Two novel ambipolar donor-acceptor type electrochromic polymers with the realization of RGB (red-green-blue) display in one polymer

Article abstract of DOI:10.1039/c4ra11628c

Two novel electrochromic monomers, 4,7-bis(4-methoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine (MOTTP) and 4,7-bis(4-butoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine (BOTTP), were synthesized and electropolymerized to give the corresponding polymers PMOTTP and PBOTTP, respectively. For the investigation of their electrochemical and electrochromic properties, the polymers were characterized using cyclic voltammetry (CV), UV-vis spectroscopy, step profiling, and scanning electron microscopy (SEM). The band gaps of the polymers were calculated based on spectroelectrochemical analysis, and were found to be 0.950 eV and 1.088 eV for PMOTTP and PBOTTP, respectively. Electrochromic investigations showed that PMOTTP and PBOTTP showed similar multichromic behaviors: saturated green color in the neutral state, highly transmissive blue in the oxidized state, and saturated red in the reduced state (red-green-blue, RGB). In addition, both polymers have excellent switching properties with more than 60% optical contrast in the NIR region and about a 0.5 s response time from neutral to oxide. Moreover, via electrochemical and spectral analyses both polymers were proven to be n-type dopable polymers. Hence, both polymers are promising materials to complete RGB electrochromic polymers for commercial applications.

Full text of DOI:10.1039/c4ra11628c

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PAPER
Two novel ambipolar donor–acceptor type
electrochromic polymers with the realization of
Cite this: RSC Adv., 2014, 4, 61537
RGB (red-green-blue) display in one polymer†
b
b
b
Hui Zhao,a Daidi Tang,ab Jinsheng Zhao, Min Wang and Jianmin Dou
*
*
Two novel electrochromic monomers, 4,7-bis(4-methoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine
(MOTTP) and 4,7-bis(4-butoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine (BOTTP), were synthesized
and electropolymerized to give the corresponding polymers PMOTTP and PBOTTP, respectively. For the
investigation of their electrochemical and electrochromic properties, the polymers were characterized
using cyclic voltammetry (CV), UV-vis spectroscopy, step profiling, and scanning electron microscopy
(SEM). The band gaps of the polymers were calculated based on spectroelectrochemical analysis, and
were found to be 0.950 eV and 1.088 eV for PMOTTP and PBOTTP, respectively. Electrochromic
investigations showed that PMOTTP and PBOTTP showed similar multichromic behaviors: saturated
green color in the neutral state, highly transmissive blue in the oxidized state, and saturated red in the
reduced state (red-green-blue, RGB). In addition, both polymers have excellent switching properties with
more than 60% optical contrast in the NIR region and about a 0.5 s response time from neutral to oxide.
Moreover, via electrochemical and spectral analyses both polymers were proven to be n-type dopable
polymers. Hence, both polymers are promising materials to complete RGB electrochromic polymers for
commercial applications.
Received 1st October 2014
Accepted 4th November 2014
DOI: 10.1039/c4ra11628c
www.rsc.org/advances
mirrors3 for cars and electrochromic displays. As a type of
important electron-rich heterocycle-based polymer, poly-
Introduction
Since the discovery of the conductivity of doped polyacetylene,1
study in the eld of conducting polymers has attracted more
and more scientists to be engaged in it. In recent decades,
conjugated polymers have shown great potential for applica-
tions in organic electronic devices, such as photovoltaic
devices,2 light-emitting diodes (LEDs),3 eld effect transistors,4
sensors,5 and electrochromic devices.6–8 Electrochromism is
dened as a reversible change in the color or optical density of a
substance induced by a change in voltage or electrical poten-
tial.9,10 Although the earliest electrochromic devices were mostly
based on inorganic oxides, such as those of tungsten, iridium,
nickel,11 etc., the use of conjugated polymers as active layers in
electrochromic devices has received enormous attention
because of their high optical contrasts,12 fast switching
times13,14 and processability,15 and the ability to ne-tune their
band gaps through structure modication.7 To date, examples
of electrochromic devices include smart windows,16 rear-view
thiophene and its derivatives are the most commonly used
conjugated polymer materials for electrochromic purposes
because of their exibility towards synthetic modications.
Red, green and blue (RGB) are the three main colors for
display technology since all other subtractive colors can be
achieved by mixing these three.17 Among the RGB colors,
neutral state green polymers with highly transmissive oxidized
states are very difficult to obtain. This is because to obtain a
neutral green polymer, two absorption bands are necessary, one
in the red region of the spectrum, and the other in the blue
region, leaving an absorption trough in the green region of the
spectrum. Furthermore, the two absorption bands should be
controlled with the same applied potential, and disappear
simultaneously upon successive oxidation to give a bleached
transmissive oxidation state.18 The band gaps of the neutral
green to transmissive switching polymers are usually less than
1.2 eV, which can only be achieved by means of a donor–
acceptor approach. Donor–acceptor systems lead to a narrower
band gap because of resonances that enable a stronger double
bond character between the donor and acceptor units.19 Since
the report of the rst neutral green polymer in 2004 (a soluble
bithiophene-thienopyrazine copolymer from the Wudl group20
that had a non-transmissive oxidation state) a few green-to-
transmissive polymers have been reported by the Reynolds
group,21 the Toppare group,22 the Cihaner group,23 etc. These
aCollege of Chemical Engineering, China University of Petroleum (East China),
Qingdao, 266580, P. R. China
bShandong Key Laboratory of Chemical Energy Storage and Novel Cell Technology,
Liaocheng University, Liaocheng, 252059, P. R. China. E-mail: j.s.zhao@163.com;
jmdou@lcu.edu.cn
† Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
spectra of the intermediates and target compounds. See DOI: 10.1039/c4ra11628c
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studies open the way for commercialization of electrochromic a Ag wire (0.02 V vs. SCE.) as the pseudo-reference electrode.
