Electrochromic π-Conjugated Copolymers Derived from Azulene, Fluorene, and Dialkyloxybenzothiadiazole

Article abstract of DOI:10.1071/CH13147

A series of random π-conjugated copolymers P1, P2, and P3 were synthesised from 1,3-dibromoazulene, 4,7-dibromo-5,6-bis(dodecyloxy)benzo-2,1,3- thiadiazole, and 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) via Suzuki coupling reactions. The copolymers

Full text of DOI:10.1071/CH13147

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 1048–1056
Full Paper
http://dx.doi.org/10.1071/CH13147
Electrochromic p-Conjugated Copolymers Derived from
Azulene, Fluorene, and Dialkyloxybenzothiadiazole
A B
,
A
A
Shaun Zhi Hao Lim,
Wei Teng Neo, Ching Mui Cho,
A
A
B
Xiaobai Wang, Angeline Yan Xuan Tan, Hardy Sze On Chan,
A C
,
and Jianwei Xu
A
Institute of Materials Research and Engineering, Agency for Science, Technology and
Research (A*STAR), 3 Research Link, Singapore 11760.
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543.
B
C
Corresponding author. Email: jw-xu@imre.a-star.edu.sg
A series of random p-conjugated copolymers P1, P2, and P3 were synthesised from 1,3-dibromoazulene, 4,7-dibromo-
5,6-bis(dodecyloxy)benzo-2,1,3-thiadiazole, and 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) via Suzuki coupling
reactions. The copolymers P1 3 had molecular weights in the range of 17000–30900 g molꢀ1 with polydispersity indexes
of 1.45–2.03. Thermal analysis showed that the polymers P1 3 had good thermal stability with decomposition
]
]
temperatures ranging from 341 – 3638C both in air and in nitrogen. Photoluminescence studies showed that polymer
P1 and P2 are weakly fluorescent with low quantum yields of 0.013 and 0.0029 for P1 borne with 30 % azulene and P2
borne with 50 % azulene in the polymer backbone, respectively. P3 borne with 70 % azulene resulted in complete
quenching of fluorescence. The electrochemical band gaps for P1–3 are very close to their corresponding optical band
gaps. Electrochromic study showed that three polymer thin films displayed the same colour change from yellowish green
at the neutral and electrochemically-reduced state to greyish brown at the electrochemically-oxidised state. In particular,
electrochromic contrasts of 17 % and 13 % for P2 and P3, respectively, were recorded in the near infrared region.
Manuscript received: 5 April 2013.
Manuscript accepted: 17 July 2013.
Published online: 6 August 2013.
Introduction
many applications such as non-emissive displays and smart
windows.[5] However, the main shortcoming for electrochromic
conjugated polymer materials is the redox stability. Some
approaches have been attempted to solve this issue, such as the
donor-acceptor (D-A) approach, which involves electron-rich
donors and electron-poor acceptors within a polymer backbone.
By controlling the composition of electron-rich and electron-
poor moieties in the polymer chain, we can control HOMO and
LUMO levels and ultimately the energy of the intraband tran-
sitions in D-A type conjugated polymers.
Currently, most electrochromic polymers are based on
3,4-alkylenedioxythiophene derivatives with neutral state col-
our spanning a full colour palette as well as transmissive
oxidised states.[9] Structural modifications and particularly the
D-A approach were utilised to design a variety of conjugated
polymers. For example, fused heterocycle units containing high
heteroatom content such as benzothiadiazole are commonly
utilised as an electron acceptor. Due to the presence of elec-
tron-withdrawing nitrogen atoms within the heterocycle, ben-
zothiadiazole derivatives are electron-deficient, acting as ideal
electron acceptors. Many D-A type conjugated polymers con-
taining benzothiadiazole derivatives have also been reported to
show high hole mobility, low band gaps, as well as wide optical
absorption bands[10] for organic field-effect transistor and
organic photovoltaic applications. In this work, following
Studies of organic conjugated polymers today have resulted in
their electronic applications such as organic light-emitting
diodes,[1] organic field-effect transistors,[2] organic photo-
voltaics,[3] as well as electrochromic devices.[4] The growing
interest of organic conjugated polymers arises from the ability
to fine-tune their properties through the modification of the
conjugated polymer structures. This allows for the polymers to
be tailored for their desired applications. Organic conjugated
polymers also possess other advantages such as mechanical
flexibility, low-cost, scalability, and processing.[5] Electro-
chromism can be defined as the reversible colour change
induced by an external voltage.[4] Since this phenomena in the
semiconductor tungsten oxide was first discovered by Deb in
1969,[6] most work has focussed on inorganic electrochromic
materials such as the oxides of molybdenum (MoO3) and tita-
nium (TiO2). Inorganic electrochromic materials are known to
have high photochemical stability, however, they are costly to
fabricate, exhibit slow response times and have low coloration
efficiency.[4,7] Incorporation of conductive conjugated poly-
mers in electrochromic devices offers many advantages, for
example, they are easier to process and have faster response
times and higher optical contrasts than inorganic electrochromic
material-based devices.[4,8] These unique characteristics
allow organic electrochromes to be potentially utilised in
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

Electrochromic p-Conjugated Copolymers
1049
a similar D-A approach, 5,6-dialkoxy-2,1,3-benzothiadiazole
was adopted to design and synthesise azulene and
fluorene-containing conjugated polymers. Azulene is a mole-
cule consisting of two fused aromatic rings, namely, a seven-
membered ring and a five-membered ring. Azulene and its
derivatives are known to have interesting electronic properties
such as low ionisation energy as well as a tendency to form
radical cations and anions.[11] In addition, azulene has a rela-
tively low oxidation potential (0.96 V vs saturated calomel
electrode, SCE)[12] as compared with similar fused aromatics
such as naphthalene, which has the same molecular formula
(1.64 V vs SCE).[13] Thus, it is expected that azulene units in
polymers are oxidised readily, which may result in interesting
electrochromic properties. Poly(azulene)[14] and azulene-
containing copolymers[15] including electrochromic conjugated
copolymers, for example, poly(azulene-alt-fluorene), have been
reported recently, however, they have shown low switching
stability of several cycles as well as slow bleaching times.[15e]
The introduction of 5,6-dialkyloxy-2,1,3-benzo[3,4]thiadiazole
as an electron acceptor to the azulene-fluorene copolymers is
hypothesised to improve the electrochromic properties of the
polymers by lowering the band gap and avoiding over-oxidation
of copolymers. Herein we report the synthesis and characterisa-
tion of a series of azulene-containing conjugated copolymers
that are composed of varying ratios of electron donor and
acceptor units.
