From aerospace to screen: Multifunctional poly(benzoxazine)s based on different triarylamines for electrochromic, explosive detection and resistance memory devices

Article abstract of DOI:10.1016/j.saa.2019.117524

Four kinds of main-chain benzoxazine polymers (PBZ) containing triarylamine (TAA) units were synthesized by Mannich reaction and characterized by ¹H nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) techniques, etc. Thermal,

Full text of DOI:10.1016/j.saa.2019.117524

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
From aerospace to screen: Multifunctional poly(benzoxazine)s based on
different triarylamines for electrochromic, explosive detection and
resistance memory devices
a,
c,
Yan Wang a, Haiying Niu b, Qingyi Lu a, Wei Zhang a, Xin Qiao a, Haijun Niu a, , Yanhong Zhang , Wen Wang
⁎
⁎
⁎
a
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, Department of Macromolecular Science and Engineering, School of Chemical, Chemical Engineering and Ma-
terials, Heilongjiang University, Harbin 150086, PR China
b
Daxinganlingshiyan Middle School, Heilongjiang Province 16500, PR China
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, PR China
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 August 2019
Received in revised form 7 September 2019
Accepted 8 September 2019
Available online 10 September 2019
Four kinds of main-chain benzoxazine polymers (PBZ) containing triarylamine (TAA) units were synthesized by
Mannich reaction and characterized by 1H nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR)
techniques, etc. Thermal, optical, photophysical and electrochemical properties were studied. The 50% of char
residue is left in N2 at 800 °C. The polymers are soluble in common organic solvents and easily spin-coated
onto indium tin oxide (ITO) coated glass substrates. All the polymers have voltage window ranging from 0 to
1.8 V, and the colors change from yellowish to dark red when voltage is applied. Meanwhile, device assembled
from polymer exhibit significant color changes. Furthermore, the polymers also have promising potential appli-
cation in explosive detection and resistance memory devices.
Keywords:
Benzoxazine
Triarylamine
© 2019 Elsevier B.V. All rights reserved.
Explosive detection
Fluorescence
Resistance memory
1. Introduction
synthesized a great deal of different kinds of polymers containing TAA
and researched their electro-optical properties deeply [16].
Electrochromic (EC) materials have caught the attention by scien-
tists for many years [1–4]. The phenomenon that the reversible color
changes provided that adequate potential is applied on the EC material
is defined as electrochromism [5–8]. EC materials have found an in-
creasingly wide utilization in many fields, such as, displays devices, elec-
tronic paper and so on [9–12]. EC materials can be classified into two big
families including inorganic EC materials such as tungsten trioxide
(WO3), and organic or polymer EC materials such as phthalocyanines,
polythiophene (PTh), polyaniline (PAn) and so on [13–15]. In recent
years, EC materials are becoming more and more intelligent, such as,
Wang's group showed the ambient-driven actuator with a commercial
acid ionomer film which can safely contact skin. Jia's team reported
that the copolymer EC film with flexible and stretchable performances
acted like natural chameleon skin. These examples reflect
electrochromism has a broader exploration prospect. The triarylamine
(TAA) derivatives have been focused on for many years due to excellent
donor and charge transport properties in Organic Light Emitting Diode
(OLED), Organic Photovoltaic Solar Cell (OPV) and EC. Many groups
As for PBZ, due to a wide range of interesting features, such as near-
zero shrinkage [17], excellent heat resisting property, low surface free
energy [18], agile molecular design, it plays an important role in the
aeronautical astronautical technologies [19,20], refractories, insulating
materials, medicine and chemical industry. BZ is a six membered het-
erocyclic compound containing N and O, which is prepared via Mannich
reaction of phenol, amines and paraformaldehyde. PBZ can be prepared
by curing precursors successfully. In view of the advantages and the
wider application prospects of PBZ, many scientists have made many at-
tempts to combine BZ with other multifunctional materials. Liou's
group has prepared flexible PBZ films containing TAA in 2014 [21],
followed by Ishida's group reported on the successful polymerization
of fused-ring BZs and naphthoxazines in 2017 [22]. And recently PBZs
containing organic/inorganic hybrid materials incorporating silicon-
based species and carbon-based species have been explored [23].
In order to combine inherent thermal advantages of BZ with
electrooptical properties, and effectively modulate the electrooptical
properties, we designed and synthetized four kinds of main-chain BZs
polymers with good film-forming capability from selected TAA-based
diamines and naphthalene-based diamines. The relationship of
structure-property is studied, and the optical band can be tuned finely
by changing the structure of TAA. These polymers also have good EC be-
havior with high optical transmittance change.
⁎
Corresponding authors.
E-mail addresses: haijunniu@hotmail.com (H. Niu), zhangyanhong@hlju.edu.cn
(Y. Zhang), wangwen@hit.edu.cn (W. Wang).
https://doi.org/10.1016/j.saa.2019.117524
1386-1425/© 2019 Elsevier B.V. All rights reserved.

