Neutrally colourless, transparent and thermally stable polynorbornenes via ring-opening metathesis polymerisation for near-infrared electroactive applications

Article abstract of DOI:10.1039/c1jm10296f

Two novel near-infrared (NIR) electrochromic polynorbornenes, poly(NBDTPA) and poly(HNBDTPA), containing electroactive chromophores, were prepared via ring-opening metathesis polymerisation from a new norbornene derivative (NBDTPA) using Grubbs' catalysts

Full text of DOI:10.1039/c1jm10296f

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PAPER
Neutrally colourless, transparent and thermally stable polynorbornenes via
ring-opening metathesis polymerisation for near-infrared electroactive
applications†
a
b
a
a
Wei-Ren Lian, Kun-Li Wang, Jyh-Chiang Jiang, Der-Jang Liaw, Kueir-Rarn Leec and Juin-Yih Laic
*
Received 20th January 2011, Accepted 25th March 2011
DOI: 10.1039/c1jm10296f
Two novel near-infrared (NIR) electrochromic polynorbornenes, poly(NBDTPA) and poly
(HNBDTPA), containing electroactive chromophores, were prepared via ring-opening metathesis
polymerisation from a new norbornene derivative (NBDTPA) using Grubbs’ catalysts followed by
hydrogen reduction. The glass transition temperatures (Tg) of poly(NBDTPA) and hydrogenated poly
(HNBDTPA) were 141 ꢀC and 91 ꢀC, respectively. The 10% weight-loss temperatures of hydrogenated
poly(HNBDTPA) and poly(NBDTPA) were up to 440 ꢀC and 397 ꢀC, respectively. Hydrogenated poly
(HNBDTPA) film showed excellent transparency (up to 91%). Poly(NBDTPA) showed a cyclic
voltammogram and electrochromic behaviour that was similar to poly(HNBDTPA). The cyclic
voltammogram of a poly(HNBDTPA) film cast onto an indium-tin oxide (ITO)-coated glass substrate
exhibited three reversible oxidation redox couples at 0.63, 0.82 and 1.23 V vs. Ag/Ag+ in acetonitrile
solution. The electrochromic characteristics of poly(HNBDTPA) showed excellent stability and
reversibility, with multi-stage colour changes from its colourless neutral form to green, light-blue and
dark-blue with applied potentials of 0 to 1.75 V. The colour switching time and bleaching time of the
poly(HNBDTPA) were 7.1 s and 4.2 s in the near-infrared region (1230 nm), and 7.4 s and 4.5 s in the
UV-vis region (420 nm), respectively.
oxides typically require more than 60 s to switch states.14
Introduction
Polymers have easier processing and higher film quality than
Electrochromism is observed in reversible redox systems that
small organic molecules,15 but traditionally, most polyimides
exhibit significant colour changes in different oxidation states.1
and polyamides have high melting or softening temperatures
The construction of organic molecules that contain multiple
and are insoluble in most organic solvents because of the
redox-active chromophores is very important for the prepara-
strong interchain interactions. These properties make them
tion of multielectrochromic materials that respond to different
generally difficult to process, thus limiting their applica-
potentials with a variety of colours.2,3 Electrochromism has
tions.16,17 In addition, polyimides exhibit intrinsic colour due to
been reported in redox-active inorganics (tungsten oxide),4
the strong charge transfer complex (CTC). Coloured polymers
organic small molecules (viologen)5 and polymer materials such
are disliked as electrochromic materials, and native colour is
as conjugated polymers,6–9 polyamides10 and polyimides.11,12
one of the disadvantages for practical use in optical devices. In
However, inorganic electrochromic materials suffer from pro-
general, electrochromic polymers that are colourless are
cessing and performance problems;13 for example, inorganic
important but not common.18,19 The large transmittance
difference between colourless and coloured states is desirable
for electrochromic materials because they can exhibit high
aDepartment of Chemical Engineering, National Taiwan University of
contrast.
