Three donor-acceptor polymeric electrochromic materials employing 2,3-bis(4-(decyloxy)phenyl)pyrido[4,3-b]pyrazine as acceptor unit and thiophene derivatives as donor units

Article abstract of DOI:10.1016/j.electacta.2014.09.022

In this study, 5,8-dibromo-2,3-bis(4-(decyloxy) phenyl) pyrido[4,3-b]pyrazine was synthesized and used as the acceptor unit, which then be coupled with three thiophene derivatives (3-methylthiophen, 3-methoxythiophen and 3,4-dimethoxythiophen) to give ris

Full text of DOI:10.1016/j.electacta.2014.09.022

Electrochimica Acta 146 (2014) 231–241
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Three donor-acceptor polymeric electrochromic materials employing
2,3-bis(4-(decyloxy)phenyl)pyrido[4,3-b]pyrazine as acceptor unit and
thiophene derivatives as donor units
b,
Hui Zhao a, Yuanyuan Wei a,b, Jinsheng Zhao b, , Min Wang
*
*
a
College of Chemical Engineering, China University of Petroleum (East China), QingDao 266580, PR China
Shandong Key laboratory of Chemical Energy Storage and Novel Cell Technology, Liaocheng University, Liaocheng 252059, PR China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 25 July 2014
Received in revised form 7 September 2014
Accepted 9 September 2014
Available online 16 September 2014
In this study, 5,8-dibromo-2,3-bis(4-(decyloxy) phenyl) pyrido[4,3-b]pyrazine was synthesized and used
as the acceptor unit, which then be coupled with three thiophene derivatives (3-methylthiophen,
3-methoxythiophen and 3,4-dimethoxythiophen) to give rise to three novel monomers, 2,3-bis(3-
methoxythiophen and 3,4-dimethoxythiophen) to give rise to three novel monomers, 2,3-bis
(4-decyloxy) phenyl)-5,8-bis(3-methylthiophen-2-yl) pyrido[4,3-b]pyrazine(M1), 2,3-bis(4-decyloxy)
phenyl)-5,8-bis(3-methoxylthiophen-2-yl) pyrido[4,3-b]pyrazine (M2) and 2,3-bis(4-decyloxy) phenyl)-
5,8-bis(3,4-dimethoxylthiophen-2-yl) pyrido [4,3-b]pyrazine(M3). The corresponding polymers P1, P2,
P3 were obtained by electropolymerization method. These polymers were characterized in terms of their
spectroelectrochemical and electrochemical properties by cyclic voltammetry and UV-Vis-NIR
spectroscopy. The band gaps of the polymers were calculated based on the spectroelectrochemistry
analysis, and were 1.39 eV, 1.136 eV and 1.25 eV for P1, P2, P3, respectively. Electrochromic investigations
showed that P1 switched between blue and colorless transmissive state, P2 switched between green to
transmissive light gray color, and P3 switched between blue to transmissive light brown color.
Electrochromic switching studies showed that all three polymers exhibit high coloration efficiency, fast
response time, and express more than 45% change in the transmittance in the near-IR region, which could
make these polymers useful in applications in NIR electrochromic devices.
Keywords:
Donor-acceptor polymers
Thiophene derivatives
pyrido[3,4-b] pyrazine
Electrochromism
ã 2014 Elsevier Ltd. All rights reserved.
1. Introduction
proper choice of the donor and acceptor groups allows one to select
the approximate HOMO and LUMO energies of the resulting
Considering the unique properties such as attainability of faster
response times, higher contrast ratios, color tenability via
structural modification and easy processibility (e.g. electrochemi-
cal deposition, spin coating and spray coating), conjugated
polymers(CPs) are bound to be promising materials of today's
world and are extensively applied in the fields of displays[1],
energy-saving “smart” windows[2], field-effect transistors[3],
sensors[4] and memory devices[5]. Recently, low band gap
conjugated polymers have received considerable interest, specifi-
cally those with band gaps below 1.5 eV [6]. One efficient way to
design low band gap polymers is to alternate electron donor (D)
and acceptor (A) units in the conjugated backbone of the polymers,
i.e. D-A approach. The major advantage of the D-A approach is that
polymer. These low band gap polymers are typically colored in
their neutral state as their * transition lies in the visible or
even the near-infrared region of the electromagnetic spectrum.
The materials become more transparent to the eye upon doping, as
p-p
the
p-p * transition is bleached and lower-energy electronic
transitions, which absorb in the IR, is developed [7].Therefore,
many different types of low band gap polymers have been
synthesized and developed for this purpose. 1,2,3-benzothiadia-
zole[8], thieno[3,4-b]pyrazine[9], 2,1,3-benzothiadiazole[10], qui-
noxaline[11,12], are mostly preferred acceptor type units to design
donor-acceptor type low band gap molecules. Quinoxaline
derivatives (Qx) have been demonstrated to be an admirable
building block for synthesis of low band gap conjugated polymers
due to their electronwithdrawing feature of two imine nitrogens in
the Qx. In terms of providing the utility of introducing substituents
easily on the 2 and 3 positions of itself, the Qx unit has a great
structure for controlling the electronic structure of the ending
polymers [13].
* Corresponding authors.
E-mail addresses: j.s.zhao@163.com (J. Zhao), wangmin1724@163.com
(M. Wang).
http://dx.doi.org/10.1016/j.electacta.2014.09.022
0013-4686/ã 2014 Elsevier Ltd. All rights reserved.

