Tuning of reversible thermochromic properties of salicylaldehyde Schiff bases through the substitution of methoxy and nitro groups

Article abstract of DOI:10.1016/j.molstruc.2019.01.012

In this study, in order to explore the new kinds of reversible thermochromic materials, four salicylaldehyde Schiff bases (BSP1-4) substituted with methoxy and nitro groups were synthesized and their reversible thermochromic properties were investigated.

Full text of DOI:10.1016/j.molstruc.2019.01.012

Accepted Manuscript
Tuning of Reversible Thermochromic Properties of Salicylaldehyde Schiff bases
through the substitution of Methoxy and Nitro Groups
Mengyu Shi, Chen Xu, Zhihong Yang, Lei Wang, Shujuan Tan, Guoyue Xu
PII:
S0022-2860(19)30012-2
10.1016/j.molstruc.2019.01.012
MOLSTR 26063
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
19 November 2018
02 January 2019
Please cite this article as: Mengyu Shi, Chen Xu, Zhihong Yang, Lei Wang, Shujuan Tan, Guoyue
Xu, Tuning of Reversible Thermochromic Properties of Salicylaldehyde Schiff bases through the
substitution of Methoxy and Nitro Groups, Journal of Molecular Structure (2019), doi: 10.1016/j.
molstruc.2019.01.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form.
Please note that during the production process errors may be discovered which could affect the
content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Tuning of Reversible Thermochromic Properties of Salicylaldehyde
Schiff bases through the substitution of Methoxy and Nitro Groups
Mengyu Shia, Chen Xua, Zhihong Yanga*, Lei Wanga, Shujuan Tana, Guoyue Xua
aCollege of Material Science and Technology, Nanjing University of Aeronautics and
Astronautics, Nanjing 210016, P. R. China.
* Email: rogeryangzhihong@163.com
Abstract
In this study, in order to explore the new kinds of reversible thermochromic
materials, four salicylaldehyde Schiff bases (BSP1-4) substituted with methoxy and
nitro groups were synthesized and their reversible thermochromic properties were
investigated. All compounds were characterized by HNMR, IR, UV-vis, TG-DSC,
and SEM techniques. Through HNMR, IR and UV-vis spectra, four target products
display different colors. The UV-Vis spectra show that the type and position of the
substituents of BSPs can control the absorption wavelength by changing the electronic
transition mode, which leads to the difference of the color of the samples. With the
increase of light colored samples, the content of the ΔE1 gradually increases until the
maximum value of 48.6. TG-DSC result indicates good thermal stability of the BSP
below 350 °C. SEM images show that the surface morphologies of four BSP samples
are different, including flake, rod and strip. The thermochromic properties of the BSP
samples were evaluated using variable temperature FT-IR spectroscopic, solid
UV-Vis spectra and response time. DFT calculations were conducted to rationalize
the optical behavior of the BSP samples in Enol and Ketone form. The results of
1

ACCEPTED MANUSCRIPT
variable temperature FT-IR support the hypothesis that the reversible thermochromic
mechanism of salicylaldehyde Schiff base is proton transfer. Among all of BSPs, the
BSP1 has the best reversible thermochromic cycle stability (ΔE2=2.6) and the shortest
response time (10s) which can be used as the new promising thermochromism
materials.
Keywords: Salicylaldehyde Schiff base, reversible thermochromism, cycle stability,
response time
1. Introduction
In recent years, thermochromic materials have attracted wide attention due to their
potential applications in industrial, medical, military and other fields [1-2]. They are
commonly used as temperature monitoring materials in the industrial field [3]. It can
be used not only as a temperature indicator coating to monitor the temperature of the
instrument [4], but also as a building coating to provide cool hue in summer and warm
tone in winter, thus reducing the energy consumption of the building environment in
any season [5-6]. Beside these, they can also be used as a temperature measurement
material in the medical field [7], for example, the clinical thermometer material and the
indication of temperature changes during the treatment. It is also reported that the
polydiacetylene microcapsule biosensor with a thermochromic effect can recognize
small molecules by the color change caused by heating, and has a strong response [8-9]
.
At the same time, the coating of thermochromic material can be used as the coating in
military technology to change the color of military equipment to match the
surrounding environment, thus reducing the probability of detection [10]. Current
2

ACCEPTED MANUSCRIPT
findings show that the synthesis of thermochromic materials is mainly concentrated
[11]
on inorganic materials such as VO2
and La1-xSrxMnO3 [12]. However, VO2 and
La1-xSrxMnO3 are not only expensive and dangerous, but also have high discoloration
temperature. In addition, our group has studied the inorganic thermochromic materials
of ZnO doped by different density Co for visible light stealth [13]. It is found that the
ZnO thermochromic materials[14] also encounter with the problems of high
discoloration temperature and long response time to heat. Therefore, the applications
of inorganic thermochromic materials has been greatly limited in the low-temperature
area.
Currently, organic thermochromic materials[15] have aroused widespread concern
because of their low discoloration temperature, short response time and variable
[16]
colors characteristics as compared to the inorganic thermochromic materials
.
Spirocyclic compounds, triphenylmethanes, fluorocarbons and Schiff bases are the
widely studied organic thermochromic materials [17-19]. Among them, because of its
ease of synthesis, great varieties in the discoloration temperatures with different
structures [20-21], and excellent performance in many fields, Schiff bases have been
attracted more and more attention in the thermochromic material area [22-23]. At
present, the Schiff bases have been widely reported in biomedical, metal complexes
[24-27]
and military applications[28]. Because Schiff bases can be used as a ligand to
combine with various metal ions, the scope of research in the biomedicine field has
been expanded[29]. For example, the combination of Schiff base with specific metal
ions can achieve molecular recognition [30-33], and Schiff base metal complexes can
3

