A precursor strategy for the synthesis of low band-gap polymers: An efficient route to a series of near-infrared electrochromic polymers

Article abstract of DOI:10.1039/c1jm10629e

A precursor strategy for the synthesis and screening of a series of conjugated donor-acceptor polymers is demonstrated by successful preparation of low band-gap polymers (P2-P5) containing triphenylamine as an electron donor and several heterocycles as acceptors, such as [1,2,5]thiadiazolo[3,4-g] quinoxaline, [1,2,5]thiadiazolo[3,4-i]dibenzo[a,c]phenazine, benzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole), and selenadiazole[3,4-f] benzo[c][1,2,5]thiadiazole, that are transformed from a single reactive polymer (P1). Polymers P2-P5 have the band gap of 1.71-1.29 eV and show the absorption and emission in the near infrared (NIR) spectral region. All the polymers are also NIR electrochromic. In particular, polymer P3 is electrochemically switchable between leaf-like green coloring and near-infrared absorbing states with an efficiency of 479 cm² C^-1 at 1310 nm or 232 cm² C^-1 at 1550 nm, making it potentially useful for electrically switchable day-to-night camouflage applications.

Full text of DOI:10.1039/c1jm10629e

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PAPER
A precursor strategy for the synthesis of low band-gap polymers: an efficient
route to a series of near-infrared electrochromic polymers†
a
Gang Qian, Hana Abu and Zhi Yuan Wang
a
ab
*
Received 11th February 2011, Accepted 14th March 2011
DOI: 10.1039/c1jm10629e
A precursor strategy for the synthesis and screening of a series of conjugated donor–acceptor polymers
is demonstrated by successful preparation of low band-gap polymers (P2–P5) containing
triphenylamine as an electron donor and several heterocycles as acceptors, such as [1,2,5]thiadiazolo
[3,4-g]quinoxaline, [1,2,5]thiadiazolo[3,4-i]dibenzo[a,c]phenazine, benzo[1,2-c:4,5-c0]bis([1,2,5]
thiadiazole), and selenadiazole[3,4-f]benzo[c][1,2,5]thiadiazole, that are transformed from a single
reactive polymer (P1). Polymers P2–P5 have the band gap of 1.71–1.29 eV and show the absorption
and emission in the near infrared (NIR) spectral region. All the polymers are also NIR electrochromic.
In particular, polymer P3 is electrochemically switchable between leaf-like green coloring and near-
infrared absorbing states with an efficiency of 479 cm2 Cꢀ1 at 1310 nm or 232 cm2 Cꢀ1 at 1550 nm,
making it potentially useful for electrically switchable day-to-night camouflage applications.
electron-withdrawing benzo[1,2-c:4,5-c0]bis([1,2,5]thiadiazole)
Introduction
(BBTD) or its selenium analogues,35–38 whose quinoidal char-
acter contributes to the low band-gap nature of these chromo-
phores.39 As the band gap level is readily changed by introducing
the different donors and acceptors, it can then be expected that
the corresponding conjugated polymers containing the BBTD
and other structurally related acceptors with a gradual change in
the electron-withdrawing ability or the LUMO level are likely to
absorb at different wavelengths and show various colors.
To obtain the corresponding polymers derived from each of
the acceptors that are structurally related to BBTD, one would
need to tediously synthesize and purify each of the acceptor
monomers and carry out a large number of polymerizations. For
making high molecular weight conjugated aromatic polymers,
such as polyfluorenes, the Suzuki cross-coupling reaction is more
favorable and widely used than other cross-coupling reactions.
However, the strong acceptors such as BBTD are rather labile to
alkaline bases and some nucleophiles (e.g., fluoride and
hydroxide anions),40 making it very difficult, if not impossible, to
polymerize the BBTD-based monomers by the Suzuki cross-
coupling reaction. Although the Stille cross-coupling reaction
could be applied for polymerization, the required trialkylstannyl
monomers are toxic and often difficult to purify. Therefore, in
order to utilize the Suzuki reaction in polymer synthesis and have
a quick access to a series of low band gap polymers based on
BBTD and its analogs for systematic studies on the structure–
property relationship and screening certain properties, an alter-
native synthetic strategy is needed.