232
H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
Scheme 1. The previously reported polymers employing pyrido[3,4-b] pyrazines as the acceptor unit for optoelectronic applications.
Pyrido[3,4-b] pyrazine is a kind compounds with the similar
corresponding polymers including P1, P2, and P3 were obtained.
Then the electrochemical and electrochromic properties of these
polymers were characterized by using cyclic voltammetry (CV),
UV-Vis-NIR spectroscopy, fluorescence spectra and scanning
electron microscopy (SEM). P1 exhibited a color change from
light blue to a colorless transmissive, while P3 switched between
blue to a transmissive brown state. It is particularly worth
mentioning here is that the polymer film of P2 showed a green
color in the neutral state and turned to transmissive gray upon
oxidation. From the results, the thiophene derivatives with
electron-donating groups can successfully tailor the electro-
chromic properties by combining a low monomer oxidation
potential and a narrow band gap as well as the high electrical
conductivity. Out of our expectation, although the electron-
donating ability of tributyl(3,4-dimethoxythiophen-2-yl) stannane
is stronger than that of tributyl(3-methoxythiophen-2-yl) stan-
nane, P3 experienced a blue shift when compared to P2, which
may be attributed to the steric hindrance effect.
molecular structure of Qx, which exhibits a more stronger
electron-withdrawing effect than that of Qx due to the presence
of the additional pyridine N-atoms. On the basis of the synthesis of
the monomers depicted in Scheme 1, polymer (A) and Polymer (B)
have been electrochemically synthesized and characterized in
terms of their electrochromic properties [7,14]. Both polymers
showed a valuable neutral green color in addition to their stable
oxidation and reduction (p- and n-type doping) processes, and the
detailed characterizations varied with the substituents in the 2 and
3 positions of pyrido[3,4-b] pyrazine ring. It has clearly been
demonstrated that alkyl side chains not only enhance the ease of
processing, but also modify the electronic properties of the
conjugated polymers [15]. The oxidation potential of the polymers,
stability of the oxidized state, and band gap can drastically be
altered upon insertion of strong electron-donating alkoxy side
chains in the polymer backbone [16].
On the other hand, 3,4-ethylenedioxythiophene (EDOT), EDOT
derivatives and thiophene are most frequently used as the donor
units for the construction of D-A-D type monomers. In one of our
recent work, the use of 3-methoxythiophene as the donor unit also
led to the formation of low band gap polymers, with low oxidation
potential, ease of electrochemical polymerization and robust
stabilities [17]. Besides 3-methoxythiophene, 3-methylthiophene
and 3,4-dimethoxythiophene are never used as the donor units
with a pyrido[3,4-b] pyrazine acceptor unit in D-A type polymers.
Based on the above considerations, in this study, we synthe-
sized three novel monomers including 2,3-bis(4-decyloxy) phe-
nyl)-5,8-bis(3-methylthiophen-2-yl) pyrido[4,3-b]pyrazine(M1),
2. Experimental section
2.1. General
All chemicals except indicated otherwise were purchased from
Aldrich Chemical as analytical grade without further purification
methods. 3,4-diamino-2,5-dibromopyridine and 1,2-bis(4-(decy-
loxy) phenyl) ethane-1,2-dione were synthesized according to the
previously reported literature methods. 1H NMR and 13C NMR
spectroscope studies were performed on a Varian AMX 400 spec-
trometer, which use tetramethylsilane (TMS) as the internal
standard. Electrochemical synthesis and experiments were carried
out in a one-compartment cell with a CHI760 Electrochemical
Analyzer controlled by a computer, employing a platinum wire
with a diameter of 0.5 mm as a working electrode, a platinum ring
as a counter electrode, and a silver wire (Ag wire) as a pseudo-
reference electrode. Scanning electron microscope (SEM) meas-
urements were taken by using a Hitachi SU-70 thermionic field
emission SEM. The thicknesses of the polymers were tested by Step
2,3-bis(4-decyloxy)
pyrido[4,3-b]pyrazine(M2) and 2,3-bis(4-decyloxy) phenyl)-5,8-
bis(3,4-dimethoxylthiophen-2-yl) pyrido[4,3-b]pyrazine(M3),
phenyl)-5,8-bis(3-methoxylthiophen-2-yl)
using 2,3-bis(4-(decyloxy) phenyl) pyrido[4,3-b]pyrazine as the
acceptor unit. The introduction of 4-(decyloxy) phenyl substituent
on the 2 and 3 positions of the pyrido[4,3-b]pyrazine ring
enhanced the solubility of the monomers, which is beneficial to
the handling of the electropolymerizaition processing in conven-
tional solvent. Through electrochemical deposition method their

H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
233
Profiler. In situ spectroelectrochemistry was performed using a
2.2. Synthesis
Varian Cary 5000 UV-Vis-NIR spectrophotometer with a measure-
ment range that extended from 200 nm in the UV to 3300 nm in the
NIR. The polymer films were potentiostatically deposited onto
transparent electrodes of ITO/glass (the active area: 0.9 cm ꢀ 3.0
cm) which placed in a quartz cuvette that allowed for placement of
the transparent electrode, Pt wire counter electrode, and Ag wire
pseudo reference. Digital photographs of the polymer films were
taken by a Canon Power Shot A3000 IS digital camera. Fluorescence
spectra were performed on a F-380 Fluorospectro Photometer
instrument combined with a computer.
2.1. 3,4-diamino-2,5-dibromopyridine (1)
The synthesis of 3,4-diamino-2,5-dibromopyridine was im-
proved on the basis of the method reported in a literature [18]. The
mixture of pyrido-3,4-diamine(2 g, 18.3 m mol) with an aqueous
HBr (48%, 30 ml) were prepared in a 250 ml three-neck round-
bottom flask with a magneton inside. After the mixture was heated
to 100 ꢁC, bromine(2.5 ml) was added dropwise, and the solution
was stirred for 5 h at 135 ꢁC. After the mixture was cooled to room
temperature, an aqueous solution of Na2S2O3, an aqueous solution
Scheme 2. Synthetic route of the monomers. (a) HBr, Br2,135 ꢁC, 5 h. (b) HAc, HBr, reflux,12 h. (c) DMF, K2CO3,1 - bromine decane, TBAB,120 ꢁC, 90 min. (d) 50 ꢁC,12 h. (e-g) Pd
(PPh3)2Cl2, tributyl(3-methylthiophen-2-yl) stannane (e), tributyl(3-methoxythiophen-2-yl) stannane (f), tributyl(3,4-dimethoxythiophen-2-yl) stannane (g), dry toluene,
reflux, 24 h.