UV-3600 UV-Vis-NIRspectrophotometer was also employed
for spectro-electrochemical studies. Photos were taken with a
Canon Digital IXUS 50 digital camera.
Synthesis of Compounds 2–4
1,2-Diamino-4,5-bis(dodecyloxy)benzene (2)
1,2-Bis(dodecyloxy)-4,5-dinitrobenzene (1) (1.0 mmol,
1 equiv.) and palladium on carbon (0.20 g, 10 wt-%) were added
to a round-bottomed flask with 100 mL of ethanol. The mixture
was stirred and purged under argon for ,1 h. Hydrazine mono-
hydrate (2.0 mL, excess) was added dropwise to the suspension.
The resultant reaction mixture was then heated up to reflux at
708C until a colourless solution with black (carbon) suspension
developed. The hot mixture was filtered using Celite and rinsed
with hot ethanol. Excess solvent from the resulting filtrate was
evaporated under reduced pressure to yield an off-white solid 2,
which was subsequently dried under vacuum for ,24 h. Com-
pound 2 is easily oxidised in air, and it was used for the next
reaction immediately after preparation. Yield: 96 %.
5,6-Bis(dodecyloxy)benzo-2,1,3-thiadiazole (3)
A Schlenk flask was first loaded with 2 (1.0 mmol, 1 equiv.)
dissolved in a mixture of dry toluene (10 mL) and dry triethy-
lamine (10 mL). The resultant mixture was degassed via five
freeze-pump-thaw cycles under liquid nitrogen. After warming
to room temperature, N-thionylaniline (0.30 mL, excess) was
added to the stirring mixture. The resulting dark red solution was
then left to stir for ,16 h before it was heated up to reflux at
1208C for ,3 h. Excess solvent was evaporated under reduced
pressure and the resulting crude product was triturated with
deionised water (40 mL). The crude product was filtered off and
the residue was then recrystallised twice using hot ethanol to
yield compound 3 as an off-white solid (0.315 g, 70 %). 1H NMR
(CDCl3): d 7.13 (s, 2H), 4.09 (t, 4H), 1.91 (m, 4H), 1.26
(m, 28H), 0.88 (t, 6H).
Experimental
Materials
All reagents and chemicals, unless otherwise stated, were
obtained from Aldrich, Alfa Aesar, or Lancaster and were
used as received. Tetrahydrofuran was distilled over sodium/
benzophenone. Triethylamine (TEA) was purified by reflux-
ing with potassium hydride followed by distillation. 9,9-
Dioctylfluorene-2,7-bis(trimethyleneborate) was recrystallised
in hexane before use. Acetonitrile (ACN) was purchased from
˚
4,7-Dibromo-5,6-bis(dodecyloxy)
Baker, dried over Type 4A 1–2 mm molecular sieve beads
benzo-2,1,3-thiadiazole (4)
(International Laboratory USA) and filtered using 0.22 mm
PVDF syringe filters (Millipore Millex-GV) before use. Indium
tin oxide (ITO, 60 O/sq) coated poly(ethylene terephthalate)
(PET) was obtained from Aldrich and used as the substrate.
To a dissolved mixture of 3 (1.0 mmol) in dichloromethane
(16 mL) and acetic acid (7 mL), was added dropwise molecular
bromine (6.0 mmol), and the resulting mixture was stirred in the
dark for ,48 h at room temperature. The reaction mixture was
then poured into a separatory funnel containing deionised water
(20 mL). The desired product was extracted with dichloro-
methane followed by washing with water (40 mL), aqueous
NaHCO3 (40 mL) and aqueous Na2SO3 (40 mL). The organic
layer obtained was dried over anhydrous MgSO4 and excess
solvent was evaporated under reduced pressure. The product
was recrystallised with hot ethanol twice to obtain compound 4
Instrumentation
1H NMR spectra were acquired on a 400 MHz DRX Bruker
NMR spectrometer in CDCl3 with TMS as the internal refer-
ence. The molecular weights of the polymers were determined
on a Waters Model 2690 gel permeation chromatography
(GPC) system using HPLC-grade THF at a flow rate of 1 mL
minꢀ1 and injection volume 10 mL with poly(methyl methac-
rylate) (PMMA) as standards. UV-Vis absorption spectra were
measured using a Shimadzu UV-3101 spectrometer. The
photoluminescence measurements were performed using a
Perkin Elmer LS55 fluorescence spectrometer with a Xenon
lamp as a light source, 10 nm Ex (excitation) and Em (emis-
sion) slits, 150 nm minꢀ1 scan rate and 1 % attenuator. Thermal
analysis was performed in a Perkin-Elmer thermogravimetric
analyser (TGA 7) in nitrogen or in air at a heating rate of
108C minꢀ1 and a TA Instruments differential scanning calo-
rimetry (DSC) 2920 at a heating rate and a cooling rate of
108C minꢀ1 in nitrogen. An Autolab PGSTAT128N potentio-
stat/galvanostat was used for cyclic voltammetry experiments
and in situ spectro-electrochemical studies. A Shimadzu
1
as a fluffy off-white solid (0.455 g, 75 %). H NMR (CDCl3):
d 4.16 (t, 4H), 1.88 (m, 4H), 1.28 (m, 28H), 0.89 (t, 6H).