It is well known that hydrogenated polynorbornene deriva-
Science and Technology, Taipei, 10607, Taiwan. E-mail: liawdj@mail.
ntust.edu.tw; liawdj@gmail.com; liawdj@ntu.edu.tw; liawdj@yahoo.com.
tw; Fax: +886-2-23781441 or +886-2-27376644; Tel: +886-2-27376638
or +886-2-27335050
bDepartment of Chemical Engineering and Biotechnology, National Taipei
University of Technology, Taipei, 10608, Taiwan
cR&D Center for Membrane Technology, Department of Chemical
Engineering, Chung Yuan University, Chung-Li, 32023, Taiwan
† Electronic supplementary information (ESI) available: Synthetic
procedures, NMR spectra, UV and PL spectra, optimised structures,
electron density contours, atomic charge distribution, and charge
differences. See DOI: 10.1039/c1jm10296f
tives are colourless, amorphous and readily solution-processed
polymers with high thermal stability and optical transparency.20–22
Ring-opening metathesis polymerisation (ROMP) offers an
effective method to polymerise norbornene monomers with
different side chain substituents.23 Grubbs’ ruthenium (Ru) car-
bene complexes efficiently catalyse ROMP under ambient
conditions, with high tolerance toward polar functional groups.24
Thus, ROMP using a Ru-based catalyst is a promising route to
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prepare polynorbornenes and hydrogenated polynorbornenes,
which are colourless, transparent and easily solution-processed.
Triphenylamine (TPA) derivatives show excellent thermal and
electrochemical stabilities, electron-donating abilities and
optoelectronic properties.25 They have been used as advanced
materials for polymer memories,26–32 electroluminescence,33
chemosensor applications,34,35 and electro-optical devices.36
However, oxidation of TPA may lead to undesired radical cation
dimerisation of the TPA moieties to produce tetraphenylbenzi-
dine.37 The addition of electron-donating substituents, such as
the alkyl group at the para-position of the phenyl group of the
TPA structure, could prevent coupling reactions and lead to
stable colouration.38 Because of a noticeable change of colour-
ation, TPA derivatives are regarded as good electrochromic
materials.39–42 On the other hand, near-infrared (NIR) electro-
chromic materials are receiving attention due to their potential
applications in optical communications,43 biomedicines,44 and
camouflage materials in warfare.45 To fabricate a flexible elec-
trochemical device, NIR-colouring organic materials are
needed.46 Materials exhibit electrochromic characteristics both in
the near-infrared and visible light regions, which could be used in
widespread applications. To the best of our knowledge, there
has been no report on the near-infrared electrochromism of
polynorbornene derivatives with pendant electroactive
chromophores.
solvent, tetrahydrofuran, was dried over sodium metal and
distilled before use.
Synthesis of N-(4-bromophenyl)-4-(2-phenylpropan-2-yl)-N-(4-
(2-phenyl- propan-2-yl)phenyl)benzeneamine (Scheme 1, TPA).
N-(4-Bromophenyl)-4-(2-phenylpropan-2-yl)-N-(4-(2-phenyl-
propan-2-yl)phenyl)benzeneamine was prepared from bis[4-
(2-phenyl-2-propyl)phenyl]amine and 1,4-dibromobenzene
(see detailed procedures in the ESI†). Yield: 62%. White solid.
1H NMR (CDCl3): d (ppm) ¼ 1.72 (s, 12H, Hd); 6.96–6.98 (d,
2H, Hg); 6.99–7.01(d, 2H, Hf); 7.13–7.16 (d, 2H, He); 7.20–
7.25 (m, 2H, Ha); 7.31–7.35 (m, 6H, Hc + Hh + Hb). 13C NMR
(CDCl3): d (ppm) ¼ 30.7 (C6), 42.4 (C5), 114 (C7), 123.7 (C9),
124.6 (C12), 125.5 (C1), 126.7 (C3), 127.6 (C8), 127.9 (C2),
131.9 (C13), 144.7 (C10), 145.4 (C11), 147.0 (C4), 150.5 (C14).