display devices. However, in the context of low cost and high Before and aer each experiment, the silver pseudo reference
performance display devices, the main demand is becoming to was calibrated versus the ferrocene redox couple and then
achieve multi-colored redox achievable states from a single adjusted to match the SCE reference potential. Electrodeposi-
polymer. To date, the examples of neutral state green polymeric tion was performed using a 0.2 M solution of tetrabutylammo-
materials have still been very rare,24 and examples with addi- nium hexauorophosphate (TBAPF6) at a scan rate of 100 mV sꢀ1
tional multicolor properties are welcome.
for 20 cycles. Scanning electron microscopy (SEM) measure-
For the construction of D–A–D type monomers, ethyl- ments were taken using a Hitachi SU-70 thermionic eld
enedioxythiophene (EDOT) and thiophene derivatives are emission scanning electron microscope. The thickness and
usually used as the donor units. Meanwhile, 1,2,3-benzo- surface roughness of the polymer lms were measured on a
triazole, quinoxaline,25–27 thieno[3,4-b]pyrazine, 2,1,3-benzo- KLA-Tencor D-100 step proler. UV-vis-NIR spectra were recor-
thiadiazole (BTD) and benzoselenadiazole (BSE)28 are the most ded on a Varian Cary 5000 spectrophotometer connected to a
preferred acceptor-type units. The [1,2,5]thiadiazolo[3,4-c]pyri- computer. A three-electrode cell assembly was used for spec-
dine (PT) heterocycle has been employed as an electron- troelectrochemistry measurements where the working electrode
decient unit in the synthesis of polymer photovoltaic mate- was indium tin oxide (ITO) glass, the counter electrode was a
rials and panchromatic organic sensitizers for dye-sensitized stainless steel wire, and a Ag wire was used as the pseudo
mesoscopic solar cells.29,30 Compared with the BTD unit, PT is reference electrode. The polymer lms for spectroelec-
a stronger acceptor20 due to the presence of the additional trochemistry were prepared using potentiostatic deposition on
pyridine N-atoms.
ITO glass slides (the active area: 1.0 cm ꢁ 2.8 cm). The thickness
The donor–acceptor type polymer (PTBT) synthesized with of the polymer lms grown potentiostatically on ITO was
benzotriazole as the acceptor unit and thiophene as the donor controlled by the total charge passed through the cell and was
unit displayed all three additive colors (RGB) at different measured using a step proler. Digital photographs of the
oxidation states, and also showed both n- and p-dopable char- polymer lms were taken using a Canon Power Shot A3000 IS
acteristics.31 The discovery of RGB displaying polymers make it digital camera. Elemental analyses are determined using a
possible for the construction of a single component electro- Thermo Finnigan Flash EA 1112, CHNS-O elemental analysis
chromic display device. Very limited progress concerning the instrument. Mass spectrometry analysis was conducted using a
synthesis of RGB displaying polymers has been made since the Bruker maXis UHR-TOF mass spectrometer.
discovery of PTBT. By employing 3-alkoxy thiophene as the
donor unit and PT as the acceptor unit, we report here the
synthesis and electrochemical properties of two newly
Synthesis procedures
designed donor–acceptor type electrochromic polymers, poly-
2,5-Dibromopyridine-3,4-diamine. A mixture of pyrido-3,4-
[4,7-bis(4-methoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyri- diamine (2 g, 18.3 mmol) with aqueous HBr (48%, 30 ml) was
dine] (PMOTTP) and poly[4,7-bis(4-butoxythiophen-2-yl)-[1,2,5]- prepared in a 250 ml three-neck round-bottom ask with a
thiadiazolo[3,4-c]pyridine] (PBOTTP). Both polymers are green magnetic stirring bar inside. Aer the mixture was heated to 100
in their neutral states, highly transmissive blue in their oxidized ꢂC, bromine (2.5 ml) was added dropwise, and the solution was
states, and saturated red in their reduced states. The effects of stirred for 5 h at 135 ꢂC. The mixture was cooled to room
the length of the alkyl chains in the donor unit on the electro- temperature, then an aqueous solution of Na2S2O3, an aqueous
chemical and spectral behaviors of the resulting polymers are solution of Na2CO3, and distilled water were added in this order
also discussed in detail.