Synthesis of Polymers P1–3
A Schlenk tube was first loaded with 4 (0.385 mmol), 9,9-
dioctylfluorene-2,7-bis(trimethyleneborate) (0.550 mmol), and
1,3-dibromoazulene (0.165 mmol) dissolved in dry and degas-
sed toluene (12 mL). A degassed aqueous solution (9.0 mL) of
K2CO3 (2.50 g, 18.0 mmol), aliquat 336Ò (1 mL), and Pd(PPh3)4
(0.007 mmol) were added. The reaction mixture was purged
under argon for ,30 min after which the reaction was run in a
closed system at 1058C for 3 days. The reaction mixture was
then poured into excess cold methanol (200 – 250 mL) with

1050
S. Z. H. Lim et al.
vigorous stirring. The obtained precipitate was filtered, washed
with cold deionised water, and dried under vacuum. The crude
product was dissolved in chloroform and filtered. The filtrate
obtained was evaporated to dryness and dissolved in a minimal
amount of toluene. The solution was added dropwise to a beaker
of cold methanol (200 mL) with vigorous stirring to re-precip-
itate the polymer. The precipitate was then filtered and washed
with acetone in a Sohxlet extractor for 2 days. The final product
was obtained after drying in a vacuum oven for ,24 h as a
greenish solid.
Results and Discussion
Synthesis and Characterisation of Acceptor and Polymers
The synthetic route leading to polymers P1 3 is outlined in
]
Scheme 1. First, the reduction reaction of 1,2-dinitro-4,5-bis
(dodecyloxy)benzene (1) was carried out in the presence of Pd/C
and hydrazine to give a quantitative yield of 1,2-diamino-4,5-bis
(dodecyloxy)benzene (2). Compound 2 underwent an intramo-
lecular cyclisation reaction using N-thionylaniline according to
the literature method with modification[16] to afford 5,6-bis
(decyloxy)benzo-2,1,3- thiadiazole (3) in 70 % yield. In contrast
to the reported method, the diamine 2 could be directly used in
the cyclisation reaction instead of converting it into its diamino
dihydrogen chloride. Compound 3 was brominated using
molecular bromine to give 4,7-dibromo-5,6-bis(dodecyloxy)
Copolymer P1
Yield: 0.369 g, 91 %; 1H NMR (CDCl3): d 8.63 – 8.65
(d, 0.6H), 8.33 (s, 0.3H), 7.70 – 7.96 (m, 6H), 7.16 (t, 0.6H),
3.94 (t, 2.8H), 2.10 – 2.17 (m, 4H), 1.57 (m, 6.8H), 1.28 – 1.38
(m 43.6H), 0.79 – 087 (d, 10.2H).
benzo-2,1,3-thiadiazole (4). Polymers P1 3 were synthesised
]
by reacting compound 4 and 1,3-dibromoazulene (6) with 9,9-
dioctylfluorene-2,7-bis(trimethyleneborate) (5) via Suzuki
coupling reaction in the presence of Pd(PPh3)4 catalyst
(Scheme 1). The resultant crude copolymers were precipitated
in methanol, followed by purification with Soxhlet extraction
using acetone as solvent. Compositions, synthetic yields,
molecular weights, polydispersity, and thermal data of the
Copolymer P2
Yield: 0.339 g, 83 %; 1H NMR (CDCl3): d 8.56 – 8.58
(d, 1H), 8.26 (s, 0.5H), 7.63 – 7.88 (m, 6H), 7.09 (t, 1H), 3.87
(t, 2H), 2.03 – 2.10 (m, 4H), 1.48 – 1.55 (m, 8H), 1.15 – 1.20
(m, 38H), 0.78 – 0.86 (m, 9H).
Copolymer P3
polymers P1 3 are summarised in Table 1. P1, P2, and P3 were
]
obtained in good yield (91, 83, and 88 %, respectively).
Yield: 0.377 g, 88 %; 1H NMR (CDCl3): d 8.56 – 8.58
(d, 1.4H), 8.263 (s, 0.7H), 7.55 – 7.86 (m, 6H), 7.08 (t, 1.4H),
3.88 (t, 1.2H), 2.04 – 2.10 (m, 4H), 1.48 – 1.57 (m, 5.2H), 1.09 –
1.15 (m 32.4H), 0.72 – 0.86 (m, 7.8H).
The polymers P1 3 were structurally characterised by 1H NMR
]
and elemental analysis (Table S1 in the Supplementary
Material), and their chemical structures and compositions
are consistent with the expected structures. As an example, the
1H NMR spectrum of the polymer P2 is illustrated in Fig. 1. A
doublet at d 8.57 and a singlet at d 8.26, corresponding to
characteristic azulene protons Ha and Hb, respectively, were
observed. Azulene protons Hc were overlapped with fluorene
aromatic protons centred at d 7.75. A triplet at d 7.09 was
assigned to azulene protons Hd. These suggested that azulene
units were intact duration polymerisation. The distinctive sig-
nals at d 2.03 from alkyl groups at the 9,9-position of the
fluorene moiety and d 3.87 from the alkoxy substituents of
compound 4 were observed (Fig. 1). Polymers P1 and P3
showed similar NMR spectra profiles to the polymer P1 and
their spectra are given in the Supplementary Material (Fig. S1).