The low band-gap conjugated polymers with absorption in the
near-infrared (NIR) region are potentially useful in NIR opto-
electronic applications, such as NIR light-emitting diodes,1–4
NIR photovoltaics,2,5–17 NIR optical attenuators,18,19 biomedi-
cals,20 and camouflage in warfare.21 The band gaps of conjugated
polymers mainly correlate to alternation of the single and double
bond length, conjugation length, coplanarity, substituent effect
and intermolecular interactions.22–25 Lowering the band gap will
result in a spectral shift to the long wavelength or even into the
NIR region. One of the common strategies is to introduce the
electron donor (D) and acceptor (A) groups into the conjugated
polymer backbone, resulting in hybridization of the electronic
levels of donor and acceptor units due to a resonance structure
with the reduced alternation in the bond length between single
and double bonds.23 Therefore, the strong D–A charge transfer is
required to lower the band gap level and realize the NIR
absorption.26–34 One of the widely used acceptors is based on
benzobisthiadiazole or the related selenium-containing and qui-
noxaline analogs, due to their strong electron-withdrawing
ability and the low LUMO level.
We have previously investigated a series of the D–p–A–p–D
type of NIR chromophores containing the strongly
aDepartment of Chemistry, Carleton University, 1125 Colonel By Drive,
Ottawa, Ontario, Canada K1S 5B6
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, China. E-mail: wayne_wang@carleton.ca
On the basis of the synthetic viability for BBTD and its
analogs, we propose to use a precursor strategy for making
a series of structurally related conjugated polymers from a single
reactive polymer. In principle, a polymer that contains the
† Electronic
supplementary
information
(ESI)
available:
Thermoproperties, optical spectra and CV measurements of these
polymers. See DOI: 10.1039/c1jm10629e
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5,6-diaminobenzothiadiazole unit can be converted to many
other polymers by performing specific reactions involving this
reactive moiety with various reagents. It is known that o-di-
aminobenzene and its derivatives can be converted to a number
of heterocyclic compounds such as benzothiadiazole, benzosele-
nadiazole, phenazine and quinoxaline in high yields.38,41,42
Accordingly, a polymer containing 5,6-diaminobenzothiadiazole
can be used as a precursor to a series of polymers containing the
electron-deficient heterocyclic units (Scheme 1).
indium tin oxide (ITO)-coated glass slides as the working elec-
trode, a platinum wire as the counter electrode and a silver wire
as the reference electrode.
4,7-Bis[4-(N-(4-bromophenyl)-N-(4-methylphenyl)amino)-
phenyl]-5,6-diamino-2,1,3-benzothiadiazole (2)
To a solution of 4,7-bis[4-(N-phenyl-N-(4-methylphenyl)amino)
phenyl]-5,6-diamino-2,1,3-benzothiadiazole (1) (0.61 g, 0.9 mmol)
in freshly distilled dichloromethane (45 mL) was added tetrabu-
tylammonium tribromide (0.96 g, 2.0 mmol). The mixture was
stirred at room temperature for 3 h. The precipitates were filtered
and washed with dichloromethane. The solid was dried under
vacuum to give the product as orange solid (0.55 g, 73%). 1H NMR
(300 MHz, CDCl3) d (ppm): 7.44 (d, 4H, J ¼ 8.3 Hz), 7.35 (d, 4H,
J ¼ 8.7 Hz), 7.20 (d, 4H, J ¼ 8.5 Hz), 7.13 (d, 4H, J ¼ 8.6 Hz), 7.09
(d, 4H, J ¼ 9.1 Hz), 7.05 (d, 4H, J ¼ 8.7 Hz), 2.34 (s, 6H).
Herein we demonstrate the feasibility of a precursor approach
to a series of low band-gap conjugated polymers (P2–P5) by
facile transformation of a single polymer (P1) containing
a reactive 5,6-diaminobenzothiadiazole unit. These D–A type
polymers have different band gaps due to a structural variation
only at the acceptor part and are all electrochromic and NIR
absorbing. Interestingly, polymer P3 is leaf-like green and can be
switched electrochemically to become NIR absorbing at the
telecommunication wavelengths (e.g., at 1310 and 1550 nm).