to get a yellow precipitate. The precipitate was separated by
ltration and washed with distilled water three times. Recrys-
tallization from a mixture of toluene–THF (v/v ¼ 5 : 1) gave 3,4-
diamino-2,5-dibrompyridine as a white occulent material
(yield ¼ 51%). Mp: 216–218 ꢂC. 1H NMR (400 MHz, DMSO,
Experimental
General
All chemicals unless indicated otherwise were purchased from d/ppm): 7.53 (s, 1H, pyridic-H), 5.99 (s, 2H, N–H), 5.03 (s, 2H,
commercial sources and used without further purication, N–H). 13C NMR (100 MHz, CDCl3, d/ppm): 139.93, 139.13,
except for tetrahydrofuran which was distilled over Na/ 129.54, 126.67, 106.22. MS (C5N3Br2H5) m/z: calcd for 266.9;
benzophenone prior to use. 2,5-Dibromopyridine-3,4- found 266.8. Anal. calcd for (C5N3Br2H5): C, 22.5; N, 15.7; Br,
diamine,32,33 4,7-dibromo-[1,2,5]thiadiazolo[3,4-c]pyridine32,33 59.9; H, 1.9. Found: C, 22.4; N, 15.7; Br, 60.0, H, 1.9.
and tributylstannane34 compounds were prepared according to
4,7-Dibromo-[1,2,5]thiadiazolo[3,4-c]pyridine. To a 100 ml
the literature methods. 1H NMR and 13C NMR spectroscopy three-neck round-bottom ask equipped with a magnetic stir-
studies were carried out on a Varian AMX 400 spectrometer and ring bar, 2,5-dibromopyridine-3,4-diamine (1.1 g, 3.8 mmol)
the chemical shis (d) were given relative to tetramethylsilane and pyridine (12 ml) were added successively under a nitrogen
as the internal standard. Electrochemical synthesis and exper- atmosphere. SOCl2 (0.7 ml) was then added dropwise, and the
ꢂ
iments were performed in a one-compartment cell with a CHI resulting mixture was stirred at 0 C until a deep pink-colored
760C Electrochemical Analyzer controlled by a computer, solution was obtained. This was followed by reux at 80 ꢂC
employing a platinum wire with a diameter of 0.5 mm as the overnight. The reaction mixture was allowed to cool to room
working electrode, a platinum ring as the counter electrode, and temperature. The reaction mixture was ltered, washed with
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water repeatedly and dried over anhydrous MgSO4. The crude
mixture was puried by column chromatography on silica gel by
eluting with hexane–dichloromethane (4 : 1, v/v) to give a yellow
1
solid. Mp: 115 ꢂC. H NMR (300 MHz, CDCl3, d/ppm): 8.552 (s,
1H, pyridic-H). 13C NMR (100 MHz, CDCl3, d/ppm): 162.79,
160.88, 153.17, 139.18, 96.22. MS (C5SN3Br2H) m/z: calcd for
294.9; found 294.7. Anal. calcd for (C5SN3Br2H): C, 20.4; S, 10.9;
N, 14.2; Br, 54.2; H, 0.3. Found: C, 20.3; S, 10.8; N, 14.3; Br, 54.3;
H, 0.3.
General procedure for the synthesis of MOTTP and BOTTP via
Stille coupling
4,7-Dibromo-[1,2,5]thiadiazolo[3,4-c]pyridine (1.00 g, 3.4 mmol)
and an excess of tributyl(3-methoxythiophen-2-yl)stannane (17
mmol) or tributyl(3-butoxythiophen-2-yl)stannane (17 mmol)
with Pd(PPh3)2Cl2 (0.238 g, 0.34 mmol) as the catalyst were
dissolved in anhydrous toluene (80 ml) at room temperature.
The solution was stirred under a nitrogen atmosphere for 30
min. The temperature was raised immediately until the solution
was at reux. The mixture was stirred under a nitrogen atmo-
sphere for 24 h, cooled and concentrated on a rotary evaporator.
Finally, the residue was puried by column chromatography on
silica gel using hexane–dichloromethane as the eluent.
4,7-Bis(4-methoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]-
pyridine (MOTTP). The crude mixture was puried by column
chromatography on silica gel by eluting with hexane–dichloro-
methane (1 : 2, v/v) to give MOTTP as a red solid (0.8 g, 67%).
Mp: 186 ꢂC. 1H NMR (400 MHz, CDCl3, d/ppm): 8.77 (s, 1H,
pyridic-H), 8.34 (d, 1H, thienyl-H), 7.77 (d, 1H, thienyl-H), 6.56
(d, 1H, thienyl-H), 6.42 (d, 1H, thienyl-H), 3.89 (d, 6H, –OCH3).
13C NMR (100 MHz, CDCl3, d/ppm): 159.55, 159.17, 146.28,
140.50, 135.27, 123.34, 120.02, 103.19, 99.27, 57.56. MS
(C13H11N3S3O2) m/z: calcd for 337.4; found 337.5. Anal. calcd for
(C13H11N3S3O2): C, 46.3; N, 12.4; O, 9.5; S, 28.5; H, 3.3. Found: C,
46.5; N, 12.4; O, 9.4; S, 28.3; H, 3.4.
Scheme 1 Synthetic route for the monomers.