234
H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
of Na2CO3, and distilled water were added in this order to get a
yellow precipitate. Then the precipitate was separated by filtration
and washed with distilled water three times. Recrystallization
from the mixture solution of toluene: THF (v:v = 5:1) gave white
4H), 1.45 (m, 4H), 1.33 (m, 24H), 0.88 (t, 6H). 13C NMR (CDCl3,
101 MHz, ppm): = 189.03, 159.95, 127.81, 121.54, 110.17, 63.94,
27.35, 25.61, 25.00, 24.75, 21.40, 18.14 (see Fig. S2 in Supporting
Informations).
d
flocculence solid, yielded: 50%.1H NMR (DMSO, 400 MHz, ppm):
d=
7.53 (s, 1H), 5.99 (s, 2H), 5.03 (s, 2H). 13C NMR (DMSO,
101 MHz, ppm): = 139.93, 139.13, 129.54, 126.67, 106.22 (see
2.3. 5,8-dibromo-2,3-bis(4-(decyloxy) phenyl) pyrido[4,3-b]pyrazine
(3)
d
Fig. S1 in Supporting Informations).
A solution of 2,5-diromopyridine-3,4-diamine (1 g, 3.74 m mol)
and 1,2-bis(4-(decyloxy) phenyl) ethane-1,2-dione (1.955 g, 3.74 m
mol) which dissolved in glacial acetic acid (60 ml) was heated to
50 ꢁC and stirred for 12 h under argon atmosphere. At the end of the
reaction, yellow sediment was observed. Then the solution was
cooled to room temperature and filtered. The separated solid was
washed with distilled water several times and dried under vacuum
oven to give a yellow flocculent precipitate, yielded: 90%. 1H NMR
2.2. 1,2-bis(4-(decyloxy) phenyl) ethane-1,2-dione (2)
The synthesis of 2 needs two steps. Firstly, a mixture of 1,2-
bis(4-methoxyphenyl) ethane-1,2-dione (3 g, 11 m mol), glacial
acetic acid (30 ml) and HBr (48%, 100 ml) were added into a
three-neck round bottom flask orderly, heated to reflux and
stirred for 12 h. Then the mixture was cooled to room
temperature and brown precipitation was observed. Then the
precipitate was separated by filtration and washed with distilled
water three times to give 1,2-bis(4-hydroxyphenyl) ethane-1,2-
dione, yield: 75%. Secondly, 1,2-bis(4-hydroxyphenyl) ethane-1,2-
dione (2.0 g, 8.2 m mol), 1 - bromine decane (4.04g, 3.78 ml),
anhydrous potassium carbonate (2.4g, 0.24 mol) and Tetra-n-
butylammonium bromide (1.33 g, 4.13 m mol) were dissolved in
DMF (120 ml). The solution was heated to 120 ꢁC and reacted for
90 min. After reaction, the solution was cooled to room
temperature, added with water, oscillated by ultrasonic instru-
ment. The cream-color deposit was filtered and washed by
distilled water several times to give the target product (2) with a
high productivity (about 90%). 1H NMR (CDCl3, 400 MHz, ppm):
(CDCl3, 400 MHz, ppm):
4H, ArH), 4.00 (t, 4H), 1.80 (m, 4H), 1.42 (m, 28H), 0.88 (t, 6H). 13
NMR (CDCl3, 101 MHz, ppm): = 161.50, 161.18, 157.85, 156.77,
d= 8.68 (s, 1H), 7.67 (m, 4H, ArH), 6.88 (m,
C
d
146.85, 146.00, 136.82, 132.14, 131.87, 129.80, 120.16, 114.75, 68.43,
32.11, 29.57, 26.24, 22.89, 14.32 (see Fig. S3 in Supporting
Informations).
2.4. Synthesis of M1, M2 and M3
5,8-dibromo-2,3-bis(4-(decyloxy) phenyl) pyrido[4,3-b]pyra-
zine (2 g, 2.65 mmol) and tributyl(3-methylthiophen-2-yl) stan-
nane (5.3 mmol) or tributyl(3-methoxythiophen-2-yl) stannane
(5.3 mmol) or tributyl(3,4-dimethoxythiophen-2-yl) stannane
(5.3 mmol) were dissolved in anhydrous toluene (80 ml) and Pd
(PPh3)2Cl2 (0.2 g,2.85 mmol) was added at room temperature.
d
= 7.92 (m, 4H, ArH), 6.94 (m, 4H, ArH), 4.02 (t, 4H), 1.80 (m,
Fig. 1. Cyclic voltammogram curves of M1 (a), M2 (b) and M3 (c) in 0.2 M TBAPF6/DCM/ACN solution at a scan rate of 100 mV s ꢂ1 respectively. j denotes the current density. E
denotes the potential.