ACCEPTED MANUSCRIPT
also act as catalysts to catalyze cell viability[34-35]. Since from the eighties of last
century, the thermochromic and photochromic properties of Schiff base[36] have been
extensively studied and many studies have shown that the thermochromic mechanism
of salicylaldehyde Schiff base is proton transfer[37-38]. The research on the
salicylaldehyde Schiff base thermochromic effect is also expected to contribute to the
development of new thermochromism materials based on the rich color and good
thermochromism properties of Schiff base.
In this work, four salicylaldehyde Schiff bases (BSPs) with different substituents
were synthesized and characterized as demonstrated in Fig. 1. The color of the
product can be effectively controlled by changing the absorption wavelength through
shifting the position of the substituent. Through the regulation of precursors, the
energy band regulation of Schiff base is realized, and the mechanism of
thermochromism is effectively revealed. At the same time, the structure and color of
BSPs, the reversible thermochromic mechanism and the properties of BSPs were
systematically studied.
2. Experimental
2.1.
Materials
The ethanol and N-N’-dimethylformamide (DMF) were purchased from Nanjing
Chemical Reagent Co., Ltd. and were used as received without further purification.
P-phenylenediamine, Salicylaldehyde (AR, 98% purity), 3-methoxysalicylaldehyde,
4-methoxysalicylaldehyde and 5-nitrosalicylaldehyde (98% purity) were purchased
from Aladdin.
4

ACCEPTED MANUSCRIPT
2.2
Methods
2.2.1 Synthesis of N, N'- Bis（salicylidene）-p-phenylenediamine (BSP1)
Synthesis of BSP1 was carried out by refluxing p-phenylenediamine and
salicylaldehyde in ethanol according to the method previously reported[27]. 10ml
ethanol solution of 1mmol (0.108g) p-phenylenediamine was stirred for 1h at 40 ºC. 2
mmol(0.244g) salicylaldehyde was dissolved in 40 ml ethanol and was slowly added
to the reaction mixture at 40 ºC and heated at 70 ºC for 4 hours. After the completion
of the reaction, the precipitate was washed three times with a large amount of ethanol,
and then the product was recrystallized with DMF. The solid was dried at 80 ºC for
24h in a vacuum oven and the resulting product was carefully collected which was the
orange-yellow solid crystallographic sample and the yield rate is around 80.0 %.
2.2.2 Synthesis of N,N'-Bis(3-methoxysalicylidene)-p-phenylenediamine (BSP2) and
N,N'-Bis(4-methoxysalicylidene)-p-phenylenediamine (BSP3)
The
N,N'-Bis(3-methoxysalicylidene)-p-phenylenediamine
(BSP2)
and
N,N'-Bis(4-methoxysalicylidene)-p-phenylenediamine(BSP3)
were synthesized
according to the methods reported in the literature[28]. The desired 2mmol(0.304g)
3-methoxysalicylaldehyde dissolved in 20 mL ethanol and the resulting mixture was
then stirred for 1h at 40 ºC. 1mmol(0.108g) p-phenylenediamine was dissolved in 30
ml ethanol and was slowly added to the reaction mixture at 40 ºC. Then mixture was
heated at 65 °C for 4 hours. The reaction mixture was washed repeatedly with ethanol
and recrystallized from DMF. The resulting product was an orange-red powder
sample, and the yield rate was around 81.0 %.
5

ACCEPTED MANUSCRIPT
The BSP3 was then prepared in the same way as BSP2. The prepared product was
the lemon yellow crystalline solid and the yield rate was around 83.0%.
2.2.3 Synthesis of N,N'-Bis(5-nitrosalicylidene) -p-phenylenediamine (BSP4)
The crude product was obtained by the above method using a similar route applied
for BSP2 and BSP3[27, 29].The reaction mixture was washed repeatedly with ethanol
and recrystallized from DMF. The product was deep yellow powder sample, and the
yield rate was around 78.0%.
2.3 Characterization of Salicylaldehyde Schiff Base (BSPs)
The 1HNMR were acquired using a spectrometer Bruker AvanceⅢ-400, operating
at 400 MHz. For the analyses, the samples were dissolved in CDCl3 at a concentration
of 10 mg /mL. Infrared absorption spectra were obtained with a Nexus 670 FTIR
spectrometer operated in the range 4000-500 cm-1. To get more information about the
structures of the BSP1-4 at different temperatures, variable temperature infrared
absorption spectra were carried out using an IR/Nicolet 6700 in the range 4000–500
cm-1, with a heating rate of 20 ºC/min. Variable temperature infrared measurements
for these four compounds were performed at 150 ºC, 155 ºC, 180 ºC and 145 ºC,
respectively. The samples were prepared in the form of pellets with KBr (sample/KBr
= 1% w/w), and a pure KBr disk was used as a reference. Electronic absorption
spectral analysis was carried out using a Shimadzu 2450 UV-Vis spectrophotometer
and the investigated wavelength were ranged from 200 to 800 nm. The 200-600 nm
spectra were obtained using four liquid samples of a same concentration with DMF as
a solvent. The 400-700 nm visible spectrum were obtained using four solid samples
6