2
Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
2. Experimental section
The synthetic process of P1 is illustrated as an example. M1 (0.50 g,
1.13 mmol), 9,9-bis(4-hydroxyphenyl)fluorene (0.40 g, 1.13 mmol)
was charged into a three-neck round-bottomed flask under nitrogen at-
mosphere, then 7.2 mL toluene-ethanol mixture (Vol:Vol = 1:2) was
added and the mixture was stirred at room temperature. When it al-
most changed into homogeneous solution, paraformaldehyde was
added (0.14 g, 4.52 mmol) into the flask, then the mixture was slowly
heated to 80 °C for 48 h. After cooled to the room temperature, it was
poured into methanol slowly. The final product was filtered and purified
by Soxhlet extractor with methanol. Yield: 71.2%, yellow powder solid,
IR (KBr, cm−1): 938 (oxazine ring), 1280 (CH2 wag). 1H NMR
(400 MHz, DMSO d6, δ, ppm): 5.3–4.5 (oxazine ring), 7.8–6.5 (aromatic
ring of benzene).
2.1. Methods, materials
A detailed description of the materials, methods are shown in the
support information.
2.2. Synthesis of monomers and polymers
The synthetic routes of the monomers (M1, M2, M3, and M4) and
polymers (P1, P2, P3, and P4) are shown in Scheme 1, in which mono-
mers were synthesized in modified way according to our reported arti-
cles [24,25].
Scheme 1. Synthetic routes of the monomers M1–M4 (a) and polymers P1–P4 (b).

Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
3
P2: Yield: 73.5%. A yellow particle, IR (KBr, cm−1): 934 (oxazine
ring), 1269 (CH2 wag). 1H NMR (400 MHz, DMSO d6, δ, ppm): 5.5–4.5
(oxazine ring), 8.1–6.7 (aromatic ring of benzene).
P3: Yield: 70.5%. A light yellow particle, IR (KBr, cm−1): 940 (oxazine
ring), 1342 (CH2 wag). 1H NMR (400 MHz, DMSO d6, δ, ppm): 5.5–4.5
(oxazine ring), 8.0–6.4 (aromatic ring of benzene).
P4: Yield: 71.5%. A dark yellow particle, IR (KBr, cm−1): 940 (oxazine
ring), 1386 (CH2 wag). 1H NMR (400 MHz, DMSO d6, δ, ppm): 5.5–4.5
(oxazine ring), 8.1–6.5 (aromatic ring of benzene).
2.3. Preparation of the polymer films and WO3 film
Firstly, 0.1 g polymer sample was dissolved into N-methyl-2-
pyrrolidinone (NMP). Then, the homogeneous solution of the polymer
sample was dripped onto the 1 cm × 5 cm of ITO, followed by being uni-
formly distributed through a spin coater at a speed of 500 rpm. Finally,
the film was dried in vacuum at 100 °C for 12 h. To give a deep explana-
tion for the test, the polymer films thicknesses were tested by an inter-
ference microscope, which of P1, P2, P3 and P4 are 95 nm, 115 nm,
100 nm and 120 nm, respectively. To improve the contrast ratio, the
WO3 electrode was introduced in the EC device. By a low-temperature
sol-gel process the ECD WO3 film on ITO was obtained and the specific
preparation process was described in detail in elsewhere [26,27].
Here, WO3 film and the polymer active film were coated on two pieces
of ITO with the same area worked as cathode and the anode (thick-
nesses are 93 nm, 98 nm, respectively.).
Fig. 2. Thermogravimetric (TGA) curves of four polymers.
Fig. 2). The 10% weight loss of these polymers decomposition tempera-
tures (Td) in N2 is in the range of 410–490 °C. Also, these polymers still
remain high char yields about 56%–67% at 800 °C. Moreover, the Td of
weight loss at 5%, 10% and 20% are summarized in Table 1. As seen in
the Table 1, the order of the four polymers char yields is: P4 N P3 N P2
N P1. The most likely reason is that the naphthalene rings in P3 and P4
makes it harder for the polymers to decompose during being heated
in high temperature region. At the same time, the more the polymers
have benzene rings in the main chain, the higher they show thermal
properties.
3. Results and discussion
3.1. Characterization of monomers and polymers
The better solubility enlarges the application area of these polymers.
10 mg of polymer was dissolved in various solvents (methanol, NMP,
dimethylsulfoxide (DMSO), tetrahydrofuran (THF), dichloromethane
and the results are summarized in Table S1. As it is seen from the
table, P3 and P4 are less soluble as compared to P1 and P2 which
might be due to the presence of more benzene rings facilitating the ag-
gregation of P3 and P4. Meanwhile the molecular weights of these poly-
The four diamine monomers were obtained by previous work of our
group. NMR FT-IR spectrometers were used to confirm the structures of
monomers and polymers.
Take P4 as an example, the peaks appear at the range of 4.5–5.5 ppm
in 1H NMR spectrum which confirms the presence of oxazine ring, and
the peaks within the scope of 6.48–8.13 ppm are representing aromatic
hydrogens. The peaks at about 940 cm−1 in IR spectra (Fig. 1) are as-
cribed to oxazine vibration. From the results of IR and NMR data confirm
that the expected structure is obtained successfully. Figs. S1, S2, and S3
also confirm the proposed structures of P1, P2, and P3.
mers including number-average ( Mn ), weight-average (Mw ), and
polydispersity index (PDI) were evaluated through gel permeation
chromatography (GPC) technique. The specific data are shown in
Table 1 which indicates these polymers have different PDI, lower degree
of polymerization (DP) and medium molecular weights. A high molec-
ular weight can be obtained by polymerization according to the synthe-
sis route, however in the GPC test; some part of the polymer
precipitated during the test so that GPC results came from those with
low molecular weight which could be soluble fractions.
3.2. Thermal properties and solubility
The thermal stability plays a positive role in the application of the
material. All polymers show similar degradation profiles (shown in
Fig. 1. The 1H NMR spectrum (a) and FT-IR spectrum (b) of P4.