H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
235
Raise the temperature gradually until the solution was refluxed
(about 123 ꢁC). The mixture was stirred under argon atmo-
sphere for 24h, cooled and concentrated by evaporating toluene
under reduced pressure. The residue was purified by column
chromatography on silica gel using hexane- dichloromethane as
the eluent. The purified product M1 is bright red solid. 1H NMR
145.60, 139.96, 133.04, 132.28, 131.94, 130.96, 130.67, 123.41,
118.44, 114.55, 100.93, 99.88, 68.34, 61.17, 60.54, 57.48, 57.61,
32.09, 29.57, 26.25, 22.86, 14.27 (see Fig. S6 in Supporting
Informations).
3. Results and discussion
(CDCl3, 400 MHz, ppm):
d= 9.04 (s, 1H), 8.39 (s, 1H), 7.77 (m, 4H,
ArH), 7.66 (m, 1H), 7.11 (s, 1H), 7.15 (s, 1H), 6.91 (m, 4H, ArH),
4.01 (m, 4H), 1.81 (m, 4H), 1.39 (m, 12H), 1.29 (m, 12H), 0.91 (t,
3.1. Synthesis of monomers
6H). 13C NMR (CDCl3, 101 MHz, ppm):
d
=160.85, 160.54, 155.32,
The synthetic route to the monomers is shown in Scheme 2.
First, 3,4-diamino pyridine was brominated at the 2 and 5 positions
with bromine in 48% HBr to yield 3,4-diamino-2,5-dibromopyr-
idine(1) [18]. Second, 1,2-bis(4-methoxyphenyl) ethane-1,2-dione
was hydroxylated in the mixed solution of HAc/HBr and then
alkylated in the presence of DMF, 1 - bromine decane, anhydrous
potassium carbonate and Tetra-n-butylammonium bromide
(TBAB) to produce 1,2-bis(4-(decyloxy) phenyl) ethane-1,2-dione
(2). 1 and 2 were mixed in ice acetic acid solution to produce a
yellow flocculent precipitate (3) by a condensation reaction.
Finally, the condensation product was reacted with corresponding
tributylstannane in the presence of a catalytic amount of Pd
(PPh3)2Cl2 to generate the end product M1, M2 and M3 with
satisfactory yields, respectively.
152.95, 151.04, 143.39, 140.76, 139.83, 138.37, 137.62, 136.25,
133.24, 132.28, 131.88, 131.02, 130.68, 128.78, 126.83, 124.86,
124.45, 114.55, 68.35, 32.12, 29.62, 26.28, 22.91, 14.34 (see
Fig. S4 in Supporting Informations). The purified product M2 is
dark red solid. 1H NMR (CDCl3, 400 MHz, ppm):
d= 9.00 (s,1H),
8.30 (d, 1H), 7.70 (m, 4H, ArH), 6.90 (m, 4H, ArH), 6.52 (d, 1H),
6.46 (d, 1H), 4.00 (m, 4H), 3.97 (d, 6H), 1.80 (m, 4H), 1.35 (m,
28H), 0.90 (t, 6H). 13C NMR (CDCl3, 101 MHz, ppm):
d= 160.90,
160.58, 159.00, 158.65, 155.53, 153.25, 150.56, 143.18, 139.95,
136.35, 132.50, 132.24, 131.82, 130.93, 130.55, 124.22, 122.58,
118.39, 114.60, 103.28, 100.99, 68.35, 57.53, 32.12, 29.58, 26.28,
22.90, 14.34(see Fig. S5 in Supporting Informations). The
purified product M3 is also dark red solid. 1H NMR (CDCl3,
400 MHz, ppm):
d= 9.6 (s, 1H), 7.7 (m, 4H, ArH), 6.87 (m, 4H,
ArH), 6.49 (s, 1H), 6.45 (s, 1H), 3.91 (s, 6H), 3.85 (s, 6H), 3.77 (m,
4H), 1.79 (m, 4H), 1.33 (m, 28H), 0.89 (t, 6H). 13C NMR (CDCl3,
101 MHz, ppm): d= 160.83, 160.50, 155.09, 152.60, 151.16, 150.96,
Fig. 2. CV curves of the P1 (a), P2 (b) and P3 (c) film at different scan rates between 25 and 300 mV sꢂ1 in the monomer-free 0.2 M TBAPF 6/ACN/DCM solution, respectively.

236
H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
3.2. Electrochemical polymerization and characterization
electrode surface as well as non-diffusional redox process [19],
as shown in the figure insert. A redox process of p-doping was
observed between 0.45 V and 1.2 V for P1, between 0.04 V and 1.0 V
for P2 and between 0.65 V and 1.3 V for P3. In addition, to
investigate the stability characteristic, the three polymer films
were deposited on a Pt electrode using CV method in TBAPF6/DCM/
ACN as described above. Then the polymer films were cycled
between doped and neutral states for 1000 times with a scan rate
of 200 mV/sꢂ1 in 0.1 M lithium perchlorate/propylene carbonate
(PC) electrolyte/solvent couple (as shown in Fig. 3). From the
figure, after 1000 cycles, P2 still remained a high stability without
any considerable charge loss (about 1.4%). Although the stabilities
of P1 and P3 are not as steady as that of P2, they also had an
acceptable stability since P1 retained 87.2% of its electroactivity
and P3 retained 76.8% of its electroactivity both after regular
1000 cycles (see Fig. S7 in Supporting Informations). Then a
conclusion can be arrived at that all three polymer films have a
long-term switching stability between redox states, especially in
the case of P2, which makes them to meet the severe requirement
for electrochromic polymers, thus to be outstanding candidates for
commercial device applications.
As described previously, polymers of this system usually not
only have low band gap but also can reveal n-doped character.
However, systems that exhibit n-type doping are less commonly
obtained due to practically inaccessible reduction potentials and
instability of organic anions, especially to water and oxygen [13].
Fig. 4 displayed the both n-doping and p-doping process of the
three polymers at a scan rate of 100 m V sꢂ1. The potentials were
swept between -0.4 and 1.1 V for p-doped spectra, -0.40 and ꢂ1.8 V
for n-doped spectra for P2. Scanning intervals were 0.0 and 1.35 V
for p-doped spectra and 0.0 and ꢂ1.8 V for n-doped spectra for both
P1 and P3. As is shown in the picture, P2 has revealed perfect n-
doping process, and displayed an E1/2 of -1.18 V. In addition, the
redox peak of the n-doping/dedoping is much stronger than the p-
doping/dedoping process. This behavior has been noted previously
in similar systems and is attributed to both n-doping and a redox
transport mechanism with the lack of a capacitive character
indicative of a localized anionic state rather than the delocalized
state required for significant n-type conductivity [14,20]. Never-
theless, the n-doping processes of P1 and P3 were too weak to be
observed. This is most likely due to the the decomposition of the
polymers at the high negative potentials, which may catalyzed by
the presence of oxygen and traceful amount of waters in the
solvents, since the measurements were conducted in the ambient
temperature environment.
All of the three polymers were polymerized on the platinum
wire (the diameter is 0.5 mm) by cyclic voltammogram (CV)
with the same potential scan rate (100 mV s ꢂ1) to characterize
both the monomer oxidation potential and the polymer
electrochemistry during film deposition. Fig. 1 displayed the
continuous CV curves of the monomers including M1, M2 and
M3 in given concentration in ACN/DCM (1:1, by volume) solution
containing 0.2 M tetrabutylammonium hexafluorophosphate
(TBAPF6) as a supporting electrolyte. The first cycle of the curve
is attributed to the oxidation of the monomer. Irreversible
monomer oxidation peak was appeared at 1.26 V for M1, 1.0 V for
M2 and 1.1 V for M3 and the onset oxidation potentials (Eonset) of
M1, M2 and M3 were calculated as 1.17 V, 0.91 V and 1.1 V,
respectively. Concomitant increase in the current intensities
indicated the characteristic behavior of the formation of
a
deposited electroactive film on the working electrode surface. As
shown in Fig. 1, the voltammograms suggested that all three
monomers showed more than one oxidation process and a broad
reduction process. The CV curves of M1 showed a cathodic peak
at around 0.75 V and the corresponding two shoulder-like
oxidation peaks were found at about 0.81 V (Fig. 1a). Similarly,
the CV curves of M2 exhibited a reduction peak at about 0.63 V
and two oxidation peaks at about 0.67 V and 0.90 V respectively
(Fig. 1b), the CV curves of M3 displayed a reduction peak at
about 1.0 V and an obvious oxidation peak at about 1.15 V
(Fig. 1c). Compared to M1, the redox potentials were decreased
for M2/M3, which mainly due to the ability to contribute
electrons of 3-methoxy/3,4-dimethoxy group on thiophene ring
is much stronger than that of 3-methyl group. However, out of
our expectation, the onset oxidation potential (Eonset) of M3 is
slightly more positive than that of M2. This may be because of
the steric-hindrance effect. As can be seen from the structure of
M3, since two methoxyl groups are located on the adjacent
location of thiophene ring, the steric-hinerance effect is greatly
strengthened. This is likely to make a dent in their conjugative
effect, which leads to a higher Eonset
.
For the sake of detecting the properties of the polymer films, the
polymer-coated electrode was removed from the monomer
solution and rinsed with DCM/ACN before being immersed in an
electrolyte solution containing 0.2 M TBAPF6 in DCM/ACN. The
polymer films were then cycled between their redox potentials
ranges at different scan rates from 300 to 25 m V sꢂ1 (Fig. 2). A
linear increase in the peak currents as a function of the scan rates
confirmed well-adhered electroactive polymer films on the
Fig. 3. The first and the 1000th CV curve of the P2 film at a scan rate of 200 mV sꢂ1
in 0.1 M lithium perchlorate/propylene carbonate (PC) electrolyte/solvent couple.
Fig. 4. The n-doping process and p-doping prosess of the three polymers at a scan
rate of 100 m V sꢂ1
.