Cyclic Voltammetry and Spectro-Electrochemistry
The solutions of polymers P1 3 (10 mg mLꢀ1 in o-xylene) were
]
spin-coated onto ITO/PET using a SCS G3P-8 spincoater to
form polymer thin films. For the cyclic voltammetry (CV)
experiments, a three-electrode configuration was used with ITO
coated PET as the working electrode, platinum wire (Metrohm)
as the counter electrode and Ag/AgCl (3 M KCl) (Metrohm) as
the reference electrode. A 0.1 M LiClO4/ACN electrolyte/sol-
vent couple was used. In situ spectro-electrochemical data was
obtained using a three-electrode cell assembly, which consists of
the ITO coated PET substrate as the working electrode, platinum
wire as the counter electrode, and Ag wire as the pseudo-ref-
erence electrode.
OC12H21
Br
OC12H21
C12H21
O
OC12H21
iii
C12H21
Br
O
C12H21
O
OC12H21
C
12H21O
i
ii
O2N
N
N
N
N
NO2
H2N
NH2
S
S
1
2
3
4
O
B
O
B
iv
4
ꢀ
ꢀ
O
O
Br
Br
C8H17
C8H17
6
5
C12H21O
OC12H21
O
P1 (x ꢁ 0.3, y ꢁ 1; z ꢁ 0.7)
P2 (x ꢁ 0.5, y ꢁ 1, z ꢁ 0.5)
P3 (x ꢁ 0.7, y ꢁ 1, z ꢁ 0.3)
z
y
x
N
N
C8H17
C8H17
S
Scheme 1. Synthetic route leading to acceptor 4 and polymers P1 3. Reagents and conditions: (i) Hydrazine
]
monohydrate, 10 % Pd/C, EtOH, argon, 708C; (ii) N-thionylaniline, triethylamine (TEA), toluene, 1208C; (iii)
bromine, acetic acid, dichloromethane, r.t.; (iv) toluene, aq. K2CO3, aliquat 336Ò, Pd(PPh3)4, 1058C, 3 d.

Electrochromic p-Conjugated Copolymers
1051
Table 1. Compositions, yields, molecular weights, polydispersity (Mw/Mn) and thermal data of
polymers P1 P3
]
Mw: the weight-average molecular weight; Mn: the number-average molecular weight; Mw/Mn: the
polydispersity index
Polymer
Feed ratio
x/y/z
Actual ratio
x/y/z
Yield [%]
Mw
Mn
Mw/Mn
Td
In air In N2
P1
P2
P3
0.30/1.00/0.70 0.30/0.99/0.72
0.50/1.00/0.50 0.52/1.04/0.45
0.70/1.00/0.30 0.71/1.02/0.27
91
83
88
62600 30900
24600 17000
41500 26400
2.03
1.45
1.57
360
359
363
350
341
356
Hc
He
Hd
RO
N
OR
∗
Ha
1
0.5
0.5
Hb
N
Rꢂ
Rꢂ
S
∗
Hc,He
fluorenyl-9,9-C(CH2-)2
OCH2
-
Hd
Hb
Ha
10
9
8
7
6
5
4
3
2
1
0
[ppm]
Fig. 1. 1H NMR spectrum for polymer P2. *Residual solvents (CHCl3 and H2O).
(a)
(b)
100
80
60
40
20
0
100
P1
P1
P2
P3
P2
90
P3
80
70
60
50
40
30
20
10
0
100
200
300
400
500
600
700
800
900
100
200
300
400
500
600
700
Temperature [ꢃC]
Temperature [ꢃC]
Fig. 2. Thermogravimetric graphs for polymers P1 3 in (a) air and (b) nitrogen. The data was recorded at a rate of 108C minꢀ1
.
]
Polymer compositions were estimated by comparing the inte-
gration area of characteristic signals at d 8.6, 3.9, and 2.1 of the
three co-monomers 6, 4, and 5, which are in good agreement
with reactant feed ratios. Molecular weights of the polymers
against poly(methyl methacrylate) standards were determined
by GPC. All polymers had relatively high molecular weights
ranging from 17000 – 30900 g molꢀ1 with reasonable polydis-
persity indexes (1.45–2.03). In addition, all polymers had good

1052
S. Z. H. Lim et al.
thermal stability with decomposition temperatures (Td) in the
range of 341 to 3638C in air and in nitrogen (Fig. 2). It was
noteworthy that all polymers showed better thermal stability in
inert atmosphere than in air. For example, the polymer P1 has
higher Td by 108C in air than in nitrogen. This may relate to the
presence of the benzothiadiazole unit in the copolymer back-
bone. This monomer acts as anti-oxidant which may be oxidised
to form benzo[2,1,3]thiadiazole 2,2-dioxide in air. The poly-
mers were also studied by DSC and no glass transitions were
detected.
UV-Vis Spectroscopy and Photoluminescence
Spectroscopy of Polymer Solution
The UV-Vis spectra and photoluminescence spectra of the
polymers P1 3, together with the reference polymer poly
]
(azluene-alt-dioctylfluorene) (P4) in THF solution were mea-
sured (Fig. 3). The UV-Vis absorption and photoluminescence
data are summarised in Table 2. The reference polymer P4
exhibited one strong p - p absorpꢀti1on band at 363 nm with molar
absorptivitity (e) of 20500 L mol cmꢀ1, a very weak shoulder
peak at 477 (e ¼ 240 L molꢀ1 cmꢀ1), and a very weak broad
band at 629 nm (e ¼ 190 L molꢀ1 cmꢀ1).[15e] In contrast, poly-
mers P1 3 showed two distinctive absorption peaks in the range
]
0.5
of 328 to 349 nm (e ¼ 28200 – 39000 L molꢀ1 cmꢀ1) and
between 392 and 407 nm (e ¼ 23200 – 24100 L molꢀ1 cmꢀ1). In
contrast to reference polymer P4, no weak broad band was
observed at long wavelengths. In comparison with the major
absorption lmax (363 nm) of the polymer P4, p-p absorption
P1
P2
P3
0.4
0.3
0.2
0.1
0
l
max for the polymers P1, P2, and P3 are red-shifted by 44, 36,
and 28 nm, respectively. This is due to the intramolecular
electron transfer between the electron donor and electron
acceptor units, which leads to the red-shifted absorption lmax
.
