylphosphine)palladium(0) and
n-butyllithium (2.5 mol/L in hexane) were procured from Energy Chemical Industry
Co., China. and lithium perchlorate (LiClO4) was obtained from Nanjing Chemical
Reagent Co., China.
Characterizations
Fourier transform infrared spectra (FTIR) of solid-state samples (KBr matrix) were
measured with a Bruker VECTOR22 spectrometer at 4 cm-1 resolutions. 1H-NMR and
13C-NMR spectra were collected on a Bruker DRX-400 spectrometer. The UV-vis
spectra were recorded on a Perkin Elmer Lambda 35 instrument with 1 nm resolution.
The emission spectra were measured using a FluoroMax Spectrofluorometer
(HORIBA JobinYvon, France) with 1 nm resolution. A solid-state electrochromic
device (Scheme 2) was fabricated by dropping the oligomer sample solution (4 mg
mL-1 in THF) and poly(methyl methacrylate)(PMMA) based gel polymer electrolyte,
respectively on two ITO glass plates to form their film and then piecing on the films
facing each other, in which the as-prepared oligomer sample acts as electrochromic
material and the PMMA-based gel polymer electrolyte made by mixing evenly right
15

amount of LiClO4, PMMA and propylene carbonate serves as ion conductor layer,
respectively.12, 15 The reversibility test for the electrochromism of the sample was
made by using the multiple cyclic voltammograms scanning from 0 to 3.0 V with a
scan rate of 100 mV s-1. Molecular weight of these oligomers were determined by gel
permeation chromatography (GPC) with Waters-244 HPLC pump and THF was used
as solvent and relative to polystyrene standards. The Agilent 6540 high resolution
mass spectrometer (HRMS) was employed for mass characterization. Cyclic
voltammetry (CV) was carried out by using a standard three-electrode system, which
consists of a platinum sheet drop-casted with oligomer sample as the working
electrode, a platinum wire as the counter electrode, Ag/AgCl as the reference
electrode,
and
acetonitrile
containing
0.1
M
tetrabutylammonium
hexafluorophosphate (TBAPF6, Aldrich) as the supporting electrolyte. CV curves
were recorded and calibrated using ferrocene as the standards, whose highest
occupied molecular orbital (HOMO) was set at 4.8 eV with respect to zero vacuum
level. The redox potential of Fc/Fc+ was measured under the same condition as
oligomer samples and was located at 0.36 V related to the Ag/AgCl electrode. The
HOMO and lowest unoccupied molecular orbital (LUMO) energy levels in eV as well
as the electrochemical energy gap (Eg in eV) of the samples were calculated
according to the following equations:33
EHOMO = -e(
ELUMO = -e(
-
-
+ 4.8) (eV) (1)
+ 4.8) (eV) (2)
where Eox and Ered were the onset of oxidation and reduction potential respectively.
Scheme 2 Sketch map of solid-state electrochromic device.
Syntheses of monomers with thiophene and indole moieties
The overall approach to the synthesis of the monomers with thiophene and indole
moieties by a Pd-catalyzed Suzuki coupling reaction is depicted in Scheme 1, and the
16