4
Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
Table 1
Thermal properties and molecular weights of the polymers.
Ta5%
°C
Ta20%
°C
Chair yieldb
(wt%)
PDI
DPd
c
c
Polymer code
Mw
Mn
(Da)
(Da)
P1
P2
P3
P4
419
380
372
400
576
493
521
550
56
59
63
67
7.0 × 103
7.6 × 103
8.2 × 103
8.6 × 103
4.2 × 103
5.0 × 103
5.2 × 103
5.6 × 103
1.67
1.52
1.58
1.53
8.3
8.2
8.7
8.4
a
The Td of polymers weight losses at 5% and 20% in nitrogen.
Measured at 800 °C in nitrogen.
Dispersity: Mw/Mn.
b
c
d
Degree of polymerization.
3.3. Optical properties
Table 2
Optical properties of the polymers.
The UV–vis and photoluminescence (PL) data are summarized in
Table 2. P2 and P4 exhibit strong UV–vis absorption at about 350 nm
in NMP solution which can be attributed to the π–π* transition of the
TAA seen from the Fig. 3(a) [28,29]. The large Stokes shift (133, 130,
187 and 169 nm for P1, P2, P3 and P4, respectively) from the Table 2,
we ascribe the Stokes shift to a certain energy loss in process of emission
compared to the process of excitation. The Stokes shift mainly depends
on the change of the molecular equilibrium configurations between the
ground state and excited state. During the transition, the greater is the
difference in molecular equilibrium configuration from ground to exited
Polymer
In solutiona
As film
λ (nm)
λ (nm)
Code
Absmax
PLmax
φ
PL (%)b
Absmax
Absonset
P1
P2
P3
P4
353
345
343
351
486
475
530
520
2.4
367
394
371
398
501
514
406
492
46
2.2
14
a
1 × 10−4 mol/L NMP solution for measured.
Quinine sulfate used as reference standard (φPL = 54.6%) for measured.
b
Fig. 3. UV–vis and PL spectra of P1 to P4 at room temperature in NMP solution (1 × 10−5 mol/L) (a). Photographs taken under 365 nm UV light illumination (b) and the CIE 1931 (x,y)
chromaticity diagram (c) of the these polymers. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
5
state, the greater does the emission energy lose due to the energy con-
suming of molecular vibrational, and the larger is the Stokes shift. As a
result, the larger Stokes shift of P3 and P4 compared with P1 and P2
may come from the large energy loss in the emission of conjugated
structures. According to the following formula (1), it can be seen that
there is a complex relationship between Stokes shift and μ, solvent.
The electron transition causes the redistribution of the electron cloud,
so the dipole moment of the ground state and excited state of molecules
are generally different, and the influence of solvent polarity molecules
on the ground state and excited state of molecules is different. We
may speculate that the Stokes shifts of P3 and P4 are larger due to the
larger dipole moment change between the ground state and excited
state compared to P1 and P2.
E1/2 values of P2 and P4 are 0.94 V (Eonset = 0.73 V), and 1.00 V (Eonset
= 0.53 V), respectively. It is generally believed that the conjugated
structure will have the narrower Eg because combination of the raised
HOMO level and the decreased LUMO. The half wave potentials and
onset potentials for the polymers are mainly related to the HOMO, not
LUMO. However, the narrower Eg could only come from the LUMO
drops, with HOMO rising slightly at the same time. Meanwhile, ac-
companied with different thickness of polymer films, different effec-
tive areas of the polymer on ITO, along with the tighter aggregation
of P2 and P4 than P1 and P3, the half wave potentials of P1 and P3
are lower than the others. Seen from Fig. 4(a), CV curves of the P2
and P4 exhibit a couple of redox peaks which may be attributed to
the lack or absence of electrochemical splitting resulting in stable
diradicals. On the contrary, the P1 and P3 exhibit two couples of
redox waves which can be explained by that the first oxidative
peaks can be attributed to oxidation of one of the N atom in a TPA nu-
cleus and the second step ones are resulted from a dication structure
formed from radical recombination [32,33]. Meanwhile, the color of
all polymers changes almost from yellowish to dark blue, finally
dark red.
2
2
~
~
Vabs−V F
≈
ðμE−μGÞ Δf þ Const
ð1Þ
4πε0hcr3w
The optical properties of the polymer were mainly studied through
the analysis of UV–vis spectra and PL spectra. The φPLs of the four poly-
mers in NMP solution were calculated by the following formula (2). In
the formula (2) φunk, Iunk, Aunk, ηunk, φstd, Istd, Astd and ηstd are PL quantum
efficiency, PL integral area excited at the excitation wavelength, absor-
bance intensity and corresponding refractive index of solution for the
sample (noted as unk) and reference standard (noted as std), respec-
The EHOMO and the optical band gap (Eg) are obtained by using
E
onset and the λonset, respectively. The detailed data are summarized
in Table 3 which experimental results of the highest occupied molec-
ular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels of these polymers are in the range from
−4.70 to −5.08 eV, and −1.65 to −2.67 eV respectively. TAA plays
an important role in the electronic structure and regulation of the
polymers. The difference in the position of the nitrogen atoms still
causes a difference in their redox potentials, which in turn leads to
a slight different HOMO and LUMO of the polymers. P1 and P4 have
close reduction potentials, resulting in the similar HOMO and
LUMO while P2 and P3 have large difference HOMO, LUMO due to
the large reduction potentials.