ACCEPTED MANUSCRIPT
previously dried in an oven at 80 ºC. Approximately 2 mg of each samples were
dispersed
in
20
mg
of
barium
sulfate.
Calorimetry
(TG-DSC) analysis was measured by a Mettler Toledo TGA Analyzer (SDTA851,
Switzerland) from 25 to 500 ºC in N2 at a heating rate of 10 ºC/ min. The morphology
was observed by a Carl Zeiss SIGMA scanning electron microscope with the
accelerating voltage of 15 kV at liquid nitrogen atmosphere.
2.4 Reversible thermochromic properties
The cover glasses (20×20mm) were prepared with cleaning by ethanol. Four
samples of equal amount (10mg) on the cover glasses were placed on the heating
platform (FD-7000，0-450 ºC), and were numbered 1, 2, 3 ,4 respectively. After that,
the platform was heated up from room temperature to its corresponding
decomposition temperature. The discoloration process of four samples were observed
and recorded when four samples were removed from the heating platform and cooled
to room temperature. The chromatic aberration（ΔE1）between the original samples
and the samples after the thermochromism was measured by a Colorimeter (X-Rite).
The operation( heating and cooling) was repeated to record chromatic aberration
（ΔE2） between the color of samples at room temperature after 50 heating-cooling
cycles and the original color of the samples. The reversible thermochromism cycle
stability can be evaluated by comparing the chromatic aberration（ΔE2. The response
time includes heating discoloration time of the samples and cooling color reversible
time of the sample. This process uses a stopwatch to record the time which refers to
the time from beginning discoloration to color stabilization and the color to be
7

ACCEPTED MANUSCRIPT
reversible when cooled.
3. Results and discussion
3.1 1HNMR spectroscopy
Table 1 shows a simplified table of the 1HNMR spectra of BSPs. The BSPs show
clear chemical structure with exact assignments of hydrogen atoms from table 1. The
chemical shift of 1H in different positions of the sample molecules are shown in the
table 1. From table 1, the chemical shift of hydrogen (C-7) with about 8.57 to 9.10
ppm can be observed for the BSP1, BSP2, BSP3, and BSP4 samples, which were
belong to -CH=N- in the products. The location of the hydroxyl group on the benzene
ring is at about 13 ppm and the methoxy H in BSP2 and BSP3 appears between
3.8-3.9 ppm. In combination with the positions and area of H, the molecular structures
of BSPs(1-4) have been determined.
3.2 Infrared spectroscopy
Fig. 2 presents IR spectroscopy of BSPs. It is obvious that IR spectroscopy show
almost identical peaks at 1610-1630 cm-1, demonstrating the similar chemical
structures with BSPs. The main difference between BSPs and precursors is the newly
formed imine structure（C=N）. In contrast to the precursor, the peaks at 1610-1630
cm-1 of the products is the newly formed imine structure, which is not contained in the
precursor. The characteristic peaks at 1622, 1615, 1613 and 1625cm-1 are attributed to
the stretching vibration of the imine structure(C=N) in the BSPs[30]. The broad peaks
at around 3300cm-1 is the absorption peak generated by the associative hydroxyl
group in the BSPs. In addition, the peaks at 1305 and 1250 cm-1 come from C-O-C
8

ACCEPTED MANUSCRIPT
stretching in BSP2 and BSP3. With the conjugation with benzene ring, -NO2
symmetry and asymmetric stretching occurs at 1385 and 1522 cm-1, while C-N
stretching occurs at 835 cm-1 for BSP 4.
3.3 Electronic absorption spectra
All BSPs molecules have the same molecular skeleton with different substituents
located on the phenolic ring. The electron absorption spectra of the BSP1, BSP2,
BSP3 and BSP4 in the 200-800nm are shown in Fig.3(a-b). Although no substituents
are connected to the phenolic structures of the BSP1 molecules, there are -OCH3 and
-NO2 substituents in BSP2, BSP3 and BSP4 at different positions (ortho, meta, or
para-positions) of the phenolic structures. Therefore, the spectral properties and
molecular structure are affected by these substituents. As shown in Fig.3(a), BSP1 has
a strong absorption peak in the 270-300 nm, which is caused by the n-π* electron
transition of the imine group (-CH=N-) in the conjugated system. The absorption peak
of benzene ring appears in the 230-250 nm. However, the -OCH3 and -NO2 of BSP2,
BSP3 and BSP4 are connected to the benzene ring in the form of auxiliary chromium
or chromophore, and change the energy level state of the π orbits. So, the absorption
peaks of BSP2, BSP3 and BSP4 conjugated chains occurred red shift, corresponding
to the strong absorption peak of 300-400 nm in Fig.3 (a).
Fig.3 (b) presents the visible spectrum of BSP’s compounds. It can be seen that the
peaks of BSP1, BSP2, BSP3 and BSP4 in the visible region appear in 400-550 nm.
According to the theory of complementary colors[15],the material absorbs light of a
9