48, 49
obtained monomers are named as M1, M2, M3, M4, M5 and M6, respectively.
The related compounds such as a2, a3, b1, b2, b3 and c1 were prepared according to
40, 50, 51
the reported literatures,
supporting information.
and the details for their synthesis were provided in
M1: a mixture of 5-bromine indole (a1) (1.29 g, 10 mmol), 3-thienylboronic acid (1.0
g, 5.0 mmol), Pd(PPh3)4 (0.29 g, 0.25 mmol) and K2CO3 (2.76 g, 20 mmol) in the
mixed solution of THF (50 mL) and H2O (10 mL) was reflux for 12 h under nitrogen
atmosphere. After cooling, 50 mL water was added and the reaction mixture was
extracted with CH2Cl2. The combined organic layer was washed with H2O and brine,
dried over anhydrous Na2SO4, and evaporated under reduced pressure successively.
After completely removing the solvent, the crude product was purified through a
silica gel chromatography column using EtOAc/petroleum ether as the eluent to
afford 0.89 g as reddish brown solid in 89% yield. FT-IR (KBr, cm-1): 3399, 3095,
1
1636, 1463, 1310, 1195, 1089, 770. H-NMR (CDCl3) δ (ppm): 8.14 (s, br, 1H), 7.87
13
(t, 1H, J = 0.8 Hz), 7.46-7.36 (m, 4H), 7.25-7.19 (m, 2H), 6.58 (m, 1H). C-NMR
(100 MHz, CDCl3) δ(ppm): 143.8, 135.3, 128.5, 127.0, 126.0, 125.1, 121.6, 119.0,
111.5, 103.1. HRMS (ESI) Calcd for C12H10NS (M+H)+: 200.0528; Found: 200.0528.
The same procedure was employed to prepare M2 to M6 except that the starting
materials were different.
M2: the starting materials were 5-bromo-1-ethyl-1H-indole (a2) (1.12 g, 5 mmol) and
3-thienylboronic acid (1.0 g, 7.5 mmol), and light yellow solid was obtained (1.12 g,
95% yield). FT-IR (KBr, cm-1): 3096, 2930, 2856, 1616, 1453, 1333, 1220, 1087, 765.
1H-NMR (CDCl3) δ (ppm): 7.67 (d, 2H, J =1.2 Hz), 7.50-7.42 (m, 2H), 7.41-7.33 (m,
3H), 7.13 (d, 1H, J =3.2 Hz), 6.51 (d,1H, J =3.0 Hz), 4.18 (m, 2H, J =7.3 Hz), 1.48 (t,
3H, J =7.3 Hz). 13C-NMR (100 MHz, CDCl3) δ(ppm): 143.9, 135.3, 129.2, 127.9,
127.8, 127.1, 125.9, 120.9, 119.0, 118.9, 109.7, 101.6, 41.3, 15.7. HRMS (ESI) Calcd
for C14H14NS (M+H)+: 228.0841; Found: 228.0844.
M3: the starting materials were 5-bromo-1-hexyl-1H-indole (a3) (1.46 g, 5.0 mmol)
and 3-thienylboronic acid (0.96 g, 7.5 mmol), and orange red oil was obtained (1.53 g,
95% yield). FT-IR (KBr, cm-1): 3098, 2931, 2846, 1615, 1480, 1380, 1205, 1088, 775.
17

1H-NMR (CDCl3) δ 7.85 (d, 1H, J = 9.0 Hz), 7.50-7.41 (m, 2H,), 7.41-7.30 (m, 3H,),
7.09 (t, 1H, J =2.7 Hz), 6.53-6.48 (m, 1H,), 4.08 (t, 2H, J =7.1 Hz), 1.88-1.76 (m, 2H),
13
1.30 (s, 6H), 0.94-0.84 (m, 3H). C-NMR (100 MHz, CDCl3) δ(ppm): 143.9, 136.6,
129.1, 128.6, 127.7, 127.0, 125.9, 120.8, 118.9, 118.8, 109.8, 101.4, 46.7, 31.6, 30.4,
26.8, 22.7, 14.2. HRMS (ESI) Calcd for C18H22NS (M+H)+: 284.1467; Found:
284.1468.
M4: the starting materials were 1H-indol-5-ylboronic acid (b1) (0.98 g, 6.0 mmol)
and 3′-bromo-2,2′:5′,2″-terthiophene (c1) (0.78 g, 2.4mmol), and yellow solid was
obtained (1.02 g, 92% yield). FT-IR (KBr, cm-1): 3413, 3090, 1636, 1463, 1310, 1195,
1
1089, 770. H-NMR (CDCl3) δ (ppm): 8.19 (s, br, 1H), 7.70 (t, 1H, J =0.7 Hz), 7.39
(d, 1H, J = 8.4 Hz), 7.26-7.18 (m, 5H), 7.11 (dd, 1H, J =1.2 Hz, J =5.1 Hz), 7.03(m,
2H), 6.90 (dd, 2H, J =3.6 Hz, J =5.1 Hz), 6.57 (m, 1H). 13C-NMR (100 MHz, CDCl3)
δ(ppm): 140.7,137.2, 136.3, 135.3, 134.7, 129.9, 128.0, 127.9, 127.85, 127.7, 127.0,
126.0, 125.4, 124.7, 124.5, 123.73, 123.71,121.6, 111.0, 103.0. HRMS (ESI) Calcd
for C20H14NS3 (M+H)+: 364.0283; Found: 364.0284.
M5: the starting materials were 1-ethyl-1H-indol-5-ylboronic acid (b2) (1.02 g, 5.4
mmol) and c1 (0.66 g, 2 mmol), and yellow oil was obtained (0.85 g, 40% yield).
1
FT-IR (KBr, cm-1): 3095, 2931, 2850, 1615, 1483, 1351, 1212, 1077, 692. H-NMR
(CDCl3) δ (ppm): 7.88 (d, 1H, J=1.3 Hz), 7.66 (d, 1H, J = 1.1 Hz), 7.39 (s, 1H), 7.28
(s, 1H), 7.18 (m, 2H), 7.10 (d, 2H, J = 3.1 Hz), 7.07 (dd, 1H, J=1.1 Hz, J = 5.1Hz),
7.00 (d, 1H, J = 3.7 Hz), 6.86 (dd, 1H, J =3.6 Hz, J =5.1 Hz), 6.47 (d, 1H, J = 2.7 Hz),
4.14 (q, 2H, J =13.0 Hz), 1.44 (t, 3H, J =7.2 Hz). 13C-NMR (100 MHz, CDCl3)
δ(ppm): 141.0, 137.3, 135.4, 134.8, 129.3, 128.9, 128.1, 127.7, 127.1, 126.2,125.5,
124.6, 123.8, 123.2, 121.9, 119.7, 109.4, 101.6, 41.2, 15.6. HRMS (ESI) Calcd for
C22H18NS3 (M+H)+: 392.0596; Found: 392.0596.
M6: the starting materials were 1-hexyl-1H-indol-5-ylboronic acid (b3) (0.74 g, 3.0
mmol) and c1 (0.78 g, 2.4 mmol), and yellow oil was obtained (1.02 g, 94% yield).
1
FT-IR (KBr, cm-1): 3106, 2927, 2858, 1618, 1486, 1369, 1231, 1044, 689. H-NMR
(CDCl3) δ 7.69 (t, 1H, J =0.7 Hz), 7.34 (d, 1H, J = 8.5 Hz), 7.25-7.20 (m, 4H),
7.15-7.10 (m, 2H), 7.06-7.02 (m, 2H), 4.13 (t, 2H, J =7.2 Hz), 1.93-1.82 (m, 2H),
18