To further comprehend the influence of molecular structure on
the electronic structures and energy levels of the polymers, the
HOMO levels and LUMO levels were calculated by using Gaussian
03W program based on B3LYP/6-31G (d, p) set level, and the data
are shown in Table 3. Seen from Fig. 4(b) the electron density distri-
bution of the HOMO is mainly located on diamine moiety meanwhile
that of the LUMO is mainly located on oxazine ring [34,35]. The the-
oretical data of the HOMO and LUMO levels are in the range from
−4.65 to −4.91 eV, and −0.97 to −1.05 eV, respectively. The se-
quence theoretical data of Eg values among the four polymers is P2
b P4 b P1 b P3 which is little different from the experiment results
order of the P2 b P1 b P4 b P3. P2 has the least Eg contrast to the P3
with the largest Eg. The little deviation between theory calculation
and experiment results from CVs due to the theoretical results are
calculated from the repetition unit not from the long chain and the
impact interaction of molecules and solvent and aggregation of poly-
mers in solvent are not considered together [36]. Also, electrochem-
ical impedance spectroscopy (EIS) technique can be used to
tively. The quinine sulfate in thin sulfuric acid was used as std (φPL
=
54.6%) for measured. From the Table 2 which P2 and P4 have the stron-
ger PL peaks than these of P1 and P3 with the φPL of P1, P2, P3 and P4
being 2.4%, 46%, 2.2% and 14%, respectively. As Fig. 3(b) and
(c) indicating that P2 and P4 emit stronger bright yellow PL and dark
yellow PL, respectively. Meanwhile, P1 and P3 exhibit weak yellow-
blue PL and weak yellow-green PL, respectively. However, the λmax of
P2 and P4 exhibit nearly 20 nm red shift compared with the P1 and
P3. The above results can be explained by the rigid and conjugated
structures of P2 and P4 are more planar and longer than these of P1
and P3 [30,31]. At the same time, it can be seen from Table 1 that the
solid films of P2 and P4 generate obvious red shift relative to the dilute
solution in the absorption spectra, which may be caused by the strong
intermolecular interaction in the solid that makes the biphenyl confor-
mation tend to be planar and increases the conjugated length of the
polymers.
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Aunk
ηunk
ηstd
φunk ¼ φstd
ð2Þ
3.4. Electrochemical properties with quantum chemistry calculation
The electrochemical data can be used for calculating energy level
and energy gap. The detailed data of the CV testing are summarized in
the Table 3. The E1/2 values of P1 are 0.69 V (Eonset = 0.52 V) and
1.00 V. E1/2 values of P3 are 0.70 V (Eonset = 0.35 V) and 0.96 V. The
Table 3
Electrochemical properties and quantum calculations of the polymers.
abs
a
peak b
electroc
electroc
d
quantume
quantume
Polymer code
λ
E
E
E
Eg
E
E
LUMO
Eqguantume
onset
onset
HOMO
LUMO
HOMO
Ag/AgCl
P1
P2
P3
P4
501
514
406
492
0.52
0.73
0.35
0.53
−4.87
−5.08
−4.70
−4.88
−2.39
−2.67
−1.65
−2.36
2.48
2.41
3.05
2.52
−4.80
−4.65
−4.91
−4.79
−0.98
−0.97
−0.98
−1.05
3.82
3.68
3.93
3.74
a
λ
onset of polymer film.
Epoenasekt: onset potential of the polymer CV curve was obtained by making a tangent line on cyclic voltammetry.
b
c
The HOMO energy levels are calculated from cyclic voltammetry referenced to oxidation potential of ferrocene (4.8 eV).
d
e
EHOMO ¼ −ðEOX−E
eV þ ð−4:8Þ eV; Efiglm = 1240/λonset; ELUMO (eV) = EHOMO + Egfilm
.
1
FcÞ
Quantum theoretical calculation of these polymers.
2

6
Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
Fig. 4. CV scanning of the polymers on the ITO/glass electrode in 0.2 mol·L−1 LiClO4/CH3CN solution at a scanning rate of 50 mV·s−1 (the arrow indicating the direction of scanning) (a).
Graphics of the electron density in the frontier molecular orbitals of repetition units (b). Impendence spectra (c) of P1, P2, P3 and P4.
determine the mechanism of conductivity of ion. The impedance
data were fitted by the ZsimDemo software. From Fig. 4(c) the curves
can be divided into two parts in which the represents charge transfer
resistance of electrons and ions locate at semicircle of high frequency
region and the oblique line of low frequency band [37]. The specific
impedance data values are summarized in Table S3. Rct is a represen-
tative datum for the EC performance. From the Table S3, All the four
polymers have relatively low resistance values, and P2 has the larg-
est resistance values compared with the other three polymers,
which means P2 has the longer response time.
is reached in Fig. 5(b). However the spectrum does not change obvi-
ously because of the TAA having been completely oxidized [38]. The
EC absorption of P1, P3 and P4 are shown in Fig. S4, Fig. S5 and
Fig. S6, respectively. From Fig. 5(d), the EC mechanism of P2 is ex-
plained by that the TPA from neutral state in yellowish turns to in-
termediate state in blue and finally changes to oxidized state in
dark red which could be attributed to the electronic transfer of
TAA → TAA2+
.