Experimental section
Polymer P1
Materials
To a mixture of 9,9-dioctyl-2,7-bis(trimethyleneboronate)fluorene
(3) (0.2792 g, 0.5 mmol) and 4,7-bis[4-(N-(4-bromophenyl)-N-(4-
methylphenyl)amino)-phenyl]-5,6-diamino-2,1,3-benzothiadiazole
(2) (0.4193 g, 0.5 mmol) were added Aliquat 336 (0.12 g), Pd(PPh3)4
(7.9 mg, 0.007 mmol), degassed toluene (7 mL) and aqueous
potassium carbonate (2 M, 3 mL), under a dry argon atmosphere.
The mixture was heated to 95 ꢁC and stirred in the dark for 48 h.
After cooling, the mixture was poured into methanol. The precip-
itate was collected by filtration and then dissolved in dichloro-
methane. The solution was washed with water, dried over
anhydrous Mg2SO4, and then concentrated to an appropriate
volume. The fiber-like brown polymer (0.5 g, 94%) was obtained by
pouring the concentrated solution into methanol. 1H NMR
(300 MHz, CDCl3) d (ppm): 7.74 (d, 2H, J ¼ 7.9 Hz), 7.61–7.55 (m,
8H), 7.47 (d, 4H, J ¼ 8.7 Hz), 7.29–7.25 (m, 8H), 7.20–7.13 (m, 8H),
2.36 (s, 6H), 2.00 (br, 4H), 1.20–1.00 (m, 20H), 0.80–0.75 (m, 10H).
All chemicals and reagents were used as received from commercial
sources without purification. Solvents for chemical synthesis were
purified by distillation. Chemical reactions were carried out under
an argon atmosphere. 4,7-Bis[4-(N-phenyl-N-(4-methylphenyl)
amino)phenyl]-5,6-diamino-2,1,3-benzothiadiazole (1)4 and 9,9-
dioctyl-2,7-bis(trimethyleneboronate)fluorene (3)43 were prepared
according to literature methods.
Methods
1H NMR spectra were recorded using a Bruker Avance 300
NMR spectrometer. The number- and weight-average molecular
weights of the polymers were determined by gel-permeation
chromatography (GPC) with a Waters 410 instrument and
polystyrene as standard and THF as eluent. Thermogravimetric
analysis (TGA) was carried out in nitrogen on a Hi-Res TGA
2950 thermogravimetric analyzer with a heating rate of 10 ꢁC
minꢀ1. Absorption and fluorescence spectra were recorded with
a Shimadzu UV-3600 spectrophotometer and a PTI fluorescence
system or a Lambda 900 Perkin-Elmer spectrophotometer,
respectively. FTIR spectra were recorded on a Bio-Rad FTS-135
spectrophotometer. Cyclic voltammetry was performed using
Polymer P2
To a suspension of polymer P1 (107 mg, 0.1 mmol) in acetic acid
(20 mL) was added 1,4-dioxane-2,3-diol (53 mg, 0.44 mmol) in
argon. The mixture was heated to 60 ꢁC overnight. Ethanol was
then added to precipitate the polymer. After washing with
ethanol and drying at 80 ꢁC under vacuum blue polymer P2 was
a
BAS 100B/W electrochemical workstation. Spectroelec-
trochemistry was performed in dry acetonitrile containing
Et4NClO4 (0.1 M) at room temperature under argon, using an
1
obtained (109 mg, 99%). H NMR (300 MHz, CDCl3) d (ppm):
8.87 (s, 2H), 7.81 (d, 4H, J ¼ 8.7 Hz), 7.74 (d, 2H, J ¼ 7.7 Hz),
7.63–7.56 (m, 8H), 7.35–7.18 (m, 16H), 2.37 (s, 6H), 2.09 (b, 4H),
1.05 (m, 20H), 0.90–0.70 (m, 10H).
Polymer P3
Following the procedure for the preparation of polymer P2, by
changing 1,4-dioxane-2,3-diol to phenanthrene-5,6-dione, poly-
mer P3 was obtained as a dark green solid. Yield: 89%. 1H NMR
(300 MHz, CDCl3) d (ppm): 9.03 (br, 2H), 8.42 (br, 2H), 8.07 (br,
4H), 7.75–7.16 (m, 30H), 2.39–2.35 (m, 6H), 2.03 (br, 4H), 1.06
(br, 20H), 0.76 (br, 10H).
Scheme 1 Transformation of 5,6-diaminobenzothiadiazole moiety in
a precursor polymer to various heterocyclic-containing polymers.