In the second step, 4,7-dibromo-[1,2,5]thiadiazolo[3,4-c]pyri-
dine was obtained through oxidation with an excess amount of
SOCl2 in pyridine as the solvent. The organotin compound was
synthesized following previous literature.35 In the last step, the
Stille coupling reaction was achieved in anhydrous toluene in
the presence of a catalytic amount of Pd(PPh3)2Cl2. The reac-
tions proceeded quite nicely to afford MOTTP and BOTTP in
satisfactory yields.
4,7-Bis(4-butoxythiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyri-
dine (BOTTP). The crude mixture was puried by column
chromatography on silica gel by eluting with hexane–dichloro-
methane (1 : 3, v/v) to give BOTTP as a red solid (0.76 g, 50%).
Mp: 149 ꢂC. 1H NMR (400 MHz, CDCl3, d/ppm): 8.78 (s, 1H,
pyridic-H), 8.38 (d, 1H, thienyl-H), 7.78 (d, 1H, thienyl-H), 6.55
(d, 1H, thienyl-H), 6.41 (d, 1H, thienyl-H), 4.03 (m, 4H, –OCH2),
1.81 (m, 4H, –CH2), 1.33 (m, 4H, –CH2), 0.99 (t, 6H, –CH3). 13C
NMR (100 MHz, CDCl3, d/ppm): 158.47, 154.97, 146.34, 139.66,
135.01, 123.89, 120.29, 103.62, 99.61, 70.22, 31.53, 19.47, 14.05.
MS (C13H23N3S3O2) m/z: calcd for 349.5; found 349.4. Anal. calcd
for (C13H23N3S3O2): C, 44.7; N, 12.0; O, 9.2; S, 27.5; H, 6.6.
Found: C, 44.5; N, 12.1; O, 9.2; S, 27.7; H, 6.5.
Electrochemical polymerization
The electrochemical properties of the monomers and their
polymers were examined using cyclic voltammetry. Both poly-
mers were deposited on Pt wire using cyclic voltammetry with a
single scan rate (100 mV sꢀ1) in an acetonitrile (ACN)–
dichloromethane (DCM) (1 : 1, by volume) solvent mixture
containing 0.2 M tetrabutylammonium hexauorophosphate
(TBAPF6) as the supporting electrolyte and 0.005 M of the
monomers. Voltammetry curves for the repeated scanning
electropolymerization of MOTTP and BOTTP are shown in
Fig. 1. The rst cycle of the CV test corresponded to oxidation of
the monomer and the onset oxidation potentials (Eonset) of
MOTTP and BOTTP are 1.06 and 1.07 V, respectively. During the
repetitive anodic potential scan, the generation of a new quasi-
reversible redox couple (Eopx ¼ 0.77 V, Erped ¼ 0.62 V for MOTTP
and Eopx ¼ 0.81, Erped ¼ 0.82 V for BOTTP) and the increase in the
Results and discussion
Synthesis of monomers
The synthesis of MOTTP and BOTTP was carried out with slight current intensity of this quasi-reversible redox peak indicated
modications of the well-established literature procedures the formation of highly electroactive polymers on the surface of
(Scheme 1). The rst step of this route involved the bromination the working electrodes. The oxidation potential of MOTTP was
of pyridine-3,4-diamine in the presence of an HBr/Br2 mixture. lower than that of BOTTP since the electron donating effect of
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Fig. 1 Cyclic voltammogram curves of MOTTP (a) and BOTTP (b) in
ACN–DCM (1 : 1) containing 0.2 M TBAPF6 at a scan rate of 100 mV sꢀ1
.
BOTTP decreased due to the sterically bulky groups introduced
by the butyl chains , resulting in a less efficient overlap between
the p-orbital of the oxygen atom and the conjugated system. It
can thus be concluded that the redox reaction is highly sensitive
to the steric and electronic effects of the substituent.35
The electrochemical behaviors of the polymer lms
(prepared on platinum wires by sweeping the potentials for
three cycles) were characterized using CV at different scan rates
between 25 and 300 mV sꢀ1 in monomer-free electrolyte solu-
tion. The CV curves of PMOTTP and PBOTTP at different scan
rates between 25 and 300 mV sꢀ1 during the p-doping process
are shown in Fig. 2a and b, respectively. A couple of redox peaks
with an oxidation potential of 0.79 V and a reduction potential
of 0.74 V were clearly observed during the p-doping process for
PMOTTP. A couple of redox peaks for PBOTTP were located at
0.91 and 0.80 V. The scan rate dependence of the peak current
density is illustrated in Fig. 2c. As seen in Fig. 2c, the peak
current densities show a linear relationship as a function of
scan rate for both polymers, which indicates that the electro-
chemical processes are not diffusion limited and are quasi-
reversible even at high scan rates.8 The CV curves of PMOTTP
and PBOTTP at a scan rate of 100 mV sꢀ1 during the n-doping
and p-doping processes are shown in Fig. 2d. A quasi-
reversible p-style doping and a quasi-reversible n-style doping
Fig. 2 CV curves of the PMOTTP (a) and PBOTTP (b) films at different
scan rates between 25 and 300 mV sꢀ1 in the monomer-free 0.2 M
TBAPF6–ACN–DCM solution. (c) Scan rate dependence of the anodic
and cathodic peak current densities for the p-doping/dedoping
process. jp.a and jp.c are the anodic and cathodic peak current densities,
respectively. (d) The CV curves of PMOTTP and PBOTTP at scan rate
100 mV sꢀ1 during n-doping and p-doping processes.