ACCEPTED MANUSCRIPT
certain color, and the material will display the complementary color of the certain
color. Combining the actual colors of BSPs, the products absorb blue-green light in
400-500 nm, which in turn show complementary color yellow to orange range. In
addition, due to the introduction of substituents, the UV-Vis absorption spectrum of
BSPs move towards the long wavelength, and its yellowness tends to decrease
(BSP3>BSP1>BSP4>BSP2).
3.4 Thermal analysis
Thermogravimetric-Differential Scanning Calorimetry curves (TG-DSC) of
salicylaldehyde Schiff bases (BSPs) are presented in Fig.4(a-d). There are
endothermic peaks in DSC curves at around 200-225°C in Fig.4(a-c). Because BSP1,
BSP2 and BSP3 undergo a phase transition at 200 ° C, 220 ° C, 225 ° C without
decomposition, this process has no decomposed weight loss platform in the TG curve.
However, the pronounced loss of weight can be observed at 370-390°C in Fig.4(a-c),
which were due to that BSP(1-3) decomposed at certain temperature condition.
Correspondingly, a remarkable endothermic peak in DSC curve is observed at
370-390°C in Fig.4(a-c). It can be seen that TG-DSC curve of BSP4 was different
from BSP(1-3), which had only one exothermic peak in DSC curves and weight loss
platform of decomposition in the TG curves at 365°C in Fig.4(d). Moreover, no other
phase transitions occurred before the BSP4 decomposition. In addition, the heat
change generated by the thermochromic process of BSPs was much smaller than the
heat change produced by the phase transition of BSPs, so the thermochromism of
BSPs was not reflected as shown in Fig.4(a-d). Therefore, it indicates that BSPs have
10

ACCEPTED MANUSCRIPT
poor thermal stability above 360° C.
3.5 Scanning electron microscopy (SEM)
In order to investigate the micromorphology of BSPs, the SEM images have been
taken on the surface of the products and are shown in the Fig.5. It can be observed
that the BSP1 has a flaky morphology, and the surface is smooth in Fig.5(a). In
Fig.5(b), the BSP2 presents rods and blocks morphology with different sizes, and the
surface is rough. The both flaky and lump shapes have been observed in Fig. 5(c) for
the BSP3. As shown in Fig.5(d), the BSP4 presents the regular and long strip-like
morphology and the surface is smooth and regular. The morphology and size of the
four products are very different.
3.6 Thermochromism and Theoretical study
3.6.1 Thermochromism
The optical images of salicylaldehyde Schiff base (BSPs) are shown in the
Fig.6(a-d). The Fig.6(a-d) present that the BSPs have the ability as a reversible
thermochromic material. The colors and temperatures of BSPs’ reversible
thermochromic are shown in Table 2. It can be seen that the salicylaldehyde Schiff
base samples have different colors in Fig.6(a-d). At room temperature, the BSP1,
BSP2, BSP3 and BSP4 are light orange, orange, lemon yellow and dark yellow,
respectively. According to results in section 3.3, the yellowness of the BSPs’ color
tends to decrease as well (BSP3>BSP1>BSP4>BSP2). During the heating process to
the temperature of 145-155 ºC, the color of BSP1-3 changed from light orange to red,
orange turned into deep orange, and the lemon yellow became orange, respectively,
11

ACCEPTED MANUSCRIPT
and the performance was restored after cooling to the room temperature. BSP4
changed color from dark yellow to light orange with heating at 180°C, and the color
of BSP4 recovered after cooling to room temperature as well. Therefore, the processes
of BSPs color changing have been proved to be the reversible thermochromism.
Combined with Table 2, BSP1-4 only have differences in color and discoloration
temperature, and the processes of color changing are similar, which are reversible
thermochromism. Moreover, the color changing of BSP3 was the largest compared
with the original color when heating, followed by the BSP1, BSP4 and BSP2.
Therefore, with the increasing of the sample light, the ΔE1 was increased.
3.6.2 Theoretical study
In order to explore the relationship between thermochromism and structure of
samples, four kinds of Salicylaldehyde Schiff base (BSPs) samples were subjected to
variable temperature infrared test to observe whether the structure of Schiff base
changed under different discoloration temperature. The results are shown in Fig.7.
Fig.7 (a-d) is the infrared spectra of four salicylaldehyde Schiff bases BSP1, BSP2,
BSP3, and BSP4 at different temperatures. It can be seen that the molecule structures
of BSPs at the discoloration temperature are different from the structures at room
temperature. The shifts of the hydroxyl peak were obvious at different the
discoloration temperatures, which were disappeared in different salicylaldehyde
Schiff bases. And the hydroxyl peak reappeared around at 3400-3500cm-1 after the
cooling. This phenomenon is caused by the proton of hydroxyl group transferring to
the nitrogen of imine when heating and called proton transfer.
12