H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
237
cases, the emission maxima experienced a bathochromic shift
relative to their excitation maxima, indicating a Stokes shift values
of 61 nm, 71 nm, and 65 nm for M1, M2 and M3 respectively. The
relatively small red shift observed for the monomers suggests that
there is little geometric change between the ground and excited
states. It is interesting to find that the excitation and emission of all
the monomers lie in the visible region, implying favorable
photoluminescence properties of soluble monomers dissolved in
DMF [21].
3.4. Morphology
Scanning electron micrographs (SEM) of polymers provide their
clear surface and bulk morphologies, which are two important
aspects closely related to their optical and electrical properties,
such as charge transport and counterions storage capability. Fig. 6
gives the SEM images of P1, P2 and P3, which were prepared
potentiostatically in the solution of 0.2 M TBAPF
ACN/DCM
6
Fig. 5. Fluorescence emission spectra of the M1 (a), M2 (b) and M3 (c) dissolved in
DMF. Inset: the fluorescence excitation spectra of the three monomers.
containing relevant monomers on ITO electrodes with a charge of
0.1 C and dedoped before characterization. P1 film exhibits a
compact structure with lots of globules pervading on the surface
and the approximate diameters of these globules are in the range of
2000–5000 nm. P2 film shows a porous structure like coral grown
with small granules, some holes are also found on the surface.
While P3 film exhibits a globular cluster structure with scattered
holes between the clusters. The morphologies can facilitate the
movement of doping anions into and out of the polymer film
during doping and dedoping, which has a great influence on the
optical and electrical properties of polymer films. In addition, the
thicknesses of the three polymer films were measured by Step
Profiler ((see Fig. S8 in Supporting Informations)). The thicknesses
3.3. Fluorescence characteristics of monomers
Fluorescence test is another method to evaluate the character-
istics of monomers. The fluorescence spectra of the monomers
dissolved in DMF are illustrated in Fig. 5. The test results showed
that all three monomers are good shiners. M1, M2 and M3
exhibited a strong excitation peak in visible region at 521 nm,
529 nm and 524 nm, respectively. While their emission peaks were
observed at 582 nm, 600 nm and 589 nm, respectively. In all three
Fig. 6. SEM images of P1 (a), P2 (b) and P3 (c) deposited potentiostatically onto ITO electrode.