1.40-1.31 (m, 6H,), 0.90 (t, 3H, J =6.6 Hz). 13C-NMR (100 MHz, CDCl3) δ(ppm):
141.0, 137.4, 136.6, 135.8, 134.8, 130.0, 128.8, 128.5, 128.1, 127.18, 127.15, 126.2,
125.5, 124.6, 123.9, 123.2, 121.9, 109.6, 101.4, 46.7, 31.6, 30.5, 26.9, 22.8, 14.2.
HRMS (ESI) Calcd for C26H26NS3 (M+H)+: 448.1222; Found: 448.1227.
Polymerization of M1 to M6
The monomer was dissolved in chloroform. A suspension of FeCl3 in chloroform of
equal volume was dropwise added to the monomer solution for 2 h at room
temperature under stirring and nitrogen atmosphere. The final concentration for the
monomer and FeCl3 was 0.05 and 0.2 M, respectively. When the addition was
complete, the mixture was stirred for an additional 6 h. The reaction mixture was
poured into methanol, and the formed precipitate was filtered off and washed with
methanol. The resulting product was then boiled in chloroform and concentrated
ammonia under stirring for 30 min. After that, the procedure of separating out the
water phase and adding in fresh ammonia was repeated four times, and then the
remaining organic phase was washed twice with ethylenediaminetetraacetic acid
(EDTA) aqueous solution (0.05 M). The organic phase was washed again with water,
then filtrated and finally poured into methanol to give the precipitate. The portions
with low molecular weight was removed by Soxhlet extraction with methanol and
diethyl ether respectively.52, 53 These obtained products from their corresponding
monomers are denoted as P1, P2, P3, P4, P5 and P6, respectively. The P1, P2 and P3
are named as thiophene oligomers, and the P4, P5 and P6 are named as terthiophene
oligomers.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No.
21374046), Open Project of State Key Laboratory of Supramolecular Structure and
Materials (SKLSSM201616) and the Testing Foundation of Nanjing University.
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Graphical abstract
23

Highlights:
The oligomers consisting of both thiophene and indole moieties were synthesized.
The conjugated oligomers show different bandgaps and absorption behaviors.
An electrochromic device based on oligomer (P3) displays a novel four-color
changes.
The oligomer of P3 exhibits high sensitivity and selectivity for Zn2+ detection.
24