ACCEPTED MANUSCRIPT
According to the previous study, the Thermochromism mechanism of
salicylaldehyde Schiff bases is related to the molecule structure during heating[31]
.
Under the excitation of heat energy, the hydroxyl proton of salicylaldehyde Schiff
bases transfers to the imine nitrogen, and then the enol structure is converted to a
ketone structure in Fig.8. Therefore, the absorption spectrum of the molecule changes
and the color changes. When cooled, the ketone structure is restored to enol structure
and the color is reversible, so the whole process is reversible thermochromism[25-26]
.
The results of variable temperature infrared spectroscopy support the proton
transfer hypothesis that salicylaldehyde Schiff base. Therefore, the thermochromic
mechanism of BSP1-4 is a classical proton transfer mechanism.
In order to further understand the thermochromic mechanism of BSP samples, the
molecular modeling was carried out using the density functional theory with B3LYP
using 6-31G basic sets implemented in a Gaussian 05 program. Fig. 9 gives the
optimal geometric configuration of the BSP 1 sample (lactone type) and its cationic
BSP' along with the calculated electron distribution in the frontier molecular orbitals
(MOS). In BSP1, the HOMO and LOMO are uniformly distributed throughout the
entire conjugated molecule. While in the case of BSP', the HOMO was retained on the
phenyl ring unit, and the LUMO was localized on the Keto salicylaldehyde structure
unit. Fig. 10 shows the energy of the MOs and the energy are corresponding to
electron transfer transition (Δ E=Homo-LUMO) in BSP1 sample. The results indicate
that the ΔE value of BSP' is considerably smaller than ΔE of the BSP, which is in
good agreement with the color from yellow to red and the red shift in the absorption
13

ACCEPTED MANUSCRIPT
spectra when BSP1is heated.
3.7 Reversible thermochromic properties
To better measure the thermochromic properties of samples, it can quantify the
thermochromism properties of BSPs as two indicators (cycle stability and response
time). The Fig.11(a-d) show a visible spectrum of original samples and the samples at
room temperature after heating-cooling cycle 50 times. It is obvious that the higher
the coincidence degree of the two curves, the better the reduction of the color after
cooling to the original color, and the better the cycle stability of the sample. It can be
seen that the coincidence of the two curves was high in Fig.11(a) and Fig.11(d),
indicating that the color of the sample at room temperature after heating-cooling cycle
50 times is basically same as the color of the original sample. There is a large
difference between the two curves in Fig.11(c), and the cycle stability of BSP3 is the
worst. Fig.11(f) shows the color difference of BSPs between the color at room
temperature after heating-cooling cycle 50 times and original color of the four
samples. It can be seen that the color of the BSP1 at room temperature after repeated
heating was similar to the original color, followed by BSP2, BSP4 and BSP3.
Fig.11 (e) shows a comparison of the heating discoloration response time and the
cooling discoloration response time of the BSPs. Whether it was the heating
discoloration or the cooling discoloration response time, the response time of the
BSP1 was the shortest, and the response time of BSP3 was the longest. Under the
same thermochromic mechanism, the response time of BSPs was different, and it can
be considered that this difference comes from the microscopic appearance of BSPs.
14

ACCEPTED MANUSCRIPT
Combined with the morphological analysis of the section 3.5, BSP1, BSP2 and BSP4
showed regular shape distribution, and BSP3 had stripes and flakes. Because of its
irregular microstructure, the response time of BSP3 was longer than that of other
samples when heated or cooled. Besides, the BSPs had a longer cooling discoloration
response time than heating discoloration response time. There was a hysteresis[32]
when the cooling color recovered, and the hysteresis may be related to the energy of
BSPs in the discoloration process. Combined with Fig.10, when the energy (heat) was
given, the molecule changed from a higher energy level structure to a lower energy
level structure, and protons transfer under energy excitation. Conversely, when the
energy was disappeared, the proton need spontaneously transfer from the N to O atom
and the spontaneous transfer of proton is a slow process. Therefore, the
comprehensive cycle stability and response time of sample, the BSP1, BSP2 and
BSP4 possessed good the thermochromic performance.
4. Conclusions
In this work, we successfully synthesized four kinds of salicylaldehyde Schiff Base
(BSPs) via the solution method and confirm the composition of these target products
by HNMR and IR results. It is demonstrated by UV-Vis spectra that the absorption
wavelength can be controlled by the type and location of the substituent of BSPs
through the variation of the transition mode of the electron, leading to the different
colors of samples. With the increase of light colored samples, the content of the ΔE1
increases until the maximum value of 48.6. Simultaneously, the thermochromic
mechanism of BSPs is mainly contributed by the proton transfer, which is strongly
15