The magnitude of spectral red-shift progressively decreases
with the decrease in percentage of the electron acceptor
(benzothiadiazole) in the polymers P1 3. The optical band gap
]
was estimated according to the onset of absorption, showing
that it gradually reduced from 2.38 eV for P1 to 2.19 eV for P3
with the decrease in percentage of the electron acceptor unit in
the polymers (Table 2).
300
350
400
450
500
550
600
Wavelength [nm]
Fig. 3. UV-Vis absorbance spectra for P1 (1.064 ꢁ 10ꢀ5 M), P2
(1.239 ꢁ 10ꢀ5 M), and P3 (1.339 ꢁ 10ꢀ5 M) in THF.
0.0010
0.0005
0
Table 2. Absorption lmax, emission lmax, optical band gap, and
emission quantum yields of polymers P1 P3
]
NA: not applicable
Polymer Abs lmax [nm]A abs lonset [nm] EgOpt [eV]B Em lmax [nm] jPL
P1
P2
P3
328 (39000),
407 (24100)
332 (36800),
399 (25900)
349 (28200),
392 (23200)
520
545
565
2.38
2.27
2.19
509
509
NA
0.013
0.0029
NA
ꢄ0.0005
ꢄ0.0010
ꢄ2.0
ꢄ1.5
ꢄ1.0
ꢄ0.5
0
0.5
1.0
1.5
Potential [V] vs Ag/AgCl
AThe values in parentheses are molar absorptivities (e) with units of
L molꢀ1 cmꢀ1
.
Fig. 4. Cyclic voltammogram of polymer P3 on ITO coated PET in 0.1 M
LiClO4/ACN at a scan rate of 50 mV sꢀ1
BOptical band gaps were calculated by the equation: Eg ¼ 1240/abs lonset
Opt
.
.
Table 3. Electrochemical properties of polymers P1 P3
]
Polymer Oxidation Reduction Eox [V]C Ered [V]C
Ip
Ea(LUMO)[eV]D EgEc [eV]E
Ep,a [V]A
Ep,c [V]B
(HOMO)[eV]D
P1
P2
P3
1.06
1.03
1.07
ꢀ1.65
ꢀ1.64
ꢀ1.64
0.99
0.85
0.85
ꢀ1.41
ꢀ1.44
ꢀ1.40
ꢀ5.39
ꢀ5.25
ꢀ5.25
ꢀ2.99
ꢀ2.96
ꢀ3.00
2.40
2.29
2.25
AAnodic oxidation peak potential.
BCathodic reduction peak potential.
CEox and Ered are the onset potentials of oxidation and reduction respectively.
DIp and Ea are the ionisation potential and electron affinity respectively; Ip ¼ –(Eox þ 4.4) eV, Ea ¼ –(Ered þ 4.4)
eV.[25]
EElectrochemical band gap ¼ Eg ¼ LUMO – HOMO.
Ec

Electrochromic p-Conjugated Copolymers
1053
RO
OR
N
OR
RO
RO
OR
N
S
C8H17
C8H17
N
N
N
N
S
C8H17
S
C8H17
C8H17
C8H17
C8H17
C8H17
C8H17
C8H17
ꢀe
ꢀe
ꢀe
ꢀe
RO
OR
OR
RO
RO
OR
N
C8H17
C8H17
N
N
N
S
S
C8H17
N
N
C8H17
C8H17
C8H17
C8H17
C8H17
S
C8H17
C8H17
RO
N
OR
N
OR
RO
RO
OR
C8H17
C8H17
N
N
S
S
C8H17
C8H17
N
N
C8H17
C8H17
C8H17
S
C8H
17 C8H17
C8H17
Scheme 2. Oxidation pathway of polymer P2
0.9
0.8
0.7
1.2V
1.0V
n
0.6
0.5
0.4
0.3
0.2
0V
ꢄ2.0V
R
R
P4: R ꢁ CH2CH(C2H5)C4H9
Chart 1.
The quantum yields, jPL, of polymers P1 3, were measured
]
in dilute THF solutions using quinine sulfate (0.1 M in H2SO4)
as a standard.[17] It was observed that the photoluminescence
quantum yields of the polymers, jPL are significantly lower than
other aromatic conjugated polymers. It was previously proposed
that the incorporation of non-protonated azulene units in the
polymer backbone could lead to a significant reduction in
photoluminescence of the polymers as the non-protonated
azulene units may act as quenchers to photoluminescence.[13b]
This explanation supports the trend observed as the jPL
decreases with the increase of azulene moieties from P1 to P3.
For instance, P1 and P2 with mole percentage of azulene of 30
and 50 %, respectively, are weakly fluorescent with a jPL of
0.013 and 0.0029; however, the polymer P3 with mole percent-
age of 70 % azulene is completely non-emissive, which is
similar to the polymer P4.
0.1
0
400
600
800
1000
1200
1400
1600
Wavelength [nm]
Fig. 5. Spectro-electrochemistry of polymer P3 film on ITO coated PET in
0.1 M LiClO4/ACN electrolyte/solvent couple.
shown in Scheme 2. Azulene exists as an electron-rich five-
membered ring fused to an electron-deficient seven-membered
ring and thus the electron removal of the polymer P2 usually
occurs at the electron-rich cyclopentadiene part of the azulene
ring to generate a cationic radical, which further rearranges to
form a closed shell structure. The formation of similar tropylium
cation radicals for azulene and its derivatives have been iden-
tified by electron spin resonance spectroscopy.[18]
The energy levels of the HOMO and LUMO of the polymers
P1 3 were estimated from the oxidation and reduction onsets in
the cyclic voltammograms and the onset of absorption lmax in
the UV-Vis spectra, and the results are given in Table 3.