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can be seen. In the reduction region, a sharp redox peak
(Eopx ¼ ꢀ0.87 V, Erped ¼ ꢀ0.78 V for PMOTTP and Eopx ¼ ꢀ0.88,
Erped ¼ ꢀ0.79 V for PBOTTP) is observed which indicates that the
polymer has an n-dopable character. Furthermore, it is notable
that the redox peaks of the n-doping/dedoping were much
stronger than those of the p-doping/dedoping process. The
results indicate that [1,2,5]thiadiazolo[3,4-c]pyridine (PT) is a
strong electron acceptor and PMOTTP and PBOTTP are good n-
type conjugated polymers. As clearly stated in literature,36
negatively doped waves in CV do not necessarily mean that a
true n-type doping process is available. Hence, additional
support must be given via spectroelectrochemistry studies.
The stability and robustness of the polymer lms are quite
important parameters for them to be amenable for use in
numerous practical applications. The stability of each polymer
lm without purging the electrolyte solution with an inert gas
was investigated in monomer-free electrolyte solution by
applying potential pulses. The polymer lms were prepared on
Pt wires by sweeping the potentials for ve cycles at 100 mV sꢀ1
in an acetonitrile (ACN)–dichloromethane (DCM) (1 : 1, by
volume) solvent mixture containing 0.2 M TBAPF6 and 0.005 M
of the monomers, and their cyclic voltammograms were recor-
ded with 1000 cycles at 200 mV sꢀ1 in monomer-free electrolyte
solution. The total charge involved during the electrochemical
process was calculated for each voltammogram cycle. Fig. 3
shows the change in the CV curves for the PMOTTP and
PBOTTP lms between the 1st and 1000th cycles. As depicted in
Fig. 3a, the loss of the total charge of PMOTTP was approxi-
mately 10% between the initial and 1000th cycles. For PBOTTP,
the loss of the total charge was approximately 11% between the
initial and 1000th cycles (see Fig. 3b). The stability test was
carried out without the exclusion of air in order to test the
environmental stability of the polymers, since the presence of
air has been supposed to have a detrimental effect on the
stability of materials.12 We suspect that the observed loss is
most probably due to the degradation reactions catalyzed by the
presence of oxygen and trace water in the environment. On the
other hand, the electropolymerization procedure under envi-
ronmental conditions may lead to the formation of oligomers,
which impedes good lm-forming properties and the adhesion
to the substrate.
Fig. 3 Electrochemical stability of PMOTTP (a) and PBOTTP (b) in
monomer-free ACN–DCM (1 : 1) containing 0.2 M TBAPF6 after 1000
switching cycles by CV.
lm during the doping and dedoping processes, in good
agreement with the good redox activity of the lms.37
The thickness and roughness of the polymer lms were
investigated using a step proler. The thickness images are
shown in Fig. 4b. As seen from Fig. 4b, the thicknesses of
PMOTTP and PBOTTP were 830 nm and 1002 nm, respectively.
The images of the step proler measurements reveal that the
polymer lms have extremely rough surfaces with detectable
pits, which is in good agreement with the morphologies shown
in the SEM images. Fig. 4b also reveals that PMOTTP has a lower
average roughness value compared to PBOTTP due to the
compact morphology of PMOTTP.
Morphology and thickness
The surface morphologies of the polymer lms were analyzed
using SEM. The lms were prepared using a constant potential
in an ACN–DCM (1 : 1, by volume) solution containing 0.2 M
TBAPF6 and 0.005 M of the relevant monomer on an ITO elec-
Optical properties of the monomers and lms
trode with the same polymerization charge of 3.0 ꢁ 10ꢀ2 C and The UV-vis absorption spectra of the monomers dissolved in
were dedoped before characterization. The SEM images of these CH2Cl2 and the corresponding dedoped polymer lms depos-
polymer lms are shown in Fig. 4a. The PMOTTP lm exhibits a ited on ITO electrodes were examined. The absorption spectra
compact morphology with a stack of granules. The diameters of of these novel monomers and lms in their neutral states are
the granules range from 4 to 24 mm. In contrast, the PBOTTP shown in Fig. 5. As the Fig. 5 shows, both monomers exhibited
lm exhibits an irregular coral-like porous structure grown with two characteristic absorption bands as the typical features of
small granules, and the approximate diameters of the granules donor–acceptor conjugated compounds, which were assigned
are in the range of 2–12 mm. These morphologies can facilitate to the p–p* transition and intramolecular charge transfer,
the movement of doping anions into and out of the polymer respectively. Two obvious absorption peaks were observed at
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Fig. 5 UV-vis absorption spectra of MOTTP and BOTTP in DCM. Inset:
absorption spectra of the corresponding polymers deposited on ITO in
their neutral states.
308 and 482 nm for MOTTP, and 309 and 487 nm for BOTTP.