238
H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
assigned to a result of J-aggregation [23]. There was a bit red shift
observed when compared M1 with M2, which can be attributed to
the strong electron-donating group of 3-methoxythiophen.
However, comparing M3 to M2, a little small blue shift was
observed, which caused by the steric hindrance effect as
mentioned previously. Similarly, an obvious bathochromic shift
of the maximum absorption wavelengths was observed in the case
of P2 when compared with both P1 and P3.
Table 1 summarized the electrochemical parameters of the
three monomers and their corresponding polymer films, contain-
ing the onset oxidation potential (Eonset), maximum absorption
wavelength (lmax), low energy absorption edges (lonset), HOMO
and LUMO energy levels and optical band gap (Eg) values. The
HOMO energy level could be achieved by using the formula
HOMO = –e(Eox,onset + 0.02 + 4.4). Herein, the number 0.02 in the
formula is a correction parameter due to the reference electrode
used in the experiments was not standard electrode. So before and
after each experiment, the silver pseudo reference was calibrated
versus the ferrocene redox couple and then adjusted to match the
SCE reference potential. For DCM: ACN=1:1(v:v), the adjusted
value is 0.02 while for DCM: ACN=1:9(v:v), the value is -0.098.
Among the three monomers, M2 possesses the lowest band gap
while M3 owns the highest band gap. For comparing, the band gaps
of the three monomers were calculated by the density functional
theory (DFT) level employing the Gaussian 09 programs. The
ground-state electron density distribution of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) are illustrated in Fig. 8. The HOMO and LUMO
orbitals largely delocalized on the aromatic rings, forming a large
conjugated system. The obtained band gap values were also listed
Fig. 7. UV–vis spectra of M1,M2 and M3 in DCM. Insert: absorption spectra of the
corresponding polymers deposited on ITO at the neutral state.
are calculated as 823 nm, 708nm and 678 nm for P1, P2 and P3,
respectively. The rugged scanning curves indicated the uneven
surfaces of the films, which in good agreement with the
morphologies given above.
3.5. Optical properties of the monomers and polymer films
Fig. 7 illustrated the UV–vis spectra of M1, M2 and M3
monomers in DCM and their corresponding polymer films at the
neutral state. The films were potentiostatically electrodeposited
onto ITO glass with the same polymerization charge of 2.5ꢀ10ꢂ2
C
in a mixed solution of DCM/ACN containing 0.005 M monomers
and 0.2 M TBAPF6 at 1.35 V, 1.1 V and 1.35 V, respectively. After
polymerization, electrochemical dedoping of the films was
respectively performed at 0 V, -0.4 V and 0 V in a solution of
DCM/ACN containing only 0.2 M TBAPF6 for a while, then washed
with ACN/DCM for several times to remove the supporting
electrolyte and oligomers/monomers.
The shape of the absorption spectra for either monomers or
their corresponding polymers is quite similar to each other.
Without any exception, all three monomers exhibited two distinct
absorption bands, which could be accounted for the strongp-p*
transition and intramolecular charge transfer band, respectively
[22]. The two absorption peaks were appeared at 310 nm and
423 nm for M1, 313 nm and 427 nm for M2, 310 nm and 412 nm for
M3. Their corresponding polymer films also displayed two
absorption bands centered at 401 nm and 704 nm for P1, at
405 nm and 822 nm for P2, at 391 nm and 656 nm for P3.
Compared the monomer with its corresponding polymers, a
bathochromic shift was observed for all three cases, which may be
Fig. 8. The optimized geometries and the molecular orbital surfaces of the HOMOs
and LUMOs for M1, M2 and M3.
Table 1
The onset oxidation potential (Eonset), absorption onsets wavelength (lonset), HOMO and LUMO energy levels and optical band gap (Eg) of the monomers and their
corresponding polymers.
Compounds
Eonset,vs. (Ag wire)(V)
lonset(nm)
Eg,op(a)(eV)
HOMO(eV)
LUMO(b)(eV)
Eg,cal (c)(eV)
M1
M2
M3
P1
1.17
0.91
1.1
0.425
-0.03
0.76
–
540
541
536
890
1092
990
–
2.298
2.294
2.315
1.39
1.136
1.25
-5.59
-5.33
-5.52
-4.727
-4.39
-5.062
–
-3.292
-3.036
-3.205
-3.37
-3.254
-3.812
–
2.812
2.773
2.898
–
–
–
–
P2
P3
P(A)(d)
1.4
(a)
Eg,op=1241/lonset, in which lonset refers to the low energy absorption edges.
LUMO energy level was obtained by using the formula LUMO=–|HOMO-Eg|, where HOMO and Eg refer to the absolute value.
Eg,cal was calculated by DFT calculations.
See the reference [7].
(b)
(c)
(d)

H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
239
in Table 1. The highest band gap is observed for M3 and lowest is for
M2, which in well consistent with the band gaps size order
calculated from the lower energy absorptions. These values were
found to be nearly 0.47 to 0.58 eV higher than the values from
experimentally data. This is mainly due to various effects such as
solvent effects and other experimental conditions. The band gap of
M3 is higher than that of both M1 and M2, which hinted that steric
hindrance of substituents on the thiophene ring of the oligomers
may have decreased the population of the quinoidal electronic
configuration and weaken intermolecular charge transfer, then
affected the optical properties of the monomers.
From the Table 1, it can be seen that all three polymer films had
relatively low band gaps which made them to be good candidates
for electronic devices. The band gap of P1 was somewhat higher
than those of both P2 and P3, which can be assigned to the poor
electronic donating ability of the methyl group on the thiophene
ring. P2 had the lowest optical band gap (about 1.136 eV) among
them. It is generally accepted that increasing the planarity and
conjugation of aromatic systems leads to a decrease in the energy
gap. Out of our expectation, P3 had a higher band gap (about
1.25 eV) than P2. This can be attributed to that the introduction of
four methoxy-group substituents on the thiophene ring causes
steric hindrance, resulting in less order and less conjugation by the
blue shift in the absorption spectra and the increase in polymer's
band gap [22]. The results above suggest that as the donor unit, 3-
methyoxythiophen maybe have a more brilliant future than 3-
methylthiophen and 3,4-dimethylthiophen do. Nevertheless, the
band gap of anyone among the three polymers is lower than that of
the previously reported polymer P(A) [7], whose band gap value
was 1.4 eV, indicating that all three polymers are promising
candidates for organic photovoltaic applications.
3.6. Electrochromic properties of the polymers
UV-vis absorption spectra of three polymer films on ITO coated
slide glass (the active area was 0.9 cm ꢀ 3.0 cm) with the same
polymerization charge of 2.5ꢀ10ꢂ2C were performed by holding
the film at the desired potential and measuring the absorbance
from 350 to 2200 nm with applied potentials ranging from -0.4 V to
1.2 V for P2 and 0 V to 1.35 V for both P1 and P3 respectively for p-
doping in a monomer-free 0.2 M TBAPF6/ACN/DCM solution.
Fig. 9 revealed spectroelectrochemistry and the corresponding
colors of electrochemically prepared three polymer films at neutral
and doped states. At neutral state, all of the three polymer films
showed two absorption peaks, which was the nature of donor-
acceptor systems. The high-energy peaks, which are attributed to
the transitions from the thiophene-based valence band to its
antibonding band, are mainly absorbing in the UV-region, with
minor tailing into the visible region, limiting the contribution of
the high-energy peak to the color perceived by the human eye. On
the other hand, the low-energy peaks, originating from the D–A
interaction, have pronounced absorption in the red part of the
visible region, making the polymers appear blue in the reduced
state. For P2, two well-separated absorption peaks were located at
408 nm and 820 nm, with a deep valley at 520 nm, which gives the
polymer a green color as the green light is not absorbed and is
allowed to pass through the film. Moreover,
p-p* interaction
between donor and acceptor determines the match between these
units and plays a crucial role on the electronic properties of the
Fig. 9. The spectroelectrochemistry of films' p-doping processes on ITO electrode as applied potentials between 0 V and 1.35 V for P1 (a), between -0.4 V and 1.1 V for P2 (b)
and between 0 V and 1.35 V for P3 (c) in the monomer-free 0.2 M TBAPF6/ACN/DCM solution.