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Polymer P435
A precursor strategy is applied to the synthesis of target
polymers in order to overcome the problem associated with the
instability of the strong acceptors under the Suzuki reaction
conditions. We reasoned that 5,6-diaminobenzothiadiazole
should be a suitable functional group to be present in a monomer
and also in a precursor polymer, since its benzothiadiazole
moiety is fairly stable under the basic conditions and its o-di-
aminobenzene moiety can be readily converted into various
electron acceptors (e.g., quinoxaline and benzothiadiazole).44
Accordingly, monomer 2 was designed and synthesized from the
known compound 1 by bromination with tetrabutylammonium
tribromide. Polymerization of monomers 2 and 3 by the Suzuki
cross-coupling reaction afforded the precursor polymer P1.
Using a specific reagent as indicated for each polymer, the
cyclization reaction of the phenylenediamine moiety in P1 pro-
ceeded smoothly to yield P2 (1,4-dioxane-2,3-diol), P3 (phen-
anthrene-5,6-dione), P4 (N-thionylaniline) and P5 (selenium
dioxide) in high yield (>87%) (Scheme 2).
To a solution of polymer P1 (106.9 mg, 0.1 mmol) in dry pyridine
(10 mL) were added N-sulfinylaniline (0.5 mL, 4 mmol) and
chlorotrimethylsilane (0.5 mL,ꢁ3.6 mmol) in argon atmosphere.
The mixture was stirred at 80 C overnight. An excess of chlor-
otrimethylsilane was then distilled off. The solution was
concentrated to 5 mL and poured to methanol. The fiber-like
dark blue polymer P4 (98 mg, 89%) was collected by filtration,
washed with ethanol and dried at 80 ꢁC under vacuum. 1H NMR
(300 MHz, CDCl3) d (ppm): 8.20 (d, 4H, J ¼ 8.5 Hz), 7.75 (d, 2H,
J ¼ 7.4 Hz), 7.63–7.56 (m, 8H), 7.34–7.16 (m, 16H), 2.37 (s, 6H),
2.06 (br, 4H), 1.06 (br, 20H), 0.80–0.75 (m, 10H).
Polymer P5
To the solution of polymer P1 (106.9 mg, 0.1 mmol) in THF
(45 mL) was added SeO2 (118 mg, 1.1 mmol) in argon. The
mixture was refluxed for 4 h. The solution was then concentrated
to 10 mL and poured to methanol. The fiber-like dark grey
polymer P5 (0.1 g, 87%) was collected by filtration, washed with
ethanol and dried at 80 ꢁC under vacuum. 1H NMR (300 MHz,
CDCl3) d (ppm): 8.13 (d, 4H, J ¼ 8.7 Hz), 7.79–7.65 (m, 2H),
7.65–7.40 (m, 8H), 7.36–7.18 (m, 16H), 2.38 (s, 6H), 2.07 (br,
4H), 1.30–1.00 (m, 20H), 0.82–0.76 (m, 10H).
Structural characterization
The chemical structures of polymers P1–P5 were characterized
by the 1H NMR and IR spectroscopic methods. A peak at
4.18 ppm in the 1H NMR spectrum (Fig. 2a) of P1 is assigned to
the amine hydrogen, which is an indicator for transformation of
the amino groups. In Fig. 2b–e, the peaks at 8.88, 9.04, 8.42, 8.08,
8.20 and 8.13 ppm are assigned to the Ha–Hf, respectively, since
these chemical shifts coincide well with those of compounds with
similar structures (d 8.85,38 9.05,38 8.44,38 8.07,38 8.184 and 8.024
ppm). Because of the presence of the strong electron-with-
drawing BBTD and SBTD, the chemical shift of the adjacent
hydrogens He and Hf in P4 and P5 shifted to a lower field. The IR
spectrum of polymer P1 exhibits the nasym(HNH) and
nsym(HNH) at 3442 and 3357 cmꢀ1, respectively. As expected,
these two bands are absent in the IR spectra of polymers P2–P5
(Fig. 3).
Results and discussion
Design and synthesis
Linking a donor and an acceptor through a p-spacer is
a common method of construction of low band gap compounds
and polymers.23,36 In particular, the D–p–A–p–D structure
promotes an effective interaction between donor and acceptor
units, which can result in a significant decrease in the energy gap.