Meanwhile, the optical band gaps (Eg) of the monomers were
precisely calculated from the low energy absorption edges
(lonset) (Eg ¼ 1241/lonset). The Eg values of MOTTP and BOTTP
based on lonset (lonset ¼ 563 nm for MOTTP, 561 nm for BOTTP)
were calculated to be 2.20 eV and 2.21, eV respectively.
Compared with MOTTP, BOTTP has a small blue shi in the low
energy absorption band and a slightly higher band gap due to
the fact that the introduction of a longer butyl chain in place of
methyl causes greater steric hindrance, resulting in less order
and less conjugation.
The UV-vis absorption spectra of the neutral-state lms are
shown in the inset of Fig. 5. The location of the absorption
maxima and intensity of the main p–p* peak give some insight
into the polymer color. As seen from the inset of Fig. 5, both
polymers presented two well-separated maximal absorption
peaks in the visible region which were assigned to the strong
p–p* transition and intramolecular charge transfer in the
neutral state. The absorption peaks were located at 435 nm and
850 nm for PMOTTP and for PBOTTP were located at 432 nm
and 848 nm, which are essential values for a neutral green state.
A minimum absorption is observed at around 514 nm for
PMOTTP, and the difference in the transmittances of the peaks
at 435 and 850 nm is excellent for producing a highly saturated
green color. PBOTTP exhibited
a
similar spectroelec-
trochemistry behavior. The Eg values of PMOTTP and PBOTTP
based on lonset (1300 nm for PMOTTP, 1140 nm for PBOTTP)
were calculated to be 0.95 eV and 1.088 eV, respectively (see
Table 1). As shown in Table 1, PMOTTP has an apparent red
shi in the low-energy absorption wavelengths and a lower
band gap than PBOTTP. It is suggested that the introduction of
the butyl group on the donor unit increased the repulsive steric
effect on neighboring repeated units, which nally brought
about a decrease in effective conjugation length in the homo-
polymer and a slightly higher band-gap.35 Moreover, the poly-
mers experienced a red shi in the absorption maxima
compared to the corresponding monomers, which is attribut-
able to the inter-chain interaction caused by p-stacking of the
Fig. 4 SEM images of (a1) PMOTTP and (a2) PBOTTP, and step profiler
images of (b1) PMOTTP and (b2) PBOTTP deposited potentiostatically
onto ITO electrodes.
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Table 1 The onset oxidation potential (Eonset), maximum absorption wavelength (lmax), absorption onset wavelength (lonset), HOMO and LUMO
energy levels and optical band gap (Eg)
vs. (Ag-wire)
lonset (nm)
Compounds
Eonset (V)
lmax (nm)
Ega (eV)
Egd (eV)
HOMOb (eV)
LUMOc (eV)
MOTTP
BOTTP
PMOTTP
PBOTTP
PBDTe
1.06
1.07
0.039
0.035
—
308 482
309 487
435 850
432 848
428 755
563
561
1300
1140
1043
2.20
2.21
0.95
1.088
1.191
1.956
2.032
—
—
—
ꢀ5.48
ꢀ5.49
ꢀ4.459
ꢀ4.455
—
ꢀ3.28
ꢀ3.28
ꢀ3.509
ꢀ3.367
—
a
b
c
Calculated from the low energy absorption edges (lonset), Eg ¼ 1241/lonset
.
HOMO ¼ ꢀe(Eonset + 4.4) (Eonset vs. SCE). Calculated by the
subtraction of the optical band gap from the HOMO level. Eg is calculated based on DFT. e Data were taken from ref. 38.
d
polymers. The parameters of 4,7-bis(2,3-dihydrothieno[3,4-b]- the monomers. The PMOTTP and PBOTTP lms were electro-
[1,4]dioxin-7-yl)benzo[c][1,2,5]thiadiazole (PBDT)38 are also lis- deposited onto ITO with the same polymerization charge of 2.0
ted in Table 1 in order to compare it with the newly prepared ꢁ 10ꢀ2 C at 1.20 V and 1.25 V, respectively. Aer polymerization,
polymers. In comparison, the optical band gaps of PMOTTP and electrochemical dedoping was carried out at 0 V in monomer-
PBOTTP are somewhat lower than that of PBDT which free ACN–DCM (1 : 1, by volume) containing 0.2 M TBAPF6,
demonstrated that PMOTTP and PBOTTP were more and then the in situ electronic absorption spectra of the polymer
outstanding D–A type conducting polymers than PBDT.38
lms upon stepwise oxidation were investigated. Fig. 7 shows
To obtain the precise electrical band gaps of the monomers, the spectroelectrochemistry and corresponding colors of the
DFT calculations were carried out on the density functional PMOTTP and PBOTTP lms. As seen from Fig. 7, both polymers
theory (DFT) level, employing the Gaussian 05 program. The exhibited two transitions due to their donor–acceptor nature in
ground-state electron density distributions of the highest the neutral state. The two transitions in the D–A type polymers
occupied molecular orbital (HOMO) and lowest unoccupied were attributed to the transitions from the thiophene-based
molecular orbital (LUMO) are illustrated in Fig. 6. The values of valence band to its antibonding counterpart (high-energy tran-
the HOMO and LUMO are ꢀ5.627 and ꢀ3.666 for MOTTP, and sition) and to the substituent-localized conduction band (low
ꢀ5.586 and ꢀ3.554 for BOTTP. The band gaps are calculated to energy transition). Hence, interactions between donor and
be 1.956 and 2.032 for MOTTP and BOTTP respectively. These acceptor units (their match) determined the energy and inten-
values were found to be lower than the values from the experi- sity of these transitions.39 The intensities of the low energy
mental data. This is mainly due to various effects, such as transitions for the polymers are apparently higher than those of
solvent effects, and to variation in the solid state compared to the corresponding high-energy transitions, as observed from
the gaseous states. However, the band gap of MOTTP based on the absorption peak in the visible region.