ACCEPTED MANUSCRIPT
supported by the in-situ IR and DFT calculations. The reversible thermochromic
properties are mainly depended on the cycling stability and response time. The higher
coincidence degree of two visible spectrum curves after cycles, the smaller ΔE2 and
the better cycle stability of samples. Among all of BSPs, BSP1 owns the best
performance, including the smallest ΔE2 (2.6), the best cycle stability and the shortest
response time (10s). Therefore, BSPs can be expected to work as new basic skeletons
to combine with different substituents, thus producing promising compounds for the
new thermochromism materials.
Acknowledgements
This work is financially supported by the Fundamental Research Funds for the
National Natural Science Foundation of China (Grant Nos: 51602154, 51403102), and
The Natural Science Foundation of Jiangsu Province, China (Grant Nos:
BK20140811), and Fundamental Research Funds for the Center Universities (Grant
Nos: NE2018103), and a Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
16

ACCEPTED MANUSCRIPT
Reference
[1] Hennerdal, L. O.; Berggren, M., Picture-to-picture switching in full-color
thermochromic paper displays. Appl. Phys. Lett. 2011, 99, 238-240.
[2] Yarimaga, O.; Jaworski, J.; Yoon, B.; Kim, J. M., Polydiacetylenes:
supramolecular smart materials with a structural hierarchy for sensing, imaging and
display applications. Chem. Commun. 2012, 48, 2469-2485.
[3] Wang S, Xu Z, Wang T, et al. Warm/cool-tone switchable thermochromic material
for smart windows by orthogonally integrating properties of pillar[6]arene and
ferrocene[J]. Nature Communications. 2018, 9,210-212.
[4]Kinuta T, Sato T, Tajima N, et al. Solid-state thermochromism observed in
charge-transfer complex composed of binaphthol and viologen[J]. Journal of
Molecular Structure. 2010, 982(1), 45-49.
[5] Zheng, S.; Xu, Y.; Shen, Q.; Yang, H., Preparation of thermochromic coatings and
their energy saving analysis. Sol. Energy 2015, 112, 263-271.
[6] Granqvist, C. G., Electrochromics and thermochromics: towards a new paradigm
for energy efficient buildings. Mater. Today 2016, 3, S2-S11.
[7] Ibarra-Rodriguez M, Muñoz-Flores B M, Dias H V, et al. Fluorescent molecular
rotors (FMRs) of organoboron compounds from Schiff bases: Synthesis, viscosity,
reversible thermochromism, cytotoxicity and bioimaging cells[J]. Journal of Organic
Chemistry. 2017, 82(5), 2375-2378.
[8] Kulčar, R.; Friškovec, M.; Hauptman, N.; Vesel, A.; Gunde, M. K., Colorimetric
properties of reversible thermochromic printing inks. Dyes and Pigments 2010, 86,
271-277.
[9] Kusmariya, B. S.; Tiwari, A.; Naikoo, G. A.; Mishra, A. P., Spectroscopic,
thermal, non-isothermal decomposition kinetics and quantum chemical computational
studies of Ni(II)- and Cu(II)-Schiff base complexes. Res. Chem. Intermed. 2016, 43,
1671-1687.
[10] Ramadan R M, Al-Nasr A K A, Ali O A M. Synthesis, spectroscopic, DFT
studies and biological activity of some ruthenium carbonyl derivatives of
bis-(salicylaldehyde)phenylenediimine Schiff base ligand[J]. Journal of Molecular
Structure. 2018, 78,1161-1163.
[11] Chen, X.; Yoon, J., A thermally reversible temperature sensor based on
polydiacetylene: Synthesis and thermochromic properties. Dyes and Pigments 2011,
89, 194-198.
[12] Li, W.; Ji, S.; Qian, K.; Jin, P., Preparation and characterization of VO2(M)-SnO2
thermochromic films for application as energy-saving smart coatings. J Colloid
Interface Sci. 2015, 456, 166-169.
[13] Fan, D.; Li, Q.; Xuan, Y., Tailoring the solar absorptivity of thermochromic
material La0.7 Ca0.2 Sr0.1 MnO3 Quant. Spectrosc. Ra 2011, 112, 2794-2800.
[14] Zhu M, Qi H, Wang B, et al. Thermochromism of vanadium dioxide films
controlled by the thickness of ZnO buffer layer under low substrate temperature[J].
Journal of Alloys & Compounds. 2018, 740,844-851.
[15] Xu Y, Li J, Hu W, et al. Thermochromism and supramolecular chirality of the
17