In this work, we chose triphenylamine (TPA) as an electron
donor, along with the four different acceptors, namely [1,2,5]
thiadiazolo[3,4-g]quinoxaline (TQ), [1,2,5]thiadiazolo[3,4-i]
dibenzo[a,c]phenazine (TDP), benzo[1,2-c:4,5-c0]bis([1,2,5]thia-
diazole) (BBTD), and selenadiazole[3,4-f]benzo[c][1,2,5]thiadia-
zole (SBTD), to construct the D–p–A–p–D repeat unit in the
main chain of conjugated polymers (Fig. 1). The 9,9-dioctyl-
fluorene moiety is introduced to extend the p-conjugation and
impart the solubility to the target polymers.
The molecular weights were determined by GPC using THF as
eluent and polystyrene as standard. The precursor P1 had
a number-average molecular weight (Mn) of 56 000 g molꢀ1 and
a polydispersity index (PDI) of 3.2 (Table 1). In comparison,
polymers P2–P4 appear to have relatively lower molecular
weights (16 000, 18 000 and 28 000 g molꢀ1, respectively), which
is mainly due to the exclusion of less soluble but higher molecular
weight fractions in THF.
Copolymers P1–P5 exhibited good thermal stability (Table 1
and Fig. S1†), as assessed by thermogravimetric analysis (TGA),
and had the onset temperatures for 5% weight loss in nitrogen
over 300 ꢁC. A small weight loss occurred at ca. 100 ꢁC,
presumably due to the presence of a small amount of solvent in
polymer samples.
Optical property
The absorption and emission spectra of polymers P1–P5 in
chlorobenzene and film are shown in Fig. 4 and S2† and the data
are summarized in Table 2. Without a strong acceptor, the
precursor P1 absorbs in the short wavelength region with
a maximum (lmax) at 440 nm and emits at 559 nm in chloro-
benzene. Because of the introduction of strong electron acceptors
Fig. 1 Chemical structures of polymers P1–P5.
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Scheme 2 Synthetic routes to polymers P1–P5. Reagents and conditions: (i) Bu4NBr3, CH2Cl2, 25 ꢁC, 3 h; (ii) Pd(PPh3)4, K2CO3 (aq, 2 M), toluene,
Aliquat 336, 95 ꢁC, 48 h; (iii) 1,4-dioxane-2,3-diol, AcOH, 60 ꢁC, overnight; (iv) phenanthrene-5,6-dione, AcOH, 60 ꢁC, overnight; (v) PhNSO, TMSCl,
pyridine, 80 ꢁC, overnight; (vi) SeO2, THF, reflux, 4 h.
Fig. 3 FT-IR spectra of polymers P1–P5.
Table 1 Molecular weights and thermal properties of polymers P1–P5
PDIa
Td/ꢁCb
a
a
Polymer
Mn (ꢂ103)
Mw (ꢂ103)
P1
P2
P3
P4
P5
56
16
18
28
179
32
34
75
3.2
2.0
1.9
2.7
NA
423
438
430
419
341
NA
NA
Fig. 2 1H NMR spectra (300 MHz, CDCl3) of polymers P1–P5.
a
Determined by gel permeation chromatography using THF as eluent
and polystyrene as standard. Temperature at which 5% weight loss is
b
observed using TGA under N2 at a heating rate of 10 ꢁC minꢀ1
.
by cyclization of the 1,2-diaminophenylene moiety in P1, the
absorption and emission peaks of polymers P2–P5 are red shifted
dramatically. The electronic absorption spectra of these four
polymers display two discrete bands (Fig. 4a): the bands in the
short wavelength region plus the shoulders ranging from 300–450
nm ascribed to the p–p* and n–p* transitions of the conjugated
aromatic segments, and those in the long wavelength region are
due to the intramolecular charge transfer (ICT) transitions
between the donors and the acceptors. The maximum absorption
wavelength (lmaxabs) varies in the order of P1 < P2 < P3 < P4 <
abs
P5. The lmax of P3 is red-shifted about 90 nm relative to P2,
due to the extension of the conjugation length in the TDP core.
Replacement of sulfur atom by selenium atom led to a further
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intermolecular charge transfer bands. The longest emission
wavelength reaches 1280 nm, falling in the telecommunication
window. Unlike many NIR dyes, the films of polymers P2–P5
emit strongly in the NIR spectral region (Fig. S2†), making them
potentially useful in light emitting diodes (LEDs).