DFT calculations is slightly lower than that of BOTTP, which is
exactly consistent with the band gaps based on optical thin polymer lms, in situ spectroelectrochemical measure-
experiments. ments were performed wherein the absorption spectra were
To further elucidate the optoelectronic properties of these
In view of the device and high-performance display appli- measured as increasingly higher oxidizing potentials were
cations, the optical properties of the polymers should be man- applied across the polymer lm. As shown in Fig. 7-a1, upon
ifested by using the changes in the optical absorption spectra oxidation of the polymer PMOTTP, the intensity of the absorp-
under various voltage pulses. Potentiostatic deposition was tion of the two p–p* transition bands (centered at 435 and 850
performed in an ACN–DCM (1 : 1, by volume) solution con- nm) started to decrease simultaneously with a concomitant
taining 0.2 M TBAPF6 as a supporting electrolyte and 0.005 M of increase in the near-IR region (beyond 1400 nm), indicating the
formation of polarons40 and bipolarons.41 In the fully oxidized
state of the polymer PMOTTP (1.2 V), the newly formed lower
energy electronic transitions lie sufficiently outside the visible
region with little tailing from the NIR, which makes the polymer
present a highly transmissive blue color in the oxidized states.
In this case, PMOTTP can switch between the neutral green
color to the highly transmissive blue color in the oxidized state.
As shown in Fig. 7-b1, the PBOTTP lm experienced a similar
trend in change to that of PMOTTP as the potential stepped
from 0 V (neutral state) to 1.25 V (oxidized state). The UV-vis
spectra for PMOTTP and PBOTTP (Fig. 7) display well-dened
isosbestic points at approx. 564 nm and 558 nm, respectively,
indicating that the PMOTTP and PBOTTP polymers were being
Fig. 6 The ground-state electron density distributions of the highest
occupied molecular orbital (HOMO) and lowest unoccupied molec-
ular orbital (LUMO).
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interconverted between two distinct forms on both occasions:
the neutral form and radical cation.39
Conducting polymers with stable negatively doped states
receive tremendous interest, since a more complicated device
structure such as those of LEDs3 and ambipolar eld effect
transistors can be attained with these materials. In addition to
reduction waves in CV, evidence for charge carrier formation
upon reduction should also be studied by spectroelec-
trochemistry. Absorption spectra of the polymers at the reduc-
tion potential of ꢀ1.6 V were investigated to characterize the
optical changes that occurred during the n-doping process and
prove the introduction of charge carriers to the conjugated
systems in the n-doped state. As shown in Fig. 7, upon the
switch from the neutral to the reduced state, several similar
changes have been seen for the UV-vis-NIR spectra of the two
polymers, including a moderate absorption increase in the NIR
region, a weakening or even disappearance of the absorption
band at around 850 nm, a red-shi in the p–p* transition band,
and the appearance of apparent and well-dened absorption
peaks at around 500 nm in the visible region. The maximum
absorption peaks were centered at 508 nm and 512 nm for
PMOTTP and PBOTTP, respectively, which gives rise to a satu-
rated red color in the reduced states of the polymers. Hence,
with the strong absorption changes in the NIR region and the
CV waves observed at negative potentials, it is clear that both
PMOTTP and PBOTTP are displaying true n-type doping
processes.37 It is interesting that these polymers are unique in
the literature to date with their highly saturated green color in
the neutral state, highly transmissive blue colour in the oxidized
state and saturated red colour in the reduced state. To further
establish the colors of the polymers, the colorimetric properties
were characterized using CIE1976 color space (L*a*b*). The
values of the relative luminance (L), hue (a) and saturation (b)
were measured in the neutral, oxidized and reduced states of
the polymers and are summarized in Table 2.
Switching properties
The stabilities, optical contrasts, and response times of the
polymer lms upon electrochromic switching between their
neutral and oxidized states have been monitored both in the
visible and NIR regions. The studies were performed with a
switching interval of 4 s at their dominant wavelengths by multi-
potential steps, switching repeatedly between their fully neutral
and oxidized states in a monomer-free ACN–DCM (1 : 1, by
volume) solution containing 0.2 M TBAPF6 as a supporting
electrolyte. The optical contrasts, response times and coloration
efficiencies of PMOTTP, PBOTTP and PBDT based on electro-
chromic switching at different given wavelengths are shown in
Table 3. PBDT was used for comparison with the new polymers.