In order to further investigate the stability of the polymer EC
films, it is necessary to obtain the transmittance change (Δ%T) and
the response time which are measured by repeating step potential
during the polymer films change from neutral to oxidized state
[39]. The EC materials are used as the smart windows with fast re-
sponse time, good film stability and the color efficiency (CE). In this
study the polymer films exhibit better EC reversibility after 500 s of
being reduced and oxidized at the given wavelength. It indicates
that the polymer films are very stable and the adhesion force to the
ITO substrate is very strong. Take P2 as an example, in the Fig. 5(e),
when the wavelength is set at 930 nm with applied voltage of 1.4 V
the coloring and the bleaching time are 2.3 s and 2.0 s, respectively.
The results are collected in Table S2 and the transmittance spectra
are provided in Fig. S7(a), (b), and (c).
3.5. EC characteristics
Fig. 5(a) shows a series of EC spectroelechemical curves of
polymers upon corresponding applied potentials (from 0.0 to
1.8 V). In the neutral state, there are strong absorption peaks at
the wavelengths range between 360 and 390 nm. With the potential
applied, new absorptions appear at wavelengths around 440 nm to
700 nm and 900 nm to 1100 nm, respectively. Take P2 for example,
there is a characteristic absorption at around 320 nm corresponding
to the π–π* transition of TAA moieties. Meanwhile, the color of the
polymer film is yellowish in the neutral state with high
transmittance as shown in Fig. 5(b). Accompanied with the voltage
increasing from 0 to 0.8 V, a new absorbance emerges at 480 nm in
Fig. 5(a), while the polymer film color changes from yellowish to
dark blue and the transmittance gradually decrease in Fig. 5(b).
Until the voltage is increased to 1.8 V, there is obvious absorbance
peaks appear at the range of 930 nm, and the color of the polymer
film changes dark red. At the same time, the lowest transmittance
3.6. ECD characteristics
To improve the contrast of the device before and after oxidation and
evaluate the application of the polymers, a solid-state device was as-
sembled by the active polymer film coated on ITO working as the
anode, tungsten trioxide (WO3) coated on ITO working as the cathode,

Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
7
Fig. 5. Electronic absorption spectra (a) transmittance spectra (b) and the color coordinate (c) proposed EC mechanism (d) dynamic changes of the transmittance and current (e); optical
switching (f) of P2 film in LiClO4/CH3CN solution upon application of different voltages. (For interpretation of the references to color in this figure, the reader is referred to the web version
of this article.)
lithium perchlorate (LiClO4) in mixture of polymethyl methacrylate
(PMMA) and propylene carbonate (PC) solution working as gel electro-
lyte with polyethylene terephthalate (PET) as spacer, respectively. The
solid-state device is easier to be assembled than the device with liquid
electrolyte, as well as the performance is more stable. Finally, the device
is sealed by epoxy resin. The schematic of device and photos of
electrochromic performance are shown in Fig. 6(a). The simple device
exhibits a more pronounced color change when the voltage was

8
Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
Fig. 6. Schematic of EC device with P2 electrode and pictures of color changing of device (a). UV–vis spectra (b) and change in transmittance (b) for solid-state ECD upon application of an
external voltage. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
increased from 0 to 0.6 V. The color of device changed from the initial
film light yellow to dark purple, and finally to the dark blue with three
color states. A simple optical characteristics test was carried out on the
solid-state device through an electrochemical workstation, and it can
be seen from the Fig. 6(b) that the UV–visible absorption of the solid-
state device shows noticeable change when different states are
displayed. In the meantime, the transmittance of the device is shown
in Fig. 6(c), and it is worth noting that the transmittance is nearly 50
in bleached state and is lowest in colored state of the ECD. Compared
with liquid electrolyte and single electrode of previous work [40], the
performances of this solid-state device before and after being applied
voltage are more excellent, and the color contrast of the device is
more obvious.
maximum of P2 in NMP appears at 475 nm. As the concentration of
TNT gradually increases, the PL intensity of polymer solution de-
creases. In Fig. 7(a) and (b), all the PL intensities of polymer solution
show a similar downward tendency which indicate that P2 can re-
spond to TNT and TNP selectively. Compared to P2 and P4, the PL in-
tensities of P1 and P3 are weak. Therefore, here P1 and P3 are not the
first choice for detecting explosives. The performance of P4 is shown
in Fig. S8(a) and (b). Fig. 7(c) and (d) reveals the linear relationship
of TNT or TNP concentration to quenching rate (I0/I − 1). And the
inset exhibits the linear fit at the concentration range from 0 μM to
40 μM of TNT or TNP. The mechanism of quenching is understood
by that TNT and TNP are strong electron acceptors, when PBZ is arriv-
ing to TNT or TNP, electrons flow from the donor to the acceptor.
Meanwhile, Stern-Volmer quenching constant (KSV) can be obtained
by formula (3) [42]. Take P2 as an example, according to formula,
KSVs of TNP and TNT are calculated to be 33,370 M−1 and
51,360 M−1, respectively. One can conclude that P2 has higher sensi-
tivity to TNT than others.
3.7. Explosives detection
TNT and TNP are the dangerous explosives which are damage to
security, environment [41], and health (including lung damage, and
skin irritation for human beings). So, to investigate the detection
properties of TNT and TNP sensors, the PL quenching experiments
was carried out to detect explosives by adding the TNT/TNP into
the solutions of these polymers. Take P2 for example, the emission
I0
¼ 1 þ KSV ½Qꢀ
ð3Þ
I

Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
9
Fig. 7. Changes in PL spectra of P2 (5 μg/mL) under regularly TNP (0–75 μg/mL) (a); TNT (0–60 μg/mL) (b) added. The dependence of PL intensities upon TNT (c) and TNP (d) concentration
(inset: calibration curves of Ksv. I and I0 are PL intensities with or without polymer solution respectively).