Incorporation of azulene units into the conjugated polymers
Electrochemical Behaviour of Polymers
and Cyclic Voltammetry (CV)
Cyclic voltammetry measurements on spin-coated polymer
films of P1 3 were conducted in ACN with lithium perchlorate
]
]
as the electrolyte and their CV data are summarised in Table 3.
All potentials were measured against the Ag/Agþ reference
couple. A typical cyclic voltammogram of polymer P2 is
illustrated in Fig. 4. The polymer P2 showed one reversible
oxidation redox wave in the range of 0.6–1.1 V under an anodic
sweep. Being a random copolymer with two major repeating
units, the probable oxidation pathway of the polymer P2 is
P1 3 leads to a considerable decrease in oxidation of potentials.
]
For example, polymer P1 (Ep.a ¼ 1.06 V vs Ag/AgCl) has a
lower anodic oxidation peak potential (by 0.53 V) than homo-
polymer poly(9,9-dihexylfluorene) (Ep.a ¼ 1.59 V[19] vs Ag/
AgCl). Similarly, the electrochemical band gap values of

1054
S. Z. H. Lim et al.
the polymers P1 3 (2.40–2.25 eV) are lower compared to
]
Table 4. Optical contrast and switching speeds of polymers P1 P3
]
NA: not applicable
homopolymer poly(9,9-dihexylfluorene) (Eg ¼ 2.86 eV[19]).
However, the band gaps of the polymers P1 3 are higher
]
Polymer
Optical contrastA [%]
Switching speedB [s]
Coloration Bleaching
UV-visC NIRD UV-visC NIRD
than that of our reported polymers, for example, P4 with a band
gap of 1.6 eV. This is because the reported band gap of the
reference polymer P4 is calculated according to the onset of
absorption of 630 nm, which belongs to the S0-S1 absorption
band. The inclusion of the azulene unit in polymer 4 resulted in
three bands at 630, 477, and 355 nm. In contrast, in this case,
when an acceptor monomer dialkoxybenzothiadiazole is includ-
ed in the polymer backbone, the band at around 600 nm
disappeared. Therefore, the band gap is estimated in terms of
the onset of the absorption band around 400 nm. If the onset of
absorption of the most intense peak of the reference polymer P4
(lmax ¼ 355 nm) is used for estimation of its band gap, the band
gap of P4 is around 2.85 eV, which is much higher than the band
UV-VisC
NIRD
P1
P2
P3
P4
–
4
6
17
–
7.1
7.0
9.1
NA
–
8
3.8
4.5
8.1
9.4
10.7
6.8
4.8
2.3
NA
2
13
NA
NA
AThin film of polymers were stepped between ꢀ 1.8 V and þ 1.8 V using a
square wave potential.
BSwitching speeds were calculated as the time required to obtain 95 % of full
switch.
CUV-Vis analysis performed at 410, 396, and 386 nm for P1, P2, and P3
respectively.
DNIR analysis performed at 1265, 1325, and 1365 nm for P1, P2, and P3
gap of the polymers P1 3. This reveals the ability of the
]
acceptor units to lower the band gaps through domination of
the LUMO level (Chart 1).
By varying the composition of azulene and benzothiadiazole
respectively.
in the polymers, the polymers P1 3 showed slight differences
]
for both oxidation and reduction peak potentials. The HOMO
and LUMO levels of P1 3 are calculated in terms of the onset of
]
in P3. Based on the onset of the p-p* transition, the spectro-
electrochemical band gaps of P1 P3 are estimated and sum-
]
marised in Table 3. The spectro-electrochemical band gaps are
found to be in good agreement with the electrochemical band
gaps. However, their values are slightly lower compared with
those determined electrochemically. This difference is due to
the interface barrier present between the polymer film and
electrode, which results in a higher potential required during
cyclic voltammetry for hole and charge injection.[18a] Hence,
this gives rise to the higher values determined for the electro-
chemical band gaps.
To probe the electrochromic properties of the polymers, a
square-wave potential step absorptiometry was utilised. The
polymer thin films were stepped between their oxidised (1.8 V)
and reduced states (ꢀ1.8 V) at 10 s intervals. The percent
transmittance changes were monitored as a function of time at
lmax and NIR region. The optical contrasts achieved for the
polymers are summarised in Table 4. It can be seen that the
electrochromic contrasts obtained at the NIR region are higher
as compared with those obtained at lmax in the visible regions. In
particular, P2 and P3 show electrochromic contrasts of 17 and
13 % respectively at the NIR region (Fig. 6). Introduction of an
electron acceptor moiety into the azulene copolymer improved
the redox stability and thus more cycles were observed, but the
gradual drop in the optical contrasts of the polymers upon
repeated cycling reveals that the stability of such polymers
needs further improvement. This is probably due to the two
electron-donating alkoxy substituents in the benzothiadiazole,
which weaken the overall electron-accepting ability of this
electron acceptor and hence lead to limited improvement. The
switching speeds of the polymers are calculated as the time
required to obtain 95 % of the full optical contrasts. It is
their oxidation and reduction waves. The electrochemical band
gaps of 2.40, 2.29, and 2.25 eV, for P1, P2, and P3 respectively,
are slightly larger than that of corresponding optical band gaps
deduced from the absorption spectra (2.38, 2.27, and 2.19 eV).