Synthesis of the norbornene monomer containing electroactive
chromophore (Scheme 1, NBDTPA). Yield: 50%. White solid. See
detailed procedures in the ESI†. 1H NMR (THF-d8): d (ppm) ¼
0.62–0.64 (Hn6; 1H), 1.21–1.22 (Hn7 and Hx7; 2H), 1.35–1.36
(Hn7 and Hx7; 2H), 1.42–1.44 (Hx1; 1H), 1.62–1.64 (H13; 24H),
1.78–1.80 (Hx6, Hx5 and Hx6; 3H), 1.86–1.88 (Hn6; 1H), 2.54–
2.56 (Hn5; 1H), 2.71–2.73 (Hn1; 1H), 2.80–2.81 (Hx4; 1H),
2.84–2.86 (Hn4; 1H), 3.35–3.42 (Hn8; 2H), 3.68–3.76 (Hx8; 2H),
5.71–5.72 (Hn3; 1H), 6.01–6.02 (Hx2 and Hx3; 2H), 6.10–6.11
(Hn2; 1H), 6.90–6.96 (H9, H12 and H10; 16H), 7.06–7.08 (H11 and
H16; 12H), and 7.10–7.26 (H15 and H14; 16H). 13C NMR (THF-
d8): d (ppm) ¼ 25.86 (Cx5), 26.33 (Cx6), 31.17 (Cn6), 31.18 (C13),
31.26 (Cn7), 38.96 (Cn5), 43.01 (Cx4), 43.19 (C21), 43.25 (Cx1),
45.88 (Cn1), 45.93 (Cn4), 50.21 (Cx7), 57.96 (Cn8), 68.26 (Cx8),
122.35 (C9), 123.56 (C12), 126.27 (C16), 126.39 (C10), 127.52 (C14),
128.26 (C11), 128.72 (C15), 133.71 (Cn3), 137.33(Cx2) 137.40 (Cx3),
138.08 (Cn2), and 142.27, 142.37, 145.25, 147.02, and 151.86 (C17,
C18, C19, C20, and C22).
In this report, colourless, transparent polynorbornene deriv-
atives containing propeller-shape triarylamines were prepared
with electron-donating pendent propyl phenyl groups, high
molecular weights and low polydispersity indices via ROMP and
hydrogenation to afford saturated polynorbornenes. The intro-
duction of the electron-donating propyl phenyl substituent can
prevent coupling reactions and lower oxidation potentials.7 In
addition, the 2,2-diphenyl propane structure in the polymer
chain can enhance thermal stability and chemical resistance.47,48
The basic properties of saturated and saturated polynorbornene
derivatives such as solubility, thermal properties, optical prop-
erties, electrochemical properties and electrochromic properties
are discussed in this investigation.
Preparation of poly(NBDTPA) (Scheme 2). Yield: 85%. Col-
ourless solid. See detailed procedures in the ESI†. 1H NMR
(THF-d8): d (ppm) ¼ 1.50–1.55 (H13), 1.14–3.34 (H1 and H4–H7),
3.70–3.80 (H8), 5.14–5.36 (H2 and H3), 6.72–7.16 (H9- H12 and
H14–H16). 13C NMR (THF-d8): d (ppm) ¼ 31.84 (C13), 37.13–
42.23 (C1 and C4-–C7), 43.21 (C21), 55.21 (C8), 122.82–128.73
(C2, C3, C9–C12 and C14–C16), 142.23–151.95 (C17–C20 and C22).
Experimental section
Materials
Hydrogenation of poly(NBDTPA) (Scheme 2). Yield: 86%.
1
Colourless solid. See detailed procedures in the ESI†. H NMR
Catalysts Ru (G1), Ru (G2) and Ru (G3) were obtained from
Sigma-Aldrich. The synthesis of 5-(aminomethyl)bicyclo[2.2.1]
hept-2-ene (NBMA, b.p. ¼ 60–61 ꢀC/11 mmHg) was accom-
plished via the Diels–Alder reaction of freshly cracked cyclo-
pentadiene and allyl amine according to the literature.49,50
Dicyclopentadiene and allyl amine were purchased from Merck.
Palladium acetate, sodium tert-butoxide, tri-tert-butylphosphine
and 1,4-dibromobenzene were purchased from Aldrich and
used as received. Bis[4-(2-phenyl-2-propyl)phenyl]amine was
purchased from Ouchi Shinko and was recrystallised from
ethanol (106–108 ꢀC). 2,6-Di-tert-butyl-4-methylphenol, p-tol-
uenesulfonylhydrazide and 1,10-bis(diphenylphosphino)ferro-
cene (DPPF) were purchased from Acros. Solvents such as
xylene and benzene were purified by typical procedures and
handled under a moisture-free atmosphere. The polymerisation
(THF-d8): d (ppm) ¼ 1.11–3.43 (H1–H7), 1.56 (H13), 3.71–3.76
(H8), 6.77–7.30 (H9–H12 and H14–H16). 13C NMR (THF-d8):
d (ppm) ¼ 21.10–21.45 (C2 and C3), 31.43 (C13), 37.13–42.11 (C1
and C4–C7), 43.87 (C21), 54.81 (C8), 123.08–130.25 (C9–C12 and
C14–C16), 142.27–152.15 (C17–C20 and C22).