240
H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
Table 2
attributes: hue (a), saturation (b) and luminance (L). The values of
the relative luminance (L) and a, b were measured at the neutral
and oxidized states of the three polymers and summarized in
Table 2. The color is less transmissive for P2 than that of P1 in
oxidized state, which due to the polaronic absorption band of P1
tailed more into the visible region. It is worth mentioning that P2
film is unique in the literature with its highly saturated green color
in the neutral state and exceptional transparency in the oxidized
state. In addition, compared with P2, both the electron poor
homologue P1 and the electron strong homologue P3 revealed
blue shifted dominant wavelengths in visible and in NIR regions
which were consistent with electrochemical data.
The colorimetry analysis of the three polymer in both neutral and oxidized states.
Polymers
P1
E,vs. (Ag wire)(V)
L
a
b
Color
0
60
65
52
58
56
50
-12
–2
-16
–1
-7
1
2
27
3
2
Blue
Colorless
Green
Light gray
Blue
Light brown
1.35
-0.4
1.1
0
P2
P3
1.35
2
2
resulting polymers since it influences the intramolecular charge
transfer in these types of polymers. For P1 and P2, the intensities
for high-energy transitions and low-energy transitions are
comparable with each other, indicating the ideal match between
the donor and acceptor units and the strong interactions between
them. Additionally, isosbestic points at around 804 nm for P1 and
957 nm for P2 confirmed the coexistence of more than one charged
species absorbing in the visible region on the polymer chain.
Upon oxidation process of the three polymer films, the
absorptions in the visible region started to decrease and new
bands were intensified due to the formation of lower energy charge
carriers such as polarons and bipolarons on the polymer backbone,
along with a color change. In-situ spectroelectrochemical studies
for the polymer films showed that during oxidation the color of the
film changed from a transimissive light blue to colorless for P1,
from green to a transparent light gray color for P2 and from blue to
a transimissive brown color for P3. Color is made up of three
Based on the discussion above, a conclusion can be drawn that
the different structures of the polymers due to the difference in
donor moiety affect not only the monomer oxidation and the
polymer redox couple potentials but also the maximum absorption
wavelengths of the corresponding transition.
3.7. Electrochromic switching studies
Optical contrast (or percent transmittance, refers to the
percentage transmittance change ( T%) at a specified wave-
4
length), switching time (or response time, defined as the necessary
time for 95% of the full optical switch), stability (or cycle life) and
coloration efficiency are very important parameters to evaluate
electrochromic materials. They were analyzed by changes that
occurred in the transmittance (increments and decrements of the
absorption band at a specified wavelength with respect to time)
Fig.10. (a) Electrochromic switching, percent transmittance change monitored at 400, 700 and 2000 nm for P1 between 0 V and 1.35 V. (b) Electrochromic switching, percent
transmittance change monitored at 408, 820 and 2000 nm for P2 between -0.4 V and 1.1 V. (c) Electrochromic switching, percent transmittance change monitored at 1400 nm
for P3 between 0 V and 1.35 V.