In addition, both optical band gaps and electrochemical band
gaps increase with an increase in acceptor moieties from P3 to
P1. This is contrary to the expected results whereby an increase
in mole percentage of acceptor moieties would result in a
decrease in band gap. A possible explanation for the observa-
tions could be the significant increase in steric effects from the
two alkyloxy side chains with the incorporation of more 5,6-bis
(dodecyloxy)benzo-2,1,3-thiadiazole acceptor moieties. The
steric effects imposed by two alkyloxy side chains considerably
affects the co-planarity of the polymers which give rise to a
shorter effective p-conjugation chain length and eventually a
higher band gap as observed.[20] Another plausible reason is that
co-monomer azulene plays a more important role in governing
the band gap than the electron acceptor. Azulene is a non-
alternate aromatic compound with peculiarly lower oxidation
potential (þ 0.96 V vs SCE) than fused aromatic compounds
such as naphthalene, and it is able to reduce the band gap when it
acts as a co-monomer in conjugated polymers. Therefore, the
higher the percentage of azulene in polymers is, the lower the
band gap.
Spectro-Electrochemistry and Electrochromic
Behaviour of Polymers
Spectro-electrochemical studies on the polymers were carried
out in a 0.1 M LiClO4/ACN electrolyte/solvent couple while
increasing the applied potential from ꢀ 2 V to about þ 1.6 V.
Upon increasing applied potentials, the absorption at lmax in the
visible region is suppressed, with a concomitant formation of the
absorption band in the near infrared (NIR) region. This is
believed to be due to the formation of low-energy polarons and
bipolarons. As a result, the colour of the polymers changed from
yellowish green at 0 and ꢀ 2.0 V at the initial neutral state and
electrochemically reduced state to greyish brown at the oxidised
state as shown in Fig. 5. These changes are observed in all three
polymers but the effect of the colour change is more pronounced
observed that the coloration speeds for P1 P3 measured at
]
both UV-Vis and NIR wavelengths are slower than those of the
azulene-fluorene copolymer. The bleaching speeds of P2 and
P3, on the other hand, are significantly faster compared with
their coloration speeds. The incorporation of small and moder-
ate amounts of acceptor units in P3 and P2 respectively helps to
improve the bleaching speeds of the polymer. Among the three
polymers, P3 displays better switching speeds with a coloration
time of 4.5 s and a bleaching time of 2.3 s at the visible and
NIR wavelengths, respectively.

Electrochromic p-Conjugated Copolymers
1055
100
(a)
(b)
105
100
95
90
90
80
85
0
100
200
300
400
500
0
100
200
300
400
Time [s]
Time [s]
Fig. 6. Square-wave potential step absorptiometry of (a) polymer P2 and (b) polymer P3 films at 1325 nm and 1365 nm respectively on an ITO
coated PET in 0.1 M LiClO4/ACN electrolyte/solvent couple between ꢀ 1.8 V and þ 1.8V with a switch time of 10 s.
Conclusions
[7] R. M. Paul Monk, D. Rosseinsky, Electrochromism and Electrochro-
mic Devices 2007 (Cambridge University Press: Cambridge).
[8] P. M. Beaujuge, S. Ellinger, J. R. Reynolds, Nat. Mater. 2008, 7, 795.
doi:10.1038/NMAT2272
A series of random p-conjugated copolymers were synthesised
from 1,3-dibromoazulene, 4,7-dibromo-5,6-bis(dodecyloxy)
benzo-2,1,3-thiadiazole,
and
9,9-dioctylfluorene-2,7-bis
[9] (a) C. M. Amb, A. L. Dyer, J. R. Reynolds, Chem. Mater. 2011, 23,
(trimethyleneborate) via Suzuki cross-coupling reactions. The
effect of electron acceptor and azulene units on the optical and
electrochemical properties was studied. Polymers showed
red-shifted absorption lmax when compared with the reference
polymer poly(azulene-alt-dioctylfluorene). Polymers P1 and P2
are fluorescent with low quantum yields of 0.013 and 0.0029,
respectively, but, P3 is non-fluorescent due to its high azulene
content. The optical band gaps are in good agreement with the
electrochemical band gaps. Subsequent electrochromic studies
revealed that incorporation of an electron acceptor into azulene-
containing conjugated polymers to some extent enhanced redox
stability. These polymers displayed a colour change from
yellowish green to greyish brown upon oxidation, with optical
contrasts of 17 and 13 % in the NIR region.
397. doi:10.1021/CM1021245
(b) P. Shi, C. M. Amb, E. P. Knott, E. J. Thompson, D. Y. Liu, J. Mei,
A. L. Dyer, J. R. Reynolds, Adv. Mater. 2010, 22, 4949. doi:10.1002/
ADMA.201002234
(c) A. L. Dyer, E. J. Thompson, J. R. Reynolds, ACS Appl. Mater.
Interfaces 2011, 3, 1787. doi:10.1021/AM200040P
[10] (a) E. Bundgaard, F. C. Krebs, Macromolecules 2006, 39, 2823.
doi:10.1021/MA052683E
(b) O. Atwani, C. Baristiran, A. Erden, G. Sonmez, Synth. Met. 2008,
158, 83. doi:10.1016/J.SYNTHMET.2007.12.013
(c) J. Subbiah, P. M. Beaujuge, K. R. Choudhury, S. Ellinger,
J. R. Reynolds, F. So, ACS Appl. Mater. Interfaces 2009, 1, 1154.
doi:10.1021/AM900116P
(d) T. L. Wang, A. C. Yeh, C. H. Yang, Y. T. Shieh, W. J. Chen,
T. H. Ho, Sol. Energy Mater. Sol. Cells 2011, 95, 3295. doi:10.1016/
J.SOLMAT.2011.07.021
[11] F. Wang, Y. H. Lai, Macromolecules 2003, 36, 536. doi:10.1021/
MA025662I
[12] L. Jo¨nsson, Acta Chem. Scand. A 1981, 35b, 683. doi:10.3891/
ACTA.CHEM.SCAND.35B-0683
[13] (a) W. C. Neikam, M. M. Desmond, J. Am. Chem. Soc. 1964, 86, 4811.
doi:10.1021/JA01076A017
Supplementary Material
The 1H NMR spectra of polymers P1 and P3 and the elemental
analysis data of polymers P1–3 are available on the Journal’s
website.