Measurements
The 1H and 13C NMR spectra were recorded on a Bruker DRX-
500 instrument operating at 500 MHz for protons and 125 MHz
for carbons. The weight-average (Mw) and number-average (Mn)
molecular weights were determined by gel permeation chroma-
tography (GPC). Five Waters (Ultrastyragel) columns 300 ꢁ 7.7
3
4
5
ꢀ
mm (guard, 500, 10 , 10 , 10 A in a series) were used for GPC
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Scheme 1 Synthesis of the norbornene monomer NBDTPA, containing triphenylamine structures.
Scheme 2 Preparation of the polynorbornenes poly(NBDTPA) and poly(HNBDTPA).
analysis with tetrahydrofuran (THF) (1 mL minꢂ1) as an eluent.
The eluents were monitored with a refractive index detector (RI
2000). Polystyrene was used as a standard. The melting
temperature (Tm) and glass transition temperature (Tg) were
measured on a DuPont 9000 differential scanning calorimeter
(TA Instruments TA 910) at a heating rate of 10 ꢀC minꢂ1 under
a steady flow of nitrogen. Thermogravimetric data were obtained
on a Perkin-Elmer TG/DTA (Diamond TG/DTA). Experiments
were carried out on approximately 3–5 mg samples at a heating
rate of 10 ꢀC minꢂ1 under nitrogen or air flowing conditions (20
cm3 minꢂ1). UV-vis spectra of the polymer films or solutions were
recorded on a JASCO V-550 spectrophotometer at room
temperature in air. Photoluminescence spectra were measured
with a Horiba Jobin Yvon FluoroMax-3 spectrofluorometer.
Cyclic voltammetry (CV) was performed with CHI model 619A
with indium-tin oxide (ITO) was used as a working electrode and
a platinum wire as an auxiliary electrode at a scan rate of 50 mV
sꢂ1 against an Ag/Ag+ reference electrode in a solution of 0.1 M
tetrabutylammonium
perchlorate
(TBAP)–acetonitrile
(CH3CN). The spectroelectrochemical cell was composed of a 1-
cm cuvette, ITO as a working electrode, a platinum wire as an
auxiliary electrode, and an Ag/Ag+ reference electrode. The
absorption spectra in the spectroelectrochemical analysis were
measured with a UV-vis-NIR spectrophotometer.
Results and discussion
Synthesis and characterisation
1
The H and 13C NMR as well as two-dimensional NMR tech-
niques were used to identify the structures of NBDTPA. The
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Table
(NBDTPA) polymerised by Ru (G1), Ru (G2), and Ru (G3)
1
Molecular weights and thermal properties of the poly
Yield (%)
Mn (GPC)
10ꢂ5
ꢁ
Tg (ꢀC)
a
b
c
Ru catalyst
Ru (G1)
Ru (G2)
Ru (G3)
20
80
85
1.33
3.52
3.91
141
141
143
a
Yield was determined gravimetrically: ratio [monomer]/[catalyst] ¼
b
1000, temperature: 30 ꢀC, reaction solvent: THF. Determined by gel
permeation chromatography (GPC) in THF using polystyrene as
a standard. c Tg was determined by DSC at a heating rate of 10 ꢀC minꢂ1
.
ROMP of NBDTPA using the Ru (G1) was attempted but
suffered from the drawbacks of poor initiation and lower
activity.51 Ru (G2) exhibited good activity to the polymerisation
of NBDTPA, but gave polymers with uncontrolled molecular
weight and broad PDI due to the slower initiation and faster
propagation rates.52 It is widely known that Ru (G3) exhibits
better control due to faster initiation and inhibits side reactions
like back-biting.53 Ru (G3) promoted higher activity than Ru
(G1) and Ru (G2) initiators. In Table 1, the number-average
molecular weights of poly(NBDTPA) obtained by Ru (G1), Ru
(G2) and Ru (G3) were, respectively, 1.33, 3.52 and 3.91 ꢁ 105
with yields of 20, 80 and 85%.