3.8. Resistance memory performance
Fig. 8(c), the electron is injected from Al electrode to the LUMO and
the hole is injected from ITO electrode to the HOMO of the P3, in
which the energy barriers are 3.29 eV and 0.11 eV, respectively. It can
be concluded that the memory device conduction process is dominated
by the hole injection energy barrier other than by the electron injection
due to the hole barrier is too high to prevent the migration of electrons.
At the negative voltage (0 to −2.2 V), the holes are gently injected from
ITO electrode into the HOMO energy of the P3. When the scan voltage is
increased to the threshold voltage, sufficient energy is supplied to the
electrons on the Al electrode to surmount the energy barrier between
Al and LUMO for injection into the active layer. At the moment, when
the voltage is further increased, since the holes and electrons have suf-
ficiently high mobility, a conductive road is formed from the Al elec-
trode to the ITO electrode, and the current passes through the road
and the device maintains the in an ON state.
These resistance memory devices are made up by a simple
sandwich-shaped structure in which the polymer film works as an ac-
tive layer, Al works as top electrode and ITO works as the bottom elec-
trode, respectively. Illustration of the resistance memory device is
depicted in Fig. 8(a). The electrical conductance memory performances
of device is showed through the current–voltage (I-V) characteristics
curves in Fig. 8(b). All the polymers show similar memory behaviors,
P3 is chosen as an example. As shown in Fig. 8(b), a typical I-V charac-
teristic curve is constituted by four steps scan with the different applied
voltage. The process is called as “write-read-erase-reread” (WRER) with
the increase of the applied voltage [43]. Firstly, the conduction process
of the “writing” describes that the resistance memory device with the
sweep comes up a sharp at −2.2 V from high resistance state (HRS) to
low resistance state (LRS) [44]. Then, during the second sweep, the de-
vice remains LRS state which is defined “reading” step in a digital mem-
ory. In the third scan during the reverse bias from 0 to 6 V, it is in HRS
state. However, the current suddenly goes downward and back to the
original low conductive state at near 4.0 V which is called “erasing”.
The fourth sweep remains “OFF” state during 0 to −8 V. I-V characteris-
tics of P1, P2 and P4 are shown in Fig. S9(a), (b) and (c), respectively.
Memory mechanism in the view of energy level is analyzed. From the
4. Conclusions
In this study, a series of new TAA-based with main-chain BZ poly-
mers were successfully facilely synthesized by Mannich reaction with
the diamines. The four polymers showed a good behavior in EC, resis-
tance memory performance and explosive detection. In EC behavior,
the polymer films showed obvious color changes that from yellowish

10
Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
Fig. 8. (a) Schematic of the memory device; (b) I-V curves of P3 and (c) energy level diagram of the ITO/P3/Al memory device.
bazole pendants: dual photoluminescence and electrochromic behavior, ACS Appl.
Mater. Interfaces 4 (2012) 1211–1218.
[2] N.E. Dov, S. Shankar, D. Cohen, T. Bendikov, K. Rechav, M. Lahav, M.E. Boom,
Electrochromic metallo-organic nanoscale films: fabrication, color range, and de-
vices, J. Am. Chem. Soc. 139 (2017) 11471–11481.
[3] R.Z. Zheng, J.Q. Zhang, C.Y. Jia, Z.Q. Wan, Y.R. Fan, X.L. Weng, J.L. Xie, L.J. Deng, A
novel self-healing electrochromic film based on a triphenylamine cross-linked poly-
mer, Polym. Chem. 8 (2017) 6981–6988.
in neutral state to dark red in final oxidized state with the voltage being
applied and the films could remain the excellent stability. Moreover,
these polymers could not only achieve explosive detection through PL
quenching mechanism, but also could be applied in digital memory de-
vices. In summary, the polymers will be promising candidates for opto-
electronic field.
[4] S.-H. Hsiao, J.-Y. Lin, Electrosynthesis of ambipolar electrochromic polymer films
from anthraquinone-triarylamine hybrids, J. Polym. Sci. Part A: Polym. Chem. 54
(2016) 644–655.
Acknowledgements
[5] W.S. Li, J.Y. Ning, M.W. Qi, Y.W. He, H. Meng, Thieno [3,2-b]thiophene-based conju-
gated copolymers for solution-processable neutral black electrochromism, Polym.
Chem. 9 (2018) 5608–5616.
This work was supported by the National Natural Science Founda-
tion of China (51773053); Heilongjiang Province Foundation for Re-
turners from Oversea (LC2018024); Heilongjiang University Graduate
Innovation Fund (YJSCX2019-078HLJU). Innovation and entrepreneur-
ship program for college students (201910212039).
[6] G.F. Cai, J.X. Wang, P.S. Lee, Next-generation multifunctional electrochromic devices,
Acc. Chem. Res. 49 (2016) 1469–1476.
[7] C. Mallet, M.L. Borgne, M. Starck, W.G. Skene, Unparalleled fluorescence of a
polyazomethine prepared from the self-condensation of an automer and its poten-
tial use as a fluorimetric sensor for explosive detection, Polym. Chem. 4 (2013)
250–254.
Declaration of competing interest
The authors declare no conflict of interest.