(b) R. J. Waltman, J. Bargon, Can. J. Chem. 1986, 64, 76. doi:10.1139/
V86-015
Acknowledgement
[14] (a) G. Tourillon, F. Garnier, J. Electroanal. Chem. 1982, 135, 173.
doi:10.1016/0022-0728(82)90015-8
We thank the Institute of Materials Research and Engineering (IMRE),
Agency for Science, Technology and Research (A*STAR) for financial
support.
(b) M. Iyoda, K. Sato, M. Oda, Tetrahedron Lett. 1985, 26, 3829.
doi:10.1016/S0040-4039(00)89262-X
(c) R. J. Waltman, J. Bargon, Can. J. Chem. 1986, 64, 76. doi:10.1139/
V86-015
References
[1] A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz, A. B. Holmes,
Chem. Rev. 2009, 109, 897. doi:10.1021/CR000013V
[2] A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo,
Chem. Rev. 2010, 110, 3. doi:10.1021/CR900150B
[3] Y. J. Cheng, S. H. Yang, C. S. Hsu, Chem. Rev. 2009, 109, 5868.
doi:10.1021/CR900182S
(d) K. G. Neoh, E. T. Kang, T. C. Tang, Polym. Bull. 1988, 19, 325.
doi:10.1007/BF00263932
(e) J. Daub, M. Feuerer, A. Mirlach, J. Salbeck, Synth. Met. 1991, 42,
1551. doi:10.1016/0379-6779(91)91897-J
(f) W. Schuhmann, J. Huber, A. Mirlach, J. Daub, Adv. Mater. 1993, 5,
124. doi:10.1002/ADMA.19930050211
[4] A. A. Argun, P. H. Aubert, B. C. Thompson, I. Schwendeman,
C. L. Gaupp, J. Hwang, N. J. Pinto, D. B. Tanner, A. G. MacDiarmid,
J. R. Reynolds, Chem. Mater. 2004, 16, 4401. doi:10.1021/
CM049669L
[5] P. M. Beaujuge, J. R. Reynolds, Chem. Rev. 2010, 110, 268.
doi:10.1021/CR900129A
(g) M. Porsch, G. Sigl-Seifert, J. Daub, Adv. Mater. 1997, 9, 635.
doi:10.1002/ADMA.19970090809
(h) F. X. Redl, O. Ko¨the, K. Ro¨ckl, W. Bauer, J. Daub, Macromol.
Chem. Phys. 2000, 201, 2091. doi:10.1002/1521-3935(20001001)
201:15,2091::AID-MACP2091.3.0.CO;2-4
(i) F. Wang, Y. H. Lai, N. M. Kocherginsky, Y. Y. Koteski, Org. Lett.
2003, 5, 995. doi:10.1021/OL0274615
[6] S. K. Deb, Appl. Opt. 1969, 8, 192.

1056
S. Z. H. Lim et al.
(j) B. Meana-Esteban, C. Lete, C. Kvarnstroem, A. Ivaska, J. Phys.
Chem. B 2006, 110, 23343. doi:10.1021/JP063181L
[16] J. Bouffard, T. M. Swager, Macromolecules 2008, 41, 5559.
doi:10.1021/MA8010679
(k) G. Nie, T. Cai, S. Zhang, J. Hou, J. Xu, X. Han, Mater. Lett. 2007,
61, 3079. doi:10.1016/J.MATLET.2006.11.004
[15] (a) S. E. Estdale, R. Brettle, D. A. Dummur, C. M. Marson, J. Mater.
[17] A. N. Fletcher, Photochem. Photobiol. 1969, 9, 439. doi:10.1111/
J.1751-1097.1969.TB07311.X
[18] (a) C. J. Cooksey, J. L. Courtneidge, A. G. Davies, P. S. Gregory,
J. C. Evans, C. C. Rowlands, J. Chem. Soc., Perkin Trans. 2 1988, 807.
doi:10.1039/P29880000807
(b) F. Gerson, M. Scholz, H. J. Hansen, P. Uebelhart, J. Chem. Soc.,
Perkin Trans. 2 1995, 215. doi:10.1039/P29950000215
[19] B. Liu, W. L. Yu, Y. H. Lai, W. Huang, Chem. Mater. 2001, 13, 1984.
doi:10.1021/CM0007048
Chem. 1997, 7, 391. doi:10.1039/A606139G
(b) F. Wang, Y. H. Lai, Macromolecules 2003, 36, 536. doi:10.1021/
MA025662I
(c) F. Wang, Y. H. Lai, M. Han, Org. Lett. 2003, 5, 4791. doi:10.1021/
OL0357346
(d) Z. Hussain, H. Hopf, L. Pohl, T. Oeser, A. K. Fischer, P. G. Jones,
Eur. J. Org. Chem. 2006, 5555. doi:10.1002/EJOC.200500733
(e) X. Wang, J. K. Ng, P. Jia, T. Lin, C. M. Cho, J. Xu, X. Lu, C. He,
Macromolecules 2009, 42, 5534. doi:10.1021/MA900847R
(f) M. Murai, E. Amir, R. J. Amir, C. J. Hawker, Chem. Sci. 2012, 3,
2721. doi:10.1039/C2SC20615C
[20] H. Yi, S. Al-Faifi, A. Iraqi, D. C. Watters, J. Kingsley, D. G. Lidzey,
J. Mater. Chem. 2011, 21, 13649. doi:10.1039/C1JM12089A