ACCEPTED MANUSCRIPT
coumarin-substituted polydiacetylene LB films[J]. J Colloid Interface Sci. 2013,
400(12), 116-122.
[16] Zhu, Y.; Xu, G.; Guo, T.; Hou, H.; Tan, S., Preparation,infrared emissivity and
thermochromic properties of Co deped ZnO by solid state reaction. J. Alloy. Compd.
2017, 720, 105-115.
[17] Zhang, F.; Guan, P.; Xiaoling, H. U., Color-changing mechanism and application
development of organic reversible thermochromic materials. Mater. Rev. 2012.
[18] White, M. A.; Bourque, A., Colorant, Thermochromic. Springer New York 2013,
1-12.
[19] Chen, J. R.; Yang, D. Y., Design and synthesis of an
o-hydroxyphenyl-containing spiropyran thermochromic colorant. Org. Lett. 2009, 11,
1769-72.
[20] Wu, D.; Fang, B.; Zhang, M.; Du, W.; Zhang, J.; Tian, X.; Zhang, Q.; Zhou, H.;
Wu, J.; Tian, Y., D-A type phenanthridine derivatives with aggregation-induced
enhanced emission and third-order nonlinear optical properties for bioimaging. Dyes
and Pigments 2018, 159, 142-150.
[21] Inabe, T.; Mitani, T. T.; Maruyama, Y., Structure and optical properties of a
thermochromic Schiff base. low-temperature structural studies of the N,
N'-disalicylidene-p-phenylenediamine and N, N'. Struct. 1989, 45, 2203-2207.
[22] Moustakali-Mavridis, I.; Hadjoudis, E.; Mavridis, A., Crystal and molecular
structure of some thermochromic Schiff bases. Acta Crystallogr. 2010, 34,
3709-3715.
[23] Panja, A.; Jana, N. C.; Adak, S.; Brandão, P.; Dlháň, L.; Titiš, J.; Boča, R., The
structure and magnetism of mono- and di-nuclear Ni(II) complexes derived from
{N3O}-donor Schiff base ligands. New J. Chem. 2017, 41, 3143-3153.
[24] Wu, Y.-h.; Chen, L.; Yu, J.; Tong, S.-l.; Yan, Y., Synthesis and spectroscopic
characterization of meso-tetra (Schiff-base substituted phenyl) porphyrins and their
zinc complexes. Dyes and Pigments 2013, 97, 423-428.
[25] Mahmoud, W. H.; Omar, M. M.; Sayed, F. N., Synthesis, spectral
characterization, thermal, anticancer and antimicrobial studies of bidentate azo dye
metal complexes. J. Therm. Anal. Calorim. 2016, 124, 1071-1089.
[26] De Araujo, E. L.; Barbosa, H. F.; Dockal, E. R.; Cavalheiro, E. T., Synthesis,
characterization and biological activity of Cu(II), Ni(II) and Zn(II) complexes of
biopolymeric Schiff bases of salicylaldehydes and chitosan. Int. J. Biol. Macromol.
2017, 95, 168-176.
[27] Zhang E, Wei Y, Huang F, et al. Synthesis, crystal structure and bioactivity of
manganese complexes with asymmetric chiral Schiff base[J]. Journal of Molecular
Structure. 2017, 1155,320-329.
[28] Santhanamoorthi, N.; Senthilkumar, K.; Kolandaivel, P., Tautomerization and
solvent effects on the absorption and emission properties of the Schiff base N,N′
-bis(salicylidene)-p-phenylenediamine-A TDDFT study. Mol. Phys. 2010, 108,
1817-1827.
[29] Xu, Y.; Xue, L.; Wang, Z. G., Synthesis, X-ray crystal structures, and
antibacterial activities of Schiff base nickel(II) complexes with similar tetradentate
18

ACCEPTED MANUSCRIPT
Schiff bases. Russ. J. Coord. Chem. 2017, 43, 314-319.
[30] Wang, K.; Ma, L.; Liu, G.; Cao, D.; Guan, R.; Liu, Z., Two fluorescence turn-on
coumarin Schiff's base chemosensors for cyanide anions. Dyes and Pigments 2016,
126, 104-109.
[31] Rahman, L. A.; Farhan, A. M.; Al-Taee, A. R.; Kadhum, A. A. H.; Mohamad, A.
B., Kinetic study on chalcones and Schiff base proton transfer reactions. Asian J.
Chem. 2016, 28, 765-770.
[32] Kandori, H., Hydration switch model for the proton transfer in the Schiff base
region of bacteriorhodopsin. BBA - Bioenergetics 2004, 1658, 72-79.
[33] Akhter, S. A.; Zaman, H. U.; Mir, S.; Dar, A. M.; Shrivastava, S., Synthesis of
Schiff base metal complexes: a concise review. Eur. Chem. Bull. 2017, 6, 335-337.
[34] Xie, Y.-Z.; Shan, G.-G.; Li, P.; Zhou, Z.-Y.; Su, Z.-M., A novel class of Zn(II)
Schiff base complexes with aggregation-induced emission enhancement (AIEE)
properties: Synthesis, characterization and photophysical/electrochemical properties.
Dyes and Pigments 2013, 96, 467-474.
[35] Zaltariov, M. F.; Cazacu, M.; Avadanei, M.; Shova, S.; Balan, M.; Vornicu, N.;
Vlad, A.; Dobrov, A.; Varganici, C. D., Synthesis, characterization and antimicrobial
activity of new Cu(II) and Zn(II) complexes with Schiff bases derived from
trimethylsilyl-propyl-p-aminobenzoate. Polyhedron 2015, 100, 121-131.
[36] Zbačnik, M.; Pičuljan, K.; Parlov-Vuković, J.; Novak, P.; Roodt, A., Four
thermochromic o-hydroxy Schiff bases of α-aminodiphenylmethane: solution and
solid state study. Crystals 2017, 7, 25.
[37] Serdiuk, I. E.; Roshal, A. D., Exploring double proton transfer: A review on
photochemical features of compounds with two proton-transfer sites. Dyes and
Pigments 2017, 138, 223-244.
[38] Mehlana, G.; Wilkinson, C.; Ramon, G.; Bourne, S. A., Reversible thermo- and
mechanochromic behaviour in a 3D hydrogen bonded discrete complex. Polyhedron
2015, 98, 224-229.
19