Appendix A. Supplementary data
[8] B. Berns, B. Tieke, Electrochromic polyiminocarbazolylenes with latent hydrogen
bonding, Polym. Chem. 6 (2015) 4887–4901.
[9] W.J. Zhang, C.R. Zhu, Z.J. Huang, C.B. Gong, Q. Tang, X.K. Fu, Electrochromic 2,4,6-
triphenyl-1,3,5-triazine based esters with electron donor-accept structure, Org.
Electron. 67 (2019) 302–310.
[10] C. Ma, M. Taya, C.Y. Xu, Smart sunglasses based on electrochromic polymers, Polym.
Eng. Sci. 48 (2008) 2224–2228.
[11] S. Beaupre, A.-C. Breton, J. Dumas, M. Leclerc, Multicolored electrochromic cells
based on poly(2,7-carbazole) derivatives for adaptive camouflage, Chem. Mater.
21 (2009) 1504–1513.
[12] S. Shao, J.J. Shi, I. Murtaza, P.P. Xu, Y.W. He, S. Ghosh, X.S. Zhu, I.F. Perepichka, H.
Meng, Exploring the electrochromic properties of poly(thieno[3,2-b]thiophene)s
decorated with electron-deficient side groups, Polym. Chem. 8 (2017) 769–784.
[13] J.C. Wu, Y.F. Han, J. Liu, Y.C. Shi, J.M. Zheng, C.Y. Xu, Electrofluorochromic and
electrochromic bifunctional polymers with dual-state emission via introducing
multiple C-H… π bonds, Org. Electron. 62 (2018) 481–490.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.saa.2019.117524.
References
[1] R. Ponnapati, M.J. Felipe, V. Muthalagu, K. Puno, B. Wolff, R. Advincula, Conjugated
polymer network films of poly(p-phenylene vinylene) with hole-transporting car-

Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 225 (2020) 117524
11
[14] D. Weng, M. Li, J.M. Zheng, C.Y. Xu, High-performance complementary
electrochromic device based on surface-confined tungsten oxide and solution-
phase N-methyl-phenothiazine with full spectrum absorption, J. Mater. Sci. 52
(2017) 86–95.
[15] C.X. Li, C.L. Liu, L. Shi, G.M. Nie, Electrochemical copolymerization of 3,4-
ethylenedioxythiophene and 6-cyanoindole and its electrochromic property, J.
Mater. Sci. 50 (2015) 1836–1847.
[31] X.F. Cheng, J.S. Zhao, C.S. Cui, Y.Z. Fu, X.X. Zhang, Star-shaped conjugated systems
derived from thienyl-derivatized poly(triphenylamine)s as active materials for
electrochromic devices, J. Electroanal. Chem. 677-680 (2012) 24–30.
[32] X.C. Ma, H.J. Niu, H.L. Wen, S.H. Wang, Y.F. Lian, X.D. Bai, W. Wang, Synthesis,
electrochromic, halochromic and electro-optical properties of polyazomethines
with a carbazole core and triarylamine units serving as functional groups, J. Mater.
Chem. C 3 (2015) 3482–3493.
[16] W.A. Cai, Q.Y. Lu, S.Z. Wang, T.D. Xiao, H.J. Niu, W. Wang, Synthesis and characteri-
zation of stable electrochromic polyimides with quinolin-8-yloxy-substituted
triphenylamine units, J. Electroanal. Chem. 801 (2017) 388–394.
[17] U. Thubsuang, S. Wongkasemjit, H. Ishida, T. Chaisuwan, Improvement in the pore
structure of polybenzoxazine-based carbon xerogels through a silica templating
method, J. Porous. Mater. 21 (2014) 401–411.
[18] N.N. Gosh, B. Kiskan, Y. Yagci, Polybenzoxazines—new high performance thermoset-
ting resins: synthesis and properties, Prog. Polym. Sci. 32 (2007) 1344–1391.
[19] X. Ning, H. Ishida, Phenolic materials via ring-opening polymerization: synthesis
and characterization of bisphenol-A based benzoxazines and their polymers, J.
Polym. Sci. Part A: Polym. Chem. 32 (1994) 1121–1129.
[20] C.H. Lin, S.L. Chang, C.W. Hsieh, H.H. Lee, Aromatic diamine-based benzoxazines and
their high performance thermosets, Polymer 49 (2008) 1220–1229.
[21] C.H. Lin, S.L. Chang, T.Y. Shen, Y.S. Shih, H.T. Lin, C.F. Wang, Flexible polybenzoxazine
thermosets with high glass transition temperatures and low surface free energies,
Polym. Chem. 3 (2012) 935–945.
[33] X.T. Wu, W. Wang, B. Li, Y.J. Hou, H.J. Niu, Y.H. Zhang, S.H. Wang, X.D. Bai, Synthesis
and electrochromic, acidochromic properties of Schiff based containing
triphenylamine and thiophene units, Spectrochim. Acta A Mol. Biomol. Spectrosc.
140 (2015) 398–406.
[34] Q.Y. Lu, W.A. Cai, H.J. Niu, W. Wang, X.D. Bai, Y.J. Hou, Novel polyamides with 5H-
dibenzo[b,f] azepin-5-yl-substituted triphenylamine: synthesis and visible-NIR
electrochromic properties, Polymers 10 (2017) 542–564.