H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
241
Table 3
thiophen derivatives (donor) were synthesized to understand the
effects of different electron donating groups on electrochemical
and optoelectronic properties. The results suggested that the
influence of donor units for D-A type polymer properties were
related to many factors such as electron-donating ability, steric
hindrance and the match between donor and acceptor units. At
neutral state, P1, P2 and P3 showed a transmissive light blue, green
and blue color, respectively. And all the three polymers are almost
colorless and highly transmissive in their oxidized states with a
strong absorption in the IR region. This property may ease several
applications for NIR electrochromic devices. Furthermore, P2 had
the lowest band gap (about 1.136 eV) among the three polymers,
which demonstrated the excellent match between the acceptor
unit and the donor one. In addition, all three polymers revealed
high optical contrast, fast switching time and satisfactory
coloration efficiency, especially in the case of P2. Taking all above
into account, the three polymers are outstanding candidates for
the potential realization of RGB-based polymer electrochromic
device applications.
The optical contrasts and response times from neutral state to oxidation state of the
three polymers.
Polymer
P1
l
/nm
Optical contrast/%
Response time/s
400
700
2000
408
820
2000
1400
1490
2000
11.1
20.1
68.1
16.7
25.9
61.3
47.7
50
0.88
0.88
0.46
0.61
0.55
0.35
2.23
–
P2
P3
P(A)
18
–
while switching the potential step wisely between the neutral
state and oxidized state with a residence time of 4 s.
Fig. 10 exhibited the optical contrast of the three polymer films
at different wavelengths. The polymer film was potentiostatically
deposited onto ITO-coated glass (active area: 0.9 cm ꢀ 3.0 cm) with
the polymerization charge of 1.5 ꢀ10 ꢂ2C in the same manner as
described above. After regular switching for 300 s, there was no
distinct loss in percent transmittance contrast observed, which
manifested the high stabilities of all three polymer films. The
optical contrast and the response times from reduced to oxidized
state of the three polymer films at different wavelengths were
summarized in Table 3. As is shown in Table 3, all three polymer
films revealed a relatively high optical contrast in NIR region which
hinted a very significant character for various NIR applications.
Comparing with P(A), our polymers have higher transmittance
values, which indicated the promising potential applications in
smart windows of polymers studied in this paper. P1 and P2 had
such fast switching time that less than 1 s both in the visible and
NIR region but the response time of P3 was more longer (over 2 s)
than that of P1 and P2. This is a further indication that the
electrochromism property of P3 is not as good as that of P1and P2.
Coloration efficiency (CE) is another key parameter to evaluate
the properties of polymer films as it describes the change in optical
absorbance at the wavelength of interest to the density of injected/
ejected charge [24]. It is defined as the change in the optical
Acknowledgements
The work was financially supported by the National Natural
Science Foundation of China (51473074, 31400044), the General
and Special Program of the postdoctoral science foundation China
(2013M530397, 2014T70861)
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.elec-
tacta.2014.09.022.
References
[1] R.J. Mortimer, A.L. Dyer, J.R. Reynolds, Displays 27 (2006) 2–18, doi:http://dx.
doi.org/10.1016/j.displa.2005.03.003.
[2] A. Azens, C.G. Granqvist, J. Solid State Electrochem. 7 (2003) 64–68, doi:http://
dx.doi.org/10.1007/s10008–002-0313–4.
[3] N. Stutzmann, R.H. Friend, H. Sirringhaus, Science 299 (2003) 1881–1884, doi:
http://dx.doi.org/10.1126/science.1081279.
density (
DOD) for the charge consumed per unit electrode area
[4] D.T. McQuade, A.E. Pullen, T.M. Swager, Chem. Rev. 100 (2000) 2537–2574, doi:
http://dx.doi.org/10.1021/cr9801014.
(D
Q) [25]. The calculating equations were given below:
ꢀ
ꢁ
[5] S. Möller, C. Perlov, W. Jackson, C. Taussig, S.R. Forrest, Nature 426 (2003) 166–
169, doi:http://dx.doi.org/10.1038/nature02070.
Tb
Tc
DOD
Q
D
OD ¼ log
and
h
¼
D
[6] W. Walker, B. Veldman, R. Chiechi, S. Patil, M. Bendikov, F. Wudl, Macro-
molecules 41 (2008) 7278–7280, doi:http://dx.doi.org/10.1021/ma8004873.
[7] M. Nikolou, A.L. Dyer, T.T. Steckler, E.P. Donoghue, Z.C. Wu, N.C. Heston, A.G.
Rinzler, D.B. Tanner, J.R. Reynolds, Chem. Mater. 21 (2009) 5539–5547, doi:
http://dx.doi.org/10.1021/cm902768q.
[8] A. Balan, D. Baran, G. Gunbas, A. Durmus, F. Ozyurt, L. Toppare, Chem.
Commun. (2009) 6768–6770, doi:http://dx.doi.org/10.1039/B912482A.
[9] A.P. Zoombelt, M.A.M. Leenen, M. Fonrodona, Y. Nicolas, M.M. Wienk, R.A.J.
Janssen, Polymer 50 (2009) 4564–4570, doi:http://dx.doi.org/10.1016/j.poly-
mer.2009.07.028.
[10] P. Sakthivel, H.S. Song, N. Chakravarthi, J.W. Lee, Y.S. Gal, S. Hwang, Polymer 54
(2013) 4883–4893, doi:http://dx.doi.org/10.1039/C4TA2585G.
[11] S. Toksabay, S.O. Hacioglu, N.A. Unlu, A. Cirpan, L. Toppare, Polymer, 55 309
(2014) 3093–3099, doi:http://dx.doi.org/10.1016/j.polymer.2014.05.029.
[12] A.T. Taskin, A. Balan, B. Epik, E. Yildiz, Y.A. Udum, L. Toppare, Electrochim. Acta
54 (2009) 5449–5453, doi:http://dx.doi.org/10.1016/j.electacta.2009.04.042.
[13] Y. Lee, Y.M. Nam, W.H. Jo, J. Mater. Chem. 21 (2011) 8583–8590, doi:http://dx.
doi.org/10.1039/C1JM10877H.
Where Tb and Tc are the transmittances before and after coloration,
respectively, and denotes the coloration efficiency (CE). The
h
coloration efficiency was measured as 94.8 cm2Cꢂ1 at 400 nm,
100.0 cm2Cꢂ1 at 700 nm and 269.7 cm2Cꢂ1 at 2000 nm for P1. And
for P2, CE was calculated as 87 cm2Cꢂ1 at 408 nm, 161.8 cm2Cꢂ1 at
820 nm, 309.6 cm2Cꢂ1 at 2000 nm. For P3, CE was counted as
507.7 cm2Cꢂ1 at 1400 nm. All the results calculated above were
reasonable coloration efficiency.
For electrochromic applications, the ability of a polymer to
switch rapidly and to exhibit a striking color change and maintain
high stability upon repeated cycles is important. Considering the
above discussion, a summing-up can be made that all three
polymer films have high optical contrast, fast switching time and
satisfactory coloration efficiency, which make them to be better
candidates in electrochromic display applications, especially in the
field of smart windows.
[14] C.J. DuBois, K.A. Abboud, J.R. Reynolds, J. Phys. Chem. B 108 (2004) 8550–8557,
doi:http://dx.doi.org/10.1021/jp037013u.
[15] G. Sonmez, I. Schwendeman, P. Schottland, K. Zong, J.R. Reynolds, Macro-
molecules 36 (2003) 639, doi:http://dx.doi.org/10.1021/ma021108x.
[16] G.E. Gunbas, A. Durmus, L. Toppare, Adv. Funct. Mater. 18 (2008) 2026–2030,
doi:http://dx.doi.org/10.1002/adfm.200701386.
[17] Z. Xu, M. Wang, J.S. Zhao, C.S. Cui, W.Y. Fan, J.F. Liu, Electrochim. Acta 125 (2014)
4. Conclusion
241–249, doi:http://dx.doi.org/10.1016/j.electacta.
[18] B.-L. Lee, T. Yamamoto, Macromolecules 32 (1999) 1375, doi:http://dx.doi.org/
10.1021/ma981013o.
In this study, three new type electrochromic polymers
containing pyrido[4,3-b]pyrazine group (acceptor) and different

242
H. Zhao et al. / Electrochimica Acta 146 (2014) 231–241
[19] M. Ak, M.S. Ak, G. Kurtay, M. Güllü, L. Toppare, Solid State Sci. 12 (2010) 1199–
1204, doi:http://dx.doi.org/10.1016/j.solidstatesciences.2010.03.013.
[20] C.J. DuBois, J.R. Reynolds, Adv. Mater. 14 (2002) 1844, doi:http://dx.doi.org/
10.1002/adma.200290016.
[23] H. Yamagata, F.C. Spano, J. Chem. Phys. 136 (2012) 184–901, doi:http://dx.doi.
org/10.1063/1.4705272.
[24] P.M. Beaujuge, J.R. Reynolds, Chem. Rev. 110 (2010) 268–320, doi:http://dx.doi.
org/10.1021/cr900129a.
[21] L.Y. Xu, J.S. Zhao, R.M. Liu, H.T. Liu, J.F. Liu, H.S. Wang, Electrochim. Acta 55
(2010) 8855–8862, doi:http://dx.doi.org/10.1016/j.electacta.2010.08.044.
[22] C.M. Amb, A.L. Dyer, J.R. Reynolds, Chem. Mater. 23 (2011) 397–415, doi:http://
dx.doi.org/10.1021/cm1021245.
[25] M. Deepa, A. Awadhia, S. Bhandari, Phys. Chem. Chem. Phys. 11 (2009) 5674–
5685, doi:http://dx.doi.org/10.1039/B900091G.