Fig. 1 1H NMR spectrum of NBDTPA in THF-d8.
Basic properties
Poly(NBDTPA) is not indefinitely stable in air. When stored
under ambient conditions for weeks, poly(NBDTPA) became
insoluble, while the polymers stored under N2 were still soluble
because of the oxidation of unsaturated C]C double bonds in
poly(NBDTPA).54 Poly(NBDTPA) and poly(HNBDTPA) are
both highly soluble in common organic solvents such as toluene,
xylene, chlorobenzene, tetrahydrofuran (THF), benzene and
1,2-dichlorobenzene, but they are partially soluble in dimethyl
sulfoxide (DMSO), N,N-dimethylacetamide (DMAc), N,N-
dimethylformꢀamide (DMF) and N-methyl-2-pyrrolidinone
(NMP) at 55 C.
ratio of NBDTPA endo- and exo-isomers was calculated to be
4 : 1 (Fig. 1). As the polymerisation proceeded, the vinylic proton
peaks of NBDTPA at 5.72–6.11 ppm disappeared in the 1H
NMR spectrum of poly(NBDTPA) (see ESI†). The vinyl proton
peaks appeared as broad signals between 5.14 and 5.36 ppm due
1
to the double bond in the polymer main chain. From the H
NMR spectrum, the resonance signals of olefinic protons of poly
(NBDTPA) between d (ppm) 5.14 and 5.36 completely dis-
appeared in Fig. 2. The aromatic resonance signals of poly
(HNBDTPA) between d (ppm) 6.77 and 7.25 still appeared,
which indicated the remaining triphenylamine side groups.
The UV-vis absorption of NBDTPA and poly(NBDTPA) both
exhibit strong absorption at 311 nm in THF solution. However,
there are no UV-vis absorption bands for NBMA in the range
250–350 nm. Thus, the UV-vis absorption at 311 nm of poly
(NBDTPA) can be assigned to a p–p* transition resulting from
the conjugated triphenylamine structure,8a and is not related to
cyclic norbornene structure (see Fig. S6†). The PL spectra of
NBDTPA and poly(NBDTPA) in THF solutions both show
a maximum band at 413 nm. The optical absorption and emission
spectra of poly(HNBDTPA) are similar to poly(NBDTPA). In
Fig. 3, the hydrogenated poly(HNBDTPA) film had excellent
transparency (up to 91%; thickness 3 mm). The onset wavelength
of poly(NBDTPA) in the solid state from the UV-vis trans-
mittance spectrum was approximately 392 nm, at which the
energy gap of poly(NBDTPA) is estimated to be 3.1 eV.
Thermal properties
In Table 1, the poly(NBDTPA) norbornenes obtained by the
three different Grubbs’ ruthenium catalysts had glass transition
Fig. 2 1H NMR spectrum of poly(HNBDTPA) in THF-d8.
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showed three reversible oxidation redox couples with E1/2 values
of 0.59, 0.77 and 1.26 V. Poly(HNBDTPA) exhibited three
similar reversible oxidation redox couples at E1/2 values of 0.63,
0.82 and 1.23 V. Both poly(NBDTPA) and poly(HNBDTPA)
showed colour changes from colourless to green then to light-
blue and finally to dark-blue at applied potentials of 0.90, 1.20
and 1.50 V in the oxidative scan, respectively. From the oxida-
tion potential relative to ferrocene/ferrocenium, which can
correspond to ꢂ4.8 eV for ferrocene below the vacuum level,58
the HOMO energy level of poly(NBDTPA) can be calculated
approximately. The HOMO energy level and LUMO energy
level of poly(NBDTPA) were ꢂ4.75 eV and ꢂ1.59 eV,
respectively.
Theoretical study of polynorbornene derivatives for oxidation
mechanism
Fig. 3 Transparent spectra and film states (thickness approximately 3
mm) of hydrogenated poly(HNBDTPA).