The optical contrast (DT%), one of the crucial factors in
appraising an electrochromic material, is dened as the percent
transmittance change at a specied wavelength between the
redox states. Fig. 8 shows the electrochromic switching
Fig. 7 (a1) p-doping: spectroelectrochemistry of PMOTTP films on ITO
electrode with applied potentials between 0 V and 1.2 V in the monomer-
free 0.2 M TBAPF6–ACN–DCM solution. (a2) n-doping: spectroelec-
trochemistry of PMOTTP films at ꢀ1.6, 0 and 1.2 V. (b1) p-doping: spec-
troelectrochemistry of PBOTTP films on ITO electrode with applied
potentials between 0 V and 1.25 V in the monomer-free 0.2 M TBAPF6–
ACN–DCM solution. (b2) n-doping: spectroelectrochemistry of PBOTTP
films at ꢀ1.6, 0 and 1.25 V.
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Table 2 The values of L, a, b for the polymers in neutral, oxidized and
reduced states
properties of PMOTTP between 0 and 1.2 V and PBOTTP
between 0 and 1.25 V at two different wavelengths in both
visible and NIR regions. The optical contrasts of PMOTTP (24%
at 430 nm, 70% at 1550 nm) and PBOTTP (32% at 435 nm, 61%
at 1500 nm) are satisfactory. Compared with the structural
analogue polymer PBDT, the newly prepared polymers have
comparable optical contrasts in both the visible and the NIR
region. The outstanding optical contrast in the NIR region is a
very signicant property for many NIR applications. Aer 1000
cycles of switching, the polymers kept working without signi-
cant loss in the percent transmittance contrast value. This
indicated that the polymers have quite high optical stabilities,
retaining 96% at 430 nm and 97% at 1550 nm for PMOTTP and
94% at 435 nm and 95% at 1500 nm for PBOTTP aer 1000
cycles of switching.
E, vs.
Polymers
PMOTTP
(Ag wire) (V)
L
a
b
Colors
0
1.2
61
60
42
63
60
43
ꢀ9
ꢀ5
43
ꢀ17
ꢀ4
25
11
Saturated green
Transmissive blue
Red
Saturated green
Transmissive blue
Red
ꢀ4
ꢀ1
13
ꢀ1
2
ꢀ1.6
PBOTTP
0
1.25
ꢀ1.6
Table 3 The optical contrasts (DT%), response times and coloration
efficiencies (CE) of PMOTTP, PBOTTP and PBDT at different
wavelengths
Response time, another of the most important characteris-
tics of electrochromic materials, is the necessary time for 95%
of the full optical switch to occur (aer which the naked eye
could not sense the color change).42 PMOTTP has excellent
switching times from neutral to oxide of 0.439 s at 430 nm and
0.438 s at 1550 nm. PBOTTP has comparable switching times of
0.510 s at 435 nm and 0.550 s at 1500 nm. In comparison, it can
be easily found that the PMOTTP lm showed a slightly faster
switch time than that of PBOTTP. The faster switching response
could be ascribed to the faster dopant ion diffusion during the
switch time of the new polymers, which is faster than that of
PBDT in NIR region and is comparable in the visible region.
The coloration efficiency (CE) is also an important charac-
teristic for electrochromic materials, which is dened as the
relation between the injected/ejected charge as a function of
electrode area (DQ) and the change in optical density (DOD) at a
specic dominant wavelength (lmax) as illustrated by the
following equation:40
Optical
Response Coloration efficiency
Compounds l (nm) contrast (DT%) time (s)
(CE, cm2 Cꢀ1
)
PMOTTP
PBOTTP
PBDTa
430
1550
435
1500
428
24
70
32
61
37
23
72
0.44
0.44
0.51
0.55
<1
76.9
249
99
259
—
755
1500
0.40
z1
—
—
a
Data were taken from ref. 39.
ꢀ
ꢁ
Tb
Tc
DOD ¼ log
DOD
DQ
h ¼
where the Tb is the transmission in the bleached state and Tc is
the transmission in the colored state. Tc and Tb values are
measured at a nominated wavelength (typically the wavelength
producing the maximum optical density change, DODl
¼
max
log(Tb/Tc)l ). DQ is the charge density, the charge ingress/
max
egress divided by the geometric electrode area of the polymer.
CE is expressed in units of cm2 Cꢀ1. The coloration efficiencies
of PMOTTP were found to be 76.9 cm2 Cꢀ1 at 430 nm and
249 cm2 Cꢀ1 at 1550 nm during the p-doping process, whereas the
values for PBOTTP were 99 cm2 Cꢀ1 at 435 nm and 259 cm2 Cꢀ1 at
1500 nm.
Conclusions
Fig. 8 (a) Electrochromic switching, percent transmittance change
monitored at 430 and 1550 nm for PMOTTP between 0 and 1.2 V. (b)
Electrochromic switching, percent transmittance change monitored at
435 and 1500 nm for PBOTTP between 0 V and 1.25 V.
Novel D–A type monomers based on [1,2,5]thiadiazolo[3,4-c]
pyridine (PT) as the acceptor unit and alkoxy thiophene as the
donor as well as their corresponding polymers have been
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Acknowledgements
This work was nancially supported by the National Natural 24 G. E. Gunbas, A. Durmus and L. Toppare, Adv. Funct. Mater.,
Science Foundation of China (51473074, 31400044), and the
2008, 18, 2026–2030.
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