Electrochemical property
The electrochemical properties of polymers P1–P5 (except for P2
due to its low solubility) are investigated by cyclic voltammetry
(CV) and the data are summarized in Table 2. The chlorobenzene
solution of each polymer with a concentration of 10 mg mLꢀ1
was used to spin coat thin films on ITO-coated glass slides. Once
the solvent was fully evaporated, CV was performed in a 0.1 M
Et4NClO4 electrolyte in acetonitrile with Pt wire counter elec-
trode and Ag/Ag+ reference electrode. The cyclic voltammo-
grams of P1 and P3–P5 are shown in Fig. S5†. The less soluble
P2 could not be cast in a film for CV experiment. Two quasi-
reversible waves are observed for all the polymers in the oxida-
tion region, which are attributed to the oxidation of TPA
segments, and the two electrons are removed successively from
the two nitrogen atoms. The oxidation potential of ferrocene/
ferrocenium (Fc/Fc+) was measured to be 0.44 V to the Ag/Ag+
electrode under the same conditions and the redox potential of
Fc/Fc+ has an absolute energy level of ꢀ4.8 eV to vacuum.45,46
Thus the HOMO levels of polymers are calculated according to
the following equation:
Fig. 4 (a) Normalized absorption and (b) fluorescence emission spectra
of polymers P1–P5 in chlorobenzene.
EHOMO ¼ ꢀe(Eox + 4.36) (eV)
(1)
red-shift (P5 vs. P4), due to the polar effect of selenium atom.42
The colors of polymers P1–P5 vary according to their absorption
characteristics, depending on the acceptor core structure in the
repeat units, and are typically orange, blue, green, grey and dark
goldenrod (Fig. S3†). It is worth noting that P3 absorbs at 380
and 720 nm, with a valley at 503 nm and has a color of a green
leaf (Fig. S4†). Unlike the absorption spectra, the emission
spectra just show one peak in the visible or near infrared spectral
region in solution. The emission wavelengths of P1–P5 vary in
the same trend as for the absorption. It is interesting to note that
polymers P2–P5 exhibit a large Stokes shift (Table 2), which
further confirms the presence of a strong ICT in these polymers.
In comparison with the absorption spectra in solution, the
absorption maxima of the polymer films are red-shifted by 7–87
nm, indicating the presence of intermolecular interaction.
Moreover, the fluorescence spectra of the polymer films display
several peaks, which may be attributed to the vibronic bands and
where Eox is the onset oxidation potential of the sample vs. Ag/
Ag+. Due to the strong electron-donating TPA and amino
groups, the precursor polymer P1 exhibits a low onset oxidation
potential (Eonsetox) at 0.31 V and the HOMO level of ꢀ4.67 eV,
which coincide well with those of polymers containing TPA and
NMe2 groups.47 Due to the presence of the D–p–A–p–D struc-
ture in P3–P5, their onset oxidation potentials increase and
HOMO levels decrease about 0.3 eV. Accordingly, the band-gap
levels of P2–P5 are in the range of 1.3–1.7 eV.
Electrochromic property
Polymers P1–P5 are all electrochromic, changing from orange,
blue, green, grey and dark goldenrod in the neutral state to grey
or sienna in the oxidized state (Fig. 5). In the oxidized state or
radical cation form, all the polymers absorb intensely in the NIR
region, with a maximum peak around 1200 nm. For P1, its color
Table 2 Optical and electrochemical data of P1–P5
lmaxabs/nma
(solution)
lmaxPL/nma
(solution)
Stokes shiftsa/
cmꢀ1
lmaxabs/nm
(film)
lmaxPL/nm
(film)
Eg optical/
eVb
Eonset
V
HOMO/
eV
LUMO/
eVc
ox
/
Polymer
P1
P2
P3
P4
P5
371, 440 (sh)d
559
834
945
1017
1072
9065
4457
3765
4173
2957
369, 527 (sh)d
NA
750,822
962, 1026, 1158
1028, 1156, 1265
1098, 1171, 1279
2.67
1.71
1.50
1.44
1.29
0.31
NA
0.63
0.52
0.51
ꢀ4.67
NA
ꢀ4.99
ꢀ4.88
ꢀ4.87
ꢀ2.00
NA
ꢀ3.28
ꢀ3.38
ꢀ3.58
608
697
714
814
626
727
744
821
a
b
Measured in chlorobenzene, excitation wavelengths are at their maximum absorption wavelengths. Optical band gap, estimated from the onset
wavelength of optical absorption spectra. Calculated from the HOMO level and optical band-gap. d Shoulder peak.