The electrochromic phenomenon is based on the redox behav-
iour of the polymer, that is, the removal of electrons from the
polymer and capture of electron by the polymer. A mechanism
based on molecular orbital theory has been reported.7,9–11 The
first electron is removed from the HOMO of the polymer to form
a single occupied molecular orbital (SOMO), and the second
electron is removed from the SOMO to form the second oxida-
tion state, and so on. All theoretical calculations in this study
were carried out using the quantum mechanical package
Gaussian 0359 based on the molecular orbital approach. The
equilibrium structure for the polymer was determined using DFT
with the B3LYP functional and the 6-31G* basis set. The
structure and optimised structure of the model compound (M1)
by B3LYP/6-31G* were chosen to study the electrochemical
behaviour of poly(NBDTPA) (see Fig. S7†). The electronic states
of the neutral structure and oxidised structures of M1 were
simulated, and the main results are summarised in Table 2. The
detailed distribution is in the Table S1†. The main atomic charge
differences were located on the 25N, 33C, 46N, 54C, 73C, 84C,
107N, 115C, 125C, 153C and 164C atoms. For the first oxidation
(losing the first electron), the 25N, 33C, 46N, 54C, 73C, 84C,
107N, 115C, 125C, 153C and 164C atoms contributed 11.3%,
4.7%, 6.8%, 4.0%, 0.6%, 3.7%, 6.0%, 4.0%, 3.9%, 0.7% and 0.7%
of an electron, respectively. For the second oxidation, the 25N,
33C, 46N, 54C, 73C, 84C, 107N, 115C, 125C, 153C and 164C
atoms contributed 3.3%, 1.3%, 5.4%, 4.1%, 1.0%, 4.0%, 6.0%,
4.1%, 4.3%, 0.9% and 1.0% of an electron, respectively. For the
third oxidation, the 46N, 54C, 73C, 84C, 107N, 115C, 125C,
153C and 164C atoms contributed 2.0%, 3.0%, 3.3%, 3.0%, 1.9%,
2.8%, 3.1%, 2.9% and 3.5% of an electron respectively, whereas
the 25N and 33C received 1.0% and 2.7% of an electron. The
simulated electron density contour of the ground state and oxi-
dised states (see Fig. S8) illustrate the electron density distribu-
tions of the HOMO (ground state) and SOMO (first oxidised
state) of M1. They were mainly located on the di(triphenyla-
mino)amine moiety. However, the electron density of the
HOMO state is more concentrated on the diphenylamine struc-
ture close to the cyclic norbornene unit, while that of the SOMO
state is slightly concentrated on the two triphenylamine moieties
far from the cyclic norbornene unit. It is obvious that the elec-
tronic density contours show that the electron lone pair of the
nitrogen atoms have strong coupling with p electrons. The
temperatures (Tg) between 138 and 142 ꢀC. In many applications,
the instability of the unsaturated polymers to chemical and
thermal degradation is a concern.55,56 In Fig. 4, the unsaturated
poly(NBDTPA) and saturated poly(HNBDTPA) showed 10%
weight-loss temperatures (Td10%) at 397 and 440 ꢀC under
nitrogen, respectively. The saturated poly(HNBDTPA) is more
stable than unsaturated poly(NBDTPA), with an increase in the
ꢀ
10% decomposition temperature of approximately 43 C under
nitrogen. The glass transition temperatures (Tg) of poly
(NBDTPA) and hydrogenated poly(HNBDTPA) were 141 and
91 ꢀC, respectively. The decrease in Tg caused by hydrogenation
was approximately 50 ꢀC. Obviously, the rotation around the
hydrogenated carbon–carbon single bond increased the mobility
of the polymer chain.57
Electrochemical properties
The electrochemical behaviours of poly(NBDTPA) and poly
(HNBDTPA) were investigated by cyclic voltammetry (CV) of
films cast on ITO-coated glass substrates as the working elec-
trode in a dry CH3CN solution containing 0.1 M TBAP as the
electrolyte. Oxidative and reductive cycles of poly(NBDTPA)
and poly(HNBDTPA) are shown in Fig. 5. Poly(NBDTPA)
Fig.
(HNBDTPA) measured under nitrogen. The temperature was raised at
a rate of 10 ꢀC minꢂ1
4 TGA curves for poly(NBDTPA) and hydrogenated poly
.
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Fig. 5 Cyclic voltammograms of poly(NBDTPA) and poly(HNBDTPA) in films cast on indium-tin oxide (ITO)-coated glass substrates in CH3CN
containing 0.1 M TBAP. The scanning rate is 0.1 V sꢂ1
.