ACCEPTED MANUSCRIPT
Figure captions:
Fig. 1 Structural representation of the salicylaldehyde Schiff bases BSPs.
Fig. 2 IR spectroscopy of the salicylaldehyde Schiff bases BSPs.
Fig. 3 (a) UV-visible absorption spectra of BSPs in DMF solvent; (b) UV-visible
absorption spectra of BSPs on solid BaSO4.
Fig. 4 TG-DSC results of BSPs.
Fig. 5 SEM images of the BSPs: (a) BSP1;(b) BSP2; (c) BSP33; (d) BSP4.
Fig.6 The reversible thermochromic properties of BSPs from room temperature to
discoloration temperature and back to room temperature: (a) BSP1; (b) BSP2; (c)
BSP3; (d) BSP4.
Fig. 7 Variable temperatures IR spectra of BSPs: (a) BSP1; (b) BSP2; (c) BSP3; (d)
BSP4.
Fig.8 Mechanism diagram of salicylaldehyde Schiff bases’ reversible
thermochromism.
Fig. 9 Optimized geometry of BSP1 and BSP1'.
Fig. 10 HOMO-LUMO orbitals of the BSP and BSP1' and their energy level diagram.
Fig.11 (a-d): the visible spectrums of original BSPs and the BSPs at room temperature
after heating-cooling cycle 50 times. (e): the heating discoloration response time and
the cooling discoloration response time of the BSPs. (f): the chromatic
aberration(△E2) of BSPs between the color at room temperature after
heating-cooling cycle 50 times and original color.
20

ACCEPTED MANUSCRIPT
Figure 1
Figure 2
21

ACCEPTED MANUSCRIPT
Figure 3
Figure 4
22

ACCEPTED MANUSCRIPT
Figure 5
Figure 6
23

ACCEPTED MANUSCRIPT
Figure 7
Figure 8
24

ACCEPTED MANUSCRIPT
Figure 9
Figure 10
25

ACCEPTED MANUSCRIPT
Figure 11
26

ACCEPTED MANUSCRIPT
Table caption
Table 1 1HNMR analysis of BSPS.
Table 2 The colors and temperatures of BSPs thermochromic.
Table 1
1HNMR signals/ ppma
Product
H-3
H-4
H-5
H-6
H-7
H-8, H-9
H-a
H-b
-
C3-H,C4-H,C5-H 7.05(d,2H) 7.40(d,2H) 6.94(t,2H) 7.43(d,2H) 8.67(s,2H) 7.37(s,4H) 13.20(s,2H)
C3-OCH3
C4-OCH3
-
7.01(d,2H) 6.91(t,2H) 7.04(d,2H) 8.68(s,2H) 7.38(s,4H) 13.61(s,2H) 3.95(s,6H)
6.52(s,2H) 7.28(d,2H) 8.57(s,2H) 7.32(s,4H) 13.75(s,2H) 3.86(s,6H)
8.45(s,2H) 9.10(s,2H) 7.35(s,4H) 13.29(s,2H)
6.50(s,2H)
-
C5-NO2
7.01(d,2H) 8.05(d,2H)
-
-
s=singlet; d=duplet; t=triplet; H-a=signal corresponding to H of the OH group； H-b=signal
corresponding to H of the MeO group.
a For assignments see Fig. 1.
Table 2
△E1（Chromatic
aberration between
color after heating and
initial color of sample）
19.5
Sample
Initial color
The color
Temperature of
after heating
thermochromic /℃
BSP1
Light orange
Orange
Red
150
155
180
145
BSP2
BSP3
BSP4
Dark orange
Orange
14.6
48.5
14.8
Lemon yellow
Dark yellow
Light orange
27

ACCEPTED MANUSCRIPT
Research Highlights

Four salicylaldehyde Schiff Base substituted with methoxy and nitro groups are
synthesized.



Their novel reversible thermochromic properties are highlighted.
The sample shows the good cycling stability and rapid response time.
The samples can be used as a basic skeleton to combine different substituents
together to produce promising compounds for the new thermochromic materials.