[35] T.C. Chu, X.P. Ju, X. Han, H.M. Du, Y. Zhang, J.S. Zhao, J.H. Zhang, Synthesis and
electrochromic properties of cross-linked and soluble conjugated polymers based
on 5,8,14,17-tetrabromoquinoxaline [2′,3′:9,19] phenanthro [4,5-abc] phenazine
as the multifunctionalized acceptor unit, Org. Electron. 73 (2019) 43–54.
[36] L.-C. Lin, H.-J. Yen, Y.-R. Kung, C.-M. Leu, T.-M. Lee, G.-S. Liou, Novel near-infrared
and multi-colored electrochromic polybenzoxazines with electroactive triarylamine
moieties, J. Mater. Chem. C 2 (2014) 7796–7803.
[37] X.C. Sun, J.K. He, Y.T. Meng, L.C. Zhang, S.C. Zhang, X.Y. Ma, J. Zhao, Microwave-
assisted ultrafast and facile synthesis of fluorescent carbon nanoparticles from a sin-
gle precursor: preparation, characterization and their application for the highly se-
lective detection of explosive picric acid, J. Mater. Chem. A 4 (2016) 4161–4171.
[38] X.L. Liu, L.Q. Kong, H.M. Du, Y. Zhang, J.S. Zhao, Y. Xie, Synthesis and electrochromic
properties of electrochromic polymers based on propylenedioxythiophene,
diketopyrrolopyrrole and benzodithiophene units, Org. Electron. 64 (2019)
223–235.
[39] Y. Zhou, X.C. Liu, X.T. Jia, D.M. Chao, Synthesis and electrochromic properties of a
graphene oxide/silicon dioxide/oligoaniline interpenetrating network composite,
New J. Chem. 43 (2019) 3829–3834.
[40] Q.Y. Lu, X. Zhang, W.A. Cai, Y. Wang, C.Y. Yang, Y. Chen, Donor-acceptor conjugated
polymers containing isoindigo block for novel multifunctional materials for
electrochromic, resistance memory, and detector devices, Sol. Energy Mater. Sol.
Cells 200 (2019) 109979–109990.
[41] C. Muthu, S.R. Nagamma, V.C. Nair, Luminescent hybrid perovskite nanoparticles as
a new platform for selective detection of 2,4,6-trinitrophenol, RSC Adv. 4 (2014)
55908–55911.
[42] Z.M. Gou, Y.J. Zuo, M.G. Tian, W.Y. Lin, Siloxane-based nanoporous polymers with
narrow pore-size distribution for cell imaging and explosive detection, ACS Appl.
Mater. Interfaces 10 (2018) 28979–28991.
[22] C.R. Arza, P. Froimowicz, L. Han, R. Graf, H. Ishida, Design, synthesis, characterization,
and polymerization of fused-ring naphthoxazine resins, Macromolecules 50 (2017)
9249–9256.
[23] M.G. Mohamed, S.W. Kuo, Functional silica and carbon nanocomposities based on
polybenzoxazines, Chem. Phys. 220 (2018) 1800306–1800319.
[24] J.W. Cai, L.N. Ma, H.J. Niu, P. Zhao, Y.F. Lian, W. Wang, Near infrared electrochromic
naphthalene-based polyimides containing triarylamine: synthesis and electrochem-
ical properties, Electrochim. Acta 112 (2013) 59–67.
[25] H.J. Niu, J.W. Cai, P. Zhao, C. Wang, X.D. Bai, W. Wang, Simple approach to regulate
the spectra of novel kinds of polyazomethines containing bulky triphenylamine:
electrochemistry, electrochromism and photophysical responsive to environment,
Dyes Pigments 96 (2013) 158–169.
[26] J. Bae, H. Kim, H.C. Moon, S.H. Kim, Low-voltage, simple WO3-based electrochromic
devices by directly incorporating an anodic species into the electrolyte, J. Mater.
Chem. C 4 (2016) 10887–10892.
[27] K.L. Zhou, H. Wang, S.J. Zhang, J.T. Jiu, J.B. Liu, Y.Z. Zhang, H. Yan, Electrochromic
modulation of near-infrared light by WO3 films deposited on silver nanowire sub-
strates, J. Mater. Sci. 52 (2017) 12783–12794.
[28] W.H. Chen, K.L. Wang, W.Y. Hung, J.H. Jiang, D.J. Liaw, K.R. Lee, J.Y. Lai, C.L. Chen,
Novel triarylamine-based alternating conjugated polymer with high hole mobility:
synthesis, electro-optical, and electronic properties, J. Polym. Sci. Part A: Polym.
Chem. 48 (2010) 4654–4667.
[43] M.-C. Tsai, C.-L. Wang, C.-Y. Lin, C.-L. Tsai, H.-J. Yen, H.-C. You, G.-S. Liou, A novel
porphyrin-containing polyimide for memory devices, Polym. Chem. 7 (2016)
2780–2784.
[29] H.-M. Wang, S.-H. Hsiao, Ambipolar, multi-electrochromic polypyromellitimides
and polynaphthaalimides containing di(tert-butyl)-substituted bis(triarylamine)
units, J. Mater. Chem. C 2 (2014) 1553–1564.
[44] Y.Q. Li, M.Y. Zhou, Z.W. Yang, Y. Li, Anthraquinone-functionalized polyurethane de-
signed for polymer electrochromic and electrical memory applications, J. Mater. Sci.
53 (2018) 15600–15613.
[30] C.X. Xu, J.S. Zhao, J.S. Yu, C.S. Cui, Ethylenedioxythiophene derivatized
polynapthalenes as active materials for electrochromic devices, Electrochim. Acta
96 (2013) 82–89.