Table 2 Atomic charge distribution of selected atoms in ground state,
first, second and third oxidation states, and the charge difference of DQ1,
DQ2 and DQ3 for M1
potentials were increased from 0 to 0.90 V (Fig. 6A), a new band
appeared at 400 nm, and a broad band at 1232 nm in the NIR
region gradually increased in intensity due to the first oxidation.
When the potential was increased to 1.20 V, corresponding to the
G.S.a
Ox1b
Ox2c
Ox3d
DQ1e DQ2f DQ3g
25 N ꢂ0.4616 ꢂ0.3481 ꢂ0.3155 ꢂ0.3262 0.1134 0.0326 ꢂ0.0107
33 C
0.1356 0.1826 0.1958 0.1693 0.0470 0.0133 ꢂ0.0266
46 N ꢂ0.4456 ꢂ0.3777 ꢂ0.3233 ꢂ0.3033 0.0679 0.0544 0.0200
54 C ꢂ0.0478 ꢂ0.0080 0.0331 0.0631 0.0398 0.0410 0.0300
73 C ꢂ0.2397 ꢂ0.2338 ꢂ0.2236 ꢂ0.1904 0.0059 0.0102 0.0332
84 C ꢂ0.0462 ꢂ0.0089 0.0309 0.0604 0.0372 0.0399 0.0295
107 N ꢂ0.4458 ꢂ0.3852 ꢂ0.3250 ꢂ0.3060 0.0605 0.0602 0.0191
115 C ꢂ0.0506 ꢂ0.0111 0.0301 0.0577 0.0395 0.0412 0.0277
125 C ꢂ0.0485 ꢂ0.0096 0.0335 0.0643 0.0389 0.0431 0.0309
153 C ꢂ0.2397 ꢂ0.2331 ꢂ0.2243 ꢂ0.1949 0.0066 0.0088 0.0294
164 C ꢂ0.2394 ꢂ0.2329 ꢂ0.2220 ꢂ0.1869 0.0065 0.0109 0.0351
a
b
c
G. S: ground state. Ox1: loss of first electron. Ox2: loss of second
¼
d
electron. Ox3: loss of third electron. DQ1
e
atomic charge
f
difference of Ox1 and G.S. DQ2 ¼ atomic charge difference of Ox2
and Ox1. g DQ3 ¼ atomic charge difference of Ox3 and Ox2.
studied model compound (M2) structure and optimised structure
by B3LYP/6-31G* were plotted (see detailed data in the ESI†) to
study the electrochemical behaviour of poly(HNBDTPA). The
electrochemical behaviour of poly(NBDTPA) is similar to that of
poly(HNBDTPA). This suggests that the redox behaviours of
poly(NBDTPA) and poly(HNBDTPA) are almost the same.
This calculated result is similar to the electrochromic experi-
mental result.
Electrochromic characteristics
For the electrochromic investigations, poly(NBDTPA) and poly
(HNBDTPA) were cast on ITO-coated glass slides, and
a homemade electrochemical cell was built from a commercial
UV-visible cuvette. The cell was placed in the optical path of the
sample light beam in a UV-vis-NIR spectrophotometer. The
results for poly(NBDTPA) film (thickness of approximately 3
mm) are presented in Fig. 6 as a series of UV-vis-NIR absorbance
curves correlated to electrode potentials. When the applied
Fig. 6 Absorption spectra of poly(NBDTPA) (thickness approximately
3 mm) thin film (in CH3CN with 0.1 M TBAP as the supporting elec-
trolyte) (A) Eappl: (a) 0.00, (b) 0.80, (c) 0.90, (d) 1.00, (e) 1.10 and (f) 1.20
V. (B) Eappl: (a) 0.00, (f) 1.20, (g) 1.30, (h) 1.40, (i) 1.50, (j) 1.60 and (k)
1.70 V.
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contrast and electrochromic reversibility. The poly(HNBDTPA)
showed higher thermal stability than poly(NBDTPA), while poly
(NBDTPA) showed a higher glass transition temperature than
poly(HNBDTPA). Both polynorbornenes had higher glass
transition temperatures than general commercial poly-
norbornenes. Thus, these characteristics suggest that these col-
ourless polynorborenes are good candidates for electrochromic
materials.
Acknowledgements
The authors would like to thank the National Science Council of
the Republic of China for the financial support of this work.
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