c
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Fig. 5 UV/Vis/NIR absorption spectra of polymer P1 and P3–P5 thin films on ITO-coated glass substrate (in CH3CN with 0.1 M Et4NClO4 as the
supporting electrolyte) in its neutral (dashed line) and oxidized states (solid line).
changes from orange to slate gray in the oxidized state. Similar to
other conjugated electrochromic polymers, various hues, tints,
shades or tones of these polymer films could be obtained by
applying different oxidation potentials.48 Moreover, if two or
three colors are mixed, the resulting color will lie on a straight
line connecting two points on the chromaticity diagram and the
mixed color position can be changed along the line by tuning the
ratio of the amounts of the two mixed colors.49
Interestingly, the colors of P1–P3 are close to secondary and
primary subtractive colors (orange, blue and green) and
thus many different colors could be obtained from the mixture of
P1–P3.
The green polymer P3 was subjected to more extensive
investigation for its visible and NIR electrochromism. The film
of P3 was made from its chlorobenzene solution by spin-coating
Fig. 6 Absorption spectra of a P3 film on ITO glass slide in 0.1 M
and the film thickness was about 80 nm. When the applied
Et4NClO4 from 0.0 V to 1.0 V.
voltage increases from 0.0 V to 1.0 V, the p–p* absorption
band at 380 nm decreases gradually (Fig. 6) and at the same
time, a new broad band appears in the NIR region at about
ITO glass was monitored at the telecommunication wavelengths
of 1310 nm and 1550 nm respectively, while the potential was
switched between 0.0 V and 1.0 V (vs. Ag wire) using square wave
potentiometry. The changes in percent transmittance (DT%)
between the neutral (at 0.0 V) and oxidized states (at 1.0 V) were
found to be 36% at 1310 nm and 30% at 1550 nm wavelengths,
respectively (Fig. 7). The response time from the neutral to
oxidized state or vice versa was found to be 2.0 s for 95% of full
optical recovery. The optical contrast kept nearly constant after
hundreds of cycles switch, indicating good stability and robust-
ness of P3.
1160 nm, due to the formation of radical cations (polarons) in
the main chain (Fig. S6†). Meanwhile, the color of film P3
changed from green to grayish brown. When the potential was
increased to a more positive value of 1.3 V, corresponding to the
second-step oxidation, the absorption band at 1160 nm dis-
appeared and a new band appeared at about 630 nm (Fig. S7†),
indicating the formation of bipolarons at a higher level of
p-doping.
The rate of change in transmittance upon oxidation/neutral
states provides a direct evaluation of the switching ability of
a material. The transmittance of the film of polymer P3 coated on
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J. Mater. Chem., 2011, 21, 7678–7685 | 7683

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Table 3 Electrochromic properties of P3
Wavelength/
nm
Contrast
(DT%)a
Attenuationb/
dB mmꢀ1
CEc/cm2
Cꢀ1
Response
time/sd
1310
1550
36
30
44
21
479
232
2.0
2.0
a
Difference in transmittance between the fully oxidized and neutral
states. Optical attenuation per film thickness in mm. Color efficiency
b
c
d
at a given wavelength. Response time to reach 95% of maximum
attenuation range during oxidation.
where L is the thickness (mm) of the electrochromic active layer.
Accordingly, the optical attenuation of P3 is calculated to be
44 dB mmꢀ1 at 1310 nm and 21 dB mmꢀ1 at 1550 nm, which is
comparable to those of PEDOT film.51
Conclusions
We have demonstrated a feasibility of the precursor strategy for
a quick access to a series of low band-gap D–A polymers con-
taining different strong acceptor units. By varying the electron
acceptor strength, the band-gap levels of polymers P2–P5 change
from 1.71 eV to 1.29 eV and therefore exhibit different colors as
orange, blue, green, grey and dark goldenrod. After being elec-
trochemically oxidized to radical cations, the color changed to
grey or sienna and all the polymers had strong NIR absorption in
the oxidized state. Polymer P3 is potentially a switchable
camouflage material for use in day and at night.
Acknowledgements
This work was supported by the Natural Sciences and Engi-
neering Research Council of Canada and the National Natural
Science Foundation of China (21074132, 20920102032 and
20834001).
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